Adsorption of Benzene, Toluene, and p-Xylene on Microporous SiO2

Adriana Dervanoski da Luz , Selene Maria de Arruda Guelli Ulson de Souza , Cleuzir da Luz , Josiane Maria Muneron de Mello , and Antônio Augusto Ulso...
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Ind. Eng. Chem. Res. 2004, 43, 1779-1787

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Adsorption of Benzene, Toluene, and p-Xylene on Microporous SiO2 M. A. Herna´ ndez* and J. A. Velasco Departamento de Investigacio´ n en Zeolitas y 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

M. Asomoza, S. Solı´s, F. Rojas, and V. H. Lara Departamento de Quı´mica, Universidad Auto´ noma MetropolitanasIztapalapa, P.O. Box 55-534, D.F., Me´ xico

Adsorption isotherms of aromatic BTX (benzene, toluene, and p-xylene) hydrocarbons on pure and Ag-doped SiO2 microporous substrates synthesized by the sol-gel procedure have been measured by the gas chromatographic technique at several temperatures in the range between 423 and 573 K. Both Langmuir and Dubinin adsorption models can reasonably fit every set of BTX adsorption data at all temperatures. The uptake degree of these hydrocarbon adsorptives on both SiO2 and Ag/SiO2 microporous substrates is temperature-dependent. Additionally, the degrees of interaction of BTX molecules with SiO2 and Ag/SiO2 microporous walls have been investigated by evaluation of the corresponding isosteric heats of adsorption (qst) by means of a Clausius-Clapeyron-type equation. The isosteric heats of adsorption of BTX hydrocarbons at different adsorbate loadings have been inferred from the adsorption isotherms and found to follow the decreasing sequence qst(benzene) < qst(toluene) < qst(p-xylene). Isosteric heats of adsorption of BTX compounds on SiO2 microporous substrates have been found, in general, to increase with an increase in the adsorbate loading as a consequence of the cohesive interactions between adsorbate molecules. Addition of Ag atoms to the SiO2 structure helps to homogenize the magnitudes of the isosteric heats of adsorption of the BTX compounds. Introduction Volatile organic compounds (VOCs) are among the most common air pollutants released by chemical, petrochemical, and related industries. VOCs are key reactants involved in photochemical reactions occurring in the atmosphere and which lead to serious environmental hazards.1 Among VOCs, benzene derivatives are especially harmful to the environment and human health.2 Stringent control of VOC emissions is one of the main objectives of the regulations introduced under the 1990 USA Clean Air Act Amendment.3 There exist several methods such as condensation, absorption, adsorption, contact oxidation, and incineration for removal and/or recovery of organic vapors. Because of their environmental implication, removal of VOCs from a bulk gas stream by an adsorption process is a common engineering practice4 that has been receiving increasing attention during the past few years.5 Among adsorbents potentially useful for efficient VOC removal, microporous inorganic materials represent one of the best alternatives to perform this task. These types of materials have found a wide variety of applications including adsorption, membrane separation processes, catalysis, development of sensors, and numerous emerging specialty applications.6 Microporous materials can be crystalline, such as zeolites, or amorphous, such as silica gels.7 Currently, the most widely utilized adsorbent for environmental cleaning is high surface area activated carbon; nevertheless, this material presents some problems, including the fact that the adsorbed molecules are very often not destroyed or decomposed (through ir* To whom correspondence should be addressed. E-mail: [email protected].

