Efficient Atomically Dispersed Co/N-C Catalysts for Formic Acid Dehydrogenation and Transfer Hydrodeoxygenation of Vanillin.

Atomically dispersed catalysts have gained considerable attention due to their unique properties and high efficiency in various catalytic reactions. Herein, a series of Co/N-doped carbon (N-C) catalysts was prepared using a metal-lignin coordination strategy and employed in formic acid dehydrogenation (FAD) and hydrodeoxygenation (HDO) of vanillin. The atomically dispersed Co/N-C catalysts showed outstanding activity, acid resistance, and long-term stability in FAD. The improved activity and stability may be attributed to the high dispersion of Co species, increased surface area, and strong Co-N interactions. XPS and XAS characterization revealed the formation of Co-N3 centers, which are assumed to be the active sites. In addition, DFT calculations demonstrated that the adsorption of formic acid on single-atom Co was stronger than that on Co13 clusters, which may explain the high catalytic activity. The Co/N-C catalyst also showed promising performance in the transfer HDO of vanillin with formic acid, without any external additional molecular H2.

Janus bifunctional periodic mesoporous organosilica.

Synthesis of a Janus periodic mesoporous organosilica material (JPMO) is presented here. In this strategy, the surface of the hollow silica material was selectively functionalized with two different bridged organic-inorganic hybrid groups. It was found that the resulting bifunctional material is able to form a stable Pickering emulsion. This new type of PMO material may be suitable for widespread applications in various fields related to material science and catalysis.

Fast and Selective Aqueous-Phase Oxidation of Styrene to Acetophenone Using a Mesoporous Janus-Type Palladium Catalyst.

A heterogeneous Janus-type palladium interphase catalyst was obtained by selective surface modification of a hollow mesoporous silica material. The catalyst comprises hydrophobic octyl groups on one side of the silica nanosheets and single-site bis-imidazoline dichlorido palladium(II) complexes on the other. The structure of this composite material has been analyzed by means of elemental analysis, atomic absorption spectroscopy, BET surface analysis, TGA, SEM and solid-state CP-MAS 13C and 29Si NMR spectroscopy. The catalyst showed extraordinary activity for the aqueous-phase oxidation of styrene to acetophenone using 30% hydrogen peroxide as the oxidant. An 88% yield of acetophenone could be achieved after 60 min.

Increasing the Complexity in the MIL-53 Structure: The Combination of the Mixed-Metal and the Mixed-Linker Concepts.

The isoreticular mixed-component concept is a promising approach to tailor the material properties of metal-organic frameworks. While isoreticular mixed-metal or mixed-linker materials are commonly synthesized, the combination of both concepts for the development of isoreticular materials featuring both two metals and two linkers is still rarely investigated. Herein, we present the development of mixed-metal/mixed-linker MIL-53 materials that contain different metal combinations (Al/Sc, Al/V, Al/Cr, Al/Fe) and different linker ratios (terephthalate/2-aminoterephthalate). The possibility of changing the metal combination and the linker ratio independently from each other enables a large variety of modifications. A thorough characterization (PXRD, ATR-IR, TGA, 1 H NMR, ICP-OES) confirmed that all components were incorporated into the framework structure with a statistical distribution. Nitrogen physisorption measurements showed that the breathing behavior can be tailored by adjusting the linker ratio for all metal combinations. All materials were successfully used for post-synthetic modification reactions with maleic anhydride.

Formic Acid-Assisted Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Bifunctional Pd Nanoparticles Supported on N-Doped Mesoporous Carbon.

Biomass-derived 5-hydroxymethylfurfural (HMF) is regarded as one of the most promising platform chemicals to produce 2,5-dimethylfuran (DMF) as a potential liquid transportation fuel. Pd nanoparticles supported on N-containing and N-free mesoporous carbon materials were prepared, characterized, and applied in the hydrogenolysis of HMF to DMF under mild reaction conditions. Quantitative conversion of HMF to DMF was achieved in the presence of formic acid (FA) and H2 over Pd/NMC within 2 h. The reaction mechanism, especially the multiple roles of FA, was explored through a detailed comparative study by varying hydrogen source, additive, and substrate as well as by applying in situ ATR-IR spectroscopy. The major role of FA is to shift the dominant reaction pathway from the hydrogenation of the aldehyde group to the hydrogenolysis of the hydroxymethyl group via the protonation by FA at the C-OH group, lowering the activation barrier of the C-O bond cleavage and thus significantly enhancing the reaction rate. XPS results and DFT calculations revealed that Pd2+ species interacting with pyridine-like N atoms significantly enhance the selective hydrogenolysis of the C-OH bond in the presence of FA due to their high ability for the activation of FA and the stabilization of H- .

