CHEM. RES. CHINESE UNIVERSITIES 2009, 25(4),

Transcription

1 CHEM. RES. CHINESE UNIVERSITIES 2009, 25(4), Synthesis, Characterization and Phase Transformation of 3D Open-framework Zinc Phosphate [Zn 6 (H 2 O)P 5 O 20 ][C 5 N 2 H 14 ][C 6 H 16 N 2 ] 0.5 3H 2 O SONG Yu 1,2, DONG Zhao-jun 1, WANG Yong-wei 2, FU Jun 3* and LI Ji-yang 1* 1. State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun , P. R. China; 2. School of Chemistry Engineering & Material, Dalian Institute of Light Industry, Dalian , P. R. China; 3. Geriatric Ward, First Hospital of Jilin University, Changchun , P. R. China Abstract With N,N -dimethylpiperazine(dmpip) as the structure-directing agent, a zinc phosphate [Zn 6 (H 2 O) P 5 O 20 ][C 5 N 2 H 14 ][C 6 H 16 N 2 ] 0.5 3H 2 O(1) with a novel three-dimensional(3d) open-framework architecture was hydrothermally synthesized. Its structure was determined by single-crystal X-ray diffraction analysis and further characterized by ICP, NMR, and TG analyses. Compound 1 crystallized in the triclinic space group P1 (No.2) with a=0.9984(2) nm, b=1.2354(3) nm, c=1.2384(3) nm, α=88.32(3), β=74.57(3), γ=75.81(3) and Z=2. The alternation arrrangement of tetrahedral Zn units[zno 4 and ZnO 3 (H 2 O)] and PO 4 units forms an anionic 3D open framework containing intersecting 16-, 10- and 8-ring channels. Water molecules, diprotonated DMPIP, and diprotonated N-methylpiperazine(MPIP) decomposed from DMPIP, are located in the channels and interact with the host framework via H-bonds. Compound 1 transforms to another new phase(compound 2) upon calcination at C with the removal of the water coordinated to Zn atoms in the lattice. The phase transformation of compound 2 was further studied. Keywords Hydrothermal; Zinc phosphate; Phase transformation Article ID (2009) Introduction Since the pioneering work of Wilson et al. [1] in 1982 on the preparation of aluminophosphate molecular sieves, open-framework metal phosphates have received much attention in recent years because of their great importance in many fields of today applied chemistry, such as catalysis, adsorption, optics, electronics and magnetics. In the last years, more than 120 new zinc phosphate structures were discovered [2 20]. Notable examples are ND-1 with 24 ring channels [13], and [NH 3 (CH 2 ) 2 NH 2 (CH 2 ) 2 NH 3 ][Zn(PO 4 ) 3 (HPO 4 )] with intersecting helical channels [14]. These materials display one-, two-, or threedimensional structures which are built mainly from ZnO 4 and PO 4 tetrahedra, and more rarely by ZnO 6 and ZnO 5 units. These materials are generally prepared under hydrothermal or solvothermal conditions in the presence of an organic amine as a structure-directing agent. The structure variety is greatly manipulated by the types of the template as well as the synthesis conditions. Interestingly, the same organic amine can direct different structures, and the different organic amines can direct the same structure. We preliminarily studied a novel zinc phosphate [Zn 6 (H 2 O)P 5 O 20 ][C 5 N 2 H 14 ][C 6 H 16 N 2 ] 0.5 3H 2 O(1) with large 16-ring channels prepared by hydrothermal combinatorial approach [21]. Here lies the synthesis, characterization and phase transformation of compound 1 prepared by conventional hydrothermal technique. 2 Experimental 2.1 Synthesis and Characterization Compound 1 was synthesized by a hydrothermal reaction of a mixture of Zn(OAc) 2 2H 2 O, H 3 PO 4, N,N -dimethylpiperazine(dmpip)(98.5%, Acros Co.), and H 2 O at a molar ratio of 1.0:3.0:5.0:227. Typically, 0.5 g of Zn(OAc) 2 2H 2 O was first dispersed in 10 ml *Corresponding author. fjjilin@yahoo.com.cn; lijiyang@jlu.edu.cn Received December 12, 2008; accepted March 9, Supported by the National Natural Science Foundation of China(No ) and the Scientific and Technological Planning Project of Jilin Province, China(No ).

