APPLICATION OF A WATER STABLE ZINC(II) GLUTAMATE METAL ORGANIC FRAMEWORK FOR PHOTOCATALYTIC DEGRADATION OF ORGANIC DYES

A water stable metal-organic framework (MOF) having structural formula {[Zn(H2O)(L)]∙xsolvent}n (L = glutamate) (1), was prepared in aqueous medium by simple mixing the Zn(II) salt and glutamate ligand at room temperature. The single crystal X-ray diffraction study indicates that 1 crystallizes in an orthorhombic system with a space group of P212121 in which the Zn(II) center adopts pseudo-octahedral geometry. The bridging interactions through carboxylate groups in 1 generates a 2D structure in 1 comprising of the microporous channels. The UV/Vis diffuse-reflection spectroscopy for 1 had been performed which indicated that MOF possess semiconducting nature with a band gap of 3.23 eV and therefore may be a potential candidate as photocatalyst. The photocatalytic behavior of 1 against photo-degratdation of organic dyes has been investigated. The possible photocatalytic activity of 1 against organic dyes have been addressed using density of states (DOS) calculations.

Generally, MOFs used to degrade dyes are having porous morphology with 2D or 3D frameworks [11].The stability and recyclability of these MOFs requires improvement.Therefore, it is important to synthesize stable MOFs that can be used for degradation of organic dyes.In particular, the selection of the ligand plays a crucial role in designing as well as construction of the stable MOFs [12][13].With these viewpoints and in our continuous pursuit for the development of new MOFs for the organic dye-degradation in the waste-water discharge.Herein, a water stable zinc-glutamate-MOF having the structural formula {[Zn(H 2 O)(L)]•xsolvent} n (L = glutamate) (1), had been chosen which was prepared instantly in aqueous medium by simple mixing the reactants at room temperature had been used as the possible photocatalyst for the degradation of organic dyes and the probable mechanism of the photocatalysis have been addressed using density of states (DOS) calculations.Additionally, the UV/Vis diffuse-reflection spectra have also been investigated.

General considerations
All the purchased chemicals were used without further purification.Powder X-ray diffraction (PXRD) data was collected on Bruker D8 ADVANCE X-ray diffractometer with Cu-Kα radiation (λ = 1.5418Å) at 50 kV, 20 mA with a scanning rate of 6°/min and a step size of 0.02°.The simulated powder patterns were calculated using Mercury 2.0.The purity and homogeneity of the bulk products were determined by comparing the simulated and experimental X-ray powder diffraction patterns.Fourier transform infrared (FT-IR) spectra as KBr pellet were measured using a Nicolet Impact 750 FTIR in the range of 400-4000 cm -1 .We followed the photocatalytic method and computational details from our current work [14].

Synthesis of {[Zn(H 2 O)(L)]•xsolvent} n (1)
The synthesis procedure of 1 was analogous to previous work [14], except that volume of H 2 O in our synthesis was increased to 1.5 times.

Photocatalytic reactions
The experiment was carried out in a typical process. 1 (0.062 g) was added into 200 mL of methyl violet (MV) and rhodamine-B (RhB) (10 mg L −1 ).The suspension solution was stirred in the dark for about 30 min.Then, the mixture was stirred continuously under UV irradiation from a 175 W pressure mercury vapor lamp (main output 365 nm).At a given interval, aliquots of the reaction mixture were periodically taken and analyzed using a UV-vis spectrophotometer.This procedure was repeated in the absence of a catalyst as a blank comparison experiment.

Computational details
The probable mechanism of the photocatalysis of organic dyes in presence of 1 has been addressed using density of states (DOS) calculations.The geometry optimization of the 1 have been done using B3LYP functional and using 6-31G** basis set for all the atoms.All the calculations were performed using Gaussian 09 program [15] and GaussSum 3.1 was used to obtain density of state (DOS) plots [16].

