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bayerite adsorbent
Accepted Manuscript High CO adsorption capacity, and CO selectivity to CO2, N2, H2, and CH4 of CuCl/ bayerite adsorbent Kanghee Cho, Jungsu Kim, Jong-ho Park, Taesung Jung, Hee Tae Beum, Dong-woo Cho, Young Woo Rhee, Sang Sup Han PII:
S1387-1811(18)30541-9
DOI:
https://doi.org/10.1016/j.micromeso.2018.10.010
Reference:
MICMAT 9148
To appear in:
Microporous and Mesoporous Materials
Received Date: 26 July 2018 Revised Date:
5 October 2018
Accepted Date: 12 October 2018
Please cite this article as: K. Cho, J. Kim, J.-h. Park, T. Jung, H.T. Beum, D.-w. Cho, Y.W. Rhee, S.S. Han, High CO adsorption capacity, and CO selectivity to CO2, N2, H2, and CH4 of CuCl/ bayerite adsorbent, Microporous and Mesoporous Materials (2018), doi: https://doi.org/10.1016/ j.micromeso.2018.10.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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High CO adsorption capacity, and CO selectivity to CO2, N2, H2, and CH4 of CuCl/bayerite adsorbent Kanghee Choa,1, Jungsu Kima,b,1, Jong-ho Parka, Taesung Junga, Hee Tae Beuma, Dong-woo
a
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Cho, Young Woo Rheeb*, Sang Sup Hana**
Climate Change Research Division, Korea Institute of Energy Research, 152 Gajeong-ro,
Yuseong-gu, Daejeon 34129, Republic of Korea
Graduate School of Energy Science and Technology, Chungnam National University, 99
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b
Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea
Both authors contributed equally to this work with first authorship.
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*Corresponding author at: Chungnam National University, Daejeon 34134, Republic of Korea. (E-mail address: [email protected])
**Corresponding author at: Korea Institute of Energy Research, Daejeon 34129, Republic of
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HIGHLIGHTS
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Korea. (E-mail address: [email protected])
• Highly selective CO adsorbent based on CuCl/bayerite composite was synthesized.
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• Adsorbent exhibited very high CO/CO2 selectivity. • High separation ability is attributed to the large surface area of bayerite. • Thermal treatment conditions for dispersion of CuCl on bayerite were optimized.
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ACCEPTED MANUSCRIPT ABSTRACT
We synthesized nanoporous CO-selective adsorbent composed of CuCl supported on bayerite. Before supporting the CuCl, the bayerite chemical was calcined at 623 K to increase
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the surface area to 469 m2 g-1. The CuCl was highly dispersed on the activated bayerite via a thermal monolayer dispersion process. The highest CO adsorption capacity was achieved at an optimal temperature of 573 K and CuCl content of 30 wt%. Thus synthesized adsorbent
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exhibited a high CO adsorption capacity of 48.5 cm3 g-1 at 293 K, but a very low CO2 adsorption capacity (2.89 cm3 g-1), resulting in CO/CO2 separation factor of 16.8. When the
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CuCl content increased to 36 wt%, the adsorbent exhibited much higher separation factor (35.5), although the CO adsorption capacity was somewhat smaller (41.6 cm3 g-1). The CO adsorption capacity and CO/CO2 selectivity of this adsorbent are larger than those of our previous CuCl/boehmite adsorbent showing CO adsorption of 34 cm3 g-1 and a CO/CO2
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separation factor of 12.4. The present adsorbent also shows very high selectivity for CO over H2, N2, and CH4. Therefore, this adsorbent is expected to show excellent CO separation performance for various industrial processes such as steam-reforming and steel-making
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which involve CO, CO2, H2, N2, and CH4.
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Keywords: Adsorption; Carbon monoxide; High CO/CO2 selectivity; Adsorbent; CuCl/bayerite
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ACCEPTED MANUSCRIPT 1. Introduction Carbon monoxide (CO) is a highly valuable chemical that is a raw material for producing various chemicals such as acetic acid, polycarbonate, isocyanates, and polyurethane [1-4]. Furthermore, it has recently receiving great interest as a main component
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of syn-gas (CO+H2) that can be used to synthesize fuel and olefins. CO is produced by steam reforming, partial oxidation, and steelmaking processes [2-6]. In these industrial processes, CO is exhausted as a mixture with CO2, N2, H2, and CH4. Owing to its toxicity, CO is
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normally burned off, thus producing heat energy. To recover this high-value component, and to reduce the generation of CO2 during CO combustion, there is a strong demand for CO
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separation technologies.
Various technologies for CO separation have been developed, such as cryogenic distillation, absorption, and adsorption processes [5-8]. Cryogenic distillation has been widely used for large-scale separation of CO, but it is a highly energy-consuming process.
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Especially, the separation of CO from gases containing N2 is significantly challenging because of the two species’ similar boiling points [2]. COSORB, a typical commercial process for CO absorption separation uses a CuAlCl4/toluene solution to absorb CO
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selectively [2]. Although this process achieves high CO purity and a high yield, there are
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environmental issues associated with disposal of the spent solvent [7,8]. Vacuum-pressure swing adsorption (VPSA) process is a promising method for CO
separation because of its relative simplicity. This process, which involves low energy consumption and low operation cost, can achieve high CO purity and high yield. In this process, Cu(I) complexes that show strong interactions with CO via π-complexation are used for CO adsorption. The s-orbitals of Cu(I) ions can form σ bonds with the π orbitals of CO molecules, and π back-bonding is simultaneously formed between the d-orbitals of Cu(I) ions and the antibonding π-orbitals of CO molecules [9-12]. For efficient CO separation, it is very
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ACCEPTED MANUSCRIPT important that the adsorbent has a high CO adsorption capacity and high CO selectivity. Both factors are significantly relevant to the selection of supporting materials for the Cu(I) [13]. To realize high dispersion of Cu(I) species and, consequently, a large CO adsorption capacity, it is necessary to use a suitable nanoporous material that exhibits a large surface area and strong
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interactions with Cu(I) complexes. Simultaneously, to enhance the selectivity for CO, the supporting material should show weak interaction with other impurities such as CO2, N2, H2, and CH4.
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Xie et al. [1] used zeolite Y and 13X zeolite as supporting materials for CuCl to prepare CO adsorbent. They physically mixed zeolites and CuCl and heated the mixture to a
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certain temperature between the Tammann temperature (the temperature at which the mobility and reactivity of the molecules in a solid become appreciable) and the melting point of CuCl. This adsorbent exhibited CO adsorption capacity higher than 2.2 mmol g-1 at 298 K. However, as it also showed a similar CO2 adsorption capacity, this adsorbent could not
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efficiently separate CO from CO2. Hirai et al. [14] prepared copper(I) halide-loaded activated carbon (AC) adsorbents using an impregnation method. Their CuCl/AC adsorbent had high CO adsorption capacity owing to the large BET surface area of AC, but it also exhibited high
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CO2 adsorption capacity owing to the natural properties (high microporosity and high CO2
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affinity) of AC [14-18].
Recently, using boehmite as a supporting material, Cho et al. developed a CO-
selective adsorbent that exhibited high CO adsorption capacity (1.56 mmol g-1 at 293 K) and high CO/CO2 selectivity (12.8) [13]. Although the CO adsorption capacity of this adsorbent is somewhat smaller than those of the previous zeolite-based adsorbents, the high CO selectivity is advantageous. The most important part of the development of this adsorbent was the use of boehmite as a supporting material. As Boehmite exhibits a large BET surface area (~270 m2 g-1) and sufficiently strong interactions with CuCl species, high dispersion of CuCl
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ACCEPTED MANUSCRIPT species can be realized. Moreover, as the interaction between boehmite and CO2, the adsorption capacity for CO2 is very low. In their report, Cho et al. claimed that high content of hydroxyl groups on the surface of boehmite likely leads to strong interactions with CuCl species and large ratio of mesopores to micropores cause very low CO2 adsorption [13]. If a
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supporting material with a larger BET surface area and more hydroxyl groups is used, the synthesized CO adsorbent may show a better CO adsorption capacity and higher CO selectivity.
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In this work, we used bayerite as a supporting material to synthesize a CO-selective adsorbent that shows higher CO adsorption capacity and CO selectivity than previous CO
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adsorbents. Pristine bayerite has a small BET surface area, but thermal activation at 623 K, increases the surface area to more than 400 m2 g-1. To gain an in-depth understanding of the advantages of using bayerite to support CuCl species, we analyzed the physical properties of the bayerite supporting material. In addition to a large surface area, bayerite is known to have
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a higher content of hydroxyl groups than boehmite. Owing to these properties, it was possible to impregnate large amounts of CuCl into bayerite via a thermal monolayer deposition route. The physical properties of the thus-synthesized adsorbent were characterized by powder X-
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ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray spectroscopy
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(SEM/EDS), and the Brunauer–Emmett–Teller (BET) method. The adsorption capacities of the new adsorbent for CO, CO2, N2, H2, and CH4 were measured using a volumetric sorption analyzer, and the CO/CO2, CO/N2, CO/H2, and CO/CH4 separation factors were calculated from the adsorption isotherms via theoretical calculation using the Langmuir-Freundlich equation.
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ACCEPTED MANUSCRIPT 2. Experimental 2.1. Preparation of adsorbent The adsorbent was prepared by impregnating CuCl (Aldrich, 97%) into high-purity synthetic bayerite (Sasol) via a thermal monolayer dispersion process, following a similar
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method to that used to synthesize the previous CuCl/boehmite-based CO adsorbent [13]. Before CuCl impregnation, bayerite was calcined at 623 K (heating rate: 3 K min-1) for 6 h under nitrogen condition. After cooling to room temperature, the calcined bayerite and CuCl
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chemical were mixed thoroughly. Finally, to obtain the adsorbent, the mixture was placed in a quartz tube under vacuum conditions, and heated to certain temperature between 523 K and
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673 K (heating rate: 3 K min-1) for 24 h.
