Synthetic CaO-based sorbent for high-temperature CO2 capture in sorption-enhanced hydrogen production

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Synthetic CaO-based sorbent for high-temperature CO2 capture in sorption-enhanced hydrogen production Piya Pecharaumporn a, Suwimol Wongsakulphasatch b,*, Thongchai Glinrun c, Atthaphon Maneedaeng d, Zulkafli Hassan e, Suttichai Assabumrungrat a a

Center of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, 10330, Thailand b Department of Chemical Engineering, Faculty of Engineering, King Mongkut's University of Technology North Bangkok, Bangkok, 10800, Thailand c Department of Petrochemical and Environmental Engineering, Faculty of Engineering, Pathumwan Institute of Technology, Bangkok, 10330, Thailand d School of Chemical Engineering, Institute of Engineering, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand e Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya, Tun Razak, 26300, Gambang, Kuantan, Pahang, Malaysia

article info

abstract

Article history:

Calcium precursor and surfactant addition on properties of synthetic alumina-containing

Received 22 April 2018

CaO-based for CO2 capture and for sorption-enhanced steam methane reforming process

Received in revised form

(SE-SMR) were investigated. Results showed that the sorbent derived from calcium D-glu-

20 June 2018

conic acid (CG-AN) offered CO2 sorption capacity of 0.38 g CO2/g sorbent, which is greater

Accepted 25 June 2018

than 0.17 g CO2/g sorbent of the sorbent derived from calcium nitrate (CN-AN). Addition of

Available online xxx

CTAB surfactant during synthesis was found to enhance CO2 sorption capacity for CG-AN but not for CN-AN sorbents. Stability tests of the modified sorbents for 10 cycles showed

Keywords:

that CG-AN-CTAB provided higher CO2 sorption capacity than CN-AN-CTAB for each cor-

Alumina-containing CaO-based sor-

responding cycle. Incorporation of CG-AN with Ni catalyst (Ni-CG-AN) using wet-mixing

bent

technique offered the longest pre-breakthrough period of 60 min for average maximum

High-temperature CO2 capture

H2 purity of 88% at 600
C and a steam/methane molar ratio of 3.

Sorption-enhanced steam methane

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

reforming H2 production

* Corresponding author. E-mail address: [email protected] (S. Wongsakulphasatch). https://doi.org/10.1016/j.ijhydene.2018.06.153 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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Introduction The demand of hydrogen is expected to continuously increase as hydrogen is considered as an ideal energy carrier [1]. It is also an important feedstock in various chemical and petroleum industries such as the production of ammonia, methanol, or refining of crude oil, etc. [2e4] At present, reforming process has been developed to use in industrial scale such as partial oxidation, dry reforming, or steam reforming, etc. Among these, steam methane reforming process (SMR) has widely been applied to produce hydrogen as this process offers high hydrogen to carbon monoxide ratio and low cost when compared to other technologies [5e8]. However, the process suffers from multi-step operation, high energy requirement, and catalyst deactivation due to severe operating conditions [1,9,10]. To minimize such disadvantages, the concept of process intensification, the combination of reaction and separation process, named “sorptionenhanced steam methane reforming (SESMR)”, has been developed and received increased attention [3,11e14]. The SESMR is operated based on Le Chatelier's principle, of which the conversion of reactants to products and the rate of forward reaction can be increased by selectively removing some of the reaction products [15]. As a consequence, by employing this concept, the limitations of conventional SMR process can be diminished; less severe operating conditions can be operated and energy efficiency can be improved as exothermic heat of adsorption could compensate to endothermic heat of reforming reaction. The process allows for the lower capital and operating costs [16,17]. In addition, it is interesting to note that, the ETP BLUE map scenario has indicated that with the use of this integrated technology, energy-related emissions of CO2 can be reduced up to 50% by 2500 compared to the level reported in 2005 [18]. One of key techniques to improve overall performances of SESMR process is the use of appropriate sorbent and catalyst. A development is the combination of an adsorbent with a catalyst into one-body. The concept of this material is to use the catalyst to speed up the reaction while the sorbent functions to selectively capture CO2, providing high H2 purity in a single step as steam reforming, water-gas shift, and carbon dioxide removal can occur simultaneously. Catalyst used for SESMR should resist coke formation by steam reforming, be inactive for side-reactions, maintain the activity at high temperature, and have high mechanical strength as well as good heat transfer properties [19]. Different types of metal oxide, e.g., platinum, nickel, rhodium, gold, etc., have been applied for SESMR process. Among these, nickel oxide has been extensively proved to be suitably used for SESMR due to its good conversion rate of methane and low cost [20e25]. Sorbent applied for SESMR should possess high sorption capacity, fast sorption/ desorption kinetics, high thermal and mechanical stability [26]. CaO has been widely used as sorbent for CO2 capture in SESMR because CaO can capture CO2 at high temperature, where the reaction of SESMR occurs, with high sorption kinetics and sorption capacity. However, loss-in-capacity problem upon multi-cycle uses due to sintering effect is the drawback of this material [27].

