Laboratory Experiment pubs.acs.org/jchemeduc
Synthesis and Characterization of Zeolite Na−Y and Its Conversion to the Solid Acid Zeolite H−Y Terence E. Warner,*,† Mads Galsgaard Klokker,‡ and Ulla Gro Nielsen‡ †
Department of Chemical Engineering, Biotechnology and Environmental Technology, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark ‡ Department of Physics, Chemistry, and Pharmacy, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark S Supporting Information *
ABSTRACT: Zeolite Y has an iconic crystal structure, but more importantly, the hydrogen modification zeolite H−Y is the classic example of a solid acid which is used extensively as a catalyst in the oil industry. This metastable compound cannot be synthesized directly, which creates an opportunity to discuss various preparative strategies with the students, such as the three-stage procedure described here. Stage I concerns the hydrothermal synthesis of zeolite Na−Y, followed by ion-exchange with an ammonium acetate solution to form zeolite NH4−Y, and the latter is subsequently converted to zeolite H−Y by thermolysis. Stages II and III may instead be performed using commercially available zeolites, Na−Y and NH4−Y, respectively, which shifts the learning objectives to structural characterization of zeolites. The characterization of the product and intermediate materials gives the students a practical insight into the applicability and limitations of powder X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and thermogravimetric analysis, and how these analytical tools complement each other. These aspects make its synthesis and characterization an ideal practical exercise for an upper-level undergraduate laboratory in inorganic or materials chemistry courses. Moreover, the methods and skills learned during this experiment enable the students to tackle more complex zeolites and related framework materials. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Materials Science, Solid State Chemistry, Synthesis, X-ray Crystallography, NMR Spectroscopy, Gravimetric Analysis
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INTRODUCTION Solid inorganic acids are an important class of materials that warrant discussion at the undergraduate level. In fact, illustrations of the Linde zeolite type-Y (or faujasite) structure are a popular motif on the front cover of textbooks concerning zeolites and heterogeneous catalysis (see Figure 1). The reason for this is that certain derivatives of the hydrogen modification, zeolite H−Y, serve as highly profitable catalysts for cracking long-chain hydrocarbons in the production of petroleum spirits. The laboratory-scale synthesis of this solid acid will appeal to students of solid-state chemistry since it is an example of a metastable inorganic compound that cannot be prepared directly from either its binary oxide components, or from an aqueous solution of its constituent ions, but instead it requires a three-stage procedure. Furthermore, it is an example of a reliable and safe hydrothermal synthesis in a closed system at ambient pressure. This experiment involves the preparation of the sodium form (zeolite Na−Y), followed by ion-exchange with an aqueous solution of an ammonium salt to create the ammoniacal form (zeolite NH4−Y), and finally thermolysis to liberate ammonia and yield the acidic form (zeolite H−Y). Each of these three stages presents unique practical challenges where the degree of success can be monitored by powder X-ray © XXXX American Chemical Society and Division of Chemical Education, Inc.
diffraction (PXRD), solid-state nuclear magnetic resonance spectroscopy (SSNMR), and thermogravimetric analysis (TGA). This material also provides a convenient forum for discussing chemical concepts like Brønsted and Lewis acids, ion-exchange, and structural modification, as well as a comparison of various common characterization techniques. The preparation of zeolites has appeared in this Journal before. Belver and Vicente2 described the synthesis and characterization of zeolite K−F from kaolin, using Fourier transform infrared (FTIR) spectroscopy and PXRD. Copperthwaite et al.3 prepared zeolite Na-ZSM-5 in a steel autoclave at 160 °C and converted it to its acid form by ionexchange with ammonium sulfate solution, followed by thermolysis at 500 °C, and then evaluated its catalytic performance for σ-xylene isomerization. Whereas Saini and Peres4 reported a templated synthesis of ZSM-5 with characterization by PXRD and infrared (IR) spectroscopy, Balkus and Ly5 reported the preparation and characterization of zeolite Na−X by IR spectroscopy and PXRD, and demonReceived: November 24, 2016 Revised: March 9, 2017
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DOI: 10.1021/acs.jchemed.6b00718 J. Chem. Educ. XXXX, XXX, XXX−XXX
