Catalysts used in heterogeneous catalytic processes generally experience a decrease inactivity over a period of time. The time taken for the activity of a catalyst to fall to anundesirable level depends on several factors. In acidic zeolites, the main cause is thedeposition of coke. The cost of catalyst deactivation is very high in industrial processes;therefore catalysts with superior time stability are very valuable. Zeolites with largepores, which give some shape-selective characteristics, particularly to high-silica zeolites,are attractive catalysts for reforming-type reactions. The time stability of these catalysts isone of the most significant factors in determining their use. While some loss of catalyticactivity is inevitable, there are various ways to reduce it.In this work, some commercial and in-house nanoporous-based catalysts, such asUSY, beta and mordenite zeolites, and mesoporous aluminosilicate molecular sieves suchas MCM-48 and SBA-15, loaded with metals and acting as mono- and bimetallicbifunctional catalysts, were used for hydroisomerisation experiments in a fixed-bedreactor at pressures between 1 and 15 bar and at feed space time ranging from 2.57 to10.26 h-1 (35.14 to 140.6 kg s mol-1) to hydroisomerise n-heptane over a temperaturerange of 210-270 °C. This study evaluates changes with time-on-stream in the catalyticactivity, product selectivity and manner of deactivation of metal-loaded zeolite andmesoporous aluminosilicate catalysts, at constant contact time of 5.13 h-1 and a hydrogento-n-heptane molar ratio fixed at 9. Various analytical techniques were used tocharacterise fresh and aged catalysts.Results show that various factors were responsible for the deactivation of severalmetal loaded micro- and mesoporous catalysts, which were thoroughly examined. Theseresults agree with literature reports that pore architecture is the most important factoraffecting coke formation and deactivation in zeolite catalysts. Thus, those catalysts withthree-dimensional pore structures lacking cavities or cages, such as zeolite beta, were bestable to resist deactivation. It was found that highly acidic bifunctional catalysts such asmordenite lost their activity rapidly during the initial few hours and that conversion thenreached a constant "level-off" value after a few hours on stream. In contrast, thosecatalysts with high Si/Al ratios and those which had been acid-leached or steamedshowed better activity, higher selectivity towards isomeric products and better timestability. Moreover, the balance between the number of metal sites and the number ofacid sites played an important role in determining the activity, selectivity and stability ofthe bifunctional catalysts. The higher metal loading improved catalytic stability, due to abetter balance and closeness of the catalytic functions. In addition, the bimetallic catalystsimproved the formation of metal particles to a great extent and reduced their sizes, whichmay have affected their selectivity and stability.
|Date of Award||1 Aug 2012|
- The University of Manchester
|Supervisor||Arthur Garforth (Supervisor)|