Alexey A. Zhokh

Non-Fickian diffusion of methanol in mesoporous media: Geometrical restrictions or adsorption-induced?

The methanol mass transfer in the mesoporous silica and alumina/zeolite H-ZSM-5 grains has been studied. We demonstrate that the methanol diffusion is characterized as a time-fractional for both solids. Methanol transport occurs in the super-diffusive regime, which is faster comparing to the Fickian diffusion. We show that the fractional exponents defining the regime of transport are different for each porous grain. The difference between the values of the fractional exponents is associated with a difference in the energetic strength of the active sites of the surface of the media of different chemical nature as well as the geometrical restrictions of the porous media. Increasing by six-fold, the pore diameter leads to a 1.1 fold increase of the fractional exponent. Decreasing by three-fold, the methanol desorption energy results into the same increasing the fractional exponent. Our findings support that mainly the adsorption process, which is defined by the energetic disorder of the corresponding surface active sites, is likely to be the driving force of the abnormality of the mass transfer in the porous media. Therefore, the fractional exponent is a fundamental characteristic which is individual for each combination of the porous solid and diffusing species.

Application of the Time-Fractional Diffusion Equation to Methyl Alcohol Mass Transfer in Silica

Non-usual behavior of methyl alcohol mass transfer in mesoporous silica is experimentally and theoretically investigated. Analysis of the experimental data in terms of the second Fick’s law and accounting of various pore geometries demonstrates no correspondence between the experimental data and theoretical solutions. We show that contrary to standard diffusion approach the experimental data are in an excellent coincidence with the solution of the time-fractional diffusion equation, obtained for boundary conditions that correspond to the experimental conditions. Obtained results reveal that methyl alcohol in mesoporous silica may exhibit anomalous features because of geometrical constraints of silica pores.

Two-Path Conversion of Methanol to Olefins on H-ZSM-5/Al2O3 Catalyst

It was shown that during the transformation of methanol on H-ZSM-5/Al2O3 catalyst with zeolite/alumina ratio 1/1 increase of the methanol partial pressure leads to an extremal increase in the formation rate of ethylene, propylene, and butylene while the formation rate of dimethyl ether increases monotonically. The yield of the olefins increases with increase of temperature on account of increase in the degree of transformation of the methanol while the dimethyl ether yield passes through a maximum. The results are explained by a kinetic scheme according to which the dimethyl ether and olefin formation follows different routes.

Effect of H-ZSM-5/Al 2 O 3 Catalyst Acidity on the Conversion of Methanol

The conversion of methanol on H-ZSM-5/Al2O3 catalyst with a 1/1 zeolite/alumina ratio by mass was studied. The presence of acid sites of different strength on the H-ZSM-5/Al2O3 catalyst was established by the NH3-TPD method. It was found that increasing the reaction temperature leads to increase of the degree of conversion of the methanol and the yield of the olefin, while the yield of dimethyl ether passes through a maximum. The obtained results are explained in terms of a proposed kinetic scheme of the process, according to which the dimethyl ether and hydrocarbons are formed on different types of acid sites.

Non-Fickian Transport in Porous Media: Always Temporally Anomalous?

The methanol transport through the porous media, e.g., pelletized zeolite-based catalyst, was investigated. Standard Fickian diffusion equation was found to be inapplicable to a description of the experimental mass transfer kinetics. The diffusion equation with the fractional-order temporal and spatial derivatives was used in order to fit the experimental data. It is demonstrated that only the diffusion equation, which contains the time-fractional derivative or both, space–time-fractional derivatives, may thoroughly describe the experimental data. The measured time-fractional order is 0.85, which reveals the non-Fickian sub-diffusive transport through the pellet. The obtained fractional order allows the application of the continuous-time random walk model as a physical ground for the explanation of the measured non-Fickian transport. Contrary, using the space-fractional diffusion equation for the experimental data description leads to the inconsistent results similarly to the standard diffusion equation. The obtained findings reveal that the diffusate flux and the flux conservation law cannot be purely space fractional.

Methanol conversion to olefins on H-ZSM-5/Al 2 O 3 catalysts: kinetic modeling

A kinetic study of the methanol-to-olefin (MTO) process was performed for alumina, zeolite H-ZSM-5, and H-ZSM-5/alumina catalyst with 1/1 zeolite/alumina ratio for the solids prepared using commercial zeolite Na-ZSM-5. The catalysts were characterized by XRD, TEM, nitrogen adsorption isotherms, and ammonia thermodesorption. A new kinetic model of the MTO reaction over the as-prepared catalyst was proposed. The model is based on the experimental data of the methanol conversion to dimethyl ether and light olefins obtained under different temperatures and methanol partial pressures. Moreover, the model accounts for the product distribution dependence on the acidity of the as-prepared catalysts. The developed model was found to be sensitive to the methanol conversion route.