Removal of gaseous sulfur and phosphorus compounds by carbon-coated porous magnesium oxide composites

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Chemical Engineering Journal 283 (2016) 1234–1243 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Removal of gaseous sulfur and phosphorus compounds by carbon-coated porous magnesium oxide composites Anh-Tuan Vu a,b, Keon Ho a, Chang-Ha Lee a,⇑ a b Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, Republic of Korea School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam h i g h l i g h t s g r a p h i c a l a b s t r a c t  Carbon-coated MgO composites were synthesized using an aerogel method  MgO/C composites had a high surface area of 723 m2/g  The sorption capacity of DMMP and 2-CEES was higher than those of MgO and AC  The composite sorbed DMMP almost twice more than 2-CEES in dry condition  Carbon layer on MgO protected active catalytic and sorption sites from H2O molecules a r t i c l e i n f o Article history: Received June 2015 Received in revised form 31 July 2015 Accepted August 2015 Available online 28 August 2015 Keywords: MgO composite Carbon Aerogel Sorption 2-CEES DMMP a b s t r a c t Carbon-coated porous magnesium oxide (MgO/C) composites were synthesized using an aerogel route for removal of dimethyl methylphosphonate (DMMP) and 2-chloroethyl ethyl sulfide (2-CEES) in dry and wet conditions The sorption capacities of the as-prepared samples for DMMP (0.23 lg/mL) and 2-CEES (0.26 lg/mL) were evaluated by breakthrough experiments in nitrogen under ambient conditions MgO/C composites exhibited a decrease in surface area with carbon content (648–723 m2/g), but had a higher surface area than MgO Under dry conditions, the sorption capacities of the MgO/C composite with a low carbon content of 6.39 wt% (MgO/C-1; 67.8 mg/g for DMMP and 35.3 mg/g for 2-CEES) were higher than those of pure MgO and activated carbon (AC) The sorption capacity of MgO/C composites decreased with an increase in carbon content and became even lower than those of MgO and AC Under humid conditions, the sorption capacities and breakthrough time of pure MgO decreased significantly and became lower than that of AC In contrast, the sorption capacities of the MgO/C-1 composite for DMMP and 2-CEES under humid conditions remained at about 91 and 86% of those measured under dry conditions, and were higher than those of AC In addition, the MgO/C composite remained reactive toward 2-CEES even under humid conditions MgO/C composites were more stable than MgO under humid conditions because of the presence of carbon-coated shells Ó 2015 Elsevier B.V All rights reserved Introduction Removal of hazardous chemicals from the environment is a critical issue from both biological and environmental standpoints ⇑ Corresponding author Tel.: +82 02 2123 2762; fax: +82 02 312 6401 E-mail address: leech@yonsei.ac.kr (C.-H Lee) http://dx.doi.org/10.1016/j.cej.2015.08.083 1385-8947/Ó 2015 Elsevier B.V All rights reserved [1,2] Environmental regulations for emissions from industries and acceptable levels of human exposure are continuously being adjusted and made more stringent The most abundant hazardous components can be classified into two categories based on their source: natural and anthropogenic hazardous materials Anthropogenic pollutants generally originate from combustion, chemical reactions, or from the A.-T Vu et al / Chemical Engineering Journal 283 (2016) 1234–1243 unsecured effluent of toxic materials Furthermore, a considerable amount of anthropogenic chemicals containing sulfur and phosphorus such as mustard, sarin, and soman agents are produced for military purposes [3] These anthropogenic chemicals are highly persistent in the environment and critically harm humans even at low concentrations Many efforts have been made both to reduce pollution and to eliminate toxic materials from the environment Adsorptive removal of toxic components from the atmosphere is one of the most attractive technologies [4] Among various candidate solid materials, metal oxides have been reported to be effective materials for adsorption and decomposition of toxic and persistent chemicals due to their high surface areas, large number of highly reactive edges, corner defect sites, unusual lattice planes, high surface-to-volume ratio, and reusability [5–11] Particularly, magnesium oxide (MgO) materials have been the focus of attention for decontamination of toxic chemicals in recent years because well-designed MgO materials have a high sorption capacity as well as effective decomposition ability Many studies have focused on methods to synthesize MgOs with high surface area, pore volume, and small crystal size to improve sorption capacity and reactivity for toxic chemicals [12–15] However, sorption capacity and reactivity of MgO are significantly reduced in the presence of water, because of the strong affinity of MgO for water [6] Furthermore, the excellent removal performance of hazardous chemicals on adsorbents under dry conditions does not guarantee effectiveness in practice, because these chemicals exist in the environment with a certain level of humidity Even though many solid materials have been reported to have higher efficiency under dry conditions than activated carbon, their performance is often equivalent or worse than that of activated carbon under humid conditions Therefore, selection of solid materials for the removal of hazardous chemicals under humid conditions has been