Carbonylation of Ethylene Oxide to β-Propiolactone: A Facile Route to

Letter pubs.acs.org/acscatalysis

Carbonylation of Ethylene Oxide to β‑Propiolactone: A Facile Route to Poly(3-hydroxypropionate) and Acrylic Acid Erin W. Dunn,† Jessica R. Lamb, Anne M. LaPointe, and Geoffrey W. Coates* Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853-1301, United States S Supporting Information *

ABSTRACT: We report an improved synthesis of poly(3hydroxypropionate) (P3HP) from ethylene oxide (EO) and carbon monoxide (CO) through the intermediate βpropiolactone (PL). The optimized carbonylation of EO resulted in high selectivity for PL using a bimetallic [Lewis acid]+[Co(CO)4]− catalyst. Anionic ring-opening polymerization of PL by organic ionic compounds to afford P3HP was also investigated. A phosphazenium carboxylate initiator displays the highest activity for the polymerization and produces polyesters with molecular weights over 100 kDa and narrow molar mass distributions. Furthermore, the known rearrangement of PL and the thermolysis of P3HP provide efficient EO-based routes to the important commodity chemical acrylic acid. KEYWORDS: epoxide carbonylation, anionic polymerization, acrylic acid, polyester, biodegradable polymer

A

desired product. Another attractive strategy is the direct carboxylation of ethylene, but unfavorable thermodynamics require the use of a superstoichiometric base to achieve high yields as the acrylate salt.6 Fermentation can also produce AA; however, the product is toxic to most potential host organisms.7 Alternatively, 3-hydroxypropionic acid made from biorenewable feedstocks can be derivatized to yield AA.8,9 Unfortunately, biosynthetic routes often suffer from low yields, are energyintensive, and require separation of the reaction product from the bacterial culture. Another promising route to AA is the pyrolysis of poly(3hydroxypropionate) (P3HP). P3HP is an important polyester that is biodegradable, biocompatible, and has good mechanical properties.10 In addition to its utility as a polymer, P3HP may be used as a safe transportation and storage medium before onsite thermolysis to AA.11 Transportation and storage have always been challenging for AA because, over time, it both oligomerizes via conjugate addition and radically polymerizes, potentially leading to violent thermal runaway. Additives are used to suppress the polymerization, but the oligomers need to be thermally or catalytically cracked to release the glacial AA at the time of use.1 P3HP is a chemically stable polymer that can be safely and easily transported before cracking on site to give high-purity glacial AA. The most common synthesis of P3HP comes from the ringopening polymerization of PL. Various initiators, including (tetraphenylporphyrinato)aluminum chloride,12 alkali-metal alkoxides,13 alkali-metal carboxylates,14 tetraalkylammonium

crylic acid (AA) is an important commodity chemical for the manufacture of fibers, coatings, adhesives, and superabsorbent polymers.1 Global annual AA production is ∼4.5 million metric tons,2 predominantly from the two-step oxidation of propylene (Scheme 1A).1 Ethylene cyanohydrin, Scheme 1. Chemical Routes to Acrylic Acid (AA)

β-propiolactone (PL), acetylene, and acrylonitrile have also been employed industrially for the synthesis of AA but are not currently economically competitive with the propylene oxidation process.1,3 Environmental and sustainability considerations have pushed for investigations into renewable routes to AA. Systems based on renewable feedstocks such as catalytic dehydration of lactic acid4 and oxidative carbonylation of ethylene5 have been developed, but the reactions suffer from low yields of the © XXXX American Chemical Society

Received: September 28, 2016 Revised: October 23, 2016

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DOI: 10.1021/acscatal.6b02773 ACS Catal. 2016, 6, 8219−8223

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ACS Catalysis

Table 1. Carbonylation of Ethylene Oxide (EO) to βPropiolactone (PL)a

(R4N+) carboxylates,15 and organolanthanides16 have been used for the polymerization of PL, although most suffer from either low molecular weight or broad molar mass distributions. Other chemical syntheses of P3HP have been reported via the copolymerization of ethylene oxide (EO) and carbon monoxide (CO)17 and macrocyclic ester polymerization;18 however, again, these routes result in low molecular weight or broad dispersities. Biosynthetic routes to P3HP have been reported, but the overall polymer yield is low.10a,19 We sought to improve a potentially renewable route to PL, P3HP, and AA from the inexpensive feedstocks ethylene oxide and carbon monoxide (Scheme 1B).20 EO and CO can be made from fossil fuels, shale gas, and renewable sources, giving this route flexibility to take advantage of lower costs and sustainability. First, carbonylation of EO to PL can be accomplished using easily prepared [Lewis acid]+[Co(CO)4]− catalysts. In initial reports, EO carbonylation suffered from low yields or side reactions to acetaldehyde and/or succinic anhydride (SA).20,21 Second, anionic polymerization of PL produces P3HP, as discussed above. We optimized the polymerization using organic ionic catalysts, yielding high molecular weight polymer with narrow dispersities (Đ). Both the carbonylation and polymerization steps are atom economical and high yielding, resulting in an efficient route to P3HP. The direct rearrangement of PL to AA using phosphoric acid3a and the thermolysis of P3HP into AA22 are known processes, suggesting that our method could be used as an alternative to propylene oxidation for the production of acrylic acid. Initially, we examined the carbonylation of EO by previously reported, well-defined bimetallic catalysts of the form [Lewis acid]+[Co(CO)4]− (Table 1). In addition to targeting full conversion of epoxide, we aimed to suppress further carbonylation of PL to SA, since this compound is known to retard the polymerization of PL.23 The salen-based catalyst [(salph)Al(THF)2]+[Co(CO)4]− (1; salph = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-phenylenediamine; THF = tetrahydrofuran)24 resulted in incomplete conversion (44%) of EO after 8 h in toluene (entry 1). [(OEP)Cr(THF)2]+[Co(CO)4]− (2; OEP = octaethylporphyrinato)25 showed higher activity but still left 26% of EO unreacted under the same conditions (entry 2 in Table 1). The most active catalyst [(pClTPP)Al(THF)2]+[Co(CO) 4 ] − (3; pClTPP = meso-tetra(4-chlorophenyl)porphyrinato)26 achieved full conversion of EO in toluene; however, 25% SA was produced (entry 3 in Table 1). Acetaldehyde was not observed in any of the reactions. On the basis of previous mechanistic studies,27 we expected solvent choice to affect the amount of double carbonylation and, therefore, screened additional solvents to minimize this side reaction. As expected, 1,4-dioxane increased the amount of SA (entry 4 in Table 1), while THF cleanly converted EO to PL using 1 mol % 3 in 8 h (entry 5 in Table 1). These results are consistent with mechanistic and solvent studies on the single and double carbonylation of propylene oxide.26 The ratedetermining step of epoxide carbonylation is the solventassisted ring closing to β-lactone (Scheme 2, top cycle). Thus, a strongly donating solvent, such as THF, will more easily bind to the metal center and assist in the transition from neutral to cationic aluminum. Conversely, the rate-determining step of the β-lactone carbonylation is SN2 ring opening by the cobaltate during which a solvent molecule dissociates from the aluminum (Scheme 2, bottom cycle). Strongly donating solvents suppress the rate of this step, which effectively shuts down the lactone

