Catalysts For Several Reactions Research Articles

Silica-Resin-Bentonite Nanocomposite and Its Application in Catalysis

Bentonites are natural clays found in abundance in deposits all over the planet and possess certain properties that make them interesting for various industrial applications. Through their activation or acid treatment, they can be used as catalysts in several reactions of interest. However, these materials form colloidal suspensions in water or in aqueous solutions, which makes their separation and recovery difficult and prevents their implementation on an industrial scale. To overcome these limitations, in the present work, a silica-resin-bentonite composite material was synthesized and activated with HNO3. The activated solids were characterized and evaluated in the catalytic reaction of solketal synthesis from glycerol and acetone. The best results were obtained for a composite containing 47 wt.% acidified bentonite at 90 °C, with a HNO3 concentration of 0.5 mol L−1, which was attributed to both its acid site density—3.9 mmol per gram of bentonite—and the acidic strength of these sites.

Molybdenum oxide and sulfide active sites for isobutane dehydrogenation with methanol as a probe molecule

Molybdenum oxides and sulfides are alternative catalysts for several reactions, including alkane dehydrogenation. Correlations have been developed between reactivity and structure, but it is difficult to compare different catalysts without determining numbers of active sites. Here, methanol temperature programmed surface reaction (TPSR) is applied to bulk MoS2 and a series of supported molybdenum oxides and sulfides used for isobutane dehydrogenation. Rates and deactivation behavior are similar between supported molybdenum oxides and sulfides. Coking increases with loading and correlates well with the fraction of Mo-based CO2-forming sites from methanol TPSR. Normalizing isobutane dehydrogenation rates by formaldehyde-forming sites unifies rates over oxide and sulfide materials. These measurements confirm earlier hypotheses that MoS2 and higher loadings of oxides and sulfides are intrinsically fast catalysts, but they have few active sites and are prone to coking. In general, methanol TPSR is proposed as an accessible route to count sites on both supported oxides and sulfides.

Differences in Catalytic Activity and Durability of Nitrogen Sites on Nitrided SBA-15 or Porous Carbon Nitride

Nitrided SBA-15 (NSBA-15) is a solid base catalyst for several reactions, in which the basicity arises from a lone pair on a nitrogen atom substituted in the silicate framework. Porous carbon nitride (PCN) formed on a SBA-15 template is also a solid base catalyst with a nitrogen atom as the active site. In this study, the catalytic activity and durability of nitrogen atoms in nitrided mesoporous silica and porous carbon nitride were compared to evaluate the origin of basicity by using the Knoevenagel condensation of benzaldehyde with active methylene compounds. With malononitrile (pKa of 11.1) as the methylene compound, both NSBA-15 and PCN showed a high product yield. Using ethyl cyanoacetate (pKa of 13.1), only NSBA-15 catalyzed the reaction, probably because the active site of NSBA-15 is a quasi-aliphatic amine and that of PCN is an aromatic amine. On the other hand, PCN showed high durability as the content of nitrogen was not decreased by the catalytic reaction and long-term storage, whereas the Si–N bond in NSBA-15 easily underwent hydrolysis.

Utility of NaMoO3F as a Precursor for Homogeneous Distribution of Cobalt Dopants in Molybdenum Oxynitrides

Molybdenum nitrides and their related compounds have been focused as a catalyst for several reactions. Although the doping into molybdenum nitrides lead to the higher catalytic activity, the simultaneous control of the morphology, the crystallinity, and the dopant state in doped MoN cannot be easily achieved due to the limitation of the synthesis method. In this study, one of the mixed anion compounds, NaMoO3 F was used as a precursor for molybdenum oxynitrides with hexagonal MoN phase. This route led to the homogeneous distribution of cobalt in the molybdenum oxynitride compared with that obtained by the other method. The cobalt-doped molybdenum oxynitride from NaMoO3 F exhibited high oxygen reduction reaction catalytic activity due to the high distribution of cobalt in the crystal. This paper proposes that the mixed anion compounds can be a unique precursor for the other materials to expand the controllability of materials toward improvement of their activity.

Activated layered double hydroxides: assessing the surface anion basicity and its connection with the catalytic activity in the cyanoethylation of alcohols.

