Vertically aligned nickel 2-methylimidazole metal-organic framework

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Vertically aligned nickel 2-methylimidazole metal-organic framework fabricated from graphene oxides for enhancing fire safety of polystyrene Yanbei Hou, Weizhao Hu, Xia Zhou, Zhou Gui, and Yuan Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01906 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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Industrial & Engineering Chemistry Research

Vertically aligned nickel 2-methylimidazole metal-organic framework fabricated from graphene oxides for enhancing fire safety of polystyrene Yanbei Houa, Weizhao Hua, Xia Zhoua, Zhou Gui∗a and Yuan Hu∗a State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China Abstract: In this work, flower-like nickel 2-methylimidazole metal-organic framework (Ni-MOF) was prepared by a solvothermal method. Vertically aligned Ni-MOF was fabricated from graphene oxide (GO) solution in the same way. The combination of GO and Ni-MOF (GOF) obviously suppressed the agglomeration of Ni-MOF sheets. As-synthesized GOF has bigger pore volume and specific surface area, which are beneficial for volatile degradation products adsorption. It is noteworthy that the addition of GOF obviously reduced the fire hazard of polystyrene (PS). More than 33% decrease in peak of heat release rate for PS/GOF composite was obtained when the content of the additives is only 1.0 wt%. Meanwhile, the reductions of total smoke and CO production were also prominent during the combustion of PS/GOF, respectively 21% and 52.3% decreases compared with that of pure PS. The synergism effects between layered GO and porous Ni-MOF realized the improved performances of PS. Thus, this work paves a feasible pathway to design efficient flame retardants for enhancing fire safety of polymers. Keywords: metal-organic framework, graphene oxide, fire safety

∗

Corresponding author. Tel/Fax: +86 551 63601669 (Z. Gui) +86 551 63601664(Y. Hu). E-mail address: [email protected](Z. Gui) [email protected] (Y. Hu).

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1. Introduction Metal organic materials (MOFs), as crystalline porous materials, possess unique physicochemical properties and have tremendous applications numerous fields

1-3

.

Despite interesting features, MOFs still have weak points, the fragile nature of the metal/ligand junctions that hinder their large-scale deployment. In addition, owing to their organic component, the initial mass loss of MOFs generally begins with heating. Two approaches have been proposed to optimize the existing design scheme: modification of MOFs and building MOF-based composites 4. The latter method is more attractive as it can not only weak the disadvantages of MOFs, but also potentially bring synergistic functions 5. Graphene and its derivatives have been proved as excellent supports for their high mechanical strength and chemical stability 6

. Recently, increasing efforts have been devoted to integrating graphene-derived

compounds into MOFs, such as Ni-doped MOF-5/reduced GO (rGO) for high-performance energy storage materials 7, MOF-5/GO composites for gases adsorption 4, and MIL-53(Fe)-rGO for wastewater treatment 8. The integrated composites may exert excellent properties. To this end, it is intriguing to explore graphitic derivatives/MOFs composites for excavating novel properties and applications. Polystyrene (PS), as a mass produced thermoplastic polymer, is widely used in ordinary life, due to its excellent mechanical durability, good processing, low density, and thermal resistance. However, its flammability and emission of a large amount of smoke and hazardous gases during combustion limit its further applications. To


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improve the fire safety of PS during combustion, significant effort has been put in academic and industrial community. It has reported that layered nanomaterials (including GO, montmorillonite) have natural barrier properties, which inhibit thermal transmission and gaseous fuel transfer, contributing to better thermostability and flame retardancy 9. To decrease the release amount of smoke and toxic gases during combustion of PS, catalytic transition metal compounds were widely studied, including cobalt oxide

10

, alpha-FeOOH

11

, and zirconium phosphate

12

. Meanwhile,

nitrogen-containing compounds are also major component of flame retardants

13

,

which function in gas phase to neutralize free radicals, and thus retard the fire spread. The ligands of zeolitic imidazolate frameworks (ZIFs, a class of MOFs) are imidazole compounds, composed by nitrogen-rich structures

14

. The unique metal

ions-N atoms structures endow them superior thermal and chemical stability. It has reported that these nitrogen-rich MOFs can be used as flame retardants to enhance fire safety of polymers

