Al ratios

MICROPOROUS MATERIALS ELSEVIER

Microporous Materials 9 (1997) 1-12

Thermochemistry of Na-faujasites with varying Si/Al ratios Ivan Petrovic *, Alexandra Navrotsky Princeton Materials

Institute and Department

of Geological and Geophysical Sciences, Princeton NJ 08544, USA

University, Princeton,

Received30 November 1995;accepted6 May 1996

Abstract The thermochemicalstability of dehydrated sodiumfaujasites,Na,Al,Si,,-,,O,, with Si/Al ratios of 1.25 to 360 (x=0 to 0.44) has been studied by high temperature solution, drop solution, and transposedtemperature drop calorimetry near 977 K. Enthalpiesof solution becomemore endothermicas the Si/Al ratio decreases,implying an exothermic (stabilizing) enthalpy of the chargecoupledsubstitutionSi4+-+A13+ Na4 in thesestructures.Deviation of the samplewith Si/Al= 1.25(13X) from the linear trend suggeststhat there may be a maximum stabilization for the compositionwith Si/Al = 1. Theseobservationsare consistentwith previous studiesof chargecoupledsubstitutionsin aluminosilicateglasses and densesodiumaluminosilicates.Usingenthalpiesof solution of SiO,, Al,O,, Na,O (derived from Na,CO, drop solution experiments)and NaAlO,, standard molar enthalpiesof formation of dehydrated framework structuresfrom elements,simpleoxides, and NaAlO, and SiOZwere determined.Implications for the synthesisof high silica materialsand the apparent thermal stability are discussed. Keywords:

Faujasite;Framework; Stability; Enthalpy; Calorimetry

1. Introduction

More than three decades after the first synthesis of faujasite Y by Breck [I], the Si/Al ratio for directly synthesized materials based on the FAU framework has been raised to only about 4 (using a template), and for practical applications, post synthesis treatment or ‘secondary’ synthesis is being used to adjust the Si/Al ratio, which greatly effects both the catalytic activity (acidity) and the thermal stability. Nowadays, faujasite catalysts

* Corresponding author at (present address): Engelhard Corporation, 101 Wood Avenue, Iselin, NJ 08830-0770, USA. Tel. + 1 908 2055003; fax. + 1 908 2055268.

account for about one half of the annual market volume in the industrial use of zeolites [2]. Fundamental understanding of the structure/ stability relations, and especially of the effect of the aluminum content on stability of faujasite type catalysts is needed to provide a basis for the successful synthesis of new materials and development of new technological processes. Although considerable attention has been given to the thermochemistry of adsorption and ion exchange [ 31, essentially no experimental data exist concerning the energetics of the basic crystalline faujasites. A simple method for estimating missing enthalpies of formation is based upon the addition of enthalpies of formation of the constituent binary oxides. Estimates yielded by this model for

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dehydrated frameworks range from within 6 kJ/mol to about 220 kJ/mol away from the experimental value [4]. La Iglesia and Aznar [5] presented a similar empirical scheme, also based on contributions from the constituent oxides, for estimating the standard Gibbs free energies of formation of zeolites. The value they reported for dehydrated 13X is in very good agreement with that obtained from dissolution experiments [6]. This approach, however, neglects contributions due to differences in framework topologies. Computational techniques and modeling have been widely used to study stabilities of aluminosilicate zeolites. Free-valence geometry molecular mechanics calculations were carried out on a sodalite cage [7]. Van Santen et al. [8] studied the role of interfacial energy in zeolite synthesis. Relative stabilities of faujasite and mordenite as a function of Al/Si ratio, and substitution energies for Al atom in siliceous zeolites have been reported [9]. Heats of formation and lattice energies of several frameworks with varying Al/Si ratio obtained from energy minimization studies have been presented [ 10,111. Aluminum substitution defect energies in the ME1 (ZSM-18) framework have been calculated to understand preferred sites for framework aluminum [ 121. In previous papers we reported calorimetric studies of structure/ stability relations in a series of high and pure silica zeolites [ 13,141, and ordered silica mesoporous molecular sieves [ 151. Recently, thermochemical study of dense and microporous aluminophosphates was completed [ 161, and thermochemical data have been obtained for moganite, a dense silica polymorph containing 4-membered rings of SiO, tetrahedra [ 171. To extend this systematic study of the structure/ stability relations in micro-/mesoporous materials, we studied the energetics of charge coupled substitutions Si4+-+A13+ +Na+ in faujasite type frameworks. Using high-temperature solution, drop solution, and transposed temperature drop calorimetry near 977 K, we obtained standard enthalpies of formation of dehydrated frameworks. In this paper, we report these results, and discuss their implications for the synthesis and apparent thermal stability of these materials.

