This invention relates to an improved process for the manufacture of hydroquinone and catechol. More particularly this invention relates to a process for the oxidation of phenol to a mixture of hydroquincne and catechol using hydrogen peroxide as the oxidant in the presence of a solid catalyst containing an organotransition metal complex.
Many processes are known in the prior art for the conversion of phenol to hydroquinone and catechol using hydrogen peroxide, H,02 as the oxidant. In U.S. Patent 3 , 929, 913 , assigned to Brichima, the catalyst used is ferrocene. Ferrous sulfate chelates are used as catalysts in the U.S. patent 3,920,756 assigned to Ube. In the Rhone-Poulenc process described by J. Varagnat in the journal of Industrial Engg. Chemistry, Product Research Development, Vol. 15, page 212 (1976), a combination of phosphoric and perchloric acids are used as catalysts. The use of molecular sieves as catalysts for the oxidation of phenol to hydroquinone and catechol using H202 as the oxidant is also known. European Patent 0266825 describes the use of crystalline gallium titanium silicates as catalysts. European Patent 0265C13 describes the use, as catalysts, of zeolites with a pore diameter between 5 and 12A. U.S. Patent 4,396,783 and U.K. Patent 2116974 both assigned to Enichem claim the use of a titanium silicate molecular sieve, TS-1, in the hydroxylation of aromatics.
In the prior art processes for the manufacture of hydroquincne and catechol, in addition to the above desired products, significant amounts of heavy oxidation products, hereinafter referred to as tar, are also produced in the process. In example 5 of U.S.
Patent 4,396,783, for instance, 21 % by weight of phenol was converted into byproduct tar. The tar originates from the further reaction of hydroquinone and catechol at the elevated temperatures during the exothermic oxidation of phenol. Any modification of the process which reduces the formation of tar will constitute a significant improvement of the process.
It is, therefore, an object of this invention to provide an improved process for the oxidation of phenol to hydroquinone and catechol wherein the production of the undesired byproduct tar is suppressed leading thereby to enhanced yields of hydroquinone and catechol.
Accordingly, the invention provides an improved process for the manufacture of hydroquinone and catechol which comprises reacting a mixture of phenol and aqueous hydrogen peroxide, in the presence of a solid catalyst consisting of an organotransition metal complex wherein some or all of the hydrogen atoms of the said organotransition metal complex have been substituted by one or more electron withdrawing groups, at a temperature below 90°C, in the presence or absence of solvents and isolating the hydroquinone and catechol from the effluents, from the reaction zone by conventional methods.
In an embodiment of the present invention the organotransition metal complex is selected from pthalocyanines and porphyrins.
In another embodiment of the present invention, the transition metal
Patent 4,396,783, for instance, 21 % by weight of phenol was converted into byproduct tar. The tar originates from the further reaction of hydroquinone and catechol at the elevated temperatures during the exothermic oxidation of phenol. Any modification of the process which reduces the formation of tar will constitute a significant improvement of the process.
It is, therefore, an object of this invention to provide an improved process for the
oxidation of phenol to hydroquinone and catechol wherein the production of the
undesired byproduct tar is suppressed leading thereby to enhanced yields of hydroquinone and catechol.
Accordingly, the invention provides an improved process for the manufacture of hydroquinone and catechol which comprises;
a) reacting a mixture of phenol and aqueous hydrogen peroxide in the molar ratio between 1 to 7 in the presence of a solid catalyst containing an organo transition metal complex such as herein described encapsulated in a solid matrix such as herein described wherein some or all of the hydrogen atoms of the said organotransition metal complex have been substituted by one or more electron withdrawing groups such as herein described,
b) isolating and separating the hydroquinone and catechol from the effluents from the reaction zone by conventional methods as herein described.
In an embodiment of the present invention the organotransition metal complex is selected from pthalocyanines and porphyrins.
In another embodiment of the present invention, the transition, the transition metal
particularly in directing certain oxidative processes. Many known pthalocyanines have been judged to suffer certain drawbacks by being deficient in the combination of properties desired for many candidate uses, such as in the oxidation of phenols and more particularly in the oxidation of phenols to hydroquinone and catechol. One major drawback of homogeneous pthalocyanine catalysts in industrial oxidation processes is the formation of aggregates in solution which significantly deactivates these catalysts.
