The Silica-pillared Montmorillonite Materials With Ordered Gallery Mesoporous Structure: Synthesis, Characterization And The Catalytic Performance

The design and synthesis of pillared clay using ion exchange and self-assembly, translating the 2D layered of clay structure into 3D porous structure is one of the most attractive research fields.Due to the special high surface area and micro-mesoporous structure,their potential applications as catalysts,supports,selective adsorbents, separating agents,and porous matrixes for encapsulation of specific functional molecules have been of great interest.In general,pillaring is achieved by direct introduction of bulk inorganic (polyoxcations) by ion exchange or organic precursors(metal alkoxides) between the interlayers of clay.Pillaring process that use metal alkoxide are facilitated by a preswelling step whereby interlayer regions are exposed to quaternary ammonium. However,preswelling procedures are problematic because they are complex, non-quantitative and require reagents and the products obtained by ion exchange have low surface area,wide pore size distribution and poor thermal stability.In this research,we have analyzed and concluded the preview works and prepared the ordered mesoporous silica-pillared montmorillonite by ion exchange,salvation, gallery tempalting,self-assembly synthetically.Based on the synthesis of ordered mesoporous silica-pillared montmorillonite,the nickel and cobalt ions have been incorporated into the gallery framework.The modern characterization methods such as powder X-ray diffraction(XRD),N₂ adsorption-desorption isotherms(BET),SEM, HRTEM,UV-vis and TPR had been used to characterize the obtained samples.The influence of the porous structure,pore size distribution and catalytic activity by the template molecules with different functional groups,cationic surfactants mixture and cationic-anionic surfactants mixture has been studied and discussed on detail.The catalytic activity of the mesoporous silica-pillared montmorillonite has been tested by cracking of coker gas oil(CGO) and plant atmosphere.Moreover,the properties of the tunable porous structure and the mechanism of pore form had also been studied. The main research and the achievements are indicated as the following:(1) The silica-pillared montmorillonite with ordered mesoporous structure was synthesized using cetyltrimethylammoniumethyl bromide as the gallery template. The product has ordered pore size(3.9nm) and high specific surface area(622 m²/g).This regularity of pore distribution is achieved by the liquid crystal mechanism of quaternary ammonium cationic surfactant between layers of montmorillonite.(2) The prepared montmorillonite hybrid materials have high specific surface area, micro-mesoporous composite structure and ordered mesopore size distribution. However,there is some difference between the prepared samples which obtained using different surfactants as template.The sample prepared by surfactant with eater group has relative lower surface area(400 m²/g).However,most of the surface area is from gallery mesopores,moreover,the mesopore size is bigger (4.3nm).The sample obtained by surfactant with beazyl group has the highest surface area(1156 m²/g).(3) The pore size of prepared silica-pillared montmorillonite can be controlled by proportion of different cationic surfactants mixture.In addition,utilization of cationic-anionic mixed surfactants contributes to increase the constituent of mesopore effectively.The silica-pillared montmorillonite can be synthesized using mixed surfactants as gallery templates to obtain certain pore size,surface area and micro-mesopore composing according to the actual need of application for catalysts or adsorption agent.(4) Because of the amorphous gallery framework,the nickel and cobalt ions can be incorporated into the silica framework between the layers of silica-pillared montmorillonite.Our experimental results have proved this.The gallery Si/Ni or Si/Co ratios can even be up to 5/1 without destroy of layered structure.The characterization of instrument proves that large number of Si-O-Ni and Si-O-Co chemical bonds exist.The metal containing silica-pillared montmorillonite has high specific surface area(300-600m²/g),larger diameter(about 3.8nm),uniform pore size distribution and high catalytic activity in compared with traditional metal-oxide-pillared montmorillonite.(5) In order to characterize the catalytic property of silica-pillared montmorillonite and metal containing silica-pillared montmorillonite,the catalysis cracking of coker gas oil(CGO) and plant atmosphere is used to estimate the catalytic properties respectively.The experimental results indicate that the bigger mesopore size of silica-pillared montmorillonite contributes to the higher conversion(Up to 90%) of CGO with high selectivity towards gasoline(71.2%) and low selectivity towards coke(Smaller than 2%).Due to the Si-Al structure of the layers,the silica-pillared montmorillonite has the acidic centers itself.The catalytic activity of silica-pillared montmorillonite is much higher than pure MCM-41 and which is up on to the Al-MCM-41(Si/Al=30).And the selectivity towards coke is lower than Al-MCM-41.Nickel metal doped silica-pillared montmorillonite has unique mesopore of 3.8nm,so its catalytic activity conversion of plant asphalt cracking (90.2%) is much higher than the Ni-ZSM-5(12.7%) and similar to the Ni-MCM-41(86.3%).The selectivity towards gaseous hydrocarbons(31.1%) and gasoline(32.5%) of nickel metal doped silica-pillared montmorillonite is near the Ni-MCM-41(15.4%and 39.7%).Furthermore,the selectivity towards coker of nickel metal doped silica-pillared montmorillonite is much lower than that of Ni-MCM-41(30%).Above all,the results indicate that prepared montmorillonite hybrid materials have high specific surface area(400-1156m²/g),micro-mesoporous composite structure and ordered mesopore size distribution.However,there is some difference between the prepared samples which obtained using different surfactants as template. The sample prepared by surfactant with eater group has relative lower surface area. However,most of the surface area is from gallery mesopores,moreover,the mesopore size is bigger(4.3nm).The sample obtained by surfactant with beazyl group has the highest surface area(1156 m²/g),but most of the surface area is from micropore which pore size is smaller than 1.5nm.The pore size of material is controllable by the chain length of surfactant.Since gallery heights of prepared silica-pillared montmorillonite are nearly double the chain length of surfactants,this implies that surfactants molecules in the gallery are arranged as molecular lamellar bilayers and that tetraethoxysilane produce firm enough silica-pillars to prop the expanded gallery after the removal of surfactants.According to the actual need of application,the pore size of prepared silica-pillared montmorillonite can be controlled by proportion of different cationic surfactants.In addition,utilization of cationic-anionic mixed surfactants contributes to increase the constituent of mesopore effectively.The silica-pillared montmorillonite can be synthesized using mixed surfactants as gallery templates to obtain certain pore size, surface area and micro-mesopore composing according to the actual need of application for catalysts or adsorption agent.Because of the amorphous gallery framework,the nickel and cobalt ions can be incorporated into the silica framework between the layers of silica-pillared montmorillonite.Our experimental results have proved this.The gallery Si/Ni or Si/Co ratios can even be up to 5/1 without destroy of layered structure.The characterization of instrument proves that large number of Si-O-Ni and Si-O-Co chemical bonds exist.The metal containing silica-pillared montmorillonite has high specific surface area(300-600m²/g),larger diameter(about 3.8nm),uniform pore size distribution and high catalytic activity in compared with traditional metal-oxide-pillared montmorillonite.The experimental results indicate that the prepared silica-pillared montmorillonite has ordered mesoporous structure.This regularity of pore distribution is achieved by the liquid crystal mechanism of quatemary ammonium cationic surfactant between layers of montmorillonite.The surfactants arrange molecular sieves in the gallery areas of montmorillonite,the tetraethoxysilane hydrolysis happens around the sieves and form the ordered mesoporous structure between the layers of clay.This research will broaden the study field of layered materials and promote the application of montmorillonite-based porous materials in various fields especially in the petro-chemical catalysis.Furthermore,the research has important theoretical and practical significance for the development of porous clay materials.