2D metal-organic frameworks endow ammonium perchlorate with enhanced thermal effect

Ye Bo-yun , Feng Chen-he , Zho Feng-qi , An Chong-wei ,c, Wng Jing-yu ,c

a Xi'an Modern Chemistry Research Institute, Science and Technology on Combustion and Explosion Laboratory, Xian 710065, PR China

b School of Environment and Safety Engineering, North University of China, Taiyuan, 030051, Shanxi, PR China

c Shanxi Engineering Technology Research Center for Ultrafine Powder, North University of China, Taiyuan, 030051, Shanxi, PR China

Keywords:MOF-5 2D MOF AP Thermal decomposition

ABSTRACT To investigate the effect of different structures of MOF on the catalytic performance of ammonium perchlorate(AP)thermal decomposition.By adding 2-methylimidazole(2-MI)as an inhibitor during the synthesis of MOF-5,a two-dimensional(2D)MOF material was prepared and characterized by SEM,EDS,XRD, FT-IR and other means.The prepared two-dimensional material was added to the AP raw material to investigate its effect on the AP thermal decomposition process.After DSC and TG test,it was found that the addition of this kind of 2D MOF material can completely eliminate the low-temperature decomposition peak of the AP-based composite and advance the high temperature decomposition peak to 296.5 °C,and the apparent heat of decomposition increase to 905.6 J/g,which has a better catalytic effect than the bulk MOF-5.In addition, through the combustion test, it was found that 2D MOF, as a burning rate catalyst, can effectively enhance the burning rate of AP based solid propellant.

1.Introduction

Metal-organic framework (MOF) is an inorganic material formed by the self-assembly of metal ions as the coordination center and organic ligands as the backbone.With the inherent advantages of high porosity and high specific surface area,and the ability to achieve structural diversity through the modulation of different types of metal nodes and ligands, MOF materials have received widespread attention in the fields of catalysis, gas adsorption and energy storage [1-5].

MOF materials are also currently used in heterogeneous catalysis.The oxygen balance of AP is as high as 34%,so it is widely used in solid propellants as an oxidant and is one of the main components in propellant formulations.And the thermal decomposition performance of AP directly affects the combustion performance of solid propellants.As a combustion catalyst to improve the combustion process of propellants, we have previously studied the promotion effect of several MOF materials on the AP thermal decomposition process and found that the generated metal oxide in situ during the combustion process can promote the combustion of the propellant, and its large specific surface area provides more catalytically active sites and adsorbs excess small molecules in the gas phase,thereby greatly improving the combustion performance of the propellant [6-9].

However, MOF materials also have a large volume problem,which is more prone to accumulation and affect their catalytic effects.Therefore, the development of a two-dimensional (2D) MOF material is considered.This material will have a higher surface area-volume ratio based on its unique two-dimensional structure,thereby overcoming the above-mentioned problem of easy agglomeration.2D MOF nanoplates not only can expose more active sites,but also facilitate the transport of gas molecules and the transfer of electrons [10-13].At present, solvents induce rapid nucleation, adding surfactants, and template methods to prepare two-dimensional MOF materials.Among them, it is most convenient and effective to use a surfactant as an inhibitor to reduce the surface energy and the total energy of the system through orientation or Van der Waals force, thereby slowing down the crystal growth rate [14-17].Guo Changyan etc.used 2-methylimidazole(2-MI) as a coordination modulator, and by adjusting the 2-MI concentration, the polarity, solubility of the solvent, and the reaction time,MOF materials of different sizes can be obtained,and the morphology of the MOF can also be improved to a certain extent[18].

In this study, 2-MI was used as an inhibitor in the synthesis of MOF-5.A two-dimensional MOF material was prepared and applied to the thermal decomposition process of AP.The catalytic effect of the two-dimensional nanosheet material on the thermal decomposition of AP was investigated, and the combustion performance of the propellant was improved.

