Preparation, Characterization And Application Of Pt Nanoparticle Catalysts For Propane Dehydrogenation

The demand of propylene, an important basic chemical raw material, growscontinuously. The price difference between propylene and propane also is rising.Propane dehydrogenation is an important process for producing propylene. At present,the bottleneck which restricts its application is the poor stability of catalyst. In thiswork, a stable nano-catalyst was prepared to relieve the deactivation problemassociated with the traditional propane dehydrogenation platinum-based catalyst.Platinum nanoparticle catalysts were prepared through a liquid-phase synthesisroute combined with ultrasonic vibration method. Effect of preparing methods,loading methods, and carriers on the structure and properties of platinumnanoparticles were investigated. Relationship between the dehydrogenationperformances with the structural properties was clarified. Results show thatpreparation conditions influence the morphologies of platinum nanoparticles.Platinum nanoparticles prepared are uniform using chemical reduction method usingNaBH4as a reducing agent. Stabilizer, PVP, prevents aggregation of platinumnanoparticles. The particle size distribution of platinum nanoparticles is narrow whenthe molar ratio of PVP to Platinum was15:1. Platinum nano-catalyst exhibits a highselectivity to propylene and a high conversion of propane when prepared by ultrasonicvibration method. The pore diameter of mesoporous alumina ranged from6to10nm.Br nsted acid sites exist on the surface of carrier. The platinum nano-catalyst preparedby ultrasonic vibration method exhibits good performance in propanedehydrogenation. The distribution of platinum nanoparticles is uniform, and theaverage diameter of particles is3nm.Effect of tin and cerium as promoters on the structure and dehydrogenationperformance of catalysts was investigated. The catalyst was characterized by using various techniques: TEM, XRD, XPS and CO-chemisorption. The tin promoterincreases the selectivity to propene of Pt/Al2O3catalyst. The dehydrogenationperformance of the catalyst prepared by reduction method is better than that ofcatalyst prepared by impregnation. The particle size distribution of platinumnanoparticles when using cerium modified alumina as the support is uniform with theaverage diameter centered at2.9nm. Cerium exists in the form of Ce3+at low content.It interacts with carrier and platinum nanoparticles, increasing the resistance tosintering.Pt@SnO2/Al2O3catalyst was obtained by liquid-phase reduction. Effect ofpreparing methods and reaction conditions was investigated. Resuts show thatPt@SnO2/Al2O3catalyst for the dehydrogenation of propane exhibits the highestactivity, stability and resistance to coke when prepared under conditions as follows:PVP as stabilizer, the molar ratio of PVP to Platinum as15:1, ethanol-water as thesolvents, platinum content of0.4wt%, the molar ratio of platinum to tin as1:1.5. Byinvestigating the influence of reaction conditions on the performance fordehydrogenation, the optimal conditions were determined as follows: reductiontemperature at500℃, reaction time of4h, space velocity of1400h-1,propane/hydrogen feed ratio of1:1and reaction temperature at580℃. The conversionof propane and selectivity to propene were23%and93%respectively after reactionof72h, which is superior to PtSn commercial catalyst.The catalysts were further optimized by introducing K, Ce promoters toPt@SnO2/Al2O3catalyst. Compared with Pt@SnO2/Al2O3catalysts, the initialconversion of the catalysts modified by Ce increased by about5.2%. The conversionof propane and the selectivity to propylene were27%and93%respectively afterreaction for200min. The Pt@SnO2/Al2O3catalyst modified by K promoter shows thebest performance, and the conversion of propane and the selectivity to propylene were24.5%and97.5%respectively. NH3-TPD, H2-TPR, Py-IR and H2-O2titration analysisshowed that the active component of catalyst was better dispersed after modified by K,Ce promoters. Besides, the interaction between the active components and carrier wasstrengthened, decreasing the amount carbon and preventing the reaction of deep dehydrogenation. Hence, the stability of the catalyst at high temperature is furtherimproved.The intrinsic kinetics of dehydrogenation reaction was studied using a fixed beddifferential reactor. Four propane dehydrogenation reaction models were deducedbased on Langmuir-Hinshelwood mechanism for heterogeneous catalytic surfacereactions. The kinetic parameters of the model were obtained by fitting the equationⅡwith experimental data. The activation energy, adsorption energy and pre-exponentialfactor were obtained according to the Arrhenius equation and Van’t Hoff equation.Statistical tests and Boudart guidelines indicate that the dynamic model is reliable.