Recent Advances in Catalysts for Methanol Synthesis via

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Ind. Eng. Chem. Res. 2003, 42, 6518-6530

REVIEWS Recent Advances in Catalysts for Methanol Synthesis via Hydrogenation of CO and CO2 Xin-Mei Liu,†,‡ G. Q. Lu,† Zi-Feng Yan,†,‡ and Jorge Beltramini*,† Department of Chemical Engineering, University of Queensland, Brisbane 4072, Australia, and State Key Laboratory for Heavy Oil Processing, Key Laboratory of Catalysis, CNPC, University of Petroleum, Dongying 257062, China

Since the start of last century, methanol synthesis has attracted great interests because of its importance in chemical industries and its potential as an environmentally friendly energy carrier. The catalyst for the methanol synthesis has been a key area of research in order to optimize the reaction process. In the literature, the nature of the active site and the effects of the promoter and support have been extensively investigated. In this updated review, the recent progresses in the catalyst innovation, optimization of the reaction conditions, reaction mechanism, and catalyst performance in methanol synthesis are comprehensively discussed. Key issues of catalyst improvement are highlighted, and areas of priority in R&D are identified in the conclusions. 1. Introduction With the rapid changes and development in modern industry, the energy and environmental issues have become two major concerns. Methanol has been a common chemical feedstock for several important chemicals such as acetic acid, methyl tert-butyl ether (MTBE), formaldehyde, and chloromethane for over 30 years.1 Methanol, being a clean liquid fuel, could provide convenient storage of energy for fuel cell applications, particularly in transportation and mobile devices. When used as a fuel, it is a cleaner energy compared with most other sources. For these reasons, methanol synthesis is still attracting interest despite the fact that the current technology process was developed a long time ago. Commercially, methanol has been produced from syngas using natural gas or coal, mainly containing CO and H2 along with a small amount of CO2.2 Methanol synthesis through hydrogenation of carbon dioxide has attracted continuous worldwide research interest in the past 15 years because of its environmental impact. The potential use of CO2, the most important greenhouse gas, as an alternative feedstock replacing CO in the methanol production has received attention as an effective way of CO2 utilization. A recent study showed that a mixture with a proper proportion of CO2 and CO not only can increase the yield of methanol but also decreases the apparent activation energy of the reaction. Of special interest is that the presence of CO2 could maintain the active copper sites in the oxidation state or prevent an over-reduction of the ZnO component3-5 when Cu/Zn catalysts are used during methanol synthesis. Therefore, the use of mixtures of CO2 and CO * To whom correspondence should be addressed. Tel.: 61 7 33654316.Fax: 61733654199.E-mail: [email protected]. † University of Queensland. E-mail: [email protected]. ‡ University of Petroleum. E-mail: [email protected] (Z.F.Y.) and [email protected] (X.-M.L.).

as reactant feedstock should be of importance in methanol synthesis. At present the key to methanol synthesis is to develop the most efficient catalyst. Many research groups are engaged in catalyst preparation using different catalyst compositions and preparation methods. New catalysts based on nickel, copper, and alloys have been reported. Recently, ultrafine particle catalysts have also been proposed. Deng et al.6 reported that the ultrafine Cu/ZnO/Al2O3 ternary catalyst prepared with an oxalate gel coprecipitation exhibited high activity. Tanaka et al.7 obtained a Cu/ZnO binary catalyst using ethylene glycol as the solvent. They found that the methanol selectivity was higher when the ratio of copper to zinc was 1 and the active sites were ascribed to the copper atoms in contact with the oxide crystallites. Nitta et al.8 reported that the proper addition of certain amounts of ZnO to the Cu/ZrO2 catalyst could greatly enhance its activity. Liaw and Chen9 obtained ultrafine Cu-based catalysts using a reduction method in which the dispersion and stability of Cu could be enhanced by doping Cr, Zr, and Th. Other groups10,11 confirmed that doping the trivalent metal ions or rare-earth elements such as La and Y onto the catalysts could improve its performance. No matter what method is used for the catalyst to possess higher activity, selectivity, and long life, it is important to obtain a catalyst with larger surface area, fine particle size, and a high dispersion of active sites. Although extensive studies have been carried out for over 2 decades, controversies still remain concerning the role of active sites involved in the catalysts, the effect of the addition of various promoters, and the reaction mechanism. 2. Catalyst Innovation for Methanol Synthesis 2.1. Active Site. Although the properties of catalysts used in the synthesis of methanol has been extensively studied for many years, Cu still remains as an impor-

10.1021/ie020979s CCC: $25.00 © 2003 American Chemical Society Published on Web 11/11/2003

