Jiachen Li ,Liguo Wang ,Yan Cao ,Chanjuan Zhang ,Peng He ,Huiquan Li

1 CAS Key Laboratory of Green Process and Engineering,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 Chemical and Biochemical Engineering,Technical University of Denmark,DK-2800 Lyngby,Denmark

3 Sino-Danish College University of Chinese Academy of Sciences,Beijing 100049,China

4 Sino-Danish Center for Education and Research University of Chinese Academy of Sciences,Beijing 100049,China

Keywords:CO2 Reduction C2+Chemicals Fuels Catalysis

A B S T R A C T The chemical utilization of CO2 is a crucial step for the recycling of carbon resource.In recent years,the study on the conversion of CO2 into a wide variety of C2+important chemicals and fuels has received considerable attention as an emerging technology.Since CO2 is thermodynamically stable and kinetically inert,the effective activation of CO2 molecule for the selective transformation to target products still remains a challenge.The welldesigned CO2 reduction route and efficient catalyst system has imposed the feasibility of CO2 conversion into C2+chemicals and fuels.In this paper,we have reviewed the recent advances on chemical conversion of CO2 into C2+chemicals and fuels with wide practical applications,including important alcohols,acetic acid,dimethyl ether,olefins and gasoline.In particular,the synthetic routes for C--C coupling and carbon chain growth,multifunctional catalyst design and reaction mechanisms are exclusively emphasized.

1.Introduction

For more than two centuries,utilization of carbon-rich fossil fuels such as coal,oil and natural gas,has greatly pushed the progressing of human civilization,economic and social development.However,the burning of fossil fuels led to the concentration of CO2in the atmosphere and reached the highest level of ～400 μl·L-1in recent years;especially the increasing demand of energy results in the rapid increase level of CO2in atmosphere,which brings about adverse climate changes and increasing global temperatures,as a result build up an irreversible and uninhabitable planet for human beings[1–5].

Meanwhile,CO2is a ubiquitous and generally available C1feedstock in the world.Therefore,the need to simultaneously increase our energy supply while reducing CO2emissions is one of the major challenges our global society is facing today.In this context,the chemical conversion of CO2from greenhouse gas into value-added products with the assistance of H2is highly desirable,which could reduce both the current CO2emission level and the dependence of chemical production on the depleting fossil fuels.

Unfortunately,carbon is in its highest oxidation state in CO2and its transformation into other valuable chemicals requires high energy input.Up to now,the transformation of CO2to C1chemicals and some important products have been studied via chemical,electrochemical and photochemistry methods,such as methane[6,7],methanol[8]formic acid[9],alkyl carbamates[10],cyclic carbonates[11–13],dimethyl carbonate[14],formic acid esters,urea,carbonic acid esters,decanoic acid esters,amino groups,salicylic acid and so on[15–19].Our group has engaged in the valorization of CO2resources via the coupling reaction with the high-energy compounds into various chemicals and fuels,e.g.alcohols,carbonates,and carbamates[20–24],in which processes the construction of C--O or C--N bonds were usually formed.The chemical utilization of CO2has also studied in cyclic carbonate synthesis via cycloaddition reaction,which shows promising application in industrial manufacture of cyclic carbonates from CO2and epoxides[25,26].For C2+chemicals(C2+chemicals are compounds which include C--C bond with two or more carbon atoms)and fuels,the formations of new C--C bonds are inevitably involved.However,the formation of C--C bonds via CO2hydrogenation is a major challenge in the field of CO2chemistry[27].As a very stable molecule(ΔfGθ=-396 kJ·mol-1),the reduction of CO2needs high energy.To convert relatively simple CO2molecules into more complex and more active C2+chemicals is a key issue[28].What's more,it is also a challenge to accurately control carbon chain growth for obtaining desired C-ranged hydrocarbons and oxygenated chemicals.Recently,the advances on the direct conversion of CO2to more valuable feedstocks,such as higher alcohols,acetic acid,light olefins and gasoline have not been exclusively reviewed.

It is known that,CO2can be hydrogenated to hydrocarbons or oxygenates mainly via two routes.One is CO mediate route,also called CO2-FTS,which refers to the conversion of CO2to CO by reverse water–gas shift(RWGS)reaction and followed by the Fischer–Tropsch synthesis(FTS)—the hydrogenation of CO to hydrocarbons[29].Another is methanol mediate route that is described as methanol that produced from CO2hydrogenation converted into other hydrocarbons[30].In addition,some papers have reported that[31,32]water formed in the process suppressed the conversion of CO2and accelerated the deactivation of catalyst;therefore,proper catalyst that could catalyze both RWGS/methanol and C–C coupling reaction is crucial,and the active sites are needed to combine both RWGS/methanol intermediate and C--C coupling because of thermodynamic stability of CO2.

Another important issue is the source of H2since the chemical conversion of CO2into valuable products should be under the assistance of H2.Thus,the production of H2is quite important.However,hydrogen is unavailability in nature and its production could fall into two main categories,conventional and renewable methods,in accordance with feedstocks[33].The first category accommodates the methods which produce hydrogen from fossil fuels,covering the methods of hydrocarbon reforming and pyrolysis[34].For the second category,hydrogen could be produced from renewable resource such as water,wind[35],tidal[36],solar energy[37]and biomass[38–41].In addition,multi-resource energy coupling is also a development trend for H2production.

In this review,we focused on stating the current understanding of CO2chemical conversion into two carbon and more than two carbon(C2+)chemicals and fuels,which are usually reduction reactions.An overview of the different routes is presented in Fig.1.

Fig.1.Overview of the CO2 conversion routes to C2+chemicals.

