Preparation of zeolitic imidazolate framework-8 /graphene oxide composites with enhanced VOCs adsorp

a b s t r a c t

Hybrid nanocomposites of zeolitic imidazolate framework-8 (ZIF-8) and graphene oxide (GO) were prepared in a methanol system at room temperature. The ZIF-8/GO composites exhibited tunable nanoscale morphology and porosity, both determined by the GO content. A series of characterization techniques confirmed the formation of strong interactions between ZIF-8 and GO in the synthesized composites. The as-synthesized ZIF-8/GO composites were composed of aggregated nano-sized particles,and exhibited a higher volatile organic compounds (VOCs) uptake capacity than normal ZIF-8 crystal owing to the synergistic effect between ZIF-8 and graphene oxide(GO). Also an increase in the content of GO resulted in enhancing in the VOCs adsorption capacity, and the maximum adsorption capacity for VOCs was up to 240 mg/g on ZIF-8/GO with the GO content of 15 wt%. The synergistic interactions of ZIF-8 and GO may provide a new path to fabricate novel ZIFs/GO composites for a wide range of applications such as adsorption.

1. Introduction

As the principle member of volatile organic compounds (VOCs),methylene chloride has become the main pollutant in the air, andhow to dispose it has attracted the social attention.Methods suchas biological treatment, distillation, adsorption, and catalyticoxidation have been used for the removal of VOCs. One of the mosteffective methods for VOCs control is adsorption, the adsorbent isimportant for enhancing VOCs uptake capacity. To find a moreeconomical and effective adsorbent to handle VOCs, many scientificresearch workers dedicated themselves to preparing materials andRecently, zeolitic imidazolate frameworks (ZIFs), the sub-familyof metal-organic frameworks (MOFs), have attracted increasingattention. Similar to traditional zeolites, ZIF-8 possesses diversestructures, where typically Zn2þ ions play the role of silicon whilethe imidazolate anions form bridges which mimic the role of oxygenin zeolite frameworks, with the ZneImeZn angle around 145 ,already obtained many significant achievements [1e5]. Theseachievements provided new solutions to VOCs pollution, but theywere not suitable to widely use in industrial application. Moreover,some of them may cause secondary pollution.

Recently, zeolitic imidazolate frameworks (ZIFs), the sub-familyof metal-organic frameworks (MOFs), have attracted increasingattention. Similar to traditional zeolites, ZIF-8 possesses diversestructures, where typically Zn2þ ions play the role of silicon whilethe imidazolate anions form bridges which mimic the role of oxygenin zeolite frameworks, with the ZneImeZn angle around 145 ,and considered to be a efficient and recyclable adsorbent due to theexceptional chemical and thermal stabilities as well as an ultrahighsurface area and abundant functionalities. This has triggered greatinterests in their promising and potential applications in gas separation[6e8] and adsorption [9e12], catalysis [13], sensors [14].

On the other hand, the graphene and graphene-based materialshave become the focus in the last few years. Graphene oxide (GO),an important precursor for graphene, has a layered structure withplenty of functional groups [15e17]. Owing to its unique structure,GO has been widely utilized in the preparation of composite materialswith promising adsorptive properties. Actually, MOFs-basedgraphite oxide composites have been investigated for various applications.For instance, Bandosz and colleagues reported the synthesisof MOF-5 and graphite oxide hybrid composites for theadsorption of ammonia [18]. The same group found that the compositesof copper-based MOF with animated GO can enhance CO2adsorption efficiency [19,20]. Rao and co-workers generated hybridcomposites of GO with ZIF-8, which exhibited tunable nanoscalemorphology and good CO2 uptake at 195 K [21]. Recently, YongdeXia and co-workers synthesized composites which showedenhanced CO2 adsorption energy and significant CO2 storage capacity[22]. However, adsorption of VOCs on the ZIFs/GO compositeshas rarely been reported. In this work, we present a simple andvalid synthesis method to produce ZIF-8/GO composites. Thesehybrid ZIF-8/GO composites largely maintain the high texturalproperties and large surface area, and the morphology and porosityof crystals are tunable via control over the amount of GO. Theresulting ZIF-8/GO composites were demonstrated to improvemethylene chloride adsorption capacity.

