Tao Jiang,Fei Xiao,Yujun Zhao,Shengping Wang*,Xinbin Ma

Key Laboratory for Green Chemical Technology,School of Chemical Engineering and Technology,Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),Tianjin University,Tianjin 300072,China

Keywords:Layered double hydroxides CO2 capture CaO-based Organic ions

ABSTRACT CO2,one of the main components of greenhouse gases,increased rapidly because of the growing use of fossil fuels.And CaO sorbents possess the capability to be used in capture of CO2 at high temperature.In the work,Ca-Al complex oxides derived from citrate and stearate intercalated layered double hydroxides were fabricated and their CO2 adsorption capacity was compared with that from CO32- intercalated layered double hydroxides.The results presented that the sorbents (Ca/Al=5) with Ca-Al-citrate layered double hydroxides as precursors performed best and displayed remarkable CO2 capture capacity of 52.0%(mass) at the carbonization temperature of 600 °C without distinct recession during cycling adsorption/desorption tests.The excellent CO2 adsorption capacity of the sorbent was ascribed to its smaller crystallite size of calcinated particles,optimized pore size distribution as well as homogeneous distributed Ca and Al in the sorbent.

1.Introduction

The anthropogenic emission of CO2is considered as the major contributor of global warming [1-3].As one of the most favorable ways to mitigate greenhouse gas impact,the process of calcium looping,which contains carbonation and calcination periods,has gained wide public attention in recent years[4-7].In this process,CaO usually captures CO2at over 550 °C and transforms into CaCO3.And then CaO can regenerate upon thermal treatment[8,9].In the process of CO2adsorption,the CaCO3product layer mainly contains Ca2+andions.During the movement ofions in the CaCO3product layer,in order to meet the principle of electrical neutrality,moves in the opposite direction in the CaCO3product layer.So,the CO2adsorption process is mainly completed by the three-step reactions at the interface of CaO-CaCO3as the Eqs.(1)-(3)show.And the diffusion of CO2and mobility of O2-are the key to the capture of CO2in the early stage of diffusioncontrolled carbonation reaction[10-12].However,the CO2uptake capacity of CaO sorbents decays rapidly during multiple adsorption/desorption cycles,which is because of the bigger molar volume of CaCO3than CaO as well as lower Tammann temperature of CaCO3(～533 °C) than the general adsorption temperature[13-16].

Hydrotalcite-like compounds,also known as layered double hydroxides (LDHs),are promising inorganic materials which can serve as precursors for CO2capture at high temperatures [17-19].There are positively charged metal-hydroxide layers with charge-balancing ions and interlaminar H2O molecules in their structure [20-22].

Due to their anti-sintering properties,the mixed oxides gained from CaO-based LDHs have been considered as favorable sorbents and received worldwide attention in recent years.For example,Changet al.has developed a sequence of techniques to synthesize Ca-Al LDHs,including conventional coprecipitation [23],sol-gel[24],AlOOH-supported [25],and reverse microemulsion [26].Among these methods,the Ca-Al mixed oxidesviathe coprecipitation technique showed the highest CO2uptake of 48.5% (mass)in 30 cycles at the adsorption temperature of 600 ℃,while CO2capture capacity of the samples prepared through the reverse microemulsion experienced a relatively larger decline from 40%to 30% (mass) during 30 cycles [23,26].

On the other hand,several studies have demonstrated that the interlayer ions exist a positive impact on capture capacity of Mg-Al LDHs derived sorbents [27-32].Upon calcination,some anions are decomposed to release gases such as CO2and H2O,which will facilitate the formation of pores in the obtained mixed oxides,thus resulting in an increase in the surface basicity and promoting the capture of CO2.Wanget al.[28] and Hutsonet al.[29]discussed influence of ions includingSO42-,Fe(CN6)4-and drew a conclusion that the amorphous complex oxides gained from Mg-Al-CO3LDHs exhibited the highest CO2capture performance.Besides,Wanget al.[28] presented the preparation of complex oxides gained from organic Mg-Al LDHs with stearate anions intercalating into the layer.Owing to the decomposition of long carbon chains,this CO2sorbent exhibited a higher CO2uptake of 5.06%-5.50%(mass)which was twice more than the adsorption value of the conventional Mg-Al-CO3LDHs.Kouet al.[30] introduced methanol to the preparation process of LDHs prior to drying,which can effectively increase the specific surface area and improve the porosity of the sorbents by substituting the water molecules of MgAl-CO3LDHs.Besides,the aqueous miscible organic solvent treatment was used to exfoliate Mg3-Al-CO3LDH into nanosheets with a flower-like morphology [31].And the exfoliated LDH structure exposed more interlayered CO2active sites.What’s more,Mg-Al LDH was prepared and further successfully modified with cetyltrimethyl ammonium bromide[32].Organic modification of LDH with long-chain anionic surfactants results in the elongation of interlayer spacing.And the addition of organically modified layered double hydroxide (OLDH) as nanomaterial enhanced the physical and chemical properties of the composites.

