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Adsorption of Gas Phase Organic Compounds by Swellable Organically Modified Silica Paul L. Edmiston,*,† Laura J. West,† Alison Chin,† Noel̈ Mellor,† and David Barth‡ †

Department of Chemistry, The College of Wooster, 943 College Mall, Wooster, Ohio 44691, United States ABS Materials, Inc., 1909 Old Mansfield Road, Wooster, Ohio 44691, United States

‡

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S Supporting Information *

ABSTRACT: Swellable organically modified silica (SOMS) is a sol− gel derived material that spontaneously expands >2.5× upon contact with organic liquids, absorbing 7.8 mL/g. Adsorption of gas-phase volatile organic compounds by SOMS was measured to examine how the capability to swell affects capacity and rate of organic vapor absorption. Static adsorption capacities of SOMS for organic vapors at saturated vapor pressure ranged from 0.7 to 1.05 g/g, which was higher than values for other sorbents (powdered activated carbon, Amberlite XAD-4, Tenax TA, OptiPore, and organophilic zeolite). Rates of adsorption by SOMS were similar to those of XAD-4, a porous polymer with similar surface chemistry, and slower than that of activated carbon. Sequential coadsorption of multiple compounds by SOMS was measured, yielding enhanced adsorption capacity attributed to adsorption-induced pore expansion. The sequential adsorption of phenol and acetone vapor (p = p0) led to a total capacity of 5.7 g/g. Adsorption of organic vapors was not selective and fully reversible in all cases. The hydrophobicity of SOMS strongly excludes water and water vapor. Fluorescence recovery after photobleaching was used to measure in-particle diffusion constants of fluorescein before and after adsorption for vapor and liquid.

1. INTRODUCTION Developing materials for the adsorption of volatile organic compounds (VOCs) from air is important for improving separations for industrial processing and environmental protection. A wide variety of porous solids have been studied for VOC capture, including activated carbon,1−3 polymeric resins,4−7 zeolites,8 organoclays,9 and hydrophobic aerogels.10−12 Development of new types of adsorbents typically addresses improvement of total adsorption capacity and rate of adsorption as primary objectives. In addition to these parameters, reversible adsorption and chemical selectivity are also important factors in developing sorbents for VOC capture. Facile desorption and subsequent sorbent regeneration reduces media consumption and allows for recovery of adsorbates, which is useful for chemical separations. Selectivity is important when removing VOCs from humid environments or separating mixtures of organic gases. Sol−gel polymerization13 provides a creative route to the synthesis of porous materials where the pore size and structure can be controlled to optimize adsorption characteristics. Diversification of chemical characteristics is controlled by the selection of di- or trialkoxysilane monomers with various organic functional groups on the silicon center. Polymerization via hydrolysis and condensation of the alkoxysilane groups results in porous materials in which the organic groups modify the pore surface chemistry. The nature of the organic group can © 2016 American Chemical Society

also control the material’s three-dimensional structure. Organic functional groups which bridge two silicon centers, for example bis(trimethoxysilylethyl)benzene (BTEB, Figure 1), are used to

Figure 1. Structure of the sol−gel precursor BTEB.

create organic−inorganic hybrid materials where properties such as homogeneous pore architecture, mechanical strength, optical clarity, and thermal stability can be controlled by the sol−gel processing conditions.14−16 Molecular scale organization promoted via interactions facilitated by the organic bridging group yield bridged polysilsesquioxanes with defined structures.17,18 In some cases, molecular-scale self-organization is combined with further structural direction by surfactant templates to yield mesoporous sorbents with a high degree of order.19,20 Even without templating, the bridging organic Received: Revised: Accepted: Published: 12068

June 23, 2016 October 25, 2016 October 31, 2016 October 31, 2016 DOI: 10.1021/acs.iecr.6b02403 Ind. Eng. Chem. Res. 2016, 55, 12068−12079

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Industrial & Engineering Chemistry Research

column breakthrough experiments were used to measure dynamic adsorption capacity. Theoretical approaches to adsorption kinetics onto sorbents from aqueous solution have been described;27 however, gas-phase adsorption to sol−gel derived materials are less numerous. Here, a pseudo-second order kinetic model was applied to model kinetic data to determine the degree of fit.28 Pseudo-second order kinetics is applicable when the rate of the physical adsorption/desorption process controls the sorption kinetics. The mathematic expression of this model was proposed by Blanchard29 and subsequently derived based on a classical Langmuirian model of adsorption. The most commonly used pseudo-second order kinetic equation was given by Ho,28 written as

groups can lead to ordering. In the instance of BTEB, the aryl ring allows for π−π interactions and microscale phaseseparation in polar solvents as the hydrophobic groups dominate the growing polymer system.21,22 Under specific conditions (i.e., concentration, catalyst type, solvent), we discovered a unique morphology from which the final pore structure was highly flexible and spontaneously expands upon absorption of organic liquids.23 Swellable organically modified silica (SOMS) is a bridged polysilsesquioxane material prepared from BTEB that can spontaneously absorb 6 times its mass of organic liquids, leading to a 2.5× total volume change of the particles upon absorption. The swelling process is fully reversible. The volumetric change concomitant with absorption leads to generation of mechanical force in excess of 100 N/g.24 Absorption of organic liquids is nonselective, i.e., solvents from methanol to hexane equally induce swelling.24 However, the hydrophobicity of SOMS prevents water from entering the pore network. Exclusion of water allows polysilsesquioxanes to be used in aqueous solutions as an adsorbent for dissolved or dispersed organic compounds.25 The flexibility of the porous organosilica matrix allows for microscale-to-macroscale expansion during adsorption of organic solutes, leading to pseudolinear adsorption isotherms.26 In the dry state, SOMS is a high surface area, porous material with hydrophobic surface chemistry due to the high aryl content. The surface chemistry of SOMS with an abundance of cross-linked aryl groups is similar to styrene-divinylbenzene copolymer resins such as Dow Optipore L-493 and Amberlite XAD-4. The difference between these organic polymeric materials and SOMS (commercial name: Osorb media) is the silica polysilsesquioxane structure and flexibility to swell in response to absorption. Although a wide array of silica or organosilica materials with variable morphologies and physical properties has been synthesized using the sol−gel process, materials that can swell are unusual. The sol−gel process begins with polycondensation, which generates a gel that is dried to form a solid termed a xerogel. Strong capillary forces during drying shrink the gel and yield a dry porous material. In almost all cases, the shrinkage upon drying is irreversible due to further chemical cross-linking. For SOMS, cross-linking is prevented by derivatization of Si− OH groups prior to drying. Flexibility of the matrix allows potential energy to be stored during shrinkage upon drying as tension. The tensioned state is maintained by intramaterial noncovalent interactions between the collapsed surfaces. Thus, the dry state can be described as a porous collapsed matrix with stored mechanical energy that can be released if an adsorbate enters the matrix and disrupts intramatrix interactions. The ability to increase pore size upon ad/absorption and release potential energy during re-expansion makes SOMS an interesting sorbent to study. Adsorption from the gas phase by SOMS was evaluated in the work reported here, especially in the context of understanding the effect that spontaneous expansion has on capacity. It was hypothesized that matrix expansion of SOMS would be triggered by capillary condensation, leading to an increased adsorption compared to traditional polymeric sorbents. The adsorption characteristics of SOMS to adsorb VOC vapors were measured in comparison to other sorbents. Equilibria and kinetics measurements were used to characterize the adsorption and desorption process. Adsorption isotherms varying vapor phase partial pressure, p, were used to characterize the equilibrium using the Freundlich adsorption model, while

