Prediction of Adsorption Equilibrium of VOCs onto Hyper-Cross

Article pubs.acs.org/est

Prediction of Adsorption Equilibrium of VOCs onto Hyper-CrossLinked Polymeric Resin at Environmentally Relevant Temperatures and Concentrations Using Inverse Gas Chromatography Lijuan Jia,† Jiakai Ma,† Qiuyi Shi,† and Chao Long*,†,‡ †

Environ. Sci. Technol. 2017.51:522-530. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/20/19. For personal use only.

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, China ‡ Nanjing University Yancheng Environmental Protection Technology and Engineering Research Institute, 888 Yingbin Road, Yancheng 22400, China S Supporting Information *

ABSTRACT: Hyper-cross-linked polymeric resin (HPR) represents a class of predominantly microporous adsorbents and has good adsorption performance toward VOCs. However, adsorption equilibrium of VOCs onto HPR are limited. In this research, a novel method for predicting adsorption capacities of VOCs on HPR at environmentally relevant temperatures and concentrations using inverse gas chromatography data was proposed. Adsorption equilibrium of six VOCs (n-pentane, n-hexane, dichloromethane, acetone, benzene, 1, 2-dichloroethane) onto HPR in the temperature range of 403−443 K were measured by inverse gas chromatography (IGC). Adsorption capacities at environmentally relevant temperatures (293−328 K) and concentrations (P/Ps = 0.1−0.7) were predicted using Dubinin−Radushkevich (DR) equation based on Polany’s theory. Taking consideration of the swelling properties of HPR, the volume swelling ratio (r) was introduced and r·Vmicro was used instead of Vmicro determined by N2 adsorption data at 77 K as the parameter q0 (limiting micropore volume) of the DR equation. The results showed that the adsorption capacities of VOCs at environmentally relevant temperatures and concentrations can be predicted effectively using IGC data, the root-mean-square errors between the predicted and experimental data was below 9.63%. The results are meaningful because they allow accurate prediction of adsorption capacities of adsorbents more quickly and conveniently using IGC data.

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INTRODUCTION

Adsorption has been proven to be an effective approach to remove VOCs from the gas streams with high removal efficiencies and simple process units.8−10 The core of adsorption process technology is to develop suitable adsorbents with high specific surface, stable physical, chemical properties, and regenerability on site, etc. Hyper-cross-linked polymeric resin (HPR) represents a class of predominantly microporous adsorbents with high surface areas and high micropore volume.11,12 Adsorption equilibrium of some VOCs onto

The continuous discharge of volatile organic compounds (VOCs) such as halohydrocarbons, alkanes, and ketones has caused severe outdoor air pollution. They may undergo photochemical reactions with nitrogen oxides in the presence of sunlight, yielding even more hazardous compounds,1,2 such as secondary organic aerosol (SOA) particles,3−7 and leading to the formation of photochemical smog and ozone depletion. With the increasing awareness of environmental protection, regulations on VOCs emission have grown more stringent worldwide. Therefore, it is urgently required to develop environmentally friendly technologies to minimize the release of VOCs into the atmosphere. © 2016 American Chemical Society

Received: Revised: Accepted: Published: 522

October 5, 2016 December 2, 2016 December 12, 2016 December 12, 2016 DOI: 10.1021/acs.est.6b05039 Environ. Sci. Technol. 2017, 51, 522−530

