Understanding and Controlling Reactivity of Unsaturated Oxygenates and Polyols on Metal Catalysts

The influence of adsorption geometry on the reduction affinity of nitroaromatics on Au(111)

Chemoselective reduction of nitro groups in multifunctional nitroaromatics is a challenging catalytic process with high interest due to the importance of the resulting anilines for the chemical industry. Molecular-level understanding of the ways by which adsorption geometry of nitroaromatics influence their affinity toward nitro reduction will enable the development of highly selective reactions. Herein, taking advantage of the well-ordered self-assembly of para- and ortho-nitrothiophenol (p-NTP and o-NTP, respectively) monolayers on Au(111), we examined the correlation between adsorption geometry and nitro reduction affinity. The anchoring geometry of NTPs and their nitro reduction affinity were determined by conducting polarized X-ray absorption spectroscopy while the influence of NTPs' adsorption geometry on the interaction with the Au surface was analyzed by density functional theory (DFT) calculations. Exposure of surface anchored p-NTPs to reducing conditions led to their reorientation from a tilt angle of 52° to 25°, which enabled strong interactions between the π system of the molecules and the Au surface. Direct correlation was identified between the surface proximity of the nitro group, its parallel position to the surface and the resulting reduction yield. The asymmetric structure of o-NTP led to a tilted adsorption geometry in which the nitro group was rotated away from the plane of the aromatic ring and therefore was positioned parallel and in high proximity to the Au surface. This positioning led to surface-bonding that involved the oxygen atoms of o-NTP. The higher surface proximity and stronger surface interactions of the nitro group in o-NTP enabled nitro reduction already at 180 °C, while in p-NTP nitro reduction was achieved only at 230 °C, due to the longer distance between the NO₂ group and the Au surface that led to weaker adsorbate-surface interactions. Thus, parallel positioning of the nitro group and high surface proximity were found as essential descriptors for nitro reduction affinity in both p-NTP and o-NTP on the Au surface. These findings provide explicit guidelines for tuning the reactant and surface properties in order to control the reactant's adsorption geometry for selective nitro reduction in multifunctional nitroaromatics.

Ni-Cu/Al2O3 from Layered Double Hydroxides Hydrogenates Furfural to Alcohols

The hydrogenation of furfural is an important process in the synthesis of bio-based chemicals. Copper-based catalysts favor the hydrogenation of furfural to alcohols. Catalytic activity and stability were higher at a Ni-to-Cu atomic ratio of 1:1 and 1.5:0.5 compared to 0.5:1.5. Here, we prepared Ni-Cu/Al2O3 hydrogenation catalysts derived from layered double hydroxides (LDHs). Catalysts calcined at 673 K and reduced at 773 K with nominal Ni/Cu atomic ratios y/x = 1.5/0.5, 1/1 and 0.5/1.5 were characterized by XRD, FESEM-EDX, H2-TPR, XPS, FAA and BET. Their activity was tested at 463 K and in a 0.05 g g−1 furfural solution in ethanol, and the space velocity in a packed-bed reactor (PBR) was 2.85 gFF gcat−1 h−1. In a slurry reactor (SSR) at 5 MPa H2 and a contact time of 4 h, conversion was complete, while it varied from 91 to 99% in the PBR. Tetrahydrofurfuryl alcohol (TFA) was the main product in the SSR, with a selectivity of 32%, 63% and 56% for Ni0.5Cu1.5Al1, Ni1Cu1Al1 and Ni1.5Cu0.5Al1, respectively. The main product in the atmospheric PBR was furfuryl alcohol (FA), with a selectivity of 57% (Ni0.5Cu1.5Al1), 61% (Ni1Cu1Al1) and 58% (Ni1.5Cu0.5Al1). Other products included furan, methylfuran, 1-butanol and 1,2-pentanediol. Ethyl tetrahydrofurfuryl ether and difurfuryl ether were also formed via the nucleophilic addition of furfural with ethanol and furfuryl alcohol.

