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High temperature CO2 adsorption by mesoporous silica supported magnesium aluminum mixed oxide
Accepted Manuscript High Temperature CO2 Adsorption by Mesoporous Silica Supported Magnesium Aluminum Mixed Oxide Aamir Hanif, Soumen Dasgupta, Anshu Nanoti PII: DOI: Reference:
S1385-8947(15)00857-8 http://dx.doi.org/10.1016/j.cej.2015.06.018 CEJ 13789
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
5 May 2015 3 June 2015 4 June 2015
Please cite this article as: A. Hanif, S. Dasgupta, A. Nanoti, High Temperature CO2 Adsorption by Mesoporous Silica Supported Magnesium Aluminum Mixed Oxide, Chemical Engineering Journal (2015), doi: http://dx.doi.org/ 10.1016/j.cej.2015.06.018
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High Temperature CO2 Adsorption by Mesoporous Silica Supported Magnesium Aluminum Mixed Oxide Aamir Hanif, Soumen Dasgupta, Anshu Nanoti* CSIR- Indian Institute of Petroleum, Dehradun 248005, India ABSTRACT Magnesium aluminum mixed oxide (Mg-Al-O) was deposited in various weight percentages into SBA-15 mesoporous silica through excess solution impregnation of magnesium and aluminum nitrate salts and subsequent high temperature calcination. In all the syntheses the Mg:Al mole ratio was fixed at 2. High temperature CO2 uptake capacity of these composites were measured in the temperature range 300-400 oC and were compared with the capacity of the unsupported mixed oxide phase with same Mg:Al mole ratio of 2. An improvement in the specific equilibrium capacity as well as uptake kinetics for CO2 was observed in the composite adsorbent after an optimum loading of the mixed oxide. The result could be explained on the basis of better accessibility of the active sites for CO2 adsorption following dispersion of the mixed oxide on to the high surface area SBA-15 support. The CO2 adsorption capacity is higher than recent reports of Mg-Al-O supported on Graphene oxide and CNT where there is additional complication of support degradation during thermal decomposition of Mg/Al salt precursors to corresponding metal oxides during adsorbent preparation.
Keywords: High temperature CO2 adsorption, Mg/Al oxide-SBA15 composites, Active phase dispersion, Adsorption isotherms, Adsorption kinetics
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1. INTRODUCTION Carbon dioxide (CO2) capture research is getting lot of attention in recent years as it has potential for providing breakthrough technologies to combat anthropogenic global warming phenomenon [1]. With the total annual CO2 emissions at nearly 36 gigatonnes, coal combustion is known to have largest contribution since coal based thermal power plants are the major concentrated source for CO2 emission [2,3]. Capture from coal fired power plants thus makes good sense for controlling the rising levels of CO2 in the atmosphere. Capture of CO2 can be done via pre as well as post combustion strategies. Higher level of CO2 concentration in the shifted synthetic (syn) gas available from pre-combustion route compared to that in the flue gas from post combustion route should favor the former route for CO2 capture. There are different possible techniques for CO2 capture from shifted syn gas like solvent absorption, membrane separation and adsorption [4]. Amongst these techniques absorption is a relatively mature technology particularly in the context of post combustion CO2 capture. However due to the requirement of low temperature for CO2 absorption and a relatively higher temperature for the spent solvent regeneration and also keeping in view that shifted syn gas from pre-combustion route is available at very high temperature (250-500 oC), the process becomes unattractive as lot of heating and cooling duties will be involved. There are also issues related with solvent degradation over repeated cycles [5]. The membrane technology has also not matured enough for the said application [6]. In this context adsorption is emerging as a promising technique. However due to high temperature of the shifted syn gas conventional adsorbents such as zeolites, activated carbons, etc. have low CO2 adsorption capacity and may not be suitable for this application. Thus there is a need to develop special class of high temperature CO2 sorbents [7,8]. Such sorbents will also
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find application in sorption enhanced water gas shift reactions for production of hydrogen [9]. The temperature of the fuel gas stream, available after the high temperature sour water gas shift reaction in the pre-combustion scheme, is in the range of 250-500oC and has CO2 partial pressure of around 15-30 bar [8]. High temperature adsorbents will have advantages over low or moderate temperature adsorbents in the context of pre-combustion CO2 capture as the former will impart greater efficiency in the capture process since cooling of the fuel gas stream prior to CO2 capture will not be required. Adsorbents based on calcium oxide [10,11] lithium zirconates [12], lithium silicates [13] and hydrotalcites [14–16] have been evaluated for CO2 capture in this temperature domain. Calcium oxide, lithium zirconates and silicates have high CO2 adsorption capacity reaching up to 11.6 mmol/g at 1 bar pressure [17]. However these materials have slow CO2 uptake kinetics and require very high temperature (above 800oC) for complete regeneration. The adsorption capacity also decreases rapidly over multiple cycles due to sintering of the active phase [18]. In comparison hydrotalcites have relatively lower CO2 capacity in the range of 0.1- 1.4 mmol/g [17]. But they are still considered a better choice as they can be regenerated with minimum temperature swing and shows better retention of capacity over many cycles of adsorptionregeneration[19,20]. Magnesium aluminum mixed oxide (Mg-Al-O) formed during high temperature decomposition of hydrotalcite is regarded as the active CO2 adsorbing phase. It is conceivable that an efficient dispersion of the Mg-Al-O phase on a high surface area support could impart better surface characteristics and accessibility of active sites for CO2 adsorption. Another advantage of high dispersion will be reduced sintering of the mixed metal oxide phase during thermal regeneration of the spent adsorbent. This should help in maintaining CO2 adsorption capacity over multi-cycle adsorption regeneration experiments. There are few earlier reports on high temperature CO2 adsorption by supported Mg-Al-O where carbon materials such
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as graphene oxide [21–23], carbon nanotubes [24] and carbon nanofibre [25] have been used as support. Due to poor interaction of mixed oxide with carbon surface an inefficient dispersion was observed even at relatively lower oxide loading. Ordered mesoporous silica SBA-15 is known to be a better support for dispersion of large number of metal oxides [26–29]. However to the best of our knowledge there is no report of SBA-15 supported Mg-Al-O mixed oxides for CO2 adsorption in mid high temperature regime (200-400 oC) as prevalent in pre-combustion carbon dioxide capture though recently mesoporous Mg-Al-O mixed oxide has shown promise as CO2 adsorbent in the lower temperature range of 60-150oC [30]. In the present work we have synthesized composites with very high dispersion of Mg-Al-O mixed oxides on SBA-15 which then have been studied for high temperature CO2 adsorption over a temperature range of 300-400 oC. The mole ratio of Mg/Al in synthesized composites has been fixed at 2 which is reported to be optimum ratio for CO2 adsorption [31]. The results are compared with previously reported Graphene oxide and CNT supported Mg-Al-O mixed oxides as well as with an unsupported Mg-Al-O phase and also with a commercial hydrotalcite material. The SBA-15 supported composite adsorbents have improved CO2 adsorption capacity and CO2 uptake kinetics. The result could be explained on the basis of better accessibility of the active sites for CO2 adsorption following dispersion of the mixed oxide on to the high surface area SBA-15 support.
