Showing papers in "Chemcatchem in 2014"

Facile Synthesis of Au/g‐C3N4 Nanocomposites: An Inorganic/Organic Hybrid Plasmonic Photocatalyst with Enhanced Hydrogen Gas Evolution Under Visible‐Light Irradiation

Abstract: Noble-metal Au nanoparticles deposited on graphitic carbon nitride polymer (g-C3N4) photocatalyst by a facile deposition–precipitation method exhibited high photocatalytic activity for hydrogen gas production under visible-light irradiation. The Au/g-C3N4 nanocomposite plasmonic photocatalysts were characterized by X-ray diffraction spectroscopy, diffuse reflectance UV/Vis spectroscopy, FTIR spectroscopy, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and photoelectrochemical measurements. We studied the effect of Au deposition on the photocatalytic activity of g-C3N4 by investigation of optical, electronic, and electrical properties. Enhanced photocatalytic activity of Au/g-C3N4 naocomposite for hydrogen production was attributed to the synergic mechanism operating between the conduction band minimum of g-C3N4 and the plasmonic band of Au nanoparticles including high optical absorption, uniform distribution, and nanoscale particle size of gold. The mechanism of te photocatalytic activity of the nanocomposite photocatalyst is discussed in detail. Deposition of Au nanoparticles on g-C3N4 was optimized and it was found that 1 wt % Au-loaded g-C3N4 composite plasmonic photocatalyst generated a photocurrent density of 49 mA cm−2 and produced a hydrogen gas amount of 532 μmol under visible light, which were more than 3000 times higher and 23 times higher, respectively, than the values of neat g-C3N4.

Dissolution of Noble Metals during Oxygen Evolution in Acidic Media

Abstract: The electrochemical production of hydrogen and hydrocarbons is considered to play a decisive role in the conversion and storage of excess amounts of renewable energy. The electrocatalysis of the oxygen evolution reaction (OER), however, faces significant challenges for practical implementation of electrolyzers. In this work, a comparative study on the activity and stability of oxidized polycrystalline noble metals during the OER is presented. All studied metals exhibit transient and steady-state dissolution. Transient dissolution takes place during oxide formation and reduction. Steady-state dissolution depends on the OER mechanism on each surface: On metals such as Ru and Au, for which oxygen from the oxide participates in the OER, the Tafel slope is low and the dissolution rate is high. In contrast, on metals for which oxygen evolves directly from adsorbed water, such as Pt and presumably Pd, the Tafel slopes are high and the dissolution rates are low. This should be considered in the design of optimal OER catalysts.

Metal–Organic Frameworks as Biomimetic Catalysts

Abstract: In this Minireview, we have summarized the recent progress of biomimetic catalysis in the field of metal-organic frameworks (MOFs) with a focus on the implantation of biomimetic active sites into a stable MOF. In addition, the potential of creating highly selective catalytic pockets and diffusion-favored hierarchical structures in MOFs has also been explored. Furthermore, we have highlighted the achievements of MOF catalysts in the applications as mimics of peroxidase, cytochromes P450, hemoglobin, and photosynthetic systems.

Highly Efficient Reversible Hydrogenation of Carbon Dioxide to Formates Using a Ruthenium PNP‐Pincer Catalyst

Abstract: The use of hydrogen as a fuel requires both safe and robust technologies for its storage and transportation. Formic acid (FA) produced by the catalytic hydrogenation of CO2 is recognized as a potential intermediate H2 carrier. Herein, we present the development of a formate-based H2 storage system that employs a Ru PNP-pincer catalyst. The high stability of this system allows cyclic operation with an exceptionally fast loading and liberation of H2. Kinetic studies highlight the crucial role of the base promoter, which controls the rate-determining step in FA dehydrogenation and defines the total H2 capacity attainable from the hydrogenation of CO2. The reported findings show promise for the development of practical technologies that use formic acid as a hydrogen carrier.

