GB2140034A

GB2140034A - Electrolytic oxidation of manganous ion

Info

Publication number: GB2140034A
Application number: GB08408702A
Authority: GB
Inventors: James Peter Millington; Iam Mccrady Dalrymple
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1983-05-18
Filing date: 1984-04-04
Publication date: 1984-11-21
Also published as: GB2140034B; GB8408702D0

Abstract

A method for the oxidation of manganous ion (Mn<2+>) to manganic ion (Mn<3+>) which method comprises electrolytically oxidising manganous sulphate in solution in sulphuric acid of a concentration no greater than 35% vol/vol H2SO4. The manganic ion solution produced by this method may be used directly to oxidise an oxidisable chemical species, such as an organic compound containing a methyl group attached to an aromatic nucleus. In this case the oxidation produces a corresponding aldehyde.

Description

SPECIFICATION Electrolytic oxidation of manganous ion The present invention relates to a method of oxidation of manganous ion to manganic ion and to the use of the manganic ion so produced for chemical oxidation.

It has long been known that it is possible to carry out oxidations of organic compounds such as xylenes to corresponding tolualdehydes by contacting the organic compound with manganic ion produced in an electrolytic cell. The electrolytic reaction employed is the oxidation of manganous sulphate to manganic sulphate. The manganic ion needs to be stabilised and previous work in the area has been based upon the belief that it is necessary to employ a concentration of sulphuric acid of about 45% vol/vol H2SO4 for this purpose. Such a concentration of sulphuric acid provides a thermodynamically stable solution of manganic ion.

Previous work using such concentrated acid solutions has employed both low concentrations of manganese sulphate which produce clear solutions and higher concentrations of manganese sulphate produce flowable pastes.

Previous workers have preferred higher concentrations of manganese sulphate of the kind which produce flowable pastes because they envisage difficulty in handling the relatively higher volume throughput that would be necessary to convey an adequate quantity of manganic ion to the reactor in which the oxidation of the organic species is to take place if solutions of manganese sulphate in sulphuric acid were employed. The maximum concentration of manganous sulphate dissolvable in sulphuric acid of a concentration of 45% vol/vol H2SO4 is about 4%.

The present inventors have now discovered that it is perfectly feasible to employ solutions of manganese sulphate in sulphuric acid for the chemical oxidation of organic species, and that the efficiency of the oxidation process generally can be very considerably increased by employing a much lower concentration of sulphuric acid than has previously been contemplated. It has been discovered that solutions of manganic sulphate in sulphuric acid are kinetically stable for wholly adequate periods of time even when the concentration of sulphuric acid is too low for the solution to be thermodynamically stable.

Accordingly, the present invention provides a method for the oxidation of manganous ion (Mn2+) to manganic ion (Mn3+) which method comprises electrolytically oxidising manganous sulphate in solution in sulphuric acid of a concentration no greater than 35% vol/vol H2SO4.

It is preferred that the concentration of sulphuric acid be about 30% vol/vol H2SO4. As the concentration of sulphuric acid is reduced the life-time of the manganic ion in the solution will decrease. However, it must be born in mind that the acid solution of manganic ion is to be employed in the oxidation of chemical species which may well be to some extent destroyed by contact with sulphuric acid. This loss of material in the chemical oxidation stage will normally be more severe at higher acid concentrations.Accordingly, it may in some instances be worthwhile to use a concentration of sulphuric acid which does not provide that level of kinetic stability for the manganic ion which would normally be preferred so as to reduce the extent of the attach of sulphuric acid upon the oxidisable species in the chemical oxidation stage, which will follow the oxidation of manganous ion to manganic ion by the method of the present invention.

The route by which manganic ion decays in the solution in sulphuric acid is by the reaction 2 Mn3+oMn2+ + Mn4+. Decay of manganic ion to manganous ion will therefore take place more quickly in more concentrated solutions. Accordingly, it is preferred that the concentration of the manganic ion sulphate in the solution be up to 0.15 molar.

