1 Introduction
Higher alcohols are important compounds with widespread applications in chemistry and the energy sector.Isobutanol,a highly important higher alcohol,is widely employed as a feedstock for the synthesis of specialty products such as pharmaceuticals and plastics in the chemical industry.In the energy sector,isobutanol is used as an important gasoline additive that increases its combustion efficiency and octane number.The demand for isobutanol as a gasoline additive will be increased in the future because of the phasing-out of methyl tert-butyl ether(MTBE)as a gasoline additive coupled with population growth and the increased demand for petrochemicals it entails.Currently,isobutanol is mainly produced by hydration of the corresponding petroleum-derived alkene(usually propylene)via acid catalysts,which has very low single-pass conversion(~5%),hampering the wider industrialization of this technology.To satisfy the increasing requirement for isobutanol,alternative isobutanol synthesis methods based on the conversion of syngas,which is more environmentally benign and economical,have aroused extensive research interest worldwide.Syngas,a gaseous mixture predominantly composed of CO,H
2,CH
4,CO
2,and H
2O,can be obtained from biomass or coal.The process of syngas-derived isobutanol synthesis has obvious advantages in terms of sustainability and is highly suitable to resource conditions in China,which has plenty of coal but little oil
[1]。
Accordingly,the first section of this review will highlight the developments in the catalysts for isobutanol synthesis in terms of catalyst category,preparation method,and isobutanol-generation mechanism.the second section will deal with the molecular aspects of Zn-Cr catalysts for isobutanol synthesis,drawing clear links between performance and catalyst microstructure.Our group's work,which focuses on the use of non-stoichiometric spinel-structure Zn-Cr catalysts for isobutanol synthesis,will be introduced in this section.the third section will discuss current challenges and trends in the field of isobutanol synthesis from syngas using Zn-Cr catalysts。
2 Isobutanol synthesis from syngas
2.1 Catalyst categories for the synthesis of higher alcohols
Various catalysts for higher-alcohol synthesis(HAS)via syngas have been evaluated.However,reports on the direct synthesis of isobutanol from syngas are quite limited
[2,3]。 Generally,HAS catalysts can be classified into four categories:modified Fischer-Tropsch synthesis(FTS)catalysts,Rh-based catalysts,Mo-based systems,and modified methanol synthesis(MS)catalysts
[3⇓~5]。 FTS catalysts(such as Cu-Co-based and Cu-Fe-based systems)are considered to be the most promising catalysts for HAS.However,the use of FTS catalysts leads to a wide distribution of linear C
1-C
6alcohols limited by the Anderson-Schulz-Flory(ASF)distribution rule,necessitating very complex separation processes to isolate the higher alcohols
[6⇓⇓⇓⇓⇓~12]。
Rh-based catalysts,which are employed on promoters(e.g.,Fe,Li,and Mn),are the only catalysts capable of producing ethanol and other C
2+oxygenates effectively from syngas
[13⇓⇓~16]。 However,their high cost and limited supply hamper their large-scale utilization。
Mo-based catalysts(e.g.,MoS
2,Mo
2C,MoO
x,and MoP)are superior to other HAS catalysts in terms of their resistance to sulfur poisoning
[17⇓⇓~20]。 However,owing to the emission of organic sulfonated compounds and H
2S,researchers have switched their focus from MoS
2-based materials to other Mo-containing catalysts in recent years
[18]。
Modified MS catalysts are most suitable for producing isobutanol because their product distribution breaks the constraint of the ASF rule,mainly comprising methanol and isobutanol.Modified MS catalysts can be divided into two groups:low-temperature Cu-based catalysts(such as Cu-ZnO-Al
2O
3,Cu/ZrO
2,and Cu/CeO
2)and high-temperature Zn-Cr catalysts.Cu-based catalysts are among the most active systems for producing isobutanol under mild conditions.However,the average carbon number for alcohols produced over low-temperature catalysts is typically low,with a higher selectivity for ethanol and propanol than that of high-temperature catalysts
[1,21⇓ ~23]。
Modified high-temperature Zn-Cr catalysts are used to produce isobutanol owing to Their excellent performances and longer catalyst life-times.their selectivities for isobutanol are generally high,with methanol and isobutanol selectivities of up to~95%.Here,the distribution weighting towards higher alcohols will simplify isobutanol separation and contribute significantly to isobutanol industrialization。
2.2 Isobutanol-generation mechanism
The mechanism of isobutanol generation has been thoroughly studied and an initial consensus has been reached.First,Graves ascribed the formation of higher alcohols to the condensation of alcohols with lower carbon numbers.Natta et al.proposed that alcohol formation entails a series of reactions involving CO insertion into alkoxide species to generate higher carboxylates,which are then reduced to alcohols.Nunan et al.
