Influence of Additives on the Solvothermal Synthesis in the Formation of Zn-MOF5

LIN Xiaoying ()， SU Ting( )， ZHONG Qinhua()， SHI Ronghui ()， LIU Minyi ()， LIU Yamin()， LU Dongfei()

1 Institute of Environmental Protection， Fujian University of Technology， Fuzhou 350118， China2 College of Ecological Environment and Urban Construction， Fujian University of Technology， Fuzhou 350118， China

Abstract: The influences of additives of NH3， HCl， KOH and CH3OH on the solvothermal synthesis of the Zn-based metal-organic frameworks (Zn-MOF5s) were investigated. Powder X-ray diffraction (PXRD)， thermal gravimetric analysis (TGA)， Fourier translation infrared spectroscope (FT-IR)， N2 adsorption/desorption at 77 K and CO2 sorption measurements were used to characterize the as-prepared Zn-MOF5s. The experimental results show that additives of NH3， CH3OH， HCl and KOH in the synthesis of the Zn-MOF5s do not change the underlying topology， but they are extremely sensitive to the pore textural properties， thus changing the CO2 adsorption capacity. Additives would lower the pore width and the surface area，and then lower the CO2 adsorption capacity of Zn-MOF5s.

Key words: Zn-MOF5s; additives; influence; CO2 adsorption capacity; nanocrystal

Introduction

Scientific studies suggest that the increased greenhouse gas concentrations in the atmosphere， particularly carbon dioxide， are a major factor in global warming.CO2capture and storage by adsorbents is an economical and relatively mature method by considering the low cost of equipment and the possible recycling uses of the captured CO2[1-2]. Metal-organic frameworks (MOFs)， a new class of crystalline materials constructed by metal-containing nodes bonded to organic bridging ligands， are promising novel adsorbents for CO2capture owing to their porous structures and high thermal stability[3-4]. Extensive research has been evoted to the synthesis and characterization of MOFs[5-9]. A common characteristic of these systems is their complex parameter space， since its small changes in composition can have a profound impact on the structures formed and thus on their properties[10]. In this paper， five highly crystalline materials Zn-MOF5s with different additives of NH3， HCl， KOH and CH3OH in reaction system were synthesised and the effects of the additives on the topology structure， pore textural properties and CO2adsorption property of MOFs were discussed.

1 Experimental

1.1 Materials

All chemicals including Zn(NO3)2·6H2O (99%)， DMF (N，N-Dimethylformamide， 99%)， BDC (benzene-1，4-dicarboxylic acid， 98.9%)， KOH， CH3OH， HCl and NH3were obtained from Aladdin Industrial Corporation (USA) and used without further purification, and 1 mol/L potassium hydr-oxide (KOH) solution was prepared.

1.2 Synthesis of Zn-MOF5s

Zn-MOF5s were synthesized following the procedure reported by Furukawaetal.[11]with some amendments. Zn(NO3)2·6H2O (0.23 g， 7.5 mmol) and BDC (0.83 g， 5.0 mmol) were mixed together in 80 mL of DMF. Then， the solution was dispersed for 30 min by ultrasonic cleaner. NH3， HCl， KOH and CH3OH (each about 0.05 mL) were added into the mixture and ultrasound irradiation for 10 min， and then the homogenous mixture was placed inside a 100 mL teflon-lined stainless steel autoclave. It was kept at 150 ℃ for 24 h in the oven to yield crystals. Then the autoclave was cooled down to room temperature naturally and the crystals were isolated by filtration. The crystals were washed with DMF three times. Then the complexes were vacuumed to -0.1 MPa at 150 ℃ for 4 h， and then cooled to room temperature. MOFs synthesized without and with the additives of NH3， HCl， KOH or CH3OH were referred to as Zn-MOF5， Zn-MOF5-NH3， Zn-MOF5-HCl， Zn-MOF5-KOH and Zn-MOF5-CH3OH, respectively.