Molybdenum Sulfides and Selenides as Possible Electrocatalysts for CO2 Reduction

Abstract: Linear scaling relations between reaction intermediates pose a fundamental limitation to the CO2 reduction activity of transition-metal catalysts. To design improved catalysts, we propose to break these scaling relations by binding key reaction intermediates to different sites. Using density functional theory, we demonstrate this principle in the active edge sites in MoS2, MoSe2, and Ni-doped MoS2. These edges show the unique property of selectively binding COOH and CHO to bridging S or Se atoms and CO to the metal atom. DFT calculations suggest a significant improvement in CO2 reduction activity over the transition metals. Our results point to the broader application of the active edge sites of transition-metal dichalcogenides in complex electrochemical processes.

Copper in Photocatalysis

Abstract: Light-induced electron transfer (CuI to CuII), oxidative addition (CuI to CuIII), or the activation of copper alkene or alkyne complexes are possible key steps that offer unique possibilities for organic synthesis, including [2+2]-cycloadditions, cross-coupling reactions or atom transfer radical additions. This Minireview provides an overview on the photophysical properties of photocatalysts based on copper and on synthetic transformations mediated by them.

Recent Advances in the Stabilization of Platinum Electrocatalysts for Fuel‐Cell Reactions

Abstract: Polymer electrolyte membrane fuel cells (PEMFCs) feature high energy densities, low operating temperatures, and low environmental impact, which make them a promising technology for power applications. As a key component of PEMFCs, Pt-based catalysts are still under widespread investigation and have shown exciting performance; however, to move towards their successful commercialization, focusing solely on their catalytic activity is not sufficient. Instead, more effort is required to improve their stability and to decrease costs. Herein, we provide a comprehensive review of current research activities that have concentrated on how to stabilize the Pt-based catalysts. We devote the most attention to the structure-optimization of the Pt-based catalysts and the development of advanced supports. The feasible strategies for structure optimization are subdivided into three groups: 1) dimension effects; 2) electronic and bifunctional effects; and 3) steric effects. Then, we discuss the techniques that have been developed for improving carbon black and for generating various types of carbon-free supports and composites supports (e.g., graphite, carbon nanotubes, new-type oxides and nitrides, and macromolecules). An outlook on the future trends and developments in this area is also provided at the end of the review.

Immobilized Transition Metals as Catalysts for Cross‐Couplings in Continuous Flow—A Critical Assessment of the Reaction Mechanism and Metal Leaching

Abstract: In the not too distant future many industrially important chemicals (including pharmaceuticals) will probably be manufactured using continuous flow technology. For a significant number of synthetic steps involved in these protocols transition metal (mostly palladium)-catalyzed carbon–carbon or carbon–heteroatom bond forming reactions (“cross-coupling chemistry”) will play an important role. Designing a process for continuous cross-coupling chemistry involves either the use of a homogeneous or of a heterogeneous (immobilized) catalyst/ligand system. In the latter case, the catalyst/ligand system is typically in the form of a packed-bed reactor, through which the reaction mixture is pumped, employing an appropriate temperature regime and residence time. Although this approach has been widely popular during the past 15 years, there is growing evidence that suggests that the use of immobilized transition metal catalysts for performing cross-coupling chemistry in continuous flow is, in fact, not very practical. As demonstrated in this review, significant leaching of the transition metal out of the packed-bed catalyst will almost inevitably occur, leading to decreased catalyst activity and contamination of the product with transition metal. This is a consequence of the well-known fact that the reaction mechanism for these kinds of transformations is (quasi)homogeneous and involves the transformation of a Pd0 species into a (soluble) PdII species. Using an immobilized catalyst in a batch protocol the transient leaching of palladium will not be immediately obvious, as, after completion of the catalytic cycle, Pd0 will typically redeposit onto the support. In contrast, in continuous flow mode, the palladium metal will progressively be “chromatographed” through the packed-bed catalyst until, ultimately, all palladium will be removed from the support. This effect typically will become only evident when long run experiments are performed. The preferred alternative, in particular for larger scale experiments, is to use a homogeneous (pre)catalyst in combination with an appropriate catalyst recycling technology.

