Dr Stylianos D. Stefanidis received training and gained valuable experience in catalyst synthesis and characterisation methods in Aston University, as well as in the University of Cordoba during a one-month research visit. ZSM-5 zeolites with varying degrees of mesoporosity were synthesised from a microporous ZSM-5 sample by desilication and characterised.
Additionally, a method for the shaping of the zeolite powders into aggregates with clay binders was developed, in order to produce catalyst particles suitable for testing in fluidised bed reactors. A practical method was developed using available laboratory equipment to produce rigid wafers of ZSM-5 catalysts with bentonite as binder, which could be crushed in a mortar and sieved to obtain particles of desired size. The potential synergistic interaction of the zeolite and bentonite phases in the aggregates was also investigated. It was observed that during the mixing of zeolite powders with bentonite, mild desilication of the zeolite took place, leading to slightly higher mesopore surface area and slightly lower micropore in the aggregates than expected. The impact on the surface and pore properties was very limited and it was reasonably not expected to impact the comparative performance of the microporous ZSM-5 aggregates vs. the mesoporous ZSM-5 aggregates during testing. The impact of mixing with bentonite on the acid sites of the zeolite phases more significant as Na+ ions from bentonite ion-exchanged with the H+ of the Brønsted acid sites during mixing, leading to their deactivation. This effect was successfully mitigated by acid washing the bentonite before mixing in order to remove Na.
The mesoporous ZSM-5 zeolites were tested for the CFP of beech wood in a Py-GC-MS-FID system and their performance was compared to a conventional microporous ZSM-5. All mesoporous ZSM-5 zeolites exhibited significantly enhanced activity and selectivity towards the formation of valuable deoxygenated products such as monoaromatic hydrocarbons compared to the microporous ZSM-5. Differences in the performance of the mesoporous ZSM-5 zeolites were observed depending on the degree of developed mesoporosity, however, the differences were not substantial and, in some cases, they were within experimental error. As such, it was concluded that mild desilication with low-concentration NaOH solutions (e.g. 0.2M) microporous ZSM-5 zeolite (SiO2/Al2O3=80) was optimal to obtain high yields of mesoporous ZSM-5 zeolites with well-developed mesopores and mesoporous surface area that significantly increased the activity and selectivity of the material in the CFP of biomass.
Moreover, aggregates of microporous and mesoporous ZSM-5 zeolites with bentonite were tested. It was found that mixing the zeolites with bentonite had a significant detrimental impact on their activity. However, the mesoporous ZSM-5 aggregates exhibited higher activity than the microporous ZSM-5 aggregates, in agreement with the trends observed when using pure zeolites and it was concluded that the aggregates could be utilised for catalyst testing in larger fluidised bed reactors without masking the differences in the performance of the microporous vs. the mesoporous zeolites. The detrimental effect of bentonite could potentially be mitigated by acid washing the bentonite to remove Na before mixing with the zeolite powders, as indicated by the work described above.
