Single-component gas phase adsorption and desorptionstudies using a tapered element oscillating microbalance

Arjan Giaya, Robert W. Thompson *

Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609-2280, USA

Received 18 January 2002; received in revised form 16 May 2002; accepted 17 May 2002

Abstract

Adsorption equilibrium data, usually obtained using volumetric or gravimetric methods, are important for the

proper design of adsorption systems. In this work, a tapered element oscillating microbalance (TEOM� series 1500

pulse mass analyzer) was used to characterize the hydrophobicity of some microporous materials, and to obtain ad-

sorption isotherms of some chlorinated volatile organic compounds (CVOCs) on those materials. The sorption ca-

pacities of samples of silicalite-1, dealuminated zeolite Y (DAY), and activated carbon for trichloroethylene,

chloroform, and water vapor were determined. A ‘‘hydrophobicity index’’ also was determined by monitoring water

desorption at elevated temperatures.

The adsorption of pure component CVOCs showed that a DAY sample had a higher limiting adsorption capacity

than the silicalite-1 sample. The opposite behavior was observed when the CVOC adsorption was in the presence of

water. These data supported the conclusion that water interferes with the adsorption of CVOCs in these materials, even

though they exhibit significant hydrophobic character.

� 2002 Elsevier Science Inc. All rights reserved.

Keywords: TEOM�; Gas adsorption isotherms; Desorption rates; Hydrophobicity

1. Introduction

The US Environmental Protection Agency(EPA) has defined volatile organic compounds(VOCs) as stable products having a vapor pressureabove 13.332 Pa (0.1 mmHg) at ambient conditions[1,2]. Chlorinated VOCs (CVOCs) are one sub-group of VOCs that contain chlorine, and are

regulated by the EPA due to their toxicity andsuspected carcinogenic properties.

Virtually all CVOCs are synthesized, ratherthan naturally occurring. CVOCs frequently aregenerated as by-products when drinking water isdisinfected by chlorination. In spite of this prob-lem, chlorination is the most commonly used waterdisinfectant in the US. However, the US EPA hasestablished the maximum allowable limit forCVOCs in drinking water, such as the trihalome-thanes (CHCl3, CHCl2Br, CHClBr2, and CHBr3),for example, which may have a combined level ofno more than 80 mg/l. Granular activated carbon

Microporous and Mesoporous Materials 55 (2002) 265–274

www.elsevier.com/locate/micromeso

*Corresponding author. Fax: +1-508-831-5853.

E-mail address: rwt@wpi.edu (R.W. Thompson).

1387-1811/02/$ - see front matter � 2002 Elsevier Science Inc. All rights reserved.

PII: S1387-1811 (02 )00428-6

(GAC) has long been the adsorbent of choice forremediation of CVOCs in drinking water [3,4].

In a previous study [5] we found that hydro-phobic zeolites can offer certain advantages overGAC, especially at very low concentrations in thepresence of liquid water or water vapor. In par-ticular, high-silica ZSM-5, silicalite-1, adsorbedmuch more of certain common CVOCs (trichlo-roethylene (TCE), chloroform (CLF), carbon tet-rachloride (CTC), and tetrachloroethylene (PCE))out of liquid water solutions than GAC at lowCVOC concentrations. While a dealuminated ze-olite Y (DAY) sample performed poorly at lowconcentrations, it adsorbed more of the CVOCsout of liquid water than the silicalite-1 or GACsamples at higher CVOC concentrations. Theyalso showed that the CVOCs appeared to form acondensed liquid phase inside the micropores, i.e.,appearing to have a normal organic liquid densityin the pores.

More recently, Giaya and Thompson reportedon a modified equation of state for water confinedin narrow slit-pores [6] and in cylindrical micro-pores [7]. One of the key results of that work [7]was to show that water can exist as a liquid phasein the pores of a ‘‘hydrophobic’’ DAY sample, butonly as a vapor in silicalite-1 pores having essen-tially the same ‘‘degree of hydrophobicity’’. Theseresults explained why silicalite-1 performed well asa CVOC adsorbent in liquid water, while DAY,penalized by the existence of liquid water in itspores, did not perform as well.