limited to strong hydrophilic adsorbents It would be highly desirable to design MgO-based composites with hydrophobic surfaces that show high removal capacity for hazardous chemicals under humid conditions Carbon-coated metal oxides have been produced using various synthesis methods for various applications A carbon-coated metal oxide was developed to improve the stability, electric conductivity, and electrochemical performance of batteries [16] Coating of a metal oxide with hydrophobic carbon could minimize the problem of water adsorption because carbon has a non-polar surface [17] In addition, various carbon-coated metal oxides have been developed, such as carbon-coated ZnO and CaO prepared by poly vinyl alcohol pyrolysis [18], ferrite nanoparticles coated with carbon prepared using a hydrothermal method [19], and carbon-coated Ni/TiO2 prepared via a hydrothermal method [20] To create MgO/carbon composites, various synthesis methods have been suggested, including chemical vapor deposition (CVD), pyrolysis of a magnesium hydroxide aerogel modified with resorcinol, precipitation with the assistance of sucrose, and one-pot assembly [17,21–23] A chemical weapon agent (CWA) is a chemical substance whose toxic properties are used to kill, injure, or incapacitate soldiers (and sometimes civilians) Therefore, the decontamination of chemical warfare agents is arguably the most challenging issue facing militaries around the world Furthermore, due to possible terror threats, protecting civilians from CWAs has become increasingly important for many governments Sulfur mustard (SM), commonly known as mustard gas, is a class of related cytotoxic and vesicant CWAs, which can cause large blistering on any exposed skin and in the lungs [24] There is no known antidote or specific treatment against SM exposure, and the current therapy is largely supportive Like some other nerve CWAs, sarin attacks the nervous system by interfering with the degradation of the neurotransmitter acetylcholine at 1235 neuromuscular junctions, and death will usually occur [25] In various studies, dimethyl methylphosphonate (DMMP; CH3PO(OCH3)2) is used as a simulant for toxic phosphorus compounds such as sarin [26] and 2-chloroethyl ethyl sulfide (2-CEES; CH3CH2SCH2CH2Cl) is often considered as a surrogate compound for SM [27,28] Therefore, development of effective materials for removing DMMP and 2-CEES could help protect the environment and humans from refractory hazardous chemicals as well as CWAs In this study, carbon-coated magnesium oxide (MgO/C) composites were synthesized via an aerogel route to remove efficiently CWAs As representative surrogates of CWA, dimethyl methylphosphonate and 2-chloroethyl ethyl sulfide were selected The sorption capacities of the MgO/C composites were measured by breakthrough experiments in gas phase under ambient dry and wet conditions (0.23 and 0.26 lg/mL DMMP and 2-CEES in N2 flow, respectively) The removal efficiency of the MgO/C composites were evaluated and compared with those of pure MgO and activated carbon (AC) Experimental section 2.1 Materials The following materials were used in this study: toluene (Aldrich, USA, 99.9%), a magnesium methoxide solution in methanol (Aldrich, 7.82%), 2-CEES (Aldrich, 98%), DMMP (Aldrich, 97%), glucose (Aldrich, 99%), activated carbon (Calgon Filtrasorb 300, coal-derived AC), N2 gas (Dae-Deok Gas, Korea, 99.999%), H2 (Dae-Deok Gas, 99.999%), and air (Dae-Deok Gas, 99.999%) All chemicals and solvents were used without further purification 2.2 Preparation of composites MgO/C composites were developed using an aerogel procedure In a typical experiment, which was conducted at room temperature, a mixture of toluene (100 mL) and magnesium methoxide solution (20 mL) was placed in a glass reactor with a stirrer Another solution was prepared by dissolving the desired amount of glucose in 2.0 mL of distilled water and this solution was slowly added using a syringe to prepare the mixture The addition of the glucose solution led to a white cloudy precipitate, but the solution became clear after a few minutes To minimize evaporation of organic solvent, the glass reactor top was covered with aluminum foil The mixture was stirred vigorously overnight at room temperature to allow completion of hydrolysis Subsequently, the hydrolysis gel was put into a high-pressure autoclave reactor The gel was first flushed and then pressurized up to 100 psi with N2 gas The autoclave reactor was gradually heated from room temperature to 265 °C at a rate of 1.0 °C/min and this temperature was remained for 10 Solvent vapors in the reactor were quickly vented to the atmosphere and flushed with N2 again to remove any remaining solvent vapors The obtained powder in the reactor was dried in an oven at 120 °C for 12 h to remove residual organic solvents Hydrated powder (before calcination) was obtained and denoted HY-MgO/C The final step was calcination Calcination has been shown to improve the textural properties and sorption capacity of MgO [12,29] In this study, the hydrated powder was calcined in a furnace under vacuum using the following steps to produce mesoporous MgO with a high surface area: (1) ramping from room temperature to 220 °C at °C/min and soaking at 220 °C for h, (2) ramping from 220 to 280 °C at 0.8 °C/min and soaking at 280 °C for h, (3) ramping from 280 to 350 °C at 0.8 °C/min and soaking at 350 °C for h, and (4) ramping from 350 to 500 °C at 0.8 °C/min 1236 A.