- Email: [email protected]
Surfactant assisted organization of an exfoliated vanadyl ortho phosphate to a mesostructured lamellar vanadium phosphate phase
Microporous and Mesoporous Materials 67 (2004) 229–234 www.elsevier.com/locate/micromeso
Surfactant assisted organization of an exfoliated vanadyl ortho phosphate to a mesostructured lamellar vanadium phosphate phase Soumen Dasgupta, Monika Agarwal, Arunabha Datta
*
Indian Institute of Petroleum, Dehra Dun 248 005, India Received 5 July 2003; received in revised form 7 November 2003; accepted 9 November 2003
Abstract The catalytically important vanadyl hydrogen phosphate hemihydrate, VOHPO4 Æ 0.5H2 O phase with strong interlayer bonding has been exfoliated for the first time in DMF-water mixture and the exfoliated hemihydrate phase has been reorganized into a novel mesostructured lamellar VPO phase having the same V–P–O connectivity as in VOHPO4 Æ 0.5H2 O, using octadecyl amine as the structure directing template. Ó 2003 Published by Elsevier Inc. Keywords: Vanadyl orthophosphate; Lamellar phase; Mesostructured; Surfactant assisted organization
1. Introduction Supramolecular assembly of surfactants and soluble V–P–O precursors has recently been used for the synthesis of mesostructured vanadium phosphate (VPO) phases. These materials are of interest because of their fascinating structures, interesting host–guest chemistry and possible use as precursors of novel nanofabricated VPO phases with potential application in catalysis. Most of the syntheses carried out so far have involved organization by Sþ I type charge interaction between a cationic surfactant (Sþ ) and negatively charged VPO precursor ions (I ) in solution with the nature of the mesoVPO phases obtained viz. hexagonal, cubic and lamellar depending upon the synthetic conditions applied [1]. A counter anion (X ) mediated, Sþ X I0 , synthesis involving the organization of a cationic long chain alkyl amine surfactant (Sþ ) and neutral VOPO4 precursors has also been reported [2]. Very recently we have demonstrated a covalent templating route, using a long chain alkyl amine as the structure directing sur-
*
Corresponding author. Tel.: +91-135-660-263; fax: +91-135-660202. E-mail address: [email protected] (A. Datta). 1387-1811/$ - see front matter Ó 2003 Published by Elsevier Inc. doi:10.1016/j.micromeso.2003.11.006
factant, for the synthesis of a mesolamellar vanadium phosphate phase [3]. The vanadyl hydrogen phosphate hemihydrate, VOHPO4 Æ 0.5H2 O phase is important as the precursor for the active vanadyl pyrophosphate catalyst used commercially for the selective oxidation of butane to maleic anhydride [4]. VOHPO4 Æ 0.5H2 O has a 2D layer structure [5] formed by extensive cross linking between the face shared vanadyl octahedra and phosphate tetrahedra. The layers are tightly bound together by network hydrogen bonds and are therefore not very amenable for intercalation reactions [6] although there is a report of intercalation of small chain alkyl amines into the layered structure of VOHPO4 Æ 0.5H2 O in non-aqueous solvents [7]. The exfoliation property of VOHPO4 Æ 0.5H2 O has not yet been investigated although the exfoliation of the vanadium (V) orthophosphate VOPO4 Æ 2H2 O, which has a comparatively much weaker interlayer interaction, has been very recently reported and has been used for the design of some novel microstructurally modified V– P–O materials and supported VPO catalysts [8] for the selective oxidation of butane to maleic anhydride. In the present work we demonstrate the hitherto unreported, exfoliation of the layered vanadium (IV) orthophosphate VOHPO4 Æ 0.5H2 O as well as the organization of this exfoliated solid into a mesostructured lamellar
230
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
phase having the same V–P–O connectivity as in VOHPO4 Æ 0.5H2 O using a long chain alkyl amine surfactant such as octadecyl amine (ODA) as the structure directing agent.
2. Experimental 2.1. Synthesis VOHPO4 Æ 0.5H2 O was synthesized following a literature method [9]. In a typical synthesis, 0.909 g (0.005 mol) of V2 O5 was dispersed in 30 ml aqueous solution of 0.695 g (0.01 mol) NH2 OH Æ HCl. Into this mixture 1.153 g of 85% H3 PO4 (0.01 mol) was added and the slurry was stirred at 80 °C for 1 h to get a clear dark blue solution (designated as SOL-1). When SOL-1 was slowly evaporated a blue pasty mass was obtained, which was aged at 120 °C in an air oven for 12 h to get a sky blue solid. This was washed several times with portions of boiling water till the washings were chloride free. The solid was then dried at 120 °C for 6 h. XRD, elemental analysis by ICP and TGA of this solid confirmed the formation of the pure VOHPO4 Æ 0.5H2 O phase. For the exfoliation experiment, 0.215 g (0.00125 mol) of finely powdered VOHPO4 Æ 0.5H2 O was dispersed into a 40 ml mixture of DMF and H2 O (1:1 V/V) and stirred at 80 °C for 2 days to get a clear dark coloured solution (designated as SOL-2). For the organization of a mesostructured lamellar vanadium phosphate phase, 0.337 g (0.00125 mol) of ODA was stirred in 25 ml of distilled water at 80 °C for 30 min to get a homogeneous dispersion, which was added, into 40 ml of SOL-2 prepared by the method as mentioned above when immediate precipitation of a pale blue solid was observed. The slurry was stirred for 6 h at 80 °C and the solid separated by filtration, thoroughly washed with distilled water and subsequently with acetone and dried overnight at 120 °C in an air oven. This solid was designated as VP-ODA. Phosphorous and vanadium content of VP-ODA was determined by ICP analysis: P ¼ 7.02%, V ¼ 11.56%. Water and amine content was determined by TGA considering mass loss below 150 °C due to the loss of water (3.34%) and mass loss in the range 150–550 °C due to the loss of incorporated amine (61%). C, H and N content determined by CHN analysis shows C ¼ 49%, H ¼ 9.3% and N ¼ 3.2%. Combining ICP, TGA and CHN data the composition of VP-ODA was determined to be (C18 H37 NH2 ) Æ (VOHPO4 ) Æ 0.5H2 O.