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2. EXPERIMENTAL 2.1 Adsorbent Synthesis Synthesis of SBA-15 support was done following a reported procedure [32]. In a typical synthesis 20 g of polyethylene oxide-block-poly propylene oxide-block polyethylene oxide based copolymer surfactant (Pluronic P123, Av. mol wt.: 5800, Sigma-Aldrich) was dissolved in 150 ml distilled water by vigorous stirring. In this solution, 600 ml of 2M HCl was added and the resultant mixture was stirred vigorously for nearly about an hour. Tetraethyl ortho silicate (Sigma Aldrich, 42 g) was added drop wise to the above solution at room temperature with continuous stirring till a slurry was formed. The slurry was stirred at 35 oC for 24 hours and then aged at 80 oC for next 12 hours. The solid was then separated by filtration, washed repeatedly with warm distilled water several times and then dried at room temperature under ambient atmosphere. The dried material was then calcined at a temperature of 550 oC in a muffle furnace under air flow of 200 ml/min for 6 hours. The temperature ramp rate for attaining the final calcinations temperature was 1oC/min. Magnesium aluminum mixed oxide having Mg:Al mole ratio 2:1 was dispersed into the calcined SBA-15 in different weight percentages by following procedure. Calculated amount of the magnesium nitrate hexahydrate and aluminum nitrate nonahydrate was dissolved in distilled water in which a definite amount of calcined SBA-15 powder was added. The resulting suspension was stirred slowly for 12 hours at room temperature and then the water was slowly evaporated at 60 oC with continuous stirring till a thick paste was obtained. This paste was then heated at 500 oC in a muffle furnace for 5 hours using a temperature ramp of 2 oC/min to attain the final temperature. With the above mentioned procedure samples with Mg:Al:Si mole ratios of 2:1:5.1; 2:1:2.18; 2:1:0.9 and 2:1:0.4 were synthesized which corresponds to 30, 50, 70 and 85 % of MgO-Al2O3
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mixed oxide in the resultant composite. The samples were named as MAS-30, MAS-50, MAS70, and MAS-85 where the numerals denote the weight percentage of MgO-Al2O3 mixed oxide. One more sample denoted as MAS-100 was synthesized in the similar way where no SBA-15 was added and thus it comprised purely of magnesium aluminum oxide. 2.2 Characterization Characterization of the synthesized adsorbents was done by PXRD, surface area, pore size distribution analysis, and SEM. The PXRD patterns were taken with the help of a Bruker (Germany) advanced difractometer using Cu Kα radiation. Pore size and BET surface area analysis were carried out in Micromeritics Tristar analyzer. Prior to surface area and pore size analysis the samples were activated by heating in a muffle furnace at 500 oC for 6 hours and then degassed under helium flow at 350 oC for 12 hours. For SEM micrographs and elemental mapping analysis Quanta (Netherlands) 200F FESEM was used.
2.3 Equilibrium isotherm and CO2 uptake rate measurements Gravimetric analyser IGA-001 (Hiden Isochema UK) was used to measure CO2 equilibrium isotherm of the synthesized adsorbents in the temperature range 300-400 oC. About 100 mg of adsorbent was loaded in a sample holder of the analyzer and pretreated at 450 oC under vacuum (10-6 millibar) for 12 hours till no further decrease in weight of the adsorbent was observed. After pretreatment of the adsorbent, the temperature of adsorption cell was lowered to target temperature of isotherm under vacuum. Further isotherm measurements were carried out by introducing CO2 at different pressures ranging from 50 millibar to 15 bar and at each pressure CO2 uptake was measured gravimetrically giving enough time to attain equilibrium. Equilibrium isotherms up to 15 bar were measured for all the synthesized adsorbents. The kinetics of CO2 uptake were monitored by measuring the adsorbent weight increase at different time intervals
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following a CO2 pulse pressurization to 50 millibar from a high vaccum level of 10-6 millibar. The CO2 uptake was monitored up to 50 minutes since beyond this time the uptake became negligible. A commercial hydrotalcite adsorbent with Mg:Al mole ratio 2:1 (Pural MG63-HT from SASOL-Germany) was also tested for high temperature CO2 adsorption for comparison purpose. For the multi-cycle study the adsorbent, after every adsorption experiment, was regenerated in situ at 450 oC under high vacuum (10-6 millibar), till it regained its original weight, before starting adsorption step of the next cycle. 3. RESULTS AND DISCUSSION Magnesium nitrate is known to decompose at around 450 oC to periclase magnesium oxide [33] whereas aluminum nitrate decomposes to an amorphous aluminum oxide over the temperature range of 200-500 oC [34] whereas mesoporous silica SBA-15 retains its ordered structure in this temperature range [35]. Thus in all our syntheses the final calcination of the as synthesized materials were carried out at 500 oC. The XRD pattern (Fig. 1) of pure Mg/Al mixed oxide (MAS-100) in the 2θ range 30-70 degree shows diffraction peaks due to magnesium oxide periclase phase (JCPDS No: 43-1022) and small amount of spinel MgAl2O4 (JCPDS NO: 822424) phase. This XRD pattern is similar to that of periclase phase obtained from calcination of hydrotalcite with Mg/Al mole ratio of 2 except that the peaks are broader suggesting smaller crystallite size of periclase phase derived from hydrotalcite calcination. However XRD (Fig. 1) of the samples namely MAS-30, MAS-50 and MAS-70 in the same 2θ range shows absence of any such peaks suggesting a complete dispersion of the Mg/Al mixed oxide phase on the mesoporous support up to weight percent loading of 70%. Similar complete disappearance of metal oxide PXRD peaks due to efficient dispersion on SBA-15 has also been reported earlier [27–29]. However when the percentage loading of the mixed oxide was increased further to 85% as in MAS-85 the peaks corresponding to the periclase phase appeared.
