Benzene Hydroxylation and Simultaneous Extraction of Phenol in Two

Jan 24, 2013 - membrane contactor (LMC)] were tested in the synthesis and separation of phenol produced by direct hydroxylation of benzene using a Fen...
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Benzene Hydroxylation and Simultaneous Extraction of Phenol in Two Membrane Contactors Made with Three-Compartment Cells Raffaele Molinari,*,† Pietro Argurio,† and Teresa Poerio‡ †

Department of Environmental and Chemical Engineering, UdR INCA, University of Calabria, Via Pietro Bucci, cubo 44/A, I-87036 Rende (CS), Italy ‡ National Research Council−Institute for Membrane Technology (ITM−CNR), c/o University of Calabria, Via Pietro Bucci, cubo 17C, I-87036 Rende (CS), Italy S Supporting Information *

ABSTRACT: Two different three-compartment membrane contactors [called solid membrane contactor (SMC) and liquid membrane contactor (LMC)] were tested in the synthesis and separation of phenol produced by direct hydroxylation of benzene using a Fenton reaction. Phenol produced in the aqueous reacting phase was extracted in the organic phase and simultaneously stripped in the basic aqueous phase. Preliminary tests on phenol recovery evidenced better performances (86.5% of phenol recovered in the strip phase) using the SMC with 0.1 M Na2SO4 in the aqueous feed phase at 35 °C. In the tests of partial oxidation, higher phenol productivity (0.62 gph gcat−1 h−1) was obtained in this last system because the high phenol flux away from the reacting phase permitted one to extract a high amount of phenol in the organic and aqueous (strip) phases. This extraction protected phenol by its subsequent oxidation. It was evidenced that the use of a third compartment containing an alkaline aqueous stripping phase permitted one to recover phenol at 100% purity. 80−97%, benzene conversion of 2−16% below 250 °C, and a phenol yield of 1.5 gph gcat−1 h−1 at 150 °C. Some papers in the literature considered H2O2 as the oxidant in benzene hydroxylation to phenol.13−16 Bianchi et al.9 reported a water−acetonitrile (1:1) biphasic reaction medium in which the produced phenol was extracted into the organic phase and the Fenton catalyst was soluble in the aqueous phase. Benzene conversion of 8.6% and a selectivity to phenol of 97% were attained in such a system. The high selectivity was obtained by reducing the contact time between phenol and the catalyst. The method proposed by Bianchi et al. operates in the liquid phase and is an attractive alternative to current multistep reactions.17,18 However, its implementation presents an important thermodynamic limitation: phenol can be more easily oxidized than benzene, and the process of direct hydroxylation also yields catechol, benzoquinone, and hydroquinone, until complete degradation of the cyclic structure of benzene or the formation of polymeric tars.19,20 As evidenced by Bianchi et al., prompt removal of the produced phenol from the reaction environment represents a key point for developing such a method. From this aspect, membranes can play an important role. Membrane reactors are very attractive because of their advantages related to the synergy between the chemical reaction and a membrane when implemented in the same device.21,22 Within this family, the membrane contactors also

1. INTRODUCTION Phenol and its derivatives, such as cyclohexanone, are very important chemical intermediates in industry. Their world production is continuously growing at 7% per annum from 7 million tonnes in 2002. In industries, phenol is widely used in the synthesis of pharmaceuticals, agrochemicals, petrochemicals, and plastics.1 Nowadays, 95% of the world phenol production is made by the so-called “cumene process”.2 Although this one has been refined, it still has two disadvantages: high energy consumption, because of multistep reactions, and the production of a large amount of acetone as the byproduct.3 This means that each enlargement of the production capacity of phenol involves finding market opportunities for a corresponding amount of acetone. A possible approach that minimizes such a difficulty consists of acetone recycling by its conversion to propylene or isopropyl alcohol, both capable of benzene alkylation. Such an approach results in a more complex production cycle. On these bases, the search for new routes for phenol synthesis became more intensive in the past decade.4−10 The approaches proposed were mainly based on the use of three oxidants: nitrous oxide (N2O), oxygen (O2), and hydrogen peroxide (H2O2). The AlphOx process, operating at 400−450 °C with N2O, permitted one to obtain a phenol productivity of 0.4 gph gcat−1 h−1 and a conversion to phenol (mol %) of 85%.11 Niwa et al.12 reported a single-stage method of benzene oxidation to phenol using a palladium membrane reactor. In their reactor, the active hydrogen species, formed by permeation from one side of the palladium membrane, produced active oxygen species on the opposite side by reacting with O2 gas. Then, the active oxygen species reacts with the adsorbed benzene on palladium, converting it into phenol. This one-step process attained a phenol selectivity of © 2013 American Chemical Society