Article

Environmental Science & Technology HPR has been investigated, such as halohydrocarbon,13,14 benzene,14−18 methyl ethyl ketone,18 and alkanes,19−21 indicating that HPR has good adsorption performance toward VOCs.18−22 However, compared with activated carbon, the adsorption equilibrium data of VOCs onto HPR are limited. Adsorption isotherm and adsorption capacity are the essential part for modeling, simulating, and optimizing the adsorption system in engineering applications. The commonly used methods for measuring VOCs adsorption isotherms include gravimetric and volumetric methods.23 However, both of the methods are time-consuming and may be affected by toxicity or the availability of the adsorbate. Recently, it has been proved that inverse gas chromatography (IGC) is a straightforward and very sensitive technique for the characterization of porous adsorbent.24−28 Compared with traditional gravimetric and volumetric methods, the smaller amount of adsorbent and the shorter experimental time were required for IGC measurements at the finite concentration to determine adsorption isotherms. However, it is noted that the adsorbentpacked column temperature of IGC such as 343−548 K,29−34 is generally higher than the operation temperature of adsorption units in practical engineering application. Moreover, the VOCs concentrations measured in IGC are far lower than the emission concentration of VOCs from manufacturing facilities.32−34 For the design of a reliable adsorption system, therefore, it is necessary to predict the adsorption isotherms at environmentally relevant temperatures and concentrations (corresponding to practical environmental systems) according to the adsorption equilibrium data obtained using IGC. It is well-known that Dubinin−Radushkevich (DR) model based on Polanyi potential theory is commonly used for describing physical adsorption of organic vapors onto microporous adsorbents.35−37 According to Polanyi potential theory, the curve of adsorbed volume against adsorption potential, which is called adsorption characteristics curve,37 is independent of temperature,38 allowing the use of DR equation without introducing any additional parameters to predict adsorption isotherms at other temperatures in terms of that at experimental temperature. Therefore, if the adsorption isotherms obtained using IGC can be described well with DR equation, it would be possible to predict the adsorption isotherms at environmentally relevant temperatures and concentrations according to IGC data. The aim of this present study is to predict the adsorption isotherms of VOCs onto HPR at environmentally relevant temperatures and concentrations using a combination of DR model and adsorption equilibrium data determined by IGC. The adsorption isotherms of several VOCs (n-pentane, nhexane, dichloromethane, acetone, benzene, and 1, 2-dichloroethane) at 403, 423, and 443 K were measured using IGC at the finite concentration, and the traditional gravimetric method was used to determine adsorption isotherms at environmentally relevant temperature (293−328 K). The accuracy of the prediction was assessed by comparing the predicted adsorption capacities with experimental capacities at environmentally relevant temperatures and concentrations. To the best of our knowledge, this is the first time that the adsorption capacities of polymeric resin for VOCs at environmentally relevant temperatures and concentrations were predicted using IGC.

Chemical Reagent Company, China (>99.5% purity). The HPR were synthesized by suspension polymerization of 4-Vinylbenzyl chloride and divinylbenzene followed by a Friedel− Crafts-type post-cross-linking reaction using 1,2-dichloroethane as solvent; the synthetic process was provided in detail in our previous study.28 By controlling the degree of post-cross-linking reaction, two resins with different pore structure parameters were obtained, and named HPR-1 and HPR-2, respectively. Characterization. Nitrogen adsorption−desorption isotherms at 77 K were measured using an ASAP 2010 (Micromeritics Instrument Co.). The specific surface area was determined using the N2 isotherms data by the means of BET equation. The micropore pore volume (Vmicro) and mesopore pore volume (Vmeso) were calculated from the N2 isotherms data by Dubinin−Radushkevich (DR) and Barrett−Joyner− Halena (BJH) methods, respectively. The pore size distributions were calculated by applying the density functional theory (DFT) to N2 isotherm data. In addition, the swelling property of HPR was studied by VOCs solvent adsorption in a column. The column was made of glass (i.d. = 5 mm) with a porous plate at the bottom. The solvent was pumped into the column by the peristaltic pump (LongerPump, China) continuously with a flow velocity of 5 mL/min. After the height of resins was constant and stable, the volume swelling ratio (r) was determined by comparing the height of dry resins (H0) and swollen resins (Ht) (eq 1). The experiment was conducted for three times to obtain an average value for all the VOCs. r = (Ht − H0)/H0

(1)

Adsorption Equilibrium: Inverse Gas Chromatograph Method (IGC). IGC experiments were carried out in a gas chromatograph (Shimadzu GC2014, Japan) equipped with a flame ionization detector. The injector and detector were stabilized in the GC system at 453 and 523 K, respectively. The adsorption experiments were operated in the column temperature range of 403−443 K under a nitrogen flow rate of 35 mL/ min. The resins (1 g) were dried at 383 K and placed in the chromatographic stainless steel column (50 cm length, 3 mm i.d.). Experiments were carried out at finite dilution region. To satisfy the finite concentration, small amounts (1−10 μL) of pure hydrocarbons (n-pentane, n-hexane, dichloromethane, acetone, benzene, and 1, 2-dichloroethane) were injected separately with a 10 μL syringe into the GC and were eluted isothermally. For each measurement, at least three repeated injections were taken, obtaining reproducible results. Adsorption isotherms were obtained from the chromatographic peaks using a characteristic-peak elution method.24 When the adsorbate density, molecular weight and the volume injected are accurately known and the corrected carrier gas flow rate, Fc, remains constant, the adsorbate partial pressure, p, is calculated as eq 2:

p=

qhiRT FcSpeak

(2)