-T Vu et al / Chemical Engineering Journal 283 (2016) 1234–1243 and soaking at 500 °C for h The magnesium oxide/carbon composite was obtained and stored in a glass bottle filled with N2 In this study, to investigate the effect of the amount of carbon in the MgO/C composites on their textural and physical properties as well as sorption capacities, three different magnesium oxide/ carbon composites were prepared: molar ratios of magnesium methoxide to glucose were 1:0.05, 1:0.09 and 1:0.18 The as-prepared composites were denoted MgO/C-1, MgO/C-2, and MgO/C-3, respectively Activated carbon (AC) and pure MgO (prepared using the same procedure to composite) were used for comparison purposes 2.3 Characterization X-ray diffraction patterns (XRD) of as-synthesized samples were recorded using an X-ray spectrometer (Ultima IV) using Cu Ka radiation (k = 1.5418 ) operated at 40 kV and 100 mA, and diffractograms were taken with a step size of 0.02° XRD patterns were recorded from 10° to 100° (2h) with a scanning step of 0.02° X-ray photoelectron spectroscopy (XPS) was performed on a Thermo VG using monochrome Al Ka as the excitation source Morphology and size of the samples were observed by transmission electron microcopy (TEM, JEM-2010), scanning electron microscopy (SEM, Hitachi 4700), and energy-dispersive X-ray spectroscopy (EDS, JSM-7100F) Textural properties were measured by N2 adsorption/desorption isotherms using a Quantachrome instrument (Autosorb iQ, version 3.0 analyzer) This instrument was also used to measure the adsorption isotherms of water vapor for the MgO and MgO/C samples Fourier transform infrared spectroscopy (FT-IR, VERTEX 70) was recorded in the wave range between 4000 and 400 cmÀ1 2.4 Dynamic breakthrough experiments for removal of DMMP and 2-CEES A schematic diagram of the dynamic breakthrough system is shown in Fig A continuous flow column reactor (water-jacket glass column), packed with as-prepared material, was used to measure the sorption capacities of the synthesized materials for DMMP and 2-CEES at ambient DMMP and 2-CEES vapors were generated from a vessel in a water bath at 25 °C by nitrogen bubbling Then, the generated vapor was diluted by another N2 stream to control experimental vapor concentration and prevent vapor condensation Gas flows were controlled by two mass flow controllers (MFCs) To confirm the homogeneous phase, the generated feed gas was passed through a mixing tank Before the breakthrough experiments, the concentration of feed gas was confirmed by an on-line gas chromatograph with a flame photometric detector (GC; Agilent 6890N) through a by-pass line of the sorption column And the on-line GC was used to monitor the concentrations of DMMP and 2-CEES at the outlet of the sorption column Additionally, the components of outlet gas samples were analyzed by a GC-MS (Agilent 7890A-5977A) In each experiment, sorbent (50 mg) was packed into a waterjacket column reactor (inner diameter: mm; length: 175 mm) Then, glass beads and glass wool were put into both ends of the column The column temperature was controlled by a water circulator and measured by a thermocouple (RTD, Pt 100 X) inserted in the column Prior to sorption, the column was activated at 150 °C with a pure N2 flow at 15 mL/min for h, and then cooled to 25 °C Subsequently, a mixture of DMMP or 2-CEES in N2 was fed into the sorption column The feed concentrations of DMMP and 2-CEES was calibrated before each breakthrough experiment and controlled within the range of ±3% When sorbent particles are tested in a breakthrough experiment, differences in the pressure drop and bed porosity can result in experimental deviations Therefore, it is important to evaluate materials using the same packing conditions as used in the breakthrough experiments [30–32] In this study, an equal amount of sample particles was packed into the column for each experiment The packing length occupied by the sorbent particles was also kept equal in every experiment to maintain the same packing density Because the reactor was a glass column, the packing length could be monitored during experiments In addition, a low fixed flow rate was used to minimize errors caused by changes in the packing density (length) in breakthrough experiments even though the experiments took longer Since the boiling point between 2-CEES and DMMP was different, it was not easy to carry out the breakthrough experiments under the same flow rate and concentration In the study, the breakthrough experiments were carried out by using a similar concentration condition for each sorbate: DMMP (concentration; 0.23 lg/mL and flow rate; 30 mL/min) and 2-CEES (concentration; 0.26 lg/mL and flow rate; 22.5 mL/min) at 25 °C Sorbent column MFC: Mass Flow Controller RTD: Resistance Temperature Detector GC: Gas Chromatography PG: Pressure Gauge Fig Schematic diagram of breakthrough apparatus for DMMP and 2-CEES sorption 1237 A.