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Jan 14, 2016 - ABSTRACT: The desorption performances of naphthalene and pyrene on mesoporous MCM-41, SBA-15, and CMK-3 sorbents are studied on ...
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Desorption of Polycyclic Aromatic Hydrocarbons on Mesoporous Sorbents: Thermogravimetric Experiments and Kinetics Study Ziyi Li,† Yingshu Liu,† Xiong Yang,*,† Yi Xing,*,‡ Chuenjinn Tsai,§ Zhanying Wang,† Quan Yang,† and Ralph T. Yang# †

School of Mechanical Engineering and ‡School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, China § Institute of Environmental Engineering, National Chiao Tung University, University Road, Hsinchu 30010, Taiwan # Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States S Supporting Information *

ABSTRACT: The desorption performances of naphthalene and pyrene on mesoporous MCM-41, SBA-15, and CMK-3 sorbents are studied on the basis of temperature-programmed desorption experiments over the temperature range of 350−800 K at different heating rates. The kinetic parameters for each sorbate−sorbent pair are determined with combined model-fitting methods. The data for naphthalene and pyrene are best fitted with the same kinetic models on MCM-41 (with 1D mesopore channels), in contrast to those on SBA-15 and CMK-3, which have micropore−mesopore structures, leading to different desorption mechanisms for these two sorbates. SBA-15 with interconnectivity between adjacent mesopores not only shows high sorption capacities but also offers diffusion advantages in desorption, which contributes to the order of the degree of ease in desorption: SBA-15 > MCM-41 > CMK-3. CMK-3, with higher microporosity and hydrophobicity, shows stronger binding with the adsorbates while still benefiting in pyrene desorption from the consecutive mesoporosity.

1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are an important class of toxic organic compounds widely found in air, water, and soil. The emission control of PAHs has attracted special interest in scientific research, because of their widespread releases, toxicological risks, and strong persistence in the environment.1−3 Various treatment processes have been investigated,4 among which adsorption is one of the most competitive technologies because of its cost-effectiveness and easy operation.5,6 In general, it has been recognized that silica gels and activated carbons are widely adopted as adsorbents.7,8 Unfortunately, some disadvantages of the traditional adsorbents, such as fire risk, pore blockage, high mass-transfer resistance, and difficulty in regeneration after adsorption of high-boiling compounds,9,10 largely challenge their applications in adsorption of large molecules. Therefore, it is imperative to find other efficient adsorbents for controlling PAH emissions. Over recent years, there have been significant advances in the synthesis of mesoporous materials, such as the mesoporous silicas MCM-4111 and SBA-1512,13 and the mesoporous carbon CMK-3.14,15 These mesoporous adsorbents have generated a great deal of interest in the area of organic pollutant capture, due to their outstanding properties such as large specific surface areas, ordered meso-structure, hydrophobic properties, and high thermal stabilities.16−18 As one of the most significant advantages over conventional adsorbents, the ordered-mesoporous nature of the support permits good diffusivity of largesized compounds through the pore space, delivering fast desorption kinetics and thus facile regeneration.19 From the application point of view, this is quite important in terms of low regeneration energy cost and stable cycling performance.20 © XXXX American Chemical Society

Accordingly, in adsorption removal of PAH pollutants, the desorption performance for the sorbate−sorbent pair is particularly relevant in the selection of adsorbents and the deployment of adsorption/desorption cycles, making it an essential characteristic to be determined in advance. The most widely used method for obtaining desorption information is based on thermogravimetric analysis (TGA), in which temperature-programmed desorption (TPD) is recorded under any heating profile.21 The key kinetic information (or, “kinetic triplet”)the activation energy, the pre-exponential factor, and the kinetic modelcan be determined with combined kinetic analysis of experimental data.22 According to this line of research, previous studies have mainly dealt with traditional adsorbents for desorption of organic pollutants, such as dibenzo-p-furane on zeolites,23 formaldehyde on Al2O3 materials,24 and dioxin25 and even PAHs20 on activated carbons. However, thus far, the existing literature on desorption of PAHs on mesoporous adsorbents is very limited. In this work, naphthalene as the simplest form of PAH with two fused benzene rings and pyrene with four rings were chosen as the adsorbates. Desorption kinetics on three representative mesoporous adsorbents, MCM-41, SBA-15, and CMK-3, were investigated. The different sorbate−sorbent pairs provide an opportunity to explore the different desorption behaviors particularly originated from geometrical structures and physical properties. First, the pristine adsorbents were Received: October 9, 2015 Revised: December 26, 2015 Accepted: January 14, 2016

