Sorbate Interaction of Organics on

Jul 23, 1997 - Silicalite has a strong affinity for some organic compounds with high polarity and is usually phobic to organics with hydroxyl groups. ...
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Langmuir 1997, 13, 4094-4101

Sorbate/Framework and Sorbate/Sorbate Interaction of Organics on Siliceous MFI Type Zeolite Ying-cai Long,* Hui-wen Jiang, and Hong Zeng Department of Chemistry, Fudan University, Shanghai 200433, People’s Republic of China Received October 18, 1996. In Final Form: May 6, 1997X Thermal analyses were used to study the sorbate/framework and sorbate/sorbate interaction on siliceous MFI type zeolite. Alkanes, aromatics, alkyl alcohols, multivalue hydroxyl alcohols, alkylamines, organic acids, acetaldehyde, ethyl ether, acetone, and ethyl acetate were investigated as the adsorbate. The adsorption and desorption behavior of high silica MFI type zeolite is greatly influenced by the presence of framework aluminum, cations, and silanol defects. There is a perfect Si-O-Si micropore surface without cations and silanol defects in the high hydrophobic zeolite silicalite. The essential host/guest interaction can be clearly studied by the thermal desorption behavior from silicalite. A general hydrophobic/organophilic property of 27 organic compounds with different functional groups on silicalite has been compared in terms of the affinity index AT determined by DTG. The physical meaning of the AT value, and the nature of the host/guest interaction are discussed. Silicalite has a strong affinity for some organic compounds with high polarity and is usually phobic to organics with hydroxyl groups. The special sorbate/framework and sorbate/sorbate interactions for certain organics can be identified from the double peaks in the DTG curves and thermal effects in the DTA curves. The formation of the double-molecule complexes of p-xylene, toluene, phenol, ethylene glycol, and ethylamine in silicalite is revealed.

Introduction MFI type zeolite is a hydrophobic zeolite and has wide applications in catalysis and separation. There are many publications related to its structure, adsorption, and desorption. Among the adsorbates usually investigated, hexane and other n-alkanes, and aromatics such as benzene, toluene, and p-xylene are the most common.1-7 Other adsorbates observed are 3-methylpentane,2 1,2,4trimethylbenzene,3 some simply substituted benzenes,5 n-butyne,8 Freon-12, NH3, CO2, propylene,9 and SO2.9,10 Several organics with various polarities, such as bromobenzene, pyridine, acetonitrile,11 methanol, ethanol, formic acid, acetic acid, ethyl acetate, ethyl ether, acetaldehyde, tetrachloromethane, trichloromethane, monochlorobenzene, and carbon disulfide12 are studied as well. The study on the adsorption of methanol, ethanol, propanol, and butanol from aqueous solution was used for extraction.13,14 Water sorption by the zeolite has been shown to vary linearly with aluminum content.1 The saturated sorption * To whom correspondence should be addressed. E-mail address: [email protected]. Fax: (+86)-21-6549-3232. X Abstract published in Advance ACS Abstracts, June 15, 1997. (1) Olson, D. H.; Haag, W. O.; Lago, R. W. J. Catal. 1980, 61, 390. (2) Olson, D. H.; Kokotallo, G. T.; Lawton, S. L.; Meier, W. M. J. Phys. Chem. 1981, 85, 2238. (3) Ma, Y. H.; Tong, T. D.; Sand, L. B.; Hou, L. Y. New Developments in Zeolite Science Technology, Proceedings of the 7th International Zeolite Conference; Murakami, Y., lijima, A., Ward, J. W., Eds.; Kadansha Ltd.; Tokyo, Japan, 1986; p 531. (4) Ruthven, D. M.; Eic, M.; Richard, E. Zeolites 1991, 11, 647. (5) Hill, S. G.; Seddon, D. Zeolites 1991, 11, 699. (6) Pope, C. G. J. Phys. Chem. 1986, 90, 835. (7) Karsli, H.; Gulfaz, A.; Yucel, H. Zeolites 1992, 12, 728. (8) Shen, D.; Rees, L. V. C. Zeolites 1991, 11, 684. (9) Long, Y.-C.; Yang, G.-R.; Sun, Y.-J. Petrochem. Technol. 1994, 23 (12), 786 (in Chinese). (10) Gollakota, S. V.; Chriswell, C. D. Ind. Eng. Chem. Res. 1988, 27, 139. (11) Thamm, H. J. Phys. Chem. 1988, 92 (1), 193. (12) Long, Y.-C.; Yang G.-R.; Sun, Y.-J. Petrochem. Technol. 1995, 24 (1), 9 (in Chinese). (13) Haegh, G. S. In Zeolites: Synthesis, Structure, Technology and Application, Studies in Surface Science and Catalysis; Drzaj, B., et al., Eds.; Elsevier: Amsterdan, Oxford, New York, Tokyo, 1985; Vol. 24, p 605. (14) Milestone, N. B.; Bibby, D. M. J. Chem. Technol. Biotechnol. 1981, 31, 732.

