ChemComm Published on 24 October 2013. Downloaded by Memorial University of Newfoundland on 12/11/2013 15:16:42.
COMMUNICATION
Cite this: DOI: 10.1039/c3cc47931e Received 15th October 2013, Accepted 23rd October 2013
View Article Online View Journal
A new method using compressed CO2 for the in situ functionalization of mesoporous silica with hyperbranched polymers† ´ pez-Aranguren,*ab Lourdes F. Vegabc and Concepcio ´ n Domingoa Pedro Lo
DOI: 10.1039/c3cc47931e www.rsc.org/chemcomm
The present work focuses on the development of a new eco-efficient method, based on the use of compressed CO2 as a solvent, reaction medium and catalyst, for the in situ polymerization of ethyleneimine inside mesoporous silica.
Porous silica is one of the most effective solid supports for the incorporation of organic groups to obtain hybrid functional materials. The modification of the porous framework of silica is of high interest for certain applications, such as catalysis, adsorption, chromatography and drug delivery.1,2 In particular, organic components containing amino groups (aminosilanes or aminopolymers) are impregnated into porous silica to obtain materials for CO2 capture and separation applications.2 Grafted hyperbranched aminopolymers are often preferred as modifiers versus aminosilanes, as they have a higher amine density, which is considered advantageous in CO2 adsorption applications.3 Current developed methods to prepare hyperbranched polyamines grafted onto mesoporous silica need the use of organic solvents, catalysts, temperature and/or long reaction times. The present work demonstrates the possibility of polymerizing ethyleneimine into mesoporous silica obtaining a highly loaded material by using compressed CO2 at low temperatures and short reaction times. The 3-membered ring ethyleneimine (Menadiona S.A.), also referred as aziridine (Fig. 1a), was chosen to impregnate the mesoporous (3.8 nm pore diameter) silica MCM-41 (ACS Materials). The aziridine gives polyethyleneimine (PEI) by ringopening polymerization, which contains primary, secondary and tertiary amines, and forms covalent bonds with the silica (Fig. 1b).4 The in situ polymerization of aziridine into mesoporous silica has been conventionally carried out by the liquid phase route, requiring the use of a large amount of organic solvents.5–7 The liquid approach has some additional disadvantages related to bulk polymerization and diffusion limitations. A free-solvent process, based on a vapor phase transport route, has been recently described.8 Although the a
Instituto de Ciencia de Materiales de Barcelona - CSIC, Campus UAB, Bellaterra, 08193, Spain. E-mail: [email protected]; Tel: +34 935863206 b MATGAS Research Center, Campus UAB, Bellaterra, 08193, Spain c ´licos, Air Products Group, C/Arago´n 300, Barcelona, 08009, Spain Carburos Meta † Electronic supplementary information (ESI) available: TGA curves and N2 adsorption isotherms. See DOI: 10.1039/c3cc47931e
This journal is
c
The Royal Society of Chemistry 2013
Fig. 1 Schematic representation of (a) aziridine ring-opening polymerization using CO2, (b) hybrid product, and (c) CO2 adsorption in primary amines through carbamate formation.
use of organic solvents was avoided in this process, still long reaction times of 24 h and temperatures close to 80 1C were required to obtain significant loadings. In this communication an effective and fast new procedure is developed for the in situ ring-opening polymerization of aziridine into porous silica by using compressed CO2 both as a reaction medium and as a catalyst. In previous studies, we have demonstrated that compressed CO2 can be used as an efficient solvent for the surface modification and functionalization of porous materials.9–12 Following a similar synthesis route, experiments were performed in this work in a high-pressure autoclave (TharDesign) of 100 mL running in the batch mode.13 The autoclave was charged with 300 mg of MCM-41 and l mL of aziridine, avoiding the physical contact between the two materials. The vessel was slowly pressurized with CO2 at ambient temperature. At ca. 1 MPa, the formation of a dense vapor cloud was visually observed together with a simultaneous increase of the temperature of 4–6 1C, evidencing the exothermic character of the ring-opening polymerization of aziridine aided by CO2 addition (Fig. 1a). It has been stated that unsubstituted aziridine reacts with CO2 to give a homopolymer in the absence of the other catalyst, i.e., CO2 is not incorporated Chem. Commun.