In which q is the injection amount (mol), hi is the particular height at corresponding concentration (μV) (shown in Supporting Information (SI) Figure S124), Speak is the area of the chromatographic peak (μV·s), R is the ideal gas constant (8.314 m3 Pa/mol K), T is the temperature of the column (K), Fc is the corrected flow rate (eq 3; cm3/min),

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MATERIALS AND METHODS Materials. N-pentane, n-hexane, dichloromethane, acetone, benzene, and 1, 2-dichloroethane were supplied by Nanjing 523

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Environmental Science & Technology ⎛ T ⎞ Fc = jFm⎜ ⎟ ⎝ Tamb ⎠

(3)

where Fm is the uncorrected flow rate (cm /min), Tamb is the ambient temperature (K), j is the James−Martin factor for the correction of gas compressibility when the column inlet (Pi) and outlet (P0) pressures are different and it is given by 3

2 ⎡ ⎤ 3 ⎢ (pi /p0 ) − 1 ⎥ j= 2 ⎢⎣ (pi /p0 )3 − 1 ⎥⎦

(4)

The amount adsorbed at different i values Vi (mol/g) could be calculated from eq 5: Vi =

qSi mSpeak

(5)

where m is the mass of polymer in the column, Si (μV·s) indicates the adsorbent holdup contribution and is the area at a peak height of hi, but it differs from Speak in the respect that it includes the adsorbent holdup time, shown as the shaded area in SI Figure S1. Adsorption Equilibrium: Gravimetric Method. The adsorption of VOCs at environmentally relevant concentrations and temperatures was determined by gravimetric method. The detailed experimental apparatus and adsorption procedure have been described previously.14 Briefly, about 0.1 g of HPR was precisely weighed out and put into the glass adsorption column. The carrier gas containing a scheduled concentration of VOCs vapor was passed through the column until the weight of the column become stable, and the equilibrium amount adsorbed was equal to the weight change of adsorbent before and after the adsorption process. After a series of equilibrium VOCs uptakes at different relative pressures, the VOCs isotherm was obtained at the destined temperature (293−328 K). Here, a high precision microbalance (AL104, Mettler Toleod, Switzerland) was adopted as the weighing device. The adsorption of npentane, n-hexane, and dichloromethane at environmentally relevant temperature were conducted on HPR-1 and acetone, benzene, 1, 2-dichloroethane on HPR-2.

Figure 1. Pore structure of two hyper-cross-linked polymeric resins (HPR-1 and HPR-2).

Table 1. Pore Structure Parameters of Hyper-Cross-Linked Polymeric Resins

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pore parameters

HPR-1

HPR-2

SBET (m2/g) Vmicro(cm3/g) Vmeso(cm3/g) average pore diameter(W, nm)

1113.2 0.53 0.31 2.56

944.4 0.43 0.29 2.48

chromatographic peaks according to the method described above. At lowering temperature, the increasing of the maximum amount adsorbed (qv) was observed; very small pressure values indicated that the surface coverage is low. Dubinin−Radushkevich (DR) model, which is based on Polanyi adsorption potential theory, is a commonly used isotherm model for physical adsorption of organic vapors onto microporous adsorbent and can be defined as the following equations:

RESULTS AND DISCUSSION Characterization of the Hyper-Cross-Linked Polymeric Resins. The N2 adsorption−desorption isotherms of HPR-1 and HPR-2 at 77 K are demonstrated in Figure 1(a). According to IUPAC classification, both of them were typical of adsorbents with a predominantly microporous structure, since the majority of pore-filling occurred at relative pressures below 0.1. The higher adsorption capacities of nitrogen on HPR-1 resulted from the higher degree of post-cross-linking reaction of HPR-1 than HPR-2. The slope of the plateau at medium relative pressures and accelerated uptake at higher relative pressure meant that they contained a proportion of mesopores and macropores. The pore size distributions of two resins are shown in Figure 1(b). It is clearly observed from Figure 1(b) that the two adsorbents showed a similar pore size distribution because of the similar synthesis method. The salient pore structure parameters are shown in Table 1. Adsorption Isotherms by IGC and DR Modeling. Adsorption isotherms of six VOCs (n-hexane, n-pentane, dichloromethane, acetone, benzene, and 1, 2-dichloroethane) onto HPR-1 at the temperature of 403, 423, and 443 K are presented in Figure 2. The isotherms were calculated from

qv = q0exp( − (ε /E)2 )

(6)

ε = RT ln(Ps/P)

(7)