dq(t ) = k 2(qe − q(t ))2 dt

(1)

and after integrating the expression with the boundary condition, q(t = 0) = 0 gives t 1 t = + q(t ) k 2qe qe

(2)

where q(t) is the amount adsorbed at time t, k2 is the rate constant, and qe = q(t → ∞). A plot of t/q(t) vs t will give a linear relationship. Use of the pseudo-second order kinetics to model gas phase adsorption is somewhat novel; therefore, a pseudo-first order model30 (eq 3) was also examined, which is generally applicable when the bulk concentration remains constant. dq(t ) = k1(qe − q(t )) dt

(3)

Measurement of the adsorption and desorption rates for a wide range of gas phase organics to SOMS was conducted. Kinetics of other sorbents were also examined in comparison.

2. EXPERIMENTAL SECTION 2.1. Materials. BTEB was obtained from Gelest and used without further purification. All solvents (>99% purity) were obtained from Fisher Scientific and used without further purification. Propane and butane gases were in compressed form from Air Products. Other chemicals and sorbents, including Optipore L-493 (macroporous polymer), DARCO activated carbon, XAD-4, Tenax TA, and organophilic molecular sieves (cat# 419095) were obtained from Aldrich. 2.2. Synthesis and Characterization of SOMS Adsorbents. The method to prepare SOMS was carried out as described previously24 (see the Supporting Information). For characterization, the sorbent was ball milled to a powder (≈200 mesh) as determined by dry sieving the material after grinding. Surface area and pore volume were measured by N2 adsorption using a Beckman Coulter SA-3100 analyzer after a 45 min outgassing time at 150 °C. Surface area was calculated using the BET method,31 and the pore size distribution was calculated using the BJH method.32 Powder X-ray diffraction (PXRD) was measured using a Rigaku ZSX Primus II instrument. X-ray diffraction was carried out on a thin layer (≈0.75 mm) of powdered material spread across a sample tray. A rotating copper anode (30 kV, 15 mA, λKα = 1.542 Å) was operated in continuous scan mode at a speed of 2 degrees/min. The sampling width was 0.02 degrees. The empty sample tray was run as a background. PXRD profiles of swollen material were measured by swelling the material with mineral oil. Swelling 12069

DOI: 10.1021/acs.iecr.6b02403 Ind. Eng. Chem. Res. 2016, 55, 12068−12079

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dried, leaving fluorescein dispersed throughout the particles. Photobleaching experiments were done using a Nikon C1si scanning confocal microscope34,35 using 488 nm excitation and emission >510 nm. The laser power was increased to the maximum to photobleach an area approximately 250 μm2. The fluorescence recovery area and a nearby nonbleached region of equivalent area were scanned every 1 s for 100 s. The recovery curves were corrected for further photobleaching during recovery by normalizing values to the fluorescence intensity of the unbleached region. The corrected recovery intensity vs time was fitted to (eq 4)36 using nonlinear regression

capacity (mL absorbed/gram of solid) was determined by first measuring the dry mass and then slowly applying acetone to the media until the first observance of residual liquid. After removal of any residual liquid, the mass of the swollen media was weighed, and the difference in mass was used to determine the volume of acetone absorbed using the density at T = 25 °C. The change in temperature at constant pressure for the uptake and swelling by mineral oil was measured by calorimetry. A sample of SOMS (5 g) was added to a sealed vacuum dewar, and the apparatus was equilibrated at 30.0 °C. Mineral oil (50 mL) pre-equilibrated to 30.0 °C was added through an open port in sufficient volume to ensure complete swelling of SOMS. The change in temperature was measured with a high precision thermocouple (Parr Instruments). The heat capacity of SOMS was measured by heating 10 g of material to 30 °C and adding it to 50 mL of 5.0 °C water in a calorimeter measuring ΔT. 2.3. Static Vapor-Phase Adsorption. Static vapor equilibrium adsorption at 25 °C was measured over time using a Pyris 1 PerkinElmer thermogravimetric analyzer. A mass of sample was placed on the balance, and a neat liquid or solid organic substance was added to the chamber to yield a static condition of saturated vapor. Mass of the sorbent was measured every 6 s until equilibrium was reached. A single sample of SOMS was used for all measurements, which was fully characterized as described above for accurate comparisons. Reproducibility was tested by repeating measurements using the same sample and also by measuring adsorption by separate batches of material synthesized by the same procedure. Intrabatch reproducibility of adsorption capacity and BET surface area was ≤4% rsd, and interbatch reproducibility was ≤7% rsd. Adsorption isotherms were measured by addition of 25 mg of sorbent into a 350 mL chamber. An exact mass of each organic substance was added, and the chamber was sealed and equilibrated at 30 °C. The amount of organic substance was such that it would completely vaporize at 1 atm. After the system had come to equilibrium, the mass gain of the sorbent was recorded. The vapor phase concentration at equilibrium was determined by mass balance and the ideal gas equation. 2.4. Dynamic Vapor-Phase Adsorption. A 25 mL/min nitrogen gas stream was passed through a 100 mg sorbent bed (particle size: 80−120 mesh) placed in a 6 mm diameter column housed in an incubator. Saturated vapors of methanol, toluene, and 2-butanone were generated by diverting the entire nitrogen flow to sparge through solutions of solvent prior to entering the sorbent bed (T = 30 °C). Organic vapor concentration in the influent and effluent vapor stream was measured by infrared spectrometry using a G-2 gas cell (10 cm path length, Internal Crystal Laboratories) fitted in a PerkinElmer 2000 FT-IR using 4 cm−1 resolution. Spectra of the effluent gas stream were taken every 3 min until breakthrough. The amount of vapor was determined by integrating the absorbance peak area of unique peaks characteristic to each compound (methanol, −OH, 3710 cm−1; toluene, C−C ring stretch 1520 cm−1; and 2-butanone CO; 1735 cm−1). Calibration curves for each vapor were prepared by vaporizing a known mass of compound into a known gas cell volume using the ideal gas equation to determine the partial pressure. The column and entire flow system were maintained at 30 °C. Capacity was calculated from the breakthrough curve by integrating under the curve.33 2.5. Fluorescence Recovery after Photobleaching (FRAP). Granular SOMS material was fully swollen by addition of a solution of 0.1 μM fluorescein in ethanol. The ethanol was