View Article Online
Communication
ChemComm
Table 1 Amine content (rN), BJH pore volume (Vp) and CO2 adsorption values for a mixture of gases (CO2 10 v% plus N2) and calculated amine efficiency, expressed as mol of adsorbed CO2 per mol of loaded amine, for samples of this work and similar materials from the literature8,14,15
Published on 24 October 2013. Downloaded by Memorial University of Newfoundland on 12/11/2013 15:16:42.
Sample
rN Vol. CO2 mol CO2 [mmol N per g] Vp [cm3 g 1] [mmol g 1]/T [1C] mol N
MCM-41 — MLPEI@MCM 6
0.92 0.21
HLPEI@MCM 8 7–9 SBA15 8
0.07 0.33–0.07
0.07/45 1C 0.95/25 1C 0.59/45 1C 0.61/45 1C 0.9–0.4/25 1C
SBA15 14 MCM41 14 SBA15 15
0.36 0.01 1.19
0.7/25 1C 0.4/25 1C 0.6/25 1C
4 6 7.4
— 0.16 0.10 0.10 0.13– 0.04 0.17 0.07 0.09
into the polymer.12 Operating temperature and reaction time were set at 45 1C and 10 min, respectively. The pressure was varied between 6 and 10 MPa for the preparation of samples MLPEI@MCM and HLPEI@MCM, respectively. The thermal behavior of functionalized samples and the loadings were determined by thermogravimetric analysis (TGA, Q5000 TA Instruments). The amine content was calculated from the TGA weight loss in the temperature range of 200–500 1C (Table 1). Besides, data were corroborated by elemental analysis for sample MLPEI@MCM, giving a loading of 5.4 mmol N per g, consistent with the value estimated by TGA of 6 mmol N per g. Fig. 2 illustrates the derivative curves of the TGA weight loss profiles measured for the synthesized samples (TGA data are shown in Fig. S1 in the ESI†). Derivative curves were compared with literature data of similar products involving PEI loaded into mesoporous SBA-15 (7–9 mmol N per g) prepared by ring-opening polymerization and following the liquid4,5 or the vapor diffusion route.8 Samples prepared using conventional routes decomposed at temperatures of 200–250 1C, while samples synthesized in this work attained a temperature of ca. 340 1C before significant decomposition. Hence, the maximum in the derivative of the decomposition profiles of samples prepared using compressed CO2 was shifted more than 100 1C to the right, indicating notable higher thermal stability than similar products reported in the literature. Pore volumes of the pristine support and prepared materials were analyzed by N2 adsorption at 196 1C (BET ASAP 2000, Micromeritics). Prior to measurements, samples were outgassed under vacuum at 120 1C for 24 h. N2 adsorption isotherms are shown in Fig. S2 in the ESI.† Treated samples showed a great
Fig. 2 Normalized derivative profiles obtained from the TGA weight loss curves of HLPEI@MCM and MLPEI@MCM samples and literature data.4,5,8
Chem. Commun.
Fig. 3
CO2 adsorption isotherms of prepared samples at 25 and 75 1C.