Modifier Effect in Silica-Supported FePO4 and Fe-Mo-O Catalysts for Propylene Glycol Oxidation

Currently, catalytic processing of biorenewable raw materials into valuable products attracts more and more attention. In the present work, silica-supported FePO₄ and Fe-Mo-O catalysts are prepared, their phase composition, and catalytic properties are studied in the process of selective oxidation of propylene glycol into valuable mono- and bicarbonyl compounds, namely, hydroxyacetone and methylglyoxal. A comparative analysis of the main routes of propylene glycol adsorption with its subsequent oxidative conversion into carbonyl products is carried out. The DFT calculations show that in the presence of adsorbed oxygen atom, the introduction of the phosphate moiety to the Fe-containing site strengthens the alcohol adsorption on the catalyst surface with the formation of the 1,2-propanedioxy (–OCH(CH₃)CH₂O–) intermediate at the active site. The introduction of the molybdenum moiety to the Fe-containing site in the presence of the adsorbed oxygen atom is also energetically favorable, however, the interaction energy is found by 100 kJ/mol higher compared to the case with phosphate moiety that leads to an increase in the propylene glycol conversion while maintaining high selectivity towards C₃ products. The catalytic properties of the synthesized iron-containing catalysts are experimentally compared with those of Ag/SiO₂ sample. The synthesized FePO₄/SiO₂ and Fe-Mo-O/SiO₂ catalysts are not inferior to the silver-containing catalyst and provide ~70% selectivity towards C₃ products, while the main part of propylene glycol is converted into methylglyoxal in contrast to the Ag/SiO₂ catalyst featuring the selective transformation of only the secondary C-OH group in the substrate molecule under the studied conditions with the formation of hydroxyacetone. Thus, supported Fe-Mo-O/SiO₂ catalysts are promising for the selective oxidation of polyatomic alcohols under low-temperature conditions.

A new reduction method based on simultaneous Ti3AlC2 support etching and metal deposition to prepare Pt catalysts for aqueous-phase selective hydrogenation of furfural to furfuryl alcohol

The new catalysts Pt/Ti3AlxC2Ty developed by a novel reduction method outperformed reference catalysts made by deposition–precipitation in the selective hydrogenation of furfural to furfuryl alcohol.

Direct Heterogenization of the Ru-Macho Catalyst for the Chemoselective Hydrogenation of α,β-Unsaturated Carbonyl Compounds

In this study, a commercially available homogeneous pincer-type complex, Ru-Macho, was directly heterogenized via the Lewis acid-catalyzed Friedel-Crafts reaction using dichloromethane as the cross-linker to obtain a heterogeneous, pincer-type Ru porous organometallic polymer (Ru-Macho-POMP) with a high surface area. Notably, Ru-Macho-POMP was demonstrated to be an efficient heterogeneous catalyst for the chemoselective hydrogenation of α,β-unsaturated carbonyl compounds to their corresponding allylic alcohols using cinnamaldehyde as a model compound. The Ru-Macho-POMP catalyst showed a high turnover frequency (TOF = 920 h^-1) and a high turnover number (TON = 2750), with high chemoselectivity (99%) and recyclability during the selective hydrogenation of α,β-unsaturated carbonyl compounds.

Infrared spectroscopic measurements of the structure of organic thin films; furfural on Pd(111) and Au(111) surfaces

This work demonstrates the use of reflection–adsorption infrared spectroscopy for continually monitoring the structure of organic thin films as a function of thickness and temperature to complement diffraction methods.

Effect of Hydrotalcites Interlayer Water on Pt-Catalyzed Aqueous-Phase Selective Hydrogenation of Cinnamaldehyde

