Jason C. Hicks

Catalytic Hydrogenation of CO2 to Formic Acid with Silica‐Tethered Iridium Catalysts

The combustion of fossil fuels as the primary source of chemical energy has led to increasing quantities of CO2 in the environment. To combat the negative impact of these releases and therefore to make the use of fossil fuels cleaner, research efforts have focused on the capture, sequestration, and transformation of CO2. [3] With sufficient input of chemical energy, CO2 can be transformed into useful products (e.g. , chemicals, fuels, and polymers). 4] Without a catalyst, these transformations are too slow to be of practical value and thus require sufficiently active, durable, and selective catalysts. A direct and promising approach to generating liquid products from CO2 is the catalytic hydrogenation of CO2 to formic acid (FA). [5] Typical unsupported, homogeneous transition-metal catalysts capable of catalyzing this reaction include Ru, Rh, Pd, Ir, and Pt in the form of halides (M Cl) or hydrides (M H). 6] There are few reports of heterogeneous catalysts for this reaction. Organic–inorganic hybrid catalysts are a class of tethered heterogeneous catalysts designed to retain the selectivity of homogeneous catalysts while being immobilized on heterogeneous supports to allow for easy separation. However, very few reports have focused on the use of tethered heterogeneous catalysts for the reduction of CO2 to liquids. Baiker and co-workers reported a co-condensation method to incorporate a transition-metal complex based on Ru, Ir, Pt, or Pd within a silica framework. As reported, Ru–phosphine hybrid catalysts showed the best activities for the synthesis of N,N-diethylformamide from CO2, H2, and diethylamine. Yu et al. reported aminosilica-tethered Ru complexes with a turnover frequency (TOF) of 1482 h 1 for CO2 hydrogenation to FA when PPh3 was added under supercritical CO2 conditions (80 8C, 18 MPa of total pressure). 10] In this study, a new silica-tethered iridium catalyst Ir-PN/SBA-15 was synthesized for the hydrogenation of CO2 to FA (Scheme 1). To the best of our knowledge, we report the first use of this organic–inorganic hybrid silica-tethered bidentate Ir complex for the hydrogenation of CO2 to FA. The precatalyst was synthesized through a multistep grafting approach by using an iminophosphine ligand tethered to mesoporous SBA-15. Once activated by H2, these new materials exhibited relatively high catalytic activities with a turnover number (TON) of 2.8 10 in 20 h (60 8C, total pressure of 4.0 MPa). The alkoxysilane-containing bidentate iminophosphine ligand o-Ph2PC6H4CH = N(CH2)3Si(OMe)3 (3) was synthesized through the Schiff base reaction of o-(diphenylphosphino)benzaldehyde [Ph2P(o-C6H4CHO)] (1) with 3-(aminopropyl)trimethoxysilane [NH2(CH2)3Si(OMe)3] (2) in anhydrous toluene (see the Supporting Information for a detailed procedure). The structure of the ligand was confirmed by P NMR, C NMR, and H NMR spectroscopy (Figures S1–S3, Supporting Information). The mesoporous silica support, SBA-15, was synthesized in a manner similar to that previously reported. N2 physisorption results showed a type IV isotherm, a BET surface area of 950 m g , and a Barrett–Joyner–Halenda (BJH) pore size of 6.2 nm (Table S1 and Figure S4, Supporting Information). Iminophosphine ligand 3 was subsequently grafted onto SBA-15 to afford PN/SBA-15 (3 a), and this species underwent metalation with IrCl3 hydrate in refluxing anhydrous ethanol to afford Ir-PN/SBA-15 (3 b) (Figure 1, see the Supporting Information for a detailed procedure). For comparison, monodentate phosphine precatalysts 4 b and 5 b and primary amine precatalyst 2 b were also synthesized (Figure 1). The unsupported analogue Ir(Cl3)Ph2PC6H4CH = N(CH2)2CH3 (denoted as Ir-PNPr ) was also synthesized for comparison to the hybrid materials (see the Supporting Information). The tethered hybrid materials were characterized by a battery of techniques including FTIR, UV/Vis, thermogravimetric analysis (TGA), N2 physisorption, inductively coupled plasma optical emission spectrometry (ICP-OES), and X-ray photoelectron spectroscopy (XPS) (see the Supporting Information). The FTIR spectra of unsupported PNPr and Ir-PNPr showed the Schiff base (CH=NR) double-bond adsorption at 1639 cm , as well as phenyl ring vibrations at 3061, 1587, 1435, 743, and 694 cm . Similar peaks also appeared in the FTIR spectra of [a] Dr. Z. Xu, N. D. McNamara, G. T. Neumann, Prof. W. F. Schneider, Prof. J. C. Hicks Department of Chemical and Biomolecular Engineering University of Notre Dame 182 Fitzpatrick Hall, Notre Dame, IN 46556 (USA) Fax: (+ 1) 574-631-8366 E-mail : jhicks3@nd.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200839. Scheme 1. Structure of the tethered bidentate Ir catalyst used in the hydrogenation of CO2.

