WO2006038893A1

WO2006038893A1 - Oxidation of phenolic compound with hydrogen peroxide generated in the presence of the phenolic compound

Info

Publication number: WO2006038893A1
Application number: PCT/SG2005/000339
Authority: WO
Inventors: Suresh Parappuveetil Sarangadharan; Raman Ravishankar
Original assignee: Agency For Science, Technology And Research
Priority date: 2004-10-06
Filing date: 2005-10-06
Publication date: 2006-04-13

Abstract

In a method for oxidizing a phenolic compound, hydrogen peroxide is generated in the presence of the phenolic compound and a catalyst for catalyzing oxidation of the phenolic compound to form a para-selectively oxidized phenolic compound. The hydrogen peroxide can be generated electrolytically. The phenolic compound can be a phenol. The oxidized phenolic compound can comprise hydroquinone. The catalyst can comprise a catalytic molecular sieve, such as a titanium silicalite-1 (TS-1) molecular sieve.

Description

OXIDATION OF PHENOLIC COMPOUND WITH HYDROGEN PEROXIDE GENERATED IN THE PRESENCE OF THE PHENOLIC COMPOUND

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority from United States Provisional Patent Application No. 60/615,959, filed October 6, 2004, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to methods of oxidation of phenolic compounds, and more particularly to methods of oxidation of phenolic compounds with hydrogen peroxide.

BACKGROUND OF THE INVENTION

[0003] Hydroquinone has a variety of applications. For example, hydroquinone can be used in medical applications, as monomer inhibitors, for making dyes and pigments, as agricultural chemicals, and for preparing a variety of polymers. Hydroquinone and its derivatives can be used in photographic applications as developer, in lithography applications, in photochemical machining, in x-ray films, and the like. Hydroquinone derivatives can also be used as antioxidants or anti-ozonants in rubber industry.

[0004] There are known processes for selectively forming hydroquinone from a mixture of phenol and hydrogen peroxide in the presence of a catalytic molecular sieve such as a titanium silicalite (TS-1). In the known processes, a solution of hydrogen peroxide is prepared and is mixed with phenol to form a mixture. The mixture is heated to a temperature of above 60 to 70 ⁰C in the presence of a TS-1 to convert phenol to hydroquinone and catechol by oxidation. Hydroquinone and catechol are respectively para- and ortho- isomers of benzenediol. For producing hydroquinone, it is desirable to increase the proportion of hydroquinone in the reaction products, that is, the para-selectivity of the production process.

[0005] Hydrogen peroxide has also been used to oxidize other aromatic compounds.

[0006] However, conventional processes for oxidizing phenolic compounds with hydrogen peroxide have some drawbacks. For example, one problem is that hydrogen peroxide is a hazardous chemical and its handling, storage, and transportation can be expensive. A problem in the conventional processes for selective oxidation of phenol is that the para-selectivity achievable in these processes is low, typically less than 70%. Another problem is that in these processes, the conversion rates of phenol often have to be lowered in order to achieve high para-selectivity. For example, in a reported study, when conversion rate was increased from 4% to 37%, the corresponding para-selectivity decreased from 82% to 70%. Typically, the phenol conversion rate is about 20% for achieving about 60% para-selectivity in these processes. When the phenol conversion rate is low, additional production cost may be incurred to recover or recycle un-reacted phenol.

[0007] Accordingly, there is a need for an improved method of selectively oxidizing a phenolic compound, which can address one or more of the problems discussed above.

SUMMARY OF THE INVENTION

[0008] Therefore, an aspect of the present invention relates to a method for oxidizing a phenolic compound, in which hydrogen peroxide is generated in the presence of the phenolic compound and a catalyst for catalyzing oxidation of the phenolic compound to form a para-selectively oxidized phenolic compound. The hydrogen peroxide can be generated electrolytically. The phenolic compound can be a phenol. The oxidized phenolic compound can comprise hydroquinone. The catalyst can comprise a catalytic molecular sieve, such as a titanium silicalite-1 (TS-1) molecular sieve.

[0009] Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the figures, which illustrate, by way of example only, embodiments of the present invention,

[0011] FIG. 1 is a schematic diagram of an electrolytic cell;

[0012] FIG. 2 is a line graph of phenol conversion rate and para-selectivity as functions of reaction time;

[0013] FIG. 3 is a three-dimensional bar graph of para-selectivity as a function of reaction time and titanium content; and

[0014] FIGS. 4 and 5 are line graphs of phenol conversion rate, para- selectivity, and hydroquinone yield as functions of reaction time.