The chemical oxidation step will generally take place satisfactorily at a temperature of 70"C or below.

Various factors effect the selection of the best working temperature. The higher the temperature the lower the half life of the manganic ion at any chosen sulphuric acid concentration. Typical half lives are set out in Table I below.

TABLE I Acid Conc. (%v/v) Temp. ("C) + Life (Mins) Mn3+ 35 60 150 35 70 44 35 80 12 30 60 70 30 70 14 25 40 270 25 60 34 Temperature also has an effect on current efficiency for the oxidation of manganese ion however which is shown in Table II.

TABLE II Current Density 1000 A/M2 Flow Rate 34.8 I/min 35% (vol/vol) H2S04 Temperature Current Efficiency 25 77.1% 35 77.7% 45 89.7% 60 75% Generally, the temperature preferred for use in both the chemical and the electro-chemical oxidation steps is from 35"C to 45"C.

The use of solutions of manganese sulphate in sulphuric acid in place of flowable pastes enables one to employ a filter press type electro-chemical cell. Such cells are capable of high current efficiencies and of a high throughput. The current efficiences quoted above are based on the use of such a cell. They are particularly suited to reactions such as the reaction of manganous ion to manganic ion which are transport dependant.

It is therefore preferred that a filter press type electro-chemical cell be employed for the electro-chemical oxidation of manganous to manganic ion in the present invention. A particularly preferred cell for this purpose is the type described in our European Patent Application publication No. 0064417.

Such a cell comprises an electrically insulated frame defining an opening and a pair of opposed electrodes occupying the opening and constituting an anode and a cathode, each electrode being sealingly engaged at its edges with the frame and separated and insulated from the other electrode of the pair by the frame. There may be a cell divider extending between the opposed electrodes also sealingly engaged in the frame. There may be a turbulance promoter, e.g., a sheet of expanded plastics mesh overlying at least the cathode electrode of the pair. A plenum inlet and a plenum outlet is provided so that the electrolyte can be made to flow swiftly over the face of the electrodes. Where the cell is divided, an inlet and outlet is provided for each of the two compartments produced by the division.

Generally, a substantial number of opposed electrodes are provided in a bi-polar stack in such a cell.

For use in this invention, the filter press cell employed is preferably one having a plurality of compartments each divided by an ion exchange membrane into anode and cathode compartments, the solution of manganese sulphate to be oxidised being circulated through the anode compartments and a solution of manganous sulphate and sulphuric acid being circulated through the cathode compartments. A suitable material for the ion exchange resin cell divider is a nafion ion exchange resin.

The preferred construction of suitable cells and the detailed manner of their operation are described in the European Patent Specification referred to above.

The present invention includes a method for the chemical oxidation of an oxidisable chemical species by manganic ion which comprises electro-chemically oxidising manganous ion to manganic ion by a method as described above and contacting the resulting manganic ion containing solution with the oxidixable species under oxidation conditions. The oxidisable species is prefereably an organic compound. It is known that manganic ion is useful in the oxidation of organic compounds containing a methyl group attached to an aromatic nucleus to the corresponding aldehyde. Examples of oxidation reactions which can be carried out by this method are the oxidation of ortho, meta and para xylenes to the corresponding tolualdehydes, of toluene to benzaldehyde and of a meta-chlorotoluene to a meta-chlorobenzaldehyde. Anethole may also be oxidised.

Preferably the organic compound to be oxidised and any solvent employed for it will be immiscible with the sulphuric acid solution of manganese ion since this will facilitate the separation of the organic reaction product from the spent solution of sulphuric acid and manganese sulphate. Naturally, it is not necessary that all of the manganic ion be used up in the chemical oxidation. Preferably, the system employed is one in which manganese sulphate acid solution is circulated between a reactor for the chemical oxidation and the electro-chemical cell in which the electro-chemical oxidation of the manganese ion takes place.After the manganese sulphate solution has reacted with the organic compound, the organic reaction products are separated from the manganese and sulphuric acid is returned to the electro-chemical cell for reoxidation and thereafter is used for further chemical oxidation. It may be necessary to subject the spent solution of manganese sulphate to one of a variety of treatments to remove contaminating materials therefrom after the chemical oxidation. The steps needed will depend upon the nature of the chemical species present in the chemical oxidation step. Generally, such techniques as vacuum stripping will be found adequate.

