A sustainable and environmentally friendly source of ethanol for transportation fuel is cellulose, which is the major component of lignocellulosic plant biomass. Hydrolysis of cellulose generates glucose, which can be fermented to ethanol. However, in general, the hydrolysis of cellulose is a difficult process, and is currently the rate-limiting step in the cellulose-to-ethanol conversion. Cellulase enzymes with very specific catalytic structures are capable of efficient cellulose hydrolysis without byproduct formation. However, enzymes are relatively slow, thermally unstable, and expensive, as they must be biologically produced are not readily reusable. The proposed research will develop and understand the function of artificial enzyme analogues that can hydrolyze cellulose and its subunits. Artificial enzyme structures will be created by molecular imprinting of cellulose oligomers onto organic-inorganic hybrid solid catalyst supports based on sol-gel methods. This strategy has the potential to combine the efficiency and specificity of protein-based enzymes with the robustness and cost effectiveness of heterogeneous solid catalysts.
In the proposed research, artificial enzyme analogues for hydrolysis of cello-oligomers and cellulose will be generated based on a molecular-imprinting sol-gel technique combined with a high-throughput synthesis and screening method. Molecular cavities generated by the imprinting process will provide specific binding domains for sub-units of cellulose as well as solid acid sites required for hydrolysis of glycosidic bonds. To potentially enhance substrate hydrolysis and promote induced fit between the molecularly imprinted catalyst and substrate, the flexibility of the sol-gel catalysts will be tuned by adding organo-silanes to the sol-gel matrix. It is hypothesized that the specificity of the molecularly-imprinted catalysts would hinder side reactions that lead to degradation of glucose, which is a major problem with conventional solid-acid catalysts. The high throughput method will be able to screen a large number (~10,000) of synthesis parameters to optimize the composition and structure of the molecularly-imprinted catalysts.
Broader Impacts
The proposed education and outreach activities are designed to stimulate student interest in biomass energy and fundamental ideas in science and engineering. Undergraduate students from underrepresented groups recruited through the Louise Stokes Alliance for Minority Participation (AMP) Summer Undergraduate Research Program will be given opportunities to participate in the research. As part of the outreach plan, the PI will work with a local cable access TV station to develop programming on the current state of energy landscape and the importance of developing renewable energy technologies. This scientific program will be aired throughout the Philadelphia metro area.
Mesoporous silica, which are silica materials that have nanometer-sized pores, has found numerous applications in catalysis, chemical sensing and separations, optics, and thermal insulation because of its high surface area, porosity, and tunable pore morphology and sizes. In particular, mesoporous silica is a very useful material for catalysis and separations due to its high thermal and mechanical stability, wide ranging physical and surface chemical properties, and relatively inexpensive and well understood synthesis. However, mesoporous silica can degrade in water even at room temperature, rendering its use in certain aqueous phase catalysis and separation applications impractical. Ionic liquids (ILs), also known as molten salts, have recently emerged as novel solvents for many different applications, particularly catalysis and separations, because of their chemical diversity, thermochemical stability, and negligible volatility, possibly serving as alternatives to aqueous solvents in applications with mesoporous silica. One such promising example is the use of ILs instead of water as solvents for the catalytic conversion of lignocellulosic biomass. Solid acids based on mesoporous silica could potentially serve as the catalyst in such IL-based biomass conversion processes. Likewise, mesoporous silica materials infiltrated with ILs have been used as supported liquid phases for the separation of various compounds as well. In order for these applications of porous silica with ILs to be feasible, mesoporous silica must maintain its structure and properties in ILs under the range of conditions subjected to. In this work, we have examined the ionothermal stability of mesoporous silica films (MSFs) in four different imidazolium ionic liquids (ILs) and compared this to their stability in liquid water. Ionothermal treatment of MSFs in anhydrous 1-butyl-3-methylimidazolium acetate, bromide, chloride, and thiocyanate reveals that MSFs are stable in these ILs up to 175 °C. Solvothermal stability of MSFs in imidazolium IL-water mixtures was also investigated, and mesoporous silica is found stable in compositions greater than 25 mol% IL. Our findings suggest that anhydrous imidazolium ILs are unable to degrade mesoporous silica, possibly making these better solvents than water for use with mesoporous silica. This NSF CBET grant also led to a transformative discovery in enabling the fabrication of crack-free nanoparticles films. Creating uniform films of nanoparticles on surfaces is a critical engineering practice that has a wide range of applications in energy storage and conversions as well as in sensing and electronic devices. However, when nanoparticle films are deposited, cracks or defects form easily, which significantly compromises the functionality of these nanoparticle films. Our group has developed a new way to avoid such cracks when depositing thin films of nanoparticles without introducing complex chemistry and processes. By sequentially depositing thin layers of nanoparticles multiple times, rather than depositing a thick layer at once, we were able to show that crack formation in nanoparticle films could be significantly suppressed. We believe the suppression of cracking in nanoparticle films is due to the formation of covalent bonds between neighboring silica nanoparticles during drying steps in between nanoparticle depositions. We showed that by using this sequential deposition method crack-free mesoporous Bragg stack reflectors exhibiting structural color could be generated. Our work in general has shown that by judiciously tailoring the processing conditions silica-based materials, which are one of the most tunable materials, can be used in a variety of applications including energy storage & conversion, biomass conversion and sensing & electronic devices. For example, by using a mixture of ionic liquid and water, it will be possible to use solid acids based on silica to catalyze biomass conversion that used to rely on expensive enzymes. Also by creating crack-free nanoparticle films, it will be possible to create highly efficient solar cells and lithium ion batteries based on nanoparticle electrodes that are defect free. Moreover, our work has presented multiple exhibitions on the benefits of nanomaterials at the Philly Materials Day events and has supported several undergraduate students who are currently pursuing advanced degrees in science and engineering. The PI has also delivered multiple Science Café lectures to general public.
