Effects of Metals and Ni₃S₂ on Reactions of Sulfur Species (HS^–, S, and S₂O₃^2–) under Alkaline Hydrothermal Conditions

The reactions of sulfur species (HS^–, S, and S₂O₃^2–) under hydrothermal conditions are an important scientific subject due to their implications in biology and geology and potential application for hydrogen production as well. In this paper, effects of representative metals and alloy such as Ni, Fe, Co, W, Hastelloy C-276 alloy, SUS 316 alloy, and sulfide mineral Ni₃S₂ on reactions of sulfur species under alkaline hydrothermal conditions were studied. The results showed that there are mainly three types of reactions among sulfur species, H₂O, and metal (or Ni₃S₂): (i) sulfidation of metals and Ni₃S₂ by sulfur species; (ii) disproportionation of sulfur species; (iii) oxidation of sulfur species by water to produce hydrogen. Co and Ni can be sulfided, and W remains unchanged in the presence of HS^–. The presence of Ni and Ni₃S₂ would significantly enhance the transformation of S₂O₃^2– into SO₃^2– and SO₄^2–. This study would give clues to sulfur chemistry within hydrothermal waters and guide the selection of the reaction pathway of sulfur species by adding metals or Ni₃S₂

Photocatalytic Oxidation of Glucose into Formate on Nano TiO₂ Catalyst

Formic acid, as an excellent hydrogen-storage material, has recently become increasingly important. Previous conversion of biomass into formic acid suffers from problems of strict operation conditions such as the use of a high concentration of H₂O₂ oxidant and alkali at higher temperatures or the precious metal catalysts. Here, a straightforward photocatalytic method to selectively convert glucose into formate is first reported. Using a nano TiO₂ catalyst, we show that glucose can be efficiently converted into formic acid with a high yield of 35% at ambient condition with 0.03 M NaOH concentration, and the proposed method also worked on xylose for formate production. The conversion of glucose into formic acid is mainly attributed to the accelerated formation of active oxidative radicals (O₂^•–, ^•OH) in the presence of hydroxyl ions, and also, hydroxyl ions adjust the charge of the TiO₂ surface and control the adsorption of glucose and the desorption of formic acid. These results open up a new approach toward economical utilization of sustainable biomass and clean solar energy for formic acid production

No Catalyst Addition and Highly Efficient Dissociation of H₂O for the Reduction of CO₂ to Formic Acid with Mn

The “greenhouse effect” caused by the increasing atmospheric CO₂ level is becoming extremely serious, and thus, the reduction of CO₂ emissions has become an extensive, urgent, and long-term task. The dissociation of water for CO₂ reduction with solar energy is regarded as one of the most promising methods for the sustainable development of the environment and energy. However, a high solar-to-fuel efficiency keeps a great challenge. In this work, the first observation of a highly effective, highly selective, and robust system of dissociating water for the reduction of carbon dioxide (CO₂) into formic acid with metallic manganese (Mn) is reported. A considerably high formic acid yield of more than 75% on a carbon basis from NaHCO₃ was achieved with 98% selectivity in the presence of simple commercially available Mn powder without the addition of any catalyst, and the proposed process is exothermic. Thus, this study may provide a promising method for the highly efficient dissociation of water for CO₂ reduction by combining solar-driven thermochemistry with the reduction of MnO into Mn

Enhancing the Synergistic Effect of Bifunctional Pd-Based Catalyst by Phosphonic Acid for Cellobiose Conversion to Sorbitol

Catalytic conversion of cellulosic biomass into polyols is an effective approach for biomass upgrading. However, the conversion involves cascade reactions which require various catalytic sites cooperation for activating specific steps, resulting in low efficiency. Here, we propose a bifunctional catalyst containing acid sites and hydrogenation capacity for polyol production with cellobiose as the model compound of cellulose. Phosphonic acids (PAs) containing either methyl (MPA) or aminomethyl (NH2MPA) tails were used to tune the Pd/Al2O3 catalyst to form monolayers, thereby modulating the surface physiochemical properties and introducing multiple catalytic sites. An increasing ratio of PdO/Pd and a strong metal–support interaction were observed after PA modification, which could boost the hydrogenation activity for cellobiose. Furthermore, Pd/Al2O3-MPA exhibited excellent hydrolytic hydrogenation activity due to the increase of Lewis acid sites with good durability under hydrothermal conditions by MPA modification, achieving a 78.5% yield of sorbitol at 180 °C for 5 h. A plausible mechanism for enhancing the synergistic effect of Pd and support over Pd/Al2O3-MPA was also proposed. This investigation provides a novel strategy for rationally designing hydrothermally stable bifunctional catalysts to utilize biomass in a more effective way

Reduction of Carbon Dioxide in Hydrothermal Cracking of Polymer Wastes

Reduction of Carbon Dioxide in Hydrothermal Cracking of Polymer Waste

Reduction of CuO into Cu with Guaiacol as a Model Compound of Lignin with a Homogeneous Catalyst of NaOH