-T Vu et al / Chemical Engineering Journal 283 (2016) 1234–1243 pressure during the breakthrough experiments was measured by two electrical pressure gauges The pressure drop in all experiments was 0.25 psi due to packing sorbent particles Breakthrough saturation took a long time in each experiment due to dense packing, the slow flow rate, and low concentrations of DMMP and 2-CEES The sorption capacities of DMMP and 2-CEES were calculated using the following equation: F  t0:5  C o m  103 100 200 110 ð1Þ AC 220 222 MgO/C-3 MgO/C-2 where F is the flow rate of the feed gas (mL/min), t0.5 is the time for 50% sorbate breakthrough (min), Co is the initial concentration of DMMP or 2-CEES (lg/mL), and m is the sorbent mass (g) The sorption equilibrium rate as well as sorbate breakthrough can be used to quantify the dynamic sorption properties of sorbents The following Yoon and Nelson equation [33] was used to fit the experimental breakthrough curves: MgO/C-1 MgO 20 40 60 80 100 2-Theta (degree)  ð2Þ The wide-angle XRD diffraction patterns of MgO, AC, and MgO/C composites are presented in Fig 2(a) Diffraction peaks were observed at 2h values of 42.43, 61.78, and 77.73 corresponding to the (2 0), (2 0), and (2 2) plans of MgO, respectively This indicated that MgO had a cubic structure with a crystal lattice parameter of a = 4.21 Å (JCPDS No 75-0447) In addition, the relatively broad and low intensity of diffraction peaks indicated a small crystallite size of 4.1 nm approximated by using the Scherrer equation [12], as presented in Table Diffraction peaks were observed at 23.02, 43.40, and 79.24 of 2h values, which could be assigned to crystal phase of commercial activated carbon (JCPDS No 82-1691) As shown in Fig 2(a), all MgO/C composites exhibited the same XRD diffraction peaks as MgO, but no characteristic peaks corresponding to carbon were detected, even though the carbon content of the composites was as high as 17.86 wt% (MgO/C-3) We deduced that the carbon in the composites was amorphous The intensity of diffraction peaks increased slightly with carbon content in the MgO/C composites The crystallite size of the composites (2.8–3.1 nm) was smaller than that of MgO, as shown in Table The chemical composition of the composites was determined using X-ray photoelectron spectroscopy (XPS) As shown in Fig (b), full-scale XPS spectra of MgO and MgO/C composites exhibited very clear MgO features The photoelectron peaks at 49.8, 92.2, and 1304 eV corresponded to Mg 2p, 2s, and 1s, respectively, and the O 1s peak was at 553.0 eV, consistent with a previous report [34] Activated carbon was obviously observed by the intense photoelectron peak C 1s at 284.6 eV The peak was also observed with a lower intensity than carbons in all the composites, indicating the successful incorporation of carbon into MgO The carbon content in the composites estimated by the C 1s peak area was 6.39, 10.68, and 17.86 wt% for MgO/C-1, MgO/C-2, and MgO/C-3, respectively, as shown in Table As expected, the carbon content Mg KLL C1s AC 2.0x10 MgO Mg2p Mg2s 1.0x10 O2s MgO/C-3 Mg KL5 KL3 Mg KL3 Mg KL1 O KL2 O KL1 1.5x10 C1s Results and discussion 3.1 Characterization O1s C KL1 (b) Mg1s where KYN represents the Yoon–Nelson rate constant (minÀ1) and t0.5 is the time for 50% sorbate breakthrough (min) The fit between experimental data and Eq (2) was found by determining the best regression coefficient (R2) obtained with different couples (KYN, t0.5) The value of t0.5 was used to evaluate the efficiency of the sorbents 2.5x10 Counts / s ln  C ¼ K YN  t À t0:5  K YN Co À C (a) Intensity (a.u.) Sorption capacity ðmg=gÞ ¼ 002 MgO/C-2 5.0x10 MgO/C-1 0.0 1400 1200 1000 800 600 400 200 Binding Energy (eV) Fig (a) XRD patterns and (b) XPS spectra of MgO, AC, MgO/C-1, MgO/C-2, and MgO/C-3 samples increased with an increase in the amount of glucose used during the synthesis N2 adsorption/desorption isotherms and pore size distributions of MgO and MgO/C composites are presented in Fig 3(a) BET surface area, pore volume, and the average pore size diameter of asprepared samples are presented in Table The isotherm curve of MgO was classified as a type IV based on the IUPAC system In addition, MgO had a sharp type H3 hysteresis loop containing a steep region associated with closure of the hysteresis loop at the relative pressure of $0.5 This suggested a mesoporous material with non-rigid aggregated particles forming slit-shape pores The isotherm of AC was classified as type I, implying a microporous material with a small average pore diameter (1.20 nm) in Table As shown in Fig 3(a), the isotherm curves of the MgO/C composites were type IV with a type H4 hysteresis loop at P/Po = 0.4–1, showing a mesoporous material The hysteresis loops of composites were smaller than that of MgO and showed a shift of the closure of hysteresis loop to a lower value The results associated with narrow slit pores, including a pore in the micropore region Pore size distributions of composites were smaller than that of MgO and were comparatively narrower with an average pore diameter in the range of 3.43–3.84 nm When glucose and water were added into magnesium methoxide solution, the homogenous gels were formed because of the interaction of glucose with Mg(OH) polymer-like gels by hydrogen 1238 A.