BETA ZEOLITES MODIFIED WITH LANTHANUM AND CERIUM OXIDES

FOR THE ISOMERIZATION OF HEXANE

Yuji Haga1, Pusparatu1, Kiyotada Aoyama1, Kenichi Komura1,

Yoichi Nishimura1, Yoshihiro Sugi1*, Jong-Ho Kim2, and Gon Seo2 1 Department of Materials Science and Technology, Gifu University, Gifu 501-1193, Japan.

e-mail: ysugi@cc.gifu-u.ac.jp 2 Department of Applied Chemical Engineering and The Research Institute for Catalysis, Chonnam National University, Gwangju, 500-757, Korea. e-mail: gseo@chonnam.ac.kr

Keywords: BEA zeolite, BEA zeolite modified with rare earth oxide, La2O3, CeO2, Isomerization of hexane, Cracking, Branched alkanes, Coke-deposition

Abstract. Influences of SiO2/Al2O3 ratio and the modification of beta zeolites (BEA) with La2O3 and

CeO2 on catalytic properties were examined in the isomerization and cracking of hexane. The

catalytic activity for all BEA zeolites was almost in the same level at 210 min after starting. The

combined selectivity for C4, C5, and C6 branched alkanes kept almost in the same level for the change

of acid amounts. The amounts of coke deposition during the catalysis were decreased with the

SiO2/Al2O3 ratio. The modification of BEA with La2O3 and CeO2 declined coke-deposition without

significant decrease of catalytic activity. These results suggest that BEA has high potentiality for the

isomerization of linear alkanes to branched alkanes, and that the prevention of coke-deposition by the

modification of zeolites with rare earth oxides is expected to develop new long life solid catalysts.

Introduction

Aromatic hydrocarbons or olefins have been regulated to be removed from gasoline pool because of

increasing environmental concern, and alkylates and/or isomerates are planned to introduce for an

alternative octane booster [1-3]. For these purposes, the isomerization of linear alkanes to branched

alkanes over zeolites has been interested in many workers [2,5,6]. Beta zeolite (BEA) has been

recognized as the most appropriate zeolite among large pore molecular sieves, because of its

relatively weak acidity and flexible three dimensional pore systems [5,6]. Most of workers have been

examined the hydroisomerization of linear alkanes using zeolite supported platinum catalysts. These

reactions proceed by bi-functional catalyses of acid and transition metal. However, it is unclear to

understand the acidic properties in the isomerization of linear alkanes over these catalysts.

BEA has a three-dimensional intersecting channel system: two mutually perpendicular straight

channels, each with a cross section of 0.76 x 0.64 nm, run in the a- and b-directions. A sinusoidal

channel of 0.55 x 0.55 runs parallel to the c-direction [7]. Recently, new methods of zeolite synthesis

using dried gels, such as a dry-gel conversion (DGC) method and solid transformation have been well

documented by some research groups [8-10]. These new methods have the advantages such as the

expansion of chemical compositions, the reduction of the SDA, high conversion of gel, fine particles

with high crystallization, etc. compared with the conventional hydrothermal synthesis method.

In this paper, we examine the isomerization of hexane over H-Beta zeolites (BEA) with some

SiO2/Al2O3 ratios, and modified BEA zeolites with La2O3 and CeO2 to know their influences on

coke-deposition in the isomerization.

Experimental

Snthesis of BEA Zeolites

A typical procedure (SiO2,100 mmol) was as follows [9]: TEAOH solution (35 wt%), 15.57 g (37

mmol), was mixed with a 25.2 wt% aqueous solution of NaOH, 1.14 g (7.2 mmol), followed by the

Materials Science Forum Vols. 539-543 (2007) pp 2323-2328Online available since 2007/Mar/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.539-543.2323

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addition of colloidal silica (Ludox HS-40), 15.02 g, containing SiO2, 6.01 g (100 mmol), and the

mixture was stirred for 30 min. Al2(SO4)3, 0.34 g (0.01 mmol), was dissolved in warm water, 30.63

ml, and added to the above mixture. The molar composition was: SiO2:0.37TEAOH:0.072

NaOH:0.01Al2O3:17H2O. Resultant mixture was stirred for further 2 h at room temperature, and then,

the gel was dried for ca. 5 h on an oil bath at 80-90°C with continuous stirring. The dried and

powdered gel was transferred to a Teflon cup (55 mm x 37 mm I.D.). This cup was placed in a

Teflon-lined autoclave (125 ml) with the support of a Teflon holder. A small amounts of external bulk

water (ca. 0.2 g per 1 g of dry gel) was added in Teflon cup, and it was placed at the bottom of the

autoclave. The crystallization was carried out at 175°C for 30 h. After the crystallization was

completed, the autoclave was cooled to room temperature. The products were washed thoroughly

with distillated water and dried at room temperature over night. The as-synthesized zeolite sample

was heated 550oC for 7 h in an air stream to remove SDA. The calcined sample was heated in the

ammonium nitrate solution at 80°C for 12 h to convert to H-form, and finally calcined at 500oC in an

air stream. The zeolites abbreviate BEA(30), BEA(66), BEA(100) and BEA(200): the value in the

parentheses show SiO2/Al2O3 ratio.

