Adsorption of Propanol Isomer Vapors on Sorbonorit B4 Activated

Article pubs.acs.org/jced

Adsorption of Propanol Isomer Vapors on Sorbonorit B4 Activated Carbon: Equilibrium and Spectroscopic Studies Dorota Downarowicz* and Tomasz Aleksandrzak Department of Chemical Engineering and Environmental Protection Processes, West Pomeranian University of Technology, Szczecin, Aleja Piastów 42, 71-065 Szczecin, Poland ABSTRACT: The adsorption equilibrium of propan-1-ol (1PN) and propan-2-ol (IPA) vapor was measured at (293.15, 313.15, 348.15, 373.15, and 393.15) K using the gravimetric method. The multitemperature Toth, Sips, inhomogeneous DA, and hybrid Langmuir−Sips models were used to fit the experimental data. Results indicate that Sips and hybrid Langmuir−Sips models provide the best correlation. The surface composition of Sorbonorit B4 (SB4) was evaluated from X-ray photoelectron, infrared, and Raman spectroscopy. It was demonstrated that the adsorbent is heterogeneous and contains various surface oxygen groups. Although both alcohols have the same molecular formula, the maximum adsorption capacity of SB4 for 1PN vapor is somewhat higher (ca. 6.13 mol·kg−1) than that of IPA (ca. 5.58 mol·kg−1). The value of the isosteric heat of adsorption for the former compound is also larger than that of the other over the whole loading range. SB4 exhibits a higher affinity for 1PN than for IPA vapor, which may be due to various availabilities of micropore volumes for both compounds caused by different molecular shapes and sizes.

1. INTRODUCTION Propanol is much less commercially important than methanol and ethanol, but recently its annual consumption significantly increased (3.9% per year).1 There are two position isomers of propanol, propan-1-ol (1PN) and propan-2-ol (IPA), that are completely miscible with water and most organic liquids. They are commonly used as solvents (half of the total global consumption of IPA and more than 40% of 1PN), and their vapors can be released into the atmosphere during production, handling, storage, and transportation.1 A loss of both alcohols can be reduced by waste gas purification in adsorption solvent recovery systems.2,3 Activated carbon is widely used in environmental systems because of its high adsorption capacity and low price. The adsorbent characteristics (surface area, pore size distribution, and surface chemistry), the nature of the adsorbate (molecular weight, molecular topology, and polarity), and adsorption process conditions have a large impact on the adsorption efficiency.4 The structural and surface heterogeneity of activated carbons can affect the process, especially for polar compound adsorption.4 The former heterogeneity type is the result of differences in pore size and shape. The latter is caused by carboxyl, carbonyl, and hydroxyl surface functional groups that are located on the edges of graphitic planes. The groups are usually polar in nature and play the role of primary adsorption centers for polar compounds and thereby cause an increase in the affinity of carbon for these species.5−7 For these reasons, the heterogeneity degree of active carbon may affect the efficiency of the process.8 © XXXX American Chemical Society

Adsorption is nearly always an exothermic process; therefore, multitemperature isotherm models are required for proper design calculations of adsorption plants. The models are useful for predicting the adsorbent capacity at any temperature and concentration and calculating the adsorption enthalpy.9 However, only some of them are suitable for adsorption equilibrium data of polar compounds on heterogeneous or mesoporous adsorbents. The main objective of this study was to investigate the effects of various molecular structures of propanol isomers (1PN and IPA) on the adsorption efficiency by SB4, a heterogeneous activated carbon. Moreover, several different multitemperature isotherm models were selected to describe the experimental equilibrium data. The maximum adsorbent capacity and the isosteric heat of adsorption were also determined. They are the crucial data for the correct design and operation of adsorption plants.

2. EXPERIMENTAL SECTION 2.1. Materials. Extruded, steam-activated carbon Sorbonorit B4 was selected as an adsorbent. It was manufactured from peat by Norit Ltd. (The Netherlands). The adsorbent consists of cylindrical pellets with a diameter of 3.7 mm and a length of 6.5 mm.9 Owing to its favorable adsorption properties (BET Received: July 4, 2016 Accepted: September 28, 2016

A

DOI: 10.1021/acs.jced.6b00583 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

surface area of 1170 m2·g−1), it is used in solvent vapor recovery applications.10 Two high-purity (99.7%) aliphatic alcohols (propan-1-ol and propan-2-ol) from Chempur (Poland) were used as adsorbates. They have the same molecular weight (60.096 g·mol−1) but different structures and therefore various physical−chemical properties. Owing to their excellent solubility in water and most organic liquids, both compounds are used in industry as reagents, solvents, and cleaning fluids. The basic properties of the compounds are presented in Table 1.