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capacities of hydrocarbons are different in Al-free ZSM-5, Na-ZSM-5, and K-ZSM-53 and are greatly influenced by zeolite crystal size and aluminum content.5,15 The silanol groups in silicalite increase the heat of sorption of 2-butyne 10 KJ/mol in comparison with the heat of sorption of n-butane.8 It was found that the solid phase changes from 4 to 6.5 molecules of p-xylene adsorbed per unit cell of ZSM-5.2 An unusual hysteresis in the adsorption isotherm of p-xylene is caused by unusual sorbent/sorbate or sorbate/ sorbate interaction.6 The formation of a bromobenzene association was found by calorimetric determination.11 The interaction energies of silicalite with methane, methanol, and water have been calculated.16 Molecular dynamics simulation of propane and methane in silicalite indicated both adsorbate/silicalite and adsorbate/adsorbate interactions.17 The sorption energies in both n-alkane-silicalite and p-xylene-silicalite systems have also been calculated by computer simulations.18,19 The most important work was the determination of the location of p-xylene in a single crystal of zeolite H-ZSM-5.20 The nature of the interaction forces between the methyl H atom in a xylene and the aromatic ring atoms in a neighbor xylene was revealed. XRD and NMR studies displayed the inversible symmetry transition of siliceous MFI zeolite at low loadings of p-xylene, benzene, pyridine, and acetylacetone.21-23 The unusual features on sorption isotherms and on TPD(TG)/DTG curves of n-hexane and n-heptane were found to be caused by a commensurate freezing in silicalite and a kind of phase transition taking place.24,25 The hydrophobic/organophilic character is based on the interaction between the guest molecules adsorbed and (15) Doelle, H.-J.; Heering, J.; Riekert, L.; Marosi, L. J. Catal. 1981, 71, 27. (16) Vigne-Maeder, F.; Auroux, A. J. Phys. Chem. 1990, 94, 316. (17) Nicholas, J. B.; Trouw, F. R.; Mertz, J. E.; Iton, L. E.; Hopfinger, A. J. J. Phys. Chem. 1993, 97, 4149. (18) Mentzen, B. F. Mater. Res. Bull. 1995, 30 (11), 1333. (19) Mentzen, B. F.; Lefebvre, F. Mater. Res. Bull. 1995, 30 (5), 613. (20) Koningsveld, H. van; Tuinstra, F.; Bekkum, H. van; Jansen, J. C. Acta Crystallogr. 1989, B45, 423. (21) Fyfe, C. A.; Kennedy, G. J.; De Schutter, C. T.; Kokotailo, G. T. J. Chem. Soc., Chem. Commun. 1984, 541. (22) Portsmouth, R. L.; Gladden, L. F. J. Chem. Soc., Chem. Commun. 1992, 512. (23) Mentzen, B. F.; Gelin, P. Mater. Res. Bull. 1995, 30 (3), 373. (24) Smit, B.; Maesen, T. L. M. Nature 1995, 374 (2), 42.