Carbothermal Reduction of Ti-Modified IRMOF-3: An Adaptable Synthetic Method to Support Catalytic Nanoparticles on Carbon

This work describes a novel method for the preparation of titanium oxide nanoparticles supported on amorphous carbon with nanoporosity (Ti/NC) via the post-synthetic modification of a Zn-based MOF with an amine functionality, IRMOF-3, with titanium isopropoxide followed by its carbothermal pyrolysis. This material exhibited high purity, high surface area (>1000 m(2)/g), and a high dispersion of metal oxide nanoparticles while maintaining a small particle size (~4 nm). The material was shown to be a promising catalyst for oxidative desulfurization of diesel using dibenzothiophene as a model compound as it exhibited enhanced catalytic activity as compared with titanium oxide supported on activated carbon via the conventional incipient wetness impregnation method. The formation mechanism of Ti/NC was also proposed based on results obtained when the carbothermal reduction temperature was varied.

Chelating Agent-Free, Vapor-Assisted Crystallization Method to Synthesize Hierarchical Microporous/Mesoporous MIL-125 (Ti)

Titanium-based microporous heterogeneous catalysts are widely studied but are often limited by the accessibility of reactants to active sites. Metal-organic frameworks (MOFs), such as MIL-125 (Ti), exhibit enhanced surface areas due to their high intrinsic microporosity, but the pore diameters of most microporous MOFs are often too small to allow for the diffusion of larger reactants (>7 Å) relevant to petroleum and biomass upgrading. In this work, hierarchical microporous MIL-125 exhibiting significantly enhanced interparticle mesoporosity was synthesized using a chelating-free, vapor-assisted crystallization method. The resulting hierarchical MOF was examined as an active catalyst for the oxidation of dibenzothiophene (DBT) with tert-butyl hydroperoxide and outperformed the solely microporous analogue. This was attributed to greater access of the substrate to surface active sites, as the pores in the microporous analogues were of inadequate size to accommodate DBT. Moreover, thiophene adsorption studies suggested the mesoporous MOF contained larger amounts of unsaturated metal sites that could enhance the observed catalytic activity.

CO2 Capture and Conversion with a Multifunctional Polyethyleneimine‐Tethered Iminophosphine Iridium Catalyst/Adsorbent

Tunable, multifunctional materials able to capture CO2 and subsequently catalyze its conversion to formic acid were synthesized by the modification of branched polyethyleneimine (PEI) with an iminophosphine ligand coordinated to an Ir precatalyst. The molecular weight of the PEI backbone was an important component for material stability and catalytic activity, which were inversely related. The amine functionalities on PEI served three roles: 1) primary amines were used to tether the ligand and precatalyst, 2) amines were used to capture CO2 , and 3) amines served as a base for formate stabilization during catalysis. Ligand studies on imine and phosphine based ligands showed that a bidentate iminophosphine ligand resulted in the highest catalytic activity. X-ray photoelectron spectroscopy revealed that an increase in Ir 4f binding energy led to an increase in catalytic activity, which suggests that the electronics of the metal center play a significant role in catalysis. Catalyst loading studies revealed that there is a critical balance between free amines and ligand-metal sites that must be reached to optimize catalytic activity. Thus, it was found that the CO2 capture and conversion abilities of these materials could be optimized for reaction conditions by tuning the structure of the PEI-tethered materials.