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tant active catalyst component, even though the nature of the active site is yet to be fully understood. It is generally accepted that the coordination, chemisorption, and activation of carbon monoxide and the homogeneous splitting of hydrogen take place on Cu0 or Cu+ and that the heterogeneous splitting of hydrogen, which provides Hδ+ and Hδ- in the catalytic process, takes place on ZnO.12 However, different points of view on the nature and valence of copper sites still exist. Some researchers13-15 pointed out that metallic Cu atoms were uniformly active for methanol synthesis. Pan et al.16 found that the activity of the catalyst is directly proportional to the surface area of metallic Cu and methanol is formed on a metallic Cu surface of a Cu-based catalyst. Similarly, Deng et al.6 reported that the catalytic activity of a Cu/ZnO/Al2O3 ternary catalyst for carbon dioxide hydrogenation increases with an increase in the metallic copper surface area, reached a maximum, and then decreased at a Cu/ZnO molar ratio of 8. Rasmussen et al.17,18 investigated methanol synthesis by hydrogenation of CO2 on Cu(100) at total pressures of 1-4 bar and reported that metallic copper is actual an active site. They formulated a kinetic model for the elementary steps of methanol synthesis. It indicated that the calculated reaction rate and the activation energy for methanol synthesis were in accord with those measured on Cu(100). They excluded the possibility of Cu+ active sites because only metallic copper was observed on the Cu(100) surface used. Similarly, Askgaard et al.19 supported the metallic Cu model by applying a kinetic model based on experimental data to the methanol synthesis reaction using a Cu single crystal. However, in the presence of CO2 and with a large fraction of the Cu0 surface covered by oxygen-containing species, the catalytic activity toward methanol synthesis is found to be independent of the Cu0 surface area.20,21 This finding can be explained by the fact that the Cu+ sites might be acting as active sites in methanol synthesis. Subsequently, Herman et al.22 reported that active Cu+ ion sites are dissolved on the surface of the ZnO matrix. They based their findings on the fact that there was not any new phase other than the normal crystal structure of Cu metal and zinc oxide that coexisted in the mixed Cu/ZnO catalysts and that the activity of the rodlike networked catalyst containing more Cu+ was higher than that of the platelets. It can be deduced that the promotion effect could only stem from the catalytic activity of a solid solution such as Cu+/ZnO and active centers of the Cu-based catalyst should be Cu+ ions dissolved in ZnO. On the basis of apparent activation energy measurements, X-ray photoelectron spectroscopy, and scannin electron microscopy results, Sheffer and King23 stipulated that activity differences among unsupported copper catalysts promoted by group IA elements could be attributed to differences in the concentration of Cu+ species. It was observed24 that the addition of alkali metals such as potassium or cesium could positively promote methanol synthesis on nonsupported Cu catalysts and with cationic Cu as the active site. Szanyi and Goodman,25 while comparing the activity for methanol synthesis on clean Cu(100) and oxidized Cu(100), concluded that the Cu ion is the active site. In another study, Van Santen et al.26 stated that anything else that stabilizes the presence of Cu+ indiscriminately enhances the activity of the catalyst.

More recently, a synergetic effect between Cu and the promoter was reported.27-33 Chen et al.27 found that the coexistence of ZnO with Cu enhanced the ability of Cu to adsorb CO and ZnO enhanced synergistically the subsequent hydrogenation to form ethanol, while Cu improved the ability of Zn2+ to adsorb H. Obviously, CO is bound to Cu sites and H to ZnO sites. It was also observed that the methoxy intermediates were first found on Zn sites. Burch et al.28-32 thought that the synergistic effect might involve the migration of spillover adsorbed species such as H+ on Cu to ZnO sites to hydrogenate the formate species to methanol. Whereas Kanai et al.33 reported that the interaction between Cu and Zn sites is achieved by the way that ZnO impacts the creation of active sites such as Cu+ by the ZnOx fractioned on the Cu surface, Fujitani and Nakamura34 proposed that the creation of a Cu-Zn surface alloy on Zn/Cu(111) could be responsible for the formation of active sites. Cu-Zn sites are basically metallic, in which the Cu atoms catalyze the hydrogenation in methanol synthesis; therefore, aurichalcite should be an excellent precursor in the preparation of Cu/ZnO catalyst. Of special interest is that copper alone could not be an independent active site in effective methanol synthesis. Electronic interactions and geometrical effects are exerted on copper sites with coexistent components in the catalysts. In fact, since 1979, Herman and coworkers22 had already indicated that the catalytic activity of the binary catalyst was at least 3 orders of magnitude greater than that of the pure copper metal or zinc oxide, the resulting differences bearing the consequence of the synergistic interaction between Cu and ZnO. Burch and co-workers clearly summarized the hypotheses about the synergistic interactions between copper and zinc oxide33-40 into six categories as follows: (a) the formation of Cu+ ions in ZnO; (b) electronic interactions between Cu and ZnO; (c) Schottky Junction effects at the Cu/ZnO interface; (d) the formation of a Cu/Zn pair; (e) a specific reaction at the Cu/ZnO interface; (f) stabilization of Cu in a morphologically active form by ZnO. Surprisingly, they concluded that direct contact between the active sites and support is not necessary for higher activity. Synergy might be due to the presence of carbon dioxide. In addition, there are other opinions about the active sites for methanol synthesis. Chen et al.27 proposed that the activity of a copper-based catalyst is directly proportional to the total copper exposed (Cu0 and Cu+) and that the activity did not depend on special copper sites. On the other hand, Jackson and Frost36,41,42 established that the active sites of CO hydrogenation over ZrO2 or Y2O3-doped ZrO2 could result from oxygen anion vacancies, in which higher mobility of oxygen vacancies represents higher reaction rate. On the condition of absence of copper, a catalyst promoted by silver and gold oxides also possessed a high activity for methanol formation. This means that the above noble metals could adjust the electronic properties of their oxides and thus modify the nature of the active centers, which results in surface oxygen vacancies. Obviously, from the above information presented, there are still controversies regarding the role of the active site of catalysts for methanol synthesis. However, synergy between the main catalyst and promoter or support does now seem to be a rather well-established phenomenon. This synergy is due to the transfer of