How Green is Biocatalysis? To Calculate is To Know

Abstract: Green chemistry aims to minimize the environmental hazards of chemical processes and their products. Ever since its introduction by Anastas, this principle has inspired researchers to critically rethink chemistry in view of its potential impact on the environment. Especially, the 12 Principles of Green Chemistry have been an inspiring guideline for this process. Biocatalysis is widely considered as one of the key technologies fulfilling the 12 principles and thereby being green chemistry per se. The reader will recognize stereotypical boilerplate statements such as “enzymes as renewable and biodegradable catalysts”, “working under environmentally benign conditions (temperature, pH, etc.)”, and “operating in water as an environmentally benign solvent” frequently found in the introductory passages to biocatalysis publications. Indeed, these are important parameters that can make biocatalysis environmentally more acceptable than “classical” chemical methods, provided the advantages are not (over)compensated by the disadvantages. Unfortunately, the latter are discussed to a much lesser extent. We also noted a certain tendency to pick a few of the 12 principles to underline the greenness of the method published, which certainly is in contrast to the envisioned use of the 12 principles as a cohesive system. We believe that the time is now to transition from just claiming environmental benefits of (bio)catalysis to quantifying the environmental impact. Indeed, the potential of biocatalysis as a tool to make chemical processes greener has been demonstrated by a limited number of studies. These studies mostly comprise the life-cycle assessment (LCA) of the processes. Unfortunately, LCAs are still rather complex and work intensive. Therefore, LCAs are appreciated by industry to evaluate existing processes (and to use the positive result as a selling argument), whereas research-oriented academic groups generally do not possess the expertise, resources, and interest to perform LCAs. As a result, sustainability issues are often addressed in the phase of manuscript preparation as described above. We believe that on the long term, careless, qualitative use of the term green chemistry will discredit the concept. Therefore, with this contribution we wish to promote the use of simple metrics to assess the environmental footprint of a given method in a semi-quantitative way. To calculate is to know (better). Sheldon proposed the E factor (environmental factor) to assess the greenness of a given reaction. The E factor denotes the amount of waste generated per product equivalent [Eq. (1)] .

Mild Heterogeneous Palladium-Catalyzed Cleavage of β-O-4′-Ether Linkages of Lignin Model Compounds and Native Lignin in Air

Abstract: A mild and robust heterogeneous palladium-catalyzed CO bond cleavage of 2-aryloxy-1-arylethanols using formic acid as reducing agent in air was developed. The cleaved products were isolated in 92–98 % yield; and by slightly varying the reaction conditions, a ketone, an alcohol, or an alkane can be generated in near-quantitative yield. This reaction is applicable to cleaving the b-O-4’-ether bond found in lignin polymers of different origin. The reaction was performed on a lignin polymer model to generate either the monomeric aryl ketone or alkane in a quantitative yield. Moderate depolymerization was achieved with native lignin at similar reaction conditions. Mechanistic studies under kinetic control indicate that an initial palladium-catalyzed dehydrogenation of the alcohol is followed by insertion of palladium to an enol equivalent. A palladium–formato complex reductively cleaves the palladium–enolate complex to generate the ketone.

The Role of Ru and RuO2 in the Catalytic Transfer Hydrogenation of 5‐Hydroxymethylfurfural for the Production of 2,5‐Dimethylfuran

