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aCII!NCI!@OIAeCT- journal of MEMBRANE

SCIENCE ELSEVIER Journal of Membrane Science 270 (2006) 101 - 107

www.elsevier.com/locatelmemsci

Surface modification of mesoporous membranes by fluoro-silane coupling reagent for CO2 separation

N. Abidi a, A. Sivade a, D. Bourret a.*, A. Larbot a , B. Boutevin b,

F. Guida-Pietrasanta b, A. Ratsimihety b

• Instilut Europeen des Membranes, UMR 5635, IEMICNRS /9/9, Route de Mende. 34293 MOnIpellier Cedex 5, France b Lahora/oire de Chimie Macromo/ecu/aire. UPRESA 5076 CNRS, £ NSCM, 8 Rue de f'Eeole lIonna/e, 34296 Motllpellier Cedex. France

Received 8 March 2004; accepted 6 June 2005 Available online 17 October 2005

Abstract

The potentia1 of hybrid organic- inorganic membranes for separaling organic molecules from air, based on solubility selective mechanism, was evaluated. Alumina and titan a membranes with average pore size near 4 nm were surface modified using trimethoxysilane fluorinated coupling reagent. The penneabilities to helium. nitrogen. methane. ethane. propane. butane and carbon dioxide were evaluated at feed pressures lying between (1.5 x 10' and 3.5 x 10' Pal 1.5 and 3.5 bar and permeate outlet near I x 10' Pa (I bar). The permeabilities of the grafted membranes generally decreased by about two to three orders of magnitude compared with the untreated membranes. The C021N2

permselectivity increased significantly in the case of the Ti02 gmfted membrane. The membranes performances were compared and the Ti02 grafteu. membrane exhibits highcr permselectivity and permeability, so that. it is a good candidate for CO2 to N l separation and CO2 to hydrocarbon separation. © 2005 Elsevier B. V. All rights reserved.

Keywords: Surface modification; F1uorosilane: Ceramic membrane: Gas permeation; Grafted membranes

1. Introduction

During the few last years, special attention has been paid to ceramic membranes for gas separation because of their thermal and chemical stabilities compared to those of poly­meric membranes [1.2]. Interesting application may include the separation of C02 from gas mixtures. which is of par­ticular interest for environmental protection. Unfortunately. the mechanism for gas transport in commercially available porous ceramic membranes with pores < I 0 nm is Knudsen diffusion. Therefore. the separation efficiency between gases having a small difference in molecular weights. such as C02 and N2. is quite low. Thus, many researchers have tried to modify the surface of ceramic membranes in order to enhance the C02/N2 separation factor by gas transport mechanisms other than Knudsen diffusion [3-12].

• Corresponding author. E-mail addrn f>: bourret@criLuniv-montp2.fr (D. Bourret).

0376·7388/$ - see front matter () 2005 Elsevier B. V. All rights reserved. doi: I 0.1 OI6lj.memsci .2005.06.054

For membranes with pores <lOnm, surface modification by covalently bonded molecular mono-layer is one of the more convenient ways to alter membrane performance. Such modification can increase the performances of the membrane by, on one hand, reducing the mean pore size, and on the other hand. by promoting an eventual specific interaction between the surface of the membrane and the permeating molecules to enhance penneation.

Miller and Koros [13] modified commercially available Membralox® mesoporous alumina membranes with an aver­age pore size of 4 nm. A vapour-phase deposition was used to link trichlorosilane coupling reagent to the alumina surface.

Paterson and co·workers [14-16J perfonned such sur­face modification using wet chemistry routes rather than vapour-phase in order to link the coupling reagent. With octadecyltrichlorosilane [14]. the C02/N2 permselectivity (i.e .. the ratio of single gas permeabilities) obtained was only 1.05. a little higher than the Knudsen ideal separation factor. 0.8. Similar behaviour than for permeation through

102 N. Abidi et al. / Journal of Membrane Science 270 (2006) 101-107

low-density polyethylene membranes was obtained by Ran­don and Paterson [15] using n-dodecylphosphate as coupling reagent (C021N2 permselectivity of 4.5 for the modified membrane compare to 2.5 or 12.6 for, respectively, high and low-density polyethylene). The best C021N2 permselectivity (10.2) was obtained [16] for a modified ,,{-A}z03 membrane with a monolayer of polydimethylsilane on its surface. In this case the transfer mechanism through the PDMS-aIumina membrane was a solubility/diffusion one.

