Synthesis of Terpineol from Alpha-Pinene Catalyzed by α-Hydroxy Acids

Abstract

We report the use of five alpha-hydroxy acids (citric, tartaric, mandelic, lactic and glycolic acids) as catalysts in the synthesis of terpineol from alpha-pinene. The study found that the hydration rate of pinene was slow when only catalyzed by alpha-hydroxyl acids. Ternary composite catalysts, composed of AHAs, phosphoric acid, and acetic acid, had a good catalytic performance. The reaction step was hydrolysis of the intermediate terpinyl acetate, which yielded terpineol. The optimal reaction conditions were as follows: alpha-pinene, acetic acid, water, citric acid, and phosphoric acid, at a mass ratio of 1:2.5:1:(0.1–0.05):0.05, a reaction temperature of 70 °C, and a reaction time of 12–15 h. The conversion of alpha-pinene was 96%, the content of alpha-terpineol was 46.9%, and the selectivity of alpha-terpineol was 48.1%. In addition, the catalytic performance of monolayer graphene oxide and its composite catalyst with citric acid was studied, with acetic acid used as an additive.

Study on the Hydration of α-Pinene Catalyzed by α-Hydroxycarboxylic Acid–Boric Acid Composite Catalysts

“…In the one-step synthesis of terpineol from α-pinene catalyzed by HCA–boric acid, terpineol had optical activity, l -pineol was produced when l -pinene was used, and d -terpineol was produced when d -pinene was used. The enantiomeric excess (ee) value of the terpineol was 71%, similar to the value reported in [ 13 ], where the ee value of terpineol was about 70%. The results of the α-pinene hydration reaction catalyzed by HCA and boric acid in the solvent-free condition are shown in Table 2 , indicating that the mandelic acid–boric acid composite catalyst had the best catalytic effect, followed by the lactic acid–boric acid composite catalyst, while the glycolic acid–boric acid composite catalyst had the worst catalytic effect.…”

“…The α-pinene hydration under the catalysis of boronic acid complexes with HCAs followed the same reaction mechanism. As indicated in a previous study [ 13 ], l -terpineol could be obtained by using l -α-pinene as the starting material, and d -terpineol could be obtained by using d -α-pinene as the starting material. In addition to isomeric byproducts, terpinyl acetate was also a significant byproduct.…”

“…As described by Meng et al [ 13 ], HCAs can catalyze the synthesis of α-pinene to terpineol but have low catalytic activity. Thus, the use of H 2 SO 4 or H 3 PO 4 is needed to improve the catalytic activity.…”

“…Because chloroacetic acid is highly corrosive and toxic, we studied the catalytic hydration of α-pinene with α-hydroxycarboxylic acid (HCA) [ 13 ]. We found that the reaction rate of α-pinene was very slow when HCA was used as a catalyst alone.…”

“…Because HCA and boric acid were easily soluble in water, the catalyst and water were in the lower layer and the product was in the upper layer after the reaction. Then, we used the byproduct of the pinene hydration reaction as an additive in the reaction, and the results showed that the selectivity of terpineol was improved [ 17 ]. Thus, the acid water containing the catalyst in the lower layer could be directly used for the next reaction.…”