APPLICATION+OF+GREEN+CHEMISTRY

__3.1 Utilizing Carbon Dioxide__
The increasing global concern of greenhouse gas emissions, particularly carbon dioxide can be greatly attributed to the consumption of fossil fuels. Most of the world’s energy sources rely on the combustion of these fossil fuels. In a generation that depends heavily on fossil resources it is necessary to implement alternative strategies of using renewable feed-stocks and reducing the amount of greenhouse emissions.

 Carbon dioxide is a non-toxic, non-flammable chemical that is being generated at an exponential rate. It would seem that CO₂ would be an excellent source of carbon in making industrial chemicals. However due to its thermodynamic stability, the conversion of carbon dioxide requires a great amount of energy. As a result, the process of converting CO₂ is an endothermic reaction (Fig. 13) that often requires specific reaction conditions and a catalyst which come at a greater cost.

Carbon based feedstock is required in the production of many important organic chemicals. Utilizing CO₂ as a raw material has only been recognized in a few processes such as the production of urea, methanol, cyclocarbonates and salicylic acid. The concept of utilizing CO₂ as a building block in reactions that are feasible and functional allows opportunities to reduce the depletion of fossil fuels.

__3.2 Catalytic Processes and Efficiency__
The use of inorganic stoichiometric processes generates more waste in comparison to catalytic processes. In evaluating atom efficiency of each process, a catalytic process provides an alternative pathway for the reaction and increases the efficiency. An example of how the 2 processes compare can be noted in the following reaction.



// 3.2.1 Synthesizing lactone //
In preparing δ-lactone, it is common to use a palladium catalyst in converting butadiene with carbon dioxide

The process of synthesizing δ-lactone using butadiene and CO₂ requires the recycling of its by-products and catalyst in order to increase the overall selectivity of the reaction. Atom efficiency refers to the amount of desired product converted from the reactants. In recycling the by-products, less waste is produced and more desired product is generated.

// 3.2.2 Hydrogenation of lactone //
While analyzing the hydrogenation of δ-lactone, it is evident that homogenous catalytic reactions require milder conditions in regards to the pressure and temperature. In contrast, heterogeneous catalytic reactions are carried out at temperatures of 200-300⁰C and pressures of up to 300 bars(3). Consequently such reactions only yield 28% whereas integrating a homogeneous catalyst yield of 68%. Using a homogenous catalysis allows an increased yield that results in a more energy efficient process. Both economic and environmental impacts are reduced.

The production of 2-ethylheptanol from δ-lactone can be derived efficiently in only 3 steps and can yield 91% of product(3). 2-ethylheptanol is applicable as a plasticiser for polymers. This demonstrates how CO₂ can serve as a viable building block for efficient reactions that require minimal derivations.

// 3.2.3 Hydroformylation of lactone //
Hydroformylation reactions of δ-lactone can be carried out with either cobalt or rhodium catalysts. Specifically Rhodium catalysts are more efficient because they involve more moderate conditions of temperature, pressure and time. The effect is increased selectivity and efficiency at a reasonable cost. These homogenous catalyzed reactions yield intermediate aldehydes useful for polymers and alcohols.

Previous Page