Pd/MIL-100 Catalyst For Aqueous Phase Phenol Selective Hydrogenation

As the second largest biomass resource,lignin has high C/O ratio and can produce more energy as a fuel.The catalytic conversion of lignin to high value-added organic chemicals has a promising future.Phenol is the most basic component of lignin,its selective hydrogenation product cyclohexanone is an important intermediate for the production of nylon.However,cyclohexanone can be easily further hydrogenated to cyclohexanol.It is important to develop an efficient catalyst with good selectivity.In this work,Pd-supported catalyst was investigated in the phenol hydrogenation reaction,Metal-Organic Framework MIL-100（Cr）with coordination unsaturated metal center was selected as the catalyst support.The effect of acidity from MIL-101 on the catalytic performance of phenol hydrogenation was studied.Pd/MIL-100（Cr）was prepared by a double-solvent impregnation method（DSM）.The catalyst was systematically characterized by XRD,FT-IR,TEM,XPS and CO chemisorption.The catalytic performance of the obtained catalyst was examined and compared with Pd/MIL-100（Cr）-IMP prepared by single-solvent impregnation.Pd/MIL-100（Cr）showed higher conversion than that of Pd/MIL-100（Cr）-IMP,which was attributed to the higher dispersion and smaller Pd grains.The effects of reaction temperature,reaction pressure,Pd/phenol molar ratio,and reaction time on the catalytic performance were investigated through sequential experiments.Phenol can be converted completely within 1 hour,with cyclohexanone selectivity of 98.3%under optimum conditions:100℃,0.13 MPa H₂pressure and Pd/phenol molar ratio of 0.012.Pd/MIL-100（Cr）can be reused 5 times without significant loss in activity.The effect of Lewis acidity of the support on phenol hydrogenation was further investigated.three metal cations Al³⁺,V³⁺and Ce³⁺with different p Ka values were selected and exchanged into the trinuclear cluster of MIL-100 through isostructural substitution to obtain bimetallic MIL-100.Pd/MIL-100（Cr/M）catalysts were prepared by DSM.The relative acidity and acid amount of bimetallic MIL-100 were characterized by pyridine adsorbed FT-IR,NH₃-TPD,and EPR.The results showed that by introducing 2^ndmetal center into the support,the Lewis acidity can be tuned.The performance Pd/MIL-100（Cr/M）catalyzed phenol hydrogenation showed similar conversion,while the selectivity of cyclohexanone was quite different.With the enhancement of Lewis acidity,the cyclohexanone selectivity increased accordingly.Pd/MIL-100（Cr/Ce）showed the highest selectivity of cyclohexanone with the highest amount of LAS.In addition,when the Lewis acid sites of MIL-100（Cr）was poisioned with pyridine,the selectivity of cyclohexanone was significantly reduced,which further explained the promoting effect of Lewis acid on the selectivity of cyclohexanone.