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Preparation and CO2 separation properties of amine-modified mesoporous silica membranes

Preparation and CO2 separation properties of amine-modified mesoporous silica membranes
Preparation and CO2 separation properties of amine-modified mesoporous silica membranes

Preparation and CO 2separation properties of amine-modi?ed

mesoporous silica membranes

Yuzuru Sakamoto a ,Kensuke Nagata b ,Katsunori Yogo

a,b,*

,Koichi Yamada

a,b,c

a Research Institute of Innovative Technology for the Earth,9-2Kizugawadai,Kizu-cho,Soraku-gun,Kyoto 619-0292,Japan

b Nara Institute of Science and Technology,Keihanna Science City,8916-5,Takayama-cho,Ikoma,Nara 630-0192,Japan c

Seikei University,Department of Materials and Life Science,3-3-1Kichijoji Kita-machi,Musashino-shi,Tokyo 180-8633,Japan

Received 28July 2006;received in revised form 25October 2006;accepted 1November 2006

Available online 15December 2006

Abstract

Mesoporous silica membranes were prepared on porous alumina supports by both hydrothermal and sol–gel spin-coating methods.SEM images showed that these techniques deposited dense mesoporous silica layers of 200and 500nm,respectively,on the alumina supports.TEM and XRD observations showed that these membranes have a highly ordered cubic structure with a pore diameter of 2nm.Gas permeation properties of these mesoporous silica membranes were governed by the Kundsen di?usion mechanism.Surface modi?cation of the pore walls of mesoporous silica membranes by grafting amino-silane greatly improved the CO 2permselectivities.The amine-modi?ed mesoporous silica membranes showed high CO 2/N 2separation properties for a mixture of 20%CO 2and 80%N 2at 373K.The CO 2/N 2selectivities of the membranes prepared using hydrothermal and sol–gel spin-coating methods were 50and 800,respectively.

ó2006Elsevier Inc.All rights reserved.

Keywords:Carbon dioxide;Mesoporous silica;Membrane;MCM-48;Aminosilane

1.Introduction

The increasing atmospheric concentration of carbon dioxide (CO 2)due to fossil fuel combustion is a serious environmental problem.Recently,CO 2capture and sequestration has attracted considerable attention as one of the options to reduce CO 2emission [1].Various pro-cesses,such as liquid solvent absorption [2],membrane separation [3]and pressure (and/or temperature)swing adsorption (P(T)SA)[4]have been proposed for separation and recovery of CO 2emitted by power plants,steel works,etc.However,the costs of CO 2separation from ?ue gases account for approximately 70–80%of the total cost of

CO 2sequestration [1].Therefore,it is important to develop a new e?cient and energy-saving technique for CO 2separation.

Membrane separation processes are generally more energy e?cient and easier to operate than other separation processes.In particular,inorganic membranes such as zeo-lite and porous oxide would be suitable materials for the separation of carbon dioxide at high temperature and high pressure,because they are more resistant to severe condi-tions than organic membranes.Polymer membranes show good a?nity for CO 2and high CO 2/N 2separation proper-ties,however,thermal expansion/mobility of polymers at high temperatures causes degradation of CO 2selectivity.Since zeolites and mesoporous silica possess well de?ned micro/meso pores,considerable attention has been focused on the production of membranes that are capable of highly selective separation of gases [5,3,6].Mesoporous silica thin membranes on porous substrates have been prepared by templating silicates with various surfactant micelles and

1387-1811/$-see front matter ó2006Elsevier Inc.All rights reserved.doi:10.1016/j.micromeso.2006.11.007

*

Corresponding author.Address:Research Institute of Innovative Technology for the Earth,9-2Kizugawadai,Kizu-cho,Soraku-gun,Kyoto 619-0292,Japan.Tel.:+81774752305;fax:+81774752318.E-mail address:yogo@rite.or.jp (K.Yogo).

https://www.wendangku.net/doc/2214715595.html,/locate/micromeso

Microporous and Mesoporous Materials 101(2007)

303–311

using dip-coating/spin-coating techniques and hydrother-mal treatment[7–15].

