Recent Progress and Opportunity of Metal Single-Atom Catalysts for Biomass Conversion Reactions

The conversion of lignocellulosic biomass into platform chemicals and fuels by metal single atoms is a new domain in solid catalysis research. Unlike the conventional catalysis route, single-atom catalysts (SACs) proliferate maximum utilization efficiency, high catalytic activity, and good selectivity to the desired product with an ultralow loading of the active sites. More strikingly, SACs show a unique cost-effective pathway for the conversion of complex sugar molecules to value-added chemicals in high yield and selectivity, which may be hindered by conventional metal nanoparticles. Primarily, SACs having adjustable active sites could be easily modified using sophisticated synthetic techniques based on their intended reactions. This review covers current research on the use of SACs with a strong emphasis on the fundamentals of catalyst design, and their distinctive activities in each type of reaction (hydrogenation, hydrogenolysis, hydrodeoxygenation, oxidation, and dehydrogenation). Furthermore, the fundamental insights into the superior actions of SACs within the opportunity and prospects for the industrial-scale synthesis of value-added products from the lignocelluloses are covered.

Directional methanolysis of kitchen waste for the co-production of methyl levulinate and fatty acid methyl esters: Catalytic strategy and machine learning modeling

To add value to ordinary kitchen waste, heterogeneous acid-base catalytic methanolysis was conducted to produce high-value liquid biofuels, methyl levulinate (ML) and fatty acid methyl esters (FAMEs). Yields of 53.3 % ML and 98.5 % FAME were achieved by methanolysis of kitchen waste under the co-catalysis of carbon-silica composite (C/Si-SO₃H) and zirconium modified ultrastable Y zeolite (Zr/USY). These target products can be easily recovered from the methanolic phase and can be purified at the end of the reaction. The collaborative combination of C/Si-SO₃H and Zr/USY exhibited higher activity than their commercial counterpart. This strategy can be applied to differently composed kitchen waste and kitchen waste with different water content. Product yields were predicted using an artificial neural network method, and the relative importance of the influencing factors was investigated by the random forest method. The systematic insight gained from this work supports the value-added utilization of kitchen waste.

Cationic Covalent Organic Framework with Ultralow HOMO Energy Used as Scaffolds for 5.2 V Solid Polycarbonate Electrolytes

Solid polymer electrolytes (SPEs) have become promising candidate to replace common liquid electrolyte due to highly improved security. However, the practical use of SPEs is still restricted by their decomposition and breakage at the electrode interfacial layer especially at high voltage. Herein, a new cationic covalent organic framework (COF) is designed and synthesized as a reinforced skeleton to resist the constant oxidative decomposition of solid polycarbonate electrolyte, which can stabilize cathode electrolyte interphase layer to develop long-term cycle solid lithium metal battery. The ultralow HOMO energy (-12.55 eV according to density functional theory (DFT) calculations), reflecting its oxidation resistance at positive potential, would be responsible for the high decomposition voltage of 5.2 V versus Li⁺ /Li of solid polycarbonate electrolyte. Furthermore, the smooth surface of interfacial layer and inhibited decomposition reaction at cathode side is confirmed in solid LiCoO₂ cell, which realizes high initial capacity up to 160.3 mAh g^-1 at 0.1 C and greatly improved stability in 4.5 V class solid polymer lithium metal battery with high capacity retention over 200 cycles. This new type of high-voltage resistant solid polymer electrolyte promotes the realization of high-voltage cathode materials and higher energy density lithium metal battery.

Overcoming Limitations in the Strong Interaction between Pt and Irreducible SiO2 Enables Efficient and Selective Hydrogenation of Anthracene

Interactions between metals and oxide supports are crucial in determining catalytic activity, selectivity, and stability. For reducible oxide supported noble metals, a strong metal-support interaction (SMSI) has been widely recognized. Herein we report the intermediate selectivity and stability over an irreducible SiO₂ supported Pt catalyst in the hydrogenation of anthracene that are significantly boosted due to the SMSI-induced formation of intermetallic Pt silicide and Pt-SiO₂ interface. The limitation in the strong interaction between Pt nanoparticles and irreducible SiO₂ has been breached by combining the strong electrostatic adsorption method and following the high temperature reduction strategy. Due to the isolated Pt active sites by Si atoms, the activated H species produced over the Pt₂Si/SiO₂ catalyst with an initial catalytic activity of 2.49 μmol/(m²/g)/h as well as TOF of 0.95 s^-1 preferentially transfer to the outer ring of anthracene to 87% yield of symmetric octahydroanthracene (sym-OHA) by subsequent hydrogenation. In addition, the Pt₂Si/SiO₂ catalyst presents an excellent stability after five cycles, which can be attributed to the fact that intermetallic Pt₂Si nanoparticles are anchored firmly onto the surface of the SiO₂ support. The discovery contributes to broaden the horizons on the SMSI effect in the irreducible oxide supported metal particle catalysts and provides guidance to design the metal-SiO₂ interface and tune the surface chemical properties in diverse application conditions.