-T Vu et al / Chemical Engineering Journal 283 (2016) 1234–1243 Table Textural properties of MgO, MgO/C composites, and activated carbon a b Sample BET surface area (m2/g) BJH mesopore volume (cc/g) SF microspore volume (cc/g) Total pore volume (cc/g) Average pore diameter (nm) Crystallite sizea (nm) wt% of carbonb MgO HY-MgO/C-1 MgO/C-1 MgO/C-2 MgO/C-3 AC 512 1243 723 689 648 1336 1.62 2.33 1.36 1.30 1.14 0.461 0.146 0.259 0.444 0.434 0.451 0.459 1.766 2.589 1.804 1.734 1.591 0.920 5.66 3.64 3.84 3.43 3.43 1.20 4.1 – 2.8 2.9 3.1 – – – 6.39 10.68 17.86 – Estimated by (2 0) XRD diffraction peak of MgO Result from XPS analysis 1200 800 dV/dr (cc/g/nm) Volume @ STP (cc/g) 1000 MgO/C-1 MgO/C-2 MgO/C-3 MgO AC (a) 600 0 10 400 15 20 25 30 Pore diameter (nm) 35 40 200 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/Po) (b) MgO Transmittance (%) 428 MgO/C-1 873 1637 1458 592 3440 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Fig (a) N2 isotherm curves (inset: pore size distribution) of AC, MgO, MgO/C-1, MgO/C-2, and MgO/C-3 samples, (b) FTIR spectra of MgO and MgO/C-1 samples bonding or MgAO bonding [35,36] (Fig S1) In addition, glucose (melting temperature: 150 °C [37]) melted and spread out on the surface of MgO during calcination The glucose on the surface of MgO resulted in improving the surface area and porosity of composite From the N2 sorption/desorption isotherms, the MgO/C composites exhibited a high surface area and pore volume However, at high molar ratio of magnesium methoxide to glucose (1:0.09 and 1:0.18), the precipitation was very significant and the gels became less homogeneous The extra carbon in the com- posites led to a decrease in BET surface area as follows: MgO/C-1 (723 m2/g) > MgO/C-2 (689 m2/g) > MgO/C-3 (648 m2/g) > MgO (512 m2/g) in Table However, the surface areas of the composites were smaller than those of AC and HY-MgO/C-1 (MgO/C-1 before calcination) BJH mesopore volumes of composites (1.14–1.36 cc/g) were smaller than that of MgO, but the SF micropore volumes (0.434–0.451 cc/g) were higher than that of MgO As mentioned previously, the composites had well-developed mesopores and some micropores Even though the micropore volumes of the composites were similar to that of AC, the average pore diameter of the composites was larger than that of AC In addition, the surface areas of the composites were much smaller than that of AC FT-IR spectra of MgO and MgO/C-1 as a representative composite are shown in Fig 3(b) The spectra were similar to each other The broad intense band at 3440 cmÀ1 and minor peak at 1637 cmÀ1 were ascribed to stretching vibrations and bending vibrations of (AOH) groups attached to the surfaces of particles [38,39] And these bands could be also ascribed to vibrational stretching modes from (AOH) groups in adsorbed water molecules [40] The bands at the low frequencies of 873, 592, and 428 cmÀ1 were attributed to stretching vibrations of AMgAOAMgAOA bonding In addition, it was reported that the synthesized metal oxides using metal alkoxides by an aerogel method contained a certain number of (AOH) groups on the surface of the materials [7] And the band at 1458 cmÀ1 was ascribed to vibration of MgAOH bonding [41,42] These indicated that a number of AOH groups from Mg(OH)2 remained on the surface of MgO and MgO/C-1 particles despite calcination at 500 °C However, no band corresponding to MgAC, C@O, or CAH bonding was observed This once again implied that isolated carbon in amorphous phase was formed after thermal decomposition of glucose in the composites under vacuum at 500 °C TEM, SEM, and SEM/EDS images of MgO and MgO/C-1 samples are shown in Fig Fig 4(a) shows MgO bulk particles of 100–150 nm in size with a rough surface that formed due to aggregation of many small particles with about nm in size, as shown in Fig 4(b), consistent with the XRD results presented in Table The morphology of the MgO/C-1 composite (Fig 4(c)) was different from that of MgO The TEM image of MgO/C-1 in Fig 4(d) showed that amorphous carbon particles were well dispersed on the surface of the aggregated MgO particles and the particle were $10 nm in size In addition, the morphology of carbon could be seen in the TEM images of MgO/C-1 sample after etching using concentrated HCl, as shown in Fig 4(e); many holes smaller than nm were evident, but the particle sizes were similar to those of MgO/C-1 before etching process To further confirm the composition and structure of the composites, SEM/EDS measurements were conducted on the MgO/C-1 sample EDS spectrum results for C, Mg, and O are shown in Fig 4(f) The elemental map for C shown in Fig 4(g) indicated good distribution of C in the composite, while the map for Mg shown in Fig 4(h) revealed aggregated Mg The SEM/EDS image shown in Fig 4(i) revealed the absence A.-T Vu et al / Chemical Engineering Journal 283 (2016) 1234–1243 1239 Fig SEM images of (a) MgO and (c) MgO/C-1; TEM images (b) MgO, (d) MgO/C-1, and (e) MgO/C-1 after etching MgO; (f) EDS spectrum of MgO/C-1; (g) and (h) elemental maps of carbon and magnesium of MgO/C-1, respectively; (i) SEM/EDS image of MgO/C-1 of the concentrated regions of specific elements; dispersion of carbon on the surface of MgO particles was the dominant finding This could be attributed to the interaction of glucose with the Mg(OH)2 polymer-like gels (Fig S1) and the melting and spreading of glucose on the surface of MgO during calcination Based on these results, it was concluded that MgO was well coated by carbon, resulting in core-shell structures However, at high molar ratios of magnesium methoxide to glucose, the carbon coated MgO particles could be mixed with extra carbons, which came from the additional amount of carbon source (glucose), as described above 3.2 Removal of DMMP Sorption behavior of as-prepared samples was determined by breakthrough curves; the curves showed the DMMP concentration at the outlet of the sorbent column as a function of time Breakthrough curves for DMMP on MgO, activated carbon, and MgO/C composites at 25 °C under dry conditions are shown in Fig 5(a) The breakthrough time of DMMP on MgO (199 min) was longer than that of the MgO/C-2 and MgO/C-3 composites (164 and 129 min, respectively) However, the saturation time