Quantitative Study of the Adsorption of Aromatic Hydrocarbons

Instituto de Ciencias de la Universidad Auto´noma de Puebla, Edificio 76, ... Universidad Auto´noma Metropolitana-Iztapalapa, P.O. Box 55-534, D.F.,...
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Ind. Eng. Chem. Res. 2005, 44, 2908-2916

Quantitative Study of the Adsorption of Aromatic Hydrocarbons (Benzene, Toluene, and p-Xylene) on Dealuminated Clinoptilolites M. A. Herna´ ndez,* L. Corona, and A. I. Gonza´ lez Departamento de Investigacio´ n en Zeolitas and Postgrado de Ciencias Ambientales, Instituto de Ciencias de la Universidad Auto´ noma de Puebla, Edificio 76, Complejo de Ciencias, Ciudad Universitaria, CP 72570 Puebla, Me´ xico

F. Rojas and V. H. Lara Departamento de Quı´mica, Universidad Auto´ noma Metropolitana-Iztapalapa, P.O. Box 55-534, D.F., Me´ xico

F. Silva Fisicoquı´mica de Materiales and Postgrado de Ciencias Ambientales, Instituto de Ciencias de la Universidad Auto´ noma de Puebla, CP 72570 Puebla, Me´ xico

Adsorption of benzene, toluene, and p-xylene (BTX) on dealuminated clinoptilolites was investigated in the temperature range of 398-498 K by way of the inverse gas chromatography method. The dealuminated zeolites were prepared via a chosen number of acid treatments of natural clinoptilolite precursors with aqueous HCl. Both natural and acid-leached substrates were characterized by X-ray diffraction, N2 adsorption at 76 K, and BTX adsorption. The Freundlich, Langmuir, and Dubinin-Astakhov adsorption models were found to approximately fit the BTX adsorption data, within the selected temperature range. The uptake degree of these aromatic hydrocarbons was temperature-dependent. Additionally, the degree of interaction between these BTX vapors and the dealuminated clinoptilolite samples was examined through the evaluation of the isosteric heat of adsorption by employing the Clausius-Clapeyron equation. Introduction Among natural zeolites, clinoptilolite is the most abundant one. This zeolite is a member of the Heulandite family, with a molar ratio Si/Al > 4. An average of 22 water molecules composes the unit cell, with Na, K, Ca, and Mg as the most common charge-balancing cations. Representative unit cell parameters for the (Na1.84K1.76Mg0.2Ca1.24)(Si29.84Al6.16O72)‚21.36H2O form are a ) 17.662 Å, b ) 17.911 Å, c ) 7.407 Å, and R ) 116.40°.1 Gas or vapor molecules can penetrate the crystalline structure through a series of intersecting channels; each layer of these channels is separated by a dense, gas-impermeable layer of tetrahedral crystals. This 2-D micropore channel system was first characterized for Heulandite. Channels A (10-membered rings) and B (8-membered rings) run parallel to each other and to the c axis of the unit cell, while channel C (8membered rings) lies along the a axis, intersecting both the A and B channels (see Figure 1). The ellipticalshaped 8- and 10-membered rings that constitute the channel system are nonplanar and, therefore, cannot be simply dimensioned. The selectivity and uptake rate of gases by clinoptilolite zeolites are influenced by the type, number, and location of the charge-balancing cations residing in the A-C channels. Specification of cation and water locations as well as coordination of these species within the structure is still under investigation. The positions adopted by exchangeable cations and channel water molecules are interdependent. Variations in the cation composition cause changes in the amount and structural distribution of water molecules; likewise, changes in the channel water content affect the positions of the channel cations.2 The micropore * To whom correspondence should be addressed. E-mail: [email protected].

Figure 1. Schematic representation of the 2-D channel arrangement in clinoptilolite. The cross-sectional dimensions (in nanometers) as well as the orientations of channels A-C are depicted.

widths inside these clinoptilolite solids can correspond to the following types: ultramicropores (diameters < 0.7 nm) and supermicropores (diameters between 2 and 0.7 nm).3 Nonetheless, the presence of pores larger than supermicropores in natural and dealuminated zeolites can be attributed to the existence of impurities or to the partial destruction of the zeolitic matrix during the dealumination process.4 Partially dealuminated zeolites have useful properties, such as improved pore volume (with respect to natural precursors), good hydrothermal stability, excellent catalytic activity (especially during cracking reactions), and fair hydrocarbon isomerization rates. The dealumination process of zeolites by hydrothermal

