III International Conference

PP-I-31MECHANISM OF N-BUTANE CONVERSION ON Cs-SALT OF PHOSPHOTUNGSTICACID: A STUDY BY IN SITU 13 C MAS NMR AND EX SITU GC-MSLuzgin M.V., Rogov V.A., Timofeeva M.N., Stepanov A.G.Boreskov Institute of Catalysis SB RAS, Novosibirsk, RussiaE-mail: luzgin@catalysis.ruUsing in situ 13 C MAS NMR and ex situ GS-MS analysis, the conversion of n-butane-1-13 C on solid Cs-salt of phosphotungstic acid Cs 2.5 H 0.5 PWO 40 was demonstrated to proceed viatwo parallel reactions: (1) the migration of 13 C-label in the n-butane molecule from methylinto methylene group and (2) skeletal isomerization into isobutane. It has been establishedthat the migration of13 C-labels proceeds via monomolecular mechanism, whereasbimolecular pathway is prevalent for the n-butane isomerization into isobutane, even in thepresence of CO.Skeletal isomerization of n-butane to isobutane has attracted much attention sinceisobutane is an important feedstock for the synthesis of methyl tert-butyl ether, a high-octanegasoline additive, and for the alkylation of butanes as well. The use of more ecologicallyfriendly solid acid catalyst for this reaction should be preferred. In addition, the acid-catalyzedisomerization of n-butane represents an important object for mechanistic studies. Indeed, asopposed to n-pentane and higher alkanes, a monomolecular carbenium ion mechanism for theisomerization of n-butane to isobutane would require formation of a very unstable primarycarbenium ion as compared with a secondary ion in the case of higher carbenium ions.It has been established earlier that skeletal isomerization of n-butane on solid acidcatalysts, sulfated zirconia and zeolite H-ZSM-5, proceeds via bimolecular mechanism [1-3],which allows to avoid the formation of primary carbenium ions as the intermediate. For thecatalysts based on solid phosphotungstic acid and its cesium salts the predominance ofintermolecular pathway for n- to iso-butane conversion is also generally accepted [4].However, basing on the observation of the selective formation of isobutane from n-butane, themonomolecular mechanism has been supposed for initial stage of n-butane conversion on Cssaltof phosphotungstic acid [5]. So, the mechanism of n-butane conversion on solid catalystsbased on phosphotungstic acid requires further comprehensive studies.In present paper, using in situ 13 C MAS NMR and ex situ GS-MS analysis, theconversion of n-butane- 13 C has been studied on solid Cs-salt of phosphotungstic acid(Cs 2.5 H 0.5 PWO 40 ) at 80-150 ºC both in the absence and in the presence of carbon monoxide.63