Investigations on post-synthetically modified UiO-66-NH2 for the adsorptive removal of heavy metal ions from aqueous solution

Accepted Manuscript Investigations on post-synthetically modified UiO-66-NH2 for the adsorptive removal of heavy metal ions from aqueous solution Hira Saleem, Uzaira Rafique, Robert P. Davies PII:

S1387-1811(15)00532-6

DOI:

10.1016/j.micromeso.2015.09.043

Reference:

MICMAT 7327

To appear in:

Microporous and Mesoporous Materials

Received Date: 10 August 2015 Revised Date:

23 September 2015

Accepted Date: 24 September 2015

Please cite this article as: H. Saleem, U. Rafique, R.P. Davies, Investigations on post-synthetically modified UiO-66-NH2 for the adsorptive removal of heavy metal ions from aqueous solution, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.09.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

Investigations on post-synthetically modified UiO-66-NH₂₂ for the adsorptive removal of heavy ACCEPTED MANUSCRIPT metal ions from aqueous solution Hira Saleem1, 2, Uzaira Rafique2 and Robert P. Davies1*

Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AZ, U.K.

2

Department of Environmental Sciences, Fatima Jinnah Women University, Rawalpindi, Pakistan.

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* E-mail: [email protected]; Tel: +44 (0)207 5945754

Abstract

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The metal organic framework UiO-66-NH2 has been post-synthetically modified to introduce thiourea, isothiocyanate and isocyanate functionalities without compromising the structural and thermal stability of the parent framework. 1H NMR and IR spectroscopies have been

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used to monitor the extent of framework functionalization. UiO-66, UiO-66-NH2 and the new

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functionalized frameworks UiO-66-NHC(S)NHMe, UiO-66-NHC(S)NHPh, UiO-66-NCS and UiO-66-NCO have been studied as adsorbents for the capture of a range of heavy metals from homoionic aqueous solution, with a view towards applications in environmental remediation. Functionalization markedly improved metal removal efficiency up to 99 % with calculated maximum adsorption capacities of 49, 117, 232 and 769 mg/g for Cd2+, Cr3+, Pb2+ and Hg2+ respectively for UiO-66-NHC(S)NHMe.

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ACCEPTED MANUSCRIPT 1. Introduction The field of metal-organic frameworks, colloquially known as MOFs, has undergone rapid growth over the past decade [1]. MOFs are defined, using IUPAC recommendations [2], as

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“metal coordination networks with organic ligands containing potential voids”. The often high or ultrahigh porosity of these materials (with reported internal surface areas up to 7000 m2 g−1) [3] make them highly attractive for a range of applications, most notably in gas

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storage [4][5, 6] and separations [7, 8] but also in drug delivery [9, 10] catalysis [11, 12], artificial photosynthesis [13], and electronic devices [14]. MOFs have also shown some

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promise for the removal and detoxification of pollutants and other hazardous molecules (including certain chemical warfare agents [15]) from the environment. The adsorptive removal of toxic or harmful gases and vapours from the gas phase using MOFs has been reported for a variety of small molecules including CO, CO2, NH3, NOX, H2S, and SOX as well as for other volatile organic compounds (VOCs) such as benzene and xylene [16, 17].

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However, the application of MOFs in the decontamination of waste-water aqueous solutions via liquid phase adsorption is more challenging and currently less widely reported upon.

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Contamination of waste water from industrial processes and agricultural and domestic activities is a growing global issue affecting the whole biosphere, with known detrimental

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impacts on human health and well-being. A number of studies have reported upon the use of MOFs for the removal of organic contaminants including the dyes methyl orange and methylene blue on MOF-235 [18], malachite green on MIL-100(Fe) [19], clofibric acid and naproxen on MIL-101(Cr) and MIL-100(Fe) [20], phenol on MIL-53(Cr) [21] and methylchlorophenoxypropionic acid (MCPP) on UiO-66 [22]. In some cases the MOFs were shown to have removal capacities and efficiencies exceeding those of conventional adsorbents such as activated carbon [20].