10.1021/ie049276w CCC: $30.25 © 2005 American Chemical Society Published on Web 03/23/2005

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treatment or acid leaching modifies the innermost structure of these substrates more than a direct chemical exchange can do if employing, for instance, SiCl4 or (NH4)2SiF6. It is generally agreed that steaming not only induces the removal of aluminum from the zeolitic framework but also gives rise to extraframework aluminum species more or less bounded to oxygen atoms and/or hydroxyl groups. The vacancies created by the removal of tetracoordinated Al atoms are filled by Si atoms proceeding from other parts of the zeolite. When a large number of Si atoms have left a given region of the zeolite, the structure may collapse. This leads to the formation of an important secondary porosity, with a wide distribution of pore diameters.5 It is, therefore, necessary to employ characterization techniques such as N2 adsorption-desorption, Hg porosimetry, and electron microscopy to detect this secondary porosity. Other analysis techniques have also been found useful for studying dealuminated zeolites, e.g., IR spectroscopy and X-ray diffraction (XRD) as well as solid-state NMR by observation of 29Si, 27Al, and 129Xe nuclei.6 Ackley et al.7,8 have written a literature review regarding the structure, composition, and ion-exchange properties of clinoptilolite, as well as the adsorption and diffusion processes of different species taking place on and through this natural zeolite. Khulbe et al.9 studied the interaction of H2S, CO, and SO2 over activated (dealuminated) clinoptilolite by electron spin resonance spectrometry. Herna´ndez et al. have performed high-resolution adsorption studies on Mexican clinoptilolites mined from different locations.10 Springuel et al. examined the properties of some dealuminated zeolites.11 In this study, it was found that the presence of molecules, cations, clays, quartz, or amorphous glassy materials inside a zeolitic structure can severely block the pores or can even cause the partial destruction of the crystalline arrangement, thus drastically lowering the adsorption capacity by diminishing the micropore volume accessible to adsorbable species. During the past few years, there has been an increased interest in the sorption properties of clinoptilolites with respect to various pollutant hydrocarbon gases ever since these zeolites constitute natural low-cost porous materials. The adsorption capacity of a zeolite seems to be a function of the Si/Al atomic ratio.12 Zeolites enriched in aluminum are hydrophilic and show little selectivity toward organic compounds, while high-silica zeolites are hydrophobic and tend to attract more strongly the organic compounds than their dealuminated counterparts.13 In this work, clinoptilolite samples mined from diverse locations in Puebla, Mexico, are studied with respect to their adsorption behavior toward benzene, toluene, and p-xylene (BTX) hydrocarbon uptake. Both natural and dealuminated clinoptilolites are employed for these sorption studies. The adsorption isotherms are measured at different temperatures by means of the gas chromatography (GC) peak maxima technique.14 The isosteric heats of adsorption at different adsorbate loadings are evaluated from the corresponding adsorption isotherm data through the Clausius-Clapeyron equation. The paper is organized as follows. The experimental details are given immediately below. These include materials, description of the clinoptilolite dealumination process, and measurement of the adsorption isotherms by the GC method. Next, an analysis and a discussion of the BTX adsorption results are both carried out. Finally, relevant conclusions of this work are presented.

Table 1. Physical Constants of BTX Adsorptivesa adsorptive

M (g mol-1)

σ (nm)

L (nm)

IP (eV)

∆Hliq (kJ mol-1)

benzene toluene p-xylene

78.11 92.14 106.17

0.65/0.65 0.65/0.89 0.98/1.05

0.730 0.850 0.940

9.3 8.8 8.5

-30.72 -33.18 -35.67

a M, molecular mass; σ, kinetic diameter; L, molecular length; IP, ionization potential; ∆Hliq, heat of liquefaction.

Table 2. Characteristics of Channel and Cation Sites in Clinoptilolite

channel

ring size/ channel axis

cation site

major cations

dimensions of channel windows (nm × nm)

A B C A

10/c 8/c 8/a 10/c

M(1) M(2) M(3) M(4)

