Correlation pubs.acs.org/IECR
Chitosan Containing Supported Ionic Liquid Phase Materials for CO2 Absorption Kaija Pohako-Esko, Matthias Bahlmann, Peter S. Schulz, and Peter Wasserscheid* Institute of Chemical Reaction Engineering, Friedrich-Alexander-University Erlangen-Nuremberg, 91058 Erlangen, Germany ABSTRACT: Here we present novel CO2 sorbents based on chitosan ionogels. The powder sorbents called inverse supported ionic liquid phase (SILP) materials were prepared by dissolving chitosan in various ionic liquids (ILs) followed by encapsulation of the ionogel droplets with nanoporous fumed silica. CO2 absorption was determined at 40 °C in the range of 200 to 5500 mbar. At 1 bar, absorption capacities of these materials were 0.1−0.8 mol kg−1; at 5 bar, values of 0.2−1.5 mol kg−1 were reached. A comparison of inverse SILP materials with and without chitosan dissolved in the applied IL indicated that the presence of chitosan increased the CO2 absorption efficiency of the materials. The aim of the study was also to compare the CO2 absorption in pure chitosan and chitosan dissolved in ILs. It was found that dissolution increases the absorption capacity of chitosan about 10 times.
■
INTRODUCTION The urgent need for reducing the atmospheric concentration of greenhouse gases has prompted the development of many new technologies to capture CO2 from exhaust gases. The industrial state-of-the-art is chemical absorption of CO2 with aqueous amines, which bind CO2 chemically and reversibly by carbamate and carbonate formation.1 The amine scrubbing process introduced in 19302 is nowadays applied in diverse processes like natural gas sweetening, treatment of flue gases and air recycling in submarines.3 In several applications, solid sorbents based on immobilized amines4,5 or amino-functionalized polymers6 are favorable due to convenient handling of the sorbent and fast absorption kinetics. However, despite the established technologies, there is still significant room for improvement prompting the development of alternative technologies. The main problematic issues related to today’s technologies for CO2 capturing with amines are the high energy input in the regeneration step, thermal degradation of amines, loss of amine absorbent due to evaporation and equipment corrosion. All these aspects reduce the efficiency of process, create additional operational costs and cause environmental issues. Ionic liquids (ILs) exhibit high CO2 solubility7 and have been extensively studied as potential absorbents in CO2 capturing processes to overcome the above-mentioned drawbacks of aqueous amine absorbents. ILs have been widely promoted as safe and easily recoverable CO2 absorbents due to their specific properties like extremely low vapor pressure, high thermal stability and nonflammability.8 Immobilized ILs, known as SILPs (supported ionic liquid phase),9 are also interesting materials for CO2 capture. However, there are several problems related to application of SILPs in CO2 capturing processes. CO2 scrubbing is typically characterized by the technical task to capture relatively large amounts of CO2 (e.g., exhaust gas from power plants contains about 10% of CO21). As the physical solubility of CO2 in the IL is limited and a direct function of the IL mass, every technical © XXXX American Chemical Society
CO2 capture system with reasonable absorption capacity needs a relatively large amount of the active sorption material. In SILP systems, the ratio of active film material to support is relatively small. Larger amounts of liquid, however, are difficult to host in the void space of porous materials without blocking transport pores and thus losing most advantages of the thin film absorption material. The influence of pore structure and filling for ILs has been discussed in details elsewhere.10 Recent studies applying SILPs for CO2 capture are focused on overcoming the above-mentioned problems.11−13 Recently, a new type of immobilized ionic liquid, so-called “inverse SILP” materials, was introduced by our group10 and others.14 Although in common SILP systems an IL layer is found on the pore walls of a porous support material (with maximum loading being the total pore volume of the support), droplets of IL are covered by silica particles in inverse SILP, thus turning the liquid into a solid (Figure 1). The concept of inverse SILP is inspired by “dry water”, which is a free-flowing powder obtained by blending water and hydrophobic silica
Figure 1. Preparation of inverse SILP and schematic representation of cross section of inverse SILP particles consisting of IL droplet covered with fumed silica particles. Received: March 3, 2016 Revised: June 5, 2016 Accepted: June 7, 2016
A
DOI: 10.1021/acs.iecr.6b00862 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Correlation
