Noble metal heterogeneous bifunctional catalysts such as Ru, Pd and Pt were reported for the hydrogenation of levulinic acid. Among them, Ru-based bifunctional catalysts have been studied more, as described in Table 1. The development of Ru-based bifunctional catalysts mainly focuses on the selection and design of supports. Different catalyst supports, such as carbon materials, resins, molecular sieves, metal oxides, metal organic frameworks, etc., have been studied for the synthesis of heterogeneous bifunctional catalysts supported on noble metals^{[22⇓⇓⇓~26][27][28,29][30⇓⇓~33][34]}.

The preparation of heterogeneous bifunctional catalysts from ruthenium supported carbon materials has attracted great attention. Ruthenium supported reduced sulfonated graphene oxide (Ru/rGO-S) as well as ruthenium supported reduced graphene oxide (Ru/rGO) were synthesized for levulinic acid hydrogenation in aqueous phase (Table 1, Entry 1 and 2)^[22]. Using water as solvent, Ru/rGO-S catalyst catalyzed the hydrogenation of levulinic acid to 4-hydroxypentanoic acid intermediate under mild reaction conditions of 50 ℃ and 2 MPa, and the intermediate was further dehydrated under the action of sulfonic acid strong acid sites to obtain GVL with a target product yield of 82.0% (Table 1, Entry 2). Meanwhile, the Ru/rGO-S catalyst exhibits a TOF value of 522 h^-1, which is higher than that of ruthenium supported catalysts reported in other studies. However, the catalyst showed a loss of activity in the cycle stability test, and the sulfonic acid function may reversibly poison the ruthenium species and reduce the hydrogenation activity of the ruthenium species. Ru/C combined with cation exchange resins (A70 and A15), niobium phosphate and oxide to form a bifunctional catalytic system^[23]. When Ru/C and A70 coexist, the GVL yield was 98.0% at 70 ° C and 0.5 MPa with water as the solvent (Table 1, Entry 3). The coexistence of the hydrogenation site and the acid site not only accelerates the hydrogenation of the carbonyl group in levulinic acid to 4-hydroxypentanoic acid, but also promotes the lactonization reaction to obtain GVL. In addition, ordered mesoporous carbon is prepared, and the carbon material is used as a carrier for loading Ru through functionalization of acidic groups such as P and S^[24]. The functional group contained in S leads to the inactivation of Ru species, and the Ru/OMC-P catalyst modified by phosphate group catalyzes levulinic acid with a GVL yield of 92.1% (Table 1, Entry 4). The acid site introduced by the phosphate group promotes the lactonization of the intermediate 4-hydroxypentanoic acid to produce GVL, and Ru/OMC-P can further catalyze the ring opening of GVL to produce valeric acid at 200 ℃. Ru/OMC-P exhibits excellent cycling stability. In addition, N-doped carbon materials have also been used as supports to support Ru for the preparation of hydrogenation catalysts. The N species in the carbon materials further increase the electron density of Ru, and the electron-rich Ru species realize the highly selective hydrogenation of levulinic acid to GVL^[23,24]. The N-doped porous carbon was modified by HfO₂, and the obtained support was loaded with Ru nanoclusters to prepare Ru/HfO₂@CN,Ru clusters, which cooperated with the generated acidic sites to catalyze the hydrogenation of levulinic acid to prepare GVL with a yield of 92.0% (Table 1, Entry 5)^[25]. Ru supported on single carbon materials and heteroatom doped carbon materials resulted in highly active ruthenium bifunctional catalysts (Table 1, Entry 6 and 7). Ru particles promoted the hydrogenation reaction, and the acid sites promoted the lactonization of 4-hydroxypentanoic acid and accelerated the hydrogenation reaction. In addition, sulfonated cation exchange resins were reported to be used as acidic supports, which were further impregnated with Ru to prepare bifunctional heterogeneous catalysts^[27]. GVL was prepared by LA hydrogenation catalyzed by Ru @ DOWEX at 70 ° C with 98.0% yield (Table 1, Entry 7). In addition to carbon-based materials and resinous supports, molecular sieves have been considered as common acidic supports for the preparation of noble metal bifunctional catalysts.

