A Two Step Postsynthetic Modification Strategy: Appending Short Chain Polyamines to Zn-NH2-BDC MOF for Enhanced CO2 Adsorption

Functionalizing metal–organic frameworks (MOFs) with amines is a commonly used strategy to enhance their performance in CO2 capture applications. As such, in this work, a two-step strategy to covalently functionalize NH2-containing MOFs with short chain polyamines was developed. In the first step, the parent MOF, Zn4O(NH2-BDC)3, was exposed to bromoacetyl bromide (BrAcBr), which readily reacts with pendant -NH2 groups on the 2-amino-1,4-benzenedicarboxylate (NH2-BDC2–) ligand. 1H NMR of the digested MOF sample revealed that as much as 90% of the MOF ligands could be functionalized in the first step. Next, the MOF samples 60% of the ligands functionalized with acetyl bromide, Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8, was exposed to several short chain amines including ethylenediamine (ED), diethylenetriamine (DETA), and tris(2-aminoethyl)amine (TAEA). Subsequent digested 1H NMR analysis indicated that a total of 30%, 28%, and 19% of the MOF ligands were successfully grafted to ED, DETA, and TAEA, respectively. Next, the CO2 adsorption properties of the amine grafted MOFs were studied. The best performing material, TAEA-appended-Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8, exhibits a zero-coverage isosteric heat of CO2 adsorption of −62.5 kJ/mol, a value that is considerably higher than the one observed for the parent framework, −21 kJ/mol. Although the boosted CO2 affinity only leads to a slight increase in the CO2 adsorption capacity in the low-pressure regime (0.15 bar), which is of interest in postcombustion carbon dioxide capture, the CO2/N2 (15/85) selectivity at 313 K is 143, a value that is ∼35 times higher than the one observed for Zn4O(NH2-BDC)3, 4.1. Such enhancements are attributed to accessible primary amines, which were grafted to the MOF ligand. This hypothesis was further supported via in situ DRIFTS measurements of TAEA-Ac-Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8 after exposure to CO2, which revealed the chemisorption of CO2 via the formation of hydrogen bonded carbamates/carbamic acid and CO2δ- species; the latter are adducts formed between CO2 and [amineH]+Br– salts that are produced during the amine grafting step.

Comprehensive study of carbon dioxide adsorption in the metal-organic frameworks M-2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)

Miguel Gonzalez, Wendy Lee Queen, Berend Smit

Analysis of the CO2 adsorption properties of a well-known series of metal-organic frameworks M-2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host-guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180 degrees, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal-O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M-2(dobdc)-CO2 adducts.

Royal Soc Chemistry2014

Uncovering the Local Magnesium Environment in the Metal-Organic Framework Mg-2(dobpdc) Using Mg-25 NMR Spectroscopy

Yifei Liu, Berend Smit

The incorporation of N,N'-dimethylethylenediamine into an expanded MOF-74 framework has yielded a material (mmen-Mg-2(dobpdc)) exhibiting "step-shaped" CO, adsorption isotherms. The coordination of mmen at the Mg open metal center is essential for the unique cooperative adsorption mechanism elucidated for this material. Despite its importance for carbon capture, there is as yet no experimental structure determination available for the underlying metal organic framework Mg-2(dobpdc). Our Mg-25 solid-state NMR data unravel the local Mg environments in several Mg2(dobpdc) samples, unambiguously confirming the formation of five coordinate Mg centers in the activated material and six-coordinate Mg centers in the solvent- or diamine-loaded samples, such as mmen-Mg2(dobpdc). A fraction of the Mg centers exhibit local disorder due to the framework deformation accompanied by the guest distributions and dynamics.

American Chemical Society2017

Biologically derived metal organic frameworks

Samantha Lynn Anderson, Kyriakos Stylianou

Metal organic frameworks (MOFs) are extended structures composed of a network of organic ligands and metal ions or clusters connected to each other via coordination bonds. The numerous choices of organic ligands and metal coordination geometries have led to the construction of porous MOFs with various compositions, network topologies, pore sizes and shapes and they possess high surface areas and low densities. The structures of MOFs can be tailor-tuned in such a way that any desired ligand or/and metal ion can be incorporated; this has given to researchers the advantage of designing MOFs for a targeted application. Within this review, we overview recent examples of a sub-class of MOFs namely biologically derived MOFs (bio-MOFs), made of multifunctional and commercially available biologically derived ligands (bio-ligands) such as: amino acids, peptides, nucleobases and saccharides and focus on their coordination chemistry with a variety of metals. Central to this review are four tables detailing the coordination modes of bio-ligands to metals, along with a visual representation of the bio-MOF that is subsequently formed. Through the detailed analysis of these structures, we highlight the structural impact of these ligands on the structure, and their contribution to the MOF properties and applications. Finally, we showcase the potential of bio-MOFs in several research areas such as CO2 capture, separation, catalysis, drug delivery and sensing. (C) 2017 Elsevier B.V. All rights reserved.

Elsevier2017