Affinity Study of Organics on Siliceous Ferrierite Type Zeolite

formic acid, acetic acid, formaldehyde, acetaldehyde, diethyl ether, acetone, methyl formate, and ethyl formate, were investigated as the adsorbat...
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Langmuir 2001, 17, 1119-1125

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Affinity Study of Organics on Siliceous Ferrierite Type Zeolite Bin Qian,† Huiwen Jiang,† Yaojun Sun,‡ and Yingcai Long*,† Department of Chemistry, Fudan University, Shanghai 200433, People’s Republic of China, and Center of Analysis and Measurement, Fudan University, Shanghai 200433, People’s Republic of China Received June 15, 2000. In Final Form: December 5, 2000

Thermal analyses were used to study the loading and the affinity of organic adsorbates on siliceous ferrierite type zeolite, which possesses perfect framework and high hydrophobic character after calcination for removing the template in the as-synthesized form. Twenty organic compounds with various functional groups, i.e., n-butane, n-pentane, n-hexane, n-heptane, methanol, ethanol, 1-propanol, 1-butanol, methylamine, ethylamine, n-propylamine, n-butylamine, formic acid, acetic acid, formaldehyde, acetaldehyde, diethyl ether, acetone, methyl formate, and ethyl formate, were investigated as the adsorbate. The AT index, defined as the value of Tdesorption - Tbp, was determined and used to compare the affinity of the organic compounds toward the zeolite. Organic molecules containing frameworkphilic functional groups such as alkylamines show high AT values. The oxygen-containing frameworkphobic functional groups such as alcohols and organic acids show lower AT values. The effect of other functional groups and the chain length of the alkyl group on the affinity are discussed. Compared with the data on high silica MFI and FAU zeolites, the AT values exhibit the same tendency for organics with the same functional groups. The influence of pore size and configuration of the zeolites on the AT values of various organic compounds adsorbed is discussed.

Introduction The crystal structure of ferrierite (FER) is closely related to the structures of the mordenite family of zeolites and is based on five-ring building units, stacked in the [001] direction. The FER framework is traversed by two types of intersecting channels, one along [001] with 10 ring openings and the other parallel to [010] with 8 ring openings.1 The structure of FER was first solved by Vaughan2 on a magnesium-containing mineral sample from Kamloops Lake, Canada, and has been confirmed by other workers.3, 4 FER possesses interesting adsorption properties5,6 and exhibits an excellent shape-selectivity in the isomerization of butene to isobutene, which is a raw material for producing MTBE (methyl tert-butyl ether), an additive in lead-free gasoline.7 The siliceous form of FER with perfect structure shows strong hydrophobic/organophilic character. The temperature of the framework collapse is higher than 1300 °C,8 showing very high stability of the zeolite. * To whom correspondence may be addressed. E-mail: [email protected]. Fax: (+86)-21-5653-3195. † Department of Chemistry. ‡ Center of Analysis and Measurement. (1) Meier, W. W.; Olson, D. H.; Baerlocher, Ch. Atlas of Zeolite Structure Types, 4th revised ed.; Published on behalf of the Structure Commission of the International Zeolite Association; Elsevier: Amsterdam, 1996; pp 106, 104, 146. (2) Vaughan. P. A. Acta Crystallogr. 1966, 21, 983. (3) Gramlich-Meier, R.; Meier, W. M.; Smith, B. K. Z. Kristallogr. 1984, 169, 201. (4) Alberti, A.; Sabelli, C. Z. Kristallogr. 1987, 178, 249. (5) van Well, W. J. M.; Cottin, X.; de Haan, J. W.; Smit, B.; Nivarthy, G.; Lercher, J. A.; van Hooff, J. H. C.; van Santen, R. A. J. Phys. Chem. B 1998, 102, 3945. (6) Long, Y. C.; Ma, M. H.; Sun, Y. J.; Jiang, H. W. Inclusion Phenom. Macrocycl. Chem. 2000, 37, 365. (7) Mooiweer, H. H.; Dejong, K. P.; Kraushaar-Czarnetzki, B.; Stork, W. H. J.; Krutzen, B. C. H. Stud. Surf. Sci. Catal. 1994, 84C, 2327. (8) Ma, M. H.; Jiang, H. W.; Long, Y. C. Acta Chim. Sin. 1998, 56, 405.

