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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 8: 1237 - 1253

DOI: https://doi.org/10.7536/PC231104

Review

Selective Hydrogenation of Acetylene: from Thermal Catalysis to Electrocatalysis, Photocatalysis and Photothermal Catalysis

  • Baisheng Pang ,
  • Yingying Xing ,
  • Ruihong Gao ,
  • Yaohua Fang ,
  • Haijun Zhang ,
  • Liang Huang , *
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  • The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
*email:

Received date: 2023-11-03

  Revised date: 2024-02-13

  Online published: 2024-04-16

Supported by

National Natural Science Foundation of China(52372030)

National Natural Science Foundation of China(U23A20559)

National Natural Science Foundation of China(52272021)

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Abstract

ethylene is one of the most important raw materials in the modern petrochemical industry.the preparation of ethylene by steam cracking of petroleum hydrocarbons generates acetylene with a volume fraction about 0.3%to 3%.These trace amounts of acetylene can poison the catalyst of the ethylene polymerization reaction.selective catalytic hydrogenation of acetylene is considered to be one of the most effective methods for removing acetylene impurities.This paper reviews the research progress of acetylene selective hydrogenation in recent years,introduces the reaction mechanism of acetylene hydrogenation,and summarizes the effects of catalyst active components,additives and carriers on the performance of acetylene selective hydrogenation.the development trend of how to further improve the performance of acetylene selective hydrogenation is discussed from the perspectives of electrocatalysis,photocatalysis and photothermal catalysis.Finally,some suggestions are proposed for the subsequent research on the selective hydrogenation of acetylene。

Contents

1 Introduction

2 Reaction mechanism of acetylene hydrogenation

3 Research progress of catalysts for thermocatalytic selective hydrogenation of acetylene

3.1 Catalyst active components and additives

3.2 Catalyst carriers

4 Trends in selective hydrogenation of acetylene

4.1 Electrocatalytic selective hydrogenation of acetylene and alkynes

4.2 Photocatalytic hydrogenation of acetylene and alkynes

4.3 Photothermal catalyzed hydrogenation of acetylene and alkynes

5 Conclusion and outlook

Key words： selective hydrogenation of acetylene; catalyst; thermal catalysis; electrocatalysis; photocatalysis; photothermal catalysis

Cite this article

Baisheng Pang , Yingying Xing , Ruihong Gao , Yaohua Fang , Haijun Zhang , Liang Huang . Selective Hydrogenation of Acetylene: from Thermal Catalysis to Electrocatalysis, Photocatalysis and Photothermal Catalysis[J]. Progress in Chemistry, 2024 , 36(8) : 1237 -1253 . DOI: 10.7536/PC231104

1 Introduction

ethylene is one of the chemicals with the largest output in modern chemical industry.it is the key polymerization raw material and reaction intermediate in chemical production.More than 75%of petrochemical products are ethylene.Therefore,ethylene plays an important role in the field of petrochemical industry.Ethylene can participate in a series of chemical reactions,such as polymerization,oxidation,alkylation,halogenation,hydration and oligomerization of ethylene,so It is widely used in the fields of synthetic plastics,rubber,fibers,pharmaceuticals,pesticides and dyes[1⇓~3]。 in recent years,the demand for ethylene has continued to increase,but the supply of ethylene market is tight,and the ethylene production technology needs to be further improved.At present,the application field of ethylene industry In China has been gradually broadened,and the breakthroughs of many new technologies have effectively promoted the rapid development of ethylene industry.the whole ethylene industry has entered a period of stable progress from the stage of rapid promotion。
Steam cracking of petroleum hydrocarbons(such as light diesel oil and naphtha)is mostly used to produce ethylene in industry[4]。 However,this method will produce acetylene with a volume fraction of 0.3%~3%,and trace acetylene will poison the catalyst(Ziegler-Natta catalyst)for ethylene polymerization[5,6]。 This will not only reduce the catalyst activity and service life,but also reduce the quality of polyethylene.Therefore,the content of acetylene needs to be reduced below 5 ppm（10-6）[7]。
The methods for removing trace acetylene from ethylene include selective catalytic hydrogenation,ammoniation,cryogenic distillation,solvent absorption,copper acetylide precipitation,complexation adsorption,and porous material adsorption[8⇓⇓⇓~12]。 Selective catalytic hydrogenation is generally used in industry to convert acetylene（C2H2）in feed gas into ethylene（C2H4）as much as possible and to minimize excessive hydrogenation of C2H4.The method has the advantages of simple process,low energy consumption,low environmental pollution,low ethylene loss,high treatment capacity and the like[13,14]。
Catalyst is the key to the selective hydrogenation of acetylene.the hydrogenation of C≡C to C=C can be promoted by selecting suitable catalysts,while the excessive hydrogenation of C≡C to C—C compounds can be inhibited.At present,the selective hydrogenation of acetylene by thermal catalysis is the most widely used in industry and is relatively mature.Heterogeneous catalysts with metal nanoparticles as the main active component have been widely used in chemical industry because of their high activity,reusability and stability[15]。 in addition,single-atom catalysts have shown excellent performance In many catalytic applications because of their abundant active sites and ultrahigh atom utilization.However,conventional heterogeneous catalysts can lead to excessive hydrogenation,thereby reducing the reaction selectivity.Therefore,improving the selectivity of ethylene products is still one of the main challenges for the selective hydrogenation of acetylene.In recent years,in order to achieve the goal of"carbon peak,carbon neutralization"and reduce greenhouse gas emissions,some researchers have applied electricity,light and photothermal catalytic technology to acetylene selective hydrogenation reaction,and devoted themselves to exploring an environmentally friendly and efficient new path of acetylene selective hydrogenation to ethylene。
in this paper,the research progress of selective hydrogenation of acetylene In recent years was reviewed.the reaction mechanism of selective hydrogenation of acetylene,the active components,promoters and supports of catalysts,and the development trend of electrocatalysis,photocatalysis and photothermal catalysis for selective hydrogenation of ethyne were introduced。

2 Acetylene hydrogenation reaction mechanism

At present,researchers generally believe that the hydrogenation reaction of acetylene follows the Horuiti-Polanyi mechanism,and the reaction process of acetylene and hydrogen is divided into the following five steps[16]。
(1)hydrogen and acetylene gradually diffuse from the gas phase to the surface of the catalyst and are adsorbed on the active sites of the catalyst;
(2)the hydrogen molecule adsorbed at the active site dissociates into two hydrogen atoms;
(3)an acetylene molecule adsorbed at the active site combines with an adjacent hydrogen atom to form an intermediate species;
(4)the intermediate species continues to combine with another adjacent hydrogen atom to form ethylene;
(5)If the adsorption capacity of ethylene molecules on the active sites is weak,it is easy to desorb from the active sites,and the vacant active sites continue to be used for hydrogen activation and acetylene hydrogenation;If the ethylene molecule cannot be desorbed from the active site in time,it will be further hydrogenated to form ethane。
the following reactions may occur during The selective hydrogenation of acetylene:[17]
主反应C2H2+H2→C2H4 ΔH=-174.3 kJ·mol-1
副反应C2H4+H2→C2H6 ΔH=-136.7 kJ·mol-1
C2H2+2H2→C2H6 ΔH=-311.0 kJ·mol-1
C2H2+nC2H2+H2→C2n+2H2n+4
2C2H2+H2→C4H6
C4H6+nC2H2+H2→C2n+4H2n+8
When the reaction temperature is high,the following side reactions also occur:
C2H4→2C+2H2 ΔH=-46.0 kJ·mol-1
C2H4→CH4+C ΔH=-125.5 kJ·mol-1
C2H2→2C+H2 ΔH=-222.7 kJ·mol-1
In the above reaction,formula(1)is the main reaction,acetylene is hydrogenated to produce ethylene,and the reaction heatΔH is-174.3 kJ·mol-1.The side reactions are more complex,among which formulas(2),(3)and(1)are competitive reactions,formulas(4)~(6)are polymerization reactions,and formulas(7)~(9)are reactions causing carbon deposition.There are two main factors affecting the selectivity of acetylene hydrogenation:one is the further hydrogenation of ethylene to ethane;Secondly,the acetylene adsorbed on the surface of the catalyst is easy to hydrodimerize to form unsaturated compounds such as 1,3-butadiene,which continue to polymerize to form polymer(green oil),blocking the micropores of the catalyst,resulting in a decrease in its specific surface area,resulting in a decrease in catalyst activity and deactivation[18]。 Generally,the main method to improve the performance of acetylene selective hydrogenation is to develop high performance catalysts to promote the main reaction smoothly and reduce the occurrence of side reactions.For example,adjusting and controlling the active components of the catalyst,adding promoters,selecting suitable carriers and changing the preparation method of the catalyst,etc.in addition,changing the ratio of hydrogen to acetylene in the feed gas or adding a certain amount of carbon monoxide(competitive adsorption with ethylene on the catalyst)in the reactor can also improve the selectivity of ethylene[19]。