DETAILED DESCRIPTION

[0015] It has been discovered that a phenolic compound such as phenol can be selectively oxidized or hydroxylated by hydrogen peroxide (HaO₂) that is generated in the presence of the phenolic compound and a suitable catalyst such as a catalytic molecular sieve. The H2O₂ may be generated electrolytically. The oxidized phenolic compound may be a para-selectively oxidized phenolic compound, such as hydroquinone and the like.

[0016] A phenolic compound can be any aromatic compound that contains at least one hydroxyl group attached to an aromatic ring or any substituted phenol group. Example phenolic compounds include hydroquinone, catechol, resorcinol, cresols, and the like. [0017] An advantage of using hydrogen peroxide generated in situ is that the problems and costs associated with handling, storage, and transporting hydrogen peroxide may be reduced or eliminated.

[0018] It has also been found that in the case of oxidation of phenol, CeH₅(OH), para-selectivity close to 100% can be achieved. Further, the conversion rate of phenol can be kept at higher than 60% when the para- selectivity is above 90%.

[0019] As used herein, para-selectivity = P_h / (P_h + Pc), where Ph and P_c are respectively the volume proportions of hydroquinone and catechol in the reaction product of phenol hydroxylation. The proportions of the reaction products may be measured with any suitable techniques such as a Gas Chromatography (GC) technique. As can be appreciated, during measurement hydroquinone may further react and form benzoquinone due to, for example, the high column and detector temperature in a GC measurement. For accurate measurement of the proportion of initially formed hydroquinone in such cases, P_h may need to include the volume proportion of any detected benzoquinone in the sample. For example, para-selectivity may be calculated as: para-selectivity = (Ph + Pb) / (Ph + Pb ⁺ Pc), where P_b is the measured volume proportion of benzoquinone.

[0020] The conversion rate of phenol for a given reaction period can be determined by measuring the phenol volume in the sample at the start and the end of the reaction period, such as using a GC technique.

[0021] In an exemplary process, H₂O₂ is generated electrolytically in the presence of a phenolic compound and a catalyst to selectively oxidize the phenolic compound with the generated H₂O₂ to form a polyhydroxybenzene such as hydroquinone. The phenolic compound can be phenol and the catalyst can be a catalytic molecular sieve.

[0022] An exemplary process is further illustrated with the aid of FIG. 1, which depicts schematically an electrolytic cell 10, exemplifying an embodiment of the present invention.

[0023] Electrolytic cell 10 has a reaction chamber 11, a cathode 12 and an anode 14. A mixture 16 is provided in chamber 11, in contact with cathode 12 and anode 14. A catalytic molecular sieve 18 is suspended in the reaction mixture 16. For example, molecular sieve 18 may include particulates immersed in mixture 16. As shown in FIG. 1 , molecular sieve 18 is included in mixture 16 and is not distinctly visible.

[0024] A gas pipe 20 is provided for supplying a gas containing oxygen, such as air, into chamber 11. It can be advantageous to direct the gas flow towards the cathode surface, as will become clear below. An optional reference electrode 22 may be provided, the use of which can be readily appreciated and understood by one skilled in the art. An output pipe 24 may be provided for extracting or withdrawing a reaction product such as a benzenediol from electrolytic cell 10. Additional input pipe lines (not shown) may be provided for transporting the mixture, or supplying additional phenol, into the cell.

[0025] Electrolytic cell 10 may include other components, as can be understood by persons skilled in the art. For example, a power source (not shown), such as a cyclic voltmeter may be provided for biasing the electrodes of the cell. Control and measuring components (not shown) may be provided for monitoring and control the operation of the cell. A heating device (not shown) may be provided for heating the mixture in the cell. Cell 10 can be a conventional electrolytic cell suitable for conducting electrochemical reactions and oxidation reactions, such as those described below. Cell 10 can be readily constructed with suitable materials by persons skilled in the art. For instance, chamber 11 may be formed of any suitable material in any suitable manner; cathode 12 and anode 14 can be made from any suitable electrodes, such as platinum electrodes. Optional reference electrode 22 may be an Ag/AgCI electrode. Other suitable materials, such as carbon, graphite, tantalum, indium oxide, lead, or metal containing ceramics, may also be used for the electrodes. The electrodes may be gas diffusion electrodes.

