Control of the Reaction Mechanism of Alkylaromatics Transalkylation by Means of Molecular Confinement Effects Associated to Zeolite Channel Architecture

Abstract

Transalkylation of alkylaromatics catalyzed by acid zeolites is a process widely employed in the petrochemical industry for upgrading aromatic fractions. The reaction mechanism is complex as it can proceed either by intermolecular alkyl-transfer involving dealkylation-alkylation steps with surface alkoxy species as reaction intermediates, or through the formation of bulkier diaryl intermediates. We have investigated how the possible formation of such bulky intermediates in the microporous channel system of different zeolite structures, together with their stabilization by confinement effects, can determine the preferential mechanism and, therefore, the selectivity of ethylbenzene disproportionation into benzene and diethylbenzene. For testing the concept, four zeolites, MCM-22 (3D MWW) with 10R pores, 12R cavities and external 12R hemicavities or "cups", DS-ITQ-2, a 2D MWW with the same 10R channels as MCM-22, no 12R cavities and much larger proportion of external "cups", a 10R ZSM-5 (MFI) and a 12R mordenite (MOR) have been used. The higher activity of DS-ITQ-2 and MCM-22 as compared to ZSM-5, at low temperature (573 k) and the high selectivity to diethylbenzene of the bidimensional material under all reaction conditions considered have been explained by means of DFT calculations. Contrary to what could be expected according to the available space at the external "cups" and at the 10R channels of the MWW structure, the bulkier diaryl intermediates are better stabilized within the 10R channel system than at the "cups" open at the external surface of the MWW materials. We show from this perspective how the channel structure and molecular confinement stabilization also explain the operating reaction mechanism in ZSM-5 and mordenite.

Effect of the catalyst in the BTX production by hydrocracking of light cycle oil

“…Disproportionation and transalkylation are the two major practical processes for the interconversion of alkylaromatics, especially for the production of dialkylbenzenes [39,40]. The contribution of these two mechanisms to the overall process depends on the relative stability of the alkylaromatic and bulkier diaryl cationic intermediates and is largely influenced by the reactant size, zeolite pores, and the presence of channel intersections or cavities [41].…”

Role of the ionic environment in enhancing the activity of reacting molecules in zeolite pores

“…A long series of investigations has shown that Brønsted acid sites (BAS) have constant acid strength for sorption and catalysis, as long as the concentrations of substituting tetrahedral atoms did not exceed a certain threshold (8,9). The high intrinsic catalytic activity of zeolites has, therefore, been attributed to the remarkable stabilization of transition states in the constraints of pores (10)(11)(12)(13)(14).…”

Effect of the chemical composition of six hydrotreated light cycle oils for benzene, toluene, ethylbenzene, and xylene production by a hydrocracking process

“…BTEX can be produced either by the direct HCK process or by disproportionation and transalkylation (DTR) processes [3,4,[27][28][29][30][31] from more complex alkyl-aromatic compounds obtained earlier by the same direct hydrocracking of naphthalene, tetralin, and indene derivatives as shown in Fig. 10.…”

“…In the case of zeolites as catalysts [28,29], the transalkylation of alkylbenzenes may proceed by intermolecular alkyl transfer involving dealkylation-alkylation steps with surface alkoxy species as reaction intermediates or through the formation of bulkier diaryl intermediates (Fig. 13) [29][30][31].…”

“…The contribution of these two mechanisms to the overall process depends on the relative stability of the alkylaromatic and bulkier diaryl cationic intermediates and is largely influenced by the reactant size, zeolite pores, and the presence of channel intersections or cavities [31]. Therefore, the cavities and pore channels must be large enough to accommodate the required transition state, especially for alkyl-aromatics with sterically hindered alkyl groups (bimolecular intermediate) [29][30][31], otherwise the formation of the relevant di-or polyalkyl aromatics should be accommodated, for example, by a dealkylation/alkylation mechanism [29]. An important condition that must be fulfilled is that the alkyl-containing aromatic molecule (the one that must lose the alkyl group) must have unlimited access to the active sites (steric constraints, catalyst choice) for effective transalkylation reactions [29].…”