Researchers at the University of Oklahoma have developed a new family of solid catalysts that can stabilize water-oil emulsions and catalyze reactions at the liquid/liquid interface. Such a recoverable catalyst that simultaneously stabilizes emulsions would be “highly advantageous” in streamlining processes such as biomass refining, in which the immiscibility and thermal instability of crude products greatly complicates purification procedures, note Crossley et al. in their paper, published 1 January in Science.
The authors deposited palladium onto carbon nanotube–inorganic oxide hybrid nanoparticles. The oxides are hydrophilic, and attracted to the water; the carbon nanotubes are hydrophobic, and prefer the organic layer.
The resulting Janus catalysts (as described by Dr. David Cole-Hamilton of University of St. Andrews, Scotland in an accompanying Perspective piece in Science), sit at the surface like a large surfactant molecule. But unlike surfactants, the nanoparticles are solids that can be easily separated out.
Rather than carrying out multiple consecutive purification steps during refining to separate out the hydrophilic by-products incompatible with fuel applications, it would be desirable to perform sequential reactions under phase-transfer conditions in a single reactor medium.
A serious drawback in such systems, however, is that the surfactants can be difficult to separate from final product mixtures. Solid particles are more easily recoverable and have also been shown in many cases to stabilize aqueous-organic emulsions, but these solid-stabilized emulsions have not been widely used in catalytic contexts. Moreover, in cases such as the refining of bio-oils in which the system is biphasic and contains up to 30% water, the most efficient way of catalyzing reactions is to place the solid catalyst at the liquid/liquid interface and to maximize the extent of interface by creating an emulsion. Otherwise, the catalyst particles will preferentially remain in the heavier phase, such as water. In that case, only the water-soluble molecules will be converted. If further conversion of water-insoluble molecules is wanted, one would need to remove them from the top of the reactor and send them to another reactor with a catalyst operating in the organic phase. Therefore, the concept of solid particles that can simultaneously stabilize an emulsion and catalyze reactions in both phases becomes an attractive proposition.
—Crossley et al.
In the study, the authors explored two preparations with nanotubes of different type, which affected the deposition of Pd. They presented results obtained for several reactions of relevance to biomass-refining chemistry: the elimination of oxygen and the condensation of small molecules. The former is needed to improve the low stability caused by the high reactivity of the oxygenated functional groups in molecules such as the phenolic compounds derived from lignin. The latter is particularly important to increase the molecular weight of those light fragments derived from the less refractory parts of the biomass (cellulose and hemicellulose).
The advantage of operating in a biphasic system, with the catalyst at the liquid/liquid interface, is the possibility of conducting the sequential reactions in a single reactor instead of two.
With solid-stabilized emulsions, a continuous process could be designed in which the two homogeneous phases coexist with the emulsion in a layered configuration: oil/emulsion/water. One can achieve full conversion on both sides of the emulsion followed by constant removal of oil-soluble products from the top layer and water-soluble products from the bottom layer while the reaction keeps occurring in the emulsion.
Our results highlight the preliminary applications of solid catalysts localized at the interface between two liquid phases. We anticipate that tailoring such emulsion-stabilizing solids with additional catalytic functional groups will facilitate a broad range of reactions.
—Crossley et al.
Steven Crossley, Jimmy Faria, Min Shen, Daniel E. Resasco (2010) Solid Nanoparticles that Catalyze Biofuel Upgrade Reactions at the Water/Oil Interface. Science Vol. 327. no. 5961, pp. 68 - 72 doi: 10.1126/science.1180769
David J. Cole-Hamilton (2010) Janus Catalysts Direct Nanoparticle Reactivity. Science Vol. 327. no. 5961, pp. 41 - 42 doi: 10.1126/science.1184556