However,it is known that the adsorption temperature of Mg-Al LDH derived sorbents is ranged from 200 to 400 °C,while the Ca-Al mixed oxides derived from Ca-Al LDH absorb CO2above the temperature of 600°C.Until now,there has been little research focusing on the preparation of Ca-Al LDHs intercalated with organic ions.Furthermore,the investigation of interlaminar ions on the performance of high-temperature CO2sorbents with Ca-Al LDHs as precursors has not been reported yet.Herein,Ca-Al LDHs with organic interlayer ions of citrate and stearate were prepared using the conventional coprecipitation method.In addition,CO2adsorption performances of Ca-Al complex oxides gained from citrate and stearate intercalated LDHs were compared with that fromintercalated LDHs.

2.Experimental

2.1.A preparation

Ca-Al LDHs intercalated with citrate ions were prepared as follows: a mixed nitrate metal solution (0.375 mol·L-1) with a Ca2+/Al3+ratio of 5 was combined with 0.05 mol·L-1sodium citrate (or stearic acid),and 50 ml NaOH (3.4 mol·L-1) was added drop-wise to the mixed liquor.The mixed liquor was then stirred at ordinary temperature for 24 h.Subsequently,the mixture was filtered and washed with deionized water thoroughly,after which products were dried at 100 °C overnight and then calcined (600 °C for 1 h).Finally,the gained complex oxides of LDHs were obtained and served as CO2sorbents.For comparison,Ca-Al LDHs intercalated with stearate ions were synthesized as mentioned above except that stearic acid replace sodium citrate and the stirring temperature of mixed liquor was 80°C,while Ca-Al LDHs intercalated withwere synthesized as mentioned above except that the NaOH and Na2CO3were added drop-wise to the mixed nitrate metal solution.

2.2.Adsorbent characterization

2.2.1.TGA

A thermogravimetric analyzer (TGA,STA449F3,NETZSCH,Germany) was used to execute the thermogravimetric analysis.The experiment was conducted in a gas stream (total flow rate of 50 ml·min-1),in which the heating rate was of 10 °C·min-1with an eventual temperature of 900 °C.

2.2.2.XRD

Powder X-ray diffraction(XRD)patterns were recorded on a D8 Focus(Bruker,Germany)with a Cu Kα(40 kV,40 mA)radiation in the diffraction angle (2θ) range of 5°-90° (2°-70°) at a scanning rate of 8 (°)·min-1(2 (°)·min-1).

2.2.3.SEM

Scanning electron microscope (SEM)/EDX was utilized to inspect surface morphology of the LDHs and gained complex oxides by a Hitachi S4800 field-emission microscope (Hitachi,Japan)operating at 10.0 kV.

2.2.4.FTIr

Fourier transform infrared spectroscopy(FTIR)was recorded on a Nicolet 6700 spectrometer (Thermo Fisher Scientific,USA) by using KBr pellets,which was equipped with a DTGS detector.

2.2.5.N2 adsorption-desorption

Textual properties of the LDHs and their derived adsorbents were determined using a Tristar 3000 analyzer (Micromeritics,USA).

2.3.CO2 adsorption analysis

CO2uptake capacity measurement of sorbents was implemented on a thermogravimetric analyzer.About 10 mg sorbent was loaded into a sample pan for each test.And the sorbent sample was firstly heated from room temperature to 800 °C and maintained for 20 min in the presence of a N2atmosphere to remove all impurities and the pre-adsorbed CO2,then cooled down to 600 °C to start the CO2adsorption/desorption cycles.Cycling testing conditions were as follows: an adsorption process was executed at 600 °C for 45 min under the atmosphere of 50% (vol)CO2/50% (vol) N2.And a desorption process was performed at 700 °C for 20 min in 100% N2(50 ml·min-1).CO2uptake capacity(% (mass)) in TGA was calculated using the following equation:

wherem0means the initial mass of the sorbent samples,mimeans the mass of the carbonated sorbents aftericycles.

3.Results and Discussion

3.1.Materials characterization

Fig.1.XRD patterns of LDHs with different interlayer anions.

