Synthesis and characterization of a novel microporous titanosilicate with a structure of penkvilksite-1M

Microporous and Mesoporous Materials 28 (1999) 511–517

Synthesis and characterization of a novel microporous titanosilicate with a structure of penkvilksite-1M Yunling Liu a, Hongbin Du a, Yihua Xu a, Hong Ding a, Wenqin Pang a,*, Yong Yue b a Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Department of Chemistry, Jilin University, Chang chun 130023, China b Wuhan Institute of Physics, The Chinese Academy of Science, Wuhan 430071, China Received 16 April 1998; accepted 25 October 1998

Abstract A novel titanosilicate which is a structural analogue of the mineral penkvilksite-1M has been synthesized in a SiO –TiO –Na O–H O–(TMA) O system. It is found that the penkvilksite-1M samples require a relatively low 2 2 2 2 2 alkalinity and appropriate titanium content. The penkvilksite-1M samples were characterized by X-ray diffraction, 29Si magic-angle spinning NMR, scanning electron microscopy, IR spectroscopy, differential thermal–thermogravimetric analysis, ion-exchange and adsorption measurements. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Synthesis; Titanosilicate; Penkvilksite-1M

1. Introduction Since the first synthesis of titanosilicate zeolite TS-1 [1] which has remarkable catalytic activity and selectivity in the aromatic hydroxylation, alkane oxidation and alkene epoxidation reactions when using dilute aqueous hydrogen peroxide as an oxidant, the synthesis of titanosilicate molecular sieves has attracted considerable research attention. So far, researchers have successfully synthesized and studied many titanium silicates such as TS-2 [2], Ti-beta [3] and Ti-MCM-41 [4]. Recently, two novel titanosilicates ETS-10 and ETS-4 [5,6 ] have been discovered. Anderson et al. have solved the structure of the microporous titanosilicate ETS-10 [7]: it has an interesting structure * Corresponding author. Fax: +86-431-894-9334.

and exhibits unique ion-exchange and adsorption properties. More recently, some new microporous titanosilicates have been synthesized. Among these are pharmacosiderite [8], GTS-1 [9], zorite [10] and UND-1 [11]. In recent years, we have been interested in the synthesis and characterization of titanosilicate materials with new structures and compositions. We have obtained several new materials, such as the novel layered titanosilicate JDF-L1 [12,13] which has potential application in catalysis, intercalation and ion exchange. Titanosilicate AM-1, which possesses the same structure as JDF-L1, was first reported by Anderson et al. [14]. During the synthesis of pure ETS-10, they obtained a co-crystallized phase AM-1 with ETS-10. Further study of AM-1 material and titanosilicates (AM-2, AM-3, AM-4) was reported by Lin et al. [15] .

1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 34 0 - 0

512

Y. Liu et al. / Microporous and Mesoporous Materials 28 (1999) 511–517

More recently, we synthesized a novel titanosilicate with the structure of the mineral penkvilksite [16 ]. There are many natural titanium silicates occurring in the Earth’s crust, but only a few such as pharmacosiderite [8] and zorite [10] have been synthesized. Penkvilksite was originally described by Bussen [17] et al. as a new titanosilicate mineral, with ideal chemical formula Na Ti Si O · 5H O 4 2 8 22 2 and space group indexed as orthorhombic or monoclinic. After the second polytype was found in the nature, the orthorhombic penkvilksite was denoted penkvilksite-2O and the monoclinic penkvilksite as penkvilksite-1M [18]. Both penkvilksite-2O and penkvilksite-1M have minor substitutions of Ca, Zr, Fe and Al for Na, Ti and Si, respectively. The structure consists of TiO 6 octahedra and SiO tetrahedra connected to each 4 other to form a three-dimensional framework. Recently, Lin et al. [15] have reported the synthesis and structural characterization of penkvilksite-2Otype titanosilicate which they named AM-3; they obtained the AM-3 phase from a Na O–K O– 2 2 SiO –TiO –H O system. 2 2 2 In this paper, we report the detailed synthesis of the penkvilksite-1M-type titanium silicate. The synthesis samples were characterized by X-ray diffraction ( XRD), 29Si magic-angle spinning (MAS) NMR, IR spectroscopy, scanning electron microscopy (SEM ), differential thermal–thermogravimetric analysis, ion-exchange and wateradsorption measurement.

lowed by fumed silica (0.6 g). The mixture was stirred until it became homogeneous, then transferred into a Teflon-lined stainless-steel autoclave, and heated at 473 K for about 20–30 days. The products were recovered by filtration, washed with water and dried at 353 K.

