Yaquan Wang

Controllable fabrication of single-crystalline, ultrafine and high-silica hierarchical ZSM-5 aggregates via solid-like state conversion

A series of single-crystalline, superfine and high-silica (SiO2/Al2O3 > 100) hierarchical ZSM-5 aggregates (denoted as HA-ZSM-5) with a tuneable crystal morphology and size, and rich intercrystalline mesoporosity, were facilely fabricated within a short reaction time using the commercially available n-hexyltrimethylammonium bromide (HTAB) as a mesopore template by a solid-like state conversion (SSC) route; the crystallization of HA-ZSM-5 is induced within 36 h at a HTAB/SiO2 ratio of 0.05 with the assistance of 2% silicalite 1 (S-1) seeds. Key factors (such as SiO2/Al2O3 ratio, seed/SiO2 ratio and HTAB/SiO2 ratio) in controlling the crystal size and morphology have been systematically investigated. The formation process was studied by monitoring the morphology, relative crystallinity and structural properties of the samples at different crystallization times. The preparation of HA-ZSM-5 zeolites may be a kinetics-controlled dissolution/induction/growth/aggregation process and the possible formation mechanism was put forward based on experimental results. The obtained samples were characterized by several techniques and the representative HA-ZSM-5 (HA-167-2S-5H) was evaluated in a methanol to propylene (MTP) reaction. Compared with the blank sample (denoted as C-ZSM-5) without HTAB addition and a commercial ZSM-5 zeolite (denoted as C1-ZSM-5), the HA-167-2S-5H sample exhibited the highest selectivity towards propylene (40.1% vs. 33.1% and 38.7%) and butylene (22.9% vs. 17.2% and 21.4%), and especially the longest catalytic lifetime (22 h vs. 6 h and 1.5 h). The remarkably enhanced catalytic performance of HA-167-2S-5H could be attributed to the synergistic effect of the excellent structural properties, appropriate acidity and good accessibility of acidic sites. In addition, the introduction of interparticle mesopores not only enhances the utilization of zeolite acid sites by the enlarged external surface but also partially suppresses side reactions and inhibits coke deposition by shortening the micropore diffusion path lengths, leading to an increased conversion.

Seed-induced synthesis of multilamellar ZSM-5 nanosheets directed by amphiphilic organosilane

Multilamellar ZSM-5 nanosheets have been prepared using silicalite-1 as a seed in the presence of commercially available dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (TPOAC). Characterization results show that these ZSM-5 nanosheets stack disorderly forming lamellar aggregates. Besides, there exists abundant interlayer space in these aggregates which penetrates the whole crystals and results in the formation of inter-connected mesopores. The methanol to gasoline (MTG) reaction has been applied as a probe reaction to evaluate the catalytic performance of ZSM-5 nanosheets. Compared with a ZSM-5 zeolite prepared without TPOAC, the multilamellar ZSM-5 nanosheets displayed an extraordinarily long MTG catalytic lifetime with a similar yield of gasoline range hydrocarbons, which could be reasonably attributed to the special morphology of the nanosheets. To reveal the crystallization mechanism of ZSM-5 nanosheets, the crystallization was stopped at different times first at 120 °C and then at 170 °C. The characterization results show that there exists a unique process of MCM-41-like mesophase formation and then transformation to crystalline MFI phases during a certain period of crystallization.