A

DOI: 10.1021/acs.iecr.5b03788 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Detailed Information of the Preparation for Each Sorbate-Loaded Samplea naphthalene

pyrene

sorbent

M0 (g)

Ms (g)

C0 (mol/L)

Msorb (g)

M0 (g)

Ms (g)

C0 (mol/L)

Msorb (g)

MCM41 SBA15 CMK3

0.237 0.239 0.231

10.3 10.4 10.4

0.1415 0.1419 0.1374

0.120 0.120 0.117

0.118 0.118 0.127

24.4 23.4 23.5

0.0189 0.0197 0.0211

0.123 0.116 0.121

a M0 is the weight of the solid adsorbate; Ms is the weight of methanol solution; Msorb is the weight of the adsorbent; C0 is the initial adsorbate solution concentration, with the known molar masses of 128.2 and 202.3 g/mol for naphthalene and pyrene, respectively, and the density of methanol of 0.79 g/mL.

based on the N2 adsorption data in a relative pressure range from 0.05 to 0.25. Low-angle X-ray diffraction (XRD) patterns of the three samples were recorded on an X-ray diffractometer (Rigaku, D/ max-2500, Japan) using Cu Kα radiation (λ = 0.1541 nm) at 40 kV and 200 mA in the 2θ range of 0.5−10° with a scanning rate of 0.5°/min. 2.3. Temperature-Programmed Desorption and Kinetic Triplet Estimation. Desorption of the naphthalene and pyrene on the three adsorbents was evaluated by the tTPD technique with a TGA (Q500, TA Instruments, New Castle, DE, USA). The TPD tests were performed at different heating rates (8, 12, 16, and 20 K/min), from 293 K to the temperature at which the residual mass curve flattened out. In each run, 20 ± 5 mg of the PAH-loaded sample was placed onto the TGA sample holder and purged with pure helium flowing at a rate of 20 mL min−1. Blank tests (12 K/min) for each pristine sorbent were also conducted, showing that residual methanol in each sample was negligible. The curves of residual mass versus temperature (TG) as well as desorption rate versus temperature (DTG) were recorded. The degree of desorption, α, can be defined as

characterized, and the PAH-loaded samples were prepared via batch adsorption. Then, the TPD for each sample was conducted, and the kinetic triplet parameters were determined by means of model-fitting methods combined with masterplots validations. Finally, detailed discussion was made to provide a comprehensive understanding of the desorption performance.

2. EXPERIMENTAL SECTION 2.1. Preparation of Polycyclic Aromatic HydrocarbonLoaded Samples. Mesoporous silicas, MCM-4111 and SBA15,13 purchased from Nanjing XFNANO Materials Tech Co., Ltd., and the mesoporous carbon, CMK-3,14 purchased from Nanjing JCNANO Materials Tech Co., Ltd., were used as the adsorbents. They are considered as model mesoporous materials that were prepared using well-established procedures.11,13,14 The adsorbents were sieved to U.S. mesh size 80− 100, washed with deionized (DI) water to eliminate impurities, dried at 383 K for 12 h to remove moisture, and finally stored in ziplock bags free of O2. Naphthalene (purity ≥ 99%) and pyrene (purity ≥ 98.5%), purchased from Sigma-Aldrich, were used as the adsorbates. Concentrated aqueous solutions of each adsorbate were prepared in methanol (HPLC grade) and were then mixed with the adsorbents in 65 mL amber glass vials with Teflonlined screw caps. Detailed information about the initial concentration for each sorbate−sorbent solution is listed in Table 1. The “pre-loaded” samples were obtained on the basis of duplicate tests using a batch equilibration technique at room temperature (293 K). The vials were sealed and shaken at 30 rpm for 48 h to reach apparent equilibrium. After the loading, the vials were placed on a bench for 1 h without disturbance to allow setting of the adsorbents. PAH-loaded samples dissolved in the solution were quickly moved into a drying oven, filtered, and kept in the air that was essentially free of moisture at 323 K for around 4 h, during which the residual methanol was removed. Bottles without any adsorbent served as blanks to monitor the losses of adsorbates during the experiments, which were found to be negligible. 2.2. Characterizations of the Sorbents. The textural properties of the mesoporous sorbents was characterized with Autosorb-1 (Quantachrome, US) at liquid nitrogen temperature (77 K). Before the measurement, the sample was outgassed at 393 K under a vacuum of 10−5 Torr for at least 12 h. The pore size distribution extending over the micro- and the mesopore ranges for each sample was obtained using the nonlocal density functional theory (NLDFT) method based on the N2 isotherms, by which the micropore, mesopore volumes, and average micropore width were also determined.26 The total pore volumes were estimated according to N2 uptake at a relative pressure (P/P0) of 0.99. The specific surface areas were estimated using the Brunauer−Emmett−Teller (BET) method

