Current Research Projects:
Simple Methodologies to Control the Spacing of Functional Groups on Surfaces - An Isolated Aminosilica Scaffold
Porous oxides such as silica are routinely surface-functionalized with organic groups to impart the solid materials with specific catalytic adsorption or other properties. The most common methodology to synthesize organic-functionalized silica materials is via the reaction of organosilanes with the surface hydroxyl groups found on most metal oxides. This method is versatile and it can be used to create myriad different types of organic species on the surface (e.g. amines, thiols, aromatics, phosphines, etc.). However, while it is easy to add specific functionalities onto the surface, it is very difficult to control the relative spacing of the functional groups on the surface and quite often, the organic groups can interact strongly with the oxide surface (e.g. basic amines or phosphines complexing with acidic hydroxyls on silica surfaces). To this end, we have developed the use of simple "spacing" or "patterning" techniques to allow the engineering of the spacing of functional groups on the surface as well as prevent unintended surface-organic interactions. Utilizing a protection/deprotection approach, we have demonstrated the ability to control the spacing of aminopropyl groups on porous silica surfaces (Pubs 11, 39) and we have shown the functional groups are uniformly reactive in subsequent reaction chemistries. We suggest the resulting aminosilica material presenting spatially positioned, uniformly reactive amine sites is an ideal material to study the mechanism of amine-based adsorption processes and for use as a host for single-site metal complex catalysts.
Catalytic Chemistry for Fine Chemical and Pharmaceutical Synthesis
Immobilized metal complexes have the potential to combine the best attributes of both homogeneous (highly active and selective) and heterogeneous (easy separation from the reaction media) catalysts. Despite this possibility, there have been few large scale industrial applications of immobilized molecular catalysts where catalyst recovery is desired. Due to the fact that the field lies at the historical interface of homogeneous and heterogeneous catalysis, the science of catalysis by immobilized organometallics has never been developed, as there have been few long term, detailed investigations of a single catalytic system. Thus, in many ways, this field of inquiry remains a highly empirical one, even after over 25 years of study. We hypothesize that this is due to the interdisciplinary nature of this subject area, where expertise in experimental materials chemistry, inorganic and organometallic chemistry needs to be complemented by modern molecular modeling and electronic structure calculations. To this end we have established a collaborative GT-UVA Focused Program in Catalysis by Immobilized Organometallics to undertake a thorough, multi-faceted investigation into these important catalytic systems. In this Department of Energy sponsored program, we are working in conjunction with the Weck and Sherrill Groups in Chemistry and Biochemistry at Georgia Tech, the Ludovice Group in Chemical and Biomolecular Engineering at Georgia Tech, and the Davis Group in Chemical Engineering at UVA. The focus of our investigations have included Pd-Pincer complexes for Heck catalysis and Mn, Co, Cr-Salen catalysts for oxidations or kinetic resolutions of epoxides. In our evaluation of immobilized Pd(II) pincer complexes, we discovered that, in contrast to numerous literature reports, these complexes are in fact precatalysts that liberate soluble, ligand-free active palladium species into solution (Pubs 19,24,31,39). Our work on salen catalysts has focused on the development of stable, recyclable polymer-supported Co-Salen complexes for the hydrolytic kinetic resolution of epoxides. Several new polymer-supported catalysts have been developed in work with Weck, with the best two systems offering reaction rates that greatly exceed those obtained with the homogeneous complex with equivalent selectivities (>99% ee at 50-55% conversion of epoxide). Salen complexes for epoxide ring-opening reactions are ideal targets for immobilization on relatively flexible supports, as the reaction transition state requires the participation of two salen complexes and the organization of two complexes can be facilitated by immobilization (Pub 34).
