Adsorption and Diffusion of n-Heptane and Toluene over Mesoporous

Article pubs.acs.org/IECR

Adsorption and Diffusion of n‑Heptane and Toluene over Mesoporous ZSM‑5 Zeolites He Zhao, Jinghong Ma,* Qiangqiang Zhang, Zhiping Liu, and Ruifeng Li* Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, China S Supporting Information *

ABSTRACT: ZSM-5 zeolites with different mesoporosities were prepared by alkaline treatment and characterized by powder XRD and nitrogen adsorption. Two C7 hydrocarbons of n-heptane and toluene were employed as probe molecules to investigate the effects of the introduction of mesopore on the adsorption and diffusion properties of ZSM-5 zeolites by comparing the experimental results of the samples treated and untreated by using NaOH. Adsorption isotherms were measured gravimetrically in the pressure range 0−32 mbar and from 293 to 338 K. The isotherms of microporous and mesoporous ZSM-5 were successfully fitted by using the Langmuir−Freundlich model and the dual-site Langmuir−Freundlich model, respectively. Henry’s constants and the initial heats of adsorption calculated from the adsorption isotherms as well as the fitting parameters displayed that the interactions between adsorbent and adsorbate were weakened after the introduction of mesopore, and the interactions between the adsorbates with microporous surface are much stronger than that between them with the mesoporous surface. Diffusion measurements were undertaken using the zero length column (ZLC) technique at partial pressure of p/p0 < 0.000 15 from 333 to 393 K. The results showed that the effective diffusion constants (Deff/R2) of the two C7 hydrocarbons increased greatly in the presence of mesopores, while the corresponding activation energy decreased due to the reduced diffusion resistance and the shortened diffusion path in the mesoporous zeolites. Also, higher and much more dramatic enhancement of the efficient diffusivities as a function of mesoporous volume for toluene relative to that for n-heptane were found, indicating that the diffusion of n-heptane is controlled by the micropore diffusion and that the diffusion of toluene is exclusively determined by mass transfer through the mesopores. than on the inside of the micropores during the reaction.9 As a result, the catalyst life of mesoporous zeolite can be prolonged. So far, the purposeful incorporation of mesopores has become an important issue of the synthesis and postsynthesis modification of zeolites to generate the transport-optimized material. Not only their very high specific surface area, accessible for adsorption and heterogeneous catalysis, but also their efficient transport properties are the key factors determining the optimal use of these materials. Therefore, a good knowledge of the intimate interaction of the molecules on the pore surfaces and the diffusion properties of the molecules is of particular importance. However, the interpenetration of the micro- and mesopore spaces leads to very special patterns in the above aspects, which are by far more complicated to be assessed than those of the purely microporous or ordered mesoporous materials. In the present work, we successfully created mesopores in ZSM-5 zeolites by applying the method of controlled desilication with NaOH, which is one of the most simple and efficient methods to generate mesopores in zeolites.10 Two C7 hydrocarbons, n-heptane and toluene, were chosen as the probe molecules to investigate the adsorption and diffusion properities of the mesoporous ZSM-5 samples, with a microporous ZSM-5 zeolite as reference. Both n-heptane and toluene are important components involved in the dehydrocyclization of n-heptane,

1. INTRODUCTION Owing to its favorable properties such as high hydrothermal stability, good shape selectivity, and excellent catalytic property, ZSM-5 zeolite with MFI type framework has been extensively used in the field of oil refining,1 petrochemical industry,2 and other chemical industries as a heterogeneous catalyst for a variety of different hydrocarbon reactions.3,4 However, the sole and small pore diameter of ZSM-5 zeolites confines the diffusion of the products and the reactants, especially when bulky hydrocarbon molecules are involved in the lower reaction rates. Furthermore, a slow mass transport to and away from the catalytic center increase the possibility of secondary reactions, with coke formation and catalyst deactivation as a consequence.5 Obviously, these drawbacks will hinder the widespread use of ZSM-5. A promising way to overcome the above shortages is to introduce a secondary pore system in the zeolites so as to form hierarchical (or mesoporous) materials,6 i.e., materials which contain both micro- and mesopores. It will promote the rate of molecular transport by the presence of mesopores, maintaining simultaneously the functionality of the materials owing to their contents of micropores. Recently, hierarchical zeolites have attracted considerable attention, because of their potential advantages in catalysis as a result of the increased external surface areas and decreased diffusion path lengths.7,8 The catalytic activity, selectivity, and accessibility toward the catalyst active sites can be enhanced as the steric limitations in converting bulky molecules are reduced and meanwhile the rate of intracrystalline diffusion is increased. It was also found that, in the presence of mesopores, coke is more inclined to deposit on the external surfaces rather © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13810

