LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Synthesis of Foam-Shaped Nanoporous Zeolite Material: A Simple Template-Based Method Vipin K. Saini and Jo~ao Pires* Department of Chemistry and Biochemistry and CQB, Faculty of Sciences, University of Lisbon, Ed. C8, Campo Grande, 1749-016 Lisbon, Portugal
bS Supporting Information ABSTRACT: Nanoporous zeolite foam is an interesting crystalline material with an open-cell microcellular structure, similar to polyurethane foam (PUF). The aluminosilicate structure of this material has a large surface area, extended porosity, and mechanical strength. Owing to these properties, this material is suitable for industrial applications such as large-scale gas separation and catalysis. As the applications of this material are significant, the underlying concepts for the design and synthesis may be used in an upper-level undergraduate laboratory in inorganic synthesis or material chemistry courses. This type of experiment can increase student understanding and integrate a variety of chemical concepts, such as targeted synthesis, characterization, and adsorption theory of gases. This laboratory experiment involves the synthesis and characterization of ZSM-5 type of zeolite foam that is synthesized from inexpensive materials and characterized by bulk density and porous volume measurements. This course would acquaint students with general characterization methods that are not normally encountered in a typical material or inorganic chemistry laboratory. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Materials Science, Physical Properties, Solids, Synthesis
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he design and synthesis of materials for specific applications is an important goal in materials chemistry. Zeolites are often used for a variety of applications1 as they are versatile materials with regular frameworks. They are highly porous crystalline solids with well-defined structures and contain silicon, aluminum, and oxygen in their framework along with cations, water, and other molecules within their pores.2 One of the biggest industrial applications of zeolites is its use in laundry detergents; in addition, they are also used in various chemical processes, refining industries, ozone abatement, medicine, and in agriculture. They are also used on industrial scale for water purification, for catalysis,3,4 and in nuclear reprocessing.5 7 However, the fine particulate nature of zeolites often poses problems in large-scale applications. These problems include health risks (associated with dust of fine particulates), high handling costs, requirement of column to fill with, high-pressure drop during separation process, and so forth. Consequently, self-supported zeolite monoliths with distributed macropores (zeolite foam) are preferred over nonshaped (particulate) zeolites for easier internal molecular diffusion and are essential for large-scale applications. It is the success of these zeolite foams that has encouraged development of these materials and studies for industrial applications. Despite wide interest in these materials in industrial and academic circles,1 7 there are few suitable experiments for undergraduate chemistry students that provide a basic exposure to the field. Therefore, an experiment is described for the synthesis of Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
ZSM-5 type zeolite foam with an open-cell microcellular structure. Students obtain a basic understanding of the characterization that is required to estimate the physical properties and performance of these materials. The characterization techniques, which are usually common in synthetic chemistry laboratories, for example, solution-phase spectroscopic techniques, are generally not applicable to the synthesis and characterization of zeolites because the extended crystalline framework renders them insoluble in any solvent. In fact, solubilization of these materials could risk the loss of their structural framework and hence their valuable physical properties. Simple and nondestructive characterization methods are used, such as bulk density and adsorption capacity measurements, which can be easily operated in traditional material chemistry laboratories. By experimentally determining the adsorption capacity of a prepared material with various vapor molecules, students gain insight into the concepts of adsorption capacity and porous volume. Structural data from ZSM-5 (as well as all other existent zeolites) can be found at the International Zeolite Association Web site.8 In addition, by reading the recent literature, students gain an understanding of current global and industrial challenges, such as the requirement for advanced adsorbent materials in
Published: October 07, 2011 276
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inorganic) around which the aluminosilicate lattice is formed so that the tunnel, or cage, size of the zeolite is translated from the size of templating cation. For example, tetrapropylammonium hydroxide (TPAOH) is generally used as the source of the templating cation for the synthesis of ZSM-5. Usually, the first step involves the hydrolysis of the silica source and template ion, which is followed by dissolution of sodium aluminate in this solution. The obtained clear solution is added to a vessel containing the polyurethane foam template. After hydrothermal treatment in the tightly sealed vessel under specific conditions, the soaked piece of polyurethane foam turns into a hard inorganic structure of similar shape and size, which is impure zeolite foam. This zeolite foam is washed and undergoes calcination to give pure zeolite foam. A pictorial representation of this procedure is given in Figure 1. The present experiment consists of two parts: the first part involves the synthesis of zeolite foam (Figure 1) and the second part consists of its characterization (see the Supporting Information for the experimental details). The zeolite foam is a ZSM-5 type, which is an aluminosilicate zeolite with a high silica and low aluminum content. Its internal structure is based on channels with intersecting tunnels. The aluminum sites in this zeolite are acidic because the substitution of Al3+ in place of the tetrahedral Si4+ silica requires the presence of an added positive charge. When the charge is supplied by H+, the acidity of the zeolite is high, which results in the reaction and catalytic chemistry of ZSM-5. This zeolite has been carefully selected due to high level of interest that these materials have received in the petroleum industry for hydrocarbon interconversion and separation applications. Finally, from a practical viewpoint, this experiment is attractive because ZSM-5 can be easily prepared from inexpensive materials that are commercially available. In the characterization part, the bulk density is determined using common instruments such as the microbalance and Vernier Caliper and the porous volume of activated samples is measured using basic glass containers and an analytical balance.