2.Synthesis of Alcohols

Various technologies are currently being taken to reduce the accumulation of CO2in the atmosphere.Among these CO2chemical transformation methods,the catalytic conversion of CO2into methanol,ethanol and higher alcohols is a potential approach from the viewpoint of high CO2conversion rates and high desired products selectivity.In addition,these alcohols can also be continuous converted to more value-added chemicals,such as high-octane gasoline and other feedstocks used for chemical industries.

Until now,much attention has been paid to CO2hydrogenation to methanol[42–45].Ethanol and higher alcohols(C2+OH)are more desirable than methanol[46,47]in many aspects,such as fuel additives,pure fuels and chemicals.However,it is a great challenge to break the high energy kinetic barriers for the formation of C--C bonds to synthesize C2+alcohol.Thus,more efficient catalytic systems are required for the conversion of CO2to ethanol and other C2+alcohol.

2.1.Synthesis of ethanol

Among varied value-added CO2hydrogenation alcohol products,ethanol is a renewable additive in the fuels and a well-known intermediate in the industries.Nevertheless,it is still a challenge to realize high selectivity to ethanol and only a few researches of direct CO2transformation to ethanol are reported,but low ethanol selectivity and yield limit its further application.

Table 1 shows a summary of CO2hydrogenation to C2H5OH over different catalysts

Table 1 Results of CO2 hydrogenation to C2H5OH over different catalysts

Kusama et al.[48,49]studied metal promoted Rh/SiO2catalysts on the hydrogenation of CO2and found that among more than 30 additives Li was the most favorable to ethanol formation.Ethanol selectivity reached up to 15.5%with 7.0%CO2conversion under optimized conditions.The CO intermediate route was verified by in situ FT-IR in their study.Furthermore,they studied Fe promoted Rh/SiO2catalysts[38].Result showed that ethanol selectivity was influenced by the amount of Fe added and ethanol selectivity of 16.0%with 16.7%CO2conversion was obtained.Mechanism research showed that Fe changed the electronic states of Rh and that Fe0promoted methanation and prevented ethanol formation,as well as CO and methanol formation.

Inui et al.[56]used combined supported Rh catalyst,Fe based modified Fisher–Tropsch catalysts,and Cu-based modified methanol synthesis catalyst,which functioned as partial reduction of CO2to CO,C--C bond formation,and--OH group insertion,respectively,to synthesize ethanol via hydrogenation of CO2.A very high ethanol space–time yields of 0.8–0.9 kg·L-1·h-1under 30%CO2conversion level was obtained,which meant that these multi-functional catalytic routes had exhibited promising application to replace the traditional ethanol synthesis route from ethene hydration.

Tompson et al.[52]synthesized a series of M/MO2C(M=Cu,Pd,Co and Fe)catalyst to evaluate CO2hydrogenation at relatively low temperatures of 135–200°C in 1,4-dioxane solvent.And the results showed that methanol was the major product at 135°C,while CH3OH,C2H5OH and C2+hydrocarbons were produced at 200°C.Different from the deposition of Cu and Pd additives promoting the CH3OH synthesis,the deposition of Co and Fe enhanced C--C coupling to produce C2–C4hydrocarbons and ethanol.Mechanism study showed that CO was the intermediate for CO2hydrogenation,and Mo2C had distinct sites for the alcohol and hydrocarbon production,and the metal deposition could alter the activities of these sites.

Fig.2.The performance of various catalysts in CO2 hydrogenation.Reaction conditions:catalyst(20 mg),H2O(2 ml),initial pressure 4.0 MPa(H2/CO2=3:1),15 h,140°C(a)or 200°C(b).The yields and selectivities are based on the number of moles of carbon[53].

More recently,Pd–Cu nanoparticles(NPs)had been utilized towards producing C2H5OH from CO2by Huang et al.[50]after changing catalyst composition and support.They found that Pd2CuNPs/P25 showed high ethanol selectivity of 92.0% and turnover frequency (TOF) of 359.0 h-1,which might result from charge transfer between Pd and Cu in the ordered Pd–Cu NPs/P25,thus increasing the reducibility of surface oxide.

Xiao et al.[53]reported a kind of non-noble cobalt catalysts(CoAlOx)for the selective hydrogenation of CO2to C2H5OH.Results presented that ethanol selectivity as high as 92.1%could be obtained at 140°C with catalyst reduced at a temperature of 600°C.Operando FT-IR spectroscopy revealed that the high ethanol selectivity over the CoAlOxcatalyst might be due to the formation of acetate from formate by insertion of*CHx,a key intermediate in the production of ethanol from CO2hydrogenation.Fig.2 gives obtained data for different catalysts at 140 and 200°C and the data confirmed the extraordinary catalytic performances of CoAlOx,and Fig.3 gives the electron microscopy characterization of CoAlOx.SEM showed that the platelike morphology still remained,and TEM showed Co particles highly dispersed on the amorphous Al2O3support.

The hydrogenation of CO2to ethanol can be regarded as including the following steps:i)reduction of CO2to CO;ii)C--C bond growth;and iii)--OH group insertion to products[56].It is generally believed that formate intermediates(*CO,*CH3,*CO,*CH3CO)are formed in the process of ethanol synthesis,which are hydrogenated into C2H5OH subsequently(proposed reaction mechanism is shown in Fig.4)[48].And the conversion of*CO into*HCO may be the rate-determining step for CO2hydrogenated to ethanol[50].

In particular,Han et al.[57]created a new ethanol synthesis route,of which CO2,H2and dimethyl ether(DME)were used as raw materials(Eq.(1)),catalyzed by a Ru–Co bimetallic catalyst with LiI as promoter.Ethanol selectivity reached to 94.1%in the liquid product as well as 71.7 C-mol%in total products.The proposed mechanism is shown in Fig.5.They speculated three reasons for this high efficiency reaction:1)Direct involvement of DME in the process of ethanol formation could decrease undesired methane by-products.2)Methanol produced could be used as a raw material for the production of DME that the selectivity of ethanol is improved.3)Ru and Co worked well together to speed up the desired reaction synergistically.