June 21, 2014 August 2, 2014 August 18, 2014 August 18, 2014 dx.doi.org/10.1021/ie502496v | Ind. Eng. Chem. Res. 2014, 53, 13810−13819

Industrial & Engineering Chemistry Research

Article

Figure 1. XRD patterns of (a) NaZSM-5(0), (b) NaZSM-5(1), and (c) NaZSM-5(2).

Figure 2. Nitrogen adsorption/desorption isotherms at 77 K and corresponding BJH pore distribution (inset) for (a) NaZSM-5(0), (b) NaZSM-5(1), and (c) NaZSM-5(2).

a useful process in aromatic and high octane number gasoline production, and these two species have served as the probe molecules for many systems.11−14 Nevertheless, the research dealing with the adsorption and diffusion properties of the mesoporous ZSM-5 zeolites are scarce. Hence, the objective of this study focuses on the exploration of the adsorption and diffusion behavior by measuring the adsorption isotherms and the ZLC desorption curves for n-heptane and toluene on the mesoporous ZSM-5, which were then compared with those of their precursor microporous ZSM-5. The adsorption capacities, Henry’s constants, and the initial heats of adsorption of the microporous and mesoporous ZSM-5 were examined to illustrate the influence of the newly generated mesopores on the adsorption potential of these samples. Also, the effective diffusion constants (Deff/R2) and the active energy (Ea) from ZLC are calculated to obtain a better understanding of the diffusion mechanism of n-heptane and toluene on the microporous and mesoporous ZSM-5. The differences in adsorption and other properties between the two C7 hydrocarbons with different kinetic diameter and shape were also compared.

Table 1. Textural Properties of ZSM-5 Samples sample

SBETa (m2/g)

Smicb (m2/g)

Sextb (m2/g)

Vmicb (cm3/g)

Vmesoc (cm3/g)

NaZSM-5(0) NaZSM-5(1) NaZSM-5(2)

382 384 400

366 302 272

16 82 128

0.15 0.12 0.11

0.02 0.12 0.23

a

Determined by the BET method. bObtained from the t-plot method. Vmeso = Vtotal − Vmic, where Vtotal is derived from the amount of nitrogen adsorbed at p/p0 = 0.99.

c

Nitrogen adsorption studies were performed at 77 K on a Quantachrome NOVA 1200e gas sorption analyzer. The specific surface area (SBET) was evaluated using the Brunauer− Emmett−Teller (BET) method. The micropore surface area (Smic) and the micropore volume (Vmic) were determined by the t-plot analysis. The mesopore volume (Vmeso) was calculated from the difference between the total pore volume (Vtotal) and the Vmic, where Vtotal was derived from the amount of nitrogen adsorbed at p/p0 = 0.99. The Barret−Joyner−Halenda (BJH) method was applied to estimate the pore size distribution from the adsorption branch of the isotherm. 2.3. Adsorption Isotherm Measurements. Adsorption isotherms of n-heptane and toluene on ZSM-5 zeolites were measured using an Intelligent Gravimetric Analyzer (IGA-002), supplied by Hiden Analytical Ltd. In this apparatus, a sensitive microbalance, which has a long-term stability of ±1 μg with a weighing resolution of 0.1 μg, was used to record the uptakes of the samples fully under the computer control. Prior to each sorption measurement, about 50 mg of sample was submitted to degas under up to 10−7 mbar and maintained at 673 K for 8−10 h until no further variations in the weight were detected. During the isotherm measurement, the vapor of the sorbate was gradually introduced into the ultrahigh vacuum system until the set pressure value was obtained. As the adsorption equilibrium at this set point was established, the vapor pressure was increased to the very next designed pressure. The mass uptake as a function of pressure was continuously recorded by computer. The isotherms measurements for n-heptane and toluene were taken in the pressure range 0−32 mbar and over the 293−323 K and the 308−338 K temperature ranges, respectively. The adsorbates used in this work were of GC grade (purity >99.9%).