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2. Experimental

2.1. Samples Sodium faujasites with silicon to aluminum ratios of 1.25 to 360 were used in this study. Samples include the commercial materials 13X and NaY (Aldrich Chemical), dealuminated zeolite Y (DAY) kindly provided by Prof. M. Davis of the California Institute of Technology, and siliceous zeolite Y (SiliceousY) Tosoh 390 HUA, a commercial material from the Tosoh Chemical Co., Japan, kindly provided by Dr. John Cook of Tosoh, USA. All samples were calcined, free of organic molecules. An additional sample of sodium zeolite Y was synthesized using the following procedure: to 36.3 g of distilled water, 5.5 g of sodium aluminate (EM Science, technical grade), 5.6 g aqueous NaOH (50 wt.% NaOH) and 5.5 g 15-Crown-5 (Aldrich Chemical, 98%) were added, and stirred vigorously until all components were completely dissolved. Next, 37.5 g of colloidal silica (Ludox HS-40) was added, and the mixture was blended at room temperature for 48 h. Then, the gel was heated in a Teflon-lined autoclave at 383 K for 7 days at autogenous pressure. The final product was recovered by filtration, and washed several times with deionized water. To remove the occluded organic molecules, the sample was calcined in air at 823 K for 6 h. To distinguish this sample from the commercial NaY, throughout the text it will be referred to as Nay(l). 2.2. Analysis The physico-chemical properties of the faujasite samples were characterized by various techniques described below. X-ray powder diffraction (XRD) data were obtained with a Scintag PAD V automated diffractometer equipped with a Ge solid-state detector, using CuKa radiation. For qualitative identification of the phases present, the patterns were collected from 5 to 75” 28 with counting times 10 s per 0.03” step. For the Rietveld analysis of SiliceousY, the sample was pressed into a pack mount, and scanned from 4 to 120” 28 with counting time 15 s per 0.02” step.

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Chemical analysis of 13X, NaY, Nay(I), and DAY samples for silicon, aluminum and sodium was performed by Galbraith Laboratories, Inc., Knoxville, TN. A specimen of SiliceousY was analyzed for aluminum and sodium by inductively coupled plasma emission spectroscopy (ICPES) using a Perkin Elmer ICP-6000 instrument. Magic angle spinning 2gSi NMR spectra were recorded on a 360 MHz Bruker AMX 360 spectrometer at a 2gSi frequency of 71.5 MHz and a spinning rate of 3.75 kHz. The 90” pulse was obtained with a tetramethylsilane (TMS) standard and was 6 us. The experiment was run using a 4 us pulse or a 60” pulse, with a 60 s delay between pulses. Magic angle spinning 27A1 spectra were obtained using the same instrument at a 27Al frequency of 93.8 MHz and a spinning rate of 7.0 kHz. The 90” pulse was obtained using 1 M Al(NO,)s as a reference, and was 5 us. A short 800 ns pulse was used with an 80 ms pulse delay for 8000 scans. Both spectra were collected at 298 K (25°C). Thermogravimetric analyses (TGA) and differential thermal analyses (DTA) were performed in static air using the Netzsch STA 409 apparatus. TGA scans were run from 298 to 983 K at a heating rate of 10 K/min, and typically, 40-50 mg of sample was used. For DTA, specimens of 13X, NaY and NaY( 1) were heated from 298 to 1573 K, and both dealuminated samples were run up to 1823 K. In all cases, the heating rate was 10 K/min, and about 30mg of sample was used. 2.3. High temperature calorimetry All high-temperature calorimetric experiments were performed using a Tian-Calvet twin microcalorimeter operating near 977 K, described in detail elsewhere [ 181. The following three types of calorimetric measurements were performed. (i) High temperature solution calorimetry was performed with molten lead borate (2PbO. B,O,) solvent. The sample (typically 15-20 mg) was suspended in a platinum holder about 5 mm above the solvent, thermally equilibrated within the calorimeter, and then dissolved in the solvent. This technique measured directly the enthalpy of solution

