• Energy Research

The stability of a 1 wt % Pt/γ-Al2O3 catalyst was tested in an ethanol/water mixture at 225 °C and autogenic pressure, conditions at which it is possible to dissolve and depolymerize various kinds of lignin, and structural changes to the catalysts were studied by means of X-ray diffraction (XRD), 27Al MAS NMR, N2 physisorption, transmission electron microscopy (TEM), H2 chemisorption, elemental analysis, thermogravimetric analysis-mass spectrometry (TGA-MS), and IR. In the absence of reactants the alumina support is found to transform into boehmite within 4 h, leading to a reduction in support surface area, sintering of the supported Pt nanoparticles, and a reduction of active metal surface area. Addition of aromatic oxygenates to mimic the compounds typically obtained by lignin depolymerization leads to a slower transformation of the support oxide. These compounds, however, were not able to slow down the decrease in dispersion of the Pt nanoparticles. Vanillin and guaiacol stabilize the aluminum oxide more than phenol, anisole, and benzaldehyde because of the larger number of oxygen functionalities that can interact with the alumina. Interestingly, catalyst samples treated in the presence of lignin showed almost no formation of boehmite, no reduction in support or active metal surface area, and no Pt nanoparticle sintering. Furthermore, in the absence of lignin-derived aromatic oxygenates, ethanol forms a coke-like layer on the catalyst, while oxygenates prevent this by adsorption on the support by coordination via the oxygen functionalities.

AbstractLignin is an attractive renewable feedstock for aromatic bulk and fine chemicals production, provided that suitable depolymerization procedures are developed. Here, we describe a tandem catalysis strategy for ether linkage cleavage within lignin, involving ether hydrolysis by water‐tolerant Lewis acids followed by aldehyde decarbonylation by a Rh complex. In situ decarbonylation of the reactive aldehydes limits loss of monomers by recondensation, a major issue in acid‐catalyzed lignin depolymerization. Rate of hydrolysis and decarbonylation were matched using lignin model compounds, allowing the method to be successfully applied to softwood, hardwood, and herbaceous dioxasolv lignins, as well as poplar sawdust, to give the anticipated decarbonylation products and, rather surprisingly, 4‐(1‐propenyl)phenols. Promisingly, product selectivity can be tuned by variation of the Lewis‐acid strength and lignin source.

AbstractThe formation of hydrocarbon pool (HCP) species during methanol‐to‐olefin (MTO) and ethanol‐to‐olefin (ETO) processes have been studied on individual micron‐sized SAPO‐34 crystals with a combination of in situ UV/Vis, confocal fluorescence, and synchrotron‐based IR microspectroscopic techniques. With in situ UV/Vis microspectroscopy, the intensity changes of the λ=400 nm absorption band, ascribed to polyalkylated benzene (PAB) carbocations, have been monitored and fitted with a first‐order kinetics at low reaction temperatures. The calculated activation energy (Ea) for MTO, approximately 98 kJ mol−1, shows a strong correlation with the theoretical values for the methylation of aromatics. This provides evidence that methylation reactions are the rate‐determining steps for the formation of PAB. In contrast for ETO, the Ea value is approximately 60 kJ mol−1, which is comparable to the Ea values for the condensation of light olefins into aromatics. Confocal fluorescence microscopy demonstrates that during MTO the formation of the initial HCP species are concentrated in the outer rim of the SAPO‐34 crystal when the reaction temperature is at 600 K or lower, whereas larger HCP species are gradually formed inwards the crystal at higher temperatures. In the case of ETO, the observed egg‐white distribution of HCP at 509 K suggests that the ETO process is kinetically controlled, whereas the square‐shaped HCP distribution at 650 K is indicative of a diffusion‐controlled process. Finally, synchrotron‐based IR microspectroscopy revealed a higher degree of alkylation for aromatics for MTO as compared to ETO, whereas high reaction temperatures favor dealkylation processes for both the MTO and ETO processes.

AbstractAlthough industrialized, the mechanism for catalytic upgrading of bioethanol over solid‐acid catalysts (that is, the ethanol‐to‐hydrocarbons (ETH) reaction) has not yet been fully resolved. Moreover, mechanistic understanding of the ETH reaction relies heavily on its well‐known “sister‐reaction” the methanol‐to‐hydrocarbons (MTH) process. However, the MTH process possesses a C1‐entity reactant and cannot, therefore, shed any light on the homologation reaction sequence. The reaction and deactivation mechanism of the zeolite H‐ZSM‐5‐catalyzed ETH process was elucidated using a combination of complementary solid‐state NMR and operando UV/Vis diffuse reflectance spectroscopy, coupled with on‐line mass spectrometry. This approach establishes the existence of a homologation reaction sequence through analysis of the pattern of the identified reactive and deactivated species. Furthermore, and in contrast to the MTH process, the deficiency of any olefinic‐hydrocarbon pool species (that is, the olefin cycle) during the ETH process is also noted.

AbstractReview: 56 refs.

Artificially mimicking aging of an equilibrium catalyst (ECAT) is an effective strategy to model the deactivation of a Fluid Catalytic Cracking (FCC) catalyst during refinery operations. Herein, we have used a correlative microscopy approach to unravel inter-particle spatial heterogeneities in artificially deactivated catalysts (DCATs) and compared them with a real-life ECAT containing on average 3800 ppm of Ni and 2300 ppm of V, and a set of density separated ECAT fractions. By doing so we could rationalize the effect of metal contaminants on catalyst acidity and pore accessibility. More specifically, the Fe, Ni, and V distributions were obtained using X-Ray Fluorescence (XRF), while Confocal Fluorescence Microscopy (CFM) after thiophene and Nile Blue A staining, respectively provided a visualization of Brønsted acid sites and accessibility distribution. We found that not only the metal poisons distribution, but also hydrothermal degradation, that affects ECATs dealumination and related acidity drop, need to be properly reproduced by artificial catalyst deactivation protocols. Fe contamination must also be taken into account since it affects matrix accessibility.

AbstractBio‐based furanics can be aromatized efficiently by sequential Diels–Alder (DA) addition and hydrogenation steps followed by tandem catalytic aromatization. With a combination of zeolite H‐Y and Pd/C, the hydrogenated DA adduct of 2‐methylfuran and maleic anhydride can thus be aromatized in the liquid phase and, to a certain extent, decarboxylated to give high yields of the aromatic products 3‐methylphthalic anhydride and o‐ and m‐toluic acid. Here, it is shown that a variation in the acidity and textural properties of the solid acid as well as bifunctionality offers a handle on selectivity toward aromatic products. The zeolite component was found to dominate selectivity. Indeed, a linear correlation is found between 3‐methylphthalic anhydride yield and the product of (strong acid/total acidity) and mesopore volume of H‐Y, highlighting the need for balanced catalyst acidity and porosity. The efficient coupling of the dehydration and dehydrogenation steps by varying the zeolite‐to‐Pd/C ratio allowed the competitive decarboxylation reaction to be effectively suppressed, which led to an improved 3‐methylphthalic anhydride/total aromatics selectivity ratio of 80 % (89 % total aromatics yield). The incorporation of Pd nanoparticles in close proximity to the acid sites in bifunctional Pd/H‐Y catalysts also afforded a flexible means to control aromatic products selectivity, as further demonstrated in the aromatization of hydrogenated DA adducts from other diene/dienophile combinations.