Even as concern mounts over energy use, energy-intensive distillation and unoptimized inorganic adsorbents dominate large scale separations as they have for decades. Thus, purifying bio-butanol from fermentation broth via distillation consumes >50% of the energy content of the fuel itself, to cite one example. Adsorption can decrease the needed energy input, but requires a paradigm shift in how selective adsorbent materials are developed.
This proposal seeks to understand the processes by which two new classes of atom-precise, but non-crystalline adsorbent materials may be constructed from intrinsically porous small molecules. In these materials, the concept of a site is directly connected to a molecule that can be synthesized with atomic precision and incorporated into material at amounts known a priori. This will allow precise testing of hypothesis regarding atomic-level site requirements for selective adsorption, free of some of the confounded interrelationships between adsorbent capacities, selectivities and structure, typical of many complex materials. In one thrust, cavity-containing oligomeric calix[n]arenes are covalently attached to support oxides through a rigid, single-atom linker such that each grafted calix[n]arene has been proven to act as one adsorption site. Specific hypotheses regarding optimal cavity volume, polarity, and H-bonding ability for the selective uptake of alcohols from aqueous solution are tested by systematic tuning of the calix[n]arene host and grafting method. In a second thrust, rigid arylene-ethynylene oligomers are grown from supports using C-C coupling chemistry at mild conditions. Oligomer collapse upon solvent changes creates a nanoporous overlayer resembling graphitic carbon, but whose structure can be systematically tuned with electron-rich or electron-poor groups. After demonstrating the generality of this novel synthesis technique, the resulting synthetic precision will be used to test the hypotheses that matching pi-acids to pi-bases across defined nanopores will lead to selective adsorption from complex mixtures. In both thrusts, the support structure is independent of and chemically distinct from the active site, enabling bulk tools such as solid state 13C NMR and diffuse-reflectance UV-visible to be surface selective. Furthermore, the designed energetic homogeneity of the adsorption sites enables bulk uptake measurements to provide molecular level insights.
The technological viability of bio-alcohols as fuels is dependent on reducing the energy required to separate them from water. In the first thrust, this proposal seeks to create new materials to understand the requirements for a selective adsorbent for low-energy butanol-water separation. Likewise, ultralow sulfur automotive fuels are contingent on removal of dibenzothiophene molecules resistant to current desulfurization technologies. In the second thrust, this proposal seeks to create new materials for the selective adsorption of such molecules. Both thrusts are easily generalizable to other targets relevant to fuels chemistry. Such challenges are of increasingly broad interest to students, industry, and the general population alike. Moreover, a strong history of undergraduate participation, direct-to-community outreach, and a group that is 50% native Spanish-speaking, will be leveraged in the culturally-diverse Chicago area to broaden participation in cutting edge science by underrepresented groups.
