Novel Fluidizable K-Doped HAc-Li4SiO4 Sorbent for CO2 Capture

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A Novel Fluidizable K-Doped HAc-Li4SiO4 Sorbent for CO2 Capture Preparation and Characterization Sai Zhang, Muhammad B.I. Chowdhury, Qi Zhang, and Hugo I. de Lasa Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03746 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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Industrial & Engineering Chemistry Research

A Novel Fluidizable K-Doped HAc-Li4SiO4 Sorbent for CO2 Capture Preparation and Characterization Sai Zhang a , Muhammad B. I. Chowdhury b, Qi Zhang a*, Hugo I. de Lasa b* a

Dept. of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China. E-mail address: * [email protected]; Tel.: +86 21 64252386

b

Dept. of Chemical and Biochemical Engineering, The University of Western Ontario, London, ON N6A 5B9, Canada. E-mail address: * [email protected]; Tel.: +1 519 6612144 Abstract

A novel fluidizable K-doped HAc-Li4SiO4 sorbent using an incipient impregnation method was prepared in this work. The produced sorbent displayed an excellent CO2 sorption capacity and stability under expected reaction conditions. Glacial acetic acid treatment was firstly used to modify the Li4 SiO4 sorbent microstructure. Following this step, an incipient impregnation method was applied to dope potassium onto the sorbent in order to further enhance the sorbent sorption capacity. This novel K-doped HAcLi4 SiO4 sorbent was characterized using X-ray diffraction, N2 adsorption-desorption, CO2 Temperature Programmed Carbonation (CO2-TPC) and CO2 Temperature Programmed Decarbonation (CO2-TPDC) analyses. The experimental results showed that the CO2 sorption capacity of the K-doped HAc-Li4 SiO4 sorbent is approximately 100 cm3 STP CO2/g sorbent. This was five times that of the Li4 SiO4 sorbent. Furthermore, the cyclic test of the K-doped HAc-Li4SiO4 sorbent demonstrated high and stable for CO2 capture. 1

Introduction

The production of carbon dioxide (CO2) from fossil fuel combustion is widely considered as a major contributor to global warming. 1 At the Paris Climate Conference (COP21) in December 2015,2 195 countries reached a comprehensive global climate agreement. This agreement sets out a global action plan to put the world on track with sustainability in order to avoid dangerous climate change by limiting global warming to well below 2 °C. Major CO2 emissions are from the fossil fuel combustion of flue gases, which usually involve high temperatures, well above 400 °C. Currently, the most commonly used CO2 capture technology is wet adsorption using an amine solution at low temperatures.3-5 However, this technology needs significant energy to regenerate the solvent. Furthermore, the solvent cost is also high. Recently, CO2 capture at high temperatures (450-700 °C) using a regenerable solid sorbent has been identified as an alternative to the low temperature CO 2 capture technology. 6-17 One of the most suitable processes for high temperature CO2 capture is considered to be the chemical looping process. It implements the CO2 solid sorbents in twin fluidized bed units with recirculation of solids having a “looping configuration”.181

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This configuration facilitates the reversible CO2 absorption and desorption processes. As a result, an essentially pure stream of CO2 can be obtained at the process outlet. This technology possesses two major advantages: a) It can use moderate to high temperatures in the 500 °C to 700 °C range; b) It can potentially be implemented in large-scale fluidized bed reactor unit.22, 23 21

Up to today, several high temperature absorption materials have been proposed and investigated as CO2 acceptors, such as CaO-based sorbents,6-9 hydrotalcite-like materials,10, 11 and alkali-based ceramics.12-17 Among these various potential CO2 capture materials, Li4 SiO4 is considered to be a promising sorbent, given its good absorption capacity, its high stability and favorable kinetics. The absorption process (forward reaction) and the desorption process (backward reaction) proceed according to the following overall stoichiometry: Li4 SiO4 (s)+CO2 (g)↔Li2 CO3 (s)+Li2 SiO3 (s)

(1)

