Catalytic Epoxidation of Propene with H2O-O2 Reactants
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Catalytic epoxidation of propene with H 2O-O2 reactants on Au/TiO 2 Manuel Ojeda and Enrique Iglesia* Au/TiO 2 catalysts form hydroperoxy species from H 2O-O2 measured using O 2 (4 kPa, Praxair, UHP) at 350 K in the 5 mixtures at near-ambient temperatures. These species can be 55 presence and absence of H 2 (4 kPa, 99.999%, Praxair) or H 2O used in the selective epoxidation of propene to propylene oxide. (0-12 kPa) using a tubular flow reactor with plug-flow hydrodynamics (He was used as a balance). Au/TiO 2 was used Propylene oxide (PO) is a useful intermediate in the as-received, while Au/Al 2O3 was treated using protocols synthesis of polyurethane, organic intermediates and solvents reported previously .12 Reactant and product concentrations 1 (e.g. propylene glycol). PO is currently produced by 60 were measured by gas chromatography (Agilent 6890GC) 2 10 chlorohydrin or hydroperoxide processes, which require using a Porapak Q packed-column (80–100 mesh, 1.82 m × toxic, corrosive, or explosive reagents, and form significant 3.18 mm) and a HP-1 capillary column (50 m x 0.32 mm; 1.05 amounts of waste by-products. Hydrogen peroxide has been µm film) with thermal conductivity and flame ionization used as an oxidant to replace alkyl hydroperoxides in PO detection, respectively. 3 synthesis. 65 Figure 1 shows propylene oxide formation rates (metal- 15 Small Au clusters (<5 nm) dispersed on Ti-containing time yield, per Au atom) at 350 K as a function of time-on- oxides, such as TiO 2 and TS-1, catalyze propene epoxidation stream on Au/TiO 2 when H 2-O2 or H 2O-O2 were used as the via the in situ formation of hydroperoxy species (*OOH) from oxidant. Propene epoxidation rates and selectivities (~95%) 4-7 -1 H2-O2 mixtures. Mechanistic studies have suggested that and catalyst deactivation rates (0.76 h first-order hydroperoxy species form on Au clusters via H2 reactions 70 deactivation rate constant) in H 2-O2 mixtures are similar to 20 with O 2; these species then react with propene on adjacent Ti those previously reported (Supplementary Information†). centers to form PO with high selectivity.8,9 This process uses Strongly-adsorbed PO-derived species have been claimed to 13 H2 as a sacrificial reductant, which predominantly converts, cause this deactivation. PO selectivities remained nearly however, to H 2O via unproductive side reactions, and leads to unchanged as deactivation occurred and conversion decreased low H 2 utilization efficiencies (30-40%, defined as PO formed 75 (Figure 2). These data, taken together with the intermediate 2 25 per H 2 consumed) and unfavorable economics. H2O2 was selectivites observed as data are extrapolated to zero recently detected during CO oxidation with aqueous systems conversion, indicate that both PO and acetone form as primary and Au catalysts 10 ; these molecules or their adsorbed products and that deactivation occurs by blocking of sites precursors may therefore form also from O 2/H 2O mixtures. without concomitant changes in the relative rates of PO and Our recent kinetic and isotopic data indicate that *OOH 80 acetone synthesis. 30 species formed from H 2O/O 2 account for the strong effect of 11 H2O on the rate of CO oxidation on Au catalysts. We provide here evidence for the formation of *OOH species from H 2O and O 2 by detecting and reporting for the first time the formation of PO via propene reactions using H 2O/O 2 35 mixtures, which appear to act as precursors for peroxide species. H 2O/O 2 mixtures form PO with selectivities as high as 80% on Au/TiO 2 at near-ambient temperatures (300-350 K) . Au/TiO2 (1.56 wt.% Au, 3.3 ± 0.7 nm mean cluster 40 diameter) was prepared by deposition-precipitation and provided by the World Gold Council. Au/Al 2O3 (0.61 wt.%, 3.5 ± 1.2 nm) was also prepared by deposition-precipitation 12 methods. HAuCl 4·xH 2O (0.24 g, Aldrich, 99.999%) was dissolved in deionized H O (80 cm 3) at 353 K. The support (5 2 Fig. 1. Rate of propylene oxide formation (expressed as 45 g, γ-Al O , Alcoa) was treated in air at 923 K for 5 h and 2 3 metal-time yield) from C H (4 kPa) and O (4 kPa) at 350 K suspended in deonized H O (120 cm 3) at 353 K. Au was 3 6 2 2 from with Au/TiO as a function of time-on-stream using H deposited onto Al O at 353 K and a pH of 7 (adjusted with 2 2 2 3 or H O as co-reactants: 4 kPa H ( ●); 1 kPa H O ( ▲), 2 kPa 0.5 M NaOH) by mixing the two solutions with stirring for 1 2 2 2 85 H O ( ■); 6 kPa H O ( ▼); 12 kPa H O ( ♦). h. Solids were filtered and washed twice with deionized water 2 2 2 Figures 1 and 2 provide evidence for the previously 50 at ambient temperature and then once at slightly higher unrecognized ability of Au/TiO to catalyze propene temperatures (323 K), and held in ambient air for 24 h. TiO 2 2 epoxidation with O and H O (instead of H ) as co-reactant. was obtained from Degussa (P25, anatase/rutile ~4). Propene 2 2 2 The synthesis of PO during water electrolysis, probably via in (4 kPa, Praxair, UHP) epoxidation rates and selectivity were 14 90 situ generation of H 2O2 or OOH species, has been reported. 