Exceptional control of carbon-supported transition metal nanoparticles using metal-organic frameworks

This report describes a versatile method to prepare metal nanoparticles supported on nanoporous carbon (M/NC3) via carbonization and carbothermal reduction (CCR) of metal-coordinated IRMOF-3 materials by post-synthetic modification (PSM) with metal precursors (i.e., Ru, W, V, and Ti). Use of IRMOF materials as templates/carbon sources led to desirable pore characteristics in the resulting materials, including high surface area (SNLDFT, 900–2000 m2 g−1) coupled with an increased mesoporosity (Vmeso/Vpore, 0.72–0.86). Formation of carbide phase metals (V8C7 and TiCxOy) was attained at 1000 °C, which is 200–300 °C lower than preparation of these carbide phases via conventional impregnation methods. Smaller sized metal nanoparticles were successfully obtained in the M/NC3 materials compared to materials prepared with un-coordinated metal impregnated IRMOF-1 (M/NC1), primarily due to the ability of IRMOF-3 to coordinate with metal precursors via PSM, leading to site isolation and minimization of aggregation of metal nanoparticles during CCR. Moreover, this coordination provided several additional benefits such as formation of ruthenium nanoparticles without encapsulation by carbon shells and formation of a WC1−x phase with enhanced thermal stability. Furthermore, all M/NC3 materials were shown to be highly active catalysts for liquid phase conversion of model compounds and derivatives of lignocellulosic biomass.

Correlating lignin structure to aromatic products in the catalytic fast pyrolysis of lignin model compounds containing β–O–4 linkages

Lignin model compounds, phenethoxybenzene, 1-methoxy-2-phenethoxybenzene, and 1-phenethoxy-4-propylbenzene were reacted over various zeolites in the vapor phase at 600 °C to elucidate the role of the zeolite type during the catalytic fast pyrolysis (CFP) of molecules containing β–O–4 linkages. The goal was to determine how different zeolites including hierarchical zeolites upgrade lignin fragments, ultimately for converting the lignin portion of lignocellulosic biomass to chemicals or fuels. Using three different catalysts, HZSM-5, H-beta, and HY, the model lignin compounds were upgraded by catalytic fast pyrolysis at 600 °C. The total acidity or Bronsted acidity of the catalysts were held similar when comparing the three different zeolite types to understand the role of each zeolite on the product distribution. The optimal catalyst for the production of benzene from all three model compounds was H-beta, but the least amount of coke was produced with HZSM-5. Compared with the analogous microporous HZSM-5 catalyst, the hierarchical materials resulted in increased liquid carbon molar yields. Although H-beta produced the highest amount of benzene with 1-methoxy-2-phenethoxybenzene, HZSM-5 was able to produce more liquid products from this model lignin fragment. To analyze the catalysts with intact model compounds, vaporization studies were conducted to prevent the thermal cleavage of the Cβ–O bond in the β–O–4 linkages. These vaporization studies resulted in lower liquid yields and higher coke yields. Thus, it is important for CFP to generate fragments of the β–O–4 linkages before catalyst interaction, and catalyst selection for the conversion of lignin should greatly depend on feed compositions.

