In situ FTIR study on the active oxygen species for the conversion of methane to methanol

In situ FTIR studies showed the generation of a peroxide species by the contact of a H₂-O₂ gas mixture or of N₂O with Fe-Al-P-O catalysts at 573 K. This oxygen species oxidized methane into methanol at 473 K, through the formation of methoxide species on the catalysts.     

Comparative studies on direct conversion of methane to methanol/formaldehyde over La–Co–O and ZrO2 supported molybdenum oxide catalysts

The selective oxidation of methane with molecular oxygen over MoO_x/La–Co–O and MoO_x/ZrO₂ catalysts to methanol/formaldehyde has been investigated in a specially designed high-pressure continuous-flow reactor. The properties of the catalysts, such as crystal phase, structure, reducibility, ion oxidation state, surface composition and the specific surface area have been characterized with the use of XRD, LRS, TPR, XPS and BET methods. MoO_x/La–Co–O catalysts showed high selectivity to methanol formation while MoO_x/ZrO₂ revealed the property for the formation of formaldehyde in the selective oxidation of methane. 7 wt MoO_x/La–Co–O catalyst gave 6.7 methanol yield (ca. 60 methanol selectivity) at 420°C and 4.2 MPa. On the other hand, the maximal yield of formaldehyde ca. 4 (47.8 formaldehyde selectivity) was obtained over 12wt MoO_x/ZrO₂ catalyst at 400 °C and 5.0MPa. 7MoO_x/La–Co–O catalyst showed higher modified H₂-consumption than 12MoO_x/ZrO₂ catalyst. The reducibility and the O^–/O^2– ratio of the catalysts may play important roles on the catalytic performance. The proper reducibility and the O^–/O^2– ratio enhanced the production of methanol in selective oxidation of methane. [MoO₄]^2– species in MoO_x/ZrO₂ catalysts enable selective oxidation of methane to formaldehyde.     

Methane partial oxidation by silica‐supported iron phosphate catalysts. Influence of iron phosphate content on selectivity and catalyst structure

Selective oxidation of methane to methanol and formaldehyde at atmospheric pressure was studied over a series of silicasupported FePO₄ catalysts, with iron phosphate content ranging from 2 to 16 wt%. Performance was evaluated over the range T=773–963 K, GHSV=25,000–65,000 h^–1, and CH₄ : O₂=1. The main products were formaldehyde, carbon monoxide and carbon dioxide. Small, but quantifiable amounts of methanol were also observed. Catalytic activity exhibited a clear dependence on the iron phosphate content. The highest selectivity and space time yield (STY) to formaldehyde and methanol were observed for 2 wt% FePO₄ on silica (STY of 622 and 25 g/kg_cat h, respectively). The selectivity–conversion pattern suggests that methane is oxidized directly to methanol and formaldehyde, and sequentially to carbon oxides. Characterization was performed by Xray powder diffraction, Xray photoelectron spectroscopy, and Mössbauer spectroscopy. Crystalline FePO₄ is observed at all loading levels, however, a significant fraction of the iron (58% at 2 wt% FePO₄) is present in an Xray amorphous phase. Mössbauer spectra suggest that this phase contains iron in fivefold coordination, and with a higher electron density relative to bulk FePO₄. The amount of this fivecoordinate phase present is roughly 1 wt% Fe, independent of total iron loading. XPS confirms the lower effective oxidation state of iron, and indicates that at low loading the surface is enriched in phosphorus relative to bulk FePO₄. It is proposed that iron in fivefold coordinate sites, isolated by phosphate groups, more selectively activates methane than crystalline FePO₄. As loading increases, so does the amount of crystalline FePO₄, which is proposed to more rapidly catalyze sequential oxidation of the selective products.     

Direct conversion of methane to higher hydrocarbons via an oxygen free,low-temperature route

The direct conversion of methane to higher hydrocarbons over a silica-supported Ru catalyst has been investigated via an oxygen free, two-step route. The reaction consists of decomposition of methane over a 3% silica-supp orted Ru catalyst at temperatures between 400 and 800 K to produce surface carbonaceous species followed by rehydrogenation of these species to higher hydrocarbons at of 368 K. It was found that the Ru/SiO₂ catalyst exhibits a trend similar to that for single-crystal Ru catalysts. However, the temperature at which a maximum in ethane selectivity occurs shifts toward a higher temperature. It was also found that the ethane yield can be optimized by changing the surface carbon coverage. Under optimum conditions a net ethane yield of about 13–15% has been realized. For this two-step reaction sequence, only a few reaction cycles could be operated without intermediate high temperature rehydrogenation and without significant loss in ethane yield. This is attributed to large amounts of inactive carbon that could not be hydrogenated at 368 K. Higher methane partial pressures were found to be desirable for this reaction. The activity of the catalyst could also be maintained at total pressures up to 10 atm.     

