Research Article www.acsami.org
CO2 Adsorption on Amine-Functionalized Periodic Mesoporous Benzenesilicas Kyohyun Sim,†,§ Nakwon Lee,†,§ Joonseok Kim,† Eun-Bum Cho,*,† Chamila Gunathilake,‡ and Mietek Jaroniec*,‡ †
Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 139-743, Korea Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, United States
‡
S Supporting Information *
ABSTRACT: CO2 adsorption was investigated on aminefunctionalized mesoporous silica (SBA-15) and periodic mesoporous organosilica (PMO) samples. Hexagonally (p6mm) ordered mesoporous SBA-15 and benzene-PMO (BPMO) samples were prepared in the presence of Pluronic P123 block copolymer template under acidic conditions. Three kinds of amine-containing organosilanes and polyethylenimine were used to functionalize SBA-15 and BPMO. Small-angle Xray scattering and nitrogen adsorption isotherms showed that these samples featured ordered mesostructure, high surface area, and narrow pore size distributions. Solid-state 13C- and 29 Si cross-polarization magic-angle spinning NMR spectra showed chemical linkage between amine-containing modifiers and the surface of mesoporous materials. The chemically linked amine-containing modifiers were found to be on both the inner and outer surfaces. N-[3-(trimethoxysilyl)propyl]ethylenediamine-modified BPMO (A2-BPMO) sample exhibited the highest CO2 uptake (i.e., ∼3.03 mmol/g measured on a volumetric adsorption analyzer) and the fastest adsorption rate (i.e., ∼13 min to attain 90% of the maximum amount) among all the samples studied. Selectivity and reproducibility measurements for the A2-BPMO sample showed quite good performance in flowing N2 gas at 40 mL/min and CO2 gas of 60 mL/min at 25 °C. KEYWORDS: CO2 adsorption, periodic mesoporous organosilica, polyethylenimine, N-[3-(trimethoxysilyl)propyl]ethylenediamine
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INTRODUCTION As compared to the last 20 000 years a significant change in the global climate occurred as a consequence of human activities such as extensive burning fossil fuels and deforesting the earth. It was shown that the annual emission of CO2 has increased by ∼80% between 1970 and 2004, and the current atmospheric CO2 concentration exceeds by ∼25% the level of 300 ppm observed in past before 1950.1 Nowadays, the reduction of CO2 emission became the key issue to slow down the global warming. Recently, various adsorption techniques and materials for CO2 capture have been developed such as zeolitic molecular sieves,2 oxides,3 metal−organic frameworks,4 activated carbons,5 and aminecontaining materials.6−9 For decades the amine-containing materials such as liquid amines10 have been used in industrial processes to capture CO2. Extensive efforts have been made toward using aqueous alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), diglycolamine, and triethanolamine (TEA) for CO2 capture.11 It was reported that these species are able to attach CO2 via reversible formation of carbamate, carbonate, and bicarbonate species.11 In this process, amine solutions mixed with ionic liquids chemically absorb ∼3− 4 mmol of CO2 per gram of amine, and reversibly release captured CO2 by heating mixture above 100 °C.12 Nevertheless, © 2015 American Chemical Society
this process possesses several disadvantages, including high cost and high energy consumption associated with evaporation of CO2. Furthermore, the effectiveness of this process is affected by oxidative degradation of amine, their low stability, and by interference of impure gases such as SO2, NO2, and NO.13,14 As a result, the research on the CO2 capture is gradually shifting from liquid amine-based scrubbing to solid adsorption to overcome the drawbacks of the former.15 Mesoporous silica is a very popular solid adsorbent for gas- and liquid-phase applications including CO2 sorption. Recently, porous solid adsorbents such as silica have been modified by the aforementioned amine-containing compounds16 to increase their affinity toward CO2. This modification is usually performed by impregnation of amines16−25 or by grafting amine-functional groups on the pore walls.26,27,31 Studies of polyethylenimine (PEI)-loaded MCM-41 mesoporous silica sorbents showed a substantial increase in the CO2 adsorption capacity for the sample with 75 wt% PEI loading under pure CO2 gas conditions.17,18 Sayari et al. investigated the effect on the pore Received: January 11, 2015 Accepted: March 5, 2015 Published: March 5, 2015 6792
DOI: 10.1021/acsami.5b00306 ACS Appl. Mater. Interfaces 2015, 7, 6792−6802
Research Article
ACS Applied Materials & Interfaces
Table 1. Physicochemical Parameters Determined from N2 Sorption Isotherms and Small-Angle X-ray Scatteringa sample
Vsp (cc/g)
Vmic (cc/g)
SBET (m2/g)
Wmax (nm)
Vt (cc/g)
dsp (nm)
a (nm)
t (nm)
SBA-15 A2-SBA-15 PEI/SBA-15 PEI/A2-SBA-15 BPMO A2-BPMO PEI/BPMO PEI/A2-BPMO
0.65 0.33 0.34 0.20 0.90 0.32 0.28 0.18
0.04 0.01 0.01 0.01 0.10 0.01 0.01