The lower concentration of sulphuric acid than has been used previously which is employed in the present invention produces a solution of greater conductivity than has previously been employed. This enables the use of lower cell voltages and the obtaining of higher current densities. It has been found that using the electro-chemical cell described in our European Patent Specification referred to above, higher current efficiencies in the electro-chemical oxidation step than have previously been reported can be obtained.

The current efficiency of the oxidation of manganese ion tends to fall with increasing current density above 1000 A/M2. Using a filter press cell, a sulphuric acid concentration of 30% (300 mls concentrated sulphuric acid/litre solution), a temperature of 30"C and a flow rate through the cell of 34 litre/min, we have achieved current efficiences of over 90%.

Most significantly however, it has been found that the lower concentration of sulphuric acid employed results in a very considerably higher overall yield of such products as tolualdehydes and benzaldehydes. Previous workers have reported an energy consumption of about 10 kilowatt hours per kilogramme for the production of para-tolualdehyde using a flowable sludge of manganese sulphate in sulphuric acid at 45% vol/vol H2SO4. It will be seen in the examples which follow that the present invention is capable of producing para-tolualdehyde at an energy consumption of less than 4 kilowatt hours per kilogramme.

Preferably, the current density employed in the present invention is between 500 and 2000 amps per square metre, for instance, 1000 amps per square metre. Using the cell described in our European Patent Specification referred to above such a current density can be obtained at a voltage of about 3.5 volts in the electro-chemical oxidation of the present invention. The use of a sulphuric acid concentration of the kind described in the prior art would restrict the current density at the voltage to less than half that figure.

The invention will be illustrated by the following Examples: EXAMPLE 1 Manganous sulphate was oxidised to manganic sulphate in sulphuric acid solution utilising a cell of the kind specifically described in European Patent Specification No. 0064417. The solution to be oxidised was circulated through the anode compartments of the cell and a solution of corresponding content was circulated through the cathode compartments of the cell.

The anode and cathode compartments of the cell were separated by a nafion ionic ion exchange membrane.

The concentration of manganous sulphate employed was 0.1 molar. Different concentrations of sulphuric acid were employed in successive runs and the results obtained are shown in the table below.

In each case, the anolyte from the cell was contacted with para-xylene to oxidise the paraxylene to para-tolualdehyde. The spent manganous sulphate solution was separated from the organic material and returned to the electro-chemical cell as anolyte. The catholyte of the cell was simply circulated through the cathode compartments.

The yield of para-tolualdehyde was measured. It was found that using 30% volume/volume H2SO4 an overall yield better that 4 kilowatt hours per kilogramme of para-tolualdehyde was obtainable. The overall current efficiency was about 80% and this consisted of about 90% current efficiency for the electro-chemical regenration step together with a yield for the organic reaction of about 90%. The yield in the organic reaction is very considerably higher than that previously reported.

TABLE H2SO4 Current efficiency kwhr/ Run Conc % in electro- Voltage mole vol/vol Chemical Oxidation Mn3+ 1 45% 51% 3.80 0.20 2 40% 65.5% 3.61 0.15 3 35% 78.8% 3.58 0.12 4 30% 90.1% 3.40 0.10 5 25% 75% 3.30 0.12 EXAMPLE 2 The procedure of Example 1 was followed to produce electrochemically a 0.1 molar solution of manganic sulphate in 35% v/v sulphuric acid at 0.12 kwhr/mole Mn3+.

