Improved Hydrothermal Stability of Mesoporous Oxides for Reactions in the Aqueous Phase

Gold nanoparticles grown on a hydrophobic and texturally tunable PDMS-like framework

A sol-gel analogue of PDMS was used to built hydrophobic siloxane supports, while confining gold nanoparticles.

High-Temperature Synthesis of Small-Sized Pt/Nb Alloy Catalysts on Carbon Supports for Hydrothermal Reactions

Catalytic biomass conversions are sustainable processes to produce value-added fuels and chemicals but need stable catalysts that can tolerate harsh hydrothermal conditions. Herein, we report a hydrothermally stable catalyst by alloying Pt with a high-melting-point metal Nb. The Pt/Nb alloy catalysts are prepared by H₂ reduction at a high temperature of 900 °C with a high-surface-area carbon black support, which can suppress metal sintering at high temperatures and thus lead to small-sized alloyed Pt/Nb particles of only 2.2 nm. Taking the advantages of surface acid property provided by the Nb sites and the size effect, the prepared C-supported small-sized Pt/Nb alloy catalysts exhibit attractive activities for the hydrogenation of levulinic acid into γ-valerolactone and the water-gas shift reaction. More significantly, benefiting from the inherent stability of high-melting-point Nb, the Pt/Nb alloy catalysts show much enhanced hydrothermal stability compared to commercial Pt/C and Ru/C catalysts.

Selective Carbon Deposition on γ-Alumina Acid Sites: toward the Design of Catalyst Supports with Improved Hydrothermal Stability in Aqueous Media

γ-Alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides under hydrothermal (HT) conditions, resulting in strong modification of its intrinsic properties. Most of the strategies that have been proposed to prevent or at least minimize its transformation into oxy-hydroxides consist in covering the alumina surface with a hydrophobic carbon layer, making it less sensitive to modifications induced by water. However, such methods necessitate high carbon contents, which significantly modifies structural and chemical properties of alumina. Here, we propose a new method based on a series of adsorption/pyrolysis cycles using sorbitol molecules previously adsorbed on specific hydration sites of the (110) faces of γ-alumina crystals. These sites, which are responsible for the dissolution of γ-alumina crystals in water, are thus selectively protected by carbon clusters, with the rest of the surface being totally exposed and accessible to adsorbates. Under HT conditions (10 h in water at 200 °C), the formation of hydroxides is almost totally suppressed by covering less than 25% of the surface with only 7 wt % carbon, which is far below the amount necessary to achieve similar results with more conventional carbon deposition methods.

Bioprivileged Molecules: Integrating Biological and Chemical Catalysis for Biomass Conversion

Further development of biomass conversions to viable chemicals and fuels will require improved atom utilization, process efficiency, and synergistic allocation of carbon feedstock into diverse products, as is the case in the well-developed petroleum industry. The integration of biological and chemical processes, which harnesses the strength of each type of process, can lead to advantaged processes over processes limited to one or the other. This synergy can be achieved through bioprivileged molecules that can be leveraged to produce a diversity of products, including both replacement molecules and novel molecules with enhanced performance properties. However, important challenges arise in the development of bioprivileged molecules. This review discusses the integration of biological and chemical processes and its use in the development of bioprivileged molecules, with a further focus on key hurdles that must be overcome for successful implementation.

Pd nanoparticles confined in mesoporous N-doped carbon silica supports: a synergistic effect between catalyst and support

Palladium nanoparticles of similar size were deposited on different supports, layers of carbon materials (with and without nitrogen doping) on the surface of a MCF (mesocellular foam) silica.

Deactivation and regeneration of carbon supported Pt and Ru catalysts in aqueous phase hydrogenation of 2-pentanone

Aqueous phase conversion of biomass-derived molecules requires development of catalysts and operating strategies that create viable operation for extended performance as necessitated for industrial applications.

Quick, selective NMR spectra of COH moieties in 13C-enriched solids

A convenient one-dimensional magic-angle spinning NMR method is presented that provides selective NMR spectra of COH moieties in uniformly ¹³C-enriched organic materials. This method, termed hydroxyl-proton selection (HOPS), eliminates the magnetization of protons directly bonded to carbons by recoupling the ¹H-¹³C dipolar interaction for a short time (∼70 μs), which also serves as a chemical-shift filter to suppress ¹H magnetization of CH₃ groups. After cross polarization to ¹³C, the signals of COH and COOH carbons are observed selectively. This makes it possible to distinguish alcohols from ethers, in particular phenols from aromatic ethers such as the furans often formed by dehydration of glucose, and carboxylic acids from carboxylates and ethers. HOPS NMR reveals that orthodiphenols are often a major component of low-temperature carbon materials. For instance, it forces the reassignment of the 143 ppm ¹³C NMR signal of hydrothermal carbon to such catecholic diphenols, while a previous NMR-based structural model had attributed this peak to a central furan-furan linkage.