In this work, we examine the inherent affinity ofthe same microporous adsorbents for two CVOCsand water by reporting their adsorption iso-therms and water desorption at elevated temper-atures. A tapered element oscillating microbalance(TEOM�), similar to ones used previously foradsorption and catalysis studies [8–18], was used inthis work. The TEOM� minimizes problems as-sociated with volumetric and gravimetric tech-niques, such as external mass transfer limitations,maldistributed flow patterns, bypassing, buoy-ancy, and other aerodynamic factors. Responsetimes are very short, and control of the gaseousenvironment around the adsorbent is quite goodwith the TEOM�. Heat transfer is facilitated bythe flowing gases.

1.1. Equilibrium data

Adsorption data are often analyzed in thecontext of isotherm equations relating equilibriumloading vs. pressure at fixed temperature. How-ever, the fact that a model fits a given experimentaldata set does not mean that the assumptions andconcepts on which the model was based necessarilygovern the given adsorption process.

Zhu et al. [11] explained the adsorption of lightalkanes in silicalite-1 based on the conventionaland double Langmuir models. The silicalite-1structure contains a three-dimensional network ofinterconnecting straight and zigzag channels. Theopening of silicalite-1 channels are, respectively,0:54� 0:56 nm2 and 0:51� 0:54 nm2 [19]. Thedifference between the channels and the intersec-tions is much larger than differences between thetwo types of channels. Therefore, Zhu et al. [11]suggested using the double Langmuir model todescribe the two types of adsorption sites in sili-calite-1. This model takes into account the geo-metrical constraint of the zeolite pore structure byassuming that the total adsorption capacity isthe sum of the maximum amount adsorbed intochannels qsat;C and in the intersections qsat;I. It alsotakes into account the different interaction poten-tials in the channels and intersections by assigningequilibrium constants for the channels KC, and theintersections, KI. Therefore:

q ¼ qsat;CKCp

1þ KCpþ qsat;I

KIp1þ KIp

ð1Þ

If channels and intersections were indistinguish-able, then Eq. (1) would simplify to the conven-tional well-known Langmuir isotherm:

q ¼ qsatKp

1þ Kpð2Þ

The conventional and double Langmuir isothermswere used in this work to analyze TCE adsorptionon silicalite-1.

1.2. Thermodynamic adsorption parameters

The equilibrium gas pressure is given by theClausius–Clapeyron equation at constant loading.

266 A. Giaya, R.W. Thompson / Microporous and Mesoporous Materials 55 (2002) 265–274

The isosteric heat of adsorption is defined by theequation:

opoT

� �q

¼ Qst

T ðV g � V aÞ ð3Þ

where Qst is the isosteric heat of adsorption, V g isthe molar volume of adsorbate in the gaseousstate, and V a is molar volume of adsorbate in theadsorbed state. Assuming an ideal gas, and ne-glecting V a when compared with V g, Eq. (3) can betransformed to:

oðln pÞoT

� �q

¼ Qst

RT 2ð4Þ

which is more accurate at high temperature andlow loadings. Plotting the adsorption data in theformat ðln pÞ vs. ð1=T Þ enables the estimation ofQst [11,20] as

Qst ¼ �Roðln pÞoð1=T Þ

� �q

ð5Þ

2. Experimental procedures

2.1. Microporous adsorbents

The microporous adsorbents used in this studywere several of the same ones used in the priorwork [5]. Specifically, this study used the samehigh-silica MFI (silicalite-1) from Union Carbide,the same DAY from Zeolyst, and the same Cen-taur� GAC from Calgon. More details of thesematerials were reported previously [5].

2.2. TEOM� pulse mass analyzer

The TEOM� series 1500 pulse mass analyzer(PMA) from Rupprecht & Patashnick (R&P) [21]is an advanced measurement device for the studyof material–gas interactions. The system incorpo-rates the TEOM�, patented by R&P, to measuresmall changes in the mass of an adsorbent while asteady flow of gases passes through it. A TEOM�

was previously used to study catalytic reactions [8–10], and to obtain adsorption data on variousadsorbents [11–18].

