GB2164935A

GB2164935A - A process for the separation of mixtures of methyl esters of meta and para cresol

Info

Publication number: GB2164935A
Application number: GB8424594A
Authority: GB
Inventors: James Peter Millington; Andrew Robert Jones; John Alan Hughes; John Edward Trotman
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1984-09-28
Filing date: 1984-09-28
Publication date: 1986-04-03
Also published as: GB8424594D0; GB2164935B

Abstract

A method of separating 3-methoxy toluene from a mixture of 3-methoxy toluene and 4-methoxy toluene comprises oxidising 4-methoxy toluene to 4-methoxy benzaldehyde using manganese dioxide and thereafter separating the 3-methoxy toluene from the resulting 4-methoxy benzaldehyde. The oxidation is preferably carried out in a stirred reaction vessel at a temperature of 45 DEG to 55 DEG C with a suspension of manganese dioxide in a solution of dilute sulphuric acid. The manganese dioxide is desirably prepared by oxidising a manganous salt in an electrochemical cell, preferably a cell as described in European Patent Specification No. 0 064 417.

Description

SPECIFICATION A process for the separation of mixtures of methyl esters of meta and para cresol It is well known that mixtures of meta and para cresol, as well as the methyl esters of these compounds, are difficult to separate, especially by distillation techniques. The boiling points of the cresols are; meta isomer 202.8"C and para isomer 201.9"C. The boiling points of the methyl esters of these cresols are 3-methoxy toluene 177.2"C and 4-methoxy toluene 176.5"C.

We have now, surprisingly, discovered that when a mixture of the methyl esters of meta and para cresol is oxidised using manganese dioxide then only the para isomer reacts, the oxidation product yielding a mixture of 4-methoxy benzaldehyde (anisaldehyde) and unchanged 3-methoxy toluene. It will be readily appreciated that these two compounds can be easily separated.

This discovery of the ability of manganese dioxide to oxidise the para isomer of the methyl ether of cresol is an important discovery and forms the basis of this invention. Accordingly, the present invention provides a method of separating 3-methoxy toluene from a mixture of 3methoxy toluene and 4-methoxy toluene which method comprises oxidising 4-methoxy toluene to 4-methoxy benzaldehyde using manganese dioxide, preferably electrochemically generated manganese dioxide, and thereafter separating the 3-methoxy toluene from the resulting 4-methoxy benzaldehyde.

The mixture of 3-methoxy toluene and 4-methoxy toluene will usually be produced by methylating a mixture of meta and para cresol, and the invention therefore provides a convenient and satisfactory method for the production of useful compounds derived from the isomeric cresols, 3-hydroxy toluene and 4-hydroxy toluene. These mixtures of the two substituted toluenes are derived from processes such as coking and coal gasification.

In carrying out the present invention, the mixture of isomeric cresols, 3-hydroxy toluene and 4hydroxy toluene maybe methylated using any standard technique such as reacting these compounds with methanol with the reaction being catalysed by acid, or by reaction with dimethyl sulphate or diazomethane. The product from this reaction, that is the mixture of 3 and 4 methoxy toluenes, results in an oil.

The oil comprising the mixture of methoxy toluenes maybe reacted in a well stirred reaction vessel at a temperature of approximately say 45 to 50"C, preferably about 50"C, with a suspension of manganese dioxide in a solution of dilute sulphuric acid. The manganese dioxide is preferably prepared by electrochemically oxidising a solution of manganous sulphate in aqueous sulphuric acid (preferably MnSO 1.0 M in H:SO 2.0 M) in an electrochemical cell. The cell is preferably undivided, ie, the anolyte and catholyte are common, the cell containing no divider.

The anode maybe made from a high hydrogen overvoltage metal such as zinc, copper, or mercury, with lead being the preferred material. The cathode may be constructed from any suitable material such as titanium, nickel, stainless steel or lead. The cell configuration is preferably such that a minimum gap is maintained between the electrodes, and the electrolyte is pumped through the cell at such a rate that the linear flow in the cell is between 10 and 50 cm/sec. Such a cell is described in our European Patent Specification No 0 064 417.

The cell is preferably operated with a current density of from 3000 A/m2 to 6000 Alml to produce an oxidising solution containing a suspension of manganese dioxide in the concentration range 0.5 to 1.0 M.

The operating parameters for the cell are critical: if the concentration of sulphuric acid is allowed to fall below 0.5 M, manganese dioxide is deposited on the anode and the cell process is stopped. At the other end of the concentration range, if the sulphuric acid concentration is allowed to raise above 2.5 M, significant amounts of manganic ion (mm"') are produced in the electrolyte. Manganic ion is a more powerful oxidising agent than manganese dioxide and its presence results in undesirable products being formed. For example, in the present case anisic acid rather than anisaldehyde will be formed:

ANISALDEHYDE

ANISIC ACID The current density should not fall below 500 A/m2 for any significant length of time as this will result in manganese dioxide deposition on the anode.Similarly, operating the cell at a high electrolyte temperature results in manganese dioxide deposition; consequently the temperature limit cell operation is preferably not greater than 15"C. The manganese dioxide produced in this way from a cell of the type described above is extremely reactive and has an activity some thirty to fifty times as great as the so called "active" manganese dioxide used in the chemical industry.

