Continuous Flow Metathesis for Direct Valorization of Food Waste: An Example of Cocoa Butter Triglyceride

The direct chemical conversion of cocoa butter triglycerides, a material available as a postmanufacture waste stream from the food industry, to 1-decene by way of ethenolysis is reported. The conversion of the raw waste material was made possible by use of 1 mol % of the [RuCl2(iBu-phoban)2(3-phenylindenyl)] catalyst. The process has been investigated in both batch and flow conditions, where the latter approach employs a Teflon AF-2400 tube-in-tube gas–liquid membrane contactor to deliver ethylene to the reaction system. These preliminary studies culminate in a continuous processing system, which maintained a constant output over a 150 min period tested.


General Experimental Considerations
The waste cocoa butter was provided by Cadbury's (now Kraft). Ethylene was purchased from BOC Ltd. Solvents and reagents were obtained from Sigma Aldrich.
Catalysts were prepared following known procedures. 1 H-NMR spectra were recorded on either a Bruker Avance DPX-400 or DRX-600 (in the case of raw cocoa butter) spectrometer with the residual solvent peak as the internal reference, resonances are reported to the nearest 0.1 ppm. NMR spectra were acquired using 30˚ flip angle and a 3.55 second recycle delay and the data was processed using the standard software baseline and phase correction algorithms. Flow experiments were conducted using a Uniqsis Flowsyn reactor system.

Procedure for batch setup
For the results described in Table 2 at one bar of ethylene pressure the following protocol was used: The catalyst (1 mol %; 0.0025 mmol) and a stirrer bar were added to a 5 mL vial in an argon-filled glove box. The vial was closed using a plastic septa cap. Next, this vial was removed from the glove box. Then, 216.7 mg cocoa butter (0.25 mmol, average M w of cocoa butter) was added into a second 5 mL vial and closed using the septa cap (outside of the glove box). The cocoa butter vial was purged with argon from a Schlenk line before addition of 1 mL of dry and oxygen-free solvent was introduced via syringe into the cocoa butter vial. The cocoa butter was dissolved and then added via syringe into the catalyst vial. The vial was then stirred with an ethylene purge from a balloon. After the allotted time the reaction was quenched with ethyl vinyl ether, before drying under reduced pressure and analysing by 1 H NMR (as described in 'Method for Determining Conversion').

For greater than 1 bar pressure of ethylene:
The charged reaction vial was then loaded into the autoclave. The autoclave was connected to an ethylene cylinder. After the allotted time the reaction was quenched with ethyl vinyl ether, before drying under reduced pressure and analysing by 1 H NMR (as described in 'Method for Determining Conversion').
*It should be noted that by conducting the same reaction using non-dried glassware, without a glovebox and with THF from a standard column type solvent purification system, conversions within 5-10% of those reported were routinely achieved.*

Flow procedure
Experimental set up is illustrated in Figure S1.

Method for Determining Conversion
Following the reaction and solvent removal, CDCl 3 was used to dissolve samples for NMR analysis, typically 100 µL of crude solution in 0.5 mL of CDCl 3 . The development of a method for the determination of conversion for this reaction was not a trivial matter. The logic used for determining the conversion by 1 H NMR spectroscopy is described in Scheme 1. GPC analysis of the starting materials showed that the oleic acid to linoleic acid ratio is 17:3. We started with the assumption that the alkene containing triglycerides underwent full ethenolysis to terminal alkene products resulting in the breaking of all double bonds (rather than partial conversion which would lead to internal alkenes from the linoleic acid). After total ethenolysis of this mixture, five products containing terminal alkene functionality would be present.
Following the removal of solvent and 1,4-pentadiene on a rotary evaporator the resulting mixture would contain four products featuring terminal alkenes in a 17:17:3:3 ratio, clearly the ratio of triglyceride terminal alkenes to lower molecular weight terminal alkenes must be 1:1.

Scheme
Scheme Scheme Scheme S S S S1 1 1 1 The linear chain hydrocarbon, terminal alkene protons all overlap in the 1 H NMR spectrum and appear in the region between 4.75-4.90 ppm. The four equivalent protons of the glycerol backbone, of both the starting material and the derived triglyceride products appear in the region between 4.00 and 4.25 ppm. Figure S2 shows the region of interest from a representative reaction. The following analysis is performed to determine conversion to terminal alkenes (not to 1-decene specifically).
Steps one to three refer to processing the integrals in the spectrum of a sample of pure