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.     

甲酸甲酯合成工艺评述

介绍了合成甲酸甲酯的六种工艺方法 ,即直接酯化法、甲醇羰基化法、甲醇脱氢法、甲醛二聚法、合成气直接合成法及二氧化碳与甲醇加氢缩合法 ,并分别进行了评述。其中已经工业化的有直接酯化法、甲醇羰基化法和甲醇气相催化脱氢法 ,另三种方法正在研究中。认为甲醇液相脱氢法具备传统气相法的所有优点 ,且反应温度低 ,能耗少 ,甲醇转化率高 ,产率亦高 ,是今后研究的主要方向     

Reaction mechanism of methyl nitrite dissociation during co catalytic coupling to dimethyl oxalate: A density functional theory study

Dissociation of methyl nitrite is the first step during CO catalytic coupling to dimethyl oxalate followed by hydrogenation to ethyl glycol in a typical coal to liquid process. In this work, the first-principle calculations based on density functional theory were performed to explore the reaction mechanism for the non-catalytic dissociation of methyl nitrite in the gas phase and the catalytic dissociation of methyl nitrite on Pd(111) surface since palladium supported on alpha-alumina is the most effective catalyst for the coupling. For the non-catalytic case, the calculated results show that the CH_3O–NO bond will break with a bond energy of 1.91 eV, and the produced CH_3O radicals easily decompose to formaldehyde, while the further dissociation of formaldehyde in the gas phase is difficult due to the strong C–H bond. On the other hand, the catalytic dissociation of methyl nitrite on Pd(111) to the adsorbed CH_3O and NO takes place with a small energy barrier of 0.03 eV. The calculated activation energies along the proposed reaction pathways indicate that(i) at low coverage, a successive dehydrogenation of the adsorbed CH_3O to CO and H is favored while(ii) at high coverage, hydrogenation of CH_3O to methanol and carbonylation of CH_3O to methyl formate are more preferred. On the basis of the proposed reaction mechanism,two meaningful ways are proposed to suppress the dissociation of methyl nitrate during the CO catalytic coupling to dimethyl oxalate.     

Reaction pathways in methanol oxidation: kinetic oscillations in the copper/oxygen system

Polycrystalline copper was used as catalyst for the selective oxidation of methanol under stoichiometric reaction conditions for oxidehydrogenation. Temperature- programmed reaction spectroscopy (TPRS) revealed a broad temperature range of reactivity with two distinct maxima for the production of formaldehyde. Phase analysis with thermogravimetry (TG) and powder X-ray diffraction (XRD) under in situ conditions showed that a phase change occurred between the two maxima for formaldehyde production from bulk Cu₂O to metallic copper. Strongly adsorbed methoxy and formate were detected by X-ray photoelectron spectroscopy (XPS) after prolonged catalytic use. A sub-surface oxygen species and surface OH were identified by XPS. A region of oscillatory behaviour was found in the temperature interval between 623 and 710 K. Multicomponent gas analysis of the reaction products with an ion-molecule reaction mass spectrometer (IMR-MS) allowed to derive a reaction sequence in which both methoxy and formate are necessary as surface species. The most selective state of the catalyst for oxidehydrogenation is the co-adsorption system methanol-oxygen. Oxidation of the surface by excess molecular oxygen leads to total oxidation. The catalyst is finally reduced by excess methanol into an inactive pure metallic form. Sub-surface oxygen segregates to the surface and initiates the activity again by enhancing the sticking coefficient for gas phase species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Low-Temperature Methanol Synthesis in Catalytic Systems Composed of Copper-Based Oxides and Alkali Alkoxides in Liquid Media: Effects of Reaction Variables on Catalytic Performance

Catalytic systems composed of copper-based oxides and alkali alkoxides are tested for low-temperature methanol synthesis in liquid phases, which involves carbonylation of methanol to methyl formate and consecutive hydrogenation of methyl formate to methanol. The effects of reaction variables on the catalytic performance are investigated under the conditions of 373-423K temperature and 1.5-5.0 Mpa pressure. The combined catalytic systems of copper chromite and potassium methoxide exhibit excellent activities for the production of methanol. Higher values of reaction temperature, initial pressure, catalyst loading, and H₂/CO ratio of the feed gas lead to higher methanol productivity. In particular, the reaction temperatures and the feed gas compositions strongly influence the catalytic performance. No methanol is formed when employing a feed gas containing 2% CO₂. The catalytic systems are deactivated in a short period even in a CO₂-free feed gas, due to the consumption of the alkoxide component. Alkali alkoxides are consumed through reactions with trace amounts of CO₂ and H₂O which are formed as by-products during the course of the runs. The results also suggest that the hydrogenation step of methyl formate over copper chromite is greatly accelerated in the presence of the alkali alkoxide.     