I(t ) = A(1 − exp(−kt )) + C

(4)

where I is the fluorescence intensity, k is the rate constant of recovery, A is the final plateau intensity of the fluorescence after recovery minus the initial intensity after photobleaching (F∞ − F0), and C is the initial intensity after photobleaching, F0. The time taken to return to half of the final intensity (t1/2 = (ln2)/k) is related to the translational diffusion coefficient D by D = 0.88ω 2 /4t1/2

(5)

where ω is the radius of the bleached area. Image analysis was done using ImageJ to measure the radius of the photobleached area and fluorescence intensity cross sections. Measurements were done in triplicate.

3. RESULTS AND DISCUSSION 3.1. Characterization of SOMS Adsorbent. Polycondensation of BTEB using fluoride catalysis under controlled conditions led to a semitranslucent porous solid with a ball pan hardness of 0.99 and a density of 0.59 g/mL. Synthesis of SOMS involves derivatization of the wet sol−gel with chlorotrimethylsilane prior to drying to end-cap residual silanols with a -Si(CH3)3 group. Derivatization has been shown to be necessary to produce the characteristic swelling behavior upon contact with organic liquids.23 The final derivatized product absorbed 7.8 mL/g acetone, leading to a ∼2.5× volume change within 2 s. The BET adsorption isotherm for dry SOMS material (Figure 2) was Type IV, indicative of a mesoporous structure with multilayer adsorption.

Figure 2. BET nitrogen adsorption and desorption isotherms for SOMS in the dry state at 77 °K. 12070


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Figure 3. Left: scanning electron micrograph of SOMS in the dry state depicting the pore structure on the surface of the material. Right: same material after thermal treatment under He at 400 °C for 1 h.

ity of the overall process but generally results in a more amorphous material. Scanning electron microscopy indicates morphology of monodisperse organosilica colloidal subunits interlinked in a disorganized fashion (Figure 3). Previous studies have shown that swelling may be due to the balloon-like swelling of the ∼50 nm cluster-like structures. However, XRD indicates there is a degree of noncrystalline ordering on the molecular scale in the dry state, as indicated by two broad peaks centered on 2θ angles of 6.2° (d = 4.45 Å) and 19.9° (d = 14.5 Å) (Figure 4). Mesoscale ordering is also supported by the

Gels that were not subject to Si−OH derivatized prior to drying did not exhibit a volumetric swelling in acetone and overall absorbed ∼10× less acetone liquid. The dry state surface area, pore volume, and average pore size were all increased with Si− OH derivatization as compared to gels where silanol groups were left unmodified (Table S1, Supporting Information). The increased pore size is attributed to the prevention of crosslinking that occurs between residual silanols upon drying when not derivatized. The porosity of SOMS in the dry state resembles cross-linked divinylbenzene (DVB) polymer sorbents such as Optipore and Amberlite XAD-4 in terms of surface area and pore volume. The surface chemistry of cross-linked DVB and SOMS are also presumed to be similar, as all share a significant aryl group content. Heating SOMS to 400 °C under helium leads to a diminishment of swelling in which uptake of acetone liquid is reduced by 40%. The high temperature annealing significantly reduces the organic content of the organosilica sorbent by about 25%, as measured by infrared spectroscopy (see the Supporting Information). The two overlapping peaks at 1034 and 1093 cm−1 from Si−O−Si stretching37 in SOMS are due to longitudinal-optic (LO)−transverse-optical (TO) splitting of the vibrational modes. LO−TO splitting has been attributed to long-range coupling of Coulomb interactions.38,39 Specifically, the antisymmetric stretching LO3−TO3 bands observed in SOMS are likely due to scattering in larger pore structures.37 The Si−OH groups were not observed, i.e., showing no notable peaks around the 3000−3600 or 950 cm−1 regions. This is due to the Si−OH groups being capped by treatment with chlorotrimethylsilane in the synthesis of SOMS. High temperature treatment may also calcine material, leading to more Si− O−Si cross-links and concomitant reduction in flexibility. However, a similar surface area and pore volume were maintained after thermal treatment (Table S1). Thus, SOMS appears to possess thermal stability under nonoxidative conditions with minimal changes up to at least 400 °C, which is superior to most polymeric sorbents. The structural stability is presumably a consequence of using a silica-based composition that is resistant to melting or structural rearrangement at high temperatures. Unlike other mesoporous sorbents that use surfactant micelles as templates to create ordered pore structures,40 no templates are used in the synthesis of SOMS. The lack of surfactants in the polymerization process reduces the complex-

Figure 4. Powder XRD diffractogram for dry SOMS compared to that of the same material swollen (4 mL/g) with mineral oil.