Understanding the interaction between organics and FER zeolite is crucial for applying the zeolite to the field of separation. Up to now, the major investigations on the adsorption onto FER were focused on alkanes and included the effects of acid sites and of the chain length on the adsorption and diffusion properties5,9,10 The FER employed in these studies had a Si/Al ratio below 50 and thus contained cations and surface defects such as silanol groups. Siliceous FER zeolite synthesized in a nonaqueous reactant system11 has almost no defects and is a good material for the investigation of interactions between the [Si-O] framework and adsorbates. The whole surface of the framework can be assumed to be actually covered by the oxygen atoms, because the radius of the oxygen atom is much larger than that of the silicon atom. The crystal structure of siliceous FER is refined in the orthorhombic space group Pnnm, and the unit cell content is [Si36O72].11,12 In the present paper, a thermogravimetric analysis/ differential thermogravimetry/differential thermoanalysis (TG/DTG/DTA) method is used to study the affinity of adsorbed organics on the framework of siliceous FER zeolite. Alkylamines, alcohols, n-alkanes, and oxygencontaining organics are investigated as the adsorbates. Varying affinities expressed by the AT value will reveal the difference of the interaction between the perfect Si-O framework of FER zeolite and the adsorbed organic compounds with various functional groups. The comparison with our previous work on high silica MFI and FAU zeolite13,14 is discussed as well. (9) Eder, F.; Lercher, J. A. J. Phys. Chem. B 1997, 101 (8), 1273. (10) van Well, W. J. M.; Cottin, X.; Smit, B.; van Hooff, J. H. C.; van Santen, R. A. J. Phys. Chem. B 1998, 102, 3952. (11) Kuperman, A.; Nadimi, S.; Oliver, S.; Ozin, G. A.; Garces, J. M.; Olken, M. M. Nature 1993, 365, 239. (12) Lewis, J. E., Jr.; Freyhardt, C. C.; Davis, M. E. J. Phys. Chem. 1996, 100, 5039. (13) Long, Y. C.; Jiang, H. W.; Zong, H. Langmuir 1997, 13, 4094.

10.1021/la000835n CCC: $20.00 © 2001 American Chemical Society Published on Web 01/25/2001

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Experimental Section Materials. The synthesis of siliceous ferrierite was performed according to the method of Kuperman et al.11 The starting raw materials were fumed silica (SiO2 ) 99.8 wt %), n-propylamine (PrNH2), pyridine (Pyr), HF/Pyr (70 wt % HF) (all from Shanghai Chemical Reagent Corp.) and distilled water. Molar ratio of the reactant mixture was 1.5SiO2:2HF/Pyr:8H2O:4PrNH2:16Pyr. Appropriate amounts of distilled water, PrNH2, and Pyr were added together in a plastic beaker. A clear solution was formed after stirring for 10 min. Then the fumed silica was added to the solution under vigorous stirring to form a uniform mixture. After agitation for 25 min, the mixture was sealed in Teflon-lined stainless steel autoclaves of 40 mL and heat-treated at 180 °C in an oven for 6-8 days. The product was filtered, washed with distilled water, rinsed with acetone, and dried at room temperature to get sample A. Sample A was calcined at 850 °C in air for 16 h to gain sample B. Characterization The scanning electron microscopy (SEM) photo of the as-synthesized sample A was taken with a Philips XL 30 scanning electron microscope. X-ray power diffraction (XRD) of the samples was carried out with a Rigaku D-MAX/II-A diffractometer. Cu KR radiation (λ ) 0.154 18 nm) was used. The scanning range was 5-35°/2θ with a scanning speed of 16°(2θ)/ min. The 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected at room temperature on a Bruker MSL-300 spectrometer. The 29Si resonance frequency used was 59.595 MHz, the width of the spectrum was 335 ppm, and the rotor was spun at 4 kHz. The radio frequency field was 37.0 kHz. The reversing angle was 60°, recycle time 2 s, and 600-3000 spectra were accumulated. Q8H8 (trimethylsilyl ester of cubic octameric silicate) was used as the second reference for the 29Si chemical shift. FT-IR spectra were recorded on a Nicolet Magna-550 spectrometer at room temperature using the KBr disk method (sample/KBr ) 1/200). The scan range was 400-2000 cm-1 with a precision of 4 cm-1. The adsorption isotherms of water, methanol, and n-hexane were measured with a Sartorius 7012 super-micro electron balance at 20 °C. The weight of the sample tested was about 100 mg, which was degassed in a vacuum at 400 °C for 40 min before adsorption. The specific surface area and the pore volume were measured from the adsorption and desorption isotherms of N2 at -196 °C using a Micromeritric ASAP 2000 instrument. The sample B tested was dehydrated at 300 °C for 6 h before determination. Measurement of the AT Value. The reagents (all from Shanghai Chemical Reagent Corp.) used as the adsorbates were analytically pure. The FER sample B used as the adsorbent was dehydrated and degassed in a vacuum system (60 Pa) at 400 °C for 30 min. The gas dissolved in the reagents was removed at the temperature of liquid nitrogen in the system as well. Dehydrated and degassed sampled B was fully saturated in the system under the saturation vapor pressure of the adsorbate at room temperature for 24-48 h, yielding a full loading of the adsorbate in the zeolite. TG/DTG/DTA analysis was carried out on a Rigaku PTC-10A thermal analyzer, 10-30 mg sample saturated in the vapor of a certain adsorbate, was used for each measurement. Any water present in the adsorbate, such as methylamine, ethylamine, formaldehyde, and acetaldehyde, was removed by KA zeolite. The temperature rises from room temperature to 500 °C at a rate of 10 °C/min at a nitrogen flow rate of 90 mL/min. The AT values, affinity index, are calculated by the following formula:13

