Preparation, characterisation and testing of catalytic polymeric membranes in the oxidation of benzene to phenol

Applied Catalysis A: General 358 (2009) 119–128

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Preparation, characterisation and testing of catalytic polymeric membranes in the oxidation of benzene to phenol Raffaele Molinari *, Teresa Poerio Department of Chemical and Materials Engineering, University of Calabria, Via P. Bucci, 44/A, I-87036 Rende, CS, Italy

A R T I C L E I N F O

A B S T R A C T

Article history: Received 29 December 2008 Received in revised form 27 January 2009 Accepted 31 January 2009 Available online 12 February 2009

Liquid-phase catalytic oxidation of benzene to phenol has been studied under mild conditions using catalytic polymeric membranes. They were prepared by dissolving the polymer polyvinylidene fluoride (PVDF) in different solvents, (dimethylacetammide (DMAc), dimethylformammide (DMF) and 1-methyl2-pyrrolidone (NMP)) and filled with two types of copper oxides catalysts. Membranes were characterised morphologically by scanning electron microscopy (SEM), pore size distribution, contact angle measurements and BET surface area. It was found that DMAc was the best solvent and that membranes filled with the CuO nanopowder catalyst presented an improved wettability to reacting mixture. Reactivity tests, performed at contact time in the range 4–19.4 s between reagents and catalyst, showed a higher phenol concentration using the PVDF membrane filled with the CuO nanopowder rather than CuO powder catalysts. Phenol productivity of 72.5 gphenol gcat1 h1 and a phenol yield of 2.3% were obtained in a single pass using a contact time with the catalyst of 19.4 s at 358 C in presence of ascorbic acid. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Catalytic polyvinylidene fluoride membranes Liquid phase benzene oxidation to phenol Copper(II) oxide catalyst Ultrafiltration membrane reactor Filled catalyst organic membrane BET surface area

1. Introduction Phenol is an important intermediate for the manufacture of petrochemicals and plastics. It is mainly produced by means of the cumene process. However, this process is complex, as it requires many steps, and have also ecological disadvantages. Hence, the study of a one-step process for the phenol production by direct oxidation of benzene is one of the most challenging tasks in oxidation catalysis. Membrane technology in oxidation reactions has received increasing attention during recent two decades. Most investigations on catalytic membranes have focused on the use of inorganic membranes because of their excellent thermal stability at high reaction temperatures. Although applications of this type of membranes concerned small molecules or decomposition reactions at high temperature, polymeric membranes can be applied in the case of low-temperature reactions with versatile applicability [1–6]. Most polymeric membranes can be easily manufactured in different shapes (e.g. hollow, spiral wound, flat sheet); they are elastic, have satisfactory diffusion and sorption coefficients and can be produced with incorporated catalysts as nanosized dispersed metallic clusters, zeolites, activated carbon or metallic

* Corresponding author. Tel.: +39 0984 496699; fax: +39 0984 496655. E-mail address: [email protected] (R. Molinari). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.01.044

complexes. The ability to produce a well-defined porous matrix that can serve as a support for a wide variety of catalytic materials gives an interesting contribute to the production of single-site catalysts in which every active site closely resemble to each other [7,8]. The membrane can select the passage of molecules of reactants across its structure, control the feeding of reactant and improve the contact between reactants and catalyst. Polymeric membranes show different affinities for different chemicals, therefore they can drive a reaction modulating the adsorption and diffusion of some components of the reaction mixture. Catalytic polymeric membranes can be prepared controlling the mechanical, chemical and thermal stabilities to yield the desired permeability and affinity for reagents and products [9]. Indeed a good porous membrane must have high permeability and excellent chemical resistance to the feed stream. The high permeability can be obtained by a high surface porosity and thin separating layer thickness. Another advantage on the employment of polymeric catalytic membranes is their ability to allow a re-use of the catalyst. All the advantages of polymeric membranes with respect to inorganic membranes can be exploited in catalytic applications provided a not very high temperature is employed. An example of this application is the one-step production of phenol in mild condition by direct hydroxylation of benzene. This is a recent subject of studies and search by several groups in the world

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[10–19], where also inorganic membranes, such as Pd membrane reactor, at a reaction temperature of 433 K [20,21], have been tested. Our group is trying to develop the process employing a membrane reactor working in mild conditions [22–24]. 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 was found. The control of contact time of phenol with the catalyst can avoid by-products formation. An innovation, with respect to the previous tested membrane reactor configuration, is the control of reactivity by inclusion of the catalyst in polymeric membranes and permeating the oxidant solution, containing the substrate, at different permeate flow rates. In this work, the polymer used as membrane material is polyvinylidene fluoride (PVDF) because of its excellent chemical resistance, particularly the oxidant resistance. The catalytic PVDF asymmetric membranes were prepared by using the phase inversion method induced by nonsolvent. The effects of different solvents on membrane morphology and catalytic properties were investigated by filling the membranes with two types of copper(II) oxide. The catalytic reaction of liquid-phase oxidation of benzene to phenol was tested assembling the membrane in a single pass ultrafiltration membrane reactor. 2. Experimental 2.1. Materials Commercially available polyvinylidene fluoride Solef1 6010 (Solvey Solexis S.A., Belgium) was used as membrane material. N,N-dimethylformammide (C3H7NO, M.W. = 73.09 g mol1, 99.5% purity, Riedel-de Hae¨n), N,N-dimethylacetammide (C4H9NO, M.W. = 87.12 g mol1, 98% purity, Fluka) and 1-methyl-2-pyrrolidone (