observation of birefringence of the gel during syneresis, which was attributed to organization via π-stacking.24 When SOMS is swollen with mineral oil, the XRD profile changes, exhibiting a single broad peak centered where d = 5.06 Å with a minor shoulder corresponding to d = 9.55 Å. Several studies of sol− gel derived materials synthesized using bridged precursors show d-spacings in 4−7 Å attributed to ladder-like structures created by incomplete cage formation.41,42 The d-spacings of 14.5 Å and greater seen in the dry state indicate that larger scale order occurs in the dry state. Materials made with aryl bridged silanes have shown sheet-like ordering with interlamellar distances of ≈20 Å.43 The disappearance in the 14.5 Å spacing upon swelling indicates that larger scale ordering within the matrix may be lost, leaving the silsesquioxane ladder as the primary organized structure in the material. Swelling is spontaneous and 12071


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Industrial & Engineering Chemistry Research generates >100 N/g of force (0.8 ± 0.1 J/g of work), indicative of ΔGswell < 0. Absorption and swelling with mineral oil is endothermic, as measured by calorimetry [ΔHswell = 5.2 ± 1.2 J/g (Cp= 3.6 J/g·K, T = 303 K)]. Thus, absorption and swelling appear assisted by entropic factors, which is supported by XRD data depicting 2−3 types of ordered structures in dry material and one major type in the swollen state. Previous data showed that SOMS nonselectively swells to the same degree upon contact with a wide variety of organic liquids,24 providing further support that absorption and swelling is driven by structural changes in the matrix rather than intermolecular actions between the adsorbate and pore surfaces. 3.2. Adsorption of Organic Vapors. The adsorption of acetone and water vapor individually at 25 °C at a saturated vapor condition, p0, under static conditions was measured for SOMS and a variety of other sorbents (Table 1). SOMS has the

collapsed state upon absorption and complete swelling to be achieved. Selectivity is an important consideration when evaluating sorbents. Exclusion of water vapor is a desirable feature because water is often present in many situations and may coadsorb and fill adsorption sites, reducing the capacity for VOCs. Water and acetone vapor adsorption capacity were measured individually for the full suite of sorbents (Table 1). SOMS was the most selective material in the adsorption of an organic vapor versus water vapor. Adsorption of acetone at equilibrium is ∼15× greater than that of water vapor, which is approximately an order of magnitude better than that of the next selective sorbent, Tenax TA. The lower capacity of SOMS to adsorb water vapor likely arises from the hydrophobicity of the organic bridging groups and Si-CH3 groups resulting from derivatization. Overall, the material appears promising for the selective adsorption of VOCs in the presence of water vapor. An interesting proof-of-concept experiment was done to further study the capability of SOMS to adsorb organics and simultaneously reject water vapor (see Supporting Information for experimental details and full results). A 10 × 80 mm sized continuously formed disk of SOMS was prepared that was used as a solid membrane at the entrance to an infrared gas cell. Propane gas saturated with water vapor (T = 25 °C) was flowed across the exterior of the disk-membrane, and the composition of the components that diffused across SOMS into the gas cell was measured by infrared spectroscopy. Propane was detected upon entering the gas cell after 5 min, growing steadily in concentration over time to steady-state after 60 min. Water was not detected even after 60 min of exposure, indicating it was fully excluded from crossing the disk membrane during the time period. Because propane is a larger molecule than water, the difference in diffusion across the pseudomembrane cannot be explained by size exclusion and thus seems to be explained by the nonpolar propane being able to be admitted into the hydrophobic pores and traverse the material, whereas water was excluded. The inability of water to be transferred through the collapsed pore structure was further studied by adding water into the pores via swelling and determining if it could escape. To add water, SOMS was fully swollen with methanol, and afterward, the swollen particles were mixed into a large volume of deionized water. When immersed in the swollen state, the methanol in the pores was exchanged for water. The particles were removed and allowed to air-dry. Even after extensive time to evaporate, an amount of water (∼0.1 gwater/gsorbent) remained entrapped in the SOMS (as confirmed by gravimetry and infrared spectrometry). In contrast, organic liquids and organic vapor-phase adsorbates were fully removed by ambient evaporation (see below). SOMS with entrapped water was further examined by exposing it to air with saturated acetone for 8 h, discontinuing acetone vapor exposure, and allowing adsorbates to evaporate at 25 °C. After evaporation, the mass of the particle had returned to the initial dry value (empty state), indicting that the water had been liberated. The result of this experiment suggests that entrapped water is free to diffuse out of the pores if there is a coadsorbate that facilitates transport, presumably through the partial opening of the collapsed pores. Selectivity of adsorption for a number of organic vapors was measured (Table 2). The static adsorption capacity of SOMS to adsorb organic vapors at the saturated vapor state was measured across a diverse range of organic compounds. For compounds with boiling points >25 °C (i.e., condensable at the

Table 1. Adsorption Capacities for Acetone and Watera sorbent SOMS SOMS, nonswell 400 °C (He) Optipore Darco PAC XAD-4 molecular sieves Tenax TA

qe water (g/g)

qe acetone (g/g)

ratio (acetone/ water)

k2 (acetone)

0.069 0.056

1.02 0.57

14.8 10.1

0.11 2.0

0.10 0.46 0.43 0.85 0.15

1.1 0.69 0.42 0.90 0.10

10.4 1.5 0.97 1.06 0.69

0.093 1.8 5.2 0.19 162

0.035

0.065

1.86

16.6

a

All measurements were replicated in at least duplicate with variance to within ±5% rsd. T = 25 °C.

highest adsorption capacity for acetone vapor among the materials studied. The amount of acetone adsorbed by SOMS at p = p0 is 1.02 g/g, equivalent to 1.29 mL acetone/gram of sorbent, which exceeds the total pore volume of the dry material by 0.11 mL/g. The excess capacity to adsorb acetone vapor appears to be facilitated by a corresponding amount of matrix expansion. Adsorption of acetone was reduced by half for the nonswelling version, which may be attributed to a nonexpandable pore structure or, more likely, an inherently smaller total BJH pore volume for the nonderivitized, nonswelling version (1.18 g/mL for swelling vs 0.46 g/mL for nonswelling). Heating SOMS to 400 °C reduced the swell and increased the mesoporous pore structure, yet despite these changes, the capacity of acetone adsorption was unchanged relative to material that can swell. These data suggest that macroscopic volumetric expansion does not play a major role in enhancing the adsorption capacity for a single component organic vapor. This is supported by the observation that little, if any, volumetric change in the particles was observed by visible inspection of SOMS particles after adsorption of a single component VOC. In contrast, exposure of SOMS to liquid acetone leads to a rapid (20 6.5 ± 0.5 0.43 ± 0.02 0.21 ± 0.01 0.17 ± 0.02 0.034 ± 0.002 0.102 ± 0.006 0.049 ± 0.02 0.09 ± 0.01 2.4 ± 0.1 0.09 ± 0.03 0.076 ± 0.003 (1.9 ± 0.2) × 10−3 0.098 ± 0.009 0.185 ± 0.006 0.109 ± 0.009

