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Center for Catalysis
0755 Gilman Hall
Ames, IA 50011
515-294-3135
vsylin@iastate.edu

Nanoscience and Catalysis

Many of the "green" feedstocks (vegetable oils, starches, wood fibers, etc.) are molecules with widely-varying properties. To successfully convert these complex feedstocks with multiple functional groups into important value-added products such as biodiesel, biodegradable polymers, and other new materials and chemicals, one has to have the ability to design highly efficient and selective catalysts with control of the chemistry that takes place at the nano-meter scale is a crucial prerequisite for developing a Green Chemical Industry in the near future.

Our goal is to: Design heterogeneous solid catalysts using nanoscience methods to realize the superb selectivity of biocatalysts and homogeneous catalysts with high turnover numbers, minimal wastes, and long lifetimes of the catalysts.

The Center for Catalysis currently supports several research projects that combine important concepts of the emerging nanoscience with catalysts design:

(1) Nanoporous Solid Catalysts for Conversion of Soybean Oil to Biodiesel

John G. Verkade, Iowa State University Department of Chemistry
Victor S.-Y. Lin, Iowa State University Department of Chemistry
George Kraus, Iowa State University Department of Chemistry

As a result of recent legislation designed to decrease vehicle emissions and the recognition that a decreased dependence on foreign sources of fuel will enhance national security, interest in the use of biodiesel (soy diesel) as an alternative fuel has accelerated.

Biodiesel has several advantages over diesel fuel made from petroleum. Biodiesel:

1. Is biodegradable in water. Up to 98% biodegrades in three weeks.

2. Contains negligible sulfur and increases lubricity.

3. Production can enhance rural revitalization.

4. Is neutral with regard to carbon dioxide emissions.

5. Use leads to significant reductions in most harmful exhaust emissions.

6. Has a high flash combustion temperature.

In addition to the above advantages of being a "green fuel", biodiesel, such as soy diesel (methyl soyate), is becoming increasingly useful in industrial solvent formulations for removing grease and resins, as carrier solvents, and as agents for shoreline oil spill clean-up. Apparently, methyl soyate is a viable alternative to traditional petroleum-derived solvents, which are of environmental concern and are under legislative pressure to be replaced by biodegradable substitutes. Furthermore, recent progress in developing ethyl soyate-based solvents (which can be made from ethanol and soy bean oil) offer several additional advantages over methyl soyate, such as a higher boiling point and lower volatile organic compounds (VOCs), for even broader commercial applications. Given that ethyl soyate is made from ethanol (which is produced by the fermentation of corn), the use of ethyl soyate would also expand the corn-based ethanol market.

Although interest in biodiesel is rapidly increasing, the process by which biodiesel is synthesized has not changed much. Currently, soy diesel (methyl soyate) is made commercially by an energy and labor-intensive process wherein soybean oil is reacted with methanol at 140-150^oF (sometimes under pressure) in the presence of sodium methoxide. This process is called "transesterification." Isolation of the desired methyl soyate from the highly caustic (toxic) catalyst and other products, such as glycerol, involves a precise neutralization process with strong acids, such as hydrochloric acid (HCl), and extensive washes with water to remove the resulting sodium chloride (NaCl) salt. Also, the glycerol must be separated from the sodium chloride salt by vacuum distillation in an energy intensive operation for this high-boiling product. Glycerol is used primarily in foods, oral care, tobacco and urethane foams. As more alkyl soyates with different alkyl functional groups, such as ethyl and isopropyl soyates, being rapidly developed to meet the growing needs of various applications, the level of difficulty in separating the corresponding catalysts, e.g., sodium ethoxide and sodium isoproxide catalysts, respectively, would unavoidably escalate due to the increasing solubility of these basic catalysts in the product solution. Clearly, the successful development of a more effective and economical method to separate the base catalysts from the products is of keen interest to further advance the techniques of biodiesel production.

