Preparation And Characterization Of Aluminophosphate Microporous Materials Templated By Tetramethylguanidine

Since the first discovery of open-framework aluminophosphates (denoted AlPO4-n) by Wilson et al. in the early 1980s, this is a landmark of the history in the development of microporous materials. Since the aluminophosphates were found, they have been received considerably interests in recent years because of the variety of structure types and framework composition as well as their applications in catalysis, adsorption, separation, and nonlinear optics. Notably, the templates for the formation of crystalline microporous aluminophosphates are normally organic ammonium or amines, and these templates are usually high-toxic and environmentally unfriendly, which severely hinders their practical production in a large scale for applications such as catalysis and separation. Therefore, the use of low-toxic and environmentally receivable templates for the synthesis of crystalline aluminophosphates are strongly desirable.Tetramethylguanidine is a kind of biologically and industrially important chemicals with relatively low-toxicity and low-cost, and many of them could be found in the products of animal metabolism. The main aim of this paper is to find new type of templates used for the synthesis of aluminophosphates materials. We have successfully synthesized AlPO-5 molecular sieve with mesopores using tetramethylguanidine as organic template. In addition, we also investigated the evolution from AlPO-5 to AlPO-21 molecular sieve synthesis templated by tetramethylguanidine.In this paper, we demonstrate a tetramethylguanidine-templated synthesis of crystalline microporous AlPO-5 for the first time. As a typical example of crystalline microporous aluminophosphates, AlPO-5 which belongs to AFI-type structure has been extensively studied due to its advantages such as excellent hydrothermal and thermal stabilities, large micropore (0.73 nm), and easy substitution of aluminum sites in the tetrahedral framework with a wide range of metal ions. The ordinary organic templates for synthesis of AlPO-5 are tetraethylammonium hydroxide (TEAOH), triethylamine (TEA), tetrapropylammonium hydroxide (TPAOH), tripropylamine (TPA), piperidine, dicyclohexylamine etc. Using tetramethylguanidine as template to synthesize crystalline microporous AlPO-5, it would be very helpful to reduce environmental pollution for the use of conventional organic amines in the synthesis of microporous aluminophosphate crystals. Additionally, the obtained product of AlPO-5 crystals with sphere-like morphology has an obvious mesoporosity, which is very favorable for mass transfer of reactants and products in catalysis and adsorption.Using tetramethylguanidine as template, we discussed the procedure and reason of the evolution from AlPO-5 to AlPO-21. The framework of AlPO-21 (AWO zeotype) consists of eight-membered ring (8-MR) channels along [001] direction, which is constructed by the alternation of Al units (including AlO4 and A1O5) and PO4 tetrahedra. Two AlO5 units are linked through an OH group. Protonated amine molecules as templates locate in the 8-MR channels and form H-bonds with the bridging oxygen atoms in the framework. Upon calcinations, AlPO-21 transforms to AlPO-25 (ATV zeotype) because of the removal of the organic species and the bridging OH groups. Using a series of testing instrument, we investigated the evolution from AlPO-5 to AlPO-21 molecular sieve synthesis templated by tetramethylguanidine. These results indicate that TMG is also a stable template for the formation of AWO structure. In the present work, we have examined the intermediate phases formed during hydrothermal synthesis of molecular sieve AlPO4-21 by using solid-state NMR techniques in combination with powder XRD and SEM. The results clearly show that the formation of AlPO4-21 includes at least three different stages: (1) upon mixing Al and P sources in the presence of the template, an amorphous aluminophosphate material was formed. (2) After the initial gel was treated at 150°C under hydrothermal conditions for about 50 min, the AlPO-5 crystals appeared. After 6 h, a pure phase of AlPO4-5 was obtained. (3) Treating the gel sample for more than 9 h resulted in the disappearance of AlPO4-5 and the formation of AlPO4-21. The crystallization of AlPO4-21 was complete after a total heating time of 24 h. The results seem to imply that AlPO4-5 is an intermediate, which directly transforms to AlPO4-21.The samples were characterized by X-ray diffraction, scanning electron microscopy, N2 adsorption-desorption, 13C NMR, NH3-TPD, fluorescence spectrum etc. According to the characterizations of the samples, the results indicate the morphology and character of the samples and display different potentials in various areas.