Surfactant-assisted intercalation of high molecular weight poly(ethylene oxide) into vanadyl phosphate di-hydrate

Materials Research Bulletin 47 (2012) 774–778

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Surfactant-assisted intercalation of high molecular weight poly(ethylene oxide) into vanadyl phosphate di-hydrate Joa˜o Paulo L. Ferreira *, Herenilton P. Oliveira Departamento de Quı´mica, FFCLRP, Universidade de Sa˜o Paulo, Av. Bandeirantes 3900, Ribeira˜o Preto, SP 14040-901, Brazil

A R T I C L E I N F O

A B S T R A C T

Article history: Received 21 June 2011 Received in revised form 10 November 2011 Accepted 3 December 2011 Available online 29 December 2011

A high molecular weight poly(ethylene oxide)/layered vanadyl phosphate di-hydrate intercalation compound was synthesized via the surfactant-assisted approach. Results confirmed that surfactant molecules were replaced with the polymer, while the lamellar structure of the matrix was retained, and that the material presents high specific surface area. In addition, intercalation produced a more thermally stable polymer as evidenced by thermal analysis. ß 2011 Elsevier Ltd. All rights reserved.

Keywords: A. Layered compounds B. Intercalation C. X-ray diffraction Hybrid materials

1. Introduction An effective combination of organic polymers with inorganic layered hosts results in a class of hybrid materials with unique physical and chemical properties and potential application in diverse fields [1–5]. Among several inorganic matrices, the layered a-vanadyl phosphate di-hydrate (a-VOPO4
2H2O) has been extensively studied because of its ability to intercalate various guests into its interlayer space, which consists of corner-sharing VO6 octahedra and PO4 tetrahedra forming V–P–O sheets that are held together by weak bonding interactions [6,7]. In addition, the oxidizing character of a-VOPO4
2H2O allows for syntheses involving redox mechanisms when this compound is placed in the presence of a proper reducing agent [8,9]. So, taking advantage of these features, it is possible to obtain several different aVOPO4
2H2O intercalation compounds in which the guest species can vary from simple ions to polymeric species [10–17]. In order to synthesize polymer/inorganic hybrid materials, approaches such as in situ intercalation/polymerization of polymeric precursors into host inorganic structures, intercalation of an organic monomer followed by polymerization, or direct polymer intercalation has been widely investigated [18,19]. Poly(ethylene) oxide (PEO) and its analogue poly(ethylene glycol) PEG have been commonly intercalated into different inorganic matrices such as clays, graphite and V2O5 by direct intercalation of poly(ethylene oxide) (PEO) or its analogue poly(ethylene glycol) (PEG) [19–23].

* Corresponding author. Tel.: +55 19 92384209; fax: +55 16 3602 4838. E-mail address: [email protected] (J.P.L. Ferreira). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.12.003

However, to date only polymers with small molecular weight have been intercalated into different layered structures [19]. In this context, herein we propose an indirect method involving surfactant molecules for the intercalation of high molecular weight organic polymers into inorganic matrices. Basically, the procedure involves intercalation of cetyltrimethylammonium bromide (CTAB) as the expansion agent in the first step, in order to obtain a suitable interlamellar distance that will allow for more facile intercalation of macromolecules. In the next step, this lamellar surfactant-containing solid is subjected to further intercalation with high molecular weight poly(ethylene oxide) under hydrothermal conditions. Therefore, the adopted procedure improves textural properties such as the specific surface area. As a consequence, this material can be employed as a potential catalyst or precursor for vanadium-phosphorous-oxide system. 2. Experimental 2.1. Chemicals Vanadium(V) oxide, phosphoric acid, cetyltrimethylammonium bromide (CTAB), poly(ethylene oxide), and the other chemicals were purchased from Acros and used as received. Water was purified using a Millipore Milli-Q System. 2.2. Synthesis VOPO4
2H2O intercalated with CTA: first, vanadium pentoxide (24 g) was reacted with phosphoric acid solution (136 mL H3PO4 + 580 mL H2O) for 16 h at 130 8C. The resulting yellow