Mesoporous Porphyrin-Silica Nanocomposite as Solid Acid Catalyst for High Yield Synthesis of HMF in Water

Solid acid catalysts occupy a special class in heterogeneous catalysis for their efficiency in eco-friendly conversion of biomass into demanding chemicals. We synthesized porphyrin containing porous organic polymers (PorPOPs) using colloidal silica as a support. Post-modification with chlorosulfonic acid enabled sulfonic acid functionalization, and the resulting material (PorPOPS) showed excellent activity and durability for the conversion of fructose to 5-hydroxymethyl furfural (HMF) in green solvent water. PorPOPS composite was characterized by N2 sorption, FTIR, TGA, CHNS, FESEM, TEM and XPS techniques, justifying the successful synthesis of organic networks and the grafting of sulfonic acid sites (5 wt%). Furthermore, a high surface area (260 m2/g) and the presence of distinct mesopores of ~15 nm were distinctly different from the porphyrin containing sulfonated porous organic polymer (FePOP-1S). Surprisingly the hybrid PorPOPS showed an excellent yield of HMF (85%) and high selectivity (>90%) in water as compared to microporous pristine-FePOP-1S (yield of HMF = 35%). This research demonstrates the requirement of organic modification on silica surfaces to tailor the activity and selectivity of the catalysts. We foresee that this research may inspire further applications of biomass conversion in water in future environmental research.

Effect of Nanoparticle Size in Pt/SiO2 Catalyzed Nitrate Reduction in Liquid Phase

Effect of platinum nanoparticle size on catalytic reduction of nitrate in liquid phase was examined under ambient conditions by using hydrogen as a reducing agent. For the size effect study, Pt nanoparticles with sizes of 2, 4 and 8 nm were loaded silica support. TEM images of Pt nanoparticles showed that homogeneous morphologies as well as narrow size distributions were achieved during the preparation. All three catalysts showed high activity and were able to reduce nitrate below the recommended limit of 50 mg/L in drinking water. The highest catalytic activity was seen with 8 nm platinum; however, the product selectivity for N₂ was highest with 4 nm platinum. In addition, the possibility of PVP capping agent acting as a promoter in the reaction is highlighted.

New catalytically active conjugated microporous polymer bearing ordered salen-Cu and porphyrin moieties for Henry reaction in aqueous solution

A catalytically active conjugated microporous polymer (SP-CMP-Cu) was facilely constructed with condensation polymerization of salen-Cu (salen = N,N'-bis(3-tertbutyl-5-formylsalicylidene) ethylenediamine) and pyrrole. The as-synthesized SP-CMP-Cu was completely characterized by powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray (EDX) analysis. The morphological features of SP-CMP-Cu were revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). According to the N2 adsorption/desorption isotherm, the Brunauer-Emmett-Teller (BET) surface area of SP-CMP-Cu was calculated to be 252 m2 g-1 with a total pore volume of 0.178 cm3 g-1. SP-CMP-Cu exhibited an outstanding catalytic performance for the Henry reaction in aqueous solutions with excellent conversion and good selectivity. Moreover, SP-CMP-Cu can be reused for up to five consecutive runs without any significant loss in its catalytic efficiency.

Spatially Separated Bifunctional Cocatalysts Decorated on Hollow-Structured TiO2 for Enhanced Photocatalytic Hydrogen Generation

Efficient charge separation can promote photocatalysis of semiconductors. Herein, a hollow-structured TiO₂ sphere decorated with spatially separated bifunctional cocatalysts was designed, which exhibited enhanced photocatalytic hydrogen generation. Ultrasmall-sized MO_x (M = Pd, Co, Ni, or Cu) nanoparticles (NPs) were first introduced into a zeolite via confinement synthesis, and then, hollow TiO₂ was fabricated by using the zeolite as a sacrificial template forming MO_x@TiO₂. Finally, Pt NPs were decorated on the outer shell, giving rise to MO_x@TiO₂@Pt, in which the MO_x NPs and Pt NPs acted as hole capturers and electron sinks, respectively. Thanks to the enhanced light harvesting of the hollow structure and improved charge separation induced by the smaller-sized cocatalysts as well as spatially separated bifunctional cocatalysts, the as-prepared PdO_x@TiO₂@Pt catalyst exhibited a superior photocatalytic hydrogen-generation property (0.45 mmol h^-1). This work demonstrates the advantage of the spatially separated bifunctional cocatalysts in enhancing the photocatalytic properties of semiconductors.

Synthesis of Intermetallic Pt-Based Catalysts by Lithium Naphthalenide-Driven Reduction for Selective Hydrogenation of Cinnamaldehyde

Intermetallic nanoparticles (NPs) with a well-defined atom binding environment and a long-range ordering structure can be used as ideal models to understand their physical and catalytic properties. In this work, several kinds of nanostructured and carbon nanotube (CNT)-supported Pt-based intermetallic compounds (IMCs) have been synthesized by one-step lithium naphthalenide-driven reduction at room temperature without the use of surfactants in light of the reduction potential of metals. In the chemoselective hydrogenation of cinnamaldehyde, the second metal in Pt-M IMCs significantly creates a suitable reaction environment through construction of a good geometric and electronic structure. The Pt₃Sn/CNT catalyst presents highly efficient and good chemoselective hydrogenation of cinnamaldehyde to cinnamyl alcohol. This can be attributed to the fact that the incorporated Sn atoms effectively dilute large Pt ensembles and increase the electron density of Pt. The in situ-formed SnO_x interfaces as Lewis acid sites facilitate the coordination of C═O bonds, enhancing the selectivity to cinnamyl alcohol. In addition, the SnO_x interface as the joint between Pt₃Sn IMCs NPs and CNTs significantly improves the stability of the catalyst in the reaction environment.