Recently, we introduced a new strategy for controlling multi-step catalytic reactions in a single vessel. Over the last several decades, with and eye towards nature’s methodology for chemical synthesis, researchers have started to develop the ability to carry out multi-step, sequential reactions in a single vessel. In many cases, these have been uncatalyzed reactions or reactions that require a single catalyst to “kick off” the first step, after which the following steps proceed rapidly. For example, in many “domino” reactions, the substrate is specifically designed so that after the first reaction is initiated, all the subsequent steps occur rapidly to give the final product. In other cases, combinations of catalysts have been used to carry out multiple sequential reactions in a single vessel. However, all previous work has reported a single system of catalysts and/or conditions that have been optimized for a particular reaction sequence. Thus, after the reaction, the catalysts inevitably end up as part of the reaction waste. This represents a very limited level of control over the reaction sequence and as a result, such catalytic sequences are of little general utility. In contrast, biological systems effectively control numerous simultaneous catalytic reactions by using the compartmentation of enzymes in different parts of the cell. In doing this, the cells are able to carry out multiple sets of catalytic sequences simultaneously and a single enzyme can participate in several different reaction cascades. This ability to use a single catalyst in many different reaction sequences has not been achieved with chemical systems. Recently, we demonstrated this for the first time by combining (i) the compartmentation concept used in biology, adopted as the sequestration of opposing catalytic sites on different solids with (ii) the ability to recover each individual catalyst after the reaction, achieved by combining magnetically separable solid catalysts with traditional gravimetrically recoverable solid catalysts. With this combination of catalysts, we were able to carry out four different catalytic sequences all using the same magnetically separable base catalysts in combination with various gravimetrically recovered catalysts. In all cases, after reaction, the individual catalysts were recovered and could be reused, in contrast to all previous reports of sequential catalytic reactions. This new approach utilizing magnetic nanoparticle catalysts represents a very versatile methodology for carrying out multiple catalytic reactions in a single pot under practical conditions. Thus, a library of magnetically and gravimetrically recoverable catalysts could be built and used in a variety of one-pot multi-step catalytic reactions giving the catalytic chemist and unprecedented level of control over multi-step, one-pot catalytic reactions. Carrying out such multi-step, one-pot reactions has the potential to eliminate separation steps in multi-step organic syntheses, substantially decreasing the cost of synthesis (Pub 36).
Polyolefins via Coordination-Insertion Polymerization
We are interested in the design and understanding of novel, solid catalytic materials for the controlled polymerization of a variety of monomers. In the area of catalytic olefin polymerization (ethylene, propylene, etc.), solid-supported transition metal complexes have been the dominant type of catalyst used in industry for a number of years. Despite this dominance, a fundamental understanding of catalyst structure and polymerization mechanism are still not well-established in most cases. In the Jones group, we are developing new synthetic protocols that allow us to create novel solid polymerization catalysts via the immobilization of single-site, homogeneous catalysts based on both early (Ti, Zr, etc.) and late (Fe, Ni, Cr etc.) transition metals on the solid surface. Because the nature of both the organometallic complex and solid support have a critical influence on polymerization properties, our research places an equal emphasis on the both aspects of the catalyst design. Additionally, we are interested in the development of novel, MAO-free activators for single-site olefin polymerizations, as methylalumoxane (MAO) represents a sizeable fraction of the catalyst cost when using single-site polymerization technologies.
Site-Isolated Molecular Catalysts on Silica Supports
Many immobilized molecular catalysts are prepared via a multi-step grafting approach, whereby the intended complex is assembled on the solid support in a manner similar to the way it is assembled in homogeneous solution. Unfortunately, most often, an array of different types of species are created on the solid surface, perhaps including the intended structure. The presence of multiple types of sites is partly a consequence of steric constraints on the surface that prevent complete construction of the intended structures. To this end, we have developed a simple molecular patterning/spacing protocol that can be used to create site-isolated molecular catalysts on the silica surface, yielding more well-defined, more active immobilized molecular catalysts. The utility of this scaffold in the creation of supported metal complex catalysts that have single-site characteristics was demonstrated in the creation of well-defined CGC-Inspired Ti and Zr ethylene polymerization catalysts. Utilization of the isolated aminosilica scaffold allows for creation of supported ethylene polymerization catalysts with 10-fold improved catalytic productivities (Pubs 17,20-21,30) and in some cases, new copolymerization activities (Pub 32).
