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Crystallisation of VOHPO4·0.5H2O in the presence of a surfactant
Applied Catalysis A: General 251 (2003) 327–335
Crystallisation of VOHPO4 ·0.5H2 O in the presence of a surfactant L. O’Mahony a , D. Zemlyanov a , Mark E. Smith b , B.K. Hodnett a,∗ a
Materials and Surface Science Institute, University of Limerick, Limerick, Ireland b Department of Physics, University of Warwick, Coventry CV4 7AL, UK
Received 28 March 2003; received in revised form 22 April 2003; accepted 27 April 2003
Abstract The formation of vanadyl hydrogen phosphate hemihydrate (VOHPO4 ·0.5H2 O) by progressive attachment of phosphorous to a vanadium oxide lamellar phase in the presence of cetyltrimethylammonium bromide (CTAB) is described. Evidence from nuclear magnetic resonance (NMR), XPS, X-ray diffraction (XRD) and scanning electron microscopy (SEM) is presented for the initial formation of a vanadium oxide lamellar structure with no long-range order to which phosphate is then progressively attached. Phosphate attachment leads to lamellar collapse with the resultant formation of the hemihydrate phase, VOHPO4 ·0.5H2 O. © 2003 Elsevier B.V. All rights reserved. Keywords: Crystallisation; Cetyltrimethylammonium bromide; Vanadium oxide; Surfactant; Phosphorous; Mesophase
1. Introduction It is widely agreed that the major component of vanadium phosphorous oxide (VPO) catalysts for n-butane oxidation to maleic anhydride is vanadyl pyrophosphate, namely (VO2 )P2 O7 [1]. Vanadyl hydrogen phosphate hemihydrate (VOHPO4 ·0.5H2 O) is the precursor for this catalyst [1–23]. The (1 0 0) plane of (VO2 )P2 O7 which is an active plane for the selective oxidation [2,3,7] is topotactically related to (0 0 1) plane of VOHPO4 ·0.5H2 O [7]. To date, a number of variations of VOHPO4 ·0.5H2 O formation have been documented which include (1) the reduction of V2 O5 with various alcohols followed by reaction with H3 PO4 [1–7]; (2) the reaction of ∗ Corresponding author. Tel.: +353-61-202246; fax: +353-61-202568. E-mail address: [email protected] (B.K. Hodnett).
V2 O5 and H3 PO4 with NH2 OH·HCl or oxalic acid with further heat treatment [2,3]; (3) hydrothermal synthesis with V2 O4 and H3 PO4 at 773 K [10]; (4) preparation of VOPO4 ·2H2 O followed by reduction with alcohols [3,6,7]. Preparation procedures are also known which involve the use of surfactants [18]. Recently, reports have appeared in which surfactants have been used in the preparation of VPO catalysts. A one-pot synthesis route proposed by Mizuno and co-workers resulted in highly crystalline VOHPO4 ·0.5H2 O. This was achieved by reacting V2 O5 and H3 PO4 with cetyltrimethylammonium chloride using vanadium metal as a reducing agent [11,18]. Surfactant-mediated preparations have also been recently described for the preparation of cubic, hexagonal and lamellar mesostructured VPO materials, all of which were unstable upon surfactant removal. These materials were determined to have no apparent long-range order and
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00362-4
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therefore crystalline VOHPO4 ·0.5H2 O formation was not observed [17,18,26]. In this paper, we describe the preparation of VOHPO4 ·0.5H2 O in the presence of cationic surfactant cetyltrimethylammonium bromide (CTAB), and present evidence for the formation of the hemihydrate precursor by attachment of PO4 3− anions onto a lamellar form of reduced vanadium oxide. This lamellar structure formed readily in a surfactant medium. 2. Experimental 2.1. Sample preparation V2 O5 (9.65 g Merck) was refluxed in a 90:10 mixture of 2-methyl propan-1-ol (180 ml, BDH Analar) and benzyl alcohol (20 ml, Riedel-de Haen) for 16 h. The dark green slurry was filtered to remove any unreacted material resulting in a dark green vanadium alcoholate with the vanadium concentration being determined by atomic absorption spectroscopy. A 10% micelle solution was prepared by adding 30 g of cetyltrimethylammonium bromide slowly to 300 ml of H2 O at 36 ◦ C under constant stirring. This procedure ensured total dissolution of CTAB. The vanadium component was introduced by slowly adding the prepared V4+ solution to the micelle solution. The molar V:CTAB ratio was 0.25:1. The temperature was kept at 36 ◦ C for 30 min after the addition of the vanadium component. Phosphorous was added in the form of crystalline ortho-phosphoric acid (o-H3 PO4 , Aldrich). The amount of phosphorous added to the system was adjusted so that the P:V ratio in solution was approximately 1.75:1 [7]. Further stirring was also required ensuring temperature control to interact the phosphorous and vanadium with micelle solution of CTAB in H2 O [20,21]. This suspension was placed in 70 ml stainless steel vessels lined with Teflon and placed in an autoclave at 120 ◦ C for times between 0 and 96 h. Autoclave times were also varied. The samples were isolated by vacuum filtration and dried at 120 ◦ C. 2.2. Sample characterisation Scanning electron microscopy (SEM) was performed using a JEOL JSM-840 electron microscope after sample coating with gold in an Edwards S150B