Layered double hydroxides (LDHs) act as catalysts in several reactions like in the cyanoethylation of alcohols with acrylonitrile to produce alkoxypropionitriles. Here we report an experimental and theoretical study in which it is shown that the experimental catalytic activity of LDHs in the cyanoethylation of 2-propanol and methanol correlates with the predicted strength of the basicity of the adsorbed surface species. First, it is shown that using activated LDHs containing Mg2+ and Al3+ (MgAl-LDH), Mg2+ and Ga3+ (MgGa-LDH), and Mg2+, Al3+ and Ga3+ (MgAlGa-LDH) great conversions to alkoxypropionitriles in high yields are obtained. Next, the basicity of these LDHs is estimated by means of the local softness, a local reactivity index calculated using density functional theory and appropriate surface models. For that, the adsorption of hydroxide and methoxide anions at the (001) surface of MgAl and MgGa-LDHs is investigated. We include LDHs containing Zn2+ and Al3+ (ZnAl-LDH) and Zn2+ and Ga3+ (ZnGa-LDH) in this part of the study to account for the effect of changing the divalent and trivalent metal composition on the basicity. It is found that hydroxide anions adsorbed on the MgGa-LDH surface and methoxide anions adsorbed on the MgAl-LDH surface are the most basic ones. This basicity trend correlates with our experimental findings about the catalytic activity of the activated LDHs. Further analyzing the connection between the LDH composition and the anion basicity, it is argued that the key steps dictating the LDH catalytic activity are the alcohol deprotonation in the cyanoethylation of 2-propanol, as it has been previously suggested, and the methoxide anion attack to the acrylonitrile double bond in the methanol cyanoethylation reaction.

The Catalytic Nature of Chevrel Phases (MxMo6S8) in Review

Chevrel Phases, MxMo6S8, are a class of heterogenous catalysts comprised of abundant materials that have been reported to out-perform conventional MoS2 catalysts in several reactions. Historically, lengthy and energy-intensive syntheses have restricted their use as catalysts, however recent developments in synthesis techniques, driven by energy storage research, have lessened these constraints. The catalytic community has been slow to adopt these advances, yet Chevrel Phases remain excellent candidates for fundamental catalytic reactions due to their unique combination of low-coordinate molybdenum atoms; tuneable oxidation state; Mo6-cluster electron-reservoirs; thermal stability, and dual Lewis acidity & basicity. This review summarises five decades of catalytic reports, identifies trends and gaps within the literature, and provides perspective on future directions for the catalytic application of Chevrel Phases. In doing so, we hope to bring these promising materials to the attention of a wider audience to facilitate improvements in energy & chemical conversion processes.

Chemically Controllable Porous Polymer–Nanocrystal Composites with Hierarchical Arrangement Show Substrate Transport Selectivity

Functional organic–inorganic hybrid materials with tunable properties are useful across many application areas, ranging from gas storage to electronics, flame retardants, separations, and catalysis. Combining polymers, with a suite of functional groups and conformational flexibility, and inorganic nanoparticles, with tunable surface chemistry and composition, yields hybrids with novel functional properties. Specifically, in catalysis, control of the electronic environment at a metal interface is paramount in determining the catalytic properties. In this contribution, we describe a modular process to prepare porous polymer–nanocrystal (NC) composites in a hierarchical, multilayered synthesis, in which multiple parameters can be accurately tuned: polymer functional groups and the corresponding pore structure, the polymer layer thickness, and the NC size, shape, and composition. This process provides for a variety of controlled materials with high surface area, tunable chemistry, and thermal and chemical stabilities. Furthermore, we demonstrate their utility for shape- and size-selective catalytic conversions both in oxidation and hydrogenation reactions, where they show increased selectivity by orders of magnitude compared to conventional polymer-supported metal catalysts. In light of the high degree of control in the composite structure, this method allows for the design and realization of catalysts for several reactions and reaction environments and for nanomaterials with other applications.

Surface acid-base properties of Cu-BTC and Fe-BTC MOFs. An inverse gas chromatography and n-butylamine thermo desorption study

Catalytic activity is usually related to the acid-base properties of the catalyst material. Metal-organic frameworks (MOFs) have been evaluated as potential catalysts for several reactions. Cu-BTC (with a well-defined crystalline structure) and Fe-BTC (with a gel-like porous structure) have both shown the presence of Lewis surface acid sites based on the observed performance in different catalytic reactions. In this work, experiments of Inverse Gas Chromatography (IGC) and n-butylamine thermo desorption analysis were carried out in order to determine the acid-base properties of both materials. The IGC measurements allow to access the acid-base properties of the material surface by using acidic, basic and amphoteric molecules. Otherwise the n-butylamine thermo desorption measurements allow the quantification of those acid and basic sites from the n-butylamine desorption curves of a sample previously saturated with that probe molecule. The Lewis acid properties obtained by both methodologies showed to be more pronounced for the Fe-BTC MOF sample. The IGC measurements exhibited values of acidity constants (KA) of 0.81 and 0.60 for Fe-BTC and Cu-BTC, respectively. Furthermore the n-butylamine thermo desorption experiments indicate total acidities of 2.06 and 0.49 mmol g−1 for Fe-BTC and Cu-BTC, respectively.