2. MATERIALS AND METHODS 2.1. Preparation of the Samples. The mesoporous ZSM5 samples were prepared via alkali-treatment by using a commercially available HZSM-5(SiO2/Al2O3 = 36, crystal diameter 2 μm) zeolite. The parent HZSM-5 sample was first treated with 0.5 or 0.7 mol/L NaOH solutions under stirring at 333 K for 6 h. Subsequently, the resulting samples were filtered and washed with deionized water until a neutral pH was obtained. Afterward, the alkali-treated zeolites and HZSM-5 precursor were ion exchanged with 0.5 mol/L NaCl aqueous solution at 303 K for 2 h, which was repeated three times. The samples were washed, dried, and then calcined in static air at 823 K to obtain the desired three ZSM-5 samples, denoted as NaZSM-5(0) (untreated with NaOH), NaZSM-5(1) (treated with 0.5 mol/L NaOH), and NaZSM-5(2) (treated with 0.7 mol/L NaOH). 2.2. Structural Characterization. X-ray powder diffraction (XRD) patterns were examined using a SHIMADZU XRD6000 diffractometer with Cu−Kα radiation operating at 40 kV and 30 mA. Crystal size, morphology, and mesoporous structure of the samples were investigated using SEM on a JEOL/ JSM-6700F scanning electron microscope. The SEM images were provided as the Supporting Information (Figure S1). 13811

dx.doi.org/10.1021/ie502496v | Ind. Eng. Chem. Res. 2014, 53, 13810−13819


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Figure 3. Adsorption isotherms of n-heptane and toluene on ZSM-5 samples at various temperatures.

2.4. Diffusion Measurements. The ZLC technique was employed to study the diffusion behavior of n-heptane and toluene on ZSM-5 samples. According to the standard ZLC method, the experiments were conducted under the low relative partial pressures of 0−0.000 15, which are considered to be within the linear region of the adsorption isotherms. Before each run, a small amount of the sample (1−2 mg) was sandwiched between two porous sintered discs and placed into the ZLC column. The sample was then activated at 573 K overnight to eliminate the impurities and moistures that were present in the adsorbent. During the measurement, the adsorbent was initially equilibrated with a known low partial

pressure of the adsorbate, which was then desorbed with a high flow rate of He purge. The effluent concentration of the sorbate was monitored by a flame ionization detector (FID), and the resulting desorption curve was recorded on the computer. The diffusion measurements for n-heptane (99.9% grade) and toluene (99.9% grade) were performed separately at 333 K, 353 K, 373 K and 353 K, 373 K, 393 K. Helium (purity >99.99%) was used as carrier gas and the flow rate was in the range of 80−100 mL/min. A purge flow rate of 80 mL/min was chosen as a standard flow rate for all experiments in this work. 2.5. Analysis of ZLC Desorption Curves. Within the linear region of the equilibrium isotherm, the ZLC desorption 13812



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curve, the normalized effluent sorbate concentration c/c0, assuming that the isothermal condition for the uniform spherical adsorbent particles and the ZLC system is perfectly mixed, is given in the following by refs 15 and 16:

(

D

)

∞ exp − βn 2 eff2 t c R = 2L ∑ 2 c0 β L L + − [ ( 1)] n n=1

where βn is given by the roots of the equations βn cot βn + L − 1 = 0

and

L=

1 FR2 3 KVsDeff

where F is the interstitial gas velocity, R the individual particle radius, K the dimensionless Henry’s law constant, and Deff /R2 the effective diffusion time constant. On the basis of the equations above, a fitting procedure called full range method17,18 was developed to extract the Deff/R2 from the experimental data.

