Hydrothermal Fabrication of High Specific Surface Area Mesoporous

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Article Hydrothermal Fabrication of High Speciﬁc Surface Area Mesoporous MgO with Excellent CO2 Adsorption Potential at Intermediate Temperatures

Wanlin Gao 1, Tuantuan Zhou 1, Benoit Louis 2 and Qiang Wang 1,*

1 College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing 100083, China; [email protected] (W.G.); [email protected] (T.Z.) 2 Laboratoire de Synthèse, Réactivité Organiques et Catalyse, Institut de Chimie, UMR 7177, Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg, France; [email protected] * Correspondence: [email protected]; Tel.: +86-136-9913-0626

Academic Editor: Simon Penner Received: 5 March 2017; Accepted: 11 April 2017; Published: 15 April 2017

Abstract: In this work, we report on a novel sodium dodecyl sulfate (SDS)-assisted magnesium oxide (MgO)-based porous adsorbent synthesized by hydrothermal method for intermediate CO2 −1 capture. For industrial MgO, its CO2 adsorption capacity is normally less than 0.06 mmol g , with a speciﬁc surface area as low as 25.1 m2 g−1. Herein, leaf-like MgO nanosheets which exhibited a disordered layer structure were fabricated by the introduction of SDS surfactants and the control of other synthesis parameters. This leaf-like MgO adsorbent showed an excellent CO2 capacity of 0.96 mmol g−1 at moderate temperatures (~300 ◦C), which is more than ten times higher than that of the commercial light MgO. This novel mesoporous MgO adsorbent also exhibited high stability during multiple CO2 adsorption/desorption cycles. The excellent CO2 capturing performance was believed to be related to its high speciﬁc surface area of 321.3 m2 g−1 and abundant surface active adsorption sites. This work suggested a new synthesis scheme for MgO based CO2 adsorbents at intermediate temperatures, providing a competitive candidate for capturing CO2 from certain sorption enhanced hydrogen production processes.

Keywords: sorption enhanced water gas shift reaction; CO2 adsorption; magnesium oxides; hydrothermal synthesis; SDS

1. Introduction

The increased atmospheric CO2 concentration and the resulting global warming and anthropogenic climate change are the price we pay for over-industrialized developments and excessive combustion of fossil fuels [1–3]. It is well accepted that the CO2 released by persistent combustion of fossil fuel is one of the main reasons for the average global temperature increase and disastrous environmental effects [4–6]. It is estimated that fossil fuels will still be the dominant supply to meet the energy demand of the future. Such a trend will make it impossible for us to halt the increase in atmospheric CO2 levels immediately [7,8]. Recently, the introduction of CO2 capture, storage (sequestration), and utilization (CSU) systems has received substantial attention because of their potential for restricting the release of anthropogenic CO2 and improving the energy utilization efficiency. Behind the technologies, suitable CO2 capturing materials are crucial for the establishment of the advanced CSU systems [5,9,10]. So was born three main approaches or processes to capture CO2 from point sources, that is pre-combustion capture, post-combustion capture and oxyfuel combustion [11]. Among these, solid adsorbents have been commonly fit for the integrated gasification combined cycles (IGCC) related

Catalysts 2017, 7, 116; doi:10.3390/catal7040116 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 116 2 of 15

pre-combustion CO2 capture processes [4,5], e.g., sorption enhanced water gas shift (SEWGS) and sorption enhanced biomass reforming (SEBR) reactions [12–15]. Among the abundant solid adsorbents which tend to capture CO2 at moderate temperatures [5,14–16], layered double hydroxides (LDHs) derived metal oxides [17–21] and magnesium oxide (MgO) based compounds [22–26] are promising for the SEWGS and SEBR technologies in the temperature range of 200-400 ◦C[10,27–30]. LDHs have the reputation of outstanding thermal stability and relatively fast CO2 adsorption kinetics. They also have the advantage of being regenerated at an intermediate temperature [31–33]. Furthermore, MgO materials have been attracting intense attention as a suitable adsorbent for effective capture of CO2 as much progress has been obtained within a short period of time [4,15,34,35]. Two factors weigh heavily against the effectiveness of pure MgO in CO2 capture performance. One is the poor thermal stability during regeneration, and the other is the lack of enough surface basic sites due to a low specific surface area (less than 25.1 m2 g−1)[23,36–38]. Due to the drawbacks of commercial MgO adsorbent, researchers adopted various methods for the fabrication of porous MgO materials with a large specific surface area [39]. Herein, we report a facile fabrication of pretty high specific surface area (>321.3 m2 g−1) mesoporous MgO by a urea hydrolysis process from magnesium nitrate aqueous solution followed by thermal decomposition of the precipitated precursors. The synthesized material has also been compared for CO2 capture capacity and kinetics with the commercial light MgO and the MgO samples obtained from thermal decomposition of commercial magnesium hydroxide. The influences of surfactant species, the ratio of ethanol and water, surfactant loading, calcination temperature, reaction time, and reaction temperature were investigated systematically to attain the maximum CO2 capture capacity of the compounds. Typically, the sodium dodecyl sulfate (SDS)-assisted MgO materials possessed the highest CO2 capture capacity of 0.96 mmol g−1 at moderate temperatures (~300 ◦C). The cycling performance of this SDS-assisted porous MgO as a CO2 adsorbent was evaluated under multiple cycles by thermogravimetric analysis (TGA) using temperature swing adsorption (TSA) with different CO2 concentrations. Finally, the optimized SDS-assisted MgO adsorbent was characterized in depth.

2. Results and Discussion

2.1. Evaluation of CO2 Adsorption Capacity To clarify the key factors determining the performance of the MgO materials, the inﬂuences of different types of surfactants, ratio of ethanol and water, SDS amount, reaction time, and reaction temperature were systematically studied using X-ray diffractometry (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analyzer (TGA) and Brunauer-Emmett-Teller (BET) analyses, as presented in detail in the following sections.

