Zeolite Chemistry II. The Role of Aluminum in the Hydrothermal

23 Zeolite Chemistry Π. The Role of Aluminum in the Hydrothermal Treatment of Ammonium-Exchanged Zeolite Y, Stabilization D. W. BRECK and G. W. SKEELS

Downloaded by AUBURN UNIV on March 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch023

Union Carbide Corp., Tarrytown Technical Center, Tarrytown, N.Y. 10591

ABSTRACT Ammonium exchanged zeolite Y was steam calcined at 573 to 1043K. Treatment with Κ F or NaCI solution removed 24 of the 56 original framework AI atoms per unit cell, which probably formed μ-trioxotrialuminum cations in the β cages. These complex cations contribute to the increased stability of the structure. Introduction Hydrothermal treatment at elevated temperature of highly ammonium ex changed zeolite Y (NH Y) produces a zeolite product that is different from a dry air- or vacuum-calcined N H Y. A high degree of crystal unity following hydrothermal treatment is maintained even at temperatures as high as 1300 K; dry-air fired N H Y loses X-ray crystallinity at temperatures near 825 K. The preparation of "stabilized Y " , the reaction stoichiometry and the physical properties of the stable and unstable products that are formed as a result of various chemical, thermal and hydrothermal treatments have been extensively studied (1,2). Kerr (3,4) proposed that hydrothermal treatment of IMH Y at elevated temperatures promotes hydrolysis of framework aluminum to form hydroxo-aluminum cations, leaving vacant tetrahedral sites in the zeolite framework (defect sites). Kerr (2) and others (5,(3) have further suggested that silicon substitution in the hydrothermally created defect sites, increasing the framework Si/ΑΙ ratio, may be the cause of the increased stability. Despite these extensive studies, the source of the increased stability has not been conclusively demonstrated. It is the purpose of this study to establish a mechanism for stabilization and to propose a model for the structure of stabilized zeolite Y. Part I of this series (7) described the use of potentiometric titrations to verify the formation of hydroxoaluminum cations, and to determine the reaction stoichiometry when N H Y is air-calcined. In the current work we have relied on the same principles, namely: 1 ) The ion exchange of hydroxoaluminum cations with aqueous NaCI will yield an acidic solution containing aluminum cations. 4

4

4

4

4

271

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

MOLECULAR

272 +

eq. 1) Al (0H)*_ + xNa+ zeolite eq.2) Α Ι ( Ο Η ) £ χ + x H 0 X

q

2

= AI(OH)* = AI(0H)

+

SIEVES—II

+

+ Na zeolite + xH

x

+

3 a m

2) Treatment of hydroxoaluminum cations with KF should produce free OH. eq.3) AI(OH) . + 3 K F . = A I F + xK + O-xlKO^ 3) Treatment of polynuclear oxoaluminum cations with NaCI will not exchange the aluminum cations, but treatment with KF should produce a basic solution. For example: x

+

+

x

a q

3 a q

4+

e

o|j

4

+

eq.4) [AI-0-AI] + 6KF + H Ο — • &olite a q ? We have carried out potentiometric titrations with HCI on the fluoridetreated samples and NaOH titrations of the samples treated with NaCI solution following the steaming of N H Y zeolite at temperatures from 573 to 1043 K. 2

A

|

p

3

+

2

K

H

2


4

Experimental The composition of the N H Y zeolite was N a ( N H ) [(AI0 ) ( S i 0 ) ] · η H 0 . One gram samples in shallow dishes (30 mm X 40 mm X 4 mm) were placed under a thermocouple in a preheated oven (3300 c m volume). Three samples were steamed at each temperature; T/K = 573, 673, 773,873, 973 and 1043. The steaming rate was 40 d m h r flowing directly over the samples. The samples were removed from the oven after two hours and cooled in a desic cator. One of the steamed samples was slurried in 50 c m of 3.4 mol dm" NaCI solution, stirred at room temperature for two hours, filtered and washed with 25 c m of the NaCI solution, and finally washed with 25 c m distilled water. The combined filtrates were titrated with 0.1 mol d m ' NaOH in one c m incre ments to a pH of about 11. A second sample was slurried in 50 c m of 3.4 mol d m ' KF solution, similarly equilibrated for two hours, and the slurry titrated with 0.1 mol d m ' HCI in one c m increments. The third sample was used to de termine the extent of NHJ removal at each calcination temperature by chemical analysis. The samples treated with NaCI solution were further analyzed by X-ray powder diffraction and infrared spectroscopy; the chemical composition (Si0 , A l 0 , M 0 ) was determined by standard wet chemical methods. The filtrate was also analyzed for S i 0 and A l 0 following the titration. 4

