Development of catalysts for green Chemistry

Catalyst is a foreign substance that will enhance the rate of  reaction by taking the reaction through an alternative path.  There are two types of catalysts.

Homogeneous catalysi: where the catalyst is in the same phase as the reaction mixture.   

Example :   Conversion of glucose using  glucose oxidase as the homogenous catalyst.

Hetergogeneous catalysis:  The catalyst is isolated from the reaction mixture. For example, during conversion of  glucose in presence of  glucose oxidase, the enzyme is immobilized so that it is separated from the reaction mixture.

Green Chemistry is defined  as " The invention, design and application of chemical produxts and processes to reduce or to eliminate the use  and generation of hazardous substances [1]. The authors developed 12 principles which are

1.Prevention  :   It is believed that prevention is better than generating the waster and treating it latter.

2.Atom Economy :  The methods devied should be such that all materials used as reactants  will have to be part of final product.

3.Less Hazardous Chemical Synthesis  :  The products generated need to be free from toxicit to health of human and environment,

4. Safer Chemicals :  The product need to meet the intended purpose while minimizing their toxicity. Chemical products should be designed to affect their desired function while minimizing their toxicity.

5. Safer Solvents and Auxiliaries :  Prevention of use of  solvents and separating agents wherever possible.

6.Design for Energy Efficiency :   Minimization of energy requirements of  chemical processes is an important aspect. Where ever possible, synthetic methods should be conducted at ambient temperature and pressure.

7.Use of Renewable Feedstocks :  Use of renewable ensures the raw material is available for longer periods of time.

8.Reduce Derivatives :  Unnecessary derivatization has to  be minimized or avoided if possible, because such steps require additional reagents and can generate waste.

9. Catalysis : Catalytic reagents will have to be superior to stoichiometric reagents.

10.Design for Degradation : Chemical products should be designed so that at the end of their function they degrade and do not exist in the environment,

         11.Real-Time analysis for Pollution Prevention : Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.

        12. Inherently Safer Chemistry for Accident Prevention :Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions and fires.

Even though Green Chemistry and sustainable chemistry are used interchangeably, there is a difference, Green chemistry suggests existence of process that is not hazardous and free from pollution,  the sustainable chemistry deals with a process that is eco friendly, concerned about growth of economy improvement in the quality of life with respect to cost benefit analysis.[2].

Green catalyst should possess higher activity, higher selectivity, efficient recovery from reaction medium, durability or recyclability, cost effectiveness.

There are two main areas of focus related to green chemistry.

• A long-term approach which involve carrying out the reactions under mild conditions.
• A short-medium-term approach (evolutionary approach), which involves improvement of current catalytic technologies and processes.

Some of the green chemistry catalysts are SCCO2[2], Hydrotropes[3], ionic liquids [4], nanocatalysts [5], perovskite type catalysts [2].

Some of the applications of these green chemistry catalysts are discussed in the following paragraphs.

Super critical CO2 is used in extraction of Cashew nut shell liquid (CNSL) from Cashew nut.  The quality of CNSL is found to be superior to other processes like solvent extraction, screw press etc [6]. But the problem associated with this technique is high operating pressures and temperatures.

Hydrotropes are compounds that increase the aqueous solubility of weakly water soluble organic compound in  water.  Examples of Hydrotropes are : Urea, tirsodium citrate etc. They have been used for dissolution of drugs in aqueous media[7]. They are also used as extraction media. The author did some unpublished work on use of trisodium citrate on extraction of phenols from tamarind seed powder.  The extraction can be conducted at room temperature. The limitations of these chemicals are that they are efficient at high concentrations.

Ionic liquids are compounds which are liquids at room temperature. They are non flammable, having less vapor pressure and re-usable. Since they have excellent solvating potential, thermal stability, they are the obvious choice of medium for chemical syntheses. Phosphonium based ionic liquids have been reported to  have been used for capturing the CO2 [4].  The major problem associated with Ionic liquids is that they are very expensive.

Nano catalysts provide high surface area ( to the tune of 20 m2/g) and hence are used for production of hydrogen and hydrogen peroxide [4]. The problem associated with them is the energy required to create such a high surface area.

Perovskites are mixed-metal oxides which are having low cost, high adaptability and thermal stability.  They are reported to have been used for photo catalytic splitting of water, They have been used for diesel soot oxidation, N2O decomposition, oxidation of CO at low temperature, oxidation of  VOCS [8].

Conclusions: Green catalysts is the important area of research and need to be studied for extending their use to industrial applications.