Interaction of Biologically Important Organic Molecules with the Unsaturated Copper Centers of the HKUST-1 Metal–Organic Framework: an Ab-Initio Study

Abstract

Metal–organic frameworks (MOFs) provide new possibilities for their potential use in catalysis, gas storage/separation, and drug delivery. In this work, a computational study is performed on the interaction of biologically important organic molecules such as caffeine, urea, niacin, and glycine with the undercoordinated copper centers of the HKUST-1 MOF. Density functional theory calculations are used to identify the adsorption sites of the organic molecules in HKUST-1 and to calculate their interaction energies. Two types of interactions are calculated: (i) strong binding via their nitrogen or oxygen atoms with the copper atoms of the paddlewheel and (ii) hydrogen bonds with the carboxylate groups of the MOF. Certain molecules such as caffeine and niacin can interact simultaneously with more than two paddlewheels, thus making the interactions even stronger. The interaction energies vary from 75 kJ mol–1 for glycine to 200 kJ mol–1 for caffeine. The confinement of the guest molecules in the cage windows of the framework can also create strong interactions. To take into account the effect of coordination with multiple paddlewheels, a very large model of the HKUST-1 needs to be used. The numbers of (i) copper sites interacting with the guest molecule and (ii) hydrogen bonds between the carboxylate groups of the MOF and the guests have a major impact on binding strength. This is important information when applying rational design to create new MOFs that should serve as drug carriers.

A DFT Screening of M-HKUST-1 MOFs for Nitrogen-Containing Compounds Adsorption

“…Adsorption Gibbs free energies on 19 M-HKUST-1 ( Figure 4 ) shows that five alkaline earth metals together with Zn, Mo, and Sn show the same trend with Cu-HKUST-1 towards three NCCs adsorption and the stability order is: NH 3 > NO 2 > NO. A similar result was observed by Supronowicz et al [ 11 ] who found NH 3 bonds to HKUST-1 stronger than NO x by DFT calculations.…”

“…HKUST-1, also known as Cu 3 BTC 2 or MOF-199, (BTC = 1,3,5 benzene tricarboxylate) has been applied for various purposes: catalysis [ 5 ], membranes [ 6 , 7 ], wastewater treatment [ 8 , 9 ], and capturing a wide variety of gases [ 10 ]. The gravimetric density of under-coordinated “open-metal” sites in HKUST-1are regarded as the main reaction sites for molecule adsorption [ 11 , 12 , 13 ]. Xiao et al [ 13 ] found that the NO adsorption capacity of HKUST-1 is as high as 9 mmol g −1 at 196 K and 1 bar pressure, which is higher than that of the previously reported porous materials.…”

A Chemical Route to Activation of Open Metal Sites in the Copper-Based Metal–Organic Framework Materials HKUST-1 and Cu-MOF-2

“…The "extracoordination" (solvent coordination) ability of the OCSs in both HKUST-1 and MOF-2 enables them to be excellent adsorbents or molecule separators for applications such as carbon dioxide sorption, 11,[28][29] water sorption, 30 amine sorption, 31 nitric oxide capture, 32 water/ethanol separation, 33 and others. 9,[19][20][22][23][24][34][35][36] To utilize that type of MOF for the aforementioned applications, an activation process to remove both pre-coordinating solvent molecules from the OCS and pore-filling guest molecules (typically the solvents used during the synthesis process) from the pores is a prerequisite step. 37 To date, five strategies 38 for effective activation have been developed: (i) thermal activation (by applying heat and vacuum; hereafter TA), [39][40][41][42][43] (ii) solvent exchange (typically performed by methylene chloride or chloroform), [44][45] (iii) freeze-drying, 46 (iv) supercritical CO 2 exchange, 45 pletely remove pore-filling solvents and it is the only known method that can remove components of both coordinated and pore-filling solvents.…”

In situ evolved gas analysis assisted thermogravimetric (TG-FTIR and TG/DTA–MS) studies on non-activated copper benzene-1,3,5-tricarboxylate

“…The dimeric Cu-containing paddle-wheel units are linked by the polydental trimesate anion and form an electrically neutral three-dimensional porous network. This arrangement results in two unsaturated copper sites per paddle-wheel, which is ready to interact with polar molecules [2,4,[6][7][8]. During the synthesis the axial coordination sites of the copper paddle-wheel units become occupied by solvent molecules, and thus each copper is able to complete its pseudo-octahedral coordination sphere [4,6,8].…”

“…That is, the structure can accommodate more water than the "stoichiometric" amount, as solvent molecules may also be stored in the pores. The solvent can be completely removed by heat treatment in vacuum: when heated to ca 180 C (also called activation [8,9]) the water molecules evaporate, leaving behind free copper sites, empty pores and thus high apparent surface area. Activation can also be followed by the naked eye, as the bright blue crystals turn dark blue.…”