2. Experimental

2.1. Materials

Zinc nitrate hexahydrate (Zn(NO3)2$6H2O, 98%) was obtainedfrom Acros. Methanol (MeOH, 98%) and 2-methyl imidazole(Hmim, 99%) were obtained from SigmaeAldrich. All materials areof analytical reagent grade and were used without any furtherpurification.

2.2. Materials preparation

2.2.1. Preparation of nanoscale ZIF-8 crystals

Nanoscale ZIF-8 was prepared according to the literature procedurereported by Wiebcke et al. [23]. In a typical synthesis,734mg of Zn(NO3)2$6H2O were dissolved in 50 ml of methanol and811 mg of 2-methyl imidazole were dissolved in another 50 ml ofmethanol. Then the zinc nitrate solution was mixed with the 2-MeIM solution and stirred for 1 h at room temperature. Overall,the metal:ligand:MeOH molar ratios were 1:4:1250. Nanoscale ZIF-8 crystals were obtained by centrifugating, thoroughly washedwith methanol and deionized water for three times and then driedat 75 C overnight.

2.2.2. Preparation of microscale ZIF-8 crystals

In a typical synthesis, 744 mg of Zn(NO3)2$6H2O were dissolved in 50 ml of methanol and 411 mg of 2-methyl imidazole weredissolved in another 50 ml of methanol. Then the zinc nitrate solutionwas mixed with the 2-MeIM solution and stirred for 1 h atroom temperature. Overall, the metal:ligand:MeOH molar ratioswere 1:2:1250. The subsequent steps were in accordance with thenanoscale ZIF-8 crystals.

2.2.3. Preparation of ZIF-8/GO hybrid nanocomposites

ZIF-8/GO hybrid nanocomposites were prepared according tothe literature procedure reported by Rao et al. [21]. Firstly, GOpowder was dispersed in methanol, followed by 4 h sonication toobtain GO suspension. Aiming to obtain the ZIF-8/GO composites,different wt% of GO suspension was added during the synthesis ofZIF-8 crystals in the stirring process. Secondly, the resultant wascentrifuged and washed for 3 times with methanol and dried at50 C. It was found that the ZIF-8/GO composites cannot be preparedsuccessfully and the samples may be physical or mechanicalmixing as the mass fraction of GO was over than 15%. The resultingproduct obtained by variation of GO content were denoted by ZIF-8/GO-X which indicated X wt% (X ¼ 2, 4, 6 10 and 15 wt %) of GO.The ZIF-8/GO-X samples were designated as ZG-X(ZG-2, ZG-4, ZG-6, ZG-10 and ZG-15) according to the mass fraction of GO.

2.3. Characterization

X-ray diffraction (XRD) patterns were measured on a BRUKERD8 ADVANCE diffractometer with Cu ka radiation at 40 KV and30 mA. The fourier-transform infrared (FTIR) spectrawere obtainedusing a SPECTRUM 400 system. The as-synthesized samples weremeasured in the wavenumber of 4000e450 cm1. Scanning electronmicroscopy (SEM) images were carried out on a Hitachi S-4300field emission scanning electronic microscope. The N2 adsorptionedesorptionisotherms were collected at 77 K on a JWBK122W.The samples were degassed at 160 C for 18 h prior to the measurement. The BET surface areawas calculated based on thedata from N2 adsorption within the range of relative pressure from0.005 to 0.3.