[26],[27]proposed that formyl species act as reactive nucleophiles that form C—C bonds upon attacking the electropositive carbon of adsorbed formaldehyde.To explain the high selectivity for isobutanol,Smith and Anderson
[28],[29]proposed that C
1and C
2species react with theαorβcarbon atom of an intermediate to form a product with one more carbon atom.β-addition is a rapid step,which explains the high selectivity for isobutanol.Thisβ-addition mechanism is widely accepted,but the two C
1intermediates involved inα-addition have not yet been determined
[24][25]。
The studies described above have greatly enhanced our understanding of the isobutanol-generation mechanism.Using chemical-enrichment and in situ diffuse reflectance infrared spectroscopy(DRIFTS),our group revealed that the isobutanol-generation mechanism comprises multi-step reactions involvingα-andβ-addition.As one of the C
1intermediates,molecularly adsorbed CO on the catalyst surface is first hydrogenated to form methanol,which is not only the initial alcohol product but also the intermediate in the formation of ethanol and higher alcohols.Ethanol is formed slowly byα-addition while propanol and isobutanol are generated rapidly viaβ-addition(i.e.,aldol condensation).Isobutanol is the terminal product because it lacks aβhydrogen atom appropriate for furtherβ-addition reaction owing to severe steric hindrance.Chemical trapping experiments have also indicated that one C
1intermediate is molecularly adsorbed CO and another C
1intermediate is formyl species
[30⇓~32][32]。 The mechanism of isobutanol formation is shown in Scheme 1。
2.3 Active sites for isobutanol synthesis on Zn-Cr catalysts
the active sites on Zn-Cr catalysts for isobutanol synthesis have been widely studied since The 1980s,and some remarkable works have been published
[3⇓~5,26,33⇓⇓ ~36]。 Nevertheless,the exact identity of the active sites remains a matter of debate.Epling and coworkers proposed that excess ZnO is the real active site for methanol and isobutanol formation,with the spinel-structure Zn-Cr acting as a support with a large specific surface area.Bertoldi et al.and Tsubaki et al.proposed that the non-stoichiometric nature of spinel-structure Zn-Cr plays a key role in methanol and higher-alcohol synthesis.All of these viewpoints on the active sites of Zn-Cr catalysts lacked direct evidence at the time of publication because of instrumentation limits.in fact,our group has dedicated much effort to investigating the real active sites of Zn-Cr catalysts for isobutanol synthesis,and this will be summarized in the following section as one of the highlights of this article
[37,38][39⇓~41][42]。
3 Non-stoichiometric spinel-structure Zn-Cr catalysts for isobutanol synthesis
3.1 Microstructure of spinel-structure Zn-Cr
Before surveying recent literature on this subject,we would like to introduce the spinel structure,specifically spinel-structure Zn-Cr.As shown in Fig.1,the chemical formula for a spinel structure is AB2O4with two types of cations located in tetrahedral and octahedral sites.The cation distribution in a spinel structure is represented as(A(1-x)Bx)IV(AxB(2-x))VIO4.Here,x=0 indicates a normal spinel structure with‘A'cations occupying the tetrahedral vacancies and‘B'cations occupying the octahedral vacancies,while x=1 indicates an inverse spinel structure with‘A'cations occupying the octahedral vacancies and half the‘B'cations occupying the tetrahedral vacancies.However,most materials with spinel structures exhibit a departure from the ideal cation distributions of normal or inverse spinel structures,where the distribution of cations between the two types of sites is affected by numerous factors such as ionic size and cationic charge.A spinel structure that deviates from ideal normal or inverse spinel distributions is defined here as non-stoichiometric.The degree of cation disorder can be expressed by the following formula:
where d%is the percentage degree of disordered cation distribution;Z is the amount of Zn cations in octahedral sites;and Zna siteis the amount of Zn cations in tetrahedral sites 。
