Materials, Synthesis

Oxygen, Nature’s Oxidant for Nature’s Feedstocks.

“Selective catalytic conversion of biobased carbohydrates to formic acid using molecular oxygen”R. Wolfel, N. Taccardi,  A. Bosmann, P. Wasserscheid, Green Chemistry, 2011, DOI: 10.1039/c1gc15434f

Graphical abstract: Selective catalytic conversion of biobased carbohydrates to formic acid using molecular oxygen

All of us have a very personal relationship to the oxidizing power of oxygen. We use oxygen to turn our food into energy, CO2 and water. There are a number of enzymes and pathways that aid this process, each aiding the reaction of food and oxygen toward the creation of CO2 and water.  Now the key to turning complex biomass into usable small molecules is the ability to control this reaction so that we can extract usable chemical building blocks without ending up back at CO2 and water. As you can see in this video over-oxidation can be a real concern.  This paper demonstrates the use of a polyoxometalate (POM) catalyst to promote the oxidation of biomass to formic acid.

An example of a Polyoxometalate structure.

I was attracted to this paper because it combines my interest in renewable feedstocks, with a longstanding fascination I have had with POMs. POMs are a class of inorganic clusters that are both beautiful (look at that symmetry) and also have been shown to have very unique properties. They are basically very small chunks of metal oxide material–smaller than nanoparticles. They can be made with a number of transition metals and have been shown to have very interesting catalytic properties including epoxidation, degradation of organic pollutants and chemical warfare agents, and selective oxidation.  POMs are attractive oxidation catalysts because of their chemical stability and resistance to auto-oxidation.

One of the daunting challenges of using biomass feedstocks is turning very large, insoluble biopolymers into usable feedstock chemicals. While formic acid is rarely identified as an important feedstock, the authors make a good attempt to rationalize its synthesis within the context of promoting a hydrogen fuel economy. I give the authors credit for really considering the potential life-cycle impacts of formic acid from petroleum vs. biomass. I don’t often see this analysis in chemistry papers, and the authors did a good job of positioning their technology.

The authors demonstrate that H5PV2Mo10O40, can be used in the presence of molecular oxygen under mild conditions to convert bio-based feedstocks into formic acid (See table 1). While the reaction will run at 80 oC using oxygen from ambient air, the rate of reaction was significantly increased by increasing the O2 pressure to 30 bar. After trying the easy targets like glucose and sucrose, they went after some more interesting biomass chemicals including cellulose, hemicellulose, lignin, and sawdust. Even with untreated sawdust the authors report a 19 wt% yield of formic acid from the dry biomass.

Table 1: Substrate Scope for the catalytic oxidation with POM

Substrate

Conversion %

FA Yeild %

Glucose

>98

47

Sorbitol

>98

56

Cellobiose

>98

47

Xylose

>98

54

Sucrose

>98

48

Glycerol

n.d.

40

Cellulose

n.d.

1

Hemicellulose

n.d.

33

Lignin

n.d.

14

Poplar Sawdust

n.d.

11

The mechanism for POM catalyzed oxidation has been explored in another interesting paper, which outlines a number of different potential pathways by which O2 can be used as an oxidant by the POM clusters and draws interesting parallels with the modes of oxidation in cellular respiration. The authors of this study demonstrated that the reaction mechanism is particularly sensitive to aldehydes in the substrate.

The potential impact of de-polymerization catalysts for biological polymers is huge. While this paper represents a fairly modest step in that direction, there are many people looking for chemical processes which break down biomass into chemical feedstocks. Currently many industrial biomass processing facilities rely on fermentation to transform biomass into chemical feedstock molecules. While elegant and potentially green, fermentation relies on large quantities of water, relatively slow processes, dilute mixtures, and complex separations. Fermentation techniques only convert the sugar/starch/cellulose fractions leaving the lignin fraction untouched. The POM catalysts target the intractable lignin fraction making this a complementary process. Future processes will need to outperform fermentation, potentially by using relatively abundant chemicals like oxygen and metal oxide catalysts.

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