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NOVEL DEALUMINATED ZEOLITE-BASED CATALYSTS FOR THE REFORMING OF HYDROCARBONS

by ZHANG, WENMIN

Abstract (Summary)
With the introduction of the reformulated Gasoline (RFG) in 1990 Clean Air Act, branched alkanes become very ideal alternative components for the new RFG, since they possess very high octane numbers and low vapor pressures. The branching of normal alkanes is playing ever-increasing role in the refineries. In response to this challenge, this dissertation investigated the performance of various 12-membered ring pore zeolites, including ZSM-12, ?-zeolite, mordenite, USY and L-zeolite, for the hydroconversion of individual probe molecules, synthetic mixtures and industrial naphthas. All zeolites were dealuminated to various extents, and loaded with 0.5 wt %Pt. A novel stepwise temperature programmed desorption of ammonia (NH 3 -STPD) technique was developed for the zeolite acidity characterization. All zeolites exhibit five distinct peaks, corresponding to variable strength of acidic sites. Coupling NH 3 -STPD, FT-IR and NMR, ammonia chemisorbed on acid sites of different strengths can be accurately determined. A 1:1 relation between the Al atoms of zeolite and ammonia molecules was discovered, thus indicating that this technique can probe accurately all acid sites for zeolites with Si/Al ? about 20. The non-dealuminated ZSM-12 (Si/Al=35) sample possesses a small number of Lewis sites while the ZSM-12 samples with higher Si/Al ratios have only Brønsted acid sites. To elucidate the reaction mechanism, a systematic study for hydroconversion of probe normal and branched octane molecules was conducted. The branched product selectivity from the conversion of normal octane increases with the increase of the Brønsted acid site strength of the zeolite. Hydroisomerization is thus favored at the expense of cracking at a higher Brønsted acid strength. Soluble coke analysis revealed that at relatively low reaction temperature compositions of the soluble coke are mostly paraffinic in nature for ZSM-12 catalyst, and both paraffinic and naphthenic for ?-Al 2 O 3 . catalyst deactivation via a successive alkylation mechanism in which carbonaceous coking precursors propagate through the continuous addition of olefinic intermediates to carbenium/ carbonium ions was thus proposed . Contrary to what one would commonly expect and previously reports, ZSM-12 was found to possess a surprisingly higher coking resistance than other large pore zeolites. This superior performance is due to its unique non-interconnecting tubular-like linear channels, which do not allow trapping/accumulation of coking precursors. In contrast, zeolites with relatively large supercages are inherently favorable to coking reactions, which in turn lead to the fast deactivation.
Bibliographical Information:

Advisor:

School:University of Cincinnati

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:zeolite catalysis hydrocarbon reforming acidity

ISBN:

Date of Publication:01/01/2001

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