AT ) Td - Tb (no unit for the AT value is required) Td is the temperature of the weight loss peak of the DTG curve of a certain organic compound adsorbed; Tb is the boiling point of the organic compound. If there are two weight loss peaks appearing in a DTG curve, the higher one is taken as Td to calculate the AT value. (14) Yang, H.; Ping, Z. H.; Niu, G. X.; Jiang, H. W.; Long, Y. C. Langmuir 1999, 15, 5382.

Figure 1. (a) SEM photo of as-synthesized siliceous FER sample A. (b) The orientation of the channels in the FER crystal. l, w, and t designate the dimension of length, width, and thickness, respectively

Results and Discussion The SEM image of sample (see Figure 1a) shows that the as-synthesized FER crystals are platelike with a size of 140 × 100 × 10 µm. The crystal shape is the same as that reported in the literature, and the orientations of the channels are shown in Figure 1b.12 A beautiful interference color of the single crystals can be observed under a polarization microscope. The XRD patterns (see Figure 2) of the zeolite appear that there are no stock defect peaks, as mentioned in the literature.15 The pattern of sample A shows orientationpreferred diffraction peaks for the platelike crystals. After calcination of sample A, the crystallinity of sample B seems to be increased, and the peaks corresponding to the diffraction of 002 and 004 were enhanced due to removal of the templates. FT-IR spectra of FER samples are shown in Figure 3. The peaks in the spectrum of sample A at 816, 754, 1220, and 1084 cm-1 are assigned to external and internal symmetric or asymmetric stretch vibrations in the Si-O tetrahedron. The peaks at 530, 597, 459, and 424 cm-1 are assigned to the framework vibrations of double-ring and T-O bend, respectively.8,16 After calcination, the peak positions of the spectrum of sample A move to higher wavenumber for sample B. A fine pattern with higher resolution appears in the range of 755-820 cm-1 corresponding to the symmetric vibration in the Si-O tetrahedron, indicating perfect framework of the siliceous FER (15) Rice, S. B.; Treacy, M. M. J.; Newsam, J. M. Zeolites 1994, 14, 335. (16) Jansen, J. C.; van der Gaag, F. J.; van Bekkum, H. Zeolites 1984, 4, 369.

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Figure 4. N2 adsorption and desorption isotherms of siliceous FER sample B (after calcination at 850 °C for 16 h).

Figure 2. XRD pattern of siliceous FER samples: a, sample A, as-synthesized; b, sample B, after calcination at 850 °C for 16 h.

Figure 5. 29Si MAS NMR spectra of siliceous FER samples: a, sample A, as-synthesized; b, sample B, after calcination at 850 °C for 16 h.