Polyolefins via Controlled-Radical Polymerization
Atom-transfer radical polymerization (ATRP) has rapidly become an important polymerization method since its discovery in 1995. ATRP utilizes a metal complex that acts as a reversible chain termination agent to drastically reduce the concentration of radicals in a polymerization, allowing for controlled monomer addition and significantly decreased termination. A key technical hurdle to widely implementing this technology on a commercial scale remains the removal of the metal complex from the product polymer after reaction. We have designed novel silica-supported CuBr complexes for ATRP and are probing the variables that impact the ability of the immobilized complexes to effectively regulate the polymerization while still allowing for efficient catalyst recovery (Pubs 13-15, 29). Additionally, we are using ATRP to design well-defined, polymer-supported metal complex catalysts for enantioselective catalysis. For example, well-defined poly(styrene)-supported Co(III)-Salen complexes are being examined.
In collaboration with Professor Joseph Schork, we are exploring the potential of novel chain transfer agents to promote the controlled radical polymerization of vinylic monomers (styrenes, methacrylates, vinyl acetate, etc.) in miniemulsions. Systems under investigation include both oil-based and water-based reversible addition fragmentation chain transfer (RAFT) polymerizations.
Continuous Polymerization Processes
Also in collaboration with Schork, we have developed unique, continuous processes for controlled radical polymerizations that utilize single tubular (Pubs 18, 26, 39) or stirred tank reactors Pubs 22, 25). In addition, by combining multiple reactors in series, we are able to tailor the structure of the polymers produced. For example, novel block copolymers that can not be made in batch processes (due to composition drift) have been prepared (Pub 22).
Environmentally Benign Polymers and Recovery/Recycle of Oligomerization Catalysts
An active area of our research is the synthesis of biodegradable polymers and polymers derived from renewable resources. In particular, we are investigating novel, non-toxic, recoverable solid catalysts for the controlled polymerization of heteroatom-containing monomers such as lactones, lactide and the copolymerizations of epoxides and carbon dioxide to produce polyesters and polycarbonates, respectively (Pubs 12, 16, 23). A major focus is on the design of well-defined, solid catalysts to replace or augment current homogeneous catalyst technology.
A particular fundamental question that we are interested in is: can oligomerization or polymerization catalysts be efficiently recovered and recycled? Our initial studies of silica-supported ATRP catalysts (and the work of others) indicate that catalysts of this type can be effectively recovered and recycled (Pubs 13-15, 29). However, this type of polymerization represents a special case where the growing polymer chain is never bond to the metal catalyst. Thus, no strong interactions between the metal and the polymer need to be broken to achieve catalyst separation from the product polymer. Currently, we are examining more typical oligomerization catalysts that utilize a coordination-insertion mechanism. In this case, we hope to learn what parameters most affect the stability and recyclability of supported catalysts.
Catalytic Conversion of Biomass and Utilization of Chemicals Derived from Biomass
Working with the Institute of Paper Science and Technology (IPST) at Georgia Tech, we are studying the conversion of woody biomass as a feedstock for chemicals and energy. Biomass, and especially woody biomass, represents a vast, under-utilized, domestic, renewable feedstock for the production of chemicals and fuels. Furthermore, unlike ethanol production from food crops such as corn, the use lignocellulosics for such applications does not place chemical and energy production in a direct competition with food production. Working with Professor, Pradeep Agrawal, we have embarked on a program investigating the one-pot conversion of wood via aqueous phase reforming (APR). Woody biomass is first partially solublized via treatment with sulfuric acid in water and in the same pot, the solublized holocelluloses are converted to hydrogen and CO2 using a reforming catalyst such as Pt/Al2O3. Essentially all of the holocellulose is solublized, leaving an acid-insoluble, lignin-rich solid residue. We are currently developing applications for the lignin fraction and evaluating the utility of our combined solublization/APR approach for the conversion of other feeds such as waste paper, glucose and glycerol.
