Original Catalyst Research Articles

Effect of stereochemistry on the catalytic activity of flavopeptides

The original flavopeptide catalyst, 3-FlC2-l-Pro-l-Tyr-l-Asp-Ado-NH-PS (Fl-PepLLL), and its diastereomers including Fl-PepDDL, Fl-PepLDL, and Fl-PepDLL were synthesized to investigate the correlation between their catalytic activity and stereochemistry. The results and related computations showed the importance of intramolecular hydrogen bonds involving the Asp residue in their 4a-hydroperoxy forms for the catalysis.

Exploring nanowire regrowth for the integration of bottom-up grown silicon nanowires into AFM scanning probes

Bottom-up grown single-crystalline silicon nanowires (SiNWs) are highly intriguing to build nanoscale probes, for instance for atomic force microscopy (AFM), due to their mechanical robustness and high aspect ratio geometry. Several strategies to build such nanowire-equipped probes were explored but their fabrication is still elaborate, time-consuming and relies partly on single-crystalline substrates. Here, we explore a new strategy to fabricate AFM probes that are equipped with single-SiNW scanning tips. The conceptual evaluation begins with a discussion on the overall design and softness of such probes based on finite-element-method simulations. For the experimental realization, SiNWs were grown by the well-established gold-catalyzed vapor–liquid–solid method employing gaseous monosilane. As-grown SiNWs were subsequently transferred onto flexible membranes and even freestanding AFM microcantilever beams via mechanical nanowire contact printing. Elongation of the deposited nanowires by so-called regrowth was triggered by reusing the original gold catalyst to yield the prospective AFM scanning tip. SiNW-equipped scanning probes were created in this manner and were successfully employed for topography imaging. Although a multitude of challenges remains, the created probes showed an overall convincing performance and a superior durability.

Cobalt Complexes of Polypyridyl Ligands for the Photocatalytic Hydrogen Evolution Reaction.

The reductive part of artificial photosynthesis, the reduction of protons into H₂, is a two electron two proton process. It corresponds basically to the reactions occurring in natural photosystem I. We show in this review a selection of involved processes and components which are mandatory for making this light-driven reaction possible at all. The design and the performances of the water reduction catalysts is a main focus together with the question about electron relays or sacrificial electron donors. It is shown how an original catalyst is developed into better ones and what it needs to move from purely academic homogeneous processes to heterogeneous systems. The importance of detailed mechanistic knowledge obtained from kinetic data is emphasized.

Development of Effective Bidentate Diphosphine Ligands of Ruthenium Catalysts toward Practical Hydrogenation of Carboxylic Acids

Abstract Hydrogenation of carboxylic acids (CAs) to alcohols represents one of the most ideal reduction methods for utilizing abundant CAs as alternative carbon and energy sources. However, systematic studies on the effects of metal-to-ligand relationships on the catalytic activity of metal complex catalysts are scarce. We previously demonstrated a rational methodology for CA hydrogenation, in which CA-derived cationic metal carboxylate [(PP)M(OCOR)]+ (M = Ru and Re; P = one P coordination) served as the catalyst prototype for CA self-induced CA hydrogenation. Herein, we report systematic trial-and-error studies on how we could achieve higher catalytic activity by modifying the structure of bidentate diphosphine (PP) ligands of molecular Ru catalysts. Carbon chains connecting two P atoms as well as Ar groups substituted on the P atoms of PP ligands were intensively varied, and the induction of active Ru catalysts from precatalyst Ru(acac)3 was surveyed extensively. As a result, the activity and durability of the (PP)Ru catalyst substantially increased compared to those of other molecular Ru catalyst systems, including our original Ru catalysts. The results validate our approach for improving the catalyst performance, which would benefit further advancement of CA self-induced CA hydrogenation.

Photocatalytic C-C Coupling from Carbon Dioxide Reduction on Copper Oxide with Mixed-Valence Copper(I)/Copper(II).

