Optimization of zeolite-β in cracking catalysts influence of crystallite size

Applied Catalysis A: General, 82 (1992) 37-50 Elsevier Science Publishers B.V., Amsterdam

37

APCAT 2212

Optimization of zeolite-p in cracking catalysts Influence of crystallite size L. Bonetto, M.A. Camblor, A. Corma* and J. Perez-Pariente Znstituto de Tecnologia Quimica, UPV-CSZS, Universidad Politkcnica de Valencia, 46071 Valencia (Spain), tel. (+34-6)3877096, fax. (+34-6)3877996 (Received 6 August 1991, revised manuscript received 11 November 1991)

Abstract

Zeolite-jl with different crystal sizes have been used as catalysts for gas-oil cracking. An optimum compromise between stability, activity and selectivity, has been found for a sample with an average crystal size of 0.40 pm. This sample, before and after steaming gives a slightly lower selectivity for gasoline and coke than a high and a low unit cell size USY seolite, respectively. The optimized seolite/3 produces more liquified petroleum gas alkenes and a relatively high i-butane yield that are useful for methyl tert. butyl ether and alkylation gasoline production. Keywords: catalytic cracking (gas-oil), FCC alkene production, FCC catalysts, gasoline, seolite-8.

INTRODUCTION

@ is a 12-member ring (12 MR) tridirectional zeolite, with two different types of channels having about 7.0 and 5.5 A [ 11. It can be synthesized within a large range of silica to-alumina ratio (12-200) [ 21, and the influence of the different synthesis variables on the rate of crystallization and the final textural characteristics has been described recently [ 3,4]. This zeolite may offer interesting opportunities as a catalyst, since it combines three important characteristics: large pores, high silica-to-alumina synthesis ratio, and a tridirectional network of pores. In addition, the dimensions of one type of pores (5.5 A) can give a certain level of shape selectivity. This has been shown to apply to the isomerization and transalkylation of xylenes [ 5,6], to the alkylation of toluene by methanol [ 71, and to the condensation of benzene and formaldehyde [ 81. Recently, it has been claimed that zeolite-8 can be used to catalyze the isomerization of Light Strain Run [ 91, or combined with BF3 it can be used to alkylate i-butane with alkenes to obtain alkylate gasolines [ 10,111. In the case of FCC, this zeolite could, in theory, be a promising catalyst, or

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0 1992 Elsevier Science Publishers B.V. All rights reserved.

38

catalyst additive, since due to its high cracking activity and low hydrogen transfer ability [ 121, it could produce high octane gasoline. When zeolite-/? was used to crack gas-oil, it was presented [ 131 that it was indeed active but produced more gases and coke than ultrastable Y zeolites (USY). Therefore, even if this zeolite is able to produce a higher octane gasoline, the octane per barrel would be lower than for low unit cell size USY zeolites due to the sensibly lower selectivity to gasoline of the former. Similar conclusions have been obtained in ulterior work for gas-oil cracking on H- zeolite$ [ 14-161. However, actual environmental restrictions, which have obliged the reformulation of the gasolines, have created the necessity of using large amounts of i-butane, C3, Cd, and C5 alkenes and i-olefins to obtain more alkylated gasoline, MTBE and MTAE. As a result, FCC cracking policy is being revised and in many cases the obtention of the C3 and C, gases, named above, is a desired objective. It is in this sense that zeolite-/I could be of much interest and use. In the present work it will be shown that zeolite$ could be optimized to produce high amounts of olefinic LPG, while giving high selectivity to gasoline and low selectivity to coke. EXPERIMENTAL

Materials Zeolite-/?I (Si/Al= 13) has been synthesized according to ref. 3, with average ~cq#al s+aof,O.;17,..Q4& a.r&6~‘jlC),~ b &lJ- AU -of tlqn,were lOO% crys:. talline as me&_iredby ‘XLray diffraction s XBD) ‘(peak’area of the 20= 22.4 b,Cu Kcu radiation). Tetraethyl ammonium-Ssamples were exchanged with NH:, [NH: ] =2.5 M, for 2 h at 353 K. Then, they were calcined in a glass fixed-bed tubular reactor by .increasing the temperature up to 823 K at 1 K/ min under nitrogen flow (200 ml/min). When 753 K were reached, the nitrogen was changed to dry air (350 ml/min) for 2 h. After this, the samples were twice exchanged and the air was calcined up to 853 K at 0.5 K/min. After this treatment all the organic template was removed from the zeolite. The steaming was carried out at 1023 K for 5 h in 100% steam. Samples were pelletized, crushed and sieved. A particle size of 0.59-0.34 mm was selected for reaction. USY zeolites were prepared in the following way: a NaY (SK-40 Union Carbide, crystallite size ca. 0.80 p) was NH: exchanged until the residual Na+ was 30.% of the original. Then, the sample was steam-calcined at 973 K for 3 h and finally twice NH,+ -exchange followed by calcination at 773 K, the final NazO content being 0.12 wt.-%. This sample was nominated as USY -1 (crystallinity 70% ) and has a framework Si/Al close to that of the starting @ample. A portion of USY-1 was

39

Fig. 1. Scanning electron micrographs of as-synthesized zeolitestop to bottom, samples: (a) 0.17~, (b) 0.40,~11, and (c) 0.70

.