Interaction of methane with surfaces of silica,aluminas and HZSM-5 zeolite. A comparative FT-IR study

Infrared investigations on the interaction of methane with silica, aluminas (, and ) and HZSM-5 zeolite have been carried out. At low temperature (173 K), methane adsorption was observed over these oxides and HZSM-5 zeolite. Our findings featured that the infrared inactive ₁ band (2917 cm^–1) of a gaseous methane molecule became active and shifted to lower frequencies (2900 and 2890 cm^–1) when it adsorbed on the surfaces of these adsorbents. Our results also demonstrate that hydroxyl groups played a very important role in methane adsorption over the acidic oxides and the HZSM-5 zeolite. When interaction between the hydroxyl groups and methane took place, the band shift of the hydroxyl groups varied with different oxides. The strength of the interaction decreased according to the following sequence, Si-OH-Al>Al-OH>Si-OH, which is in accordance with the order of their acidities. At higher temperatures, methane interacted quite differently with various oxides and HZSM-5 zeolite. It has been observed that the hydroxyl groups of silica, -alumina and HZSM-5 zeolite could exchange with CD₄ at temperatures higher than 773K, while those on -alumina could exchange at a temperature as low as 573 K. Another interesting observation was the formation of formate species over Al₂O₃ (both and ) at temperatures higher than 473 K. The formate species would decompose to CO₂, or produce carbonate at much higher temperatures. Formation of formate species was not observed over silica and HZSM-5 under similar conditions, -Al₂O₃ did not adsorb or react with methane in any case.     

Methane coupling at low temperatures on Ru(0001) and Ru(11¯20) catalysts

The conversion of methane to higher hydrocarbons on single crystal Ru catalysts has been investigated using combined elevated-pressure kinetic measurements/surface science studies. The reaction consists of activation of methane on Ru(0001) and Ru(11¯20) surfaces to produce carbonaceous intermediates at temperatures between 350 and 700 K and rehydrogenation of these species to ethane and propane at 370 K. It is found that under the reaction conditions employed, the maximum yield in ethane/propane production occurs at 500 K on both surfaces. Influence of the hydrogenation temperature on the production of ethane and propane is also examined. On Ru(0001), the yields of ethane and propane maximize at = 400 K, whereas no maximum yield was observed on Ru(11 0) in the 300–500 K temperature range. Under optimum reaction conditions, hydrocarbon products consist of 16% ethane and 2% propane. High-resolution electron energy-loss spectroscopy (HREELS) has been used to identify various forms of hydrocarbonaceous intermediates following methane decomposition. An effort is made to relate the hydrocarbon intermediates identified by HREELS to the gas phase products observed in the elevated pressure experiments.     

Atom-economical reduction of carbon monoxide to methanol catalyzed by soluble transition metal complexes at low temperatures

Hydrocarbons and methanol are considered the preferred products of catalytic reduction of carbon monoxide derived from clean natural gas. In this paper, we focus on atom-economical synthesis of methanol catalyzed by soluble transition metal complexes as single-site catalyst precursors under mild reaction conditions. Of the metal systems reported in the literature, Ni complexes activated by alkoxide bases affected homogeneous CO reduction to methanol at low temperatures (80°–130°C) and low pressures (2000–5000kPa) to achieve CO conversion and methanol selectivity of >90 and >95, respectively. The involvement of mono- and multi- nuclear Ni species that readily interconvert under methanol producing conditions is invoked. A process based on such an active catalyst system would position methanol to be considered an inexpensive feedstock for the synthesis of other derivatives.To whom correspondence should be addressed. E-mail: devinder.mahajan@stonybrook.edu     

Theoretical and Experimental Studies of CoGa Catalysts for the Hydrogenation of CO<Subscript>2</Subscript> to Methanol

Methanol is an important chemical compound which is used both as a fuel and as a platform molecule in chemical production. Synthesizing methanol, as well as dimethyl ether, directly from carbon dioxide and hydrogen produced using renewable electricity would be a major step forward in enabling an environmentally sustainable economy. We utilize density functional theory combined with microkinetic modeling to understand the methanol synthesis reaction mechanism on a model CoGa catalyst. A series of catalysts with varying Ga content are synthesized and experimentally tested for catalytic performance. The performance of these catalysts is sensitive to the Co:Ga ratio, whereby increased Ga content results in increased methanol and dimethyl ether selectivity and increased Co content results in increased selectivity towards methane. We find that the most active catalysts have up to 95% CO-free selectivity towards methanol and dimethyl ether during CO₂ hydrogenation and are comparable in performance to a commercial CuZn catalyst. Using in situ DRIFTS we experimentally verify the presence of a surface formate intermediate during CO₂ hydrogenation in support of our theoretical calculations.