Ruthenium-supported ordered mesoporous MCM-49 molecular sieve catalyst was prepared by deposition-precipitation method, and was used for the hydrogenation of levulinic acid to GVL^[28]. Ru/MCM-49 catalyst was used to catalyze the hydrogenation of levulinic acid to 4-hydroxypentanoic acid, and then 4-hydroxypentanoic acid was further converted to GVL under the action of acid sites in MCM-49 molecular sieve. Among them, the Lewis acid site promotes the lactonization of 4-hydroxypentanoic acid. GVL yield reached 93.4% at 160 ° C, 2.5 MPa, and 0.5 H (Table 1, Entry 8). In addition, the Mn-modified bimetallic Ru-Mn/MCM-49 catalyst was further prepared to promote the activation of hydrogen by adding Mn to increase the electron density of Ru^[29]. The introduction of Mn further increased the ratio of Lewis acid to Bronsted acid, resulting in a GVL yield of 98.0% at a reaction temperature of 160 ° C (Table 1, Entry 9). The bifunctional catalyst prepared by loading Ru on the mesoporous molecular sieve as a catalyst carrier can significantly improve the hydrogenation activity of the catalyst for levulinic acid and obtain GVL with higher yield.

In addition, other mesoporous oxide supports were further reported to support Ru to prepare bifunctional catalysts for the hydrogenation of levulinic acid to GVL. Ruthenium confined spherical mesoporous silica or zirconium modified spherical mesoporous silica catalysts (Ru/ZrSMS and Ru/SMS) were synthesized for the hydrogenation of levulinic acid and its esters (Table 1, Entry 10 and 11). Ultra-small Ru nanoparticles promoted the hydrogenation of levulinic acid to 4-hydroxypentanoic acid.The acid sites provided by Zr promote the dehydration lactonization of the intermediate to produce GVL. Under the reaction conditions of 70 ℃, 0.5 MPa hydrogen and 3 H, the Ru/ZrSMS catalyst achieves 94.5% GVL yield (Table 1, Entry 10)^[30]. The addition of acidic molecular sieve catalyst (H-Y₃₀,H-beta₂₅ as well as H-ZSM-5₃₀) further promoted the production of GVL, and the acidic sites accelerated the dehydration lactonization reaction. Among them, benzene sulfonic acid and heteropolyacid can poison Ru particles, thus reducing the catalytic activity. The type and number of acid sites can affect the activity of metal active sites and the reaction rate of lactonization. Other mesoporous oxides such as (AlO) (ZrO) 0.1, SBA-15, and TiO₂ were further used as acidic supports to support ruthenium to prepare Ru-based bifunctional catalysts (Table 1, Entry 12 ~ 14). The cooperation of Ru nanoparticles and acidic sites promoted the hydrogenation of levulinic acid to produce 4-hydroxypentanoic acid under the action of ruthenium species, and then the lactonization of 4-hydroxypentanoic acid under the action of acidic sites produced higher yield of GVL^[31][32][33].

In addition, other types of supports, such as other metal-organic frameworks (Ru/MIL-101-Cr), cross-linked sulfonated polyethersulfone (Ru/SPES), and hydroxyapatite (Ru/HAP), have been reported to prepare ruthenium-based bifunctional catalysts (Table 1, Entry 15-17), which efficiently catalyze the hydrogenation of levulinic acid to GVL at 70 ° C in aqueous phase^[34][35][36]. Other bifunctional catalysts supported on noble metals such as Pd and Pt have also been reported (Table 1, Entry 18 ~ 20). Ru-based bifunctional catalysts show better catalytic activity than other noble metals. Metal species and acidic sites cooperate to catalyze the hydrogenation and dehydration of levulinic acid, resulting in higher yields of GVL^{[36⇓⇓⇓~40][36,41]}.