Scaleb

m.

in1

l-8 . From

40 TABLE 1 Physicochemical characteristics of the zeolite catalyst samples

d (pm)

UC

(A)

Si-0 (cm-‘)

Si/AP

1068 1073 1072

13.ld 12.9d 13.4d

As-synthesized 0.17 0.40 0.70

After activation treatment 0.17 1092 0.40 1090 0.70 1091 USY-1 24.38 1066 After steaming 0.17 0.40 0.70 USY-2 24.28

10.6’

1100 1104 1100 1080

35.2’

AI/UC TEA+/ucb

Na (wt.-%)

K (wt.-%)

Total wt. loss (%)b

22.9 21.3 21.3

4.5 4.6 4.4

4.2 4.7 4.4

0.9 0.1 0.0

0.6 0.5 0.6

16.5

0.0 0.0 0.0 -

0.0 0.0 0.0 0.11

0.0 0.0 0.0 -

5.3

0.0 0.0 0.0 -

0.0 0.0 0.0 0.12

0.0 0.0 0.0 -

“Framework. bFrom TGA-DTA analysis. ‘From unit cell size and Fichtner-Schmittler’s equation [ 171. “From chemical analysis. TABLE 2 Characteristics of the Arabian light vacuum gas-oil Density (g/cm3) at 288 K=0.8965; nitrogen=562 ppm; density (g/cm3) at 333 K=0.868; KUOP=11.5; sulfur (wt.-%) =2.2; conradson carbon (wt.-%) =0.16 Distillation curve IBP” 507

(K)

5

lo

20

30

40

50

60

70

80

90

573

592

623

644

665

684

703

723

744

770

FBP” 821

“Initial boiling point. bFinal boiling point.

steamed (100% steam) at 1023 K for 5 h, and the resultant sample was named USY -2 (crystallinity 60% ) . The unit cell constant of the USY samples was determined by XKD using Cu Ka radiation and following ASTM procedure D-3942-80. The estimated standard deviation was 5 0.01 A. The number of aluminium per unit cell was calculated from the Fichtner-Schmittler equation [ 171. Crystallinity of the USY samples was calculated by comparing the peak height of the (5,3,3 ) peak

41

in the sample, with that in the NaY SK-40, taken as 100% crystalline. The characteristics of all zeolites used in this work are given in Table 1. An Arabian light vacuum gas-oil was used as feed, and the properties are given in Table 2. Catalytic experiments The cracking catalysts were prepared by diluting either USY or the/3-sample in a silica matrix. The experiments were performed in a fixed-bed tubular reactor within the ASTM D-3907-87 specifications. Prior to each experiment the catalyst was stripped with nitrogen (100 ml/min) at 755 and 793 K (reaction temperature) for 20 min. The catalyst-to-oil ratio (zeolite/gas-oil) was varied between 0.2 and 0.9 g g-’ by changing the weight of the gas-oil feed (5-0.6 g) in order to obtain different levels of conversion. Time-on-stream was always 60 s. Conversion is defined here as the sum of gases, coke, gasoline ( < 483 K ) and diesel (483 < T-c 593 K). The catalyst was regenerated “in situ” after each experiment, by passing 100 ml/min of air through the reactor at 503 K for 5 h. Gases were analyzed by gas chromatography and separated by Porapak Q plus silica column. Liquids were analyzed by simulated distillation. The amount of coke was measured from the weight of water and carbon dioxide generated during regeneration. RESULTS AND DISCUSSION

When zeolite-j? is synthesized following the procedure described in the original patent [ 21 and ulterior scientific papers, small crystallites IO.20 pm are obtained. When this is used for cracking gas-oil, zeolitej3 produces a much lower selectivity to gasoline, and a higher selectivity to C, + C, and coke than ultrastable zeolites, with either high and low unit cell sizes [ 131. It was shown previously [ 13,141, that the small crystallites zeolites$ were relatively unstable to the conventional calcination procedures used for activation, and a loss of properties occurred during this process. This limitation could be overcome by preparing the zeolite with a larger crystal size. However, the crystal size has to be optimized since too large crystallites, whilst being more thermally and hydrothermally stable, are less active and selective due to higher diffusion limitations, as has been shown to occur on zeolite Y [ 181. To study this, three samples were synthesized with the same silica-to-alumina ratio, but with crystallite size in a narrow distribution centered at ca. 0.17,0.40 and 0.70 pm, as can be seen from the scanning electron microscopy (SEM ) pictures (Fig. 1) , and the histograms (Fig. 2 ) obtained after accounting the crystallites. The XRD patterns of the zeolites-pbefore and after steam-