, M.W. = 99.13 g mol1, 99.8%

purity, Carlo Erba Reagents) were used as polymer solvents. Ultrapure water Millipore ELIX 5, was used as coagulation bath. Acetonitrile (CH3CN, M.W. = 41.05 g mol1, purity 99.7%, LabScan), acetic acid (C2H4O2, M.W. = 60.05 g mol1, 99.5% purity, Fluka), ascorbic acid (C6H8O6, M.W. = 176.13 g mol1, Fluka), hydrogen peroxide (H2O2, 30 wt% in water, Sigma–Aldrich) and benzene (C6H6, M.W. = 78.11 g mol1, 99.8% purity, Carlo Erba Reagents) were used to prepare the reacting solutions. Phenol (C6H5OH, M.W. = 94.11 g mol1, 99.99% purity, Sigma–Aldrich) was used for calibration curve. Copper(II) oxides (CuO, M.W. = 79.54 g mol1, 99% purity, Sigma–Aldrich) in two types, powder and nanopowder form, were used as catalysts. The particles size of the powder copper oxide was smaller than 5 mm. The mean particles size of the nanopowder copper oxide was equal to 33 nm. 2.2. Preparation of catalytic polymeric membranes and polymeric membranes Flat-sheet PVDF catalytic membranes were prepared by a phase inversion process. In this process the precursor polymer is transformed from a liquid to a solid state in a controlled manner. The solidification process is very often initiated by the liquid – liquid demixing. At a certain stage during demixing, one of the liquid phases (that one with high polymer concentration) will solidify so that a solid matrix is formed. In the phase inversion process induced by a nonsolvent the liquid mixture consisting by a polymer and a solvent is cast as a thin film upon a support (e.g. a glass plate) and then it is immersed in a coagulation bath containing the nonsolvent. The solvent diffuses into the coagulation bath whereas the nonsolvent will diffuse into the casted film.

After a given period of time, the exchange of the solvent and nonsolvent has proceeded so far thus the mixture becomes thermodynamically unstable and demixing takes place. Finally a solid polymeric film is obtained with an asymmetric structure. Different factors affect membrane morphology. These are: type of polymer, type of solvent and nonsolvent, composition of casting solution, composition of coagulation bath, temperature of the casting solution and the coagulation bath, evaporation time. A small change in one of these parameters can change the overall structure and consequently affect membrane performance. For the preparation of membranes, dimethylacetammide (DMA), dimethylformammide (DMF) and 1-methyl-2-pyrrolidone (NMP) were the solvents and distilled water was the nonsolvent. A solution (20 wt%) was prepared by dissolving the polymer in each solvent by magnetic stirring at room temperature (about 25  1 8C); then the CuO powder (or CuO nanopowder) at 16 wt% was added and the resulting mixture was stirred for an additional 24 h. It was then cast on a glass plate, by setting the knife gap at 250 mm, and exposed 30 s in air before immersing it in a coagulation bath. The plate with the thin film casting mixture was firstly immersed for 10 min in the coagulation bath containing distilled water at 25 8C, secondly, the precipitate (formed) membrane was removed from the coagulation bath and leached under running water for 24 h to remove residues of solvents. Then the membranes were dried in an oven at 60 8C. The same preparation procedure was employed to prepare the polymeric membranes without the catalyst. The prepared membranes without catalyst will be termed PVDFx (x varies from 1 to 3, where the x number indicate the type of solvent used for membrane preparation). In particular x equal to 1 means DMF, x equal to 2 DMAc and x equal to 3 NMP. The membranes filled with the copper oxide powder and copper oxide nanopowder will be termed PVDFxCuOp and PVDFxCuOnanop, respectively. Also in this case x indicates the solvent used for membrane preparation. 2.3. Membrane and catalyst characterisation The prepared membranes were characterised by the following methods to obtain some information useful to their use in the membrane reactor: (1) Membrane thickness was measured using a digital micrometer (Carl Mahr D7300 Esslingen a.N.) averaging 5 measurements. (2) Cross-section of the membranes was observed by means of FEI QUANTA 200 scanning electron microscope (SEM). Membrane samples were broken in liquid nitrogen to keep unaltered the film structure. (3) Contact angles to water and to reacting solution droplets on the membrane surfaces were measured by the sessile drop method using a CAM 200 contact angle meter (KSV Instruments, Helsinki, Finland). Contact angles to reacting solution droplets were only measured for the tested membranes (DMAc). (4) Pore size distribution was determined by a Capillary Flow Porometer (CFP 1500 AEXL). The porometry tests were performed in dry-up/wet-up mode. This one is the most commonly used; it is performed running the dry phase before the wet phase and it usually yields the best dry data. The dry sample is installed in the sample chamber with an effective area of 3.14 cm2. An increasing flow is applied, causing the pressure and the flow rate through the sample to increase. Data points are taken at each equilibrium point as long as the pressure and flow rates are increased. As the pressure increases, smaller and smaller pores are opened in the sample. After this run the dry samples were removed and were immersed in the wetting fluid Porewick1 from PMI, surface tension = 16  105 J  cm1 (16 dyne  cm1), for at least 15 min and then placed in the sample chamber. Then, a

