A mechanistic basis for the effect of aluminum content on ethene selectivity in methanol-to-hydrocarbons conversion on HZSM-5

Abstract

Increasing crystallize size or aluminum content in MFI-type zeolites independently enhances the propagation of the aromatics-based methylation/dealkylation cycle relative to that of the olefinsbased methylation/cracking cycle in methanol-to-hydrocarbons (MTH) conversion and consequentially results in higher ethene selectivity. We report that ethene selectivity increases monotonically with increasing aluminum content for ZSM-5 samples with nearly identical crystallite size as a follow-up to our recent report detailing the effects of crystallite size (J. Catal. 321 (2015) 23) on MTH selectivity. We also propose that the confected effects of crystallite size and acid site density on MTH selectivity can be correlated using a descriptor that represents the average number of acid sites that an olefin-precursor will interact with before elution.

Recent trends and fundamental insights in the methanol-to-hydrocarbons process

“…An increase of Al content enhances the chance of reactant molecules to interact with each other, thus increasing the chance to form aromatics and ethylenethe product of the aromatic cycle. 97 The observed trends also account for the improved selectivity to propylene for post-synthetically modified catalysts with different elements as well demetalated zeolites. It has been extensively shown that phosphatation and incorporation of alkaline-earth metals lead to a significant reduction of Brønsted acidity and its effect can be regarded as an increase in the Si/Al ratio.…”

Nature and Location of Carbonaceous Species in a Composite HZSM-5 Zeolite Catalyst during the Conversion of Dimethyl Ether into Light Olefins

“…The group of Bhan [19,20] has proposed different selectivity ratios in order to quiantify the importance of each cycle, e.g., the ethylene/isobutane ratio is representative of the arene/olefin cycle ratio [19]. Thus, Figure 5 displays the values of ethylene/isobutane ratio at different temperatures for time on stream values of 0 and 18 h. As observed, the arene cycle dominates at lower temperatures and longer time on stream values.…”

“…In this type of zeolites and zeotypes, DTO mechanism is assumed to occur through the "dual cycle mechanism" (similar to MTO), with olefins and arenes (alkyl-benzenes) being the intermediates in the oligomerization-cracking and olefin/aromatic methylation-dealkylation routes [19]. The pore topology of HZSM-5 zeolite makes the arene cycle particularly dominating for highly acid catalysts [20]. Regarding the arene cycle, the species involved decrease their DTO activity when they are highly methylated or alkylated [21].…”

Kinetic and Deactivation Differences Among Methanol, Dimethyl Ether and Chloromethane as Stock for Hydrocarbons

“…The lines of these figures correspond to experiments using different n BAS /Fat TOS = 20 min whereas the points represent variable TOS at n BAS /F = 196.3 μmol BAS h mol CH3 À 1 . Those indexes previously used in the literature [20,23,56,57] enable to study the dual cycle mechanism on the MTH reaction and compare the effect of the conversion or the co-feeding of different species. In this work, we have used the same mechanistic concepts to compare the MTH, DTH and CTH reactions.…”