Modulations Of Active Site Structure Of Layered Double Hydroxides And Their Catalytic Performance Toward Aldol Condensation Reaction

Aldol condensation is an important facet of synthetic chemistry used in many industrial processes,which produces a variety of value-added chemicals with versatile commercial applications.At present,the liquid alkali as homogeneous catalyst has been commonly used in aldol reactions due to high activity and cost effectiveness.However,several problems remain unsolved:generation of large amounts of waste liquid,environmental pollution and equipment corrosion;moreover,the product separation is a big conundrum,which results in further polycondensation of product.Recently,solid base materials as heterogeneous catalyst are regarded as a substitutional candidate to resolve the above problems encountered in the application of liquid alkali.Nevertheless,due to restrictions of materials exploration and research methods,fine control over basic site is rather difficult to obtain solid basic catalysts with optimized activity and desired stability.Moreover,understanding on structure of basic sites and catalysis mechanism is rather lacking,which is a bottleneck for the enhancement of catalytic performances.Hence,in this dissertation,on the basis of the structure memory effect of LDH precursors,activated layered double hydroxides as solid base catalysts were prepared by regulating structure/property of alkaline sites on the surface(type,strength,quantity and electronic structure),and their catalytic performances toward aldol condensation of isobutyraldehyde(IBD)with formaldehyde(FA)to produce hydroxypivalaldehyde(HPA)were studied in detail.Furthermore,both in situ characterization techniques and theoretical calculations were performed to determine the active structures of these LDH solid basic catalysts,with the intention of understanding the active sites structure and to augment the catalytic property.This study demonstrates new ideas and exploration for the structural design and preparation of solid basic catalysts,which can be potentially applied in aldol condensation of IBD and FA.The main research contents and results are as follows:1.Modulations of active basic sites in CaAl-LDH as a solid base catalyst toward aldol condensationIn order to modulate alkaline site structure of LDH solid basic catalysts,rehydrated CaAl-layered double hydroxides(denoted as re-CaxAl-LDH)with OH-as interlayer anion,were prepared via a two-step procedure:calcination of CaAl-LDH precursor to produce mixed metal oxide(MxAl-MMO),followed by a further rehydration treatment.The resulting re-Ca4Al-LDH sample exhibits a largely enhanced catalytic performance(HPA yield of 61.5%and product formation rate of 52.5 mmol·g-1·h-1)toward aldol condensation of IBD with FA,significantly higher than conventional solid base catalysts and comparable to liquid alkali catalysts.A combination study including XPS,EXAFS,deuterated chloroform-FTIR spectra and DFT calculation verifies that the host layer of re-CaxAl-LDH contains distorted Ca(OH)6 octahedron with an additional Ca-OH coordination that provides weak Bronsted basic site.Studies on the structure-property correlation and desorption energy of product reveal that the weak Bronsted basic site serves as active center to adsorb a-C-H of aldehyde group and accelerate the product desorption,accounting for the largely promoted HPA selectivity.A facile and cost effective approach for the preparation of LDHs-based solid basic catalysts is demonstrated,which can be used as a promising candidate in green catalysis of aldol condensation reactions.2.CaMnAl-hydrotalcite solid basic catalyst toward aldol condensation reaction:a promotion effect of MnThe rehydrated CaMnAl-layered double hydroxides as a heterogeneous basic catalyst were prepared based on the memory effect of LDH precursors.The resulting re-Ca4Mn0.5A1-LDH exhibits a largely enhanced catalytic performance toward aldol condensation of isobutyraldehyde with formaldehyde(HPA yield of 70.3%and product formation rate of 90.3 mmol·g-1·h-1),rather close to the level of liquid alkali catalysts(HPA yield:73.2%).A combination study including XPS,EXAFS and deuterated chloroform-FTIR spectra verifies that re-Ca4Mn0.5Al-LDH contains predominant Mn? species;the resulting Ca-O-Mn? structure with a highly-exposed Ca2+ s-orbital and strengthened interaction with 7-fold OH-,provides a weakened Br(?)nsted basic site compared with the reference sample re-Ca4Al-LDH.Studies on the structure-property correlation based on experimental investigations and DFT calculations reveal that the decreased strength of weak Br(?)nsted basic site modified by Mn? further accelerates the product desorption,accounting for the largely promoted HPA selectivity.Therefore,this thesis provides an effctive strategy to strikingly enhance catalyic performance via introducing promoter,which supplies a novel process for the design and preparation of high performance LDH solid base catalysts towards aldol condensation reactions.