Article pubs.acs.org/Langmuir
Interactions of NO2 with Zr-Based MOF: Effects of the Size of Organic Linkers on NO2 Adsorption at Ambient Conditions Amani M. Ebrahim,† Benoit Levasseur,† and Teresa J. Bandosz*,‡ †
Department of Chemistry, The City College of New York, 160 Convent Ave, New York, New York 10031, United States The Graduate School of CUNY, 160 Convent Ave, New York, New York 10031, United States
‡
ABSTRACT: Zirconium-based metal organic framework (Zr-MOF), UiO-66 and UiO-67, were synthesized and used as adsorbents of NO2 at ambient temperatures in either dry or moist conditions. The samples were characterized before and after exposure to NO2 by X-ray diffraction, scanning electron microscopy, N2- adsorption at 77 K, thermal analysis, and infrared spectroscopy. The results indicate the important effect of a ligand size on the adsorption of NO2 on Zr-MOF materials. While the large size of the 4,4-benzenebiphenyldicarboxylic acid (BDPC) ligand has a positive impact on the adsorption of NO2 on UiO-67 in moist conditions, the opposite effect is found in dry conditions. The large pore volume of UiO-67 enhances the adsorption of moisture and formation of nitric and nitrous acids. The small pore sizes of UiO-66 favor the NO2 removal in dry conditions via dispersive forces. Upon interaction of NO2 molecules with the Zr-MOF in dry conditions, the bond between the organic linker and metallic oxide center is broken, leading to the formation of nitrate and nitrite species. Moreover, organic ligands also contribute to the NO2 reactive adsorption via nitration reaction.
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benzene, acetone, and N,N-dimethylformamide (DMF).8 The enhanced stability of Zr-based MOFs is linked to their strong Zr−O bonds within the secondary building unit (SBU) and to the high degree of interlinking of these SBUs.13 The structural features of zirconium-based MOFs and their stability suggest their suitability as nitrogen oxides’ adsorbents. These pollutants are byproducts of industrial manufacturing processes, mobile exhaust, fossil fuel combustion, and power generating sources. An increase in the concentration of NOx molecules is considered as one of the main causes of photochemical smog formation, which is dangerous to humans and makes damage to materials.14 Therefore, the removal of nitrogen oxides has become an important target in environmental catalysis. Various adsorbents such as activated carbon modified with sewage sludge,15,17 wood-based activated carbon,16,18,19 graphite oxides and iron composites,17 and zeolites20,21 have been used for the removal of NO2 at ambient conditions. A selective catalytic reduction (SCR) process is now a common way to reduce NOx emission. Most catalysts used are based on ion-exchanged zeolites and supported metals on various metal oxides. The poor stability of metal ion-exchanged zeolites and deactivation of basic metal oxides in the presence of water are drawbacks of the catalysts applied in the reduction process.22 Another disadvantage of using SCR is the high
INTRODUCTION Metal organic frameworks (MOFs), also known as porous coordination polymers (PCPs), organic porous materials (MOPMs), porous coordination networks (PCNs), or metal organic materials (MOMs), are a class of crystalline materials having infinite network structures built with multitopic organic ligands and metal ions.1 It is known that the properties of the final materials are determined by synthesis conditions.2 For example, an increase in a reaction temperature helps to attain suitable crystallinity. 3 The first MOF structures were synthesized by Yaghi and co-workers.4 They contained Zn4 Cu,6 Cr,5 Fe,6 and V7 as metal centers. The deficiency of these MOFs was in their low hydrothermal and chemical stability.9,9 The thermal stability of MOFs is usually limited to 350−400 °C. Recently, the synthesis of stable porous high specific surface area Zr-MOFs, UiO-66, and UiO-67 has been reported.10 The inner core of these materials consists of Zr6O4(OH)4 units.10 While UiO-66 is built from hexamers of eight-coordinated ZrO6(OH)2 and 1,4-benzenedicarboxylate (BDC-6.8 Å) linkers, UiO-67 has 4,4-benzenebiphenyldicarboxylate (BPDC11.59 Å) linkers. The porosity of UiO-66 consists of narrow triangular windows with a free diameter close to 6 Å,11,12 octahedral cavities with a diameter of 11 Å, and tetrahedral cavities with a diameter of 8 Å.10 In UiO-67 the longer linker implies that both the tetrahedral and octahedral cages are of wider dimensions, and thus octahedral cavities of 16 Å and tetrahedral cavities of 12 Å are present.9 The structural integrity of these materials is preserved in solvents such as water, © XXXX American Chemical Society
Received: July 17, 2012 Revised: September 25, 2012
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dx.doi.org/10.1021/la302869m | Langmuir XXXX, XXX, XXX−XXX
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Article