Figure 3. FT-IR spectra of FER zeolite samples: a, sample A, as-synthesized; b, sample B after calcinationfor 16 h in air.

zeolite. There are some angular distortions of the Si-O tetrahedron in sample A, which are caused by the proximity of the guest pyridine molecules. During the calcination at 850 °C, the templates are removed, and the interaction between the template and the framework disappears, leading to the cell constriction, accompanied by smaller Si-O-Si bond angles and longer T-O bond.12 The adsorption and desorption isotherms of nitrogen on sample B are shown in Figure 4. Almost no hysteresis loop appears on the curves of desorption. The Langmuir surface area and pore volume calculated are 380.8 m2/g and 0.151 mL/g, respectively. The 29Si MAS NMR spectra of siliceous FER samples are shown in Figure 5. Three broader peaks at -112.5 (T2), -116.1 (T4), and -117.3 ppm (T5) appear in the spectrum of sample A. The 29Si MAS NMR spectrum of sample B exhibits high resolution so that all of five peaks

can be distinguished. The peaks at -111.1, -111.6, -115.4, -115.6, and -116.4 ppm, correspond to T3, T2, T5, T1, and T4, respectively, the five unequal locations of Si in FER.12 At the same time, no resonance peak appears at about -103 ppm, related to Si-OH or Si (1Al). The difference between sample B and sample A in chemical shift is probably generated by the interaction between the templates and the Si-O framework of the zeolite, which results in the overlap of Si-O-Si resonance peaks and the movement of its chemical shift to higher field. Figure 6 shows the vapor adsorption isotherms of water, methanol, and n-hexane on sample B at 20 °C. The loadings are 0.0096, 0.066, and 0.061 mL/g for water, methanol, and n-hexane, respectively, at p/p0 ) 0.2. This proves the high hydrophobic/organophilic property of the sample. The characterizations above confirm that siliceous FER possesses perfect structure while aluminum and Si-OH defect on the framework are absent. The quality of the sample is good enough for the investigation in this study. Desorption Behavior on Siliceous FER Zeolite. 1. Alkylamines and n-Alcohols. The TG/DTG/DTA curves of methylamine, ethylamine, n-propylamine, and nbutylamine on siliceous FER (sample B) are shown in Figure 7. There are two remarkable endothermic peaks

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Figure 6. The adsorption isotherms of water, methanol, and n-hexane on siliceous FER zeolite at 20 °C. Figure 8. TG/DTG/DTA curves of n-alcohols on siliceous FER zeolite.

Figure 7. TG/DTG/DTA curves of n-alkylamines on siliceous FER zeolite: a, TG; b, DTG; c, DTA; V, endothermic effect (these symbols have the same meaning in the following figures).

on the DTA curve and double peaks of weigh loss on the DTG curve of methylamine in the temperature range of 80-140 °C. Meanwhile, only one peak of weight loss appears on the DTG curves for the other amines and one obvious endothermic peak on the DTA curves. The peak temperature for the desorption of alkylamines is in the rang of 80-150 °C. No marked endothermic peak is observed on the DTA curves, and only one peak of weight loss appears on the DTG curves in the temperature range of 50-130 °C for n-alcohols (see Figure 8). The weight loss peak on the DTG curve is at 51, 90, 125, and 125 °C for methanol, ethanol, 1-propanol, and 1-butanol, respectively. It seems that the values of Td, the temperature for desorption of the compounds adsorbed, are approaching 125 °C with increase in carbon numbers of the alcohols from three to four. This fact indicates that the influence and polarity interaction of the functional group become faint and the dispersion interaction between the oxygen atoms on the zeolite framework and the alkyl group in the alcohols becomes dominative. 2. Normal Alkanes. The TG/DTG/DTA curves of n-butane, n-pentane, n-hexane, and n-heptane on siliceous FER are shown in Figure 9. A single peak of desorption is exposed in the temperature range of 70-150 °C on the DTG curves, much higher than the boiling point (Tb) of

Figure 9. TG/DTG/DTA curves of n-alkanes on siliceous FER zeolite.

