Solid transformation of zeolite NaA to sodalite

Microporous and Mesoporous Materials 130 (2010) 303–308

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Solid transformation of zeolite NaA to sodalite Lianhui Ding a,*, Hong Yang a, Parviz Rahimi a, Oladipo Omotoso b, Wally Friesen b, Craig Fairbridge b, Yu Shi a, Siauw Ng a a b

National Centre for Upgrading Technology (NCUT), CanmetENERGY (NRCan), 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada CanmetENERGY (NRCan), 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada

a r t i c l e

i n f o

Article history: Received 24 August 2009 Received in revised form 24 October 2009 Accepted 20 November 2009 Available online 26 November 2009 Keywords: Solid transformation Zeolite–sodalite Zeolite NaA

a b s t r a c t Sodalite was successfully synthesized by solid-state transformation of NaA in the presence of excess NaCl. The transformation conditions, effect of cation exchange, and treatment agents were investigated. The transformation mechanism is proposed as zeolite A ? carnegieite ? sodalite. When the temperature is above a threshold value, the transformation is abruptly accelerated. K- and Cs-exchanged zeolite A are transformed more rapidly than NaA. Sodalite could not be formed when the NaA was treated with KCl. Crown Copyright Ó 2009 Published by Elsevier Inc. All rights reserved.

1. Introduction The mineral sodalite with the chemical composition Na8[AlSiO4]6Cl2 was first described in 1811 by Thompson, and its structure was solved in 1930 by Pauling [1]. The framework of sodalite is completely made up of regular truncated octahedral cages called sodalite cages. These cages consist of six four-member rings and eight six-member rings. Therefore, the largest pore size is about 2.4 Å, the smallest among all zeolites. In addition to many studies on the theoretical and experimental principles of zeolite chemistry, there have been extensive investigations on industrial applications of sodalite in the fields of pigments, synthesis of nanocomposites and special host matrices for quantum dot materials, and waste management [2–7]. Aluminosilicate sodalites can be synthesized by low-temperature condensation reactions in basic solution [8–10], high-temperature, solid-state sintering [11], and structure transformation [12,13]. The appropriate synthesis route is usually determined by the stability of the cavity anion with respect to temperature and base. Sodalites are normally synthesized with sodium as the extra-framework cation, but ion-exchange allows many substitutions (e.g., Li, Ag, K.) to be performed. Solid transformation of the zeolite via converting zeolite A to zeolite–sodalite is a good technique for synthesizing sodalite when some of the anion host precursors are difficult to encapsulate in the sodalite cages under the synthesis conditions, while entering relatively easily into the cages of zeolite A. Studies on * Corresponding author. Tel.: +1 780 987 8713. E-mail address: [email protected] (L. Ding).

high-temperature phase transformation of zeolite NaA with various degrees of exchange of Li+ revealed that a the presence of a large number of Li+ ions per unit cell accelerated the thermal transformation of the zeolite framework to an amorphous state [14]. Above 730 °C, four phases (carnegieite, nepheline, b-eucryptite, and c-eucryptite) were identified. Although Weller and Wong [15] did mention the synthesis of sodalite by the structure transformation of zeolite Linde 4A in their study on the characterization of sodalite by neutron diffraction and solid-state NMR, the conversion mechanism was not systematically investigated. By studying the thermal transformation of Li+-, K+-, Cs+-, þ and NHþ 4 -exchanged zeolite X, NH4 -exhanged mordenite, and the X-ray amorphous materials obtained by ball milling of the above crystalline phases, Kosanovic et al. [16] demonstrated that most crystalline zeolites transformed to other crystalline phases without formation of amorphous intermediates. In some cases of Li+- and K+-exchanged zeolite A, an amorphous intermediate was formed and proceeded into secondary crystalline phases other than zeolite–sodalite. Many useful molecular sieve zeolites are known to be metastable in their hydrothermal synthesis environments. The metastable transformation of molecular sieve zeolite NaA to hydroxysodalite (HS) in hydrothermal systems has been demonstrated [17]. These previous studies on solid transformation focused on NaA or cation-exchanged NaA. There have been almost no studies aimed at selective formation of sodalite via the solid transformation of NaA in excess NaCl. The objective of the present work is to investigate the synthesis of sodalite by solid transformation of zeolite A, including the effects of K+, Na+, and Cs+ ion-exchange and treatment conditions on the transformation.