J.P.L. Ferreira, H.P. Oliveira / Materials Research Bulletin 47 (2012) 774–778

2.3. Characterization Elemental analysis (carbon, nitrogen and hydrogen) was performed using an Elemental Analyzer from CE Instruments, model EA-1110, by means of the Dynamic Flash Combustion method. Thermogravimetric data were registered on a Thermal Analyst equipment model 2100-TA in air atmosphere, at a 10 8C min 1 heating rate. The powder X-ray diffraction (PXRD) patterns were recorded on a SIEMENS D5005 diffractometer using a graphite monochromator and CuKa emission lines (1.541 A˚, 40 kV, 40 mA). The data were collected at room temperature over the range 28  2u  508, with a step of 0.0208. Fourier-transform infrared spectra (FTIR) were acquired from 4000 to 400 cm 1 on a Bomem MB 100 spectrometer, and the samples were dispersed in KBr and pressed into pellets (2%, w/w). Specific surface area was determined using the Brunauer–Emmett–Teller (BET) method with N2 adsorption (Quantachrome, NOVA 1200). Scanning electronic microscopy (SEM) studies were accomplished on a ZEISS – DSM 940 microscope, operating at 20 kV. 3. Results and discussion Fig. 1a–c depicts the typical powder X-ray diffraction patterns for the VOPO4/PEO, VOPO4/CTA, and VOPO4/CTA/PEO intercalated compounds and the vanadyl phosphate matrix (Fig. 1, insert). The PXRD patterns of all the intercalated compounds displayed broad peaks with low intensity, suggesting a decrease in crystallinity as compared to the starting matrix. The PXRD patterns of the obtained materials were consistent with a lamellar structure related to the presence of 0 0 1 reflection. In addition, the shift of the 0 0 1 reflection to lower 2u values indicated an increase in the interlayer spacing, thus confirming intercalation of the organic species into the matrix d-spacing equal to 1.30, 1.49, and 2.84 nm for VOPO4/ PEO; VOPO4/CTA, and VOPO4/CTA/PEO, respectively, as compared to the d-spacing of 0.75 nm for initial VOPO4
2H2O. After the hydrothermal reaction between VOPO4
2H2O and PEO

(001)

1000cps

Intensity/a.u.

250cps

001

Intensity/a.u.

product was collected by filtration and washed with acetone, followed by drying at room temperature (24 8C) under vacuum and characterization by powder X-ray diffraction and infrared spectroscopy. Thermogravimetric analysis revealed that the H2O/ VOPO4 ratio was 2. In order to obtain the intercalation compound, the resulting solid (0.50 g) was reacted with 0.3 M cetyltrimethylammonium bromide ethanolic solution for 7 days inside a steel reactor at 70 8C, 4 atm, under inert atmosphere (N2). Finally, the intercalated compound (blue) was recovered by centrifugation and washed with acetone. After each washing, the solid was separated from the liquid by centrifugation, and the product was allowed to dry at room temperature (24 8C). It should be noted that powder Xray diffraction and Fourier-Transform infrared data revealed that there was no reaction between the matrix and the surfactant molecules under mild conditions (1 atm and room temperature). Poly(ethylene oxide) intercalation into VOPO4/CTA: VOPO4/CTA (0.07 g) was poured into 25 mL PEO (300,000 g mol 1) aqueous solution placed inside a reactor. The reaction was conducted at 70 8C, 4 atm, under inert atmosphere (N2), for two days. A dark solid product, named VOPO4/CTA/PEO, was obtained and separated by filtration, washed with acetone, and finally dried in vacuum. For comparison purposes, a direct reaction between the VOPO4
2H2O and PEO (300,000 g mol 1) was carried out in the absence of intercalated cetyltrimethylammonium cation, under the same conditions mentioned above. The dark solid was designated VOPO4/PEO. It must be emphasized that there was no reaction between the matrix and the organic polymer under soft conditions, as indicated by the powder X-ray diffraction pattern and the Fourier-Transform infrared spectrum.

775

VOPO4.2H2O

(002)

002

(101)

003

10

20

(003)

30

2theta/degree

001

002

003

40

(c) (b)

001

(a) 10

20

30

40

2theta/degree Fig. 1. CuKa X-ray diffraction patterns of the VOPO4/PEO (a), e VOPO4/CTA (b) and VOPO4/CTA/PEO (c).