α=

m 0 − mt m0 − m f

(1)

where m0 is the initial weight, mt is the weight at time t, and mf is the final weight. The general equation for the solid state desorption rate under the nonisothermal condition is written as27 dα dα =β = A × e−Ea / RT × f (α) dt dT

(2)

where Ea is the activation energy, A is the Arrhenius preexponential factor, f(α) is the differential function for the reaction model, β is the heating rate, T is the absolute temperature, and R is the gas constant. To analyze the kinetics of the desorption processes, model fitting was performed in the conversion region where the activation energy was approximately constant and a single-step kinetic model could be used.28 Initial estimations of activation energy, Ea,k, and the preexponential factor, Ak, are first needed as references for subsequent use of model-fitting method. These can be done by using the Kissinger method:29 ⎛A R⎞ ⎛ β ⎞ Ea, k ⎛ 1 ⎞ ln⎜ 2 ⎟ = ln⎜⎜ k ⎟⎟ − ⎜ ⎟ R ⎝ Tmax ⎠ ⎝ Tmax ⎠ ⎝ Ea, k ⎠

(3)

Tmax is the temperature for desorption rate peak; values of Ea,k and ln Ak are obtained by plotting ln(β/(Tmax)2) as a function of 1/Tmax. The Kissinger method tends to give reliable results when the degree of the desorption for the desorption rate peak, B

DOI: 10.1021/acs.iecr.5b03788 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. Algebraic Expressions of f(α) and g(α) for the Reaction Models Considered in the Present Work symbol

model

f(α)

g(α)a

(1/2)α−1 [−ln(1 − α)]−1 (3/2) (1 − α)4/3[(1 − α)−1/3 − 1]−1

α2 α + (1 − α) ln(1 − α) [(1 − α)−1/3 − 1]2

Diffusion Models D1 D2 D3 Am P4 R2 R3 F0 (R1) F1 (A1) F2 F3 a

1-D diffusion 2-D diffusion 3-D diffusion (Z-L-T equation)

Nucleation Models random nucleation and growth of nuclei (JMA equation; mb = 2, 3/2, 4/3, 1, 2/3, m(1 − α)[−ln(1 − α)]1−1/m 1/2, 1/3 and 1/4) power law (2/3)α−1/2 Geometrical Contraction Models phase-boundary controlled reaction (contracting area, i.e., bidimensional shape) 2(1 − α)1/2 phase-boundary controlled reaction (contracting area, i.e., tridimensional shape) 3(1 − α)2/3 Reaction-Order Models zero-order 1 first-order 1−α second-order (1 − α)2 third-order (1 − α)3

[−ln(1 − α)]1/m α3/2 1 − (1 − α)1/2 1 − (1 − α)1/3 α −ln(1 − α) (1 − α)−1 − 1 0.5[(1 − α)−2 − 1]

g(α) = ∫ α0 dα/f(α). bParameter of JMA equation.

αmax, does not vary with β, which should be checked in practical uses.30 To select the appropriate kinetic model for each desorption process, a model-fitting method was performed by fitting different models to the experimental data of α versus T. One of the most popular methods is based on the Coats and Redfern equation31 ⎡ AR ⎛ ⎤ ⎛ g (α ) ⎞ E 2RT ⎞⎥ ⎟⎟ − a, i ln⎜⎜ i 2 ⎟⎟ = ln⎢ i ⎜⎜1 − ⎢⎣ βEa, i ⎝ Ea, i ⎠⎥⎦ RT ⎝ T ⎠ ⎡ AR ⎤ Ea, i ≅ ln⎢ i ⎥ − ⎢⎣ βEa, i ⎥⎦ RT

(4)

where the subscript i refers to the recommended reaction model as listed in Table 2 and gi(α) is the integral form for the model selected. The nonisothermal desorption data for naphthalene and pyrene on MCM-41, SBA-15, and CMK-3 were fitted. Plotting ln[g(α)/T2] versus 1/T gives Ea,i and ln Ai from the slope and intercept for one model toward a desorption process. The kinetic triplet with a high fitting correlation (R2) and the closest pair of Ea,i and ln Ai to the pair of Ea,k and ln Ak from the Kissinger method (eq 3) could be selected as the best one for the investigated TPD process.