reversible dissociative chemisorption) but instead are only weakly held at the surface.8 For this reason, it is worth trying to investigate the performance of other types of microporous adsorbents, which can have stronger interactions toward the adsorbed molecules, with this being one of the more important reasons for employing microporous silica as the adsorbent in this work. Sol-gel processing represents a low-temperature method for the development of microporous, typically amorphous, inorganic structures.9 Porous silica is one of the various forms of amorphous silica. Other forms are nonporous SiO2 precipitates, silica hydrogels, and pyrogenic materials such as aerosil, the opal mineral, etc. These substances vary considerably in their appearance, hardness, and degree of hydration, but all can be considered as polycondensation products of orthosilisic acid, Si(OH)4. The usual methods of preparation are precipitation with acids from silicate solutions and hydrolysis of silicon derivatives such as silicon tetrachloride or tetraethoxysilane (TEOS).10 The silica gel surface includes silanol (tSiOH) and siloxane (tSiOSit) groups attached to it. It is known that many times silanol groups act as adsorption sites. In the case that an adsorbate can interact with silanol groups, there is also the possibility that the adsorbate molecules can interact with two or more silanol groups at a time.11 SiO2 synthesized by the sol-gel method is stable toward steam treatment up to 700 °C, mechanical grinding, and also mild acid treatment.6 Because of its large surface area (a structural parameter that can attain values sometimes larger than 960 m2 g-1), significant adsorption capacity, high thermal/hydrothermal stability, and rigid microporous structure, SiO2 has a high potential for practical use as an adsorbent for both small and

10.1021/ie0204888 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/26/2004

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bulky adsorbate molecules.12 An additional valued characteristic of SiO2 adsorbents is the possibility of introducing active metal species (e.g., Ag, Pt, etc.) into the structure during the sol-gel synthesis process. Therefore, it is of great scientific and practical interest to explore the adsorption properties of microporous SiO2 with respect to different adsorbates. However, studies about the adsorption of hydrocarbons on silica substrates, particularly the bulkiest molecules, are still scarce. The present work has been undertaken with the aim of measuring the adsorption isotherms of a series of BTX hydrocarbons (viz., benzene, toluene, and p-xylene) on a series of SiO2 and Ag/SiO2 microporous adsorbents. With this purpose in mind, BTX hydrocarbon adsorption on microporous SiO2 solids has been measured through the gas chromatographic (GC) peak maxima technique13 at different temperatures. Under this scope, hydrocarbon adsorption data proceeding from pure and Ag-doped SiO2 substrates were analyzed according to Langmuir and Dubinin-Astakhov approaches; adsorption energies and isosteric heats of adsorption were then determined for each adsorbent-adsorbate pair. N2 sorption at 76 K was also performed on the same substrates to measure surface areas and pore volumes. Experimental Section Materials. TEOS and alcohol (methanol, ethanol, or propanol) of analytical grade (purity g 99.8 wt %; Aldrich) were employed for the synthesis of SiO2 substrates, while high-purity N2 gas (>99.99%; Linde) was also selected to complement the textural studies of microporous silica. A reference macroporous solid (which is needed for the estimation of micropore volumes from different comparison methods) was mined from Tehuaca´n, a region in the State of Puebla, Mexico. X-ray diffraction (XRD) identifies this reference substrate as R-quartz. High-purity aromatic hydrocarbons, benzene (99 wt %; Aldrich), toluene (99.5 wt %; Aldrich), and p-xylene (99 wt %; Aldrich), were used as adsorptives. All chemicals were used as supplied without further purification. High-purity helium (99.99%; Linde) was used for GC studies. Synthesis of Microporous SiO2. Silica monoliths were synthesized starting from precursor mixtures consisting of TEOS dissolved in alcohol (methanol, ethanol, or propanol); afterward, each one of these alkoxide-alcohol solutions was left to react with water. Specifically, gels were prepared as follows: TEOS was first dissolved in the selected alcohol and then mixed with deionized water according to a 1:4:2.5 ratio; the reactant mixtures were prepared inside glass columns that were maintained at 303 K for several days, during which the sol-gel hydrolysis-condensation reactions took place. The resulting sols were kept in the original glass columns so that gelation occurred eventually (i.e., a period of a few months was usually required). The ensuing gel was then dried in situ at 373 K for 48 h so that cylindrical monoliths of microporous silica were finally attained. The specimens that were prepared, namely, SiO2-A (methanol being used as the TEOS solvent), SiO2-B (TEOS dissolved in ethanol), and SiO2-C (propanol being the TEOS solvent), were stable, with all of them displaying a significant surface area to pore volume quotient, a characteristic that facilitates the occurrence with great effectiveness of important physical and chemical processes. The Ag/SiO2-A speci-