1387-1811/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.11.025

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L. Ding et al. / Microporous and Mesoporous Materials 130 (2010) 303–308

2. Experimental 100x10

3

2.1. Solid transformation of zeolite A 80

Counts

All chemicals were purchased from Sigma Aldrich, and used as received. Zeolite NaA (20 g, Linde LTA, chemical composition Na12[Al12Si12O48]) was ion-exchanged with 500 mL 0.5 N KCl aqueous solution three times at 80 °C (3 h each time). Between the ionexchanges, the slurry was washed with deionized water. After the last exchange, the cake was washed until free of Cl. The ion-exchange degree was greater than 85% (chemical composition: K>10.2Na<1.8[Al12Si12O48]). Using the same method, zeolite NaA was ion-exchanged with 100 mL 0.2 N CsNO3 aqueous solution twice at room temperature (24 h each time). The ion-exchange degree was roughly 45% (chemical composition: Cs5.4Na6.6[Al12Si12O48]). Zeolite NaA or ion-exchanged KA and CsA (4 g) was mechanically mixed with 16 g NaCl, KCl, or LiCl in a mortar for 30 min. The mixture was calcined in a furnace at 800–900 °C (ramp: 30 °C/min from room temperature) for 2–24 h and then washed with deionized water until free of Cl. The washed zeolites were oven-dried at 110 °C for approximately 12 h. The samples were designated as S-Na (or K or Cs)-x-y, where Na or K or Cs stands for Na- or K- or Cs-exchanged zeolite A and x is the treatment temperature and y is the treatment time.

Na-A-900-6

60 Na-A-800-6

40

20 Na-A

20

40

60

80

2θ (º) Fig. 1. XRD profiles of NaA transformation at 800 and 900 °C in absence of NaCl.

250x10

3

carnegiete

Na-sodalite

200 S-Na-900-6

2.2. Characterization of the zeolites

Z% ¼ 100%  P=P s where Z% refers to the percentage of phase Z (NaA, carnegieite, sodalite etc.); P and Ps are the total areas of the five strongest peaks of the phase Z in the XRD profiles of the tested sample and standard pure zeolite Z respectively. High-resolution SEM (HRSEM) was performed using a Hitachi S4800 field-emission scanning electron microscope (FESEM) capable of operating at accelerating voltages ranging from 0.5 kV to 30 kV. Powder samples were mounted on conductive carbon adhesive tabs for convenience of SEM observation. Thermogravimetry (TG) and differential thermal analysis (DTA) were performed simultaneously using a Mettler-Toledo TGA/SDTA851e module; 20–30 mg of powder sample was placed in a 70 lL alumina crucible and purged for 15 min at ambient temperature in N2 (50 ml/min), then heated from ambient temperature to 1273 K at a rate of 10 K/min in the same gas mixture. 3. Results and discussion 3.1. Effect of temperature on NaA–NaCl transformation to sodalite Fig. 1 illustrates XRD profiles of NaA transformation at 800 °C and 900 °C without the addition of excess NaCl. In the absence of a source of excess sodium, NaA was transformed completely to hexagonal nepheline (NaAlSiO4) above 800 °C. In the presence of excess NaCl, carnegiete (NaAlSiO4) becomes the more stable phase as indicated in Ref. [18]. In this study, when the mixture of NaA and NaCl was treated at 800 °C and above, carnegeite was formed, and subsequently transformed to sodalite. The XRD profiles of a treatment sequence are given in Fig. 2 and the rates of sodalite formation for the various treatments are given in Table 1 and Fig. 3.

Counts

The solid products were characterized by powder XRD using a Bruker D8 Advance diffractometer equipped with twin Co X-ray monochromating parabolic mirrors on the incident and diffracted beam sides. The percentage of each phase was calculated by the following equation:

S-Na-900-4

150 S-Na-900-2

S-Na-900-1

100

S-Na-800-24 S-Na-800-6

50

S-Na-800-2 Na-A

0 20

40

60

80

2θ (º) Fig. 2. XRD profiles of NaA–NaCl mixture showing the transition from NaA to sodalite as a function of heat treatment.

