Thermolytic molecular precursor routes to Cr/Si/Al/O and Cr/Si/Zr/O catalysts for the oxidative dehydrogenation and dehydrogenation of propane

Abstract

Cothermolyses of ( t BuO) 3CrOSi(O t Bu) 3 ( 1) and either [Al(O t Bu) 3] 2 ( 2) or Zr[OCMe 2Et] 4 ( 3) as solutions in n-octane resulted in formation of gels that yielded Cr/Si/Al/O or Cr/Si/Zr/O xerogels upon drying. These materials have high surface areas after calcination at 773 K (ranging from 150 to 450 m 2 g −1 ) and do not exhibit phase separation of Cr 2O 3 domains after calcination at 1473 K (by PXRD). The as-synthesized Cr/Si/M materials contain mostly octahedral Cr 3+ species with small amounts of tetrahedral Cr 6+ chromates (by DRUV–vis and XANES spectroscopies). Only Cr 6+ monochromate centers, with smaller amounts of di- and polychromate species, were detected after calcination at 773 K under O 2. These catalysts are active for the oxidative dehydrogenation (ODH) of propane; however, they exhibit poor selectivities for propylene production. In contrast, these catalysts exhibit high selectivities for propylene (>95%) in the nonoxidative dehydrogenation (DH) of propane with conversions for the best catalyst above 35% at 723 K.