Metal-organic Framework Hybrid and Structured Adsorbents for Carbon Capture

Abstract

Metal-organic frameworks (MOFs) are promising adsorbents for CO2 capture due to their high specific surface area, porosity, chemical versatility, and ease of structural modification. However, MOFs face drawbacks, including sensitivity to water vapor, and challenges in handling and processing, which hinder their industrial potential. Hybridizing MOFs with other materials enhances their performance properties and scalability. Additionally, structuring microcrystalline MOF powders into millimeter-sized forms while preserving porosity and functionality represents an effective yet challenging approach to improving the processing and handling. This thesis aims to address these limitations through hybridization and structuring strategies, contributing to the practical application of MOFs for CO2 capture.

In the first part of this study, a water-stable MOF (MIL-101(Cr)) was hybridized with graphene oxide (GO) yielding a hybrid adsorbent with enhanced CO2 capture performance, and the hybrid was then structured into mechanically robust spherical poly(acrylonitrile)-based beads of millimeter size exhibiting hierarchical porosity using a novel, simple, and scalable strategy. The powdered hybrid adsorbent with an optimum GO loading of 6 wt.% exhibited a 55% higher CO2 adsorption capacity and a 48% higher CO2/N2 selectivity than those of the parent MOF at 298 K and 1 bar, while the structured analogue provided high dispersion of the MOF@GO powder and preserved the CO2 adsorption performance and porosity characteristics of the original MOF@GO.

The second part of this study involves structuring moisture-stable and scalable microcrystalline MOFs (UiO-66 and ZIF-8) into readily processable, millimeter-sized hierarchically porous structured adsorbents with ultrahigh MOF loadings (∼90 wt %). These structured composite beads retained the physicochemical properties and separation performance of the pristine MOF crystal particles. Structured UiO-66 and ZIF-8 exhibited high specific surface areas of 1130 m2 g–1 and 1431 m2 g–1, respectively. The structured UiO-66 achieved a CO2 adsorption capacity of 2.0 mmol g–1 at 1 bar and a dynamic CO2/N2 selectivity of 17 for a CO2/N2 gas mixture with a 15/85 volume ratio at 298 K. Furthermore, the structured adsorbents exhibited excellent cyclability in static and dynamic CO2 adsorption studies, making them promising candidates for practical application.

In the third part of the study, MOF@MOF core-shell hybrid adsorbents were developed by coating a hydrophilic HKUST-1 core, known for its high CO2 uptake capacity, with a superhydrophobic ZIF-8 shell through a facile in-situ growth method. The resulting core-shell hybrid adsorbent exhibited low moisture affinity, achieving up to a 70% reduction in water vapor adsorption capacity compared to pure HKUST-1. It also demonstrated a CO2/N2 selectivity of 41.4 for a binary gas mixture containing 15 vol.% CO2 and 85 vol.% N2 at 1 bar and 298 K, which is 73% higher than that of HKUST-1 and 211% higher than that of ZIF-8. The reduced water vapor affinity and excellent CO2 capture performance of the core-shell hybrid adsorbents, combined with their cyclability, underscore their potential for practical CO2 capture applications.

The final part of this study evaluates and compares the performance of the structured adsorbents for commercial-scale CO2 capture applications using a dynamic simulation of a cyclic vacuum/pressure swing adsorption (V/PSA) process aimed at capturing and compressing CO2 from the flue gas of a coal-fired power plant. Parametric sensitivity analyses were conducted, and the V/PSA process was optimized to maximize CO2 purity and recovery while minimizing costs. Structured UiO-66 achieves a cost of $72.50 per ton of CO2 captured with 95% purity and 88.8% recovery, decreasing to $60.60 per ton with 80% recovery. Structured MIL-101@GO costs $75.00 per ton for 96.4% purity and 88.6% recovery, while structured ZIF-8 costs $80.20 per ton with 91.2% purity and 74.4% recovery. This study highlights the importance of using cost as a primary evaluation metric for structured adsorbents, integrating data from these adsorbents into system-level modelling, and optimizing cyclic adsorption processes with detailed equipment sizing and column scheduling to assess their feasibility for commercial-scale CO2 capture.

Date of Award	26 Nov 2024
Original language	American English
Supervisor	Ludo Dumee (Supervisor)