Vanillyl alcohol, a lignin derived aromatic diol, is a potential platform chemical for the production of renewable epoxy thermosets. A bio-based bisphenolic analogue, bisguaiacol (BG), was synthesized via electrophilic aromatic condensation of vanillyl alcohol and guaiacol, from which diglycidyl ether of BG (DGEBG) was prepared. In addition, three single aromatic diglycidyl ethers were synthesized from vanillyl alcohol (DGEVA), gastrodigenin (DGEGD), and hydroquinone (DGEHQ). All epoxies were characterized via 1H NMR, 13C NMR, FTIR, MS, and GPC. Dynamic mechanical analyses of the epoxies by themselves or blended with a commercial epoxy resin and cured with a diamine were conducted to determine the effect of the methoxy and methylene moieties on polymer properties. The thermomechanical results indicate that the methoxy lowers the glass transition temperature (T g) yet increases the glassy storage modulus at 25 °C, while the methylene spacer between the aromatic ring and the epoxide further lowers the T g in cured epoxy–amine systems.
The conversion of methanol to olefins on zeolites and zeotypes is an industrially important process, yet the mechanistic details remain unresolved. Various cyclic alkenyl and alkadienyl carbenium ions have been proposed as active intermediates for light olefin production, but direct experimental evidence demonstrating the transformation of these hydrocarbon pool species with product formation is lacking. In this contribution, the interpretations of UV–vis and IR spectra are advanced to provide detail on the species present during catalysis; selected individual species are generated inside the catalyst pores, and their reactivity is tracked. The basis for the interpretation is a multitude of reference spectra of acyclic and cyclic dienes, trienes, and corresponding protonated species in the liquid phase (organic medium or sulfuric acid) or sorbed on dealuminated or acidic zeolites, augmented by trends extracted from these spectra. Accordingly, spectroscopic signatures indicate the presence of both polymethylbenzenium ions and alkyl-substituted cyclopentenyl cations during the conversion of methanol on H-ZSM-5 at 300 °C. To elucidate their role, alkyl-substituted cyclopentenyl cations are generated from acyclic polyenes adsorbed on H-ZSM-5 at room temperature. Cyclization and cleavage are monitored via changes in the electronic and vibrational absorption spectra with increasing temperature. The formation of ethene, propene, and butenes from alkyl-substituted cyclopentenyl cations, in the absence of methylbenzenium ions, is unambiguously demonstrated by temperature-programmed reaction (TPReact) spectroscopy coupled with online gas-phase product analysis. In the UV–vis range, the wavelength of maximum absorption, λmax, of the alkyl-substituted cyclopentenyl cation is linearly dependent on the total number of carbons in the cation. A blue shift in the position of λmax of these cyclopentenyl cations during the TPReact indicates a loss of three carbons, which matches the average size of the concomitantly produced olefins.
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.
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