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EXPERIMENTAL PROCEDURE The reader is advised to consult the detailed account of the experimental procedure given in the Supporting Information. As an overview, the experiment is divided into three stages that when run consecutively result in the formation of the solid acid, zeolite H−Y: • Stage I (3 days): Hydrothermal synthesis of zeolite Na− Y from a strongly basic aqueous solution of sodium aluminate and sodium silicate at 100 °C. • Stage II (4 days): Synthesis of zeolite NH4−Y by ionexchange of zeolite Na−Y with an aqueous solution of ammonium acetate under reflux at about 100 °C. • Stage III (1 day): Synthesis of zeolite H−Y by thermal deammoniation of zeolite NH4−Y at 400 °C. The experimental procedure is quite flexible in that these stages can be run independently from one another if required. For instance, a commercial source of zeolite Na−Y can be used as the starting reagent, whereupon stages II and III are performed to convert it to zeolite H−Y. Or the instructor may choose to adopt just stage I to demonstrate a reliable hydrothermal synthesis of zeolite Na−Y using commonly available reagents and equipment. The timing in stage I is crucial and extends over 3 consecutive days, but some of the individual steps are quite quick and can be performed by the instructor if needed, or by making appointments with a few students to insert or remove samples from the oven. Stage II normally extends over 4 days, but can tolerate certain breaks. Stage III involves a short handling time and has a heat treatment lasting 24 h. Alternatively, this step can be illustrated by TGA only, which clearly shows release of zeolitic water from zeolite NH4−Y over the range from 30 to 140 °C, and loss of ammonia over the range from 140 to 400 °C (see Figure 2).
Figure 1. Left: An illustration of the faujasite structure emphasizing the interconnection of the sodalite or β-cages (blue) via the hexagonal prisms (yellow) which give rise to the supercage (large central cavity). The cubic unit cell (a = 24.7862 Å) is outlined in dashes.1 Right: A detail of the crystal structure of anhydrous zeolite Na−Y, Na56[Al56Si136O384], at room temperature, showing the positions of the Na sites in the β-cage (blue), hexagonal prism (yellow), and supercage (red). The experimentally determined site occupation factors are 0.45 ± 03, 0.60 ± 03, and 0.85 ± 3, respectively.1 Note that the Na sites in the supercage lie close to the four exposed (i.e., unconnected) hexagonal faces of each β-cage. Drawn using data from ICSD 188157.
strated its Co2+ ion-exchange properties in the context of water purification. However, the use of solid-state NMR spectroscopy for characterization of zeolites has not been reported in this Journal to our knowledge, although there is a description of 29Si SSNMR for characterization of cement,6 the use of 31P SSNMR for characterization of phosphate glasses,7 and the use of 2H SSNMR for studies of molecular dynamics.8 Blatter and Schumacher9 described a method for preparing zeolite Na−Y, but the focus was on its conversion to a highsilica faujasite with a Si/Al ratio ≳100. Our procedure is aimed to prepare the zeolite in its acidic form and thereby illustrate hydrothermal synthesis, ion-exchange, and thermolysis. The synthesis is divided into three stages with an option of either continuing with the intermediate products−as obtained from the preceding stages−or switching to a commercial source of zeolite Na−Y or zeolite NH4−Y, as fits within the curriculum and the objective of the laboratory class. Emphasis is also given to the structural information that can be obtained by different techniques, thereby illustrating the application of PXRD, SSNMR, and TGA. Stage I involves the hydrothermal synthesis of zeolite Na−Y as based on the method described by Ginter,10 but was modified by us to operate on a smaller scale which is more suitable for undergraduate laboratory classes. The addition of further details ensures that the procedure is now easier to follow and lies within a more practical time frame. Furthermore, we have a greater emphasis on characterization of the products. PXRD is used to confirm the formation of zeolite Na−Y. Stage II involves the exchange of the Na+ ions in zeolite Na−Y for NH4+ ions, whereby the choice of ammonium salt is discussed. 27 Al and 29Si SSNMR were used for determination of the coordination state of aluminum and the Si/Al atomic ratio of the zeolite, respectively. Finally, stage III involves the thermal deammoniation of zeolite NH4 −Y under a controlled atmosphere. Measurement of the mass of the material before and after deammoniation enables the degree of conversion from zeolite NH4−Y to zeolite H−Y to be ascertained.
Figure 2. Thermogravimetric profile for the as-prepared nominally anhydrous zeolite NH4−Y under a nitrogen atmosphere with a heating rate of 1 °C/min.
