Investigation on Titanium Silicalite‑1 Zeolite Synthesis Employing ATPAOH as an Organic Structure Directing Agent

Tetrapropylammonium hydroxide (TPAOH) as an organic structure directing agent (OSDA) is of great importance for the preparation of titanium silicalite-1 (TS-1) zeolite. In this paper, we employed a new OSDA, allyltripropylammonium hydroxide (ATPAOH), in the synthesis process and successfully synthesized ATS-1 zeolite (MFI type). Compared with traditional OSDA TPAOH, one of TPAOH’s propyl groups is substituted by an allyl group, which endows ATPAOH with unique properties. On the one hand, ATPAOH accelerates the crystallization rate of titanium silicalite zeolite remarkably due to the strong interaction between Ti species and ATPAOH during the crystallization period. On the other hand, ATPAOH is beneficial for the formation of isolated 6-coordinated Ti species, thus leading to the generation of lower amount of anatase. Owing to its abundant active Ti species, ATS-1 prepared by ATPAOH as OSDA exhibits a much better catalytic performance for the cyclohexanone ammoximation reaction than TS-1 prepared by TPAOH as OSDA

Modulating the Microenvironment of Silanols in Pure-Silicon Zeolites for Boosting Vapor-phase Beckmann Rearrangement of Cyclohexanone Oxime

Vapor-phase Beckmann rearrangement of cyclohexanone oxime (CHO) to ε-caprolactam (CPL) is still difficult to commercialize at the industrial scale due to its relatively low catalytic activity and poor lifetime. Herein, we synthesized a series of pure-silicon zeolites (including MFI, MEL, and −SVR) with three-dimensional 10-member-ring topolgies, diverse silanol status, and hierarchical porosity to investigate the synergistic effects of inner diffusivity and reactivity. S-1 zeolite of MFI-type topology with plentiful silanol nests exhibits a more preferable catalytic performance in terms of CHO conversion (99.7%) and CPL selectivity (89.7%), much higher than those of MEL- and −SVR-type zeolites mainly due to their diverse silanol distribution. With the construction of hierarchical porosity, S-1-P shows improved CPL selectivity of 94.1% owing to the enhanced diffusivity to shorten the retention time of the reactant and product molecules. The reaction mechanism and network have been further revealed by density functional theory (DFT) calculations and experimental designs, which indicate that silanol nests are major active sites due to their suitable interaction with CHO rather than terminal silanols. Particularly, the microenvironments of silanols can be modulated by alcohol solvents, ascribed to their different charge transfer and steric hindrance. Consequently, S-1-P shows superior CPL selectivity of 97.3% in ethonal solvents, which have higher adsorb energy of −0.627 eV with silanol nests than other alcohols. The present study not only provides a fundamental guide for the design of zeolite catalysts but also provides a reference for modulating the microenvironment of active sites according to the catalytic mechanism

Transformation from NaA to MCM-49 Zeolite and Its Catalytic Alkylation Performance

The transformation from NaA (LTA) to MCM-49 (MWW) zeolite was achieved in the synergism of hexamethyleneimine (HMI), NaOH, and SiO₂, in spite of no common composite build units between LTA (<i>lta</i>, <i>sod</i>, and <i>d4r</i>) and MWW (<i>mel</i> and <i>d6r</i>) structure. NaA (SiO₂/Al₂O₃ = 2.0) was employed as the parent zeolite. The samples prepared at different crystallization stages were characterized by XRD, SEM, ²⁹Si/²⁷Al/¹³C MAS NMR, and STEM-EDS to investigate the intermediates during the transformation from NaA to MCM-49. As shown in SEM and STEM-EDS images, MCM-49 was proposed to be transformed gradually from the exterior to the interior of NaA, which was clearly observed by the core (LTA, low SiO₂/Al₂O₃)–shell (MWW, high SiO₂/Al₂O₃) coexisting zeolites as intermediates. With high relative crystallinity and the uniform sizes of crystals, the final MCM-49 was featured by Si enrichment on the external surface, which was proved by the shell (SiO₂/Al₂O₃ = 45.4) wrapping around the core (SiO₂/Al₂O₃ = 22.0). For transformed H-MCM-49 zeolite, the uniform sizes of crystals and the increase of total acid sites contributed to better accessibility of active centers, which achieved simultaneous improvement in ethylene conversion and ethylbenzene selectivity in the liquid-phase alkylation of benzene with ethylene