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species among the catalyst components, which need not even be in direct physical or chemical contact.28 The interaction among various catalyst compositions results in differences in the morphology and electronic effect of each catalyst investigated. Accordingly, it may cause them to have small particle size and larger surface area. Small particle size can reduce the diffusion resistance of reactant molecules entering the catalyst pore, while the active site at the inner pore can be adequately used. This means that small particle size can promote higher activity. The larger surface area can improve the dispersion of the active component, which resists the sintering among the metal particles. Simultaneously, a change of the electronic effect can modulate the adsorption strength between the reactant molecules and the catalyst surface, which strongly influences catalyst selectivity. Namely, the electronic effect indirectly affects the catalyst selectivity. In addition, the synergy among the components can achieve the transformation of the hydrogen atom to carry out a hydrogenation reaction. This is discussed in detail next in the Support Effect section. 2.2. Support Effect. An appropriate support not only provides a good configuration but also must have some function in modulating the interactions between the primary components of the catalyst and promoter. In addition, the basicity/acidity characteristics of the catalyst can also be affected. Metal oxides are the more common supports used for the preparation of a methanol synthesis catalyst. Their properties greatly affect the catalyst activity as a result of the aspects mentioned in the previous section. Of all metal oxides ZnO, ZrO2, and SiO2 are the most typical materials used as supports in methanol synthesis catalysts. 2.2.1. ZnO. ZnO is an n-type semiconductor, with a wurtzite structure in which Zn+ ions are located in tetrahedral sites in a close-packed array of O2- ions. Its surface reactivity is defined by particle morphology.43 It possesses lattice oxygen vacancies, consisting of an electron pair (2e-) in a ZnO lattice structure, which is considered to be one kind of active site for methanol synthesis. Zn2+-2e- pairs move interstitially, leaving cation and anion lattice vacancies, which is beneficial to the adsorption and transformation of the reactant. Simultaneously, zinc oxide can improve copper dispersion of a Cu-based catalyst. Traditionally, zinc oxide is a good hydrogenation catalyst that activates hydrogen by heterogeneous splitting, giving rise to ZnH and OH. Furthermore, hydrogen spillover has been observed with ZnO acting as a reservoir of hydrogen for the hydrogenation of CO over Cu surfaces. Dennison et al.44 observed that hydrogen is adsorbed on ZnO to a much greater extent when Cu is present in the catalyst. On the other hand, Burch and co-workers28 found that hydrogen spillover from Cu to ZnO occurs very rapidly from a partially oxidized Cu surface but only to a very small extent from a fully reduced copper surface. Moreover, the hydrogen atoms were trapped at surface defects or at interstitial sites of ZnO, but they were not held too strongly. This means that a possible role of ZnO might serve as a reservoir to provide H atoms for subsequent hydrogenation of adsorbed reaction intermediates. Similarly, Spencer45 thought that synergistic effects are the result of H spillover produced from ZnO to Cu metal. A morphology effect was also postulated to state the role of the zinc oxide support in a Cu-based catalyst.

Figure 1. Model of the active site for methanol synthesis over a physical mixture of Cu/SiO2 + ZnO/SiO2 (from ref 49).

The ZnO support may optimize the dispersion of the Cu particles and stabilize many active sites by attenuating the unavoidable agglomeration of Cu particles, which takes place during a long-term operation, and by restraining Cu particles to be poisoned by feed gas impurities. Yoshihara and Campbell46 using a batch microreactor attached to an ultrahigh-vacuum chamber for pre- and postreaction surface analysis determined the relations between the surface area of Cu(110) single crystal and catalytic activity. They were able to prove that the active sites in methanol synthesis on Cu/ZnO catalysts are metallic Cu and the role of the ZnO support is to stabilize more metallic Cu sites. On the other hand, Ovesen et al.47 correlated the changes in particle morphology and the number of oxygen vacancies at the Zn-O-Cu interface. They showed that the interaction between copper and zinc oxides is important for the dynamic spreading of the copper particles on the support. To further challenge the above explanation, Fujitani and co-workers48,49 suggested that the ZnO support is also an active component in methanol synthesis. They observed that Zn species could migrate to the Cu surface in a physical mixture of Cu/SiO2 and ZnO/SiO2 upon reduction with H2. The quality of the active sites was not changed, but the quantity of the active sites increased with the increasing reduction temperature. This meant that the role of ZnO is to create Cu-Zn active sites on the Cu surface. Moreover, the catalytic activity of a Zn-deposited Cu(111) surface was much higher than that of Cu(111) alone. They also postulated a model for the function of ZnO sites during methanol synthesis. As we can see in Figure 1, the model explains that the role of ZnO was not to improve the morphology of Cu sites but to create the Cu-Zn active sites for methanol synthesis. In this context, ZnO modifies the electronic properties of Cu sites by an electron exchange and interaction with Cu particles. At the same time, ZnO have the ability to adsorb poison species that are present in syngas streams. Traditionally, copper catalysts are extremely sensitive to very low levels of sulfur poisoning. ZnO as a support could effectively remove H2S components from the gas


feed by forming zinc sulfide, which is a favorable thermodynamic reaction in methanol synthesis conditions. Twigg and Spencer50 corroborated that Cu/ZnO/ Al2O3 catalysts retained a higher activity and even accumulated quite a large amount of sulfur. With an average of 2% sulfur, the methanol synthesis activity of Cu/ZnO/Al2O3 catalysts was approximately dropped to 80% of the fresh catalyst. In contrast, Cu/Al2O3 catalysts were completely deactivated with only 0.2% sulfur in the feed. In addition, ZnO can form a special interface or surface defects between Cu and ZnO, which might be an active surface domain. As a basic oxide, ZnO can partially neutralize the acidity of Al2O3, preventing the transformation of methanol to dimethyl ether.50 2.2.2. ZrO2. The fluorite-type oxides, such as ceria, zirconia, and thoria, have face-centered-cubic (fcc) crystal structure in which each tetravalent metal ion is surrounded by eight equivalent nearest O2- ions forming the vertexes of a cube. Oxygen vacancies that serve as active sites for some reactions are created when a fluorite oxide is doped by divalent or trivalent impurity ions.51 Zirconium oxide has also been investigated as a promoter or support for the methanol synthesis catalyst.52,53 Zirconia, a strong and thermal-resistant material, is a very promising catalyst support because it bears high stability under reducing or oxidizing atmospheres;53-56 it was observed that catalytic activity using ZrO2 as the support is better than that using Al2O3, SiO2, and other materials. On the other hand, CuO can be uniformly dispersed on a ZrO2 surface, forming a specific interface that might be favorable for the methanol synthesis reaction. The synergistic effect between copper and ZrO2 was the subject of many studies. Liu et al.,57 using aerogel zirconia as the support, found that the binding energy of Cu 2p3/2 of CuO/ZrO2 is higher than that of the pure CuO. Moreover, the lower the copper oxide loading, the larger the binding energy is. They also showed that CuO and ZrO2 were not simply physically mixed, but some interactions between them occurred, changing the electronic distributions. X-ray diffraction (XRD) characterization51 showed that, on ZrO2 with no copper loading, the major peak found is mainly the monoclinic-ZrO2 crystal phase. After loading of CuO, the tetragonal-ZrO2 crystal phase appears in strong peaks and increases with copper loading. The monoclinic-ZrO2 crystal phase even fully disappears when CuO loading is higher than 10 wt %. This means that an interaction between copper oxide and zirconium oxide should exist, which inhibits the phase transformation of ZrO2. This metal-support interaction is due to the oxygen vacancies of the support, which causes a geometric effect that can affect the dispersion and alter the morphology of the supported metal. Koeppel and Baiker58,59 reported that efficient methanol synthesis catalysts were found to consist of microcrystalline copper particles that are stabilized through interaction with an amorphous zirconia matrix, resulting in a high interfacial area. The activity of methanol formation using zirconia as the support is slightly lower than when zinc oxide is used as the support. Even though zirconia is still a more preferred support for the methanol synthesis catalyst, more and more researchers are inclining to investigate its catalytic effect because of its special structure and stability.