2.Experimental section

2.1.Materials

Ammonium perchlorate (NH4ClO4, AP, 250-380 μm) was purchased from Xilong Chemical Co., Ltd.Reagent-grade Zn(NO3)2·6H2O,1,4-benzenedicarboxylic acid(H2BDC),ethyl alcohol,DMF and ethyl acetate were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.And 2-methylimidazole (2-MI)and chloroform were got from Shanghai Macklin Biochemical Technology Co., Ltd.

2.2.Preparation of MOF-5

In this paper,the hydrothermal method is used to prepare MOF-5.The preparation process is consistent with our previous work[7].The specific experimental process is as follows:Put 30 mL of DMF in which Zn(NO3)2·6H20 and H2BDC were dissolved in the reaction kettle.The reaction kettle was heated in a muffle furnace at 120°C for 20 h and then cooled to room temperature naturally.After filtering and washing, soak in chloroform, filter and dry to obtain cubic MOF-5 crystals [7].

2.3.Preparation of 2D MOF

A certain proportion of Zn(NO3)2·6H2O, H2BDC and 2-MI were uniformly dissolved in 30 mL DMF,and then the reaction kettle was kept in a muffle furnace at 120°C for 20 h.After cooling to room temperature, washed with DMF and Soak in chloroform for 12 h.Finally, filter and dry to obtain 2D MOF.

2.4.Preparation of AP/MOF

The AP and MOF materials were mixed by a solvent-nonsolvent method.The MOF material (5 wt%) was uniformly dispersed in ethyl acetate (non-solvent), AP (95 wt%) was dissolved in DMF(solvent), then ethyl acetate was added dropwise to DMF,magnetically stirred for 20 min, and filtered.Wash and dry to obtain AP/MOF mixture.

2.5.Characterization

The surface morphology and size of the samples were observed using a scanning electron microscope (SEM, Zeiss Sigma 300).The crystal structures of the samples were analyzed using an X-ray diffractometer(XRD,DX-2700)at a voltage of 40 kV and a current of 30 mA using Cu-Kα radiation.The structure was analyzed by Fourier transform infrared spectrometer(FTIR-650).The Brunaure-Emmett-Teller (BET) surface area of the samples was obtained using a specific surface and pore size analyzer (Micromeritics ASAP 2460,USA).

The thermal decomposition properties of the samples were analyzed by a differential scanning calorimeter (DSC, France Setaram Corporation).DSC test conditions: mass, 0.5 mg; heating rate, 5°C/min, 10°C/min, 15°C/min, and 20°C/min; nitrogen atmosphere,30 mL/min.In addition,a thermogravimetric analyzer(TG,Mettler Toledo)was used to test the thermogravimetric curve of the sample.TG test conditions:heating rate,10°C/min;nitrogen atmosphere, 50 mL/min.Ignition combustion experimental conditions:The nickel-chromium alloy was placed at the bottom inside a 70 μl crucible and 25 mg of solid propellant was loaded into the crucible to become the ignition specimen.Ignition was performed in an air atmosphere with a nickel-chromium wire heated by instantaneous current, and the ignition combustion process was recorded using a high-speed photographic instrument (i-SPEED 221).

3.Results and discussion

As shown in Fig.1,MOF-5 is mostly a cubic structure with pores on the surface and an average particle size of about 20 μm-30 μm.It is more like a network-type skeleton formed by a series of small sheets connected to each other.After the addition of the 2-MI ligand, it was found that the structure of the MOF changed from the original three-dimensional cube to a two-dimensional sheetlike structure,the surface was smoother than the original,and still with some pores.This showed that after adding 2-MI, MOF-5 stopped growing in the vertical direction and formed a sheet-like structure.

The EDS mapping images (Fig.2.) reflected the uniform distribution of the corresponding metal elements on the surface,and the content of O decreased after the addition of 2-MI, indicating that some 2-MI replaced the coordination of H2BDC, thereby suppressing the MOF to continue in the vertical direction.