[0026] Cell 10 may be constructed utilizing techniques disclosed in US 4,455,203 to S. Stucki, issued June 19, 1994; US 5,972,196 to O.J. Murphy, issued October 26, 1999; and European patent application publication no. 0277841 , published August 10, 1988, the contents of each of which are incorporated herein by reference.

[0027] Mixture 16 may include an electrolytic medium suitable for producing hydrogen peroxide (H₂O₂) through an electrochemical reaction at cathode 12 when an electric current is induced in the medium through cathode 12. For example, mixture 16 may include a fluid electrolyte such as water. The fluid can be purged with a gas containing oxygen molecules (O₂) so that it is oxygen rich or saturated. The fluid may be acidic and have a pH of 1 to 5. A suitable acid such as a sulfuric acid may be used for adjusting the pH value of the mixture. The electrolyte may also be an organic acid such as an aqueous acetic acid. The electrolyte may include quaternary ammonium salts such as tetrabutylammonium sulphate and tetrabutyl ammonium perchlorate. The electrolyte may also be a suitable ionic liquid.

[0028] Certain solvents and acids may not be suitable in some applications. For example, for oxidation of phenol, acids such as HCI and HNO₃ and solvents such as aromatic solvents and olefin containing organic compounds may not be suitable.

[0029] Mixture 16 contains a phenolic compound such as phenol. For ease of description it is assumed below that the phenolic compound is phenol, but it should be understood that mixture 16 may contain another suitable phenolic compound.

[0030] The amount of phenol in mixture 16 may vary. For example, when the mixture also contains the electrolytic medium, the weight ratio of phenol and the electrolytic medium may be from about 1 :10 to about 1:25, such as about 1 :20. [0031] As mentioned, mixture 16 may also include molecular sieve 18. A molecular sieve is typically a material that exhibits selective adsorption properties, which is capable of separating components of a mixture on the basis of molecular size and shape. For example, zeolites can be used as molecular sieves. As used herein, "zeolite" refers to any porous oxide structure that has well-defined pore structures due to a high degree of crystallinity. For example, a zeolite is typically a crystalline, porous aluminosilicate.

[0032] Molecular sieve 18 can be a titanium silicalite (TS-1), which is a ZSM-5 zeolite having Ti atoms incorporated into its framework. ZSM-5 zeolite has a MFI type crystal structure with pore/channel sizes from about 0.5 to about 1.2 nm.

[0033] Molecular sieve 18 has suitable pore/channel sizes for allowing oxidation of phenol by hydrogen peroxide to selectively form hydroquinone in the pores/channels. The TS-1 material may have various weight percentage of titanium content. For example, it may have from about 0.01% to about 3% of titanium. In some applications, it may be advantageous if the titanium content is from about 0.3% to about 0.6%, such as 0.45%. The molecular sieve may have an MFI crystalline structure and pore sizes of about 0.51 to 0.56 nm. In some applications, the TS-1 sieve may need to have crystallinity in the range of about 60% to about 100%. A crystallinity above about 90% may be advantageous in some applications.

[0034] Catalytic molecular sieves, in particular, TS-1 molecular sieves, suitable for the present process can be manufactured using conventional techniques, such as those disclosed in U.S. patent application publication no. 2001/0021369, published September 13, 2001 ; US 6,475,465 to Min Lin et al., issued November 5, 2002; US 5,233,097 to L. T. Nemeth et a/., issued August 3, 1993; US 4,410,501 to M. Taramasso, issued October 18, 1983; A. Thangaraj et al., "Studies on the synthesis of titanium silicalite, TS-1 ," Zeolites, (1992), vol. 12, pp. 943-950; and A.J. H. P. van der Pol et al., "Why are some titanium silicalite-1 samples active and others not?" Applied Catalysis A, (1992), vol. 92, pp. 113-130, the contents of each of which are incorporated herein by reference.

[0035] In alternative embodiments, a different catalytic molecular sieve may be used. For example, the molecular sieve may include another metal silicalite or metal silicate. The metal silicate may include at least one metal selected from a transition metal and a rare earth metal. Suitable transition metals include Ti, V, Cr, Fe, Co, Cu, Zn, Zr, Mo, Nb, Ta, and the like. Suitable rare earth metals include La, Ce, Sm, and the like. These alternative molecular sieves can be formed according to known processes as will be understood by persons skilled in the art.

