(594f) Acetic Acid Extraction Using Amine Grafted OMS: Effects Due to Degree of Amine Methylation

Cellulosic biomass represents a potential feedstock for the production of sustainable fuels, but currently requires a large energy input to be depolymerized into sugars suitable for enzymatic digestion. Organic acid side products, with the majority being acetic acid, produced during depolymerization of cellulose are a large energy sink for the process because they must be selectively removed in order to raise the pH to a range that enzymes can efficiently produce alcohols. Liquid amine sorbents have been traditionally used for this due to their inherent basicity, but suffer from a large energy penalty during separation. Additionally, the nature of the extraction is not fully understood with many competing molecular forces at play. By grafting amine functional groups on high-surface area supports, the separation could be significantly less energy intensive while maintaining high uptake and be used as a model substrate to provide a better mechanistic understanding of the extraction process.

In this work, amine grafted ordered mesoporous silica(OMS) was used to extract acetic acid and the acid uptake of primary, secondary, tertiary, and quaternary methylated amines were compared. SBA-15 and MCM-41 were used as the OMS supports due to their pore uniformity and well-characterized ordered structure. Amines were grafted on the OMS surface by post-synthetic grafting of their respective (3-amino)propyltrimethoxysilane with the exception of the quaternary amine, which was synthesized by first grafting (3-iodo)propyltrimethoxysilane on the surface, then reacting with trimethylamine in a dry environment. OMS structure was confirmed by x-ray diffraction and nitrogen adsorption; then thermo-gravimetric analysis was used to confirm loading of amine groups.

The amine grafted OMS materials were tested for uptake at various concentrations that are relevant to bio-fuels, low concentrations are found in acid hydrolysis and high in pyrolysis cellulose depolymerization, and an uptake isotherm was obtained. Results showed that primary and quaternary amines had the highest uptake with uptake values for both equal to 0.21 mmol acetic acid/g SBA-15. For secondary and tertiary loaded amines, the uptake values were 0.185 and 0.18 mmol/g, respectively. These values were recorded for a 0.5%, by volume, acetic acid in water solution and 0.5 mmol of amine loaded/g SBA-15 for all samples. Uptake as a function of the mesopore size, ligand loading, and pH will also be described. Additionally, uptake was shown to significantly decrease for all amines with increasing salt concentration and a significant decrease in uptake was observed with increasing pH. Competing uptake of varying alkyl chain length organic acids was also tested and it was found that the most hydrophobic acid was preferentially bond to the grafted amine groups.

Ongoing work, which will also be described, are findings from pulsed-field gradient NMR. These experiments will provide a complimentary microscopic picture through which to interpret the macroscopic uptake measurements. From this the self-diffusion coefficient of acetic acid was calculated and binding properties were investigated when amine grafted OMS was introduced to the system. Further, by using diffusion ordered spectroscopy these same characteristics were probed in the presence of multiple organic acids.