Synthesis、Characterization And Performance Of MCM-41 Molecular Sieve With Connectivity Of Mesopore

M41S family of mesoporous materials include MCM-41 having a hexagonal array with one-dimensional pore structure, MCM-48 of three-dimensional structure and MCM-50 of lamellar structure. It is well known that MCM-41 has the well-ordered hexagonal array structure of one-dimensional(1D) mesopore channels. Owing to its high surface area(1000 m2/g), well-defined regular pore structure, narrow pore size distribution, large pore volume and tunable pore size(1.6-50 nm), high thermal and mechanical stability, MCM-41 has been widely explored for many important applications such as adsorption and separation, ion exchange, catalysis and drug delivery and so on. The three-dimensional structure of MCM-48 provide more favorable mass transfer kinetics than the unidirectional pore system of MCM-41. Therefore, in the macromolecule separation, adsorption, catalysis and as a supported catalyst, MCM-48 or Al-MCM-48 seems more appropriate than MCM-41 or Al-MCM-41. So it is necessary to introduce a certain amount of crossing-pore between the one-dimensional channel pores of MCM-41 to become three-dimensional pore structure, it should be conducive to the diffusion of molecules, reduce the diffusion resistance, improve diffusion rates.Considering above aim, in this work, fumed silica was firstly organic-functionalized by organsilane; then LC(D)-Si-MCM-41 and L-Al-MCM-41 samples with connectivity of mesopore have been synthesized by using organic functionalized fumed silica as silica source.Herein, MCM-41 molecular sieve samples with connectivity of mesopore have been synthesized by using different amount of organic functionalized fumed silica as silica source, tetramethylammonium hydroxide as alkali source, Cetyl trimethyl ammonium bromide (CTAB) (dodecyl trimethyl ammonium bromide, DTAB) as template, following with molar rate:1SiO2:0.25CTAB(or 0.25DTAB):0.20TMAOH:40H2O. The samples were characterized by powder X-ray diffraction, N2 adsorption/desorption, Scanning electron microscopy (SEM), Transmission electron micrographs (TEM) and intelligent gravimetric analyzer (IGA). It was found that, the MCM-41 synthesized by using fumed silcia and organic functionalized fumed silica, increases the surface area from 1010m2/g(C-Si-MCM-41) up to 1452m2/g(LC-Si-MCM-41), the specific surface area is increased by 43%(MCM-41 samples have been synthesized by using dodecyl trimethyl ammonium bromide as template, increases the surface area from 917m/g up to 1292m/g, the specific surface area is increased by 40%). The increase of specific surface area can be attributed to LC(D)-Si-MCM-41 samples have a certain amount of complementary crossing pores between one-dimensional channel stuctrure. As can be seen from the transmission electron microscopy (TEM), TEM photos of LC(D)-Si-MCM-41 sample is significantly different from the MCM-41’s TEM photos synthesized using the conventional method, there are a certain amount of complementary crossing pores between one-dimensional channel stuctrure of the LC(D)-Si-MCM-41 samples. The amount of crossing pores between the one-dimensional pore determines specific surface area of MCM-41 (the different amount of organic functionalized fumed silica). The NLDFT adsorption branch pore size distribution obtained the argon adsorption isotherms showed that, for the LC(D)-Si-MCM-41 sample synthesized by using organic functionalized fumed silica, the crossing pores between one-dimensional channel is mainly mesopores.In addition, from adsorption isotherms of n-heptane, as compared with the C(D)-Si-MCM-41 sample synthesized by using fumed silica as the silicon source, the mesoporous structure of LC(D)-Si-MCM-41 sample synthesized by using organic functionalized fumed silica as the silicon source (silane coupling agent is Y-5669) is more conducive of adsorption and diffusion of n-heptane.From conversion rate of benzyl chloride in the benzylation reaction of benzyl chloride with para-xylene, it can be drawn that, as compared with the Al-MCM-41 sample sample synthesized by using fumed silica as the silicon source, L-Al-MCM-41 sample synthesized by using organic functionalized fumed silica as the silicon source, has a large difference for conversion rate. Especially after a reaction of 120 minutes, conversion rate of benzyl chloride was 47% for Al-MCM-41 sample; under the same reaction conditions, benzyl chloride the conversion rate was 87% for L-Al-MCM-41 sample, which is 1.85 times of conversion rate of the benzyl chloride for Al-MCM-41 sample. More importantly, the complementary secondary pore structure connected with the main mesoporous channels accelerate the difiusion of reactants and products in the pores of the catalyst, can effectively prevent the occurrence of side reactions, mitigate the catalyst coking and inactivation.Accordingly, from the above experimental data, we can confirm that LC(D)-Si-MCM-41 and L-Al-MCM-41 sample synthesized by using organic functionalized fumed silica as the silicon source having the large crossing pores connected with the main mesoporous channels and the higher surface area, which play an important role in controlling adsorption and desorption at a low adsorbate concentration level and catalytic reaction.