men was synthesized in the following way. The required quantity of AgNO3, to reach a SiO2 substrate containing 5 wt % Ag, was added to the volume of water involved in the SiO2-A synthesis; otherwise, the conditions selected for the preparation of this latter specimen were the same as those related to the production of silverfree solids. The Ag-containing substrate was labeled as Ag/SiO2-A (i.e., methanol was employed to dissolve TEOS). As will eventually be evident, the silica materials synthesized in this work are likely suitable for hightemperature industrial applications. Additionally, the Ag/SiO2-A sample could involve specific catalytic or medical applications (i.e., microbicide effects) and is endowed with a gray color; details of a similar synthesis procedure have been previously described.14 Sorption Experiments. All nitrogen adsorption isotherms were measured at the boiling point of liquid nitrogen (76 K at the 2200 m altitude of Me´xico City) in an automatic volumetric adsorption system (Micromeritics ASAP 2000). Nitrogen adsorption isotherms were routinely determined in the interval of relative pressures, p/p0, extending from 10-3 to 0.995. The saturation pressure, p0, was continuously registered in the course of the adsorption-desorption measurements. Powder particle sizes corresponding to 60-80 mesh were sampled from all specimens under analysis. Prior to the sorption experiments, samples were outgassed at 623 K for 20 h at a pressure of less than 10-6 Torr. Chromatographic BTX adsorption experiments were carried out in a Shimadzu GC-14A gas chromatograph equipped with a thermal conductivity detector. The chromatographic columns (i.d. ) 5 mm, length ) 50 cm) were made of glass and packed with the 60-80 mesh silica samples to be studied. Adsorbate pulses of different intensities were injected into packed columns kept at a chosen temperature, and the elution chromatogram of each pulse was registered until the recorder pen once again reached the baseline. Prior to the adsorption experiment, adsorbents were pretreated in situ under a flow of He carrier gas at 573 K for 8 h. Each hydrocarbon was injected separately in order to measure its corresponding retention time inside the appropriate adsorbent column. The isosteric heats of adsorption of the BTX hydrocarbons were calculated from sorption data evaluated at different temperatures. Calculation Methods. Brunauer-Emmett-Teller (BET), Langmuir, and external surface areas15 of the materials under study were evaluated by analyzing nitrogen adsorption data at 76 K in the relative vapor pressure (p/p0) range extending from 0.04 to 0.2. The total pore volume was estimated on the basis of the volume adsorbed at a relative pressure of about 0.95; i.e., the Gurvitsch rule16 was applied. The BarrettJoyner-Halenda pore size distributions of the silica adsorbents under study (i.e., SiO2-A, -B, and -C) indicate that all of these substrates show an almost complete absence of large pore sizes (i.e., mesopores and macropores); nevertheless, the Ag/SiO2-A microporous sample depicts the presence of a certain amount of mesopores of approximately 3.3 nm size. Micropore volumes, W0, were calculated through the RS plot method17 and considering a range extending from RS ) 0.5 to 1.75 (RS is Sing’s standard reduced parameter defined as the ratio between the volume adsorbed at the current p/p0 against the volume adsorbed at p/p0 ) 0.4). As mentioned before, a macroporous silica (R-SiO2, i.e., R-quartz) was selected as the adsorbent of reference that is needed

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1781

to perform the RS plot method. The detailed texture properties and microporosity extent of the samples under study have been reported elsewhere.18 From BTX adsorption data at low pressures, it was possible to evaluate Henry constants (KH) at different temperatures for SiO2-A and Ag/SiO2-A substrates according to the following expression:

KH ) lim pf0

( ) a amp

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 weight; σ ≡ kinetic diameter; L ≡ molecular length; IP ≡ ionization potential; ∆Hliq ≡ heat of liquefaction.

