xylose is the main component of hemicellulose,and the current research on Xylose is mainly focused on the preparation of FAL,Furfuryl alcohol(FFA)and its derivatives^[10]。 Catalytic conversion of xylose to polyols is one of the most promising conversion routes due to the abundance of hydroxyl groups in xylose.Xylose can be catalyzed by acid to form a sugar intermediate(Figure 1A),which is then hydrodeoxygenated to form 1,2-PeD,in which xylose may be dehydrated to form an intermediate FAL,which is then hydrogenolyzed to Tetrahydrofurfuryl alcohol(THFA)or FFA,and finally hydrogenated to form 1,2-PeD(Figure 1b)^[20⇓~22]。

At present,there are relatively few reports on the catalytic conversion of xylose to 1,2-PeD.for noble metal catalysts,ruthenium is the active metal For the preparation of 1,2-PeD,which has high selectivity in the hydrogenation of biomass-derived products,but its high cost and limited reserves limit large-scale industrial application^[23]。 Ordomsky et al.Used Ru/C in conjunction with Amberlyst-15 acid catalyst in water/2-methyltetrahydrofuran system,the reaction conditions were 438 K and 2.5 MPa H₂pressure for 6 H,and the yield of 1,2-PeD was less than 10%,and the total pentanediol could reach about 30%^[24]。 In the reaction process,Amberlyst-15 catalyzed the dehydration of xylose to FAL intermediate,and Ru/C further promoted the hydrogenation reaction in the presence of 2-methyltetrahydrofuran,resulting in the ring opening of FAL to pentanediol.This reaction has not been systematically studied,and the yield of 1,2-PeD still needs to be improved.Wang et al.Studied the one-pot conversion of xylose to 1,2-PeD catalyzed by a metal-acid dual catalyst composed of Ru/C+Nb₂O₅^[22]。 In the catalytic process,xylose is dehydrated to produce a sugar intermediate,and the sugar intermediate is unstable and cannot be captured,so that further dehydration occurs,and FAL is generated at an acid site(Lewis acid)provided by the Nb₂O₅or is converted to 1,2-PeD under the synergistic catalysis of a hydrogenation site provided by the acid and Ru/C.The highest yield of 1,2-PeD was 19.1%in water/cyclohexane/γ-valerolactone system at 423 K and 3 MPa H₂pressure for 4 H.In this system,the hydrogenation product increases with the increase of the concentration of hydrogenation sites,and a stronger hydrogenation process can be completed,thus obtaining a higher 1,2-PeD selectivity.In addition,the catalyst with high Lewis acid sites can reduce the activation energy of xylose dehydration in the reaction process,which is also an important factor to improve the selectivity of 1,2-PeD 。

Xirong uses metal-acid bifunctional catalyst Ru/C+NbOPO₄to catalyze xylose to prepare 1,2-PeD.When acid and hydrogenation catalyst coexist,by constructing cyclohexane-water two-phase system,mass transfer resistance is introduced to promote xylose dehydration.The addition ofγ-valerolactone(GVL)to the biphasic reaction system(the solvent ratio of GVL/water was 7/3)not only effectively inhibited the hydrogenation of xylose to xylitol,but also reasonably regulated the side reaction of xylose and the hydrogenation ability of sugar intermediates^[20]。 Under the conditions of 423 K,3 MPa H₂and 4 H,the conversion of xylose increased to 100%and the selectivity of 1,2-PeD reached 31.4%,but the reaction system was complex and not widely applicable.Xu Jie et al.disclosed a method for preparing 1,2-PeD from xylose catalyzed by Rh or Pd supported metal catalyst^[25]。 The highest 1,2-PeD selectivity of 46%was obtained with RhMo/SiO₂as catalyst and water as solvent at 433 K,4 MPa H₂and 4 H.The invention not only realizes obtaining the 1,2-PeD with high selectivity in the water phase,but also reduces the reaction cost and the separation difficulty.In order to further improve the selectivity of 1,2-PeD,the added acid catalyst was changed.When the acid catalyst was solid acid Amberlyst-15,the selectivity of 1,2-PeD could reach 51%under the same other conditions.However,the reaction system of this method was complex,and there were some problems such as the use of acid catalyst,which was corrosive to equipment,and the poor stability and difficult regeneration of supported hydrogenation catalyst 。

Wang Nailiang et al.Studied a core-shell dehydration-hydrogenation bifunctional catalyst to produce diols from xylose^[26]。 In the invention,solid acid is utilized to dehydrate xylose first,and then a path reaction of active metal hydrogenation is carried out,so that the reaction of directly hydrogenating xylose into xylitol is inhibited,thereby improving the yield of dihydric alcohol;The acid and hydrogenation active components are coated by S-1 molecular sieve to improve the stability and regeneration performance of the catalyst.The core-shell Nb-Ru bifunctional catalyst,water as solvent,at 443 K and 1 MPa H₂pressure,gave 100%conversion of xylose and 80%total yield of ethylene glycol,propylene glycol and 1,2-PeD.The catalyst has an advanced structural design,and the reaction process route can efficiently convert xylose to obtain diol with a high yield 。

For non-noble metal catalytic systems,there are some problems,such as low hydrogenation activity,harsh reaction conditions,catalyst instability and easy deactivation.For this reason,Liang yuan et al.Studied the one-pot preparation of 1,2-PeD from xylose catalyzed by encapsulated catalyst in water phase^[10,21]。 The best catalytic effect was obtained with Co@NC catalyst calcined at 600°C with water as solvent under the reaction conditions of 473 K,3 MPa H₂and 3 H,and the conversion of xylose could reach more than 95%and the yield of 1,2-PeD could reach 27.4%.The activity of Co@NC catalyst did not change significantly after five cycles,and the catalyst had excellent stability.The reaction mechanism is that the linear carbon chain structure of xylose is retained,and the C—O bond is selectively cleaved to obtain 1,2-PeD^[10]。 Under the combined action of the acidic sites of the catalyst(provided by Co^δ+)and the metal hydrogenation sites,1,2-PeD can be directly obtained from xylose via selective hydrodeoxygenation(Figure 1A )^[21]。

It is known from many research reports that different active metals have different adsorption and conversion properties for different functional groups,which leads to different products obtained from different active metal reaction systems^[20]。 At present,there are many side reactions in the catalytic conversion of xylose,and the selectivity to pentanediol is not high.In the precious metal system,the metal-acid dual catalyst is mostly used to catalyze the preparation of 1,2-PeD from xylose.The xylose is dehydrated to FAL under the action of the acid catalyst(the Lewis acid provided plays an important role),and then further hydrogenated under the action of the active metal,which can reduce the production of by-product xylitol in the reaction process,thus obtaining a high yield of 1,2-PeD.In the metal-acid dual catalyst system,the combination of Ru-based hydrogenation catalyst and solid acid catalyst(such as Amberlyst-15 and Nb₂O₅)has been studied more.Ru is an active noble metal with strong hydrogenation ability,which has obvious advantages in the catalytic preparation of 1,2-PeD from xylose.Solid acid is preferred for acid catalyst because solid acid is easier to recover and separate than liquid acid,and its corrosiveness is relatively low.Among them,Amberlyst-15 acid catalyst has a unique internal structure,which allows xylose to enter the pore and react with the active site to produce FAL;The high Lewis acid site of Nb-based acid catalyst makes the conversion of xylose to 1,2-PeD show a good dehydration reaction,which inhibits the direct hydrogenation of xylose to xylitol,thus improving the selectivity of 1,2-PeD^[20]。 Although it shows high catalytic activity,the high cost,the complex reaction system and the acid used in the catalyst are corrosive to the equipment,which is not conducive to large-scale preparation.When acid and hydrogenation catalyst coexist,the complex conversion process involving—OH,C=C,C—C,C=O and other functional groups also affects the selectivity of reaction products.Compared with precious metals,non-precious metal systems are less studied.Although their cost is low,their hydrogenolysis activity is not high,and they are prone to C-C bond cleavage at high temperatures,which makes it difficult to retain long-chain alcohols such as 1,2-PeD^[10]。 Recently,It was found that non-noble metal Co@NC catalyst catalyzed xylose to produce 1,2-PeD(yield 27.4%),and the acid sites of the catalyst and the hydrogenation sites of metal Co cooperated to catalyze xylose hydrodeoxygenation to produce 1,2-PeD,indicating that metal-acid dual catalysts are still worthy of study.Although It is not as good as precious metals,it is also a major breakthrough in non-precious metals and provides a new idea.it is still necessary to further study the production process of pentanediol from xylose with high efficiency.the activity and selectivity of chemical bond hydrodeoxygenation are still the key issues.it is necessary to develop non-precious metal catalysts with simple reaction system,mild reaction conditions and high yield^[21]。

Furfural(FAL)is a very rich source,which can be prepared from raw materials rich in hemicellulose and cellulose by catalytic conversion.FAL is also the only biomass platform molecule produced industrially on a large scale,with an annual output of more than 400,000 tons,which can be converted into a variety of important chemicals and fuels^[28]。 Because FAL molecules have unsaturated C=C and C=O bonds,high value-added fine chemicals such as 1,2-PeD and 1,5-PeD can be synthesized by further catalytic hydrogenolysis^[4,6,29]。 FAL has the advantages of renewable and natural carbon skeleton.Compared with the traditional production process of pentanediol from petroleum,the hydrogenolysis of FAL to pentanediol has broad development prospects,and the reaction process is simpler^[4]。 the current methods for hydrogenolysis of FAL to pentanediol can be divided into two main types according to their reaction paths:the first is that FAL is completely hydrogenated to THFA,and pentanediol is obtained by hydrogenation after the C—O bond in the tetrahydrofuran ring of THFA is broken,in which the product is mostly 1,5-PeD(Figure 2a)^[4]; the second is that FAL hydrogenation produces an FFA intermediate,which further cleaves the C—O bond in the furan ring of FFA,and then hydrogenates to pentanediol,whose main product is 1,2-PeD(Figure 2B)^[4,27]。 All reactant atoms in the above preparation process are utilized,and the atom economy is high,which is an effective way to achieve efficient and sustainable production of 1,2-PeD and 1,5-PeD^[29]。

at present,the noble metal catalysts used in the preparation of pentanediol by FAL hydrogenolysis At home and abroad mainly focus on one or two noble metals in the VIII subgroup,such as Pt,Pd or Rh(Table 1)^{[30⇓⇓⇓~34][35⇓~37][38,39]}。 In addition,such supported noble metal catalysts with inert materials(such as SiO₂orγ-Al₂O₃,etc.)as supports,or low-valent metal oxides(such as Re,Mo,W and V,etc.)as co-catalysts have also been widely studied^[1]。