decrease of the BJH pore volume when compared to that of bare MCM-41 (Table 1), suggesting the successful incorporation of the PEI inside the pores. The medium loaded MLPEI@MCM sample showed an adsorption isotherm that still preserved significant mesoporosity, while the highly loaded HLPEI@MCM sample exhibited a very low Vp and an almost flat isotherm, indicating potential pore blocking. CO2 adsorption occurs by a chemical reaction with primary and secondary amines forming a carbamate through an exothermic acid–base reaction, which strongly depends on both temperature and CO2 pressure (Fig. 1c). The CO2 adsorption of the prepared samples was evaluated using, primarily, an adsorption isotherm analysis in pure CO2 and, further, preliminary cyclic microbalance tests in a CO2–N2 gas mixture. CO2 adsorption isotherms were collected up to 100 kPa at 25 and 75 1C (Fig. 3). Open porosity in the sorbent after polymer functionalization has been described as an important characteristic that makes the material particularly attractive for adsorption.15,16 However, the almost total loss of Vp in sample HLPEI@MCM (Table 1) did not totally hinder the CO2 adsorption at any of the studied temperatures, as a relatively high performance was still observed for this sample, even at low adsorption pressures. This apparent contradiction is related to the higher temperature used for the CO2 adsorption measurements (25 or 75 1C) compared to 196 1C used for N2 adsorption, at which PEI chains are expected to behave as rigid materials. At 25 1C, the MLPEI@MCM sample showed a CO2 adsorption value of 1.56 mmol CO2 per g at 100 kPa, which is nearly twice the capacity of the HLPEI@MCM sample under similar conditions, 0.89 mmol CO2 per g. Increasing the temperature to 75 1C resulted in a significant enhancement in the CO2 adsorption of the highly loaded sample HLPEI@MCM to a value of 1.43 mmol CO2 per g. The opposite behavior was observed for MLPEI@MCM: the adsorption capacity was reduced to 0.89 mmol CO2 per g by increasing the temperature. The increase of the temperature is expected to affect CO2 adsorption in two mutually divergent ways. First, adsorption is enhanced by increasing polymer mobility and CO2 diffusivity. Second, adsorption is hindered by shifting the equilibrium of the carbamate formation in the exothermic reaction (Fig. 1c). For sample HLPEI@MCM, with potential pore blocking, the increase in CO2 diffusivity with temperature seemed to be the principal effect and, thus, CO2 adsorption was enhanced by increasing the adsorption temperature. In contrast, for sample MLPEI@MCM, CO2 adsorption diminished with a similar temperature increase, which fundamentally worked against the formation of carbamate molecules. This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 24 October 2013. Downloaded by Memorial University of Newfoundland on 12/11/2013 15:16:42.
ChemComm
Communication
Fig. 4 CO2 adsorption cycles (solid lines) and adjusted profiles (dotted lines) with adsorption performed in a flow of dry N2–CO2 (10 v%) at 45 1C and desorption in N2 at 105 1C.
The study of temperature swing in CO2 adsorption–desorption cycles was performed using a microelectronic recording balance (IMS HP HT Microbalance, based on a magnetically coupled Rubotherm Gmbh microbalance) under a flow of CO2 (10 v%) and N2. The samples were first dried and decarbamated by passing N2 for 180 min at 105 1C. CO2 adsorption was measured at 45 1C after 60 min. The CO2 capture performance of the prepared materials was compared with data of similar products, involving SBA-15 and MCM-41 mesoporous silica, reported by Jones et al. and obtained from the in situ polymerization of aziridine following either the standard liquid-phase method14,16 or the vapour-phase transport route,8 and with hybrids formed by the physical impregnation of low molecular weight PEI.15 Reported values of amine loading were 6–9 mmol N per g, independent of the synthesis method. These values are similar to the ones measured in this work (Table 1), indicating that the compressed CO2 method is an effective way to produce hyperbranched PEI loaded products. Samples studied in this work gave CO2 adsorption values in the order of 0.6 mmol CO2 per g at 45 1C. The CO2 adsorption capacity of sample MLPEI@MCM was also evaluated at 25 1C, showing a higher CO2 adsorption value, close to 1 mmol CO2 per g. The CO2 reported adsorption capacities of literature products under