3 Study on Thermal Catalysts for Acetylene Selective Hydrogenation

Currently,thermal catalytic acetylene selective hydrogenation is commonly used for The purification of acetylene in ethylene streams.the thermal catalytic acetylene selective hydrogenation catalyst is mainly composed of an active component,an additive and a carrier。

3.1 Catalyst active component and promoter

the active component of the catalyst,also known as the main catalyst,is the main body of the multi-component catalyst,and is also the key substance for catalysis.different active sites have Different catalytic properties in the reaction.At present,the active components of acetylene selective hydrogenation catalysts are mainly metals of VIII group and IB group.in the selective hydrogenation of acetylene,the catalytic activity of various metals can be roughly arranged in the following order:Pd>Pt>Ni~Rh>Co>Fe>Cu~Au[20]。 Among them,Pd and Ni are the most widely used catalytic active components in the selective hydrogenation of acetylene because of their high catalytic hydrogenation activity。

3.1.1 Pd-based catalyst

Monometallic Pd catalyst can achieve high acetylene conversion,but the selectivity to ethylene is very low[21,22]。 Relevant studies have found that the dispersion of Pd active components can be improved by optimizing the preparation method of the catalyst,thus affecting the performance of the Pd catalyst[23⇓~25]。 Li et al.Prepared Pd/Al2O3catalysts with different Pd loadings by the impregnation method usingγ-Al2O3as the support and Pd(NO3)2as the palladium source,and treated them by non-thermal RF plasma to study their effects on the performance of acetylene selective hydrogenation[26]。 The results show that the plasma treated sample exhibits higher acetylene reactivity and ethylene selectivity in the whole temperature range(25~65℃)compared with the sample heat-treated at 120℃in air.The results of X-ray photoelectron spectroscopy(XPS)and differential scanning calorimetry(DSC)（H2-DSC）in hydrogen atmosphere confirmed that the plasma treatment at room temperature not only reduced Pd precursor to metallic state to a large extent,but also enhanced the interaction between Pd active component and Al2O3support,thus promoting the hydrogenation of acetylene and the desorption of ethylene 。
Zhou et al.Used impregnation method to obtain PdZn intermetallic compound(PdZn/ZnO)by reduction at 400°C with Pd(NO3)2as palladium source and ZnO as carrier[27]。 the results show that the PdZn/ZnO catalyst has a special Pd-Zn-Pd spatial arrangement(as shown in Figure 1),and shows excellent catalytic performance at 80℃,with an acetylene conversion of 92%and an Ethylene selectivity of 89%.Density functional theory(DFT)calculations show that the special Pd-Zn-Pd arrangement in PdZn/ZnO is the origin of its excellent catalytic performance,that is,acetylene is adsorbed on two adjacent Pd sites with strongσbonds,which is easy to be adsorbed and activated,and hydrogenated to ethylene;Ethylene,on the other hand,adsorbs on isolated Pd sites with weakπbonds,which is beneficial to its desorption from the catalyst surface and avoids excessive hydrogenation to ethane。
图1 具有Pd-Zn-Pd空间排布的PdZn金属间化合物纳米结构催化乙炔选择性加氢示意图[27]

Fig. 1 Schematic diagram of selective acetylene hydrogenation catalyzed by PdZn intermetallic nanostructure with Pd-Zn-Pd ensembles[27]. Copyright 2016, American Chemical Society

atomically dispersed catalysts are currently a research hotspot and also play an important role in the selective hydrogenation of acetylene.Among them,single-atom catalysts have attracted much attention due to their Atomically dispersed active sites,high atom utilization,unique and controllable coordination environment,excellent catalytic activity and selectivity[28]。
Based on the previous study of PdZn intermetallic compounds,Zhou et al.Found that the construction of supported single-atom catalysts with atomically dispersed active centers is another effective way to obtain high selectivity[29]。 They prepared Pd/ZnO catalyst by incipient wetness impregnation method and studied the effect of different Pd loadings on the performance of acetylene selective hydrogenation.The supported single-atom catalyst（Pd1/ZnO SACs）was obtained by decreasing the metal Pd content(1 wt%→0.1 wt%→0.01 wt%)and reducing at 100°C.High resolution electron microscopy(HREM)results show that the particle size of the metal nanoparticles decreases significantly when the Pd loading decreases from 1 wt%to 0.1 wt%,and the Pd nanoparticles are dispersed on the surface of the support ZnO in a monatomic state on the surface of the 0.01 wt%-Pd/ZnO catalyst.The catalytic performance test results showed that the 0.01 wt%-Pd/ZnO catalyst exhibited more excellent catalytic performance in the selective hydrogenation of acetylene,with comparable ethylene selectivity and higher activity compared with their previously reported PdZn intermetallic catalyst.At 80°C,the acetylene conversion is close to 100%and the ethylene selectivity is greater than 80%.In addition,X-ray absorption spectroscopy and electron spectroscopy show that the Pd active species are more electropositive with the decrease of Pd loading.This is attributed to the fact that the Pdδ+active site of the Pd1/ZnO single-atom catalyst is easy to interact electrostatically with acetylene,which activates acetylene to produce ethylene,and promotes the adsorption of ethylene in the form of weakπbond,thus facilitating its desorption from the catalyst surface and obtaining higher selectivity 。
Hu et al.Used Pd(NH3)4(NO3)2as palladium source and carbon nanotube(CNT)as support to synthesize a Pd single-atom catalyst(Pd1-N8/CNT)attached to the Lewis base of N8by cyclic voltammetry,and the Pd single atom was stabilized on the terminal N of N8(as shown in Fig.2 )[30]。 The experimental results of acetylene selective hydrogenation showed that the catalyst exhibited remarkable catalytic performance at 40℃,with acetylene conversion of 83%and ethylene selectivity of 98%.Acetylene temperature-programmed desorption and DFT calculations show that,unlike the dissociation of H2on the metal sites of traditional catalysts,H2dissociates on the N sites close to Pd on Pd1-N8/CNT catalysts,Moreover,the charge transfer between Pd and N8prevents Pd agglomeration,and the synergistic effect is beneficial to the formation of ethylene and prevents the excessive hydrogenation of acetylene to ethane,which is the key to the high ethylene selectivity of the catalyst 。
图2 乙炔在Pd1-N8/CNT上的选择性加氢机理图[30]

Fig. 2 Selective hydrogenation mechanism of acetylene on Pd1-N8/CNT[30]. Copyright 2021, Elsevier

However,the synthesis of stable and highly loaded Pd-based single-atom catalysts,especially on oxide supports,remains a great challenge.Guo et al.Reported that a simple encapsulation strategy was used to prepare Pd single-atom catalysts,that is,the CeO2support was synthesized by co-precipitation method,then Pd was introduced onto the CeO2support by adsorption method,and then the Pd/CeO2catalyst was reduced at different temperatures[31]。 Diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS)and X-ray photoelectron spectroscopy(XPS)characterization showed that Pd nanoparticles were more likely to have strong metal-support interaction(SMSI)with the CeO2support than Pd single atoms.Therefore,by controlling the SMSI state,i.e.,under high-temperature reduction at 600°C,the Pd nanoparticles were completely covered by the CeO2support,while the Pd single atoms were exposed on the surface,resulting in a Pd1/CeO2single-atom catalyst.This method significantly improves the ethylene selectivity of Pd catalyst in the selective hydrogenation of acetylene,and the ethylene selectivity is significantly improved to 85%at 160℃when acetylene is completely converted.On the one hand,the isolation of Pd atom is beneficial to reduce the weakπ-bond adsorption strength of ethylene;On the other hand,the physical coverage of Pd nanoparticles by the support inhibits ethylene adsorption and promotes its desorption.In addition,the SMSI effect suppresses the aggregation of Pd atoms after reduction at high temperature,thus ensuring the stability of the catalyst 。
From the selective hydrogenation of acetylene,it can be seen that the active sites of the catalyst play a key role in the whole hydrogenation process,and the active sites have a significant impact on the activity and selectivity of the catalyst[32]。 the above results indicate that the ethylene selectivity can be improved while ensuring high catalytic activity by optimizing the preparation method of the catalyst and regulating the spatial arrangement of the metal sites to change the dispersion state,atomic size and electronic state of the active component of the Pd-based catalyst.In addition,single-atom catalysts have excellent catalytic activity and ethylene selectivity,but their large-scale preparation is still a challenge。