Hard X-ray-based techniques for structural investigations of CO2 methanation catalysts prepared by MOF decomposition.

Thermal decomposition of metal-organic framework (MOF) precursors is a recent method to create well-dispersed metal centers within active catalyst materials with enhanced stability, as required for dynamic operation conditions in light of challenges caused by the renewable energy supply. Here, we use a hard X-ray-based toolbox of pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) analysis combined with X-ray diffraction and catalytic activity tests to investigate structure-activity correlations of methanation catalysts obtained by thermal decomposition of a Ni(BDC)(PNO) MOF precursor. Increasing the decomposition temperature from 350 to 500 °C resulted in Nifcc nanoparticles with increasing particle sizes, alongside a decrease in Ni2+ species and strain-induced peak broadening. For lower temperatures and inert atmosphere, Ni3C and NiO phases co-existed. A graphitic shell stabilized the Ni particles. Compared to an inert atmosphere, reducing conditions led to larger particles and a faster decomposition of the MOF precursor. Catalytic studies revealed that the decomposition at an intermediate temperature of 375 °C in 5% H2/He is the best set of parameters to obtain high specific surface areas while maintaining particle sizes that feature many active Ni centers for the formation of CH4.

The introduction of functional side groups and the application of the mixed-linker concept in divalent MIL-53(Ni) materials.

Metal-organic framework materials with functional side groups are commonly used for various purposes like post-synthetic modification reactions or the tuning of pore geometries. Additionally, the mixed-linker concept, in which different linkers are statistically distributed over equivalent lattice positions, is a versatile approach to adjust the number of functional groups within the framework. However, neither functional side groups nor the mixed-linker approach have been used in MIL-53 materials with divalent metal ions yet. In the present work, we report on the synthesis of MIL-53(Ni)-Br(100), which contains only 2-bromoterephthalate as a linker molecule, and mixed-linker MIL-53(Ni)-Br(50) containing both terephthalate and 2-bromoterephthalate. These two materials represent the first functionalized and mixed-linker MIL-53 materials in combination with a divalent metal. Consequently, the possibilities to tailor the properties of the still rarely used divalent MIL-53 materials were expanded.

Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal Metal-Organic Frameworks.

Metal-organic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixed-metal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixed-metal metal-organic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixed-metal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder X-ray diffraction, X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixed-metal FeCuBTC material with HKUST-1 structure, which contained bimetallic Fe-Cu paddlewheels as well as monometallic Cu-Cu and Fe-Fe units under optimized synthesis conditions. The in-depth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixed-metal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixed-metal metal-organic framework materials and the thorough characterization thereof is particularly important to derive structure-property or structure-activity correlations.

Synthetic Strategies and Structural Arrangements of Isoreticular Mixed-Component Metal-Organic Frameworks.

In recent years, the synthesis of mixed-metal and mixed-linker metal-organic frameworks with multiple metals and/or linker molecules combined in one framework has become a growing field of interest. These mixed-component or multivariate metal-organic framework materials provide the possibility to introduce multiple functionalities inside one framework. The interaction of guest molecules with different functionalities in the same material is a promising approach in the fields of gas storage, separation, catalysis and drug delivery. Furthermore, the combination of different components may lead to synergistic effects that cannot be achieved otherwise. These mixed-component approaches open up new pathways to an even larger range of possible customizations in the field of metal-organic frameworks.

Synthesis and post-synthetic modification of amine-, alkyne-, azide- and nitro-functionalized metal-organic frameworks based on DUT-5.

Functionalized 4,4'-biphenyldicarboxylic acid molecules with additional amine, alkyne, azide or nitro groups were prepared and applied in the synthesis of novel metal-organic frameworks and mixed-linker metal-organic frameworks isoreticular to DUT-5. The properties of the frameworks could be tuned by varying the number of functional groups in the materials and the amine groups were employed in post-synthetic modification reactions without changing the framework structure or significantly decreasing the porosity of the materials.

