C-13 CP/MAS AND H-2 NMR-STUDY OF TERT-BUTYL ALCOHOL DEHYDRATION ON H-ZSM-5 ZEOLITE - EVIDENCE FOR THE FORMATION OF TERT-BUTYL CATION AND TERT-BUTYL SILYL ETHER INTERMEDIATES

The dehydration reaction of tert-butyl alcohol, selectively labelled with C-13 in CH3 or C-O groups (t-BuOH[2-C-13(1)] and t-BuOH[1-C-13]), as well as selectively deuterated in methyl groups (t-BuOH[2-H-2(9)]), was studied on H-ZSM-5 zeolite simultaneously with C-13 CP/MAS and H-2 solid state NMR. When adsorbed and dehydrated on zeolite at 296 K, t-BuOH[2-C-13(1)] and t-BuOH[1-C-13] give rise to identical C-13 CP/MAS NMR spectra of oligomeric aliphatic products. This is explained in terms of the fast isomerization of the tert-butyl hydrocarbon skeleton via the formation of tert-butyl cation as the key reaction intermediate. An alkoxide species, most probably tert-butyl silyl ether (t-BuSE), was also detected as the "side" reaction intermediate. This intermediate was stable within the temperature range 296-373 K and decomposed at 448 K to produce additional amounts of final reaction products, i.e. butene oligomers. NMR data point to the existence of equilibria between the initial tert-butyl alcohol, tert-butyl cation and butene that is formed from the intermediate carbocation.