3.1.2 Catalyst promoter

Generally,most Pd catalysts show high catalytic activity but low selectivity to ethylene,and the ethylene selectivity will decrease sharply with the extension of the reaction cycle[33,34]。 Therefore,in order to improve the catalytic performance of the catalyst,it is a feasible method to introduce a second metal component as a co-catalyst to regulate and modify the crystal structure and electronic structure of the catalyst.Although the cocatalyst itself is not active,it can improve the activity,selectivity,toxicity resistance and stability of the catalyst,which will be described from two aspects of theoretical calculation and experiment[35⇓~37]。
Liu used density functional theory calculations to study the hydrogenation of acetylene on Pd-doped Ag nanoclusters[38]。 The simulation results show that acetylene and ethylene molecules adsorb weakly on the Ag55cluster,and H2cannot bind to any site of the cluster.Therefore,the catalytic hydrogenation of acetylene and ethylene cannot be performed on the Ag55cluster.However,H2and acetylene can be adsorbed at the vertex and edge sites of the Ag54Pd1cluster at the same time,and the reaction energy barrier of acetylene hydrogenation to ethylene is lower than 0.58 eV,which indicates that acetylene hydrogenation is easier.However,the generated ethylene molecules are easy to adsorb on the top of the Pd doping atoms and occupy the only adsorption site of H2,so ethylene can not be further hydrogenated into ethane 。
Ma et al.Studied the hydrogenation reaction mechanism of acetylene over Pd/Cu monoatomic alloy(SAAs)and monolayer Pd supported Cu(111)(1 ML Pd/Cu(111))model catalysts in detail by first-principles calculations[39]。 The results show that the SAAs catalyst has higher ethylene selectivity for acetylene hydrogenation than 1 ML Pd/Cu(111),but the acetylene conversion activity is lower than that of 1 ML Pd/Cu(111).The high ethylene selectivity of SAAs is mainly attributed to the easy desorption of ethylene and the moderate dissociation activity of H2molecules.The main factor for the lower ethylene selectivity on 1 ML Pd/Cu(111)is its higher ability to promote C—H/C—C bond cleavage,leading to the formation of carbonaceous deposits and polymers such as benzene,which reduce the selectivity to ethylene 。
Dodangeh et al.Developed a new nanocatalyst for selective hydrogenation of acetylene to ethylene[40]。 mesoporous carbon nitride(MCN)was synthesized using Mesoporous molecular sieve SBA-15 as template,ethylenediamine and carbon tetrachloride as raw materials,and used as catalyst support to load Pd and Ag/Pd(0.5∶1,1∶1 and 3∶1)active components.the results show that the addition of Ag can improve the acetylene conversion and ethylene selectivity of the catalyst.When the optimal Ag/Pd ratio is 1∶1,the catalytic performance is excellent,the acetylene conversion is 99.8%,and the ethylene selectivity is 98.1%.Ag-Pd/MCN(1∶1)nanocatalyst has excellent catalytic hydrogenation performance,mainly due to the high specific surface area and total pore volume of MCN,and the uniform distribution of surface active sites;It has good ethylene selectivity,which may be due to the fact that the Ag modifier reduces the adsorption strength of Pd active sites to ethylene and prevents the excessive hydrogenation of ethylene。
Yang et al.Synthesized a PdCu/C bimetallic catalyst according to previous studies[41][42,43]。 PdCu nanoparticles were obtained by heating in oil bath at 200°C using Pd(OAc)2and Cu(OAc)2as metal sources,and then PdCu/C bimetallic catalyst was obtained by redispersing PdCu nanoparticles in cyclohexane and adding carbon as a carrier.It was heat-treated at different temperatures in an H2atmosphere to improve the ethylene selectivity without loss of activity.The PdCu/C catalyst heat-treated at 375℃showed better ethylene selectivity(86%)at 100%acetylene conversion.The average size of PdCu nanoparticles was maintained in the range of 6.6–6.8 nm,and no obvious segregation was observed.The results of X-ray diffraction(XRD)and high-resolution transmission electron microscopy(HRTEM)show that the crystal structure of PdCu/C heat-treated at 375℃changes from face-centered cubic structure to body-centered cubic structure(see Figure 3).Due to the existence of crystal transformation,isolated Pd sites are formed and uniformly and stably dispersed on the support surface.In situ extended X-ray absorption fine structure(EXAFS)results show that the Pd sites are well isolated by Cu atoms,increasing the proportion ofπ-bonded ethylene.The d-band center analysis of XPS shows that the crystal phase transformation leads to the shift of the d-band center,which weakens the adsorption of ethylene on the Pd site,which is also verified by DFT calculation.This study provides a promising method for improving the hydrogenation selectivity of solid bimetallic catalysts without loss of activity 。
图3 (A) 不同温度热处理下 PdCu 催化剂的 XRD 图谱，(B) PdCu-375 样品的 X 射线衍射 Rietveld 精修图谱，(C) 采用RIR法测定PdCu样品的面心立方/体心立方结构(FCC/BCC)比值，(D) PdCu/C、(E) PdCu/C-250和(F) PdCu/C-375的HRTEM图像[41]

Fig.3 (A) XRD patterns of PdCu catalysts with different heat treatment temperatures. (B) Pattern of X-ray diffraction Rietveld refinement for the PdCu-375 samples. (C) FCC/BCC ratio of PdCu samples by the RIR method. HRTEM images of PdCu/C (D), PdCu/C-250 (E), and PdCu/C-375 (F)[41]. Copyright 2021, American Chemical Society

It can be seen that the ethylene selectivity and stability of the Pd-based catalyst can be effectively improved by adding promoters to modify the Pd-based catalyst.the addition of promoter can improve the performance of the catalyst mainly through the geometric effect and electronic effect,which can not only improve the dispersion of active centers,but also effectively weaken the adsorption of ethylene,avoid excessive hydrogenation,and thus improve the selectivity to ethylene。