2 408 CHEM. RES. CHINESE UNIVERSITIES Vol.25 of distilled water by stirring, followed by addition of 0.46 ml of H 3 PO 4 (85%, mass fraction). Finally, 1.55 ml of DMPIP was added to it. The mixture was stirring until it was homogeneous. The reaction mixture with a ph value of ca. 6.0 was sealed in a Teflon-lined stainless steel autoclave and then heated at 180 C for 4 d. The resulting product consisting of large plate-like single crystals was filtered, washed with distilled water and then air-dried. The X-ray diffraction(xrd) patterns were recorded on a Siemens D5005 diffractometer with Cu Kα radiation(λ= nm). Inductively coupled plasma(icp) analysis was performed on a Perkin-Elmer Optima 3300DV spectrometer. Elemental analysis was conducted on a Perkin-Elmer 2400 elemental analyzer. A NETZSCH STA 449C unit was used to carry out the thermogravimetric analysis(tga) in air at a heating rate of 20 C/min. The magicangle-spinning(mas) NMR spectra were recorded on a Varian Unity-400 Spectrometer. The 31 P MNR spectra were taken at MHz at a spinning rate of 7 khz. Chemical shifts were referenced to 85%(mass fraction) H 3 PO 4 for 31 P. A total of 4000 scans were taken for 31 P. The recycle delay time for 31 P was 1.0 s. The 13 C CP NMR spectra were taken at MHz at a spinning rate of 4 khz. Chemical shifts were referenced to HMB for 13 C. 2.2 Crystallography A suitable single crystal with dimensions 0.36 mm 0.25 mm 0.24 mm was carefully selected under a polarizing microscope and glued to a thin glass fiber. The intensity data were collected on a Siemens diffractometer fitted with a Bruker SMART CCD detector under graphite-monochromated Mo Kα radiation (λ= nm). Data processing was accomplished with the SAINT processing program. The structure was solved by the direct method and refined on F 2 by full-matrix least squares using SHELXTL97 [22]. Zinc and phosphorus atoms were first located, and oxygen, carbon and nitrogen atoms were found in Fourier difference maps. The diprotonation of both DMPIP and MPIP is suggested on the basis of charge balance. The hydrogen atoms residing in the amine molecules were located geometrically. All non-hydrogen atoms were refined with anisotropic thermal parameters. The details of data collection and refinement are given in Table 1. Table 1 Crystal data and structure refinement parameters for compound 1 Empirical formula C 8 H 30 N 3 O 24 P 5 Zn 6 Formula weight Temperature/K 293(2) Wavelength/nm Crystal system Triclinic Space group P1 a/nm (2) b/nm (3) c/nm (3) a/(º) 88.32(3) β/(º) 74.57(3) γ/(º) 75.81(3) Volume/nm (5) Z, calculated density/(mg m 3 ) 2, Absorption coefficient/mm F(000) 1092 Crystal size 0.36 mm 0.25 mm 0.24 mm θ range for data collection 1.65º to 27.45º Limiting indices 12 h 12, 15 k 16, 16 l 16 Reflections collected/unique 11032/6712[R(int) = ] Completeness to θ=23.36º 99.6% Refinement method Full-matrix least-squares on F 2 Data/restraints/parameters 6712/0/415 Goodness-of-fit on F Final R indices[i>2σ(i)] * R 1 =0.0333, wr 2 = R indices(all data) * R 1 =0.0508, wr 2 = Largest diff. peak and hole/ and (e nm 3 ) * R 1 = [ΔF/ (F o )]; wr 2 =( [w(f 2 o -F 2 c )])/ [w(f 2 o ) 2 ] 1/2, w=1/σ 2 (F 2 o ). 3 Results and Discussion Fig.1 shows the XRD patterns of compound 1, the experimental pattern is in agreement with the simulated one generated based on single-crystal structural data, indicating the phase purity of the product. Inductively coupled plasma(icp) analysis shows that compound 1 contains 33.70% and 13.80% of Zn and P, respectively(calcd % and 14.10%, respectively). The elemental analysis shows the C, H, and N contents(%) of 9.37, 2.80 and 3.87(calcd. 8.73, 2.70 and 3.82), respectively. Fig.1 Experimental and simulated XRD patterns of compound 1