2.2.3. SiO2. As a catalyst support, silica possesses predominant properties, such as acid-base nature, porosity texture, and stable thermal stability. It is commonly used for catalyst preparation by a wetness impregnation method. However, copper supported on high-purity silica was found to be nearly inactive in methanol synthesis60-62 and selectivity toward methanol was also low. Most of the catalysts prepared using silica as the support needed the addition of other metal oxides as the promoter to improve its activity.34,62,63 2.3. Promoter Effect. Because methanol synthesis on a Cu-based catalyst is a structurally sensitive reaction, it is useful to modify its performance using various promoters, which could improve the activity, selectivity, or stability of the catalyst. It is well-known that Al2O3 is the widely used third component in a Cu-based catalyst because it is a good promoter. It not only can form zinc aluminate to prevent the agglomeration of active sites but also accelerates the adsorption and activation of CO because of its disorder and defect surface domain. Furthermore, it can also stabilize the highly dispersed Cu/ZnO structure. Chen et al.11 reported that doping with trivalent metal ions such as Al3+, Sc3+, and Cr3+ could also enhance the formation of monovalent cationic defects on the crystal surface of ZnO, which might accelerate both enrichment and stabilization of Cu+ on the surface during the reduction and reaction processes. Simultaneously, the doping of Sc3+ positively increases the specific surface area and greatly enhances the methanol yield. Chromia in a skeletal copper catalyst could prevent the rearrangement of Cu sites and improve the pore structure and active surface of the catalyst.64 An important observation is that loading chromia on skeletal copper can partially inhibit the reverse water gas shift reaction, improving the bulk activity and selectivity for methanol synthesis from CO2 and H2 and accelerating simultaneously the reduction of CuO species. Doping copper catalyst with Pd showed a promotion effect similar to that of Cr and positively prevented the rearrangement of Cu species. In addition, the formation of a special intersurface or surface defects between Cu and ZnO might contribute to the fast adsorption and high stability effects of a highly dispersed Cu/ZnO structure.65 Zirconia also acts as a favorable promoter in methanol synthesis as explained previously. The dissolution of zirconium ions in a copper catalyst results in the presence of a new copper oxide phase and promotes the formation of Cu+ ions,66 which enhances the activity of the catalyst for methanol synthesis at low temperatures and pressures. The optimum amount of ZrO2 is 15 wt %, resulting in maximum CO conversion and optimum methanol selectivity (see Figure 2). This means that the addition of ZrO2 improves the surface area of Cu species and adjusts the ratio of Cu+/Cu0 in the surface, increasing specifically CO conversion. Other elements such as B, Ga, Co, and Mg were also used as copper-based catalyst promoters in methanol synthesis.9,67-69 They can modify the physical-chemical properties just like the above elements. Recently, many researchers are engaged in searching for new promoters to improve the performance of the methanol synthesis catalyst. However, a magic promoter does not seem to be in sight yet. 2.4. Methods of Catalyst Preparation. As mentioned previously, it is well-known that methanol syn-


Figure 3. TEM photographs of the catalyst prepared with different precipitators (from ref 74). Figure 2. Effect of the ZrO2 content in Cu/ZnO/ZrO2 catalysts on CO conversion (s) and methanol selectivity (t), respectively (from ref 66).

thesis catalysts are structurally sensitive. The differences in preparation methods as well as pretreatment conditions showed remarkable influences on the structure of the catalysts, ultimately affecting the catalytic performance. Catalyst preparation methods were extensively reviewed as follows: 2.4.1. Coprecipitation Methods. Coprecipitation methods are the most widely used methods to prepare copper-based catalysts. Precipitators such as sodium carbonates and oxalates are added to a solution of the desired salt concentration to coprecipitate and form the catalyst precursors. Then, the coprecipitates are separated by centrifugal or evaporation methods. 2.4.1.1. Effect of the Reaction Conditions. Previous studies70-73 indicate that precipitation parameters strongly affect the performance of a CuO-ZnO-based catalyst, in which Cu/ZnO catalysts are prepared using precursors such as hydrozincite Zn5(CO3)2(OH)6, aurichalcite (Cux,Zn1-x)5(CO3)2(OH)6, malachite Cu2(CO3)(OH)2, rosasite (Cux,Zn1-x)2(CO3)(OH)2, or their mixtures.70,71 Only catalysts prepared using the aurichalcite precursor are more active to CO hydrogenation. Interestingly, the structure of the precursor varied with the Cu/Zn ratio. At a low ratio (about 30/70), the precursor produced using a solution mixture of nitrates consists only of the aurichalcite phase, while a mixture of aurichalcite and malachite phases is found at higher ratios. In the coprecipitation, the pH value plays an important role in determining the composition of the precipitates. When the precipitation was conducted at pH ) 7, the precipitates mainly consisted of the malachitelike phase and the catalysts were more active. Precipitation at pH ) 6 favored the formation of hydroxynitrate, which led to less active catalysts. It seems that the pH value exerts its effect through alteration of the phase composition of the precursors. The temperature plays also a key factor during the preparation of the optimum catalyst precursor. Wu et al.73 suggested that the temperature of coprecipitation should be less than 313 K. The crystallite size of the precipitate prepared at higher than 313 K was slightly larger, reflecting in a decrease of the catalytic activity. It was also stressed that temperature also affects the precipitation kinetics of the precursors. 2.4.1.2. Effect of the Precipitation Species. Koeppel et al.59 reported that the catalyst prepared using