3.1.3 Ni-based catalyst

Ni and Pd are in the same group VIII,which show high reactivity for many hydrogenation reactions,and the cost of Ni metal is low,so it has attracted wide attention of researchers.Similar to Pd-based catalysts,Ni-based catalysts have higher hydrogenation activity but poor selectivity to ethylene.Therefore,it is usually necessary to add other elements to modify Ni-based catalysts to improve their selectivity and stability[44⇓~46]。
Chen et al.Prepared Ni/SiO2catalyst and NixIn/SiO2bimetallic catalyst with different Ni/In ratios by impregnation method using SiO2as support,studied the effect of In addition on acetylene selective hydrogenation performance,and analyzed the deactivation phenomenon of the catalyst during the reaction[47]。 The results show that the NixIn/SiO2has higher acetylene conversion,ethylene selectivity and stability than the Ni/SiO2when the Ni/In ratio is appropriate.This is attributed to the geometric isolation of the inert In atom from the active Ni atom and the charge transfer from the In atom to the Ni atom,both of which are beneficial to reducing the adsorption strength of ethylene and inhibiting the C—C hydrogenolysis and the polymerization of acetylene and intermediate compounds.In general,Ni6In/SiO2and Ni10In/SiO2have better catalytic performance,with acetylene conversion of 100%and ethylene selectivity of>60%at 180°C.However,with the further increase of In content,the acidity of NixIn/SiO2is enhanced due to the charge transfer between In and Ni,and the polymerization of acetylene and ethylene becomes more severe.In terms of catalyst stability,the phase transformation of metal Ni to nickel carbide is the main reason for the deactivation of Ni/SiO2,and the introduction of In can inhibit the formation of nickel carbide;However,with the further increase of In content,the main reason for the deactivation of NixIn/SiO2catalyst is the formation of carbon deposition due to the increase of acidity.In the 36 H stability test,the acetylene conversion of both Ni6In/SiO2and Ni10In/SiO2was maintained at 100%,and the ethylene selectivity was>60%,which was significantly better than that of Ni/SiO2and other NixIn/SiO2.This finding can provide valuable information for the rational design of Ni-based catalysts for the selective hydrogenation of acetylene 。
Wang et al.Also used SiO2as support to prepare Ni/SiO2catalyst and NixGa/SiO2bimetallic catalyst with different Ni/Ga atomic ratios by impregnation method[48]。 The results of acetylene selective hydrogenation experiments show that the selectivity of NixGa/SiO2to ethylene is higher than that of Ni/SiO2,which is due to the formation of Ni-Ga alloy and Ni3Ga intermetallic compound（Ni3Ga IMC）during the reaction.The charge transfer from Ga to Ni is beneficial to reducing the adsorption strength and adsorption amount of ethylene on Ni atom,inhibiting the over-hydrogenation reaction,C—C bond hydrogenolysis and polymerization reaction,thus improving the ethylene selectivity.In the reaction process,the metallic Ni in the Ni/SiO2is easy to transform into the less active NiCxphase,while the formation of Ni-Ga alloy and Ni3Ga IMC can inhibit carbon from entering the Ni lattice,which is beneficial to improve the stability of the catalyst.In general,the stability of NixGa/SiO2increases first and then decreases with the increase of Ga content,and the catalytic performance of Ni5Ga/SiO2is the best.At 180℃,the conversion of acetylene and the selectivity to ethylene in the Ni5Ga/SiO2were 100%and 75%~81%,respectively,in 120 H and 68~120 H.In addition,with the further increase of Ga content,the acid amount and acid strength of the catalyst tend to increase,which promotes the polymerization and carbon deposition,thus leading to the deactivation of the catalyst 。
For supported metal catalysts,the subsurface layer significantly affects the adsorption and reaction behavior of surface sites through electronic and geometric effects.The subsurface carbon in PdCxhas been shown to be an effective promoter in the selective hydrogenation of acetylene[49⇓⇓⇓⇓~54]。 Wang et al.Designed a top-down strategy and successfully synthesized a bimetallic carbide Ni3ZnC0.7/C catalyst[55]。 As shown in Fig.4,the NiZn-ZIF-8/LHS precursor was synthesized from Zn(NO3)2·6H2O,Ni(NO3)2·6H2O and 2-methylimidazole in ethanol/water mixed solvent,and then the precursor was calcined under N2atmosphere to obtain the Ni3ZnC0.7/C catalyst.The experimental results showed that the Ni3ZnC0.7/C catalyst gave 100%conversion of acetylene and 85%selectivity to ethylene.In the stability test at 160°C for 10 H,the acetylene conversion of the Ni3ZnC0.7/C catalyst was higher than 95%,and the ethylene selectivity remained at about 85%.Therefore,the introduction of subsurface carbon in the selective hydrogenation of acetylene can significantly improve the selectivity of ethylene.Bader charge analysis showed that the subsurface carbon separated the surface Ni3clusters into two types.One is the electron depletion Ni3(C)with subsurface C atoms,and the other is the electron accumulation Ni3(Zn)with subsurface Zn atoms.DFT calculations show that Ni3(Zn)promotes the desorption of ethylene in the C2pathway and hinders the formation of C—C bond in the C4pathway,thus improving the selectivity of ethylene 。
图4 Ni3ZnC0.7/C催化剂合成示意图[55]

Fig. 4 Schematic synthesis of Ni3ZnC0.7/C catalyst[55]. Copyright 2021, American Chemical Society

the above study shows that as a non-precious metal catalyst,Ni-based catalyst is one of the candidates for the selective hydrogenation of acetylene,but it also has the problem of low selectivity.At present,the most commonly used method is to introduce Zn,Ga,In and other metals to form alloys with Ni,and to control the adsorption strength of Ni for ethylene by changing its electronic structure,so as to improve the ethylene selectivity of Ni-based catalysts.However,with the increase of the content of the second metal,the polymer will continue to generate and accumulate,and the carbon deposition reaction will occur,which will lead to the deactivation of the catalyst。
in general,Pd-based catalysts have high hydrogen activation performance and good application prospects In the selective hydrogenation of alkynes,but It will lead to excessive hydrogenation of alkynes,so Pd-based catalysts are often modified by electronic and geometric effects.Among the non-precious metal catalysts,Ni-based catalysts have high catalytic activity and low cost,but their selectivity to ethylene is low,and their stability is not high,often accompanied by the formation of green oil and carbon deposits,so it is necessary to add other elements to improve their selectivity and stability.it is considered to be one of the effective methods to improve ethylene selectivity by adding a small amount of promoter to form bimetallic catalysts with Pd or Ni to control their electronic density of States.In addition,single-atom catalysts have high atom utilization and unique electron-geometry structure,which make them show excellent catalytic performance in the selective hydrogenation of acetylene。

3.2 Catalyst support

catalyst carrier is one of the components of supported catalyst,which makes the catalyst have specific physical properties.the carrier has two kinds of functions:physical function and chemical function.the physical function is to stabilize,disperse and support the active component;the chemical function is to react with the active component to produce new compounds,provide active centers,and enhance the toxicity resistance of the catalyst[56⇓⇓~59]。 Commonly used carriers are alumina,silica,titania and activated carbon。
TiO2is widely used as the carrier of acetylene selective hydrogenation catalyst because of its stable performance,low cost,non-toxic and good anti-coking performance.Gao et al.Synthesized TiO2nanorods containing 89%(101)crystal plane（TiO2-101）and 77%(001)crystal plane TiO2nanosheets（TiO2-001）by hydrothermal method,and then supported Pd nanoparticles on different crystal planes of TiO2by impregnation method to obtain the catalyst[60]。 The effect of Pd/TiO2-101 and Pd/TiO2-001 catalysts on the performance of acetylene selective hydrogenation was investigated.The results showed that the Pd/TiO2-101 catalyst exhibited better acetylene conversion(92%)and ethylene selectivity(57%).TEM and CO chemisorption results show that Pd nanoparticles on TiO2-101 support have smaller particle size(1.53 nm)and higher dispersion(15.95%);However,the particle size of Pd nanoparticles on TiO2-001 support was larger(4.36 nm)and the dispersion was lower(9.06%).The smaller Pd particle size and its higher dispersion on the Pd/TiO2-101 catalyst enable it to have more reactive sites,thus promoting its catalytic activity.These results indicate that the structure and catalytic performance of the Pd/TiO2catalyst can be adjusted by controlling the crystal plane of the TiO2support,which greatly deepens the understanding of the selective hydrogenation of acetylene catalyzed by Pd/TiO2 。
Compared with TiO2,Al2O3has stable properties and lower cost,but there are acidic centers on the surface of Al2O3,which is easy to initiate polymerization.In recent years,researchers have done a lot of modification research based on Al2O3support,focusing on reducing its surface acidity to achieve better catalytic performance[61⇓~63]。 Zhang et al.Modified Al2O3with glucose and glucosamine hydrochloride as the precursors of C and C/N,respectively,which were labeled as CA and CNA[64]。 Then,Au supported catalysts supported on Al2O3and Al2O3modified by C or C/N co-doping,labeled as Au/Al2O3,Au/CA and Au/CNA,respectively,were prepared by a modified deposition-precipitation method,and their catalytic performance for acetylene hydrogenation was investigated.The results showed that the conversion frequency(TOF)of Au/CNA at 250℃was 5 times higher than that of Au/Al2O3and Au/CA,while the TOF of Au/CA was almost not improved.Therefore,it is speculated that the improved catalytic performance of Au/CNA is related to the interaction between N on the CNA support and Au nanoparticles.HRTEM and C2H2-TPD analysis showed that the enhanced activity of Au/CNA was independent of Au particle size and acetylene adsorption.Combined with Fourier transform infrared spectroscopy(FT-IR),XPS and other analyses,they suggested that the N species in the organic matrix on the surface of Al2O3played a promoting role in the selective hydrogenation of acetylene.XPS analysis showed that electron transfer occurred from N to Au,resulting in the enhancement of electron density of Au.Therefore,it is speculated that the high active site utilization of Au/CNA is related to the electron transfer between Au and N.This study provides a new way for the efficient catalytic hydrogenation application of organic/inorganic composites 。
layered double hydroxides(LDHs)are typical two-dimensional Layered inorganic materials,which can form the corresponding mixed metal oxides after heat treatment.This mixed metal oxide has tunable surface basicity and acidity,exchangeable cations between layers,and high adsorption capacity for immobilization of active species[65]。 These characteristics make it a good support for heterogeneous catalysis.Gao et al.First prepared a layered double hydroxide(ZnAl-LDH)precursor by co-precipitation method,and then treated the precursor at 350℃in air for 4 H to obtain the corresponding mixed metal oxide（ZnO-Al2O3）[66]。 Then,Pd,Ag nanoparticles were loaded onto Al2O3and ZnO-Al2O3supports by the impregnation method.The experimental results of acetylene selective hydrogenation showed that Pd-Ag/ZnO-Al2O3exhibited higher ethylene selectivity(77.6%)based on the complete conversion of acetylene,which was 3.92 and 1.22 times higher than that of Pd/Al2O3and Pd-Ag/Al2O3,respectively.On the one hand,the ZnO-Al2O3metal oxide derived from ZnAl-LDH has larger pore size,which is beneficial to the desorption of ethylene;On the other hand,the introduction of Ag effectively inhibited the occurrence of oligomerization 。
molecular sieves and metal-organic frameworks(MOFs)are considered to be ideal designable catalyst supports due to their regular microporous structure and good thermal stability.Gong et al.Synthesized a high-density Pd nanoparticle catalyst by atomic layer deposition(ALD)using mesoporous silica Molecular sieve MCM-41 as a support[67]。 As shown in fig.5,TiO2was first modified on the surface of MCM-41 by ALD,and then Pd was deposited in the mesopores of MCM-41 by ALD,which was named Pd/TiO2/MCM-41.The spatial confinement effect of the internal channels of the molecular sieve leads to a very narrow size distribution(2–3 nm)of Pd nanoparticles,which effectively prevents the sintering of Pd particles during high temperature treatment.High temperature treatment under H2atmosphere can induce SMSI between Pd and TiO2,and the fine mesopores on MCM-41 help to maintain the size and morphology of Pd nanoparticles.In the experiment of acetylene selective hydrogenation,the Pd/TiO2/MCM-41 catalyst showed excellent catalytic performance at room temperature and atmospheric pressure,with acetylene conversion of 83.2%and ethylene selectivity of 85.1%.The excellent performance of the Pd/TiO2/MCM-41 catalyst is due to the fact that small Pd nanoparticles are uniformly dispersed in the mesopores of MCM-41,thus providing more Pd active sites for the catalytic reaction.The SMSI between Pd and TiO2not only helps to improve the stability of the catalyst,but also improves the selectivity to ethylene 。
图5 原子层沉积法制备Pd/TiO2/MCM-41示意图[67]

