Zeolite ModificationâDirect Fluorination - ACS Symposium Series

3 Zeolite Modification—Direct Fluorination

Downloaded by NORTH CAROLINA STATE UNIV on September 10, 2012 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.ch003

BRENT M . LOK, F. P. GORTSEMA, and T. P. J. IZOD

CELESTE A . MESSINA, H . RASTELLI,

Union Carbide Corporation, Tarrytown Technical Center, Tarrytown, NY 10591

Dilute fluorine gas (0-20%) can be used to treat zeolites at near-ambient temperature and pressure. Most of the resulting materials retain very high c r y s t a l l i n i t y even after 600°C postcalcination for two hours. Both framework infrared spectra and X-ray powder diffraction patterns clearly show structural dealumination and s t a b i l i zation. The hydrophobic nature of the fluorine-treated and 600°C-calcined material i s shown by a low water adsorption capacity and selective adsorption of n-butanol from a 1 v o l . % n-butanol-watersolution. Fluorination also changes the catalytic activity of the zeolite as measured by an n-butane cracking method.

Over the past thirty years, zeolite science has grown into a major branch of chemistry. A large number of new zeolite materi a l s have been made by both direct hydrothermal synthesis and by post-synthesis modification. This has led to a large number of new applications in such diverse fields as catalysis, adsorption and ion exchange. In synthesis alone, over one hundred new zeolites have been produced. These are divided into two major classes: ones with low s i l i c o n to aluminum ratios (Si/Al10). The low s i l i c o n to aluminum ratio zeolites have hydrophilic surfaces, a large number of exchangeable cations and acid sites with moderate to high strength. On the other hand, the high s i l i c o n to aluminum ratio zeolites have an organophilic surface, a relatively small number of exchangeable cations and a relatively small number of acid sites with high strength. Many low s i l i c o n to aluminum ratio zeolites can be structura l l y modified to have a high s i l i c o n to aluminum ratio and, thus, in many respects, behave like a high silicon to aluminum ratio 0097-6156/83/0218-0041$06.00/0 © 1983 American Chemical Society In Intrazeolite Chemistry; Stucky, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


42

INTRAZEOLITE

CHEMISTRY

zeolite. The known methods of synthesis of these materials i n clude mineral or weak organic acid extraction of acid-stable zeolites such as c l i n o p t i l o l i t e (1) and mordenite (2-7J, hydrothermal treatment (8^-9) and reaction with organic complexing agents (EDTA)(10) or acetylacetone (11) or chromic salts (12). Very recently, zeolites have also been modified by chlorine (13) and chlorine-related compounds at high temperature (14, 15). Hie known modification methods can be further c l a s s i f i e d as either l i q u i d or vapor phase treatments. The acid washing, organic complexing agent extraction and chromic salt treatments f a l l into the f i r s t class while the steaming and the chlorine and related compounds reactions belong to the second class. Fundamentally, a l l the modification methods follow the same principle, that i s , to remove structural aluminum. After the removal, a structural defect site i s usually created. A widely accepted hypothesis i s that this structural defect site i s a hydroxyl nest (1). It has been further hypothesized that under the proper conditions, a s i l i c a molecule would be inserted into the vacant s i t e and, thus, anneal the structural defect (16). The source of the s i l i c a can be either the treating reagent (14) or the zeolite sample i t s e l f through rearrangement of s i l i c a from another part of the framework or from a s i l i c a impurity within the sample. (Note that i n the chromic salt case, chromia rather than s i l i c a i s claimed to be inserted i n the structure.) Because of the dealumination and the s i l i c a insertion, the treated samples are usually found to have a higher framework s i l i c o n to aluminum ratio and a higher thermal s t a b i l i t y than the untreated materials. Furthermore, hydrophobic surface properties usually result from substantial dealumination (7). We believe the use of a direct gaseous phase fluorination process for modifying the surface and structure of zeolites to be a new process (18)• The literature does contain references to the use of hydrogen fluoride (20, 22, 23), boron t r i f l u o r i d e (21, 24), aluminum monofluoride (sic) (19) and s i l i c a difluoride (sic) (19) to treat the surface of a zeolite to obtain higher catalytic activity. However, the use of fluorine gas to modify both surface and structure has not been reported before. The purpose of this paper i s to report results of fluorination of zeolites and to describe the process involved i n such a treatment. Detailed results on fluorine-treated zeolites and their unusual properties, both adsorptive and catalytic, w i l l be discussed i n forthcoming papers. Experimental Treatment Conditions Table I l i s t s the zeolite samples used i n this study along with their chemical composition and source of manufacture. Either ammonium ion-exchanged or acid-washed zeolites were employed to