Powder XRD data were obtained using a Rigaku D/MAX-IIIA diffractometer with Cu Ka radiation (l=0.15418 nm). 29Si MAS NMR was recorded at 79.5 MHz on a Bruker MSL-400 spectrometer with a magnetic field strength of 9.7 T. A magic-angle spinning speed of 4 kHz was used for 29Si. The acquisition parameters adopted were as follows: pulse width, 4 ms; recycle delay, 4 s. The chemical shifts were relative to external standards of tetramethylsilane. A Hitachi X-650B scanning electron microscope was used for SEM experiments. Thermogravimetry and differential thermal analysis were performed on a PerkinElmer TGA 7 thermogravimetric analyzer and a DTA-1700 differential thermal analyzer, respectively, under a flow of N at a heating rate of 2 10 K min−1. IR spectra were recorded on a Nicolet 5DX FT IR spectrometer using the KBr pellet technique. Adsorption was measured isothermally on a Cahn-2000 electronic recording balance. Chemical analysis of the crystallized products was carried out with a Leeman ICP-AES instrument.

2. Experimental

3. Results and discussion

2.1. Synthesis of titanium silicates

3.1. Synthesis

Tetrabutyltitanate (98%), fumed silica (99.9%), tetramethylammomium hydroxide ( TMAOH, 25%) and sodium hydroxide (96%) were used as reagents. The chemical composition of the initial gel was 3.7SiO :5.9TiO :1.0Na O:100H O:0.6 2 2 2 2 ( TMA) O. In a typical preparation, the following 2 procedure was followed: sodium hydroxide (0.27 g) was first dissolved in distilled water (12 ml ) and then Ti(OC H ) (5.1 g) was slowly 4 94 added to the solution under vigorous stirring, to which aqueous TMAOH (2.5 ml )was added, fol-

3.1.1. Effect of the TiO /SiO ratio 2 2 The effect of TiO /SiO on the synthesis of 2 2 penkvilksite-1M products is shown in Fig. 1. We find that the titanium content is an important factor for the formation of penkvilksite-1M. An appropriate titanium content in the gel is found to be essential in the crystallization of the pure products (in our synthesis of the pure penkvilksite-1M sample the TiO /SiO ratio is 2 2 1.53). A higher (TiO /SiO >2.0; Fig. 1, curve d) 2 2 or lower ( TiO /SiO <0.24; Fig. 1, curve b) tita2 2

2.2. Characterization

513

Y. Liu et al. / Microporous and Mesoporous Materials 28 (1999) 511–517

Fig. 1. XRD patterns of the as-synthesized penkvilksite-1M showing the influence of TiO /SiO ratios on the crystallization. 2 2 Curves a–d correspond to TiO /SiO ratios of 0.06, 0.24, 1.53 2 2 and 2.0, respectively.

Fig. 2. XRD patterns of the as-synthesized penkvilksite-1M showing the influence of OH−/SiO ratios on the crystallization. 2 Curves a–d correspond to OH−/SiO ratios of 0.6, 1.0, 2.2 and 2 4.5, respectively.

nium content results in the formation of amorphous phases or quartz impurities. Table 1 lists the TiO /SiO molar ratios in the 2 2 reaction mixture and in the crystalline products. At low Ti concentration, the rates of nucleation and crystallization are rapid. With the increases in the titanium content in the reaction mixture, the rate of crystallization decreases, and the crystallization time increases from 8 to 35 days. These results are consistent with those reported for the titanium silicates TS-1 and TS-2. During their crystallization, the increase in titanium content prolongs their crystallization time.