similar conditions were found to be 0.4–0.9 mmol CO2 per g at 25 1C. Hence, the PEI hybrid materials prepared here are as efficient with regard to CO2 adsorption as related materials prepared by different routes. It should be noted that under dry conditions, the amine efficiency of this class of materials for CO2 adsorption is in the order of 0.1–0.2 mol CO2 per mol N (Table 1), which are substantially lower than the ones observed for supported aminosilanes (0.3–0.5 mol CO2 per mol N).2 This is related to the presence of a considerable amount of tertiary amines in hybrid PEI products (Fig. 1b). The CO2 uptake for the prepared sorbents and the further regeneration process have been analyzed under the principles of temperature swing adsorption (Fig. 4). CO2 adsorption was carried out at 45 1C in a mixture of dry CO2 (10 v%) and N2, while CO2 desorption for solvent regeneration was performed at 105 1C under a flow of pure N2. The CO2 adsorption was easily reversed with temperature and no appreciable loss in efficiency was observed after 20 cycles (only the first 6 cycles are shown in
This journal is
c
The Royal Society of Chemistry 2013
the figure), indicating the high thermal stability of the prepared hybrid products, as required for any industrial solid adsorbent. Differences in the adsorption kinetics were observed for both samples by analyzing the slopes of the adsorption cycles in Fig. 4. Sample MLPEI@MCM showed a higher slope for the adsorption part of the cycle than sample HLPEI@MCM. The slow adsorption kinetics found for a highly loaded HLPEI@MCM sample indicated diffusion restrictions due to potential pore blocking. Mesoporous silica has been successfully functionalized with a hyperbranched amine polymer by in situ polymerization of ethyleneimine using compressed CO2 as the solvent and the catalyst. This turns out to be a very fast method, with processing time in the order of minutes, which produces high amine contents, in the order of 6–8 mmol N per g, in a thermally stable compound (>300 1C). Although experiments in this work were performed at pressures of 6–10 MPa, after optimization, similar results could be obtained by using compressed CO2 at pressures as low as 0.5–1 MPa, which is expected to reduce the process cost. The innovative components of the work rely not only on the improved characteristics of the processing method (less time consuming, no use of organic solvents or catalysts, soft reaction temperatures), but also on the superior characteristics of the obtained products, particularly those related to thermal stability. The hybrid products showed a standard CO2 adsorption capacity. ´licos, This work has been partially financed by Carburos Meta Air Products Group, by the Spanish government under project CEN2008-1027, and by the Catalan Government, SGR2009-666.
Notes and references 1 C. Perego and R. Millini, Chem. Soc. Rev., 2013, 42, 3956. 2 S. Choi, J. H. Drese and C. W. Jones, ChemSusChem, 2009, 2, 796. 3 C. H. Yu, C. H. Huang and C. S. Tan, Aerosol Air Qual. Res., 2012, 12, 745. 4 J. M. Rosenholm, A. Penninkangas and M. Linden, Chem. Commun., 2006, 3909. 5 J. C. Hicks, PhD thesis, Georgia Institute of Tecnology, 2007. 6 J. C. Hicks, J. H. Drese, D. J. Fauth, M. L. Gray, G. Qi and C. W. Jones, J. Am. Chem. Soc., 2008, 130, 2902. 7 J. H. Drese, S. Choi, R. P. Lively, W. J. Koros, D. J. Fauth, M. L. Gray and C. W. Jones, Adv. Funct. Mater., 2009, 19, 3821. 8 W. Chaikittisilp, S. A. Didas, H. J. Kim and C. W. Jones, Chem. Mater., 2013, 25, 613. 9 E. Loste, J. Fraile, M. A. Fanovich, G. F. Woerlee and C. Domingo, Adv. Mater., 2004, 16, 739. ´ia-Gonza ´lez, J. Saurina, J. A. Ayllo ´n and C. Domingo, 10 C. A. Garc J. Phys. Chem. C, 2009, 113, 13780. ´pez-Aranguren, J. Fraile, L. F. Vega and C. Domingo, 11 S. Builes, P. Lo J. Phys. Chem. C, 2012, 116, 10150. 12 K. Soga, S. Hosoda and S. Ikeda, Macromol. Chem. Phys., 1974, 175, 3309. ´pez-Aranguren, J. Saurina, L. F. Vega and C. Domingo, 13 P. Lo Microporous Mesoporous Mater., 2012, 148, 15. 14 J. H. Drese, S. Choi, S. A. Didas, P. Bollini, M. L. Gray and C. W. Jones, Microporous Mesoporous Mater., 2012, 151, 231. 15 Y. Kuwahara, D. Y. Kang, J. R. Copeland, N. A. Brunelli, S. A. Didas, P. Bollini, C. Sievers, T. Kamegawa, H. Yamashita and C. W. Jones, J. Am. Chem. Soc., 2012, 134, 10757. 16 J. H. Drese, S. Choi, R. P. Lively, W. J. Koros, D. J. Fauth, M. L. Gray and C. W. Jones, Adv. Funct. Mater., 2009, 19, 3821.