© 1997 American Chemical Society

Sorbate/Framework and Sorbate/Sorbate Interaction

the zeolite framework. Hydrophobicity (h) has been defined as the ratio of the weight loss at 150 °C to the weight loss at 400 °C for a dealuminated zeolite Na-Y.26 A hydrophobicity index (HI) has been introduced. It was defined as Xtoluene/Xwater where X is the loading, i.e., the mass of adsorbed compound per mass of dry adsorbent (such as ZSM-5, silicalite-1, and zeolite Y). It can be used to compare the hydrophobic properties and the affinities for hydrocarbons.27,28 Actually the organic compounds with various functional groups can be adsorbed onto a silicon-rich zeolite with very different affinity.14,29 The sorption and desorption behavior is strongly influenced by a small amount of cations and defects. The hydrophobicity (h) and the hydrophobicity index (HI) cannot be used to compare the affinity for some organic compounds with different functional groups on silicalite-1, which possesses a perfect framework and Si-O-Si surface without defects of Si(1Al) and Si-OH. It is also difficult to use them to investigate the sorbate/framework and sorbate/sorbate interactions. A term of organophilicity is too simple to generalize the sorption and desorption character of silicalite. It is necessary to introduce a new concept for characterizing the hydrophobic/organophilic property. The new affinity index AT has been recently defined as AT ) Td - Tb, where Td is the temperature of the weight loss peak of the DTG curve and Tb is the boiling point of a certain adsorbate.30-32 The index is convenient for comparing the degree of the host/guest interaction. There are numerous applications of silicalite based on its hydrophobic/organophilic character. It can be used as an adsorbent and a material of a zeolite membrane in the following process: (1) separation of isomers; (2) elimination of organic contamination in environment protection; (3) extraction of ferment products. Desorption is the control step in the process of separation. The affinity index of organics can be used to estimate the operation temperature and to choose the desorbent in process 1. The organics have priority to be adsorbed into the zeolite micropore system of silicalite from waste water and waste gas, or from ferment solution in process 2 and process 3. The operations of desorption from a zeolite adsorbent or pervaporation from a zeolite membrane are usually in a heated flow of air or in vacuum. The AT value will provide an approximate temperature of desorption in the process of adsorption separation, and an operation temperature in the process of membrane separation. In this paper we will report the different behaviors of sorption and desorption, and the related microstructural characteristics of silicon rich MFI type zeolite. The index AT will be used to compare the affinity order of 27 organics. The physical meaning of the AT value, the general host/ guest interaction, and its nature will be discussed. The (25) Well, W. J. M. van; Wolthuizen, J. P.; Smit, B.; Hooff, J. H. C. van; Santen, R. A. van. Angew. Chem., Int. Ed. Engl. 1995, 34 (22), 2543. (26) Anderson, M. W.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1449. (27) Berke, C. H.; Kiss, A. Chem.-Ing.-Tech. 1991, 63, 623. (28) Weitkamp, J.; Kleinschmit, P.; Kiss, A. In Proceedings of the 9th International Zeolite Conference, Montreal 1992; Von Ballmoos, R., et al., Eds.; Butterworth-Heinemann: Woburn, MA, 1993; Vol. VII, p 79. (29) Milestone, N. B.; Bibby, D. M. J. Chem. Technol. Biotechnol. 1983, 34A, 73. (30) Long, Y.-C.; Jiang, H.-W.; Zeng, H. J. Fudan Univ. 1994, 33 (1), 101. (31) Long, Y.-C.; Jiang, H.-W.; Zeng, H. Acta Chim. Sin. 1996, 54, 545 (in Chinese). (32) Long, Y.-C.; Jiang, H.-W.; Zeng, H. Progress in Zeolite and Microporous Materials, Studies in Surface Science and Catalysis; Proceedings of the 11th International Zeolite Conference; Chon, H., et al., Eds.; Elsevier Science B.V.: Amsterdam, 1997; Vol. 105, p 787.

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special sorbate/framework and sorbate/sorbate interactions for some organics will be discussed as well. Experimental Section Materials. Zeolite samples used in this study were hydrothermally synthesized in an ethylamine/Na2O/SiO2/H2O system using water glass as a source of silicon.33 The molar ratios of the reactant were 0.01 Na2O/1.0 ethylamine/1.0 SiO2/15 H2O. The reactant was sealed in a 2 L stainless steel autoclave and hydrothermally reacted at 180 °C for 18 h without stirring. The as-synthesized zeolite product was washed, filtered, dried with an IR lamp, and then calcined in an oven at about 600 °C for 2 h to remove the organic template. The detemplated sample was exchanged with 0.5 N NaCl solution at 95 °C for 4 h and washed, filtered, and then dried to get sample A (Na-ZSM-5). Sample A was treated with 0.5 N HCl solution at 95 °C for 4 h and washed, filtered, dried, and calcined at 550 °C for 2 h to get sample B (H-ZSM-5). After treatment with HCl solution, the content of Na2O in sample B decreases to 0.03 wt %. Sample B was further dealuminated at 800 °C for 100 h in a flow of air saturated with water steam in a quartz tube to get sample C (silicalite). The flow rate of water vapor was controlled at 105 ( 5 mL/h with a micropump. Characterization of Samples. The microphoto of the assynthesized MFI zeolite sample was taken with a Shimadzu S-520 scanning electron microscope. XRD patterns were collected at room temperature using a D-MAX/II A X-ray powder diffractometer with Cu KR radiation in the 2θ range 5-45° at a scanning speed of 8°(2θ)/min and in the 2θ ranges 24-25°, 29-30°, and 48-49° at a scanning speed of 0.5°(2θ)/min. The IR spectra were run on a NICOLET FT-IR-50X IR spectrometer in the scanning range 400-1600 cm-1 for a sample of KBr disks and in the scanning range 3200-4000 cm-1 for a sample of self-supported disks. 29Si MAS NMR spectra were recorded at room temperature using a Bruker MSL-300 spectrometer. The 29Si resonance frequency used was 59.595 MHz, and the rotor was spun at 3.0 KHz. The radio frequency field was 37.0 KHz, corresponding to a π/3 pulse width of 4.5 µs, and the recycle time was 2 s. Q8M8 was used as a second reference for the 29Si chemical shift. The adsorption isotherms in the vapor of n-hexane, benzene, p-xylene, and water were measured at 25 °C with a Sartorius-7012 supermicroelectron balance in vacuum. The specific surface area and the pore volume were determined with N2 adsorption isotherms at 77.35 K using a Micromeritric ASAP 2000 instrument. Adsorption and Determination of the AT Value. Reagents used as adsorbate were chromatographic grade or analytically pure. The zeolite sample tested was previously dehydrated in vacuum at 550 °C for 2 h. Adsorption in vapor of a guest compound was carried out in a fixed bed of the zeolite sample at room temperature for more than 24 h in order to fully saturate the sample. When the guest compound was a solid at room temperature, the adsorption was done at higher temperature to get vapor from the melt. TG/DTG/DTA measurements were carried out by using a PTC-10A thermal analyzer with an air flow of 70 mL/min at a rate of 5 °C/min from room temperature to 700 °C. About 10 mg of a zeolite sample were used in a test. The sensitivities of TGA and DTA used were 0.01 mg and (25 µV, respectively. The adsorption data Aw and Au, which denote the percent loading by weight (wt %) and the molecular number loaded per unit cell, respectively, were calculated from TGA. Td data were obtained from the temperature of the weight loss peak in the DTG curves. Tb is the boiling point of the guest compounds under standard conditions and can be found in any handbook of physical chemistry. If there are two weight loss peaks appearing in a DTG curve, the higher one is taken as Td to calculate the AT value.