Non-noble metals were further studied to replace noble metals for the preparation of non-noble bifunctional catalysts for the hydrogenation of levulinic acid and its esters to produce GVL. Monometallic nickel-based, cobalt-based, copper-based, as well as mixed bimetallic bifunctional catalysts were reported for the conversion of levulinic acid and its esters, as described in Table 2^{[42⇓⇓⇓⇓⇓⇓⇓⇓⇓~52][53⇓⇓⇓~57][58⇓~60][60⇓⇓⇓~64]}. Carbon materials, molecular sieves and oxides are commonly used as catalyst supports to impregnate non-precious metals to prepare bifunctional catalysts^{[42⇓⇓~45][46][47⇓⇓~50]}.

Nickel-based bifunctional catalysts have been widely studied for the hydrogenation of levulinic acid and its esters to produce GVL. Ni/HZSM-5 bifunctional catalyst was prepared for gas phase hydrogenation of levulinic acid using nickel particles supported on molecular sieve as acidic carrier^[46]. The introduction of nickel species promotes the formation of acidic sites, especially Lewis acid sites. Under the reaction condition of 320 ℃, the nickel particles promoted the hydrogenation of carbonyl group in levulinic acid to 4-hydroxypentanoic acid, the Lewis acid sites promoted the anhydrolactonization of 4-hydroxypentanoic acid to GVL (98.6%, Table 2, Entry 5), the Bronsted acid sites in nickel aluminate had synergistic effect with the metal hydrogenation sites in the catalyst, and the Bronsted acid sites stabilized the metal species^[46]. Nickel-based bifunctional catalysts can promote the hydrogenation of levulinic acid and its esters to produce GVL at higher reaction temperatures, and the synergistic catalysis of Lewis acid and Bronsted acid sites with nickel species achieves high GVL yields. In addition, other nickel-based bifunctional catalysts were synthesized for the liquid-phase hydrogenation of levulinic acid, and the catalysts showed excellent catalytic activity by selecting organic solvents such as dioxane and isopropanol as reaction media (Table 2, Entry 1 ~ 11). Nickel species can improve the stability of GVL and avoid further ring opening of GVL to produce downstream products, so it is widely used as a non-precious metal active species^[50].

Cobalt-based and copper-based bifunctional catalysts have been further studied compared to nickel-based catalysts. Co-supported N-doped carbon material (Co @ NC-700) catalyzed the hydrogenation of levulinic acid in 1,4-dioxane to prepare GVL, and the synergistic effect of metallic cobalt and acidic sites achieved a GVL yield of 100.0% (Table 2, Entry 12). Cobalt supported Y zeolite catalyst was synthesized for the hydrogenation of levulinic acid to produce GVL. Lewis acid sites are beneficial to improve the selectivity of GVL, and Bronsted acid sites can further promote the ring-opening reaction of GVL at higher temperature to produce valeric acid and other products^[54]. The Co/Y catalyst was used to catalyze the hydrogenation of levulinic acid to give GVL (80.0%, Table 2, Entry 13) in a fixed bed reactor at 200 ℃. Other inorganic metal oxides such as silica and alumina were used as supports to support cobalt species to prepare heterogeneous bifunctional cobalt-based catalysts (Table 2, Entry 14-17). Co-Al and 2.1 Co-0.9 Mg-Al catalysts with adjustable hydrogenation sites and acid-base sites were used for the hydroconversion of ethyl levulinate in aqueous phase, and the two catalysts achieved 98.0% and 97.0% GVL yields, respectively, at 150 ° C (Table 2, Entry 16 and 17)^[57]. The hydrogenation of ethyl levulinate to ethyl 4-hydroxypentanoate is promoted by the synergistic catalysis of the hydrogenation site and the acid site, the lactonization of ethyl 4-hydroxypentanoate is promoted by the Bronsted acid site, and the Lewis acid site can improve the adsorption of the catalyst to the reaction substrate and the intermediate, so that GVL with high yield is obtained. At the same time, the acidic site cooperates with the cobalt active site to further promote the ring opening of GVL in isopropanol to produce 1,4-pentanediol. Therefore, the distribution of acid sites and cobalt particles in cobalt-based bifunctional catalysts needs to be precisely regulated to obtain GVL as the target product.

