(538a) 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

Abstract

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.