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 new low-temperature methanol synthesis method: Mechanistic and kinetics study of catalytic process

Mechanism and kinetics of catalytic process for a new low-temperature methanol synthesis on Cu/ZnO catalysts from syngas (CO/CO₂/H₂) using catalytically active alcohol promoters were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Two intermediate species, adsorbed formate species and alkyl formate species, were formed in this synthesis process. 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 readily reduced to form methanol at 443 K and 1.0 MPa, and the hydrogenation rate of 2-propyl formate was found to be quicker than that of ethyl formate. As a promoter, 2-propanol exhibited a higher activity than ethanol in the reaction of the low-temperature methanol synthesis.     

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.     

On the mechanism of methanol synthesis and the water-gas shift reaction on ZnO

Zinc oxide catalyses both methanol synthesis and the forward and ‘everse water-gas shift reaction (f- and r- WGSR). Copper also catalyses both reactions, but at lower temperatures than ZnO. Presently the combination of Cu and ZnO stabilized by Al₂O₃ is the preferred catalyst for methanol synthesis and for the f- and r- WGSR. On Cu, the mechanism of methanol synthesis is by hydrogenation of an adsorbed bidentate formate [1] (the most stable adsorbed species in methanol synthesis), while the f- and r- WGSR proceeds by a redox mechanism. The f-WGSR proceeds by H₂O oxidizing the Cu and CO, reducing the adsorbed oxide and the r-WGSR proceeds by CO₂ oxidising the Cu and H₂, reducing it [2–5]. Here we show that the mechanisms of both reactions are subtly different on ZnO. While methanol is shown to be formed on ZnO through a formate intermediate, it is a monodentate formate species which is the intermediate; the f- and r-WGS reactions also proceed through a formate – a bidentate formate - in sharp contrast to the mechanism on Cu.     

Deactivation of Cu/ZnO catalyst during dehydrogenation of methanol

The deactivation of Cu/ZnO catalyst during methanol dehydrogenation to form methyl formate has been studied. The Cu/ZnO catalyst was seriously deactivated under the reaction conditions: various temperatures of 493, 523 and 553 K, atmospheric pressure and methanol GHSV of 3000 ml (STP)/g-cat h. The weight loss due to reduction of ZnO in the Cu/ ZnO catalyst was monitored by a microbalance. X-ray induced Auger spectroscopy of Zn(L₃M_4,5M_4,5) showed the increase in the concentration of metallic Zn on the catalyst surface after the reaction. Temperature-programmed reduction (TPR) of the Cu/ZnO catalyst with methanol demonstrated that the reduction of ZnO in Cu/ ZnO was suppressed by introduction of CO₂ into the stream of helium-methanol. As the concentration of CO₂ in the feed gas increased, the weight loss of the Cu/ZnO catalyst due to the reduction of ZnO decreased. The deactivation of the Cu/ZnO catalyst in the methanol dehydrogenation was also retarded by the addition of CO₂. In particular, oxygen injection into the reactant feed regenerated the Cu/ ZnO catalyst deactivated during the reaction. Based on these observations, the cause of deactivation of the Cu/ZnO catalyst has been discussed.     

The Partial Oxidation of CH3OH to CO2 and H2 Over a Cu/ZnO/Al2O3 Catalyst

The partial oxidation of CH₃OH to CO₂ and H₂ over a Cu/ZnO/Al₂O₃ catalyst has been studied by temperature-programmed oxidation (TPO) using N₂O and O₂ as the oxidant. Post-reaction analysis of the adsorbate composition of the surface of the catalyst was determined by temperature-programmed desorption (TPD). The temperature dependence of the composition of the mixture of products formed by TPO was shown to depend critically on the partial pressure of the oxidant, with the highest partial pressure of oxygen used (10% O₂ in He, 101 kPa—the CH₃OH partial pressure was 17% throughout), producing marked non-Arrhenius fluctuations on temperature programming. Unsurprisingly, therefore, the adsorbate composition of the catalyst revealed by post-reaction TPD was also found to be determined by the partial pressure of the oxidant. Using high partial pressures of oxidant (5% and 10% O₂ in He, 101 kPa), the only adsorbate detected was the bidentate formate species adsorbed on Cu. Lowering the oxygen partial pressure to 2% in He (101 kPa) revealed a catalyst surface on which the bidentate formate on Cu was the dominant intermediate with the formate on Al₂O₃ also being present. A further lowering of the partial pressure of the oxidant, obtained by using N₂O as the oxidant (2% N₂O in He, 101 kPa), resulted in a surface on which the formate adsorbed on ZnO was the dominant adsorbate with only a small coverage of the Cu by the bidentate formate.     