2.1.1. The Influence of Different Types of Surfactant on CO2 Capture With the introduction of surfactants as templates, the synthesized MgO nanomaterials exhibited a specific morphology and a high specific surface area. In order to evaluate the effect of the surfactant types, different MgO samples were synthesized using a variety of surfactants, including poly (ethylene glycol)-poly (propylene glycol)-poly(ethylene glycol) triblock copolymer (P-123), polyvinylpyrrolidone (PVP), cetyl trimethyl ammonium bromide (CTAB), and sodium dodecyl sulfate (SDS). Meanwhile, the CO2 adsorption capacity of non-assisted MgO, MgO attained through thermal decomposition of commercial Mg(OH)2 (coded as MgO-TD) as well as light MgO were studied simultaneously and compared as the control. Table1 summarizes the synthetic conditions, texture parameters, and CO 2 adsorption capacities of different types of surfactant-assisted MgO. The adoption of a hydrothermal synthesis method with the introduction of various surfactants led to a marked enhancement in the CO2 uptake of the MgO materials. Except for the sample of light MgO, all other freshly prepared MgO compounds showed a much higher BET specific surface area (>194.3 m2 g−1) and a larger pore volume (>0.30 cm−3 g−1). The addition of surfactants gave rise to a significant increase in specific Catalysts 2017, 7, 116 3 of 15 surfaceCatalysts area 2017 from, 7, 116 194.3 to 341.4 m2 g−1 and pore volume from 0.30 to 0.49 cm−3 g−1, respectively.3 of 15 Figure1a shows the uptake of CO 2 by non-assisted MgO, MgO-TD, light MgO and MgO particles MgO and MgO particles assisted with various surfactants◦ when exposed at 300 °C to pure CO2. assisted with various surfactants when exposed at 300 C to pure CO2. Typically, the light-MgO was Typically, the light-MgO◦ was first calcined at 450 °C for 60 min under a flow of high-1 purity N2 (20 first calcined at 450 C for 60 min under a flow of high purity N2 (20 mL min ) to avoid intrinsic mL min‒1) to avoid intrinsic adsorption of CO2 on the surface of light-MgO. It is clear that SDS- adsorption of CO2 on the surface of light-MgO. It is clear that SDS-assisted MgO exhibited the highest assisted MgO exhibited the highest CO2 uptake of 0.92 mmol g−1, which is much higher than that of CO uptake of 0.92 mmol g−1, which is much higher than that of light MgO (0.05 mmol g−1). For the 2light MgO (0.05 mmol g−1). For the SDS-added sample, although its CO2 adsorption capacity per unit −2 SDS-addedsurface area sample, (0.0028 although mmol m− its2) was CO lower2 adsorption than that capacity for the surfactant-free per unit surface sample area (0.0044 (0.0028 mmol mmol m−2), m ) −2 wasits lower specific than surface thatfor area the was surfactant-free higher, thus resulting sample in (0.0044 a much mmol higher m total), itsCO specific2 capture surface capacity. area For was higher,the practical thus resulting application, in amuch the total higher CO2 capture total CO capacity2 capture is more capacity. important. For theBesides, practical the MgO-TD application, with the 2 −1 −1 totalthe CO highest2 capture specific capacity surface is morearea of important. 363.40 m g Besides, yet displayed the MgO-TD inferior with CO2 theuptake highest of 0.29 specific mmol surfaceg . 2 −1 −1 areaThe of 363.40intrinsic m reasong yet for displayedsuch phenomenon inferior is CO still2 uptakeunclear ofto 0.29us. Although, mmol g th.e The specific intrinsic surface reason area for suchand phenomenon pore size are is believed still unclear to be totwo us. of Although,the parameters the specificthat influence surface the area CO2 and capture pore capacity, size are some believed other parameters including chemical impurity, surface defects, etc. may have quite an influence [40]. to be two of the parameters that influence the CO2 capture capacity, some other parameters including chemical impurity, surface defects, etc. may have quite an influence [40]. Table 1. Synthetic conditions, texture parameters and CO2 adsorption capacities of different types of Thesurfactant-assisted XRD patterns of MgO. all samples are also provided for better explanation, as shown in Figure1b. The characteristic peaks of MgO (JCPDS 45-0946) at 2θ = 42.89◦ and 62.41◦ are clearly observed, BET Ethanol Hydrothermal Calcination Pore Average CO2 revealing the formation of well-crystallized MgO. TableSurface1 summarizes the averageCrystalline size of crystalline Sample Surfactant /Water Conditions Conditions Volume Pore Size Uptake domains calculated from XRD patterns according to ScherrerArea formula. Surfactant-assistedSize (nm) samples Ratio (°C and h) (°C and h) (cm3 g−1) (nm) (mmol g−1) have the average crystallite sizes of less than 8 nm, much(m2 g smaller−1) than that of light MgO (32.4 nm) for Surfactant MgO 1:1 120 and 12 400 and 5 194.3 0.30 3.05 1.3 0.86 the in templating-free with various surfactants. The introduction of surfactant appears to be beneficial for theMgO formation P-123 of small 1:1 MgO 120 nanocrystal and 12 structure. 400 and 5 Among 305.7 all the 0.42 samples, 2.78 CTAB and 2.1 SDS showed 0.78 MgO PVP 1:1 120 and 12 400 and 5 302.3 0.36 2.40 2.0 0.86 almost identical CO2 uptake. Although it seemed that CTAB yielded better textural properties in terms MgO CTAB 1:1 120 and 12 400 and 5 341.4 0.49 2.90 3.2 0.91 ofMgO specific surface SDS area 1:1 and pore 120 and volume, 12 no 400 much and 5 difference 321.7 was 0.40 observed 2.51 with the SDS 7.1 assisted 0.92 one. MgO- In this work,- our aim 1:1 is to optimize - the CO 4002 capture and 5 capacity 363.4 of surfactant-assisted 0.53 2.91 MgO 6.0 materials 0.29 to a TD great extent while SDS-assisted MgO materials happened to exhibit the highest CO2 uptake, which is Light - 1:1 - - 25.1 0.17 13.22 32.4 0.05 theMgO reason why we chose SDS as the representative surfactant for further investigation.

Figure 1. (a) The influence of different types of surfactants on the CO2 capture capacity; (b) XRD Figure 1. (a) The inﬂuence of different types of surfactants on the CO2 capture capacity; (b) XRD spectra of MgO compounds with different types of surfactants. spectra of MgO compounds with different types of surfactants.