[0036] The molecular sieve and some components of mixture 16, such as phenol, may also be prepared as disclosed in the following references: US 5,493,061 to P. Ratnasamy and S. Sivasanker, issued February 20, 1996; US 4,982,013 to M. Gubelmann and P-J. Tirel, issued January 1 , 1991 ; US 5,714,641 to M. Costantini et al., issued February 3, 1998; US 5,434,317 to M. Costantini et al., issued July 18, 1995; A. Tuel et al. "Hydroxylation of phenol over TS-1 : surface and solvent effects," Journal of Molecular Catalysis, (1991), vol. 68, pp. 45-52 ("Tuel"), the contents of each of which are incorporated herein by reference.

[0037] Mixture 16 may be prepared in any suitable manner as can be understood by one skilled in the art.

[0038] In operation, cathode 12 and anode 14 may be biased so that an electric potential difference (voltage) is established between them to induce an electric current in mixture 16. The applied voltage may vary depend on the application. For example, in an exemplary embodiment for converting phenol to hydroquinone with a TS-1 molecular sieve, the potential difference between cathode 12 and anode 14 may be about 1.1 V, and the electric current may have a current density from about 350 to about 700 mA/cm², such as about 400 to 450 mA/cm². As can be appreciated, reference electrode 22 may be used to provide a stable and reproducible potential reference.

[0039] As can be understood, when the cathode 12 and anode 14 are electrically biased and an electric current is induced in electrolytic cell 10, hydrogen peroxide can be electrolytically produced or generated at cathode 12 through an electrochemical reaction. For example, when an aqueous liquid such as water is used as the electrolyte and mixture 16 contains sufficient oxygen gas (O₂), H₂O₂ may be produced at cathode 12 in the electrochemical reaction:

O₂ + 2H⁺ + 2e- ^ H₂O₂. (1)

In the meantime, water may be decomposed at anode 14 by the reaction:

H₂O ► Vz O₂ + 2H⁺ + 2e . (2)

[0040] The overall electrochemical reaction is thus:

H₂O + V₂ O₂ ► H₂O₂. (3)

[0041] Since oxygen molecules are consumed at cathode 12, to maintain continued production of H₂O₂, oxygen molecules may need to be supplied into mixture 16, continuously or at regular intervals. For example, air may be fed into mixture 16 through pipe 20 to supply oxygen molecules to the region adjacent cathode 12. The air may be fed at a constant rate. It may be advantageous if air is fed at a sufficient rate so that sufficient supply of oxygen near cathode 12 is maintained, as can be understood by a person skilled in the art. In some embodiments, air may be supplied at a sufficient rate so that the mixture near cathode 12 is at or near oxygen saturation. As can be appreciated, if oxygen is depleted near cathode 12, hydrogen gas may be formed electrolytically. However, hydrogen formation may be suppressed if the mixture adjacent cathode 12 is saturated or nearly saturated with air (oxygen) because the formation of H₂O₂ is thermodynamically favored over the formation of hydrogen gas. Thus, with sufficient supply of oxygen at cathode 12 and a high enough electric current through cathode 12, H₂O₂ can be continuously generated at a desired rate at cathode 12. The rate of H₂O₂ generation may be controlled, as will be further discussed below.

[0042] The formed H₂O₂ can then oxidize the phenol in mixture 16 to form benzenediols such as hydroquinone and catechol in a hydroxylation reaction.

[0043] For optimal reaction conditions, the mixture may be at a temperature of from about 50 to about 80⁰C. In some applications, a temperature of about 65 to about 70 ⁰C may be advantageous. The mixture may be heated to the desired temperature, in any suitable manner as can be understood by one skilled in the art. In some applications, heat generated by in-cell reactions may be sufficient. Typically, however, external heating may be needed.