Avance 500 NMR spectrometer. The solution state 31 P NMR experiments were done at 31 P resonance frequency of 121.49 MHz with a p=2 pulse of 9 ls. Phosphoric acid (85%) was used as an external reference. Each spectra was recorded after adding 5000 FID scans to get the optimum signal to noise ratio. The 13 C CPMAS experiments were performed at resonance frequencies of 125.75 MHz for 13 C and 500.13 MHz for 1 H applying a standard CP sequence with TPPM decoupling [10] scheme during the acquisition for most efficient heteronuclear decoupling. A p=2 pulse of 5 ls was applied to the protons to create the transverse 1 H magnetization at the spinning frequency of 8 kHz which was sufficient for removing the anisotropic part of various spin interactions, specifically for CSA. Contact pulses of 250 ls were applied in both 1 H and 13 C channels with Hartmann–Hann matching condition at 50 kHz rf field for polarization transfer in rotating frame under spin-lock condition. In the pulse sequence, 1 H spin temperature alteration was done with alternatively shifting rf phase by 180° for 1 H rf pulse and receiver phase cycling for CYCLOPS was implemented for all spectral recordings. The number of scans of FID was varied from 1000 to 3500 in order to get optimum signal to noise ratios. All 13 C spectra were processed with glycine as the external reference. In order to obtain more detailed information about internal methylene peaks, Bruker’s interactive solid-state deconvolution method was applied. For the individual peaks with their respective isotropic chemical shift values (diso ), appropriate Gaussian/Lorentzian convolution factors were applied for each spectrum. Individual peak positions, line widths and peak intensities were used as variable in iterations till the best fit for each spectra were obtained. The X-ray powder diffractograms (XRD) were recorded at room temperature on a GE XRD 9530 Dif and fractometer using CuKa radiation (k ¼ 1:5406 A) were scanned in the 2h range 2° to 50° at a rate of 1°/ min. TEM were recorded on a JEOL JEM 3010 transmission electron microscope at an accelerating voltage of 200 kV. The infra-red (FTIR) spectra were recorded on a Perkin Elmer 1760X spectrometer with the sample powder diluted in KBr (1%). Typically 100 scans with a resolution of 2 cm1 were collected for each sample. Thermogravimetric analysis (TGA) was carried out using a Mettler TG-50 model at a heating rate of 10 °C/ min and an airflow of 150 ml/min. Phosphorous and vanadium analysis of the samples were performed with an ICP-AESPS 3000 UV Leeman Labs, Inc. USA inductively coupled plasma spectrometer.
2.2. Characterization
3. Results and discussion
Nuclear magnetic resonance (NMR) experiments were performed at room temperature on a Bruker
Evidence for the exfoliation of VOHPO4 Æ 0.5H2 O is provided by the 31 P NMR of SOL-2 (Fig. 1) which
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
231
Fig. 1. 31 P NMR spectra of SOL-1 an SOL-2 (exfoliated solution of VOHPO4 Æ 0.5H2 O).
shows a sharp signal (Dm1=2 ¼ 40:1 Hz) centred at 1204.5 ppm. In contrast, the 31 P NMR spectrum of SOL-1, the precursor solution of VOHPO4 Æ 0.5H2 O which is believed [11] to contain different phosphates of the aquated vanadyl (IV) cation shoes a broad peak (Dm1=2 ¼ 2129:6 Hz) also centred at 1204.5 ppm. The position of the 31 P NMR signal indicates the presence of vanadium (IV) phosphates and the broadness of the signal in the case of SOL-1 suggests the presence of different species with slight variation in the coordination environment of the phosphorous. On the other hand, the presence of a sharp peak at 1204.5 ppm in the case of SOL-2 indicates the presence of a single type of vanadium (IV) phosphate species. At the same time the fact that the position of this peak is identical to that of SOL-1 which on evaporation gives rise to the VOHPO4 Æ 0.5H2 O phase suggests that the phosphated vanadium (IV) species present in SOL-2 has a V–P connectivity similar to that present in VOHPO4 Æ 0.5H2 O. The XRD pattern (Fig. 2) of the solid organized from the exfoliated solution of VOHPO4 Æ 0.5H2 O (VP-ODA) shows two strong peaks at d ¼ 3:62 nm (100%) and d ¼ 1:81 nm (35%) and a low intensity peak at d ¼ 1:21 These diffraction peaks are distinctly assignable to A.