The coupling reagent used by Hyun et aI. [17] was phenyl­triethoxysilane having a special functional group (phenyl radical) with a high affinity for C02. The C021N2 perms­electivity was 2.15 and a mixed Knudsen/surface diffusion was deduced for C02 transfer mechanism through the mod­ified membrane.

Mejean et al. [18] performed surface modification of commercially available ,,{-Al203 membrane (mean pore size 5 nm) with fluorinated triethoxysilane and dodecyltri­ethoxysilane. The C021N2 permselectivity were 4.0 for flu­orinated triethoxysilane and 5.3 for dodecyltriethoxysilane. A mixed Knudsen/surface diffusion was deduced for C02 transfer mechanism through the modified membranes.

More recently, Javaid et al. [19] modified commercially available Membralox® mesoporous alumina membranes with an average pore size of 5 and 12 nm using solutions of alkyl trichlorosilanes in toluene. Their approach seems to be ideal for creating solubility-selective membranes because of the ability to deliver a large number of desired chemical groups while still remaining a relatively high free volume.

The main objective of this work was to synthesize organic-inorganic hybrid membranes comprised of fluori­nated triethoxysilane grafted to the surface of commercially available mesoporous membranes, and evaluate their perfor­mance for C021N2 separation. Fluorinated coupling reagents were preferred to alkyl ones because in each case C02 has a high solubility, but alkanes have poor solubility in fluorinated compounds [18]. Thus, for separation of a mixture . of C02, alkanes and N2, ceramic membranes modified by fluorinated coupling reagent will be more efficient due to large C021N2 and low C02/alkanes permselectivities. However, some large C02/alkanes permselectivities were obtained due to the fact that a multilayer surface diffusion can occurs for butane in the case of ,,{-A}z03 modified membrane.

In this paper, commercially available mesoporous Ti02 and ,,{-Al203 membranes were surface-modified with fluo­rinated coupling reagent. The fluorinated-coupling reagent, CsF 17C2~Si(OCH3h, was delivered in chloroform and . grafted to the surface via SiOH groups formed by hydrolysis of triethoxy groups with surface water. The membrane was tested for permeance to methane, ethane, propane, butane, carbon dioxide, helium and nitrogen. Helium was used in order to test Knudsen flow for the membrane. Methane, ethane, propane, butane and carbon dioxide were used to eval­uate C021N2 and C02/alkanes permselectivities. Section 2 provides the details of the fluorinated coupling reagent syn­thesis, membrane elaboration and permeation testing. The

results are presented in Section 3, with additional discussion. Section 4 contains our conclusions.

2. Experimental

2.1. Fluorinated coupling reagent synthesis

The fluorinated-coupling reagent used in this work was (3,3,4,4,5,5,6,6,7,7,S,8,9,9,10,10,10-heptadecafluorodecy1)­trimethoxysilane, CsF17C214Si(OCH3h. It was synthesized in two steps by the following reactions [2S]:

(i) Hydrosilysation of CgF 17CH2=CH2

CSF17CH=CH2 + HSiCl3 --+ CgF17C2H4SiCb

(ii) Methoxylation of trichlorosilane

CgF17C2H4SiCI3 + CH30H --+ CgF17C2H4Si(OCH3)3

2.2. Membrane synthesis procedure

The mesoporous Ti02 membrane used in this study was provided by "Cerasiv" (France). The membrane had a 1.1 J.Lm thick active layer as measured by SEM. An average pore size of 4 nm was deduced from BET. The active layer was deposited on the inner surface of a Zr02-aA1203 multi-layer support, which had a much larger pore size (200 nm for the intermediate layer and SOO nm for the supporting one as given by SEM analysis). Thus, this macroporous support had no significant resistance to gas permeation compare to the active layer.

The "{-Ah03 membrane was purchased to the "Exekia­Pall" (France). The membrane had a 3 /-Lm thick active layer with a porosity of 40%. An average pore size of 4 nm was measured for this active layer by BET. The outer support was made with ex-alumina with much larger pore size (200 nm for the intermediate layer and 800 nm for the supporting one as given by SEM analysis) and had no significant effect on gas permeation compare to the active layer.