Figure 1. Powder X-ray diffraction patterns of MCM-41, SBA-15, and CMK-3.

nm, respectively. This also explains the higher angle for the XRD peaks on MCM-41 compared to SBA-15 and CMK-3 due to the smaller hexagonal unit cell size (a0 = 2d110/√3). Displaying similar peak diffraction angles as those on SBA-15 as the template, CMK-3 shows a more ordered structure corresponding to the higher intensity of (1 0 0) reflection compared to the other two materials. The 77 K nitrogen adsorption−desorption isotherms and pore size distributions (PSDs) for the mesoporous adsorbents are shown in Figure 2, panels a and b, respectively. The PSDs for the adsorbents after three blank TPD cycles (at 12 K/min) were also characterized as shown in Figure S1 in the Supporting Information, which differs little from the corresponding pristine samples, indicating good stabilities of the adsorbents during TPD cycling, up to 800 K in dry atmosphere. The type IV isotherm with a capillary condensation step at intermediate relative pressure for each sample indicates uniform mesoporosity.34 Significant differences in their pore distributions could be observed. MCM-41 holds the primary mesopore with

3. RESULTS AND DISCUSSION 3.1. Structural Properties of Adsorbents. The low-angle XRD patterns of MCM-41, SBA-15, and CMK-3 are depicted in Figure 1. The three materials display hexagonal arrangement of mesopores (p6m), as indicated by the well-defined peaks indexed as (1 0 0), (1 1 0), and (2 0 0) diffractions.32,33 It is well-known that MCM-41 is featured with unconnected and regular mesopores as one-dimensional (1D) channels,11 while SBA-15 has complementary micropores and small mesopores in the silica walls connecting the primary 1D mesopore channels to form a 2D hexagonal array.9,13 As a negative replica structure of SBA-15,15 CMK-3 can be described as a network of carbon rods with ordered mesopores in between and micropores in the walls of the nanorods.14 The d spacing values of the (100) peaks, obtained from Bragg’s law, nλ = 2d110 sin 2θ, where n = 1 and λ is 0.1541 nm (wavelength of the Cu Kα X-ray), for the present MCM-41, SBA-15, and CMK-3 are 3.85, 8.65, and 8.24 C

DOI: 10.1021/acs.iecr.5b03788 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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3.2. Thermogravimetric Desorption Results. The TPD curves of the residual mass (W) and the mass loss rate (dW/ dT) for naphthalene on three adsorbents are shown in Figure 3,

Figure 2. (a) 77 K Nitrogen adsorption/desorption isotherms and (b) the corresponding pore size distributions of MCM-41, SBA-15, and CMK-3.

the peak size at 2.8 nm, whereas SBA-15 and CMK-3 exhibited coexisting structures of micropores/small or large mesopores and primary mesopore. The textural parameters including the specific surface area (SBET), pore volumes, and mesopore diameter (dp,NLDFT) based on NLDFT method are listed in Table S1. The SBET of CMK-3 (1594.7 m2 g−1) is higher than those of SBA-15 (865.2 m2 g−1) and MCM-41 (973.7 m2 g−1), owing to its abundant micropores formed during the carbonization process. The pore wall thickness, Tw, was calculated by taking the difference between a0 and dp,NLDFT (Tw = a0 − dp,NLDFT) with the assumption that mesopores in all samples are presented in a perfect hexagonal structure.35 According to the synthesis mechanism, the pores and the pore walls of SBA-15 should correspond to the framework and the pores of CMK-3, respectively. As can be seen from Table S1, despite the similar unit cell sizes (a0), SBA-15 has the peak pore size at 7.31 nm with Tw of 2.68 nm, which is different from CMK-3 at 3.79 nm with Tw of 5.73 nm.36

Figure 3. TPD curves of naphthalene on the mesoporous adsorbents: (a) residual mass; (b) mass loss rate.

panels a and b, and those for pyrene in Figure 4, panels a and b, respectively. As mentioned, these PAH−sorbent samples were obtained via batch adsorption equilibrated after 48 h. The total weight loss for each sample was the almost the same from four different heating rates. This indicates that complete weight loss was obtained for all samples below 850 K. The desorption amounts of both naphthalene and pyrene are smaller on MCM41, corresponding to smaller mass losses than on the other two adsorbents, clearly ascribed to its absence of microporosity, which provides higher adsorption potential for adsorbates than in mesorpores. At much higher solution concentrations, more naphthalene should have been adsorbed compared to pyrene. As a result, the adsorbed amounts of pyrene are comparable with those of naphthalene on MCM-4 or even higher on CMKD

