Controlling the Physical Properties of Magnesium Oxide Using a

Article pubs.acs.org/IECR

Controlling the Physical Properties of Magnesium Oxide Using a Calcination Method in Aerogel Synthesis: Its Application to Enhanced Sorption of a Sulfur Compound Anh-Tuan Vu, Shunbo Jiang, Yo-Han Kim, and Chang-Ha Lee* Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, Korea ABSTRACT: To synthesize mesoporous magnesium oxide (MgO) with a high surface area and pore volume, a calcination procedure was developed for hydrated MgO prepared by the same aerogel method. The sorption capacities of as-prepared MgO compounds were evaluated by breakthrough experiments to remove methylmercaptan (291 μmol/mol) from methane. The surface area of nanosized MgO varied from 296.6 to 503.5 m2/g, depending on the calcination procedure. The physical properties and sorption capacity of MgO were affected by the final temperature, step, and heating rate during calcination, but the sorption capacity could not be evaluated only by the surface area and pore volume. MgO prepared by the four-step calcination method, which had two additional steps in the significant decomposition temperature range, had 2−11 times higher sorption capacity than MgO by other calcination methods. This suggests that optimizing the calcination method is as important as searching for a new synthesis route.

1. INTRODUCTION Magnesium oxide (MgO) has been of great interest because of its excellent physical properties, such as its large specific surface area, high pore volume, and narrow pore-size distribution. Therefore, mesoporous MgO has been widely used in many fields, such as adsorption, catalysis, toxic waste removal, paint, and superconducting products.1−4 As an effective catalyst in many organic reactions2,5,6 and as a catalyst support,7 nanoscale MgO should have a sufficiently large surface area and the right pore-size distribution to allow the diffusion of active species.8 Well-designed MgO has shown high performance in liquid and gas phases as a promising adsorbent.9−11 Natural gas contains certain sulfur compounds such as hydrogen sulfide, mercaptans, and tetrahydrothiophene.12,13 Removing sulfur compounds from natural gas has always been a critical problem in many industries because of environmental issues and catalyst poisoning.14−17 Furthermore, sulfur compounds need to be removed from municipal gas used for hydrogen generation at hydrogen stations, fuel cells, etc. To either dissociate or remove sulfur compounds from effluent gases, MgO is one of the candidate materials used in industrial applications.18,19 Various synthesis methods for nanocrystalline MgO have been developed, including sol−gel,20 hydrothermal,3 laser vaporization,21 chemical gas-phase deposition,22 microwaveassisted synthesis,4 and combustion aerosol synthesis.23 MgO with a high surface area has been reportedly developed by an aerogel method.24 In the previous study, the performances of MgO compounds prepared by polyol-meditation thermolysis, hydrothermal, and aerogel methods were compared.25 When the same calcination method was applied, the aerogel MgO had the highest surface area and sulfur sorption capacity among the as-synthesized MgO compounds. Also, a MgO/SiO2 composite contributed to improving the sorption capacity following the same aerogel method. © XXXX American Chemical Society

Many studies have focused on the development of synthesis methods3,9−11 and composites5,6,8,20 to improve the performance of MgO compounds. However, there are no studies on detailed calcination methods even though most of them require a calcination step. The calcination procedure and temperature are important factors in the synthesis of MgO compounds because textural properties and magnesium hydroxide decomposition can be significantly affected during calcination. In this study, the effects of the calcination procedure (final temperature, heating rate, and steps) on the physical properties and sorption capacity of MgO were thoroughly investigated. Mg(OH)2 was synthesized by a series of aerogel procedures, and various calcination methods were designed to enhance the physical properties of MgO particles, such as the surface area, pore volume, and crystal size. The sorption capacities of MgO compounds prepared by various calcination methods were evaluated by breakthrough experiments to remove methylmercaptan from methane as a model fuel mixture. The optimized calcination procedure for aerogel MgO was suggested in terms of the pore structural properties and sorption capacity.

2. EXPERIMENTAL SECTION 2.1. Preparation of Hydrated MgO (HY-MgO). Magnesium hydroxide or HY-MgO was prepared by an aerogel procedure. A total of 16 g of Mg(CH3O)2 (Aldrich, USA) and 80.0 g of toluene (Aldrich, USA, 99.9%) were put into a glass reactor and stirred for 30 min. Triply distilled water (1.5 mL) was added dropwise to the stirring solution in four aliquots (0.5, 0.5, 0.4, and 0.1 mL, respectively) using a syringe. A cloudy white precipitate was observed with the addition of each Received: May 6, 2014 Revised: July 24, 2014 Accepted: August 6, 2014

A

dx.doi.org/10.1021/ie5018546 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

Table 1. Calcination Procedure for HY-MgO and the Corresponding Physical Properties of MgO Samples

a

sample no.