We have classes of 20−24 students working in pairs with a background in chemistry (major or minor) as well as chemical engineering. They prepare five inorganic compounds by different synthesis methods, e.g., solid-state reaction, hydrothermal, and metathetic reactions in aqueous media, in a two week laboratory course (six classes of 4 h duration) on a rotational basis. Furthermore, characterization and investigation of the properties of these materials are also central. Thus, their materials include the preparation of the classic high-Tc superconductor, YBa2Cu3O7−δ, with a demonstration of the Meissner effect;11 a layered vanadyl phosphate, VVOPO4·2H2O, subsequently converted to VIVO(HPO4)·0.5H2O by oxidation of 2-butanol to butanone; and Vaska’s complex, IrCl(CO)[P(C6H5)3]2, followed by addition of hydrogen. In addition, we give hands-on demonstrations of the different experimental B
DOI: 10.1021/acs.jchemed.6b00718 J. Chem. Educ. XXXX, XXX, XXX−XXX
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equipment (PXRD, NMR, TGA, and ESR) as the characterization of the products is an essential part of the course. Hence, our students have to plan their experiments carefully. The limiting factor in the synthesis of zeolite Na−Y is the turbine mixer, which in our experience can be shared between three groups on 1 day. We ask the students to propose a method for the synthesis of zeolite H−Y by thermal treatment based on the TGA data (Figure 2) due to a time restraint. The Supporting Information provides a complete set of these data, as one or more of the instruments may not be available for the user.
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CHEMICAL REAGENTS The following chemical reagents were purchased from SigmaAldrich: • Anhydrous sodium aluminate, NaAlO2 technical grade (50−56 wt % Al2O3, 40−45 wt % Na2O). • Sodium silicate solution, (NaOH)x(Na2SiO3)·yH2O reagent grade (10.6 wt % Na2O, 26.5 wt % SiO2). • Sodium hydroxide pellets, NaOH ACS reagent ≥97.0%, for preparation of a 100 mL stock solution of 5 M NaOH and a 500 mL stock solution of 0.025 M NaOH. • Ammonium acetate, CH3CO2NH4 ≥ 98%.
Figure 3. Powder X-ray diffraction pattern (Cu Kα1 radiation) for anhydrous zeolite Na−Y as prepared. JCPDS 74-2761 anhydrous zeolite Na−Y, Na56[Al56Si136O384], is shown in red for comparison.
directly from the isotropic chemical shift and also identification of potential Al impurities in the samples, which may occur during the conversion from zeolite Na−Y to zeolite H−Y (see Figure 4 and Figure S5).
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HAZARDS The usual laboratory safety precautions should be exercised, particularly when handling hot solutions containing sodium hydroxide, and when operating the turbine mixer. The plastic bottles must not be heated above 100 °C, or else they will deform. Technical assistance with operating the tube furnace and gas-flow attachments would be prudent.
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RESULTS
Stage I: Synthesis of Anhydrous Zeolite Na−Y
The 10 g sample of anhydrous zeolite Na−Y has a theoretical formulation of Na56[Al56Si136O384], with a SiO2/Al2O3 mole ratio of 4.9. Only plastic equipment should be used in this step as the highly alkaline solution corrodes glassware. A small batch of seed gel is prepared from a strongly basic solution and then added to the feedstock gel to accelerate the crystallization of the desired zeolite Na−Y. The timing and proper use of a highshear turbine mixer (1600 rpm) is crucial for preparing the feedstock gel, especially during the addition of the aged seed gel. Disposable 100 mL transparent polypropylene bottles with screw caps are ideal for the crystallization step when performed in a conventional drying oven (or similar) at 100 °C. Their transparency is essential for observing the marked reduction in turbidity within the supernatant liquor, which is used to indicate that the reaction is complete. Moreover, performing the synthesis at 100 °C removes the need for an autoclave. The dried product material is analyzed by PXRD (2θ: 5− 50°) using Cu Kα radiation, and compared with JCPDS 742761 for the anhydrous zeolite Na−Y, Na56[Al56Si136O384], in order to confirm the formation of zeolite Na−Y (see Figure 3). PXRD is not the ideal analytical tool for determining the Al/Si ratio in the zeolite (by comparing the cell constants) because of the uncertainty in the sample’s zeolitic water content which likewise affects the magnitude of the cell constant. Thus, we use 29 Si SSNMR for determination of the Si/Al ratio, which is obtained by simple deconvolution of the 29Si magic-angle spinning (MAS) SSNMR spectra. 27Al MAS NMR allows for determination of the coordination of aluminum in the zeolite
Figure 4. (a) 27Al MAS NMR spectrum of zeolite Na−Y showing the single resonance from tetrahedrally coordinated Al in the zeolite framework. (b) Experimental 29Si MAS NMR spectrum (black) and a simulation (blue) of zeolite Na−Y with the individual resonances (red) and the difference between the simulated and experimental spectra. The numbers indicated the number of Al in the second coordination sphere for each 29Si resonance. SSNMR spectra were recorded at 14.1 T (600 MHz NMR spectrometer) using 15 and 7 kHz spinning speed for 27Al and 29Si, respectively.
