The conversion of methanol to olefins (MTO) is an important technology in upgrading natural gas feedstocks to chemical intermediates. The active site and structural diversity present within Bronsted acidic zeolite and zeotype catalysts have been shown to influence product selectivities, and time-on-stream stability [1]. Methanol dehydration to dimethyl ether is a step that occurs during the induction period of MTO catalysis [2], and is a versatile probe reaction of the acid strength and confining environments present within porous solid acids, for which data has been collected and calibrated on a wide variety of solid acid catalysts of known structure and composition [3]. First-order rate constants depend on the acid strength, defined rigorously by deprotonation energy (DPE), and the size of the confining environment, because dimethyl ether formation transition states are larger in size and more positively charged than relevant hydrogen-bonded methanol monomer intermediates [3]. In contrast, zero-order rate constants depend only on acid strength, because transition states and relevant protonated methanol dimer intermediates are similar in size, but differ in cationic charge distribution [4]. Among CHA zeolites (SSZ-13), rate constants depend on the fraction of Al in paired configurations (Al-O(-Si-O)x-Al) with x = 1, 2) [5], because different reaction mechanisms appear to prevail on isolated and paired sites in small pore zeolites, as observed by the formation of surface methoxy groups (in situ IR, 1457 cm-1) during steady-state methanol dehydration catalysis that mediate stepwise (dissociative) dehydration pathways.
The distribution of Al atoms between channel (~0.5 nm) and intersection (~0.7 nm) void environments and the proximity of Al sites in MFI zeolites (ZSM-5) depends on the synthesis conditions used [6]. We explore this phenomena here by adapting concepts from charge density mismatch theory [7] by manipulating total Al content and the ratio of Na+ and tetrapropylammonium (TPA+) in synthesis media to crystallize ZSM-5 with different proportions of paired Al sites, and different amounts of Al located in channel and intersection void environments. Divalent cobalt ion exchange isotherms, UV-Visible spectroscopy, and temperature programmed desorption of ammonia on Co2+-exchange ZSM-5, were used to quantify the fraction of paired Al, which increased with total Al content when only using TPA+ in synthesis solutions, and also with increasing Na+/TPA+ ratio at constant Al content. Methanol dehydration zero-order rate constants (per H+, 415 K) did not vary with H+ content, or the fraction of H+ sites in paired configurations, reflecting a constant acid strength for all site arrangements in the MFI framework. The invariance in zero-order rate constant among MFI samples contrasts the findings among CHA zeolites, for which rate constants increased systematically with the fraction of H+ sites in paired configurations [5]. First-order rate constants (per H+, 415 K) varied among the MFI samples studied here, reflecting the effects of the structure directing agents on biasing the distribution of sites towards MFI channels or their intersections. Results indicate that smaller charge-dense cations, such as Na+, bias the siting of Al in smaller voids of MFI zeolites. Manipulation of zeolite synthesis parameters to bias the acid site location and proximity in MFI zeolites, aided by quantitative characterization tools of active site arrangement and their catalytic behavior, can diversify the catalytic opportunities possible with this framework [8].
References:
[1] E. Gallego et al., Chem. Eur. J. (2018) DOI: 10.1002/chem.201803637.
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[8] B. Knott, C. Nimlos et al., ACS Catal. (2018) 770.