BEA zeolites modified with cerium and lanthanum oxides were synthesized according to BEA

zeolites by mixing their nitrates in the starting gels.

Catalytic experiments

Catalytic isomerization and cracking of hexane were carried out using 9 mm (OD) quartz tubular

down flow reactor. BEA zeolite (1 g; 18/32 meshes) was placed between two layers quartz wool, and

heated in a stream of 20 ml/min of nitrogen at 550°C for 1 h before introducing hexane. The reaction

was performed at temperature at 350-650°C. The products were analyzed with on-line gas

chromatographs using fused silica capillary columns with FID detector. The capillary columns are

CP-Al2O3/KCl (HP, 50 m x 0.53 mm, 10 µm film thickness) for the analysis of C1-C6 hydrocarbons,

and HR-1 column (GL Sciences, Tokyo, Japan) for C7+ hydrocarbons, benzene, toluene, and xylenes.

The conversion of hexane and the selectivity for the products were calculated on carbon basis:

Fed hexane (mol/mn) = ∑Areai x Ci x Fi/Ni

(Fed hexane – Areao x Cox Fo/No) Conversion (%) = x 100

Total amount

(Areai x Ci x Fi/Ni) Yieldi of each component (%) = x 100

Fed hexane

Selectivity for each component (%) = (Yieldi / Conversion) x 100

Areai = GC area of each component

Areao = GC area of hexane

Methylpentanes (2- and 3-methylpentanes and 2,2-dimethylbutane), methylbutanes (2- and

3-methylbutanes and 2,2-dimethylpropane), and 2-methylpropane were abbreviated as bC6, bC5, and

bC4, respectively. Combined selectivity for the isomerization is sum of bC6, bC5, and bC4: bC5 and

bC4 were cracked after their isomeriation of hexane.

Results and Discussion

Properties of BEA Zeolites

BEA zeolites. XRD patterns of typical BEA zeolites in this study are shown in Fig. 1(a). The

crystal sizes were less than 1 µm. Si-MASNMR spectra of BEA zeolites had clear AlO-Si(O-Si-)3

peaks assigned as framework tetrahedral aluminum along with Si(O-)4 peaks in all BEA zeolites.

2324 THERMEC 2006

Al-MASNMR of BEA(30) shows very small extra-framework aluminum; however, only aluminum

in frameworks was observed for BEA(100). These results show that most of aluminum in BEA(100)

are isomorphously substituted with silica in the framework, and work as acid sites.

NH3-TPD profiles of BEA zeolites with different SiO2/Al2O3 ratio were shown in Fig. 1(b). Two

desorption peaks (l- and h-peaks) are recognized in all BEA samples, and the h-peaks corresponding

to Brønsted acidity, have almost the same temperature at around 300°C although the acid amounts are

proportional to SiO2/Al2O3 ratio of BEA zeolites. Surface areas of the BEA zeolites are 220-300 m2/g,

and pore volumes are 50-70 ml/g.

BEA zeolites modified with lanthanum and cerium oxides. XRD patterns of La2O3 and CeO2

modified BEA zeolites with SiO2/Al2O3 ratio of 100-130 are shown in Fig. 1. The amounts of rare

earth oxides in silica were shown by the molar ratios of rare earth to silica. The patterns were not

significantly changed by the modification with the oxides, and there were no peaks assigned to the

bulk oxides. These results show that these oxides are highly dispersed. However, the intensity of some

peaks decreased to some extents: the crystallinity of BEA zeolites should be declined slightly by the

modification with large amount of oxides. The decrease of surface area also accompanies the slight

decline of crystallinity of BEA zeolites by the modification with these oxides.

Figure 3 shows NH3-TPD profiles of La2O3 and CeO2 modified BEA zeolites (La-BEA and

Ce-BEA). The h-peaks have the same peak temperatures as BEA zeolites; however, the acid amounts

were decreased gradually with the increase in the amount of these oxides. Judging from h-peaks, the

Fig. 1. XRD patterns of BEA zeolites (a) and TPD profiles (b). SiO2/Al2O3=100-130.

50 100 200 300 400 500

Temperature (oC)

SiO2/Al2O3

:200

100

66

30

SiO2/Al2O3

200

100

66

30

2θθθθ (deg)

2 10 20 30 40 50

(a) (b)

Fig. 2. XRD patterns of La2O3 and CeO2 modified BEA zeolites. SiO2/Al2O3=100-130.