(1)

where a represents the amount adsorbed on the solid walls at pressure p and am is the monolayer capacity evaluated from the Langmuir equation16

θ)

kp a ) am 1 + kp

(2)

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

1 1 1 ) + a am amkp

(3)

Standard adsorption energies19 (-∆U0) were then found from the temperature dependence of Henry constants, a relationship that is assumed to be consistent with a traditional van’t Hoff form:

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

(4)

Figure 1. XRD patterns of SiO2 samples: (a) SiO2-A obtained by the sol-gel procedure; (b) R-quartz, reference silica.

where ∆U0 ) ∆H0 - RT, with ∆H0 being the standard adsorption enthalpy, R the universal gas constant, and K0 van’t Hoff’s preexponential factor. Micropore BTX filling can be described by means of the Dubinin-Raduskevich (DR) adsorption equation20

[ ( )]

A W ) exp W0 βE0

2

(5)

where W is the volume of liquidlike adsorbate 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. Choosing each one of the BTX adsorptives as the reference adsorbate would result in β ) 1. The isosteric heat of adsorption, qst (kJ mol-1), at different adsorbate loadings can be evaluated from the adsorption isotherm data through a Clausius-Clapeyron-type equation:21

qst

[∂ ∂Tln p] ) RT a

2

(6)

where p and T are the equilibrium vapor pressure and temperature at a given adsorbate loading (a). Results and Discussion Some of the physical properties of the adsorptives used in this work are listed in Table 1. Characterization of SiO2 Substrates. XRD Analysis. XRD analysis of SiO2 specimens (see Figure 1)

Figure 2. Nitrogen sorption isotherms at 76 K on SiO2 samples: (a) macroporous R-quartz; (b) Ag/SiO2-A: (c) SiO2-C; (d) SiO2-B; (e) SiO2-A.

supply typical patterns related to microporous SiO2 and R-SiO2 reference materials. The amorphous SiO2 structure of the synthesized SiO2 adsorbent was confirmed by observing the characteristic wide diffractogram displayed by this material, while the crystalline lattice of the R-SiO2 reference material depicted a succession of sharp peaks. Nitrogen Adsorption. N2 adsorption isotherms at 76 K on SiO2 samples are shown in Figure 2. All of these isotherms correspond to a type I,15 something that is indicative of the microporous nature of the whole series of silica substrates. Important textural parameters of these porous solids are given in Table 2. The shapes of the adsorption isotherms also reflect the high degree of cohesive interaction existing between adsorbate molecules (i.e., a cooperative phenomenon that is responsible for the rounded-knee aspect of the isotherms arises during micropore filling). BET constants (CBET) are negative for the four types of silica adsorbents. This is

1782 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Table 2. Textural Parameters of SiO2 Adsorbentsa adsorbent

ASL (m2 g-1)

ASt (m2 g-1)

C(B)

V∑ (cm3 g-1)

R-SiO2 SiO2-C SiO2-B SiO2-A Ag/SiO2-A

4.28 821.5 832.8 976.5 473.1

2.67 641.5 655.5 762 252.9

240 -59 -30 -1007 -1735

0.005 0.293 0.308 0.370 0.205

a A SL ≡ Langmuir specific surface area; ASt ≡ t plot specific surface area; C(B) ≡ BET constant; VΣ ≡ volume adsorbed at p/p0 ) 0.95 and expressed as volume of liquid N2.

Table 3. Micropore Volumes W0 (cm3 g-1), Calculated by the rS Method, That Are Related to Ultramicropores and Supermicropores Constituting SiO2 Adsorbentsa adsorbent SiO2-A SiO2-B

micropore type

W0,RS

adsorbent

Um Sm Um Sm

0.172 0.366 0.138 0.305

SiO2-C Ag/SiO2-A

micropore type

W0,RS

Um Sm Um Sm

0.127 0.290 0.050 0.208

a Um ≡ ultramicropores; Sm ≡ supermicropores; W 0,RS ≡ RS plot micropore volume.

Figure 4. Adsorption isotherms of aromatic BTX hydrocarbons on SiO2-A at different temperatures.