Tfinal (°C)

heating rate (°C/min)

1 2 3 4 5 6

400 450 500 550 500 500

1 1 1 1 0.8 1

7

450

0.8

calcination procedure (temperature, °C; soak time, h) 25 → 220: 5 h → Tfinal: 5 h (two-step)

25 → 220: 5 h → 500: 5 h (two-step) 25 → 220: 5 h → 280: 1 h → 350: 2 h → 500: 5 h (four-step) 25 → 220: 5 h → 280: 1 h → 350: 2 h → 450: 5 h (four-step)

surface area (m2/g)

pore volumea (cm3/g)

mean mesopore diametera (nm)

491.1 439.7 296.6 201.2 411.5 362.1

1.35 1.17 0.70 0.61 1.78 1.00

5.2 5.0 4.9 4.7 8.8 4.9

503.5

1.31

4.7

BJH adsorption.

was evaluated from the desorption branch of the N2 isotherm using the Barrett−Joyner−Halenda (BJH) model. 2.4. Breakthrough Experiments for Methylmercaptan Removal by MgO. Figure 1 shows a schematic diagram of the

water drop. To minimize evaporation of the organic solvent, the top of the reactor was covered with aluminum foil. Also, the mixture was stirred for about 12 h at room temperature for full hydrolysis. The hydrolyzed gel solution was put into a 300 mL autoclave, which was flushed with N2 gas for about 5 min to remove air from the head space. The reactor was pressurized to 100 psi with N2 gas and gradually heated to 265 °C at 1 °C/ min by a proportional−integral−derivative controller (about 4 h). The reactor temperature was held at 265 °C for 10 min and flushed with N2 again after the pressure was released. After the reactor reached room temperature, the product powder was placed in a dry oven at 120 °C for 12 h to remove the residual organic solvents. HY-MgO was obtained as a white powder. 2.2. Calcination Procedure. One of the important factors in synthesizing nanomaterials is the calcination procedure. As a reference calcination method, the following two-step calcination procedure was applied to HY-MgO:24 (1) a ramp from room temperature to 220 °C at 1 °C/min and a soak at 220 °C for 5 h; (2) a ramp from 220 to 500 °C at 1 °C/min and a soak at 500 °C for 5 h. MgO prepared by this procedure is called sample no. 3 in Table 1 and used as a reference. Various calcination procedures were developed to improve the physical properties of MgO, as assessed by thermogravimetric analysis (TGA) of Mg(OH)2 in a N2 flow from 25 to 500 °C. Another critical factor in nanomaterial synthesis is the final calcination temperature. If the temperature is too low, thermal decomposition of magnesium hydroxide does not complete. On the other hand, if the temperature is too high, the nanobrucite structure might suddenly collapse. Therefore, we tested various temperatures (400, 450, 500, and 550 °C) to find the optimum final temperature for calcination. In addition, to evaluate the effect of the heating rate on the morphological features of the final MgO crystals, two different heating rates (0.8 and 1.0 °C/ min) were applied for calcination. The detailed calcination methods with sample numbers are listed in Table 1. 2.3. Characterization. Powder X-ray diffraction (XRD) patterns for MgO were recorded by a Rigaku (D/Max-2000H) X-ray diffractometer with a copper target using Cu Kα radiation (λ = 0.154 nm) at 40 kV and 100 mA. The diffractograms were taken with a step size of 0.02°. The morphology and size of the prepared materials were assessed by transmission electron microscopy (TEM; JEM-2010). The thermal behavior of Mg(OH)2 powder was analyzed by TGA in a N2 flow from 25 to 500 °C with a heating rate of 10 °C/min. N2 adsorption− desorption isotherms were measured at 77 K on a Quantachrome ASiQwin system. The specific surface area (SBET) was determined from the linear part of the Brunauer− Emmett−Teller (BET) equation, and the pore-size distribution

Figure 1. Schematic diagram of the breakthrough experimental apparatus for methylmercaptan sorption from methane.

