Synthesis of MCM-41/ZIF-67 Composite for Enhanced Adsorptive Removal of Methyl Orange in Aqueous Solution

ZIF-67 and MCM-41/ZIF-67 composites were successfully synthesized with water solvent at room temperature. The amounts of MCM-41 added during synthesis were varied at 2.5, 5 and 10 (%w/w) toward the amount of ZIF-67, and the obtained solids were denoted as MC (2.5)/ZIF-67, MC (5)/ZIF-67, and MC (10)/ ZIF-67, respectively. The X-ray diffraction (XRD) patterns of ZIF-67 and the composites showed characteristic peaks at 2θ of 7.32, 10.36, 12.69, 14.66, and 16.40°, similar to that of reported ZIF-67. The Fourier transform infra-red (FT-IR) spectra of all solids showed absorption bands at the same wavenumbers as reported for ZIF-67. The results of surface morphology analysis using scanning electron microscope (SEM) have shown that ZIF-67 and the composites have a cube shape, which is characteristic for the ZIF-67 standard. N2 adsorption-desorption data showed that the specific surface area of ZIF-67 and MC (5)/ZIF-67 were 1079.2 and 1011.2 m2/g, respectively, lower than that of MC (10)/ZIF-67 (1250.6 m2/g). However, results of thermal gravimetric analysis (TGA) showed that the thermal stability of MC (10)/ZIF-67 reached 357°C, higher than that of ZIF-67 (325°C). Performance of the composites as adsorbent of methyl orange (MO) in aqueous solution showed that the MC (5)/ZIF-67 had the highest adsorption capacity of 167.635 mg/g, and followed the pseudosecond-order adsorption kinetics and Langmuir isothermal adsorption.


Introduction
Organic dyes are pollutants that can provide toxic effects on microbial populations and can be toxic or carcinogenic to organisms and mammals [1]. Azo dyes are a group of synthetic organic dyes, which are most widely used in the textile, cosmetics, and food coloring industries, so they are found in industrial waste water. The release of waste water containing dyes, with different chemical compositions and toxicity, into waterways can cause environmental damage [2]. Methyl orange (MO) is one of the cationic dyes, which are widely used in the coloring industry. If the MO concentration in wastewater reaches 500 ppm, it can cause serious health

Synthesis of ZIF-67 and composite MCM-41/ZIF-67
ZIF-67 and MCM-41/ZIF-67 composites were synthesized with a molar ratio of metal:ligand of 1:11.6. The synthesis of ZIF-67 was begun by dissolving 6.09 g of 2-methylimidazole (MeIM) in 13.5 mL deionized water. Triethylamine (TEA) added was 0.5 mL and the amount of MCM-41 added was varied at 2.5, 5 and 10 w/w% toward the amount of the obtained ZIF-67. Then, CoCl 2 ·6H 2 O solution was made by dissolving 1.52 g of CoCl 2 ·6H 2 O in 10 mL deionized water. CoCl 2 ·6H 2 O solution was mixed with 2-methylimidazole solution in a vial bottle and stirred for 20 min. Furthermore, the reaction mixture was centrifuged at 3000 rpm for 30 min. The obtained solids were washed with deionized water, followed by washing with methanol. Finally, the solids were dried in an oven at a temperature of 120°C for 16 hours.

Characterization ZIF-67 and MCM-41/ZIF-67 composites
The crystallinity of ZIF-67 and MCM-41/ZIF-67 composites were determined by XRD with CuKα radiation (λ = 1.5406 Å), with 40 kV accelerated voltage, and a current of 30 mA. The 2θ angle was recorded from 3 until 50°. Analysis of the existence of functional groups on ZIF-67 and MCM-41/ZIF-67 composites was carried out by FTIR spectrophotometer at wavenumbers of 4000-400 cm −1 . Analysis by scanning electron microscope (SEM) has been done to determine the surface morphology of synthesized material. TEM images were obtained using a JEOL JEM1400 transmission electron microscope at an accelerating voltage of 200 kV. Measurement of surface area and pore size distribution of the materials were carried out using surface area analyzer based on adsorption-desorption isotherm using nitrogen gas, and the samples were degassed at 150°C for 5 hours (3 × 10 −3 Torr) before being analyzed. Thermogravimetric analysis (TGA) has been used to determine the thermal stability, and the samples were heated from temperature of 35-700°C with an air gas flow.

Adsorption experiment
Methyl orange (MO) adsorption test on the effect of contact time was carried out using 10 mg of adsorbent in 100 mg/L of MO solution. The mixture was stirred at 300 rpm for 5 until 40 min. In addition, the adsorption test was also carried out on the effect of the initial concentration and concentration of MO was varied from 50 to 330 mg/L. After adsorption process, the solution was centrifuged at a rate of 3500 rpm for 10 min to separate the adsorbent from the solution. Concentration of MO was measured using a UV-Vis spectrophotometer at the maximum wavelength. Adsorption capacities of adsorbent and %removal of MO were calculated using the formula: where C o and C e are initial and equilibrium concentrations of MO in mg/L, W is the mass of adsorbent in g, and V is the volume of solution in mL.

