Role of Lewis and Brønsted Acid Sites in the Dehydration of Glycerol over Niobia

The role of Lewis and Brønsted sites in the dehydration of glycerol on niobium oxide and Na⁺-exchanged niobium oxide is investigated using FTIR spectroscopy supported by DFT calculations. Glycerol is impregnated on the catalysts at room temperature using an <i>ex-situ</i> method. Under high vacuum conditions, glycerol forms a stable multidentate alkoxy species through its primary hydroxyl groups with the Lewis sites. When coordinated this way, the primary C–O bonds are polarized, favoring dehydration in this position to form hydroxyacetone. In contrast, dehydration of the secondary alcohol group is kinetically favored over Brønsted acid sites in the absence of steric constraints. The primary product of this reaction, 1,3-propenediol, is further dehydrated to acrolein. When more than a monolayer of glycerol is impregnated on niobia, monoaromatic compounds are also formed on the surface upon heating

Surface Interactions of C₂ and C₃ Polyols with γ‑Al₂O₃ and the Role of Coadsorbed Water

The formation of surface species from two- and three-carbon polyols on γ-Al₂O₃ in the presence and absence of coadsorbed water is investigated. Aqueous-phase adsorption isotherms indicate that competitive adsorption between water and polyol inhibits the uptake of the polyol molecules on γ-Al₂O₃ and that the polyol with the most hydroxyl groups, glycerol, experienced the greatest uptake. Deuterium solid echo pulse NMR measurements support the fact that glycerol strongly interacts with γ-Al₂O₃ in the presence of physisorbed water and that ethylene glycol interacts with γ-Al₂O₃ only after the physisorbed water has been removed. In situ high-vacuum FT-IR analysis combined with DFT simulations demonstrate that glycerol readily forms a multidentate alkoxy species through its primary hydroxyl groups with coordinatively unsaturated Al atoms of γ-Al₂O₃ in the presence of physisorbed water. This surface species exhibits a bridging alkoxy bond from one of its primary hydroxyl groups and a strong interaction with the remaining primary hydroxyl group. FT-IR analysis of 1,3-propanediol on γ-Al₂O₃ also demonstrates the formation of a multidentate alkoxy species in the presence of coadsorbed water. In contrast, polyols with hydroxyl groups only on the one- and two-carbon atoms, ethylene glycol, and 1,2-propanediol do not form alkoxy bonds with the γ-Al₂O₃ surface when coadsorbed water is present. These polyols will form alkoxy bonds to γ-Al₂O₃ when coadsorbed water is removed, and these alkoxy species are removed when water is readsorbed on the sample. The formation of strongly bound, stable multidentate alkoxy species by ethylene glycol and 1,2-propanediol on γ-Al₂O₃ is prevented by steric limitations of vicinal alcohol groups

Effect of Temperature, Pressure, and Residence Time on Pyrolysis of Pine in an Entrained Flow Reactor

High-pressure biomass gasification is poorly understood at heating rates of practical significance. This paper addresses this knowledge gap by performing pyrolysis of pine at high temperatures (600–1000 °C) and high pressures (5–20 bar) in an entrained flow reactor. Heating rates of 10³–10⁴ °C/s are achieved with solids residence time ranging from 4 to 28 s. The pyrolysis chars, gases, and tars are characterized using several techniques: N₂ and CO₂ physisorption, elemental analyses, SEM, XRD, micro-GC, FTIR-MS, and GCxGC-TOF-MS. The evolution of gases at high pressure is studied by pyrolyzing pine in PTGA at 800 °C between 5 and 30 bar. Pyrolysis pressure, temperature, heating rate, and residence time dramatically influence the physical and chemical properties of char, mainly through differences in the release of volatiles, evolution of char morphology, and carbonization of the char skeleton. The surface area and pore properties of chars correlate with the development of graphite-like structures in the carbon matrix. The gas composition from both the PTGA and PEFR shows that CO, CO₂, H₂, and CH₄ are the major light gases evolved, whereas C₂–C₄ hydrocarbons, oxygenates, and benzene are the minor light gas species observed. The formation of polynuclear aromatic tars at the longest residence times appears to occur via gas phase molecular weight growth reactions. The knowledge of char structure evolution developed in this paper will help us better understand char gasification kinetics which is important for the design of gasifiers