0.991 0.999 0.999 0.999 0.999 0.995 0.999 0.999 0.999 0.999 0.999 0.999 0.999a 0.999 0.999 0.999

a

Steady state rate following initial fast adsorption step with k2 = 0.2 to qe = 0.02

temperature of the experiment), the equilibrium capacities were relatively similar: qe 0.7−1.05 g/g. It was concluded that SOMS is nonselective in the adsorption of organic vapors. Limited selectivity is not surprising given that the adsorption is principally facilitated by noncovalent van der Waals interactions. Consequentially, the interaction strength is relatively weak, dominated initially by sorbate−surface interactions and then sorbate−sorbate interactions as multilayers form. Adsorption at ambient conditions is fully reversible in all cases. For example, acetone vapor was adsorbed and desorbed three times with complete desorption without significant loss in capacity (Figure 5). Incomplete desorption is observed for other sorbents such as activated carbon, where organic compounds become strongly adsorbed in micropores and a fraction of the population adsorbed becomes chemically/ sterically hindered to desorb.44,45 Desorption of VOCs from SOMS does not appear irreversibly inhibited by chemical or

log qe = log kF + 1/n log Ce

(6)

where kF and 1/n are the Freundlich capacity and intensity parameters, respectively. Fits were reasonable for all organic vapors tested except for methanol, which appeared to have two distinctly different adsorption regimes (Figure 7). Deviation from linearity was observed when methanol vapor was less than 130 mg/L. The slope (1/n) > 4 for methanol at low concentration is suggestive of relatively weak, cooperative adsorption. Adsorption by all other organic vapors were fit to the model, and the parameters are indicative of media with high capacity (as related to kF) and modest adsorption intensities with 1/n values ranging from 0.57 to 1.27. However, fits for most of the isotherms had regressions between 0.92 and 0.98, indicating that the Freudlich model was capable of providing useful values for comparative purposes but appeared incomplete for describing adsorption to SOMS. Adsorption isotherms were measured where Ce was in the range 0.05−0.9 p0, which would be considered on the higher end of VOC concentrations. Extrapolation of the adsorption isotherms predicts significant capacity, potentially as high as 4 g/g. The swelling behavior of

Figure 5. Mass of adsorbed acetone to SOMS divided by sorbent mass at a specific time (qt) as measured under conditions of applied acetone vapor (p = p0) and when no acetone is present in air (p = 0). Conditions: T = 25 °C, static conditions. 12073


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Industrial & Engineering Chemistry Research Table 3. Freundlich Isotherm Parameters for the Adsorption of Organic Vapors to SOMS Freundlich parameters vapor

k (mg g−1 (g/L)n)

1/n

R2

Ce range (mg/L)

pentane heptane toluene xylene acetone trichloroethylene methanol, low methanol, high

430 ± 10 3890 ± 120 1580 ± 50 4900 ± 150 1510 ± 30 3700 ± 100

0.57 ± 0.06 0.81 ± 0.09 0.67 ± 0.06 0.67 ± 0.09 1.1 ± 0.1 1.3 ± 0.2 4.8 ± 0.5 0.89 ± 0.02

0.935 0.963 0.937 0.923 0.942 0.916 0.984 0.999

20−2890 14−55 4−195 4−65 20−660 100−590 35−130 130−300

measured for a range of VOCs (Table 2). Kinetic data was fit to a pseudo-second rate model, which is typically used for modeling liquid phase adsorption, but fit the gas-phase data measured here with regressions of >0.99. Gas phase adsorption is typically not fit with a pseudo-second order expression, as the rate of adsorption is often diffusion limited. However, with SOMS it was found that the kinetics of the adsorption process are rate limiting, and the data could not be fit to a first order equation. For comparison purposes, the rate of acetone adsorption to other sorbents was also fit to the pseudo-second order expression (Table 2) with good fits (r2 > 0.99, Figure 8).

Figure 6. Static adsorption isotherms (g/g vs Ce) for xylene (■), toluene (△), methanol (○), and acetone (●) vapor to SOMS at T = 25 °C.

Figure 8. Pseudo-second order fits of adsorption to SOMS over time presteady state for octane, hexane, toluene, and acetone. Vapor concentration p = p0. Conditions: T = 25 °C, static conditions. Inset: raw gravimetric increase in mass vs time from vapor adsorption; a = acetone; h = hexane; t = toluene; o = octane.

When adsorption rates (k2) across the various organics tested are compared, the rate of adsorption to SOMS generally correlates with the vapor pressure of adsorbate, indicating little selectivity in adsorption rate with respect to chemical identity. Again, methanol is an outlier, as it had one of the slower rates of adsorption, indicating more polar vapors may have slower mass transfer rates into the hydrophobic pores. Adsorption rate was also slow for aromatic compounds, which may be due to π−π interactions between the aryl groups of sorbent and adsorbate limiting the rate of uptake. Compared to other media, adsorption rates to SOMS were slower, for instance k2,adsorb of acetone for PAC is 40× greater. Slower rates of uptake to SOMS may be due to a number of factors such as particle size differences or barriers to adsorption. Previous results have shown that SOMS in the dry state possesses a collapsed microporous structure that may restrict diffusion. The larger particle size of SOMS relative to that of PAC could also explain the slower relative rate of adsorption. Poor heat transfer by the matrix may also lead to a thermodynamic barrier44 where

Figure 7. The same data in Figure 6 plotted to determine fit to the Freundlich isotherm for xylene (■), toluene (△), methanol (○), and acetone (●) at T = 25 °C.