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ACCEPTED MANUSCRIPT The removal of heavy metal contaminants from wastewater streams presents a particular issue, since unlike many organic pollutants these inorganic pollutants are not susceptible to biological degradation. Nevertheless, MIL-100(Fe) has been reported for the removal of arsenic (As5+) from aqueous solutions by Huang and co-workers [23]. In addition, a number

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of novel MOFs containing pendant thiol or thioether functionalities have been prepared by Xu and co-workers and shown to be efficient mercury (Hg2+) scavengers from aqueous solutions [24-26]. However, the synthesis of these thiol functionalised MOFs (and in some

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cases their precursor organic ligands) is often non-facile. Alternative approaches in which thiol containing groups are incorporated into known MOF structures through a postsynthetic

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modification (PSM) step have also been reported. Qiu and co-workers prepared thiol functionalised HKUST-1 via reaction of HKUST-1 with dithioglycol [27]. More recently Dong, Liu et al. reported upon the synthesis of a thiol functionalised MIL-101(Cr) network by means of a palladium catalysed PSM step [28]. However, despite these recent advances

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there is still a need for the development of more robust (in particular improved water stability), more cost effective and easier to prepare or functionalise materials for heavy metal

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capture applications.

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In this work a post-synthetic modification strategy is used to optimise UiO-66 MOFs for applications in heavy metal extraction by incorporation of pendent sulphur-containing groups within the pores of the material. UiO-66 is a well-known Zr-based cubic framework comprised of cationic Zr6O4(OH)4 nodes and 1,4-benzenedicarboxylate (BDC) organic linkers [29]. Moreover, UiO-66 possesses a high degree of porosity and can be prepared from commercially available and affordable precursors [30]. Post-synthetic modification of UiO66-NH2 has previously been reported to prepare functionalised UiO-66 framework materials

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ACCEPTED MANUSCRIPT with applications in catalysis [31, 32], hydrogen [33] or carbon dioxide [34] gas capture, and the adsorptive removal of dyes [35] or nitrogen containing compounds [36, 37]. The unusually high water and pH stability of UiO-66 makes it an ideal candidate for water treatment applications [30]. We now report upon the extension of this family of MOFs to new

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sulphur-functionalised derivatives and their properties as scavengers for heavy metal ions

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from aqueous solutions.

2.1 Synthesis of UiO-66-NH2

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2. Experimental Section

Starting materials and solvents were purchased from commercial suppliers (Sigma-Aldrich

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and Alfa Aesar) and used without further purification. ZrCl4 (0.54 mmol) was added to a mixture of DMF (5 mL) and concentrated HCl (1 mL) and sonicated for about 20 minutes until the salt was completely dissolved. A separate mixture of ligand (2-aminoterephthalic

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acid, 0.75 mmol) and DMF (10 mL) was also sonicated for 20 minutes, and the two mixtures

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were then combined and sonicated again for a further 10 minutes. The resultant solution was heated at 80 ⁰C for 24 hours. After cooling to room temperature the resulting microcrystalline powder was filtered and washed with DMF to remove any unreacted ligand. The solid was isolated by centrifugation, soaked in water (15 mL) for 24 hours, and then methanol (3x15 mL) for 3x24 hours to exchange the DMF and facilitate the removal of any formic acid present in the pores. The sample was filtered to remove all residual solvent and the resulting solid was vacuum dried at 100 ⁰C until a final pressure of 100 mTorr was obtained.

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ACCEPTED MANUSCRIPT 2.2 Procedure for post-synthetic modification of UiO-66-NH2 In a typical reaction activated UiO-66-NH2 (ca. 60 mg) was suspended in a mixture of 0.2 mL MeOH and 1.8 mL CH3Cl and treated with 2-10 equivalents of isothiocyanate at 55 ⁰C

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for 24 hours. The solvent system employed (10 % MeOH in CH3Cl) was used in order to inhibit crystallisation of any dialkylthiourea side products [38]. For the synthesis of UiO-66NCS and UiO-66-NCO, THF (2 mL) was used as the solvent system. After the reaction was

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complete the sample was washed thoroughly with CH3Cl to extract any by-products. The solids were then centrifuged, isolated and soaked in fresh solvent for 24 hours (3 times),

obtained.

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2.2 Characterisation of the MOFs

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before vacuum drying at 100 ⁰C for up to 12 hours until a final pressure of 100 mTorr was

NMR analysis: NMR spectroscopy was performed on digested samples at 25 °C using a Bruker AV400 spectrometer. In a typical experiment approximately 10 mg of MOF sample

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was digested by sonication in either 570 µL of 48 % aqueous HF solution in DMSO-d6, or alternatively 600 µL of 1 M NaOD in D2O. After dissolution of the material the solution was