For the supported Pt catalyst system,Xu et al.Used a co-precipitation method to prepare a Pt/Co₂AlO₄bifunctional catalyst for the hydrogenation of FAL to pentanediol^[31]。 The conversion of FAL was about 100%,and the yields of 1,2-PeD and 1,5-PeD were 9.3%and 27.2%,respectively,when the reaction was carried out at 413 K and 1.5 MPa H₂for 24 H in ethanol.It was found that CoO_x(mainly Co³⁺)was responsible for the adsorption of C=C bond and the ring opening of furan ring to generate intermediate FFA,while highly dispersed Pt metal was responsible for the subsequent hydrogenation to further obtain the target product pentanediol^[31]。 Subsequently,the Pt/Co₂AlO₄catalyst was modified by the addition of Li metal,which increased the basicity of the catalyst surface,thereby inhibiting the dehydration reaction of the intermediate FFA and increasing the yields of 1,2-PeD and 1,5-PeD to 16.2%and 34.9%,respectively,under the same conditions.Moreover,the catalyst had good stability and could maintain almost the same catalytic performance in the first three cycles^[1]。 Ma et al.Reported the production of pentanediol by hydrogenolysis of FAL catalyzed by Pt/CeO₂catalyst^[30]。 The conversion of FAL was 100%,the yield of 1,2-PeD was 65%,and the yield of 1,5-PeD was low(8%)when water was used as solvent at 443 K and 1 MPa H₂pressure for 1.5 H^[1]。 It was found that water could significantly promote the ring-opening process of FFA and enhance the production of 1,2-PeD,especially under alkaline conditions,the isomerization and polymerization pathways of FFA were inhibited,resulting in high yields of 1,2-PeD^[30]。 However,water can promote the polymerization of FFA at high temperature,and carbon deposition is easy to occur on the surface of the Pt/CeO₂catalyst.Therefore,Tong et al.Further studied that the yield of 1,2-PeD was 59.9%when isopropanol was used as solvent under the mild conditions of 438 K and 1.5 MPa H₂for 4 H,and found that the catalyst was not significantly deactivated after five cycles,and its stability was significantly improved^[32]。 Mizugaki et al.explored the use of Pt/HT(basic hydrotalcite)supported catalyst to catalyze the hydrogenolysis of FAL to produce pentanediol^[33]。 The conversion of FAL and the selectivity of 1,2-PeD and 1,5-PeD were 100%,73%and 8%,respectively,at 423 K for 4 H under 3 MPa H₂pressure in isopropanol^[1]。 The catalyst showed good stability,and after three times of recycling,the activity of the catalyst and the selectivity of the main product 1,2-PeD did not lose significantly,and the yield only decreased slightly.In addition,it was found that the hydrogenolysis of FAL catalyzed by Pt/MgO and Pt/γ-Al₂O₃was not as good as that catalyzed by Pt/HT in the yield of pentanediol under the same conditions^[33]。 Recently,Bai Xiaowei et al.Prepared a multifunctional catalyst Pt/ReO_x/TiO₂with high activity by supported deposition,and explored its reaction performance for the hydrogenolysis of FAL to prepare pentanediol^[34]。 When the mass ratio of Pt/Re in the catalyst was 3∶4,the conversion of FAL was 100%,the highest selectivity of 1,5-PeD was 29.72%,and the selectivity of 1,2-PeD was 7.28%in isopropanol at 403 K and 6 MPa for 8 H.It was found that an appropriate Pt/Re mass ratio was beneficial to the cleavage of the C—O bond of THFA and the hydrogenation of furan ring to pentanediol,and the addition of acidic oxidation promoter ReO_xenhanced the hydrogenolysis ability of FAL,resulting in a higher activity of the catalyst.Therefore,Pt noble metal plays a key role in the cleavage of C—O bond in the catalytic hydrogenolysis of FAL molecule^[4]。

For supported M-Ir-ReO_x/SiO₂(M=Pd and Rh),Liu et al.Used a"one-pot"method to catalyze FAL to prepare pentanediol by adding a second metal to enhance the hydrogenation activity of the catalyst.A two-step reaction was carried out by using a Pd(0.66 wt%)-Ir-ReO_x/SiO₂catalyst.With water as the solvent,FAL was first completely converted to THFA at 313 K and 8 MPa H₂for 8 H,and then the reaction was continued at 373 K for 72 H,so that the C—O bond of the intermediate THFA was broken to 1,5-PeD,and the yield reached^[35]。 After the catalyst was reused for 4 times,the conversion of FAL and the yield of 1,5-PeD remained almost unchanged,indicating that the catalyst had good stability.Subsequently,they used the same process to prepare 1,5-PeD from FAL catalyzed by Rh-doped Ir-ReO_x/SiO₂.When the Rh-doped amount was 0.66 wt%and the FAL concentration was 50 wt%,the yield of 1,5-PeD was 71.1%^[38]; When the FAL concentration was diluted to 10 wt%,the reaction was carried out at 313 K/373 K,8 MPa H₂and 8 H/32 H,the yield of 1,5-PeD was increased to 78.2%,and the yield of 1,2-PeD was very low(1.2%).Compared with Pd-Ir-ReO_x/SiO₂,the hydrogenolysis activity of Rh-Ir-ReO_x/SiO₂for 1,5-PeD formation from THFA at high temperature is higher,and higher 1,5-PeD can be obtained from FAL in a shorter time,but the catalyst is more prone to metal aggregation during regeneration,resulting in reduced stability 。

In addition,for other supported Rh catalyst systems,Pisal et al.Catalytically hydrogenated FAL to 1,2-PeD by using a hydrothermal synthesis Rh/OMS-2 metal-base two-site catalyst^[39]。 The well-dispersed Rh provides metal sites on the catalyst and cooperates with the basic sites provided by the support OMS-2 to carry out the furan ring-opening reaction,thereby promoting diol formation.The best results were obtained when the loading of Rh was 1 wt%and methanol was used as solvent at 433 K and 3 MPa H₂for 8 H,where the conversion of FAL could reach 99.6%and the selectivity of 1,2-PeD was 87%.The catalyst showed a slight decrease in conversion after four runs,indicating good activity,selectivity and reusability^[39]。

for other supported Pd catalyst systems,Date et al.Studied supported HMS and Pd-Au bimetallic catalysts with different Pd/Au atomic ratios on SBA-15 For one-pot hydrogenation of FAL to 1,2-PeD^[36]。 It was found that the introduction of Ti improved the ring-opening pathway to form pentanediol and had a positive effect on the production of 1,2-PeD by the catalyst.Lewis acid sites and Bronsted acid sites were found on the surface of 2Pd-2Au/Ti-SBA-15 catalyst,and the concentration of acid sites was lower than that of 2Pd-2Au/Ti-HMS,which may be due to the high dispersion of metal on the surface of the support and the coverage of acid sites on the support^[36]。 The FAL conversion and 1,2-PeD selectivity of 93%and 59%,respectively,were obtained over 2Pd-2Au/Ti-SBA-15 catalyst at 433 K for 5 H under 2 MPa H₂pressure with isopropanol as solvent,while 88%FAL conversion and 51%1,2-PeD selectivity were obtained over 2Pd-2Au/Ti-HMS catalyst under the same conditions^[36]。 after the catalyst supported on HMS was reused for the third time,the conversion of FAL and the selectivity of 1,2-PeD decreased;However,the catalyst supported on SBA-15 became more stable due to the morphology of the support,which avoided the leaching of active sites and maintained the mesoporous structure,so that the selectivity to 1,2-PeD remained unchanged despite a slight decrease in conversion observed after the third cycle,indicating that the catalyst supported on SBA-15 showed the best performance in terms of stability after multiple cycles.in addition,Date et al.Prepared a Pd/(MMT-K 10)metal-acid dual-site catalyst and used it in FAL hydrogenation to prepare 1,2-PeD,and found that the hydrogenation activity of the metal and the Bronsted acid sites provided by the support montmorillonite clay(MMT-K 10)played an important role in promoting the ring opening of the intermediate FFA to form 1,2-PeD^[37]。 When the Pd loading was 3 wt%,the catalyst showed excellent performance,and FAL was almost completely converted at 493 K and 3.5 MPa H₂for 5 H with isopropanol as the solvent,and the selectivity of 1,2-PeD was up to 66%.The FAL conversion decreased slightly after the second reuse of the catalyst,which may be due to some handling loss of the catalyst in the recycle experiment;And that selectivity of 1,2-PeD also decrease,which may be due to a decrease in the density of acid sites or a decrease in the activity of the catalyst with carbon material deposite on the surface^[37]。

Compared with noble metals,the yield of non-noble metal catalysts for hydrogenolysis of FAL to pentanediol is not high enough,and some reaction conditions are more stringent.At present,there are few studies on non-noble metal catalysts,mainly focusing on Cu,Co and Ni^{[1][40,41][42,43][44]}。

For Cu-based catalysts,Adkins et al.Prepared 1,5-PeD by catalytic hydrogenolysis of FAL using a CuCr₂O₄catalyst,and only a low 1,5-PeD yield of 20%was obtained at 448 K and 10~15 MPa H₂^[40]。 the reaction conditions are harsh,and the use of copper and chromium as catalysts is not environmentally friendly,so further research on green and efficient catalysts is needed.in addition,Cui Jian et al.Studied the method of one-step hydrogenation of FAL to 1,2-PeD catalyzed by composite catalyst containing copper oxide In a continuous fixed bed^[41]。 98%conversion of FAL and 48%selectivity of 1,2-PeD can be obtained in methanol at 473 K and 8 MPa H₂.Although the cost of catalyst is low and the process is short,the reaction conditions and equipment are harsh,which limits its industrial development^[4]。

For Co-based catalysts,Sulmonetti et Al.Studied the preparation of 1,5-PeD from FAL over Cu-Co-Al catalysts^[42]。 When the catalyst was 0.25 Cu-2.75 Co-Al,FAL was almost completely converted into intermediate FFA in ethanol as solvent at 413 K under 4 MPa H₂pressure for 8 H,and 1,5-PeD with a yield of 33%could be obtained,which was slightly lower than that when FFA was used as substrate(38%),and this very small decrease may be due to the small change of substrate coverage or surface properties when aldehyde was introduced as substrate.Then,Gavila et al.Prepared CoAl-spinel nanoparticles by liquid feed flame spray pyrolysis(L-F FSP)and carried out reduction activation at different temperatures to study the hydrogenation process of FAL under mild conditions.The reduction of spinel at higher temperatures(i.e.,700 and 850℃)led to the direct formation of diols(i.e.,1,5-PeD and 1,2-PeD)from FAL^[43]。 The highest yield of 1,5-PeD of 30%was obtained with 40 mg catalyst at reduction temperature of 700°C in isopropanol as solvent at 150°C and 3 MPa H₂for 8 H.It was found that the hydrogenation/hydrogenolysis activity of FAL was attributed to the surface acidity and the formation of metallic Co nanoparticles on the surface after reduction at high temperature.Repeated experiments were performed on this catalyst,however only in the first two runs the intermediate FFA was further hydrogenated and only 9%THFA and 4%1,5-PeD were obtained.The cause of catalyst deactivation is most likely due to a combination of features such as carbon deposition,sintering of metal nanoparticles,and bulk migration/surface transformation.Although this method is relatively novel,it is still necessary to further explore the mechanism of catalyst deactivation and develop better Co-containing catalysts to improve stability and selectivity 。

For Ni-based catalysts,Shimazu's group has studied the method of hydrohydrolysis of FAL catalyzed by Ni-M(M=Y or La)to produce 1,5-PeD.Through the formation of intermediate THFA,the C—O bond of THFA is cleaved to obtain 1,5-PeD with extremely high selectivity^[44]。 When the Ni/La molar ratio was 2.5 and the hydrogen pre-reduction temperature was 573 K,100%FAL conversion and 53.5%1,5-PeD yield could be obtained with Ni-La(2.5)HT573 composite as the catalyst and isopropanol as the solvent at 423 K and 2 MPa H₂for 72 H.Recently,Kurniawan et al.Proposed a one-pot direct conversion of FAL to 1,5-PeD using a Ni-CoO_x-Al₂O₃mixed metal-metal oxide catalyst^[11]。 The conversion of FAL was 100%,the yield of 1,5-PeD was 47.5%,and the yield of 1,2-PeD was 19.2%in ethanol at 433 K and 3 MPa H₂for 6 H.The study shows that the interaction between Ni⁰and oxygen vacancy site CoO_x(O_v-CoO_x)is beneficial to the dissociative adsorption of H₂molecules on Ni⁰and the spillover of hydrogen to O_v-CoO_x.Hydrogenation of the C—O group of FAL produces FFA,which adsorbs at the Co^δ+center of the O_v-CoO_xsite in aη¹-(O)-alcohol configuration,and finally C—O bond cleavage produces 1,5-PeD.The 1,5-PeD yield gradually decreased from 47.5%to 27.0%,while the THFA yield gradually increased from 24.5%to 54.0%after the catalyst was repeated four times.The catalyst deactivation was mainly due to the oxidation of metal Ni⁰and O_v-CoO_xsites,and the initial activity of the catalyst was recovered by dry reduction of the spent catalyst after the fourth run,indicating that the catalyst has high stability and reusability.Although there is still a big gap compared with the noble metal Ir and Rh-based catalysts,it provides a new idea for the production of 1,5-PeD using economical and environmentally friendly non-noble metal catalysts 。

