Photooxidation of ethylene over Cu-modified and unmodified silica

Silica catalyzes photooxidation of ethylene to carbon dioxide and modification of copper on silica results in the lower reaction rate and partial production of ethylene oxide. The reaction does not proceed by the light irradiation through a color filter (λ > 280 nm). ESR measurement indicates that radical oxygen species assignable T-shape Si −O3− can be produced on silica by UV irradiation at 77 K. The same species are also found on silica modified with copper by UV irradiation while the quantity is smaller. When silica and silica modified with copper are UV-irradiated in presence of oxygen and ethylene at 77 K, formation of methyl radicals is confirmed only on silica while ethyl radicals are observed with both samples. We suggest that copper stabilizes C2 species and prevents cleavage of C− C bonding.


INTRODUCTION
The selective oxidation of hydrocarbons to their oxides is one of the goals of chemical industries.However, the direct oxidation with molecular oxygen often results in the low selectivity of the desired compounds.The process using a photocatalyst offers a unique route to the oxidation, but overoxidation is accompanied with the high oxidation potential of the photocatalysts such as TiO 2 .
Silica has been believed as an inert or poor catalyst for most of reactions because its surface is neutral or perhaps slightly acidic.However, some chemically active sites are known on the silica surface, e.g., ethanol can be dehydrogenated to acetaldehyde over highly dehydrated silica [1][2][3][4].Ogata et al. reported that photocatalytically active sites are present on silica and photooxidation of carbon monoxide proceeds on the surface [5].Partial oxidation of propylene to the epoxide takes place over some silica-based photocatalysts containing metal oxides, and coordinately unsaturated sites such as Si(OSi) 3 were proposed [6,7].We have also studied these peculiar active sites of silica for photocatalytic reaction.In the present work we will show that the photooxidation of ethylene to ethylene oxide can take place over silica, and it is significantly promoted by modification with a slight amount of copper while it is suggested that the sites on silica is rather important to the epoxidation.
The photoreaction was carried out under a static condition.Prior to the reaction, the sample (150 mg) was usually heated in O 2 at 673 K for 1 h, then, evacuated at 973 K for 1 h to 10 −4 Pa.The powder was spread on the flat bottom of a quartz reaction cell (12.5 cm 2 ) connected to a conventional vacuum system.After introduction of ethylene (0.5 kPa, 18 µmol) and oxygen (0.5 kPa, 18 µmol) the catalyst was irradiated using high-pressure Hg lamp (100 W) through water filter and a color filter (λ > 280 nm) at 273 K.The photoreaction was carried out under a static condition.The gas-phase products were collected with liquid nitrogen trap after irradiation and analyzed with a gas chromatograph (Shimadzu GC-8A).
ESR spectra were measured at 77 K with a Bruker ESP-300E X band-spectrometer.Adsorption of ethylene and oxygen was carried out in a vacuum system equipped with a Baratron vacuum gauge.The magnetic field was corrected by the reference of DPPH (diphenylpicrylhydrazyl).