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analysed using both 1H and 13C NMR spectroscopies. ATR-FTIR analysis: Approximately 5-10 mg of MOF sample was dried at 100 °C for at least 2 hours before analysis and the spectra were collected on a Perkin Elmer spectrum 100 series spectrometer with a universal ATR sampling. Powder X-ray diffraction analysis: Diffraction data for the MOF materials was obtained using a Panalytical X’Pert Pro MPD powder X-ray diffractometer with Cu-Kα radiation using

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ACCEPTED MANUSCRIPT a step size of 0.03° in 2θ and a 2θ range of 5-45°. Approximately 15 mg of microcrystalline MOF sample was dried at 100 °C for at least 2 hours before analysis. BET surface area analysis: The surface areas and pore volumes were determined by N2

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sorption at -196 °C using a Micromeritics 3Flex sorption analyser. Prior to the measurements, the samples were degassed overnight at 120 °C using an external sample degas system (Micromeritics, VacPrep 061) and then further degassed in-situ for 5 hours at 120 °C and 0.1

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mmHg. Surface areas were obtained using the BET equation, micropore volumes using the tplot, and the total pore volume was calculated from the isotherm at P/P₀=0.99.

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Thermogravimetric Analysis: Approximately 10 mg of MOF was analysed under a stream of air using a Perkin Elmer Pyris 1 TGA in the temperature range 25 to 800 ⁰C. The scan rate was 5 ⁰C per minute. Differences in initial and final weight of all the samples were recorded

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and the data is shown as a weight loss percentage.

2.5 Protocol for Adsorption Experiments

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Time dependent batch experiments were conducted at time intervals of 5, 10, 30, 60, 120 and

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240 minutes. Each experiment was repeated several times in order to verify consistent results. In each experiment six samples, each containing 15 mg of MOF dispersed using magnetic stirring in 15 mL of homoionic metal solution in a Falcon tube, were studied in parallel. After the allotted time the solution was separated from the MOF using a centrifuge. Residual metal ion concentration in the liquid phase was determined using inductively coupled plasmaoptical emission spectroscopy. The same experiment was repeated with Cd2+, Cr3+, Hg2+ and Pb2+ homoionic solutions at concentrations of 10, 50, 100 and 200 mg/L.

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3. Results and Discussion

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3.1 Synthesis and characterisation of functionalised MOFs The starting point for all new sulphur–functionalised UiO-66 MOFs in this work is UiO-66NH2 [39]. The UiO-66-NH2 framework is isostructural to UiO-66 but contains 2aminoterephthalate linkers in place of the terephthalate linkers in UiO-66, thus introducing

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NH2 groups within the pores of the MOF. These primary amine groups are suitable targets for

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further post-synthetic functionalization. UiO-66-NH2 was prepared from ZrCl4 and 2aminoterephthalic acid in DMF following the protocol of Farha et al. [40]. The product was obtained as fine powder, centrifuged, soaked several times in methanol to exchange the DMF and vacuum dried (see experimental section). UiO-66-NH2 consists of a face-centred arrangement of cubic Zr6O4(OH)4 inorganic clusters that coordinate to the aminoterephthalate

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linkers to give large porous tetrahedral and octahedral units with the amino groups pendant within the pores [39]. Covalent post-synthetic modification of UiO-66-NH2 was targeted on

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the amine groups via reaction with a series of isothiocyanates, thiophosgene or diphosgene in

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an attempt to prepare six novel functionalised UiO-66 MOFs (Scheme 1).

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Attempted post-synthetic functionalization of UiO-66-NH2: (i) CSCl2, THF, 55 °C, 18 hours,

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Scheme 1:

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(ii) CH3NCS, CHCl3, 55 °C, 3 days, (iii) C6H5NCS, CHCl3, 55 °C, 3 days, (iv) ClCOOCCl3, THF, 25 °C, 4

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hours, (v) C6H11NCS, CHCl3, 55 °C, 5 days, (vi) (CH3)3CNCS, CHCl3, 55 °C, 5 days.

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The degree of MOF post-synthetic functionalization was determined by digesting the MOF using either strong acid or base, followed by 1H NMR spectroscopic analysis. Although NMR characterisation of MOFs through digestion is a known method to monitor post-synthetic modification reactions [38], the high chemical stability of the UiO-66 framework made such an approach challenging for the derivatives studied in this work. The use of hydrofluoric acid in deuterated DMSO with extensive sonication has previously been reported for the digestion of UiO-66-NO2, UiO-66-Br, UiO-66-NH2 and their derivatives [41]. However, for the new MOFs prepared in this work this also led to degradation of the sulphur-containing 8

ACCEPTED MANUSCRIPT functionalities making quantitative analysis difficult. Attempts therefore moved to the use of 1 M NaOD in D2O which gave much cleaner 1H NMR spectra without further ligand degradation (Figure 1). Chavan et al. have used similar methodology to digest MOFs of UiO-66 topology with mixed linkers [42]. However, as far as we are aware this is the first

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report using the NaOD digestion method to provide evidence of post-synthetic modification

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on UiO-66-NH2.

