Regeneration of Deactivated Hollow Titanium Silicalite Zeolite from Commercial Ammoximation Process by Encapsulating Amorphous TiO₂-SiO₂Nanoparticles inside Zeolite Crystal

Abstract

An efficient regeneration method to encapsulate the poisonous amorphous TiO 2 -SiO 2 nanoparticles within zeolite crystal via post-synthesis has been developed. It is demonstrated that the encapsulation of Ti-rich nanoparticles can reduce the accessibility of H 2 O 2 molecules to acid sites of Ti-rich nanoparticles, thus lowering the inefficient decomposition of H 2 O 2 oxidant during catalytic reactions. Furthermore, the regenerated HTS zeolite is of smaller particle size than conventional TS-1 zeolite due to the existence of deactivated HTS as crystal seed during hydrothermal synthesis. Consequently, the regenerated HTS zeolite shows as high catalytic performance as fresh TS-1 zeolite in phenol hydroxylation reaction with 30 wt% H 2 O 2 aqueous solution as oxidant. Importantly, this study exploits a novel concept on updating the lifetime of industrial zeolite catalyst, which is of both academic and practical significance.Cyclohexanone ammoximation is an important process for the production of e-Caprolactam, which is one basic chemical intermediate to prepare nylon-6 monomer for synthesizing polymers, plastics and so on. [1][2][3][4][5][6][7] In this route, cyclohexanone oxime is originally produced via the ammoximation of cyclohexanone, and then the oxime is converted to e-Caprolactam through a Beckman rearrangement reaction. Noteworthy, the conventional HPO process (hydroxylamine-phosphate-oxime) is very complex and energy-intensive, with the formation of huge hazard by-products and wastes. To overcome these drawbacks, an environmental-friendly cyclohexanone ammoximation process had been developed by Enichem in 1986, using titanium substituted MFI-type zeolite (TS-1) as catalyst under mild conditions. [8][9][10][11][12][13] On the effect of tetrahedral framework Ti species, oximes can be directly obtained through the ammoximation of ketones with H 2 O 2 and NH 3 , which displays high ketone conversion and product selectivity. Most importantly, this approach is considered as a completely "green chemistry" process, due to no formation of hazard pollutants and no usage of stoichiometric reagents.However, the traditional TS-1 zeolite usually shows low catalytic activity and stability, due to the mismatch of the hydrolysis and nucleation rates of organic Si and Ti sources during hydrothermal crystallization process,. On the other hand, the narrow microporous channels restrict reagents access to active sites during catalytic reactions. For updating the catalytic performance, hollow titanium silicate (HTS) zeolite was synthesized by M. Lin et al., by using a "dissolution-recrystallization" treatment in 1990s. [14][15][16][17][18] It is demonstrated that HTS zeolite presents much higher catalytic activity and stability than conventional TS-1 zeolite in many oxidation reactions, owing to the existence of intracrystalline cavities and the reincorporation of Ti species into framework positions. Furthermore, cyclohexanone ammoximation catalyzed by HTS zeolite has been developed and commercialized by SINOPEC, China. This "...

Environmental-Friendly Catalytic Oxidation Processes Based on Hierarchical Titanium Silicate Zeolites at SINOPEC

“…In addition, in our previous works, the irreversible deactivation of HTS zeolite has been investigated by using multiple characterization methods and catalytic evaluations [64][65][66]. It is confirmed that the dissolution of framework Si atoms and the removal of framework Ti atoms cause the formation of acidic amorphous TiO 2 -SiO 2 nanoparticles, leading to the inefficient decomposition of H 2 O 2 molecules during catalytic oxidation processes.…”

Reaction Pathways in the Oxidation Decomposition of Hydroxylamine over a Titanosilicates/H2O2 System

“…It is widely accepted that nonframework Ti species in titanosilicates include amorphous SiO 2 –TiO 2 mixed binary oxides and crystalline anatase, − which are harmful to the reaction system by accelerating the decomposition of H 2 O 2 via the free radical mechanism. − Ammoximation occurs in an alkaline environment, and the gradual dissolution of the framework Si atoms results in the migration of framework Ti atoms to extra-framework positions, which is the main reason for the generation of amorphous SiO 2 –TiO 2 mixed binary oxides and the irreversible deactivation of catalyst. − We investigated the effects of TS-1 containing only framework isolated tetrahedrally coordinated Ti species, SiO 2‑ TiO 2 mixed binary oxides, and crystalline anatase on the oxidation decomposition of NH 2 OH, respectively.…”