Where qv is the volume adsorbed capacity (mL/g), E is the characteristic adsorption energy (kJ/mol), q0 is the limiting micropore volume (mL/g), ε is the adsorption potential (kJ/ mol), calculated by eq 7. For a given adsorbent, the parameter q0 of DR equation has been shown to relate to the microporous structure of the adsorbent and is generally assumed to be a constant regardless of which adsorbates are used. Since the swelling of HPR is negligible for the adsorption of VOCs on HPR-1 at lower relative pressure measured by IGC, q0 was assumed to be equal 524

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Figure 2. Adsorption isotherms of six VOCs on HPR-1 at 403, 423, and 443 K by IGC: (a) n-pentane, (b) n-hexane, (c) dichloromethane, (d) acetone, (e) benzene, and (f) 1, 2-dichloroethane.

Prediction of the Adsorption Capacities on HPR-1 at Environmentally Relevant Temperatures and Concentrations. It is known that the characteristic adsorption curves (qv vs ε) are independent of temperatures. Therefore, the adsorption capacities at environmentally relevant temperatures (293−328 K) and concentrations (p/p0 = 0.1−0.7) on HPR-1 would be predicted using the adsorption equilibrium data obtained by IGC at higher temperature (403, 423, and 443 K). The parameters of q0 and E of DR equation were set to the microporous volume of HPR-1 (Vmicro = 0.53) and the fitting E values of adsorption equilibrium data at the high temperatures (listed in SI Table S2), respectively. The predicted adsorption capacities of n-pentane, n-hexane, and 1, 2-dichloromethane on HPR-1 at environmentally relevant temperatures and concentrations are shown in Figure 4. A big discrepancy was observed between the experimental and predicted adsorption capacities. Figure 4(d) clearly shows that the predicted adsorption capacities were lower than the experimental results. For VOCs adsorption on HPR, the previous studies have found that the fitting parameter q0 by experimental data was greater than the microporous volume of resins determined by N2 adsorption isotherms at 77 K when the higher relative pressure of VOCs was adsorbed.13,14,19,21 The possible reason is that HPR can swell strongly after adsorbing organic solvents,41,42 resulting in that the actual microporous volume is larger than the measured value based on N2 adsorption data. Davankov et

to Vmicro calculated by N2 adsorption isotherms at 77 K. The fitting results of adsorption isotherms using DR equation for every temperature are listed in SI Table S1. Clearly, the experimental data were well fitted by the DR equation with the correlation coefficient R2 larger than 0.845. In addition, according to the Polanyi adsorption potential theory, plots of adsorbed volume qv against ε will form an adsorption characteristic curve, and can be employed to examine whether the Polanyi adsorption potential theory mechanistically captures the adsorption process of compounds by adsorbent.39,40 The Figure 3 presents plots of qv against the adsorption potential (ε) of six VOCs on HPR-1. It is shown clearly that the data at three temperatures fell essentially onto a single curve for each adsorbate. The characteristic adsorption curves of six VOCs on the HPR-1 were fitted with DR equation. It is learned from Figure 3 that the fitting curves were in good agreement with the experimental data. The fitting parameters were listed in SI Table S2. The above results suggested the mechanistic usefulness of Polanyi adsorption potential theory to VOCs adsorption on HPR-1 and the feasibility of using DR equation to describe the adsorption isotherms of VOCs. Moreover, it is pivotal because it allowed to predict adsorption isotherms at environmentally relevant temperature without introducing any additional parameters according to adsorption equilibrium data obtained by IGC method. 525

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Figure 3. Adsorption characteristic curves of six VOCs on HPR-1: (a) n-pentane, (b) n-hexane, (c) dichloromethane, (d) acetone, (e) benzene, and (f) 1, 2-dichloroethane.

al.43 and Urban et al.44 proposed that the swelling property of HPR in solvent is due to the strong inner stress. The inner stress could form an additional strong driving force for the polymer networks to be swollen. Therefore, the solvent uptake by HPR could arise from both micropore filling and swelling of the polymer network.43 That is to say, the predicted adsorption capacities based on the micropore volume (Vmicro) may be lower than the experimental adsorption capacities. Therefore, the volume swelling ratio (r) of HPR was introduced and r· Vmicro was used instead of the Vmicro for predicting the adsorption capacities of HPR-1 for VOCs at environmentally relevant temperatures and concentrations using the DR equation. The swelling ratio of HPR-1 and HPR-2 after adsorbing different VOCs is listed in SI Table S3. After the parameters q0 of DR equation was set to r·Vmicro, the predicted adsorption capacities of n-pentane, n-hexane and dichloromethane at environmentally relevant temperatures and concentration are shown in Figure 5. It is clear that the predicted adsorption capacities were consistent with the experimental results. The

