Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Bioderived Acrylates from Alkyl Lactates via Pd-Catalyzed Hydroesterification Gereon M. Yee, Marc A. Hillmyer, and Ian A. Tonks* Department of Chemistry, University of Minnesota−Twin Cities, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
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ABSTRACT: The conversion of methyl and ethyl lactate to their corresponding alkyl 2-(propionyloxy)propanoate esters was achieved by Pd catalyzed hydroesterificative coupling with carbon monoxide (CO) and ethylene (C2H4) at moderate temperatures and CO/C2H4 pressures. A screening of reaction conditions showed that the reaction could be carried out at low loadings of catalyst, which was generated in situ from inexpensive and commercially available reagents. High conversions and product yields were obtained in a variety of solvents and even under neat conditions. Product analysis identified transesterification to be the primary competing reaction, which could be mitigated by changing solvents, as well as minimizing the amount of excess acid present in solution. Pyrolysis of methyl and ethyl 2-(propionyloxy)propanoate transformed these esters into their respective acrylates suitable for subsequent polymerization, along with propionic acid as a valuable coproduct. KEYWORDS: Carbonylation, Alkoxycarbonylation, Hydroesterification, Pyrolysis, Lactic acid, Alkyl lactates, Acrylates, Catalysis
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INTRODUCTION Inexpensive feedstocks derived from petrochemicals have sustained the production of plastic materials on a massive scale, but the limited nature of these resources has necessitated the development of synthetic routes derived from renewable resources. A key challenge lies not only in finding a viable route from renewable feedstocks but also in developing an overall process that itself is sustainable, of low environmental impact, and economically competitive with traditional petroleum products.1 In this regard, lactic acid has shown particular promise given its ready availability from carbohydrates via fermentation,2−6 and its facile conversion into a number of commodity products including polylactide.7,8 Much of the work in this area has focused on the direct conversion of lactic acid and the corresponding alkyl lactates into acrylic acid and acrylate esters, respectively, which are important monomers used in the production of a number of compounds that are generally derived from hydrocarbon feedstocks.9−19 The direct dehydration of lactic acid using alkali and alkali earth metal catalysts has been known since the 1950s, and has experienced a revival in recent years.20 However, these routes generally suffer from limited conversions and yields.21−24 Early work from the 1930s and 1940s also showed that pyrolysis of alkyl 2-acetoxypropanoate derivatives, which can be obtained from alkyl lactates directly by acetylation, gives the corresponding alkyl acrylates in varying yields with acetic acid as the coproduct in an atom economic process (Scheme 1).25−32 The nature of the alkyl (R) group of the starting lactate was shown to significantly affect the yield of the acrylate ester obtained.33,34 Newer work © XXXX American Chemical Society
in this area has focused on the nickel catalyzed acetylation of lactide (the cyclic dimer of lactic acid) with acetic acid to give 2-acetoxypropionic acid, which can be subsequently pyrolyzed to give acrylic acid or converted to the methyl ester for production of methyl acrylate.35 This route is attractive in that it gives high yields of acrylic acid or methyl acrylate from lactide, and utilizes readily available nickel(II) nitrate and nickel(II) acetate as the acetylation catalysts, but requires somewhat harsh conditions. Alternatively, the conversion of alkyl lactates to their corresponding alkyl 2-(propionyloxy)propanoate derivatives by way of Pd catalyzed hydroesterification with carbon monoxide (CO) and ethylene (C2H4) (Scheme 1) utilizes inexpensive starting materials and would produce acrylate esters and propionic acid upon pyrolysis. This route has the added benefit of generating propionic acid as a coproduct, which has important uses as a food and feed preservative.36 Moreover, hydroesterification reactions are well studied and are effective with a number of different alcohol-containing nucleophiles.37−40 Typical Pd catalyzed hydroesterification reactions often proceed at moderate temperatures (80−120 °C), and can be carried out under neat and acid-free conditions.41,42 Additionally, such a route would allow for the possible installation of greater chemical complexity at the formed ester group, and the potential to further tune the pyrolysis reaction and thus the coproducts generated. We Received: May 22, 2018 Revised: July 2, 2018
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DOI: 10.1021/acssuschemeng.8b02359 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering Scheme 1. Routes to Acrylate Esters from Alkyl Lactates
Figure 1. Representative GC-FID chromatogram of the reaction mixture corresponding to entry 2 in Table 1.
chromatogram of the reaction mixture, which upon isolation and independent synthesis were shown to correspond to products resulting from the transesterification of 1a with EtOAc to give the corresponding acetate of 1a (and presumably ethanol), as well as the self-condensation of 1a to give alkyl lactate oligomers. By analogy, reactions with ethyl lactate (1b) also led to the formation of the desired ethyl 2(propionyloxy)propanoate (2b) as the major product (see SI for characterization details). We varied the catalyst loadings to better understand the effect of each component of the catalyst system on the conversions of 1a and 1b, as well as the yields of 2a and 2b and their respective side-products (Tables 1 and 2). Pd loadings developed a Pd catalyzed hydroesterification of methyl and ethyl lactate (1a and 1b, respectively) to the corresponding alkyl 2-(propionyloxy)propanoates (2a and 2b), utilizing a simple and robust catalyst system. Pyrolytic decomposition of 2a and 2b gave methyl and ethyl acrylate, respectively.
Table 1. Effect of Pd(OAc)2, TsOH·H2O, and PPh3 Loadings on Conversion of Methyl Lactate (1a) to Methyl 2-(propionyloxy)propanoate (2a)a
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RESULTS AND DISCUSSION The hydroesterification reaction outlined below in Scheme 2 was attempted with a catalyst loading of 1% Pd(OAc)2/4%
Entry
Pd/H+/L (%)
Time (h)
Conversion (%)b
Product Yield (%)b
Byproduct Yield (%)b
1 2 3 4 5 6 7 8 9 10c 11c 12c
0.5/4/16 1/4/16 4/4/16 0.5/4/16 1/4/16 4/4/16 1/0.5/16 1/1/16 1/16/16 1/4/0.5 1/4/1 1/4/4
16 16 16 4 4 4 4 4 4 4 4 4
77 >99 68 48 53 34 4 13 >99 72 72 26
48 79 56 24 47 31 0 7 32 99 94 31 44 61 15 13 65 26 21 19
80 91 84 13 31 48 2 5 19 1 3 13
13 7 7 10 5 4
