Synthesis gas (syngas) fermentation is a potential pathway for converting biomass to a variety of fuels and chemicals such as ethanol, methane, and acetic acid due to its low cost and potentially high efficiency.[1] In syngas fermentation, carbon monoxide is the sole carbon source from which the microorganisms produce their desired chemicals. However, the CO solubility in the aqueous media is low, which minimize the CO-liquid mass transfer rate and limits the overall product yield.[2] For syngas fermentation to be successful, the CO-liquid mass transfer rates must be improved. However, limited attention has been paid to unique approaches for enhancing CO-liquid volumetric mass transfer coefficients. In general, gas-liquid mass transfer is influenced by reactor type, power input, and system additives. Olle et al.[3] reported the oxygen-liquid mass transfer could be enhanced by up to 600% in an agitated sparged reactor when magnetite (Fe3O4) nanoparticles coated with oleic acid and a surfactant were added to the systems. Others have also shown that gas-liquid mass transfer rate can be enhanced by adding micro- or nanoparticles to the system.[4-6] In addition, the introduction of electrolytes can inhibit bubble coalescence, thereby increasing the gas-liquid interfacial area.[7]

As reviewed by Beenackers and van Swaaij,[8] the enhancement in gas-liquid mass transfer by adding particles was influenced greatly by particle size and surface characteristics (e.g., hydrophobic or hydrophilic). Mesoporous silica materials (MCM41) have extensive adsorption capacity due to their large BET surface area (~1000 m2/g), which can also be readily functionalized with organic groups to adjust the surface characteristics such as the hydrophobicity or hydrophilicity and acidity or basicity.[9-11] Hence, this material is a good candidate for enhancing the CO-liquid mass transfer rate.

MCM41 nanoparticles were prepared following the procedures of Deng et al.[12] TEM images showed MCM41 nanoparticles had a spherical morphology with a diameter of ~250 nm, which is supported by particle size distribution results. The BET surface area of MCM41 nanoparticles is ~1200 m2/g. Various organic groups such as methyl (Me), carboxylpropyl (CP), aminepropyl (AP), mercaptopropyl (MP), and mercaptoundecyl (MU) are grafted to the MCM41 nanoparticles to adjust their surface characteristics. After grafting Me groups, functionalized nanoparticles (MeMCM41) had the similar morphology and size as the MCM41 nanoparticles and the BET surface area was ~1300 m2/g.

Various electrolytes solutions were also investigated. Manganese (II) sulfate monohydrate and cobalt (II) sulfate heptahydrate were purchased from Sigma-Aldrich; cobalt (II) nitrate was purchased from Acros organics; manganese (II) chloride tetrahydrate, nickel (II) sulfate hexahydrate, nickel (II) nitrate hexahydrate, nickel chloride hexahydrate, cupric (II) nitrate, magnesium sulfate heptahydrate, ferrous (II) sulfate heptahydrate, and copper (II) sulfate pentahydrate were purchased from Fisher scientific.

The CO-water volumetric mass transfer enhancement results showed that silica particle size and preparation procedure greatly affected the enhancement. When the surfactant involved in the MCM41 synthesis was removed by extraction, the enhancement increased from 1 to 1.55 as the nanoparticles concentration increased from 0 to 0.4 wt%. For large silica particles with diameters of 1.4 and 7 µm, the maximum enhancement was 1.29 and 1.01, respectively. In addition, MCM41 nanoparticles after calcinations at 500oC showed a lower enhancement of ~1.11. These results suggested the particles size affected the CO-water mass transfer rate and surface hydroxyl groups played an important role in the CO-water mass transfer enhancement.

Mercaptopropyl or mercaptoundecyl groups functionalized MCM41 nanoparticles with the functional group loading of 5% (molar ratio) showed a continuous increase in enhancement with increase in nanoparticle concentration, with the highest enhancement of 1.9 when the nanoparticle concentration was 0.4 wt%, which is larger than those samples with other functional groups. In addition to hydrophobicity differences, the mercaptan groups on the MPMCM41 and MUMCM41 naoparticle surface seemed to contribute to the adsorption of CO. In-situ CO adsorption FT-IR spectra indicated the adsorbed CO species on MPMCM41 nanoparticles was more stable than that on MCM41 and NPMCM41 nanoparticles. Hence, it is hypothesized that when mercaptopropyl or mercaptoundecyl groups were grafted to the MCM41 nanoparticles, the nanoparticles adsorbed more CO molecules upon contacting the CO bubbles and they released CO molecules into the water, which resulted in the extra enhancement of the CO-water volumetric mass transfer coefficients compared to pure MCM41 nanoparticles. The enhancement in CO-water mass transfer coefficient (kL) also indicated the addition of MCM41 nanoparticles both increased the CO-water interfacial area (a) and CO-water mass transfer coefficient (kL).

When different electrolytes were added to the system at the concentration of 5 wt%, the CO-water mass transfer was enhanced by 1.6~4.7, depending on the electrolyte type, with copper sulfate providing the strongest enhancement of 4.7. For those electrolytes with the same cations, sulfate electrolytes showed better enhancement than other cations such as chloride and nitrate electrolytes. In addition, it is believed that the anion type gave stronger effect than the cation type. The enhancement resulted from the inhibition of bubble coalescence by electrolytes. In addition, the water-air interface had a preferential orientation, with the oxygen atoms outermost.[13] As a result of this orientation, an electrical double layer was established at the surface, with the outermost portion of the double layer being negative and the innermost part being positive. With the positive portion of the double layer directed into the solution, the anions preferentially accumulated near the interface, leading to a stronger effect by anions on the CO-water mass transfer enhancement compared to cations.

The CO-water mass transfer coefficient enhancement (Ek) results suggested the enhancement to the CO-water mass transfer rate by adding nanoparticles or electrolytes is attributed to one of two reasons. First, the addition of nanoparticles enhanced both the CO-water mass transfer coefficient (kL) and the CO-water interfacial area (a). Second, electrolyte addition only enhanced the CO-water interfacial area by suppressing bubble coalescence.

References:

Acknowledgement:

This material is based upon work supported by the Board of Regents, State of Iowa, through the Battelle Infrastructure and Platform Grants Program.

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