Synthesis and characterization of bimetallic metal-organic framework Cu-Ru-BTC with HKUST-1 structure.

The bimetallic metal-organic framework Cu-Ru-BTC with the stoichiometric formula Cu2.75Ru0.25(BTC)2·xH2O, which is isoreticular to HKUST-1, was successfully prepared in a direct synthesis using mild reaction conditions. The partial substitution of Cu(2+) by Ru(3+) centers in the paddlewheel structure and the absence of other Ru-containing phases was proven using X-ray absorption spectroscopy.

Salen-based coordination polymers of manganese and the rare-earth elements: synthesis and catalytic aerobic epoxidation of olefins.

Treatment of N,N'-bis(4carboxysalicylidene)ethylenediamine (H(4)L), with MnCl(2)⋅(H(2)O)(4), and Ln(NO(3))(3)⋅(H(2)O)(m) (Ln = Nd, Eu, Gd, Dy, Tb), in the presence of N,N-dimethylformamide (DMF)/pyridine at elevated temperature resulted (after work up) in the formation of 1D coordination polymers {[Ln(2)(MnLCl)(2)(NO(3))(2)(dmf)(5)]⋅4 DMF}(n) (1-5). In these coordination polymers the rare earth ions are connected through carboxylate groups from Mn-salen units in a 1D chain structure. Thus, the Mn-salen complex acts as a "metalloligand" with open coordination sites. All compounds were used as catalysts in the liquid-phase epoxidation of trans-stilbene with molecular oxygen, which resulted in the formation of stilbene oxide. Since the choice of the lanthanide had virtually no influence on the performance of the catalyst, only the manganese-gadolinium was studied in detail. The influence of solvent, catalyst concentration, reaction temperature, oxidant, and oxidant flow rate on conversion, yield, and selectivity was analyzed. A conversion of up to 70%, the formation of 61% stilbene oxide (88% selectivity), and a TON of 84 were observed after 24 h. A hot filtration test confirmed that the reaction is mainly catalyzed through a heterogeneous pathway, although a minor contribution of homogeneous species could not be completely excluded. The catalyst could be reused without significant loss of activity.

Aerobic epoxidation of olefins catalyzed by the cobalt-based metal-organic framework STA-12(Co).

A Co-based metal-organic framework (MOF) was investigated as a catalytic material in the aerobic epoxidation of olefins in DMF and exhibited, based on catalyst mass, a remarkably high catalytic activity compared with the Co-doped zeolite catalysts that are typically used in this reaction. The structure of STA-12(Co) is similar to that of STA-12(Ni), as shown by XRD Rietveld refinement and is stable up to 270 °C. For the epoxidation reaction, significantly different selectivities were obtained depending on the substrate. Although styrene was epoxidized with low selectivity due to oligomerization, (E)-stilbene was converted with high selectivities between 80 and 90 %. Leaching of Co was low and the reaction was found to proceed mainly heterogeneously. The catalyst was reusable with only a small loss of activity. The catalytic epoxidation of stilbene with the MOF featured an induction period, which was, interestingly, considerably reduced by styrene/stilbene co-epoxidation. This could be traced back to the formation of benzaldehyde promoting the reaction. Detailed parameter and catalytic studies, including in situ EPR and EXAFS spectroscopy, were performed to obtain an initial insight into the reaction mechanism.

Identification of the active species generated from supported Pd catalysts in Heck reactions: an in situ quick scanning EXAFS investigation.

Quick scanning extended X-ray absorption fine structure (QEXAFS) studies in the subsecond time scale have been performed to gain insight into the reaction mechanism of Heck-type C-C coupling reactions in the presence of supported Pd-based catalysts. Using a specially designed in situ EXAFS cell, both the solid catalyst and the liquid reaction mixture during the reaction of phenyl bromide (PhBr) with styrene were monitored. Soluble Pd species were only, but rapidly, detected in the liquid reaction phase once the reaction temperature of 150 °C was reached. At the same time, the conversion of PhBr started, and during the following "active phase" of the catalyst hardly any changes in the corresponding EXAFS and XANES spectra were observed. The present species could be identified as colloidal Pd(0) clusters with a size of ∼2 nm estimated from the corresponding EXAFS spectra. The QEXAFS mode not only allowed monitoring rapid changes in the second time scale but also permitted minimization of effects caused by the heterogeneity of the systems. When the reaction rate started to decrease, pronounced changes in the EXAFS spectra were observed, which were attributed to an increased formation of bromo-palladates ([PdBr(4)](2-), [Pd(2)Br(6)](2-)). In addition to the liquid-phase species, significant changes were observed for the solid catalyst that was also probed in situ during the reaction. The originally oxidized Pd catalyst was efficiently reduced upon heating. Additionally, growth of the supported Pd particles was observed by both EXAFS and STEM. The above results confirm the role of the soluble molecular Pd species as the catalytically active species and clarify their conjunction with the in situ formed Pd colloids. Furthermore, the investigation demonstrates the potential of the QEXAFS not only for monitoring rapid changes during catalysis but also for gaining deeper insight into the mechanism of such complex industrially important systems under relevant reaction conditions.