Figure 1:

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H NMR spectra of digested MOF samples using NaOD in D2O (1 M). Peaks marked with

black circles represent new peaks associated with the modified linker.

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ACCEPTED MANUSCRIPT The NMR spectrum of the digested parent UiO-66-NH2 MOF displays resonances at 6.95, 7.03 and 7.46 ppm which are characteristic of the three distinct protons on the amine functionalised aromatic ring of the aminoterephthalate linker (Figure 1). In the case of the modified MOFs, a new set of peaks was observed in the NMR spectra of UiO-66-

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NHC(S)NHMe, UiO-66-NHC(S)NHPh, UiO-66-NCS, and UiO-66-NCO, all of which exhibit a downfield shift in the aromatic resonances consistent with successful functionalization. No new peaks were observed for UiO-66-NHC(S)NHtBu or UiO-66-

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NHC(S)NHCy indicating that modification was unsuccessful in these cases. Thus, the 1H NMR spectrum of UiO-66-NHC(S)NHMe (Figures 1 and S1) displays a singlet at 3.55 ppm,

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attributable to methyl group on the newly introduced thiourea functionality. In addition, three new aromatic resonances at 7.41, 7.49 and 7.73 ppm are observed, corresponding to the three aryl protons of the modified linker. Similarly UiO-66-NHC(S)NHPh exhibits resonances for the newly introduced thiourea phenyl group at 6.73, 6.85 and 7.11 ppm, with the three

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aromatic protons of the modified linker appearing at 7.16, 7.52 and 8.26 ppm (Figures 1 and S2). In both cases the resonances due to residual unfunctionalised linker remain unchanged in chemical shift from those observed in the digested parent UiO-66-NH2 framework (Figure 1).

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Comparison of the integration of the aromatic resonances attributable to the newly modified

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linker with that of the unfunctionalised aminoterephthalic acid resonances, allows the conversion rate for each post-synthetic modification reaction to be calculated (see Table 1). The transformation of the amine groups in UiO-66-NH2 to thiourea groups was observed to depend significantly upon the steric bulk of the isothiocyanate substituent (Table 1). Methyl isothiocyanate afforded the highest conversion rates of the parent UiO-66-NH2 (up to 73 % of connecting ligands being functionalised) whilst phenyl isothiocyanate gave up to 30 % conversion. The bulkier tert-butyl isothiocyanate and cyclohexyl isothiocyanate reagents failed to react even under more forceful conditions (Scheme 1). 10

ACCEPTED MANUSCRIPT In the case of UiO-66-NH(C)SNHMe and UiO-66-NH(C)SNHPh the degree of modification increased with reaction time up to a maximum of approximately 73% conversion after 3 days (Table 1). Prolonged reaction times gave no further improvement in the conversion ratio. The bulkier tert-butyl and cyclohexyl isothiocyanate reagents failed to give any observable

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reaction with UiO-66-NH2 even when using up to 10 equivalents of the isothiocyanate with reaction times of up to five days. It is likely that the increased steric bulk associated with the cyclohexyl and tert-butyl groups prevents the reagents from entering the pores of the network

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and thus prohibits post-synthetic modification. Cohen and co-workers have previously reported similar findings for the amine functionalised IRMOF-3 on treatment with tert-butyl

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isocyanate [38].

Modification of UiO-66-NH2 was also extended to introduce isothiocyanate and isocyanate groups into the MOFs using thiophosgene and diphosgene reagents respectively. Both of these reactions gave reasonable conversions even after 18 and 4 hours respectively (Table 1).

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The 1H NMR chemical shifts of the phenyl protons in the new modified linker in UiO-66NCS appear at 7.25, 7.58 and 8.24 ppm, whereas those in UiO-66-NCO are present at 7.16,

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7.54 and 8.18 ppm (see Figures 1, S5 and S6).

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Table 1: Conversion rates for covalent modification of UiO-66-NH2 Reagent (equiv.)