sputter coater. Powder X-ray diffraction (XRD) was performed using a Philips X-ray diffractometer with Geiger–Muller counter using Cu K␣ radiation filtered through nickel. Solid-state nuclear magnetic resonance (NMR) studies were performed on Bruker MSL 300 and Chemagnetics CMX 360 spectrometers equipped with magnetic fields of 7.05 and 8.45 T, respectively. Doty 4 mm magic angle spinning (MAS) probes were used at both fields. Magic angle spinning rates between 16.5 and 18 kHz were used at 8.45 T while slower spinning rates were used at 7.05 T. The 13 P spectra were collected at a frequency of 121.497 MHz while 51 V were collected at 94.66 MHz. All MAS spectra were recorded using a single pulse program with a 90◦ tip angle. Static spectra were recorded using an Oldfield echo sequence 90◦ –τ–180◦ with extended phase cycling to overcome probe ringing and baseline distortion. Sufficient recycle delays were employed to ensure that the spectra collected were fully relaxed. All 51 V spectra were referenced with respect to -NaVO3 (solid) in which the isotropic chemical shift (δiso ) was determined to be −510.4 ppm [24]. All 31 P spectra were referenced to ammonium dihydrogen phosphate (NH4 H2 PO4 ) to which the isotropic chemical shift (δiso ) 0.8 ppm was assigned with respect to aqueous 85% o-H3 PO4 . The XPS data were obtained by a Kratos AXIS 165 spectrometer using monochromatic Al K␣ radiation (hν = 1486.58 eV) and fixed analyser pass energy of 20 eV. The atomic concentrations of the chemical elements in the near-surface region were estimated after the subtraction of a Shirley type background, taking into account the corresponding atomic sensitivity factors.
3. Results The X-ray diffraction patterns of the precursors prepared in the presence of surfactants (Fig. 1) show the evolution of phases during the synthesis of VOHPO4 ·0.5H2 O. In this figure, the phosphorous component (o-H3 PO4 ) of the mixture was added at time = 0. The first features to emerge were a number of peaks corresponding to large d-spacing values ranging from 34 to 8 Å, which is consistent with the formation of a lamellar phase. The phases, which appeared up to 24 h of reaction, did not exhibit any long-range order. At 48 h and beyond VOHPO4 ·0.5H2 O started
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Fig. 1. Phase evolution during synthesis of VOHPO4 ·0.5H2 O in the presence of surfactant (V:CTAB molar ratio of 0.25:1; P:V = 1.75:1).
Fig. 2. XRD patterns of vanadium oxide phase developed at 120 ◦ C without added o-H3 PO4 (V:CTAB molar ratio of 0.25:1; P:V = 1.75:1).
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to appear [1–21,24,25] with lamellar collapse occurring after 48 h of reaction. Reflections corresponding to d-spacing of 8.59 and 6.58 Å were observed for the synthesis times beyond 24 h, but the corresponding phases these correspond to were not identified. This synthesis was repeated in the absence of phosphoric acid but with a higher V:CTAB molar ratio
(Fig. 2). Again a phase with lamellar characteristics similar to a VOx phase previously reported for vanadium oxide was observed [27,28]. Three strong reflections at relatively small angles were observed corresponding to d-spacing values of 34.4, 17.8 and 11.6 Å of the (0 0 1), (0 0 2) and (0 0 3) planes of a lamellar structure. This phase was stable up to 48 h
Fig. 3. SEM images of precursors collected after varying times throughout hemihydrate formation.
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in the synthesis medium. When o-H3 PO4 was added at this V:CTAB molar ratio hemihydrate formed after 8 h. In general, lamellar phase was observed to transform into the hemihydrate phase more rapidly as the V:CTAB molar ratio increased. Fig. 3 presents SEM images for samples collected at synthesis times between 4 and 96 h. At early reaction times (below 12 h) the average particle size is 100–150 m. On further reaction, particle size reduction is observed resulting in a platelet size of 2 m for VOHPO4 ·0.5H2 O recovered after 96 h of reaction. A point of interest is that the hemihydrate formed in these conditions never takes up the rosette like appearance often observed with VPO catalysts prepared in alcohol without surfactants. The SEM evidence in Fig. 3 is consistent with a breaking up of the original large lamellar particles as the synthesis time increases. Indeed, EDXA for the phosphorous and vanadium of these images (Fig. 4) indicates an increasing P:V ratio as the synthesis time is allowed to develop. This finding is further confirmed by the data in Table 1 of P:V atomic ratios measured by XPS analysis. The XPS data points to a strong increase in P:V atomic ratios as the synthesis time progresses, with changes occurring in the proportions of V5+ :V4+ :V3+ , with V4+ species dominating at extended reaction times. The vanadium species represented here as 3+ is still
Fig. 4. P:V ratio measured by EDXA for the synthesis times indicated.