Due to our continued research in this area we observed that the organotransition metal complexes used as catalysts are solids insoluble in phenol or the reaction products arising from oxidation of phenol. Hence they do not undergo aggregation or change of phase during the oxidation wherein such changes are known to lead to catalyst deactivation problems.
Another drawback of pthalocyanines used in the prior art as catalysts for phenol oxidation is their low oxidative stability which is due to the easy oxidisability of the hydrogen atoms attached to the nucleus of the pthalocyanines.
We have found that the oxidative stability as well as the catalytic activity of the metal pthalocyanines used as catalysts in the oxidation of phenol are enhanced by replacing the hydrogens from the pthalocyanines by electron withdrawing groups like the halogens, nitro or cyano groups thereby rendering the metal ions easier to reduce leading to an improved oxidation activity and stability of the cata-
lysts during the reaction.
There are a total of 16 hydrogen atom positions on such pthalocyanine molecules which can in principle, be substituted by other substituents. We have observed that when some or all of the hydrogen atoms of the said pthalocyanines are substituted by one or more electron withdrawing groups such as halogen, nitro or cyano groups or mixtures of such groups there is substantial improvement in selectivity and yield of hydroquinone and catechol.
In yet another advantageous embodiment of the present invention, the organotransition metal complex may be encapsulated in a solid matrix. Due to the greater dispersion of the organotransition metal complex catalyst in solid matrices and the consequent enhanced stability of the structural integrity of the catalyst significant process advantages like greater activity, stability and easy recovery and recycla-bility of the catalyst are observed. Examples of such solid matrices include inorganic oxide like silica, alumina, molecular sieves, zeolites and the like as well as organic polymeric material.
It is an advantageous feature of the process of the present invention that due to the high activity the catalysts used herein, the oxidation reaction can be carried out at temeratures much below those used in the prior art and preferably below 90°C, thereby leading to much lower yields of undesired side products like tars and benzoquinones.
It was noted during the course of developing the process of the present invention that the concentration of hydrogen peroxide in the reaction mixture was a major parameter influencing the exothermicity of the reaction process; higher the H202 concentration, more exothermic was the reaction leading to higher concentrations of tar. On the other hand, too low concentrations of H202 in the reaction mixture lead to very low concentrations of the dihydroxy benzenes in the product necessitating an inordinate expenditure of energy in the recovery and recycle of the unreacted phenol.
Hence, in one embodiment of the process of the present invention, the molar ratio of phenol to hydrogen peroxide in the reactant mixture is preferably between 1 and 5.
Another factor that influences the formation of tar during the reaction process was found to be the concentration of the oxidation active sites of the oxidation catalyst especially in the downstream portion of the catalyst bed wherein significant quantities of the products dihydroxybenzenes are present. These dihydroxybenzenes are the precursors for the formation of the undesired tar. It may be speculated that these dihydroxy benzenes are oxidised over the oxida-• tion active sites to quinones which undergo further conversion to tar. Hence, while the oxidation active sites are essential in those regions of the catalyst bed wherein the oxidation of phenol is the desired reaction, it is desirable to lower their concentration in those regions of the catalyst bed wherein significant concentrations of the
dihydroxybenzenes are present. Accordingly, it is advantageous to have a high concentration of the oxidation active sites in the initial or front end portion of the catalyst bed and a correspondingly lower concentration of the oxidation active sites in the catalyst at the lower end of the catalyst bed. More appropriately, a progressively decreasing concentration of the oxidation active sites in the catalyst bed may be used.
While the process of the present invention may be practiced using phenol and aqueous H202 in the absence of any solvent, it may, under certain conditions, be preferable to dissolve both phenol and H202 in a solvent and carry out the oxidation reaction. Solvents which may be used advantageously include H20, tertiary butyl alcohol, acetone, methanol, acetonitrile, and the like. When such solvents are used the concentration (wt.%) of phenol and H202 in the reaction mixture may vary from 5 to 95 % and 5 to 50 %, respectively.
It has been found that when the process of conversion of phenol to a mixture of hydroquinone and catechol is carried out in accordance with the above mentioned features and embodiments of this invention, there is a significant reduction in the amount of by-product tar formation.
The details of the present invention is described in the examples given below which are provided by the way of illustration only and therefore should not be construed to limit the scope of the invention.