the normal alkanes. The phenomenon is similar to that of hydrophobic MFI and high silica FAU zeolite,13,14 which possess strong affinity with the hydrocarbons. An inconspicuous endothermic effect appears on the DTG curves of n-butane and n-heptane. 3. Oxygen-Containing Organics. Figure 10 exhibits the TG/DTG/DTA curves of acetone, diethyl ether, methyl formate, and ethyl formate. There is one peak of weight loss on the DTG curves in the temperature range of 70130 °C for these organics and one endothermic peak on the DTA curves for methyl formate and ethyl formate. There is a strong endothermic peak for formic acid, weaker endothermic peaks for acetic acid and for formaldehyde, and an inconspicuous endothermic effect for acetaldehyde shown on the DTA curves (see Figure 11). A single peak of desorption appears on the DTG curves in the temperature range of 70-160 °C for these organics. Physical Meaning of the AT Value. The affinity existing between the adsorbate and the framework of siliceous zeolite is denoted by the AT value, which is defined as the difference between the desorption temperature (Td) and boiling temperature of the adsorbate (Tb). The

Organics on Zeolite

Figure 10. TG/DTG/DTA curves of oxygen-containing organics on siliceous FER zeolite.

Figure 11. TG/DTG/DTA curves of aldehydes and organic acids on siliceous FER zeolite.

adsorption of organic molecules in the micropores can approximately be treated as physical capillary condensation because there are no strong electrostatic centers and active centers in the zeolite. If a liquid (adsorbate) is immersionally wetting (i.e., philic) a capillary surface, the capillary ascent phenomenon will cause the boiling (i.e., violent desorption) of the liquid condensed within the capillary to occur at higher temperature than its boiling point in the free state at normal atmospheric pressure. Obviously, if a liquid (adsorbate) is nonimmersionally wetting (i.e., phobic) a capillary surface, the capillary depression phenomenon will cause the boiling temperature of the liquid condensed within the capillary to be lower than its boiling point under normal conditions. Thus the

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AT value is positive if the micropore surface of the zeolite attracts the organic molecule. The AT value is negative if the micropore surface of the zeolite is phobic with the adsorbate. AT Value of Organics. The AT values, the adsorption amount (Aw, wt %) and the loading of the unit cell (Au) of organics are listed in Table 1. 1.The AT Values of Alkylamines and n-Alcohols. The AT values of alkylamines on siliceous FER are 143, 117, 94, and 42 for methylamine, ethylamine, n-propylamine, and n-butylamine, respectively. This suggests that the hydrogen bond formed with oxygen atoms on the zeolite framework is stronger than that formed with nitrogen atoms of the amine molecules themselves. With increasing molecular weight (MW), the AT values decrease from 143 to 42. The phenomenon is probably caused by the increase of the dispersion forces, which bring about the enhancement of the interaction between the alkylamine molecules themselves with the MW increase. By contrast, the AT values are distinctly lower for methanol, ethanol, 1-propanol, and 1-butanol, which are -13, 12, 28, and 7, respectively. It seems that the phobic character of the interaction between the -OH groups of alcohols and oxygen atoms on the framework of the zeolite causes the low AT values. In fact, the AT values (see Table 1) of alkylamines are much higher than those of alcohols, indicating the strong interactions between the molecules of amines and the framework or/and the strong interaction among amine molecules themselves in the channel of the zeolite. Otherwise, the higher values of Tb for alcohols caused by the hydrogen bond between alcohol molecules in the liquid state must be considered as well. 2. Normal Alkanes. The AT values of n-butane, n-pentane, n-hexane, and n-heptane are 87, 41, 55, and 51, respectively. The higher AT values for alkanes show that the interaction between the alkanes and O2- of the framework is much stronger than that among the molecules of the adsorbate themselves in liquid state, which is dominated by dispersion forces. The phenomenon can be explained as follows: The geometry of normal alkanes can match well with the construction of the framework based on the close bond angle (all about 109°) of C-C-C, H-C-H, and O-Si-O and the close bond distance (all about 1.5-1.6 nm)12,13 of Si-O and C-C. The normal alkane can freely rotate along the C-C axis. It provides a greater opportunity to form a strong interaction between the mass hydrogen atoms of normal alkanes and O2- on the framework of the zeolite by a sub-hydrogen bond. The AT value of n-butane is distinctly higher than that of other alkanes, showing a stronger interaction between butane and the framework, which is probably caused by the higher ratio of H/C in the molecule of butane. The stronger interaction is further confirmed by the endothermic effect shown on the DTA curve of butane. Meanwhile, the AT values of alkanes exhibit a periodic fluctuation, which is probably caused by the change of the periodic configuration with the increase of carbon numbers of the molecule adsorbed in the channel. If the number is even, the probable configuration of the molecules is in the form of /\/ or /\/\/. If the number is odd, the configuration is in the form of /\/\ or /\/\/\ The affinity of the alkane molecule to the framework may be stronger in the former situation, leading to higher AT value. 3. Oxygen-Containing Organics. The AT values of acetone and diethyl ether are 64 and 42, respectively. The oxygen atom in these organics exhibits an electrondrawing effect, resulting in a stronger interaction between the hydrogen atoms connected with the carbon atoms of