In both cases, the tubes bad an outer diameter of 1.0 cm and an inner diameter of 0.7 cm. For our experiments, we cut the tubes into 15 cm long pieces using a diamonded saw.

After cutting, the membranes were soaked in ethanol for 1 h and then, in bi-distilled water for 1 h at ambient temper­ature (ethanol was obtained from Merck with purity >99%). The membranes were, then, dried in an oven at 120°C dur­ing 1 day in order to leave only surface hydroxyl groups and part of the first hydration layer. After the heat treatment, the membranes were stored in a glove box.

Solution (10-3 moll-I) of the fluorinated-coupling reagent in anhydrous chloroform (Aldrich) was prepared in a glove box. The tubular membrane was filled with the solu­tion and hermetically closed. At that point, it was remove from the glove box and the solvent was allowed to evaporate though the membrane at 20°C. During this evaporation, the

N. Abidi et al. / Joumal of Membrane Science 270 (2006) 101- /07 103

membrane was manually turned over and over each 5 min. After complete evaporation of the solvent (i.e., all the solvent was removed from the membrane), the membrane was soaked in pure solvent for I h, this treatment was repeated three times in order to remove all the remaining free (physisorbed) fluorinated-coupling reagent.

2.3. Procedure for gas permeation

Only pure gas permeation measurements were run in this study. The tubular membrane was held in a cylindrical hous­ing with feed and retentate openings at either end of the tube and a penneate opening on the housing side. At each ends, rubber gaskets were held tight on the tube side by screw caps to insure sealing. Before each measurement the reten­tate exit was dead-ended and primary vacuum was held in the cylindrical housing for 10 min. Then, gas was introduced in the cylindrical housing through feed and retentate openings. This washing operation was made three times before mea­surements. During an experiment, gas was provided to the feed inlet and the retentate exit was held in the dead-ended position. Thus, all of the gas was forced to flow across the membrane and through the penneate outlet. The penneate outlet was connected to a bubble flow meter that exhaust in an extractor hood, so that the penneate pressure was always atmospheric. The pressure of the feed was adjusted with a pressure reducer and measured with a pressure gauge. The pennselectivities were deduced from the ratio of single gas penneabilities.

The gases used were helium, nitrogen, carbon dioxide, methane, ethane, propane and butane (purity of, at least, 99.9%) supplied by Air Liquide (France). Volumetric flow measurements were perfonned at feed pressures ranging between (1.5 x lOS and 3.5 x lOs Pal (1.5 and 3.5 bar) by 0.5 x lOs Pa (0.5 bar) step. The values of ambient tempera­ture and pressure measured during experiments were used to conven volumetric flow rate to molecular flow assuming ideal gas behaviour. For each mean pressure, the permeance was calculated by dividing the molar flow rate by the mem­brane flow area and the difference between feed and ambient (permeate) pressures. The penneability was, then, deduced by multiplying the penneance by the active layer thickness.

3. Transport mechanisms

The transpon of gas through a porous membrane can occur by several mechanisms [20). When the mean free path of the gas molecules is much smaller than the pore diameter, viscous flow occurs and the penneability, Fo, is given by [21]:

(I)

where e is the porosity, q the gas viscosity, Tv the tonuos­ity factor for the viscous flow, rp the pore radius, R the gas

constant, T the absolute temperature and Pmea, is the mean pressure in the membrane. When the mean free path of the gas molecules is much larger than the pore diameter Knudsen diffusion occurs:

(2)

where Tk is the tonuosity factor for Knudsen flow and v is the gas mean molecular velocity given by:

V= (:~r2 (3)

where M is the gas molecular weight. In ceramic membranes with pore diameters between 3 and

30 nm Knudsen diffusion is the "nonnal" mechanism and the separation is proponional to the inverse square root of the molecular weights. Nevenheless, since porous membranes have a pore-size distribution, gas transpon may simultane­ously occur by both viscous and Knudsen flow mechanisms leading to:

(4)

so that, CI and C2 can be deduced from the Fo =f{Pmea,) plot. Surface flow is always added and can be considered,

in a first approximation, as the flow of a two-dimensional adsorbed gas, which runs parallel and independent of the gas flow. The ratio of the flux through the gas phase with respect to the surface flux can reach values of three or more but, in general, is smaller than one. Several models have been devel­oped to describe surface flow [22]:

(i) The hydrodynamic model (GiUiland) where the adsorbed gas is considered as a liquid film which glides across the surface under the pressure gradient.