Stage II: Conversion of Anhydrous Zeolite Na−Y to Zeolite NH4−Y
This stage involves the conversion of a 5 g sample of anhydrous zeolite Na−Y to zeolite NH4−Y with a theoretical formulation of (NH4)56[Al56Si136O384]. The solid acid, zeolite H−Y, cannot be prepared directly from an aqueous solution as it is unstable under the acidic conditions needed.12 Conversion of zeolite Na−Y to zeolite H−Y can be achieved indirectly by ionexchange of sodium ions for ammonium ions at near-neutral pH, followed by drying and calcining the anhydrous zeolite NH4−Y to expel ammonia, leaving protons bonded to the oxygen atoms. Identification of the specific oxygens in the zeolite to which these protons are attached as well as the interactions and synergism of the Brønsted and Lewis acid groups serve as stimulating points for discussion in class.12−15 The ion-exchange is based quantitatively on the weighed-out mass of the zeolite Na−Y in its anhydrous form in order to C
DOI: 10.1021/acs.jchemed.6b00718 J. Chem. Educ. XXXX, XXX, XXX−XXX
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framework structure.12 This is readily achieved by preheating the material under a flowing nitrogen atmosphere at a temperature below the deammoniation temperature. Our students spread about 4 g of dried zeolite NH4−Y in an alumina boat (SRX110 Almath Ltd.) to form a thin powder bed of about 20 cm2 in area. This was inserted into a tube furnace and heated to 100 °C under flowing nitrogen to achieve full dehydration. The heating was then continued in situ to 400 °C for 4 h under flowing nitrogen to ensure a rapid removal of the ammonia from the work-tube, with appropriate venting of the exit gas from the laboratory. Our students recorded the precise mass of the material immediately before and after this heating cycle, so as to minimize the adsorption of moisture from the air that would otherwise result in an inaccurate interpretation of the mass changes. The mass change is consistent with the TGA data and correlates to a loss of ammonia equivalent to a full conversion of zeolite NH4−Y to zeolite H−Y. The dried anhydrous product material is characterized by PXRD to confirm that the faujasite framework structure is still intact, although this analysis is unable to determine the extent of the conversion of zeolite NH4−Y to zeolite H−Y since these two phases are isostructural with rather similar cell constants. Solidstate 27Al and 29Si MAS NMR is again used to probe possible dealumination and monitor the Si/Al ratio.
minimize the uncertainty concerning its zeolitic water content, even though this material is returned to an aqueous environment in stage II. Regarding the ammonium salt to be selected for the ion-exchange, aqueous ammonia solution is inappropriate as it is a weak base yielding merely 0.004 M NH4+ ions at 25 °C. Kühl12 recommended using an ammonium salt solution with a pH close to or slightly above 7, such as ammonium acetate.13 Partial conversion to zeolite NH4−Y can be achieved at room temperature. However, a more efficient ion-exchange requires higher temperature (80−100 °C) and multiple ion-exchanges with fresh ammonium salt solutions; even then, the ionexchange rarely exceeds 96%.16 The remaining 4% of the Na+ ions reside in the supercage within the faujasite structure (Figure 1).17 However, their incorporation into an amorphous phase, that would be undetectable by PXRD, is another possibility, and worthy of discussion in class. Although countercurrent exchange generally offers a greater efficiency compared with batch exchange, we did not adopt this approach because of the added practical complexity. We concluded that an optimal set of conditions involves using 2 M ammonium acetate aqueous solution at 100 °C with one repeated exchange, which resulted in a conversion factor of 91% according to 23Na MAS NMR (not shown). In all cases, there is a compromise between maximizing the incorporation of NH4+ ions into the zeolite while avoiding dealumination and a collapse of the aluminosilicate framework. There is, however, plenty of opportunity for the students to explore these parameters further. The dried product material is characterized by PXRD (2θ: 5−50°) using Cu Kα radiation to check that the faujasite framework structure is still intact. However, PXRD cannot be used to distinguish zeolite NH4−Y from zeolite Na−Y as the cell constants are nearly identical due to the similar ionic radius of NH4+ and Na+. In addition, 27Al MAS NMR shows a partial dealumination of the zeolite, as evidenced by formation of octahedrally coordinated Al [δiso(27Al) ≈ 0−10 ppm (Figure S5a)]. Moreover, the changes in the relative intensities of resonances in the 29Si NMR spectrum also show a lower Al content (Figure S6).