The PMA consisted of three major hardwarecomponents: the sensor unit, the control unit, andthe interface computer. A gas delivery and gasanalysis system accompanied the PMA. Fig. 1shows a schematic diagram of the Sensor Unit ofthe PMA, which was constructed of a special pro-prietary glass and housed the tapered element. Oneend of the tapered element was fixed in place, whilethe other end, which held the sample, was allowedto hang free. The carrier gas from the gas manifoldpassed through the sample. A purge gas flow pas-sed down around the tapered element to collect andsweep the reactor effluent out of the system. Thetapered hollow element oscillated at its naturalfrequency by applying a mechanical force througha driver controlled by a sensitive electronic feed-back amplifier. Experimental conditions werecontrolled via the LabVIEW� program.

A gas stream with the desired composition waspassed through the sample, providing good con-tact between the solid and the adsorbate. As ad-sorption occurred in the sample, its mass changed.When the mass of the materials being tested in-creased, the oscillating frequency of the taperedelement decreased, and vice versa. The system wascapable of monitoring frequency changes occur-ring in time intervals of 0.1, 0.2, 0.4 or 0.8 s (userselectable). The mass change between two time

Fig. 1. Schematic presentation of tapered element, the key el-

ement of the TEOM� (adapted from Ref. [21]).

A. Giaya, R.W. Thompson / Microporous and Mesoporous Materials 55 (2002) 265–274 267

intervals, 0 and 1, was determined using the fol-lowing formula:

Dm ¼ K0

1

f 21

�� 1

f 20

�ð6Þ

where K0 is a calibration constant unique to theinstrument’s tapered element, determined experi-mentally, f0 is the natural oscillating frequency attime 0, and f1 is the natural oscillating frequencyat time 1.

Once the frequency was measured by the controlunit and passed to LabVIEW�, the raw mass datawere calculated using the formula given above. Inour experiments, the normalized standard devia-tion usually varied in the range of ð2–4Þ � 10�6.Also, a stable baseline (typically a drift on the orderof 0.01 mg/30 mg over a 10 h period) reflected thestable performance of the TEOM�.

The TEOM� was used to perform temperature-programmed water desorption experiments as wellas to collect adsorption equilibrium data at con-stant temperature and pressure. In both cases,approximately 30–50 mg of adsorbent were loadedin the sample holder. Quartz wool was used tokeep the solid particles firmly in the bed, since aloosely packed bed would prevent the tapered el-ement from oscillating at its natural frequency.Helium was used as purge gas (�50 cm3/min),carrier gas, and as the diluting agent (total carrierand dilute gas flow 25 cm3/min). The gas flowswere controlled via MKS mass flow controllers.The instrument constant, K0, was determined at(or very close to) the experimental conditions. Itwas found that K0 changes with temperature andflow rates were insignificant, even though the os-cillating frequencies were affected.

2.3. Temperature-programmed water desorption

In these experiments adsorbent samples of in-terest were kept in a desiccator for over 24 h. Then,a certain amount of hydrated material was quicklyloaded into the tapered element.

Since the TEOM� measures the mass change,not the absolute mass, establishing a baseline wasrequired. If dry helium was passed through thehydrated sample, then water would start desorbingfrom the sample, and the mass would decrease

immediately. This problem was avoided by flowinghelium saturated with water vapor, instead of dryhelium. However, large mass fluctuations wereobserved, probably due to capillary condensationbetween particles in the bed. Thus, after the sam-ple was loaded into the tapered element, no carriergas was passed through the sample, the instrumentwas allowed to stabilize (usually 5–15 min), and areference line was obtained.

Following this time, dry helium was introducedand immediately the oscillating frequency of thetapered element changed, stemming from two fac-tors. One was the desorption of water, whichchanged the mass of the sample bed, while thesecond effect was the change in the gas environmentin the sample bed. When the air present originallyin the sample bed was replaced by helium, thedensity of the gas changed, which caused the os-cillating frequency to change. The effect of alteringthe gas environment on the oscillating frequencywas quantified by running a reference test.