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Table 1. Adsorption and AT Values on Siliceous FER Zeolite

a

guest molecule

M

Aw (wt %)

*Au (uc)

Tb(bp) (°C)

Td(weight loss peak) (°C)

AT (Td - Tb)

n-butanea n-pentane n-hexane n-heptane methanol ethanol 1-propanol 1-butanol methylamine ethylamine n-propylamine n-butylamine formic acid acetic acid formaldehyde acetaldehyde diethyl ether acetone methyl formate ethyl formate

58.12 72.15 86.18 100.21 32.04 46.07 60.10 74.12 31.08 45.08 59.11 73.14 46.03 60.05 30.03 44.05 74.12 58.08 60.05 74.08

5.1 6.4 5.0 7.7 5.3 6.8 4.2 5.1 7.2 5.0 3.1 3.3 12.2 10.0 6.2 6.2 6.2 4.5 7.1 9.5

1.90 1.92 1.25 1.66 3.58 3.20 1.51 1.49 5.01 2.40 1.13 0.98 5.73 3.60 4.47 3.06 1.81 1.69 2.56 2.78

0 36 69 98 64 78 97 118 -6 16 49 77 101 118 -20 20 35 56 32 54

87 78 124 149 51 90 125 125 85, 137 133 143 119 117 152 113 78 77 120 96 123

87 42 55 51 -13 12 28 7 143 117 94 42 16 34 133 58 42 64 64 69

Measured at a pressure of 4000 Pa.

the adsorbed molecule and O2- of the framework by a sub-hydrogen bond. The effect results in relatively high AT values of the compounds. The AT values of methyl formate and ethyl formate are 64 and 69, respectively, indicating strong interaction between the organics and the framework. The AT values of formic acid and acetic acid are 16 and 34, respectively, since carboxylic groups exist in the molecules, leading to the lower AT value. A strong and a weak endothermic peak are seen on the DTA curves for the organic acids (see Figure 11). The strong endothermic effect may be produced by a dimer formed in the channel of the zeolite due to hydrogen bonding between the molecules of formic acid. Apparently, the effect of the carboxylic group weakens with increase of the MW. The AT values of formaldehyde and acetaldehyde are 133 and 58, respectively. Only an endothermic peak is seen on the DTA curve of formaldehyde. This indicates a strong interaction between formaldehyde and O2- of the framework with a sub-hydrogen bond. Perhaps oligomerization occurs in the channels. As far as acetaldehyde is concerned, the interaction between the molecule and the framework becomes weak perhaps due to a steric effect and lower polarity of the compound. The AT values of some organics with close MW are quite different due to various functional groups in the molecules. For example, the MW of 1-propanol, acetic acid, and methyl formate is about 60. The AT values are 28, 34, and 64 for 1-propanol with hydroxyl group, acetic acid with carboxyl group, and methyl formate with methoxycarbonyl group, respectively. Obviously, the AT values of the compounds increase with increasing affinity of the functional group with O2- on the framework. As another example, the MW of 1-butanol, diethyl ether, and ethyl formate is about 74, and the AT values are 7, 42, and 69, respectively. The affinity of the functional group to the framework increases in the order of hydroxyl, ether link and ethoxycarbonyl, leading to increasing the AT value in the same order. The polarity of diethyl ether is weak. The polarity interaction between the molecules of diethyl ether is negligible in the liquid state, leading to a Tb close to that of n-pentane with a MW close to it. The interaction between the framework and the molecules of diethyl ether or of n-pentane in the channel is dominated by dispersion forces. Consequently, AT values for both compounds are the same. Comparison with High Silica MFI Zeolite and High Silica FAU Zeolite. Some data of adsorption and

Table 2. Comparison of Adsorption/Desorption Properties of High Silica FER, MFI, and FAU Zeolite siliceous FER

silicalite-113

high silica FAU14

guest molecule n/1000T

AT

n/1000T

AT

n/1000T

AT

n-hexane ethanol 1-propanol methylamine ethylamine formic acid acetic acid acetone

