Characterisation and catalytic properties of dealuminated zeolite-Y: A comparison of ammonium hexafluorosilicate and hydrothermal treatments

H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.) Catalysis by Microporous Materials

Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.

147

Characterisation and Catalytic Properties of Dealuminated Zeolite-Y: A Comparison of Ammonium Hexafluorosilicate and Hydrothermai Treatments A. P. Matharu*, L. F. Gladden* and S. W. Carr? *Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, UK ?Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, The Wirral, Merseyside, L63 3JW, UK

This paper reports a comparison of the structure and catalytic properties of zeolite-Y when dealuminated by hydrothermal and ammonium hexafluorosilicate treatments. Initial results have shown that dealumination using ammonium hexafluorosilicate is highly sensitive to many of the reaction parameters such as reaction temperature, rate of AHFS addition, reaction time, pH, framework Si/A1 ratio and also to the extent of washing of the final product. Further, degrees of dealumination beyond 50% are difficult to obtain without causing decreases in framework crystallinity. Structural characterisation of the samples has been performed using X-ray diffraction, X-ray fluorescence spectroscopy, nitrogen adsorption and 27A1 MAS NMR. The catalytic properties of the samples were studied using the n-butane cracking reaction. In future work, analysis of the cracking data will be performed as well as a detailed comparison of the deactivation processes occurring within the samples. 1. I N T R O D U C T I O N The importance of acid sites in cracking catalysts such as zeolites is well known. The dependence on the number, density and strength of these sites on the framework aluminium content of zeolites is a complex relationship [ 1,2]. However, the major contribution to the activity of zeolite catalysts has been attributed to the framework Si/A1 ratio [3,4]. Highly siliceous zeolites can be synthesised, such as the case with ZSM-5 [5], or alternatively made from existing zeolites by processes of dealumination to produce "second-generation" zeolites. Many methods exist to produce such second-generation zeolites in which the Si/A1 ratio is increased. Amongst these techniques are treatment with steam [6], SIC14 vapour [7,8], phosgene [9], nitrosyl chloride [10], boron trichloride [11], chelating agents [12] and one of the most recent treatments; ammonium hexafluorosilicate (AHFS) [ 13,14]. This latter treatment differs from many of the other methods in that it is carried out at a lower temperature and in the aqueous phase. One important aspect of the AHFS treatment is that the product formation is sensitive, to a high degree, to many of the experimental parameters such as reaction temperature, rate of AHFS addition, reaction time, pH, framework Si/A1 ratio and also to the extent of washing of the final product [ 15]. The mechanism of dealumination by the AHFS method is believed to proceed via the isomorphous substitution of aluminium for silicon within the framework. This kinetic process has to be carefully controlled to prevent the silicon being withdrawn from the framework too quickly as this will result in framework collapse and lead to poor crystallinity of the final product. Xia et al [16] carried out work into the reaction

148 mechanism from which they suggested that the presence of H30 + ions as well as F- ions was necessary for the dealumination process to proceed. They also reported that zeolites prepared by the AHFS treatment contained small amounts of structural fluorine which cannot be removed by washing. This fluoride content may form destructive HF when the zeolite is heated during calcination processes. He et al [ 17] showed that the aluminium removal rate is far greater than the silicon insertion rate and that the silicon and aluminium exchange are nonstoichiometric, thereby suggesting that crystal collapse can occur if experimental conditions are not carefully controlled. Further studies by Wang et al [ 18] have also reported the presence of non-uniform aluminium extraction arising from AHFS treatments.

