Adsorptive Water Removal from Primary Alcohols and Acetic Acid

Article pubs.acs.org/jced

Adsorptive Water Removal from Primary Alcohols and Acetic Acid Esters in the ppm-Region Christoph Pahl, Christoph Pasel,* Michael Luckas, and Dieter Bathen Universität Duisburg-Essen, Lotharstraße 1 47057, Duisburg, Germany S Supporting Information *

ABSTRACT: Dry organic solvents are important for many industrial sectors. Adsorptive water removal is one technique to obtain highly pure solvents. However, in-depth knowledge of the parameters influencing adsorption behavior is still fragmentary. This paper presents a systematic investigation of water adsorption from alcohols (C1 to C6) and acetic acid esters (methyl acetate to n-butyl acetate) of different chain lengths. Zeolites of types 3A and 4A are used as adsorbents. The impact of size exclusion on adsorption properties is analyzed. The water adsorption isotherms on zeolite 3A from solvents with a large critical molecular diameter are similar to the water vapor isotherm as expected from literature data. In case of smaller solvent molecules (methanol, ethanol, 1-propanol) a significantly lower water adsorption capacity is found on zeolite 3A. In case of all solvents on zeolite 4A water adsorption is lower than water vapor adsorption even if the estimated molecular diameter is larger than the reported window aperture of the zeolite cage. It is discussed to what extent water and solvent are capable to coadsorb and compete for adsorption sites. Basmadjian6 published a literature review on adsorptive drying of gases and liquids. He postulates that the isotherm for water removal from organic solvents on molecular sieves is independent from the solvent, as long as the solvent molecules cannot be adsorbed in the pores; that is, there should be only one isotherm for a certain type of molecular sieve. As none of the isotherms he reviewed coincided perfectly with the water vapor isotherm, Basmadjian attributed the differences to reasons like insufficient regeneration of zeolites, nonequilibrium in measurements, coadsorption of solvent molecules in macropores, and errors in water concentration measurement. Several authors investigated the adsorptive water removal from alcohols with molecular sieves. Teo and Ruthven7 studied the system water/ethanol/zeolite 3A, Jain and Gupta8 examined water/2-propanol/zeolite 4A, and Paderewski and Gabrus9 described water/1-butanol/zeolite 4A. All authors found nearly rectangular shaped isotherms typical for adsorption on zeolites. The objective of this work is to provide a systematic investigation of water adsorption from solvents to demonstrate the influence of molecular adsorptive and adsorbent properties on the adsorption capacity. Linear primary alcohols and acetic acid esters are common solvents which are often demanded with very low water content. The molecular structure of the solvents was systematically varied to describe the dependence of adsorption from size exclusion effects and polarity.

1. INTRODUCTION Optical, electronic, pharmaceutical, and chemical industries show increasing demand for highly pure organic solvents. In this context the removal of water in the lower ppm- and ppbrange is a major challenge since water is always present (e.g., air humidity) and leads to catalyst poisoning and undesired side reactions. One technique to remove water is by adsorption on zeolites, silica gels, or aluminas. Despite single technical solutions already existing, systematic measurements of influencing parameters are still missing. Literature Review. Burfield et al.1−5 investigated solvent drying with different adsorbents. Table 1 summarizes the data. Burfield demonstrated that the attainable water content decreases with the decreasing water solubility and permittivity of the solvent.1 Table 1. Water Content of Dried Organic Solvents1−5 adsorbent 1,4-dioxane benzene acetonitrile pyridine triethylamine methanol ethanol 2-butanol

Al2O3

silica gel

1400 ppm 0.2 ppm 1700 ppm 1306 ppm 223 ppm

1200 ppm 0.1 ppm 1300 ppm 926 ppm 451 ppm

zeolite 3A

27 ppm 117 ppm 34 ppm 99 ppm 99 ppm 14 ppm

© 2012 American Chemical Society

zeolite 4A 26 ppm 0.06 ppm 500 ppm 106 ppm 33 ppm 440 ppm 401 ppm

Received: April 10, 2012 Accepted: July 2, 2012 Published: August 7, 2012 2465

dx.doi.org/10.1021/je3005694 | J. Chem. Eng. Data 2012, 57, 2465−2471

Journal of Chemical & Engineering Data

Article

A review of literature data1−9 and screening tests in our laboratory have shown that the best drying performance is reached with zeolites. The use of zeolites with different pore size diameters provides an additional opportunity to characterize size exclusion effects. On that basis water adsorption from alcohols (C1 to C6) and from acetic acid esters (methyl acetate to n-butyl acetate) on zeolites 3A and 4A was examined.