In addition to the separation issue, the current base-catalyzed transesterification methods also have several other limitations. As producers look for feedstocks other than soybean oil, such as waste restaurant oils and rendered animal fats, they have observed a larger amount of free fatty acids (FFAs) contained in the feedstocks. FFAs can not be converted into biodiesel using the current base-mediated processes.

As depicted in figure 1, we covalently attached a series of transesterification catalysts onto the pore surface of an organically functionalized mesoporous silica nanosphere material. As outlined below, this structurally well-defined mesoporous silica material provides several advantages to the current homogeneous catalysts.

These uniformly shaped silica particles (particle size = 250 nm in diameter) consist of arrays of hexagonal porous channels with tunable pore diameters ranging from 2 to 10 nm (Figure 1 shows the SEM (a) and TEM (b) images of a typical mesoporous silica nanosphere). In contrast to the polydispersive amorphous sol-gel silicas, the uniform particle morphology and large pore sizes of these nanospheres will allow fast diffusion of soybean oil molecules to the catalytic sites situated inside the porous channel and thereby enhance the rate of product conversion and release. In addition, the heterogeneous nature of this catalytic system provides several other advantages, such as easy separation and high reusability.

The large accessible surface area of the mesoporous silica nanospheres (> 600 m²/g, which is at least 2 or 3 times larger than the surface area per gram of most polymer and sol-gel silica materials) provides an advantageous platform for high catalyst loading to improve the efficiency of the transesterification. Also, the hydrophobic organic functional groups render additional incentive (i.e., a hydrophobic solvation effect) for the hydrophobic reactant (soybean oil) to enter into the porous channels lined with the attached catalyst.

(a)

 

(b)

(c)

Figure 1. (a) A scanning electron microscopy (SEM) image of the mesoporous silica nanosphere. (b) A transmission electron microscopy (TEM) image of the cross-section of a mesoporous silica nanosphere showing the highly regular hexagonal lattice units of the porous framework. (c) Schematic representation of the catalytic system for methyl soyate and glycerol formation.

(2) Multi-functionalized Nanoporous Silica Catalyst for Chemical- and Regio-Controlled Polymerization

Victor S.-Y. Lin, Iowa State University Department of  Chemistry
Marek Pruski, Ames Laboratory
Aaron Sadow, Iowa State University Department of  Chemistry

Properly functionalized and structurally ordered nanoporous materials, such as MCM-type silicas and aluminas, with tunable pore size and narrow pore-size distributions have attracted much attention for their potential applications as new catalytic scaffolds to direct and orient the conformation of desired products. For example, recent interest in encapsulation and/or synthesis of conducting polymers within the channels of MCM-41 has highlighted the future application of these materials as directional electron/energy transfer nanodevices. However, several fundamental obstacles, such as low content and lack of the structural alignment of these polymer materials within the porous hosts, have yet to be overcome.

We recently synthesized a Cu²⁺-functionalized MCM-41 nanoporous silica material, with different pore diameters, which can oxidatively catalyze the formation of a highly conjugated poly(phenylene butadiynylene) polymer (PPB)² as depicted in Figure 2. Fluorescence and ¹³C solid state NMR provided spectroscopic evidence that the PPB polymer synthesized by the this mesoporous nanocatalyst exhibited an unprecedented high degree of alignment and electronic/photophysical properties compared with those of the PPB polymers synthesized with Cu²⁺ catalysts in homogeneous solutions.

Figure 2. Schematic representation of Cu²⁺-functionalized nanoporous silica catalyst (Cu-MCM) for oxidative polymerization of 1,4-diethynylbenzene into conjugated poly(phenylene butadiynylene) (PPB).

Other catalytic functional groups have been incorporated into the nanoporous silica matrix for other conformationally controlled polymerizations, such as stereochemically controlled polymerization of poly-lactic acid (PLA). As depicted in Figure 3, these nanoporous catalysts will serve as a new generation of nano-extruders for production of various polymer materials based on bio-renewable feedstocks.

Figure 3. Schematic representation of nanoporous catalyst system functionalized with stereochemical selective groups (selectors) and catalytic groups for sterechemically controlled polymerization of biorenewable feedstocks.