Catalysts 2017, 7, 116 4 of 15

The XRD patterns of all samples are also provided for better explanation, as shown in Figure 1b. The characteristic peaks of MgO (JCPDS 45-0946) at 2θ = 42.89° and 62.41° are clearly observed, revealing the formation of well-crystallized MgO. Table 1 summarizes the average size of crystalline domains calculated from XRD patterns according to Scherrer formula. Surfactant-assisted samples haveCatalysts the average2017, 7, 116 crystallite sizes of less than 8 nm, much smaller than that of light MgO (32.44 nm) of 15 for the in templating with various surfactants. The introduction of surfactant appears to be beneficial for the formation of small MgO nanocrystal structure. Among all the samples, CTAB and SDS showed Table 1. Synthetic conditions, texture parameters and CO2 adsorption capacities of different types of almost identical CO2 uptake. Although it seemed that CTAB yielded better textural properties in surfactant-assisted MgO. terms of specific surface area and pore volume, no much difference was observed with the SDS

Hydrothermal Calcination 2BET Pore Average assisted one. In this work,Ethanol/Water our aim is to optimize the CO capture capacity of surfactant-assistedCrystalline CO Uptake MgO Sample Surfactant Conditions Conditions Surface Area Volume Pore Size 2 Ratio Size (nm) (mmol g−1) materials to a great extent while SDS-assisted(◦C and h) (◦C andMgO h) materials(m2 g−1) happened(cm3 g−1) (nm)to exhibit the highest CO2 uptake,MgO whichSurfactant-free is the reason1:1 why 120 andwe 12 chose 400 and SDS 5 as 194.3 the representative 0.30 3.05 surfactant 1.3 for 0.86further MgO P-123 1:1 120 and 12 400 and 5 305.7 0.42 2.78 2.1 0.78 investigation.MgO PVP 1:1 120 and 12 400 and 5 302.3 0.36 2.40 2.0 0.86 MgO CTAB 1:1 120 and 12 400 and 5 341.4 0.49 2.90 3.2 0.91 MgO SDS 1:1 120 and 12 400 and 5 321.7 0.40 2.51 7.1 0.92 MgO-TD - 1:1 - 400 and 5 363.4 0.53 2.91 6.0 0.29 2.1.2.Light The MgO Influence - of the Ratio 1:1 of Ethanol - to Water - on CO 25.12 Capture 0.17 13.22 32.4 0.05

Apparently, the ethanol to water ratio is one of the key parameters that influences the CO2 2.1.2. The Influence of the Ratio of Ethanol to Water on CO2 Capture capture performance of MgO samples. Figure 2a shows the CO2 capture capacity of the synthesized MgO withApparently, different the ratios ethanol at 300 to water°C. Based ratio isupon one ofthe the data, key parametersone can see that that influences the CO2the uptake CO2 captureincreased withperformance the increase of MgOin water samples. content. Figure The2a showsdecline the in CO CO22capture uptake capacity with the of addition the synthesized of ethanol MgO may with be ◦ attributeddifferent to ratios a slow at hydrolysis 300 C. Based rate upon and the a slightly data, one lower can seesolubility that the of CO SDS.2 uptake The structural increased withchanges the of MgOincrease particles in water were content. also investigated The decline by in COXRD2 uptake analyses, with as the shown addition in ofFigure ethanol 2b. may All bediffraction attributed peaks to a slow hydrolysis rate and a slightly lower solubility of SDS. The structural changes of MgO particles of the samples can be indexed to the MgO (JCPDS 45-0946). Notably, two small characteristic peaks were also investigated by XRD analyses, as shown in Figure2b. All diffraction peaks of the samples of magnesium carbonate (MgCO3) at 2θ ≈ 32.20° and 53.35° (JCPDS 08-0479) were observed, can be indexed to the MgO (JCPDS 45-0946). Notably, two small characteristic peaks of magnesium indicating a little MgCO3 was incorporated◦ in ◦MgO. When the ethanol content was high, a slower carbonate (MgCO3) at 2θ ≈ 32.20 and 53.35 (JCPDS 08-0479) were observed, indicating a little hydrolysis rate and a slightly lower solubility of SDS were favorable for the growth of MgO with a MgCO3 was incorporated in MgO. When the ethanol content was high, a slower hydrolysis rate and a higherslightly crystalline lower solubility degree. ofThe SDS slight were favorableshift to a forhigher the growth angle ofmight MgO be with due a higher to the crystalline contraction degree. of the latticeThe constant slight shift when to a the higher crystalline angle might degree be duewas to high the contraction[41,42]. It should of the lattice be noted constant that whenthe specific the surfacecrystalline area increased degree was steadily high [41 ,from42]. It 58.7 should to be321.3 noted m2 that g−1 thewith specific the rise surface in water area increased content, steadilywhile the averagefrom 58.7pore to size 321.3 is mon2 ga− gradual1 with the decline rise in waterfrom 23.52 content, to while1.88 nm, the averagewhich is pore consistent size is on with a gradual the CO2 uptakedecline data, from as 23.52shown to in 1.88 Table nm, 2. which is consistent with the CO2 uptake data, as shown in Table2.

Figure 2. (a) The inﬂuence of ratios of ethanol and water on the CO2 capture capacity; (b) XRD spectra of MgO compounds with different ratios of ethanol and water. Catalysts 2017, 7, 116 5 of 15

Figure 2. (a) The influence of ratios of ethanol and water on the CO2 capture capacity; (b) XRD spectra of MgO compounds with different ratios of ethanol and water.

Catalysts 2017Table, 7, 1162. Synthetic conditions, texture parameters and CO2 adsorption capacities of sodium dodecyl 5 of 15 sulfate (SDS)-assisted MgO with different ratios of ethanol and water.

