Performance Optimization Of NiMo/Al₂O₃ Catalyst For Coke Oven Gas Thiophene Hydrodesulfurization

Coke oven gas is rich in components such as H₂,CH₄,CO and CO₂ and it is a high quality raw material for the synthesis of natural gas,methanol,ammonia,oil,glycol and hydrogen extraction.Coke oven gas including some trace sulphides（H₂S,CS₂,COS,C₄H₄S,mercaptans,thioethers,etc.）can be toxic to downstream synthesis catalysts.Of these,the five-membered heterocyclic structure of thiophene is the most stable.This thesis therefore addresses the problems of low activity and selectivity of thiophene hydrogenation catalysts for industrial applications.Therefore NiMo/Al₂O₃ catalysts were used as the basis of the study.In the first,the effect of a single carbonaceous atmosphere of coke oven gas on the selectivity of the catalysts for hydrogenation is explored.Secondly,the catalyst performance is optimally tuned.The mechanism of the influence of carrier pore structure and surface acidity and alkalinity on thiophene hydrodesulfurization（HDS）activity and sulphurization selectivity was elucidated by combining various characterization and analysis tools.The study details and conclusions are as follows:（1）The effect of a single atmosphere on the activity and selectivity of thiophene HDS over NiMo/Al₂O₃ catalysts was investigated.Under N₂ and CH₄ atmospheres,the HDS activity was high.At a reaction temperature of 230°C,the conversion of C₄H₄S is more than 95%.HDS activity was lowest under C₂H₄ atmosphere.At 350°C,the conversion of C₄H₄S was only about90%.This is probably due to the coking and carbonisation of C₂H₄ on the catalyst surface.This prevented the adsorption of thiophene on the catalyst surface resulting in the lowest catalyst HDS activity.In addition,the CO,CO₂ and C₂H₄ atmospheres have the highest number of by-products and COS formation.In particular,the CO atmosphere is more likely to cause COS formation;the C₂H₄ atmosphere tends to produce C₂H₅SH.（2）NiMo/Al₂O₃ catalysts with different pore volumes of Al₂O₃were prepared as supports.The performance of the HDS reaction was evaluated under simulated coke oven gas atmosphere.The experimental results show that the pore volume has a strong influence on the conversion and selectivity of thiophene HDS.The catalysts with larger pore volumes had higher HDS activity and higher selectivity for the main product H₂S,and lower selectivity for the by-products COS,C₂H₅SH and（C₂H₅）₂S.At 270°C,the conversion of C₄H₄S for catalyst AⅠwas more than 80%.In contrast,the conversion of C₄H₄S for catalyst A _V was only about 65%.Characterization and analysis showed that the larger the pore volume of the catalyst,the weaker the interaction force between the active metal components Ni and Mo and the support.The surface of the catalyst is more likely to form Ni-Mo-S active phase precursor NiMo O₄.The reaction gas can enter the catalyst pore channel at a faster rate,making it easier for C₄H₄S to diffuse into the active sites on the catalyst surface.Therefore the larger the pore volume the higher the HDS activity of the catalyst.（3）The NiMo/Al₂O₃ catalysts modified with alkaline earth metals（Mg,Ca,Sr,Ba）and rare earth metals（La,Ce）were investigated.The NiMo/Al₂O₃ catalysts modified with Mg showed the highest HDS activity and low by-product selectivity.The HDS activity of the catalysts modified with La and Ce was increased.At the same time,the by-product selectivity was reduced by the introduction of La into the catalysts.The modification of the catalysts with different metals resulted in effective modulation of the acidic and basic sites on the catalyst surface.The increase in acidity is more favourable to the breaking of the C-S bond on the C₄H₄S ring.The HDS activity of the catalyst is increased and the selectivity of the by-products is reduced.It also inhibits the polymerisation of Mo O₃ species and promotes the formation of Ni-Mo-S active sites,improving HDS activity and selectivity.