structural characterization of polyetherimide-based carbon molecular sieve membranes

Structural Characterization of Polyetherimide-Based Carbon Molecular Sieve Membranes

Mehran G. Sedigh, Maryam Jahangiri, Paul K. T. Liu, Muhammad Sahimi, and Theodore T. TsotsisDept. of Chemical Engineering, University of Southern California, Los Angeles, CA 90089

( )Supported carbon molecular sie®e membranes CMSMs were prepared by the pyroly-( )sis of a polyetherimide PEI polymeric precursor. The membranes were characterized

by scanning electron microscopy, energy dispersi®e spectroscopy, and micropore analy-sis, using gas adsorption techniques to relate their microstructure characteristics to theirtransport and separation characteristics. The analysis shows that preparation conditionsdetermine whether the carbon molecular sie®e layer forms within or outside the g -alumina layer of the substrate. Micropore CO and N adsorption analysis of both2 2

( ) ] 10supported and unsupported CMSMs showed a sharp peak at about 3.6 ] 3.8 =10m using the Hor®ath-Kawazoe method. For the supported CMSMs it was obser®ed thatconsecuti®e coatingrcarbonization steps reduced the pore ®olume in the micro- andmesoporous regions, without greatly affecting the ®olume of pores in the range of( ) ] 103.5 ] 6 =10 m. The reduction of the pore ®olume in the micro- and mesoporousregions is accompanied by an increase in the separation factor and a decrease in thepermeance after each coatingrcarbonization cycle. Micropore analysis of a membrane,

( )whose performance had degraded, indicated that its ®olume of pores between 3.6 ] 6=10 ] 10 m had drastically decreased from the corresponding ®alue of the as-preparedmembranes.

Introduction

Ž .The use of carbon molecular sieve membranes CMSMshas been studied in the last few years as a promising alterna-tive technology to more energy-demanding conventional pro-cesses for gas separation applications. A potential such appli-cation involves separation of an adsorptive component such

Ž .as CO or a light hydrocarbon C }C from its mixture2 2 4with less adsorptive components at low to moderate tempera-tures and feed pressures. CMSMs are prepared either through

Žthe carbonization of preexisting polymeric substrates such as.hollow fibers, or self-supporting thin films or through the

carbonization of films deposited on underlying macro- andmesoporous supports by pyrolysis, typically in an inert atmo-sphere. The pyrolysis is carried out at temperatures rangingfrom 773 K to 1,273 K. Depending on the conditions, thepyrolysis process removes part of the heteroatoms, originally

Correspondence concerning this article should be addressed to T. T. Tsotsis.Present addresses of: M. G. Sedigh, Cypress Semiconductor, Research and Devel-

opment Div., 3901 North First St., San Jose, CA 95134; P. K. T. Liu, Media andProcess Technology, Inc., 1155 William Pitt Way, Pittsburgh, PA 15238.

present in the polymer structure, while leaving behind across-linked and stiff, mostly carbon matrix. The CMSMs soprepared have an amorphous microporous structure createdby the evolution of gases as a result of the rearrangement ofthe molecular structure of the starting polymeric precursorduring the pyrolysis process.

Of importance for membranes, including CMSMs, are theirtransport and separation characteristics. These characteris-tics, typically studied by transport experiments, not only re-veal the potential of these membranes for a range of separa-tion applications, but also provide a better understanding ofthe permeation and separation mechanisms in a given mem-brane under various operating conditions. In the last few years

Ž .several groups, including ours Sedigh et al., 1998, 1999 , havestudied the transport and separation characteristics of differ-ent types of supported or unsupported CMSMs for several

Žgaseous mixtures for a brief review of the results of these.studies see, for example, Sedigh et al., 1999 . The motivation

behind all these studies is to optimize the membrane prepa-ration process, to develop membranes with a higher separa-

November 2000 Vol. 46, No. 11AIChE Journal 2245

tion factor and permeance, better operability and stability,and less of a production cost. To accomplish this goal it isimportant to be able to relate the transport and separationcharacteristics of these membranes to their structural charac-teristics. This involves knowing the structural and transportcharacteristics of these membranes as they go through thedifferent stages of preparation. Such an understanding is thekey to achieving a reliable optimized preparation techniqueand more reproducible results.

Several experimental techniques have been used throughthe years in order to gain a better understanding of the struc-ture of the CMSMs. Different types of electron microscopy,

Ž .including scanning electron microscopy SEM and transmis-Ž .sion electron microscopy TEM , are among the most com-

mon techniques. They have been utilized to provide a pictureof the cross section as well as of the surface texture of both

Žsupported and unsupported CMSMs Rao and Sircar, 1993;Limkov et al., 1994a,b; Chen and Yang, 1994; Hayashi et al.,1995, 1996, 1997; Acharya et al., 1997; Steriotis et al., 1997;

.Sedigh et al., 1999 . These techniques are useful to some ex-tent, in evaluating the overall quality of the selective layer,the presence of any macro-rmesoscopic cracks or pinholes,and for determining the apparent thickness of the permselec-tive layer. However, electron microscopy techniques, due tothe limitation of the electron beam width, cannot probe fea-ture sizes in the microporous range. They are therefore un-able to provide meaningful information regarding the pore-size characteristics of the microporous permselective carbonlayer.