BET Ethanol Hydrothermal Calcination Pore Average CO2 Table 2. Synthetic conditions, texture parametersSurface and CO2 adsorption capacitiesCrystalline of sodium dodecyl Sample /Water Conditions Conditions Volume Pore Size Uptake sulfate (SDS)-assisted MgO with different ratiosArea of ethanol(m2 and water. Size (nm) Ratio (°C and h) (°C and h) (cm3 g−1) (nm) (mmol g−1) g−1) MgO- 100% Hydrothermal Calcination BET Pore Average Ethanol/Water 120 and 12 400 and 5 58.7 0.69 23.52 Crystalline 18.6 0.04CO Uptake SampleSDS ethanol Conditions Conditions Surface Area Volume Pore Size 2 Ratio Size (nm) (mmol g−1) MgO- (◦C and h) (◦C and h) (m2 g−1) (cm3 g−1) (nm) 40:1 120 and 12 400 and 5 185.8 0.81 8.67 5.3 0.36 MgO-SDSSDS 100% ethanol 120 and 12 400 and 5 58.7 0.69 23.52 18.6 0.04 MgO-SDSMgO- 40:1 120 and 12 400 and 5 185.8 0.81 8.67 5.3 0.36 10:1 120 and 12 400 and 5 272.4 0.64 4.73 2.8 0.44 MgO-SDSSDS 10:1 120 and 12 400 and 5 272.4 0.64 4.73 2.8 0.44 MgO-SDS 100% water 120 and 12 400 and 5 321.3 0.30 1.88 5.4 0.96 MgO- 100% 120 and 12 400 and 5 321.3 0.30 1.88 5.4 0.96 SDS water 2.1.3. The Influence of the SDS Amount on CO2 Capture 2.1.3. The Influence of the SDS Amount on CO2 Capture To study the effect of the SDS content on CO2 adsorption, SDS-assisted MgO nanomaterials were preparedTo with study different the effect dosages of the SDS of SDScontent as on 0.08, CO 0.1,2 adsorption, 0.5, and 1SDS-assisted g, respectively. MgOAs nanomaterials shown in Figure were 3a, prepared with different dosages of SDS as 0.08, 0.1, 0.5, and 1 g, respectively. As shown in Figure 3a, the synthesized MgO compounds with different SDS contents showed a steady increase in CO2 capture capacitythe synthesized from 0.85 MgO to 0.96 compounds mmol g− 1with. Table different3 clearly SDS reflects contents that showed all the a samplessteady increase exhibited in CO a high2 capture capacity from 0.85 to 0.96 mmol g−1. Table 3 clearly reflects that all the samples exhibited a specific surface area of more than 321.3 m2 g−1 and a relatively small average pore size of less than high specific surface area of more than 321.3 m2 g−1 and a relatively small average pore size of less 1.88 nm. The crystal phase of MgO particles was further confirmed by X-ray diffraction. Therefore, the than 1.88 nm. The crystal phase of MgO particles was further confirmed by X-ray diffraction. results suggest that the SDS amount has merely little effect on the CO capture capacity. Therefore, the results suggest that the SDS amount has merely little effect2 on the CO2 capture capacity.

Figure 3. (a) The influence of SDS amount on the CO2 capture capacity; (b) XRD spectra of MgO Figure 3. (a) The inﬂuence of SDS amount on the CO2 capture capacity; (b) XRD spectra of MgO compounds with different amounts of SDS. compounds with different amounts of SDS.

Table 3. Synthetic conditions, texture parameters and CO2 adsorption capacities of SDS-assisted MgO with different loadings.

Hydrothermal Calcination BET Pore Average Ethanol/Water Loading Crystalline CO Uptake Sample Conditions Conditions Surface Area Volume Pore Size 2 Ratio (g) Size (nm) (mmol g−1) (◦C and h) (◦C and h) (m2 g−1) (cm3 g−1) (nm) MgO-SDS 100% water 0.08 120 and 12 400 and 5 348.3 0.36 2.04 4.9 0.85 MgO-SDS 100% water 0.1 120 and 12 400 and 5 321.3 0.30 1.88 5.4 0.96 MgO-SDS 100% water 0.5 120 and 12 400 and 5 501.3 0.64 2.55 2.1 0.93 MgO-SDS 100% water 1.0 120 and 12 400 and 5 413.4 0.39 2.31 10.0 0.90 Catalysts 2017, 7, 116 6 of 15

Table 3. Synthetic conditions, texture parameters and CO2 adsorption capacities of SDS-assisted MgO with different loadings.

BET Ethanol Hydrothermal Calcination Pore Average Crystall CO2 Loading Surface Sample /Water Conditions Conditions Volume Pore Size ine Size Uptake (g) Area Ratio (°C and h) (°C and h) (cm3 g−1) (nm) (nm) (mmol g−1) (m2 g−1) MgO- 100% 0.08 120 and 12 400 and 5 348.3 0.36 2.04 4.9 0.85 SDS water MgO- 100% 0.1 120 and 12 400 and 5 321.3 0.30 1.88 5.4 0.96 SDS water MgO- 100% 0.5 120 and 12 400 and 5 501.3 0.64 2.55 2.1 0.93 CatalystsSDS 2017water, 7, 116 6 of 15 MgO- 100% 1.0 120 and 12 400 and 5 413.4 0.39 2.31 10.0 0.90 SDS water 2.1.4. The Inﬂuence of Reaction Time on CO2 Capture 2.1.4. The Influence of Reaction Time on CO2 Capture The effect of reaction time was then investigated based on the sample with 0.1 g SDS. As shown in FigureThe4a, effect with of prolonging reaction time the was reaction then investigated time, the CObased2 capacityon the sample steadily with increased0.1 g SDS. fromAs shown 0.94 to 0.96in mmol Figure g 4a,−1 betweenwith prolonging six and the 12 reaction h and subsequentlytime, the CO2 capacity there was steadily a noticeable increased decline from 0.94 from to 0.96 to 0.55mmol mmol g− g1 −between1 between six 12and to 12 72 h h.and Figure subsequently4b shows there the was XRD a patternsnoticeable of decline the synthesized from 0.96 to samples. 0.55 −1 Whenmmol the g reaction between time 12 wasto 72 no h. longer Figure than 4b shows 18 h, allthe samples XRD patterns formed of mainly the synthesized MgO (JCPDS samples. 45-0946) When with the reaction time was no longer than 18 h, all samples formed mainly MgO (JCPDS 45-0946) with some MgCO3 (JCPDS 08-0479) impurity observed. With a further prolonging of the reaction time, the some MgCO3 (JCPDS 08-0479) impurity observed. With a further prolonging of the reaction time, the MgCO3 content steadily increased. Particularly for the 72 h sample, the majority phase became MgCO3 MgCO3 content steadily increased. Particularly for the 72 h sample, the majority phase became and the sharp and intense peaks indicated their highly crystalline nature. The major determinant lies MgCO3 and the sharp and intense peaks indicated their highly crystalline nature. The major in the degree of the urea hydrolysis process. With longer reaction time, the formation of MgCO was determinant lies in the degree of the urea hydrolysis process. With longer reaction time, the formation3 preferred, which led to a decreased CO capture capacity. Table4 demonstrates clearly that all the of MgCO3 was preferred, which led to a 2decreased CO2 capture capacity. Table 4 demonstrates clearly 2 −1 samplesthat all possess the samples high possess speciﬁc high surface specific area surface in the area range in ofthe 204.0 range to of 375.2 204.0 mto 375.2g , m and2 g‒ small1, and small average poreaverage size of pore 1.82 size to 2.19 of 1.82 nm. to 2.19 nm.

Figure 4. (a) The influence of reaction time on the CO2 capture capacity; (b) XRD spectra of MgO Figure 4. (a) The inﬂuence of reaction time on the CO2 capture capacity; (b) XRD spectra of MgO compounds at different reaction times. compounds at different reaction times.

Table 4. Synthetic conditions, texture parameters and CO2 adsorption capacities of SDS-assisted MgO with different reaction times.

