Jian Yu

Effect of inorganic salt addition during synthesis on pore structure and hydrothermal stability of mesoporous silica

Abstract Mesoporous materials were synthesized without and with the addition of salts such as NaCl and NH 4 Cl and were characterized using different analytical techniques. The hydrothermal stability was investigated by water treating these mesoporous materials at 373 K for various times in closed bottles. Addition of salts resulted in transformation of mesophase, which depends strongly on the kinds and concentration of salts added. When added with NaCl, the mesoporous materials experienced phase transformations in the sequence MCM-41→KIT-1→MCM-41→KIT-1→amorphous phase as the NaCl/surfactant molar ratio increased. However, addition of NH 4 Cl led to the formation of disordered mesoporous materials. Mesoporous materials with high hydrothermal stability could be prepared by addition of a suitable amount of salts and the materials maintained their initial mesoporous structure after hydrothermal treatment at 373 K for at least 120 h in closed bottles. For mesoporous materials without salt addition or with addition of salts with unsuitable molar ratios, mesopores would collapse and the pore size distribution became very large after hydrothermal treatment.

Preparation of high thermal stability MCM-41 in the low surfactant/silicon molar ratio synthesis systems

The preparation of a mesoporous molecular sieve MCM-41 both with bimodal framework and textural mesopores and with high thermal stability that results from a thicker wall has been achieved by decreasing surfactant/silicon ratio of synthesis gel. The resultant material retains porosity and large BET surface area even after thermal treatment at 1273 K for 1 h in air.

Synthesis of nanostructured mesoporous silica materials containing manganese

The nanostructured mesoporous silica MCM-41 materials with different amounts of manganese have been synthesized under basic conditions at room temperature. The characteristics of the Mn-containing mesoporous samples were investigated by XRD, FTIR, N2 adsorption, 29Si MASNMR and HREM techniques. It has been shown that the (100)peak intensity of XRD patterns became weakened and the specific surface area of materials decreased monotonously with the increase of Mn contents. FTIR results indicated that Mn ions could be incorporated into the Siue5f8O framework and lead to the vibration of Siue5f8Oue5f8Mn bond. 29Si MASNMR results suggested that the values of [SiO4]tetrahedra in the framework decreased with the increase of Mn ions addition, resulting in the decrease of Q4Q3 ratio. It could be seen from HREM images that nanostructured framework of Mn-modified mesoporous silica was distorted, and lamellar structure co-existed with hexagonal structure in some area.

Temperature control in the synthesis of cubic mesoporous silica materials

Abstract The cubic mesoporous silica MCM-48 materials have been synthesized by temperature control, and the properties of samples were investigated by using XRD, HREM and N 2 adsorption–desorption techniques. XRD results show that the mesophases synthesized at low temperature (≤75°C) have a poorly ordered hexagonal structure, while hydrothermal treatment at higher temperature (≥95°C) led to cubic MCM-48 mesophases. The effect of temperature on different mesophases can be due to the polymerization and condensation of silicate species at different temperatures, which results in the formation of curvatured hexagonal or cubic phase. HREM and N 2 isotherms of MCM-48 materials indicates well-ordered three-dimensional meso-channels.

Synthesis of titanium-doped ordered porous zirconium oxide with high-surface-area

Well ordered and titanium doped porous zirconium oxide with a high surface area was successfully synthesized by the surfactant templating method. X-ray powder diffraction, nitrogen adsorption, HRTEM, and UV-VIS spectroscopy methods were adopted for the characterization of the synthesized materials. Evidence showed that titanium had been incorporated into the inorganic wall of zirconium oxide during synthesis, and titanium was found not to affect the surface area at low temperatures but to substantially enhance the thermal stability of the materials. A specific surface area as high as 400 m(2)/g was obtained at a 10 mol% titanium doping level after calcining at 773 K. A significant red shift of the UV-VIS spectrum of the porous material by titanium doping was observed, which was attributed to the contribution of the Ti(IV) in the zirconia framework

Room temperature synthesis of mesoporous aluminosilicate materials

Mesoporous aluminosilicate materials with the MCM-41 structure and atomic Si/Al ratio of 8 have been synthesized at room temperature using aluminum chloride hexahydrate and tetraethyl orthsilicate as the sources of materials. The products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption and Al-27 magic-angle-spinning (MAS) NMR. XRD results show that thermal stability of the product can be improved when aluminosilicate MCM-41 is synthesized following a procedure of pH adjustment from 11 similar to 12 to 5 similar to 6 with hydrochloric acid. NMR analysis clearly shows that both 4- and 6-coordinate aluminum is present in the calcined products. High-resolution electron micrograph first exhibits the presence of nanocrystallites, which could be regions of segregated alumina, in amorphous aluminosilicate phase

Effect of composition on the sintering and microstructure of diphasic mullite gels

Abstract Diphasic gels with alumina/silica ratio of 68/32, 71.8/28.2, 74/26, and 76/24 were prepared and used to study the effect of the precursor composition on the mullite formation process, densification behavior, and microstructure development. Alumina contents of mullites in the samples sintered at high temperature (1650°C) for 4 h or at low temperature (1550°C) for longer time increased with the increase in alumina/silica ratio of the gels. A sharp decrease in densification rate was observed on transition from the mullite+silica-rich liquid phase field to the mullite solid solution. This decrease was associated with elimination of the viscous flow of the amorphous silica phase. However, the densification rate was higher for the samples with excess alumina than for the 74/26 sample. Both in the silica-rich sample (68/32) and the stoichiometric mullite sample (71.8/28.2), elongated mullite grains were observed. Such elongated grains grew at the expense of the surrounding smaller mullite grains. In the 74/26 and 76/24 samples, mullite grains were equiaxed. Furthermore, in the latter, elongated α-Al 2 O 3 grains were observed.