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at the calorimeter temperature. (ii) In drop solution calorimetry, a small pellet weighing 15-20 mg was dropped from room temperature into the molten lead borate solvent in the calorimeter at 977 K. The enthalpy of the drop solution contains the heat effect associated with heating the sample from room to calorimeter temperature plus the heat of solution of the sample. (iii) Transposed temperature drop calorimetry involves experiments in which sample ( 15-20 mg), either as a pellet or wrapped in a crimped platinum capsule, was dropped from room temperature into the calorimeter at 977 K, without the presence of solvent. If no chemical change occurs, the measured heat effect corresponds to the heat content, (Hg7’ - H2g8), of the sample, otherwise both heat content and the enthalpy of the chemical change are included. Solution and transposed temperature drop calorimetry of platinum capsules were carried out in static air, drop solution and transposed temperature drop calorimetry of small pellets were done under flowing argon atmosphere with flow rate of ~40 cm3/min. Solution and transposed temperature drop calorimetry require that samples persist with no chemical or structural changes at the calorimetric temperature for 6-8 h (until the glass assembly achieves thermal equilibrium) and about 1 h, respectively. To ensure that faujasite samples satisfy this condition, their apparent thermal stability at 977 K was examined prior to calorimetry by performing a dummy run as follows. The specimen was X-rayed, then loaded into the calorimeter, equilibrated under the corresponding calorimetric conditions (with lead borate solvent present for solution calorimetry), removed from the calorimeter, and X-rayed again. A comparison of the starting and final X-ray diffraction patterns demonstrated the persistence or non-persistence of a given material at 977 K over the time period required for calorimetric experiments.

3. Results and discussion

3.1. Characterization X-ray powder diffraction patterns of the faujasites studied are shown in Fig. 1. All samples

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Fig. 1. Powder X-ray diffraction patterns of faujasites: (a) 13X, (b) Nay, (c) Nay(l), (d) DAY, and (e) SiliceousY.

showed very good crystallinity. Rietveld analysis of the pattern of SiliceousY yielded Rwp =0.071, and the refined structure is in an excellent agreement with that of ZDDAY (zero defect dealuminated Y) reported by Hriljac et al. [ 191, implying very good quality of this material. Chemical analyses are summarized in Table 1. Silicon to aluminum ratios obtained from wet

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analysis range from 1.25 to 360. In the case of SiliceousY the SiO, content was not determined directly, but was back-calculated using Al and Na contents, and the TGA weight loss. Though this sample is not completely aluminum free, in all the following considerations it will be treated as a pure silica material with a MW as that of silicon dioxide (60.0843). In all but the DAY sample, the framework aluminum is completely charge balanced by sodium. Magic angle spinning 27A1 NMR showed no evidence of octahedral aluminum in samples 13X, NaY and NaY( 1), and their “Si spectra confirmed the framework Si/Al ratios obtained from wet chemical analyses. The 27A1 spectrum of DAY (see Fig. 2) showed the presence of nonframework aluminum (peak near 1.5 ppm), which implies that the actual framework Si/Al ratio in this material is somewhat higher than that determined from wet chemical analysis. From the relative integrated peak areas it appears that the nonframework Al species constituted 29.6% of the total aluminum, and the Si/Al (framework) was 226. However, the total aluminum was only 0.27 wt.%, which is an order of magnitude less than that determined by wet chemical analysis (see Table 1). Thus, the NMR did not appear to account for all the aluminum in this sample. For this reason, the Si/Al= 14.7 will be used for the DAY sample in all further discussions. Because of the very low aluminum content in both dealuminated samples, their 2gSi spectra could not be used to confirm their Si/Al ratios obtained from chemical analyses. The spectrum of SiliceousY is shown in Fig. 3. The single sharp peak near - 108 ppm (corresponding to Si(OSi),) is consistent with the results of the Rietveld refinement, and further confirms a very good quality of this material.

Table 1 Results of wet chemical analysis, “Si and 27A1 MAS NMR and thermogravimetric analysis to 1023 K Faujasite

Si (Wt.%)

Al (wt.%)

Na (wt.%)

Si/Al

Si/Af from NMR

Octahedral Al

wt. loss (%)

13x NaY Nay(l) DAY SiliceousY

17.23 22.57 25.12 42.94 44.75

13.20 8.62 8.33 2.31 0.12

10.75 7.43 6.45 0.58 0.01

1.25 2.51 2.90 14.7 a =360

1.29 2.60 2.75 na na

no no no yes no

22.54 24.38 21.78 5.86 3.97

’ Actual framework Si/AI is higher (see Discussion in the text).

