Conversion of the CO and CO2 mixture to alcohols and hydrocarbons by hydrogenation under the influence of the water-gas shift reaction,a thermodynamic consideration

GUO Shu-jia，WANG Han，QIN Zhang-feng，LI Zhi-kai，WANG Guo-fu，DONG Mei，FAN Wei-bin，WANG Jian-guo,2

（1.State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China；2.University of Chinese Academy of Sciences, Beijing 100049, China）

Abstract: Due to the intervention from the water-gas shift (WGS) reaction (or the reverse one (RWGS)),the hydrogenation of CO (or CO2) into alcohols and hydrocarbons often displays rather high selectivity to CO2 (or CO),which makes it rather puzzling to evaluate such conversion processes by using the relatively low selectivity to the target products.Herein,a thermodynamic consideration is made to elaborately evaluate the effect of the WGS/RWGS reaction on the hydrogenation of CO,CO2,and their mixture to typical alcohols (e.g. methanol) and hydrocarbons (e.g. ethene).The results indicate that for the hydrogenation of CO (or CO2),although the WGS (or RWGS) reaction,acting as a communicating vessel connecting CO and CO2,may have a severe influence on the equilibrium conversion of CO (or CO2),forming a large amount of CO2 (or CO),it only has a relatively minor impact on the C-based equilibrium yield of the target alcohol/hydrocarbon product.The hydrogenation of CO shows a higher C-based equilibrium yield for the target product than the hydrogenation of CO2,while the overall C-based equilibrium yield of target product for the hydrogenation of the CO and CO2 mixture just lies in between.For the hydrogenation of the CO and CO2 mixture,although the equilibrium conversion of CO and CO2 may vary greatly with the change in the feed composition,the relation between the overall C-based equilibrium yield of the target product and the feed composition is rather simple;that is,the overall C-based equilibrium yield of alcohol/hydrocarbon product decreases almost lineally with the increase of the CO2/(CO+CO2) molar ratio in the feed.These results strongly suggest that the mixture of CO and CO2 is credible in practice for the production of alcohols and hydrocarbons through hydrogenation,where the overall C-based yield should be used as the major index for the hydrogenation of CO,CO2,and their mixture.

Key words: thermodynamic consideration；CO2 hydrogenation；CO hydrogenation；water-gas shift reaction；overall C-based yield；methanol；ethene；CO and CO2 mixture

The conversion of CO and CO2into alcohols and hydrocarbons through hydrogenation is now considered as a potential measure in the exploitation of renewable carbon resources and sustainable production of fuels and chemicals[1-15].In virtue of the enormous efforts from numerous scientists and engineers,the research and development of appropriate catalysts for the CO/CO2hydrogenation have achieved great success in recent years.In particular,the application of bifunctional oxide-zeotype (OX-ZEO) composite catalyst can breach the Anderson-Schulz-Flory (ASF) rule for the product distribution in the Fischer-Tropsch synthesis (FTS) and then greatly elevate the selectivity to the target alcohol/hydrocarbon products,which excites extensive interest in the CO/CO2hydrogenation[1-15].

The hydrogenation of CO (or CO2) is usually,if not always,accompanied by the water-gas shift (WGS)reaction (or the reverse one (RWGS)),in spite of the hard effort devoted in the exploration of special catalysts that are highly active in the CO/CO2hydrogenation to alcohols/hydrocarbons but inactive in the WGS/RWGS reaction.Due to the intervention from the WGS (or RWGS) reaction,the hydrogenation of CO (or CO2) into alcohols and hydrocarbons often displays rather high selectivity to CO2(or CO) and relatively low selectivity to the target product.Accordingly,the reaction results for the CO (or CO2)hydrogenation are often reported in the product selectivity in which the product of CO2from the WGS reaction (or CO from the RWGS reaction) is excluded,in order to make the reaction results good-looking;however,this also makes the evaluation of such conversion processes rather confusing.Meanwhile,to resist the WGS reaction for CO hydrogenation (or the RWGS reaction for CO2hydrogenation),addition of certain amounts of CO2(or CO) in the feed are proposed;however,such a compromise measure arouses a conjecture whether the CO and CO2mixture is appropriate or even better than pure CO or CO2for the production of alcohols/hydrocarbons from CO/CO2through hydrogenation.