1.3 Characterizations

Thermal stability of Zn-MOF5s was checked by thermal gravimetric analysis (TGA) (STA409PG， NETZSCH， Germany). The samples were loaded into a pan and heated to 800 ℃ at a rate of 10 ℃ /min. The air gas flow rate was maintained at 50 mL/min. Fourier translation infrared spectra (FT-IR) (Nicolet 6700， Thermo Scientific，USA) were obtained to check the stability of the functional groups on the organic ligands. The spectrum was scanned from 400 to 4000 cm-1with a resolution of 4 cm-1. Powder X-ray diffraction (PXRD) patterns were obtained by D8 Advance-Broker axes with Cu-Ka radiation (=0.154 06 nm)， accelerating voltage and current of 40 kV and 40 mA， respectively. A surface area and porosimetry analyzer (ASAP2020 Micromeritics, USA) was used to determine N2adoption isotherms as well as the porosities and specific surface areas of Zn-MOF5s. The samples were evacuated at 150 ℃ for 6 h prior to the adsorption measurements under high vacuum. Surface area was determined by the Brunauer-Emmett-Teller (BET) method， and pore size distribution was derived from the Barrett-Joyner-Halenda(BJH) method.

1.4 CO2 adsorption

The CO2adsorption and performance of the Zn-MOF5s were studied with thermal gravimetric analyzer. This entailed degasifying the samples under the stream of N250 mL/min and the temperature was increased to 150 ℃ at a rate of 5 ℃/min and this temperature was kept for 1 h. This process was to outgas the amount of CO2， water and other molecules which were adsorbed at room temperature. Then pass into a stream of CO2at 100 mL/min for 60 min under keeping the stream flow rate of N250 mL/min and the temperature 27 ℃. Isothermal CO2capture tests were carried out to evaluate the suitability of the samples．

2 Results and Discussion

2.1 PXRD analysis

Figure 1 shows the PXRD patterns of samples. The PXRD patterns of the samples show that the Bragg diffraction angles in samples are essentially identical and perfectly match with the stimulated pattern from the established crystal structure data， indicating that the as-synthesized samples have the correct structure with good crystallinity and the topology structure is well maintained after adding the additives，i.e., NH3， HCl， KOH or CH3OH during the synthesis. According to Fig.1， the diffraction peaks of Zn-MOF locate at 2θare 6.8°， 9.7°， 13.5°， 15.4°，etc.， and the position of the peak is consistent with the MOF5literature report[12]. However， it can be seen that there are obvious peaks at 25° of Zn-MOF5-KOH. The peak intensities are somewhat different with respect to the additives. The intensive peaks that appeared at small 2θangles are characteristics of microporous materials， which possess numerous tiny pores or cavities that are in accordance with published data[13].It means additives of NH3， HCl， KOH and CH3OH would change the pore textural properties. The PXRD patterns of Zn-MOF5-NH3does show the two main peaks (2θ=13.5° and 2θ=20.5°)， with the remaining other small peaks were not so sharp and clear. It is indicated that its pore size can be adjusted with addition of NH3， HCl， KOH or CH3OH during the synthesis of Zn-MOF5s.

Fig.1 PXRD patterns of samples

2.2 FT-IR analysis

FT-IR spectra for samples are depicted in Fig.2. The samples of Zn-MOF5， Zn-MOF5-CH3OH， Zn-MOF5-KOH， Zn-MOF5-HCl and Zn-MOF5-NH3exhibit similar FT-IR spectra， suggesting they have similar functional groups[14]. All samples contain a broad band around 3 400 cm-1， which can be assigned to O—H stretching. The strong bands， at 1 654， 1 593 and 1 400 cm-1， can be assigned to the vibrational stretching frequencies of the framework (O—C—O)， confirming the presence of dicarboxylate linker in the Zn-MOF5s. In the region between 1 300 cm-1and 700 cm-1， several bands are observed that can be assigned to the out-of-plane vibrations of BDC[15-16]. The FT-IR spectrum of the samples indicates that there are no obvious chemical changes in the as-prepared Zn-MOF5s.