dynamic breakthrough system. The experiments were performed to evaluate the sorption capacity of MgO. Methylmercaptan (291 μmol/mol) in methane was used as an adsorbate to simulate sulfur compounds in municipal gas. MgO powder (0.10 g) was packed into a column (inner diameter, 7 mm; length, 110 mm). The column was activated at 150 °C with a pure methane flow of 5 mL/min for 2 h and purged with pure methane at 15 mL/min and 15 °C for 1 h. Then, the mixture of methylmercaptan (291 μmol/mol) and methane was fed into the column at 15 mL/min and 15 °C. The column pressure during breakthrough experiments was measured by an electrical pressure gauge. Both ends of the column were packed with glass wool and glass beads, and a metal sieve was placed between the adsorbents and glass beads. When sorbent particles were tested in a breakthrough experiment, the difference in the pressure drop and bed porosity through the column resulted in experimental deviation. In this study, an equal amount of MgO particles was packed into the column for each experiment. The packing length occupied by the sorbent particles was also equal in every experiment. A slow flow rate was used to minimize the change in the packing density (length). As a result, the pressure drop was about 0.2 bar at 15 mL/min and 15 °C because of the dense packing of the sorbent particles. To compare the sorption capacity among the as-prepared MgO compounds, breakthrough experiments were performed in fixed conditions (15 mL/min and 15 °C). Therefore, a pressure drop of about B



Article

too high temperature may collapse, significantly reducing the pore volume and surface area. Therefore, to preserve the physical properties and sorption capacity, the calcination step and temperature for HY-MgO had to be carefully controlled. The thermal behavior of the Mg(OH)2 sample in a N2 flow from 25 to 500 °C provided the necessary information to develop the calcination procedure. TGA and differential scanning calorimetry (DSC) of HYMgO in a N2 flow were performed from 25 to 500 °C, and the curves are presented in Figure 3. The weight decreased slightly

0.2 bar was observed in all packed columns. Breakthrough saturation took a long time in every experiment because of the dense packing, slow flow rate, and low methylmercaptan concentration. The gas flow was controlled by two mass-flow controllers for methane and the methylmercaptan/methane mixture. The column temperature was controlled by a water circulator and measured by a thermocouple (RTD, Pt 100 Ω) inserted in the column. The concentration of the sulfur compound in the effluent gas from the column packed with MgO was measured by an online gas chromatograph (Agilent 6890N) with a flame photometric detector. The sorption (mmol/g) of methylmercaptan was calculated by the following equation: sorption (mmol/g) =

F (mL/min) × t * (min) × C (μmol/mol) 22.4 (L/mol) × m (g) × 106

(1)

where t* is the time for 50% sorbate breakthrough, F is the flow rate of a mixture gas (15 mL/min), C is the methylmercaptan concentration (291 μmol/mol), and m is the sorbent mass (0.1 g).

3. RESULTS AND DISCUSSION 3.1. Characterization of HY-MgO. Figure 2 illustrates the N2 adsorption−desorption isotherm and the pore-size disFigure 3. TGA/DSC curves of HY-MgO.

in the temperature range of 25−220 °C in the TGA curve probably because of the evaporation of physically sorbed water or toluene residue after drying. Also, the corresponding endothermic peaks in the DSC curve were observed at 50, 90, and 140 °C. Weight loss was quite notable from 300 to 400 °C with a sharp endothermic curve at 350 °C because of decomposition of HY-MgO by the following equation: Mg(OH)2 → MgO + H 2O

(2)

Weight loss from 280 to 450 °C was 30.1%, which was in good agreement with the theoretical value of 31% for reaction (2). However, over the entire heat treatment range (50−500 °C), weight loss was 39.7% because of not only the desorption of physically sorbed water and toluene residue but also the loss of some −OCH3 groups remaining on the powder surface after hydrolysis and drying.26 After desorption of the physically sorbed molecules, weight loss started at about 280 °C and sharp thermal cracking was observed from 300 to 400 °C, with the highest slope of weight loss at around 350 °C. Therefore, it is desirable that calcination at 350 °C be carefully controlled. 3.2. Effect of Calcination Variables on the Sorption Capacity. 3.2.1. Effect of the Final Calcination Temperature. Different final calcination temperatures were applied to Mg(OH)2 from the same preparation, described in section 2.1. As shown in Table 1, the calcination procedure was (1) a ramp from room temperature to 220 °C at 1 °C/min and a soak at 220 °C for 5 h, and (2) a ramp from 220 °C to Tfinal at 1 °C/min and a soak at Tfinal for 5 h (400, 450, 500, and 550 °C corresponding to sample nos. 1−4 in Table 1, respectively). Sample no. 3 was used as a reference material, as discussed in section 2.2.