However,as regards the application of mesoporous membranes for CO2separation,since CO2is a heavier molecule than other coexistent gas components such as H2,H2O,N2,CO and CH4,porous CO2selective mem-branes cannot operate by simple Knudsen di?usion.There-fore,CO2permselectivity of porous membranes would be attained by enhancing the a?nity of pore walls for CO2 adsorption over that of the other gases.

Organically functionalized mesoporous silica has recently attracted considerable attention because of the wide range of applications as adsorbents and catalysts [16,17].Since mesoporous silica has uniform,large pores as well as a high surface area,a large number of active sites or adsorption sites can be introduced uniformly on its pore

walls by surface modi?cation with organosilane molecules.

We have already studied surface modi?cation and func-tionalization of the pore walls of various mesoporous sili-cas that are specialized for CO2capture and separation; and have reported that the CO2adsorption capacities of aminosilane-modi?ed SBA-15mesoporous silicas in the presence of water vapor were comparable to those in the absence of water vapor[18].

If such a material is prepared as a thin?lm,the molec-ular movement of functional groups connected to the rigid inorganic pore wall is restricted,and it seems that a decrease in CO2selectivity due to an increase in the free volume by thermal expansion/movement of the polymeric material in the high temperature region can be restricted.

However,it is hard to utilize SBA-15or MCM-41,which have2D tube-like structures,as membranes for gas separa-tion because the pore channels of these mesoporous silica membranes are not oriented perpendicular to the surface of the substrate.On the other hand,pore channels of mesoporous silica with cubic or3D structures,such as MCM-48or SBA-16,do not depend on the direction,and so are expected to work as e?ective gas separation layers.

In this work,mesoporous silica membranes with a cubic structure on porous Al2O3substrate were synthesized by two kinds of methods,hydrothermal treatment and spin coating.Amine modi?cation of the internal surface of mesoporous silica membrane was performed by chemical grafting of amino-silane in order to increase the CO2 a?nity of the membranes.Moreover,the CO2separation performance of the membranes was evaluated by the per-meability of CO2and N2using a CO2/N2mixed gas.

2.Experimental

Mesoporous silica membranes were prepared by hydro-thermal treatment and spin-coating of silica sol,abbrevi-ated as MS/Al2O3(H)and MS/Al2O3(S),respectively. These membranes were modi?ed by chemical grafting of 3-aminopropyltriethoxysilane(APS),leading to the forma-tion of APS/MS/Al2O3(H)and APS/MS/Al2O3(S).A prep-aration scheme for these mesoporous silica membranes is given in Fig.1.

2.1.Preparation of mesoporous silica membrane by hydrothermal treatment

MCM-48membrane was prepared as follows:The sub-strate used for membrane deposition was an asymmetric porous Al2O3disk(B18.5mm,thickness3mm)consisting of a dense layer with an average pore diameter of70nm and a substrate layer with a pore diameter of700nm(Noritake Co.,Ltd.).The alumina substrate was soaked in1M HNO3 for24h.It was then washed with distilled water three times, followed by acetone,and then dried at383K for24h.A mixture of55ml of water,1.0g of NaOH and7.3g of cetyltrimethylammoniumbromide(CTAB)was stirred at 323K until the CTAB was completely dissolved.The solu-tion of CTAB and NaOH was cooled to room temperature and10.4g of tetraethoxysilane(TEOS)was added to the solution and it was stirred for30min.,leading to formation of the silica sol.The molar ratio of the silica sol was TEOS:CTAB:H2O:NaOH=0.4:1.0:61:0.5.The porous alumina substrate was then immersed in the silica sol for 90min.The silica sol and the alumina disk were transferred to a te?on vessel,and the support was placed horizontally in this for72h at a temperature of363K.Excess precipitate on the alumina disk was carefully removed from the surface with a te?on spatula.The product was thoroughly washed with distilled water and then dried in an oven at333K. Finally,mesoporous silica membrane(MS/Al2O3(H))was obtained by calcination at773K with a heating and cooling ratio of0.5K/min.An MCM-48powder was also prepared by the same method,without porous Al2O3substrate as a reference.