Fig. 2. XRD patterns of VOHPO4 Æ 0.5H2 O and the mesolamellar solid (VP-ODA) organized from SOL-2 using ODA as the structure directing surfactant.
the 0 0 1, 0 0 2 and 0 0 3 reflections of a mesostructured lamellar solid with a basal spacing of 3.62 nm. In addition in the 2h region 14°–36°, it is evident from the expanded portion of the diffractogram that a few weak lines due to the VOHPO4 Æ 0.5H2 O phase are also present. This observation indicates that the inorganic layer of VP-ODA is composed of a weakly crystalline VOHPO4 Æ 0.5H2 O phase. The formation of a lamellar solid is also evident from the TEM micrograph of VP-ODA (Fig. 3), which shows a parallel stacking of layers with an interlayer spacing of 3.58 nm, which corresponds well with the basal spacing of 3.62 nm obtained from the XRD data. A close examination of the TEM of VP-ODA shows that its layer thickness is 1.2 nm and the interlayer separation is 2.4 nm. The interlayer separation is slightly smaller than the molecular length of the fully stretched octadecyl amine molecule, which is in keeping with the observation from NMR data that some gauche domains exist in the intercalated amine. On the other hand a layer
232
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
Fig. 3. TEM of the mesolamellar solid VP-ODA.
thickness of 1.2 nm in contrast to 0.57 nm in VOHPO4 Æ 0.5H2 O would seem to suggest that the exfoliated layers of VOHPO4 Æ 0.5H2 O while organizing in the presence of ODA do not give rise to a phase identical to the VOHPO4 Æ 0.5H2 O phase. At the same time, both the 31 P NMR and the FTIR data indicate that VP-ODA has a V–P–O connectivity similar to that present in the layers of VOHPO4 Æ 0.5H2 O. In essence therefore, it can be speculated that the inorganic matrix of VP-ODA consists of VPO layers similar to those present in VOHPO4 Æ 0.5H2 O but the stacking of these layers is different from that present in the hemihydrate phase and gives rise to a disordered layer matrix with thickness of 1.2 nm. The FTIR spectra (Fig. 4a) of VOHPO4 Æ 0.5H2 O in the P–O, V–O stretching and bending regions (500–1300 cm1 ) of VOHPO4 Æ 0.5H2 O is characterized by the presence of the following strong bands [12]: (cas (PO3 )), (1205, 1107 and 1045 cm1 ); (dip (P–OH)), (1136 cm1 ); (c (V@O)), (976 cm1 ); (c (P–OH)), (928 cm1 ); (dOOP (P–OH)), (645 cm1 ); (d (OPO)), (548, 529 cm1 ). The band structure of VP-ODA in the 500–1300 cm1 region though less well resolved, is essentially similar to that of the hemihydrate phase with minor shifts in position of some of the bands, indicating that the P–O–V connectivity in VP-ODA is similar to that of VOHPO4 Æ 0.5H2 O. VP-ODA: (1200, 1102 and 1051 cm1 ); (1134 cm1 ); (982 cm1 ); (925 cm1 ); (638 cm1 ); (542 and 525 cm1 ). This is in agreement with XRD data and the fact that the band structure of VP-ODA is not very well resolved is in keeping with the weak crystallinity of the VOHPO4 Æ 0.5H2 O phase in VP-ODA as evident from its XRD pattern. The presence of a shoulder at 940 cm1 in VP-ODA is characteristic of a vanadyl group coordinatively bound to the amine head group [13] of the incorporated ODA. In the N–H stretching region, (Fig. 4b) the asymmetric stretching band [14] at 3334
Fig. 4. FTIR spectra of VOHPO4 Æ 0.5H2 O, VP-ODA and pure ODA. (a) 400–2000 cm1 region, (b) 2000–4000 cm1 region.
cm1 of pure ODA is shifted to 3200 cm1 in VP-ODA suggesting strong coordinate covalent interaction of the NH2 head with the vanadyl group [15]. The asymmetric stretching cas (CH2 ) and bending d (CH2 ) vibrations of the methylene chains of the incorporated amine are known to be sensitive to the conformational disorder of the interior methylene groups in surfactants with an increase of the disordered gauche conformation in the methylene chain leading to small hypsochromic shifts in the cas (CH2 ) vibration and to small bathochromic shifts in the d (CH2 ) vibration [16]. In pure octadecyl amine cas (CH2 ) and d (CH2 ) vibrations are observed at 2916 and 1470 cm1 respectively while in VP-ODA the corresponding bands are shifted to 2921 and 1467 cm1 indicating the presence of some conformational disorder in the incorporated ODA molecules in VP-ODA.
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
233
ppm in ODA, due to the carbon atom (C1 ) adjacent to the amine head group and the carbon (C2 ) next to it, are not visible as distinct peaks in VP-ODA and appear as broad peaks in the deconvoluted spectrum, supporting the observation from FTIR studies, of strong interaction of the amine head group with the VPO matrix. Also, the broadening of the peak at 14 ppm due to the terminal methyl group (C18 ) of the octadecyl amine indicates interaction of the end methyl group with the VPO matrix, suggesting a monolayer arrangement of the octadecyl amine molecules in VP-ODA. 4. Conclusions
Fig. 5. Solid state 13 C CP-MAS NMR spectra of pure ODA and the mesolamellar vanadium phosphate VP-ODA. (Inset: Deconvoluted spectra of VP-ODA.)
