In Situ Generation of Active Sites in Olefin Metathesis

Abstract

The depth of our understanding in catalysis is governed by the information we have about the number of active sites and their molecular structure. The nature of an active center on the surface of a working heterogeneous catalyst is, however, extremely difficult to identify and precise quantification of active species is generally missing. In metathesis of propene over dispersed molybdenum oxide supported on silica, only 1.5% of all Mo atoms in the catalyst are captured to form the active centers. Here we combine infrared spectroscopy in operando with microcalorimetry and reactivity studies using isotopic labeling to monitor catalyst formation. We show that the active Mo(VI)-alkylidene moieties are generated in situ by surface reaction of grafted molybdenum oxide precursor species with the substrate molecule itself gaining insight into the pathways limiting the number of active centers on the surface of a heterogeneous catalyst. The active site formation involves sequential steps requiring multiple catalyst functions: protonation of propene to surface Mo(VI)-isopropoxide species driven by surface Brønsted acid sites, subsequent oxidation of isopropoxide to acetone in the adsorbed state owing to the red-ox capability of molybdenum leaving naked Mo(IV) sites after desorption of acetone, and oxidative addition of another propene molecule yielding finally the active Mo(VI)-alkylidene species. This view is quite different from the one-step mechanism, which has been accepted in the community for three decades, however, fully consistent with the empirically recognized importance of acidity, reducibility, and strict dehydration of the catalyst. The knowledge acquired in the present work has been successfully implemented for catalyst improvement. Simple heat treatment after the initial propene adsorption doubled the catalytic activity by accelerating the oxidation and desorption-capturing steps, demonstrating the merit of knowledge-based strategies in heterogeneous catalysis. Molecular structure of active Mo(VI)-alkylidene sites derived from surface molybdena is discussed in the context of similarity to the highly active Schrock-type homogeneous catalysts.

Olefin Metathesis by Group VI (Mo, W) Metal Compounds

“…only 5% of tungsten atoms are active in olein metathesis. This value is similar to what had been reported for MoO 3 supported on silica [53] and that proposed by Wachs et al for WO 3 on silica [51]. Unfortunately, no mechanism was proposed to explain the transformation of the mono-oxo tungsten (IV) species into the bisiloxy-oxo-carbene (≡SiO) 2 (O)W═CHR (R = H, CH 3 ).…”

Silica-Supported Tungsten Neosilyl Oxo Precatalysts: Impact of the Podality on Activity and Stability in Olefin Metathesis

“…50,51 In all cases, among the various theories available to compute chemical shielding tensors, the gauge including atomic orbital (GIAO) method has been adopted for the numerous advantages it presents. 52−57 The same methodology was used in previous studies involving grafted systems, showing that theoretical results are fairly accurate with respect to experimental values with an error lower than 15% for 29 Si, 31,32 10% for 31 P 58 and 17 O, 13,23,30,35 and 5% for 1 H and 13 C. 59 Typically, in order to compare our calculations with experimental values, 17 O chemical shielding has been converted to chemical shift using the usual equation δ iso = σ iso ref − σ iso sample , where σ iso ref is the isotropic 17 O chemical shielding of the liquid water. In a continuation of our previous studies, 13,23,30,35 an internal reference is used for the calibration of the σ iso ref value: σ iso ref = 292.2 ppm.…”

“…[W(CSiMe 3 )(CH 2 SiMe 3 ) 3 ] was synthesized following the literature procedure. 17 O-Enriched [WO-(CH 2 SiMe 3 ) 3 Cl] was prepared using 90 wt % 17 O-labeled water. Propylene (Air Liquide, 99.5%) was dried and deoxygenated before use by passing it through freshly regenerated molecular sieves (3 Å) and R-3-15 catalysts (BASF).…”

“…In a continuation of our previous studies, 13,23,30,35 an internal reference is used for the calibration of the σ iso ref value: σ iso ref = 292.2 ppm. The 17 O quadrupolar coupling constant C Q and the asymmetry parameter η Q , which describes the interaction between the nuclear quadrupolar moment of the oxygen nuclei and the electric field gradient (EFG) arising at these sites, are calculated from the EFG tensor eigenvalues V 11 , V 22 , and V 33 .…”

“…Recently, it has been proposed for the isoelectronic MoO 3 /SiO 2 olefin metathesis catalyst that the active site may also be a monopodal species, which can more easily lead to the active carbene by assistance of the acidic Mo-OH proton. 17 This stirs up the debate about the nature of the active site in supported group VI metathesis catalysts. It is evident that the podality will directly affect the electrophilicity and rigidity/flexibility around the metal center that may have an impact on the catalytic activity.…”