Special Issue: Enrico Drioli Festschrift Received: Revised: Accepted: Published: 10540

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Figure 1. Scheme of the three-compartment SMC.

phase increased phenol extraction in the organic phase but also increased reaction kinetics, thus promoting black solid (tar) formation and reducing the phenol productivity (0.64 gph gcat−1 h−1). The use of different operating pH values and different organic acids delayed and in some tests avoided tar formation but also gave a reduction of the phenol selectivity and then productivity. The use of iron(0) did not avoid precipitate formation, evidencing that it was caused by the use of ironbased catalysts, as observed by Bremner et al.25 Therefore, different catalysts, such as vanadium-based vanadyl(IV) acetylacetonate (VAAC) and vanadium(III) chloride (VC), were tested.26 Obtained results evidenced improved productivity (0.97 vs 0.78 gph gcat−1 h−1) using VC compared to VAAC. No black solid formation was observed, but the system productivity was significantly lower than that obtained using an iron(II) catalyst. Considering these results, in the present work, two new membrane contactor configurations have been tested in the one-step benzene hydroxylation to phenol. Each contactor is made by two membranes, which separate three compartments containing three immiscible phases: the aqueous reacting phase and organic (only benzene) phase (as in our previous work), plus a third basic aqueous phase acting as a stripping agent. Operating in this way, phenol extracted in the organic phase is simultaneously stripped in the basic aqueous phase, thus performing the process of product recovery. In the meantime, the phenol concentration difference between the aqueous reacting phase and the organic extracting phase increases, facilitating phenol recovery. The substantial difference between the two membrane contactor configurations consists of the types of membranes separating the phases: (i) in one system, called solid membrane contactor (SMC), two solid poly-

permit one to combine membrane separation and catalytic reaction in one unit operation. In 2006, we proposed the use of a biphasic membrane contactor in which a flat-sheet membrane separates two compartments containing two immiscible phases: an acidic aqueous phase (containing an iron catalyst and H2O2 as the oxidant) and an organic phase (only benzene).23 The benzene phase acts both as the reserve supply of the substrate to be oxidized, because benzene permeates across the hydrophobic membrane and reacts at the aqueous interface, and as the extracting phase, because the phenol produced permeates back across the membrane and dissolves in benzene, where it is protected by subsequent oxidations. Very high phenol selectivity in the organic phase (on the order of 99%) and interesting phenol productivity (3.60 gph gcat−1 h−1) were obtained thanks to phenol extraction in the organic phase. The limiting factor was the low rate of phenol permeation across the membrane, which was not so high as to permit the complete extraction of the produced phenol from the aqueous phase to the organic phase. Then the oxidation proceeds, producing overoxidized products such as benzoquinone, biphenyl as a trace, and a black precipitate in the reacting phase (tar). These products should be avoided in view of large-scale application, where concentration polarization and fouling phenomena could decrease the membrane performance. In order to avoid/reduce tar formation and promote phenol extraction in the organic phase, three different strategies can be applied: (i) suitable modification of the chemical conditions; (ii) use of different catalysts; (iii) development of different membrane contactor configurations. In our previous work,24 we evidenced that the use of dissolved salts such as sodium sulfate in the aqueous reacting 10541

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Figure 2. Scheme of the three-compartment LMC.

Ammonium molybdate [(NH4)6Mo7O24; purity 99%] from Sigma-Aldrich was used as the catalyst in the iodometric method. Starch (C12H22O11) was used as the indicator in the iodometric method. On the basis of our previous work,23 a hydrophobic microfiltration polypropylene porous membrane (Accurel, manufactured by Membrana, thickness 142 μm; pore size 0.2 μm; porosity 70%) was used in the SMC. 2.2. Apparatus and Methods. Experimental tests were performed in two different laboratory-made three-compartment membrane contactors. The first one was the three-compartment SMC schematized in Figure 1. It was constituted by three cells containing an organic phase and two aqueous phases, separated by two hydrophobic flat-sheet polypropylene membranes (exposed surface area = 28.3 cm2). Each cell had a volume of 130 mL and was equipped with two channels for sample withdrawal and for the addition of reagents. The three compartments were mechanically stirred and maintained at a fixed temperature by immersion in a thermostatic bath. The aqueous stripping phase was a 0.1 M NaOH solution in both transport and oxidation tests. The aqueous feed phase was a solution containing only phenol at 2 g L−1 concentration at pH 2.8 during the masstransport tests; instead, it contained the catalyst, the acid, the oxidant, and a salt during the oxidation reaction tests. The organic phase was only benzene, acting both as the substrate to be oxidized and as the extracting solvent where phenol, initially contained or formed during reaction in the aqueous feed phase, was extracted. In our recent work,26 it was found that the polypropylene membrane can be little oxidized. Thus, a three-compartment LMC, schematized in Figure 2, where the three phases are separated by two liquid membranes, could contribute to limit overoxidation reactions thanks to easier/faster phenol transport from the feed donor phase to the stripping receiving phase. The LMC system is constituted by a Teflon block in which two cells of equal size with a volume of 125 mL were obtained. The two aqueous feed and stripping phases were placed in these two cells and were magnetically stirred. Above both of them, there was a horizontal channel that was able to accommodate 125 mL of the organic phase, giving at the interface the liquid

