Synthesis of acrylic acid and its esters with Se-containing catalysts

– Se-containing catalysts of organic and inorganic nature were explored for the synthesis of acrylic acid and its esters in different solvents. Newly synthesized Se-containing microgel catalyst shown the best catalytic performance. With microgel catalyst different acrylate monomers can be produced with high yield simply by varying solvent nature.


Introduction
Acrylic acid and acrylate esters are widely used in industrial purposes for textile, adsorbent polymer, paints, adhesives manufacture [1]. Main method to produce acrylic acid is 2 step synthesis, that include propylene oxidation to acrolein with further acrolein oxidation to acrylic acid [2]. Acrylic acid can be used without further treatment or esterified to acrylate esters. Another method to produce acrylate esters is one-step oxidative esterification of acrolein [3]. Previously reported results [3,4] shown, that up to 85 % of methyl acrylate yield can be reached by this method. But to achieve such result, either reaction temperature has to be high (250 ºC) and catalyst, that contain hazardous heavy metals, has to be used [4] or reaction can be performed at moderate temperature (25 ºC) but reaction has to last 40 h [3].
To improve process of acrolein oxidation to acrylic acid or oxidative esterification to acrylate esters, reaction should be performed under mild conditions, during reasonable time, with minimum wastes and with high yield of product. Selenium catalysts shown to be effective in oxidation processes [5], and together with hydrogen peroxide as oxidant can provide good results under mild and green conditions [6]. For this purposes, we studied process of acrolein oxidation with hydrogen peroxide and selenium catalysts of organic and inorganic nature.
Se-modified microgel consists of diselenide crosslinker, incorporated into poly(Nvinylcaprolactam) microgels together with a permanent N,N´-methylenebis(acrylamide) crosslinker. Microgel, as polymer colloid with open porous structure, surface activity and solvent resistance, can provide unique chemical functionality and catalytic activity of modified microgel.
Catalytic performance of Se-containing catalysts was evaluated by the yields of methyl acrylate and acrylic acid for the reaction performed in methanol (Fig. 2). All tested catalysts proved to be catalytically active in oxidative esterification of acrolein and allowed to reach up to 90% total yield of reaction. Regardless the catalyst, methyl acrylate yield was much higher than acrylic acid. This results are in good correlation with results in work [3], where authors explain higher yield of ester due to easier formation of intermediate compound in ester synthesis than in carboxylic acid synthesis. Diphenyl diselenide, X-linker and H2SeO3 shown similar results. Semodified microgel shown the highest yield of reaction -89 %. To study the effect of solvent nature on the synthesis of acrylic acid and acrylate esters, Semodified microgel was used as the most effective catalyst. In methanol medium, ester yield and ester:acid ratio was the highest -83 % and 12:1 respectively. With increase of hydrocarbon chain in alcohol molecule (C2H5-OH and C4H9-OH) ester:acid ratio decreases, but ester yield is still higher than acid. In dioxane medium only acrylic acid was formed with 79 % yield.

Conclusion
Acrylic acid and acrylate esters can be easily synthesised in one step reaction from acrolein with yield up to 79 % and 83 % respectively with Se-modified microgel catalyst. Products of the reaction and their ratio can me modified by chosen solvent.