According to Eq. (1), the Li4 SiO4 can react with CO2, forming Li2CO3 and Li2 SiO3 during absorption process. On this basis, lithium orthosilicate may potentially display up to 8.3 mmole STP CO2/g sorbent which is equivalent to 182 cm3 STP CO2/g of sorbent. Meanwhile, due to the reversibility of the reaction, the formed Li2CO3 can decompose to desorb CO2 and form Li4 SiO4 species, as a result.17 The CO2 absorption in the Li4 SiO4 sorbent may involve two different reaction stages. First, there is a fast step which may be controlled by the reaction rate between CO2 and Li4SiO4. This may be followed by a slow stage where CO2 absorption rates may be controlled by species diffusion through the product layer.17 The influence of the diffusion controlled CO2 absorption may be increased via reduction of pore size, pore volume and surface area. This sorbent structural changes may limit the application of Li4 SiO4 based sorbents in large scale units and should be minimized. Furthermore, the selection of particles with an adequate morphology (porosity and pore network) and size may allow the application of Li4 SiO4 in fluidized bed units.20, 21, 24, 25 In our previous work, a Li SiO sorbent was modified using glacial acetic acid. A 4 4 detailed description of preparation procedures and characterization methods were reported.20,21 Thus, it was demonstrated that the acid treatment can stabilize the microstructure of the Li4SiO4 sorbent, which subsequently enhances the absorptiondesorption cyclic performance. One should notice that a glacial acetic acid Li4SiO4 sorbent leads to a relatively modest CO2 absorption capacity with 2.2 mmole STP CO2/g of sorbent or 50 cm3 STP CO2 /g sorbent. Thus, by looking for further CO2 capacity enhancements, one can consider in addition, (1) metal doping,26 and (2) eutectic doping.27-31 It is our belief that eutectic doping holds the most promise. For instance, adding potassium on Li4 SiO4 can significantly increase the sorbent’s sorption capacity.27-31 Furthermore, it was demonstrated in our previous work,30-31 that the mechanical mixing of Li2CO3. K2CO3 2


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and SiO2 powders calcined at 750 °C may allow the preparation of a K-doped Li4 SiO4 sorbent. In this work however, a different approach is attempted by using an incipient wetness method. This sorbent preparation method combines a glacial acetic acid treatment and an incipient K-doping technique. This method yields a new Li-based sorbent with KLi3 SiO4 species. As far as we are aware of, this is the first time that a KLi3 SiO4 species sorbent has been reported in the technical literature. The prepared novel sorbent is extensively evaluated under the simulated carbonation-decarbonation reactor conditions. The new sorbent displays the following features: a) a higher CO2 absorption versus that other sorbents proposed by our research team, 20, 21 b) a notable CO2 absorption stability under several absorption-desorption cycles, c) a valuable CO2 absorption in the 400-500 °C range. Experimental All the chemicals used in this work were reagent grade and were used as received without further purification. The 99.9 wt% lithium orthosilicate (Li4SiO4) was purchased from Alfa Aesar, Massachusetts, USA. The 99.7 v/v% glacial acetic acid (HAc) and the 99 wt% potassium carbonate (K2CO3) were obtained from Aldrich Chemical Co., Wisconsin, USA. The 10 v/v% carbon dioxide (CO 2) with nitrogen as the balance was purchased from Praxair Canada Inc. 2.1 Sorbent Preparation: The preparation of the glacial acetic acid treated Li4SiO4 sorbent (marked as HAcLi4 SiO4 sorbent) included the following steps: (a) Li4 SiO4 was mixed with glacial HAc with a molar ratio of 1:10; (b) the mixture was stirred at room temperature for 1hr; (c) the mixture was calcined under air flow from room temperature to 620 °C for 2 hr. The HAc-Li4 SiO4 sorbent was then further modified with potassium carbonate (K2CO3) using incipient doping as follows: (a) A K2CO3 saturated solution was added dropwise into the HAc-Li4 SiO4 sorbent (with a molar ratio of 0.12 under vacuum); (b) The resulting paste was calcined under air flow from room temperature to 620 °C for 2 hr. The K-doped Li4SiO4 sorbent was designated K-doped HAc-Li4 SiO4 sorbent. Figure 1 reports the preparation method of the K-doped HAc-Li4 SiO4 sorbent of the present study, describing the different steps. Figure 1 also describes the changes of sorbent morphology through the sorbent preparation method.