Different performances for SESMR are found to be varied depending upon type and amount of catalyst or sorbent as well as operating condition. For example, Martavaltzi et al. [28] developed a hybrid material by combining sorbent with catalyst for SESMR using Ni-CaO-Ca12Al14O33. The results showed that 16% of Ni on the hybrid catalytic sorbent showed 90% H2 yield at 650
C and a steam to carbon ratio of 3.4. Kim et al. [1] studied the use of one-body catalytic sorbent, which was composed of CaO, Ca12Al14O33 and 7% wt Ni-metallic, for SESMR process operated at 630
C, atmospheric pressure and a steam to carbon ratio of 3. The results showed that the one-body catalytic sorbent was observed to be very stable and maintained at ca. 94e95% H2 during sorption enhanced period of around 70 min. Radfarnia and Iliuta [29] developed Al-stabilized CaO-nickel hybrid sorbent-catalyst using wet-mixing method with various nickel loadings, including 12, 18, and 25%wt. The hybrid catalytic sorbent with 25%wt NiO loading offered the best performance for H2 production, of which average CH4 conversion and H2 production efficiency of 99.1% and 96.1%, respectively, can be obtained at 650
C and a steam to carbon ratio of 4. Xu et al. [30] investigated the performance of bi-functional catalysts Ni/CaO-Al2O3 prepared by sol-gel method with different CaO/Al2O3 mass ratios on SESMR process. The results showed that the bi-functional catalysts containing CaO/Al2O3 mass ratio of 6 or 8 showed H2 production of 87% for 50 consecutive SESMR runs at 650
C, 0.1 MPa, and H2O/CH4 molar ratio ¼ 2. This result was claimed to be due to high surface area, small Ni particle size, and uniform distribution of Ni, CaO, and Ca5All6 O14. Zhao et al. [31] investigated the performance of Ni/CaOCaZrO3 bi-functional catalysts on SESMR process. By varying amount of Ni loading and CaO content, the results showed that Ni:CaO:CaZrO3 ¼ 15:60.3:24.7 (%wt) provided the best SESMR performance in terms of activity and stability; 96% H2 could be obtained for pre-breakthrough period and 73% for post-breakthrough period. Although the development of one-body hybrid catalytic sorbent has been carried out by several researchers; however, such development is still a challenging task as an appropriate combination of catalyst and sorbent should be considered in order to satisfy the condition where both sorption and reaction conditions of the overall integrated process can be carried out in a single step. In this work, we are interested in improving properties of CaO-based sorbent for hightemperature CO2 capture and found in sorption-enhanced steam methane reforming process. Properties of CaO-based sorbent have been shown to depend on synthesis method [32], calcium precursor [33], additives [34], synthesis temperature [34], stirring rate [34], etc. Among these, the effect of calcium precursor and the addition of surfactant are subjects of investigation in this study as these two parameters have been found to affect morphologies, crystal structure, and porosities of precipitated calcium carbonate significantly [34e36]. The performances of our synthetic aluminacontaining CaO-based sorbents were investigated for hightemperature CO2 sorption in terms of sorption capacity and sorbent re-usability. Then, one-body catalyst/sorbent materials were synthesized and tested for H2 production via SESMR process.

Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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Materials and method Materials Calcium D-gluconic acid (C12H22CaO14 99%), calcium nitrate tetrahydrate (Ca(NO3)2$4H2O 99%), aluminum nitrate nonahydrate precursor (Al(NO3)3$9H2O) and 2-propanol were used to synthesize CaO-based sorbent. Nickel nitrate hexahydrate (Ni(NO3)2$6H2O) was used as catalyst precursor. Commercial calcium oxide (CaO) purchased from Riedel-deHaen and commercial alumina (Al2O3) purchased from Sigma Aldrich were used as commercial sorbent and support, respectively, for comparison purpose. Cationic surfactant, cetyltrimethylammoniumbromide (CTAB), was used to modify properties of CaO-based sorbent.

