Methanol synthesis and reverse water-gas shift kinetics over clean polycrystalline copper

The kinetics of simultaneous methanol synthesis and reverse water-gas shift from CO₂/H₂ mixtures have been measured at low conversions over a clean polycrystalline Cu foil at pressures of 5 bar. An absolute rate of 1.2 × 10^–3 methanol molecules produced per second per Cu surface atom was observed at 510 K, with an activation energy of 77 ± 10 kJ/mol. The rate of CO production was 0.12 molecules per second per Cu surface atom at this temperature, with an activation energy of 135 ± 5 kJ/mol. The rates, normalized to the metallic Cu surface area, are equal to those measured over real, high-area Cu/ZnO catalysts. The surface after reaction was examined by XPS and TPD. It was covered by almost a full monolayer of adsorbed formate, but no other species like carbon or oxygen in measurable amounts. These results prove that a highly active site for methanol synthesis on real Cu/ZnO catalysts is metallic Cu, and suggest that the rate-determining step in methanol synthesis is one of the several steps in the further hydrogenation of adsorbed formate to methanol.     

Methylcyclohexane conversion over ZSM-11 zeolite

The effect of various modifiers on the performance of a commercial Cu/ZnO/Al₂O₃ catalyst in methanol synthesis from CO₂/H₂ and CO/H₂ at 523 K and 30 bar has been studied. Several modifiers improved significantly the rate of methanol formation from CO₂/H₂, while all modified catalysts showed decreased rates for the synthesis from CO/H₂ in comparison with the unmodified Cu/ZnO/Al₂O₃ catalyst. The synthesis rates from both CO₂/H₂ and CO/H₂ correlated with the oxygen coverage of copper surface measured after the reaction by N₂O titration.     

On the Issue of the Active Site and the Role of ZnO in Cu/ZnO Methanol Synthesis Catalysts

The problem concerning the active site and the role of ZnO in Cu/ZnO-based methanol synthesis catalysts can be consistently explained based on the literature results by distinguishing CO₂ and CO hydrogenations. Although only metallic copper has some activities for methanol synthesis by the hydrogenation of CO₂, Cu-Zn alloying in Cu particles is responsible for the major promotional role of ZnO in industrial Cu/ZnO-based catalysts. The morphology effect reported in the literature will probably appear for the system of highly dispersed Cu particles supported on ZnO. As for the hydrogenation of CO, Cu⁺ species or Cu-O-Zn sites are the active sites for methanol synthesis. The spillover effect of the Cu-ZnO system is not significant compared to the effect of ZnO on the creation of the Cu-O-Zn site.     

CO2 Hydrogenation to Methanol Over Cu/ZnO/Al2O3 Catalysts Prepared by a Novel Gel-Network-Coprecipitation Method

A novel gel-network-coprecipitation process has been developed to prepare ultrafine Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from CO₂ hydrogenation. It is demonstrated that the gel-network-coprecipitation method can allow the preparation of the ultrafine Cu/ZnO/Al₂O₃ catalysts by homogeneous coprecipitation of the metal nitrate salts in the gel network formed by gelatin solution, which makes the metallic copper in the reduced catalyst exist in much smaller crystallite size and exhibit a much higher metallic copper-specific surface area. The effect of the gel concentration of gelatin on the structure, morphology and catalytic properties of the Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from hydrogenation of carbon dioxide was investigated. The Cu/ZnO/Al₂O₃ catalysts prepared by the gel-network-coprecipitation method exhibit a high catalytic activity and selectivity in CO₂ hydrogenation to methanol.     

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa. On Cu/SiO₂ catalyst formate species were initially formed which were subsequently hydrogenated to methanol. During the reaction a steady state concentration of formate species persisted on the copper. Additionally, a small quantity of gaseous methane was produced. In contrast, the reaction of CO₂ and H₂ on ZnO/SiO₂ catalyst only resulted in the formation of zinc formate species: no methanol was detected. The interaction of CO₂ and H₂ with Cu/ZnO/SiO₂ catalyst gave formate species on both copper and zinc oxide. Methanol was again formed by the hydrogenation of copper formate species. Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis. Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism.     

