Selective propionylation of veratrole to 3,4-dimethoxypropiophenone using zeolite H-beta catalysts

Abstract

The catalytic liquid phase propionylation of veratrole (VT) (1,2-dimethoxybenzene, 1,2-DMB) is investigated over a number of zeolite catalysts. Zeolite H-beta is found to be an effective catalyst for the selective conversion of VT to 3,4-dimethoxypropiophenone (3,4-DMPP). The SiO2/Al2O3 ratio of zeolite H-beta is higher (SiO2/Al2O3) = 26 than the H–Y and RE (70.6)-Y (SiO2/Al2O3 = 4.1). The performance of zeolite H-beta is compared with that of conventional catalyst, AlCl3. The conversion of VT, the rate (turn over frequency, TOF) of VT conversion (10−4 s−1 mol−1 Al), turnover number (TON) (mol of VT converted per mol Al in the catalyst) and the selectivity to 3,4-DMPP over H-beta after 1 h reaction time and at 403K are found to be 40.8 wt.%, 18.8 × 10−4 s−1 mol−1 Al, 67.8 and 89 wt.%, respectively. For comparison, the conversion of VT, TOF, TON and selectivity to 3,4-DMPP over AlCl3 under identical reaction conditions are estimated to be 48.5 wt.%, 3.6 × 10−4 s−1 mol−1 Al, 1.3 and 68.9 wt.%, respectively. The acidity of the zeolite catalysts is measured by the temperature programmed desorption (TPD) of ammonia. The effects of acidity of the zeolite catalysts, reaction time, catalyst concentration, reaction temperature and VT to propionyl chloride (PC) molar ratio on the catalyst performance are examined in order to optimise the conversion of VT and selectivity to 3,4-DMPP. The conversion of VT increases with the increase in reaction time, catalyst concentration and reaction temperature, whereas it decreases with the increase VT/PC molar ratio. The propionylation of 1,3-dimethoxybenzene (1,3-DMB) and 1,4-dimethoxybenzene (1,4-DMB) is also investigated over zeolite H-beta catalyst. The zeolite H-beta was recycled two times with a marginal decrease in catalytic activity, but without the loss of 3,4-DMPP selectivity and crystallinity of H-beta. Mechanistically, the PC is first polarised (CH3CH2COδ+, Clδ−) by the zeolite catalyst. The generated electrophile (CH3CH2CO+) attacks the ring of VT, resulting in the formation of 3,4-DMPP.