2.2. Adsorbent characterization and adsorption measurements
The surfaces areas of pristine bayerite, activated bayerite, and the CuCl-loaded
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adsorbent, which were thermally treated at different temperatures, were assessed by BET (Tristar 3020, Micromeritics) using N2 adsorption at liquid N2 temperature. The pore size distributions of the samples were determined from the N2 adsorption isotherms using the
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Barrett–Joyner–Halenda (BJH) method. Before the measurements, the samples were
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degassed for 12 h at 473 K.
The structures of the adsorbent and bayerite samples were determined by powder
XRD (Rigaku, D/MAX–2500) using Cu Kα radiation with a step width of 0.02o and a step time of 1 s. The morphology and the elemental distribution of the adsorbent were characterized by SEM/EDS (Hitachi, S–4800). After thermally treating the adsorbent at 573 K for 12 h under vacuum, CO and CO2 adsorption isotherms were measured by volumetric method at 293 K (Tristar 3020,
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ACCEPTED MANUSCRIPT Micromeritics). Adsorption separation factor was calculated from the ratios of the amounts of
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adsorbed CO and CO2 at 100 kPa.
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ACCEPTED MANUSCRIPT 3. Results and discussion The XRD patterns of pristine and the bayerite samples calcined at various temperatures are shown in Fig. 1. The XRD pattern of pristine bayerite shows representative peaks of the bayerite crystalline phase at 2θ = 18 o, 20 o, 27 o, and 40o. The XRD pattern of the
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bayerite sample calcined at 523 K is similar XRD, except for the appearance of a very weak peak centered at 2θ = 14.5o. This result indicates that although there is some transformation of bayerite during calcination at 523 K, the pristine crystalline phase is largely maintained.
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The bayerite samples calcined at temperatures higher than 573 K exhibited completely different XRD patterns, which included peaks corresponding to η- and θ-alumina. It is well
temperatures higher than 573 K.
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known that bayerite is gradually transformed to η- and θ-alumina by thermal treatment at
To determine the optimal temperature for inducing a large surface area in bayerite, pristine bayerite was calcined at various temperatures between 523 K and 773 K, and the pore
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textural properties of these samples were characterized by N2 physisorption. The pristine bayerite sample was characterized after degassing at 323 K, which is much lower than the calcination temperatures used for the other bayerite samples, and the other samples calcined
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at higher temperatures were analyzed after degassing at 523 K. Fig. 2 shows the N2 sorption
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isotherms of the calcined bayerite samples, and Fig. S1 shows the corresponding BJH pore size distributions. The BET surface areas and pore volumes of the samples are summarized in Table 1. The BET surface area of the pristine bayerite sample was only 10 m2 g-1, which was much smaller than those of the calcined bayerite samples. The BET surface area of the bayerite sample calcined at 523 K was somewhat smaller (397 m2 g-1) than those of the samples calcined at temperatures between 573 K and 673 K, which were all quite similar (~ 460 m2 g-1). The bayerite sample calcined at 623 K showed the largest BET surface area (469 m2 g-1), which is approximately 70% larger than that of boehmite [13]. The BET surface area
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ACCEPTED MANUSCRIPT of the sample calcined at 777 K was reduced to 307 m2 g-1. This decrease can be explained by condensation of the pore walls into thicker frameworks during calcination at high temperatures. The pore volumes of the bayerite samples increased gradually with the increase in calcination temperature. This increase was due to the generation of large void spaces
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during the transformation and condensation of the bayerite framework.
As mentioned above, for bayerite, 623 K was found to be the optimal temperature for obtaining a large BET surface area, which should be advantageous for the dispersion of CuCl
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species. The amount of CuCl dispersed on activated bayerite is likely to be greater than that on boehmite that was tested as a supporting material for CuCl in a previous study [13]. From
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the viewpoint of the BET surface area and the degree of CuCl dispersion, bayerite should be more suitable than boehmite as a supporting material for CO-selective adsorbents with high CO adsorption capacities.
We tested the activated bayerite sample prepared by calcination at 623 K as a
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supporting material to prepare CO-selective adsorbents. As described in the experimental section, a certain amount of CuCl was impregnated into the activated bayerite via thermal monolayer deposition (TMD). The potential of the activated bayerite as supporting material
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was evaluated by measuring the CO adsorption capacity at 293 K under 100 kPa of CO. In
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addition to the CO adsorption capacity, the CO2 adsorption capacity was measured and the CO selectivity to CO2 was calculated. To determine the optimal conditions to obtain an adsorbent with the maximum CO adsorption capacity and CO/CO2 separation factor, the temperature of the TMD step was varied from 523 K to 673 K, and the amount of CuCl was varied from 24 wt% to 48 wt%. Fig. 3 shows the CO and CO2 adsorption isotherms, measured at 293 K, for a series of CO adsorbents prepared by dispersing CuCl (30 wt%) at various temperatures. The amounts of CO and CO2 adsorbed by the samples under 100 kPa of each gas are summarized in Table
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ACCEPTED MANUSCRIPT 2. As shown in Fig. 3 and Table 2, the largest CO adsorption capacity (48.5 cm3 g-1) was obtained for the adsorbent prepared by impregnating CuCl at 573 K. This sample shows an unusual feature on the sorption isotherm: there is a sudden increase of CO adsorption capacity at 70 kPa. This phenomenon was reproduced several times, but further study is
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necessary to understand the reason for this result. According to a previous report [13], 573 K is between the Tammann temperature and the melting temperature of CuCl, and this temperature is sufficiently high to achieve thermal impregnation of CuCl with high dispersion.
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In that previous work, 573 K was found to be the optimal temperature for TMD of CuCl on boehmite supporting material. By contrast, the adsorbent prepared by thermally impregnating
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CuCl into bayerite at 523 K exhibited a somewhat smaller CO adsorption capacity (36 cm3 g), likely because 523 K is insufficiently high to mobilize CuCl, so the dispersion of CuCl in
this sample might be lower than that in the sample prepared at 573 K. The CuCl/bayerite samples prepared by impregnating CuCl at temperatures higher than 573 K also exhibited
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smaller CO adsorption capacities than the sample prepared by impregnating CuCl at 573 K (see Table 2). This phenomenon was also observed for the CO adsorbents prepared using boehmite as a supporting material [13]. The decrease in CO adsorption capacity with the
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increase in impregnation temperature can be explained by decreases in the surface area and
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the number of silanol groups on the surface during the thermal treatment at high temperature. This systematic investigation confirmed that the optimal temperature for CuCl impregnation into bayerite to maximize CO adsorption capacity is 573 K. The CO adsorption capacity of the adsorbent prepared by thermally impregnating
30wt% CuCl into bayerite at 573 K (CuCl/bayerite) is 43% larger than that of the adsorbent prepared using boehmite (CuCl/boehmite) under the same conditions. In contrast, the CuCl/bayerite and CuCl/boehmite adsorbents exhibited similar CO2 adsorption capacities (2.89 cm3 g-1 and 2.9 cm3 g-1, respectively). As a result, the CuCl/bayerite adsorbent is likely
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ACCEPTED MANUSCRIPT to be more suitable for CO/CO2 separation than the previous CuCl/boehmite sample. To the best of our knowledge, the previous CuCl/boehmite adsorbent exhibited the highest reported CO/CO2 separation factor (12.4) [13]. Nevertheless, the present CuCl/bayerite adsorbent, in which only the type of supporting material is different, shows a much higher CO/CO2
applying bayerite as a new type of supporting material.
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separation factor of 16.8. Thus, this improved CO/CO2 separation performance was due to
Activated bayerite and boehmite are known to have similar chemical compositions
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(AlOx(OH)y; x<1.5, y<3) [19]. Nevertheless, CuCl/bayerite shows a significantly larger CO adsorption capacity than CuCl/boehmite. The main reason for the increased CO adsorption
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capacity of CuCl/bayerite is likely to be the larger surface area of the bayerite supporting material, which is advantageous for realizing high dispersion of CuCl. To elucidate the correlation between the surface area and the CO adsorption capacity, we compared the CO adsorption capacities of four different samples prepared by thermal impregnation of 30wt%
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CuCl at 573 K into bayerite samples pre-activated at different temperatures (523 K, 573 K, 623 K, and 673 K). We had already confirmed that the surface area of the bayerite material can be finely controlled by varying the calcination temperature. Fig. 4 shows the CO and CO2
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adsorption isotherms of the four adsorbents, measured at 293 K. The adsorbents prepared by impregnating CuCl on bayerite samples activated at 523 K, 573 K, 623 K, and 673 K
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exhibited CO adsorption capacities of 32.2, 38.9, 48.5, and 38.1 cm3 g-1, and CO2 adsorption capacities of 1.62, 2.71, 2.89, and 3.49 cm3 g-1, respectively (Table 2). The CO adsorption capacities increased in the same order as the BET surface areas of the bayerite supporting materials. This result indicates that the high CO adsorption capacity of the CuCl/bayerite adsorbent can be attributed to the large surface area of the bayerite material. The CO2 adsorption capacities of the samples increased gradually with increasing temperature for the pre-activation treatment.
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ACCEPTED MANUSCRIPT According to a previous report [1], the CO adsorbent prepared using zeolite 13X as a supporting material exhibits a higher CO adsorption capacity (49 cm3 g-1) than CuCl/boehmite (34 cm3 g-1) or even CuCl/bayerite (48.5 cm3 g-1), which can be explained by the larger surface area of the zeolite (620 m2 g-1). However, the zeolite-based CO adsorbent
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also exhibited a large CO2 adsorption capacity (43 cm3 g-1 at 298K under 100 kPa of CO2) owing to the natural properties (microporosity and highly polar pore surfaces) of the zeolite. Therefore, this zeolite-based adsorbent is not suitable for CO/CO2 separation. Compared with
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the zeolite-based adsorbent, the notable points of the CuCl/bayerite adsorbent are its quite similar high CO adsorption capacity and very low CO2 adsorption capacity.
