Alkane Adsorption on Microporous SiO2 Substrata. 1. Textural

262

Energy & Fuels 2003, 17, 262-270

Alkane Adsorption on Microporous SiO2 Substrata. 1. Textural Characterization and Equilibrium M. A. Hernańdez* and J. A. Velasco Departamento de Investigacio´ n en Zeolitas, Instituto de Ciencias de la Universidad Auto´ noma de Puebla, Me´ xico, Edificio 76, Complejo de Ciencias, Ciudad Universitaria, CP 72570, Puebla, Puebla, Me´ xico, and Posgrado de Ciencias Ambientales, Instituto de Ciencias de la Universidad Auto´ noma de Puebla, Me´ xico

M. Asomoza, S. Solı´s, F. Rojas, and V. H. Lara Departamento de Quı´mica, Universidad Auto´ noma MetropolitanasIztapalapa, Iztapalapa, Me´ xico

R. Portillo and M. A. Salgado Facultad de Ciencias Quı´micas, Universidad Auto´ noma de Puebla, Me´ xico, Received May 28, 2002

Microporous silica solids of SiO2 and Ag/SiO2 (a microporous SiO2 substrate containing 5 wt % Ag) are synthesized by the sol-gel method and analyzed through several characterization techniques. The type I shapes of N2 sorption isotherms at 76 K on these materials indicate large micropore contents as well as important cohesive interactions between adsorbed N2 molecules, i.e., a gradual cooperative filling process becomes more and more evident as the concentration of adsorbate within the porous structure increases. In addition, negative values of the BET constant are found thus indicating that the adsorption mechanism on these solids can be better described in terms of a volume filling process rather than in terms of a multilayer formation phenomenon. The microporosity existing in these substrata is studied through Sing’s Rs-plots. The adsorption capacities of n-alkanes (i.e. n-hexane, n-heptane, n-octane, and n-nonane) on SiO2 and Ag/SiO2 substrata are measured by gas chromatography (GC) at different temperatures. The uptake of every hydrocarbon sorptive, on both SiO2 and Ag/SiO2 substrata, is found to be temperaturedependent. Additionally, the intensities of attractive interactions between hydrocarbon ad molecules and the surfaces of the above solids are evaluated for each adsorbate type from determinations of the isosteric heats of adsorption.

Introduction The continuous enhancement of industrialization and transport necessities demands large amounts of oil derivatives with the consequent vast pollution incidence. The substances that mainly cause atmospheric contamination are polluting agents such as gases and solids that can exist either mixed or suspended in the atmospheric air.1 There are several hundreds of classified polluting substances in the atmosphere. Within these substances, volatile organic compounds (VOCs) stand out due to their high toxicities. VOCs pollutants have acquired a great deal of attention during recent years. VOCs include a large number of chemical species that are toxic for our health and which are one of the main sources of dangerous photochemical reactions in the atmosphere thus representing a variety of environmental threats. On the other hand, the recovery of this type of com* To whom correspondence should be addressed. E-mail: mighern@ siu.buap.mx. (1) Won, D.; Corsi, R. L.; Rynes, M. Environ. Sci Technol. 2000, 34, 4193-4198.

pounds represents important commercial and safety advantages due to the possibility of entrapping VOCs inside adsorbents endowed with large sorption capacities; safety enhancement aspects are evident during the transport and dosing of VOCs since these operations become more reliable when these volatile substances are stored within porous solids. The main VOCs pollution sources are directly or indirectly related to oil and its byproducts. Some very important polluting sources of this kind are for instance: (i) automobile exhaust emissions due to incomplete hydrocarbon combustion (aromatics, olefins, and paraffins), (ii) vapor releases associated to the paint, dye, lacquer and varnish industries (alkanes and cycloalkanes), (iii) gas fumes given off from storage tanks, (iv) solvent vapors emanating from paints as well as from liquids utilized in cleaning or degreasing operations (hexane, cyclohexane, and aromatic compounds derived from toluene and xylene), (v) adhesives (methyl ethyl ketones, naphtha type derivatives, trichororoethane), (vi) aerosols, and (vii) discharges originated by

10.1021/ef020119o CCC: $25.00 © 2003 American Chemical Society Published on Web 02/06/2003

