Chemical and Structural Parameter Connecting Cavity Architecture, Confined Hydrocarbon Pool Species, and MTO Product Selectivity in Small-Pore Cage-Based Zeolites

Abstract

The catalysts used in the methanol-to-olefins reaction are considered dual systems comprising an inorganic zeolite framework and organic compounds hosted inside that act as co-catalyst. The influence of zeolite cavity architecture on the preferential stabilization of cationic intermediates involved in the paring and side-chain routes of the hydrocarbon pool mechanism is analyzed by means of DFT calculations, catalyst testing and 13 C NMR spectroscopy for some small-pore cage-based zeolites. A correlation between the degree of methylation of the entrapped methylbenzenium (MB +) cations and the selectivity to ethene and propene is found experimentally and explained in terms of the electronic distribution of the first intermediate of the paring route. A deep understanding of the reaction mechanism and of the specific host-guest interactions taking place inside zeolite catalysts allows establishing a quantitative parameter that is indicative for the contribution of the paring route and therefore the C3 = /C2 = ratio in the MTO reaction.

Directed transforming of coke to active intermediates in methanol-to-olefins catalyst to boost light olefins selectivity

“…As an archetypical co-catalysis reaction, catalytic performance in MTO is manipulated by the organic intermediates confined in zeolite channels or cavities through the sophisticated hydrocarbon pool mechanism 3 , 7 – 9 . These hydrocarbon pool species (HCPs), typically including the methyled-benzene carbocations 10 , 11 and cyclopentadienyl species 12 , 13 , are decisive for light olefins selectivity, owning to the altering of acidity 14 , reaction paths 15 , kinetics 8 , 9 , molecular transport 16 , and among others. However, the HCPs are also coke precursors that can readily evolve to polycyclic aromatic hydrocarbons (PAHs), the typical coke species, through cyclization 17 and cross-linked mechanism 18 , 19 , accelerating catalyst deactivation 20 .…”

“…Some methods have been proposed to modulate the product selectivity via tailoring the HCPs confined in zeolites for MTO reactions. By use of mimics of the HCPs as organic structure-directing agents (OSDAs) to synthesize CHA and RTH-related zeolites, it is possible to maximize the host–guest interaction between framework and polymethyl aromatic intermediates to promote the propylene/ethylene ratio 8 , 11 . However, precise design and synthesis of targeted zeolites with specific OSDAs are extremely challenging 23 , especially the HCPs slightly different in structures may significantly change the product selectivity 8 .…”

Elucidating Zeolite Channel Geometry–Reaction Intermediate Relationships for the Methanol‐to‐Hydrocarbon Process

“…[4][5][6][7] As ar esult, significant research efforts have been focused on elucidating the influence of zeolite topology,that is,channels, cages and cavities,o nt he ultimate product distributions and coking behaviors. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Yet, no direct experimental evidence has been presented for the effect of channel geometry on the reaction intermediates.…”

Impact of Zeolite Framework Composition and Flexibility on Methanol‐To‐Olefins Selectivity: Confinement or Diffusion?

“…A linear relationship was found between the C 3 = /C 2 = ratios measured for different zeolites under different reaction conditions and the E int(7/5) parameter corresponding to each structure, thus confirming the confinement effect associated to cage topology as the factor governing the MTO product selectivity. [24] However, the pure silica cluster models used in that study did not take into account the possible effect of acid site concentration or distribution, nor the influence of framework composition on the MTO reactivity. In this sense, it is interesting to compare the catalytic behavior of pairs of zeolites (aluminosilicates) and SAPOs (silicoaluminophosphates) with the same framework crystallographic structure but different framework composition, such as for instance H-SSZ-13 and H-SAPO-34 with the CHA structure, or H-SSZ-39 and H-SAPO-18 with the AEI structure ( Figure 1).…”

“…Thus, a product distribution consisting of % 50 % propene and similar amounts of ethene and butene, around 20 % each, have been reported for the two AEI-based catalysts independently of their framework composition. [21,22,[24][25][26][27][28] In contrast, CHA-type catalysts tend to produce more ethene and much less propene and butene, but there are differences in product distribution associated to framework composition. H-SSZ-13 always produces more ethene than propene, 45 % and 35 % respectively, while the opposite relationship is always found for H-SAPO-34.…”