Conditions

Conversion

UiO-66-NHC(S)NHMe

CH3NCS (2)

CHCl3, 55 °C, 3 days

73%

UiO-66-NHC(S)NHPh

C6H5NCS (4)

CHCl3, 55 °C, 3 days

30%

UiO-66-NHC(S)NHCy

C6H11NCS (10)

CHCl3, 55 °C, 1-5 days

0%

UiO-66-NHC(S)NHtBu

tBuNCS (10)

CHCl3, 55 °C, 1-5 days

0%

UiO-66-NCS

CSCl2 (2)

UiO-66-NCO

ClCOOCCl3 (2)

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Target MOF

53%

THF, 25 °C, 4 hours

68%

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THF, 55 °C, 18 hours

Additional evidence for modification was obtained using Infrared spectroscopy (Figure 2).

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The characteristic bands attributed to the amine group in UiO-66-NH2 are observed at 3486 cm-1 ʋsym(NH2), 3375 cm-1 ʋasym(NH2), 1367 and 1257 cm-1 ʋ(Car-N) [43]. Successful post-

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synthetic modification of UiO-66-NH2 led to a prominent reduction in the intensity of the stretching bands related to the primary amine group (Figure 2). This was particularly evident for UiO-66-NHC(S)NHMe which has a conversion rate (see earlier) and thus a significantly diminished concentration of primary amine groups in the network. More importantly, two new bands are observed at 1085 and 1047 cm-1 for this MOF. These are indicative of the presence of the thiourea group, and can be assigned to ʋ(C=S) and ʋ(C-N) stretching modes respectively. In the spectrum of UiO-66-NHC(S)NHPh the thiourea ʋ(C=S) and ʋ(C-N)

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ACCEPTED MANUSCRIPT stretching modes are visible at 1080 and 1035 cm-1 respectively. Assignment of these bands

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is supported by the work of Mingyan and co-workers [44].

Figure 2: ATR-FTIR of UiO-66-NH2 and modified MOF samples. (a): MOF skeletal modes region. (b):

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Enlargement of N-H stretching region. (c): Enlargement of ʋ(C=S) region.

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The IR spectra of UiO-66-NCS and UiO-66-NCO show new vibrational bands due to the introduction of isothiocyanate and isocyanate functionalities. These appear as characteristic absorptions at 2122 or 2283 cm-1 and are attributable to ʋas(N=C=S) or ʋas(N=C=O) modes respectively. These absorptions are therefore comparable in wavenumber to those assigned by Cohen et al. [45] for isothiocyanate (2100 cm-1) and isocyanate (2279 cm-1) bearing MILs.

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ACCEPTED MANUSCRIPT UiO-66-NHC(S)NHMe

UiO-66-NCS

UiO-66-NCO

UiO-66-NHC(S)NHPh

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Figure 3:

25

35

2θ

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Relative Intensity

UiO-66-NH₂

PXRD patterns of UiO-66-NH2 and post-synthetically modified samples.

Powder X-ray diffraction on the modified MOFs show that the crystallinity and cubic

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framework structure of the MOFs is retained for all the modified samples, as evidenced by the consistent peak positions and relative intensities (Figure 3).

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The post-synthetically modified MOFs were also characterised by thermal gravimetric analysis, TGA (see Figure S7). All of the newly modified MOFs showed good thermal

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stability with an initial weight loss of about 10-15 % at ~100 ⁰C. This initial weight loss can be accredited to the loss of residual solvent from the pores of framework. A further weight loss at ~475 ⁰C is observed for all MOFs due to framework decomposition. Moreover, an additional weight loss event is observed at around 220 ⁰C for the newly functionalised MOFs which is not present in the parent UiO-66-NH2 samples. This can be attributed to the thermal decomposition of the thiourea group.

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ACCEPTED MANUSCRIPT The dinitrogen sorption isotherms of the MOFs have also been measured (Figure S8) and the Brunauer-Emmett-Teller (BET) and Langmuir surface areas of all the samples calculated (Table 2). In these experiments the samples were first degassed overnight at 120 °C before surface area determination via dinitrogen adsorption at 77 K. Given that the sorption

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behaviour for MOFs can differ significantly based upon their handling and pre-treatment procedures, the parent UiO-66 and UiO-66-NH2 MOFs were also analysed for comparison

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using an identical protocol.

