Synthesis and Characterization of UiO-66-NH2 MetalâOrganic

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Materials and Interfaces

Synthesis and Characterization of UiO-66-NH2 MetalOrganic Framework Cotton Composite Textiles Meagan Bunge, Aaron Davis, Kevin Neal West, Christy Wheeler West, and T. Grant Glover Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01010 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Industrial & Engineering Chemistry Research

Synthesis and Characterization of UiO-66-NH2 Metal-Organic Framework Cotton Composite Textiles Meagan A. Bunge, Aaron B. Davis, Kevin N. West, Christy Wheeler West, T. Grant Glover* Department of Chemical and Biomolecular Engineering, University of South Alabama 150 Jaguar Dr., SH4136, Mobile, Alabama 36688, United States *E-mail: [email protected] Abstract This work details the development of a cotton fabric functionalized with UiO-66-NH2 MetalOrganic Framework (MOF). The materials were made by seeding the growth of the MOF on the cotton by first bonding zirconium (Zr) to the surface of the fabric utilizing cyanuric chloride modified with a thiol. After seeding the fabrics with Zr, UiO-66-NH2 was grown on the fabric using a hydrothermal method. Several different routes of attaching Zr to cyanuric chloride were examined. Scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD) data are consistent with UiO-66-NH2, and the fabrics have surface areas between 45-125 m2/g depending on the synthetic conditions used to produce the materials. The functionalized cotton reacts with dimethyl 4-nitrophenyl phosphate (DMNP), a chemical nerve agent simulant, as monitored by UV-Vis spectroscopy. The results illustrate that MOFfiber composites can be created using natural fibers and the resulting composites provide similar chemical warfare agent (CWA) simulant reactivity as observed on MOF-polymer composites. Introduction Adsorbent materials are frequently designed and screened to determine the effectiveness of the adsorbent to capture toxic gas and neutralize chemical warfare agents (CWAs).1–8 Of these materials, Metal-Organic Frameworks (MOFs) provide a unique opportunity to tailor the functionality of a material, and it has been shown computationally that hundreds of thousands of MOFs can be constructed and experimentally thousands of MOFs have been synthesized.9–11 Additionally, MOFs have shown particularly unique reaction and adsorption chemistry when interacting with toxic compounds when utilized in packed-bed filtration applications.2 The unique MOF reaction chemistry observed in packedbeds is appealing, and if MOFs could be attached to textiles, fibers, and surfaces the reactions observed in a packed-bed may be useful in garments, personal protective equipment, tents and other devices. Additionally, the reticular nature of MOFs has provided methods to attach MOFs to surfaces that, in some cases, are not available to traditional adsorbent materials, such as carbon.12–30 A variety of methods have been used to functionalize fibers with MOFs, such as atomic layer deposition (ALD), electrospinning, layer-by-layer methods, solvothermal synthesis, methods using reactive dyes, and the direct growth of MOFs on fibers.12–22 These techniques have been used to modify different types of fibers including synthetic polymers, cotton, silk, and nanofibers. Of these results the works that have shown that MOF-functionalized fibers have been used to neutralize chemical warfare agents are of particular interest regarding the design of fibers for defense applications.23,24,30 Two manuscripts have detailed the growth of MOFs on cotton textiles, and in both cases, these works have focused on the growth of Cu-BTC on the fibers.18,20 In the work of Bunge et al. the fibers are shown to have surface areas between 671 and 976 m2/g, electron micrographs showed defined crystals on the surface, and powder X-ray diffraction (PXRD) data were consistent with Cu-BTC.20 The data also

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showed that the adsorption of ethane and ethylene on the fiber was consistent with the adsorption properties of the unbound MOF. Unfortunately, Cu-BTC is not stable in ambient air and is therefore not practical for the development of fibers that can neutralize chemical weapons. A large number of water stable MOFs have been reported in the literature, and among them, zirconium (Zr) based MOF materials in particular have been shown to neutralize CWAs and agent simulants.31–36 The Zr-based UiO-66 MOFs are of interest because when designing MOFs for fibers that will be utilized in defense applications, because this series of MOFs has been shown to be mechanically stable, stable after exposure to water in both liquid and vapor phases, and stable to numerous solvents including acetone and chloroform.37–42 In addition to having broad stability to solvents, UiO-66 can be tailored with different functional groups, and UiO-66-NH2 has been shown to neutralize organophosphorus nerveagents and stimulants, such as dimethyl 4-nitrophenyl phosphate (DMNP), and adsorb toxic gases including ammonia (NH3) and cyanogen chloride (CNCl). 23,24,32,43,44 Moreover, the reactivity of UiO-66NH2 and DMNP is maintained even when UiO-66-NH2 is added to polyvinylidene fluoride (PVDF), attached to nonwoven polypropylene (PP) fibrous mats, or attached to silk the resulting composites exhibit reactivity towards CWAs.21,23,24 Although polypropylene and PVDF provide a reasonable basis for a composite, cotton materials are more commonly used in commercial textiles and the application of UiO-66-NH2 to cotton fibers to neutralize toxic compounds has not been detailed. Just recently, a paper was published detailing the growth of UiO-66 on cotton via a process that places the cotton in a scouring solution prior to MOF growth.46 This work is promising, but questions remain about the control of MOF growth, the growth of UiO-66-NH2 on the fabric, the impact of synthesis conditions on the surface area of the fabrics, the efficacy of attachment methods relative to simply placing unmodified cotton fabric in the MOF synthesis solution, and reactions between the fabric CWA simulants. Therefore, the purpose of this work is to synthesize UiO-66-NH2 on cotton textiles using solution chemistry and to subsequently evaluate the activity of MOF fiber composite material to neutralize a chemical warfare agent simulant. A reactive dye method (RDM) that has been previously used to attach quantum dots, Cu-BTC, and gold nanoparticles to both nylon and cotton textiles is used to attach Zr to cotton to seed MOF growth.20 The concept is presented in Figure 1, where cyanuric chloride, a common covalent bonding agent used in reactive dyeing of textiles, is attached to the cotton surface, and a binder is used to attach zirconium, which in turn is used to seed the MOF growth.45,46 Several variations of the synthesis methods can be considered when examining the synthetic concept. For example, different molecules can be used as the binding agent for Zr, other UiO-66-NH2 synthesis procedures can be examined, and various sources of zirconium can be used. This work examines each of these synthetic variations, and detailed results are presented for the conditions that produced optimal results. Experimental Because of the wide variety of experimental parameters that were considered, several synthetic methods were used. For brevity, only the procedures that produced the optimal results are presented in this section, and the remaining procedures are reported in the Supporting Information (SI). This work specifically examined the growth of UiO-66-NH2 on fibers using three different MOF synthetic methods, three different binder groups used to attach Zr, and three different sources of zirconium.


