Journal of Ceramic Processing Research. Vol. 10, Special 1, pp. s9~s13 (2009)

s9

J O U R N A L O F

CeramicProcessing Research

Application of photoacoustic spectroscopy to spatially resolved, non-destructive

measurements of organics distributed in zeolite membranes

Chang Hyun Koa, Jong-Seong Baeb, Jeong Hyun Yeumc, Namhyun Kangd, Yeong-Do Parke, Young-Seok Kimf,

Jae-Won Leeg, Sankar Nairh and Weontae Ohi,*aGreenhouse Gas Research Center, Korea Institute of Energy Research, Daejeon 305-343, KoreabNano-Surface Technology Research Team, Korea Basic Science Institute, Busan 609-735, KoreacDepartment of Natural Fiber Science, Kyungpook National University, Daegu 702-701, KoreadDepartment of Materials Science and Engineering, Pusan National University, Busan 609-735, KoreaeDepartment of Advanced Materials, Dong-eui University, 614-714, KoreafDongnam Technology Service Division, Korea Institute of Industrial Technology, Busan 618-230, KoreagEnergy Materials Center, Korea Institute of Ceramic Engineering & Technology, Seoul 153-801hSchool of Chemical & Biomolecular Engineering, Georgia Institute of Technology, GA 30332, USAiDepartment of NanoEngineering, Electro Ceramic Center, Dong-eui University, Busan, 614-714, Korea

Step-scan photoacoustic (SS PA) FT-IR experiments were performed to investigate the distribution of guest moleculesembedded in a MFI zeolite membrane. Concentration profiles of guest molecules in a membrane were determined usingtheoretically-developed analysis methods combined with experimental SS PA FT-IR data. For an actual application, wedemonstrated spatially resolved, in situ, nondestructive measurement of the concentration distributions of mixed organicmolecules penetrating through a membrane system. In addition, it is shown that SS PA spectroscopy can be applied toinvestigate the sorption/desorption behaviors of volatile organic molecules in a membrane system.

Key words: Membrane, Photoacoustic, Nanoporous material, Non-destructive measurement, Concentration profile, Adsorption,Desorption.

Introduction

Nanoporous membranes deposited from materials suchas zeolites or mixed oxides are receiving much attentionfor industrially-important applications of separations,catalytic membrane reactors and templates for nanostructuredmaterials [1-4]. Their attractiveness is due to the existenceof a well-defined, ordered nanopore structure [1, 4].Knowledge of the membrane composition (includingconcentration of guest molecules) and transport propertiesas a function of depth is highly desirable. Quantitative probesof the cross-sectional structure (e.g., electron microscopy,energy-dispersive X-ray analysis, and confocal microscopy)are destructive and/or ex situ. Besides, the study of membranetransport phenomena depends heavily on theoretical models(e.g., the Maxwell-Stefan equations) [5]. We are developinga spatially-resolved, quantitative, non-destructive, in situ,measurement technique for the concentration profile andthe transport of organics distributed in a membrane, basedon step-scan photoacoustic spectroscopy (SS PAS) [6, 7].The most important application in this study is the

transport-model-independent analysis of membrane transportby simultaneous measurement of the permeant concentrationprofile, membrane thickness, and trans-membrane flux.

In SS PAS experiments, IR radiation modulated at anacoustic frequency (5-1000 Hz) is absorbed by a sampleand converted into a thermoacoustic signal originatingfrom a cumulative region of the sample down to a certaindepth, m

s (sampling depth or thermal diffusion length).

Because the sampling depth, ms is directly related to

the modulation frequency, f as ms= (α/πf)1/2 [8-10],

depth-dependent thermoacoustic signals can be obtainedby varying the modulation frequency. Here α is the thermaldiffusivity (m2/s) of the material. Step-scan interferometerallows the chosen modulation frequency to provide thesame sampling depth over the entire spectral range.

Here we report non-destructive, in situ measurementsof concentration profiles of organic molecules distributedthrough a nanoporous zeolite membrane using SS PAS.In addition, we experimentally demonstrate that SS PAScan be applied to study the adsorption/desorption propertiesof volatile organic molecules in a membrane system.

