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Ind. Eng. Chem. Res. 2001, 40, 204-209

Adsorption of Carbon Dioxide onto Hydrotalcite-like Compounds (HTlcs) at High Temperatures Zou Yong, Vera Mata, and Alı´rio E. Rodrigues* Laboratory of Separation and Reaction Engineering, Faculty of Engineering, University of Porto, Rua dos Bragas, 4050-123 Porto, Portugal

The adsorption capacities of carbon dioxide on six commercial hydrotalcite-like compounds and the main factors (aluminum content, anion type, water content, and heat treatment temperature) influencing their adsorption capacity at high temperatures have been investigated using a gravimetric technique. There is an optimum aluminum content and heat treatment temperature for the adsorption capacity. The carbonate anion favors adsorption of carbon dioxide compared to OH-, and a low content of water also improves the adsorption capacity. The carbon dioxide adsorption capacity is mainly dependent on the microporous volume, interlayer spacing, and layer charge density of the hydrotalcite-like compounds. 1. Introduction The removal and recovery of carbon dioxide from hot gas streams is becoming increasingly significant in the field of energy production. The combustion of fossil fuels, such as coal or natural gas, releases large volumes of carbon dioxide to the environment, which has become one of the most serious global environmental problems.1-3 The topic is also very important in other areas, such as natural gas treatment,4 production of hydrogen gas,5 and the aerospace industry.8 There are several options for reducing carbon dioxide emissions, including substituting nuclear power for fossil fuels, increasing the efficiency of fossil plants, and capturing the carbon dioxide prior to emission into the environment. All of these techniques have the attractive feature of limiting the amount of carbon dioxide emitted into the atmosphere, but each has economic, technical, or societal limitations.9 The removal and recovery of carbon dioxide from power plant fuel gases is considered to be one of the effective approaches for reducing total carbon dioxide emissions from the energy point of view.3 A number of techniques can be used for the separation of carbon dioxide from fuel gas streams. The large-scale separation of carbon dioxide by absorption is a commercial operation used throughout the world. Other techniques exist that could be considered for energyrelated applications, such as cryogenic separation, membrane separation, and adsorption processes, such as pressure swing adsorption (PSA), vacuum swing adsorption (VSA), and temperature swing adsorption (TSA). Pressure swing adsorption is well suited for the removal and recovery of carbon dioxide from any hot fuel gas that contains carbon dioxide. The PSA process can be operated at elevated temperatures, typically the temperature of the fuel gas source, to remove most of the carbon dioxide, and it overcomes the need for cooling the fuel gas to ambient temperature prior to the removal of carbon dioxide.10 Therefore, in the last two decades, active research efforts have been directed toward the * To whom correspondence should be address. E-mail: [email protected]. Telephone: 351-22-2041671. Fax: 351-222041674.

separation of carbon dioxide by the PSA process, and this technique has been industrialized. However, to use the PSA process for the removal and recovery of carbon dioxide from hot fuel gas streams at elevated temperatures, the first and most important issue is to find the appropriate adsorbent. The adsorbent must have (1) high selectively and adsorption capacity for carbon dioxide at high temperature, (2) adequate adsorption/ desorption kinetics for carbon dioxide under operating conditions, (3) stable adsorption capacity for carbon dioxide after repeated adsorption/desorption cycles, and (4) adequate mechanical strength of adsorbent particles after cyclic exposure to high-pressure streams.3,11 Recently, it was reported that hydrotalcite-like compounds (HTlcs) could well meet the above requirements, and such compounds are one of the most promising adsorbents for the sorption-enhanced reaction process (SERP) for hydrogen production.5,6 A cost reduction of 25-30% compared with conventional methane steam reforming was reported by Hufton et al.7 However, there is very little information concerning the adsorption of carbon dioxide onto HTlcs at elevated temperatures in the open literature. Hydrotalcite-like compounds belong to a large class of anionic and basic clays, also known as layered double hydroxides (LDH). They are composed of positively charged brucite-like [Mg(OH)2] layers with trivalent cations substituting for divalent cations at the centers of octahedral sites of hydroxide sheets whose vertexes contain hydroxide ions, and each -OH group is shared by three octahedral cations and points toward the interlayer regions. The excess positive charge of HTlcs is compensated for by anions and water molecules present in the interstitial positions. They can be represented by the general formula [(M2+1-xM3+x(OH)2)x+‚ (An-x/n‚mH2O)x-], where M2+ ) Mg2+, Ni2+, Zn2+, Cu2+, or Mn2+; M3+ ) Al3+, Fe3+, or Cr3+; An- ) CO32-, SO42-, NO3-, Cl-, or OH-; and x is normally between 0.17 and 0.33, but there is no limitation. The structure of these compounds12 can be visualized in Figure 1a, b. These materials have received considerable attention in recent years because they are used in a wide range of applications, such as catalysts, precursors, and supports of catalysts; ion exchangers; filters; decolorising agents; industrial adsorbents; polymer stabilizers; optical hosts;