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Presence of distinct SBA-15 phase could be detected in the composites up to mixed oxide loading of 70 weight % as evident from low angle XRD data in the 2θ region 0.5-2 degree (Fig. 2). The d100 peak maxima shifts gradually towards lower 2θ value as the mixed oxide loading was increased to 70 %. This indicates gradual expansion of hexagonal lattice parameter of SBA-15 probably due to onset of disorganization in the mesoporous structure. This may be attributed to high degree of mixed oxide loading. It is observed that at 85% loading of mixed oxide (MAS85) the peak due to SBA-15 is very diffuse indicating nearly complete absence of the ordered mesoporous structure. The nitrogen adsorption-desorption isotherms (Fig. 3) and BJH pore size distribution data (Fig. 4) indicates pore narrowing, substantial decrease of pore volume and shifting of pore size maxima to lower values with increased loading of Mg-Al mixed oxide on SBA-15 support. This is indicative of deposition of mixed oxide inside the mesopores of SBA-15. At higher mixed oxide loadings of 70% and 85% the pores are nearly completely blocked. The surface area and pore volume data of the synthesized materials are given in Table 1. Both surface area and pore volume also decreases gradually with increasing loading of the mixed oxide. The wall thickness of SBA-15 hexagonal channels (Table 2) was calculated from the PXRD d100-spacing data and BJH adsorption pore diameter using formulas reported elsewhere [36]. As expected the wall thickness of SBA-15 hexagonal channels increases with increased weight loading of mixed oxide into SBA-15. The SEM micrographs show that the Styrofoam like morphology of MAS-30 and MAS-50 (Figure 5) are very similar to that of pure SBA-15 phase indicating deposition of the mixed oxide phase on the internal channel surface of mesoporous silica up to 50 weight % loading of the Mg/Al mixed oxide. As the loading of the mixed oxide was further increased to 70% level
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(MAS-70) a secondary growth of crystallites, having thin plate type morphology typical of Mg/Al mixed oxide, appeared over SBA-15 crystallites. SEM of MAS-85 shows absence of typical SBA-15 structure and appearance of larger and thicker plate like growth similar to that of the pure mixed oxide phase MAS-100. SEM observation thus supports XRD data that at 85% mixed oxide loading the SBA-15 structure disappears completely and diffraction peaks of periclase phase becomes visible. Further SEM-EDX elemental mapping Mg, Al and Si of MAS30 (Figure 6) shows that the dispersion of the active Mg/Al mixed oxide phase is homogenous on the SBA-15 support. Carbon dioxide isotherms of all the synthesized adsorbents were measured at 350 oC. Both Langmuir and a chemisorption model proposed by Lee et al [37] were used to predict the experimental data for the optimized MAS-70 composite with highest CO2 adsorbing capacity, shown in Figure 7. The Lee model predicts the experimental data better especially at higher pressures (Figure 7) thus this model is used for the prediction of isotherms of all other adsorbents as shown in Figure 8. The equation for Lee et al [37] model is ݊∗ =
mK େ P[1 + ሺa + 1ሻK ୖ P ୟ ] [1 + K େ P + K େ K ୖ P ሺୟାଵሻ ]
Where n* is the carbon dioxide capacity (mol/Kg) at pressure P (bar), ‘a’ is a dimensionless constant, KC and KR are the equilibrium parameters of direct surface interaction and complexation respectively and ‘m’ is monolayer capacity. The chemisorption model proposed by Lee et al, [37] assumes that the adsorption involves two simultaneous steps viz direct interaction of CO2 with adsorbent surface and secondary complexation of CO2 molecules with that of the adsorbed CO2 phase. The complexation may become more prevalent at the higher pressures and at lower pressures only surface interaction
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plays major role in adsorption. Probably for the same reason there is some deviation observed at lower values from the model. The various model parameters are enlisted in the Table 3. At 350 oC and 15 bar Carbon dioxide pressure the order of the carbon dioxide capacity in synthesised materials varies as SBA15
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The effect of temperature on CO2 adsorption capacity by the highest adsorbing composite MAS70 is shown in Figure 9. It is observed that the capacity remains similar at temperature range of 300-350 oC. However a fall in capacity is observed at 400 oC. This may be probably due to the reason that desorption of chemisorbed carbon dioxide from the adsorbent is more predominant at 400oC than at 300 and 350oC as reported by Hutson et al [38]. A comparison of the of CO2 uptake kinetics between the composite MAS-70 and pure mixed oxide MAS-100 was made at 350 oC and 50 mbar pressure. It is observed that at the end of nearly one hour the average CO2 uptake rate of MAS-70 was 1.2 times more than uptake rate of MAS-100. Since the kinetic comparison was made at very low pressure at which only adsorbate surface interaction is prevalent a single parameter pseudo second order kinetics model [39] was used to predict the experimental kinetic data. The equation for this model is. ݍ௧ = ݍ ൬1 −
1 ൰ ݇ݍ ݐ+ 1
Where qt = CO2 uptake in mmol/g at time ‘t’, k is rate constants of CO2 interaction with adsorbent surface. This model equation fits well with our experimental kinetic data as shown in Figure 10. The rate constants obtained from the model fit also show that the rate of adsorption on MAS-70 (k=5.81mmol.g-1.min) is higher than that of adsorption rate constant of MAS-100 (k=5.12 mmol.g-1.min). The stability of carbon dioxide capacity over multiple cycles was tested (Figure 11) with the adsorbent MAS-70 at 50 millibar and temperature of 300 oC. After every adsorption cycle the adsorbent was regenerated at 450 oC under evacuation for 2 hrs till the initial weight of the adsorbent was regained. The capacity almost stabilized over four cycles of regeneration and adsorption. After the second cycle no further decrease in capacity was observed. 4. Conclusion
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Dispersion on magnesium aluminum mixed oxide on high surface area mesoporous silica leads to improvement in carbon dioxide adsorption capacity as well as kinetics. The capacity of the composite MAS-70 is 46% and 100% more than that of pure mixed oxide and commercial hydrotalcite respectively. Compared to other carbon material supported magnesium aluminum oxides the SBA-15 supported Mg-Al-O mixed oxide has higher adsorption capacity and unlike carbon supported mixed oxides can be calcined without the need of inert atmosphere. Kinetics of composite is also nearly 1.2 times faster than that of the pure mixed oxide. The faster kinetics and higher equilibrium capacity is expected to increase the dynamic capacity also. Future work will focus on developing a PTSA based process for high temperature CO2 capture based on these adsorbents.