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Table 1 XPS analysis data for the synthesis times indicated Synthesis time (h)
P:VSURF atomic ratio
V5+ (at.%)
V4+ (at.%)
V3+ (at.%)
0 48 96
0.11 0.81 1.18
51 29 32
29 58 63
20 13 5
a topic of discussion and will be discussed in due course. Fig. 5 presents 31 P MAS NMR spectra for surfactant-mediated materials collected at a V:CTAB molar ratio of 0.25:1. Initially (0 h), sharp 31 P signals with chemical shifts at −4 and +27 ppm were observed showing a sharp resolved line characteristic of solution phase phosphorous species as well as phosphorous bound to vanadium in the 5+ state [24,25]. With increased reaction time (48 h) chemical shifts between 200 and 1200 ppm were observed indicating that the phosphorous sites within the system are linked with vanadium in 5+ and 4+ states. At later reaction times (96 h), chemical shifts can be observed up to 2000 ppm relating to phosphorous interaction with vanadium in the 4+ state alone. The most intense signal occurring between 1650 and 1700 ppm is related to VOHPO4 ·0.5H2 O formation [24,25]. The 31 P shifts allow these conclusions to be drawn; when vanadium is diamagnetic (V5+ ) the 31 P shifts appear in the typical diamagnetic range; in the V4+ state, there is an unpaired electron, resulting in a much larger paramagnetic shift [29]. In contrast, the 51 V signal in MAS and static spectra in Fig. 6 show signal intensity being reduced with synthesis time indicating an increase in V4+ content within the system under study. This loss of signal results from the unpaired electron on the vanadium itself causing such large broadening (and also possibly causing a large shift) of the vanadium signal from the nucleus on the same atom. It is effectively lost from the NMR spectrum. At initial reaction times, although some splitting sidebands are observed under MAS conditions (spinning speed, νr = 18 kHz) the residual line width of the component parts of the spectra is quite broad indicating considerable amorphous character since the chemical shift dispersion must be large [29,30]. This is in agreement with X-ray diffraction shown at initial reaction times in Fig. 1. On further
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Fig. 5.
31 P
MAS NMR spectra of materials collected at a V:CTAB molar ratio of 0.25:1 (P:V = 1.75:1).
reaction (48 h), line narrowing was observed consistent with the formation of VPO crystalline phases. This can be observed after 48 h of reaction, which is in agreement with X-ray diffraction results outlined in Fig. 1 and 31 P MAS NMR spectra in Fig. 5, showing the predominant phase VOHPO4 ·0.5H2 O being formed by this time of reaction.
4. Discussion The experimental evidence clearly points to a sequence whereby initially a lamellar vanadium oxide phase with an inter lattice spacing of 30–35 Å forms very quickly in the synthesis medium. This lamellar structure forms in the absence of phosphorous
(o-H3 PO4 ) as indicated in Fig. 2. When the phosphorous component is present it initially appears to be a solution like species, as indicated by the very sharp 31 P MAS NMR signals at chemical shifts at −4 and +27.2 ppm, which do not have any prominent sidebands (Fig. 5). The slightly broadened peak at 27.2 ppm is indicative of phosphorous bound to vanadium in the 5+ states. With progressive reaction times, the nature of the phosphorous species within the surfactant-mediated system alters considerably, with progressive shifts of the signal position. With increased reaction times (48 h) lines with chemical shifts between 200 and 1200 ppm were observed which suggests that the phosphorous within the system is interacting with both vanadium in the 5+ and 4+ states. XPS results indicate that both of these
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Fig. 6.
51 V
333
MAS NMR (A) and static (B) spectra of materials collected at a V:CTAB molar ratio of 0.25:1; (∗) indicates isotropic shift.