Alkane Adsorption on SiO2 Substrata

plastic industries (chlorated compounds).2 It is also pertinent to mention that adsorption processes constitute a current valuable option to remove VOCs from process gas streams.3 In industrial applications many solids that possess pores near to molecular dimensions (i.e., micropores) are often used as selective adsorbents, because of the physicochemical specificity advantages these substrata provide in contrast to usual mesoporous adsorbents. Adsorbents of these selective properties include activated carbons, zeolites, and microporous silica. Porous silica is one of the various forms of amorphous silica. Other forms are nonporous SiO2 precipitates; silica hydrogels; pyrogenic materials, such as aerosil; and mineral solids, such as opal; etc. These substances vary considerably in their appearance, hardness, and hydration degree, but all of them can be considered as polycondensation products of orthosilisic acid, Si(OH)4. The usual methods to prepare these siliceous substrata are precipitation with acids from silicate solutions or hydrolysis of silicon derivatives such as tetraethoxysilane (TEOS).4 The silica gel surface contains silanol (t Si-O-H) and siloxane (tSi-O-Sit) groups. It is known that, in most cases, silanol groups act as adsorption sites. When adsorbate molecules interact with silanol surface groups, there is also the possibility that these molecules can interact not only with one but also with two or more silanol groups at a time.5 Unlike mesoporous SiO2 solids obtained from colloidal dispersions of silicic acid,6 SiO2 synthesized from the sol-gel method is stable toward (i) thermal treatments up to 700 °C, (ii) gentle steam exposures, (iii) mechanical grinding, and also toward (iii) mild acid treatment. Because of their significant surface areas, considerable adsorption capacities, remarkable thermal/hydrothermal stabilities and pore structures, microporous SiO2 materials synthesized by the sol-gel technique have a high potential for practical uses such as adsorption of either small or bulky adsorbate molecules.7 Therefore, it should be of great scientific and practical interests to explore the adsorption properties of sol-gel synthesized microporous silica with respect to different adsorbates. Furthermore, studies about hydrocarbon adsorption, particularly those associated to bulky molecules, on microporous SiO2 substrata are still scarce. Adsorptions of N24 and CO2 on microporous SiO2 have been thoroughly investigated. A few studies have also been reported on the adsorption of hydrocarbons such as butane and butene8 on this kind of solids. Gordon et al.9 have carefully investigated the adsorption of nalkanes on microporous SiO2. Limpo et al.10 have (2) Lordgooei, M.; Rood, M. J.; Abadi, M. R. Environ. Sci Technol. 2001, 35, 613-619. (3) Bathen, D.; Traub, H. S.; Simon, M. Ind. Eng. Chem. Res. 1997, 36, 3993-3994. (4) Brinker, C. J.; Wallace, S.; Raman, N. K.; Sehgal, R.; Samuel, J.; Contakes, S. M. In Access in Nanoporous Materials; Pinnavaia, T. J., Thorpe, M. F., Eds.; Kluwer Academic Publishers: Boston, Ma, 1996; pp 123-139. (5) Suzuki, T.; Tamon, H.; Okazaki, M. In Proceedings of Fundamentals of Adsorption V; Le Van, M. D., Ed.; Kluwer Academic Publishers: Boston, MA, 1996; pp 897-904. (6) Suzuki, M. In Proceedings of Fundamentals of Adsorption V, Le Van, M. D., Ed.; Kluwer Academic Publishers: Boston, MA, 1996; pp 3-14. (7) Asomoza, M.; Solis, S.; Hernandez, M. A. In Proceedings of Second International Conference on Silica Science and Technology; Haidar, B., Ed.; Institut de Chimie des Surfaces et Interfaces (ICSICNRS): Mulhouse, France, 2001; pp 345-348.