Experimental

A macroporous substrate for membrane growth was

*Corresponding author: Tel : +82-51-890-1721Fax: +82-51-890-2631E-mail: wtoh2005@deu.ac.kr

s10 Chang Hyun Ko, Jong-Seong Bae, Jeong Hyun Yeum, Namhyun Kang, Yeong-Do Park, Young-Seok Kim, Jae-Won Lee, Sankar Nair and Weontae Oh

made from a-alumina (Alcoa A-16) powder, and thesynthesis of the silicalite (MFI) seed suspension and themembrane growth on the seeded substrate were conductedas described in previous papers [6, 11]. A zeolite MFImembrane was hydrothermally grown on a silicaliteseed-deposited a-alumina substrate by secondary growth;A growth solution of composition 76.6 g DI water:0.235 g KOH (Fisher Scientific): 1.08 g TPABr (Aldrich):3.77 g TEOS was used. The membrane was grown for48 h, in which TPA molecules (structure-directing agent,SDA) were evenly distributed along with the membranethickness. The membrane was washed repeatedly withhot deionized water after growth. In order to removeTPA the hydrothermally-grown membrane was calcinedat 500 oC for 6 h.

A sample cell equipped with the calcined membranewas set up as shown in Fig. 1 to carry out SS PASexperiments on an IFS-55 FT-IR spectrometer (Bruker)with an MTEC 300 photoacoustic detection module.Helium gas (99.9995%) was used as PA signal transfergas and a carbon-black substrate as a standard reference.Mixed vapors of two isomers (normal- and cyclo-hexanes)were used and transferred into a membrane cell withHe carrier gas for simultaneous permeation and SSPAS measurements. The mixed hexanes/He stream wascirculated in the upper chamber, whereas pure He wascirculated in the lower chamber to sweep away thepermeating molecules. The partial pressure (P/Psat) of guestmolecules was adjusted to be 0.14 by a combination ofa saturated mixed hexanes/He stream with the pure Hediluent. The feed rate of mixed vapors/helium mixed gasesand the sweep rate to remove the permeant gases werefixed at 3.3 mL/minute.

For the adsorption/desorption experiments of guestmolecule the partial pressure (P/P

sat) of the guest (p-xylene)was also adjusted by the amount of He gas in the mixture.A reference spectrum was collected at the same partial

pressure in which a sample spectrum was obtained. Thefeed rate of p-xylene/helium mixed gases and the sweeprate were fixed at 3.3 mL/minute, whereas desorptionmeasurements were subsequently conducted with a purehelium gas purge of 60 mL/minute after the adsorption run.

The spectra were collected over 4000-400 cm−1 witha resolution of 8 cm−1 and double-sided interferograms.Modulation frequencies were chosen in the range of 16-488 Hz and with an amplitude of 4λHeNe. For the sorption/desorption measurements of p-xylene, the modulationfrequency was fixed at 101 Hz to constantly set a samplingdepth through the entire experiments. After the SS PASexperiments, the membrane was cross-sectioned andimaged by a FE-SEM (Hitachi S800, 10 kV).

Results and Discussion

In the case of PAS measurements on the guest-sorbedmembrane, guest vapors mixed with He carrier gas werefed into the upper region (I) and then pure He gas enteringinto the bottomed region (II) swept out the permeantsas shown in Fig. 1. The signal was detected by a sensitivemicrophone and transformed to an IR spectrum. In orderto analyze the depth-resolved photoacoustic signals ofthe membrane sample, some analytical expressions forphotoacoustic signal intensity were developed in previousstudies [6, 7]; for a thermally-homogeneous and optically-heterogeneous sample, the PA signal intensity (Q) froma certain sampling depth (m) can be expressed as:

(1)

where K is a system-dependent constant, β0 is theoptical absorption coefficient of the host membrane, β1

the optical absorption coefficient per unit concentrationof the guest species for a guest-adsorbed membrane, andC(x) the depth-dependent concentration of the guest species.

Q Kβ0 x– μ⁄( )exp0

μ

∫ dx Kβ1 C x( ) x– μ⁄( )exp xd0

μ

∫+≈

Fig. 1. Schematic of the SS PAS experiment and cross-sectioned MFI zeolite membrane (adapted from Ref. 7): (a) frequency-modulated IRbeam (b) acoustic wave; Region I is feed side and IV sweep side. MFI membrane (II) was grown and calcined on a macroporous α-aluminasubstrate (III). The right-side schematic shows the N-discretized sublayers of the membrane, in which guest molecules are assumed to beuniformly distributed in each sublayer.

Application of photoacoustic spectroscopy to spatially resolved, non-destructive measurements of organics... s11

If we assume that the membrane with a continuouslyvarying distribution of guest molecules is discretized intoN sublayers as shown in the right-sided schematic ofFig. 1, PA signal intensity (Q) reduces to:

(2)

Here the first and the second terms are contributedfrom membrane and guest molecules, respectively. Therefore,the ratio of Q(guest) and Q(membrane), R(mN)acc isexpressed by:

(3)

where C’(i) represents a reduced guest concentrationin each layer and is expressed by:

(4)

Then we can estimate the accumulative ratio of Qguest

and Qmembrane by an hierarchical calculation. With extremesimplification such as a uniform distribution of guestorganics along with a membrane thickness, however, theratio of signal intensity from the guest and the hostmembrane reduces to be a constant as Qguest/Qmembrane =β1C0/β0. We already demonstrated that the concentration

profiles of the guest-adsorbed membrane were successfullyestimated and analyzed with the above equations developedby previous studies [6, 7].