RESULTS AND DISCUSSION
Table 1 shows the product distribution for the photooxidation of C 2 H 4 over SiO 2 and CuO x /SiO 2 catalysts.Ethylene is mainly photooxidized to carbon dioxide over silica evacuated at 973 K.Although a small amount of ethylene oxide whose selectivity was 2% in the ethylene conversion of 26% was detected after the irradiation for 0.8 h, no ethylene oxide was detected after the reaction for 3 h.On the other hand, the products of partial oxidation such as acetaldehyde and ethylene oxide were detected even with silica containing only 0.05 wt% of copper, the rate of ethylene conversion was drastically decreased in comparison with unmodified silica.Filtered light irradiation of which wavelength was longer than 280 nm also led to the no formation of products on SiO 2 and even CuO x /SiO 2 catalysts.It would be possible that the photoactive sites on silica are directly modified with copper, however, the quantity of copper is too small.In addition, in the content of copper does not result in further decreasing in the conversion.Hence, it is rather supposed that the photoactive sites exist on silica surface and copper deactivates the surface oxygen species such as O 3 − .
As shown in Figure 1, the yields of carbon dioxide and ethylene oxide increased under the UV irradiation on 0.1 wt% CuO x /SiO 2 , but the formation of carbon dioxide was depressed after the irradiation for 3 h when oxygen in the gas-phase was almost exhausted.It means that the formation of ethylene oxide independents of the amounts of gas-phase oxygen.The oxidation state of copper on silica can be monitored by the color of the catalyst.The color of the catalyst, which was yel-Reaction Time/h lowish after the evacuation at 973 K, was not changed obviously during the reaction, appearing that the oxygen in ethylene oxide is not mainly from copper oxide on the surface.ESR spectra depicted in Figure 2 were recorded with SiO 2 and 0.1 wt% Cu/SiO 2 after UV irradiation in the presence of oxygen at 77 K for 30 min.Both samples gave very similar ESR signals which were axially symmetrical with g // = 2.0089 and g ⊥ = 2.0043, while the intensity for SiO 2 was ca.2.5 times that for 0.1 wt% CuO x /SiO 2 which corresponds to the reaction rate (see Table 1).Hence, these radical species probably relate to the photoactivity, and the active sites in CuO x /SiO 2 are considered to be the same as in SiO When SiO 2 and 0.1 wt% CuO x /SiO 2 were UV irradiated in the presence of oxygen and ethylene at 77 K for 60 min, both samples gave broad ESR signals consisting of six lines with the intensity ratio of 1 : 3 : 6 : 6 : 3 : 1 (g = 2.0030, a = 26.0G) with the signal of the radical oxygen species (see Figure 3).The ESR signal of a radical species containing five hydrogen atoms such as the ethyl radical consists of twelve lines with the intensity ratio of 1 : 2 : 3 : 1 : 6 : 3 : 3 : 6 : 1 : 3 : 2 : 1, and the hyperfine-splitting parameters were reported to be a α = 22.4 G and a β = 26.9G [11].Since the parameters are close to each other, hybridization of the lines may take place and the signal looks like the result of splitting with five equivalent hydrogen atoms.Thus, the formation of the ethyl radical can be estimated from the ESR signal while further elucidation is still necessary.In the spectrum for SiO 2 , four sharp lines with an intensity ratio of 1 : 3 : 3 : 1 (g = 2.0031, a = 23.4G) were also present.This sharp signal can be attributed to the methyl radical of which the ESR parameters were reported as g = 2.0026 and a = 23.0G [11].Hence, it is supposed that cleavage of the C−C bonding easily takes place on SiO 2 which causes poor selectivity of products due to the partial oxidation of ethylene although the photooxidation sites in SiO 2 are very similar to those in CuO x /SiO 2 .Copper may stabilize the surface C 2 species and prevent further oxidation of ethylene oxide, etc., to carbon dioxide.

CONCLUSIONS
Silica catalyzes the photooxidation of ethylene and modification with copper promotes the partial oxidation of ethylene mainly to ethylene oxide.It should be emphasized that copper stabilizes the surface C 2 species and also prevents formation of the O 3 − ion which may induce the formation of carbon dioxide.
Although detailed mechanisms will be the subject of our future work, it is proposed as a plausible mechanism that the T-shaped Si − O 3 − may take place and the formation of carbon dioxide and the other adsorbed oxygen species, such as O − and O, etc., may be a possible intermediate, by which ethylene is epoxidated.In this paper, we have demonstrated the function of copper during the photooxidation of ethylene.

Figure 1 .
Figure 1.Time profiles of the photooxidation of ethylene into carbon dioxide (a) and ethylene oxide (b) over 0.1 wt% CuO x /SiO 2 catalyst evacuated at 973 K.

Table 1 .
Product distribution for the photooxidation of C 2 H 4 over SiO 2 and CuO x /SiO 2 catalysts evacuated at 973 K for 1 h before the reaction.Period of UV irradiation, 3 h.