Structure description of {[Zn(H 2 O)(L)]•xsolvent} n (1)
Single crystal X-ray diffraction analysis reveals that 1 crystallizes in an orthorhombic system with a space group of P2 1 2 1 2 1 .The title compound has been firstly reported by Kempe et al. [17].The asymmetric unit comprises of crystallagraphically unique Zn(II) ion, a coordinated water molecule and a L ligand.The Zn(1) is pseudo-octahedrally coordinated by one nitrogen atom (N1) from amino group and one oxygen atom (O3) bridging carboxyl group at the axial position, two oxygen atoms (O2 and O5) from bridging carboxyl group and chelating carboxyl group and one oxygen atom (O4) from a coordinated water molecule making up the basal plane (Figure 1a) [14,[16][17][18].The bridging interactions are responsible for organization of the metal atoms into a 2D structure (Figure 1b) with microporous channels.The PXRD patterns and FTIR spectra revealed that the single crystal and the rapidly synthesized 1 were similar with previous ones (Figure 2a and Figure 2c) [16][17][18].The single crystals of 1 remain unchanged in air for a very long time.Powder X-ray diffraction (XRD) patterns have unambiguously demonstrated that the crystallinity and framework integrity of 1 can be well retained even when 1 was kept in boiling water for 12 h (Figure 2b).Also, 1 also remains unchanged in water for a period of 2 months (Figure 2b).The exceptional stability of 1 can be attributed to the strong Zn-N bond.Undoubtedly, the pronounced stability of 1 build a nice foundation for its further application.

Diffuse-reflectance UV/Vis spectroscopy
The solid state UV-vis absorption spectrum of 1 is presented in Figure 3 which display a band with maxima at ca. 325 nm, which can be attributed to π-π* transitions of the ligand or ligandto-metal charge transfer [19][20].The absorption (α/S) data were calculated from the reflectance using the Kubelka-Munk function: α/S = (1-R) 2 /2R, where α is the absorption, S is the scattering coefficient, and R is the reflectance at a given energy.The energy band gap (E g ) was obtained by extrapolation of the linear portion of the absorption edges and the estimated value comes to be 3.23 eV thereby indicating semiconducting nature of 1.The band gap magnitude of 1 indicated that it may possess the potentials as semiconductor materials and may find applications as photocatalyst in photocatalytic reactions [21][22][23].

