Phosphazene-based covalent organic polymers as metal-free catalysts with improved H2 generation from NaBH4 in methanol with superior catalytic activity and re-generation ability

Abstract

Here, we report phosphazene-based covalent organic polymer (COP) structure synthesis from branched polyethyleneimine (PEI) and phosphonitrilic chloride (PNC) as PEI-PNC COP. The zeta potential value of the prepared COP structures, PEI-PNC measured as + 18.6 ± 1.1 mV and increased to + 37.3 ± 1.3 mV after simple protonation with HCl treatment as PEI-PNC+ . The specific surface area of PEI-PNC structures was measured as 50.9 m2/g. Interestingly, it was found that the PEI-PNC COP catalyzed NaBH4 alcoholysis reaction completed in 14 min, whereas the PEI-PNC+ COP catalyzed reaction was completed in 3.5 min with the same amount of H2 generation, about 253 ± 1 mL. The hydrogen generation rate (HGR) value for the PEI-PNC+ catalyzed reaction was calculated as 3744 ± 234 mL H2 x (catalyst amount (g) x min)−1 at 25 °C. The activation energy value for the PEI-PNC+ catalyzed reaction was calculated as 20.9 kJ/mol that is comparable low and/or better than metal-based catalysts used for the same purposes. Moreover, the PEI-PNC+ COP structures provide more than 50% activity after 10 consecutive usages and can readily regeneratable via simple HCl treatment. The regeneration ability of the catalyst afforded 100 consecutive uses without a significant loss of catalytic activity.