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

Abstract

In this study, seven types of α-hydroxycarboxylic acids were selected to form composite catalysts with boric acid, and their catalytic properties were studied using the catalytic hydration of α-pinene. The results showed that the composite catalyst of boric acid and tartaric acid had the highest catalytic activity. With an α-pinene, water, acetic acid, tartaric acid, and boric acid mass ratio of 10:10:25:0.5:0.4, the reaction temperature was 60 °C, the reaction time was 24 h, the conversion of α-pinene was 96.1%, and the selectivity of terpineol was 58.7%. The composite catalyst composed of boric acid and mandelic acid directly catalyzed the hydration of α-pinene in the absence of a solvent. Under the optimal conditions, the conversion of α-pinene reached 96.1%, and the selectivity of terpineol was 55.5%. When the composite catalyst catalyzed α-pinene to synthesize terpineol in one step, the terpineol was optically active, and terpineol synthesized using the two-step method with the dehydration of p-menthane-1,8-diol monohydrate was racemic. These composite catalysts may offer good application prospects in the synthesis of terpineol.

Effect of Boric Acid on the Ionization Equilibrium of α-Hydroxy Carboxylic Acids and the Study of Its Applications

“…When an HCA containing multiple carboxyl groups, such as tartaric acid, malic acid, and citric acid, is ionized in an aqueous solution, if only first-order ionization is taken into account and it is assumed that only single-or double-ligand complex products with negative charges are formed with boric acid, then its ionization equilibrium constant can be obtained from Formulas (5) and (10). The acidity coefficient of HCA after adding boric acid was calculated by Formulas ( 7) and (12), and the corresponding acidity coefficients at the apexes of the curves in Figures 4-6 are shown in Table 1. When boric acid was added to the HCA aqueous solutions, the acidity coefficients of the solutions with monoligand and diligand complexes are shown as pKa values.…”

“…In addition, the electrical conductivity and rotation of the complex formed by tartaric acid and boric acid are increased [ 10 , 11 ]. The hydration reaction of α-pinene catalyzed by HCA and boric acid can increase the conversion rate of α-pinene [ 12 ]. In the synthesis of isobornyl acetate and isoborneol catalyzed by HCA and boric acid composite catalysts from camphene, HCA and boric acid show significant synergistic catalytic effects [ 13 ].…”

Complexation of Boronic Acid with Chiral α-Hydroxycarboxylic Acids and the Ability of the Complexes to Catalyze α-Hydroxycarboxylic Acid Esterification

“…Our previous work showed as catalysts for the hydration reaction of α-pinene, the synergistic catalytic abilities of BA, phosphoric acid, and sulfuric acid with HCA decreased sequentially [ 27 , 29 ]. This was possibly related to the fact that BA was more likely to form a complex with HCA, which could better promote the forward shift of the ionization equilibrium of HCA.…”

“…Then, methyl tartrate underwent ester exchange with palmitic acid to generate methyl palmitate, and turpentyl tartrate was hydrolyzed to obtain terpineol. Because the intermediate hydroxycarboxylic acid methyl ester was more easily generated than hydroxy acid terpinyl ester and borneol ester, when BA and HCA were used as catalysts, the yield of palmitic acid methyl ester could reach 98%, the yield of terpineol was about 55%, and the yield of borneol was less than 40% [ 27 , 28 ].…”

“…As a result, citric acid-BA complexes have been used to improve the dimensional and thermal stability of bamboo materials [ 25 ]. Composite catalysts composed of BA and HCA were found to exert a good synergistic catalytic effect during the catalysis of the esterification reaction of camphene and the hydration reaction of pinene [ 26 , 27 ]. In a previous study [ 28 ], we analyzed and measured the effects of complexation reactions between BA and HCAs on the ionization equilibrium of the HCAs.…”