Investigating the high-temperature chemistry of zeolites: Dehydrogenation of zeolites and ammonia-SCR of copper exchanged small-pore zeolites.

摘要

In this thesis, we show using mass-spectrometry temperature-programmed-desorption (MS-TPD) that the product of heating high-silica H-zeolites is predominantly hydrogen. Using electronicstructure calculations we also show a plausible path for the formation of hydrogen from zeolite Bronsted acid sites and propose that the reaction should lead to the formation of [AlO4/ h]0 sites in the zeolite. These [AlO4/h]0 sites can act as nonacidic one-electron acceptors of adsorbed molecules and could react further to form a more stable species. As such these sites could play an important role in the catalytic chemistry of hydrocarbons at high temperatures.;It is proposed that the disappearance of hydrogen from the hydroxyl nests is accompanied by the formation of bisperoxysilyl groups (Si-O-O-Si). Electronic structure calculations are also employed to support the energetic feasibility of this reaction mechanism. Silicalite-1 samples made with tetraethyl-orthosilicate show a number of differences in their IR and UV-Vis spectra compared to samples made with other silica sources, and it appears that the source of silica plays an important role in the structure of the internal defects.;Nitrogen oxides (NOx) are a major atmospheric pollutant produced through the combustion of fossil fuels in internal combustion engines. Copper-exchanged zeolites are promising as selective catalytic reduction (SCR) catalysts for the conversion of NO into N2 and O2. Previously, it has been shown that when fresh, Cu-ZSM-5 has high NH3-SCR activity, however, ZSM-5 zeolites are highly susceptible to dealumination during steaming, which results in a lost of SCR activity. Whereas, recent reports have shown the enhanced performance of Cu-CHA catalysts over other zeolite frameworks in the NO decomposition of exhaust gas streams.;In the present study, Rietveld refinement of variable-temperature XRD synchrotron data obtained for Cu-SSZ-13 and Cu-SSZ-16 is used to investigate the location of copper cations in the zeolite pores and the effect of temperature on these sites and on framework stability. The XRD patterns show that the thermal stability of the zeolite SSZ-13 is increased significantly when copper is exchanged into the framework compared with the acid form of the zeolite, H-SSZ-13. Cu-SSZ-13 is also more thermally stable than Cu-SSZ-16. From the refined diffraction patterns, the atomic positions of framework atoms, copper locations and occupancies, and thermal displacement parameters were determined as a function of temperature for both zeolites. Copper is found in the cages coordinated to three oxygen atoms of the six-membered rings.;These results lead us to investigate the NH3-SCR activity of the small-pore zeolites, Cu-SSZ-13, Cu-SSZ-16, and Cu-SAPO-34. These copper exchanged smallpore zeolites have high SCR activity between 150-500°C and are shown to be much more hydrothermally stable than the medium-pore zeolite, Cu-ZSM-5. The degree of copper exchange, the dimensionality of the framework, and heteroatom framework substitution all impact the SCR activity and hydrothermal stability of the materials. Of the small-pore zeolites tested, Cu-SSZ-13 and Cu-SAPO-34 display superior SCR performance, both before and after high-temperature hydrothermal treatment.;Overall, the results of this thesis bring about new ideas as to what happens to zeolite systems at high-temperatures. The decomposition of Bronsted acid sites and hydroxyl nests within zeolite frameworks are startling finds that reevaluate previously held decomposition mechanisms within zeolites. By exchanging copper into small-pore zeolites, we showed that there is a resulting increase in the thermal stability of the material. The findings here also provide evidence that the pore size of the zeolite framework plays a crucial role in the stability of the material. Lastly, future recommendations are given for ways in which to utilize the properties of these unique materials. (Abstract shortened by UMI.)