Energy & Fuels, Vol. 17, No. 2, 2003 263

reported studies on the characterization of silica materials synthesized from TEOS and calcined at several temperatures. Narayan et al.11 have studied the porosity of silica solids. These latter materials consist of tailormade pore sizes and shapes and they are particularly important in applications where molecular recognition is needed, such as shape-selective catalysts, molecular sieves, chemical sensors, and selective adsorbents. The present work has been undertaken with the aim of measuring the adsorption isotherms of a series of n-alkane hydrocarbons (viz. hexane, heptane, octane, and nonane) on SiO2-A and Ag/SiO2-A microporous adsorbents. With this purpose in mind, hydrocarbon adsorption isotherms on microporous SiO2 solids at different temperatures were measured through the gas chromatographic (GC) peak maxima technique.12 In this paper, we are also reporting isosteric heats of adsorption evaluated at different n-alkane loadings. Finally, we examine sorption capacities and isosteric heats adsorption in terms of the structural properties of our assortment of silica aerogels. Experimental Section Materials. High-purity He (> 99.998%) and analytical grade n-alkanes were used for gas chromatographic (GC) adsorption studies. High-purity N2 (> 99.99%) was also selected to complement the textural studies of microporous silica. A reference macroporous solid, which is needed for the estimation of micropore volumes through the Rs-method, was mined from Tehuacań, a region in the state of Puebla, Mexico. X-ray diffraction identifies this reference substrate as R-quartz. Synthesis of microporous SiO2. Silica monoliths were synthesized starting from reacting mixtures of tetraethyl orthosilicate (TEOS), alcohol (methanol, ethanol, propanol)) and water. Gels were prepared as follows: TEOS is first dissolved in the selected alcohol and then mixed with deionized water according to 1: 4: 2.5 ratios, respectively, and poured inside a glass column that was maintained at 303 K for several days during which the hydrolysis-condensation reactions took place. The resulting sol was kept in the original glass column so that gelation occurred eventually. The 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 during the sol-gel process), SiO2-B (ethanol being used as the TEOS solvent), and SiO2-C (propanol being used as the TEOS solvent) were thermally stable while possessing significant surface area to pore volume quotients, a characteristic that gives place to important physical and chemical properties. The Ag/SiO2 specimen was synthesized in the following way. The required quantity of AgNO3 to reach a 5 wt % Ag containing silica substrate was added to the water used in the SiO2-A synthesis; otherwise, the conditions for the preparation of this specimen were the same as those related to the attainment of silver-free solids. The Ag-containing substrate was labeled as Ag/SiO2-A. The silica materials synthesized in this work are suitable for high-temperature industrial applications. The Ag/SiO2 sample is endowed with (8) Jaguiello, J.; Bandosz, T. J.; Putyiera, K.; Schwarz, J. A. In Proceedings of Fundamentals of Adsorption V; Le Van, M. D., Ed.; Kluwer Academic Publishers: Boston, MA, 1996; pp 417-424. (9) Gordon, P. A.; Glandt, E. D. in Proceedings of the Fundamentals of Adsorption VI; Meunier, F., Ed.; Elsevier: Paris, 1998; pp 105110. (10) Limpo, J.; Bautista, M. C. In Proceedings of the Characterization of Porous Solids III; Rouquerol, J., Rodriguez-Reinoso, F., Sing, K. S. W., Unger, K. K., Eds.; Elsevier: Netherlands, 1994; pp 429437. (11) Narayan, K. R.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682-1701. (12) Choudary, V. R.; Mantri, K. Langmuir 2000, 16, 7031-7037.

264 Energy & Fuels, Vol. 17, No. 2, 2003

Herna´ ndez et al. and calculated as volume of liquid.15 Pore size distributions (PSD) of the materials under study were evaluated by classical methods with the result of having microporous adsorbents endowed with a notable absence of both meso and macropores. Micropore volumes W0 were calculated through the Rs-plot method16 considering an Rs range from 0.5 to 1.75 (Rs is Sing’s standard reduced parameter defined as the ratio of the volume adsorbed at current p/p0 against the volume adsorbed at p/p0 ) 0.4). A macroporous silica (R-SiO2, i.e., R-quartz) was used as the adsorbent of reference that is needed to perform the Rs-plot method. All data corresponding to the adsorption of hexane, heptane, octane, and nonane on SiO2 samples could be very well fitted to the Langmuir adsorption equation thus indicating a homogeneous repartition of the adsorption sites on the silica substrata. This premise is consistent with the fact that adsorption data associated to all adsorptive hydrocarbon molecules gave excellent fits with the Langmuir equation:17