Solid state 13 C CP-MAS NMR spectra (Fig. 5) of pure ODA and VP-ODA indicate a broadening of the spectrum of VP-ODA compared to ODA and can be attributed to the decreased mobility of the incorporated amine molecules in the constrained environment of the interlayer region of VP-ODA. 13 C NMR peaks in the chemical shift range 32–34 ppm correspond to the ordered (all trans) domain while peaks in the range 29–31 ppm are due to the disordered (an nearly equilibrium mixture of trans and gauche) domain of the interior methylene groups of the alkyl amine molecules [17]. The relative ratio of the area intensity of peaks due to the disordered (Igþt ) and ordered (It ) domains in ODA and VP-ODA, estimated by deconvolution of their NMR spectra, showed an increase of the Igþt =It ratio from 0.06 in ODA to 0.28 in VP-ODA indicating more disorder in the alkyl tail of octadecyl amine molecules in VP-ODA in agreement with infrared data. The peaks at isotropic chemical shifts of 43 and 39
In essence we have shown for the first time the exfoliation of the catalytically important layered VOHPO4 Æ 0.5H2 O phase and the subsequent organization of these exfoliated layers into a mesostructured lamellar vanadium phosphate by the templating action of octadecyl amine as the structure directing surfactant. Apart from the exfoliation of a VPO phase with strong interlayer interactions, the interesting and novel feature of this approach to the synthesis of mesostructured VPO phases is that it is possible to obtain a mesolamellar phase having the same V–P–O connectivity as the starting vanadium phosphate phase. This we believe, is achievable only because the mesostructured phase is obtained from a solution containing exfoliated layers of the VOHPO4 Æ 0.5H2 O phase. It may be mentioned here that the organization of mesolamellar phases from solutions containing V4þ species and phosphoric acid [1– 3] do not give rise to mesostructured phases with the V– P–O connectivity present in VOHPO4 Æ 0.5H2 O. It is also interesting that the incorporated amine as evident from NMR and IR data, does not have an all trans configuration, as generally assumed by previous workers, but also contains gauche domains in the internal methylenes. Our present results therefore provide a novel approach to obtaining mesostructured VPO compounds and this may be significant keeping in view the structure dependent catalytic application of layered vanadium phosphate phases in the selective oxidation and ammoxidation of light paraffins and alkyl aromatics. We believe a similar strategy could be applicable to the synthesis of novel mesostructured phases from other layered materials. The organization process can be schematically represented as follows:
234
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
Acknowledgements One of the authors (S.D.) is grateful to the C.S.I.R., India for the award of a Senior Research Fellowship.
References [1] T. Abe, A. Taguchi, M. Iwamoto, Chem. Mater. 7 (1995) 1429; J.E. Haskouri, M. Roca, S. Cabrera, J. Alamo, A. Beltran-Porter, D. Beltr an-Porter, M.D. Marcos, P. Amor os, Chem. Mater. 11 (1999) 1446; N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater. 13 (2001) 179; M.A. Carreon, V.V. Guliants, Micropor. Mesopor. Mater. 55 (2002) 297. [2] J.E. Haskouri, S. Cabrera, M. Roca, J. Alamo, A. Beltran-Porter, D. Beltr an-Porter, M.D. Marcos, P. Amoros, Inorg. Chem. 38 (1999) 4243. [3] S. Dasgupta, M. Agarwal, A. Datta, J. Mater. Chem. 12 (2002) 162. [4] G. Centi, F. Trifiro, J.R. Ebner, V.M. Franchetti, Chem. Rev. 88 (1988) 55. [5] M.E. Leonowicz, J.W. Johnson, J.F. Brody, H.F. Shannon Jr., J.M. Newsam, J. Solid State Chem. 56 (1985) 370. [6] T. Nakato, Y. Furumi, N. Terao, T. Okuhara, J. Mater. Chem. 10 (2000) 737.
[7] V.V. Guliants, J.B. Benziger, S. Sundaresan, Chem. Mater. 6 (1994) 353. [8] N. Yamamoto, T. Okuhara, T. Nakato, J. Mater. Chem. 11 (2001) 1858; N. Hiyoshi, N. Yamamoto, T. Okuhara, Chem. Lett. (2001) 484; Y. Kamiya, S. Ueki, N. Hiyoshi, N. Yamamoto, T. Okuhara, Catal. Today 78 (2003) 281. [9] T. Shimoda, T. Okuhara, M. Misono, Bull. Chem. Soc. Jpn. 58 (1985) 2163. [10] A.E. Bennett, J.H. Ok, R.G. Griffin, S. Vega, J. Chem. Phys. 96 (1992) 8624. [11] E. Alberico, G. Micera, Inorg. Chim. Acta 215 (1994) 225. [12] G. Busca, F. Cavani, G. Centi, F. Trifiro, J. Catal. 99 (1986) 400. [13] P. Gendraud, M.E. de Roy, J.P. Besse, Inorg. Chem. 35 (1996) 6108. [14] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1975; C.J. Pouchart, The Aldrich Library of Infrared Spectra, Aldrich Chemical Company, Inc., Milwaukee, 1981. [15] L. Benes, R. Hyklova, J. Kalousova, J. Votinsky, Inorg. Chim. Acta. 177 (1990) 71. [16] R.A. Vaia, R.K. Teukolsky, E.P. Giannelis, Chem. Mater. 6 (1994) 1017; J. Do, A.J. Jacobson, Chem. Mater. 13 (2001) 2436. [17] W. Gao, L. Reven, Langmuir 11 (1995) 1860; L.-Q. Wang, J. Liu, G.J. Exarhos, B.C. Bunker, Langmuir 12 (1996) 2663; L. Wang, J. Liu, G.J. Exarhos, K.Y. Flanigan, R. Bordia, J. Phys. Chem. B. 104 (2000) 2810.