2

136

8

4

4 8

2

56

2

3

3

1

3

3

3

3

3

3

3

3

3

3

2

2

3

2

2

2

3

Results The potentiometric titration curves for the NaCI-treated samples showed that very little acidity was produced. The titration endpoints, calculated as HVunit cell in the NaCI treatment, or OH/unit cell in the KF treatment, together with partial unit cell compositions calculated from the chemical analyses, are shown in Table I. Chemical analyses of the titrated filtrates showed that a negli gible amount of silica was removed from the zeolites and that only a small amount of hydroxoaluminum cations were ion-exchanged during NaCI treatment. Chemi cal analyses of the zeolite phase showed the absence of fluoride ion. There was extensive cation deficiency, even after steaming at 573 K, with the deficiency increasing as the steaming temperature was increased. Coincident with the cation deficiency observed in the NaCI-treated samples, a substantial amount of base



135.8

135.8

135.8

135.8

135.8

135.8

135.8

NONE

573

673

773

873

973

1043

3

in

24.37

24.41

24.36

24.45

24.52

24.57

25,71

0

4

5. ) Cation d e f i c i e n c y / u . c . = 56.2 - (Col. 4. + C o l . 5)

4. ) Removed A l / u . c . = 56.2 - C o l . 4

4

3. ) The sample of untreated N H Y contained 48.0 N H

4

+

T

cations per unit cell and 7.6 N a cations per unit c e l l .

A

ENDPOINTS

= Crystallinity of NaCI-treated sample, based on the intensities of 5 reflections, relative

to the untreated sample of N H Y .

2. ) I/Is

69

15.13

4.09

46.0

0.9

9.2

55.3

81

31.84

3.64

42.6

3.7

9.9

52.5

73

48.22

3.64

44.4

2.0

9.8

54.2

77

3.87

43.90

69 75

7.39

38.9

-

100

2

"/is "'

X-Ray

42.76

-

5.23

30.2

-

OH " / u . c .

H V U . C .

-

KF Treatment

NaCI Treatment

-

u.c.

4

C A L C U L A T E D FROM TITRATION

- 3

-

NONE

deti c i e n c y

46.1

0

A d d i t i o n a l cation

2.3

7.8

53.9

2.8

2.6

23.3

14.4

NONE

Al/u.c?

55.6

3.)

M /u.c.

53.4

53.6

56.2

Al / u . c .

1. ) Based on S i / u . c . = 135.8

Si/u.c.

T/K

Removed

U N I T C E L L C O M P O S I T I O N , NaCI T R E A T E D S A M P L E S

or titrated in K F , 3-425 mol d m

TABLE 1 1.) Unit cell compositions, titration data, and X-ray data calculated for steamed N H Y treated in NaCI, 3-425 mol dm


