Mesoporous solids hydrothermal stability

The deposition precipitation (DP) method has been successfiilly applied in order to functionalize the mesoporous silica SBA-15 with platinum. [Pt(NH3)4](OH)2 was used as the platinum precursor. The applicability of the SBA-15 for the DP is discussed in terms of sufficient hydrothermal stability, and the SBA-15, prepared at 100 °C was found to be a suitable support. The impact of the impregnation was monitored by means of Na adsorption-desorption and X-ray diffraction measurements. Platinum surface species coexisting with platinum nanoparticles on impregnated solids after a mild calcination at 300 °C was detected by UV-Vis diffuse reflectance. Reduction of the bulk surface specie occurs at 60 °C whilst a dispersive phase, which interacts stronger with the support, is reduced at around 120 °C as shown by T emperature-Programmed-Reduction (TPR). 

The applicability of the Pt deposition precipitation technique (DP) on mesoporous silica has been evaluated and discussed. A detailed synthesis procedure is given, and a suitable support from the SBA-15 family has been identified. The material synthesized at the conditions described here was clearly able to withstand the severe conditions of the DP treatment, indicating improved hydrothermal stability. The incorporation of the active species was accomplished without compromising the structural integrity of the parent material, as monitored by XRD and N2-sorption measurements. Using UV-Vis diffuse reflectance spectroscopy we were able to detect the platinum surface complex that coexists with platinum nanoparticles on the impregnated solid. 

Since many properties of crystalline oxides, e.g., acidity, hydrothermal stability, etc., are the essential features exploited in commercial applications of these oxides, it is not unexpected that the ordered, mesoporous materials have not yet found much commerical use. The large void volumes, pore sizes and surface areas of the ordered, mesoporous materials provide advantages over microporous solids in certain areas of application but issues such as stability remain. Thus, if crystalline, extra-large pore solids could be prepared in the pore size and void volume ranges of the mesoporous materials, they would be immediately commercialized. The question remains as to why crystalline materials of this size range have not been synthesized. Navrotsky et al. have shown that pure silica, ordered, mesoporous silicas are energetically very close to pure silica, crystalline... 

The thermal (at 1153 K) and hydrothermal stability (at 933 K) of Si-, A1-, La- and Ce-MCM-41 were studied. The solids were synthesized at 373 K. XRD data indicated that Si-MCM-41 presented the more resolved XRD pattern, therefore the most ordered porous structure. XRD and N2 sorption showed that the thermal treatment leads to a structure degradation with La-MCM41 showing the higher stability and Ce-MCM-41 suffering a total structure collapse. In the hydrothermal treatment, Ce-MCM-41 maintained above of 80% of its Sbet area and 55% of its primeiry mesoporous volume. In those thermally conditions La-MCM-41and Al-MCM-41 were more stable than Si-MCM-41. The intensity of the band at 960 cm in the FT-IR spectra can be well related with the order of porous array in the prepared MCM-41 solids. 

The diversity of ordered porous solids increases at an astonishing rate, particularly among the readily crystallised MOFs, and continues to olfer novel materials properties. There is no obvious barrier to the synthesis of a myriad of new zeolite, zeotype or hybrid structures. Challenges remain, however. For zeolitic aluminosilicates, the 10 A pore size restriction remains an important barrier, and an enantiomerically pure zeolite is still out of reach. For nonsilicate crystalline microporous solids, thermal and hydrothermal stability, rather than framework geometry, limit their applicability, since fully crystalline germanates and carboxylates with pores in the mesoporous range now exist, and these solids have enormous specific surface areas. In these hybrid solids the ability to choose chirality in the building units indicates that it will be possible to prepare these in chiral form the first examples have already been prepared. 

Another approach to introduce mesoporous channels to give better access of reactant molecules to the microporous regions is to assemble zeolite nanoparticles around micellar templates, in a modification of the standard route to mesoporous silicas. Reported examples include structures that possess walls made out of nanocrystals of zeolites such as Beta or ZSM-5. These composite solids possess enhanced hydrothermal stabilities and acidities compared to mesoporous solids with fully amorphous walls. The improved properties are attributed to the presence of the zeolite fragments, because zeolites are known to have higher acidity and hydrothermal stability than amorphous silica/... 

The dimensions and accessibility of pores of zeohtes and microporous solids are confined to the subnanometer scale (<1.5 run), which hmits their applications when processing bulky molecules. Mesoporous materials with pore sizes ranging from 2 to 50 nm overcome these limitations. In contrast with microporous zeolites, these materials lack atomic ordering (crystallinity) in their silica walls as these are usually amorphous. The attractive properties of ordered mesoporous materials include well-defined pore system high surface area and pore sizes narrow pore size distribution tunable up to 100 nm existence of micropores in the amorphous wall (for thicker wall materials) existence of various wall (framework) compositions obtained from direct synthesis, or posttreatment or modification high thermal and hydrothermal stabilities if properly prepared or treated and various controllable regular morphologies on different scales from nanometers to micrometers. 

Ordered mesoporous silica seems to be an ideal hard template, which can be used as a mold for other mesostructures with various compositions, such as ordered mesoporous carbon and metal oxides. Mesoporous silicas with various different structures are available, and silica is relatively easily dissolved in HF or NaOH. Alternatively, mesoporous carbons with a solid skeleton structure are also suitable choices as hard templates due to their excellent structural stability on thermal or hydrothermal and chemical treatment. A pronounced advantage of carbon is the fact that it is much easier to remove than silica by simple combustion. The nanocasting synthesis of mesoporous carbon by using mesoporous silica as template will be discussed in detail in the section on mesoporous carbon. In many cases, silica is unsuitable for synthesizing framework compositions other than carbon, since the leaching of the silica typically affects the material which is filled into the silica pore system. 

FIGURE 2.96 H MAS NMR spectra of microsized (solid lines) and nanosized (dashed lines) HZSM-5 zeolites initial and after hydrothermal treatment at different temperatures. (Adapted from Micropor. Mesopor. Mater., 50, Zhang, W., Han, X., Liu, X., and Bao, X., The stability of nanosized HZSM-5 zeolite A high-resolution solid-state NMR study, 13-23, 2001, Copyright 2001, with permission from Elsevier.)... 

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