Example 1
10 g of phenol and 10 g of acetonitrile were taken in a three-necked 150 ml flask fitted with a condenser and a thermometer. 1 g of tetra deca copper chlorophthalocyanine powder was added to it. The mixture was heated to 7 0°C in an oil bath and 2.7 8 g of an aqueous solution of H202 (26 wt.%) was added continuously with stirring. The reaction was continued for 6 hours during which the temperature of the reaction mixture was found to increase upto 86°C and decrease subsequently to the bath temperature. The reaction mixture was analysed with a gas chromatograph (Hewlett Packard 5 880 A, employing a capillary column, 50x0.25 mm, crosslinked methyl silicone gum) . The tar content in the product was estimated by thermogravimetric analysis carried out in an inert gas atmosphere as the weight % of material remaining after the loss of the dihydroxy benzenes. The products were separated by fractional distillation for analysis.
The product analysis at the end of 6 hours was 3.0 % hydroqui-none, 9.2 % catechol, 0.4 % benzoquinone and 0.6 % tars.
Example 2
10 g of phenol and 90 g of water were taken in a three-necked 150 ml flask fitted with a condenser and a thermometer. 1 g of solid tetra nitro copper phthalocyanine was added to it. The mixture was heated to 70°C in an oil bath and 2.78 g of an aqueous solution of H2O2, (26 wt.%) was added continuously with stirring. The
reaction was continued for 6 hours. The temperature of the reaction mixture was not allowed to increase excessively. The reaction mixture was cooled by immersing a cold finger. By the use of this cooling arrangement, the maximum temperature reached was reduced to 74°C. The reaction mixture was analysed with a gas chromatograph (Hewlett Packard 5880 A, employing a capillary column, 50 x 0.25 mm, crosslinked methyl silicone gum) . The tar content in the product was estimated by thermogravimetric analysis carried out in an inert gas atmosphere as the weight % of material remaining after the loss of the dihydroxy benzenes. The products were separated by fractional distillation for analysis.
The product yield was : 3.1 % hydroquinone; 10.4 % catechol; 0.1 % benzoquinone and 0.7 % tars.
Example 3
The oxidation of phenol was next carried out in a fixed bed reactor as follows : 50 g of the tetra cyano iron phthalocyanine catalyst was compacted into pellets (2 mm X 4 mm) and loaded into a 20 mm dia. glass reactor. The reactants, viz., a solution of 50 % phenol in acetonitrile and a solution of 26 % H202 in water were passed through a preheater kept at 60°C at the rate of 100 g and 28 g per hour and then through the catalyst bed; no further heat was supplied to the reactor which was well insulated. The temperature of the different axial zones of the catalyst bed was
measured with the help of a moving thermocouple kept inside a thermowell. At steady state conditions, it was noticed that the temperature of the bed increased to 87°C at a point approximately in the middle of the catalyst bed.
The product was collected for 1 hour and the reaction mixture was analysed with a gas chromatograph (Hewlett Packard 5880 A, employing a capillary column, 50 x 0.25 mm, crosslinked methyl silicone gum). The tar content in the product was estimated by thermogravimetric analysis carried out in an inert gas atmosphere as the weight % of material remaining after the loss of the dihydroxy benzenes. The products were separated by fractional distillation for analysis.
The product mixture contained 5.0 % hydroquinone, 6.9% catechol, no benzoquinone and 0.9 % tars.
Example 4
Hexachloro manganese porphyrin was loaded in a reactor in a 3-bed arrangement such that each bed contained 2 0 gram of the catalyst and the beds were separated by zones of inert material. The zones were cooled by cooling coils wrapped around them. 120 gm of mixture of phenol and methanol in the weight ratio of 90:10 and 33.6 gms. of an aqueous solution of hydrogen peroxide (26 wt%) were heated to 60°C in a preheater and passed through the cata-
lyst beds every hour. The results are given below :
Bed 1 Bed 2 Bed 1
Inlet temp. 60°C 63°C 61°C Outlet temp. 788C 74°C 65°C
The combined product was analysed after one hour of operation. The reaction mixture was analysed with a gas chromatograph (Hewlett Packard 5880 A, employing a capillary column, 50 x 0.25 mm, crosslinked methyl silicone gum). The tar content in the product was estimated by thermogravimetric analysis carried out in an inert gas atmosphere as the weight % of material remaining after the loss of the dihydroxy benzenes. The products were separated by fractional distillation for analysis.