274

MOLECULAR

SIEVES—II

was produced by the K F treatment, which decreased with steaming temperatures above 873 K. The hydroxyl region of the infrared spectra of the NaCI-treated samples is shown in Figure 1. The infrared sample wafers were vacuum-activated at 473 Κ to remove only physically adsorbed H 0 . All samples show a very broad OH band (3745-3000 cm' ) assigned to (OH) nests in tetrahedral coordination situa ted in vacant framework sites (g). The broad OH band is found in the sample steamed at 573 K, and remains uniform in size until steaming temperatures above 873 Κ are achieved, whereupon it is reduced in size. Superimposed upon the broad OH band are two additional OH bands normally observed with steamed or "deep bed"-treated N H Y (9,10,!!). The lar gest OH band is initially found at 3587 cm" following steam treatment at 573 K, shifting to higher wave numbers with increasing steam temperature. The smaller OH band is initially found as a shoulder on the larger OH band at about 3670 cm" , becoming more distinct with possibly a doublet at about 3685 cm" , as the steam ing temperature is increased to 873 K, but finally disappearing after steaming at 1043 K. The infrared wafer of the 873 K-steamed sample was vacuum-activated at 773 Κ and subsequently exposed to 60 torr benzene vapor at ambient tempera ture, re-activated at 473 K, then exposed to 200 torr of ammonia gas at ambient temperature. The spectra are shown in Figure 2. Following activation at 773 K, the 3685 cm' OH band has shifted to 3695 cm" , the 3600 cm" band is sub stantially reduced and the broad OH band is eliminated. With both benzene and ammonia, the 3695 cm" band is completely eliminated. Vacuum activation at 473 Κ following adsorption restores the 3695 cm" band. In a subsequent experiment, a sample of N H Y was steamed at 873 K, sus pended in 3.4 mol d m ' NaCI solution and the slurry titrated to pH 11 with 0.1 mol d m ' NaOH. The infrared hydroxyl region of the washed product was examined and both the 3685 cm" and the 3600 cm' OH bands were found to be unaffected by the additional NaOH treatment. 2

1

4

4

1


1

1

1

1

1

1

1

4

3

3

1

1

Discussion In Part I of this series (J) it was shown that during dry air calcination of N H Y in a shallow bed, 16 hydroxoaluminum cations (in a unit cell containing 56 aluminum atoms) are formed. The stepwise reaction first produces 16 AI(OH) and 16 defect sites in the zeolite framework. As deammination progresses, the AI(OH) subsequently reacts to produce AMOH)^ and water, then AI(OH) and water, and finally at ~823 K, [AI-O-AI] , or μ-oxodialuminum cations. The formation of these species was confirmed by the substantial acidity produced in the NaCI titrations, and the moderate amount of base produced in the KF titra tions. In the current work, very little acid is produced and very few hydroxoalumi num cations are exchanged when steamed N H Y is treated with NaCI solution. On the other hand, treatment with KF solution produces a quantity of base nearly three times that produced by the 16 hydroxoaluminum cations found in dry aircalcined N H Y. The hydroxyl region infrared spectra do not show the presence of substantial OH groups to account for it. Interpretation of the data for dry airfired N H Y ascribed the cation deficiency in the NaCI-treated samples to either hydrolyzed hydroxoaluminum cations trapped in the sodalite cage, or to μ-oxodialuminum cations. With the steamed samples this interpretation no longer holds, 4

3

2+

3

4+

4

4

4


23.

BRECK

AND

SKEELS

Ammonium-Exchanged Zeolite Y

275


STEAMING TEMP., T/K

3587 cm" WAVELENGTH IN cm"

1

Figure 1. Hydroxyl infrared spectra of steamed NHiY following treatment with 3.4 mol dm' NaCI; wafers activated in vacuum at 473 K. 3


276

MOLECULAR

SIEVES—II

since nearly three times the requisite amount of base is produced for 8 [ A I - 0 - A I ] cations, and not nearly enough base is produced for 30-45 A I ( O H ) species. Extensive dealumination should also have other effects on the zeolite properties, such as a very large O H band due to ( O H ) nests and even extensive crystal degradation (12). The presence of steam during stabilization should inhibit the reaction of

4 +

3

4

AI(OH)

+ 2

to form A I ( O H )

2 +

and H 0 . Stabilization of A I ( O H ) 2

+ 2

would permit

further hydrolysis of framework aluminum to occur, increasing the quantity of AI(OH) formed. The charge due to 24 framework aluminum atoms is balanced +

2

by 24 A I ( O H )

+ 2

. The 16 A I ( O H )