of MgO (514 min) was shorter than that of MgO/C-2 and MgO/C-3 composites (589 and 534 min, respectively) Therefore, the breakthrough shapes of MgO/C-2 and MgO/C-3 were a little wider than that of MgO In contrast, although there was not a significant difference in breakthrough time between MgO/C-1 and MgO, the saturation time of MgO/C-1 was much longer than that of MgO, MgO/C-2, and MgO/C-3, as shown in Table The breakthrough curve of DMMP on activated carbon was steepest, while the breakthrough and saturation times were shortest for the other samples even though the activated carbon had a much higher surface area than the other sorbents (see Table 1) Since the molecular diameter of DMMP (0.57 nm) [43] is smaller than the pore size diameters of MgO, MgO/C composites, and activated carbon, the DMMP molecules can penetrate into the pores and sorb on the surface of the samples And the sorption of DMMP was attributed to physical sorption on active sites by MgAO interaction [44], as shown in Fig S2 Therefore, the surface area and pore volume are the important factors that significantly contribute to the sorption capacity of the sorbents In regard to the surface area and total pore volume, the sorption capacity of DMMP calculated from t0.5(exp) was as follows: MgO/C-1 (67.8 mg/g) > MgO/C-2 (43.7 mg/g) > MgO (42.2 mg/g) > MgO/C-3 (34.5 mg/g) > activated carbon (30.4 mg/g) This clearly showed that sorption capacity decreased with an increase in carbon content, and that it could be considerably improved by coating MgO with a small amount of carbon (MgO/C-1) The breakthrough curve shape as well as slope can be affected by sorption affinity and rate [30,32,45] As shown in Fig 5(a), activated carbon and as-prepared composites had different breakthrough curve shapes from one other This can be explained by the different affinities and concentration propagations of DMMP on each sorbent material It was reported that the molecular diameters of DMMP and 2-CEES were 0.57 and 0.69 nm, respectively [43,46] Considering the pore sizes of AC and MgO in Table 1, the sorption affinity was a more important factor than mass transfer resistance The breakthrough curve slopes of all the composites were wider than those of MgO and AC, and became steeper with an increase in carbon content This implied that the sorption affinity of DMMP on all the composites was weaker than those of MgO 1240 A.-T Vu et al / Chemical Engineering Journal 283 (2016) 1234–1243 1.0 (a) 0.8 C/Co 0.6 0.4 MgO/C-1 MgO/C-2 MgO/C-3 MgO Activated carbon Model 0.2 0.0 200 400 600 800 Time (min) 1.0 (b) 0.8 C/Co 0.6 0.4 MgO/C-1 MgO Activated Carbon Model 0.2 0.0 200 400 600 800 Time (min) Fig Comparison of breakthrough curves for DMMP sorption on sorption column packed by (a) MgO, MgO/C-1, MgO/C-2, MgO/C-3 or activated carbon in dry condition, and (b) MgO, MgO/C-1 or activated carbon in humid condition (30 %RH) and AC because the pore sizes of the composites were still larger than that of AC Since the difference in the pore sizes of all the composites was small, the increased sorption affinity of the composites stemmed from increased carbon content and the sorption affinity approached that of AC even though the sorption capacity decreased In addition, the Yoon–Nelson model could predict the experimental breakthrough curves for all sorbents with R2 > 0.99 The rate constants (KYN) obtained from model fitting were as follows: AC (0.029 minÀ1) > MgO (0.019 minÀ1) > MgO/C-3 À1 À1 (0.015 ) > MgO/C-2 (0.014 ) > MgO/C-1 (0.009 minÀ1) Together, the results indicated that MgO/C composites prepared using an aerogel method had improved BET surface area and micropore volume than MgO As carbon content in the composites increased, the BET surface area and BJH mesopore volume decreased Sorption capacity with an increase in carbon content was similar or smaller than that of MgO The contribution of improved surface area in the MgO/C composites to sorption capacity was limited MgO/C-1 composite had the longest breakthrough and saturation times as well as highest sorption capacity among the as-prepared composites MgO/C-1 was therefore selected for further evaluation under humid conditions As mentioned previously, the fact that an as-prepared composite has a high removal capacity for toxic chemicals under dry conditions compared to MgO and AC is not sufficient for its practical application, because toxic chemicals normally exist in humid conditions To evaluate the effect of water vapor on the sorption of DMMP, the removal efficiencies of MgO, MgO/C-1, and AC were re-evaluated by using the feed gas of DMMP in N2 at a relative humidity of 30% (30% RH) The other conditions for these breakthrough experiments under humid conditions were the same as those used for experiments performed under dry conditions Breakthrough curves of DMMP on MgO, MgO/C-1, and activated carbon at 30% RH are shown in Fig 5(b) The breakthrough curve shapes of all test sorbents were similar to those obtained under dry conditions, but a reduction in the sorption capacity of MgO was clearly observed The breakthrough and saturation times of MgO under humid conditions (44 and 309 min, respectively) were much shorter than those under dry conditions As a result, sorption capacity under humid conditions decreased significantly to 23.3 mg/g, corresponding to 56% of sorption capacity under dry conditions (Table 2) We ascribed this to the sorption of water vapor on active sites of MgO particles The effect of humidity on