DOI: 10.1021/acs.iecr.5b03788 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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naphthalene. Thus, the binding energies for pyrene were higher than that of naphthalene, as expected from the larger molecular size of pyrene (C16H10) than naphthalene (C10H8) due to higher polarizability and closer fitting in the micropores. Morphologically, the pyrene molecule is formed by two naphthalene molecules combined together by sharing a carbon atom on each benzene ring of naphthalene in their molecular planes. In addition, higher hydrophobicity of pyrene than naphthalene regarding the hydrophobic surfaces of the adsorbents also contributes to stronger binding energy significantly. On the basis of the above complex factors, the desorption behaviors of the strongly bound PAHs on these adsorbents merit further kinetic analysis. 3.3. Estimation of the Kinetic Triplet. The initial estimations of the Ea,k and ln Ak for each investigated desorption process were conducted using the Kissinger method (eq 3) based on values of Tmax, and the results are summarized in Table S2. The values of αmax for each sample at different heating rates were also listed, showing its negligible variation with heating rate and, thus, the applicability of the Kissinger method. Subsequently, the nonisothermal desorption data for each process were fitted with different kinetic models (Table 2) using the Coats and Redfern method (eq 4). Twenty pairs of Ea,i and ln Ai for the kinetic models demonstrate a strong correlation: ln Ai = bEa,i + c.39 This is called “compensation effect”, a relationship often observed for heterogeneous solidstate kinetics,40 based on the intersection of all the Arrhenius plots of the selected kinetic models at a single temperature referred to as the isokinetic temperature (Tiso), where b = 1/ RTiso and c = lnkiso (kiso is an artificial isokinetic rate constant).27,41 The compensation relationships (ln Ai versus Ea,i) at different heating rates for the desorption of naphthalene and pyrene are shown in Figure 5, panels a and b, respectively, and the related parameters are shown in Table S9 of the Supporting Information. As would be expected, Tiso as a fundamental materials characteristic slightly increases with the heating rate. It also ranges around the values of Tmax for each sample and follows the same order as Tmax indicative of desorption difficulty following the order SBA-15 < MCM-41 < CMK-3, for both naphthalene and pyrene. According to the fitting results using the Coats and Redfern method, the models D3, A4/3, A1, AE1.5, AE2, AE3, F2, and F3 (only for pyrene on CMK-3) are selected for comparisons because of their high squared correlation coefficient (R2 > 0.98). Values of Ea,i, ln Ai, and R2 for each of these models for each sample at different heating rates are given in Tables S3−S8 of the Supporting Information. As can be seen from the tables, although these selected models fit the experimental data well, they exhibited great difference in values of Ea,i and ln Ai. For each investigated desorption case, the best model giving the closest pair of Ea,i and ln Ai to that of Ea,k and ln Ak and the average values of the resulting kinetic parameters (denoted Ea and ln A) are listed in Table 3. For validation of the models obtained, another model-fitting method called “master plot”42 was used by transforming the experimental kinetic data into master plots that are defined as the plots of g(α)/g(0.5) versus α and comparing them with the theoretical master plots drawn by assuming various kinetic models. The integrated form of the kinetic rate equation is obtained from eq 2 as43

Figure 4. TPD curves of pyrene on the mesoporous adsorbents: (a) residual mass; (b) mass loss rate.

3, although still lower on SBA-15, which is likely due to some inaccessible micropores for pyrene with a larger molecular size. These can be related to the higher affinity for pyrene than naphthalene in mesopores and the higher hydrophobicity of pyrene than naphthalene (water solubility at 298 K of pyrene, ∼0.15 mg/L,and of naphthalene, ∼30 mg/L), resulting in stronger interactions of pyrene with the hydrophobic surfaces on these adsorbents. Particularly for CMK-3, the carbon-based surfaces show stronger π−π interactions with pyrene molecules due to their greater π-electron-rich property and flat conformation.37,38 As can be seen from Figures 3b and 4b, the increase in the heating rate leads to slight shifts of the maximal desorption rate to lower values and of Tmax to higher values. Sharper desorption peaks and lower Tmax values for SBA-15 indicate the desorption of PAHs on it were faster and easier as compared to MCM-41 and CMK-3. On all of these adsorbents, the temperatures that were required for desorption were higher for pyrene than for

g (α ) = E

∫0

α

dα AE p (x ) = f (α ) Rβ

(5) DOI: 10.1021/acs.iecr.5b03788 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research g (α ) p(x ) = g (0.5) p(0.5)

(7)

x0.5 = E/RT0.5, and T0.5 is the temperature corresponding to 50% conversion. The experimental master plots can be obtained by plotting p(x)/p(x 0.5 ) versus α, with the predetermined value of Ea as well as the T as a function of α. Comparisons of the experimental master plots for naphthalene and pyrene on adsorbents with the theoretical master plots depending on the best fitted models obtained above are presented in Figure 6, panels a and b, respectively. By following

Figure 5. Compensation relationships (ln Ai vs Ea,i) at different heating rates for the desorption of (a) naphthalene and (b) pyrene.