Abstract: We have previously shown that 2,5-dimethylfuran (DMF) can be produced selectively from 5-hydroxymethylfurfural in up to 80 % yield via catalytic transfer hydrogenation with 2-propanol as a hydrogen donor and Ru/C as a catalyst. Herein, we investigate the active phase of the Ru/C catalyst by using extended X-ray absorption fine structure, X-ray photoelectron spectroscopy, and high-resolution TEM analyses. The results reveal that RuO2 is the dominant phase in the fresh (active) catalyst and is reduced to metallic Ru during the reaction with the hydrogen produced in situ from 2-propanol. The deactivation of the catalyst is correlated with the reduction of the surface of RuO2. Reactivity studies of individual phases (bulk RuO2 and reduced Ru/C catalysts) indicate that RuO2 mainly catalyzes the Meerwein–Ponndorf–Verley reaction of 5-hydroxymethylfurfural that produces 2,5-bis(hydroxymethyl)furan and the etherification of 2,5-bis(hydroxymethyl)furan or other intermediates with 2-propanol and that the reduced Ru/C catalyst has moderate hydrogenolysis activity for the production of DMF (30 % selectivity) and other intermediates (20 %). In contrast, a physical mixture of the two phases increases the DMF selectivity up to 70 %, which suggests that both metallic Ru and RuO2 are active phases for the selective production of DMF. The oxidation of the reduced Ru/C catalyst at different temperatures and the in situ hydrogen titration of the oxidized Ru/C catalysts were performed to quantify the bifunctional role of Ru and RuO2 phases. The mild oxidation treatment of the Ru/C catalyst at 403 K could activate the catalyst for the selective production of DMF in up to 72 % yield by generating a partially oxidized Ru catalyst.

The Rise of Magnetically Recyclable Nanocatalysts

Abstract: Magnetic nanoparticles (MNPs) have attracted a great deal of interest in recent years, owing to their remarkably different and unique properties, which include a high surface area, easy recovery, and nanosize. The unique functional surface of MNPs allows the immobilization of homogeneous species, including metals, organoligands/organocatalysts, and N-heterocyclic carbenes (NHC). 2] Traditionally, in various catalytic processes, nanoparticles have been stabilized on the surface of various supports such as porous materials (e.g. silicates, zeolites, alumina, carbons, etc.), until recent developments in nanoscience and nanotechnology paved the way to the synthesis of magnetic nanocatalysts, which are often an excellent choice in heterogeneous catalysis. From a sustainable chemistry viewpoint, MNPs were initially employed as relevant alternatives to conventional inert supports, to facilitate catalyst separation and recovery using simple magnets. In this way, time-consuming and tedious filtration/separation/isolation protocols could be significantly simplified into a one/two-step method for the catalyst recovery and reuse (Scheme 1). Advances in the field of MNPs further expanded their potential into design of novel nanomaterials based on fundamental understanding. The design of MNPs involved a number of strategies such as surface modification, grafting, self-assembly, and nanocasting, which together offered significant new alternatives. These MNPs possess an important advantage compared to conventional supported systems: 1) versatility ; 2) ease and simplicity of separation; 3) improved catalyst reusability (more stable catalysts) and reduction of waste; 4) enhanced catalytic activities ; 5) different selectivities to products; 6) access to previously challenging chemistries (e.g. aqueous processes). 7] Various types of MNPs have also been developed in recent years for important catalytic applications. These include magnetite-supported organocatalysts, metal nanoparticles (Pd, Ni, Cu, Ni, Co, Au, etc.), NHC and chiral catalysts which have been extensively investigated in various organic transformations (Scheme 2 and 3). The advantages of these MNPs from the sustainable chemistry perspective are clear. Expensive and hardly reusable organoligands/organocatalysts can be

Photocatalytic Reduction of Carbon Dioxide over Self‐Assembled Carbon Nitride and Layered Double Hydroxide: The Role of Carbon Dioxide Enrichment

Abstract: A self-assembly of carbon nitride (C3N4) and layered double hydroxide (LDH) was constructed by electrostatic interaction. The pristine nitrate-intercalated Mg-Al-LDH is turned to carbonate LDH through anion exchange during the photoreduction of CO2 in aqueous solution. The carbonate anions enriched in the interlayer of LDH exhibit a remarkably high reduction efficiency to CH4 in the presence of a C3N4 photoabsorber and Pd cocatalyst.