In the previous work[15],we have evaluated the feasibility,limit,and suitable reaction conditions for the production of alcohols and hydrocarbons from CO and CO2through hydrogenation from the view of thermodynamics.In this work,by using the same method,the effect of the WGS/RWGS reaction on the hydrogenation of CO,CO2,and their mixture to typical alcohols (e.g.methanol) and hydrocarbons (e.g.ethene)was elaborately investigated.The results illustrated that although the WGS/RWGS reaction brings on rather high equilibrium selectivity to CO2/CO,it has only a relatively minor impact on the C-based equilibrium yield of the target alcohol/hydrocarbon product for the CO/CO2hydrogenation.The mixture of CO and CO2is really creditable for the production of alcohols and hydrocarbons in practice through hydrogenation,where the overall C-based yield should be used as the major index for the hydrogenation of CO,CO2,and their mixture.Such observation should be of great significance in exploring more efficient catalysts and processes for the CO/CO2hydrogenation to defined alcohol or hydrocarbon products.

1 Methods

As well known,methanol and ethene can be synthesized from CO,CO2,and their mixture by hydrogenation via the reactions:

where the WGS reaction:

or the reverse one (RWGS):

occurs as a nonindependent reaction[15].

Following the similar procedures reported previously[15-17],the equilibrium conversion of CO and CO2(XCOandXCO2) as well as the overall C-based equilibrium yield of methanol and ethene (YMeOHandYC2H4,taking both CO and CO2as the C-containing reactant) were calculated:

whereNCO,inandNCO2,inare the molar quantity of CO and CO2,respectively,in the initial reaction mixture,whileNCO,eq,NCO2,eq,NMeOH,eq,andNC2H4,eqare the molar quantity of CO,CO2,methanol,and ethene,respectively,in the equilibrium reaction mixture.For the hydrogenation of individual CO and CO2,the selectivity to CO2and CO (SCO2andSCO) was also determined,respectively:

For all the equilibrium calculation,the nonideality of the gaseous mixture was considered using the Soave-Redlich-Kwong (SRK) equation of state[18,19].In addition,the constitution of the reaction mixture was initialized according to the stoichiometric molar ratio of H2/CO and/or H2/CO2.That is,depending on the feed CO2/(CO+CO2) molar ratio (z),the initial reaction mixture has a H2/CO/CO2molar ratio of (2+z)/(1-z)/z.For all the calculations,the reaction pressure between 0.1 and 8 MPa and temperature between 150 and 600 °C were considered,in order to make comparison with the practical processes for the CO/CO2hydrogenation and the related experimental results reported in the literature.

2 Results

2.1 Effect of the WGS reaction on the hydrogenation of individual CO to methanol and ethene

As mentioned previously[15],an ideal methanol synthesis process from CO by hydrogenation does not have water in the products (Equation (1)),for which the consideration of the WGS reaction may not be necessary.However,water is a co-product for the CO hydrogenation to higher alcohols and hydrocarbons(e.g.ethene,Equation (3)) and the WGS reaction may inevitably occur in tandem with the CO hydrogenation(Equation (5)),which yields CO2and inevitably has a significant impact on the yield of the target product.

As shown in Figure 1(a),for the CO hydrogenation to ethene,the intervention from the WGS reaction does produce considerable amount of CO2and the equilibrium selectivity to CO2reaches 42.2% at 0.1 MPa and 425 °C and 27.4% at 8 MPa at 600 °C.However,it is surprising to note that the WGS reaction only has a relatively minor impact on the Cbased equilibrium yield of ethene for the CO hydrogenation,although it may conduce to a rather high equilibrium selectivity to CO2.

Figure 1 (a) Equilibrium yield of ethene and selectivity to CO2 for the hydrogenation of CO to ethene via the reaction of 2CO+4H2=C2H4+2H2O;(b) equilibrium yield of methanol and selectivity to CO for the hydrogenation of CO2 to methanol via the reaction of CO2+3H2=CH3OH+H2O;and (c) equilibrium yield of ethene and selectivity to CO for the hydrogenation of CO2 to ethene via the reaction of 2CO2+6H2=C2H4+4H2O.The first reaction has a H2/CO molar ratio of 2 in the initial reaction mixture of H2 and CO,whereas the later two reactions have a H2/CO2 molar ratio of 3 in the initial reaction mixture of H2 and CO2.The solid lines are for the individual CO or CO2 hydrogenation reactions alone,whereas the dashed lines are for those having the intervention from the WGS/RWGS reaction of CO+H2O=CO2+H2

