420
H. Cui et al. / Catalysis Communications 12 (2011) 417–420
3.4. Reaction mechanism
Based on our studies and other researchers' work [6,9,10], a pro-
posed mechanism for epoxidation of styrene with molecular oxygen
over the Co2+ catalyst is illustrated in Fig. 4. DMF molecules
were coordinated to the cobalt cations which are adsorbed on the
surface of aminopropyl-functionalized SBA-15, forming DMF-LCo(II).
The resultant cobalt complex was coordinated to molecular oxygen
to form a cobalt superoxo complex DMF-LCo(III)OO• I, which led
to oxidative addition to the C C double bond of styrene molecules
to give the styrenic olefin II. Through migratory insertion, II was
rearranged into the intermediate III. III then collapsed to the cyclic
peroxide radical IV and the cobalt catalyst DMF-LCo(II) was
regenerated. The intermediate IV could react via two different path-
ways. One is that IV underwent thermal decomposition to benzal-
dehyde and formaldehyde, and the other reaction is that IV could
react with another molecule of styrene to generate epoxide and
by-products (isomeride, over-oxidation product). In the catalytic
reaction, formaldehyde was not detected in the obtained liquid
products. However, we introduced the tail gas into water to trap
the possibly escaped formaldehyde with the unreacted oxygen, and
formaldehyde was detected in the water after reaction. Clearly, this
confirmed the presence of formaldehyde.
Fig. 3. Recycling investigations of Co-SBA-15-20-5.2 in epoxidation of styrene with Oa2,
(■) styrene conversion, and (▲) epoxide selectivity. aReaction condition is the same as
that in Table 2.
Catalyst recycling experiments were performed with repeated use
of Co-SBA-15-20-5.2 at 100 °C for 6 h. The styrene conversion slightly
decreased after the first cycle, and a small part of cobalt ions in the
catalyst was lost (Fig. 3). But the catalytic performance changed little
from the second cycle. The styrene conversion and epoxide
selectivity were kept more or less constant (88.0% and 63.0%
respectively). Any visible amount of cobalt in the solution was not
detected by the chemical analyses, indicating little leaching of Co2+
from the catalyst. This suggests the stability and recyclability of the
catalyst.
4. Conclusions
Cobalt-containing SBA-15 was prepared by the adsorption
method. The prepared material still retained highly ordered structure.
The cobalt introduced was highly dispersed in the aminopropyl-
functionalized mesoporous silica. The cobalt ions that were adsorbed
onto the functionalized SBA-15 showed excellent catalytic perfor-
mance in the epoxidation of styrene with O2. O2 or air was a suitable
oxidant over the catalyst. The catalyst possessed better stability and
recyclability in the reaction.
Acknowledgement
We are grateful for the financial support of the National Key
Technology R&D Program (no. 2007BAB24B04).
References
[1] C. Qi, L. Qiu, K. Lam, C. Yip, Z. Zhou, A. Chan, Chem. Commun. (2003) 1058–1059.
[2] J. Jiang, R. Li, H. Wang, Y. Zheng, H. Chen, J. Ma, Catal. Lett. 120 (2008) 221–228.
[3] D. Dhar, Y. Koltypin, A. Gedanken, S. Chandrasekaran, Catal. Lett. 86 (2003)
197–200.
[4] M. Krishna, M. Shivanand, L. Bharat, V. Nettem, V. Raksh, Catal. Commun. 11
(2010) 638–642.
[5] Q. Tang, Y. Wang, J. Liang, P. Wang, Q. Zhang, H. Wan, Chem. Commun. (2004)
440–441.
[6] T. Pruβ, D. Macquarrie, J. Clark, Appl. Catal. A 276 (2004) 29–37.
[7] M. Reetz, K. Toiiner, Tetrahedron Lett. 36 (1995) 9461–9464.
[8] X. Lu, Q. Xia, D. Zhou, S. Fang, A. Chen, Y. Dong, Catal. Commun. 11 (2009)
106–109.
[9] J. Sebastian, J. Jinka, R. Jasra, J. Catal. 244 (2006) 208–218.
[10] Q. Tang, Q. Zhang, H. Wu, Y. Wang, J. Catal. 230 (2005) 384–397.
[11] H. Zhan, Q. Xia, X. Lu, Q. Zhang, H. Yuan, K. Su, X. Ma, Catal. Commun. 8 (2007)
1472–1478.
[12] G. Xu, Q. Xia, X. Lu, Q. Zhang, H. Zhan, J. Mol. Catal. A: Chem. 266 (2007) 180–187.
[13] Sujandi, E.A. Prasetyanto, S.–.C. Han, S.–.E. Park, Bull. Korean Chem. Soc. 27 (2006)
1381–1385.
[14] H. Cui, Y. Zhang, Z. Qiu, L. Zhao, Y. Zhu, Appl. Catal. B: Environ. 101 (2010) 45–53.
[15] D. Macquarrie, D. Jackson, Chem. Commun. (1997) 1781–1782.
[16] H. Yoshitake, T. Yokoi, T. Tatsumi, Chem. Mater. 15 (2003) 1713–1721.
[17] B. Porsch, J. Kratka, J. Chromatogr. 543 (1991) 1–7.
[18] V. Basiuk, A. Chuiko, J. Chromatogr. 521 (1990) 29–42.
[19] X. Wang, K. Lin, J. Chan, S. Cheng, J. Phys. Chem. B 109 (2005) 1763–1769.
[20] A. Pui, Croat. Chem. Acta 75 (2002) 165–173.
Fig. 4. Proposed mechanism for epoxidation of styrene with O2 over Co2+ catalyst. L in
DMF-LCo2+ means SBA-15-x-y.