n

R2 = 1 −

∑i = 1 (qmeas, i − qcalc, i)2 n

∑i = 1 (qmeas, i − qmean)2

(2)

As the nonlinear regression analysis does not require any transformation of the Langmuir equation, the parameters are fitted to original experimental data. This procedure avoids a bias in any concentration region.10 Initial parameter values for the fit were taken from a linearized Langmuir-equation. To evaluate the quality of the fit two errors and a standard deviation are defined as follows:

2. EXPERIMENTAL WORK AND ERROR DISCUSSION Between 0.1 g and 0.5 g of adsorbent were filled into an Erlenmeyer flask with 50 mL of solvent. The flasks were tightly sealed and shaken in an incubator at 20 °C and 150 rpm until equilibrium was reached. Thermodynamic equilibrium was achieved in preliminary tests for all systems with varying compositions after 3 to 4 days. To ensure complete equilibration at the isotherm measurements, we added a safety margin so that the flasks were eventually left in the incubator for up to 7 days. A blind sample, consisting of a flask only filled with solvent, was used to detect potential water contamination from the ambient atmosphere. Before the experiments all adsorbents were prepared thermally according to manufacturer’s guidelines. Sample flasks were dried in an oven prior to use. The water content before and after adsorption was measured by coulometric Karl Fischer titration which is reliable down to about 1 ppm. The equilibrium water concentrations range from about 5 ppm to 1000 ppm. All handling of samples and measurements were carried out in a glovebox purged with dry nitrogen to prevent contamination. To ensure reproducibility and accuracy of experiments, extensive repetition experiments were performed. For each isotherm point three flasks were prepared. From each flask three samples were taken and measured. The adsorbent load q is derived from a mass balance where mads is the adsorbent mass, msolv is the solvent mass, and c0 and c1 are the initial and equilibrium water concentrations of the solution, respectively. For small amounts adsorbed the solvent concentration of the bulk solution may be assumed constant, and the load may be calculated with eq 1:

Error 1 =

Error 2 =

N

1 N

∑

1 N

∑

(|qcalc, i − qmeas, i|) ·100 qmeas, i

i=1 N

(3)

(qcalc, i − qmeas, i) ·100 qmeas, i

i=1

(4)

N

Std. Dev. =

Δqi =

∑i = 1 (Δqi − Error 2)2 N−1

(qcalc, i − qmeas, i) qmeas, i

(5)

·100 (6)

Error 1 represents the deviation between measured and fitted values without differentiating between positive and negative deviations. Error 2 takes into account the direction of deviation. Therefore in error 2 negative and positive deviations may cancel out. If error 1 is equal to error 2, measured and fitted data systematically deviate to one side only. The standard deviation represents the statistical spread of the deviation.

(1)

3. THEORETICAL CONSIDERATIONS Equilibrium Model. During the adsorption experiments three phases coexist: the adsorbate phase, the solvent phase, and the gas phase above the liquid in the flask. The adsorbate phase and the solvent phase are in contact and in thermodynamic equilibrium. So are the liquid phase and the gas phase which is illustrated in Figure 1. Neglecting surface phenomena it follows that also the gas phase and the adsorbate phase are in thermodynamic equilibrium. Using extended Raoult’s law we can transfer our isotherms to a relative

where q is the adsorbent load (g water/100 g adsorbent), msolv and mads are the weighed mass of solvent and adsorbent (kg), respectively, and c0 and c1 are the initial and equilibrium concentrations of water (g water/kg solvent), respectively. A standard deviation of sσ = 5 % was obtained for Karl Fischer measurements. Taking into account Gaussian error propagation at the adsorbent load calculation, an estimated standard deviation of sσ = 10 % was found for the adsorbent load. The measured standard deviation for the isotherm points was in the range sσ = 1 to 7 % except for very small equilibrium water concentrations (below 15 ppm) where the standard deviation was up to 20 %. The experiments lead to nearly rectangular shaped adsorption isotherms (see section 4). Therefore a nonlinear Langmuir regression was used to fit the experimental data. According to Schulthess and Dey10 a value for R2 → 1 was defined as a target for the fit.

