Many agricultural materials can benefit from acid-catalyzed conversion into more economically valuable products such as cellulosic ethanol and biodiesel. An ideal catalyst would provide fast conversion rates, minimal degradation into waste products, and energy efficient, inexpensive recycling and re-use capabilities.
Traditional liquid phase acid catalysts have disadvantages such as requiring environmentally hazardous and/or expensive post-catalytic separation or neutralization. Solid phase (heterogeneous) acid catalysts have the potential to improve the economics and environmental sustainability of acid-catalyzed conversion processes. Superparamagnetic nanoparticles provide an additional facile method of separating the solid acid catalysts.
The particle cores have diameters that are smaller than the magnetic domains of the material, allowing them to be pulled from solution by the presence of a moderate external magnetic field, and return to a non-magnetic state when the external magnetic field is removed. This may be especially useful for the catalysis of agricultural materials which may contain solid matter whose separation from the catalyst would otherwise be difficult.
Agricultural materials have significant heterogeneity on both the molecular and macroscopic levels. A useful catalyst must be stable and active in a variety of harsh chemical environments, particularly those of low pH and high temperature. Previous work employed nanoparticles whose acidic ligands were attached to the nanoparticle cores through either silicon-carbon bonds or through carboxylate-magnetite bonds. Although hydrolysis of cellulose and cellobiose increased in the presence of the acid functionalized nanoparticles, the acid groups bound to the nanoparticles were found to leach into solution.
For enhanced catalyst stability, bonds between the ligands and the magnetic cores are protected by steric blocking groups. The silica bonds are protected primarily through the presence of isobutyl and methyl groups on the nanoparticle surface, while the carboxylate-magnetite bonds are protected by extending the lengths of the alkyl chains that separate the nanoparticle surface from the active acidic moieties. By preventing water molecules from contacting the silica and carboxylate-magnetite bonds, dissociation of the acidic ligands can be avoided.
Preliminary work details the stability of various sterically protected bonds between the ligands and nanoparticle cores.