(560d) Chemical Modification of Hydrothermal Chars Using Mechanical Energy
Hydrothermal carbonization has potential for synthesis of materials with catalysis, water purification, and energy storage applications. Functionalization of the carbon surface will be required to increase effectiveness for a specific application. Mechanical methods have potential for chemical modification of hydrothermal chars with reduced environmental burdens and costs compared to traditional chemical methods. In this work, we produced hydrothermal char by heating a glucose solution to 180°C at autothermal pressure. The synthesized hydrothermal chars were then subjected to ball milling at times ranging from 30 min to 5 hours. Structural changes were evaluated using a suite of methods. Combustion analysis methods were used to confirm that the hydrothermal char underwent some combination of dehydration, decarbonylation, and decarboxylation reactions during milling with the net result being transformation into a chemical composition approximating low-rank coal. IR spectroscopy performed using both DRIFTS and ATR techniques found that aromatic C-C, aromatic C-H, and carbonyl features increased with milling time, whereas alkyl and furanic features decreased. Mass spectrometry confirmed a transition from alkyl characteristics to aromatic and black carbon characteristics in parallel with an increase in carbonyl and carboxylic acid features. Raman microscopy was used to assess mid-range order associated with localized carbon structures including alkyl side chains, bridge-head carbons, and aromatic domains. X-ray diffraction confirmed that the milled hydrothermal char remained highly amorphous. Taken together, these various data sets suggest a chemical mechanism consisting of surface oxidation in parallel with radical formation due to breaking of weak aliphatic bonds followed by recondensation into stable aromatic structures. The radical formation/recondensation mechanism was then investigated using electron spin resonance, which confirmed that ball milling initially increases radical concentrations by about a factor of 10 in the first 30-60 min followed by a gradual decrease over the next several hours. In future work, we will use the fundamental information gathered in this study to design processes for functionalizing hydrothermal char for specific applications.