Green syntheses of Silicalite-1 using hydroxyl free radicals and cellulose nanocrystals

Abstract

Zeolites play a crucial role in the various fields, but their synthesis rely on expensive organic templates and the subsequent removal of these templates through calcination generates environmentally detrimental gases. Therefore, research efforts are needed to enhance the efficiency and cost-effectiveness of zeolites synthesis.
As hydroxyl free radicals (•OH) are able to accelerate the crystallisation process of zeolites and cellulose nanocrystals (CNCs) can be green templates for zeolites syntheses, this dissertation developed a novel method for Silicalite-1 zeolites synthesis with less organic template usage through the incorporation of •OH generated by sodium persulfate (SPS) and CNCs. The effects of the organic template usage, •OH and CNCs on the yield, crystalline structure, pore structure and morphology of the produced Silicalite-1 zeolites were investigated. The presence of •OH could increase the yield of Silicalite-1 zeolites but reduce the specific surface area significantly, and the presence of CNCs was able to mitigate the loss of specific surface area. It was found that the presence of •OH and CNCs promoted the synthesis of Silicalite-1 with only 50% usage of the organic template, achieving higher Silicaclite-1 yield of 74% and higher relative crystallinity of 85% compared to the conventional Silicalite-1 synthesis with 100% usage of the organic template (the yield was 44% and the relative crystallinity was 72%). In addition, the optimal surface area of using 50% TPAOH with the synergy effect of •OH and CNCs was higher than the case that synthesized with 100% TPAOH (460 m2 /g vs. 401m2 /g). Si-O-Si bond-breaking and bond-remaking were identified in the Silicalite-1 synthesis process and no significant difference was observed in the absence and presence of •OH and CNCs. The developed Silicalite-1 zeolites were used as the supports for making Ni supported catalysts for carbon dioxide (CO2) hydrogenation to methane (CH4). And comparable CO2 conversion (i.e., 80% vs. 82%) were achieved for both catalysts (supported with Silicalite-1 zeolites synthesised using 50% of the organic template in the presence of •OH and CNCs and synthesised using 100% of the organic template in the absence of •OH and CNCs, respectively).

Date of Award	Nov 2024
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Xiaolei Fan (Supervisor) & Xiaoxia Ou (Supervisor)