Figure 1. Equilibrium model to compare gas phase and liquid phase isotherms.

q=

msolv ·(c 0 − c1) mads

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humidity scale to compare them with the water vapor isotherm on the zeolite: yH O p φH O = 2 = x H2OγH∞O 2 2 ps,H O (7) 2

where φ is the relative humidity, yH2O is the mole fraction in the gas phase, xH2O is themole fraction in the liquid phase, ps,H2O is the saturation vapor pressure (Pa), p is the total pressure (Pa), and γH∞2O is the limiting activity coefficient. Estimation of Critical Molecular Diameters. In microporous zeolite systems size exclusion considerations play an important role on the discussion of adsorption capacities. Thus, it is useful to estimate the molecular size of the compounds involved. The critical molecular diameter is defined as the diameter of the smallest cylinder that circumscribes the molecule in its equilibrium conformation.11 Since literature does not provide reliable experimental data for molecular diameters the critical diameters were estimated by a model calculation. Koresh and Soffer12 propose a model which accesses a molecular volume from the experimental molar volume of the liquid. To account for the void volume the liquid is modeled by a densest packing of equal spheres, resulting in the molecular sphere volume Vm,dp: Vm,dp =

M π · ρNA 3 2

Figure 2. Water adsorption isotherms from linear alcohols on zeolite 3A.

propanol. The load for adsorption from 1-butanol is 13.8 wt %, while for 1-pentanol and 1-hexanol 15.9 wt % are found. Isotherms for Water Adsorption from Branched Alcohols. In Figure 3, we present the experimental isotherm data of water

(8) 3

where Vm,dp is the molecular volume (m ), M is the molar mass (g·mol−1), ρ is the liquid density (g·m−3), and NA is Avogadro’s constant (mol−1). To obtain critical molecular dimension from the volume, we have to assume a molecular shape. For linear alcohols and esters a spherocylinder (radius r and length l) should be appropriate. These approximations lead to: 4 3 πr + πr 2l = Vm,dp 3

Figure 3. Water adsorption isotherms from branched alcohols on zeolite 3A.

(9a)

where a is the major axis and b is the minor axis of the ellipsoid. The critical molecular diameter is now dcrit = 2b.

adsorption from the branched alcohols 2-propanol and 2butanol on zeolite 3A. The maximum adsorbent load attained in the experimental water concentration range is 15 wt % which is in good agreement with the maximum loads of adsorption from the linear alcohols. Isotherms for Water Adsorption from Esters. Figure 4 compares the adsorption isotherms for the removal of water from acetic acid esters with zeolite 3A. All isotherms are very close together. Maximum loads are between (15.7 and 16.1) wt % which is in the same range as for the linear and branched

4. RESULTS 4.1. Adsorption Isotherms on Zeolite 3A. Isotherms for Water Adsorption from Linear Alcohols. Figure 2 shows the experimental isotherm data of water adsorption from linear alcohols (homologous series methanol−1-hexanol) on zeolite 3A. The adsorbent load (wt %) is plotted versus the equilibrium water concentration of the solution (ppmw). The symbols represent experimental data. The lines indicate the isotherms fitted to the Langmuir equation. The isotherms for water adsorption from methanol, ethanol, and 1-propanol are distinctly different from the isotherms for adsorption from 1-butanol, 1-pentanol, and 1-hexanol which are close to each other. The maximum adsorbent load attained in the experimental water concentration range is 1.8 wt % for the solvent methanol, 7.25 wt % for ethanol, and 9 wt % for 1-

Figure 4. Water adsorption isotherms from acetic acid esters on zeolite 3A.