In Intrazeolite Chemistry; Stucky, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

LOK ET AL.

3.

Zeolite Modification by Fluorination

43

avoid the formation of cation fluorides i n the pore system. Both precalcined and uncalcined samples were used. The results i n d i cate that the properties of treated products are similar regardless of their calcination state. Table I CHEMICAL COMPOSITION AND SOURCE OF VARIOUS MATERIALS TESTED


Chemical Compositions (wt %) Zeolite

Al20^

a

LZ-105 LZ-105*> H-zeolon Erionite

c

4.28 3.73 10.9 18.6

Si02

Naj2£

(NH ) 0 4

7

Mole Ratio NH^/Al

Si/Al

M/Al 1.10 0.01 0.03 0.12

0.32

This lab This lab Norton This lab

Source

90.2 2.86 95.0 0.03 89.3 0.17 77.2 2.00 ((Na+K) 0) 78.8 0.98

3.1

17.9 21.6 7.0 3.53

2.2

3.53

0.06

0.23

This lab

72.0 68.8

7.9 8.1

3.58 3.0

0.20

-

0.91 0.81

This lab This lab

4.3

3.09

0.01

0.41

This lab

2

a

E r i o n i t e 18.8 NH , TMA-Q 17.1 NH , K-L 19.5 4

4

0.04 3.6 (K 0) 74.8 0.14 2

NH Y 4

a. b. c. d.

C

20.6

20 wt % alumina bonded. Acid washed and 20 wt % alumina bonded. Mild steaming followed by NHj exchange with hot NH C1 solution. Mild steaming and NH C1 exchange repeated. 4

4

The powder sample, i n 5 to 20 gram quantities, was placed i n a Teflon container. Then, to remove the bulk water, the sample was evacuated at 60°C i n a stainless steel reactor chamber (volume - 50 dm ) equipped with a thermocouple, a pressure gauge, three gas inlets and a vacuum outlet. The three gas inlets were used to introduce fluorine, nitrogen and oxygen. The desired amount of fluorine gas (0-20 volume %) was introduced after the reactor chamber had f i r s t been f i l l e d with a predetermined amount of nitrogen and oxygen. The total gas pressure i n the reactor was always s l i g h t l y lower than one atmosphere pressure for safety reasons. The concentration of a particular gas was measured i n pressure percent by means of a pressure gauge and then converted to volume percent by assuming the ideal gas law. The sample was kept i n the reactor chamber i n contact with fluorine for a specified period and then the reactor chamber was evacuated and, subsequently, flushed with nitrogen gas. During the treatment process, the reactor was maintained at ambient or 60°C. However, the actual sample temperature, as measured by a thermocouple i n contact with the powder sample, was found to be much higher than the reactor temperature due to the exothermic nature of the fluorination reaction. To desorb fluorine and related gases, 3


INTRAZEOLITE

44

CHEMISTRY

samples were either post-activated at 150°C under vacuum or flushed with dry nitrogen for ten minutes. The treated samples were optionally postcalcined at 600°C for 2 hours and stored i n zipper-seal plastic bags and kept i n a desiccator. Table II l i s t s the treatment conditions.


