Study On The Catalytic Synthesis Of Methoxytoluene Using Dimethyl Carbonate As Methylating Reagents

The catalytic synthesis of methoxytoluene via the reaction of cresol with dimethyl carbonate (DMC) as methylating reagent under vapor-solid phase conditions was studied.Thermodynamic calculations demonstrated that the gaseous reaction of o-cresol with DMC was endothermic and high temperature was favored. The theoretic conversion of cresol reached 100% as temperature was up to 400 K.The catalytic activities of active carbon (AC) loaded with alkali metal salts by vacuum impregnation were studied. KOH/AC (10 wt%) showed the highest activity. A 98% conversion of p-cresol with a 99.6% selectivity of p-methoxytoluene was achieved at the reaction conditions: temperature 548 K, pressure 1.1 MPa, GHSV 1845 h^-1, n(DMC)/n(p-cresol) 1.5:1. Although KOH/AC exhibited the excellent catalytic activity for the methylation at the experimental conditions, it was found to deactivate quickly after about 8 h reactions. The possible reason was related to significant reduction in surface area and pore volume of the catalyst as indicated by characterization results.Using NaY loaded with KOAc (2 mmol/g) by vacuum impregnation and microwave radiation treatment, a o-cresol conversion of 83% with a 99.7% selectivity of o-methoxytoluene was obtained at the optimized reaction conditions: temperature 548 K, pressure 0.6 MPa, GHSV 870 h^-1, n(DMC)/n(p-cresol), 1. Both vacuum impregnation and microwave radiation treatment improved the dispersion of KOAc on the surface of NaY, and the catalysts showed good thermal stability between 450-600 K as indicated by XRD and TG-DTA characterization. Under the optimum conditions the activity of the catalyst was stable within 30 h reaction. Characterization results (XFD, SEM, CO₂-TPD and N₂ absorption) suggested that surfaces of the catalysts after the tests were covered by carbon deposition from DMC decomposition at high temperature and pores of the catalyst were jammed, which caused the activity lost.The comparative investigation for the synthesis of methoxytoluene(o-, m-, p-) over KOAc/NaY catalyst at the optimized conditions showed that a similar reaction behavior with conversions of cresols all higher than 76% and selectivities of methoxytoluene above 99%. It is also demonstrated that DMC is a much more efficientmethylating agent than methanol for the O-methylation of cresol over KOAc/NaY (2 mmol/g).