The crystalline structures of samples intercalated with carbonate,stearate and citrate anions(annotated as Ca-Al-CO3/stearate/citrate-LDH),were studied by XRD analysis.Fig.1 displayed the characteristic peaks of LDHs corresponding to the basal planes of(003) and (006),which were observed for all these three samples.By substituting C17H35COO-and C5H7O5COO-for,the reflection peak corresponding to (003) plane shifted to a much lower value,from 11.5° for Ca-Al-CO3-LDH to 1.9° for Ca-Al-stearate-LDH and 7.1° for Ca-Al-citrate-LDH,respectively.Calculatedviathe Bragg’s law,the distance of internal layer was increased from 0.80 nm for Ca-Al-CO3-LDH to 4.65 nm for Ca-Al-stearate-LDH and 1.24 nm for Ca-Al-citrate-LDH,respectively.In fact,during the synthesis of hydrotalcite-like compounds,the organic anions were intercalated in the interlaminar space as well as absorbed onto the surface of LDH materials [33].On one side,the inter-layer distance would be enlarged by the intercalation of organic ions,leading to the aggregation between particles and thus decreasing the specific surface area.On the other side,the adsorption of organic anions onto the surface would inhibit the growth of LDH particles,thus making the specific surface area bigger.Therefore,it can be concluded that the effect of intercalation of stearate ions was stronger than that of surface adsorption,thus forming relatively longer distance of internal layer.And the effect of surface adsorption of citrate ions was more remarkable compared to the effect of intercalation,leading to the shorter distance of internal layer.Moreover,Fig.1(c) shows that the characteristic peaks of LDH were broad and the intensities were weak,demonstrating the lower crystallinity as well as a smaller crystal size of Ca-Al--citrate-LDH.

Fig.2.XRD patterns of Ca-Al mixed oxides derived from LDHs with different interlayer anions.

Fig.3.FTIR spectra of the prepared LDHs with different interlayer anions.

Fig.4.TGA analysis of LDHs with different interlayer anions.

The characteristic peaks of LDHs disappeared after the sample was calcined at 600 ℃and then the as-synthesized LDH precursors transformed into Ca-Al complex oxides.It can be seen in Fig.2 that the crystallization phase of CaO was detected among the calcined LDHs intercalated with carbonate,stearate and citrate anions,respectively.Besides,intense peak reflections of crystallization phase of CaCO3were still observed,which may be due to the chemical reaction of CaO with CO2during the cooling process of calcination/storage stage of the sorbents or the imperfect decomposition of CaCO3.In addition,the peak reflection of Al compound was not found in XRD patterns,which indicated that Al compound was highly dispersed or amorphous in synthesized CaO sorbent samples.For the as-synthesized LDH derived sorbents,CaO phase is the active component to uptake CO2,while the Al compound is the inert component to prevent the sintering of CaO.Finally,crystallite size of CaO and CaCO3was tabulated in Table 1,utilizing the Debye-Scherrer formula.Crystallite sizes of CaO of these three kinds of samples were comparable.Nevertheless,the crystallite size of CaCO3of the citrate and stearate sample were smaller than that of the carbonate sample.This suggests that the decomposition of citrate and stearate anions could result in smaller crystallite size of the calcinated products,which was agreement with the results of Wanget al[28].Furthermore,the citrate sample possessed the smallest crystallite size of CaCO3,which was only 19.2 nm.This was attributed to the combination of citrate anions with Ca2+to form the chelate firstly,postponing the precipitation process,which further forming smaller LDH particles,thus making the calcinated particles smaller [34].

Table 1 Crystallite sizes of CaCO3 and CaO in the mixed oxides derived from Ca-Al LDHs with different interlayer anions

Fig.3 showed the FTIR spectra of the Ca-Al-LDHs intercalated with carbonate,stearate and citrate anions,respectively.The bands which present strong overlap between 840 and 650 cm-1were ascribed to the metal-oxygen stretches,corresponding to lattice vibration modes.The existence of the interlayer citrate and stearate ions is proved by two intense bands at about 1398-1581 cm-1corresponding to the symmetric and asymmetric stretching modes of the carboxylate groups,while Ca-Al--carbonate-LDH showed adsorption bands ofat 1412 cm-1[35,36].In addition,the Ca-Al-stearate-LDH showed adsorption bands at 2845 and 2914 cm-1,which were ascribed to the C-H stretching vibration for the existence of the-CH3and-CH2groups of long chain stearate ions.The presence of the broad band centered at 3450-3550 cm-1is related with water and hydroxyl O-H stretch,which is universal among layered double hydroxides[37].