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Weight losses determined from thermogravimetric analyses (TGA) are listed in Table 1. Dealuminated samples, because of their hydrophobicity, were not subject to any special treatment prior to TGA runs; the samples of 13X, NaY and NaY ( 1) were equilibrated with atmospheric moisture in laboratory conditions (55% relative humidity) until constant weight. As expected, the weight loss increases with increasing Al content, reaching a maximum for NaY, but for 13X a small decrease is observed. This is somewhat surprising behavior, however, similar findings were previously reported by Li and Rees [20]. The weight losses were later used to correct the calorimetric data. 3.2. Calorimetry-charge coupled substitutions Si4’-+A13’ +Na+ ,““,““,““,““,~“,,““I.“,,~

80.0

40.0

0.0

-40.0

wm

Fig. 2. Magic angle spinning 27A1 NMR spectrum of dealuminated Y (DAY) sample showing presence of non-framework aluminum (peak near 1.5 ppm).

100.0

I”“/““I”“I”~‘,“~‘I”~‘,“‘~I”“,“’ 0.0

-100.0

-200.0

-300.0

wm

Fig. 3. Magic angle spinning ?Si NMR spectrum of SiliceousY (peak near - 108 ppm).

Tests of the apparent thermal stability revealed that, using the usual procedure of loading the sample into the calorimeter (~30 min loading followed by 6-6.5 h of thermal equilibration), all samples except 13X persisted without structural collapse at 977 K. Thus, NaY, NaY( l), DAY and SiliceousY could be used both for solution and transposed temperature drop calorimetry. The XRD pattern of 13X showed almost complete amorphization (see Fig. 4a). A previous powder XRD study [3] indicated that structural collapse of 13X is first observed after 16 h at 933 K, and that the structure is 50% decomposed at 1043 K. In an effort to minimize the time at 977 K, the loading procedure was modified such that the sample was first dehydrated for 2.5 h at 600 K using the thermal gradient in the calorimeter furnace, and only after thermal equilibration at 908 K, was it fully inserted into the calorimeter. This shortened the time at 977 K to less than 3.5 h. The XRD spectrum exhibited only a minor amorphous bump (see Fig. 4b), so the sample was included both in solution and transposed temperature drop calorimetric measurements. Thermochemical data are presented in Table 2. Both enthalpies of solution and heat contents refer to dehydrated materials. All data are calculated per two oxygen mole assuming complete charge balancing with sodium only (13X, Nay, NaY( 1 ), DAY), and treating SiliceousY as a pure silica

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(“)

Fig. 4. Powder X-ray diffraction patterns of the 13X sample after dummy run: (a) using ‘normal’ loading procedure, and (b) after modified loading.

Table 2 Enthalpies of solution and heat contents at 977 K, and molar weights of dehydrated faujasite samples Na,Al,Si,, -x,O, (molar weights refer to two oxygen mole) Faujasite

x

13x

0.44

NaY NaY(l) DAY SiliceousY

0.28 0.26 0.06 0

MW 11.15&0.61 9.47f1.16 b 4.40 + 0.48 3.09 kO.54 - 13.43 *0.53 - 14.07 +0.26

53.14kO.57

69.7141

49.91 &Oh2 48.10&0.49 42.73kO.31 40.66 + 0.22

66.2123 65.7746 61.3974 60.0843

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All values in Table 2 have been corrected for the weight loss as determined from TGA. In Fig. 5 enthalpies of solution are plotted as a function of aluminum content together with the results obtained previously on Na,Al&, -xjO2 glasses [ 21,221 and dense sodium aluminosilicates [21,23,17]. The general shape of the curves of enthalpy of solution vs. Al/(Al+ Si) are similar for all three systems. The heats of solution become increasingly endothermic in all cases for x60.5, and they tend to exhibit a maximum near x=0.5. This is most pronounced in glasses, where data exist to x=0.56, and is suggested in faujasites, where x,,, =0.44. This maximum implies an exothermic heat of mixing for the reaction of charge coupled substitution: xNaAl0,