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Figure 1. Schematics of the K-doped HAc-Li4 SiO4 preparation method 2.2 Sorbent Characterization: The sorbent specific Brunauer-Emmett-Teller (BET) surface area, pore volume and the Barrett-Joyner-Halenda (BJH) pore size distribution were determined using nitrogen adsorption and desorption isotherms at 77K. A Micromeritics ASAP 2010 was applied to perform the N2 (99.995 vol% pure; obtained from Praxair, Canada) adsorption-desorption analysis. Before the measurements, the samples were degassed at 200 °C for 6 hrs. A Micromeritics DFT Plus software from Windows TM was used to calculate pore volume and pore size distribution by applying a Density Functional Theory (DFT) method. XRD results provide the “finger prints” of the crystalline phases in the sorbent.Xray diffraction (XRD) patterns of the sorbents were analyzed using a Rigaku Diffractometer (Ultima IV) unit with a CuKα1+ Kα2 equal to 1.54184 Å radiation. The XRD instrument was operated at 45 kV and 160 Ma, using a normal scan rate of 10° per minute (equivalent to 0.5° in the 2θ scale) spanning from 10° and 80°. X-rays were collimated using a 1° divergent angle, and a 0.2 mm receiving slit. XRD peaks were identified using the Jade 6.0 Software. On this basis, the KLi3SiO4 was unambiguously identified in the present study. The CO2 absorption-desorption sorbent capacity and the “inversion temperature” were established, as in previous studies of our research team, using CO2 Temperature Programmed Carbonation-Decarbonation analyses (TPC-TPDC) and thermodynamic considerations.20, 21 Regarding the TPC-TPDC experiments developed in this research, they were carried out using a Micromeritics Autochem 2920. 100-150 mg of the fresh sorbent sample was calcined at 690 °C for 1hr under N2 flow. This allowed the removal of moisture, absorbed CO2 and bonded impurities. The TPC runs were performed by flowing a stream of 10 vol% CO2 in N2 using a rate of 50 ml/min. The temperature was raised from room temperature to the desired higher thermal level at a 5 °C/min rate. A 4


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Thermal Conductivity Detector (TCD) was used to record the changes in carbon dioxide concentrations in the gas stream at the bed exit. Recorded CO2 peaks were integrated to establish the CO2 consumed or released during absorption and desorption processes. 3 Results and Discussion 3.1 CO2-TPD Analysis

Commercial Li4 SiO4 was first microstructurally modified by glacial acetic acid treatment in order to obtain a HAc-Li4SiO4 sorbent.20, 21 Then, the HAc-Li4 SiO4 sorbent was further modified by doping K, using the incipient doping method, to obtain a Kdoped HAc-Li4 SiO4. These three sorbents were first tested using TPC-TPDC to analyze the CO2 sorption performance. In this work, the temperature ramp was 5 °C/min. This method allows carbonation and decarbonation studies with sorbent particles in contact with a CO2 containing stream. With the prepared K-doped HAc-Li4SiO4, the CO2 capacity was established under dynamic conditions. This was considered critical to provide the necessary information for the simulation and the design of a CO2 capture system. The selected time scale was comparable to the one expected in large-scale dense fluidized beds. This system is expected to be integrated via twin fluidized bed reactors in the context of an electricity production power plant. Figure 2 compares the absorption-desorption performances of Li4 SiO4, HAcLi4 SiO4 and K-doped HAc-Li4 SiO4 sorbents. It can be seen that the K-doped HAcLi4 SiO4 sorbent displayed a better CO2 sorption-desorption capacity than other two sorbents of previous studies,20, 21 while experimentally studied using TPC-TPDC with a 5 °C/min heating ramp.

Figure 2. CO2 sorption capacities of different sorbents: (a) Li4SiO4; (b) HAcLi4 SiO4; (c) K-doped HAc-Li4 SiO4 5



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Regarding Li4 SiO4 sorption CO2 properties, one can observe in Figure 2, a Temperature Programmed Carbonation (TPC) peak with a maximum at 550 °C and a Temperature Programmed Decarbonation (TPDC) peak with a maximum at 620 °C. Thus and on the basis of an integrated TPC peak, one can establish a 20 cm3 STP CO2/g of sorbent capacity for the Li4SiO4. One should also notice that after the glacial acetic acid treatment, the CO2 sorption capacity of the HAc-Li4SiO4 is increased to 49 cm3 STP CO2/g of sorbent. Furthermore, and with respect to the novel K-doped HAcLi4 SiO4 sorbent, this material displays TPC-TPDC peaks consistently yielding a 99.5 cm3 STP CO2/g sorbent capacity. This represents approximately 5 times the CO2 capacity of the original commercial Li4 SiO4 sorbent and twice of the HAc-Li4 SiO4 sorbent. Furthermore, it was also found using TPC-TPDC analyses that the K-doped HAcLi4 SiO4 sorbent presents a double-step absorption. Firstly, there is a large sorption peak with a 75cm3 STP CO2/ g sorbent capacity and a maximum absorption at 460 °C. Then, there is a second CO2 smaller absorption peak that appears to have a 25 cm3 STP CO2/ g sorbent capacity. The maximum of this TPC peaks was observed at 550 °C. This is in contrast with the Li4SiO4 and HAc-Li4 SiO4 sorbents which display single CO2 absorption peaks at the 500-600 °C higher thermal levels. Regarding the observed 99.5 cm3 STP CO2/ g sorbent capacity of the new K-doped HAc-Li4 SiO4, this represents an encouraging 54% of the maximum CO2 absorption capacity for a orthosilicate based sorbent, as calculated by stoichiometry (Equation (1)). There is the expectation that these large CO2 absorption peaks in the 400-500 °C range can be traced to a new orthosilicate phase where a Li atom is substituted by a K atom. Thus, and in order to confirm this, a detailed sorbent characterization was performed as is reported in the upcoming sections of the present manuscript. To our knowledge, this is the first contribution where a highly performing K-doped HAcLi4 SiO4 CO2 sorbent is reported in the technical literature. 3.2 XRD Analysis Figure 3 reports the XRD patterns of the fresh Li4SiO4, HAc-Li4 SiO4 and K-doped HAc-Li4 SiO4 samples.