[0044] Hydrogen peroxide may be generated for any suitable period of time. For example, the length of the reaction period may be chosen to optimize the yield of the desired oxidized phenolic compound, or to achieve a desired balance between para-selectivity and the phenol conversion rate. For instance, the electric current may be induced for a period of time sufficient to achieve a desired rate of conversion, such as above 60%. The period of time in some particular applications may be about six hours. The reaction time may need to be limited in some applications to achieve high para-selectivity, or for other considerations such as production cost. The reaction time may need to be selected to balance different considerations. For example, in some particular applications, if the reaction time is too short, such as less than about 4 hours, the conversion rate and the para-selectivity may be low; and if the reaction time is too long, such as more than about 7 hours, while conversion rate may increase, the para-selectivity may decrease. Thus, in some applications, a reaction time of about six hours may be advantageous. As can be understood, in some applications such as during large scale batch production, the reaction mixture may need to be cooled quickly at the end of the reaction period to optimize the para-selectivity. [0045] As can be understood, the presence of the catalytic molecular sieve 18 can catalyze selective formation of benzenediol. In particular, it is expected that due to the pore shape and size of the molecular sieve such as a TS-1 molecular sieve, oxidation of phenol in the pores/channels of the sieve mainly forms hydroquinone and p-benzoquinone. Thus, high para-selectivity can be achieved. For example, in exemplary processes of the present invention, para-selectivity from 90% to 95%% have been achieved with phenol conversion rates higher than about 60%.

[0046] The increased para-selectivity and conversion rate can be attributed to the in situ generation of H₂O₂. Generation of H₂O₂ in situ is advantageous for at least the following reasons.

[0047] Without being limited to a particular theory, it is believed that when H₂O₂ is generated in situ, the pores and channels in the molecular sieve are less likely to be blocked by bulky reaction by-products so that formation of the oxidized phenolic compound such as hydroquinone at internal active sites can proceed for a long period of time.

[0048] It has been hypothesized that hydroxylation of phenol with H₂O₂ over a TS-1 sieve proceeds in two stages (see Tuel, supra). In the initial stage, phenol is mostly and immediately oxidized on the active sites on the external surface of the sieve. In this stage, mainly catechol is formed, because it is the most thermodynamically favored isomer. After a short period such as a few minutes, perhaps due to rapid poisoning of the active sites on the external surface, formation of benzenediols on the external surface practically stops. In the subsequent stage, oxidation of phenol mainly occurs inside the pores and channels of the sieve. In conventional processes, to increase the conversion rate of phenol, the relative concentration of H₂O₂ needs to be increased. In such a case, since the initial concentration of H₂O₂ is high, the oxidation reaction can proceed quickly. As the oxidation of phenol is exothermic, the temperature of the reaction mixture rises. At sufficiently high temperatures, hydroquinone and catechol can further react with H₂O₂, forming heavy by-products, referred to as tar. Other by-products may also form. The bulky by-products formed can block access to the internal active sites. Further, some hydroquinone formed will be consumed. As a result, the para-selectivity is limited and low. It has been shown that the higher the concentration of H₂O₂ in the mixture, the more tar is formed. However, if the H₂O₂ concentration is lowered, the conversion rate of phenol will decrease. In some conventional processes, to ensure effective utilization of the oxidant (H₂O₂) the phenol to oxidant ratio is kept high, typically about 5:1. As a result, the conversion rate of phenol is low.

[0049] By contrast, in an exemplary process of the present invention, the oxidant H₂O₂ can be continuously generated in situ and the rate of H₂O₂ generation can be controlled so that H₂O₂ concentration in the reaction mixture at any give time is low. Since a lower H₂O₂ concentration means slower reaction rates for both the heat-generating oxidation of phenol and the reaction between formed hydroquinone and H₂O₂, formation of tar is slower. Consequently, continued access to the internal active sites can be maintained and less hydroquinone formed is consumed, resulting in high para-selectivity even when the ultimate phenol conversion rate is high. As will be further discussed below, under proper reaction conditions, the para-selectivity in exemplary processes of the present invention may be higher than 90% when phenol conversion rate is at least about 65%.

[0050] Further, in situ generation of hydrogen peroxide may have at least two other benefitial consequences. First, it is known that TS-1 catalyzes effectively under mild conditions using dilute hydrogen peroxide as oxidant. When generated in situ, the oxidant concentration in the reaction mixture is effectively dilute, resulting in a high ratio of TS-1/H₂O₂ favorable for oxidation. Secondly, superoxide anions can be generated inside the air trapped sieve channels under electrochemical conditions. It is possible that active titanium- peroxo species, which are expected to be responsible for oxidation of phenol, can be formed directly from superoxide anions. [0051] In addition, lower hydrogen peroxide concentration leads to reduced decomposition of hydrogen peroxide.