Fig. 5 Schematic diagram of Pd/TiO2/MCM-41 prepared by atomic layer deposition[67]. Copyright 2019, Elsevier

The above results show that the selection of appropriate support can promote the uniform dispersion of active components on the surface of the catalyst,and improve the thermal stability and selectivity of the catalyst.The size,morphology and dispersion of the active component can be improved by controlling the crystal face,acidity and pore structure of the support,and the catalytic activity of the catalyst can be enhanced.In addition,Al2O3and SiO2commonly used in selective hydrogenation of acetylene in industry are acidic catalyst supports,which usually need to be modified to reduce their acidity and reduce the formation of oligomers,so as to improve the catalytic performance of the catalyst.A comparative summary of the performance of different catalysts in the thermal catalytic selective hydrogenation of acetylene is shown in Table 1 。
表1 Performance Comparison of Different Catalysts in Thermal Catalytic Acetylene Selective Hydrogenation

Table 1 Performance comparison of different catalysts in thermally catalyzed selective hydrogenation of acetylene

Carrier Active constituent Assistant Synthesis method Reaction temperature/℃ Acetylene conversion/% Ethylene selectivity/% ref
γ-Al2O3 Pd Impregnation 105 80.9 76.9 26
ZnO Pd Zn Impregnation 80 92 89 27
ZnO Pd Incipient wetness impregnation 100 100 >80 29
CNT Pd Cyclic voltammetry 40 83 98 30
CeO2 Pd Co-precipitation and adsorption 160 100 85 31
MCN Pd Ag Template method 200 99.8 98.1 40
C Pd Cu Impregnation 100 100 86 41
SiO2 Ni In Impregnation 180 100 >60 47
SiO2 Ni Ga Impregnation 180 100 75~81 48
C Ni Zn Impregnation pyrolysis 160 100 85 55
TiO2 Pd Hydrothermal and impregnation 60 92 57 60
CNA Au Deposition-precipitation 300 24 95 64
ZnO-Al2O3 Pd Ag Co-precipitation and impregnation 90 100 77.6 66
TiO2/MCM-41 Pd Atomic layer deposition 21 83.2 85.1 67

4 Development Trend of Acetylene Selective Hydrogenation

at present,the thermal catalytic selective hydrogenation of acetylene is the most widely used In industry and is relatively mature.However,the traditional thermal catalytic selective hydrogenation of acetylene needs to consume a lot of fossil energy and has a high cost;Higher requirements for instruments and equipment;the reaction temperature is relatively high.in recent years,due to the energy crisis and environmental pollution caused by fossil fuels,sustainable development has attracted great attention all over the world,so the development of renewable clean energy has become a consensus.Among them,electrocatalysis and photocatalysis are very active research fields at present,which have gradually attracted the attention of researchers in the fields of energy and catalysis.the reaction conditions of electrocatalysis and photocatalysis hydrogenation are mild,the reaction can be carried out at normal temperature and normal pressure,and fossil fuels are replaced by electric energy and solar energy,thereby being beneficial to reducing greenhouse gas emission,being environment-friendly,and saving energy.In the following,the selective hydrogenation of alkynes reported recently is discussed from the aspects of electrocatalysis,photocatalysis and photothermal catalysis。

4.1 Lectrocatalytic selective hydrogenation of alkyne

for The selective hydrogenation of alkynes,the electrocatalytic hydrogenation process is a direction that researchers continue to explore.the electrocatalytic selective hydrogenation can be carried out at normal temperature and normal pressure without external hydrogen supply,and the process is simple;reaction start and stop and product selectivity regulation can be quickly realized by adjusting the potential,and the operation is simple and convenient;renewable energy(solar energy,wind energy)can be used to provide electricity to achieve carbon neutrality in the whole process,which helps to reduce greenhouse gas emissions.Compared with the traditional thermal catalytic method,the method directly uses water as a hydrogen source at normal temperature and pressure,and the Reaction conditions are safer.the combination of electrocatalytic hydrogenation and Renewable electric energy provides an environmentally friendly,inexpensive and efficient new route For acetylene hydrogenation to ethylene。
Wang et al.Reported an efficient electrocatalytic process for the selective hydrogenation of acetylene using a Cu catalyst at room temperature and atmospheric pressure[68]。 Cu microparticles were prepared by solvothermal method in ethylene glycol using CuSO4·5H2O as precursor and carbon as carrier.Compared with thermal catalysis,this process has the advantage of being operable under mild conditions,and in combination with water electrolysis based on renewable electrical energy,hydrogen is generated in situ in the hydrogenation reaction,thus avoiding additional H2supply,as shown in Fig.6[69,70]。 this is an environmentally friendly and efficient route for the selective hydrogenation of alkynes.the hydrogen evolution reaction was inhibited by optimizing the Cu catalyst to expose more active surfaces,which was beneficial to the preferential adsorption and hydrogenation of acetylene.in This process,the selectivity of the reduction product is regulated by adjusting the electrode potential,and the mass transfer is promoted by coating a microporous gas diffusion layer on the carbon support.the high ethylene faradaic efficiency of 83.2%at−0.6 V vs RHE and V vs RHE current density avoids excessive acetylene hydrogenation to ethane.the faradaic efficiency and the overall production rate of ethylene were greatly improved by using the continuous flow method In the electrocatalytic acetylene selective hydrogenation process compared with the previously reported closed reaction system[71⇓⇓⇓~75]。 In addition,the Cu catalyst showed good stability In the continuous test for 100 H.In situ X-ray absorption near edge structure(XANES),Fourier transform attenuated total reflection infrared spectroscopy(ATR-FTIR)characterizations combined with first-principles calculations show that electron transfer from the Cu surface to acetylene enhances acetylene adsorption and hydrogenation,while suppressing the competitive hydrogen evolution reaction(HER)and promoting ethylene desorption,leading to highly selective ethylene production.the electrochemical hydrogenation proceeds via an electron-coupled proton transfer pathway,which requires a higher activation energy for further hydrogenation compared with desorption of the produced ethylene,thus effectively inhibiting excessive hydrogenation to ethane.This efficient electrocatalytic process provides a green route for the development of energy-saving and environment-friendly ethylene production methods。
图6 传统热催化与电催化乙炔选择性加氢工艺的特点比较[68]