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successively increasing pressure is applied across the membrane sample using nitrogen as pressurising gas. At a certain pressure, the surface tension of the pore filling liquid in the largest pores is exceeded. The liquid is displaced and gas flow through the open pore can be monitored. Further pressure increase causes successive liquid displacement in smaller pores until all pores are open. By comparing the gas flow rate of a dry and wet sample at the same pressure, the percentage of flow (Q) passing through the pores larger than or equal to the specified may be calculated from the pressure size relationship.   ðwet flowÞh ðwet flowÞl Q¼   100 ðdry flowÞh ðdry flowÞl where Q is the percentage flow rate, l and h are the lower pressure limit and the higher pressure limit, respectively. The measured flow rate data allow the calculation of the pore size distribution (D). D¼

ðQ  Q L Þ ðdL  dÞ

where d is the maximum pore diameter and L is the previous value. (5) The permeation properties for PVDF2, PVDF2CuOp and PVDF2CuOnanop membranes were tested in an ultrafiltration membrane reactor fed with the reacting solution (acetonitrile– benzene mixture 8:1 (v/v), containing hydrogen peroxide 1:1 molar ratio with benzene) at different transmembrane pressures (range from 0.1 to 0.6 bar) in order to control the contact time of reagents and products with the catalyst. (6) BET (Brunauer–Emmet–Teller) measurements were performed with a Micromeritics Asap 2010 apparatus. It was determined the surface area, pore volume, average pore size of the catalysts (CuOp and CuOnanop), the tested catalytic membranes (PVDF2CuOp, PVDF2CuOnanop) and the bare membrane PVDF2. The sample tube was filled with a known amount of catalyst or membrane and degassed by heating at 350 8C (150 8C for the membrane) and evacuated for at the least 8 h. The volume of nitrogen adsorbed onto the sample at the temperature of liquid nitrogen (190 8C) was measured as a function of the relative pressure. 2.4. Experimental plant, calculation of contact time and procedure of catalytic tests The catalytic tests were performed in the experimental plant schematised in Fig. 1. This is an ultrafiltration membrane reactor,

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where a peristaltic pump fed the permeation cell with the feed solution containing the reagents. The membrane in the permeation cell had an exposed surface area of 4 cm  6 cm = 24 cm2. This type of reactor permits to control the contact time between the feed solution and the catalyst by changing the permeate flow rate operating at different transmembrane pressures checked by a manometer. Calculation of contact times were performed using the following equations: v¼

Permeate flow rate ðmL=minÞ cm3 =min cm ¼ ¼ min Membrane surface ðcm2 Þ cm2

where v is the permeation velocity of the reacting solutions across the membrane Contact time ¼

Membrane thickness ðcmÞ vðcm=minÞ

The catalytic tests in the membrane reactor were carried out employing the following procedure. A solution containing 10 mL of an acetonitrile–benzene mixture 8:1 (v/v) ratio and hydrogen peroxide 1:1 molar ratio with benzene was pumped in the permeation cell at different transmembrane pressures corresponding to different contact times of feed solution with the catalyst. The permeate solution was collected and analysed for determining the concentrations of phenol and other by-products. Every test was repeated twice. Other catalytic tests were performed using the CuOnanop catalyst and the PVDF2CuOnanop membrane in an open batch reactor to evaluate the best catalytic system. The amount of CuOnanop catalyst was the same of that one entrapped in the PVDF2CuOnanop membrane (0.625 g); in addition 4 mmol of ascorbic acid, an acetonitrile–benzene mixture 8:1 (v/v) ratio and hydrogen peroxide 1:1 molar ratio with benzene were used. The batch reactor was thermostated at 35 8C and stirred by a magnetic bar. In the batch catalytic tests carried out using the PVDF2CuOnanop, the membrane was immersed in the solution before described and magnetically stirred. The time course of the tests was monitored for 1800 s. 2.5. Analyses in solution Phenol and oxidation by-products in the organic phase were detected by a high performance liquid chromatography (HPLC) Agilent 1100 series using an Agilent ZORBAX Eclipse XDB-C8 (4.5 mm  150 mm, 5 mm) column. The mobile phase consisted of an acetonitrile/water solution 80/20 (v/v) fed with a flow rate of 1.0 mL min1. The column pressure was 49 bar and the injection volume was 20 mL. The oxidation products, after extraction with diethyl ether, were also analysed at GC–MS (QP 2010S-Shimadzu) in order to collect more information on the type of products. The results were reported as phenol yield, defined as: Phenol yield (%) = Ph/Bz  100, where Ph is the mol numbers of phenol in the permeate, and Bz is the mol number of benzene in the feed solution in the membrane reactor. The phenol selectivity, in the catalytic tests carried out in the batch reactor, was calculated as: phenol selectivity (%) = Ph/ (Bq + Ph + Biph)  100 where Ph, Bq and Biph are the mol numbers of phenol, benzoquinone and biphenyl detected in the reacting mixture, respectively. 3. Results and discussion 3.1. Membrane morphology and catalyst distribution

Fig. 1. Scheme of the membrane reactor: feed (1); peristaltic pump (2); manometer (3); permeation cell (4); membrane (5); permeate (6); thermostatic bath (7).

