Highly Selective Methylation of Ethylene to Propylene within the Confined Side Pockets of Mordenite Zeolite

Abstract

Ethylene methylation with C1 reagents provides a viable route for propylene production and enables flexible adjustment of olefin output ratios in industrial processes (e.g., naphtha steam cracking and methanol-to-olefins). However, this process faces the long-standing challenges of low propylene selectivity and rapid catalyst deactivation. Herein, a highly selective and stable catalyst for ethylene methylation to propylene was developed by precisely regulating the acid site distribution in mordenite (MOR) zeolite, where the acid sites in the 12-membered ring (12-MR) channels were passivated using pyridine, while those located in the confined 8-membered ring (8-MR) side pockets remained accessible. This spatial control of acid site distribution allowed the exclusive occurrence of ethylene methylation within the sterically confined pockets, effectively suppressing the side reactions requiring larger space (e.g., methanol-to-hydrocarbons reaction and olefin oligomerization). Remarkably, the optimized pyridine-modified MOR catalyst achieved an unprecedented propylene selectivity of 97% and exhibited exceptional stability with no sign of deactivation during a 70 h test. In situ Fourier transform infrared (FT-IR) spectroscopy, theoretical calculations, and isotope labeling experiments were utilized to elucidate the mechanism of ethylene methylation and establish the reaction network within the confined pockets. It is anticipated that the side pockets of the MOR zeolite, which can be considered as an angstrom-scale reactor, would provide more opportunities for the precise assembly of small organic molecules.

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(a) Comparison of the catalytic performance of ethylene methylation with DME over different zeolite catalysts (TOS = 2 h). (b) GC-MS chromatograms of the organic species extracted from the spent catalysts after 24 h of reaction. Reaction conditions: T = 563 K, P _total = 101.3 kPa, P(DME) = 3.3 kPa, P(C₂H₄) = 33 kPa, GHSV = 1200 mL/g_cat/h, N₂ as the balance gas. The peak at the retention time of 9.65 min in panel b corresponds to the internal standard (hexachloroethane).

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Representative MD simulation snapshots and corresponding density distributions of (a, d) ethylene, (b, e) propylene, and (c, f) butylene within the MOR zeolite framework throughout the simulations.

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(a, b) FT-IR spectra and ¹H MAS NMR spectra of H-MOR (Si/Al = 16) before and after pyridine modification. (c, d) The catalytic performance of ethylene methylation with DME over MOR (Si/Al = 16) with different acid site distributions and over Py-HMOR with different Si/Al ratios, respectively. Reaction conditions: T = 563 K, P _total = 101.3 kPa, P(DME) = 3.3 kPa, P(C₂H₄) = 33 kPa, GHSV = 1200 mL/g_cat/h, and N₂ as the balance gas.

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(a–c) Ethylene methylation over Py-HMOR (Si/Al = 16) under different reaction conditions and (d) the stability test of the catalyst. Reaction conditions for (a): P _total = 101.3 kPa, P(DME) = 3.3 kPa, P(C₂H₄) = 33 kPa, and GHSV = 1200 mL/g_cat/h; for (b): T = 563 K, P(DME)/P(C₂H₄)/P(N₂) = 1/10/19.7, GHSV = 1200 mL/g_cat/h; for (c): T = 563 K, P _total = 101.3 kPa, P(DME) = 1–4 kPa, P(C₂H₄) = 33 kPa, or P(DME) = 3.3 kPa, P(C₂H₄) = 0–33 kPa, GHSV = 1200 mL/g_cat/h; for (d): T = 563 K, P _total = 101.3 kPa, P(DME) = 3.3 kPa, P(C₂H₄) = 33 kPa, GHSV = 1200 mL/g_cat/h. N₂ as the balance gas.

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In situ FT-IR spectra of Py-HMOR (Si/Al = 16) and the corresponding mass spectra of the effluent gas from the IR cell following the sequential introduction of DME (a, b) and ethylene (c, d). Experimental conditions: T = 523 K, P _total = 101.3 kPa, DME (1% in He, 5 mL/min), C₂H₄ (2% in He, 5 mL/min), and He purging (20 mL/min) for 15 min.

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Gibbs free energy surface (inside) for the proposed ethylene methylation reaction mechanism (outside) within the 8-MR side pockets of the MOR zeolite.

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Isotopic distribution of products from reactions of ethylene with DME over Py-HMOR (Si/Al = 16). In all panels: top: ¹³C-ethylene feed; bottom: ¹²C-ethylene feed.

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Reaction network of ethylene methylation with DME in the confined 8-MR side pockets of the Py-HMOR catalyst.