ammonium carbonate as a precipitator showed lower activity than the catalyst made by hydroxide precipitation. Acetate-modified precursors resulted in dramatically increased activity compared to the oxide precursor. This showed that the catalyst synthesized under slightly acidic conditions had a copper surface area similar to those of the samples prepared under alkaline conditions while distinctly larger than those of the samples obtained under neutral precipitation conditions. This means that the pH value in coprecipitation is very important in order to obtain an optimum catalyst composition. A homogeneous Cu and Zn distribution over spherical particles was obtained when oxalate was used as the precipitator, whereas irregular particles with Cu aggregates on the surface of the catalyst were found when conventional carbonate salts were used as coprecipitation agents.74 The transmission electron microscopy (TEM) photographs in Figure 3 clearly showed the different shapes of the catalysts prepared using different precipitator agents. If the precipitator is sodium carbonate, the catalyst cannot exclude the sodium ions, which remain in the catalyst, reducing the total Cu surface area and activity of the catalyst. It is worth noting that the CuO/ZnO ratio obtained for the catalyst prepared using carbonate as the precipitator is higher than that obtained with oxalate as the precipitator, which makes the reduction of catalyst with carbonate easier than that with oxalate. This means that the process of isomorphous substitution between Cu and Zn actually occurred during coprecipitation with carbonate and that ZnO exists mainly as amorphous particles in the catalyst prepared using oxalate. Methanol synthesis activity was also shown to be higher than that of carbonate by an order of magnitude. 2.4.1.3. Solvent Effect. Figure 4 shows the morphology of the precursor prepared with different solvents.75 The precursor obtained using deionized water as the solvent mainly consisted of large amorphous clusters. However, a little quasispherical crystal was also observed. Precursors obtained using dimethylformamide exhibited a column-shaped crystal phase, whereas in the case of using ethanol as the solvent, the precursor was found to be mostly amorphous. When diethylene glycol was employed as the solvent, the resulting precursor exhibited both amorphous clusters and a barlike crystal phase. From the experimental findings, it can be stated that different coprecipitation mechanisms actually exist and the significant differences found in the structure and morphology of the precursors are the consequence of the different solvents employed during the precipita-


Figure 4. TEM photographs of precursors prepared with different solvents (from ref 75).

tion step. Overall, coprecipitation using ethanol as the solvent was found to be the most effective route for the preparation of a highly active and selective catalyst. The XRD patterns shown in Figure 5 illustrate that the catalysts prepared with different solvents possess different particle sizes. A broad diffraction peak for calcined CuO/ZnO/Al2O3 catalysts prepared using ethanol as the solvent is observed. This is indicative of the catalyst consisting of uniform ultrafine particles. N2 adsorption data showed that the surface area of the catalyst prepared using ethanol as the solvent is much higher than that using water.6 Consequently, copper dispersion and the crystallite size of the catalyst with ethanol are much higher than those of catalysts with other solvents. TEM (Figure 6) clearly revealed that very small quasispherical particles are found in the catalyst with ethanol. The effects of different solvents on the texture and morphology of the catalyst are ascribed to their surface tension, viscosity, and dielectric constant.75 The weaker surface tension of the solvent can mitigate the serious shrinkage in the volume and resist the microstructural collapsing of the precursor of the catalyst. The catalyst prepared with the weaker surface tension solvent would have large surface area and volume, which is beneficial

to the dispersion of the active sites. The viscosity influences the nucleation process of the particles. The more viscous the solvent used, the faster nucleation and the slower the nucleus growth rate. According to the nucleation theory, fast nucleation and the slower the nucleus growth lead to fine crystallites. Hence, the more viscous solvent can synthesize the finer crystallites. All in all, the solvent plays an important role in the preparation of the catalyst. The solvent with small surface tension and large viscosity contributes to the preparation of the disordered and isomorphous substitution precursor, which will be of benefit to the activity of the catalyst.75 2.4.2. Other Preparation Methods. The most common way to prepare metal-supported catalysts, the impregnation technique, can also be used to synthesize methanol catalysts. The characteristics of the impregnated catalysts are determined by the precursor used and the impregnation conditions. However, for the case of methanol synthesis catalysts, the performance of the catalyst prepared using this technique is not as good as that when a coprecipitate method is employed.69 Silica is commonly used as the support by the impregnation method. Burch and Chappell37 confirmed that Al2O3, ZrO2, Ga2O3, and ZnO can be used as the

6524 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 Table 1. Effect of the Heating Rate for the Calcination on the Crystallite Sizes of CuO and ZnO (from Reference 78) heating rate/(K min-1) 1 2 5 10 20

Figure 5. Profile of the XRD patterns of calcined CuO/ZnO/Al2O3 prepared with different solvents (from ref 73).

promoter of Cu/SiO2 catalysts where B2O3 and In2O3 can be poison components. ZnO was proposed to be the reservoir for atomic hydrogen and to promote hydrogen spillover. Gotti and Prins76 found that Ca and La oxides bear strong promoting effects on the silica-supported Cu and that CO2 is the main carbon source for methanol.

CuO/nm

ZnO/nm

3.9 4.6 8.8

3.5 3.3 4.3 4.6 10.6

Toyir and co-workers69 prepared Cu-Ga/ZnO and CuGa/SiO2 catalysts to verify the effect of the support and promoter on the conversion of CO2. Liaw and Chen9 using a reducing technique prepared the Cu-based ultrafine catalysts. The depositionprecipitation method of ZrO2-supported catalysts and the uniform gelation method for Cu/ZnO/Cr2O3/Al2O3/ Ga2O3 were also investigated.57,77 2.4.3. Pretreatment Effect. The structure and catalytic performance of the catalysts also depends on calcination and subsequently reduction.51,78 The lower heating rate of calcination benefits the formation of fine particle size catalysts. Fujita et al.78 observed an increase in the particle size of CuO or ZnO catalyst prepared using aurichalcite when the heating rates were increased from 2 to 20 K min-1 (listed in Table 1). UVvis diffusion reflectance spectra showed that the absorption edge of the samples at lower heating rates shifts toward a lower wavenumber. This indicates that at lower heating rates it is feasible to synthesize ultrafine catalysts. The effects of the heating rate on the CuO or ZnO particle size should be attributed to the interaction of water, which could accelerate the growth of metal oxide crystallites. When calcination was carried out at a faster heating rate, the decomposition of the precursor would proceed in a short period. Accordingly, the metal oxide will interact with water at higher partial pressure so that calcination at higher heating rates leads to an increase in the CuO or ZnO size. The reduction is necessary to obtain optimum catalyst performance. Unfortunately, the reduction process is exothermic, which could accelerate the agglomeration of surface active sites, especially at higher heating rate

Figure 6. TEM photographs of calcined precursors prepared with different solvents (from ref 73).


