IRREVERSIBLE ADSORBATE THERMODYNAMICS ON ZEOLITE SURFACES

Author

Ajibola Lawal, University of Massachusetts AmherstFollow

Author ORCID Identifier

https://orcid.org/0000-0001-8726-6570

AccessType

Open Access Dissertation

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

Wei Fan

Third Advisor

Scott Auerbach

Subject Categories

Catalysis and Reaction Engineering | Thermodynamics

Abstract

The design of solid acid catalysts has greatly benefitted from understanding adsorbate energetics (enthalpy and entropy), where the relative stability of surface species dictates the overall catalytic turnover frequency. Different techniques exist for zeolite characterization to detail functionality and adsorbate interaction with the Brønsted acid site. Despite these techniques, information on the adsorption thermodynamics of adsorbates irreversibly bound to a Brønsted acid site, representative of reaction intermediates and transition states, particularly entropic contributions, are relatively limited. Existing approaches to measure an adsorbate’s entropy require the adsorption/desorption process to equilibrate. The challenge arises from the requirement of establishing equilibrium between an adsorbate and Brønsted acid site to measure thermodynamic information. Irreversible bound adsorbates, however, do not equilibrate on relevant timescales given their negligible rate of desorption, cannot attain adsorption/desorption equilibrium, limiting the applicability of existing methods. To overcome this issue, the concept of adsorption assisted desorption through the co-adsorption of multiple irreversible bound adsorbates has been leveraged. Relative thermodynamic information can be obtained when exposing a Brønsted acid site to multiple irreversible bound adsorbates, which collectively behave as competitive adsorbates that attain an adsorption/desorption equilibrium. While one irreversible bound adsorbate does not equilibrate due to limited desorption, a binary mixture of irreversible bound adsorbates can continuously displace one another from an Brønsted acid site and attain adsorption-desorption equilibrium. We developed a method that leverages on adsorption-assisted-desorption, where we measure the adsorption thermodynamics of alkylamines on Brønsted acid sites on H-MFI Zeolite. Through adsorption-assisted desorption, whereby distinct alkylamines facilitate desorption from Brønsted acid sites, we demonstrate that equilibrated states are achieved. By varying relative vapor phase partial pressures and temperatures, we demonstrate the ability to experimentally measure the adsorption enthalpy and entropy of alkylammonium adsorbates on mostly isolated Brønsted acid sites in H-ZSM-5 (Si/Al = 140). Across a homologous family of sec-alkylamines (C₃-C₅) adsorbed on isolated Brønsted acid sites, a fixed contribution to the enthalpy (19 ± 4 kJ mol CH₂^-1) and entropy (25 ± 4 J mol CH₂^-1 K^-1) of adsorption per methylene unit of was found to exist, likely resulting from electrostatic interactions between the alkyl chain and surrounding pore environment. We investigated, the effect of orientation and size of primary alkylamines, providing quantitative understanding of the trends of adsorption energetic for these transition-state like molecules. There is a consistent adsorption energetic increment for a homologous series of primary alkylamines, with additional methyl (-CH₂) group. We also utilize the energetic information to discuss adsorption orientation preference on a C3 and C5 backbone, showing the change in preference from a terminal orientation to central orientation with increased catalytic temperature, and suggest its influence on cracking reactions. Using a single descriptor of alkyl side-chain length we describe the difference in adsorption enthalpy and adsorption entropy of multiple alkylamines, measuring a consistent loss in entropy of 27 ± 11 J mol CH₂^-1 K^-1 and enthalpic stabilization of 17 ± 4 kJ mol CH₂^-1 suggesting that the dominating alkyl-chain length is stabilized in the same fashion, within the pore environment with each added n-CH_2. Having established the adsorption energetics of these alkylamines without the influence of proximate sites and measured the overall adsorption thermodynamics of these irreversible bound species over mostly isolated sites. We use the adsorbate thermodynamics methodology, to investigate the interplay of proximate sites for these alkylamines. We measure proximate sites on the H-MFI framework, by exchanging parent form (H-MFI), with divalent Co²⁺. Co-MFI residual H⁺ site count is compared with H-MFI Brønsted site count to accurately estimate proximate sites. We demonstrate this concept of lateral interactions between neighboring alkylammonium species by manipulating Al content to control the fraction of Al pairs in H-MFI, revealing a linear trend between relative adsorption energetics and the fraction of Al pairs onto which alkylamines adsorb. This electrostatic interactions and stabilization for larger alkylamines is reduced linearly with increasing fraction of Al-pairs at lower Si/Al where lateral interactions from proximate sites diminishes relative differences. This work has established differences in adsorption energetics for transition state like molecules to interpret and understand trends for zeolite catalyzed reactions. We also detail the thermodynamic effect of proximate sites, by showing that one can rationalize the decreased stabilization through electrostatic interactions, as lateral interaction between adsorbates happens at these proximate sites. This informs us about the energetic effect of proximate sites and its catalytic consequences. Providing relevant thermodynamic information to tailor-make zeolites for different industrially relevant chemistries.

DOI

https://doi.org/10.7275/36062618

Recommended Citation

Lawal, Ajibola, "IRREVERSIBLE ADSORBATE THERMODYNAMICS ON ZEOLITE SURFACES" (2024). Doctoral Dissertations. 3059.
https://doi.org/10.7275/36062618 https://scholarworks.umass.edu/dissertations_2/3059

Creative Commons License

This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.