At the end of the temperature desorption run,the sample was cooled, then dehumidified air dis-placed helium as the carrier gas, and a referenceline was obtained. Then helium was used again asa carrier gas. The noted change in frequency wasdue solely to the change in the gas environment,since there was no mass change taking place. This‘‘apparent mass change’’ was subtracted from thetemperature desorption run to get the actual masschange due only to water desorption.

After determining the apparent mass changedue to the gas environment change, the water de-sorption amount was determined as follows. Withdry helium flowing through the sample, the tem-perature was increased gradually according to apreset program (usually 5 �C/min) up to 550 �C.

Fig. 2 shows a typical temperature profile andweight change curve for an activated carbonsample undergoing desorption. For the tempera-ture profile shown in Fig. 2, the mass curve labeled‘‘hydrated sample’’ appears to go through a min-imum. This is an artifact of the TEOM� technique,since the instrument does not measure the massdirectly. The instrument constant, K0, did not varyappreciably with changes in temperature and gascomposition. However, these changes caused theoscillating frequency to change [21], which was

268 A. Giaya, R.W. Thompson / Microporous and Mesoporous Materials 55 (2002) 265–274

interpreted by the software as a mass change.Therefore, it was necessary to run blank tests toidentify the frequency change due to temperaturechanges of the element and gas flows. Fig. 2 alsoshows the apparent weight change of a dry sample(blank) run using an activated carbon sample. Theactual water loss during this run was found bysubtracting the blank (dry) run signal from thehydrated sample signal, and is shown in Fig. 2 aswell. The temperature-programmed water desorp-tion of two other samples (silicalite-1 and DAY)were carried out in the same manner, and areshown in Section 3.

2.4. Equilibrium adsorption experiments

Adsorption isotherms were obtained for thethree adsorptives (water, TCE, and CLF) and thethree adsorbents (silicalite-1, DAY, and a CalgonCentaur� GAC). The adsorptives were introducedinto the system by purging the carrier gas througha saturator containing pure liquid adsorptive. Thepartial pressure of the adsorptive in the gas streamgoing through the sample bed was controlled bymixing the carrier gas with the diluent (helium)stream. The total flowrate (carrier and diluent) waskept constant. The temperature of the saturatorwas maintained at the desired value with a Fisherscientific heater–cooler system model 9101, equip-ped with a pump for circulating an ethylene gly-

col–water mixture. The partial pressure of theadsorptive in the effluent stream was calculatedbased on the temperature of the saturator (hencethe saturation pressure of the adsorptive at thattemperature) and the carrier and diluent flowrates. In our experiments with CVOCs, the lowestvalue of partial pressure was limited by the lowesttemperature that could be reached in the saturator,and the lowest flow that could reliably be mea-sured with the mass flow controllers available.

Adsorption experiments were started with aclean sample and pure helium as a carrier gas.Then, a small fraction of the carrier gas was di-verted through the saturator. This step change inthe partial pressure of the adsorptive was followedby adsorption in the sample bed, as reflected in theincreased weight of the sample. The partial pres-sure was kept constant until the mass of the samplebed was constant. At that point, the fraction of thecarrier gas going through the saturator was chan-ged to a higher value, and the mass uptake wasmonitored again.

Fig. 3 shows a typical TCE adsorption experi-ment. In this figure the mass changes and the gasflowrates were plotted (‘‘TCE’’ denotes the flow-rate of the carrier gas passing through the TCEsaturator, and ‘‘He’’ denotes the same for the dil-uent stream). Partial pressures were calculated

Fig. 2. Weight change during a temperature-programmed de-

sorption run using the TEOM�. The actual weight loss was

computed by subtracting the hydrated sample curve from the

dry sample, both of which were measured by the TEOM�.

Fig. 3. Adsorption of TCE on the silicalite-1 sample. The solid

line is the mass change reported by the TEOM�; the dotted

lines are the flowrates of the carrier gas going through the

saturator (TCE) and diluent helium stream (He).

