Pieter Vanelderen

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.

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.

Ternary Ag/MgO-SiO2 catalysts for the conversion of ethanol into butadiene.

Ternary Ag/Magnesia-silica catalysts were tested in the direct synthesis of 1,3-butadiene from ethanol. The influence of the silver content and the type of silica source on catalytic performance has been studied. Prepared catalysts were characterized by (29) Si NMR, N2 sorption, small-angle X-ray scattering measurements, XRD, environmental scanning electron microscopy with energy dispersive X-ray analysis (ESEM/EDX), FTIR spectroscopy of adsorbed pyridine and CO2 , temperature-programmed desorption of CO2 and UV/Vis diffuse reflectance spectroscopy. Based on these characterization results, the catalytic performance of the catalysts in the 1,3-butadiene formation process was interpreted and a tentative model explaining the role of the different catalytically active sites was elaborated. The balance of the active sites is crucial to obtain an active and selective catalyst to form 1,3-butadiene from ethanol. The optimal silver loading is 1-2 wt% on a MgO-silica support with a molar Mg/Si ratio of 2. The silver species and basic sites (Mg−O pairs and basic OH groups) are of prime importance in the 1,3-butadiene production, catalyzing mainly the ethanol dehydrogenation and the aldol condensation, respectively.

The active site of low-temperature methane hydroxylation in iron-containing zeolites

An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O; however, despite nearly three decades of research, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species—α-Fe(ii) and α-O—are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive ‘spectator’ iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function—producing what is known in the context of metalloenzymes as an ‘entatic’ state—might be a useful way to tune the activity of heterogeneous catalysts.

Spectroscopy and Redox Chemistry of Copper in Mordenite

Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the literature lacks appropriate spectroscopic and structural information on such Cu-rich zeolite samples. Higher copper loadings increase the complexity of the copper identity and their location in the zeolite host, but they also provide the opportunity to create novel Cu sites, which are perhaps energetically less favorable, but possibly more reactive and more suitable for catalysis. In order to address the different role of each Cu site in catalysis, we here report a combined electron paramagnetic resonance (EPR), UV/Vis-NIR and temperature-programmed reduction (TPR) study on highly copper-loaded MOR. Highly resolved diffuse reflectance (DR) spectra of the CuMOR samples were obtained due to the increased copper loading, allowing the differentiation of two isolated mononuclear Cu(2+) sites and the unambiguous correlation with extensively reported features in the EPR spectrum. Ligand field theory is applied together with earlier suggested theoretical calculations to determine their coordination chemistry and location within the zeolite matrix, and the theoretical analysis further allowed us to define factors governing their redox behavior. In addition to monomeric species, an EPR-silent, possibly dimeric, copper site is present in accordance with its charge transfer absorption feature at 22200 cm(-1), and quantified with TPR. Its full description and true location in MOR is currently being investigated.

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 FeIV═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 3dz2 orbital with the higher lying 4s.