The Preparation And Characterization Of Mesoporous Modified Montmorillonites

Pillared clay is a kind of the mineral materials, which is synthesized by pillaring inorganic polycations or organic ionic groups (molecules) into clay minerals. Modified clay is a two-dimensional mineral material with large surface area, high thermal stability, strong surface acidity and variable pore size, therefore it is widely used in such fields as catalyst, sewerage treatment, antibacterial material, photic functional material, ionic conductor, pharmacy, nano-complex preparation and nuclear scrap treatment. With the development of nanotechnology, nano-grade pillared montmorillonite has attracted much more attention. However, recent researches mainly focus on the preparations and applications of microporous pillared montmorillonite, and the prepared technique and method are still in the traditional level. The study of mesoporous pillared montmorillonite with widely applied potentials is in the initial stage. Therefore it is a significant scientific topic to improve and develop the traditional technique and method of the preparation and high productive efficiency of mesoporous modified montmorillonite.In this paper, CTAB-Al-Mts were synthesized by intercalating cetyltrimethylammonium bromide ( CTAB ) and Al₁₃ polycations into montmorillonite through precursor method, and CTAB-Si-Mts were synthesized by pillaring hexadecyltrimethylammonium bromide ( CTAB ) and tetraethyl orthosilicate (TEOS) into montmorillonite through gallery-templated method. The structure, configuration, porous property and mechanism of CTAB-Al-Mts and CTAB-Si-Mts were characterized by the powder X-ray diffraction (XRD ) , Fourier transform infrared spectroscopy (FTIR) , thermal analysis (DTA-TG) , chemical composition, nuclear magnetic resonance spectroscopy (²⁷Al MAS NMR and ²⁹Si MAS NMR) , porous analysis (BET) .This work was supported by the National Nature Science Foundation of China (Grant No. 40372029) , the National Nature Science Foundation of Guangdong Province (Grant No. 05103410, 030471) .Main conclusions of this work are drawn as follows:1. Based on the XRD results, the concentration of CTAB has a significant effect on the basal spacing of organo-inorgano modified montmorillonites. Generally, the basal spacings increase with the increase of CTAB concentrations.2. TG-DTG and chemical analysis show that there are CTAB compositions and hydroxy-Al cations compositions in the interlayer of CTAB-Al-Mts.With the increasing of CTAB concentration, the mass losses of CTAB-Al-Mts increase. There is an exchanged ionic reaction between the CTAB cations and the hydroxy-Al cations.The SiO₂ content in the CTAB-Si-Mts increases with the increase of CTAB concentrations, and the CTAB templating reaction makes TEOS enter into the interlayer easily and form the inside and outside silicon structure in the clay interlayers. After calcination a hexagons structure similar to MCM-41 is formed.3. FTIR results reveal that the hydrophobicity of CTAB-Al-Mts increases with the increasing of CTAB contents. In CTAB-Si-Mts the interlayered H₂O contents quickly decrease after high-temperature calcination to remove the template, the Al-O(OH) octahedral structures destroy, and no CTAB or TEOS exists in the interlayer.4. With the increasing of the hydrothy-Al cations (or decreasing of CTAB contents) , the chemical shifts of Al^â…£ and Al^â…¥ move toward higher frequency. The rate of the Al^â…£: Al^â…¥ increases. With the increasing of CTAB concentration and hydroxy-Al cations, the chemical shift of the Q² goes near to Q³, the Si-O layered structure changes into the Si-O net structure as well.5. Analyses of porous characteristic indicate that the precursor and the templeting methods can produce mesoporous modified montmorillonites. By the templating method relative large average pore diameter mesoporous montmorillonite materials can be made. This implies the reaction and effect of CTAB and hydroxyl-Al cations are defferent from those of CTAB and TEOS. The porous structure of samples prepared by the precursor method is same as that of montmorillonites, but the porous structure of samples prepared by the templeting method is silimar to the MCM-41, which is made of the hexyon porous structure formed by the Si-O layer and completely destroyed clay layers.6,The results of adsorption experiments show that the adsorption abilities of the samples to PO₄^3- and p-nitrophenol are different. The pH values of solutions have an effect on the adsorption rate of samples to the PO₄^3-, and the alkaline condition improves the adsorption rate of p-nitrophenol. The adsorption curves of samples to PO₄^3- belong to the constant partition isotherm classification with the same shape and characterization, implying mesoporus materials adsorption characteristic. The adsorption curves of samples to p-nitrophenol mainly belong to high affinity isotherm classification implying ionic exchanged chemical adsorption characteristic, and the shapes are different for different samples.