Synthesis and Characterization of Microporous Coordination Polymers as Adsorbents for CO2 Capture.

Abstract

The accumulation of CO2 in the Earth’s atmosphere is an important environmental issue, and coal-fired power plants are significant contributors to anthropogenic CO2 emissions. The capture of CO2 from these facilities via amine-scrubbing has been well-investigated, but deemed to be too costly for widespread deployment. An adsorption-based process that relies on microporous coordination polymers (MCPs) may offer an efficient route to CO2 capture. In particular, MCPs of the M/DOBDC series (DOBDC = 2,5-dioxido-1,4-benzenedicarboxylate; M = Zn, Ni, Co, and Mg) have previously been shown to exhibit exceptional CO2 uptake at low partial pressures under static conditions; the best performer, Mg/DOBDC, exhibits a CO2 uptake of 23 wt% at 0.1 atm and 25 °C, superior to the 12 wt% uptake of zeolite 13X, a benchmark candidate for CO2 capture, under similar conditions.

In this dissertation, the performance of the M/DOBDC materials was examined under flow conditions with mixtures of N2/CO2 and N2/CO2/H2O as surrogates for flue gas; after hydration and subsequent thermal regeneration Co/DOBDC retained 85% of the pristine CO2 capacity while Mg/DOBDC, retained only 15% of the pristine capacity. These data indicated that improving material stability in the presence of H2O was a critical issue to confront in designing new MCPs for CO2-capture, and two approaches were explored to confront this problem.

The first approach entailed enhancing the CO2 uptake of a H2O-stable MCP, MIL-100(Al), via post-synthetic modification with proximal amines, namely those in tris(2-aminoethyl)amine (tren). This yielded a new MCP, tren@MIL-100(Al), which exhibited an initial CO2 capacity of 8.4 wt% under dry conditions; the capacity diminished after several cycles. When fully hydrated, tren@MIL-100(Al) had a CO2 capacity of 3.7 wt%, an improvement over M/DOBDC materials, which displayed no capacity under this condition.

A second approach involved the synthesis of an MCP based on the acetylacetonylate coordination functionality, which is known to generate H2O-stable metal complexes. Simple geometric design principles were executed, wherein a ditopic linker bearing acetylacetonylate units was combined with Al3+ to yield a new MCP, Al/BAB. The material represents successful deployment of acetylacetonylate as a coordination functionality in MCPs, thus opening a new subset of these materials.