10.1021/ie000238w CCC: $20.00 © 2001 American Chemical Society Published on Web 11/21/2000

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 205 Table 1. Main Chemical and Physical Properties of Three Commercial Hydrotalcite-like Compounds from CONDEA Chemie Gmbh (Germany) property chemical description form Al2O3:MgO (%) L.O.I.a (%) thermal stability surface area (m2/g) 3h/550 °C 3h/950 °C 3h/1150 °C pore volume (mL/g) loose bulk density (g/mL) particle size < 25 µm (%) particle size < 45 µm (%) particle size < 95 µm (%) Al2O3 (Mg-dop, 100%) (%) MgO (Mg-dop, 100%) (%) a

PURAL MG30

PURAL MG50

PURAL MG70

aluminum magnesium hydroxide powder 70:30 35

aluminum magnesium hydroxide powder 50:50 40

aluminum magnesium hydroxide powder 30:70 45

271 110 30 0.55 0.44 27.1 49.7 91.4 70.9 29.1

228 90 25 0.45 0.58 32.9 61.5 97.1 49.3 50.7

201 120 65 0.35 0.59 59.2 92.9 100 29.2 70.8

L.O.I ) loss on ignition.

The objective of this work is to study the adsorption behavior of carbon dioxide onto several commercial HTlcs, as well as the main factors influencing the adsorption capacity at high temperatures, to provide the experimental and theoretical basis for developing HTlcs as adsorbents for carbon dioxide. This study is part of a research program on coupled reaction/separation processes in which it is intended to remove carbon dioxide in a steam reforming process and so to increase the equilibrium conversion and work at lower temperatures than are conventionally used. The chemical modification of HTlcs and the adsorption/desorption kinetics of carbon dioxide at high temperature and in the presence of steam will be studied in future work. 2. Experimental Section

Figure 1. Structure models for hydrotalcite-like compounds [(M2+1-xM3+x(OH)2)x‚(An-x/n‚mH2O)x-] (HTlcs). (a) 3-D structure12, (b) 2-D structure.

and ceramic precursors. However, up to now, the main applications of HTlcs are as catalysts and precursors of catalysts.13-18 Only a few papers on HTlcs as adsorbents for carbon dioxide have been reported.5,19-21

2.1. Materials and Reagents. In this study, six HTlcs from two companies were used. CONDEA Chemie Gmbh (Germany) provided PURAL MG30, PURAL MG50, and PURAL MG70; EXM696, EXM701, and EXM911were provided by Su¨d-Chemie AG Company (Germany). Representative characteristics of the six HTlcs samples are reported in Tables 1 and 2, respectively. The carbon dioxide (N48, 99.998% purity) was from Air Liquide (France). 2.2. Apparatus and Procedure. 2.2.1. Apparatus. A schematic diagram of the experimental apparatus for the measurement of single adsorption isotherms is shown in Figure 2. It has three major sections: a weighing system, a gas-providing system, and a data acquisition system. Weighing measurements were performed with a microbalance (A) (CI-Robal, Wilshire, UK) in which a cage with samples inside is suspended in one of its arms (B). 2.2.2. Procedure. Experiments for the single adsorption isotherms were carried out as follows. A small amount of HTlc sample (60-80 mg) was introduced into the microbalance basket, and the sample was submitted to a controlled temperature ramp of 4-5 K/min under vacuum conditions (total pressure of ∼2 mbar) until the required temperature was reached. The sample was kept at the required temperature and ∼2 mbar for 3-4 h until no further variations in the weight were detected. The required temperature of the single adsorption isotherms is fixed in the oven, and the system is kept waiting until steady conditions are achieved. The experimental isotherm starts from the vacuum condition

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Figure 4. Effect of the anion type on the adsorption of carbon dioxide on HTlcs (anion CO32-, EXM696 and EXM911; anion OH-: MG30, MG50, and MG70) at 300 °C.

