remarks on studies for direct production of phenol.pdf

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. 2010; 5: 191206Published online 10 September 2009 in Wiley InterScience(www.interscience.wiley.com) DOI:10.1002/apj.369

Special Theme Review

Remarks on studies for direct production of phenolin conventional and membrane reactors

Raffaele Molinari* and Teresa PoerioDepartment of Chemical and Materials Engineering, University of Calabria, Via P. Bucci, 44/A, I-87036 Rende (CS)-Italy

Received 29 December 2008; Revised 5 March 2009; Accepted 18 March 2009

ABSTRACT: The great interest in the oxidation reaction of benzene to phenol is linked to some disadvantages of the

cumene process, such as environmental impact, production of an explosive intermediate, a multi-step process (which

involves (1) difficulty to achieve high phenol yield, in relation to the benzene used and (2) high capital investment),

and a high acetone production as a co-product which results in an over supply in the market.

In this paper, we discuss various studies concerning a new approach based on a one-step and acetone-free method

for phenol production. Particular attention is devoted to phenol production processes using various configurations of

membrane reactors (MRs) and a photocatalytic membrane reactor (PMR). In particular, the biphasic MR allowed to

achieving high selectivity values (9798%).The described studies have been classified according to oxidant type such as N 2O, O2, and H2O2. Each of them

shows that direct oxidation of benzene to phenol is a difficult task and further efforts are needed to search and replace

the three step traditional process of converting benzene into phenol with a process of direct oxidation. 2009 Curtin

University of Technology and John Wiley & Sons, Ltd.

KEYWORDS: cumene process; one-step phenol production; membrane reactor; nitrous oxide; oxygen; hydrogen

peroxide; phenol by photocatalysis

INTRODUCTION

Phenol is an important raw material for the synthesisof petrochemicals, agrochemicals, and plastics. Exam-ples of employment of phenol as an intermediate areproduction of bisphenol A, phenolic resins, caprolac-tam, alkyl phenols, aniline, and other useful chemicals.Current worldwide capacity for phenol production isnearly 7 million metric tonnes per year.[1] Today, almost95% of the worldwide phenol production is based onthe so called cumene process (described afterwardsin more detail) which is based on a three-step process.Despite its great success, the cumene process has somedisadvantages: poor ecology, an explosive intermedi-

ate (cumene hydroperoxide (CHP)), and a multistepscharacter, which makes difficult to achieve high phenolyields with respect to benzene. For example, as reportedby Niwa et al .[2] the benzene conversion is about 20%in the first step (concerning the production of cumenefrom benzene and propylene) using the traditional sup-ported phosphoric acid catalyst, while in the second stepit is about 25% for the oxidation of cumene to CHP

*Correspondence to: Raffaele Molinari, Department of Chemicaland Materials Engineering, University of Calabria, Via P. Bucci,44/A, I-87036 Rende (CS)-Italy. E-mail: [email protected]

with air. In the third step the decomposition of CHP tophenol and acetone with sulphuric acid gives phenol in

a yield over 93%. Accordingly, in this traditional pro-cess, the one-pass yield of phenol, based on the amountof benzene initially used, is less than 5%.

The presence of acetone, produced as by-product ina 1 : 1 stoichiometry, is the greater inevitable disadvan-tage. This problem brings serious issues since the eco-nomics of this process significantly depends on the mar-ketability of the acetone by-product.[3] In order to solvethis problem the Mitsui Company ventured into phenolproduction with a modified cumene process making useof acetone recycling. The acetone recycling includedtwo additional steps to convert acetone into propylene,

via hydrogenation and dehydration, which is then reusedin the first step of the cumene process. The Mitsui 5-steptechnology increases the process complexity.

The search for new routes for phenol productionbased on the direct benzene oxidation became moreintensive in the last decade. Many studies emphasizethe innovative potentialities and the emerged role of themembrane reactors (MRs)[46] for improving existingindustrial processes and for introducing new produc-tion methodologies. MRs can improve the efficiency ofchemical conversion processes reducing reactants con-sumption and by-product formation (and also reducing

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92 R. MOLINARI AND T. POERIO Asia-Pacific Journal of Chemical Engineering

polluting emissions). Higher energy efficiency, modu-larity, and easy scale-up are some other advantages ofMRs with respect to conventional fixed bed reactors.[7]

In this work, after an in depth description of thecumene process, the main studies for alternative meth-ods of phenol production are presented. Particular atten-tion is devoted to the phenol production processes stud-ied by our research group using various configurations

of MRs and a photocatalytic membrane reactor (PMR).The described studies have been classified according

to oxidant type, such as N2O, O2, and H2O2, employedfor the direct oxidation of benzene to phenol in bothtraditional and membrane processes.

CUMENE PROCESS

The aim of all the described innovative processesbeing replacement of the multi-step processes of phenolproduction, a wide description of the cumene process is

reported below, in order to use it as a comparison forother processes.

The cumene process was developed and first realisedin the former Soviet Union, where in 1949 a firstindustrial plant was put into operation. The sameprocess, sometimes referred to as Hocks process, wasdeveloped independently in the Western Countries. Thefirst plant was commissioned in 1952 in Canada.[8]

Between 1939 and 1945 the cumene was producedto be used for aviation fuel during the World WarII. In 1989, more than 90% of cumene demand wasfor use as an intermediate for the phenol production.

The first step of the cumene process is the alkylationof benzene with propylene to cumene. This alkylationreaction is performed using different processes. Aprocess, named SPA (solid phosphoric acid) process,currently licensed by UOP, uses as catalyst a complexmixture of orthosiliconphosphate and polyphosphoric

acid supported on kieselguhr. The small amounts ofwater, that continuously are fed into the reactor inorder to maintain the desired level of activity, developH3PO4 causing some downstream corrosion. Pressures

and temperatures employed in the SPA process are inthe range 2940 bar and 200260 C, respectively. Atypical reactor output yield contains 94.8 wt% cumene,3.1 wt% diisopropylbenzene (DIPB) and 2.1 wt% heavy

aromatics. More than 40 SPA plants have been licensedworldwide.[8] Another process developed by Monsanto

uses an AlCl3 and hydrogen chloride catalyst. Thisprocess permitted to obtain the highest overall yield:

almost 99 wt% based on benzene and 98 wt% basedon propylene. SPA and AlCl3 processes offer differentfeatures but both suffer from a variety of drawbackssuch as high environmental impact, corrosive catalystsand formation of oligomers and other impurities.