Fig. 2 shows the SS PA spectra of hexanes adsorbedin a MFI membrane at a modulation frequency 101 Hz.In superimposed two spectra of normal- and cyclo-hexanes,characteristic signals (dotted line) of n-hexane can bedistinguished from those (solid line) of c-hexane as shownin the insets of Fig. 2(a). The characteristic IR signalsof the hexane/MFI complex are assigned on the basis ofthe literature [12, 13]; C-H bendings and stretchings ofn- and c-hexane are strongly apparent at 1300-1500,and 2800-3000 cm−1. The vibrations of the membraneframework are in a range 1250-800 cm−1. In order toanalyze quantitatively the concentrations of guest moleculesas a function of depth, signals from a n-hexane shouldbe isolated from those of c-hexane. A series of SS PASobtained from a n-hexane-adsorbed MFI membrane arepresented in Fig. 2(b), in which PA spectra were measuredat modulation frequencies from 27 to 488 Hz. Twocharacteristic signals of n-hexane assigned as signals Aand B were deconvoluted as shown in the inset of Fig. 2(b),and the ratios (B/A) were found to be 0.349 ± 0.018regardless of modulation frequency. This result indicatesthat the signal from c-hexane could be separated from thesignal component (A) of n-hexane, which was overlappedwith that of c-hexane, on the SS PA spectrum collectedfrom mixed vapors of n- and c-hexanes.

Fig 3(a) shows the representative SS PAS data measuredwith a binary mixed vapor of n- and c-hexanes (5/5 v/v)at modulation frequencies from 19 to 499 Hz. Theexperiments were conducted at a steady-state conditionwith a feed partial pressure (P/Psat) of 0.14. As themodulation frequency increases, the sampling depth andthe PA signal intensity correspondingly decreases. In orderto analyze these spectra, the integrated intensities of the1383, and 1465 cm−1 (n- and c-hexanes) and 797 cm−1

(membrane) were used for the quantitative analysis. Each

Q Kβ0μn 1 1 e⁄–( ) Kβ1 C i( )μn eμi 1–

– μn

⁄e

μi

–( ) μn

⁄–( )

i 1=

N

∑+≈

R μN( )accQguest

Qmembrane

---------------------⎝ ⎠⎛ ⎞

N

=

Kβ1μN C i( )i 1=

N

∑ eμi 1–

μN

⁄–

eμiμN

⁄–

–( )

Kβ0μN 1 1 e⁄–( )-----------------------------------------------------------------------=

β1β0-----

⎝ ⎠⎛ ⎞

C i( )i 1=

N

∑ eμi 1–

μN

⁄–

eμiμN

⁄–

–( )

1 1 e⁄–( )-------------------------------------------------------=

C' i( )β1β0-----

⎝ ⎠⎛ ⎞C i( )=

Fig. 2. (a) A representative SS PA spectra collected at a modulation frequency of 101 Hz from the hexane-adsorbed MFI membrane; dottedand solid lines indicate the spectra taken from n- and c-hexane-adsorbed membranes, respectively. The partial pressure (P/Psat) of vaporizedhexane was 0.14. The insets of this figure show the enlarged parts of characteristic signals of hexane isomers. (b) A series of SS PA spectraof a n-hexane-adsorbed MFI membrane collected at various modulation frequencies from 27-488 Hz. This spectral range belongs to thedotted square of Fig. 2(a). The inset shows the representative peak deconvolutions of signal components A and B.

s12 Chang Hyun Ko, Jong-Seong Bae, Jeong Hyun Yeum, Namhyun Kang, Yeong-Do Park, Young-Seok Kim, Jae-Won Lee, Sankar Nair and Weontae Oh

