Youssef Belmabkhout, Rodrigo Serna-Guerrero and Abdelhamid Sayari*

Adsorption of CO2-containing gas mixtures over amine-bearing pore-expanded MCM-41 silica:
application for CO2 separation

Youssef Belmabkhout, Rodrigo Serna-Guerrero and Abdelhamid Sayari*

Department of Chemistry and Department of Chemical and Biological Engineering

University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada

Tel.: 1 (613) 5625483

Fax.: 1 (613) 5625170

Corresponding author; Email:

1. Materials

The pore-expanded mesoporous support was prepared in two steps based on a procedure described elsewhere(Sayari et al. 1998; Kruk et al. 2000).Briefly, MCM-41 type silica was synthesized at 100 C using CTAB as structure directing agent. A pore expansion procedure followed, using DMDA as swelling agent at 120 C for 3 days(Sayari et al. 1998; Sayari et al. 1999.After removal of the surfactant template and expander agent by calcination, the obtained product was labeled PE-MCM-41.

Incorporation of the amine functionality was achieved via surface grafting following a procedure described elsewhere(Harlick and Sayari, 2007).A sample of PE-MCM-41 was loaded into a multi-neck glass flask containing 150 mL of toluene. Once a homogeneous mixture was obtained, 0.3 mL per gram PE-MCM-41 of distilled deionized water was added and left stirring for 30 min. The glass flask was then submerged in a silicon oil bath set at 85 C using a temperature controlled stirring hotplate with an external temperature probe. Triaminesilane (3 mL per gram of silica) was subsequently added to the mixture and left stirring for 16 h. The material was filtered and washed with copious amounts of toluene, then pentane. Finally, the recovered solid was dried at 100 °C in a natural convection oven for 1 h and was labeled TRI-PE-MCM-41.

Fig. S1. N2 adsorption desorption isotherm of TRI-PE-MCM-41

2. Experimental set-up for single gas adsorption measurements

Fig.S2.Schematic diagram of the Rubotherm gravimetric-densimetric set-up (source: Rubotherm GMBH).

Adsorption equilibrium measurements of pure gases were performed using a Rubotherm gravimetric-densimetric apparatus (Bochum, Germany), schematically represented in figure S2. It is composed mainly of a magnetic suspension balance (MSB) and a network of valves, mass flowmeters and temperature and pressure sensors.The MSB overcomes the disadvantages of other commercially available gravimetric instruments by separating the sensitive microbalance from the sample and the measuring atmosphere(Dreisbach et al. 2003).In a typical adsorption experiment, about 0.5 – 1 g adsorbent is placed in a basket suspended by a permanent magnet through an electromagnet. The cell in which the basket is housed is then closed. The sample is outgassed at 348 K – 423 K at a residual pressure 10-4 mbar. Adsorption measurements may be performed in a wide range of pressure (0 – 60 br) and temperature (298 – 423 K). The clean (outgassed) adsorbent is exposed to flowing gas at constant temperature at a rate of 100 ml/min.The change in the weight of the adsorbent sample as well as the pressure and temperature were monitored continuously until the thermodynamic equilibrium was reached.The gravimetric method allows the direct measurement of the reduced CO2 adsorbed amount .Correction for the buoyancy effect is required to determine the excess adsorbed amount(Belmabkhout et al. 2004; Dreisbach et al. 2003)using equation 1, where Vadsorbent and Vss refer to the volume of the adsorbent and the volume of the suspension system, respectively. These volumes are determined using the helium isotherm method by assuming that helium penetrates in all open pores of the materials without being adsorbed(Belmabkhout et al. 2004, Sircar 2002).The density of the gas is determined experimentally using a volume-calibrated titanium cylinder. By weighing this volume in the gas atmosphere, the local density of the gas is also determined. Simultaneous measurement of adsorption capacity and gas phase density as a function of pressure and temperature is thus possible.

(1)

3. Experimental set-up for column breakthrough measurements

The experimental set-up used for dynamic breakthrough measurements isshown in Figure S3. The gas manifold consisted of three lines fitted with mass flow controllers of precision 1%. Line “A” is used to feed an inert gas, most commonly nitrogen, to activate the sample before each experiment. The other two lines, “B” and “C” feed a mixture of CO2 and other gases like CH4. Whenever required, gases flowing throughlines “B” and “C” may be mixed before entering a column packed with 40-60 mesh particles of TRI-PE-MCM-41 using a four-way valve. The stainless steel column was 120 mm in length with4.2 mm of inner(6.4 mm outer)diameter. The downstream effluent was monitored using a Pfeiffer Thermostar mass spectrometer. The detection limit of CO2 was estimated to be below 10 ppm. In a typical experiment, the adsorbent was treated at 373 K or 423 K for 2 hours under nitrogen flow of 100 mL/min, then cooled to room temperature and the gas flow was switched to the desired gas mixture at the same flow rate.The level of humidity was controlled in a similar manner as described for equilibrium measurements using distilled-deionized water in a temperature-controlled glass saturator.The complete breakthrough of CO2 and other species was indicated by the downstream gas composition reaching that of the feed gas.

The adsorption capacity was estimated from the breakthrough curves produced by the MS response using the following equation (2):

(2)

Figure S3. Set-up for column breakthrough measurements

where nadsi is the dynamic adsorption capacity of any gas i, F is the total molar flow, C0i is the concentration of the gas i entering the column, W is the mass of adsorbent loaded in the column, and tni is the stoichiometric time corresponding to gas i, which is estimated from the breakthrough profile according to Equation 3:

(3)

where C0i and CAi are the concentrations of any gas i upstream and downstream the column, respectively(Claudino et al. 2004).

References

Belmabkhout, Y., Frère, M., De Weireld, G. Meas. Sci. Technol. 15 (2004)848-858.

Claudino, A., J. L Soares, J.L., Moreira, R.F.P.M., Jose,H.J. Carbon 42 (2004) 1483-1490.

Dreisbach, F., Seif, R., Losch, H.W.J. Therm. Anal. Calorim. 71(2003)73-82.

Harlick, P.J.E A. Sayari, A. Ind. Eng. Chem. Res. 46 (2007) 446-458.

Kruk, M., Jaroniec, M., Sayari, A. Microporous Mesoporous Mater.35-36 (2000) 545-553.

Sayari, A., Kruk, M., Jaroniec, M., Moudrakovski, I.L. Adv. Mater. 10 (1998) 1376-1379.

Sayari, A., Yang, Y., Kruk, M.,Jaroniec, M. J. Phys. Chem. B. 103 (1999)3651-3658.

Sircar, S.: S. In: K. Kaneko (Eds.), Proceedings of Fundamental of Adsorption 7. IK International., ChibaCity, 2002, pp. 656-663.

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