μm) to a homogeneous bed and well packed into a glass column (internal diameter 9 mm). The concentrations of NO2 and NO in the outlet gas were measured using an electrochemical sensor (RAE Systems, MultiRAE Plus PGM-50/5P). The adsorption capacity of each adsorbent was calculated in milligram per gram of adsorbent bed (mg/g) by integration of the area above the breakthrough curve. The tests were conducted in both moist (relative humidity 71.0%) and dry conditions by diluting NO2 with air. The measurements were stopped at the concentrations of NO2 and NO of 20 and 200 ppm, respectively (the limits of the gas sensors). After the breakthrough tests, all samples were exposed to a flow of carrier air only (180 mL/min) to evaluate the strength of NO2 adsorption. The suffix -ED is added to the name of the samples after exposure to NO2 in dry conditions and -EM for samples after exposure to NO2 in moist conditions. Surface pH. The samples were first dried, then a 0.2 g sample of a dry adsorbent was added to 10 mL of deionized water, and the suspension was stirred overnight to reach equilibrium. Then the pH was measured using Fischer Scientific Accumet Basic pH meter. Adsorption of Nitrogen. Nitrogen isotherms were measured at −196 °C using an ASAP 2010 (Micromeritics). Before each analysis initial and exhausted samples were dried and degassed at 120 °C. The surface area, SBET, the total pore volume, Vt, the micropore volume Vmic (calculated from the t-plot), and the mesopore volume, Vmes, were calculated from the isotherms. SEM. Scanning electron microscopy (SEM) was performed on Zeiss Supra 55 instrument with a resolution of 5 nm at 30 kV. Analyses were performed on a sample powder previously dried. For some samples, a sputter coating of a thin layer of gold was performed to avoid specimen charging. XRD. To obtain crystallographic structure of the materials, XRD was employed. Using standard powder diffraction procedures, the adsorbents (fresh and exhausted) were ground with DMF in an agate mortar and smear mounted onto a glass slide, and X-ray diffraction was measured on a Philips X’Pert Xray diffractometer using Cu Kα radiation with a routine power of 1600 W (40 kV, 40 mA). Thermal Analysis. Thermogravimetric (TG) curves and their derivatives (DTG) were obtained using a TA Instruments thermal analyzer, Q600. The samples (initial and exhausted moist and dry) were previously dried in oven at 100 °C to remove moisture and then heated up to 1000 °C, with a heating rate 10 °C/min under a nitrogen flow of 100 mL/min. FTIR. Fourier transform infrared (FTIR) spectroscopy was carried out using a Nicolet Magna-IR 830 spectrometer using the attenuated total reflectance method. The spectrum was generated, collected 32 times, and corrected for the background noise. The experiments were done on powdered samples.
corrosiveness and high toxicity of ammonia, which is used as a reducing agent.22 Various MOFs have been studied for pollution removal.6,8,10,23−28 Petit and Bandosz investigated ammonia removal on the copper-based MOF and its composites with graphite oxide.24 Metal organic frameworks, MOF-5 and MIL(Fe), were also used as filtration media for the adsorption of ammonia.25,26 Moreover, the removal of ammonia on MOF was studied by Saha and Deng27 and Yaghi and co-workers.7 The latter group also investigated uptake of hydrogen, nitrogen, argon, carbon dioxide, and methane gases on different MOFs.28 Zloeta and co-workers studied the effect of MOF functionalization on the hydrogen sorption capacity.29 Copper-based MOF/graphite oxide nanocomposites were explored as NOx30 and H2S31 adsorbents at ambient conditions. Adsorption of xylene and hexane isomers on UiO-66 was addressed by Rodriguez and coworkers.11 Owing to UiO-66 stability in a water suspension, this material was also used as a photocatalyst for hydrogen generation.32 The structural stability of MOFs upon water adsorption is an important asset for potential applications of these materials as gas adsorbents and storage media because H2O is very difficult to be fully removed from industrial gas sources,33 and it is always present in ambient air. While several studies have been conducted on the application of zirconium-based MOFs as adsorbents,8,11 research on the application of these materials as adsorbents at atmospheric pressure and ambient temperature is still not common owing to rather limited industrial applications.34 Nevertheless, these conditions are common for ambient air purification, and there is a need to remove toxic/corrosive gases from confined spaces owing to the damage they may cause to humans and to the detrimental effects on the sophisticated electronic instruments. Therefore, the objective of this study is to present the effects of the different ligands of UiO on NOx adsorption in moist and dry conditions. Understanding the behavior of UiO-66 and UiO-67 as NO2 adsorbents under high relative humidity is an important task which may lead to the development of efficient air filtering media.