Surfactant assisted organization of an exfoliated vanadyl ortho phosphate to a mesostructured lamellar vanadium phosphate phase Soumen Dasgupta, Monika Agarwal, Arunabha Datta
*
Indian Institute of Petroleum, Dehra Dun 248 005, India Received 5 July 2003; received in revised form 7 November 2003; accepted 9 November 2003
Abstract The catalytically important vanadyl hydrogen phosphate hemihydrate, VOHPO4 Æ 0.5H2 O phase with strong interlayer bonding has been exfoliated for the first time in DMF-water mixture and the exfoliated hemihydrate phase has been reorganized into a novel mesostructured lamellar VPO phase having the same V–P–O connectivity as in VOHPO4 Æ 0.5H2 O, using octadecyl amine as the structure directing template. Ó 2003 Published by Elsevier Inc. Keywords: Vanadyl orthophosphate; Lamellar phase; Mesostructured; Surfactant assisted organization
1. Introduction Supramolecular assembly of surfactants and soluble V–P–O precursors has recently been used for the synthesis of mesostructured vanadium phosphate (VPO) phases. These materials are of interest because of their fascinating structures, interesting host–guest chemistry and possible use as precursors of novel nanofabricated VPO phases with potential application in catalysis. Most of the syntheses carried out so far have involved organization by Sþ I type charge interaction between a cationic surfactant (Sþ ) and negatively charged VPO precursor ions (I ) in solution with the nature of the mesoVPO phases obtained viz. hexagonal, cubic and lamellar depending upon the synthetic conditions applied [1]. A counter anion (X ) mediated, Sþ X I0 , synthesis involving the organization of a cationic long chain alkyl amine surfactant (Sþ ) and neutral VOPO4 precursors has also been reported [2]. Very recently we have demonstrated a covalent templating route, using a long chain alkyl amine as the structure directing sur-
*
Corresponding author. Tel.: +91-135-660-263; fax: +91-135-660202. E-mail address: [email protected] (A. Datta). 1387-1811/$ - see front matter Ó 2003 Published by Elsevier Inc. doi:10.1016/j.micromeso.2003.11.006
factant, for the synthesis of a mesolamellar vanadium phosphate phase [3]. The vanadyl hydrogen phosphate hemihydrate, VOHPO4 Æ 0.5H2 O phase is important as the precursor for the active vanadyl pyrophosphate catalyst used commercially for the selective oxidation of butane to maleic anhydride [4]. VOHPO4 Æ 0.5H2 O has a 2D layer structure [5] formed by extensive cross linking between the face shared vanadyl octahedra and phosphate tetrahedra. The layers are tightly bound together by network hydrogen bonds and are therefore not very amenable for intercalation reactions [6] although there is a report of intercalation of small chain alkyl amines into the layered structure of VOHPO4 Æ 0.5H2 O in non-aqueous solvents [7]. The exfoliation property of VOHPO4 Æ 0.5H2 O has not yet been investigated although the exfoliation of the vanadium (V) orthophosphate VOPO4 Æ 2H2 O, which has a comparatively much weaker interlayer interaction, has been very recently reported and has been used for the design of some novel microstructurally modified V– P–O materials and supported VPO catalysts [8] for the selective oxidation of butane to maleic anhydride. In the present work we demonstrate the hitherto unreported, exfoliation of the layered vanadium (IV) orthophosphate VOHPO4 Æ 0.5H2 O as well as the organization of this exfoliated solid into a mesostructured lamellar
230
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
phase having the same V–P–O connectivity as in VOHPO4 Æ 0.5H2 O using a long chain alkyl amine surfactant such as octadecyl amine (ODA) as the structure directing agent.
2. Experimental 2.1. Synthesis VOHPO4 Æ 0.5H2 O was synthesized following a literature method [9]. In a typical synthesis, 0.909 g (0.005 mol) of V2 O5 was dispersed in 30 ml aqueous solution of 0.695 g (0.01 mol) NH2 OH Æ HCl. Into this mixture 1.153 g of 85% H3 PO4 (0.01 mol) was added and the slurry was stirred at 80 °C for 1 h to get a clear dark blue solution (designated as SOL-1). When SOL-1 was slowly evaporated a blue pasty mass was obtained, which was aged at 120 °C in an air oven for 12 h to get a sky blue solid. This was washed several times with portions of boiling water till the washings were chloride free. The solid was then dried at 120 °C for 6 h. XRD, elemental analysis by ICP and TGA of this solid confirmed the formation of the pure VOHPO4 Æ 0.5H2 O phase. For the exfoliation experiment, 0.215 g (0.00125 mol) of finely powdered VOHPO4 Æ 0.5H2 O was dispersed into a 40 ml mixture of DMF and H2 O (1:1 V/V) and stirred at 80 °C for 2 days to get a clear dark coloured solution (designated as SOL-2). For the organization of a mesostructured lamellar vanadium phosphate phase, 0.337 g (0.00125 mol) of ODA was stirred in 25 ml of distilled water at 80 °C for 30 min to get a homogeneous dispersion, which was added, into 40 ml of SOL-2 prepared by the method as mentioned above when immediate precipitation of a pale blue solid was observed. The slurry was stirred for 6 h at 80 °C and the solid separated by filtration, thoroughly washed with distilled water and subsequently with acetone and dried overnight at 120 °C in an air oven. This solid was designated as VP-ODA. Phosphorous and vanadium content of VP-ODA was determined by ICP analysis: P ¼ 7.02%, V ¼ 11.56%. Water and amine content was determined by TGA considering mass loss below 150 °C due to the loss of water (3.34%) and mass loss in the range 150–550 °C due to the loss of incorporated amine (61%). C, H and N content determined by CHN analysis shows C ¼ 49%, H ¼ 9.3% and N ¼ 3.2%. Combining ICP, TGA and CHN data the composition of VP-ODA was determined to be (C18 H37 NH2 ) Æ (VOHPO4 ) Æ 0.5H2 O.