(ii) The hopping model where molecules hop over the sur­face from site to site.

(iii) The random walk hopping model which is the most fre­quently used. It is based on a two-dimensional Fick's law and leads to:

Fo = C3D, (~;) rpl (5)

where D, is the diffusion coefficient and x, is the fractional surface occupation of the mobile specimen. The term "p I expresses the increase of internal surface when decreasing rp, specific interactions are lumped in C3.

Multilayer adsorption occurs when the relative pressure PIP, (P, is the condensation pressure) is increased so thai a species is adsorbed in several layers. This is an extension of the monolayer adsorption and surface diffusion models can be extended to include the multilayer diffusion [23J. Due to monolayer and subsequently multilayer diffusion. the per­meability will increase until a maximum is reached. This maximum occurs as capillary condensation slans and a liq­uid meniscus is formed. After this point, the permeability

104 N. Abidi el al. / Journal of Membrane Science 270 (2006) 101-107

decreases due to the liquid transport contribution (the perme­ability for vapour flow is much higher than for liquid flow). This qualitative picture was illustrated by Lee and Wang using six possible modes for a cylindrical capillary in case of mul­tilayer diffusion and capillary condensation [241·

If the pore size is decreased (rp < 3 nm), the contribution of the surface flow increases while the gas phase diffusion contribution decreases. There will be a gradual change to solution-diffusion type of transport as present in most of the non-porous polymer membranes or to mechanisms like those in microporous membranes where adsorption plays an impor­tant role [25,261.

4. Results and discussion

4.1 . Original membranes

All the measurements were run at pressure difference lying from 0.1 x 105 to 0.5 x I as Pa (0.1 to 0.5 bar). The plots of the molar fluxes against the pressure difference were always observed to be linear for both Ti02 and jI-A1203 membranes. Thus, the permeabilities were obtained from linear regression of these data and were constant in the pressure range f>Pequal to (0.1-0.5) x I as Pa. These permeabilities plotted against the reciprocal of the square root of the molecular weights of the gases are linear and so the mechanism for the gas permeation is simple Knudsen flow (Fig. I).

At 20 °C, the nitrogen permeability of the unmodi­fied jI-AI20) membrane was constant at 13.4 x 10- 12

mols- I m- I Pa- I, this value is rather smaller than those of Randon and Paterson [15] and lavaid et al. [19] for com­mercially available 5 nm jI-alumina membranes (i.e., respec­tively. 24 x 10- 12 and 41.6x 10- 12 mol s- 1 m- Ipa- I). Such differences are related to the pore size difference and the tortuosity factor, T, which is linked to the pores distribu­tion and shape factor. For the Ti02 membrane, the nitrogen permeability at 20 °C was 25.5 x 10- 12 mol s-I m- I Pa- I, thus its value of £iT is higher than that of the jI-A1203 membrane. Thi s is confirmed by SEM analysis of the active

' .. a, 'II! 6

.~

'0 .§.4

• He(AI, O,) • N2(AI, O,) x C02(AI,O,) t,. C3H8(AI,O,) <> He(TiOJ o N2(TIO,' + C02(TiO,) o C3H8(TiO~)

2 4 5 6

Fig. I. Penneabilities to He. N2. C02. C3Hg against Win (M: molec ular weigh!) for TiOl and Al20 3 bare membranes.

Picture I. SEM surface analysis of the TiOz membrane (the straighlline is

300 nm long).

layers were one can see larger voids in the Ti02 membrane (Picture I) than in the jI-AIz03 membrane (Picture 2). These voids are not linked together so that, the overall permeability is increased but the flow remains a Knudsen one (i.e., only the tortuosity factor, T, decreases).

4.2. Grafted membranes

4.2.1. TiOz membrane Fig. 2 shows the permeabilities of the different gases as a

function of the mean pressure in lhe membrane. For all gases the permeability is higher than that of Helium. This result suggests that , even if some Knudsen flow still remains, other mechanisms, certainly related to the surface modification, occur for these gases.