3.1.2. Effect of the OH−/SiO ratio 2 Fig. 2 and Table 2 show the influence of the OH−/SiO molar ratio in the original reaction 2 mixture on the crystallization of penkvilksite-1M. We see that the OH−/SiO ratio plays an important 2 role in the formation of penkvilksite-1M. As shown in Table 2, we can obtain pure penkvilksite-1M samples at a lower OH−/SiO ratio. As the concen2 tration of sodium hydroxide increases, a zorite impurity phase is crystallized with the penkvilksite-1M (see Fig. 2). In another study we synthesized the zorite-type titanosilicate with TMAOH as a template at a high OH−/SiO ratio 2

Table 1 Influence of TiO /SiO ratio on the crystallization of penkvilksite-1Ma 2 2 TiO /SiO in gel 2 2

TiO /SiO in the product 2 2

Time (days)

Crystal phase

0.06 0.24 1.53

0.018 0.033 0.181

8 17 35

penkvilksite-1M +quartz penkvilksite-1M +quartz penkvilksite-1M

a 3.7SiO :2.0NaOH:100H O:1.2TMAOH; T=473 K. 2 2

514

Y. Liu et al. / Microporous and Mesoporous Materials 28 (1999) 511–517

Table 2 Influence of the OH−/SiO ratio on the crystallization of 2 penkvilksite-1Ma OH−/SiO

Product

2

0.4 0.6 1.0 2.2 4.6

penkvilksite-1M penkvilksite-1M penkvilksite-1M+zorite penkvilksite-1M+amorphous penkvilksite-1M+unknown

K+ or Na++K+) systems, depending on the content of alkali metal ions in the gel. In the presence of K+, ETS-10 was readily obtained, while with Na+ ions alone in the gel, penkvilksite-1M and ETS-4 were synthesized. The former was favored at low Na+ concentration and the latter at high Na+ concentration. 3.2. Characterization

a 3.7SiO :5.9TiO :100H O:1.2TMAOH; T=473 K; 2 2 2

[10]. On further increasing OH−/SiO ratio, e.g. 2 when OH−/SiO >2.2, the gel will form an amor2 phous phase or an unknown phase with a small amount of penkvilksite-1M. In summary, we can prepare pure penkvilksite-1M samples at a low O H−/SiO =0.2–0.6. 2 3.1.3. Effect of the alkali metal cations In the synthesis of titanosilicate penkvilksite-1M, we found that alkali metal ions have an important effect on its formation. In fact, several titanosilicates were produced in SiO –TiO –M O–H O–( TMA) O (M=Na+ or 2 2 2 2 2

3.2.1. XRD analysis The XRD pattern of the as-synthesized penkvilksite-1M is listed in Table 3. The index of powder patterns was carried out with the program powd. 12. The indexed unit-cell parameters ˚, for penkvilksite-1M are a=8.9492(6) A ˚ , c=7.3904(5) A ˚ , a=90.00°, b= b=8.7282(9) A 112.83°, c=90.00°; monoclinic system. For the mineral penkvilksite-1M (space group P2 /c) ˚ , b=8.727(3) A ˚ , c=7.387(3) A ˚ , 1b= a=8.956(4) A 112.74(3)° [18]. Hence the synthesized product pattern is indexed satisfactorily with these parameters. This indicates that the product is the analogue of the mineral penkvilksite-1M.

Table 3 Powder XRD pattern of the as-synthesized penkvilksite-1M h

k

l

˚) d (A obs

˚) d (A calc

I/I o

h

k

l

˚) d (A obs

˚) d (A calc

I/I

1 1 −1 0 2 1 −1 −2 1 2 2 1 −1 −2 −1 0 4 4 3 1 −4

0 1 1 2 0 1 0 0 2 2 1 3 3 3 1 4 0 1 2 4 1

0 0 1 0 0 1 2 2 1 0 1 0 1 1 3 0 0 0 1 1 3

8.2849 6.0121 5.3170 4.3700 4.1235 3.9816 3.7107 3.3434 3.1211 2.9934 2.8755 2.7374 2.6702 2.4199 2.3536 2.1853 2.0618 2.0061 2.0061 1.9588 1.9001

8.2480 5.9948 5.3025 4.3641 4.1240 3.9780 3.6939 3.3378 3.1224 2.9974 2.8735 2.7437 2.6671 2.4192 2.3546 2.1821 2.0620 2.0068 2.0089 1.9608 1.9016

100 1.1 1.2 0.2 19.8 1.3 0.4 1.5 1.0 0.0 3.4 1.4 1.6 0.3 0.1 0.9 0.6 1.2 1.2 0.1 0.1