species are present. X-ray diffraction of the samples synthesised for more than 24 h shows phosphorous uptake and collapse of the lamellar structure with the resultant transformation to VOHPO4 ·0.5H2 O. At later reaction times (96 h), 31 P chemical shifts can be observed up to 2000 ppm with the most intense line being centred at approximately 1650–1700 ppm which
is characteristic of phosphorous linked to vanadium in the 4+ state in VOHPO4 ·0.5H2 O. There two specific questions to be considered at this point (1) whether this vanadium oxide lamellar phase collapses on phosphorous interaction due to lamellar surface charge neutralisation leading to hemihydrate formation or (2) whether the vanadium
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oxide lamellar phase re-dissolves and then reacts further with the phosphorous component within the system. A key factor is that hemihydrate formation is pH dependent and only occurs at pH values below 2.0 and will be discussed in future work. This factor is compatible with a model whereby PO4 3− anions exchange with the Br− anions of the inter-planar surfactant. Bromine was detected by EDXA at initial synthesis times, but became less intense as the phosphorous signal increased. The phosphorous NMR signal at short synthesis times had the characteristics of a solution phase species. Thus, phosphate builds up inside the lamellar vanadium oxide phase. Note that the P:V ratio shown in Fig. 4 for the synthesis time of 48 h is nearly identical to the value recorded after 96 h. However, VOHPO4 ·0.5H2 O was not observed by XRD until after 48 h of reaction (see Fig. 1). With sufficient phosphorous incorporation, the hemihydrate phase starts to develop, maintaining the general lamellar or platelet type appearance of the vanadium oxide lamellar structure (see Fig. 3). EDXA, NMR and XPS evidence are consistent with the attachment of increased levels of phosphorous to the solid phase as synthesis time progresses with the average oxidation state of vanadium moving closer to V4+ . This process, which is essentially viewed as slow, would eventually lead to shrinking of the layer spacing observed in VOHPO4 ·0.5H2 O upon expulsion of the surfactant. This expulsion possibly generated by disruption of the surfactant bilayer when PO4 3− replaces Br− may be the driving force for reduction in particle size with retention of the particle morphology that accompanied the hemihydrate formation.
5. Conclusion VOHPO4 ·0.5H2 O has been formed by the progressive attachment of phosphorous to a vanadium oxide lamellar phase. Phosphorous attachment to the positively charged lamella was found to cause collapse. Only upon lamellar collapse, hemihydrate formation was observed. This transformation is pH dependent and occurs below the pH value of 2.0. This transformation has been observed to accelerate on increased V:CTAB molar ratios and will be discussed at a later date.
Acknowledgements The authors acknowledge the financial support from the Materials and Surface Science Institute, University of Limerick. The collaboration between Limerick and Warwick was made possible through a Framework 5 Marie Curie Training Site Grant to Warwick. Mark E. Smith thanks the EPSRC for funding NMR equipment at Warwick. Leonard O’Mahony would like to thank Dr. Andrew Howes for his important contribution to this research. References [1] B.K. Hodnett, Heterogeneous Catalytic Oxidation, Wiley, New York, 2000 (Chapter 5). [2] B.K. Hodnett, Catal. Rev. Sci. Eng. 27 (1985) 373. [3] G. Centi, F. Trifiiro, J.R. Ebner, V.M. Franchetti, Chem. Rev. 88 (1988) 5. [4] E. Bordes, Catal. Today 1 (1987) 499. [5] G.J. Hutchings, Appl. Catal. 72 (1991) 1. [6] H. Igarashi, K. Tsuji, T. Okuhara, M. Misono, J. Phys. Chem. 97 (1998) 7065. [7] H.S. Horowitz, C.M. Blackstone, A.W. Sleight, G. Teufer, Appl. Catal. 38 (1988) 193. [8] M. Abon, K.E. Bere, A. Tuel, P. Delichere, J. Catal. 156 (1995) 28. [9] B.K. Hodnett, P. Permanne, B. Delmon, Appl. Catal. 6 (1983) 231. [10] C.C. Toradi, J.C. Calabrese, Inorg. Chem. 23 (1984) 1308. [11] H. Hatayama, M. Misono, A. Taguchi, N. Mizuno, Chem. Lett. (2000) 884. [12] N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater. 13 (2001) 179. [13] J.EI. Haskouri, M. Roca, S. Cabrera, J. Alamo, A. Beltran-Porter, D. Beltran-Porter, M. Dolores Morcos, P. Amoros, Chem. Mater. 11 (1999) 1446. [14] T. Doi, T. Miyake, Chem. Commun. (1996) 1635; T. Doi, T. Miyake, Chem. Mater. 6 (1994) 353. [15] V.V. Guliants, J.B. Benziger, S. Sundaresan, I.E. Wachs, J.M. Jehng, J.E. Roberts, Catal. Today 28 (1996) 275. [16] M.A. Carreon, V.V. Guliants, Microporous Mesoporous Mater. 55 (2002) 297. [17] M.A. Carreon, V.V. Guliants, Catal. Today 78 (2003) 303. [18] N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater. 13 (2001) 179. [19] T. Abe, A. Taguchi, M. Iwamoto, Chem. Mater. 7 (1995) 1429. [20] G. Centi, Catal. Today 16 (1993) 1–4. [21] D. Farrusseng, A. Julbe, M. Lopez, C. Guizard, Catal. Today 56 (2000) 211. [22] M.T. Sananes, A. Tuel, J.C. Volta, J. Catal. 145 (1993) 251. [23] M.T. Sananes, A. Tuel, G.J. Hutchings, J.C. Volta, J. Catal. 148 (1994) 395.
