Rideal-type reaction of formate species with alcohol: A key step in new low-temperature methanol synthesis method

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study the reaction mechanism of the formate adsorbed species with ethanol to form the ethyl formate on Cu/ZnO catalyst surface in a novel low-temperature methanol synthesis process. The results indicate that the formate adsorbed species were firstly formed by CO/CO₂/H₂ adsorbed on Cu/ZnO catalyst, followed by rapid reaction with ethanol to form ethyl formate. It was found that the species reacted with formate adsorbed species were ethanol in gas phase rather than adsorbed ethoxy species. The reaction of the adsorbed formate species with ethanol on Cu/ZnO catalyst surface proceeded according to Rideal-type mechanism, not Langmuir–Hinshelwood mechanism.     

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.     

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.     

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.     

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.     

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.     

Simultaneous MS-IR Studies of Surface Formate Reactivity Under Methanol Synthesis Conditions on Cu/SiO<Subscript>2</Subscript>

The coverages and surface lifetimes of copper-bound formates on Cu/SiO₂ catalysts, and the steady-state rates of reverse water-gas shift and methanol synthesis have been measured simultaneously by mass (MS) and infrared (IR) spectroscopies under a variety of elevated pressure conditions at temperatures between 140 and 160 °C. DCOO lifetimes under steady state catalytic conditions in CO₂:D₂ atmospheres were measured by ¹²C–¹³C isotope transients (SSITKA). The values range from 220 s at 160 °C to 660 s at 140 °C. The catalytic rates of both reverse water gas shift (RWGS) and methanol synthesis are ~100-fold slower than this formate removal rate back to CO₂ + 1/2 H₂, and thus they do not significantly influence the formate lifetime or coverage at steady state. The formate coverage is instead determined by formate’s rapid production/decomposition equilibrium with gas phase CO₂ + H₂. The results are consistent with formate being an intermediate in methanol synthesis, but with the rate-controlling step being after formate production (for example, its further hydrogenation to methoxy). A 2–3 fold shorter life time (faster decomposition rate) was observed for formate under reactions conditions, with both D₂ and CO₂ present, than in pure Ar or D₂ + Ar alone. This effect, due in part to the effects of the coadsorbates produced under reaction conditions, illustrates the importance of using in situ techniques in the study of catalytic mechanisms. The carbon which appears in the methanol product spends a longer time on the surface than the formate species, 1.8 times as long at 140 °C. The additional delay on the surface is attributed in part to readsorption of methanol on the catalyst, thus obscuring the mechanistic link between formate and methanol.     

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.     

The promoting effect of alcohols in a new process of low-temperature synthesis of methanol from CO/CO2/H2

The catalytic promoting effects of eleven different alcohols, as reaction medium, on the synthesis of methanol from feed gas of CO/CO₂/H₂ on Cu/ZnO solid catalyst were investigated. Added alcohol altered the reaction route to realize a low-temperature synthesis method where formate was an intermediate. Many alcohols showed catalytic promoting effect for methanol formation at temperature as low as 443 K, remarkably lower than that in the present industrial ICI process.     

The effect of ZnO in methanol synthesis catalysts on Cu dispersion and the specific activity

The effect of ZnO in Cu/ZnO catalysts prepared by the coprecipitation method has been studied using measurements of the surface area of Cu, the specific activity for the methanol synthesis by hydrogenation of CO₂, and XRD. Although the Cu surface area increases with increasing ZnO content (0–50 wt%) as is generally known, the specific activity of the Cu/ZnO catalysts with various weight ratios of Cu:ZnO is greater than that of a ZnO-free Cu catalyst. These facts clearly indicate that the role of ZnO in Cu/ZnO catalysts can be ascribed to both increases in the Cu dispersion and the specific activity. The XRD results indicate the formation of a Cu–Zn alloy in the Cu particles of the Cu/ZnO catalysts, leading to the increase in specific activity. It is thus considered that the Cu–Zn surface alloy or a Cu–Zn site is the active site for methanol synthesis in addition to metallic copper atoms that catalyze several hydrogenation steps during the methanol synthesis. Furthermore, the advantage of the coprecipitation method through a precursor of aurichalcite is ascribed to both improvements in the Cu surface area and the specific activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Intrinsic kinetics of one-step dimethyl ether synthesis from hydrogen-rich synthesis gas over bi-functional catalyst