To overcome the limitations of the SPA process,UOP began searching for a new cumene catalyst. More

than 100 different catalyst materials were screened,

including mordenites, MFIs, Y-zeolites, amorphous sil-icaaluminas, and -zeolite. UOP selected the mostpromising catalyst, based on -zeolite, for cumene pro-duction and then began to optimize the process designaround this new catalyst. The result of this study was the

UOP Q-Max process. The Q-Max process representsa new generation of cumene technology based upon ahighly selective zeolitic catalyst. It provides excellentcumene product quality (99.97 wt% purity) and near-stoichiometric cumene yield (>99.7 wt%).

The first Q-Max process unit came on stream atJLM Chemicals, Blue Island, IL, USA in August

1996. The JLM project was a revamp of an existingUOP Catalytic Condensation unit (SPA catalyst). AQ-Max unit consists (Fig. 1) of an alkylation reactor,a distillation section, and a transalkylation reactor.[10]

The alkylation reactor is divided into four catalystbeds contained in a single reactor shell. Propylene

Propane

Depropanizercolumn

Benzene

Benzenecolumn

DIPBColumn

Cumenecolumn

Heavies

Alkylationreactor

PropylenBenzene

Cumene

Transalkylationreactor

Figure 1. Q-MAX process scheme for cumene production (Elaborated from Schmidt 2005).[10].

2009 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2010; 5: 191206DOI: 10.1002/apj


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Asia-Pacific Journal of Chemical Engineering PRODUCTION OF PHENOL IN CONVENTIONAL AND MEMBRANE REACTORS 19

Oxidation

Air

ConcentrationDecomposition/Neutralisation

Acetone Purification

Phenolpurification

Acid Neutralizing

agent

Acetone

Phenol

PhenolRecovery

Cumene

AMS HydrogenationCumene(AMS)

Cumene

Residue

Figure 2. Sunoco/UOP Phenol process scheme for phenol and acetone production (Elaborated fromSchmidt 2005).[10].

feed and a mixture of fresh and recycle benzene arecharged into the alkylation reactor, where the propylenereacts to form mainly cumene. The stream from the

alkylation reactor flows into the depropanizer column,which removes the propane that entered the unit with the

propylene feed. The bottoms stream of the depropanizercolumn is sent to the benzene column where excessbenzene is collected overhead and recycled. The bottom

stream of the benzene column is sent to the cumenecolumn where cumene produced is recovered overhead.The bottom stream from the cumene column, containingmostly DIPB, is sent to the DIPB column where it isrecovered and recycled to the transalkylation reactor.In this reactor, DIPB and benzene are converted toadditional cumene. The effluent from the transalkylation

reactor is then sent to the benzene column.Other processes for the cumene production, licensed

by Dow Chemical, Mobil Bagder, CdTech, and Eni,have been converted to more efficient and environmen-

tally friendly zeolite-based processes.[9]

The last steps of the cumene process are the cumene

conversion to CHP and then the decomposition ofconcentrated CHP to phenol and acetone using theSunoco/UOP Phenol process (Fig. 2).

The key process steps in the Sunoco/UOP Phenolprocess include (1) liquid-phase oxidation of cumeneto CHP performed using oxygen from air; (2) concen-

tration of CHP and recycling of unreacted cumene tooxidation; (3) decomposition of concentrated CHP tophenol and acetone, accompained by dehydration ofdimethylphenylcarbinol (DMPC) to alphamethylstyrene(AMS) catalyzed by sulphuric acid at 60100 C; (4)

neutralization of the decomposition catalyst; (5) frac-tionation of the neutralized decomposition product for

recovery of acetone, phenol, AMS, and residue; (6)recovery and purification of acetone and phenol, andrejection of by products as heavy residue; and (7) AMShydrogenation back to cumene for recycle to oxidation,or AMS refining for sale as a by product.

The high overall yield from oxidation and decompo-sition achieved is 1.31 cumene wt/phenol wt without tarcracking.[10]

Despite its great success, the cumene process hassome disadvantages such as the production of anexplosive intermediate (CHP); indeed the oxidation ofcumene to CHP occurs at conditions close to its flamma-bility limits and CHP is a potentially unstable material,which can violently decompose under certain condi-tions. It is a multi-step process, which makes it difficultto achieve high phenol yields in relation to the benzeneused and which leads to a high capital investment. Itrequires the use of aggressive media (dilute sulphuricacid at 60100 C) and has a high acetone productionas a co-product which results in an over supply in the

market.

[11]

Besides, current predictions show that phe-nol demand will be significantly higher than the demandof acetone and that the co-product supply of acetone willsoon exceed the demand of acetone. Phenol plants usingthe cumene process need expensive control equipmentsand involve increased operation complexity owing tothe complex and highly energy consuming three-stagecumene process. As a result, a great number of mod-ern phenol companies are searching for a manufacturingmethod independent on acetone production. The abovearguments justify the great interest to approach a one-step and acetone-free method for the phenol production.

ONE-STEP OXIDATION USING N2OAS OXIDANT

A new route for producing phenol directly from benzenewas the use of N2O as an oxidizing agent in thegas phase. The mechanism of oxidation of benzene tophenol by means nitrous oxide, a very selective oxidant,is still controversial.

According to Panov et al .[8] the so-called -sites,which are formed in an iron-containing zeolite matrix



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can dissociate N2O at low temperature (300C)[1223].

