Experimental and DFT Calculated IR Spectra of Guests in Zeolites: Acyclic Olefins and Host–Guest Interactions

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 2,6-dimethyl-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 gas-phase calculations.