3 No.4 SONG Yu et al. 409 Compound 1 crystallized in the triclinic space group P1 (No.2) with a=0.9984(2) nm, b=1.2354(3) nm, c=1.2384(3) nm, α=88.32(3), β=74.57(3), γ=75.81(3) and Z=2. As shown in Fig.2, each asymmetric unit contains 47 non-hydrogen atoms, of which 32 belong to the framework and 15 to the guest. There are six crystallographically independent zinc atoms and five crystallographically independent phosphorus atoms. All Zn atoms are tetrahedrally coordinated by oxygen atoms. Zn1 and Zn4 each makes three Zn O P bonds and one Zn O Zn bond; Zn3 and Zn6 each makes four Zn O P bonds; Zn2 makes two Zn O P bonds, one Zn O Zn bond and one terminal Zn OH 2 bond, and Zn5 makes two Zn O P bonds and two Zn O Zn bonds. The Zn O Zn bond is formed through a µ 3 -O oxygen atom. It is noted that ZnO 3 (H 2 O) coordination exists in compound 1. Similar case has been found in zinc phosphate Zn 4 (PO 4 ) 3 (H 2 O)(C 2 H 5 NH 3 ) [23]. The Zn O bond distances in Zn O P linkages are in a range of (3) (3) nm, which are typical for those Fig.2 Asymmetric unit of compound 1 with thermal ellipsoids at 50% probability in known zinc phosphates [2 21]. The Zn O bonds in Zn O Zn and Zn OH 2 linkages have larger bond lengths, which are in a range of (3) (3) nm. All P atoms share four oxygen atoms with adjacent Zn atoms. The P O bond distances are in the range of (3) (3) nm. 31 P NMR spectrum, as shown in Fig.3(A), shows four signals at δ 8.9, 6.2, 5.1 and 4.4, respectively, which are the characteristic of the tetrahedrally coordinated P atoms. Fig.3 31 P MAS NMR spectra(a) and 13 C CP NMR spectra(b) of compounds 1(a) and 2(b) The alternation rearrangement of tetrahedral Zn units[zno 4 and ZnO 3 (H 2 O)] and PO 4 units forms an open-framework with the macroanion of [Zn 6 (H 2 O)P 5 O 20 ] 3. The charge balance is achieved by diprotonated N,N -dimethylpiperazine(dmpip) and N-methylpiperazine(MPIP). MPIP might be decomposed from DMPIP during the hydrothermal synthesis. The decomposition or fragmentation of the organic amines have been found in (Al 2 P 4 O 16 )[NH 3 (CH 2 ) 3 NH 3 ] 2 [NH 3 (CH 2 ) 2 NH 3 ] [24]. The open framework of compound 1 consists of intersecting 16-, 10- and 8-ring channels along the [100][Fig.4(A)], [010] [Fig.4(B)] and [001] directions[fig.4(c)], respectively. Fig.4 Polyhedral views of the framework structure of compound 1 showing intersecting 16-, 10- and 8-ring channels along the [100](A), [010](B) and [001](C) directions, respectively Each 16-ring channel accommodates two diprotonated DMPIP and six water molecules. The diprotonated MPIP molecules reside in the 10-ring channels. The 13 C NMR spectrum of compound 1 is shown in Fig.3(B). It displays four peaks at δ 51.8, 49.0, 43.8, and 38.3, which supports the existence of diprotonated DMPIP and MPIP in the product. Each N atom of DMPIP and MPIP forms one strong H-bond to the framework oxygen atom. The N O distances are in a range of nm. Fig.5(A) shows the TG-DTA curves of compound 1 in air in a temperature range of C. It

4 410 CHEM. RES. CHINESE UNIVERSITIES Vol.25 shows four stages of mass loss occurring at C, C, C, and C, respectively. The first two mass losses in a total of 6.08%(mass fraction) correspond to the loss of the water molecules in the channels and the coordinated water molecules to the Zn atom in the lattice(calcd. 6.56%). The third one of 10.02% corresponds to the decomposition of MPIP(calcd. 9.30%), while the last one of 5.72%(mass fraction) corresponds to the decomposition of DMPIP(calcd. 5.29%). The DTA curve shows two strong exothermic peaks at and C, respectively, corresponding to the decomposition of MPIP and DMPIP, respectively. The thermal analysis for compound 1 was also performed in nitrogen. In contrast to the results in air, only one stage of mass loss of 14.62% is observed at C, which is attributed to the desorption of both MPIP and DMPIP(calcd %). Accordingly only one endothermic peak at C is observed. Fig.6 XRD patterns of compound 1 calcined at 150(a), 200(b), 300(c) and 350 C(d) C. Furthermore, the TG curve of compound 2 shows no mass loss before 400 C, indicating no water molecules exist in the product[fig.5(b)]. Therefore, it is believed that the loss of the coordinating water molecules to the Zn atoms in the lattice is responsible for the phase transform. Compositional analyses for compound 2 give that the contents(%) of Zn, P, C, H and N are 36.9, 15.6, 9.8, 2.33 and 4.01, respectively. The results are consistent with the expected empirical formula of [Zn 6 (H 2 O)P 5 O 20 ] [C 5 N 2 H 14 ][C 6 H 16 N 2 ] 0.5 based on the formula of compound 1 upon the removal of the water molecules. The TG curve of compound 2 shows two stages of mass loss occurring at C and C, respectively. The first mass loss of 10.20% corresponds to the decomposition of MPIP(calcd. 