Synthesis of alumina-containing CaO-based sorbent In this work, alumina-containing CaO-based sorbents were synthesized by wet-mixing method with the use of two different precursors: calcium nitrate tetrahydrate and calcium D-gluconic acid. The sorbents synthesized from calcium nitrate tetrahydrate were designated as CN-AN and those synthesized from calcium D-gluconic acid were designated as CGAN. All sorbents were synthesized with pre-determined controlled composition of CaO:Ca12Al14O33 equal to 70:30 %wt. For CN-AN, 27.6 g of calcium nitrate tetrahydrate and 7.11 g of aluminum nitrate nonahydrate were added into a 190-ml de-ionized water, which were mixed with 32.5 ml of 2propanol. The solution was continuously stirred at 75
C for 1 h and then allowed to dry at 120
C in an oven for 18 h. The obtained powder was collected from the oven and calcined at 500
C for 3 h in air. Distilled water was later added into the resulting powder and then dried at 120
C for 2 h and calcined at 900
C for 1.5 h in air to produce CaO-Ca12Al14O33. The same procedure was conducted for CG-AN with the use of 21.5 g of calcium D-gluconic acid instead of calcium nitrate tetrahydrate and 3 g of aluminum nitrate nonahydrate.

Synthesis of alumina-containing CaO-based sorbent modified by surfactant Firstly, the desired concentration of CTAB surfactant, being 1, 3, 5, 7 or 10 mM, corresponding to 0.082, 0.246, 0.410, 0.574 or 0.820 g, respectively, was added into 190-mL de-ionized water mixed with 32.5 mL of 2-propanol. The solution was stirred at room temperature until CTAB was totally dissolved. Then, 3 g of aluminum nitrate nonahydrate and 21.5 g of calcium Dgluconic acid were added into the solution. After that, the solution was heated up to 75
C and continuously stirred for 1 h and then allowed to dry at 120
C in an oven for 18 h. The powder was collected from the oven and calcined at 500
C for 3 h in air. Distilled water was added into the resulting powder and then dried at 120
C for 2 h, followed by calcination at 900
C for 1.5 h in air. The obtained powder was named as CGAN-CTAB XX mM. The same method was applied for calcium nitrate precursor CN-AN-CTAB XX mM with the use of 27.6 g of calcium nitrate tetrahydrate and 7.11 g of aluminum nitrate nonahydrate.

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Synthesis of one-body catalyst/sorbent material To synthesize one-body material, Ni was incorporated with alumina-containing CaO-based sorbent by wetness impregnation method. In this work, 12.5%wt of Ni was desired to use as catalyst for SESMR followed Chanburanasiri et al. [22] To prepare the material, 12.5%wt-nickel nitrate hexahydrate (Ni(NO3)2.6H2O) was firstly dissolved in 75 ml of de-ionized water. After that, the 2-g synthetic alumina-containing CaO-based sorbent was added into nickel solution. Then, the sample was stirred for 30 min at 100
C, dried at 100
C overnight, and calcined at 900
C for 1.5 h in air. The resulting material was designated as Ni-CGAN or Ni-CN-AN in case of using alumina-containing CaObased sorbent without surfactant modification and as Ni-CG-AN-CTAB or Ni-CN-AN-CTAB in case of using alumina-containing CaO-based sorbent with surfactant modification. To gain an insight into the role of CaO-based alumina sorbent on hydrogen production, the effect of synthesis method of CaO-based sorbent were investigated. Simplified sol-mixing method was selected to compare with wet-mixing. For sol-mixing technique, firstly, calcined calcium D-gluconic acid (C12H22CaO14) (at 900
C, 1.5 h) and aluminum nitrate nonahydrate were added into the 190-ml de-ionized water mixed with 32.5 ml of 2-propanol solution. The mixture solution was continuously stirred at 75
C for 1 h and then allowed to dry at 120
C in an oven for 18 h. The powder was collected from the oven and calcined at 500
C for 3 h in air. Distilled water was added to the resulting powder to form a paste which was then dried at 120
C for 2 h and calcined at 900
C for 1.5 h in air. To combine with Ni, 12.5%wt -nickel nitrate hexahydrate (Ni(NO3)2.6H2O) was firstly dissolved in 75 ml of de-ionized water. After that, the 2-g synthetic alumina-containing CaO-based sorbent was added into nickel solution. Then, the sample was stirred for 30 min at 100
C, dried at 100
C overnight, and calcined at 900
C for 1.5 h in air. The resulting material was designated as Ni-CG-AN solmixing.

Characterization of synthetic materials Crystallinity of the synthetic materials was characterized by X-ray powder diffraction (XRD), which was determined by D8 Advance of Bruker AXS using Cu Ka radiation (l ¼ 1.5406  A). The pattern was recorded for 2q ranging from 10 to 80
with an increasing step of 0.04
and a scan speed of 0.5 min1. Surface texture of the material was measured by N2 adsorption/desorption technique using Micromeritics Chemisorp 2750. The sample was degassed at 200
C for 3 h prior to conducting the measurement at 196
C. The results of N2 isotherms was used to estimate surface area with the use of BET (Brunauer-Emmett-Teller) method. Total pore volume was estimated from the amount of N2 adsorbed at relative pressure of 0.99. Pore diameter and pore size distribution of the sample was examined by the BJH (Barrett-Joyner-Hallenda) method [37,38]. Morphology of the samples was detected by scanning electron microscope, SEM (JEOL JSM 6360 LV).

Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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Sorption and SESMR performance tests In this work, performances of the synthetic aluminacontaining CaO-based sorbent on CO2 sorption and of the one-body catalyst/sorbent on the production of H2 via SESMR process were conducted using a fixed-bed reactor. Quartz tube with ID ¼ 10 mm, OD ¼ 12 mm, and length ¼ 500 mm was used as the fixed-bed reactor. For CO2 sorption test, sorbent was packed in the reactor, which was supported by quartz wool. Before conducting CO2 sorption experiment, the sorbent was pretreated by purging 50-mL/min pure N2 at 850
C for 1 h. Then, CO2 sorption test was carried out by switching the gas to 10 mL/min CO2 flow (15% v/v, balanced N2) and reducing the temperature to the desired CO2 sorption temperature of 600
C. The sorption duration was carried out until full capacity of the sorbent, which was observed by CO2 concentration at the outlet equals to that at the inlet. An amount of CO2

sorption capacity was measured by a mass spectrometer. Multiple cycles of sorption/desorption were repeated in order to test the ability to retain CO2 sorption capacity. The desorption test was conducted by purging 50 mL/min of pure N2 at 850
C for 0.5 h. CO2 sorption capacity was calculated from breakthrough curves using the following equation [39]:
.  CO2 sorption capacity gCO2 gsorbent 3 2  Z t CCO2 ;out 4FCO ;in  1  A, dt5 2 CCO2 ;in 0 ¼ W

(1)

where FCO2 is feed flow rate of CO2 (g min1), CCO2 ;in and CCO2 ;out are concentrations of CO2 at the inlet and outlet of the reactor (%), respectively, t is sorption time (min), W is weight of the sorbent (g), A is calculated from

Fig. 1 e XRD patterns of modified sorbents by different concentrations of CTAB; a) CN-AN and b) CG-AN. Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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0

1 yCO2 @ A A¼ , 1þ
ytotal;in 1  yCO2 yN2

(2)

where yN2 is volumetric flow rate of N2 (ml min1), ytotal;in is total volumetric flow rate of feed stream (ml min1), yCO2 is volume fraction of CO2 in the outlet stream. SESMR was conducted by packing one-body catalyst/sorbent material in the same reactor as carried out with the CO2 sorption test. Before running the reaction test, the one-body catalyst/sorbent was pretreated at 850
C for 1 h with N2 (30 mL/min) and the catalyst sites were reduced in a flow of H2/ N2 (50% H2) at 850
C for 1 h. A 10-mL/min CH4 was fed into the reactor with the co-flow N2 of 20 mL/min and steam of 30 mL/ min, which would yield the steam-to-methane ratio of 3. In our experiments, the inlet steam was generated by heating liquid water at 150
C, which was controlled by wrapped thermotape along the injection pipe. The reaction was conducted isothermally at 600
C under atmospheric pressure. Product compositions were analyzed by gas chromatography (GC-8A, Shimadzu) equipped with a thermal conductivity

detector (TCD) using molecular sieve 5A and Parapak-Q packed column. The performances of one-body catalyst/sorbent were examined in terms of CH4 conversion and product compositions. The conversion of CH4 is defined as: CH4 conversion ð%Þ ¼


 FCH4;in  FCH4;out FCH4;in

 100

(3)

where FCH4;in and FCH4;out are molar flow rates of methane at the inlet and the outlet, respectively.

Results and discussions Effect of calcium precursor and surfactant addition on CaObased alumina sorbents for CO2 capture XRD patterns of modified alumina-containing CaO-based sorbents synthesized by using different precursors and CTAB concentrations are shown in Fig. 1. Typical diffraction peaks assigned to CaO at 2q ¼ 28.2, 32.3, 37.4, 53.8, 64.3, 67.6 and 79.1


