Estudo do efeito dos parâmetros de síntese da Sn-MCM-41 e Sn-SBA-15 em suas propriedades estruturais, texturais e catalíticas na isomerização da glicose

Abstract

The 5-hydroximethilfurfural (5-HMF) is a platform molecule that has great potential to originate a wide range of compounds with potential interest in biorefinery, as well as compounds currently produced exclusively from non-renewable carbon sources. The 5-HMF can be obtained via dehydration of hexoses, but with low selectivity (< 30%). On the other hand, fructose, a glucose isomer, can be dehydrated to 5-HMF with high selectivity (> 70%). Recently it has been discovered that using a combination of Lewis acids and Brønsted acids, in the right proportion, allows the direct conversion of glucose into 5-HMF with selectivities higher than 60%. In this process, the Lewis acid isomerizes the glucose to fructose, which is subsequently dehydrated by the Brønsted acid. While several brønsted acid catalysts are known to be efficient in the dehydration of fructose, Sn-Beta zeolite appears almost exclusively as a heterogeneous Lewis acid catalyst for glucose isomerization. However, Sn-Beta has a long, complicated synthesis with questionable reproducibility. Thus, in this study, we proposed the use of mesoporous amorphous stannosilicates as catalysts of glucose isomerization. The effects of the synthesis parameters of mesoporous stannosilicates Sn-MCM-41 and Sn-SBA-15 were evaluated, in the face of its textural, structural and catalytic properties in the isomerization of glucose. The characteristic structures were obtained for all silicates, although the use of an organic base and higher concentrations of acid decreased the ordering respectively for samples MCM-41 and SBA-15. It was observed the presence of tetrahedral tin, indicating insertion in the framework, besides penta and hexacoordinated tin species. The isotherms obtained are as expected for these materials. Changes in pH and Si:Sn ratio didn’t present great effects on the MCM-41 cell parameters and wall thickness, although thermal treatment reduced the last. Variations in synthesis parameters have more visible effects for SBA-15 silicates. The materials in general showed values of selectivity for fructose above 75%, which can be considered very good. It is worth mentioning MCM-41 synthesized with TMAOH showed these values even with larger conversions. The most promising materials (TMAOH 12.9-3% Sn-25 ° C; NH4OH 12.9-3%-50 ° C and SBA15-0.07-3%) were studied more deeply in relation to their catalytic properties. The kinetic tests and indicated that in general the reaction goes into equilibrium at 110 minutes. Only exchange of the organic solvent GVL and THF (polar aprotic) to methanol (polar protic) resulted in significant changes in the behavior of the rates of glucose conversion and selectivity, being GVL the most interesting solvent.