The heterogeneous hydrogenation of α,β-unsaturated compounds requires understanding of the structure-activity relationship of metallic catalysts in consideration of solvent-mediated processes. In this work, a CoAl hydrotalcites (CoAl-HTs)-supported Pt nanoparticle catalyst is employed to study the effect of solvent water and HTs interlayer water on the aqueous-phase selective hydrogenation of cinnamaldehyde (CALD). Pt/Co₂Al₁-HTs catalyst displays 5075 h^-1 of specific reaction rate and 89% of C═O hydrogenation selectivity at 80 °C under 20 bar of H₂. Combined results of isotope-labeling experiments and theoretical DFT calculations demonstrate that the water-mediated hydrogen-exchange pathway exists in the reaction with a relatively lower-energy barrier in comparison to the direct H₂-dissociated hydrogenation pathway. The results also reveal that the interlayer water species of HTs support participate in the hydrogen-exchange reaction. Based on the H₂-D₂ exchange results, these HTs interlayer water species can promote H₂ activation and dissociation processes and thus accelerate the CALD hydrogenation reaction even under solvent-free conditions.

How Catalysts and Experimental Conditions Determine the Selective Hydroconversion of Furfural and 5-Hydroxymethylfurfural

Furfural and 5-hydroxymethylfurfural stand out as bridges connecting biomass raw materials to the biorefinery industry. Their reductive transformations by hydroconversion are key routes toward a wide variety of chemicals and biofuels, and heterogeneous catalysis plays a central role in these reactions. The catalyst efficiency highly depends on the nature of metals, supports, and additives, on the catalyst preparation procedure, and obviously on reaction conditions to which catalyst and reactants are exposed: solvent, pressure, and temperature. The present review focuses on the roles played by the catalyst at the molecular level in the hydroconversion of furfural and 5-hydroxymethylfurfural in the gas or liquid phases, including catalytic hydrogen transfer routes and electro/photoreduction, into oxygenates or hydrocarbons (e.g., furfuryl alcohol, 2,5-bis(hydroxymethyl)furan, cyclopentanone, 1,5-pentanediol, 2-methylfuran, 2,5-dimethylfuran, furan, furfuryl ethers, etc.). The mechanism of adsorption of the reactant and the mechanism of the reaction of hydroconversion are correlated to the specificities of each active metal, both noble (Pt, Pd, Ru, Au, Rh, and Ir) and non-noble (Ni, Cu, Co, Mo, and Fe), with an emphasis on the role of the support and of additives on catalytic performances (conversion, yield, and stability). The reusability of catalytic systems (deactivation mechanism, protection, and regeneration methods) is also discussed.

Defining Pt-compressed CO2 synergy for selectivity control of furfural hydrogenation

The development of a sustainable methodology for catalytic transformation of biomass-derived compounds to value-added chemicals is highly challenging. Most of the transitions are dominated by the use of additives, complicated reaction steps and large volumes of organic solvents. Compared to traditional organic solvents, alternative reaction media, which could be an ideal candidate for a viable extension of biomass-related reactions are rarely explored. Here, we elucidate a selective and efficient transformation of a biomass-derived aldehyde (furfural) to the corresponding alcohol, promoted in compressed CO2 using a Pt/Al2O3 catalyst. Furfural contains a furan ring with C[double bond, length as m-dash]C and an aldehyde group, and is extremely reactive in a hydrogen atmosphere, resulting in several by-products and a threat to alcohol selectivity as well as catalyst life. The process described has a very high reaction rate (6000 h-1) with an excellent selectivity/yield (99%) of alcohol, without any organic solvents or metal additives. This strategy has several key features over existing methodologies, such as reduced waste, and facile product separation and purification (reduced energy consumption). Combining the throughput of experimental observation and molecular dynamics simulation, indeed the high diffusivity of compressed CO2 controls the mobility of the compound, and eventually maintains the activity of the catalyst. Results are also compared for different solvents and solvent-less conditions. In particular, combination of an effective Pt catalyst with compressed CO2 provides an encouraging alternative solution for upgradation of biomass related platform molecules.