Every membrane in Figs. 2–4 shows the so-called asymmetric morphology being characterised by a thin skin and a porous bulk that comprises large voids extending to the central or even towards the bottom region of the membrane. The upper half of the cross

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Fig. 2. Cross section images of PVDF membranes prepared with DMF: (a) without catalyst (PVDF1); (b) with copper oxide powder catalyst (PVDF1CuOp); (c) with copper oxide nanopowder catalyst (PVDF1CuOnanop).

section is composed by parallel finger-like voids, whereas the lower half is composed by small independent cellular pores that are enclosed in the polymer matrix. These morphological features are typically derived from liquid–liquid-phase demixing process [25,26]. Figs 2c, 3c and 4c show the cross sections of PVDFxCuOnanop membranes in backscattering mode: a uniform catalyst distribution is observed in these catalytic polymeric membranes prepared using the copper oxide nanopowder. Backscattered electrons (BSE) consist of high-energy electrons originating in the electron beam, that are reflected or backscattered out of the sample. Backscattered electrons may be used to detect contrast among areas with different chemical compositions, especially when the average atomic number of the various regions is different, since the brightness of the BSE image tends to increase with the atomic number.

In contrast, no uniform catalyst distribution was observed in PVDF catalytic membranes prepared using the copper oxide powder (Figs. 2b, 3b and 4b); indeed, large particles not homogeneously dispersed in the polymeric matrix are visible. 3.2. Pore size distribution measurements Another factor of interest for the membrane characterisation is the pore size distribution. Indeed the permeability is related to the distribution and determines fluid flow through the membrane. In general the pores in ultrafiltration and microfiltration membranes do not have the same size but exist as a distribution of sizes. The pore size distributions of the membrane samples are shown on the bar graphs reported in Figs. 5–7. In Fig. 5 the pore size distributions of membranes prepared using the DMF solvent in the casting

Fig. 3. Cross section images of PVDF membranes prepared with DMAc: (a) without catalyst (PVDF2); (b) with copper oxide powder catalyst (PVDF2CuOp); (c) with copper oxide nanopowder catalyst (PVDF2CuOnanop).

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Fig. 4. Cross section images of PVDF membranes prepared with NMP: (a) without catalyst (PVDF3); (b) with copper oxide powder catalyst (PVDF3CuOp); (c) with copper oxide nanopowder catalyst (PVDF3CuOnanop).

solution are reported. The PVDF1 membrane had a larger pore distributions in the range from 0.02 to 0.11 mm. The PVDF1CuOp membrane had a pore size distribution in the range from 0.02 to 0.11 micrometer with a maximum distribution in the range from 0.02 to 0.025 mm. The PVDF1CuOnanop membrane had a pore distribution in the range from 0.02 to 0.13 with a maximum distribution in the range 0.02–0.03 mm. In Fig. 6 the pore size distributions obtained for the membranes prepared using the DMAc in the casting solution are reported. From data analysis it is evident that using the DMAc solvent a narrow pore size distribution was obtained especially for PVDF2CuOp and PVDF2CuOnanop membranes. Indeed, in the PVDF2CuOnanop membrane pores with dimensions in the range from 0.02 to 0.13 mm are present, but a maximum distribution in a narrow range (from 0.02 to 0.04 mm) is observed. For PVDF2CuOp membrane the range of maximum distribution is also present between 0.02 and 0.04 mm. Fig. 7 shows the pore size distribution

of the membranes obtained using the NMP solvent. For each one of these membranes a larger pore size distribution was obtained. Adding CuO catalysts during membrane formation caused a decrease of pore sizes with respect to the bare membranes. 3.3. Contact angle and thickness measurements The contact angle between water droplets and membrane is an important parameter to measure surface hydrophilicity [27,28]. In general, surface hydrophilicity is higher while its contact angle is smaller than 908. It can be seen from Table 1, that copper oxide particles cause a decrease of the contact angle bottom side where the catalyst sedimentation took place, with respect to top side, during membrane formation. Indeed, a contact angle bottom side, equal to 70.238 was measured for PVDF1 membrane (without the catalyst), while contact angles equal to 59.798 and 60.198 were measured for PVDF1CuOp and PVDF1CuOnanop membranes (with

Table 1 Contact angles between the membrane and water or the reacting solution droplets (in parenthesis) and thickness of PVDF membranes. PVDF membranes

Without catalyst PVDFx (x = 1, 2, 3)a

With copper oxide nanopowder PVDFxCuOnanop

Solvents

Contact angle (8)

Contact angle (8)

Thickness (mm)

Contact angle (8)

DMF

Top side 95.87 () Bottom side 70.23 ()

47

Top side 89.49 () Bottom side 59.79 ()

104

Top side 90.61() Bottom side 60.19 ()

69

DMAC

Top side 92.49 (39.82) Bottom side 82.27 ()

58

Top side 79.45 (34.05) Bottom side 48.02 ()

116

Top side 91.10 (24.63) Bottom side 76.10 ()

81

NMP

Top side 117.4 () Bottom side 82.30 ()

117

Top side 102.8 () Bottom side 75.05 ()

107

Top side 101.9 () Bottom side 56.92 ()

117

a

1 = DMF; 2 = DMAc; 3 = NMP.