Chem. Commun.
COMMUNICATION
Cite this: DOI: 10.1039/c3cc47931e Received 15th October 2013, Accepted 23rd October 2013
View Article Online View Journal
A new method using compressed CO2 for the in situ functionalization of mesoporous silica with hyperbranched polymers† ´ pez-Aranguren,*ab Lourdes F. Vegabc and Concepcio ´ n Domingoa Pedro Lo
DOI: 10.1039/c3cc47931e www.rsc.org/chemcomm
The present work focuses on the development of a new eco-efficient method, based on the use of compressed CO2 as a solvent, reaction medium and catalyst, for the in situ polymerization of ethyleneimine inside mesoporous silica.
Porous silica is one of the most effective solid supports for the incorporation of organic groups to obtain hybrid functional materials. The modification of the porous framework of silica is of high interest for certain applications, such as catalysis, adsorption, chromatography and drug delivery.1,2 In particular, organic components containing amino groups (aminosilanes or aminopolymers) are impregnated into porous silica to obtain materials for CO2 capture and separation applications.2 Grafted hyperbranched aminopolymers are often preferred as modifiers versus aminosilanes, as they have a higher amine density, which is considered advantageous in CO2 adsorption applications.3 Current developed methods to prepare hyperbranched polyamines grafted onto mesoporous silica need the use of organic solvents, catalysts, temperature and/or long reaction times. The present work demonstrates the possibility of polymerizing ethyleneimine into mesoporous silica obtaining a highly loaded material by using compressed CO2 at low temperatures and short reaction times. The 3-membered ring ethyleneimine (Menadiona S.A.), also referred as aziridine (Fig. 1a), was chosen to impregnate the mesoporous (3.8 nm pore diameter) silica MCM-41 (ACS Materials). The aziridine gives polyethyleneimine (PEI) by ringopening polymerization, which contains primary, secondary and tertiary amines, and forms covalent bonds with the silica (Fig. 1b).4 The in situ polymerization of aziridine into mesoporous silica has been conventionally carried out by the liquid phase route, requiring the use of a large amount of organic solvents.5–7 The liquid approach has some additional disadvantages related to bulk polymerization and diffusion limitations. A free-solvent process, based on a vapor phase transport route, has been recently described.8 Although the a
Instituto de Ciencia de Materiales de Barcelona - CSIC, Campus UAB, Bellaterra, 08193, Spain. E-mail: [email protected]; Tel: +34 935863206 b MATGAS Research Center, Campus UAB, Bellaterra, 08193, Spain c ´licos, Air Products Group, C/Arago´n 300, Barcelona, 08009, Spain Carburos Meta † Electronic supplementary information (ESI) available: TGA curves and N2 adsorption isotherms. See DOI: 10.1039/c3cc47931e
This journal is
c
The Royal Society of Chemistry 2013
Fig. 1 Schematic representation of (a) aziridine ring-opening polymerization using CO2, (b) hybrid product, and (c) CO2 adsorption in primary amines through carbamate formation.
use of organic solvents was avoided in this process, still long reaction times of 24 h and temperatures close to 80 1C were required to obtain significant loadings. In this communication an effective and fast new procedure is developed for the in situ ring-opening polymerization of aziridine into porous silica by using compressed CO2 both as a reaction medium and as a catalyst. In previous studies, we have demonstrated that compressed CO2 can be used as an efficient solvent for the surface modification and functionalization of porous materials.9–12 Following a similar synthesis route, experiments were performed in this work in a high-pressure autoclave (TharDesign) of 100 mL running in the batch mode.13 The autoclave was charged with 300 mg of MCM-41 and l mL of aziridine, avoiding the physical contact between the two materials. The vessel was slowly pressurized with CO2 at ambient temperature. At ca. 1 MPa, the formation of a dense vapor cloud was visually observed together with a simultaneous increase of the temperature of 4–6 1C, evidencing the exothermic character of the ring-opening polymerization of aziridine aided by CO2 addition (Fig. 1a). It has been stated that unsubstituted aziridine reacts with CO2 to give a homopolymer in the absence of the other catalyst, i.e., CO2 is not incorporated Chem. Commun.