This would produce (O) species responsible forthe selective oxidation of benzene to phenol. Otherresearchers attribute the catalytic activity to surfaceacidity only (Bronsted sites or extra framework alumina-based Lewis acid sites).[2427] The first ones to use N2Oas oxidant for the oxidation of benzene to phenol were,in 1983, Iwamoto et al .[28] This reaction achieved, at

550 C, selectivity over vanadium catalyst exceeding70%. In 1988, Suzuki et al .[24] in Japan (Tokyo Instituteof Technology), Gubelmann et al .[26] in France (RhonePoulenc Co.), and Kharitonov et al .[29] in Russia at theBoreskov Institute of Catalysis (BIC), discovered inde-pendently, that ZSM-5 zeolites are the best catalysts forthis reaction.

On this basis zeolites of various chemical composi-tions were tested: AlSi[3034], GaSi[35,36], TiSi[37],and Si (silicalite).[38,39].

The new phenol synthesis process based on zeolitecatalyst was developed jointly by Solutia Inc. (formerly

a chemical business unit of Monsanto) and the BIC.The phenol process, called the AlphOx process afterthe name of-oxygen, was successfully transferred toa pilot plant that was built by Solutia in Pensacola(Florida). The reaction parameters of AlphOx pilotplant, operating at 400450 C with a contact time of12 s are reported in Table 1. In Fig. 3, a flowsheet

Table 1. Reaction parameters of the AlphOx pilotplant.

Catalyst Fe-ZSM5

Phenol productivity (kg/kgcath) 0.4Benzene conversion to phenol (mol%) 9798N2O conversion to phenol (mol%) 85

of the AlphOx pilot plant[40] shows a simple adiabaticreactor, where the reaction is performed.

This process shows several advantages compared tothe cumene process, i.e. one step only, no acetone by-product, and no explosive intermediates. Despite theadvantages, it is associated with serious problems: theprohibitively high cost of N2O that could hinder itsindustrial application[4] and the rapid deactivation of the

catalyst that results in lower yield and short lifespan ofthe catalyst.[41,42] The major cause of catalyst deactiva-tion is the accumulation of phenol inside the pores ofZSM-5 crystals with consequent coke formation.

ONE-STEP OXIDATION USING O2AS OXIDANT

The most available chemical element on the Earth is theoxygen (53.6 atomic %) and from economical and envi-ronmental point of view it represents an attractive and

challenging agent for selective oxidation of hydrocar-bons. Generally, the selective oxidation of hydrocarbonswith oxygen can be performed in two different ways:activation of oxygen to generate the active oxygenspecies or activation of substrate. The substrate acti-vation is very difficult for the hydroxylation of benzeneto phenol, because the bond energy of the CH bondof benzene is about 472.2 kJ/mol[43] which is muchhigher than that of other kinds of CH bond.[44]. Inorder to perform the oxidation reaction, the activationof oxygen is necessary. One method to generate the acti-vated oxygen species in the benzene hydroxylation reac-tion is the reductive activation process that reducesmolecular oxygen by using reducing agents such asH2, CO, NH3, ascorbic acid, dithioalcohols, zinc, andiron.[4,4551] Hydrogen as reducing agent was used in

Figure 3. A flowsheet of the AlphOx pilot plant (Elaborated from Parmon et al., 2005).[40].



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numerous studies summarized by Kuznetova et al .[52]

In these works, catalysts of Pt or Pd and transition metaloxides were utilised with a mixture of O2 and H2 in theliquid phase providing 90% selectivity and 0.10.6%conversion of benzene to phenol. Kuznetsova et al .[52]

employed the oxygen-hydrogen mixture over silica sup-ported bi-component catalysts containing group VIIImetals and heteropoly compounds, obtaining a ben-

zene conversion of 4.4% and a phenol production of0.96 mmol/h.

Ehrich et al .[53] reported that in gas phase, usingplatinum catalysts, benzene conversion was equal to1.0% (Selectivity, Sphenol = 97%) at 413 K, whereaspalladium catalysts reached a conversion of only 0.2%(Sphenol = 86%) for the same contact time and temper-ature.

Although oxygen is an ideal oxidant, oxygen andhydrogen mixture is dangerous. In the literature alter-native ways to avoid explosion, and successfully con-trol the hydroxylation process, involve supplying oxy-

gen and hydrogen separately. These modes are basedon the use of a membrane reactor that is an effec-tive method to produce active oxygen species.[2,5457].Indeed, Niwa et al .[2] reported a single stage methodof benzene oxidation to phenol using a palladium MR(Fig. 4).

In this reactor, hydrogen and oxygen are separatelysupplied on opposite sides of the membrane. The activehydrogen species, formed by the permeation from oneside of the Pd membrane, produces active oxygenspecies on the opposite side by reacting with oxygengas. Then, the active oxygen species reacts with theadsorbed benzene on Pd and the benzene is directly

converted into phenol.This one-step process attained phenol formation

selectivities of 8097% and benzene conversions of216% below 250 C (phenol yield: 1.5 kg/kg of cata-lyst per hour at 150 C).

Otsuka et al .[58,59] were the first to study another kindof membrane reactor, a fuel cell, to perform the one-step oxidation of benzene to phenol using H2 O2 fuelcell both in liquid phase and in gas phase. The fuel cells

H2

O2

H

H

OH

Benzene

PalladiumMembrane

Phenol

H

Figure 4. Scheme of the reductive oxidationof benzene to phenol using catalytic palladiummembrane (Elaborated from Itoh et al. 2003).[57].

are electrochemical reactors that allow direct conversionof the chemical energy of a fuel into electricity. In theH2 O2 fuel cell system reactor, oxygen is activated bythe permeated hydrogen from the electrolyte membrane(Fig. 5).

Further studies were carried out by Cai et al .[56] usinga H2 O2 proton exchange membrane fuel cell (PEMFC)with Nafion1 membrane as the electrolyte. In this

system it was found that phenol was the only productdetected with a yield equal to 0.35% at 100 mA/cm2 at80 C.