Acknowledgement The authors thank Mr. Swapnil Divekar for discussions and help during experiments. Aamir Hanif is grateful to University Grants Commission, India for financial support in the form of a research fellowship and to Director, CSIR-Indian Institute of Petroleum for providing research facilities to carry out this work. Corresponding Author *Email: [email protected]. Phone: +91-135-2525727 Fax: +91-135-2660098 References
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Table 1. Surface area and pore volume of adsorbents. Pore volume calculated at p/po=0.99
Sample Code
BET surface (m2/g)
area Langmuir Surface area Total pore volume (m2/g) (cc/g)
SBA-15
707.69
1068.34
1.34
MAS-30
363.21
544.45
0.72
MAS-50
212.63
318.41
0.43
MAS-70
178.33
267.09
0.37
MAS-85
165.05
248.53
0.29
MAS-100
127.18
191.37
0.17
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Table 2: Lattice parameters and wall thickness of SBA-15 in synthesized Composites
Sample
d100
Lattice parameter ‘a0’ (nm)
(nm)
BJH Adsorption Pore Diameter ‘Φ ’ (nm)
Wall Thickness Wt(nm)
SBA-15
9.8076
11.32
7.96
3.36
MAS-30
9.8076
11.32
7.54
3.78
MAS-50
11.9281
13.44
7.80
5.97
MAS-70
12.0915
13.96
7.84
6.11
MAS-85
-
nd
7.04
--
MAS-100
-
nd
4.94
--
(n.d= not detected), unit cell parameter a0=2(d100/•3), Wt=a0-Φ
17
Table 3: Model fitting parameters obtained after fitting Lee et al model with CO2 adsorption isotherms at 350oC.
Adsorbent
Kr (bar-a) 0.127
m (mol/Kg) 0.167
a
SBA-15
Kc (bar-1) 5.913
MAS-30
2.991
0.018
0.244
1.439
MAS-50
2.142
0.008
0.267
1.785
Pural MG-63 HT
2.869
0.009
0.308
1.600
MAS-70
2.839
0.037
0.598
1.283
MAS-85
2.815
0.016
0.511
1.466
MAS-100
6.965
0.042
0.369
1.374
1.159
18
Table 4. Comparison of synthesized composite MAS-70 with other best literature reports on supported Mg-Al mixed oxides under similar conditions of Pressure (1 bar) and Temperature (300 oC).
Active Phase
Support
Mg-Al mixed oxide
SBA-15
Mg-Al mixed oxide
BET Surface area of Calcined Composite (m2/g) 178
Adsorptive Feed Gas
CO2 capacity (mmol/g)
Reference
CO2
0.46
Present work
Graphene 149 oxide
80%N2 +20%CO2
0.44
[21]
Mg-Al mixed oxide
Graphene Not given oxide
CO2
0.30
[22]
Mg-Al mixed oxide
Graphene 199 oxide
CO2
0.38
[23]
Mg-Al mixed oxide
Carbon nanotube
20%CO2 +80%N2
0.16
[24]
142
19
Figure 1. Wide angle PXRD of synthesized materials. The diamond shapes denote periclaseMgO phase and star shapes MgAl2O4 spinel phase. (a) SBA-15 (b) MAS-30 (c) MAS-50 (d) MAS-70 (e) MAS-85 (f) MAS-100 (g) Pural MG-63HT Figure 2. Low angle PXRD of synthesized adsorbents. (a) SBA-15 (b) MAS-30 (c) MAS-50 (d) MAS-70 (e) MAS-85 (f) MAS-100 and (g) Pural MG-63HT Figure 3. N2 adsorption desorption isotherms of synthesized adsorbents at 77K. Figure 4. BJH pore size distribution of synthesized adsorbents. Figure 5. SEM micrographs of synthesized adsorbents. Figure 6. SEM-EDX Mapping of MAS-30 (a) Si (b) Mg (c) Al (d) SEM of mapped region Figure 7. Fitting of experimental isotherm data for composite MAS-70 at 350 oC by the Langmuir and the chemisorption model proposed by Lee et al Figure 8. Experimental and predicted carbon dioxide isotherms for synthesized adsorbents at 350 oC. Points denote experimental data and lines predicted plots for (a) MAS-70 (b) MAS-85 (c) MAS-100 (d) Pural MG-63HT (e) MAS-50 (f) MAS-30 (g) SBA-15 Figure 9. Isotherms of MAS-70 at different temperatures.
Figure 10. Kinetic data fitted with a pseudo second order kinetic model.
Figure 11. Adsorption of CO2 on MAS-70 over multiple cycles at 350oC and 50 millibar pressure.
21
Figure 1
22
Figure 2
23
Figure 3
24
Figure 4
25
Figure 5
26
-2µm-
Figure 6
27
Figure 7
28
Figure 8
29
Figure 9
30
Figure 10
31
Figure 11
32
Research Highlights • • • • •
Dispersion of active Mg-Al-O mixed oxide phase on high surface area ordered mesoporous silica support. The highly dispersed composite sorbent shows improved CO2 capacity in temperature range of 300-400oC. Sorption Kinetics of the dispersed composite is also improved. Sorbent is regenerable with stable cyclic capacity. Synthesized sorbents have better CO2 capacity than reported Graphene oxide and CNT supported Mg-Al hydrotalcites.