Thickness (mm)

With copper oxide powder PVDFxCuOp

Thickness (mm)

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Fig. 5. Pore size distributions of PVDF membranes prepared with DMF: (a) without catalyst; (b) with copper oxide powder catalyst; (c) with copper oxide nanopowder catalyst.

Fig. 6. Pore size distributions of PVDF membranes prepared with DMAc: (a) without catalyst; (b) with copper oxide powder catalyst; (c) with copper oxide nanopowder catalyst.

the catalyst), respectively. As a consequence of the enhanced hydrophilic character of the composite membranes, also considering that acetonitrile is a polar solvent, the permeate flow rate of the feed solution should increase with respect the non-filled membrane. The values of contact angles between the membrane and the reacting solution droplets, reported in parenthesis in Table 1, confirm the high affinity of the reacting solution for the surface of the membrane filled with the CuO. However, contact angles equal to 39.828, 34.058 and 24.638 for the top side of PVDF2, PVDF2CuOp and PVDF2CuOnanop, were observed confirming the high hydrophilicity. Indeed, the solution droplets were instantaneously adsorbed by the bottom side surface of the PVDF2CuOp and PVDF2CuOnanop, allowing not accurate contact angles measurements. Thus, they are not reported in Table 1. The membrane thickness (Table 1), was always smaller than the cast thickness

setted for all membranes at 250 mm, as expected. For instance, employing the DMF solutions, the membrane thickness varied from 47 micron for the membrane without the catalyst (PVDF1) to 104 mm, for that one containing the copper oxide powder (PVDF1CuOp), and to 69 mm for that one containing the copper oxide nanopowder (PVDF1CuOnanop). Similar thickness trends ware obtained for the membranes made using the DMAc solutions. Indeed, the membrane thickness varied from 58 mm for the membrane without catalyst (PVDF2), to 116 mm, for that one containing the copper oxide powder (PVDF2CuOp), and to 81 mm for that one containing copper oxide nanopowder (PVDF2CuOnanop). It can be observed, for all the membranes, that thickness is higher for the membrane filled with the copper oxide powder owing to the greater particle sizes than nanopowder catalyst. Furthermore, a not homogeneous powder catalyst distribution in

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Fig. 8. Permeate flow rate vs transmembrane pressure feeding the membrane cell with the reacting solution (PVDF membrane prepared with DMAc solvent (= PVDF2) with and without the catalysts).

[29,30]. This can be interpreted as follows: PVDF is a hydrophobic polymer and its hydrophilicity can be improved significantly by adding the CuO particles, which have hydrophilic character. Thus, the permeate flow rates, at transmembrane pressure equal to 0.6 bar, for the PVDF2CuOnanop membrane increased from 0.5 mL min1 for the PVDF2 membrane to 2.9 for the PVDF2CuOnanop. That corresponds to contact times of 16.7 and 4 s, respectively, calculated by equations in Section 2.4. This trend was not evidenced when powder particles of copper oxide were used, indeed at 0.6 bar the value of permeate flow rates for the PVDF2CuOp membrane was the same of PVDF2 membrane and both decreased at lower pressure. The same permeate flow rates, despite the enhanced hydrophilic character for the PVDF2CuOp membrane, were due to a greater thickness and a smaller pores dimension of the PVDF2CuOp respect to PVDF2 membranes. The advantage to entrap the CuO nanoparticles in the polymer matrix is surely the control of contact time of substrate with the catalyst but also the improvement of some properties of membranes such as membrane permeability. From Fig. 8 it can be seen that the fluxes of reacting solution increase with increasing the pressure for all membranes, thus calculated contact times decrease with pressure increasing (Fig. 9). This variation of membrane permeability permits to change the contact times of the PVDF2CuOnanop catalytic membrane from high to low values (19 to 4 s) by changing the transmembrane pressure from 0.1 to 0.6 bar thus controlling the successive oxidation of the product (phenol). The characterisation results (SEM, contact angle, permeability) indicate that the Fig. 7. Pore size distributions of PVDF membranes prepared with NMP: (a) without catalyst; (b) with copper oxide powder catalyst; (c) with copper oxide nanopowder catalyst.

the polymeric matrix is observed (see Figs. 2b, 3b and 4b). The higher membrane thickness of the membrane prepared with NMP is caused by a very different morphology as can be seen by comparing the SEM microphotographs (see Figs. 2–4). The optimal membrane thickness in terms of mechanical resistance (observed during membrane detachment from the glass plate) and high permeability (see Fig. 8) was obtained using DMAc as solvent. 3.4. Permeate flow rate Permeate flow rate through the membrane was affected by the addition of CuO particles and by morphological properties such as membrane thickness and pore size (see Table 1 and Fig. 6). The addition of the nanosized inorganic filler led to an increase in the PVDF membrane permeability in agreement with other authors

Fig. 9. Contact time of the reacting solution crossing the PVDF2 type membranes at different transmembrane pressures.