conversion (%) entry 1 2 3 4 5 6c 7d

catalyst (mol %) 1 2 3 3 3 3 3

(1.0) (1.0) (1.0) (1.0) (1.0) (0.5) (0.2)

solvent

PL

SA

toluene toluene toluene 1,4-dioxane THF THF THF

44b 74b 75 43 >99 >99 96b

99 >99 >99 >99 27

3.5 40.7 64.1 33.5 67.3 59.4 105.8 135.0 20.9

1.2 1.2 1.1 1.3 1.1 1.1 1.3 1.3 1.1

a

General reaction conditions: [lactone]:[catalyst] = 1000:1 (Mn,theo = 72.1 kDa) and [lactone] = 1.0 M in THF, Trxn = 24 °C, quenched by addition of acetic acid. bDetermined by 1H NMR spectroscopy of crude reaction mixture. cDetermined by GPC calibrated with polystyrene standards in CHCl3 at 30 °C. dReaction conditions: [PL]:[catalyst] = 2000:1 (Mn,theo = 144.1 kDa). eReaction conditions: [PL]:[catalyst] = 4000:1 (Mn,theo = 288.2 kDa). 8221

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the chloride-containing 7a resulted in low conversion to P3HP, even after 2 h, because of poor nucleophilicity (entry 1 in Table 3). The more sterically bulky triphenylacetate anion (7b) achieved full conversion after 2 h, but the resulting material had a low Mn (entry 2 in Table 3). Use of a benzoate anion (7c) resulted in full conversion in 1 h (entry 3 in Table 3), whereas the more electron-deficient perfluorobenzoate anion (7d) required 2 h and resulted in decreased molecular weight and a broader dispersity (entry 4 in Table 3). Complex 7e, which contains a bulky pivalate anion, resulted in the highest activity, achieving full conversion within just 30 min (entry 5 in Table 3). All of the anions with higher nucleophilicity resulted in narrow dispersities, indicating fast initiation. It is unclear why the anion would affect the properties of the polymer after initiation, but our group has previously observed similar effects with phosphazenium cocatalysts in the polymerization of epoxides.44 The 1H NMR spectra indicated that all complexes were clean, but we cannot rule out an undetected trace impurity as the source of the inconsistency. Pivalate was also tested with [PPN]+ (8) and showed comparable results to 4, although the activity and Mn were inferior to the phosphazenium system 7e (entry 6 in Table 3). Decreasing the amount of 7e led to an increase in the molecular weight of the P3HP (entries 7 and 8 in Table 3). Complex 7e was also tested for the ring-opening polymerization of the bulkier methyl-substituted β-butyrolactone monomer but low activity was observed (entry 9 in Table 3). In conclusion, we have successfully improved a two-step procedure to turn the inexpensive feedstocks ethylene oxide and carbon monoxide into high molecular weight poly(3hydroxypropionate) with narrow molar mass distributions. Improved polymerization conditions and inexpensive starting materials provide a promising avenue for the development of P3HP into an industrially relevant polymer. This process also represents a new, nonpropylene based route to the important commodity chemical acrylic acid using known methods of PL rearrangement or pyrolysis of P3HP.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b02773. Experimental procedures, NMR spectra, GPC chromatogram, and mechanistic experiments (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Currently at: Unilever, Trumbull, CT 06611, USA.

Notes

The authors declare the following competing financial interest(s): G.W.C. is a cofounder of, and has an equity stake in, the company Novomer, which is commercializing epoxide carbonylation technology.

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ACKNOWLEDGMENTS We thank Drs. Scott Allen and Kevin Noonan for advice regarding research and drafting of this manuscript. We thank the DOE (No. DE-FG02-05ER15687) and Novomer for support of this research, and J.R.L. thanks the NSF (No. DGE-1144153) for a graduate fellowship. 8222

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