Results and Discussion Structural Properties of Samples. Scanning electron microscopy (SEM) (see Figure 1) shows that the assynthesized MFI zeolites are in a prismatic form of single crystals with a size of 6 × 15 µm2. The XRD patterns (see Figure 2) indicate that the samples are a pure phase of (33) Long, Y.-C.; Sun, Y.-J.; Wu, T.-L.; Wang, L.-P.; Qian, M.; Fei, L. CN application No. 92 1 13807.5 (1992) (in Chinese).

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Figure 1. SEM photo of as-synthesized MFI type zeolite.

Figure 3. 29Si MAS NMR spectra of (A) Na-ZSM-5, (B) H-ZSM5, and (C) silicalite.

Figure 2. XRD patterns of (A) Na-ZSM-5, (B) H-ZSM-5, and (C) silicalite.

MFI type zeolite with a high crystallinity. Sample A(NaZSM-5) and sample B(H-ZSM-5) possess orthorhombic symmetry. Sample C belongs to monoclinic symmetry according to the diffraction peak doublets at 24.4°, 29.2°, and 48.6°/2θ. 29Si MAS NMR spectra show that the SiOH peak at -103 ppm and the Si(1Al) peak at -106 ppm, which appear in the patterns of sample A and sample B, disappear in the pattern of sample C (see Figure 3). The small amount of framework aluminum in sample A and sample B came from the silica source as an impurity in the water glass. The 29Si resonance with high resolution indicates a symmetry transition from orthorhombic to monoclinic, accompanied by a perfection of the framework upon steam treatment (see Figure 3C). In the process of preparing sample C, the dealuminum and desilanol effect of steam treatment can also be detected by the FT-IR

spectra. A [TO4] external asymmetrical stretching vibration at 1226 cm-1 and an internal asymmetrical stretching vibration at 1095 cm-1 move to higher frequencies (see Figure 4). At the same time the vibrations of silanol groups at about 3730 and 3450 cm-1 disappear in the spectra of sample C (see Figure 5). 27Al MAS NMR, 29Si MAS NMR, XRD, FT-IR, and water adsorption were used to investigate the aluminum state of high silica H-ZSM-5 after treatment with steam at high temperature. It has been found that the newly formed extraframework aluminum species is in a form of Si-O-Al dispersed on the outer surface of the zeolite.34 The porous system of the zeolite is still open. All these facts indicate that sample C possesses a perfect crystalline structure, whose framework is constructed by Si-O-Si without Si(1Al) and Si-OH defects. This is the typical character of silicalite-1, a member of the MFI family of zeolites.1,35-38 The n-hexane sorption volume of sample C is 0.177 mL/g (∼7.8 molecules/unit cell) at P/P0 ) 0.2 and 0.182 mL/g (∼8.0 molecules/unit cell) at P/P0 ) 0.5, measured by a sorption isotherm at 25 °C. The benzene and p-xylene sorptions of sample C are 0.121 mL/g (∼7.9 molecules/unit cell) and 0.166 mL/g (∼7.8 molecules/unit cell) at P/P0 ) 0.2, respectively.12 The water sorption is 6.7 wt % for sample B and 0.4 wt % for sample C at P/P0 ) 0.8, determined by sorption isotherms. It indicates that sample C is highly hydrophobic. The BET surface areas (34) Long, Y.-C.; Jin, M.-Y.; Sun, Y.-J.; Wu, T.-L.; Wang, L.-P.; Fei, L. J. Chem. Soc., Faraday Trans. 1996, 92 (9), 1647. (35) Sun, Y.-J.; Huang, Y.-F.; Wu, T.-L.; Wang, L.-P.; Fei, L.; Yang, H.; Long, Y.-C. Acta Chim. Sin. 1994, 52, 573 (in Chinese). (36) Flanigen, E. M.; Patton, R. L. USP, 4073 565 (1978). (37) Fify, C. A.; O’Brien, J. H.; Strobl, H. Nature 1987, 326 (19), 281. (38) Wu, E. L.; Lawton, S. L.; Olson, D. H.; Rohrman, A. C., Jr.; Kokotallo, G. T. J. Phys. Chem. 1979, 83 (21), 2777.