33
Graphical abstract
34
S1385-8947(15)00857-8 http://dx.doi.org/10.1016/j.cej.2015.06.018 CEJ 13789
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
5 May 2015 3 June 2015 4 June 2015
Please cite this article as: A. Hanif, S. Dasgupta, A. Nanoti, High Temperature CO2 Adsorption by Mesoporous Silica Supported Magnesium Aluminum Mixed Oxide, Chemical Engineering Journal (2015), doi: http://dx.doi.org/ 10.1016/j.cej.2015.06.018
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High Temperature CO2 Adsorption by Mesoporous Silica Supported Magnesium Aluminum Mixed Oxide Aamir Hanif, Soumen Dasgupta, Anshu Nanoti* CSIR- Indian Institute of Petroleum, Dehradun 248005, India ABSTRACT Magnesium aluminum mixed oxide (Mg-Al-O) was deposited in various weight percentages into SBA-15 mesoporous silica through excess solution impregnation of magnesium and aluminum nitrate salts and subsequent high temperature calcination. In all the syntheses the Mg:Al mole ratio was fixed at 2. High temperature CO2 uptake capacity of these composites were measured in the temperature range 300-400 oC and were compared with the capacity of the unsupported mixed oxide phase with same Mg:Al mole ratio of 2. An improvement in the specific equilibrium capacity as well as uptake kinetics for CO2 was observed in the composite adsorbent after an optimum loading of the mixed oxide. The result could be explained on the basis of better accessibility of the active sites for CO2 adsorption following dispersion of the mixed oxide on to the high surface area SBA-15 support. The CO2 adsorption capacity is higher than recent reports of Mg-Al-O supported on Graphene oxide and CNT where there is additional complication of support degradation during thermal decomposition of Mg/Al salt precursors to corresponding metal oxides during adsorbent preparation.
Keywords: High temperature CO2 adsorption, Mg/Al oxide-SBA15 composites, Active phase dispersion, Adsorption isotherms, Adsorption kinetics
1
1. INTRODUCTION Carbon dioxide (CO2) capture research is getting lot of attention in recent years as it has potential for providing breakthrough technologies to combat anthropogenic global warming phenomenon [1]. With the total annual CO2 emissions at nearly 36 gigatonnes, coal combustion is known to have largest contribution since coal based thermal power plants are the major concentrated source for CO2 emission [2,3]. Capture from coal fired power plants thus makes good sense for controlling the rising levels of CO2 in the atmosphere. Capture of CO2 can be done via pre as well as post combustion strategies. Higher level of CO2 concentration in the shifted synthetic (syn) gas available from pre-combustion route compared to that in the flue gas from post combustion route should favor the former route for CO2 capture. There are different possible techniques for CO2 capture from shifted syn gas like solvent absorption, membrane separation and adsorption [4]. Amongst these techniques absorption is a relatively mature technology particularly in the context of post combustion CO2 capture. However due to the requirement of low temperature for CO2 absorption and a relatively higher temperature for the spent solvent regeneration and also keeping in view that shifted syn gas from pre-combustion route is available at very high temperature (250-500 oC), the process becomes unattractive as lot of heating and cooling duties will be involved. There are also issues related with solvent degradation over repeated cycles [5]. The membrane technology has also not matured enough for the said application [6]. In this context adsorption is emerging as a promising technique. However due to high temperature of the shifted syn gas conventional adsorbents such as zeolites, activated carbons, etc. have low CO2 adsorption capacity and may not be suitable for this application. Thus there is a need to develop special class of high temperature CO2 sorbents [7,8]. Such sorbents will also
2
find application in sorption enhanced water gas shift reactions for production of hydrogen [9]. The temperature of the fuel gas stream, available after the high temperature sour water gas shift reaction in the pre-combustion scheme, is in the range of 250-500oC and has CO2 partial pressure of around 15-30 bar [8]. High temperature adsorbents will have advantages over low or moderate temperature adsorbents in the context of pre-combustion CO2 capture as the former will impart greater efficiency in the capture process since cooling of the fuel gas stream prior to CO2 capture will not be required. Adsorbents based on calcium oxide [10,11] lithium zirconates [12], lithium silicates [13] and hydrotalcites [14–16] have been evaluated for CO2 capture in this temperature domain. Calcium oxide, lithium zirconates and silicates have high CO2 adsorption capacity reaching up to 11.6 mmol/g at 1 bar pressure [17]. However these materials have slow CO2 uptake kinetics and require very high temperature (above 800oC) for complete regeneration. The adsorption capacity also decreases rapidly over multiple cycles due to sintering of the active phase [18]. In comparison hydrotalcites have relatively lower CO2 capacity in the range of 0.1- 1.4 mmol/g [17]. But they are still considered a better choice as they can be regenerated with minimum temperature swing and shows better retention of capacity over many cycles of adsorptionregeneration[19,20]. Magnesium aluminum mixed oxide (Mg-Al-O) formed during high temperature decomposition of hydrotalcite is regarded as the active CO2 adsorbing phase. It is conceivable that an efficient dispersion of the Mg-Al-O phase on a high surface area support could impart better surface characteristics and accessibility of active sites for CO2 adsorption. Another advantage of high dispersion will be reduced sintering of the mixed metal oxide phase during thermal regeneration of the spent adsorbent. This should help in maintaining CO2 adsorption capacity over multi-cycle adsorption regeneration experiments. There are few earlier reports on high temperature CO2 adsorption by supported Mg-Al-O where carbon materials such
3
as graphene oxide [21–23], carbon nanotubes [24] and carbon nanofibre [25] have been used as support. Due to poor interaction of mixed oxide with carbon surface an inefficient dispersion was observed even at relatively lower oxide loading. Ordered mesoporous silica SBA-15 is known to be a better support for dispersion of large number of metal oxides [26–29]. However to the best of our knowledge there is no report of SBA-15 supported Mg-Al-O mixed oxides for CO2 adsorption in mid high temperature regime (200-400 oC) as prevalent in pre-combustion carbon dioxide capture though recently mesoporous Mg-Al-O mixed oxide has shown promise as CO2 adsorbent in the lower temperature range of 60-150oC [30]. In the present work we have synthesized composites with very high dispersion of Mg-Al-O mixed oxides on SBA-15 which then have been studied for high temperature CO2 adsorption over a temperature range of 300-400 oC. The mole ratio of Mg/Al in synthesized composites has been fixed at 2 which is reported to be optimum ratio for CO2 adsorption [31]. The results are compared with previously reported Graphene oxide and CNT supported Mg-Al-O mixed oxides as well as with an unsupported Mg-Al-O phase and also with a commercial hydrotalcite material. The SBA-15 supported composite adsorbents have improved CO2 adsorption capacity and CO2 uptake kinetics. The result could be explained on the basis of better accessibility of the active sites for CO2 adsorption following dispersion of the mixed oxide on to the high surface area SBA-15 support.