Na+, Ca2+ Ca2+, Na+ K+ Mg2+

0.72 × 0.44 0.47 × 0.41 0.55 × 0.40 0.72 × 0.44

Experimental Section Materials. High-purity aromatic hydrocarbons, benzene (99 wt %, Aldrich), toluene (99.5 wt %, Aldrich), and p-xylene (99 wt %, Aldrich), were used as adsorptives. All of these chemicals were used as supplied without further purification. High-purity helium (99.99%, Linde) was used for GC determinations. Some important physical properties of the adsorptives employed in this work are listed in Table 1. Note that the kinetic diameters (σ) of the adsorptive molecules15 are comparable to the sizes of the A-C channels of the clinoptilolite crystal structure;7 see Table 2. Dealumination of Clinoptilolite. The clinoptilolites selected for this work are Mexican natural zeolites proceeding from Tehuaca´n in the State of Puebla, Mexico. The CLINA label accounts for the natural sample, which is free of any chemical treatment. Samples of dealuminated clinoptilolite zeolites were prepared at laboratory scale by means of an acid-leaching process16 of the CLINA precursor; this leaching process caused the exchange of polyvalent cations by protons and the removal of impurities. This modification procedure consisted of a series of washing cycles of natural clinoptilolite samples (mesh 60-80) with dilute HCl; this was followed by prolonged rinses with deionized water. Each of the steps of the HCl washing procedure sample was made at 50 °C during 6 h with 1 N HCl. A sample labeled as CLIDA1 was prepared from CLINA, after a sample of the natural zeolite was treated once with HCl (the CLIDA1 labeling follows after a number of washings with HCl). Likewise, a sample branded as CLIDA2 ensued from CLINA after treating a sample of the natural zeolite twice with HCl. Experimental Measurement Techniques. (i) XRD. XRD patterns were determined by means of a Siemens D-500 diffractometer employing nickel-filtered Cu KR radiation. (ii) N2 Adsorption. All N2 adsorption isotherms were measured at the boiling point of liquid N2 (76 K at the 2200-m altitude of Mexico City) in an automatic volumetric adsorption system (Quantachrome AutoSorb1LC). N2 adsorption isotherms were determined in the interval of relative pressures, p/p0, extending from 10-5 to 0.995. The saturation pressure, p0, was continuously registered in the course of the adsorption-desorption measurements. Powder particle sizes corresponding to mesh 60-80 were sampled from all specimens under analysis. Prior to the sorption experiments, samples

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were outgassed at 623 K during 20 h at a pressure lesser than 10-6 Torr. The Brunauer-Emmett-Teller (BET),17 Langmuir,18 and t (external) surface areas19 of the clinoptilolites under study were evaluated from N2 adsorption data in the p/p0 range extending from 0.04 to 0.2. The total pore volume, VΣ, was estimated through the Gurvitsch rule18 on the basis of the volume adsorbed at the relative pressure p/p0 ≈ 0.95 and calculated as volume of liquid. Information about the pore size distributions (PSDs) of the zeolites under study were obtained from the differential curves of comparison plots (DCCP) method.20 The microporosity inherent to our clinoptilolites was studied by way of the high-resolution RS-plot method, while considering a range extending from RS ) 10-7 to 0.7 (RS is Sing’s standard reduced parameter defined as the ratio of the volume adsorbed at the current p/p0 to the volume adsorbed at p/p0 ) 0.4).21 The adsorption isotherms of BTX hydrocarbons on CLINA and CLIDA adsorbents were measured at different temperatures by the GC technique14 using helium (35 cm3 min-1) as the carrier gas. The adsorption equilibrium pressures ranged from 0 to 210 Torr for CLINA and from 0 to 800 Torr for CLIDA, respectively. BTX adsorptive pulses of different amounts were injected into clinoptilolite-packed glass columns (i.d. ) 5 mm; length ) 50 cm) at a known temperature, and the elution chromatogram of each pulse was recorded continuously until the recorder pen reached the baseline once more. The detailed experimental arrangement and procedures involved in the pulse experiments have been described elsewhere.14 All of the experiments for determining the adsorption isotherms were carried out in a Shimadzu GC-14A gas chromatograph equipped with a thermal conductivity detector. Before the adsorption experiment was carried out, the adsorbents were pretreated in situ under a flow of the carrier gas at 573 K during 8 h. The BTX adsorptives were injected separately into the column in order to measure their corresponding retention times. Calculation Methods. Data corresponding to the adsorption of BTX compounds on clinoptilolite samples were fitted to standard Freundlich, Langmuir, and Dubinin-Asthakov (DA) isotherm models through linear regression in order to determine the adsorption parameters pertinent to each of the above approaches. The Freundlich adsorption equation22 can be written as

a ) kfp1/n

(n > 1)

(1)

where a is the amount adsorbed (mmol g-1), kf is the Freundlich adsorption constant, and n is an exponential factor. From BTX adsorption data at low pressures, it is possible to evaluate Henry’s constants (KH) at different temperatures for CLINA and CLIDA clinoptilolites according to the following expression:

KH ) lim pf0

( ) a amp

(2)

where a represents the amount adsorbed at pressure p and am is the monolayer capacity evaluated from the Langmuir equation:23

θ)

a kp ) am 1 + kp

(3)

Here k ) KH, something that can be tested graphically by plotting 1/a versus 1/p:

1 1 1 + ) a am amkp

(4)

Standard adsorption energies (-∆U0)24 can be found from the temperature dependence of KH, a relationship that is consistent with a traditional van’t Hoff form:

∂ ln KH ∆U0 ) ; KH ) K0 exp(-∆U0/RT) ∂T RT 2

(5)

In this expression, it should be noted that ∆U0 ) ∆H0 + RT, with ∆H0 being the standard adsorption enthalpy, R the universal gas constant, and K0 van’t Hoff’s preexponential factor. The total micropore volumes of the clinoptilolites can be calculated from BTX isotherm data analyzed according to the DA adsorption equation:25

[ ( )]

A W ) exp W0 βE0

3

(6)

In the above equation, W is the volume of the adsorbate retained inside the micropore structure at a given relative vapor pressure p/p0, W0 is the total micropore volume, A ) RT ln(p0/p) is the thermodynamic adsorption potential, E0 is the characteristic adsorption energy (which is a function of the adsorbent), and β is an affinity coefficient that permits the comparison between the adsorption potential of the test adsorbate and that of a reference adsorbate. When our BTX substances are chosen as the reference adsorbates during measurements involving each one of these substances, then it results that β ) 1 in all cases. The saturation pressure values p0 at different temperatures were evaluated from the Thek-Stiele equation.26 The isosteric heat of adsorption, qst (kJ mol-1), at different adsorbate loadings was evaluated from the adsorption isotherm data through a Clausius-Clapeyron type equation.27

qst(a)

[∂ ∂Tln p] ) RT a

2

(7)

Here p and T are the equilibrium pressure and adsorption temperature at a given adsorbate loading a. Results and Discussion X-ray Analysis. The XRD patterns of the CLINA and CLIDA samples were typical of clinoptilolite zeolites as described by Treacy et al.28 The XRD patterns (Figure 2) showed evidence of the presence of some quartz (2θ ∼ 27°). A comparison of the XRD pattern of the CLIDA zeolites with that of the R quartz (a reference sample mined from Tehuaca´n, Mexico29) clearly indicated that the acid modification procedure of zeolitic samples yielded an increase in the intensity of the peak corresponding to quartz. This behavior can be related to the progressive dealumination that the crystalline structure suffered as the number of acid treatments increased while quartz moieties remained mostly unaffected by

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Figure 2. XRD patterns of clinoptilolite zeolites. A sample of R quartz (2θ ≈ 27.5°) is used as the reference. The C labels represent clinoptilolite signals, while the Q labels mean quartz.

Figure 3. (a) N2 adsorption isotherms at 76 K on clinoptilolite zeolites. (b) PSDs of CLINA and CLIDA zeolites calculated from the DCCP method.

the HCl leaching action. In general, all zeolites (including the natural substrate) exhibited good crystallinity and, consequently, reasonably sharp diffraction patterns, especially those due to the CLIDA specimens. N2 Adsorption. N2 adsorption isotherms at 76 K on CLINA and CLIDA zeolites are shown in Figure 3a. This figure portrays the evolution of the shapes of the N2 isotherms with respect to the number of HCl washings;

