Shrinkage hydrothermal

Table III shows XRD and porosimetry data for calcined USY and AFS zeolites. All samples show shrinkage of the unit cell to comparable values following calcination. As a result, calcined samples are compared at similar silica-alumina framework ratios. All calcined samples have well developed microporous structures and comparable total pore volumes. These porosimetry data confirm that the hydrothermally dealuminated materials contain a significant fraction of mesopores relative to chemically dealuminated materials. The extensive washing given to AFS-1 results in higher micropore surface area and volume compared to AFS-2 and suggest that AFS-2 contains occluded fluoroaluminate and fluorosilicate compounds within the microporous structure.

Similar arguments relative to the importance of hydrogen bonding in collagen fibrillar structure are also derived from a comparison of the shrinkage temperatures and compositions of mammalian and fish skin collagens. The latter, which possesses (see Section II, 5) fewer hydrox-ylic side chains than does the former, also has less hydrothermal stability (92). 

Figure 2 (left) depicts the variance of lattice constant with hydrothermal aging temperature. VA samples showed the largest decrease in lattice constant due to the rapid dealumination by V. CaAf samples had little lattice shrinkage. The lattice constant variance of Fe/Y and Ni/Y were similar to each other, and much smaller to the V/Y samples. 

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... 

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