SELECTIVE C-ALKYLATION OF PHENOL WITH METHANOL OVER CATALYSTS DERIVEDFROM COPPER-ALUMINUM HYDROTALCITE-LIKE COMPOUNDS

Copper aluminium hydrotalcites (CuA1-HT) with a Cu/Al atomic ratio of 2, 3 and 4 (CuAl 2.0-HT, CuA1 3.0-HT, CuAl 4.0-HT) were synthesised by the coprecipitation method using a mixture of NaOH/Na2CO3 as precipit ant. The pure crystalline hydrotalcite (HT) phase was detected by X-ra y diffraction when the copper-aluminium hydrotalcite was prepared with out ageing (CuAl 3.0-WAHT). Vapour phase alkylation of phenol with met hanol was carried out over CuAl-HT calcined at 450 degrees C/8 h in ai r (CuA1-CHT). The activity and selectivity of these catalysts were com pared with those of magnesium-aluminium calcined hydrotalcites (MgAl 4 .0-CHT) from our earlier report. MgAl 4.0-CHT showed higher selectivit y for 2,6-xylenol whereas CuA1-CHT was selective for o-cresol. Among t he CuAl-CHT studied, CuAl 3.0-CHT showed higher phenol conversion at l ow temperatures (250-350 degrees C). The catalyst derived from pure co pper-aluminium hydrotalcite (CuAl 3.0-WACHT) showed lowest phenol conv ersion compared to other copper based catalysts obtained from a mixtur e of HT and malachite phases. The selectivity for o-cresol decreased i n the order CuAl 3.0-CHT > CuA1 2.0-CHT > CuAl 3.0-WACHT approximate t o 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 methan ol 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 CuA1-CHT could be attributed to the higher acidit y of these catalysts as evidenced by the higher cyclohexanol dehydrati on activity.