Investigating the interplay of hydrogen transfer, protolytic cracking, and dehydrogenation reactions over faujasite zeolites by using isooctane conversion as a probe†

Abstract

Isooctane can serve as a useful probe to investigate hydrogen transfer reactions in faujasite zeolites. Its unique nature, containing both quaternary and ternary carbons, allows for appreciable protolytic cracking and hydrogen transfer reaction rates under modest reaction conditions where zeolite structural integrity is preserved. We show how a simple plot of C₄ alkane selectivity vs. C₁ selectivity allows for visible quantification of the tradeoffs between protolytic cracking and hydrogen transfer pathways. Similarly, we illustrate how deviations from a linear relationship between these two products can be used to quantify dehydrogenation rates. The role of metal cations such as Na, Ca, and Co as titrants was explored to modify the rates of these reaction pathways, enabling quantitative assessment of cation titration at not only promoting new pathways, but selectively titrating sites that are most active for protolytic cracking. We further contrast this simple approach in ternary diagrams that visually depict the contributions of catalyst modifications and reaction conditions on the three parallel reactions, revealing that Na selectively titrates sites responsible for protolytic cracking while Co promotes dehydrogenation reactions. The effects of isooctane conversion, reaction time on stream, and isooctane partial pressure on isooctane cracking selectivity are discussed. We have also applied the quantitative analysis derived from the isooctane cracking probe to explain the product distribution of n-heptane cracking, an example of a linear alkane that exhibits a greater variety of products. This comparison demonstrates that the trends observed in isooctane cracking for a series of catalysts are translatable to n-heptane cracking, highlighting the practical application of the isooctane cracking probe to more complex feeds. Finally, we reveal that the incorporation of Lanthanum, often added to Y zeolites in industrial catalytic cracking operations, enhances both hydrogen transfer rates and dehydrogenation rate of alkanes. This comprehensive approach improves our understanding of catalyst performance and reaction mechanisms while offering insight for practical applications.