+ (1 - x)SiO,

+ Na,Al,Si,,

-x,O,

(1) The process of dissolving these framework aluminosilicate structures in molten lead borate to form a dilute (co.5 wt.-%) solution breaks up the framework into isolated species, presumably silicate and aluminate tetrahedra and sodium cationic species dissolved in the borate matrix. Thus, the enthalpy of solution can be considered a measure of the strength of bonding, at least in a relative sense when comparing various compositions. 40

a Errors are reported as two standard deviations of the mean. b This value was obtained by combination of transposed temperature drop and drop solution calorimetry of small pellets.

material. Corresponding molecular weights of dehydrated materials are also listed in Table 2. Enthalpies of solution were obtained for all samples from solution calorimetry and, in the case of 13X, also from drop solution and transposed temperature drop calorimetry of small pellets. Since there is almost 2 kJ/mol difference between the two values of 13X (consistent with partial amorphization), the drop solution value will be used in the following thermochemical calculations.

/

0.0

0.1

0.2

0.3

/

-13x

0.4

0.5

0.6

AI/(AI+Si)

Fig. 5. Enthalpies of solution in lead borate solvent near 977 K of Na,Al&, -xIO2 faujasites, sodium aluminosilicate glasses, and dense sodium aluminosilicates as a function of Al content.

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Following the formalism of Roy and Navrotsky [24], we can define a heat of stabilization for the Si4+-+A13+ +Na+ charge coupled substitution as follows:

(2)

For ~~0.5, where the increase in enthalpy of solution is approximately linear with increasing x, AHstab is essentially defined by the negative of the slope of AH,,, vs. x. The average AHstab for faujasites determined from the linear least squares fit is - 59.81 kJ/mol, which is very similar to values for glasses: -62.55 kJ/mol, and slightly less stabilizing than for dense silicates: -75.94 kJ/mol. The systematics here imply that the stabilization energy may increase somewhat with increasing density, with only a small difference between glasses and faujasites. Values as determined for individual faujasite samples are given in Table 3. The rather significant deviation of the DAY specimen is attributed to its somewhat lower sodium content than is required for full charge balancing, and uncertainty in the true framework Si/Al ratio. Since x is very small (0.06), even a small inaccuracy in this value will translate to a relatively large deviation of AHstab (kJ/mol) from other values. Additional uncertainty is introduced by AH,,, of DAY, which may be affected by its non-stoichiometric composition. The disagreement between NMR and chemical analysis for total Al content may also suggest that this sample is somewhat problematic. The strong exothermic heat for the substitution is contrary to the constant volume Table 3 Relative stabilization of the substitution Si4’ +A13+ Na,Al,Sio - x,O2 dehydrated faujasite type structures Si/Al

+Naf

x ;~~~~;)

~360~ 14.7 2.90 2.51 1.25

0 0.06 0.26 0.28 0.44

a Calculated using Eq. (2). b Sample treated as a pure silica material.

N/A - 10.67 -66.00 - 65.96 -53.50

in

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energy minimization findings of Jackson et al. [ 91, who reported a substitution energy of 37.54 eV for a single Al atom, implying destabilizing character of the substitution. The nature of the non-framework charge-balancing cation in this simulation, however, is not clear. A previous study [24] of the charge-coupled substitutions Si4+ +A13+ + l/nM”+ in glasses confirmed regular systematics for the entire alkali and alkaline-earth series. The increase in enthalpy of solution becomes more pronounced with decreasing field strength (or increasing basicity) of the non-framework cation, and can be related to the ability of the non-framework cation to perturb the bridging oxygen. Later it was also shown that this perturbation can be modeled by ab initio molecular orbital calculations on small clusters [25]. DeYoreo et al. [ 261 found that AHstab for substitutions in xNaTO,-( 1 -x)SiO, glasses (T = Al, Ga, Fe, B) becomes less exothermic in the order Al, Ga, Fe, B, and correlated it with the flexibility of the T-O-T angle as measured by the width of the minimum in the potential energy. These observations and suggested similar systematics of the energetics of the Si4+ -+A13+ + Na+ substitution in dense, glassy and microporous aluminosilicates provide a basis for a speculation that similar behavior of the energetics of substitutions as observed in glasses may be expected also in microporous zeolitic materials with corresponding composition. Such systematics can be used as a powerful predictive tool for studying both isomorphous framework substitutions and various ion-exchanged forms in the fast growing field of molecular sieves. The heat contents, (H9,, - Hz&, were measured by transposed temperature drop calorimetry using platinum capsules as described above. All samples were preheated at 993 K prior to the measurements to drive off the water, then tightly crimped in platinum and stored in a desiccator. Throughout the experiment, all samples maintained constant weight, implying that neither additional weight loss nor re-hydration took place. The zeolite specimen typically contributed about 75% to the total measured heat effect, the platinum about 25%. Fig. 6 shows heat contents plotted vs. Al/(Al+ Si). A linear trend of increasing heat content with

