Thermomorphic Polyethylene-Supported Organocatalysts for the Valorization of Biomass and CO2 – ThermoPESO

Thermomorphic polyethylene (PE) exhibits extremely different solubilities over a narrow range of temperatures. PE becomes completely soluble in hot non-polar solvents, but it is totally insoluble in the same solvents at room temperature. Applied to catalysis, catalysts supported onto thermomorphic PE would combine some advantages of homogeneous catalysis, with reactions carried out above the solubilization point, to those of heterogeneous catalysis, since a simple lowering of the temperature allows precipitation of the catalyst, and thus easy separation and recovery. However, to the best of our knowledge, thermomorphic polyethylene has never been used as a support for organocatalysts. The main scientific barrier is the synthesis of such hybrid organic/polymeric catalysts that requires different expertise to be successful. To lift this barrier, the project will be gathering organic chemists from the “Institut de Chimie et Biochimie Moléculaires et Supramoléculaires” (ICBMS) and polymer chemists from the laboratory of “Chemistry, Catalysis, Polymers and Processes” (C2P2) to design new catalytic species incorporating an organic catalyst and a polymeric part.

Several batches of iodine end-functionalized polyethylene (PE-I) were prepared with Mn of 770 to 1800 g mol-1. Imidazolium type organocatalysts were grafted onto the PE derivatives by varying the length of the PE chain, the nature of the substituents and the counterion of the catalysts. These species were characterized by NMR, MALDI-ToF-MS, SEC and DSC.
The study of the catalytic activity of the supported PE-based organocatalysts was carried out on the reaction of formation of cyclic carbonates from epoxides and CO2. This work demonstrates that organocatalysts are active at low catalytic loading (from 0.1 to 4 mol%) and at low CO2 pressure (5-10 atm) under solvent-free conditions. Under these conditions, a range of terminal and internal epoxides were converted to corresponding cyclic carbonates in high yields and selectivities. Unlike conventional supported organocatalysts, the use of thermomorphic polyethylene as a support does not decrease the catalytic activity. The recyclability of the catalyst was studied and validated over five runs. In addition, the integrity of the organocatalysts is preserved after recycling.
Subsequently, organocatalysts supported on thermomorphic PE were used to prepare carbonates from CO2 and epoxides derived from vegetable oils. A range of epoxidized fatty acids, and epoxidized rapeseed oil, was converted to carbonates in good yields (75-96%, 9 examples). Recycling studies showed that the catalyst can be recovered and recycled over 10 runs without significant loss of activity. The developed organocatalysts constitute the first recyclable catalytic species for this valuable transformation, which allows valorising molecules derived from biomass and CO2.

The proof of concept being obtained and validated on several known transformations, we are currently interested in the use of organocatalysts supported on thermomorphic polyethylene for the study of new transformations in which we could take advantage of the remarkable properties of these species.

[1] “A thermomorphic polyethylene-supported imidazolium salt for the fixation of CO2 into cyclic carbonates”. K. Grollier, N. D. Vu, K. Onida, A. Akhdar, S. Norsic, F. D’Agosto, C. Boisson, N. Duguet, Adv. Synth. Catal. 2020, 362, 1696–1705.
DOI: 10.1002/adsc.202000032

[2] “Thermomorphic polyethylene-supported organocatalysts for the valorization of vegetable oils and CO2”. A. Akhdar, K. Onida, N. D. Vu, K. Grollier, S. Norsic, C. Boisson, F. D’Agosto, N. Duguet, Adv. Sustainable Syst. 2021, 5, 2000218.
DOI: 10.1002/adsu.202000218

The valorization of biomass and CO2 has now become a necessity in the long term and offers a great challenge to chemists and biochemists. In this context, catalytic transformations are playing a crucial role and organocatalysis can present several distinct advantages: organocatalysts do not rely on heavy metals, exhibit low toxicity and can mimic enzymes’ activity. However, the use of organocatalysts has barely been investigated for the transformation of biomass or CO2. This could be explained by the fact that organocatalysis has inherent drawbacks such as high catalyst loadings, moderate stability, difficult recoverability and recyclability. To tackle these issues, the ThermoPESO project aims at developing thermomorphic polyethylene-supported organocatalysts. The main interests for developing such catalytic species are to obtain thermally robust, recoverable and reusable organocatalysts that will enable efficient and selective transformations of biomass and CO2 without compromising the catalytic efficiency.