Hydrothermal Calcination BET Pore Average Ethanol/Water Loading Crystalline CO Uptake Sample Conditions Conditions Surface Area Volume Pore Size 2 Ratio (g) Size (nm) (mmol g−1) (◦C and h) (◦C and h) (m2 g−1) (cm3 g−1) (nm) MgO-SDS 100% water 0.1 120 and 6 400 and 5 369.1 0.35 1.90 11.6 0.94 MgO-SDS 100% water 0.1 120 and 12 400 and 5 321.3 0.30 1.88 5.4 0.96 MgO-SDS 100% water 0.1 120 and 18 400 and 5 276.6 0.40 2.19 20.3 0.83 MgO-SDS 100% water 0.1 120 and 48 400 and 5 204.0 0.19 1.86 55.0 0.79 MgO-SDS 100% water 0.1 120 and 72 400 and 5 375.3 0.34 1.82 43.3 0.55

2.1.5. The Influence of the Reaction Temperature on CO2 Capture It is fairly clear that the formation of intermediates is largely affected by the reaction temperatures. Therefore, SDS-assisted MgO particles have been methodically prepared under different temperatures Catalysts 2017, 7, 116 7 of 15 at 100, 120, 140, and 160 ◦C, respectively, to further explore the influence of the reaction temperature. −1 Figure5a shows that the CO 2 uptake went up from 0.45 to 0.96 mmol g as the temperature increased from 100 to 120 ◦C and then declined from 0.96 to 0.15 mmol g−1 from 120 to 160 ◦C. To better understand the circumstances, we also explored the structural changes of all samples synthesized in the temperature range of 100-160 ◦C by XRD analyses. With the reaction temperature lower than ◦ 140 C, all samples formed mainly MgO (JCPDS 45-0946) with some MgCO3 (JCPDS 08-0479) impurity observed. As the diffraction peaks of MgCO3 became stronger with elevated temperatures, the MgCO3 ◦ content steadily increased. The 160 C sample presented the majority phase of MgCO3 with the sharp and intense peaks indicative of their highly crystalline nature. It is observed that MgO is the main components of the samples synthesized at low temperatures. It is interesting that specific surface ◦ area had good correlation with the CO2 capture capacity (Table5). For the 100 C sample, the slow hydrolysis rate at lower temperatures is favorable to form large particles. The higher crystallinity gave rise to a decline in specific surface area, which shows good consistent with the CO2 capture capacity. The samples fabricated at elevated temperatures are mainly MgCO3. The specific surface area is rather lowCatalysts due 2017 to, the 7, 116 high decomposition temperature, which coincides well with the CO2 capture capacity.8 of 15

Figure 5. (a) The influence of reaction temperature on the CO2 capture capacity; (b) XRD spectra of Figure 5. (a) The inﬂuence of reaction temperature on the CO2 capture capacity; (b) XRD spectra of MgO compounds at different reaction temperatures. MgO compounds at different reaction temperatures.

2.2. Characterization of SDS-Assisted MgO Table 5. Synthetic conditions, texture parameters, and CO2 adsorption capacities of SDS-assisted MgO withTo clarify different the reaction details temperatures.of the SDS-assisted MgO, XRD, BET, FTIR and SEM analyses were utilized to characterize the MgO sample synthesized with 0.1 g SDS at 120 °C for 12 h. XRD patterns (Figure Hydrothermal Calcination BET Pore Average Crystalline Ethanol/Water Loading CO Uptake 5b)Sample showed that the synthesizedConditions SDS-assistedConditions samplesSurface consisted Area ofVolume a largePore portion Size ofSize MgO along2 with Ratio (g) (mmol g−1) (◦C and h) (◦C and h) (m2 g−1) (cm3 g−1) (nm) (nm) a smaller part of MgCO3. It is noteworthy that urea will hydrolyze within the aqueous magnesium MgO-SDS 100% water 0.1 100 and 12 400 and 5 106.8 0.50 9.44 63.4 0.45 nitrateMgO-SDS solution100% water to form different 0.1 120 precursors and 12 400 under and 5 different 321.3 operating 0.30 conditions. 1.88 The 5.4 specific surface 0.96 MgO-SDS 100% water 0.1 140 and 12 400 and 5 181.7 0.22 2.40 50.1 0.69 areaMgO-SDS of the 100%SDS-assisted water 0.1MgO was 160 and calculated 12 400 andusing 5 the 41.1Brunauer-Emmett-Teller 0.07 3.49 (BET) 50.7 method 0.15 and the pore volume distribution was calculated by the Barrett-Joyner-Halenda (BJH) method. Figure 6a illustrates the N2 adsorption isotherms of the SDS-assisted samples. The isotherms were classified as type IV with a sharp H3 hysteresis loop, which reveals the existence of slit-type interparticle mesopores in the as-synthesized samples. This type of hysteresis loop is usually spotted on solids comprised of aggregates or agglomerates of particles forming slit shaped pores with a non-uniform size and/or shape [43]. The SDS-assisted MgO sample possesses a high specific surface area of 321.3 m2 g‒1, a high pore volume of 0.30 cm3 g−1, and a narrow micropore size distribution, with the majority centered at 1.88 nm (Figure 6b). It is obvious that the template SDS contributes tremendously to the fabrication of the porous structure in leaf-like MgO. The morphological changes of SDS-assisted MgO materials during the thermal treatment were also examined using SEM analyses. Figure 7a,b indicates that a well-organized lamellate structure of the precursors crystallites is noticeable. The disordered thin-layer morphology of SDS-assisted MgO nanosheet is highly likely correlated with the considerable disorganization caused by the removal of interlayer water molecules after the calcination process, as shown in Figure 7c,d. Besides, well- developed hexagonal prisms crystallites of MgO-TD (Figure 7e,f) are observed, which provides a good explanation in the great variations of CO2 uptake between the SDS-assisted MgO nanosheet and the MgO-TD due to the significant differences in their morphology.