Tailoring the Morphology of g‐C3N4 by Self‐Assembly towards High Photocatalytic Performance

Abstract: Molecular self-assembly is the spontaneous association of stable aggregates with particular function or structure formed by noncovalent bonds under equilibrium conditions. These noncovalent interactions among molecules include ionic interactions, van der Waals interactions, hydrophobic interactions, hydrogen bonds and p–p interactions. Of these, hydrogen bonding is the most widely used interaction to form supramolecular aggregates, owing its reversibility, directionality, saturability, and specificity. In spite of the relatively low energy of a single hydrogen bond, multiple hydrogen bonds can produce stable supramolecular aggregates through superposition and synergy. For example, a melamine molecule forms three hydrogen bonds with a cyanuric acid molecule, yielding highly stable connections and thus creating a well-known supramolecular aggregate named cyanuric acid–/melamine (CA·M) complex. Meanwhile, ionic interaction is also important to form supramolecular aggregates because of its nonsaturability, nondirectionality yet relatively strong noncovalent interactions. As each type of noncovalent interactions has unique advantages, it is essential to combine different types of noncovalent interactions in a single supramolecular aggregate system to create tailorable morphologies and desired functionalities. For example, Nair and Weck reported a highly functionalized copolymer via the self-assembly process combining hydrogen bonding with ionic interactions. Molecular self-assembly has been widely used to synthesize organic materials with specific morphologies, such as g-C3N4. As a metal-free, polymeric semiconductor, g-C3N4 has recently become especially attractive because of its excellent sunlight harvesting capability and rich pyridine-like nitrogen, both of which are essential for water splitting, pollutants degradation, as well as a variety of redox reactions. Unfortunately, there are a number of formidable challenges, including lowvisible light absorption, surface area and electroconductivity, and unideal grain boundary effects, which negatively affect the photocatalytic activity. Of many approaches to overcome these challenges, we summarized the following two major methodologies to develop g-C3N4 with targeting morphology towards high photocatalytic activity. First, hard templates were created to obtain uniform structure and high surface area; this method is respectable but it often consists of complex steps and needs HF to remove hard template, harmful to the environment. Second, molecular self-assembly has been recently developed as a more effective and environmentally friendly method. Shalom et al. started from cyanuric acid and melamine to produce supramolecular aggregates, which were subsequently used to prepare ordered and hollow carbon nitride leading to excellent photoactivity. It was noticed that the hydrogen bonding between cyanuric acid and melamine played a vital role in forming a particular morphology. As specific morphologies often imply high photoactivity, it is of significance to develop new strategies tailoring the morphologies of g-C3N4. Although ionic interactions are particularly useful in the formation of supramolecular aggregates, they have not yet been employed for the development of g-C3N4 of We report a novel method for the preparation of graphitic carbon nitride (g-C3N4) with various morphologies through self-assembly and calcination, which starts from the raw materials melamine, urea, and cyanuric acid. The hollow to wormlike morphologies of g-C3N4 could be readily tailored by adjusting the molar ratio of melamine to urea; with increase in the molar ratio from 3:1 to 1:3, a morphology transformation was observed. The morphologies were tailored by self-assembly of the aggregates by hydrogen bonding and ionic interactions. Correspondingly, an increased BET surface area from 49.6 to 97.4 mg 1 was observed. If used as a photocatalyst in degrading rhodamine B (RhB) under visible-light irradiation, these gC3N4 samples demonstrated 7 to 13 times higher performance than conventional bulk g-C3N4. The high performance was attributed to the unique morphology that provided not only high specific surface area but low recombination losses of photogenerated charges.

Direct Reductive Amination of Ketones: Structure and Activity of S-Selective Imine Reductases from Streptomyces.

Abstract: The importance and structural diversity of chiral amines is well-demonstrated by the myriad nonenzymatic methods for their chemical production. In nature, the production of amines is performed by transamination rather than by reduction of an imine precursor derived from the corresponding ketone. Imine reductases, however, show great potential in the reduction of cyclic imines that are stable towards hydrolysis in aqueous reaction media. Here, we report the catalytic activity of two S-selective imine reductases towards 3,4-dihydroisoquinolines and 3,4-dihydro-β-carbolines and their activity in the direct reductive amination of ketone substrates. The crystal structures of the enzyme from Streptomyces sp. GF3546 in complex with the cofactor NADPH and from Streptomyces aurantiacus in native form have been solved and refined to a resolution of 1.9 A.