The Toth isotherm model is an empirical expression used to describe monolayer adsorption, which was developed to improve Langmuir isotherm fittings. The modified form of the model developed by Taqvi et al.13 is expressed as p q = qmT (bT + pnT )1/ nT (1) where bT (Pa·nT−1) is the parameter of the equation defined as ⎛ n ΔH ⎞ ⎟ bT = b0T exp⎜ − T ⎝ RT ⎠

Table 1. Physical−Chemical Properties of 1PN and IPA11 property

1PN

IPA

density of liquid at 293.15 K/kg·m−3 boiling point/K saturation pressure at 293.15 K/Pa liquid molar volume/cm3·mol−1 heat of vaporization/kJ·mol−1

802 370.3 1993 74.939 41.69

783 355.4 4450 76.784 39.87

(1a) −1

q is the adsorption capacity (mol·kg ), qmT is the maximum adsorption capacity (mol·kg−1), p is the equilibrium pressure of the adsorbate (Pa), T is the temperature (K), ΔH is the heat of adsorption (J·mol−1), R is the universal gas constant (8.314 J· mol−1·K−1), and b0T (Pa·nT−1) and nT are the parameters of the equations. The exponential term of eq 1a comprises the heterogeneity parameter nT, thus the isosteric heat of adsorption ΔH can be considered to be a loading- and temperature-independent parameter. The model can be applied over the whole range of adsorbate concentration (both low- and high-pressure limits); therefore, the Clausius−Clapeyron equation is satisfied.13 The Sips isotherm model is a combination of the Langmuir and Freundlich equations. At low adsorbate concentrations, the model is reduced to the Freundlich isotherm and thus does not follow Henry’s law. At high concentrations, it predicts a monolayer adsorption capacity characteristic of the Langmuir isotherm. The Sips model can be given by14

2.2. Apparatus and Procedure. The adsorption isotherm measurements of 1PN and IPA vapors on SB4 activated carbon were conducted using an IGA-002 intelligent gravimetric analyzer (Hiden Isochema Ltd., U.K.). It is a precise computer-control microbalance system with a resolution of 0.1 μg and an uncertainty of ±1 μg. The balance and pressure control system were fully thermostated to 0.2 K to eliminate changes in the external environment. A sample of activated carbon with a mass of ca. 80 mg was placed in the thermostated reactor chamber with accurate temperature control (±0.1 K). Prior to isotherm measurements, the SB4 sample was outgassed to constant weight at high vacuum (10−6 Pa) at 393.15 K for 2 h. The measurements were conducted at (293.15, 313.15, 348.15, 373.15, and 393.15) K and pressures of up to 4450 Pa for IPA and 2430 Pa for 1PN. A comprehensive description of the methodology of isotherm determination with IGA-002 is presented elsewhere.12 The elemental composition and chemical states of the SB4 sample were determine by X-ray photoelectron (XPS), infrared (IR), and Raman spectroscopy. XPS measurements were performed with an XPS PHI 5000 VersaProbe (ULVAC-PHI, Japan/USA) using Al monochromatic X-ray radiation (15 kV and 25 W). XPS analysis was conducted at high vacuum (UHV) (the base pressure inside the analysis chamber was 10−2 to 10−7 Pa). One unbroken granule of the activated carbon (spot size 100 μm × 100 μm) was selected for analysis. Widescan spectra in the binding energy range of 0 to 1400 eV were taken with a 0.4 eV step interval while narrow spectra were measured with a 0.1 eV step interval. The spectra were analyzed and processed using CasaXPS software. Raman spectra were acquired on an inVia Raman microscope (Renishaw) at an excitation wavelength of 785 nm. IR spectra were acquired on a Nicolet 6700 FT-IR spectrometer. The samples were prepared in KBr tablets. Duplicate analyses were made to confirm the resulting observations.

q = qmS

(bSp)nS 1 + (bSp)nS

(2) −1

−1

The temperature dependence of the qmS (mol·kg ), bs (Pa ), and ns parameters of the equations can be expressed as ⎛ ⎛ T ⎞⎞ qmS = q0S exp⎜⎜q1S⎜1 − ⎟⎟⎟ T0 ⎠⎠ ⎝ ⎝

(2a)

⎛ Q ⎛T ⎞⎞ bS = b0S exp⎜ S ⎜ 0 − 1⎟⎟ ⎠⎠ ⎝ RT0 ⎝ T

(2b)

