Self-pillared Pentasil Research Articles

Enhanced Molecular Transport in Hierarchical Silicalite-1

Fundamental understanding of the mass transport of petrochemical and biomass derived molecules in microporous and mesoporous solid catalysts is important for developing the next generation of heterogeneous catalysts for traditional hydrocarbon processing including biomass pyrolysis and upgrading. Hierarchical zeolites with both micropores and mesopores exhibit enhanced mass transport and unique catalytic performance in reactions involving large molecules. However, quantitative description of mass transport in such materials remains elusive, owing to the complicated structure of hierarchical pores and difficulty in the synthesis of the materials with controllable structures. In this work, zero length column chromatography (ZLC) was used to study temperature-dependent diffusion of cyclohexane in silicalite-1, self-pillared pentasil (SPP) zeolite, and three-dimensionally ordered mesoporous imprinted (3DOm-i) silicalite-1. The samples were synthesized with controllable characteristic diffusion lengths from micrometer scale (ca. 20 μm) to nanometer scale (ca. 2 nm), allowing systematic study of the effect of mesoporosity on the mass transport behavior of hierarchical zeolites. The results show that the introduction of mesoporosity can indeed significantly facilitate the mass transport of cyclohexane in hierarchical silicalite-1 by reducing diffusional time constants, indicating rapid overall adsorption and desorption. However, when the length scale of the material approaches several nanometers, the contribution from the surface resistance, or "surface barrier", to overall mass transfer becomes dominant.

Kinetics and Mechanism of Benzene, Toluene, and Xylene Methylation over H-MFI

The methylation of benzene, toluene, para-xylene, and ortho-xylene over MFI structured H-ZSM-5 and mesoporous self-pillared pentasil (H-SPP) with dimethyl ether (DME) at low conversions (<0.1%) and high DME:aromatic ratios (>30:1) showed linear rate dependencies on aromatic pressure and zero dependence on DME pressure for benzene and toluene. These results are consistent with studies performed for olefin methylation, and are indicative of a zeolite surface covered in DME-derived species reacting with benzene or toluene in the rate-determining step. Saturation in the reaction rate was observed in xylene pressure dependence experiments (at 473 K, <5 kPa xylene); however, enhancement in the reaction rate was not observed when comparing ∼1 μm crystallite H-ZSM-5 and 2–7 nm mesopore H-SPP, indicating that xylene methylation proceeds in the absence of diffusion limitations. Simultaneous zero-order rate dependencies on xylene and DME pressures are described by a model based on adsorption of xylene onto a surface methylating species. This model is consistent with observed secondary kinetic isotope effects (kH/kD = 1.25–1.35) and extents of d0, d3, and d6 DME formation in the effluent because of isotopic scrambling between unlabeled and d6 DME when co-fed with aromatics over H-ZSM-5. Post-reaction titration of surface species with water after desorption of physisorbed intermediates showed a 1:1 evolution of methanol to Al present in the catalyst, indicating the presence and involvement of surface methoxides during steady-state methylation of aromatics species.