Article pubs.acs.org/EF
Silica Nanoparticles as Supports for Regenerable CO2 Sorbents Sergio Meth,*,†,‡ Alain Goeppert,† G. K. Surya Prakash,† and George A. Olah† †
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, California 90089-1661, United States ‡ Chemical Engineering, Federal University of Pampa Bagé, Rio Grande do Sul, Brazil ABSTRACT: This paper describes the capture of CO2 by polyethylenimine (PEI) physically adsorbed on the surface of silica nanoparticles. The resulting material proved to be not only regenerable but also having the right qualities needed for a practical utilization, including absorption not only from high concentration streams like flue gases and natural gas wells but also from the air itself. Polyethyleneglycol (PEG) was used as a coadditive. The effect on CO2 capture of temperature, supports, and concentrations of polyethylenimine and polyethyleneglycol was evaluated. Based on the results, the PEI and PEG concentrations were optimized to obtain a material with optimal CO2 absorption characteristics. The obtained material demonstrated the practical possibility to capture CO2 using an inexpensive, easy-to-prepare, and regenerable absorbent.
1. INTRODUCTION Since the beginning of the Industrial Revolution two centuries ago, humankind has been burning fossil fuels to generate energy. Initially, coal fuelled the unprecedented expansion and development of civilization. In the late 19th century, petroleum oil, followed by natural gas in the 20th century, started to supplement coal. Today, together they constitute about 80% of the energy consumed in the world.1 The utilization of these nonrenewable sources is associated with a number of problems. One is the fact that burning fossil fuels on such a large scale has increased the level of atmospheric carbon dioxide by more than 30% in the last three centuries. Since there is no indication that an alternative cheap energy source will be available on the enormous scale needed to replace fossil fuels in the near future, it is expected that the CO2 concentration in air will continue to increase. Carbon dioxide is considered to be a major greenhouse gas causing the so-called global warming of Earth.2 Therefore, one of the most important environmental priorities is to find an economical solution to capture carbon dioxide. Once captured, the CO2 can be sequestered in geological formations or under the sea (carbon capture and sequestration, CCS). However, CCS is only a temporary solution plagued with problems. Another alternative is to use the captured CO2 as a raw material for the synthesis of organic compounds through chemical or electrochemical reduction using alternative power sources such as solar, wind, or nuclear energy (carbon capture and recycling, CCR).3−9 1.1. CO2 Capture Technologies. The existing CO2 separation/capture technologies10−15 are considered to be too expensive and energy intensive for large scale CO2 separation. It is therefore important to develop new technologies to reduce costs.7 Since the current cost of the CO2 separation and capture step is about three times more expensive than the sequestration step, more attention has to be focused in reducing the total cost by improving the former. The U.S. Department of Energy (DOE) estimates US$10/ton of CO2 captured to be an economical cost acceptable for the industry.16 © 2012 American Chemical Society
Besides environmental problems, there are also instances in the industry where it is highly desirable to separate CO2 from various streams.17 For example, natural gas has often an acidic fraction (mostly CO2) that has to be separated (gas sweetening) before the gas can be sent to the consumer.11 Another industrial situation is the synthesis gas used as raw material for the ammonia production. This gas contains hydrogen and nitrogen, and it is essential that any carbon dioxide present be eliminated so that the desired reaction can effectively take place. There are several techniques for the separation of CO2 from gas streams.7,11,17 These techniques are based on different physical and chemical properties including absorption into a liquid solvent, adsorption onto a solid, cryogenic separation, or permeation through membranes.18,19 Eventually, even the CO2 in the air will have to be captured.3,20−23 Among these techniques, the most widely used technology and also the oldest is the CO2 absorption/desorption process based on aqueous amine solutions.7,11,24 The most commonly used amines for CO2 capture in the liquid phase are monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), and methyldiethanolamine (MDEA). Certain sterically hindered amines, like 2amino-2-methyl-1-propanol, also are considered as absorbents due to their high CO2 loading properties.7 The process to remove acid components (like H2S and CO2) from a gas flow using aqueous amine solutions was developed over 70 years ago for removing acidic impurities out of natural gas streams.24 This process uses MEA and, until today, remains (together with DEA) the most widely used because it has a fast rate of reaction with CO2, allowing the use of a shorter absorption tower compared to other, less reactive, amines. Several companies (among them initially Fluor Daniel Inc., Dow Chemical Co., Kerr McGee Chemical Corp., and ABB Received: February 22, 2012 Revised: April 9, 2012 Published: April 11, 2012 3082
dx.doi.org/10.1021/ef300289k | Energy Fuels 2012, 26, 3082−3090
Energy & Fuels
Article