Effects of zirconia promotion on the activity of Cu/SiO2 for methanol synthesis from CO/H2 and CO2/H2

The effect of zirconia promotion on Cu/SiO₂ for the hydrogenation of CO and CO₂ at 0.65 MPa has been investigated at temperatures between 473 and 573 K. With increasing zirconia loading, the rate of methanol synthesis is greatly enhanced for both CO and CO₂ hydrogenation, but more significantly for CO hydrogenation. For example, at 533 K the methanol synthesis activity of 30.5 wt% zirconia-promoted Cu/SiO₂ is 84 and 25 times that of unpromoted Cu/SiO₂ for CO and CO₂ hydrogenation, respectively. For all catalysts, the rate of methanol synthesis from CO₂/H₂ is higher than that from CO/H₂. The apparent activation energy for methanol synthesis from CO decreases from 22.5 to 17.5 kcal/mol with zirconia addition, suggesting that zirconia alters the reaction pathway. For CO₂ hydrogenation, the apparent activation energies (~12 kcal/mol) for methanol synthesis and the reverse water-gas shift (RWGS) reaction are not significantly affected by zirconia addition. While zirconia addition greatly increases the methanol synthesis rate for CO₂ hydrogenation, the effect on the RWGS reaction activity is comparatively small. The observed effects of zirconia are interpreted in terms of a mechanism which zirconia serves to adsorb either CO or CO₂, whereas Cu serves to adsorb H₂. It is proposed that methanol is formed by the hydrogenation of the species adsorbed on zirconia.     

Investigating the effect of metal oxide additives on the properties of Cu/ZnO/Al2O3 catalysts in methanol synthesis from syngas using factorial experimental design

The Cu/ZnO/Al₂O₃ catalysts, prepared by co-precipitation method, have been modified by adding small amount of Mn, Mg, Zr, Cr, Ba, W and Ce oxides using design of experiments (1/16 full factorial design). The structure and morphology of catalysts were studied by X-ray diffraction (XRD) and BET. Performance of the prepared catalysts for CO/CO₂ hydrogenation to methanol was evaluated by using a stainless steel fixed-bed reactor at 5 MPa and 513 K. The oxide additives were found to influence the catalytic activity, dispersion of Cu, Cu crystallite size, surface composition of catalyst and stability of catalysts during their operations. The results showed that the Mn and Zr promoted catalysts have high performance for methanol synthesis from syngas.     

Comments on “The effect of ZnO in methanol synthesis catalysts on Cu dispersion and the specific activity” [by T. Fujitani and J. Nakamura]

Fujitani and Nakamura recently reported on the effect of ZnO on Cu/ZnO methanol synthesis catalysts (Catal. Lett. 56 (1998) 119). Having measured the methanol synthesis activity of a series of Cu/ZnO catalysts of different Cu/ZnO ratios, they reported a linear relationship between the copper metal area and the methanol yield (implying a fixed value of the copper specific activity) and paradoxically they also reported a volcano-type relationship between the copper specific activity in methanol synthesis and the ZnO content. This paradox is resolved by showing that their Cu/ZnO catalysts fall into two groups: (i) the low-surface-area copper catalysts which have a specific activity of 10 mg CH₃OH/m²-Cu h and (ii) the high-surface-area copper catalysts which have specific activity of 14.8 mg CH₃OH/m²-Cu h. These different specific activities derive from different surface morphologies of the copper in these catalysts. This revised version was published online in November 2006 with corrections to the Cover Date.     

The difference in the active sites for CO2 and CO hydrogenations on Cu/ZnO-based methanol synthesis catalysts

The effect of Zn in copper catalysts on the activities for both CO₂ and CO hydrogenations has been examined using a physical mixture of Cu/SiO₂+ZnO/SiO₂ and a Zn-containing Cu/SiO₂ catalyst or (Zn)Cu/SiO₂. Reduction of the physical mixture with H₂ at 573–723 K results in an increase in the yield of methanol produced by the CO₂ hydrogenation, while no such a promotion was observed for the CO hydrogenation, indicating that the active site is different for the CO₂ and CO hydrogenations. However, the methanol yield by CO hydrogenation is significantly increased by the oxidation treatment of the (Zn)Cu/SiO₂ catalyst. Thus it is concluded that the Cu–Zn site is active for the CO₂ hydrogenation as previously reported, while the Cu–O–Zn site is active for the CO hydrogenation.     

Partial Oxidation of Ethanol to Hydrogen over Ni–Fe Catalysts

A study has been conducted to identify the influence of zirconia phase and copper to zirconia surface area on the activity of Cu/ZrO₂ catalysts for the synthesis of methanol from either CO/H₂ or CO₂/H₂. To determine the effects of zirconia phase, a pair of Cu/ZrO₂ catalysts was prepared on tetragonal (t-) and monoclinic (m-) zirconia. The zirconia surface area and the Cu dispersion were essentially identical for these two catalysts. At 548 K, 0.65 MPa, and H₂/CO_x= 3 (x = 1, 2), the catalyst prepared on m-ZrO₂ was 4.5 times more active for methanol synthesis from CO₂/H₂ than that prepared on t-ZrO₂, and 7.5 times more active when CO/H₂ was used as the feed. Increasing the surface area of m-ZrO₂ and the ratio of Cu to ZrO₂ surface areas further increased the methanol synthesis activity. In situ infrared spectroscopy and transient-response experiments indicate that the higher rate of methanol synthesis from CO₂/H₂ over Cu/m-ZrO₂ is due solely to the higher concentration of active intermediates. By contrast, the higher rate of methanol synthesis from CO/H₂ is due to both a higher concentration of surface intermediates and the more rapid dynamics of their transformation over Cu/ZrO₂.     