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DAY

4o

y

-I 0.0

SiliceousY I

I

I

I

I

0.1

0.2

0.3

0.4

0.5

AI/(AI+Si)

Fig. 6. Heat content at 977 K plotted versus Al/(AI + Si).

increasing aluminum content is observed. This increase can be attributed to the changing composition, specifically to the increasing number of nonframework sodium atoms present in the structure (i.e. increasing number of total atoms per two oxygens). In high silica zeolites, the relative enthalpy (H9,, - HZ& decreases with decreasing framework density [ 131. Similar comparison among sodium aluminosilicates with nearly identical composition (~~0.25) shows that the heat content at 977 K may increase with decreasing framework density: low albite -NaAlSi,Os(46.82 kJ/mol) < albite glass (48.11 kJ/mol)% NaY( 1) (48.10 kJ/mol). Data for albite and albite glass, adjusted to a two oxygen mole, were taken from Robie et al. [27].

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oxides and the molar enthalpies of formation from oxides obtained in this study (see below) we calculated the standard molar enthalpies of formation of dehydrated faujasite frameworks from the elements. The following enthalpies of formation of binary oxides were used: -910.70 kJ/mol for SiOZ (a-quartz), taken from CODATA [29], - 1675.69 kJ/mol for cl-Al,O, (corundum) and -417.98 kJ/mol for Na,O, both from JANAF tables [ 301. The enthalpies of formation of dehydrated faujasite frameworks from elements are listed in Table 4 and plotted as a function of aluminum content in Fig. 7. The plot shows a strong linear Table 4 Standard molar (per two oxygen mole) enthalpies of formation of dehydrated sodium faujasite frameworks (Na,Al,Si+,,O,) from elements, oxides, and from sodium aluminate and quartz Si/Al

Enthalpy of formation (kJ/mol) from elements 1.25

- 1012.4 -975.5 -968.8

2.51 2.90 14.7 ~360"

oxides

NaAlO,/SiO,

-41.79

-3.40 -2.21 0.02 13.02 13.06

-26.64 -22.66 1.78 13.06

-911.1 -897.6

a Sample treated as pure silica. -880

3.3. Enthalpies offormation - implications for synthesis and thermal stability According to the study of Navrotsky et al. [28], high temperature solution calorimetry with lead borate solvent can be used to determine enthalpies of formation of both anhydrous and volatilebearing phases. Recently, this technique has been successfully used to study the relative stabilities of several dense and microporous aluminophosphates [ 161. Using thermodynamic values of the constituent

-1000 -

13x \

-1020 0.0

I 0.1

I 0.2

I 0.3

'

I 0.4

0.5

AI/(AI+Si)

Fig. 7. Standard molar enthalpies of formation from elements of dehydrated faujasite frameworks as a function of Al content.

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dependence of A@ on Al/(Al + Si). The general trend seen is that the enthalpies become more negative as the aluminum content increases. This overall stabilization is consistent with the exothermic heat of the Si4+ -Al3 +Na+ substitution as discussed above. Energetics here imply that even without effects of hydration, faujasite frameworks become more stable, with respect to the elements, with increasing aluminum content. Considering that the exothermic contribution from hydration significantly increases with increasing Al/Si [ 3 11, the dependence of the total heat of formation on Al content will be even stronger for hydrated materials. The apparent discrepancy between increasing energetic stability and decreasing thermal stability is discussed below. Previous computational studies [ 10,111 reported less negative A@ from elements of dehydrated frameworks with increasing Al content (destabilizing nature of the substitution) for several structures including faujasite, and when hydration was included, only a very weak dependence was seen for faujasite with a minimum at Al/Si ~0.4. In both cases, sodium ions were used to charge balance framework aluminums. By contrast, Mabilia et al. [7] reported that the sodalite cage becomes more stable as the aluminum content increases. In this study, the presence of the non-framework cations was taken into account by adjusting the partial atomic charges of the oxygens bonded to Al atoms, so that the overall charge was zero. Since A@ values from elements include contributions from constituent oxides, they are in general large exothermic values, including for the metastable phases. To gain better insight into the structure/stability relations it may be more useful to consider Am from oxides, or in this case specifically, from SiO, and NaA102, instead. The standard molar enthalpies of formation from the oxides were determined from the thermochemical data of the constituent oxides (see Table 5) and calorimetric data obtained in this study for Na,Al,Si,, - XjO2 faujasites (see Table 2). For the reaction involving the formation of the Na,Al& - XjO2 framework from simple oxides at 298 K: -,O,

