No abstract

Sn-β zeolites prepared using different recipes feature very different catalytic activity for aqueous phase glucose isomerization, suggesting the presence of different active sites. A systematic study of the morphology and atomic-level structure of the materials using DNP NMR spectroscopy in combination with first principles calculations allows for the discrimination between potential sites and 2 leads to a proposal of specific structural features that are important for high activity. The results indicate that the materials showing highest activity posses a highly hydrophobic, defect-free zeolite framework. Those materials show so-called closed and associated partially hydrolyzed Sn(IV)-sites in the T6 and T5/T7 lattice position. On the other hand post-synthetically synthesized Sn-b samples prepared in two steps via dealumination and subsequent solid-state ion-exchange from Al-b show significant lower activity which is associated with a hydrophilic framework and/or a lower accessibility and lattice position of the Sn sites in the zeolite crystal. Further we provide a method to distinguish between different Sn sites based on NMR cartography using chemical shift and chemical shift anisotropy as readily measurable parameters. This cartography not only allows identifying the nature of the active sites (closed, defect-open and hydrolyzed-open), but also their position within the BEA framework.

Dynamic nuclear polarization surface enhanced NMR (DNP-SENS), Mössbauer spectroscopy, and computational chemistry were combined to obtain structural information on the active-site speciation in Sn-β zeolite. This approach unambiguously shows the presence of framework Sn(IV)-active sites in an octahedral environment, which probably correspond to so-called open and closed sites, respectively (namely, tin bound to three or four siloxy groups of the zeolite framework).

Significance The Phillips catalyst—CrO x /SiO 2 —produces 40–50% of global high-density polyethylene, yet several fundamental mechanistic controversies surround this catalyst. What is the oxidation state and nuclearity of the active Cr sites? How is the first Cr–C bond formed? How does the polymer propagate and regulate its molecular weight? Here we show through combined experimental (infrared, ultraviolet-visible, X-ray near edge absorption spectroscopy, and extended X-ray absorption fine structures) and density functional theory modeling approaches that mononuclear tricoordinate Cr(III) sites immobilized on silica polymerize ethylene by the classical Cossee–Arlman mechanism. Initiation (C–H bond activation) and polymer molecular weight regulation (the microreverse of C–H activation) are controlled by proton transfer steps.

We describe the reactivity of well-defined chromium silicates toward ethylene and propane. The initial motivation for this study was to obtain a molecular understanding of the Phillips polymerization catalyst. The Phillips catalyst contains reduced chromium sites on silica and catalyzes the polymerization of ethylene without activators or a preformed Cr-C bond. Cr(II) sites are commonly proposed active sites in this catalyst. We synthesized and characterized well-defined chromium(II) silicates and found that these materials, slightly contaminated with a minor amount of Cr(III) sites, have poor polymerization activity and few active sites. In contrast, chromium(III) silicates have 1 order of magnitude higher activity. The chromium(III) silicates initiate polymerization by the activation of a C-H bond of ethylene. Density functional theory analysis of this process showed that the C-H bond activation step is heterolytic and corresponds to a σ-bond metathesis type process. The same well-defined chromium(III) silicate catalyzes the dehydrogenation of propane at elevated temperatures with activities similar to those of a related industrial chromium-based catalyst. This reaction also involves a key heterolytic C-H bond activation step similar to that described for ethylene but with a significantly higher energy barrier. The higher energy barrier is consistent with the higher pKa of the C-H bond in propane compared to the C-H bond in ethylene. In both cases, the rate-determining step is the heterolytic C-H bond activation.

A detailed exploration of the three proposed mechanisms (associative, dissociative, and interchange) for the activation of Grubbs–Hoveyda-type precatalysts is performed, using DFT (B3LYP) calculations. The effects induced by the nature of the reacting alkene, the bulk of the chelating alkoxy group, and the presence of substituents in the Hoveyda ligand are taken into account. Results show that, while the associative mechanism has always high energy barriers, neither the dissociative nor the interchange mechanism can be ruled out for the first step of the activation process. In fact, the preference for one or the other mechanism seems to be influenced, to a large extent, by the nature of the chelating alkoxy group in such a way that small OR groups tend to favor the interchange pathway. Moreover, for all considered Grubbs–Hoveyda-type precatalysts, the highest transition structure corresponds to the Hoveyda ligand decoordination at the end of the cross-metathesis process. It is worth noting that this is observed regardless of the initial alkene coordination pathway (dissociative or interchange), precursor nature, and substrate and, thus, the rate-determining transition structure in all considered cases is the final alkene decoordination process. In contrast, the highest transition structure for the activation process of the phosphine-containing complexes is the initial phosphine dissociation, for which the reacting alkene is not yet involved. Overall, although the interchange mechanism may also have a role, the present calculations show that the different sign of the experimentally measured activation entropies is more likely associated with a change in the nature of the rate-determining transition structure rather than in a change of the nature of the elementary steps.

Grafting molecular precursors on partially dehydroxylated silica followed by a thermal treatment yields silica-supported M(III) sites for a broad range of metals. They display unique properties such as high activity in olefin polymerization and alkane dehydrogenation (M = Cr) or efficient luminescence properties (M = Yb and Eu) essential for bioimaging. Here, we interrogate the local structure of the M(III) surface sites obtained from two molecular precursors, amides M(N(SiMe)) vs siloxides (M(OSi(OBu))·L with L = (THF) or HOSi(OBu) for M = Cr, Yb, Eu, and Y, by a combination of advanced spectroscopic techniques (EPR, IR, XAS, UV-vis, NMR, luminescence spectroscopies). For paramagnetic Cr(III), EPR (HYSCORE) spectroscopy shows hyperfine coupling to nitrogen only when the amide precursor is used, consistent with the presence of nitrogen neighbors. This changes their specific reactivity compared to Cr(III) sites in oxygen environments obtained from siloxide precursors: no coordination of CO and oligomer formation during the polymerization of ethylene due to the presence of a N-donor ligand. The presence of the N-ligand also affects the photophysical properties of Yb and Eu by decreasing their lifetime, probably due to nonradiative deactivation of excited states by N-H bonds. Both types of precursors lead to a distribution of surface sites according to reactivity for Cr, luminescence spectroscopy for Yb and Eu, and dynamic nuclear polarization surface-enhanced Y NMR spectroscopy (DNP SENS). In particular, DNP SENS provides molecular-level information about the structure of surface sites by evidencing the presence of tri-, tetra-, and pentacoordinated Y-surface sites. This study provides unprecedented evidence and tools to assess the local structure of metal surface sites in relation to their chemical and physical properties.

Heterolytic C-H bond activation of ethylene on Cr III-O has been proposed as the initial step in olefin polymerization on (≡SiO) 3 Cr III active sites of molecularly-defined analogues of the Philips catalyst. Here, by using realistic amorphous periodic models that account for structural complexity, strain, and active site heterogeneity of welldefined silica-supported Cr III-based polymerization catalysts, we show that this activation step is significantly favored due to the strain present on highly dehydroxylated silica. Furthermore, we find that initiation by insertion of ethylene into the Cr-O bond is even more favorable, especially for more strained sites, while both mechanisms can compete for less strained and thereby less active sites. Our results suggest a competing dual pathway for ethylene polymerization on Cr III sites and are consistent with a distribution of active sites, the experimentally observed broad distribution of polymer molecular weight, and the increased polymerization activity upon high-temperature calcination in Phillips catalysts.