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Influence of the Dehydrogenation Function on Propene Aromatization Catalysis Over Physical Mixtures of PtZn/SiO2 and H-MFI
This work studies propene aromatization reaction on H-MFI (Si/Al = 40) and physical mixtures of H-MFI (Si/Al = 40) and PtZn/SiO2 (2 wt% Pt, 3 wt% Zn) at 723 K - 823 K and 3 kPa C3H6. The influence of PtZn alloy (dehydrogenation function) is investigated on the product distribution and selectivity of metal-acid catalyzed propene aromatization. Typical product distribution consists of methane, ethane, ethene, propane, C4-C6 alkanes and alkenes, and benzene, toluene, xylene (BTX). On comparing the BTX carbon selectivity over the two catalysts at first equivalent space velocity and then equivalent propene conversion, higher BTX selectivities are observed on PtZn+H-MFI than H-MFI in both the cases. The higher BTX selectivities were previously attributed in the literature to the dehydrogenation pathway on the metal function. However, space velocity is an inadequate descriptor of reaction progress because the conversion of reactants can be different at same space velocity. Similarly, propene conversion is an incomplete descriptor for reaction progress because intermediates such as ethene and C4-C6 hydrocarbons react to form higher molecular weight hydrocarbons and subsequent aromatics as the reaction progresses. Such reactive hydrocarbons were lumped together as reactive intermediates and the remaining hydrocarbons were classified as non-reactive species or products. When BTX selectivities over PtZn-H-MFI and H-MFI are compared at equivalent temperature and equivalent conversion of all the reactive intermediates, both the catalysts exhibit similar BTX selectivities, suggesting that the presence of the dehydrogenation metal function doesn’t influence the selectivity towards BTX products. Further, we hypothesize cyclohexene as an intermediate in aromatic formation and use cyclohexene conversion as a probe reaction to understand how aromatics are formed over Brønsted acid sites and PtZn alloy. Cyclohexene conversion results at 723 K and 823 K shows the presence of an alternate route of aromatic formation via dehydrogenation of cycloalkenes, and this dehydrogenation pathway has an order of magnitude higher rates than the hydride transfer route on Brønsted acid sites. Further, we propose dominant reaction pathways of C1 – BTX hydrocarbon formation on H-MFI and bifunctional PtZn+H-MFI. Finally, we discuss the implications of using PtZn+H-MFI on developing a commercial propylene aromatization process and provide our recommendations for chemical and fuel production. In summary, these findings reveal previously unknown mechanistic details of metal bifunctionality for propene aromatization catalysis.
Engineering Research Center for Innovative and Strategic Transformation of Alkane Resources - CISTAR
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