(3)

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Table 5 Enthalpies of solution and heat contents at 977 K of oxides (Na,O, A1,03 (corundum), Si02 (synthetic quartz)), and of monosodium aluminate (NaAlO,) used in calculations of the enthalpies of formation of faujasites Oxide Na,O

A&OS SiO, NaAlO,

Source

;\kkL~;a

;‘k’fZl

- 172.21) 3.26 32.97 kO.86 b -4.35kO.12 19.66 +0.46

)

1% 301

57.85 75.12 44.00 64.44

this work, [30] ]17,331

DW’I

’ Errors are reported as two standard deviations of mean. b Average of 31 measurements done in this laboratory.

the following thermochemical cycle is used to calculate the standard molar enthalpy of formation: x/2Na,O

(s, 298 K)+x/2Na,O

(s, 977 K) AH1 (4)

x/2Na,O

(s, 977 K)+x/2Na,O

(soln., 977 K) AH2 (5)

x/2A1203 (s, 298 K)-*x/2Al,O,

(s, 977 K) AH, (6)

x/2Al,O,

(s, 977 K)+x/2Al,O,

(soln., 977 K) AH4 (7)

(1 -x)SiO,

(s, 298 K)+(l

-x)SiOZ

(s, 977 K) AH, (8)

(1 -x)SiO, A& Na,Al,Si,

(s, 977 K)+(

1 -x)SiO,

(soln., 977 K) (9)

-XO2 (s, 977 K)+Na,Al,Si,

-XO2 (10)

(s, 298 K) -AH7 x/2Na,O

(soln., 977 K) +x/2Al,03

+( 1 -x)SiO,

(soln., 977 K)

(soln., 977 K)+NaAl,Si,-,O,

(s, 977 K) -AH8 x/2Na,O

x/2Na,O + x/2 Al,O, + ( 1 -x) SiO, +Na,Al,Si,

Materials

0, A@

+ x/2Al,O,

(11) + ( 1 - x)SiO, +Na,Al,Si,

_X (12)

10

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from which

*OI(5

AH; = AH, + AH2 + AH, + AH4 + AH, s

i- AH6 - AH7 - AH8

(13)

where AHI, AH,, AH5, and AH, represent the heat contents (Hgy7- Hz&, and AH,, AH4, AHs, and AH, refer to the enthalpies of solution at 977 K. All calculations are based on a two oxygen mole, and the enthalpies are reported correspondingly. To the best of our knowledge, with the exception of the SiliceousY, this work represents the first determination of the enthalpies of formation for synthetic faujasite-type zeolites. To calculate enthalpies of formation from SiO, and NaA102, heat contents and enthalpies of solution of Na,O and Al,O, (Eqs. 4-7) in the above thermochemical cycle are substituted by corresponding values of NaAlO, (see Table 5), and the enthalpy of formation is calculated according to Eq. (14)

A@ = AH, + AHlo + AH, + AH6 -AH,

9 (1997)

3E s. 5 'Z

IO 0 _____

l-12

A 0 ------------*a-------

-----A

-10

-

c g z 5 2 E w

-20

-

0 0

-3o-

-40

-

0.0

0

0.1

0.2

0.3

0.4

0.5

AI/(AI+Si) Fig. 8. Standard molar enthalpies of formation of dehydrated faujasite frameworks as a function of aluminum content (solid circles ~ from simple oxides Na,O, Al,O,, 50,; open triangles - from NaAIO, and SiOJ

