Shape-Selective Synthesis of Alkylcyclopentenyl Cations in Zeolites and Spectroscopic Distinction of Constitutional Isomers

Abstract

Alkylcyclopentenyl cations belong to the long-lived intermediates that make up the “hydrocarbon pool” during the catalytic conversion of methanol on zeolites, and recent works show that such cations contribute to olefin and aromatics formation. From liquid phase chemistry, two types of alkylcyclopentenyl cations are known and distinguished by the substituent at the central carbon (C-2) of the allylic system: a more stable type with a methyl group at the C-2 and a less stable type with hydrogen at the C-2. The following three linked objectives are pursued: (i) IR spectroscopic distinction of the different substitution patterns at the allylic system of alkylcyclopentenyl cations, (ii) the role of the zeolite framework in determining the substitution pattern, and (iii) identification of alkylcyclopentenyl cations from precursors relevant to methanol-to-olefins conversion. UV–vis and IR spectroscopy are applied in situ to characterize alkylcyclopentenyl cations produced by adsorption of a pentaalkylcyclopentadiene or by adsorption and thermally induced cyclization of 2,6-dimethyl-2,4,6-octatriene. Prior knowledge of electronic spectra is combined with DFT-computed and experimental IR spectra to establish the frequency of the C–H vibration of the C-2 hydrogen-substituted type, and a characteristic red shift of the asymmetric allylic stretching vibration (Δν ≈ −20 to −30 cm–1) after replacing hydrogen by methyl at the central carbon of the allylic system. Although DFT demonstrates that both types of ions fit into medium- and large-pore zeolites and that the C-2 methyl-substituted type is thermodynamically favored even in the pores of the considered zeolites, formation of alkylcyclopentenyl cations is found by UV–vis and IR spectra to be shape-selective. The bulkier C-2 methyl-substituted type is detected in large-pore zeolites (MOR, BEA) and in the intersections of medium-pore zeolites (MFI), whereas in channels of medium size (TON), the less bulky C-2 hydrogen-substituted type is exclusively formed. The type of ion formed and its overall size are dictated by the zeolite framework and are independent of the precursor; the same type of alkylcyclopentenyl cation as found through cyclization of dimethyloctatriene could be generated from ethene.

A new mechanistic proposal for the aromatic cycle of the MTO process based on a computational investigation for H-SSZ-13

“…37 The extent to which olefin and aromatic cycle contribute to the overall rate of olefin production is still under debate and likely varies for different catalysts such as H-ZSM-5, 33,[40][41][42] H-BEA, 10,13,[43][44][45] H-SAPO-34 19,46,47 and H-SSZ-13. [48][49][50] While some experimental observations involve cyclopentenyl cations, 42,[51][52][53][54][55][56] all detailed mechanistic proposals for the aromatic cycle start from aromatic polymethylbenzenes. Two mechanistic variants have been proposed for the production of olefins via the aromatic cycle, the side-chain and the paring mechanism (see Scheme 1).…”

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

“…Carbocations are also important intermediates in other zeolitecatalyzed reactions, such as poly-ethylated benzenium ions in the ethanol to olefins (ETO) reaction as the ''sister-reaction'' of MTO, 42,46,75,149,314 cyclic carbenium ions in 1-ethylcyclohexene isomerization, 213 alkylated naphthyl carbenium ions in the conversion of ethene to propene, 315 acylium ion in DME/ methanol carbonylation 102 and Friedel-Crafts acylation, 316,317 and alkylated phenyl carbenium ions in alkylaromatic transalkylation. 117,276,[318][319][320][321][322] Chowdhury et al 149 employed spectroscopic approaches to validate the reaction and deactivation mechanism of the ETO process in H-ZSM-5, where alkyl side chain carbenium ions, alkylated phenyl carbenium ions, and poly-arenium ions were detected as the intermediates to produce alkenes by operando UV-Vis diffuse reflectance spectroscopy.…”

“…117,276,[318][319][320][321][322] Chowdhury et al 149 employed spectroscopic approaches to validate the reaction and deactivation mechanism of the ETO process in H-ZSM-5, where alkyl side chain carbenium ions, alkylated phenyl carbenium ions, and poly-arenium ions were detected as the intermediates to produce alkenes by operando UV-Vis diffuse reflectance spectroscopy. Hernandez et al 213 explained the mechanism of 1-ethylcyclohexene isomerization in the EUO zeolite, where the ethylcyclohexenium ion would be formed by the protonation of 1-ethylcyclohexene with BAS, and the subsequent ring contraction-expansion reactions would involve a PCP + -like transition state with the character of carbonium ions. Moreover, the insightfully theoretical calculations indicated that the different stabilizations of the ethylcyclohexenium ion by the intersection, 12MR pockets and 10MR channels would result different catalytic activity from the free energy surfaces.…”

“…185,209 Besides the PMB + -based aromatic cycle, methylated naphthyl and anthracyl carbenium ions can also act as HCP species to produce light alkenes, especially in some zeolites or molecular sieves with a large cavity. 180,210–213 Therefore, methylated naphthyl and anthracyl compounds are interfacing compounds between the active HCP species and coke precursors (Fig. 21a), and additionally methylated naphthyl carbenium ions have a similar size to the traditional heptaMB + and are not larger than the cavity of CHA (Fig.…”

Shape-Selectivity of Cyclopentenyl Cation Isomerization: Investigating Kinetic Control in Medium-Pore Zeolites

“…Summary of results from Hernandez et al Experimental IR data are consistent with methyl-substituted form in H-MOR, H-substituted form in H-ZSM-22, and both methyl- and H-substituted forms in H-ZSM-5, while DFT-computed free energies predict that the methyl form is the most stable in all cases.…”