sorption was not significant for activated carbon The rate constant, KYN (0.035 minÀ1), increased under humid conditions, but breakthrough and saturation times decreased slightly The relative decrease in sorption capacity (24.6 mg/g) was much smaller than that of MgO as shown in Table As a result, the sorption capacity of AC became similar to that of MgO, but the breakthrough time was longer due to strong adsorption affinity under humid conditions As expected from the AC results, the sorption of DMMP by carbon-coated MgO (MgO/C-1) was not significantly affected by the presence of water vapor The breakthrough and saturation times of DMMP for the MgO/C-1 sample were 179 and 744 min, respectively, showing very little decrease in comparison with dry conditions Correspondingly, the sorption capacity under humid conditions (61.5 mg/g) was 91% of that under dry conditions and approximately 2.5-fold higher than those of activated carbon and MgO under humid conditions In addition, the rate constant (KYN) Table Breakthrough and saturation times, and sorption capacity of DMMP in dry and humid condition (30 %RH) Sample Condition tb (min) ts (min) t0.5 (fit) t0.5 (exp) KYN (minÀ1) R2 Sorption capacity (mg/g) AC MgO MgO/C-1 MgO/C-2 MgO/C-3 AC MgO MgO/C-1 Dry Dry Dry Dry Dry Humid Humid Humid 159 199 189 164 129 139 39 179 369 514 784 589 534 344 309 744 224 312 493 330 250 186 166 444 221 306 491 317 243 179 169 446 0.029 0.019 0.009 0.014 0.015 0.035 0.016 0.010 0.996 0.998 0.990 0.997 0.993 0.990 0.994 0.992 30.4 42.2 67.8 43.7 34.5 24.6 23.3 61.5 tb and ts are breakthrough and saturation times, respectively t0.5 (fit) and t0.5 (exp) are the times for 50% sorbate breakthrough obtained from fitting and interpolation for experimental data, respectively 1241 A.-T Vu et al / Chemical Engineering Journal 283 (2016) 1234–1243 The result is also supported by the water adsorption isotherms shown in Fig The interaction sorption capacity of MgO with water vapor was much stronger and larger than that of the MgO/C-1 composite This implied that the hydrophobicity of the carbon shell effectively protected the MgO crystals from water vapor 200 Mg(OH)2 Intensity (a.u.) 220 (d) 222 3.3 Removal of 2-CEES Mg(OH)2 (b) (a) 20 40 60 80 100 2-Theta (degree) Fig XRD patterns before and after DMMP sorption in humid condition, respectively: (a) and (b) for MgO, and (c) and (d) for MgO/C-1 under humid conditions was also similar to that under dry conditions, as shown in Table MgO and the MgO/C-1 composite showed different behaviors for sorption of DMMP according to the presence of water vapor The breakthrough and saturation times as well as sorption capacity of MgO were significantly lower than those obtained under dry conditions, while these values did not differ much for the MgO/C-1 composite Therefore, a carboncoated MgO composite was successfully developed to remove DMMP efficiently under both dry and humid conditions To investigate the effect of water vapor on the crystalline structure of MgO and the MgO/C-1 composite, both sorbents were analyzed by XRD after DMMP sorption under humid conditions The XRD patterns before and after sorption are compared in Fig No significant changes in the peak intensity, full width at half maximum (FWHM), or peak position of (2 0), (2 0), and (2 2) crystal plans of either sorbent was observed after DMMP sorption under humid conditions MgO underwent substantial conversion to Mg (OH)2 crystallite, leading to the appearance of Mg(OH)2 diffraction peaks This was due to the reaction of MgO with water In contrast, no significant diffraction peaks of Mg(OH)2 were observed for the MgO/C-1 composite This implied that it was difficult for MgO to react with water molecules in the composite Identical breakthrough experiments as performed with DMMP were carried out for the feed gas of 2-CEES (0.26 lg/mL) in N2 at 22.5 mL/min The breakthrough curves of 2-CEES on the MgO and MgO/C-1 sorbents under dry conditions are presented in Fig (a) Each sorbent had two breakthrough curves: a 2-CEES curve and a reacted product curve The composition of outlet gas from the breakthrough column for 2-CEES sorption was analyzed by a GC-MS Vinyl ethyl sulfide (CH3CH2SCH@CH2) and 2-CEES were detected at 6.7 and 12.2 of retention times, respectively, as shown in Fig S3 The sorption and decomposition of 2-CEES on sorbents result from the reactive site of the isolated (AOH) groups (Fig S4) Breakthrough times, saturation times, and sorption capacities are listed in Table 1.0 2-CEES/MgO product/MgO 2-CEES/MgO/C-1 product/MgO/C-1 model 0.8 0.6 0.4 0.2 0.0 100 200 300 400 500 Time (min) 1.0 2-CEES/MgO 2-CEES/MgO/C-1 product/MgO/C-1 model 900 800 (a) C/Co (c) MgO/C-1 MgO 0.8 (b) 0.6 600 C/Co Volume @ STP (cc/g) 700 500 0.4 400 300 0.2 200 100 0.0 0 0.0 0.2 0.4 0.6 0.8 Relative pressure (P/Po) Fig Water vapor adsorption isotherms of MgO and MgO/C-1 1.0 100 200 300 400 Time (min) Fig Comparison of breakthrough curves for 2-CEES sorption on sorption column packed by (a) MgO and MgO/C-1 in dry condition, and (b) MgO and MgO/C-1 in humid condition (39 %RH) 1242 A.