NMR OBSERVATION OF INDANYL CARBENIUM ION INTERMEDIATES IN THE REACTIONS OF HYDROCARBONS ON ACIDIC ZEOLITES

In situ C-13 MAS-NMR was used to study the reactions of [alpha-C-13]st yrene, [beta-C-13]styrene, and [alpha-C-13]-alpha-methylstyrene on the acidic zeolites HZSM-5, HY, and H-mordenite. Neither the methylphenyl carbenium ion nor the dimethylphenylcarbenium ion were observed as per sistent species by NMR, even at low temperatures. However, styrene rea cted on zeolite HY to form an appreciable yield of the 3-methyl-1-phen ylindanyl cation, and this cation was stable up to ca, 323 K. The mech anism by which this cation formed was apparently hydride transfer from neutral 1-methyl-3-phenylindan to transient (i.e., not observed) meth ylphenylcarbenium ion. The analogous 3,3-dimethyl-1-phenylindanyl cati on did not form as a persistent species when alpha-methylstyrene was a dsorbed on HY; we attribute this negative result to the relative diffi culty of intermolecular methyl transfer vs hydride transfer. Styrene d imers cracked on HZSM-5 at ca. 433 K to form a significant yield of th e 1-methylindanyl cation, which was stable indefinitely in the zeolite at 298 K. Further heating converted this cation into naphthalene, the ultimate product of dimerization, cracking, skeletal isomerization, a nd dehydrogenation. A very low yield of the analogous 1,3,3-trimethyli ndanyl cation formed in the reactions of alpha-methylstyrene on HZSM-5 ; we attribute the disparity in yields between the less bulky and more bulky cation to the steric effect of the pore size. The yields of bot h methylindanyl cations on zeolite HY were very low or negligible. Thi s observation may reflect the fact that HZSM-5 is a slightly stronger acid than HY. The above results are introduced and discussed in the co ntext of our recently proposed reclassification of typical zeolites aa s strong acids rather than superacids. We also show that the observati on of indanyl cations but not simple phenylcarbenium ions as persisten t species in high concentration is an expected consequence of the rela tive acid strengths of the zeolites and carbenium ions. The principal components of the C-13 chemical shift tensor of the charged carbon of the 3-methyl-1-phenylindanyl cation were delta(11) = 310, delta(22) = 286, and delta(33) = 68 ppm; for the 1-methylindanyl cation these valu es were delta(11) = 359, delta(22) = 320, and delta(33) = 76 ppm.