The performance and structural study of CuNi alloy catalysts for methanol synthesis

Novel CuxNiy/γ-Al₂O₃ alloy catalysts were prepared and studied for methanol synthesis from CO/CO₂ hydrogenation. The structure of the catalysts was characterized using N₂ adsorption–desorption, X-ray diffraction, transmission electron microscopy, scanning transmission electron microscopy, X-ray photoelectron spectroscopy and temperature-programmed reduction. The characterization results demonstrated the formation of CuNi alloy. The strong interaction between Cu and Ni had a promotion effect for methanol synthesis. Cu₃Ni₇/γ-Al₂O₃ catalyst exhibited the highest formation rate of CH₃OH, which is 5.86 mmol/g h, much higher than the commercial Cu/ZnO/Al₂O₃ catalyst at the same reaction conditions.     

Methanol synthesis on a Cu(100) catalyst

The synthesis of methanol on a Cu(100) single crystal surface was studied between 500–550 K and at pressures between 44–102 kPa using a gas mixture of CO₂/CO/H₂ = 1/2/12. The specific reaction rates found for methanol synthesis were approximately an order of magnitude lower than those rates previously reported for silica supported, Cu-based catalysts. Furthermore the rates observed for the Cu(100) catalyst are estimated to be several orders of magnitude smaller than those rates found for ZnO supported Cu catalysts at comparable reaction conditions. The very low concentration of ionic copper species on the surface is thought to be responsible for the low activity of the Cu(100) catalyst.     

Methanol synthesis over a Zn-deposited copper model catalyst

Methanol synthesis by the hydrogenation of CO₂ over Zn-deposited polycrystalline Cu was studied using surface science techniques. The Zn sub-monolayer was oxidized by the reaction mixture during the reaction at 523 K, leading to the formation of ZnO species. The kinetic results definitely showed that the ZnO species on the Cu surface promoted the catalytic activity of methanol formation, where the activity of Cu increased by a factor of 6 at the Zn coverage of 0.17. A volcano-shaped curve was obtained for the correlation between the Zn coverage and the catalytic activity, which was very similar to the correlation curve between the oxygen coverage and the specific activity for methanol formation previously obtained for the Cu powder catalysts. The role of ZnO in Cu/ZnO based catalysts was ascribed to the stabilization of Cu⁺ species by the ZnO moieties on the Cu surface.     

Spectroscopic and Kinetic Analysis of a New Low-Temperature Methanol Synthesis Reaction

The spectroscopy and kinetics of a new low-temperature methanol synthesis method were studied by using in situ DRIFTS on Cu/ZnO catalysts from syngas (CO/CO₂/H₂) using alcohol promoters. The adsorbed formate species easily reacted with ethanol or 2-propanol at 443 K and atmospheric pressure, and the reaction rate with 2-propanol was faster than that with ethanol. Alkyl formate was easily reduced to form methanol at 443 K and 1.0 MPa, and the hydrogenation rate of 2-propyl formate was found to be faster than that of ethyl formate. 2-Propanol used as promoter exhibited a higher activity than ethanol in the reaction of the low-temperature methanol synthesis.     

A quantum-chemical study of adsorbed nonclassical carbonium ions as active intermediates in catalytic transformations of paraffins. I. Protolytic cracking of ethane on high silica zeolites

Coprecipitated Cu-ZrO₂ catalysts were found to show higher selectivity to methanol in CO₂ hydrogenation than conventional Cu-ZnO catalysts. Addition of ZnO to Cu-ZrO₂ catalysts of Cu/ZrO₂ = 1 (weight ratio) greatly enhanced the activity at lower temperatures, while keeping the high methanol selectivity of Cu-ZrO₂ catalysts. A remarkable increase in the Cu dispersion with increased amount of added ZnO explains the increased activity at lower temperatures, while the reforming of methanol to CO is accelerated by ZnO at higher temperatures, leading to a lowered yield of methanol. It is suggested that ZrO₂ rather than ZnO in the ternary systems plays a more effective role for the selective formation of methanol.     