Figure 1. Steps involved in the synthesis of zeolite foam (ZSM-5).
pollution control and separation and purification of valuable gas mixtures.
’ EXPERIMENTAL OVERVIEW The formation of zeolite is generally a kinetically controlled process. Normally, the reaction time is long (for example, in nature this reaction may take millions of years), but the use of hydrothermal conditions during the reaction can effectively enhance the reaction rates. The present method of zeolite-foam synthesis exhibits excellent ease and versatility in the design of the material. Using this method, the size and shape of the interconnecting macropores, the thickness and morphologies of the films surrounding the macropores, and the threedimensional structures of the zeolite foam can easily be manipulated. The basic experimental procedure of zeolite-foam synthesis is similar to the general hydrothermal process, which is used commonly for the synthesis of zeolites on various scales. The interesting aspect as well as the advantage of this procedure resides in the use of polyurethane foam (PUF) as a template during hydrothermal synthesis. The reaction takes place in presence of a PUF template inside a tightly sealed vessel. PUF is an inexpensive and commercially available material, although it can also be synthesized via a procedure developed by Pinto.9 The PUF template provides sites for the seeding of zeolite crystals and therefore allows faster nucleation of zeolite on the surface than in the bulk. Compared to earlier approaches,10 this method is simple and requires short time periods. Generally, these types of aluminosilicate zeolites can be easily synthesized at temperatures below 100 °C in alkaline solutions. However, these syntheses are performed at even higher temperatures under autogenous pressure in tightly sealed vessels to reduce the reaction time, to control morphologies, and to optimize compositions. The elevated reaction temperature and high pressure result in a dense phase of zeolites. Zeolites are generally synthesized from solutions of sodium aluminate and a suitable silica source such as TEOS (tetraethyl orthosilicate) or sodium silicate.11 The type of zeolite formed depends on (i) the reactants used, (ii) the synthesis conditions applied (such as temperature, time, and pH), and (iii) the templating ion. The templating ion is usually a cation (organic or
’ HAZARDS Polyurethane foam (PUF) dust may cause eye and lung irritation. TEOS may cause irritation to eyes and skin and it is harmful if inhaled in excess. The TPAOH is categorized as a corrosive base; therefore, it may cause damage if swallowed and may cause irritation to eyes and skin. The sodium aluminate is also hazardous in case of skin contact (corrosive, irritant), of eye contact (irritant), of ingestion and, of inhalation. Because the reaction involves a hydrothermal step, special care should be taken regarding temperature of the Schott Duran bottle and related burn risks and students should be careful with the temperature of oven, as these bottles are strictly advised to use up to 140 °C only. It is, therefore, suggested that in the end of the reaction sufficient time should be given to Schott Duran glass bottle to cool-down to room temperature. Use of gloves is strictly advised during removal of zeolite foam from Schott Duran glass bottle and also during washing. Calcination step also needs special care where students should be advised to remove the sample from furnace only when it cools down to room temperature. ’ DISCUSSION This is a suitable experiment for upper-level undergraduate students in inorganic synthesis or material chemistry courses. The reagents required to carry out this experiment are economical 277