propylene membranes were used to separate the three compartments; (ii) in the other one, called liquid membrane contactor (LMC), phase separation was based on the immiscibility of the solvents used (water and benzene); the interfaces between the two organic/water phases represent practically the two liquid membranes across which phenol permeates. Experimental work was carried out in two consecutive steps. Preliminarily, the transfer rates of the two membrane contactor configurations were determined by carrying out transport tests at 25 and 35 °C, also investigating the influence of salts dissolved in the aqueous phases. Then, benzene hydroxylation to phenol was carried out in both reactors, evidencing the main differences.

2. EXPERIMENTAL SECTION 2.1. Materials. Benzene (C6H6; purity 99.8%) from Carlo Erba Reagenti was used as the substrate and organic extracting phase. Phenol (C6H5OH; purity 99.99%), benzoquinone (C6H4O2; purity 99.9%), and biphenyl (C12H10; purity 99.99%) from Sigma-Aldrich were used for analytical calibrations. Iron(II) sulfate (FeSO4·7H2O; purity 99%) from SigmaAldrich was employed as the catalyst. Hydrogen peroxide [H2O2; 30% (w/w) solution in water] from Sigma-Aldrich was used as the oxidant. Sodium chloride (NaCl; purity 99.9%) and sulfate (Na2SO4; purity 99.0%) from Sigma-Aldrich were used to modulate the ionic strength of the reacting aqueous phase. Acetic acid (CH3COOH; purity 96%, d = 1.84 g mL−1) from Sigma-Aldrich was used to achieve acidic pH in the aqueous phase. Ultrapure water was obtained from Milli-Q equipment by Millipore. Diethyl ether (C4H10O; purity 99.8%) from Carlo Erba was used in analyses for the extraction of organics from the aqueous solution. Sodium thiosulfate (Na2S2O3; purity 99.9%) and potassium iodide (KI; purity 99%) from Sigma-Aldrich were used for H2O2 titration with the iodometric method. 10542

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Instead, in the catalytic tests, the results were elaborated on, defining the following parameters: conversion of H2O2 to phenol Xox (%) = [(nph,aq + nph,org + nph,strip)/mmol of H2O2 reacted] × 100, where nph,aq, nph,org, and nph,strip are respectively the amount (mmol) of phenol detected in the aqueous feed, organic, and strip phases at the end of each catalytic test (t = 240 min); selectivity to phenol in the i phase Sph,i (%) = [nph,i/ (nph,i + nbq,i)] × 100, where nph,i and nbq,i are respectively the amount (mmol) of phenol and benzoquinone detected in the i phase; growth rate of phenol amount in the i phase ri (mmol L−1 h−1) = Δphi/Δt, where Δphi is variation of the phenol concentration in the i phase obtained in the time interval Δt; phenol flux from the feed to the organic phase Jf→o (mmol h−1 m−2); phenol flux from the organic to the strip phase Jo→s (mmol h−1 m−2); phenol productivity Pph (gph g cat−1 h−1) = mass (g) of phenol produced per gram of catalyst per unit of time.