In addition, copper supported on oxide supports was used as a bifunctional catalyst to catalyze the conversion of levulinic acid and its esters to produce GVL. The Cu/Zr-Al-3 catalyst was synthesized for the conversion of levulinic acid to produce GVL,Cu⁰ promoted the dissociation of hydrogen to produce active hydrogen species, which further attacked the carbonyl group to give 4-hydroxypentanoic acid, and the acidic site further promoted the dehydration lactonization of the intermediate to give GVL (100.0%, Table 2, Entry 18)^[58]. The presence of oxygen deficiency in the Cu/Zr-Al-3 catalyst also promotes the adsorption of levulinic acid as well as intermediates^[57]. The layered double hydroxide-derived CuAl catalyst catalytically converted the hydrogenation of levulinic acid at 110 ° C under reaction conditions to give a GVL yield of 95.3% (Table 2, Entry 19), with copper as the active species to produce active hydrogen species by activating hydrogen,This further enables the hydrogenation of the carbonyl group in levulinic acid to give 4-hydroxypentanoic acid, and the Bronsted acid site promotes the lactonization of 4-hydroxypentanoic acid to produce GVL and stabilize GVL, resulting in a higher GVL yield^[59]. At the same time, the basic sites generated by magnesium species in the catalyst can further neutralize the acidic sites, improve the stability of copper species, and promote the further ring opening of GVL to obtain 1,4-pentanediol. The copper-based bifunctional catalyst shows high hydrogenation activity and catalyzes the hydrogenation of levulinic acid and its esters to produce GVL (Table 2, Entry 18-20), but the produced GVL can be further converted into compounds such as 1,4-pentanediol or 2-methyltetrahydrofuran through ring opening, thereby reducing the overall yield of GVL^[60].

In order to further develop non-noble metal bifunctional catalysts with high activity and stability, the second metal was further introduced to prepare bimetallic bifunctional catalysts. Catalysts such as Ni/Cu/Al/Fe,Ni_4.59Cu₁Mg_1.58Al_1.96Fe_0.70,Ni₂Co₁P,Cu-Ni/Al₂O₃-ZrO₂ and Cu-Ni/Al₂O₃ have been reported (Table 2, Entry 21 – 25). The bimetallic active sites promote the adsorption of levulinic acid and activation hydrogenation to 4-hydroxypentanoic acid. The acidic sites on the catalyst further promote the dehydration reaction to realize the lactonization of the intermediate to produce GVL^{[61][62][63][64][65]}. Compared with the single metal bifunctional catalyst, the binary metal bifunctional catalyst also showed excellent hydrogenation activity of levulinic acid and its esters, and the bimetallic active sites could further promote the adsorption and activation of hydrogen and accelerate the hydrogenation reaction rate.

In the non-noble metal bifunctional catalyst, the non-noble metal active species and the acid site can promote the hydrogenation of levulinic acid and its ester on the metal site and the lactonization of the intermediate on the acid site to obtain a high yield of GVL. Non-noble metal catalysts catalyze the hydrogenation of levulinic acid and its esters to produce GVL in gas phase or liquid phase, but compared with noble metal bifunctional catalysts, non-noble metal bifunctional catalysts have more stringent reaction conditions, requiring higher reaction temperature and hydrogen pressure. At the same time, due to the low stability of non-noble metal bifunctional catalysts in water phase, the reaction solvents are mainly concentrated in organic solvents such as dioxane and alcohols^{[42⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~65]}. In order to realize the preparation of GVL from levulinic acid and its esters under greener reaction conditions, non-noble metal bifunctional catalysts with high hydrothermal stability need to be further developed. At the same time, bifunctional catalysts were further studied to catalyze levulinic acid transfer hydrogenation to prepare GVL, avoiding the use of external hydrogen, and replacing high-pressure hydrogen in existing studies by using suitable hydrogen donors.

Noble metal or non-noble metal supported acid supports were used to prepare bifunctional catalysts as described in Table 3. Metal species, such as Ru, Ni, and Cu, catalyze hydrogen transfer from hydrogen donors (Table 3, Entry 1-8), and acidic sites in acidic supports promote dehydration reactions to achieve lactonization^{[66,68⇓~70]}.