Photocatalysis
The photocatalytic activities of 1 was evaluated by using the MOF as photocatalyst in the photodegradation of MV/RhB dyes (0.032 g L -1 ) in water and irradiating the samples under a 250 W Hg lamp.The degradation ratios of MV/RhB dyes in water were monitored by observing the intensity of the characteristic absorption band of MV/RhB dyes (Figure 4a-b).The UV-Vis spectroscopy indicated that no new absorption band appeared in the UV-vis absorption spectrum thereby indicating the complete decomposition of MV/RhB dyes in water.The calculation results show that the conversion rates of MV and RhB are 73.3% and 55.1%, respectively.For the sake of comparison, the total catalytic degradation efficiency of the control experiment (without the use of catalyst 1) had also been carried out.The degradation rates of MV and RhB were merely 10.8% and 15.6%, respectively within 100 min under the same condition.The absorbance of MB without any catalysis under UV irradiation decreased in the beginning, but the rate of decomposition became slow later time.This observation is consistent with the previous reports [24][25][26].The current results indicated that UV-vis light may induce the MOFs/organic ligands (L) to produce O/N-metal charge transfer by promoting an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).The charge transfer excited state (MOFs/or L*) can be deactivated by oxidizing the contaminant directly and/or oxygenating water molecules into ˙OH radicals to complete the photocatalytic process [25].In addition, the photo-stability of 1 was monitored using PXRD analysis during the course of the photocatalytic reactions (Figure 2a), which indicated that there was no appreciable morphological changes in the MOF.Also, 1 can be reused for three catalytic cycles without significant photocatalytic efficiency decay and obvious loss of crystallinity (Figure 1a).From these result we infer that 1 can be a robust catalyst for MV and RhB photodegradation.
To investigate the kinetics of MV/RhB photocatalytic degradation by 1, experimental data can be described by using the well-known Langmuir-Hinshelwood model as expressed by ln (C/C 0 ) = −kt (k = apparent reaction rate constant) (Figure 4c).C 0 is the initial concentration of MV/RhB, t is the reaction time, and C is the concentration of RhB/MV at the reaction time t.The plot of ln (C 0 /C) and irradiation time (t) is approximately linear and approximated the firstorder kinetic equation (Figure 4c).The calculated apparent rate constant (k) for the photodegradation of Rh B and MV in the presence of 1 are 0.33×10 -2 and 1.09×10 -2 min -1 , respectively.Thus, 1 could be an efficient photo-catalyst for degrading MV in comparison to RhB [27][28][29].
The most likely photocatalytic degradation mechanism of organic dyes in the presence of 1 has been addressed with the aid of band structure calculations on 1 which is based on DFT method [30][31].As presented in Figure 5, the valence band lying just below the Fermi level of 1 is mainly contributed by the carboxylate oxygen of the glutamate moiety with meager contribution coming from Zn(II), coordinated nitrogen and carbon centers.Also, the conduction band lying just above the Fermi level in the range of 1.46 to 2.10 eV has been derived from carbon center with slight admixture from the oxygen centers.Therefore, the electronic transition in 1 mainly takes place from the carboxylate oxygen with meagre contributions from the Zn(II), nitrogen center to carbon center.In the typical photocatalytic process, the sample 1 can be excited to produce electron-hole pairs under visible light irradiation and as band structure calculations reveal that the hole moves to Zn(II) centers and the electron migrates to glutamate entity.The generation of holes on the Zn(II) centers will correspond to its oxidation which is now capable to oxidize the dye to re-reduce back to Zn(II) again.

CONCLUSION
The water stable zinc(II)-based MOF showed semiconducting nature with a band gap of 3.23 eV as evidenced by the UV/Vis diffuse-reflection spectroscopy and hence used as photocatalyst for the photo-degradation of methyl violet and Rhodamine B dyes.The DOS calculations reveals that under irradiation of 1 the hole moves to Zn(II) centers and the electron migrates to carbon centers.The generation of holes on the Zn(II) centers will correspond to its oxidation which is now capable to oxidize the dye to re-reduce back to Zn(II) again.Such types of MOFs prove to be a good catalyst for organic dyes and definitely such systems will be worthy to develop new and versatile materials photocatalyts.

Figure 1 .
Figure 1.(a) view of the local coordination geometry of metal center; (b) view of layer packing diagram along the bc-plane.

Figure 2 .
Figure 2. Powder XRD profiles of 1 (a) as-synthesized samples, dehydrated samples and after being used as photocatalyst to photo-degrade dyes; (b) soaked in water medium for different time length; (c) IR spectra of 1 at different medium and conditions.

Figure 3 .
Figure 3. (a) UV-vis diffuse-reflectance spectra of 1.(b) Solid-state optical diffuse-reflection spectra of 1 derived from diffuse reflectance data at ambient temperature.The intercept of the extrapolated absorption edge on the energy scale (x axis) gives the band gap of the samples.

Figure 3 .
Figure 3. (a) UV-vis diffuse-reflectance spectra of 1.(b) Solid-state optical diffuse-reflection spectra of 1 derived from diffuse reflectance data at ambient temperature.The intercept of the extrapolated absorption edge on the energy scale (x axis) gives the band gap of the samples.

Figure 4 .
Figure 4. (a) and (b) UV-vis absorption spectra of the MV and RhB solution during the decomposition reaction under 250W Hg lamp irradiation in the presence of 1, respectively; (c) the plot displaying photocatalytic degradation kinetics of MV/RhB with different concentration of 1.

Figure 5 .
Figure 5. Density of states (DOS) and partial DOS for 1.