membrane. The compositions of the aqueous and organic phases were identical with that ones used in the SMC. Phase separation was maintained thanks to the immiscibility of the solvents used (water and benzene). Phenol permeates through the interfaces between the phases, which practically represent two liquid membranes, with an exposed surface area of 4.5 cm × 4.5 cm = 20.3 cm2. The reactor was closed at the top by a Plexiglas cover, which was covered by an organic-resistant film to avoid attack by the vapors of the organic solvent. The cover was equipped with three holes: a central one, to insert a mechanical shaker for organic phase stirring, and two holes in correspondence with the two cells for sample withdrawal and for the addition of the reagents (i.e., the oxidant during the oxidation tests). The stirring intensity in the LMC was set to a maximum value to avoid mixing of the phases. This limitation was not present in the case of SMC, where the three phases were separated by two solid membranes that physically avoid phase mixing. A stainless steel coil, connected to a thermostatic bath with the tubes passing across the cover, allowed one to maintain isothermal conditions. In both SMC and LMC, the catalytic oxidation reaction takes place at the interface membrane−aqueous feed phase. Produced phenol permeates across the solid or liquid membranes, and it is extracted in the organic phase (benzene), thus avoiding the use of other organic solvents. Phenol extracted in the organic phase is then stripped in the aqueous alkaline phase, where it is converted to sodium phenolate, avoiding its backdiffusion because it is insoluble in the organic phase, thus performing the process of simultaneous product recovery, purification, and concentration. During the experimental tests, samples of the feed, organic, and stripping phases were withdrawn every 30 min and analyzed. Phenol and oxidation byproducts were detected by high-performance liquid chromatography (Agilent 1100 Series instrument) using a Gemini 5u-C18 (4.60 mm × 250 mm, 110 Å) column by UV readings at 254 nm wavelength. The mobile phase consisted of a 50/49/1 (v/v/v) acetonitrile/water/acetic acid solution fed to a flow rate of 1.0 mL min−1. The column pressure was 104 bar and the injection volume 20 μL. Identification of the oxidation products was performed by analyzing the solutions with gas chromatography−mass spectrometry (GC−MS; QP2010S, from Shimadzu) using an Equity 5 column. A pH meter (WTWInolab Terminal Level 3) with a SenTix 81 (WTW) glass pH electrode was used for pH measurements. Mass-transport tests were performed in both membrane contactors using (i) an aqueous solution with a phenol concentration of 2 g L−1 at pH 2.8 as the feed phase, (ii) benzene as the organic phase, and (iii) a 0.1 M NaOH solution as the aqueous stripping phase. Transport tests had a duration of 390 min. The influence of temperature (25 and 35 °C) and the use of different salts (NaCl and Na2SO4) dissolved in the aqueous feed phase at different concentrations, in order to increase its ionic strength and thus promote phenol extraction in the organic phase, were considered. The obtained results were elaborated on, defining the following parameters: extraction percentage in the i phase (i = org or strip) Ei = phi/(phfeed + phorg + phstrip) × 100, where phi, phfeed, phorg, and phstrip are respectively the phenol concentration in the i phase, feed, organic, and strip phases at the end of each experimental test (t = 390 min).

3. RESULTS AND DISCUSSION 3.1. Mass-Transport Tests. 3.1.1. Mass-Transport Tests without Salts. The results obtained in the transport tests in both membrane contactors using the same composition of the phases, at 25 and 35 °C, without salts are summarized in Figures 3 and 4. It can be observed that, for the LMC, extraction percentages do not change significantly with temperature, while a clear enhancement in the strip is obtained using the SMC.

Figure 3. Extraction percentages obtained in the organic and strip phases in the transport tests in both LMC and SMC at T = 25 °C (aqueous feed phase, phenol concentration = 2 g L−1, pH 2.8; organic phase, benzene; aqueous stripping phase, 0.1 M NaOH).

Considering that phenol was initially present in the feed aqueous phase, the sum Estrip + Eorg is the phenol percentage that crossed the feed/organic interface in each system. This sum is about the same (76% ± 3%) at 25 °C for both systems and at 35 °C for the LMC. Instead, a significant increase of this percentage is obtained for the SMC at 35 °C (91.2%). This can be explained considering the concentration profiles (Figure 5) through the liquid membrane (LM) and the solid membrane (SM) interfaces. Indeed, in the case of the LM interface, only mass transfer in the two boundary layers has to be considered, plus the partition coefficient (Kd) at the interface. The partition coefficient is an equilibrium parameter defined as the ratio between the phenol concentration in the organic phase and that in the aqueous phase at equilibrium (Kd = phorg,eq/phaq,eq). In the considered temperature range (25−35 °C), the partition coefficient does not vary significantly. For the SM interface, the 10543

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The results obtained during transport tests at 25 and 35 °C without and with NaCl or Na2SO4 salts are reported in Figures 6 and 7.