Figure 3. RS plots for N2 adsorption on SiO2 samples: (a) Ag/ SiO2-A; (b) SiO2-C; (c) SiO2-B; (d) SiO2-A.

associated with the fact that adsorption in micropores22 is far from following a BET filling mechanism. At p/p0 > 0.4, the incidence of a plateau (saturation zone) in the isotherm that extends from relatively low relative pressures up to p/p0 ∼ 1 is observed. This characteristic strongly suggests that, besides the absence of mesopores and macropores, adsorption in microporous silica resembles the mechanism of volume filling. Micropore volumes associated with supermicropores (i.e., a pore width of between 0.7 and 2 nm) and ultramicropores23 (a pore width of less than 0.7 nm) are listed in Table 3; these results proceed from RS plots analyzed in a way that will be explained afterward. Adsorption capacities can be expressed in terms of different parameters such as Langmuir’s monolayer saturation limit am or Gurvitsch’s limiting adsorption volume VΣ.16 In general, it seems that the adsorption capacity of our SiO2 substrata increases according to the nature of ROH that is present during the sol-gel synthesis procedure. Sorption capacities of microporous SiO2 substrata exhibit the following decreasing order: SiO2-A > SiO2-B > SiO2-C > Ag/SiO2-A. The RS plots shown in Figure 3 are constructed from isotherm data through procedures described in the literature.24 In these RS plots, it is possible to observe two well-defined zones of behavior, thus indicating the existence of the two kinds of micropores mentioned above inside the SiO2 structures. The first zone is attributable to the occupation of the largest micropores

(i.e., supermicropores, Sm), while the other characteristic zone is related to the adsorbate occupation of the finest micropores (ultramicropores, Um). These latter voids are believed to be rigid interparticle pores caused by corpuscle agglomeration23 and which appear during the drying process of the silica gel microstructure.25 From the results shown in Tables 2 and 3, it can be established that SiO2-A presents the largest micropore volume (W0), specific surface area (AS), and total pore volume (VΣ). For this reason, this solid was chosen as the precursor in order to synthesize the Ag-doped solid substrate that was labeled as Ag/SiO2-A. BTX Adsorption. Isotherms of the adsorption of BTX hydrocarbons at different temperatures on the SiO2 adsorbents are presented in Figures 4 and 5, and their corresponding Langmuir plots are presented in Figures 6 and 7. The pulse maxima method13 was utilized for evaluating the adsorption isotherms from the pulse data at the desorption edges of the elution curves for the BTX adsorbates on the SiO2 adsorbents. The amount of BTX adsorptive taken up by the silica substrates showed a strong thermal dependence26 in the range of temperatures studied. In Figure 5, the adsorption isotherms of the BTX hydrocarbons on Ag/SiO2 are depicted in the previously described range of temperatures (423-573 K). These isotherms show a behavior similar to that shown by the corresponding isotherms on the SiO2-A silver-free precursor adsorbent (see Figure 4). It should be mentioned, however, that the adsorption process in microporous adsorbents of the type studied here is favored by the increase of the temperature (i.e., there arises an activated diffusion process). This kind of effect is due to the diffusion of adsorbate molecules through very narrow constrictions into the cavities beyond.16 The adsorption isotherms of BTX hydrocarbons exhibit type I nearly rectilinear isotherms, thus meaning a strong interaction

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1783

Figure 5. Adsorption isotherms of aromatic BTX hydrocarbons on Ag/SiO2-A at different temperatures.

Figure 7. Langmuir plots for the adsorption of aromatic BTX hydrocarbons on Ag/SiO2-A at different temperatures.

Figure 8. Variation of the adsorption saturation limit (am) with temperature for aromatic BTX hydrocarbons on SiO2 adsorbents: (a) SiO2-A; (b) Ag/SiO2-A.

Figure 6. Langmuir plots for the adsorption of aromatic BTX hydrocarbons on SiO2-A at different temperatures.

between the SiO2 surface and the BTX hydrocarbon molecules. These types of isotherms are similar to straight-line type I isotherms, which have been reported for the adsorption of n-alkanes on zeolites ZSM-527 or of hydrocarbons on slit-shaped micropores of activated carbon fibers.28 The temperature dependence of the monolayer adsorption capacity (am) of each adsorptive, derived from the Langmuir plots, is shown in Figure 8 for both types of SiO2 materials. The am versus T trend is basically ascending for BTX adsorptives on both SiO2-A and Ag/