To realize the evolution of C2+ hydrocarbons like C2H4 from CO2 reduction in photocatalytic systems remains a great challenge, owing to the gap between the relatively lower efficiency of multielectron transfer in photocatalysis and the sluggish kinetics of C-C coupling. Herein, with Cu-doped zeolitic imidazolate framework-8 (ZIF-8) as a precursor, a hybrid photocatalyst (CuOX@p-ZnO) with CuOX uniformly dispersed among polycrystalline ZnO was synthesized. Upon illumination, the catalyst exhibited the ability to reduce CO2 to C2H4 with a 32.9% selectivity, and the evolution rate was 2.7 μmol·g-1·h-1 with water as a hole scavenger and as high as 22.3 μmol·g-1·h-1 in the presence of triethylamine as a sacrificial agent, all of which have rarely been achieved in photocatalytic systems. The X-ray absorption fine structure spectra coupled with in situ FT-IR studies reveal that, in the original catalyst, Cu mainly existed in the form of CuO, while a unique Cu+ surface layer upon the CuO matrix was formed during the photocatalytic reaction, and this surface Cu+ site is the active site to anchor the in situ generated CO and further perform C-C coupling to form C2H4. The C-C coupling intermediate *OC-COH was experimentally identified by in situ FT-IR studies for the first time during photocatalytic CO2 reduction. Moreover, theoretical calculations further showed the critical role of such Cu+ sites in strengthening the binding of *CO and stabilizing the C-C coupling intermediate. This work uncovers a new paradigm to achieve the reduction of CO2 to C2+ hydrocarbons in a photocatalytic system.

Enantioselective preparation of mechanically planar chiral rotaxanes by kinetic resolution strategy

Asymmetric synthesis of mechanically planar chiral rotaxanes and topologically chiral catenanes has been a long-standing challenge in organic synthesis. Recently, an excellent strategy was developed based on diastereomeric synthesis of rotaxanes and catenanes with mechanical chirality followed by removal of the chiral auxiliary. On the other hand, its enantioselective approach has been quite limited. Here, we report enantioselective preparation of mechanically planar chiral rotaxanes by kinetic resolution of the racemates via remote asymmetric acylation of a hydroxy group in the axis component, which provides an unreacted enantiomer in up to >99.9% ee in 29% yield (the theoretical maximum yield of kinetic resolution of racemate is 50%). While the rotaxane molecules are expected to have conformational complexity, our original catalysts enabled to discriminate the mechanical chirality of the rotaxanes efficiently with the selectivity factors in up to 16.

Product selective reaction controlled by the combination of palladium nanoparticles, continuous microwave irradiation, and a co-existing solid; ligand-free Buchwald–Hartwig aminationvs.aryne amination

We have developed a continuous microwave irradiation-assisted Buchwald–Hartwig amination using our original Pd nanoparticle catalyst with a copper plate as a co-existing metal solid.

Catalytic Synthesis of Optically Active Polycyclic Heterocyclic Compounds

In basic pharmaceutical sciences to achieve drug development, research on the efficient chemical synthesis of small molecules having cyclic skeletons is important. We have been engaged in the development of artificial catalysts for asymmetric ring formation reactions that exclusively synthesize right-handed or left-handed cyclic compounds and have achieved the construction of optically active cyclic skeletons using our original catalysts. The synthesis of biologically active compounds was facilitated through six-membered ring construction by Diels-Alder reaction of Danishefsky diene; however, no asymmetric variant of the reaction has been achieved. We approached this unresolved issue using multi-coordinated lanthanide metals. A new chiral lanthanide catalyst was developed, and the catalytic asymmetric Diels-Alder reaction of Danishefsky diene was realized for the first time. By modifying the chemical structure of Danishefsky diene, we applied the lanthanide catalyst to the syntheses of polycyclic compounds and biologically active compounds. We achieved the asymmetric synthesis of natural products, antibacterial and antimalarial compounds, and an anti-obesity drug lead compound. Moreover, the novel catalyst exhibited higher performance than the previously reported ones. The latest generation of the catalyst can be handled stably in air at room temperature. Furthermore, we succeeded in the development of new catalysts by focusing on the properties of its metal precursors, such as nickel and indium, and achieved the construction of polycyclic skeletons by using these catalysts.

Using Mn based on lightweight expanded clay aggregate (LECA) as an original catalyst for the removal of NO2 pollutant in aqueous environment