The physicochemical properties of spinel-structure materials can be tailored by adjusting The cation distribution between tetrahedral and octahedral vacancies.Hence,methods to determine cation distributions in spinel-structure Zn-Cr are essential.The Rietveld method is widely used to determine crystal structure details such as lattice parameters and cation distribution
[43⇓⇓⇓⇓⇓⇓~50]。 Using powder X-ray diffraction(PXRD)refinement by the Rietveld method,Attfield et al.discovered that the Li cations in spinel-structure Li
0.33Ti
2O
4are only located at tetrahedral sites while Ti cations are located at two octahedral positions.The migration of Ti cations was explained as being due to equalizing the filling of octahedral vacancies between close-packed oxide layers to minimize cation-cation repulsions.The micro-structures of K
2Ta
2O
6and Fe
3P
5SiO
19were also investigated by the Rietveld method,and their photocatalytic and electrocatalytic performances were explained well using the results of Rietveld simulations
[51][45][49]。
Research into cation distribution in spinel-structure Zn-Cr was performed by Mancic et al.,who reported that the lattice parameters of a spinel structure are closely associated with its non-stoichiometry and surface defects.This study also indicated that some Zn ions enter the Cr sub-lattice and others become interstitial in the spinel structure,while oxygen vacancies are formed in place of interstitial Zn
[52]。
the Rietveld method has some inherent limitations in the simulation of cation distribution for non-stoichiometric spinel-structure Zn-Cr.Specifically,to achieve reasonable accuracy,the peak strengths of the XRD patterns need to reach a very high level,which requires much longer testing times and more stringent experimental parameters.Furthermore,the XRD patterns of non-stoichiometric spinel-structure Zn-Cr could never reach this level owing to the low crystallinity associated with disordered cation distribution。
In addition to the Rietveld method,synchrotron radiation is a powerful technique to detect the chemical environment of central atoms.Both X-ray absorption near edge spectroscopy(XANES)and extended X-ray absorption fine structure(EXAFS)provide valuable information about coordination numbers and distances
[53,54]。 Accordingly,this technology has been used recently to determine cation distribution in spinel structures,including Li-Ti,Cu-Fe,and Zn-Fe
[51][55][56⇓~58]。 However,there are a limited number of reported studies that deal with the analysis of cation distribution in spinel-structure Zn-Cr by XANES and EXAFS。
3.2 Influence of cation distribution in spinel-structure Zn-Cr
in general,most spinel-structure samples show a departure from the ideal distributions,and the cation distribution In the two polyhedral vacancies is affected by numerous factors including ionic size,sample particle size,and cationic charge
[51,54,59,60]。 Marinkovic et al.prepared spinel-structure Zn-Cr using a mechanically activated ZnO-Cr
2O
3system and studied its microstructure using the Rietveld method.They also found that excess ZnO can enter the spinel lattice and further generate non-stoichiometric spinel structures.Compared with tetrahedral vacancies,the extra Zn cations prefer to locate in octahedral vacancies because the Zn cations in octahedral vacancies are more stable than those in tetrahedral vacancies.They attribute this phenomenon to the large difference in the crystal lattice energies of ZnO and Cr
2O
3,which are~4107 and~15 186 kJ/mol,respectively.Our group has also dedicated considerable effort to investigating the factors influencing cation distribution in spinel-structure Zn-Cr,including annealing temperature,the Zn/Cr element ratio,preparation method,and alkali metal,using both X-ray absorption spectroscopy(XAS)refinement and the Rietveld method.More importantly,we have focused on the relationship between isobutanol production and cation distribution in spinel-structure Zn-Cr,which has not been extensively studied by other researchers.Our research design,strategy,and approach will also be discussed below
[61][62][63,64][65][66]。
3.2.1 Influence of calcination temperature and Zn/Cr ratio on cation distribution in spinel-structure Zn-Cr