Figure 1. Elution chromatogram of n-hexane from adsorbent SiO2-A at different temperatures. a gray color and details of a similar synthesis procedure have been previously described.13 Measurements. X-ray diffraction (XRD) spectra were determined by means of a Siemens D-500 diffractometer employing nickel-filtered KR radiation. All N2 adsorption isotherms were measured at the boiling point of liquid nitrogen (76 K in Me´xico City) in an automatic volumetric adsorption system (Micromeritics ASAP 2000). N2 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 during 20 h at a pressure lesser than 10-6 Torr. Chromatographic n-alkane sorption experiments were carried out in a GC-14A Shimadzu gas chromatograph equipped with a thermal conductivity detector. The chromatographic columns were made of glass and packed with the 60-80 mesh silica samples to be studied. Prior to the adsorption experiment, adsorbents were pretreated in situ under a flow of the He carrier gas at 573 K during 8 h. Each n-alkane was injected separately in order to measure its corresponding retention time inside the appropriate adsorbent column. Adsorption isotherms of n-hexane, n-heptane, n-octane and n-nonane on SiO2-A and Ag/SiO2-A samples were evaluated at different temperatures: 423-523 K for SiO2-A and 423573 K for Ag/SiO2-A by applying the GC peak maxima method12 and employing He (35 cm3 min-1) as the carrier gas. Equilibrium pressures ranged between 10-3 and 180 Torr. Adsorbate pulses of different intensities were injected into SiO2-A and Ag/SiO2-A packed columns (i.d. ) 5 mm; length ) 50 cm) kept at a chosen temperature and the elution chromatogram of each pulse was registered until the recorder pen reached once more the base line (see Figure 1). The isosteric heats of adsorption of the n-alkanes were calculated from sorption data evaluated at different temperatures. Calculation Methods and Procedures. BET and Langmuir specific surface areas14 of the materials under study were evaluated from N2 adsorption data at 76 K in the relative pressure range from 0.04 to 0.2. The total pore volume was estimated on the basis of the volume adsorbed at p/p0 ∼ 0.95, (13) Asomoza, M.; Dominguez, M. P.; Solis, S.; Lara, V. H.; Bosch, P.; Lopez, T. Mater. Lett. 1998, 36, 249-253. (14) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierrotti, R.; Rouquerol, J.; Siemienieswka, T. Pure Appl. Chem. 1985, 57, 603-619.

θ ) a/am ) KL p/(1 + KL p)

(1)

where a (mmol g-1) is the specific equilibrium amount adsorbed, at pressure p and temperature T, and am is the limiting molar saturation capacity of the monolayer. The quantity amKL (KL being the Langmuir constant) is equal to KH (the Henry constant) while am and amKL can be both evaluated graphically by plotting 1/a versus 1/p, i.e.:

(1/a) ) (1/am) + (1/amKL)(1/p)

(2)

Standard adsorption energies (-∆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/∂T) ) ∆U0/RT2;

KH ) K0e-∆U0/RT

(3)

where R is the universal gas constant and K0 is van’t Hoff’s preexponential factor. The isosteric heat of adsorption qst (kJ mol-1) at different adsorbate loadings can be evaluated from the adsorption isotherms data through a Clausius-Clapeyron type equation: 18

[∂ ln p/∂T]a ) + qst(a)/RT2

(4)

where p and T are the equilibrium pressure and temperature at a given adsorbate loading.

Results and Discussion Characterization of SiO2. XRD analysis of SiO2 specimens (see Figure 2) supply typical patterns related to SiO2-A and R-SiO2 reference material. The amorphous SiO2 structure of the synthesized SiO2-A 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 3. These isotherms correspond to type Ib19 something that is (15) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982; p 113. (16) Sing, K. S. W. in Principles and Applications of Pore Structural Characterization; Haynes, J. M., Rossi-Doria, P., Eds.; Arrowsmith: Bristol, 1985; pp 1-11. (17) Dunne, J. A.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1997, 13, 4333-4341. (18) Rudzinski, W.; Everett, D. H. Adsorption of Gases on Heterogeneous Surfaces, Academic Press: San Diego, 1992; p 7.



Table 1. Textural Parameters of SiO2 Adsorbents, Calculated through Different Methods of Analysisa adsorbents R-SiO2 SiO2-C SiO2-B SiO2-A

VB, cm3 STP g-1

VL, cm3 STP g-1

Vt, cm3 STP g-1

AsB, m2 g-1

AsL, m2 g-1

Ast, m2 g-1

CB

V∑, cm3 g-1

0.616 122.6 140.9 164.9

0.985 188.7 191.2 224.3

0.654 157.4 163.3 187

2.680 533.6 613.4 717.7

4.28 821.5 832.8 976.5

2.67 641.5 655.5 762

240 -59 -30 -1007

0.005 0.293 0.308 0.370

a R-SiO is the reference material. V and As , are the BET monolayer capacity and specific surface area, respectively. V and As are 2 B B L L the Langmuir monolayer capacity and specific surface area, respectively. Vt and Ast are the ordinate and external surface area calculated from de Boer’s t-plot. CB is the BET constant, and VΣ is the volume adsorbed close to saturation (p/p0 ∼ 0.95), calculated as volume of liquid (Gursvitch rule).

