Diffuse reflectance infrared Fourier transform spectroscopy of adsorbed carbon monoxide is used along with X-ray absorption spectroscopy to study the effect a second alloying metal (Zn, Cu) has on the electronic state and local structure of rhodium on the surfaces of Rh/HZSM-5 zeolite catalyst. It is established that introducing copper and zinc helps improve the stability of rhodium toward aggregation (the formation of clusters) under conditions of the oxidative carbonylation of methane into acetic acid. Compared to monometallic catalyst Rh/HZSM-5, where single atom rodium sites are partially aggregated into clusters, the proportion of Rh° is halved in the case of Rh–Zn/HZSM-5, and Rh clustering does not occur in the case of Rh‒Cu/HZSM-5. The stabilizing effect of Cu is due to the interaction between copper and rhodium cations on the surface of zeolite.

In a slurry reactor, the formation of compaction products on Mg/HZSM-5 catalysts, irrespective of the SiO2 /Al2O3 molar ratio in the zeolite, proceeds predominantly on strong acidic sites. The composition of compaction products (mostly trimethyl- and tetramethylbenzenes) virtually does not change with a growth of the molar ratio; however, their amount decreases, which is related to a growth of the mesopore volume with an increase in the SiO2 /Al2O3 ratio. This leads to a decrease in diffusion limitations and contribution of secondary reactions and enhances the removal of coke precursors from the zeolite surface, thus promoting the catalyst activity (the conversion of DME increases twofold). The composition of reaction products changes only slightly with a growth of the SiO2 /Al2O3 molar ratio, the total selectivity to lower olefins is ca. 70 wt.%. A rapid loss of the Mg/HZSM-5 activity with extending the operation time in a slurry reactor is caused not by coking, but rather by «clogging» of the catalyst with decomposition products of the dispersion medium (polydimethylsiloxane).

Direct synthesis of liquid hydrocarbons from СО 2 and Н 2 on a combined bifunctional catalyst consisting of ZnAlO x or ZnCrO x (oxide catalysts for the synthesis of hydrocarbons from СО and Н 2 ) and HZSM-5 zeolites with different SiO 2 /Al 2 O 3 molar ratios was studied. The physicochemical characteristics of the zeolites, namely, acidity evaluated by temperature-programmed ammonia desorption, porosity, and specific surface area, were examined. The catalyst performance was studied on a micro-pilot installation in the flow recirculation mode at 340°С and a pressure of 10 MPa. The ZnAlO x /ZnZSM-5(40) catalyst exhibits the highest selectivity to С 5+ hydrocarbons, which is associated with the presence of strong Brønsted acid sites on its surface.

The review integrates and systematizes literature data on the use of single-atom catalysts in methane chemistry, with the emphasis on the most recent results. The single-atom catalysts are heterogeneous catalysts of the latest generation in which single metal atoms supported on an inorganic material act as the active sites. The features of CH4 activation on the surface of these catalysts are considered and compared with the behaviour of other known heterogeneous catalysts containing metal nanoclusters or active metal nanoparticles in relation to the direct oxidative and non-oxidative conversion of methane into various chemicals. The efficiency of application of single-atom catalysts of various compositions and types (supported single metal atoms, mono- and polymetallic monoatomic contacts, complex disperse compositions, etc.) is considered for reactions involving methane, including dry, steam and oxidative reforming, partial oxidation to methanol, oxidative carbonylation and carboxylation to acetic acid, non-oxidative and oxidative methane coupling to ethane/ethylene, methane dehydroaromatization, and methylation of benzene with methane. The new prospects opened up in methane chemistry by using single-atom catalysts are discussed. The bibliography includes 307 references.