the CLINA and CLIDA1 substrates render IUPAC30 type IV isotherms (with extremely narrow hysteresis loops), while a type I isotherm is proper for the CLIDA2 specimen (the hysteresis loop of this sample corresponds to a IUPAC type H4). Distinctive features of these types of isotherms are as follows: (i) the extent of microporosity in dealuminated clinoptilolites increases, in general, with the number of acid treatments; the plateaus of the N2 isotherms corresponding to CLIDA clinoptilolites reach increasing heights according to the accessible microporosity depicted by each zeolite; and (ii) the existence of a low-pressure hysteresis region is evident for the CLIDA2 specimen. It is also important to note that CLIDA2 possesses a microporous volume several times larger than that of CLINA. Therefore, acid treatment of high-silica natural clinoptilolites can render adsorbents of enhanced accessible pore volumes via the mechanism of decationation and dealumination and also by dissolution of any amorphous materials blocking the entrances to the A-C channels of the clinoptilolite structure. The cation blocking effects at pore entrances in natural clinoptilolites are thus diminished by acid treatment, then lowering the cation-exchange capacity of the resultant substrates by leaching out Al3+ from framework positions and introducing H+ into the remaining cation sites of the natural precursor.6 Important textural parameters of CLINA and CLIDA clinoptilolites are listed in Table 3. For the CLIDA clinoptilolites, the BET equation CB constants are sometimes negative, and this can be explained by the fact that multilayer adsorption in micropores is not a plausible model therein. The isotherm of the CLINA zeolite shows an upward deviation at high relative pressures due to multilayer formation and capillary condensation taking place in the mesopores (secondary porosity) of this sample.10 Figure 3b shows the curves calculated by the DCCP method;20 this procedure consists of determining the derivative of the t plot (i.e., a curve of the adsorbed volume versus the thickness, t, of the adsorbed layer) to give a straightforward (approximate) idea of the main pore sizes of an adsorbent. In this figure, CLINA exhibits a shallow multimodal distribution with pore size maxima occurring at 0.255, 0.573, 1.431, and 2.145 nm. In contrast, CLIDA2 exhibits a sharp multimodal distribution with pore size maxima happening at 0.335, 0.566, and 2.072 nm. For CLIDA1, representative pore size results could not be obtained by means of the DCCP method because of the flat aspect of the resultant curve. The above DCCP characteristics suggest the following reflections concerning the porous structures of our clinoptilolite adsorbents. (i) CLINA pore entrances are closed to the uptake of N2 molecules; this is likely due to the presence of bulky ionic species at these entrance points. (ii) CLIDA2 is an open microporous structure in which primary and secondary micropore filling mechanisms take place.10 The existence of a low-hysteresis loop associated with this substrate can mean that microporous channels are either interconnected to each other by thin capillaries or surrounded by narrow cracks that are due to the dealumination process. The CLIDA1 solid represents a transitional substrate with porous characteristics between those of the CLINA and CLIDA2 structures. BTX Adsorption. The dynamic chromatographic method was used for evaluating the adsorption isotherms from injection pulse data proceeding from the

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Table 3. Textural Parameters of Natural (CLINA) and Dealuminated (CLIDA) Clinoptilolite Zeolites Determined from N2 Adsorptiona zeolite

ASB (m2 g-1)

ASL (m2 g-1)

ASt (m2 g-1)

CB

p/p0 BET plot

VΣ (cm3 g-1)

W0 (cm3 g-1)

CLINA CLIDA1 CLIDA2

11.74 37.39 162.2

19.01 54.40 230

11.74 26.42 108.4

41 -80 -390

0.10-0.31 0.10-0.31 0.14-0.31

0.030 0.028 0.299

0.029 0.223

a A , BET specific surface area; A , Langmuir specific surface area; A , t-plot surface area; C , BET constant; p/p0, the relative SB SL St B pressure range used for the BET plot; VΣ, volume adsorbed at p/p0 ) 0.95 and expressed as the volume of liquid N2 (Gursvitch rule);17 W0, volume micropore estimated from Sing’s RS method.21

elution curves of the adsorptives on the zeolites under study. These BTX adsorption isotherms were measured in a region of low adsorbate concentrations; hence, adsorption on the mesopore surface can be practically neglected. Consequently, the parameters that characterize the microporous structure13 of our clinoptilolite adsorbents have been calculated from the BTX adsorption isotherms, without introducing any correction for adsorption on the mesopore surface. BTX adsorption isotherms at different temperatures on CLINA, CLIDA1, and CLIDA2 zeolites are presented in parts a-c of Figure 4, respectively; the corresponding Freundlich and Langmuir linearized plots are shown in parts a and b of Figure 5, respectively. In Figure 4a-

Figure 4. Adsorption isotherms of BTX compounds on CLINA and CLIDA zeolites: (a) benzene; (b) toluene; (c) p-xylene.

c, two important things can be observed: (i) the adsorbed amounts of BTX substances on CLINA and CLIDA1 are not several times lesser (as in the case of N2 adsorption) but are comparable to the BTX uptakes that are taking place on CLIDA2, and (ii) the amount adsorbed decreases with temperature. The rather similar uptakes of BTX molecules inside CLINA and CLIDA substrates can be associated with an activated diffusion process. This kind of effect is due to the existence of steric barriers that hinder the diffusion of adsorptive molecules through very narrow channels,18 thus requiring significant activation energies (i.e., sufficiently large temperatures) to permeate these capillaries into the cavities beyond. The existence of activated diffusion has been conceived theoretically for the case of hydrocarbon adsorptives31 entering microporous structures analogous to those studied in this work. The decrease of the amount adsorbed with temperature is a characteristic typical of physisorption; as the temperature increases, the trapping of adsorptive molecules becomes more difficult by reason of their mounting kinetic energy. Finally, for comparison reasons, it is relevant to mention that the adsorption of light hydrocarbon molecules by microporous adsorbents32 typically renders

Figure 5. (a) Freundlich and (b) Langmuir plots for the adsorption of aromatic BTX hydrocarbons on CLINA and CLIDA zeolites at different temperatures: full symbols, benzene; open symbols, toluene; partially filled symbols, p-xylene.

