Salts Effects On The Synthesis Of SBA-3 In A Weak Acid System

SBA-3 refers to a mesoporous silica molecular sieve has a hexagonal structure with p6mm space group, synthesis in strong acid medium (pH<0), using cationic quaternary ammonium salt surfactant as the template agent. Although it has the similar pore symmetry to mesoporous MCM-41 molecular sieve which uses the same template synthesis in alkaline system, it has been paid much attention in some research fields such as catalysis, adsorption, separation and hard template synthesis of novel porous materials etc, because of abundant micropores in the mesoporous wall of SBA-3 and the feature that its preparation method can adjust the material’s macro morphology easily. Therefore, nowadays SBA-3 has become the main body of a very promising material in the research field of the study on the mesoporous molecular sieve synthesis and application.At present, the study of SBA-3 has a defect that it can be only synthetized in the light of strong acidic medium provided by strongly acidic medium such as the high concentration of HC1, H2SO4, HNO3 and HBr (pH<0). Meanwhile, it is difficult to form an ordered mesoporous phase or only can obtain the amorphous gel by using the weak inorganic or organic acids, such as H3PO4 or HAc, as silicon source hydrolysis catalyst. In order to solve the above problems, this study firstly uses H3PO4 as the catalyst of silica source hydrolysis and brings in the inorganic salt (including NaCl, NaNO3, Na2SO4 and NaBr) which only can change the synthesis in ionic strength and without affecting the pH values to synthesize mesoporous SBA-3 molecular sieve with high quality. At the same time, this study makes use of various kinds of characterization techniques, including systems of XRD, the adsorption of N2, SEM and TEM etc, to do the research to see how synthesis parameters, such as the concentration of phosphoric acid, inorganic salt types and concentration, template agent of alkyl chain length, agitation and static processing etc, and conditional variant influence the order of SBA-3 mesoporous structure. On the basis, this study makes a further investigation to see the effect on mesoporous SBA-3 phase structure by bringing in phosphate. Moreover, it firstly uses the sol-gel method that acid hydrolysis of silicon source is used at first and then salt is put in order to induce condensation to synthesizes mesoporous SBA-3 molecular sieve with highly ordered, then elaborates its formation mechanism. Undoubtedly, when the highly ordered of mesoporous SBA-3 molecular sieve can control the preparation efficiently in gentle acidic system, it not only reduces the cost of materials preparation and synthesis difficulty, but also decreases the environment pollution and the damage of equipments. It is obvious that the moderate synthetic medium is more helpful to pack the guest molecules with biological activity directly and will certainly expand the applicable scope of mesoporous materials. Meanwhile, it must be helpful to control the structure, function and morphology of mesoporous materials better according to the physical-chemical process of synthesizing SBA-3 in a weak acid (0<pH<2) system and the profound understanding of the mechanism of action in each part. The main results of the research are as follows:1. For the first time, using highly ordered mesoporous SBA-3 molecular sieve to control synthesis within a wide phosphate and the concentration of inorganic salt has become true by bringing in inorganic sodium salt such as NaCl, NaNO3, Na2SO4 or NaBr respectively in the system of TEOS-CTAB-H3PO4-H2O which is very difficult to form ordered mesoporous phase. It was found that the changes during the induction period of synthesis system conforms with the Hofmeister anion order, namely the extension of the induction period according to the sequence of NO3-<Br-<Cl-<SO42-, with the differences of various anions on surfactant micelle template affinity. At the same time, it can adjust the surface area, pore volume and pore size of the preparation of materials, respectively ranging from 1214.6 to 1467.6 m2/g, from 0.72 to 0.92 cm3/g and from 2.1 to 2.7 nm. The SBA-3 of high quality can be synthesized in the range of a wider phosphoric acid and inorganic salt concentration because NO3- and Br- have a stronger affinity for micelle template than Cl- and SO4-. Moreover, the increase of template agent alkyl chain length would significantly improve the ordered structure of the synthesis of mesoporous SBA-3. What’s more, compared stirring reaction with static treatment, there is not a significant change of induction period, but the morphology of the material becomes more regular.2. On the basis of the above research, the author makes a further study on how phosphate such as (NH4)3PO4, Na2HPO4 and NaH2PO4 affects the structure of synthetic product phase in the system of TEOS-CTAB-H3PO4-H2O. It was found that it seems that the changes of the acid and alkaline hydrolysis of phosphate may significantly affect the pH range of the system, and also may make the synthetic process more complicated after forming the buffer solution with H3PO4. Therefore, it is not good to synthesize an ordered mesoporous SBA-3. In order to maximally reduce the influence of the pH value on the system by bringing in phosphate, the author firstly proposes and uses Sol-gel method to control the synthesis of the mesoporous SBA-3 with highly quality which can provide abundant experimental basis to make a better understanding of the synthesis mechanism of mesoporous material in an acid system.3. According to the results of the research, the driving force formed with an ordered mesoporous SBA-3 does not completely depend on the concentration of H+ of the synthetic system, but also is influenced by the ionic strength. Besides the high acid (pH<0), it also brings a high ionic strength. On the other hand, in a tender acidic system (0<pH<2), it is possible to make the guest molecules with biological activity conveniently go through the main body of the material of mesoporous structure which has a profound significance by bringing in the idea of using inorganic salt to synthesize SBA-3 with high quality.