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To further increase the CO adsorption capacity of the CuCl/bayerite adsorbent, we varied the content of CuCl (12, 24, 30, 36, and 48 wt%) impregnated at 573 K in the bayerite supporting material pre-activated at 623 K. Fig. 5 shows the CO and CO2 adsorption isotherms, measured at 293 K, of the prepared CO adsorbents, and the CO and CO2
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adsorption amounts measured under 100 kPa of each gas are summarized in Table 2. As shown in Fig. 5 and Table 2, the CO adsorption capacity gradually increased from 19.2 cm3 g-1 to 48.5 cm3 g-1 as the CuCl content increased from 12 wt% to 30 wt%. However, the CO
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adsorption capacity decreased gradually to 41.6 and 31.4 cm3 g-1 when the CuCl content increased further to 36 and 48 wt%, respectively. In contrast, the CO2 adsorption capacity
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gradually decreased from 10.1 cm3 g-1 to 2.95, 1.17, and almost 0.00 cm3 g-1, as the CuCl content was increased from 12 wt% to 30, 36, and 48 wt%, respectively. Consequently, the adsorbent with the highest CO adsorption capacity (48.5 cm3 g-1 at 30 wt% CuCl) exhibited an impressively high CO/CO2 separation factor of 16.8. An even higher CO/CO2 separation factor (35.5) was achieved by increasing the CuCl content to 36 wt%, but the CO adsorption capacity decreased somewhat. Between 30 wt% and 36 wt% CuCl, it is possible to choose between two apexes (i.e., CO adsorption capacity and CO/CO2 selectivity).
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ACCEPTED MANUSCRIPT The results of this systematic investigation indicate that CuCl is highly dispersed on bayerite when the CuCl content is lower than 30 wt%. Therefore, up to 30 wt% of CuCl content, the CO adsorption capacity can be increased by increasing the CuCl content on the surface of the bayerite supporting material, as CuCl strongly interacts with CO molecules.
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The increase in the CO adsorption capacity (from 19.2 to 48.5 cm3 g-1; 2.51 times) corresponds exactly to the increase in the CuCl content (from 12 to 30 wt%; 2.50 times). However, when the CuCl content increased higher than 30 wt%, an excess amount of CuCl
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starts to form several layers on the bayerite, blocking some portion of pores of the bayerite. In contrast, the decrease in the CO2 adsorption capacity is inversely proportional to the CuCl
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content, indicating that CO2 is preferentially adsorbed on the surface of bayerite rather than on CuCl component. The blocking some portion of the pores at CuCl contents greater than 30 wt% is also the reason to the decrease of the CO2 adsorption capacity. As mentioned above, the CuCl/bayerite adsorbent prepared by impregnating 30 wt%
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CuCl at 573 K on pre-activated (at 623 K) bayerite shows great performance for CO adsorption and CO/CO2 separation, owing to the high dispersion of CuCl on the large surface area of bayerite. To confirm the high and uniform dispersion of CuCl, we performed an
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SEM-EDS analysis of this CuCl/bayerite sample. Fig. 6 shows SEM images of the pre-
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activated bayerite, CuCl powder, and the CuCl/bayerite adsorbent, as well as corresponding elemental (Al and Cu) mapping images. The SEM and EDS images show that the bayerite particles and CuCl particles have quite different morphologies. The bayerite particles (particle diameter: 40-70 µm on average) appeared roundish overall, but a high-magnification SEM image (inset) revealed that the bayerite particles consisted of aggregates of many tiny particles (particle diameter: 20-50 nm) possessing inter-particle mesopores. The CuCl particles (particle diameters: 40-80 µm) were agglomerates of crystal-like (octahedral) small particles. The EDS elemental mapping results showed that Al and Cu species were only
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distributions of Al and Cu species in all the particles. This result indicates that almost all the CuCl particles melted at 573 K during the TMD step and filled the mesopores of the neighboring bayerite particles.
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A high CO adsorption capacity and high CO/CO2 separation factor make the CuCl/bayerite adsorbent advantageous for CO separation in various fields where CO is
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normally present with significant amounts of CO2. For example, Linz-Donawitz gas (LDG) and blast furnace gas (BFG), which are generated during steel-making process, are composed of 16-22% of N2, 60-67% of CO, and 16-18% of CO2, and 44-60% of N2, 20-30% of CO, and 16-25% of CO2, respectively [6]. The CuCl/bayerite adsorbent is expected to show great
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performance in CO separation from LDG and BFG. The aforementioned CO/CO2 separation factor, which was evaluated by dividing the CO adsorption capacity measured at 293 K under 100 kPa by the CO2 adsorption capacity measured under the same conditions, is an indirect
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value that can be used to predict the separation performance of the adsorbent. As this factor is
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derived from the adsorption capacity under single-component condition, it is necessary to calculate the CO selectivity of the adsorbent over CO2 under mixed gas condition. We applied the ideal adsorbed solution theory (IAST) model to calculate the selectivity for CO over CO2. We calculated the selectivity of the CuCl/bayerite adsorbent for CO over CO2 under several pressures of mixed gas by using the CO and CO2 adsorption isotherms measured for each single component gas. We calculated the CO/CO2 selectivities for the two CO adsorbents prepared by impregnation with 30 wt% and 36 wt% CuCl; these adsorbents were selected because they exhibited the maximum CO adsorption capacity and the
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ACCEPTED MANUSCRIPT maximum CO/CO2 selectivity, respectively. First, we applied the Langmuir-Freundlich equation and the Langmuir equation to fit the CO and CO2 adsorption isotherms, respectively, and to evaluate theoretically the CO/CO2 selectivity of the CO adsorbents in a mixture of CO and CO2 gases. For both samples, in the case of CO adsorption isotherm fitting, the
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adsorption amounts experimentally measured in the pressure range of 0~74 kPa were selectively used for the calculation of CO/CO2 selectivity, even though the calculation does not include the pressure range showing sudden increase of the CO adsorption capacity. In
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contrast, in the case of CO2 adsorption isotherm fitting, the adsorption amounts experimentally measured in the pressure range of 0~100 kPa were used. The Langmuir-
follows:
=
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Freundlich equation, which was used for isotherm fitting and calculating the selectivity, is as
, where P is gas pressure at adsorption equilibrium.
( )
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Using the above equation, the isotherm parameters, qsat, b, and n were calculated for the two CO adsorbents (Table S1). Considering the R2 value obtained for each isotherm fitting, the isotherm fitting was quite reliable. Based on the isotherm parameters, the adsorption
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capacities of the adsorbents for CO and CO2 can be theoretically estimated at the target pressure, and the CO/CO2 selectivity of the adsorbents in CO/CO2 mixtures was evaluated
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using the equation of ‘(XCO/XCO2)/(PCO/PCO2)’. To calculate the CO selectivity using the IAST model, we assumed that the mixed gas was composed of only CO and CO2, at a volume ratio of 1:1.
In Fig. 7, which shows the results of the IAST calculation, CO/CO2 selectivity is plotted with respect to the pressure of the mixed gas (1 ~ 250 kPa). The separation of CO from LDG and BFG based on the VPSA process is normally operated under the gas pressure lower than 300 kPa. The LDG and BFG gas which is exhausted at almost atmospheric pressure is pressurized up to 300 kPa during the adsorption step of the VPSA process, in
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ACCEPTED MANUSCRIPT order to enhance working capacity of CO adsorbent and, consequently, the CO yield. However, owing to the high cost of pressurization and the decrease in product selectivity, further pressurization is not preferred. Considering the partial pressures of CO and CO2 in LDG and BFG, the pressure range of the CO/CO2 gas mixture in the present IAST calculation
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covers the normal pressure range of the CO VPSA process. As shown in Fig. 7, 30 wt% CuCl/bayerite and 36 wt% CuCl/bayerite samples exhibited CO selectivity of 1,112 and 1,982 at 1 kPa of mixed gas (1:1 CO:CO2, volume ratio), respectively; these values are
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notably higher than previous results [11-13]. As expected, the 36 wt% CuCl/bayerite sample shows a much higher CO/CO2 selectivity than the 30 wt% CuCl/bayerite sample. Increasing
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the pressure of the mixture to 100 kPa decreased the CO/CO2 selectivity to 125 for 30 wt% CuCl/bayerite and to 139 for 36 wt% CuCl/bayerite; the CO/CO2 selectivity decreased further to 92 and 78, respectively, when the pressure was increased to 250 kPa. The decrease in the selectivity with increasing pressure is a typical feature in IAST calculation. The CO
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adsorption amount increased steeply at low pressure, but increased gradually more slowly with increasing pressure, which is a typical feature of π-complexation adsorbents. However, the CO2 adsorption amount increased continuously with increasing CO2 pressure. Therefore,
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the CO/CO2 selectivity decreased with increases in the pressure of the mixed gas.
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Nevertheless, the CO/CO2 selectivities of the two CO adsorbent samples are still higher than those of previous CO adsorbents reported elsewhere [11-13]. CO sources generated in various fields sometimes also include significant amounts of
N2, CH4, and H2. To separate only CO from these sources via a VPSA process using CuCl/bayerite adsorbent, the adsorbent should have high selectivity for CO over N2, CH4, and H2, as well as selectivity for CO over CO2. To evaluate the selectivity for CO over N2, CH4, and H2, we measured the N2, CH4, and H2 adsorption isotherms of the CuCl/bayerite sample at 293 K (Fig. 8). Compared with the adsorption of CO2 or CO, the CuCl/bayerite sample
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ACCEPTED MANUSCRIPT adsorbed negligible amounts of N2 and CH4, and a very small amount of H2 (1.01 cm3 g-1) under 100 kPa at 293 K. The present adsorbent showed very high CO selectivity factors to H2, N2, and CH4 (separation factors: CO/H2=48.9, CO/N2=542, CO/CH4=187). As the N2, CH4, and H2 adsorption amounts were are very small, it was difficult to use IAST to calculate
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CO/N2, CO/CH4, and CO/H2 selectivity values. In any case, the selectivities for CO over H2, N2, and CH4 are expected to be much larger than the selectivity for CO over CO2. Thus, we expect that this new CuCl/bayerite adsorbent will show outstanding performance for
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adsorption-based CO separation from various CO sources.
