Selective Catalytic Epoxidation–Hydration of α-Pinene with Hydrogen Peroxide to Sobrerol by Durable Ammonium Phosphotungstate Immobilized on Imidazolized Activated Carbon

Abstract

It is presented that the activated carbon was carboxylated with hydrogen peroxide and then acylated with 2-methylimidazole to prepare the porous carbon support with a surface imidazolated modification. Through the adsorption of phosphotungstic acid on the fundamental site of an imidazolyl group and then adjusting the acid strength with the ammonia molecule, a catalytic carbon material immobilized with ammonium phosphotungstate (AC-COIMO-NH4PW) was obtained, which was used to catalyze a one-pot reaction of convenient α-pinene and hydrogen peroxide to sobrerol. The bifunctional active site originated from the dual property of ammonium phosphotungstate, as the oxidant and acid presenting a cooperatively catalytic performance, which effectively catalyzes the tandem epoxidation–isomerization–hydration of α-pinene to sobrerol, in which the solvent effect of catalysis simultaneously exists. The sobrerol selectivity was significantly improved after the acid strength weakening by ammonia. Monomolecular chemical bonding and anchoring of ammonium phosphotungstate at the basic site prevented the loss of the active catalytic species, and the recovered catalyst showed excellent catalytic stability in reuse. Using acetonitrile as the solvent at 40℃ for 4 h, the conversion of α-pinene could reach 90.6%, and the selectivity of sobrerol was 40.5%. The results of five cycles show that the catalyst presents excellent stability due to the tight immobilization of ammonium phosphotungstate bonding on the imidazolized activated carbon, based on which a catalytic-cycle mechanism is proposed for the tandem reaction.

Ammonium Phosphotungstate Bonded on Imidazolized Activated Carbon for Selective Catalytic Rearrangement of α-Epoxypinane to Carveol

“…The FTIR spectrum and XRD of activated carbon (AC), phosphotungstic acid supported on imidazolized AC (AC-COIMI-HPW), and ammonium phosphotungstate anchored by imidazolized AC (AC-COIMI-NH4PW) is shown in Figure 1A. The FTIR spectrum of AC in Figure 1A showed that the absorption peaks at 1608 cm −1 and 1400 cm −1 attributed to C=C stretching vibration of the cyclic aromatic skeleton in AC, and there is the characteristic absorption peak of -OH linked at the edge of fused polycyclic aromatic in the bulk of AC at 3130cm In our previous work, it was found that imidazolated nitrogenated activated carbon anchoring phosphotungstate had excellent catalytic performance for benzyl alcohol [24], and the reaction of α-pinene with hydrogen peroxide on ammonium phosphotungstate immobilized on imidazolated nitrogenated activated carbon could selectively convert to sobrerol [25]. In present work, ammonium phosphotungstate anchored by imidazolated nitrogenous activated carbon was developed to catalyze α-epoxypinane ring-opening rearrangement, so as to improve the selective synthesis of carveol and provide a new green [26] catalytic material system from biomass based activated carbon for the preparation of high-value carveol from plentiful and cheap turpentine.…”

“…In our previous work, it was found that imidazolated nitrogenated activated carbon anchoring phosphotungstate had excellent catalytic performance for benzyl alcohol [24], and the reaction of α-pinene with hydrogen peroxide on ammonium phosphotungstate immobilized on imidazolated nitrogenated activated carbon could selectively convert to sobrerol [25]. In present work, ammonium phosphotungstate anchored by imidazolated nitrogenous activated carbon was developed to catalyze α-epoxypinane ring-opening rearrangement, so as to improve the selective synthesis of carveol and provide a new green [26] catalytic material system from biomass based activated carbon for the preparation of high-value carveol from plentiful and cheap turpentine.…”