Ionothermal synthesis of magnesium-containing aluminophosphate molecular sieves and their catalytic performance.

Owing to their promising catalytic and adsorptive properties, microporous materials, often referred to as molecular sieves or open-framework materials, have attracted more and more attention in different research fields. Among them, microporous aluminiophosphate molecular sieves (designated AlPO4-n) have been extensively studied by many authors, owing to their wonderfully complex structures and broadly applications since their discovery in 1982. However, because the frameworks of these materials are neutral and lack of Brønsted acid sites, sometimes they can not be used in catalysis directly. Nevertheless, the lattice Al and/or P atoms in the AlPO4-n frameworks can be partially replaced by silicon (designated (SAPO-n) and/or other metal elements (designated MeAPO-n), to form frameworks with Brønsted acid sites and/or catalytically active metal centers. Commonly, AlPO4-n and MeAPO-n compounds are often crystallized from a gel under hydrothermal or solvothermal conditions. Recently, a novel synthetic route (ionothermal synthesis) provides a potential route to making these microporous materials. Ionothermal synthesis is the use of an ionic liquid or eutectic mixture as the reaction solvent and if necessary, the structure-directing agent in the synthesis of materials. Ionic liquids are special molten salts typically containing organic cations and inorganic anions. The peculiar properties of ionic liquids endow the ionothermal synthesis many interesting features and potential advantages over the traditional methods of molecular sieve synthesis. For example, the ionothermal synthesis can take place at ambient pressure because of the vanishingly small vapor pressure of ionic liquids, which eliminates the safety concerns associated with high pressures. In addition, the excellent microwave-absorbing property of ionic liquids allows the ionothermal synthesis being carried out under microwave conditions, leading to the rapid crystal-growth rate and high product selectivity. Although many AlPO4-based molecular sieves including some novel-framework topologies have been prepared using ionothermal method, the amount of the MeAPO-n structures synthesized ionothermally is relatively small. To date, only several SAPO-n and CoAPO-n 15] structures could be obtained by using this new synthetic technique. Moreover, the research on the applications of these MeAPO-n compounds to catalysis or gas adsorption has not been reported in the literature. Herein we demonstrate that the ionothermal synthesis method can be applied to the synthesis of MeAPO-n materials with excellent catalytic performance. In the present work, a magnesium compound was introduced into the reaction system of AlPO4-n and several Mg-containing aluminophosphate molecular sieves were successfully synthesized in 1-butyl 3-methylimidazolium bromide ionic liquid ([bmim]Br). Results are presented about the physical and chemical characterization (such as X-ray diffraction: XRD, X-ray fluorescence: XRF, scanning electron microscopy: SEM, Brunauer–Emmett–Teller: BET, NH3-temperature programmed desorption: NH3-TPD, thermogravimetric: TG, and NMR analysis) of the MAPO samples obtained with different Mg contents and amine as well as their catalytic evaluation in the hydroisomerization of n-docecane. Table 1 summarizes the details of the synthesis conditions and the products obtained ionothermally. Only the AlPO4tridymite dense phase could be obtained at 170 8C for 3 day without the addition of metal elements (Table 1, AlPO). [a] Dr. L. Wang, Dr. Y.-P. Xu, B.-C. Wang, Prof. Z.-J. Tian, Prof. L.-W. Lin Dalian National Laboratory for Clean Energy State Key Laboratory of Catalysis Dalian Institute of Chemical Physics, Chinese Academy of Sciences 457 Zhongshan Road, Dalian, 116023 (China) Fax: (+86)411-84379151 E-mail : [email protected] [email protected] [b] Prof. S.-J. Wang, Prof. J.-Y. Yu Department of Chemical Engineering Dalian Institute of Light Industry Dalian, 116034 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200801383.