Selective C-alkylation of phenol with methanol over catalysts derived from copper-aluminium hydrotalcite-like compounds

Abstract

Copper aluminium hydrotalcites (CuAl-HT) with a Cu/Al atomic ratio of 2, 3 and 4 (CuAl 2.0-HT, CuAl 3.0-HT, CuAl 4.0-HT) were synthesised by the coprecipitation method using a mixture of NaOH/Na2CO3 as precipitant. The pure crystalline hydrotalcite (HT) phase was detected by X-ray diffraction when the copper-aluminium hydrotalcite was prepared without ageing (CuAl 3.0-WAHT). Vapour phase alkylation of phenol with methanol was carried out over CuAl-HT calcined at 450°C/8 h in air (CuAl-CHT). The activity and selectivity of these catalysts were compared with those of magnesium-aluminium calcined hydrotalcites (MgAl 4.0-CHT) from our earlier report. MgAl 4.0-CHT showed higher selectivity for 2,6-xylenol whereas CuAl-CHT was selective for o-cresol. Among the CuAl-CHT studied, CuAl 3.0-CHT showed higher phenol conversion at low temperatures (250–350°C). The catalyst derived from pure copper-aluminium hydrotalcite (CuAl 3.0-WACHT) showed lowest phenol conversion compared to other copper based catalysts obtained from a mixture of HT and malachite phases. The selectivity for o-cresol decreased in the order CuAl 3.0-CHT> CuAl 2.0-CHT> CuAl 3.0-WACHT ≈ CuAl 4.0-CHT> MgAl 4.0-CHT at low phenol conversion levels (15–20%). A suitable reaction pathway for the alkylation of phenol with methanol over these catalysts has been proposed after a careful examination of the reaction sequence. The selective C-alkylation to give o-cresol and 2,6-xylenol over CuAl-CHT could be attributed to the higher acidity of these catalysts as evidenced by the higher cyclohexanol dehydration activity.