DEUTERIUM SOLID-STATE NMR-STUDY OF THE MOLECULAR MOBILITY AND DEHYDRATION OF TERT-BUTYL ALCOHOL ON ZEOLITE H-ZSM-5

The molecular mobility and dehydration reaction of tert-butyl alcohol, selectively deuteriated in the methyl groups (t-BuOH[2 - H-2(9)], dTB A), absorbed on H-ZSM-5 zeolite was studied using H-2 NMR spectroscopy . At 173-298 K two modes of fast anisotropic motion were observed for the adsorbed alcohol: rotation of CD3 groups around the C-C bonds and rotation of the entire (CD3)(3)C fragment around the C-O bond. The inf luence of the walls of the H-ZSM-5 channels on the geometry of adsorbe d dTBA is small, the increase in the CD3-C-O angle not exceeding 2.7 /- 1.2 degrees compared with the same angle in solid dTBA. This is exp lained by location of the alcohol molecules at channel intersections o f the zeolite, whose dimensions exceed those of the dTBA molecule. The lifetime of the dTBA molecule at these adsorption sites exceeds 1 x 1 0(-5) s. The observed reaction products are deuteriated water with an unusual H-2 NMR Lineshape and two types of butene oligomers: less mobi le species with the lineshape typical of solid-state 2H NMR and more m obile species with a liquid-like lineshape. The number of more mobile species increases with increase in temperature. In addition, H-2 NMR i ndicates the presence of tert-butyl groups in the reaction products. F or oligomers with a liquid-like lineshape, the diffusion coefficient D is 3 x 10(-13) m(2) s(-1) at 373 K, whereas for oligomers with al sol id-like lineshape D much less than 5 x 10(-14) m(2) s(-1) at 173-373 K . The diffusion coefficient for the t-BuOH molecule was estimated as D much less than 2 x 10(-14) m(2) s(-1) within the temperature range 17 3-296 K.