Deuterium Solid-State NMR Study of the Molecular Mobility and Dehydration of tert-Butyl Alcohol on Zeolite H-ZSM-5
Magnetic Resonance in Chemistry
, E-ISSN: 1097-458X
||2H NMR, Dehydration, Diffusion, H‐ZSM‐5 zeolite, Molecular mobility, tert‐Butyl alcohol
Stepanov Alexander G.
Maryasov Alexander G.
Romannikov Vyacheslav N.
Zamaraev Kirill I.
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences
The molecular mobility and dehydration reaction of tert-butyl alcohol, selectively deuteriated in the methyl groups (t-BuOH[2 - H-2(9)], dTBA), 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 influence of the walls of the H-ZSM-5 channels on the geometry of adsorbed 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 explained by location of the alcohol molecules at channel intersections of the zeolite, whose dimensions exceed those of the dTBA molecule. The lifetime of the dTBA molecule at these adsorption sites exceeds 1 x 10(-5) s. The observed reaction products are deuteriated water with an unusual H-2 NMR Lineshape and two types of butene oligomers: less mobile species with the lineshape typical of solid-state 2H NMR and more mobile species with a liquid-like lineshape. The number of more mobile species increases with increase in temperature. In addition, H-2 NMR indicates the presence of tert-butyl groups in the reaction products. For 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 solid-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 173-296 K.