60th Annual Report on Research 2015

The project aims to develop a synthetic approach to tune the band gap of the metal-organic frameworks (MOFs) for catalytic organic transformation reactions. The specific tasks laid out in the project are:

(1) Synthesize a series of MOFs with different side groups substituted on the organic linker via a one-step method or a post-synthesis modification process.

(2) Identify the correlation of the optical band-gap and electronic structure of the organic framework with the substituted side groups.

(3) Investigate the photocatalytic activity of MOFs toward the aerobic organic transformation.

2. Significant Results

Both our previous experimental and theoretical results have proved that the side group functionalization could significantly tune the optical band gap of BDC-Zr-MOF. To further understand the influences of varying side groups on the electronic structure of MOF materials, a density functional theory approach coupled with Boltzmann transport equation within the relaxation time approximation was employed in our recent study to investigate the charge mobility and density for MOFs with different functional side groups, as shown in Table 1. From the elastic constant calculation, functionalization does not significantly increase the stiffness of MOF materials. It was confirmed that the delocalized π electrons in the aromatic ring of the linker account for band-like conduction along the organic linker. Side group substitution could lead to the increased deformation potential and thus enhance the electron-phonon scattering, which inversely decreases the charge mobility. NO₂ group function results in the largest deformation, and thus the lowest mobility of charge carriers in MOF. BDC-MOF has the largest mobility but not significantly larger than other designs. [Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP03920G]

The most significant finding in our theoretical study is that the functionalization actually increases the number of states near the LUMO by tailoring the spectral layout of low lying unoccupied orbitals. Figure 1 gives a visualization of the DOS as a function of strain positions for MOF with different side functional groups. The lines outline the HOMO, LUMO and Fermi positions, respectively. It is interesting that the energy levels converge as the crystal is compressed. The data provided in this figure were used to determine the density of states. Results illustrate that functionalization accounted for the increased charge density, and highlighted the most advantageous avenue for increasing the conductivity in MOF based materials. It was concluded that in order to design MOF based materials with conductivity values on the order of moderately doped semiconductors the transport must be tunneling based along short chains with functionalization that contributes to high carrier concentrations. This prediction could prove important for increasing the overall conductivity of MOFs. [Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP03920G]

The photocatalytic activity of MOF with different side groups was experimentally tested on the organic transformation reaction, as shown in Figure 2. Benzyl alcohol was used as the substrate, and DMF as the solvent. Cool white light lamps (>400 nm) were used as the visible light source. In the dark, NH₂-MOF does not show the activity to drive the organic transformation reaction. Upon the visible light irradiation (>400nm, 2c), NH₂-MOF demonstrates the best performance for the catalytic reaction, which indicates that the catalytic activity stems from the light excitation. Negligible product was detected when using H-MOF. NO₂-MOF demonstrates the moderate reaction conversion efficiency. This trend is comparable to the light absorption capability as shown in Figure 2c, which satisfies our proposed functionalization design for more light harvesting. In the reaction, all Zr-MOF materials proved the 100% selectivity with the sole product benzaldehyde.

Au was also incorporated in MOF porous matrix. Combining the HRTEM image (Figure 2e) and XPS spectrum (Figure 2f), ultra-small Au clusters were uniformly confined in the micropores of MOF materials. The incorporation of Au clusters in MOF help the increase of photocatalytic activity for organic transformation under visible light irradiation (Figure 2d). This primary illustration bestows MOF materials as attractive substrate for guest catalysts with more options in catalysis.

Briefly, our results indicate that side group functionalization could vary the band gap of MOF materials, and thus extend the light absorption into the visible light range by the proper substitution, which benefits the photocatalytic activity of MOF materials in an extended light spectrum. Furthermore, the theoretical calculation predicts the design rules for MOF functionalization to obtain high carrier concentration and overall conductivity of MOF materials, which is of critical importance for photocatalytic applications.

3. Impacts of Research

The ACS Petroleum Research Funds have helped the PI with exploring MOF catalysts, an exciting new area. The research will lay the foundation for my future success in this emerging area. The outcome of project promotes the development of green chemistry techniques to reduce the ecological impact of chemical processes. In addition, the interdisciplinary nature of this project drives students to get trained in materials science, electrochemistry and organic chemistry. The research has stimulated students’ interests in chemistry, engineering and physical science.

Table 1. Predicted elastic constant, deformation potentials, and charge mobility and density for three MOF designs at 300K. [Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP03920G]

Figure 1. Density of states plots for the three MOF designs (a, H-BDC; b, NH₂-BDC; c, NO₂-BDC) as a function of strain values. There is a linear slope for both the HOMO and LUMO across the range of strain for all three MOF designs. [Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP03920G]

Figure 2. Photocatalytic organic transformation with MOF. (a) The proposed organic transformation reaction from Benzyl alcohol to benzaldehyde; (b) the schematic illustration for the testing cell; (c) UV-VIS absorption spectra for MOFs and the light source spectrum range; (d) photocatalytic organic transformation performance; (e) HRTEM image for Au-MOF, and (f) XPS spectrum for Au⁰ in MOF.