(300,000 g mol 1), the diffraction peak moved toward low 2u region, revealing formation of a new layered structure in the case of VOPO4/PEO (Fig. 1a). However, the PXRD pattern of this solid presented a broad peak, suggesting poor crystallinity. This unexpected result led us to repeat the synthetic procedure, but similar PXRD patterns were obtained again. Here, it is very important to mention that other works have failed to intercalate PEO into the vanadyl phosphate matrix even in the case of very low molecular weight polymers [19]. As for the VOPO4/CTA sample (Fig. 1b), the interlayer spacing calculated from 2u angles was d0 0 1  d0 0 2  d0 0 3  1.50 nm (d0 0 1 = 1.49 nm, d0 0 2 = 0.76 nm, d0 0 3 = 0.51 nm), reflecting the extent of intercalation as well as the regularity of the stacking of the vanadyl layers. In other words, VOPO4/CTA still consisted of parallel and continuous layers with homogeneous spacing, despite the poor crystallinity. Furthermore, the PXRD pattern indicated that the host’s framework was preserved after the reaction, which is coherent with occurrence of a topotactic reaction. As already mentioned, the VOPO4 interlayer spacing increased from 0.75 to 1.49 nm upon insertion of the surfactant CTA (Dd = 0.749 nm). Taking into account that the length of the fully stretched CTA+ is about 2.37 nm, and that the cross-sectional diameter of the head of the CTA+ ion [(CH3)3N+–] is about 0.5 nm, from the increase in the basal distance it is reasonable to assume that the guest species is oriented in parallel with the VOPO4 sheets, in a bilayer arrangement. In addition, diffraction peaks relative to CTA were absent from the PXRD pattern (Fig. 1b), indicating that the guest species is mainly located between the inorganic sheets instead of being adsorbed onto the matrix. After reaction of VOPO4/CTA with PEO (300,000 g mol 1), the PXRD peaks of the product moved toward lower 2u region (Fig. 1c) as compared to VOPO4/CTA and VOPO4/PEO. The d-spacing increased from 1.49 nm in the case of VOPO4/CTA to 2.94 nm in the case of VOPO4/CTA/PEO, as the diffraction angle shifted from 5.92 to 3.00 for the d0 0 1 peak, respectively. The diffraction peaks were not equidistant, evidencing that the sample presents a slightly distorted lamellar structure. Comparison of the samples VOPO4/ CTA and VOPO4/CTA/PEO demonstrated that PEO intercalation did not significantly change the crystalline structure of the VOPO4 matrix. It should be noted that the low intensity peak at 5.05 suggests the presence of a second interlayer domain arising from the stress on the framework during the intercalation process. As for the strongest peak (d0 0 1), not only does it indicate an increase in the interlayer distance, but it also shows that the intensity is higher in the presence of PEO. Thus, the presence of CTA facilitated PEO

J.P.L. Ferreira, H.P. Oliveira / Materials Research Bulletin 47 (2012) 774–778

776

10 % 4000

3000

2000

δ(O-P-O) δ(C-C)

δ(V-O-P)

νs(P-O)

ν(V-OH)

ν(V=O)

νas(P-O) ou ν(C-O-C)

ν(CH2)

ν(O=C=O) νs(C-H)

νas(C-H)

+

ν(N-H) em NH4

(c)

ν(CH2)

(b)

ν(O-H)banda larga

Transmitance / %

(a)

1000 -1

Wavenumber / cm

Fig. 2. FF-IR spectrum of VOPO4
2H2O (a), VOPO4/PEO (b) and e VOPO4/CTA/PEO (c).

