Novel schemes for the development of catalysts with high selectivity for hydrocarbon processing.
Argonne National Laboratory
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Manufacturing chemicals for industrial processes often involves use of a catalyst, which speeds up the chemical reaction involved and lessens the amount of energy required. Ideally, the catalyst has high selectivity for the desired end products and a practical lifetime. In collaboration with researchers from Ames Laboratory, Iowa State University, and Northwestern University, a basic research group at Argonne National Laboratory is finding novel catalytic systems with which to more cheaply and efficiently manufacture products derived from methane in the natural gas being generated from underground shale gas deposits. It is also identifying new routes to making higher performance catalysts for hydrogenation.
The research on three newly developed catalytic systems includes:
• An iridium-containing catalyst supported on a microporous metal- organic framework. This material facilitates the borylation of methane in as phase, which has application in the oil and gas industries.
• A platinum-containing catalyst on a zinc-modified silica support for chemoselective hydrogenation of functionalised nitro-aromatics to aromatic amines in a liquid or gas phase. This product has application in a variety of industries.
• A vanadium-containing catalyst on silica support for hydrogenation of alkenes and alkynes in a liquid phase.
This basic research encompasses computer modelling of catalytic mechanisms, catalyst synthesis, catalyst characterisation in advanced analytical facilities, and state of the art catalyst testing. The latter makes use of Argonne’s High-Throughput Research Facility, which provides highly automated technology not found in the private sector that accelerates the discovery of materials and screening of process conditions.
Iridium-containing catalyst for activation of methane
In the last two decades, the energy industry has been transformed by the widespread use of fracking; that is, the extraction of shale gas by hydraulic fracturing in shale formations buried deep underground. Shale gas is natural gas found in shale rock formations created hundreds of millions of years ago. This capability to tap previously inaccessible shale gas deposits has created an abundant source of methane and other hydrocarbon gases in North America, which can be chemically converted to a mixture of carbon monoxide and hydrogen that can be used as a low cost feedstock for making synthetic chemicals such as methanol or ammonia.
The first of the catalysts to be discussed, iridium on a microporous metal-organic framework support, expedites the conversion of the methane in natural gas to methanol. Methane constitutes the largest fraction of natural gas. Given the large reservoir of natural gas in shale formations within the US, methane could be a low cost and abundant starting material for the manufacture of value added chemicals and fuel. While methane combusts at very high temperatures (approximately 2000°C), it is one of the more difficult hydrocarbons to transform into other products at lower temperatures because of its strong hydrogen-carbon bonds. Metal-catalysed borylation has recently emerged as a promising route for the catalytic functionalisation of methane. A major challenge in this regard is selective borylation towards the desired monoborylated product.
Since the 1960s, zeolites have been commonly used as a supporting material to perform catalysis. Zeolites are microporous crystalline minerals that often include silicon, aluminum, and oxygen. They are commonly used as commercial adsorbents and catalysts and have a cage-like framework in which reactant molecules can become trapped. However, if the molecules are too big to fit inside the framework, no catalysis will occur. Metal-organic frameworks are attractive alternative candidates for performing shape selective catalysis because they are structurally tunable. They can be synthesised with pore and aperture sizes tailor-made for targeted molecules.
Argonne built upon earlier work from different research teams. They showed how they could introduce a boron based compound, in a process called borylation, and thereby found a promising route for transition metal-catalysed methane activation under much less demanding chemical conditions than would otherwise be possible. The teams separately observed the borylation process, yielding products that were both monoborylated (technologically valuable) and bisborylated (undesired). The catalytic reaction scheme is represented in Figure 1. By inserting an iridium based catalyst inside the metal- organic framework, our team has been able to produce a reaction that formed only the monoborylated product because the pores of the metal-organic framework were too small for the bisborylated product to form.
As part of this research, the Argonne team investigated a metal-organic framework first developed at the University of Oslo, Norway, and referred to as ‘Universitetet i Oslo-67’ (UiO-67). This highly stable material is composed of Zr6 inorganic nodes and 4,4’-biphenyl dicarboxylate organic linkers with an aperture size of 0.8 nm and an octahedral cavity of 1.1 nm3 in solvent accessible volume. The team prepared a shape selective catalyst by the mixed linker synthesis of the UiO-67-Mix and subsequent metallation with an iridium-containing precursor, [Ir(1,5-cyclooctadiene)μ-Cl)]2 (see Figure 2).
Borylation of methane was conducted in the Argonne high pressure, high throughput reactor with the heterogeneous catalyst UiO-67-Mix-Ir and, for comparison, a homogeneous catalyst that consisted of the iridium catalyst [Ir(1,5-cyclooctadiene)μ-Cl)]2 plus phenanthroline. Initial methane borylation was performed in cyclohexane at 150°C and 34 atm of CH4 gas for 14 hours, employing 5 mol% of iridium catalyst. Under this catalytic condition, the UiO-67-Mix-Ir system fully converts bis(pinacolborane) and yields about 17% of monoborylated methane with a turnover number of 32, which is slightly higher than the value for the homogeneous catalyst sample. Under this condition, the major product is a monoborylated cyclohexane. Remarkably, the selectivity ratio of monoborylated versus diborylated methane products is 15:1 with the heterogeneous catalyst. In comparison, the homogeneous catalyst gives a monoborylated methane yield of only 5.6%. Thus, the heterogeneous catalyst exhibits both higher yield and selectivity for monoborylated methane than the homogeneous catalyst. Importantly, the team found that MOF UiO-67-Mix without iridium had no effect on methane borylation.
The team also carried out detailed optimisation of the catalytic reaction conditions using UiO-67-Mix-Ir as the catalyst by screening several different C-H inert solvents. Using C6D12 as the solvent led to bis(pinacolborane) conversion of 19% and a much lower yield of 3.3% for monoborylated methane. Conversely, when the common solvent tetrahydrofuran was employed, 98% conversion of the borylation reagent was achieved; however, the major product was borylated tetrahydrofuran, with only 3.8% yield of monoborylated methane. Among the solvents tested, dodecane gave the best results: a conversion of >99%, turnover number of 67, a monoborylated methane yield of 19.5%, and no detectable amount of diborylated methane or borylated dodecane (<1%), which amounts to an extraordinarily high selectivity of >99% for the monoborylated product.
Experiment strongly suggests that the UiO-67-Mix-Ir structure with metal-organic framework allows bis(pinacolborane) and methane molecules to react inside the pores to yield monoborylated methane while preventing the formation of the thermodynamically more favoured diborylated methane. In the next phase of our research, the team plans to activate methane with the same chemistry, but will substitute Earth-abundant metals such as iron, cobalt, nickel and copper for iridium, which is rare and expensive.
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