2θθθθ (deg)

2 10 20 30 40 50

Ce-BEA

:0.0170

0.0123

0.0087

0.0042

0.0000

CeO2/SiO2

0.0059

0.0038

0.0026

0.0000

0.0009

La2O3/SiO2

2 10 20 30 40 50

La-BEA

Materials Science Forum Vols. 539-543 2325

acid strength of BEA zeolites was not much influenced by the amount of aluminum and/or rare earth

oxides; however, the acid amount decreased with decreasing in aluminum and increasing in CeO2.

These results suggest that these oxides deactivate strong acid sites of BEA zeolites because of their

basic characters. Surface areas of BEA zeolites are 160-320 m2/g, and pore volumes are 40-75 ml/g.

Isomerization over BEA

BEA zeolites. Figure 4 shows the influences of SiO2/Al2O3 ratio of BEA zeolites on the

isomerization of hexane at 400°C. Data were taken after 210 min from starting, because the

significant decrease of the conversion occurred in early stages. The activities of these BEA zeolites

were not much changed with the increase of SiO2/Al2O3 ratio although the decline of the activity was

much for the zeolites with the low ratio due to their strong acidity: the almost same conversions were

observed for all BEA zeolites after 210 min from starting The combined selectivities for branched

alkanes (bC6+bC5+bC4) and their isomer distribution were in the similar level for all BEA zeolites.

The amounts of the coke deposited during the catalysis were decreased with the increase of

SiO2/Al2O3 ratio. These results suggest that the numbers of active sites worked over the zeolites are in

the similar level because of rapid deactivation of strong acid sites by coke-deposition. Time on stream

for BEA(100) was shown in Fig. 4(b). Although deactivation occurred slowly with reaction time, the

selectivity for bC6 over BEA(100) increased with reaction time corresponding to the decrease of the

Fig. 4. The influences of SiO2/Al2O3 of BEA zeolites on the isomerization of hexane. (a) Conversion and coke-deposition. (b) Time on stream over BEA(100). Reaction conditions: temperature, 400

oC; fed hexane,

2.29 mmol/min, carrier gas (N2), 0.89 mmol/min; W/F, 8.17 g.h/mol. Data: Conversion and selectivity: 210 min; Coke: 260 min after starting.

30 60 100 2000

10

20

30

40

50

60

bC6 bC5 bC4

0

2

4

6

8

10

Coke

Conv.

SiO2/Al2O3 (mol/mol)

:bC4

bC5

bC6:Conv.

0

10

20

30

0

20

40

60

80

0 50 100 150 200 250

Reaction time (min)

(b)(a)

Fig. 3. TPD profiles of La2O3 and CeO2 modified BEA zeolites. SiO2/Al2O3=100-130.

Temperature (oC)

50 100 200 300 400 500

Ce-BEA

CeO2/SiO2

0.0170

0.0123

0.0087

0.0042

0.0000

::Acid (mmol/g)

:0.103

0.097

0.149

:0.135

0.148

:Acid (mmol/g)

:0.097

0.097

0.125

0.149

0.148

La2O3/SiO2

0.0059

0.0038

0.0026

0.0009

0.0000

50 100 200 300 400 500

La-BEA

2326 THERMEC 2006

selectivity for bC4; however, the selectivity for

bC5 remained constant with the time.

It is important to elucidate the catalysis with

constant amount of acid sites in order to confirm

how the amount of acid sites is influenced on

isomerization of hexane. Figure 5 shows the

influences of SiO2/Al2O3 ratio on the

isomerization, where aluminum amounts in BEA

zeolites kept constant with diluting by silica.

Catalytic features of the isomerization were

resembled to all BEA zeolites with the different

SiO2/Al2O3 ratio: the catalytic activities remained

almost constant with the change of the ratio, and

the combined selectivities for branched alkanes

were also in the similar levels of 45-50% in the

isomerization for all BEA zeolites. The amounts

of the coke-deposition during the catalysis were

resembled to the catalysis with the same catalyst

amounts. These results show that BEA zeolites

with the different SiO2/Al2O3 ratio have almost the same acidic and catalytic properties for the

isomerization.

BEA zeolites modified with La2O3 and CeO2. As discussed above, the coke-deposition was

observed during the isomerization of hexane over BEA zeolites. Coke-deposition should occur at the

early stages, and the active acidic sites for the isomerization have similar character for BEA zeolites

with the different SiO2/Al2O3 ratio. The prevention of coke-deposition is one of keys for long-life

catalysts. We examine the modification of BEA zeolites (SiO2/Al2O3=100-130) with La2O3 and CeO2

to prevent the deactivation by coke-deposition. Acid amounts of the BEA zeolites were decreased

with the addition of La2O3 and CeO2; however, these oxides did not change the acid strength.