Catalysts 2017, 7, 116 8 of 15

2.2. Characterization of SDS-Assisted MgO To clarify the details of the SDS-assisted MgO, XRD, BET, FTIR and SEM analyses were utilized to characterize the MgO sample synthesized with 0.1 g SDS at 120 ◦C for 12 h. XRD patterns (Figure5b) showed that the synthesized SDS-assisted samples consisted of a large portion of MgO along with a smaller part of MgCO3. It is noteworthy that urea will hydrolyze within the aqueous magnesium nitrate solution to form different precursors under different operating conditions. The specific surface area of the SDS-assisted MgO was calculated using the Brunauer-Emmett-Teller (BET) method and the pore volume distribution was calculated by the Barrett-Joyner-Halenda (BJH) method. Figure6a illustrates the N2 adsorption isotherms of the SDS-assisted samples. The isotherms were classified as type IV with a sharp H3 hysteresis loop, which reveals the existence of slit-type interparticle mesopores in the as-synthesized samples. This type of hysteresis loop is usually spotted on solids comprised of aggregates or agglomerates of particles forming slit shaped pores with a non-uniform size and/or shape [43]. The SDS-assisted MgO sample possesses a high specific surface area of 321.3 m2 g−1, a high pore volume of 0.30 cm3 g−1, and a narrow micropore size distribution, with the majority centered at 1.88 nm (Figure6b). It is obvious that the template SDS contributes tremendously to the fabrication of

Catalyststhe porous 2017, 7 structure, 116 in leaf-like MgO. 9 of 15

FigureFigure 6. 6.(a()a Nitrogen) Nitrogen adsorption-desorption adsorption-desorption isotherms isotherms and and ( b) pore size distributions of of SDS-assisted SDS-assisted MgOMgO samples samples generated generated hydrothermally hydrothermally and and calcined at 400 ◦°CC for 55 h.h.

The morphological changes of SDS-assisted MgO materials during the thermal treatment were also examined using SEM analyses. Figure7a,b indicates that a well-organized lamellate structure of the precursors crystallites is noticeable. The disordered thin-layer morphology of SDS-assisted MgO nanosheet is highly likely correlated with the considerable disorganization caused by the removal of interlayer water molecules after the calcination process, as shown in Figure7c,d. Besides, well-developed hexagonal prisms crystallites of MgO-TD (Figure7e,f) are observed, which provides a good explanation in the great variations of CO2 uptake between the SDS-assisted MgO nanosheet and the MgO-TD due to the signiﬁcant differences in their morphology.

Figure 7. SEM images of (a,b) precursors; (c,d) MgO nanosheets after calcination at 400 °C for 5 h; (e,f) MgO-TC of different magnifications, respectively.

Based on SEM analyses, the possible mechanism is proposed to explain the formation of the SDS- assisted MgO nanosheet in Scheme 1. The surfactants are usually utilized to prepare metal oxide nanoparticles, in which the polar groups directly interact with the particles surface and strongly influence their shape [44]. SDS is an anionic surfactant which can be adsorbed on the surface of precursors and can act as a directing agent by the interaction of negatively charged head groups with the magnesium ion. During the calcination of precursors, the surfactant molecules decomposed and induced the formation of small MgO nanocrystals with plenty of mesopores. The morphology was

Catalysts 2017, 7, 116 9 of 15

FigureCatalysts 6.2017 (a), 7Nitrogen, 116 adsorption-desorption isotherms and (b) pore size distributions of SDS-assisted9 of 15 MgO samples generated hydrothermally and calcined at 400 °C for 5 h.

FigureFigure 7. SEM 7. SEM images images of ( ofa,b (a), bprecursors;) precursors; (c (c,d,d)) MgO MgO nanosheetsnanosheets after after calcination calcination at 400 at 400◦C for °C 5 for h; 5 h; (e,f) MgO-TC(e,f) MgO-TC of different of different magnifications, magniﬁcations, respectively. respectively.

BasedBased on SEM on SEM analyses, analyses, the possible the possible mechanism mechanism is proposed is proposed to to explain explain the the formation formation of of the the SDS- assistedSDS-assisted MgO nanosheet MgO nanosheet in Scheme in Scheme 1. The1. The surfacta surfactantsnts are are usually utilized utilized to prepareto prepare metal metal oxide oxide nanoparticles,nanoparticles, in which in which the the polar polar groups groups directly directly interactwith with the the particles particles surface surface and and strongly strongly influenceinﬂuence their their shape shape [44]. [44 SDS]. SDS is isan an anionic anionic surf surfactantactant whichwhich can can be be adsorbed adsorbed on theon surfacethe surface of of precursorsprecursors and andcan canact actas a as directing a directing agent agent by by the the in interactionteraction of of negatively negatively charged charged head head groups groups with with the magnesiumthe magnesium ion. ion. During During the the calcination calcination of of prec precursors,ursors, the the surfactant surfactant molecules molecules decomposed decomposed and and inducedinduced the formation the formation of small of small MgO MgO nanocrystals nanocrystals withwith plenty plenty of of mesopores. mesopores. The The morphology morphology was was well preserved in the MgO nanomaterials during the thermal treatment as indicated in Figure7c,d. Amines are well-recognized for their capacity to bind carbon dioxide. As urea was used in the synthesis of SDS-assisted MgO nano-materials, a representative vibrational FTIR spectrum of the MgO nano-materials was presented in Figure8d to deny the presence of surface urea/amine species. The adsorption band at 1456 cm−1 corresponds to the asymmetric stretch of carbonate ions. The peaks at 1647 and 1087 cm−1 were assigned to the asymmetric and symmetric stretch of bidentate carbonates, respectively. The three distinct FTIR bands in the SDS-assisted MgO were chemisorbed on the surface of MgO. The characteristic peak of the OH group at 3398 cm−1 was observed as well. The existence of these weak peaks for various carbonate species may be resulted from the adsorption of the atmospheric CO2 on the surface of MgO [45,46]. Moreover, as there was no sign of the NH stretching vibration from 3150 to 3450 cm−1 [47,48], the absence of surface urea/amine species was clearly veriﬁed. Besides, no residual surfactants were detected. Catalysts 2017, 7, 116 10 of 15 Catalysts 2017, 7, 116 10 of 15 well preserved in the MgO nanomaterials during the thermal treatment as indicated in Figure 7c,d. Amineswell preserved are well-recognized in the MgO nanomaterials for their capacity during to the bind thermal carbon treatment dioxide. as Asindicated urea wasin Figure used 7c,d.in the synthesisAmines ofare SDS-assisted well-recognized MgO for nano-materials, their capacity toa rebindpresentative carbon dioxide. vibrational As urea FTIR was spectrum used in ofthe the MgOsynthesis nano-materials of SDS-assisted was presented MgO nano-materials, in Figure 8d toa redenypresentative the presence vibrational of surface FTIR urea/amine spectrum ofspecies. the MgO nano-materials was presented in Figure 8d to deny the presence of surface urea/amine species. The adsorption band at 1456 cm−1 corresponds to the asymmetric stretch of carbonate ions. The peaks The adsorption band at 1456 cm−1 corresponds to the asymmetric stretch of carbonate ions. The peaks at 1647 and 1087 cm−1 were assigned to the asymmetric and symmetric stretch of bidentate carbonates, at 1647 and 1087 cm−1 were assigned to the asymmetric and symmetric stretch of bidentate carbonates, respectively. The three distinct FTIR bands in the SDS-assisted MgO were chemisorbed on the surface respectively. The three distinct FTIR bands in the SDS-assisted MgO were chemisorbed on the surface of MgO. The characteristic peak of the OH group at 3398 cm−1 was observed as well. The existence of of MgO. The characteristic peak of the OH group at 3398 cm−1 was observed as well. The existence of these weak peaks for various carbonate species may be resulted from the adsorption of the these weak peaks for various carbonate species may be resulted from the adsorption of the atmospheric CO2 on the surface of MgO [45,46]. Moreover, as there was no sign of the NH stretching atmospheric CO2 on the surface of MgO [45,46]. Moreover, as there was no sign of the NH stretching −1 vibrationCatalystsvibration 2017from from, 7, 1163150 3150 to to 3450 3450 cmcm− 1 [47,48],[47,48], thethe absence ofof surfacsurfacee urea/amineurea/amine species species was was clearly10 clearly of 15 verified.verified. Besides, Besides, no no residual residual surfactants surfactants werewere detected.detected.