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EXPERIMENTAL SECTION
Materials. All chemicals (DMF (N,N-dimethylformamide, 99%), BDC (benzene-1,4-dicarboxylic acid, 98.9%), BDPC (4,4-benzenebiphenyldicarboxylate, 98.9%), DCM (dichloromethane, 99%), and ZrCl4)) were supplied by either Sigma-Aldrich, VWR, or Alfa Aesar and used without further purification. Synthesis of Zr-MOF and UiO-66/UiO-67. The synthesis of ZrMOFs was carried out following the procedure described by Chavan and co-workers35 with some amendments. UiO-66(Zr) was prepared by dissolving (10 mmol) of zirconium tetrachloride, ZrCl4, and BDC (10 mmol) in 300 mL of DMF. Then the reagents were transferred to a 500 mL round-bottom flask, and 50 mL of DMF was added. The system was heated at 120 °C under shaking for 24 h. A resulting white product was filtered off, washed with DMF to remove the excess of the unreacted organic linker, and immersed in a dichloromethane solution for solvent exchange. The sample was again filtered and dried under vacuum at 120 °C for 26 h. The same drying method was used in the synthesis of HKUST,30,31 and it led to the total removal of the DMF solvent. The described above synthesis procedure was followed for the synthesis of UiO-67, but BDPC was used as an organic linker. Methods. NO2 Breakthrough Capacity. The NO2 breakthrough capacities were measured in a laboratory-scale, fixed-bed system, at room temperature, in dynamic conditions. In a typical test, 1000 ppm NO2 in nitrogen diluted with air went through a fixed bed of an adsorbent with a total inlet flow rate of 225 mL/min. The adsorbent was well mixed with 2 mL of nonreactive glass beads (diameter 0.5−1
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RESULTS AND DISCUSSION The measured NO2 breakthrough curves and desorption curves are shown in Figure 1. NO concentration curves are collected in Figure 2. In dry conditions, the breakthrough curve for UiO67 is steeper than that for UiO-66. In moist conditions, an opposite trend is noticed. The variation in the shape of the breakthrough curves for both materials in dry and moist conditions suggests differences in the mechanism of adsorption. The steepness of the curves is associated with the fast kinetics of adsorption. The results suggest that during the duration of the breakthrough experiments the mechanism of adsorption changes, especially for UiO-66 ED and UiO-67 EM, and water seems to affect differently the performance of both adsorbents. The calculated NO2 adsorption capacities for each sample and the percentages of NO released are summarized in Table 1. As seen, the material with BDC organic linker adsorbs 73 mg/g of NO2 in dry conditions, and the sample with BDPC is able to retain about 10% more NO2 in the same conditions. On the surface of both materials about ∼25% of NO2 is converted to NO. Interestingly, in moist conditions UiO-67 adsorbs much B
dx.doi.org/10.1021/la302869m | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
molecules in a water film. That water can also contribute to the reactions in that nanospace. These sequence of the adsorption process was also observed by Ahrens and co-workers on ironexchanged zeolites. 20 Their measured NO 2 adsorption capacities were between 10 and 180 mg/g depending on the type of zeolite and the amount of water present in the system. A decrease in the surface pH of our materials after NO2 adsorption in moist conditions suggests the formation of acidic species in the pore system (Table 1). A greater decrease in the pH for UiO-67 than for UiO-66 indicates that more or stronger acid is formed on the surface of the former sample. It is well-known that in an adsorption process surface area and porosity are of paramount importance. Nitrogen adsorption isotherms are plotted in Figure 3, and the
Figure 1. NO2 breakthrough curves for the UiO-66 and UiO-67 materials.
Figure 3. Nitrogen adsorption isotherms for UiO-66 and UiO-67 before and after exposure to NO2. Figure 2. NO release curves for the UiO-66 and UiO-67 materials.
parameters of the porous structure calculated from these isotherms are listed in Table 2. The shape of the isotherms
Table 1. NO2 Breakthrough Capacities, % of NO Released, and pH Values before and after Exposure to NO2 in Moist (EM) and Dry (ED) Conditions
Table 2. Parameters of the Porous Structure Derived from the Nitrogen Isotherms for the Samples before and after Exposure to NO2
pH
samples
NO2 capacity [mg/g]
NO released [%]
UiO-66 ED UiO-66 EM UiO-67 ED UiO-67EM
73 40 79 118
>30 ∼25 >30