Fig.3.Electron microscopy characterization of CoAlOx-600.a)SEM image.b)TEM image(inset:size distribution of the Co NPs).c)HAADF-STEM image.d–f)EDX elemental maps for CoAlOx-600(d),Al(e),and Co(f).g,h)HRTEM images.i)FFT image of the Co NPs corresponding to the HRTEM image in(h).j)EELS result.The pink circles highlighted the cobalt particles[53].

Fig.4.The proposed reaction mechanism of ethanol synthesis from CO2+H2[48].

Fig.5.The proposed mechanism of ethanol synthesis from CO2,H2 and CH3OCH3[57].

Furthermore,Han et al.[58]developed another new synthesis route from paraformaldehyde,CO2and H2(Eq.(2)),which were accelerated over a Ru–Co bimetallic catalyst.The selectivity of ethanol reached to 50.9 C-mol%.They verified that the synergize of paraformaldehyde hydrogenation,RWGS reaction and methanol homologation contributed to the outstanding result of the reaction,which was also promoted by the cooperation of Ru and Co catalyst.Detailed mechanism is shown in Fig.6.

Another promising technique that utilizes CO2is via electroreduction way.Sun et al.[59]reported a high-efficiency metal-free catalyst(Ndoped mesoporous carbon)for CO2electroreduction with almost 100%ethanol selectivity and 77% faraday efficiency. They speculated that high electricity density contributed to the dimerization of CO*as well as formation of ethanol.The detailed reaction energy was shown in Fig.7.

Fig.6.The synergy of the reactions for ethanol synthesis from paraformaldehyde,CO2 and H2.Ru*and Co*represent the active species of Ru and Co,respectively[58].

Fig.7.Calculated reaction energy diagrams for the electroreduction of CO2 to ethanol over the pyridinic and pyrrolic N sites[59].

As one of the important oxygenated chemicals,ethanol has wide applications in fuel additive and intermediate for synthetic chemistry.At present,the synthesis of ethanol from CO2has attracted growing interests and witnessed rapid progress both in the thermal and electrocatalysis routes.To make the production of ethanol from CO2into practice,more work should be done in the development of highly active and selective heterogeneous catalyst system.The precise design of the non-noble catalysts with the functionalities of CO2activation and subsequent transformation towards ethanol should be paid great attention.Moreover,the systematic study of the mechanism for C--C bond formation is very helpful for understanding the pathway for the selective formation of ethanol.Although great efforts still needed to be deserved,the breakthrough of ethanol synthesis from sustainable CO2in pilot scale is believed to be achieved in the near future.

2.2.Synthesis of higher alcohols(C2+OH)

Up to now,research progress is mainly focused on the synthesis of methanol and ethanol.Higher alcohols(C2+OH)are more desirable in many cases,especially as fuels and fuel additives.However,the synthesis of C2+OH via CO2hydrogenation is obviously a challenge.The main difficulty of this process is concentrated on the catalyst[60].Heterogeneous catalysts have the disadvantages of low activity,low C2+OH selectivity and high reaction temperature[61].Therefore,new catalysts with high catalytic performance are needed to be developed.

Han et al.[61]studied the synthesis of C1–5OH from CO2hydrogenation via Ru–Rh bimetallic homogeneous catalyst promoted by LiI in DMI solvent.They found that the catalyst system could work effectively and the C2+alcohols could be obtained at 160°C,which is the lowest temperature reported so far for producing C2+alcohols via CO2hydrogenation.Under the optimized conditions,the alcohols selectivity reached to as high as 96.4%.Noteworthy to say that the alcohols generated by their catalytic system included not only the linear but also branched alcohols,which suggested that the reaction pathway is different from the reported CO2hydrogenation in the literature.

Han et al.[60]reported Ru3(CO)12–Co4(CO)12homogeneous bimetallic catalyst with bis(triphenylphosphoranylidene)ammonium chloride(PPNCl)as co-catalyst and LiBr as the promoter for the CO2hydrogenation.Under this bromide promoted catalytic system,alcohols selectivity achieved as high as 90.8%.This outstanding performance of the catalytic system resulted from the synergized effect of Ru,Co,PPNCl,and LiBr.The bromide promoted Ru catalyst catalyzed the methanol to produce ethanol;Co behaved while accelerating the C2+alcohols.The coordination between the active Ru center(Ru*)and Clfrom PPNCl enhanced the electron density of the metal center,which would facilitate the oxidative addition step as well as the hydrogenation step.

Pt/Co3O4heterogeneous catalysts were also applied to convert CO2into higher alcohols by some researchers.Han et al.[62]firstly used Pt/Co3O4catalyst on CO2hydrogenation to produce higher alcohols(C2–C4),and the effect of water was investigated.Results showed that,88.1%alcohol selectivity could be reached under Pt/Co3O4catalyst at 220°C with water as solvent and DMI(1,3-dimethyl-2-imidazolidinone)as co-solvent.They discovered that water took part in the reaction by providing a hydrogen source,and the C2+alcohols are formed via methanol intermediate route.Li et al.[63]paid more attention to the morphology effect of Co3O4on higher alcohol synthesis from CO2hydrogenation.They found that the Co3O4with both of the nanoplate and nanorod structures performed relatively higher selectivity during 50 h under mild reaction condition.The highest selectivity of C2+OH is 26.7%at 190°C.The Pt/Co3O4-p was much easily reduced,while part of Co3O4was reduced to CoO and metallic Co in the reaction condition formed new active site(Pt-Co/Co3O4-x).