The activation of oxygen by carbon monoxide wasreported by Tani et al .[60] who developed a direct syn-thetic method for producing phenol from benzene usingair in presence of CO and molybdovanadophosphoricacids as catalysts, obtaining a phenol yield of 27.3%along with a small amount of 1,4-benzoquinone (3.4%yield) and a benzene conversion of 41.6%. Variousstudies in which ascorbic acid was used as reducingagent for the benzene hydroxylation are summarized in

Table 2.Direct oxidation of benzene to phenol by O2 with-out any reducing agent would be of great interestfor the industry. Some studies, indeed, have beenreported using heteropolyacid (HPA) catalysts hav-ing both strong acidity and redox properties.[68,69]

Passoni et al .[70] performed the benzene hydroxyla-tion using molybdovanadophosphoric HPAs, producing12 mmol of phenol with a selectivity of 75%. However,after 4 h of reaction time, the HPA was irreversiblyreduced and fresh addition of HPA promoted unselec-tive oxidation.

ONE-STEP OXIDATION USING H2O2AS OXIDANT

Hydrogen peroxide, H2O2, is a very attractive oxidantfor liquid-phase reactions. It can oxidize organic com-pounds with the generation of water as the only theoreti-cal co-product. It is relatively cheap,


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Figure 5. Reaction scheme for the phenol synthesis using a fuel cell reactor(Elaborated from Cai et al. 2005).[56].

Table 2. Catalytic systems reported in the literature using ascorbic acid as reducing agent.

Catalyst Conditions

Yield ofphenol

(%)

Benzeneconversion

(%)Selectivity

(%) References

CuO/Al2O3 (Cu = 3 wt%) 80 vol% acetic acid, T = 80C,

PO2 = 1 atm, benzene = 22.5 mmol,ascorbic acid = 4 mmol

1.3 [61]

VCl3 50% Water/CH3CN, T = 50C, PO2 =

1 atm, benzene = 11.3 mmol, ascorbicacid = 0.1 mmol

3.6 98.1 [62]

CuO/Al2O3 (Cu = 2 wt%) 80 vol% acetic acid, T = 30C,


2.5 [63]

Polyoxometalate [(C4H9)4N]m Benzene : acetone: sulfolane : water = 9.2 91.8 [64][PW11CuO39(H2O)]nH2O 1 : 7 : 1 : 1, T = 50

C, PO2 = 10 atm,

benzene = 11.3 mmol, ascorbic acid =0.8 mmol

V/Al2O3 80 vol% acetic acid, T = 30C,


8.4 [65]

VOx/CuSBA-15 Acetic acid: water = 2 : 1 T = 80C,

PO2 = 7 atm, benzene = 11.3 mmol,ascorbic acid = 11.9 mmol

27 100 [66]

LaOx/HZSM5 (0.5 wt%) 80 vol% acetic acid, T = 80 C,PO2 = 4 atm, benzene = 5.6 mmol,ascorbic acid = 1 mmol

4.2 [67]

Recently, Bianchi et al .

[85]

reported a wateracetonitrile (1 : 1) biphasic reaction medium in whichthe produced phenol was extracted into the organicphase and the Fenton catalyst was soluble in the aque-ous phase. The selectivity of the benzene hydroxylationwas obtained by reducing the contact time betweenphenol and catalyst. The oxidation of benzene and itsderivatives by Fentons reagent has been known for along time.[89] The classical oxidation process utilizesthe reaction of aqueous iron(II) with hydrogen peroxideto generate hydroxyl radicals, which are then used tocarry out the substrate oxidation.

However, the selectivity of this process is rather poorsince phenol is more reactive towards oxidation than

benzene itself, and formation of oxidation by-products

(catechol, hydroquinone (HQ), benzoquinones (BQ),biphenyl, and tars) usually occurs.

Molinari et al .[90,91], in order to avoid the phenol

over-oxidation, employed a different configuration of

membrane reactors. The major advantage to use themembrane technology in catalytic processes is the

separation of products from the reaction mixture. The

product separation is one of the most common tasks

in using a membrane to build a reactor and permits



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Table3.

CatalyticsystemsreportedintheliteratureusingH2O2

asoxidant.

Catalyst

Conditions

Yield

of

phenol

(%)a

Benzene

conversion(%)

Selectivity(%)b

References

Pyridinemodifiedmolybdovanadophosphate

PyPMo10V2O40

Solvent:aceticacid/CH3CN;T=

80C

Benzene:11.28mmol;H2O2:33.95mmol

20.5

98

[72]

Vanadiumsubstitutedheteropolym

olybdic

acid

Solvent:aceticacid;

T=

70C

H4PMo11VO40

Benzene:22.50mmol;H2O2:47mmol

10.1

93

[73]

Vanadium-substitutedheteropolym

olybdates

Solvent:aceticacid;

T=

25C

MixtureofH4PMo11VO40,H5PM

o10V2O40,


26.6

91.2

[74]

H6PMo9V3O40

Mesoporoustitanosilicate

Solvent:CH3CN;T=

60C

Ti-MCM-41

Benzene:4.8mmol;

H2O2:32.6mmol

70

98

[75]

ModifiedTitanio-Silicalite

Solvent:sulfolane;T

=

100C

TS-1B

ratioH2O2/Benzene

=

0.1

8.6

9

4c

[76,77]

Cu2+-substitutedmolecularsieves

Solvent:CH3CN/H2O;T=

60C

Cu-AlPO4

Benzene:10mmol;H2O2:27.4mmol

28

28

10

0d

[78]

Iron-porphyrinencapsulatedinPM

AA

(Polymethacrylicacid)

Solvent:Benzene/H2O2;T=

70C


0.13

100

[79]

Waterwashedmanganesenodule

leached

residue(WMNLR)Powdercon

taining:

Mn,Fe,Al2O3,SiO2,H2O

Solvent:aceticacid;

T=

25C


12.7

98

[80]

Ferrictri(dodecanesulfonate)Fe(D

S)3

Solvent:Ionicliquid

(1-n-octyl-3-ethylimidazolium

hexafluorophosphate

(OMImPF6);T=50

C

54

100

[81]




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Table3.