Table II FLUORINE TREATMENT CONDITIONS

Sample # 1 2 3 4 5 6 7 8 9 10 11 12 a. b. c. d. e. f.

F % 2 Vol %

Zeolite a

LZ-105 LZ-105 H-zeolon H-zeolon H-zeolon Erionite Erionite NH , TMA-Q NH K-L NH , Y NH , Y NH Y b

0

d

4

4#

c

4

c

4

C

4

10 5 10 5 1 5 5 5 5 2 2 1

Treatment Conditions O% 2 Duration Temp Vol % (min) (°C) Type 2 5 2 0^ 09 5 5 5 5 09 09 0*

10 15 10 30 45 5 5 5 5 5 15 15

60 60 60 25 25 60 60 60 60 60 25 25

F Content (wt %)

e

2

Severe Severe Severe Severe Mild Severe Severe Severe Severe Mild Mild Mild

N.A. 2.4*

3.5* 10.0 N.A. 4.6 N.A. 4.4 4.8 N.A. 2.2 N.A.

20 wt % alumina bonded. Acid washed and 20 wt % alumina bonded. Mild steaming followed by NHj exchange with hot NH C1 solution. Mild steaming and NH C1 exchange repeated. F content analyzed after fluorination. F content analyzed after 600°C calcination and 2 1/2 hr. soxhlet extraction subsequent to fluorination. g. Samples treated i n a flow-through reactor with premixed F - N mixture. N.A. - Not analyzed. 4

4

2

2

2

2

Characterization For sample characterization, a Philips X-ray diffractometer employing Cul^ radiation was used over a 26 range of 4° to 56° at a scan rate of 2 degrees/min. I. R. spectra were obtained from 0.5 i n . diameter KBr pellets (for framework spectra) and s e l f supported wafers (for hydroxyl region spectra). Two concentrations were usually employed in KBr pellets: 0.25 mg sample per 350 mg KBr and 1 mg sample per 350 mg KBr. Spectra were recorded with a Nicolet model 7199 FT-IR spectrometer. Gas phase adsorption studies on treated samples were carried out using a McBainBakr gravimetric adsorption system with quartz springs. Samples were activated at 350°C, l x l O " torr for about 16 hours before 4


3.


LOK ET AL.

45

the adsorption measurement. A binary liquid (1 vol % n-butanol in water) shake test (25) was conducted by mixing 0.5 g of the sample with 5 cm or 0.25 g of the sample with 2.5 cm of the nbutanol-H 0 solution i n a glass v i a l . The slurry was well shaken mechanically for one and a half to two hours. The mother liquor was analyzed by gas chromatography. A delta hexane loading test (26) was used as one of the measurements of surface hydrophobicity and organophilicity. Water was adsorbed onto the sample at near saturation pressure (^20 torr) and ambient temperature. Then, when the water adsorption reached equilibrium, 50 torr of n-hexane vapor was introduced. The additional amount adsorbed i s termed the delta hexane loading. Finally, the catalytic activity was measured by a 2 mole % nbutane cracking method described i n detail on another paper (27). 3

