Preparation Of Pd-nanofiber Catalyst Based On Alkyne Hydrogenation

Selective hydrogenation was widely used in petrochemical and fine chemical fields.Olefins are important raw materials for the production of various compounds,including dye intermediates.The key to realize selective hydrogenation is the preparation of high-efficiency catalyst.Nano catalysts containing Pd are considered to be efficient and available catalysts,but the selectivity is poor due to the side reactions caused by excessive hydrogenation.It is usually modified by a second metal or organic ligand to improve its chemical selectivity.However,the improvement of selectivity is usually at the expense of catalytic activity.The relationship between equilibrium catalytic activity and product selectivity is still the focus of scholars' research.Therefore,it is imperative to develop a Pd catalyst with simple preparation and high reaction efficiency.In this study,the hollow carbon nanofiber encapsulated Pd nano catalyst was prepared by the combination of electrospinning technology and in-situ reduction.The hollow structure,the size of Pd nanoparticles and the electronic structure of Pd surface were accurately regulated by screening spinning materials and changing carbonization conditions.Through the interaction between support and metal and spatial confinement effect,high catalytic rate,high selectivity and inhibition of excessive hydrogenation caused by prolonged reaction time were realized.Firstly,the composite nanofiber membrane was prepared by electrospinning with PAN,PMMA and PVP polymers as spinning materials.After all samples were pre oxidized to improve the thermal stability of the composite nanofiber shell,the template was removed by high-temperature carbonization technology and Pd nanoparticles were reduced to obtain hollow carbon nanofiber encapsulated Pd catalyst,and the hollow nitrogen doped carbon nanofiber encapsulated Pd catalyst was prepared in situ.The size of Pd nanoparticles was accurately regulated by controlling the carbonization conditions,and the structure of the catalyst was systematically characterized.When the carbonization temperature was 600?800 ?,the particle size of Pd decreased with the increase of temperature.At the temperature of 800?1000 ?,the particle size increased with the increase of temperature,and the catalyst with the smallest Pd nanoparticles(about 5.82nm)was obtained at the carbonization temperature of 800 ?.When the synthesized catalyst was applied to phenylacetylene hydrogenation,its catalytic selectivity(> 90%)was significantly higher than that of commercial Lindlar catalyst(84%),and had high selectivity retention ability.Even if the reaction time was extended to 180 minutes,the selectivity of the catalyst was still greater than 73%,compared with 56% of Lindlar catalyst.In addition,the catalyst obtained at 800 ? had the best performance for selective hydrogenation of phenylacetylene,that is,the conversion of phenylacetylene was greater than 99%,and the selectivity of styrene was 96.3%.Secondly,the synergy between electron transfer and spatial confinement effect between carrier metal nanoparticles was studied at 800 ? carbonization temperature.Under the conditions of similar metal loading,similar metal particle size,the same electronic environment and the same catalytic environment,the spatial position of active nanoparticles was changed.By encapsulating the hollow Pd catalyst(Pd@HCNF)Compared with surface supported Pd catalyst(Pd/HCNF)applied to various alkyne hydrogenation reactions,the existence and function of hollow confinement effect are proved.The effect of confinement on the substrate accumulation of small molecules can improve the catalytic rate;The mass transfer inhibition effect on large molecules will reduce the catalytic rate;Zeolite like action on larger molecules selectively catalyzes the substrate.In terms of catalytic stability,after five cycles,the conversion and selectivity of Pd/HCNF catalyst decreased by 4.0% and 5.2% respectively,Pd@HCNF catalyst decreased by only 0.5% and 1.3%,showing the outstanding advantages of encapsulated catalyst in preventing the loss of active metal and maintaining catalytic stability.