The reaction kinetics of the dimethyl ether synthesis from hydrogen-rich synthesis gas over bi-functional catalyst was investigated using an isothermal integral reactor at 220–260°C temperature, 3–7 MPa pressure, and 1,000–2,500 mL/g·h space velocity. The H₂/CO ratio of the synthetic gas was chosen between 3 : 1 and 6 : 1. The bi-functional catalyst was prepared by physically mixing commercial CuO/ZnO/Al₂O₃ and γ-alumina, which act as methanol synthesis catalyst and dehydration catalyst, respectively. The three reactions, including methanol synthesis from CO and CO₂ as well as methanol dehydration, were chosen as independent reactions. The Langmuir-Hinshelwood kinetic models for dimethyl ether synthesis were adopted. Kinetics parameters were obtained using the Levenberg-Marquardt mathematical method. The model was reliable according to statistical and residual error analyses. The effects of different process conditions on the reactor performance were also investigated.     

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.     

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.     

Structural change of Cu/ZnO by reduction of ZnO in Cu/ZnO with methanol

The reducibility of ZnO was investigated in the temperature range of 523–623 K in a stream of a reducing agent such as H₂, CO, and methanol. ZnO was reduced only in the presence of copper in the vicinity of ZnO with CO and methanol, but it was not reduced with H₂. Methanol was a stronger reducing agent in the reduction of ZnO than CO, while CO was stronger in the reduction of CuO than methanol. Two types of brass were observed resulting from the reduction of ZnO in the Cu/ZnO sample by XRD. Zanghengite brass started to be formed at 573 K in addition to α-brass which was observed at the temperature above 523 K in the temperature range of 523–623 K during the ZnO reduction with methanol. The carbon monoxide chemisorption showed that the copper surface areas decreased during the reduction of ZnO with methanol. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

Effects of space velocity on methanol synthesis from Co2/Co/H2 over Cu/ZnO/Al2O3 catalyst

The space velocity had profound and complicated effects on methanol synthesis from CO₂/CO/H₂ over Cu/ZnO/Al₂O₃ at 523 K and 3.0MPa. At high space velocities, methanol yields as well as the rate of methanol production increased continuously with increasing CO₂ concentration in the feed. Below a certain space velocity, methanol yields and reaction rates showed a maximum at CO₂ concentration of 5–10%. Different coverages of surface reaction intermediates on copper appeared to be responsible for this phenomenon. The space velocity that gave the maximal rate of methanol production also depended on the feed composition. Higher space velocity yielded higher rates for CO₂/ H₂ and the opposite effect was observed for the CO/H₂ feed. For CO₂/CO/H₂ feed, an optimal space velocity existed for obtaining the maximal rate.     

In situ IR studies on the mechanism of methanol synthesis from CO/H2 and CO2/H2 over Cu-ZnO-Al2O3 catalyst

Methanol synthesis from CO/H₂ and CO₂/H₂ was carried out at atmospheric pressure over Cu/ZnO/Al₂O₃ catalyst. The formation and variation of surface species were recorded by in situ FT-IR spectroscopy. The result revealed that both CO and CO₂ can serve as the primary carbon source for methanol synthesis. For CO/H₂ feed gas, only HCOO-Zn was detected; however, for CO₂/H₂, both HCOO-Zn and HCOO-Cu were observed, and without CH₃O-Cu. HCOO-Zn was the key intermediate. A scheme of methanol synthesis and reverse water-gas shift (RGWS) reaction was proposed.     

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.