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Journal of Chemical Education and readily available. To maintain hydrothermal conditions, the reaction is carried out in a Schott Duran glass bottle. These types of bottles are autoclavable up to 140 °C in standard conditions and are therefore an inexpensive alternative of expensive autoclave instrument. The reaction can be carried out within any standard variable temperature oven. Before starting the experiment, students should be ready with a cylindrical piece of polyurethane foam (which can fit easily in 100 mL Schott Duran bottle without squeezing) and all the required reagents: TPAOH, TEOS, and sodium aluminate. The preparation of synthetic reaction mixture, its soaking into the foam template, and its loading into the Schott Duran glass bottle can be attained in one 3 4 h laboratory session (see the Supporting Information for the time management), after which the bottle sample is subjected to hydrothermal treatment in oven for two days. After an interval of two days, a 4 5 h laboratory session is required for washing, drying, and calcination of the obtained structure. Once the final sample is ready, the characterization, bulk density and porous volume measurement, can be carried out in a 2 3 h laboratory. Previous studies on the synthesis have shown that the hydrothermal reaction temperature and reaction time have a significant affect on the physical characteristics of prepared sample.12 However, the rate of increase in temperature in the hydrothermal reaction is also critical. During the course of synthesis, the PUF template is being dissolved simultaneously due to the highly alkaline medium. Thus, a fast rise in temperature would speed up the destruction of the PUF template, and it may collapse before it gets replaced by the initial layer of aluminosilicate. If the oven used in laboratory does not have an editable rate program, the students need to manually increase the temperature using small temperature intervals, for example 5 10 °C/min. The synthesized zeolite sample is characterized for its physical properties such as adsorption capacity and bulk density (see the Supporting Information for the details). The adsorption capacity measurement involves adsorption of vapor molecules into the pore system of the dehydrated zeolite. Students should use more than one type of vapor molecules. For example, students may try a homologous series of compounds with varying sizes, as this would also help them to understand the size-exclusion phenomenon related to zeolites. The students should present the data from these experiments in a tabular form as shown in Table 1 in the Supporting Information, where the values of adsorption capacity occupied by vapor molecules of different sizes are compared. Each student group (including 2 3 students) writes a lab report, handed in the session after the experimental work is complete. Student groups should be encouraged to share their data with other groups. The PUF provides the surface for the nucleation (seeding) of zeolite particles that ultimately cover the entire surface of PUF framework and make a thin layer that acquires the shape of foam. Therefore, it would be interesting if students tried this synthesis in the absence of the PUF template. This reaction would yield a thick zeolite monolith settled in the bottom of Schott Duran glass bottle. An increase in hydrothermal reaction temperature causes an increase in the film thickness,12 which clearly would also increase the bulk density of prepared foam sample. The bulk density is an important physical property of porous materials; it is the density of the material including the pores.13 Therefore, students should focus on the variation in bulk density by variation in temperature and compare their results with the results provided in Table 2 in the Supporting Information. Students should
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compare the density of PUF with the prepared samples of the zeolite foam. They will discover that density of PUF is more than respective zeolite foam, which results because the wall structure in the zeolite foam is very fine and the thickness is low.
