Identifying zeolite catalysts that can simultaneously optimize p-xylene selectivity and feed utilization is critical to toluene alkylation with methanol (TAM). Here, we show that zeolite MCM-22 (MWW) has an exceptional catalyst lifetime in the TAM reaction at high operating pressure, conversion, and selectivity. We systematically probe the catalytic behavior of active sites in distinct topological features of MCM-22, revealing that high p-xylene yield and catalyst stability are predominantly attributed to sinusoidal channels and supercages, respectively. Using a combination of catalyst design and testing, density functional theory, and molecular dynamics simulations, we propose a spatiotemporal coke coupling phenomenon to explain a multistage p-xylene selectivity profile wherein the formation of light coke in supercages initiates the deactivation of unselective external surface sites. Our findings indicate that the specific nature of coke is critical to catalyst performance. Moreover, they provide unprecedented insight into the synchronous roles of distinct topological features giving rise to the exceptional stability and selectivity of MCM-22 in the TAM reaction.

Diels−Alder (DA) reactions of furans yield oxanorbornene derivatives which can be converted to a variety of molecules, ranging from molecules of biological interest to naturally occurring organic compounds, and to aromatics via dehydration, a promising alternative for the synthesis of aromatics from renewables. With furan being one of the less reactive dienes, the development of Lewis acidic heterogeneous catalysts, without the shortcomings of the traditional homogeneous catalysts, is critically important. Herein, we use computational chemistry to study the DA reaction of furan and methyl acrylate in three zeotypic Lewis acids, Sn-, Zr-, and Hf-BEA. We find that all three exhibit the same ability to enhance the electrophilic character of the dienophile and promote modest charge transfer from the diene. Despite being moderately Lewis acidic, they still achieve a reduction of about 12.5 kcal/mol in the activation energy relative to the reaction in the absence of catalyst.

We calculate the optical conductivity of an anisotropic two-dimensional system with Rashba spin-flip excitation within the Kubo formalism. We show that the anisotropic Rashba effect caused by an external field changes significantly the magnitude of the spin splitting. Furthermore, we obtain an analytical expression for the longitudinal optical conductivity associated with inter-band transitions as a function of the frequency for an arbitrary polarization angle. We find that the diagonal components of the optical conductivity tensor are direction-dependent and the spectrum of optical absorption is strongly anisotropic with an absorption window. The height and width of this absorption window are very sensitive to the system anisotropy. While the height of absorption peak increases with increasing effective mass anisotropy ratio, the peak intensity is larger when the light polarization is along the armchair direction. Moreover, the absorption peak width becomes broader as the density of state mass or Rashba interaction is enhanced. These features can be used to determine parameters relevant for spintronics through the optical absorption spectrum.

Density functional theory electronic structure calculations were used to explore the mechanism for the Diels-Alder reaction between 2,5-dimethylfuran and maleic anhydride (MA). Reaction paths are reported for uncatalyzed and Lewis and Brønsted acid-catalyzed reactions in vacuum and in a broad range of solvents. The calculations show that, while the uncatalyzed Diels-Alder reaction is thermally feasible in vacuum, a Lewis acid (modeled as Na(+)) lowers the activation barrier by interacting with the dienophile (MA) and decreasing the HOMO-LUMO gap of the reactants. A Brønsted acid (modeled as a proton) can bind to a carbonyl oxygen in MA, changing the reaction mechanism from concerted to stepwise and eliminating the activation barrier. Solvation effects were studied with the SMD model. Electrostatic effects play the largest role in determining the solvation energy of the transition state, which tracks the net dipole moment at the transition state. For the uncatalyzed reaction, the dipole moment is largely determined by charge transfer between the reactants, but in the reactions with ionic catalysts, there is no simple relationship between solvation of the transition state and charge transfer between the reactants. Nonelectrostatic contributions to solvation of the reactants and transition state also make significant contributions to the activation energy.