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 investigated by gas chromatography (GC) and in situ H-1 and C-13 MAS NMR spectroscopy under continuous-flow conditions using a new MAS NMR microreactor with cylindrical catalyst bed. At reaction temperatures of T = 373 K and T = 393 K a propan-2-ol conversion of 50 and 100%, respectively, 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 NMR spectroscopy, the initial step of the reaction was found to be the physisorption of propan-2-ol on Bronsted acid sites. A formation of isopropoxy species could be excluded by C-13 MAS NMR spectroscopy. H-1 MAS NMR spectroscopy indicated that the Bronsted acid sites of the zeolites 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 protonation 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 propan-2-ol conversion on a partially dealuminated zeolite HY, this by-reaction is promoted by extra-framework aluminium species. The formation of coke precursors which caused C-13 MAS NMR signals at 10-50 ppm is explained by an oligomerization of propene. In situ C-13 MAS NMR experiments carried out under a continuous flow of propene showed that the above-mentioned coke precursors are also formed on partially rehydrated zeolite HY. (C) 1997 Academic Press.