at present,the research on selective hydrogenation of FAL to pentanediol At home and abroad is mainly focused on noble metal catalysts,and many metal-acid/base bifunctional supported catalysts have been developed by impregnation method.Pt,Pd and Rh noble metals show excellent hydrogenation activity for the furan ring of furan compounds such as FAL,and then they adjust the acidity and alkalinity of the catalyst surface by loading different carriers,so as to obtain the selectivity of different main products.According to the research conclusion described above,the combination of most metal Pt and alkaline support is beneficial to obtain 1,2-PeD,because the active particles of Pt change under alkaline conditions,which can inhibit the formation of side reactions and promote the rapid ring opening of FAL to intermediate FFA,thus further hydrogenating to 1,2-PeD^[33]。 In addition,the combination of metal Pd and acidic support can also produce better selectivity of 1,2-PeD,which is due to the synergistic effect of the hydrogenation reaction of metal Pd and the Bronsted acid site provided by acidic support to produce intermediate FFA,and then the target product is obtained by hydrogenation^[37]。 However,the combination catalyst of Pt,Pd and Rh with the addition of acidic oxidation promoter ReO_xis more conducive to obtaining 1,5-PeD,mainly because the incorporation of ReO_xincreases the synergistic effect between Re and other metals,thus enhancing the catalytic activity,promoting the opening of furan ring of FAL to form intermediate THFA,and finally hydrogenating to 1,5-PeD^[35]。 However,in order to develop a more cost-effective and sustainable chemical industry,it is necessary to replace precious metals with cheap and abundant non-precious metal catalysts,but there are relatively few studies on non-precious metals.Therefore,the direct conversion of FAL to pentanediol on the design of heterogeneous catalysts is still a great challenge,and the following problems need to be solved:(1)researchers have designed many metal-acid/base two-site catalysts,For noble metal systems,the high cost of noble metals and oxophilic metal oxides(e.g.,Rh,Ir,and Re,etc.)hinders the large-scale practical application of renewable pentanediol production^[11]。 However,the yield of one-pot conversion of FAL to pentanediol over non-precious metal catalysts is relatively low,and there are still some problems such as harsh reaction conditions and poor catalyst stability;(2)At present,the reaction pathway of hydrogenolysis of FAL to pentanediol is hydrogenation of the C=O bond of FAL to FFA or complete hydrogenation to THFA,and then selective cleavage and ring opening of the C—O bond In the furan ring of FFA or THFA to pentanediol.Although some studies have produced pentanediol in high yields through a multi-step route,additional energy-intensive,time-consuming,and expensive separation techniques are required to recover reaction intermediates(such as FFA and THFA),solvents,and spent catalysts.in view of the above problems,it is necessary to further develop an efficient(simplified one-pot),inexpensive and stable non-noble metal catalyst for the hydrogenolysis of FAL to produce pentanediol.Recent studies have shown that non-noble metal catalysts with a variety of metals or combinations of metals and metal oxides can obtain higher yields of pentanediol.for example,the non-noble Ni-based catalysts reported by Shimazu's group and Kurniawan provide a new idea for the catalytic conversion of FAL to 1,5-PeD.the Ni-based catalyst shows strong hydrogenation activity,can be used for hydrogenation reaction and hydrogenolysis reaction,is one of the promising candidates for preparing pentanediol without noble metal,and can explore the synergistic effect of more metals on the basis of the Ni-based catalyst,as well as the exploration of better reaction conditions,supports(acidic or basic)and solvent types to optimize the catalyst,which is very important for the realization of non-precious metal catalysts to efficiently catalyze the cleavage of the C—O bond of the furan ring of FAL and obtain high yields of pentanediol^[44][11]。

Catalytic hydrogenation of FFA can produce a variety of products,including the ring-opening hydrogenation products 1,2-PeD and 1,5-PeD.The cleavage of the C—O bond of FFA may be caused by the active metal,the carrier,or the synergistic effect of the two.When the furan ring of FFA selectively cleaves the C₂—O₁bond,it can produce the intermediate product THFA,which can be further hydrogenated to 1,5-PeD(Fig.3A),while when the C₅—O₁bond of FFC is cleaved,it can directly open the ring to produce 1,2-Pe^[1]。 So far,a variety of catalysts for FFA catalytic hydrogenation to pentanediol have been studied,and the technology is becoming more and more mature,mainly according to the different catalyst systems(noble metal and non-noble metal)。

At present,the research focuses on the noble metal catalysts for the catalytic conversion of FFA to pentanediol,mainly Pt and Ru(Table 2)^{[30,33,45][2,46⇓⇓~49]}。

For the supported Pt catalyst system,Mizugaki et al.studied the FAL hydrogenolysis performance of Pt/HT catalyst,and further explored the direct catalytic hydrogenolysis of FFA to prepare 1,2-PeD^[33]。 1,2-PeD was obtained in 80%yield in isopropanol at 423 K under 3 MPa H₂pressure for 4 H.It is found that polar hydrogen species are generated between the surface basic sites of HT and Pt nanoparticles,and these polar hydrogen species attack the adsorbed furan ring,resulting in selective cleavage of the C—O bond,followed by Pt hydrogenation to give 1,2-PeD^[33]。 Wang's group reported the preparation of 1,2-PeD from FFA catalyzed by Pt/CeO₂catalyst supported by CeO₂nanocrystals with different fine shapes in alcohol solvent.It was found that the synergistic effect of CeO₂support and metal Pt could effectively adjust the chemical state of Pt by oxygen vacancies on the surface of support,and the chemical state（Pt⁰）of Pt was beneficial to the cleavage of C—O bond in furan ring,which played a key role in improving the catalytic activity and 1,2-PeD selectivity of the catalyst^[45]。 The 1,2-PeD yield could reach 77.1%over 4.5%Pt/CeO₂-C catalyst at 438 K under 2 MPa H₂for 24 H.The catalyst showed excellent catalytic performance after being reused for 5 times without deactivation or change in product distribution.In addition,Wang et al.Prepared Pt/Al₂O₃and Pt/La-Al₂O₃catalysts by impregnation method to catalyze the hydrogenolysis of FFA to produce 1,2-PeD^[30]。 Under the reaction conditions of Pt/Al₂O₃as catalyst,water as solvent,443 K,1 MPa H₂and 1.5 H,19.2%1,2-PeD can be obtained,and the low yield is due to the production of a large amount of cyclopentanone(24.2%),which makes the carbon balance poor(63.9%).The reason may be that there are some weak acid sites on the Al₂O₃,which leads to the unsatisfactory polymerization.Therefore,carriers with acidic sites are not considered suitable for the selective formation of 1,2-PeD from FFA in the aqueous phase.However,the yield of 1,2-PeD can be increased to 41.6%when the Al₂O₃is modified by lanthanum(La)and treated at 1000℃,which may be due to the inhibition of isomerization and polymerization under alkaline conditions.Therefore,the effect of water on the selective ring opening of FFA to 1,2-PeD is not unique to Pt/CeO₂,and other carriers without acidic sites may also have similar functions 。

For the supported Ru catalyst system,the hydrogenation of FFA to pentanediol using Lewis acid catalyst is easy to polymerize,so Zhu Yulei's team investigated the preparation of 1,2-PeD from FFA catalyzed by Ru-based catalysts supported on different alkaline carriers^[2]。 The Ru/MnO_xcatalyst was found to have the best catalytic performance,and the conversion of FFA and the selectivity of 1,2-PeD were 89.2%and 41.4%,respectively,when water was used as solvent at 423 K and 1.5 MPa H₂pressure for 4 H.The selectivity of 2-PeD gradually increased to 42.1%after 6 H,and the reason for the higher yield may be that MnO_xin the form of Mn(OH)₂can effectively inhibit the polymerization of FFA in the aqueous phase,and the reaction conditions of low pressure and high temperature are conducive to the formation of 1,2-PeD^[2]。 the selectivity of 1,2-PeD slightly decreased from 42.1%to 38.6%when the catalyst was continuously reused for four times,which may be due to the further hydrogenolysis of 1,2-PeD and the production of 2-(hydroxymethyl)-tetrahydrofuran-2-ol^[2]。 In addition,Pd and Rh catalysts loaded with MnO_xwere also prepared in this study,and it was found that the hydrogenolysis performance was not as good as that of Ru/MnO_xcatalyst,which may be due to the fact that the main product produced under Pd and Rh catalysis was THFA^[1]。 In addition,Claus et al.Studied the hydrogenolysis of FFA to 1,2-PeD catalyzed by Ru/Al₂O₃catalyst,in which the basic additive Na₂CO₃was used to reduce the polymerization in the reaction^[46]。 A conversion of 100%of FFA and a selectivity of 32%of 1,2-PeD can be obtained after 1 H of reaction at 473 K and 10 MPa H₂pressure using water as solvent^[46]。 It was found that revealing the formation of polyfurfuryl alcohol deposits during the reaction by ATR spectroscopy could help to improve the efficiency and safety of FFA hydrogenolysis.Then,Wang et al.Prepared Ru-based catalysts for the hydrogenolysis of furan compounds in aqueous phase,and found that the metal/basic bifunctional Ru-Mn/CNTs(multi-walled carbon nanotubes)catalyst had the best catalytic effect on the preparation of 1,2-PeD from FFA^[47]。 A conversion of 86.2%of FFA and a selectivity of 20.1%of 1,2-PeD can be obtained in water at 423 K under 4 MPa H₂pressure for 4 H.The results show that the excellent catalytic performance of Ru-Mn/CNTs comes from the synergistic effect of the three components in the catalyst,in which the basic sites of Mn-OH groups produced by the surface rehydration of Mn species in aqueous environment play an important role,thus promoting the hydrogenolysis reaction of FFA.And the catalyst exhibited a very stable catalytic performance,with no deactivation observed in five cycle tests^[47]。 Wang Xincheng et Al.explored the preparation of pentanediol by hydrogenolysis of FFA catalyzed by Ru-based catalyst supported on Mn-Al binary composite support^[49]。 Ru/Mn_0.67Al_0.33was the best catalyst among them.99.6%of FFA conversion and 48.2%of 1,2-PeD selectivity were obtained in 10%furfuryl alcohol solution at 423 K under 2 MPa H₂for 10 H.The catalyst could be reused for 11 times without obvious deactivation;Under solvent-free conditions,the selectivity of 1,2-PeD was still as high as 42.3%when the mass ratio of catalyst to FFA was 1∶25,and the catalyst could be reused for 7 times.Overall,the catalyst had good stability^[49]。 It was found that water as a solvent could promote the furan ring opening reaction of FFA,increase the reaction rate of FFA catalytic hydrogenolysis,and thus improve the selectivity of 1,2-PeD;Moreover,the addition of Al₂O₃in the carrier system can not only effectively increase the specific surface area,but also help to attach abundant MnO_xon the surface of the catalyst,which can significantly improve the performance of the catalyst^[49]。 in addition,Yamaguchi et al.Studied the hydrogenolysis of FFA to 1,2-PeD over supported Ru catalysts In various solvents^[48]。 Among all the tested combinations,Ru/MgO in water showed the best selectivity for 1,2-PeD,and the conversion of FFA and the selectivity of 1,2-PeD were 100%and 42%,respectively,after 1 H of reaction at 463 K and 3 MPa H₂pressure^[48]。 it was found that the selectivity of 1,2-PeD was determined by some characteristics of the support,and for MgO,It may be due to its basic sites that enhance the hydrogenolysis of 1,2-PeD by FFA。

Due to the high price of precious metals,the research on the preparation of pentanediol by hydrogenolysis of FFA has gradually shifted to non-precious metal catalysts,such as non-precious metal Cu,Co and Ni catalytic systems(Table 3)^{[40,50⇓⇓~53][42,54][44,55]}。