The NO2 gas is one of the by- products of incineration processes that is the main source of the atmospheric pollutants, So the removal of NO2 is essential for reducing the environmental contamination. In this study, the oxidation of NO2 and producing nitric acid by Mn based on LECA, (Mn/LECA), as a novel nanocatalyst was investigated in aqueous solution. The progress of the treatment reaction in a Semi-batch reactor (SBR) was monitored by conductometry. The Mn/LECA was prepared and characterized by Fourier Transform Infra-Red (FTIR) spectroscopy, Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), and Energy-Dispersive X-ray spectroscopy (EDX) techniques. The Box−Behnken design (BBD) was employed to optimize the operational variables, including the amounts of nanocatalyst, temperature, the concentration of NO2, and the time of reaction in NO2 oxidation. The graphical response surfaces were applied to obtain the optimal circumstances. The Analysis of variance (ANOVA) presented a great value of determination coefficient (R2 = 0.9945, Radj2=0.9881 and Rpred2=0.969) and acceptable prediction second-order regression model. The highest efficacy in the oxidation of NO2 was 94.3%, at a primary temperature of 30 °C, amounts of Mn/LECA at 9 g/L, time of reaction at 60 min, and concentration of NO2 at 3 mg/L. The reusability and durability of the catalyst were good, and the oxidation efficiency of NO2 was 91 and 89.6% after 120 and 180 min, in second and third cycles, respectively.

Molecular engineering of dispersed nickel phthalocyanines on carbon nanotubes for selective CO2 reduction

Electrochemical reduction of CO2 is a promising route for sustainable production of fuels. A grand challenge is developing low-cost and efficient electrocatalysts that can enable rapid conversion with high product selectivity. Here we design a series of nickel phthalocyanine molecules supported on carbon nanotubes as molecularly dispersed electrocatalysts (MDEs), achieving CO2 reduction performances that are superior to aggregated molecular catalysts in terms of stability, activity and selectivity. The optimized MDE with methoxy group functionalization solves the stability issue of the original nickel phthalocyanine catalyst and catalyses the conversion of CO2 to CO with >99.5% selectivity at high current densities of up to −300 mA cm−2 in a gas diffusion electrode device with stable operation at −150 mA cm−2 for 40 h. The well-defined active sites of MDEs also facilitate the in-depth mechanistic understandings from in situ/operando X-ray absorption spectroscopy and theoretical calculations on structural factors that affect electrocatalytic performance. Widespread deployment of electrochemical CO2 reduction requires low-cost catalysts that perform well at high current densities. Zhang et al. show that methoxy-functionalized nickel phthalocyanine molecules on carbon nanotubes can operate as high-performing molecularly dispersed electrocatalysts at current densities of up to −300 mA cm–2.

Hydrophobic electrocatalyst for the enhanced activity of oxygen reduction reaction through controllable liquid/gas/solid interface

Slow gas consumption process has been considered as one of the main limiting factors to enhance the activity of oxygen reduction reaction (ORR). The superwettable materials can overcome the limitation of underliquid gas transport in electrocatalysis, however, most of the preparation process are complex and high cost. Herein, an electrocatalyst for ORR is synthesized by loading ZIF-67 on carbon black, then the hydrophobicity is bestowed on the catalyst and controlled by adjusting the amount of 1H,1H,2H,2H-perfluorodecyltriethoxysilane, aimed to preparing the carbon-based electrocatalyst with hydrophobic surface (Cal-CoZIF-XE2B/PFDS). Continuous and abundant cavitations on the catalyst surface ensure the rapid diffusion of oxygen to the three phase contact area of the reaction system. Compared with the unmodified catalyst, the hydrophobic catalyst has larger diffusion-limited current density (from 4.4 mA/cm2 of the original catalyst to 5.2 mA/cm2 of the modified catalyst) and higher half-wave potential (from 0.81 V of the original catalyst to 0.83 V of the modified catalyst). Furthermore, the as-prepared electrocatalysts which obtained with low cost and simple method, presenting durability and stability against alkaline and acidic electrolyte solutions and providing a general and reliable strategy for the study of ORR catalyst.

One-step preparation of a N-CNTs@Ni foam electrode material with the co-production of H2 by catalytic reforming of N-containing compound of biomass tar

Carbon nanotubes (CNTs) can be formed during catalytic reforming of hydrocarbons with H2 co-production over Ni-based catalysts. Normally, the formed CNTs need to be separated from original catalysts in order to be coated on the substrates to make high performance electrodes. To simplify the traditional preparation process, a novel one-step method for simultaneously producing a CNTs electrode material and H2 was proposed. A Ni-Fe modified Ni foam (Ni-Fe/NF) was chosen as both catalyst and electrode substrate to in situ grow nitrogen-doped carbon nanotubes (N-CNTs) during catalytic reforming of pyrrole. Pyrrole was selected as an N-containing model compound of biomass tar. The results showed that Ni-Fe nanoparticles on Ni-Fe/NF catalyst remarkably improved the H2 production and pyrrole conversion, which increased by more than 20 times than those of fresh Ni foam. Besides, a significant amount of multi-walled CNTs with more than 40 graphite layers grew on the surface of spent catalyst and their diameters were mainly from 10 nm to 20 nm. N element was detected on the formed CNTs and the Graphitic-N was the main type of N-doping. The spent catalyst (N-CNTs@Ni foam) can be directly used as a composite electrode material with high performance and stability. Its electrochemical capacitance reached 855 mF/cm2 at a current density of 5 mA/cm2 and remained over 95% after 10,000 cycles.