The molecular length is obtained from bond lengths and vander-Waals-radii. The critical molecular diameter is dcrit = 2r. Branched alcohols are approximated by an ellipsoidal shape. For a prolate ellipsoidal molecule we have:

4 πab2 = Vm,dp 3

(9b)

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Table 2. Langmuir Parameters and Approximation Errors for Adsorption on Zeolite 3A solvent

qmon/(g/100 g)

b/kg·g−1

R2

error 1/%

error 2/%

std. dev./%

methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol 2-propanol 2-butanol 2-propanol 2-butanol 2-propanol 2-butanol

1.8 7.25 9 13.8 15.9 15.9 15.2 15.04 15.2 15.04 15.2 15.04

0.01 0.005 0.031 0.030 0.020 0.024 0.014 0.015 0.014 0.015 0.014 0.015

0.96 0.98 0.98 0.99 0.99 0.99 0.99 0.98 0.99 0.98 0.99 0.98

6.0 13.1 6.1 5.8 7.6 13.0 4.7 9.1 4.7 9.1 4.7 9.1

−1.4 7.8 1.5 −0.7 3.5 −6.5 2.3 4.6 2.3 4.6 2.3 4.6

7.7 18.5 7.5 8.6 12.5 25.0 6.6 17.1 6.6 17.1 6.6 17.1

identical. The maximum loads are 17 wt % which is in accord with the data for linear alcohols. Isotherms for Water Adsorption from Esters. The isotherms for water removal from esters on zeolite 4A are plotted in Figure 7. The isotherms are close together. The maximum loads range from (16 to 18) wt %. The initial slope is steeper than in case of the alcohols.

alcohols. The initial slope is steeper compared to the isotherms for water adsorption from alcohols. The Langmuir parameters of the measured isotherms and the errors characterizing the regression quality are listed in Table 2. 4.2. Adsorption Isotherms on Zeolite 4A. Isotherms for Adsorption from Linear Alcohols. Figure 5 depicts the water

Figure 5. Water adsorption isotherms from linear alcohols on zeolite 4A.

Figure 7. Water adsorption isotherms from acetic acid esters on zeolite 4A.

adsorption isotherms from linear alcohols on zeolite 4A. For methanol and ethanol the isotherms as well as the maximum loads differ considerably from the data of the other solvents. The maximum loads for methanol and ethanol are (9 and 14) wt %, respectively. For 1-propanol to 1-hexanol the maximum loads range from (16 to 18) wt %. Isotherms for Water Adsorption from Branched Alcohols. In Figure 6, we show the isotherm data for water removal from branched alcohols on zeolite 4A. The isotherms are nearly

In Table 3 the Langmuir parameters are presented along with the errors required to evaluate the regression quality. All experimental data are provided in the Supporting Information. 4.3. Critical Molecular Diameters. The approximation results for the critical molecular diameters of the solvents used in this work are summarized in Table 4. The estimated critical diameter of methanol is in agreement with literature.13 The diameters for the other alcohols given in literature14 are larger than our estimates. For the esters no literature data were found. For the window aperture of the α-cage of zeolite 3A and 4A the data sheets of the zeolite manufacturer give values of 3.2 Å and 3.8 Å, respectively. Comparing the diameters from Table 4 to the reported window apertures one can discuss size exclusion on zeolites. The diameter of methanol is smaller than the window aperture of zeolite 3A, so coadsorption of methanol on zeolite 3A should be expected. The diameters for all of the other alcohols given in literature are larger than 3.8 Å so that no coadsorption would be expected. However, the estimates give rise to more differentiated conclusions. The estimated diameter of ethanol is equal to the window aperture of zeolite 3A, while the diameter of 1-propanol is only slightly larger. The coadsorption of these solvents should also not be excluded.

Figure 6. Water adsorption isotherms from branched alcohols on zeolite 4A. 2468



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Table 3. Langmuir Parameters and Approximation Errors for Adsorption on Zeolite 4A alcohol

qmon/(g/100 g)

b/kg·g−1

R2

error 1/%

error 2/%

std. dev./%

methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol 2-propanol 2-butanol methyl acetate ethyl acetate n-propyl acetate n-butyl acetate

9.0 14.3 16.3 16.9 17.8 17.8 16.73 17.18 16.06 16.4 17.13 18.29

0.0009 0.004 0.0067 0.008 0.007 0.008 0.007 0.0064 0.0317 0.0459 0.0274 0.0397

0.96 0.95 0.98 0.94 0.98 0.99 0.99 0.97 0.98 0.99 0.97 0.97

5.9 2.6 6.4 11.6 9.8 3.5 6.0 6.7 5.0 6.2 8.7 13.0

1.5 −0.05 2.8 5.7 6.4 1.0 3.1 2.8 2.1 −3.0 2.7 6.1

10.5 3.5 8.9 22.5 20.7 4.8 8.6 8.4 9.4 9.1 14.5 22.1

Table 4. Approximated Critical Molecular Diameters of the Solvents solvent

dcrit/Å calculated

dcrit/Å from literature

methanol ethanol 1-propanol 1-butanol 1-pentanol 1-hexanol methyl acetate ethyl acetate n-propyl acetate n-butyl acetate 2-propanol 2-butanol

