Emiel Emiel Hensen

Highly efficient and robust Au/MgCuCr2O4 catalyst for gas-phase oxidation of ethanol to acetaldehyde

Gold nanoparticles (AuNPs) supported on MgCuCr2O4-spinel are highly active and selective for the aerobic oxidation of ethanol to acetaldehyde (conversion 100%; yield ∼95%). The catalyst is stable for at least 500 h. The unprecedented catalytic performance is due to strong synergy between metallic AuNPs and surface Cu(+) species. X-ray photoelectron spectroscopy shows that Cu(+) is already formed during catalyst preparation and becomes more dominant at the surface during ethanol oxidation. These Cu(+) species are stabilized at the surface of the ternary MgCuCr2O4-spinel support. Further kinetic measurements indicate that the Cu(+) species act as sites for O2 activation.

Catalytic depolymerization of lignin in supercritical ethanol.

One-step valorization of soda lignin in supercritical ethanol using a CuMgAlOx catalyst results in high monomer yield (23 wt%) without char formation. Aromatics are the main products. The catalyst combines excellent deoxygenation with low ring-hydrogenation activity. Almost half of the monomer fraction is free from oxygen. Elemental analysis of the THF-soluble lignin residue after 8 h reaction showed a 68% reduction in O/C and 24% increase in H/C atomic ratios as compared to the starting Protobind P1000 lignin. Prolonged reaction times enhanced lignin depolymerization and reduced the amount of repolymerized products. Phenolic hydroxyl groups were found to be the main actors in repolymerization and char formation. 2D HSQC NMR analysis evidenced that ethanol reacts by alkylation and esterification with lignin fragments. Alkylation was found to play an important role in suppressing repolymerization. Ethanol acts as a capping agent, stabilizing the highly reactive phenolic intermediates by O-alkylating the hydroxyl groups and by C-alkylating the aromatic rings. The use of ethanol is significantly more effective in producing monomers and avoiding char than the use of methanol. A possible reaction network of the reactions between the ethanol and lignin fragments is discussed.

Ethanol as capping agent and formaldehyde scavenger for efficient depolymerization of lignin to aromatics

Obtaining renewable fuels and chemicals from lignin presents an important challenge to the use of lignocellulosic biomass to meet sustainability and energy goals. We report on a thermocatalytic process for the depolymerization of lignin in supercritical ethanol over a CuMgAlOx catalyst. Ethanol as solvent results in much higher monomer yields than methanol. In contrast to methanol, ethanol acts as a scavenger of formaldehyde derived from lignin decomposition. Studies with phenol and alkylated phenols evidence the critical role of the phenolic –OH groups and formaldehyde in undesired repolymerization reactions. O-alkylation and C-alkylation capping reactions with ethanol hinder repolymerization of the phenolic monomers formed during lignin disassembly. After reaction in ethanol at 380 °C for 8 h, this process delivers high yields of mainly alkylated mono-aromatics (60–86 wt%, depending on the lignin used) with a significant degree of deoxygenation. The oxygen-free aromatics can be used to replace reformate or can serve as base aromatic chemicals; the oxygenated aromatics can be used as low-sooting diesel fuel additives and as building blocks for polymers.

Synergy between Lewis acid sites and hydroxyl groups for the isomerization of glucose to fructose over Sn-containing zeolites: a theoretical perspective

The mechanism of glucose isomerization to fructose catalyzed by Lewis acidic Sn sites in the framework of MOR, BEA, MFI and MWW zeolites was investigated by periodic DFT calculations. The main focus was on the influence of the nature of the active site and the zeolite topology on the rate-controlling hydride shift step. A general finding is that the Sn-catalyzed isomerization of glucose is strongly promoted by proximate hydroxyl groups. These hydroxyl groups can derive from co-adsorbed water molecules or internal silanols. The cooperative action of such proton donors with the Lewis acidic Sn sites results in more effective compensation of the negative charge developing on the O1 atom of glucose during the rate-controlling hydride shift reaction step. The variation in the shape of the micropores with a zeolite topology affects the mode and strength of carbohydrate adsorption, which is dominated by van der Waals forces. Their influence on the intrinsic reactivity of intrazeolite Sn sites is small. We propose that higher glucose adsorption energy in the narrower micropores of 10-membered ring zeolites (e.g., Sn-MFI and Sn-MWW) adversely affects the intrachannel diffusion compared to that in the zeolites with larger pores. The high catalytic performance of Sn-MWW towards glucose transformation is due to the lower barrier for the hydride shift step resulting from the presence of a relatively strong acidic bridging silanol group next to the Lewis acidic Sn site.