(Continued).

Catalyst

Conditions

Yield

of

phenol

(%)a

Benzene

conversion(%)

Selectivity(%)b

References

Vanadium(IV)complexeswith

N-hydroxyiminodicarboxylicacids,

Ca[V(hida)2][hida=

ON(CH

2COO)2]

Solvent:CH3CN;T=

20C

Benzene:5mmol;H

2O2:4mmol

3.2

[82]

Na3VO4

Solvent:aceticacid;

T=

55C

Benzene:270mmol;H2O2generatedfrom

Na2O2=

270mm

ol

23.1

[83]

Oxovanadium(V)hydroxamate

Solvent:CH3CN;T=

80C

C26H20N2O5ClV

Benzene:22.50mmol;H2O2:53mmol

75

100

[84]

FeSO4,5-carboxy-2-metylpyrazine-N-oxide

Solvent:H2O/(CH3C

N-Benzene);T=

3550C

Benzene:180mmol;H2O2:18mmol

8.6

9

7c

[85]

Copperhydroxyphosphate

Solvent:acetone;T=

60C

Cu2(OH)PO4


30.8

79.8

[86]

Clay-supportedvanadiumoxide

Solvent:aceticacid;

T=

60C

Benzene/H2O2=

1.4

14

94

[87]

Fe3+

Al2O3

Solvent:CH3CN/H2O;T=

60C


27

27

100

[88]

aPhenolyield(%)=

molphenol/mol

initialbenzene

100.

bSelectivity(%)=

(molphenol/molofalloxidationproductsdetected)

100.

cCalculatedasmolofphenol/molof

reactedbenzene100.

dCalculatedbydividingtheyieldwit

htheconversion.



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C6H6

C6H5OH

Aqueousphase

Hydrophobicmembrane

TC

Aqueous

phase

Organic

phase

Thermostated

bath

Membrane

Stirrers

C6H6+ H2O2 + cat. C6H5OH + H2O

Organicphase

Figure 6. Scheme of the biphasic membrane reactor.

to obtain advantages in terms of yield and selectivityin equilibrium-limited reactions and in consecutive

catalytic reactions.

DIRECT OXIDATION OF BENZENE INCATALYTIC MEMBRANE REACTORS USINGH2O2 AS OXIDANT

Biphasic membrane reactor

The first approach studied[89] in our work was the liquid-phase catalytic benzene oxidation using a biphasic sys-tem (Fig. 6). The two phases consist of an aqueous

phase, containing the catalyst (FeSO4) and hydrogenperoxide (oxidant), and an organic phase containinginitially only benzene. They were placed in a two com-partment cell separated by a hydrophobic membrane toavoid phase mixing during the oxidation reaction and toextract phenol from the reaction at ambient conditions.

This system showed a high selectivity to phenol,avoiding its over-oxidation in the organic phase. Indeed,benzene permeation, across the membrane from theorganic phase, feeds the reaction interface, and then theformed phenol permeates back across the membrane tothe organic phase taking shelter from over oxidation.

The high value of phenol selectivity (98%) is obtainedthanks to its extraction into the organic phase, thereby

avoiding further contact with the catalyst, which issoluble in the aqueous phase.

To optimize the performance of the biphasic mem-brane reactor the following parameters were studied:(1) feeding-mode and amount of hydrogen peroxide,(2) various organic acids to promote phenol extractioninto the organic phase, (3) different kinds of membraneas perm-selective barrier for separating the two phases.

A constant amount of hydrogen peroxide of 18 mmolwas added by three different methods: (1) one stepmode, meaning that all H2O2 was added at the begin-ning of the experimental run; (2) pump top mode, i.e. by

adding the H2O2 slowly in 4 h by means of a peristalticpump with a flow rate of 4.2 ml h1; and (3) pump bulkmode, similar to the previous one but the difference wasthe oxidant injection in the bulk of the aqueous phaseof Fig. 6.

By adding the hydrogen peroxide slowly (in 4 h)into the aqueous phase (mode iii)), a higher conver-sion based on hydrogen peroxide (96.8%) and phenolproduction (13.3 g l1) were achieved (Table 4).

This difference can be explained with the highreactivity of phenol in the aqueous phase: indeed, whenthe oxidant is added slowly, the interfacial reaction is



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Table 4. Influence of the addition of hydrogen peroxide mode on the benzene hydroxylation in the biphasicmembrane reactor.

Mode of hydrogenperoxide feeding

Selectivity tophenola (%)

Benzeneconversion to

phenolb (%)

Hydrogen peroxideconversion to phenolc

(%)

Phenol concentration

g1

One step 98 0.49 39.25 5.1Pump top 98 0.81 65.06 8.5Pump bulk 98 1.20 96.78 13.3

a selectivity to phenol = [mmol phenol/(mmol phenol + mmol biphenyl + mmol benzoquinone)] 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.

Table 5. Influence of hydrogen peroxide amount on the benzene hydroxylation in the biphasic membrane reactor.

Amount of hydrogenperoxide

Selectivity tophenol (%)

Benzeneconversion tophenol (%)

Hydrogen peroxideconversion to phenol

(%)

Phenol concentration

g l1

9 98 0.25 40.87 2.718 98 1.20 96.78 13.3

36 98 1.12 45.17 11.8

the limiting step, so that phenol can permeate throughthe membrane without over oxidation caused byfurtherattack from hydroxyl radical.

Instead, when the hydrogen peroxide is fed by onestep mode, the permeation of produced phenol acrossthe membrane becomes the limiting step, so that acertain amount of phenol suffers over oxidation. Thusa higher by-product formation in the aqueous phasetakes place, resulting in lower phenol production and

uncontrolled consumption of oxidant.Regarding to the amount of hydrogen peroxide added

in the aqueous phase, the results evidenced an optimalamount of 18 mmol (Table 5). Indeed, an increase from9 to 18 mmol increased the benzene and hydrogenperoxide conversion to phenol and phenol production.