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Figure 3. XRD patterns of the (a) Li4SiO4; (b) HAc-Li4 SiO4 and (c) K-doped HAcLi4 SiO4 It can be seen in Figure 3 that the peaks of the three samples showed different intensities. The fresh Li4 SiO4 sample contained mainly crystalline phases of Li4 SiO4 with trace amounts of Li2CO3. It is interesting to note that the HAc-Li4SiO4 sorbent, the characteristic XRD for Li4 SiO4 with Li2CO3 and Li2 SiO3 also being detected. Furthermore, for the K-doped HAc-Li4 SiO4 sorbent, the following species were detected in the XRD diffractogram: Li4 SiO4, KLi3 SiO4, LiKCO3 and K2CO3. The Li2 SiO3 was observed with minor XRD intensity peaks. Regarding these findings and while there were expectations of having K-doped Li4 SiO4 sorbents,28-31 this is the first contribution where a KLi3 SiO4 phase is unambiguously XRD identified. As already discussed in Section 3.1, when using CO2-TPC and CO2-TPDC, the KLi3 SiO4 displayed a significantly augmented sorbent sorption capacity. This CO2 capacity enhancement comes together with a sorption peak in the lower 400-500 °C range. This peak can be assigned to a KLi3 SiO4 phase. To confirm this, separate CO2-TPC runs were performed and interrupted at a 7



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convenient temperature. Samples were cooled down, as soon as possible, under nitrogen flow. Figure 4 shows the XRD patterns of a K-doped HAc-Li4 SiO4 sorbent after performing a CO2-TPC process, which was interrupted at 500 °C.

Figure 4. XRD patterns of K-doped HAc-Li4 SiO4 after temperature programmed CO2 absorption. The run was interrupted at 500 °C with the sample being cooled at room temperature under N2 flow as quickly as possible (30 minutes) It can be observed in Figure 4 that peaks for the KLi3SiO4 species, disappeared after the sorbent absorbed CO2 at temperatures lower than 500 °C. Meanwhile, the XRD patterns indicated that Li2CO3, Li2SiO3 and K2CO3 were formed during the CO2-TPC process. Thus, it can be concluded, that the first CO2 absorption peak of the K-doped HAc-Li4 SiO4 sorbent, as presented in Figure 2, can be attributed to the KLi3 SiO4. As a result, the CO2 absorption process in the KLi3 SiO4 sorbent, can be described via the following equation: 2KLi3 SiO4 (s)+2CO2 (g)=K2 CO3(s)+Li2 CO3 (s)+2 Li2 SiO3 (s)

(2)

Regarding the reverse of Equation (2), it is well known that CO 2 sorption can be enhanced in Li4 SiO4 by facilitating CO2 diffusion.25, 27-33 In our previous work, 30, 31 a K-doped Li4 SiO4 sorbent was prepared by mixing Li2CO3, SiO2 and K2CO3 followed by calcination at 750 °C for 6 hr. Results showed that doping K into Li4 SiO4 enhances CO2 sorption, with a possible explanation being the formation of an Li2CO3 and K2CO3 eutectic phase. 27, 32, 33 3.3 BET Analysis N2 adsorption-desorption experiments were performed to determine the microstructural characteristics of the Li4SiO4, the HAc-Li4 SiO4 and the K-doped HAcLi4 SiO4 samples. Figure 5 reports the nitrogen isotherms as well as pore size distributions. It can be observed that all the isotherms are of Type 3 with a H3 hysteresis loop. This indicates that the lithium orthosilicate species aggregate with plate-like particles and slit-like pores.34 One can notice in Figure 5 that after HAc treatment, the surface area of the sorbent 8


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is increased from 2.09 m2/g to 4.06 m2/g. One can also observe that incipient doping of the HAc-Li4SiO4, slightly decreases the sorbent surface area to 3.57 m2/g. Thus, one can argue that modifications with HAc and potassium addition provide extra surfaces for CO2-sorbent interactions, improving as a result, the CO2 absorption rates.

