The physicochemical and catalytic (CO2 hydrogenation) characteristics of Mo-containing catalysts were studied. The catalysts containing 8 and 15 wt

The physicochemical and catalytic (CO₂ hydrogenation) characteristics of Mo-containing catalysts have been studied. Catalysts with an oxide content of Mo 8 and 15 wt% were prepared by impregnation with ammonium paramolybdate γ-Al₂O₃ followed by drying and calcining at 500°C. The introduction of Mo oxide reduces the pore volume of the support and increases their average size, which indicates the distribution of the deposited molybdenum oxide in the pores of the support. According to X-ray diffraction data, the calcined catalyst contains practically no crystalline MoO₃ phase. According to the Raman spectra, oxygen-containing formations are present on the catalyst surface, in which Mo atoms are tetrahedrally and octahedrally coordinated with respect to oxygen atoms. The impregnated MoO₃ oxide is partially reduced by hydrogen during linear heating starting from 320°C. Hydrogenation of CO₂ (gas of composition, vol.%: 30.7 CO₂, 68 H₂, rest. N2, sample 0.5 g) was studied in the mode of linear heating up to 400°C. The main reaction is the reverse reaction of CO steam reforming. The contribution of the methanation reaction to CO₂ hydrogenation is small. An increase in temperature and pressure has a positive effect on CO₂ conversion. With an increase in pressure from 1 to 5 MPa, the CO content increases approximately twofold. In the hydrogenation of CO₂, γ-Al₂O₃, preheated in a flow of H₂ to 400°C, also exhibits noticeable activity, although significantly lower compared to Mo-containing catalysts. With increasing pressure, the activity of aluminium oxide and Mo-containing catalysts, increases.

The adsorption, temperature-programmed desorption, and carbonylation of dimethyl ether (DME) in the presence of mordenite and ferrierite (SiO2/Al2O3 ≈ 20, Zeolyst International) is studied. The effect of introducing Cu, Co, and Mg cations by ion exchange is discussed. Dimethyl ether carbonylation is conducted at 200°С, a pressure of 3 MPa, and a space velocity of 8000 mL g–1 h–1 in the following mixture (vol

Mordenite and ferrierite (Zeolyst International, H-form, SiO2/Al2O3 ≈ 20) were used to study the adsorption, thermally programmed desorption, and carbonylation of dimethyl ether (DME). The behavior of mordenite with Cu, Co, and Mg cations introduced by ion exchange has also been studied. DME carbonylation was carried out at 200°C, pressure 3 MPa, space velocity 8000 ml g–1 h–1 in a mixture, vol. %: ⁓2.2 DME, 92.8–95.5 CO, rest. N2. After the induction period, the methyl acetate content is about 4–5 times higher for mordenite compared to ferrierite. The formation of water, methanol and hydrocarbons was observed in small quantities. The introduction of Cu, Co, Mg cations into mordenite by ion exchange (single ion exchange, cation/Al ratio no more than 35%) not only increased the stability, but also increased the activity in the DME carbonylation reaction. It was found that an increase in the content of copper (from 1.19 to 2.23 wt %) and Mg (from 0.62 to 1.8 wt %) differently affects the activity. In the case of copper, an increase in activity was observed, while in the case of magnesium, the activity decreased. Preliminary reduction of copper-exchange mordenite leads to a decrease in activity and the appearance of metallic copper particles on the surface of mordenite crystallites. According to in situ diffuse reflectance infrared spectroscopy, the introduction of magnesium cations by triple ion exchange leads to a noticeable decrease in the number of Brønsted acid sites (BACs) in both the 12-MR and 8-MR channels of mordenite. The catalytic characteristics of ferrierite practically do not change when copper and magnesium are introduced by ion exchange.

