PROPIONATES

The present invention relates to a method of producing a-Lkylene monoacetates and monopropionates of high purity and in good yields. Such esters are suitable for use as organic ester hardeners, especially for cold silicates used for bonding sands in the produc- tion of foundry cores.

Methods of preparing mono esters of ethyl glycol and propylene glycol particularly of the acrylates and methacrylates have been disclosed in several prior publications, including British patent specification Serial Nos 1133522, 1120301, 1107234, 1082883, and 105461 . The problem with most of these prior art methods is that the final product is a mixture of mono- and diesters, apart from the free acids and alcohols, which are very difficult to separate without resorting to complex fractional distillation techniques. Such expedients not only reduce the overall yield of the monoester recovered but also adversely affect the purity of the product and the economics ofthe process.

It has now been found that the desired monoesters may be produced .in a substantially pure state in relatively high yields without using a complex purification step. Accordingly, the present invention is a process for producing al ylene glycol monoesters by reacting in the liquid phase a molar excess of ethylene oxide or propylene oxide with acetic acid or propionic acid at elevated temperature in the presence of a catalyst comprising a chromium salt of a saturated or an unsaturated aliphatic carboxylic acid containing between 1 and 10 carbon atoms.

The carboxylic acid used in preparing the chromium salt catalyst is suitably selected from formic, acetic, butyric, pentanoic, hexanoic, octanoic, 2-ethylhexanoic and decanoic acids and mixtures thereof. Whichever salt is used, it should preferably be soluble in the reaction medium. Chromium octanoate is a particularly preferred example of such a catalyst.

The chromium salt may be prepared in situ, by heating chromium hydroxide with the carboxylic acid for a short time in order to effect solution. The reaction is then continued in the usual manner. The amount of chromium salt employed in the reaction may vary between 0.1 and _ by weight of the reactant acid employed.

A slight molar excess of the olefin oxide is used in the reaction mixture. Thus, for example the molar ratio of olefin oxide to the.reactant carboxylic acid in the reaction mixture is preferably between 1.05 and 1.2.

The reaction of the present invention may be carried out at a temperature between 40 and 120 C. The reaction pressure is preferably above atmospheric, for example between 20-40 psig. The process of the present invention may be operated batch-wise or continuously. The course of the reaction may be followed by measuring the acidity of the reaction mixture from time to time and hence determining the content of unreacted acid.

The monoacetates and monopropionates of ethylene glycol and propylene glycol need to have low levels of residual acidity eg less than 0.1 of acetic acid for foundry uses. Such low levels of acidity are obtained at the reaction stage using the process of the present invention whilst maintaining the alkylene glycol monoester content at around 90 . The product may be separated from the catalyst by a simple distillation which may be carried out at reduced pressure.

The invention is further illustrated with reference to the following Examples: Example 1 Preparation of Propylene Glycol Monoacetate

To a suitable stirred, heated autoclave were charged 60 pt

by weight of acetic acid and _\Λ pt by weight of chromium octanoate. The vessel was sealed and evacuated and purged with nitrogen several times to ensure removal of air. The vacuum was finally broken with nitrogen to atmospheric pressure. The vessel contents were heated to 90 C and at this temperature the addition of 4l8 pt by weight (20$ excess) of propylene oxide was commenced at a rate of approximately 1 pt by weight per minute. The temperature was maintained at 90-100 C and the maximum pressure recorded was 28 psig. After 1 hourfe reaction the acidity was less than 0.5$, the reactor and contents were cooled to 50°C and vacuum applied for 30 min to remove excess propylene oxide. The crude product was distilled to give a 91$ yield of a product containing 93$ propylene glycol monoacetate and an acidity of 0.02$ as acetic acid. Example 2

Preparation of Propylene Glycol Monopropionate

To a suitable stirred, heated autoclave were charged 444 pt by weight of propionic acid and 4.5 pt by weight of chromium octanoate. The vessel was sealed and evacuated and purged with nitrogen several times to ensure removal of air. The vacuum was finally broken with nitrogen to atmospheric pressure. The vessel contents were heated to 90 C and at this temperature the addition of 383 pt by weight (10$-excess) or propylene ^" oxide was commenced at approximately 15 pt by weight per minute. The temperature was maintained at 90-100 C and the maximum pressure recorded was 34 psig. After 1.7 hr reaction the acidity was less than 0.2$, the reactor and contents were cooled to 0 C and vacuum applied for 30 minutes to remove excess propylene oxide. The crude product was distilled to give a 90$ yield of a product containing 97$ propylene glycol monopropionate with an acidity of 0.04$ as propionic acid.