This solution was used to oxidise p-tert. butyltoluene to p-tert. butyl benzaldehyde at a temperature of 70"C. The yield was 77% of p-tert. benzaldehyde based upon the manganic ion added. This corresponds to an overall energy consumption 4.2 kilowatt hours per kilogramme of aldehyde.

EXAMPLE 3 The procedure of Example 2 was followed to oxidise pchlorntoluene to p-chlorobenzaldehyde at a temperature of 60'C. The yield was 75% of p-chlorobenzaldehyde based upon the manganic ion added. This corresponds to an overall energy consumption of 4.5 kilowatt hours per kilogramme of aldehyde.

EXAMPLE 4 The procedure of Example 2 was followed to oxidise 2,4-dichlorotoluene to 2,4-dichlorobenzaldehyde at a temperature of 60 C. The yield was 76% of 2,4-dichlorobenzaldehyde based upon the manganic ion added. This corresponds to an overall energy consumption of 3.6 kilowatt hours per kilogramme of aldehyde.

Claims

1. A method for the oxidation of manganous ion (Mn2+) to manganic ion (Mn3+) which method comprises electrolytically oxidising manganous sulphate in solution in sulphuric acid of a concentration no greater than 35% vol/vol H2S04.

2. A method as claimed in claim 1 wherein the concentration of sulphuric acid is about 30% vol/vol H2SO4.

3. A method as claimed in claim 1 or claim 2 wherein the concentration of manganic ion in the solution is up to 0.1 so.

4. A method as claimed in any preceding claim wherein the oxidation is carried out at a temperature of from 35"C to 40"C.

5. A method as claimed in any preceding claim wherein the reaction is carried out in a filter press type electro-chemical cell.

6. A method as claimed in claim 5 wherein the cell comprises a plurality of compartments, each being divided by a cationic ion exchange membrane into anode and cathode compartments, the solution of manganese sulphate to be oxidixed being circulated through the anode compartments and a solution of manganous sulphate and sulphuric acid being circulated through the cathode compartments.

7. A method for the oxidation of manganous ion to manganic ion substantially as hereinbefore described in run 3 or run 4 of Example 1.

8. A method for the chemical oxidation of an oxidisable chemical species by manganic ion (Mn3+) which comprises electro-chemically oxidising manganous ion (hun2+) to manganic ion by a method as claimed in any preceding claims and contacting the resulting manganic ion containing solution with the oxidisable species under oxidation conditions.

9. A method as claimed in claim 8 wherein the oxidisable species is an organic compound.

10. A method as claimed in claim 9 wherein the organic compound contains a methyl group attached to an aromatic nucleus and the oxidation produces a corresponding aldehyde.

11. A method as claimed in claim 10 wherein the oxidation is of a xylene to the corresponding tolualdehyde, of toluene to benzaldehyde or of m-chlorotoluene to m-chlorobenzaldehyde or is an oxidation of anethole.

1 2. A method as claimed in any one of Claims 8 to 11 wherein after the oxidation of the organic compound the solution of manganous ion in sulphuric acid is separated from the reaction product and is returned to the electro-chemical cell and re-oxidised to produce manganic ion which is then used for the oxidation of more of the organic compound.

1 3. A method as claimed in any one of claims 8 to 12 wherein the reaction of the manganic ion with the organic compound is carried out at a temperature of from 35"C to 40"C.

14. A method for the chemical oxidation of xylene to the corresponding tolualdehyde substantially as hereinbefore described in run 3 or run 4 of Example 1.

1 5. A method for the chemical oxidation of p-tert. butyl toluene to the corresponding p-tert.

butyl benzaldehyde substantially as hereinbefore described in Example 2.

1 6. A method for the chemical oxidation of p-chlorotoluene to the corresponding pchlorobenzaldehyde substantially as hereinbefore described in Example 3.

1 7. A method for the chemical oxidation of 2,4-dichlorotoluene to 2,4-dichlorobenzaldehyde substantially as herein before described in Example 4.

18. An oxidised chemical species whenever produced by a method as claimed in any one of Claims 8 to 14.