⎛ T⎞ nS = n0S + n1S⎜1 − 0 ⎟ ⎝ T⎠

(2c)

where q0S (mol·kg−1), q1S, b0S (Pa−1), QS (J·mol−1), n0S, and n1S are the parameters of the equations and T0 is the reference temperature (293.15 K). The nS parameter in eq 2 characterizes the heterogeneity of adsorbent−adsorbate systems. The hybrid Langmuir−Sips isotherm model is more complex in comparison to the previous equations. It is a combination of the Langmuir and Sips isotherms. The model is given by15 ⎡ b p b2LSp nLS ⎤ ⎥ q = qmLS⎢ 1LS + 1 + b2LSp nLS ⎦ ⎣ 1 + b1LSp

(3)

The temperature dependency of the hybrid isotherm parameters is as follows:

3. MULTITEMPERATURE ISOTHERM MODELS In the literature, several multitemperature isotherm models, such as Toth, Sips, inhomogeneous DA, and hybrid Langmuir− Sips, are proposed for the description of the heterogeneous adsorbent−adsorbate equilibrium.9

⎛q ⎛ 1 1 ⎞⎞ qmLS = q0LS exp⎜⎜ 1LS ⎜ − ⎟⎟⎟ T0 ⎠⎠ ⎝ R ⎝T B

(3a)



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⎛ Q ⎛1 1 ⎞⎞ b1LS = b01LS⎜⎜ − 1LS ⎜ − ⎟⎟⎟ R ⎝T T0 ⎠⎠ ⎝

(3b)

⎛ Q ⎛1 1 ⎞⎞ b2LS = b02LS⎜⎜ − 2LS ⎜ − ⎟⎟⎟ R ⎝T T0 ⎠⎠ ⎝

(3c)

⎛ n ⎛1 1 ⎞⎞ nLS = n0LS⎜⎜ − 1LS ⎜ − ⎟⎟⎟ T0 ⎠⎠ ⎝ R ⎝T −1

(3d)

−1

−1

where qmLS (mol·kg ), b1LS (Pa ), b2LS (Pa·nLS ), nLS, q0LS (mol·kg−1), q1LS (J·mol), b01LS (Pa−1), Q1LS (J·mol−1), b02LS (Pa· nLS−1), Q2LS (J·mol−1), n0LS, and n1LS (J·mol−1) are the parameters. The inhomogeneous DA isotherm model is a combined form of two Dubinin−Astahov expressions (DA1 + DA2)16 ⎡ ⎛ ⎛ ⎞n1DA ⎞ ⎛ ⎛ A ⎞n2DA ⎞⎤ A ⎢ q = qmDA exp⎜⎜ −⎜ ⎟ ⎟⎟ + exp⎜⎜ −⎜ ⎟ ⎟⎟⎥ ⎢⎣ ⎝ ⎝ E1 ⎠ ⎠ ⎝ ⎝ E2 ⎠ ⎠⎥⎦ −1

(4)

Figure 1. Wide-scan XPS spectra of the SB4 sample.

−1

where E1 (J·mol ), E2 (J·mol ), n1DA, and n2DA are the parameters of the equations. Parameter A (J·mol−1) in eq 4 is the adsorption potential that is defined as ⎛p ⎞ A = RT ln⎜ 0 ⎟ ⎝ p⎠

using the CasaXPS software. An iterative least-squares fitting algorithm was used to decompose the peaks, with the curves being taken as 30% Gaussian and 70% Lorenzian. An asymmetric shape of the C 1s peak (Figure 2) indicates the

(4a)

where p0 (Pa) is the saturation pressure. The qmDA (mol·kg−1) parameter is written as a function of temperature qmDA = q0DA exp( −γDA(T − T0)) −1

(4b)

−1

where q0DA (mol·kg ) and γDA (K ) are the parameters of the equation. The last two adsorption isotherm equations were developed for the adsorption of condensable vapors on porous adsorbents. Both isotherm equations have two regions: the former is linear or favorable and depends on the Langmuir (or DA1) isotherm parameter, and the latter depends on the Sips (or DA2) isotherm parameter and greatly affects the capillary condensation region.16

4. RESULTS AND DISCUSSION 4.1. Chemical Structure of the Activated Carbon Surface. Figure 1 presents wide-scan XPS spectra in the binding energy (BE) range of 0 to 1400 eV for a SB4 sample. The spectra show two strong peaks, C 1s and O 1s, corresponding to the major components: carbon and oxygen. The C 1s peak extends from 282 to 293 eV, whereas the O 1s peak extends from 528 to 538 eV. Furthermore, the wide spectra exhibit ghost peaks: S 2p (BE from 163 to 173 eV) and Si 2p (BE from 98 to 107 eV). Both elements (sulfur and silicon) are present as impurities in the X-ray source.17 The narrow, high-resolution scan XPS spectra were recorded for all peaks. They were used for quantitative analysis of the elemental composition of the activated carbon surface. The spectra show the presence of 94% carbon, 5% oxygen, 0.5% sulfur, and 0.5% silicon. Because of the low content of the latter two elements, the results of the quantitative analysis of the XPS spectra are not shown in this article. The possible compounds on the SB4 surface were determined by a deconvolution of the narrow XPS peaks

