Surface Modified Pd/g-C₃N₄ For The Selective Hydrogenation Of Isoprene

With the increasing production of ethylene,the high value-added utilization of byproducts from the naphtha thermal cracking process for ethylene production has received a lot of attention.Selective hydrogenation process is the most important way to utilize these by-products efficiently.The operational efficiency of selective hydrogenation processes is influenced by many factors,among which catalysts are one of the most important influencing factors,whose morphology,size,structure and surface properties have a critical impact on catalytic performance.The introduction of suitable surface organic ligands can not only stabilize the metal nanoparticles on the catalysts but also synthesize nanoparticles of specific morphology.The precise coordination of organic ligands at the metal interface can change the surface chemistry properties of the loaded nanocatalysts,which in turn can have promoting effect on the catalytic process.Thanks to the development of atomic-scale characterization techniques,the effects brought by surface ligands are gradually gaining deeper understanding.In order to effectively improve the atomic utilization of noble metals,this study controlled the surface properties of the catalysts by interfacial modification with two different organic ligands.And the selective hydrogenation activity of isoprene was tested.A series of characterization tools were used to sy stematically investigate the mechanism of the effect of different surface ligands on the catalytic hydrogenation reaction,and the main studies are as follows:(1)The incorporation of palladium into the carriers was fabricated by photoreduction deposition method.The high resolution transmission electron microscopy showed that the palladium nanoparticles could be uniformly dispersed on the surface of carbon nitride with a particle size of about 3 nm.Acrylic acid was introduced on the catalyst surface by photoinduced grafting(PIG)method.SEM and TEM results showed that the palladium nanoparticles were not exfoliated after the modification of acrylic acid,and the dispersion and the particle size was basically the same as that of undecorated catalyst.The catalytic performance of selective hydrogenation was investigated using isoprene as a simulated oil.XPS and CO in situ IR showed that the acrylic acid did not change the electronic state of the palladium nanoparticles.The isoprene-TPD demonstrated isoprene can be adsorbed on the catalyst and the adsorption intensity would increase with increased time of PIG.Meanwhile,hydrogen spillover experiments and proton migration experiments of Pd/CNA-4 showed that the acrylic acid ligand could accelerate the transfer of H activated on the metal.However,the excess surface ligand may have a non-negligible steric effect on the neighboring Pd nanoparticles,which may shield the active center and hinder the adsorption of the reactants.The combination of these factors results in a "volcanic"trend in the hydrogenation performance of the catalyst.(2)In the catalytic process,the nature of the surface ligand itself may also affect the catalytic performance such as hydrophobicity.Based on this,a photo-grafting method was used.to modify the carbon nitride loaded palladium with oleylamine.Thermogravimetric(TG)analysis and Fourier infrared(IR)spectroscopy showed that oleylamine was successfully grafted on the catalyst,and elemental analysis indicated that the time of photoinduced grafting was correlated with the oleylamine loading content.The hydrogenation performance of isoprene with different modification times was examined.The results showed that the activity gradually increased with the amount of oleylamine,with Pd/CNO-6 reaching 85%activity and 95%selectivity.The XPS,in situ CO IR studies and the results of water contact angle tests demonstrated that the oleylamine modification will not only change the electronic state of the metal and affect the adsorption but also enhances the surface hydrophobicity,increases the dispersion in hydrophobic solution,and effectively enriches the hydrophobic substrate on the catalyst surface,thus providing a high selective hydrogenation performance.