EFFECT OF CRYSTALLITE SIZE ON THE ACTIVITY AND POISON RESISTANCE OF A SHAPE-SELECTIVE ZEOLITE

Using a series of constant SiO2/Al2O3 ratio ZSM-5 crystallites, which varied over almost 2 orders of magnitude in size, we demonstrate the cracking of n-dodecane to be what one expects for a first-order reaction limited by diffusion within the zeolite pores. At 630 K and 35 atm of hydrogen, the intrinsic activity of the crystals is independent of crystals sizes while the observed activity is adequately described by using effectiveness factors derived from classical reaction-diffusion theory. Deactivation by coke deposition is also independent of crystallite size and is only a function of time on stream. Shape selectivity is demonstrated by poisoning with a relatively bulky nitrogen-containing compound (5,6-benzoquinoline), which diffuses more slowly than n-dodecane due to steric constraints. The poisoning is well described as a shell progressive phenomenon with an active core of zeolite and a poisoned outer shell. Thus, resistance to catalyst poisoning increases with the square of the crystallite size.