Pt agglomeration and entombment in single channel zeolites: Pt/LTL

This paper presents a transmission electron microscopy study of the role of Pt agglomeration and channel blockage in the deactivation of Pt/K-zeolite L aromatization catalysts. A Z-contrast imaging study of Pt particle size distributions shows that there is an increase in the number of channel blockages by Pt with increasing catalyst reaction time. Modeling shows that when agglomeration proceeds so that there is a likelihood of two or more blockages per channel, significant quantities of Pt become entombed. This problem is exacerbated by long zeolite L channel lengths. Conversely, if the channels are too short, Pt seepage to the external surfaces becomes significant and the rate of coke production increases, again resulting in loss of accessible Pt. Ideally, the best Pt/KL catalysts have all of the Pt inside the channels in a finely dispersed state. It is argued that there exists an optimum 'length-loading' window for Pt/KL catalysts:0.25≤wl≤0.5where w is the percentage of Pt by weight in the zeolite, and l is the channel length (μm). The upper limit minimizes the deleterious effects of multiple channel blockages by ensuring that, on average, there is not enough Pt in the channels to form two or more blockages. The lower limit guards against Pt seepage to the exterior surfaces when channel lengths are too short.