New Insight into Platinum Dissolution from Nanoparticulate Platinum‐Based Electrocatalysts Using Highly Sensitive In Situ Concentration Measurements

Abstract: Time- and potential-resolved electrochemical Pt dissolution from commercial Pt and prepared PtCu alloy nanoparticulate catalysts have been studied under potentiodynamic conditions in 0.1 M HClO4 by using on-line inductively coupled plasma mass spectrometry (ICP-MS). For the first time the exact amount of dissolved Pt per cycle has been measured on real electrocatalysts. Results show clearly that Pt dissolution depends on the particle size: approximately seven times as much Pt is released into the solution from commercial 3 nm Pt particles as from a commercial 30 nm Pt sample. The stability of our prepared PtCu electrocatalyst is higher than that of a commercial 3 nm electrocatalyst, which is, however, still slightly lower than that of a commercial 30 nm Pt electrocatalyst.

Fe2O3@LaxSr1−xFeO3 Core–Shell Redox Catalyst for Methane Partial Oxidation

Abstract: Efficient and environmentally friendly conversion of methane into syngas is a topic of practical relevance for the production of hydrogen, chemicals, and synthetic fuels. At present, methane-derived syngas is produced primarily through the steam methane reforming processes. The efficiencies of such processes are limited owing to the endothermic steam methane reforming reaction and the high steam to methane ratio required by the reforming catalysts. Chemical looping reforming represents an alternative approach for methane conversion. In the chemical looping reforming scheme, a solid oxygen carrier or “redox catalyst” is used to partially oxidize methane to syngas. The reduced redox catalyst is then regenerated with air. The cyclic redox operation reduces the steam usage while simplifying the heat integration scheme. Herein, a new Fe2O3@LaxSr1−xFeO3 (LSF) core–shell redox catalyst is synthesized and investigated. Compared with several other commonly investigated iron-based redox catalysts, the newly developed core–shell redox catalyst is significantly more active and selective for syngas production from methane. It is also more resistant toward carbon formation and maintains high activity over cyclic redox operations.

Cascade of Liquid‐Phase Catalytic Transfer Hydrogenation and Etherification of 5‐Hydroxymethylfurfural to Potential Biodiesel Components over Lewis Acid Zeolites

Abstract: We report a one-step process for the production of diesel fuel from biomass-derived 5-hydroxymethylfurfural (HMF). The reaction proceeds through the sequential transfer hydrogenation and etherification of HMF to 2,5-bis(alkoxymethyl)furan, a potential biodiesel additive, catalyzed by a Lewis acid zeolite, such as Sn-Beta or Zr-Beta. An alcohol is used as a hydrogen donor and as a reactant in etherification. This cascade reaction can selectively produce high yields of the biodiesel additive (>80 % yield) from HMF with the Sn-Beta catalyst and secondary alcohols, such as 2-propanol and 2-butanol.

Selective Conversion of Cellulose to Hydroxymethylfurfural in Polar Aprotic Solvents

Abstract: Herein, we report a new reaction pathway to produce hydroxymethylfurfural (HMF) from cellulose under mild reaction conditions (140–190 °C; 5 mM H2SO4) in polar aprotic solvents (i.e. THF) without the presence of water. In this system, levoglucosan is the major decomposition product of cellulose, followed by dehydration to produce HMF. Glucose, levulinic acid, and formic acid are also produced as a result of side reactions with water, which is a by‐product of dehydration. The turnover frequency for cellulose conversion increases as the water content in the solvent decreases, with conversion rates in THF being more than twenty times higher than those in water. The highest HMF yield from cellulose was 44 % and the highest combined yield of HMF and levulinic from cellulose was 53 %, which are nearly comparable to yields obtained in ionic liquids or biphasic systems. Moreover, the use of a low boiling point solvent, such as THF, facilitates recovery of HMF in downstream processes.