Ind. Eng. Chem. Res., Vol. 44, No. 9, 2005 2913 Table 4. Freundlich Parameters for the Adsorption of BTX Hydrocarbons on CLINA and CLIDA Zeolites benzene

toluene

kf × (mmol g-1 Torr-1)

n

249.00 103.40 78.60 46.30 30.20 45.30 39.00

2.897 2.351 2.258 1.916 2.063 1.849 1.860

23.80 22.60 117.80 67.40

1.806 1.674 2.283 1.968

26.40 37.30

1.715 1.925

104

adsorbent

T (K)

CLIDA2

398 423 448 473 498 398 423 448 473 498 398 423 448 473 498

CLIDA1

CLINA

p-xylene

kf × (mmol g-1 Torr-1)

n

170.80 148.60 106.30 71.90 67.40 38.60

2.462 2.719 2.655 2.358 2.470 1.708

70.80 68.30 78.60 141.40 106.60 138.50 101.30 118.20

2.185 2.241 2.562 2.344 2.234 2.548 2.414 2.704

104

kf × 104 (mmol g-1 Torr-1)

n

101.20 101.90 91.50 57.60 54.40 40.90 53.00 54.40 69.50 40.30 91.10 79.40 99.80 91.00 99.30

1.828 2.278 2.492 2.077 2.269 1.444 1.818 1.941 2.255 1.964 1.714 1.919 2.310 2.366 2.413

Table 5. Langmuir Parameters for the Adsorption of BTX Compounds on Clinoptilolites benzene sample

T (K)

CLIDA2

398 423 448 473 498 398 423 |448 473 498 398 423 448 473 498

CLIDA1

CLINA

toluene

p-xylene

KH × (mmol g-1 Torr-1)

am (mmol g-1)

KH × (mmol g-1 Torr-1)

am (mmol g-1)

KH × 104 (mmol g-1 Torr-1)

am (mmol g-1)

29.10 13.50 11.00 8.093 5.339 9.015 8.394 6.145 4.986

0.217 0.160 0.134 0.136 0.062 0.147 0.110 0.089 0.080

22.00 17.50 13.80 9.035

0.228 0.147 0.107 0.109 0.084 0.191

19.8 13.4

0.165 0.155

18.30 15.60 18.00 10.50 7.795 11.30 8.160 8.132 9.302 6.061 23.00 15.20 16.80 16.30 17.00

0.372 0.153 0.086 0.107 0.087 0.561 0.244 0.175 0.120 0.120 0.326 0.208 0.131 0.105 0.112

104

8.566 9.739

0.070 0.069

Table 6. Standard Adsorption Energies (-∆U0) of BTX Adsorptives on CLIDA2 adsorptive

ln KH × 106

-∆U0 (kJ mol-1)

p-xylene toluene benzene

1.035 17.236 29.774

25.900 16.118 13.771

type I isotherms. Furthermore, BTX adsorption isotherms on CLIDA2 are similar to those of n-alkanes on ZSM-5 zeolites and silicalite.33,34 Freundlich Model. The Freundlich model can be fitted extremely well to most BTX adsorption data, although is not suitable for benzene adsorption at 448 K on CLIDA1 and CLINA or for toluene adsorption at 423 K on CLIDA1. The values of the Freundlich parameters (kf and n) related to BTX adsorption on CLINA-CLIDA zeolites are listed in Table 4. Langmuir Model. The Langmuir approach works reasonably well for all but one case: that corresponding to toluene adsorption at 473 K on CLIDA2. The values of the Henry and Langmuir constants (i.e., KH and am) are listed in Table 5. The temperature dependence of the monolayer adsorption capacity (am), derived from the Langmuir plots, is shown in Figure 6a. In general, for all zeolites am decreases with an increase in the temperature for all adsorbates, especially for p-xylene. In this latter case, the drastic decrease in am with T can be related to the considerable steric restrictions that the relatively large (compared to the sizes of benzene and toluene molecules) p-xylene molecules suffer when trying to penetrate into

