CONVERSION OF PROPAN-2-OL ON ZEOLITES LANAY AND HY INVESTIGATED BY GAS-CHROMATOGRAPHY AND IN-SITU MAS NMR-SPECTROSCOPY UNDER CONTINUOUS-FLOW CONDITIONS

The conversion of propan-2-ol on zeolites HY and LaNaY has been invest igated by gas chromatography (GC) and in situ H-1 and C-13 MAS NMR spe ctroscopy under continuous-flow conditions using a new MAS NMR microre actor with cylindrical catalyst bed. At reaction temperatures of T = 3 73 K and T = 393 K a propan-2-ol conversion of 50 and 100%, respective ly, and the formation of propene, diisopropyl ether, and small amounts of acetone was determined by GC. Applying in situ H-1 and C-13 MAS NM R spectroscopy, the initial step of the reaction was found to be the p hysisorption of propan-2-ol on Bronsted acid sites. A formation of iso propoxy species could be excluded by C-13 MAS NMR spectroscopy. H-1 MA S NMR spectroscopy indicated that the Bronsted acid sites of the zeoli tes LaNaY and HY were hydrated by water molecules in the first part of the induction period. These water molecules were formed in result of the propan-2-ol dehydration. The strong low-field shift of the H-1 MAS NMR signals of the hydrated Bronsted acid sites is due to a partial p rotonation of adsorbed water molecules. At T = 393 K, a significant C- 13 MAS NMR signal of strongly bonded acetone molecules appeared at 220 ppm in the spectra of zeolites LaNaY and HY. As demonstrated by propa n-2-ol conversion on a partially dealuminated zeolite HY, this by-reac tion is promoted by extra-framework aluminium species. The formation o f coke precursors which caused C-13 MAS NMR signals at 10-50 ppm is ex plained by an oligomerization of propene. In situ C-13 MAS NMR experim ents carried out under a continuous flow of propene showed that the ab ove-mentioned coke precursors are also formed on partially rehydrated zeolite HY. (C) 1997 Academic Press.