Figure 2. XPS spectra of C 1s on the SB4 surface.

presence of several carbon species of different binding energies. On the basis of BE values, the following components were identified: aromatic (graphitic character) carbons at 284.6 eV (B peak), aliphatic carbons at 285 eV (C peak), hydroxyl/ phenolic groups at 286.1 eV (peak D), carbonyl groups at 287.6 eV (E peak), and carboxyl groups at 289.8 eV (F peak). An Xray satellite (shakeup) peak at 291.6 eV (G peak) was also observed. It is induced by a cloud of π electrons in materials with high concentrations of sp2 carbons (π → π* transitions occurring in aromatic rings).18 An additional A peak at 283.9 eV was required for the complete C 1s curve fit. All peak positions agree to within 0.1 eV. The relative composition of each functional group present on the SB4 surface was calculated as the ratio of its peak area to C



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sharp peaks at about 1605 cm−1 and about 1314 cm−1 and two relatively broad peaks around 1542 cm−1 and around 1284 cm−1, namely, G1, D1, G2, and D2, respectively. The Raman spectrum showed excellent curve fitting (accuracy factor ca. 0.99). The G1 and D1 peaks are assigned to the graphitic structure of the activated carbon because such vibrations arise from E2g in-plane vibrational mode and A1g in-plane breathing mode, respectively, exhibited in the spectra of the basal plane of polycrystalline graphite.20 The G2 and D2 peaks are attributed to the amorphous structure of the activated carbon.21 The peak intensity ratio of G2 to G1 (IG2/IG1) is a useful parameter expressing the relative content of the disordered part to the ordered part in the activated carbons22 and was calculated to be 0.65. IR spectroscopy was used to determine the surface groups on activated carbon, and the results are presented in Figure 5. The

the total area of the C 1s signal. It was found that nonfunctionalized (aromatic and aliphatic) carbons are the main surface components of SB4. They accounted for 66% of the total area of the C 1s peak region. The concentrations of both components are similar (35.4% aromatic and 30.7% aliphatic carbons). The concentrations of functional carbon atoms present in C−OH, CO, and COOH groups were 12.4, 4.9, and 4.7%, respectively. Unidentified peak A constituted 3.8% of the total peak C 1s area, whereas shakeup intensities constituted 2.2%. The deconvolution of the O 1s spectrum (Figure 3) demonstrated the presence of two oxygen group contributions

Figure 3. XPS spectra of O 1s on the SB4 surface. The A and B peaks correspond to oxygen in the double- and single-bonded states.

Figure 5. IR spectra of the SB4 surface.

with binding energies of 531.7 ± 0.1 eV (A peak) and 533.4 ± 0.1 eV (B peak). The A peak with lower binding energy was identified as oxygen doubly bound to carbon (CO in carbonyl or carboxyl groups). The B peak corresponded to oxygen with a single bond to carbon (C−O in alcohol groups). The contributions of A and B peaks to the total O 1s peak area were 3.2 and 1.9%, respectively. Figure 4 presents the Raman spectrum with Gaussian fitting of the activated carbon. The spectrum consists of two relatively

spectrum exhibits a broad and intense band at approximately 3450 cm−1, related to stretching vibrations of hydroxyl groups. The band at 2350 cm−1 corresponds to CO 2 in the environment. The peaks at 1630 and 1530 cm−1 are assigned to CC aromatic skeletal stretching and CC stretching bands, respectively. The peak at 1388 cm−1 is associated with C−H asymmetric bending. The band at 1235 cm−1 arises from the C−O asymmetric stretching of aromatic ethers, esters, or phenols. The band at 1095 cm−1 is related to C−O in carboxylic acids, alcohols, and esters. The bands in the range of 400 to 700 cm−1 indicate C−C stretching vibrations.23 The identified oxygen functional groups may play the role of primary active centers. Adsorbed alcohol molecules can be bound to them and form clusters located on the carbon surface as well as at the entry of micropores.8,18 The high content of the hydrophilic surface groups was identified as a major cause of higher adsorption capacities of both alcohol vapors compared to those of nonpolar compounds.19 Evidence for this can be found in the adsorption equilibrium results presented in section 4.2. 4.2. Modeling of Experimental Equilibrium Data. The adsorption equilibrium data of IPA and 1PN on SB4 at (293.15, 313.15, 333.15, 348.15, 373.15, and 393.15) K are presented in Tables 2 and 3. Each experiment was performed three times, and then the data were averaged.