The reduction of CuO with a lignin model compound, guaiacol, is investigated, to develop a new and green method for both Cu smelting and the utilization of lignin. The results showed that CuO can be completely reduced into Cu and guaiacol selectively converted to carboxylic acids such as fumaric, maleic, acetic, and formic acids at a mild temperature of 250 °C. The presence of NaOH has a significant effect on enhancing the reduction of CuO and the selective yield of carboxylic acids. Two possible reaction pathways were proposed for the reduction of CuO with guaiacol. One pathway involves the oxidative decomposition of guaiacol into carboxylic acids. The other reaction pathway proceeds with the polymerization of guaiacol into oligomers, which can be reused for CuO reduction due to their easy hydrolyzation to monomers under hydrothermal conditions

Facile Synthesis of Dimethyl Succinate via Esterification of Succinic Anhydride over ZnO in Methanol

An efficient esterification of succinic anhydride (SA) to dimethyl succinate (DMS) using ZnO in methanol was investigated for the first time. ZnO is commercial, low cost, noncorrosive and environmentally friendly metallic oxide and can promote remarkably the production of DMS from SA in excellent yields with high selectivity. The highest yield of DMS of 100% was achieved at the temperature of 140 °C for 10 h. The results indicated that reaction temperature and time had significant influences on the yield of DMS. Mechanism study found that ZnO was first dissolved to form a Zn species, and then redeposited to ZnO during the reaction, which was crucial for SA conversion. The redeposited ZnO could be reused at least 4 times without the loss of the activity. Moreover, the reaction of ethanol and SA was also effective to give excellent yield at the optimal conditions. The present study demonstrates a promising greener way for the synthesis of DMS from biomass-derived SA

Selective Electrocatalytic Reduction of Nitrite to Dinitrogen Based on Decoupled Proton–Electron Transfer

The development of denitrification catalysts which can reduce nitrate and nitrite to dinitrogen is critical for sustaining the nitrogen cycle. However, regulating the selectivity has proven to be a challenge, due to the difficulty of controlling complex multielectron/proton reactions. Here we report that utilizing sequential proton–electron transfer (SPET) pathways is a viable strategy to enhance the selectivity of electrochemical reactions. The selectivity of an oxo-molybdenum sulfide electrocatalyst toward nitrite reduction to dinitrogen exhibited a volcano-type pH dependence with a maximum at pH 5. The pH-dependent formation of the intermediate species (distorted Mo­(V) oxo species) identified using operando electron paramagnetic resonance (EPR) and Raman spectroscopy was in accord with a mathematical prediction that the p<i>K</i>_a of the reaction intermediates determines the pH-dependence of the SPET-derived product. By utilizing this acute pH dependence, we achieved a Faradaic efficiency of 13.5% for nitrite reduction to dinitrogen, which is the highest value reported to date under neutral conditions

Winterization of Vegetable Oil Blends for Biodiesel Fuels and Correlation Based on Initial Saturated Fatty Acid Constituents

Winterization is a simple method to remove saturated fatty acid contents in biodiesel fuels for improving their cold flow properties. In this work, biodiesel fuels with different initial long-chain (C16 and above) saturated fatty acid constituents (<i>S</i>_i) were prepared from blends of palm, canola, and corn oils. The prepared biodiesels were treated at various winterization temperatures (<i>T</i>_w) to investigate the effect of <i>T</i>_w and <i>S</i>_i on the final saturated fatty acid constituents (<i>S</i>_w) of the winterized biodiesel fuel. Optical microscopy showed that ball-like crystals formed with fluid regions at moderate cooling rates (−6 °C/h) could allow solid–liquid separation by filtration. A saturated fatty acid reduction ratio, <i>R</i>_s, defined as (<i>S</i>_i – <i>S</i>_w)/<i>S</i>_i × 100, was used with the experimental results on large samples (ca. 600 mL) to develop a correlation for winterization temperature as <i>T</i>_w (°C) = 0.659 <i>S</i>_i (wt%) – 0.104 <i>R</i>_s (wt%) – 10.197. The correlation can provide estimation of the required winterization temperature for reducing a specified ratio of fatty acids in a biodiesel fuel that mainly contains long-chain fatty acids from the initial saturated fatty acid constituents. When used with literature relationships for cold filter plugging point (CFPP) and <i>S</i>_w, estimation of the CFPP of winterized biodiesel fuels is possible without requiring actual winterization treatment

New Method for Highly Efficient Conversion of Biomass-Derived Levulinic Acid to γ‑Valerolactone in Water without Precious Metal Catalysts

γ-Valerolactone (GVL) is receiving increasing attention because of its significant characteristics of an ideal sustainable liquid fuel. In this work, a novel and nonprecious metal catalytic method of the hydrogenation of biomass-derived levulinic acid (LA) into GVL by water splitting is first reported. Commercially available nonprecious metals of Fe, Ni, Cu, Cr, and Mo exhibited significantly catalytic activities in the hydrogenation of LA into GVL. Over 90% yield of GVL from LA can be obtained at a relatively low temperature of 180 °C, and an excellent 98% yield of GVL was achieved over the Fe catalyst at 250 °C. Catalyst Fe is stable and still keeps the high catalytic activity after recycles. The extraordinary catalytic activity of the general Fe powder is probably because of the role of hot water and also a synergistic role of the Fe and ZnO or Zn/ZnO. Also, the reactor wall of a reactor made of the stainless steel material acted as a significant catalyst, and a considerably high GVL yield of 56% can be obtained even without any addition of catalyst. This method is simple, highly efficient, and requires neither gaseous hydrogen nor precious metal catalyst, which is key for the practical application