3. RESULTS AND DISCUSSION 3.1. Characterization. The XRD patterns of three ZSM-5 samples are depicted in Figure 1. It shows that the typical MFI structure remains in the NaZSM-5(1, 2) zeolites after the NaOH treatment, only with a slight decrease in the intensity of most peaks, compared with that of NaZSM-5(0). Pore structures of the samples were analyzed by using N2 adsorption/desorption isotherms at 77 K (Figure 2). NaZSM-5(0) exhibits a type-I isotherm according to the IUPAC classification, confirming its solely microporous structure. NaZSM-5(1) and NaZSM-5(2) show a combination of type I and type IV isotherms, with the appearance of the micropore filling at low pressure and the hysteresis loops at higher pressures, which indicates clearly the presence of mesoporosity. The BJH pore size distribution curves display a relatively wide pore size distribution with mesopore diameters varying from 3 to 30 nm, most centering in the range 5−10 nm. From pore structure parameters listed in Table 1, it is seen that the BET surface area of NaZSM-5(1) and NaZSM-5(2) do not change significantly as compared with that of the NaZSM5(0) sample, while a considerable mesoporous surface area and mesoporous volume are obtained for the NaZSM-5(1, 2) samples. Treatment of the ZSM-5 zeolites with aqueous NaOH solutions is known to generate extensive intracrystalline mesoporosity in zeolites by a selective extraction of framework silicon. From the above results and SEM images (Figure S1 in the Supporting Information), it is clear that the anticipated mesoporous ZSM-5 zeolites were successfully prepared via alkaline treatment.19 3.2. Adsorption Results. 3.2.1. Adsorption Equilibrium Isotherms. Adsorption isotherms are measured for n-heptane and toluene on three ZSM-5 samples at different temperatures, as is presented in Figure 3. It shows that the adsorption isotherms of n-heptane and toluene nicely exhibit similar adsorption tendency on the same samples. The isotherms for both adsorbates on NaZSM-5(0) display a typical type-I adsorption isotherm, which reaches the saturation quickly at low pressures corresponding to the micropore filling, followed by a plateau at high relative pressures. However, the adsorption isotherms of both NaZSM-5(1) and NaZSM-5(2) show a different trend from that of NaZSM-5(0), that is, with the initial increase in

Figure 4. Comparisons of adsorption isotherms of n-heptane and toluene on different ZSM-5 samples at 308 K.

uptakes at low pressures, mainly caused by the filling of micropores, followed by continual adsorption amounts increases as the pressure increases to higher relative pressures. Obviously, the upward deviation of the adsorption amounts of NaZSM5(1) and NaZSM-5(2) at higher pressures is associated with the adsorption of newly created mesopores. As for the adsorption capacities of the three ZSM-5 samples, it is observed that the adsorbed amounts of both n-heptane and toluene within the micropore filling region decrease with the micropore volume (Vmic) of the samples decreasing. The order is as follows: NaZSM-5(2) < NaZSM-5(1) < NaZSM-5(0). 13813



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The situation is the same with the adsorption results of n-heptane and toluene on microporous silicalite11 and on ZSM-5.12 It is wellknown that the molecules hosted in the narrow pores of ZSM-5 have a strong interaction with the zeolite pore walls, the strength of which interaction is associated with the size, polarity, and shape of the adsorbate. The estimated pore size of the channel of ZSM-5 is 0.54−0.56 nm, while the kinetic diameters for n-heptane and toluene are 0.43 and 0.59 nm, respectively. Obviously, the steric limitation from the micropores of ZSM-5 for n-heptane can be ignored. While toluene possesses larger size than the micropore size of ZSM-5, even a slight tortuosity or shrinkage of the pore mouth could stop the molecule from entering the channels of ZSM-5. Besides, the larger size of toluene restricts the orientations that it can adopt in the confined space within the micropores, thus the adsorption of toluene is localized to some extent. For NaZSM-5(1) and NaZSM-5(2), although the mass uptakes of n-heptane are still higher than those of toluene in the micropore filling region, the adsorption amounts of toluene increase rapidly when single and multilayer sorption on the mesoporous surface has occurred and finally surpass the adsorption amounts of n-heptane. That is, the adsorption amounts for toluene are higher than that for n-heptane at higher relative pressure. This result pertains to the interactions between adsorbate and adsorbent. So the adsorption capacity is not only related to the size of both adsorbate and adsorbent but also to the interaction between them. In the mesopores, however, the steric restriction can be neglected for both n-heptane and toluene to make the adsorption capacity depend mainly on the adsorbent−adsorbate interaction. 3.2.2. Henry’s Constants and Initial Heats of Adsorption. As usual, Henry’s constants reflect the interaction between the adsorbates with the surface of the adsorbents in the linear region of the adsorption isotherms. In this work, Henry’s constants for n-heptane and toluene were calculated from the isotherm data at p/p0 < 0.0004. According to the Virial equation, ln(p/q) = −ln(KH) + A1q + A2q2 + A3q3..., as q gets closer to zero, the plot of ln(p/q) vs the loading q should approach a line, and the Henry’s constant (KH) can be derived from the intercept −ln(KH).20 Table 2 summarizes the values of KH for n-heptane and toluene on the ZSM-5 samples. It shows that in the three samples, Henry’s constants of both adsorbates are in the order NaZSM5(2) < NaZSM-5(1) < NaZSM-5(0), which is a decreasing trend with the mesoporous volume decrease. In addition, it is obvious that the Henry’s constants of n-heptane are higher than that of toluene at the same temperatures. Similar trends were observed for the adsorption of n-heptane and toluene on silicalite and