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Pure Powder MOF Synthesis To produce UiO-66-NH2 three methods were utilized: 1) the procedure of Katz et al. and in this work is referred to as the “HCl addition” method where HCl is added as a modulator to increase MOF yield47 2) a low acid method by Walton and Tulig where the formation of HCl is avoided in the MOF synthesis37 and 3) a procedure published by DeCoste et al. that was a variation of the Cavka et al. method, which is the first reported UiO-66 synthesis.39,40 Synthesis of MOF Cotton Textile Composites A scheme is shown in Figure 1 where cysteamine (an amino thiol) is used to bind Zr to the cotton. Each of the synthetic procedures examined in this work considers a variation of this scheme. For the synthetic section below, and in some references in the manuscript, as well as each of the synthetic sections listed in the SI, the section header details the following: “the binder molecule, the zirconium source, and the MOF synthesis method” Cysteamine, Zirconium Chloride, and HCl Addition Zirconium Seeding Step Nine swatches measuring approximately 0.5” x 0.5”, were cut from a white cotton Faded Glory brand Tshirt. These fabrics were washed with soap (manufacturer and details in SI) and water and then rinsed with chloroform (CHCl3). Then the fabric swatches were placed in a 100 mL beaker containing a 65 °C solution of 5 g of sodium carbonate dissolved in 50 mL of water and allowed to stir for 5-10 mins. In an Erlenmeyer flask, 1.88 g of cyanuric chloride was dissolved in 40 mL of chloroform with stirring. The fabric was added to the flask, capped with a vented stopper and allowed to stir for 1 hour at room temperature. The fabric was then taken out and added to a beaker that contained 20 mL of water and 0.5 g of cysteamine. After stirring for 22 hours, the fabric was collected and washed with water and chloroform. Zirconium chloride (ZrCl4), 0.2 g, and 10 mL of N,N-dimethylformamide (DMF) were added to a 20 mL vial and sonicated for approximately 5 mins. The washed fabric was added to the vial and stirred for three days. MOF Synthesis Step After adding Zr to the surface, the MOF synthesis step was carried out. Two 20 mL vials were used; one vial contained 0.125 g ZrCl4, 5 mL DMF, and 0.3 mL of HCl and the second vial included 0.135 g of 2-aminoterephthalic acid and 10 mL DMF. The fabric was added to the first vial with ZrCl4 and sonicated for 20 mins, while the second vial stirred on a stir plate. Once the first vial was finished sonicating, the contents from the second vial were added and sonicated for another 20 mins. After sonication, the vial was placed in the oven to bake for 24 hours at 120 °C. The sample was then removed from the oven, allowed to cool, and collected via vacuum filtration. Modified fibers and any MOF particles that did not attach to the fibers were washed with DMF and then ethanol. Both the fabric and the powder were collected in separate vials. The vials were left open to allow the products to dry. Control: MOF Fabric without RDM using HCl Addition method As before, nine 0.5” x 0.5” swatches were cut from a cotton T-shirt. They were then added to a 20 mL vial containing 0.125 g zirconium chloride, 5 mL DMF, and 0.3 mL HCl. This vial was sonicated for 20 mins. In a separate 20 mL vial, 0.135 g of 2-aminoterephthalic acid and 10 mL DMF were combined. After sonicating the first vial, the contents of the second vial were added to the solution and sonicated another 20 mins. Next, the vial was placed in a 120 °C oven and baked for 24 hours. After baking, the vial was removed and allowed to cool before filtering. During filtration the sample was