104

7.868 11.90 10.80 11.40 27.20 17.30 21.90 23.90 21.60

0.116 0.106 0.080 0.161 0.163 0.142 0.090 0.092

the zeolitic structure. Figure 6b shows plots of the standard adsorption energy -∆U0 versus the molecular

Figure 6. Adsorption parameters of aromatic BTX hydrocarbons on CLIDA2 zeolite: (a) variation of the adsorption saturation limit (am) with temperature; (b) standard adsorption energy (-∆U0) versus molecular weight of adsorbed BTX compounds; (c) temperature dependence of Henry’s law constants (kH).

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Ind. Eng. Chem. Res., Vol. 44, No. 9, 2005 Table 7. DA Parameters for the Adsorption of Benzene, Toluene, and p-Xylene on CLIDA2 T (K)

a (mmol g-1)

-E0 (kJ mmol-1)

R corr. coeff.

398 423 448 473 498

0.373 0.366 0.397 0.587 0.327

Benzene 28.767 24.805 25.239 22.616 25.627

398 423 448 473 498

0.297 0.247 0.231 0.280 0.264

Toluene 24.450 28.698 29.668 27.832 30.686

0.999 0.999 0.999 0.999 0.999

398 423 448 473 498

0.313 0.214 0.186 0.247 0.230

p-Xylene 17.99321 24.04616 27.92661 24.50593 28.194

0.999 0.998 0.998 0.999 0.998

0.997 0.999 0.999 0.999 0.999

Table 8. Isosteric Heats of Adsorption (-qst, kJ mol-1) of Benzene, Toluene, and p-Xylene on CLINA, CLIDA1, and CLIDA2 Zeolites adsorbate CLIDA2 CLIDA1 CLINA Si-MCM41 benzene toluene p-xylene

Figure 7. (a) DA plots for the adsorption of BTX hydrocarbons on CLIDA2 zeolite at different temperatures. (b) Variation of the saturation adsorption filling amounts (a0) of BTX hydrocarbons and characteristic adsorption energy E0 at different temperatures in the micropores of the CLIDA2 zeolite.

weights of BTX compounds adsorbed on the CLIDA2 substrate. Larger -∆U0 values arise as the molecular mass of the aromatic adsorptive is increased. This means that the greatest adsorption interaction of CLIDA2 occurs with the p-xylene molecules (Table 6); this kind of result has been previously observed.35 Figure 6c shows the temperature dependence of Henry’s constants for BTX adsorption on the CLIDA2 zeolite. DA Model. The adsorption potential, A, in micropores can be significantly larger than that associated with adsorption on mesopores because of the overlapping of potential wells emanating from opposite walls of the micropores; consequently, the smallest micropores (ultramicropores) are filled at very low relative pressures (corresponding to high adsorption potentials), and then the larger micropores (supermicropores) are filled subsequently.25 The linearized DA equation renders good straight lines for BTX adsorption on the CLIDA2 zeolite; see part a of Figure 7. The adsorption saturation capacity of the micropores, designated as a0, and the characteristic adsorption energy, designated as -E0, have been determined from the above plots together with the regression coefficients R; see Table 7. The behavior observed there is related to the effect of the molecular sieve displayed by these zeolitic adsorbents. Isosteric Heat of Adsorption. The trends of the isosteric heat of adsorption (qst) as a function of the

75.197 63.295 61.110

18.243 4.712 18.410

4.855 1.225 4.942

52.540 60.673 64.755

SiO2

Ag/SiO2

59.324 66.763 63.699 89.702 81.986 100.070

amount adsorbed of BTX adsorptives on CLINA and CLIDA clinoptilolites are presented in Figure 8 and in Table 8. In this same table, sorption data of BTX adsorptives on some other adsorbents that have been previously studied are also reported for comparison.22,36 The qst values related to the CLIDA2 substrate corresponds to the following sequence: qst(benzene) > qst(toluene) > qst(p-xylene). This succession is consistent with the ionization potentials of the BTX adsorptives: benzene (9.3 eV) > toluene (8.8 eV) > p-xylene (8.5 eV)15 (Table 1). The enthalpies of vaporization of BTX compounds are as follows: benzene, 34.91 kJ mol-1; toluene, 43.67 kJ mol-1; p-xylene, 42.76 kJ mol-1. Therefore, because the qst values proceeding from BTX adsorption on CLIDA2 are larger (Figure 8) than the enthalpies of vaporization of these compounds, the adsorptive molecules interact strongly both with the surface and with the neighboring adsorbate molecules. Hence, the primary and secondary micropore mechanism fillings seem to be taking place during the BTX adsorption on CLIDA2: first, there occurs a strong interaction between adsorbate molecules and the adsorbent surface;

Figure 8. Variation of the isosteric heat of adsorption (-qst) of benzene, toluene, and p-xylene hydrocarbons with adsorbate loading on CLINA, CLIDA1, and CLIDA2 zeolites.