’ ADDITIONAL AND COMPLEMENTARY ACTIVITIES If powdered X-ray diffraction (XRD) is available, a diffractogram (X-ray diffraction pattern) of the prepared zeolite (after having been crushed) can be obtained. This experimental XRD pattern can be compared with one provided by International Zeolite Association.8 This would identify the structure as ZSM-5 zeolite. However, for the sake of convenience, the diffractogram of ZSM-5 has been provided in Supporting Information. The surface area of the prepared zeolite sample can also be measured if an automated sorption analyzer is available. The details of procedures for such purpose have been discussed elsewhere.14 If students obtained a nitrogen adsorption isotherm, it can be compared with original data. For this purpose, an excel sheet has been provided in Supporting Information where nitrogen adsorption isotherm data, plot, and calculations for surface area have been demonstrated. The zeolite foam produced from this experimental procedure can be tested in the laboratory to determine other important properties. For example, ion exchange is one of the characteristic properties of a zeolite. A simple and colorful experiment to test the ion-exchange capabilities of prepared zeolite foam has been published in this Journal.15 In such experiments, students may use colored salt of any transition metal to monitor the reaction; for example, the pink colored solution of cobalt chloride is a good option. Using instrumental techniques, such as IR spectroscopy, selective adsorption of alcohols by the zeolite foam can also be monitored.16 The monolithic disc of zeolite obtained in absence of PUF foam can be used to study the permeation of absorbable and nonadsorbable gases through structure.17 The catalytic application of various zeolites has been proposed by various authors to introduce basic concepts of physical chemistry and thermodynamics. Following one such procedure,18 the catalytic properties of the prepared zeolite foam can be used to investigated the conversion of methanol to hydrocarbon. ’ SUMMARY This experiment is expected to have a high pedagogical value owing to the general skills in nanoporous material development and characterization that the students acquire. This experiment promotes the level of overall comprehension and understanding of basic concepts of inorganic and physical chemistry and encourages student-based ideas on directions the experiments could be extended if they desired to study these and other related materials in further detail. ’ ASSOCIATED CONTENT
bS
Supporting Information A student handout with detailed experimental instructions containing guidelines for synthesis, characterization, and time management; characterization data from adsorption capacity measurements and bulk density measurements; two additional activities. This material is available via the Internet at http:// pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We are grateful to the Center and to the Department of Chemistry and Biochemistry of the Faculty of Sciences, University of Lisbon for resources. V.K.S. acknowledges Fundac-~ao para a Ci^encia e a Tecnologia (FCT) for postdoctoral grants SFRH/BPD/34872/2007. ’ REFERENCES (1) Mumpton, F. A. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3463–3470. (2) Chao, P.-Y.; Chuang, Y.-Y.; Ho, G. H.; Chuang, S.-H.; Tsai, T.-C.; Lee, C.-L.; Tsai, S.-T.; Huang, J.-F. J. Chem. Educ. 2008, 85, 1558–1561. (b) Belver, C.; Vicente, M. A. J. Chem. Educ. 2006, 83, 1541–1542. (c) Williams, D. J.; Huck, B. E.; Wilkinson, A. P. Chem. Educator 2002, 7, 33–36. (d) Pietra, T. J. Chem. Educ. 2002, 79, 492–493. (e) Coker, E. N.; Davis, P. J.; Kerkstra, A.; van Bekkum, H. J. Chem. Educ. 1999, 76, 1417–1419. (f) Mastersa, A. F.; Maschmeyer, T. Microporous Mesoporous Mater. 2011, 142 (2 3), 423–438. (3) Breck, D. W. Zeolite Chemistry and Catalysis; ACS Monograph 171; American Chemical Society: Washington, DC, 1974. (4) Weitkamp, J. Solid State Ionics 2000, 131 (1 2), 175–188. (5) Heck, R. M.; Gulati, S.; Farrauto, R. J. Chem. Eng. J 2001, 82, 149–156. (6) Cybulski, A.; Moulijn, J. A. Structural Catalysts and Reactors; Marcel Dekker: New York, 1998. (7) Kulprathipanja, S., Ed. Zeolites in Industrial Separation and Catalysis; Wiley-VCH Verlag GmbH & Co.: Weinheim, 2010. (8) International Zeolite Association Web site. http://www. iza-online.org (accessed Sep 2011). (9) Pinto, M. J. Chem. Educ. 2010, 87, 212–215. (10) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974; pp 725 755. (11) Lee, Y.; Lee, J. S.; Park, Y. S.; Yoon, K. B. Adv. Mater. 2001, 13, 1259–1263. (12) Robson, H., Ed. Verified Synthesis of Zeolitic Materials, 2nd revised ed.; Elsevier: Amsterdam, 2001. (13) D1622-98, Standard Test Method for Apparent Density of Rigid Cellular Plastics; American Society for Testing and Materials: West Conshohocken, PA, 1998. (14) Sumida, K.; Arnold, J. J. Chem. Educ. 2011, 88, 92–94. (15) Balkus, K. J.; Ly, K. T. J. Chem. Educ. 1991, 68, 875–877. (16) Cooke, J.; Henderson, E. J. J. Chem. Educ. 2009, 86, 606–610. (17) Lito, P. F.; Magalh~aes, A. L.; Silva, C. M. J. Chem. Educ. 2009, 86, 976–979. (18) Bibby, D. M.; Johnston, P.; Orchard, S. W.; Copperthwaite, R. G.; Hutchings, G. J. J. Chem. Educ. 1986, 63, 632–634.
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