For Cu-based catalysts,in 1931,Adkins et al explored the use of CuCr₂O₄catalysts to catalyze the hydrogenolysis of FFA to produce pentanediol^[40]。 It was found that the furan ring in furanol was easily cleaved,and high yields of 1,2-PeD(40%)and 1,5-PeD(30%)were obtained at 448 K under 10~15 MPa H₂pressure for 11.5 H^[1]。 The catalyst can be used repeatedly and will not be deactivated rapidly in the process of use,but the existence of Cr element in the catalyst will have potential pollution hazards to the environment,and the catalytic reaction needs to be carried out under ultra-high pressure H₂（10~15 MPa）,which is a strict condition,so it is urgent to design a high-performance non-noble metal catalyst without Cr^[1]。 Liu et al.Studied and synthesized Cu-Mg₃AlO_4.5bifunctional catalyst for the preparation of pentanediol from FFA,which is Cr-free,efficient and inexpensive^[50]。 With 10 wt%Cu loading,100%FFA conversion,nearly 51%1,2-PeD and 29%1,5-PeD yields were obtained in ethanol at 413 K and 6 MPa H₂for 24 H.The catalytic activity and the product selectivity are greatly improved due to the cooperative effect of the highly dispersed metal Cu and the alkaline Mg₃AlO_4.5,so that the performance of the catalyst is obviously better than that of a commercial CuCr₂O₄catalyst and a Co and Ni-based non-noble metal catalyst supported on the same carrier,and is even better than that of some Ru,Pt and Rh-based noble metal catalysts^[1]。 At the same time,the FFA conversion and the selectivity to 1,2-PeD and 1,5-PeD did not decrease significantly after the catalyst was repeatedly operated for 6 times,showing good recycling stability and convenient recycling.In addition,Liu et al.Prepared a series of Cu-Al₂O₃bifunctional catalysts with different Cu contents by co-precipitation method,and selected the optimal Cu loading of 10 wt%from the Cu contents of 2–30 wt%,at which the Cu-Al₂O₃catalyst showed the best activity for pentanediol production^[51]。 The conversion of FFA and the selectivity of 1,2-PeD and 1,5-PeD were 60.4%,48.6%and 22.7%,respectively,at 413 K and 6 MPa H₂for 8 H with ethanol as solvent^[1]。 The synergy of the highly dispersed Cu particles with the Lewis acid sites of the Al₂O₃support and the combination of active copper particle size and surface acidity play an important role in obtaining high yields of pentanediol.The catalyst does not significantly reduce the conversion of FFA or the selectivity of pentanediol in five cycles,and has high durability 。

In order to further explore the relationship between the FFA hydrogenolysis performance of Cu-based catalysts and the catalyst composition,Sulmonetti et Al.Synthesized Cu-Co-Al mixed metal oxides through the calcination of layered double hydroxide(LDH),and explored the process of FFA liquid phase hydrogenolysis catalyzed by Cu-Co-Al mixed metal oxides to produce pentanediol^[42]。 A 98%FFA conversion,18.9%1,2-PeD selectivity and 44.7%1,5-PeD selectivity can be obtained over a small amount of Cu-modified 0.1 Cu-2.9 Co-Al catalyst in ethanol at 433 K under 4 MPa H₂for 2 H.It was found that the high yield of pentanediol was mainly due to the synergistic effect and mutual balance between the Co²⁺and the active metal（Cu⁰and Co⁰）.XPS analysis showed that there were a large number of Co²⁺and a small amount of Co⁰and Cu⁰on the surface of the catalyst,which indicated that during the reaction,A large amount of cobalt oxide containing Co²⁺contributes to the ring opening of the furan ring of FFA,while the presence of metal Co⁰and Cu⁰on the surface promotes further hydrogenation,which makes the catalyst have high catalytic activity,thus obtaining high conversion of FFA and high selectivity of 1,5-PeD^[1][42]。 Gao et al.Prepared a series of Cu-LaCoO₃catalysts with different copper loadings for FFA hydrogenolysis to produce pentanediol^[56]。 The catalyst with 10 wt%Cu loading exhibited the best catalytic activity under pre-reduction conditions containing 5%H₂-95%N₂^[29,56]。 FFA was almost completely converted to 1,5-PeD and 1,2-PeD with 40.3%and 15.2%yields,respectively,over the catalyst in ethanol at 413 K and 6 MPa H₂for 2 H^[29,56]。 When the catalyst was recycled four times,the conversion of FFA was almost unchanged,while the selectivity of pentanediol decreased slightly.It was found that the performance of the catalyst was greatly affected by the pre-reduction conditions and the metal Cu content in the catalyst,and the synergistic effect between the balanced Cu⁰and CoO sites was the main factor to achieve high yield of pentanediol^[29]。

In addition,Claus et al.Studied the catalytic activity of various commercial Cu-based catalysts in the aqueous hydrogenolysis of FFA,and found that the catalysts containing amphoteric ZnO or ZrO₂in the system had better 1,2-PeD selectivity^[52]。 The conversion of FFA and the selectivity of 1,2-PeD and 1,5-PeD were 28%,34%and 10%,respectively,when the catalyst composed of 60CuO40ZrO₂was used to catalyze the reaction of FFA at 443 K,9 MPa H₂pressure and aqueous solution containing Na₂CO₃as solvent for 6 H^[52]。 Although the addition of Na₂CO₃to the reaction reduces the amount of polymer formed by the acid-catalyzed side reaction in the aqueous solution to improve 1,2-PeD selectivity,the carbon balance is still low.Although the activity of Cu-based catalysts is quite low,studies on the interface between copper and support materials and the effect of support on catalytic performance show that commercial copper catalysts are still promising alternatives to precious metal catalysts.Zhu et al.Designed and synthesized a highly dispersed Cu@MgO-La₂O₃catalyst to improve the performance of Cu-based catalysts in the ring-opening hydrogenolysis of FFA to pentanediol^[53]。 The catalyst has a core-shell structure and bifunctional metal and basic sites,and the FFA conversion of 94.9%,1,2-PeD selectivity of 67.1%and 1,5-PeD selectivity of 18.8%can be obtained at 413 K and 6 MPa H₂for 24 H in isopropanol.The catalyst has good stability and recyclability,and the selectivity of pentanediol decreases slightly after six times of reuse.Combined with various characterization techniques and studies,it was found that MgO was beneficial to increase the specific surface area and improve the dispersion of Cu particles,while La₂O₃was the main adsorption site of FFA in the strong basic support,and the synergistic effect of highly dispersed Cu nanoparticles and basic sites was beneficial to improve the catalyst activity and selectivity to pentanediol.Combined with the current research results of FFA hydrogenolysis catalyzed by Cu-based catalysts,it was found that nanoparticles Cu and alkaline supports with high C—O bond hydrogenolysis activity and low C—C bond cleavage activity were beneficial to the stable adsorption of reactants and intermediates 。

For Co-based catalysts,Huber et al.Reported that Co/TiO₂catalysts can be stabilized by strong metal-support interaction(SMSI)to catalyze the hydrogenolysis of FFA aqueous solution,and the SMSI effect enables the catalyst to selectively cleave the C—O bond in the furan ring,thereby generating 1,5-PeD from FFA^[54]。 Among them,the catalyst calcined at 873 K and then reduced has the best catalytic performance.Under the conditions of weight hourly space velocity（WHSV）=5.8 h⁻¹,temperature of 413 K and 2.34 MPa H₂pressure,the conversion of FFA and the selectivity of 1,5-PeD are 99.0%and 30.3%,respectively,while the yield of 1,2-PeD is very small(selectivity is only 2.4%),and the yield of 1,5-PeD remains unchanged after three cycles^[1,54]。 It was found that the calcination and reduction temperatures of the catalyst were the key factors determining its stability,the hydrogenolysis activity of FFA,and the selectivity to pentanediol^[1]。

For Ni-based catalysts,Shimazu et al.Catalyzed FFA to pentanediol with Ni-Y₂O₃composite catalyst.In the reaction process,part of FFA was first converted into intermediate THFA,and then THFA was further hydrogenated to form 1,5-PeD^[55]; The formation of a small amount of 1,2-PeD is caused by the direct ring opening of another part of FFA.The Ni⁰-Y₂O₃boundary was found to be active and selective in the hydrogenation of C—O bonds in furan compounds.The Ni-Y₂O₃catalyst calcined at 623 K(Ni/Y molar ratio of 2.5)was the most favorable for the production of 1,5-PeD,and the yields of 1,5-PeD were 41.9%and 47.8%at 423 K for 24 H when the H₂pressure was 2 and 3 MPa,respectively,while almost no 1,2-PeD was produced(the yield was less than 2% )^[1,55]。 Reuse of Ni-Y₂O₃（2.5）-623 was carried out under optimal conditions to evaluate the stability of the catalyst.Although the conversion of FFA remained at 100%,the yield of 1,5-PeD decreased to 15.3%in the second run,probably due to the poisoning of the Ni⁰-Y₂O₃boundary,which reduced the hydrogenolysis ability of C—O bond 。

At present,for the preparation of pentanediol by FFA catalytic hydrogenolysis,researchers have created many multi-metal systems for furan conversion,and it has been found that the active metal particle size in the catalyst and the synergistic effect between the metal and the support are the key factors to improve the FFA hydrogenolysis conversion and pentanediol yield^[1]。 In the aspect of noble metal catalysts,most studies are the combination of bifunctional catalysts of noble metal Pt,Ru and alkaline support,which have stronger catalytic activity,and the hydrogenolysis ability of FFA is improved by adjusting the amount of acid and alkali of the catalyst through the added support.It shows higher selectivity of single pentanediol product,and has better hydrogenolysis effect In organic solvent,which obviously promotes the direct ring-opening process of FFA,mainly producing 1,2-PeD and very little 1,5-PeD,but these catalysts have higher cost.In the non-noble metal system,the results show that the combination of metal Cu and basic support has more catalysts than acidic and neutral supports,and the combination of metal Cu and basic support has more tendency to produce 1,2-PeD.This is because Cu can increase the metal active sites on the surface of the catalyst and promote the direct ring opening of FFA to obtain 1,2-PeD under the synergistic effect of acidic/basic sites provided by other supports^[42]; However,Co-based and Ni-based catalysts have higher selectivity for 1,5-PeD,which may be due to the different selective cleavage of the C—O bond of the furan ring in FFA during the reaction(Fig.3),resulting in different main products.Although the cost of non-precious metal use is relatively low,there is still much room for improvement in product selectivity,which needs further study^[1]。 In addition,since the production of pentanediol involves a ring-opening reaction,a long reaction time and harsh reaction conditions are required.Therefore,there are still many problems in the hydrogenolysis of FFA,such as the high cost of noble metal catalysts,the limited conditions in the reaction process,or the low selectivity of pentanediol.It is still urgent to develop an effective and environmentally friendly non-noble metal catalyst under mild conditions to effectively convert FFA into value-added pentanediol.It has been found that the Cu@MgO-La₂O₃catalyst with excellent stability can obtain high yield of pentanediol through the synergistic effect of bifunctional metal and basic sites,which is due to the high C—O bond hydrogenolysis activity and low C—C bond cleavage activity on nano-Cu particles,and La₂O₃is the main adsorption site of FFA in strong basic support^[53]。 this provides a new idea for the preparation of pentanediol using non-noble metals,and new catalyst systems can be developed on This basis to adjust the selectivity of the target product in the future。

Compared with FAL/FFA,THFA has higher stability due to the presence of hydroxymethyl and cyclic ether bonds in its molecular structure,and has an easily controlled adsorption configuration,which leads to the cleavage of THFA cyclic ether(C—O)bonds and further hydrogenation to obtain higher yields of pentanediol^[29]。 During hydrogenolysis,different cleavage positions of the THFA cyclic ether bond give rise to different products,such as 1,5-PeD or 1,2-PeD(Fig.4)^[14,29]。 In general,1,5-PeD is more difficult to produce than 1,2-PeD,because the steric effect leads to the need for more activation energy to produce 1,5-PeD.Therefore,how to obtain 1,5-PeD with high yield has become a hot research topic^[29]。

At present,the research on efficient noble metal catalysts for hydrogenolysis of THFA to produce pentanediol is mainly based on noble metals such as Rh,Ir and Pt.For loading other carriers(such as C,SiO₂,ZrO₂,etc.)on such noble metals and adding ReO_x,Catalysts of low-valent metal oxides such as MoO_x,WO_x,and VO_xare more widely explored(Table 4 )^{[57⇓⇓⇓⇓⇓⇓⇓~65][60,66,67][68⇓⇓~71][1]}。