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Table 1. Structural Defects and Adsorption of Zeolite ZSM-5 and Silicalite sample

Si(1Al) (%) (-106 ppm)

Si-OH (%) (-103 ppm)

SiO2/Al2O3

toluene

Na-ZSM-5 H-ZSM-5 silicalite-1

5.2 3.1 0

6.2 8.2 0

152 256 ∞

11.3 12.4 16.4

adsorption (mL/100 g) p-xylene benzene ethylbenzene 14.2 15.8 16.6

11.1 11.9 12.2

11.5 12.5 11.8

H 2O 5.0 8.2 0.6

Figure 6. TGA/DTG/DTA spectra of aromatics on H-ZSM-5 (A) and Na-ZSM-5 (B): (a) benzene, (b) toluene, (c) ethylbenzene, (d) p-xylene. (- - -) TG; (‚‚‚) DTA; (s) DTG; (V) endothermal effect (these symbols have the same meaning in following figures). Figure 4. FT-IR spectra for framework vibration of (A) NaZSM-5, (B) H-ZSM-5, and (C) silicalite.

Figure 5. FT-IR spectra for Si-OH vibration of (A) Na-ZSM5, (B) H-ZSM-5, and (C) silicalite.

of samples B and C are 376.0 and 380.2 m2/g, respectively, measured by N2 sorption at 77 K. The Langmuir surface area is 500.8 m2/g for sample B and 532.3 m2/g for sample C. The pore volume is 0.1851 mL/g for sample B and 0.1838 mL/g for sample C, calculated from the N2 sorption isotherm at P/P0 ) 0.9744. These sorption data indicate that samples B and C have an ideal porous system,

consistent with references.1,2,7,25,39 The crystallinity is 100%. The quality of these samples is good enough for the investigation in this study. Structural Defects and Adsorption. Table 1 tabulates the contents of Si(1Al) and Si-OH in the Gaussian line shape fits by computer from 29Si MAS NMR spectra, and the sorption of water and aromatics measured from TGA/DTG/DTA curves (see Figures 6 and 7). The sorption amounts of benzene, p-xylene, and water on silicalite are almost the same as the former data determined from isotherms. Evidently, the adsorption of aromatics increases and the adsorption of water decreases as the SiO2/ Al2O3 ratio increases. The hydrophobic/organophilic property greatly changes with the contents of aluminum and silanol defects. Silicalite (sample C) has the best hydrophobicity. Thermal Desorption Behavior of Aromatics. The weight loss continues up to 600 °C with a tailing phenomenon in the TGA curves of Na-ZSM-5 and H-ZSM-5 shown in Figure 6. It is reasonable to believe that the silanol defects and cations act as a strong electrostatic sorption center in H-ZSM-5 and Na-ZSM-5. There is an exothermic effect in the DTA curves in the temperature range 200-600 °C due to a catalytic effect with desorption of aromatics from these samples. The exothermic peaks appear at about 300 °C for ethylbenzene-Na-ZSM-5, p-xylene-Na-ZSM-5, and p-xylene-H-ZSM-5 as well. On (39) Suzuki, I.; Nambq, S.; Yashima, T. J. Catal. 1983, 81, 485.

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Figure 8. TGA/DTG/DTA spectra of alkylamines on silicalite.

Figure 7. TGA/DTG/DTA spectra of hydrocarbons on silicalite.

the contrary, the temperatures for complete desorption of these compounds from silicalite are lower than 300 °C, and the desorption occurs in a narrow region without catalytic effect (see Figure 7). So far no exothermic effect in the DTA curves is found. On the other hand, a difference is seen in the DTG curves for toluene and ethylbenzene. There are two peaks in the DTG curves of toluene-HZSM-5 and toluene-silicalite. Only one peak appears in the DTG curves of toluene-Na-ZSM-5. In the case of ethylbenzene, one peak occurs in the curve of silicalite, and two peaks appear in the curves of Na-ZSM-5 and H-ZSM-5. These facts indicate that the cations and silanol groups also strongly influence the desorption behavior based upon different host/guest interactions. The hydrophobicity of the siliceous zeolite is based on the absence of exchangeable cations and the substitution of aluminum atoms in the framework by silicon atoms. The influence of the structural defects on the hydrophobicity is not negligible. The hydrophobic/organophilic property is a character of the siliceous zeolite and is a generalization of the difference between organics and water upon adsorption and desorption. It is a reflection of an interaction between the micropore surface of the zeolite and certain organic molecules. In order to reveal the nature of the interaction, it is necessary to exclude an influence of cations and the structural defect of Si-OH. The experiments above indicate that the behavior of thermal desorption from silicalite is an essential reflection of the interaction between the perfect surface and the adsorbate, since there is no influence. That is why we used silicalite (sample C) to determine the AT values and to investigate the sorbate/ framework and sorbate/sorbate interactions. The AT

