GAM-3: a zeolite formed from AlPO4-5 via multistep structural changes

GAM-7: an organic-inorganic hybrid layered aluminophosphate crystal formed by zeolite transformation

The interzeolite conversion (IZC) of metallo- and aluminophosphate zeolites was demonstrated using a non-cyclic secondary amine, diisopropylamine, as the organic structure-directing agent (OSDA). The IZC of AlPO4-5 with AFI topology could successfully produce an unknown highly crystalline phase, named GAM-7. In metallo-aluminophosphates (metal-AlPOs), SAPO-5 with a small amount of SiO2 was also interconverted to a single crystal GAM-7, designated as [Si]GAM-7. However, in other metal-AlPOs, the corresponding GAM-7 crystals were formed as either an intermediate or a mixture of plural crystals. Additionally, it was found that GAM-7 could also be obtained by hydrothermal synthesis (HTS), but not synthesized from a silico-aluminophosphate hydrogel, indicating that [Si]GAM-7 can be prepared by only the IZC technique. The obtained GAM-7 crystal was well-characterized to elucidate the framework structure, and this revealed that GAM-7 was a new organic-inorganic hybrid layered aluminophosphate having a terminal POH group in the corrugated layered sheets, and the OSDA molecule between the layers was robustly interacting with an acidic POH site by a hydrogen bond. Furthermore, we have performed the tracking of pH values during the transformation from AlPO4-5 into GAM-7 and have argued a reasonable consideration for the IZC process.

Two-dimensional microporous GAM-6 formed by the interzeolite conversion of CoAPO-5

The interzeolite conversion of CoAPO-5 with piperidine assistance yielded a new microporous zeotype, GAM-6. Characterization and structural elucidation revealed that GAM-6 has a 3-ring local structure and two-dimensional 10-ring pore channels, and shows reasonable microporosity and solid acidity.

Recognizing the Minimum Structural Units Driving the Crystallization of SAPO-34 in a Top-Down Process

Recognizing the structure and nature of the nuclei for zeolites crystallization on an atomic level is of great importance, which can provide guidance on the control of crystallization kinetics and the rational synthesis of zeolites. However, it remains a long-standing challenge due to the difficulty in characterization of amorphous precursor with limited crystal nuclei. Herein, a top-down synthesis system was designed for SAPO-34 molecular sieve and well investigated. A clear precursor solution with abundant SAPO-34 crystal nuclei was obtained under a depolymerization-dominant condition. The species in the liquid precursor were identified by FT-ICR MS, solid-state MAS NMR and atomic pair distribution function analyses. In combination with various designed experiments, it is revealed that both the formation of small species containing Si-O-Al bonds and reaching a certain concentration, is crucial for driving the crystallization of SAPO-34, rather than structural units with specific spatial conformation. This work provides an important understanding on the (pre)nucleation of SAPO-34 and sheds light on the synthesis control of SAPO molecular sieves.

Vibrational characteristics of aluminum-phosphate compounds by an experimental and theoretical approach

Aluminum phosphates are materials with relatively wide potential applications in many industries. The vibrational features of selected compounds were established on Raman and infrared spectroscopy. The experimentally determined spectra are compared to those calculated by ab initio methods. This gives a unique possibility of a proper assignment of the experimental spectral features to specific modes of vibration. In the results, it was evidenced that the spectra are characterized by two specific intense bands in the mid- and high-frequency range due to the P–O–P and P–O bonds in [PO₄] tetrahedron vibrations. The position of the high-frequency band is related to the number of bridging oxygen atoms connecting [PO₄] tetrahedrons in the unit cell. Additionally, the differences in the spectra were evidenced as a result of different polymorphic forms of the selected compounds. Therefore, the results may be useful in determining the phase composition of polyphase materials or structural features of aluminum–phosphate glasses and glass–ceramic materials.

On the key role of aluminium and other heteroatoms during interzeolite conversion synthesis

Interzeolite conversion, a synthesis technique for several zeolite frameworks, has recently yielded a large amount of high-performing catalytic zeolites. Yet, the mechanisms behind the success of interzeolite conversion remain unknown. Conventionally, small oligomers with structural similarity between the parent and daughter zeolites have been proposed, despite the fact these have never been observed experimentally. Moreover, recent synthesis examples contradict the theory that structural similarity between the parent and daughter zeolites enhances interzeolite conversion. In this perspective it is proposed that heteroatoms, such as aluminium, are key players in the processes that determine the successful conversion of the parent zeolite. The role of Al during parent dissolution, and all consecutive stages of crystallization, are discussed by revising a vast body of literature. By better understanding the role of Al during interzeolite conversions, it is possible to elucidate some generic features and to propose some synthetic guidelines for making advantageous catalytic zeolites. The latter analysis was also expanded to the interconversion of zeotype materials where heteroatoms such as tin are present.

The crucial roles of aluminium in driving and controlling interzeolite conversion, a useful catalyst synthesis protocol, are put under scrutiny.