SiO2-A samples. The temperature dependence of Henry constants (KH) for BTX adsorption on SiO2-A and Ag/ SiO2-A adsorbents is shown in Figure 9; in every case, the KH tendency is preponderantly to increase with temperature. The corresponding values of am and KH are listed in Tables 4 and 5. Figure 10 shows the plots of the standard adsorption energy ∆U0 versus molecular weight of BTX compounds adsorbed on SiO2-A and Ag/SiO2-A samples. ∆U0 values indicate that a greater interaction occurs between adsorbent and adsorbate molecules as the molecular mass of the adsorptive is enlarged (i.e., as the number of carbon atoms in the aromatic molecule increases); this fact has been previously observed.29 The DR equation applied to each isotherm provides fair straight lines (Figures 11 and 12); the adsorption

1784 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

Figure 10. Variation of the standard energy of adsorption (∆U0) with the molecular weights of BTX hydrocarbons: (a) SiO2-A; (b) Ag/SiO2-A.

Figure 9. Temperature dependence of Henry’s law constants for the adsorption of aromatic BTX hydrocarbons on silica samples: (a) SiO2-A; (b) Ag/SiO2-A. Table 4. Henry Constants, Langmuir Monolayer Capacities, and Standard Adsorption Energies of BTX Adsorptives on the SiO2-A Substrate adsorbate benzene toluene p-xylene

T (K)

am (mmol g-1)

KH × 103 (Torr-1)

-∆U0 (kJ mol-1)

423 473 523 423 473 523 423 473 523

0.235 0.423 0.359 0.207 0.403 0.405 0.3 0.240 0.436

6.1 4.43 15.5 8.53 5.67 20.1 5.37 15.2 34

16.14 14.63 34.00

a a m ≡ monolayer capacity; KH ≡ Henry constant; -∆U0 ≡ standard adsorption energy.

Table 5. Henry Constants, Langmuir Monolayer Capacities, and Standard Adsorption Energies for Adsorption of BTX Adsorptives on Ag/SiO2-A adsorbate benzene

toluene

p-xylene

T (K)

am (mmol g-1)

KH × 103 (Torr-1)

-∆U0 (kJ mol-1)

423 455 523 573 423 455 523 573 423 455 523 573

0.409 0.381 0.362 0.712 0.258 0.266 0.464 0.681 0.288 0.214 0.580 0.599

0.933 2.47 7.47 5.43 2.11 5.35 7.63 11.0 1.76 10.16 9.56 29.0

31.05 Figure 11. DR plots for the adsorption of BTX hydrocarbons on SiO2-A at different temperatures. 33.73

the fractional filling coverage reaches a value θ ) 1/e. The isosteric heat of adsorption attained when θ ) 1/e, i.e., qst,θ)1/e, can be expressed as

41.67

qst,θ)1/e ) βE0 + ∆Hliq

saturation amounts in the micropores, designated as a0 (mmol g-1), and E0, the characteristic adsorption energy (kJ mmol-1), could be determined from DR plots. Values of a0 and E0 are listed in Tables 6 and 7. The βE0 parameter in the DR equation is associated with the isosteric heat of adsorption (qst) that is attained when

(7)

where ∆Hliq is the heat of liquefaction30 (∆Hliq values for each BTX hydrocarbon are shown in Table 1) and qst is thermodynamically related to qd (the differential heat of adsorption) by the expression qst ) qd - RT. Consequently, a change in qst corresponds to one of similar magnitude in qd. Around θ ) 1/e, the qst value of each adsorbate is scarcely affected by a change in the

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1785

Figure 13. Variation of the isosteric heat of adsorption (qst) of BTX hydrocarbons with adsorbate loading on SiO2 adsorbents: (a) SiO2-A; (b) Ag/SiO2-A.

