Mechanistic insights for enhancing activity and stability of Nb-incorporated silicates for selective ethylene epoxidation

Abstract

Significant ethylene epoxidation activity was observed over niobium (Nb) incorporated mesoporous silicate materials Nb-KIT-5, Nb-MCM-48, and Nb-TUD-1, with hydrogen peroxide (H 2 O 2) as oxidant and methanol (MeOH) as solvent under mild operating conditions (35 °C and 50 bars). No CO 2 as byproduct was detected at these conditions. The measured ethylene oxide (EO) productivity over Nb-TUD-1 materials (342-2539 g EO h-1 kg-1 Nb) spans a greater range than those observed with Nb-KIT-6 (234-794 g EO h-1 kg-1 Nb), Nb-KIT-5 (273-867 g EO h-1 kg-1 Nb) and Nb-MCM-48 (71-219 g EO h-1 kg-1 Nb) materials at similar operating conditions. However, significant H 2 O 2 decomposition and Nb leaching were observed in all cases. Computational studies employing minimal models of the catalytically active sites, suggest how the Brønsted acidity may lead to these detrimental pathways. Indeed, lowering the metal loading to significantly reduce the Brønsted acidity results in a dramatic increase in H 2 O 2 utilization towards EO formation (4304 g EO h-1 kg-1 Nb). The increased EO productivity either matches or surpasses that observed on the conventional Ag-based heterogeneous catalyst (with O 2 as oxidant) as well as a Re-based homogeneous catalyst (with H 2 O 2 as oxidant). These results are paving the way for further computational and experimental investigations aimed at the rational design of improved epoxidation catalysts that reduce H 2 O 2 decomposition and metal leaching to practically viable levels.

Metal-Incorporated Mesoporous Silicates: Tunable Catalytic Properties and Applications

“…The selectivity for EO is in the range of ~80% over W-KIT-6 and 46–73% with Nb-KIT-6. The observed EO yields are comparable with those reported for homogeneous Re-based (1610–4970 gEO/h/kg Re) [ 57 ] and conventional heterogeneous Ag-based catalysts (700–4400 gEO/h/kg Ag) [ 58 , 59 ]. However, H 2 O 2 decomposition (~80%) and metal leaching caused by Brønsted acid sites in these catalysts are major drawbacks [ 54 ].…”

“…Clearly, the challenge is to synthesize M-KIT-6 materials that are exclusively Lewis acidic by passivating the Brønsted acid sites. To address the stability of KIT-6, and to improve the utilization of H 2 O 2 , we studied Nb-TUD-1 as a catalyst and found that it showed superior activity (~2500 g EO/h/kg Nb) compared to other Nb- or W-based silicates [ 59 ]. The EO productivity was further improved to ~4000 g EO/h/kg Nb by decreasing the Brønsted acid sites achieved by lowering the Nb loading.…”

“…The EO productivity was further improved to ~4000 g EO/h/kg Nb by decreasing the Brønsted acid sites achieved by lowering the Nb loading. Nevertheless, about 60% of Nb was leached after initial run [ 59 ]. To further thwart the Brønsted acidity, Nb-TUD-1 was capped with ion pair agents or covalent bonding groups [ 60 ].…”

“…In the liquid phase epoxidation involving H 2 O 2 , it is widely accepted that the tetrahedrally-coordinated metal (Lewis site) in the silica matrix forms a metal peroxo complex by reaction with H 2 O 2 which then interacts with olefinic substrates to form the corresponding epoxide [ 54 ]. Computational chemistry studies of the interaction of H 2 O 2 on a simple niobyl silicate site (Nb=O, representative of NbO 4 units) and niobium hydroxide (Nb-OH, representative of Brønsted acid sites) silicate indicate that the presence of Brønsted acid sites causes H 2 O 2 decomposition with concomitant Nb leaching [ 59 ]. Indeed, the design of catalysts with reduced Brønsted acidity increases H 2 O 2 utilization towards epoxide formation while simultaneously reducing metal leaching [ 60 , 66 ].…”

Epoxidation of Ethylene by Whole Cell Suspension of Methylosinus trichosporium IMV 3011

“…Direct oxidation of ethylene to epoxyethane with molecular oxygen is a significant synthetic method for both industry and academia. During the last decades, many research works have been focused on epoxidation reactions of ethylene by supported catalysts such as transition metal complexes and metal nanoparticles [3][4][5][6].…”