values determined in air flow are therefore typical and can be used in separation practice. Physical Meaning of AT Values. Considering that there are no strong electrostatic sorption centers on the micropore surface of the perfect silicalite framework, the adsorption of organic molecules can approximately be treated as a physical capillary condensation. If a liquid is immersionally wetting (i.e., philic) a capillary surface, the capillary ascent phenomenon will cause the boiling (i.e., violent desorption) temperature of the liquid condensed within the capillary to be higher than its boiling point in the free state at normal atmospheric pressure. Obviously, if a liquid is nonimmersionally wetting (i.e., phobic) a capillary surface, the capillary depression phenomenon will cause the boiling temperature of the liquid within the capillary to be lower than its boiling point under normal conditions. It is reasonable to believe that the AT value is positive if the micropore surface of silicalite is playing a role in the philicity with the organic molecule. The AT value is negative if the micropore surface is phobic with the adsorbate. No unit for the AT values is required while comparing the philicity or the phobicity of the interaction. Thermal Effect on Desorption of Organics from Silicalite. Typical TGA/DTG/DTA patterns of aromatics, n-alkanes, alkylamines, alkyl alcohols, multivalue hydroxyl alcohols, and other organics with an oxygen atom are shown in Figures 7-10. The temperatures for complete desorption of these compounds are lower than 300 °C. There is no significant thermal effect on desorption for most hydrocarbons. An endothermic effect can be found from the DTA curves of silicalite for p-xylene, alkyl alcohols, multivalue hydroxyl alcohols, alkylamines, and formic acid, respectively. AT Value of Organics. The AT values and the adsorption data of organics are listed in Table 2. The AT values are 60-90 for the saturated hydrocarbons, which indicates the organophilic property of silicalite. The AT values are 6-13 for benzene, toluene, ethylbenzene, and p-xylene but -91 for naphthalene. This fact indicates that silicalite has weaker affinity with the unsaturated

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Table 2. Adsorption and AT Values of Zeolite Silicalite guest molecule

M

Aw (wt %)

Au/unit cell

molecule size (nm)

Tb (bp) (°C)

n-pentane n-hexane n-heptane benzene toluene ethylbenzene p-xylene naphthalene ethanol 1-propanol 1-pentanol phenol ethylene glycol 1,2-propanediol 1,3-propanediol glycerol methylamine ethylamine n-propylamine pentylamine n-hexylamine formic acid acetic acid acetaldehyde ethyl ether acetone ethyl acetate

72.15 86.18 100.21 78.12 92.15 106.17 106.17 128.19 46.07 60.11 88.15 94.11 62.07 76.11 76.11 92.11 31.06 45.07 59.11 87.11 101.19 46.03 60.05 44.05 74.12 58.08 88.10

10.40 12.77 11.30 10.70 14.21 10.20 14.29 6.80 11.93 12.90 14.08 12.20 15.90 13.04 14.44 6.30 10.68 12.23 11.11 13.90 13.51 22.33 18.23 13.46 11.03 12.57 11.48

8.3 8.5 6.5 7.9 8.9 5.5 7.8 3.1 14.9 12.4 9.2 7.5 14.8 9.9 10.9 4.0 19.8 15.6 10.8 9.1 7.7 28.0 17.5 17.6 8.6 12.5 7.5

0.90 1.03 1.16 0.58 0.91 1.03 1.32 1.16 0.69 0.82 1.07 0.84 0.85 0.82 0.99 0.99 0.50 0.63 0.75 1.10 1.13 0.36 0.69 0.44 0.85 0.62 1.18

36.1 69 98.4 80.1 110.6 136.2 138.3 218 78.5 97.4 137.3 181.7 198.9 189 213.5 290 -6.3 16.6 47.8 104.4 130 100.7 117.9 20.8 34.5 56.2 77.06

Td (weight loss peak) (°C) 79, 63, 32, 94,

78, 104,

72, 107,

103 127 185 90 124 149 144 127 75 110 141 144 144 134 166 193 154 144 94 121 194 86 128 137 87 95 138

AT (Td - Tb) 67 58 87 10 13 13 6 -91 -4 13 4 -38 -55 -55 -48 -97 160 127 46 15 64 -15 10 116 53 39 61

Figure 10. TGA/DTG/DTA spectra of organics with an oxygen atom.