From the mechanism point of view,in the process of higher alcohol synthesis,methanol is generally considered to form in the first step,which is further converted into ethanol.And these small alcohols act as intermediate to generate larger alcohols subsequently[61].Han et al.[60]considered that CO was also formed during the first step,and they put forward the proposed mechanism as shown in Fig.8.Ru catalyst catalyzed the CH3OH to generate C2H5OH,and Co catalyst([Co])mainly enhanced the production of C2+OH in the reaction.

Fig.8.Proposed mechanism of C2+OH synthesis from CO2 hydrogenation[60].

Electrocatalytic reduction of CO2to prepare carbon-based chemical feedstock is an effective way to solve the long-term storage of renewable energy.However,since the reaction involves the transfer of multiple CO2molecules and more than 12 electrons,the electrochemical reduction of CO2to produce multi-carbon alcohol is still full of challenges.Sargent et al.[64]reported a class of core–shell vacancy engineering catalysts that utilize sulfur atoms in the nanoparticle core and copper vacancies in the shell to achieve efficient electrochemical CO2reduction to propanol and ethanol(Fig.9).Results showed that the alcohol-to-ethylene ratio increased more than six-fold compared to that of bare-copper nanoparticles,and achieved C2+alcohol production rate of(126±5)mA·cm-2with a selectivity of(32±1)%Faradaic efficiency.Their research provided new ideas for the selective production of high energy density liquid alcohol fuels,e.g.engine fuels.

3.Synthesis of Acetic Acid

Carboxylic acids are important basic chemicals in industry and human life.Great progress had been made in formic acid synthesis from CO2hydrogenation.The carboxylic acids with two or more C atoms(C2+)are more useful in many cases.However,synthesis of C2+carboxylic acids using CO2as raw material is challenging.The reported routes suffered from obvious disadvantages,such as low selectivity,low activity,higher reaction temperature and use of expensive and/or toxic reactants.Acetic acid is a significant bulk substance,which is commonly produced by methanol carbonylation.In this process,methanol reacted with carbon monoxide to produce acetic acid according to Eq.(3)[65]:

While,acetic acid generated directly from CO2and CH4is another route developed in recent years in view of chemical utilization of CO2.The chemical reaction equation is shown in Eq.(6).Although this reaction possessed 100%atom efficiency,it is thermodynamically unfavorable.Thus,it is necessary to find efficient catalytic system to activate these two molecules for this reaction.Actually,some researchers have already conducted research work on catalytic system,for example,zeolite catalytic system,which has uniform molecular size pores,acid catalytic activity,good thermal stability and hydrothermal stability,and can catalyze many reactions with high activity and high selectivity.Huang et al.first reported the co-conversion of CH4and CO2to acetic acid using the Cu/Co-based metal oxide heterogeneous catalyst.However,the selectivity for acetic acid was only 28%and the product distribution was broad.

Wang et al.[66]further reported a co-conversion of methane and CO2to produce acetic acid over the bifunctional Zn/H–ZSM-5 catalyst(zinc-modified H–ZSM-5 zeolite)at the range of 523–773 K,and also explored the mechanism.Results showed that zinc methyl species(–Zn–CH3)were formed by activation of CH4from zinc site and followed with the insertion of Zn–C bond into CO2.The final acetic acid formation was through transfer of proton,where Br?nsted acid sites played the vital role and the zeolite framework served as a platform for stabilizing and transferring these Br?nsted protons.

Park et al.[67]reported a synthesis method of CH3COOH from CO2and CH4over Cu-M+-ZSM-5 catalyst(M+=Li+,Na+,K+and Ca++)which is continuously fed in flow micro-reactor system.They demonstrated that cation ZSM-5 catalysts loaded with Cu nanoparticles could simultaneously activate both methane and CO2.Acetic acid sustained a yield of 395 μmol·(g cat)-1·h-1for 10 h.Accordingly,the formation rate of CH3COOH was obtained in the order of K ＞Na ＞Ca ＞Li.

Fig.9.Catalyst design and structural characterization.a)Preparation of ethanol and propanol by electrochemical reduction of CO2 using Cu2S–Cu–V core–shell–vacancy copper.b–c)Structural characterization of V–Cu2S.d–g)Structure Characterization of Cu2S–Cu–V.

Curet-Arana et al.[68]studied the mechanism of co-conversion of CH4and CO2into acetic acid systematically over MFI zeolite exchanged with Be,Co,Cu,Mg,Mn,and Zn using density functional theory(DFT).Calculations and the results are shown in Fig. 10. The whole process could be explained as below:activation of CH4led to the formation of(–Cu–CH3),while the activation of CO2helped the formation of surface carbonate species over cation sites(Steps 1–3):insertion of CO2into the(–Cu–CH3)produced surface acetate species(–Cu–OOCCH3)as a reaction intermediate which abstracted the proton to form acetic acid(Steps 4–5).

Recently,Han's group reported a significant progress in synthesizing acetic acid from methanol,CO2and hydrogen(Eq.(5))over Ru–Rh bimetallic catalysts using imidazole as ligand and LiI as promoter in 1,3-dimethyl-2-imidazolidinoe(DMI)solvent[69].Outstanding yield of 77.0%acetic acid with 22.5 h-1TOF was obtained under 200°C and 12 h.More recently,they explored a monometallic catalytic system including RH2(CO)4Cl2,4-methylimidazole(4-MI),LiCl,and LiI,which had advantages of low corrosiveness and easy preparation compared with previously Ru–Rh bimetallic catalyst system[70].The TOF as well as the yield of acetic acid under Rh2(CO)4Cl2system reached up to 26.2 h-1and 81.8%,respectively,which is more effective than the bimetallic system.

They also conducted experiments to clarify the mechanism,and the results indicated that the reaction did not proceed via the CO route and CO2took part in the reaction directly.As shown in Fig.11,LiI reacted with methanol to form CH3I,then CH3I was oxidatively added to the Rh active species,then CO2was inserted into CH3RhI to form the CH3COORhI;at last,acetic acid was formed by reductive elimination from CH3COORhI with H2,and LiOH reacted with HI to form LiI and H2O.In this reaction,LiI promoted acetic acid formation,and LiCl activated catalyst and contributed to the stability of the catalyst,meanwhile Rh2(CO)4Cl2and 4-MI were applied as the ligand.During this process,CO2was directly inserted into the intermediate[69,70].

