Platinum Confined Within MIL-101(Cr) For Enhanced Catalytic Selective Hydrogenation Of The Carbonyl Bond In α,β-unsaturated Aldehydes

The selective hydrogenation of α,β-unsaturated aldehydes(UAL)is an important route to synthesize unsaturated alcohols.Unsaturated alcohols are important intermediates in the manufacture of fine chemicals such as fragrances and pharmaceutical compounds.This reaction involves a complex reaction process involving the competitive reduction of C=O and C=C bonds of α,β-unsaturated aldehydes,where C=C bonds are thermodynamically easier to hydrogenate than C=O bonds.Therefore,it is quite challenging to improve the selectivity of C=O bond hydrogenation in the reaction to produce unsaturated alcohols(UOLs).In order to obtain highly selective C=O bond hydrogenation products,the construction of catalysts with high activity,high selectivity and good stability becomes the key to this reaction.The research in this thesis is devoted to the design and synthesis of stable Pt-based catalysts using the confinement of metalorganic frameworks(MOFs).Taking the selective hydrogenation of cinnamaldehyde and furfural as a model reaction,combined with various analysis and characterization methods,the Relationship between structure and activity of Pt catalysts supported on modified MIL-101(Cr)MOF materials were explored.Noble metal nanoparticles based catalysts tends to aggregate easily during the catalytic reaction process because of its high surface energy,which is not conducive to the dispersion of noble metals on the support.In this study,MIL-101(Cr)supported Pt particle catalysts were synthesized using polyols(polyvinylpyrrolidone-PVP,ethylene glycol-EG,and polyethylene glycol-PEG)as reducing agent,modifier,and stabilizer.Pt/MIL-101(Cr)(PEG)catalyst in selective hydrogenation of furfural to furfural alcohol compared with Pt/MIL-101(Cr)(EG)and Pt/MIL-101(Cr)(PVP)catalysts showed good catalytic hydrogenation activity.The excellent catalytic hydrogenation activity and reaction rate of the Pt/MIL-101(Cr)(PEG)catalyst are attributed to the charged-rich Pt(metallic Pt0)species and,to a certain extent,the steric effect induced by narrow pores of MIL-101(Cr).Using amine(-NH2)functionalized MIL-101(Cr)-NH2 as an active support,a Pt nanoparticle-based catalyst encapsulated within the pores of amine-functionalized MOFs was synthesized by polyol reduction method.The as-prepared 3%Pt/MIL-101(Cr)-NH2(x)catalysts retained the inherent properties of MIL-101(Cr)-NH2(x)supports such as crystallinity,surface area,pore texture,and surface acidity.Remarkably,the NH2 modified MIL-101(Cr)-NH2(x)supported Pt-based catalysts(3%Pt/MIL-101(Cr)-NH2(x))improved the selective hydrogenation of carbonyls(C=O)bond in CAL and FFL.Compared with other catalysts reported in literature,the selectivity towards the C=O bond is above 80%under mild conditions.The improved catalytic performance was credited to the nitrogen(N)heteroatom of the NH2 group existing in the MIL-101(Cr)-NH2 framework,and somewhat to the steric effect induced by mesopores of MIL-101(Cr)NH2.The N heteroatoms not only contribute to the highly uniform dispersion and stable confinement of small-sized Pt nanoparticles(≈2nm)within the pores of MIL-101(Cr)NH2,but also transfer its valence electrons to the vacant d orbitals of Pt nanoparticles leading to more new interfacial electrophilic and nucleophilic sites,which are beneficial for selective hydrogenation of C=O bond.Besides,the steric effect induced by mesopores of MIL-101(Cr)-NH2,encaging Pt nanoparticles,can also enhance the selective adsorption of the C=O bond to interact with the catalyst active sites,resulting in higher selective hydrogenation of C=O bond.Using MIL-101(Cr)as an active support,a serial of Pt-Co intermetallic nanoparticleIMNs catalyst supported by MIL-101(Cr)((3%Pty%Co/MIL-101(Cr),y=1,3,5)was designed and prepared by polyol reduction method.The structure-activity relationship was investigated by XRD,N2 adsorption-desorption,FTIR spectroscopy,SEM,TEM,XPS,CO chemisorption,NH3-TPD,XANES and EXAFS.Studies have shown that cobalt-modified 3%Pty%Co/MIL-101(Cr)catalysts can improve the selective catalytic hydrogenation of cinnamaldehyde hydrogenation-CAL to cinnamyl alcohol-COL when compared to 3%Pt/MIL-101(Cr)catalyst under mild conditions.It was found that 95%conversion of CAL with 91%selectivity to COL can be reached with 3%Pt3%Co/MIL101(Cr).In addition,the catalyst can achieve 97%furfural-FFL conversion and 94%furfuryl alcohol-FOL selectivity.This high activity and selectivity towards the C=O bond hydrogenation is attributed to the electronic and geometric effects arising from the charge transfer between Co and Pt in the uniformly dispersed Pt-Co IMNs catalyst.In addition,various characterization results revealed that the addition of Co to the IMNs can promote the Lewis acid sites that facilitate the polarization of the charge-rich C=O bonds and their adsorption via their oxygen atom,and also generate new interfacial acid sites,which are beneficial for enhanced selective hydrogenation of C=O bond.