The oxidation of the mixture of methoxy toluenes reformed to above is conveniently carried out as follows. The suspension of manganese dioxide prepared as described above is reacted in a well stirred flask at 45 to 55"C, preferably 50"C, with the mixture of 3 and 4 methoxy toluene. Reaction is compieted in a short time (less than two minutes) and may be observed by the disappearance of the black suspension of manganese dioxide from the mixture.

The two layers are then separated and the aqueous layer containing manganous sulphate and sulphuric acid is conveniently returned to the cell for regeneration and the organic layer retained in the reaction flask. The process may be repeated until all the 3-methoxy toluene in the mixture has been converted to anisaldehyde.

The invention is further illustrated with reference to the following examples.

EXAMPLE I Ten litres of a solution containing 1 M of manganous sulphate dissolved 2 M sulphuric acid was electrolysed in a DEM Cell, as described in our European Patent Specification No 0 064 417, using a lead anode (area 0.05 m2) and a stainless steel cathode with an interelectrode gap of 2mm, at a current density of 5000 A/m7 at a cell potential of 4.5 V for 24 minutes. The resultant solution on analysis was found to contain 0.5 M of a suspension of manganese dioxide, equivalent to a current efficiency of 70%.

The suspension was then placed in a 20 1 flask equipped with a high speed stirrer heated to 50"C, and 610 g of a mixture containing equal parts of 3 and 4 methoxy toluene added. The stirring was continued for a further five minutes by which time all the manganese dioxide had disappeared. The stirrer was then stopped and the organic layer separated and analysed.

The products and yields are shown below: 4-methoxy benzaldehyde 306g 90% yield 4-methoxy benzoic acid 17g 5% " High MW material lOg 3% 3-methoxy toluene 305g 100% recovery Example II The reaction was identical to the previous example but in this case the electrolytic circuit was charged with 5 1 of oxidising solution and as before 610 g of the mixture.

The products and yields are as shown below: 4-methoxy benzaldehyde 107g 98% yield 4-methoxy benzoic acid 2g 1% High MW material 2g 1% Unchanged 4-methoxy toluene ISOg Unchanged 3-methoxy toluene 300g 98% recovery It is to be understood that in a further aspect of the invention there is provided a method of making active manganese dioxide by the electrochemical process described above.

Claims

1. A method of separating 3-methoxy toluene from a mixture of 3-methoxy toluene and 4methoxy toluene which method comprises oxidising 4-methoxy toiuene to 4-methoxy benzaldehyde using manganese dioxide and thereafter separating the 3-methoxy toluene from the resulting 4-methoxy benzaldehyde.

2. A method as claimed in Claim 1, wherein the mixture of 3-methoxy and 4-methoxy toluenes is produced by methylating a mixture of meta and para cresol.

3. A method as claimed in Claim 1 or Claim 2, wherein the mixture of methoxy toluenes is reacted in a well stirred reaction vessel at a temperature of 45" to 55"C with a suspension of manganese dioxide in a solution of dilute sulphuric acid.

4. A method as claimed in any one of the preceding clams, wherein the manganese dioxide is prepared by oxidising a solution of manganous sulphate in aqueous sulphuric acid in an electrochemical cell.

5. A method as claimed in Claim 4, wherein the anode of the electrochemical cell is a lead anode and the cathode is a titanium, nickel, stainless steel or lead cathode.

6. A method as claimed in either Claims 4 or 5, wherein the electrochemical cell is a cell as described in European Patent Specification No 0 064 417.

7. A method as claimed in Claim 6, wherein the electrolyte is pumped through the cell at a rate of from 10 to 50 cm/sec.

8. A method as claimed in Claim 6 or Claim 7, wherein the cell is preferably operated with a current density of from 3000 to 6000 A/m7 whereby an oxidising solution is produced containing a suspension of manganese dioxide in the concentration range 0.5 to 1.0 M.

9. A method as claimed in any one of Claims 5, 4 to 8, wherein the cell is operated at a temperature of not greater than 15' C.

10. A method as claimed in Claim 1 and substantially as hereinbefore described in either of the specific examples.

11. Anisaldehyde when prepared by the oxidation of 3-methoxy toluene using manganese dioxide as the oxidising agent.

12. 4-methoxy toluene when recovered from a mixture of 3-methoxy and 4-methoxy toluene when the 3-methoxy toluene has been oxidised using a method as claimed in any one of the Claims 1 to 10.

13. A method of making active manganese dioxide wherein said manganese dioxide is prepared by oxidising a solution of manganese sulphate in aqueous sulpuric acid in an electrochemical cell.