The CuNi-0.4Al/AC bifunctional catalyst was prepared for the catalytic transfer hydrogenation of levulinic acid, and the GVL yield was 97.2% at 220 ° C for 2 H (Table 3, Entry 4)^[69]. The CuNi alloy is used as a hydrogenation site to activate isopropanol to generate active hydrogen species, thereby realizing the hydrogenation of carbonyl in levulinic acid to obtain 4-hydroxypentanoic acid, and the subsequent acid site promotes the lactonization reaction of 4-hydroxypentanoic acid to generate GVL. Ni₃P-CePO₄ catalyst was synthesized for catalytic transfer hydrogenation of levulinic acid to produce GVL, and the distribution of acidic sites and basic sites in the catalyst was controlled by changing the Ce/Ni ratio^[70]. Under the reaction conditions of 180 ° C and 2 H, the Ni₃P-CePO₄(0.1) catalyst achieved 99.9% levulinic acid conversion and 89.9% GVL yield (Table 3, Entry 5). The Lewis acid site activates the carbonyl group in levulinic acid, the Lewis base site catalyzes the dissociation of the hydroxyl group in isopropanol to give an active hydrogen species, isopropanol is converted to acetone, the unsaturated carbonyl group interacts with the metal site and reacts with the adjacent active hydrogen to give 4-hydroxypentanoic acid, and the subsequent acid site further promotes the anhydrolactonization reaction to produce GVL^[70]. In the catalytic transfer hydrogenation process of the metal-supported bifunctional catalyst, the metal site or the Lewis base site can promote the hydrogen donation of the hydrogen donor, the active hydrogen species realize the hydrogenation of levulinic acid to obtain 4-hydroxypentanoic acid, and the subsequent acidic site promotes the further lactonization of the intermediate to obtain GVL (fig. 3).

Acid site as well as basic site tunable bifunctional catalysts were prepared by metal heteroatom modified molecular sieves (Table 3, Entry 9 and 10). Zr-beta was synthesized for Meerwein-Ponndorf-Verley (MPV) reduction of levulinic acid to prepare GVL. Under the reaction conditions of 118 ℃ and 10 H, the conversion of levulinic acid in isopropanol solvent reached 100.0%, and the yield of GVL reached 96.0% (Table 3, Entry 9)^[74]. A Lewis acid site and a basic site in Zr-beta promote MPV reduction reaction of an unsaturated carbonyl group in levulinic acid to generate 4-hydroxypentanoic acid,The subsequent weak acid sites promote the anhydrolactonization of 4-hydroxypentanoic acid to produce GVL, and too many Bronsted acid sites can promote the esterification of levulinic acid with isopropanol, or promote the occurrence of other side reactions, affecting the yield of GVL. There are abundant acid sites in the modified molecular sieve catalyst, among which Lewis acid sites can replace the traditional metal sites to promote the MPV reduction reaction to realize the hydrogenation of unsaturated carbonyl groups in levulinic acid and its esters, and basic sites can promote the MPV reduction together with Lewis acid sites^[75]. In addition, 4-hydroxypentanoic acid acts as an intermediate to produce GVL by lactonization at the Bronsted acid site.