Fig. 2 e SEM images of CN-AN sorbents modified by adding different CTAB concentrations; a) CN-AN without CTAB and b) CN-AN-CTAB 1 mM c) CN-AN-CTAB 3 mM d) CN-AN-CTAB 5 mM e) CN-AN-CTAB 7 mM and f) CN-AN-CTAB 10 mM. Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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and those of Ca12Al14O33 inert support at 2q of 18, 30, 34 and 57
, are observed for all CN-AN and CG-AN samples investigated here, which is in good agreement with those reported by  rio et al. [32] and Zhou et al. [33]. Cesa Morphologies of CN-AN and CG-AN photographed by SEM are shown in Figs. 2a and 3a, respectively. CN-AN shows uniform distribution of octahedral particles, whereas connected small rounded particles are observed with CG-AN. Particle size of CN-AN is larger than CG-AN; CN-AN has particle size of 0.5 mm in average whereas particle size of CG-AN is 0.3 mm in average. The smaller particle size observed with CGAN could be attributed to the impurity D-gluconic acid  (C6H12O2 7 ) is larger than nitrate (NO3 ); the larger molecular size could obstruct the interaction between calcium and oxygen or calcium and alumina, resulting in weak interaction between such molecules during precipitation and leading to the formation of loosely packing of precipitated particles CaOCa12Al14O33 (see Fig. 4). Larger particle size with larger void between particles was observed when CTAB was added into the CN-AN; 1 mm for CN-

AN-CTAB 1e5 mM and 3 mm for CN-AN-CTAB 7e10 mM. In contrast, particle size of CG-AN-CTAB sorbents remain unchanged but larger void between particles is observed (see Figs. 2bef and 3bef, respectively). The void between particles, which makes the sorbent to be structured with pore, is a result of structure directing by CTAB micelles as shown by the proposed mechanism pictured in Fig. 4. The micelles of CTAB in the solution hindered the interaction between precipitated solids during precipitation; when precipitated solids are washed or heated during calcination, the micelles are removed from the system and left spaces between particles. Increasing CTAB concentration leads to larger void space. This result could be because a number of pore generated by CTAB micelles combined into a single large void space when a number of micelles are increased in the system as shown in Fig. 4b and d [40]. Comparison between CN-AN-CTAB and CGAN-CTAB for each corresponding CTAB concentration, the results show that CN-AN-CTAB sorbents possess lower BET surface area and pore volume than those of CG-AN-CTAB sorbents, whereas, pore diameter of CN-AN-CTAB sorbents

Fig. 3 e SEM images of CG-AN sorbents modified by adding different CTAB concentrations; a) CG-AN without CTAB and b) CG-AN-CTAB 1 mM c) CG-AN-CTAB 3 mM d) CG-AN-CTAB 5 mM e) CG-AN-CTAB 7 mM and f) CG-AN-CTAB 10 mM. Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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are larger than those of CG-AN-CTAB sorbents. This could probably be due to the effect of molecular structure of calcium precursor; calcium nitrate precursor has smaller molecular size than that of calcium D-gluconic acid, short range interaction would occur for the case of calcium nitrate, leading to dense packing of particles after calcination as evidenced by SEM images depicted in Figs. 2 and 3.

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Amount of CO2 sorption capacity of all CN-AN sorbents are provided in Fig. 5a. The results show that all sorbents provide similar CO2 sorption capacity of ca. 0.2 g CO2/g sorbent. This could be due to similar surface texture of all sorbents (Table 1), leading to comparative CO2 sorption capacity. For the case of CG-AN sorbents (Fig. 5b), higher CO2 sorption capacity is observed with CG-AN-CTAB when compared to those of CN-

Fig. 4 e Proposed mechanisms of CaO-based sorbent formation for a) CN-AN-CTAB at low CTAB concentration, b) CN-ANCTAB at high CTAB concentration, c) CG-AN-CTAB at low CTAB concentration, d) CG-AN-CTAB at high CTAB concentration. Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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Fig. 5 e CO2 sorption capacity of a) CN-AN and b) CG-AN sorbents with and without modification by adding CTAB. Sorption condition: atmospheric pressure, sorption temperature ¼ 600
C, 15%v/v CO2 (N2 balanced).

AN-CTAB. This could be attributed to higher surface area and smaller interconnected particle size of the sorbent. When CTAB concentration is increased from 1 mM to 5 mM, CO2 sorption capacity increases from 0.42 to 0.46 g CO2/g sorbent and then the sorption capacity declines to 0.38 g CO2/g sorbent when CTAB concentration is further increased to 10 mM. The increase of sorption capacity at low concentration of CTAB could be attributed to an enhancement of active surface CaO due to void space generated by CTAB micelles and small particle size of the sorbent as discussed previously, whereas the reduction of sorption capacity at higher CTAB concentration could be due to sintering effect. Compared to other solid

sorbents, such as Li4SiO4 [41], Li8ZrO6 [42], Li2ZrO3 [42], hydrotalcite [43], or Na2ZrO3 [44], etc., as shown in Table 2, our results show that alumina-containing CaO-based sorbents offer good performance on high-temperature CO2 capture. Although sorption capacity is an important factor on determining performance of the sorbent; however, for industrial applications, re-usability of the material is another essential property that is needed to be considered. In this study, stability of the synthetic alumina-containing CaObased sorbents were investigated using CN-AN-CTAB 3 mM and CG-AN-CTAB 3 mM as the previous results showed that these sorbents provide good performance on CO2 sorption

Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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Table 1 e BET surface area, pore volume, and pore size diameter of modified CN-AN and CG-AN calcium-based sorbents with different CTAB concentrations. Sample

CN-AN CN-AN-CTAB 1 mM CN-AN-CTAB 3 mM CN-AN-CTAB 5 mM CN-AN-CTAB 7 mM CN-AN-CTAB 10 mM CG-AN CG-AN-CTAB 1 mM CG-AN-CTAB 3 mM CG-AN-CTAB 5 mM CG-AN-CTAB 7 mM CG-AN-CTAB 10 mM

Surface area (m2/g)

Pore volume (cm3/g)

Pore size diameter (nm)

7.3 5.6 6.3 7.5 5.2 6.3 11.3 12.4 14.1 12.1 12.0 7.6

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.03

8.8 9.4 10.8 15.6 9.7 10.7 6.8 4.2 4.6 7.8 7.2 8.5

capacity. The results of CO2 sorption capacity for 10 cycles run are shown in Fig. 6. The CG-AN-CTAB 3 mM sorbent shows higher sorption capacity than CN-AN-CTAB 3 mM for each corresponding cycle but greater decrease upon multiple repeated cycles is observed. The greater reduction might be due to particles of CG-AN-CTAB 3 mM collapsed and aggregated during heating upon multiple-cycle tests, causing the reduction of CaO active surface. This hypothesis is evidenced by SEM images shown in the inset in Fig. 6 that particles of CNAN-CTAB are likely unchanged whereas particles of CG-ANCTAB become larger after 10 cycles test.

One-body hybrid catalytic sorbent for sorption-enhanced steam methane reforming Effect of calcium precursor and the addition of CTAB surfactant As seen in the previous section, CN-AN-CTAB 3 mM and CGAN-CTAB 3 mM are good sorbents for CO2 capture. In this section, both sorbents were combined with Ni catalyst to use as one-body catalyst/sorbent for SESMR process. Fig. 7 shows

Table 2 e Summary of optimal CO2 sorption parameters for various sorbents. Sorbent CaO-Ca12Al14O33 Li4SiO4 Li8ZrO6 K-doped Li2ZrO3 Potassium modified hydrotalcite (K-HTC) Na2ZrO3 CaO-Ca12Al14O33

Sorption temperature (
C) PCO2 Sorption technique CO2 sorption capacity (bar) (g CO2/g sorbent) 690 600 800 550 400 800 600

0.15 1 1 0.25 0.5 1 0.15

TGA TGA TGA TGA CI Microbalance TGA Fixed-bed

0.37 0.37 0.35 0.22 0.026 0.22 0.46

Ref. [22] [35] [36] [37] [38] [39] This work

Fig. 6 e Stability test for CG-AN-CTAB 3 mM and CN-AN-CTAB 3 mM modified sorbents. Sorption condition: atmospheric pressure, 600
C, 15%v/v CO2 (N2 balanced). Regeneration condition: atmospheric pressure, 850
C under pure N2. Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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Fig. 7 e XRD patterns of hybrid multi-functional materials: NiO/CaO-Ca12Al14O33.

XRD patterns of Ni-CN-AN, Ni-CG-AN, Ni-CN-AN-CTAB 3 mM, and Ni-CG-AN-CTAB 3 mM. The main peaks representing CaO-Ca12Al14O33 are observed similar to those found in the case of solely alumina-containing CaO-based sorbent reported previously. The additional peaks representing NiO are observed at 2q of 43 and 79.1
, and are in consistent with those s et al. [45]. SEM images (Fig. 8) show that Nireported by Moise CN-AN has particle size of ca. 0.5 mm and larger particle size of ca. 0.8 mm is observed with Ni-CG-AN. The addition of CTAB leads to an increase of particle size for Ni-CG-AN-CTAB 3 mM

to be 1.5 mm, whereas the particle size of Ni-CN-AN-CTAB 3 mM remains nearly the same, ca. 0.5 mm, when compared to the unmodified sorbent Ni-CN-AN. Fig. 9 shows product compositions (dry basis) obtained from SESMR using our one-body catalyst/sorbent. For the case of using Ni-CN-AN (Fig. 9a), three regions of H2 production, pre-breakthrough, breakthrough, and post-breakthrough are observed. In the pre-breakthrough period, H2 purity of ca. 93% could be produced for 30 min, which is higher than the production from SMR at equilibrium ca. 75%. The higher H2