3.0 3.2 3.4 3.5 3.5 3.6 3.9 4.2 4.4 4.6 5.0 5.1

3.013 4.514 4.914 4.914 4.914

Figure 8. Comparison of vapor and liquid phase isotherms for water adsorption on zeolite 3A (non-coadsorbing solvents). 5.614

load of the vapor isotherm still increases, while the liquid phase isotherms seem to be only slightly inclining. This illustration supports Basmadjian’s thesis since there is no evidence for more than one single isotherm for water removal from organic liquids that do not coadsorb in zeolite pores. The vapor isotherm is a rough estimation of maximum capacity for water adsorption from many liquid alcohols and esters on zeolite 3A with an error of ± 10 % remaining. A different picture is found comparing the liquid phase isotherms of the alcohols with a small critical diameter (methanol, ethanol, 1-propanol) with the water vapor isotherm, as shown in Figure 9. The water load of the adsorbent in case of liquid phase adsorption is significantly lower which may be attributed to considerable coadsorption of the solvent reducing capacity for the water molecule. From methanol to ethanol to 1-propanol the water capacity increases with increasing

According to the estimates, all linear alcohols have a diameter smaller than 3.8 Å and thus should be capable to coadsorb on zeolite 4A. Also the diameter of methyl acetate is not much larger than the α-cage window aperture of zeolite 4A. However, the critical diameters of the long chain esters and the branched alcohols are too large to expect coadsorption on any of the zeolites.

5. DISCUSSION In his literature review6 Basmadjian postulates there should only be one theoretical isotherm for water removal from organic solvents with molecular diameters larger than the window aperture of the used zeolite. Furthermore this isotherm should be identical with the water vapor isotherm on the zeolite. The obvious similarity of the water removal isotherms from 1-butanol to 1-hexanol and from the esters found in this work suggests a careful examination of Basmadjian’s thesis. 5.1. Adsorption Behavior of Zeolite 3A. Figure 8 compares isotherms transferred according to eq 7 for alcohols from 1-butanol to 1-hexanol and esters from methyl acetate to butyl acetate with the water vapor adsorption isotherm for the zeolite 3A used in the experiments. The adsorbent water load is plotted versus relative humidity. The blue points represent the data of the water vapor isotherm, while the other isotherms are displayed in gray to make comparison easier. All isotherms of water adsorption from the solvents are quite steep. All isotherms show maximum adsorbent loads in the same order of magnitude. For a relative humidity up to 1 % no significant difference between the liquid phase and the vapor isotherms is determined. Toward higher relative humidity the

Figure 9. Comparison of vapor and liquid phase isotherms for water adsorption on zeolite 3A (strongly coadsorbing solvents). 2469



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Figure 10. Additionally, the capacities of water adsorption from methanol and ethanol are higher than on zeolite 3A (compare Figures 9 and 11). Presumably, on zeolite 4A the solvent coadsorption is driven back at increasing water concentration. 5.3. Impact of Solvent Coadsorption. The experimental data give several hints that solvent coadsorption takes place to a much stronger degree than expected from the comparison of molecular diameters and window apertures. For all solvents the initial slopes of the water isotherms are a little steeper on zeolite 3A than on zeolite 4A. However, at higher water concentrations zeolite 4A has a higher capacity than zeolite 3A. We assume that solvent coadsorption is stronger on zeolite 4A at very low water concentrations owing to its larger window aperture. But at higher water concentrations the adsorbed solvent is more and more substituted by water molecules. This will be plausible if one considers that zeolite 3A is produced from zeolite 4A by replacing sodium ions with larger potassium ions. On account of their larger ionic radius potassium ions exhibit weaker interactions with water molecules than sodium ions. For that reason the water capacity at vapor phase adsorption is slightly higher on zeolite 4A. Secondary alcohols have a branched structure and a large molecular diameter. They should be incapable to coadsorb on either of the zeolites. By the example of zeolite 4A, Figure 12