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Crystallisation of VOHPO4 ·0.5H2 O in the presence of a surfactant L. O’Mahony a , D. Zemlyanov a , Mark E. Smith b , B.K. Hodnett a,∗ a
Materials and Surface Science Institute, University of Limerick, Limerick, Ireland b Department of Physics, University of Warwick, Coventry CV4 7AL, UK
Received 28 March 2003; received in revised form 22 April 2003; accepted 27 April 2003
Abstract The formation of vanadyl hydrogen phosphate hemihydrate (VOHPO4 ·0.5H2 O) by progressive attachment of phosphorous to a vanadium oxide lamellar phase in the presence of cetyltrimethylammonium bromide (CTAB) is described. Evidence from nuclear magnetic resonance (NMR), XPS, X-ray diffraction (XRD) and scanning electron microscopy (SEM) is presented for the initial formation of a vanadium oxide lamellar structure with no long-range order to which phosphate is then progressively attached. Phosphate attachment leads to lamellar collapse with the resultant formation of the hemihydrate phase, VOHPO4 ·0.5H2 O. © 2003 Elsevier B.V. All rights reserved. Keywords: Crystallisation; Cetyltrimethylammonium bromide; Vanadium oxide; Surfactant; Phosphorous; Mesophase
1. Introduction It is widely agreed that the major component of vanadium phosphorous oxide (VPO) catalysts for n-butane oxidation to maleic anhydride is vanadyl pyrophosphate, namely (VO2 )P2 O7 [1]. Vanadyl hydrogen phosphate hemihydrate (VOHPO4 ·0.5H2 O) is the precursor for this catalyst [1–23]. The (1 0 0) plane of (VO2 )P2 O7 which is an active plane for the selective oxidation [2,3,7] is topotactically related to (0 0 1) plane of VOHPO4 ·0.5H2 O [7]. To date, a number of variations of VOHPO4 ·0.5H2 O formation have been documented which include (1) the reduction of V2 O5 with various alcohols followed by reaction with H3 PO4 [1–7]; (2) the reaction of ∗ Corresponding author. Tel.: +353-61-202246; fax: +353-61-202568. E-mail address: [email protected] (B.K. Hodnett).
V2 O5 and H3 PO4 with NH2 OH·HCl or oxalic acid with further heat treatment [2,3]; (3) hydrothermal synthesis with V2 O4 and H3 PO4 at 773 K [10]; (4) preparation of VOPO4 ·2H2 O followed by reduction with alcohols [3,6,7]. Preparation procedures are also known which involve the use of surfactants [18]. Recently, reports have appeared in which surfactants have been used in the preparation of VPO catalysts. A one-pot synthesis route proposed by Mizuno and co-workers resulted in highly crystalline VOHPO4 ·0.5H2 O. This was achieved by reacting V2 O5 and H3 PO4 with cetyltrimethylammonium chloride using vanadium metal as a reducing agent [11,18]. Surfactant-mediated preparations have also been recently described for the preparation of cubic, hexagonal and lamellar mesostructured VPO materials, all of which were unstable upon surfactant removal. These materials were determined to have no apparent long-range order and
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00362-4
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therefore crystalline VOHPO4 ·0.5H2 O formation was not observed [17,18,26]. In this paper, we describe the preparation of VOHPO4 ·0.5H2 O in the presence of cationic surfactant cetyltrimethylammonium bromide (CTAB), and present evidence for the formation of the hemihydrate precursor by attachment of PO4 3− anions onto a lamellar form of reduced vanadium oxide. This lamellar structure formed readily in a surfactant medium. 2. Experimental 2.1. Sample preparation V2 O5 (9.65 g Merck) was refluxed in a 90:10 mixture of 2-methyl propan-1-ol (180 ml, BDH Analar) and benzyl alcohol (20 ml, Riedel-de Haen) for 16 h. The dark green slurry was filtered to remove any unreacted material resulting in a dark green vanadium alcoholate with the vanadium concentration being determined by atomic absorption spectroscopy. A 10% micelle solution was prepared by adding 30 g of cetyltrimethylammonium bromide slowly to 300 ml of H2 O at 36 ◦ C under constant stirring. This procedure ensured total dissolution of CTAB. The vanadium component was introduced by slowly adding the prepared V4+ solution to the micelle solution. The molar V:CTAB ratio was 0.25:1. The temperature was kept at 36 ◦ C for 30 min after the addition of the vanadium component. Phosphorous was added in the form of crystalline ortho-phosphoric acid (o-H3 PO4 , Aldrich). The amount of phosphorous added to the system was adjusted so that the P:V ratio in solution was approximately 1.75:1 [7]. Further stirring was also required ensuring temperature control to interact the phosphorous and vanadium with micelle solution of CTAB in H2 O [20,21]. This suspension was placed in 70 ml stainless steel vessels lined with Teflon and placed in an autoclave at 120 ◦ C for times between 0 and 96 h. Autoclave times were also varied. The samples were isolated by vacuum filtration and dried at 120 ◦ C. 2.2. Sample characterisation Scanning electron microscopy (SEM) was performed using a JEOL JSM-840 electron microscope after sample coating with gold in an Edwards S150B
