There is great current interest to design multifunctional micro- or mesoporous materials. A main goal of this project is the synthesis of organic-inorganic hybrid materials Most of the research is directed towards carboxylic acids as the organic complexing agent. The effort also involves the more complex phosphonic acids. The strategy is to prepare pillared layered materials in which the inorganic portion constitutes the layer and the organic the pillars. The products can have high surface areas (~400m2/g), with small pores (~7-8 diameter) up to mesopores (~25 diameter). The pillars can be functionalized with sulfonic acid, carboxylic acid, amino, etc. groups. Furthermore, the pillars can be further separated by small spacers which themselves carry functional groups. Thus, they can be prepared as ion exchangers, as specific complexants, catalysts and as luminescent materials. When the metal is four valent, the particles formed are nanoparticles in which insufficient Bragg reflections to solve their crystal structures are obtained. The mystery of their structure is further compounded by the fact that the pores grow without the use of templates or spacer groups and are often larger than the interlayer distances and yield type I gas sorption isotherms. This is unprecedented as the pore formation depends upon the solvent used. Therefore, we employ a variety of tools such as extended x-ray absorption fine structure and high resolution electron microscopy as well as in-situ methods to probe their structures along with the more usual infra-red, visible, nuclear magnetic resonance, and neutron scattering and gas sorption methods to characterize the pores. The research is also directed towards molecules that breathe by using flexible alkyl, polyether and polyimine pillars separated by high levels of spacer groups. Thus students learn to synthesize unusual materials and employ cutting edge spectroscopic tools as well as the latest crystal structure methodology.
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Research in chemistry often leads to unexpected results. There exist a large number of mineral-like compounds that contain pores. These pores are of the order of 4-13 Angstroms, where one Angstrom is about a hundred-millionth of an inch. So these pores are the sizes of small molecules. These mineral-like materials are principal catalysis used in petroleum refining and many other useful reactions. Therefore, they are of great interest to chemists and chemical engineers. This project aims to produce porous materials from layers of inorganic material bonded together with organic (carbon based) pillars. Visualize a parking garage with the floor and ceiling made of inorganic minerals connected by the pillars that are carbon based. Samples with pores that range from 7 angstroms to 25 angstroms are synthesized. The pillars can be made highly reactive by attaching acid or basic groups. They can be made to conduct protons for use in fuel cell membranes and to carry out a variety of useful reactions. One such reaction converts relatively useless compounds to those useful to the pharmaceutical industry. They can also separate molecules by size and finally a variety that breathes by using flexible pillars I sbeing synthesized. They swell when taking up solvent and deflate when emptied. A large number of uses for these fascinating materials is predicted. Research on these materials provides students with special skills in synthesis and techniques of handling nano-sized particles and state of the science spectroscopic X-ray, neutron and electron microscopy methodology. A partnership with a historically African-American University augments the already high diversity (Hispanic, women, Puerto Rican) of our research group.
Our studies during the past five years have been involved with materials that are layered. A simple example is that of a stack of pancakes, each pancake being a single layer. However, our materials consist of very small particles termed nanoparticles. One nanometer is smaller than one ten millionth of an inch. One such class of compounds that we have prepared are pillared. That is, the layers are connected by molecules that tie the layers together as shown schematically in Figure 1. This pillaring creates porosity because the pillars space themselves in an irregular fashion. Furthermore, we can attach small molecules to the layers that change the way the pillars are arranged resulting in greater porosity. Porosity allows these compounds to sorb fluids. If the pillars are hydrophobic, meaning they reject water and prefer organic or oily fluids, such fluids are preferentially taken up. Hydrophilic pillars are just the opposite in that they prefer water. By adjusting the character of the pillars and the small spacer molecules allows separation of different kinds of mixtures to be effected. We have also changed the behavior of these materials by adding reactive groups to the pillars such as strong acids or bases which allows them to behave as catalysts. A catalyst is a compound that allows difficult reactions to proceed under milder conditions or speeds up the rate of a reaction. They are indispensable for many industrial processes. We have demonstrated a number of such catalytic reactions. Proton conductors that may find use in fuel cells were obtained by functionalizing the pillars with sulfonic acid groups. Another class of compounds that are layered, the zirconium phosphates, are not pillared but the layers form stacks as shown in Figure 2. With these compounds it is possible to insert molecules between the layers. The layers may also be separated from each other by a process of swelling. These particles can be prepared as nanoparticles or of any size up to large crystals. Such features allow us to prepare compounds with a hugh range of different properties. For example, we prepared compounds termed Janus particles. Janus is the Roman God of doorways having two faces one facing forward and one backwards. Our layered materials have reactive surfaces, so we can bond a very hydrophobic compound to the surface. Then, by separating the layers the top and bottom layers are Janus particles because the side opposite to the hydrophobic one is studded with hydrophilic groups. Many uses for such compounds include emulsifiers, sensors, surfactants, and drug delivery. Although many other groups have prepared Janus particles of different kinds, none of these processes lend themselves to mass production at a reasonable price. We are trying to prepare our layered compounds so that the number of layers is small, say 4 or 6. In this case 33 to 50% of the particles will be Janus and can be separated from the others. We also have several other methods to attain mass production but they are proprietary as we now have an industrial firm supporting this work. We have also filed for a preliminary patent. Another procedure that we have used with these layered materials is to create electron transfer reactions. For example, we bond a highly hydrophobic compound to the outer top and bottom surfaces. Then we place a molecule that is an electron donor in between the layers and attach an electron acceptor to the hydrophobic groups on the outer surfaces. By exposing the particles to the appropriate ultraviolet radiation electrons transfer from the donor to the acceptor. We are now attempting to utilize these reactions in circuits that can be used as switches. We shall conclude this narrative with some applications to medicine. We have placed insulin between the layers of zirconium phosphate (ZrP). The idea is to create a pill that can be taken orally and enter the blood stream where the insulin is released slowly. To do this the particles need to be protected from the acid in the stomach and released in the blood. This will require special methods of surface functionalization. The opposite is required in the case of cancer drug delivery. We have been able to place the very active anticancer drug cisplatin between the ZrP layers as well as the potent breast cancer drug doxorubicin. We have found by in-vitro tests with several types of breast cancer cells that a high rate of cell destruction occurs in 48 hours with doxorubicin. The next step is to go further with live animals (mice) for which have we decorated the surface of the ZrP particles with PEGS and biomolecules to avoid damage to healthy tissue and also to direct the particles to the tumors or cancerous cells. This work is in progress. However, we are no longer supported by NSF.