In situ FTIR studies of methanol adsorption and dehydrogenation over Cu/SiO2 catalyst

In situ FTIR spectroscopy was used to identify the adsorbed species and the intermediates during methanol dehydrogenation over Cu/SiO₂ catalyst, and a schematic reaction network was proposed. Methoxy species on copper, which were derived from adsorbed methanol, dehydrogenated into formaldehyde. Then several competitive pathways took place. The adsorbed formaldehyde could desorb to the gas phase, or react with another adsorbed methoxy group to form methyl formate, and/or undergo further dehydrogenation to CO and H₂. Carbon monoxide formed from the decomposition first adsorbed on high-index planes of copper, and then on low-index planes as the reaction progressed. With the increase of temperature, the concentration of formaldehyde and CO in gas phase increased, and that of methyl formate decreased.     

Creation of the active site for methanol synthesis on a Cu/SiO2 catalyst

The effect of ZnO/SiO₂ in a physical mixture of Cu/SiO₂ and ZnO/SiO₂ on methanol synthesis from CO₂ and H₂ was studied to clarify the role of ZnO in Cu/ZnO-based catalysts. An active Cu/SiO₂ was prepared by the following procedure: the Cu/SiO₂ and ZnO/SiO₂ catalysts with a different SiO₂ particle size were mixed and reduced with H₂ at 523-723 K, and the Cu/SiO₂ was then separated from the mixture using a sieve. The methanol synthesis activity of the Cu/SiO₂ catalyst increased with the reduction temperature and was in fairly good agreement with that previously obtained for the physical mixture of Cu/SiO₂ and ZnO/SiO₂. These results indicated that the active site for methanol synthesis was created on the Cu/SiO₂ upon reduction of the physical mixture with H₂. It was also found that ZnO itself had no promotional effect on the methanol synthesis activity except for the role of ZnO to create the active site. The active site created on the Cu/SiO₂ catalyst was found not to promote the formation of formate from CO₂ and H₂ on the Cu surface based on in situ FT-IR measurements. A special formate species unstable at 523 K with an OCO asymmetric peak at ~1585 cm^-1 was considered to be adsorbed on the active site. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methanol synthesis from carbon dioxide at atmospheric pressure over Cu/ZnO catalyst. Role of methoxide species formed on ZnO support

Methanol synthesis from CO₂ and H₂ was carried out over a Cu/ZnO catalyst (Cu/Zn = 3/7) at atmospheric pressure, and the surface species formed were analyzed by diffuse reflectance FT-IR spectroscopy and temperature programmed desorption method. Two types of formate species and zinc methoxide were formed in the course of the reaction. Zinc methoxide was readily hydrolyzed to methanol. H₂O formed through the reverse water gas shift reaction was suggested to be involved in the hydrolysis of zinc methoxide.     

Spectroscopic evidence for adsorption sites located at Cu/ZnO interfaces

FTIR spectra are reported of CO and formic acid adsorption on a series of Cu/ZnO/SiO₂ catalysts. Peaks due to linear CO adsorbed on copper diminished in intensity as the loading of ZnO was increased. This behaviour was explained in terms of ZnO island growth on the copper surface. Similarly, reduction of the copper concentration while maintaining a constant ZnO loading also resulted in further attenuation in bands ascribed to CO chemisorbed on copper. Formic acid exposure to a Cu/SiO₂ sample produced a formate species displaying a _as(COO) mode at 1585 cm^–1. Addition of a small quantity of ZnO to the catalyst resulted in substantial promotion of formate growth, which was accompanied by a shift (and broadening) of the _as(COO) vibration to 1660–1600 cm^–1. Since further ZnO incorporation poisoned formate creation it was concluded that formate species bonded to Cu and Zn sites located at interfacial positions had been formed. The role of such species in methanol synthesis is discussed.     

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.     

On the mechanism of the methanol synthesis involving a catalyst based on zirconia support

The nature of the pivotal intermediate during the synthesis of methanol from CO₂/H₂, in the presence of ZnO/ZrO₂ aerogel catalyst is envisaged. The kinetic studies performed using in situ FTIR spectroscopy of the species formed on the surface of the catalyst in the absence and in the presence of hydrogen show that the initial reactive adsorbed species formed from C0₂ gas is the unidentate carbonate species. Its hydrogenation into the formate species is much faster than the hydrogenation of the formate species into methoxyl species. The comparison is based on a quantitative measurement of the rate constant of the hydrogenation of the various species. The results explain that during the C0₂/H₂ reaction only formate and methoxyl species are observed.     