In recent years advances in scanned probe microscopyŽ .techniques, such as scanning tunneling microscopy STM and

Ž .atomic-force microscopy AFM , have made it possible to lookat the surface of microporous materials in greater detail thanit was ever possible before using electron microscopy tech-

Žniques Hoffman et al., 1994; Economy et al., 1995; Rao and.Sircar, 1996 . These techniques certainly provide a better un-

derstanding of surface roughness and texture. However,scanned probe microscopy techniques have their own short-comings. Notable among them is the sensitivity of the probe

Žto surface roughness. Since the probe tip has a certain size a.few angstroms typically , it does not penetrate into openings

Ž .pores that are smaller than its size. It therefore does notprobe the microscopic details of the surface. In addition,scanned probe microscopy techniques are known to createimage artifacts. Both electron and probe microscopy tech-niques have the distinct disadvantage of being local tech-niques, and thus not providing overall properties and charac-teristics of microporous materials.

In addition to microscopy techniques, there is anothergroup of techniques available for structural characterizationof microporous materials. These techniques are based on ad-sorption or condensation of different types of probe moleculeswithin the material’s porous structure at a specified pressureand temperature. Adsorption-based techniques have beenwidely used for obtaining the surface area, pore volume, meanpore-size, and pore-size distribution of many different micro-

Žporous materials, including activated carbons Inagaki andNakashima, 1992; Ehrburger et al., 1992; Miura et al., 1993;Koresh, 1993; Kumita et al., 1995; Setoyama et al., 1996;

. Ž .Yoshizawa et al., 1996 , and carbon molecular sieves CMSŽMiura et al., 1993; Jagiello et al., 1995; Nakashima et al.,

.1995; Steriotis et al., 1997 . The main advantage of thesetechniques is that, unlike the microscopy techniques, they arenot local techniques capable only of probing a thin layer nearthe surface. Instead, they are capable of generating overallcharacterization data for the microporous materials of inter-est. These data can then be used for generating a completepicture of the pore network in terms of its pore volume, sur-face area, and pore-size distribution. The adsorption-basedtechniques are, however, limited in the resolution of the poresizes by the size of the probe molecules that are accessibleinto the pore structure. For most membrane applications,however, this does not present a limitation. The main disad-vantage of the adsorption-based techniques is in choosing theappropriate pore model for the interpretation of the adsorp-tion data. Since one does not have a priori knowledge of thepore structure, this makes it difficult to estimate the absolutesize parameters.

From the preceding discussion it should be clear that mi-croscopy and adsorption techniques have their own individ-ual limitations. Applied in combination, however, they pro-vide the best available tools for the characterization of micro-porous materials. We have applied both techniques for thestructural characterization of the CMSMs that have been

Ž .prepared by our group Sedigh et al., 1999 . These are pre-Ž .pared by the pyrolysis of polyetherimide PEI polymeric pre-

cursors supported on meso-rmacroporous supports. The ob-jective of the study was to obtain a better picture of the struc-ture of these membranes, and to create a connection be-tween the structure and the membrane’s separation andtransport characteristics.

Experimental TechniquesMembrane preparation

To prepare the supported CMSMs from PEI precursors,support substrates consisting of cylindrical ceramic tubesŽ .0.007 m ID, 0.010 m OD, and 0.045 m long , manufacturedby U.S. Filter, were dip-coated in a PEI solution. These sup-port tubes have a thin layer of g-alumina, with an average

y9 Žpore diameter of 5=10 m on the inside of the tube Sedigh.et al., 1998 . The average argon permeance, which was used

as a yard stick for appropriateness of substrates, was in thew y6 y5 x 3 2range of 3.481=10 ]3.481=10 m rm ?Pa ? s. To pre-

pare the PEI solution with proper concentration, Ultem 1000Ž . Ž .supplied by GE was dissolved in 1,2 dichloroethane DCE .As is explained later in the following discussion, the concen-tration of the polymeric precursor solution has a significanteffect on where the selective carbogenic layer forms, and onits characteristics. Therefore, for the preparation of theCMSM it is important to select the polymeric precursor solu-tion with the appropriate concentration. Unless otherwisenoted, for all the membranes referred to in this article a 6 wt.% PEIrDCE solution was used for coating the initial sub-strate, and a 2% solution was used for all the following coat-ing stages. After coating, the membrane was dried in air forone day, and was then tested with He and N rAr to make2sure that the layer is free of any cracks and pinholes. Detailsof the coating method and preliminary tests were described

Ž .in detail elsewhere Sedigh et al., 1999 .After coating with the PEI film, the membrane was car-

Ž .bonized in the presence of flowing Ar 99.997% pure in a

November 2000 Vol. 46, No. 11 AIChE Journal2246

cylindrical furnace, 0.0508 m diameter and 0.6096 m long,externally heated and controlled by a programmable OmegaCN3000 controller. The membrane was placed between twothermocouples about 0.05 m apart with the tip of the thermo-couples close to the outside surface of the membrane to makesure that no temperature gradient exists along the axis of the

Ž . y6 3membrane. An Ar flow rate of 1]1.33 =10 m rs was usedto remove the gases evolved during carbonization and tomaintain the inert atmosphere.