2.2.Preparation of mesoporous silica membrane using spin-coating of silica sol

Al2O3substrate was immersed in liquid para?n for12h at room temperature,in order to prevent silica sol from

304Y.Sakamoto et al./Microporous and Mesoporous Materials101(2007)303–311

percolating into pores of the substrate.After immersion,the surface of the Al2O3substrate was wiped with tissue paper to remove excess liquid para?n.Silica sol for spin-coating was prepared as follows[10]:a mixture of3.50g of EtOH,4.16g of TEOS and1.80g of diluted HCl(0.056M,pH=1.25) was stirred for1h.0.72g of CTABr and3.50g of EtOH were added to the mixture.The molar ratio of silica sol was CTABr:TEOS:H2O:HCl:EtOH=0.1:1:0.28:22.

Four hundred microliters of silica sol was dropped onto the Al2O3substrate by spin-coating at4000rpm.Thus, mesoporous silica membrane(MS/Al2O3(S))was obtained by calcination of the silica sol deposited on the Al2O3sub-strate at773K.

2.3.Surface modi?cation of mesoporous silica membrane by chemical grafting

The pore walls of the two types of mesoporous silica membranes obtained were modi?ed by chemical grafting of3-aminopropyltrimethoxysilirane(APS)in order to improve the CO2a?nity of the pore wall.

Five milliliters of APS was dropped onto the surface of the mesoporous silica membrane(MS/Al2O3(H)or MS/ Al2O3(S))placed in the?ask,and heated at423K for 24h under an Ar?ow,in order to prevent hydrolysis and condensation of the APS.Amine-modi?ed mesoporous silica membranes(APS/MS/Al2O3(H)and APS/MS/ Al2O3(S))were obtained after washing with toluene to remove unreacted APS and drying at333K.

The pore size distribution of MCM-48and APS/MCM-48was calculated by the BJH method using adsorption isotherms of N2at77K.MCM-48had a sharp pore size dis-tribution approximately2.2nm.After amine modi?cation, the strong peak at2.2nm was not observed,while small peak at 1.2nm was appeared.It was shown APS was grafted uniformly on the surface of MCM-48pore walls.

2.4.Characterization

Structural analyses of these mesoporous silica mem-branes were conducted by X-ray di?raction(XRD),and the pore diameters of the membranes were evaluated by transmission electron microscopy(TEM).Surface observa-tions and thickness measurements of the membranes were performed with a scanning electron microscope(SEM).

XRD patterns were recorded from2h=1.5–10on an X-ray di?ractometer(Rigaku RINT2000)using Cu K a X-ray(40kV and40mA).

TEM observation was performed on a HITACHI HF-2000electron microscope(200kV).Membrane sam-ples were scratched by a cutter and the obtained powder was dispersed in1ml of ethanol.After1h,a few drops of suspension were deposited on a microgrid and dried in air.The microgrid was evacuated for1h and TEM obser-vation was carried out.

SEM micrographs were obtained with a HITACHI S-5000electron microscope(10kV).Mesoporous silica membranes which had been broken into pieces were placed on a holder and coated with Pt–Pd by ion sputtering(HIT-ACHI E102)with a current of15mA at0.05Torr for 2min.

2.5.Gas permeability measurement of the membranes

Gas permeability measurements of the membranes were carried out using a di?erential pressure type of gas permeability device GTR-20XFR(Yanagimoto factory).

A schematic diagram of the experimental set-up for separa-tion measurements is shown in Fig.2.Single gas perme-ation measurements of the membranes were carried out using H2,CH4,N2,Ar,CO2and SF6at room temperature. The feed side of the membrane was kept at150kPa,and permeate side was evacuated by vacuum pump.The perme-ate side of the membrane was evacuated by vacuum pump. Evaluation of the CO2separation properties of the amine-modi?ed mesoporous silica membranes was carried out using a mixture of CO2/N2gas(CO2/N2=20/80).The pressure drop between the feed side and the permeate side was kept at150kPa.The permeated gas was analyzed by a gas chromatograph equipped with a TCD detector (G2800T,Yanagimoto factory).

In order to avoid adsorbed water a?ects the permeation results,the membrane was evacuated at373K for24h before measurement.The CO2/N2mixture was introduced after the membrane had been heated at the measuring tem-perature for1h.The gas permeation measurements were carried out after1h.The reproducibility of the permeabil-ity was con?rmed by the deferent membrane samples and repetition of the measurement.

