| Methyl tert-butyl ether(MTBE)has been widely used as a high octane gasoline additive and antiknock.However,in industry,MTBE contains various sulfides,such as Dimethyl sulfide(DMS)and Dimethyl disulfide(DMDS),which make the sulfur content in MTBE seriously exceeds the standard.Nevertheless,when desulfurizing DMDS from MTBE by adsorption,there will be a strong competitive adsorption effect between MTBE and DMDS on the traditional complex adsorbents.To address this phenomenon,this study has developed a zeolite adsorbent with core-shell composite structure,in which the core phase is a Y zeolite and the shell phase is a silicalite-1 pure silicon zeolite.The theoretical basis of this study is as the following:first,the molecular size of MTBE and DMDS is quite different,and the desulfurization of DMDS can be completed by physical shape selection and chemical adsorption;second,the pore channel of silicalite-1 pure silicon zeolite is single and the pore size is smaller;third,Y zeolite have extremely large void structure,which can provide sufficient adsorption active sites and storage space.A series of Y@silicalite-1 core-shell composite zeolite adsorbents after Y zeolite being modified were prepared in this study,and XRD,SEM,TEM,FT-IR,Raman,N2 adsorption-desorption,ICP-OES,SEM-EDS,27Al MAS NMR were used to characterize the surface morphology and structural characteristics of the composite zeolite adsorbent;the desulfurization performance of the core-shell composite zeolite was evaluated by both static adsorption experiment in a static closed vessel and dynamic adsorption experiment in a micro fixed bed adsorption reactor;also,the adsorption process and mechanism of shape selective adsorption of DMDS in MTBE by Y@silicalite-1 core-shell composite zeolite adsorbents were studied.At beginning,the NaY@silicalite-1 core-shell composite zeolite were prepared by hydrothermal synthesis with NaY as core zeolite and silicalite-1 as shell zeolite,and the optimum synthesis conditions of silicalite-1 pure silicon zeolite shelled on NaY zeolite were investigated to be tetraethyl orthosilicate(TEOS)/tetrapropylammonium hydroxide(TPAOH)/ethanol(EtOH)/deionized water(H2O)/NaY=20 g:19g:17g:87g:5g.The NaY@silicalite-1 core-shell composite zeolite prepared under this condition owns the best shell coverage with a shell thickness of about 100?400 nm.When used as adsorbent,the maximum sulfur adsorption capacity is about 20.711 mgs/gadsorbents.While the adsorption capacity of NaY zeolite was negative when it was used as adsorbent alone,suggesting that MTBE and DMDS did exist competitive adsorption effect on NaY zeolite,and the adsorbent preferentially adsorbs MTBE,resulting in the increase of DMDS concentration in MTBE solution;The desulfurization performance of NaY and silicalite-1 zeolites prepared by physical mixing method in MTBE was also investigated;results showed that the adsorbent prepared by physical mixing had no obvious selective adsorption performance and still preferred adsorption MTBE.The adsorption process of DMDS on NaY@silicalite-1 core-shell composite zeolite adsorbent can be described as following:when NaY@silicalite-1 core-shell composite zeolite adsorbent contact MTBE and DMDS at the same time,MTBE molecules with larger molecular size will be blocked outside the whole core-shell zeolite by silicalite-1,while DMDS molecules with smaller molecular size can diffuse rapidly within the silicalite-1 shell pore channel and pass smoothly through the shell pore channel and finally be adsorbed inside the NaY zeolite by forming Na-S bonds until the adsorption saturation is reached.To improve the desulfurization performance of NaY@silicalite-1 core-shell composite zeolite adsorbent,considering that the reason why the core zeolite(NaY)can adsorb DMDD is that it can form Na-S bond to complete the adsorption and storage of sulfur,Na+ ions in NaY were replaced by Cu2+ ions with stronger adsorption capacity by ion exchange,respectively from CuCl2(aq),Cu(NO3)2(aq)and CuSO4(aq).Though the crystallinity of Y zeolites decreased due to lattice defects after modification by Cu2+ions,the structural characteristics of Y zeolites were not changed.In addition,the existence of Cu-S bond formed by the combination of Cu2+ and sulfur atom in DMDS can be clearly detected by raman spectrum,and a Cu2+ion can bond with a DMDS molecular.Taking the CuY zeolite obtained by Cu2+ion exchange as the core,the CuY@silicalite-1 core shell composite zeolite adsorbent was prepared according to the aforementioned core-shell forming conditions;and the core-shell zeolite CuY@silicalite-1 modified by CuCl2(shell thickness of about 100?200 nm)showed the best desulfurization performance with a highest sulfur adsorption capacity of 32.882 mgs/gadsorbents,much higher than that of NaY@silicalite-1.The application of core-shell composite structured zeolite adsorbent for DMDS removal in MTBE determines that the shape selection performance of shell zeolite as well as the adsorption and storage capacity of core zeolite are indispensable,but generally,the shape selection performance of shell zeolite is more important in the desulfurization process.In the follow-up exploration,the surface of Y zeolite was modified by desilication with sodium hydroxide(NaOH)treatment to increase its outer surface area and provide more loading positions for silicalite-1 tiny grains to grow into larger particles smoothly and finally agglomerate into silicalite-1 shells.The total surface area of the NaYOH decreased due to the desilication of NaOH on the NaY zeolite after alkali treatment,but the external surface area increased,which provided favorable conditions for the growth of silicalite-1 crystals on the surface of YOH.Meanwhile,the total pore volume of NaYOH also increased due to the desilication,provide YOH zeolite a larger volume for its adsorption and storage in the application for the removal of DMDS from MTBE.The silicalite-1 pure silicon shell of CuYOH@silicalite-1 prepared by CuYOH can achieve almost a complete coverage with a shell thickness of less than 100 nm,and the sulfur adsorption capacity of the CuYOH@silicalite-1 modified by CuCl2 can be further increased to 37.073 mgs/gadsorbents.In view of the mechanism and desulfurization effect in alkali treatment,the hydrochloric acid(HCl)treatment of Y zeolite is further carried out in this study.The dealumination after acid treatment can increase the outer surface and expose more silicon atoms on the outer surface of Y zeolite.The effect of these exposed silicon atoms is equivalent to silicalite-1 pure silicon grains attached to the surface of Y zeolite during hydrothermal process to provide more growth cores for the growth of pure silicon crystals,which can theoretically obtain more perfect,denser,and more shape selective silicalite-1 pure silicon zeolite shell.Results showed that the silicalite-1 shell was more uniform and dense,and the thickness of the shell was thinner of about 30-100 nm.However,due to the high strength of hydrochloric acid treatment to remove the framework aluminum of Y zeolite,a large number of micropores in Y zeolite disappeared,and the pore volume decreased seriously,so that the sulfur adsorption capacity of CuYH@silicalite-1 core-shell composite zeolite modified by CuCl2 was only 34.585 mgs/gadsorbents,slightly lower than that of the CuYOH@silicalite-1 adsorbent prepared by alkali treatment. |
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