Table 2. Henry’s Constants (KH) and Heats of Adsorption at Zero Coverage (Qst) on ZSM-5 Samples for n-Heptane and Toluene sample

sorbate

T (K)

KH (mmol/g mbar)

NaZSM-5(0)

n-heptane n-heptane n-heptane toluene toluene toluene n-heptane n-heptane n-heptane toluene toluene toluene n-heptane n-heptane n-heptane toluene toluene toluene

293 308 323 308 323 338 293 308 323 308 323 338 293 308 323 308 323 338

190 36.0 8.40 32.4 7.93 2.65 104 25.0 7.07 20.2 6.10 2.38 52.0 16.4 5.44 9.72 3.82 1.82

NaZSM-5(1)

NaZSM-5(2)

Qst (kJ/mol) 81.4

72.3

70.6

61.7

59.2

48.4

Table 3. Langmuir−Freundlich Fitting Parameters of n-Heptane and Toluene on NaZSM-5(0) Zeolite T(K)

qs (mmol g−1)

293 308 323

1.14 1.11 1.04

308 323 338

1.06 1.00 0.92

b0 (mbar

−1

n-heptane 26.9 15.9 8.07 toluene 10.8 6.57 3.90

)

n0

R2

1.16 1.10 1.08

0.9970 0.9961 0.9934

1.18 1.12 1.02

0.9942 0.9959 0.9954

However, the adsorption amounts within the single and multilayer adsorption range for the mesoporous adsorption increase with the mesopore volume (Vmeso) increasing, which results in the increase of adsorption amounts at higher pressures. Moreover, in order to evaluate the adsorption potential for n-heptane and toluene, the adsorption amounts of these two sorbates on the three samples are compared under the same relative pressure and at the same temperature (see Figure 4). It is observed that for NaZSM-5(0), the saturation adsorption amounts of n-heptane are higher than that those of toluene.

Table 4. DSLF Fitting Parameters of n-Heptane and Toluene on NaZSM-5(1) and NaZSM-5(2) Zeolites samples

adsorbate

T (K)

qs1 (mmol g−1)

b1 (mbar−1)

n1

qs2 (mmol g−1)

b2 (mbar−1)

n2

R2

NaZSM-5(1)

n-heptane

293 308 323 308 323 338 293 308 323 308 323 338

1.06 1.05 0.99 0.92 0.88 0.83 1.02 0.97 0.92 0.86 0.84 0.81

17.7 8.18 5.08 6.55 4.25 3.22 9.21 5.87 3.23 5.68 3.61 2.78

1.43 1.40 1.38 1.65 1.58 1.47 1.54 1.51 1.46 1.71 1.67 1.59

1.03 1.00 0.93 1.86 1.71 1.62 1.89 1.84 1.80 2.82 2.70 2.61

0.0065 0.0060 0.0055 0.023 0.016 0.010 0.0041 0.0035 0.0029 0.020 0.014 0.0083

0.72 0.89 0.97 1.28 1.28 1.27 0.67 0.82 0.90 1.33 1.37 1.33

0.9987 0.9983 0.9976 0.9974 0.9967 0.9967 0.9978 0.9980 0.9981 0.9950 0.9955 0.9858

toluene

NaZSM-5(2)

n-heptane

toluene

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Figure 5. Experimental data (symbols) and theoretical ZLC curves (lines) for n-heptane and toluene at different temperatures in ZSM-5 samples.

ZSM-5.11,12 It is well-known that the forces involved in physical adsorption include both the van der Waals forces (dispersion−repulsion) and the electrostatic interaction. When the molecule adsorbs on the surface of the zeolite by the dispersion force, the close-range repulsion between the adsorbate and the adsorbent (Born repulsion) appears with the overlapping of electron clouds. Therefore, it can be reasonably concluded that the higher Henry’s constants of n-heptane on the zeolite surface compared to those of toluene with larger molecular size have been caused by the smaller repulsion force between the interacting pair. The isosteric heats of adsorption at zero coverage Qst were also examined to assess further the adsorption strength of

n-heptane and toluene on the three ZSM-5 samples. In terms of Henry’s constant, Qst can be expressed as21