Figure 2. N2 adsorption−desorption isotherm of HY-MgO (inset: pore-size distribution).

tribution of HY-MgO [Mg(OH)2]. According to IUPAC classification, its isotherm curve was type IV with a H3 hysteresis loop. This result implied that HY-MgO was a nonrigid aggregate of platelike particles, giving rise to slit-shape pores. The isotherm curve also contained a steep region associated with closure of the hysteresis loop at a relative pressure of 0.4−0.5. The hysteresis loop developed to a relative pressure (P/P0) of almost 1. The hysteresis loop clearly revealed that HY-MgO was a mesoporous material with a high BET surface area (1025.6 m2/g) and a high BJH pore volume (3.05 cm3/g) before calcination. The pores were mainly 4−6 nm, which was obtained from the desorption branch of the N2 isotherm by the BJH method. Treating HY-MgO with high temperature can create nanocrystalline MgO particles. However, the morphology at C



Article

which differed from the others. The results indicate that the breakthrough characteristics of MgO at these two temperatures were not exactly proportional to the decrease in the surface area and pore volume. However, high calcination temperature, such as 500 or 550 °C, may improve the sorption affinity of methylmercaptan because the shape of the breakthrough curve became steeper.27,28 Therefore, considering a sudden decrease of the surface area and pore volume in Table 1, the nanobrucite structure of MgO might suddenly collapse at the high temperature range. To optimize the breakthrough time and sorption capacity, a final calcination temperature of 450 °C was suggested. 3.2.2. Effect of the Calcination Steps. As mentioned in Figure 3, the TGA curve of HY-MgO indicated two important temperatures, 280 and 350 °C, which correspond to the beginning of decomposition and of significant weight loss, respectively. To stabilize the MgO structure, calcination steps at each of these temperatures (sample no. 6) were added to the procedure used for sample no. 3. As shown in Table 1, the two steps, soaking for 1 h at 280 °C and soaking for 2 h at 350 °C, were added to the previous two-step calcination between 220 and 500 °C at 1 °C/min (resulting in a four-step calcination). The four-step calcination (no. 6) increased the surface area (362.1 m3/g) and pore volume (1.0 cm3/g) compared with the two-step calcination (no. 3, 296.6 m2/g and 0.7 cm3/g in Table 1). As shown in Figure 5 and Table 2, the sorption performance

The breakthrough curve and sorption capacity of methylmercaptan on as-prepared MgO compounds are shown in Figure 4 and Table 2. The breakthrough and saturation times

Figure 4. Breakthrough curves of methylmercaptan on MgO with twostep calcination (Table 1). Final calcination temperature: 400 °C (no. 1), 450 °C (no. 2), 500 °C (no. 3), and 550 °C (no. 4).

Table 2. Breakthrough Times and Sorption Capacities of MgO Compounds Prepared by Different Calcination Procedures sample no. 1 (twostep) 2 (twostep) 3 (twostep) 4 (twostep) 5 (twostep) 6 (fourstep) 7 (fourstep)

breakthrough time (min)

saturation time (min)

sorption capacity (mmol/g)

130

350

0.53

200

500

0.79

100

260

0.37

50

250

0.14

130

350

0.50

150

300

0.49

480

900

1.50

clearly increased, showing the same shape as that of the breakthrough curve, as the final calcination temperature increased from 400 to 450 °C. Also, the sorption capacity naturally increased from 0.53 to 0.79 mmol/g. The surface area and pore volume became smaller as the final calcination temperature increased (no. 1, 491.1 m2/g at 400 °C; no. 2, 439.7 m2/g at 450 °C, as shown in Table 1). The results imply that variation in the breakthrough time and sorption capacity was opposite to the change of the surface area in this temperature range. The surface area and pore volume of MgO compounds decreased steeply as the final calcination temperature increased from 450 °C (no. 3, 500 °C; no. 4, 550 °C, in Table 1). Compared with the results at 450 °C, the breakthrough time and sorption capacity decreased significantly and the breakthrough curve became steeper as the calcination temperature increased. In addition, the decrease in the breakthrough time of MgO calcined at 500 °C (no. 3) was relatively small, considering the decreased surface area. The breakthrough curve of MgO at 550 °C (no. 4) tailed off after increasing,

Figure 5. Breakthrough curves of methylmercaptan on MgO with twostep (no. 3) and four-step (no. 6) calcination procedures.

parameters of MgO prepared by a four-step calcination (no. 6), such as the breakthrough time, saturation time, and sorption capacity, were higher than those prepared by a two-step calcination (no. 3). The shape of the breakthrough curve for sample no. 6 was similar to that for sample no. 3 despite the additional calcination steps. In addition, the improvement in the sorption performance was approximately proportional to the increase in the surface area, as shown in Tables 1 and 2. These results imply that the final calcination temperature affected the sorption affinity, capacity, and structure of MgO. The additional calcination steps, with the same final calcination temperature and heating rate, may not contribute to the sorption affinity but do so to the sorption capacity and structure. However, as shown in Figure 4 and Tables 1 and 2, the sorption capacity and surface area of sample no. 6 were D



