Investigation of Crystallization of Molecular Sieve AlPO₄-5 by the Dry Gel Conversion Method

Abstract

Crystallization of AlPO 4 -5 (AFI), a representative aluminophosphate-based molecular sieve with AFI topology, was examined under dry gel conversion, in particular, vapor phase transport (VPT) conditions. The VPT approach allows one to "freeze" crystallization at different stages and, therefore, to capture several reactive intermediates, which provide "snapshots" of the crystallization process. These intermediates were characterized by powder X-ray diffraction, Raman spectroscopy, and a combination of several solid-state NMR techniques. The results show that the reactive intermediates have two key features: (1) They bear certain structural characteristics of the target framework. (2) They are fragile and held together by weak bonding interactions. Hydrogen bonding apparently plays a key role in the formation of intermediates and their subsequent transformation to the final AFI structure. An AlPO 4 -5 crystallization mechanism under VPT conditions was proposed.

Highly efficient crystallization of AlPO4-5 accelerated by a rotating hydrothermal synthesis route

“…† Obviously, the vecoordinated Al species are prominent, which is about 70% of the total Al species in the sample. This is different from the static hydrothermal synthesis route 23 and DGC route 24 reported, in which four-coordinated Al is prominent. It implies that the stable coordination state of Al species in the initial reaction gel is inuenced by the crystallization routes; [23][24][25] ve-coordinated Al(OP) 4 (OH 2 ) species is more easier to be formed under rotating hydrothermal condition, for comparison, fourcoordinated Al(OP) 4 is easier to be formed under static synthesis route, including static "hydrothermal or DGC" route.…”

“…This is different from the static hydrothermal synthesis route 23 and DGC route 24 reported, in which four-coordinated Al is prominent. It implies that the stable coordination state of Al species in the initial reaction gel is inuenced by the crystallization routes; [23][24][25] ve-coordinated Al(OP) 4 (OH 2 ) species is more easier to be formed under rotating hydrothermal condition, for comparison, fourcoordinated Al(OP) 4 is easier to be formed under static synthesis route, including static "hydrothermal or DGC" route. The peak shape of 27 Al MAS NMR spectra for samples R-180(5)-60 and R-180(8)-60 has no obvious difference with that of starting reaction gel, implying that intermediate phases R-180(5)-60 and R-180(8)-60 have the similar Al chemical environment with that of initial gel.…”

“…In the 31 P MAS NMR spectrum of R-180(10)-60, a narrow peak centered at around À29 ppm is observed, which is ascribed to P(OAl) 4 environment and consistent with the reported result. [20][21][22] The 31 P MAS NMR peak shape of R-180(20)-60 looks nearly identical with that of R-180(10)-60, which is consistent with the reported results, [23][24][25] implying that R-180(10)-60 has the similar P chemical environment with well-crystallized AlPO 4 -5 (R-180(20)-60).…”

Investigation of water assisted phase transformation process from AlPO4-5 to AlPO4-tridymite

“…1 H MAS NMR spectra (Figure 4) indicate a broad peak at 7.2 ppm belonging to strong hydrogen bonded proton between protonated amine and aluminophosphate exchanging with water, and the peak shift to downfield continuously to 9.0 ppm at 150 min. [31][32][33] At the same time, the broad peak near 5.2 ppm becomes distinguishable, which is also attributed to protonated amine as this peak shift to downfield at 6.2 ppm when the temperature is lowered to 25 C (see supporting information Figure S2). However, the 7.2 ppm peak is more sensitive to temperature, and it disappears when the temperature decreased to room temperature.…”

“…31 P MAS NMR spectra (Figure 3) show the main peak of AlPO4-5 at -29 ppm with broad shoulder at -24 ppm, the broad peak between -10 and -20 ppm is attributed to partially condensed P in amorphous phase, and minor dissolved phosphate complex at -0.2 ppm exchanges with phosphate partially connected with aluminum. [28,29] After 130 min, the sharp peak at -0.2 ppm change into broad peaks near -7 ppm with enhanced peak intensity, which is attributed to aluminophosphate complexes/terminal connected phosphate. [29,30] The upfield shift of the peak means the condensation reaction between Al and P occurs or the exchange between solution phosphate and partial connected phosphate is enhanced.…”