Wu et al.investigated cation distribution in spinel-structure Zn-Cr As a function of annealing temperature using XANES and EXAFS for the first time.as shown in Fig.2,They found that cation distribution disorder decreases with increasing annealing temperature.the sample annealed at 750℃(ZC750)has a normal spinel structure while the degree of disorder distribution of the sample annealed at 300℃(ZC300)reaches 0.24.they ascribed the variation in cation distribution to the different particle sizes of samples annealed at temperatures.Their study demonstrated the close relationship between cation distribution disorder in spinel-structure Zn-Cr and annealing temperature,and it provided a method to determine the cation distribution in spinel-structure Zn-Cr,which has been invaluable to this field of study.However,the studies above focused on tailoring the microstructure of spinel-structure Zn-Cr and did not involve any consideration of catalyst performance
[59]。
The annealing temperature and element ratio of non-stoichiometric spinel-structure Zn-Cr for isobutanol synthesis was first studied by our group
[62,63]。 The Raman and XAS spectra of spinel-structure Zn-Cr prepared at different annealing temperatures and the Zn/Cr element ratios are shown in Figure 3.For ZC-0.5-700,the“ZC”indicates Zn and Cr elements,“0.5”represents the Zn/Cr ratio,and“700”represents the annealing temperature.The rest of the catalysts are labelled using the same convention.The peaks at 558 and 688 cm
-1in the Raman spectra are related to cation distribution,and the results show that the cation distribution distortion is more prominent in catalysts annealed at lower temperatures and with higher Zn/Cr ratios.The intense Raman band at 688 cm
-1is attributed to the symmetric Cr-O stretching vibration of CrO
6groups 。
The broadness of the band at 688 cm-1is related to cation-anion bond lengths and polyhedral distortion in the spinel-structure Zn-Cr.Such distortion is more prominent in samples calcined at lower temperatures and with higher Zn/Cr ratios.The band at 558 cm-1is unexpected,and this unexpected band indicates a higher level of disordered cation distribution 。
the XAS spectra of spinel-structure Zn-Cr including the XANES and EXAFS signals are also shown in Fig.3.the changes in the features labelled A and B in Fig.3b,i.e.,feature A increasing in amplitude while feature B decreases,result from some Zn cations transferring from tetrahedral sites to octahedral sites,leading to more serious cation distribution disorder.the intensity of the pre-edge peak in Cr K-edge XANES in Fig.3c results from the enhancement of the p-d orbital mixing,which is allowed in tetrahedral symmetry but forbidden in octahedral symmetry.Thus,the increase of the pre-edge peak intensity in Fig.3c reflects the number of Cr ions occupying tetrahedral sites(i.e.,cation distribution disorder),which decreases in the order ZC-0.8-400>ZC-0.65-400>ZC-0.5-400>ZC-0.5-550>ZC-0.5-700.as shown in Figure 3d,besides the two normal-intensity peaks centred at~1.6 and~3.0Å,an additional weaker peak appears at 2.6Åin the R-space signals for the Zn K-edge,which clearly results from cation distribution disorder.the new peak intensity increases with decreasing annealing temperature and increasing Zn/Cr ratio,which is the same trend As that for cation disorder distribution。
Our group also simulated the cation distribution results obtained by the Rietveld method for XRD patterns and multiple-edge refinement for Zn K-edge EXAFS spectra.Typical simulation results are shown in Fig.4 and Table 1.the lattice parameter for spinel-structure Zn-Cr increases with decreasing annealing temperature and increasing Zn/Cr element ratio,resulting from more serious cation distribution disorder.the degree of cation distribution disorder increases from ZC-0.5-700 to ZC-0.8-400 from 0%to 27%,respectively,by multiple-edge refinement,and 0.36%to 21.5%,respectively,by the Rietveld method.Hence,a clear conclusion can be drawn from the qualitative and quantitative analysis,i.e.,that cation distribution disorder becomes more serious with decreasing annealing temperature and increasing Zn/Cr element ratio。
