Insights in starch acetylation in sub- and supercritical CO2

An in-depth study on the acetylation of starch with acetic anhydride (Ac2O) and sodium acetate (NaOAc) as the catalyst in pressurized carbon dioxide (scCO2) in a broad pressure range (8–25 MPa) and a temperature of 90 °C is provided. Highest degrees of substitution (DS) of 0.29 (1 h reaction time) and 0.62 (24 h reaction time) were found near the critical point of the mixture (15 MPa). The phase behavior of the system CO2, starch and acetic anhydride (Ac2O) was studied in a high pressure view cell. The critical points were a clear function of the temperature and increased from the range of 9.4–10 MPa to 14.5–14.8 MPa when going from 50 to 90 °C (Ac2O mole fraction at the critical point in the range of 0.08–0.09). Acetylation experiments with a range of starch particles sizes showed a clear relation between the DS and the particle size.

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► Starch acetylation in scCO2 leads to products with a wide range of DS (DSmax = 0.62).
► Pressure is a very suitable process parameter to tune the DS.
► The phase behavior of starch/CO2/Ac2O mixtures was studied in a view cell.
► A clear relation between the product DS and the particle size was observed.