Furthermore, the permeability of butane increases from 4 x 10- 14 to 14 X 10- 14 mol m- I s-I Pa- I when Pmc." is equal to 0.85 x 105 and 1.1 x 105 Pa, respectively. This fast increase is characteristic of a multilayer surface diffusion. Thus, for Pmc." <0.85 x 105 Pa monolayer surface diffu­sion is the main mechanism for butane transport and for Pmc," > 0.85 x 105 Pa a multilayer surface diffusion occurs.

Picture 2. SEM surface analysis of the A1203 membrane (the straight line is 300 nm long).

N. Abidi el 01. / Journal o/Membrane Science 270 (2(X)6) 101- 107 t05

:I: He

ON2

OCH4

I>C2H6

DC3H8

-C4Hl0

.C02

••••••••• ••••

2

Me.n Pressurexl0-5 (Pa)

3

Fig . 2. Penneabilities to He. N2, C02, CH4. C2H6. C]Hs. C4HlO gases for the Ti02 grafted membrane.

Methane, ethane and propane have approximately the same permeability than butane at Pme• n <0.85 x 105 Paand exhibit a slight increase until P mean = 2.25 x 105 Pa. So, it can be con­cluded that a monolayer surface diffusion is also the main mechanism for these gases. These results suggest that the Knudsen diffusion pan of the permeability still remains but decreases greatly due to lower value of (elrk) rp geometric factor and that this reduction is not balanced by the surface flow enhancement.

However, the only surface flow is not sufficient to explain the high C02 permeability and it is necessary to also consider the gas transport by a solubilisation-diffusion mechanism as in dense polymer membranes. Effectively, after surface graft­ing most of the pore the CSFI7C2l4Si= chains can block necks. In this situation, the gas transport through the pore necks and at the surface of Ti02 grains is controlled by a solubilisation-diffusion mechanism as in dense polymer membranes. Thus, the overall permeability is the sum of var­ious contributions:

(i) FOk for Knudsen diffusion trough the empty voids which remains connected. Nevertheless, few empty pores still remain in the membrane so that the tor­tuosity is drastically increased and the (£irk) rp

geometric factor lowered. This contribution depends on the square root of the molecular weight so that FOk(He)> FOk(CH4) > FOk(N2) > FOk(C2H6) > FQk(C02) = FOk(C3HS) > FQk(C4 H 10)·

(ii) Fom for a monolayer surface diffusion, which is affected by specific interactions between the fluorinated sur­face and the gas molecules, such interactions, are lumped in the constant C3 of Eq. (5). This contri­bution will increase when the solubility of the gas molecules in the fluorinated chain increases that is to say, when the interactions between the fluorinated surface and the gas molecules increase. C02 is very soluble in fluorinated compounds, alkanes are slightly soluble, Nz and He very slightly soluble 118] so that FOm(C02) >> Fom(alkanes) > Fom(Nz) > FOm(He).

(iii) FOnlUt for a multilayer surface diffusion, which is affected by specific interactions between the fluorinated

surface and the gas molecules, but also by the condens­abiljty of the gas. Here, only C4H to is to be considered.

(iv) FOp forthe gas transport through the pore necks and at tbe surface ofTi02 grajns. This mechanism depends greatly on the solubility of the gas in the fluorinated chain, so that FOp(COz)>> FOp(alkanes) > FOp(N 2) > FOp(He). On the other hand, the diffusion step of this mechanism is improved by the higher mobility of the fluorinated chains.

For each gas the permeability is the sum of these individual contributions, but some of them are jnsignificant for some gases.

4.2.2. AlzOj membrane Butane has too low gas flow to be measured with our

experimental device (minimum permeability detectable for butane: I x 10- 16 mol m- 1 s- 1 Pa- I) and so we can suggest that capillary condensation occurs in this case. This is only possible if the connected pores are smaller than those in the TiOz membrane.