Table 3. Kinetic Triplet for Each Desorption Process Obtained by the Coats and Redfern Method naphthalene kinetic triplet

MCM-41

model Ea (kJ/mol) ln A (min−1)

A2/3 73.55 22.16

pyrene

SBA-15 CMK-3 A1 59.76 17.92

A1/3 77.92 17.31

MCM-41 A2/3 129.04 30.19

SBA-15 CMK-3 A4/3 54.87 11.82

F3 87.03 18.77

Figure 6. Comparisons of the experimental master plots of p(x)/ p(x0.5) versus α with theoretical master plots of g(α)/g(0.5) versus α for the desorption of (a) naphthalene and (b) pyrene.

where x = E/RT and p(x) is the Arrhenius temperature integral based on the approximation formula as44 p(x ) =

⎞ e −x ⎛ 1 ⎜ ⎟ ⎝ x 1.002x + 1.874 ⎠

the master plot method, the kinetic agreement during the whole course of each desorption case exhibits the same result as that obtained by the Coats and Redfern method: A2/3, A1, and A1/3 for naphthalene and A2/3, A4/3, and F3 for pyrene, on MCM-41, SBA-15, and CMK-3, respectively, which confirms our previous conclusions. 3.4. Kinetic Analysis: Johnson−Mehl−Avrami (JMA) Model Parameters and Desorption Mechanisms. From

(6)

From the integral kinetic equation at infinite temperature (eq 5), we can obtain the following equation using a reference point at α = 0.5: F

DOI: 10.1021/acs.iecr.5b03788 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

For CMK-3, the A1/3 model for naphthalene corresponding to the mechanism of diffusion control suggests more significant role of pore diffusion resistance in desorption. This can be ascribed to the steric restrictions for naphthalene in abundant micropores and to the highly hydrophobic surfaces on CMK-3 showing stronger interactions for naphthalene with a nonpolar molecular structure. The reaction-order kinetic model (F3) for pyrene as a compound with even higher molecular hydrophobicity and flat conformation than naphthalene might result from its higher affinity in terms of the stronger π−π interaction. As would be expected, being subject to greater adsorbate− substrate interaction and thus tighter sorption with strong binding energy, pyrene tended to exhibit desorption in more like a decomposition reaction as the kinetic mechanism, which is different from those for other cases. On the other hand, the highly ordered structure of consecutive mesopores in CMK-3 would contribute to more facile diffusion for desorbing adsorbates from the pore surfaces to the bulk phase, as indicated by the lower Ea and ln A for pyrene compared to MCM-41 without such structural advantages. It needs to be stressed that values of Ea and ln A for naphthalene are slightly higher on CMK-3 than on MCM-41, owing to the intensive sorption by micropores as defects in walls of the carbon. Although both CMK-3 and SBA-15 have coexisting structures of micropores and mesopores, the ways of displaying the structural advantages for desorption on the two materials are different. In SBA-15, transversal micropores and narrow mesopores embedded in walls of mesopores act not only as available adsorption spaces for gases but also as the interconnections between adjacent mesopore channels.54 The 2D connectivity is believed to facilitate the mass transfer for the PAH adsorbates being desorbed from either micropores or mesopores, that is, to help diffusion inside the entire porous structure of SBA-15,55 except for some very small and deep pores related to the surface roughness. By contrast, for CMK-3 as the inverse structure of SBA-15, the micropores formed during carbonization processes locate inside the carbon walls. The interior microporosity enhances the adsorption of PAHs adsorbates as shown above, but is hard to provide facile diffusion as in SBA-15, because the releasing adsorbates are more likely to be hindered due to the strong surface binding or the potential clogging effects during collisions within the disorder structure. These results suggest that advanced materials design with proper existence of micropores (both amounts and locations) in connection with mesopores would benefit the practical use of sorbents in terms of reaching a balance between strong adsorption and facile desorption.