3


2

Results To i l l u s t r a t e the dealumination and structure stabilization in treated zeolites, severely treated and postcalcined ones are chosen as examples. The degree of dealumination, nevertheless, can be controlled by varying the treatment conditions. X-Ray Diffraction For LZ-105 (28), there i s very l i t t l e observable change i n the X-ray spectra before and after treatment. F u l l retention of c r y s t a l l i n i t y i s evident. However, reduction i n intensity for the peak at 29 = 24.4° i s noted. Such an intensity change i s also very prominent i n the spectrum of fluoride s i l i c a l i t e , a s i l i c a polymorph synthesized i n the presence of a fluoride salt (29). For H-zeolon, erionite and NH4Y, there are noticeable changes in both peak intensity and peak positions, especially for those treated under severe conditions (Figures l a , b,c). The substant i a l increase i n peak intensities for low angle peaks observed i n the X-ray spectra of fluorine treated H-zeolon, erionite and NH Y i s also found i n the X-ray spectra of the hydrothermally treated samples (9). In addition to the intensity changes, a l l the X-ray peaks for the fluorine-treated samples are slightly shifted to higher 26 values compared to the untreated materials. Shifts to higher 26 values usually imply a decrease i n unit c e l l size associated with dealumination and structure stabilization. Similar peak shifts have also been observed for high temperaturesteamed samples (9)• For both NH4,K-L and NH , TMA-ft, the X-ray diffraction spectra (Figure Id, e) indicate that both zeolites retain most of their c r y s t a l l i n i t y after fluorine treatment and 500°C calcination. Except for some minor changes i n peak intensity for fluorine-treated NH ,K-L, there i s no evidence of peak intensity and position change as found for other treated zeolites. After 600°C calcination i n a i r , a further loss of c r y s t a l l i n i t y i s seen for fluorine-treated NH4,K-L. However, NH ,TMA-fi loses v i r t u a l l y a l l crystallinity. 4

4

4

4


INTRAZEOLITE CHEMISTRY

(a) H-ZEOLON

jwJJ 5% F TREATED AND 600°C CALCINED


2

(b) ERIONITE*** UNTREATED

JLJ

ij 5% F TREATED 2

(c) NH4Y*** UNTREATED

UUJ u (d) NH4, TMAfi

2% F TREATED 2

UNTREATED

5% F TREATED AND 5O0 C CALCINED 2

o

(e) NH4, KL

5% F TREATED AND 500°C CALCINED 2

4

8 12 18 20 24 28 32 36 40 44 48 52 56 28

Figure 1. X-Ray d i f f r a c t i o n s p e c t r a o f f l u o r i n e - t r e a t e d

zeolites


3.

LOK ET AL.


47

I. R. Spectra 1

Framework Region (400 cm""

1

- 1600

cm" )

It i s well known in the literature (30) that a s h i f t of certain I. R. bands to higher wave numbers i s evidence of dealumination. Since LZ-105 has a r e l a t i v e l y high Si/Al ratio (Si/Al 21.6), the s h i f t s of band positions are small. In addition to the band position shifts i n fluorine-treated LZ-105, sharpening of peaks at 628 and 587 cm" occurs, indicative of structure s t a b i l i zation. For a l l the 600°C-calcined fluorine-treated H-zeolon and NH Y zeolites, band position shifts and band sharpening are prominent in the I. R. framework spectra as shown in Figures 2a and 2b, respectively. These samples show as much as 20 cm up-shift in the 1078, 799 and 637 cm" bands for H-zeolon and 1059 cm" band for NH Y zeolite. The s p l i t t i n g of the band at 579 cm" and the shifting and s p l i t t i n g of the band at 637 cm" into two bands at 594 and 571 cm" and 669 and 658 cm" and the appearance of multiple bands i n the region of 800 cm" to 700 cm" are observed for the fluorine-treated H-zeolon. A doublet near 1200 cm" and shoulders at 795 and 485 cm" appear i n the fluorine-treated NH Y spectrum. These prominent changes strongly confirm that substant i a l dealumination and structure stabilization occur for H-zeolon and NH Y zeolite after fluorination and calcination. The spectrum for the 600°C-calcined fluorine-treated erionite sample shows substantial s h i f t s in band positions, but band sharpening i s less obvious. The bands at 1082, 792, 578, 470 and 438 cm" are shifted to 1098, 814, 585, 477 and 444 cm" , respectivel y , after fluorine treatment and 600°C calcination. The large shifts observed are evidence of dealumination. The s p l i t t i n g of the 1082 cm" band into a doublet located at 1098 and 1085 cm" and some degree of band sharpening imply structure stabilization for fluorine treated erionite. Finally, I. R. spectra for fluorine-treated and then 500°Ccalcined N H , K - L and NH ,TMA-Q (Figures 2c and 2d) show an ups h i f t of band positions, but loss of spectral resolution. Thus, the I. R. results indicate that the two zeolites undergo structural dealumination by fluorine treatment and subsequent calcination, but both show no evidence of structure s t a b i l i z a t i o n .