Specifically,higher annealing temperatures lead to significant crystallite growth,and the particle size dramatically affects cation distribution.In theory,most atoms in large particles are located in the internal bulk structure and thus situated in the most thermodynamically stable sites,generating a normal spinel structure.However,for small particles,a larger proportion of atoms are situated on the surface of the catalyst and the surface chemical environment is influenced by dangling bonds.The results of multiple characterizations and simulations revealed that the atoms on the surface prefer to be in disordered distributions,which are in the lowest-energy state and thus more stable.Specifically,surface atom coordination is distorted by dangling bonds including adsorbed oxygen or hydroxide ions,leading to more extensive cation distribution disorder.In the Zn2+-rich samples,the excess Zn atoms will enter the spinel structure to generate non-stoichiometric spinel structures.Furthermore,the extra Zn cations prefer to locate in octahedral vacancies,aggravating the level of distribution disorder because cations located in octahedral vacancies are more stable than those in tetrahedral vacancies owing to the large difference in the crystal lattice energies of ZnO and Cr2O3 。
3.2.2 Influence of alkali metal on cation distribution in spinel-structure Zn-Cr
in addition to annealing temperature and Zn/Cr ratio,the alkali metal loaded on the catalyst surface also affects cation distribution in non-stoichiometric spinel-structure Zn-Cr.Tsubaki et al.employed Li,Na,K,and Cs to surface-modify Zn-Cr catalysts and evaluated their performances in isobutanol synthesis.They reported that the K-modified catalyst exhibited better performance and demonstrated that non-stoichiometric spinel-structure Zn-Cr plays a crucial role in isobutanol synthesis.Based on their research,our group further investigated the influence of different K promoter contents on Zn-Cr catalysts for isobutanol synthesis and found that the K promoter affects cation distribution between tetrahedral and octahedral sites and thus isobutanol synthesis
[42][66]。 in this study,We estimated the cation distribution of the Zn-Cr spinel with different contents of K promoter by the Rietveld method,and the results showed that the numbers of Zn cations in octahedral sites and Cr cations in tetrahedral sites increased linearly from 13.79%to 22.01%as the K promoter content increased from 0%to 6%.we also discovered that the K promoter increases the number of surface dangling bonds(such as—OH)and stabilizes the disordered cation structure.the crucial role of the K promoter in isobutanol synthesis from syngas is shown in Fig.5。
3.2.3 Influence of excess ZnO on cation distribution in spinel-structure Zn-Cr
despite the ideal Zn/Cr ratio in spinel-structure Zn-Cr being 0.5,Giamello and co-workers prepared a Zn-Cr material with a Zn/Cr molar ratio of 1 and only identified a single non-stoichiometric spinel phase.They speculated that excess ZnO may migrate into the spinel structure to further aggravate the degree of cation distribution disorder as well as forming a non-stoichiometric spinel structure.Marinkovićet al.,Vaccari et al.,and Calvin et al.reached the same conclusion that,Despite the strong affinity of Zn cations for tetrahedral vacancies,spinel-structure Zn-Cr prepared with excess Zn species by various methods show significant amounts of Zn cations loaded into octahedral vacancies to form non-stoichiometric structures.However,there is something undeniably conjectural about such conclusions because these studies were performed decades ago without the powerful technology required to fully investigate the microstructure of non-stoichiometric spinel-structure Zn-Cr
[41][61][67][68]。
to simplify our research,our primary studies focused on the cation distribution in non-stoichiometric spinel-structure Zn-Cr while deliberately eliminating the effect of excess ZnO by using Zn/Cr ratios lower than 0.8,because This is the boundary point at which a catalyst may contain extra ZnO by coprecipitation.However,the optimal Zn/Cr ratio for a Zn-Cr catalyst for isobutanol formation from syngas is 1.2.Once the ratio of Zn/Cr reaches 1.2,besides the non-stoichiometric Zn-Cr spinel structure,a wurtzite ZnO phase will appear.this prompted us To ask“what is the role of excess ZnO for isobutanol synthesis from syngas?”