Figure 5. N2 adsorption-desorption isotherms and pore size distribution of (a) Li4 SiO4; (b) HAc-Li4 SiO4 and (c) K-doped HAc-Li4 SiO4 Regarding pore size distribution and according to the Barrett-Joyner-Halenda (BJH) plot, the pore sizes of all the sorbents of this study were distributed in the 300 to 2500 Å mesopore-macropore range. On the basis of the above, it can be safely hypothesized that the first treatment with glacial acetic acid changes the Li4SiO4 morphology, yielding a Li4 SiO4 with a larger specific surface area. Furthermore, the K incipient doping with K2CO3, followed by drying, gives a K2CO3 phase on the HAc-Li4SiO4. Calcinations of the resulting powder at 620 °C, allow the K2CO3 and the Li4 SiO4 to react with each other, with this leading to: a) a partial substitution of Li by K, forming a KLi3 SiO4-Li4 SiO4 blend, b) a largely preserved Li4 SiO4 pore structure. A schematic diagram of the KLi3 SiO4, synthesis process is reported in Figure 1(b). 3.4 Stability Tests The stability of a sorbent is crucial when applying the sorbent for CO2 capture from fuel-fired power station at high temperatures. Figure 6(a) compares the CO2 capacity of different sorbents over repeated cycles under a 10 vol% CO2 atmosphere. It can be seen that both HAc-Li4SiO4 and K-doped HAc-Li4 SiO4 sorbent presented excellent stability, while the CO2 capacity of the Li4SiO4 sorbent decreased dramatically during the absorption-desorption processes. A conversion decay X of the sorbent during cycling was defined in this work as follows: C

X= CN 1

(3)

where CN and C1 are the CO2 sorption capacities of the sorbent at the “N” cycle and at the first cycle, respectively. The conversion decay of the three different sorbents were calculated using equation (3), as shown in Figure 6(b). For the K-doped HAc-Li4SiO4 sorbent, the CO2 sorption capacity of the 10th cycle decreased only by 5% compared 9



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with that of the first cycle, indicating its high stability. In the case of the HAc-Li4 SiO4 sorbent, although the sorption capacity declined slightly during absorption-desorption process, its CO2 capacity at the 10th cycle remains 85% percent of the first cycle. Regarding the Li4SiO4 sample, it can be seen that the CO2 sorption ability decreased dramatically. At the 10th cycle, the CO2 sorption capacity of Li4 SiO4 sample was only 40% that of the first cycle. The conversion decay of different samples during cycling, further proved that the K-doped HAc-Li4SiO4 sorbent is highly stable.

Figure 6. (a) CO2 sorption capacities and (b) X of Li4 SiO4, HAc-Li4 SiO4 and K-doped HAc-Li4 SiO4 during ten absorption-desorption cycles: (Temperature ramp: 5 °C/min; Gas composition: 10 vol% CO2 in nitrogen; flow rate: 50 ml/min). Note: direction of the arrows describes successive runs: from run 1-10. In order to further clarify and compare the different Li4SiO4 based sorbent in this work, table 1 summarized the quantitative information of CO2 absorption capacities during cycling. Table 1. CO2 absorption capacities for the Li4SiO4 based sorbents of the present study. Sorbent

CO2 sorption capacities (cm3 STP CO2/ g sorbent) during cycles 1

K-doped

2

3

4

5

6

7

8

9

10

99.5

98.1 97.5

96.9

96.5 96.3

95.3 94.9

94.8 94.6

HAc- Li4 SiO4

49

47.7 45.5

45.2

44.7 44.3

43.7 43.1

42.5 42.2

Li4 SiO4

20

16.9 14.6

13.3

12.8 12.2

10.0

8.5

HAc-Li4 SiO4 9.8

8

Figure 7 reports the XRD diffractograms for the used Li4SiO4 sorbents after 1 and 10 cycles of CO2-absorption and CO2-desorption. 10


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Figure 7. XRD patterns of Li4 SiO4 (a) after 1 cycle and (b) after 10 cycles Figure 7 shows that both Li2CO3 and Li2SiO3 are already present after one complete absorption-desorption cycle. Furthermore, their XRD peak intensity becomes of even larger magnitude after ten CO2 absorption-desorption cycles. This can be attributed to the formation of a Li2SiO3 and a Li2CO3 shell on the particle surface after successive CO2 absorption-desorption cycles. It is hypothesized that these species block sorbent pores leading to incomplete regeneration of the Li4 SiO4 sorbent at 690 °C. These findings were consistent with the results of Figure 6, showing a progressive decline of the CO2 absorption capacity for Li4SiO4 with the number of cycles. Figure 8 reports the XRD diffractograms for the glacial acetic acid modified lithium orthosilicate (HAc-Li4 SiO4) after 1 and 10 CO2 absorption-desorption cycles.

