Hydrothermally stable heterogeneous catalysts for biorenewable-derived molecule conversions to chemicals

Anderson, Jason
Major Professor
Brent H. Shanks
Committee Member
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Chemical and Biological Engineering

The biorenewables filed seeks to emulate the petroleum model for chemical production; from a select few chemicals a plethora of products are produced. This emulation of the modern petroleum refinery-henceforth called the biorefinery-would allow greater penetration of biorenewable feedstocks into the typically petroleum based chemicals industry. Of great important to this idea is the development of catalysts capable of handling the conditions inherent to biorenewable feedstocks. Water presents a significant challenge to today's catalysis. Using biorenewables, especially sugars, forces the processing to be in the condensed phase as they have little to no volatility. Water is the solvent of choice with most bio-based systems. In addition, the reactions desired to create the chemicals (like esterification) will also create water. This work sought to overcome this difficulty by developing new catalysts with a hydrothermally stable scaffold and active group.

From this work the development of the carbon catalyst is shown. First, I investigated of the hydrothermal stability of the current carbon catalysts in literature. Second, some model compound work showed which active site configurations, if possible, would increase the hydrothermal stability of the acid catalyst. The last two are papers developing the first and second generation of hydrothermally stable acid catalysts. This work increases the possibility of chemicals derived from biorenewables.

Hydrothermal stability of carbon based acid catalysts synthesized by sulfonating carbohydrates pyrolyzed at moderate temperatures (300-600°C) has been reported previously. To test the effect of carbon structure on hydrothermal stability, we produced catalysts by dry pyrolysis at 350ºC and 450ºC or by hydrothermal carbonization, followed by sulfonation with fuming sulfuric acid, as well as by direct sulfonation of glucose. The catalysts were characterized by BET, titration, Raman spectroscopy, TGA, XPS, reaction testing, and 13C solid state NMR. Catalysts were hydrothermally treated and then analyzed for sulfur retention and catalytic activity. The lower temperature carbon catalysts showed the best stability, however all showed significant activity loss. Solid state NMR characterized the structural details to attempt to correlate functional groups to hydrothermal stability of catalyst active sites. Structural models generated from NMR data showed that the most stable catalysts contained a significant fraction of furan rings and hardly any polycondensed aromatic rings.

Development of heterogeneous catalysts for the biorenewables industry requires catalyst materials that are resistant to hydrothermal degradation. Unlike metal oxides and silica, carbon materials are recalcitrant to hydrothermal conditions. However, for solid-acid sulfonated carbon materials, there are conflicting reports on the stability of the sulfonic-acid groups on the aromatic rings for commercial applications. Currently, incomplete understanding remains about the relationship between hydrothermal stability and the immediate electronic hybridization of the carbon atoms adjacent to the sulfonic-acid active group. To test this relation systematically, model compounds containing sulfonic acid groups linked to aromatic, alkane, or cycloalkane carbon atoms were subjected to hydrothermal conditions (100°, 130°, and 160°C DI water up to 24 h). The structural integrity of the compounds was monitored with solution NMR. While the aromatic-sulfonic compounds degrade readily, the changes in the molecules with alkyl sulfonic acid linkages are negligible. Therefore, a hydrothermally stable sulfonic-acid catalyst needs to contain the sulfur attached via alkyl linkers.

We combined research showing typical electrophilic substitution methods for sulfonated carbon catalysts to be inadequate with initial testing of model compounds and a proof of concept of glucose and taurine. This use of the Malliard reaction resulted in a catalyst stable under hydrothermal conditions but initially in colloidal form. Since this is undesireable in industrial processing, we sought to further stabilize the carbon backbone with the addition of more glucose. We found that the ratio of the glucose to the glucose taurine mixture is not as important as the ion used for the precursor. The potassium ion increased the amount of sulfur on the carbon catalyst, thereby increasing the reaction rate on a mass basis. These catalysts suffer from low surface area so we supporting them on SBA-15 and mesoporous carbon nanoparticles. With these two supports, the catalysts showed good activity on a similar sulfur basis.

From previous research the Maillard reaction was successfully used to create hydrothermally stable carbon catalysts through pyrolysis synthesis. The Maillard reaction was used to create a new catalyst through a hydrothermal synthesis. The combination of glucose and taurine in a hydrothermal synthesis creates a solid that retains the sulfur-from the active group-even better than through pyrolysis synthesis. The synthesis temperatures ranged from 200-300ºC and it was found that the most stable catalysts were synthesized at 250ºC. The catalytic activity seemed insensitive to differences in the changes of the glucose to taurine ratio from 1:1 to 2:1 at the 250ºC synthesis. At the 200ºC synthesis temperature, the activity is not stable through the hydrothermal testing and at the 300ºC synthesis temperature; the sulfur retention is not as stable as the catalysts synthesized at 250ºC.

From this work the development of the carbon catalyst is shown. First, the initial work showed the hydrothermal instability of the current carbon catalysts in literature because of their attachment of the sulfonic acid through an aromatic carbon. Second, model compounds showed an active site configuration connecting the sulfonic acid to the backbone through an aliphatic carbon, if possible, would increase the hydrothermal stability of the acid catalyst. The last two are papers developing the first and second generation of hydrothermally stable acid catalysts whereby glucose and taurine are used to make a catalyst through the Millard reaction. This work increases the possibility of chemicals derived from biorenewables.