molecular diameter and decreasing polarity of the solvent. Coadsorption is strongest for methanol which is the smallest and most polar solvent. 5.2. Adsorption Behavior of Zeolite 4A. Adsorption on a zeolite of type 4A was measured to get a deeper insight into the influence of molecular dimensions on adsorption properties. Due to its larger window aperture a more pronounced coadsorption and a lower water capacity at the end of the measured range was expected. Figure 10 displays the transferred liquid phase isotherms for alcohols from 1-propanol to 1-hexanol and esters from methyl

Figure 10. Comparison of vapor and liquid phase isotherms for water adsorption on zeolite 4A (weakly coadsorbing solvents).

acetate to butyl acetate on zeolite 4A together with the water vapor isotherm. Compared to the adsorption on zeolite 3A (see Figure 8) it is evident that the isotherms on zeolite 4A are less steep. Contrary to expectations the water capacities at the end of the humidity range are not lower but rather slightly higher than in case of adsorption on zeolite 3A. The liquid phase isotherms reach a load of (15 to 17) wt %, while for vapor adsorption about 20 wt % are observed. Compared to zeolite 3A there is a marked gap between the capacities of liquid phase adsorption and vapor phase adsorption. As found for zeolite 3A there are also solvents of considerably lower water adsorption capacity on zeolite 4A. Figure 11 points out the deviations of the water vapor isotherm and the liquid phase isotherms for water adsorption from methanol and ethanol. As on zeolite 3A, coadsorption is strongest for the smallest and most polar solvent methanol. Surprisingly, the isotherm for adsorption from 1-propanol is not clearly below the water vapor isotherm so it is included in

Figure 12. Comparison of adsorption isotherms from secondary alcohols and esters on zeolite 4A.

illustrates that the isotherms for adsorption from these solvents are significantly below the ester isotherms, above all at low humidity. This behavior cannot be explained without assuming coadsorption of the alcohols. Rigorous size exclusion relying on simple diameter estimations is not found. Aside from the errors supposed to be made at diameter estimation, we have to take into account that neither the molecules nor the zeolite windows have strictly stiff geometries. The molecular shape of liquid and gas molecules changes because of bond vibration and rotation. As for the zeolites, an impact of atomic vibrations on the window geometry may also be presumed. Owing to incomplete ion exchange zeolite 3A still has windows of type 4A where sodium ions have remained. Finally one has to consider that both the zeolites contain mesoporous binder such that in any case some adsorption capacity is available which is not subject to size exclusion. To examine the significance of solvent coadsorption for adsorptive drying more thoroughly, adsorption capacity data of solvents on zeolites are required. Until now, for these systems only few data are available which besides are contradictory. Simo et al.15 examined the adsorption of water and ethanol

Figure 11. Comparison of vapor and liquid phase isotherms for water adsorption on zeolite 4A (strongly coadsorbing solvents). 2470



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vapor mixtures on a fixed bed of zeolite 3A by breakthrough experiments. They report very low capacities for ethanol adsorption on the zeolite. Wang et al.16 have measured adsorption isotherms of water and ethanol vapor on zeolite 3A by a gravimetric apparatus. At a relative humidity of 70 % the ethanol capacity was about 50 % of the water capacity. Possibly the contact time of breakthrough experiments is insufficient to reach adsorption equilibrium in zeolite systems. In this context adsorption measurements from the vapor phase close to saturation are planned in a gravimetric equilibrium apparatus to obtain capacity data of pure solvents and of mixtures of (solvents + traces of water). A comparison of data for zeolites with and without binder material should deliver additional information on the role of binder content.