For the supported Rh-MO_x（Re,Mo,and W)catalyst system,Tomishige et al.Developed a Rh-ReO_x/SiO₂catalyst,which was used to catalyze the hydrogenolysis of THFA to produce 1,5-PeD^[58]。 The catalyst(Re/Rh molar ratio of 0.5)was prepared at 393 K,8 MPa H₂ 。

and water as solvent,the conversion of THFA and the selectivity of 1,5-PeD were 96%and 80%,respectively^[58]。 However,the major product of selective hydrogenolysis of THFA by Rh/SiO₂was 1,2-PeD under the same conditions.It was found that the introduction of ReO_xcould not only control the main product distribution,but also interact with the-CH₂OH group of THFA to further generate a methoxy group with strong adsorption ability,which could promote the hydrohydrolysis of THFA,thus improving the selectivity of 1,5-PeD^[4]。 Subsequently,the research group also prepared a series of catalysts,and found that the catalysts using Ir,Re,W,Mo or V oxides combined with Rh had high catalytic activity for the preparation of pentanediol from THFA^[60,61]。 Among them,94.2%THFA conversion and 90.3%1,5-PeD selectivity were obtained over Rh-MoO_x/SiO₂catalyst with Mo/Rh molar ratio of 0.13 at 373 K for 24 H in 8 MPa H₂and water as solvent^[61]。 The results showed that the surface of Rh metal particles in the catalyst was attached by MoO_x,and the interaction between them increased the active interfacial area of the reaction,which made the hydrogenolysis of THFA have high selectivity and activity^[61]。 Comparatively speaking,the activity of Rh-ReO_x/SiO₂is lower than that of Rh-MoO_x/SiO₂,and the THFA conversion of the former decreases after five times of reuse,which may be due to the leaching of Re,resulting in low stability,while the latter has no significant loss in conversion and product selectivity.Chen et al.Studied the hydrogenolysis of THFA to 1,5-PeD catalyzed by bimetallic catalyst with metal Rh modified ReO_x/C^[59]。 When the Re/Rh ratio was 0.25,water was used as the solvent to catalyze THFA at 373 K and 8 MPa H₂,the yield of 1,5-PeD was up to 94%,which was higher than that of Rh-ReO_x/SiO₂（86%）.The characterization results show that the ReO_xclusters attached to the surface of Rh metal particles can cause the direct synergistic interaction between ReO_xand Rh metal atoms,thus promoting the hydrogenolysis of THFA^[63]。 Subsequently,Dumesic et al.Studied the hydrogenolysis of THFA to 1,5-PeD using Rh-MO_x/C(M=Re,Mo)catalyst^[64]。 It was found that the low-valent acidic oxophilic promoter（ReO_xor MoO_x）could effectively promote the hydrogenolysis reaction at the active site of the metal catalyst,and it was also found that the key to obtain higher 1,5-PeD selectivity was the adjustment of the molar ratio between the oxophilic promoter and Rh metal^[1,64]。 When the Re/Rh molar ratio was 0.5,47.2%THFA conversion and 97.2%1,5-PeD selectivity could be achieved over the 4 wt%Rh-ReO_x/C catalyst at 393 K under 3.4 MPa H₂pressure for 4 H.The catalyst was reduced with H₂for 4 H(523 K)in a continuous flow reaction system to obtain better catalytic activity and stability.In contrast,under the same conditions,the hydrogenolysis conversion of THFA catalyzed by 4 wt%Rh-MoO_x/C was slightly increased to 51.6%,but the 1,5-PeD selectivity was slightly decreased to 91.3%when the Mo/Rh molar ratio was 0.1^[1]。 the catalytic activity showing this small change may be due to the difference in the acidity of the hydroxylate produced by the low valent metal oxide^[1]。 In addition,the catalyst Rh/C containing only a single metal showed a lower catalytic performance,the conversion of THFA was only 8.5%,the selectivity of 1,5-PeD was 59.1%,and the selectivity of 1,2-PeD was even lower,which was 20.7%,which was obviously not as good as the hydrogenolysis activity of the catalyst with added promoter^[64]。 Based on the experimental data and the theoretical calculation of density functional theory,it is found that the acidic catalyst is helpful to promote the cleavage and ring opening of C—O,which is due to the strong combination of metal Re and the attached oxygen atom,which abstracts the electrons from the hydroxyl group of THFA and the alkoxide combination of Rh,and then the alkoxide forms acidic carbocation,which further causes the ring opening;Acid catalysis can also promote the combination of dehydration reaction and metal hydrogenation reaction^[4]。

In addition,for other supported Rh-based catalysts,Chatterjee et al.explored the use of Rh-MCM-41 catalyst to catalyze the hydrogenolysis of THFA to produce 1,5-PeD in supercritical CO₂^[57]。 The results showed that the conversion and selectivity were significantly affected by the change of process variables such as CO₂,H₂pressure,temperature and reaction time.When the metal loading was about 1%,the conversion of THFA was 80.5%and the selectivity of 1,5-PeD was up to 91.2%under the optimum reaction conditions（14 MPa CO₂,4 Mpa H₂and 20 H).However,supercritical CO₂requires high pressure in the reaction,which has more stringent requirements for equipment and higher energy consumption,thus limiting its large-scale application in industry^[4]。 Mu et al.Used a mixed catalyst system of Rh/SiO₂and MoO₃to catalyze the hydrogenolysis of THFA to 1,5-PeD^[62]。 When the amount of Rh/SiO₂was fixed at 2 G and the MoO₃was increased to 0.2 G,27.9%THFA conversion as well as up to 80%1,5-PeD selectivity could be achieved at 393 K,6 MPa H₂and 20 H with water as the solvent.The results showed that with the increase of MoO₃content,the conversion of THFA and the yield of 1,5-PeD increased gradually,and the activity of the catalyst in aqueous solution also increased,mainly because water and MoO₃produced H_xMoO₃during the reaction,and the Mo−OH sites attached to the acidic surface could effectively promote the C—O bond cleavage of THFA,thereby improving the catalytic activity^[29]。 In addition,the catalyst showed excellent reusability and could be recycled more than four times without significant loss of 1,5-PeD yield.Gondre et al.Studied a series of bifunctional catalysts with different oxophilic cocatalysts(Re,W,and Mo)supported on Rh-ZrO₂,which were used to prepare 1,5-PeD by hydrogenolysis of THFA^[65]。 The study reported that the order of 1,5-PeD yield obtained by each catalyst was Rh-Mo/ZrO₂,Rh-W/ZrO₂,and Rh-Re/ZrO₂from large to small,and the reason may be that the difference of supports affected the metal-support interaction(Zr-M−OH),which affected the Bronsted acidity.Among them,90%selectivity of 1,5-PeD can be obtained with 5%Rh-4%Mo/ZrO₂catalyst in 5 wt%THFA aqueous solution at 393 K and 8 MPa H₂for 24 H.This reaction demonstrates the bifunctional character of the catalyst,in which Rh particles bring metal sites,metal Mo creates Bronsted acid sites,and THFA adsorbs at sites on the surface(such as Zr-M−OH)via—CH₂OH groups on the Bronsted acid 。

For supported Ir-MO_x（M=Mo,Re,W,and V),Tomishige et al.Studied the reaction of hydrogenolysis of THFA to 1,5-PeD catalyzed by Ir-ReO_x/SiO₂catalyst,and found that the catalyst had a higher TOF ratio,higher activity,and comparable selectivity compared with Rh-ReO_x/SiO₂,which was attributed to the stronger hydrogenation ability of Rh/SiO₂,which was beneficial to the attack of C—O bond by hydride formed by hydrogen molecules due to its small hindrance^[60]。 When the molar ratio of Re to Ir is 2,the conversion of THFA is about 94.0%and the selectivity of 1,5-PeD is 87.0%at 373 K and 8 MPa H₂for 8 H with water as solvent.In addition,in the hydrogenolysis of THFA,the catalytic activity of monometallic Rh/SiO₂and Ir/SiO₂is not ideal,but it can be found that Rh/SiO₂can obtain 1,2-PeD with high selectivity,which is due to the dissociation of H₂molecules into active hydrogen and the attack of C—O bond with low steric hindrance^[60]; However,Ir/SiO₂shows a different situation,the catalytic activity of this catalyst is lower,and the active hydrogen species in the reaction is different from that of Rh/SiO₂,which makes it more inclined to produce 1,5-PeD^[4]。 Next,the addition of M(M=Mo,Re,and W)to Ir-SiO₂was found to significantly increase the catalytic activity,and the most active of the three Ir-MO_x/SiO₂catalysts(M/Ir molar ratio of 0.25)was Ir-ReO_x/SiO₂,followed by Ir-MO_x/SiO₂and Ir-WO_x/SiO₂^[1]。 The direct production of 1,5-PeD from THFA catalyzed by Ir-ReO_x/SiO₂is superior to that from FAL catalyzed by"one-pot"method with two-stage temperature control,and the yield of 1,5-PeD from the former is about 20%higher than that from the latter^[38,60]。 Subsequently,Li Ning et al.Prepared a Ir-MO_x/SiO₂catalyst and used it to catalyze the selective hydrogenolysis of THFA to 1,5-PeD in a continuous flow reactor^[66]。 Among them,4 wt%Ir-MoO_x/SiO₂showed the best catalytic performance when the Mo/Ir molar ratio was 0.13.The conversion of THFA was 75%and the selectivity of 1,5-PeD was 65%at 393 K and 6 MPa H₂for 6 H in water.In addition,the average particle size of Ir particles did not change significantly after the catalyst was used for 30 H,indicating that Ir was very stable in the reaction system,while the conversion of THFA decreased,which may be due to the leaching of Mo species during the reaction.Next,Pholjaroen et al.Employed VO_xmodified Ir/SiO₂(molar ratio V/Ir=0.10)catalyst for THFA selective hydrogenolysis to 1,5-PeD,and showed better performance than V-modified other noble metal(Rh,Ru,Pt,and Pd)catalysts,which could achieve high THFA conversion and 1,5-PeD selectivity even at low reaction temperature or system pressure^[67]。 When the optimal V/Ir atomic ratio was about 0.10,the conversion of THFA was 57.0%and the selectivity of 1,5-PeD could reach 89%over 4 wt%Ir-VO_x/SiO₂catalyst at 353 K and 6 MPa H₂for 6 H.The catalyst has poor stability due to the leaching of V element during the reaction process.This defect can be improved by adding mineral acid(or organic acid)to the THFA aqueous solution.Because the solubility of vanadate in acidic environment is lower,the addition of acid can better reduce the leaching of V element^[67]。 The modification of vanadium on the Ir-VO_x/SiO₂catalyst has no significant effect on the average particle size of Ir,and the close synergistic effect of vanadium oxide and Ir particles promotes the reduction of VO_xat lower temperature,and the interaction between partially reduced and separated VO_xattached to Ir particles greatly improves the catalytic effect of the catalyst^[67]。

