Mechanism of branched carbon-chain formation from CO and H₂over oxide catalysts. Part 1.—Adsorbed species on ZrO₂and CeO₂during CO hydrogenation

Abstract

The adsorbed species on ZrO, and CeO, during CO hydrogenation forming branched carbon chains, especially isobutene, (over ZrO,), have been investigated by chemical trapping, in situ IR, and solid-state NMR methods.These indicated that methoxide and formate were present as the surface species on ZrO, . CO hydrogenation on ZrO, with pre-adsorbed methoxide or formate showed that the pre-adsorption of methoxide promotes the formation of the higher hydrocarbons containing mainly isobutene but that formate retards the reaction. Chemical trapping experiments on CeO, after CO hydrogenation at 523 and 673 K led to the formation of methane together with methanol, indicating the presence of methyl, p-methylene, or carbene as a surface species. q2-Formaldehyde was suggested to be the precursor of the methyl, p-methylene, or carbene species, the latter of which in turn gives C, species by insertion of CO. Following the products of CO hydrogenation and chemical trapping on CeO, at 523 K with time indicated that an aldol-condensation-type reaction leads to C, and branchedchain C, products from C, oxyhydrocarbon.

Desorption of Dimethyl Ether from a Methoxy Species Formed on a CeO2 Surface

“…CeO 2 catalyst with a specific surface area of 29 m 2 g -1 was used for TPD experiments. The preparation of the CeO 2 catalyst has been described elsewhere. − TPD experiments were performed using a conventional flow system with N 2 carrier gas (60 cm 3 min -1 ). The CeO 2 catalyst (2.0 g) in a quartz reactor was heated at 873 K for 1 h and then was exposed to DME added to N 2 carrier gas through a mass flow valve at each adsorption temperature.…”

“…[1][2][3][4][5][6][7] We have been exploring the CO-H 2 reaction over CeO 2 and ZrO 2 catalysts which selectively produce isobutene. [6][7][8][9][10] The surface methoxy species were observed on these catalysts during the CO-H 2 reaction by NMR, and the addition of dimethyl ether (DME) to the reaction remarkably enhanced the isobutene production since the dissociative adsorption of DME provided the surfaces with the methoxy species as intermediates. [8][9][10] The dissociative chemisorption of DME on metal oxides forms surface methoxy species as shown in Scheme 1.…”

“…[6][7][8][9][10] The surface methoxy species were observed on these catalysts during the CO-H 2 reaction by NMR, and the addition of dimethyl ether (DME) to the reaction remarkably enhanced the isobutene production since the dissociative adsorption of DME provided the surfaces with the methoxy species as intermediates. [8][9][10] The dissociative chemisorption of DME on metal oxides forms surface methoxy species as shown in Scheme 1. 5,11 Recently, we have reported the behavior of the methoxy species formed by the adsorption of DME on CeO 2 , where the terminal-or bridged-OCH 3 molecules formed on the surface are thermally converted into another type of µ-OCH 3 .…”

“…In the reaction, surface methoxy species on metal oxides are considered to be important intermediates leading to hydrocarbons. For this reason, surface methoxy species on metal oxides have been extensively investigated. − We have been exploring the CO−H 2 reaction over CeO 2 and ZrO 2 catalysts which selectively produce isobutene. − The surface methoxy species were observed on these catalysts during the CO−H 2 reaction by NMR, and the addition of dimethyl ether (DME) to the reaction remarkably enhanced the isobutene production since the dissociative adsorption of DME provided the surfaces with the methoxy species as intermediates. − The dissociative chemisorption of DME on metal oxides forms surface methoxy species as shown in Scheme . , Recently, we have reported the behavior of the methoxy species formed by the adsorption of DME on CeO 2 , where the terminal- or bridged-OCH 3 molecules formed on the surface are thermally converted into another type of μ-OCH 3 …”

Adsorption and reaction of formaldehyde on thin-film cerium oxide

“…Cerium oxide catalysts are also of interest in other chemical processes such as the production of functionalized hydrocarbons. Many of these processes involve the production and interconversion of simple C 1 molecules such as methane, methanol, formaldehyde and formic acid [3]. Investigations of adsorption, decomposition and production of these C 1 molecules on well-defined cerium oxides provide insights for the understanding of the mechanisms in the actual industrial reactions.…”