Tuning functional sites and thermal stability of mixed-linker MOFs based on MIL-53(Al).

The MIXMOF concept has been applied to the metal-organic framework compound MIL-53(Al). The random incorporation of two different linker molecules (benzene-1,4-dicarboxylate and 2-aminobenzene-1,4-dicarboxylate) in the framework structure and its influence on thermal stability were proven using several complementary techniques (XRD, ATR-IR, MAS-NMR and TG).

Platinum nanoparticles: the crucial role of crystal face and colloid stabilizer in the diastereoselective hydrogenation of cinchonidine.

The preparation of stable metal nanoparticles requires a strong interaction between the (organic) stabilizer and the metal surface that might alter the catalytic properties. This behavior has been described as "poisoning" since the stabilizer normally decreases the catalytic activity due to site blocking. Here we show a striking influence of the stabilizer on the selectivity in the hydrogenation of cinchonidine (CD) over poly(acrylic acid) (PAA)-stabilized Pt nanoparticles with well-defined shape distributions. In the hydrogenation of the heteroaromatic ring of cinchonidine in toluene, the diastereomeric excess of the (S)-hexahydrocinchonidine increased upon increasing Pt{111}/Pt{100} ratio, but this distinct shape selectivity was observed only after the oxidative removal of PAA at 473 K. The use of the as-prepared nanoparticles inverted the major diastereomer to R, and this isomer was formed also in acetic acid. This striking change in the diastereoselectivity indicates that poly(acrylic acid), which remains on the Pt surface after preparation, interacts with CD during hydrogenation almost as strongly as the solvent acetic acid. The PAA stabilizer plays a dual role: it allows one to control the size and shape of the nanoparticles during their synthesis, and it affects the rate and diastereoselectivity of the hydrogenation of CD probably through a "surface-localized acidification".

Genesis of coordinatively unsaturated palladium complexes dissolved from solid precursors during Heck coupling reactions and their role as catalytically active species.

The Forum Article critically summarizes investigations and discussions on the nature and role of potential active species in C-C coupling reactions of the Heck type using catalyst systems with "ligand-free" inorganic salts, simple inorganic complexes, and supported and nonsupported (colloidal) Pd particles. From a series of experiments and reports, it can be concluded that the "active species" is generated in situ in catalytic systems at higher temperature conditions (>100 degrees C). In all heterogeneous systems with solid Pd catalysts, Pd is dissolved from the solid catalyst surface under reaction conditions by a chemical reaction (complex formation and/or oxidative addition of the aryl halide), forming extremely active coordinatively unsaturated Pd species. Pd is partially or completely redeposited onto the support at the end of the reaction when the aryl halide is used up. The Pd dissolution-redeposition processes correlate with the reaction rate and are strongly influenced by the reaction conditions. Skilled preparation of the catalyst and careful adjustment of the reaction conditions allowed the development of highly active heterogeneous catalysts (Pd/C, Pd/metal oxide, and Pd/zeolite), converting aryl bromides and aryl chlorides in high yields and short reaction times. Reaction conditions have been developed allowing the conversion of bromobenzene with turnover numbers (TONs) of 10(7) and even of unreactive aryl chlorides (chlorobenzene and chlorotoluene) in high yields with simple "ligand-free" Pd catalyst systems like PdCl2 or Pd(OH)2 in the absence of any organic ligand. Simple coordinatively unsaturated anionic palladium halide (in particular, bromo) complexes [PdXn](m-) play a crucial role as precursor and active species in all ligand-free and heterogeneous catalyst systems and possibly in Heck reactions at all.