Advances in porous and nanoscale catalysts for viable biomass conversion

Solid catalysts with unique porosity and nanoscale properties play a promising role for efficient valorization of biomass into sustainable advanced fuels and chemicals.

Slowing the Kinetics of Alumina Sol-Gel Chemistry for Controlled Catalyst Overcoating and Improved Catalyst Stability and Selectivity

Catalyst overcoating is an emerging approach to engineer surface functionalities on supported metal catalyst and improve catalyst selectivity and durability. Alumina deposition on high surface area material by sol-gel chemistry is traditionally difficult to control due to the fast hydrolysis kinetics of aluminum-alkoxide precursors. Here, sol-gel chemistry methods are adapted to slow down these kinetics and deposit nanometer-scale alumina overcoats. The alumina overcoats are comparable in conformality and thickness control to overcoats prepared by atomic layer deposition even on high surface area substrates. The strategy relies on regulating the hydrolysis/condensation kinetics of Al(^s BuO)₃ by either adding a chelating agent or using nonhydrolytic sol-gel chemistry. These two approaches produce overcoats with similar chemical properties but distinct physical textures. With chelation chemistry, a mild method compatible with supported base metal catalysts, a conformal yet porous overcoat leads to a highly sintering-resistant Cu catalyst for liquid-phase furfural hydrogenation. With the nonhydrolytic sol-gel route, a denser Al₂ O₃ overcoat can be deposited to create a high density of Lewis acid-metal interface sites over Pt on mesoporous silica. The resulting material has a substantially increased hydrodeoxygenation activity for the conversion of lignin-derived 4-propylguaiacol into propylcyclohexane with up to 87% selectivity.

Protective Carbon Overlayers from 2,3-Naphthalenediol Pyrolysis on Mesoporous SiO₂ and Al₂O₃ Analyzed by Solid-State NMR

Hydrothermally stable carbon overlayers can protect mesoporous oxides (SiO₂ and Al₂O₃) from hydrolysis during aqueous-phase catalysis. Overlayers made at 800 °C by pyrolysis of 2,3-naphthalenediol deposited out of acetone solution were analyzed by solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. Power absorption due to sample conductivity was prevented by diluting the sample in nonconductive and background-free tricalcium phosphate. While pyrolysis on SiO₂ produced a predominantly aromatic carbon film, at least 15% of nonaromatic carbon (sp³-hybridized C as well as C=O) was observed on γ-Al₂O₃. These species were not derived from residual solvent, according to spectra of the same material treated at 400 °C. The sp³-hybridized C exhibited weak couplings to hydrogen, short spin-lattice relaxation times, and unusually large shift anisotropies, which are characteristics of tetrahedral carbon with high concentrations of unpaired electrons. Moderate heat treatment at 400 °C on SiO₂ and Al₂O₃ resulted in yellow-brown and nearly black samples, respectively, but the darker color on Al₂O₃ did not correspond to more extensive carbonization. Aromatic carbon bonded to hydrogen remained predominant and the peaks of naphthalenediol were still recognizable; however, some of the chemical shifts differed by up to 5 ppm, indicating significant differences in local structure. On SiO₂, additional sharp peaks were detected and attributed to 1/3 of the 2,3-naphthalene molecules undergoing fast, nearly isotropic motions.

Improved hydrothermal stability of Pd nanoparticles on nitrogen-doped carbon supports

Carbon supports have been shown to provide better hydrothermal stability than alumina or silica supports, thus attracting more attention for aqueous-phase biomass conversion reactions.

Stability of Pd nanoparticles on carbon-coated supports under hydrothermal conditions

Hydrothermal stability is one of the major challenges facing heterogeneous catalysis in biomass conversion to chemicals or fuels.

From sequential chemoenzymatic synthesis to integrated hybrid catalysis: taking the best of both worlds to open up the scope of possibilities for a sustainable future

Here an overview of all pathways that integrate chemical and biological catalysis is presented. We emphasize the factors to be considered in order to understand catalytic synergy.