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Figure 8. XRD patterns of HAc-Li4 SiO4 (a) after 1 cycle and (b) after 10 cycles Figure 8 shows a dominant Li4 SiO4 phase after the first cycle with no contribution from Li2CO3 and Li2 SiO3 species. Furthermore, only traces of Li2CO3 and Li2SiO3 were detected after ten CO2 absorption-desorption cycles. Figure 9 describes the XRD diffractograms for the K-doped HAc-Li4SiO4 sorbent following both 1 and 10 CO2 absorption-desorption cycles.

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Figure 9. XRD patterns of K-doped HAc-Li4 SiO4 (a) after 1 cycle and (b) after 10 cycles One can notice in Figure 9, dominant Li4 SiO4 and KLi3 SiO4 species for sorbent samples, both after 1 and 10 cycles. Li2CO3 and Li2 SiO3 were still being observed at the trace level, with this showing a stable microstructure. Other interesting observations from the XRD diffractograms were the KLi3 SiO4 and Li4 SiO4 peak intensity ratios after the 1st and the 10th cycle. These ratios were 1.42 and 1.40 respectively, showing a stable sorbent structure. Table 2 summarizes the BET surface area, the average pore diameter and the pore volume for three samples: a) fresh sorbent samples, b) sorbent samples after the first CO2 absorption-desorption cycle, c) sorbent samples after 10 CO2 absorptiondesorption cycles. Table 2. Physiochemical properties of the sorbents sorbents

BET surface area (m2 /g)

Average pore diameter (Å)

Fresh

After 1 cycle

After Fresh 10 cycles

Li4SiO4

2.09

1.46

0.72

243

240

HAc-Li4 SiO4

4.06

3.86

3.80

232

K-doped HAcLi4SiO4

3.57

3.42

3.28

210

Pore volume (cm3/g)

After After Fresh 1 10 cycle cycles

After 1 cycle

After 10 cycles

230

0.013 0.009

0.002

210

175

0.024 0.022

0.016

220

240

0.014 0.020

0.022

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One can notice in Table 2 that N2 adsorption-desorption showed a reduction of the specific surface area for Li4SiO4, with these changes becoming less significant for both HAc-Li4 SiO4 and K-doped HAc-Li4 SiO4. Similar variations were noticed for the average pore size and pore volume. Thus, for the Li4 SiO4 sorbent, it appears that there is a reduction of pore volume and specific surface area, which can be associated with a change of the pore sorbent network. However, these morphological changes were observed to a smaller extent for the HAc-Li4 SiO4 sorbent. Finally, for the K-doped HAc-Li4 SiO4 sorbent, the following was observed: a) the average pore diameter and the pore volume slightly increased, b) the specific surface area decreased modestly. One should note that in our previous work,31 a solid state method was implemented to synthesize the K-doped Li4 SiO4 sorbent. A temperature over 700 °C was used in the sorbent preparation which resulted in a dense morphology of the sorbent. In this respect, the pore size distribution of the fresh K-doped Li4 SiO4 sorbent was between 1 nm and 8 nm. This approach led to a K-doped Li4SiO4 sorbent with a 45 cm3 STP CO2/g sorbent capacity. However, and after three CO2 absorption-desorption cycles, the pore size broadened to 1nm to 56nm, yielding a CO2 capacity that increased to a 100 cm3 STP CO2/g sorbent range. While in principle this approach was promising, it involved solidsolid reactions, at temperatures higher than the Li4SiO4 melting point. Given this, it was anticipated that close control of structural sorbent properties in large scale samples for industrial use, could be problematic. It was in this context, that an alternative method, combining acid treatment and incipient wetness K-doping, was proposed in the present study. As a result, and on the strength of the findings reported here, it can be concluded that the acid treatment can favorably modify the Li4SiO4 sorbent microstructure. Furthermore, if one follows this with incipient K-doping, one sees that K is incorporated in the sorbent microstructure securing: a) a high CO2 sorbent capacity in the 100 cm3 STP CO2/g sorbent range from the first cycle already, b) a good stability of the sorbent capacity over multiple cycles (more than 10 cycles), and c) a favorable CO2 capture in the 400-500 °C desirable industrial application temperature range. Conclusions 

A novel fluidizable K-doped HAc-Li4 SiO4 sorbent was successfully synthesized by combining glacial acetic acid treatment and an incipient Kdoping method.