Advances in Nanostructured Metal-Encapsulated Porous Organic-Polymer Composites for Catalyzed Organic Chemical Synthesis

Porous organic polymers (POPs) are of growing research interest owing to their high surface areas, stabilities, controllable chemical configurations, and tunable pore volumes. The molecular nanoarchitecture of POP provides metal or metal oxide binding sites, which is promising for the development of advanced heterogeneous catalysts. This article highlights the development of numerous kinds of POPs and key achievements to date, including their functionalization and incorporation of nanoparticles into their framework structures, characterization methods that are predominantly in use for POP-based materials, and their applications as catalysts in several reactions. Scientists today are capable of preparing POP-based materials that show good selectivity, activity, durability, and recoverability, which can help overcome many of the current environmental and industrial problems. These POP-based materials exhibit enhanced catalytic activities for diverse reactions, including coupling, hydrogenation, and acid catalysis.

Computational Study of MoN2Monolayer as Electrochemical Catalysts for Nitrogen Reduction

Metallic two-dimensional materials are emerging as promising catalysts for several reactions. This work reports a computational study of molybdenum nitride (MoN2) nanosheet as a catalyst for ammonia synthesis at room temperature under the scheme of density functional theory. Pure MoN2 shows excellent performance for dinitrogen adsorption and activation, but large energy input is needed to refresh the surface following the reaction. Iron (Fe) doping can effectively improve it to achieve full nitrogen reduction with a calculated overpotential of 0.47 V, indicating that Fe-doped MoN2 is a promising catalyst for electrochemical synthesis of ammonia from water and air.

Au and Pt nanoparticle supported catalysts tailored for H2 production: From models to powder catalysts

The use of metal nanoparticles (NPs), including Au and Pt, supported over oxides has been pivotal, and is ever increasing in enabling catalytic reactions which target the production of hydrogen. We review here the most recent works pertaining to the fundamental understanding of the structure, morphology, growth, characterization, and intrinsic phenomenological properties of Au- and Pt-based catalysts that influence the reactivity and selectivity to target hydrogen production. We draw on surface science and theoretical methods of model and powder catalysts using high resolution imaging, spectroscopy, scattering experiments, and theoretical studies. Based on these insights we identify key aspects of studies of supported metal nanoparticle (NP) catalysts for several reactions. The main focus of this review is on the intersection of catalytic chemistry related to the water-gas shift (WGS), oxygenate steam reforming (OSR), and solar-assisted reactions (SAR).

Unraveling the structure sensitivity in methanol conversion on CeO2: A DFT + U study

Methanol decomposes on oxides, in particular CeO2, producing either formaldehyde or CO as main products. This reaction presents structure sensitivity to the point that the major product obtained depends on the facet exposed in the ceria nanostructures. Our density functional theory (DFT) calculations illustrate how the control of the surface facet and its inherent stoichiometry determine the sole formation of formaldehyde on the closed surfaces or the more degraded by-products on the open facets (CO and hydrogen). In addition, we found that the regular (100) termination is the only one that allows hydrogen evolution via a hydride–hydroxyl precursor. The fundamental insights presented for the differential catalytic reactivity of the different facets agree with the structure sensitivity found for ceria catalysts in several reactions and provide a better understanding on the need of shape control in selective processes.

Tuning the Fe(II)/(I) formal potential of the FeN4 catalysts adsorbed on graphite electrodes to the reversible potential of the reaction for maximum activity: Hydrazine oxidation

In classical volcano correlations in electrocatalysis, the shape of (log io) plots versus the binding energy of the reactant to active sites, the maximum activity is associated to a situation where the surface coverage of reactant species is equal to 0.5 corresponding to a free energy of adsorption of the reactant equal to zero. When studying electrochemical reactions catalysed by macrocyclic transition metal complexes adsorbed on graphite electrodes, volcano correlations are found when plotting the activity versus the formal potential of the catalyst for several reactions, assuming that the formal potential is somehow related to the free energy of the adsorption of the reacting molecule. In this work we offer an interpretation different from that given in the literature for these correlations for the electrooxidation of hydrazine. The highest catalytic activity is achieved when the formal potential of the catalyst approaches the N2H4/N2 reversible potential. In contrast, if log i (normalized for the actual surface concentrations of Fe(II) active sites at constant potential) is plotted versus the difference between the Fe(II)/(I) formal potential of the catalysts and the standard potential of the N2H4/N2 reaction, the correlation is linear with a slope close to 2RT/F.