A further increase to 36 mmol gave the followingresults: (1) the benzene conversion decreased (1.12 vs1.20) promoting over-oxidation of a small amount ofphenol; (2) the hydrogen peroxide conversion decreasedat half of its optimal value (45.2 vs 96.8); and(3) the phenol production decreased slightly (11.8 vs

13.3 g l

1

).This behaviour can be explained considering thattoo much oxidant in the aqueous phase provokes anincrease of oxidant capacity of the system. Thus thefraction of produced phenol that did not permeate in theorganic phase through the membrane was converted toover oxidation products which remained in the aqueousphase. This result confirms that the oxidant must be fedin appropriate amount and slowly.

Acidic pH is necessary to promote phenol extrac-tion in the organic phase. In particular, acetic acid(pKa = 4.76) and trifluoroacetic acid (pKa = 0.6)

were considered to investigate the influence of acidicstrength on phenol transport in the organic phase. Next,ascorbic acid was tested because it could promote phe-nol extraction and it should be able to find and neutralizethe hydroxyl radicals in excess. The results (Table 6)showed that best system performance was obtained byusing acetic acid.

The influence of porous membrane material (poly-propylene (PP), polytetrafluoro ethylene (PTFE)

hydrophobic; polyacrylonitrile (PAN) hydrophilic) wasalso investigated to compare system performance inchanging membrane interface where oxidation reactiontakes place (Table 7). When a hydrophobic support isemployed, the reaction happens at membrane interfaceon the aqueous side, while with hydrophilic supportthe oxidation takes place at membrane interface on theorganic side. Obtained results showed that hydrophobicPP porous support was the best.

Summarizing, the results showed that iron(II) sul-phate as the catalyst, 18 mmol of hydrogen peroxidepumped in 4 h in the bulk of the aqueous phase as

oxidant, acetic acid for pH control, and polypropy-lene hydrophobic porous support as separation barriergave the best system performance in terms of phenolproductivity (38.2 mmol gcat

1 h1), selectivity to phe-nol (98%), benzene conversion to phenol (1.2%), andhydrogen peroxide conversion to phenol (96.8%). Thedrawback of this system was the low rate of phenolextraction into the organic phase. Indeed, the phenolthat does not cross the membrane rapidly to reach theorganic phase reacts further to generate over-oxidationproducts and forms a black solid. Further studies are inprogress to solve or reduce this problem.



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Table 6. Influence of acid type on the benzene hydroxylation in the biphasic membrane reactor.

AcidSelectivity tophenol (%)

Benzene conversion tophenol (%)

Hydrogen peroxideconversion to phenol (%)

Phenol

concentration g l1

Acetic 98 1.20 96.78 13.3Ascorbic 98 0.30 49.06 3.2Trifluoroacetic 98 0.22 17.82 1.5

Table 7. Influence of membrane materials on the benzene hydroxylation in the biphasic membrane reactor.

Membranematerial

Selectivity tophenol (%)

Benzene conversion tophenol (%)

Hydrogen peroxideconversion to phenol (%)

Phenol

concentration g l1

PP 98 1.20 97 13.3PTFE 98 0.84 67 8.8PAN 98 0.22 36 2.4

(a) (b)

Figure 7. Cross section images of PVDF membranes prepared with DMAc and(a) copper oxide nanopowder catalyst (PVDF2CuOnanop); (b) copper oxide powder catalyst(PVDF2CuOp).

Ultrafiltration membrane reactor

A further approach based on the control of the contacttime of phenol with the catalyst was investigated inorder to avoid/reduce by-product formation.[91] Thecatalyst was entrapped in polymeric membranes and thesolution containing the reactants (the oxidant and thesubstrate) permeated through the membrane at differentpermeation flow rates.

Various flat-sheet polyvinylidene fluoride (PVDF)catalytic membranes were prepared by the phase inver-sion process induced by a nonsolvent. Dimethylacetam-mide (DMAc), dimethylformammide (DMF), and 1-methyl-2-pyrrolidone (NMP) were used as solvents anddistilled water as the nonsolvent. For each solvent apolymer solution (20 wt%) in the solvents was pre-pared at room temperature; then the catalyst, a CuOpowder or CuO nanopowder (16 wt%), was added andthe solution was cast on a glass plate to obtain themembrane. By scanning electron microscopy (SEM) auniform catalyst distribution was observed in all the

polymeric membranes prepared using the copper oxidenanopowder (Fig. 7a). In contrast, no uniform catalystdistribution was observed in PVDF membranes pre-pared using the copper oxide powder (Fig. 7b). Themembrane properties were affected by the additionof CuO particles in terms of morphological proper-ties such as membrane thickness and pore size, and interms of membrane performance such as the membranepermeability.

The membrane characterization tests evidenced thatthe best solvent was the DMAc. The prepared catalyticmembranes were tested using an ultrafiltration mem-brane reactor (Fig. 8).

It is composed of an ultrafiltration membrane where aperistaltic pump fed a dead-end permeation cell with thefeed solution. The permeation cell containing the mem-brane had an exposed surface area of 4 cm 6 cm =24 cm2. This system permits to control the contact timeof feed solution with the catalytic membrane by vary-ing the permeate flow rates changing the transmembranepressure.



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Feed

TC

Permeate

Catalyticmembrane

Figure 8. Scheme of the ultrafiltration membranereactor.

Calculation of contact times were performed by usingthe following equations:

Contact time =Membrane thickness (cm)

v (cm min1)

v =Permeate flow rate (cm3min1)

membrane surface (cm2)=

cm

min(2)

where v is the permeation velocity of the reactingsolutions across the membrane.

A typical procedure for oxidation tests in the mem-brane reactor was as follows. A solution containing100 ml of an acetonitrile-benzene mixture of 8 : 1 v/vratio and hydrogen peroxide of 1 : 1 molar ratio withbenzene was pumped into the permeation cell at differ-ent transmembrane pressures corresponding to differentcontact times.

The permeated solution was collected and analysed

for determining the concentrations of phenol and by-products.