The review analyzes the specific features of methyl acetate synthesis by dimethyl ether carbonylation in the presence of zeolite catalysts. The structural characteristics of zeolites, in particular, mordenite and ferrierite, that affect carbonylation are discussed. The bridging hydroxyl groups (Al–OH–Si) of zeolites function as Brønsted acid sites and interact with dimethyl ether. The carbonylation reaction is characterized by an induction period, during which dimethyl ether molecules interact with Brønsted acid sites to form methoxy groups. The incorporation of CO into the methoxy group leads to the formation of an acetyl intermediate. Methyl acetate is formed due to the interaction between a dimethyl ether molecule and an acetyl intermediate. The reaction is facilitated by the confinement effect characteristic of small zeolite pores, in particular, eight-membered pockets of mordenite. Methyl acetate is synthesized at moderate temperatures (about 200°C) in a CO–dimethyl ether mixture at a high selectivity and a significant dimethyl ether conversion. The occurrence of hydrocarbon formation side reactions, along with the methyl acetate synthesis target reaction, is observed; the side reactions decrease the on-stream stability of the catalyst. In the case of mordenite, these reactions are attributed to the sites present in the twelve-membered channels. The on-stream stability of the catalysts can be increased by using special techniques for neutralizing the deactivating sites of the zeolite. The effect can be achieved by pyridine adsorption or ion exchange of the respective protons of the mordenite structure for organic or metal cations. The introduction of zinc ions and, additionally, copper ions significantly inhibits the activity of the mordenite sites on which the side reactions occur. A new area of research is the use of multifunctional catalysts that mediate methyl acetate synthesis directly from synthesis gas.

A precursor CuO/ZnO/Al2O3 catalyst for methanol synthesis has been prepared at room temperature by introducing a ternary salt solution into the excess of sodium carbonate solution following the reverse co-precipitation method. The catalyst was tested for the synthesis of methanol from synthesis gas. The composition (vol. %) has been presented: CO, 22; CO2, 5.8; N2, 5.5; H2, balance. The methanol productivity was recorded to be 2.7 kg kgcat–1 h–1 at a temperature of 260 °С, a pressure of 3 MPa, and a space velocity of 61,700 l (kgcat)–1 h–1. The possibility of regenerating the activity of a catalyst subjected to conditions of artificial aging (overheating in a syngas environment) has been tested: approximately 92% of the initial activity could be restored. Physicochemical studies were conducted using the thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), the scanning electron microscopy (SEM), and the X-ray diffraction (XRD) techniques using the Cu, Zn, and Al oxides obtained following the co-precipitation method. X-ray studies revealed that an amorphous phase was obtained from the Al oxide that was synthesized following both the direct and reverse co-precipitation methods. The calcination temperature was 300 °C. Individual oxides of Cu and Zn obtained following the reverse co-precipitation method form crystalline phases when calcined at 300 °C. When all three ingredients are present, calcination of the sample at 300 °C helps obtain an X-ray amorphous structure of the catalyst characterized by a high specific surface area. High-temperature carbonates are formed from samples prepared using a ternary mixture of nitrate salts when these are calcined at 300 °C.

A ZnO/Al2O3 catalyst synthesized by impregnating gamma-alumina from a zinc nitrate solution and calcination at 400°C is studied in the hydrogenation of carbon oxides. During heating in a stream of a 2.4% H2/N2 mixture to 400°С, ZnO undergoes partial reduction. The activity of the reduced catalyst is studied in a range of 300–400°С at 5 MPa and a space velocity of 6000 NL $${\text{kg}}_{{{\text{cat}}}}^{{ - 1}}$$ h−1. The main product of CO hydrogenation is methanol. In addition, the methanol dehydration and CO methanation reactions occur. Water formed during methanol dehydration provides the formation of CO2 via the CO steam reforming reaction. With an increase in temperature from 300 to 400°C, the selectivity for oxygenates (methanol and dimethyl ether, in terms of methanol) decreases from ~74 to 56%, while the selectivity for hydrocarbons (methane, ethane, ethylene, propane) increases from 1 to 14%. The main products of CO2 hydrogenation are CO and H2O. The formation of oxygenates and a small amount of methane, in addition to CO, is observed. Water formed in a significant amount during CO2 hydrogenation adversely affects the dehydration of methanol. In methanol synthesis at 240°C, the catalyst exhibits an insignificant activity in the case of using H2/CO and almost no activity in the case of H2/CO2. Data on CO and CO2 hydrogenation in the presence of ZnO/Al2O3 are consistent with the results for precipitated ZnO. In addition, at a pressure of 3 or 5 MPa and a temperature of 344 or 364°C, the content of oxygenates in the case of CO hydrogenation is 4–5 times higher than that in the case of CO2 hydrogenation. Analysis of the dependence of the relative selectivity for oxygenates on the contact time leads to the conclusion that, in the presence of zinc oxide, methanol is formed from both CO and CO2.

