Process for the continuous preparation of heavy alkyl acrylates
The present invention relates to a process for the continuous synthesis of heavy alkyl acrylates by a transesterification reaction using hydrozincite as a heterogeneous catalyst.
1. A process for the continuous synthesis of (meth)acrylic esters of formula (I):
in which R is a hydrogen atom or a methyl group, and R 1 is a linear or branched alkyl radical, or a cycloaliphatic, aryl, alkylaryl or arylalkyl radical, including from 4 to 40 carbon atoms, or a linear or branched alkyl radical containing at least one heteroatom and from 3 to 40 carbon atoms, comprising the step of reacting an alkyl (meth)acrylate of formula (II):
in which R has the abovementioned meaning and R 2 is a linear or branched alkyl group containing from 1 to 3 carbon atoms, with an alcohol of formula (III):
R 1 —OH (III)
in the presence of at least one polymerization inhibitor and hydrozincite as heterogeneous catalyst,
wherein the catalyst content is between 0.01 mol_% and 0.5 mol % relative to the alcohol.
2. The process as claimed in claim 1 which is a transesterification reaction taking place in a transesterification reactor stirred at a temperature of 80-150° C., a reaction mixture exiting the reactor comprising the heavy alkyl acrylate with, as light products, heavy alcohol, light alcohol and unreacted light acrylate, and, as heavy products, the polymerization inhibitor(s) and heavy reaction products,
further comprising the step of subjecting said reaction mixture to a liquid/solid separation step to separate out the catalyst.
3. The process as claimed in claim 2 , in which said (meth)acrylic ester of formula (I) is purified according to the steps below:
a. sending said reaction mixture to a first distillation column (C1) having a top and tail and under a pressure of 1000 to 20,000 Pa, and performing a first distillation therein, producing:
at the top, a head stream comprising unreacted starting materials, and a minor fraction of heavy products (Michael adducts), and
at the tail, a stream composed essentially of the heavy alkyl acrylate;
b. sending the tail stream from the first distillation column (C1) to a second distillation column (C2) having a top and a tail and under a pressure of 1000 to 20,000 Pa, and performing a second distillation producing:
at the top, the desired heavy alkyl acrylate,
at the tail, the polymerization inhibitor, and also heavy reaction products.
4. The process as claimed in claim 3 , comprising an additional step of sending the head stream from the first distillation column (C1) to the transesterification reactor in order to recycle the starting materials.
5. The process as claimed in claim 2 , in which said liquid/solid separation step involves a technique selected from the group consisting of: filtration, electrofiltration, absorption, centrifugation and decantation, the catalyst content of product thus treated being less than 500 ppm.
6. The process as claimed in claim 2 in which said catalyst is introduced into the transesterification reaction as a suspension in the reaction mixture or is placed in a fixed bed in the transesterification reactor.
7. The process as claimed in claim 1 in which said polymerization inhibitor comprises at least one N-oxyl derivative and at least one polymerization inhibitor chosen from phenol compounds and phenothiazine compounds in a mass ratio of between 1 and 10, limits included.
8. The process as claimed in claim 7 , in which the N-oxyl compound is chosen from 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), and derivatives thereof, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4-OH-TEMPO), 4-oxo-2,2,6,6-tetramethylpiperidine 1-oxyl (4-Oxo-TEMPO), and mixture(s) thereof.
9. The process as claimed in claim 1 in which said polymerization inhibitor is chosen from phenol compounds and phenothiazine compounds.
10. The process as claimed in claim 1 in which the polymerization inhibitor is chosen from the group consisting of phenothiazine, 4-methyl-2,6-tert-butylphenol, hydroquinone, hydroquinone methyl ether, p-aminophenol, p-nitrosophenol, 2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol, 2-methyl-4-tert-butylphenol, 4-methyl-2,6-tert-butylphenol (or 2,6-tert-butyl-p-cresol) or 4-tert-butyl-2,6-dimethylphenol, hydroquinone (HQ), and hydroquinone methyl ether (HQME).
11. The process as claimed in claim 1 in which a mass content of polymerization inhibitor is from 0.001% to 0.5% in the reaction mixture.
12. The process as claimed in claim 1 in which a mass content of Michael adducts in the tail stream from the first distillation column (C1) is less than or equal to 5%.
13. The process as claimed in claim 1 in which the (meth)acrylic ester of formula (I) is dimethylaminoethyl acrylate.