The XRD patterns (Fig.3(a)) of the 2D MOF-5 obtained with 2-MI fully matches with the MOF-5 pattern, both them have characteristic points at 2θ = 6.8°, 9.6°, 13.8°, 15.4°, 19.8°, and 29.8°,which is consistent with the simulated MOF-5 XRD patterns(CCDC#938392)and those reported in the Refs.[19-23],indicating that the MOF structure was not affected by 2-MI.The FT-IR spectra results further confirmed that the crystal structure of the 2D MOF almost retained its original nature after the addition of 2-MI(Fig.3(b)).MOF-5 has strong peaks at 1386 cm-1and 1581 cm-1,which may be due to the tensile vibration of carboxylate anions present in the material.It is speculated that the broad band of 3000 cm-1-3600 cm-1is the characteristic peak of -OH, which originates from the water in the metal coordination.In addition,2D MOF has characteristic peaks caused by C-H stretching vibration on the imidazole cation near 3096 cm-1,and some absorption peaks in the range of about 2700 cm-1-3000 cm-1are caused by C-H stretching vibration of methyl group, which means besides maintaining the original MOF structure on the 2D MOF surface,there is also exists 2-methylimidazole.

The specific surface area and pore size distribution of MOF-5 and 2D MOF are shown in Fig.4.The specific surface area of MOF-5 is 421.89 m2/g, and its adsorption isotherm shows a typical type I isotherm.The N2adsorption phenomenon corresponds to the microporous material of MOF-5,indicating that the external surface area of MOF-5 is relatively small and its adsorption is mainly controlled by the microporous volume.After the addition of 2-MI,the cubic structure changed into a two-dimensional structure,resulting in the micropores of the sample becoming mesopores with a pore size of 10-20 nm and the specific surface area increased to 540.63 m2/g.Consequently, more active sites were exposed and the adsorption amount increased, which was favorable to the effective contact between the catalyst and the reaction gas.

Fig.1. SEM images of (a), (b) MOF-5 and (c), (d) 2D MOF.

Fig.2. EDS mapping images of (a) MOF-5 and (b) 2D MOF.

We all know that MOF-5 is a three-dimensional rigid skeleton formed by [ZnO4]6+inorganic group and organic group[O2C-C6H4-CO2]2-formed by four Zn2+and one O2-.And its with a large pore volume and good thermal stability.Pablo [24]has proposed the growth mechanism of MOF-5, and believed that the framework of MOF-5 is developed through the related processes of nucleation and dispersing different sublayers with different stability.These sublayers rely on the existence of non-framework substances for successful crystal growth, and pointed out that the crystal growth of MOF involves the direct addition of monomer BDC units and simple solvated Zn2+, or zinc BDC-based units, rather than complex secondary structural units,and its growth rate in the＜100> direction is slower than ＜110> direction.Zheng Chunman[25]also believed that the growth direction of MOF-5 along the＜100> regional axis was the slowest, and finally formed a cubic shape composed of six ＜100>faces.He found that during the solvothermal synthesis of MOF-5, the earliest formed Zn5(OH)8(NO3)2·2H2O nanosheets, and these nanosheets are clustered together and with the help of H2BDC adsorbed on the surface,they are stacked in the direction of ＜100> to form microplates.These microplates are further loosely stacked to form layered composite particles.A large amount of H2BDC is inserted into these layered particles,and instead of theanion,the phase transition from Zn5(OH)8(NO3)2·2H2O-H2BDC complex to MOF-5 finally forms the final porous cube, as shown in Fig.5.

Fig.3. (a) XRD patterns of samples; (b) FT-IR spectra of MOF-5 and 2D MOF.

Fig.4. Adsorption isotherm and Pore size distribution curve of (a) MOF-5 and (b) 2D MOF.

2-MI is a kind of competitive ligand, when quantitative 2-MI is added during the synthesis process of MOF-5, the lone pair of electrons on its nitrogen atom will soon compete with H2BDC to coordinate metal ions and delay its growth on the ＜100>surface.At the same time, it can also serve as the basis for deprotonation of carboxylic acid ligands,and attach to the surface of the MOF sheet,stabilize the MOF nanosheets and limit its growth in the vertical direction, eventually lead MOF- 5 shape to become a twodimensional MOF sheet structure, which is consistent with that shown in the SEM images.This two-dimensional structure will help expose more active sites on its surface, thereby increasing the contact surface of the reactants with the catalytically active centers.