The carbonization protocol involved raising the tempera-Ž .ture slowly 0.0167 Krs }to prevent crack formation during

carbonization}and holding it constant first at 623 K for 1,800s, and then at 873 K for 14,400 s. Subsequently, the mem-brane was cooled to 453 K at a rate of 0.0333 Krs, and toroom temperature at a rate of 0.0833 Krs. The coatingrcarbonization procedure was repeated as many times as re-quired to modify the selective layer, in order to achieve thedesired membrane separation factor and permeance. We havealso prepared unsupported pyrolyzed PEI samples. Thesewere prepared according to the same carbonization proce-dure by placing a weighed amount of PEI in a Coors ceramiccrucible in between the thermocouples inside the furnace.

Membrane characterizationŽ .The permeation of single gases CH , CO , Ar, and H4 2 2

and mixtures of the same gases through the carbonized mem-brane was measured after each carbonization step, in orderto evaluate the membrane’s transport characteristics. Detailsof the transport characterization system and the experimen-

Žtal techniques we followed are described elsewhere Sedigh.et al., 1999 .

SEM and EDS studies were performed at the Center forŽ .Electron Microscopy and Microanalysis CEMMA of the

University of Southern California, using a Cambridge 360Ž .SEM instrument Cambridge Instruments, UK . The noncon-

ductive samples, that is, alumina and polymer-coated sup-ports, were prepared for SEM by gold deposition on the sur-face using a DC sputtering technique at 53320 Pa with an Arplasma. Supported CMSMs were studied without any goldsputtering due to their semiconductive carbon layer. For theEDS studies of the polymer-coated substrates, however, thesurface was sputtered with high-purity graphite to prevent theinterference of the gold peak that could have been caused bygold sputtering.

Gas adsorption experiments were performed in a staticmode using an ASAP 2010 micropore analyzer from Mi-cromeritics, Inc., Norcross, GA. The system was equippedwith a turbomolecular pump that was able to create a vac-uum down to 1.33=10y3 Pa. A special analysis port togetherwith a bigger-diameter sample tube were designed and built.This enabled us to measure the pore volume, surface area,and pore-size distribution of the membranes after each car-bonization or degradation step, without the need to crushingthem in order to place them in the standard sample tube.Incremental volume dosing instead of the pressure table,which measures the gas uptake within a certain interval closeto a prespecified relative pressure, was followed in the lowrelative pressure region, that is, for less than 0.1. This is inorder to ensure the accuracy of the measured pore-size dis-tribution in that region of the isotherms. The desired dosing

volume is usually obtained by a preliminary run to measuretotal pore volume at a relative pressure of 0.1. The dosingvolume was found by dividing the measured pore volume bythe total number of points required in that region of theisotherm. Following this method the sample is dosed by acertain amount of gas in each step, which therefore generatesa more accurate pore-size distribution in the low-pressure re-gion, where all the micropores are found. The samples were

Ždegassed on the analysis port after being partially degassed.on the degassing port to take advantage of the ultrahigh vac-

uum created by the molecular drag pump. The degassing ofthe CMSM usually took about 2 days at 473]513 K, while ittook about 3 days to degas the pyrolyzed powder, due to thevery microporous nature of the powder and the diffusion lim-itation of the desorbed gases through the tight pore networkof the powder. Moreover, to eliminate the problems thatmight be caused by helium entrapment inside the microporesduring free-space measurements, the tests were alwaysstopped after the free-space measurement, and the sampleswere degassed under the same conditions for as long as itwas required. The second degassing step took approximately1 to 2 days for the supported CMSMs and CMS prepared bypyrolysis of PEI.

The N adsorption tests were performed at 77 K, and an2isothermal jacket around the sample tube maintained the levelof liquid nitrogen constant for up to one day. For longer runs,which was normally the case for CMSMs and CMS, both thesample Dewar and the cold trap Dewar needed to be refilled.In this case the system was stopped for about one hour inorder to establish equilibrium before resuming the run. TheCO adsorption tests were performed at 273 K using an ice-2water isothermal bath. Compared to the liquid nitrogen tem-perature, this high temperature greatly enhanced the diffu-sion process through the pore network and shortened the du-ration of the analysis. However, due to the very high vapor

Ž 6 .pressure of CO at 273 K, 3.481=10 Pa , and the physical2Ž 5limitations of the system maximum pressure of 1.233=10

.Pa , we were unable to extend the CO analysis beyond a2relative pressure of 0.03. According to the HK model used inour calculations, this relative pressure corresponds to porewidths of up to 6=10y10 m. Therefore, despite its associateddifficulties and long run duration, the N adsorption test was2utilized in routine applications in order to obtain the overallPSD both in the micro- and mesoporous regions.

