Performance Modulation Of Boron Active Sites In The Fe Doped Octahedral Molecular Sieve PKU-1 Catalyzed Glycerol Ketonization Reaction

In the design and study of solid acid catalysts researchers tend to focus on catalysts containing transition metal（TM）active centers and lack the exploration of catalysts containing non-metallic active centers.This is because non-metallic elements lack the electronic flexibility of TM.However,combining non-metallic elements with TM elements can greatly enhance the catalytic performance of non-metallic sites.For example,triple-liganded boron compounds are widely used as Lewis acid centers and exhibit comparable or even superior catalytic performance to TM active sites.Meanwhile,with the rapid development of biodiesel industry,the production of glycerol as a by-product is increasing year by year,and how to convert it into high value-added products has become the key to promote the sustainable development of biodiesel industry.From numerous studies,it is found that acid-catalyzed glycerol ketonization reaction is one of the most promising reactions for glycerol resource utilization,but there are still many problems in this reaction process that need to be solved.Based on this,we applied catalysts with non-metallic acid centers to glycerol ketonization reactions,which not only can promote the sustainable development of biodiesel industry,but also facilitate the research and development of non-metallic active site catalysts.In this paper,PKU-1 materials were prepared by hydrothermal synthesis,and Fe³⁺was introduced into the backbone to improve the catalytic activity of the boron active site,while the effect of sample crystallinity on the catalytic performance was systematically investigated by optimizing the synthesis conditions.The nanophase hydrophobic Fe_xAl_1-x-PKU-1 catalyst was developed for the problems of interfacial limitation and catalyst deactivation due to water absorption.The main conclusions were obtained as follows.A series of Fe_xAl_1-x-PKU-1 catalysts were obtained by introducing Fe³⁺in the hydrothermal synthesis using octahedral molecular sieve PKU-1 as the parent material,and were used in the glycerol ketonization reaction to study the effect of Fe doping amount on the boron active sites.Characterization analysis revealed that the skeletal structure and surface functional group species of Fe_xAl_1-x-PKU-1 were very stable,but the substitution of some Al sites in the skeleton by Fe³⁺homocrystals seriously affected the surface composition,specific surface area and acid properties of the catalysts.The promoting effect of Fe³⁺in Fe_xAl_1-x-PKU-1 on the surface boron active site was discussed for the first time in conjunction with the mechanism of action of M-E type catalysts.It was found that the material obtained a maximum glycerol conversion of92.8%and selectivity of 95.1%for Solketal by reacting at 318 K,a glycerol/acetone mass ratio of 1/5 and a catalyst feed of 5.0%by mass of glycerol for 3 h.In the previous study we found that the exposure of boron active sites on the surface of Fe_xAl_1-x-PKU-1 is closely related to its crystallinity and surface morphology.Therefore,we chose the best catalytic activity of 18%Fe doped PKU-1 as the standard and prepared a series of samples with different crystallinity by optimizing the temperature and microenvironmental acidity during the hydrothermal synthesis.The characterization analysis revealed that the crystalline quality of the material would seriously affect the electron transfer between Fe and boron species in the backbone,thus affecting its catalytic activity.The activity evaluation yielded the maximum acid content and acid strength of Fe_xAl_1-x-PKU-1（FWHM=0.237）prepared at 513 K and 200μL of HNO₃ addition,which yielded 96.3%glycerol conversion and 98.5%Solketal selectivity in the reaction.To overcome the problems of multi-phase mass transfer and catalyst deactivation due to water uptake in glycerol ketonization reaction,we designed and developed nanophase Fe_xAl_1-x-PKU-1 materials prepared by using Al₂O₃（20 nm）as Al source and modified with CTAB hydrophobicity to obtain catalytic materials with hydrophobic surface.The experimental study and related characterization revealed that the hydrophobic functional groups grafted on the catalyst surface would cover part of the boron active site leading to the decrease of acid content,but the increase of hydrophobicity effectively overcame the problem of catalyst deactivation due to water absorption.Therefore,considering the dual effect of acid content and hydrophobicity,the nanophase Fe_0.2Al_0.8-PKU-1（CTAB=0.04 g）obtained by optimizing the experimental conditions can obtain 85.3%glycerol conversion and 97.1%Solketal selectivity in the crude glycerol ketonization reaction.