Figure 4. Raman spectra with Gaussian fitting of the SB4 surface. D



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Table 2. Experimental Isotherm Data for IPA vapor on SB4a p/Pa

q/mol·kg−1

6.1 22.2 41.9 87.9 131.8 180.6 221.7

1.79314 2.65835 3.56034 4.17887 4.38594 4.51695 4.58312

4.7 19.9 42.9 91.1 135.0 179.9 222.2 269.6

0.52174 1.44560 2.01398 2.73343 3.18889 3.48773 3.67707 3.83303

4.9 21.3 43.8 86.9 133.6 180.3 221.9 267.8

0.33704 0.79394 1.12578 1.54231 1.88105 2.14857 2.34405 2.52567

5.3 21.7 43.6 91.8 133.3 178.1 223.6 269.8

0.24436 0.51454 0.71570 1.01580 1.22008 1.40352 1.56004 1.70077

5.0 23.3 42.3 90.2 136.8 182.5 221.4 265.9

0.1440 0.2789 0.3728 0.5559 0.6838 0.7737 0.8675 0.9499

4.4 20.5 43.9 87.1 132.2 178.2 222.8 268.1

0.07526 0.15473 0.23083 0.33131 0.41449 0.48547 0.54581 0.60111

p/Pa

q/mol·kg−1

293.15 K 4.64294 4.69079 4.73107 4.76378 4.79453 4.90156 4.97677 313.15 K 314.7 3.95498 356.5 4.03231 400.8 4.10796 446.2 4.17122 667.6 4.36992 894.5 4.48621 1 340.1 4.62058 268.3 313.5 360.2 404.8 450.6 671.9 895.3

p/Pa

were determined with the Levenberg−Marquardt method using Statistica 12.5 software. The goodness of fit of a model to the experimental data was evaluated on the basis of the average relative error (ARE)12

q/mol·kg−1

1 1 2 2 3 3

341.8 787.7 233.3 678.7 124.4 568.8

5.07868 5.15763 5.22249 5.28045 5.33411 5.39760

1 2 2 3 3 4 4

788.6 233.8 680.1 125.0 563.5 012.7 462.7

4.70922 4.77322 4.82348 4.86891 4.90338 4.93596 4.96604

333.15 K 313.0 2.68595 359.2 2.82087 404.4 2.93988 443.3 3.02862 667.9 3.42493 895.8 3.68011 1 334.7 3.98037

1 2 2 3 3 4 4

785.8 233.6 679.4 125.2 563.1 017.2 458.3

4.15214 4.26452 4.34574 4.40677 4.45246 4.49577 4.53095

348.15 K 314.0 1.80645 361.6 1.92020 404.5 2.04270 443.8 2.12295 668.3 2.51762 895.3 2.81219 1 341.2 3.21411

1 2 2 3 3 4 4

781.4 230.8 677.8 124.1 570.1 015.9 462.3

3.47924 3.66524 3.80631 3.91331 3.99529 4.06563 4.12318

373.15 K 311.7 1.02736 355.8 1.09573 401.2 1.16201 445.7 1.22240 668.2 1.48082 892.7 1.69619 1 336.2 2.03394

1 2 2 3 3 4 4

783.8 230.6 676.6 123.4 569.3 014.7 462.2

2.30052 2.51587 2.69426 2.8441 2.97385 3.08332 3.18037

393.15 K 313.5 0.65109 359.1 0.69783 404.5 0.74099 449.1 0.78116 672.2 0.95329 895.8 1.09779 1 340.3 1.33297

1 2 2 3 3 4 4

787.8 233.5 679.3 124.6 570.6 015.9 462.8

1.52687 1.69157 1.8377 1.96638 2.08361 2.1892 2.28767

N

ARE =

100 ∑ N i=1

qexp − qcal i

qexp

i

i

(5)

