Preparation Of Ca-based Layered Double Hydroxides And Their Catalytic Performance Toward Aldol Condensation Reaction

Serving as an important aspect of synthetic chemistry applied in numerous industrial processes,base-catalyzed aldol condensation reactions have caused considerable attention to produce various value-added chemicals with all kinds of industrial applications.Among these aldol condensation processes,the reaction of isobutyraldehyde(IBD)with formaldehyde(FA)to produce hydroxypivalaldehyde(HPA)is an important intermediate for the production of neopentyl glycol(NPG),which has been widely used in the manufacturing of polyesters,lubricants,plasticizers,and synthetic resin coatings.Generally,homogeneous catalysts such as triethylamine,KOH and NaOH,have been used in this aldol reaction,but suffer from catalyst separation and environmental pollution.Based on the idea of environment-friendly catalysis,heterogeneous basic catalysts play a more pivotal role,as their employment in industrial processes offers opportunities to facilitate separation,avoid corrosion and pollution issues.However,compared with homogeneous basic catalysts(the highest level of HPA yield:73.5%),the HPA yield of solid basic catalysts is not satisfactory(only 4.3%-50%),and the catalytic activity needs to be further improved.Therefore,how to design and prepare novel solid basic catalysts with high activity and stability through elaborate control over basic site properties remains a big challenge.In this thesis,taking advantage of the structure memory effect of layered double hydroxides(LDHs),rehydrated Ca4Al1-xMxLDHs(M=Ga?In)catalysts were prepared and applied in aldol condensation reaction,i.e.,isobutyraldehyde(IBD)reacts with formaldehyde(FA)to obtain hydroxypivalaldehyde(HPA).Furthermore,both in situ characterization techniques and theoretical calculations were performed to determine the active structures of these LDHs solid basic catalysts,and the regulation mechanism of the doping of metal elements with large ion radius on the fine structure of catalysts was investigated.This study demonstrates an exploration for the structure design and preparation of novel solid basic catalysts,which can be used as a prospective candidate in green catalysis of aldol condensation reactions.The main research contents and conclusions of this thesis are as follows:1.Preparation of re-Ca4Al1-xMx-LDHs(M=Ga and In)and their catalytic performance toward aldol condensationRehydrated Ca4Al1-xMx-LDHs(M=Ga and In)materials were prepared by virtue of the structural topotactic transformation and memory effect of Ca4Al1-xMx-LDHs precursors.XRD patterns and SEM images verify the successful preparation of re-Ca4Al1-xMx-LDHs materials.By changing the x value of Ca4Al1-xMx-LDHs precursors(from 0 to 0.15),seven samples were obtained.Among these samples,the re-Ca4Al0.90Ga0.10-LDHs catalyst displays the optimal catalytic performance with a IBD conversion of 91.0%and HPA yield of 72.0%,which significantly exceeds the re-Ca4Al-LDHs catalyst(HPA yield:60.0%)and even close to the catalytic performance of liquid basic catalysts(HPA yield:73.5%).2.Studies on structure-activity relationship of re-Ca4Al1-xMx-LDHs(M=Ga and In)toward aldol condensation reactionOn the basis of the above results,the structure of re-Ca4Al1-xMx-LDHs catalyst was studied by a series of characterization experiments and DFT calculations,and the structure-activity correlation was established.CDCl3-FTIR characterization verifies that the incorporation of Ga element enhances the concentration of 7-coordinated Ca-OH group(weak basic site),which is the intrinsic active site in this reaction.EXAFS measurements reveal that the incorporation of Ga with moderate ionic radii,leads to a lattice expansion of hydrotalcite laminates and the increase of Ca-O bond length.XPS and DFT calculations show that the alkaline strength of the catalyst does not change with the incorporation of Ga or In.Investigations on the structure-property correlation indicate that Ga as a structural promoter induces a moderate expansion of the laminate lattice,which results in a significant increase in the concentration of weak basic sites in re-Ca4Al0.90Ga0.10-LDHs,accounting for its high catalytic activity.This work investigates geometry effects on the structure of Ca-based hydrotalcites,and provides a new route for the design and preparation of high performance solid basic catalysts toward aldol condensation reaction.