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On the mechanism of the selective oxidation of methanol over elemental silver

We analysed previously the interaction of silver with oxygen and characterised three different atomic oxygen species. The present communication uses TPRS data to assign a chemical function to each of these three species prepared on a sample of practical electrolytic silver particles. With stationary and instationary conversion experiments close to practical conditions we confirmed the conclusions from the TPRS data also to hold qualitatively for the stationary operating catalyst. Surface oxygen was found to react in an oxydehydrogenation reaction with adsorbed methanol with a significant selectivity to total oxidation. Sub-surface oxygen catalyses the dehydrogenation of the adsorbed methanol with no selectivity to total oxidation. Dissolved atomic oxygen from the bulk replenishes both surface species via sub-surface oxygen. The interconversion of all three species at the high reaction temperatures required to overcome the barriers for the formation and motion of the various atomic oxygen species limits the overall selectivity of the formaldehyde production.     

Fullerene C60 Supported on Silica and γ-Alumina Catalyzed Photooxidations of Alkenes

Deposition of fullerene C₆₀ (2% w/w) on silica and -alumina provokes a two orders-of-magnitude increase of its activity for the liquid-phase photooxidation of 2-methyl-2-heptene. Kinetic studies concerning the above photooxidation showed a first-order dependence of the reaction rate on the alkene concentration. The corresponding reaction-rate constant was found to be higher in the case where -alumina was used as carrier. The nature of the carrier does not influence the mechanism and the selectivity of the reaction. High dispersion of the supported fullerene is achieved on the surface of the carriers, which increase the fullerene light absorbance especially in the visible range.     

Shape selective catalysis in methylation of toluene: Development,challenges and perspectives

Toluene methylation with methanol offers an alternative method to produce p-xylene by gathering methyl group directly from C1 chemical sources. It supplies a “molecular engineering” process to realize directional conversion of toluene/methanol molecules by selective catalysis in complicated methylation system. In this review, we introduce the synthesis method of p-xylene, the development history of methylation catalysts and reaction mechanism, and the effect of reaction condition in para-selective technical process. If constructing p-xylene as the single target product, the major challenge to develop para-selective toluene methylation is to improve the p-xylene selectivity without, or as little as possible, losing the fraction of methanol for methylation. To reach higher yield of p-xylene and more methanol usage in methylation, zeolite catalyst design should consider improving mass transfer and afterwards covering external acid sites by surface modification to get short “micro-tunnels” with shape selectivity. A solid understanding of mass transfer will benefit realizing the aim of converting more methanol feedstock into para-methyl group.

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The chemisorption of methanol on Cu films on ZnO(000¯1)-O

The interactions of methanol with well-defined Cu films on the oxygen-terminated ZnO(000¯1)-O surface have been studied, mainly using temperature programmed desorption (TPD). The Cu films, which were from submonolayer to multilayer in coverage, had been structurally characterized in previous studies using XPS, LEIS, ARXPS, LEED and work function measurements, and by CO, H₂O and formic acid adsorption. On clean Cu films methanol is adsorbed reversibly, desorbing at 200–260 K from atom-thick Cu islands, and at 155 K from multilayer islands preannealed to 550 K. In this respect, the atom-thin islands resemble Cu(110) sites and multilayer islands resemble Cu(111), consistent with behavior of other adsorbates. On oxygen-predosed multilayer films (preannealed to 600 K), methanol reacts to form methoxy species which decompose at 395 K to yield formaldehyde and hydrogen in TPD, also like Cu(111). Multilayer films preannealed to >750 K show a decrease in the peak area for methoxy decomposition which correlates with the loss of Cu area due to severe clustering. Oxygen-predosed Cu islands which are but one Cu atom thick show no clear evidence for a methoxy state in TPD. This suggests that oxygen atoms on such atom-thin Cu islands are poor Brønsted bases relative to O_a on bulk Cu surfaces, consistent with results for adsorbed water. Results on high-area Cu/ZnO catalysts are discussed in the light of these new results.     

Preparation,characterisation and activity of an iron/sodalite catalyst for the oxidation of methane to methanol

An iron sodalite catalyst similar to that reported to have good selectivity for the oxidation of methane to methanol by Lyons et al. has been prepared, tested and characterised. In a limited range of temperature at 34 bar total pressure the selectivity of the catalyst to methanol is a little better than that of an empty silica glass reactor. Before reaction, X-ray powder diffraction and Mössbauer spectroscopy confirm the presence of Fe(III) in the sodalite framework. After reaction Mössbauer spectroscopy identifies Fe(II) and also small particles of iron(III) oxide, < 1 m in size.     

