Synthesis and characterization of [Cr,Al]-ZSM-5 zeolites

[Cr x ,Al y ] silicates with MFI-type structure were synthesized from synthesis gels with a Si/(Cr + Al) molar ratio of 23, and compositions of chromium and aluminum varying from 0 to 4. The solids were characterized by chemical analysis, XRD, infrared spectroscopy, magic angle spinning n.m.r., SEM-TEM, N 2 adsorption, and e.s.r. All solids were highly crystalline materials. The e.s.r. study shows the presence of Cr(III) in different magnetic environments, depending upon the treatments of the samples. Extensive oxidation of occluded Cr(III) clusters to Cr(VI) species occurred after calcination, whereas the presence of structural and nonstructural Cr(V) species was also detected. It was observed that the presence of chromium ions in the synthesis gel increases the amount of structural aluminum in the zeolites, which produces [Cr,Al] silicates with acidity and activity in the n -heptane cracking reaction, higher than conventional Al silicates.