2.4. Adsorption measurement

The experimental system for the fixed-bed operation is shownin Fig. 1, and the fixed-bed was made of a quartz tube with 5 mminner diameter and 10 cm in height. Adsorption measurement wascarried out in the fixed bed using a continuous flow reactor at roomtemperature. The obtained samples were loaded in a quartz tube.The flow meter was used to adjust the flow rates of air in order tokeep the concentration of methylene chloride remained. Methylenechloride was used as the methylene chloride source, the initialconcentration was 0.48 mg/ml. The flow rate of gaseous mixturewas 20 ml/min. To obtain the breakthrough curve of the adsorptionexperiment, the gas concentration of inlet and outlet of the tubewas detected by a gas chromatography (GC).

3. Results and discussion

3.1. Characterization of the as-synthesized ZIF-8 crystals and ZGcomposites

As shown in Fig. 2, compared with an XRD the patterns fromsimulated data, the XRD patterns obtained from Nano-ZIF-8 andthe patterns taken from Micro-ZIF-8 all showed pure ZIF-8 structurewithout any other crystalline phase. And there were no significantdifference between the peaks of Micro-ZIF-8 and the peaksof Nano-ZIF-8. The XRD patterns of the ZG-X composites withdifferent GO content also showed the reflection of ZIF-8 crystalline.With an increase of GO content, a new broad peak appeared andheightens at 2q ¼ 8.5close to the characteristic peaks of ZIF-8,which could be attributed to GO. The small angle shift comparedto that of GO indicated the intercalation of ZIF-8 crystals betweenthe GO sheets. The ZG-X exhibited the only new peak from pristineGO without any other new ones regardless of GO content. Even ifthe content of GOwas up to 15%, the XRD patterns of ZG-X were stillsimilar with the XRD patterns of pure ZIF-8. The results indicatedthat GO sheets may have the strong interaction with ZIF-8 crystalsas an integral part of ZIF-8.

What's more, the FTIR spectra also proved the existence of thestrong interactions between ZIF-8 and GO sheets in the ZG-Xcomposites. All the ZG-X composites exhibited the similar FTIRspectra to pure ZIF-8. As shown in Fig. 3, there were no peaks orbands were consistent with in the as-synthesized ZG-X compositescompared to the CaO stretching vibrations at 1790 cm1 and theCeO vibrations at 1068 cm1 in pure GO. The majority of theabsorption bands for ZG-X composites were consistent with pristineZIF-8 such as the bands at 1250e1500 cm1 which wereassociated with the imidazole ring stretching while the bandsbelow 1250 cm1 were assigned with the out-of-plane bending andin-plane bending of the imidazole. The strong peak at 1541 cm1could be assigned as the CaN stretch mode while the bands at 1126and 936 cm1 could be assigned as the CeN stretching of theimidazole units. The results clearly indicated that the strong interactionswere formed between ZIF-8 and GO in the ZG-Xcomposites.

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Fig. 1. Schematic diagram of the adsorption experimental system.

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Fig. 2. XRD patterns of ZIF-8 and ZIF-8/GO nanocomposites.

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Fig. 3. FTIR spectra of GO, ZIF-8 and ZG-X composites.

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Fig. 4. Representative SEM images of as-synthesized ZIF-8 sample and ZG-X composites:

(a) Micro-ZIF-8, (b) Nano-ZIF-8, (c) ZG-2, (d) ZG-4, (e) (f) ZG-6, (g) (h) ZG-10, (i) (j) ZG-15.

The morphologies of ZIF-8 and ZG-X composites were examinedby SEM images and all samples showed aggregated nanoparticles.The pure Micro-ZIF-8 and Nano-ZIF-8 showed hexagonalmorphology with the particle sizes in the range of 1000e1500 nmand in the range of 100e200 nm as shown in Fig. 4 (a) and (b),respectively. Similar hexagonal morphology was found on the ZG-Xcomposites and the dense GO sheets act as a platform for thenanoscale ZIF-8 crystals growth (Fig. 4 (g), (h), (i), (j)). As can beseen from the SEM images, the particle size of Nano-ZIF-8 was adjustable by controlling the amount of GO. The particle size of ZG-2 which was similar with the pure ZIF-8 was about 200 nm. Withthe increasing the content of GO, the particle size of ZIF-8 decreasesin ZG-4. Upon increasing the GO content to 4 wt%, the particle sizedecreased to 130 nm in ZG-6. Actually, when the GO content in theZG-15 composites further increased up to 15 wt%, the averageparticle size in the resulting ZG-15 sample decreased to 60 nm. Itcan be inferred that the GO sheets act as both size-controlling andstructure-directing agents for ZIF-8 crystals. Fig. 5.