4 2 −5 −2 1 −4 3 4 −6 0 4 −6 6 −4 6 −5 5 3 −7 −6 −3

2 3 1 2 3 3 4 0 0 6 2 2 0 5 1 4 3 6 1 4 6

0 2 1 4 3 3 1 2 2 0 2 2 0 1 0 3 1 0 3 2 3

1.8646 1.7725 1.7328 1.7024 1.6473 1.6191 1.5711 1.5205 1.4929 1.4550 1.4376 1.4115 1.3744 1.3744 1.3578 1.3306 1.3177 1.2857 1.2521 1.2317 1.2173

1.8644 1.7722 1.7327 1.7009 1.6478 1.6189 1.5707 1.5216 1.4913 1.4547 1.4368 1.4112 1.3747 1.3741 1.3579 1.3512 1.3173 1.2858 1.2527 1.2312 1.2176

0.3 0.6 0.5 0.4 0.5 0.3 0.1 0.7 0.2 0.1 0.1 0.1 1.6 1.6 0.3 0.2 0.1 0.2 0.2 0.2 0.1

o

515

Y. Liu et al. / Microporous and Mesoporous Materials 28 (1999) 511–517

3.2.2. NMR results The 29Si MAS NMR spectra of as-synthesized penkvilksite-1M is shown in Fig. 3. The product gives two resonance at −95.7, and −101.1 ppm. In the earlier work we observed three signals with chemical shifts d of −85.2, −96.0 and −101.6 ppm, and we mistakenly assigned the peaks at −101.6 to the impurities phase [16 ]. In fact, penkvilksite-1M contains two Si sites of the type Si (2Si, 2Ti) and Si (3Si, 1Ti). We assign the resonances at −95.7 and −101.7 ppm to the Si (2Si, 2Ti) and Si (3Si, 1Ti) environments, respectively. This result is similar to that in the Lin’s report about penkvilksite-2O [15]. We know that, despite the different space group symmetries, penkvilksite-1M and penkvilksite-2O have the same atoms, labeled in the same way, in the asymmetric unit. They differ only in the stacking of the same building blocks [18].

Fig. 4. Scanning penkvilksite-1M.

electron

micrograph

of as-synthesized

3.2.3. SEM A scanning electron micrograph (Fig. 4) shows that the crystals appear as thin plate with diameters of ca. 20×50 mm. 3.2.4. IR spectroscopy Fig. 5 shows the IR spectrum of the penkvilksite-1M in the region 1400–400 cm−1. The

Fig. 5. IR spectra of as-synthesized penkvilksite-1M.

Fig. 3. 29SiMAS NMR spectra of penkvilksite-1M

absorption bands are at 1147, 1066, 980, 930, 897, 789, 739, 683, 586, 557, 463 and 432 cm−1. According to the framework vibration models of Flanigen et al., the IR absorption spectrum of penkvilksite-1M can be assigned as follows: 1147, 1066, 980, and 789, 739, 683 cm−1 are attributable respectively to the asymmetric and symmetric stretching vibrations of the framework TO poly4 hedra, while 586, 557, 463, 432 cm−1 may be due to the vibrations of the T-O bending modes.

516

Y. Liu et al. / Microporous and Mesoporous Materials 28 (1999) 511–517

3.2.5. Thermal properties Thermogravimetric analysis (Fig. 6, curve a) and differential thermal analysis ( Fig. 6, curve b) for as-synthesized penkvilksite-1M indicates that there is a weight loss of 8.4% between 87 and 245°C. Elemental analysis shows that the as-synthesized penkvilksite-1M occludes no organic molecules, so this weight loss is associated with the removal of water located on the external surface and in the pores. In addition, the mass loss (8.4%) is consistent with the calculated value (8.6%) of four H O molecules per formula of the compound 2 Na Ti Si O · 4H O [18]. 4 2 8 22 2 The thermal stability of the products treated at different temperatures presented in Fig. 7 show that below 873 K the structure of penkvilksite-1M

Fig. 6. TG–DTA curves for penkvilksite-1M.