For the supported Pt-WO_xcatalyst system,Feng et al.Studied the reaction process of hydrogenolysis of THFA to 1,5-PeD over a bifunctional catalyst composed of Pt nanoparticles and WO₃/ZrO₂.The relationship between the content of WO₃and the performance of the catalyst was evaluated.With the increase of the content of WO₃,the number of Bronsted acid sites in the catalyst increased.When the WO₃content was 5%,better catalytic activity and 1,5-PeD yield could be obtained^[69]。 The conversion of THFA was 56%and the selectivity of 1,5-PeD was 65%at 423 K and 5 MPa H₂for 5 H.From the characterization,it is known that the W-(OH)-Zr site formed at the interface between the WO₃monolayer domain and ZrO₂plays an important role in the hydrogenolysis of THFA to 1,5-PeD.The balance of adsorption sites and the number of W-(OH)-Zr sites on ZrO₂promotes the formation of primary alumina species and secondary carbides,thus promoting the formation of 1,5-PeD.In addition,Chen Changlin's group prepared Pt/Y₂O₃-WO₃-ZrO₂catalysts with different Y₂O₃contents by impregnation-calcination method to catalyze the hydrogenolysis of THFA to 1,5-PeD^[71]。 It can be seen from this study that the introduction of Y₂O₃greatly increases the acidity of the catalyst,and the catalytic activity of the catalyst is also enhanced.When the Y₂O₃content is 1 wt%,the catalyst has the maximum acid amount,the maximum Pt dispersion,the best reduction performance and the best catalytic activity,and the conversion of THFA is 88%and the selectivity of 1,5-PeD is 77.3%under the conditions of substrate in the form of 40%THFA aqueous solution,glycerol mass space velocity of 0.2 h⁻¹,423 K and 4 MPa H₂pressure for 100 H.Wang's group used a facile modified sol-gel method to develop WO₃（WO₃@SiO₂）embedded in mesoporous SiO₂framework,and then used the impregnation method to load 5 wt%Pt to prepare Pt/WO₃@SiO₂catalyst,which was used to catalyze the hydrogenolysis of THFA to 1,5-PeD^[68]。 The conversion of THFA was 82.9%and the selectivity of 1,5-PeD was 72.9%when water was used as solvent at 493 K and 6 MPa H₂for 24 H.It has been shown that in aqueous solution,WO_xwill be converted to H_xWO₃,and the synergistic effect of H_xWO₃possessing Bronsted acidic sites and active metals can effectively promote the hydrogenolysis of THFA,thus showing excellent catalytic performance^[70]。

Compared with noble metals,non-noble metal catalysts require more stringent reaction conditions when catalyzing the cleavage of the C—O bond of THFA cyclic ethers.Even so,their activity and selectivity are still far inferior to those of noble metal catalysts,and there are few studies on the hydrogenolysis of THFA catalyzed by non-noble metal catalysts(Table 5)^[1]。

Tomishige et al.Reported the use of commercial CuCr and Raney Ni catalysts to catalyze the hydrogenolysis of THFA to produce pentanediol^[58]。 After 4 H of reaction at 453 K and 8 MPa H₂,the conversion of THFA was less than 1%,almost no conversion,and the total selectivity of pentanediol was also very low,less than 30%.Despite the high reaction temperature and the large amount of catalyst,their catalytic activity was still very low,indicating that these conventional hydrogenolysis catalysts were not suitable for the selective production of pentanediol from THFA^[1]。 Huber et al.Used Ni/Al₂O₃catalyst for aqueous hydrogenolysis of THFA to prepare pentanediol^[23]。 With 20 wt%Ni loading,the catalyst reacted at 523 K and 5.5 MPa H₂,resulting in 17.3%THFA conversion and 59.2%1,5-PeD selectivity,respectively,but no 1,2-PeD was produced^[1]。 It was found that there was a strong interaction between Ni and the tetrahydrofuran ring,which led to the cleavage of the C—O bond of the tetrahydrofuran ring.In addition,the conversion of THFA and the selectivity of 1,5-PeD increased with the increase of Ni loading from 5 wt%to 20 wt%^[1,23]。 Then,Shimazu et al.Further studied the hydrogenolysis of THFA catalyzed by Ru-modified Ni-Y₂O₃composite catalyst on the basis of hydrogenolysis of FFA and FAL catalyzed by Ni-Y₂O₃and Ni-La(OH)₃,respectively,to improve the hydrogenation activity of the catalyst by doping a small amount of noble metal^[44,55][72]。 The conversion of THFA and the yield of 1,5-PeD were 93.4%and 86.5%,respectively,at 423 K and 2 MPa H₂for 40 H in isopropanol with 1 wt%Ru loading,which was significantly higher than that of the Ni-Y₂O₃catalyst^[1]。 The results show that the addition of Ru affects the absorption of hydrogen and forms a Ru-Ni⁰-Y₂O₃interface,which can effectively promote the cleavage of C—O bond of THFA to form 1,5-PeD^[1,72]。 In addition,Borgna et al.Designed and prepared a series of Ni-WO_x/SiO₂catalysts and used them in the hydrogenolysis of THFA to produce 1,5-PeD^[73]。 Under the conditions of 523 K,3.4 MPa H₂pressure and water as solvent,the conversion of THFA and the selectivity of 1,5-PeD were 28.7%and 47.3%,respectively,after 4 H^[73]。 The results show that the tungsten oxide in the catalyst provides a high concentration of Lewis acid sites,which can react with water to form H_xWO₃and convert Lewis acid sites into Bronsted acid sites,thus enhancing the hydrogenolysis activity of THFA and the selectivity of 1,5-PeD^[73]。 After three runs of the catalyst,the conversion of THFA and the selectivity of 1,5-PeD decreased.By changing the loading of Ni and W,it was found that the reason for the loss of catalytic activity may be the weight loss of the catalyst due to the weak interaction between Ni and silicon support,and the weight loss of Ni and W increased with the increase of W loading.However,it was found that the mass loss of Ni and W decreased with the increase of W loading,which indicated that the formation of intermetallic compound phase might be related to its better anti-leaching stability^[73]。

At present,the research on the selective hydrogenolysis of THFA to pentanediol at home and abroad mainly focuses on the preparation of 1,5-PeD,and the catalysts are mainly noble metals,while other non-noble metal catalysts have also received more and more attention.In the preparation of noble metal catalysts,considering the advantages of THFA such as excellent molecular structure and stability,many researchers usually prefer to add noble metals(such as Rh,Ir and Pt)to oxyphilic metal oxides(such as ReO_x,MoO_xor WO_x）)and load other carriers(such as C,SiO₂,ZrO₂,etc.)to form a combination of bimetal-acid catalysts,which can be used to catalyze the hydrogenolysis of THFA to obtain high yields of 1,5-PeD^[29]。 The results show that the higher catalytic activity may be due to the adsorption of THFA with the OH group on the active site of the oxophilic metal linkage,which increases the acidic site of the catalyst with the addition of oxophilic metal oxide in aqueous solution.Then other metals catalyze the hydrogenolysis of C—O to obtain the target product.the physical mixture of noble metals and oxophilic metals has low activity,so close chemical contact and strong interaction of metal oxides are essential^[65]。 However,noble metals are expensive,and in some cases,THFA ring opening requires harsh reaction conditions,such as high H₂pressure(6–8 MPa)and long reaction time(8–24 H),which also increase the production cost of 1,5-PeD.In addition,a series of non-noble metal catalysts such as Cu and Ni-based catalysts have also been applied to THFA ring opening,among which Ni-based catalysts are relatively better for the preparation of 1,5-PeD by aqueous hydrogenolysis of THFA,and the combination of metal-acid catalysts(such as Ni-WO_x/SiO₂catalyst)has also been reported,indicating that the synergistic effect of active metal and acid sites is essential for the hydrogenation of THFA to 1,5-PeD.There are few studies on non-precious metal systems,and there are some problems,such as the low activity or selectivity of the target product 1,5-PeD,the complex reaction path,and the adverse effect of more by-products on the selectivity of the product.In general,the reaction mechanism of THFA hydrogenolysis to mixed oxides over metal-based catalysts is very complex,involving the dissociation of C—O,O—H,or C—H bonds and the subsequent hydrogenation step.Previous studies have only provided the cleavage mechanism of the C—O bond on the secondary carbon atom with a higher degree of substitution,resulting in the formation of 1,5-PeD,but have not considered the activation of the primary carbon atom with a lower degree of substitution and the C—O bond on the side chain.So far,the research on the hydrogenolysis of THFA at different positions of the C-O bond inside and outside the furan ring is also relatively limited,most of which are noble metals,and the dependence of the selectivity of the product on the catalyst is still vague.Therefore,in order to achieve higher yield of pentanediol,it is still a challenge to develop an efficient and stable non-noble metal catalytic system for THFA hydrogenolysis under mild conditions 。

LA(Levulinic acid,LA),made from cellulose,is an important source of C₅compounds and one of the most important renewable platform chemicals^[74]。 In recent years,efforts have been devoted to the conversion of LA into high-value chemicals such as 1,4-PeD,a useful monomer for the production of polyesters and a feedstock for the synthesis of many other chemicals and fuels^[74]。 However,there are few reports on the preparation of 1,4-PeD catalyzed by LA,most of which are efficient synthesis of 1,4-PeD by selective hydrogenation of LA,EL(Ethyl levulinate,EL)or GVL^[77]。 the specific reaction process is as follows:(a)1,4-PeD is prepared by direct hydrogenation of LA;(B)The ketone group of LA is reduced to 4-Hydroxypentanoic acid(HPA),HPA is cyclized to another structurally stable chemical GVL as an intermediate,and GVL is hydrogenated to 1,4-PeD;(C)LA/EL is hydrogenated to GVL,and GVL is further hydrogenated to 1,4-PeD(Figure 5)^[74⇓~76]。

At present,The noble metal catalysts for the preparation of 1,4-PeD catalyzed by LA have high activity,but they are expensive and easy to be deactivated.the noble metals used are Ru,Rh,Ir and Pt(Table 6)^{[74,78⇓⇓~81][82][83][75]}。

For Ru-based catalysts,Geilen et al.Developed a catalyst composed of a ruthenium precursor such as Tris(acetylpyruvate)ruthenium[Ru(acac)₃]as the active site and 1,1,1-tris(diphenylphosphinomethyl)ethane(Triphos)as the ligand to catalyze the hydrogenolysis of LA to produce 1,4-PeD^[78]。 Under the reaction conditions of 433 K and 10 MPa H₂for 18 H,the yield of 1,4-PeD can reach 95%.Under the same reaction conditions,a slightly lower catalytic activity was observed compared to complex(Triphos)Ru(CO)(H)₂as catalyst,obtaining 1,4-PeD in 73%yield.Next,Phanopoulos et al.Used a ruthenium-N-triphosphorus complex as a catalyst precursor to catalyze the preparation of 1,4-PeD from LA^[79]。 Under relatively mild conditions(423 K,6.5 MPa H₂and 25 H),LA was almost completely converted and 1,4-PeD was obtained in up to 99%yield.In conclusion,it was found that the Ru/TriPhos catalytic system could effectively reduce the conditions for deep hydrogenation of LA and achieve higher yields of 1,4-PeD,showing great potential for highly selective hydrogenation/dehydration pathways for LA conversion.However,as a homogeneous catalyst,Ru/TriPhos is not easy to separate and recycle,which also limits its wide application,and further research and improvement are needed in the recycling of the catalyst^[81]。 in addition,Corbe-Demailly et al.Reported that the bimetallic Ru-Re catalytic system could efficiently and selectively hydrogenate LA to 1,4-PeD In water^[80]。 It was found that the catalytic conversion of ester group could be promoted by the synergistic effect of acidic sites and metal active sites,and the Ru/Re ratio had a significant effect on the activity of the catalyst and the selectivity of 1,4-PeD^[81]。 When 1.9%Ru-3.6%Re/C was used as the catalyst,the highest yield of 82%of 1,4-PeD was obtained at 413 K and 15 MPa H₂for 28 H.Cui et al.Explored an efficient green process for direct conversion of LA to 1,4-PeD using a Ru-MoO_x/AC catalyst in a continuous fixed-bed reactor^[74]。 It has been shown that the presence of Mo is very important for the hydrogenation of C=O in carbonyl or carboxyl groups,and the synergistic effect of Ru and Mo is the key reason for the high efficiency of the catalyst.When the Mo/Ru atomic ratio of the catalyst was 0.25,the conversion of LA was 99.9%and the yield of 1,4-PeD was 96.7 mol%in water at 343 K and mild 4 MPa H₂.The catalyst not only has high activity,but also has good stability,which can be used for 200 H without deactivation^[74]。