3.Results and discussion

3.1.Characterization of mesoporous silica membranes prepared using hydrothermal treatment

The XRD patterns of MS/Al2O3(H)and APS/MS/ Al2O3(H)are shown in Fig.3.As the XRD peaks of

Y.Sakamoto et al./Microporous and Mesoporous Materials101(2007)303–311305

MS/Al 2O 3(H)at 2h =2.8,3.2,5.2were obtained at the same positions as for the MCM-48powder sample,it was obvious that an MCM-48layer was formed on the Al 2O 3support.In addition,no change in the peak position was observed for APS/MS/Al 2O 3(H),and this result showed that the structure of MCM-48was retained after amine modi?cation.

An SEM image of the MS/Al 2O 3(H)surface is shown in Fig.4(a).MS/Al 2O 3(H)has a rough surface,however,the Al 2O 3substrate is completely covered by the deposited MCM-48layer.A cross-sectional view of MS/Al 2O 3(H)is shown in Fig.4(b).It can be seen that MCM-48was depos-ited on both the surface and the inside of the substrate:MCM-48particles were deposited on the surface,but also in the spaces between the alumina particles.Although the interface between MCM-48and Al 2O 3substrate was not clear,a dense MCM-48layer of approximately 500nm was observed from the surface into the substrate.In addi-tion,the grain boundaries of Al 2O 3particles of the sub-strate are not seen clearly,which suggests that very ?ne MCM-48particles were also deposited deep inside the Al 2O 3substrate.

An SEM image of the APS/MS/Al 2O 3(H)surface is shown in Fig.5(a).As APS/MS/Al 2O 3(H)has a smooth surface with no cracks,it is suggested that the amino-sila-ne(APS)reagent reacted with not only the pore walls of MCM-48but also the surface of the MS/Al 2O 3(H)mem-brane.In the cross-sectional image of APS/MS/Al 2O 3(H)shown in Fig.5(b),a dense layer approximately 2000nm thick is observed.As the dense layer of APS/MS/Al 2O 3(H)is 4times as thick as that of MS/Al 2O 3(H),it was consid-ered that APS reagent reacted not only with the dense

Fig.4.SEM images of mesoporous silica membrane (MS/Al 2O 3(H))prepared by hydrothermal treatment:(a)top view;(b)cross-section.

Fig.5.SEM images of mesoporous silica membrane (APS/MS/Al 2O 3(H))prepared by chemical modi?cation with APS of the pore wall of MS/Al 2O 3(H):(a)top view;(b)cross-section.

306Y.Sakamoto et al./Microporous and Mesoporous Materials 101(2007)303–311

MCM-48layer but also with the ?ne MCM-48particles deposited in the spaces among Al 2O 3particles deeper than 500nm from the membrane surface.As a result,the dense layer of amine-modi?ed mesoporous silica increased to 2000nm.

A typical TEM image of MS/Al 2O 3(H)is shown in Fig.6.The highly ordered mesoporous structure with a diameter of ca.2nm is observed over a wide area of the sample.From these XRD,SEM and TEM observations,it was shown that MS/Al 2O 3(H)membrane possesses a highly ordered mesoporous structure similar to that of MCM-48powder,and that MCM-48was deposited not only on the surface but also inside the substrate,namely,in the spaces between Al 2O 3particles.

3.2.Characterization of mesoporous silica membranes prepared by the spin-coating technique

XRD patterns of MS/Al 2O 3(S)and APS/MS/Al 2O 3(S)are shown in Fig.7.XRD peaks of MS/Al 2O 3(S)at 2h =2.6,2.9and 5.0were obtained.The peak positions

were shifted to slightly lower angles (0.2°)than those of the samples prepared using the hydrothermal treatment and MCM-48powder.The d -spacings of the peaks around 2h =3for MCM-48and MS/Al 2O 3(S)were 28.20and 29.83respectively.Therefore,the structure of MS/Al 2O 3(S)was not consistent with MCM-48and MS/Al 2O 3(H).How-ever,no change in the peak position was observed after amine modi?cation,and the structure of mesoporous silica was retained.

A TEM image of MS/Al 2O 3(S)is shown in Fig.8.A uniform cubic mesoporous structure with a diameter of ca.2nm,very similar to that of MS/Al 2O 3(H)(Fig.6),was observed over a wide area of MS/Al 2O 3(S).