The nucleation and crystal growth rates can be predicted from the induction time (the time before any sodalite phase is detected by XRD) and the slopes of the crystallization curves, respectively. Almost no induction time was required at temperatures of 800 °C and 900 °C, suggesting that nucleation was very rapid (almost no nucleation time was needed). At 800 °C, the sodalite crystal growth rate was low (with only 29% sodalite formed after 6 h). Within 6 h, 94% of NaA was converted to sodalite at 850 °C and over 99% of the NaA converted to sodalite at 900 °C (Table 1). This is consistent with the phase transformation observed at approximately 820 °C, above the melting point of NaCl (Fig. 4). The diffraction patterns show that transformation to sodalite proceeds through carnegiete. The phase composition changes with the treatment time at 800 °C and 900 °C are shown in Figs. 5 and 6, respectively. At both temperatures, transformation of NaA to carnegiete occurs concurrently with the formation of sodalite from carnegiete. At 900 °C (Fig. 6), NaA transformed completely to carnegiete within 2 h. Many molecular sieve zeolites are known to be metastable in their hydrothermal synthesis environments. Zeolite NaA can be transformed to hydroxysodalite (HS) in hydrothermal systems [17]. Although the solid transformation currently studied is different from the hydrothermal system, the same

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L. Ding et al. / Microporous and Mesoporous Materials 130 (2010) 303–308 Table 1 Effect of treatment temperature on the transformation. NaA

Treatment temperature, °C Treatment time, h Phase composition, wt% Zeolite A: NaA Carnegieite (NaAlSiO4) Sodalite (Na4Al3Si3O12Cl) Cell parameter of zeolite A (Å)

100

24.571

S-Na-8006

S-Na-8506

S-Na-9006

800 6

850 6

900 6

69 2.3 29 24.635

BDL 6.1 93.9

BDL 0.3 99.7

Percentage of various phases, %

Phase name

80

NaA

70 60 50 40

Sodalite

30 20 10

Carnegetite 0 0

100

5

10

15

20

25

Time, hr

o

900 C

Fig. 5. Phase composition change with treatment time at 800 °C.

60

120

o

100

800 C

40

20

0 0

5

10

15

20

25

Calcination time, h Fig. 3. Crystallization curves of sodalite when treated at 800 and 900 °C.

Percentage of various phases, %

Sodalite content,wt %

80

Sodalite 80

60

40

20

Carnegetite 0 2.5 2.0 1.5

NaA+NaCl NaCl NaA Sodalite

NaA 0

1

2

3

4

5

6

7

Time, hr Fig. 6. Phase composition change with treatment time at 900 °C.

Delta T

1.0

formation of amorphous aluminosilicate intermediates was not observed. The proposed transformation sequence is:

0.5 0.0

zeolite A ! carnegieite ! sodalite

-0.5 -1.0 -1.5 -2.0 750

800

o

850

900

Temperature, C Fig. 4. Amplified DTA profiles of NaA, NaA + NaCl, NaCl, and sodalites.

conclusion, that sodalite is a more stable phase than zeolite NaA, still applies. In several studies, the framework has been observed to collapse to form an amorphous phase, followed by growth of a new crystalline phase [14,16,19]. In this study, for the NaA + NaCl system, the

The carnegieite phase was identified in the entire transformation process. The transformation rate of a metastable zeolite A phase depended on the rates of formation of intermediates and carnegieite, as well as the crystallization rate of sodalite. During the solid transformations at both temperatures, no significant amounts of the intermediates were identified, suggesting that the crystallization of carnegieite and sodalite proceeded rapidly, and at a rate similar to that of the formation of the intermediates. The transformation of zeolite A to carnegieite became the ratedetermining step and, therefore, temperature had a significant impact on the transformation. The morphologies of the samples thermally treated with the mixtures of NaA and NaCl at various temperatures and for various times are shown in Fig. 7. At 800 °C, from the SEM images, large amounts of bars or sheets were observed. All these bars or sheets were probably NaCl crystals that had not melted or remained

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Fig. 7. SEM micrographs of zeolite A, samples treated at 800 °C for 2, 6, and 24 h and at 900 °C for 1, 2, 4, and 6 h, and zeolite–sodalite.

unconverted zeolite A crystals. Further studies need be conducted to identify these bars and sheets. At higher temperature (900 °C),

well above the melting point of NaCl (801 °C), NaCl crystals melt quickly, and the solid transformation proceeded rapidly. As a

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L. Ding et al. / Microporous and Mesoporous Materials 130 (2010) 303–308

result, the treated samples showed the appearance of zeolite–sodalite after 1 h. It was assumed that the activation energies of the breaking of ‘‘external” Si–O–Si and Si–O–Al bonds of the zeolite are lower than the activation energies of rotation and translation of primary and secondary building units of the zeolite framework. LTA contains double 4R connecting sodalite cages. The ring tension is likely to be highest in the small 4R [20]. Less energy is needed to break the 4R bonds, and zeolite A is broken to sodalite cages. These sodalite cages are easily crystallized to sodalite through carnegieite. 3.2. Effect of cation exchange of NaA on the transformation

3.3. Effect of treatment reagents on the NaA transformation

Fig. 8. XRD profiles of NaA treated with NaCl, KCl, and LiCl at 800 °C for 6 h.

