Postsynthesis hydrothermal treatment

The DTG curve for uncalcined DS-AD sample, which did not undergo the postsynthesis hydrothermal treatment, exhibited major weight change peaks at about 450 and 500 K [19],... 

POSTSYNTHESIS HYDROTHERMAL TREATMENT OF DOUBLE-MESOPORE SILICA MOLECULAR SIEVES IN WATER... 

The synthesis procedure for DMS silicas was described elsewhere [3.4]. The postsynthesis hydrothermal treatment was carried out by replacing the mother liquor with pure water after the normal synthesis. Typically, the as-synthesized DMS silica was mixed with a certain amount of pure water, which was equivalent to about 20ml of water per gram of dried uncalcined samples, transferred into an autoclave, and kept in the oven maintained at 373K and 423K, respectively, for different periods of time. Finally, the solid product was filtered, washed, dried, and calcined as in the normal preparation. 

Important trends of double-mesopore structural development resulting from the water-treatment are revealed in Figure 2 by the N2 adsorption-desorption isotherms and the corresponding BJH pore size distribution based on the desorption branch for the representative samples mentioned above. The isotherm of the normally synthesized DMS simple shows a typical irreversible type IV adsorption isotherm with two separate, well-expressed HI hysteresis loops as defined by lUPAC at relative pressures p/po of 0.2-0.45 and that of 0.8-1.0, respectively. The first condensation step on the isotherm at p/pQ=0.2-0.45 is similar to that for usual MCM-41 materials, however, obviously, this inflection at higher relative pressures differs completely from that of previously-synthesized mesoporous materials in the aspect of their effects on the mesoporous frameworks of the product, namely, this material is of a clear double mesopore size distribution. After 1 day of postsynthesis hydrothermal treatment, the properties of the samples changed dramatically. Compared with the normally synthesized DMS sample, the water-treated sample at 373K shows more steep adsorption steps at 0.25-0.4p/po and 0.8-1.0p/po, respectively, suggesting that double-... 

To further examine the effect of the postsynthesis hydrothermal treatment on the extent of double-mesopore framework cross-linking, we employed the Si MAS NMR. It can be seen from Figure 3 that the Q4 (Si(OSi)4) to Q3 (Si(OSi)3(OH)) ratio of the water-treated (at 373K) uncalcined DMS is higher than that of the normally prepared one. The high portion of... 

To improve the meso-structural order and stability of the mesoporous silica ropes, a postsynthesis ammonia hydrothermal treatment (at 100 °C) was invoked. As indicated by the XRD profile in Fig. 3A, 4-5, sharp features are readily observed in ammonia hydrothermal treated samples. Moreover, after the post-synthesis ammonia treatment, the sample also possesses a sharp capillary condensation at p/po 0.35(Fig. 3B) corresponding to a much narrower BJH pore size distribution of ca. 0.12 nm (at FWHM). In other words, the mesostructures are not only more uniform but also more stable when subjected to the post-synthesis treatment. The morphology of the silica ropes remained unchanged during the ammonia hydrothermal process. The mesostructures remain intact under hydrothermal at 100 °C in water even for extended reaction time (> 12 h). 

Another method to tailor pore size of mesoporous solids is to perform restructuring upon hydrothermal treatment. Applying aging treatments at different temperatures and for prolonged periods (from 24 hours up to several days) can efficiently modulate the nature of the mesophase. Such a treatment can either be performed directly in the mother liquor or at a different pH in fresh solutions (typically water or alcohol). For example, Kushalani et al. (185) proved that sUiceous MCM-41-type mesophases could be restmctured at elevated temperatures in its mother liquor resulting in pore size expansion from 3.7 to 5.9 nm. Also, postsynthesis treatments often improve the thermal stabUity of samples normally obtained at room temperature (with less shrinkage... 

To conclude, when it comes to hydrothermal treatment, a lot of structural modification can occur depending on the experimental conditions. The key parameter in these structural changes is the flexibility of the silicate network. Apart from hydrothermal treatment, numerous other postsynthesis treatments have been described in order to reduce the pore size [21] or improve structural order, hydrothermal stability, and other characteristics of the mesoscopic materials [lOd]. 

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