[0052] Any or all of the above factors may contribute to the increased para- selectivity in the electrocatalytic process.

[0053] As now can be appreciated, in some applications, the rate of hydrogen peroxide generation may impact on the para-selectivity and the yield of the desired end product. As can be understood, the rate of hydrogen peroxide generation may be controlled by altering a number of production conditions, such as temperature, electrolyte concentration, and the biasing voltage. To obtain the optimal para-selectivity or product yield, the rate of hdyrogen peroxide generation may be adjusted, such as by altering one or more of these conditions. The optimal rate for a particular application may be readily determined, such as through test runs, as can be understood by persons skilled in the art. In some applications, it may be advantageous if the rate of hdyrogen peroxide generation is from about 7 mmol/hour to about 13 mmol/hour.

[0054] After a desired reaction time, such as about six to seven hours, the desired reaction product, such as hydroquinone, may be extracted or withdrawn from cell 10, such as through pipe 24. Extraction and withdrawal of the products can be carried out in any suitable manner, as can be understood by one skilled in the art. For example, a desired product in a liquid mixture may be extracted by evaporate the solvent. Solid products may be physically separated from the reaction mixture and dissolved in a suitable solventm, and the desired product is then extracted by evaporate the solvent.

[0055] In different embodiments, the production process may be in batches or continous, as can be understood by persons skilled in the art. The production process can also be automated.

[0056] As now can be appreciated, the exemplary process of selective oxidation of phenol to form hydroquinone and catechol can be modified. For example, the catalytic molecular sieve can have a support on which TS-1 or its substitutes can be formed. The support can include a film or a membrane coated over a substrate. The substrate can be made of any suitable material, such as polymers, stainless steel, Teflon, glass, porous silica, alumina, titania, vanadia, ceria and the like.

[0057] In different embodiments, the electrolytic medium in the electrolytic cell may be separated from the mixture containing phenol, but the hydrogen peroxide is still generated adjacent to the mixture so that the generated hydrogen peroxide can oxidize the phenol in the mixture. It is not necessary that the reaction mixture directly contacts the electrodes such as the cathode. For example, the reaction mixture and the cathode may be separated by a suitable membrane. Further, the molecular sieves may be replaced with a different catalyst that can selectively catalyze the oxidation of phenol. It is also possible to deposit the catalyst such as a molecular sieve on the cathode, or otherwise incorporate the catalyst in the cathode. The catalyst may be incorporated into a membrane disposed at or adjacent to the cathode. The catalyst may also be in the forms of pellets, or coated on a mesh.

[0058] The in situ electrolytic generation of hydrogen peroxide technique may also be used to convert other compounds to polyhydroxybenzenes. For example, in the above described processes, the phenol in the mixture may be replaced with a different aromatic compound for producing a desired polyhydroxybenzene. The aromatic compound can be a phenol with more than one hydroxyl groups or a substituted phenol. For example, the hydrogen atom of a hydroxyl group in a phenol may be substituted with another group. Suitable modifications to other aspects of the process may be needed depending on the particular phenolic compound used and the desired product. For example, the molecular sieve may be replaced with another suitable catalyst for selective conversion of the phenolic compound to the desired product. Such modifications can be understood by persons skilled in the art. Similar to the exemplary processes described earlier, hydrogen peroxide may be electrolytically generated in a mixture including the aromatic compound and the catalyst, to selectively convert the aromatic compound to the desired polyhydroxybenzene.

[0059] As can be understood, hydrogen peroxide may also be generated in situ using a non-electrolytic technique. For example, hydrogen peroxide may be generated by reacting suitable precursor chemicals, such as in enzymatic generation of hydrogen peroxide and catalytic reaction of molecular hydrogen with oxygen.

[0060] Hydrogen peroxide generated in situ may also be advantageously used in oxidation of different aromatic compounds to form a phenolic compound, such as in oxidation of benzene to phenol, toluene to cresol, or the like. The technique can also be used in ammoxidation of cyclohexanone to an oxime, epoxidation of olefinics, or the like.

[0061] The invention is further illustrated by the following non-limiting examples.

EXAMPLE I

[0062] Phenol, C₆H₅(OH), was oxidized according to the exemplary process described above and illustrated in FIG. 1.

[0063] Specifically, the cathode and anode of the electrolyte cell were made of platinum electrodes and the reference electrode was made of Ag/AgCI.