The new K-doped HAc-Li4 SiO4 sorbent was unambiguously identified using XRD analysis.



Experimental CO2 TPC-TPDC results showed that this fluidizable K-doped HAc-Li4 SiO4 sorbent possesses both an excellent CO2 sorption capacity and stability in the 400-500 °C range.



According to the CO2 TPC-TPDC data, the CO2 sorption capacity of the K14


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doped HAc-Li4 SiO4 sorbent reached 99.5 cm3 STP CO2/g sorbent. This is five times that of the original commercial Li4 SiO4 sorbent and twice of the HAc-Li4 SiO4 sorbent. 

The developed K-doped HAc-Li4SiO4 sorbent was demonstrated to be a stable sorbent with a high CO2 absorption performance. Its operation is considered compatible with its application in large-scale fluidized CO2 absorption-desorption cyclical processes.

Acknowledgements We would like to acknowledge the financial support of the China Scholarship Council, National Natural Science Foundation of China (Grant 21176080) and Natural Science Foundation of Shanghai (16ZR1408200) who awarded a Graduate Visiting Scholarship to Sai Zhang. We are also grateful to the Natural Sciences and Engineering Research Council (NSERC Discovery Grant-Prof. H. de Lasa) who financially contribute to cover the material and the supply expenses for the present study. References (1) Lackner, K. S. A Guide to CO2 Sequestration. Science 2003, 300, 1677. (2) U.N. Framework Convention on Climate Change, Adoption of the Paris Agreement Home Page. https://unfccc.int/resource/docs/2015/cop21/eng/l09r01.pdf (accessed September 2016). (3) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. PostCombustion CO2 Capture Using Solid Sorbents: A Review. Ind. Eng. Chem. Res. 2012, 51, 1438. (4) Mason, J. A.; McDonald, T. M.; Bae, T. H.; Bachman, J. E.; Sumida, K.; Dutton, J. J.; Kaye, S. S.; Long, J. R. Application of a High-Throughput Analyzer in Evaluating Solid Adsorbents for Post-Combustion Carbon Capture via Multicomponent Adsorption of CO2, N2, and H2O. J. Am. Chem. Soc. 2015, 137, 4787. (5) Didas, S. A.; Kulkarni, A. R.; Sholl, D. S.; Jones, C. W. Role of Amine Structure on Carbon Dioxide Adsorption from Ultradilute Gas Streams such as Ambient Air. ChemSusChem 2012, 5, 2058. (6) Chanburanasiri, N.; Ribeiro, A. M.; Rodrigues, A. E.; Arpornwichanop, A.; Laosiripojana, N.; Praserthdam, P.; Assabumrungrat, S. Hydrogen Production via Sorption Enhanced Steam Methane Reforming Process Using Ni/CaO Multifunctional Catalyst. Ind. Eng. Chem. Res. 2011, 50, 13662. (7) Sultana, K. S.; Tran, D. T.; Walmsley, J. C.; Rønning, M.; Chen, D. CaO Nanoparticles Coated by ZrO2 Layers for Enhanced CO2 Capture Stability. Ind. Eng. Chem. Res. 2015, 54, 8929. (8) Rahman, R. A.; Mehrani, P.; Lu, D. Y.; Anthony, E. J.; Macchi, A. Investigating the Use of CaO/CuO Sorbents for in Situ CO2 Capture in a Biomass 15