Figure 4. Extraction percentages obtained in the organic and strip phases in the transport tests in both LMC and SMC at T = 35 °C (aqueous feed phase, phenol concentration =2 g L−1, pH 2.8; organic phase, benzene; aqueous stripping phase, 0.1 M NaOH). Figure 6. Extraction percentages obtained in the strip phase in the transport tests with and without salts in both LMC and SMC at 25 °C (aqueous feed phase, phenol concentration = 2 g L−1, pH 2.8, 0.1 M NaCl or Na2SO4; organic phase, benzene; aqueous stripping solution, 0.1 M NaOH).

additional resistance is the membrane thickness. Mass transfer in both boundary layers and inside the membrane is of the diffusive type. The mass-transfer coefficient (k = D/l) depends on the temperature (through D, the diffusion coefficient) and layer (or membrane) thickness (l). Thus, in the case of the SMC working at 35 °C, the increase of the temperature (from 25 to 35 °C) and the stirring effect at the SM interfaces (reduced layer thickness, l) both play on the rise of k, increasing significantly the extraction percentage. Besides, it can be observed that the phenol percentage recovered in the strip phase is higher in the SMC, for both temperatures considered, than that using the LMC (60.8% vs 49.0% and 77.3% vs 46.5% at 25 and 35 °C, respectively). Moreover, the percentage of phenol present in the organic phase is higher in the case of LMC, highlighting a greater difficulty to recover it in the strip. This behavior is most likely due to the worst conditions of agitation of the organic phase in the LMC because of the limited stirring intensity to avoid phase mixing. 3.1.2. Mass-Transport Tests with Salts. On the basis of our previous work,24 it is expected that the presence of dissolved salts in the aqueous feed phase increases phenol extraction in the organic phase and then in the strip phase.

Figure 7. Extraction percentages obtained in the strip phase in the transport tests without and with salts in both LMC and SMC at 35 °C (aqueous feed phase, phenol concentration = 2 g L−1, pH 2.8, 0.1 M NaCl or Na2SO4; organic phase, benzene; aqueous stripping solution, 0.1 M NaOH).

Figure 5. Schematization of the concentration profiles through the LM and SM feed/organic phase interfaces. 10544

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rates and fluxes in Tables S1 and S2 in the Supporting Information (SI).

It can be observed that the extraction percentage in the strip phase increased significantly when 0.1 M NaCl or Na2SO4 was added for both membrane contactors. Besides, the advantage of using the SMC is confirmed by the higher recovery of phenol in the strip phase. Furthermore, an extraction percentage significantly increasing with the temperature in the case of the SMC is observed. Better results, as expected, were obtained using Na2SO4 (86.5% vs 80.1%) compared to NaCl. This trend can be explained by salt solvation phenomena. Indeed, these salts in the aqueous feed phase are completely dissociated, permitting the formation of more or less solvated ions. This solvation leads to a decrease in the amount of water molecules available for solvation of phenol, so it acquires more affinity for the organic phase and decreases its concentration in the aqueous phase.27,28 Because the same salt concentration gives a higher amount of ions dissolved in the aqueous feed phase in the case of Na2SO4, phenol is more easily extracted from the feed phase and finally transported in the strip phase. In Table 1, it is also evidenced that phenol extraction and recovery in the strip phase do not increase by increasing the NaCl concentration from 0.1 to 0.2 M in the aqueous feed phase.

Figure 8. Phenol concentration in the organic and strip phases versus time during the catalytic tests with NaCl in the LMC [aqueous feed phase, pH 2.8 (0.19 mL of acetic acid), 0.095 g of iron(II) sulfate, 0.1 M NaCl; organic phase, benzene; aqueous stripping phase, 0.1 M NaOH; T = 35 °C].

Table 1. Extraction Percentage in the Strip Phase in the Transport Tests at Different NaCl Concentrations in the Aqueous Feed Phase (Aqueous Feed Phase, Phenol Concentration = 2 g L−1, pH 2.8; Organic Phase, Benzene; Aqueous Stripping Phase, 0.1 M NaOH; T = 35 °C) Estrip (%) NaCl (mol L−1)