Figure 2. Schematic diagram of the experimental apparatus for measuring adsorption equilibrium isotherm by a gravimetric technique.

Figure 3. Effect of aluminum content on the adsorption of carbon dioxide on HTlcs (MG30, MG50, and MG70) at 300 °C, 1bar.

of nearly 2 mbar (adsorption path), and then small amounts of the carbon dioxide are introduced step by step until atmospheric pressure is reached. Each experimental point in the adsorption isotherm was repeated three times to verify data reproducibility. 3. Results and Discussion 3.1. Effect of Aluminum Content. The adsorption capacities of carbon dioxide onto PURAL MG30 (70%Al2O3), MG50 (50% Al2O3), and MG70 (30% Al2O3) samples having different aluminum contents at 300 °C and 1 bar are shown in Figure 3. The results show that the amounts of adsorbed carbon dioxide on the three HTlcs increased when the amount of aluminum was decreased from 70% (MG30) to 50% (MG50). However, MG50 has a higher aluminum content than MG70, but the amount of adsorbed carbon dioxide on MG50 is slightly higher than that on MG70. The reason is that incorporated aluminum has two functions: one is that the density of the layer charge in HTlcs increased with

increasing the aluminum content, which is favorable for adsorption of carbon dioxide;20 the other leads to a decrease in the interlayer spacing of HTlcs and a reduction in the number of high-strength carbon dioxide adsorption sites in the HTlcs with increasing the aluminum content.22-24 Therefore, there is an optimum aluminum content in the HTlcs for adsorption of carbon dioxide, in agreement with results reported by Gang et al.20 3.2. Effect of the Type of Anion. Figure 4 shows the results of adsorption capacity for carbon dioxide on HTlcs with two kinds of anion at 300 °C. The results show that the amounts of adsorbed carbon dioxide on HTlcs containing CO32- (EXM samples) are higher than those on HTlcs containing OH- (MG samples). The main reasons are that (1) the carbonate ion CO32- is larger than the hydroxide ion OH-, having a larger interlayer spacing (0.765 nm) compared with that of OH- (0.755 nm),14 and (2) the charge of the CO32- is higher than that of the OH-. Therefore, HTlcs containing CO32have more void space in their interlayer regions and can accommodate more carbon dioxide gas.20 3.3. Effect of the Water Content. EXM 701 is prepared from EXM 696 by calcination at elevated temperature. This material, with a very low content of water, is very hygroscopic, as shown in Table 2. The adsorption capacities of carbon dioxide on EXM696 and EXM701 samples at 300 °C are shown in Figure 5. The results show that the adsorption capacity of EXM696 is higher than that of EXM701, particularly at low pressure (P < 0.05 bar). The reason is that there is a very low content of M2CO3 (M ) Na, K) in the HTlcs, because of the preparation process. When there is some water in the HTlcs, carbon dioxide will undergo the reaction 25 M2CO3 + CO2 + H2O ) 2MHCO3, which is favorable for adsorption of carbon dioxide and enhances the adsorption capacity of carbon dioxide. This result indicates that a low water content in the HTlcs can enhance the adsorption capacity for carbon dioxide at high temperature. This effect is somewhat related to the fact that the adsorption capacity for CO2 is enhanced by the moisture content of the gas stream.26 3.4. Effect of the Temperature. Figures 6 and 7 show the results of adsorption capacity of carbon dioxide on EXM696 and EXM911 at room temperature (20 °C) and higher temperatures (200 and 300 °C), respectively. The results show that (1) the adsorption capacities of EXM696 at the three temperatures are all higher than those of EXM911 and (2) the order of the adsorption capacities of EXM696 and EXM911 at the three temperatures is all Q(300 °C) > Q(20 °C) > Q(200 °C), which deserves some comment.

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 207 Table 2. Main Chemical and Physical Properties of Three Commercial Hydrotalcite-like Compounds from Su 1 d-Chemie AG Company (Germany) property

EXM696

EXM701

EXM911

chemical description form density (g/mL) pH (5% suspension, water) moisture (2h,120 °c) (%) surface area (m2/g) Al2O3 (%) MgO (%) CO2 (%)

basic magnesium aluminum hydroxy carbonate powder 2.2 8.6 4 74 20.8 33.8 11

basic magnesium aluminum hydroxy carbonate powder 2.7 12.3 1 180 32 58.4 8.3

basic magnesium aluminum hydroxy carbonate powder 2.1 9.5 2 17 22.4 31.7 9.6

Figure 5. Adsorption of carbon dioxide on EXM696 and EXM701 at 300 °C: effect of the water content.