The Mechanism of Glucose Isomerization to Fructose over Sn‐BEA Zeolite: A Periodic Density Functional Theory Study

The isomerization of glucose to fructose in the presence of Sn-containing zeolite BEA (beta polymorph A) was studied by periodic DFT calculations. Focus was placed on the nature of the active site and the reaction mechanism. The reactivities of the perfect lattice Sn(IV) site and the hydroxylated SnOH species are predicted to be similar. The isomerization activity of the latter can be enhanced by creating an extended silanol nest in its vicinity. Besides the increased Lewis acidity and coordination flexibility of the Sn center, the enhanced reactivity in this case is ascribed to the reaction environment that promotes activation of the confined sugar intermediates through hydrogen bonding. The resulting multidentate activation of the substrate favors the rate-determining hydrogen-shift reaction. These findings suggest the important role of defect lattice sites in Sn-BEA for catalytic glucose isomerization.

Efficient Tandem Synthesis of Methyl Esters and Imines by Using Versatile Hydrotalcite‐Supported Gold Nanoparticles

Efficient basic hydrotalcite (HT)-supported gold nanoparticle (AuNP) catalysts have been developed for the aerobic oxidative tandem synthesis of methyl esters and imines from primary alcohols catalyzed under mild and soluble-base-free conditions. The catalytic performance can be fine-tuned for these cascade reactions by simple adjustment of the Mg/Al atomic ratio of the HT support. The one-pot synthesis of methyl esters benefits from high basicity (Mg/Al = 5), whereas moderate basicity greatly improves imine selectivity (Mg/Al = 2). These catalysts outperform previously reported AuNP catalysts by far. Kinetic studies show a cooperative enhancement between AuNP and the surface basic sites, which not only benefits the oxidation of the starting alcohol but also the subsequent steps of the tandem reactions. To the best of our knowledge, this is the first time that straightforward control of the composition of the support has been shown to yield optimum AuNP catalysts for different tandem reactions.

Mechanism of CO2 hydrogenation to formates by homogeneous Ru-PNP pincer catalyst : from a theoretical description to performance optimization

The reaction mechanism of CO2 hydrogenation by pyridine-based Ru-PNP catalyst in the presence of DBU base promoter was studied by means of density functional theory calculations. Three alternative reaction channels promoted by the complexes potentially present under the reaction conditions, namely the dearomatized complex 2 and the products of cooperative CO2 (3) and H2 (4) addition, were analysed. It is shown that the bis-hydrido Ru-PNP complex 4 provides the unique lowest-energy reaction path involving a direct effectively barrierless hydrogenolysis of the polarized complex 5*. The reaction rate in this case is controlled by the CO2 activation by Ru–H that proceeds with a very low barrier of ca. 20 kJ mol−1. The catalytic reaction can be hampered by the formation of a stable formato-complex 5. In this case, the rate is controlled by the H2 insertion into the Ru–OCHO coordination bond, for which a barrier of 65 kJ mol−1 is predicted. The DFT calculations suggest that the preference for the particular route can be controlled by varying the partial pressure of H2 in the reaction mixture. Under H2-rich conditions, the former more facile catalytic path should be preferred. Dedicated kinetic experiments verify these theoretical predictions. The apparent activation energies measured at different H2/CO2 molar ratios are in a perfect agreement with the calculated values. Ru-PNP is a highly active CO2 hydrogenation catalyst allowing reaching turnover frequencies in the order of 106 h−1 at elevated temperatures. Moreover, a minor temperature dependency of the reaction rate attainable in excess H2 points to the possibility of efficient CO2 hydrogenation at near-ambient temperatures.