root-mean-square error (RMS %), determined via eq 8, was 5.36% and much lower than that (21.41%) when the parameters q0 of DR equation was set to Vmicro. The results demonstrate the effectiveness of predicting the adsorption capacities at environmentally relevant temperatures and concentrations by IGC data using DR equation, assuming that the parameters q0 of DR equation was equal to r·Vmicro. ⎡ k ⎛ q − q ⎞2 ⎤ vcal ⎥ 1 ⎢ ⎜ vexp ⎟ RMS% = 100 ∑⎜ ⎢ ⎟⎥ k ⎢ 1 ⎝ qv ⎠ ⎥⎦ cal ⎣

(8)

Prediction of Adsorption Capacities on HPR-2 at Environmentally Relevant Temperatures and Concentrations. Because the pore structure of HPR are affected by many factors, such as reaction temperature, reaction time, and the dose of pore-forming,45,46 the pore size distribution and salient properties parameters of HPR may be different even if the same monomer and cross-linking agent are used during the synthesis. Therefore, it would be very valuable if the adsorption 526

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Figure 4. Predicted adsorption capacities of (a) n-pentane, (b) n-hexane, (c) dichloromethane, and (d) comparison of the experimental and predicted adsorption capacities when the parameters q0 of DR equation was set to Vmicro.

Figure 5. Predicted adsorption capacities of (a) n-pentane, (b) n-hexane, and (c) dichloromethane, and (d) comparison of the experimental and predicted adsorption capacities when the parameters q0 of DR equation was set to r·Vmicro.

however, their pore diameters were different. We can find that the average pore diameter of HPR-1 was larger than HPR-2. It is known that the characteristic adsorption energy, E of DR

capacities of one resin for VOCs can be predicted by the IGC data on another resin. It is learned from Figure 1 that the pore size distributions of HPR-1 was similar to that of HPR-2, 527

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Figure 6. Predicted adsorption capacities of (a) acetone, (b) benzene, (c) 1, 2-dichloroethane, and (d) comparison of the experimental and predicted adsorption capacities when the parameters q0 of DR equation was set to r·Vmicro, and the characteristic adsorption energy E HPR‑2 = WHPR‑1· E HPR‑1/WHPR‑2.

equation, is a function of pore size distribution of adsorbent.47 For the adsorption of a given adsorbate on the different adsorbents, it is suggested that the characteristic adsorption energy, E, is inversely related to the average width of the pores (W), that is, W·E = C (C is constant).48 In this study, therefore, for the adsorption of a given adsorbate on HPR-2, the fitting parameter E of DR equation can be obtained from the relation of WHPR‑2·EHPR‑2 = WHPR‑1·EHPR‑1. So, the adsorption capacities of acetone, benzene, and 1, 2-dichloroethane on HPR-2 at environmentally relevant temperatures and concentrations were also predicted by IGC data on HPR-1 at 403, 423, and 443 K using DR equation, where q0 = r·Vmicro and E HPR‑2= WHPR‑1·E HPR‑1/WHPR‑2. Figure 6 shows that the predicted adsorption capacities of acetone, benzene and 1, 2-dichloroethane on HPR-2 were in agreement with the experimental data with RMS lower than 9.63%, conforming the feasibility of the predicted method by IGC data combined with DR equation. As a comparison, the parameters q0 = Vmicro and E HPR‑2= WHPR‑1·E HPR‑1/WHPR‑2 were also used to predict the adsorption capacities of acetone, benzene and 1, 2-dichloroethane on HPR-2. From the SI Figure S3, it is found that there was an obvious discrepancy predicted between the predicted and experimental adsorption capacities with large RMS of 27.80%.

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to calculate adsorption isotherms. Figure S2: Adsorption isotherms of six VOCs measured by gravimetric method at 293−328 K. Figure S3: The predicted adsorption capacities of acetone, benzene, 1, 2-dichloroethane when the parameters q0 of DR equation was set to Vmicro (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 25 89680380; e-mail: [email protected]. ORCID

Chao Long: 0000-0003-0876-2213 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially funded by National Natural Science Foundation of China (No. 51578281). REFERENCES

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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.est.6b05039. Table S1 and S2: Fitting parameters of DR equation. Table S3: Swelling ratio of two adsorbents. Figure S1: Principles of the characteristic-point elution method used 528

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DOI: 10.1021/acs.est.6b05039 Environ. Sci. Technol. 2017, 51, 522−530

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