sputter coater. Powder X-ray diffraction (XRD) was performed using a Philips X-ray diffractometer with Geiger–Muller counter using Cu K␣ radiation filtered through nickel. Solid-state nuclear magnetic resonance (NMR) studies were performed on Bruker MSL 300 and Chemagnetics CMX 360 spectrometers equipped with magnetic fields of 7.05 and 8.45 T, respectively. Doty 4 mm magic angle spinning (MAS) probes were used at both fields. Magic angle spinning rates between 16.5 and 18 kHz were used at 8.45 T while slower spinning rates were used at 7.05 T. The 13 P spectra were collected at a frequency of 121.497 MHz while 51 V were collected at 94.66 MHz. All MAS spectra were recorded using a single pulse program with a 90◦ tip angle. Static spectra were recorded using an Oldfield echo sequence 90◦ –τ–180◦ with extended phase cycling to overcome probe ringing and baseline distortion. Sufficient recycle delays were employed to ensure that the spectra collected were fully relaxed. All 51 V spectra were referenced with respect to -NaVO3 (solid) in which the isotropic chemical shift (δiso ) was determined to be −510.4 ppm [24]. All 31 P spectra were referenced to ammonium dihydrogen phosphate (NH4 H2 PO4 ) to which the isotropic chemical shift (δiso ) 0.8 ppm was assigned with respect to aqueous 85% o-H3 PO4 . The XPS data were obtained by a Kratos AXIS 165 spectrometer using monochromatic Al K␣ radiation (hν = 1486.58 eV) and fixed analyser pass energy of 20 eV. The atomic concentrations of the chemical elements in the near-surface region were estimated after the subtraction of a Shirley type background, taking into account the corresponding atomic sensitivity factors.
3. Results The X-ray diffraction patterns of the precursors prepared in the presence of surfactants (Fig. 1) show the evolution of phases during the synthesis of VOHPO4 ·0.5H2 O. In this figure, the phosphorous component (o-H3 PO4 ) of the mixture was added at time = 0. The first features to emerge were a number of peaks corresponding to large d-spacing values ranging from 34 to 8 Å, which is consistent with the formation of a lamellar phase. The phases, which appeared up to 24 h of reaction, did not exhibit any long-range order. At 48 h and beyond VOHPO4 ·0.5H2 O started
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Fig. 1. Phase evolution during synthesis of VOHPO4 ·0.5H2 O in the presence of surfactant (V:CTAB molar ratio of 0.25:1; P:V = 1.75:1).
Fig. 2. XRD patterns of vanadium oxide phase developed at 120 ◦ C without added o-H3 PO4 (V:CTAB molar ratio of 0.25:1; P:V = 1.75:1).
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to appear [1–21,24,25] with lamellar collapse occurring after 48 h of reaction. Reflections corresponding to d-spacing of 8.59 and 6.58 Å were observed for the synthesis times beyond 24 h, but the corresponding phases these correspond to were not identified. This synthesis was repeated in the absence of phosphoric acid but with a higher V:CTAB molar ratio
(Fig. 2). Again a phase with lamellar characteristics similar to a VOx phase previously reported for vanadium oxide was observed [27,28]. Three strong reflections at relatively small angles were observed corresponding to d-spacing values of 34.4, 17.8 and 11.6 Å of the (0 0 1), (0 0 2) and (0 0 3) planes of a lamellar structure. This phase was stable up to 48 h
Fig. 3. SEM images of precursors collected after varying times throughout hemihydrate formation.
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in the synthesis medium. When o-H3 PO4 was added at this V:CTAB molar ratio hemihydrate formed after 8 h. In general, lamellar phase was observed to transform into the hemihydrate phase more rapidly as the V:CTAB molar ratio increased. Fig. 3 presents SEM images for samples collected at synthesis times between 4 and 96 h. At early reaction times (below 12 h) the average particle size is 100–150 m. On further reaction, particle size reduction is observed resulting in a platelet size of 2 m for VOHPO4 ·0.5H2 O recovered after 96 h of reaction. A point of interest is that the hemihydrate formed in these conditions never takes up the rosette like appearance often observed with VPO catalysts prepared in alcohol without surfactants. The SEM evidence in Fig. 3 is consistent with a breaking up of the original large lamellar particles as the synthesis time increases. Indeed, EDXA for the phosphorous and vanadium of these images (Fig. 4) indicates an increasing P:V ratio as the synthesis time is allowed to develop. This finding is further confirmed by the data in Table 1 of P:V atomic ratios measured by XPS analysis. The XPS data points to a strong increase in P:V atomic ratios as the synthesis time progresses, with changes occurring in the proportions of V5+ :V4+ :V3+ , with V4+ species dominating at extended reaction times. The vanadium species represented here as 3+ is still
Fig. 4. P:V ratio measured by EDXA for the synthesis times indicated.