⎛ ∂ln KH ⎞ ⎟ = −⎜ ⎝ ∂T ⎠ RT Q st

2

As can be seen from the results listed in Table 2, the initial isosteric heats Qst for both n-heptane and toluene on the three ZSM-5 samples decrease with increasing mesoporosities. This sequence resembles that of Henry’s constants. Furthermore, the value of Qst for n-heptane on either NaZSM-5(1,2) or NaZSM5(0) is higher than the corresponding values for toluene, indicating the affinity between the zeolite surface and n-heptane 13815



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respectively, the adsorption sites located in the micropores and mesopores. As mentioned above, the deviations of the parameter n from unity can describe the heterogeneity of the zeolite surface. On the basis of the values of parameters n0 obtained from the L−F model of n-heptane and toluene on NaZSM-5(0) (Table 3), it is clear that the surface of NaZSM-5(0) is practically homogeneous for both the adsorbates. Whereas, both n1 and n2 listed in Table 4 give larger values deviating from unity and increase with the increasing mesoporosity, it is therefore apparent that the introduction of mesopores sacrifice the homogeneity of the samples. With regard to the parameter b, first, the values of b0 for n-heptane on NaZSM-5(0) are higher than that for toluene at same temperature, which is similar to Henry’s constants. Similarly, the values of b1 for n-heptane on NaZSM-5(1, 2) are higher than those for toluene. Contrarily, the values for b2 of n-heptane are smaller than those of toluene. A possible explanation is that the steric hindrance imposed by the micropore size limitation diminished in mesopores, and the repelling force doses not dominate the adsorption of the adsorbate toward the zeolite surface anymore, thus toluene was therefore observed to possess a higher b2 than n-heptane. Second, the values of b1 for both n-heptane and toluene on mesoporous NaZSM-5(1, 2) samples are much higher than b2, indicating that the interactions between adsorbates with the microporous surface are much stronger than those between them with the mesoporous surface. In addition, the comparison between b0 and b1 displays that the interaction between either n-heptane or toluene with the microporous surface is weakened with the increasing mesoporosity of the samples. 3.3. Diffusion Results. Figure 5 displays the experimental data and theoretical ZLC curves at different temperatures for n-heptane and toluene in the three ZSM-5 samples. All the experimental data agree well with the fitting lines, which confirm the validity of the applied theoretical model. The diffusion parameters extracted from the theoretical fitting are presented in

is higher than between the zeolite surface and toluene. In accordance with the previous reported results, the limiting heat of adsorption for n-heptane on ZSM-522 and silicalite-123 turned out to be 79.6 and 83.4 kJ/mol, whereas that for toluene on silicalite-1 was about 65 kJ/mol.24 3.2.3. Modeling of Equilibrium Isotherms. The adsorption equilibrium data were fitted by the Langmuir−Freundlich (LF) isotherm model for NaZSM-5(0) and the dual-site Langmuir− Freundlich (DSLF) isotherm models for NaZSM-5(1, 2), respectively. Apparently, the fitting curves shown in Figure 3 shows that experimental data abides by these models. The fitting parameters are presented in Tables 3 and 4. LF model can be expressed as follows: q = qs

b0p(1/ n0) 1 + b0p(1/ n0)

where q and qs are, respectively, the adsorbed amount at equilibrium pressure p and the saturation adsorption capacity. Parameters b0 and n0 characterize, respectively, the adsorbate−adsorbent interaction and the system heterogeneity. If n0 is equal to unity, the model is reduced to the Langmuir model, thus it can be regarded as the parameter characterizing the deviations from an ideal homogeneous surface. Owing to the larger external surface area of the mesoporous NaZSM-5(1, 2), the adsoption sites in the part of established mesopore structure (external surface area) cannot be neglected. On the basis of the hypotheses of two kinds of adsorption sites, the DSLF model can be expressed as follows: q = qs1

b1p(1/ n1) 1 + b1p(1/ n1)

+ qs2

b2p(1/ n2) 1 + b2p(1/ n2)

where qs1 and qs2 are the saturation adsorption capacities of two kinds of adsorption sites (sites 1 and 2) at the equilibrium pressure p, and b1 and b2 together with n1 and n2, characterize the adsorbate−adsorbent interaction and the system heterogeneity for sites 1 and 2, respectively, where sites 1 and 2 represent, Table 5. ZLC Fitting Data for n-Heptane on ZSM-5 Samples sample NaZSM-5(0)

NaZSM-5(1)

NaZSM-5(2)