washed with DMF then ethanol. Both the fabric and the MOF powder that was not attached to the fibers were collected and stored in different vials. This fabric is designated as the control fabric, while plain cotton has no MOF or RDM. The plain cotton was used as received without any washing or modifying. Replicate MOF Synthesis Step The impact of repeating the MOF growth step on the properties of the composite was also examined. In particular, the MOF growth step was repeated on fabrics prepared using the cysteamine, zirconium chloride, and with the HCl addition MOF growth method. To accomplish this task, after the previously mentioned modification steps, six swatches were exposed to the MOF synthesis step again following the same MOF procedure as described previously. One vial had 0.125 g ZrCl4, 5 mL DMF, and 0.3 mL HCl. The remaining pieces of cotton that had been previously modified were added to this vial and sonicated for 20 mins. A second vial with 0.135 g 2-aminoterephthalic acid and 10 mL DMF was combined and stirred while the first vial sonicated. Once vial one was finished sonicating, the second vial was added to it and sonicated for another 20 mins. After, the vial was placed in the oven and baked for 24 hours at 120 °C. The vial was then removed from the oven and allowed to cool prior to filtration. The sample was filtered and washed with DMF and ethanol; the fabric sample and powder were collected in different vials and allowed to air dry in the open vials. This same process was repeated again with three of the remaining pieces of fabrics to produce swatches having a total of three MOF synthetic steps. Characterization of Samples Powder X-ray Diffraction PXRD was taken on a Proto AXRD Benchtop Powder Diffraction System. Samples were scanned from 5-50° with a step size of 0.02° and a dwell time of 10 seconds with 20 mA and 30 kV. Care was taken to prepare the fabrics for PXRD analysis to minimize the error associated with sample thickness. In some cases, the fabric would curl after modification with MOF and was difficult to lay flat for PXRD analysis. Therefore, a sample holder for the instrument approximately 1” in diameter with an exterior wall approximately 0.25” high forming a cup like shape was used. To prepare the samples, a piece of clay was placed in the cup and a zero-background disk placed on top of the clay. Then fabric samples were placed on a zero-background disk, and the plate was pressed down into the clay, so the fabric sample would remain as flat as possible to run the analysis. Sample preparation may have a significant impact on the analysis of fabric samples depending on how the sample is prepared. Adsorption Isotherms A Micromeritics ASAP 2020 was used to measure BET surface area using N2 at 77K. Samples were outgassed at 150 °C with a ramp rate of 1 °C/min overnight while under vacuum on a Schlenk line and transferred to the 2020 sample holders under a UHP nitrogen environment. The Rouquerol correlation was used for all BET area calculations.48 Scanning Electron Microscopy An Evo 50 scanning electron microscopy (SEM) operating in variable pressure mode was used to collect images of the fibers at 500X and 1500X magnification. Samples were collected without gold sputtering.


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Evaluation of Chemical Agent Simulant Reactivity A Thermo Evolution 300 UV-Vis Spectrometer was used to test the reactivity of the fabrics with DMNP. A buffer solution was made by adding 0.5183 g of 4-ethylmorpholine into a 10 mL volumetric flask and then filling with water. Fabric pieces were cut into smaller pieces for this experiment. A vial with a fabric sample and 1 mL of the buffer solution was stirred for 30 mins. Approximately 4.5 µL of DMNP was then added to the vial. The conversion of the DMNP was evaluated throughout the reaction by collecting 20 µL aliquots periodically from the mixture and added to a beaker containing a new 10 mL buffer solution as prepared previously. The reaction was conducted at room temperature without temperature control. To calibrate, 4-nitrophenol and sodium hydroxide were mixed to form sodium 4nitrophenoxide. The calibration curve was used to find the molar absorptivity, which was then used to calculate the concentration of p-nitrophenoxide from the Beer-Lambert equation during the DMNP reaction. Attenuated Total Reflectance-Fourier Transform Infrared Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) data was collected on a Thermoscientific Nicolet iS50 FT-IR diamond tip anvil. A background scan of eight scans were ran before each sample was analyzed. Each fabric was scanned for 200 scans with at a resolution of 4. Thermogravimetric Analysis A NETZSCH TG 209 F1 was used for thermogravimetric analysis (TGA). Samples were heated with a ramp rate of 3 °C/min until reaching 800 °C while under air flow. Results were used to determine the percent of MOF on the fibers as previously described in the literature.23 Results and Discussion Numerous synthesis methods were examined, and the results are shown in Table 1 and 2. The HCl addition method was studied because the addition of HCl as a modulator in the synthesis of UiO MOFs increases MOF yield.47 However, to prevent excessive damage to the fibers from the HCl, only a small amount, 0.3 mL 0.645 M, was used during MOF synthesis. The UiO MOF synthesis process of Tulig and Walton that does not form HCl during synthesis was also examined in an effort to limit the exposure of the cotton to HCl.37 Of the methods examined, some methods produced results with poor PXRD patterns with broad peaks, or in some cases no peaks, indicating poor MOF formation. Likewise, a range of surface areas, 15261 m2/g, was observed for the different methods examined. The results from these experiments are summarized in Table 1. The table shows that it is possible to precipitate high surface area Zr material on cotton that are not UiO-66-NH2. Also, the method of Cavka, produced materials with UiO-66-NH2 PXRD patterns, but samples obtained with this method were more inconsistent than those obtained using acid modulation. Also, as noted on the table PXRD data was not collected for the [glycine, zirconium propoxide, and low acid] method because this sample produced large clumps of MOF deposits on the fibers, which would skew the results of both the PXRD and BET analysis, and this method was not examined further. Although not studied in detail, a single sample was produced using [glycine, zirconium chloride, and HCl addition] that had similar surface area and PXRD pattern as the similarly prepared cysteamine sample indicating that glycine could also be used as a binding agent. The best results, as identified with BET and PXRD experiments, are summarized in Table 2 and were obtained when [cysteamine, zirconium chloride, and HCl addition] and [4-aminobenzoic acid, zirconium chloride, and



