catalysts 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.