Fig.5.SEM images of LDHs with different interlayer anions: (a) citrate,(b) carbonate,(c) stearate,and their derived mixed oxides: (d) carbonate,(e) citrate,(f) stearate.

Fig.6.Adsorption/desorption isotherms((a),(c)) and pore size distributions ((b),(d)) of LDHs with different interlayer anions and Ca-Al LDHs derived mixed oxides with different interlayer anions.

Fig.4 presented the TGA analysis of Ca-Al-LDHs intercalated with carbonate,stearate and citrate anions,which all exhibited that there are three-stage steps of mass loss.The first stage within the scope of 100-200°C is mainly because of the transfer of physically adsorbed interlaminar water.The elimination of hydroxyls on the layer and interlaminar ion groups led to the second stage within the scope of 200-500 °C.And decomposition of CaCO3resulted in the final step above 600°C.It was obvious that the mass loss of these three samples followed the trend of Ca-Al-stearate-LDH ＞Ca-Al-citrate-LDH ＞Ca-Al-CO3-LDH,which was owing to the different molecular weight of stearate,citrate,and carbonate anions.

The morphology of samples intercalated with carbonate,stearate and citrate anions and derived absorbents were scrutinized by SEM.As shown in Fig.5(a),(b),(c),all of the synthetic LDHs possessed plate-like structure.Especially,the morphology of Ca-Al--citrate-LDH was relatively porous and the platelets were small,implying the lower crystallinity which further corroborated the XRD data.Compared to the regular hexagonal plate-like structure of Ca-Al-CO3-LDH,Ca-Al-stearate-LDH displayed a draped and cross-linked plate-like morphology,and its platelets were much larger.

Fig.7.Initial capacity comparison of Ca-Al mixed oxides derived from LDHs with different interlayer anions.

After calcination at 600 °C,the Ca-Al-LDHs intercalated with carbonate,stearate and citrate anions were transformed into Ca-Al composite components.Fig.5(d),(e),(f) displayed that Ca-Al-citrate-LDH derived complex oxides possessed a much looser structure compared to that derived from Ca-Al-CO3-LDH.However,the particles of Ca-Al-stearate-LDH derived complex oxides aggregated severely,which may be related to the draped and cross-linked plate-like morphology of Ca-Al-stearate-LDH.

Fig.8.CO2 adsorption capacities with the number of cycles of Ca-Al mixed oxides derived from LDHs with different interlayer anions.

Typical N2adsorption/desorption isotherms for Ca-Al-stearate/CO3/citrate-LDH and their derived complex oxides were presented in Fig.6(a) and (c),inferring characteristics of types II and IV(IUPAC classification)and exposing the existence of mesoporous and macroporous structures in the six samples.The specific surface area of Ca-Al-citrate-LDH,Ca-Al-CO3-LDH and Ca-Al-stearate-LDH were 23.6,15.8 and 20.8 m2·g-1,respectively.Fig.6(b)showed the pore size distribution of Ca-Al LDHs with various interlayer anions.It’s obviously that there were two ranges of pore distribution inside the carbonate-LDH,including 3-4 and 20-50 nm.And it could be found that predominant pore sizes of the stearate-LDH were within the scope of 3-7 and 10-50 nm,while that of citrate-LDH was mainly mesoporous within the scope of 3-10 nm as well as the macroporous in the range of 50-80 nm.It’s noted that the apparent change occurred on pore distribution of the derived oxides samples intercalated with different organic ions after calcination process.As seen in Fig.6(d),the pore distribution of the cirtrate sample turned to the scope of 5-25 nm,which was probably due to the thermal decomposition of citrate,forming the corresponding pore structure.Especially,comprared with stearate-LDH without calcination,the pore volume of the stearate sample declined visibly,of which the specific surface area went down to 9.9 m2·g-1(felled by half),hinting that the collapse of the pore structure resulted by the decomposition of stearate.In comparison,there was little change on the pore distribution of the carbonate sample,of which the specific surface area reduced slightly to 14.6 m2·g-1.According to literatures,the higher the proportion of macropores (＞50 nm) of the particles,the smaller the surface area and pore volume of adsorbents,causing the less reaction sites for CO2sorption.Meanwhile,high percentage of mesopores (2-50 nm) of the sorbents could equilibrate gas diffusion and lead to adequate surface area for carbonation of CaO,which could be also conductive to enhancing the structural stability of the CaO sorbents [38,39].Therefore,it was expected that the citrate sample would display better CO2adsorption capacity.