Owing to the moderately exothermic and molecularity-invariant nature of the WGS reaction[15],the WGS reaction extent at equilibrium is independent of pressure but considerably suppressed at a higher reaction temperature.In contrast,for the CO hydrogenation to ethene,a lower reaction temperature and/or a higher pressure are thermodynamically favorable to a higher C-based equilibrium yield of ethene,due to its strongly exothermic and moleculedecreasing nature.Meanwhile,the equilibrium selectivity to CO2is also greatly governed by the amount of water formed from the CO hydrogenation as well as the content of CO leftover in the reaction mixture.The amount of water in the reaction mixture is proportional to the yield of target product (e.g.ethene),whereas the content of CO is just to the contrary.At a low temperature and/or a high pressure,the high yield of ethene leaves less CO available to participate in the WGS reaction (in spite of a large amount of water is generated by the CO hydrogenation);in contrast,at a high temperature and/or a low pressure,the low yield of ethene is accompanied by the formation of less water to drive the WGS reaction (even now there is a large amount of CO surplus in the reaction mixture).Accordingly,at a given pressure,the equilibrium selectivity to CO2displays a maximum with the increase of the reaction temperature (Figure 1(a));in addition,the maximum equilibrium selectivity to CO2shifts to a higher temperature in position but decreases in magnitude,with the increase of the reaction pressure.

Taking all these factors into account,for the CO hydrogenation to ethene,at a low temperature,the intervention from the WGS reaction leads to a certain decrease in the C-based equilibrium yield of ethene,whereas at a high temperature,the presence of the WGS reaction may be even somewhat beneficial to a higher C-based equilibrium yield of ethene (Figure 1(a)).For example,at 1 MPa and 375 °C,although the consideration of the WGS reaction brings on an equilibrium selectivity of 29.3% to CO2,the C-based equilibrium yield of ethene decreases only from 74.8%to 67.8%.In contrast,at 1 MPa and 450 °C,the intervention from the WGS reaction results in an equilibrium selectivity of 33.9% to CO2,but the Cbased equilibrium yield of ethene even increases slightly from 52.3% to 55.4%.At all events,compared with the great influence of the WGS reaction on the equilibrium selectivity to CO2,it only has a relatively minor impact on the C-based equilibrium yield of ethene for the CO hydrogenation,in particular at a high reaction pressure.

2.2 Effect of the RWGS reaction on the hydrogenation of individual CO2 to methanol and ethene

For the hydrogenation of CO2to alcohols (e.g.methanol,Equation (2)) and hydrocarbons (e.g.ethene,Equation (4)),the RWGS reaction (Equation (6)) may take place inescapably as a parallel side reaction that competes with the main reaction and generates CO and water.As mentioned previously[15],the RWGS reaction is moderately endothermic and molecularity-invariant;accordingly,the RWGS reaction extent at equilibrium is then independent of pressure but considerably enhanced at a higher reaction temperature.As shown in Figure 1(b),(c),the selectivity to CO increases considerably with the increase of reaction temperature and/or the decrease of reaction pressure.Meanwhile,similar to the CO hydrogenation,the hydrogenation of CO2to alcohols and hydrocarbons is strongly exothermic and molecule-decreasing;as a result,the Cbased equilibrium yield of methanol and ethene decreases significantly with the increase of reaction temperature and/or the decrease of reaction pressure.

Nevertheless,as illustrated in Figure 1(b),(c),at a sufficiently low temperature and/or a high pressure,the impact of the RWGS reaction on the C-based equilibrium yield of methanol and ethene is relatively minor,in spite of its great influence on the equilibrium selectivity to CO.For example,for the CO2hydrogenation to methanol at 225 °C and 8 MPa,although the equilibrium selectivity to CO reaches 5.6%,the C-based equilibrium yield of methanol only decreases to 35.9% upon the competition from the RWGS reaction,in comparison with the value of 37.1% where the RWGS reaction is inactivated.In contrast,for the CO2hydrogenation to ethene at 350 °C and 3 MPa,the competition from the RWGS reaction only leads to a rather minor decrease of the C-based equilibrium yield of ethene to 63.4%,compared with the value of 64.1% where the RWGS reaction is not allowed.It should be stated at the same time that at an adequately high temperature (e.g.＞ 250 °C for the CO2hydrogenation to methanol at 3 MPa and ＞ 400 °C for the CO2hydrogenation to ethene at 3 MPa),the RWGS reaction in competition inevitably has a severe impact on the main reaction and leads to a significant decrease in the C-based equilibrium yield of methanol and ethene (Figure 1(b),(c)).