HCl addition] samples were used to grow UiO-66-NH2 on cotton. These conditions produced the most defined UiO-66-NH2 PXRD patterns and surface areas that were higher than the controls. The growth of UiO-66-NH2 via some methods has been shown to be quite sensitive to experimental conditions.47 Therefore, it is not surprising that when cotton is added to these synthetic processes that some methods are less effective at producing a MOF functionalized textile. The use of acid modulators has been shown to increase MOF defects; however, the presence of defects in the MOF crystal was less of a concern because these defects sites may in fact facilitate reaction and mass transfer of organophosphate species, such as CWAs or CWA simulants. For comparison, a control fabric was produced by placing the as-received cotton without any RDM or surface treatments in the MOF hydrothermal reaction solution. This control provides an indication of how many MOF crystals are intercalated on the MOF fibers simply by allowing the MOF to form in the presence of a tortuous fiber. All other synthetic combinations produced some combination of results, but only with comparison to the control can the impact of the method be assessed. To gain insight about the changes of the fiber surface during the fiber modification process, a universal indicator was used to monitor the pH of fibers after each step of RDM, and a thiol indicator, Ellman’s Reagent (5,5’-Dithiobis-(2-nitrobenzoic acid)), was used to test the fibers after the cysteamine step, as shown in Figure 2. As described in the experimental procedure, several pieces of MOF fabric were made at one time by having multiple pieces of fabric in each reaction vial. To test pH a single piece was removed at each step during the process. The pH scale of the indicator is: 4.0 –Red, 5.0 –Orange Red, 6.0 –Yellow, 7.0 –Yellow-Green, 8.0 –Green, 9.0 –Blue, and 10.0 –Violet. The selected swatch was placed in a 20 mL vial and drops of indicator were placed on the fabric. The pH indicator was also applied to a plain piece of cotton unwashed and washed before the RDM process to gather the baseline pH of the cotton. The changes in the observed pH are consistent with the synthetic process, with the cotton after the sodium carbonate step showing a violet color consistent with a basic pH. Continuing through the process, the fabric changes to a yellow color after the addition of cyanuric chloride, and yellow/green after the addition of cysteamine. The cotton becomes more acidic after soaking in the zirconium solutions. To test for the presence of thiols, 0.5 g of Ellman’s Reagent and 10 mL of water were combined and stirred in a 20 mL vial. After the cysteamine step, a piece of fabric was taken out and the mixture was dripped onto the fabric. To evaluate the binding of the cysteamine to the cotton, an additional swatch was washed with soap and water and chloroform prior to the thiol test. The purpose of the two washing steps was to ensure that unbound starting material, both water soluble cysteamine and water insoluble cyanuric chloride, were removed prior to the test. This test was also completed on a plain piece of cotton as a control and after the cyanuric chloride step, as shown in Figure 2b. The yellow change in color of the RDM sample confirmed the presence of sulfur via cysteamine on the fiber. The color of the thiol indicator is evenly distributed across the swatch providing some indication about the uniformity of the thiols on the surface of the fabric. To evaluate the reproducibility of the method, all steps were completed in triplicate, and the surface area results for the modified fabrics and controls are as shown in Table 2. Multiple rounds of MOF synthesis were applied to the fibers to increase the loading of the MOF on the fabric swatches. Up to three steps of MOF procedures were applied to the fabric pieces. BET surface area and PXRD patterns were collected after each round of MOF synthesis to monitor the changes to the fabrics characteristics. Fibers functionalized with Zr using cyanuric chloride, cysteamine, and one MOF synthesis step produced surface areas higher than the control and had PXRD patterns consistent with UiO-66-NH2 pure powders.


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The PXRD peaks are broad due to the MOF being bound to the amorphous cotton and possible defects in the MOF structure. Repeating the MOF synthesis step increased the BET surface area and produced a modest increase in the PXRD pattern, as shown in Table 2 and Figure 3. To identify the effect of the RDM to facilitate MOF growth on the fibers, two-sided hypothesis testing was completed using the surface areas of the samples and a 95% confidence interval. As detailed elsewhere, this test examines data against the standard normal random variable, Z, and considers the data statistically valid when Z ≤ -1.96 or Z ≥ 1.96.49 Equation 1 solves for Z where ӯ is the mean of the modified sample, µ is the mean of the control sample, σ is the sample standard deviation of the control sample, and n is the number of sample runs. A two-sided test was used to avoid assuming that the RDM MOF process would increase surface area during each MOF synthesis step. It is possible that subsequent MOF synthesis steps would reduce the surface area if MOF crystals are removed from the surface or the fibers are damaged. The BET surface area of all samples was collected once except for control run 2 with two MOF synthesis steps, which was run twice. The two runs were averaged and displayed in Table 2. =