2 +

cations in dry air-fired N H Y condensed to 4

4 +

8 [ A I - 0 - A I ] during dehydroxylation (one in each sodalite cage), and the 24 AI(OH) formed in the steam-calcined Y condense to q [AI-O-AI- Ô-AO ' or μ-trioxotrialuminum cations, with one cation located in each sodalite cage. +

0

3

+


2

Ί 3+

This cationic cluster would not be ion-exchangeable in NaCI solution and, hence, acid could not be produced. A considerable amount of base would be produced in the K F treatment. p— Ο Π3 + eq.5)

[AI-O-AI-O-AIJ

+3H 0 + 9KF

•

2

zeolite soin. 3K +3AIF +6KOH (OH/AI=2) zeolite soin. soin. Each cluster would consist of three aluminum atoms in Site Γ bridged through three oxygen atoms located in Site II '. If one of the 0 oxygen atoms at a framework defect site were to move slightly, into a Site I Γ position, each of the aluminum atoms in the cluster would be in octahedral coordination. This would be achieved as shown in Figure 3, using three oxygen atoms of the 6-ring opening into the hexagonal prism, as well as sharing the four oxygen atoms in Site ΙΓ. This model is consistant with the structure suggested by Maher et al. for their Structure I, a self-steamed N H Y (5), and with the structure of Bennett and Smith suggested for ultrastable Y (13), as well as with the x-ray fluorescence data of KUhl (14) for stabilized Y . When this model was applied to the actual titration data derived from N H Y steamed up to 873 K, very close agreement was found between the expected and the measured free OH produced in the KF titrations, as shown in Table II. Above 873 K, the measured free OH decreases while the cation deficiency remains high. At the same time the unit cell size increases by 0.05A before decreasing again at the higher steaming temperature. This increase in unit cell is accompanied by a shift of framework infrared frequencies to lower wavenumbers, indicating a lower Si/ΑΙ framework ratio. The broad OH band due to (OH) in framework tetrahedral vacancies also decreases in size. With temperatures at and above 873 K, the mobility of framework oxygen atoms has been shown to be appreciable (15,16). One can envision movement of +

3

4

4

4

4


BRECK


23.

A N D SKEELS

Ammonium-Exchanged Zeolite Y

277

Figure 2. Effect of C H and NH on the 3600 cm and 3695 cm OH infrared bands found in steamed NH^Y fououHng treatment in 3.4 mol dm NaCI. (1) NH+Y steamed at 873 K, vacuum activated at 773 K; (2) 60 torr C H at RT; (3) vacuum activated at 473 K; (4) 200 torr NH ; (5) vacuum activated at 773 K. 6

6

1

3

1

3

6

I 3618 cm"

A - FRAMEWORK 0

Figure 3.

4

6

3

IN NORMAL POSITION

Β - FRAMEWORK 0

4

MOVED INTO SITE II

Illustration of [Al-0-Al-0-Al] of stabilized Y in the sodalite unit. Small circles, Al; large circles, oxygen. 3+


Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977. 43.90 1.19

1.15

,μ-oxodialuminum cations

1.04

48.22

(I)

(2)

31.84 1.57

0.52

49.86 31.84

16.66

973

(3) The Calculated OH value is based on the suggested aluminum species

(2) from AI(OH)

+

(1) from AI(OH)2 , μ-trioxotrialuminum cations

RATIO, C A L C U L A T E D / M E A S U R E D V A L U E

MEASURED OH/uc 42.76

(1) 49.98

(D 52.20

(I)

>

49.12

(3

S U G G S T E D ALUMINUM S P E C I E S

C A L C U L A T E D OH/uc

873

773

673

STEAMING T E M P . , T / K

TITRATED IN K F SOLUTION

E X P E C T E D VS. MEASURED OH FORMATION IN STEAMED NH4Y

T A B L E II


23.