Fig. 6 Comparison of the features of traditional thermocatalytic and electrocatalytic acetylene selective hydrogenation processes[68]. Copyright 2021, Springer Nature

Bu et al.Prepared Cu dendrite catalyst by electrochemical deposition using KOH solution containing Cu ions as electrolyte[76]。 The electrochemically deposited Cu dendrites exhibited ethylene selectivity of up to 97%in a crude ethylene stream containing 1×104ppm acetylene at room temperature and atmospheric pressure with water as the hydrogen source,enabling continuous production of a polymer-grade ethylene stream(4 ppm acetylene)at a large space velocity of 9.6×104mL·gcat-1·h-1with excellent long-term stability.DFT calculations and Raman spectroscopy studies show that the excellent electrocatalytic performance of Cu catalyst for acetylene selective hydrogenation is mainly due to its strong adsorption of acetylene and easy desorption of ethylene.Compared with other metals(Ag,Au,Pd,Ni),Cu is more beneficial to the adsorption of acetylene and the desorption of ethylene 。
NHC-metal complexes have shown outstanding potential in catalysis of homogeneous organic reactions(e.g.,alkyne hydrogenation,olefin metathesis)due to the strongσ-donor and strong chelating effect of N-heterocyclic carbene(NHC).the strongσ-electron donating effect of the NHC ligand greatly enriches the electron density at the chelating metal site[77⇓~79]。 Therefore,to suppress the side reactions such as hydrogen evolution,over-hydrogenation,and carbon-carbon coupling in the electrocatalytic selective hydrogenation of acetylene,NHC-metal complexes are ideal candidate catalysts.Zhang et al.Developed an NHC-metal complex with an electron-rich metal center as an electrocatalyst for the selective hydrogenation of acetylene[80]。 Among a series of NHC-metal complexes(Cu,Ag,Au,and Pd),NHC-Cu exhibits an ethylene faradaic efficiency of over 98%under pure acetylene flow.Even in the crude ethylene feedstock containing 1%acetylene（1×104ppm）,the specific selectivity of NHC-Cu exceeded 99%in the 100 H stability test,the space velocity was up to 9.6×105mL·gcat-1·h-1,and the conversion frequency was 2.1×10-2s-1,which was obviously better than that of the thermal catalyst reported so far.Electrochemical Raman analysis and DFT calculations show that the electron-rich Cu sites in NHC-Cu are beneficial to the adsorption of electrophilic acetylene and the desorption of nucleophilic ethylene,and inhibit the side reactions in the electrocatalytic selective hydrogenation of acetylene.This work not only opens a new window for NHC-metal complexes in the field of electrocatalysis,but also provides new ideas for the design of selective hydrogenation catalysts 。
Ma et al.Synthesized nitrogen-doped carbon-supported Ni single-atom catalyst(SA-Ni-NC)by pyrolysis of Ni-doped zeolitic imidazolate framework-8(ZnNi-ZIF)precursor[81]。 The SA-Ni-NC exhibited a high ethylene faradic efficiency of 91.3%and a large current density of V vs RHE at−0.6 V vs RHE in alkaline aqueous solution and pure acetylene flow.Even（1×104ppm）in the crude ethylene stream with 1%acetylene impurity,SA-Ni-NC still exhibited 97.4%acetylene conversion.DFT calculations and in situ electrochemical Raman analysis show that the excellent electrocatalytic performance of SA-Ni-NC is attributed to the weakπadsorption of ethylene on a single Ni atom,which is beneficial to the rapid desorption of ethylene,thus avoiding its excessive hydrogenation and finally promoting the selective hydrogenation of acetylene to ethylene 。
the adsorption configuration of the reaction intermediate directly affects the selectivity of the product,which can be effectively improved by controlling the formation of weakπadsorption of ethylene intermediate on the surface of Cu-based catalyst[82]。 the active site steric isolation strategy has been shown to be an effective way to control The adsorption configuration of intermediates in thermal catalytic systems[83⇓~85]。 Inspired by the above progress,Jiang et al.Proposed to apply the active site steric isolation strategy to the electrocatalytic selective hydrogenation of acetylene[86]。 N-doped carbon-supported Cu single-atom catalyst(Cu SA/NC)was prepared by a simple impregnation pyrolysis method.CuSA/NC catalyst can（40~120 mA·cm-2）in a wide range of current density.In pure acetylene flow,the faradic efficiency of ethylene can reach 87.5%,and that of 1,3-butadiene is less than 1%.CuSA/NC catalyst has a good inhibition effect on side reactions such as over-hydrogenation and C-C coupling.DFT calculations show that the activation barriers of acetylene over-hydrogenation path and C—C coupling path on isolated Cu sites are higher than those on Cu(111)surface,and the adsorption of ethylene on isolated Cu sites becomes weaker.DFT calculations combined with experiments show that the excellent performance of Cu SA/NC in the electrocatalytic selective hydrogenation of acetylene is attributed to the weakπadsorption of ethylene intermediate and the high activation barrier of C—C coupling on isolated sites.This study provides a comprehensive understanding of active site steric isolation strategies for suppressing side reactions in the electrocatalytic selective hydrogenation of acetylene 。
In addition,An et al.Reported a novel aqueous Zn-C2H2cell,which not only can reduce acetylene to ethylene through a unique discharge mechanism,but also can generate：C2H2+Zn+H2O→C2H4+ZnO[87]。 As shown in Fig.7,this Zn-C2H2cell uses Cu dendrite on gas diffusion electrode as the positive electrode,Zn as the negative electrode,and 1 mol·L-1KOH aqueous solution as the anode electrolyte.Under pure acetylene flow,this Zn-C2H2cell has an open circuit potential of 1.14 V and a peak power density of 2.2 mW·cm-2,which exceeds that of the reported Zn-CO2cell[88⇓⇓⇓⇓⇓⇓~95]。 Even with an ethylene stream containing 1%acetylene（1×104ppm）,the Zn-C2H2cell exhibited 99.97%acetylene conversion with 95%selectivity to ethylene and ultimately produced a polymer-grade ethylene feed containing only~3 ppm acetylene.The design of this Zn-C2H2cell is generally applicable to the reduction of other alkynes,such as propyne and butyne,etc.This study provides an effective strategy for the design of green ethylene purification and functional batteries 。
图7 水系Zn-C2H2电池示意图[87]

Fig. 7 Schematic diagram of aqueous Zn-C2H2 battery[87]. Copyright 2022, John Wiley and Sons

Compared with the traditional thermal catalytic hydrogenation process,the Electrocatalytic alkyne selective hydrogenation reaction has significant selectivity,superior stability and lower cost,indicating that the Electrocatalytic alkyne selective hydrogenation technology has great potential.electrocatalytic acetylene selective hydrogenation uses water as hydrogen source,the reaction conditions are mild,and the ethylene selectivity is high,so it is one of the good choices to replace the thermal catalytic acetylene selective hydrogenation.electrocatalytic selective hydrogenation of acetylene can only show excellent performance at low overpotential,but low overpotential will lead to too obvious side reactions such as C—C coupling and excessive hydrogenation,resulting in poor selectivity of the target product.Therefore,it is also necessary to design appropriate catalyst structures,develop efficient electrocatalysts,and suppress related side reactions to improve the selectivity of target products。