1 In contrast, catalytic epoxidation of propene or other synthesis rates, however, are independent of H 2O pressure substrates with H 2O-O2 reactants have not been reported. PO 60 above 2 kPa, while acetone synthesis rates increased synthesis rates with H2O-O2 reactants are significantly lower monotonically with H 2O pressure. than with H 2-O2, but the former avoid the significant loses of 5 costly H 2 co-reactants via its unproductive pathways to form H2O instead of OOH species (hydrogen efficiency ~29 %). Both H2O and O 2 are needed to form active hydroperoxy species required for PO synthesis. Neither C 3H6-O2 nor C 3H6- H2O reactants formed detectable products on Au/TiO 2 at 350 10 K. PO synthesis also requires the presence of both Au and Ti sites. We did not detect PO from C 3H6-O2-H2 or C 3H6-O2-H2O reactants on either TiO 2 or Au/Al 2O3 catalysts. Au and Ti sites must reside in reasonable proximity, since PO was not detected when C 3H6-O2-H2 or C 3H6-O2-H2O reactants were 15 contacted with a physical mixture Au/Al 2O3 and TiO 2 (mass ratio 1:1, 0.125-0.250 mm aggregates). The rate of PO formation (extrapolated to zero time-on- stream) increased slightly from 0.5 to 0.7 mol h -1 (g-at Au) -1 when the H2O partial pressure in equimolecular C 3H6-O2 Fig. 2. Selectivity (carbon basis) to propylene oxide obtained 20 mixtures (4 kPa) increased from 1 to 2 kPa (Figure 1); higher with Au/TiO 2 at 350 K at different propene conversion levels H2O pressures (up to 12 kPa), however, did not influence reaction rates. 65 changed by catalyst deactivation (4 kPa C 3H6; 4 kPa O 2; 4 kPa H2; 1-12 kPa H 2O). The PO synthesis rates reported here are inconsistent with the use of gaseous H 2O2 reactants as intermediates and The data in Figures 1 and 2, taken together, suggest that 25 indicate that PO is formed instead from propene and adsorbed maximum PO yields are achieved at intermediate H 2O hydroperoxide species. H 2O2 pressures in equilibrium with -18 -19 - 70 pressures. While rates are lower than with H 2 as co-reactants, O2/H 2O (4 kPa/12 kPa) are 7x10 kPa (K eq = 1.48x10 kPa 1/2 the use of H 2O avoids the need for H 2 and its predominant ; 350 K). At this pressure, the frequency of H2O2 collisons loss via unproductive combustion side reactions. with Au clusters (Supplementary Information†) would be -6 -1 -1 In summary, we provide evidence here for the 30 5x10 mol h (g-at Au) , a value much lower than required -1 unprecedented ability of Au/TiO catalysts to form to maintain the observed epoxidation rates (~0.7 mol h (g-at 2 -1 75 hydroperoxy surface species from O 2/H 2O mixtures and for Au) ), consistent with an inadequate supply of H 2O2(g) as the reactive species and with PO synthesis via propene reactions their involvement in oxidation reactions, such as selective conversion of propene to PO at near-ambient temperatures with hydroperoxy surface species instead of H 2O2. The (350 K). These pathways provide an attractive strategy to 35 involvement of bound hydroperoxy species requires, in turn, replace H by H O in propene epoxidation reactions. We atomic proximity between the sites that form *OOH (Au) and 2 2 80 expect that these hydroperoxy intermediates will prove useful those that consume it via reactions with propene (Ti), possibly 4 for other epoxidation reaction upon more rigorous evaluation at Au-TiO 2 interfaces, as proposed earlier and consistent with the absence of epoxidation turnovers on physical and optimization. M. Ojeda acknowledges financial support from the 40 mixtures of Au/Al 2O3 and TiO 2. 15 European Union (Marie Curie Actions). This work was also Density functional theory (DFT) suggests that OOH species 85 supported by the Director, Office of Basic Energy Sciences, can form from H 2O and O 2 on Au 8 clusters to form [O 2·H 2O] Chemical Sciences Division of the US Department of Energy complexes in which protons are shared between H2O and O 2 to ultimately form adsorbed (HOO*) complexes.