(674d) Mesoporous Nb-Silicates As Ethylene Epoxidation Catalysts: Novel Strategies to Passivate Bronsted Acidity

Ethylene oxide (EO) is one of the widely used chemical intermediates with current global production capacity of 20.5 million metric tons/yr. The present Ag-based catalytic process has a large carbon footprint due to CO₂ formation as byproduct. The search for alternate EO technology that suppresses such CO₂ formation continues to receive attention. Recently, we reported that Nb-TUD-1 catalysts are highly active and selective (â?¥99%) toward EO formation with no CO₂ formation as byproduct.¹ However, H₂O₂ (oxidant) decomposition and significant Nb leaching are major drawbacks that must be overcome for this catalytic process to be practically viable. Computational studies suggest that passivation of Brønsted acid sites can reduce H₂O₂ decomposition and metal leaching.

This report focuses on developing improved strategies to neutralize Brønsted acid sites including base treatment and covalent capping of catalysts. The effects of such improved catalysts on catalyst performance (epoxidation activity, catalyst recyclability, H₂O₂ utilization and resistance to metal leaching) were systematically investigated. Different capping agents were investigated with Nb-TUD-1 catalysts, aimed at virtually eliminating the Brønsted (B) acidity of the catalyst while retaining only the Lewis (L) acidity. Capping of Nb-OH and Si-OH groups in the catalyst using two organic bases (tetraethylammonium hydroxide and di-isopropylethyl amine) significantly enhanced H₂O₂ utilization efficiency (70% and 90%, respectively), but the resistance to metal leaching was only moderately improved. In order to increase hydrolytic stability, four different capping groups, viz., methyl (-Me), trimethylsilyl (-SiMe₃), t-butyl (-CMe₃) and benzyl (-CH₂Ph), with moderate to high hydrolytic stability were investigated as capping groups. The capped catalyst using benzyl group shows the best performance in terms of reducing H₂O₂ decomposition and metal leaching. It displays significantly reduced H₂O₂ decomposition (0.0-0.26 %) after a 5 h run. It also shows very high EO selectivity (~99 %) and a dramatic reduction in Nb leaching (< 3% after 4 cycles compared to >75% after one cyce with the non-benzylated catalyst). The benzylated catalyst was recycled 4 times with minimum loss of catalytic activity. These results clearly suggest that passivation of Brønsted acid sites either by base treatment or by capping with suitable organic groups increases EO selectivity, H₂O₂ utilization efficiency and resistance to metal leaching compared to the untreated parent catalyst. The benzylated Nb-TUD-1 catalyst in particular shows immense potential to be a viable epoxidation catalyst.

References:

1. Yan, W.; Ramanathan, A.; Patel, P. D.; Maiti, S. K.; Laird, B. B.; Thampson, W. H.; Subramaniam, B. J. Catal. 336 (2016) 75-84.