Heteronuclear Metal-organic Frameworks; Adsorption and Luminescence Aspects

Synthesis of heteronuclear metal organic frameworks (1 & 2) using 4-mercaptobenzoic acid (H2MBA) and 2,6-naphthalenedicarboxylic acid (2,6-NDA) was carried out followed by successful characterization with FTIR, SEM, EDX, H-NMR, TGA and PXD. Brunauer-Emmett-Teller (BET) and luminescence studies of these MOFs were also investigated.


Introduction
Metal-organic frameworks (MOFs) have lead to innumerable research and gained attention in the scientific fields like chemistry, physics, materials science and other bordering research areas, etc [1]. These MOFs have potentially accessible voids, which make them ideal materials for a wide range of uses like gas storage, separation and catalysis [2]. Recently, metal-organic frameworks (MOFs) were used as adsorbents due to having unique textural properties that could be determined by analysis of their isotherms. Brunauer, Demming, Demming, and Teller (BDDT) categorized these adsorption isotherms into five types [3,4] The design, structure and properties of these MOFs can be tuned by modifying organic linkers [5].
Current direction is to design heterometallic or mixed-metal MOFs that contain different metal cations. Initially simple bimetallic MOFs were obtained by combination of coordination complexes (acting as a secondary building unit, or SBU) with a metal salt to construct 3D frameworks [6][7][8]. Complex heterometallic MOFs have also been reported recently and contain larger number of cations, or two types of SBUs [9]. However, a detailed description and characterization of these heterometallic systems in different applications is still superficial.
Keeping in view recent trends towards heteronuclear MOFs, in this paper, we report the synthesis, morphological, luminescence and gas adsorption studies of two heteronuclear MOFs (1 & 2) using 4mercaptobenzoic acid (H2MBA) and 2,6-naphthalenedicarboxylic acid (2,6-NDA) as organic linkers. USA) and energy-dispersive X-ray spectroscopy (EDS) was carried out by using EDAX system (Ametek, Inc. Materials Analysis Division, USA). TGA was done by SDT Q600 instrument (range 30 o to 800°C, heating rate of 10ºC/min). Diffractometer (Equinox 2000, Thermoscientific, USA, CuKα1 radiation) was used to obtain diffraction patterns. BET analysis was completed by using Tri Star II 3020 version 2.00. Luminescence spectra were recorded by using PerkinElmer multimode plate reader at excitation wavelength 320 nm.

Results and discussions
Heteronuclear MOFs (1 & 2) were obtained in good yields (62-68 %) by reacting bismuth and lead salts with respective organic linker. MOF (1) was orange colored while MOF (2) was colorless. Both the MOFs were air stable solids and soluble only in DMSO suggesting that they possess polymeric or aggregated framework structure. The bonding modes of H2MBA and H2NDA with mixed metal ions were established by comparing FT-IR spectra of MOFs (1) & (2) with free linkers i.e H2MBA and H2NDA. The FT-IR spectra exhibited the presence of strong absorption peaks between 1557-1392 cm -1 for (1) and 1508-1397 cm -1 for (2). This can be attributed to the asymmetric and symmetric COO stretching vibrations, respectively. The separations (Δυ) between υasym(COO) and υsym(COO) were found to be 165 cm -1 and 111 cm -1 indicating coordination through carboxylate group via monoanionic bidentate mode [20]. A shift of band was found at 1010 cm -1 in MOF (1) (1099 cm -1 for H2MBA linker) which indicates the coordination through C=S group as well. 1 H-NMR spectra of MOFs (1 & 2) were recorded in DMSO-d6 and are represented in (Figures 1 & 2). These spectra lack the presence of -COOH signal thus indicating deprotonation and subsequent coordination with Bi +3 and Pb +2 ions [21,22]. The other signals such as multiplets in the region 7.31-7.68 ppm for (1) and 8.02-8.65 ppm for (2), assigned to protons of aromatic rings, are in agreement with proposed structures. 3.1. Powder X-ray diffraction studies X-ray powder diffraction is very useful to learn about crystal structure, phase type and material purity. Powder X-ray diffraction patterns of MOFs (1) and (2) are shown in (Figures 3 & 4). These heteronuclear MOFs were less crystalline than the homonuclear Pb-MBA and Bi-NDA MOFs [23,24].  No contributions of other crystalline phases (in addition to Bi and Pb-containing phases) was detected in the diffraction patterns. The main diffraction peaks for heteronuclear MOF (1) were observed at 2θ = 11.15º, 19.17º, 22.37º and 28.95º and were indexed with monoclinic (C12/m1) system having unit cell parameters (a = 6.6726 Å, b = 6.1108 Å, c = 3.3001 Å, β = 110.412º).

Scanning electron microscopy and EDS
The morphologies of these MOFs (1 & 2) were analyzed by SEM and their micrographs are shown in (Figures 5a & 6a). The SEM image of the MOF (1) displayed homogeneous irregular nanocrystallites aggregated clusters and is comparable to the morphology of MM-MOF-74 [M2(DOT); DOT = dioxidoterephthalate] [25]. The SEM image of MOF (2) displayed irregular shaped particles. These images clearly illustrated that the incorporation of two different metal ions lead to significant changes in their morphology which was observed in case of homonuclear Pb-MBA and Bi-NDA MOFs [23,24] The chemical composition of MOFs (1) and (2) was studied by EDS and the spectra are presented in (Figures 5b & 6b). Quantitative analysis of the EDX (1) revealed the existence of bismuth (33.35 wt %), lead (22.86 wt %), carbon (19.54 wt %), oxygen (13.66 wt %) and sulphur (10.59 wt %). Similarly, EDX spectrum of (2) showed the existence of bismuth (28.24 wt %), lead (24.03 wt %) and oxygen (14.83 wt %). These results clearly depicted the incorporation of both bismuth and lead metal ions into the organic framework thus confirming the successful synthesis of heteronuclear MOFs (1 & 2).

Thermal gravimetric analysis
Thermal gravimetric analysis of heteronuclear MOFs (1 & 2) were carried out to check their thermal stability and the respective TGA curves are presented in (Figure 7). Initial weight loss was observed between 50-150°C in both MOFs and can be attributed to the release of the hydrated water and methanol molecule. Second decomposition was observed in the range 200-250°C for (1) and 215-300°C for (2) corresponding to the release of DMF molecule [26]. Further decomposition started at 250°C for (1) and at 300°C for (2). This goes up to 520°C and then the curve becomes almost flat leading to formation of respective metal oxide.

Photo-physical studies
Luminescence properties of heteronuclear MOFs (1 & 2) were studied in solid state at room temperature. The data obtained is graphically represented in (Figures 8 and 9).

Conclusions
In this paper, two heteronuclear MOFs have been synthesized and structurally characterized. MOF (1) exhibited very appreciable luminescence than MOF (2) and could be considered as promising material in light devices. Moreover, these MOFs could also be considered as effective adsorbents for nitrogen gas as well as for other pollutants due to their better pore textural properties.