At all events,as clarified previously[15],the hydrogenation of CO2to alcohols and hydrocarbons is thermodynamically preferable at a lower reaction temperature and/or a higher pressure.In addition,the C-based equilibrium yield of methanol and ethene,even under the influence of the RWGS reaction,is usually still much higher than the level at which most of the currently available catalysts can arrive for the CO2hydrogenation[15,20].It suggests that current research on the CO2hydrogenation to alcohols and hydrocarbons is still greatly restricted by the kinetic factors;there is still adequate margin left for one to enhance the efficiency of CO2hydrogenation processes through designing and preparing more efficient catalysts[1-4,7,15,20].

2.3 Hydrogenation of the CO and CO2 mixture to methanol

The addition of certain amount of CO2(or CO) in the feed is often proposed to resist the WGS reaction for the CO hydrogenation (or to counteract the RWGS reaction for the CO2hydrogenation).On this ground,the effect of the feed composition of the CO and CO2mixture on the overall C-based equilibrium yield of methanol (taking both CO and CO2as the C-containing reactant,Equation (9)) as well as the equilibrium conversion of CO and CO2was systematically considered,as demonstrated in Figures 2 and 3.

Apparently,the WGS or RWGS reaction acts vividly on the equilibrium conversion of CO and CO2(Figure 2),illustrating that the WGS or RWGS reaction as a communicating vessel connecting CO and CO2has a significant influence on the composition of equilibrium reaction mixture (viz.,the relative equilibrium conversion of CO and CO2).Meanwhile,the overall C-based equilibrium yield of methanol for the hydrogenation of the CO and CO2mixture with different feed compositions (CO2/(CO+CO2)=0,0.25,0.5,0.75,and 1) resembles each other in the varying pattern with the reaction temperature and pressure,although a higher content of CO2in the reaction mixture in general leads to a lower overall Cbased equilibrium yield of methanol.In particular,the case of CO2/(CO+CO2)=0 represents the hydrogenation of individual CO to methanol,where the WGS reaction cannot be considered and the C-based equilibrium yield of methanol is actually identical to the equilibrium conversion of CO.In contrast,the hydrogenation of individual CO2to methanol at CO2/(CO+CO2)=1 displays a rather lower C-based equilibrium yield of methanol than the individual CO hydrogenation,accompanied by a considerably high equilibrium selectivity to CO.

Figure 2 Overall C-based equilibrium methanol yield,CO conversion,and CO2 conversion for the hydrogenation of the CO and CO2 mixture to methanol via the reactions of CO+2H2=CH3OH and CO2+3H2=CH3OH+H2O,where the water-gas shift (WGS)reaction of CO+H2O=CO2+H2 as a nonindependent reaction occurs inevitably;depending on the CO2/(CO+CO2) molar ratio (z),the initial reaction mixture has a H2/CO/CO2 molar ratio of (2+z)/(1-z)/z.For the pure CO2 hydrogenation (z=1),the equilibrium selectivity to CO was displayed instead of the equilibrium CO conversion

The influence of the feed CO2/(CO+CO2) molar ratio (z) on the overall C-based equilibrium methanol yield,CO conversion,and CO2conversion for the hydrogenation of the CO and CO2mixture into methanol at a typical reaction temperature (225 °C) and pressure (5 MPa) is further demonstrated in Figure 3.Apparently,although the equilibrium conversion of CO and CO2may vary greatly with the feed CO2/(CO +CO2) molar ratio,the overall C-based equilibrium yield of methanol decreases almost lineally with the increase of the feed CO2/(CO+CO2) molar ratio;that is,a higher content of CO in the reaction mixture is thermodynamically conducive to a higher overall Cbased equilibrium yield of methanol.At all events,the overall C-based equilibrium yield of methanol increases considerably with the decrease of the reaction temperature and/or the increase of reaction pressure.