the desorption kinetics results (Table 3), it is found that all investigated processes except pyrene-on-CMK-3 follow the JMArate equation,45,46 g(α) = [−ln(1 − α)]n in integral form, which belongs to the nucleation and growth model type. The JMA equation has often been used to describe the reaction kinetics of phase transformation and decomposition.47 The exponent n in the equation is a factor related to the specific mode.48 For instance, when n < 0.5, the mechanism is diffusioncontrolled; when 0.5 ≤ n ≤ 1, the mechanism approaches phase-boundary control; and when n > 1, the mechanism is controlled by nucleation and growth.49,50 Adsorption of naphthalene and pyrene in the mesopores and embedded micropores with different steric restrictions indicates that their releases into bulk gas during desorption were controlled by different mechanisms. In particular, the effects of the pore connectivity (the way the pores are connected in space) on desorption remain to be clarified. For MCM-41, the desorptions for naphthalene and pyrene share the same n value in the JMA model (A2/3) indicative of the phase-boundary control mechanism.51 This could be closely associated with the phase transformation for adsorbates from adsorbed solid to bulk gas. The mechanism did not change with the adsorbate molecular size, which could be attributed to the unconnected and regular cylindrical mesopores in MCM-41 material. In this simple porous structure, the existences of the adsorbed structures covering the surface are more likely to be consistent for different adsorbates, as long as they fit the size of the pores. As indicated by the characterization result in Figure 2b, the pores in MCM-41 concentrated in 2.5−4.5 nm. Either naphthalene with a molecular size of 0.655 nm or pyrene with a molecular size of 0.816 nm52 ought to enter the pores and reach adsorption sites. The values of Ea and ln A are much higher for pyrene than for naphthalene on MCM-41, indicating higher difficulty in desorption. The main reason for this could be the higher adsorption potential for pyrene than for naphthalene, because of the proximity of adsorbate molecules and pore walls. Additionally, desorbed in the 1D channel with only one way in and out, the escaping molecules with larger size were more easily blocked or hindered during the mass transfer processes. For SBA-15, the kinetic model for naphthalene (A1) indicates the mechanism of phase-boundary control and was changed for pyrene (A4/3) with a higher value of n indicating the mechanism of nucleation and growth. The different mechanisms suggest different existing forms of naphthalene and pyrene adsorbed on SBA-15, as can be judged from the wide pore size distribution including the mesopore peaking at 7.31 nm (Table S1). The two PAHs with different molecular sizes favor the adsorption in pores with different size ranges, in terms of the preferential adsorption due to proximity of adsorbates and pore walls. As a result, different steric restrictions for the adsorbates in pores led to different desorption kinetic mechanisms. In contrast to the case of MCM-41, the values of Ea and ln A are lower for pyrene than for naphthalene on SBA-15. As these kinetic parameters are lumped characteristics for the entire desorption processes, this could be attributed to the smaller adsorbed amount of pyrene as seen from Figure 4. In addition, the existence of intrawall connections in the regular mesoporous structures53 is able to provide less resistance for diffusion during desorption. This effect, for SBA-15 material with certain amounts of small mesopores in addition to micropores as lateral channels, could be more significant on larger-sized and less-adsorbed pyrene molecules.

4. CONCLUSIONS We conducted temperature-programmed desorption of two model PAHs on typical mesoporous adsorbents for kinetic studies and desorption mechanism originated from geometrical structures and physical properties. Applying combined kinetic analysis based on the experimental data gives reliable kinetic results, identifying JMA kinetic models A2/3, A1, and A1/3 for naphthalene and JMA models A2/3 and A4/3 and reaction-order model F3 for pyrene, on MCM-41, SBA-15, and CMK-3 sorbents, respectively. MCM-41 with simple structure of 1D mesopore channels exhibited similar desorption behaviors for both adsorbates while delivering higher kinetic parameters (Ea and ln A) for pyrene due to its higher affinity with surfaces and larger possibility of blockage in pore. With coexisting micropore−mesopore structures that favor adsorbates differG

DOI: 10.1021/acs.iecr.5b03788 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