15

. However, van der Waals force of the metal-organic complex

favored its aggregation and exhibited inferior dispersion in matrix, implying the functions of MOFs were not well taken. It has confirmed that better dispersion state of additives in polymer matrix can endow polymeric composite superior performances 16-18

. Integration of graphene oxide and nitrogen-rich MOFs to form composites can

suppress the aggregation of MOFs, but also potentially obtain novel construction and performance of GO/MOFs. Meanwhile, the catalytic metal ions existed in MOFs endow the composites much more attractions. Therefore, the inclusion of GO/MOFs composites into polymers probably contributes enhanced performances to polymeric



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materials. Here we synthesized nickel 2-methylimidazole MOF (Ni-MOF) with flower-like morphology and GO/Ni-MOF (GOF) with vertically oriented channels. The combination of GO and nitrogen-containing Ni-MOF potentially enlarges both advantages and thus broad the application fields of their composites. The thermostability, flame retardancy, and the smoke release of PS composites were measured by thermogravimetric analysis and cone calorimeter. Moreover, the influence of the additives on the release of CO during combustion was investigated by the steady-state tube furnace (SSTF). The mechanism to illustrate reduced fire hazards of PS was also discussed. 2. Experimental 2.1 Materials The chemicals used in this study were of analytical reagent grade and used with no further

treatment.

Nickel

(II)

nitrate

hexahydrate,

2-methylimidazole,

N,

N-dimethylformamide (DMF) and anhydrous methanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Polystyrene (158 K) was obtained from BASF-YPC Co., Ltd. (China). 2.2 Preparation of Ni-MOF and GOF Ni-MOF and GOF were synthesized by a solvothermal method. Briefly, nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, 0.58 g) and 2-methylimidazole (0.66 g) were respectively dissolved in 35 mL anhydrous methanol. With magnetic stirring, a bottle green solution was formed by adding ligand solution dropwise into Ni(NO3)2·6H2O


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methanol solution. Transformed the mixed solution into an autoclave (80 mL) and held temperature at 180 oC for 6 h. The sample was centrifuged and washed several times with methanol. Ni-MOF, state brown powder, was obtained after drying in oven. Methanol, used in all steps, was recollected by rotary evaporation. Graphene oxide (GO) was synthesized by a modified Hummers method, which was referred to our previous work

19

. Then dissolved metal salt in methanol solution of

GO (4 mg/mL). The followed steps were similar with preparation process of Ni-MOF. The obtained GOF powder with dark gray color was carefully collected for later use. 2.3 Preparation of PS and its composites PS composites with GO, Ni-MOF and GOF (1 wt%) were prepared by solution mixing method. In general, the previously obtained additives were ultrasonic dispersed in a certain volume of N, N-dimethylformamide to yield a homogeneous dispersion. The calculated PS was dissolved in the dispersion and treated with sonication for 60 min. Then the mixture was poured into distilled water to exchange solvent and dried at 90 oC for 12 h to completely remove the solvent. The samples were cut into pellets and finally hot-pressed into sheets. 2.4 Characterization Morphology of the sample was studied by a PHILIPS XL30E scanning electron microscope (SEM). Transmission electron microscopy (TEM) (JEM-2100F, Japan Electron Optics Laboratory Co., Ltd.) was employed and the accelerating voltage was 200 kV. X-ray diffraction (XRD) was performed using a Japan Rigaku D/Max-Ra rotating-anode X-ray diffractometer equipped with a Cu-Ka tube and a Ni filter (l =



