A comparative study of methyl-tert-butyl ether synthesis on zeolites HY, HBeta, HBeta/F and HZSM-5 by in situ MAS NMR spectroscopy under flow conditions and on-line gas chromatography

Investigating the formation of methyl-tert-butyl ether (MTBE) on zeolites H Y, HBeta, HBeta/F and HZSM-5 by in situ C-13 MAS NMR spectroscopy under flo w conditions, signals of alkoxy species have been observed at 77-90 ppm. On -line gas chromatographic analysis of the reactant and product molecules le aving the MAS NMR rotor reactor yielded that the formation of these alkoxy species is in line with high yields of MTBE and vice versa. Methanol adsorp tion experiments performed by in situ H-1 MAS NMR spectroscopy under flow c onditions indicated that methanol molecules adsorbed on the highly active z eolite HBeta are characterized by a low protonation degree. Based on the ex perimental results of in situ H-1 MAS NMR spectroscopy and on-line gas chro matography, a mechanism of the synthesis of MTBE on acidic zeolites is prop osed in which the reaction of isobutene with bridging OH groups leading to the formation of isobutoxy species with 'carbenium-like properties' plays a key role. (C) 2000 Elsevier Science B.V. All rights reserved.