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Improving the economic feasibility of the biorefinery through catalysis engineering: enhancing the catalyst performance and optimizing valuable product yields

Periodic Reporting for period 1 - ECOCAT (Improving the economic feasibility of the biorefinery through catalysis engineering: enhancing the catalyst performance and optimizing valuable product yields)

Reporting period: 2018-01-15 to 2020-01-14

The growing energy demand globally, in combination with environmental concerns and concerns with the dependency on finite fossil energy resources and energy security, has spurred research into alternative energy resources. Lignocellulose is a non-edible, abundant and low-cost form of biomass that can be converted to a liquid renewable energy carrier (bio-oil) via a process known as fast pyrolysis. Bio-oil is a complex mixture of oxygenates derived from biomass and has some unfavourable properties that make its conversion to transportation biofuels very challenging. By incorporating a solid catalyst in the fast pyrolysis process, the biomass-derived products can be partially deoxygenated and a bio-oil with more favourable properties can be produced that can be upgraded to transportation fuels. The disadvantage of using a catalyst is the formation of byproducts such as coke, water and permanent gases. In addition, the need to periodically replace the catalyst has a significant impact on the economics of the process and the cost of the products.

The aim of ECOCAT was to improve on the economics of catalytic fast pyrolysis by reducing catalyst-related operating costs. This can be achieved by the development of a more active catalyst with superior selectivity towards key valuable products. The most commonly used catalyst for catalytic fast pyrolysis is the ZSM-5 zeolite, a catalyst with a unique microporous structure that is very effective for the production of valuable monoaromatic hydrocarbons and for the minimisation of solid byproducts (coke). However, the microporous structure of ZSM-5 may not be optimal for the conversion of the large biomass-derived compounds that are formed during. As such, adding a degree of mesoporosity in the ZSM-5 may be beneficial for the cracking of the large compounds into smaller intermediates, which can then diffuse through the micropores to be converted to valuable deoxygenated products.

In ECOCAT, ZSM-5 zeolites with varying degrees of added mesoporosity were synthesised. The mesoporous ZSM-5 zeolites were both more active and more selective than microporous ZSM-5 and gave consistently higher yields of desirable products.
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.
ECOCAT helped Dr S.D. Stefanidis to broaden his experimental and theoretical skills in catalyst engineering, characterisation methods, catalytic processes and biofuels. This will help him achieve his professional goal to obtain a leading position in a university or in the industry in Europe and establish his own research group that will innovate in the field of bioenergy, biomass conversion, catalytic processes and catalysis engineering.

The Fellow also had the opportunity to expand his collaboration network by meeting peers at conferences he attended and by working with and learning from colleagues in Aston University and the University of Cordoba. Aston University also benefited from working with Dr S.D. Stefanidis who supervised MEng projects, assisted academic staff in the supervision of PhD students and contributed with his expertise to the improvement of the lab’s catalyst characterisation techniques and biomass catalytic pyrolysis facilities.

The methods and catalytic materials developed complement and provide insight into earlier reported state-of-the-art results. The results contribute to the improvement of the economics of catalytic fast pyrolysis by providing a more active catalyst that offers increased yields of valuable renewable products, hence increasing the EU Research Area competitiveness in the world. The results have been disseminated in the proceedings of three international conferences and presented in one summer school; PYRO 2018 in Kyoto, Japan on June 3-8, 2019; Pyroliq 2019 in Cork, Ireland on June 16-21, 2019; ABC Salt Summer School 2019 in Birmingham, UK on 12-14 August, 2019; CatBior V in Turku, Finland on September 23-27, 2019. The presentations drew the attention of fellow researchers and representatives of the catalyst and biofuel industry.