Industrial & Engineering Chemistry Research particles at high speed.15 In contrast to the common SILP materials, the inverse SILP systems are characterized by a significantly larger ratio of IL to support. Thus, more IL and active sorption ingredients dissolved therein can be made available for absorption processes still keeping the solid nature of the sorbent and rather short diffusion distances as key advantages of the technology. In our previous study, inverse SILP materials have shown very promising CO2 absorption performances and fast absorption kinetics due to the high dispersion of the IL stabilized between the silica particles. Combining amino-functional ILs and fumed silica, absorption efficiencies measured for inverse SILPs were 1.5−3 mol CO2 kg−1 inverse SILP (0.5−0.8 mol CO2 mol−1 IL) at 40 °C and 1 bar.10 In the current study, we expand on the use of inverse SILP materials for CO2 capture technologies by reporting on novel solid sorbents prepared from chitosan ionogels. High CO2 absorption efficiencies are expected by combining physisorption of CO2 by the IL solvent and chemisorption by chitosan. Economic considerations and green chemistry principles directed our development of new CO2 sorbent materials toward the usage of chitosan, a natural amino-functional biopolymer. Chitosan is the deacetylated derivative of chitin, which is the second most abundant biopolymer in nature after cellulose (Figure 2).16 It makes chitosan a highly interesting ingredient for environmental friendly, renewable and inexpensive future CO2 absorption technologies.17−20
Figure 3. ILs applied in this study to dissolve chitosan: 1-ethyl-3methylimidazolium acetate ([EMIM][OAc]), 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-ethyl-3-methylimidazolium methyl methylphosphonate ([EMIM][Me(Me)PO3]), 1-ethyl-3-methylimidazolium 2-methoxyethyl methylphosphonate ([EMIM][Me(EG)1(Me)PO3]), 1-ethyl-3-methylimidazolium 2-(2-methoxy-ethoxy)-ethyl methylphosphonate ([EMIM][Me(EG)2(Me)PO3]), 1-butyl-3-methylimidazolium 2-methoxyethyl methylphosphonate ([BMIM][Me(EG) 1 (Me)PO 3 ]), 1-butyl-3-methylimidazolium 2-(2-methoxyethoxy)-ethyl methylphosphonate ([BMIM][Me(EG)2(Me)PO3]).
yl methylphosphonate ([EMIM][Me(EG)1(Me)PO3]) and 1ethyl-3-methylimidazolium 2-(2-methoxy-ethoxy)-ethyl methylphosphonate ([EMIM][Me(EG)2(Me)PO3]). Using the named ILs, inverse SILPs with and without chitosan were prepared. CO2 absorption in developed materials was studied by measuring absorption isotherms at 40 °C in the pressure range of 200−5500 mbar.
Figure 2. Structure of chitosan.
■
The main obstacle for technical applications of biopolymers is their typically low solubility in most conventional organic solvents due to strong inter- and intramolecular hydrogen bonds. ILs are known as powerful solvents for biopolymers.21−25 Cellulose solubilities of up to 30 wt % have been reported in various ILs.22 Despite its structural similarity, chitosan solubility in ILs is significantly lower and solutions with more than 10 wt % of chitosan are difficult to achieve.26−28 Dissolving biopolymer in ILs leads to gelation, and therefore the obtained material is called ionogel.29 Chitosan ionogels have great potential as sorbents for CO2 as the both components, IL and chitosan, can contribute to the capture of CO2. Conventional ILs absorb CO2 through physical absorption mechanism only. Higher absorption capacities can be realized by incorporation of chemically active functional groups into the IL structure30 or by mixing the IL with amines.31,32 Following the analogy, a combination of chitosan and ILs appears promising. There are only a few examples about usage of chitosan ionogels for capture of CO2.33−35 However, in the mentioned studies a significant increase in the absorption capacity of IL due to the addition of a small amount of chitosan is reported. ILs presented in Figure 3 were tested for dissolution of chitosan in the current study. On the basis of their ability to dissolve chitosan, only four from these ILs were chosen for preparation of inverse SILP: 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]), 1-ethyl-3-methylimidazolium 2-methoxyeth-
EXPERIMENTAL SECTION Materials and Methods. [BMIM]Cl was purchased from Solvent Innovation (Merck). [EMIM][OAc], acetonitrile and chitosan (molecular weight 5000−190 000 Da, deacetylation 75−85%) were purchased from Sigma-Aldrich. [EMIM][Me(Me)PO3] was received from BASF AG. [EMIM][Me(EG)1(Me)PO3], [EMIM][Me(EG)2(Me)PO3], [BMIM][Me(EG)1(Me)PO3] and [BMIM][Me(EG)2(Me)PO3] were synthesized using a method described previously.36 All ILs were dried at about 10−2 mbar and 60 °C for at least 16 h until the water content determined by Karl Fischer coulometric titration (Metrohm, 756 KF Coulometer) was