Results and DiscussionScanning electron microscopy and energy dispersi©espectroscopy

In order to understand the effect of the coatingrcarboniza-tion cycle on the substrate and to find out where the selectivecarbon film is formed and how it modifies the underlyingsupport structure, the cross section of the membrane at dif-ferent stages of the coatingrcarbonization cycle were studiedby electron microscopy. As noted previously, for most of thesamples investigated the first coating utilized a 6% PEI in

ŽDCE solution. For subsequent coatings after the first coat-.ing had been carbonized we utilized a 2% PEIrDCE solu-

tion. The concentrations of the polymeric precursor solutionsand the carbonization procedures followed are the same asthose used for the membranes prepared for the transport and


separation, and gas adsorption characterization experiments.For a number of membranes we also performed the firstcoating using PEI solutions of a higher concentration. Thegoal for these experiments was to identify conditions for whichthe polymeric film will mostly form inside or outside the top-most support substrate layer.

Ž .Figure 1a is a low-magnification 1K SEM picture of across section of the original alumina substrate after it hadbeen gold-sputtered. There are four distinct regions in thisSEM image. The bottom part, with a coarse, granular struc-

Žture, is the macroporous a-alumina layer, with a reported by. y5the manufacturer average pore diameter of about 1.5=10

m. This layer, which is about 1.5=10y3 m thick, provides themechanical strength for the alumina membrane. It is coveredby two other a-alumina layers that appear to have finer gran-

Ž .ular structures, with reported by the manufacturer averagepore diameters of 8=10y7 m and 2=10y7 m, respectively. InFigure 1a, one observes a thin layer on top of these two lay-

Ž . y6ers, with an average thickness of 2]3 =10 m. This layer,

Figure 1. SEM images of the cross section of the alu-( ) ( )mina substrate: a 1 K magnification; b 10

K magnification.

Figure 2. SEM image of the cross section of the poly-( )mer coated substrate: a 1 K magnification;

( )b 10 K magnification.

which at this SEM resolution looks smooth and nonporous, isthe g-alumina layer which, according to the manufacturer, hasan average pore diameter of 5=10y9 m. Figure 1b is a

Ž .higher-magnification 10K picture of the same membranecross section. Although gold-sputtering tends to smooth outsurface features, it is still possible to recognize that the g-alumina layer is indeed porous.

Figures 2a and 2b show the cross section of the membraneafter it had been dip-coated once in a 6 wt. % PEI in DCEsolution at 1K and 10K magnification, respectively. It can beobserved in Figure 2a that the polymer solution has pene-trated beyond the inner g-alumina layer and into the underly-ing 2=10y7-m and 8=10y7-m average pores-size a-aluminalayers. Comparing Figure 1a with Figures 2a and 2b, it is clearthat as a result of the dip-coating process, the void volume inthe g-alumina layer is filled with the polymer. Moreover, it

Žcan be seen from Figure 2b within the resolution in this SEM.picture for the sample dip-coated with the 6% solution there

is no distinct polymer film on the top of the substrate.


Whether the polymeric film completely penetrates the under-lying g-alumina layer or mostly stays on the top depends onthe concentration of the polymeric solution and the condi-

Žtions of dip-coating and, of course, the nature of the under-lying substrate}however, this goes beyond the scope of this

.article . To investigate this issue we have found it helpful tocouple electron microscopy with EDS.

In order to verify the effect that the concentration of thepolymeric solution has on the position of the polymeric layer,we have coated a number of U.S. Filter tubes with polymericsolutions of varying concentration. To investigate the effectof concentration alone we have kept the other dip-coatingconditions constant. The membranes were left in the solutionfor 180 s, after which time they were pulled out at a constant

rate of 2=10y4 mrs. After being dried for 2 days, all themembranes were sputtered with high-purity graphite. Replac-ing gold with graphite removes possible interference from thegold peak in the EDS spectra, and at the same time creates aconductive surface, which is required for electron spec-troscopy.

Figure 3 shows the EDS results obtained by scanning a1.5=10y5=1.5=10y5 m2 surface of three different samplesprepared by the procedure described earlier using 6 wt. %,10 wt. % and 15 wt. % PEI in DCE solutions. The analysis inFigure 3 was done using an electron beam generated by a10-kV accelerating voltage. There is a distinct Al peak, repre-senting the g-alumina substrate, for the sample that was

