Adsorption of CO2 on MgAl layered double hydroxides: Effect of intercalated anion and alkaline etching time?

Yan-Yan Feng(馮艷艷), Xiao-Di Niu(牛瀟迪), Yong-Hui Xu(徐永輝), and Wen Yang(楊文),?

1Department of Chemistry and Bioengineering,Guilin University of Technology,Guilin 541004,China

2Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials,

Department of Chemistry and Bioengineering,Guilin University of Technology,Guilin 541004,China

Keywords: CO2 adsorption,MgAl-LDHs,one-pot hydrothermal method,intercalated anion,alkaline etching

1. Introduction

In recent years,with the fast speed of population growth and social development, human beings need more energy,along with the continuous progress of modernization and automation.[1]Up to date, most of the energy comes from fossil resources; however, the inadequate combustion of fossil resources and other irregular use of fossil resources not only wasted the earth’s precious resources,but also produced a large number of toxic and harmful gases and fuel waste,resulting in serious environmental pollution.[2]The burning of fossil resources has produced a large amount of CO2, which can cause obvious greenhouse effect and other serious environmental problems on earth.[3–5]This phenomenon has attracted worldwide attention,and in order to reduce CO2emission, the relevant factories have to install and design the process technology for CO2capture. Hence, it is necessary to strengthen the research on CO2adsorption.

There are many solid adsorbents to deal with CO2, but more efficient methods with fast adsorption rate and low cost are needed. According to the adsorption temperature,CO2adsorbents can be divided into three types: low-temperature adsorbent(adsorption temperature below 200?C),[6–8]mediumtemperature adsorbent (adsorption temperature between 200 and 400?C),[9]and high-temperature adsorbent (adsorption temperature above 400?C).[10]For low-temperature adsorption,great efforts have been devoted to advancing the capture performance. Guo et al.[6]prepared a series of porous activated carbons derived from sugarcane bagasse,and compared with the physically activated carbons,the NaOH-activated carbon showed high dynamic CO2uptake of 1.31 mmol/g at 60?C under 10%CO2flowing gas.Verrecchia et al.[7]investigated the three main factors affecting the synthesis of zeolites from coal fly ash, and then achieved the adsorbent with excellent CO2adsorption performance as compared to commercial 13X.Chen et al.[8]synthesized the premodified Li/Al hydrotalcite impregnated with polyethylenimine(PEI),and with PEI loading of 40%,the functionalized adsorbent obtained the highest adsorption capacity of 1.723 mmol/g at 50?C.

Among abundant low-temperature solid adsorbents,MgAl layered double hydroxides (MgAl-LDHs) have got a great deal of attention. LDHs, consisting of positively charged layers and interlayer anions, belong to anionic layered compounds. Owing to the mobility and strong interchangeability of interlayer anions,[11–13]LDHs have been applied in many fields,such as adsorption,[14–16]catalysis,[17–19]electrochemistry,[20,21]and flame retardant.[22,23]In terms of adsorption applications, MgAl-LDHs could not only remove CO2from industrial exhaust,but also collect the anionic pollutants in the environment and gas pollution,etc. To date,numerous research on MgAl-LDHs has focused on improving the preparation methods,including changing the molar ratio of Mg/Al,[24]modifying with alkali metals,replacing the intercalation anions,[25]and so on.[26,27]Among various preparation methods, the co-precipitation method is the most commonly used. However, for the obtained adsorbents, it is disadvantageous to CO2adsorption due to their small specific surface area and stacked structure. Therefore, it is of great significance to promote the CO2uptake of MgAl-LDHs with loose and porous structure.

In the present study, MgAl-LDHs are obtained via the one-pot hydrothermal method. Due to the amphoteric nature of Al species, NaOH is used to remove the Al species in the LDHs,[28]and appropriate treating time with NaOH solution would contribute meaningfully to forming some nanopores and increasing the available specific surface area, which ultimately expose more effective adsorption sites for CO2uptake.Consequently, we investigate the effect of intercalated anion and alkaline etching time on the structure and morphology of MgAl-LDHs for use in CO2uptake. Adjusting the intercalated anion and alkaline etching time of LDHs can tailor the structural characteristics of the resultant adsorbents,which in turn tune their adsorption performances,for MgAl-LDHs with high specific surface area and large pore volume are beneficial to the CO2adsorption process. After that,the adsorbents are characterized by x-ray diffraction (XRD), N2adsorption,scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR), respectively, followed by adsorption measurement of CO2. In order to explore the adsorption mechanism,the adsorption data are fitted by the firstorder,pseudo-second-order and Elovich models,respectively.In general, this work would provide meaningful guidance for designing MgAl-LDHs to improve CO2adsorption performance.