Performance for the catalytic CO oxidation over the Mo0.5Sn9Co90.5O2 catalyst

In this study, carbon monoxide catalytic oxidant MoSnCoOx was prepared by coprecipitating method to investigate the effect of MoO3 addition amount on catalyst activity. Meanwhile, the sulfur and water resistance of the catalysts were researched by simulating the industrial conditions of sintering smoke emission. The samples were studied using BET, XRD, SEM and H2-TPR techniques. The results revealed that catalyst added with 0.5% MoO3 had better activity than the original Sn9Co91O2 catalyst and its efficiency reached 98.04% at 70 °C. After 21 h reaction with sulfur and water in the flue gas, the catalyst efficiency only decreased to 89.75%. According to the results of characterization, Mo0.5Sn9Co90.5O2 catalyst could better resist sulfur and water, because the presence of MoO3 not only reduced the crystallinity of the catalyst, but it also increases the specific surface area and oxidation capacity.

Selective Cl-Decoration on Nanocrystal Facets of Hematite for High-Efficiency Catalytic Oxidation of Cyclohexane: Identification of the Newly Formed Cl-O as Active Sites.

Understanding the structure-reactivity relationship at the atomic scale is of great theoretical importance for rational design of highly active catalysts, which has long been a central concern in catalysis communities and interface science. Herein, we developed a high-efficiency catalyst for catalytic oxidation of C6H12 by poststructural decoration on well-defined single-crystal facets of hematite. Especially for Cl-decorated {012} facets, the conversion and KA oil selectivity are improved about 3.4 times and 2 times, respectively. A better catalytic performance of the newly formed active site is derived from the charge difference between Cl and the neighboring outmost O atoms, which is affected by the geometric and electronic structures of the original catalyst surface. Based on the experimental results and the theoretical analysis, we concluded that the contribution of various O terminations to Cl-decoration follows the order O(I) > O(III) > O(II). Cl-decorated {001} facets show the highest intrinsic activity, whereas Cl-decorated {012} facets show the best catalytic performance because of their more active sites for Cl-decoration.

Constructing defect-rich V2O5 nanorods in catalytic membrane electrode for highly efficient oxidation of cyclohexane

Understanding the surface defect chemistry of oxides has proven to be highly important in electrochemical oxidation. V2O5 nanorods (NRs) were loaded onto porous Ti membranes, then treated with H2 to generate reduced defect-rich V2O5 NRs/Ti (r-V2O5 NRs/Ti) electrodes. These r-V2O5 NRs/Ti electrodes were used to construct an electrocatalytic membrane reactor (ECMR) for the electro-oxidation of cyclohexane (CHA) to cyclohexanone (K) and cyclohexanol (A). The electrocatalytic results showed that the r-V2O5 NRs/Ti membrane electrode exhibited superior catalytic performance compared with the original V2O5 NRs catalyst. 55.5% conversion and more than 99% selectivity were obtained at 30 °C and ambient pressure without any oxidant, which is superior to most catalysts reported in the literature. More importantly, density functional theory (DFT) calculations confirmed that the existence of oxygen vacancies on the r-V2O5 NRs/Ti electrode led to the lower band gap facilitated electron transition, resulting in enhanced conductivity and superior electrochemical activity.

The ultrasonication boosts the surface properties of CoFe2O4/C nanoparticles towards ORR in alkaline media