Figure 6 shows the effects of the modification of BEA zeolite zeolites (SiO2/Al2O3=100-130)

with La2O3 on the isomerization, where data were taken after 210 min from starting. The amounts of

Fig. 6. The effects of modification of BEA zeolite with La2O3 on conversion and coke-deposition in the isomerization of hexane. SiO2/AlO3(BEA): 100-130. Reaction conditions: temperature, 400

oC; fed

hexane, 2.29 mmol/min, carrier gas (N2), 0.89 mmol/min; W/F, 8.17 g.h/mol. Data: Conversion and selectivity: 210 min; Coke: 260 min from starting.

bC6 bC5 bC4

0.0000 0.0009 0.0026 0.0038 0.00590

10

20

30

40

50

60

0

2

4

6

8

10

La2O3/SiO2 (mol/mol)

Conv.

Coke

Fig. 7. The effects of modification of BEA zeolite with CeO2 on conversion and coke-deposition in the isomerization of hexane. SiO2/AlO3(BEA): 100-130. Reaction conditions: temperature, 400

oC; fed

hexane, 2.29 mmol/min, carrier gas (N2), 0.89 mmol/min; W/F, 8.17 g.h/mol. Data: Conversion and selectivity: 210 min; Coke: 260 min from starting.

0

10

20

30

40

50

60

bC6 bC5 bC4

0.0000 0.0042 0.0087 0.0123 0.0170

10

20

30

40

50

60

0

2

4

6

8

10

CeO2/SiO2 (mol/mol)

Conv.

Coke

bC6 bC5 bC4

Coke

Conv.

SiO2/Al2O3 (mol/mol)

30 60 100 2000

10

20

30

40

50

60

0

2

4

6

8

10

Fig. 5. The influences of SiO2/Al2O3 of BEA zeolites on conversion and coke-deposition in the isomerization of hexane at constant amount of aluminum. Reaction conditions: temperature, 400

oC;

fed hexane, 2.29 mmol/min, carrier gas (N2), 0.89 mmol/min; W/F, 8.17 g.h/mol. Data: Conversion and selectivity: 210 min; Coke: 260 min from starting.

Materials Science Forum Vols. 539-543 2327

the coke deposited during the catalysis were declined significantly with the increase in the amounts of

La2O3 although the conversion and the selectivity for combined branched remained almost constant.

The effects of the modification of BEA zeolites (SiO2/Al2O3=100-130) with CeO2 were shown in

Fig. 7. The similar effects observed for La2O3 were observed by the addition of CeO2 to the BEA

zeolites: the coke-deposition was decreased with the increase of the amounts of CeO2 although higher

amounts were necessary for effective prevention of coke-deposition. In these cases, the activity and

the selectivity for each branched products were almost in the same level by the addition of CeO2.

These results show that the modification of BEA zeolites with La2O3 and CeO2 effectively

declines the coke-deposition during isomerization and cracking of hexane. This should be due to the

decrease of coke-deposition at strong acid sites, because these oxides deactivate them by their basic

characters.

Conclusion

Influences of SiO2/Al2O3 ratio and rare earth oxides modification of Beta zeolites (BEA) on catalytic

properties were examined in the isomerization and cracking of hexane. The catalytic activity was

almost in the same level after 210 min for all BEA zeolites, although the conversion decreased in the

early stages. The selectivity for the isomerization was not much influenced by the amount of acid

sites; the combined selectivity for branched alkanes (2- and 3-methylpentanes, 2,2-dimethylbutane, 2-,

and 3-methylbutanes, 2,2-dimethylpropane, and 2-methylpropane) kept in the same level by the

change of SiO2/Al2O3 ratio. The amounts of coke deposited during the reaction were decreased with

the ratio. These results suggest that active species for all BEA zeolites have similar acidic characters.

The modification of BEA zeolites with rare earth oxides, La2O3 and CeO2, declined

coke-deposition without significant decrease of the activity and the selectivity of products. This

should be due to the decrease of coke-deposition at strong acid sites, because La2O3 and CeO2

deactivate them by their basic characters.

The prevention of coke-deposition by the modification of zeolites with rare earth oxides is a

promising way to develop the long life catalysts for the solid acid catalysis.

A part of this work was financially supported by a Grant-in Aid for Scientific Research (B) 16310056,

the Japan Society for the Promotion of Science (JSPS), and by Research Project under The

Japan-Korea Basic Scientific Cooperation Program, JSPS and the Korea Science and Engineering

Foundation (KOSEF). Pusparatu is grateful to the Japanese Government (MEXT) Scholarship

Program

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