Porphyrin-Based Metal-Organic Frameworks as Adsorbents for Ambient Temperature Nitrogen Dioxide Removal


Student thesis: Doctoral Thesis

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Award date15 Jun 2021


Nitrogen oxides (NOx) are major air pollutants that cause huge damage to the environment and people’s health. The immoderate atmospheric emission of NOx contributes to the formation of photochemical smog, fine particulate matter (PM2.5), acid rain, as well as many human diseases, such as asthma and emphysema. The development of techniques to reduce NOx emissions is therefore important. Among seven types of NOx, nitrogen dioxide (NO2) is the most prevalent form of NOx and is largely produced from industrial flue gas and automobile exhaust gas. Although some mature technologies, e.g., selective catalytic reduction (SCR) applicable for NOx conversion at high temperatures (250-600 oC), have been widely used in industry, the control and abatement of ambient NO2 emission is still quite a complex and challenging issue.

Selective gas adsorption by solid adsorbents, which is based on the host-guest interactions, represents a promising approach for NO2 removal because of its potentially high working capacity, wide range of working temperatures, and ability to recover the captured species for further use. A few types of porous solid adsorbents, such as silica, carbon materials, zeolites, and metal-organic frameworks (MOFs) and their composites, have been attempted for NO2 capture suitable for ambient conditions. However, as NO2 is a highly reactive and corrosive gas, it is quite challenging for people to develop adsorbents that simultaneously possess high NO2 adsorption capacity, satisfactory selectivity, and good regenerability in real cases.

In the presented thesis, I attempt to address this dilemma by resorting to a series of porphyrin-based MOFs (PMOFs), which are typical MOFs featuring high surface area, strong adsorption sites, and robust material stability, especially towards the water and some acid gases, e.g., NO2. The unique features of PMOFs lie in that they possess not only stable metal nodes but also versatile metal centers at the center of a porphyrin ring imparting special interactions, e.g., π-backbonding, which endows them with great potential as highly efficient and robust NOx adsorbents. In this regard, I firstly synthesized two major types of PMOF by tuning their constituent metal nodes as potential NO2 adsorbents. The first three types of PMOFs using trivalent aluminum (Al3+) and its homologous family of gallium (Ga3+) and indium (In3+) as metal nodes were Al-PMOF, Ga-PMOF, and In-PMOF; the second two types of PMOFs using tetravalent zirconium (Zr4+) as metal nodes were PCN-222 and PCN-224. Especially, all the five PMOFs possess the same type of adsorption sites, i.e., the bridging hydroxyl (μ3-OH) group which can bind NO2 via hydrogen (H)-bonding, as verified by the in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO2 adsorption. The NO2 adsorption capacities of PMOFs were evaluated by the dynamic column breakthrough experiments with 100 ppm NO2 gas under ambient conditions. Among them, Al-PMOF exhibited the highest NO2 adsorption capacity of 1.85 mmol/g due to its highest density of μ3-OH sites. PCN-224 ranked as the second “strong” NO2 removal adsorbent with a capacity of 1.36 mmol/g due to its decent μ3-OH density and ultrahigh surface area (above 2500 m2/g), followed by Ga-PMOF (1.13 mmol/g), In-PMOF (0.90 mmol/g), and PCN-222 (0.52 mmol/g). Thus, it was concluded that the overall NO2 adsorption capacity was mainly decided by the density of μ3-OH site in PMOFs. The structural integrity of Al-PMOF after NO2 adsorption was well retained, as validated by its powder X-ray diffraction (PXRD) patterns and porosity measurement. The development of Al-PMOF for NO2 removal solves the structure degradation problem faced by most MOFs as NO2 adsorbents, thereby showing its great potential as a NO2 adsorbent in real deNOx applications.

Apart from the study of metal nodes as active NO2 adsorption sites in PMOFs, the exploration of the other important adsorption site – the metal center in the porphyrin ring was implemented based on a biomimetic concept, i.e., the π-backbonding interactions between metalloporphyrins and specific π-acidity gas molecules. I selected the previous superior Al-PMOF as a platform and inserted the first-row transition metals (TMs), e.g., nickel (Ni2+), cobalt (Co2+), copper (Cu2+), and zinc (Zn2+), providing backdonation ability with empty s orbitals and electrons occupying d orbitals, into the parent Al-PMOF to form a series of Al-PMOF(M). The NO2 adsorption experiments confirmed that the inserted Ni was the most effective TMs in enhancing NO2 adsorption and exhibited the higher adsorption affinity toward NO2 than another three Al-PMOF(M). Al-PMOF(Ni) showed the highest NO2 capacity of 2.30 mmol/g and good regenerability above 90 % after three adsorption-desorption cycles. Combined in-situ DRIFTS, synchrotron PXRD, and density functional theory (DFT) calculations revealed the adsorption mechanism upon adsorption - NO2 partially transformed to N2O4 and interacted with TMs via moderate π-backbonding and Al-OH nodes via H-bonding. This π-backbonding dominated interaction between NO2 and inserted TMs in Al-PMOF was different from the conventional Lewis acid-base interaction which usually happens in MOF adsorbents. This work presented PMOFs as a platform to tailor π-backbonding adsorbents.

In the above two chapters, I have studied the function of single metal nodes and the role of the incorporated single metal center in PMOFs for NO2 adsorption. Although superior NO2 adsorbents which show both high NO2 adsorption capacity and good regenerability were obtained, the evolution and release of nitric oxide (NO) is still a problem as NO also belongs to toxic NOx gases, which often accompanies the NO2 adsorption process owing to the disproportionation reaction. In this context, I resorted to designing multivariate PMOFs (MTV-PMOFs) which consist of bi-functional TMs in the porphyrin ring, namely, one type of TM bears strong NO2 adsorption ability and the other type of TM enables NO to be adsorbed strongly and retain its release. Herein, I developed Al-PMOF as the precursor for the fabrication of Al-PMOF(NixCoy) MTV-PMOFs by precise control of the molar ratio of Ni (strong NO2 adsorption site) and Co (strong NO adsorption site) as inserted TMs. The increased structural diversity and complexity of MTV-PMOFs conferred not only significantly increased NO2 adsorption capacity but also markedly decreased NO release amount. The Al-PMOF(Ni1Co1) exhibited the highest NO2 adsorption capacity of 3.66 mmol/g among other counterparts. The NO retention ability of Al-PMOF(Ni1Co1) was more than 44 times higher than that of the single TM incorporated PMOF (Al-PMOF(Ni)). It was also observed that the Ni/Co molar ratio in MTV-PMOFs was linearly related to the amount of NO released. The synergetic effect of inserted Ni and Co in MTV-PMOFs was corroborated by comparing the NO2 adsorption and NO retention performance of Al-PMOF(Ni1Co1) and the physical mixture of Al-PMOF(Ni) and Al-PMOF(Co) (1:1) as the NO2 capacity and NO retention time of the former was approximately 1.6 times higher and 1.5 times longer than the latter, excluding the simple sum of the NOx removal performance of Al-PMOF(Ni) and Al-PMOF(Co). This synergetic effect was firstly established in MTV-PMOFs and helped to achieve ideal NOx adsorbent exhibiting both high NO2 as well as NO adsorption performance.

Overall, it was demonstrated that PMOFs are promising NO2 adsorbents. The decent NO2 adsorption capacity, high regenerability, and low NO release amount, all substantiated that PMOFs are good candidate for NO2 removal under ambient conditions. Fundamental studies of PMOFs which identify their multiple and versatile adsorption sites have afforded new insights for the design of next-generation adsorbents for ambient NOx removal.