Overcoming ammonia synthesis scaling relations with plasma-enabled catalysis

AbstractCorrelations between the energies of elementary steps limit the rates of thermally catalysed reactions at surfaces. Here, we show how these limitations can be circumvented in ammonia synthesis by coupling catalysts to a non-thermal plasma. We postulate that plasma-induced vibrational excitations in N2 decrease dissociation barriers without influencing subsequent reaction steps. We develop a density-functional-theory-based microkinetic model to incorporate this effect, and parameterize the model using N2 vibrational excitations observed in a dielectric-barrier-discharge plasma. We predict plasma enhancement to be particularly great on metals that bind nitrogen too weakly to be active thermally. Ammonia synthesis rates observed in a dielectric-barrier-discharge plasma reactor are consistent with predicted enhancements and predicted changes in the optimal metal catalyst. The results provide guidance for optimizing catalysts for application with plasmas.Plasma catalysis holds promise for overcoming the limitations of conventional catalysis. Now, a kinetic model for ammonia synthesis is reported to predict optimal catalysts for use with plasmas. Reactor measurements at near-ambient conditions confirm the predicted catalytic rates, which are similar to those obtained in the Haber–Bosch process.

Composition-directed FeXMo2−XP bimetallic catalysts for hydrodeoxygenation reactions

The development of task-specific bimetallic phosphide catalysts can be accomplished by exploiting the electronic and bi-functional effects of multiple metal combinations, thus providing materials with tunable catalytic properties. Here, we present the modulation of metal compositions (i.e., Fe and Mo) in the synthesis of FeXMo2−XP (0.88 ≤ X ≤ 1.55), leading to a series of iso-structural, orthorhombic FeXMo2−XP catalysts via reduction at 750 °C. Hydrodeoxygenation of phenol was selected as a probe reaction to showcase the effect of metal composition on the catalytic performance. In particular, catalysts with Fe compositions between 0.99 and 1.14 (i.e., Fe0.99Mo1.01P and Fe1.14Mo0.86P) exhibited high selectivities to C–O bond cleavage of phenol with H2 to form benzene. The catalysts with the highest selectivities to C–O scission also exhibited the highest acidity as determined from NH3 temperature programmed desorption experiments. Density functional theory (DFT) calculations indicate the high Lewis acidity for the ∼1 : 1 Fe : Mo compositions resulted from a greater charge separation between metallic species and P species. These compositions led to greater selectivities to benzene due to desired coordination environment of the phenol on catalytic surface, as evidenced by both DFT calculations and a time on stream study using a benzonitrile poison. Enhanced TOFs were also observed with catalysts exhibiting greater Lewis acid character, which reduce the activation energy required to cleave the C–O bond of phenol, as evidenced by DFT calculations. This structure–property study highlights the effects of metal composition in bimetallic phosphides to enhance the activity and selectivity for C–O bond cleavage reactions.

Enhanced CO2 capture capacities and efficiencies with N-doped nanoporous carbons synthesized from solvent-modulated, pyridinedicarboxylate-containing Zn-MOFs

This paper describes the pyrolysis of pyridinedicarboxylate-containing Zn-based metal–organic frameworks (MOFs) to form nanoporous carbons with accessible N dopants to adsorb CO2. The optimal materials were synthesized using N-heterocycle additives to control the amount of coordinated DMF in the base MOF structure, thereby increasing its thermal stability prior to pyrolysis.

Mechanistic insights into hydrodeoxygenation of phenol on bimetallic phosphide catalysts

Catalytic hydrodeoxygenation (HDO) of phenolics is a necessary step for upgrading bio-oils to transportation fuels. Bimetallic catalysts offer the potential of increased activities and selectivities for desired products. Adding non-metallic elements, such as phosphorous, allows for charge distribution between the metal and nonmetal atoms, which improves Lewis acid character of catalytic surfaces. This work utilizes experimental and density functional theory (DFT) based calculations to identify potential C–O bond cleavage pathways and product selectivities for HDO reactions on FeMoP, RuMoP, and NiMoP catalysts. Our work demonstrates that FeMoP catalyst favors direct deoxygenation pathway due to a lower activation energy barrier for C–O bond cleavage whereas RuMoP and NiMoP catalysts promote ring hydrogenation first, followed by the cleavage of C–O bond. The Bader charge analysis indicates that for these catalytic systems Moδ+ site bears a large positive charge which acts as a Lewis acid site for HDO reactions. Overall, we find that trends in the experimental product selectivities are in good agreement with that predicted with DFT calculations.