Enhanced Hydrothermal Stability of γ‑Al₂O₃ Catalyst Supports with Alkyl Phosphonate Coatings

In this study, monolayers formed from organophosphonic acids were employed to stabilize porous γ-Al₂O₃, both as a single component and as a support for Pt nanoparticle catalysts, during exposure to hydrothermal conditions. To provide a baseline, structural changes of uncoated γ-Al₂O₃ catalysts under model aqueous phase reforming conditions (liquid water at 200 °C and autogenic pressure) were examined over the course of 20 h. These changes were characterized by X-ray diffraction, NMR spectroscopy, N₂ physisorption, and IR spectroscopy. It was demonstrated that γ-alumina was rapidly converted into a hydrated boehmite (AlOOH) phase with significantly decreased surface area. Deposition of alkyl phosphonate groups on γ-alumina drastically inhibited the formation of boehmite, thereby maintaining its high specific surface area over 20 h of treatment. ²⁷Al MAS NMR spectra demonstrated that hydrothermal stability increased with alkyl tail length despite lower P coverages. Although the inhibition of boehmite formation by the phosphonic acids was attributed primarily to the formation of Al₂O₃–PO_<i>x</i> bonds, it was found that use of longer-chain octadecylphosphonic acids led to the most pronounced effect. Phosphonate coatings on Pt/γ-Al₂O₃ improved stability without adversely affecting the rate of a model reaction, catalytic hydrogenation of 1-hexene

Surface Interactions of Glycerol with Acidic and Basic Metal Oxides

The surface species formed by glycerol on γ-Al₂O₃, TiO₂ anatase, ZrO₂, MgO, and CeO₂ both in the presence and in the absence of bulk water are investigated with infrared spectroscopy. The acid–base properties of the metal oxides are characterized by pyridine and CO₂ adsorption/temperature-programmed desorption. The metal oxides studied provide a distribution of strengths of Lewis acid sites as well as strengths and types of basic sites which afford insight into the role of these various sites in polyol/metal oxide surface interactions. Even in the presence of bulk water, glycerol forms a bridging alkoxy bond through a primary alcohol group to two coordinatively unsaturated metal atoms and participates in a Lewis acid/base interaction between the oxygen atom of the other primary alcohol and a coordinatively unsaturated metal atom that is also involved in the alkoxy bond. These interactions only occur with metal oxides which contain strong Lewis acid sites. A quantitative correlation between the C–O stretching frequencies of the chemisorbed groups and the electronegativity of the metal atoms is established. Glycerol experiences an additional surface interaction via a hydrogen-bonding interaction between its secondary alcohol group and a relatively weak basic surface oxygen atom. Stronger base sites are blocked by adsorbed water or CO₂. In the absence of strong Lewis acid sites, in the case of MgO, hydrogen-bonding interactions between glycerol and surface hydroxyls are the dominant means of interaction

Important Roles of Enthalpic and Entropic Contributions to CO₂ Capture from Simulated Flue Gas and Ambient Air Using Mesoporous Silica Grafted Amines

The measurement of isosteric heats of adsorption of silica supported amine materials in the low pressure range (0–0.1 bar) is critical for understanding the interactions between CO₂ and amine sites at low coverage and hence to the development of efficient amine adsorbents for CO₂ capture from flue gas and ambient air. Heats of adsorption for an array of silica-supported amine materials are experimentally measured at low coverage using a Calvet calorimeter equipped with a customized dosing manifold. In a series of 3-aminopropyl-functionalized silica materials, higher amine densities resulted in higher isosteric heats of adsorption, clearly showing that the density/proximity of amine sites can influence the amine efficiency of adsorbents. In a series of materials with fixed amine loading but different amine types, strongly basic primary and secondary amine materials are shown to have essentially identical heats of adsorption near 90 kJ/mol. However, the adsorption uptakes vary substantially as a function of CO₂ partial pressure for different primary and secondary amines, demonstrating that entropic contributions to adsorption may play a key role in adsorption at secondary amine sites, making adsorption at these sites less efficient at the low coverages that are important to the direct capture of CO₂ from ambient air. Thus, while primary amines are confirmed to be the most effective amine types for CO₂ capture from ambient air, this is not due to enhanced enthalpic contributions associated with primary amines over secondary amines, but may be due to unfavorable entropic factors associated with organization of the second alkyl chain on the secondary amine during CO₂ adsorption. Given this hypothesis, favorable entropic factors may be the main reason primary amine based adsorbents are more effective under air capture conditions