Recent publications on the mechanism of methanol synthesis on Cu-containing catalysts were critically analyzed. The following mechanisms can be distinguished based on the key intermediates: formate, carbonate, carboxyl, and formyl. A stepwise mechanism of conversion of CO2 and CO into methanol was proposed taking into account the available experimental and calculated data. According to this mechanism, hydrogenation of CO2 starts with interaction of a CO2 molecule with dissociated hydrogen chemisorbed on the copper surface, forming monodentate formate, which easily transforms into bidentate formate. Subsequent hydrogenation of bidentate formate through a series of intermediates forms methanol. Another possible route of CO2 conversion in the presence of dissociated hydrogen is formation of carboxyl, which is converted into methanol via several intermediates, including formyl. If CO is present in syngas, its role is to remove the OH groups from the surface via the surface carboxyl (*COOH). Then carboxyl can undergo the following transformations: decomposition to СО2 and Н* and hydrogenation via formyl to methanol. The appearance of carbonate intermediates on the copper surface observed by IR spectroscopy at low pressures is not related to the mechanism of methanol synthesis. Some of the results of experiments obtained in the study of transition states require additional studies.

The production of dimethyl ether from synthesis gas involves the use of bifunctional catalysts, which mediate the synthesis and dehydration of methanol. The effect is provided by the presence of a methanol synthesis component and a dehydration component in the catalyst; the components can be synthesized independently. The published data on the synthesis of a CuО/ZnО/Al2О3 methanol synthesis catalyst by precipitation from a nitrate solution are conflicting; therefore, it is relevant to study the effect of the procedure and conditions for the precipitation of copper, zinc, and aluminum nitrate solutions with sodium carbonate on the activity of the above catalyst. Data on the phase composition of the precipitates after a heat treatment and the activity of the synthesized catalysts in methanol synthesis suggest that the presence of nitrate groups in the precipitates leads to a decrease in catalytic activity. A method to prepare a methanol synthesis catalyst at room temperature by introducing a solution of copper, zinc, and aluminum salts into a sodium carbonate solution (reverse coprecipitation) is proposed. This method provides the level of activity of the resulting samples directly in methanol synthesis that is close to the level of activity of a commercial methanol synthesis catalyst. Dimethyl ether synthesis in the presence of bifunctional catalysts, in which the methanol synthesis component is prepared by different methods and the dehydration component is commercial gamma-Al2O3, is studied. In the presence of the synthesized samples, the methanol selectivity is almost identical; however, the productivities with respect to oxygenates and dimethyl ether are significantly different.

The effect of the space velocity on the characteristics of bifunctional catalysts with different dehydrating components in the synthesis of methanol/dimethyl ether (DME) from syngas with a composition (vol %): CO 19.1, CO2 5.9, N2 5.5, H2 (the rest) at 260°C and 3 MPa was studied. The commercial catalyst of methanol synthesis MegaMax 507 (CuO/ZnO/Al2O3) was used as the methanol component of the catalyst; the dehydrating agent was active gamma-alumina and quartz glass (for comparison), which is inert in methanol dehydration. At space velocities of less than 20 000 L(kgcat h)–1, the CO conversion and oxygenate productivity (based on C1) for the samples with alumina are higher than for the samples with quartz glass. The effect was explained by the fact that at partial conversion of methanol into DME, the rate of the reverse reaction of methanol with water decreases. This ultimately leads to an increase in the yield of methanol. The preferability of the one-stage synthesis over the two-stage synthesis in the production of oxygenates depends on the load on gas, the difference in the productivity passing through a maximum at increased load on gas.

The adsorption of butene-1 on Beta zeolite (H form) is studied via flow-adsorption calorimetry. Upon feeding a mixture of 2 vol % of butene-1 in nitrogen over the pre-calcined zeolite (500°C) at room temperature, an exothermic effect is observed that is associated with the adsorption and transformations of butene, particularly its isomerization to cis- and trans-butenes-2. The thermal desorption of adsorbed butene‑1 results in formation of hydrocarbon products showing that oligomerization proceeds during adsorption. It is found that zeolite pretreated with moist nitrogen adsorbs water up to 9.2 wt %. A weak exothermic effect is observed when butene-1 is adsorbed on this rehydrated zeolite, due apparently to the physical adsorption of butene-1. When the rehydrated zeolite is held for long periods of time in a stream of a butene/nitrogen mixture, cis-butene-2 is detected at the reactor outlet, indicating the gradual replacement of water with butene-1 on the active sites of zeolite.

