Isomorphous substitution induced ionothermal synthesis of magnesium aluminophosphate zeolites in fluoride-free media

Abstract

Magnesium aluminophosphate zeolites (MgAPOs) with AEL, AFI and ATS structures were ionothermally synthesized in a fluoride-free system depending on the structure of the ionic liquids and the concentration of magnesium, where the incorporation of magnesium into the framework induced their crystallization, resembling that of fluoride ion in aluminophosphate analogues formation.Crystalline microporous aluminophosphates (AlPOs) based on strict alternation of AlO 4 and PO 4 tetrahedra are known as a prominent family of zeolitic materials. 1 Unlike aluminosilicate zeolite analogues, the framework of aluminophosphates is generally neutral, which limits their catalysis and ion-exchange functionality. Fortunately, the lattice Al and/or P can be isomorphously replaced by silicon and/or other metal atoms to form an anion zeolitic framework with Brønsted acid sites and/ or catalytically active metal centers, i.e. silicon-or metalsubstituted aluminophosphates, 2 which are of great signi-cance for their established and potential catalytic applications, for instance, paraffin isomerization and methanol to olen process. 3 In recent years, ionothermal synthesis, a method that uses ionic liquid as both solvent and template, has exhibited unique features and potential advantages over traditional hydrothermal and solvothermal synthesis in preparing aluminophosphate-based zeolites, as well as metal-organic frameworks and inorganic-organic hybrids. 4 For example, ionic liquid's negligible vapor pressure allows the synthesis in atmospheric vessels and the safer use of microwave heating; 5 ionic liquid's different chemistry and structural variety associated with the use of amines displays great potential to prepare zeolitic materials with novel structures and compositions. 6 Till now, several heteroatom-, including Co-, Mg-, Fe-, Mn-, Zn-, substituted aluninophosphate zeolites have been obtained by this method. 6b,7 Remarkably, a new cobalt-substituted aluminophosphate named SIZ-7 (SIV structure) with 3-dimensional 8-membered ring channels of zeolitic structure was ionothermally prepared by Morris et al.; 6b MgAPOs with AEL and AFI structures were successfully prepared by Wang et al. in uoride-containing ionothermal media, exhibiting excellent catalytic performance in the isomerisation of long-chain paraffin. 7a During the ionothermal synthesis of these materials, HF is always used and considered as one of the very important synthesis variables. In addition to the role of promoting the crystallization kinetics, uoride appears to be important in obtaining the full condensed structures of AlPOs without hanging P-O bonds, which is always stable for removal of the templates and thus to create the porosity. 4a Furthermore, Ma et al. proposed that during the synthesis of AlPOs, uoride could incorporated into the AFI structure and plays the role of structure-directing agent. 8 However, uoride in the synthesis media due to its toxicity and causticity is not desirable from an industrial viewpoint. Moreover, it was also reported that HF...

Co‐Structure‐Directing Effect in Ionothermal Synthesis of Extra‐Large‐Pore Aluminophosphate Zeotype with −CLO Topology

“…At a ratio of 0, the AEL structure formed after a short induction time and reached maximum crystallinity after a crystallization time of 10 h, at which time another phase with CHA structure, containing 3D 8‐ring intersecting channels, appeared with gradually improved crystallinity along the crystallization time axis. The phase transformation from AEL to CHA structures could be ascribed to the formation of dimethylimidazolium cations derived from IL decomposition through transalkylation of the alkyl groups in the presence of fluoride . Notably, no −CLO structure was observed during the crystallization process in the absence of HDA.…”

“…The phase transformation from AEL to CHA structures could be ascribed to the formation of dimethylimidazolium cations derived from IL decompositiont hrough transalkylation of the alkyl groups in the presence of fluoride. [35,36] Notably,n o ÀCLO structure was observed during the crystallization process in the absence of HDA. At ar atio of 0.5, RUB-A2 was formed insteadoft he AEL structure in the initial stage of crystallization and, with increasing crystallization time, RUB-A2 was gradually transformed into the ÀCLO structure.…”