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Fig. 5. Nitrogen sorption isotherms measured at 77 K for sample Micro-ZIF-8 andNano-ZIF-8.

Textural parameters such as BET surface area, Langmuir surfacearea, pore volume and pore size of the Micro-ZIF-8, Nano-ZIF-8 andZG-X composites were obtained from N2 adsorptionedesorptionmeasurements at the same condition. All seven samples exhibitedN2 adsorptionedesorption isotherms of a mode of type I. The isothermsof synthesized ZG-X composites displayed a steep rise underlow relative pressure, indicating those composites aremicropore dominated materials, while a second slight rise at highrelative pressure indicated the existence of mesopores, which are inagreement with the pure ZIF-8 material. In addition, as summarizedin Table 1, the surface areas of the ZG-X composites weretunable by controlling over the GO content. The BET surface areas ofresulting Micro-ZIF-8 and Nano-ZIF-8 were 1185.8 and 1768.9m2/g,and the Langmuir surface areas were 1324.7 and 1961.9 m2/g,respectively. The high surface area of our ZIF-8 samples showedthat the crystals have fully developed microstructure with highcrystallinity. The BET surface area of the ZG-X compositesdecreased compared to pristine ZIF-8 with increasing GO content.Thus, the surface areas of ZG-2, ZG-4, ZG-6, ZG-10 and ZG-15 were860.6, 778.1, 722.1, 605.3, 559.3 m2/g, respectively. The decrease insurface area of the ZG-X composites may be due to an increasingproportion of nonporous GO which can block the pore channels ofZIF-8. However, the degree of reduction of ZG-X was smaller thanthat earlier reported [21].

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As summarized in Table 2, with an increase in the GO content,the medium pore size distribution of the ZG-X composites showeda slight increase. The micropore volume decreased slightly from0.27 in ZG-2 to 0.20 cm3/g in ZG-15. This might be due to the factthat the surface area of GO in the composite is smaller than ZIF-8,resulting in less surface area per unit weight of the materials.And the micropore volume in ZG-X composites was far less than0.40 in Micro-ZIF-8 and 0.58 cm3/g in Nano-ZIF-8. Unsurprisingly,the change in micropore volume was consistent with the porevolume in ZG-X and pure ZIF-8.

The conclusion from textural parameters and SEM imagesclearly demonstrated that the ZG-X composites are not just aphysical mixture. There may be not only a new porosity betweenGO and ZIF-8 units, but also new structures where GO sheets couldbe embedded within ZIF-8 crystals. Fig. 6.

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Fig. 6. Nitrogen sorption isotherms measured at 77 K for the ZG-X composites.

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Fig. 7. Breakthrough curve of adsorption of methylene chloride by Nano-ZIF-8 andMicro-ZIF-8 samples (C0 ¼ 0.48 mg/ml).