View Article Online
Communication
ChemComm
Table 1 Amine content (rN), BJH pore volume (Vp) and CO2 adsorption values for a mixture of gases (CO2 10 v% plus N2) and calculated amine efficiency, expressed as mol of adsorbed CO2 per mol of loaded amine, for samples of this work and similar materials from the literature8,14,15
Published on 24 October 2013. Downloaded by Memorial University of Newfoundland on 12/11/2013 15:16:42.
Sample
rN Vol. CO2 mol CO2 [mmol N per g] Vp [cm3 g 1] [mmol g 1]/T [1C] mol N
MCM-41 — MLPEI@MCM 6
0.92 0.21
HLPEI@MCM 8 7–9 SBA15 8
0.07 0.33–0.07
0.07/45 1C 0.95/25 1C 0.59/45 1C 0.61/45 1C 0.9–0.4/25 1C
SBA15 14 MCM41 14 SBA15 15
0.36 0.01 1.19
0.7/25 1C 0.4/25 1C 0.6/25 1C
4 6 7.4
— 0.16 0.10 0.10 0.13– 0.04 0.17 0.07 0.09
into the polymer.12 Operating temperature and reaction time were set at 45 1C and 10 min, respectively. The pressure was varied between 6 and 10 MPa for the preparation of samples MLPEI@MCM and HLPEI@MCM, respectively. The thermal behavior of functionalized samples and the loadings were determined by thermogravimetric analysis (TGA, Q5000 TA Instruments). The amine content was calculated from the TGA weight loss in the temperature range of 200–500 1C (Table 1). Besides, data were corroborated by elemental analysis for sample MLPEI@MCM, giving a loading of 5.4 mmol N per g, consistent with the value estimated by TGA of 6 mmol N per g. Fig. 2 illustrates the derivative curves of the TGA weight loss profiles measured for the synthesized samples (TGA data are shown in Fig. S1 in the ESI†). Derivative curves were compared with literature data of similar products involving PEI loaded into mesoporous SBA-15 (7–9 mmol N per g) prepared by ring-opening polymerization and following the liquid4,5 or the vapor diffusion route.8 Samples prepared using conventional routes decomposed at temperatures of 200–250 1C, while samples synthesized in this work attained a temperature of ca. 340 1C before significant decomposition. Hence, the maximum in the derivative of the decomposition profiles of samples prepared using compressed CO2 was shifted more than 100 1C to the right, indicating notable higher thermal stability than similar products reported in the literature. Pore volumes of the pristine support and prepared materials were analyzed by N2 adsorption at 196 1C (BET ASAP 2000, Micromeritics). Prior to measurements, samples were outgassed under vacuum at 120 1C for 24 h. N2 adsorption isotherms are shown in Fig. S2 in the ESI.† Treated samples showed a great
Fig. 2 Normalized derivative profiles obtained from the TGA weight loss curves of HLPEI@MCM and MLPEI@MCM samples and literature data.4,5,8
Chem. Commun.
Fig. 3
CO2 adsorption isotherms of prepared samples at 25 and 75 1C.