Oxidative coupling of methane over LaCoO3−δ-based mixed oxides

The catalytic activity of LaCoO_3–-based mixed oxides for the oxidative coupling of methane has been tested by TPR and cyclic reaction. Characterization has been done by XRD, TGA and Mössbauer spectrometry. It is likely that the perovskite-crystal structure containing hypervalent metal ions has an important role and that unique structural oxygen species in the perovskite contribute to the partial oxidation of methane.     

Methane Aromatization over 2 wt% Mo/HZSM-5 in the Presence of O2 and NO

In the non-oxidative aromatization reaction (temperature = 770 C, flow rate = 34 ml min^-1), 2 wt% Mo/HZSM-5 deactivated after 4 h due to severe coking. We observed that with a suitable amount of O₂ (5.3 vol%) in the methane feed, the catalyst could last for more than 6 h with a ca. 4% yield of aromatics at 770 °C. Depending on the concentration of O₂ or the reaction temperature, there are three reaction zones in the catalyst bed: (i) methane oxidation; (ii) methane reforming; and (iii) methane aromatization. CO and H₂ produced in the first two zones are accountable for stability amelioration of the catalyst. The addition of NO exhibited similar effects on the reaction. Further increase in O₂ (8.4 vol%) or NO (14.2 vol%) concentration would result in CO and CO₂ being the predominant carbon-containing products; C₂H₄ and C₂H₆ were generated in small amounts and no aromatics were detected.     

Catalytic oxidation of methanol to methyl formate over silver – a new purpose of a traditional catalysis system

Catalytic reaction was performed in the unregarded temperature region over silver catalysts with long catalytic lifetime for the conversion of methanol to methyl formate. O-saturated or O-saturated silver catalysts were studied individually to identify the roles of O, O in the oxidative esterification of methanol over an unsupported polycrystalline silver catalyst. A synergic process is proposed based on the coexistence of -oxygen species and -oxygen species on the surface of polycrystalline silver at about 573 K.     

The partial gas‐phase hydrogenation of benzene to cyclohexene on supported and coated ruthenium catalysts

The conversion of benzene to useful products such as cyclohexene is of industrial interest because of the expected surplus of benzene due to its substitution in gasoline by other nonpolluting components in the next years. Therefore, the partial gasphase hydrogenation of benzene to cyclohexene at atmospheric pressure was performed in order to develop catalysts as an alternative to those used in liquidphase hydrogenation. Two types of rutheniumcontaining catalysts were investigated, viz. supported catalysts with different support materials and coated catalysts with electrolytically formed alumina as support. In order to yield the desired cyclohexene the presence of methanol as a reaction modifier was necessary in the gas phase during the reaction. The hydrogenation on supported Ru catalysts gave selectivities of about 35%, while on coated Ru catalysts selectivities up to 45% were obtained at conversion degrees of 5%. Improved catalyst performance, especially higher selectivity and yield, was obtained at increased partial pressure of methanol and hydrogen and by addition of copper as second metal in the oxide layer of the coated catalysts.     

Pyrolysis behavior of poly[(n-propylamino/methylamino)borazine]

A processable poly[(n-propylamino/methylamino)borazine] (PPAB) has been pyrolyzed in Ar to study its thermal decomposition behavior. The structural evolution and chemical composition change during pyrolysis were characterized by chemical analysis, thermal gravimetry–mass spectrometry (TG–MS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicated that the polymer-to-ceramic transition of PPAB involved two steps. Below 400 °C, the gas species were mainly methane and methylamine, while from 400 to 900 °C those were methane and n-propylamine. The PPAB displayed a ceramic yield of 52 wt% at 1000 °C and the pyrolyzed product was amorphous boron nitride (BN) with a small quantity of carbon impurity, in presence as CC and CN bonds. Moreover, for the pyrolyzed product, further heat treatment resulted in the occurrence of a transformation from amorphous to turbostratic.     

Novel Catalytic Properties of Rh Sulfide for the Synthesis of Methanol from CO + H2 in the Presence of H2S

Rh sulfide yielded 800 gkg-cat^–1h^–1 of methanol at 593 K and 5.1 MPa from CO + H₂ (syngas) even in the presence of H₂S 100 ppm in concentration. The obtained space-time yield of methanol was comparable with that obtained with a commercial Cu/Zn/Al catalyst at a conventional reaction condition (523 K and 5.1 MPa) from a feed containing both syngas and CO₂.