[0064] The electrolytic mixture was prepared by mixing about 0.18 mmol of sulfuric acid, about 10.6 mmol of phenol and about 20 ml of water.

[0065] The molecular sieve used was TS-1 with a Ti concentration of about 0.45 wt%.

[0066] After TS-1 and the mixture were placed in the electrolytic cell. The mixture and the TS-I sieve were sonicated for about 2 to 5 minutes in the cell to uniformly disperse the reactants and the catalyst. The pH of the mixture was adjusted to about 1.2 to 1.8. The mixture was purged with air with an external air pump directing the flow of air towards the surface of the cathode.

[0067] The cathode and anode were biased to a potential difference of about 1.1 V, with a cyclic voltmeter. The electric current through the cathode had a current density of about 400 to 450 mA/cm².

[0068] The mixture was heated, with external heating, to a temperature of about 65 to about 70 ⁰C.

[0069] The reaction products were withdrawn from the cell at regular intervals and were analyzed by a Gas Chromatography (GC) technique with a HP-5 capillary column and a Flame Ionization Detector (FID).

[0070] The oxidation products were identified against standard samples by GC-mass spectrometry.

[0071] The measured phenol conversion rate and para-selectivity are shown in FIG. 2. The squares indicate measured phenol conversion rates and the circles indicate measured para-selectivity. The dotted lines are respectively fits of the measured data to a smooth curve. As can be seen, the para-selectivity remained relatively high within about 7 hours of reaction time. The conversion rate increased to about 75% after about seven hours. At about 6 hours of reaction time, the para-selectivity is still above 90% while the conversion rate has increased to above 60%.

EXAMPLE Il

[0072] The oxidation reaction was effected as described in Example I, except that the content of Ti in the molecular sieve was varied, with values of 0.06, 0.3, 0.45, and 1.5 wt %, and the mixture was heated to a temperature of about 65 ⁰C.

[0073] The phenol conversion rate and para-selectivity measured were dependent on the Ti content, as shown in Table I and FIG. 3. As can be seen, the optimal content of Ti is about 0.45 wt%.

EXAMPLE III

[0074] The oxidation reactions were carried out as in Example I, except that the electrodes biasing voltage was varied from 1 V to 5 V.

[0075] The effects of biasing voltage are illustrated in FIGS. 4 and 5, where the circles indicate measured phenol conversion rate, the squares indicate measured yield of hydroquinone, and the triangles indicate measured para-selectivity. As can be seen, while the conversion rates and the para- selectivity are similar at both biasing voltages, the hydroquinone yield is much higher at biasing voltage of 1.1 V than at 5 V.

[0076] Without being limited to a particular theory, this effect may be attributed to anodic oxidation of phenol which is enhanced by higher biasing voltage. Further, when the biasing voltage was 5V, the corresponding current density was about 300 to 450 mA/cm² and the heat generated by the reaction alone is sufficient to heat the reaction mixture to a temperature of about 65 to 70 ⁰C, without any external heating. Such locally generated heat may promote reactions of formed hydroquinone with hydrogen peroxide, thus reducing the ultimate yield.

[0077] External heating was required to keep the mixture at a temperature of about 65 to about 70 ⁰C when the biasing potential was from 1 to 1.2V.

EXAMPLE IV

[0078] The oxidation reactions were carried out as in Example I, except that the pH of the reaction mixture was varied. After about six hours of reaction time, the phenol conversion rate for a neutral mixture was about 1 to 2%, and about 30% for a basic mixture. For the basic mixture, the para- selectivity was about 70% and the main reaction product was maleic anhydride. [0079] Thus, it may be advantageous to use an acidic mixture for obtain optimal results.

[0080] As discussed, the exemplary embodiments of the present invention can be advantageous in comparison with conventional processes of selective oxidation of phenolic compounds. In the exemplary processes described herein, the para-selectivity can be significantly increased with high phenolic compound conversion rate, such as higher than 60%. Further, the production cost may be decreased due to increased conversion rate and the elimination of hazards and costs associated with handling, storage and transportation of hydrogen peroxide produced remotely.

[0081] Other features, benefits and advantages of the embodiments described herein not expressly mentioned above can be understood from this description and the drawings by those skilled in the art.

[0082] The contents of each reference cited above are hereby incorporated herein by reference.

[0083] Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.

Table I.