Selectivity in Hydrogenation Catalysis with Unsaturated Aldehydes: Parallel versus Sequential Steps

A high-flux molecular beam setup has been used to characterize the kinetics of the steady-state catalytic hydrogenation of unsaturated aldehydes, specifically of crotonaldehyde, promoted by platinum surfaces under single-collision conditions. Surprisingly, in addition to the hydrogenation of the individual single bonds, to yield the saturated aldehyde and the unsaturated alcohol, the formation of the saturated alcohol, the product of the hydrogenation of both C═C and C═O bonds, was detected as well. This indicates that the dual hydrogenation reaction is a primary pathway and not the result of secondary hydrogenation of the other products as commonly assumed. Moreover, an increase in the partial pressure of the reactant was found to shift the reaction selectivity from the saturated alcohol to the saturated aldehyde without significantly affecting the selectivity toward the production of the unsaturated alcohol. We explain these observations by proposing a mechanism involving the parallel formation of several monohydrogenated intermediates on the surface.

Selective hydrogenation of unsaturated aldehydes over Pt nanoparticles promoted by the cooperation of steric and electronic effects

The selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols can reach high selectivity and activity at room temperature using Pt nanoparticles immobilized on a non-porous Al₂O₃ support stabilized by aspartic acid.

Efficient Decarbonylation of Furfural to Furan Catalyzed by Zirconia-Supported Palladium Clusters with Low Atomicity

Decarbonylation of furfural to furan was efficiently catalyzed by ZrO₂ -supported Pd clusters in the liquid phase under a N₂ atmosphere without additives. Although Pd/C and Pd/Al₂ O₃ have frequently been used for decarbonylation, Pd/ZrO₂ exhibited superior catalytic performance compared with these conventional catalysts. Transmission electron microscopy and X-ray absorption fine structure measurements revealed that the size of the Pd particles decreased with an increase in the specific surface area of ZrO₂ . ZrO₂ with a high surface area immobilized Pd as clusters consisting of several (three to five) Pd atoms, whereas Pd aggregated to form nanoparticles on other supports such as carbon and Al₂ O₃ despite their high surface areas. The catalytic activity of Pd/ZrO₂ was enhanced with a decrease in particle size, and the smallest Pd/ZrO₂ was the most active catalyst for decarbonylation. When CeO₂ was used as the support, a decrease in Pd particle size with an increase in surface area was also observed. Single Pd atoms were deposited on CeO₂ with a high surface area, with a strong interaction through the formation of a Pd-O-Ce bond, which led to a lower catalytic activity than that of Pd/ZrO₂ . This result suggests that zero-valent small Pd clusters consisting of more than one Pd atom are the active species for the decarbonylation reaction. Recycling tests proved that Pd/ZrO₂ maintained its catalytic activity until its sixth use.

Selective Hydrogenation of Furfural to Furfuryl Alcohol in the Presence of a Recyclable Cobalt/SBA-15 Catalyst

The hydrogenation of furfural to furfuryl alcohol was performed in the presence of a Co/SBA-15 catalyst. High selectivity (96 %) at a conversion higher than 95 % is reported over this catalytic system. As the conversion of furfural to furfuryl alcohol occurs over metallic Co sites, the effect of reduction temperature, H2 pressure, and reaction temperature were studied. Optimum reaction conditions were: 150 °C, 1.5 h, 2.0 MPa of H2 . The catalyst was recyclable, and furfuryl alcohol was recovered with a purity higher than 90 %. The effect of the solvent concentration was also studied. With a minimum of 50 wt % of solvent, the selectivity to furfuryl alcohol and the conversion of furfural remained high (both over 80 %). Likewise, the activity of the catalyst is maintained even in pure furfural, which confirms the real potential of the proposed catalytic system. This catalyst was also used in the hydrogenation of levulinic acid to produce γ-valerolactone selectively.

Self-supported composites of thin Pt–Sn crosslinked nanowires for the highly chemoselective hydrogenation of cinnamaldehyde under ambient conditions

PtSn/SnO₂crosslinked nanowires with forming Pt–Sn alloy were highly chemoselective for hydrogenation of cinnamaldehyde to cinnamyl alcohol in good recyclability and stability under ambient conditions.

Factors affecting activity and selectivity in the oxidation of glycerol promoted by platinum catalysts

Selectivity in glycerol oxidation with Pt catalysts depends strongly on reaction conversion because of poisoning by the accumulated products.