/√

Eq. 1

From the results, the cysteamine modified fibers with two MOF synthesis steps had the greatest impact on fiber area and demonstrate that the RDM MOF growth method increases fibers surface area with a 95% confidence. However, when modified fabric went through a third MOF synthesis step, RDM was shown to not have a significant impact on the fibers compared to the control because the Z value was not in the valid range. The swatches produced using the 4-aminobenzoic acid produced results similar to the cysteamine samples. A single round of the MOF procedure obtained PXRD patterns consistent with the pure powder and had higher surface area compared to the control sample. There was an increase in surface area after the second MOF synthesis and the crystallinity also improved. When a third MOF synthesis step was applied to the fabrics, the surface area increased and the PXRD patterns exhibited crystallinity, but relative to the control this synthesis step was not effective. Table 2 displays the surface area results from the runs. Hypothesis testing indicates that the RDM steps before the MOF process were a critical step to improve the loading of MOF particles on the modified fibers. These results emphasize the importance of comparing the modified fibers to a control sample because even without modification using a RDM, the control fabric acquires a significant surface area from MOF particles simply being intercalated in the fibers of the cotton. To estimate the loading of MOF on the fibers a surface area method was used utilizing the following equation:

= wt % of fabric that is MOF ∗ 1MOF powder surface area 6

78

Eq. 2

In this case, the surface area of the MOF fabric composite can be measured, and it can be assumed that the area of the fabric without MOF is negligible. Therefore, any area can be attributed to the wt % of the MOF powder added. For example, a fabric with surface area of 180 m2/g utilizing a MOF with 1800 m2/g would contain 10 wt % MOF. A plot can be produced based on this expression by varying the MOF wt % from 0 to 100% to illustrate the impact of MOF area on total fabric area. To illustrate how UiO-66NH2 fabrics might compare to other functionalized fibers, three representative materials with different



surfaces areas, UMCM-1, Cu-BTC, and ZnO nanoparticles, were included on this plot, as shown in Figure 4. UMCM-1, Cu-BTC, and ZnO nanoparticle fibers were not prepared. Rather, the data shown in the plot is solely a function of the common literature reported surface area of the UMCM-1, Cu-BTC, and ZnO nanoparticles powders, and is used to illustrate how the surface area of a fabric composite is dependent on the area of the attached structure. Typically, in these experiments, samples were 0.5 in2 and 3-5 swatches were used for surface area analysis. Therefore, the total contribution to the surface area by the swatches by geometry and neglecting any area of the fibers within the body of the swatch is 0.0013 m2/g and is small relative to the measured surface area by nitrogen porosimetry. When the porosity of the pure fabric swatches was measured by nitrogen porosimetry the results were within the error the adsorption measurement of the 2020. The variation of BET surface area for different sample preparations of UiO-66-NH2 is shown Table 1. It is also possible that the wt % MOF could be estimated by heating the composite materials until all the zirconium in the MOF has been reduced to zirconium oxide, and then use the structure of the MOF to determine the mass of MOF that would produce the measured amount of zirconium oxide. In this case, it is convenient to assume the MOF formed on the fabric is defect free. Measurement of the MOF wt % by monitoring the mass change in the fabric is also possible, but it is difficult because of the water retained by the fabric from synthesis and/or ambient air exposure. In all three approaches assumptions are required that may impact the calculated wt %. However, to provide a point of comparison between the surface area technique and heating the fabric to recover the zirconium oxide, the modified cotton that went through two MOF synthetic steps was also heated in the TGA and the method discussed by Peterson et al. was used to calculate the MOF wt % loading.23 Using this method, the 2X MOF fabric contains 17 wt % MOF, which is approximately 10% higher than when calculated using the surface area method described above. To provide the most conservative estimate of MOF loading, the surface-area based calculation of 7.5 wt % was to describe the 2X MOF sample and the surface area technique was used to calculate the wt % for all fabric samples. The TGA data used for the wt % loading calculation are provided in the SI. From the three rounds of MOF synthesis, UiO-66-NH2 modified fibers exhibited between 5-29 wt % loadings on the fibers. The surface area of the composite is appropriately dependent on the weight of the cotton, and the cotton has negligible surface area relative to the MOF. During the course of these experiments, UiO-66-NH2 was produced as a pure powder and characterized. In addition, powdered UiO66-NH2 that did not attach to the surface of the fibers collected at the bottom of the synthesis vials. In the analysis above, the surface area of the powder that did not attach to the cotton was used to calculate the wt % MOF for all the fabrics because it was assumed that the powder that was not attached to the fibers was a more accurate representation of the MOF that was on the fiber versus MOF powder prepared in solution without the fabric being present. In general, the surface area of the MOF powder that did not attach to the fibers and the surface area of the MOF powder produced in solution without fabric present were similar and only varied by 5-10%. To investigate the changes in the fabric surface after MOF growth, ATR-FTIR data were collected and are shown in Figure 5 When comparing the blank fabric with the thiol treated fabric there are changes to the IR spectrum. The stretching at 1518 cm-1 (C=N) and 789 cm-1 (C-Cl) indicates the presence of the cyanuric chloride core. The stretching of the N-H and S-H (2650 (w) νS-H 1627 (w) δN-H 1518 (w) νC=N 890 (w) δS-H 789 (w) νC-Cl ) coupled along with the positive Elleman’s test confirms the presence of cysteamine.