Oxidative conversion of methane to С2 hydrocarbons using a mixture of CO2 with a small amount of O2 as an oxidant was studied. The use of zeolite catalysts with monoatomic rhodium distribution allows the reaction to be performed at low temperatures and pressures (380–450°С, 0.1–3.0 MPa) in the gas-phase mode. When performing the oxidative conversion of methane in the flow-through mode, the use of the monoatomic catalytic system containing an additional doping component (Zn, Cu, Mg) allows the ethane yield to be increased by 60

Zeolite catalysts for the conversion of dimethyl ether to light olefins with a monoatomic distribution of rhodium are studied via infrared spectroscopy of the diffuse reflection of adsorbed carbon monoxide and X-ray absorption spectroscopy. The zeolite is preliminarily treated with ultrasound to obtain a monatomic distribution of the active component on the support’s surface, and a polymer (chitosan hydrochloride) is used as the medium for dispersing rhodium at the stage of impregnation. A sample prepared via the traditional impregnation of zeolite with an aqueous solution of rhodium chloride is studied for purposes of comparison. It is shown that rhodium in the structure of zeolite treated with ultrasound is in the form of isolated metal centers whether it is deposited with or without a polymer. Synthesis with chitosan results in a more disperse distribution of rhodium on the outer surface of the zeolite and greater oxidizing ability of the catalyst.

The paper shows that introducing rhodium into zeolite significantly enhances the catalytic performance of the system. Rh/IK(UST), a catalyst sample distinguished by an increased concentration of strong acid sites, exhibited the highest activity and selectivity towards acetic acid (AA). In the presence of this catalyst, the yield of AA more than doubled, and the AA/methanol ratio increased by more than an order of magnitude, compared to the other Rh-modified catalysts. A synergistic effect of strong Brønsted acid sites and Rh single-atom sites was found. Their proximity plays a key role in this mechanism.

The influence of the method used for introduction of Rh into a ZSM-5 zeolite-based catalyst and on its physicochemical and catalytic properties exhibited in dimethyl ether conversion to light olefins was evaluated. Use of chitosan as a medium for rhodium dispersing affords a 10% increase in the dimethyl ether conversion while maintaining the selectivity for light olefins at a level of 75%, which is most likely due to location of rhodium predominantly on the zeolite surface, as well as to its high dispersion. The method of rhodium introduction also affects the amount of the formed coke deactivating the catalysts. Through the use of chitosan as a medium for rhodium dispersing and of a rotary evaporator in the zeolite impregnation stage the amount of the coke on the catalyst surface was reduced. The presence of chitosan as a polymer matrix during modification of the ultrasonically treated zeolite leads to a 6% decrease in the yield of the target reaction products (ethylene and propylene), which is associated with increased coke formation.

Zinc-isomorphously substituted HZSM-5 (Zn/HZSM-5(iso)) surpasses Zn/HZSM-5(i.e) sample obtained by ion exchange in its catalytic performance in the conversion of a mixture of DME + syngas (in different compositions) to liquid hydrocarbons. The uniform distribution of highly dispersed zinc species over zeolite crystals provides high selectivity of Zn/HZSM-5(iso) to liquid hydrocarbons (90 wt%). New ZnOH+ active sites of medium strength with mild hydrogenation properties explain the low yield of arenes (4.7 wt%) and the high yield of i-alkanes (72.5 wt%). The absence of methanol in the product stream indicates its dehydration to DME, which is known to have a higher reactivity or its participation in the methylation of alkenes to form methyl-substituted alkanes. A larger mesopores volume in Zn/HZSM-5(iso) promotes a lower degree of coke precursors polycondensation. Therefore, this allows the use of mild oxidative regeneration conditions. The Zn/HZSM-5(iso) catalyst retains its efficient and stable operation for 72 h, with high conversion of DME and high selectivity for liquid hydrocarbons. The resulting hydrocarbon mixture can be used as a base component for environmentally friendly fuels.

In the production of light olefins from dimethyl ether, a nanozeolite catalyst modified with lanthanum and zirconium exhibited lower activity and selectivity than a magnesium-loaded zeolite catalyst. This is due to a higher percentage of strong acid sites in La–Zr/HZSM-5, which promoted secondary reactions and decreased the selectivity towards light olefins. Furthermore, the difference in the catalytic properties of the samples modified with Mg and La–Zr can be explained by the different concentrations of the methanol product: the methanol content was markedly higher in the presence of La–Zr/HZSM-5 than with Mg/HZSM-5. To gain better insight into the role of methanol in the synthesis of light olefins from dimethyl ether, the relative activity of Brønsted acid sites on the catalyst surface in the conversion of methanol and dimethyl ether was comparatively assessed using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It was demonstrated that at high temperatures (above 260°C) the conversion of both DME and methanol occurs by the oxonium–ylide mechanism. However, there are differences in the formation routes of primary intermediates: formaldehyde from methanol, and ketene from dimethyl ether.