Fig.9.SEM-EDX of Ca-Al-citrate LDH derived mixed oxides (after 17 cycles).

Fig.10.CO2 adsorption capacities with the number of cycles of Ca-Al mixed oxides derived from LDHs with different Ca/Al ratios.

Fig.11.SEM images of Ca-Al-citrate-LDHs derived mixed oxides with different Ca/Al ratios: (a) Ca/Al=2,(b) Ca/Al=5,(c) Ca/Al=7.

3.2.CO2 capture capacities of the derived composite oxides

3.2.1.Effect of interlayer anions

The CO2capture performances of LDHs derived sorbents with various interlayer anions including citrate,stearate and carbonate(annotated as sorbent-citrate,sorbent-stearate,sorbent-CO3,respectively) were determined utilizing a thermogravimetric analyzer.And Fig.7 displayed CO2uptake performance curves in the initial cycle of the complex oxides.It was observed that the sorbent-citrate owned the highest adsorption performance of 52.0% (mass),while sorbent-stearate had the lowest CO2uptake of 27.8% (mass).Since the valence of stearate was lower than that of other anions and the molecular weight of stearate was the highest,the additive amount of stearate for the LDH synthesis was largely higher than that of the other two anions,which suggested that stearate as an overweight anion could disrupt the layered structure during calcination [40].As mentioned in Section 3.1,the specific surface area of the stearate sample felled by half compared with that of the stearate-LDH sample without calcination,hinting that the collapse of the pore structure resulted by the decomposition of stearate.Hence,the relatively small surface area resulted in the poor performance of the stearate-LDH sorbent.As shown in Fig.8,a decline from 38.2%to 31.7%(mass) in CO2capture performance could be viewed on the sorbent-CO3during the 17 cycles,while the sorbent-citrate and sorbent-stearate displayed no capacity deactivation.An interesting observation for sorbent-citrate and sorbent-stearate was the increase of adsorption performance during the initial five cycles,implying the self-activation phenomenon resulted from the phase change of hard skeleton to soft skeleton and reconstructing of sorbents [41].

The distribution of Ca and Al of sorbent-citrate after 17 adsorption-desorption cycles was evaluated using SEM/EDX measurements.Fig.9 demonstrated that the first diagram was backscattered electron image of the entire chemical elements,while the bright spots in the other three represented the distribution of Ca,Al,and O.As can be seen,the distribution of Ca and Al on the surface of sorbent-citrate was uniform.Generally speaking,Al components typically work as durable barriers to reduce the sintering of the sorbent particles.Hence,the high-dispersion Al inside CaO particles could effectively improve the thermostability of the sample at high adsorption and desorption temperatures even during multiple cycles.

3.2.2.Effect of Ca/Al ratios

The CO2adsorption capacities of Ca-Al-citrate-LDHs derived sorbents samples with various Ca/Al ratios were presented in Fig.10.It was noted that the CO2uptake increased from 20.9% to 52.0% (mass) as the Ca/Al ratio rose to 5:1 from 2:1 in the first cycle.When Ca/Al ratio reached 7:1,the adsorption performance fell to 44.9% (mass) instead.Additionally,all of the sorbents showed favorable stability during the 17 adsorption-desorption cycles.

The morphologies of the Ca-Al-citrate-LDHs derived sorbents with various Ca/Al ratios were characterized using SEM.As displayed in Fig.11,the 5:1 mixed oxides exhibited a looser and porous structure when compared to that of 2:1 and 7:1 mixed oxides.It was obvious that the surface porosity followed the trend of 5:1＞7:1 ＞2:1,which was well consistent with the trend of CO2uptake values.

4.Conclusions

Mixed oxides with citrate and stearate intercalated Ca-Al-LDHs as precursors were synthesizedviathe co-precipitation process for the first time.Compared with sorbent-stearate and sorbentcarbonate,sorbent-citrate with Ca/Al ratio of 5 showed the excellent CO2capture capacity of 52.0% (mass) and excellent durability after 17 cycling tests at the adsorption temperature of 600°C.SEM/EDX characterization of the spent sorbent-citrate indicated that CaO and Al2O3binary complex oxides were highly scattered even after multiple cycles,providing a promising anti-sintering ability of the sorbent.The excellent CO2adsorption performance of sorbent-citrate was also attributed to the smaller crystallite size of calcinated particles and its mesoporous structure.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support by National Key Research and Development Program of China (2017YFB0603300),the Program for New Century Excellent Talents in University (NCET-13-0411) and the Program of Introducing Talents of Discipline to Universities (B06006)is gratefully acknowledged.