A. Giaya, R.W. Thompson / Microporous and Mesoporous Materials 55 (2002) 265–274 269

from the flowrates; the adsorbed amounts werecalculated from the changes in the mass curve.

In a few instances, TCE or CLF were intro-duced by injecting (small) pure liquid in the maingas manifold via an ISCO syringe pump, model100D. In these cases, the carrier gas flowrate waskept constant, and the partial pressure was calcu-lated from the flowrate of pure liquid entering intothe manifold. Adsorption experiments reportedhere were performed at atmospheric pressure, andthe temperature was varied from 25–75 �C.

3. Results and discussion

3.1. Temperature-programmed water desorption

The water temperature desorption data for sil-icalite-1, DAY, and Centaur� activated carbon(GAC) are shown in Fig. 4. From this figure onecan see that all these materials are hydrophobic,adsorbing from 1–5% water by weight. The defi-nition of the hydrophobicity index suggested ear-lier by Giaya et al. [5] was

hi ¼ Vt � V>150

Vtð7Þ

where Vt is the total pore volume of the material,usually determined by N2 adsorption, and V>150 isthe volume of water desorbed at temperaturesabove 150 �C using the bulk liquid density for

adsorbed water. An alternative definition of ‘‘hy-drophobicity’’ using the ‘‘fraction of the porevolume not filled by water’’ at a fixed temperatureand pressure was provided recently [22]. While notprecisely the same as that definition, the expressionfor hi in Eq. (7) is rather similar. Using Eq. (7) andthe water temperature desorption data obtainedwith the TEOM�, hydrophobicity index values, hi,were calculated and are shown in Table 1.

Temperature-programmed water desorptiondata are routinely obtained using thermogravi-metric analysis (TGA). Hydrophobicity indexvalues determined based on TGA data, reportedpreviously [5], are shown in Table 1 as well forcomparison. Based on the data shown in this table,water desorption data obtained from the TEOM�

agreed well with the data obtained with the moreroutinely used TGA.

These data show that all three of these micro-porous materials are about equally hydrophobic,based on this definition. These results alone wouldnot explain why water, both liquid and vapor, in-terfered significantly with the DAY sorption ca-pacity for CVOCs while water was not a significantfactor when using silicalite-1 for CVOC sorption[5]. However, the analysis of water in microporousmaterials recently demonstrated that liquid watermay exist inside DAY pores while only watervapor will exist in the pores of silicalite-1 [7]. Infact, both the strength of fluid–wall interactionsand the micropore diameter are key factors. Thefluid–wall interactions, based on hydrophobicity,were almost the same for these three materials.

3.2. Trichloroethylene adsorption

Adsorption isotherms for TCE on silicalite-1and DAY are shown in Figs. 5 and 6, respectively.

Fig. 4. Temperature-programmed water desorption of DAY,

silicalite-1, and Centaur� activated carbon samples.

Table 1

Values of the hydrophobicity index for silicalite-1, DAY, and

GAC based on temperature-programmed water desorption

measured by TGA [5] and TEOM�

Sample Vt (cm3/g) hi (TEOM�) hi (TGA)

DAY 0.38 0.993 0.994

Silicalite-1 0.21 0.984 0.982

GAC 0.51 0.977 0.972

Vt was determined by BET N2 adsorption.

270 A. Giaya, R.W. Thompson / Microporous and Mesoporous Materials 55 (2002) 265–274

The adsorption data were obtained at 1:01� 105

Pa (1 atm) total pressure and several temperatures.The adsorption capacities of silicalite-1 and DAYfor TCE from the vapor phase at 25 �C comparedwell with their adsorption capacities for TCE fromaqueous solutions when the TCE was present inwater at relatively high concentrations. This sup-ports the conclusion given earlier [5] that, even inthe presence of water, the pores are filled pre-dominantly by organic molecules at sufficientlyhigh CVOC concentration.

