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Preparation, characterisation and reactivity of polydimethylsiloxane membranes for selective oxidation of benzene to phenol
Desalination 200 (2006) 673–675
Preparation, characterisation and reactivity of polydimethylsiloxane membranes for selective oxidation of benzene to phenol Raffaele Molinari*, Teresa Poerio, Pietro Argurio Department of Chemical Engineering and Materials, University of Calabria Via P. Bucci, Cubo 45/A, I-87030 Rende (CS), Italy Tel. +39 0984 496699; Fax +39 0984 496655; email: [email protected] Received 28 October 2005; accepted 3 March 2006
1. Introduction The one-step production of phenol by direct hydroxylation of benzene is a recent object of studies and searches. Indeed, the oxidation reaction has low selectivity since the phenol is more reactive towards oxidation than benzene, and substantial formation of by-products such as biphenyl and further oxidation compounds is found. The polydimethylsiloxane (PDMS) is one of the most studied polymer for catalytic reactions. This high permeable elastomer is prepared easily and combine a fairly high thermal (up to 250°C) and mechanical stability with chemical resistance [1]. The entrapment of zeolite crystals into rubbery polymers has been widely studied for application in pervaporation with encouraging results in terms of permeability and selectivity. However, few studies on hybrid membranes applied in catalytic reactions have been performed [2]. NaX zeolite, with FAU topology is used commercially as an adsorbent and catalyst. This zeolite is usually synthesised in the sodium *Corresponding author.
form with a Si/Al ratio in the range 1.0–1.5 that confers a strong hydrophilic character. NaX has pores and supercages of 7.4 and 13 Å, respectively. Such dimensions can provide a preferential channel for the extraction of phenol from organic phase thus taking it shelter from over oxidation. In this work the prepared dense PDMS membranes, pure and hybrid (charged with zeolite), have been characterised with SEM observations and then tested in a liquid-phase catalytic benzene oxidation to phenol by using a biphasic membrane reactor.
2. Results and discussions The filler (NaX) used in this study was purchased by Aldrich. Zeolite crystals before using were activated at 500°C and stored into a dryer to avoid water adsorption. The PDMS (Sylgard® 184 silicone elastomer) was supplied by Dow Corning Co. in a kit containing a base and a curing agent. The solvent used for preparing all membrane samples was chloroform (CHCl3) by Carlo Erba reagenti (purity > 99.5%).
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.473
674
R. Molinari et al. / Desalination 200 (2006) 673–675
(a)
(b)
Fig. 1. SEM cross section of different membranes (a) PDMS, (b) PDMS–NaX.
The results of preliminary tests, reported in Fig. 2 and Table 1, show an higher phenol concentration in the organic phase obtained by 8 Phenol conc. [g L–1]
PDMS membrane samples were prepared by pouring, in a Teflon plate, a solution containing the curing agent and the base with a ratio 1:10 (w/w) dissolved in CHCl3 stirring magnetically for 5 h. The solutions were evaporated overnight and then placed in an oven for 5 h at 150°C to allow the cross-linking of the material; finally the formed film was peeled from the Teflon plate. Also hybrid films were prepared by casting on a Teflon plate from the polymer solution in which the zeolite filler was dispersed. After a first evaporation step in air at room temperature, the films were cured in an oven for 6 h at 150°C. The cross-section images of PDMS and PDMS– NaX membranes are shown in Fig. 1(a) and (b), respectively. Fig. 1(b) shows a not homogeneous distribution of the crystals due to their sedimentation towards the membrane side in contact with the Teflon plate. This behaviour depends on: (i) viscosity of the suspension, (ii) difference of density between particles and polymer, (iii) interactions among filler particles and polymer. Those membranes were tested in a membrane reactor made with a two compartment cells to separate the organic and the aqueous phases. The system, operated at T = 35°C was composed by: (i) aqueous phase: 130 mL of ultrapure water containing 0.41 mmol of FeSO4, 18 mmol of hydrogen peroxide as the oxidant, and 4 mmol of acetic acid; (ii) organic phase: 130 mL of benzene corresponding to 1485.82 mmol; (iii) membrane placed between the two compartments to separate the phases.