For other noble metal catalysts,Li et al.Studied the hydrogenolysis of LA to 1,4-PeD catalyzed by Mo modified Rh/SiO₂catalyst^[82]。 It was found that the high efficiency of this catalyst can be attributed to the synergism between Rh and Mo species in close contact.In addition,from the reaction of some model compounds,it was also found that the modification of Mo promoted the hydrogenation of carboxyl groups,which was also the reason for the high yield of 1,4-PeD.Among them,the 4%Rh-MoO_x/SiO₂（Mo/Rh=0.13）catalyst,using water as solvent,at 353 K and 6 MPa H₂for 12 H,the LA conversion was almost 100%,and the 1,4-PeD yield was 70%.At the same time,the catalyst was tested continuously for 30 H,and no obvious change in the activity of the catalyst and the selectivity of the product was observed,indicating that the catalyst has excellent performance and good stability.Wang et al.Reported the direct hydrogenation of LA to 1,4-PeD by heterogeneous catalyst Ir-MoO_x/SiO₂^[83]。 When the Mo/Ir atomic ratio is 0.13,the performance of the 4%Ir-MoO_x/SiO₂catalyst is the best.The close contact of Mo species with Ir particles promotes their reduction by hydrogen spillover,and the synergistic effect of Ir particles and the partially reduced isolated MoO_xspecies attached to them(the synergistic effect existing between Ir and Mo species in close contact),may be responsible for the excellent catalytic performance of this catalyst.100%conversion of LA and 42.3%yield of 1,4-PeD can be obtained at 373 K and 6 MPa H₂using water as solvent.The catalyst has low toxicity and can be reused,which is beneficial to practical application.Mizugaki et al.Developed a hydroxylapatite-supported platinum-molybdenum bimetallic catalyst(Pt-Mo/HAP),which was used for the preparation of 1,4-PeD by selective hydrogenolysis of LA^[75]。 It was found that the synergistic effect between Pt nanoparticles and molybdenum oxide could easily promote the hydrogenation of intermediates HPA and GVL to 1,4-PeD,and the Pt/Mo ratio in the catalyst significantly affected the product selectivity.Setting the Pt/Mo ratio from 1 to 20 and using water as the solvent,the reaction was carried out under the mild conditions of 403 K and 5 MPa H₂for 12 H.It was found that the highest yield of 1,4-PeD could reach 93%when the Pt/Mo ratio was 4,which was significantly higher than that of other Pt/Mo ratios,such as the yield of 1,4-PeD was only about 70%when the ratio was 2.The Pt-Mo/HAP catalyst was easily separated from the reaction mixture and was recyclable,with no significant loss of activity or selectivity after being reused twice 。

Due to the low cost and small investment of the non-noble metal catalyst,the production energy consumption can be effectively reduced,the economic benefit is improved,and the industrial production is facilitated^[77]。 Therefore,non-noble metal catalysts for LA/EL to prepare 1,4-PeD have better research prospects,which include Cu,Co,etc.(Table 7)^{[76,81,84⇓⇓~87][88,89]}。

Cu-based catalysts are widely used in ester hydrogenation catalysts because the interaction between Cu-based catalysts and the electron-rich oxygen of the ester bond C=O weakens the C—O bond of the adjacent ester bond^[81]。 It is generally believed that Cu-Cr catalyst can promote the activation of ester group,so researchers have conducted in-depth research on LA catalytic conversion^[81]。 Hixon et al.Studied the hydrogenolysis of LA catalyzed by Cu-Cr catalyst,and the 1,4-PeD yield was only 44%at 573 K and 20 MPa H₂pressure.However,Cu-Cr catalyst has potential pollution to the environment and requires harsh conditions of ultra-high pressure H₂,thus losing the possibility of its promotion in industrial application^[84]。 Zhao et al.Prepared a new type of inexpensive non-precious metal Cu-based catalyst by liquid phase chemical reduction method,and catalyzed the direct hydrogenation of LA(2 wt%~2.4 wt%)to 1,4-PeD on 30%FeCuB catalyst^[85]。 Using 1,4-dioxane as solvent,the conversion of LA was 100%,the yield of 1,4-PeD was 85.1%,and the remaining product was GVL at 473 K under 5 MPa H₂pressure for 6 H.The catalyst has the advantages of simple preparation,low cost,no toxicity,harmlessness,convenience for large-scale production,magnetism,and easy recovery and reuse after use,which indicates that the synthesis process has high potential economic value^[85]。

In addition,Ren et al.Reported a non-precious ternary framework CuZnAl catalyst for direct hydrogenation of EL to 1,4-PeD^[86]。 Using 1,4-dioxane as solvent,the conversion of EL was 100%and the yield of 1,4-PeD was 98%at 433 K and 6 MPa H₂for 6 H.Although the catalyst has high activity and selectivity,the leaching of Cu in the liquid medium usually leads to the deactivation of the catalyst.It was found that Zn in the framework alloy is an important promoter for regulating the selectivity of 1,4-PeD production,not only greatly improving the conversion of EL to GVL,but also accelerating the process of lactone ring opening to produce diols.In addition,parameters such as catalyst metal loading,solvent,H₂pressure and reaction time are also important factors affecting the preparation of 1,4-PeD.Li Fuwei et al.Used bimetallic catalysts to achieve the conversion of levulinic acid compounds to 1,4-PeD with high activity and selectivity^[76]。 The hydrogenolysis of EL catalyzed by CuCo/Al₂O₃(where Cu:28 wt%,n_Cu:n_Co=1)catalyst was carried out in a tubular fixed-bed reactor with 1,4-dioxane as solvent at 413 K and 1.8 mL/H EL(20%)feed flow rate for 24 H,and the yield of 1,4-PeD reached 88%.However,when butyl levulinate was used as the substrate,CuCo/Al₂O₃(with Cu:37 wt%,n_Cu:n_Co=2)catalyst gave the highest yield of 92%of 1,4-PeD at 433 K for 12 H with 5 MPa H₂.The method is economical and environment-friendly,does not need to use precious metal,does not need to add acid/alkaline additives,has low catalyst preparation cost,small investment,simple reaction system and good stability,and is suitable for industrial production of 1,4-PeD 。

However,the additional steps to convert LA to EL are costly and require careful handling of toxic reaction conditions and the use of organic solvents^[87]。 Therefore,some researchers have developed more efficient and robust non-noble metal-based catalysts to catalyze the conversion of LA.Karanwal et al.Employed a trimetallic Cu-Ni-Zn/H-ZSM-5 catalyst to directly convert LA to 1,4-PeD via an efficient one-pot process in aqueous medium^[87]。 Under mild reaction conditions(403 K,2.5 MPa H₂and 6 H),LA was almost completely converted,and a high yield of 93.4%of 1,4-PeD could be obtained.The large number of Lewis acid sites of the catalyst enhances the adsorption of LA,and the adsorbed LA is converted to GVL at the Bronsted acid sites of H-ZSM-5,and then the hydrogenation reaction is carried out at the Cu-Ni alloy sites.GVL then absorbs the Lewis acid site,is activated,undergoes ring opening,and is finally hydrogenated to 1,4-PeD at the Cu-Ni alloy site.It was found that the presence of Zn promoter effectively inhibited the growth of Cu-Ni alloy nanoparticles(NPs)on the surface of H-ZSM-5,and the Zn-promoted H₂spillover on Cu-Ni alloy NPs enhanced the hydrogenation of LA to 1,4-PeD.The catalyst showed good stability and recoverability for up to five consecutive cycles with little loss of activity,probably due to the inhibition of coke formation,the small extent of catalyst sintering,and the absence of metal leaching 。

Since Co is also active for hydrogenation,Co-based catalysts are often considered as an alternative to Cu-based catalysts^[88]。 Cen et al.Developed a highly active and selective cobalt catalyst,Co/ZrO₂,for EL hydrogenation to 1,4-PeD^[89]。 Using 1,4-dioxane as solvent,100%conversion of EL and 78%selectivity of 1,4-PeD could be obtained at 463 K and 8 MPa H₂for 8 H^[89]。 The 1,4-PeD yield gradually decreased after the Co/ZrO₂was reused for four times,but the catalyst performance could be recovered after calcination and reduction,and the catalyst could be recycled for 12 times,and no obvious leaching of Co element was observed,showing a very high catalytic stability.The reaction temperature is high and the yield of 1,4-PeD is moderate.Therefore,there is still room for the development of cobalt-based catalysts with high activity at low reaction temperatures^[88]。 Shao et Al.Synthesized Co-Mg-Al catalyst by coprecipitation method,and studied the hydrogenation reaction of 1,4-PeD on EL^[88]。 Using 2.1Co-0.9 Mg-Al as catalyst,isopropanol as solvent,at 423 K and 4 MPa H₂for 10 H,the conversion of EL was 100%and the yield of 1,4-PeD was 98%^[88]。 the results show that the catalyst exhibits excellent catalytic activity,which is mainly due to the synergistic effect between hydrogenation and acid sites.Lewis acid sites improve the adsorption of substrates and reaction intermediates.Bronsted acid sites and hydrogenation sites can effectively catalyze lactonization and GVL conversion.and the aluminum-containing layered double hydroxide(LDH)structure is also beneficial to the structural stability of the catalyst,which inhibits the sintering of Co species,so that the catalyst has good recyclability,can be reused four times,and shows excellent activity for the production of 1,4-PeD。

to sum up,the production of 1,4-PeD by LA/EL is a continuous reaction with GVL as the intermediate,and the selectivity to GVL or 1,4-PeD depends on the specific catalytic performance of the catalyst for the basic reactions such as hydrogenation,lactonization and ring-opening of GVL,which are closely related to the formulation of the catalyst,the distribution of hydrogenation sites,the distribution of acid-base sites,and the characteristics of the pore structure^[88]。 LA/EL catalysts for the preparation of 1,4-PeD are mainly divided into noble metal and non-noble metal catalytic systems.For noble metals,Ru-based systems have been studied more,and noble metal catalysts promoted by reducible metal oxides(such as MoO_xand ReO_x）)have been reported to have high activity,which may be due to the fact that the acidic sites of the catalyst can activate the carbonyl group of GVL,and the binding of active metals to the acidic sites is very effective for the hydrogenation of LA/EL to 1,4-PeD.In the noble metal system,on the one hand,the depth of LA hydrogenation can be controlled by adjusting the loading amount of the metal,on the other hand,the acidity of the molecular sieve carrier can be adjusted,so that the interaction force between the metal and the carrier is more suitable for the selected reaction path.So as to obtain a higher target product 1,4-PeD,and also has potential advantages in recovery,separation,simple operation and the like,but the high cost of the precious metal restricts the large-scale production of the 1,4-PeD^[90]。 In terms of non-precious metal catalysts,It has been reported that Cu and Co are active In catalyzing the conversion of LA/EL to 1,4-PeD,among which the multi-metal system has been studied more,and the catalytic effect is relatively good.This may be due to the formation of alloy between multiple metals,which increases the active metal sites and enhances the hydrogenolysis of LA/EL.In addition,the acidic sites provided by the catalyst support or other metals can also promote the ring opening of GVL,thus obtaining a higher yield of 1,4-PeD.Although the non-noble metal system is cheap,there are still some problems,such as catalyst deactivation,instability and harsh reaction conditions caused by metal ions leached from the catalyst in the liquid medium.Therefore,It is of great significance to develop transition metal-based catalysts to catalyze LA/EL to produce 1,4-PeD.In order to flexibly adjust the selectivity for GVL and 1,4-PeD,supports with tunable acidic or basic sites and porous structures should be considered.it has been reported that Co-Mg-Al catalyst can catalyze EL to prepare 1,4-PeD(98%)with high yield,and LDH is used as a precursor to induce strong interaction between Co and the support.it is beneficial to the dispersion of Co and the formation of a developed porous structure,thus forming abundant acidic sites.the synergistic effect of the hydrogenation sites and acidic sites of the metal can effectively catalyze the lactonization reaction and the transformation of GVL^[88]。 This provides some new ideas for the conversion of EL to 1,4-PeD,and also proves that the synergistic effect of hydrogenation and acidic sites is beneficial to the development and preparation of highly active catalysts for the selective conversion of biomass derivatives。