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ACCEPTED MANUSCRIPT 4. Conclusion We synthesized a highly selective CO adsorbent based on a CuCl/bayerite composite. Under the optimal preparation conditions, 30 wt% CuCl was thermally impregnated via a TMD process at 573 K into the bayerite supporting material, which was pre-activated at 623
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K. This sample exhibited an excellent CO adsorption capacity of 48.5 cm3 g-1 at 293 K under 100 kPa of CO and a CO/CO2 separation factor of 16.8. The CO adsorption capacity of the present CuCl/bayerite is much larger than that of the previous CO adsorbent that used
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boehmite as a supporting material, and even CO/CO2 separation factor is somewhat larger than that of the previous adsorbent [13]. The CO/CO2 separation factor of the CuCl/bayerite
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adsorbent dramatically increased to 35.5 when the content of CuCl was increased to 36 wt%. The previous adsorbent using boehmite exhibited quite high CO adsorption capacity, and especially the best recorded CO/CO2 separation factor [13]. To the best of our knowledge, the CO/CO2 separation factor of the present CuCl/bayerite adsorbent is now the highest reported
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value. The CO/CO2 selectivity under mixed gas conditions calculated using the IAST model is much larger than the CO/CO2 separation factor. Assuming a 1:1 volume ratio of CO and CO2 in the mixed gas, the CO/CO2 selectivity of 36 wt% CuCl/bayerite adsorbent was 1,982
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under 1 kPa, 139 under 100 kPa, and 78 under 250 kPa of mixed gas at 293 K. The high CO
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adsorption capacity of CuCl/bayerite is due to the large surface area of bayerite, which can disperse CuCl species well, as confirmed by the uniform dispersion observed by SEM-EDS analysis. This adsorbent showed negligible adsorption capacities for N2, CH4, and H2, which normally coexist with CO in various CO sources generated from various fields, such as steelmaking and steam reforming processes. Overall, the new CuCl/bayerite adsorbent developed in this work will show great performance as a CO adsorbent for selective CO separation from various CO sources.
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ACCEPTED MANUSCRIPT 5. Acknowledgments This work was conducted under the framework of Research and Development Program of the
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Korea Institute of Energy Research (KIER, B8-2426-02).
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ACCEPTED MANUSCRIPT 6. References [1] Y. Xie, J. Zhang, J. Qiu, X. Tong, J. Fu, G. Yang, H. Yan, Y. Tang, Zeolites modified by CuCl for separating CO from gas mixtures containing CO2, Adsorption 3 (1996) 27-32.
impregnated with metal halide, AIChE J. 42 (1996) 422-430.
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[2] H. Tamon, K. Kitamura, M. Okazaki, Adsorption of carbon monoxide on activated carbon
[3] S. Hou, C. Chen, C. Chang, C. Wu, J. Ou, T. Lin, Firing blast furnace gas without support fuel in steel mill boilers, Energy Convers. Manage. 52 (2011) 2758-2767.
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[4] A. Chauvel, G. Lefebvre, Petrochemical processes, 2nd ed., Editions Technip, Paris, 1989.
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[5] Y.-I. Lim, J. Choi, H.-M. Moon, G.-H. Kim, Techno-economic comparison of absorption and adsorption processes for carbon monoxide (CO) separation from Linze-Donawitz gas (LDG), Korean Chem. Eng. Res. 54 (2016) 320-331.
[6] W.-H. Chen, M.-R. Lin, T.-S. Leu, S.-W. Du, An evaluation of hydrogen production from
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the perspective of using blast furnace gas and coke oven gas as feedstocks, Int. J. Hydrogen Energy 36 (2011) 11727-11737.
[7] F. Kasuya, T. Tsuji, High purity CO gas separation by pressure swing adsorption, Gas
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Sep. Purification 5 (1991) 242-246.
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[8] F. Gao, Y. Wang, S. Wang, Selective adsorption of CO on CuCl/Y adsorbent prepared using CuCl2 as precursor: Equilibrium and thermodynamics, Chem. Eng. J. 290 (2016) 418427.
[9] R.T. Yang, Adsorbents: Fundamentals and applications, John Wiley & Sons, New Jersey, 2003. [10] Y. Wang, R.T. Yang, J.M. Heinzel, Desulfurization of jet fuel by π-complexation adsorption with metal halides supported on MCM-41 and SBA-15 mesoporous materials, Chem. Eng. Sci. 63 (2008) 356-365.
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ACCEPTED MANUSCRIPT [11] J.W. Yoon, T.-U. Yoon, E.-J. Kim, A.-R. Kim, T.-S. Jung, S.-S. Han, Y.-S. Bae, Highly selective adsorption of CO over CO2 in a Cu(I)-chelated porous organic polymer, J. Hazard. Mater. 341 (2018) 321-327. [12] A.-R. Kim, T.-U. Yoon, S.-I. Kim, K. Cho, S.S. Han, Y.-S. Bae, Creating high CO/CO2
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selectivity and large CO working capacity through facile loading of Cu(I) species into an iron-based mesoporous metal-organic framework, Chem. Eng. J. 348 (2018) 135-142.
[13] K. Cho, J. Kim, H.T. Beum, T. Jung, S.S. Han, Synthesis of CuCl/Boehmite adsorbents
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that exhibit high CO selectivity in CO/CO2 separation, J. Hazard. Mater. 344 (2018).
[14] H. Hirai, K. Wada, M. Komiyama, Active carbon-supported copper(I) chloride as solid
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adsorbent for carbon monoxide, Bull. Chem. Soc. Jpn. 59 (1986) 2217-2223. [15] M. Ishioka, T. Okada, K. Matsubara, Formation and characteristics of vapor grown carbon fibers prepared in Linz-Donawitz converter gas, Carbon 30 (1992) 975-979. [16] Y. Huang, Y. Tao, L. He, Y. Duan, J. Xiao, Z. Li, Preparation of CuCl@AC with high
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CO adsorption capacity and selectivity from CO/N2 binary mixture, Adsorption 21 (2015) 373-381.
[17] J. Ma, L. Li, J. Ren, R. Li, CO adsorption on activated carbon-supported Cu-based
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adsorbent prepared by a facile route, Sep. Purif. Technol. 76 (2010) 89-93.
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[18] T.C. Golden, W.C. Kratz, F.C. Wilhelm, Highly dispersed cuprous compositions, US Patent No. 5,126,310, 1992. [19] X. Krokidis, P. Raybaud, A.-E. Gobichon, B. Rebours, P. Euzen, H. Toulhoat, Theoretical study of the dehydration process of boehmite to γ-alumina, J. Phys. Chem. B 105 (2001) 5121-5130.
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ACCEPTED MANUSCRIPT Tables Table 1. Porous texture properties of various bayerite material and CuCl/bayerite composites.
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Pre-activation Vtotb CuCl content SBETa temperature (wt%) (m2 g-1) (cm3 g-1) (Thermal treatment temperature) 1c 0 10 0.04 2 523 K 0 397 0.11 3 573 K 0 465 0.17 4 623 K 0 469 0.22 5 673 K 0 454 0.25 6 773 K 0 307 0.27 7 623 K (573 K) 30 302 0.14 a SBET (m2 g-1) is BET surface area was calculated from edges obtained at relative pressure (P/P0) between 0.05 and 0.3 of the isotherms using BET equation. b Vtotal is pore volume calculated at P/P0 = 0.95 c This bayerite sample was not activated by thermal treatment, but it was degassed at 323 K before N2 sorption measurement.
Table 2. CO and CO2 adsorption capacity and CO/CO2 separation factor of CuCl/bayerite composites.
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Thermal treatment CuCl content Adsorbed CO Adsorbed CO2 temperature CO/CO2 (wt%) (cm3 g-1) (cm3 g-1) (pre-activation temperature) 1 523 K (623 K) 30 36.0 2.66 13.5 2 573 K (623 K) 30 48.5 2.89 16.8 3 623 K (623 K) 30 30.1 3.77 7.98 4 673 K (623 K) 30 25.1 4.26 5.89 5 573 K (523 K) 30 32.2 1.62 19.7 6 573 K (573 K) 30 38.9 2.71 14.4 7 573 K (673 K) 30 38.1 3.49 10.9 8 573 K (623 K) 12 19.2 10.1 1.90 9 573 K (623 K) 24 31.9 4.54 7.02 10 573 K (623 K) 36 41.6 1.17 35.5 11 573 K (623 K) 48 31.4 0.00 -a a The CO/CO2 selectivity was not calculated, because the CO2 adsorption capacity of this sample is almost negligible.
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Figures
Fig. 1. XRD patterns of pristine bayerite and the treated bayerite samples which were
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calcined at various temperatures.
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Fig. 2. N2 sorption isotherms (taken at 77 K) of pristine bayerite and the treated bayerite
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samples which were calcined at various temperatures.
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Fig. 3. (a) CO and (b) CO2 adsorption isotherms (taken at 293 K) of the CuCl/bayerite samples which were prepared by impregnating 30 wt% CuCl at various temperatures on bayerite (pre-activated at 623 K). The numbers written in the legend of the figure are the
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temperatures for the impregnation of the CuCl.
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Fig. 4. (a) CO and (b) CO2 adsorption isotherms (taken at 293 K) of the CuCl/bayerite samples which were prepared by impregnating 30 wt% CuCl at 573 K on various bayerite materials pre-activated at various temperatures. The numbers written in the legend of the
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Fig. 5. (a) CO and (b) CO2 adsorption isotherms (taken at 293 K) of the CuCl/bayerite samples which were prepared by impregnating various amount of CuCl on a bayerite material (pre-activated at 623 K) at 573 K. The number in the legend of the figure is the weight
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percent of CuCl to total weight of the composite of CuCl and bayerite.
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Fig. 6. SEM micrographs of activated bayerite, CuCl powder, and CuCl/bayerite which was synthesized by impregnating 30 wt% of CuCl at 573 K, and EDS elemental mapping images
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of Al and Cu for the CuCl/bayerite sample.