PDMS PDMS–NaX 6 4 2 0 0
30
60
90
120
150
180
210
240
Time [min]
Fig. 2. Phenol concentration in the organic phase versus the time by using the two membranes. Table 1 Selectivity, benzene and hydrogen peroxide conversion by using the two membranes Membranes
Selectivity Benzene H2O2 to phenol conversion conversion (%)a to phenol (%)b to phenol (%)c
PDMS 99.85 PDMS–NaX 99.57 a
0.47 0.21
37.99 11.45
Selectivity to phenol = [mmol phenol/(mmol phenol + mmol biphenyl)] ´ 100. b Benzene conversion to phenol = (mmol phenol/mmol benzene initial) ´ 100. c Hydrogen peroxide conversion to phenol = (mmol phenol/mmol hydrogen peroxide initial) ´ 100.
R. Molinari et al. / Desalination 200 (2006) 673–675
using the PDMS membrane. In particular phenol concentrations equal to 4.96 and 2.27 g L–1 are obtained after 240 min by using PDMS and PDMS–NaX membranes, respectively. In Table 1 the results are reported in terms of selectivity and benzene and hydrogen peroxide conversion to phenol. The best values are obtained for the system that employs the PDMS membranes without the zeolite.
675
results show that PDMS membranes gave the best system performance in terms of selectivity to phenol (99.85%), benzene conversion to phenol (0.47%), and hydrogen peroxide conversion to phenol (37.99%). Research is in progress to improve further the system performance.
Acknowledgements The authors thank the MIUR within the FIRB 2003 programme for the financial support.
3. Conclusion Polydimethylsiloxane (PDMS) has been used successfully as pure material to prepare membranes and as host matrix to prepare hybrid membranes, by a dry phase inversion process. The filler used in the hybrid membrane was NaX zeolite crystals. The morphological characterisation by SEM showed a not homogeneous distribution of crystals for the hybrid membrane. Both membranes (hydrophobic) were used in a biphasic membrane reactor obtaining a selective oxidation of benzene to phenol. The reported
References [1]
[2]
I.F.J. Vankelecom, K.A.L.Vercruysse, P.E. Neys, W.A. Diedrik Tas, K.B.M. Janssen, P.P. KnopsGerrits and P.A. Jacobs, Novel catalytic membranes for selective reactions, Top. Catal., 5 (1998) 125–132. I.F.J. Vankelecom, R.F. Parton, M.J. Casselman, J.B. Uytterhoeven and P.A. Jacobs, Oxidation of ciclohexane using FePcY-zeozymes occluded in polydimethylsiloxane, membranes, J. Catal., 163 (1996) 457–464.
Preparation, characterisation and reactivity of polydimethylsiloxane membranes for selective oxidation of benzene to phenol Raffaele Molinari*, Teresa Poerio, Pietro Argurio Department of Chemical Engineering and Materials, University of Calabria Via P. Bucci, Cubo 45/A, I-87030 Rende (CS), Italy Tel. +39 0984 496699; Fax +39 0984 496655; email: [email protected] Received 28 October 2005; accepted 3 March 2006
1. Introduction The one-step production of phenol by direct hydroxylation of benzene is a recent object of studies and searches. Indeed, the oxidation reaction has low selectivity since the phenol is more reactive towards oxidation than benzene, and substantial formation of by-products such as biphenyl and further oxidation compounds is found. The polydimethylsiloxane (PDMS) is one of the most studied polymer for catalytic reactions. This high permeable elastomer is prepared easily and combine a fairly high thermal (up to 250°C) and mechanical stability with chemical resistance [1]. The entrapment of zeolite crystals into rubbery polymers has been widely studied for application in pervaporation with encouraging results in terms of permeability and selectivity. However, few studies on hybrid membranes applied in catalytic reactions have been performed [2]. NaX zeolite, with FAU topology is used commercially as an adsorbent and catalyst. This zeolite is usually synthesised in the sodium *Corresponding author.
form with a Si/Al ratio in the range 1.0–1.5 that confers a strong hydrophilic character. NaX has pores and supercages of 7.4 and 13 Å, respectively. Such dimensions can provide a preferential channel for the extraction of phenol from organic phase thus taking it shelter from over oxidation. In this work the prepared dense PDMS membranes, pure and hybrid (charged with zeolite), have been characterised with SEM observations and then tested in a liquid-phase catalytic benzene oxidation to phenol by using a biphasic membrane reactor.
2. Results and discussions The filler (NaX) used in this study was purchased by Aldrich. Zeolite crystals before using were activated at 500°C and stored into a dryer to avoid water adsorption. The PDMS (Sylgard® 184 silicone elastomer) was supplied by Dow Corning Co. in a kit containing a base and a curing agent. The solvent used for preparing all membrane samples was chloroform (CHCl3) by Carlo Erba reagenti (purity > 99.5%).