LMC

SMC

0.1 0.2

64.2 63.6

80.1 79.9

In summary, transport tests evidenced that better performances were obtained using the SMC with 0.1 M Na2SO4 in the aqueous feed phase at 35 °C. 3.2. Catalytic Tests. From previous results and considering the operating conditions determined in our previous works,23,24 the catalytic tests were carried out in both membrane contactors at 35 °C using for each of the three phases the following: (i) an aqueous solution at pH 2.8, containing 0.19 mL of acetic acid, 0.095 g of iron(II) sulfate, and 0.1 M salt (NaCl or Na2SO4) as the aqueous feed phase; (ii) benzene as the organic phase; (iii) a 0.1 M NaOH solution as the aqueous stripping phase. Because the presence of the counterion could influence the catalytic performance, the oxidation tests were carried out with both NaCl and Na2SO4. Catalytic tests had a duration of 240 min. Organics in the aqueous reacting phase were analyzed after quantitative extraction with diethyl ether at the end of each run. A fixed amount of H2O2 (18 mmol, which was the optimum in the previous works) was manually fed in the aqueous phase slowly (with a micropipet, step-by-step mode) in 4 h at time intervals of 5 min (0.375 mmol per step). The obtained results were elaborated on using the parameters reported in the Apparatus and Methods section. 3.2.1. Catalytic Tests with NaCl. The results obtained during the catalytic tests with NaCl in both LMC and SMC are reported as the phenol concentration in the organic and strip phases versus time in Figures 8 and 9 and as phenol growth

Figure 9. Phenol concentration in the organic and strip phases versus time during the catalytic tests with NaCl in the SMC [aqueous feed phase, pH 2.8 (0.19 mL of acetic acid), 0.095 g of iron(II) sulfate, 0.1 M NaCl; organic phase, benzene; aqueous stripping phase, 0.1 M NaOH; T = 35 °C].

Better performances of the SMC can be observed, confirming the observation done in the mass-transport tests. Indeed, higher mass-transfer rates, and then higher concentrations in the feed and strip phases (see Figures 8 and 9), were obtained in the SMC for all of the test runtimes. It is noticeable that at 30 min Jo→s (phenol flux from the organic phase to the strip phase) is 1.83 mmol h−1 m−2 in the case of SMC (see Table S2 in the SI), while no phenol transport from the organic phase to the strip phase is observed in the LMC (Table S1 in the SI) because of the poor agitation of the organic phase to avoid phase mixing. For both membrane contactors, rorg was higher than rstrip at the beginning of the catalytic tests because the concentration gradient at the feed/organic interface was high, while at the organic/strip interface, it was practically zero. Thus, phorg rapidly increases, while phstrip remains zero until the concentration gradient at the organic/strip interface promotes phenol stripping. 10545

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During the catalytic runs, rorg and rstrip increase in both membrane contactors up to 180 min. After that time, a decrease of rorg (from 210.0 to 32.5 mmol L−1 h−1) was observed in the SMC because the concentration gradient between the reacting and organic phases decreases. Indeed, around 180−210 min, phorg versus time has an inflection point owing to the reaching of the same mass-transport rates. At 240 min, it practically reaches a plateau because Jf→o becames comparable to Jo→s. In the case of LMC, not very high concentrations were achieved because of lower mass-transfer rates (Figure 8). phorg and phstrip increase in the time interval 0−240 min, but at higher time (not shown in the figure), an inflection point and a plateau in the phenol concentration in the organic phase were also observed in the LMC. In Table 2, a summary of the results obtained in the catalytic tests with 0.1 M NaCl in both membrane contactors is

subsequent oxidation is limited and the productivity is increased. 3.2.2. Catalytic Tests with Na2SO4. The results obtained in the catalytic tests with Na2SO4 in both contactors are reported as the phenol concentration in the organic and strip phases versus time in Figures 10 and 11 and as phenol fluxes in Tables S3 and S4 in the SI.

Table 2. Summary of the Results Obtained in the Catalytic Tests in Both Membrane Contactors with NaCl and Na2SO4 [Aqueous Feed Phase, pH 2.8 (0.19 mL of Acetic Acid), 0.095 g of Iron(II) Sulfate, 0.1 M NaCl or Na2SO4; Organic Phase, Benzene; Aqueous Stripping Phase, 0.1 M NaOH; T = 35 °C; Time = 240 min] NaCl nph,aq (mmol) nph,org (mmol) nbq,org (mmol) nph,strip (mmol) nbq,strip (mmol) Xox (%) Sph,org (%) Sph,strip (%) Pph (gph gcat−1 h−1)

Figure 10. Phenol concentration in the organic and strip phases versus time during the catalytic tests with Na2SO4 in the LMC (aqueous feed phase, pH 2.8 (0.19 mL of acetic acid), 0.095 g of iron(II) sulfate, 0.1 M Na2SO4; organic phase, benzene; aqueous stripping phase, 0.1 M NaOH; T = 35 °C).