Fig.2 FT-IR spectra of samples

2.3 Physical properties of the Zn-MOF5s

Pore textural properties of the Zn-MOF5s crystals，were calculated from the N2adsorption and desorption isotherms shown in Fig.3. Except for Zn-MOF5-HCl， the other curves type-I isotherm according to the IUPAC classification[17]characteristic for the microporous solids[18]. Zn-MOF5-HCl shows typical IV isotherms， with characteristic hysteresis loops at relative pressures above 0.9， exhibiting both meso-and microporosity. The Zn-MOF5-HCl crystal has a layer shape while the other four crystals have cubic geometry. The N2uptakes on the Zn-MOF5， Zn-MOF5-NH3， Zn-MOF5-KOH， Zn-MOF5-CH3OH and Zn-MOF5-HCl crystals at 0.1 MPa and 77 K are 357， 322， 280， 261and 275 cm3/g， respectively. The corresponding BET surface are 1 060， 851， 815，777 and 683 m2/g， respectively.These results suggest that the addition of NH3, KOH, CH3OH or HCI in the solvothermal synthesis affects the generation of pores and reduces the surface area of MOF. Moreover, the decrease in surface area may be due to residual additives in the pore.

Fig.3 N2 adsorption-desorption isotherms

Table 1 summarizes the BET surface area (SBET), Langmuir surface area (SLangmuir), pore volumes(Vmic) and pore diameter(D) of the Zn-MOF5s crystals calculated from the N2adsorption data using the ASAP-2020 built-in software. The results suggest that the generation of porosity is influenced by the addition of NH3， KOH， CH3OH or HCl in the solvothermal synthesis and additives can lower the surface area of MOFs. This may be due to the addition residual in the pore， which reduces the surface area.

Table 1 Textural properties of samples

2.4 SEM analysis

In order to characterize the morphology and porous texture， the SEM images of the samples are given in Fig.4. It can be seen from the pictures that the sample of Zn-MOF5(Fig.4(a))， Zn-MOF5-NH3(Fig.(c)) and Zn-MOF5-KOH(Fig.4(d)) are regular cubic structures， corresponding to previous literature[19]. However， the structure of Zn-MOF5-CH3OH and Zn-MOF5-HCl is different from Zn-MOF5. The former is irregular cubic structure and the latter is sheet structure. Besides we can also see macropores on the surface of Zn-MOF5， Zn-MOF5-NH3and Zn-MOF5-KOH. These results suggest that the morphology and porous texture is influenced by the addition of CH3OH or HCl in the solvothermal synthesis.

(a)

(b)

(c)

(d)

(e)

2.5 TGA analysis

The TGA analysis for the samples is shown in Fig.5. There are three weight-loss steps. The first weight loss appearing before 100 ℃ is due to the evaporation of surface adsorbed water and guest molecules on MOFs. The second weight loss at 150-400 ℃ is attributed to the decomposition of DMF. No distinct weight loss appears in 30-150 ℃， which means the activated temperature (150 ℃) is applicable to remove the residual solvent in the pores. After that， the samples did not show any significant weight change up to 220 ℃ and then gradually started to lose the weight. This result confirms the stability of the framework at higher temperature and there were almost no impurities in the structure. The third stage of weight loss at 400-550 ℃ is ascribed to the decomposition of Zn-MOF to ZnO. According to weight loss rate， Zn-MOF5-KOH and Zn-MOF5-NH3have slightly higher thermal stability than the other MOFs， ascribing to alkaline regents (NH3and KOH) accelerate deprotonation of ligand， then more rigid structures have been constructed.

Fig.5 TGA analysis of samples

2.6 CO2 adsorption

As shown in Fig.6，CO2uptakes on the Zn-MOF5， Zn-MOF5-HCl， Zn-MOF5-NH3， Zn-MOF5-KOH and Zn-MOF5-CH3OH， at 27 ℃ and 0.1 MPa are approximately 8.6%， 5.3%， 6.3%， 6.8%， 5.5% by weight, respectively. The CO2adsorption capacities on the Zn-MOF5-NH3and Zn-MOF5-KOH are higher than on the Zn-MOF5-HCl and Zn-MOF5-CH3OH.

Fig.6 CO2adsorption kinetics of samples

3 Conclusions

In summary， five Zn-MOF5s with addition of NH3， KOH， CH3OH or HCl in the solvothermal synthesis were prepared and the additives influence on the formation of the Zn-MOF5s and CO2uptake capacity were investigated. The results show that the additives in the synthesis of Zn-MOF5s can reduce the surface area. The TGA analysis suggests that additives will change the thermal stability of Zn-MOF5s. NH3and KOH enhance thermal stability of Zn-MOF，while HCl and CH3OH lower the thermal stability. This study may have general implication for the other MOFs materials with improved thermal stability and surface area.