Second-Sphere Effects on Methane Hydroxylation in Cu-Zeolites

Two [Cu₂O]²⁺ cores have been identified as the active sites of low temperature methane hydroxylation in the zeolite Cu-MOR. These cores have similar geometric and electronic structures, yet different reactivity with CH₄: one reacts with a much lower activation enthalpy. In the present study, we couple experimental reactivity and spectroscopy studies to DFT calculations to arrive at structural models of the Cu-MOR active sites. We find that the more reactive core is located in a constricted region of the zeolite lattice. This leads to close van der Waals contact between the substrate and the zeolite lattice in the vicinity of the active site. The resulting enthalpy of substrate adsorption drives the subsequent H atom abstraction stepa manifestation of the “nest” effect seen in hydrocarbon cracking on acid zeolites. This defines a mechanism to tune the reactivity of metal active sites in microporous materials

Identification of α‑Fe in High-Silica Zeolites on the Basis of ab Initio Electronic Structure Calculations

α-Fe is the precursor of the reactive Fe^IVO core responsible for methane oxidation in Fe-containing zeolites. To get more insight into the nature and stability of α-Fe in different zeolites, the binding of Fe­(II) at six-membered-ring cation exchange sites (6MR) in ZSM-5, zeolite beta, and ferrierite was investigated using DFT and multireference ab initio methods (CASSCF/CASPT2). CASPT2 ligand field (LF) excitation energies of all sites were compared with the experimental DR-UV–vis spectra reported by Snyder et al. From this comparison it is concluded that the 16000 cm^–1 band of α-Fe, observed in all three zeolites, can uniquely be assigned to a high-spin square-planar (SP) Fe­(II) located at a 6MR with an Al–Si–Si–Al sequence, where the Al atoms are positioned opposite in the ring and as close to each other as possible. The stability of such conformations is also confirmed by the binding energies obtained from DFT. The bands at 10000 cm^–1 in the experimental spectra, assigned to spectator Fe­(II), are attributed to six-coordinated trigonal-prismatic Fe­(II) species, as calculated for the γ-site in ZSM-5. The entatic effect of the zeolite lattice on the stability of the SP sites was investigated by making use of the unconstrained Fe­(II) model complex FeL₂ (with L = [Al­(OH)₄]⁻). The SP conformer is approximately 2 kcal/mol more stable than the tetrahedral form, indicating that the SP coordination environment of α-Fe is not imposed by the zeolite lattice but rather electronically preferred by Fe­(II) in the environment of four O ligands. A significant contribution to the stability of the SP conformer is provided by mixing of the doubly occupied 3d_<i>z</i>² orbital with the higher lying 4s

Spectroscopic Definition of the Copper Active Sites in Mordenite: Selective Methane Oxidation

Two distinct [Cu–O–Cu]²⁺ sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu–O–Cu active site geometry, having a ∠Cu–O–Cu ∼140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu–O–Cu]²⁺ active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu–O–Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu–O–Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites

Fast and Selective Sugar Conversion to Alkyl Lactate and Lactic Acid with Bifunctional Carbon–Silica Catalysts

A novel catalyst design for the conversion of mono- and disaccharides to lactic acid and its alkyl esters was developed. The design uses a mesoporous silica, here represented by MCM-41, which is filled with a polyaromatic to graphite-like carbon network. The particular structure of the carbon-silica composite allows the accommodation of a broad variety of catalytically active functions, useful to attain cascade reactions, in a readily tunable pore texture. The significance of a joint action of Lewis and weak Brønsted acid sites was studied here to realize fast and selective sugar conversion. Lewis acidity is provided by grafting the silica component with Sn­(IV), while weak Brønsted acidity originates from oxygen-containing functional groups in the carbon part. The weak Brønsted acid content was varied by changing the amount of carbon loading, the pyrolysis temperature, and the post-treatment procedure. As both catalytic functions can be tuned independently, their individual role and optimal balance can be searched for. It was thus demonstrated for the first time that the presence of weak Brønsted acid sites is crucial in accelerating the rate-determining (dehydration) reaction, that is, the first step in the reaction network from triose to lactate. Composite catalysts with well-balanced Lewis/Brønsted acidity are able to convert the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into ethyl lactate in ethanol with an order of magnitude higher reaction rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly, the ability to tailor the pore architecture further allows the synthesis of a variety of amphiphilic alkyl lactates from trioses and long chain alcohols in moderate to high yields. Finally, direct lactate formation from hexoses, glucose and fructose, and disaccharides composed thereof, sucrose, was also attempted. For instance, conversion of sucrose with the bifunctional composite catalyst yields 45% methyl lactate in methanol at slightly elevated reaction temperature. The hybrid catalyst proved to be recyclable in various successive runs when used in alcohol solvent