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Table 2 BET specific surface area, pore volume and pore size of the catalysts and the tested catalytic membranes. Sample

BET specific surface area (m2 g1)

Pore volume (cm3 g1)

Pore diameter (nm)

CuOp CuOnanop PVDF2 PVDF2CuOp PVDF2CuOnanop

0.76  0.02 24.5  2.5 8.60  0.9 2.88  0.3 9.12  0.9

0.0017 0.0687 0.0789 0.0193 0.0528

35.3  3.5 9.9  1.0 24.5  2.5 17.3  1.7 18.9  1.8

membrane prepared whit DMAc solvent (PVDF2), seems more suitable for this application considering membrane morphology, catalyst distribution in the polymer matrix, membrane thickness, mechanical resistance and permeability variation. This type of membrane with the entrapped catalyst was used to perform the catalytic tests. 3.5. Surface area measurements Table 2 lists the BET surface area and the pore volume of the catalysts, tested catalytic membranes and bare membranes. From these results, as expected, a higher surface area for the catalyst with a smaller particles dimension (CuOnanop catalyst) was obtained; in particular the BET surface area was equal to 24.53 and 0.76 m2 g1 for the CuO catalyst as nanopowder and powder, respectively. The dispersion of CuO nanoparticles in the polymer matrix did not modify the pore volume of the polymeric membrane, this means that the nanopowder was homogeneously dispersed in the polymer matrix without influencing pores formation of the membranes. The BET surface area, average pore diameter and pore volume of PVDF2CuOp membrane were 2.88 m2 g1, 17.3 nm and 0.0193 cm3 g1, respectively, which were smaller than those of the bare membrane. Thus copper oxide powder influences the pore formation of the membranes.

yield (2.3%) was obtained using the PVDF2CuOnanop membrane at 35 8C using a reacting solution containing ascorbic acid and a contact time equal to 19.4 s. Also for the PVDF2CuOp membrane, the best catalytic system was obtained at 35 8C in presence of ascorbic acid. The smaller phenol yield (1.7%) using the PVDF2CuOp membrane, despite the longer contact time (66.8 s), was caused by the non-homogeneous dispersion of CuO catalyst in the polymeric matrix. Using the ascorbic acid in the reacting solution (Fig. 11), the phenol concentration increased by increasing the contact time while the by-products such as benzoquinone and biphenyl were detected as traces. The study of the oxidation mechanism by Fenton-type reagents is very complex. A plausible mechanism for benzene oxidation to phenol catalyzed by CuO in presence of ascorbic acid is reported in Scheme 1. The reactions in this scheme represent some of the multiple parallel processes that occur immediately after OH generation by reaction (2). Hydroxyl radicals initiate the nonselective oxidation processes of organic and inorganic species present in the system through mechanisms that include hydrogen moving away, oxygen addition and radical formation. Reaction (6) indicates the possible way of reaction of Cu(II) for new generation of Cu(I). Reactions (2), (3), (4) and (5) show the role of OH radical, by generation or

3.6. Reactivity of catalytic membranes in the ultrafiltration membrane reactor and hypothesis of a kinetic equation Reactivity of the PVDF2 catalytic membranes were tested in the ultrafiltration membrane reactor schematised in Fig. 1. The preliminary catalytic tests were performed at two temperatures (35 and 50 8C); the effect of acetic and ascorbic acid addition (4 mmol each one) in the reacting media was also studied. These compounds should interact with the formation of hydroxyl radicals thus controlling the selectivity of the partial oxidation. The results of the experimental tests, feeding the reacting solution at different contact times and different temperatures, are reported in Figs. 10 and 11 showing phenol yield for both the PVDF2CuOp and PVDF2CuOnanop membranes. Best result in term of phenol

Fig. 10. Phenol yield (%) at various contact times employing the PVDF2CuOp membrane.

Fig. 11. Phenol yield (%) at various contact times employing the PVDF2CuOnanoOp membrane.

Scheme 1. Proposed reaction pathway for benzene oxidation to phenol.

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consumption. Therefore, the overall reaction rate can be expressed as follows: d½OH ¼ k2 ½H2 O2   ½CuðIÞ  k5 ½OH  ½Benzene  k3 ½H2 O2  dt  ½OH  K 4 ½OH½CuðIÞ

(7)

Assuming the steady state condition for hydroxyl radicals:   d½OH ¼0 (8) dt the concentration of these radicals can be expressed in the form: ½OH ¼

k2 ½H2 O2 ½CuðIÞ k5 ½Benzene þ k3 ½H2 O2  þ k4 ½CuðIÞ

(9)

On the other hand, the phenol is formed by hydroxycyclohexadienyl radicals (HCD) (6) and its rate of formation of formation can be written as:

Fig. 12. Phenol selectivity (%) obtained in the catalytic tests carried out in the batch reactor.

d½Phenol ¼ k6 ½HCD  ½CuðIIÞ dt

nanop systems, in absence of ascorbic acid, because a longer contact time is necessary in order the reacting solution permeates through the membrane. By comparing the obtained results at 35 8C with and without acetic acid, for the PVDF2CuOp systems, it was evident that the high phenol yield was obtained in presence of acetic acid (1.38 vs 0.77%). This different trend, as reported in literature (although it is not clearly understandable) [34], was probably due to an efficient use of H2O2, in presence of acetic acid, for the phenol formation. Instead, comparing the two different catalytic systems in presence of acetic acid at 35 8C, it is observed that the slow step favours the system that allows a longer contact time with the catalyst, that is the PVDF2CuOp system which gives a phenol yield of 1.39 vs 0.53%. At higher temperature (50 8C) the phenol concentration decreased for the two catalytic membranes. One of the reasons of this decline can be ascribed to the increase of hydrogen peroxide decomposition in the reacting solution [35].

