Nanoparticle Electrocatalysts for the Oxidation of Waste Lignin

Lignin is a large organic molecule and a primary waste from the pulping industry and biorefineries [1, 2].  Currently, lignin has little value other than as a fuel to generate heat.  Lignin can, however, be used as a renewable raw material for a variety of value-added chemicals.  Unfortunately, catalytic processes are difficult to implement for the degradation of waste lignin because selective oxidation is difficult and these processes produce an unacceptable amount of solid residue like char [2, 3].

Electrochemical processes, on the other hand, have several advantages over catalytic processes, including the use of mildly basic solvents as electrolytes, and control over the rate of reaction and the products.  Thus, the electrochemical oxidation of waste lignin would be an ideal process to utilize this renewable raw material.

While there have been efforts related to the electrochemical oxidation of lignin, most work has focused either on expensive platinum electrodes or on low surface area, large metal electrodes.  However, nanoparticle electrocatalysts can be developed that enhance the rate of electrochemical reaction through enhancement of electrocatalyst surface area and porosity.

We will report on our efforts to develop nanoparticle electrocatalysts with specific compositions and architectures for the electrochemical oxidation of lignin in weak alkaline solution.  For example, Ni-Cu alloy nanoparticle electrocatalysts are suitable for the electro-oxidation of lignin in basic solution, as shown in Figure 1.

Figure 1.  Electrochemical oxidation of lignin on Ni-Cu alloy nanoparticle electrocatalysts.  Baseline is 1 M NaOH in the absence of lignin.

 

As shown in Figure 1, oxidation peaks in the presence of lignin occur around 0.03 V and 0.45 V vs. NHE that do not occur in the absence of lignin.  Figure 1 indicates that lignin may undergo a complicated, multi-step oxidation process.  For example, lignin may oxidize to one product at a lower potential, and that product may in turn oxidize to yet another product at higher potential.  The oxidation potential and oxidation time (i.e., charge passed) affects the product distribution.

One of the most important aspects of the electrochemical oxidation of lignin in alkaline solution is the identification of the oxidation products.  We will present GC/MS data that identify lignin oxidation products.  This process could provide an economic way of reducing the organic waste from the pulping and biorefinery industries.

References.

 

1.  A. French, “’Lignin’ May Be the Magic Word,” Innovation: America’s Journal of Technology Commercialization, June/July 2011.

2.  C. Grestini, “Conversion of Lignin: Chemical Technologies and Biotechnologies,” EuroBioRef Summer School, Universita Degli Studi di Roma, Castro Marina, Lecce, Italy, September 21, 2011.

3.  M. P. Pandey and C. S. Kim, “Lignin Depolymerization and Conversion: A Review of Thermochemical Methods,” Chemical Engineering and Technology, 34, 327-331 (1999)