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0.1542 nm). X-ray photoelectron spectroscopy (XPS) was carried out with a VGESCALB MK-II electron spectrometer (Al Ka excitation source at 1486.6 eV). N2 adsorption–desorption isotherms were measured using a Micromeritics ASAP 2460 analyzer at 77 K. Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., USA) under air flow of 25 ml·min-1 and heated from 25 oC to 800 oC at a linear heating rate of 20 oC·min-1. The cone calorimeter (Stanton Redcroft, UK) tests were performed according to ISO 5660 standard procedures. Each specimen of dimensions 100 × 100 × 3 mm3 was wrapped in aluminium foil and exposed horizontally to an external heat flux of 35 kW·m-2. The SSTF tests were measured according to ISO TS 19700 20. Typically, 20 g of samples were spread evenly over a quartz boat, which was fed into the furnace at 650°C at around 40 mm· min-1. 3. Results and discussion 3.1 Morphological and structural characterizations The morphologies of Ni-MOF, GO, and GOF were characterized by SEM (Fig. 1a-c) and TEM (Fig. 1d-f). Flower-like Ni-MOF composed by disordered nanosheets can be easily distinguished from Fig. 1a, d. Layered GO can be identified by the wrinkles and distinct outline observed in Fig. 1b, e, indicating the successful exfoliation of graphite powder. Ni-MOF nanosheets vertically grew on the surfaces of GO, obtained oriented growth structure (Fig. 1c). It can be observed from Fig. 1d and f that the agglomeration of Ni-MOF sheets is effectively prevented by this oriented growth. The nano-sized channels, composed by Ni-MOF sheets, potentially enlarge


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the pore volume and superficial area of GOF, which is beneficial for the catalysis and gas adsorption. Fig. 2a shows the XRD patterns of GO, Ni-MOF, and GOF. Typical spectrum of GO demonstrates its successful synthesis process has been conducted

21

. The

characteristic (002) peak of GO was located at about 10.4o, agreed with the previously report

22

. The high-intensities peak at 11.5o distinctly confirmed the crystalline

structure of Ni-MOF. The characteristic peak intensity of Ni-MOF in XRD curves of GOF is relatively lower than that of Ni-MOF and importantly indicating that the vertically aligned structure inhibits the stacking of Ni-MOF. The new peak appeared at 23.8o is corresponding to (002) peak of reduced GO (rGO)

23

, implying thermal

reductive reaction was proceeded during the preparation process of GOF. XPS measurements were carried out on the Ni-MOF and GOF (Fig. 2b, c) to characterize their elemental compositions. The nickel, nitrogen and carbon present in the survey spectra of Ni-MOF. Comparably, oxygen element only appears in GOF, which is derived from GO. To figure out the effect of GO on the valence state of nickel ions, the curves of Ni 2p were selected and plotted in Fig.2c. The peaks lactated at 855.1 and 872.5 eV, are corresponding to vibration of Ni 2p1/2 and Ni 2p3/2, distinctly confirmed the presence of +II formal oxidation number of nickel 24, which is well agreement with the valence of the metallic elements previously reported

14, 25

.

The same peak locations illustrate that the introduction of GO has little effect on the ion valence of nickel. The distribution of elements in Ni-MOF and GOF was detected by EDX element mapping analysis and the results were shown in Fig. S1 and Fig. 2d



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respectively. The distributions of all elements are well agreement with the shape of original images. Compared with Ni-MOF, the homodisperse of nickel and nitrogen elements indicate that the inclusion of GO significantly suppresses the agglomeration of Ni-MOF. The specific surface area and pore volume of Ni-MOF and GOF were investigated by nitrogen adsorption-desorption experiments, as shown in Fig. 3. The nitrogen adsorption-desorption isotherms of the synthesized Ni-MOF and GOF samples both exhibit H3-type hysteresis loops (P/Po > 0.4), which indicates the presence of mesopores consisted by multilayered structure

26

. The pore diameter distribution

curves of Ni-MOF and GOF are portrayed in Fig. 3c, d. The two curves are composed by sharp peaks at 2-3 nm and broad peaks at 35 and 10 nm respectively for Ni-MOF and GOF, which further confirms the existence of mesoporous structures. The Brunauer-Emmett-Teller (BET) surface area of Ni-MOF is 65.6 m2·g-1, while is much lower than that of GOF (156.7 m2·g-1). Meanwhile, the pore volumes of Ni-MOF and GOF are 0.41 and 0.96 m3·g-1, respectively. It is reasonable to believe that the introduction of GO inhibits the aggregation of Ni-MOF platelets, thus leading to bigger pore volume and surface area. The results are consistent with electron microscopy surveys and make GOF a potential gas absorber and carrier for chemical reaction. 3.2 TGA of the additives and the analysis of degradation products The thermal oxidation behaviors of the additives were investigated by TGA equipment and the results were shown in Fig. 4a. On the onset stage (

Vertically aligned nickel 2-methylimidazole metal-organic framework

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