Figure 12. DR plots for the adsorption of BTX hydrocarbons on Ag/SiO2-A at different temperatures. Table 6. DR Parameters for BTX Adsorption on SiO2-Aa

adsorbate benzene toluene p-xylene

T (K)

a0 (mmol g-1)

W0 (cm3 g-1)

-E0 (kJ mmol-1)

qst,θ)1/e (kJ mmol-1)

R corr. coeff

423 473 523 423 473 523 423 473 523

0.113 0.726 0.645 0.091 0.561 1.806 0.130 0.219 2.037

0.010 0.069 0.062 0.010 0.066 0.214 0.018 0.031 0.293

11.309 9.744 13.714 11.238 9.827 11.297 9.576 11.993 11.544

41.629 40.064 44.034 44.418 43.007 44.477 45.246 47.663 47.214

0.999 0.997 0.999 0.997 0.998 0.998 0.999 0.999 0.999

a a ≡ DR micropore saturation adsorption; W ≡ DR micropore 0 0 volume; E0 ≡ characteristic adsorption energy; R ≡ linear correlation coefficient.

Table 7. DR Parameters for BTX Adsorption on Ag/SiO2-A

adsorbate benzene

toluene

p-xylene

T (K)

a0 (mmol g-1)

W0 (cm3 g-1)

-E0 (kJ mol-1)

qst,θ)1/e (kJ mol-1)

R corr. coeff.

423 455 523 573 423 455 523 573 423 455 523 573

0.054 0.127 0.390 1.435 0.036 0.120 0.803 1.310 0.050 0.061 2.333 1.456

0.005 0.012 0.037 0.138 0.004 0.014 0.095 0.155 0.007 0.008 0.009 0.209

12.059 12.392 13.757 13.561 12.667 11.830 11.770 14.152 9.870 14.684 9.991 14.758

42.779 43.112 44.477 44.281 45.847 45.01 44.95 47.332 45.54 50.354 45.661 50.428

0.999 0.998 0.996 0.996 0.999 0.996 0.997 0.999 0.994 0.999 0.997 0.999

adsorption temperature, as is shown in Tables 6 and 7. Therefore, the magnitude of the isosteric heat of adsorption evaluated at a coverage θ ) 1/e, i.e., qst,θ)1/e, reflects the adsorption energy involved during micropore filling. From Tables 6 and 7, it can be seen which kind of

relationship exists between the saturation micropore adsorption and qst,θ)1/e. qst values for SiO2 adsorbents containing an important amount of ultramicropores are higher than those corresponding to SiO2 substrates made mostly by supermicropores. The higher qst,θ)1/e is for an SiO2 adsorbent, the bigger the adsorption energy is because of an increased overlapping between adsorption potentials arising from opposite wall sides of the micropore void. Isosteric Heat of Adsorption. The isosteric heat of adsorption qst (kJ mol-1) of BTX on SiO2-A and Ag/ SiO2-A at different adsorbate loadings was evaluated from the adsorption isotherm data (see Figures 4 and 5) by means of a Clausius-Clapeyron-type equation and is depicted in Figure 13. For both SiO2-A and Ag/SiO2-A samples, these heat values conform to an order of qst of p-xylene < toluene < benzene and are owed in great part to the interactions among the hydrogen alkyl atoms of BTX compounds and the silanol groups (Si-OH) attached to the silica surfaces. By observation of Figure 13, it seems that the surface of the Ag/SiO2 microporous material is more energetic and heterogeneous than that associated with the pure silica microporous material; this assertion is based on the fact that for homogeneous surfaces a diminution of qst with the amount adsorbed31 follows about the same pattern as that seen in Figure 13 for the Ag/SiO2 substrate. This Ag-doped material is consistent with a model of a quasi-homogeneous surface,32 in which some atoms of a doping material (i.e., Ag atoms) are distributed at random across the surface of the SiO2 substrate, thus modifying the adsorption properties with regard to those of the dopant-free SiO2 surface. BTX adsorption energetics may also influence the ionization potential of the adsorptives. The ionization potentials of pure BTX adsorptives are in the following order: benzene (9.3 eV) > toluene (8.8 eV) > p-xylene (8.5 eV). See Table 1. An increase in the molecular weight of the adsorptive can also result in an enhancement of the isosteric heat of adsorption because of the increased dispersion forces. However, in the present case, the increase in the isosteric heats of adsorption is too large to be caused merely by the increased dispersion forces. The presence of rising numbers of methyl groups attached to the aromatic rings of the adsorbates causes an increase in the