Fig. 8 e SEM images of a) Ni-CN-AN, b) Ni-CG-AN, c) Ni-CN-AN-CTAB 3 mM, d) Ni-CG-AN-CTAB 3 mM. Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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Fig. 9 e Gas product compositions (dry basis) of a) Ni-CN-AN, b) Ni-CG-AN, c) Ni-CN-AN-CTAB 3 mM, d) Ni-CG-AN-CTAB 3 mM, and e) NieAl2O3,commercial from SESMR. Preparation of one-body catalyst/sorbent materials: wet-mixing. Operating conditions: atmospheric pressure, reaction temperature of 600
C, steam/methane molar ratio of 3, and total flow rate ¼ 60 ml/min.

Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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Table 3 e BET surface area, pore volume, and pore size diameter of modified Ni-CN-AN and Ni-CG-AN. Sample

Ni-CN-AN Ni-CN-AN-CTAB 3 mM Ni-CG-AN Ni-CG-AN-CTAB 3 mM Ni-CG-AN sol-mixing

Surface area (m2/g)

Pore volume (cm3/g)

Pore size diameter (nm)

12.4 16.9

0.02 0.05

9.9 16.8

13.8 13.3

0.04 0.04

10.7 10.5

16.0

0.03

7.7

not observed for Ni-CN-AN-CTAB 3 mM; H2 production gradually decreases from 88% and reaches equilibrium (Fig. 9c). The result of no pre-breakthrough period would be due to sintering effect; a number of void space generated by CTAB modified alumina-containing CaO-based sorbents are collapsed at high temperature, leading to the hindrance of either CaO or Ni active surfaces. On the other hand, Ni-CG-ANCTAB 3 mM offers 30 min of pre-breakthrough period and H2 produced due to sorption-enhanced reaction is ca. 90%. The observation of pre-breakthrough period could be attributed to the contribution of high CO2 sorption capacity of CG-AN-CTAB 3 mM sorbent in sorption-enhanced reaction, whereas shorter pre-breakthrough period could be due to particle agglomeration caused from high temperature reaction.

Effect of synthesis method production obtained during pre-breakthrough period indicates the enhancement of H2 production due to the effect of simultaneous CO2 removal by the alumina-containing CaObased sorbent. After pre-breakthrough period, H2 content gradually decreases, while CO, CO2, and CH4 contents increase, indicating that the sorbent is gradually saturated and the effect of sorption-enhanced reaction becomes less effective. This period is called breakthrough period. After 75 min, the alumina-containing CaO-based sorbent is saturated with CO2, the effect of sorption-enhanced reaction is no longer pronounced and the SESMR becomes SMR, which is referred to as post-breakthrough period. In case of Ni-CG-AN (Fig. 9b), longer pre-breakthrough period of 60 min with lesser H2 production of ca. 88% is observed when compared to Ni-CN-AN. The result of longer pre-breakthrough period could probably be due to higher CO2 sorption capacity of Ni-CG-AN as a result of void space between particles as well as high surface active exposure (see Table 3). In case of CTAB modified aluminacontaining CaO-based sorbent, pre-breakthrough period is

Two combination techniques, wet-mixing and sol-mixing, were selected to investigate the effect on H2 production. Fig. 10 shows XRD pattern of one-body catalyst/sorbent materials prepared by wet-mixing and sol-mixing methods. The results show similar compositions and phase formation for both materials as similar XRD patterns representative of CaO, Ca12Al14O33, and NiO are observed. SEM images of Ni-CG-AN wet-mixing and Ni-CG-AN sol-mixing shown in the inlet of Fig. 10 reveal Ni-CG-AN sol-mixing possesses larger aggregated particles with larger void space compared to Ni-CG-AN wet-mixing. Fig. 11 shows product compositions obtained from SESMR using one-body catalyst/sorbent materials prepared by wetmixing and sol-mixing methods. The results show adsorption period of 30 min with the maximum H2 purity of 94% is obtained from Ni-CG-AN sol-mixing whereas longer prebreakthrough time of 60 min with lower maximum H2 purity of 88% is observed with Ni-CG-AN wet-mixing. The

Fig. 10 e XRD pattern of hybrid multi-functional materials: 12.5%Ni-CG-AN-wet mixing and 12.5%Ni-CG-AN sol-mixing.

Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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improvement of maximum H2 purity observed with the sorbent prepared by sol-mixing method might be because CaO, which was transform from calcination of calcium D-gluconic acid at 900
C, reacted with H2O and became Ca(OH)2. Because of the volume increase from CaO to Ca(OH)2 and the expansion of particle caused by the exothermic hydration of CaO, the aggregates crack and swell and the regular hexagonal crystalloids of Ca(OH)2 are generated, leading to larger active surface of the material [46,47]. This result is inconsistent with surface texture examined by BET as shown in Table 3 that higher surface area is observed with Ni-CG-AN sol-mixing.