decrease of the BJH pore volume when compared to that of bare MCM-41 (Table 1), suggesting the successful incorporation of the PEI inside the pores. The medium loaded MLPEI@MCM sample showed an adsorption isotherm that still preserved significant mesoporosity, while the highly loaded HLPEI@MCM sample exhibited a very low Vp and an almost flat isotherm, indicating potential pore blocking. CO2 adsorption occurs by a chemical reaction with primary and secondary amines forming a carbamate through an exothermic acid–base reaction, which strongly depends on both temperature and CO2 pressure (Fig. 1c). The CO2 adsorption of the prepared samples was evaluated using, primarily, an adsorption isotherm analysis in pure CO2 and, further, preliminary cyclic microbalance tests in a CO2–N2 gas mixture. CO2 adsorption isotherms were collected up to 100 kPa at 25 and 75 1C (Fig. 3). Open porosity in the sorbent after polymer functionalization has been described as an important characteristic that makes the material particularly attractive for adsorption.15,16 However, the almost total loss of Vp in sample HLPEI@MCM (Table 1) did not totally hinder the CO2 adsorption at any of the studied temperatures, as a relatively high performance was still observed for this sample, even at low adsorption pressures. This apparent contradiction is related to the higher temperature used for the CO2 adsorption measurements (25 or 75 1C) compared to 196 1C used for N2 adsorption, at which PEI chains are expected to behave as rigid materials. At 25 1C, the MLPEI@MCM sample showed a CO2 adsorption value of 1.56 mmol CO2 per g at 100 kPa, which is nearly twice the capacity of the HLPEI@MCM sample under similar conditions, 0.89 mmol CO2 per g. Increasing the temperature to 75 1C resulted in a significant enhancement in the CO2 adsorption of the highly loaded sample HLPEI@MCM to a value of 1.43 mmol CO2 per g. The opposite behavior was observed for MLPEI@MCM: the adsorption capacity was reduced to 0.89 mmol CO2 per g by increasing the temperature. The increase of the temperature is expected to affect CO2 adsorption in two mutually divergent ways. First, adsorption is enhanced by increasing polymer mobility and CO2 diffusivity. Second, adsorption is hindered by shifting the equilibrium of the carbamate formation in the exothermic reaction (Fig. 1c). For sample HLPEI@MCM, with potential pore blocking, the increase in CO2 diffusivity with temperature seemed to be the principal effect and, thus, CO2 adsorption was enhanced by increasing the adsorption temperature. In contrast, for sample MLPEI@MCM, CO2 adsorption diminished with a similar temperature increase, which fundamentally worked against the formation of carbamate molecules. This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 24 October 2013. Downloaded by Memorial University of Newfoundland on 12/11/2013 15:16:42.
ChemComm
Communication
Fig. 4 CO2 adsorption cycles (solid lines) and adjusted profiles (dotted lines) with adsorption performed in a flow of dry N2–CO2 (10 v%) at 45 1C and desorption in N2 at 105 1C.
The study of temperature swing in CO2 adsorption–desorption cycles was performed using a microelectronic recording balance (IMS HP HT Microbalance, based on a magnetically coupled Rubotherm Gmbh microbalance) under a flow of CO2 (10 v%) and N2. The samples were first dried and decarbamated by passing N2 for 180 min at 105 1C. CO2 adsorption was measured at 45 1C after 60 min. The CO2 capture performance of the prepared materials was compared with data of similar products, involving SBA-15 and MCM-41 mesoporous silica, reported by Jones et al. and obtained from the in situ polymerization of aziridine following either the standard liquid-phase method14,16 or the vapour-phase transport route,8 and with hybrids formed by the physical impregnation of low molecular weight PEI.15 Reported values of amine loading were 6–9 mmol N per g, independent of the synthesis method. These values are similar to the ones measured in this work (Table 1), indicating that the compressed CO2 method is an effective way to produce hyperbranched PEI loaded products. Samples studied in this work gave CO2 adsorption values in the order of 0.6 mmol CO2 per g at 45 1C. The CO2 adsorption capacity of sample MLPEI@MCM was also evaluated at 25 1C, showing a higher CO2 adsorption value, close to 1 mmol CO2 per g. The CO2 reported adsorption capacities of literature products under similar