Table 2. Micropore Volumes W0 (cm3 g-1) Corresponding to Supermicropores and Ultramicropores Present in SiO2 Adsorbents and Calculated by the rs Methoda sample SiO2-A SiO2-B SiO2-C

micropore type

W0

sm um sm um sm um

0.134 0.372 0.129 0.313 0.122 0.296

a W is given in cm3 liquid per gram of adsorbent; sm means 0 supermicropores, and um means ultramicropores (finest micropores).

Figure 2. XRD patterns of SiO2 samples. (a) SiO2-A specimen and (b) macroporous R-quartz used as reference material.

Figure 3. N2 sorption isotherms at 76 K on SiO2 samples. (a) SiO2 reference solid, (b) SiO2-C, (c) SiO2-B, and (d) SiO2-A.

indicative of the microporous nature of the whole series of silica substrata. Important textural parameters of these substrata are given in Table 1. The shapes of the adsorption isotherms also reflect the high degree of attractive interaction existing between adsorbate molecules (i.e. a cooperative phenomenon that is responsible

of the rounded knee aspect of the isotherms arises during micropore filling). BET constants (CBET) are negative for the three types of silica adsorbents. This is associated to the fact that adsorption in micropores20 is far from a BET filling mechanism. At p/p0 > 0.4, it is observed the incidence of a plateau (saturation zone) in the isotherm that extends from relatively low relative pressures up to p/p0 ∼ 1. This characteristic strongly suggests that, besides of the absence of mesopores and macropores, adsorption in microporous silica resembles the mechanism of volume filling. Micropore volumes associated to supermicropores (i.e., pore width between 0.7 and 2 nm) and ultramicropores (pore width lesser than 0.7 nm) are listed in Table 2; 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 as or Gurvitsch’s limiting adsorption volume VΣ. In general, it seems that the adsorption capacity of our SiO2 substrata increases according to the amount 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. The Rs plots shown in Figure 4 are constructed from isotherm data trough procedures described in the literature.19 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 (19) Sing, K. S. W. In Third International Conference on Fundamentals of Adsorption, Mersmann, A. B., Scholl, S. E., Eds.; Engineering Foundation Location: New York, 1989; p 70. (20) Remy, M. J.; Poncelet, P. J. Phys. Chem. 1995, 99, 773-779.

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Figure 4. Rs-plots for SiO2 samples. (a) SiO2-C, (b) SiO2-B, and (c) SiO2-A.

by corpuscle agglomeration21 and that appear during the drying process of the silica gel microestructure.12 From the results shown in Table 2, 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 in order to obtain a new kind of solid substrate that was labeled as Ag/ SiO2-A. n-Alkane Adsorption. Adsorption isotherms of nalkanes (hexane, heptane, octane, and nonane) at different temperatures on the SiO2 adsorbents are presented in Figures 5 and 6 and their corresponding Langmuir plots are portrayed in Figures 7 and 8. From the adsorption isotherms on adsorbent SiO2-A, Figure 5, it is observed that there is a preferred adsorption of nonane, i.e., this adsorbate is retained in a greater proportion than any of the other adsorbates between a given pressure range. It can be observed: (i) that the increase in the temperature causes a decrease in the adsorption capacity of the silica specimens toward these hydrocarbons and (ii) that the separations among these curves are almost completely lost at a temperature of around 523 K. This last effect can be attributed to the greater mobilities of the adsorbed alkane molecules inside the SiO2 micropores as the temperature is raised. Nevertheless, there also exist the possibility of having adsorbate-adsorbate interactions and this phenomenon depends on how many molecules are distributed throughout the SiO2 microstructure. From these figures, it is observed that the adsorption process is favored by a decrease of temperature. The adsorption isotherms of n-alkanes on Ag/SiO2-A are shown in Figure 6 in the previously mentioned temperature range (i.e., 423-573 K). These isotherms show behaviors similar to those followed by the isotherms corresponding to the precursor SiO2-A adsorbent. Nonetheless, at temperatures lower than 458 K, the intensities of these sorption processes are favored by a decrease of the temperature. n-alkanes exhibit (21) Kaneko, K. In Equilibria and Dynamics of Gas Adsorption on Solid Surface; Rudzinski, W., Steele W., Zgrablich, G., Eds.; Elsevier: Amsterdam, 1994; pp 679-714 (682).