1786 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Table 8. Mean Isosteric Heats of Adsorption (kJ mol-1) of BTX Adsorbates on Assorted SiO2 Adsorbents adsorbates adsorbent

benzene

toluene

p-xylene

Ag/SiO2-A SiO2-A Si-MCM-41 NaBeCaLita

29.8 28.1 52.5 21.5

38.5 33.1 60.7 23.7

43.6 40.8 64.8 29.8

electron density of the aromatic ring and consequently provokes a decrease of the ionization potential according to the number of methyl groups of the adsorptive. This phenomenon produces a differentiation between the magnitudes of the BTX isosteric heats of adsorption occurring in either SiO2 or Ag/SiO2 microporous adsorbents just in the same way as that shown in Figure 13. In the case of the pure SiO2 microporous substrate, there exist high interaction energies between hydrogen atoms attached to the methyl groups of the BTX adsorbates and the oxygen atoms of the silanol groups adhered to the microporous walls of the silica adsorbent. On the other hand, in the case of the Ag/SiO2 adsorbent, besides the interaction energy between the hydrogen atoms of the BTX methyl groups of the adsorptives and the oxygen atoms of the superficial silanol groups, there is an additional interaction with silver atoms that is distributed along the walls of the micropores. For the pure SiO2 adsorbent, this interaction energy increase is approximately 8.7 kJ mol-1 per each additional methyl group on going from benzene to toluene and 3.9 kJ mol-1 when moving from toluene to p-xylene. In turn, for the Ag/SiO2 sample, there is an increase of 5.0 kJ mol-1 when going from benzene to toluene and of 7.7 kJ mol-1 on changing from toluene to p-xylene. The mean isosteric heats of adsorption (qst, kJ mol-1) (averaged over the range of different adsorbate loadings studied) for the adsorption of benzene, toluene, and p-xylene on the adsorbents studied here are presented in Table 8. In the same table there are also mean qst values corresponding to the uptake of BTX adsorptives on adsorbents previously reported.27,33 Conclusions From the present studies on the adsorption of aromatic BTX hydrocarbons on pure and Ag-doped SiO2 microporous adsorbents, the following conclusions can be drawn. 1. The adsorption isotherms of all aromatic BTX hydrocarbons follow both Langmuir and DR adsorption models. 2. The differences between the heats of adsorption of BTX adsorptives on microporous pure and Ag-doped SiO2 adsorbents are related to (i) the electron density of each aromatic BTX ring and (ii) the interaction energies between the hydrogen atoms attached to the methyl groups of the BTX adsorbates and the oxygen and silver atoms that are located on the silanol groups or anchored on the walls of the micropores, respectively. 3. The SiO2-A microporous surface is relatively heterogeneous from an energetic point of view if compared to the more homogeneous character of the Ag/SiO2 substrate. Additionally, the presence of Ag atoms within the silica structure homogenizes the surface energy, turning it more specifically toward the nature of the adsorptive employed.

Nomenclature a ) amount adsorbed per unit mass of adsorbent am ) Langmuir monolayer capacity a0 ) micropore saturation adsorption ASB ) BET specific surface area ASL ) Langmuir specific surface area ASt ) external surface area calculated through the t method CB ) BET constant E0 ) characteristic adsorption energy KH ) Henry constant IP ) ionization potential L ) length M ) molecular weight P ) pressure qst ) isosteric heat of adsorption (kJ mol-1) qst,θ)1/e ) isosteric heat of adsorption evaluated at θ ) 1/e Sm ) supermicropores (pores of width between 0.7 and 2 nm) Um ) ultramicropores (pores of width lesser than 0.7 nm) VΣ ) volume adsorbed close to the saturation pressure (p/ p0 ∼ 0.95) and calculated as volume of liquid (Gursvitch rule) W0 ) micropore volume R-SiO2 ) reference material RS ) Sing’s Rs parameter s ) critical diameter θ ) a/am, coverage ∆U0 ) standard adsorption energy ∆Hliq ) heat of liquefaction

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Received for review June 28, 2002 Revised manuscript received September 29, 2003 Accepted November 5, 2003 IE0204888