Hydroconversion of hydrocarbons over HZSM5 and Mo-HZSM5 catalysts: A FTIR and flow reactor study

The interactions of n-heptane, benzene, and toluene with HZSM5 and Mo-HZSM5 catalysts were studied by FTIR spectroscopy. The results from the FTIR stu dy on the interactions of single molecules with these catalysts are compare d with the product distribution results obtained during the hydroconversion of a model mixture of n-heptane-benzene-toluene over HZSM5-alumina and Mo- HZSM5-alumina catalysts. The FTIR study indicates that there are interactio ns of heptane, benzene, and toluene with both the strongly acidic internal OH's and the less acidic silanol groups in HZSM5. These interactions seem t o be a precursor for the formation of carbenium ions, which are the active species in hydroconversion reactions. The incorporation of Mo into HZSM5 ca uses a strong decrease in the number of external silanols, suggesting that some of the impregnated Mo species are located at the external zeolite surf ace. It is also evident from the FTIR results that the incorporation of Mo does not entirely destroy or exchange the internal OH's of HZSM5. The addit ion of Mo to the zeolite, in addition to causing a decrease in the overall conversion of heptane, causes a marked simplification in the pattern of the main reaction products. The cracking and alkylation functionalities of the catalyst are almost suppressed, whereas a significant increase in skeletal isomerization is observed. This suggests, in line with the FTIR experiment s, that the incorporation of Mo into the HZSM5-alumina catalyst causes a de crease in the availability of the strong Bronsted acid sites located in the cavities of the zeolite.