Despite the TEM observation that both MS/Al 2O 3(H)and MS/Al 2O 3(S)membranes have highly ordered meso-porous structures,however,the XRD peak intensity of MS/Al 2O 3(S)was stronger than that of MS/Al 2O 3(H).This means that a dense layer of highly ordered mesostructure was produced on top of the alumina support of MS/Al 2O 3(S).

An SEM image of the MS/Al 2O 3(S)surface is shown in Fig.9(a).The Al 2O 3substrate seems to be homogeneously covered by a smooth surface of mesoporous silica layer without cracks or pinholes.A cross-sectional image of MS/Al 2O 3(S)is shown in Fig.9(b).A dense layer with a thickness of 300nm is observed.

On the other hand,in the case of spin-coating of silica sol on unpretreated porous Al 2O 3substrate (without immersion in liquid para?n),no dense mesoporous silica layer was obtained.As the viscosity of the silica sol of the composition for cubic structure formation was low,it was easily spread into the pores of the alumina support.As a result,random deposition of the silica on and into the alumina substrate was observed by SEM (Fig.10(a)and (b)).

An SEM image of the APS/MS/Al 2O 3(S)surface is shown in Fig.11(a).APS/MS/Al 2O 3(S)has a smooth surface,as does MS/Al 2O 3(S).In the cross-sectional image of APS/MS/Al 2O 3(S)(Fig.11(b)),the thickness

of

Fig. 6.TEM image of MS/Al 2O 3(H)prepared by hydrothermal treatment.

Fig.8.TEM image of MS/Al 2O 3(S).

Y.Sakamoto et al./Microporous and Mesoporous Materials 101(2007)303–311307

APS/MS/Al 2O 3(S)is 300nm.In this case,the 300nm thickness of MS/Al 2O 3(S)was maintained even after amine modi?cation.These observations suggest that the reaction with APS proceeded only on the dense mesoporous silica layer.The morphology of dense layers was di?erent between the two types of the membranes (S and H).It is due to the di?erence of synthesis process.In the case of hydrothermal treatment,dense silica layer formation pro-ceeded by the nucleation and deposition of the MCM-48particles on the Al 2O 3substrate,therefore some MCM-48

particles were also deposited in the deep layer of the Al 2O 3substrate.On the other hand,in the case of spin-coating preparation,ordered mesoporous silica layer was produced through the solvent evaporation from the silica sol dispersed on the alumina top surface.3.3.Evaluation of single gas permeability

In order to evaluate the uniformness of the two types of membranes,measurements of single gas permeation of

the

Fig.9.SEM images of MS/Al 2O 3(S)prepared by spin-coating

technique.

Fig.10.SEM images of silica sol deposited on the Al 2O 3substrate without immersion in liquid

para?n.

Fig.11.SEM images of APS/MS/Al 2O 3(S)prepared by the chemical grafting of MS/Al 2O 3(S)with APS:(a)top view;(b)cross-section.

308Y.Sakamoto et al./Microporous and Mesoporous Materials 101(2007)303–311

mesoporous silica membranes were performed with various gases,SF6,CO2,Ar,N2,CH4and H2.The plots of perme-ance versus the reciprocal of the square root of the molec-ular weight(MW)of the gases are shown in Fig.12.Both membranes showed a linear relationship between perme-ance and MWà1/2,so the gas permeation was governed by the Knudsen di?usion mechanism and mesoporous sil-ica membranes had a highly ordered structure which had neither large cracks nor pinholes.Although the MS/ Al2O3(H)layer was thicker than that of MS/Al2O3(S), the gas permeability of MS/Al2O3(H)was much higher than that of MS/Al2O3(S).In the case of the hydrothermal treatment,deposition of mesoporous silica particles occurred in the porous Al2O3substrate,so that many grain boundaries and pores larger than4nm were present in the MS/Al2O3(H).

