In situ characterization of catalysts active in partial oxidations: TS-1 and Fe-MFI case studies A. Zecchina a,b, *, S. Bordiga a,b , G. Spoto a,b , A. Damin a , G. Berlier a , F. Bonino a , C. Prestipino a and C. Lamberti a,b,c a Department of Inorganic, Physical and Material Chemistry, University of Turin, Via P. Giuria 7, 10125 Torino, Italy E-mail: Zecchina@ch.unito.it b INSTM Research unit of Turin University, Torino, Italy c INFM UdR di Torino Universita `, Torino, Italy A concise review of the firmly established knowledge and of the unresolved problems concerning the structure and the reactivity of Ti and Fe sites in TS-1 and Fe-MFI partial oxidation catalysts is given. Some new experimental and theoretical results are also described. KEY WORDS: TS-1; Fe-MFI; partial oxidation reactions; in situ spectroscopies 1. Introduction In recent years, two interesting classes of partial oxidation catalyst have attracted particular interest in the scientific community: (i) the Ti-based heterogeneous [1–17] and homogeneous catalysts [18–23]; and (ii) the Fe-based heterogeneous [24–31] and homogeneous cat- alysts [32–37]. In particular TS-1 [1–12], and Fe-silicalite and Fe-ZSM-5 [30,31,38] have proved their utility and selectivity in several partial oxidation reactions using hydrogen peroxide and N 2 O as oxidants. As far as TS-1 is concerned, the most relevant oxi- dation reactions [1–12] are summarized in scheme 1. Note that some of these reactions form the basis of industrial processes. As far as Fe-silicalite and Fe-ZSM- 5 are concerned, beside the oxidation of methane, they are active catalysts in the oxidation of benzene to phe- nol. Scheme 2 shows the possible future role of Fe-MFI catalysts in the adipic acid process [38]. The outstanding properties of these heterogeneous catalysts in terms of yield and selectivity, the presence of homogeneous counterparts [18–23,32–37] and the similarity of some of them (the Fe-based ones) to enzymatic catalysts [39,40] has stimulated a great deal of investigations concerning the structure, valence, coordination state, nuclearity of the catalytic sites and reaction mechanisms. In this work we give a brief overview on the most important achievements and the unresolved problems concerning these two catalysts. Due to space limitations and the broadness of the subject, this review has no presumption to be exhaustive. 2. Experimental TS-1 ðSi=Ti ¼ 50Þ has been synthesized in EniChem laboratories following a procedure described in the original patent [1]. Fe-silicalite ðSi=Fe ¼ 90Þ has been synthesized at the University of Milano following the hydrothermal method described by Ratnasamy and Kumar [41]. The structural and catalytic peculiarities of TS-1 synthesized by EniChem are well known, while those of Fe-silicalite have been thoroughly investigated in two previous papers [42,43] by Forni’s group. Ti k-edge XANES and EXAFS experiments have been performed using the EXAFS3 and EXAFS 13 beamlines of the LURE DCI storage ring using a Si(311) and Si(111) monochromator, respectively. Fe k-edge XANES and EXAFS experiments have been performed using the GILDA BM8 beamline of the ESRF storage ring using for both parts of the X-ray absorption spec- trum a Si(311) monochromator. All X-ray absorption measurements have been performed at room tempera- ture, with the only exception of the TS-1/H 2 O/H 2 O 2 system, which has been measured at liquid nitrogen temperature to stabilize the active complex. Diffuse reflectance UV–vis spectra were obtained on a Varian Cary 5 spectrometer. To limit the computational demand, in our calcu- lations, the cluster/embedded cluster ONIOM [44] scheme has been adopted on a cluster obtained by cutting a fraction of the MFI framework around the T1 site [45]. The final ONIOM energy (as such as gradients, and so on) is defined as: EðONIOMÞ¼ EðHMÞþ½EðLRÞ EðLMÞ, where E(LR) is the energy of the whole cluster calculated at (RHF/3- 21G(H,N,Si,O), 3-21G*(Ti)) low (L) level, E(LM) is the M energy at (RHF/3-21G(H,N,Si,O), 3-21G*(Ti)) and E(HM) is the M energy obtained at (B3-LYP/6- 311+G(d,p)(H,N,Si,O), ppp(Ti)) high (H) level. Here, ppp represents the basis-set employed for Ti atoms: it is composed of effective small core pseudo-potentials [46]+LANL2DZ [46]+Ahlrichs’ ‘‘p’’ polarization function [47]. *To whom correspondence should be addressed. Topics in Catalysis Vol. 21, Nos. 1–3, October 2002 (# 2002) 67 1022-5528/02/1000-0067/0 # 2002 Plenum Publishing Corporation