42

1

0.6

crystal sixe (y 1

crystal size Cp1 ,,% crystals (cl 20

Fig. 2. Crystal size distribution of as-synthesized p, (b) 0.40 F, and (c) 0.70,um.

jkamples,

with average crystal sizes: (a) 0.17

ing are given in Fig. 3. The percentage of crystallinity retention during steaming are reported in Table 3. The stability increases with increasing crystallite size, but this increase is not very high. Cracking behavior of the unsteamed samples When the samples were used to crack gas-oil, results from Fig. 4a show that a maximum in activity is obtained for the sample with a 0.40-e crystal size. This sample represents an optimum between stability and diffusion limitations. Results from Fig. 4b show that the minimum for coke production is also

43

(b)

Fig. 3. XRD pattern of zeolite-j3 samples with different crystal size, from top to bottom: 0.70 p, 0.40 ,um and 0.17 ,um; (a) fresh, (b) steamed catalysts. TABLE 3 Crystallinity retained after steaming of &w.nples Crystal size.

(%I

(PI 0.17 0.40 0.70

95

98.

100

,.

:_.

,.

achieved for this crystal size, while the selectivity for C, + C, is minimum. Results from Fig. 4a, also show that, before steaming, the zeolite$ with a 0.40pm crystallite size gives only a slightly lower conversion than a USY-1 zeolite with 24.38 A unit cell size, for which conversion is maximum [ 191, and may correspond to the zeolite present in a fresh catalyst. In addition, this zeolite-/I gives almost the same gasoline selectivity, the same coke, more gases and less diesel, than the USY-1 zeolite (Fig. 5). However, the shape of the curves of yield versus conversion, indicate that the m&ability of both, diesel and gasoline appear at a lower conversion on the fi than on the USY-1 sample. This indicates that the former zeolite gives more recracking or, what is equivalent, is more selective than Y zeolite for cracking diesel and

44 conversion

70

1;:

(%)

Y\ /O “\

55-

0.4

0.2

0.0

0.6

crystal

size

1.0

0.6 (pm)

selectivity

1

(b)

0.06

Cl+C2

“--

I

. 0.2

0.4

0.6

0.0

1

o.ooL0 0.2

0.4

0.6

0.8

1

0.4

0.6

0.8

1

0.55-

0.06

0

0.2

Coke

d

1 0.061

0.04’

I

“h’

I 0

0.2

0.4

crystal

0.6

0.8

1

size (~1

Fig. 4. (a) Influence of crystal size at cat./oil=0.40 on total conversion, and comparison with USY - 1./I?, q ; and USY - 1,O. (b ) Influence of crystal size on selectivity to gases, C, + C2, gasoline, diesel and coke on zeolite-8, Cl; and USY-1, 0 at 70% conversion level.

45

yield (%I

40

50

60

70

80

30

40

50

60

70

80

Diesel

30

40

50

60

0’

30

70

80

30

40

50

60

70

80

I 40

50

60

conversion

70

80

(%I

Fig. 5. Selectivity to gases, C1 + C2, gasoline, diesel and coke on zeolite-j3 0.40 pm (0 ) and USYl(O).

gasoline than for cracking bigger molecules in the gas-oil range. This is a consequence of the smaller pore dimensions of p, which are shape selective for larger molecules. The yields to gases on p with different crystal size and on the USY-1 zeolites are given in Fig. 6. All zeolites-/I produce more alkenes in gases than USY-1. However, better results are obtained with the sample with bigger cry&all&es. Maximum yields of butenes and i-butane were obtained with /I with a crystal size of 0.40 pm, while maximum propene was obtained for a 0.70-pm crystal size. It appears then that /I-samples produce a higher yield of LPG alkenes than USY-1 as a result of the lower hydrogen transfer ability of the former [ 121, but also as a result of its higher recracking activity. One should remember that

46

-

beta

0.17

beta

0.4

Propane

Propylene

i-Butane

n-Butane

Butene

Fig. 6. Yield of LPG gases at 70% conversion level, on the different crystal size /3, and USY-1.