Avance 500 NMR spectrometer. The solution state 31 P NMR experiments were done at 31 P resonance frequency of 121.49 MHz with a p=2 pulse of 9 ls. Phosphoric acid (85%) was used as an external reference. Each spectra was recorded after adding 5000 FID scans to get the optimum signal to noise ratio. The 13 C CPMAS experiments were performed at resonance frequencies of 125.75 MHz for 13 C and 500.13 MHz for 1 H applying a standard CP sequence with TPPM decoupling [10] scheme during the acquisition for most efficient heteronuclear decoupling. A p=2 pulse of 5 ls was applied to the protons to create the transverse 1 H magnetization at the spinning frequency of 8 kHz which was sufficient for removing the anisotropic part of various spin interactions, specifically for CSA. Contact pulses of 250 ls were applied in both 1 H and 13 C channels with Hartmann–Hann matching condition at 50 kHz rf field for polarization transfer in rotating frame under spin-lock condition. In the pulse sequence, 1 H spin temperature alteration was done with alternatively shifting rf phase by 180° for 1 H rf pulse and receiver phase cycling for CYCLOPS was implemented for all spectral recordings. The number of scans of FID was varied from 1000 to 3500 in order to get optimum signal to noise ratios. All 13 C spectra were processed with glycine as the external reference. In order to obtain more detailed information about internal methylene peaks, Bruker’s interactive solid-state deconvolution method was applied. For the individual peaks with their respective isotropic chemical shift values (diso ), appropriate Gaussian/Lorentzian convolution factors were applied for each spectrum. Individual peak positions, line widths and peak intensities were used as variable in iterations till the best fit for each spectra were obtained. The X-ray powder diffractograms (XRD) were recorded at room temperature on a GE XRD 9530 Dif and fractometer using CuKa radiation (k ¼ 1:5406 A) were scanned in the 2h range 2° to 50° at a rate of 1°/ min. TEM were recorded on a JEOL JEM 3010 transmission electron microscope at an accelerating voltage of 200 kV. The infra-red (FTIR) spectra were recorded on a Perkin Elmer 1760X spectrometer with the sample powder diluted in KBr (1%). Typically 100 scans with a resolution of 2 cm1 were collected for each sample. Thermogravimetric analysis (TGA) was carried out using a Mettler TG-50 model at a heating rate of 10 °C/ min and an airflow of 150 ml/min. Phosphorous and vanadium analysis of the samples were performed with an ICP-AESPS 3000 UV Leeman Labs, Inc. USA inductively coupled plasma spectrometer.
2.2. Characterization
3. Results and discussion
Nuclear magnetic resonance (NMR) experiments were performed at room temperature on a Bruker
Evidence for the exfoliation of VOHPO4 Æ 0.5H2 O is provided by the 31 P NMR of SOL-2 (Fig. 1) which
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
231
Fig. 1. 31 P NMR spectra of SOL-1 an SOL-2 (exfoliated solution of VOHPO4 Æ 0.5H2 O).
shows a sharp signal (Dm1=2 ¼ 40:1 Hz) centred at 1204.5 ppm. In contrast, the 31 P NMR spectrum of SOL-1, the precursor solution of VOHPO4 Æ 0.5H2 O which is believed [11] to contain different phosphates of the aquated vanadyl (IV) cation shoes a broad peak (Dm1=2 ¼ 2129:6 Hz) also centred at 1204.5 ppm. The position of the 31 P NMR signal indicates the presence of vanadium (IV) phosphates and the broadness of the signal in the case of SOL-1 suggests the presence of different species with slight variation in the coordination environment of the phosphorous. On the other hand, the presence of a sharp peak at 1204.5 ppm in the case of SOL-2 indicates the presence of a single type of vanadium (IV) phosphate species. At the same time the fact that the position of this peak is identical to that of SOL-1 which on evaporation gives rise to the VOHPO4 Æ 0.5H2 O phase suggests that the phosphated vanadium (IV) species present in SOL-2 has a V–P connectivity similar to that present in VOHPO4 Æ 0.5H2 O. The XRD pattern (Fig. 2) of the solid organized from the exfoliated solution of VOHPO4 Æ 0.5H2 O (VP-ODA) shows two strong peaks at d ¼ 3:62 nm (100%) and d ¼ 1:81 nm (35%) and a low intensity peak at d ¼ 1:21 These diffraction peaks are distinctly assignable to A.