GVL is a non-toxic and biodegradable biomass-based platform compound with a sustainable supply of raw materials for the production of carbon-based chemicals and energy,which is considered to have a good application prospect^[12]。 more and More researchers have begun to pay attention to the application of GVL downstream products,such as 1,4-PeD,an important fine chemical intermediate^[12]。 At present,there are three ways of GVL ring-opening cleavage,and the main products are different due to the different positions of the ring-opening cleavage.When the C-O bond of the lactone bond in GVL is cleaved,1,4-PeD is the main product(Fig.6a).It has also been suggested that the direct hydrogenation of the carbonyl group of GVL results in the formation of an intermediate,2-hydroxy-5-methyltetrahydrofuran,followed by further cleavage of the C-O bond on the hydroxyl side of the intermediate,resulting in 4-hydroxypentanal or 5-hydroxy-2-pentanone,which can be converted to 1,4-PeD by hydrogenation(Fig.6B)^[12][91]。

At present,most of the studies on the preparation of 1,4-PeD by GVL are focused on the hydrogenation of LA to GVL,and the further hydrogenation of GVL to 1,4-PeD,while there are few noble metal systems for the direct catalytic conversion of GVL to 1,4-PeD.For noble metal Ru,LüJinkun studied the hydrogenation of GVL to 1,4-PeD catalyzed by different Ru-based catalysts,and the GVL conversion of 14.4%,21%and 14.4%,and the 1,4-PeD selectivity of 78.1%,86.7%and 86.7%were obtained by using framework ruthenium,Ru/AC and Ru/Al₂O₃catalysts,respectively,with water as the solvent at 393 K and 6 MPa H₂^[81]。 it was found that the interaction between the framework Ruthenium and the weak acid site of the supported ruthenium catalyst and the active site of the metal could promote the cleavage of the C—O bond of GVL to produce 1,4-PeD;ruthenium has no acidic site,so It has no similar catalytic effect^[81]。 And on the basis of experiments and calculations,we can know that the catalytic activity of these catalysts from big to small is:framework ruthenium,Ru/C and Ru/Al₂O₃,which shows that framework ruthenium has the best performance in the aspect of ester activation^[81]。

non-noble metal catalysts have low activity and poor stability,and there are some studies on them at present.the reported Non-noble metals include Cu,Ni and Co(Table 8),but The application potential needs to be further explored^{[12,91⇓⇓⇓⇓⇓⇓~98][99][100,101]}。

Cu is a promising active component in the selective production of 1,4-PeD by GVL^[12]。 For Cu-based catalysts,Du et al.Studied a series of Cu/ZrO₂catalysts for GVL to produce 1,4-PeD^[92]。 When the Cu loading was 30 wt%,the GVL conversion and 1,4-PeD selectivity of the 30-Cu/ZrO₂-OG catalyst were 97%and 99%,respectively,at 473 K,6 MPa H₂and 6 H with ethanol as the solvent,and the catalyst showed excellent stability with only a slight decrease in GVL conversion and 1,4-PeD selectivity after three cycles^[92]。 Xu et al.Further studied the direct conversion of GVL to 1,4-PeD in ethanol by Cu-based catalysts on the basis of the conversion of LA to GVL^[93]。 With 30 wt%Cu loading and 60 wt%Ti loading,a high 1,4-PeD selectivity(96%)but low GVL conversion(27%)could be obtained over Cu(30%)-TiO₂(60%)/ZrO₂-CP-600 catalyst at 473 K,5 MPa H₂and 6 H.Therefore,it is feasible to convert GVL to 1,4-PeD in ethanol with excellent selectivity using supported Cu-based catalysts,in which the surface acidic sites play a key role in the hydrogenolysis mechanism of the lactone 。

Sun et al.Used a Cu/ZnO catalyst for GVL hydrogenation to 1,4-PeD in a continuous flow reactor^[91]。 When the Cu loading was 40 wt%,the 40-Cu/ZnO-CP catalyst reacted at 413 K,1.5 MPa H₂,and the H₂flow rate was 90 cm³/min,the conversion of GVL was 82.3%and the selectivity of 1,4-PeD was 99.2%.The results showed that more 1,4-PeD was obtained at low temperature in the catalytic process,because 1,4-PeD was further converted to 2-methyltetrahydrofuran and 1-pentanol at high temperature.In addition,higher calcination temperature will reduce the surface area and catalytic activity of the catalyst,and the most suitable calcination temperature of the catalyst is 500℃,which shows excellent catalytic activity and can remain stable within 10 H.Zhai et al.Used metal organic chemical vapor deposition(MOCVD)to prepare Cu/MgO catalyst for GVL to produce 1,4-PeD^[94]。 The study showed that higher H₂pressure was favorable for GVL hydrogenation,but compared with H₂pressure,the effect of temperature on 1,4-PeD selectivity was more significant.The 18%Cu/MgO catalyst with 1,4-dioxane as solvent showed excellent catalytic activity at 473 K and 10 MPa H₂pressure for 10 H,obtaining 90.5%conversion of GVL and 94.4%selectivity of 1,4-PeD,and the catalyst still showed good stability after three cycles without significant loss of activity.Wu et al.Prepared a highly efficient Cu_x/Mg_3−xAlO nanocatalyst with controllable bifunctional catalytic sites for the selective hydrogenation of GVL to 1,4-PeD^[95]。 It was found that the conversion of GVL was 93%and the selectivity of 1,4-PeD was>99%over the Cu_1.5/Mg_1.5AlO bifunctional catalyst in 1,4-dioxane at 433 K for 12 H at 5 MPa H₂,and the catalyst could be recycled five times at 5 H under the same other conditions.The activity decreased slightly(GVL conversion decreased from about 64%to about 60%),while the selectivity of 1,4-PeD did not decrease significantly,showing excellent activity,selectivity and stability,mainly because the active metal Cu in the catalyst has a high degree of dispersion and can interact with the basic sites on the surrounding surface^[95]。

Liang et al.Prepared a Cu-based catalyst with LaCoO₃as the carrier and used it for the hydrogenation of GVL^[96]。 With 30 wt%Cu loading,92.5%conversion of GVL and 92.1%selectivity of 1,4-PeD could be obtained over Cu/LaCoO₃catalyst at 513 K under 7 MPa H₂pressure for 10 H.The method has the advantages of simple operation,mild reaction conditions,low cost,and high stability of the catalyst.Simakova et al.Used homogeneous deposition-precipitation method to prepare Cu/SiO₂catalyst for GVL hydrogenation to 1,4-PeD^[97]。 The highest 1,4-PeD selectivity(67%)was obtained with n-butanol as solvent at 403 K and 1.3 MPa H₂for 9 H,together with 32%GVL conversion.The activity of the catalyst decreased slightly with the increase of operation time(9 H)due to the partial oxidation of copper during the reaction,but it could be recovered after 2 H of reduction with H₂at 523 K,indicating that the catalyst had good stability.Liu Qiang et al.Prepared Cu/Al₂O₃and a series of Cu-Zn/Al₂O₃catalysts with different Zn/Cu molar ratios by impregnation and co-precipitation methods,and used them for GVL hydrogenolysis to produce 1,4-PeD^[12,98]。 The conversion of GVL was 98%and the selectivity of 1,4-PeD was less than 20%when 1,4-dioxane was used as solvent at 473 K and 4 MPa H₂for 3 H with Cu/Al₂O₃catalyst;However,when the Zn_1.5Cu/Al₂O₃（Zn/Cu molar ratio of 1.5)catalyst was used for 2 H,the conversion of GVL was 91%and the selectivity of 1,4-PeD was 97%.When the Cu/Al₂O₃catalyst was reused for 5 times,the conversion of GVL decreased from 98%to 50%,and the selectivity of 1,4-PeD increased;In contrast,the Zn_1.5Cu/Al₂O₃catalyst could be reused 10 times,the conversion of GVL decreased from 91%to 80%,and the selectivity of 1,4-PeD remained stable^[12]。 This indicates that the introduction of Zn can effectively increase the selectivity and optimize the reusability of the Cu/Al₂O₃catalyst,mainly because the addition of Zn can weaken the Lewis acid sites on the surface of the catalyst,inhibit the dehydration reaction of 1,4-PeD,reduce the formation of by-product 2-methyltetrahydrofuran,and then improve the selectivity of 1,4-PeD^[12]。 In addition,it was found that the catalytic performance of the catalyst and the selectivity of the product were affected by the reduction temperature,and a higher product yield could be obtained at a reduction temperature of 440℃,in which the Cu_0.3Zn/Al₂O₃catalyst gave 62%conversion of GVL and 98%selectivity of 1,4-PeD at 473 K,4 MPa H₂and 2 H^[12]。

For Ni-based catalysts,Zhang et al.Reported heterogeneous MoO_xmodified Ni catalyst for GVL selective hydrogenolysis to 1,4-PeD,which was derived from molybdate（Mo₇O₂₄⁶⁻）intercalated NiAl-LDH precursor^[99]。 The results showed that the number of surface-defective MoO_x(0<x<3)species increased with the increase of reduction temperature,and the surface-defective MoO_xspecies could significantly promote the adsorption and activation of carbonyl groups in GVL,and then promote the cleavage of C=O bond and its adjacent C—O bond,thus increasing the selectivity of products^[12,99]。 In particular,the Ni-MoO_x/Al₂O₃catalyst obtained at the reduction temperature of 600°C with mesitylene as the solvent under mild reaction conditions(433 K,4 MPa H₂,and 4 H)achieved 82%GVL conversion as well as 94.0%overall yield of 1,4-PeD and 2-methyltetrahydrofuran,with 78%1,4-PeD selectivity.In addition,when the reduction temperature was 500℃,70%GVL conversion and 92%1,4-PeD selectivity were obtained over Ni-MoO_xcatalyst in water at 433 K under 4 MPa H₂pressure for 4 H,and the hydrogenolysis of GVL in water presented a more sustainable and environmentally friendly chemical process 。

for Co-based catalysts,Srimani et al.Investigated PNNH-based cobalt complex as a catalyst For GVL hydrogenation,pointing to an unprecedented ester hydrogenation mechanism involving an enolate intermediate,indicating selectivity toward enolizable esters^[100]。 1,4-PeD was obtained in 50%yield over（PNNH）CoCl₂catalyst at 403 K,5 MPa H₂and 70 H.Zhu et al.Prepared a Co/ZrO₂catalyst(Co metal loading of 15 wt%)and applied it to the liquid-phase hydrogenation of GVL to produce 1,4-PeD^[101]。 Co/ZrO₂with Co²⁺exhibited high catalytic performance at 750°C calcination,achieving 86.1%GVL conversion and 97.2%1,4-PeD selectivity at 438 K under 5 MPa H₂pressure conditions.In addition,the cycle stability of the Co/ZrO₂-CP-750 catalyst was tested,and the conversion rate of GVL decreased slightly from 77.5%to 69.9%after three cycles,showing good stability 。

at present,the research on GVL hydrogenation is still immature At home and abroad,and the catalytic hydrogenation of GVL to 1,4-PeD is selectively carried out on homogeneous or heterogeneous catalysts.However,from the point of view of industrial implementation,homogeneous catalysts are inferior to heterogeneous catalysts due to the difficulty in separating the catalyst from the reaction medium^[97]。 in general,the catalysts for hydrogenolysis of GVL to 1,4-PeD include both noble metal and non-noble metal systems.in the precious metal system,although the yield is slightly higher in some cases,its large-scale application in industry is still not feasible in the short term,mainly because the precious metal is very scarce in the earth's crust and requires high procurement costs^[102]。 However,non-precious metal systems have been widely studied due to their low manufacturing cost,especially transition metal Cu-based catalysts,which have shown reasonable catalytic activity for the hydrogenation of C=O and have been widely used in the hydrogenation of organic compounds with carbonyl functional groups.Cu-based catalysts can use the acidic or basic sites provided by the support or the synergistic effect between heterogeneous metals to improve the activity of the catalyst and obtain higher yields,but they also have some disadvantages,such as the special treatment of metal complexes and the difficulty of catalyst separation^[12]。 It has been proved that the support of Cu-based catalysts has different abundance of acidic or basic sites,which may affect the conversion of reactants and the selectivity of products.Tuning the relative abundance of hydrogenation sites and acidic/basic sites is an important strategy to regulate the selectivity of the target product,and the development of catalysts with tunable active sites is of great significance for the production of 1,4-PeD^[102]。