Scheme 1. Schematic illustration of the formation mechanism of SDS-assisted MgO nanosheets. SchemeScheme 1. 1.SchematicSchematic illustration illustration of of the the formation formation mechanism ofof SDS-assisted SDS-assisted MgO MgO nanosheets. nanosheets.

Figure 8. CO2 uptake over repeated cycles by SDS-assisted MgO particles of (a) CO2 adsorption in 100% CO2 for 30 min at 300 °C and desorption in 100% N2 for 30 min at 350 °C; (b) CO2 adsorption in FigureFigure 8. 8.COCO2 uptake2 uptake over over repeated repeated cycles cycles by by SDS-assisted MgOMgOparticles particles of of (a ()a CO) CO2 adsorption2 adsorption in in 100% CO2 for 30 min at 300 °C and desorption in 100% N2 for 30 min at 400 °C; (c) CO2 adsorption in ◦ ◦ 100%100% CO CO2 for2 for 30 30 min min at at 300 300 °CC and and desorption desorption in 100%100% NN22 forfor 3030 min min at at 350 350 C;°C; (b ()b CO) CO2 adsorption2 adsorption in in 50% CO2 and 50% N2 for 30 min at 300 °C and desorption in 100% N2 for 30 min at 400 °C; (d) FTIR ◦ ◦ 100%100% CO CO2 for2 for 30 30 min min at at 300 300 °CC and and desorption desorption in 100% NN22 forfor 3030 min min at at 400 400 C;°C; (c ()c CO) CO2 adsorption2 adsorption in in spectra of SDS-assisted MgO nanomaterial.◦ ◦ 50%50% CO CO2 and2 and 50% 50% N N2 for2 for 30 30 min min at at 300 300 °CC and and desorption inin 100%100% N N22for for 30 30 min min at at 400 400C; °C; (d )(d FTIR) FTIR spectraspectra of ofSDS-assisted SDS-assisted MgO MgO nanomaterial. nanomaterial.

2.3. Cycling Stability Tests

High regenerability under repeated cycles of CO2 adsorption and desorption is an obligatory requirement for CO2 adsorbents practically utilized in CSU systems. Therefore, the CO2 adsorption/desorption cycling stability of the synthetic SDS-assisted MgO was comprehensively examined via typical TSA procedures with circulation times fixed at 20 cycles. Figure 8a,b show the continuous adsorption/desorption cycling stability of the synthetic SDS-assisted MgO performed at Catalystsdifferent2017 desorption, 7, 116 temperatures (350 or 400 °C). During the first few cycles, the CO2 capture11 of 15 capacity of SDS-assisted MgO particles degraded slightly, and in later cycles the uptake displays an excellent regenerability in terms of both rate and extent of uptake of CO2. Notably, the comparatively obvious decline in CO uptake initially occurred at 350 ◦C may be attributed to the incomplete obvious decline in CO2 uptake initially occurred at 350 °C may be attributed to the incomplete decomposition of MgCO due to the unsuitable desorption temperature. It is noteworthy that although decomposition of MgCO3 3 due to the unsuitable desorption temperature. It is noteworthy that the CO uptake of SDS-assisted MgO nanoparticles has met a gradual decline of 0.1 mmol g−1 although2 the CO2 uptake of SDS-assisted MgO nanoparticles has met a gradual decline of 0.1 mmol when exposed to 50% CO atmosphere during the adsorption period, as shown in Figure8c, the g−1 when exposed to 50% CO2 2 atmosphere during the adsorption period, as shown in Figure 8c, the −1 adsorption capacity becamebecame considerablyconsiderably stablestable withwith aa reversiblereversible capacitycapacity ofof moremore thanthan 0.40.4 mmolmmol gg−1.. In general, it can be concluded that the CO adsorption performance became weakened to a certain In general, it can be concluded that the CO22 adsorption performance became weakened to a certain extent with the decrease in CO concentration. Figure8b indicated that the CO capture capacity of extent with the decrease in CO22 concentration. Figure 8b indicated that the CO22 capture capacity of −1 the SDS-assistedSDS-assisted MgOMgO samplesample eventuallyeventually reachesreaches aa steadysteady valuevalue ofof 0.50.5 mmolmmol gg−1 without further significantsignificant degradation,degradation, suggesting suggesting a greata great prospect prospect of application of application in the sorption-enhanced in the sorption-enhanced hydrogen productionhydrogen production processes [processes49,50]. [49,50]. Additionally, we have explored the main reason for thethe capacitycapacity decaydecay during several cycles using bothboth BET BET and and SEM SEM techniques. techniques. After adsorption/desorptionAfter adsorption/desorption process process for 20 times, for the20 disorderedtimes, the thin-layerdisordered morphology thin-layer morphology of the SDS-assisted of the SDS- MgOassisted nanosheet MgO (Figure nanosheet9a) was (Figure totally 9a) destroyed was totally into piecesdestroyed and into a high pieces desorption and a high temperature desorption required temperature promoted required agglomeration promoted of agglomeration leaf-like particles of leaf- into alike block particles mass (Figureinto a 9blockb). Besides, mass (Figure BET analysis 9b). Besides, showed BET that afteranalysis 4 cycles, showed the specificthat after surface 4 cycles, area the of 2 −1 SDS-assistedspecific surface MgO area nanomaterial of SDS-assisted showed MgO a downtrend nanomaterial from showed 321.3 to a 260downtrend m g , whichfrom 321.3 shows to good 260 consistencym2 g−1, which with shows the good observed consistency SEM images. with the The observed results SEM demonstrated images. The that results the main demonstrated reason for that the capacitythe main decay reason observed for the in capacity the sorbents decay over observed repeated in cycles the sorbents was the aggregationover repeated due cycles to the grievouswas the desorptionaggregation condition due to the leading grievous to an desorption irreversible condition decline inleading active to sites an onirreversible the surface decline and severe in active damage sites toon thethe specificsurface morphology.and severe damage to the specific morphology.