In short summary,the synthesis of acetic acid from CO2in the current reports generally employed homogeneous catalyst system.Iodide compounds usually were involved in the reaction system,which has imposed serious equipment corrosion during reaction process.In terms of the production cost,sustainability and safety,heterogeneous catalyst system needed to be developed for this important route.Alternatively,with the depletion of fossil energy,the chemical valorization of CO2into acetic acid will exhibit promising application in industrial scale owing to the atom efficiency,abundant carbon source and environmental friendliness.

4.Synthesis of Dimethyl Ether(DME)

Dimethyl ether(DME)is an important chemical and has received significant attention as a clean fuel source because of its lower NOxemission and its near-zero smoke evolution compared to traditional diesel fuels.Generally speaking,there are two technologies for the production of DME from CO2:1)a two-step process(dehydration of methanol that produced from CO2hydrogenation),as shown in Eqs.(6–7)[71–75].2)Onestep process:a single step using bifunctional catalyst for both methanol synthesis and methanol dehydration in one reactor[76].

Fig.10.An ostensible mechanism for the Co-Conversion of CH4 and CO2 into acetic acid over Cu-M+–ZSM-5 Zeolite:(Steps 1–3)activation of CH4 leads to the formation of(–Cu–CH3),while the activation of CO2 helps in the formation of surface carbonate species over cations site.(Steps 4–5):insertion of CO2 into the(–Cu–CH3),produces surface acetate species(–Cu–OOCCH3)as a reaction intermediate which abstracts the proton to form acetic acid[68].

Methanol intermediate route:

Fig.11.Proposed mechanism of the reaction over the Rh-based catalytic system[70].

Two-step route is mainly focused on the catalyst for CO2hydrogenation and methanol dehydrogenation.According to current researches,Cu–Zn–Al(CZA)based catalysts are commonly used for methanol synthesis.And different additives are used to modify the conversion rate of CO2[77–79].While for the second step,dehydrogenation of methanol to DME is usually catalyzed by acid catalysts,such as,γ-alumina[73,75],ZSM-5[74]and so on.

Compared with two-step method,the one-step method has remarkable result because of low methanol concentration in the reactor,which could change thermodynamic equilibrium of the reaction[76].One-step synthesis of DME from CO2hydrogenation can break the thermodynamic restriction that exits for methanol synthesis and consequently improve the CO2conversion.Moreover,the direct method is more economical compared to the indirect one because of eliminating the cost of methanol purification and higher conversion of methanol.Therefore,one-step process is the development trend of catalytic synthesis of DME from CO2[80].

Up to now,the one-step process focuses on CuO–ZnO–Al2O3/HZSM-5,CuO–ZnO–ZrO2/γ-alumina,CuO–Fe2O3–ZrO2/HZSM-5 and so on.The process adopted bifunctional catalysts composed of both methanol synthesis active site and a methanol dehydration active site[81].

Wu et al.[82]used CuO–ZnO–Al2O3/HZSM-5 catalysts to conduct reaction of one-step producing DME from CO2.CuO–ZnO–Al2O3catalysts were made by urea–nitrate combustion,which were merged together with HZSM-5 subsequently.Results showed that the amount of urea had a great influence on the structure of the catalyst.With 40%stoichiometry of urea added,CuO–ZnO–Al2O3/HZSM-5(40CZAH)catalyst with smaller CuO and Cu grain size,higher BET and copper surface areas,and lower reduction temperatures were obtained,which resulted in 15%DME yield and 30.6%CO2conversion.

Fig.12.General DME synthesis scheme.

Mota et al.[83]prepared impregnated catalysts(Cu·ZnO supported on γ-alumina)to study one-step DME synthesis from CO2and the highest selectivity reached up to 35%,under the operating conditions of higher than 270°C and 5 MPa.They deduced that the formation of DME is mainly controlled by catalyst's metal activity instead of the acid function.

It is worth to mention that Ateka et al.[84]used CuO–ZnO–ZrO2(CZZr)/SAPO-18 and CuO–ZnO–MnO(CZMn)/SAPO-18 to replace the conventional CZA/γ-Al2O3catalyst to produce DME from CO2.They found that CZZr and CZMn metal functionalities exhibited similar behavior during the methanol synthesis step and showed higher DME yield and selectivity compared with CZA.Thereby,favoring the valorization of CO2under moderate conditions(3 MPa and H2/COx=3).These results are promising for future studies that focus on optimizing the operating conditions with the objective of CO2valorization(Fig.12).

Qin et al.[85]reported synthesis of DME from CO2over Cu–Fe–Zr/HZSM-5 catalyst in the range of 240°C–280°C.The highest CO2conversion obtained was around 30%at 260°C,3.0 MPa.They put forward that methanol synthesis was the speed control of the reaction and the proposed mechanism was shown in Fig.13.They illustrated that the formation of H2CO was rate control steps of methanol synthesis.The process model was considered to be an accurate description of intrinsic kinetic of DME synthesis from carbon addition.

5.Synthesis of Olefins

5.1.Light olefins(C2–C4)

Another promising route to exploit CO2is the hydrogenation to high value added light olefins(C2–C4),which are one of the most important intermediates in organic chemical process and are essential feedstocks in petrochemical industry.Currently,they are mainly produced by hydrocarbon fluid catalytic cracking(FCC)and steam cracking of petroleum[86],as well as by alkane dehydrogenation[87].The constantly growing demand of light olefins,which has caused the global production capacity to double over the past 15 years,asks for new synthesis processes based on low-cost feedstock.