On the other hand,in the case of the spin-coating method,the prepared mesoporous silica layer was dense and uniform,so that the permeance of MS/Al2O3(S)was lower than that of MS/Al2O3(H).Although mesopores with a3D structure should be formed under these condi-tions according to a report by Besson et al.[10],the XRD pattern of MS/Al2O3(S)(Fig.7)shows that it has a cubic structure like that of MCM-48.It is thought that liquid para?n oozed out of the pores of the Al2O3sub-strate and that the molar ratio of silica sol was changed when the silica sol was deposited on the pretreated Al2O3.An ine?ectual structure for gas permeance such as a2D structure parallel to the Al2O3substrate might be formed around the interface of the substrate and the mesoporous silica layer because the composition of the silica sol was changed by the dissolution of liquid para?n.

3.4.Evaluation of CO2separation from a mixed gas

In order to evaluate the CO2separation performance with a mixed gas,the permeability of CO2and N2from a CO2/N2mixture(CO2=20%)was measured using a di?er-ential pressure-type permeability device(Fig.2).The results of the CO2/N2mixed gas permeation tests for Al2O3substrate and mesoporous silica membranes are summarized in Table1.The Al2O3substrate did not show CO2selectivity and the CO2/N2selectivity a(CO2/N2)was 1.The gas permeabilities of MS/Al2O3(H)and MS/ Al2O3(S)decreased by one order and a(CO2/N2)was0.8, suggesting that the gas permeation was governed by the Knudsen di?usion mechanism.Although the gas perme-ance of the amine-modi?ed mesoporous silica membranes was decreased compared with the unmodi?ed membranes, both the modi?ed membranes(APS/MS/Al2O3(H)and APS/MS/Al2O3(S))showed high CO2permselectivity and the a(CO2/N2)values were50and800,respectively,at 373K.It is considered that the CO2a?nity of mesoporous silica membranes was improved by the amine modi?cation.

The plots of CO2and N2permeance versus temperature for APS/MS/Al2O3(S)are shown in Fig.13.The perme-ance of CO2and N2increased with increasing temperature. The a(CO2/N2)of APS/MS/Al2O3(S)also rose up to800at 373K,and the CO2/N2selectivity decreased above373K.

In our previous work[18–20],it was con?rmed that CO2-adsorbed aminosilane-modi?ed mesoporous silica SBA-15was completely regenerated by heating up to 423K.In addition,IR spectra of surface species formed on amine-modi?ed SBA-15by the amine-CO2reaction: absorption bands at1628cmà1and1563cmà1attributed to ammonium carbamate,and absorption bands at 3439cmà1attributed to N–H vibration of ammonium car-bamate,showed that the CO2adsorption proceeds through the formation of ammonium carbamate(Eq.(1))

2R-NH2+CO2!R-NHCOOà+tH3N-Re1T

Table1

CO2separation properties of membrane samples

Membrane Temperature(K)Permeance(mol/m2s Pa)CO2selectivity a(CO2/N2)

CO2N2

Al2O3substrate a298 1.1·10à6 1.1·10à6 1.0

MS/Al2O3(H)a298 1.0·10à7 1.4·10à70.8

MS/Al2O3(S)a298 3.4·10à8 4.1·10à80.8

APS/MS/Al2O3(H)b373 4.7·10à108.3·10à1250

APS/MS/Al2O3(S)b373 1.0·10à9 1.2·10à12800

Test gas:CO2/N2=20/80.Measurements were carried out after a1h and b24h.

Y.Sakamoto et al./Microporous and Mesoporous Materials101(2007)303–311309

Therefore,it is obvious that CO2was permeated through these amine-modi?ed mesoporous silica membranes by re-peated adsorption and desorption on the amino-groups as in Eq.(1).The maximum of CO2selectivity at around 373K is reasonable if we consider the balance between the adsorption and desorption and the surface di?usion of the CO2molecule.

The plots of the change in the gas permeation perfor-mance of amine-modi?ed mesoporous silica membranes at373K over time are shown in Fig.14.In the case of APS/MS/Al2O3(H),CO2permeance was almost constant between3and6·10à10(mol/m2s Pa)and N2permeance was also constant at1·10à12(mol/m2s Pa)until20h,then the N2permeance reached1·10à11(mol/m2s Pa). The CO2/N2permselectivity decreased from340to50with an increasing N2permeance.This might be attributed to structural changes resulting from APS deposited on the outer surface or between the grain boundary of the meso-porous silica layer.It is suggested that thermal expansion or increased mobility of the APS chain at high tempera-tures causes the formation of free space which allows N2 permeation.