where qexp is the experimental adsorption capacity, qcal is the calculated adsorption capacity, and N is the number of experimental points. The curves for different equations were used for the selection of the appropriate isotherm models. As indicated by the ARE error values in Table 4 for the IPA− SB4 system, the hybrid Langmuir−Sips and Sips models give a better fitting of the equilibrium data (ARE < 3.9%) than do the other models (ARE from 5.4 to 8.3%). However, for the 1PN− SB4 system, the hybrid Langmuir−Sips model fit the data better (ARE = 4.1%) than the other models (ARE from 5.6 to 7.0%). Figures 6−9 show that all simulated isotherm curves agree well with the experimental results. It can be also observed that the adsorbate amounts decrease with increasing temperature, which is indicative of physical adsorption. The adsorption isotherms are classified as type I, according to the IUPAC classification.24 However, in high-pressure ranges, the isotherms at 293.15 K show some deviations from monolayer adsorption. This is typical of adsorbents with a large porous content. The structural heterogeneity of SB4 is confirmed by the results of the pore size distribution analysis using the Barrett−Joyner− Halend method (BJH) presented elsewhere.25 The analysis showed that SB4 contained three groups of pores: micropores (ca. 70%), mesopores (ca. 29%), and macropores (0.01%). Micropore diameters from 6 to 10 Å were dominant.25 A comparison of the equilibrium data reveals that the adsorption capacity for 1PN vapor is higher than that of IPA at all temperatures. This is confirmed by the maximum capacity values (qm) estimated from the Toth model (Table 4), which are roughly 6.126 and 5.576 mol·kg−1, respectively. Although they have the same molecular formula, isomeric propanol molecules have different shapes and sizes. It is the effect of various arrangements of the hydroxyl group along the carbon chains. Three-dimensional structures of propanol molecules were generated with MarvinSketch software (version: 16.2.1.0, ChemAxon). They are represented by a ball-and-stick model, in which red spheres represent oxygen atoms and bright-gray and dark-gray spheres represent carbon and hydrogen atoms, respectively. A molecular projection area was selected to describe their geometrical structures. It is a surface occupied by a separate molecule on a flat adsorbent surface. It can be described by the following parameters: minimum and maximum projection areas and projection radiuses. They were computed for the lowest-energy conformers using ChemAxon’s geometrical descriptors plug-in. As Table 5 shows, the minimum projection area of the 1PN molecule is greater than that of IPA by 1.9 Å2, whereas its minimum radius is lower by 0.28 Å. Moreover, the ratios of maximum to minimum projection radiuses of 1PN and IPA molecules are 1.46 and 1.14, respectively. This indicates that the former molecule has an elongated shape and the other is more compact. Therefore, a straight chain of the 1PN molecule can be arranged parallel along the pore walls and joined closely

a

Standard uncertainties are u(T) = 0.1 K and u(p) = 0.003p Pa. The combined expanded uncertainty is Uc(q) = 0.00038 mol·kg−1 (level of confidence = 0.95).

The four isotherm models presented in section 3 were used to correlate the experimental data. The results are presented in Figures 6−9. Their parameter values are given in Table 4. They E



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Table 3. Experimental Isotherm Data for 1PN Vapor on SB4a p/Pa