Figure 5. Equilibrium isotherms of n-alkanes at different temperatures on adsorbent SiO2-A. (a) n-hexane, (b) n-heptane, (c) n-octane, and (d) n-nonane.

isotherms similar to asymptotic type I sorption curves, something that has been previously reported for the adsorption of n-alkanes on ZSM-5 zeolites.22 Saturation Langmuir limits (am), Henry constants (KH), and adsorption energies (∆U0) together with their corresponding linear correlation coefficients R (calculated from the pertinent linear plots) for the adsorption of n-hexane, n-heptane, n-octane, and n-nonane on the SiO2-A and Ag/SiO2 adsorbents are all listed in Tables 3 and 4. The temperature dependence of Henry constants can be seen in Figure 9. The increase in sorption energy with carbon number follows a growing pattern (Figure 10), at least up to nonane. Saturation Limit. The temperature dependences of the saturation limit, derived from the Langmuir plots, are shown in Figure 11 for the cases of samples SiO2-A and Ag/SiO2-A, respectively. The trends are somewhat more consistent when saturation limits are expressed as mmol g-1, rather than as weight percent. The saturation limit decreases with rising temperature. These results are consistent with the hypothesis that at a sufficiently high temperature a limiting uptake will be approached. Such result would reveal conformity with the ideal Langmuir model since, for sufficiently large molecules, steric restrictions imposed by the pore system greatly reduce the time of residence and therefore the interaction potential between neighboring admolecules. (22) Sing, K. S. W.; Roberts, A. R.; Tripathi, V. Langmuir 1987, 3, 331-335.



Figure 6. Equilibrium isotherms of n-alkanes at different temperatures on adsorbent Ag/SiO2-A. (a) n-hexane, (b) nheptane, (c) n-octane, and (d) n-nonane.

Isosteric Heat of Adsorption. Variations of qst (kJ mol-1) according to the amounts of n-alkanes adsorbed on SiO2-A are shown in Figure 12. The heats of adsorption of n-hexane, n-heptane, n-octane and n-nonane on this SiO2-A adsorbent conform the following sequential order: n-hexane < n-heptane < n-octane < n-nonane. For every alkane adsorbate, the heat of adsorption increases with rising adsorbate loading. Analysis of these results reveal that, for the case of hexane adsorbed on SiO2-A, there is a slight increase in qst when the amount of adsorbed substance is raised. This fact may be attributed to the adsorbate-adsorbent interactions due to the high energetic interaction that arises between n-alkane hydrogen atoms and oxygen atoms of the silanol groups attached to the channel walls.23 However, as the length of the n-alkane chain increases the growth of qst with coverage is still more pronounced. The isosteric heats of adsorption for nheptane, n-octane, and n-nonane increase from an initial value (at a coverage θ ) a/am of about 0.015) up to a maximum amount occurring at around 80% of the maximum uptake (see Figure 12). Furthermore, the initial qst value (at θ ) 0.015) is larger the longer is the n-alkane chain length. The overall sorption potential (ΨTot), for a nonpolar adsorbate on a high silica adsorbent at low θ, can be considered as the sum of nonspecific potential interactions arising from dispersion (ΨD) (23) Suzuki, T.; Tamon, H.; Okazaki, M. In Proceedings of Fundamentals of Adsorption V; Le Van, M. D., Ed.; Kluwer: Boston, MA, 1996; pp 905-912.