4
2. EXPERIMENTAL 2.1 Adsorbent Synthesis Synthesis of SBA-15 support was done following a reported procedure [32]. In a typical synthesis 20 g of polyethylene oxide-block-poly propylene oxide-block polyethylene oxide based copolymer surfactant (Pluronic P123, Av. mol wt.: 5800, Sigma-Aldrich) was dissolved in 150 ml distilled water by vigorous stirring. In this solution, 600 ml of 2M HCl was added and the resultant mixture was stirred vigorously for nearly about an hour. Tetraethyl ortho silicate (Sigma Aldrich, 42 g) was added drop wise to the above solution at room temperature with continuous stirring till a slurry was formed. The slurry was stirred at 35 oC for 24 hours and then aged at 80 oC for next 12 hours. The solid was then separated by filtration, washed repeatedly with warm distilled water several times and then dried at room temperature under ambient atmosphere. The dried material was then calcined at a temperature of 550 oC in a muffle furnace under air flow of 200 ml/min for 6 hours. The temperature ramp rate for attaining the final calcinations temperature was 1oC/min. Magnesium aluminum mixed oxide having Mg:Al mole ratio 2:1 was dispersed into the calcined SBA-15 in different weight percentages by following procedure. Calculated amount of the magnesium nitrate hexahydrate and aluminum nitrate nonahydrate was dissolved in distilled water in which a definite amount of calcined SBA-15 powder was added. The resulting suspension was stirred slowly for 12 hours at room temperature and then the water was slowly evaporated at 60 oC with continuous stirring till a thick paste was obtained. This paste was then heated at 500 oC in a muffle furnace for 5 hours using a temperature ramp of 2 oC/min to attain the final temperature. With the above mentioned procedure samples with Mg:Al:Si mole ratios of 2:1:5.1; 2:1:2.18; 2:1:0.9 and 2:1:0.4 were synthesized which corresponds to 30, 50, 70 and 85 % of MgO-Al2O3
5
mixed oxide in the resultant composite. The samples were named as MAS-30, MAS-50, MAS70, and MAS-85 where the numerals denote the weight percentage of MgO-Al2O3 mixed oxide. One more sample denoted as MAS-100 was synthesized in the similar way where no SBA-15 was added and thus it comprised purely of magnesium aluminum oxide. 2.2 Characterization Characterization of the synthesized adsorbents was done by PXRD, surface area, pore size distribution analysis, and SEM. The PXRD patterns were taken with the help of a Bruker (Germany) advanced difractometer using Cu Kα radiation. Pore size and BET surface area analysis were carried out in Micromeritics Tristar analyzer. Prior to surface area and pore size analysis the samples were activated by heating in a muffle furnace at 500 oC for 6 hours and then degassed under helium flow at 350 oC for 12 hours. For SEM micrographs and elemental mapping analysis Quanta (Netherlands) 200F FESEM was used.
2.3 Equilibrium isotherm and CO2 uptake rate measurements Gravimetric analyser IGA-001 (Hiden Isochema UK) was used to measure CO2 equilibrium isotherm of the synthesized adsorbents in the temperature range 300-400 oC. About 100 mg of adsorbent was loaded in a sample holder of the analyzer and pretreated at 450 oC under vacuum (10-6 millibar) for 12 hours till no further decrease in weight of the adsorbent was observed. After pretreatment of the adsorbent, the temperature of adsorption cell was lowered to target temperature of isotherm under vacuum. Further isotherm measurements were carried out by introducing CO2 at different pressures ranging from 50 millibar to 15 bar and at each pressure CO2 uptake was measured gravimetrically giving enough time to attain equilibrium. Equilibrium isotherms up to 15 bar were measured for all the synthesized adsorbents. The kinetics of CO2 uptake were monitored by measuring the adsorbent weight increase at different time intervals
6
following a CO2 pulse pressurization to 50 millibar from a high vaccum level of 10-6 millibar. The CO2 uptake was monitored up to 50 minutes since beyond this time the uptake became negligible. A commercial hydrotalcite adsorbent with Mg:Al mole ratio 2:1 (Pural MG63-HT from SASOL-Germany) was also tested for high temperature CO2 adsorption for comparison purpose. For the multi-cycle study the adsorbent, after every adsorption experiment, was regenerated in situ at 450 oC under high vacuum (10-6 millibar), till it regained its original weight, before starting adsorption step of the next cycle. 3. RESULTS AND DISCUSSION Magnesium nitrate is known to decompose at around 450 oC to periclase magnesium oxide [33] whereas aluminum nitrate decomposes to an amorphous aluminum oxide over the temperature range of 200-500 oC [34] whereas mesoporous silica SBA-15 retains its ordered structure in this temperature range [35]. Thus in all our syntheses the final calcination of the as synthesized materials were carried out at 500 oC. The XRD pattern (Fig. 1) of pure Mg/Al mixed oxide (MAS-100) in the 2θ range 30-70 degree shows diffraction peaks due to magnesium oxide periclase phase (JCPDS No: 43-1022) and small amount of spinel MgAl2O4 (JCPDS NO: 822424) phase. This XRD pattern is similar to that of periclase phase obtained from calcination of hydrotalcite with Mg/Al mole ratio of 2 except that the peaks are broader suggesting smaller crystallite size of periclase phase derived from hydrotalcite calcination. However XRD (Fig. 1) of the samples namely MAS-30, MAS-50 and MAS-70 in the same 2θ range shows absence of any such peaks suggesting a complete dispersion of the Mg/Al mixed oxide phase on the mesoporous support up to weight percent loading of 70%. Similar complete disappearance of metal oxide PXRD peaks due to efficient dispersion on SBA-15 has also been reported earlier [27–29]. However when the percentage loading of the mixed oxide was increased further to 85% as in MAS-85 the peaks corresponding to the periclase phase appeared.