to answer this question,We prepared a series of Zn-Cr catalysts with a Zn/Cr ratio of 1.2 by different methods including The combustion method(marked as Comb-ZC),impregnation method(Impr-ZC),coprecipitation method(Copr-ZC),and fractional precipitation method(Frac-ZC),and investigated the effect of ZnO in different states for isobutanol synthesis.the percentage of excess ZnO in the catalysts was estimated by the linear combination fit(LCF)method using Zn K-edge spectra.the LCF results are presented in Fig.6.the percentage of ZnO decreases from 20.4%to 13.2%in the order Comb-ZC>Impr-ZC>Copr-ZC>Frac-ZC,with a corresponding increase in the non-stoichiometric spinel-structure Zn-Cr content from 79.6%to 86.8%.the decrease in ZnO is most likely due to it entering the Zn-Cr spinel structure,which increases cation distribution disorder by generating more non-stoichiometric Zn-Cr.we also found that excess ZnO with smaller particles dramatically enhances the synergic effects between ZnO and non-stoichiometric spinel-structure Zn-Cr,which contributes significantly to isobutanol synthesis performance.The effects of different ZnO states in isobutanol synthesis from syngas is illustrated in Fig.7。
3.3 Relationship between cation distribution disorder and isobutanol production
in summary,our group has dedicated much effort to aggravating the degree of cation distribution disorder by numerous methods,including controlling the annealing temperature and adjusting the Zn/Cr ratio.Furthermore,a K promoter was loaded on the surface of the catalyst to stabilize the disordered cation structure.our group also simulated the results of cation distribution analysis by the Rietveld method for XRD patterns and multiple-edge refinement for Zn K-edge EXAFS spectra.Then,we prepared a curve showing the relationship between isobutanol production and the level of cation distribution disorder.As shown in Fig.8a,isobutanol production exhibits a linear relationship with the degree of cation distribution disorder in spinel-structure Zn-Cr,confirming the strong association between them.This linear relationship was reported for the first time by our group。
the performances and stabilities of the best Zn-Cr catalysts for isobutanol synthesis were determined,and the results are shown in Fig.8b and 8c
[69]。 The Zn-Cr catalysts present excellent stabilities under the harsh reaction conditions of 10 MPa,400℃,and a space velocity of 10 000 h
-1.The deactivation mechanism of Zn-Cr-based catalysts for isobutanol formation from syngas was further studied,and some typical causes of deactivation,including sintering,coke deposition,the formation of inactive species,and the loss of active metals,were found to be slight and did not explain the deactivation of the catalyst.As shown in Fig.8d,the structural change,especially the weakness of the cation distribution disorder after extended reaction periods,is the primary reason for the deactivation of the Zn-Cr catalyst,which is consistent with our previous research and further demonstrates the crucial role of cation distribution in non-stoichiometric spinel-structure Zn-Cr for isobutanol synthesis from syngas.Therefore,non-stoichiometric spinel-structure Zn-Cr is not thermodynamically stable.Thus,the Zn-Cr catalyst structure slowly changes from a metastable structure to a thermodynamically stable structure over the duration of the 1000-h stability test,decreasing the level of cation distribution disorder 。
3.4 Tailoring physicochemical properties via cation distribution
the physicochemical properties of spinel-structure samples can be tailored by controlling the cation distribution between tetrahedral and octahedral vacancies.Hence,we investigated the changes of multiple physicochemical properties as affected by cation distribution and attempted to explain the linear relationship between isobutanol production and the level of cation distribution disorder。
Irrespective of the method used to aggravate the degree of cation distribution disorder or stabilize the metastable non-stoichiometric spinel-structure,which included preparation method,loading of the K promoter,adjusting Zn/Cr ratio,and changing the annealing temperature,the changes in physicochemical properties are similar.As shown in Fig.9a and 9b,we found that the binding energy values of Zn and Cr ions increase with increasing degree of cation distribution disorder,resulting from the formation of a non-stoichiometric spinel structure with more defects and oxide vacancies on the catalyst surface.Furthermore,it has been found that structures with a higher degree of cation distribution disorder have smaller particle sizes(~6 nm in diameter in our research),meaning that a larger fraction of the metal cations are located on the surface.A high number of dangling bonds on the surface of a catalyst affects its bonding environment and cation distribution.the results of characterization and simulation indicated that the lowest-energy configurations of the samples are satisfied by the distribution of disordered cations。
the K promoter was loaded on all the catalyst surfaces,as shown in Fig.9c.The K on the catalyst surface increases the number of dangling bonds,including
and O
ads,from~29%to 52%,which can adsorb molecules to the vacancies on the catalyst surface and play crucial roles in stabilizing the disordered metastable state.The redox properties of the catalysts were evaluated by H
2-TPR(Fig.9d).The H
2consumption increases with increasing cation distribution disorder.Although the Zn
2+and Cr