After adding MOF-5 and 2D MOF materials to AP raw materials,the morphology of their composites are shown in Fig.6.It can be observed that both MOF materials are mixed with AP particles,but it can be clearly seen that the bulk MOF-5 is simply mixed with AP,but the contact area of them is a little.Some MOF-5 cubes are even scattered around the AP material alone;while the two-dimensional MOF films are more closely attached to the surface of the AP material, exposing more catalytically active sites, and increasing the contact area of active sites and AP crystals.It can also be seen from the EDS mapping images that zinc is evenly distributed on the composite,indicating that the 2D MOF nanosheets are mixed in the AP crystal,but the content is less.Besides,after the addition of two MOF materials, the XRD characteristic peaks of the AP-based composites are almost the same as those of the AP raw materials,indicating that the addition of MOF would not affect the AP crystal,as shown in Fig.5(a).

Fig.5. Formation mechanism of MOF-5 cube and 2D MOF sheet.

Fig.6. SEM images of (a), (b)AP/MOF-5; (c), (d)AP/2D MOF and (e) EDS mapping images of 2D MOF.

As shown in Fig.7(a), the DSC curves showed that the thermal decomposition of AP is a continuous and complex process, and always consisting of three stages[26-30].Firstly,the endothermic peak at 245°C corresponds to the transformation of AP from oblique to cubic crystal.Secondly,the two exothermic peaks are the low-temperature decomposition peak and high-temperature decomposition peak, respectively.The low-temperature decomposition usually occurs below 350°C.It is mainly a solid-state reaction to generate some oxidizing intermediates such as ClO3,ClO,H2O,O2and some nonoxidized NH3.NH3will adhere to the AP surface and inhibit the further decomposition of AP.As the temperature increases, AP undergoes high-temperature decomposition,which is mainly a gas-phase reaction process,generating N2O,NO,Cl2,H2O,O2and other gases.The TG curve of AP(Fig.7(b))has the same thermal decomposition trend as the DSC curve of AP.It is divided into two weight loss stages.The weight loss in the first stage is 22%,corresponding to the low-temperature decomposition peak of AP, and the weight loss in the second stage is 78%, indicating that the thermal decomposition of AP is mainly concentrated in the high-temperature decomposition process and releases a large amount of heat.

After adding a small amount (5%) of MOF-5 to the AP raw material, it can be clearly seen that both the high temperature decomposition peak and the low temperature decomposition peak of the composite are significantly advanced.The LTD peak was advanced from 311.8°C to 288.1°C, while the HTD stage was weakened and approached the low-temperature decomposition stage, from 409.7°C of the raw material to 322.5°C, which was advanced nearly 87°C,and the apparent decomposition heat(ΔH)also increased from the original 576 J/g-815.8 J/g.Based on previous researches[7],we know that the catalytic effect of MOF-5 is mainly due to the large amount of Zn2+supported in MOF which is in the electron-deficient stage,and it’s conducive to the adsorption of lone pair electrons in AP products.Zn2+can attract the lone pair electrons to facilitate the break of the N-X bond,and some nitrogen oxides produced by AP decomposition can also react with Zn,releasing a large amount of NO2, and effectively preventing NH3,HClO4from re-condensing when they were cold;on the other hand,the MOF material itself has a large specific surface area and a large number of pores, which is conducive to the adsorption of small molecules of gas,thereby speeding up the decomposition process.

Fig.7. (a) DSC and (b) TG of samples.

The addition of 2D MOF material completely eliminated the low-temperature decomposition stage of the AP-based composite,the high-temperature decomposition peak was advanced to 296.5°C, and the apparent decomposition heat was 905.6 J/g,indicating that its catalytic effect on the thermal decomposition process of AP is better than that of MOF-5 cube.It can also be clearly seen from the TG curves that the entire decomposition process of the composite has changed from two weightless stages to one,which corresponds to the only high-temperature decomposition stage in the DSC curves.To better understand the catalytic performance of 2D MOF, a summary of recent highly active MOF-type catalysts in the literature is shown in Table 1.The comparison reveals that 2D MOF/AP has the lowest thermal decomposition peak temperature (296.5°C), indicating that 2D MOF has superior catalytic performance and is advantageous in reducing the HTD of AP.