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In the UiO-66-NH2 fabric IR spectrum, the C-Cl stretch from cyanuric chloride is still present and confirms that the covalently bound anchor was not lost during the MOF synthesis. The strong stretching at 1575 cm-1 is due to the C=O of the MOF linker, as well as the new IR signal at 1502 cm-1 (C=C). The new band at 485 cm-1 is attributed to the new Zr-O bond formed during the MOF synthesis (1575 (st) νC=O 1502 νC=C 1256 (m) δC-N 770 (st) δN-H 485 (st) νZr-O). The direct monitoring and quantification of the reaction of the OH groups on the fabric can be difficult. Specifically, while many chemical species and bonds can be readily identified as powders and solutions using technique such as NMR and FTIR, when these species are placed on the surface of a fabric, and in particular a cellulose fabric, identification becomes more difficult due to the signals produced by the cellulose. Additionally, it can be difficult to distinguish between materials entrained in the texture of the fibers and materials covalently bond to the surface. Moreover, it can be difficult to identify the extent to which OH groups have reacted because the OH region in the FTIR data is broad due to the large number of OH groups in cellulose and adsorbed water. SEM micrographs were used to observe changes to the fiber surface. Figure 6 exhibits the comparison of the plain cotton fabric 6a and a sample with two MOF steps using cysteamine 5b at 500X magnification. A change in surface texture is apparent between the two samples. Fibers of the plain fabric are smooth compared to the UiO modified fibers, where deposits are observed; however, the UiO66-NH2 material does not uniformly coat the surface of the fibers. Images were collected without gold sputtering and in variable pressure mode of the SEM. The non-uniform coverage of the fiber maybe occurring because of gaps in fiber coverage from each of the synthetic steps in the process used to produce the MOF fibers. These gaps may occur where cyanuric chloride did not bind to the cotton, where cysteamine did not react to the cyanuric chloride, where Zr did not attach to the thiol, or where unexpected reactions occurred, such as crosslinking of cotton fibers via cyanuric chloride in which case the second chlorine of the cyanuric chloride is consumed by reaction with another fiber and is no longer available for reaction with cysteamine. It is also possible that a portion of the coating is removed during the solvent washing step. In this work, a set of synthetic conditions needed to produce a UiO-66-NH2 cotton were identified, and it is likely that these conditions could be optimized to increase MOF coverage as observed by SEM and identify which of the above items are likely to impact surface coverage, but this is beyond the scope of the current work. To evaluate the ability of these modified fibers to neutralize CWAs, the reaction between the CWA simulant, DMNP, was monitored in aqueous solution.50 UiO-66 and UiO-66-NH2 have been shown to hydrolyze DMNP previously and it is a convenient simulant because the reaction can be monitored with UV-Vis spectroscopy.23,24,33,51 In contact with the modified fibers, DMNP reacts and forms pnitrophenoxide as shown in Figure 7. For these experiments a buffer solution was prepared, the MOF fabric was added to the solution, DMNP was then added to the fabric/buffer solution, and the reactivity was monitored periodically for 90 mins. For comparison, UV-Vis measurements were taken for this reaction in the presence of plain cotton, DMNP in buffer solution only, and pure MOF powder. Figure 8a shows the spectra of the reaction monitored in the presence of plain cotton fabric and Figure 8b show results in the presence of the modified fibers using cysteamine with one MOF step synthesis for the DMNP reaction. Plain cotton fibers exhibited little reaction with DMNP as shown in Figure 8a. From Figure 8b, the p-nitrophenoxide absorbance band at approximately 407 nm increased while the DMNP absorbance band around 275 nm decreased throughout the reaction. The results are consistent with the literature that also shows a decreasing absorbance band for DMNP and an increasing absorbance band for p-nitrophenoxide during the course of the reaction.24,30,32,33 The production of p-



nitrophenoxide is visibly observed with the increased absorbance band at the end of the reaction, which is consistent with the observed yellow color of the reaction solution as shown in Figure 9.24 The Beer-Lambert Law was used to calculate the concentration of p-nitrophenoxide, which was used to calculate the percent conversion over time, which is shown in Figure 10. Consistent with the surface area of the fibers, the fibers that were exposed to two rounds of MOF synthesis show a higher conversion of DMNP. By assuming first-order reaction kinetics, the natural log of the DMNP concentration was used to calculate the half-life for the reaction as shown in Figure 11. In these experiments, the fabric that underwent two MOF synthetic steps contained approximately 7.5 wt % MOF and the sample that was exposed to one MOF synthetic step contained 4.6 wt % MOF providing approximately 1.6 and 1.1 mg of MOF in each experiment as determined using the method discussed in Figure 4. However, these masses are estimates and will depend proportionally on the surface area used in the calculation of the wt % loading. Figure 12 shows the mass of the MOF and the half-life of DMNP for each sample. The dependence of the half-life on the sample mass is shown with the greatest sample mass of pure MOF powder producing the shortest half-life. Also, the 2X MOF and the 1.6 mg pure powder MOF runs have the same half-life indicating that the MOF on the fabric is behaving similarly to the pure powder and is not inhibited by being bound on the surface. The half-life data for the pure cotton should be interpreted carefully as the UV-Vis spectra showed only the loss of DMNP over time and limited production of pnitrophenoxide, as seen by the absence of a strong p-nitrophenoxide signal. The reduction of DMNP signal occurs because the DMNP is absorbed into the cotton fabric. The production of p-nitrophenoxide was detected and the reaction may be occurring between the cotton surface or with the water/buffer solution. To help identify whether the cotton or buffer was forming the p-nitrophenoxide, the spectra of a DMNP water/buffer solution were also collected as a control. At the end of 90 mins of monitoring the DMNP buffer solution was yellow and a weak signal for p-nitrophenoxide was obtained. As a minor point, the DMNP signal of the buffer only solution increased slightly over the length of the experiment as the viscous DMNP mixed throughout the aqueous solution. It is likely that the increase in DMNP signal was not observed in the other experiments as the DMNP reaction was occurring more quickly in the presence of the MOF than the buffered solution. The half-life obtained in the buffer solution (491 mins) versus the plain cotton in buffered solution (433 mins) indicates that the cotton may have a minor contribution to the half-life, likely via surface OH groups. However, the difference between these two experiments may be within the statistical variance of the experiment, and more detailed experiments would be needed to quantify the impact of plain cotton on a reaction of DMNP in buffer. In either case, DMNP half-lives of both the plain cotton and the buffer-only solution are both several times longer than the MOF modified fibers. The half-life times are comparable to those in the literature; however, a detailed comparison is difficult with differences in sample mass and reaction conditions. More broadly, the results show that a uniformly coated fiber is not required to obtain reactivity for this CWA simulant, that a natural fiber can be utilized as a supporting substrate, and that a solution chemistry route is available to modify fibers with MOFs. Moreover, the results indicate that the use of a natural cotton fiber substrate may cause additional complexities in CWA solution experiments as the cotton adsorbs target reagent, which may be further complicated if the OH groups on the surface of the cotton react with the targeted reagent.