Ruthenium Nanoparticles in Ionic Liquids – A Saga

Ionic liquids (ILs) are excellent media for the generation and stabilisation of metallic nanoparticles (NPs). Their ionic character coupled with 3-D structural pre-organisation in the liquid state, serves to direct the growth of transition metal NPs generated in situ, and to subsequently protect and stabilise them. Until now, many different NPs have been successfully synthesised within these media, however much attention has been paid to Ru-NPs. These have been prepared with small sizes and narrow size distributions by reduction of organometallic compounds with molecular hydrogen as well as decomposition of transition-metal complexes in the zero-valent state. These stable Ru-NPs immobilised in the ILs have proven to be efficient green catalysts for several reactions in multiphase conditions, including important energy-related processes such as biomass refinement. Furthermore, they present potential novel materials for use in the production of smarter electronic devices. In this review, the synthesis, stabilisation and size-control of Ru-NPs via various methods in different ILs is discussed, followed by their varied application in catalysis and potential in new fields. Keywords: Ionic liquids, Ruthenium, Nanoparticles, Catalysis

Lattice-Matched Bimetallic CuPd-Graphene Nanocatalysts for Facile Conversion of Biomass-Derived Polyols to Chemicals

A bimetallic nanocatalyst with unique surface configuration displays extraordinary performance for converting biomass-derived polyols to chemicals, with potentially much broader applications in the design of novel catalysts for several reactions of industrial relevance. The synthesis of nanostructured metal catalysts containing a large population of active surface facets is critical to achieve high activity and selectivity in catalytic reactions. Here, we describe a new strategy for synthesizing copper-based nanocatalysts on reduced graphene oxide support in which the catalytically active {111} facet is achieved as the dominant surface by lattice-match engineering. This method yields highly active Cu-graphene catalysts (turnover frequency = 33-114 mol/g atom Cu/h) for converting biopolyols (glycerol, xylitol, and sorbitol) to value-added chemicals, such as lactic acid and other useful co-products consisting of diols and linear alcohols. Palladium incorporation in the Cu-graphene system in trace amounts results in a tandem synergistic system in which the hydrogen generated in situ from polyols is used for sequential hydrogenolysis of the feedstock itself. Furthermore, the Pd addition remarkably enhances the overall stability of the nanocatalysts. The insights gained from this synthetic methodology open new vistas for exploiting graphene-based supports to develop novel and improved metal-based catalysts for a variety of heterogeneous catalytic reactions.

MCl2(ampy)(dppf) (M = Ru, Os): Multitasking Catalysts for Carbonyl Compound/Alcohol Interconversion Reactions

The easily accessible complexes [MCl2(dppf)(ampy)] (M = Ru (cis-1), Os (trans-2); dppf = 1,1′-bis(diphenylphosphino)ferrocene; ampy = 2-aminomethylpyridine) in the presence of base (NaOiPr, KOtBu) are efficient catalysts for several reactions involving carbonyl compounds and alcohols. The derivatives 1 and 2 catalyze the selective transfer hydrogenation of aldehydes and ketones to alcohols with 2-propanol using 0.1–0.005 mol % of catalyst at 82 °C with TOF values up to 3.0 × 105 h–1. These compounds (0.1–0.02 mol %) promote the hydrogenation of aldehydes and ketones in EtOH or a MeOH/EtOH mixture at 30–90 °C (5 atm of H2) and the acceptorless dehydrogenation of alcohols to ketones in tBuOH at 130–145 °C (0.4 mol %). Complexes 1 and 2 (0.5 mol %) catalyze the racemization of chiral alcohols at 70 °C in 2-propanol and the isomerization of allylic alcohols to ketones in tBuOH at 70–120 °C (1 mol %). In addition, 1 and 2 (0.5 mol %) promote the α alkylation of α-tetralone with primary alcohols (EtOH, nPrOH, and nBuOH) at 120 °C in a tBuOH/toluene mixture (1/2 v/v). Complex 2 is easily obtained in 93% yield from [OsCl2(PPh3)3], dppf, and ampy in toluene.