Overall, both the silicalite-1 and the DAYsamples had a high inherent affinity for pure TCE.At lower partial pressures, silicalite-1 adsorbedmore TCE than DAY, probably due to its smallerpore diameter which enhanced the TCE–wall in-teraction forces. At higher partial pressures at agiven temperature, the adsorption capacity wasslightly higher for DAY than for silicalite-1. Thisis consistent with the fact that the DAY porevolume is larger than that of silicalite-1. The ad-sorption capacity for both materials decreasedwith increasing temperature.

Silicalite-1 and DAY adsorption capacities forTCE from pure vapor phase were not as differentas adsorption of TCE from aqueous solutions,especially at low TCE concentrations in water.This reiterates the strong influence that water hadon adsorption in DAY when present as a vapor ora liquid.

The TCE adsorption on silicalite-1 data werefitted to the conventional and double Langmuirmodel equations (1) and (2), respectively. The re-sults of these fits also are shown in Fig. 5. Thedouble Langmuir model fits the data better, how-ever, this does not prove that adsorption of TCEon silicalite-1 happens as described by the doublesite Langmuir model. It was expected that thedouble Langmuir model would give a better fitthan the conventional single site Langmuir model,since it has more fitting parameters.

3.3. Chloroform adsorption

CLF adsorption isotherms of silicalite-1 andDAY samples at 25 �C are shown in Fig. 7. Theseisotherms have features similar to the TCE ad-sorption isotherms on these two materials. Thelimiting adsorption capacity of DAY was higherthan that of silicalite-1. However, silicalite-1 wasmore selective for CLF at low partial pressures.

3.4. Water adsorption

Water adsorption isotherms on silicalite-1 at 25,35, 50 and 75 �C are shown in Fig. 8. As alreadynoted by the temperature-programmed water de-sorption data, the silicalite-1 sample was very hy-drophobic. The adsorption capacity of silicalite-1

Fig. 5. Pure-component TCE vapor adsorption isotherms of

silicalite-1 at 25, 35, 50, and 75 �C. Curves are the best fit of theconventional and double Langmuir models.

Fig. 6. Pure-component TCE vapor adsorption isotherms of

DAY at 25, 35, and 50 �C.

A. Giaya, R.W. Thompson / Microporous and Mesoporous Materials 55 (2002) 265–274 271

for water was about an order of magnitude lowerthan its capacity for TCE and CLF. The valuesreported here at 25 �C in Fig. 8 are comparable tothose shown earlier for water vapor adsorption onhigh-silica MFI zeolite, also at 25 �C [22].

It is interesting to note that, based on pure gasphase adsorption, the silicalite-1 and DAY sam-ples were both very hydrophobic, as demonstratedby their low affinity for water. And, they both hadhigh adsorption capacities for TCE and CLF. Yet,these two materials behaved quite differently whenadsorption of these CVOCs out of liquid watersolutions was studied [5]. As noted in more recent

work [6,7] the different performance in the pres-ence of water at low CVOC concentrations wasundoubtedly due to the interference of liquid waterin the DAY pores and the elimination of liquidwater from the pores of silicalite-1. The differentbehavior was due to the �2 �AA difference in porediameters of the two zeolites, and the slightly dif-ferent fluid–wall interaction parameters.

3.5. Isosteric heats of adsorption

From the isotherm data of adsorption of TCEon silicalite-1 and DAY at various temperatures,the isosteric heats of adsorption were determinedusing Eq. (5). Qst values vs. loading are presentedin Fig. 9.

It is observed that for silicalite-1, the isostericheat of adsorption increased monotonically withloading. This suggests that as loading increased,adsorbate–adsorbate interactions became moresignificant with increased crowding, thus contrib-uting to an increase in the isosteric heat of ad-sorption. In the case of TCE adsorption on DAY,the isosteric heat of adsorption stayed more or lessconstant except at loadings close to the saturationcapacity. The essentially constant value of Qst forTCE adsorption on DAY resulted from the largerpore diameter and pore volume compared to thoseof silicalite-1. The more open structure resulted inless crowding of adsorbate molecules, and moresignificant adsorbate–wall interactions at inter-mediate loadings.