Fig. 2. XRD patterns of VOHPO4 Æ 0.5H2 O and the mesolamellar solid (VP-ODA) organized from SOL-2 using ODA as the structure directing surfactant.
the 0 0 1, 0 0 2 and 0 0 3 reflections of a mesostructured lamellar solid with a basal spacing of 3.62 nm. In addition in the 2h region 14°–36°, it is evident from the expanded portion of the diffractogram that a few weak lines due to the VOHPO4 Æ 0.5H2 O phase are also present. This observation indicates that the inorganic layer of VP-ODA is composed of a weakly crystalline VOHPO4 Æ 0.5H2 O phase. The formation of a lamellar solid is also evident from the TEM micrograph of VP-ODA (Fig. 3), which shows a parallel stacking of layers with an interlayer spacing of 3.58 nm, which corresponds well with the basal spacing of 3.62 nm obtained from the XRD data. A close examination of the TEM of VP-ODA shows that its layer thickness is 1.2 nm and the interlayer separation is 2.4 nm. The interlayer separation is slightly smaller than the molecular length of the fully stretched octadecyl amine molecule, which is in keeping with the observation from NMR data that some gauche domains exist in the intercalated amine. On the other hand a layer
232
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
Fig. 3. TEM of the mesolamellar solid VP-ODA.
thickness of 1.2 nm in contrast to 0.57 nm in VOHPO4 Æ 0.5H2 O would seem to suggest that the exfoliated layers of VOHPO4 Æ 0.5H2 O while organizing in the presence of ODA do not give rise to a phase identical to the VOHPO4 Æ 0.5H2 O phase. At the same time, both the 31 P NMR and the FTIR data indicate that VP-ODA has a V–P–O connectivity similar to that present in the layers of VOHPO4 Æ 0.5H2 O. In essence therefore, it can be speculated that the inorganic matrix of VP-ODA consists of VPO layers similar to those present in VOHPO4 Æ 0.5H2 O but the stacking of these layers is different from that present in the hemihydrate phase and gives rise to a disordered layer matrix with thickness of 1.2 nm. The FTIR spectra (Fig. 4a) of VOHPO4 Æ 0.5H2 O in the P–O, V–O stretching and bending regions (500–1300 cm1 ) of VOHPO4 Æ 0.5H2 O is characterized by the presence of the following strong bands [12]: (cas (PO3 )), (1205, 1107 and 1045 cm1 ); (dip (P–OH)), (1136 cm1 ); (c (V@O)), (976 cm1 ); (c (P–OH)), (928 cm1 ); (dOOP (P–OH)), (645 cm1 ); (d (OPO)), (548, 529 cm1 ). The band structure of VP-ODA in the 500–1300 cm1 region though less well resolved, is essentially similar to that of the hemihydrate phase with minor shifts in position of some of the bands, indicating that the P–O–V connectivity in VP-ODA is similar to that of VOHPO4 Æ 0.5H2 O. VP-ODA: (1200, 1102 and 1051 cm1 ); (1134 cm1 ); (982 cm1 ); (925 cm1 ); (638 cm1 ); (542 and 525 cm1 ). This is in agreement with XRD data and the fact that the band structure of VP-ODA is not very well resolved is in keeping with the weak crystallinity of the VOHPO4 Æ 0.5H2 O phase in VP-ODA as evident from its XRD pattern. The presence of a shoulder at 940 cm1 in VP-ODA is characteristic of a vanadyl group coordinatively bound to the amine head group [13] of the incorporated ODA. In the N–H stretching region, (Fig. 4b) the asymmetric stretching band [14] at 3334
Fig. 4. FTIR spectra of VOHPO4 Æ 0.5H2 O, VP-ODA and pure ODA. (a) 400–2000 cm1 region, (b) 2000–4000 cm1 region.
cm1 of pure ODA is shifted to 3200 cm1 in VP-ODA suggesting strong coordinate covalent interaction of the NH2 head with the vanadyl group [15]. The asymmetric stretching cas (CH2 ) and bending d (CH2 ) vibrations of the methylene chains of the incorporated amine are known to be sensitive to the conformational disorder of the interior methylene groups in surfactants with an increase of the disordered gauche conformation in the methylene chain leading to small hypsochromic shifts in the cas (CH2 ) vibration and to small bathochromic shifts in the d (CH2 ) vibration [16]. In pure octadecyl amine cas (CH2 ) and d (CH2 ) vibrations are observed at 2916 and 1470 cm1 respectively while in VP-ODA the corresponding bands are shifted to 2921 and 1467 cm1 indicating the presence of some conformational disorder in the incorporated ODA molecules in VP-ODA.
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
233
ppm in ODA, due to the carbon atom (C1 ) adjacent to the amine head group and the carbon (C2 ) next to it, are not visible as distinct peaks in VP-ODA and appear as broad peaks in the deconvoluted spectrum, supporting the observation from FTIR studies, of strong interaction of the amine head group with the VPO matrix. Also, the broadening of the peak at 14 ppm due to the terminal methyl group (C18 ) of the octadecyl amine indicates interaction of the end methyl group with the VPO matrix, suggesting a monolayer arrangement of the octadecyl amine molecules in VP-ODA. 4. Conclusions
Fig. 5. Solid state 13 C CP-MAS NMR spectra of pure ODA and the mesolamellar vanadium phosphate VP-ODA. (Inset: Deconvoluted spectra of VP-ODA.)