Increased Diels-Alderase activity through backbone remodeling guided by Foldit players.

Computational enzyme design holds promise for the production of renewable fuels, drugs and chemicals. De novo enzyme design has generated catalysts for several reactions, but with lower catalytic efficiencies than naturally occurring enzymes. Here we report the use of game-driven crowdsourcing to enhance the activity of a computationally designed enzyme through the functional remodeling of its structure. Players of the online game Foldit were challenged to remodel the backbone of a computationally designed bimolecular Diels-Alderase to enable additional interactions with substrates. Several iterations of design and characterization generated a 24-residue helix-turn-helix motif, including a 13-residue insertion, that increased enzyme activity >18-fold. X-ray crystallography showed that the large insertion adopts a helix-turn-helix structure positioned as in the Foldit model. These results demonstrate that human creativity can extend beyond the macroscopic challenges encountered in everyday life to molecular-scale design problems.

On the structural and surface properties of transition-metal nanoparticles in ionic liquids

Ionic liquids (ILs), in particular imidazolium-based ILs, have proven to be suitable media for the generation and stabilisation of soluble metal nanoparticles (NPs). Indeed, transition-metal NPs with small sizes, narrow size distribution and different shapes have been prepared by reduction of organometallic compounds with molecular hydrogen, decomposition of transition-metal complexes in the zero-valent state, metal bombardment or simple transfer for previously prepared water- or classical organic solvent-soluble colloids to the ILs. The formation and stabilisation of NPs in these highly hydrogen bonded organised supramolecular fluids occur with the re-organisation of the hydrogen bond network and the generation of nanostructures with polar and non-polar regions, including the NPs. The IL forms a protective layer, which is probably composed of imidazolium aggregates located immediately adjacent to the nanoparticle surface, which provides both steric and electronic protection against aggregation and/or agglomeration. These stable transition-metal NPs immobilised in the ILs have proven to be efficient green catalysts for several reactions in multiphase conditions and also novel materials for chemical sensors. In this critical review, the structural/surface properties of these soluble metal NPs dispersed in ILs and their application in catalysis and as chemical sensors are discussed, with particular attention paid to the stabilisation models proposed to explain the stability and properties of these metal NPs (219 references).

Relationship between the Surface Properties and the Catalytic Performance of Al-, Ga-, and AlGa-Pillared Saponites

A saponite from Ballarat, CA was intercalated with solutions containing only-Al, only-Ga, and two mixed AlGa-polycations, with various Al/Ga ratios. The pillared solids were obtained by calcination of the intercalated precursors at two temperatures, 300 and 500 °C. The physicochemical characteristics of the solids were studied by several techniques (XRD, N2 adsorption at −196 °C, TG, DTA, FT-IR and UV−vis−NIR) and also by means of their behavior in various acid-catalyzed reactions. All the solids showed layered, ordered structure with basal spacings of ca. 18 Å and high specific surface areas between 247 and 387 m2/g (intercalated solids), 236 and 320 m2/g (calcined at 300 °C), and 201 and 331 m2/g (calcined at 500 °C). The materials were tested as catalysts in several reactions, including liquid-phase (rearrangement of α-pinene to camphene and methanol addition to 3,4-dihydropyran) and gas-phase processes (dehydration of the alcohols 2-methyl-3-butyn-2-ol and 2-propanol and cumene conversion). The catalytic behavior of the solids was discussed in terms of their surface properties and synthesis procedure.

Phosphine‐Functionalized Dendrimers for Transition‐Metal Catalysis

Abstract Dendrimers are well-defined hyperbranched macromolecules with characteristic globular structures for the larger systems, of which their use has been explored for various applications including catalysis. Dendritic catalysts potentially combine the advantages of both homogeneous and heterogeneous catalysis since the soluble dendritic catalyst can be separated from the product-stream by nano-filtration. In addition, dendritic effects on transition-metal catalysis can be expected, depending on the position of the catalytic site(s) as well as the spatial separation of the catalysts within the dendritic framework. We have prepared both core- and periphery-functionalized dendritic catalysts that are sufficiently large to enable separation by nano-filtration techniques. Here we review our findings using these promising novel transition metal-functionalized dendrimers as catalysts in several reactions. To cite this article: J.N.H. Reek, C. R. Chimie 6 (2003).