(10)

But, considering generation and consumption of HCD and assuming no accumulation of free radicals, it is obtained: d½HCD ¼ k5 ½Benzene½OH  k6 ½HCD  ½CuðIIÞ ¼ 0 dt

(11)

from which k5 ½Benzene½OH ¼ k6 ½HCD  ½CuðIIÞ

(12)

and then d½Phenol ¼ k5 ½Benzene½OH dt

(13)

Substituting expression (9) in (13), it is obtained: d½Phenol ¼ k5 ½Benzene dt k2 ½H2 O2 ½CuðIÞ  k5 ½Benzene þ k3 ½H2 O2  þ k4 ½CuðIÞ

3.7. Reactivity of CuOnanop and PVDF2CuOnanop in the batch reactor (14)

A systematic study will be necessary for determining the constant k2–k3–k4–k5 and then the validity of Eq. (14) derived from the reaction steps (2)–(6) of Scheme 1. When ascorbic acid and hydrogen peroxide are present Cu(II) species are rapidly reduced to Cu(I), which then reacts with hydrogen peroxide (Scheme 1) regenerating Cu(II). The hydrogen peroxide can both reduce Cu2+ and oxidize Cu+ [31,32]. The PVDF2CuOnanop catalytic system was more active than PVDF2CuOp. This could be ascribed to both the large surface area and the higher dispersion of metal oxide species in the polymeric matrix (see Table 2). In absence of ascorbic acid the following mechanism can be proposed: kI

CuðIIÞ þ H2 O2 @ CuOOHþ þ Hþ

kII

CuOOHþ !CuðIÞ þ O2  þ Hþ slow

kIII

CuðIÞ þ H2 O2 !CuðIIÞ þ OH þ OH

(I)

(II) (III)

In the first step (Eq. (I)), complexation of Cu(II) by H2O2 leads to the formation of copper hydroperoxide. Decomposition of the copper hydroperoxide to form Cu(I) and O2 happens in the slow step (Eq. (II)) [33]. The other steps remain the same as those given for the first pathway (Scheme 1). The slow step explains the best results for the PVDF2CuOp systems with respect to PVDF2CuO-

Some catalytic tests in batch were performed employing the best experimental conditions obtained for the PVDF2CuOnanop membrane in the ultrafiltration membrane reactor. The results in terms of phenol selectivity (%) are reported in Fig. 12. They confirm the high phenol selectivity (100%); indeed, the by-products, benzoquinone and biphenyl, were detected in traces for the first 600 s of test. After this time the phenol selectivity decreased to 83% at 1800 s. This value was still very high compared to the value (48%) obtained using the CuOnanop catalyst not entrapped in the polymeric matrix. This amazing result is probably caused by a modified interaction among the substrate, products and catalyst inside the polymeric matrix. Besides, it should be considered that the substrate (benzene) was preferentially adsorbed by the membrane with respect to phenol. This last one, after its formation, is released in the solution avoiding the formation of over-oxidation products thus obtaining high values of selectivity with respect to the free catalyst. 4. Conclusions Characterisation of the prepared catalytic membranes containing CuO powder and CuO nanopowder catalysts, using the PVDF polymer, showed an asymmetric structure, a uniform distribution of CuO nanopowder and an increase of wettability to reacting mixture of the membrane filled with CuO powder catalyst. The different solvents (DMF, DMAc and NMP) produced membranes with different morphological structure and chemical-physical properties suitable to be used in an ultrafiltration membrane

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reactor. Among all the prepared membranes it was found that DMAc was the best solvent. Every membrane was also characterised with permeability tests for determining and controlling the contact time between the catalyst and the reacting mixture; it was found the possibility to change the contact time in a range between 4 and 19.4 s. The BET measurements confirmed the best catalyst dispersion in the PDVF2CuOnanop without loss of surface area when entrapped in the polymeric membrane. A higher phenol concentration in the catalytic tests on the UF membrane reactor using the PVDF membrane prepared with DMAc filled with the CuO nanopowder rather than CuO powder catalysts was found. A phenol yield (mol%) of 2.3 was obtained using a contact time with the catalyst of 19.4 s at 35 8C in presence of 4 mmol of ascorbic acid. By-products such as benzoquinone and biphenyl were detected as traces. Use of an ultrafiltration membrane reactor in the direct oxidation of benzene to phenol seems an interesting perspective for controlling the partial oxidation and reducing by-products formation. Being encouraging the results of this explorative study further research is in progress to improve the system performance by testing other more efficient catalysts able to increase the phenol yield and to maintain high selectivity for long time. Acknowledgements The authors thank the MIUR within the FIRB 2005-2008 programme for the financial support. References [1] J.M. Zheng, J.M. Sousa, D. Mendes, L.M. Madeira, A. Mendes, Catal. Today 118 (2006) 228–236. [2] J. Huang, L. El-Azzami, W.S.W. Ho, J. Membr. Sci. 261 (2005) 67–75. [3] S. Ziegler, J. Theis, D. Fritsch, J. Membr. Sci. 187 (2001) 71–84. [4] C. Liu, Y. Xu, S. Liao, D. Yu, Y. Zhao, Y. Fan, J. Membr. Sci. 137 (1997) 139–144. [5] D. Fritsch, K.V. Peinemann, J. Membr. Sci. 99 (1995) 29–38. [6] H. Gao, Y. Xu, S. Liao, R. Liu, J. Liu, D. Li, D. Yu, Y. Zhao, Y. Fan, J. Membr. Sci. 106 (1995) 213–219.