4.2 Photocatalytic hydrogenation of alkyne

Photocatalytic hydrogenation is a promising method using photogenerated electrons and holes to participate in the reaction.Photocatalytic alkyne hydrogenation uses abundant and clean solar energy to provide energy for selective hydrogenation of alkynes to olefins,with mild reaction conditions,simple operation,low energy consumption and no secondary pollution.Photocatalytic alkyne hydrogenation is expected to provide a more cost-effective and sustainable strategy。
Lian et al.Prepared Pt/TiO2catalyst by loading Pt on TiO2by photodeposition method[96]。 The particle size distribution and optical properties of Pt supported on the surface of TiO2were characterized by TEM and UV-vis absorption spectroscopy.The results show that the loading of Pt particles is about 5 nm,and the loading of Pt enhances the absorption intensity of TiO2in the visible region.The results of photocatalytic hydrogenation of phenylacetylene showed that the Pt/TiO2catalyst had high conversion of phenylacetylene(phenylacetylene was completely converted after 8 H irradiation)and high styrene selectivity(91.3%)within 6 H under room temperature and atmospheric pressure with methanol as hydrogen source and 385 nm single wavelength irradiation.By optimizing the loading of Pt,the conversion of phenylacetylene was 92.4%and the selectivity of styrene was 91.3%when the loading of Pt was 1 wt.However,the traditional thermal catalytic hydrogenation uses hydrogen as the hydrogen source,and although the conversion rate of phenylacetylene is 100%,the product is ethylbenzene from excessive hydrogenation(Fig.8A,B).This is mainly due to the fact that during the photocatalytic reaction,the electrons on the conduction band of TiO2migrate to the Pt particles,resulting in an increase in the electron density of Pt,and the high electron density on the surface of Pt particles is conducive to the desorption of styrene,an intermediate product of hydrogenation(as shown in Figure 8C).Therefore,the over-hydrogenation reaction does not occur in the photocatalytic hydrogenation process,and the styrene selectivity is high.At the same time,it was found that the Pt/TiO2photocatalyst also had high selectivity for the hydrogenation of other types of alkynes,indicating that the selective hydrogenation of alkynes over Pt/TiO2photocatalyst is universal 。
图8 （a）以甲醇悬浮液为氢源，苯乙炔在Pt/TiO2上光催化加氢为苯乙烯的示意图；（b）以H2作氢源的Pt/TiO2的比较实验；（c）苯乙炔在Pt/TiO2上选择性加氢反应的光催化反应机理[96]

Fig. 8 (a) Schematic of photocatalytic hydrogenation of phenylacetylene to styrene on Pt/TiO2 using methanol suspension as a hydrogen source; (b) the comparative experiment over the Pt/TiO2 with H2 as hydrogenation source; (c) Proposed photocatalytic reaction mechanism for selective hydrogenation of phenylacetylene over Pt/TiO2[96]. Copyright 2020, Elsevier

Arcudi et al.Reported a photocatalytic reaction system using cobalt(Ⅲ)meta-tetra(4-sulfophenyl)porphyrin as a catalyst(CoTPPS),ruthenium terpyridine（[Ru(bpy)3]2+）or a lower cost organic semiconductor(porous carbon nitride)as a photosensitizer,and sodium ascorbate(NaAsc)as a sacrificial agent[97]。 The reaction was carried out at room temperature,and H2O was used as the proton donor,which avoided the use of H2as the hydrogen source in the traditional hydrogenation reaction.The results of photocatalytic hydrogenation experiments showed that the photocatalytic reaction system could realize the selective hydrogenation of acetylene to ethylene in the non-competitive(in the absence of ethylene)/competitive(in the presence of ethylene and acetylene)conditions under 450 nm single wavelength irradiation,and the reaction selectivity was as high as 99%.Close to 100%conversion was achieved under near-industrial reaction conditions(simultaneous presence of ethylene and acetylene).Moreover,compared with the current thermal catalytic hydrogenation reaction,the photocatalytic reaction system has great advantages in selectivity and sustainable development 。
Wei et al.Prepared Pd/TiO2catalyst by impregnation method using TiO2as support and Pd(OAc)2as palladium source[98]。 The experimental results showed that the Pd/TiO2catalyst exhibited excellent catalytic performance for the selective hydrogenation of 1,3-butadiene driven by the light intensity of 66 mW·cm-2at room temperature,using water as the hydrogen source to generate hydrogen atoms in situ by photocatalytic water splitting.The conversion of 1,3-butadiene was 99%and the selectivity of butene was 100%at room temperature for 180 H.The electron transfer characteristics of TiO2and Pd/TiO2were analyzed in situ by electrochemical impedance spectroscopy,and the modification of Pd on TiO2greatly reduced the impedance under illumination,because Pd improved the mobility of photogenerated electrons on the surface of TiO2,thus preventing the recombination of photogenerated electrons and holes.DFT calculations show that the hydrogen atoms generated on Pd tend to hydrogenate 1,3-butadiene rather than generate H2.In addition,as shown in Fig.9,the fixed-bed continuous reaction process used in the experiment can shorten the reaction time of gas reactants and the contact time between the product and the catalyst,which provides a great advantage for obtaining high olefin selectivity,so it has high catalytic selectivity and stability for butene 。
图9 光辅助丁二烯半加氢固定床反应系统示意图[98]

Fig. 9 The diagram of fixed-bed reaction system for the light-assisted butadiene semi-hydrogenation[98]. Copyright 2022, John Wiley and Sons

Compared with the thermal catalytic hydrogenation route,which has large hydrogen consumption,high temperature and unsatisfactory olefin selectivity,the photocatalytic alkyne hydrogenation uses water or methanol as the hydrogen source,the reaction can be carried out at normal temperature and pressure,and the reaction can be started and stopped quickly and the product selectivity can be regulated by adjusting the light intensity,so the operation is simple and easy to control.However,most of the photocatalysts reported so far can only produce photocatalytic activity under ultraviolet light excitation,and the utilization of visible light is low.Therefore,a lot of research efforts are still needed to research and develop selective hydrogenation photocatalysts that can be driven by visible light。

4.3 Hydrogenation of alkynes by photothermal catalysis

photothermal catalysis is an emerging research area involving The integration of thermal and photocatalytic processes.This is different from conventional thermal catalysis,because photogenerated carriers can be directly transferred to the orbitals of adsorbed molecules to promote their desorption,dissociation,or activation,thereby initiating chemical reactions and generating completely different reaction pathways.Photothermal catalysis can be roughly divided into three categories:the first is light-assisted thermal catalytic reaction,in which light only plays the role of providing a heat source,and in terms of reaction mechanism,the essence of the reaction is a thermal catalytic process;the second is heat-assisted photocatalytic reaction,in which light plays a decisive role in the reaction process,and heating can promote the reaction rate;the third is Photothermal coupling catalysis[99]。
Zhou et al.Successfully synthesized an atomically dispersed Pd single-atom catalyst（Pd1/N-graphene)on a nitrogen-doped graphene support using a freeze-drying assisted method[100]。 The Pd1/N-graphene catalyst was heated to 125°C under photothermal conditions(full-spectrum Xe lamp irradiation),and in the presence of excess ethylene,the Pd1/N-graphene exhibited excellent catalytic performance with 99%acetylene conversion and 93.5%selectivity to ethylene.This remarkable catalytic performance is due to the large number of Pd active sites on the catalyst surface and the weak adsorption energy of dispersed Pd single atoms on ethylene,which prevents the hydrogenation of ethylene to ethane.In addition,when the Pd loading was 2.3 wt%and the light intensity was 0.54 W·cm-2,the Pd1/N-graphene catalyst showed excellent durability for the selective hydrogenation of acetylene at 125℃for 24 H,which was attributed to the strong local coordination effect of N atoms on Pd atoms,which inhibited the agglomeration of Pd.High-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)and EXAFS analysis also confirmed the Pd-N4coordination on the N-doped graphene support.At the same time,the catalytic performance of Pd1/N-graphene catalyst under photothermal heating and direct heating was compared,and it was found that the conversion of acetylene and the selectivity of ethylene were basically the same under the two heating methods,indicating that the light only provided a heat source in this process,which belonged to the light-assisted thermal catalytic reaction.The results demonstrate the feasibility of acetylene photothermal hydrogenation to ethylene for the first time,and provide a reference for the development of other important chemical conversion of photothermal catalytic systems in industry 。
Guo et al.Adopted a simple but versatile strategy to prepare Pd1/TiO2single-atom catalysts using the SMSI effect[101]。 The Pd/TiO2catalyst was prepared by ball milling,and then the ethylene selectivity of the TiO2supported Pd catalyst was improved by selectively encapsulating a small amount of coexisting Pd nanoclusters/nanoparticles through the SMSI effect.Due to the good photocatalytic performance of TiO2,the integrated photothermal catalysis can greatly improve the activity of the catalyst.Selective hydrogenation of acetylene can be achieved at temperatures as low as 70°C under photothermal catalysis.Under the same temperature,the results of acetylene selective hydrogenation under dark reaction and light reaction conditions show that the introduction of light can significantly improve the conversion of acetylene(from 20%to 80%),indicating that photogenerated charges may be involved in the reaction process.Under light irradiation,the conversion of acetylene increased abruptly,rather than gradually,indicating that it was a photothermal coupling catalytic process.Moreover,the kinetic analysis shows that the apparent activation energy（22.9 kJ·mol-1）of photothermal catalysis is lower than that of thermal catalysis（33.3 kJ·mol-1）,which means that the reaction path under light irradiation is different from that of traditional thermal catalysis.The experimental study combined with DFT calculations showed that the photoinduced electron transfer from TiO2to the neighboring Pd atom promoted the activation of acetylene,thus favoring the photothermal catalytic selective hydrogenation reaction.This work provides a promising strategy for controlling the catalytic performance of acetylene selective hydrogenation,and also opens a new window for photothermal catalytic selective hydrogenation 。
Photothermal catalysis is based on the synergy between photochemical and thermochemical reaction pathways,which can significantly improve the catalytic activity and modulate the catalytic reaction pathway and selectivity.Moreover,the coupling of photocatalytic and thermocatalytic processes overcomes the problems of low photocatalytic activity and high reaction barrier of thermocatalysis,which provides a promising strategy for improving the activity and selectivity of hydrogenation,oxidation,and CO2reduction.However,up to now,compared with thermal catalysis,there are few studies on photothermal catalysis for selective hydrogenation of acetylene,and there are still many problems.First of all,the phenomenon of poor catalyst stability also exists in the photothermal catalytic reaction[102]。 Therefore,The modification of catalyst to inhibit its deactivation is the focus and difficulty of current research.in addition,the mechanism of how solar energy changes the reaction pathway in the photothermal catalytic system remains to be explored.the development of appropriate in situ characterization techniques to observe the evolution of reactant molecules on the surface of catalysts during photothermal reaction is of great value for explaining the mechanism of photothermal catalysis.the performance comparison of different catalysts in electrocatalytic,photocatalytic and photothermal catalytic selective hydrogenation is shown in Table 2。
表2 Performance Comparison of Different Catalysts in Electrocatalytic, Photocatalytic and Photothermal Catalytic Selective Hydrogenation