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CONCLUSIONS
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ASSOCIATED CONTENT
After performing these experiments, our students became more aware of the interconnected issues involved in the hydrothermal synthesis of zeolite H−Y, most notably the need for seeding during the process of forming the parent zeolite Na−Y, and the parameters governing the ion-exchange and deammoniation processes, especially the distinct changes in sample texture and separation of phases, as observed during the key steps in part I, which facilitate this learning. Whereas hydrothermal synthesis is routinely performed in inorganic and materials research groups, this technique is not common in the teaching laboratory, and consequently, our students were generally intrigued by the approach. Moreover, our students gained valuable experience with interpretation of PXRD, TGA, and SSNMR data, including the strengths and limitations of these techniques, especially how PXRD and SSNMR complement each other, a discussion which can be expanded to the characterization of other porous materials and/ or glasses, as well as silicate chemistry. Furthermore, the synthesis of zeolite H−Y is an excellent starting point in the lectures on heterogeneous catalysis, and may be combined with a description of, for example, metal organic frameworks or porous aluminum phosphates.
Stage III: Deammoniation of Anhydrous Zeolite NH4−Y to Zeolite H−Y
This final stage involves the deammoniation of a 4 g sample of anhydrous zeolite NH4−Y to zeolite H−Y with a theoretical formulation of H56[Al56Si136O384]. Anhydrous zeolite NH4−Y can be converted to zeolite H−Y by thermolysis which expels ammonia from the ammonium ions leaving behind protons attached to the zeolite framework structure. Our students found it instructive to perform thermogravimetric analysis (TGA) on their zeolite NH4−Y (prepared in stage II) in order to establish the optimal conditions for this heating profile. Figure 2 shows that the mass loss occurs in three steps, corresponding to the three endotherms in the differential thermal analysis reported by Chu,16 which are attributed to the following processes: • 20−150 °C: desorption of zeolitic water. • 150−250 °C: dissociation of NH3 from NH4+ ions in the hexagonal prism and β-cage. • 250−400 °C: dissociation of NH3 from NH4+ ions in the supercage. The zeolite NH4−Y must be thoroughly dehydrated before the deammoniation stage, or else the damp ammonia expelled during the deammoniation will cause hydrolysis of the
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00718. Detailed and comprehensive description of the three stages of the preparative procedure, the analyses of the product and intermediate materials by solid-state nuclear magnetic resonance spectroscopy, and powder X-ray diffraction (PDF, DOC) D
DOI: 10.1021/acs.jchemed.6b00718 J. Chem. Educ. XXXX, XXX, XXX−XXX
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(16) Chu, P. The deammoniation Reaction of Ammonium Y Zeolite. J. Catal. 1976, 43 (1−3), 346−352. (17) Lim, W. T.; Seo, S. M.; Wang, L.; Lu, G. Q.; Heo, N. H.; Seff, K. Single-Crystal Structures of Highly NH+4-exchanged, Fully Deaminated, and Fully Tl+-exchanged Zeolite Y (FAU, Si/Al = 1.56), all Fully Dehydrated. Microporous Mesoporous Mater. 2010, 129 (1), 11− 21.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Terence E. Warner: 0000-0001-8397-6030 Ulla Gro Nielsen: 0000-0002-2336-3061 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Danny Kyrping, Lars B. Hansen, and Torben H. Jensen for technical assistance. U.G.N. and M.G.K. are grateful for financial support from the Villum Foundation via the “Villum Young Investigator Program” Grant VKR022364 (U.G.N. and SSNMR equipment) and 600 MHz NMR (Villum Center for Bioanalytical Services, 600 MHz NMR instrument).
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REFERENCES
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