Catalytic Conversion of Cellulose into Levulinic Acid by a Sulfonated Chloromethyl Polystyrene Solid Acid Catalyst

Abstract: A novel solid acid catalyst, sulfonated chloromethyl polystyrene (CP) resin (CP‐SO3H‐1.69), was synthesized by partially substituting chlorine groups (Cl) of CP resin with sulfonic group(SO3H). This new type solid acid contains not only acid sites, but also cellulose‐binding sites (Cl). A high yield of levulinic acid up to 65.5 % was obtained by converting microcrystalline cellulose over CP‐SO3H‐1.69. The high catalytic activity of CP‐SO3H‐1.69 was attributed to high amount of sulfonic group and chlorine on the catalyst, which is essential to keep the catalyst with great affinity to substrate.

Homogeneous Photocatalytic Reduction of CO2 to CO Using Iron(0) Porphyrin Catalysts: Mechanism and Intrinsic Limitations

Abstract: A photochemical catalytic reduction of CO2 was performed in an organic solvent with iron(0) porphyrins as homogeneous molecular catalysts under visible light irradiation. With modified tetraphenylporphyrins consisting of internal phenolic groups, the photochemical process led to the production of CO, with H2 as a minor product. High catalytic selectivity for CO formation and turnover numbers up to 30 were obtained. Degradation of the catalyst occurred at longer irradiation times, along with decreased selectivity. Furthermore, addition of a weak acid, which increased the reduction efficiency under electrochemical conditions, led to rapid deactivation of the catalyst. With the unmodified tetraphenylporphyrin as catalyst, we observed lower performance and higher proportion of H2, which highlighted differences in the reduction pathways followed. A combination of a spectroscopic study and product analysis performed under various conditions led to detailed reduction mechanisms and helped pave the way for designing durable photocatalytic systems.

Nanoporous Polymers: Bridging the Gap between Molecular and Solid Catalysts?

Abstract: The combination of the advantageous properties of molecular and solid catalysts is considered the “Holy Grail” in catalysis research. Great potential is provided by nanoporous polymers. Chemically well‐defined moieties in combination with a high stability render these materials suitable as catalyst supports for liquid‐phase and even aqueous‐phase catalytic processes, especially regarding the transition from fossil resources to renewable resources. In this Minireview, recent developments are summarized, covering the three main approaches: solid metal‐free organocatalysts, immobilized molecular catalyst species, and supported metal nanoparticles and clusters. Their potential is evaluated and the question as to whether nanoporous polymers can bridge the gap between homogeneous and heterogeneous catalysis is critically discussed.

Superparamagnetic CuFeO2 Nanoparticles in Deep Eutectic Solvent: an Efficient and Recyclable Catalytic System for the Synthesis of Imidazo[1,2‐a]pyridines

Abstract: Superparamagnetic CuFeO2 nanoparticles were prepared and characterized by XRD, SEM, TEM, FRIR spectroscopy, and vibrating sample magnetometry techniques. The catalytic activity of CuFeO2 nanoparticles was investigated in the one-pot, three-component reaction of 2-aminopyridines, aldehydes, and alkynes using citric acid–dimethylurea melt as a green solvent. An array of imidazo[1,2-a]pyridines were obtained in good to excellent yields. The catalytic system can be successfully reused six times with keeping its catalytic performance.

Selective catalytic reduction of NOx with NH3 over a Cu-SSZ-13 catalyst prepared by a solid state ion exchange method

Abstract: A novel solid state method was developed to synthesize Cu-SSZ-13 catalysts with excellent NH3-SCR performance and durable hydrothermal stability. After the solid state ion exchange (SSIE) process, the SSZ framework structure and surface area was maintained. In-situ DRIFTS and NH3-TPD experiments provide evidence that isolated Cu ions were successfully exchanged into the pores, which are the active centers for the NH3-SCR reaction.