7
Presence of distinct SBA-15 phase could be detected in the composites up to mixed oxide loading of 70 weight % as evident from low angle XRD data in the 2θ region 0.5-2 degree (Fig. 2). The d100 peak maxima shifts gradually towards lower 2θ value as the mixed oxide loading was increased to 70 %. This indicates gradual expansion of hexagonal lattice parameter of SBA-15 probably due to onset of disorganization in the mesoporous structure. This may be attributed to high degree of mixed oxide loading. It is observed that at 85% loading of mixed oxide (MAS85) the peak due to SBA-15 is very diffuse indicating nearly complete absence of the ordered mesoporous structure. The nitrogen adsorption-desorption isotherms (Fig. 3) and BJH pore size distribution data (Fig. 4) indicates pore narrowing, substantial decrease of pore volume and shifting of pore size maxima to lower values with increased loading of Mg-Al mixed oxide on SBA-15 support. This is indicative of deposition of mixed oxide inside the mesopores of SBA-15. At higher mixed oxide loadings of 70% and 85% the pores are nearly completely blocked. The surface area and pore volume data of the synthesized materials are given in Table 1. Both surface area and pore volume also decreases gradually with increasing loading of the mixed oxide. The wall thickness of SBA-15 hexagonal channels (Table 2) was calculated from the PXRD d100-spacing data and BJH adsorption pore diameter using formulas reported elsewhere [36]. As expected the wall thickness of SBA-15 hexagonal channels increases with increased weight loading of mixed oxide into SBA-15. The SEM micrographs show that the Styrofoam like morphology of MAS-30 and MAS-50 (Figure 5) are very similar to that of pure SBA-15 phase indicating deposition of the mixed oxide phase on the internal channel surface of mesoporous silica up to 50 weight % loading of the Mg/Al mixed oxide. As the loading of the mixed oxide was further increased to 70% level
8
(MAS-70) a secondary growth of crystallites, having thin plate type morphology typical of Mg/Al mixed oxide, appeared over SBA-15 crystallites. SEM of MAS-85 shows absence of typical SBA-15 structure and appearance of larger and thicker plate like growth similar to that of the pure mixed oxide phase MAS-100. SEM observation thus supports XRD data that at 85% mixed oxide loading the SBA-15 structure disappears completely and diffraction peaks of periclase phase becomes visible. Further SEM-EDX elemental mapping Mg, Al and Si of MAS30 (Figure 6) shows that the dispersion of the active Mg/Al mixed oxide phase is homogenous on the SBA-15 support. Carbon dioxide isotherms of all the synthesized adsorbents were measured at 350 oC. Both Langmuir and a chemisorption model proposed by Lee et al [37] were used to predict the experimental data for the optimized MAS-70 composite with highest CO2 adsorbing capacity, shown in Figure 7. The Lee model predicts the experimental data better especially at higher pressures (Figure 7) thus this model is used for the prediction of isotherms of all other adsorbents as shown in Figure 8. The equation for Lee et al [37] model is ݊∗ =
mK େ P[1 + ሺa + 1ሻK ୖ P ୟ ] [1 + K େ P + K େ K ୖ P ሺୟାଵሻ ]
Where n* is the carbon dioxide capacity (mol/Kg) at pressure P (bar), ‘a’ is a dimensionless constant, KC and KR are the equilibrium parameters of direct surface interaction and complexation respectively and ‘m’ is monolayer capacity. The chemisorption model proposed by Lee et al, [37] assumes that the adsorption involves two simultaneous steps viz direct interaction of CO2 with adsorbent surface and secondary complexation of CO2 molecules with that of the adsorbed CO2 phase. The complexation may become more prevalent at the higher pressures and at lower pressures only surface interaction
9
plays major role in adsorption. Probably for the same reason there is some deviation observed at lower values from the model. The various model parameters are enlisted in the Table 3. At 350 oC and 15 bar Carbon dioxide pressure the order of the carbon dioxide capacity in synthesised materials varies as SBA15
10
The effect of temperature on CO2 adsorption capacity by the highest adsorbing composite MAS70 is shown in Figure 9. It is observed that the capacity remains similar at temperature range of 300-350 oC. However a fall in capacity is observed at 400 oC. This may be probably due to the reason that desorption of chemisorbed carbon dioxide from the adsorbent is more predominant at 400oC than at 300 and 350oC as reported by Hutson et al [38]. A comparison of the of CO2 uptake kinetics between the composite MAS-70 and pure mixed oxide MAS-100 was made at 350 oC and 50 mbar pressure. It is observed that at the end of nearly one hour the average CO2 uptake rate of MAS-70 was 1.2 times more than uptake rate of MAS-100. Since the kinetic comparison was made at very low pressure at which only adsorbate surface interaction is prevalent a single parameter pseudo second order kinetics model [39] was used to predict the experimental kinetic data. The equation for this model is. ݍ௧ = ݍ ൬1 −
1 ൰ ݇ݍ ݐ+ 1
Where qt = CO2 uptake in mmol/g at time ‘t’, k is rate constants of CO2 interaction with adsorbent surface. This model equation fits well with our experimental kinetic data as shown in Figure 10. The rate constants obtained from the model fit also show that the rate of adsorption on MAS-70 (k=5.81mmol.g-1.min) is higher than that of adsorption rate constant of MAS-100 (k=5.12 mmol.g-1.min). The stability of carbon dioxide capacity over multiple cycles was tested (Figure 11) with the adsorbent MAS-70 at 50 millibar and temperature of 300 oC. After every adsorption cycle the adsorbent was regenerated at 450 oC under evacuation for 2 hrs till the initial weight of the adsorbent was regained. The capacity almost stabilized over four cycles of regeneration and adsorption. After the second cycle no further decrease in capacity was observed. 4. Conclusion
11
Dispersion on magnesium aluminum mixed oxide on high surface area mesoporous silica leads to improvement in carbon dioxide adsorption capacity as well as kinetics. The capacity of the composite MAS-70 is 46% and 100% more than that of pure mixed oxide and commercial hydrotalcite respectively. Compared to other carbon material supported magnesium aluminum oxides the SBA-15 supported Mg-Al-O mixed oxide has higher adsorption capacity and unlike carbon supported mixed oxides can be calcined without the need of inert atmosphere. Kinetics of composite is also nearly 1.2 times faster than that of the pure mixed oxide. The faster kinetics and higher equilibrium capacity is expected to increase the dynamic capacity also. Future work will focus on developing a PTSA based process for high temperature CO2 capture based on these adsorbents.