2. Experimental details

2.1. Materials

In experiment, magnesium nitrate hexahydrate (99.0%,Mg(NO3)2·6H2O), aluminum nitrate nonahydrate (99.0%,Al(NO3)3·9H2O), magnesium chloride hexahydrate (98.0%,MgCl2·6H2O), aluminum chloride hexahydrate (97.0%,AlCl3·6H2O), magnesium acetate tetrahydrate (99.0%,Mg(CH3COO)2·4H2O), aluminum acetate (Al(CH3COO)3),urea (99.0%, H2NCONH2), and sodium hydroxide (96.0%,NaOH) were analytical grade and purchased from Xilong Chemical Co. Ltd., Guangzhou, China. All the chemicals were used without further purification.

2.2. Preparation of the samples

2.2.1. Preparation of MgAl-LDHs intercalated with different anions

2.2.2. Preparation of the adsorbents alkaline-etched by 3.0 mol/L NaOH

The adsorbents alkaline-etched by 3.0 mol/L NaOH were prepared as follows.[28]A 50 mL NaOH solution with 0.2 g of MgAl(Cl)was fully stirred at room temperature for 30 min,followed by alkaline etching reaction at 95?C for 3,6 and 9 h,respectively. Finally, the product was washed with water and ethanol for several times and dried at 100?C overnight,named as MgAl(Cl)-3,MgAl(Cl)-6,and MgAl(Cl)-9,respectively.

2.3. Characterization

XRD patterns of the samples were characterized by a power x-ray diffractometer(PANalytical X’Pert3)with Cu Kαradiation.[21]The pore structure of the samples was obtained using N2physisorption at 77 K by a Micromeritics ASAP Tristar II 3020 equipment (Micromeritics Instrument Corporation). Prior to the analysis, the sample was degassed at 150?C for 3 h. The morphology of the samples was observed using a field-emission scanning electron microscope(SEM, Hitachi SU5000). FT-IR spectra were conducted on a Fourier transform infrared spectrometer(Nicolet iS10,American Thermo Scientific Company)in the wavelength range of 400–4000 cm?1.

2.4. CO2 adsorption capacity evaluation

The CO2adsorption performance of the samples was tested by a thermogravimetric analyzer (SDT Q500, TA).[31]Appropriately 10 mg of the sample was put into an alumina pan, and He was injected as a protective gas with a gas flow of 100 mL/min. Next, the sample was raised to 75?C with a heating rate of 5?C/min. After stabilizing at 75?C for 30 min,the gas was switched to CO2with a gas flow of 100 mL/min,and the adsorption process was kept at 75?C for 60 min. The adsorption amount of CO2was calculated according to the weight change of the sample during the adsorption process.

The first-order (Eq. (1)),[32]the pseudo-second-order(Eq. (2)),[33]and Elovich (Eq. (3))[34]kinetic models are applied over the as-prepared adsorbents,and the kinetic models are expressed as follows:

where qe,1(qe,2) is the adsorption capacity of MgAl-LDHs at equilibrium (mg/g); qtis the adsorption capacity (mg/g)of CO2by the adsorbent at time t (min); k1(1/min), k2(g/(mg·min)) and k are the first-order, pseudo-second-order and Elovich rate constants, respectively; β is the relationship between surface coverage and activation energy.

3. Results and discussion

3.1. Characterization analyses

The XRD patterns of the samples are presented in Fig.1.Characteristic diffraction features of the LDHs structure appear in all MgAl-LDHs samples at 2θ of 11.5?, 23.6?,35.0?,39.6?and 47.1?, corresponding to the reflections of (003),(006), (009), (015), and (018), respectively.[35]The sample MgAl(Cl) synthesized with chloride salts as precursors shows higher crystallinity than the samples MgAl(NO) and MgAl(Ac), suggesting that the crystallinity is affected by the precursor nature. In addition, the characteristic peaks of Mg5(CO3)4(OH)2·4H2O(brucite)are observed for MgAl(Ac)using acetate salts as precursors.As for the alkaline-etched adsorbents shown in Fig.1(b), the characteristic peak of LDHs also appears, and the characteristic peaks at 2θ of 23.6?and 47.1?change from single peak to double peaks. As the samples undergo alkaline etching, the characteristic peaks of Mg6Al2(OH)18·4.5H2O would obviously appear, while the characteristic peaks of Mg4Al2(OH)14·3H2O decrease,revealing that the Al species are partly removed. With the further increase of the alkaline etching time, the characteristic peaks of Mg4Al2(OH)14·3H2O are gradually weakened.