Characterization of materials
The effect of the addition of MCM-41 on characteristic of the obtained MCM-41/ZIF-67 composites has been determined from the results of characterization using XRD, FTIR, SEM-EDX, nitrogen adsorption-desorption, and TGA. The XRD patterns of ZIF-67 and MCM-41/ZIF-67 composites are shown in Figure 1. As can be seen, the characteristic peaks appear at 2θ of 7.32, 10.36, 12.69, 14.66, and 16.40°, respectively, which are in accordance with the results of previous studies [22]. Likewise, the XRD patterns of the MCM-41/ZIF-67 composites have shown the same pattern as pure ZIF-67. The addition of MCM-41 during the ZIF-67 synthesis, however, has resulted in a reduction in crystallinity of the obtained composites, which may be attributed to amorphous properties of mesoporous silica [24].
FT-IR analysis has been carried out to identify the functional groups of the obtained materials, and the results are shown in Figure 2. Each sample exhibit the presence of a sharp absorption band at a wavenumber of 423 cm −1 , due to the Co▬N stretching vibration, indicating a bond between Co and N of the 2-methylimidazole ligand [25]. An absorption band at 693 cm −1 may be attributed to the bending vibration of the ring 2-metilimidazole [26]. Absorption peaks at wavenumbers of 755 and 1577 cm −1 may be resulted from out-of-plane vibrations and stretching vibrations of C〓N in 2-methylimidazole ligands. The occurrence of C▬N bending vibration has been identified from the appearance of a peak at wavenumber of 992 cm −1 , while stretching vibration from C▬N is identified at wavenumber 1141 cm −1 . Absorption bands at wavenumbers 2926 and 3134 cm −1 appear due to stretching vibration of C▬H sp 3 aromatic ring on 2-methylimidazole and stretching vibration of C▬H sp 2 on aliphatic hydrocarbon chains [25]. The characteristic peak of MCM-41 that appears at wavenumber of 1050 cm −1 , as a result of Si▬O▬Si asymmetric vibration, is seen in the MC(5)/ZIF-67 composite spectrum and appears more pronounced in the MC(10)/ZIF-67 composite. This observation has indicated the existence of MCM-41 in the composite.
Surface morphology of ZIF-67, as is shown in Figure 3(a), has a cubic shape, similar to that reported by Qian et al. [18]. Likewise, the MC(10)/ZIF-67 composite also has a uniform surface morphology, as is shown in Figure 3(b). The results of SEM showed that the presence of mesoporous silica resulted in the increase in particle size, while the cube shape and more uniform surface morphology remained unchanged. The SEM images may indicate that the growth of ZIF-67 has taken place  inside the pores as well as on the surface of mesoporous silica. The phenomenon is reinforced by TEM image of the MC(10)/ZIF-67 composite, in which the characteristic hexagonal pore shape of MCM-41 is not seen any more in Figure 3(c).
Nitrogen adsorption-desorption isotherms and pore size distributions of ZIF-8, and the composites are presented in Figure 4(a) and (b), respectively, and the results of BET surface area and pore size are summarized in Table 1.
As shown in Figure 4(a), type I isotherm adsorption curves of ZIF-67 and the composites have indicated that adsorption occurred in micropores of the samples that have high surface area. At a relatively higher pressure (P/P 0 = 0.85-1), the amount of gas absorbed increased and a narrow vertical hysteresis loop was observed on all samples, which may be due to the presence of interparticle mesoporous. Based on the BET equation, the highest surface area was observed for  MC(10)/ZIF-67 (1250.6 m 2 /g). The high surface area may result from the formation of ZIF-8 particles inside the MCM-41 mesopores so as to produce new micropores with high surface area [27]. The pore distribution curves of ZIF-67 and the composites based on the BJH method are shown in Figure 4b, in which microporous and mesoporous distributions have been observed ( Table 1). It can be concluded that the addition of mesoporous silica into the ZIF-8 synthesis results in the increase in the pore volume and pore diameter of the obtained composites (Figure 4b).
The results of TGA, as is shown in Figure 5, confirmed two-stage process of decomposition of ZIF-67 and MC(10)/ZIF-67 composite.
At a temperature range of 35-325°C, a mass decrease of 7.68% was observed, as a result of the loss of guest molecules of solvent or ligands that occupied the pores or surface of the samples [18]. Furthermore, a sharp decrease of 60.05% mass was observed in the temperature range of 325-429°C, which considered as decomposition of 2-methylimidazole ligand and damage of ZIF-67 structure. In addition, it can be seen in Figure 5 that the thermal stability of the MC(10)/ZIF-67 was slightly higher than that of ZIF-67. The observation was in accordance with the result reported by Bhatt et al. [28]. Also, higher remaining mass of the composite as compared to the ZIF-67 was due to presence of SiO 2 from the MCM-41 added.

