Insights into Structure Direction of Microporous Aluminophosphates: Competition between Organic Molecules and Water

Temperature-dependence of the influence of the position-2-methyl group on the structure-directing effect of piperazine in the synthesis of open-framework aluminophosphates

The temperature-dependence of the influence of the position-2-methyl group on the structure-directing effect of piperazine in the synthesis of open-framework aluminophosphates was investigated. By heating the initial mixture with the composition of Al₂O₃:1.5 P₂O₅:R:125 H₂O at 160 °C, where R is the structure-directing agent of 2-methylpiperazine or piperazine, layered aluminophosphates APMeP150 (R = 2-methylpiperazine) and AP2pip (R = piperazine) were obtained. When the same initial reaction mixture was heated at 190 °C, layered aluminophosphates APMeP200 (R = 2-methylpiperazine) and AP2pip (R = piperazine) were crystallized. APMeP200 and AP2pip have the same inorganic sheet topology. We investigated the crystallization processes of the four open-framework aluminophosphates and found that increasing the heating temperature slowed the crystallization of the initial mixtures. The non-bonding interactions between the inorganic layers of the four open-framework aluminophosphates and the experimental or theoretically generated structure-directing agents were calculated. The possible starting points of the crystallization of the four open-framework aluminophosphates were analysed and compared. The temperature-dependence of the influence of the position-2-methyl group on the structure-directing effect of piperazine revealed that the structure-directing effect, the most important factor in the synthesis of zeolites and related open-framework materials, is determined by multiple factors. The structural parameters of the same compound can be affected by the synthesis conditions.

Time resolved in situ X-ray diffraction study of crystallisation processes of large pore nanoporous aluminophosphate materials

Time resolved high-resolution X-ray powder diffraction was utilized to obtain detailed changes in the crystal structure parameters during the hydrothermal crystallization process of the nanoporous aluminophosphate AlPO-5 (AFI) structure. This in situ study offered not only the influence of metal ions on the onset of crystallization and estimation of the activation energy of the process, but also allowed us to determine in detail the changes in lattice parameters during this process. More importantly the time-resolved study clearly showed the lattice expansion in the divalent metal ions substituted system right from the on-set of crystallization process, compared to the one without any dopant ions, which suggest that an amorphous or poorly crystalline network is formed prior to crystallization that contains the large divalent ions (compared to Al(iii), the substituting element), which is in agreement with the combined XAS/XRD study reported earlier. A mechanism based on this and the earlier study is suggested.

Nascent zeolite frameworks grown from amorphous gels – identification and prospects for crystal engineering

From the standpoint of designing microporous frameworks with desired pore diameter and dimensionality, and, especially, the optimization of crystal habit, the crystal engineering of new zeolites must be based on concepts/procedures different from those appropriate for the design of organic crystals. This is because the structure building units proposed by Barrer and co-workers are probably not instrumental for framework construction, thus eliminating the important ‘synthon’ approach used for the construction of innovative organic solids. With some variant of the Flanigen model for crystal growth via one SiO2 unit at a time, the best approach to zeolite crytal growth engineering appears to occur indirectly via structure directing agents that can also be modified to influence the emergent crystal habit. Prospects for identifying frameworks emerging from synthesis gels are also discussed in this review, revealing that the use of radial distribution functions is less informative than for the analysis of silicate glasses.