(2) Burfield, D. R.; Lee, K.; Smithers, R. H. Desiccant Efficiency in Solvent and Reagent Drying. A Reappraisal by Application of a Novel Method for Solvent Water Assay. J. Org. Chem. 1977, 42, 3060−3065. (3) Burfield, D. R.; Smithers, R. H. Desiccant Efficiency in Solvent and Reagent Drying. 3. Dipolar Aprotic Solvents. J. Org. Chem. 1978, 43, 3966−3968. (4) Burfield, D. R.; Smithers, R. H. Desiccant Efficiency in Solvent and Reagent Drying. 7. Alcohols. J. Org. Chem. 1983, 48, 2420−2422. (5) Burfield, D. R.; Smithers, R. H.; Tan, A. Desiccant Efficiency in Solvent and Reagent Drying. 5. Amines. J. Org. Chem. 1981, 46, 629− 631. (6) Basmadjian, D. The Adsorptive Drying of Gases and Liquids. Adv. Drying 1984, 3, 307−357. (7) Teo, W. K.; Ruthven, D. M. Adsorption of Water from Aqueous Using 3-A Molecular Sieves. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 17−21. (8) Jain, A. K.; Gupta, A. K. Adsorptive Drying of Isopropyl Alcohol on 4 A Molecular Sieves: Equilibrium and Kinetic Studies. Sep. Sci. Technol. 1994, 29, 1461−1472. (9) Paderewski, M.; Gabrus, E. Adsorptive Drying of Organic Liquids in a Vibratory Agitator. Inz. Chem. I Proc. 1996, 17, 449−460. (10) Schulthess, C. P.; Dey, D. K. Estimation of Langmuir Constants using Linear and Nonlinear Least Squares Regression Analysis. Soil Sci. Soc. Am. J. 1996, 60, 433−442. (11) Ruthven, D. M. Molecular Sieve Separations. Chem. Ing. Tech. 2011, 83, 44−52. (12) Koresh, J.; Soffer, A. Estimation of Cross-sectional Diameters of Non-spherical Molecules. J. Chem. Soc., Faraday Trans. I 1980, 76, 2472−2485. (13) Ullmann’s Modeling and Simulation; Wiley VCH: Weinheim, 2007. (14) Choudary, V. R.; Nayak, S. V.; Mamman, A. S. Diffusion of Straight- and Branched-Chain Liquid Compounds in H-ZSM-5 Zeolite. Ind. Eng. Chem. Res. 1992, 31, 624−628. (15) Simo, M.; Sivashanmugam, S.; Brown, C. J.; Hlavacek, V. Adsorption/Desorption of Water and Ethanol on 3A Zeolite in NearAdiabatic Fixed Bed. Ind. Eng. Chem. Res. 2009, 48, 9247−9260. (16) Wang, K.-S.; Liao, C.-C.; Chu, R. Q.; Chung, T.-W. Equilibrium Isotherms of Water and Ethanol Vapors on Starch Sorbents and Zeolite 3A. J. Chem. Eng. Data 2010, 55, 3334−3337.

6. SUMMARY AND CONCLUSION This work provides a systematic investigation of the parameters influencing water adsorption from solvents in the trace range. The adsorbents were zeolites of type 3A and 4A which had the best performance in screening experiments. Water adsorption isotherms were measured from alcohols and esters from about 5 ppm to 1000 ppm. All isotherms found were well approximated by a Langmuir fit. On zeolite 3A the isotherms of water adsorption from solvents with a large critical molecular diameter exhibit a close similarity to the water vapor isotherm. For these systems the water vapor isotherm is a good estimate for water maximum capacity in case of solvent drying. As for smaller solvent molecules (methanol, ethanol, 1-propanol), a significantly lower water adsorption capacity compared to water vapor adsorption indicates coadsorption of the solvent governed by molecular diameter and polarity. On zeolite 4A water adsorption capacity from all solvents is clearly lower than the capacity of water vapor adsorption. As found for zeolite 3A, methanol and ethanol have a considerably lower water adsorption capacity compared to water vapor adsorption. It is striking that for many solvents the uptake of water from the solvent by the zeolite is lower than the uptake of water vapor even if the estimated molecular diameter is larger than the reported window aperture of the zeolite cage. Further adsorption capacity measurements of solvent vapors and (solvent + water) vapor mixtures on different zeolites in a gravimetric equilibrium apparatus should disclose to what extent these findings may be explained by solvent coadsorption in the cages and on the binder material.

■

ASSOCIATED CONTENT

S Supporting Information *

Tables of experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 0049 203 379 3119. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Burfield, D. R.; Gan, G.; Smithers, R. H. Molecular Sieves Desiccants of Choice. J. Appl. Chem. Biotechnol. 1978, 28, 23−30. 2471


Adsorptive Water Removal from Primary Alcohols and Acetic Acid

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