A possible approach to this problem is to chemically bind amine based absorbents on solid supports. The grafting of amines and polyamines to the surface of solids, especially silicas and alumino-silicas, has been fairly well investigated.29−33 The reports in the literature show results always below 80 mg CO2/ g and, in most cases, below 60 mg CO2/g of absorbent. The possibility of using amines and polyamines physically adsorbed (deposited) on solids has also been studied mainly on silica, polymers, and alumina, but a majority of the prepared absorbents employed supports with particle sizes above 1 μm (a typical silica gel, for example, has a particle size in the range 60−200 μm). A number of reports also used relatively small molecules like MEA, DEA, or polyethylenimine of low molecular weights (PEI-LMW, MW ∼ 600). Due to their low boiling points, these adsorbents using MEA and DEA were found to be unstable, releasing considerable amounts of material during the regeneration step.34−36 This shows the importance of using an amine with very low or even no volatility to avoid contamination of the gas stream and loss of activity over time. Satyapal et al.37 describes the development of a sorbent based on PEI impregnated on a high surface area polmethylmethacrylate polymer. This solid was used in the space shuttles to remove CO2 from the cabin atmosphere and release it into space in a pressure swing absorption (PSA) mode. The capacity is approximately 40 mg CO2/g absorbent at 50 °C and 0.02 atm CO2.38−40 The preferred supports described in the associated patents are polymeric in nature. Xu et al.41−44 describe the preparation and performances of a solid absorbent consisting of PEI deposited on the mesoporous MCM 41, described by them as “molecular basket” absorbents. Absorption capacities for CO2 for MCM-41-PEI with different loadings of PEI were measured with pure CO2. The best result was achieved with MCM-41-PEI containing 75 wt % PEI with 133 mg CO2/g sorbent at 75 °C. With MCM-41-PEI containing 50 wt % PEI, the optimal value was obtained at 75 °C with 112 mg CO2/g sorbent. In a subsequent publication, using more solvents during the sorbent preparation step, Xu et al.42 improved the absorption to 126 mg CO2/g with the MCM-41 sorbent containing 50% PEI. At the same temperature, silica gel−PEI containing 50 wt % PEI absorbed only 78 mg CO2/g absorbent despite the high surface area (550 m2/g) of the employed silica gel. More recently, mesoporous capsules containing PEI or tetraethylenepentamine (TEPA) were also shown to have high CO2 absorption capacities.45 In a contribution from our group, we demonstrated a material composed of PEI on precipitated silica with an absorption capacity of up to 200 mg CO2/g absorbent.46,47 It is important to emphasize that the amines described here can exhibit a better CO2 absorption through the addition of ethylene glycol and polyethylene glycol (PEG). This has been known for many years.48 Huntchinson,11 in 1939, described such a system. It was shown that it is possible to remove contaminants, including carbon dioxide, from gas streams using liquid absorbents based on dialkyl ether of polyethylene glycol solvent.49,50 This was followed by the discovery that when PEG was mixed with amines, a synergetic effect in the removal of acidic components including carbon dioxide and hydrogen sulfide from gas mixtures through gas−liquid interface was observed.50 1.2. Chemistry of CO2 Absorption on Amines. Sterically unhindered primary alkanolamines react with CO2 to form a carbamate. In the absence of water, 2 mol of amine are
Lummus Crest Inc.) have adapted this MEA based process to capture CO2 from industrial gases. However, these aqueous amine solution processes have several limitations that are drawbacks in their use on a large scale as required by the energy industry. Because it is necessary to prevent excessive corrosion and high viscosity, aqueous solutions containing only 10−30 mass % of MEA have to be used. The rest is water, which is not active for CO2 absorption. During the endothermic regeneration step, the water portion (70−90 mass %) of the solution has also to be heated, making the process very energy demanding. Other amines, being secondary and hindered, like DEA, DIPA, and AMP, have more moderate rates of reaction with CO2 and also have, to some extent, a tendency for corrosion and solvent degradation. On the other hand, MDEA (a tertiary amine) reacts with CO2 at a slow rate. This happens because the pKa values are different depending on the nature and structure of the amines (primary, secondary, or tertiary), therefore providing different affinities for an acid gas such as CO2.25 An interesting approach is the combination of several alkanolamines to obtain favorable characteristics while minimizing undesirable ones. Companies that work in this field such as UOP or Dow Chemical Co. have developed some of these blended alkanolamines solutions, in general, MDEA based solutions containing MEA or DEA. We can point out that the main advantage of using amine solutions is that they are a very well established technology with high selectivity to acid components (CO2, H2S, etc.) present in mixtures of gases. However, there are several drawbacks: high energy requirement for the regeneration step, corrosion problems, limited loadings of the active amines for CO2 absorption, and limited lifetime due to degradation by oxidation of the amine. An additional problem is the requirement for a large absorption tower with pressure drops in the gas flow associated with this kind of equipment. Some of the problems associated with aqueous amine solution have been overcome through the immobilization of these amines on porous solids26,27 and desorbing the CO2 from the absorbent by vacuum and/or heating during the regeneration step.11,17 Even before this approach, solids had already been used for the separation of CO2 by physical adsorption. These solids include