N2adsorption isotherms of the samples are depicted in Fig.2. All the samples exhibit typical type-III isotherms with low N2uptake at low relative pressure(P/P0)and high N2uptake at high P/P0. For MgAl-LDHs intercalated with different anions, the adsorption volume of MgAl(Cl)is highest among the three samples, while the adsorption volume of MgAl(Ac)is lowest. With the alkaline etching of NaOH,the N2adsorption capacity depicts a reverse U-shaped trend. As the alkaline etching time increases,the N2uptake of MgAl(Cl)-3 gradually increases,and the adsorption capacity of MgAl(Cl)-6 reaches the maximum. With further increase of the alkaline etching time, the adsorption capacity of MgAl(Cl)-9 obviously decreases. The above results indicate that the alkaline etching time has a significant influence on the pore structure of the MgAl-LDHs.

Fig.1. XRD patterns of the samples.

Fig.2. N2 adsorption isotherms of the samples.

Figure 3 displays pore size distributions (PSD) of the samples computed with the Barrett–Joyner–Halenda (BJH)method. As shown in Fig.3, all the samples possess a distribution of pores within the diameters of 0–50 nm, confirming the formation of mesoporous materials,which is attributed to the stacked structure of LDHs. It is noted that the mesopore volume of MgAl(Cl) within 2–50 nm is higher than that of MgAl(Ac)and MgAl(NO),in good agreement with the N2adsorption isotherms,and is further expanded after alkaline etching treatment. With the alkaline etching time of 9 h,the pores of MgAl(Cl)-9 collapse and the corresponding mesopore volume declines. For the alkaline-etched adsorbents, MgAl(Cl)-6 possesses huge number of micropores and mesopores, revealing that alkaline etching could make the significant contribution of micropores and mesopores,especially mesopores.This also explains why the sample MgAl(Cl)-6 achieves an enhanced CO2adsorption capacity.

Figure 4 presents the Brunauer–Emmett–Teller specific surface area (BET SSA) and pore volume (PV) of the samples. The variation of intercalated anions leads to differences in the BET SSA and PV. Among the intercalated adsorbents,the MgAl(Cl)using chloride salts as precursors displays the highest BET SSA and PV,indicating that the porosity of MgAl(Cl)is well-developed. Compared with others,the MgAl(NO)sample has the lowest BET SSA,being 86.5%and 66.0% of the MgAl(Ac) and MgAl(Cl), respectively. The alkaline etching of NaOH results in high surface areas and large pore volumes,and the MgAl(Cl)-6 sample achieves the largest BET SSA of 28.13 m2/g and PV of 0.0765 cm3/g.

Fig.3. Pore size distributions of the samples obtained by the BJH adsorption branch.

SEM characterization results of the samples are shown in Fig.5. Sheet-like LDHs with smooth surface of MgAl(NO)and MgAl(Cl)using chloride salts and nitrate salts as precursors can be observed in Figs. 5(a) and 5(c), while disorderly stacked structure of MgAl(Ac)with acetate salts as precursors can be seen from Fig.5(b). The different morphologies presented by the MgAl-LDHs with different precursors may be due to the nature of intercalated anions. After alkaline etching treatment, the layered structure begins to be destroyed, suggesting that the Al species of the layered structure are partly removed, along with the increase of BET SSA and PV of the modified LDHs,which would be conducive to the exposure of the active sites, so as to improve the adsorption performance of the adsorbent. Nevertheless, the longer the alkaline etching time, the more serious the structural damage, and with the alkaline etching time being 9 h, the sheet-like structure of MgAl(Cl)-9 is severely crushed and aggregated, resulting in poor dispersibility, which is consistent with the results of N2adsorption analysis.

Fig.4. BET specific surface area(a)and pore volume(b)of the samples.

Fig.5. SEM images of the samples: (a)MgAl(NO),(b)MgAl(Ac),(c)MgAl(Cl),(d)MgAl(Cl)-3,(e)MgAl(Cl)-6,and(f)MgAl(Cl)-9.

Fig.6. FT-IR spectra of the samples.

3.2. CO2 adsorption performances of the adsorbents

The effect of intercalated anion and alkaline etching time on CO2adsorption for MgAl-LDH adsorbents was performed,and in order to analyze the adsorption mechanism of CO2on the adsorbent, the adsorption data are fitted by the firstorder kinetic equation,the pseudo-second-order kinetic equation and the Elovich model,respectively. Figure 7(a)exhibits the CO2adsorption data of the MgAl-LDHs with different precursors.Obviously,the intercalated anion could influence CO2adsorption behavior,and it is meaningful to apply the suitable precursor to prompt CO2adsorption performance of MgAl-LDHs. Among the three samples, the adsorbent MgAl(Cl)possesses the highest CO2adsorption capacity. The impact of intercalated anion for MgAl-LDHs on CO2adsorption behavior and adsorption kinetics is illustrated in Figs.7(b)–7(d)and Table 1. Compared with the first-order kinetic equation and the Elovich model, the correlation coefficient R2for the pseudo-second-order kinetic equation is close to 1.0,suggesting that the fitting curves are in good agreement with the adsorption data and the adsorption process is more suitable to be described by the pseudo-second-order kinetic equation.

