Ni-based Thioetherification Catalysts For Removing Mercaptans From Liquefied Petroleum Gas At Low-Temperature

Thioetherification reaction has been widely used in refineries for sweetening from fluid catalytic cracking?FCC?gasoline,however,there is no report on such process employed in removing mercaptan from Liquefied petroleum gas?LPG?up to date.Nowadays,Ni-based catalysts have gained wide application in thioetherification reaction due to their good sulfur tolerance and low cost,nevertheless,Ni catalysts usually have relatively lower activities at low temperature.This study is aimed to develop new Ni-based thioetherification catalysts with high activities at low temperature starting with monometallic Ni/Al₂O₃ catalysts for mercaptan removal from LPG.First,the right Ni content of monometallic Ni/Al₂O₃ was determined,on the basis of optimized Ni/Al₂O₃ catalyst,a series of Fe,Mo,V and Cu incorporated Ni/Al₂O₃ catalysts were prepared,and the influence of second metals on the surface species properties and the thioetherification activities of the catalysts were investigated,the results showed that the incorporation of all the second metals increased the reducibility of Ni species on Ni/Al₂O₃ catalysts,among which,the addition of Cu inhibited the thioetherification activity of Ni/Al₂O₃ while the incorporation of Fe,Mo and V enhanced the thioetherification activities of Ni/Al₂O₃,especially Fe promoted Ni/Al₂O₃ exhibited the highest activity at lower temperature?80 ??.Based on the above bimetallic NiFe/Al₂O₃ system,the influence of incorporated Fe contents on the properties of the surface Ni active species as well as the activities of mercaptan removal of Ni/Al₂O₃ catalysts was further systematically studied and the results indicated that the promoting effects of Fe species include two vital aspects:?i?partial Fe species acting as a textural promoter strongly interact with the surface of Al₂O₃,which prevents the formation of the NiAl2O4 phase and thus increases the amount of Ni oxides weakly interacting with Al₂O₃;Ni oxide species are mainly present in the form of isolated NiO weakly interacting with Al₂O₃ and NiO in close contact with Fe2O3?NiFe₂O₄?,both of which can be easily converted into catalytically active NiSx and NiS?FeS?,respectively,after sulfidation;and?ii?partial Fe species serving as an electron-donor transfer electron density to the closely contacted NiS in NiS?FeS?species,leading to the weakened Ni-S bond strength and thereby contributing to higher activity of NiS?FeS?.These two promoting effects of Fe endow the resulting bimetallic catalysts with significantly enhanced low-temperature activity.Among the bimetallic catalysts,14Ni8Fe/Al₂O₃ showed the superior thioetherification activity with lower olefin saturation activity,giving the conversions of CH3 SH and C2H5 SH as high as 97.6% and 95.6% at 80 ? from LPG.To further improve the low-temperature activity of Ni Fe/Al₂O₃,P,La and Ca elements were selected as modifiers of Al₂O₃ supports,corresponding supported NiFe catalysts were prepared,and the influence of Al₂O₃ modifications on the surface active species and low-temperature activities of the resulting NiFe catalysts were investigated.The results showed that the thioetherification activity of NiFe/Al₂O₃ is greatly affected by the introduction of P,La and Ca elements: as compared to that of NiFe/Al₂O₃,at 70 ?,NiFe/Al₂O₃-P exhibited enhanced thioetherification activity,while NiFe/Al₂O₃-La and NiFe/Al₂O₃-Ca had obviously decreased thioetherification activity.The characterization results indicated that there are three types of sulfided Ni species in NiSx active phases in NiFe catalysts,namely,with Ni-S,Ni-S2 and Ni-Sn coordinations,with Ni in Ni-S2 groups being the most ideal for mercaptan activation.The introduction of P increases the quantity of Ni species in Ni-S2 coordinations and promotes the thioetherification activity of NiFe/Al₂O₃,while the addition of La and Ca decreases the quantity of Ni species with such coordination and thus inhibits the activity of the resultant catalysts.To better the adaptability of the NiFe/Al₂O₃ catalyst to different feestocks,a series of Al₂O₃ supported Fe containing nickel phosphide catalysts were synthesized,and the effects of Al₂O₃ properties and P/Ni mole ratios on the structure and catalytic activity were investigated.The results showed that the NiFeP catalyst using alumina AH?520?obtained by calcining aluminum hydroxide at 520 ? as support had excellent catalytic performance for mercaptan removal,mainly related to the different textural properties and electron states of the Ni⁰ species of the NiFeP catalysts supported on the different aluminas.It was also found that the thioetherification performance,textural properties,and the reducibility and dispersion of the surface active species of the Ni FeP catalysts were affected by the incorporation of P with different contents in the P/Ni mole ratio range investigated,and the introduction of P species inhibited the sulfidation of the catalyst;NiFeP catalyst with a Ni/P atomic ratio of 0.28 had the highest activity,which probably correlated with its Ni⁰ species content and electron density property.Furthermore,the effects of activation methods on the activity of the NiFeP catalysts were also studied.The assessment and characterization results showed that catalyst NiFeP?S400-R?activated by sulfidation at 400 ? and reduction at 560 ? had extremely high mercaptan removal activity,giving the conversions of CH3 SH and C2H5 SH as high as 99.5% and 98.8% from LPG at 65 ?.The reason is that the reduction step at 560 ? used in activating the catalyst facilitates the exposure of Ni⁰ species covered by sulfur species and more importantly the removal of sulfur atoms strongly bonded to Ni atoms,resulting in the formation of sulfur vancies,adjusting the electron density of Ni⁰ species,and generating more medium Ni⁰ sites.