Na2SO4

SMC

LMC

SMC

LMC

0.51 0.59 0.16 0.20 0.00 7.23 78.7 100 0.32

0.11 0.08 0.01 0.04 0.00 1.27 88.4 100 0.06

0.71 0.86 0.20 0.93 0.00 13.87 81.3 100 0.62

0.54 0.21 0.00 0.10 0.00 4.68 100.0 100 0.21

reported. A higher phenol amount (mmol) in all three phases (aqueous, organic, and strip) using the SMC can be observed. The conversion of H2O2 to phenol Xox was higher in the case of SMC. However, the low Xox obtained values evidenced masstransfer limitation of the overall reaction/permeation process, which means a high amount of the oxidant was consumed in subsequent oxidations. In both of the contactors, the selectivity to phenol in the strip phase was 100% because benzoquinone, which was the only byproduct found in the organic phase by GC−MS analyses, was not detected in the strip phase. This result is very important, indicating that the desired product can be recovered from the reacting system (catalyst + oxidant + substrate) at a very high purity level. Good selectivity to phenol in the organic phase was also obtained in both SMC and LMC. Benzoquinone, biphenyl (at trace levels), and tars were the byproducts found in the aqueous reacting phase. Benzoquinone was the only byproduct extracted in the organic phase because of its higher concentration gradient with respect to biphenyl. The highest selectivity value obtained in the LMC (88.4% vs 78.7%) is misleading because the productivity obtained in this system is very low compared to that obtained in the SMC (0.06 vs 0.32 gph gcat−1 h−1). This behavior is justified by the best transport performances already observed in the mass-transport tests and confirmed now in the oxidation tests. Practically, considering that the produced phenol extracted faster in the SMC,

Figure 11. Phenol concentration in the organic and strip phases versus time during the catalytic tests with Na2SO4 in the SMC (aqueous feed phase, pH 2.8 (0.19 mL of acetic acid), 0.095 g of iron(II) sulfate, 0.1 M Na2SO4; organic phase, benzene; aqueous stripping phase, 0.1 M NaOH; T = 35 °C).

It can be observed that also in this case the SMC gives better performances compared to the LMC. Indeed (see Figures 10 and 11), higher phenol concentrations in both the organic (620 vs 155 mg L−1) and strip phases (676 vs 76 mg L−1), and therefore higher mass-transfer rates, were obtained in the SMC. As expected, the values of both Jf→o and Jo→s, obtained using 0.1 M Na2SO4, are higher than those obtained using 0.1 M NaCl for both membrane contactors because Na2SO4 gives a higher ionic strength, thus promoting phenol extraction from the aqueous reacting phase. As a consequence, the inflection point of the phenol concentration in the organic phase (phorg) versus time, previously observed around 180−210 min using NaCl in the SMC, is now obtained around 90−120 min using Na2SO4 (see Figure 11). After 150 min, phorg remains 10546

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in the aqueous phase for both membrane contactors. However, better performances were obtained using SMC with 0.1 M Na2SO4 in the aqueous feed phase at 35 °C because it gives an higher ionic strength and phenol is more easily extracted from the feed phase and finally transported in the strip phase. In the second stage of the work, benzene hydroxylation to phenol was carried out in both reactors at 35 °C. The best results were obtained using SMC with 0.1 M Na2SO4, confirming that observed during the mass-transport tests. In particular, higher phenol concentrations in both the organic (620 vs 155 mg L−1) and strip phases (676 vs 76 mg L−1), and therefore higher mass-transfer rates, were obtained in that system compared to LMC. As a consequence, higher phenol productivity (0.62 gph gcat−1 h−1) was also obtained because the highest phenol flux allowed faster extraction of the produced phenol in the organic and strip phases, thus avoiding its subsequent oxidations. In both contactors, the selectivity to phenol in the strip phase was 100%, evidencing that the use of a third compartment, containing an alkaline aqueous stripping phase, permitted one to recover the product at 100% purity level. Tar formation in the aqueous reacting phase was not avoided. The use of different catalysts could solve this problem.