* - In this case, the hydroquinone yield was low because most of the reaction products were undesirable products such as maleic anhydride, succinic acid and polyphenols impurities, although the phenol conversion rate and para- selectivity were high.

Claims

WHAT IS CLAIMED IS:

1. A method for oxidizing a phenolic compound, comprising:

generating hydrogen peroxide in the presence of said phenolic compound and a catalyst for catalyzing oxidation of said phenolic compound to form a para-selectively oxidized phenolic compound.

2. The method of claim 1 , wherein said oxidized phenolic compound comprises a hydroxylated phenolic compound.

3. The method of claim 1 or 2, wherein said oxidized phenolic compound comprises a polyhydroxybenzene.

4. The method of any one of claims 1 to 3, wherein said phenolic compound is phenol.

5. The method of any one of claims 1 to 3, wherein said phenolic compound is selected from phenols and substituted phenols.

6. The method of any one of claims 1 to 3, wherein said phenolic compound is selected from hydroquinone, catechol, resorcinol, and cresols.

7. The method of any one of claims 1 to 5, wherein said oxidized phenolic compound comprises hydroquinone.

8. The method of any one of claims 1 to 7, wherein said generating comprises generating said hydrogen peroxide electrolytically.

9. The method of any one of claims 1 to 8, wherein said phenolic compound and said catalyst are mixed, forming a mixture.

10. The method of claim 9, wherein said generating comprises: providing an electrolytic cell comprising said mixture and an electrolytic medium for producing said hydrogen peroxide when an electric current is induced in said electrolytic cell; and inducing said electric current in said electrolytic cell to produce said hydrogen peroxide.

11. The method of claim 10, further comprising supplying oxygen molecules into said electrolytic cell so as to maintain production of said hydrogen peroxide.

12. The method of claim 11 , wherein said supplying oxygen molecules comprises feeding air into said electrolytic cell.

13. The method of any one of claims 10 to 12, wherein said electrolytic medium comprises a fluid.

14. The method of claim 13, wherein said electrolytic medium comprises water.

15. The method of claim 14, wherein said electrolytic medium comprises a sulfuric acid.

16. The method of any one of claims 13 to 15, wherein said fluid is acidic.

17. The method of any one of claims 13 to 16, wherein said fluid has a pH value of about 1.2 to 1.8.

18. The method of any one of claims 10 to 17, wherein said mixture comprises said electrolytic medium, and the weight ratio of said phenolic compound and said electrolytic medium in said mixture is from about 1 :10 to about 1 :25.

19. The method of any one of claims 10 to 18, wherein said weight ratio of said phenolic compound and said electrolytic medium in said mixture is about 1 :20.

20. The method of any one of claims 10 to 19, wherein said electric current has a current density from about 400 to about 450 mA/cm².

21. The method of any one of claims 9 to 20, wherein said mixture is at a temperature from about 65 ⁰C to about 70 ⁰C.

22. The method of any one of claims 9 to 21 , further comprising heating said mixture.

23. The method of any one of claims 1 to 22, wherein said hydrogen peroxide is generated for a period of time sufficient to oxidize at least 60% of said phenolic compound.

24. The method of any one of claims 1 to 23, wherein said hydrogen peroxide is generated for about six hours.

25. The method of any one of claims 1 to 24, wherein said hydrogen peroxide is generated at a rate from about 7 mmol/hour to about 13 mmol/hour.

26. The method of any one of claims 1 to 25, further comprising extracting said oxidized phenolic compound.

27. The method of any one of claims 1 to 26, wherein said catalyst comprises a catalytic molecular sieve.

28. The method of claim 27, wherein said molecular sieve comprises titanium silicalite-1 (TS-1).

29. The method of claim 28, wherein said TS-1 comprises from about 0.3 to about 0.6 wt% of titanium.

30. The method of claim 28, wherein said TS-1 comprises about 0.45 wt% of titanium.

31. The method of claim 27, wherein said molecular sieve comprises a metal silicate.

32. The method of claim 31 , wherein said metal silicate comprises a transition metal.

33. The method of claim 32, wherein said transition metal is selected from Ti, V, Cr, Fe, Co, Cu, Zn, Zr, Mo, Nb, and Ta.

34. The method of any one of claims 31 to 33, wherein said metal silicate comprises a rare earth metal.

35. The method of claim 34, wherein 'said rare earth metal is selected from La, Ce, and Sm.