On the Mechanism of Lewis Acid Catalyzed Glucose Transformations in Ionic Liquids

A complementary computational and experimental study of the reactivity of Lewis acidic CrCl2, CuCl2 and FeCl2 catalysts towards glucose activation in dialkylimidazolium chloride ionic liquids is performed. The selective dehydration of glucose to 5‐hydroxymethylfurfural (HMF) proceeds through the intermediate formation of fructose. Although chromium(II) and copper(II) chlorides are able to dehydrate fructose with high HMF selectivity, reasonable HMF yields from glucose are only obtained with CrCl2 as the catalyst. Glucose conversion by CuCl2 is not selective, while FeCl2 catalyst does not activate sugar molecules. These differences in reactivity are rationalized on the basis of in situ X‐ray absorption spectroscopy measurements and the results of density functional theory calculations. The reactivity in glucose dehydration and HMF selectivity are determined by the behavior of the ionic liquid‐mediated Lewis acid catalysts towards the initial activation of the sugar molecules. The formation of a coordination complex between the Lewis acidic Cr2+ center and glucose directs glucose transformation into fructose. For Cu2+ the direct coordination of sugar to the copper(II) chloride complex is unfavorable. Glucose deprotonation by a mobile Cl− ligand in the CuCl42− complex initiates the nonselective conversion. In the course of the reaction the Cu2+ ions are reduced to Cu+. Both paths are prohibited for the FeCl2 catalyst.

Bis-N-heterocyclic carbene aminopincer ligands enable high activity in Ru-catalyzed ester hydrogenation

Bis-N-heterocyclic carbene (NHC) aminopincer ligands were successfully applied for the first time in the catalytic hydrogenation of esters. We have isolated and characterized a well-defined catalyst precursor as a dimeric [Ru2(L)2Cl3]PF6 complex and studied its reactivity and catalytic performance. Remarkable initial activities up to 283,000 h(-1) were achieved in the hydrogenation of ethyl hexanoate at only 12.5 ppm Ru loading. A wide range of aliphatic and aromatic esters can be converted with this catalyst to corresponding alcohols in near quantitative yields. The described synthetic protocol makes use of air-stable reagents available in multigram quantities, rendering the bis-NHC ligands an attractive alternative to the conventional phosphine-based systems.

A computational DFT study of CO oxidation on a Au nanorod supported on CeO2(110): on the role of the support termination

Possible reaction paths for CO oxidation on ceria-supported Au nanoparticle catalysts were modeled by placing a Au nanorod on a CeO2(110) surface. The results are discussed against experimental and computational data in the literature for Au/CeO2 with emphasis on the role of the ceria surface termination and involvement of ceria lattice oxygen atoms. Three CO oxidation mechanisms were modeled using density functional theory calculations: (i) reaction of adsorbed CO with ceria lattice O atoms (Mars–van Krevelen mechanism), (2) reaction of adsorbed CO with co-adsorbed O2 (co-adsorption mechanism) and (3) dissociation of adsorbed O2 followed by CO oxidation (stepwise mechanism). All three candidate mechanisms are relevant to CO oxidation catalysis as they exhibit nearly similar overall reaction barriers. The Mars–van Krevelen mechanism is consistent with experimental findings on the involvement of lattice O atoms in CO oxidation. This mechanism is prohibitive for CeO2(111) because of too high oxygen vacancy formation energy. Besides, the specific surface termination of CeO2(111) prevents O2 adsorption at its interface with Au due to repulsive interactions with the lattice O atoms. Molecular O2 adsorption is possible on CeO2(110) because of the presence of Ce4+ ions in the top layer of the surface. O2 adsorption can occur on a defective Au/CeO2(111) surface (J. Am. Chem. Soc., 2012, 134, 1560), because exposed Ce3+ ions are available. However, it is established here that O2 dissociation will heal the vacancies and deactivate Au supported on the CeO2(111) surface. The importance of Mars–van Krevelen and stepwise mechanisms in CO oxidation by Au/CeO2 strongly depends on the surface plane of the ceria support.