Fig. 7. Pure-component CLF vapor adsorption isotherms on

silicalite-1 and DAY samples at 25 �C.

Fig. 8. Water vapor adsorption on silicalite-1 at 25, 35, 50, and

75 �C.

Fig. 9. Isosteric heat of adsorption of TCE vapor on silicalite-1

and DAY.

272 A. Giaya, R.W. Thompson / Microporous and Mesoporous Materials 55 (2002) 265–274

Also, the isosteric heats of adsorption for wateron silicalite-1 were computed as before and plottedin Fig. 10. It is interesting to note that the heats ofadsorption of water on silicalite-1 were essentiallyconstant with loading, and were lower than theheats of adsorption of TCE shown in Fig. 9. Thelower values stemmed from the fact that the ad-sorption loadings were an order of magnitudelower than that of TCE, and therefore the adsor-bate–wall interactions dominated over adsorbate–adsorbate interactions, as in the case with TCE.Additionally, the inherent nature of the silicalite-1surface is hydrophobic and ‘‘organophilic’’, whichresulted in relatively lower adsorbate–wall inter-action energies.

The reason the isosteric heats of adsorption forTCE on DAY and water on silicalite-1 were es-sentially constant, in addition to the prominenceof the adsorbate–wall interactions, was due to theuniformity of the adsorption sites in these twostructures. The similarity of adsorption sites couldbe said to be about equivalent in the range ofloading levels reported in these experiments. Atmuch lower loadings one would expect to finddecreasing values of the isosteric heats of adsorp-tion with increased loading as the stronger siteswere occupied first. At higher loading levels,adsorbate–adsorbate interactions become moresignificant. This behavior has been observed forTCE and other CVOCs on silicalite-1 previously[23].

The values reported for Qst for TCE and wateradsorbed on silicalite-1 obtained here through

adsorption isotherms via the TOEM� are in agood agreement with the isosteric heats of ad-sorption obtained by Garrot et al. [23] using acalorimeter. However, Garrot et al. [23] reportedvalues over a wider range of adsorbed loadinglevels. Additionally, the values reported for the Qst

of water vapor on silicalite-1 here are very close tothose reported previously [22] in their Fig. 8.

4. Conclusions

A TEOM� was used to obtain temperature-programmed water desorption data. The dataobtained with the TEOM� were identical to thewater desorption data obtained with a conven-tional TGA instrument. All three samples studiedhere (silicalite-1, DAY and Centaur� activatedcarbon) had a very low affinity for water. Theirinherent hydrophobicity was quite high as com-puted by our definition [5], i.e., by Eq. (7).

The TEOM� also was used to generate ad-sorption isotherms of gases on microporous solids.The amount of CVOCs adsorbed at saturation wasvery close to the void volume of silicalite-1 andDAY. Both silicalite-1 and DAY had high af-finities for TCE and CLF vapors. These resultsreinforced the prior conclusions regarding thedisruptive influence of water in DAY pores, inspite of its inherent hydrophobicity [5,7].

Values of isosteric heats of TCE adsorption onsilicalite-1 and DAY were obtained based on ad-sorption isotherms at three or four different tem-peratures. These values seem to agree well withvalues obtained with a calorimeter [23]. Isostericheats of adsorption for water on silicalite-1 weresomewhat lower and essentially constant, andconsistent with values reported previously.

Acknowledgements

The authors acknowledge support from theNational Science Foundation through grant DGE-9355019, and the support of Zodiac Pool Care,Inc. of Pampano Beach, FL. The authors acknowl-edge Prof. R. Datta for the use of the TEOM�.We acknowledge Dr. F. Syed and Dr. M. Serban

Fig. 10. Isosteric heat of adsorption of water vapor on silica-

lite-1.

A. Giaya, R.W. Thompson / Microporous and Mesoporous Materials 55 (2002) 265–274 273

for assistance with the TEOM� measurements andfor helpful discussions. Finally, we acknowledgeProf. Steven Suib at the University of Connecticutfor the use of TGA facilities.

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