Solid state 13 C CP-MAS NMR spectra (Fig. 5) of pure ODA and VP-ODA indicate a broadening of the spectrum of VP-ODA compared to ODA and can be attributed to the decreased mobility of the incorporated amine molecules in the constrained environment of the interlayer region of VP-ODA. 13 C NMR peaks in the chemical shift range 32–34 ppm correspond to the ordered (all trans) domain while peaks in the range 29–31 ppm are due to the disordered (an nearly equilibrium mixture of trans and gauche) domain of the interior methylene groups of the alkyl amine molecules [17]. The relative ratio of the area intensity of peaks due to the disordered (Igþt ) and ordered (It ) domains in ODA and VP-ODA, estimated by deconvolution of their NMR spectra, showed an increase of the Igþt =It ratio from 0.06 in ODA to 0.28 in VP-ODA indicating more disorder in the alkyl tail of octadecyl amine molecules in VP-ODA in agreement with infrared data. The peaks at isotropic chemical shifts of 43 and 39
In essence we have shown for the first time the exfoliation of the catalytically important layered VOHPO4 Æ 0.5H2 O phase and the subsequent organization of these exfoliated layers into a mesostructured lamellar vanadium phosphate by the templating action of octadecyl amine as the structure directing surfactant. Apart from the exfoliation of a VPO phase with strong interlayer interactions, the interesting and novel feature of this approach to the synthesis of mesostructured VPO phases is that it is possible to obtain a mesolamellar phase having the same V–P–O connectivity as the starting vanadium phosphate phase. This we believe, is achievable only because the mesostructured phase is obtained from a solution containing exfoliated layers of the VOHPO4 Æ 0.5H2 O phase. It may be mentioned here that the organization of mesolamellar phases from solutions containing V4þ species and phosphoric acid [1– 3] do not give rise to mesostructured phases with the V– P–O connectivity present in VOHPO4 Æ 0.5H2 O. It is also interesting that the incorporated amine as evident from NMR and IR data, does not have an all trans configuration, as generally assumed by previous workers, but also contains gauche domains in the internal methylenes. Our present results therefore provide a novel approach to obtaining mesostructured VPO compounds and this may be significant keeping in view the structure dependent catalytic application of layered vanadium phosphate phases in the selective oxidation and ammoxidation of light paraffins and alkyl aromatics. We believe a similar strategy could be applicable to the synthesis of novel mesostructured phases from other layered materials. The organization process can be schematically represented as follows:
234
S. Dasgupta et al. / Microporous and Mesoporous Materials 67 (2004) 229–234
Acknowledgements One of the authors (S.D.) is grateful to the C.S.I.R., India for the award of a Senior Research Fellowship.
References [1] T. Abe, A. Taguchi, M. Iwamoto, Chem. Mater. 7 (1995) 1429; J.E. Haskouri, M. Roca, S. Cabrera, J. Alamo, A. Beltran-Porter, D. Beltr an-Porter, M.D. Marcos, P. Amor os, Chem. Mater. 11 (1999) 1446; N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater. 13 (2001) 179; M.A. Carreon, V.V. Guliants, Micropor. Mesopor. Mater. 55 (2002) 297. [2] J.E. Haskouri, S. Cabrera, M. Roca, J. Alamo, A. Beltran-Porter, D. Beltr an-Porter, M.D. Marcos, P. Amoros, Inorg. Chem. 38 (1999) 4243. [3] S. Dasgupta, M. Agarwal, A. Datta, J. Mater. Chem. 12 (2002) 162. [4] G. Centi, F. Trifiro, J.R. Ebner, V.M. Franchetti, Chem. Rev. 88 (1988) 55. [5] M.E. Leonowicz, J.W. Johnson, J.F. Brody, H.F. Shannon Jr., J.M. Newsam, J. Solid State Chem. 56 (1985) 370. [6] T. Nakato, Y. Furumi, N. Terao, T. Okuhara, J. Mater. Chem. 10 (2000) 737.
[7] V.V. Guliants, J.B. Benziger, S. Sundaresan, Chem. Mater. 6 (1994) 353. [8] N. Yamamoto, T. Okuhara, T. Nakato, J. Mater. Chem. 11 (2001) 1858; N. Hiyoshi, N. Yamamoto, T. Okuhara, Chem. Lett. (2001) 484; Y. Kamiya, S. Ueki, N. Hiyoshi, N. Yamamoto, T. Okuhara, Catal. Today 78 (2003) 281. [9] T. Shimoda, T. Okuhara, M. Misono, Bull. Chem. Soc. Jpn. 58 (1985) 2163. [10] A.E. Bennett, J.H. Ok, R.G. Griffin, S. Vega, J. Chem. Phys. 96 (1992) 8624. [11] E. Alberico, G. Micera, Inorg. Chim. Acta 215 (1994) 225. [12] G. Busca, F. Cavani, G. Centi, F. Trifiro, J. Catal. 99 (1986) 400. [13] P. Gendraud, M.E. de Roy, J.P. Besse, Inorg. Chem. 35 (1996) 6108. [14] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1975; C.J. Pouchart, The Aldrich Library of Infrared Spectra, Aldrich Chemical Company, Inc., Milwaukee, 1981. [15] L. Benes, R. Hyklova, J. Kalousova, J. Votinsky, Inorg. Chim. Acta. 177 (1990) 71. [16] R.A. Vaia, R.K. Teukolsky, E.P. Giannelis, Chem. Mater. 6 (1994) 1017; J. Do, A.J. Jacobson, Chem. Mater. 13 (2001) 2436. [17] W. Gao, L. Reven, Langmuir 11 (1995) 1860; L.-Q. Wang, J. Liu, G.J. Exarhos, B.C. Bunker, Langmuir 12 (1996) 2663; L. Wang, J. Liu, G.J. Exarhos, K.Y. Flanigan, R. Bordia, J. Phys. Chem. B. 104 (2000) 2810.