Integrated NMR and Ab Initio Study of Acetonitrile in Zeolites: A Reactive Complex Model of Zeolite Acidity

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Abstract

Experimental and theoretical methods have provided a detailed view of the mechanism of proton transfer from a zeolite to a weak base. The NMR behavior of acetonitrile in acidic zeolites differs significantly from that in superacid solution and implies a different structure for the protonated species. 13C and 1H chemical shifts are strongly and reversibly temperature dependent for acetonitrile in zeolite HZSM-5 - in the high-temperature range used for most catalytic applications of zeolites. The experimental results were first used to propose a qualitative model in which progressive proton transfer occurs with increasing temperature, resulting in a partial charge on the nitrile carbon. Ab initio Hartree-Fock-Roothaan calculations using finite clusters to model the acid site were carried out to refine this model and test alternate hypotheses. The minimum energy conformation for the acetonitrile-zeolite cluster in a large channel is a hydrogen-bonded complex. The effects of increased temperature and reduced channel size were simulated by introducing constraints prior to the geometry optimization in a manner so as to sample higher energy conformations and close contact with the channel wall. Those calculations yielded a family of structures describing proton transfer from the zeolite to the weak base within the complex. Hydrogen-bonded acetonitrile bends at higher energies; for bend angles near 165°, the energies for zeolite protonation and N protonation cross. The accuracy of the calculated structures was tested by using ab initio methods to calculate the 1H and 13C chemical shifts of the clusters and related models. Only bent structures accounted for the 13C shifts, and calculated 1H shifts for the cluster models were in complete agreement with experiment. The (reversible) reaction between the Bronsted acid site and the weak base acetonitrile should be viewed as a cooperative or concerted process, rather than simple proton transfer from a superacid to a base. Changes in the structure of the base drive proton transfer as the complex moves along the reaction coordinate. The temperature-dependent NMR behavior of acetonitrile in various zeolites is sensitive to differences in acid strength and zeolite framework. Relative energies derived from Hartree-Fock-Roothaan calculations were used to construct potential wells for quasi-one-dimensional models of zeolites with different pore diameters. These models rationalize the different temperature dependence in the medium and large pore zeolites and suggest that confinement of the weak base near the acid site is an important aspect of reactivity in acidic zeolites. The integration of experiment and calculation demonstrated here is a more rigorous alternative to the use of qualitative chemical shift arguments to deduce structure, and it permits closure between theory and experiment.

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