SOMS can provide this capacity; however, in practice, the vapor phase concentrations of a single component vapor may not be high enough to access the potential adsorption capacity of the fully swollen state. 3.3. Rate of Adsorption. Kinetic measurements of organic vapor adsorption were conducted as means of further understanding the adsorption process of SOMS. Rate of adsorption under static conditions at 25 °C (p = p0) was 12074


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the mass transfer kinetics affects adsorption capacity. Breakthrough curves have a slow rise in effluent concentration that do not reach complete saturation (C/C0 = 1) even at long time scales for toluene and butanone (see the Supporting Information for data). Capacity was approximately 0.25−0.50 times the static adsorption capacity. The affinity and capacity were lowest for methanol, which corresponds to previous data that adsorption of methanol has a slower rate of static adsorption compared to that of other VOCs. Adsorption of the semipolar butanone is the highest, better than that of toluene. Toluene should have one of the stronger affinities to SOMS because adsorption can be facilitated by both van der Waals and π−π interactions. Toluene has the slowest static adsorption rate, which appears to impact the dynamic adsorption capacity. The dynamic adsorption capacity for toluene is half as high as the capacity under static conditions (0.52 vs 0.92 g/g). Whether differences in the rate of adsorption can be used to improve selectivity will require further study. 3.6. Coadsorption of Organic Vapors. Mixtures of vapors are common in industrial settings, so the adsorption of two species was studied in a unique manner by the sequential adsorption of one vapor, and while still loaded, exposing the laden SOMS to the vapor of a second organic substance alone. Sequential adsorption of pure phenol vapor in air followed by pure acetone vapor in air was performed to determine the total capacity of SOMS when two compounds are adsorbed. The purpose was to evaluate if matrix expansion provides enhanced capacity upon sequential events of organic vapor adsorption. SOMS was first exposed to saturated phenol vapor for 11.6 h, leading to a loading of 0.68 g/g phenol/sorbent (Table 4).

localized heat created during adsorption or capillary condensation cannot be rapidly dissipated into the bulk matrix. To better understand how pore flexibility affects the rate of adsorption, the kinetics of nonswelling organosilica were measured using acetone vapor. Interestingly, the adsorption rate constant for underivatized (nonswelling) SOMS is an order of magnitude faster than that of the swelling form. The 400 °C heated and partially calcined material, which still swells, has a rate of adsorption that is slightly less than SOMS despite being more mesoporous. Thus, the flexibility of the matrix to expand appears to have significant effect on the adsorption rate. The kinetics of acetone desorption were measured. Interestingly, the reversible desorption of acetone from SOMS was 20 times faster than adsorption (for example, acetone: k2,on = 0.109 ± 0.009; k2,off = 2.5 ± 0.3). Adsorption by SOMS further differs from Optipore and PAC, as these standard sorbents both exhibit a faster adsorption than desorption rate. Differences in rate of adsorption and desorption by SOMS hint that the pore matrix may be structurally different after adsorption, which is not surprising given the volumetric flexibility of the material. 3.4. Adsorption Model. Given the ability of the pores in SOMS to expand and contract, development of a model of adsorption that accounts for volume change is warranted. A model of adsorption for sorbents that demonstrate a “C” type (linear) isotherm was put forth by Giles et al.,46 where the process of adsorption is multistep, involving the “disentanglement” of the adsorbing substrate by an absorbate. kdisentangle

(solute in vapor) ⇐==⇒ (intermediate state) k re‐entangle

Table 4. Static Adsorption Capacities for Coadsorption of Acetone to SOMS at T = 25°Ca

kadsorb

⇐⇒ (solute in adsorbed state )} kdesorb

Experimentally, the kinetics of the mass gain upon adsorption (k2,on) would be controlled by both kdisentangle and kadsorb in addition to the concentration gradient between each state. What is unknown is whether the rate of disentanglement involves a mass change in the sorbent; in other words, is the intermediate state a solute bound state. Assuming solute interacts with sorbent to create an intermediate state, the rate of disentanglement appeared to be the rate limiting step. Disentanglement rate, perhaps better described as pore opening, would be controlled by (i) the time it takes the conformation of SOMS to adopt the intermediate state and (ii) the concentration gradient between the vapor concentration and concentration in the intermediate state. The concentration gradient between the vapor-intermediate state in the adsorption direction is likely shallower during adsorption when [vapor] = p0 compared to the desorption direction where [vapor] = 0, especially if it assumed the number of intermediate states is small (i.e., intermediate state volume is small) and sites fill rapidly in the presence of solute vapor. Under these conditions, kdisentangle would be limiting in the direction of adsorption whereas kdesorb would be limiting in the off direction. Thus, SOMS appears well-suited for slow, high capacity adsorption of VOC vapors and rapid and complete regeneration. Further research is needed to better understand the rapid and reversible desorption mechanism, which may be useful to exploit in industrial applications where fast regeneration is desirable. 3.5. Dynamic Adsorption. The dynamic adsorption capacity was measured for methanol, toluene, and butanone at high concentrations (p = p0, T = 30 °C) to determine how

a

initial adsorbate

qe1 (g/g)

qe2,acetone (g/g)

k2

none phenol tetramethylbenzene

N/A 0.68 0.73

1.0 5.0 2.7

0.11 0.0083 0.013

Variance to within ±5% rsd.

Exposure to phenol vapor was discontinued followed by immediate exposure to acetone vapor at p0. The low vapor pressure of phenol limited the extent of desorption during the time period required to adsorb acetone. Adsorption of acetone to the phenol laden sorbent (Figure 9) led to a further 470% increase in total mass (5.4 g/g total loading), exceeding the adsorption capacity for acetone by empty SOMS (1.02 ± 0.04 g/g). During the adsorption of acetone, the particles remained free of any free liquid on their surface and volumetrically increased ∼2× in size. The 4.7-fold increase in acetone adsorption capacity by SOMS saturated with phenol could be attributed to a manifestation of Raoult’s law if it is presumed that adsorbed acetone exists as a condensed liquid due to capillary condensation and phenol acts as a solute. Accordingly, the phenol and acetone could be modeled as a condensate solution, and the partial vapor pressure of the acetone in the mixture would be lower than that of pure acetone, pushing the equilibrium to a higher amount of total adsorbed acetone. The hypothesis that enhanced coadsorption capacity is due to vapor pressure depression was tested by measuring the acetone adsorption capacity as a function of the phenol preloaded state (Figure 10). Although acetone adsorption capacity increases 12075


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Table 5. Static Adsorption Capacities for Coadsorption Vapors to SOMS after Preadsorption of Phenol Vapors at T = 25°Ca

a

second adsorbate

qe1,phenol (g/g)

qe2 (g/g)

k2

acetone isopropanol pentane

0.54 0.66 0.55

3.6 2.8 0.47

0.015 0.016 0.97

Variance to within ±6% rsd.