Zirconium oxide samples synthesized and modified with Al, Si, W oxides were studied as dehydrating components of bifunctional catalysts for synthesis of dimethyl ether. ZrO2 modified with Al and Si oxides is inactive in the dehydration of methanol, but its dehydrating activity becomes comparable with the activity of γ-Al2O3 upon introduction of WO3. According to the data on methanol adsorption at 46°C, the adsorption capacity of ZrO2 modified with SiO2 is comparable with the capacity of γ-Al2O3, and that of ZrO2 modified with WO3 is four times lower. The difference in the catalytic and adsorption behavior of the samples is accounted for by the data of in situ high-temperature diffuse reflection IR spectroscopy. A high content of weak Brønsted acid centers is observed on the ZrO2 surface, with the interaction of these with methanol resulting in the methoxylation of the catalyst surface. When W oxide is deposited onto zirconium oxide, the surface has strong Brønsted centers involved in the reaction of methanol dehydration. A coprecipitation from solution of nitrate salts of Cu, Zn, La with Na carbonate, followed by the procedures of filtration, washing, and thermal treatment, yielded a catalyst for synthesis of methanol, the activity of which is close to that of the industrial catalyst.

The influence of the flow rate on parameters of bifunctional catalysts with different dehydrating components in the synthesis of methanol/dimethyl ether (DME) from synthesis gas (19.1 vol.% CO, 5.9 vol.% CO2, 5.5 vol.% N2, rest H2) at 260 °C and 3 MPa was studied. The commercial catalyst Megamax 507 was used as the methanol agent and active γ-alumina as dehydrating agent, quartz glass, which is inert to methanol dehydration, being used for comparison. At flow rate below 20,000 L·(kgcat·h)–1, the conversion of CO and oxygenate capacity (calculated as C1) was higher over alumina-containing samples than over quartz glass. This observation was accounted for by a decrease in the rate of the reverse reaction of methanol with water at the partial conversion of methanol to DME to lead eventually to an increase in the methanol yield. The benefit of the one-stage synthesis against two-stage synthesis of oxygenates depended on the load: as the load increased, the difference in the capacity went through maximum.

Several bifunctional catalysts for dimethyl ether (DME) synthesis from syngas are prepared on the basis of commercial methanol-synthesis Megamax 507 catalyst. Commercial HZSM-5 zeolites with a SiО2/Al2О3 ratios of 23, 80, and 307 and γ-alumina were used as dehydration components. Physicochemical characteristics of zeolites and alumina are studied: temperature-programmed desorption of ammonia, the porosity, and the specific surface area. The activity of catalyst in DME synthesis is studied in a microcatalytic flow-type setup at a pressure of 3 MPa in a temperature range of 200–260°С with a productivity based on syngas of up to 30000 L $${\text{kg}}_{{{\text{cat}}}}^{{ - 1}}$$ h–1. The composition of syngas was (vol %): CO, 21; CO2, 6; Н2, 67; N2, 6 . It is shown that zeolites, especially with silica/alumina ratios of 23 and 80, are more active than alumina in methanol dehydration to DME, but in the presence of these zeolites, traces of hydrocarbons were detected at 260°С. The zeolite with a silica/alumina ratio of 307 is the most interesting of the studied zeolites. Hydrocarbons are almost not formed on it, and its activity in methanol dehydration is somewhat higher than that of alumina. The behavior of the methanol-synthesis component of the bifunctional catalyst is studied: the apparent activation energy of methanol synthesis and the degree of approaching to equilibrium are estimated depending on the catalyst load.

Heterogeneous catalytic reactions of dimethyl ether (DME) with various compounds (alkenes, aromatic compounds, CO, etc.) are surveyed. Analysis of published data allows the conclusion that the formation of products generally involves surface intermediates produced by the interaction of DME with Brønsted acid sites. There is no formation of water in this case, suggesting that DME can be preferred to methanol in some cases. Surface intermediates CH*3 which are bound to the oxygen atoms of the zeolite lattice (methoxides) and retain their reactivity in the case of temperature elevation to 473 K have been identified using IR, UV, and in situ high-resolution solid-state NMR spectroscopy. Based upon the data on the state of intermediates that are formed from DME on the surface of heterogeneous catalysts, a series of catalytic reactions involving DME, namely, methylation of alkenes and aromatic compounds, carbonylation, synthesis of ethanol, and partial oxidation resulting in a set of compounds have been considered. Some reactions, such as carbonylation of DME by synthesis gas, synthesis of ethanol, and synthesis of dimethoxymethane and polyoxymethylene dimethyl ether, are of industrial interest.