Figure 7. Effects of the temperature on the catalytic activity of methanol synthesis on different compositions of Cu/ZnO/Al2O3 catalysts: (9) 10/80/10; (b) 30/60/10; (O) 60/30/10; (- -) calculated formation rate of methanol based on thermodynamic equilibrium (from ref 82).

or high temperature. Fujita et al.79 reported that the lower reduction temperature is beneficial for improving copper dispersion and activity for the methanol synthesis and that using methanol as the reducing agent is better than using hydrogen. During reduction with methanol, endothermic dehydrogenation to methyl formate inhibits any temperature increase that can damage the catalyst structure. 3. Optimization of the Reaction Conditions 3.1. Temperature. It is well-known that the reaction rate for CO/CO2 hydrogenation is always increased with increasing temperature. However, because it is an exothermic reversible reaction, the product distribution will be controlled by both thermodynamics and kinetics. Its equilibrium constant decreases with an increase in the temperature. Thus, higher temperature is a disadvantage to the yield of methanol. Zhang et al.80 reported that methanol selectivity decreases slowly in the lower temperature range and quickly at higher temperature. On the other hand, Bill and co-workers81 observed that the methanol yield increases with temperature up to 493 K; then it started to decrease above 493 K despite a continuous increase in the conversion of CO2. Traditionally, the reaction temperature reported is below 573 K. In this temperature range, the methanol yield is relatively high. Sun et al.82 investigated the effects of the temperature on the catalytic activity of methanol synthesis on Cu/ZnO/Al2O3 catalysts (as shown in Figure 7). It was observed that there is a maximum of methanol yield at lower temperature, which is an indication of the shifting from kinetics- to thermodynamics-limited reactions. The maximum methanol yields under higher temperature were close to the thermodynamic equilibrium value. Methanol synthesis exhibits different optimum operation temperature ranges when different catalysts were used. Liaw and Chen9 reported that a 20% mixture of Zr-Cu is a good catalyst in the lower temperature range of 473-498 K, while a 20% mixture of Cr-Cu is the better one in the range of 498-523 K. The Brookhaven National Laboratory in USA tested a range of temperatures of 373-403 K using a mixture of NaOH, alcohol, and acetate as the catalyst. In addition, Tsubaki et al.83 successfully decreased the reaction temperature to 423443 K by changing the reaction pathways through use of methanol as the reaction solvent. The space yield was up to 0.17 kg of MeOH/L‚h at 443 K and 30-50 bar. To obtain a higher one-pass conversion of CO or CO2, the tendency is to lower the reaction temperature. There-

fore, there have been many attempts toward the development of an active catalyst to be able to operate at low temperature, where equilibrium is rather favorable.84 3.2. Pressure. The methanol synthesis process has been operated at high pressures since its invention by BASF in the 1920s. With the development of newer catalysts, the operating pressure was obviously decreased. In the late 1960s, ICI started operation at 50100 atm using a Cu-ZnO/Al2O3 catalyst. Deng et al.6 reported that an oxalate-coprecipitated ultrafine Cu/ ZnO/Al2O3 catalyst can be operated at 20 atm and methanol selectivity is rather high. Simultaneously, Cu/ hydrophobic silica catalyst prepared by incipient wetness impregnation could also be operated at the pressure of 20 atm. Kilo et al.67 reported that methanol synthesis could be successfully carried out at 17 atm using Cu/ZrO2 catalysts modified by chromium and manganese oxides. Meanwhile, Chen et al.11 even operated this process at 10 atm using Cu/ZnO/M2O3. Considering the cost and tendency of the methanol synthesis process, it should be operated at as much lower pressure range as possible. Therefore, the search for novel catalysts that can allow the synthesis to operate at even lower pressure conditions is continuing. 3.3. Space Velocity. Space velocity is another important factor that affects the methanol synthesis reaction. Generally, an increase in the space velocity can decrease the rate of conversion, but the yield of methanol and turnover frequency (TOF) of methanol formation increases with an increase in the hourly space velocity.82 It is interesting to note that the space velocity has different effects when using different reactant gases.85 In the hydrogenation of CO over a Cu/ZnO/ Al2O3 catalyst, CO conversion decreased rapidly with increasing space velocity, while methanol selectivity remained unchanged. However, during CO2 hydrogenation, conversion decreased much more slowly and methanol selectivity increased with the space velocity. Using a Cu/ZrO catalyst, Koeppel et al.59 also investigated the influence of the space velocity on the selectivity of methanol. They obtained results similar to those of other researchers. Simultaneously, they concluded that the selectivity to methanol tends toward a finite value as the contact time approaches zero. All of these findings show that the choice of optimum space velocity should be determined not only by the conversion and selectivity but also by the type of reactant gas. 4. Deactivation of the Catalysts Copper-based catalysts are sensitive to a variety of deactivation factors. The thermal stability of Cu is only superior to Ag among the commonly used metal catalysts. Heating can cause easy sintering and agglomeration of copper-based catalysts that have to be used in a relatively low and narrow temperature range.50,86 To improve the life of the copper catalyst, it is essential to add a suitable promoter to increase its thermal stability. Unfortunately, the loss in activity in the early stage of the process is so great that it is very difficult to choose the right promoter. Wu et al.73,87 added a small amount of colloidal silica-to-copper catalyst to suppress the agglomeration of copper and zinc sites, consequently improving greatly the stability of the catalyst. The life of the catalyst for methanol synthesis is relevant to the operating pressure, in which the catalyst deactivation rate becomes higher at higher pressures. ZnAl2O4 support could be modified by introducing some