Figure 9. SEM images of (a,b) SDS-assisted MgO nanoparticles after adsorption/desorption for 20 Figure 9. SEM images of (a,b) SDS-assisted MgO nanoparticles after adsorption/desorption for 20 cycles. cycles.

3. Materials and Methods

3.1. Materials

Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, 99%), urea (CO(NH2)2, 99%), and sodium dodecyl sulfate (SDS) were purchased from Sinopharm Chemical Reagent Co Ltd (Shanghai, China). All chemicals were used as received without further puriﬁcation. Deionized water was utilized in all the experimental processes. Catalysts 2017, 7, 116 12 of 15

3.2. Sample Preparation Samples of SDS-assisted porous MgO were prepared by urea hydrolysis assisted precipitation of magnesium hydroxide from magnesium nitrate aqueous solution followed by thermal decomposition of the precipitated magnesium hydroxide. Typically, 3 g of urea (0.05 mol CO(NH2)2) and appropriate amounts of magnesium nitrate hexahydrate (2.56 g of Mg(NO3)2·6H2O for maintaining urea:Mg mole ratio of 5) were mixed in 50 mL of water with the addition of certain amounts of sodium dodecyl sulfate (0.1 g SDS) and stirred vigorously in a Teﬂon lined stainless steel autoclave at room temperature. The autoclave was then treated hydrothermally in a preheated oven at 120 ◦C for 12 h. The resulting precipitates were separated by ﬁltration and repeatedly washed with DI water several times and dried at 70 ◦C overnight in the oven. Dried powders were next ground in a crucible, and calcined at 400 ◦C for 5 h in air. In all cases, the temperature of the furnace was increased at a very slow rate of 1 ◦C min−1 until the desired temperatures were attained.

3.3. Sample Characterization Phase compositions and crystallographic structures of the samples were examined by X-ray diffractometry (XRD: Shimadzu XRD-7000 diffractometer, power 40 KV, 30 mA) using Cu Kα radiation in the 2θ range from 5◦ to 80◦ with a scanning rate of 5◦ min−1. As there is no diffraction peak observed in the range from 5◦ to 80◦, we only presented the XRD patterns in the range of 20◦ to 80◦ to make them clearer. Size and morphology of the sample grains were measured via scanning electron microscopy (SEM, S-4800). The Brunauer-Emmett-Teller (BET, Builder SSA-7000) method was utilized to calculate the speciﬁc surface area of the prepared samples using the adsorption branch acquired at a relative pressure (P/P0) range of 0.05–0.30. Molecular speciation of the samples was explored by Fourier transform infrared spectroscopy (FTIR: Bruker Vertex 70 spectrophotometer).

3.4. Evaluation of CO2 Adsorption Capacity

CO2 uptake by the samples was determined from variations in sample weight under a flow of 100% dry CO2 at atmospheric pressure as measured with a thermogravimetric analyzer (TGA, Q50 TA Instrument). The flow rates of N2 and CO2 were controlled by a mass flow controller (MFC). All samples were pre-calcined at 450 ◦C for 1 h under a flow of 100% high purity nitrogen −1 (20 mL min ) giving consideration to remove the preabsorbed species (atmospheric CO2, water, etc.). The measurements started by switching the gas from nitrogen to CO2 with a constant flow of high −1 ◦ purity CO2 (1 atm, 40 mL min ) at 300 C for 4 h. With regard for the regenerability of the samples ◦ under repeated cycles, CO2 adsorption was performed at 300 C in the flow of 100% CO2 for 30 min and desorption was performed at 350 ◦C or 400 ◦C in the flow of 100% nitrogen for 30 min. The cycles of adsorption and desorption were repeated 20 times

4. Conclusions This work sheds light on the synthesis and utilization of an original SDS-assisted MgO-based porous adsorbent for CO2 capture at intermediate temperatures. The mesoporous MgO adsorbents are successfully synthesized by a simple urea hydrolysis method from aqueous magnesium nitrate solution with a fully disordered layer structure. With the assistance provided by the surfactant SDS the fabricated materials exhibited a uniform ordered narrow mesopores distribution. Comparison of CO2 capture performances indicated that with the same surfactant SDS, the SDS content had little effect on CO2 capture capacity. The effect of ethanol to water ratios, reaction time, and reaction temperature have been thoroughly investigated. The highest performance was found to dissolve SDS in pure DI water. The superlative reaction temperature was 120 ◦C and at least 12 h was needed for the urea hydrolysis process to be completed comprehensively. The XRD patterns provided a clearer understanding of the reaction mechanism. Typically, the SDS-assisted MgO materials possessed the −1 2 −1 highest CO2 capture capacity of 0.96 mmol g with a high speciﬁc surface area of 321.3 m g Catalysts 2017, 7, 116 13 of 15

◦ at moderate temperatures (~300 C). We suspect that the high CO2 capture capacity should be related to both the high speciﬁc surface area and high surface defects or active adsorption sties, etc. The regenerability of the newly fabricated nanomaterials has been further investigated using a −1 TSA process, which demonstrated a pretty excellent reversible CO2 uptake of more than 0.5 mmol g through multi cycles. The mesoporous MgO materials which possess awesome reactivity and stability in multi cycle processes show signiﬁcant potential to halt the increasing global CO2 emissions.

Acknowledgments: This work was supported by the Fundamental Research Funds for the Central Universities (2016ZCQ03), Beijing Excellent Young Scholar (2015000026833ZK11), the National Natural Science Foundation of China (51622801, 51572029, and 51308045), and the Xu Guangqi grant. Author Contributions: Q.W. conceived and designed the experiments; W.G. performed the experiments and analyzed the data; T.Z. and B.L. contributed reagents/materials/analysis tools; W.G. and Q.W. wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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