Acknowledgement The authors thank Mr. Swapnil Divekar for discussions and help during experiments. Aamir Hanif is grateful to University Grants Commission, India for financial support in the form of a research fellowship and to Director, CSIR-Indian Institute of Petroleum for providing research facilities to carry out this work. Corresponding Author *Email: [email protected]. Phone: +91-135-2525727 Fax: +91-135-2660098 References
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Table 1. Surface area and pore volume of adsorbents. Pore volume calculated at p/po=0.99
Sample Code
BET surface (m2/g)
area Langmuir Surface area Total pore volume (m2/g) (cc/g)
SBA-15
707.69
1068.34
1.34
MAS-30
363.21
544.45
0.72
MAS-50
212.63
318.41
0.43
MAS-70
178.33
267.09
0.37
MAS-85
165.05
248.53
0.29
MAS-100
127.18
191.37
0.17
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Table 2: Lattice parameters and wall thickness of SBA-15 in synthesized Composites
Sample
d100
Lattice parameter ‘a0’ (nm)
(nm)
BJH Adsorption Pore Diameter ‘Φ ’ (nm)
Wall Thickness Wt(nm)
SBA-15
9.8076
11.32
7.96
3.36
MAS-30
9.8076
11.32
7.54
3.78
MAS-50
11.9281
13.44
7.80
5.97
MAS-70
12.0915
13.96
7.84
6.11
MAS-85
-
nd
7.04
--
MAS-100
-
nd
4.94
--
(n.d= not detected), unit cell parameter a0=2(d100/•3), Wt=a0-Φ
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Table 3: Model fitting parameters obtained after fitting Lee et al model with CO2 adsorption isotherms at 350oC.
Adsorbent
Kr (bar-a) 0.127
m (mol/Kg) 0.167
a
SBA-15
Kc (bar-1) 5.913
MAS-30
2.991
0.018
0.244
1.439
MAS-50
2.142
0.008
0.267
1.785
Pural MG-63 HT
2.869
0.009
0.308
1.600
MAS-70
2.839
0.037
0.598
1.283
MAS-85
2.815
0.016
0.511
1.466
MAS-100
6.965
0.042
0.369
1.374
1.159
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Table 4. Comparison of synthesized composite MAS-70 with other best literature reports on supported Mg-Al mixed oxides under similar conditions of Pressure (1 bar) and Temperature (300 oC).
Active Phase
Support
Mg-Al mixed oxide
SBA-15
Mg-Al mixed oxide
BET Surface area of Calcined Composite (m2/g) 178
Adsorptive Feed Gas
CO2 capacity (mmol/g)
Reference
CO2
0.46
Present work
Graphene 149 oxide
80%N2 +20%CO2
0.44
[21]
Mg-Al mixed oxide
Graphene Not given oxide
CO2
0.30
[22]
Mg-Al mixed oxide
Graphene 199 oxide
CO2
0.38
[23]
Mg-Al mixed oxide
Carbon nanotube
20%CO2 +80%N2
0.16
[24]
142
19
Figure 1. Wide angle PXRD of synthesized materials. The diamond shapes denote periclaseMgO phase and star shapes MgAl2O4 spinel phase. (a) SBA-15 (b) MAS-30 (c) MAS-50 (d) MAS-70 (e) MAS-85 (f) MAS-100 (g) Pural MG-63HT Figure 2. Low angle PXRD of synthesized adsorbents. (a) SBA-15 (b) MAS-30 (c) MAS-50 (d) MAS-70 (e) MAS-85 (f) MAS-100 and (g) Pural MG-63HT Figure 3. N2 adsorption desorption isotherms of synthesized adsorbents at 77K. Figure 4. BJH pore size distribution of synthesized adsorbents. Figure 5. SEM micrographs of synthesized adsorbents. Figure 6. SEM-EDX Mapping of MAS-30 (a) Si (b) Mg (c) Al (d) SEM of mapped region Figure 7. Fitting of experimental isotherm data for composite MAS-70 at 350 oC by the Langmuir and the chemisorption model proposed by Lee et al Figure 8. Experimental and predicted carbon dioxide isotherms for synthesized adsorbents at 350 oC. Points denote experimental data and lines predicted plots for (a) MAS-70 (b) MAS-85 (c) MAS-100 (d) Pural MG-63HT (e) MAS-50 (f) MAS-30 (g) SBA-15 Figure 9. Isotherms of MAS-70 at different temperatures.
Figure 10. Kinetic data fitted with a pseudo second order kinetic model.
Figure 11. Adsorption of CO2 on MAS-70 over multiple cycles at 350oC and 50 millibar pressure.
21
Figure 1
22
Figure 2
23
Figure 3
24
Figure 4
25
Figure 5
26
-2µm-
Figure 6
27
Figure 7
28
Figure 8
29
Figure 9
30
Figure 10
31
Figure 11
32
Research Highlights • • • • •
Dispersion of active Mg-Al-O mixed oxide phase on high surface area ordered mesoporous silica support. The highly dispersed composite sorbent shows improved CO2 capacity in temperature range of 300-400oC. Sorption Kinetics of the dispersed composite is also improved. Sorbent is regenerable with stable cyclic capacity. Synthesized sorbents have better CO2 capacity than reported Graphene oxide and CNT supported Mg-Al hydrotalcites.
33
Graphical abstract
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