Figure 3 Overall C-based equilibrium methanol yield,CO conversion,and CO2 conversion varied with the feed CO2/(CO+CO2)molar ratio (z) for the hydrogenation of the CO and CO2 mixture into methanol at 225 °C and different pressures (left) and at 5 MPa and different temperatures (right),via the reactions of CO+2H2=CH3OH and CO2+3H2=CH3OH+H2O,where the water-gas shift(WGS) reaction of CO+H2O=CO2+H2 as a nonindependent reaction occurs inevitably;depending on the CO2/(CO+CO2) molar ratio (z=0-1),the initial reaction mixture has a H2/CO/CO2 molar ratio of (2+z)/(1-z)/z

2.4 Hydrogenation of the CO and CO2 mixture to ethene

For the hydrogenation of the CO and CO2mixture to ethene,as displayed in Figures 4 and 5,similar results are obtained.That is,although the equilibrium conversion of CO and CO2may vary greatly with the feed composition (CO2/(CO+CO2)=0,0.25,0.5,0.75,and 1),the overall C-based equilibrium yield of ethene(taking both CO and CO2as the C-containing reactant,Equation (10)) for the hydrogenation of the CO and CO2mixture with different feed compositions resembles each other in the varying pattern with the reaction temperature and pressure (Figure 4).In like manner,the overall C-based equilibrium yield of ethene increases considerably with the decrease of the reaction temperature and/or the increase of reaction pressure.It is noteworthy that at two sides of CO2/(CO+CO2)=0 and 1,representing the hydrogenation of individual CO and CO2,respectively,the WGS/RWGS reaction here has a severe impact on the main reaction to ethene and leads to a considerably high equilibrium selectivity to CO2and CO,respectively.

Figure 4 Overall C-based equilibrium ethene yield,CO conversion,and CO2 conversion for the hydrogenation of the CO and CO2 mixture to ethene via the reactions of 2CO+4H2=C2H4+2H2O and 2CO2+6H2=C2H4+4H2O,where the water-gas shift (WGS)reaction of CO+H2O=CO2+H2 as a nonindependent reaction occurs inevitably;depending on the CO2/(CO+CO2) molar ratio (z),the initial reaction mixture has a H2/CO/CO2 molar ratio of (2+z)/(1-z)/z.For the pure CO hydrogenation (z=0),the equilibrium selectivity to CO2 was displayed instead of the equilibrium CO2 conversion,whereas for the pure CO2 hydrogenation (z=1),the equilibrium selectivity to CO was displayed instead of the equilibrium CO conversion

Similarly,at a typical reaction temperature (350°C) and pressure (3 MPa),as demonstrated in Figure 5,in spite of the great variance in the equilibrium conversion of CO and CO2with the feed composition,the overall C-based equilibrium yield of ethene decreases lineally with the increase of the feed CO2/(CO+CO2) molar ratio;that is,a higher content of CO in the reaction mixture is thermodynamically preferable to a higher overall C-based equilibrium yield of ethene.In addition,in comparison with the hydrogenation of CO/CO2to methanol,the hydrogenation of CO/CO2to ethene (with a much lower Gibbs free energy[15]) shows a much higher overall C-based equilibrium yield for the target product under the same reaction conditions.

Figure 5 Overall C-based equilibrium ethene yield,CO conversion,and CO2 conversion varied with the feed CO2/(CO+CO2) molar ratio (z) for the hydrogenation of CO and CO2 mixture into ethene at 350 °C and different pressures (left) and at 3 MPa and different temperatures (right),via the reactions of 2CO+4H2=C2H4+2H2O and 2CO2+6H2=C2H4+4H2O,where the water-gas shift (WGS)reaction of CO+H2O=CO2+H2 as a nonindependent reaction occurs inevitably;depending on the CO2/(CO+CO2) molar ratio (z=0-1),the initial reaction mixture has a H2/CO/CO2 molar ratio of (2+z)/(1-z)/z

3 Discussion

Above results demonstrate that the WGS/RWGS reaction does have certain influence on the CO/CO2hydrogenation to alcohols and hydrocarbons.An ideal solution to this issue is naturally to design a catalyst that is only active to the hydrogenation of CO/CO2to the defined products but absolutely inactive to the WGS/RWGS reaction.This is however rather difficult and may be even impossible in practice.

Fortunately,for the hydrogenation of CO (or CO2),although the WGS (or RWGS) reaction,acting as a communicating vessel connecting CO and CO2,may have a severe influence on the equilibrium conversion of CO (or CO2) as well as the equilibrium selectivity to CO2(or CO),it only has a relatively minor impact on the C-based equilibrium yield of the target alcohol/hydrocarbon product under appropriate reaction temperatures and pressures.