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next, a cohesive interaction between adsorbate molecules is developed. The fact that the magnitudes of the isosteric heats of adsorption are different from one adsorbent to another can be related to the high interaction energy depicted by the hydrogen atoms of the benzene rings and methyl groups of the adsorptives toward the oxygen atoms of the zeolites and also to the molecular sieve effect displayed by each adsorbent. The isosteric heat of adsorption of benzene on CLINA and CLIDA1 is associated with nearly uniform surfaces ever since the adsorption on these substrates is smaller than that taking place on CLIDA2. This behavior can be attributed to the slow diffusion of the adsorptives inside the zeolitic structure mainly because of the partial blocking of pore entrances by the cations (M+) and also by the residual material that remains at the entrances of channels A-C. According to this, it can be expected that the available adsorption space and the different sizes of the entrance windows play major roles in the magnitudes of the isosteric heats of BTX adsorption on CLINA and CLIDA1. Therefore, progressive HCl treatment of CLINA develops into wider and wider pore entrances by cation exchange and larger and larger micropore volumes, so that these two effects allow an increased adsorption and a cohesive interaction between adsorbed molecules. On the basis of the two latter effects, acid leaching of CLINA ensures the participation of a greater number of adsorption centers in dealuminated zeolites. Another possible effect that acid leaching produces is the transformation of ultramicropore entities into supermicropore cavities, in which secondary (cohesive) adsorption is likely to take place.

a0 ) micropore adsorption saturation amount ASB ) BET specific surface area ASL ) Langmuir specific surface area ASt ) t-plot external surface area A ) RT ln(p0/p), the thermodynamic adsorption potential CB ) BET constant DCCP ) Differential Curves of Comparison Plots E0 ) characteristic adsorption energy IP ) ionization potential KH ) Henry’s constant K0 ) van’t Hoff’s preexponential factor kf ) Freundlich adsorption constant L ) molecular length M ) molecular weight 1/n ) exponential factor in the Freundlich equation p ) equilibrium pressure p/p0 ) relative vapor pressure R ) universal gas constant R ) correlation coefficient T ) temperature qst ) isosteric heat of adsorption VΣ ) volume adsorbed at p/p0 ) 0.95 and expressed as the volume of liquid N2 (Gurvitsch rule) W0 ) micropore volume, estimated by Sing’s RS method W ) volume of liquidlike adsorbate inside the micropore Greek Letters σ ) kinetic diameter ∆Hliq ) heat of liquefaction ∆U0 ) standard adsorption energy ∆H0 ) standard adsorption enthalpy β ) affinity coefficient

Literature Cited Conclusions Acid leaching of natural high-silica clinoptilolites can render efficient microporous adsorbents for the adsorption of aromatic compounds. Large, blocking cations at pore entrances are substituted by protons, then providing ready access to aromatic molecules into the channels of clinoptilolite; the concomitant dealumination of the structure that is brought by acid treatment enhances the channel widths, thus creating supermicropore structures. The adsorption isotherms determined at different temperatures of aromatic BTX hydrocarbons on CLIDA2 comply satisfactorily well with Freundlich, Langmuir, and DA adsorption models. The heats of adsorption of the aromatic hydrocarbons increase with an increase in the adsorbate loading, then pointing to the existence of lateral interactions between adsorbed molecules. The difference among the heats of adsorption of BTX adsorptives on natural and acid-treated clinoptilolites indicates that the nature of the adsorptive molecules, due to the presence (or absence) of methyl groups, substantially affects the degree of interaction between the hydrogen atoms of the adsorptives and the oxygen atoms of the zeolites. The magnitudes of the isosteric heat of adsorption are different for each adsorbent and are related to the above interactions as well as to the effect of the molecular sieve displayed by each adsorbent. Nomenclature a ) amount adsorbed at pressure p am ) monolayer capacity evaluated from the Langmuir equation

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Received for review August 10, 2004 Revised manuscript received February 18, 2005 Accepted February 22, 2005 IE049276W