Article

ramp from 350 to 450 °C at 0.8 °C/min and a soak at 450 °C for 5 h. The surface area of sample no. 7 was the highest of all MgO compounds in the study, as shown in Table 1. The surface area and pore volume of sample no. 7 were higher than those of sample no. 6, which was prepared at a four-step calcination with a different final calcination temperature and heating rate. In addition, the surface area and pore volume of sample no. 7 were higher than those of sample no. 2, which was calcined by the two-step method and the same final calcination temperature. Figure 7 shows the breakthrough curves of the optimized MgO (no. 7), sample no. 3 (two-step calcination at 500 °C)

lower than those of MgO compounds calcined at lower final temperatures (samples nos. 1 and 2) regardless of the calcination steps. 3.2.3. Effect of the Heating Rate. The heating rate is an important factor that can affect a material’s structure. In this study, a heating rate of 0.8 °C/min from 220 to 500 °C was applied to sample no. 5 with the two-step calcination procedure in Table 1. The result was compared with that of the reference sample (no. 3; heating rate of 1 °C/min). MgO calcined at a lower heating rate (no. 5) had much higher surface area (411.5 m2/g) than did sample no. 3. Moreover, the pore volume (1.78 cm3/g) and mesopore diameter (8.8 nm) of sample no. 5 were much better than those of sample no. 3, as shown in Table 1. The surface area of sample no. 5 almost approached that of MgO prepared at 400 °C (no.1), but the pore volume was highest in the study. In addition, MgO calcined at a lower heating rate (no. 5) had a longer breakthrough time and higher sorption capacity than MgO calcined at a higher heating rate (no. 3), as shown in Figure 6.

Figure 7. Breakthrough curves of methylmercaptan on MgO (no. 3, synthesized as a reference; two-step calcination at 500 °C), MgO (no. 2; two-step calcination at 450 °C), and optimized MgO (no. 7).

and sample no. 2 (two-step calcination at 450 °C). The breakthrough time (480 min) and saturation time (900 min) of sample no. 7 were much longer than those of sample nos. 2 and 3. As a result, the sorption capacity of sample no. 7 (1.50 mmol/g) was almost 2 and 4 times higher than those of sample nos. 2 and 3, respectively. The shape of the breakthrough curve for sample no. 7 was similar to that of no. 2, which was calcined at the same final temperature. Therefore, the shape of the breakthrough curve was confirmed as strongly dependent on the final calcination temperature. In addition, even though the surface area and pore volume of sample no 0.7 were comparable with those of sample no. 1, the sorption performance of sample no. 7 was much higher than that of any other sample (Table 1). This result indicates that the performance of MgO prepared even by the same synthesis method cannot be simply evaluated by the surface area and that the calcination method can significantly affect the structure, sorption capacity, and affinity. 3.3. Physical Characterization. TEM images of HY-MgO and optimized MgO are presented in Figure 8, which were magnified at 50K, 150K, and 400K for parts a−c of Figure 8, respectively. HY-MgO had a thin hexagonal plate shape, and its size ranged from 50 to 150 nm (Figure 8a). TEM images of optimized MgO (Figure 8b,c) showed the MgO bulk aggregated with nanoparticles of smaller than 5 nm, which supported the XRD and BET crystal size results. The aggregated MgO bulk was 100−200 nm in size, which agreed with the size of the HY-MgO hexagonal plates. Figure 9 shows the XRD patterns of two MgO samples (nos. 7 and 3). Both samples had three marked diffraction peaks at

Figure 6. Breakthrough curves of methylmercaptan on MgO with heating rates of 1.0 °C/min (no. 3) and 0.8 °C/min (no. 5).

Sample no. 5 prepared by a two-step calcination had a similarly shaped breakthrough curve as sample no. 1 (two-step calcination at 400 °C), but the sorption capacity was smaller, as shown in Table 2. Moreover, even though sample no. 5 had a higher surface area than sample no. 6, which had a four-step calcination and the same final temperature, the sorption capacity was almost the same as that of sample no. 6. Therefore, the heating rate in the temperature range of significant MgO decomposition (Figure 3) was critical to formation of the physical structure as well as the sorption affinity of MgO, even with the same final temperature. 3.2.4. Optimum Calcination Procedure. According to the above results, the following four-step calcination method with a final calcination temperature of 450 °C and a heating rate of 0.8 °C/min in the significant decomposition range was the optimized calcination to transform HY-MgO into MgO (no. 7 in Table 1). Thus, the optimized calcination procedure was (1) a ramp from room temperature to 220 °C at 1 °C/min and a soak at 220 °C for 5 h, (2) a ramp from 220 to 280 °C at 0.8 °C/min and a soak at 280 °C for 1 h, (3) a ramp from 280 to 350 °C at 0.8 °C/min and a soak at 350 °C for 2 h, and (4) a E



Article

Figure 8. TEM images of (a) Mg(OH)2 and (b and c) optimized MgO (no. 7): The scale bars are (b) 50 and (c) 10 nm.

