Vertical Aluminophosphate Molecular Sieve Crystals Grown at Inorganic-Organic Interfaces

Abstract

Tubular aluminophosphate molecular sieve crystals were grown at an organic interface with their channels (7 angstroms in cross section) vertical to the substrate. To induce surface nucleation and oriented growth of AIPO4-5 crystals, organophosphonate layers cross-linked with Zr(lV) were assembled on a gold substrate and the modified substrate was immersed in a hydrothermal bath containing reagents for the synthesis of the molecular sieve. Reflection-absorption infrared studies demonstrated the stability of the phosphonate layers under these conditions. Drastic changes in the morphology of the surface-grown crystals from spherical agglomerates to vertical needles to thin tilted needles could be achieved by adjusting the water content of the synthesis bath. Nitrogen sorption in these structures on a piezoelectric device confirmed the presence of zeolitic microporosity. The films obtained in all of our experiments exhibited x-ray diffraction (XRD) patterns corresponding to that of the AFI structure, that is, AIPO4-5. Grazing-angle incidence (1.5°) XRD patterns of the surface-grown AlPO4-5 crystals (Fig. 3) monolayer to its poor thermal stability at acidic pH. The RAIR spectrum shows that MUP monolayers are not stable at 180°C for longer than 2 hours. When the added zirconyl layer was presented to the A1PO4-5 synthesis bath, no crystal growth was observed either, suggesting a lack of electrostatic interactions. The templating agent DABCO also plays an important role. When it was substituted with tripropylamine, no A1PO4-5 growth occurred even though the bulk synthesis proceeded without difficulty. This effect could be related to the dipositive charge on protonated DABCO and associated surface interactions.We were also interested in the nature of the strong interactions between the aluminophosphate nuclei and the phosphonate surface. We performed quenching reactions under bulk synthesis conditions to explore the association between the phophonate surface and zeolite crystal faces. Ethylphosphonic acid (H2PO3CH2CH3) (25 mmol) was added to the bulk AlPO4-5 crystallization mixture after 40 min, and the resulting suspension was placed back into the oven at 180'C for another 8 hours. The SEM and XRD data show that addition of the "quenching agent" blocks further crystal growth. The size and morphology of quenched crystals are similar to those from a regular synthesis stopped after 40 min. These results suggest that the phosphonic acid groups have a strong affinity to the growing faces of the AlPO4-5 crystals. The 10011 faces appear to interact most effectively with the phosphonate surface, thus favoring the vertical orientation of the c axes.We found that the morphology and size of surface-grown A1PO4-5 crystals can be controlled by varying the temperature and the water content of the synthesis bath. We obtained semispherical aggregates at 1500C and lower water content (d = 60). Dilution of the system produces larger hexagonal prismatic

Catalytic Activity of 0.4 % Rhodium Supported Microporous AlPO4-5 Catalyst for NO-CO-C3H6-O2 Reactions

“…1 shows XRD patterns of as-synthesized AlPO 4 -5, calcined AlPO 4 -5 and 0.4 % Rh loaded AlPO 4 -5. The XRD patterns of these samples correspond well with that for reported AlPO 4 -5 structure and do not show any impurity [21][22][23][24][25][26]. Upon calcination, a little increase in intensities of XRD patterns were observed due to little increased particle size.…”

Ionothermal Synthesis of Aluminophosphate Molecular Sieve Membranes through Substrate Surface Conversion

“…[26] The orientation of AEL molecular sieve membranes could be modified by adjusting the addition of HF. AEL consists of ten-membered-ring channels parallel to the c axis of the crystal, which is the fastest growing axis during the ionothermal procedure.…”

Zeolite Membranes

“…They have demonstrated the high molecular selectivity of chemical sensors based on this method. The same authors discovered [87,88] that single component organophosphonate films could promote the growth of oriented molecular sieves, specifically zincphosphate and aluminophosphate crystals. It was found that the surface crystallization requires only very small aggregates of phosphate groups.…”