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Conclusion Cotton fabrics were modified with UiO-66-NH2 by growing the MOF on the fiber. The process was initiated by binding Zr to the surface of the fiber using cyanuric chloride. A variety of synthetic methods were examined including variations in the MOF synthesis process and variations in the Zr binding steps. The best results were obtained when an HCl modulated UiO-66-NH2 synthesis process was used in combination with either cysteamine or 4-aminobenzoic acid to bind Zr to the surface of the fabrics. Production of the MOF cotton swatch and then repeating the MOF synthesis step increased the surface area of the fabrics but repeating the MOF synthesis process three times did not provide any meaningful increase in surface area relative to the control fabric. The control experiment, where cotton is placed unmodified into the MOF synthesis solution, shows that a significant amount of UiO-66-NH2 can be added to the fibers simply by intercalation of the crystals in the cotton, and underscores the importance of control experiments when functionalizing textiles. The functionalized fabrics react with DMNP producing half-life times comparable to other MOF fiber composites and illustrate that a uniform fiber coating is not required to have a high fabric surface area or react with CWA simulants. With the other reported UiO-66-NH2 fibers being produced via ALD and electrospinning, this is, to the best of our knowledge, the first production of a cotton UiO-66-NH2 fiber and demonstration of its reaction with CWA simulants.



Page 12 of 30

Figure 1. This scheme shows cotton fabric being functionalized with Zr using a reactive dye method. The Zr functionalized cotton is then placed into a hydrothermal UiO-66-NH2 reaction and MOF is grown on fabric (step not shown).


Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Results obtained when fibers are produced using different binders, zirconium sources, and UiO-66-NH2 methods. In most cases these methods did not have PXRD peaks with enough definition to identify the material as UiO-66-NH2. The Cavka method produced functionalized materials but quality samples were not obtained consistently. Binder

Zirconium Source

UiO-66-NH2 Method

BET Fabric Area (m2/g)

UiO-66-NH2 PXRD Pattern

Cysteamine

Zirconium Acetate

HCl Addition

63-67

no

Glycine

Zirconium Acetate

HCl Addition

23, 37, 70

no

Glycine

Zirconium Chloride

HCl Addition

46

yes

Cysteamine

Zirconium Acetate

Low Acid

70, 79

no

Cysteamine

Zirconium Propoxide

Low Acid

70

no

Glycine

Zirconium Acetate

Low Acid

31, 60, 131

no

Glycine

Zirconium Propoxide

Low Acid

231, 261

--

Cysteamine

Zirconium Chloride

Cavka

15, 54, 83

yes



Page 14 of 30

a Plain Cotton

Soap & water wash

Na2CO3

Cyanuric Chloride

Cysteamine

Zr Acetate

DMF & ZrCl4

b Plain Cotton

Cyanuric Chloride

Cysteamine

Cysteamine washed

Figure 2. (a) A universal pH indicator was used to monitor changes in fabric pH during each step of the RDM Zr seeding process. In all cases, the observed pH is consistent with the synthetic steps. To ensure that the cysteamine was bound to the fabric, (b) a thiol indicator was applied to the fabric. The indicator has been applied to plain cotton, fabric functionalized with cyanuric chloride, with cysteamine, and with cysteamine that was also washed with soap and water and chloroform. The indicator shows the cysteamine is not removed with washing and provides uniform coverage on the fabric swatch.


Page 15 of 30

Simulation Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pure Powder 3X MOF 2X MOF 1X MOF Plain Cotton 10

20

30

40

50

2θ (Degree)

Figure 3. PXRD data for the plain cotton, one, two, and three steps of MOF synthesis, and pure powder are shown. The MOF modified fibers were made using cysteamine and ZrCl4. The UiO-66-NH2 peaks on the modified fibers become more defined after each additional MOF step.



Page 16 of 30

Table 2. Control, cysteamine, and 4-aminobenzoic acid fabric surface areas and statistical confidence data. Z is the standard normal random variable, µ is the mean of the control sample, σ is the sample standard deviation of the control, and ӯ is the mean of the modified sample.