9.70%), and the second one of 5.13% corresponds to the decomposition of DMPIP(calcd. 5.51%). The 31 P and 13 C NMR spectra of compound 2 are shown in Fig P NMR spectrum shows four similar signals to those of compound 1, indicating the tetrahedral coordination environment of P atoms has little change during the phase transformation from compound 1 to compound 2. The 13 C NMR spectrum also shows four similar signals to those of compound 1, confirming the existence of both DMPIP and MPIP in the structure of compound 2. Fig.5 TG-DTA curves of compounds 1(A) and 2(B) in air The thermal behavior of compound 1 is further studied by XRD analysis for samples calcined at various temperatures(fig.6). It is found that compound 1 transformed to a new phase, denoted as compound 2, upon calcination at C. No phase transform occurred upon calcination before 200 C, whereas above 400 C an amorphous phase was formed because of the decomposition of the template molecules in the product. Based on TG analysis the coordinated water molecules to the Zn atoms are lost in a range of 4 Conclusions With N,N -dimethylpiperazine(dmpip) as the structure-directing agent, zinc phosphate [Zn 6 (H 2 O) P 5 O 20 ][C 5 N 2 H 14 ][C 6 H 16 N 2 ] 0.5 3H 2 O(1) with novel 3D open-framework architecture has been hydrothermally prepared. It consists of intersecting 16-, 10- and 8-ring channels along [100], [010] and [001] directions, respectively. The structure is made up of tetrahedral Zn units[zno 4 and ZnO 3 (H 2 O)] and PO 3 4 units. It is found that compound 1 transforms to a new phase

5 No.4 SONG Yu et al. 411 (compound 2) upon the removal of the coordinated water molecules to zinc atoms in the lattice via calcination. To complete the coordination sphere of these zinc atoms, the zinc atoms have to link to other adjacent atoms to form a new bond. This might be the cause of the phase transformation. Further structural analysis of compound 2 is under the way. References [1] Wilson S. T., Lok B. M., Messina C. A. et al., J. Am. Chem. Soc., 1982, 104, 1146 [2] Natarajan S., Chem. Commun., 2002, 780 [3] Ng H. Y., Harrison W. T. A., Microporous Mesoporous Mater., 2001, 50, 187 [4] Yu J. H., Wang Y., Shi Z., et al., Chem. Mater., 2001, 13, 2972 [5] Liu W., Liu Y. L., Shi Z., et al., J. Mater. Chem., 2000, 10, 1451 [6] Choudhruy A., Natarajan S., Rao C. N. R., Inorg. Chem., 2000, 39, 4295 [7] Harmon S. B., Sevov S. C., Chem. Mater., 1998, 10, 3020 [8] Gier T. E., Bu X. H., Feng P., et al., Nature, 1998, 395, 154 [9] Harrison W. T. A., Phillips M. L., Chem. Commun., 1996, 2771 [10] Harrison W. T. A., Gier T. E., Stucky G. D., et al., Chem. Mater., 1996, 8, 145 [11] Alberti G., Bartocci M., Santarelli M. et al., Chem. Commmun., 1996, 2771 [12] Harrison W. T. A., Phillips M. L. F., Angew. Chem., Int. Ed., 1995, 34, 1745 [13] Yang G. Y., Sevov S. C., J. Am. Chem. Soc., 1999, 121, 8389 [14] Neeraj S., Natarajan S., Rao C. N. R., Chem. Commun., 1999, 165 [15] Neeraj S., Natarajan S., Rao C. N. R., Chem. Mater., 1999, 11, 1390 [16] Neeraj S., Natarajan S., Rao C. N. R., New J. Mater., 1999, 23, 303 [17] Neeraj S., Natarajan S., Rao C. N. R., J. Solid State Chem., 2000, 150, 417 [18] Ayyappan S., Bu X. H., Cheetham A. K., et al., Chem. Commun., 1998, 2181 [19] Ayyappan S., Cheetham A. K., Natarajan S., et al., Chem. Mater., 1998, 10, 3764 [20] Natarajan S., Vaidhanathan R., Rao C. N. R., et al., Chem. Mater., 1999, 11, 1633 [21] Song Y., Yu J. H., Li Y., et al., Chem. Commun., 2002, 1720 [22] Sheldrick G. M., SHELXL Program, Version 5.1, Siemens Industrial Automation, Inc., Madison, 1997 [23] Song T., Hursthouse M. B., Chen J. S., et al., Adv. Mater., 1994, 6, 679 [24] Sugiyama K., Hiraga K., Yu J. H., et al., Acta Cryst. Sect. C, 1999, 55, 1615