55 12 28 143 117 16 34 64

88.5 155.2 129.2 206.6 162.5 291.7 182.3 130.2

58 -4 13 160 127 -15 10 39

94.8 163.0 135.9 407.9 177.1 311.5 188.1 147.9

51 -15 -12 160 150 -16 -2 15

39.9 88.7 42.0 139.2 66.6 159.2 127.1 46.9

the AT values on high silica FAU, MFI, and siliceous FER zeolite are listed in Table 2. FAU zeolite is a connected cages system with a 12 oxygen member windows of 0.74 nm and 0.3 cm3/g pore volume. MFI zeolite possesses 10 oxygen member ring straight channels of 0.56 × 0.53 nm and 10 oxygen member ring zigzag channels of 0.51 × 0.55 nm. The pore volume is about 0.19 cm3/g.1 The siliceous FER possesses a 10 oxygen member ring elliptical straight channel and a 8 oxygen member ring straight channel with pore sizes of 0.54 × 0.44 nm and 0.30 × 0.46 nm, respectively.12 The pore volume is about 0.15 cm3/g. Most organic adsorbates are adsorbed in the 10 ring channel of FER zeolite.5,8,9 The adsorption capacity per 100 g and the adsorption density per 1000T14 of FER are less than those of FAU and MFI zeolite for all adsorbates. The AT values on these high silica zeolites with different topology exhibit completely the same tendency with respect to organics with the same functional group and various MW. Evidently, two kinds of functional groups play a role: phobic and philic (attractive) to Si-O framework of the zeolites. A molecule containing an alkyl or amino group shows strong affinity to the Si-O framework, leading to higher a AT value. For example, the AT values of alkanes and alkylamines are in the range of 40-90 and 40-150. Meanwhile, a molecule containing a phobic group such as hydroxyl or carboxylic acid group shows weaker affinity to the framework, leading to lower AT value. Thus, the AT values of alcohols and organic acids are in the range of -16 to 34. It seems that the pore size of the zeolite also influences the AT value. The effect of the functional group in the adsorbed molecule on the AT value becomes weaker with the decrease of the pore size. When the pore size decreases,

Organics on Zeolite

the AT value increases for the molecule containing the frameworkphobic group and the AT value decreases for the molecule containing the frameworkphilic group. For example, the AT value of ethylamine is 150, 127, and 117, and that of ethanol is -15, -4, and 12 on FAU, MFI, and FER zeolite, respectively. The picture is different for n-alkanes. For example, the AT values for n-hexane are 55, 58, and 51 on FER, MFI, and FAU zeolite, respectively. The AT value is in the order of AT(MFI) > AT(FER) > AT(FAU). This phenomenon is probably explained as follows: MFI zeolite possesses a zigzag channel which is commensurate with the nalkanes.17,18 The configuration of zigzag channel in MFI zeolite plays an essential role in “locking” the fluid into a specific structure.17 This spatial effect leads to the increase of the AT values. Conclusion 1. Siliceous FER zeolite possesses a perfect structure and high hydrophobic/organophilic character on adsorption/desorption. (17) Smit, B.; Maesen, T. L. M. Nature 1995, 374, 42. (18) van Well, W. J. M.; Wolthuizen, J. P.; Smit, B.; van Hooff, J. H. C.; van Santen, R. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 2543.

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2. Alkyl and amino groups in the adsorbed organic compounds show a frameworkphilic character, whereas hydroxyl and carboxylic acid groups show a frameworkphobic character on siliceous FER zeolite. 3. The AT value, the affinity index, exhibits periodic fluctuation for n-alkanes with increasing number of carbons. 4. The AT values of siliceous FER, high silica MFI, and FAU zeolite exhibit almost the same tendency to organics with the same functional group and varying molecular weights. 5. The effect of functional group of the adsorbed molecule on the AT value changes with the variation of the pore size in the zeolite. With the pore size decrease, the AT value increases for the organic molecule with a frameworkphobic functional group and the AT value decreases for the organic molecules with a frameworkphilic functional group. Acknowledgment. This work is supported by National Natural Science Foundation of China (Project 20073010) and the Doctoral Foundation of the State Education Ministry. LA000835N