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Table 1 XPS analysis data for the synthesis times indicated Synthesis time (h)
P:VSURF atomic ratio
V5+ (at.%)
V4+ (at.%)
V3+ (at.%)
0 48 96
0.11 0.81 1.18
51 29 32
29 58 63
20 13 5
a topic of discussion and will be discussed in due course. Fig. 5 presents 31 P MAS NMR spectra for surfactant-mediated materials collected at a V:CTAB molar ratio of 0.25:1. Initially (0 h), sharp 31 P signals with chemical shifts at −4 and +27 ppm were observed showing a sharp resolved line characteristic of solution phase phosphorous species as well as phosphorous bound to vanadium in the 5+ state [24,25]. With increased reaction time (48 h) chemical shifts between 200 and 1200 ppm were observed indicating that the phosphorous sites within the system are linked with vanadium in 5+ and 4+ states. At later reaction times (96 h), chemical shifts can be observed up to 2000 ppm relating to phosphorous interaction with vanadium in the 4+ state alone. The most intense signal occurring between 1650 and 1700 ppm is related to VOHPO4 ·0.5H2 O formation [24,25]. The 31 P shifts allow these conclusions to be drawn; when vanadium is diamagnetic (V5+ ) the 31 P shifts appear in the typical diamagnetic range; in the V4+ state, there is an unpaired electron, resulting in a much larger paramagnetic shift [29]. In contrast, the 51 V signal in MAS and static spectra in Fig. 6 show signal intensity being reduced with synthesis time indicating an increase in V4+ content within the system under study. This loss of signal results from the unpaired electron on the vanadium itself causing such large broadening (and also possibly causing a large shift) of the vanadium signal from the nucleus on the same atom. It is effectively lost from the NMR spectrum. At initial reaction times, although some splitting sidebands are observed under MAS conditions (spinning speed, νr = 18 kHz) the residual line width of the component parts of the spectra is quite broad indicating considerable amorphous character since the chemical shift dispersion must be large [29,30]. This is in agreement with X-ray diffraction shown at initial reaction times in Fig. 1. On further
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Fig. 5.
31 P
MAS NMR spectra of materials collected at a V:CTAB molar ratio of 0.25:1 (P:V = 1.75:1).
reaction (48 h), line narrowing was observed consistent with the formation of VPO crystalline phases. This can be observed after 48 h of reaction, which is in agreement with X-ray diffraction results outlined in Fig. 1 and 31 P MAS NMR spectra in Fig. 5, showing the predominant phase VOHPO4 ·0.5H2 O being formed by this time of reaction.
4. Discussion The experimental evidence clearly points to a sequence whereby initially a lamellar vanadium oxide phase with an inter lattice spacing of 30–35 Å forms very quickly in the synthesis medium. This lamellar structure forms in the absence of phosphorous
(o-H3 PO4 ) as indicated in Fig. 2. When the phosphorous component is present it initially appears to be a solution like species, as indicated by the very sharp 31 P MAS NMR signals at chemical shifts at −4 and +27.2 ppm, which do not have any prominent sidebands (Fig. 5). The slightly broadened peak at 27.2 ppm is indicative of phosphorous bound to vanadium in the 5+ states. With progressive reaction times, the nature of the phosphorous species within the surfactant-mediated system alters considerably, with progressive shifts of the signal position. With increased reaction times (48 h) lines with chemical shifts between 200 and 1200 ppm were observed which suggests that the phosphorous within the system is interacting with both vanadium in the 5+ and 4+ states. XPS results indicate that both of these
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Fig. 6.
51 V
333
MAS NMR (A) and static (B) spectra of materials collected at a V:CTAB molar ratio of 0.25:1; (∗) indicates isotropic shift.
