285141 Hydrothermal Deactivation of Electrophilically Sulfonated Carbon Catalysts
Hydrothermal Deactivation of Electrophilically Sulfonated Carbon Catalysts
Jason M Anderson1, Robert Johnson2, Klaus Schmidt-Rohr2, Brent H Shanks1
1 Department of Chemical Engineering, Iowa State University, IA, USA
2 Department of Chemistry Iowa State University, IA, USA
One of the technical limitations to convert biomass feedstocks into chemicals is the development of heterogeneous acid catalysts that are hydrothermally stable. Carbon based acid catalysts produced by sulfonating pyrolyzed sugars have been reported to have promising stability. However, ambiguities exist in describing why some electrophilically sulfonated carbons are claimed to be hydrothermally stable. In the current work, glucose was used to make four different sulfonated carbon materials: dry pyrolysis (350°C and 450°C for ~10 hours in N2), hydrothermal carbonization (200°C for ca. 19 hours in liquid water), and direct sulfonation glucose (150°C for 2 hours) by mixing with fuming sulfuric acid. The hydrothermal carbon char was also sulfonated via benzene sulfonic acid radical for comparison. The catalysts were characterized with BET physisorption, titration, Raman spectroscopy, TGA, XPS, reaction testing (esterification), and solid state NMR. The hydrothermal stability of the catalysts was tested by treatment with 160°C liquid water for 24, 48, and 72 hours. Although they gave a >66% reduction in activity, the hydrothermal carbonization and 350°C carbon catalysts showed the best hydrothermal stability after three hydrothermal treatments. The sulfur on all the catalysts was found to leach off during hydrothermal treatment (from ICP analysis of the filtrate from hydrothermal testing). The sulfur weight percentages from elemental analysis of the catalysts were found not to strongly correlate with activity. This suggests there may be differences of the availability or chemical nature of the sulfur active groups.
Standard characterization methods were unsuccessful in providing a structural explanation the hydrothermal stability. Solid state NMR provided structural details that indicated the most stable catalysts contained a significant fraction of furanic carbons, which might be influential in the greater stability of the sulfonic acid groups. Current electrophilic sulfonation techniques appeared to be inadequate for synthesis of hydrothermally stable acid catalysts.