The mixing sensitivity of toluene and ethylbenzene sulfonation using fuming sulfuric acid studied in a rotor-stator spinning disc reactor

The mixing sensitivity was studied for the sulfonation of two alkylbenzenes (ArH's), toluene and ethylbenzene, by using a rotor-stator spinning disc reactor (SDR). Fuming sulfuric acid (FSA), containing 30 wt. % free sulfur trioxide, was used as a sulfonating agent, which resulted in a fast, exothermic, and viscous liquid-liquid reaction system. Experiments were conducted by varying the SDRs’ rotational speed between 50 and 9000 rpm. In addition, different molar flow ratios (ArH:FSA) were investigated, ranging from 0.8 to 3.2, while operating at a maximum residence time of 7.1 s. The selectivity towards mono-sulfonated products was found to increase significantly at a higher rotational speed. This was observed consistently for both alkylbenzenes and all molar flow ratios, confirming the mixing sensitivity of the two alkylbenzene sulfonation reactions. Furthermore, it was proposed that the mixing sensitivity arises from a competition that exists between the desired and highly reactive primary sulfonation and the much less reactive consecutive sulfonation step to disulfonic acids.