Solvent-free synthesis of crystalline ZSM-5 zeolite: Investigation of mechanochemical pre-reaction impact on growth of thermally stable zeolite structures

摘要

In this study, we investigate the synthetic tunability of a solvent-free synthesis of crystalline ZSM-5 zeolite. Conventional zeolite syntheses use hydrothermal reactions to facilitate reagent dissolution and atomic level mixing that lead to zeolite crystallization at moderate temperatures near 200 degrees C. In solvent-free reactions, a reagent mixing step is very important to a successful synthesis of homogeneous and structurally stable zeolites. Key to synthetic success in our ZSM-5 zeolite syntheses is utilization of extended grinding that facilitates identifiable chemical exchange reactions prior to thermal processing. This mechanochemical pre-reaction step allows several key energy intensive or costly synthetic variables to be reduced and still yield thermally stable ZSM-5 products. A grinding induced exchange reaction yields a strongly basic ammonia environment as paste formed by reagents and released waters of hydration. This locally basic environment likely contributes to formation and assembly of tetrahedral structure building components (MO4 where M = Si, Al) that assemble into extended zeolite pore structures around the amine template. Specific synthetic conditions examined here for their influence on stable crystalline ZSM-5 growth include variation in reaction temperature (120-180 degrees C), reaction time (4-22 h) and amount of organic template (0-15 wt%). With the appropriate mechanochemical prereaction chemistry and reagent intimate mixing, crystalline and thermally robust ZSM-5 is formed at temperatures as low as 160 degrees C in 10 h with as little as 5 wt% organic template. The as-formed zeolite shows high relative crystallinity and is structurally stable during higher temperature 550 degrees C air calcination to remove the organic template from the internal pores and yields zeolites with surface areas near 300 m(2)/g. The decrease in organoamine template content is noteworthy as it is an expensive non-recyclable molecular template and lower template use appears to increase zeolite crystallite size. The synthesized ZSM-5 materials shows expected catalytic activity in biomass conversion of cellulose and glucose into hydroxymethylfurfural (HMF). Glucose showed higher catalytic degradation (biomass conversion 72-79% with 33-42% yield of HMF) than the cellulose (biomass conversion 55-58% with 33-38% yield of HMF).