Process for making n-butyl esters from butadiene

Details Process for making n-butyl esters from butadiene Process for making n-butyl esters from butadiene

2001-04-30

2002-10-15

Shah, Mukund J. (Department: 1624)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

C560S247000

Reexamination Certificate

active

06465683

ABSTRACT:

The present invention relates to a process for making n-butyl esters by reacting butadiene with a carboxylic acid in the presence of a Brønsted or Lewis acid catalyst to form the unsaturated ester which is subsequently hydrogenated to form the saturated ester.
BACKGROUND OF THE INVENTION
It is known that n-butyl esters such as n-butyl acetate can be produced by a number of routes. For instance, the hydroformylation of propylene in the presence of acetic acid is a method which gives a mixture of n-butyl acetate and iso-butyl acetate. This method however requires a source of syngas which increases capital costs. An alternative method is to react ethylene with vinyl acetate in the presence of an acid catalyst followed by the hydrogenation of the resultant unsaturated ester. A further method is the reaction of ethylene with ethanol in the presence of a base catalyst to form butanol and the reaction thereof with acetic acid to form butyl acetate. In addition, all these methods rely on the use of either relatively expensive feedstocks such as ethylene and n-butanol or involve multiple reaction stages or expensive catalysts and separation stages. The acid catalysed addition of butadiene to acetic acid using ion-exchangeresin catalysts having bulky counterions to improve the reaction selectivity to two isomeric C
4
butenyl acetates is disclosed in several patents viz., U.S. Pat. No. 4,450,288 (alkyl pyridinium), U.S. Pat. No. 4,450,287 (quaternary ammonium), U.S. Pat. No. 4,450,289 (quaternary phosphonium). The main objective of these patents is stated to be the production of secondary butenyl acetate. However, there is no mention in these documents of the isolation of n-but-2-enyl acetate or the production of n-butyl acetate. Butadiene is a relatively inexpensive by-product of the refining process and is a potential feedstock for making butyl esters. It is commercially available either as a purified chemical or as a constituent of a hydrocarbon cut. For example, as a constituent of a mixed C
4
stream obtained from naptha stream cracking. Typically such streams contain species such as butane, 1-butene, 2-butene, isobutane and isobutene in addition to butadiene. It is advantageous that a process utilising butadiene can use such streams. However, butadiene is also in equilibrium with 4-vinyl cyclohexene, a Diels Alder dimer of butadiene. This dimer can be thermally cracked back to butadiene:
So any process involving the use of butadiene as feedstock needs to take this reversible reaction into consideration.
EP-A-84133 describes a process for the production of unsaturated alcohols and/or esters of unsaturated alcohols. The reference describes the reaction between conjugated dienes and water or aqueous carboxylic acids. The resulting product, is a complex mixture of unsaturated isomeric alcohols and esters.
SUMMARY OF THE INVENTION
It has now been found that saturated n-butyl esters and secondary butyl esters can be synthesised without resort to either (a) the hydroformylation route from propylene or (b) the use of vinyl acetate or ethylene feedstocks in relatively simple stages.
According to a first aspect of the present invention, a process is provided for making a butyl ester from butadiene, this process comprising:
a. reacting butadiene with a saturated aliphatic monocarboxylic acid to form a mixture of n-butenyl and secondary butenyl esters,
b. separating the n-butenyl ester from the secondary butenyl ester, and
c. hydrogenating the n-butenyl ester separated in step b) in the presence of a catalyst to the corresponding n-butyl ester.
The butadiene employed in step a) may be employed in the form of a substantially pure butadiene. Alternatively, a hydrocarbon mixture comprising butadiene may be employed. In one embodiment a raw (e.g. crude or depleted) C
4
stream comprising butadiene, isobutene, 1 and 2-butenes and butane is employed. Such a stream may comprise up to 60% butadiene.
The secondary butenyl ester separated in step b) may be: i) recycled to step a), ii) hydrogenated in the presence of a catalyst to produce sec-butyl ester, iii) thermally cracked to produce the starting butadiene and a saturated aliphatic monocarboxylic acid; or iv) further reacted.
A preferred embodiment of the present invention is a process for making a butyl ester from butadiene, said process comprising:
a. reacting butadiene or a hydrocarbon fraction comprising butadiene with a saturated aliphatic monocarboxylic acid to form a mixture of n-butenyl and secondary butenyl esters,
b. separating the n-butenyl ester from the secondary butenyl ester,
c. recycling the secondary butenyl ester thus recovered to step a), and
d. hydrogenating the n-butenyl ester in the presence of a catalyst to the corresponding n-butyl ester.
In the present process, the saturated, aliphatic carboxylic acid suitably has 1-6 carbon atoms and is preferably acetic acid. Thus, the present process can be readily adapted to the reaction of butadiene with acetic acid to form a mixture of n-butenyl acetate (also known as crotyl acetate) and secondary butenyl acetate, the latter being separated and preferably recycled to the initial stage and the n-butenyl acetate (crotyl acetate) being catalytically hydrogenated to form n-butyl acetate.
The reaction is suitably carried out in the liquid or mixed liquid/gas phase in the presence of a solvent. It is not essential that both reactants dissolve completely in the solvent. However, it is an advantage if the solvent chosen is such that it is suitably capable of dissolving both the reactants. Specific examples of such solvents include hydrocarbons such as decane and toluene and oxygenated solvents such as butyl acetate or excess carboxylic acid reactant and recycled higher esters such as C
8
acetates recycled sec-butenyl acetate. The use of excess carboxylic acid as a reactant can be advantageous when this chemistry is used to extract butadiene from an impure stream, as it facilitates reaction at high conversion of butadiene, or in process terms high efficiency of removal of butadiene. Currently the removal or recovery of butadiene from refinery streams requires a separate processing stage.
The reactions taking place in a preferred embodiment of the invention can be represented graphically by the following equation:
n-Butyl Carboxylate by the Addition of Carboxylic Acids to Butadiene
The reactions, and in particular, the addition reaction between butadiene and the carboxylic acid (step a), may be carried out using a homogeneous or heterogeneous catalyst. Heterogeneous catalysts may be advantageous in certain cases as they can facilitate the separation of the reaction product from the reaction mixture, and/or allow the catalyst to be conveniently separated from reaction by-products (mostly high boiling point butadiene oligomeric species). The preferred catalysts are based on strong acid ion-exchange resins (e.g. Amberlyst 15®, Amberlite IR120®) with a proportion of the acidic sites exchanged with bulky counterions such as tetra-phenylphosphonium counterions. Typically these counterions account for less than 10% of the available acidic sites.
The heterogeneous catalyst phase can be a partially or fully insoluble liquid phase (e.g. acidic ionic liquids, liquid acidic polymers and partially solvated polymers) or a solid (e.g. HY zeolite, strong acid macroreticular, macronet and gel ion-exchange resins and heteropolyacids of tungsten or molybdenum which have been ion-exchanged and/or supported on a carrier material). In addition to Amberlyst 15® mentioned above, other suitable examples of heterogeneous catalysts include fluorinated ion-exchange resins like Nafion®, phosphoric acid functionalised polymers, and acidic oxides such as HY zeolites.
In certain cases the activity of heterogeneous catalysts may decrease after prolonged periods of use. This may be due to blockage of active sites by butadiene oligo- and polymerisation products. In such cases, it may be advantageous to carry out the process of the present invention in homogeneous phase. Suitable homogeneous catalysts include sulphonic acids,

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