In Situ UV−vis and EPR Study on the Formation of Hydroperoxide Species during Direct Gas Phase Propylene Epoxidation over Au/Ti-SiO₂Catalyst

Abstract

In recent years, there have been great experimental and theoretical advances in the understanding of the epoxidation of propylene by O(2) and H(2) over Au supported on titanium-containing oxidic supports; however, thus far spectroscopic evidence of reacting species for proposed mechanisms has been lacking. Hydroperoxide species have been postulated as an intermediate responsible for the epoxidation of propylene with O(2) and H(2). In order to obtain direct evidence for the different type of active oxygen species, in situ UV-vis and EPR measurements were carried out during the epoxidation of propylene with O(2) and H(2) over a Au/Ti-SiO(2) (Ti/Si = 3:100) catalyst. It was determined that the adsorbed species of oxygen (O(2)(-)) resided on Au, more likely at a perimeter site, and it led to the formation of titanium hydroperoxo species. These results support the possible mechanism of formation of these hydroperoxo species via H(2)O(2) produced from O(2) and H(2) adsorbed on the Au surfaces.

Aerobic epoxidation of propene over silver (111) and (100) facet catalysts

“…[2][3][4][5][6][7] Interesting alternative routes for the production of propylene oxide are based on direct epoxidation of propylene with molecular oxygen, [8][9][10] in a similar way as ethylene oxide is nowadays industrially produced, [11][12][13][14][15] or otherwise with hydrogen peroxide formed "in situ" on gold and palladium based catalysts. [16][17][18][19] In ethylene epoxidation, selectivities higher than 80% are obtained using Ag/α-alumina catalysts, and increased attention has been paid to silver based catalyst for production of propylene oxide by direct epoxidation using molecular oxygen. However, selectivity towards propylene oxide during epoxidation with air is usually lower than 40% at 10% conversion, [6][7][20][21] which makes industrial implementation not viable.…”

Towards selective catalytic oxidations using in situ generated H2O2

“…We conclude that this is related to the initial formation of a titanium hydroperoxo/superoxo species following addition of H 2 O 2 (noted by a change in catalyst colour from white to yellow upon contact with H 2 O 2 ) and that this species endures long after "free" H 2 O 2 in solution has decomposed. These titanium hydroperoxo species have previously been widely reported using EPR and UV-Vis analysis [67][68][69] and we assume they can act as stoichiometric oxidising agents following their formation.…”

Propane reacts with O2 and H2 on gold supported TS-1 to form oxygenates with high selectivity

“…The steady-state Au PR position (539.4 nm) was higher than that observed under H 2 flow (534.1 nm), confirming the presence of adsorbed O 2 on Au during reaction. Although an adsorbed superoxide O 2 À species has been observed by electron paramagnetic resonance spectroscopy, 16 to the best of our knowledge, this is the first in situ experimental evidence of adsorbed oxygen on Au supported on a titanosilicate. Propane conversion and selectivities on Au/TS-1 appeared to track with spectroscopic changes (see ESIw); however, to prove that the observed species are true intermediates would require the use of transient spectroscopic techniques.…”

“…Changes occur in several regions ( Fig. 1 and ESIw): (1) a band at about 540 nm due to a plasmon resonance (PR) of Au nanoparticles 15 (Au is mostly metallic as evidenced by the absence in the Au L 3 -edge XANES of a near-edge resonance around 11 920 eV due to oxidized Au, see ESIw); (2) band growth at 320-410 nm and 210-240 nm due to Ti-hydroperoxo species; 12,16 and (3) a band decrease at 250-300 nm due to tripodal Ti sites, Ti(OSi) 3 -OHÁ(H 2 O) 2 . 12 These assignments are supported by density functional theory (DFT) calculations (see ESIw).…”