Loss Mass / %

intercalation into the interlayer domain, leading to an intercalated nanocomposite. In other words, the CTA introduced a hydrophobic character into the lamellar domain of the VOPO4 matrix, thereby favoring PEO intercalation by matching of the PEO polarity with the polarity of CTA. Besides, the entropy gained by desorption of solvent molecules compensated the reduction in entropy elicited by confinement of the intercalated polymeric chain [24]. The infrared spectra of VOPO4
2H2O, VOPO4/PEO, and VOPO4/ CTA/PEO recorded between 4000 and 400 cm 1 are shown in Fig. 2. In the case of the VOPO4
2H2O matrix (Fig. 2a), the vibrational bands at 1160 and 956 cm 1 are assigned to the P–O stretching vibration of the PO4 tetrahedral group and the V–O stretching of the vanadyl group (V5 5O). The bands at 678 and 567 cm 1 correspond to V–O–P and O–P–O bending vibrations, respectively [6,25,26]. The broad bands near 1614 and 3300 cm 1 are associated with water molecules weakly bound to the matrix. The band at 3559 cm 1 is ascribed to the O–H vibrational mode related to water molecules bound to the vanadium site (V–OH2). As for CTA, its spectrum presented the characteristic bands at 3018, 2912, 2847, and 1472 cm 1, attributed to the n(NH4+), nas(C–H), ns(C–H), and s(CH2) vibrational modes, respectively [24]. Concerning pure poly(oxide-ethylene), its spectrum displayed a large broad band relative to asymmetric CH2 stretching between 3000 and 2750 cm 1, with two narrow bands appearing at 2739 and

100 95 90 85 80 75 70 65 60 55 50 45 40

o

Rate =10 C/min, N2 VOPO4.2H2O VOPO4/PEO VOPO4/CTA VOPO4/CTA/PEO

I 100

II 200

III 300

400

500

600

700

o

Temperature / C Fig. 3. Thermogravimetric for VOPO4
2H2O, VOPO4/PEO, VOPO4/CTA and VOPO4/ CTA/PEO.

2693 cm 1. Additionally, there were several peaks in the 1500– 700 cm 1 region, associated with the main vibrational modes sa(CH2), na(COC), and ra(CH2) at 1467, 1109, and 843 cm 1, respectively [27]. With respect to the spectrum of VOPO4/CTA/PEO (Fig. 2c), there were no significant shifts in the bands relative to the organic components CTA and PEO as compared to the pure ones. The bands lying in the region between 850 and 1200 cm 1 were difficult to interpret because phosphates and V–O species absorb at nearly the same frequencies, not to mention the superposition with the vibrational modes of PEO. Nevertheless, in the 500– 750 cm 1 range it was possible to note that the vibrational modes of the matrix (678 cm 1 and 567 cm 1) shifted toward higher wavenumber due to constraints imposed by the intercalation. Moreover, the broadening of these bands suggested that there are perturbations to the structure arising from host–guest interactions. However, the overall framework of the matrix was maintained, corroborating the X-ray data. Fig. 3 illustrates the thermogravimetric curves of VOPO4
2H2O, VOPO4/PEO; VOPO4/CTA, and VOPO4/CTA/PEO. For VOPO4
2H2O (Fig. 3a), there were two main weight loss events. The first extended to around 125 8C and is attributed to loss of weakly bound water. The second went on up to around 280 8C and is ascribed to release of intramolecular water and water molecules bound to vanadium. This thermogravimetric curve revealed that the formula of the vanadyl phosphate is VOPO4
2.0H2O. The thermal decomposition of the VOPO4 intercalation compounds generally proceed through the following steps: (i) desorption of the physically adsorbed water, (ii) removal of the interlayer water, (iii) release of coordinated water molecules, and (iv) decomposition of the organic components (CTA and PEO). These steps start from room temperature and continue up to ca. 500 8C. Together with the elemental chemical analysis data, thermogravimetric weight loss values were used to determine the composition of the prepared samples (Table I). The minor amount of nitrogen found in VOPO4/ CTA/PEO suggested that VOPO4/CTA surfactant molecules were released upon PEO insertion, which is consistent with the proposed procedure. Besides, it was reasonable to expect that residual CTA molecules would remain in the hybrid material, as observed in Table 1. Comparing the thermogravimetric curves of VOPO4/PEO (Fig. 3b) and VOPO4/CTA/PEO (Fig. 3d), it was noted that decomposition of the organic species occured at a higher temperature (from 250 8C to 330 8C), reflecting the effect of the two-dimensional constraints imposed by the rigid inorganic sheets on the organic molecules. Furthermore, this thermal stability can