Among numerous studies,the production of light olefins by the hydrogenation of CO2,would be highly desirable from the viewpoint of CO2utilization.However,until now,only few studies have been reported on the selective hydrogenation of CO2to C2–C4olefins.The conversion of CO2to lower olefins(C2–C4)also includes two routes:1)Fischer-Tropsch pathway(RWGS reaction(Eq.(8))and the CO hydrogenation to olefins(FTO,Fischer–Tropsch to olefins.Eq.(9));and 2)methanol mediated process(MTO,methanol to olefins Eqs.(2)and(10)).

Fig.13.The process simulation of the methanol synthesis reaction from CO2 hydrogenation on Cu(111)via the formate pathway(brown,Cu;gray,C;red,O;white,H)[85].

RWAS:

FTO:

Combined reaction:

Iron-based catalysts are valid on RWGS and in favor of CO hydrogenation to olefins.Han et al.[88]studied dynamic structure of Fe-based catalyst(α-Fe2O3and γ-Fe2O3)for CO2hydrogenation using a combination of operando Raman spectroscopy and X-ray Diffraction coupled with online gas chromatography.Results indicated that the structure of iron oxides went through a process of α-Fe2O3(γ-Fe2O3■α-Fe3O4(γ-Fe3O4)■α-Fe(γ-Fe)■χ-Fe5C2(θ-Fe3C)during activation process.And both iron carbides of χ-Fe5C2and θ-Fe3C showed high activities.χ-Fe5C2exhibited higher selectivity to lower olefins because of its higher effective barrier,but weak alkene hydrogenation ability.However,θ-Fe3C showed stronger chain growth probability than χ-Fe5C2because of its strong CO2adsorption,which could enhance the chaingrowth of adsorbed carbonaceous species.Fig.14 gives the detailed structure performance.

In addition,many researchers have studied various supporters,such as K-OMS-2 nanofiber[89],potassium[90–92],ZIF-8[93],and MIL-53(Al)[93],to modify iron based catalyst.

Suib et al.[89]developed a new kind of porous cryptomelane-type octahedral molecular sieve manganese oxide(K-OMS-2)supported Fe nonocatalysts for light olefin production in both CO2and CO hydrogeneration.High selectivities to light olefins(C2-/C-:1.8–13.4)and catalytic activities(87.2%CO conversion and 45%CO2conversion)at temperature of 593 K in both CO and CO2hydrogenation are owing to the synergistic effect of iron carbides,potassium promoters,and manganese oxide supports.

Song et al.[90]also adopted K to promote Fe-based catalysts(Fe–Co/Al2O3)for the CO2hydrogenation to olefin.They found that lower olefins were the majority among C2+hydrocarbons when the atomic ratio of K/Fe was 1.Adding K would diminish the weakly adsorbed hydrogen on the metal surface while increasing absorbed CO2over catalyst surface,which led to a decrease in further hydrogenation of desired produced olefins and thus increased in the light olefins.In addition,they also found that Fe–Co bimetallic catalyst showed better property than Fe–Mn catalyst when both of them were promoted by K under the same conditions(573 K,1.0 MPa).Pathway analysis showed that the light olefins were formed via CO or CO-liked intermediate in CO2hydrogenation.

Visconti et al.[91]focused on the morphology of K-promoted bulk Fe-based catalyst.They prepared catalysts via ammonium glycolate complexes decomposing,potassium carbonate impregnating,drying and calcinating.They found that a careful control of the calcination process allowed to achieve high surface area and spinel structure that facilitated its reduction and carburization.The K–Fe catalyst with maximum surface area as well as greatest spinel structure fraction,could lead to a majority of light olefins in the product pool(300°C,0.5 MPa,H2/CO2inlet ratio=3,GHSV=2700 cm3·h-1·(g cat)-1).This good result could be ascribed to the maximization of type II sites(active in CO activation and C--C bond formation)instead of type I sites(active in WGS/RWGS process).

Wang et al.[92]found that CO2could react with hydrogen to form methane and C2–C4olefins catalyzed by an Fe/ZrO2catalyst.Afterwards they used Na+,K+,or Cs+to modify Fe/ZrO2catalyst,and K+with a proper concentration(0.5 wt%–1.0 wt%)showed the best performance to lower olefins with 12%C2–C4yield.The possible reason might be the production of χ-Fe5C2active phase facilitated by K+,which was good for light alkenes.

Hu et al.[93]also used metal organic frameworks(MOFs)with different sizes and morphologies to support Fe-based catalyst.They found that the larger crystals of ZIF-8 supporter performed lower selectivity of olefins,due to the influence of secondary hydrogenation reaction in the process of internal diffusion.

The direct CO2hydrogenation to hydrocarbons proceeds via a modified Fischer–Tropsch synthesis process.However,this process favors the hydrogenation of surface adsorbed intermediates,resulting in the ready formation of methane with a decrease in chain growth.Thus,it remains a grand challenge to simultaneously achieve high selectivity for lower olefins and low selectivity for CH4.

Fig.14.Scheme of structure-performance relationship of α-Fe2O3 and γ-Fe2O3 catalysts for CO2 hydrogenation during activation and reaction processes[88].

Fig.15.A possible reaction mechanism for the conversion of CO2 into methanol on the Zn–Ga–O catalyst[98].

The indirect route of lower olefin production from CO2includes conversion of CO2into methanol and olefin production from CH3OH(MTO)(Eq.(11)).Until now,MTO route has attracted attention from many researchers.While,the water formed in the process of methanol synthesis will impede the reaction seriously as well as the activity of catalyst[94].Thus,there is a need to find high efficiency catalyst to enhance catalytic activity and stability.