silica gel, alumina, activated carbon, or even porous materials with pore size controlled by crystal structure such as zeolites. Zeolite11,28 based adsorbents, in particular, show, at low temperature (e.g., room temperature), high absorption capacities for CO2 (zeolite 13X, 160 mg CO2/g; zeolite 4A, 135 mg CO2/g at 25 °C in pure CO2). However, these adsorbents show fast decline in adsorption capacities with increasing temperature. Furthermore, because the gases are only physically adsorbed on these solids, the separation factors between different gases (e.g., CO2/N2) are low. There are some characteristics that we should come to expect for a practical CO2 selective solid sorbent such as high absorption capacity, long-term stability, and regenerability with no or minimal degradation. The temperature difference between the exothermic absorption step and the endothermic desorption step should be reasonably low to minimize energy input during the regeneration process. The solid absorbent should be a powder able to be used eventually in a fluidized bed. Finally, from a practical point of view, it is important to prepare the absorbent from easily available and relatively inexpensive raw materials. 3083
dx.doi.org/10.1021/ef300289k | Energy Fuels 2012, 26, 3082−3090
Energy & Fuels
Article
necessary to capture 1 mol of CO2, as can be seen in the following example with MEA. The heat of absorption for this reaction (−458 cal/g CO2, exothermic) is high,11 leading to a high-energy consumption for the thermal regeneration step, which typically occurs at a temperature of about 120 °C. To reach this temperature, it is necessary to heat not only the active component, alkanolamine, but also the water fraction:
Table 1. Approximate Integral Heats of Solution for Absorption of H2S and CO2 in Alkanolamine Solutionsa integral heat of solution (cal/g acid gas)
In presence of water, the reaction is somewhat different, practically doubling the absorption capacity because water has the capacity of hydrolyzing the carbamate and freeing one of the amino groups. The problem is that since the carbamate ion has some stability, this reaction does not take place spontaneously.
amine
H2S
CO2
MEA (monoethanolamine) DEA (diethanolamine) DGA MDEA (methyldiethanolamine) TEA (triethanolamine) DIPA (diisopropanolamine)
342 283 375 289 239 264
458 389 456 319 258 400
a
Based on total heat released when acid gas is absorbed from a mole ratio of 0 to about 0.4 mol acid gas/mol amine at 38 °C with typical commercial amine concentrations.11
desorption, are examined. The aim was to obtain a solid material in a powder form with a good absorption capacity, without high energy requirements for the regeneration step and with good mechanical properties allowing its utilization for example in a fluidized bed. The optimization of the composition aimed at obtaining the best absorbing capacity without losing the required mechanical properties was pursued throughout this work. Not only high CO2 levels were considered but also levels as low as 360 ppm, similar to the present concentration of CO2 in the air (390 ppm).
R‐NH 2 + CO2 + H 2O ⇄ R‐NHCOO− + H3O+
The mechanism is different for secondary amines like DEA, where CO2 initially reacts to form a carbamate, which may also react further to form a bicarbonate product in the presence of high CO2 concentrations. It allows 1 mol of CO2 to be captured by only 1 mol of amine. The heat of reaction (absorption) of 363 cal/g CO2 is still high,11 but it is lower than in the case of MEA, which makes the energy requirements for the desorption step relatively less demanding .
2. EXPERIMENTAL SECTION Commercially available silicas, fumed silica (Sigma, surface area 390 ± 40 m2/g, primary particle size 0.007 μm), precipitated silica (Hi-SiL T600, PPG Industries, surface area 170 ± 15 m2/g, particle size 3.5−7.5 μm), and silica gel (EMD, 60−200 mesh) were used as supports. The manufacturers gave the characteristics of the materials, and some of these data were confirmed during the course of our work. The polyethylenimines: low molecular weight (PEI-LMW, MW ca. 800), high molecular weight (PEI-HMW, MW ca. 25 000), poly(ethylene glycol) (PEG, avg. MW ca. 400), and all other reagents were purchased from Aldrich (unless otherwise stated) and used as received. Commercial gases and special gas mixtures were bought from Gilmore and Air Gas Companies. All gases used in the experiments were dry. TGA (thermogravimetric analysis) experiments were carried out using a Shimadzu TGA-50 thermogravimetric analyzer. DSC (differential scanning calorimetry) measurements were done using a Shimadzu DSC-50 differential scanning calorimeter. SEM (scanning electron microscopy) pictures were taken on a Cambridge 360 electron microscope. The samples were previously covered by a thin gold layer that was sputtered coated. BET (Brunauer−Emmett−Teller) analysis (including surface area, porous volume, and porous distribution) were carried out on a PMA BET Sorpto metter model 201A using nitrogen as adsorbate and −196 °C as the testing temperature. Samples for TEM (transmission electron microscopy) were embedded in EPO-fix (from Electron Microscopy Sciences), sectioned, and viewed on a Philips EM 420 transmission electron microscope. The preparation of the materials was carried out by impregnation of the supports in two steps: in the first step, a methanolic solution of polyethyleneglycol (about 10%) was added stepwise under stirring to a suspension of the support in methanol (proportion 1 g/20 mL). After mixing for an hour at room temperature, the solvent was removed from the mixture by heating at 50 °C under vacuum on a rotovap, followed by dynamic vacuum treatment overnight (
