Monitoring the coordination of aluminium during microporous oxide crystallisation by in situ soft X-ray absorption spectroscopy

Abstract

An in situ cell, which is capable of obtaining time resolved soft X-ray data (200 eV , E , 3000 eV) under hydrothermal conditions has been developed and used to study the selfassembly processes occurring during microporous aluminophosphate crystallization.Many of the useful physical properties associated with zeotypes, such as ion-exchange capacity, molecular sieving and shape selectivity, originate from the presence of nanometre-sized channels and/or cages within their structure. Although they occur within the Earth's crust, a large demand on their usage has triggered major research efforts on the preparation of synthetic analogues of these minerals, the alteration of their properties by manipulation of their framework and the synthesis of new topologies altogether. 1 However, the self-assembly steps by which such important materials form are still not well understood and thus the synthesis of new topologies is still something of a hit and miss process. [2][3][4] Understanding the critical stages in microporous materials crystallisation is then clearly the first step towards realising the possibility of rational microporous materials design for specific purposes e.g. tailoring a material's properties for a specific catalytic reaction.Time-resolved in situ experiments using a variety of analytical techniques have been performed in an attempt to obtain such information. [2][3][4] The majority of these studies have concentrated on using scattering techniques to follow these processes since they are sensitive to structural ordering in the system. However, since selfassembly processes involve the ordering of atomic or molecular species, spectroscopic techniques can also yield important insight since they are more sensitive. For example, it has been possible to follow the behaviour of a small amount of a hetero-atom (an example being transition metal ions, which can be considered as a 'probe' species) to obtain information on these self-assembly processes. [5][6][7] However, such information can be considered at best indirect and is only possible to observe with hetero-atoms, which give a strong spectroscopic signature and which do not influence the phase formation processes unduly. It is far more useful therefore to follow the crystallisation behaviour of the major structural elements, although this is more difficult to achieve since their spectroscopic signatures are often comparatively weak or less detailed than the corresponding transition metal ion-substituted systems. Perhaps, the most relevant technique to obtain such molecular insight into the assembly processes under hydrothermal conditions is NMR, since it is particularly sensitive to changes in the local structure of the zeolite/zeotype atomic species. Indeed, it has been successfully employed to obtain detailed information regarding the formation of microporous systems. [8][9][10] However, the interpretation of NMR data is not always straightforward due to quadrupolar effects or line broadening as a result of interactions with paramagnetic species, such a...

Molecular engineering of microporous crystals: (VI) Structure-directing effect in the crystallization process of layered aluminophosphates

“…Recently, the crystallization processes of aluminophosphate molecular sieves were studied using a new strategy/method and by monitoring the crystallization process with in situ or ex situ characterization techniques [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. However, the understanding of the crystallization process of microporous aluminophosphates and the structure-directing effect of organic additives at the molecular level is still insufficient, and the data are sometimes contradictory [27].…”

In Situ UV Raman Spectroscopic Study on the Synthesis Mechanism of AlPO‐5

“…By combining the information obtained from the previously studies, [5,6,15] we propose the following synthesis mechanism for AlPO-5 ( Figure 4). In the very early stages, the aluminates and phosphates react to form amorphous aluminophosphate gel phase with some of the octahedral AlO 6 species, thereby transforming into tetrahedral AlO 4 species.…”

Catalyst Characterization—Heterogeneous

“…Growth of these nuclei into macroscopic crystals then occurs by addition of the primary units to the crystal surface. The interesting observation is that, despite the differences in structures and compositions, many aspects of porous material crystallization are similar suggesting a common formation mechanism (161,163,(192)(193)(194)(195).…”