Preparation Of Intermetallic Compounds And Their Catalytic Performance Towards Selective Hydrogenation Of ?,?-unsaturated Aldehydes

Selective hydrogenation of ?,?-unsaturated aldehydes serves as a significant chemical transformation in pharmaceutical,food and fine chemicals industries for the production of unsaturated alcohols.Normally,saturated aldehydes are predominant products since C=C bond undergoes a more thermodynamically favorable hydrogenation than C=O bond;therefore,how to increase the hydrogenation selectivity of C=O bond in ?,?-unsaturated aldehydes to obtain unsaturated alcohols has attracted considerable attention in fundamental research and industrial applications.In recent years,intermetallic compounds(IMCs)with unique geometric structures and electronic effects have shown good catalytic performance toward selective hydrogenation reactions.However,several problems remain unsolved in the catalytic reaction of IMCs for the selective hydrogenation of ?,?-unsaturated aldehydes.First,IMCs catalysts with fine structure/property(electronic and geometric structure)toward selective hydrogenation reactions cannot be strategically engineered and prepared,which substantially influences the resulting catalytic performance.Second,understandings of intrinsic active site,catalytic reaction pathway and reaction mechanism are rather lacking,as a result of the limitations of characterization techniques.Third,stable and universal catalytic materials,which can be used as a promising candidate in selective hydrogenation reactions of ?,?-unsaturated aldehydes,are urgently needed in practical applications.In this work,a series of supported IMCs(Co-In,Co-Ga,Ni-Sn,Ni-Sb and Ni-Bi)were obtained by virtue of the topotactic transformation process of layered double hydroxides(LDHs)precursors,owing to their versatility in chemical composition and structural architecture.The resulting IMCs exhibited highly-efficient catalytic performance(activity,selectivity,stability and universality)towards selective hydrogenation reactions of ?,?-unsaturated aldehydes.In addition,the combination of in situ experimental characterizations and theoretical calculations for establishing structure-property correlation,offers an effective approach for investigations on intrinsic active site and reaction mechanism.This study demonstrates new explorations for the structure design and preparation of IMCs catalysts based on LDHs precursors,which can be potentially applied in selective hydrogenation reactions.The specific main research contents are as follows:1.Preparation of Co-based IMCs and their catalytic performance toward selective hydrogenation of cinnamaldehydeTwo Co-based IMCs(CoIn3 and CoGa3)were synthesized via topotactic transformation from LDHs precursors,which displayed surprisingly high catalytic performance toward the hydrogenation reaction of ?,?-unsaturated aldehydes(C=O bond vs.C?C bond).The conversion of all these three samples(Co,CoIn3 and CoGa3)is close to 100%(100?,2 MPa),but the selectivity toward unsaturated alcohol(cinnamyl alcohol,COL)increases in the following order:Co<CoIn3<CoGa3.Especially,the CoGa3 IMC exhibits the optimal catalytic performance with a conversion of 99%and selectivity of 96%,which is superior to previously reported Co-based catalysts and even comparable to the level of noble metals catalysts(e.g.,Pt,Au,or Pd).A combination research including XPS,XANES and in situ CO-IR spectrum substantiates electron transfer from Ga or In to Co,leading to the formation of Co-Ga or Co-In coordination.FT-IR characterization and DFT calculation study verify that the electropositive elements(Ga and In)in IMCs serve as the active sites and facilitate the adsorption of polarized C=O bond;whilst the adsorption of C=C bond on the Co site is extremely depressed,which is responsible for the highly enhanced selectivity toward the hydrogenation of C=O bond.This work reveals the key role of the functional group adsorption in determining the hydrogenation selectivity of a,p-unsaturated aldehydes.2.Preparation of Ni-based IMCs and studies on reaction mechanism of selective hydrogenation of furfuralThree Ni-Sn IMCs(Ni3Sn1,Ni3Sn2 and Ni3Sn4)were obtained via structural transformation from ultrathin nanosheets of Ni2Al-LDHs precursor,which exhibited surprisingly enhanced catalytic behavior toward selective hydrogenation(C=O)of furfural to furfuryl alcohol.Notably,Ni3Sn2 IMC displays the optimal catalytic performance(99%selectivity)compared with monometallic Ni(2%selectivity).A combination study based on in situ FT-IR and Bader charge reveals the electron transfer from Sn to Ni,facilitating the adsorption of C=0 bond on Ni top sites while inhibits the adsorption of C=C bond.Both experimental studies(catalytic evaluations and in situ FT-IR)and theoretical calculations(DFT calculations and microkinetic modeling)verify a vertical adsorption configuration of furfural on Ni3Sn2,followed by the first hydrogenation on C atom in C=O bond(rate-determining step)and the second hydrogenation on 0 atom in C=O bond.A detailed investigation on structure-selectivity correlation is verified by virtue of establishing the adsorption configuration of substrate and the reaction pathway,which would pave way for rational design and development of efficient heterogeneous catalysts toward selective hydrogenation reactions.3.Studies on universality of Ni-based IMCs toward selective hydrogenation of ?,?-unsaturated aldehydesWe further extended a unique and facile synthesis of monodisperse nanoscale Ni-based IMCs(Ni,Sb1 and Ni1Bi1)via structural transformation from ultrathin nanosheets LDHs precursors.Notably,Ni1Bi1 IMC serves as a highly efficient catalyst for selective hydrogenation of various ?,?-unsaturated aldehydes(crotonaldehyde,3-methyl-2-butenal,2-pentenal,furfural,5-hydroxymethylfurfural and cinnamaldehyde)to produce corresponding?,?-unsaturated alcohols(catalytic selectivity:97.2%,93.2%,94.5%,95.7%,97.7%and 98.9%,respectively).In addition,no obvious decrease in activity and selectivity of Ni1Bi1 IMC catalyst was found after six-time recycling use,showing a good stability.A combination study including STEM,XANES,in situ FT-IR and DFT calculation substantiate a separation effect of active Ni sites by Bi sites with a rugged surface atomic arrangement.More importantly,in situ FT-IR measurements and DFT calculations confirm that ?,?-unsaturated aldehydes undergo a vertical line adsorption configuration through terminal oxygen in C=O attaching to the top Ni in Ni1Bi1 IMC,resulting in the hydrogenation of C=O group and the selectivity towards the final products(unsaturated alcohols).This work demonstrates a successful paradigm for the exploration of LDHs-based IMCs catalysts via a detailed investigation on structure-property correlation,which can be potentially used as a promising candidate in the selective hydrogenation reactions.