J.P.L. Ferreira, H.P. Oliveira / Materials Research Bulletin 47 (2012) 774–778 Table 1 Results of elemental analysis of the intercalation compounds. Sample

Nitrogen (%)

Carbon (%)

Hydrogen (%)

CTAB (mol)

VOPO4/PEO VOPO4/CTA VOPO4/CTA/PEO

– 0.4 0.1

8.7 11.4 34.1

2.0 3.6 6.5

– 0.1 0.03

be attributed to the stabilizing effect arising from increased interactions between the polymeric chains. Therefore, the intercalation stabilized the organic polymer, affecting the initial decomposition temperature. The SEM images of VOPO4
2H2O, VOPO4/PEO, VOPO4/CTA, and VOPO4/CTA/PEO are represented in Fig. 4. VOPO4
2H2O consisted of irregular stacked square platelets (Fig. 4a) with a length of approximately 15–30 mm and thickness less than 0.4 mm. The SEM image of the VOPO4/PEO sample (Fig. 4b) demonstrated that the crystallites partially retained the platy morphology of the initial VOPO4
2H2O matrix after the intercalation process. Despite the warped shape and cracks originating from the mechanical stress and interlayer expansion, the image reflected a lamellar structure for VOPO4/PEO, as verified by PXRD data. It should be emphasized that the PEO was also present around the matrix particles. Reaction with CTA resulted in a material in which the corners of the square VOPO4 became rounded. Moreover, VOPO4/CTA did not present the initial plate-like morphology, despite its lamellar structure (Fig. 4c). A exfoliation-reconstruction process probably occurred, i.e., there was delamination of the stacked sheets by swelling of the interlayer spaces in the medium, followed by restacking of the exfoliated layers. In the case of VOPO4/CTA intercalation with PEO (Fig. 4d), this process started when the matrix swelled upon contact with the solvent, followed by PEO insertion into the interlamellar domain. During the reconstruction, the PEO units were then inserted into the host’s structure. However, the SEM

777

image suggested that for VOPO4/CTA/PEO this exfoliation–reconstruction process was not at all complete, since PEO was also present on the surface and edges of the inorganic host. Nevertheless, it is noteworthy that the amount of PEO present in VOPO4/ CTA/PEO was larger than the quantity that would have been obtained by its direct incorporation into VOPO4; i.e., without the assistance of the CTA molecules, as discussed above. Therefore, the surfactant-assisted intercalation methodology indeed allowed for incorporation of high molecular weight polymers, in our case, poly(ethylene oxide). It should be noted that reactions of layered a-vanadyl phosphate dihydrate with surfactant molecules and later reaction of the matrix containing surfactant molecules with poly(ethylene oxide) led to materials with higher specific surface area as compared to the matrix alone (15 m2/g), more specifically 45 m2/g for VOPO4/CTA and 220 m2/g for VOPO4/CTA/PEO. BET results showed that the surface area increased in the presence of the guest species, being remarkable for the ternary hybrid material. Therefore, the replacement of surfactant molecules by poly(ethylene oxide) culminated in a material with improved of specific surface area. Besides, the obtained surface area was consistent with the morphology revealed by SEM. In addition, this surface area value was comparable to other values reported for the VPO system synthesized via the solvothermal method [28] and via thermal treatment of vanadyl n-butylphosphate [29], indicating that this method is convenient for the preparation of materials with high surface area that can be used as a VPO catalytic system. 4. Conclusion Layered poly(ethylene oxide)/vanadyl phosphate has been synthesized by using a surfactant molecule, which increases the distance between the matrix layers, thereby improving polymer intercalation. During this process, surfactant molecules are

Fig. 4. Scanning electron micrographs of the VOPO4
2H2O (a), VOPO4/PEO (b), VOPO4/CTA(c) and VOPO4/CTA/PEO (d).