100 90 80 70

Mass Loss, wt%

For a certain SiO2/Al2O3 ratio and zeolite type, thermally induced, solid phase transformations mainly depend on the ionic radius of the cations [14]. Therefore, the solid transformations of the K- and Cs-exchanged NaA were studied. The phases and their compositions obtained from XRD are summarized in Table 2. K+- and Cs+- exchanged zeolite A were more readily converted to carnegieite and sodalite after thermal treatment at 800 °C for 6 h, than NaA. The transformation rate of the ion-exchanged zeolites in the presence of excess NaCl decreased in the order: CsA > KA > NaA. From experiment section, it can be noted that the degrees of Cs+ and K+ ion-exchange are 45% and 85% respectively. Although no study was conducted on the effect of the degree of ion-exchange on the transformation rate, it can be reasonably concluded that the discrepancy in transformation rate of the CsA and KA is mainly due to the difference of the cations form of zeolite A, and higher Cs+ and K+ ion-exchanges will favor the acceleration of the transformation rate.

60

NaA NaA+NaCl NaCl KCl NaA+KCl LiCl NaA+LiCl Sodalite

50 40

In order to investigate the effect of treatment reagents on the transformation, NaA was mixed with NaCl, KCl, and LiCl, and then treated at 800 °C for 6 h. The phase composition from XRD, TG, and DTA results for NaA, KCl, NaCl, and their mixtures with zeolite NaA are given in Table 3 and Figs. 8–10, respectively. The weight losses

30 20 0

200

400

600

800

o

Temperature, C

Table 2 XRD results of solid transformation of K- and Cs-exchanged NaA. Phase Name

S-Na-800-6

S-K-800-6

S-Cs-800-6

Treatment temperature, °C Treatment time, h Phase composition, wt% Zeolite A: NaA Zeolite A: (Na, K)A Zeolite A: (Na, Cs)A_ Carnegiete (NaAlSiO4) Sodalite (Na4Al3Si3O12Cl) Cell parameter of zeolite A (Å)

800 6

800 6

800 6

10

69 58 2.0 40 24.725(2)

54 3.0 43 24.637(3)

Delta T

2.3 29 24.635(3)

Table 3 Effect of treatment reagents on the transformation of NaA.

Treatment temperature, °C Treatment time, h Phase composition, wt% Zeolite A: NaA Zeolite A: (Na,K)A a-Li5AlSi2O8 Carnegiete (NaAlSiO4) b-eucryptite (LiAlSiO4) Sodalite (Na4Al3Si3O12Cl) Sodalite (Li8Al6Si6O24Cl2)

Fig. 9. TG profiles of NaCl, KCl, LiCl, NaA, NaA + NaCl, NaA + KCl, NaA + LiCl, and sodalite.

NaA + NaCl

NaA + KCl

NaA + LiCl

800 6

800 6

800 6

0 NaA NaA+NaCl NaCl KCl NaA+KCl LiCl NaA+LiCl Sodalite

69 100 2.3

0

29

0

32 0 41 27

-10 600

700

800

900

o

Temperature, C Fig. 10. DTA profiles of NaCl, KCl, LiCl, NaA, NaA + NaCl, NaA + KCl, NaA + LiCl, and sodalite in 600–900 °C range.