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Gasifier. Energy Fuels 2015, 29, 3808. (9) Donat, F.; Florin, N. H.; Anthony, E. J.; Fennell, P. S. Influence of HighTemperature Steam on the Reactivity of CaO Sorbent for CO 2 Capture. Environ. Sci. Technol. 2012, 46, 1262. (10) Yong, Z.; Mata, V.; Rodrigues, A. E. Adsorption of Carbon Dioxide onto Hydrotalcite-like Compounds (HTlcs) at High Temperatures. Ind. Eng. Chem. Res. 2001, 40, 204. (11) Wu, Y. J.; Li, P.; Yu, J. G.; Cunha, A. F.; Rodrigues, A. E. High-Purity Hydrogen Production by Sorption-Enhanced Steam Reforming of Ethanol: A Cyclic Operation Simulation Study. Ind. Eng. Chem. Res. 2014, 53, 8515. (12) Wang, C.; Dou, B.; Song, Y.; Chen, H.; Xu, Y.; Xie, B. High Temperature CO2 Sorption on Li2 ZrO3 Based Sorbents. Ind. Eng. Chem. Res. 2014, 53, 12744. (13) Jo, H. G.; Yoon, H. J.; Lee, C. H.; Lee, K. B. Citrate Sol-Gel Method for the Preparation of Sodium Zirconate for High-Temperature CO2 Sorption. Ind. Eng. Chem. Res. 2016, 55, 3833. (14) Yin, Z.; Wang, K.; Zhao, P.; Tang, X. Enhanced CO2 Chemisorption Properties of Li4SO4, Using a Water Hydration-Calcination Technique. Ind. Eng. Chem. Res. 2016, 55, 1142. (15) Zhang, Q.; Shen, C.; Zhang, S.; Wu, Y. Steam Methane Reforming Reaction Enhanced by a Novel K2CO3-Doped Li4 SiO4 Sorbent: Investigations on the Sorbent and Catalyst Coupling Behaviors and Sorbent Regeneration Strategy. Int. J. Hydrogen Energy 2016, 41, 4831. (16) Kato, M.; Yoshikawa, S.; Nakagawa, K. Carbon Dioxide Absorption by Lithium Orthosilicate in a Wide Range of Temperature and Carbon Dioxide Concentrations. J. Mater. Sci. Lett. 2002, 21, 485. (17) Qi, Z.; Daying, H.; Yang, L.; Qian, Y.; Zibin, Z. Analysis of CO 2 Sorption/Desorption Kinetic Behaviors and Reaction Mechanisms on Li4 SiO4. AIChE J. 2013, 59, 901. (18) Rodríguez, N.; Alonso, M.; Abanades, J. C. Experimental Investigation of a Circulating Fluidized-Bed Reactor to Capture CO2 with CaO. AIChE J. 2011, 57, 1356. (19) Diego, M. E.; Arias, B.; Grasa, G.; Abanades, J. C. Design of a Novel Fluidized Bed Reactor to Enhance Sorbent Performance in CO2 Capture Systems Using CaO. Ind. Eng. Chem. Res. 2014, 53, 10059. (20) Chowdhury, M. B. I.; Quddus, M. R.; de Lasa, H. I. CO2 Capture with a Novel Solid Fluidizable Sorbent: Thermodynamics and Temperature Programmed Carbonation-Decarbonation. Chem. Eng. J. 2013, 232, 139. (21) Quddus, M. R.; Chowdhury, M. B. I.; de Lasa, H. I. Non-isothermal Kinetic Study of CO2 Sorption and Desorption Using a Fluidizable Li4 SiO4. Chem. Eng. 16


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Figure and table captions: Table 1. CO2 absorption capacities for the Li4SiO4 based sorbents of the present study. Table 2. Physiochemical properties of the sorbents. Figure 1. Schematics of the K-doped HAc-Li4 SiO4 preparation method. Figure 2. CO2 sorption capacities of different sorbents: (a) Li4SiO4; (b) HAcLi4 SiO4; (c) K-doped HAc-Li4 SiO4. Figure 3. XRD patterns of the (a) Li4SiO4; (b) HAc-Li4 SiO4 and (c) K-doped HAcLi4 SiO4. Figure 4. XRD patterns of K-doped HAc-Li4 SiO4 after temperature programmed CO2 absorption. The run was interrupted at 500 °C with the sample being cooled at room temperature under N2 flow as quickly as possible (30 minutes). Figure 5. N2 adsorption-desorption isotherms and pore size distribution of (a) Li4 SiO4; (b) HAc-Li4 SiO4 and (c) K-doped HAc-Li4 SiO4. Figure 6．(a) CO2 sorption capacities and (b) X of Li4 SiO4, HAc-Li4 SiO4 and Kdoped HAc-Li4SiO4 during ten absorption-desorption cycles: (Temperature ramp: 5 °C/min; Gas composition: 10 vol% CO2 in nitrogen; flow rate: 50 ml/min). Note: direction of the arrows describes successive runs: from run 1-10. Figure 7. XRD patterns of Li4 SiO4 (a) after 1 cycle and (b) after 10 cycles. Figure 8. XRD patterns of HAc-Li4 SiO4 (a) after 1 cycle and (b) after 10 cycles. Figure 9. XRD patterns of K-doped HAc-Li4SiO4 (a) after 1 cycle and (b) after 10 cycles.

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Novel Fluidizable K-Doped HAc-Li4SiO4 Sorbent for CO2 Capture

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