The product mixture contained 3.5 % hydroquinone; 10.6 % catechol and 1.6 % tars.
Example 5
Tetra deca chloro cobalt phthalocyanine was loaded in a reactor in a 3-bed arrangement such that each bed contained 20 gram of the catalyst and the beds were separated by zones of inert material. The zones were cooled by cooling coils wrapped around them. The temperature at the inlet of each bed was maintained at 62+1
°C with the help of the cooling arrangement. 20 gm of mixture of phenol and water in the weight ratio of 90:10 was heated in a preheater to 60°C and passed through the catalyst beds every hour. 11.2 gms. of an aqueous solution of hydrogen peroxide (26 wt%) was injected individually at the top of each bed ( per hour ) without preheating. The phenol water mixture was injected in one lot at the inlet of the first bed.
The reaction mixture was analysed with a gas chromatograph (Hewlett Packard 5880 A, employing a capillary column, 50 x 0.25 mm, crosslinked methyl silicone gum). The tar content in the product was estimated by thermogravimetric analysis carried out in an inert gas atmosphere as the weight % of material remaining after the loss of the dihydroxy benzenes. The products were separated by fractional distillation for analysis.
The combined product of one hour had 6.2 % hydroquinone; 10.1 % catechol and 0.9 % tars.
Example-6
In an autoclave, 10 g of phenol and 10 g of acetonitrile were taken. 1 g of tetra deca chloro copper pthalocyanine encapsulated in the aluminosilicate zeolite Y was added. The mixture was heated to 70 °C and 2.78 g of an aqueous solution of hydrogen peroxide (26 % wt) was added continuously with stirring for 6 hrs. After 6 hrs the autoclave was cooled. The products were
analysed with a gas chromatograph. The tar content in the product was estimated by thermogravimetric analysis carried out in an inert gas atmosphere as the wt% of material remaining after the loss of dihydroxy benzenes.
The reaction mixture was analysed with a gas chromatograph (Hewlett Packard 5880 A, employing a capillary column, 50 x 0.25 mm, crosslinked methyl silicone gum). The tar content in the product was estimated by thermogravimetric analysis carried out in an inert gas atmosphere as the weight % of material remaining after the loss of the dihydroxy benzenes. The products were separated by fractional distillation for analysis.
The product analysis at the end of 6 hrs was 4.2 wt% hydroqui-none, 8.5 wt% of catechol, 0.6 wt% of benzoquinones and 0.8 wt% of tars.

We Claim:
1. An improved process for the manufacture of hydroquinone and catechol which
comprises.
a) reacting a mixture of phenol and aqueous hydrogen peroxide in the molar ratio
between 1 to 7 in the presence of a solid catalyst containing an organo transition
metal complex such as herein described encapsulated in a solid matrix such as
herein described wherein some or all of the hydrogen atoms of the said
organotransition metal complex have been substituted by one or more electron
withdrawing groups such as herein described, at a temperature below 90°C
optionally in presence of solvents such as herein described,
b) isolating and separating the hydroquinone and catechol from the effluents from
the reaction zone by conventional methods as herein described.
2. An improved process as claimed in claim 1 wherein the organotransition metal
complex is a phthalocyanine or porphyrin.
3. An improved process as claimed in claim 1 wherein the preferable molar ratio of
phenol and aqueous hydrogen peroxide is 1 to 5.
4. An improved process as claimed in claims 1-3 wherein the transition metal is
selected from iron, cobalt, copper, chromium, manganese or mixtures thereof.
5. An improved process as claimed in claim 1 wherein the said electron withdrawing
group is selected from the halogens, the nitro group, the cyano group or mixtures
thereof.
6. An improved process as claimed in claims 1 to 5 wherein the solvent is selected
from tertiary butyl alcohol, acetonitrile, water, methanol, acetone and the like and
mixtures thereof.
7. An improved process as claimed in claims 1-6 wherein the solid matrix used is an
inorganic oxide such as silica, aluminia, aluminosilicates, molecular sieves or an
organic polymer.
8. An improved process for the manufacture of hydroquinone and catechol
substantially as hereinabove described with reference to the Examples.