Ionothermal Synthesis of an Aluminophosphate Molecular Sieve with 20‐Ring Pore Openings

Abstract

Crystalline porous materials with large or extralarge pores continue to be of particular significance in both industry and academia for their potential applications in shape-selective catalysis and adsorption/separation. Of these zeolitic materials, especially aluminosilicateand aluminophosphate-based molecular sieves are of prime interest because of their high stability associated with their widespread use in many established process and emerging applications. The materials VPI-5 (VFI framework type, 18-ring) and UTD-1 (DON framework type, 14-ring) were the first extra-large pore (pores constructed of more than 12 Tatoms) aluminophosphate and aluminosilicate materials discovered. The oxide frameworks are built up by corner-sharing [AlO4] and [PO4] tetrahedra as well as [AlO4] and [SiO4] tetrahedra. In the search for materials with even larger pores, an anionic open-framework aluminophosphate JDF-20 (20-ring) was reported; however, it could not be classified as a zeolite because its framework (with an Al/P ratio of 5:6) is unstable upon removal of the occluded protonated templates by calcination. Larger pore openings were also achieved using Ge or Ga as the framework Tatom in a high amount, for example in ECR-34 (ETR framework type, 18-ring), ITQ-33 (18-ring), cloverite (-CLO framework type, 20ring), and ITQ-37 (30-ring). In this context, the use of Ge or Ga as framework atoms as well as fluoride has been found to facilitate the formation of a double four-ring (D4R) unit. This is in agreement with the prediction by Brunner and Meier that structures with extra-large pores should contain a large number of threeand four-membered rings. Ionothermal synthesis, in which ionic liquids act as both the solvent and template, is a novel method that has attracted great interest in the synthesis of zeolitic and other porous materials. Besides the advantage of experimenting at ambient pressure, ionic liquids offer different chemistry and structural variety associated with the use of additional amines as structure-directing agents (SDA), and therefore open up new vistas for the synthesis of new porous materials. Herein, we report the ionothermal synthesis of the first aluminophosphate molecular sieve with 20-ring pore openings, denoted as DNL-1 (Dalian National Laboratory Number 1). This molecular sieve was confirmed as a structural analogue to the gallophosphate molecular sieve cloverite by using a combination of Rietveld refinement of powder X-ray diffraction (PXRD) data and NMR analysis. Moreover, in comparison to cloverite, DNL-1, as-synthesized and calcined, exhibits excellent stability. DNL-1 was synthesized in the ionic liquid 1-ethyl-3methylimidazolate bromide ([emim]Br) with 1,6-hexanediamine (HDA) as the co-SDA. The detailed synthetic procedure is described in the Experimental Section. The assynthesized DNL-1 material displays uniformly globular agglomerates of grainlike nanocrystals with a diameter of about 20 mm (see the Supporting Information). Analysis by energy dispersive X-ray spectroscopy (EDX) indicates the P/ Al/F molar ratio of approximately 3:3:1. The inductively coupled plasma (ICP) analysis gives the content (wt%) of Al 16.50 and P 16.65. The elemental and thermogravimetric (TG) analyses show the content (wt%) of C 9.72, N 3.64, H 3.29, and a total weight loss of 34%. Combined with the results of the structure refinement (see below), the chemical formula of DNL-1 was determined as j (C6N2H18)104(C6N2H11)80(H2O)910 j [Al768P768O2976(OH)192F288]. Using the initial structure model from cloverite, the Rietveld refinement of as-synthesized DNL-1 was successfully performed in space group Fm 3c with refined unit cell parameter a= 51.363(1) , which is comparable to that of cloverite a= 51.713 , considering the smaller ionic radius of Al. Similar results were observed in the all-silica and Gecontaining polymorph C of zeolite Beta. Figure 1 shows the very good agreement between observed and calculated PXRDpatterns, taking into account the limited signal to noise ratio, in particular for the data collected at a high angle which can be reflected from the expected R factor of 14.5%. These results adequately confirm that DNL-1 is a pure aluminophosphate analogue of the -CLO structure. The skeletal model of the refined framework structure is shown in Figure 2. The framework of DNL-1 shows the general features of the -CLO structure: 1) two nonintersecting three-dimensional channel systems with 20-ring and 8-ring windows, respectively, 2) four terminal hydroxy groups (Al [*] Y. Wei, Prof. Z. Tian, R. Xu, Dr. H. Ma, R. Pei, Prof. W. Zhang, Prof. Y. Xu, Dr. L. Wang, K. Li, Dr. B. Wang, G. Wen, Prof. L. Lin State Key Laboratory of Catalysis Dalian National Laboratory for Clean Energy Dalian Institute of Chemical Physics Chinese Academy of Sciences, Dalian 116023 (China) Fax: (+86)411-843-79151 E-mail: tianz@dicp.ac.cn