Performance comparison of the developed one-body hybrid catalytic sorbent for sorption-enhanced steam methane reforming Our investigations have shown that synthesis of aluminacontaining CaO-based sorbents with the use of different calcium precursors or the addition of structure-directing agent affects properties of the sorbent, which in turn, affects CO2 sorption capacity and H2 production. A key finding from our studies is that, for the application to SESMR process, combining sorbent and catalyst into one-body material is not a direct combination of sorption and catalyst performances. As seen, although CG-AN-CTAB 3 mM sorbent offers the highest CO2 sorption capacity; however, when the sorbent is combined with NiO, sorption-enhanced reaction period has less impact when compared to the unmodified sorbent. This could possibly be due to better distribution of Ni active site on the surface of the sorbent; dense packing of sorbent particles could support Ni better than the loose one, which could result in higher active catalytic sites for steam reforming reaction. However, it is worth to note that performance of hydrogen production depends upon not only properties of multi-functional material but also operating conditions of SESMR process. Table 4 summarizes H2 production from SESMR process using different sorbent sources, different synthesis methods, and different operating conditions reported in the literature compared with our results. Consistence with other works, high H2 purity can be obtained with the use of CaO-based sorbent. Our developed materials show that high H2 purity can be produced at milder operating conditions, i.e., lower temperature and lower steam to methane ratio (S/C).

Fig. 11 e Gas product compositions (dry basis) of a) Ni-CGAN wet-mixing and b) Ni-CG-AN sol-mixing from SESMR. Operating conditions: atmospheric pressure, reaction temperature of 600
C, steam/methane molar ratio of 3, and total flow rate ¼ 60 ml/min.

Table 4 e Comparison of H2 production using multi-functional materials containing CaO as sorbent. Catalyst 12.5 wt%Ni/Al2O3þCaO 16%NiO-CaO-Ca12Al14O33 18%Ni-CaO-CaZrO3 (CTAB)

Source of calcium oxide Calcium oxide Calcium acetate Calcium acetate

12.5%Ni-CaO-Ca12Al14O33 Calcium D-gluconic acid 12.5%Ni-CaO (CTAB)-Ca12Al14O33 Calcium nitrate 12.5%Ni-CaO-Ca12Al14O33 12.5%Ni-CaO (CTAB)-Ca12Al14O33 a

Condition CaO synthesis Sorption-enhanced Maximum Ref. method period H2 concentration a
T( C) S/C 600 650 650

3 3.4 4

600 600 600 600

3 3 3 3

Commercial Sol-mixing Wet-mixing with sonication Wet-mixing and hydration Wet-mixing and hydration

10 min 110 min 18 min

65% 90% 92%

[22] [27] [28]

60 min 45 min 30 min e

88% 90% 93% 88%

This work This work

S/C ¼ steam to methane ratio.

Please cite this article in press as: Pecharaumporn P, et al., Synthetic CaO-based sorbent for high-temperature CO2 capture in sorptionenhanced hydrogen production, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.153

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Conclusions We have investigated the effect of calcium precursor, the addition of structure-directing agent, and synthesis method of CaO-based alumina materials on high-temperature CO2 capture in sorption-enhanced steam methane reforming. Different calcium precursors provided different polymorphs and also the ability to adsorb CO2. The sorbents derived from calcium D-gluconic acid (CG) offered higher CO2 sorption capacity than calcium nitrate (CN) in mixed CO2/N2 system (15% v/v CO2) at 600
C due to their higher surface area and smaller particle size. Modification properties of alumina-containing CaO-based sorbents by addition of CTAB showed an improvement of CO2 sorption capacity in case of using calcium D-gluconic acid as calcium precursor. However, modification of alumina-containing CaO-based sorbent by CTAB did not provide favorable role in H2 production by SESMR as shorter sorption-enhanced reaction period was observed when compared to the unmodified one. An incorporation of the synthetic alumina-containing CaO-based sorbent with Ni to use as one-body catalyst/sorbent for SESMR showed that incorporating 12.5% Ni with CG-AN by wet-mixing method provides the longest pre-breakthrough period of 60 min with 88% H2 at 600
C and a steam/methane molar ratio of 3.

Acknowledgements The authors would like to thank the Ratchadapisek Sompoch Endowment Fund (2016), Chulalongkorn University (CU-59003-IC) and the Thailand Research Fund (DPG5880003) for funding supports.

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