Diffusion of Mixtures of Light Alkanes and Benzene in Nano-Sized H‑ZSM5

The joint transport of linear C4–C6 alkanes and benzene has been studied over nanosized H-ZSM5 crystals by fast-time-resolved infrared spectroscopy. The internal and external concentration profiles during the coadsorption of alkanes and aromatics were directly monitored for a zeolite in which transport is controlled by the pore entrance step. The detailed kinetic analysis revealed a decrease in the adsorption and pore entrance rates of light hydrocarbon molecules upon addition of benzene by about 10–30%. This was caused by both a reduction in the rate constants and in the coverage at the external surface. On the other hand the apparent diffusion coefficient of benzene decreased up to 80% in comparison to the pure component. This decrease is attributed to a competitive adsorption at sites near the pore mouth. The extent increased with decreasing chain length of the cosorbed alkane, which corresponds to a simultaneous decrease in occupation of pore mouth sites by light hydrocarbons

Methanol Usage in Toluene Methylation with Medium and Large Pore Zeolites

The reaction of toluene methylation was investigated with four acidic zeolites of different pore geometries: the medium pore zeolites H-ZSM5 and H-ZSM11 as well as the large pore zeolites H-MOR and H-BEA. The methylation, methanol consumption, light hydrocarbon formation, and disproportionation rates for the reaction of toluene, <i>p</i>-xylene, and 1,2,4-trimethylbenezene with methanol were determined. The products of toluene methylation (e.g., xylenes and trimethylbenzenes) were readily methylated further in both medium and large pore zeolites. A considerably higher fraction of methanol was used to form light hydrocarbons with the medium pore zeolites than with large pore zeolites. This was related to the fact that the dealkylation of light hydrocarbons from highly methylated aromatics became more favorable relative to methylation at an earlier stage, that is, after fewer methyl groups were added to the aromatic ring. Increasing the effective residence time of bulky aromatic molecules with medium pore zeolites, modified either by coating the surface with tetraethyl orthosilicate or by increasing the intracrystal pore length, converted a larger fraction of methanol to light hydrocarbons via methylation and subsequent dealkylation of light hydrocarbons

Accurate Adsorption Thermodynamics of Small Alkanes in Zeolites. Ab initio Theory and Experiment for H‑Chabazite

Heats of adsorption of methane, ethane, and propane in H-chabazite (Si/Al = 14.4) have been measured and entropies have been derived from adsorption isotherms. For these systems quantum chemical ab initio calculations of Gibbs free energies have been performed. The deviations from the experimental values for methane, ethane, and propane are below 3 kJ/mol for the enthalpy, and the Gibbs free energy. A hybrid high-level (MP2/CBS): low-level (DFT+dispersion) method is used to determine adsorption structures and energies. Vibrational entropies and thermal enthalpy contributions are obtained from vibrational partition functions for the DFT+dispersion potential energy surface. Anharmonic corrections have been evaluated for each normal mode separately. One-dimensional Schrödinger equations are solved for potentials obtained by (curvilinear) distortions of the normal modes using a representation in internal coordinates

Nature and Location of Cationic Lanthanum Species in High Alumina Containing Faujasite Type Zeolites

The nature, concentration, and location of cationic lanthanum species in faujasite-type zeolites (zeolite X, Y and ultrastabilized Y) have been studied in order to understand better their role in hydrocarbon activation. By combining detailed physicochemical characterization and DFT calculations, we demonstrated that lanthanum cations are predominantly stabilized within sodalite cages in the form of multinuclear OH-bridged lanthanum clusters or as monomeric La³⁺ at the SI sites. In high-silica faujasites (Si/Al = 4), monomeric [La(OH)]²⁺ and [La(OH)₂]⁺ species were only found in low concentrations at SII sites in the supercages, whereas the dominant part of La³⁺ is present as multinuclear OH-bridged cationic aggregates within the sodalite cages. Similarly, in the low-silica (Si/Al = 1.2) La–X zeolite, the SI′ sites were populated by hydroxylated La species in the form of OH-bridged bi- and trinuclear clusters. In this case, the substantial repulsion between the La³⁺ cations confined within the small sodalite cages induces the migration of La³⁺ cations into the supercage SII sites. The uniquely strong polarization of hydrocarbon molecules sorbed in La–X zeolites is caused solely by the interaction with the accessible isolated La³⁺ cations