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Table 2: Calculated surface areas of the parent MOFs and their post-synthetically modified derivatives

SBET

SLangmuir

(m2/g)

(m2/g)

1578

1767

UiO-66-NH2

1293

1514

UiO-66-NHC(S)NHMe

470

540

UiO-66-NHC(S)NHPh

760

953

UiO-66-NCS

534

683

300

304

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UiO-66

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Sample

UiO-66-NCO

A BET surface area of 1578 m2/g was determined for UiO-66 (Table 2) which is similar to literature values reported for this MOF (1580 m2/g) [40]. The BET surface area of UiO-66NH2 was found to be 1293 m2/g which is slightly higher than the literature reported value of 1200 m2/g [40]. The size of the substituent and the degree of post-synthetic modification both play a significant role in dictating the available volume for the nitrogen molecules within the 15

ACCEPTED MANUSCRIPT MOF structure. Thus the surface area of UiO-66-NH2 is reduced relative to that of UiO-66 due to the decrease of internal void space on introduction of the amine groups. In the same way UiO-66-NHC(S)NHMe shows a reduced BET surface area of 470 m2/g when compared to UiO-66-NH2 that can be attributed to conversion of the amine groups to bulkier methyl

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thiourea groups. UiO-66-NHC(S)NHPh has a calculated BET surface area of 760 m2/g and is therefore more porous than UiO-66-NHC(S)NHMe despite the increased steric demands of the phenyl group. This can be explained by the much lower degree of modification –30% for

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UiO-66-NHC(S)NHPh compared to 73% for UiO-66-NHC(S)NHMe (see Table 1). Despite the fact that the isothiocyanate and isocyanate groups in UiO-66-NCS and UiO-66-NCO do

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not provide much steric bulk, these MOFs exhibit significantly reduced BET surface areas of 534 and 300 m2/g respectively. Nevertheless similar observations of much reduced surface areas for isothiocyanate and isocyanate functionalised MOFs have been reported by Cohen et al. on post-synthetic modification of MIL-53(Al) to give MIL-53(Al)-NCO and MIL-53(Al)-

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NCS [45].

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3.2 Batch equilibrium metal cation adsorption studies

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The ability of the new post-synthetically functionalised MOFs to remove metal ions from aqueous solutions was investigated for a series of toxic heavy metal ions (Pb2+, Cr3+, Cd2+, and Hg2+). Their adsorption performance was then compared to that of unmodified UiO-66 and UiO-66-NH2 samples. Adsorption experiments were performed in Falcon tubes using the batch protocol with 15 mg of MOF sample in 15 mL metal ion aqueous solutions and metal ion concentrations in the range 10 to 200 mg/L (see experimental section for full details). After treatment the solution was centrifuged and inductively coupled plasma-optical emission spectroscopy was employed to quantify the metal concentration remaining in the liquid 16

ACCEPTED MANUSCRIPT phase. The metal removal percentage was calculated using Equation 1 where Ci and Cₑ represent the initial and equilibrium metal ion concentrations (mg/L) respectively. The results for the parent and modified MOFs are shown in Figure 4.  ₑ 

x100

(1)

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   % 

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100

Cd²⁺

40

UiO-66-NHC(S)NHMe

UiO-66-NH₂₂

Pb²⁺

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UiO-66

0

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20

Hg²⁺

UiO-66-NCO

60

UiO-66-NCS

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Cr³⁺

UiO-66-NHC(S)NHPh

Metal Removal (%)

80

Figure 4: Percentage metal removal (%) by MOF sample. Extraction conditions: sample solution, 15 ml of a 100 mg/L of target metal ion; MOF, 15 mg; extraction time, 240 mins.

A marked increase in the adsorptive removal of all the metal cations studied was observed for the post-synthetically modified MOFs. This can be attributed to the newly introduced functional groups within their pore structures. In particular, the incorporation of sulphurcontaining groups appears to be crucial for complexation of the soft metal cations, with the heavy metal extraction performance of UiO-66-NCO significantly below that of UiO-6617

ACCEPTED MANUSCRIPT NCS. The removal efficiency for all four of the metal ions studied was best with UiO-66NHC(S)NHMe or UiO-66-NCS, giving up to a 25-fold improvement in metal extraction over UiO-66 and UiO-66-NH2. Thus treatment of a 100 mg/L mercury(II) aqueous solution with UiO-66-NHC(S)NHMe led to a 99.4% reduction in mercury(II) concentration to give a final

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value of just 0.63 mg/L. In comparison, a similar experiment with UiO-66 yielded only a 4.0 % reduction to give a final mercury(II) concentration of 96.04 mg/L. The high conversion rate for the post-synthetic modification step leading to UiO-66-NHC(S)NHMe (Table 1)

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combined with the low steric demands of the methyl thiourea group give this MOF the

forward for further investigations.