Since ultrasonication was reported to be used for exfoliation of layered materials in water (Science-ref. 31), the method has never been explored in non-layered materials, in particular, the effect of ultrasonication on the surface property of materials. In this work, CoFe2O4/C nanoparticles were synthesized and processed using ultrasonic treatment in water. Through the ultrasonic treatment, the electrocatalytic activity of the CoFe2O4/C nanoparticles towards ORR was improved significantly with a higher mass activity (5.05 mA mg−1) than the original CoFe2O4/C (2.75 mA mg−1) in O2-saturated 0.1 M KOH solution. The half-wave potential on the original CoFe2O4/C was also shifted to the positive side by 60 mV. Furthermore, the treated CoFe2O4/C catalyst exhibits a constant half-wave potential with better onset potential after 2000 cycles, and a 50 mV of half-wave potential on the original catalyst was moved to negative under same test condition. The analysis from characterizations reveals that the enhanced ORR performance of the treated CoFe2O4/C resulted from the Co2+ and Fe3+ enriched surface with more cations being occupied in the tetrahedral sites than the octahedral sites after ultrasonic treatment. In addition, compared to the original CoFe2O4/C catalyst, the specific surface area of the treated CoFe2O4/C was improved 1.8 times, mesoporous grown to microspores with 2.2 times increased volumes, which has provided higher active sites and accelerated transport between O2 and electrolyte during the ORR process. The ORR via the 4-electron transfer pathway on the treated CoFe2O4/C catalyst, while the 2-electron transfer process is favoured on the original CoFe2O4/C catalyst. This further signifies that the ultrasonic process had significantly influence on the electrochemical properties of the CoFe2O4/C catalyst.

Charging up cobalt for hydroformylation

Catalysis Hydroformylation reactions are applied at massive scale in the chemical industry to transform olefins into aldehydes. The original catalysts were neutral cobalt complexes. Hood et al. now report that positively charged cobalt complexes, stabilized by chelating phosphine ligands, show higher activities at lower pressures than their neutral counterparts, approaching the activities of precious rhodium catalysts. These charged catalysts are particularly adept at accelerating the reactions of internal olefins while avoiding decomposition. Spectroscopic studies implicate the involvement of 19-electron intermediates in the catalytic cycle. Science , this issue p. [542][1] [1]: /lookup/doi/10.1126/science.aaw7742

Highly active cationic cobalt(II) hydroformylation catalysts.

The cobalt complexes HCo(CO)4 and HCo(CO)3(PR3) were the original industrial catalysts used for the hydroformylation of alkenes through reaction with hydrogen and carbon monoxide to produce aldehydes. More recent and expensive rhodium-phosphine catalysts are hundreds of times more active and operate under considerably lower pressures. Cationic cobalt(II) bisphosphine hydrido-carbonyl catalysts that are far more active than traditional neutral cobalt(I) catalysts and approach rhodium catalysts in activity are reported here. These catalysts have low linear-to-branched (L:B) regioselectivity for simple linear alkenes. However, owing to their high alkene isomerization activity and increased steric effects due to the bisphosphine ligand, they have high L:B selectivities for internal alkenes with alkyl branches. These catalysts exhibit long lifetimes and substantial resistance to degradation reactions.

Integrating pyrolysis and ex-situ catalytic reforming by microwave heating to produce hydrocarbon-rich bio-oil from soybean soapstock

The composite catalysts were synthesized with SiC powder and ZSM-5 and were characterized by Brunauer-Emmett-Teller, X-ray diffraction, thermogravimetric analysis, pyridine-infrared spectroscopy, and scanning electron microscopy. The catalysts showed a high heating rate and excellent catalytic performance for pyrolysis vapors, and the product fractional distribution and chemical compositions of bio-oil in a tandem system (microwave pyrolysis and microwave ex-situ catalytic reforming) was examined. Experimental results confirmed the quality of bio-oil produced by the microwave-induced catalytic reforming was better than that product through electric heating. Additionally, 36.94 wt% of bio-oil was obtained using the catalyst with 20%ZSM-5/SiC under the following conditions: feed-to-catalyst ratio, 2:1; pyrolysis temperature, 550 °C; and catalytic temperature, 350 °C. The selectivities of hydrocarbons reached up to 75.88%. After five cycles, the activity of the regenerated composite catalyst was retained at 95% of the original catalyst.

Nickel nanoparticles loaded on ceria oxide spheres as catalyst for dry reforming of methane

In this paper, nickel particles were loaded on ceria oxide flowers dispersed on a framework of inorganic fibres as catalyst for dry reforming of methane. The tests of dry reforming of methane were performed at 700-800°C. With the Ni/Ce flowers, at 700°C, 750°C and 800°C initial CH4 conversion and after 100 h were 85-63%, 93-88%, 96-87%, respectively. Without the dispersion of ceria oxide flowers, CH4 conversion was 96-70% at 800°C. The samples were reactivated after 100 h of reforming at 800°C and then the reforming reaction was performed again for another 100 h. It was found that the conversion of CH4 after reactivation was higher and more stable than the original catalyst by a few per cent.