Study On The Construction Strategy Of High Dispersion Lewis Acid Adsorption Sites For Mesoporous Molecular Sieve Desulfurization

The silicon-aluminum type microporous zeolites are usually used as the adsorbents in adsorption desulfurization technology,the reason is that the silicon-aluminum type microporous zeolites have the ideal theoretical adsorption capacity of sulfur by virtue of its own acid capacity.However,there are a serious of catalytic conversion reactions between olefin and thiophene sulfides because of Br?nsted acid in silicon-aluminum type microporous zeolites.The macromoleculer sulfides produced by the catalytic conversion reaction block the pore channel of the microporous zeolites,which leads to the dilemma that the actual sulfur capacity of the adsorbent is much lower than the theoretical value.The modification of silicon-based mesoporous molecular sieves provides a new way to solve the above problems.On the one hand,mesoporous molecular sieves have larger surface area,pore volume and pore diameter.On the other hand,silica-based mesoporous molecular sieves are non-acidic active sites,and abundant silica-hydroxyl groups on their surfaces provide ideal conditions for the directional construction of active sites.According to the HSAB principle,thiophene is a soft Lewis base,and its corresponding optimal adsorption activity site should be soft Lewis acid.Therefore,it has a great significance to study constructing Lewis acid sites on mesoporous molecular sieve for developing mesoporous desulfurization adsorbent.In this paper,two silicon-based mesoporous molecular sieves,MCM-41and SBA-15,were used as carriers to support metal active species by means of post-grafting,composite grafting and pH-regulating impregnation.Respectively,a series of desulfurization adsorbents,such as Al-MCM-41,Ce-Al-SBA-15 and Ce-SBA-15,were prepared.Texture properties of adsorbents were characterized with X-ray diffraction?XRD?,Ar adsorption-desorption,N₂adsorption-desorption,Fourier transform infrared spectroscopy?FTIR?,high resolution transmission electron microscope?HRTEM?and element map imaging technology?EDX?.Then the dispersion of metal species and its effect on physical properties of mesoporous materials were investigated.The acid properties of the adsorbents were characterized by both ammonia temperature-programmed desorption?NH₃-TPD?and FTIR spectra of pyridine adsorption-desorption?Py-FTIR?.Then the active sites of Lewis acid in the adsorbents were investigated.Finally,the ability of adsorbent to remove thiophene sulfide was systematically evaluated by static adsorption experiment,fixed bed adsorption penetration experiment and extraction experiment.Firstly,Al-MCM-41 adsorbents with different Al contnts were prepared by post-grafting method,and the effect of Al species loading on the Lewis acid active site construction and desulfurization performance of adsorbents were investigated.Physical-chemical characterization and desulfurization performance of adsorbents show that:the ordered mesoporous channels of MCM-41 and the highly dispersed species of Al were maintained.By regulating the introduction of Al species,two Lewis acid active sites?L₁ in the form of three coordination and L₂ in the form of five coordination?and one Br?nsted acid active site were constructed respectively.Low content aluminum species is conducive to the formation of L₁?Lewis?acid center,high content aluminum species is conducive to the formation of L₂?Lewis?acid center.The adsorption thiophene performance of L₂ acid center was far stronger than L₁acid center,and L₁ acid center was slightly stronger than Br?nsted acid center.Competitive adsorption and catalytic conversion of olefin and thiophene take place in the Br?nsted acid center,and the latter is dominated.The existence of L₂ acid center greatly promoted the catalytic conversion reaction.The adsorption of macromolecular sulfide increased the saturated adsorption capacity of Al-MCM-41 adsorbent.Secondly,CeNa-AlSBA-15,CeH-AlSBA-15 and H-AlSBA-15adsorbents were prepared by composite grafting method,and the influence of different metal species on the Lewis acid active site construction and desulfurization performance of adsorbents were investigated.The results of physical-chemical characterization and desulfurization performance of adsorbents show that:the ordered mesoporous channel of SBA-15 was maintained.Besides,Ce species and Al species were highly dispersed and effectively anchored in SBA-15.Lewis acid active sites with different morphology,such as Al-OH,Ce-OH and Na⁺,were constructed in the adsorbents,and the Ce-OH active site was the best thiophene adsorption active site.H-AlSBA-15 adsorbent has the maximum acid content,but CeH-AlSBA-15 adsorbent has the maximum thiophene removal capacity.This phenomenon indicating that the type of acid activity center has a much greater impact on the thiophene removal performance than the acid content.Finally,Ce-SBA-15 adsorbents were prepared by pH-regulating impregnation method,and the effects between physical-chemical properties and desulfurization performance of adsorbents by different solvent pH values were investigated.The results showed that Ce species were affected by strong electrostatic adsorption and oxidation under acidic solvent environment.The optimal solvent pH is 3,Ce-SBA-15?3?adsorbent has the maximum acid content and sulfide adsorption capacity.No macromolecular sulfides were detected in the tests of adsorption breakthrough experiment with simulated oils containing olefins and thiophene.The results showed that there was no Br?nsted acid activity site in the adsorbent,and the Lewis acid activity site was reasonably speculated to be the constructed.