Adsorption kinetic studies
Kinetic study of MO adsorption was carried out using 20 mL of MO solution with a concentration of 100 mg/L. The contact times were varied in the range of 5-40 min, while the weight of adsorbent was 10 mg. The effect of contact time on MO adsorption for ZIF-67, MC(5)/ZIF-67, and MC(10)/ZIF-67 materials is illustrated in Figure 6. It can be seen that adsorption capacities increased with the increase in contact time to reach a maximum value after 25 min, after which a more stable adsorption capacity was observed until 50 min. The results indicated that the empty space of pores and the surface of adsorbent have been completely filled with adsorbate [29].
The kinetics data were analyzed using pseudofirst-order and pseudosecondorder kinetic models using Eqs. (3) and (4).   Based on the calculation of adsorption kinetics with Eqs. (3) and (4), graph plots were performed on each of the adsorption kinetics models and the obtained graphs are shown in Figure 7. MO adsorption kinetics model of ZIF-67, MC(5)/ZIF-67, and MC(10)/ZIF-67 adsorbents were determined by comparing the R 2 values of each graph of the adsorption kinetics as shown in Table 2. It can be seen that correlation coefficient of the pseudosecond-order plot is higher than the pseudofirst-order plot. The results indicated that the mechanism of adsorption in all materials observed was as a result of chemisorption [30].
Adsorption of MO at different initial concentrations was carried out at the optimum contact time obtained of 25 min with MO concentration in the range of 50-330 mg/L. As is shown in Figure 8, the adsorption capacity increased with the increase in initial concentration of MO and the equilibrium of adsorption capacity was observed at an initial concentration of 285.20 mg/L. By further increasing the initial MO concentrations, a stable equilibrium adsorption capacity was obtained. The MC(5)/ZIF-67 showed highest Q e value of 167.64 mg/g, compared to other adsorbents. The results of adsorption studies have been in accordance with the results of N 2 adsorption-desorption analysis, in which the higher the pore size of the adsorbent, the higher is the adsorption capacity. The largest pore size of MC(5)/ ZIF-67, as a result of adding 5%w/w mesoporous silica during the ZIF-67 synthesis, has improved its performance in MO adsorption [26]. The large pore size of the MC(5)/ZIF-67 allowed MO molecules to easily diffuse in to the pores and interacted electrostatically with cobalt ions from ZIF-67.

Isotherm adsorption
The study of adsorption isotherms was based on Langmuir and Freundlich isotherms models as calculated in Eqs. (5) and (6): Plots of adsorption isotherm obtained from the calculation are presented in Figure 9. The calculated correlation coefficient for each adsorbent used approaches  Table 3.

Langmuir and Freundlich isotherm parameters for the adsorption of MO.
with the value of 1 in the Langmuir model ( Table 3). These results indicated that the adsorption process occurred in accordance with the Langmuir adsorption isotherm model, which led to the assumption that adsorption occurred on a specific homogeneous surface of the adsorbent, which had the same energy for monolayer adsorption [31].   As an anionic dye, MO contains SO 3 group in its molecular structure. The adsorption of MO by ZIF-67 and MCM-41/ZIF-67 composites can occur as a result of the electrostatic interaction between the SO 3 group and the positive charge of cobalt ion from ZIF-67 [12]. In addition, the imidazole rings of ZIF-67 may interact with the benzene rings from MO molecules through π-π stacking [16,19]. The molecular interaction of MO with ZIF-67 is illustrated in Figure 10.

Conclusions
ZIF-67 and MCM-41/ZIF-67 composites have been successfully synthesized at room temperature in aqueous solution with addition of TEA as a deprotonating agent. The results of characterization using XRD indicated that both ZIF-67 and composites exhibited the same diffractogram patterns, with characteristic peaks at 2θ of 7.32, 10.36, 12.69, 14.66, and 16.40°, similar to that of reported ZIF-67. In addition, the absorption bands of FTIR spectra of all composites appeared at the same wavenumber as observed for synthesized and the reported ZIF-67. SEM images showed similar cube shapes of surface morphology between ZIF-67 and MC(10)/ ZIF-67 composites. The results of analysis by adsorption-desorption N 2 showed that the highest surface area of 1250.6 m 2 /g was observed for MC(10)/ZIF-67. The highest adsorption capacity toward MO in aqueous solution of 167.6 mg/g was achieved by MC(5)/ZIF-67 adsorbent, and followed a pseudosecond-order adsorption kinetics and Langmuir adsorption isotherm.