1

4

1

1

1

4

1

1

1

1

1

1

1

4

4

1

1

1

1

4

4

OH Region (3100 cm"

1

- 3900

1

cm" )

Before treatment, acid-washed LZ-105 shows two d i s t i n c t bands i n the OH region I. R. spectrum: one at 3745 cm and the other at 3615 cm" . The former i s attributable to OH groups on terminal zeolite surfaces or amorphous impurity material. The 3615 cm" band i s believed to be related to the Bronsted acid sites in LZ-105. After fluorination and 600°C calcination, both the 3615 and 3745 cm" bands disappear. 1

1

1

American Chemical Society Library 1155 16th St. N. % Washington, C. G., 20031 In Intrazeolite Chemistry;D. Stucky, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INTRAZEOLITE

H-Zeolon


(a)

CHEMISTRY

i

i

i

i

i i

1 1

i

i

i

i

i... i

i

18001700 1600 15001400 1300 120011001000 900 800 700 600 500 400 WAVENUMBERS

i

Figure 2. Framework i n f r a r e d s p e c t r a of f l u o r i n e - t r e a t e d z e o l i t e s


LOK

3.

ET

49


AL.

In the H-zeolon case, two different types of hydroxy1 groups are found i n the zeolite (Figure 3a). One band at 3745 cm" s p l i t s after fluorination into two bands: one s t i l l at 3745 cm" with much lower intensity and the other at 3713 cm" . Then upon calcination at 500°C, the two peaks become one peak at 3745 cm" with a substantially reduced intensity. The other hydroxyl group band at 3624 cm" decreases i n intensity after fluorination and then completely disappears after calcination at 600°C. Similar observations of reduced intensity for the fluorinetreated sample and nearly complete elimination of bands for the treated and 600°C-caicined sample are found for erionite and NH4Y (Figure 3b). Since fluorine-treated NH ,K-L and NH,TMA-ft lose c r y s t a l l i n i t y upon 600°C calcination, the OH region spectra were taken after 500°C calcination. Both show substantial reduction i n intensity for Bronsted acid OH groups (bands in the region of 3630-3680 cm" ) and some reduction of intensity for the 3745 cm" band. 1

1

1

1


1

4

4

1

1

McBain-Bakr Adsorption Oxygen Adsorption at -183°C The adsorption of oxygen i s used here as another measure of total c r y s t a l l i n i t y (Table III). LZ-105, H-zeolon, erionite, NH ,TMA-fl, NH ,K-L and NH Y a l l show 10-20% loss i n their t o t a l oxygen capacities. There are at least two explanations for such a reduction. F i r s t , the t o t a l pore volume i s lowered because the total c r y s t a l l i n i t y after fluorination decreases s l i g h t l y . Second, the fluorine treatment results i n the entrapment of fluoride compounds such as MF, A I F 3 , A1F (0H), A1F(0H)2# etc. i n the pore systems. 4

4

4

2

Water Adsorption at 25°C The adsorption of water i s used i n this study as one of the measures of surface hydrophobic!ty. As the data i n Table IV show, the fluorine-treated and subsequently calcined LZ-105, Hzeolon, erionite and NH Y a l l have low water capacity as compared to their untreated counterparts. The reduction in water capacity (>50 vol % at 4.6 torr) i s appreciably greater than the reduction in oxygen capacity (

Zeolite ModificationâDirect Fluorination - ACS Symposium Series

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Zeolite ModificationâDirect Fluorination - ACS Symposium Series