The sizes of the MgO crystals (D, nm) were calculated from the (200), (220), and (222) diffraction peaks using the Scherrer equation31

DXRD =

Kλ B cos θ

(3)

where K is a dimensionless constant (0.94 for a cubic crystal MgO), the θ and B variables are the angles between the incident and diffracted beams and the line broadening at halfmaximum (rad), respectively. The optimized MgO crystal (no. 7) was smaller than the reference MgO crystal (no. 3), as shown in Table 3. This difference may stem from the sintering Table 3. Crystallite Size and Surface Area of Optimized MgO (No. 7; Four-Step Calcination at 450 °C) and Reference MgO (No. 3; Two-Step Calcination at 500 °C) crystallite size (nm)a

Figure 9. XRD patterns of (a) optimized MgO (no. 7) and (b) MgO (no. 3, reference).

MgO sample no.

(200)

(220)

(222)

SBET (m2/g)

particle size (nm)b

ψ

7 3

4.4 4.6

5.0 5.1

7.0 7.5

503.5 296.6

3.3 5.7

0.22 0.97

a

Determined by the XRD pattern. bDetermined by the BET surface area.

(200), (220), and (222) diffraction maxima. The diffraction peaks had a standard pattern of the pure cubic phase of MgO with a crystalline lattice parameter of a = 4.22 Å, which agrees with the JCPDS data (75-0447).29 These results indicate that both the reference MgO sample (no. 3) and the optimized MgO sample (no. 7) had a single-phase cubic structure. However, the optimized MgO (four-step calcination at 450 °C) had slightly higher peak intensities than did the reference MgO (two-step calcination at 500 °C). This result implies that the optimized MgO (no. 7) is a better cubic-phase crystal.10,30

of MgO nanoparticles at higher calcination temperature and heating rate for the reference MgO (500 °C; 1 °C/min) than for the optimized MgO (450 °C; 0.8 °C/min). The N2 adsorption−desorption isotherms of the optimized MgO (no. 7) and reference MgO (no. 3) are illustrated in Figure 10. According to the IUPAC classification, both samples were mesoporous materials with type IV isotherms. Both the optimized MgO (no. 7) and reference MgO (no. 3) had a F



Article

MgO (no. 7) was smaller than that of the reference MgO (no. 3, 5.7 nm). In addition, ψ = (DBET/DXRD)3, which reflects the partial sintering of the primary crystallites.32 The results clearly showed that the optimized MgO (no. 7) had less sintering than did the reference MgO (no. 3) in Table 3. In summary, the four-step calcination with a heating rate of 0.8 °C/min and a final calcination temperature of 450 °C contributed to the formation of regular pores and the improvement of the sorption capacity. In addition, as shown in Table 1, a high calcination temperature and a high heating rate in the decomposition range can cause significant collapse of the pore structure, thus reducing the pore volume and surface area. The decreased pore volume and surface area may reduce the sorption properties.

4. CONCLUSIONS Mesoporous MgO sorbent with high performance was synthesized by controlling the calcination procedure. A comparison was made on the sorption performance of methylmercaptan (291 μmol/mol) in methane on the MgO compounds prepared at various calcination methods. Increasing the final temperature led to decreasing surface area but improving sorption affinity. The sorption capacity increased with an increase in the final calcination temperature up to 450 °C but then began to decrease with a further increase in the final temperature. Thus, the performance of MgO cannot be simply evaluated by its surface area and pore volume. However, at the same final calcination temperature, the differences in the shape of the breakthrough curve among MgO compounds were relatively small, regardless of the heating rate and calcination steps. Controlling the heating rate and two steps in the temperature range of significant decomposition improved the physical properties, such as the surface area, pore volume, and crystal size as well as the sorption capacity of methylmercaptan. Also, the sorption capacity (1.50 mmol/g) of methylmercaptan on the optimized MgO calcined by a four-step procedure with a final temperature of 450 °C and a heating rate of 0.8 °C/min was 2−11 times higher than that of MgO compounds calcined by other methods. For improvement of the efficiency and applicability, porous supporters and/or promoters have been applied to MgO. Controlling the calcination step can contribute to improving the textual properties and sorption affinity of MgO. Therefore, further studies are needed to verify whether the calcination method can also have significant effects on MgO-based composites.