species are present. X-ray diffraction of the samples synthesised for more than 24 h shows phosphorous uptake and collapse of the lamellar structure with the resultant transformation to VOHPO4 ·0.5H2 O. At later reaction times (96 h), 31 P chemical shifts can be observed up to 2000 ppm with the most intense line being centred at approximately 1650–1700 ppm which
is characteristic of phosphorous linked to vanadium in the 4+ state in VOHPO4 ·0.5H2 O. There two specific questions to be considered at this point (1) whether this vanadium oxide lamellar phase collapses on phosphorous interaction due to lamellar surface charge neutralisation leading to hemihydrate formation or (2) whether the vanadium
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oxide lamellar phase re-dissolves and then reacts further with the phosphorous component within the system. A key factor is that hemihydrate formation is pH dependent and only occurs at pH values below 2.0 and will be discussed in future work. This factor is compatible with a model whereby PO4 3− anions exchange with the Br− anions of the inter-planar surfactant. Bromine was detected by EDXA at initial synthesis times, but became less intense as the phosphorous signal increased. The phosphorous NMR signal at short synthesis times had the characteristics of a solution phase species. Thus, phosphate builds up inside the lamellar vanadium oxide phase. Note that the P:V ratio shown in Fig. 4 for the synthesis time of 48 h is nearly identical to the value recorded after 96 h. However, VOHPO4 ·0.5H2 O was not observed by XRD until after 48 h of reaction (see Fig. 1). With sufficient phosphorous incorporation, the hemihydrate phase starts to develop, maintaining the general lamellar or platelet type appearance of the vanadium oxide lamellar structure (see Fig. 3). EDXA, NMR and XPS evidence are consistent with the attachment of increased levels of phosphorous to the solid phase as synthesis time progresses with the average oxidation state of vanadium moving closer to V4+ . This process, which is essentially viewed as slow, would eventually lead to shrinking of the layer spacing observed in VOHPO4 ·0.5H2 O upon expulsion of the surfactant. This expulsion possibly generated by disruption of the surfactant bilayer when PO4 3− replaces Br− may be the driving force for reduction in particle size with retention of the particle morphology that accompanied the hemihydrate formation.
5. Conclusion VOHPO4 ·0.5H2 O has been formed by the progressive attachment of phosphorous to a vanadium oxide lamellar phase. Phosphorous attachment to the positively charged lamella was found to cause collapse. Only upon lamellar collapse, hemihydrate formation was observed. This transformation is pH dependent and occurs below the pH value of 2.0. This transformation has been observed to accelerate on increased V:CTAB molar ratios and will be discussed at a later date.
Acknowledgements The authors acknowledge the financial support from the Materials and Surface Science Institute, University of Limerick. The collaboration between Limerick and Warwick was made possible through a Framework 5 Marie Curie Training Site Grant to Warwick. Mark E. Smith thanks the EPSRC for funding NMR equipment at Warwick. Leonard O’Mahony would like to thank Dr. Andrew Howes for his important contribution to this research. References [1] B.K. Hodnett, Heterogeneous Catalytic Oxidation, Wiley, New York, 2000 (Chapter 5). [2] B.K. Hodnett, Catal. Rev. Sci. Eng. 27 (1985) 373. [3] G. Centi, F. Trifiiro, J.R. Ebner, V.M. Franchetti, Chem. Rev. 88 (1988) 5. [4] E. Bordes, Catal. Today 1 (1987) 499. [5] G.J. Hutchings, Appl. Catal. 72 (1991) 1. [6] H. Igarashi, K. Tsuji, T. Okuhara, M. Misono, J. Phys. Chem. 97 (1998) 7065. [7] H.S. Horowitz, C.M. Blackstone, A.W. Sleight, G. Teufer, Appl. Catal. 38 (1988) 193. [8] M. Abon, K.E. Bere, A. Tuel, P. Delichere, J. Catal. 156 (1995) 28. [9] B.K. Hodnett, P. Permanne, B. Delmon, Appl. Catal. 6 (1983) 231. [10] C.C. Toradi, J.C. Calabrese, Inorg. Chem. 23 (1984) 1308. [11] H. Hatayama, M. Misono, A. Taguchi, N. Mizuno, Chem. Lett. (2000) 884. [12] N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater. 13 (2001) 179. [13] J.EI. Haskouri, M. Roca, S. Cabrera, J. Alamo, A. Beltran-Porter, D. Beltran-Porter, M. Dolores Morcos, P. Amoros, Chem. Mater. 11 (1999) 1446. [14] T. Doi, T. Miyake, Chem. Commun. (1996) 1635; T. Doi, T. Miyake, Chem. Mater. 6 (1994) 353. [15] V.V. Guliants, J.B. Benziger, S. Sundaresan, I.E. Wachs, J.M. Jehng, J.E. Roberts, Catal. Today 28 (1996) 275. [16] M.A. Carreon, V.V. Guliants, Microporous Mesoporous Mater. 55 (2002) 297. [17] M.A. Carreon, V.V. Guliants, Catal. Today 78 (2003) 303. [18] N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater. 13 (2001) 179. [19] T. Abe, A. Taguchi, M. Iwamoto, Chem. Mater. 7 (1995) 1429. [20] G. Centi, Catal. Today 16 (1993) 1–4. [21] D. Farrusseng, A. Julbe, M. Lopez, C. Guizard, Catal. Today 56 (2000) 211. [22] M.T. Sananes, A. Tuel, J.C. Volta, J. Catal. 145 (1993) 251. [23] M.T. Sananes, A. Tuel, G.J. Hutchings, J.C. Volta, J. Catal. 148 (1994) 395.
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