Gregory T. Neumann

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.

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.