of the zinc component as zinc spinel or by doping other promoter, which is not sufficiently refractory as a support at 100 bar. The catalyst deactivation caused by poisonous compounds is rarely a problem in methanol synthesis. The poisonous components such as sulfur and chlorine are effectively removed in feed pretreatment. Of special consideration is that copper is very sensitive to the sulfur, chlorine, and other compounds, in which the presence of a trace of them can cause a great loss of the activity of the catalysts. Methane, long-chain paraffins, or higher alcohols are usually formed along with methanol formation, as a result of the presence of other transition metals and acidic or alkaline impurities in copper catalysts. Twigg and Spencer50 reported that iron, cobalt, and nickel catalysts have considerable activity of methanation, which can cause the formation of methane and other long-chain paraffins. Simultaneously, the deposition of the alkaline or acidic impurities can respectively result in the production of higher alcohol and high molecular weight waxes.

both methanol formation rates for the individual CO and CO2 hydrogenations were much lower than that for CO/CO2 mixture hydrogenation and the methanol formation rate for CO hydrogenation is lower than that for CO2 at 490 K. This means that CO2 remarkably promoted methanol formation and it is also a carbon source in methanol synthesis. However, most researchers thought that both CO and CO2 could be hydrogenated to methanol and the predominant reaction depended on the operating conditions. Although the reaction for methanol synthesis has been extensively investigated for nearly 4 decades, the reaction pathway involving the copper-based catalyst is still a topic of intensive debate. For a Cu/ZnO catalyst, a bifunctional mechanism is currently accepted. It is assumed that CO2 is hydrogenated on the Cu sites to yield methanol and water and that subsequent gasphase transport from the Cu sites leads to the displacement of relatively stable species from ZnO. However, quite different bifunctional mechanism routes were also postulated. Saussey and Lavalley92 proposed the following routes:

5. Reaction Mechanism for Methanol Synthesis

ZnO OH(surf) + CO f HCOO(a)

Methanol synthesis via hydrogenation of CO or CO2 is a moderately exothermic reaction:

CO(g) + 2H2(g) f CH3OH (∆H298 ) -91 kJ mol-1,

HCOO(a) + 2H(a) f CH3O(a) + O(surf)

(4) (5)

Cu CO2 + 3H2 f CH3OH + H2O

(6)

ZnO CH3O(a) + H2O f CH3OH + OH(surf)

(7)

-1

∆G298 ) -25.34 kJ mol ) (1) CO2(g) + 2H2(g) f CH3OH + H2O (∆H298 ) -49.5 kJ mol-1, ∆G298 ) 3.30 kJ mol-1) (2) Reaction (1) and (2) are all favored thermodynamically at low temperatures, which limits its one-pass conversion. To avoid the accumulation of a large amount of reaction heat, one-pass conversion of the reactant gas is only 15-25% in the commercial process. It is necessary to set up a recycling reactor to reuse the reactant gas that does not convert and increase the production cost. Simultaneously, the water gas shift reaction [reaction (3)] occurs as a side reaction in conventional processes, consuming water that reaction (2) forms, which might positively contribute to the methanol production.

H2O + CO f H2 + CO2 (∆H298 ) -41.2 kJ mol-1, ∆G298 ) -28.6 kJ mol-1) (3) Combination of the water gas shift reactions results in a strong driving force, which dramatically increases the synthesis gas conversion.88 Many research projects have been carried out to elucidate the role of CO2 in methanol formation. Klier35 stated that CO2 only adjusts or controls the surface composition, oxidation state, and dispersion of CuO in the catalyst but not as a direct reactant. On the contrary, Chinchen et al.89,90 suggested that carbon dioxide is the sole carbon source for methanol synthesis. Simultaneously, Zhang et al.91 used CO/ H2, CO2/H2, and CO/CO2/H2 respectively as reactant gases to investigate the function of CO2. They found that

From this mechanism, it is assumed that the gas-phase intermediate is H2O and methoxy species are hydrated on the ZnO sites. This is consistent with the observation that small amounts of water in the gas feed increase the catalyst activity. Fakley and co-workers93 proposed another mechanism in which methanol reacts with methanoates on the zinc oxides to produce methyl methanoate, which ultimately hydrogenated when in contact with copper:

Cu CO2 + 3H2 f CH3OH + H2O

(8)

ZnO OH(surf) + CO f HCOO(a)

(9)

HCOO(a) + CH3OH f HCOOCH3 + OH(surf)

(10)

Cu HCOOCH3 + 4H(a) f 2CH3OH

(11)

This route could explain the appearance of traces of methyl methanoate in the product stream during industrial synthesis. In addition, this bifunctional mechanism can better explain the synergy between the copper and zinc oxide. Another bifunctional mechanism is proposed by Herman et al.22 whereby the nondissociative Cu+ sites chemisorb and activate carbon monoxide meanwhile the hydrogen was activated on the zinc oxide surface. The model is illustrated in Figure 8. The initial step in the synthesis of methanol over a Cu/ZnO catalyst is adsorption and activation of CO on the Cu+ sites and of hydrogen on the surrounding ZnO surface sites. The heterogeneous splitting of hydrogen and the bonding of CO to cationic Cu+ sites represent an electrophilic attack of the carbon end of CO by protons and a nucleophilic attack on the oxygen end of CO by hydride ions. It was suggested that the hydrogenolysis of the Cu-CH2OH bond is the rate-limiting step.


Figure 9. Changing the reaction course from a high-temperature ICI process (a) to a new low-temperature route (b) by addition of alcohol (from ref 83). Figure 8. Mechanism model for methanol synthesis (from ref 22).