Hydrodeoxygenation of Phenol as a Model Compound for Bio‐oil on Non‐noble Bimetallic Nickel‐based Catalysts

Abstract: Ni/HZSM‐5 catalysts for the hydrodeoxygenation of bio‐oil were modified with Cu and Co. A first test series with phenol as a model revealed that the presence of Ni is essential for high activity. However, modification with Cu deteriorated the catalytic performance significantly whereas an admixture of Co increased the activity and selectivity toward the target hydrocarbons benzene and cyclohexane. Characterization studies performed by using TEM, X‐ray photoelectron spectroscopy, and XRD techniques elucidated detrimental effects of the second metals: both Cu and Co form alloys with Ni; however, Cu caused a loss of active sites as well as tended to segregate at the surface whereas Co made the particles smaller and strongly stabilized the active Ni sites, that is, Ni dispersion increased. A NiCo/HZSM‐5 catalyst (10 wt % of each metal content) showed 99 % hydrocarbon selectivity at complete phenol conversion. Additional studies using intermediate products such as benzene, cyclohexene, and C6 oxygenates as feeds highlight the effect of the catalyst composition on key reaction steps.

Synthesis and Activity of Plasmonic Photocatalysts

Abstract: Plasmonic photocatalysts, which have intensive light absorption and high charge‐separation efficiencies, are regarded as promising candidates to solve energy and environmental issues in the future. In this Review, we summarize recent developments in the synthesis, activity, and mechanism of plasmonic photocatalysts, with the aim of stimulating improvements in photocatalytic activities and promoting the development of photocatalysis. The materials systems, energy‐transfer mechanisms, and factors that influence the photocatalytic activities of plasmonic photocatalysts are discussed. Some perspectives for the future development of the design of highly efficient plasmonic photocatalysts are proposed.

Whole-Cell Biocatalysis in Deep-Eutectic-Solvents/Aqueous Mixtures

Abstract: Whole-cell biocatalysis with the use of baker’s yeast is demonstrated in different mixtures of water with deep eutectic solvents (DESs; choline chloride/glycerol, 1:2 mol/mol). Enantioselective ketone reduction is observed for long reaction times (>200 h), which suggests that the whole cells remain stable in these neoteric solvents. By changing the proportion of the DES added, a complete inversion of enantioselectivity is observed, from approximately 95 % enantiomeric excess (ee) (S) in pure water to approximately 95 % ee (R) in the pure DES. Presumably, some (S)-oxidoreductases present in baker’s yeast are inhibited by DESs.

Stable Performance of Ni-Catalysts in Dry Reforming of Methane at High Temperatures for an Efficient CO2-Conversion into Syngas

Abstract: The catalytic performance of a Ni/MgAlOx catalyst was investigated in the high temperature CO2 reforming of CH4. The catalyst was developed using a Ni, Mg, Al hydrotalcite-like precursor obtained by co-precipitation. Despite the high Ni loading of 55 wt%, the synthesized Ni/MgAlOx catalyst possessed a thermally stable microstructure up to 900 °C with Ni nanoparticles of 9 nm. This stability is attributed to the embedding nature of the oxide matrix, and allows increasing the reaction temperature without losing active Ni surface area. To evaluate the effect of the reaction temperature on the reforming performance and the coking behavior, two different reaction temperatures (800 and 900 °C) were investigated. At both temperatures the prepared catalyst showed high rates of CH4 consumption. The higher temperature promotes the stability of the catalyst performance due to mitigation of the carbon formation.

The Importance of Catalyst Wettability

Abstract: The contribution of catalyst wettability for catalytic performance has been ignored for a long time. In this Concept, we have briefly summarized recent works on the importance of catalyst wettability for improving catalytic activity, product selectivity, and catalyst stability. Suitable catalyst wettability is favourable for several reasons: 1) to enrich the reactants, leading to enhancement of the catalytic activity; 2) to inhibit side-reactions, giving desirable products; 3) to reduce the poisoning of active sites and damage to catalyst framework, resulting in an increase of catalyst stability. These advantages of suitable catalyst wettability will be very important for designing and developing novel heterogeneous catalysts in the future.