These results are interesting in that the volumetric matrix expansion allows for a solid-phase SOMS particle to be loaded with species that can significantly enhance adsorption of a target vapor via a specific type of bonding interaction without loss of adsorption capacity. In the case of the phenol−acetone system, a total maximum adsorbed mass of 5.7 g/g was accommodated by the volumetric expansion. Despite the amount of phenol and acetone adsorbed, the particles remained dry by visual inspection. 3.7. FRAP Diffusion Measurements. Understanding diffusion within materials can be useful in understanding adsorption dynamics. Because SOMS is optically transparent, it is possible to use optical methods to follow the diffusion of spectroscopic probes. FRAP is a method of directly measuring the diffusional rate of a fluorescent probe molecule. Fluorescein was doped into the samples by swelling and drying the material followed by FRAP measurements. In the dry condition, fluorescein was found to be immobile due to the adsorption to pore surfaces and a lack of a fluid to diffuse. Upon adsorption of isopropanol vapor at T = 25 °C, the probe was free to diffuse, and almost complete fluorescence recovery of the 3 μm bleached region was observed after 30 s (Figure 11). When vapors are absorbed by SOMS, a nonliquid dopant (i.e., fluorescein) was free to diffuse. Near-complete recovery in photobleaching across the entire area indicates that the SOMS is continuously porous, at least after adsorbing organic vapors. Diffusion of entrapped fluorescein is attributed to dissolution of the probe in condensed adsorbate within interlinked pores. The diffusion coefficient was 1.0 × 10−8 cm2/s, which indicates a comparable hindered mean free path when compared to other sol−gel derived materials (Table 6). Fully swelling the material with liquid increases the diffusion rate fourfold, presumably by increasing the pore openings. Overall, the data indicate SOMS is continuously porous, and the rate of diffusion is directly related to the swollen state. These data also support the hypothesis that the kinetics of adsorption is modulated by change in pore size caused by ad/absorption of organic vapors and liquids. In the fully swollen state, the rate of diffusion of fluorescein is strongly controlled by the viscosity of the solvent, as observed by the expected reduction in diffusivity when swollen with mineral oil. This indicates that in the fully swollen state, pore openings are relatively wide enough to prevent localized confinement. The transparency of organosilica materials makes additional optical studies an intriguing opportunity for future studies of mass transport in porous media, especially because the pore size can be controlled by varying the swell state.

Figure 9. Adsorption of acetone vapor (wt) over time divided by mass of SOMS (w0). Acetone vapor was held constant at p = p0 with and without the preadsorption of 0.56 g/g of phenol at T = 25 °C under static conditions.

Figure 10. Mass of adsorbed acetone at equilibrium per gram of SOMS (qe) as a function of phenol coadsorbate preloading condition (q1). Acetone vapor concentration p = p0. T = 25 °C, static conditions.

with the phenol loading state, a plot of acetone capacity vs acetone:phenol mole ratio is not linear, suggesting that soluteinduced vapor pressure depression plays only a partial role in the enhanced adsorption. To further understand the high degree of adsorption capacity, the initial sorbate was changed from phenol to tetramethylbenzene, which have similar boiling points (182 vs 192 °C, respectively) but different polarities. Acetone remained as the second sorbate. Adsorption capacity of acetone in the tetramethylbenzene system was still high but substantially reduced (2.68 g/g) compared to that of acetone coadsorbed with phenol (Table 4). These data suggest that a specific interaction between acetone and phenol enhances adsorption presumably through hydrogen bonding to the hydroxyl group of phenol. Strong hydrogen bonding between acetone and phenol has been observed.47 In a second set of experiments, phenol was used as the initial adsorbate, and the identity of the higher vapor pressure secondary binding organic adsorbate was varied from acetone to either isopropanol or pentane, which led to adsorption capacities of 2.78 and 0.47 g/ g, respectively (Table 5). Substantial decreases in adsorption capacity of pentane, which can interact only with phenol via weaker van der Waals forces, indicated that the enhanced coadsorption of acetone and isopropanol is likely due to sorbate−sorbate hydrogen bonding or dipole−dipole interactions.

4. CONCLUSIONS Swellable organically modified silica is a unique, mechanically flexible sorbent that resembles other polymer sorbents in the empty collapsed state but volumetrically expands to allow for enhanced capacity at high vapor phase concentrations. 12076


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Figure 11. Fluorescence intensity profiles of bleached spots 5, 8, 15, and 35 s after photobleaching for fluorescein in SOMS: A, adsorbed into dry SOMS without other adsorbates; B, swollen by addition of 4 mL/g mineral oil; C, during exposure to isopropanol vapor (saturated conditions, p = p0); D, swollen with 4 mL/g isopropanol liquid.

Table 6. FRAP Recovery Curve Fits and Diffusion Coefficients for Fluorescein

a

sample

A

C

k (s−1)

X2

+isopropanol vapor +isopropanol liquid +mineral oil swollen dry SOMS fluorescein in water fluorescein in sol−gel 1 fluorescein in sol−gel 2 steroids in sol−gel 3

0.92 0.97 0.58 no diffusion

0.00 0.00 0.09

0.062 0.105 0.018

0.30 0.01 0.02

D (cm2 s−1)a

ref

(1.2 ± 0.2) × 10−8 (3.1 ± 0.2) × 10−8 (4.9 ± 0.2) × 10−9 4.25 × 10−6 2.0 × 10−8 2.0 × 10−7 1.0 × 10−7to 1.0 × 10−9

48 49 50 51

Average of three trials at unique locations on different particles.