-T Vu et al / Chemical Engineering Journal 283 (2016) 1234–1243 Table Breakthrough and saturation times, and sorption capacity of 2-CEES in dry and humid condition (39 %RH) Sample Condition tb (min) ts (min) t0.5 (fit) t0.5 (exp) KYN (minÀ1) R2 Sorption capacity (mg/g) MgO MgO/C-1 MgO MgO/C-1 Dry Dry Humid Humid 202 222 42 152 392 397 297 327 290 305 194 262 288 302 190 260 0.035 0.036 0.025 0.049 0.994 0.998 0.996 0.999 33.7 35.3 22.2 30.4 tb and ts are breakthrough and saturation times, respectively t0.5 (fit) and t0.5 (exp) are the times for 50% sorbate breakthrough obtained from fitting and interpolation for experimental data, respectively The breakthrough curve shapes of 2-CEES on MgO and the MgO/ C-1 composite were similar with a rate constant (KYN) of 0.035 and 0.036 minÀ1, respectively The breakthrough time, saturation time, and sorption capacity of 2-CEES on MgO was 202 min, 392 min, and 33.7 mg/g, respectively The corresponding values for the MgO/C-1 composite (222 min, 397 min, and 35.3 mg/g) were slightly higher than those of MgO, as shown in Table Compared to the results obtained for DMMP, the sorption rate constant was faster, but the saturated sorption amount was smaller In view of the reactivity shown in Fig 8(a), decomposition of 2-CEES on MgO was greater than of the MgO/C-1 composite The reacted product appeared before 2-CEES breakthrough in MgO, but the reacted product took much longer to appear for the MgO/ C-1 sample due to penetration of 2-CEES and product molecules into the carbon-coated shells of the composite particles As mentioned in Fig 3(b), it was again confirmed that (-OH) groups remained on the surface of MgO and that these groups played a role in the reaction of 2-CEES with MgO [6,11,47,48] The reaction could be explained by the following equation: ðOHÞAMgOA þ CH2 ClACHASACH2 ACH3 ! ClAMgOA þ CH2 @CHASACH2 ACH3 þ H2 O ð3Þ The sorption and reaction of 2-CEES on MgO and MgO/C-1 composites in the presence of water vapor (39% RH) are shown in Fig (b) The breakthrough shape of 2-CEES on MgO was wider with a decreased rate constant (KYN) of 0.025 minÀ1, implying that the sorption affinity of 2-CEES for MgO was significantly lower under humid conditions than dry conditions The breakthrough and saturation times of 2-CEES on MgO under humid conditions also decreased steeply to 42 and 297 min, respectively As a result, the sorption capacity was 22.2 mg/g and the change in sorption behavior under humid conditions was the same as that obtained for DMMP, as shown in Tables and Furthermore, no reacted product was observed owing to sorption of water molecules on catalytic active sites of MgO In contrast, the breakthrough shape of the MgO/C-1 composite became steeper with a higher rate constant (KYN) of 0.049 minÀ1 than that obtained under dry conditions The breakthrough and saturation times of the MgO/C-1 composite were 152 and 324 min, respectively, representing a smaller decrease than seen for MgO The sorption capacity of the composite under humid conditions was about 86% of that under dry conditions, and was much larger than that of MgO, as shown in Table Furthermore, for the MgO/C-1 composite, vinyl ethyl sulfide from the reaction of 2-CEES with MgO was still detected, although levels of this product concentration were lower than that obtained under dry conditions High sorption capacity and reactivity of 2-CEES on the MgO/C-1 composite under humid conditions confirmed again that the carbon in the MgO/C composite worked as a hydrophobic shell and protected MgO from water sorption Conclusions Carbon-coated MgO composites were prepared via an aerogel route with glucose as a carbon precursor to efficiently remove 2-CEES and DMMP MgO/C composites had higher surface areas and microspore volumes and lower mesopore volumes and crystallite sizes than MgO As the glucose amount in the synthesis step increased, the carbon content of the MgO/C composites increased and the surface area and mesopore volume decreased because too much amount of glucose addition led to forming carbon out of the surface of MgO The MgO/C composite with 6.39 wt% carbon showed the highest sorption capacity for DMMP (67.8 mg/g at 0.23 lg/mL) and 2-CEES (35.3 mg/g at 0.26 lg/mL) among the assynthesized composites under dry conditions The sorption capacities of MgO and the MgO/C composites were higher than that of AC, even though the surface area of AC was the highest In addition, the sorption capacity of MgO/C composites decreased with an increase in carbon content MgO/C composites with more than 10 wt% carbon had lower sorption capacity than MgO, even though their surface areas were larger than that of MgO This implied that the contribution of the surface area to sorption capacity was limited, but the sorption affinity played an important role in determining sorption capacity Under humid conditions, the sorption capacities of DMMP and 2-CEES on MgO decreased significantly Furthermore, MgO lost reactivity toward 2-CEES due to the sorption of water molecule on catalytic active sites In contrast, carbon content of the composite allowed effective protection of sorption and reaction of DMMP and 2-CEES from water vapor The sorption capacities of DMMP and 2-CEES on the MgO/C-1 sample under humid conditions were 61.5 mg/g and 30.4 mg/g, about 91% and 86% of those under dry conditions, respectively The carbon shell protected the sorbent composite from water vapor Because aerogel MgO with higher mesopore volume than AC can have a higher sorption affinity and capacity for CWA molecules than AC, a carbon thin layer coating of MgO is the most promising way to produce the sorbents with higher sorption capacity and reactivity than AC in humid condition Acknowledgements We would like to acknowledge the financial support from the R&D Convergence Program of MSIP 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