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.     

Methanol dehydrogenation in the liquid phase with Cu-based solid catalysts

Studies of methanol dehydrogenation in the liquid phase with Cu-based solid catalysts have shown (i) the product (methylal) is totally different from that in the gas-solid phase reaction (methyl formate) even using the same catalyst, and (ii) the differences in the calcination atmosphere of the catalyst (N₂ versus dry air) have a marked effect on the product selectivity (methylal versus methyl formate). These results are in contrast to the gas-phase reaction (com-monly methyl formate). he results are discussed on the basis of surface species characterized with XPS and XRD.     

Vapor phase dehydrogenation of methanol to methyl formate in the catalytic membrane reactor with Cu/SiO2/ ceramic composite membrane

The Cu/SiO₂/ceramic composite membrane was prepared on the SiO₂/ceramic mesoporous membrane by an ion exchange method, and vapor phase dehydrogenation of methanol to methyl formate in the catalytic membrane reactor was investigated. It showed much better performance in the catalytic membrane reactor than that in the fixed-bed reactor under the same reaction conditions. At 240 °C, 57.3% conversion of methanol and 50.0% yield of methyl formate were achieved in the catalytic membrane reactor and only 43.1% conversion of methanol and 36.9% yield of methyl formate were achieved in the fixed-bed reactor.     

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.     

Dehydrogenation of methanol over copper-containing catalysts

A comparative study of copper-containing catalysts with different chemical and phase compositions is performed to determine conditions for the implementation of the vapor phase and highly selective dehydrogenation of methanol to methyl formate or syngas. A thermodynamic analysis of the reaction is also performed. It is shown that Cu⁰ nanoparticles formed in the course of reductive activation reveal different selectivities with respect to the formation of methyl formate from methanol or its dehydrogenation with formation of syngas. By correctly selecting the catalyst composition and process conditions, high (90–100%) selectivity with respect to either methyl formate or syngas can be attained. Catalysts based on Cu–Zn hydrosilicate of the zincsilite type and on CuAlZn aurichalcite are highly selective in the process of methyl formate formation. An estimation based on experimental data shows that the productivity of Cu/SiO₂ catalyst, the one most effective in dehydrogenation to syngas, is as high as 20 m³/h of syngas at a methanol vapor pressure of 1 atm, a temperature of 200°C, and a contact time of 0.5 s.     

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.     

Low Temperature Water–Gas Shift/Methanol Steam Reforming: Alkali Doping to Facilitate the Scission of Formate and Methoxy C–H Bonds over Pt/ceria Catalyst

Doping Pt/ceria catalysts with the Group 1 alkali metals was found to lead to an important weakening of the C–H bond of formate and methoxy species. This was demonstrated by a shift to lower wavenumbers of the formate and methoxy ν(CH) vibrational modes by DRIFTS spectroscopy. Li and Na-doped Pt/ceria catalysts were tested relative to the undoped catalyst for low temperature water–gas shift and methanol steam reforming using a fixed bed reactor and exhibited higher catalytic activity. Steaming of formate and methoxy species pre-adsorbed on the catalyst surface during in-situ DRIFTS spectroscopy suggested that the species were more reactive for dehydrogenation steps in the catalytic cycle for the Li and Na-doped catalysts relative to undoped Pt/ceria. However, with increasing atomic number over the series of alkali-doped catalysts, the stability of a fraction of the carbonate species was found to increase. This was observed during TPD-MS measurements of the adsorbed CO₂ probe molecule by a systematic increase of a high temperature peak for a fraction of the CO₂ desorbed. This result indicates that alkali-doping is an optimization problem—that is, while improving the dehydrogenation rates of methoxy and formate species, the carbonate intermediate stability increases, making it difficult to liberate the CO₂. Infrared spectroscopy results of CO adsorbed on Pt and ceria suggest that the alkali dopant is located on, and electronically modifies, both the Pt and ceria components. The results not only lend further support to the role that methoxy and formate species play as intermediates in the catalytic mechanisms, but also provide a path forward for improving rates by means other than resorting to higher noble metal loadings.     

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.     