Fig. 8. H O adsorption isotherm for penkvilksite-1M at 293 K. 2

remains. However, it collapses and converts into a dense phase at 973 K. The results show that the as-synthesized penkvilksite-1M has a fine thermal stability. 3.2.6. Ion exchange property The ion-exchange behavior of penkvilksite-1M was measured in 1 M KNO solution at 80°C with 3 stirring for 24 h. It was found that 18% of the potassium ions could be exchanged into the penkvilksite-1M. The ion-exchange capacity of K+ is 0.72 mequiv g−1, which is lower than that of other minerals. This may be due to the short reaction time employed or because the sodium ions are located in the framework of the penkvilksite-1M. 3.2.7. Isothermal adsorption of water The water-adsorption isotherm at 293 K for penkvilksite-1M is show in Fig. 8. Prior to the measurement the sample was dehydrated at 473 K and 10−3 Torr for 2 h and then cooled to room temperature under vacuum. A Brunauer– Emmett–Teller adsorption experiment shows type I adsorption behavior and indicates that the adsorption capacity of water is 6.0 wt% at P/P =0.2. 0 4. Conclusions

Fig. 7. XRD patterns of penkvilksite-1M as synthesized (curve a) and calcined for 2 h at 573 (curve b), 673 (curve c) 773 (curve d) 873 (curve e) and 973 K (curve f ).

In conclusion, a titanosilicate penkvilksite-1M analogue was synthesized under hydrothermal

Y. Liu et al. / Microporous and Mesoporous Materials 28 (1999) 511–517

conditions. The TiO /SiO and OH−/SiO ratios 2 2 2 play important roles in the formation of penkvilksite-1M. The as-synthesized penkvilksite1M samples can adsorb water and have an ionexchange capacity. The investigation of thermal properties shows that synthesized penkvilksite-1M has a fine thermal stability. The successful synthesis of penkvilksite-1M provides possibilities for the synthesis of other new types of titanium silicate materials by varying various synthesis factors such as alkali metal ions, TiO /SiO , OH−/SiO ratios 2 2 2 and organic templates.

Acknowledgements The authors acknowledge the financial support of the National Nature Science Foundation of China and the Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University.

References [1] M. Taramasso, G. Perego, B. Notri, US Patent 4 410 501, 1983.

517

[2] J.S. Reddy, R. Kumar, P. Ratnasany, Appl. Catal. 58 (1990) L1. [3] M.A. Camblor, A. Corma, J. Perez-Paariente, Zeolites 13 (1993) 82. [4] T. Blasco, A. Corma, M. Tnavarro, J. Perez-Pariente, J. Catal. 156 (1995) 65. [5] S.M. Kuznicki, US Patent 4 863 202, 1989. [6 ] S.M. Kuzinicki, US Patent 4 938 939, 1990. [7] M.W. Anderson, O. Terasaki, T. Ohsuna, A. Phillippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Nature 367 (1994) 347. [8] W.T.A. Harrison, T.E. Gier, G.D. Stucky, Zeolites 15 (1995) 408. [9] D.M. Chapman, A.L. Roe, Zeolites 10 (1990) 730. [10] H. Du, F. Zhou, W. Pang, Y. Yue, Microporous Mater. 7 (1996) 73. [11] X. Liu, M. Shang, J.K. Thomas, Microporous Mater. 10 (1997) 273. [12] H. Du, M. Fang, J. Chen, W. Pang, J. Mater. Chem. 6 (1996) 1827. [13] M.A. Roberts, G. Saanker, J.M. Thomas, R.H. Jones, H. Du, J. Chen, W. Pang, R. Xu, Nature 381 (1996) 401. [14] M.W. Anderson, O. Terasaki, T. Ohsuna, P.J.O. Malley, A. Phillippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Philos. Mag. B 71 (1995) 813. [15] Z. Lin, J. Rocha, P. Brandao, A. Ferreira, A.P. Esculcas, J.D. Pedrosa de Jesus, A. Philippou, M.W. Anderson, J. Phys. Chem. B 101 (1997) 714. [16 ] Y. Liu, H. Du, F. Zhou, W. Pang, J. Chem. Soc., Chem. Commun., (1997) 1467. [17] I.V. Bussen, Yu.P. Men’shikov, A.A.N. Mer’ov, A.P. Nedorezova, Yu.I. Uspenksay, A.P. Khomyakov, Dokl. Earth Sci. Sect. 217 (1975) 126. [18] S. Merlino, M. Pasero, G. Artioli, A.P. Khomyakov, Am. Miner. 79 (1994) 1185.