Investigation of the Effects of Non-Thermal Plasma and Microwaves on Mordenite

Abstract

Zeolites constitute a multi-billion pound industry, with no indication of diminishing in value. Preliminary studies at the University of Central Lancashire discovered that the regeneration of a sample of coked mordenite via microwave plasma extended the catalytic life of the zeolite when used in the toluene disproportionation reaction. As a joint-venture between the University of Central Lancashire and Johnson Matthey, this research was designed with the purpose of investigating the effects of microwaves and plasma to determine whether they could be used as novel methods of zeolite modification.
The application of microwaves in synthetic zeolite chemistry is well established, however the application of microwaves for post-synthesis modification of zeolites is much more limited. Microwave regeneration has been reported to display higher efficiencies and reduced regeneration times compared to conventional regeneration. However, there are, to this author’s knowledge, no studies performed on the regeneration of coked zeolites using microwaves. Dealumination of zeolites using microwaves is also very limited, with results reporting faster dealumination rates and a more pronounced decrease in crystallinity compared with conventional methods.
The application of plasma in zeolite chemistry is limited to zeolite calcination, regeneration and surface modification. Plasma regeneration of a coked zeolite is, to this author’s knowledge, limited to a single study, where focus was on removing the carbon. There are limited studies on the effect of plasma on zeolite acidity. Results suggest plasma can be used to increase the density of Brønsted acid sites, which can alter product selectivity within a reaction.
This research investigated the effects of microwaves and plasma on the zeolite mordenite. Microwaves, microwave plasma and dielectric barrier discharge plasma were applied to samples of virgin and coked mordenite. Using toluene disproportionation as the probe reaction, changes in catalytic activity were observed. Characterisation methods including pyridine and collidine infrared studies, ammonia temperature programmed desorption and solid state nuclear magnetic resonance were used to explain the changes in catalytic activity. Results showed microwave plasma regeneration extended the catalytic life of mordenite due to the destruction of Brønsted acid sites caused by dealumination, without loss of crystal structure. Whilst the loss of Brønsted acid sites was also seen in microwave treated mordenite and microwave plasma treated virgin mordenite, it was accompanied by the loss of catalyst crystallinity.
These results have shown that microwave plasma can be used to fully regenerate coked mordenite and can dealuminate the sample without loss of catalyst crystallinity. In the toluene disproportionation reaction, this reduces the amount of cracking which occurs, subsequently leading to less coke deposition and therefore an extended catalytic life.