Effect of Humidity on the CO₂ Adsorption of Tertiary Amine Grafted SBA-15

Aminosilica materials are promising candidates for CO₂ capture from dilute streams such as ambient air and flue gas. Most aminosilica sorbents are constructed using primary and/or secondary amines, which have been shown to primarily react with CO₂ to form alkylammonium carbamates and related structures. While ammonium bicarbonate formation is known to occur in aqueous amine solutions in the presence of CO₂, there has been conflicting evidence of its formation on solid supported analogues. To probe if the ammonium bicarbonate species can exist on solid supported amines, tertiary amines, which are known to form bicarbonates in aqueous solution, are grafted onto mesoporous silica SBA-15, and the materials are further characterized using in situ FTIR spectroscopy and solid-state NMR spectroscopy in the presence of humid and dry CO₂. Dry and humid CO₂ capacities for these sorbents are also evaluated using fixed bed experiments and thermogravimetric analysis. This work shows that ammonium bicarbonates can exist on solid supported amines but also demonstrates that tertiary amines are poor CO₂ sorbents under the conditions employed

¹⁵N Solid State NMR Spectroscopic Study of Surface Amine Groups for Carbon Capture: 3‑Aminopropylsilyl Grafted to SBA-15 Mesoporous Silica

Materials composed of high-porosity solid supports, such as SBA-15, containing amine-bearing moieties inside the pores, such as 3-aminopropylsilane (APS), are envisioned for carbon dioxide capture; solid-state ¹⁵N NMR can be highly informative for studying chemisorption reactions. Two ¹⁵N-enriched samples with different APS loadings were studied to probe the identity of the pendant molecules and structure of the chemisorbed CO₂ species. ¹⁵N cross-polarization magic-angle spinning NMR provides unique information about the amines, whether they are rigid or dynamic, by measuring contact time curves and rotating frame, T_1ρ(¹⁵N), relaxation. Both carbamate and carbamic acid are formed; carbamic acid is shown to be less stable than carbamate. After desorption, a steady state for the chemisorbed reaction product is reached, leaving behind carbamate. ¹⁵N NMR monitors the evolution of the species over time. During desorption, APS is regenerated, but the ammonium propylsilane intensity does not change, leading us to conclude that carbamic acid desorbs, while carbamate (to which ammonium propylsilane is ion paired) persists. A secondary ditehtered amine present does not react with CO₂, and we posit this may be due to its rigidity. These findings demonstrate the versatility of solid-state NMR to provide information about these complex CO₂ reactions with solid amine sorbents

Nature and Location of Cationic Lanthanum Species in High Alumina Containing Faujasite Type Zeolites

The nature, concentration, and location of cationic lanthanum species in faujasite-type zeolites (zeolite X, Y and ultrastabilized Y) have been studied in order to understand better their role in hydrocarbon activation. By combining detailed physicochemical characterization and DFT calculations, we demonstrated that lanthanum cations are predominantly stabilized within sodalite cages in the form of multinuclear OH-bridged lanthanum clusters or as monomeric La³⁺ at the SI sites. In high-silica faujasites (Si/Al = 4), monomeric [La(OH)]²⁺ and [La(OH)₂]⁺ species were only found in low concentrations at SII sites in the supercages, whereas the dominant part of La³⁺ is present as multinuclear OH-bridged cationic aggregates within the sodalite cages. Similarly, in the low-silica (Si/Al = 1.2) La–X zeolite, the SI′ sites were populated by hydroxylated La species in the form of OH-bridged bi- and trinuclear clusters. In this case, the substantial repulsion between the La³⁺ cations confined within the small sodalite cages induces the migration of La³⁺ cations into the supercage SII sites. The uniquely strong polarization of hydrocarbon molecules sorbed in La–X zeolites is caused solely by the interaction with the accessible isolated La³⁺ cations

Stability of Pt/γ-Al₂O₃ Catalysts in Lignin and Lignin Model Compound Solutions under Liquid Phase Reforming Reaction Conditions

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