Bell94

Fisher and conceived a further bifunctional mechanism for methanol synthesis from CO/H2 on Cu/ ZrO2/SiO2 in which formate species form on zirconia and undergo stepwise hydrogenation to methylenebisoxy, methoxide, and finally methanol, with atomic hydrogen being supplied through spillover from Cu. The reductive elimination of methoxide is slow relative to other steps in the reaction mechanism. The suppressed rate of methanol synthesis from CO as compared to CO2 hydrogenation is the result of the absence of water formation, which prevents the more simple release of methoxide by hydrolysis. At the same time, the authors compared the rates of methanol synthesis on Cu/ZrO2/ SiO2 and Cu/SiO2 catalysts. They found that the enhanced methanol rate on Cu/ZrO2/SiO2 as compared with that on Cu/SiO2 could be attributed to the reaction proceeding through the lower energy formate route on Cu/ZrO2/SiO2, while proceeding through the energy formyl mechanism on Cu/SiO2. On the other hand, Tsubaki et al.83 altered the traditional reaction path to a low temperature using an organic alcohol (ROH). The new route can use syngas containing CO2 and/or H2O and can be operated at significantly low temperature and low pressure, such as 423-443 K and 30-50 bar. It was assumed that carbon dioxide and water are utilized as intermediates in the novel pathway. The reaction route is composed of several steps as listed below.

CO + H2O f CO2 + H2

(12)

CO2 + 1/2H2 + Cu f HCOOCu

(13)

HCOOCu + ROH f HCOOR + CuOH

(14)

HCOOR + 2H2 f ROH + CH3OH

(15)

CuOH + 1/2H2 f H2O + Cu

(16)

In the reaction path, Cu represents the catalytic center of copper-based catalysts and ROH is the accompanying alcohol. Here, alcohol acts as the catalytic liquid medium because it was not consumed while the overall reaction is accomplished. Involvement of alcohol in the reaction changes the reaction route from a to b (shown in Figure 9). Methanol synthesis95,96 from pure CO and H2 involves carbonylation of methanol to methyl formate followed

by heterogeneous hydrogenolysis of methyl formate to form two methanol molecules. The net result is the reaction of hydrogen with carbon monoxide to give methanol via the intermediate methyl formate.

CO + CH3OH f HCOOCH3

(17)

HCOOCH3 + 2H2 f 2CH3OH

(18)

CO + 2H2 f CH3OH

(19)

For the hydrogenation of CO2 to methanol, the pathways are similar to those of CO. It also proceeds via formate and methoxy intermediates. Fujitani et al.97 reported that formate coverage on the Cu/ZnO catalysts is proportional to the TOF for methanol formation and that the formate hydrogenation is the rate-limiting step. The reaction pathways can be shown as below:

CO2 + 1/2H2 f HCOO(a)

(20)

HCOO(a) + 2H(a) f CH3O(a) + O(a)

(21)

CH3O(a) + H(a) f CH3OH

(22)

6. Discussion Methanol synthesis from syngas hydrogenation is a proven process for chemical industries and for the production of environmentally clean fuels. However, its production process still poses great challenges. On the one hand, the syngas conversion per pass is severely limited by reaction thermodynamics. To resolve the limitations of thermodynamics on the conversion and methanol yield, one of the most effective ways is to remove the reaction heat from the reaction system. Another measure is to develop an effective catalyst that has a good performance at low temperatures. While the nature of the active site and effects of the support and promoter are still under investigation, the reaction mechanism still remains an open question. Only when these issues are truly overcome can the optimum catalyst with perfect activity and selectivity be synthesized. Conversely, to develop an optimum catalyst, great effort must be devoted to the elucidation of the active site and the effects of the support and promoter. Of most importance is that the methanol catalyst should possess higher metal dispersion, larger surface area, and ultrafine particle, which could inhibit the deactivation


caused by agglomeration of the active sites and renders higher conversion by accelerating the reactant diffusion. Because the methanol synthesis catalysts are structure-sensitive catalysts, the differences in preparation methods, preparation conditions, and pretreatment have considerable influences on the structures of the catalysts, which finally lead to the disparities in the catalytic performance. Moreover, to hinder the agglomeration or fast growth of the particles during the reduction, the exothermic heat in the reduction of the catalyst should be quickly taken away. Fujita’s method79 is worth noting. The ultrafine catalyst is potentially one way to optimize the catalyst in the future because of the highest activity requirements for the catalyst components to coexist in a fine interdispersion. As mentioned above, some groups6,9,75,82,98 have already prepared ultrafine catalyst particles that exhibited much higher catalytic activity and selectivity for methanol synthesis. In addition, the effect of the feed composition on the nature of the catalyst surface and on the catalyst activity must be further investigated. The carbon source for methanol synthesis is another current controversy. Using the isotopic labeling studies, some groups76,89,99,100 confirmed that methanol can mainly form from CO2 and the function of CO in the hydrogenation of CO and CO2 is to scavenge the adsorbed oxygen to produce surface CO2. It was realized that CO was the dominant reactant for producing methanol. To enhance the performance of the catalyst and obtain the optimum methanol yield, the feed composition needs to be further optimized on the basis of the surface science technology. Furthermore, it is essential to prove the role of CO2 and CO in the syngas hydrogenation reaction. 7. Conclusion There is an increasing interest in methanol synthesis from hydrogenation of syngas because of the importance of natural gas and fuel cell technologies. A vast volume of literature exists on the process and catalysts; however, the roles of the active site, the effects of the support and promoter, and the reaction mechanism are still not fully understood. In this paper, we have discussed these issues in light of the latest advances in this area. The recent prevalent view of the active site is the synergy between the primary catalyst and support or promoter, and most researchers accept the bifunctional mechanism. Because the catalyst for methanol synthesis is sensitive to the structure of the catalyst, the preparation method, preparation conditions, and component significantly influence its performance. The primary preparation method centers around the coprecipitation method, and the Cu-based catalyst is widely used in this process. The following key issues are identified as important to the successful development of new catalysts for methanol synthesis. (1) An optimum catalyst should have a high specific surface area and ultrafine or nanostuctured particles of the metal active sites. (2) It is highly desirable to understand the surface molecule interaction mechanism in elucidating the methanol synthesis kinetics. (3) It is important to systematically characterize the nature of the active sites and interactions among active components, support, and promoter to be able to tailor the structure of the catalyst. (4) The key to increasing the one-pass conversion is

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Received for review December 2, 2002 Revised manuscript received September 19, 2003 Accepted September 22, 2003 IE020979S

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