In contrast,in the case of APS/MS/Al2O3(S),CO2 permeance was almost constant under these reaction conditions(in the range of1–3·10à9(mol/m2s Pa))and 10times as high as that of APS/MS/Al2O3(H).It is consid-ered that the movement of the functional amino-group connected to the rigid inorganic pore wall of the dense mes-oporous silica layer is restricted,and,as a result,it seems that the decrease in CO2selectivity due to an increase in the free volume in the high temperature region can be avoided in the APS/MS/Al2O3(H)membrane.

In addition,although the single gas permeance of unmodi?ed MS/Al2O3(H)was much higher than that of MS/Al2O3(S),the CO2permeability of APS/MS/Al2O3(S) was much higher than that of APS/MS/Al2O3(H).It is suggested that selective adsorption of CO2towards the dense separation layer enhanced the CO2permeability and N2permeation might be hindered by the adsorbed CO2molecules.

From these observations,it is suggested that the reac-tion of APS proceeds only with a dense mesoporous silica layer,and a thin CO2separation layer of amine-modi?ed mesoporous silica was successfully prepared on top of Al2O3substrate using spin coating of silica sol.

4.Conclusions

Mesoporous silica membranes were successfully pre-pared on porous alumina supports by hydrothermal treat-ment and spin-coating of silica sol.From the TEM and XRD observations,it was shown that these membranes have a highly ordered cubic structure with a pore diameter of ca.2nm.Surface modi?cation of the pore walls of mes-oporous silica membranes by grafting amino-silane greatly improved the CO2permselectivities.After amine modi?ca-tion,the thickness of the dense layer of MS/Al2O3(H) increased to2000nm,while no obvious change was observed for MS/Al2O3(S).Immersion of the porous alu-mina substrate in liquid para?n before spin-coating of sil-ica sol was e?ective at preventing the sol from percolating into pores of the substrate,causing formation of a dense layer.APS/MS/Al2O3(S)showed higher CO2permeability and selectivity than APS/MS/Al2O3(H)because of the thinner dense separation layer formed.Both of the amine-modi?ed membranes showed extremely high CO2 permselectivity and their a(CO2)values were50and800, respectively,at373K.It was con?rmed that amine-modi-?ed mesoporous silica layer works as an e?ective CO2 separation layer.

310Y.Sakamoto et al./Microporous and Mesoporous Materials101(2007)303–311

Acknowledgement

This work was supported by the Ministry of Economy, Trade and Industry of Japan.

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不久,他命人起草了废除科举制度的诏书,准备颁发下去。书还 未下达,地位略低于平章的参政许有王,又出来反对废除科举制度。 他对伯颜说:“如果废除科举考试制度,世上有才能的人都会怨恨的。”伯颜针锋相对地说:“如果继续实行科举考试制度,世上贪赃 枉法的人还要多。”许有王反驳说:“没有实行科举考试制度的时候,贪赃枉法的人也不是很多吗?”伯颜讽刺他说:“我看中举的人中有用 之材太少,只有你参政一个人能够任用!”许有王不服气,举出很多当 时中举的高官来反驳伯额。伯颜当然不会改变自己的观点,于是两人 争论得非常激烈。第二天,满朝文武被召到祟天门听读皇帝下达的废 除科举制席的诏书,许有王还特地被侮辱性地通知在班首听读。看来,皇帝特意要让这个反对者将诏书听得明白些。许有王心里非常不愿意,但又惧怕得罪皇帝遭到祸害,只好勉强跪在百官前列听读诏书。听读 完诏书后,百官纷纷回府,许有王满脸不高兴地低头走路。 有个名叫普化的御史特地走到他边上,凑着他的耳朵冷嘲热讽他说:“参政,你这下成为过河拆桥的人啦。这话的意思是,你许参政 是靠科举当官的,现在宣读皇上关于废除科举制度诏书,你跪在最前面,似乎是废除科举制度的领头人,就像一个人过了桥后就把桥拆掉 一样。许有王听了又羞又恨,加快步伐离开。之后他借口有病,再也 不上朝了。

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