Figure 7. Corresponding Langmuir plots for adsorption of n-alkanes on SiO2-A, at different temperatures. (a) n-hexane, (b) n-heptane, (c) n-octane, and (d) n-nonane.

and repulsion (ΨR) forces, as well as from electrostatic interactions associated with the polarizability of the adsorbate molecule (Ψp):

ΨTot ) ΨD + ΨR + Ψp

(5)

For n-alkanes (i.e., nonpolar molecules) the mean polarizability R of the molecules increases with chain length,24 and therefore the ΨD potential, which is a function of the polarizability, and qst should increase with n-alkane chain length. The ΨD term for each pair of adsorbate-adsorbent interacting atoms can be assumed to be additive; therefore, n-alkane-SiO2-A adsorption generally produces a continuous increase of qst with carbon number. This growth in qst with carbon number is due to forces of attraction between adsorbate molecules. The increases are approximately of: (i) 9 kJ mol-1 per additional CH2 group on going from hexane to heptane, (ii) 27 kJ mol-1 on moving from heptane to octane, and (iii) 9 kJ mol-1 on changing from octane to nonane. With respect to qst values of n-heptane, n-octane, and n-nonane on SiO2-A, it is observed that the degree of interaction between the adsorbent surface and the corresponding hydrocarbon molecule is greater if compared to n-hexane The following qst sequence can be established, for instance, when the adsorption uptake (24) Richards, R. E.; Rees, L. V. C. Zeolites 1987, 3, 335-340.

268


Herna´ ndez et al. Table 4. Saturation Limit of Langmuir am, Henry’s Law Parameter (KH), the Adsorption Energies ∆U0 for the Adsorption of n-Alkanes on Adsorbent Ag/SiO2-Aa adsorbate hexane

heptane

octane

nonane

a m, -∆U0, tempera- amKL ) KH, 10-3 Torr mmol g-1 kJ mol-1 ture, K 423 443 458 473 523 573 423 443 458 473 523 573 423 443 458 473 523 573 423 443 458 473 523 573

3.051 1.442 1.387 0.712 0.420 0.351 4.180 1.648 1.690 0.882 0.538 0.473 7.634 2.517 2.776 1.222 0.720 0.502 12.483 4.154 3.343 1.896 0.856 0.371

0.218 0.256 0.190 0.198 0.215 0.228 0.286 0.348 0.229 0.185 0.169 0.157 0.290 0.565 0.230 0.172 0.150 0.132 0.627 0.646 0.431 0.184 0.146 0.181

63.254

82.595

154.804

260.638

R 0.997 0.999 0.997 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.996 0.996 0.999 0.999 0.999 0.998 0.996 0.987 0.999 0.997 0.991 0.999 0.998 0.997

a K is Henry’s law constant, a is saturation limit of Langmuir, H m ∆U0 is the van’t Hoff adsorption energy, and R is the linear correlation coefficient.

Figure 8. Corresponding Langmuir plots for n-alkanes Ag/ in SiO2-A. (a) n-hexane, (b) n-heptane, (c) n-octane, and (d) n-nonane. Table 3. Saturation Langmuir Limits am, Henry Constants (KH), and Adsorption energies (∆U0) for the Sorption of n-Alkanes on Substrate SiO2-Aa adsorbate hexane heptane octane nonane

am, -∆U0, tempera- amKL ) KH , ture, K 10-3 Torr-1 mmol g-1 kJ mol-1 423 473 523 423 473 523 423 473 523 423 473 523

6.531 2.484 2.251 5.598 3.120 3.008 7.973 3.268 2.468 16.667 4.522 2.663

0.145 0.153 0.097 0.195 0.124 0.085 0.252 0.137 0.083 0.279 0.140 0.089

12.809 23.758 25.316 48.432

R 0.990 0.997 0.986 0.999 0.996 0.997 0.999 0.998 0.989 0.999 0.997 0.992

a K is Henry’s Law constant, a is the Langmuir saturation H m limit, ∆U0 is the adsorption energy, and R is the linear correlation coefficient.

g-1:

is fixed at a ) 0.09 mmol n-nonane > n-octane > n-heptane > n-hexane. That is, an increase in the length of the linear hydrocarbon adsorbate molecule significantly affects the interaction potential between each of these molecules and the surface of the SiO2-A adsorbent. For all n-alkane adsorbates, qst increases with raising adsorbate loading and the shapes of qst versus coverage plots are indicative of an adsorption phenomenon that is roughly occurring on an energetically uniform (with respect to the distribution of adsorption sites) surface or microporous volume.

Figure 9. Temperature dependence of Henry’s law constants for n-alkanes on SiO2 adsorbents. (a) Ag/SiO2-A and (b) SiO2A.

