Boosting CO2 chemical fixation over MOF-808 by the introduction of functional groups and defective Zr sites.

CO2 cycloaddition has emerged as a promising approach for producing value-added cyclocarbonates and mitigating greenhouse gas emissions. Although MOF-808 serves as a stable catalyst for cycloaddition, its limited activity constrains broader applications. Through the modification with a fluoride group via a ligand exchange method, F-MOF-808-1.5 exhibits exceptional performance, achieving a 98.8% conversion with 97.8% selectivity to epichlorohydrin carbonate-marking a substantial 100% improvement compared to pristine MOF-808. The defective Zr sites and the electron-withdrawing groups synergistically promote the ring opening of epoxides. Furthermore, the catalyst demonstrates high stability over multiple reaction cycles. Notably, without adding solvents and co-catalysts, F-MOF-808-1.5 outperforms most reported MOF-based catalysts.

Low-Valent Manganese Atoms Stabilized on Ceria for Nitrous Oxide Synthesis.

Nitrous oxide, N2 O, exhibits unique reactivity in oxidation catalysis, but the high manufacturing costs limit its prospective uses. Direct oxidation of ammonia, NH3 , to N2 O can ameliorate this issue but its implementation is thwarted by suboptimal catalyst selectivity and stability, and the lack of established structure-performance relationships. Systematic and controlled material nanostructuring offers an innovative approach for advancement in catalyst design. Herein low-valent manganese atoms stabilized on ceria, CeO2 , are discovered as the first stable catalyst for NH3 oxidation to N2 O, exhibiting two-fold higher productivity than the state-of-the-art. Detailed mechanistic, computational and kinetic studies reveal CeO2 as the mediator of oxygen supply, while undercoordinated manganese species activate O2 and facilitate N2 O evolution via NN bond formation between nitroxyl, HNO, intermediates. Synthesis via simple impregnation of a small metal quantity (1 wt%) predominantly generates isolated manganese sites, while full atomic dispersion is achieved upon redispersion of sporadic oxide nanoparticles during reaction, as confirmed by advanced microscopic analysis and electron paramagnetic resonance spectroscopy. Subsequently, manganese speciation is maintained, and no deactivation is observed over 70 h on stream. CeO2 -supported isolated transition metals emerge as a novel class of materials for N2 O production, encouraging future studies to evaluate their potential in selective catalytic oxidations at large.

Ceria-Supported Gold Nanoparticles as a Superior Catalyst for Nitrous Oxide Production via Ammonia Oxidation.

The production of nitrous oxide, N2 O, via NH3 oxidation is not on a practical scale due to the lack of a suitable catalyst. Instead, it is produced via thermal decomposition of NH4 NO3 , rendering N2 O too costly and limiting its prospective uses. Herein, we report CeO2 -supported Au nanoparticles (2-3 nm) as a highly selective catalyst for low-temperature NH3 oxidation to N2 O, exhibiting two orders of magnitude higher space-time yield than the state-of-the-art Mn-Bi/α-Al2 O3 and remarkable stability over 70 h on stream. The reaction proceeds via a Mars-van Krevelen mechanism, with the density of interfacial Auδ+ species and the oxygen storage capacity of CeO2 identified as the key performance descriptors. The latter could be enhanced by cobalt doping, improving the catalytic activity and setting a new benchmark for N2 O productivity. These findings establish NH3 oxidation as an efficient process for N2 O manufacture and facilitate its broader utilization in selective oxidations.

Pt/ZrO2 Prepared by Atomic Trapping: An Efficient Catalyst for the Conversion of Glycerol to Lactic Acid with Concomitant Transfer Hydrogenation of Cyclohexene.

