Deep Catalytic Adsorptive Desulfurization Of Diesel Using Bi-functional Titania-silica Materials

The combustion of sulfur-containing diesel produces sulfur oxides,which induces environmental problems such as acid rain and haze,and endangers human health.As a consequence,all countries worldwide require that the refining industry strictly control the sulfur content and produce low-sulfur/zero-sulfur diesel,which has brought great challenge to the oil refining industry.Among the various non-hydrotreating technique developed in recent years,adsorption desulfurization is attractive due to its mild operating conditions and simple equipment.However,the challenge on adsorption desulfurization is the competitive adsorption of polycyclic aromatic hydrocarbon and other components in fuel,resulting in low desulfurization efficiency of real diesel.In order to overcome the bottleneck of low adsorption desulfurization efficiency resulted from strong competitive adsorption of diesel components,the novel coupled catalytic oxidation-adsorption approach was proposed in this work.By doing so,the deep desulfurization capacity and selectivity of real diesel increased dramatically and low-sulfur/zero-sulfur diesel can be produced,which is superior than those applied with single adsorption.Based on the coupled catalytic oxidation-adsorption mechanism,a systematic study on the deep desulfurization of diesel over bi-functional titania-silica materials was carried out.A bi-functional material TiO₂/SBA-15 with both catalytic and adsorptive active sites was developed and investigated for desulfurization of model diesel systematically,and the enhanced desulfurization performance by the coupled catalytic oxidation-adsorption mechanism is further elaborated.The results showed that TiO₂/SBA-15 can reduce the sulfur content from 100 to 3ppm from model diesel,and the sulfur removal of TiO₂/SBA-15 reached>97%in<0.5 h.The maximum sulfur capacity reached up to 20 mg-S/g-A,>20 times of that under single adsorption mechanism.It should be mentioned that the adsorption capacity of TiO₂/SBA-15 was also higher than many reported desulfurization adsorbents,such as activated carbon and MOFs at high specific surface areas.The coupled catalytic oxidation-adsorption desulfurization mechanism was further illustrated.DBT was converted to corresponding sulfone DBTO₂ over the catalytic sites of TiO₂,followed by the selective adsorption of DBTO₂ over the adsorption site of Si-OH on TiO₂/SBA-15 surface.With the integration of the two types of active sites,deep desulfurization of diesel was achieved.This wotk further designed bi-functional TiO₂/silica-gel materials with low-cost and superior stability,and systematically investigated the coupled desulfurization performance and mechanism for real diesel.The results showed that the surface acidity in TiO₂/silica-gel materials played a key role in the desulfurization performance of TiO₂/silica-gel material.The path of DBT oxidation was further simulated.The oxidation is activated or initiated by the generation of free radical,which then attracts DBTs and oxidizes them to sulfones.The oxidizability of radical and radical-sulfur interactions are more critical to desulfurization efficiency.Cumene hydroperoxide were screened as the one with the highest oxidative desulfurization capacity.The dynamic desulfurization performance of TiO₂/silica gel using fixed-bed was also systematically investigated.The results indicated that the outlet sulfur concentration can reach as low as<10 ppm?national V?and 1ppm?national IV?,and the oil treating capacity can reach60 and 130ml-oil/g-Sorbent,respectively.This work systematically optimize the fixed bed reactor and process parameters such as height-diameter ratio,oxygen-sulfur ratio,reaction temperature,and two-stage dynamic model of coupled desulfurization was also built:C=A t?t_B C=?C₀*ek?t-???/?1+ek?t-??? t>t_BThe recycling stability of TiO₂/silica gel materials was investigated by solvent elution-air heat regeneration method.The regeneration results showed that no loss on desulfurization capacity was observed on adsorbents after 10 adsorption-regeneration recycles,suggesting both of the catalytic and adsorptive sites on the adsorbents can be fully recovered in multiple cycles,and the adsorption capacity still kept 1.2 mg-S/g-A.In addition,preliminary experimental explorations showed that Ce doping can further reduce the regeneration temperature by 50°C.A semi-continuous oxidation/adsorption-regeneration desulfurization process for fuel based on catalytic oxidation-adsorption coupled mechanism and the developed low-cost,high-stability bi-functional TiO₂/silica-gel material was proposed in this work.The series of catalytic oxidation-adsorption desulfurization research published in Fuel?2016?and AIChE Journal?2018?,provides an effective and economic approach for high-selectivity,ultra-deep desulfurization of low sulfur diesel,and is potential to be applied in an industrial setting.