New Supported Pd and Pt Alloy Catalysts for Steam Reforming and Dehydrogenation of Methanol

The catalytic performances of supported Group 810 metal (Co, Ni, Ru, Pd, Ir and Pt) catalysts for steam reforming of methanol, CH₃OH + H₂O CO₂ + 3H₂, and dehydrogenation of methanol to methyl formate, 2CH₃ OH HCOOCH₃ + 2H₂, are markedly affected by the kinds of supports as well as the metals used. The selectivity for steam reforming and the formation of methyl formate was markedly improved when Pd or Pt were supported on ZnO, In₂O₃ and Ga₂O₃. The combined results of temperature-programmed reduction, XRD, XPS and AES revealed that Pd-Zn, Pd-In, Pd-Ga, Pt-Zn, Pt-In and Pt-Ga alloys were formed upon reduction. Over the catalysts having an alloy phase, the reactions proceeded selectively, whereas over the catalysts having a metallic phase, methanol was decomposed to carbon monoxide and hydrogen predominantly. It was shown that the reactivity of formaldehyde intermediate over the Pd and Pt alloys was markedly different from that over metallic Pd and Pt. Over Pd and Pt alloys, aldehyde species were stabilized and transformed into carbon dioxide and hydrogen or methyl formate by nucleophilic addition of water or methanol, respectively. By contrast, over metallic Pd and Pt, aldehyde species were rapidly decarbonylated to carbon monoxide and hydrogen.     

Formaldehyde,methanol and methyl formate from formic acid reaction over supported metal catalysts

The catalytic gas-phase decomposition of formic acid was studied over Au/Al₂O₃ and Pt/SBA-15 to investigate the formation of products other than H_2, CO₂, CO and H₂O. Formaldehyde, methanol and methyl formate were detected and identified as secondary products. Two calculation methods for H₂ selectivity (direct and indirect) were compared and assessed in terms of their validity at different temperatures, establishing that a direct quantification of H₂ is necessary for correct results. Based on selectivity trends of all the detected products a reaction scheme is proposed for the decomposition of formic acid and formation of formaldehyde, methanol and methyl formate.     

Methanol oxidation over model cobalt catalysts: Influence of the cobalt oxidation state on the reactivity

X-ray photoelectron and absorption spectroscopies (XPS and XAS) combined with on-line mass spectrometry were applied under working catalytic conditions to investigate methanol oxidation on cobalt. Two cobalt oxidation states (Co₃O₄ and CoO) were prepared and investigated as regards their influence on the catalytic activity and selectivity. In addition adsorbed species were monitored in the transition of the catalyst from a non-active state, to an active one. It is shown that the surface oxidation state of cobalt is readily adapted to the oxygen chemical potential in the CH₃OH/O₂ reaction mixture. In particular, even in oxygen-rich mixtures the Co₃O₄ surface is partially reduced, with the extent of surface reduction following the methanol concentration. The reaction selectivity depends on the cobalt oxidation state, with the more reduced samples favouring the partial oxidation of methanol to formaldehyde. In the absence of oxygen, methanol effectively reduces cobalt to the metallic state, also promoting H₂ and CO production. Direct evidence of methoxy and formate species adsorbed on the surface upon reaction was found by analysing the O 1s and C 1s photoelectron spectra. However, the surface coverage of those species was not proportional to the catalytic activity, indicating that they might also act as reaction inhibitors.     

Temperature-programmed surface reactions of methanol on commercial Cu-containing catalysts

Temperature-programmed reaction spectroscopic studies reveal two main transformation routes of methanol adsorbed on commercial Cu-containing catalysts. First the reverse methanol synthesis reaction (hydrolysis) CH₃OH+H₂O=CO₃+3H₂; a second route is not connected with CH₃OH synthesis and it includes bimolecular interaction of methanol giving methyl formate. The conversion of the latter compound results in the formation of CO, and other intermediates often postulated in methanol synthesis.     

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.     

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.     

Modeling of surface intermediates under forced periodic conditions applied to CO2 methanation

Forced oscillations of the carbon dioxide concentration in a hydrogen feed were applied to the carbon dioxide methanation reaction with simultaneous gas phase and surface analysis. The response of the gas phase and surface species to these repeated dynamic conditions show a time shift following the sequence CO₂ → formate → CO → methane, but can be modeled by a simple model involving one single adsorbed surface intermediate and a reservoir in equilibrium with this intermediate. The dynamic behavior of this intermediate is very close to the variation of the IR absorption band associated with formate and carbonate species.