Organic-Inorganic Functionalization Of MCM-41 Mesoporous Molecular Sieves For Desulfurization Of C₄ Hydrocarbons

C₄ hydrocarbons are important raw materials for different kinds of chemical products,such as alkylation gasoline and methyl tert-butyl ether?MTBE?.However,different kinds of sulfides,including dimethyl disulfide?DMDS?,tert-butyl mercaptan?TBM?,and dimethyl sulfide?DMS?are usually present in the C₄ hydrocarbons.The presence of these sulfides can not only lead to the exorbitant sulfur content of subsequent products but also corrode the equipment and poison the catalysts.Therefore,it is necessary to remove these sulfides from C₄ hydrocarbons.Hydrodesulfurization?HDS?is the conventional process to remove sulfur compounds.However,the HDS process can lead to the saturation of valuable olefins,which is a loss of olefins.Therefore,the HDS process is not suitable for the desulfurization of C₄ hydrocarbons.Adsorptive desulfurization is a promising method for the production of ultraclean fuels due to the advantages of simpleness,convenience,and low operation costs.Among solid adsorbents,MCM-41 mesoporous molecular sieves are widely used in the field of adsorption-separation and catalysis because of high surface area and pore volume and low diffusion resistance of products and reactants in the pore.In this thesis,different methods were used to modify MCM-41 mesoporous molecular sieves.Both inorganic and organic components were incorporated into molecular sieves to prepare novel composite materials with stable performance and controlled functions.The prepared organic-inorganic composite materials were characterized and investigated for the removal of sulfur compounds in C₄ hydrocarbons.The modification methods and mechanism of MCM-41 mesoporous molecular sieves were studied.It is of great theoretical and practical significance for the preparation of novel composite materials and deep understanding of desulfurization process.Grafting method and combined impregnation-grafting method were used for the modification of MCM-41.Active metal component was introduced by taking advantage of the coordination function of amino group from aminosilanes.The results of FT-IR,13 C CP-MAS NMR,and 29 Si MAS NMR indicate the successful grafting and structure integrity of aminosilanes.With the increasing content of grafted aminosilanes,the content of coordinated active component increased.However,the surface area,pore volume,and pore diameter decreased,which affected the desulfurization results.The sulfur capacities of samples for different sulfides decrease in the order TBM,DMS,DMDS.The main desulfurization mechanisms for TBM include physical adsorption by modified molecular sieves,formation of direct S-M bonds with CuO and Cu²⁺,and formation of thiolates by reactions with CuO and Cu²⁺.Comparing with impregnated CuO on molecular sieves,the metal ions coordinated by aminosilanes show efficient desulfurization results for TBM due to the spatially isolated active component.The formation of thiolates plays a significant role for the removal of TBM.DMS and DMDS are mainly removed by physical adsorption due to poor reactivity and polarity.And CuO plays a more dominant role than Cu²⁺ for the removal of DMS and DMDS.For better dispersion of incorporated active components,in situ synthesis and combined in situ synthesis-grafting method were used for the modification of MCM-41.The results of low angle XRD,FT-IR,and H2-TPR indicate the incorporation of Cu into the framework of MCM-41 after modification.However,the results of wide angle XRD show that the Cu in the gel during the synthesis process are not entirely incorporated into the framework of MCM-41.Part of the Cu exists as CuO phase.The?40?Cu-MCM-41 synthesized through the in situ synthesis method shows better desulfurization performance than MCM-41-Cu?40?prepared by the incipient wetness impregnation method.This is because of that the isolated Cu species in the framework and well-dispersed CuO species can provide effective active sites for desulfurization.For the comprehensive desulfurization of CuO and Cu²⁺,MCM-41?Cu?was obtained by selective impregnation of CuO on the external surface of MCM-41.Then,MCM-41?Cu?-NH2-Cu was obtained with coordination of Cu²⁺ by grafted aminosilanes on the internal surface.Because of the activity of metal ions for TBM in the channel,MCM-41?Cu?-NH2-Cu shows better desulfurization performance than MCM-41?Cu?for the removal of TBM.The metal ions coordinated by amino group provide almost no adsorption sites for DMS and DMDS.And the physical adsorption on internal surface of MCM-41?Cu?-NH2-Cu decreased.So MCM-41?Cu?shows better desulfurization performance than MCM-41?Cu?-NH2-Cu for DMS and DMDS.TBM can be adsorbed on both the external and internal surface of MCM-41?Cu?-NH2-Cu.However,DMS and DMDS are selectively adsorbed on the external surface of MCM-41?Cu?-NH2-Cu.To compare with grafting method,aminosilanes were introduced into MCM-41 by co-condensation method.Then the metal ions were coordinated by the amino group of aminosilanes.With the increasing content of aminosilanes,the ordered structure of synthesized molecular sieves decreased.More amino groups which can coordinate metal ions are introduced into MCM-41 when the content of aminosilanes is high.But the block of channels is severe.The decrease of surface area,pore volume,and pore diameter can affect the desulfurization results.Mercaptans with small molecular size and steric hindrance are easier to enter the channels of prepared composite materials.However,mercaptans with small molecular size are easily oxidized to disulfides,which can affect the desulfurization results.The desulfurization performances of modified MCM-41 by different methods were compared.Because of the combination of good desulfurization performance of Cu²⁺ coordinated by amino groups on the internal surface for TBM and desulfurization performance of CuO on the external surface for TBM and disulfides,modified MCM-41 samples by selective functionalization show better desulfurization performance for the removal of TBM.The isolated Cu species in the framework and well-dispersed CuO species can provide effective active sites for desulfurization.So modified MCM-41 samples by in situ synthesis method show better desulfurization performance for the removal of DMS and DMDS.