Rejection of water over organic vapors is substantial, although adsorption of organic vapors was nonselective. Synergy was observed for multicompound adsorption: if there is strong interaction between sorbates, total loading capacities exceeded 5 g/g, facilitated by the volumetric expansion of SOMS. Adsorption by SOMS was fully reversible in all cases. Kinetics of adsorption for a wide range of materials were measured, and it was found that the flexibility of the matrix resulted in reduced rates of adsorption, presumably due to adsorption induced conformation changes that slowed uptake but enhanced capacity. Desorption rates from SOMS were faster that adsorption rates and similar to those of rigid sorbents. FRAP measurements indicated that solute diffusion coefficients are similar to those of other continuously porous gel-state organosilica. Overall, SOMS is a high capacity, volumetrically expanding sorbent with moderate affinity and high reversibility. It can be distinguished from other sorbents by the unusually high rejection of water, the ability to be preloaded with other

substances to promote adsorption of specific VOCs, and reversibility that does not require elevated temperatures.

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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.6b02403. Physical properties of sorbents, synthesis method, infrared spectra, photographs of swelling, dynamic adsorption experimental data, and diffusion of propane vs water through a SOMS disk (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: (330)-263-2113. Funding

The research was funded in part by the National Science Foundation (Grant CBET-0930371) 12077


Article

Industrial & Engineering Chemistry Research Notes

(18) Bantignies, J. L.; Vellutini, L.; Maurin, D.; Hermet, P.; Dieudonné, P.; Wong Chi Man, M.; Bartlett, J. R.; Sauvajol, J. L.; Moreau, J. J. E. Insights into the self-directed structuring of hybrid organicinorganic silicas through infrared studies. J. Phys. Chem. B 2006, 110, 15797. (19) Kapoor, M. P.; Yang, Q.; Inagaki, S. Self-assembly of biphenylene-bridged hybrid mesoporous solid with molecular-scale periodicity in the pore walls. J. Am. Chem. Soc. 2002, 124, 15176. (20) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. Porous polysilsesquioxanes for the adsorption of phenols. Environ. Sci. Technol. 2002, 36, 2515. (21) Boury, B. Self-association in hybrid organic-inorganic siliconbased material prepared by surfactant-free sol-gel of organosilane. In Silicon Based Polymers; Springer: Netherlands, 2008; pp 233−246. (22) Moreau, J. J. E.; Pichon, B. P.; Wong, M. W. C.; Bied, C.; Pritzkow, H.; Bantignies, J.-L.; Dieudonné, P.; Sauvajol, J.-L. A better understanding of the self-structuration of bridged silsesquioxanes. Angew. Chem., Int. Ed. 2004, 43, 203. (23) Burkett, C. M.; Edmiston, P. L. Highly swellable sol−gels prepared by chemical modification of silanol groups prior to drying. J. Non-Cryst. Solids 2005, 351, 3174. (24) Burkett, C. M.; Underwood, L. A.; Volzer, R. S.; Baughman, J. A.; Edmiston, P. L. (2008). Organic−inorganic hybrid materials that rapidly swell in non-polar liquids: nanoscale morphology and swelling mechanism. Chem. Mater. 2008, 20, 1312. (25) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. Porous polysilsesquioxanes for the adsorption of phenols. Environ. Sci. Technol. 2002, 36, 2515. (26) Edmiston, P. L.; Underwood, L. A. Absorption of dissolved organic species from water using organically modified silica that swells. Sep. Purif. Technol. 2009, 66, 532. (27) Plazinski, W.; Rudzinski, W.; Plazinska, A. Theoretical models of sorption kinetics including a surface reaction mechanism: A review. Adv. Colloid Interface Sci. 2009, 152, 2. (28) Ho, Y. S.; McKay, G. Pseudo-second order model for sorption processes. Process Biochem. 1999, 34, 451−465. (29) Blanchard, G.; Maunaye, M.; Martin, G. Removal of heavy metals from waters by means of natural zeolites. Water Res. 1984, 18, 1501. (30) Azizian, S. Kinetic models of sorption: a theoretical analysis. J. Colloid Interface Sci. 2004, 276, 47−52. (31) Brunauer, S.; Emmett, P. H.; Teller, E. J. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 1938, 60, 309. (32) Barrett, E. P.; Joyner, L. G.; Halenda, P. H. The determination of pore volume and area distributions in porous substances I. Computations from nitrogen isotherms. J. Am. Chem. Soc. 1951, 73, 373. (33) Murillo, R.; Garcıa, T.; Aylón, E.; Callén, M. S.; Navarro, M. V.; López, J. M.; Mastral, A. M. Adsorption of phenanthrene on activated carbons: Breakthrough curve modeling. Carbon 2004, 42, 2009. (34) Blonk, J. C. G.; Don, A.; Aalst, H. V.; Birmingham, J. J. Fluorescence photobleaching recovery in the confocal scanning light microscope. J. Microsc. 1993, 169, 363. (35) Seiffert, S.; Oppermann, W. Systematic evaluation of FRAP experiments performed in a confocal laser scanning microscope. J. Microsc. 2005, 220, 20. (36) Presley, J. F. In Methods Express, Cell Imaging; Stephens, D., Ed.; Scion Publishing Ltd: Bloxham, 2006; pp 119−142. (37) Almeida, R. M.; Pantano, C. G. Structural investigation of silica gel films by infrared spectroscopy. J. Appl. Phys. 1990, 68, 4225. (38) Payne, M. C.; Inkson, J. C. Longitudinal-optic-transverse-optic vibrational mode splittings in tetrahedral network glasses. J. Non-Cryst. Solids 1984, 68, 351. (39) De Leeuw, S. W.; Thorpe, M. F. Coulomb splittings in glasses. Phys. Rev. Lett. 1985, 55, 2879. (40) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. An ordered mesoporous organosilica hybrid material with a crystal-like wall structure. Nature 2002, 416, 304.

The authors declare the following competing financial interest(s): The process to prepare the materials described within has been patented by The College of Wooster and licensed to ABS Materials, Inc. The corresponding author has a financial interest in ABS Materials, Inc.

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ACKNOWLEDGMENTS The authors thank the Keck Center for Nanoscale Structure and Dynamics at the University of Arizona for use of the confocal microscope facilities. Gratitude is extended to the laboratory of Dr. Umit Ozkan at The Ohio State University, Department of Chemical Engineering for use of equipment for thermal treatment and useful discussions.

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