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Fig. 7. CO/CO2 selectivity of (a) 30 wt% CuCl/bayerite sample and (b) 36 wt%
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CuCl/bayerite sample under various pressure of the mixture gas composed of CO and CO2
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(1:1 volume ratio), which was theoretically calculated using IAST model.
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Fig. 8. Adsorption isotherms of 30 wt% CuCl/bayerite adsorbent for CO, CO2, N2, CH4, and
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H2 gases, which were taken at 293 K.
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HIGHLIGHTS
• Highly selective CO adsorbent based on CuCl/bayerite composite was synthesized. • Adsorbent exhibited very high CO/CO2 selectivity. • High separation ability is attributed to the large surface area of bayerite.
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• Thermal treatment conditions for dispersion of CuCl on bayerite were optimized.
S1387-1811(18)30541-9
DOI:
https://doi.org/10.1016/j.micromeso.2018.10.010
Reference:
MICMAT 9148
To appear in:
Microporous and Mesoporous Materials
Received Date: 26 July 2018 Revised Date:
5 October 2018
Accepted Date: 12 October 2018
Please cite this article as: K. Cho, J. Kim, J.-h. Park, T. Jung, H.T. Beum, D.-w. Cho, Y.W. Rhee, S.S. Han, High CO adsorption capacity, and CO selectivity to CO2, N2, H2, and CH4 of CuCl/ bayerite adsorbent, Microporous and Mesoporous Materials (2018), doi: https://doi.org/10.1016/ j.micromeso.2018.10.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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High CO adsorption capacity, and CO selectivity to CO2, N2, H2, and CH4 of CuCl/bayerite adsorbent Kanghee Choa,1, Jungsu Kima,b,1, Jong-ho Parka, Taesung Junga, Hee Tae Beuma, Dong-woo
a
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Cho, Young Woo Rheeb*, Sang Sup Hana**
Climate Change Research Division, Korea Institute of Energy Research, 152 Gajeong-ro,
Yuseong-gu, Daejeon 34129, Republic of Korea
Graduate School of Energy Science and Technology, Chungnam National University, 99
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b
Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea
Both authors contributed equally to this work with first authorship.
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*Corresponding author at: Chungnam National University, Daejeon 34134, Republic of Korea. (E-mail address: [email protected])
**Corresponding author at: Korea Institute of Energy Research, Daejeon 34129, Republic of
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HIGHLIGHTS
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Korea. (E-mail address: [email protected])
• Highly selective CO adsorbent based on CuCl/bayerite composite was synthesized.
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• Adsorbent exhibited very high CO/CO2 selectivity. • High separation ability is attributed to the large surface area of bayerite. • Thermal treatment conditions for dispersion of CuCl on bayerite were optimized.
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ACCEPTED MANUSCRIPT ABSTRACT
We synthesized nanoporous CO-selective adsorbent composed of CuCl supported on bayerite. Before supporting the CuCl, the bayerite chemical was calcined at 623 K to increase
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the surface area to 469 m2 g-1. The CuCl was highly dispersed on the activated bayerite via a thermal monolayer dispersion process. The highest CO adsorption capacity was achieved at an optimal temperature of 573 K and CuCl content of 30 wt%. Thus synthesized adsorbent
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exhibited a high CO adsorption capacity of 48.5 cm3 g-1 at 293 K, but a very low CO2 adsorption capacity (2.89 cm3 g-1), resulting in CO/CO2 separation factor of 16.8. When the
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CuCl content increased to 36 wt%, the adsorbent exhibited much higher separation factor (35.5), although the CO adsorption capacity was somewhat smaller (41.6 cm3 g-1). The CO adsorption capacity and CO/CO2 selectivity of this adsorbent are larger than those of our previous CuCl/boehmite adsorbent showing CO adsorption of 34 cm3 g-1 and a CO/CO2
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separation factor of 12.4. The present adsorbent also shows very high selectivity for CO over H2, N2, and CH4. Therefore, this adsorbent is expected to show excellent CO separation performance for various industrial processes such as steam-reforming and steel-making
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which involve CO, CO2, H2, N2, and CH4.
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Keywords: Adsorption; Carbon monoxide; High CO/CO2 selectivity; Adsorbent; CuCl/bayerite
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ACCEPTED MANUSCRIPT 1. Introduction Carbon monoxide (CO) is a highly valuable chemical that is a raw material for producing various chemicals such as acetic acid, polycarbonate, isocyanates, and polyurethane [1-4]. Furthermore, it has recently receiving great interest as a main component
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of syn-gas (CO+H2) that can be used to synthesize fuel and olefins. CO is produced by steam reforming, partial oxidation, and steelmaking processes [2-6]. In these industrial processes, CO is exhausted as a mixture with CO2, N2, H2, and CH4. Owing to its toxicity, CO is
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normally burned off, thus producing heat energy. To recover this high-value component, and to reduce the generation of CO2 during CO combustion, there is a strong demand for CO
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separation technologies.
Various technologies for CO separation have been developed, such as cryogenic distillation, absorption, and adsorption processes [5-8]. Cryogenic distillation has been widely used for large-scale separation of CO, but it is a highly energy-consuming process.
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Especially, the separation of CO from gases containing N2 is significantly challenging because of the two species’ similar boiling points [2]. COSORB, a typical commercial process for CO absorption separation uses a CuAlCl4/toluene solution to absorb CO
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selectively [2]. Although this process achieves high CO purity and a high yield, there are
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environmental issues associated with disposal of the spent solvent [7,8]. Vacuum-pressure swing adsorption (VPSA) process is a promising method for CO
separation because of its relative simplicity. This process, which involves low energy consumption and low operation cost, can achieve high CO purity and high yield. In this process, Cu(I) complexes that show strong interactions with CO via π-complexation are used for CO adsorption. The s-orbitals of Cu(I) ions can form σ bonds with the π orbitals of CO molecules, and π back-bonding is simultaneously formed between the d-orbitals of Cu(I) ions and the antibonding π-orbitals of CO molecules [9-12]. For efficient CO separation, it is very
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ACCEPTED MANUSCRIPT important that the adsorbent has a high CO adsorption capacity and high CO selectivity. Both factors are significantly relevant to the selection of supporting materials for the Cu(I) [13]. To realize high dispersion of Cu(I) species and, consequently, a large CO adsorption capacity, it is necessary to use a suitable nanoporous material that exhibits a large surface area and strong
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interactions with Cu(I) complexes. Simultaneously, to enhance the selectivity for CO, the supporting material should show weak interaction with other impurities such as CO2, N2, H2, and CH4.
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Xie et al. [1] used zeolite Y and 13X zeolite as supporting materials for CuCl to prepare CO adsorbent. They physically mixed zeolites and CuCl and heated the mixture to a
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certain temperature between the Tammann temperature (the temperature at which the mobility and reactivity of the molecules in a solid become appreciable) and the melting point of CuCl. This adsorbent exhibited CO adsorption capacity higher than 2.2 mmol g-1 at 298 K. However, as it also showed a similar CO2 adsorption capacity, this adsorbent could not
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efficiently separate CO from CO2. Hirai et al. [14] prepared copper(I) halide-loaded activated carbon (AC) adsorbents using an impregnation method. Their CuCl/AC adsorbent had high CO adsorption capacity owing to the large BET surface area of AC, but it also exhibited high
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CO2 adsorption capacity owing to the natural properties (high microporosity and high CO2
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affinity) of AC [14-18].
Recently, using boehmite as a supporting material, Cho et al. developed a CO-
selective adsorbent that exhibited high CO adsorption capacity (1.56 mmol g-1 at 293 K) and high CO/CO2 selectivity (12.8) [13]. Although the CO adsorption capacity of this adsorbent is somewhat smaller than those of the previous zeolite-based adsorbents, the high CO selectivity is advantageous. The most important part of the development of this adsorbent was the use of boehmite as a supporting material. As Boehmite exhibits a large BET surface area (~270 m2 g-1) and sufficiently strong interactions with CuCl species, high dispersion of CuCl
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supporting material with a larger BET surface area and more hydroxyl groups is used, the synthesized CO adsorbent may show a better CO adsorption capacity and higher CO selectivity.
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In this work, we used bayerite as a supporting material to synthesize a CO-selective adsorbent that shows higher CO adsorption capacity and CO selectivity than previous CO
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adsorbents. Pristine bayerite has a small BET surface area, but thermal activation at 623 K, increases the surface area to more than 400 m2 g-1. To gain an in-depth understanding of the advantages of using bayerite to support CuCl species, we analyzed the physical properties of the bayerite supporting material. In addition to a large surface area, bayerite is known to have
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a higher content of hydroxyl groups than boehmite. Owing to these properties, it was possible to impregnate large amounts of CuCl into bayerite via a thermal monolayer deposition route. The physical properties of the thus-synthesized adsorbent were characterized by powder X-
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ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray spectroscopy
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(SEM/EDS), and the Brunauer–Emmett–Teller (BET) method. The adsorption capacities of the new adsorbent for CO, CO2, N2, H2, and CH4 were measured using a volumetric sorption analyzer, and the CO/CO2, CO/N2, CO/H2, and CO/CH4 separation factors were calculated from the adsorption isotherms via theoretical calculation using the Langmuir-Freundlich equation.
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method to that used to synthesize the previous CuCl/boehmite-based CO adsorbent [13]. Before CuCl impregnation, bayerite was calcined at 623 K (heating rate: 3 K min-1) for 6 h under nitrogen condition. After cooling to room temperature, the calcined bayerite and CuCl
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chemical were mixed thoroughly. Finally, to obtain the adsorbent, the mixture was placed in a quartz tube under vacuum conditions, and heated to certain temperature between 523 K and
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673 K (heating rate: 3 K min-1) for 24 h.
2.2. Adsorbent characterization and adsorption measurements
The surfaces areas of pristine bayerite, activated bayerite, and the CuCl-loaded
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adsorbent, which were thermally treated at different temperatures, were assessed by BET (Tristar 3020, Micromeritics) using N2 adsorption at liquid N2 temperature. The pore size distributions of the samples were determined from the N2 adsorption isotherms using the
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Barrett–Joyner–Halenda (BJH) method. Before the measurements, the samples were
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degassed for 12 h at 473 K.