The preliminary catalytic tests were performed at twotemperatures (35 and 50 C); the effect of acetic andascorbic acid addition in the reacting media was alsostudied.

The obtained results (Tables 8 and 9) showed a higherphenol concentration using the PVDF membrane filledwith the CuO nanopowder rather than CuO powdercatalysts. In particular a phenol yield of 2.3 (%)was obtained in a single pass using a contact timewith the catalyst of 19.6 s at 35 C. The by-products

such as benzoquinone and biphenyl were detected astraces.Further research is in progress to improve system

performance by testing other more efficient catalystsable to increase the phenol yield and to maintain highselectivity value for longer time.

Table 8. Phenol Yield (%)a at different contact times using the catalytic membranes prepared with DMA copperoxide nanopowder catalyst (PVDF2CuOnanop).

Contacttime (s)

Yield (%)at 35 C

Yield (%)at 50 C

Yield (%)at 35 C with

acetic acidb

Yield (%)at 35 C withascorbic acidc

4 0.19 0.34 0.24 0.254.5 0.21 0.27 0.22 0.275.1 0.25 0.27 0.21 0.366.9 0.29 0.25 0.19 0.4110.6 0.43 0.24 0.17 0.8819.4 0.44 0.53 0.18 2.3

a Phenol yield (%) = Ph/Bz 100, Ph is the mol numbers of phenol in the permeated solution, Bz is the mol number of benzene in the feedsolution, Bz = 113 mmoles.b 4 mmol of acetic acid.c 4 mmol of ascorbic acid.

Table 9. Phenol yield (%)a at different contact times using the catalytic membranes prepared with DMA copperoxide powder catalyst (PVDF2CuOp).

Contacttime(s)

Yield (%)at 35 C

Yield (%)at 50 C

Yield (%)at 35 C with

acetic acidb

Yield (%)at 35 C withascorbic acidc

33.4 0.1 0.34 0.98 1.147.7 0.1 0.27 1.13 1.366.8 0.1 0.27 1.38 1.7

a Phenol yield (%) = Ph/Bz 100, Ph is the mol numbers of phenol in the permeated solution, Bz is the mol number of benzene in the feedsolution, Bz = 113 mmoles.b 4 mmol of acetic acid.c 4 mmol of ascorbic acid.



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ONE-STEP CONVERSION OF BENZENE TOPHENOL WITH COUPLED PHOTOCATALYTICAND MEMBRANE TECHNOLOGIES

A direct synthesis of phenol from benzene was alsostudied using a photocatalytic reaction coupled with amembrane. In photo-catalysis the only difference withconventional catalysis is the mode of activation of thecatalyst that happens by a photonic activation.[92,93]

Indeed the absorption of a photon by semiconduct-ing solids excites an electron (e) from the valenceband to the conduction band if the photon energy, h,equals or exceeds the band gap of the semiconduc-tor/photocatalyst. Simultaneously, an electron vacancyor a positive charge, called a hole (h+), is also gener-ated in the valence band (Fig. 9). Ultraviolet (UV) ornear-ultraviolet photons are typically required for thiskind of reactions.

Generally, the hole oxidizes water to hydroxyl rad-icals and the electron can be donated to an electron

acceptor such as an oxygen molecule (leading to forma-tion of superoxide radical) or a metal ion (with a redoxpotential more positive than the band gap of the pho-tocatalyst). In particular, for the benzene hydroxylationthe formed hydroxyl radical adds directly to benzeneto produce a hydroxycyclohexadienyl (HCHD) radicalwhich rapidly undergoes an H-atom abstraction by oxi-dants (O2, Fe

3+, Cu2+, etc.) leading to phenol.Several photocatalytic systems using TiO2 as het-

erogeneous photocatalyst and polyoxometalate (POM)as homogeneous photocatalyst were studied by Parket al .[94] investigating on the effects of various elec-

tron acceptors such as O2, Fe3+

, H2O2, Ag+

, N2O, andsurface-modified TiO2 (platinization, fluorination, andsilica loading). They observed that phenol productionyield and selectivity were enhanced with the addition

Band Gap(Eg)Band Gap(Eg)

Conduction band

Valence band

e-

h+

OH-/H2OOHOH

O2

O2-O2-

UVUV

Figure 9. Band-gap diagram: formation of holes(h+) and electrons (e) upon UV irradiation ofsemiconductor surface.

of Fe3+, H2O2, or Fe3+ + H2O2 or modifying the sur-

face of the catalyst, but the highest yield observed inthis study was obtained with the addition of POM toTiO2 suspension.

Selective photo-oxidation of liquid benzene was stud-ied, also, using cation-exchanged zeolites dispersedin C6H6/CH3CN/H2O mixtures at room temperatureby using molecular oxygen,[95] Mo complexes with

Mo1 Mo4 nuclearities grafted on mesoporous silicaFSM-16[96], and bis- and tris-(bipyridine) Ru complexesgrafted on mesoporous FSM-16[97] using hydrogen per-oxide as an oxidant under the irradiation of UV-light.

One step oxidation in a PhotocatalyticMembrane Reactor

The photocatalytic process coupled with a membranesystem for the direct hydroxylation of benzene wasstudied by Molinari et al .[98] In a PMR, the one-step

synthesis of phenol and its simultaneous separationhappen. The experimental plant used (Fig. 10) wasrealized by connecting a photocatalytic batch reactorwith the biphasic separation system described in Fig. 6in which the aqueous reactive phase was constituted by700 ml of an acidic solution containing suspended TiO2as catalyst and benzene as substrate.

Preliminary dark reactions and photolytic tests wereperformed on benzene dispersions in water, in absenceof UV light and the catalyst, respectively. They showedthat the oxidation reaction occurs in a true photocat-alytic regime. Besides, by means of batch tests, the

influence of some photocatalytic parameters (initial sub-strate amount, catalyst concentration, pH, etc.) werestudied in order to choose the optimal operative con-ditions to employ in the photo membrane contactor(PMC).