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overall best performance. Based on these promising results UiO-66-NHC(S)NHMe was taken

Equilibrium studies can be used to determine the maximum adsorption efficiency of the adsorbents and assess their potential practical applications. The adsorption statistics are usually conveniently represented by adsorption isotherms relating the mass of solute

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adsorbed per unit mass of adsorbent to the solute concentration in the solution at equilibrium [46]. Aqueous solutions of Cd2+, Cr3+, Hg2+ and Pb2+ in the concentration range 10 to 200

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mg/L were used to generate Langmuir isotherms for UiO-66-NHC(S)NHMe. The maximum adsorption capacity of the MOF for each of the heavy metal cations was calculated from the

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isotherms. These calculations assume that complete monolayer coverage of the metal cation on the adsorbent surface leads to saturation. According to this model binding sites should be homogeneous with equivalent adsorption energies and there are no interactions between adsorbed species [23]. The linear equation of the Langmuir isotherm is given in Equation 2 where qm is the maximum adsorption capacity (mg/g) and KL is the Langmuir constant corresponding to the energy of adsorption which is calculated from the linear plot of Ce/qe versus Ce (Figure 5). Mass balance Equation 3 was used to evaluate the amount of metal

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ACCEPTED MANUSCRIPT adsorbed per unit mass of adsorbent (q, mg/g), where V is the volume of metal solution (L), and W is the amount of MOF material used (g) (Table S2).





  

+



(2)



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%

!ₑ  "# − "ₑ

(3)

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Figure 5: Langmuir isotherm plots for UiO-66-NHC(S)NHMe

The experimental data is in very good agreement with the Langmuir isotherm model with correlation values greater than 0.99 for all the metals studied (see Supplementary information). The maximum adsorption capacities (qm) calculated for Cd2+, Cr3+, Hg2+ and Pb2+ cations on UiO-66-NHC(S)NHMe are 49, 117, 769 and 232 mg/g respectively. These 19

ACCEPTED MANUSCRIPT values are significantly higher than those reported for other conventional adsorbents [47, 48, 49] and are comparable to or exceed values reported for other highly porous functionalised MOFs: for example Morsali and co-workers [50] have calculated qm values for TMU-5 of 43

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HKUST-1 to exhibit a qm value of 714 mg/g for Hg2+.

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(Cd2+), 127 (Cr3+) and 251 (Pb2+) mg/g and Qiu et al. [27] showed thiol functionalised

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4. Conclusions

UiO-66 frameworks have been modified and studied to address environmental issues of heavy metal pollution. Thiophosgene, diphosgene, and a series of isothiocyanates were employed in the post-synthetic modification of UiO-66-NH2 to give, for the first time, UiO-

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66 frameworks with pendent sulphur containing groups within the pores of the material. The degree of functionalization was monitored using 1H NMR and IR spectroscopies, whilst PXRD studies and TGA analysis confirmed the structural and thermal integrity of the

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functionalised MOFs. In comparison to UiO-66 and UiO-66-NH2 the porosity of the new functionalised materials (as measured using BET surface area calculations) is somewhat

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reduced. Nevertheless, all novel functionalised MOFs showed very good or excellent adsorption capacities for the heavy metal cations Cd2+, Cr3+, Hg2+ and Pb2+, performing significantly better that the parent UiO-66 and UiO-66-NH2 MOFs. Of the new MOFs reported, UiO-66-NHC(S)NHMe showed the best capacity for heavy metal removal, especially with mercury(II) cations. This can be explained by the high conversion rate (73%) in the amine to methyl thiourea post-synthetic modification step used to prepare this MOF, combined with the low steric demands of the methyl thiourea group. Adsorption equilibrium isotherms were obtained for UiO-66-NHC(S)NHMe and were shown to fit well with the 20

ACCEPTED MANUSCRIPT Langmuir isotherm model with correlation coefficients of >0.99. The maximum adsorption capacities (qm) for Cd2+, Cr3+, Hg2+ and Pb2+ on UiO-66-NHC(S)NHMe were calculated to be 49, 117, 769 and 232 mg/g respectively. Our results indicate that incorporation of thiourea groups dramatically improves the heavy metal removal efficiency of UiO-66 frameworks due

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to the high affinity of sulphur for heavy metal ions. Existing MOF materials which have been reported for heavy metal capture mostly require challenging preparative routes and / or exhibit poor stability in aqueous solutions. In contrast, the high maximum adsorption

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capacities for heavy metals exhibited by these new modified UiO-66 MOFs, coupled with their relatively straightforward synthesis and the known high chemical stability of the UiO-66

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family of MOFs, make them of interest for further studies in the treatment of industrial waste

Acknowledgements

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effluents.