The variation of the isosteric heats of adsorption for the adsorption of n-alkanes on the Ag/SiO2-A adsorbent is also depicted in Figure 12. Analysis of this figure indicates that, for n-hexane, the adsorbent shows a slight decrease in qst; with coverage, this behavior is a characteristic of nonuniform surfaces. This decrease of qst with coverage can be attributed to interactions of the Ag+ ion with the oxygen atoms of the silanol groups of the micropore walls with the hydrogen atoms of methyl


Figure 10. Variation of the limiting energy of adsorption with carbon number of n-alkanes. (a) SiO2-A and (b) Ag/SiO2 -A.


Figure 12. Variation of the isosteric heat of adsorption qst (kJ mol-1) of n-alkanes on SiO2 samples. (a) n-hexane, (b) n-heptane, (c) n-octane, and (d) n-nonane. Open symbols denote adsorbent SiO2-A while full symbols denote adsorbent Ag/SiO2-A. Table 5. Isosteric Heat of Adsorption qst (kJ mol-1) of n-Alkanes on SiO2 Samples adsorbent/ adsorbate

hexane

heptane

octane

nonane

SiO2-A Ag/ SiO2-A

39.119 58.209

48.165 71.337

75.627 87.242

84.648 99.739

n-hexane. This fact confirms that an increase in the number of carbons atoms in the hydrocarbon chain affects the degree of interaction between these sorptive vapors and the adsorbent. The increases are approximately 13 kJ mol-1per additional CH2 group on going from n-hexane to n-heptane, of 16 kJ mol-1 on moving from n-heptane to n-octane and again 13 kJ mol-1 when changing from n-octane to n-nonane. The results of these estimations are listed in Table 5. Conclusions

Figure 11. Variation of saturation limit (am, mmol g-1) with temperature for adsorption of n- alkanes on the adsorbents: (a) SiO2-A and (b) Ag/SiO2-A.

group. It is observed that there are specific adsorbentadsorbate interactions and that the magnitude of heat decreases with the increase of the amount adsorbed, manifesting behavior typical of an energetically heterogeneous adsorbent.25 For n-heptane, n-octane, and n-nonane, behaviors similar to the behavior of n-hexane can be seen; however, the heats of adsorption of these hydrocarbons are larger compared with the heat of adsorption of n-hexane. The following sequence can be established, for example when the amount of adsorbed substance corresponds to a ) 0.045 mmol g-1: n-nonane> n-octane > n-heptane> (25) Cao, D. V.; Sircar, S. Ind. Eng. Res. 2001, 40, 156-162.

1. Based on the texture analysis, evaluated by N2 adsorption at 76 K, it can be seen that the SiO2-A adsorbent presents the most convenient values of textural properties (i.e., surface area and total pore volume) with respect to those regarding the SiO2-B and SiO2-C substrata. These adsorbents present an absolute absence of large pore sizes (i.e., meso- and macropores). Microporosity analyses of these samples indicate high micropore contents in all SiO2 specimens studied. The SiO2-A adsorbent shows two kinds of micropores in the microporous structure coexisting: a collection of supermicropores (sm) and a group of ultramicropores (um). 2. Due to this latter characteristic, the SiO2-A adsorbent was chosen to be prepared in the presence of AgNO3, to render an Ag/SiO2 adsorbent that is selective with respect to the retention of n-alkanes. It is established that there are some noteworthy differences between the results obtained with Ag/SiO2-A and the precursor SiO2 -A adsorbent. 3. It is confirmed that adsorption of the n-alkanes is favored by a decrease of the temperature, since at 573

270


K the separations among the isotherms of the hydrocarbons are almost totally lost while at 423 K sorption differences are notable. 4. The patterns of the isosteric heats of adsorption versus coverage for n-alkanes in the SiO2-A adsorbent correspond to interactions on quasi-uniform surfaces or volumes while those on the Ag/SiO2-A substrate match interactions on heterogeneous surfaces or volumes. The influence of Ag deposited on a precursor SiO2 adsorbent is evident especially with respect to the

Herna´ ndez et al.

increases of the heats of the adsorption of the n-alkanes studied. Acknowledgment. Thanks are given to National Council of Science and Technology of Me´xico (CONACyT) for financial support under the projects “Caracterizacioń y Usos de So´lidos Porosos Naturales” Reference 960502003. EF020119O

Alkane Adsorption on Microporous SiO2 Substrata. 1. Textural

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