Figure 8. Decarbonation behavior of HTlcs at various temperatures:29 amount of CO2 evolved from HTlcs at various temperatures.

Figure 6. Adsorption of carbon dioxide on EXM696 at 20, 200, and 300 °C.

Figure 7. Adsorption of carbon dioxide on EXM911 at 20, 200, and 300 °C.

Experiments for the single adsorption isotherms are carried out in a vacuum and started when no variations in sample weight are observed. Actually, before the measurement is started, the sample is heated to the desired temperature and is subjected to thermal decomposition of the HTlcs. It was reported24,27-28 that HTlcs have different thermal decomposition behaviors in the different stages of calcination. Figure 8 shows the

decarbonation behaviors of HTlcs at various temperatures.29 When heated to 200 °C, hydrotalcite-like compounds become dehydrated, and the product still retains a layer structure. A significant rearrangement of the octahedral brucite-type layer occurs with the migration of the M3+ cation out of the layer to tetrahedral sites in the interlayer, and the d spacings decrease progressively with increasing temperature.24,28 Because of the decrease in the d spacings, the HTlcs have less void space in their interlayers and can accommodate less carbon dioxide gas.20 Meanwhile, the amounts of adsorbed carbon dioxide on the surface of the HTlcs decrease with increasing temperature; in particular, the low-strength basic sites (bicarbonate) disappeared above 100 °C.30 Therefore, the adsorption capacities of EXM696 and EXM911 at 200 °C are lower than those at 20 °C. At temperatures above 300 °C, the above unfavorable mechanism for carbon dioxide adsorption is still present; however, the dehydroxylation between OH groups of contiguous layers and the decarbonation processes occur above 300 °C. The former produces a structure modification of the layers consisting of a change in the M3+ cation environment from octahedral to tetrahedral coordination. The latter removes the carbonate anion as CO2 from the HTlcs; could partially destroy the layers, giving rise to holes; forms micropores in the HTlc decomposition products, and increases the surface area and pore volume of the products,24 which is very important for the adsorption of carbon dioxide. Therefore, the process enhances the adsorption capacity of carbon dioxide on the product, and so the adsorption capacities of EXM696 and EXM911 at 300 °C are higher than those at 20 °C. The evolution of the HTlc structure described above is illustrated in Figure 9.28 The process

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which meets the requirement for the sorption-enhanced reaction process.11 HTlcs can be directly used as adsorbents for the removal and recovery of carbon dioxide from power plant fuel gases. The HTlcs containing CO32- show higher adsorption capacities than those containing OH-. The presence of a low water content in the HTlcs is favorable for the adsorption of carbon dioxide. The aluminum content in substituted HTlcs and the heat treatment temperature strongly affect the adsorption capacity, and there is an optimum aluminum content and heat treatment temperature when HTlcs are to be used as adsorbents for carbon dioxide at elevated temperatures. Acknowledgment This work was supported by Fundac¸ a˜o Para a Cieˆncia e Tecnologia, Ministe´rio da Cieˆncia e Tecnologia, Portugal (Postdoctoral Fellowship PRAXISXXI/BPD/14816/ 1997). Literature Cited

Figure 9. Structure models for products in the different stages of calcination of HTlcs:28 (a) the starting hydrotalcite-like compounds, (b) the dehydrated intermediate, and c) the decomposed materials.

of heat treatment of the HTlcs has two functions. One is forming micropores in the decomposition product of the HTlcs, which is favorable for adsorption of carbon dioxide; the other has an opposite effect, such as causing the d spacings and the amounts of adsorbed carbon dioxide on the surfaces of the HTlcs to decrease with increasing temperature. Combining Figure 8 with the fact that a low water content in the HTlcs enhances the adsorption capacity of carbon dioxide (see section 3.3), it is expected that an optimum temperature for the heat treatment of HTlcs should exist. 4. Conclusions Two types of commercial hydrotalcite-like compounds were studied as adsorbents for carbon dioxide at elevated temperatures. Except for PURAL MG30, all of the samples have an adsorption capacity for carbon dioxide higher than 0.30 mmol/g at 300 °C and 1 bar,

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Received for review February 14, 2000 Revised manuscript received September 19, 2000 Accepted September 20, 2000 IE000238W