conditions were found to be 0.4–0.9 mmol CO2 per g at 25 1C. Hence, the PEI hybrid materials prepared here are as efficient with regard to CO2 adsorption as related materials prepared by different routes. It should be noted that under dry conditions, the amine efficiency of this class of materials for CO2 adsorption is in the order of 0.1–0.2 mol CO2 per mol N (Table 1), which are substantially lower than the ones observed for supported aminosilanes (0.3–0.5 mol CO2 per mol N).2 This is related to the presence of a considerable amount of tertiary amines in hybrid PEI products (Fig. 1b). The CO2 uptake for the prepared sorbents and the further regeneration process have been analyzed under the principles of temperature swing adsorption (Fig. 4). CO2 adsorption was carried out at 45 1C in a mixture of dry CO2 (10 v%) and N2, while CO2 desorption for solvent regeneration was performed at 105 1C under a flow of pure N2. The CO2 adsorption was easily reversed with temperature and no appreciable loss in efficiency was observed after 20 cycles (only the first 6 cycles are shown in
This journal is
c
The Royal Society of Chemistry 2013
the figure), indicating the high thermal stability of the prepared hybrid products, as required for any industrial solid adsorbent. Differences in the adsorption kinetics were observed for both samples by analyzing the slopes of the adsorption cycles in Fig. 4. Sample MLPEI@MCM showed a higher slope for the adsorption part of the cycle than sample HLPEI@MCM. The slow adsorption kinetics found for a highly loaded HLPEI@MCM sample indicated diffusion restrictions due to potential pore blocking. Mesoporous silica has been successfully functionalized with a hyperbranched amine polymer by in situ polymerization of ethyleneimine using compressed CO2 as the solvent and the catalyst. This turns out to be a very fast method, with processing time in the order of minutes, which produces high amine contents, in the order of 6–8 mmol N per g, in a thermally stable compound (>300 1C). Although experiments in this work were performed at pressures of 6–10 MPa, after optimization, similar results could be obtained by using compressed CO2 at pressures as low as 0.5–1 MPa, which is expected to reduce the process cost. The innovative components of the work rely not only on the improved characteristics of the processing method (less time consuming, no use of organic solvents or catalysts, soft reaction temperatures), but also on the superior characteristics of the obtained products, particularly those related to thermal stability. The hybrid products showed a standard CO2 adsorption capacity. ´licos, This work has been partially financed by Carburos Meta Air Products Group, by the Spanish government under project CEN2008-1027, and by the Catalan Government, SGR2009-666.
Notes and references 1 C. Perego and R. Millini, Chem. Soc. Rev., 2013, 42, 3956. 2 S. Choi, J. H. Drese and C. W. Jones, ChemSusChem, 2009, 2, 796. 3 C. H. Yu, C. H. Huang and C. S. Tan, Aerosol Air Qual. Res., 2012, 12, 745. 4 J. M. Rosenholm, A. Penninkangas and M. Linden, Chem. Commun., 2006, 3909. 5 J. C. Hicks, PhD thesis, Georgia Institute of Tecnology, 2007. 6 J. C. Hicks, J. H. Drese, D. J. Fauth, M. L. Gray, G. Qi and C. W. Jones, J. Am. Chem. Soc., 2008, 130, 2902. 7 J. H. Drese, S. Choi, R. P. Lively, W. J. Koros, D. J. Fauth, M. L. Gray and C. W. Jones, Adv. Funct. Mater., 2009, 19, 3821. 8 W. Chaikittisilp, S. A. Didas, H. J. Kim and C. W. Jones, Chem. Mater., 2013, 25, 613. 9 E. Loste, J. Fraile, M. A. Fanovich, G. F. Woerlee and C. Domingo, Adv. Mater., 2004, 16, 739. ´ia-Gonza ´lez, J. Saurina, J. A. Ayllo ´n and C. Domingo, 10 C. A. Garc J. Phys. Chem. C, 2009, 113, 13780. ´pez-Aranguren, J. Fraile, L. F. Vega and C. Domingo, 11 S. Builes, P. Lo J. Phys. Chem. C, 2012, 116, 10150. 12 K. Soga, S. Hosoda and S. Ikeda, Macromol. Chem. Phys., 1974, 175, 3309. ´pez-Aranguren, J. Saurina, L. F. Vega and C. Domingo, 13 P. Lo Microporous Mesoporous Mater., 2012, 148, 15. 14 J. H. Drese, S. Choi, S. A. Didas, P. Bollini, M. L. Gray and C. W. Jones, Microporous Mesoporous Mater., 2012, 151, 231. 15 Y. Kuwahara, D. Y. Kang, J. R. Copeland, N. A. Brunelli, S. A. Didas, P. Bollini, C. Sievers, T. Kamegawa, H. Yamashita and C. W. Jones, J. Am. Chem. Soc., 2012, 134, 10757. 16 J. H. Drese, S. Choi, R. P. Lively, W. J. Koros, D. J. Fauth, M. L. Gray and C. W. Jones, Adv. Funct. Mater., 2009, 19, 3821.
Chem. Commun.