Figure 10. N2 adsorption−desorption isotherms of MgO (no. 3, reference) and optimized MgO (no. 7).

typical H3 hysteresis loop, which was the same as that of HYMgO with nonrigid aggregates of platelike particles forming slit-shape pores (Figure 2). However, for both MgO compounds, the difference between the adsorption and desorption isotherms was much smaller than that for HYMgO. The relative pressure range of the hysteresis loop for sample no. 7 was similar to that for HY-MgO. The hysteresis loop of sample no. 3 started at a lower relative pressure but almost disappeared at a high relative pressure. Figure 11 shows the pore-size distribution of sample nos. 7 and 3. Most pores in both samples are between 3 and 7 nm, but

■

Figure 11. Pore-size distributions of MgO (no. 3, reference) and optimized MgO (no. 7).

*Tel: + 82-02-2123-2762. Fax: + 82-02-312-6401. E-mail: [email protected]. Notes

the pore-size distribution curve of sample no. 7 had a higher peak than that of sample no. 3. The theoretical particle size was also calculated from the BET surface area, assuming a spherical particle, from the following equation: DBET =

6000 ρS

AUTHOR INFORMATION

Corresponding Author

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was financially supported by KCRC/Ministry of Education, Science and Technology, South Korea.

(4)

where DBET is the equivalent particle diameter (nm), S is the specific surface area (m2/g), and ρ is the density (MgO, 3.58 g/ cm3).32 In Table 3, the particle size (3.3 nm) of the optimized

REFERENCES

(1) Rajagopalan, S.; Koper, O.; Decker, S.; Klabunde, K. J. Nanocrystalline metal oxides as destructive adsorbents for organo-

G



Article

phosphorus compounds at ambient temperatures. Chem.Eur. J. 2002, 8, 2602. (2) Choudary, B. M.; Mulukutla, R. S.; Klabunde, K. J. Benzylation of aromatic compounds with different crystallites of MgO. J. Am. Chem. Soc. 2003, 125, 2020. (3) Ding, Y.; Zhang, G.; Wu, H.; Hai, B.; Wang, L.; Qian, Y. Nanoscale magnesium hydroxide and magnesium oxide powders: Control over size, shape, and structure via hydrothermal synthesis. Chem. Mater. 2001, 13, 435. (4) Makhluf, S.; Dror, R.; Nitzan, Y.; Abramovich, Y.; Jelinek, R.; Gedanken, A. Microwave-assisted synthesis of nanocrystalline MgO and its use as a bacteriocide. Adv. Funct. Mater. 2005, 15, 1708. (5) An, H.; Gao, Z.; Zhao, X.; Wang, Y. MgO-PbO catalyzed synthesis of diethylene glycol bis(allyl carbonate) by transesterification route. Ind. Eng. Chem. Res. 2011, 50, 7740. (6) Choudhary, V. R.; Mulla, S. A. R.; Uphade, B. S. Oxidative coupling of methane over supported La2O3 and La-promoted MgO catalysts: Influence of catalyst−support interactions. Ind. Eng. Chem. Res. 1997, 36, 2096. (7) Li, W.-C.; Lu, A.-H.; Weidenthaler, C.; Schüth, F. Hardtemplating pathway to create mesoporous magnesium oxide. Chem. Mater. 2004, 16, 5676. (8) Paul, M.; Pal, N.; Mondal, J.; Sasidharan, M.; Bhaumik, A. New mesoporous magnesium−aluminum mixed oxide and its catalytic activity in liquid phase Baeyer−Villiger oxidation reaction. Chem. Eng. Sci. 2012, 71, 564. (9) Li, X.; Xiao, W.; He, G.; Zheng, W.; Yu, N.; Tan, M. Pore size and surface area control of MgO nanostructures using a surfactanttemplated hydrothermal process: High adsorption capability to azo dyes. Colloids Surf., A 2012, 408, 79. (10) Zhou, J.; Yang, S.; Yu, J. Facile fabrication of mesoporous MgO microspheres and their enhanced adsorption performance for phosphate from aqueous solutions. Colloids Surf., A 2011, 379, 102. (11) Bhagiyalakshmi, M.; Lee, J. Y.; Jang, H. T. Synthesis of mesoporous magnesium oxide: Its application to CO2 chemisorption. Int. J. Greenhouse Gas Control 2010, 4, 51. (12) de Angelis, A. Natural gas removal of hydrogen sulphide and mercaptans. Appl. Catal., B 2012, 113−114, 37. (13) Ryzhikov, A.; Hulea, V.; Tichit, D.; Leroi, C.; Anglerot, D.; Coq, B.; Trens, P. Methyl mercaptan and carbonyl sulfide traces removal through adsorption and catalysis on zeolites and layered double hydroxides. Appl. Catal., A 2011, 397, 218. (14) Kim, J. H.; Ma, X.; Zhou, A.; Song, C. Ultra-deep desulfurization and denitrogenation of diesel fuel by selective adsorption over three different adsorbents: A study on adsorptive selectivity and mechanism. Catal. Today 2006, 111, 74. (15) Song, C.; Ma, X. New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Appl. Catal., B 2003, 41, 207. (16) Koriakin, A.; Ponvel, K. M.; Lee, C.-H. Denitrogenation of raw diesel fuel by lithium-modified mesoporous silica. Chem. Eng. J. 2010, 162, 649. (17) Kwon, J.-M.; Moon, J.-H.; Bae, Y.-S.; Lee, D.-G.; Sohn, H.-C.; Lee, C.-H. Adsorptive desulfurization and denitrogenation of refinery fuels using mesoporous silica adsorbents. ChemSusChem 2008, 1, 307. (18) Slack, A. V.; Hollinden, G. A. Sulfur dioxide removal from waste gases, 2nd ed.; Noyes Data Corp.: Park Ridge, NJ, 1975. (19) Klabunde, K. J.; Stark, J.; Koper, O.; Mohs, C.; Park, D. G.; Decker, S.; Jiang, Y.; Lagadic, I.; Zhang, D. Nanocrystals as stoichiometric reagents with unique surface chemistry. J. Phys. Chem. 1996, 100, 12142. (20) Xu, B.-Q.; Wei, J.-M.; Wang, H.-Y.; Sun, K.-Q.; Zhu, Q.-M. Nano-MgO: novel preparation and application as support of Ni catalyst for CO2 reforming of methane. Catal. Today 2001, 68, 217. (21) El-Shall, M. S.; Slack, W.; Vann, W.; Kane, D.; Hanley, D. Synthesis of nanoscale metal oxide particles using laser vaporization/ condensation in a diffusion cloud chamber. J. Phys. Chem. 1994, 98, 3067.