Zeolites and Porous Silica Materials
The Jones group is involved in the engineering of porous silica for applications in catalysis and separations. Silica is our support of choice for catalyst immobilization as its particle size, porosity and surface chemistry can be easily manipulated to produce well-defined, desired structures. Furthermore, silica is environmentally benign, inexpensive and chemically inert in most media, making it an ideal support material. In many cases, well-defined hexagonal or cubic mesoporous silicas such as MCM-41, MCM-48 or SBA-15 are synthesized and utilized as ideal model hosts for catalytic and adsorption applications.
We are also working in the area of zeolite synthesis and surface modifications. The Jones group has the capacity to prepare virtually any known molecular sieve. Working with Professors Bill Koros and Sankar Nair, we are interested in the synthesis, functionalization and optimization of surface-modified zeolites for mixed-membrane applications.
Inorganic-Organic Hybrid Materials for Separations
Separation processes consume an enormous amount of the energy used in chemical processing. We are interested in the development of new, low-energy separation processes based on functionalized oxide materials. Two such activities are described here.
There is growing political pressure world-wide to cut down on CO2 emissions in an attempt to slow the rate of global warming. Much of the CO2 emitted comes from gas or coal-fired power plants. The huge scale on which CO2 is produced at these sites and the fact that the gases are emitted at single point sources (as opposed to at millions of points spread across the country as in the case of automobiles) makes these streams potentially viable targets for CO2 capture strategies. A common, traditional CO2 capture strategy is to use aqueous solutions of secondary amines for the absorptive separation of CO2 from methane, for example. After absorption, the amines are regenerated by CO2 removal induced by a temperature swing. However, the vast amount of energy to heat an aqueous solution in a temperature swing process makes this technology too expensive for flue gas applications. An alternative that has attracted attention in recent years is adsorptive separations using amine-functionalized oxides or polymers in a fixed bed or other configuration. However, very little is known about the mechanism of CO2 adsorption on these solids and as a consequence, the rational design of better adsorbents is not straightforward. We are working with the National Energy Technology Laboratory (NETL) in Pittsburgh to evaluate our family of differently "spaced" aminosilica materials in fundamental studies of the CO2 adsorption mechanism. In particular, we hope to elucidate the role of single primary amines vs. multi-amine clusters in the capture of gas phase CO2 in the presence of water vapor. In parallel, we are working with the NETL and Prof. Eric Beckman of the University of Pittsburgh to address an additional problem with amine-functionalized solid CO2 adsorbents - the typically low CO2 capacities that these materials display. New adsorbent materials with high densities of amine sites are being prepared, and initial results from our laboratories indicate that we can increase the amine capacity of the solids almost by a factor of 10 using new synthesis methodologies. Ultimately, we hope to generate both new practical adsorbent materials with high CO2 capacities and good stability and a mechanistic understanding of the adsorption process.
Working with Professors Koros and Nair, we are developing mixed-matrix membranes for gas separations. Membrane separation processes represent attractive, low energy alternative to traditional separation methodologies such as distillation. Inorganic thin film membranes based on zeolites have been developed for a number of years. Using the molecular discrimination capabilities of the crystalline, microporous zeolites, excellent separations can be achieved with in some cases sub-angstrom level discrimination between permeants. However, pure zeolite membranes are incredibly expensive to prepare and this has inhibited widespread use of these separation devices to date. In contrast, polymer membranes can be cheaply processed into large surface area hollow fiber modules, making them a relatively low cost alternative. However, the range of separations that can be achieved with pure polymer membranes is somewhat limited. Hence, recent research has focused on combining zeolite molecular sieves with processable polymer matrices to ideally combine the best attributes of polymer (processability) and inorganic (great molecular discrimination) into a single membrane. The Koros group has years of experience in working with these types of membranes, but an 'Achilles heal' of the technology has been achieving a good compatibilization between the inorganic sieve and organic polymer. In many cases, a phase separation occurs at the interface. We are combining our expertise in zeolites with our background in organic modification of oxide surfaces to attack the zeolite-polymer interface problem.