Figure 9. TGA/DTG/DTA spectra of alcohols, phenol, and multivalue hydroxyl alcohols on silicalite.

hydrocarbons than with the saturated hydrocarbons. The AT value of -4 for ethanol increases to 13 for 1-propanol. The AT value decreases by about 55 when one hydroxyl group is added to an alkyl alcohol with the same bulk structure, such as from 1-propanol to 1,2-propanediol, from

1,3-propanediol to glycerol, and from ethanol to ethylene glycol as well. The AT value decreases by about 40 from benzene to phenol. The negative AT values for alkyl alcohols with multivalue hydroxyl groups indicate that silicalite is usually phobic to the organics with hydroxyl groups. The AT values are 160, 127, and 116, respectively, for methylamine, ethylamine, and acetaldehyde, which are much higher than those for other alkylamines and organics with oxygen atoms. It is possible that there is a double-molecule associate existing in the channel intersections of the zeolite. Subhydrogen Bond between Hydrocarbons and Framework O2-. It is a simple way to investigate the degree of general interaction between a zeolite (i.e., the

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host) and an adsorbate (i.e., the guest). During temperature programming, the temperature for mass desorption of a guest molecule from a micropore (i.e., Td, the temperature of the weight loss peak in the DTG curve) is related to the interaction between the surface of silicalite and the molecules. The framework of silicalite is constructed by [SiO4] tetrahedra linked by sharing O2-. Since the radius of the oxygen atom is much larger than that of the silicon atom, the whole surface of the framework is actually covered with O2-. The distance between two neighboring atoms of oxygen in the silicalite framework is 0.149 nm, which is very close to 0.154 nm, the distance between two neighboring atoms of carbon in saturated hydrocarbons. The bond angle of C-C-C or H-C-H in saturated hydrocarbons is 109°23′, which is equal to the bond angle of O-Si-O in the framework of silicalite. The C-H group of saturated hydrocarbons can freely rotate along the C-C axis. It provides a greater opportunity to play a role in the interaction between the mass hydrogen atoms of saturated hydrocarbons and the O2- in the framework with a subhydrogen bond. It leads Td to be much higher than Tb for most saturated hydrocarbons adsorbed as a monomolecular layer within the micropore of the zeolite. The influence of the number of carbon atoms on the interaction is not obvious. The weak subhydrogen bond increases the AT values but cannot induce a visible thermal effect of desorption. Well et al. determined the temperatures of maximum mass loss in pure helium flow for n-alkanes on silicalite using the TPD technique.25 We can calculate the AT values from the temperatures measured by Well as follows (n-alkane/AT): butane/83; pentane/60; hexane/87; heptane/96; octane/95; nonane/ 87. These data are basically consistent with our data determined in air flow and prove our deduction. The number of hydrogen atoms in an aromatic molecule is less than that in a molecule of saturated hydrocarbon with the same number of carbon atoms. The aromatic ring is in a plate form. The hydrogen atoms on the ring cannot rotate along the C-C axis. The opportunity of playing the role of the interaction between framework O2- and the molecules adsorbed with subhydrogen bonds for aromatics is much less than that for saturated hydrocarbons. This leads to a lower AT value for aromatics. The AT value decreases to negative because of a stronger polarity of the hydroxyl groups in molecules for multivalue hydroxyl alcohols, which are repulsed by the framework O2-. This is the reason that silicalite is usually phobic to organics with hydroxyl groups. Effect of Associate on AT Value. Restricted by micropore size in the structure, the molecules of p-xylene can form an associate with a hydrogen bond in the channel intersections of MFI zeolite.20 A hydrogen atom of the methyl group in the molecule of p-xylene combines with an aromatic ring in a neighbor molecule of p-xylene to form the associate with a hydrogen bond. The situation is similar to that in the pure crystal of p-xylene and may cause the visible thermal effect on the desorption in the DTA curve.40 The plate molecules of naphthalene more easily form a tight crystal structure in the free state. The boiling point of naphthalene is 218 °C, much higher than that of n-nonane (C9H20, bp ) 150.7 °C), which has a similar molecular weight. The dimensions of the naphthalene molecule are 0.58 × 1.16 nm2. The adsorption of naphthalene is about three molecules per unit cell. It is impossible for the molecules to form an associate with each other within the micropores of silicalite. The (40) Long, Y.-C.; Sun, Y.-J.; Zeng, H.; Gao, Z.; Wu, T.-L.; Wang, L.-P. J. Inclusion Phenom., in press.