Table 2 Comparison of the performance of different catalysts in electrocatalytic,photocatalytic and photothermal catalytic selective hydrogenation

Catalyst Synthesis method Hydrogen donors Reaction condition Catalytic performance ref
Electrocatalysis Cu/C Solvothermal Water T=25 ℃; V=4.8 mL·min-1 $F E_{C_{2} H_{4}}$=83.2% 68
Cu/C Electrochemical deposition Water T=25 ℃;
$V_{\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)} / V_{\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)}$=1:99;
SV=9.6×104 mL·gcat-1·h-1		 $F E_{C_{2} H_{4}}$=97% 76
NHC-Cu Impregnation Water T=25 ℃;
$V_{\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)} / V_{\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)}$=1:99;
SV=9.6×105 mL·gcat-1·h-1		 $F E_{C_{2} H_{4}}$=99%;
TOF=2.1×10-2 s-1		 80
SA-Ni-NC Impregnation Water T=25℃;
$V_{\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)} / V_{\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)}$=1:99;
SV=2.4×104 mL·gcat-1·h-1		 $C_{C_{2} H_{2}}$=97.4%;
$F E_{C_{2} H_{4}}$=91.3%;
TOF=22.9 h-1		 81
Cu SA/NC Impregnation pyrolysis Water T=25 ℃; V=10 mL·min-1 $F E_{C_{2} H_{4}}$=87.5% 86
Zn-C2H2 Electrochemical deposition T=25 ℃;
$V_{\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)} / V_{\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)}$=1:99		 $C_{C_{2} H_{2}}$=99.97%;
$S_{c_{2} H_{4}}$=95%		 87
Photocatalysis Pt/TiO2 Photodeposition method Methanol T=25 ℃; PLE=0.5 mmol CPLE=92.4%;
SSTE=91.3%		 96
CoTPPS Hydrogen T=25 ℃;
$V_{\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)} / V_{\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)}$=1:30		 $C_{C_{2} H_{2}}$=100%;
$S_{c_{2} H_{4}}$=99%		 97
Pd/TiO2 Impregnation Hydrogen T=25 ℃;
SV=3604 mL·gcat-1·h-1		 $C_{C_{4} H_{6}}$=99.0%;
$S_{C_{4} H_{8}}$=98.0%		 98
Photothermal catalysis Pd1/N-graphene Freeze-drying Hydrogen T=125 ℃;
$V_{\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)} / V_{\left(\mathrm{H}_{2}\right)} / V_{\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)}$=1:20:20;
SV=1.2×103 mL·gcat-1·h-1		 $C_{C_{2} H_{2}}$=99%;
$S_{c_{2} H_{4}}$=93.5%		 100
Pd/TiO2 Ball milling Hydrogen T=120 ℃;
$V_{\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)} / V_{\left(\mathrm{H}_{2}\right)} / V_{\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)}$=1:10:20;
SV=1.2×105 mL·gcat-1·h-1		 $C_{C_{2} H_{2}}$=99%;
$S_{c_{2} H_{4}}$=64.5%		 101

5 Conclusion and outlook

in a word,selective catalytic hydrogenation of acetylene is one of the most effective methods to remove trace acetylene from ethylene.Pd-based catalysts are still the most widely used catalysts In industry.However,the activity and stability of Pd-based catalysts need to be further improved due to their low selectivity,coking deactivation and high price.Therefore,many researchers have systematically studied the active components,promoters and supports of acetylene selective hydrogenation catalysts,with the main purpose of changing the state of the active center of the catalyst through geometric effects and electronic effects.Its essence is to change the adsorption and desorption intensity of acetylene and ethylene on the active center,and to find a suitable balance point,so as to further improve the ethylene selectivity and stability of the catalyst.How to further improve the ethylene selectivity and stability of the catalyst while improving the catalytic activity of acetylene is still the focus of future research。
the research mainly focuses on the following aspects:First,the carrier is an important component of the loading of active components and additives.the support plays a role in stabilizing,dispersing and supporting the catalyst,and the structure of the support itself and the surface acidity and alkalinity will affect the performance of the catalyst,so the modification of the support is an important way to improve the catalytic performance.Secondly,Pd-based catalysts are widely used in the selective hydrogenation of acetylene,and finding other non-precious metals to replace or reduce the amount of precious metals is an effective way to reduce the production cost of industrial catalysts in the future.Thirdly,the introduction of a second,third or more metal components as promoters can effectively improve the activity,selectivity,toxicity resistance and stability of the catalyst。
electrocatalytic and photocatalytic hydrogenation of alkynes is the new direction and trend of acetylene selective hydrogenation in the future.electrocatalysis and photocatalysis technologies have the advantages of low cost,environmental friendliness and mild experimental conditions in terms of energy and atom economy,which are feasible alternatives to the existing acetylene selective hydrogenation technology.However,the current research results in these two directions are still relatively few,and there are still some problems and challenges:the catalytic efficiency of Electrocatalysis is not high,and it often needs low overpotential to show excellent performance,but low overpotential will lead to too obvious side reactions;the light absorption efficiency of photocatalysis is low,and most of them need to be excited by ultraviolet light to produce photocatalytic activity,and the utilization rate of visible light is low.and both of them have the problems of low selectivity and poor stability of the catalyst.Therefore,the main research directions of future Electrocatalytic and photocatalytic acetylene selective hydrogenation are as follows。
(1)Developing efficient catalysts is an important way to improve the efficiency of electrocatalytic selective hydrogenation of acetylene;the design and preparation of catalysts with specific absorption wavelength range is the key to solve the low efficiency of photocatalytic acetylene selective hydrogenation。
(2)that catalytic activity of the catalyst is improve by optimizing the synthesis proces and regulating the active component,the size,the structure,the morphology and the like of the catalyst.the activity and stability of the catalyst can be improved and its service life can be prolonged by alloying,monoatomization,doping,structural regulation,surface modification,coating or compounding.In addition,it is possible to further improve the selective hydrogenation efficiency by regulating the gas affinity/hydrophobicity of the catalyst to the reactant gas and hydrogen。
(3)the control of catalytic reaction conditions is also the key to improve the reaction activity and selectivity.the selectivity of the target product can be improved and the selective synthesis of specific products can be realized by controlling the potential,regulating the light intensity and optimizing the reaction solvent.In order to improve the efficiency of selective hydrogenation,it is also worth exploring to design a catalytic system with multi-field coupling of light,electricity,heat and magnetism。

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