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This work was financially supported by Imperial College London and the International Research Support Initiative Program of Higher Education Commission, Pakistan. Thanks to

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Camille Petit for collection of the gas sorption data and Paul Lickiss for useful discussions.

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Novel functionalised Zr-based MOFs prepared using post-synthetic modification

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Pendant sulphur-containing groups have been introduced into the pores of the MOFs Functionalization markedly improved heavy metal removal efficiency

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High performance material with facile preparative route and good aqueous stability

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Electronic Supplementary Information

Investigations on post-synthetically modified UiO-66-NH₂₂ for the

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adsorptive removal of heavy metal ions from aqueous solution Hira Saleem1, 2, Uzaira Rafique2 and Robert P. Davies1* 1

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Department of Chemistry Imperial College London South Kensington London UK SW7 2AZ

Department of Environmental Sciences Fatima Jinnah Women University Rawalpindi Pakistan.

Contents 1

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* Corresponding Author E-mail: [email protected] Fax: +44 870 1300438 Tel: +44 207 5945754

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H NMR Analysis ...................................................................................................................................... 2

Thermogravimetric Analysis .................................................................................................................... 5 Surface Area Analysis .............................................................................................................................. 5

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Adsorption Analysis Data ......................................................................................................................... 6

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NMR Analysis

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Figure S1. 1H NMR spectrum of UiO-66-NHC(S)NHMe digested with 1 M NaOD.

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1H

Figure S2. 1H NMR spectrum of UiO-66-NHC(S)NHPh digested with 1 M NaOD. 2

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Figure S3. 1H NMR spectrum of UiO-66-NHC(S)NHtBu digested with 1 M NaOD showing no evidence of modification.

Figure S4. 1H NMR spectrum of UiO-66-NHC(S)NHCy digested with 1 M NaOD showing no evidence of modification.

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Figure S5. 1H NMR spectrum of UiO-66-NCS digested with 1 M NaOD.

Figure S6. 1H NMR spectrum of UiO-66-NCO digested with 1 M NaOD.

4

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Thermogravimetric Analysis 100

UiO-66-NH₂ UiO-66-NHC(S)NHMe

90

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UiO-66-NHC(S)NHPh UiO-66-NCS

Weight %

80

UiO-66-NCO

70

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60 50

100

200

300

400

500

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40 0

600

700

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Temperature °C

Figure S7. Thermogravimetric analysis of post-synthetically modified MOF samples under air.

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Surface Area Analysis

UiO-66 UiO-66-NH₂ UiO-66-NHC(S)NHPh

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400

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Quantity adsorbed (cm³/g)

500

UiO-66-NCS UiO-66-NCO UiO-66-NHC(S)NHMe

100

0

0

0.2

0.4

0.6

0.8

1

P/Pₒ

Figure S8: Effect of functionalisation on dinitrogen sorption behaviours of modified and unmodified MOF samples. 5

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Adsorption Analysis Data Table S1: Equilibrium isotherm parameters to calculate maximum adsorption capacity of modified MOF, UiO-66-NHC(S)NHMe. Extraction conditions: sample solution, 15 ml of target metal ions; MOF, 15 mg of UiO-66-NHC(S)NHMe; extraction time, 240 mins.

Cr³⁺⁺

Hg²⁺⁺

Cₑ/qₑ

10 50 100 200 10 50 100 200 10 50 100 200 10 50 100 200

2.61 25.28 65.17 156.21 2.09 12.46 32.80 106.26 0.05 0.29 0.63 1.44 0.59 3.32 8.99 33.79

7.39 24.72 34.83 43.79 7.91 37.54 67.20 93.74 9.95 49.71 99.37 198.56 9.41 46.68 91.01 166.21

0.353 1.023 1.871 3.568 0.263 0.332 0.488 1.134 0.005 0.006 0.006 0.007 0.063 0.071 0.099 0.203

qm (mg/g)

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qₑ (mg/g)

49

118

769

233

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Pb²⁺⁺

Cₑ (mg/L)

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Cd²⁺⁺

Cₒ (mg/L)

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Target metal ion

6