778

J.P.L. Ferreira, H.P. Oliveira / Materials Research Bulletin 47 (2012) 774–778

replaced with guest species, while the structural integrity of the matrix is retained. In addition, intercalation induces higher thermal stability for the polymer, as evidenced by thermal analysis. Besides, the end product contains low amount of surfactant molecules. Moreover, it was found that this procedure leads to materials with surface area comparable to those obtained by solvothermal and thermal treatment methods, making the resulting hybrids, suitable for catalysis. The use of this strategy proved to be viable for intercalation of an organic polymer with high molecular weight into the lamellar structure of the matrix, with structure retention, and can be used as a new approach for the preparation of new materials, without affecting the structural integrity of the matrix. Acknowledgements The authors thank CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico) and FAPESP (Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo) for financial support. CAPES (Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior) is gratefully acknowledged for fellowships. References [1] C. Sanchez, B. Julian, P. Belleville, M. Popall, J. Mater. Chem. 15 (2005) 3559. [2] C. Sanchez, H. Arribart, M.M.G. Guille, Nat. Mater. 4 (2005) 277. [3] P. Gomez-Romero, Adv. Mater. 3 (2001) 163.

[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

F. Mammeri, E. Le Bourhis, L. Rozes, C. Sanchez, J. Mater. Chem. 15 (2005) 3787. M. Alexandre, P. Dubois, Mater. Sci. Eng. R: Rep. 28 (2000) 1. J.W. Johnson, A.J. Jacobson, J.F. Brody, S.M. Rich, Inorg. Chem. 21 (1982) 3820. J. Kalousova, J. Votinsky, L. Benes, K. Melanova, V. Zima, Collect. Czech. Chem. Commun. 63 (1998) 1. V. Zima, L. Benes, K. Melanova´, M. Vlcek, J. Solid State Chem. 163 (2002) 281. A. Chauvel, M.E. de Roy, J.P. Besse, et al. Mater. Chem. Phys. 40 (1995) 207. V. Zima, L. Benesˇ, K. Melanova, Solid State Ionics 106 (1998) 285. N. Kinomura, T. Toyama, N. Kumada, Solid State Ionics 78 (1995) 281. P. Cˇapkova´, M. Trchova´, V. Zima, H. Schenk, J. Solid State Chem. 150 (2000) 356. A. De Stefanis, S. Foglia, A.A.G. Tomlinson, J. Mater. Chem. 5 (1995) 475. K. Mela´nova´, L. Benesˇ, V. Zima, J. Votinsky´, J. Solid State Chem. 157 (2001) 50. K. Goubitz, P. Capkova´, K. Mela´nova´, W. Molleman, H. Schenk, Acta Crytallogr., Sect. B 57 (2001) 178. V. Zima, K. Mela´nova´, L. Benes, M. Trchova´, P. Capkova´, P. Matejka, J. Chem. Eur. 8 (2002) 1703. L. Benes, V. Zima, K. Mela´nova´, M. Trchova´, P. Capkova´, B. Koudelka, P. Matejka, Chem. Mater. 14 (2002) 2788. R. Bissessur, et al. J. Solid State Sci. 8 (2006) 531. K. Melanova´, L. Benes, V. Zima, R. Vahalova´, Chem. Mater. 11 (1999) 2173. P. Aranda, E. Hitzky-Ruiz, Appl. Clay Sci. 15 (1999) 119. A. Kelarakis, E.P. Giannelis, Polymer 52 (2011) 2221. S. Chia-Chi, S. You-Hwei, J. Colloid Interface Sci. 332 (2009) 11. E.M. Guerra, J.K. Ciuffi, H.P.Oliveira, J. Solid State Chem. 179 (2006) 3814. D. Ratna, S. Diveka, A.B. Samui, B.C. Chakraborty, A.K. Banthia, Polymer 47 (2006) 4068. G. Matsubayashi, H. Nakajima, Chem. Lett. 1 (1993) 31. A. Espina, C. Trobajo, S.A. Khainakov, J.R. Garcia, A.I. Bortun, J. Chem. Soc., Dalton Trans. 5 (2001) 753. B.L. Papke, M.A. Ratner, D.F. Shiriver, J. Phys. Chem. Solids 42 (1981) 493. A.A. Rownaghi, Y.H. Taufiq-Yap, F. Rezaei, Chem. Eng. J. 155 (2009) 514. Y. Kamiya, E. Nishikawa, A. Satsuma, M. Yoshimune, T. Okuhara, Micropor. Mesopor. Mater. 54 (2002) 277.