Long et al.

situation leads Td to decrease to 127 °C, and the AT value decreases to -91. The endothermic effects on desorption for alcohols or phenol possibly come from a deassociation of the associate, which is formed in combination with neighboring molecules with hydrogen bonds within the channel intersections. Abnormal AT Values. Amine groups are also polar with little negative electricity, and their polarities are weaker than that of hydroxyl groups. So the AT values of alkylamines are usually positive. The molecular sizes of methylamine and ethylamine are 0.50 and 0.63 nm, respectively. It is easy to form a double-molecule associate by combining NH2 groups with an adjacent molecule with a stronger hydrogen bond in the channel intersections. The double-molecule associate may lead the AT value to be abnormally high for methylamine and ethylamine and cause the visible endothermic effect on the desorption. The molecules of pentylamine can not form doublemolecule associates in the channel intersections because of their larger molecular size. The hydrogen bond, which exists between two neighboring molecules in the free liquid state, diminishes, since the molecule of pentylamine is isolated by the zeolite channels. The diminution of the hydrogen bond makes the Td close to the boiling point of n-hexane, which has a similar molecular weight to that of pentylamine. The endothermic effects of desorption for alkylamines are a sign of forming associates with hydrogen bonds in the channel intersections. The AT value is abnormally high for acetaldehyde. This is also possibly caused by a double-molecule associate forming with a hydrogen bond at the channel intersections. The tendency of the AT values is abnormal for n-heptane (l ) 1.16 nm), 1-pentanol (l ) 1.07 nm), and pentylamine (l ) 1.13 nm) in comparison with n-hexane, 1-propanol, and n-propylamine, respectively. There are four channel intersections per unit cell of silicalite. The space of the intersection can be seen as a cross of four channels with lengths of 1.05 nm and cross sections of 0.54 × 0.56 nm2. If a molecule has a length 1.05 nm is not easily adsorbed at the intersections. In this situation, the interaction between the alkyl groups and the framework O2-, and the interaction between the polar functional groups of the molecules will change more. It leads the AT values to become abnormal. Double Peaks in DTG Curves. There are double peaks in the DTG curves of n-hexane, n-heptane, p-xylene, toluene, phenol, ethylene glucol, ethylamine, and nhexylamine. The peaks are at 79 and 127 °C for n-hexane and at 63 and 185 °C for n-heptane. These data are close to those reported in the literature.25 “Since the first desorption step of n-heptane takes place before that of n-hexane, this first mass loss should be determined by entropic effects rather than by energetic effects.” This phenomenon implicates that a kind of phase transition takes place during the filling of silicalite, arising as a consequence of the interplay between the length of the zigzag pores and the length of the alkanes.24 The double peaks of weight loss are at 107 and 194 °C for nhexylamine. The temperature of first peak is lower than that of pentylamine at 121 °C. There is a probability that the filling of different length molecules leads to the phase transition, as for the situation of n-hexane and n-heptane. The difference between alkanes and alkylamines is in the thermal effect of desorption. In the case of p-xylene (Au ) 7.8), there is a p-xylene/ p-xylene complex formed in the channel intersections. Upon desorption, the p-xylene molecules in the channel

Sorbate/Framework and Sorbate/Sorbate Interaction

intersections leave the zeolite first through the straight channels with a high diffusion rate. In this step the energy of the hydrogen bonding between the hydrogen atoms of the methyl groups and the carbon atoms of the aromatic rings has to be overcome, so an endothermic effect occurs. Then the rest of the p-xylene molecules in the zigzag channels will leave, with a lower diffusion rate, without any evident heat effect. A symmetry transition from O2(P212121) to O1(Pnma) occurred at the first peak.40 The desorption of toluene (Au ) 8.9) is similar to that of p-xylene, but the thermal effect is not visible. The first peak is at 32 °C, much lower than the 94 °C for p-xylene. This is because the bonding in the toluene/toluene complex is weaker than that in the p-xylene/p-xylene complex in silicalite. The similar desorption behavior in the DTG curves also occurs in phenol (Au ) 7.5), ethylene glycol (Au ) 14.8), and ethylamine (Au ) 15.6). The sorption of these organics is close to 8 or 16 molecules/unit cell. The first peak in the DTG curves can be assigned to the deassociation of the adsorbate/adsorbate complex, accompanying an endothermic effect. There are double peaks of endothermic effect in the DTA curve of ethylamine (see Figure 8). This indicates that two species of stable associate are probably formed in the zeolite. One is composed of 16 molecules per unit cell, and the other of 8 molecules per unit cell.

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The first peak in the DTA curve belongs to deassociation of the former, and the second peak belongs to deassociation of the latter. Conclusion Cations and silanol groups in siliceous MFI zeolite have great influence on the adsorption and desorption of organics. The essential host/guest interaction can be clearly displayed in the perfect framework of silicalite. There are three types of interaction occurring in the organics-silicalite system: (1) a general sorbate/framework interaction, leading to different hydrophobic/organophilic properties, which can be compared in terms of the AT value determined by DTG; (2) a phase transition caused by the different fillings in zigzag channels with various lengths of alkanes and alkylamines; and (3) a formation of an associate in the zeolite, which can be recognized with the appearance of double peaks in the DTG curves and with the thermal effect of desorption. Acknowledgment. This study was supported by the Chinese National Science Foundation (Grant CH 29236120-03). LA961009A