47905-G10 Biomimetic Synthesis of Heterogeneous Catalysts
This project has enhanced the development of bio-inspired nanocatalysts that are functional under non-traditional conditions of an aqueous medium at room temperature for C-coupling reactions. Three peptides were proposed to prepare Au, Pd, and SiO2 materials in a composite structure; however, proof of their individual nanoparticle production activity, as well as their cross-reactivity, was required prior to composite fabrication. By using the Pd4 peptide (sequence: TSNAVHPTLRHL), isolated via phage display techniques in collaboration with Dr. Rajesh R. Naik of the Air Force Research Laboratory, we were able to prepare Pd nanoparticles of 1.9 ± 0.3 nm in diameter. The particles were synthesized in the following manner.
First, the peptide was dissolved in solution to which 3.3 equivalents of K2PdCl4 were added. The solution was quickly agitated and then allowed to stand for 30 min to ensure complexation of the Pd2+ to the peptide, which is anticipated to occur through the numerous amines of the sequence. Such events were monitored by UV-vis spectroscopy, which suggested metal-ion/peptide binding via development of a ligand to metal charge transfer band at 217 nm.
Second, the materials were fully reduced employing a 10-fold excess of NaBH4 to induce the nucleation and growth of the Pd nanoparticles. After reduction, a broad absorbance with increasing intensities towards lower wavelengths was observed, consistent with Pd nanoparticles, which was confirmed using transmission electron microscopy. The nanoparticles are produced with the peptides bound to the surface through the two histidines at the 6 and 11 positions. Theoretical modeling suggests that they bind and orient themselves with respect to the surface and other bound sequences to form a kink-like peptide structure. As a result of this surface structure, a significant portion of the metallic Pd nanoparticle surface may be revealed to solution to maximize interactions with reagents. This exposure is required to result in high catalytic reaction rates.
To explore this theory, homogeneous C-coupling reactions were initially studied. Such reactions were anticipated to ascertain 1) the feasibility of catalytic reactivity from these materials, 2) basic conditions for their catalytic use, and 3) a base of comparison for future homo- and heterogeneous reaction designs employing environmentally and energy friendly conditions.
Initial studies were completed by coupling 4-iodobenzoic acid with phenyltin trichloride to produce biphenylcarboxylic acid as a model system. Since the materials are produced using bio-based methods, biologically and environmentally friendly conditions of a water-based solvent at room temperature were processed. Initially, 0.50 mol% Pd was used to catalyze the reaction, which resulted in a 100% product yield in 24 h; however, a full analysis of the catalytic loading was subsequently conducted. Since Pd is a precious metal, high reaction yields using low catalyst loadings are desirable to minimize both the absolute amount of Pd required and any potential loss that may occur.
From this analysis, quantitative reaction yields were achieved at loadings of ≥0.005 mol% Pd. Note that this value represents the total Pd concentration in solution and not the concentration of Pd nanoparticles. Beyond the loading analysis, determination of the catalytic turnover frequency (TOF), which is a measure of the rate of the reaction, was conducted that indicated a value of 3207 ± 269 mol product(mol Pd x h)^-1. This TOF value, which was studied at a 0.05 mol% Pd loading, is in line with the high product yields at low concentrations. Together, the low concentrations of catalyst in the reaction mixture with high TOF values, both of which are significantly enhanced as compared to many materials typically employed for Stille coupling, are likely the result of the peptide surface structure.
Additionally, the selected reaction conditions suggest that the materials can be employed as model catalytic systems to determine optimal structure-function relationships for environmentally and energy friendly nanocatalysts. The nanoparticles were also determined to be reactive across a variety of substrates including iodo and bromo compounds for the aryl halide, acidic and alcohol functionalities, and with both aryl and vinyl tin species to control the final product.
Future research is now focused in two specific areas: 1) identifying the effects of the peptide surface for controlling the reactivity of the materials via rational design and 2) developing schemes to produce heterogeneous-based catalytic materials.
Funding from the ACS-PRF has been instrumental in the research gains made by the group. These research results have recently been published in ACS Nano, with further studies presently under review elsewhere. The funds have provided for the continuing research activities of three graduate students, which will progress into the new academic year. It is anticipated that multiple research directions will be generated from these seed funds.