Ž .coated with the 6 wt. % solution Figure 3a . There are no


such peaks, however, for the samples that were coated withŽ .the 10 wt. % and 15 wt. % solutions Figures 3a and 3b . We

estimate that the electron beam, generated by the 10-kV ac-celerating voltage, penetrates about 10y6 m deep. That theAl peak is completely masked, in the cases of the films cre-ated by the 10 wt. % and 15 wt. % polymeric solutions, isindicative that the film that forms on the top of the g-aluminalayer is at least 10y6 m thick. The very visible peak for thecase of the 6 wt. % solution is indicative that the film in thiscase, if present at all, is significantly thinner than 10y6 m.Figure 4 provides the EDS spectra of the same two 10 wt. %and 15 wt. % samples taken with electron beams generatedwith a 20-kV accelerating voltage, for which we estimate theyhave penetrated roughly 2=10y6 m into the sample. In Fig-ure 4a the Al peak is clearly visible, suggesting that the thick-ness of the polymeric film on the top of the g-alumina layer isconsiderably less than 2=10y6 m. The Al peak size for the15 wt. % sample is less visible, however, suggesting that thepolymer film on top of the g-alumina layer for this case isthicker than that for the 10 wt. % sample, probably about2=10y6 m. These observations are also in qualitative agree-ment with SEM observations of the same samples, thoughwith the latter technique it is much more difficult to pin-point precisely where the polymer film ends and the g-aluminarpolymer layer begins. One conclusion that one drawsfrom these studies is that it is possible to control the positionand thickness of the polymer film by controlling the concen-tration of the polymer solution. A further detailed discussionon this issue goes beyond the scope of this article, however,and will be the subject of a future article on the topic ofreproducibility in the preparation technique of CMSMs.

Figure 5 shows the cross section of the membrane after thefirst round of coatingrcarbonization. As noted previously, theimage is taken without the need for gold-sputtering, since thestructure is conductive by itself due to the presence of thecarbon on the surface. After carbonization, at this resolution,the structure closely mimics the initial support structure, see

ŽFigure 1a note that Figure 5 is upside down when compared.to Figures 1 and 2 . Carbonization appears to have loosened

up the tight structure that was observed after the polymerŽcoating notice the difference in the a-alumina layer in Fig-

.ures 5 and 2a . Figures 6a and 6b show the top g-aluminaŽlayer after it had been coated and carbonized once Figures

. Ž .6a and twice Figure 6b , respectively. Comparing Figures 6aand 6b, it is obvious that the carbonized polymer clearly mod-ifies the microstructure of the g-alumina layer, a result that

Žwas also confirmed by gas adsorption experiments see be-.low . Moreover, comparing Figures 6a and 6b, it is clear that

the additional coatingrcarbonization cycle creates a morecompact and smooth layer, consistent with the observationŽ .Sedigh et al., 1999 that the gas permeance decreases aftereach coatingrcarbonization cycle.

Gas adsorption experimentsTo obtain a better picture of the microstructure of the

CMSMs, and to follow the changes in their pore volume andstructure as these membranes go through various preparationsteps, and in order to relate these to transport and separa-tion characteristics, we have utilized gas adsorption tech-niques. To establish a basis for comparison, we first studied

Figure 4. EDS spectrum of the polymer-coated sam-ples.

Ž .Accelerating voltage 20 kV. Coated with a 10% by weightŽ .PEIrDCE solution; b 15% by weight PEIrDCE solution.

the bare support substrates. Figure 7 represents the pore-sizeŽ .distribution PSD of the original U.S. Filter substrate, ob-

tained by N adsorption at 77 K and analyzed by the BJH2Ž .method Webb and Orr, 1998 using the desorption branch

Ždata the adsorption isotherm is also shown in an insert inthe figure, as well as in Figure 8 in order to provide a ‘‘mod-eless’’ picture of the adsorption characteristics during mem-

. y9brane synthesis . There is a peak at about 5.4=10 m, whichŽ . y6results from the 2]3 =10 -m-thick g-alumina layer, previ-

ously referred to. It is important to note that the weight of


the g-alumina layer, where gas adsorption mainly takes place,Ž y3.is a very small 1.0]2.0=10 fraction of the total weight of

the support substrate. Since the weight of the g-alumina layerwas not precisely known, we opted instead to express thesorption volume in the cumulative and differential pore-volume calculations with respect to the total weight of the0.05-m support tube. As a result, although the gas volumes

Žmeasured are well above the resolution of the instrument and.therefore the resulting PSDs are highly accurate , the calcu-

lated absolute values, reported for cumulative and differen-tial pore volumes, are rather low.

Figures 8a and 8b compare the PSD of a supported CMSM,prepared by three consecutive coatingrcarbonization cycles,with that of the original support substrate, both in the

Ž y9 y8 .mesoporous 2=10 m]5=10 m and in the microporousŽ y9 . Ž-2=10 m regions. This membrane had an ideal ratio

.of single gas permeabilities CO rCH separation factor of2 4about 57, with a CO permeance of 3.22=10y7 m3rm2?Pa ? s.2The gas adsorption analysis in the microporous region wasperformed using N at 77 K with incremental volume dosing.2In the literature there are models that combineadsorption]desorption isotherms with network models of

Ž .pore space and percolation theory Sahimi, 1995 . Thesemodels are usually very accurate, but are very involved andrequire an extensive set of data, especially for the desorptionprocess, in order to be accurate. We do not currently havesuch an extensive data basis, however, and therefore the re-sults were analyzed using the HK method, which assumes a

Ž .slit-shaped pore geometry Webb and Orr, 1998 , likely to bea good assumption for the CMSM, but not so for the originalsubstrate. As can be seen in Figure 8a, for the supported