The structures of the adsorbent and bayerite samples were determined by powder
XRD (Rigaku, D/MAX–2500) using Cu Kα radiation with a step width of 0.02o and a step time of 1 s. The morphology and the elemental distribution of the adsorbent were characterized by SEM/EDS (Hitachi, S–4800). After thermally treating the adsorbent at 573 K for 12 h under vacuum, CO and CO2 adsorption isotherms were measured by volumetric method at 293 K (Tristar 3020,
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adsorbed CO and CO2 at 100 kPa.
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ACCEPTED MANUSCRIPT 3. Results and discussion The XRD patterns of pristine and the bayerite samples calcined at various temperatures are shown in Fig. 1. The XRD pattern of pristine bayerite shows representative peaks of the bayerite crystalline phase at 2θ = 18 o, 20 o, 27 o, and 40o. The XRD pattern of the
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bayerite sample calcined at 523 K is similar XRD, except for the appearance of a very weak peak centered at 2θ = 14.5o. This result indicates that although there is some transformation of bayerite during calcination at 523 K, the pristine crystalline phase is largely maintained.
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The bayerite samples calcined at temperatures higher than 573 K exhibited completely different XRD patterns, which included peaks corresponding to η- and θ-alumina. It is well
temperatures higher than 573 K.
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known that bayerite is gradually transformed to η- and θ-alumina by thermal treatment at
To determine the optimal temperature for inducing a large surface area in bayerite, pristine bayerite was calcined at various temperatures between 523 K and 773 K, and the pore
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textural properties of these samples were characterized by N2 physisorption. The pristine bayerite sample was characterized after degassing at 323 K, which is much lower than the calcination temperatures used for the other bayerite samples, and the other samples calcined
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at higher temperatures were analyzed after degassing at 523 K. Fig. 2 shows the N2 sorption
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isotherms of the calcined bayerite samples, and Fig. S1 shows the corresponding BJH pore size distributions. The BET surface areas and pore volumes of the samples are summarized in Table 1. The BET surface area of the pristine bayerite sample was only 10 m2 g-1, which was much smaller than those of the calcined bayerite samples. The BET surface area of the bayerite sample calcined at 523 K was somewhat smaller (397 m2 g-1) than those of the samples calcined at temperatures between 573 K and 673 K, which were all quite similar (~ 460 m2 g-1). The bayerite sample calcined at 623 K showed the largest BET surface area (469 m2 g-1), which is approximately 70% larger than that of boehmite [13]. The BET surface area
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during the transformation and condensation of the bayerite framework.
As mentioned above, for bayerite, 623 K was found to be the optimal temperature for obtaining a large BET surface area, which should be advantageous for the dispersion of CuCl
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species. The amount of CuCl dispersed on activated bayerite is likely to be greater than that on boehmite that was tested as a supporting material for CuCl in a previous study [13]. From
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the viewpoint of the BET surface area and the degree of CuCl dispersion, bayerite should be more suitable than boehmite as a supporting material for CO-selective adsorbents with high CO adsorption capacities.
We tested the activated bayerite sample prepared by calcination at 623 K as a
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supporting material to prepare CO-selective adsorbents. As described in the experimental section, a certain amount of CuCl was impregnated into the activated bayerite via thermal monolayer deposition (TMD). The potential of the activated bayerite as supporting material
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was evaluated by measuring the CO adsorption capacity at 293 K under 100 kPa of CO. In
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addition to the CO adsorption capacity, the CO2 adsorption capacity was measured and the CO selectivity to CO2 was calculated. To determine the optimal conditions to obtain an adsorbent with the maximum CO adsorption capacity and CO/CO2 separation factor, the temperature of the TMD step was varied from 523 K to 673 K, and the amount of CuCl was varied from 24 wt% to 48 wt%. Fig. 3 shows the CO and CO2 adsorption isotherms, measured at 293 K, for a series of CO adsorbents prepared by dispersing CuCl (30 wt%) at various temperatures. The amounts of CO and CO2 adsorbed by the samples under 100 kPa of each gas are summarized in Table
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necessary to understand the reason for this result. According to a previous report [13], 573 K is between the Tammann temperature and the melting temperature of CuCl, and this temperature is sufficiently high to achieve thermal impregnation of CuCl with high dispersion.
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In that previous work, 573 K was found to be the optimal temperature for TMD of CuCl on boehmite supporting material. By contrast, the adsorbent prepared by thermally impregnating
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CuCl into bayerite at 523 K exhibited a somewhat smaller CO adsorption capacity (36 cm3 g), likely because 523 K is insufficiently high to mobilize CuCl, so the dispersion of CuCl in
this sample might be lower than that in the sample prepared at 573 K. The CuCl/bayerite samples prepared by impregnating CuCl at temperatures higher than 573 K also exhibited
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smaller CO adsorption capacities than the sample prepared by impregnating CuCl at 573 K (see Table 2). This phenomenon was also observed for the CO adsorbents prepared using boehmite as a supporting material [13]. The decrease in CO adsorption capacity with the
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increase in impregnation temperature can be explained by decreases in the surface area and
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the number of silanol groups on the surface during the thermal treatment at high temperature. This systematic investigation confirmed that the optimal temperature for CuCl impregnation into bayerite to maximize CO adsorption capacity is 573 K. The CO adsorption capacity of the adsorbent prepared by thermally impregnating
30wt% CuCl into bayerite at 573 K (CuCl/bayerite) is 43% larger than that of the adsorbent prepared using boehmite (CuCl/boehmite) under the same conditions. In contrast, the CuCl/bayerite and CuCl/boehmite adsorbents exhibited similar CO2 adsorption capacities (2.89 cm3 g-1 and 2.9 cm3 g-1, respectively). As a result, the CuCl/bayerite adsorbent is likely
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ACCEPTED MANUSCRIPT to be more suitable for CO/CO2 separation than the previous CuCl/boehmite sample. To the best of our knowledge, the previous CuCl/boehmite adsorbent exhibited the highest reported CO/CO2 separation factor (12.4) [13]. Nevertheless, the present CuCl/bayerite adsorbent, in which only the type of supporting material is different, shows a much higher CO/CO2
applying bayerite as a new type of supporting material.
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separation factor of 16.8. Thus, this improved CO/CO2 separation performance was due to
Activated bayerite and boehmite are known to have similar chemical compositions
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(AlOx(OH)y; x<1.5, y<3) [19]. Nevertheless, CuCl/bayerite shows a significantly larger CO adsorption capacity than CuCl/boehmite. The main reason for the increased CO adsorption
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capacity of CuCl/bayerite is likely to be the larger surface area of the bayerite supporting material, which is advantageous for realizing high dispersion of CuCl. To elucidate the correlation between the surface area and the CO adsorption capacity, we compared the CO adsorption capacities of four different samples prepared by thermal impregnation of 30wt%
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CuCl at 573 K into bayerite samples pre-activated at different temperatures (523 K, 573 K, 623 K, and 673 K). We had already confirmed that the surface area of the bayerite material can be finely controlled by varying the calcination temperature. Fig. 4 shows the CO and CO2
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adsorption isotherms of the four adsorbents, measured at 293 K. The adsorbents prepared by impregnating CuCl on bayerite samples activated at 523 K, 573 K, 623 K, and 673 K
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exhibited CO adsorption capacities of 32.2, 38.9, 48.5, and 38.1 cm3 g-1, and CO2 adsorption capacities of 1.62, 2.71, 2.89, and 3.49 cm3 g-1, respectively (Table 2). The CO adsorption capacities increased in the same order as the BET surface areas of the bayerite supporting materials. This result indicates that the high CO adsorption capacity of the CuCl/bayerite adsorbent can be attributed to the large surface area of the bayerite material. The CO2 adsorption capacities of the samples increased gradually with increasing temperature for the pre-activation treatment.
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ACCEPTED MANUSCRIPT According to a previous report [1], the CO adsorbent prepared using zeolite 13X as a supporting material exhibits a higher CO adsorption capacity (49 cm3 g-1) than CuCl/boehmite (34 cm3 g-1) or even CuCl/bayerite (48.5 cm3 g-1), which can be explained by the larger surface area of the zeolite (620 m2 g-1). However, the zeolite-based CO adsorbent
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also exhibited a large CO2 adsorption capacity (43 cm3 g-1 at 298K under 100 kPa of CO2) owing to the natural properties (microporosity and highly polar pore surfaces) of the zeolite. Therefore, this zeolite-based adsorbent is not suitable for CO/CO2 separation. Compared with
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the zeolite-based adsorbent, the notable points of the CuCl/bayerite adsorbent are its quite similar high CO adsorption capacity and very low CO2 adsorption capacity.
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To further increase the CO adsorption capacity of the CuCl/bayerite adsorbent, we varied the content of CuCl (12, 24, 30, 36, and 48 wt%) impregnated at 573 K in the bayerite supporting material pre-activated at 623 K. Fig. 5 shows the CO and CO2 adsorption isotherms, measured at 293 K, of the prepared CO adsorbents, and the CO and CO2
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adsorption amounts measured under 100 kPa of each gas are summarized in Table 2. As shown in Fig. 5 and Table 2, the CO adsorption capacity gradually increased from 19.2 cm3 g-1 to 48.5 cm3 g-1 as the CuCl content increased from 12 wt% to 30 wt%. However, the CO
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adsorption capacity decreased gradually to 41.6 and 31.4 cm3 g-1 when the CuCl content increased further to 36 and 48 wt%, respectively. In contrast, the CO2 adsorption capacity
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gradually decreased from 10.1 cm3 g-1 to 2.95, 1.17, and almost 0.00 cm3 g-1, as the CuCl content was increased from 12 wt% to 30, 36, and 48 wt%, respectively. Consequently, the adsorbent with the highest CO adsorption capacity (48.5 cm3 g-1 at 30 wt% CuCl) exhibited an impressively high CO/CO2 separation factor of 16.8. An even higher CO/CO2 separation factor (35.5) was achieved by increasing the CuCl content to 36 wt%, but the CO adsorption capacity decreased somewhat. Between 30 wt% and 36 wt% CuCl, it is possible to choose between two apexes (i.e., CO adsorption capacity and CO/CO2 selectivity).