3+cations in spinel-structure Zn-Cr are very difficult to reduce,all the catalysts are in a metastable state,and this state allows a catalyst to be reduced much more easily.After H
2reduction,the active species,i.e.,free-state Zn
2+inside the non-stoichiometric spinel structure in octahedral sites,will appear and catalyse isobutanol synthesis.Appropriate adsorption strength of the reactant CO is crucial for isobutanol production;weak adsorption of CO(αpeak in Fig.10a)will not induce the initial reaction,and strong adsorption of CO(γpeak)will cause the dissociation of CO and form by-products(such as CH
xspecies).Thus,cation distribution disorder can be used to tailor CO adsorption strength and moderate CO adsorption(βpeak),which increases with increasing degree of cation distribution disorder.We discovered that moderate CO adsorption allows reaction with surface hydroxyl group to form formate species,determined from peaks at 1375 cm
-1(σ(CH))and 1600 cm
-1(υ
as(CO
2-)),which are the most stable intermediates in the synthesis of methanol
[70,71]。 The alkalinity of the catalysts,estimated by CO
2-TPD(Fig.10b)demonstrates that the amount of CO
2adsorption increases with increasing degree of cation distribution disorder.As described above,isobutanol formation consists ofα-addition andβ-addition.β-Addition is an aldol condensation and it is strongly influenced by the alkalinity of the catalyst.The enhanced alkalinity induced by cation distribution is another significant factor in isobutanol formation 。
4 Conclusions and outlook
Isobutanol is a fundamental chemical intermediate and clean fuel additive.isobutanol synthesis from coal or biomass syngas,which is a viable way to resolve the issue of energy safety,is highly suitable to the resource conditions of China,which has plenty of coal but little oil.Zn-Cr-based catalysts have been widely used to produce Isobutanol owing to their longer usable life-times,less severe coke deposition,and simple alcohol product distribution.This mini review presents the recent progress in Isobutanol synthesis using Zn-Cr catalysts,focusing on the crucial role of cation distribution in non-stoichiometric spinel-structure Zn-Cr.Several strategies for aggravating cation distribution disorder were introduced as highlights,including adjusting the Zn/Cr element ratio,annealing temperature,preparation method,K-promoter loading,and the use of excess ZnO.Two quantitative methods including Rietveld analysis of PXRD patterns and multiple-edge refinement for Zn K-edge EXAFS spectra can determine the cation distribution in spinel-structure Zn-Cr.Isobutanol production shows a linear relationship with the degree of cation distribution disorder because it dramatically affects the physicochemical properties of spinel-structure Zn-Cr,such as particle size,alkalinity,adsorption of CO,and oxygen state on the catalyst surface。
Despite significant progress in the design and fabrication of Zn-Cr materials for isobutanol synthesis from syngas in the past decade,several challenges remain for Zn-Cr based catalysts,such as their low activities and selectivities,as well as uncertainty surrounding the formation mechanism of isobutanol(especially that regarding the C2 alcohol intermediate).To address this challenge,future research efforts are required in the following areas
[72⇓⇓~75]。
(1)Significant challenges remain in increasing the activity and selectivity of Zn-Cr catalysts because serious side reactions still occur and lead to high selectivity of CHxspecies and CO2during isobutanol formation at low space velocity.How to suppress these side reactions,especially the water-gas shift reaction,and enhance the activity of catalysts still require further exploration 。
(2)Although our group identified a linear relationship between isobutanol production and the degree of cation distribution disorder for the first time,significant challenges remain in identifying the factors directly influencing isobutanol formation.Many researchers have proposed that oxygen vacancies and defects on non-stoichiometric spinel structures play crucial roles in activating the reactants.However,it is difficult to gain accurate information on oxygen vacancies and defects.Computational chemistry is a powerful tool for building structures that can control reactions at the atomic level,affording a feasible solution to this issue。
(3)the small specific surface area of Zn-Cr catalysts can inhibit their activity and make it difficult to investigate the mechanism and active sites For isobutanol formation because heterogeneous catalysis only involves surface atoms.the use of a suitable carrier with a large specific surface area and excellent chemical properties may solve this issue.for example,multi-walled carbon nanotubes(CNTs)have huge application potential,especially in catalysis,because of their unique geometrical structure and rich electronic properties.A researcher may load a Zn-Cr catalyst on the inner surfaces of CNTs to utilize their large surface areas and excellent physicochemical properties,thereby increasing catalyst activity.Furthermore,designing a novel catalyst system and exploring changes to catalyst structure and the reaction pathway of C1 active intermediates during the reaction process also need to be studied more in the future。
National Natural Science Foundation of China(21706218)
National Natural Science Foundation of China(21908182)
Natural Science Foundation of Shaanxi Province(2019JQ-920)
Youth Innovation Team of Shaanxi Universities
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