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.473
674
R. Molinari et al. / Desalination 200 (2006) 673–675
(a)
(b)
Fig. 1. SEM cross section of different membranes (a) PDMS, (b) PDMS–NaX.
The results of preliminary tests, reported in Fig. 2 and Table 1, show an higher phenol concentration in the organic phase obtained by 8 Phenol conc. [g L–1]
PDMS membrane samples were prepared by pouring, in a Teflon plate, a solution containing the curing agent and the base with a ratio 1:10 (w/w) dissolved in CHCl3 stirring magnetically for 5 h. The solutions were evaporated overnight and then placed in an oven for 5 h at 150°C to allow the cross-linking of the material; finally the formed film was peeled from the Teflon plate. Also hybrid films were prepared by casting on a Teflon plate from the polymer solution in which the zeolite filler was dispersed. After a first evaporation step in air at room temperature, the films were cured in an oven for 6 h at 150°C. The cross-section images of PDMS and PDMS– NaX membranes are shown in Fig. 1(a) and (b), respectively. Fig. 1(b) shows a not homogeneous distribution of the crystals due to their sedimentation towards the membrane side in contact with the Teflon plate. This behaviour depends on: (i) viscosity of the suspension, (ii) difference of density between particles and polymer, (iii) interactions among filler particles and polymer. Those membranes were tested in a membrane reactor made with a two compartment cells to separate the organic and the aqueous phases. The system, operated at T = 35°C was composed by: (i) aqueous phase: 130 mL of ultrapure water containing 0.41 mmol of FeSO4, 18 mmol of hydrogen peroxide as the oxidant, and 4 mmol of acetic acid; (ii) organic phase: 130 mL of benzene corresponding to 1485.82 mmol; (iii) membrane placed between the two compartments to separate the phases.
PDMS PDMS–NaX 6 4 2 0 0
30
60
90
120
150
180
210
240
Time [min]
Fig. 2. Phenol concentration in the organic phase versus the time by using the two membranes. Table 1 Selectivity, benzene and hydrogen peroxide conversion by using the two membranes Membranes
Selectivity Benzene H2O2 to phenol conversion conversion (%)a to phenol (%)b to phenol (%)c
PDMS 99.85 PDMS–NaX 99.57 a
0.47 0.21
37.99 11.45
Selectivity to phenol = [mmol phenol/(mmol phenol + mmol biphenyl)] ´ 100. b Benzene conversion to phenol = (mmol phenol/mmol benzene initial) ´ 100. c Hydrogen peroxide conversion to phenol = (mmol phenol/mmol hydrogen peroxide initial) ´ 100.
R. Molinari et al. / Desalination 200 (2006) 673–675
using the PDMS membrane. In particular phenol concentrations equal to 4.96 and 2.27 g L–1 are obtained after 240 min by using PDMS and PDMS–NaX membranes, respectively. In Table 1 the results are reported in terms of selectivity and benzene and hydrogen peroxide conversion to phenol. The best values are obtained for the system that employs the PDMS membranes without the zeolite.
675
results show that PDMS membranes gave the best system performance in terms of selectivity to phenol (99.85%), benzene conversion to phenol (0.47%), and hydrogen peroxide conversion to phenol (37.99%). Research is in progress to improve further the system performance.
Acknowledgements The authors thank the MIUR within the FIRB 2003 programme for the financial support.
3. Conclusion Polydimethylsiloxane (PDMS) has been used successfully as pure material to prepare membranes and as host matrix to prepare hybrid membranes, by a dry phase inversion process. The filler used in the hybrid membrane was NaX zeolite crystals. The morphological characterisation by SEM showed a not homogeneous distribution of crystals for the hybrid membrane. Both membranes (hydrophobic) were used in a biphasic membrane reactor obtaining a selective oxidation of benzene to phenol. The reported
References [1]
[2]
I.F.J. Vankelecom, K.A.L.Vercruysse, P.E. Neys, W.A. Diedrik Tas, K.B.M. Janssen, P.P. KnopsGerrits and P.A. Jacobs, Novel catalytic membranes for selective reactions, Top. Catal., 5 (1998) 125–132. I.F.J. Vankelecom, R.F. Parton, M.J. Casselman, J.B. Uytterhoeven and P.A. Jacobs, Oxidation of ciclohexane using FePcY-zeozymes occluded in polydimethylsiloxane, membranes, J. Catal., 163 (1996) 457–464.