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L. Ding et al. / Microporous and Mesoporous Materials 130 (2010) 303–308

Table 4 Weight loss at various temperature ranges (wt%). Sample

<250 °C

250–700 °C

700–1000 °C

NaA LiCl LiCl + NaA NaCl NaA + NaCl KCl NaA + KCl Sodalite

16.01 39.53 29.93 0.09 3.63 0 3.62 2.45

3.48 0.98 1.61 0.24 0.70 0.37 0.63 1.85

0.22 44.89 40.77 21.27 21.41 23.89 19.20 0.56

15

10

Delta T

5

0

NaA NaA+LiCl LiCl sodalite

-5

-10

intermediates to Li–sodalite becomes the rate-determining step, although the mechanism is the same for the two systems. NaA was converted to a-Li5AlSi2O8 and b-eucryptite (Li(AlSiO4) followed by crystallization of sodalite (lithian). A similar amount of sodalite was produced at 800 °C for both NaCl (29 wt%) and LiCl (27 wt%). The melting points of LiCl, NaCl, and KCl are 605, 801, and 776 °C, respectively. LiCl can exchange for Na in NaA in solution at low temperatures as well as after melting at 605 °C. An increase in the number of lithium cations per unit cell accelerates the thermal transformation of the zeolite. Structurally, sodalite is based upon simple truncated octahedra (or b-cages) linked in three dimensions. The compositional formula of the framework is expressed as [Al6Si6O24]6. Only three monovalent cations (e.g., Na+) per sodalite cage are required for charge compensation of the negatively charged framework, and an excess mineral salt unit (e.g., NaCl) is typically incorporated during the synthesis. Inside the cages of such salt-bearing sodalites, a central anion is surrounded tetrahedrally by four sodium cations. During the solid transformation of NaA, the salt-bearing compounds used were LiCl, NaCl, or KCl. Under the conditions investigated, KCl does not appear suitable for incorporation into the cage to form sodalite. Although the melting point of KCl (776 °C) lies between those of NaCl (801 °C) and LiCl (605 °C), and the transformation temperatures (>800 °C) are higher than its melting point, NaA can not be converted to sodalite when mixing with KCl, suggesting that the sizes of cations rather than the melting points play an important role in determining whether NaA is able to be transformed. The exact reason behind this need be further studied. 4. Conclusions

-15 600

700

800

900

o

Temperature, C Fig. 11. Amplified DTA profiles of NaA, NaA + LiCl, LiCl, and sodalites.

at various temperature ranges are calculated and summarized in Table 4. In order to clearly illustrate the phase changes of NaA + NaCl, and NaA + LiCl systems, the DTA profiles between 550 and 900 °C were amplified and are presented in Figs. 4 and 11, respectively. The TG curves indicate the major weight loss occurred between 25 and 250 °C and 700 and 1000 °C. The weight loss is continuous although DTA shows several endotherms. The weight loss between 25 and 250 °C is caused mainly by dehydration (free and hydrated H2O) and dehydroxylation, which are indistinguishable since they are channel constituents bound by the strong hydrogen bonding in the channel. Residual hydroxyl groups are lost between 250 and 700 °C corresponding to a weight loss of 0.63–3.48%. For pure NaA, the exothermic peak at 880 °C without any weight loss is due to the structural change of NaA. The exothermic peaks at about 820 °C for the NaCl + NaA system and 780–800 °C for the LiCl + NaA system correspond to the structure collapse of the zeolite, and the temperatures are significantly reduced in the presence of alkaline chlorides. The melted alkaline chlorides are easily diffused into the channels or cages of the zeolites, and lead to the destruction of the framework structures. When KCl and LiCl were used to treat NaA instead of NaCl, no Na–sodalite was formed with KCl (Fig. 8). Some K+ exchanged for Na, and caused an increase in the lattice parameter from 24.571 Å of NaA to 24.693 Å. The increase in lattice parameter is still less than the 24.725 Å obtained for K-exchanged NaA at 80 °C. For LiCl treatment, a reaction pathway similar to that for NaCl treatment was observed. However, for NaA + LiCl system, NaA was more easily transformed to its intermediates (a-Li5AlSi2O8 and b-eucryptite (LiAlSiO4), and the transformation of the

Sodalite can be synthesized through solid transformation of NaA in the presence of excess NaCl. The nucleation rate of sodalite is very high and little induction time is needed. When the treatment temperature is above a threshold value, the transformation is abruptly accelerated, and sodalite can be formed quickly. The mechanism of the thermal transformation of zeolite 4A is proposed in the following sequence:

zeolite A ! carnegieite ! sodalite for NaCl and; zeolite A ! a  Li5 AlSi2 O8 ! b-eucryptite ! sodalite for LiCl K- and Cs-exchanged zeolite A was transformed more rapidly than NaA. However, no sodalite could be formed when NaA was treated with KCl. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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