q/mol·kg−1

p/Pa

5.6 12.4 24.1 48.6 74.3 99.2 123.1

0.76299 1.87882 3.35832 4.23336 4.55414 4.71803 4.81857

146.7 172.5 196.8 223.3 246.7 367.7 488.9

5.4 12.2 23.9 49.0 72.2 98.1 122.1 147.3

1.21575 1.75598 2.30997 3.00007 3.38168 3.66628 3.85718 4.00916

5.6 12.1 24.7 49.6 74.0 96.9 121.3 147.0

0.61650 0.90428 1.23748 1.70109 2.02669 2.27080 2.48758 2.67812

172.1 197.0 220.1 246.0 365.3 489.7 733.3

6.5 10.0 23.8 50.1 73.9 98.3 123.4

0.37060 0.49326 0.74086 1.05542 1.26393 1.44703 1.60874

172.7 196.8 221.3 245.1 364.5 489.2 732.6

q/mol·kg−1

p/Pa

q/mol·kg−1

p/Pa

q/mol·kg−1

732.7 975.2 1 217.9 1 460.8 1 703.7 1 946.9

5.40617 5.50736 5.59685 5.68109 5.78995 5.98609

148.2

1.75174

975.7 1 219.3 1 462.6 1 705.9 1 949.9 2 191.9 2 435.7

4.94487 5.01642 5.07453 5.12336 5.16639 5.20448 5.23978

6.1 12.1 25.8 51.1 75.9 96.7 120.6 145.5

0.19682 0.27510 0.39489 0.55182 0.67000 0.75379 0.84319 0.92424

170.1 194.5 219.7 244.8 363.7 485.7 732.0

333.15 K 2.83643 2.97382 3.09240 3.19784 3.57351 3.83098 4.14096

973.6 1 219.0 1 462.4 1 705.7 1 949.6 2 193.1 2 436.4

4.32169 4.44610 4.53504 4.60332 4.65808 4.7039 4.74291

6.8 12.0 23.0 50.4 74.5 98.5 123.7 147.3

0.12305 0.16103 0.22699 0.33252 0.41418 0.47381 0.53062 0.57850

348.15 K 1.87847 1.99384 2.10050 2.18284 2.58297 2.88217 3.28707

971.3 1 216.4 1 460.8 1 704.6 1 949.0 2 192.0 2 436.5

3.55191 3.74839 3.89614 4.01203 4.10572 4.18176 4.24584

5.4 12.8 25.0 49.9 75.4 99.7 119.7 147.3

0.06729 0.10391 0.14204 0.20084 0.25289 0.29243 0.35526 0.39485

293.15 K 4.88784 4.94696 4.99304 5.03087 5.06882 5.19185 5.27936 313.15 K 172.9 4.12998 194.3 4.21225 219.7 4.29325 244.6 4.36009 363.5 4.57155 489.1 4.70146 732.0 4.84928

p/Pa

q/mol·kg−1

373.15 K 0.99759 1.06652 1.13215 1.19374 1.44139 1.65371 2.00222

971.4 1 216.0 1 460.0 1 704.8 1 948.3 2 190.7 2 434.6

2.26284 2.48453 2.67081 2.82855 2.96256 3.07999 3.18415

172.6 196.7 221.3 246.5 367.3 489.3 734.5

393.15 K 0.62707 0.66834 0.71030 0.74951 0.90604 1.04096 1.27390

973.8 1 215.7 1 462.1 1 705.1 1 949.6 2 192.3 2 434.2

1.4528 1.61159 1.75333 1.88151 1.99637 2.10088 2.19926

174.0 191.4 222.7 248.9 368.2 490.0 734.1

413.15 K 0.42654 0.45296 0.48643 0.51188 0.61478 0.70438 0.85406

977.5 1 220.0 1 463.1 1 706.6 1 951.2 2 192.9 2 437.5

0.9796 1.09297 1.19383 1.28331 1.36762 1.44577 1.52096

p/Pa

q/mol·kg−1 348.15 K

a

Standard uncertainties are u(T) = 0.1 K and u(p) = 0.003p Pa. The combined expanded uncertainty is Uc(q) = 0.00038 mol·kg−1 (level of confidence = 0.95).

Figure 6. Experimental and correlated isotherms for 1PN adsorption onto SB4 at various temperatures: ●, 293.15 K; ○, 313.15 K; ◆, 333.15 K; ◊, 348.15 K; ■, 373.15 K; □, 393.15 K; −, hybrid model; and − − , inhomogeneous DA.

Figure 7. Experimental and correlated isotherms for 1PN adsorption onto SB4 at various temperatures: ●, 293.15 K; ○, 313.15 K; ◆, 333.15 K; ◊, 348.15 K; ■, 373.15 K; □, 393.15 K; −, Toth model; and − − , Sips model.

together or to functional surface groups by hydrogen bonding.26,27

The isosteric heat of the adsorption (Qst) was used to characterize the strength of adsorbate−adsorbent and adsorF



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Table 4. Adsorption Isotherm Parameters and Average Relative Errors for SB4 model Toth

Sips

hybrid Langmuir−Sips

Figure 8. Experimental and correlated isotherms for IPA adsorption onto SB4 at various temperatures: ●, 293.15 K; ○, 313.15 K; ◆, 333.15 K; ◊, 348.15 K; ■, 373.15 K; □, 393.15 K; −, hybrid model; and − − , inhomogeneous DA.

inhomogeneous DA

parameters and errors

1PN

IPA

qm/mol·kg−1 b0T/kPa·nT−1 nT ΔH/kJ·mol−1 ARE/% q0S/mol·kg−1 q1S b0S/Pa−1 QS/kJ·mol−1 T0/K n0S n1S ARE/% q0LS/mol·kg−1 q1LS/kJ·mol−1 b01LS/Pa−1 Q1LS/kJ·mol−1 b02LS/Pa·nLS−1 Q2LS/kJ·mol−1 n0LS n1LS/kJ·mol−1 ARE/% q0DA/mol·kg−1 γDA × 103/K−1 E1/kJ·mol−1 n1DA E2/kJ·mol−1 n2DA ARE/%

6.1264 172.84593 0.47858 60.22427 6.99 5.81192 0.10855 0.09933 24.129860 293.15 0.6365 0.02377 4.04 3.22082 992.88895 0.13847 58.66638 0.20150 17.90099 0.37479 0.36158 3.25 2.84099 0.0006 15.29095 3.32631 16.30093 1.3282 5.02

5.57624 877.10504 0.54814 56.97768 8.32 5.29042 0.04937 0.05606 23.18569 293.15 0.75691 −0.39711 4.07 2.98602 0 0.05422 55.10143 0.28877 24.99368 0.30792 2.02346 3.18 2.61019 0.695394 14.865794 3.563258 18.219779 1.68251 5.43

