Rh-Based Mixed Alcohol Synthesis Catalysts: Characterization and Computational Report

Abstract

Executive SummaryThe U.S. Department of Energy is conducting a program focused on developing a process for the conversion of biomass to bio-based fuels and co-products. The lignocellulosic biomass feedstock is first gasified to produce a product known as synthesis gas (syngas), which consists of H 2 and CO. The syngas stream is subsequently converted thermochemically within a temperature range of 240 to 330°C and at elevated pressure (e.g., 1200 psig) over a catalyst. Ethanol is the desired reaction product, although other side compounds are produced, including C 3 to C 5 alcohols; higher (i.e., greater than C 1 ) oxygenates such as methyl acetate, ethyl acetate, acetic acid and acetaldehyde; and higher hydrocarbon gases such as methane, ethane/ethene, propane/propene, etc. Saturated hydrocarbon gases (especially methane) are undesirable because they represent a diminished yield of carbon to the desired ethanol product and represent compounds that must be steam reformed at high energy cost to reproduce CO and H 2 . Ethanol produced by the thermochemical reaction of syngas could be separated and blended directly with gasoline to produce a liquid transportation fuel. Additionally, higher oxygenates and unsaturated hydrocarbon side products such as olefins also could be further processed to liquid fuels.The goal of the current project is the development of an Rh-based catalyst with high activity and selectivity to C 2 + oxygenates. Since the program began in 2005, significant improvements in activity and selectivity to the Rh-based catalyst have been achieved through empirical catalyst screening (e.g., support and promoter screening) and operational parametric screening. This report chronicles an effort to characterize numerous supports and catalysts to identify particular traits that could be correlated with the most active and/or selective catalysts. Characterization techniques were chosen to identify the properties of a variety of supports and catalysts. Carbon and silica were the general classes of supports analyzed. Analysis procedures used for several supports are identified below:• Ash concentration of the carbon supports • Elemental composition via inductively coupled plasma• Surface area (Brunauer-Emmett-Teller) and pore size distribution (Barrett-Joyner-Halenda) via N 2 adsorption and desorption• Surface acidity measurements of the silica supports via NH 3 desorption• Carbon support surface functionality characterization via thermogravimetric analysis with mass spectrometer analysis of the effluent gas• Diffuse reflectance infrared Fourier transform spectroscopy analysis of selected carbon supports to identify surface functionality.Generally, analyses provided guidance in the selection of acceptable catalyst supports. For example, supports with high surface areas due to a high number of micropores were generally found to be poor at producing oxygenates, possibly because of mass transfer limitations of the products formed out of the micropores.A number of prepared catalysts (i.e., with Rh and other metal p...

Effect of Collective Dynamics and Anharmonicity on Entropy in Heterogenous Catalysis: Building the Case for Advanced Molecular Simulations

“…AIMD studies have demonstrated that the dynamics (and, thus, the entropy) of metal clusters are strongly influenced by the nature of the support material and of the adsorbates for clusters on the order of 1 nm or less. − These studies showed that strong dynamic fluctuations were linked with phase formation during catalysis, ,− particle sintering, facilitated transport on metal cluster surfaces and the formation of transient reactive single atom sites . To illustrate, in Figure we show the structural dynamics of Au 20 supported on neutral, oxidized, and reduced rutile TiO 2 (110) from AIMD trajectories at 700 K, as characterized by the distribution of Au atoms from the COM of the cluster.…”

Understanding Structure–Property Relationships of MoO3-Promoted Rh Catalysts for Syngas Conversion to Alcohols

“…In each ALD cycle, the sample was exposed to Mo(CO) 6 (Sigma-Aldrich) for 35 s (5 s pulse time, 30 s soak time) and O 3 for 5 s, and the precursors were purged from the reactor with N 2 for 60 s between each dose. The Mo(CO) 6 precursor was held at 40 °C and the delivery line was heated to 55 °C to ensure adequate vapor pressure of Mo(CO) 6 . The O 3 was formed from O 2 by an AC-2025 ozone generator (In USA Inc.), yielding a feed gas composition of 10% O 3 /90% O 2 .…”

“…The catalytic conversion of coal, natural gas, or biomass-derived synthesis gas (syngas, CO + H 2 ) to higher oxygenates (C 2+ oxy) is a promising path toward the long-term production of higher value fuels and chemicals. − Rh-based catalysts are among the most widely studied for higher alcohol synthesis (HAS) due to the moderate intrinsic selectivity of Rh toward higher oxygenates. − Thermodynamic studies have determined conditions under which HAS is viable, but no commercial catalyst with appropriate activity and selectivity has been developed due to the kinetically favorable side reactions that shift selectivity away from the desired products. , In particular, methanol and hydrocarbon synthesis pathways compete with C 2+ oxy formation depending on how CO binds to the catalyst surface. Surfaces which favor CO dissociation convert syngas to hydrocarbons, whereas surfaces which allow associative CO adsorption yield methanol .…”