Highly Thermostable Ni-based Catalysts For Syngas Methanation

Natural gas is a vital component of the world's energy supply.It's not only a clean fuel,which is widely used in heating,vehicle,electricity generation,but also a feedstock for the production of plastics and organic chemicals.The increasing demand and the shortage of natural gas as well as the unpredictable price of oil has made the production of synthetic natural gas(SNG)from coal more attractive.Methanation catalyst is the key for the conversion of coal-derived syngas to SNG.Therefore,our focus is to develop the moderate temperatue methanantion catalysts with high activity and stability,to meet the demand of the SNG process.There are three main types of methanation catalysts:MCR-2X series(HALDOR TOPSOE),CRG series(Johnson Matthey)and G1 series(BASF).However,all of these catalysts need to be imported from foreign big companies,and development of our own SNG catalysts has become more imperative..The major challenge with SNG catalysts is serious deactivation of traditional Ni/Al2O3 catalysts,due to the sintering of nickel on catalysts.Here,the thermodynamic analysis of carbon monoxide methanation with H2/CO ratio equal to 3.0 under different pressure and temperature was first conducted using Gibbs free energy minimizing method,which provides the thermodynamic baseline for the evaluation of experimental data.We prepared MgAl2O4 support and Ni/MgAl2O4 catalysts and evaluated their the activity and stability in carbon monoxide methanation reaction,and directly compared with the traditional Ni/y-Al2O3 catalyst.MgO promoted NiAl2O4 catalysts(noted as NiAl2O4-MgO)show better carbon monoxide methanation reaction activity and stability than NiAl2O4,Ni/MgAl204 and Ni/Al2O3 catalysts prepared by impregnation.These catalysts were characterized by XRD,N2 physisorption,TG-DTA,H2-TPR,SEM and TEM ect.The mainly results are summaerized as follows:1.MgAl2O4 and ?-Al2O3 supported Ni-based catalystsWe synthesized MgAl2O4 materials by ethanol-hydrolysis with magnesium nitrate hexahydrate and aluminum isopropoxide at 150? and then calcined at 800?.The Ni/MgAl2O4 and Ni/Al2O3 catalysts were then prepared by wet impregnation method for testing in carbon monoxide methanation.Under conditions of 3Mpa,450 ? and 40mL/min(WHSV=12000mL g-1 h-1),the CO conversion and CH4 selectivity of two series catalysts were about 99.6%and 96%respectively.In order to better evalaute the catalyst stability,the CO conversions were all adjusted below equilibrium.The results show that 7.5%Ni/MgAl2O4 and 7.5%Ni/?-Al2O3 are stable for CO methanation.In general,Ni/MgAl2O4 catalyst is more stable than Ni/y-Al2O3 catalyst with the same nickel loading,probably due to stronger metal-support interaction of Ni/MgAl2O4 than that of Ni/y-Al2O3.This conclusion is confirmed by the results based on XRD,N2 physisorption,H2-TPR,SEM etc.analysis.It should be noted that over Ni/MgAl2O4 catalyst,the selectivity of CH4 decreases to 50%,while that of C2H6,C3H8 and CO2 increases significantly,with CO2 up to 30%especially at low CO conversion.2.MgO promoted NiAl2O4 catalystsMgO promoted NiAl2O4 catalysts(denoted as NiAl2O4-xMgO,x mean mole ratio of magnesium to nickel,equal to 0,0.5,1,or 1.5)prepared by solvent assisted hydrolysis and wet impregnation were also studied for carbon monoxide methanation.After exposing to hydrogen atmosphere at 800? for 3 hours,the inactive Ni-Al spinel was found to restructure to a highly active catalyst with nano-sized(2-20nm)nickel particles.The more of MgO impregnated into NiAl2O4,the smaller of nickel particles of catalyst after reduction in hydrogen at high temperature.After reduction in hydrogen at 800? activity tests of the NiAl2O4-xMgO catalysts were carried out under conditions of H2/CO=3.0,3Mpa,450 ? and 40mL/min(WHSV=120000mL g-1 h-1),the CO conversion and CH4,CO2 selectivity of two series catalysts were 99.6%and 96%,respectively.Durability tests of the NiAl2O4-xMgO catalysts were carried out under conditions of H2/CO=3.0,3Mpa,450 and 40mL/min(WHSV=240000mL g-1 h-1).The most robust catalyst is NiAl2O4 and NiAl2O4-MgO,where nearly no nickel sintering occurred as confirmed by XRD measurement.However,nickel particles in NiAl2O4-0.5MgO and NiAl2O4-1.5MgO were found to have severe sintering.