Steam reforming of ethanol over Pt/ceria with co-fed hydrogen

Metal/ceria catalysts are receiving great interest for reactions involving steam conversion, including CO for low-temperature water–gas shift, and the conversion of chemical carriers of hydrogen, among them methanol, and ethanol. The mechanism by which ROH model reagents are activated on the surface of the Pt/partially reduced ceria catalyst was explored using a combination of reaction testing and infrared spectroscopy. In this particular investigation, the activation and turnover of ethanol were explored and compared with previous investigations of methanol steam reforming and low-temperature water–gas shift under H₂-rich conditions, where the surface of ceria is in a partially reduced state. Under these conditions, activation of ethanol was found to proceed by dissociative adsorption at reduced defect sites on ceria (i.e., Ce surface atoms in the Ce³⁺ oxidation state), yielding an adsorbed type II ethoxy species and an adsorbed H species, the latter identified to be a type II bridging OH group. In the presence of steam, the ethoxy species rapidly undergoes molecular transformation to an adsorbed acetate intermediate by oxidative dehydrogenation. This is analogous to the conversion of type II methoxy species to formate observed in previous investigations of methanol steam reforming. In addition, although formate then decomposes in steam to CO₂ and H₂ during methanol steam reforming, in an analogous pathway for ethanol steam reforming, the acetate intermediate decomposes in steam to CO₂ and CH₄. Therefore, further H₂ production requires energy-intensive activation of CH₄, which is not required for methanol conversion over Pt/ceria.     

The effect of CuO–ZnO–Al2O3 catalyst structure on the ethanol synthesis from syngas

CuO–ZnO–Al₂O₃ catalysts were prepared by complete liquid phase technology with different addition sequences. The results indicated that the catalyst prepared by the addition of (C₃H₇O)₃Al to Cu(NO₃)₂ and Zn(NO₃)₂ solutions shows excellent ethanol selectivity at the initial stages of reaction, reaching approximately 40%. X-ray photoelectron spectroscopy results showed that ethanol synthesis requires a higher Cu⁺ content and higher Cu/Zn ratio on the catalyst surface. The temperature-programmed reduction test revealed strong interactions between Cu species and zinc or aluminum oxide. The increase in the difficulty of catalyst reduction indicated higher ethanol selectivity.     

Revealing the effect of synergistic interaction between ZnO and ZnCr2O4 on the syngas aromatization

Zn-Cr-based catalysts are widely used as oxide catalysts for syngas aromatization, and it is difficult to study the synergistic effect of ZnO and Zn-Cr spinel due to the complex system of non-stoichiometric Zn-Cr spinels. In order to reveal the synergistic effect, we physically mixed ZnO and ZnCr₂O₄ with definite structure to avoid ambiguous structure of non-stoichiometric Zn-Cr spinels. The results showed that the introduced ZnO affected the oxygen vacancies generation and promoted the activation of CO and H₂, leading to an increase of oxygenates compared to the sole ZnCr₂O₄. Due to the synergy of ZnO and ZnCr₂O₄, the xZZC catalysts could produce more adsorbed species than the ZnCr₂O₄ catalyst, while the ZnCr₂O₄/ZSM-5 catalyst was more difficult to convert formate species. The ZnO in xZZC/ZSM-5 decreased the formate adsorption strength, which favored the continued conversion of formate and further realized the enhanced pulling effect on CO conversion.     

Methanol Synthesis

Methanol, like ammonia, is one of the key industrial chemicals produced by heterogeneous catalysis. As with the original ammonia catalyst (Fe/K/Al₂O₃), so with methanol, the original methanol synthesis catalyst, ZnO, was discovered by Alwin Mittasch. This was translated into an industrial process in which methanol was produced from CO/H₂ at 400?°C and 200 atm. Again, as with the ammonia catalyst where the final catalyst which is currently used was achieved only after exhaustive screening of putative “promoters”, so with methanol, exhaustive screening of additives was undertaken to promote the activity of the ZnO. Early successful promoters were Al₂O₃ and Cr₂O₃ which enhanced the stability of the ZnO but not its activity. The addition of CuO was found to increase the activity of the ZnO but the catalyst so produced was short lived. Current methanol synthesis catalysts are fundamentally Cu/ZnO/Al₂O₃, having high CuO contents of?~60?% with ZnO?~?30?% and Al₂O₃?~?10?%. Far from promoting the activity of the ZnO by incorporation of CuO, the active component of these Cu/ZnO/Al₂O₃ catalysts is Cu metal with the ZnO simply being involved as the preferred support. Other supports for the Cu metal, e.g. Al₂O₃, MgO, MnO, Cr₂O₃, ZrO₂ and even SiO₂ can also be used. In all of these catalysts the activity scales with the Cu metal area. The original feed has now changed from CO/H₂ to CO/CO₂/H₂ (10:10:80), radiolabelling studies having provided the unlikely discovery that it is the CO₂ molecule which is hydrogenated to methanol; the CO molecule acts as a reducing agent. The CO₂ is transformed to methanol on the Cu through the intermediacy of an adsorbed formate species. These Cu/ZnO/Al₂O₃ catalysts now operate at?~230° and between 50 and 100 atm. This important step change in the activity of methanol synthesis has resulted in a significant reduction in the energy required to produce methanol. The “step change” however has been incremental. It has been obtained on the basis of fundamental knowledge provided by a combination of surface science techniques, e.g. LEED, scanning tunnelling microscope, TPD, temperature programmed reaction spectroscopy, combined with catalytic mechanistic studies, including radiolabelling studies and chemisorption studies including reactive chemisorption studies, e.g. N₂O reactive frontal chromatography.     