-AH, (14)

where AH, and AH,, are heat content and enthalpy of solution, respectively, of NaAlO,. Results for both the enthalpy of formation from oxides and from quartz and sodium aluminate are summarized in Table 4 and plotted in Fig. 8 as a function of aluminum content. Several interesting features are noted in this plot. First, both dependences extend from endothermic to exothermic field, and they both become more exothermic as the Al/(Al+Si) increases. Second, for pure silica composition they merge to the same endothermic value of 14.06 kJ/mol. The Al/(Al+Si) ratio at which they cross the ‘zero’ line is different, and is shifted towards lower values for A- from oxides. From the synthesis point of view, it may be interesting to note that the Si/Al at which A@ (SiO,/NaAlO,) changes from exo- to endothermic (Si/A1=3), roughly corresponds to the current limit for direct synthesis. The energetics here may imply that any faujasite material (at least in the case of dehydrated sodium frameworks) with Si/Al higher than ~3-3.5 would be metastable with respect to quartz and sodium aluminate, and with Si/Al higher than ~9 also with respect

to binary oxides. For catalytic cracking applications, the optimum zeolite component has been reported to be SiOz/A1203 of about 14 [34]. Since hydration of the charge balancing non-framework cation is an exothermic process, thus making the total A& more negative (stabilizing), we speculate that for hydrated faujasites energetics may be a favorable factor for direct synthesis of materials with or near optimum Si/Al of about 7. Fig. 9 shows the apparent thermal stability of faujasites as a function of Si/Al ratio. The temperature of the first exothermic peak in the DTA trace was taken as the temperature at which structural collapse occurred. Results obtained in this work are in very good agreement with those reported previously [ 35,361. Apparent thermal stability significantly increases with increasing Si/Al, with a smooth increase of the decomposition temperature for Si/Al= 1.23-7, followed by a rather steep increase for Si/Al = 14.7, and essentially no further change between Si/Al= 14.7 and 360. Both dealuminated samples collapsed near 1800 K. This contrast between increasing thermodynamic stability (in terms of enthalpy of formation) and decreasing thermal stability with increasing aluminum content can be considered in terms of the

I. Petrovic,

A. Navrotsky

/ Microporous

Materials

9 (1997)

I-12

11

4. Conclusions

l

l

cristobalite

0

1100

0

5

10

15

thiswork

360

Si/Al Fig. 9. Apparent %/Al ratio.

thermal

stability

of faujasites

as a function

1 1 of

decomposition products and their energetics. Examination of the decomposition products showed amorphization in the case of 13X, NaY and NaY( 1 ), and transformation to cristobalite of both dealuminated samples. Enthalpy of solution data (Fig. 5) show that the stability of glasses increases with decreasing %/Al even more than in faujasites. This means that the energy difference between faujasites and glasses (decomposition products) of equivalent composition increases with decreasing Si/Al, thus affecting their thermal stability. Dealuminated materials transform directly to cristobalite, which requires opening of the 4-membered rings of Si04 tetrahedra present in the faujasite structure, but absent in cristobalite. Since the activation energy for breaking the Si-0 bond is high, and only thermal energy is available, this may explain high decomposition temperature and its insensitivity to the Si/Al ratio of these samples. Also, Al-O bonds are weaker and break more easily, both thermodynamically and in terms of activation energy, than Si-0. Furthermore, the Na may play a role by ‘pulling’ on the oxygen in the breaking T-O-T bonds. All these factors cooperate to make the thermodynamically metastable faujasites decompose more readily, in a kinetic sense, to denser assemblages, with increasing aluminum content.

High temperature calorimetry has been used to study stability of sodium faujasites as a function of increasing Al content. Enthalpies of solution show that, at 977 K, stability increases with increasing aluminum content. This observation is consistent with previous studies of dense and glassy sodium aluminosilicates, but is contrary to the results obtained from computational energy minimization studies. The standard molar enthalpies of formation of dehydrated faujasites from the elements, binary oxides, and quartz and sodium aluminate have been obtained. The general trend in the enthalpies of formation is, that with increasing Al/Si they become more negative, implying increasing stability. Results suggest that, if enthalpies of hydration were included in the enthalpies of formation, the energetics may be favorable for the direct synthesis of high-silica faujasites with 4
Acknowledgment We thank Prof. M.E. Davis of CalTech and Dr. J. Cook of Tosoh, USA, for providing the samples of dealuminated faujasites, and M. Simpson of Princeton University for help with the synthesis. We also thank Dr. S. Schramm and C.E. Chase of Mobil Research and Development Co. for collecting the MAS-NMR spectra, and M. Borcsik from Princeton University for ICPES analysis. This work was supported by the US Department of Energy (Grant DE-FG 02-85ER13437) and by the New Jersey Commission on Science and Technology.

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