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ACCEPTED MANUSCRIPT The results of this systematic investigation indicate that CuCl is highly dispersed on bayerite when the CuCl content is lower than 30 wt%. Therefore, up to 30 wt% of CuCl content, the CO adsorption capacity can be increased by increasing the CuCl content on the surface of the bayerite supporting material, as CuCl strongly interacts with CO molecules.
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The increase in the CO adsorption capacity (from 19.2 to 48.5 cm3 g-1; 2.51 times) corresponds exactly to the increase in the CuCl content (from 12 to 30 wt%; 2.50 times). However, when the CuCl content increased higher than 30 wt%, an excess amount of CuCl
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starts to form several layers on the bayerite, blocking some portion of pores of the bayerite. In contrast, the decrease in the CO2 adsorption capacity is inversely proportional to the CuCl
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content, indicating that CO2 is preferentially adsorbed on the surface of bayerite rather than on CuCl component. The blocking some portion of the pores at CuCl contents greater than 30 wt% is also the reason to the decrease of the CO2 adsorption capacity. As mentioned above, the CuCl/bayerite adsorbent prepared by impregnating 30 wt%
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CuCl at 573 K on pre-activated (at 623 K) bayerite shows great performance for CO adsorption and CO/CO2 separation, owing to the high dispersion of CuCl on the large surface area of bayerite. To confirm the high and uniform dispersion of CuCl, we performed an
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SEM-EDS analysis of this CuCl/bayerite sample. Fig. 6 shows SEM images of the pre-
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activated bayerite, CuCl powder, and the CuCl/bayerite adsorbent, as well as corresponding elemental (Al and Cu) mapping images. The SEM and EDS images show that the bayerite particles and CuCl particles have quite different morphologies. The bayerite particles (particle diameter: 40-70 µm on average) appeared roundish overall, but a high-magnification SEM image (inset) revealed that the bayerite particles consisted of aggregates of many tiny particles (particle diameter: 20-50 nm) possessing inter-particle mesopores. The CuCl particles (particle diameters: 40-80 µm) were agglomerates of crystal-like (octahedral) small particles. The EDS elemental mapping results showed that Al and Cu species were only
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ACCEPTED MANUSCRIPT present in the bayerite and CuCl samples, respectively. However, the SEM image of the CuCl/bayerite sample shows that most of the particles have overall morphologies similar to those of bayerite particles. The SEM image rarely shows particles with the morphology of the CuCl particles. The EDS elemental mapping image of CuCl/bayerite shows uniform
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distributions of Al and Cu species in all the particles. This result indicates that almost all the CuCl particles melted at 573 K during the TMD step and filled the mesopores of the neighboring bayerite particles.
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A high CO adsorption capacity and high CO/CO2 separation factor make the CuCl/bayerite adsorbent advantageous for CO separation in various fields where CO is
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normally present with significant amounts of CO2. For example, Linz-Donawitz gas (LDG) and blast furnace gas (BFG), which are generated during steel-making process, are composed of 16-22% of N2, 60-67% of CO, and 16-18% of CO2, and 44-60% of N2, 20-30% of CO, and 16-25% of CO2, respectively [6]. The CuCl/bayerite adsorbent is expected to show great
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performance in CO separation from LDG and BFG. The aforementioned CO/CO2 separation factor, which was evaluated by dividing the CO adsorption capacity measured at 293 K under 100 kPa by the CO2 adsorption capacity measured under the same conditions, is an indirect
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value that can be used to predict the separation performance of the adsorbent. As this factor is
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derived from the adsorption capacity under single-component condition, it is necessary to calculate the CO selectivity of the adsorbent over CO2 under mixed gas condition. We applied the ideal adsorbed solution theory (IAST) model to calculate the selectivity for CO over CO2. We calculated the selectivity of the CuCl/bayerite adsorbent for CO over CO2 under several pressures of mixed gas by using the CO and CO2 adsorption isotherms measured for each single component gas. We calculated the CO/CO2 selectivities for the two CO adsorbents prepared by impregnation with 30 wt% and 36 wt% CuCl; these adsorbents were selected because they exhibited the maximum CO adsorption capacity and the
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ACCEPTED MANUSCRIPT maximum CO/CO2 selectivity, respectively. First, we applied the Langmuir-Freundlich equation and the Langmuir equation to fit the CO and CO2 adsorption isotherms, respectively, and to evaluate theoretically the CO/CO2 selectivity of the CO adsorbents in a mixture of CO and CO2 gases. For both samples, in the case of CO adsorption isotherm fitting, the
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adsorption amounts experimentally measured in the pressure range of 0~74 kPa were selectively used for the calculation of CO/CO2 selectivity, even though the calculation does not include the pressure range showing sudden increase of the CO adsorption capacity. In
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contrast, in the case of CO2 adsorption isotherm fitting, the adsorption amounts experimentally measured in the pressure range of 0~100 kPa were used. The Langmuir-
follows:
=
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Freundlich equation, which was used for isotherm fitting and calculating the selectivity, is as
, where P is gas pressure at adsorption equilibrium.
( )
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Using the above equation, the isotherm parameters, qsat, b, and n were calculated for the two CO adsorbents (Table S1). Considering the R2 value obtained for each isotherm fitting, the isotherm fitting was quite reliable. Based on the isotherm parameters, the adsorption
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capacities of the adsorbents for CO and CO2 can be theoretically estimated at the target pressure, and the CO/CO2 selectivity of the adsorbents in CO/CO2 mixtures was evaluated
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using the equation of ‘(XCO/XCO2)/(PCO/PCO2)’. To calculate the CO selectivity using the IAST model, we assumed that the mixed gas was composed of only CO and CO2, at a volume ratio of 1:1.
In Fig. 7, which shows the results of the IAST calculation, CO/CO2 selectivity is plotted with respect to the pressure of the mixed gas (1 ~ 250 kPa). The separation of CO from LDG and BFG based on the VPSA process is normally operated under the gas pressure lower than 300 kPa. The LDG and BFG gas which is exhausted at almost atmospheric pressure is pressurized up to 300 kPa during the adsorption step of the VPSA process, in
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ACCEPTED MANUSCRIPT order to enhance working capacity of CO adsorbent and, consequently, the CO yield. However, owing to the high cost of pressurization and the decrease in product selectivity, further pressurization is not preferred. Considering the partial pressures of CO and CO2 in LDG and BFG, the pressure range of the CO/CO2 gas mixture in the present IAST calculation
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covers the normal pressure range of the CO VPSA process. As shown in Fig. 7, 30 wt% CuCl/bayerite and 36 wt% CuCl/bayerite samples exhibited CO selectivity of 1,112 and 1,982 at 1 kPa of mixed gas (1:1 CO:CO2, volume ratio), respectively; these values are
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notably higher than previous results [11-13]. As expected, the 36 wt% CuCl/bayerite sample shows a much higher CO/CO2 selectivity than the 30 wt% CuCl/bayerite sample. Increasing
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the pressure of the mixture to 100 kPa decreased the CO/CO2 selectivity to 125 for 30 wt% CuCl/bayerite and to 139 for 36 wt% CuCl/bayerite; the CO/CO2 selectivity decreased further to 92 and 78, respectively, when the pressure was increased to 250 kPa. The decrease in the selectivity with increasing pressure is a typical feature in IAST calculation. The CO
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adsorption amount increased steeply at low pressure, but increased gradually more slowly with increasing pressure, which is a typical feature of π-complexation adsorbents. However, the CO2 adsorption amount increased continuously with increasing CO2 pressure. Therefore,
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the CO/CO2 selectivity decreased with increases in the pressure of the mixed gas.
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Nevertheless, the CO/CO2 selectivities of the two CO adsorbent samples are still higher than those of previous CO adsorbents reported elsewhere [11-13]. CO sources generated in various fields sometimes also include significant amounts of
N2, CH4, and H2. To separate only CO from these sources via a VPSA process using CuCl/bayerite adsorbent, the adsorbent should have high selectivity for CO over N2, CH4, and H2, as well as selectivity for CO over CO2. To evaluate the selectivity for CO over N2, CH4, and H2, we measured the N2, CH4, and H2 adsorption isotherms of the CuCl/bayerite sample at 293 K (Fig. 8). Compared with the adsorption of CO2 or CO, the CuCl/bayerite sample
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ACCEPTED MANUSCRIPT adsorbed negligible amounts of N2 and CH4, and a very small amount of H2 (1.01 cm3 g-1) under 100 kPa at 293 K. The present adsorbent showed very high CO selectivity factors to H2, N2, and CH4 (separation factors: CO/H2=48.9, CO/N2=542, CO/CH4=187). As the N2, CH4, and H2 adsorption amounts were are very small, it was difficult to use IAST to calculate
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CO/N2, CO/CH4, and CO/H2 selectivity values. In any case, the selectivities for CO over H2, N2, and CH4 are expected to be much larger than the selectivity for CO over CO2. Thus, we expect that this new CuCl/bayerite adsorbent will show outstanding performance for
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adsorption-based CO separation from various CO sources.