T (K) 333 353 373 333 353 373 333 353 373

L 21.1 17.5 11.3 31.7 21.5 13.7 27.5 18.8 11.8

Deff/R2 (s−1)

β 2.99 2.96 2.87 3.04 3.00 2.92 3.03 2.98 2.88

5.15 1.08 2.28 7.79 1.61 3.00 1.17 2.08 3.79

× × × × × × × × ×

−5

10 10−4 10−4 10−5 10−4 10−4 10−4 10−4 10−4

Deff (m2 s−1) 5.15 1.08 2.28 7.79 1.61 3.00 1.17 2.08 3.79

× × × × × × × × ×

10−17 10−16 10−16 10−17 10−16 10−16 10−16 10−16 10−16

Ea (kJ/mol) 38.4

34.8

30.4

Table 6. ZLC Fitting Data for Toluene on ZSM-5 Samples sample NaZSM-5(0)

NaZSM-5(1)

NaZSM-5(2)

T (K) 353 373 393 353 373 393 353 373 393

L 33.3 22.6 18.9 28.2 21.1 20.0 26.6 17.8 15.9

β 3.05 3.00 2.98 3.03 2.99 2.98 3.02 2.97 2.95

Deff/R2 (s−1)

Deff (m2 s−1)

−4

−16

1.51 2.42 3.75 3.16 4.82 7.28 4.63 6.92 9.90 13816

× × × × × × × × ×

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

1.51 2.42 3.75 3.16 4.82 7.28 4.63 6.92 9.90

× × × × × × × × ×

10 10−16 10−16 10−16 10−16 10−16 10−16 10−16 10−16

Ea (kJ/mol) 26.2

24.1

21.9



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Tables 5 and 6. As is clear from Tables 5 and 6, all the L values obtained from the theoretical fittings are significantly higher than 5, which are considered a point of transition from the equilibrium-controlled to the diffusion-controlled regime.25

Moreover, the desorption curves for n-heptane on NaZSM5(1) at 333 K performed at three different flow rates present similar long time slopes (see Figure 6). It indicates the adequacy of the experimental results to the proposed model for which the intracrystalline diffusivity should be independent of the flow rate change.26 To sum up the diffusion parameters for the two sorbates presented in Tables 5 and 6, it is seen that the Deff of n-heptane and toluene in the three samples are of the same magnitude between 10−16 and 10−17 m2/s, which are comparable to the reported results.13 The comparison of the diffusion results reveals obviously that the effective diffusivity values of both n-heptane and toluene within the measured range of temperature increase with the introduction of mesopores into zeolite. The variations of the effective diffusion constants for n-heptane and toluene with mesopore volume at 373 K are compared in Figure 7. A strong correlation exists obviously between the mesopore volume and the Deff/R2. For each species, the Deff/R2 monotonically increases with the mesoporosity of the ZSM-5 zeolites, suggesting that mesopores play an important role in accelerating the overall diffusion process over the adsorbents. In microporous NaZSM-5(0), the Deff/R2 for n-heptane is slightly smaller than that for toluene. While in NaZSM-5(1) and NaZSM-5(2), a much more remarkable increase in effective diffusion constant as a function of Vmeso for toluene is observed compared with that for n-heptane, e.g., at 373 K, the Deff/R2 of toluene is about 1.8 times higher than that of n-heptane over NaZSM-5(2). On the basis of the Henry’s constants of n-heptane and toluene on NaZSM-5(0), NaZSM-5(1), and NaZSM-5(2) obtained above, it can be found that the interaction of n-heptane with zeolites is stronger than that of toluene, thus the slower diffusion of n-heptane is at least partially due to the stronger adsorption between n-heptane and the micropore walls.27 Additionally, it is clear that both micropore and mesopore are involved in the diffusion process of both n-heptane and toluene; therefore, it is most likely that the diffusion of n-heptane is mainly controlled by the micropore diffusion while the mesopore diffusion is a dominant mechanism for the toluene transport. With the increasing temperature, the mobility of the adsorbate molecules is enhanced, thus leading to an increase of the diffusion rate. The relationships between the temperature and the Deff/R2, which are given by the Arrhenius plots for n-heptane and toluene, are shown in Figure 8 and the activation

Figure 6. Desorption curves for n-heptane from NaZSM-5(1) at 333 K at different flow rates.

Figure 7. Effective diffusivity at 373 K and activation energy in ZSM-5 samples for n-heptane and toluene as a function of mesopore volume.

