Molecular Vibrations and Shape-Selectivity: A Computational Model of Biofuel Precursors in Zeolites

Author

Babgen Manookian, University of Massachusetts AmherstFollow

Author ORCID Identifier

https://orcid.org/0000-0002-6273-0995

AccessType

Open Access Dissertation

Document Type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemistry

Year Degree Awarded

2022

Month Degree Awarded

May

First Advisor

Scott M. Auerbach

Subject Categories

Computational Chemistry | Materials Chemistry | Physical Chemistry

Abstract

We have used Density Functional Theory (DFT) to model acyclic and cyclic olefins in acidic zeolites. We have studied the impact of host-guest interactions between adsorbed molecules and zeolite frameworks through the lens of molecular vibrations and shape-selectivity. This work considered three zeolite frameworks with varying pore structures and environments: large pore zeolite HMOR and medium pore zeolites HZSM-5 and HZSM-22. A key finding is that for acyclic olefins in acidic zeolites there exists two regimes of host-guest interaction: a strong interaction leading to protonation and a weak interaction between charged guest and zeolite framework. We found that these interactions manifest in the IR spectra such that protonation leads to significant changes in band position for allylic vibrations, v_am(C=C─C⁺), and in contrast these band positions are left substantially unchanged due to the weaker Coulombic interaction. These results indicate that to model acyclic olefins in acidic zeolites one only need to consider the protonated state in the gas phase. We worked in close collaboration with zeolite experimentalists E. Hernandez and F. Jentoft at the University of Massachusetts Amherst Chemical Engineering department to investigate the presence of shape-selectivity during the formation of alkylcyclopentenyl cations from acyclic precursors in acidic zeolites. We incorporated DFT models and configurational sampling to establish band positions associated with allylic stretching v_as(C=C─C⁺) in cyclopentenyl cations. We found that the band position of this stretch was sensitive to the substitution pattern on the allylic system of the ring, such that a methyl substitution instead of a hydrogen at the center carbon (C-2) resulted in a ~ 20 cm^-1 red-shift in the IR band. Our collaborative efforts also found that the formation of these alkylcyclopentenyl cations in zeolites is shape-selective; the C-2 methyl-substituted alkylcyclopentenyl cation forms in larger pore HMOR whereas in medium pore zeolites, HZSM-5 and HZSM-22, the C-2 hydrogen-substituted alkylcyclopentenyl appears to be the main product. We performed DFT-based thermodynamics calculations and found that the relative stability of the methyl-substituted alkylcyclopentenyl cation remained unchanged in all three zeolites. This suggests that the formation of these alkylcyclopentenyl cations is not under thermodynamic control. We used DFT calculations to build a microkinetic model of the isomerization of alkylcyclopentenyl cations in HZSM-5 and HZSM-22 zeolites, using both finite-temperature dynamics and zero-Kelvin path methods to compute barriers. We found that the isomerization leading to experimentally relevant alkylcyclopentenyl cations with C-2 methyl (T-type) or hydrogen (K-type) substitutions occurs through a multi-pathway reaction network. We found that the pathways were similar in the two zeolites, but the populations at equilibrium differed such that one T-type product formed in HZSM-5 and two formed in HZSM-22 with evidence of kinetic control of product formation.

DOI

https://doi.org/10.7275/28563594

Recommended Citation

Manookian, Babgen, "Molecular Vibrations and Shape-Selectivity: A Computational Model of Biofuel Precursors in Zeolites" (2022). Doctoral Dissertations. 2551.
https://doi.org/10.7275/28563594 https://scholarworks.umass.edu/dissertations_2/2551