Factors determining the suitability of zeolite BEA as para-selective nitration catalyst

The nitration of toluene and 2-nitrotoluene with acetyl nitrate was investigated using several solid acid catalysts. In the nitration of toluene, enhanced para-selectivity was observed only with zeolite BEA. For the nitration of 2-nitrotoluene, all the catalysts displayed good para-selectivity, but only BEA led to high yields. The superior performance of BEA was due to the fact that only with this zeolite did the heterogeneously catalyzed reaction compete successfully with the homogeneous nitration in the liquid phase. Geometrical considerations showed that the high para-selectivity of BEA was not caused by classical transition-state selectivity but was induced by the reaction of the arene with a surface-bound acetyl nitrate complex. The existence of this complex was supported by NMR measurements, which showed that its formation is a specific property of zeolite BEA.