Ž . y10CMSM there is a sharp peak at about 3.6]3.8 =10 m,

Ž . y10with a substantially smaller peak in the 6]8 =10 -m rangeŽthe presence of peaks in the microporous region is signifi-cant; however, the absolute values for the pore sizes reportedmust be taken with a ‘‘grain of salt,’’ since they are much

.dependent on the model used for data interpretation . Thesevalues are both absent in the initial substrate. Figure 8b showsthe PSD distribution of CMSMs in the mesoporous region.One can notice in this figure that the original g-alumina layer

y9 Žpeak has shifted from 5.4 to 3.8=10 m the presence ofthe carbogenic layer also increases the amount of N ad-2

.sorbed . This observation is consistent with the results of theSEM studies, which show that, under this preparation proto-col, the selective layer forms, mostly inside the g-aluminalayer.

In order to provide a connection between the microporousstructure of the supported CMSM and the original polymericprecursor, gas adsorption experiments were carried out withpowders of the pyrolyzed PEI prepared as described earlier.The pore size distribution of one such powder sample is shownin Figure 9. In this figure we include gas adsorption datausing both CO and N as probe gases at 273 and 77 K, re-2 2spectively. As can be seen, the results of both tests confirmthe presence of a dominant peak at;3.7=10y10 m. Table 1summarizes the data of the total pore volume and BET sur-face area obtained by the CO and N adsorption experi-2 2ments. The CO adsorption experiment at 273 K results in2both higher pore volume and surface area, because CO has2a higher affinity than N for adsorption on carbogenic mate-2rials, and its diffusion at 273 K inside the microporous net-work of CMS materials is considerably faster than that of N277 K. It is interesting to note, however, that both tests indi-cate the presence of the same peak at about 3.7=10y10 m.

Figure 5. SEM image of the cross section of the membrane after carbonization: 1 K magnification.


( ) ( )Figure 6. SEM image of the cross section, 10 K magnification: a after first carbonization; b after second car-bonization.


Figure 7. Pore-size distribution and adsorption iso-therm of support substrate.

In addition, the N adsorption analysis at 77 K also revealed2Žthe presence of a smaller shoulder peak as previously noted,

due to experimental limitations, in our system about, 6=10y10

m is the upper limit of application for the CO adsorption2experiments, and as a result a similar peak could not be de-

.tected using CO at 273 K . A similar shoulder peak was also2previously observed with the supported CMSM, though forthe powders this peak is shifted to about 10y9 m.

We have also used gas adsorption experiments to study theeffect that each additional cycle of coatingrcarbonization hason the microstructure of the resulting membranes. The re-sults for two cycles of coatingrcarbonization for one of thesupported membranes that were prepared are shown in Fig-ure 10. Table 2 summarizes the transport characteristics interms of the single gas permeance and separation factor forthe same membrane after each coatingrcarbonization step.The CO rCH ideal separation factor increased from 1.5 to2 412.5 as the CO permeance decreased by about 50% in value.2By comparing the adsorption data in Figure 10 in the regionof pore diameters under 6=10y10 m, the conclusion is thatthere is little difference between the first and second coat-ingrcarbonization cycles. After the second coatingrcarboni-zation cycle the primary microporous peak remains un-changed, while there is a slight decrease in the peak’s porevolume from 1,750 to 1,600 m3rkg ?m. However, there is adifference between the PSD after the first and second cycles,in that there is a reduction of the pore volume of the poresbigger than 6=10y10 m, extending all the way to the meso-porous region. In our opinion, this explains the increase inseparation factor from 1.5 to 12.5 and also the lowering ofthe permeance.

The direct relationship that exists between the membrane’smicrostructure and its transport characteristics was also stud-ied for the membrane of Figure 8 after its transport charac-teristics had been degraded due to prolonged exposure toheavy oil vapors. Table 3 summarizes the separation factorand CO permeance of this membrane in its original state2

Figure 8. PSD and adsorption isotherm of the CMSM(and PSD of support substrate H CMSM, `

) ( )support substrate : a microporous region;( )b mesoporous region.

and after it had degraded. Table 3 shows both the separationfactor and the CO permeance of the membrane decreased2as a result of degradation of the membrane. Upon the com-pletion of the transport experiments it was theorized that thisis likely to be the result of micropore volume clogging, that is,of the selective pores of the membrane with an average width

Ž . y10of about 3.6]3.8 =10 m, due to the adsorption of or-ganic vapors. This view is consistent with N adsorption data2for both the fresh membrane and the membrane that hadbeen degraded, which are shown in Figure 11. Note the sig-

Ž .nificant reduction of the N adsorption peak in the 3.6]3.82=10y10-m region for the degraded membrane as comparedwith the original fresh membrane. One also concludes that,to have undergone the evacuation of the sample in the micro-


(Figure 9. PSD of the pyrolyzed PEI at 873 K. H CO2)adsorption, ` N adsorption .2

Table 1. Results of Adsorption Experiments

CO N2 23 y4 y5Ž .Pore volume m rkg 1.01=10 8.2=102 5 5Ž .BET surface m rkg 2.45=10 1.573=10

pore analyzer without being desorbed, whatever has cloggedthis micropore region is strongly held there. It should be clearfrom the preceding discussion that there is a close tie be-tween the micro-rmesostructure of the CMSM and its trans-port and separation characteristics. Pore-size distribution andpore volume are a significant indicator of membrane per-formance in terms of its ability to separate gaseous species,which are slightly different in their molecular kinetic diame-ters.

