Structure and reactivity of dehydroxylated bronsted acid sites in H-ZSM-5 zeolite: generation of stable organic radical cation and catalytic activity for isobutane conversion
Date
2011
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Publisher
University of Delaware
Abstract
Zeolites are crystalline aluminosilicate materials that have wide application in industry as solid acid catalyst. Since zeolites have high acidity, high surface area as well as the ability to do shape selectivity, they are used primarily as a solid catalyst in oil refining and petrochemical industries, in processes such as hydrocracking, fluid catalytic cracking (FCC). Bronsted acid sites are described as a hydroxyl group bridged between Al and Si (Al-OH-Si). It is well known that Bronsted acid sites are significant in a number of hydrocarbon processes, such as alkane cracking and isomerization and many other. The Bronsted acid sites of zeolites are decomposed at high temperatures, usually above 600 ̊C. This high temperature condition is commonly found in the fluidized catalytic cracking where catalyst is recycled forth and back between the riser and the regenerator under an oxidative atmosphere. The process of decomposition of hydroxyl group from the initial structure is called dehydroxylation. The dehydroxylation is believed to proceed via a dehydration mechanism of the acid sites. This heterolytic pathway of Bronsted acid site decomposition has been the accepted dehydroxylation path for low-silica zeolites for decades, although the molecular details of the structure remaining inside the zeolites are still unknown. However, our group reported that hydrogen is also formed during the dehydroxylation process. Our group has also proposed a new pathway to explain the decomposition of Bronsted acid sites of high-
silica zeolites and the formation of [AlO4]0 sties in zeolites. Oxidized zeolites are known to extract electrons from molecules having small ionization potential. Since oxidation of zeolites is considered to lead to the dehydroxylation of Bronsted acid sites, we suggest that the dehydroxylated Bronsted acid sites are responsible for the electron-transfer process. Using naphthalene as a probe molecule, it can be shown that the new sites have the ability to extract an electron from naphthalene and form stable radical cations. We investigated the formation of these new sites by thermal treatment and oxidation treatment. A series of UV/vis spectra showed that after naphthalene radical cations were generated, single-electron transfers back into the ZSM-5 framework to form a stable electron-hole pair and reform the naphthalene neutral molecule. Using ammonia TPD, IR spectra, and UV/vis spectra of the sample with different Si/Al ratios, the structure of the new generated sites was characterized. These observations suggest that the most common site generated is different depending on each treatment. The activation of small alkanes over acid sites has been investigated extensively because of its relevance to technologically important processes such as fluidized catalytic cracking in petroleum refineries, but also because C-H and C-C bond activation is of fundamental scientific interest. The reactivity and selectivity of newly generated sites is investigated using isobutane conversion. The conversions of the samples, which were treated by high temperature treatment and oxygen treatment for dehydroxylation, are greater than the conversions of the acid catalyst. When conversion is low, the product distribution is limited to the monomolecular cracking of the C-C bond and dehydrogenation of the C-H bond. The cracking-to-dehydrogenation ratio significantly increases after dehydroxylation treatments. Several groups have proposed that carbonium or carbenium ion intermediates on Bronsted acid sites in zeolites play the key role in the activation of isobutane. However, in this thesis, we proposed that the presence of redox sites resulted in radical cation chemistry instead of protolytic chemistry in the propane and isobutane cracking process.