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ACCEPTED MANUSCRIPT 4. Conclusion We synthesized a highly selective CO adsorbent based on a CuCl/bayerite composite. Under the optimal preparation conditions, 30 wt% CuCl was thermally impregnated via a TMD process at 573 K into the bayerite supporting material, which was pre-activated at 623
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K. This sample exhibited an excellent CO adsorption capacity of 48.5 cm3 g-1 at 293 K under 100 kPa of CO and a CO/CO2 separation factor of 16.8. The CO adsorption capacity of the present CuCl/bayerite is much larger than that of the previous CO adsorbent that used
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boehmite as a supporting material, and even CO/CO2 separation factor is somewhat larger than that of the previous adsorbent [13]. The CO/CO2 separation factor of the CuCl/bayerite
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adsorbent dramatically increased to 35.5 when the content of CuCl was increased to 36 wt%. The previous adsorbent using boehmite exhibited quite high CO adsorption capacity, and especially the best recorded CO/CO2 separation factor [13]. To the best of our knowledge, the CO/CO2 separation factor of the present CuCl/bayerite adsorbent is now the highest reported
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value. The CO/CO2 selectivity under mixed gas conditions calculated using the IAST model is much larger than the CO/CO2 separation factor. Assuming a 1:1 volume ratio of CO and CO2 in the mixed gas, the CO/CO2 selectivity of 36 wt% CuCl/bayerite adsorbent was 1,982
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under 1 kPa, 139 under 100 kPa, and 78 under 250 kPa of mixed gas at 293 K. The high CO
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adsorption capacity of CuCl/bayerite is due to the large surface area of bayerite, which can disperse CuCl species well, as confirmed by the uniform dispersion observed by SEM-EDS analysis. This adsorbent showed negligible adsorption capacities for N2, CH4, and H2, which normally coexist with CO in various CO sources generated from various fields, such as steelmaking and steam reforming processes. Overall, the new CuCl/bayerite adsorbent developed in this work will show great performance as a CO adsorbent for selective CO separation from various CO sources.
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ACCEPTED MANUSCRIPT 5. Acknowledgments This work was conducted under the framework of Research and Development Program of the
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Korea Institute of Energy Research (KIER, B8-2426-02).
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ACCEPTED MANUSCRIPT 6. References [1] Y. Xie, J. Zhang, J. Qiu, X. Tong, J. Fu, G. Yang, H. Yan, Y. Tang, Zeolites modified by CuCl for separating CO from gas mixtures containing CO2, Adsorption 3 (1996) 27-32.
impregnated with metal halide, AIChE J. 42 (1996) 422-430.
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[2] H. Tamon, K. Kitamura, M. Okazaki, Adsorption of carbon monoxide on activated carbon
[3] S. Hou, C. Chen, C. Chang, C. Wu, J. Ou, T. Lin, Firing blast furnace gas without support fuel in steel mill boilers, Energy Convers. Manage. 52 (2011) 2758-2767.
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[4] A. Chauvel, G. Lefebvre, Petrochemical processes, 2nd ed., Editions Technip, Paris, 1989.
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[5] Y.-I. Lim, J. Choi, H.-M. Moon, G.-H. Kim, Techno-economic comparison of absorption and adsorption processes for carbon monoxide (CO) separation from Linze-Donawitz gas (LDG), Korean Chem. Eng. Res. 54 (2016) 320-331.
[6] W.-H. Chen, M.-R. Lin, T.-S. Leu, S.-W. Du, An evaluation of hydrogen production from
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the perspective of using blast furnace gas and coke oven gas as feedstocks, Int. J. Hydrogen Energy 36 (2011) 11727-11737.
[7] F. Kasuya, T. Tsuji, High purity CO gas separation by pressure swing adsorption, Gas
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Sep. Purification 5 (1991) 242-246.
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[8] F. Gao, Y. Wang, S. Wang, Selective adsorption of CO on CuCl/Y adsorbent prepared using CuCl2 as precursor: Equilibrium and thermodynamics, Chem. Eng. J. 290 (2016) 418427.
[9] R.T. Yang, Adsorbents: Fundamentals and applications, John Wiley & Sons, New Jersey, 2003. [10] Y. Wang, R.T. Yang, J.M. Heinzel, Desulfurization of jet fuel by π-complexation adsorption with metal halides supported on MCM-41 and SBA-15 mesoporous materials, Chem. Eng. Sci. 63 (2008) 356-365.
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ACCEPTED MANUSCRIPT [11] J.W. Yoon, T.-U. Yoon, E.-J. Kim, A.-R. Kim, T.-S. Jung, S.-S. Han, Y.-S. Bae, Highly selective adsorption of CO over CO2 in a Cu(I)-chelated porous organic polymer, J. Hazard. Mater. 341 (2018) 321-327. [12] A.-R. Kim, T.-U. Yoon, S.-I. Kim, K. Cho, S.S. Han, Y.-S. Bae, Creating high CO/CO2
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selectivity and large CO working capacity through facile loading of Cu(I) species into an iron-based mesoporous metal-organic framework, Chem. Eng. J. 348 (2018) 135-142.
[13] K. Cho, J. Kim, H.T. Beum, T. Jung, S.S. Han, Synthesis of CuCl/Boehmite adsorbents
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that exhibit high CO selectivity in CO/CO2 separation, J. Hazard. Mater. 344 (2018).
[14] H. Hirai, K. Wada, M. Komiyama, Active carbon-supported copper(I) chloride as solid
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adsorbent for carbon monoxide, Bull. Chem. Soc. Jpn. 59 (1986) 2217-2223. [15] M. Ishioka, T. Okada, K. Matsubara, Formation and characteristics of vapor grown carbon fibers prepared in Linz-Donawitz converter gas, Carbon 30 (1992) 975-979. [16] Y. Huang, Y. Tao, L. He, Y. Duan, J. Xiao, Z. Li, Preparation of CuCl@AC with high
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CO adsorption capacity and selectivity from CO/N2 binary mixture, Adsorption 21 (2015) 373-381.
[17] J. Ma, L. Li, J. Ren, R. Li, CO adsorption on activated carbon-supported Cu-based
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adsorbent prepared by a facile route, Sep. Purif. Technol. 76 (2010) 89-93.
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[18] T.C. Golden, W.C. Kratz, F.C. Wilhelm, Highly dispersed cuprous compositions, US Patent No. 5,126,310, 1992. [19] X. Krokidis, P. Raybaud, A.-E. Gobichon, B. Rebours, P. Euzen, H. Toulhoat, Theoretical study of the dehydration process of boehmite to γ-alumina, J. Phys. Chem. B 105 (2001) 5121-5130.
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ACCEPTED MANUSCRIPT Tables Table 1. Porous texture properties of various bayerite material and CuCl/bayerite composites.
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Pre-activation Vtotb CuCl content SBETa temperature (wt%) (m2 g-1) (cm3 g-1) (Thermal treatment temperature) 1c 0 10 0.04 2 523 K 0 397 0.11 3 573 K 0 465 0.17 4 623 K 0 469 0.22 5 673 K 0 454 0.25 6 773 K 0 307 0.27 7 623 K (573 K) 30 302 0.14 a SBET (m2 g-1) is BET surface area was calculated from edges obtained at relative pressure (P/P0) between 0.05 and 0.3 of the isotherms using BET equation. b Vtotal is pore volume calculated at P/P0 = 0.95 c This bayerite sample was not activated by thermal treatment, but it was degassed at 323 K before N2 sorption measurement.
Table 2. CO and CO2 adsorption capacity and CO/CO2 separation factor of CuCl/bayerite composites.
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Thermal treatment CuCl content Adsorbed CO Adsorbed CO2 temperature CO/CO2 (wt%) (cm3 g-1) (cm3 g-1) (pre-activation temperature) 1 523 K (623 K) 30 36.0 2.66 13.5 2 573 K (623 K) 30 48.5 2.89 16.8 3 623 K (623 K) 30 30.1 3.77 7.98 4 673 K (623 K) 30 25.1 4.26 5.89 5 573 K (523 K) 30 32.2 1.62 19.7 6 573 K (573 K) 30 38.9 2.71 14.4 7 573 K (673 K) 30 38.1 3.49 10.9 8 573 K (623 K) 12 19.2 10.1 1.90 9 573 K (623 K) 24 31.9 4.54 7.02 10 573 K (623 K) 36 41.6 1.17 35.5 11 573 K (623 K) 48 31.4 0.00 -a a The CO/CO2 selectivity was not calculated, because the CO2 adsorption capacity of this sample is almost negligible.
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Figures
Fig. 1. XRD patterns of pristine bayerite and the treated bayerite samples which were
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calcined at various temperatures.
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Fig. 2. N2 sorption isotherms (taken at 77 K) of pristine bayerite and the treated bayerite
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samples which were calcined at various temperatures.
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Fig. 3. (a) CO and (b) CO2 adsorption isotherms (taken at 293 K) of the CuCl/bayerite samples which were prepared by impregnating 30 wt% CuCl at various temperatures on bayerite (pre-activated at 623 K). The numbers written in the legend of the figure are the
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temperatures for the impregnation of the CuCl.
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Fig. 4. (a) CO and (b) CO2 adsorption isotherms (taken at 293 K) of the CuCl/bayerite samples which were prepared by impregnating 30 wt% CuCl at 573 K on various bayerite materials pre-activated at various temperatures. The numbers written in the legend of the
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figure are the temperatures for the pre-activation of the bayerite materials.
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Fig. 5. (a) CO and (b) CO2 adsorption isotherms (taken at 293 K) of the CuCl/bayerite samples which were prepared by impregnating various amount of CuCl on a bayerite material (pre-activated at 623 K) at 573 K. The number in the legend of the figure is the weight
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percent of CuCl to total weight of the composite of CuCl and bayerite.
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Fig. 6. SEM micrographs of activated bayerite, CuCl powder, and CuCl/bayerite which was synthesized by impregnating 30 wt% of CuCl at 573 K, and EDS elemental mapping images
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of Al and Cu for the CuCl/bayerite sample.
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Fig. 7. CO/CO2 selectivity of (a) 30 wt% CuCl/bayerite sample and (b) 36 wt%
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CuCl/bayerite sample under various pressure of the mixture gas composed of CO and CO2
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(1:1 volume ratio), which was theoretically calculated using IAST model.
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Fig. 8. Adsorption isotherms of 30 wt% CuCl/bayerite adsorbent for CO, CO2, N2, CH4, and
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H2 gases, which were taken at 293 K.
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HIGHLIGHTS
• Highly selective CO adsorbent based on CuCl/bayerite composite was synthesized. • Adsorbent exhibited very high CO/CO2 selectivity. • High separation ability is attributed to the large surface area of bayerite.
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• Thermal treatment conditions for dispersion of CuCl on bayerite were optimized.