MTO:

SAPO-34 is a promising industrial catalyst mainly because of its high selectivity to C2H4and C3H6.Liu et al.[95]and Sun et al.[96]reported a bifunctional catalyst composed of In-Zr oxide for CO2activation and SAPO-34 zeolites for C--C coupling,which could convert CO2to C2–C4olefins with 80%–90%selectivity.The design of this bifunctional catalyst could break through the thermodynamic limitations in the process of methanol synthesis,because of the equilibrium shift resulted from methanol consumed[95].The adding of zirconium was in favor of methanol formation from CO2hydrogenation because of the new building vacancies as well as resisting sintering of oxide particles.Additionally,no significant deactivation of catalyst was observed within 150 h,which was potential for application in industry[96].

Li et al.[97]also produced ZnZrO/SAPO tandem catalyst to catalyze MTO reaction.CHxO species(not only methanol)via CO2hydrogenation were generated firstly over ZnZrO,which were transferred onto SAPO zeolite subsequently.

Wang et al.[98]employed a bifunctional catalyst consisting of SAPO-34 and ZnGa2O4to catalyze CO2to lower olefins,with 86%selectivity and 13%CO2conversion at 370°C.They gave the possible reaction mechanism shown in Fig.15 and speculated that the oxygen vacancies on the surface of ZnGa2O4were used to activate CO2to form formate and methoxide species,which were formed into methanol intermediate subsequently.

In addition to chemical routes to use CO2,electrochemical reduction of CO2to desired products was also studied by many researchers.For example,Sargent et al.[99]reported CO2electroreduction to ethylene over copper electrocatalyst reaching to 70%faradaic efficiency at-0.55 V versus a reversible hydrogen electrode(RHE)(Fig.16).Recently,Bell et al.[100]reported copper catalyst for electrochemical reduction of CO2.Specially,they substituted pure copper for CuAg bimetallic electrodes,which was found to show higher selectivity to multi-carbon products.Higher selectivity was on account of selective inhibition of hydrogen evolution.And it was caused by compressive strain in the process of CuAg surface alloy formation.A small quantity of Ag could promote the selectivity to multi-carbon oxygenated Cu surface in the reaction of CO2electroreduction.Cu nanocatalyst has advantages in the electrocatalytic reduction of CO2to C2+products due to good ability of C--C coupling.

Fig.16.Structure and performance of the polymer-based gas diffusion electrode.(A)Schematic illustration of the graphite/carbon NPs/Cu/PTFE electrode.(B)Cross-sectional SEM image of a fabricated graphite/carbon NPs/Cu/PTFE electrode.(C)SEM image of Cu nanoparticles sputtered on the PTFE membrane.(D)Comparison of ethylene faradaic efficiencies on graphite/carbon NPs/Cu/PTFE and graphite/Cu/PTFE electrodes for CO2-RR in 7 mol·L-1 KOH electrolyte.Values are means,and error bars indicate SD(n=3 replicates)[99].

Karen et al.[101]developed a metal-ion battery recycling method to regulate the crystal surface of Cu nanocatalysts based density functional theory(DFT).And a highly selective electrocatalytic reduction of CO2to produce C2+products could be achieved.They also constructed a copper–ion battery with the highest faraday efficiency of 60%,an H2ratio of less than 20%in the product,as well as a C2+current density of about 40 mA·C·m-2.Gewirth et al.[102]developed a method of converting CO2into multi-carbon hydrocarbons and oxygenates.When the alloy film contained 6%Ag,CO2electroreduction showed the better performance,and the Faradaic efficiency of ethylene and ethanol reached to 60%and 25%,respectively.In addition,Minteer et al.[103]studied bioelectrochemical CO2reduction to produce ethylene and propene over a metalloenzyme.

In summary,there are two synthesis routes of lower olefin(C2–C4)process via CO2hydrogenation.One is Fischer-Tropsch pathway(RWGS reaction(Eq.(1))and the CO hydrogenation to olefins(FTO,Fischer–Tropsch to olefins.Eq.(10)).In this route,Fe based catalysts show good performance.Another important process is methanol mediated process(MTO,methanol to olefins Eqs.(2)and(11)).Zeolites are often employed into this route.For electrochemical route to utilize CO2,copper electrode and alloy are often employed as electrodes.

5.2.Higher olefins

It is worth to mention that Guo et al.[82]created a bio-promotersmodified catalyst for CO2hydrogenation into alkenes.The catalyst was composed of iron carbides as well as alkali promoters from corncob,based on the synergistic catalysis occurring in biomass enzyme.Compared with chemical promoters,potassium derived from bio-accelerator showed a stronger migration ability during CO2hydrogenation,because of enriched K+on the surface facilitate carbides formation,which increase C-C coupling activity.The specific reaction mechanism was shown in Fig.17.The experimental results showed alkene selectivity of 72%and 50.3%C4–C18alkene selectivity.

6.Synthesis of Gasoline

Gasoline(C5–C11hydrocarbons)is a significant transportation fuel used widely all over the world[28].C5–C11hydrocarbons are generally obtained in three ways:1)by refining petroleum 2)from syngas by FTS process;3)MTG(methanol to gasoline)process[104].Converting CO2to fuels not only contribute to CO2emission reduction,but also provide valuable fuels.However,because of the chemical inert and thermodynamically stable of CO2,it is a challenge to transform CO2to long chain product,as C5–C11hydrocarbons.

CO2can be hydrogenated to hydrocarbons by either direct or indirect route.The direct CO2hydrogenation(CO2-FT)is often described as the combination of the reduction of CO2to CO via reverse water–gas shift(RWGS)reaction and subsequent hydrogenation of CO to hydrocarbons via Fischer–Tropsch synthesis(FTS).

