Reaction Mechanisms of the Zeolite-Catalyzed Transformations of C9 Alkylaromatics

Abstract

Transformations of alkylaromatic hydrocarbons are the “heart” of the industrial petrochemistry using zeolite catalysts as the most important large-scale catalysts. For example, m-xylene isomerization and transalkylation of 1,2,4-trimethylbenzene (1,2,4-TMB) with toluene processes using zeolite-based catalysts are utilizting all over the world for the commercial production of p-xylene, which is mainly used as a raw material for the production of polyester fibers. Especially C9 alkylaromatic hydrocarbons involving 1,2,4-TMB and iso-propylbenzene (iPB) are major feedstocks for the production of highly valuable para-dialkylbenzenes, such as p-xylene and p-di-iso-propylbenzene. When a zeolite catalyst is considered, a deep understanding of reaction pathways is necessary not only for a detailed interpretation of the catalytic behavior of zeolite catalysts but also for selecting the zeolite structures potentially usable in a given reaction. Observation and identification of organic species (i.e., reactants, intermediates, and products) involved in catalytic process are surely important in this context, since they provide a clue to reasonably clarify this complex issue at molecular scale. In this study, we have investigated the reaction mechanisms of C9 alkylaromatics conversions (i.e., 1,2,4-TMB disproportionation and iPB disproportionation) with respect to the structural and physicochemical properties in zeolite catalysts. Furthermore, as an extension of our work on the mechanisms of zeolite-catalyzed conversions of C9 alkylaromatics, we have focused on the transalkylation of C9 alkylaromatics (i.e., 1,2,4-TMB and iPB) with toluene into the xylenes and cymenes, respectively. 1. While 1,2,4- TMB disproportionation is one of the potential technologies for p-xylene production, its reaction intermediates have neither been experimentally observed nor identified yet. Here we present gas chromatography-mass spectrometry (GC-MS) evidence that not only pentamethylated diphenylmethane (5mDPM) derivatives but also hexamethylated diphenylmethane (6mDPM) ones are serving as key reaction intermediates of 1,2,4-TMB disproportionation over large-pore zeolites. We also propose a new bimolecular diphenylmethane-mediated reaction pathway for the formation of tetramethylbenzenes (TeMBs) and xylenes over large-pore zeolites based on the GC-MS results obtained. Comparison with the GC-MS results from LaNa-Y after transalkylation reactions of three different TeMB isomers with three xylene and three TMB isomers, respectively, allows us to rationally identify most of the seven 5mDPM and three 6mDPM derivatives at their isomer level. A combination of GC-MS and DFT calculation results demonstrates that the bimolecular 1,2,4-TMB disproportionation over LaNa-mordenite, a practically one-dimensional 12-ring zeolite with regard to this reaction, is a new example of transition state shape selectivity in zeolite catalysis. 2. The catalytic properties of a series of large-pore (H-Y, H-beta, H-mordenite, and H-UZM-35) and medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites are compared in iPB disproportionation. Among the zeolite catalysts studied here, H-UZM-35 with a three-dimensional framework consisting of one type of straight 12-ring channels and two types of tortuous 10-ring channels was found to show a comparable di-iso-propylbenzenes (DiPBs) yield to that of H-beta with two intersecting 12-ring channels, the best catalyst tested for this reaction so far. GC-MS analysis of used zeolite catalysts demonstrates that while mono-iso-propylated 2,2-diphenylpropane derivatives are serving as real reaction intermediates of iPB disproportionation over large-pore zeolites, mono-iso-propenylated 2,2-diphenylpropane species, which contains a double bond in the alkyl chain, are intermediates of its side reaction. Unlike that of other aromatic hydrocarbons such as m-xylene, ethylbenzene, and n-propylbenzene, the formation of di-iso-propylated derivatives was not observed as reaction intermediates. A new bimolecular diphenylpropane-mediated reaction pathway, which includes both intermediates of main and side reactions of iPB disproportionation, is proposed based on the experimental and theoretical results. 3. We report the catalytic properties of a series of large-pore (H-Y, H-beta, H-mordenite, and H-UZM-35) and medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites with different framework structures for the transalkylation of 1,2,4-TMB with toluene. H-NU-87 with intersecting 10- and 12-ring channels, but whose access to the inner part of the crystal can only occur through the 10-ring pores, was found to show a significantly higher xylenes yield and catalyst stability than the cage-based, large-pore zeolite H-Y, the current commercial transalkylation catalyst. GC-MS analyses of the used zeolite catalysts reveal that the type of diphenylmethane derivatives serving as key reaction intermediates of 1,2,4-TMB-toluene transalkylation is strongly influenced by the pore architecture of the zeolite catalyst, indicative of transition state shape selectivity. A bimolecular diphenylmethane-mediated reaction mechanism for this transalkylation has been proposed based on the overall GC-MS results of this work and further confirmed by DFT calculation results. 4. The catalytic activities of four large-pore (H-Y, H-beta, H-mordenite, and H-UZM-35) and three medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites for the transalkylation of iso-propylbenzene (iPB) with toluene are investigated. Among the zeolite catalysts employed here, H-UZM-35 with a 12 × 10 × 10-ring channel system was found to exhibit a comparable cymenes yield to that of H-beta with a 12 × 12 × 12-ring channel system, the most widely studied catalyst for this reaction. GC-MS analysis reveals that monomethylated 2,2,-diphenylpropane species, whose existence has not been experimentally verified yet, are serving as the main reaction intermediates of the bimolecular iPB-toluene transalkylation. Also, the intrazeolitic build-up of dimethylated 2,2-diphenylpropane and 2-methylphenyl-2-iso-propylphenylpropane species, which must be involved in the formation of 2-tolylpropanylium cations and thus in the simultaneous consumption and production of the reactant molecules (i.e., toluene and iPB), was observed. The formation of these three different groups of diphenylpropane species, which has been further supported by the DFT calculation results, allowed us to propose a new bimolecular reaction mechanism for this transalkylation. To our knowledge, our study is the first example where the repetitive mechanism is ascertained in zeolite-catalyzed reactions.