2. EXPERIMENTAL The preparation of the hydrothermally dealuminated samples is described below; the NH4-Y parent sample was kindly donated by Crosfield Chemicals. Sample H1 was prepared by steaming NH4-Y at 425 ~ for one hour. This regime gave optimum steamed dealuminated zeolite Y. The sample was then treated with 5wt% ammonium sulphate solution (adjusted to pH 8 with NH 3 solution) and aged at 80 ~ for one hour. Sample H2 was prepared by ion exchange of NH4-Y with a 5wt% ammonium sulphate solution (adjusted to pH 8 with NH 3 solution) and aged at 80 ~ for one hour. This material was then steamed at 450 ~ for one hour and finally ion exchanged again as above. Sample H3 was prepared from NH4-Y by twice ion exchanging with ammonium sulphate solution (as above) and then steaming at 520 ~ for 30 minutes. Finally, this sample was ion exchanged again as earlier. Sample H4 was prepared as sample H3 except that it was steamed at 710 ~ for 30 minutes. A second series of samples was prepared using an ammonium hexafluorosilicate treatment, the samples are referred to using the identifier FS. Samples of the NH4-Y were slurried in 2M ammonium acetate (98% assay, BDH, UK) solution. Aqueous AHFS (99%, Advocado Chemicals, UK) was then added slowly via a metered syringe pump at a reaction temperature of 90 ~ under vigorous stirring. After addition of the AHFS the reaction mixture was maintained at 90 ~ under stirring for the duration of a specific reaction time. Throughout the process the pH was maintained between 5-7. The reaction mixture was then filtered and washed thoroughly with 4 litres of distilled-deionised water at 80 ~ and the zeolite was then dried in an oven at 80 ~ for 12 hours. Table 2 shows the AHFS treatment regime for each sample produced. The structural properties of the samples were characterised by X-ray diffraction (XRD), Xray fluorescence spectroscopy (XRFS), nitrogen adsorption and 27A1 MAS NMR. The acid properties of the zeolite were also investigated using n-butane cracking as a test reaction. 3. RESULTS AND DISCUSSION

3.1. X-ray Diffraction Experiments were performed using a Phillips PW-3010 automated powder diffractometer with CuKot radiation (40mA, 40kV). Measurements of the crystal unit cell size (u.s.c.) [19] and hence the framework Si/A1 ratio ((Si/A1)IV) [20] as well as the crystallinity of the zeolites were obtained, and are given in Table 1, for the parent and hydrothermally treated zeolites, and in Table 3 for the AHFS treated zeolites. As expected, both the u.c.s (framework Si/A1 ratio) and crystallinity reflect the degree of steam treatment in the hydrothermally treated

149 Table 1 Characterisation of parent (NH4-Y) and hydrothermally (H) treated materials. Sample

2.15

Crystallinity (%) 100

Surface Area (m2/~) 932

NH4-Y *

Chemical s~gl 2.54

(Si/A1)IV

HI

2.65

3.77

87

798

H2

3.06

4.02

84

78O

H3

2.78

5.76

77

729

H4

2.77

7.38

74

681

* Na20 content 2.3% Table 2 Treatment regime used for the preparation of AHFS dealuminated materials. Sample

AHFS Rate (ml/min)

Reaction Time (hrs)

Total AHFS (ml)

FS1.363

AHFS Concentration (mol/dm 3) 0.50

1.000

18

46

FS2.363

(I.25

1.000

18

46

FS3.363

0.50

1.000

24

46

FS5.363

0.50

5.000

18

46

FS6.363

0.50

10.000

18

46

FS7.363

0.50

1.000

18

46

FS8.363

0.50

1.000

5

93

FS9.363

0.5(I

0.250

3

93

FS 10.363

0.50

0.125

7

93

zeolites; the most severely steamed sample, H4, still maintained 70% of its original crystallinity. In the case of AHFS treated samples high levels of crystallinity were maintained at the expense of lower levels of aluminium removal. From Tables 2 and 3 it is seen that the rate of AHFS addition does not affect the product Si/A1 ratio or crystallinity. The greatest influence appears to be reaction time and the concentration of the AHFS solution, in agreement with the work of Garral6n et al [ 14]. Overall, treatment by AHFS appears to be an optimisation process between crystallinity and aluminium extraction. Controlling the experimental parameters in

150 order to bring this about is difficult and the most highly dealuminated sample produced in these trials was that with a Si/A1 ratio of 6.4 and crystallinity of 48%. The effects of using a lower concentration of AHFS as in the FS2.363 sample resulted in the product not exhibiting aluminium removal at all, however a small degree of silicon enrichment did occur as shown by the increase in the chemical Si/A1 ratio. The data for the AHFS treated samples suggest that the washing regime was adequate since XRFS results (chemical Si/A1 in Tables 1 and 3) for washed and further washed samples gave identical bulk Si/A1 ratio and crystallinity values. These observations are also consistent with there being no evidence of impurities in the AHFS treated zeolites after washing, as determined by XRD. These data suggest that extra-framework silicon in these materials, as detected by XRFS, exists not as AHFS but probably as SiO 2 deposits as suggested by Wang et al [ 18] or as other fluorinated species.