[7] M.G. Buonomenna, E. Drioli, R. Bertoncello, L. Milanese, L.J. Prins, P. Scrimin, G. Licini, J. Catal. 238 (2006) 221–231. [8] V. Parvulescu, C. Constantin, G. Popescu, B.L. Su, J. Mol. Catal. A Chem. 208 (2004) 253–256. [9] E. Gallo, M.G. Buonomenna, L. Vigano`, F. Ragaini, A. Caselli, S. Fantauzzi, S. Cenini, E. Drioli, J. Mol. Catal. A Chem. 282 (2008) 85–91. [10] T. Miyahara, H. Kanzaki, R. Hamada, S. Kuroiwa, S. Nishiyama, S. Tsuruya, J. Mol. Catal. A Chem. 176 (2001) 141–150. [11] M. Ishida, Y. Masumoto, R. Hamada, S. Nishiyama, S. Tsuruya, M. Masai, J. Chem. Soc. Perkin Trans. 2 (1999) 847–853. [12] Y.K. Masumoto, R. Hamada, K. Yokota, S. Nishiyama, S. Tsuruya, J. Mol. Catal. A Chem. 184 (2002) 215–222. [13] D. Bianchi, M. Bertoli, R. Tassinari, M. Ricci, R. Vignola, J. Mol. Catal. A Chem. 200 (2003) 111–116. [14] D. Bianchi, R. Bortolo, R. Tassinari, M. Ricci, R. Vignola, Angew. Chem. Int. Edit. 39 (2000) 4321–4323. [15] V.I. Sobolev, K.A. Dubkov, E.A. Paukshtis, L.A. Pirutko, M.A. Rodkin, A.S. Kharitonov, G.I. Panov, Appl. Catal. A Gen. 141 (1996) 185–192. [16] S. Perathoner, F. Pino, G. Centi, G. Giordano, A. Katovic, J.B. Nagy, Top. Catal. 23 (2003) 125–136. [17] M. Tani, T. Sakamoto, S. Mita, S. Sakaguchi, Y. Ishii, Angew. Chem. Int. Edit. 44 (2005) 2586–2588. [18] A. Kubacka, Z. Wang, B. Sulikowski, V.C. Corberan, J. Catal. 250 (2007) 184–189. [19] Y. Li, Z. Feng, R.A. van Santen, E.J.M. Hensen, C. Li, J. Catal. 255 (2008) 190–196. [20] N. Itoh, S. Niwa, F. Mizukami, T. Inoue, A. Igarashi, T. Namba, Catal. Commun. 4 (2003) 243–246. [21] K. Sato, T. Hanaoka, S. Hamakawa, M. Nishioba, K. Kobayashi, T. Inoue, T. Namba, F. Mizukami, Catal. Today 118 (2006) 57–62. [22] R. Molinari, T. Poerio, P. Argurio, Catal. Today 118 (2006) 52–56. [23] R. Molinari, T. Poerio, P. Argurio, Desalination 200 (2006) 673–675. [24] R. Molinari, T. Poerio, P. Argurio, Italy Patent CZ2006A000029 (2006), to University of Calabria. [25] M. Mulder, Basic Principles of Membrane Technology, Kluver, Dordrecht, 1991, pp. 367–368. [26] L.P. Cheng, H.Y. Shaw, J. Polym. Sci. Pol. Phys. 38 (2000) 747–754. [27] L. Palacio, J.I. Calvo, P. Pranados, A. Hernandez, P. Vaisanen, M. Nystrom, J. Membr. Sci. 152 (1999) 189–201. [28] J.T.F. Keurentjes, J.G. Harbrecht, D. Brinkman, J.H. Hanemaajer, M.A. Cohen Stuart, H. Van’t Riet, J. Membr. Sci. 47 (1989) 333–344. [29] L. Yan, Y.S. Li, C.B. Xiang, Polymer 46 (2005) 7701–7706. [30] A. Bottino, G. Capannelli, A. Comite, Desalination 146 (2002) 35–40. [31] H. Sigel, C. Flierl, R. Griesser, J. Am. Chem. Soc. 91 (1969) 1061–1064. [32] Y. Luo, K. Kustin, I.R. Epstein, Inorg. Chem. 27 (1988) 2489–2496. [33] J.F. Perez-Benito, J. Inorg. Biochem. 98 (2004) 430–438. [34] Y.-k. Masumoto, R. Hamada, K. Yokota, S. Nishiyama, S. Tsuruya, J. Mol. Catal. A Chem. 184 (2002) 215–222. [35] D. Mantzavinos, Trans IChemE 81 (2003) 99–106.