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Development of Bi-Functional Zeolite-Based Catalysts for Methane Dehydroaromatization

Lai, Yungchieh (2016) Development of Bi-Functional Zeolite-Based Catalysts for Methane Dehydroaromatization. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Abstract

Benzene is one of the most important organic intermediates in the petrochemical industry. Direct methane dehydroaromatization (DHA) under non-oxidative environment has been shown to be a promising pathway to produce benzene since Mo/HZSM-5 was reported as a viable catalyst in 1993. The reaction over this catalyst is generally accepted to proceed via a synergistic mechanism between the metal sites (i.e. MoOxCy) and the Bronsted acid sites (BAS) of HZSM-5. The high benzene selectivity is attributed to the similarity in pore size of ZSM-5 with the dynamic molecular size of benzene. However, diffusion of large benzene molecules inside the zeolite micropores also enhances the chance for undesired further reactions on the BAS, resulting in coking and hence deactivation of the catalyst. While the catalyst can be regenerated via burn-off of the coke in an oxidative environment, this burn-off also results in oxidation of the Mo species, and thus requires a (re-)activation period to convert Mo back into its active oxicarbide form. This catalyst deactivation via coking combined with the lengthy reactivation of the catalyst pose a significant hurdle for economic viability of the process.
The objective of this research is to develop a new bi-functional zeolite-based (metal) catalyst that is highly stable while maintaining the high benzene yield from DHA. Towards this goal, we aimed to investigate systematically how the catalytic performance is affected by changes in i) metal sites and ii) porosity of HZSM-5.
To study the impact of the metal site in the zeolite, Zn-HZSM-5 and Fe-HZSM-5 catalysts with different metal dispersion was prepared via different synthetic routes and the structural properties of the catalysts were correlated to their catalytic performance. We found that only well dispersed metal species located within the micropores of the zeolite are active for DHA, confirming the proposed reaction mechanism of bi-functional metal/ZSM-5 catalysts in the literature. Furthermore, for Fe-HZSM-5, we found that atomically dispersed iron is thermally most stable and result in no detectable coke formation (on the metal sites). This observation thus yields a valuable guideline for the preparation of DHA catalysts and can potentially help to resolve the primary issues of state-of-the-art Mo/HZSM-5 catalysts: thermal stability and coking resistance.
As a second target, the micro- and meso-porosity of HZSM-5 was systematically changed by introducing different types and quantities of “porogens” (metal oxide or carbon black NPs) during zeolite hydrothermal crystallization. We found that coke formation within micropores is suppressed if a controlled degree of mesoporosity is introduced into the catalysts, while too much mesoporosity again enhances coke formation, presumable due to “excess space” and BAS for hydrocarbon formation. Moreover, with increasing mesoporosity the microporosity of the zeolite is decreased, which results in lower benzene selectivity (due to the absence of shape/size selectivity in the mesopores). Overall, one hence obtains optimal performance by balancing minimization of coke formation with high benzene selectivity by introducing a controlled (small) amount of mesoporosity into HZSM-5.


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Details

Item Type: University of Pittsburgh ETD
Status: Unpublished
Creators/Authors:
CreatorsEmailPitt UsernameORCID
Lai, Yungchiehyul69@pitt.eduYUL69
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairVeser, Götzgveser@pitt.eduGVESER
Committee MemberEnick, Robert Mrme@pitt.eduRME
Committee MemberMillstone, Jilljem210@pitt.eduJEM210
Committee MemberKeith, JohnJAKEITH@pitt.eduJAKEITH
Date: 25 January 2016
Date Type: Publication
Defense Date: 1 September 2015
Approval Date: 25 January 2016
Submission Date: 1 October 2015
Access Restriction: 5 year -- Restrict access to University of Pittsburgh for a period of 5 years.
Number of Pages: 178
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Chemical Engineering
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: zeolite, ZSM-5, Mo/HZSM-5, methane, aromatization, nanomaterial
Date Deposited: 25 Jan 2016 20:40
Last Modified: 15 Nov 2016 14:30
URI: http://d-scholarship.pitt.edu/id/eprint/26168

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