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43890-B5
Gemini Surfactant Clay Intercalates for Triphase Catalysis

Nahid Shabestary, Southern Illinois University (Edwardsville)

We are pursuing triphase catalysis (TC) which is a unique type of phase transfer catalysis (PTC) using clay as solid phase and support for the catalyst.  We have used hectorite clay from the smectite clay family (layered swelling clay) to support the catalysts through intercalation chemistry.  We have developed various novel clay supported catalysts in our research using Gemini surfactants.  Gemini surfactants essentially consist of two surfactants attached by a spacer group close to the two head groups.  

Fig. 1. Schematic representation of a Gemini surfactant.

Gemini surfactants have the technological advantages of lower critical micelle concentration (CMC), higher surface tension and superior wetting properties than achievable with traditional monomeric surfactants.  We have synthesized and characterized Gemini dicationic compounds C₁₆-C₂-C₁₆, C₁₆-C₄-C₁₆, C₁₆-C₆-C₁₆, C₁₂-C₄-C₁₂, C₁₂-C₆-C₁₂ for application in triphase catalytic reactions using toluene as organic phase and salt water as aqueous phase.  The nucleophilic displacement reaction is given below:

Fig. 2.  A triphase catalytic reaction.  The reactants from organic and aqueous phase come to the surface of the solid phase (Gemini intercalated clay) and react. 

While our main goal is to study the effect of Gemini-hectorite clay complex on the kinetics of the displacement reaction, we are also interested in a clear understanding of the factors that determine the often very strong selectivity of the clay for certain ions and the propensity of the hectorite clay to intercalate Gemini surfactants.  For this reason, we have used X-ray powder diffraction to examine the intercalation of Gemini surfactants with different space and tail length, focusing particularly on the space occupied by non-charge-neutralizing moieties.  The important difference from the traditional quaternary ammonium surfactant with one cation is that the Gemini surfactants carry a double charge and also provide the prospect of testing the effect of surfactant charge on the intercalation reaction.

Fig. 3. Intercalated Gemini surfactants in the interlamellar space of hectorite clay.

Gemini-clay complexes are also interesting in regards to their swelling in the presence of solvents such as toluene and aqueous systems.  The swelling properties of various Gemini-clay intercalates in the presence of a single solvent system has been studied by different groups.  Based on some of our preliminary results (X-ray diffraction as well as reaction kinetics), we tend to believe that some of the Gemini surfactants have a greater affinity for adsorbing solvents and swelling.  This may help us to be able to tailor clay-Gemini catalysts with more superior catalytic behavior in triphase catalytic reactions.

By studying X-ray diffraction patterns and measured basal spacing we are seeing some differences which must be related to length of the spacer or tail group of the Gemini surfactants intercalated in the interlamellar spacing of the clay.  In our preliminary work regarding triphase catalytic reactions, we have seen some differences in the rate of the displacement reaction.  However, we have no clear understanding or solid justification for the differences being observed to date.  In this respect, we decided to repeat some of our work to see if we can verify and confirm our earlier results.  Gemini synthesis, characterization, as well as clay purification and clay intercalation reactions take a vast amount of time; however we felt it necessary to take that route. 

I feel that the impact of this interdisciplinary area of research at SIUE is significant.  In particular, this area of research has been a great source of experience for my undergraduate students.  Neha Parikh and Danielle Reed have worked on the Gemini synthesis, clay purification and intercalation chemistry.  They have also done some preliminary work towards the kinetics of the displacement reaction.  Danielle presented a poster at the 2007 ACS Midwest Regional Meeting, Kansas City, MO, November 7 – 10, and both Danielle and Neha each presented a poster at the 235^th ACS National meeting, New Orleans, LA April 6-10, 2008.  The impact of this project on my research and my career has also been profound as I have since been tenured and promoted to Associate Professor.

Currently, we are preparing for more experiments to determine the rate of the nucleophilic displacement reaction using various Gemini-clay catalysts that we have developed in our laboratory and are also planning to investigate the effect of stirring rate on the triphase displacement reaction.

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