In order to further evaluate the catalytic performance of the prepared samples,MOF-5 and 2D MOF were added to the AP-based solid propellant as the ignition rate catalysts, and the combustion process was photographed by high-speed photography, and the ignition delay time and combustion process are shown in Fig.8.The ignition delay of the solid propellant without catalyst was as high as 132.6 ms,and the ignition delay of the solid propellant with MOF-5 and 2D MOF decreased to 18 ms and 21 ms.The reason for this difference may be the different thermal decomposition onset temperatures of the catalyst/AP.In addition, MOF-5 and 2D MOF contribute to the increase of combustion rate by catalyzing and thus accelerating the thermal decomposition of AP,which is manifested by the decrease of combustion duration.The shortest combustion time was observed in the sample with 2D MOF(928 ms),which was lower than that of the sample with MOF-5 (1293 ms) and the sample without catalyst (1440 ms).It is shown that the thermal decomposition process of AP is closely related to the combustion process of solid propellant, and the DSC and TG curves show that the onset decomposition temperature of 2D MOF/AP is slightly higher compared with MOF-5/AP, but its thermal decomposition process is a continuous phase with a higher thermal decomposition rate.Similarly, the combustion results showed that the solid propellant with 2D MOF added had the fastest combustion rate, indicating that the catalytic performance of 2D MOF was superior to that of MOF-5.

Based on the proton transfer theory of AP thermal decomposition and previous work, the corresponding catalytic mechanism was proposed,as shown in Fig.9.The initial decomposition of AP is the process of proton transfer from NH4+to ClO4-, followed by the formation of gaseous NH3and HClO4,but the decomposition rate is faster than the sublimation rate in the low-temperature decomposition stage,resulting in the adsorption of unoxidized NH3 on the crystal surface, which hinders the decomposition of AP.After adding catalyst, 2D MOF with a large specific surface area continued to adsorb NH3and other gases,which promoted further decomposition of AP.With the increase in temperature, NH3and HClO4were thermally decomposed into gases such as Cl2, O2, NO,N2O and HCl.2D MOF loaded with a large amount of unsaturated Zn2+could react with the nitrogen oxides in the product gas and release a large amount of NO2, thus accelerating the reaction.The results of the catalytic performance analysis show that the twodimensional MOF materials have better catalytic performance than the MOF cubes.The reason is not only because the catalytic properties of the MOF material itself,it also benefited from the twodimensional sheet structure of the MOF prepared in this experiment.As shown in Fig.8,on the one hand,the formation of a twodimensional MOF sheets can not only expose more catalytically active sites, but its two-dimensional sheet-like structure and the presence of crystal defects are also more conducive to its contact with AP crystals;on the other hand,the addition of 2-MI in 2D MOFmaterials can also be used as a basic catalytic site to participate in the reaction,and the high energy possessed by the imidazole ring is also conducive to increasing the energy of the entire complex,thereby promoting the rapid progress of AP thermal decomposition.

Table 1Summary of thermodynamic parameters of the samples.

Fig.8. Combustion process of (a) AP; (b) AP/MOF-5 and (c) AP/2D MOF.

Fig.9. Catalytic mechanism diagram of AP’s thermal decomposition.

4.Conclusions

In summary, the addition of the 2-MI ligand makes it compete with the H2BDC ligand and coordinate with Zn2+on the surface of MOF-5, preventing further connection of the nanosheets and restricting their growth in the vertical direction to form a 2D MOF sheet structure.This structure not only exposes more catalytically active sites,but also more conducive to adhere to the surface of the AP crystal,and 2-MI itself can also be used as a basic catalytic site,thereby achieving a better catalytic effect than the MOF-5 cubes.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.