Article pubs.acs.org/IECR
Understanding the Performance of New Amine-Functionalized Mesoporous Silica Materials for CO2 Adsorption Pedro López-Aranguren,†,‡ Santiago Builes,‡ Julio Fraile,† Lourdes F. Vega,‡,§ and Concepción Domingo*,† †
Instituto de Ciencia de Materiales de Barcelona (ICMABCSIC), Campus de la UAB, 08193 Bellaterra, Spain MATGAS Research Center, Campus de la UAB, 08193 Bellaterra, Spain § Carburos Metálicos S.A., Air Products Group, C/Aragón 300, 08009 Barcelona, Spain ‡
ABSTRACT: The current work builds on previous works on grafting of organosilanes on mesoporous silica supports using a supercritical CO2 anhydrous method, and it extends them by examining the capacity of supercritically prepared aminosilica hybrid products for CO2 adsorption and its separation from mixtures with other gases. The CO2 adsorption capacity under dry conditions of mesoporous silica gel and MCM-41 functionalized with a monoaminosilane was evaluated first by recording CO2 adsorption isotherms and next by performing microbalance cyclic adsorption/desorption experiments at 25 and 45 °C. CO2 adsorption and desorption rates were also studied as a function of the aminosilane loading. CO2 adsorption results were compared to similar data available in the literature, showing improved performance for the supercritically prepared products related to the CO2 adsorption efficiency and cyclic regenerability.
1. INTRODUCTION One of the most important methods proposed to mitigate anthropogenic CO2 emissions is to adsorb and separate CO2 from diluted sources, such as gases emitted from fossil fuel combustion. For this purpose, the use of functionalized solid porous sorbents, based on silica supports modified with primary and secondary amine molecules or polymers,1 is nowadays considered as a less-energy-intensive alternative technology than conventional methods based on aqueous alkanolamine absorbents.2−5 Aminosilica materials exhibit a high adsorption capacity, good CO2 selectivity in gas mixtures, and fast adsorption and desorption rates.6,7 Three important parameters significantly affect the performance, cost, and environmental acceptability of different amine-functionalized silica options: (i) the amine and silica chemical composition and the hybrid structure, (ii) the interaction between the amine and the silica in the hybrid product, and (iii) the aminosilica synthesis method. Periodic mesoporous silicas are the most commonly studied solid supports;8,9 however, their use for industrial applications may result in costly materials.10 Mesoporous disordered silica gels, with similar sorption properties, are available at a significantly lower cost than the periodic materials.11 In both types of supports, aminosilicas can be obtained either by physical impregnation of the matrix with amine-containing small molecules12 or polymers or by grafting via organosilanes or tethered hyperbranched aminopolymers.13−17 Aminosilanes are usually grafted on the surface of hydroxylated silica supports by admixing the components in toluene.18−35 As a clean alternative, silanization can be carried out using an organic solvent-free process based on the use of supercritical carbon dioxide (scCO2), which has been often applied to alkylsilanes,36−42 and extended in our group to aminosilanes and hyperbranched aminopolymers.43−45 The essence of the supercritical method consists of adequately © 2014 American Chemical Society
choosing the experimental conditions to moderate carbamate formation,46 which occurs by the reaction between the amine and the scCO2 solvent.47 In this work, the monoamine 3(methylamino)propyltrimethoxysilane was chosen to form the hybrid supports, since it has a higher solubility than other di- or triamines in scCO2.48 The supercritical loading route has the additional advantage of being a green process.47,49 For the design and cost analysis of the supercritically prepared hybrid systems, a detailed understanding of CO2 and CO2/N2 separation is crucial.47,50−52 Hence, the present work scrutinizes the high adsorption capacity and effectiveness of supercritically prepared hybrid products that are compared to literature data for similar materials prepared using conventional methods. The influence of the material surface area and pore characteristics on the long-term performance is also discussed, as well as the adsorption and desorption rates.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. 3-(Methylamino)propyltrimethoxysilane (MAP; Sigma-Aldrich) was used for functionalization (Figure 1a). Two different supports were investigated: periodic MCM-41 (ACS Materials) and disordered silica gel (Cleancat Iberamigo S.A.) with pore diameters (Pd) of 4 and 9 nm and surface areas (Sa) of 1127 and 440 m2 gp−1, respectively (Table 1). Substrate aminosilanization was performed in scCO2 by using a procedure described elsewhere.53 Samples chosen for the CO2 adsorption study are depicted in Table 1. For a better identification of the experimental conditions used for sample preparation, sample labels from ref 47 are also added to Table 1. Received: Revised: Accepted: Published: 15611
July 23, 2014 September 6, 2014 September 12, 2014 September 12, 2014 dx.doi.org/10.1021/ie502945r | Ind. Eng. Chem. Res. 2014, 53, 15611−15619
Industrial & Engineering Chemistry Research
Article
Figure 1. (a) Reactions taking place between the secondary amine −NH−CH3 and CO2 for carbamate formation. Schematic representation of experimental supports and their functionalization with MAP: (b) periodic MCM-41 and (c) CC silica gel. Note that pore sizes are not at scale; the mean pore diameters of MCM-41 and CC are 4 and 9 nm, respectively.
