Chemistry and Chemical Biology ETDs

Publication Date

Fall 11-13-2019

Abstract

Ionic tetrapodal ligands with colinear coordinating arms are very effective for designing hybrid porous materials with unusual structure and properties. The novelty of this research work lies in the utilization of a unique borate ligand that leads to charge-separated MOF structure with tailor designed properties. Borate ligands being tetrahedral afford 3D materials and the negative charge of borate anion can compensate the positive charge of metal ion in the framework. The borate ligands designed in this research consists of quaternary ammonium cation and anionic borate with four pyridine arms. These ligands upon coordination with Cu(I), Cu(II), Ag(I) and Co(II) metal cations formed six different charge-separated MOFs (UNM-1 to UNM-6).

Chapter 1 covers a brief review on the design, synthesis, classification and applications of MOFs. Additionally, MOF interpenetration, control and applications of interpenetration are discussed with examples.

Chapter 2 is about the synthesis, structural characterization and application of a charge-separated diamondoid UNM-1 MOF assembled from tetrakis(4-pyridyltetrafluorophenylethynyl)borate (T1) and Cu(I) metal cation. UNM-1 MOF structure displays 4-fold interpenetration, resulting in high environmental stability, and at the same time possesses relatively large surface area (SABET = 621 m2/g) due to the absence of free ions. Gas adsorption measurements revealed temperature-dependent CO2 adsorption/desorption hysteresis and large CO2/N2 ideal selectivities up to ca. 99 at 313 K and 1 bar, suggesting potential applications of this type of charge-separated MOFs in flue gas treatment and CO2 sequestration.

In Chapter 3, synthesis and single-crystal structural characterization of four new charge-separated MOFs (UNM-2, UNM-3, UNM-4 and UNM-5) based on two tetrapodal borate ligands: (T1) and tetrakis(4-pyridyltetrafluorophenyl)borate (T2) having coordinating arms of different lengths and pyridine groups at the end of each arm are discussed. Coordination of these tetrapod with Cu(I)/Cu(II), and Ag(I) ions under specific conditions led to a series of new charge-separated MOFs in single crystalline forms. UNM-2/UNM-3 in monoclinic C 2/c space group, are derived from Cu(NO3)2 upon coordination with T1. On the other hand, coordination of T2 with Cu(CH3CN)4BF4 and AgBF4 respectively formed UNM-4 and UNM-5 in monoclinic I 2/a space group. These MOFs possesses several degrees of interpenetration that are correlated with the arm lengths of ligands. All these MOFs, except UNM-3 are 3D, 2-fold interpenetrated crystalline frameworks. UNM-3 is 1D framework containing coordinated solvent molecules in the crystal structure.

In Chapter 4, synthesis of UNM-6, characterization, post-synthetic ionic exchange, applications in chemical fixation of CO2 and CO2/N2 adsorption are discussed. UNM-6 is synthesized via solvothermal synthesis from the assembly of T2 with Co(NO3)2. UNM-6 is crystallized in cubic crystal system with P -4 3 n space group and is 4-fold interpenetrated, 3D framework containing coordinated nitrate ion in the crystal structure. The nitrate ion in UNM-6 is replaced with different anions like halides, cyanide and azide by post-synthetic ionic exchange reaction. Furthermore, UNM-6 and bromide exchanged Br-UNM-6 MOFs are used as heterogenous catalysts in the cycloaddition reaction of CO2 to epichlorohydrin without co-catalyst. Both catalysts can be easily separated from the product, very efficient and stable, with ability to be reused multiple times. Additionally, UNM-6 showed very high CO2/N2 separation selectivity of 1022 at 313 K under 1 atm pressure.

In summary, this dissertation highlights the versatility of tetrapodal borate ligands in engineering charge-separated MOFs with diverse structures and controlled functionality.

Language

English

Keywords

MOFs, Interpenetration, Selectivity, Gas separation, Ionic exchange, Catalysis

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Yang Qin

Second Committee Member

Brian Gold

Third Committee Member

Christine Mai Le

Fourth Committee Member

Gayan Rubasinghege

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