Different mixed metal oxide bifunctional catalysts have been reported, such as SnO₂/SBA-15, GluPC-Zr, ZrO₂(10)/SBA-15, ZrFeO (1:3) -300, ZrO₃, Zr₁Fe₁-150, and Mn₂CoO_x (Table 3, Entry 11 – 17). By regulating the types of oxides, the distribution of acidic sites and basic sites in the catalyst can be changed, and then the MPV reduction of levulinic acid and its esters and the subsequent lactonization of 4-hydroxypentanoic acid can be regulated^{[76][77][78][79][80][81][82]}. The SnO₂/SBA-15 was prepared to prepare GVL by catalytic transfer hydrogenation of levulinic acid with isopropanol as solvent at 110 ℃ for 8 H, the conversion of LA was 85. 0%, and the yield of GVL was 80. 8% (Table) The Lewis acid sites in the 3,Entry 11),SnO₂/SBA-15 promoted the MPV reduction reaction to realize the hydrogenation of levulinic acid to obtain 4-hydroxypentanoic acid, and the Bronsted acid sites promoted the subsequent dehydration lactonization reaction of 4-hydroxypentanoic acid to obtain GVL^[76]. In addition, the FeZrO_x catalyst was synthesized to prepare GVL by catalytic transfer hydrogenation of ethyl levulinate, and the conversion of ethyl levulinate was 100.0% and the yield of GVL was 87.2% at 230 ℃ for 3 H with ethanol as solvent (Table 3, Entry 14)^[79]. The FeZrO_x catalyst has abundant acid sites and basic sites, and the basic sites and Lewis acid sites promote the hydrogen donation of the alcohol, so that the hydrogenation of levulinic acid and levulinic acid ester is realized through MPV reduction reaction to obtain an intermediate, and the acid sites further catalyze the lactonization of the intermediate to obtain GVL. The acidic site and the basic site in the Zr₁Fe₁-150 catalyst cooperate to catalyze levulinic acid to prepare GVL, and the product yield is 96.7% at 200 ℃ for 1 H (Table 3, Entry 16)^[81]. In addition to alcohols, formic acid was used as a hydrogen donor, and a GVL yield of 77.0% was obtained 1 h,Mn₂CoO_x catalyst at 230 ° C (Table 3, Entry 17)^[82]. Therefore, a bifunctional catalyst composed of mixed acidic and basic oxides is used, the basic sites in the catalyst and a Lewis acid promote catalytic transfer hydrogenation, and other Bronsted acid or weak acid sites promote the lactonization reaction of 4-hydroxypentanoic acid and esters thereof, so that different sites cooperate to catalyze the hydrogenation of levulinic acid and esters thereof to prepare GVL. In the process of hydrogen transfer hydrogenation, alcohols, especially isopropanol, are more used as solvents than hydrogen donors such as formic acid to realize the transfer hydrogenation of levulinic acid and its esters to produce GVL.

In addition, bifunctional catalysts such as metal cation-modified MOF catalysts (ZrF MOFs), cation-modified corrosion acid catalysts (Zr-humic acids), MOF-derived bifunctional catalysts (UiO-66-S₆₀), and heteropolyacid and metal-modified MOF composite catalysts (HPW @ MOF-808) have been further studied (Table 3, Entry 18 – 21)^{[83][84][85][86]}. The bifunctional catalyst is obtained by compounding different acidic materials and modifying an acidic carrier with metal, and has adjustable Lewis acid, Bronsted acid and basic sites. The bifunctional catalyst realizes the hydrogen donation of a hydrogen donor by means of a Lewis acid and a basic site to promote the MPV reduction reaction, promotes the transfer hydrogenation of levulinic acid and esters thereof to obtain intermediates such as 4-hydroxypentanoic acid or amyl ester, and the subsequent Bronsted acid site promotes the lactonization reaction of the intermediates to obtain GVL^[84].

Compared with the direct hydrogenation of levulinic acid and its esters to prepare GVL, the catalytic transfer hydrogenation reaction avoids the use of high-pressure hydrogen, and uses hydrogen donors such as alcohols and formic acid to replace high-pressure hydrogen, which is simpler, more feasible and more sustainable. Isopropanol is used more as a hydrogen source than other hydrogen donors, as the hydroxyl groups on the branched chain in isopropanol are more likely to break down to produce active hydrogen species^[87,88]. In addition, formic acid is used as a hydrogen source and a solvent, and since formic acid is acidic and can corrode a reaction apparatus, it is more preferable to use an alcohol as a hydrogen source^[89]. The reaction rate of catalytic transfer hydrogenation is slower than that of direct hydrogenation, just as the adsorption activation of hydrogenation sites for hydrogen is easier than the hydrogen donation of hydrogen donors. Subsequently, transfer hydrogenation catalysts with higher activity and stability need to be further developed.