Metal-Organic Frameworks as Potential Platforms for Carbon Dioxide Capture and Chemical Transformation

Author

Wenyang Gao, University of South FloridaFollow

Graduation Year

2016

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemistry

Major Professor

Shengqian Ma, Ph.D.

Committee Member

Jianfeng Cai, Ph.D.

Committee Member

John N. Kuhn, Ph.D.

Committee Member

Julie P. Harmon, Ph.D.

Keywords

Microporous Materials, Gas Separation, Heterogeneous Catalysis, CO2 Utilization

Abstract

The anthropogenic carbon dioxide (CO₂) emission into the atmosphere, mainly through the combustion of fossil fuels, has resulted in a balance disturbance of the carbon cycle. Overwhelming scientific evidence proves that the escalating level of atmospheric CO₂ is deemed as the main culprit for global warming and climate change. It is thus imperative to develop viable CO₂ capture and sequestration (CCS) technologies to reduce CO₂ emissions, which is also essential to avoid the potential devastating effects in future. The drawbacks of energy-cost, corrosion and inefficiency for amine-based wet-scrubbing systems which are currently used in industry, have prompted the exploration of alternative approaches for CCS. Extensive efforts have been dedicated to the development of functional porous materials, such as activated carbons, zeolites, porous organic polymers, and metal-organic frameworks (MOFs) to capture CO₂. However, these adsorbents are limited by either poor selectivity for CO₂ separation from gas mixtures or low CO₂ adsorption capacity. Therefore, it is still highly demanding to design next-generation adsorbent materials fulfilling the requirements of high CO₂ selectivity and enough CO₂ capacity, as well as high water/moisture stability under practical conditions.

Metal-organic frameworks (MOFs) have been positioned at the forefront of this area as a promising type of candidate amongst various porous materials. This is triggered by the modularity and functionality of pore size, pore walls and inner surface of MOFs by use of crystal engineering approaches. In this work, several effective strategies, such as incorporating 1,2,3-triazole groups as moderate Lewis base centers into MOFs and employing flexible azamacrocycle-based ligands to build MOFs, demonstrate to be promising ways to enhance CO₂ uptake capacity and CO₂ separation ability of porous MOFs. It is revealed through in-depth studies on counter-intuitive experimental observations that the local electric field favours more than the richness of exposed nitrogen atoms for the interactions between MOFs and CO₂ molecules, which provides a new perspective for future design of new MOFs and other types of porous materials for CO2 capture. Meanwhile, to address the water/moisture stability issue of MOFs, remote stabilization of copper paddlewheel clusters is achieved by strengthening the bonding between organic ligands and triangular inorganic copper trimers, which in turn enhances the stability of the whole MOF network and provides a better understanding of the mechanism promoting prospective suitable MOFs with enhanced water stability.

In contrast with CO₂ capture by sorbent materials, the chemical transformation of the captured CO₂ into value-added products represents an alternative which is attractive and sustainable, and has been of escalating interest. The nanospace within MOFs not only provides the inner porosity for CO₂ capture, but also engenders accessible room for substrate molecules for catalytic purpose. It is demonstrated that high catalytic efficiency for chemical fixation of CO₂ into cyclic carbonates under ambient conditions is achieved on MOF-based nanoreactors featuring a high-density of well-oriented Lewis active sites. Furthermore, described for the first time is that CO₂ can be successfully inserted into aryl C-H bonds of a MOF to generate carboxylate groups. This proof-of-concept study contributes a different perspective to the current landscape of CO₂ capture and transformation. In closing, the overarching goal of this work is not only to seek efficient MOF adsorbents for CO₂ capture, but also to present a new yet attractive scenario of CO₂ utilization on MOF platforms.

Scholar Commons Citation

Gao, Wenyang, "Metal-Organic Frameworks as Potential Platforms for Carbon Dioxide Capture and Chemical Transformation" (2016). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/6503