The hydrogenation of CO displays a higher Cbased equilibrium yield for the target alcohol/hydrocarbon product than the hydrogenation of CO2,while the overall C-based equilibrium yield of the target product for the hydrogenation of the CO and CO2mixture just lies in between.For the hydrogenation of the CO and CO2mixture to alcohols/hydrocarbons,in spite of the great variance in the equilibrium conversion of CO and CO2along with the feed composition,the overall C-based equilibrium yield of target alcohol/hydrocarbon product decreases almost lineally with the increase of the feed CO2/(CO+CO2)molar ratio.

As the composition of the equilibrium reaction mixture varies greatly with the reaction temperature,pressure,and feed composition,the addition of CO2in the feed for the CO hydrogenation (or CO for the CO2hydrogenation) to resist the WGS reaction (or the RWGS reaction) may be less effective.However,the influence of the feed composition on the overall Cbased equilibrium yield of the target alcohol/hydrocarbon product is much simpler;that is,a higher content of CO2in the reaction mixture of CO and CO2leads to a lower overall C-based equilibrium yield of the target alcohol/hydrocarbon product.Meanwhile,current results suggest that the mixture of CO and CO2is practical and may be even more efficient and cost-effective to produce alcohols and hydrocarbons from CO/CO2through hydrogenation,in comparison with pure CO or CO2.

At present the hydrogenation of CO (or CO2) to alcohols/hydrocarbons is usually evaluated by the conversion of CO (or CO2) as well as the selectivity to target product compared with the selectivity to CO2(or CO).To make the reaction results good-looking,the reaction results for the CO (or CO2) hydrogenation are often reported in the product selectivity in which the product of CO2from the WGS reaction (or CO from the RWGS reaction) is excluded,which makes the evaluation of such conversion processes rather puzzling.

On account of the inescapability of the WGS or RWGS reaction in the hydrogenation of CO,CO2,and their mixture,as well as the much simple relation between the overall C-based equilibrium yield of the target alcohol/hydrocarbon product and the feed composition (viz.,the feed CO2/(CO+CO2) molar ratio),it is then strongly proposed that the overall Cbased yield should be used as the major index to evaluate the reaction processes for the hydrogenation of CO,CO2,and their mixture.In addition,as the mixture of CO and CO2is credible for the production of alcohols and hydrocarbons in practice through hydrogenation,greater effort should then be devoted in the design and development of efficient catalysts that are highly active for the hydrogenation of both CO and CO2to the target alcohol/hydrocarbon products rather than in the possibly hopeless obstruction of the WGS/RWGS reaction.

In addition,the use of the CO and CO2mixture for hydrogenation can also spare a lot in the separation and purification of feed gas;furthermore,the reaction tail gas after extracting the target product,which is surely a mixture of CO and CO2,can be directly recycled to refeed the reactor without strenuous separation and refinement.

4 Conclusions

An elaborate thermodynamic analysis is used to investigate the effect of the WGS or RWGS reaction on the hydrogenation of CO,CO2,and their mixture to typical alcohols (e.g.methanol) and hydrocarbons (e.g.ethene).

The results indicate that although the WGS (or RWGS) reaction may bring on rather high equilibrium selectivity to CO2for the CO hydrogenation (or to CO for the CO2hydrogenation),it has only a relatively minor impact on the C-based equilibrium yield for the target alcohol/hydrocarbon product under appropriate reaction conditions.The hydrogenation of CO displays a higher C-based equilibrium yield for the target product than the hydrogenation of CO2,while the overall C-based equilibrium yield of the target alcohol/hydrocarbon product for the hydrogenation of the CO and CO2mixture just lies in between.

Although the equilibrium conversion of CO and CO2as well as the composition of equilibrium reaction mixture may vary greatly with the change in the feed composition,the relation between the overall C-based equilibrium yield of the target alcohol/hydrocarbon product and the feed composition is much simple;that is,the overall C-based equilibrium yield of alcohol/hydrocarbon product decreases almost lineally with the increase of the CO2/(CO+CO2) molar ratio in the feed.

These results strongly suggest that the mixture of CO and CO2is credible for the production of alcohols and hydrocarbons in practice through hydrogenation,where the overall C-based yield should be used as the major index for the hydrogenation of CO,CO2,and their mixture.In addition,the use of the CO and CO2mixture for hydrogenation can spare a lot in the separation and purification of the feed gas and the recycling of the reaction tail gas.However,hard efforts may still be necessary in the design and development of efficient catalysts that are highly active for the hydrogenation of both CO and CO2to the target alcohol/hydrocarbon product.