Control: MOF Fabric without RDM MOF run

Run 1 (m2/g)

Run 2 (m2/g)

Run 3 (m2/g)

µ (m2/g)

STD (σ)

1x 2x 3x

35 68 159

44 72 131

35 72 106

38 71 132

5 2 27

Cysteamine MOF run

Run 1 (m2/g)

Run 2 (m2/g)

Run 3 (m2/g)

ӯ (m2/g)

STD

Z

RDM

1x 2x 3x

47 78 91

48 75 73

84 103 246

60 85 137

21 15 95

7.22 11.00 0.30

Effective Effective Not Effective

4-Aminobenzoic acid MOF run

Run 1 (m2/g)

Run 2 (m2/g)

Run 3 (m2/g)

ӯ (m2/g)

STD

Z

RDM

1x 2x 3x

51 123 160

54 54 90

65 71 104

57 83 118

7 36 37

6.22 9.00 -0.91

Effective Effective Not Effective


Page 17 of 30

300 2

Surface Area of Fabric (m /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250 200 150 UMCM-1 Cu-BTC UiO-66-NH2 ZnO nanoparticles

100 50 0 5

10

15

20

25

30

Weight % MOF added to Fabric Figure 4. The weight percent loading of MOFs and nanoparticles on composite fibers. This plot was generated to provide a convenient method to estimate wt % of MOF loading on the fibers. MOFs other than UiO-66-NH2 and nonporous ZnO nanoparticles are shown to illustrate the appropriate dependence of composite area on the surface area of the attached structure. As noted in the text, surface area alone cannot be used to indicate the presence of MOF crystals on the fibers because amorphous, high surface area, Zr materials can be precipitated on fibers during hydrothermal synthesis processes.



100

90 % Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

80

70

60 4000

c

Plain Cotton Thiol Fabric 2X MOF Fabric

3500

3000

2500

2000

1500

1000

500

-1

Wavenumbers (cm )

Figure 5. IR Spectra of plain cotton, cotton that had been through the cyanuric chloride and thiol steps (Thiol Fabric), and a 2X MOF fabric sample that was produced using cysteamine and ZrCl4.


Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

c

Figure 6. SEM images of (a) plain cotton and (b) modified fibers both at 500X magnification where the scale bar shown is 50 µm. When the white box from image (b) is magnified an additional 3X (c) the area shows decorations of MOF on the fibers. The modified fibers have gone through two steps of MOF synthesis steps using cysteamine. Fibers have PXRD patterns consistent with UiO-66-NH2.



Figure 7. DMNP reaction producing p-nitrophenoxide.24


Page 20 of 30

Page 21 of 30

0.4 Initial 2.5 mins 5 mins 7.5 mins 10 mins 15 mins 20 mins 30 mins 40 mins 50 mins 60 mins 90 mins

DMNP

0.3

0.2

0.1

0.0 250

300

350 400 Wavelength (nm)

450

500

1.2

(b)

Initial 2.5 mins 5 mins 7.5 mins 10 mins 15 mins 20 mins 30 mins 40 mins 50 mins 60 mins 90 mins

1.0 Absorbance

(a)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

DMNP

0.6

p-nitrophenoxide

0.4 0.2 0.0 250

300

350 400 Wavelength (nm)

450

500

Figure 8. Spectra image of (a) plain cotton and (b) MOF/cotton composite fibers synthesized using cysteamine to bind Zr to the fabric and completing one UiO-66-NH2 MOF synthesis step to grow the MOF on the cotton fiber.



a

b

Figure 9. (a) At the beginning of the process, the sample is placed in the buffer solution with no DMNP. The sample stirred for 30 mins before adding the DMNP. (b) Once the DMNP was added, the sample was allowed to stir for a 1.5 hrs to observe the reaction shown in.


Page 22 of 30

Page 23 of 30

70 60

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 40 30

Plain Cotton 1X MOF 2X MOF Pure Powder 1.6 mg Pure Powder 2.4 mg DMNP/Buffer Only

20 10 0

20

40

60

80

100

Time (mins)

Figure 10. Percent conversion of DMNP to p-nitrophenoxide for plain cotton, pure powder, as well as fibers produced with one and two MOF synthesis steps.



-9.8

-10.0 ln[DMNP]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

-10.2

Plain Cotton 1X MOF 2X MOF Pure Powder 1.6 mg Pure Powder 2.4 mg DMNP/Buffer Only

-10.4

-10.6 0

20

40

60

80

100

Time (mins) Figure 11. Half-lives of various textiles, UiO-66-NH2 powder, and when only DMNP placed in the buffered solution. The results show an appropriate dependence on MOF mass.


Page 25 of 30

600

Half-Life (mins)

500

3.0 Half-Life (mins) MOF mass (mg)

2.5

400

2.0

300

1.5

200

1.0

100

0.5

0

0.0 Plain Cotton

1X MOF Cotton

2X MOF MOF Powder MOF Powder Water/Buffer Run 2 Run 1 Only Cotton Sample

Figure 12. Half-life of plain cotton, 1X MOF cotton, 2X MOF cotton, water/buffer only, and two different MOF powder samples are shown as well as the MOF mass of each sample.


MOF Mass (mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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(4) (5) (6)

(7) (8)

(9) (10) (11) (12) (13) (14) (15)

(16)

(17)

(18)

(19)

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ASSOCIATED CONTENT Supporting Information Available: Additional procedures used to synthesize samples and TGA data are included in the Supporting Information.

500

CWA Simulant Half-Life (mins)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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CC

200

Cotton

100

Plain Cotton

Water & Buffer

MOF Cotton

Pure Powder



TOC Image 110x59mm (72 x 72 DPI)


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Synthesis and Characterization of UiO-66-NH2 MetalâOrganic

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