Table 3 Characterisation of AHFS treated materials. Sample

n/d

Chemical Si/A1

(Si/A1)IV

Crystallinity (%)

Surface Area (m2/g)

FS1.363

4.01

2.98

89

780

FS2.363

2.61

2.15

116

1015

FS3.363

4.36

2.80

77

675

FS5.363

3.94

2.80

87

803

FS6.363

4.19

2.98

87

700

FS7.363

3.11

3.11

90

744

FS8.363

8.12

6.38

51

n/d

FS9.363

5.51

3.72

78

n/d

FS10.363

5.80

4.56

52

n/d

not detelTnined

3.2 Nitrogen Adsorption Nitrogen adsorption experiments were performed using a Micromeritics ASAP 2000 sorption apparatus. Surface areas and pore volume distributions were calculated using the BET [21] and the BJH [22] methods, respectively. Prior to analysis, the samples were outgassed at 400 ~ for 12 hours. Results of the nitrogen adsorption study show that dealumination by both hydrothermal and AHFS treatment results in materials which differ in textural properties when compared with each other and with the parent material. Figure 1 shows the nitrogen

151 adsorption/desorption isotherms for the parent material NH4-Y, hydrothermally treated (H1) and AHFS treated (FS6.363) samples. The isotherm for NH4-Y is the typical Type-I response for a microporous material [23]. The presence of a small amount of hysteresis between the adsorption and desorption sections of the isotherm is evidence of a highly microporous material with only a minimal contribution from mesopores. In contrast, the hydrothermally treated sample showed significant adsorption/desorption hysteresis, indicative of the existence of mesopores within the sample. The sorption capacity of the AHFS treated sample (FS6.363) is seen to be much reduced relative to that of the parent material. Also a small hysteresis does exist, which is very similar in nature to that of the parent material, suggesting that the parent microporous structure has been maintained with little evidence of site defect introduction. One interesting feature of the treatment regime for FS2.363 is that the product displayed an increase in the total surface area and a decrease in the mean pore size, but maintained the micropore component of total surface area. This can probably be attributed to silicon insertion into site defects, resulting in site repair within the zeolite framework. The lower adsorption capacity of this zeolite may be due to the presence of oxy-fluorinated aluminium species trapped within the cages of the zeolite which are not removed by washing as suggested by Akporiaye et al [24]. The hysteresis in the high pressure region of the isotherm may be due to a "disturbance" of these fluorinated species which settle back into their respective sites in the low pressure regions of the isotherm. These conclusions are supported by a Dubinin-Radushkevitch [25] analysis, shown in figure 2. In the case of the parent material (NH4-Y) and the AHFS treated samples evidence of a uni-modal micropore distribution is obtained. In contrast, both micro- and meso-porosity are observed for the hydrothermally treated samples. The mesopore volume increases with the severity of the hydrothermal treatment. This behaviour is typical of hydrothermally dealuminated samples.

~" 320

2.55

~D 281

2.45

m~'~242

I

[ ~ NH4-Y FS6.363 H1

2.35

"~m~203 2.25 <164

~ 2.15

1250r0'. ' '0'.3' ' '0'.5" ' "0:8' ' '1.0 ;> Relative Pressure (p/ps)

. . . .

0-------0 I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

i

. . . .

I

. . . .

I

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 [Log(p/p)]2

Figure 1. Nitrogen adsorption isotherms Figure 2. Dubinin-Radushkevitch (D-R) for NH4-Y, HI and FS6.363 samples, plots of NH4-Y, HI and FS6.363 samples. 3.3 27AI MAS NMR Measurements

27A1 magic-angle-spinning NMR spectra were recorded at a spectrometer frequency of 104.2 MHz on a Chemagnetics 400 MHz spectrometer. The samples were spun at 10 kHz and an rf pulse of 0.2 ms was used (~/2 = 2ms) with a recycle delay of 0.3s. The 27A1 chemical shifts (ppm) are reported relative to the 27A1 resonance of Al(H20)63+. NMR spectra for the hydrothermally treated (H2) and AHFS treated (FS9.363) samples are shown in figures 3 and

152 4, respectively. In figure 4 the resonance at 60 ppm corresponds to tetrahedral framework aluminium. The resonance at 3-4 ppm suggests the presence of a small amount of extraframework aluminium. This extra-framework aluminium may not necessarily be octahedrally co-ordinated but may be present as fluorine-complexed aluminium. This is currently being investigated using 19F NMR. The hydrothermally treated sample gives a tetrahedral aluminium resonance which is much broader than that observed for the the AHFS treated sample; this feature in the spectrum is probably due to the presence of non-framework tetrahedral aluminium. The presence of a high degree of extra-framework aluminium is also consistent

..._____/

/ 'N.,..

ppm

ppm '

1 O0

0

- 1O0

Figure 3.27A1 MAS NMR spectrum of H2.

