Preparation Of Ni-Based Catalysts And Their Catalytic Performance Towards Selective Hydrogenation Of Furfural

With the rapid development of human society,environmental problems have become increasingly prominent,and the development and utilization of renewable biomass resources have attracted great attention.Furfural is an important biomass platform compound obtained by hydrolysis of lignocellulose,whose conversion products have broad applications in petrochemical,environmental energy and other fields.A variety of high value-added products（e.g.,2-methylfuran and furfuryl alcohol）can be obtained by selective hydrogenation of furfural,but how to improve the selectivity of target product remains a challenge.Compared with the high cost of noble metal catalysts,nickel-based catalysts have attracted much attention because of their cost effectiveness and excellent catalytic hydrogenation activity.At present,nickel-based catalysts show strong reduction ability for C═O bond and C═C bond in furfural,but the following problems and deficiencies in directed hydrogenation remain unresolved:（1）It is difficult to control the particle size of nickel-based catalyst particles to ensure the uniformity of active structure,so as to improve the catalytic selectivity;（2）The lack of understanding of active site structure and the disclosure of the structure-activity relationship limit the design and preparation of high-performance catalysts.In view of the above key problems,based on the rich adjustability of the structure and composition of layered double hydroxides（LDHs）,this thesis realizes a fine regulation of Ni nanoparticle size by adjusting the thickness of LDHs precursor laminate as well as the process of structural topology transformation,and obtains high-efficiency catalysts for selective hydrogenation of furfural.The structure-activity correlation between the active site structure and catalytic performance of the catalyst was studied by electron microscopy,X-ray spectroscopy and infrared spectroscopy,and the reaction mechanism of selective catalytic hydrogenation of furfural was revealed.In this thesis,a new strategy for regulating the particle size of nickel based catalyst is proposed,which provides practical explorations for the design,preparation and catalytic performance regulation of hydrogenation catalyst.The main research contents of this thesis are as follows:1.Study on nickel-based catalysts with tunable particle size and their performance for selective hydrogenation of furfuralNi Mg Al-LDHs were synthesized through the aqueous miscible organic solvent treatment（AMOST）.A series of precursor materials with different thickness were obtained by adjusting the stripping time（denoted as Ni Mg Al-LDHs-t,t=0,1,4,8 h）,with the laminate thickness of 28.7 nm,18.2 nm,9.6nm and 5.1 nm confirmed by AFM characterization.Four supported nickel-based catalysts（denoted as Ni/MMO-t,t=0,1,4,8 h）were prepared by reducing LDH precursors in a H₂ atmosphere.XRD and TEM characterizations verify a uniform dispersion of Ni metal particles,with particle size of 12.8 nm,10.1 nm,7.1 nm and 4.5 nm,respectively.The data fitting analysis shows that the thickness of Ni Mg Al-LDHs-t laminate has a linear positive correlation with the Ni particle size of Ni/MMO-t（R²=0.97）.In the selective hydrogenation of furfural,all the four catalysts display a 100%conversion of furfural.Among them,Ni/MMO-0 and Ni/MMO-8 samples exhibit the highest yields of 2-methylfuran（85.3%）and furfural alcohol（89.1%）,respectively.In addition,these two catalysts maintain stable catalytic activity and selectivity after five catalytic cycles.2.Study on structure-activity correlation of selective hydrogenation of furfural catalyzed by Ni/MMOBased on the work of above part,the effect of metal particle size on the catalytic performance for selective hydrogenation was studied through a variety of characterization techniques,and the catalytic reaction mechanism was further revealed.Combined with XPS,CO-DRIFT and XAFS experimental measurements,it is confirmed that with the decrease of Ni particle size,the proportion of Ni⁰ decreases whilst the proportion of Ni^δ+increases accordingly.The time curve of catalytic performance shows that the selective hydrogenation of furfural obeys the following process for the four samples:C═O bond undergoes hydrogenation to produce furfural alcohol,followed by the cleavage of C–OH bond to generate 2-methylfuran.In situ infrared spectroscopy was used to explore the adsorption configuration of reactants and reaction intermediate species.In the presence of Ni sample with a small particle size（Ni/MMO-8）,furfural molecule experiences activation adsorption with aη¹（O）configuration at the Ni^δ+site at the interface;and therefore the hydrogenation of C═O bond occurs,so as to obtain furfuryl alcohol.In the case of Ni/MMO-0 sample with a large particle size,furfural molecule undergoes adsorption via aη²（C,O）mode,in which O atom in the carbonyl group is adsorbed on Ni^δ+whilst the C atom is attached to Ni⁰.The C═O group first experiences activation reduction to produce furfural alcohol,followed by the subsequent breakage of C–OH group to produce 2-methylfuran.This work provides a method to control the particle size of Ni-based catalysts,which show potential applications in selective hydrogenation reaction.