signal component was deconvoluted as described in ourprevious study [6]. R(mN)acc (eq. 3) is obtained from theaccumulated ratio of signal intensities on the guestspecies and the host membrane, using the SS PA spectrumcorresponding to each sampling depth. After this, themixed signals of guest species (n- and c-heanxes) shouldbe separated from each other. The extraction of the guestconcentration along with sampling depth followed theprocedure described in our previous study [7]. At thesmallest μ, C’(1) is obtained. C’(1) is used for deter-mination of C’(2). This procedure is iterated until theinterface of the membrane and substrate is reached. Wehave prepared two more binary mixed vapors of n- andc-hexanes with 2/8 and 8/2 (v/v) and analyzed them asdescribed above. The concentration profiles of a seriesof binary mixed vapors (n- and c-hexanes) are presentedin Fig. 3(b). As mentioned in the literature [1, 2, 11],MFI zeolite membranes grown on macroporous aluminasubstrates have nanopore channels perpendicularly alignedto the substrate. Therefore, n-hexane is expected to penetratefaster than c-hexane by considering the cross-sectionalsteric effect of the n-hexane isomer. The concentrationprofiles of Fig. 3(b) supported this concept; n-hexane (opensquare symbols and their lines) exhibits higher concentrationsthan c-hexane (closed circle symbols and their lines)through the entire depth. The concentration of n- and c-hexanes gradually decreases over a depth of 100 μm. Asone component loading in a mixture composition increases,the entire profile shifts upwards due to the increased chemicalpotential of the organic vapor on the feed and sweep-outsides. These profiles were obtained from SS PAS datameasured during in situ permeation experiments on theguest molecules-adsorbed membrane without any destructionof the membrane sample. These concentration profiles showthat we can investigate the detailed information on transportbehavior on the molecules adsorbed in a membrane.

Fig. 4(a) shows the PA spectra of a p-xylene-adsorbedMFI zeolite membrane, which has been calcined at 500 oCfor 6 h to remove TPA molecules. The dotted line spectrumin this figure was obtained from the calcined membrane.As the time of p-xylene adsorption increased, C-H stretchings

(2800-3100 cm−1) from p-xylene grew strongly. Thegrowth of signal intensities is due to the increase of p-xylene adsorption. The investigation of the adsorption/desorption characteristics of p-xylene on MFI zeolitemembrane was conducted in situ during the PAS meas-urements with feeding of a p-xylene-mixed He carriergas as shown in Fig 1(a). The signal intensity ratios ofp-xylene/membrane as a function of time were plottedin Fig. 4(b) and 4(c), in which we found that it tookover 20 h to fully saturate p-xylene into this membranebut the adsorbed gases have immediately begun to bedesorbed from the membrane on the desorption run,resulting in stabilization at a low intensity ratio in 5 h.However, the intensity ratio does not reach zero. Thisindicates that the adsorbed p-xylenes remain in themembrane at a desorption time of over 25 h. In order toremove the strongly-bound adsorbed molecules, high extra

Fig. 3. (a) A series of SS PA spectra of a binary system (n- and c-hexanes)-adsorbed MFI membrane collected at various modulationfrequencies from 19-488 Hz. The binary system consists of n- and cyclo-hexane mixture (5/5 v/v). Two dotted lines indicate the A and Bsignal components of Fig. 2. (b) Concentration profiles of n- and c-hexanes permeating in a membrane. This figure shows the open squaresymbols and their lines for n-hexane, and closed circle ones for c-hexane. The continuous curves are only a guide for the eye.

Fig. 4. (a) A series of SS PA spectra collected at different p-xylenesorption on the calcined MFI membrane; the dotted line spectrumwas obtained from the pure calicined MFI membrane. (b) and (c)the signal intensity ratios of p-xylene/membrane as a function oftime during the sorption and the desorption processes, respectively;the solid lines are only guides to show the curve change.

Application of photoacoustic spectroscopy to spatially resolved, non-destructive measurements of organics... s13

energy such as heat, and pressure is supplied to themembrane. Conclusively we found from these results thatthe adsorption of p-xylene into a MFI membrane graduallyincreased to reach the saturation level, but desorptionof p-xylene from the membrane promptly appeared andleveled off in a few hours.

Conclusions

We have demonstrated the spatially-resolved, non-de-structive measurements on guest molecules adsorbed ina MFI zeolite membrane. The characterization method usedin this study has successfully analyzed the concentrationdistribution of each guest component on the mixed vaporsof n- and c-hexanes adsorbed in a membrane. In addition,it has found that the PA experiment is applicable toinvestigate the adsorption/desorption behavior of guestmolecules on a membrane system. The adsorption of p-xylene into a MFI membrane was relatively slower thanits desorption from the membrane. In particular the useof PAS is highly desirable in understanding transport innanoporous materials and membranes because this methodis available to monitor in situ the adsorption/desorptionprocesses in the membrane system.

Acknowledgements

This work was supported by the Korea Research

Foundation Grant funded by the Korean Government(MOEHRD, Basic Research Promotion Fund) (KRF-2006-311-D00384). The authors also acknowledge thepartial support from ECC at Dong-eui University as aRIC program of ITEP under MKE and Busan City.

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