Figure 8. Arrhenius plots for n-heptane and toluene from ZSM-5 zeolites (a) NaZSM-5(0), (b)NaZSM-5(1), and (c) NaZSM-5(2). 13817



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*E-mail: rfl[email protected].

energies derived from the plots are compiled in Tables 5 and 6. Contrary to the trend of diffusivity values (see Figure 7), the activation energy for both n-heptane and toluene in NaZSM5(1) and NaZSM-5(2) samples is substantially lower than those for them in NaZSM-5(0). Such observation conforms to the general pattern of higher activation energy involving diffusion in a microporous adsorbent and of lower activation energy for diffusion in a mesoporous adsorbent. Lower activation energy in a mesoporous zeolite related to the pure microporous zeolite can just be ascribed to the introduction of mesopores in the zeolite system with both micropore and mesopore. From the results above, the presence of mesopores leads to an obvious enhancement of the intracrystalline diffusivities and a decrease of activation energy of both n-heptane and toluene in zeolite crystals, which can be attributed to the shortened diffusion path and the reduced diffusion resistance after the introduction of mesoporous into the ZSM-5 zeolite.

Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS In this work, ZSM-5 zeolites with different mesoporosities were prepared by alkali-treatment. The adsorption and diffusion properties of n-heptane and toluene on these mesoporous samples were investigated, with microporous ZSM-5 zeolite as reference. It is concluded that both the micro- and mesopores are involved in the adsorption and transport processes of adsorbates, and the introduction of the mesopores has significant influence on both the adsorption and diffusion efficiencies of both n-heptane and toluene on ZSM-5 zeolites. Adsorption isotherms of microporous NaZSM-5(0) zeolite were successfully fitted by using the Langmuir−Freundlich model, whereas the dual-site Langmuir− Freundlich model fitted the isotherms data over the mesoporous NaZSM-5(1, 2) very well. At low pressures, the adsorption capacities for each sorbate are put in order of NaZSM-5(2) > NaZSM-5(1) > NaZSM-5(0), as the mesoporosity increases, while the opposite trends occurred at higher pressures. Henry’s constants and the initial heats of adsorption exhibit decreasing trends with the introduction of mesopores into the zeolites. The ZLC experiments showed that, due to the smaller diffusion resistance and shorter diffusion path, the effective diffusivities are enhanced greatly and the diffusion activation energies are reduced for both n-heptane and the toluene in the presence of mesopores, which is different from pure microporous ZSM-5 zeolite. The differences in size and the polarity of n-heptane and toluene result in their different adsorption and diffusion properties. Higher KH and Qst as well as lower diffusivities for n-heptane than those for toluene are observed. Meanwhile, a much more dramatic enhancement of the effective diffusivities as a function of mesoporous volume for toluene compared to that for n-heptane indicates that the diffusion of n-heptane is considered to be controlled by the micropore diffusion and that the diffusion of toluene is exclusively determined by the mass transfer through the mesopores.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of NaZSM-5 samples as noted in text. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. 13818



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(22) Denayer, J. F.; Souverijns, W.; Jacobs, P. A.; Martens, J. A.; Baron, G. V. High-temperature low-pressure adsorption of branched C5-C8 alkanes on zeolite beta, ZSM-5, ZSM-22, Zeolite Y, and Mordenite. J. Phys. Chem. B 1998, 102, 4588−4597. (23) Sun, M. S.; Talu, O.; Shah, D. B. Adsorption equilibria of C5-C10 normal alkanes in silicalite crystals. J. Phys. Chem. 1996, 100, 17276− 17280. (24) Pope, C. G. Sorption of benzene, toluene, and p-xylene on sillcalite and H-ZSM-5. J. Phys. Chem. 1986, 90, 835−837. (25) Jiang, M.; Eić, M. Transport properties of ethane, butanes and their binary mixtures in MFI-type zeolite and zeolite-membrane samples. Adsorption 2003, 9, 225−234. (26) Gunadi, S. Brandani. Diffusion of linear paraffins in NaCaA studied by the ZLC method. Microporous Mesoporous Mater. 2006, 90, 278−283. (27) Vinh-Thang, H.; Huang, Q.; Malekian, A.; Eić, M.; Trong-On, D.; Kaliaguine, S. Diffusion characterization of a novel mesoporous zeolitic material. Adsorption 2005, 11, 421−426.

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Adsorption and Diffusion of n-Heptane and Toluene over Mesoporous

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