Fig.7. CO2 adsorption isotherms of the samples prepared with different precursors fitted by the first-order kinetic model,the pseudo-secondorder kinetic model,and the Elovich kinetic model.

In the initial stage of adsorption, the adsorption rate of the samples is faster; however,as the adsorption process progresses, the adsorption rate slows down and the adsorption equilibrium would be reached after a period of time. It is found that the CO2adsorption process of the samples could be divided into two stages: rapid surface reaction stage and CO2diffusion controlling stage. In the rapid surface reaction stage, the large slope of the curve indicates the fast adsorption rate. Among the intercalated samples,the adsorption rate of MgAl(Ac) is higher, and thus its CO2adsorption capacity hardly changes after 15 min. The rapid reaction stage is the important stage of adsorption, that is, CO2reacts with the alkaline binding sites on the adsorbent surface; with the progress of adsorption,the adsorption site of the adsorbent is gradually covered,which slows down the adsorption rate. As the adsorption process of CO2for MgAl(NO)and MgAl(Ac)reaches saturation, the adsorption capacity of MgAl(Cl) still increases,ascribing to the developed pore structure and abundant alkaline adsorption sites. In addition, it should be noted that the crystallinity of MgAl-LDHs has a non-negligible effect on CO2adsorption performance, and high crystallinity leads to favorable CO2uptake.

Figure 8 shows CO2uptake of alkaline-etched MgAl(Cl)adsorbents. It can be seen that the adsorption performance of the adsorbents is significantly improved after alkaline etching, and the sample MgAl(Cl)-6 with alkaline etching time of 6 h has the highest adsorption amount of 16.3 mg/g. As the alkaline etching time continues to extend, the CO2uptake of MgAl(Cl)-9 sharply decreases, due to the collapse of pore structure and the fragmentized sheet-structure. Hence,the CO2adsorption performance is greatly influenced by alkaline etching time,and appropriate alkaline etching time can advance the contact between CO2molecules and the adsorbent. The fitting results of the kinetic models (displayed in Figs. 8(b)–8(d) and Table 1) show that both the first-order and pseudo-second-order kinetic models are applicable, and thus the adsorption process includes physical adsorption and chemical adsorption; however, the correlation coefficients of the pseudo-second-order kinetic model are slightly higher than those of the first-order kinetic model, indicating that the adsorption process of the adsorbent on CO2is more obedient to the pseudo-second-order kinetic model than to the first-order kinetic model. The sample MgAl(Cl)-6 owns the fastest adsorption rate of 0.041 g/(mg·min), while MgAl(Cl)without alkaline etching has the lowest adsorption rate of 0.020 g/(mg·min), consistent with CO2adsorption capacities of the alkaline-etched samples, which indicates that alkaline etching treatment is conducive to the internal diffusion of CO2during the adsorption process.

Fig.8. CO2 adsorption isotherms of the samples prepared with various alkaline etching times fitted by the first-order kinetic model, the pseudo-second-order kinetic model,and the Elovich kinetic model.

Table 1. Adsorption parameters from kinetic models of CO2 adsorption data.

4. Conclusions

MgAl layered double hydroxides have been synthesized by the one-pot hydrothermal method to investigate the effect of intercalated anion and alkaline etching time on CO2adsorption. By means of XRD,N2adsorption,SEM,FT-IR and CO2adsorption analyses,the results demonstrate that the adsorbent MgAl(Cl)using chloride salts as precursors shows a high crystallinity,sheet-like LDHs with smooth surface and developed pore structures.In contrast,MgAl(Ac)employing acetate salts as precursors displays a poor crystallinity, disorderly stacked structure and unsatisfactory pore structure; and correspondingly,MgAl(Cl)possesses the highest CO2uptake among the three intercalated samples. With alkaline etching of NaOH,the adsorption performance of the adsorbents is significantly improved, and MgAl(Cl)-6 with alkaline etching time of 6 h has the largest adsorption amount of 16.3 mg/g, which could be ascribed to well-developed porosity. The alkaline adsorption active sites over the surface of the adsorbent are fully exposed,which is conducive to the combination of acid gas CO2with it, thereby enhancing the CO2capture. As the alkaline etching time further increases, the CO2adsorption capacity of MgAl(Cl)-9 obviously reduces, mainly due to the collapse of pore structure and the fragmentized sheet-structure. Therefore, this work would provide a valuable idea for the rational design of MgAl-LDHs for enhancing CO2adsorption.