Table 1. Sample Amine Loading (ρN and ρmN)a sample MCM-41 1_MCM-41/MAP 2_MCM-41/MAP 3_MCM-41/MAP 4_MCM-41/MAP 5_MCM-41/MAP 6_MCM-41/MAP CC 1_CC/MAP 2_CC/MAP 3_CC/MAP
naming ref 47
ρNb [mmol gp−1]
ρmNc [molecules nm−2]
1-MAP@MCM41 5-MAP@MCM41 11-MAP@MCM41 12-MAP@MCM41 13-MAP@MCM41 10-MAP@MCM41
0.40 0.96 1.81 2.43 3.42 3.99
0.2 0.5 1.1 1.5 2.4 2.9
1-MAP@CC 4-MAP@CC 6-MAP@CC
0.63 2.02 2.63
0.8 3.2 4.3
Sa [cm3 g−1]
Pv [cm3 gp−1]
1127 916 837 681 554 23 10 440 351 142 117
0.92 0.78 0.73 0.57 0.47 0.03 0.01 0.96 0.87 0.37 0.26
a
Naming rules used in this work are pooled with those used in ref 47. Pv and Sa values are also shown before and after substrate functionalization. ρN is given in millimoles of nitrogen (1 N is equivalent to 1 amine group) per gram of hybrid product. cρmN corresponds to the molecular surface density, in number of amine molecules per square nanometer of pristine substrate. b
carried out at 105 °C in a flow of N2 for 90 min. A minimum of 10 cycles was applied to each sample.
2.2. Characterization. Data on the amine loading and pore structure of the synthesized materials are required to study in detail their CO2 adsorption capacity. Those parameters, determined from thermogravimetric analysis and low-temperature N2 adsorption/desorption experiments, were obtained in a previous work44 and are summarized here in Table 1. The CO2 adsorption isotherms were recorded up to 25 kPa at 25 °C using a Micromeritics ASAP 2020 analyzer. Samples were first outgassed under reduced pressure at 120 °C for 20 h. The study of the CO2 adsorption/desorption cyclic behavior was performed using a microelectronic recording balance (IMS HP HT microbalance, based on a magnetically coupled Rubotherm GmbH microbalance) with a cell of 100 mL. Measurements were performed at atmospheric pressure and with a total flow of 200 standard cm3 min−1. The samples were first dried and decarbamated by passing N2 at 105 °C for 180 min. Then, they were cooled to the desired adsorption temperature (Tads = 25 or 45 °C) and CO2 adsorption was initiated by switching the N2 purge gas to a CO2/N2 mixture (10/90 vol %) maintained for 60 min. The desorption step was
3. RESULTS AND DISCUSSION 3.1. Chemical and Textural Characteristics of Hybrid Products. MCM-41 has a periodic hexagonal array of unidirectional noninterconnected pores of 4 nm diameter (Figure 1b), while amorphous silica gels have interconnected pores, with a mean diameter of 9 nm, in a complex random and tortuous pore structure (Figure 1c). Aminosilanes were supercritically grafted on the internal surface of these mesoporous silica supports by their reaction with surface silanols, as represented in Figure 1b,c. Different loading degrees were reached modifying the experimental conditions.54 As denoted in Table 1, some samples were obtained with relatively low loadings (1_MCM-41/MAP, 2_MCM-41/MAP, and 1_CC/MAP), medium values (3_MCM-41/MAP, 4_MCM41/MAP, and 2_CC/MAP), and high amine content (5_MCM-41/MAP, 6_MCM-41/MAP, and 3_CC/MAP). Before conducting proper CO2 adsorption measurements, it is advantageous to have a deep knowledge of the textural 15612
dx.doi.org/10.1021/ie502945r | Ind. Eng. Chem. Res. 2014, 53, 15611−15619
Industrial & Engineering Chemistry Research
Article
volume (Table 1), CO2 accessibility and adsorption were not hindered (Figure 2a). This apparent contradiction to the N2 adsorption results is explained by the higher temperature used during the CO2 adsorption measurements (25 °C) compared to the −196 °C employed for the N2 adsorption tests. At the low N2 temperature, the grafted amine chains are expected to behave as rigid materials, thus limiting gas diffusion. 3.3. CO2 Adsorption and Separation from N2. For the supercritically prepared samples, the ability of selectively adsorbing CO2 from a mixture with N2 was examined at 25 and 45 °C in a microbalance (Table 2). Adsorption values were taken from the first adsorption/desorption cycle.
characteristics of the mesoporous supports. For the studied matrixes, both the pore volume and surface area measured from N2 adsorption at low temperature decreased after aminosilane functionalization (Table 1). At similar loadings, the decrease in the pore volume was more dramatic for the MCM-41/MAP samples than for the CC/MAP products, reflecting the smaller pore diameter of the MCM-41 support. Samples 5_MCM-41/ MAP and 6_MCM-41/MAP, situated in the high loading range, had an almost null pore volume due to pore blocking, while the highly loaded 3_CC/MAP sample still presented an open void space corresponding to ca. 30−40% of the initial value. The presence of a significant porosity in the sorbent after amine functionalization has been described as an important characteristic for CO2 adsorption at ambient temperature.55 Hence, pore blocking, although not being determinant, should be taken into account when analyzing the adsorption behavior. 3.2. CO2 Adsorption Isotherms. CO2 uptake at low pressures in aminosilicas occurs mainly by chemisorption; the CO2−amine interaction described in the literature is a temperature reversible acid−base reaction (Figure 1a).45,56 For primary and secondary amines, the formed species are carbamate ion pairs. The CO2 adsorption isotherms of MCM41 and CC pristine supports showed a low sorption capacity (