BRECK

Ammonium-Exchanged Zeolite

A N D SKEELS

Y

279

oxygen atoms along the framework, and framework silicon atoms flipping through one position in a chain reaction (17). This could be described as migration of the tetrahedral vacancies, as proposed by Peri (6). However, the data of the present experiments indicate some of the non-framework aluminum is also reinserted into the framework, causing an increase in unit cell size and resulting in less base being produced in the KF titrations. The base produced in KF solution following steaming at 1043 Κ corresponds to 8[AI-0-AI] species. This model is consis tant with the X-ray structure data of Bennett and Smith (13), who found density equivalent to 16 AI atoms in Site Γ and at least 5 oxygen atoms in Site U when ultrastable Y was heated at 973 K. The infrared spectra of steamed N H Y show that very few hydroxyl groups are present in the zeolite, particularly when the optical densities of the spectral bands are compared to the OH bands in air- or vacuum-activated N H Y. In addi tion to the broad OH band assigned to hydroxyl nests, there are two relatively small hydroxyl bands observed with stabilized Y which differ in position from airor vacuum-activated N H Y. Most authors seem to agree that the relative positions of the infrared bands are near 3600 and 3700 cm" , but they have disagreed on structural assignments and relative acidity (S,9,1Q,1J). Based on the current study and the proposed model for stabilized Y , it seems reasonable to conclude that the OH groups responsible for the 3600 cm' band are in relatively inaccessible positions, possibly associated with the μ-trioxotrialuminum cations inside the sodalite cage. The 3600 cm" band is affected slightly by benzene and N H , and not all by NaCI or dilute NaOH solution and is, therefore, not strongly acidic. Steaming above 873 K, where framework annealing takes place, nearly eliminates this hydroxyl group. The OH groups responsible for the 3695 cm' band are definitely accessible and are completely eliminated by both benzene and N H . They are not strongly acidic, since both NaCI and dilute NaOH solution do not affect the size of the OH band. This hydroxyl group may be associated with the defect centers in the zeolite framework, since steaming at 1043 Κ completely eliminates the infrared band. 4+

4


4

4

1

1

1

3

1

3

Literature Cited 1) Breck, D. W., "Zeolite Molecular Sieves: Structure, Chemistry and Use", (Wiley-lnterscience, New York, 1974), p. 474-483, 507-518. 2) Kerr, G. T., Adv. Chem. Ser., Amer. Chem. Soc. (1973) 121, Mol. Sieves, Internat. Conf. 3rd, 219. 3) Kerr, G. T., J. Phys. Chem. (1967) 71, 4155. 4) Kerr, G.T., J. Catal. (1969) 15, 200. 5) Maher, P. K., Hunter, F. D., and Scherzer, J., Adv. Chem. Ser., Amer. Chem. Soc. (1971) 101, Mol. Sieves, Internat. Conf. 2nd, 266. 6) Peri, J. B., Proc. 5th Internat. Congress on Catalysis (1972), 329. 7) Breck, D. W., and Skeels, G. W., Proc. 6th Internat. Congress on Catalysis (1976). 8) Bennett, J. M., Breck, D. W., and Skeels, G. W., to be published. 9) Scherzer, J., and Bass, J. L., J. Catal. (1973) 28, 101. 10) Ward, J. W., J. Catal. (1970) 19, 348. 11) Jacobs, P. Α., and Uytterhoeven, J. B., J. Catal. (1971) 22, 193. 12) Kerr, G. T., J. Phys. Chem. (1968) 72, 2594. 13) Bennett, J. M., and Smith, J. V., Private communication.


280

MOLECULAR SIEVES—II


14) KUhl, G. H., Mol. Sieves, Internat. Conf. 3rd, Recent Progress Reports (Leuven University Press, 1973), 227 15) Antoshin, G. V., Minachev, Kh.M., Sevastjanov, Ε. N., and Kondratjev, D. Α., Russ. J. Phys. Chem. (1970) 44, 1491. 16) Peri, J. B., J. Phys. Chem. (1975) 79,1582. 17) Lacy, E. D., Acta Cryst. (1964) 18, 149.


Zeolite Chemistry II. The Role of Aluminum in the Hydrothermal

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