_by kank

cf8ckipg an $hd

i’;lbbi;te+

++c,qe & tlp,g~h#, dkene

&

pro&,e&

$eoel, or g~c@e8pctioq, fi&s$ei,,

‘if

h

&;‘e

a shorter al-

j’&cr&&&~~

will produce two alkenes. This effect when accompanied by low hydrogen transfer should produce high yields of C3 and C, alkenes. Unfortunately, our gas analysis system did not allow us to obtain a separation of i-butene and nbutenes good enough to be quantified separately. In this case it appears that, from the point of view of a fresh catalyst, the zeolite-/l with an average crystal size of 0.40 p, shows a clear advantage on USY zeolite for producing short chain alkenes, while the gasoline penalty from the FCC is relatively small. However, taking into account its possibilities for producing raw materials for MTBE, and the increase in potential gasoline yield gain from the FCCU feed, zeolite$l looks like an attractive material. Zeolite-j3 in the equilibrium catalyst The impact of the fresh catalyst is sometimes considerable, a major part of the conversion and selectivity comes from the equilibrium catalyst. To study the behavior of an equilibrated zeolite-p, gas-oil cracking was performed on the /&xunple after steaming at 1023 K for 5 h under 100% steam, and the results

I’

47

(4 50

conversion (%I .

45-

40-

q-

II-

35/ /O

30-

25 0.2

0.0

0.6

0.4

1.0

0.6

crystal size (p 1 selectivity

(b)

/ O.lOi

0

0.2

0.4

0.6

0.6

1

0

0.2

0.4

0.2

0.4

0.6

0.6

1

0

0.2

0.4

0.6

0.6

1

0.6

1

OBO ‘--

I _-__I 0

0.05’

0.6

I 0

0.2

0.4

crystal

0.6

0.6

1

size (p)

Fig. 7. (a ) Influence of crystal size at &./oil =:0.60 on totaleonversion after steam treatment and comparison with USY -2. A 0; and USY -2,O. (b ) Influence of crystal size after steam treatment on selectivity to gases, C, +C,, gasoline, diesel and coke at 70% level conversion on zeolite-/3, q; and USY-2 a.

m

beta 0.4

beta 0.17

Propane

Propylene

i-Butane

n-Butane

Butene

Fig. 8. Yield of gases at 70% conversion level after steam treatment on different crystal size /I and USY -2.

were compared with equilibrated USY-2 (24.28 A). Results from Fig. 7a show that conversion increases by increasing the crystal size from 0.17 to 0.40 ,um. However, a further increase in the crystal size to 0.70 pm does not produce a gain in conversion. A higher conversion is observed with the USY-2 sample. Nevertheless, taking into account the influence of the unit cell size on the gasoil cracking activity of USY-zeolites [ 191 it appears that the activity of the zeolitej3 would be equivalent to that of a USY with a unit cell size of 24.2424.26 A. This shows that from the point of view of the zeolite component, /3 could be as active as the USY zeolite used in high octane catalysts. Therefore, a FCC catalyst prepared with a zeolite-/I with the optimum crystal, combined with an active matrix could give the same gas-oil conversion as the actual existing high octane catalysts, based on low unit cell USY zeolite. However, if conversion is defined as the sum of gasoline plus gases plus coke, as it is normally done, then the 0.40 pm j? is as active as the USY-2 zeolite. This is probably a result of the stronger acidity shown by zeolite#, which gives a higher turnover frequency for cracking than USY samples [ 121. The important role that other factors, in addition to crystallite size, such as instance acidity play in the case of zeolit..e$ can be inferred by considering the fact that the activity

49

of the remaining crystalline material, is much higher for the 0.40-pm sample than for the other two /&samples. Investigations on this matter are underway. From the point of view of selectivity, if the optimum crystal size of p is used, it can be seen from the results given in Fig. 7b, that a high selectivity to gasoline is obtained. This is practically the same result as that obtained with USY-2 with 24.28 A, and therefore is higher than the selectivity expected if a lower unit cell size USY zeolite is used. The higher amount of gases formed corresponds to a lower yield of diesel, as the coke produced practically the same for /3 as for USY-2 zeolite. The yield to the different gases produced (Fig. 8) show much higher values for C3 and C, alkenes and i-butane on the p samples. CONCLUSIONS

fi-Zeolite synthesized within a narrow crystal size distribution around 0.40 pm shows an optimum behavior from the point of view of gas-oil cracking activity and selectivity. When prepared in this way zeolite$ is slightly less active than USY zeolites if Light Cycle Oil is included in the conversion. If conversion is defined as the sum of gases plus coke plus gasoline, its activity can be even higher than that of the corresponding USY. In this case, using the optimum zeolite$, it is possible, while obtaining almost the same gasoline yield, to produce a much greater amount of C3 and C, alkenes due to its low hydrogen transfer activity, which can be used for producing gasoline alkylate and MTBE. In conclusion, a high yield and a high octane gasoline can be, in the end, obtained, by using an optimized zeolite-p in a FCC catalyst. ACKNOWLEDGEMENTS

Financial support from the Spanish CICYT, project MAT 91-1152 is gratefully acknowledged. L.B. thanks the M.E.C. of Spain for a scholarship.

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