adsorption on activated carbons at very low concentrations. Ind. Eng. Chem. Res. 2003, 42, 155. (7) Valderrama, C.; Gamisans, X.; De las Heras, X.; Farran, A.; Cortina, J. L. Sorption kinetics of polycyclic aromatic hydrocarbons removal using granular activated carbon: Intraparticle diffusion coefficients. J. Hazard. Mater. 2008, 157, 386. (8) Mastral, A. M.; Garcia, T.; Callen, M. S.; Navarro, M. V.; Galban, J. Removal of naphthalene, phenanthrene, and pyrene by sorbents from hot gas. Environ. Sci. Technol. 2001, 35, 2395. (9) Kosuge, K.; Kubo, S.; Kikukawa, N.; Takemori, M. Effect of pore structure in mesoporous silicas on VOC dynamic adsorption/ desorption performance. Langmuir 2007, 23, 3095. (10) Yun, J. H.; Choi, D. K.; Kim, S. H. Adsorption of organic solvent vapors on hydrophobic Y-type zeolite. AIChE J. 1998, 44, 1344. (11) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Sheppard, E. W. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 1992, 114, 10834. (12) Kruk, M.; Jaroniec, M.; Ko, C. H.; Ryoo, R. Characterization of the porous structure of SBA-15. Chem. Mater. 2000, 12, 1961. (13) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science 1998, 279, 548. (14) Ryoo, R.; Joo, S. H.; Jun, S. J. Synthesis of highly ordered molecular sieves via template-mediated structural transformation. J. Phys. Chem. B 1999, 103, 7743. (15) Vinu, A.; Hossain, K. Z.; Kumar, G. S.; Ariga, K. Adsorption of L-histidine over mesoporous carbon molecular sieves. Carbon 2006, 44, 530. (16) Gibson, L. T. Mesosilica materials and organic pollutant adsorption: part A removal from air. Chem. Soc. Rev. 2014, 43, 5163. (17) Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M. Ordered mesoporous carbons. Adv. Mater. 2001, 13, 677. (18) Serrano, D. P.; Calleja, G.; Botas, J. A.; Gutierrez, F. J. Adsorption and hydrophobic properties of mesostructured MCM-41 and SBA-15 materials for volatile organic compound removal. Ind. Eng. Chem. Res. 2004, 43, 7010. (19) Wu, Z.; Zhao, D. Ordered mesoporous materials as adsorbents. Chem. Commun. 2011, 47, 3332. (20) Li, Z.; Liu, Y.; Yang, X.; Xing, Y.; Wang, Z.; Yang, Q.; Yang, R. T. Desorption kinetics of naphthalene and acenaphthene over two activated carbons via thermogravimetric analysis. Energy Fuels 2015, 29, 5303. (21) Park, J. H.; Yang, R. T. Predicting adsorption isotherms of lowvolatile compounds by temperature programmed desorption: iodine on carbon. Langmuir 2005, 21, 5055. (22) Perez-Maqueda, L. A.; Criado, J. M.; Gotor, F. J.; Malek, J. Advantages of combined kinetic analysis of experimental data obtained under any heating profile. J. Phys. Chem. A 2002, 106, 2862. (23) Xi, H. X.; Li, Z.; Zhang, H. B.; Li, X.; Hu, X. J. Estimation of activated energy and isotherm of low-volatility dioxin by TPD technique. Sep. Purif. Technol. 2003, 31, 41. (24) Chen, D.; Qu, Z.; Sun, Y.; Wang, Y. Adsorption−desorption behavior of gaseous formaldehyde on different porous Al2O3 materials. Colloids Surf., A 2014, 441, 433. (25) Yang, R. T.; Long, R. Q.; Padin, J.; Takahashi, A.; Takahashi, T. Adsorbents for dioxins: a new technique for sorbent screening for lowvolatile organics. Ind. Eng. Chem. Res. 1999, 38, 2726. (26) Ravikovitch, P. I.; Neimark, A. V. Characterization of micro-and mesoporosity in SBA-15 materials from adsorption data by the NLDFT method. J. Phys. Chem. B 2001, 105, 6817. (27) Gotor, F. J.; Criado, J. M.; Malek, J.; Koga, N. Kinetic analysis of solid-state reactions: the universality of master plots for analyzing isothermal and nonisothermal experiments. J. Phys. Chem. A 2000, 104, 10777. (28) Monazam, E. R.; Spenik, J.; Shadle, L. J. CO2 desorption kinetics for immobilized polyethylenimine (PEI). Energy Fuels 2014, 28, 650.

ently according to their sizes, both SBA-15 and CMK-3 showed gaps between desorption mechanisms for naphthalene and pyrene, but interestingly in different ways. SBA-15 with micropores and narrow mesopores as interconnections between primary mesopores facilitated diffusion more significantly for larger-sized and less-adsorbed pyrene, resulting in lower kinetic parameters than naphthalene. This structural advantage is expected to contribute significantly to the obtained desorption easiness order of SBA-15 > MCM-41 > CMK-3. As compared to the mesosilicas, CMK-3 with greater microporosity and surface hydrophobicity showed stronger bonding with the PAH adsorbates, particularly for pyrene with higher polarizability, molecular hydrophobicity, and flat conformation, leading to strong π−π interactions, which might make desorption process somewhat like a decomposition reaction (F3 as kinetic model). However, the pyrene desorption on it could still benefit from the consecutive mesoporous structures, as indicated by the lower kinetic parameters compared to MCM-41.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03788. Textural parameters for the mesoporous adsorbents in section 3.1; pore size distributions of the adsorbents after three TPD cycles in section 3.1; fitting results for desorption kinetic models based on the Kissinger method and the Coats and Redfern method along with parameters of compensation effect in section 3.3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(X.Y.) Phone: +86-10-52072257. Fax: +86-10-62329145. Email: [email protected]. *(Y.X.) Phone: +86-10-62332206. Fax: +86-10-62347649. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (Grant 51478038) is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.iecr.5b03788 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX