Comparative Study of the Catalytic Stability of Bifunctional

Nanoporous-based Catalysts in n-Heptane Hydroisomerisation

Faisal Alotaibi (Presenting Author), King Abdulazeez City for Science & Technology, (KACST) National Center for Petrochemical Technology, Riyadh, Saudi Arabia,

Catalysts used in heterogeneous catalytic processes generally experience a decrease in activity over a period of time. The time taken for the activity of a catalyst to fall to an undesirable level depends on several factors. In acidic zeolites, the main cause is the deposition of coke. The cost of catalyst deactivation is very high in industrial processes; therefore catalysts with superior time stability are very valuable. Zeolites with large pores, which give some shape-selective characteristics, particularly to high-silica zeolites, are attractive catalysts for reforming-type reactions. The time stability of these catalysts is one of the most significant factors in determining their use. While some loss of catalytic activity is inevitable, there are various ways to reduce it.
In this work, some commercial and in-house nanoporous-based catalysts, such as USY, beta and mordenite zeolites, and mesoporous aluminosilicate molecular sieves such as MCM-48 and SBA-15, loaded with metals and acting as mono- and bimetallic bifunctional catalysts, were used for hydroisomerisation experiments in a fixed-bed reactor at pressures between 1 and 15 bar and at feed space time ranging from 2.57 to 10.26 h-1 (35.14 to 140.6 kg s mol-1) to hydroisomerise n-heptane over a temperature range of 210-270 °C. This study evaluates changes with time-on-stream in the catalytic activity, product selectivity and manner of deactivation of metal-loaded zeolite and mesoporous aluminosilicate catalysts, at constant contact time of 5.13 h-1 and a hydrogen-to-n-heptane molar ratio fixed at 9. Various analytical techniques were used to characterise fresh and aged catalysts.
Results show that various factors were responsible for the deactivation of several metal loaded micro- and mesoporous catalysts, which were thoroughly examined. These results agree with literature reports that pore architecture is the most important factor affecting coke formation and deactivation in zeolite catalysts. Thus, those catalysts with three-dimensional pore structures lacking cavities or cages, such as zeolite beta, were best able to resist deactivation. It was found that highly acidic bifunctional catalysts such as mordenite lost their activity rapidly during the initial few hours and that conversion then reached a constant â??level-offâ? value after a few hours on stream. In contrast, those catalysts with high Si/Al ratios and those which had been acid-leached or steamed showed better activity, higher selectivity towards isomeric products and better time stability. Moreover, the balance between the number of metal sites and the number of acid sites played an important role in determining the activity, selectivity and stability of the bifunctional catalysts. The higher metal loading improved catalytic stability, due to a better balance and closeness of the catalytic functions. In addition, the bimetallic catalysts improved the formation of metal particles to a great extent and reduced their sizes, which may have affected their selectivity and stability.