;

'

'

I

1 O0

'

'

'

'

I

'

'

'

'

I

'

'

- 1 O0

Figure 4. 27A1MAS NMR spectrum of FS9.363.

with the relatively intense octahedral aluminium resonance. These data support the conclusions of the nitrogen adsorption analysis, in that the AHFS treatment retains the structural integrity of the zeolite to a much greater extent compared to the hydrothermal treatment. 3.4.

n-Butane

Catalytic

Cracking

The catalytic activity of the zeolites was investigated using the n-butane reaction. 200mg samples of each zeolite were taken from batches calcined at 500 ~ in an air flow of 50ml/min for three hours, and loaded into a quartz plug-flow reactor of internal diameter 10mm. The samples were activated at 400 ~ in a helium flow of 30ml/min for one hour. 2 mol% of nbutane in helium was then passed over the catalyst at a flowrate of 30 ml/min. 20~tl samples of the product gas were then taken at specific time intervals and passed through a gas chromatograph (Philips PU 4500) for analysis. The products were resolved using a GraphpacGC column with 0.19% picric acid at an oven temperature of 29 ~ in combination with a flame ionization detector at 250 ~ Temperature programmed activity profiles for parent, hydrothermally and AHFS treated zeolites showed that the hydrothermally treated zeolites yield a much greater activity than do the parent or AHFS treated materials. Selectivity profiles of the catalyst also showed significant differences between the AHFS and steam dealuminated samples. Figures 5 a and b show the

153 propane and other product selectivities for the steam (H3) and AHFS (FS9.363) treated zeolites as a function of n-butane conversion. The value of n-butane conversion represents the steady state conversion of n-butane to products at a given temperature. Thus, these graphs also indicate the change in product selectivity with increasing operating temperature of the reactor. In the case of the hydrothermally treated zeolites the propane selectivity increases with respect to temperature, whilst the inverse occurs in the AHFS treated samples. This observation suggests that different temperature-dependent mechanisms [26-28] are occurring in zeolites dealuminated by different treatments. (a)

(b) 12

90

~" 10

10

88 87

888 86 ~

"~" 6

84 ~ <

~

4

82 `<,~ ~

2

80 "~

0

86 ~ 85 ~

~ 6 ~ 4

0

o

5

10

15

20

25

30

n-Butane Conversion (%)

35

84 ~" `< 83 ,~ 82 ~ 0'''2"''z['''6"''8""

lb

n-Butane Conversion (%)

Figure 5. Product selectivity for n-butane cracking over (a) hydrothermally-treated sample H3, and (b) AHFS-treated sample FS9.363. The reaction products are methane (0), ethane and ethene (A), propane (o), isobutane (m), but-l-ene (.) and but-2-ene (o). 4. C O N C L U S I O N High degrees of dealumination are difficult to achieve using AHFS compared with those obtained via hydrothermal treatment, because of loss of framework crystallinity. The characterisation techniques used here have shown that silicon enrichment occurs during the AHFS treatment, leading to higher bulk Si/A1 ratios. 27A1 MAS NMR appears to show the presence of aluminium species other than those tetra- or octahedrally co-ordinated. These may be the fluorinated aluminium species mentioned in earlier works. The textural properties of AHFS treated zeolites are not changed relative to the parent material in contrast to the steam dealuminated zeolites, where the introduction of secondm~y mesopores occurs. The cracking activity of AHFS treated material is greater than that of the parent zeolite, but not as great as that of the steam dealuminated samples. Selectivity studies have shown that different reaction mechanisms occur within the zeolite depending on the method of dealumination used.

A CKN O W L E D G EMEN TS APM wishes to acknowledge financial support for this work from EPSRC, Unilever Research (Port Sunlight, UK) and The Isaac Newton Trust (Trinity College, Cambridge), and

154 Dr Heyoung He of the Department of Chemistry, University of Cambridge, for performing the 27A1 MAS NMR measurements. Dr D Rawlence of Crosfield Chemicals is acknowledged for helpful discussions and general support.

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