(3ab) Catalytic Synthesis Gas Conversion to Produce Chemical Products From Non-Petroleum Resources

The production of many fuel and chemical products begins with petroleum.  As concerns over the price of and access to petroleum increase, interest in natural gas, coal, and biomass resources as an alternative feed stocks also increases.  All three can all be reformed or gasified to produce synthesis gas (syngas) a reactive mixture of CO and H­₂.  While the production of syngas has been commercialized, the development of simple and cost effective processes to utilize syngas to make a wide range of products is still under investigation.

I will present on three strategies to utilize syngas to synthesize chemical products from syngas by 1) reactions with H₂, 2) reactions with CO, 3) reactions with both H₂ and CO.  In each of these cases, emphasis was placed on novel processes and catalysts that lead to higher selectivities to the desired products while also simplifying processing conditions relative to more traditional methods.

1) Direct synthesis of H₂O₂ from H₂ and O₂ over near-surface alloys

Commercial production of H₂O₂ separates the O­­₂ reduction and H₂ oxidation steps by oxidizing and reducing an anthraquinone instead, which produces the desired H₂O₂ but requires separation and recycling of the athraquinone catalyst.  The two steps are separated to avoid the thermodynamically favored formation of H₂O.  The direct synthesis of H₂O₂ over a heterogeneous catalyst holds promise by simplifying the process and eliminating many costly separation steps, if high selectivity to H₂O₂ over H₂O₂ can be achieved.  I am investigating Au/Ni near-surface alloys (NSAs) using periodic, self-consistent Density Functional Theory (DFT) calculations.  While Au may have higher O-H bond formation activity, it is a poor O₂-dissociation catalyst, and likewise Ni is very effective at O₂-dissociation but not O₂ hydrogenation.  NSAs can combine the high activity of one metal with the higher selectivity of another, less-reactive metal catalyst, and Au atoms substituted into the surface of Ni (111) have already been shown to possess higher steam reforming selectivity than pure Ni by impeding the formation of carbon. 

2) Carbon-carbon bond formation in syngas derivatives

Syngas contains no carbon-carbon bonds, whereas many desirable products contain one or more C-C bonds.  This represents a challenge to finding alternatives to petroleum, which already contains C-C bonds.  Many C₁ molecules containing no C-C bonds such as methanol, formaldehyde, and dimethoxymethane (DMM) can be made readily from syngas today.  Coupling C₁ compounds by solid acid-catalyzed carbonylation reactions can produce desirable fuel and chemical products, such as ethylene glycol (monoethylene glycol – MEG), a polyester monomer currently produced from petroleum products.  A high pressure liquid-phase reaction to produce MEG from syngas derivatives was known which gave very poor selectivity at low pressure.  I discovered a novel low-pressure gas-phase process to couple DMM and CO to produce methyl methoxyacetate (MMAc), a MEG precursor, with high selectivity using acidic zeolites.  Among common zeolites, H-FAU was found to possess the highest selectivity because its large cage structure inhibited the rate of the undesired disproportionation reaction.  The Si/Al ratio of the zeolite, which determines the density of acid sites within the catalyst pore structure, has a strong effect on the rate of MMAc formation.  By reducing the density of acid sites, the rate of MMAc formation increased for both FAU and acidic ZSM-5 (MFI) up to Si/Al ratios of ~30, and then did not increase further.  At a Si/Al ratio of 30, the active sites in the zeolites are placed far enough from each other so as to avoid interactions between them.  An extensive FTIR spectroscopic investigation provided evidence for the proposed mechanism, which was also supported by DFT calculations performed by a colleague. 

3) Supported liquid-phase Rh hydroformylation catalysts

Hydroformylation reactions are used to upgrade olefins to their next higher homolog aldehydes.  Propene hydroformylation produces n- and isobutanal, and homogeneous Rh catalysts are preferred for their tunable selectivity between isomers.  The process depends on efficient capture and recycling of expensive Rh-phosphine catalysts, and so a heterogeneous catalyst and gas-phase process is preferred.  I investigated Rh particles supported on SiO₂ for gas-phase propene hydroformylation activity.  It became apparent that while butanals where produced with 100% selectivity the addition of triphenylphosphine to the catalyst was essential.  Kinetic rate data together with FTIR and NMR evidence led to the discovery that the phosphine was melting and producing a liquid phase inside the SiO₂ pores that was facilitating the dissolution of the Rh to produce essentially the same Rh-phosphine catalysts as the commercial homogenous process.  In this way, a homogeneous catalyst was supported inside a solid support, combining the high selectivity of the liquid-phase process with the simplicity and high throughput of the gas-phase process.