ConclusionsSupported PEI-based CMSMs were studied in an attempt

to relate their transport properties to their structural charac-teristics. A number of structural characterization experimentswere performed with the CMSMs, the support substrates, aswell as with the pyrolyzed PEI powder. SEM and EDS analy-ses were utilized in order to identify the position of the poly-meric precursor film and of the final carbon layer within thesupport’s pore structure. Where the carbon layer is locateddepends on the concentration of the initial polymeric precur-sor and on the rest of the dip-coating conditions.

To confirm the SEM observations, and in order to relatethe membrane’s transport and separation characteristics toits microstructure, micropore volume analysis using the gasadsorption technique was also performed. N adsorption in2the microporous region revealed the presence of a sharp peak

Ž . y10at about 3.6]3.8 =10 m. This peak is absent in the origi-nal support substrate. These are the pores which, in ouropinion, are responsible for the molecular sieving propertiesof the supported CMSM. Micropore analysis of pyrolyzed PEIpowders has shown a similar peak, indicating that this peak is

Figure 10. PSD of CMSM after first and second car-(bonization, ` — first carbonization, H

) ( )—second carbonization : a microporous( )region, b mesoporous region.

Table 2. Transport Characteristics

1st Carbonization 2nd Carbonizationy6 y6CO permeance 2.4505=10 1.149=102

CO rCH sep. fac. 1.5 12.52 4

Permeance: m3rm2?Pa ? s.

Table 3. Membrane Decrease Due to Its Degradation

Original Membrane After Degradationy7 y8CO permeance 3.202=10 3.829=102

CO rCH sep. fac. 57 4.42 4

Permeance: m3rm2?Pa ? s.


( )Figure 11. PSD of original CMSM ` , and degraded( )membrane H in the microporous region.

an intrinsic characteristic of the pyrolyzed polymer. It wasalso observed that after each coatingrcarbonization step,

Ž . y10while the magnitude of the 3.6]3.8 =10 -m peak onlyslightly changed, there was a considerable reduction in thepore volume in the tail of the PSD curve in the micro- andmesoporous regions, which in our opinion indicates a reduc-tion in the number of micro- and mesoscopic size cracks andpinholes. This is probably one of the main reasons for obtain-ing higher separation factors and lower permeances after eachadditional coatingrcarbonization cycle. The study of a mem-

Žbrane, whose properties had degraded due to exposure to.organic vapors , provided similar clues. It was observed, for

example, that after the membrane had degraded, the poreŽ . y10volume in the 3.6]6 =10 -m range drastically decreased,

and both the separation factor and permeance of the mem-brane simultaneously decreased. The reduction of the porevolume was mainly attributed to the adsorption of large or-ganic compounds.

AcknowledgmentWe are grateful to the National Science Foundation and the U.S.

Department of Energy for partial support of this study.

Literature CitedAcharya, M., B. A. Raich, H. C. Foley, M. P. Harold, and J. J. Lerou,

‘‘Metal Supported Carbogenic Molecular Sieve Membranes: Syn-Ž .thesis and Applications,’’ Ind. Eng. Chem. Res., 36, 2924 1997 .

Chen, Y. D., and R. T. Yang, ‘‘Preparation of Carbon MolecularSieve Membrane and Diffusion of Binary Mixtures in the Mem-

Ž .brane,’’ Ind. Eng. Chem. Res., 33, 3146 1994 .Economy, J., M. Daley, E. J. Hippo, and D. Tandon, ‘‘Elucidating

the Pore Structure of Activated Carbon Fibers through DirectŽ .Imaging using Scanning Tunneling Microscopy STM ,’’ Carbon, 33,

Ž .344 1995 .Ehrburger, P., N. Pusset, and P. Dziedzinl, ‘‘Active Surface Area of

Ž .Microporous Carbons,’’ Carbon, 30, 1105 1992 .Hayashi, J., M. Yamamoto, K. Kusakabe, and S. Morooka, ‘‘Simulta-

neous Improvement of Permeance and Permselectivity of 3,3X,4,4X-

Biphoenyltetracarboxylic-Dianhydride-4,4 X-Oxydianiline PolyimideMembrane by Carbonization,’’ Ind. Eng. Chem. Res., 34, 4364Ž .1995 .

Hayashi, J., H. Mizuta, M. Yamamoto, K. Kusakabe, S. Morooka,and S. Sush, ‘‘Separation of EthanerEthylene and PropanerPro-pylene Systems with a Carbonized BPDA-PPX ODA Polyimide

Ž .Membrane,’’ Ind. Eng. Chem. Res., 35, 4176 1996 .Hayashi, J., H. Mizuta, M. K. Yamamoto, K. Usakabe, and S. Mo-

rooka, ‘‘Pore Size Control of Carbonized BPDA-PPX ODA Poly-imide Membrane by Chemical Vapor Deposition of Carbon,’’ J.

