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.

We performed experimental and periodic density functional theory (DFT) IR spectroscopy to investigate the adsorption of acyclic olefins over both acidic and nonacidic zeolites. Two conjugated polyenes, 2,4-dimethyl-1,3-pentadiene (I) and 2,6dimethyl-2,4,6-octatriene (II) were studied to probe organic intermediates that can be formed during methanol conversion and lead to deactivating species known collectively as "coke." We computed vibrational spectra using zeolite-adsorbed and gas-phase models for both neutral and protonated forms of I and II and compared these DFT results to diffuse reflectance IR Fourier transform (DRIFT) spectra of zeolite−guest systems. Our experimental and computational results are precise enough to pinpoint the surprising fact that the gauche s-cis conformation of species I is the major conformer during adsorption over dealuminated zeolite β. Computed zeolite-adsorbed spectra of the protonated species I and II best represent the DRIFT spectra obtained after the adsorption of the olefins on HMOR at 20 °C, with computed bands at 1543 and 1562 cm −1 for molecules I + and II + , respectively, attributed to the allylic stretching mode, ν(C=C−C + ). These computed band frequencies are within 6 cm −1 of experimental data and confirm that the interaction between neutral acyclic olefins and acidic zeolites leads to protonation of the olefin. A comparison of computed spectra of the protonated species in the gas phase to those in the zeolite indicates that the electrostatic interaction between alkenyl and alkadienyl cations and negative zeolite framework does not significantly impact the position of the allylic stretching bands. These results highlight that computed spectroscopy and thermodynamics coupled with experimental spectra can be used to elucidate complex mixtures in zeolites, and certain spectral features of adsorbed olefins can be accurately modeled by gasphase calculations.

We performed periodic density functional theory (DFT) calculations to study the dynamics of alkyl substituents on cyclopentenyl cations in medium-pore acid zeolites, H-ZSM-5 and H-ZSM-22. Our study seeks to shed light on how zeolite shape selectivity can influence key reaction intermediates in the methanol-to-hydrocarbon process, which can lead to both value-added products and coke species. We considered the isomerization of 4-ethyl-4,5,5-trimethylcyclopentenyl cation (A), which has been shown by Hernandez et al. [ACS Catal. 2021, 11, 12893–12914] to lead to IR spectra and alkyl substitution patterns that vary with zeolite pore structure, in contrast to DFT-predicted thermodynamics. Here, we investigate the role of kinetic control on zeolite shape-selectivity by computing exhaustive DFT dynamics of substituent rearrangement in medium-pore zeolites starting with cation A. We used the Rule Input Network Generator (RING) code to enumerate isomerization pathways from A to five product isomers that differ mainly in the presence or absence of an alkyl substituent on the central allylic carbon of the cyclopentenyl ring, yielding a reaction network with a total of 24 distinct species. We combined metadynamics and climbing-image nudged elastic band (CI-NEB) methods to compute the free-energy landscapes, including barriers for all species in H-ZSM-5 and H-ZSM-22 zeolites. Integrating kinetic equations for the reaction network in the two zeolites predicts that equilibrium product distributions are obtained after 103 s in H-ZSM-5, while 108–109 s is required for equilibration in H-ZSM-22, suggesting the clear possibility of kinetic control depending on zeolite structure.