Solid-state nuclear magnetic resonance studies of the reactions of olefins within zeolite catalysts

Abstract

Olefins are one of several classes of organic compounds which undergo acid-catalyzed reactions within acidic zeolite catalysts. Reactions of olefins are of considerable interest in heterogeneous catalysis since they constitute a large part of the many reactions which occur during catalytic cracking. Many mechanisms for these reactions have been proposed, but none have been verified by direct experimental evidence. This dissertation introduces the application of solid-state 13C NMR as an in situ technique to study intrazeolitic reactions. The technique provides insight into the mechanism of olefin and diene oligomerization and the role of this and other reactions in catalyst deactivation. Novel sample preparation methods developed as part of this research are described in detail. The mechanism of propene oligomerization within zeolite HY was revealed using in situ 13C MAS NMR. The use of low-temperature sample preparation techniques helped in identifying the reactive intermediates involved in oligomerization. Moreover, reactive species not involved in the oligomerization but present in the sample at room temperature were identified. Differences between the oligomerization of butadiene within zeolites HY and HZSM-5 were uncovered by studying these catalyst/adsorbate systems. Special sample preparation techniques for this highly reactive diene were developed. The nature of the intrazeolitic product was directly affected by catalyst channel size, and provided a possible route for catalyst deactivation. Two instrumental techniques were investigated as methods for characterizing zeolites deactivated by carbonaceous deposit buildup. The factors governing the spin dynamics of the solid-state 1H-13C cross polarization experiment were studied to determine the reliability of quantitative measurements of 13C CP/MAS spectra of deactivated zeolite samples. Thermal desorption-gas chromatography-mass spectrometry of adsorbed species from deactivated zeolites was found unsuitable as an alternative to solvent extraction for identifying individual compounds of the deposit.