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Author ORCID Identifier
https://orcid.org/0000-0002-8262-4196
AccessType
Campus-Only Access for Five (5) Years
Document Type
dissertation
Degree Name
Doctor of Philosophy (PhD)
Degree Program
Chemical Engineering
Year Degree Awarded
2024
Month Degree Awarded
February
First Advisor
Omar Abdelrahman
Second Advisor
Friederike Jentoft
Third Advisor
Wei Fan
Fourth Advisor
Joseph DuChene
Subject Categories
Catalysis and Reaction Engineering
Abstract
Solvents are frequently involved in catalytic conversions over solid acid catalysts, and it is widely acknowledged that the presence of solvent can significantly alter the reaction kinetics. However, how solvents participate and affect a catalytic cycle remains debated. Water is one of the most commonly used solvents, and numerous researches have focused on interpreting the effect of water. However, challenges in deciphering the effect of water arises from the highly non-ideal thermodynamics of the aqueous phase, exacerbated by the use of probe chemistries that already involve water as a reactant or product, which could mask the independent influence of water on the catalytic cycle. Herein, the vapor phase Hofmann elimination of tert-butylamine (TBA) was used as a water-free and purely Brønsted acid-catalyzed probe chemistry to systematically investigate the solvent effect of water on zeolite-catalyzed reactions. Kinetic measurements were carried out in a continuous-flow packed bed reactor with controlled partial pressures of TBA and solvent in the feed, and an on-line gas chromatograph was used to quantify the reaction rates. A significant decrease in the rate of Hofmann elimination over H-ZSM-5 zeolite was observed when water vapor was co-fed with TBA, while such rate inhibition disappeared once water was removed from the feed. Through a combination of kinetic measurements, in-situ spectroscopy, and temperature-programmed surface reaction, it was revealed that the rate inhibition was not related to changes in TBA surface fractional coverage or intrinsic reaction energetic barrier. Based on the results, a possible mechanism of cooperative adsorption theory was proposed for the solvent effect, which was quantitatively verified by microkinetic modeling. With the understanding achieved with water over H-ZSM-5 zeolite, the scope of research was extended to other types of solvents and heteroatom in zeolite framework, so as to examine whether the proposed theory is generally applicable. Twenty solvents varying in molecular size, polarity and proton affinity were tested over H-ZSM-5 zeolite, and various degrees of inhibition were observed. A roughly linear relationship between the degree of rate inhibition and solvent dipole moment was observed, which indicated that the solvent polarity might dictate the extent of solvation of the Brønsted acidic proton. Additionally, an updated kinetic model was implemented and verified to better describe the effect of various solvents. In contrast to the aluminosilicate zeolites, phosphorus-containing zeolites exhibit completely different response to the presence of water. Feeding water significantly enhanced the catalytic activity of phosphorus-containing Beta (P-BEA) zeolite for 2-propanol dehydration and TBA Hofmann elimination. Using a series of in-situ and ex-situ characterization methods, including amine titration, in-situ pyridine IR spectroscopy and solid-state 31P NMR, the activity enhancement was demonstrated to be a result of polyphosphate hydrolysis which dramatically increased the Brønsted acid site density. In addition, the use of a sufficiently basic nitrogen-containing molecule was found to be critical to access the highest possible density of acid sites, which greatly exceeds that achievable through hydrolysis using water alone.
DOI
https://doi.org/10.7275/36066650
Recommended Citation
Chen, Han, "Understanding Solvation of Brønsted Acid Sites with Hofmann Elimination of Alkylamines" (2024). Doctoral Dissertations. 3038.
https://doi.org/10.7275/36066650
https://scholarworks.umass.edu/dissertations_2/3038
Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.