Heterogeneous Catalytic Conversion of Biobased Chemicals into Liquid Fuels in the Aqueous Phase

Different biobased chemicals are produced during the conversion of biomass into fuels through various feasible technologies (e.g., hydrolysis, hydrothermal liquefaction, and pyrolysis). The challenge of transforming these biobased chemicals with high hydrophilicity is ascribed to the high water content of the feedstock and the inevitable formation of water. Therefore, aqueous-phase processing is an interesting technology for the heterogeneous catalytic conversion of biobased chemicals. Different reactions, such as dehydration, isomerization, aldol condensation, ketonization, and hydrogenation, are applied for the conversion of sugars, furfural/hydroxymethylfurfural, acids, phenolics, and so on over heterogeneous catalysts. The activity, stability, and reusability of the heterogeneous catalysts in water are summarized, and deactivation processes and several strategies are introduced to improve the stability of heterogeneous catalysts in the aqueous phase.

Periodic mesoporous organosilicas derived from amphiphilic bulky polymethylsiloxane

Depending on the synthetic conditions (surfactant, acid, base, and presence or absence of TEOS), the sol–gel polymerization of functionalized polyorganosiloxane affords various textured silica-based materials.

Renewable Feedstocks: The Problem of Catalyst Deactivation and its Mitigation

Much research has been carried out in the last decade to convert bio-based feedstock into fuels and chemicals. Most of the research focuses on developing active and selective catalysts, with much less attention devoted to their long-term stability. This Review considers the main challenges in long-term catalyst stability, discusses some fundamentals, and presents options for their mitigation. Three main challenges are discussed: catalyst fouling, catalyst poisoning, and catalyst destruction. Fouling is generally related to the deposition of insoluble components present in the feed or formed by degradation of the feed or intermediates. Poisoning is related to the deposition of electropositive contaminants (e.g. alkali and alkaline earth metals) on acid sites or of electronegative contaminants (e.g. N and S) at hydrogenation sites. Catalyst destruction results from the thermodynamic instability of most oxidic supports, solid acids/bases, and hydrogenation functions under hydrothermal conditions.

Palladium nanoparticles confined in thiol-functionalized ordered mesoporous silica for more stable Heck and Suzuki catalysts

The influence of confinement of Pd nanoparticles on thiol functionalized silica structures (modified MCF, SBA-15, plugged SBA-15 and Aerosil) was explored for the Heck and the Suzuki reactions.

Low-temperature aqueous-phase reforming of ethanol on bimetallic PdZn catalysts

Bimetallic PdZn catalysts supported on carbon black (CB) and carbon nanotubes (CNTs) were found to be selective for CO-free H₂ production from ethanol at low temperature (250 °C).

Quantitative solid-state 13C NMR with signal enhancement by multiple cross polarization

A simple new method is presented that yields quantitative solid-state magic-angle spinning (MAS) (13)C NMR spectra of organic materials with good signal-to-noise ratios. It achieves long (>10ms) cross polarization (CP) from (1)H without significant magnetization losses due to relaxation and with a moderate duty cycle of the radio-frequency irradiation, by multiple 1-ms CP periods alternating with (1)H spin-lattice relaxation periods that repolarize the protons. The new method incorporates previous techniques that yield less distorted CP/MAS spectra, such as a linear variation ("ramp") of the radio-frequency field strength, and it overcomes their main limitation, which is T1ρ relaxation of the spin-locked (1)H magnetization. The ramp of the radio-frequency field strength and the asymptotic limit of cross polarization makes the spectral intensity quite insensitive to the exact field strengths used. The new multiCP pulse sequence is a "drop-in" replacement for previous CP methods and produces no additional data-processing burden. Compared to the only reliable quantitative (13)C NMR method for unlabeled solids previously available, namely direct-polarization NMR, the measuring time is reduced by more than a factor of 50, enabling higher-throughput quantitative NMR studies. The new multiCP technique is validated with 14-kHz MAS on amino-acid derivatives, plant matter, a highly aromatic humic acid, and carbon materials made by low-temperature pyrolysis.

Stability of amorphous silica-alumina in hot liquid water

Herein, the hydrothermal stability of amorphous silica-alumina (ASA) is investigated under conditions relevant for the catalytic conversion of biomass, namely in liquid water at 200 °C. The hydrothermal stability of ASA is much higher than that of pure silica or alumina. Interestingly, the synthetic procedure used plays a major role in its resultant stability: ASA prepared by cogelation (CG) lost its microporous structure, owing to hydrolysis of the siloxane bonds, but the resulting mesoporous material still had a considerable surface area. ASA prepared by deposition precipitation (DP) contained a silicon-rich core and an aluminum-rich shell. In hot liquid water, the latter structure was transformed into a layer of amorphous boehmite, which protected the particle from further hydrolysis. The surface area showed relatively minor changes during the transformation. Independent of the synthetic method used, the ASAs retained a considerable concentration of acid sites. The concentration of acid sites qualitatively followed the changes in surface area, but the changes were less pronounced. The performance of different ASAs for the hydrolysis of cellobiose into glucose is compared.