3.2. Adsorption studies

Figs. 7 and 8 shows the breakthrough curves obtained at asimilar inlet concentration for all synthetic adsorbents. The columncharacteristics obtained from the breakthrough curves were shownin Table 3. In this section, the parameters such as flow rate and beddepth were fixed at 20 ml/min and 10 cm, respectively. The timeneeded for breakthrough point (time when the outlet concentrationis 2% of the inlet concentration, tr), the equilibrium point (timewhen the outlet and inlet concentrations are nearly identical, te),the equilibrium adsorption capacity (Mtf) and finally the degree ofcolumn utilization (C/C0) are shown in Table 3. According to theexperiment results, it was found that the higher the content of GOand the higher pore size of the ZG-X composites, the longer thebreakthrough time and the longer the equilibrium time whichmean the better adsorbing performance. It can be speculated thatthe methylene chloride molecules find more difficulty in diffusingin ZG-2 with a narrow microporosity causing a decrease in theadsorption capacity and equilibrium time than ZG-X compositeswith higher GO content. It can be found in Table 3 that the equilibriumtime of Micro-ZIF-8 and Nano-ZIF-8 was about 163 and170 min and their adsorption capacity was 69 and 138 mg/g,respectively. The Nano-ZIF-8 performed better than the Micro-ZIF-8 owing to higher surface area. Generally, VOCs adsorption capacitiesof porous materials were in direct proportion to samplessurface area. However, the adsorption capacities of five ZG-Xcomposites samples which have the lower surface area were allhigher than that of the pristine ZIF-8. Not only that, the ZG-Xcomposites exhibited good methylene chloride adsorbing capacityand methylene chloride uptake increasing with the increasing ofGO content. The result was consistent with the early report aboutthe CO2 adsorption capacity of ZG-X composites [21,23]. The ZG-2was a good adsorbent with an uptake capacity of 139 mg/g at theequilibrium time of 162 min, while the ZG-4 showed a methylenechloride uptake capacity of 156 mg/g at the equilibrium time of169 min. ZG-6 exhibited methylene chloride uptake of about182 mg/g at the equilibrium time of 192 min which increasesfurther to 211 mg/g at the equilibrium time of 215 min in ZG-10under same condition. The adsorption capacity of ZG-15 at theequilibrium time of 225 min which possessed the lowest surfacearea was the highest of all, up to 240 mg/g. As for the degree ofcolumn utilization, it was observed that it was at high level for allthe samples and no relationship was found between this parameterand the textural properties of the adsorbents. According to thecolumn characteristics that based on the experimental breakthroughcurves, the unusual uptake can be attributed to the synergisticeffect of GO and ZIF-8 as the latter with different functionalgroups provided specific interaction sites for methylene chloridemolecule, and the pore sizes of ZG-X adsorbents are moreapproximate to the size of methylene chloride molecule. The synergisticeffect may become the most important factor in adsorptionthan the others such as surface area, pore volume and pore size, sothe ZG-X composites have a better performance in methylenechloride uptake. Therefore, the synthesized method of GO into thecomposites can effectively affect the textural properties andconsequently perform remarkably on the methylene chloride uptakecapacities. To the best of our knowledge, this is the first reportof using ZIF-8/GO nanocomposites as the adsorbents of methylenechloride with tunable porosity and remarkable adsorption capacity.

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4. Conclusion

In conclusion, preparation of pure ZIF-8 crystals and ZIF-8/GOhybrid nanocomposites was carried out in a methanol system.The formation of strong interactions between ZIF-8 and GO in thesynthesized composites was confirmed by the XRD, FTIR, SEMimages and N2 sorption measurement. It can be inferred that ZIF-8is stabilized on the GO sheet surfaces through functional groupsand the textual properties of the ZG-X composites are tunable bycontrolling over the concentration of GO. The GO sheets act as astructure-directing agent for the growth of ZIF-8 nanocrystalsthrough coordination modulation. The surface areas and pore volumesof the ZG-X composites are smaller than those of the nanosizedZIF-8 and micro-sized ZIF-8, while the ZG-X compositesexhibited excellent performance with high methylene chlorideuptake compared to the pure ZIF-8. With the increase of the contentof GO, the methylene chloride adsorption capacity enhanced,and the maximum adsorption capacity was up to 240 mg/g on ZG-15. We speculated that the significant methylene chloride uptakeresulted from the strong interactions between some groups of GOand methylene chloride molecules. The synergistic interactions ofZIF-8 and GO may provide a vital path for the fabrication ofmultifunctional MOFs/GO composites for various applications.

Acknowledgment

This work was supported by the National Natural ScienceFoundation of China No. 21376026.

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