(22) Matthews, J. S.; Just, O.; Obi-Johnson, B.; Rees, W. S. CVD of MgO from a Mg(β-ketoiminate)2: Preparation, characterization, and utilization of an intramolecularly stabilized, highly volatile, thermally robust precursor. Chem. Vap. Deposition 2000, 6, 129. (23) Helble, J. J. Combustion aerosol synthesis of nanoscale ceramic powders. J. Aerosol Sci. 1998, 29, 721. (24) Utamapanya, S.; Klabunde, K. J.; Schlup, J. R. Nanoscale metal oxide particles/clusters as chemical reagents. Synthesis and properties of ultrahigh surface area magnesium hydroxide and magnesium oxide. Chem. Mater. 1991, 3, 175. (25) Kim, Y.-H.; Tuan, V. A.; Park, M.-K.; Lee, C.-H. Sulfur removal from municipal gas using magnesium oxides and a magnesium oxide/ silicon dioxide composite. Microporous Mesoporous Mater. 2014, 197, 299. (26) Diao, Y.; Walawender, W. P.; Sorensen, C. M.; Klabunde, K. J.; Ricker, T. Hydrolysis of magnesium methoxide. Effects of toluene on gel structure and gel chemistry. Chem. Mater. 2002, 14, 362. (27) Lim, S.-H.; Woo, E.-J.; Lee, H.; Lee, C.-H. Synthesis of magnetite−mesoporous silica composites as adsorbents for desulfurization from natural gas. Appl. Catal., B 2008, 85, 71. (28) Koriakin, A.; Kim, Y.-H.; Lee, C.-H. Adsorptive desulfurization of natural gas using lithium-modified mesoporous silica. Ind. Eng. Chem. Res. 2012, 51, 14489. (29) Reddy, M. B. M.; Ashoka, S.; Chandrappa, G. T.; Pasha, M. A. Nano-MgO: An efficient catalyst for the synthesis of formamides from amines and formic acid under MWI. Catal. Lett. 2010, 138, 82. (30) Inoue, M.; Hirasawa, I. The relationship between crystal morphology and XRD peak intensity on CaSO4·2H2O. J. Cryst. Growth 2013, 380, 169. (31) Rakmak, N.; Wiyaratn, W.; Bunyakan, C.; Chungsiriporn, J. Synthesis of Fe/MgO nano-crystal catalysts by sol−gel method for hydrogen sulfide removal. Chem. Eng. J. 2010, 162, 84. (32) Rezaei, M.; Khajenoori, M.; Nematollahi, B. Synthesis of high surface area nanocrystalline MgO by pluronic P123 triblock copolymer surfactant. Powder Technol. 2011, 205, 112.

H


Controlling the Physical Properties of Magnesium Oxide Using a

Recommend Documents