Direct synthesis of dimethyl ether from carbon-monoxide-rich synthesis gas: Influence of dehydration catalysts and operating conditions

Various dehydration catalysts were studied in the synthesis of dimethyl ether (DME) directly from carbon-monoxide-rich synthesis gas under a series of different reaction conditions. The investigated catalyst systems consisted of combinations of a methanol catalyst (CuO/ZnO system) with catalysts for methanol dehydration based on γ-Al₂O₃ or zeolites and γ-Al₂O₃ was identified as the most favorable dehydration catalyst. Various reaction parameters such as temperature, H₂/CO ratio and space velocity were studied. The impact of water on Cu/ZnO/Al₂O₃-γ-Al₂O₃ catalysts was investigated and no deactivation could be observed at water contents below 10% during running times of several hours. A running time of several days and a water content of 10% led to a significant increase of CO conversion but the water gas shift reaction became dominating and CO₂ was the main product. After termination of water feeding significant deactivation of the catalyst system was observed but the system returned to high DME selectivity. Catalyst stability and the influence of CO₂ in the gas feed were studied in experiments lasting for about three weeks. The presence of 8% of CO₂ caused an approximately 10% lower CO conversion and an about 5% lower DME selectivity compared to the reaction system without CO₂.     

Influence of titania on zirconia promoted Cu/SiO2 catalysts for methanol synthesis from CO/H2 and CO2/H2

The effects of adding mixtures of titania and zirconia on the methanol synthesis activity and selectivity of Cu/SiO₂ were investigated. The synthesis of methanol from both CO/H₂ and CO₂/H₂ mixtures was examined at 0.65 MPa and temperatures between 448 and 573 K. For CO hydrogenation, the addition of ZrO₂ alone increased the methanol synthesis activity of Cu/SiO₂ by up to three-fold. Substitution of a portion of the ZrO₂ by TiO₂ decreased the methanol synthesis activity of the catalyst relative to that observed when only ZrO₂ is added. ZrO₂ addition also enhanced the methane synthesis activity by as much as seven fold. In the case of CO₂ hydrogenation, the maximum methanol synthesis activity is achieved when a 50/50 wt% mixture of ZrO₂ and TiO₂ is added to Cu/SiO₂. Neither the presence of the oxide additive nor its composition had any effect on the activity of the reverse water–gas-shift reaction, which suggests that this reaction proceeds only on Cu. The observed effects of ZrO₂ and TiO₂ on the catalytic activity of methanol synthesis from CO and CO₂, and methane synthesis from CO, are interpreted in terms of the strength and concentration of acidic and basic groups on the surface of the dispersed oxide.     

A new method of low-temperature methanol synthesis on Cu/ZnO/Al2O3 catalysts from CO/CO2/H2

A new synthesis method of low-temperature methanol proceeded on Cu/ZnO/Al₂O₃ catalysts from CO/CO₂/H₂ using 2-butanol as promoters. The Cu/ZnO/Al₂O₃ catalysts were prepared by co-impregnation of r-Al₂O₃ with an aqueous solution of copper nitrate and zinc nitrate. The total carbon turnover frequency (TOF), the yield and selectivity of methanol were the highest by using the Cu/ZnO/Al₂O₃ catalyst with copper loading of 5% and the Zn/Cu molar ratio of 1/1, which precursor were not calcined, and reduced at 493 K. The activity of the catalysts increased due to the presence of the CuO/ZnO phase in the oxidized form of impregnation Cu/ZnO/Al₂O₃ catalysts. The active sites of the Cu/ZnO/Al₂O₃ catalyst for methanol synthesis are not only metallic Cu but also special sites such as the Cu–Zn site, i.e. metallic Cu and the Cu–Zn site work cooperatively to catalyze the methanol synthesis reaction.     

Synergy at a distance in the synthesis of methanol over copper catalysts

Physical mixtures of Cu/SiO₂ and ZnO/SiO₂ catalysts have been studied for the synthesis of methanol from CO/CO₂/H₂ at 250 °C and 10 bar pressure. It is found that the activities are very much higher for the physical mixtures than would be expected from the activities of either of these two catalysts in isolation. The results suggest that the high activity of conventional Cu/ZnO/Al₂O₃ catalysts may arise from a synergy between the Cu and ZnO phases.     

Evidence for the migration of ZnOx in a Cu/ZnO methanol synthesis catalyst

The behavior and role of ZnO in Cu/ZnO catalysts for the hydrogenations of CO and CO₂ were studied using XRD, TEM coupled with EDX, TPD and FT-IR. As the reduction temperature increased, the specific activity for the hydrogenation of CO₂ increased, whereas the activity for the hydrogenation of CO decreased. The EDX and XRD results definitely showed that ZnO _x (x = 0–1) moieties migrate onto the Cu surface and dissolve into the Cu particle forming a Cu-Zn alloy when the Cu/ZnO catalysts were reduced at high temperatures above 600 K. The content of Zn dissolved in the Cu particles increased with reduction temperature and reached 18% at a reduction temperature of 723 K. The CO-TPD and FT-IR results suggested the presence of Cu⁺ sites formed in the vicinity of ZnO _x on the Cu surface, where the Cu⁺ species were regarded as an active catalytic component for methanol synthesis.