Inhibiting the Dealkylation of Basic Arenes during n-Alkane Direct Aromatization Reactions and Understanding the C₆ Ring Closure Mechanism

Abstract

Pt/KL is the widely accepted catalyst for aromatizing n-hexane by 1,6 ring closure but encounters deactivation issues when aromatizing higher carbon-number feeds; undergoing extensive dealkylation to give unwanted CH4 and unselective products, as well as over-aromatization to form coke. Here, we report the use of a non-acidic MFI zeolite support, containing excess K + beyond ion exchange capacity, well dispersed Pt, and high Pt presence inside the pores, for maximising direct n-alkane aromatization selectivity. TGA, catalyst deactivation studies, and characterizations show that the smaller pore sizes and lack of large cages in the MFI support sterically inhibit coke formation inside the pores (0% compared to 4.9% over Pt/KL for n-octane aromatization), which also reduces dealkylation of ethylbenzene and o-xylene under mild conditions to give a more selective product distribution, 86% selectivity by weight towards C6 ring closure compared to 27% for Pt/KL. Additionally, using NH3-TPD, XPS,CO-DRIFTS, and STEM, we show the contribution of excess K + as an inhibitor of strong acid sites, an indirect Pt electron promoter through improving metal support interaction, and Pt dispersant. This work highlights the alternative use of well-understood zeolitic supports for the highly selective aromatization of n-heptane and n-octane by 1,6 ring closure, increasing the number of potential streams that can undergo direct aromatization, and providing a suitable alternative to Pt/KL.

The Cross-Coupling of Methane with Nonoxidative Hydrocarbons

“…A detailed mechanism was proposed by Jarvis and Harrhy et al for the aromatization of n-octane, one of the heavier and most abundant alkanes found in naphtha [74]. Although this study was not investigated under a methane environment, the mechanism for aromatization of paraffins still holds and provides valuable insight into the role of single-atom Pt sites.…”

“…The resulting C 6 bimetallocycle, formed over Pt nanoparticles, undergoes subsequent dehydrogenation to form a bimetallo-ethylcyclohexene followed by bimetallo-ethylcyclohexadiene and finally a bimetalloethylcyclohexatriene. Note that all these steps occur on the same site, and no desorption and readsorption is thought to occur due to the absence of cyclopentenes in the product [74], which would form if the ethylcyclohexene were to desorb and readsorb as shown in the previous subsection. Furthermore, the absence of free intermediates in the product stream of low temperature reactions strengthens this hypothesis.…”

“…Bimetallocycle mechanism for the reaction of n-octane over Pt/KMFI (KX) (blue). Pt cluster mechanism for the reaction of n-octane over Pt/ MFI (magenta)[74]…”

Direct Systems: Methane Dehydroaromatization (MDA) and the Oxidative Coupling of Methane (OCM)

“…In addition to the benefits of acidity obtained through using zeolites like ZSM-5, the MFI type structure exhibited by ZSM-5 provides a unique morphology which is believed to promote aromatic formation through shape selectivity [22]. Below is shown the detailed structure of ZSM-5 as well as other commonly used supports in the petrochemical industry, to help demonstrate the nature of its potential shape selectivity as one of the many reasons why ZSM-5 is used in aromatization reactions (Fig.…”

Highly Selective Skeletal Isomerization of Cyclohexene Over Zeolite-Based Catalysts for High-Purity Methylcyclopentene Production

“…[16] On the other hand, the presence of K in the zeolite framework structure also leads to enlarged micropores and uniform morphology of zeolite catalysts, which is also reported in previous studies. [17,18] Then, more e cient ion exchange process can be realized and the acidity can be more effectively controlled, which is proved by high Na content in NaUZSM-5 and corresponding low acidity. The changes of acidity, structure and morphology make combined contributions to the favorable methylcyclopentene selectivity.…”