A series of heterogeneous catalysts consisting of highly dispersed Pt nanoparticles supported on nanosized ZrO2 (20 to 60 nm) was synthesized and investigated for the one-pot transfer hydrogenation between glycerol and cyclohexene to produce lactic acid and cyclohexane, without any additional H2. Different preparation methods were screened, by varying the calcination and reduction procedures with the purpose of optimizing the dispersion of Pt species (i.e., as single-atom sites or extra-fine Pt nanoparticles) on the ZrO2 support. The Pt/ZrO2 catalysts were characterized by means of transmission electron microscopy techniques (HAADF-STEM, TEM), elemental analysis (ICP-OES, EDX mapping), N2-physisorption, H2 temperature-programmed-reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Based on this combination of techniques it was possible to correlate the temperature of the calcination and reduction treatments with the nature of the Pt species. The best catalyst consisted of subnanometer Pt clusters (<1 nm) and atomically dispersed Pt (as Pt2+ and Pt4+) on the ZrO2 support, which were converted into extra-fine Pt nanoparticles (average size = 1.4 nm) upon reduction. These nanoparticles acted as catalytic species for the transfer hydrogenation of glycerol with cyclohexene, which gave an unsurpassed 95% yield of lactic acid salt at 96% glycerol conversion (aqueous glycerol solution, NaOH as promoter, 160 °C, 4.5 h, at 20 bar N2). This is the highest yield and selectivity of lactic acid (salt) reported in the literature so far. Reusability experiments showed a partial and gradual loss of activity of the Pt/ZrO2 catalyst, which was attributed to the experimentally observed aggregation of Pt nanoparticles.

Multifunctional Heterogeneous Catalysts for the Selective Conversion of Glycerol into Methyl Lactate.

Multifunctional catalytic systems consisting of physical mixtures of Au nanoparticles (2-3 nm) supported on metal oxides and Sn-MCM-41 nanoparticles (50-120 nm) were synthesized and investigated for the selective conversion of glycerol to methyl lactate. The Au catalyst promotes the oxidation of glycerol to trioses, whereas the solid acid Sn-MCM-41 catalyzes the rearrangement of the intermediate trioses to methyl lactate. Among the supported Au nanoparticles, Au/CuO led to the highest yield and selectivity toward methyl lactate, while the Sn-MCM-41 nanoparticles showed much better catalytic performance than a benchmark solid acid catalyst (USY zeolite). The activity of the multifunctional catalytic system was further optimized by tuning the calcination temperature, the gold loading in the Au/CuO catalyst, and the Au/Sn molar ratio, reaching 63% yield of methyl lactate (ML) at 95% glycerol conversion. This catalytic system also showed excellent reusability. The catalytic results were rationalized on the basis of a detailed characterization by means of TEM, N2-physisorption, UV-vis spectroscopy, and by FT-IR using probe molecules (CO and ethanol).

Transformation of cellulose and its derived carbohydrates into formic and lactic acids catalyzed by vanadyl cations.

The transformation of cellulose or cellulose-derived carbohydrates into platform chemicals is the key to establish biomass-based sustainable chemical processes. The systems able to catalyze the conversion of cellulose into key chemicals in water without the consumption of hydrogen are limited. We report that simple vanadyl (VO(2+)) cations catalyze the conversions of cellulose and its monomer, glucose, into lactic acid and formic acid in water. We have discovered an interesting shift of the major product from formic acid to lactic acid on switching the reaction atmosphere from oxygen to nitrogen. Our studies suggest that VO(2+) catalyzes the isomerization of glucose to fructose, the retro-aldol fragmentation of fructose to two trioses, and the isomerization of trioses, which leads to the formation of lactic acid under anaerobic conditions. The oxidative cleavage of C-C bonds in the intermediates caused by the redox conversion of VO2(+)/VO(2+) under aerobic conditions results in formic acid and CO2. We demonstrate that the addition of an alcohol suppresses the formation of CO2 and enhances the formic acid yield significantly to 70-75 %.

Chemical synthesis of lactic acid from cellulose catalysed by lead(II) ions in water.

The direct transformation of cellulose, which is the main component of lignocellulosic biomass, into building-block chemicals is the key to establishing biomass-based sustainable chemical processes. Only limited successes have been achieved for such transformations under mild conditions. Here we report the simple and efficient chemocatalytic conversion of cellulose in water in the presence of dilute lead(II) ions, into lactic acid, which is a high-value chemical used for the production of fine chemicals and biodegradable plastics. The lactic acid yield from microcrystalline cellulose and several lignocellulose-based raw biomasses is >60% at 463 K. Both theoretical and experimental studies suggest that lead(II) in combination with water catalyses a series of cascading steps for lactic acid formation, including the isomerization of glucose formed via the hydrolysis of cellulose into fructose, the selective cleavage of the C3-C4 bond of fructose to trioses and the selective conversion of trioses into lactic acid.