Ž .Memb. Sci., 124, 243 1997 .Hoffman, W. P., M. B. Fernandez, and M. B. Rao, ‘‘Characterization

of Porosity in Nanoporous Carbons Using Scanning Tunneling Mi-Ž .croscopy,’’ Carbon, 32, 1383 1994 .

Inagaki, M., and M. Nakashima, ‘‘Irreversible Adsorption of CarbonŽ .Dioxode Gas on an Activated Carbon,’’ Carbon, 30, 1135 1992 .

Jagiello, J., T. J. Bandosz, K. Putyera, and J. A. Shwarz, ‘‘MicroporeStructure of Template}Derived Carbons Studied Using Adsorp-tion of Gases with Different Molecular Diameters,’’ J. Chem. Soc.

Ž .Faraday Trans., 91, 2929 1995 .Koresh, J. E., ‘‘New Molecular Probe Technique for the Assessment

of Ultramicroporous Structure,’’ J. Chem. Soc. Faraday Trans., 89,Ž .2059 1993 .

Kumita, M., J. Ozaki, J. Kobayashi, M. Ito, F. Watanda, and M.Hasatani, ‘‘Characteristics of Helium Adsorption on MicroporousSolids Under Cryogenic Conditions,’’ J. Chem. Eng. Jpn., 28, 159Ž .1995 .

Linkov, V. M., R. D. Sanderson, and E. P. Jacobs, ‘‘Highly Asym-Ž .metrical Carbon Membranes,’’ J. Memb. Sci., 95, 93 1994a .

Linkov, V. M., R. D. Sanderson, and E. P. Jacobs, ‘‘Carbon Mem-branes from Precursors Containing Low-Carbon Residual Poly-

Ž .mers,’’ Polymer Inter., 35, 239 1994b .Miura, K., J. Hayashi, T. Kawaguchi, and K. Hashimoto, ‘‘A Shape-

Selective Catalyst Utilizing a Molecular Sieving Carbon with ShapeŽ .Pore Distribution,’’ Carbon, 31, 667 1993 .

Nakashima, M., S. Shimada, M. Inagaki, and T. A. Centeno, ‘‘On theAdsorption of CO by Molecular Sieve Carbons: Volumetric and2

Ž .Gravimetric Studies,’’ Carbon, 33, 1301 1995 .Rao, M. B., and S. Sircar, ‘‘Nanoporous Carbon Membranes for Sep-

aration of Gas Mixtures by Selective Surface Flow,’’ J. Memb. Sci.,Ž .85, 253 1993 .

Rao, M. B., and S. Sircar, ‘‘Performance and Pore Characterizationof Nanoporous Carbon Membranes for Gas Separation,’’ J. Memb.

Ž .Sci., 110, 109 1996 .Sahimi, M., Flow and Transport in Porous Media and Fractured Rock,

Ž .Chap. 5, VCH, Weinheim, Germany 1995 .Sedigh, M. G., W. J. Onstot, L. Xu, W. L. Peng, T. T. Tsotsis, and

M. Sahimi, ‘‘Experiments and Simulation of Transport and Sepa-ration of Gas Mixtures in Carbon Molecular Sieve Membranes,’’ J.

Ž .Phys. Chem. A, 102, 8580 1998 .Sedigh, M. G., L. Xu, T. T. Tsotsis, and M. Sahimi, ‘‘Transport and

Morphological Characteristics of Polyetherimide-Based CarbonMolecular Sieve Membranes,’’ Ind. Eng. Chem. Res., 38, 3367Ž .1999 .

Setoyama, N., K. Kaneko, and F. Rodriguez-Reinoso, ‘‘Ultramicro-pore Characterization of Microporous Carbon by Low Tempera-

Ž .ture Helium Adsorption,’’ J. Phys. Chem., 100, 10331 1996 .Steriotis, T., A. Beltsios, C. Mitropoulos, W. Kanellopoulos, S. Ten-

nison, A. Wiedeman, and V. Keiderling, ‘‘On the Structure of anAsymmetric Carbon Membrane with a Novolax Resin Precursor,’’

Ž .J. Appl. Poly. Sci., 64, 2323 1997 .Webb, P. A., and C. Orr, Analytical Methods in Fine Particle Technol-

Ž .ogy, Micromeritics Instrument Corporation, Norcross, GA 1998 .Yoshizawa, W., M. Yamada, M. Shiraishi, K. Kaneko, and N.

Setryama, ‘‘Evaluation of Accessible and Inaccessible Micro-porosities of Microporous Carbons,’’ J. Chem. Soc. Faraday Trans.,

Ž .92, 2297 1996 .

Manuscript recei®ed Feb. 15, 2000, and re®ision recei®ed May 30, 2000.


structural characterization of polyetherimide-based carbon molecular sieve membranes

Documents