A Study For Catalytic Performance Regulation Of Ir-based Catalysts For Selective Hydrogenation Of Crotonaldehyde

α,β-unsaturated alcohols are important chemical raw material and reaction intermediate of perfumes,pharmaceutical as well as some fine chemical products.In homogeneous system,the method of selective reduction of α,β-unsaturated aldehydes/ketones is used to yield the corresponding unsaturated alcohols at the expense of using expensive metal hydrides(e.g.,NaBH4 or LiAlH4),which,however,suffered from harsh reaction conditions,difficult isolation and purification of product.However,continuous production of α,β-unsaturated alcohols via heterogeneous catalytic selective hydrogenation process is of great benefit including production efficiency,environmental friendliness and economic advantages.As a kind of simpleα,β-unsaturated aldehyde,only the methyl group is attached to one side of C=C bond for crotonaldehyde.Compared to large long-chain substituents such as phenyl,it is difficult to hinder adsorption of C=C bond for methyl by steric effect.So hydrogenation of thermodynamically unfavorable C=O bond is particularly difficult for crotonaldehyde.Therefore,selective hydrogenation of crotonaldehyde is a research topic with important industrial value and academic value.The gas selective hydrogenation of crotonaldehyde chosen as target reaction was evaluated in a fixed-bed microreactor.This dissertation is centered around preparation of supported Ir-based catalysts to explode the effects of support,metal salt type,and metal loading on the selective hydrogenation performance of Ir-based catalysts.Using silica as the carrier,Ir/SiO2 prepared with iridium acetylacetone had the best catalytic performance.The selectivity and TOF value of Ir/SiO2 with high loading of Ir(≥1 wt%)are at the optimal level,but the slow deactivation still exists.In order to solve the deactivation problem caused by CO poisoning and surface coverings on Ir/SiO2 catalysts during hydrogenation,mixing trace amounts of oxygen into reaction system was used to eliminates those surface coverings on the active sites in situ by the oxidation process.While maintaining high selectivity of crotyl alcohol,no obvious deactivation occurred during the catalyst evaluation of hundreds of hours.The catalyst structure and surface species were investigated by XRD,TEM,H2-TPR,XPS,CO-TPD,FT-IR,dynamic O2/CO pulse adsorption and other characterizations.The results showed that Ir nanoparticles on the catalyst surface remain zero valence before and after the hydrogenation.High selectivity of crotyl alcohol will be attributed to the adsorption of crotonaldehyde on the surface of Ir particles by C=O bond.A part of crotonaldehyde on the active sites of Ir particles surface will decarbonylate to form CO2 which tightly occupy the active sites resulting in the decline of catalytic activity.CO adsorbed on particle surface is neither easy to form CO2 via disproportionation and then leave the surface,nor form H2O and CH4 by hydrogenation at low temperature(<423 K).Mixing trace amounts of oxygen in reaction system will preferentially accelerate the oxidation of CO strongly absorbed on the surface to form CO2 which quickly desorb leading to activation of poisoned metal sites and promotion for the adsorption and activation of H2.