Table 5. Projection Area Parameters of 1PN and IPA Molecules

Figure 9. Experimental and correlated isotherms for IPA adsorption onto SB4 at various temperatures: ●, 293.15 K; ○, 313.15 K; ◆, 333.15 K; ◊, 348.15 K; ■, 373.15 K; □, 393.15 K; −, Toth model; and − − , Sips model.

bate−adsorbate interactions. It was calculated on the basis of the multitemperature adsorption isotherms using the Clausius− Clapeyron equation14 ⎛ ⎞ ⎜ ∂ ln p ⎟ Q st = −R ⎜ 1 ⎟ ⎝∂ T ⎠

()

Figure 10 shows that the Qst curves for 1PN and IPA have increasing trends up to loadings of 0.7 and 1.8 mol·kg−1, respectively. The Qst value for IPA increases from 49.4 to 54.2 kJ·mol−1 before it drop by 2.5 kJ·mol−1 and then increases to 59.2 kJ·mol−1. For 1PN adsorption, it increases from 56.1 to 57.2 kJ·mol−1, then is almost constant in the loading range from 0.7 to 3 mol·kg−1, and is close to the ΔH value in the Toth equation (ΔH = 60.2 kJ·mol−1). At higher coverages, its value increases to 62.9 kJ·mol−1. The initial increase in the isosteric heat of adsorption with an increase in propanol loading indicates that lateral interactions between adsorbate molecules are dominant over adsorbate−adsorbent interactions.29 Figure

(6) −1

where Qst is the isosteric heat of adsorption (J·mol ). As can be seen from Figure 10, the Qst value depends on the amount of adsorbate. It is indicated that activated carbon has a heterogeneous surface, and the process occurs on various adsorption sites. G



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decisive role in the process.27,30 In contrast to this, propanol isomer adsorption behavior is dependent on the participation of the dispersion interaction (alkyl group) and hydrogen bonding (hydroxyl group).5 The contribution of particular interactions for such polar compounds depends on their molecular polarity as well as chain lengths and structures31 The slightly different shape of the 1PN vapor isotherm compared to that of IPA may be due to stronger dispersion forces between the former alcohol molecules and the adsorbent. This results in higher Qst values for 1PN, as indicated in Figure 10. It can be the effect of th adsorption efficiency of propanol vapors in activated carbon beds.

5. CONCLUSIONS Adsorption equilibrium measurements for two propanol isomers (propan-1-ol and propan-2-ol) on Sorbonorit B4 (SB4) activated carbon were conducted at five temperatures in the range of 293.15 to 393.15 K. Four multitemperature isotherm models (Toth, Sips, inhomogeneous DA, and hybrid Langmuir−Sips) were used to fit the experimental data. Calculation indicated that the hybrid Langmuir−Sips model provides the best correlation (ARE < 4%) for both adsorbent− adsorbate systems. The maximum adsorption capacities of SB4 for 1PN vapor is higher than that of IPA by ca. 0.55 mol·kg−1. A comparison between experimental and literature equilibrium data suggests that both types of alcohol vapor are more effectively adsorbed by SB4 than is benzene vapor. This may be due to the abundance of surface oxygen groups identified by XPS and IR and Raman spectroscopy. Different types of interactions may also affect the adsorption efficiency. Because of the complexity of this issue, further studies are necessary to fully elucidate the adsorption mechanism for heterogeneous SB4.

Figure 10. Dependence of the isosteric heat of adsorption versus adsorbent loading. ○, 1PN; ●, IPA.

10 suggest that the various interactions may be responsible for the complex adsorption behavior in SB4. More details in this regard can be found elsewhere.6,25−29 Figure 11 shows a comparison between the experimental isotherms of both types of alcohol vapor at 313.15 K and the results obtained by Yun and Choi for benzene vapor.28

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

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Figure 11. Experimental adsorption isotherms on SB4 at 313.15 K. ■, 1PN; ○, IPA; and ◆, benzene.

As can be seen, in the relative pressure range of p/p0 > 0.02, the adsorbent shows a greater adsorption capacity for both propanol isomer vapors than for benzene. It appears that this is the effect of the interactions with an oxygen functional group present on an SB4 surface.27,29 The isotherms vary in shape, which reveals different types of interactions occurring during adsorption. The sharper initial part of the benzene isotherm compared to other curves suggests that a dispersion interaction between their nonpolar molecules and adsorbent surface plays a H



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Adsorption of Propanol Isomer Vapors on Sorbonorit B4 Activated

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