A first set of experiments was performed varying theinitial volume of benzene added to the TiO2 suspensionto investigate the influence of the amount of substrateon the rate of phenol production. The obtained results

TB PR

L

P

MC

aq phase org phase

Figure 10. Scheme of the PMC: PR, photoreactorsystem; MC, membrane contactor; L, UV lamp; TB,thermostatic bath; P peristaltic pump.



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showed that oxidation kinetics did not depend onthe undissolved substrate but on its concentration insolution (460 50 mg l1 with 5 and 10 ml vs 6010 mg l1 with 1 ml after 2 h), showing the need torealize a system that provides benzene continuously.

Besides, by changing the pH of the aqueous sus-pensions it was observed that the system productivityincreased at alkaline values, due to a lower phenol

adsorption on the catalyst surface which reduced itssubsequent oxidation reactions. Moreover, on using sul-phuric or hydrochloric acid to reduce the pH, it wasobserved that the oxidation was also influenced by thetype of anion present. In particular, the sulphate ionincreased the productivity but reduced the selectivity ofthe process. Negligible variations were obtained in therate of phenol production in a range of catalyst concen-tration of 0.11 g l1, with values of 0.13 vs 0.18 mgl1 min1, respectively, in the first 100 min.

By means of extraction and transport tests the effi-ciency of the realized system was verified in terms of

flux and extraction degree of phenol in the organicphase. From the slope of the equilibrium isotherm(25 C) of the water/benzene system, a distribution coef-ficient of phenol between the two phases of 2.1 wasobtained, while an extraction percentage value (E%)of 24 2% was measured at the steady state (after360 min) in the transport tests. Although these valueswere lower than those of other extractants reported inthe literature, a constant benzene concentration of about200 50 mg l1 was measured in the aqueous phaseassuring a constant feeding of the substrate to the reac-tive environment.

In a first set of photooxidation tests in the PMC the

effect of the pH of the aqueous phase on the efficiencyof the process was investigated.

The data obtained at pH values of 5.5 and 3.1 showedthat the more acidic pH condition allows to obtain aslight increase in the phenol production in the aqueousphase (Fig. 11) and a constant flux (after 2 h) in theorganic phase of 1.27 mmol h1 m2 (compared to thevalue of 1.06 mmol h1 m2 obtained at pH 5.5).

0

4

8

12

16

20

0 100 200 300

Time (min)

400 500

Cphenol(mgL-1)

pH 3.1pH 5.5

Figure 11. Phenol concentration in the aqueous phaseversus the time in the photocatalytic oxidation testsperformed in the PMC at pH 3.1 and 5.5.

During the reaction runs, three main intermediateproducts were observed in the high performance liquidchromatography (HPLC) chromatograms. Two of themwere identified by GC-MS (gas chromatography-massspectrometry) measurements as BQ and HQ, whilethe other oxidation product (Ox3.6) is under study.By the area of these by-products it was observedthat the membrane system allowed to maintain HQ

in the aqueous phase, but it was not able to rejectthe other oxidation products, which pass completelyinto the organic phase due to their greater solubilityin that phase. However, the acidic pH led to a lowerextent of formation and extraction of these intermediates(Fig. 12).

Another set of experiments was performed investi-gating the effects of some parameters, such as catalystconcentration and light intensity, on the photocatalyticand separation performance of the PMC. Comparing theresults obtained with higher TiO2 amounts and lowerirradiation power with those using 0.1 g l1 of catalyst

and an irradiation of 6.0 mW cm

2

(Fig. 13), only anegligible increase of phenol production and extractionwas observed with a catalyst concentration of 1 g l1,while a marked decrease of the system efficiency wasfound with a light intensity of 4.7 mW cm2.

pH 5.5

0

60

120

180

HQ Ox3.6 BQ

Ar

ea(mAU)

Aqueous phase

Organic phase pH 3.1

HQ Ox3.6 BQ

Figure 12. Comparison of the HPLC peaks of three inter-mediates in the two phases measured in the photocatalyticoxidation tests at pH 3.1 and 5.5.

0

5

10

15

20

Aqueous phase Organic phase

Cphenol(mgL-1)

CAT0.1 I6.0 CAT1.0 I6.0 CAT0.1 I4.7

Figure 13. Phenol concentration in the aqueous andorganic phases in oxidation experiments in the PMC (CTiO2 =0.1 g l1 and I = 6.0 mW cm2; CTiO2 = 1.0 g l

1 and I =6.0 mW cm2; CTiO2 = 0.1 g l

1 and I = 4.7 mW cm2).



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CONCLUSIONS

The cumene process for making phenol is today unsus-tainable in terms of environmental impact and energy-consumption. In this context much effort is devotedto new processes that produce phenol using a directroute with high yield and selectivity. Direct oxida-tion has been studied by many authors in both gas

and liquid phases, under low and high pressure, andwith and without catalysts, making this reaction morestudied both for the variety of approaches and num-ber of tested catalysts; nevertheless, relatively few cat-alytic processes have successfully been developed. Oneexample of advanced direct oxidation process is theAlphox process developed by Solutia based on nitrousoxide oxidation of benzene with phenol selectivities of97100% at 100% N2O conversion. However, the cat-alyst, at present, requires frequent regeneration drivingup capital costs. When oxygen was used as the oxidant,it usually needs reducing agents to activate the oxy-

gen and this not only consumes the oxidant but alsoincreases the possibility of explosion. An interestingprocess based on the oxygen as oxidant is representedby the employment of a palladium membrane reactorusing O2/H2 as the oxidant, where a high phenol yieldwas obtained and the possibility explosion was avoided.The development of biphasic systems using H2O2 asoxidant has allowed to getting high selectivity values(9798%) thanks to the selective role of the membrane.Currently, further efforts are in demand to search andreplace the three step traditional process to convert ben-zene into phenol with a process of direct oxidation.However, too much resistance is encountered to sub-

stitute the old with the new.

Acknowledgements

The authors thank the MIUR within the FIRB 20052009 programme for the financial support.

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