Fig. 3 shows the permeabilities of the different gases as a function of the mean pressure in the membrane. Nitrogen, methane and propane exhibit the same permeabiliy lying between 0.2 x 10- 14 and 0.28 x 10- 14 mol m- I s- t Pa- I

when P me.n is equal to 0 .75 x 105 and 2.25 x lOS Pa, respec­tively. These values are lower than that of helium but are not in the Knudsen ratio, so we can deduce that the mono­layer surface diffusion is smaller in the Al203 grafted mem­brane than in the TiOz one. This situation can be explained by a diminution of the number of " partially filled pores". Such hypothesis also explajns that the permeabilities are lower in the AI20 3 grafted membrane than in the Ti02 one.

For ethane, due to the typical fast rise of the gas flow, we must suppose that the surface flow is important and so that we are just at the beginning of a multilayer surface diffusion.

To explain the high COz permeability it is necessary to consider, as for Ti02 membrane, not only the monolayer sur­face diffusion but also a solubilisation-djffusion mechanism as in dense polymer membranes.

.. 10 ": f(;N2 •••••••••• ";'''!

o ClHI "e; b.C2H6 @!i"F@ (; xH. 1l!:tA# S- oc ... • 5

~ ;t( )( )( )( ;t( :I: 'b ~

)( ;t(;t(;t(;t(;t( .. ~ ®hM~~M 1:lCCC :;; .. .. 0 E 0 2 3 .. Q.

Mean Pressurexl0·' (Pa)

Fig. 3. Pemleabilities to He. N2. C0 2. C~. C2H6. CJHs. C" H to gase'\ fo r the A1zO] grafted membrane.

106 N. I&bidi et al. / JoumaJ of Membrane Science 270 (2006) 101-107

14

12 N

l: 10 ~ > 8

~ a; 8 .. E 4 :.

2

o ·15

X

-

O'* __ (lI:)2)

DIff ....... [17] • .6"""" ..... (11] X,. .... Mt27! 1Ill .......... "'11 -L.toger ..... ~)t' .. +u,..- ..... fC,.II'~

• Oflrl6l_1NJOlI ._-._-.-0:.:

fj.

+ 0 •• ·14 ·13 ·12 ·11 ·10 ·9

log(Pennoabilily (mol..".m·' .Pa'n

Fig. 4. Pennstlectivities Fo(COz)IFo(N2) against the C~ permeability, Fo(COz) compared 10 the literature data.

4.3. Comparison WiTh previous works

In Fig. 4. the permselectivities FO(C02)IFo(N2) against Fo(C~) are given fortwo polymers and several grafted mem­branes reported in the literature.

Compared to polyethylene membrane, our grafted Ti02 membrane exhibits a significantly higher permitivity for the same permselectivity; on the other hand. the Al203 grafted membrane has higher permitivity but less permselectivity.

With polydimethyl-siloxane (PDMS), which is one of the best polymers currently used for solubility-based membrane separation [27], the AI20J grafted membrane exhibits a sig­nificantly lesser permselectivity for about the same selectivity and permeability. Unlike. the Ti02 grafted membrane has higher pennselectivity but lesser permeability than PDMS membrane, so that, these membranes exhibit rather similar performances.

The comparison of our Ah03 grafted membrane with others studies also on grafted Ah03 membranes [l4, 16, 17] shows that these grafted membranes exhibit rather similar performances. Unlike. ourTi02 grafted membrane has higher permselectivity and permeability, so that. this membrane is the best candidate for C021N2 separation.

These results show that the Ti02 grafted membrane is a good candidate for C02 to N2 separation . Furthermore. this membrane allows a good separation of C02 from alkanes, so that it will be more efficient than PDMS for separation of CO2 from mixtures with N2 and alkanes.

5. Conclusions

In previous work in the literature, the surface of inorganic mesoporous membranes were chemically modified using dif­ferent chains oftrichlorosilanes.ln this work. we have shown that modifications of the same kind can be achieved using trimethoxy fluoro silanes. We further demonstrated that the gas permeation properties of such membranes can be con­trolled through the nature of the substrate and oligomer choice. This approach seems ideal for creating solubility

selective membranes for separation of C02 from mixtures with N2 and alkanes. This property is essentially due to the solubility differences of these different gases in fluorinated compounds.

Furthermore, the Ti~ grafted membrane exhibits a mul­tilayer diffusion with butane. So that, with the same alkane chain were butane is more soluble than in our fluorinated chain, one can expect higher butaneIN2 separation efficiency.

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