Sun et al.[105]succeeded in direct hydrogenation of CO2to produce fuels using multifunctional Na–Fe3O4/HZSM-5 catalyst which provided three types of active sites:Fe3C4,Fe5C2and acid sites,which catalyzed RWGS,olefins synthesis and oligomerization isomerization aromatization respectively(Fig.18).Results showed that 22%CO2conversion with 78%gasoline selectivity,and 4%methane selectivity were obtained.Changing zeolite type and/or catalyst's integration manner would lead to different gasoline compositions.What was worth mentioning is that low ratio of H2/CO2in feed gas was permitted by this multifunctional catalyst,which decreased hydrogen cost.Moreover,the catalyst showed stability for 1000 h.These meant that CO2-gasoline route over Na–Fe3O4/HZSM-5 catalyst had broad application prospects.

However,the indirect route is generally performed via using different reactors with syngas,and/or forming intermediate of methanol[105].Sun et al.[28]reported a bifunctional catalyst consisting of In2O3and HZSM-5,which could convert CO2into C5+hydrocarbons with 78.6%selectivity and only 1%CH4selectivity at 13.1%CO2conversion.They speculated that methanol formed from CO2hydrogenation at the site of oxygen vacancy over the surface of indium oxides catalyst,which was subsequently transformed into hydrocarbons inside HZSM-5(Fig.19).In addition,the number of active vacancies determines the activity of catalyst.That meant using a proper support and/or introducing modifiers could enhance catalytic activity.Industry test also showed a good result of pellet catalyst,which meant a potential application in the industries.

In contrast,the direct route is more economically and environmentally benign since the direct route usually produces CO and light paraffins as major products owing to weak CO hydrogenation activity and over-hydrogenation of olefins.Gasoline-range hydrocarbons are generally produced from refining of petroleum,or from syngas via FTS process,or from methanol-to-gasoline(MTG)process[19].So far,there has been no report on highly selective synthesis of gasoline from direct CO2hydrogenation.The key to this process is to search for a highly efficient catalyst.

7.Conclusions

Fig.17.Reaction scheme for CO2 hydrogenation to α-olefins.The existences of biopromoters weaken secondary hydrogenation of olefins and strengthen the reaction activity of C--C coupling[82].

Fig.18.Reaction scheme for CO2 hydrogenation to gasoline-range hydrocarbons.The CO2 hydrogenation reaction over Na–Fe3O4/Zeolite multifunctional catalyst takes place in three steps:(1)an initially reduced to CO intermediate via RWGS,(2)a subsequent hydrogenation of CO to α-olefins intermediate via FTS and(3)the formation of gasoline-range hydrocarbons via the acid-catalyzed oligomerization,isomerization and aromatization reactions[105].

Undoubtedly,the chemical utilization of CO2has received significant and increasingly attention worldwide as an important carbon source with the gradual depletion of fossil energy.The utilization of CO2is being developed towards the environment friendly,diversified and efficient processes.The reduction of CO2to C2+chemicals and fuels,e.g.alcohols,acetic acid,dimethyl ether,olefins and gasoline,has witnessed rapid growth in recent years.Definitely,the new emerging green catalytic technology will play an important role in the efficient utilization of CO2as cheap and easily available carbon oxygen sources.In order to improve the efficiency of the catalytic process,new catalytic material with high efficiency is needed to be developed.The CO2conversion was restrained by reaction thermodynamics,the intrinsic catalytic activity must be excellent;simultaneously the selectivity should be improved.Additionally,the lifetime of the catalyst also could be long enough to meet the requirement of the industrialization.The development of the reduction catalyst with the good activity and durability is the prerequisite of the industrialization.Meanwhile,the in-depth understanding of the reaction mechanisms relevant to C=O bond activation and C--C bond on the catalyst surface are crucial for developing more effective catalyst system.Besides the development of catalyst with remarkable activity and stability,the design of specific reactor is inevitable for the enlargement of the technology.

Fig.19.Molecular-level mechanism for CO2 hydrogenation into hydrocarbons.a)Energy profile from DFT calculations for CO2 hydrogenation to CH3OH on the In2O3(110)surface(D and P stand for defective and perfect surfaces with and without the oxygen vacancy,respectively).b)Schematic of the hydrocarbon-pool mechanism for CH3OH conversion into hydrocarbons inside HZSM-5.c)Schematic for the formation of CH3OH from CO2 at the oxygen vacancy site on the In2O3 catalyst surface,which involve four major steps:(1)CO2 adsorption at the oxygen-vacancy site,(2)sequential hydrogenation of the adsorbed CO2 species to CH3OH,(3)CH3OH desorption,which leads to the surface without the oxygen vacancy,and(4)hydrogenation of the surface to regenerate the oxygen-vacancy site.d)Schematic for hydrocarbon formation from CH3OH at the acidic site inside the pores of the HZSM-5 catalyst via the hydrocarbon-pool mechanism[28].

In some extent,the chemical utilization of CO2to chemicals and fuels is beyond the scope of the field of CO2chemistry,the feasibility of the practical application of this route is dominantly depending on the source of cheap hydrogen.From a practical view-points,the comprehensive and economical assessment of varied production routes via hydrogenation of diverse CO2resources as raw materials should be clarified.It should be noted that the storage and remote transfer of hydrogen are difficult so far.In particular,the reduction transformation of CO2with H2into readily stored and transported chemicals and fuels is highly desired.The integration of multiple sources of energy,e.g.solar energy,wind energy and nuclear energy,could potentially provide abundant and complementary H2for CO2reduction.Importantly,CO2-free H2source is beneficial for neat CO2mitigation,which could be realized together with the development of renewable energy or atomic energy in the future.

The transformations of CO2to important chemicals and fuels have demonstrated enormous potential and now are considered as an important route for CO2chemical utilization on large scale.At present,most of the new-developed technologies are still in the stage of laboratory study and engineering demonstration.The comprehensive evaluation of the catalyst efficiency,durability,cost and energy integration on the industrial demonstration scale has great significance for the practical applications.