A Fourier-transform infrared study of CO2 and CO2/H2 interactions with caesium-doped copper catalysts

FTIR spectra are reported of CO₂ and CO₂/H₂ on a silica-supported caesium-doped copper catalyst. Adsorption of CO₂ on a “caesium”/silica surface resulted in the formation of CO₂ ⁻ and complexed CO species. Exposure of CO₂ to a caesium-doped reduced copper catalyst produced not only these species but also two forms of adsorbed carboxylate giving bands at 1550, 1510, 1365 and 1345 cm⁻¹. Reaction of carboxylate species with hydrogen at 388 K gave formate species on copper and caesium oxide in addition to methoxy groups associated with caesium oxide. Methoxy species were not detected on undoped copper catalyst suggesting that caesium may be a promoter for the methanol synthesis reaction. Methanol decomposition on a caesium-doped copper catalyst produced a small number of formate species on copper and caesium oxide. Methoxy groups on caesium oxide decomposed to CO and H₂, and subsequent reaction between CO and adsorbed oxygen resulted in carboxylate formation. Methoxy species located at interfacial sites appeared to exhibit unusual adsorption properties.     

A DRIFTS study of the morphology and surface adsorbate composition of an operating methanol synthesis catalyst

The nature of the species adsorbed on a Cu/ZnO/Al₂O₃ catalyst while it was producing methanol has been elucidated in this study using DRIFTS. The species are carbonates, formate, CO, oxygen atoms ( 2% of a monolayer) and methoxy on the Cu and methoxy on the ZnO. The frequencies observed for the C-O stretch on Cu, 2076, 2092, 2105 and 2132 cm^–1, have revealed the morphology of the copper component of the operating catalyst. The surface of the copper is predominantly the (111) face ( 65%) (the 2076 cm^–1 peak) with the (755) (the 2092 cm^–1 peak) and the (311) (the 2105 cm^–1 peak) faces occupying roughly 20% and 15%, respectively, of the copper area. The 2132 cm^–1 peak derives from CO adsorbed on Cu⁺ site on the copper which is 2% of a monolayer.     

Difference in the reactivity of acetaldehyde intermediates in the dehydrogenation of ethanol over supported Pd catalysts

Catalytic performances of supported Pd catalysts for the dehydrogenation of ethanol were greatly modified upon the formation of Pd alloy phases. Over Pd–Zn, Pd–Ga and Pd–In alloys, acetaldehyde was selectively produced at lower conversion levels. With the increased conversion level, ethyl acetate was produced at the expense of acetaldehyde. The selectivities for the ethyl acetate formation exceeded that over a Cu/ZnO catalyst. Over metallic Pd, the decomposition of ethanol, C₂H₅OH → CO + CH₄ + H₂, occurred to a considerable extent. It was shown that the reactivity of acetaldehyde species over the Pd alloys was markedly different from that over metallic Pd. Over the Pd alloys, acetaldehyde species were stabilized and transformed into ethyl acetate by the nucleophilic addition of ethanol. By contrast, over metallic Pd, aldehyde species were rapidly decarbonylated to methane and carbon monoxide. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Pretreatments of Cu/ZnO-Based Catalysts on Their Activities for the Water–Gas Shift Reaction

The effects of the pretreatments of Cu/ZnO-based catalysts prepared by a coprecipitation method on their activities for the water–gas shift reaction at 523K were investigated. The activity of a Cu/ZnO/ZrO₂/Al₂O₃ catalyst for the water–gas shift reaction was less affected by calcination at temperatures ranging from 673-973K and by H₂ treatment at 573 or 723K than that of a Cu/ZnO/Al₂O₃ catalyst. The catalyst activity could be correlated mainly to the Cu surface area of the catalyst.