Recent achievements in the development of new methods for producing acetic acid (AA) from methane using heterogeneous catalysts are summarized and systematized. Modern heterogeneous-catalytic processes of methane conversion to AA via syngas and alternative one- and two-step AA production procedures via “low-temperature” oxidative methane conversion (via oxidative coupling, oxyhalogenation, oxidation into methanol, or oxidative transformations of СН 4 in the presence of carbon oxides) are considered. The major attention is paid to the one-step AA synthesis by methane oxydation with carbon dioxide (by carboxylation reaction). Specific features of heterogeneous catalysts recently developed for this reaction are discussed.

BACKGROUND The goal of this study was to investigate the deactivation of ZSM-5 in the DTO process (dimethyl ether to light olefins) in a slurry reactor (SLR) in polydimethylsiloxane dispersion medium compared with its deactivation in a fixed-bed reactor (FBR), and to unveil reasons behind the rapid deactivation of the catalyst in the SLR. RESULTS Under suspension conditions coke formation proceeds more slowly compared with fixed-bed conditions: The amount of coke formed under suspension conditions is less than under fixed-bed conditions, 1.4 mg versus 1.7 mg, respectively. Also, in terms of the chemical composition the coke is lighter (xylenes and trimethylbenzenes versus C4,5,6-substituted benzenes). Using thermogravimetric analysis and gas chromatography/mass spectrometry pyrolytic system, it was shown that the decomposition products of the dispersion medium contribute to the blocking of micropores and the rapid deactivation of the catalyst in SLR. The localization of coke in both SLR and FBR occurs primarily in the microporous channels of the zeolite (>60%). In SLR, the weight fraction of coke on the external catalyst surface was 11% higher than that on the FBR. This finding can be attributed to the blocking of micropores by decomposition products of the dispersion medium. CONCLUSION Under suspension conditions, coke formation in the DTO process proceeds more slowly compared with fixed-bed conditions. The rapid loss of catalyst activity in the SLR was related not to coke formation, but instead to the blocking of catalyst pores by products of thermal decomposition of the dispersion medium. For the same reason, in SLR the weight fraction of coke on the external surface of the catalyst was higher than that in the FBR. (c) 2021 Society of Chemical Industry (SCI).

Nanosized zeolites prepared by in situ seed-induced synthesis and samples prepared by ion exchanging commercial zeolites were successfully synthesized and modified with zinc. Their catalyst performance was investigated in the dimethyl ether aromatization at high pressures (up to 10 MPa) and various WHSV. The use of in situ seed-induced zeolite leads to a higher yield of arenes (39.1%) against 35.5% over the conventional sample. Zinc modification reduces the total yield of aromatics (by 22%), primarily BTX arenes (by 60%), compared to the parent zeolite. Such behavior correlates with a decrease in acidity and B/L ratio after modification.

Oxidative conversion of methane to acetic acid at 150°С and a pressure of 6.5 MPa with the assistance of carbon monoxide on rhodium-modified ZSM-5 zeolites with different SiO2/Al2O3 molar ratios was studied. A decrease in SiO2/Al2O3 from 300 to 30 leads to a 2.7-fold increase in the acetic acid yield. Such behavior of the catalytic system correlates with an increase in the acidity, primarily with an increase in the density of Brønsted acid sites on the zeolite surface. Introduction of 0.1–0.5 wt % rhodium into the zeolite increases the acetic acid yield by a factor of more than 3. The highest acetic acid yield (0.53 mmol/gcat) is attained on the zeolite with SiO2/Al2O3 = 33 modified with 0.5 wt % Rh. The methane transformation to acetic acid occurs while involving both the acid sites of the zeolite and the sites formed by reactive rhodium particles.

The effect of radiation-chemical treatment of the Rh*chitosan composite on the morphology, as well as on the textural and catalytic properties of the catalytic systems based on HZSM-5 zeolite modified with rhodium and chitosan was evaluated. A transmission electron microscopic examination showed that radiation-chemical reduction led to the formation of smaller rhodium nanoparticles in the Rh*chitosan composite and to their finely dispersed distribution over the zeolite catalyst surface, responsible for enhancement of the activity and operational stability of the catalyst in dimethyl ether conversion to light olefins.