practically constant and the growth rate in the organic phase rorg decreases from 215.0 to 20.0 mmol L−1 h−1 (see Table S4 in the SI). In Table 2, a summary of the results, obtained in the catalytic tests with 0.1 M Na2SO4, is reported and compared with those obtained with 0.1 M NaCl. It can be observed that Na2SO4 gives better results than NaCl in both systems because phenol extraction is favored. However, the not very high Xox values confirm that also in this case the overall reaction/permeation process is mass-transfer-limited, and a high amount of the oxidant was consumed in subsequent oxidations. In both contactors, the selectivity to phenol in the strip phase is 100%, as was already seen for NaCl, confirming the advantage in using the proposed three-compartment membrane contactor, which means that the desired product can be recovered from the reacting system at a very high purity level. Higher phenol productivity (0.62 gph gcat−1 h−1) was obtained in the SMC with Na2SO4 because the highest phenol flux permitted one to extract a higher amount of phenol to the organic and strip phases, thus protecting it by subsequent oxidations. The low phenol productivity obtained in the present work in comparison with that obtained in the previous work23 using the two-compartment cell without salt (0.62 vs 3.60 gph gcat−1 h−1) indicates that the addition of salts in the aqueous reacting phase increases phenol extraction but also increases reaction kinetics, promoting subsequent oxidation reactions. This behavior can be explained considering the results obtained by Le Troung et al.,29 who studied the effects of sulfate and chloride ions on the oxidation rate of ferrous ion by H2O2. In particular, the authors evidenced that in the presence of sulfate or chloride the oxidation rate of FeII was faster than that in their absence and also depended on the ion concentrations. Furthermore, the sulfate and chloride ions may affect the efficiency of FeII/H2O2 and FeIII/H2O2 systems owing to complex formation with FeII and FeIII.30 It must also be observed that, compared to our previous work,24 the use of a third compartment, containing an aqueous stripping phase, and the use of salts permitted one to recover the product at 100% purity.

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ASSOCIATED CONTENT

S Supporting Information *

Growth rate of phenol and phenol fluxes versus time during the catalytic tests with NaCl in the LMC (Table S1), growth rate of phenol and phenol fluxes versus time during the catalytic tests with NaCl in the SMC (Table S2), growth rate of phenol and phenol fluxes versus time during the catalytic tests with Na2SO4 in the LMC (Table S3), and growth rate of phenol and phenol fluxes versus time during the catalytic tests with Na2SO4 in the SMC (Table S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +39 0984 496699. Fax: +39 0984 496655. E-mail: r. [email protected].

4. CONCLUSIONS The synthesis and separation of phenol, by direct partial oxidation of benzene, can be carried out in membrane reactors able to perform a simultaneous product recovery. Two new membrane contactor configurations, in which two solid (SMC) or liquid (LMC) membranes separate three compartments containing (i) an aqueous reacting phase, where benzene hydroxylation to phenol happens, (ii) an organic phase (only benzene) acting also as substrate reservoir, where the produced phenol is extracted, and (iii) a third basic aqueous phase, acting as a stripping agent, have been tested. Preliminarily, mass-transport tests were carried out at 25 and 35 °C to determine the mass permeation flux of the two membrane reactors and the influence of salts dissolved in the feed aqueous phase. The obtained results evidenced better phenol recovery in the strip phase using SMC (60.8% vs 49.0% at 25 °C and 77.3% vs 46.5% at 35 °C). Probably the worst conditions of agitation of the organic phase in the case of LMC, because of the limited stirring intensity to avoid phase mixing, caused the observed greater difficulty to recover phenol in the strip phase. The extraction percentage in the strip phase increased significantly when 0.1 M NaCl or Na2SO4 was added

Notes

The authors declare no competing financial interest.

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LIST OF SYMBOLS aq = aqueous phase Δphi = variation of the phenol concentration in the i phase (mg L−1) Ei = extraction percentage in the i phase (%) gcat = mass of the catalyst (g) gph = mass of phenol (g) i = i phase (i.e., aq, org, or strip) Jf→o = phenol flux from the feed to the organic phase (mmol h−1 m−2) Jo→s = phenol flux from the organic to the strip phase (mmol h−1 m−2) Kd = partition coefficient LMC = liquid membrane contactor nbq,i = amount of benzoquinone in the i phase (mmol) nph,i = amount of phenol in the i phase (i.e., aq, org, or strip) (mmol) org = organic phase dx.doi.org/10.1021/ie302942r | Ind. Eng. Chem. Res. 2013, 52, 10540−10548

Industrial & Engineering Chemistry Research

Article

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phi = phenol concentration in the i phase (i.e., aq, org, or strip) (mg L−1) Pph = phenol productivity (gph gcat−1 h−1) ri = growth rate of the phenol concentration in the i phase (mmol L−1 h−1) SMC = solid membrane contactor Sph,i = selectivity to phenol in the i phase (%) strip = strip phase Xox = conversion of H2O2 to phenol (%)

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