Investigating the impacts of concentrated alkali metal poisons on the catalytic pyrolysis of biomass toward continuous processing of contaminated solid feedstocks

Catalytic pyrolysis of lignocellulosic biomass, waste plastics, and other solid waste resources over acid zeolites yields valuable aromatics, olefins and fuel-range hydrocarbons. However, zeolite catalyst deactivation by the concentrated alkali metals in these feedstocks is a critical technical challenge limiting the implementation of such strategies at scale. Further complicating this challenge is the lack of fundamental, scientific knowledge around the behavior of concentrated alkali metals in zeolite catalysts, as the petroleum industry where zeolites are utilized most prevalently operates at low levels of metal contaminants. To bridge this knowledge gap and probe the possibility of operating biomass conversion processes at higher levels of alkali contaminants (>1000 ppm), we incorporated controlled amounts of potassium into ZSM-5 catalysts via wet impregnation with KNO3. The pristine framework had a silicon to aluminum ratio of 26 with a corresponding Bronsted acid site density 956.7 mmol/g as determined by ammonia titration and temperature programed desorption. We show that as potassium accumulates up to 2 wt% on the catalyst, the Brønsted acid site density decreases proportionally, consistent with the known tendency of alkali metal ions to titrate acid sites. However, when only 50% of the acid sites are titrated (1 wt% potassium), the carbon yield of aromatics from pine wood biomass decrease from 18% to nearly zero, suggesting a change in the per-site selectivity of the remaining acid sites. To investigate this effect, we carried out reaction kinetics studies using ethanol dehydration into hydrocarbons as a probe into the rates of dehydration, carbon-carbon coupling and hydride transfer reactions. We demonstrate that the per-proton rates of reverse-beta scission reactions (carbon-carbon coupling) decrease rapidly as potassium accumulates in the framework, explaining the decreased aromatic yields from biomass over partially exchanged ZSM-5. We furthermore carried out x-ray photoelectron spectroscopy experiments, finding that the binding energies of O1s electrons in framework oxygens decreased with incorporation of up to 10 wt% potassium, suggesting a change in the electronegativity of anionic oxygens at Brønsted acid sites. We show that the per-proton activity of reverse beta scission reactions correlate with this decrease in framework electronegativity as potassium accumulates in the catalyst. We interpret these results to suggest that potassium ions polarize surrounding framework oxygens and weaken the binding energy of more electronegative olefins at nearby acid sites. Consistent with our hypothesis, we show that ethanol dehydration at an elevated temperature of 500°C yields less aromatics with increasing potassium content in the ZSM-5; and that this yield decrease is proportional to the O1s binding energy determined by XPS. We demonstrate that processing ZSM-5 with up to 10 wt% potassium in concentrated HNO3 removes all but 0.5 wt% of K+ ions, regenerating a corresponding amount of framework surface area and acid site density without removing detectible amounts of framework Al nor destroying the ZSM-5 crystal structure. However, the electronegativity decrease effected by wet impregnation of potassium is only partially reversed by acid leeching, suggesting that some initial catalyst deactivation impacts may not be fully reversible.