Studies in Surface Science and Catalysis, volume 154 241 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

NANOSIZED p ZEOLITES AND THEIR COMPOSITES AS CATALYSTS FOR ACYLATION AND ALKANE

ISOMERIZATION

Wang, Y., Min, E. and Mu, X.

SINOPEC Research Institute of Petroleum Processing, P O Box 914-22, Beijing 100083, P.R. China. E-mail:wvrxyg(g)sina.com

ABSTRACT

Zeolite p is a high-silica, large pore crystalline aluminosilicate material, it has a high potential for use as a catalyst in various hydrocarbon conversion reactions. When the size of zeolite crystallites is on the nanoscale it has a significant influence on their catalytic performance in a range of reactions. For nano-sized zeolite crystallites however, separation is always a problem in their preparation and furthermore aggregation can be a serious problem in respect of actual application of the materials. In this paper we describe novel processes used to prepare nanosized zeolite P and its composite, respectively. Zeolite p and the composite were characterized by XRD, TEM, EDAX, N2 adsorption and NH3-TPD. The results show that nanocrystalline zeolite p exhibits the highest activity in acylation of 2MN(2-methoxynaphthalene), and that the composite material P/AI2O3 is a suitable support for palladium in the isomerization of n-octane in the presence of hydrogen.

Keywords: Nanocrystalline; zeolite P; Sol-gel; composite; acylation; isomerization

INTRODUCTION

Zeolite P is a high-silica, large pore crystalline aluminosilicate material first synthesized by Wadlinger et al. in 1964 [1]. The framework structure of P possesses three dimensional 12-membered ring pores with an interconnected channel system. By virtue of its high-silica content and relatively high acid strength it has a high potential, comparable to that of zeolite Y and ZSM-5, for use as a catalyst in various hydrocarbon conversion reactions such as Friedel-Crafts acylation and hydroisomerization [2,3]. At present, zeolites commercially used generally are of large crystals, however, use of very small zeolite crystals in the nanosized range is desirable since mass transfer limitations are less significant. Furthermore, deactivation could proceed more slowly with such materials because of their larger specific external area. Synthesis of nanocrystalline zeolite p has received much attention [4-6].

Although nanocrystalline zeolites exhibit attractive properties, there are still two key problems remaining unsolved. One is the difficulty of recovering nanocrystalline materials from the stable colloidal suspensions. The other is aggregation of nanosized zeolites, which results in a large part of the zeolite being in the interior and unavailable for reaction. Composites with embedded nanosized zeolite particles separately may overcome the above limitations [7-9].

In this paper, we describe a novel process used to synthesize nanosized zeolite p. This method reduces both the crystallization time and temperature, and will be introduced in industry. We also describe a novel method used to produce nanosized zeolite P composites. This method not only facilitates the separation of the nanosized zeolite p catalyst but also improve catalyst efficiency. The physicochemical characterization and catalytic properties of the samples are described in detail.

EXPERIMENTAL

Preparation

Nanocrystalline zeolite P Synthesis mixtures were prepared with the following reagents in the laboratory: commercial solid silica

gel, pseudoboehmite, sodium hydroxide, 30% aqueous tetraethylammonium hydroxide (TEAOH), and distilled water. The gel was homogenized by stirring, and transferred to a stainless-steel autoclave for

242

crystallization. A two-stage crystallization procedure was employed [10]. At the first low temperature stage, aluminosilicate gels can be formed on the external surface of solid silica particles. Si02 in the internal of solid particles enters into crystalline area constantly till silica particles dissolve completely, and the nucleis of zeolite p form; the nucleis of zeolite grow into particle at the second high temperature stage. During crystallization period, the gel was stirred continually at 150 rpm. After specific hours, the solids were separated from the mother liquor by centrifugation at 14000 rpm, washed with deionized water and dried at 373 K for 2 h. The as-prepared zeolite was converted to the ammonium form by ion exchange with 1.0 M NH4CI (4 times), and the resulting material dried at 373 K.

Three different zeolite samples denoted as PL, PM and PN were synthesized by varying the crystallization time and temperature of two stages (Table 1). These samples were then used for studying the characteristics of zeolite with different crystal size.

Table 1. Synthesis Conditions of the Zeolite Samples .̂

PL

PM

PN

First stage Temperature/°C 120 115 115

Time/h 24 24 24

Second stage Temperature/°C

148 125 125

Time/h 48 24 3

'Common conditions: SiO2/Al2O3=30, OH7SiO2=0.12, TEAOH/SiO2=0.09, H20/Si02=6.5.

Nanocrystalline zeolite ji/AhOs composites Nanosized zeolite P/AI2O3 composite was prepared by sol-gel method. First, y - AI2O3 was hydrolyzed

to alumina sol. Then the sol was rapidly added to a zeolite P colloidal solution prepared as described above to form a gel. The gelation rate was controlled by altering the pH value and the concentration of the zeolite. After the gel was dried, nanosized zeolite P/AI2O3 composite was obtained. Composites with various content of zeolite P were prepared by varying the ratio of zeolite P to alumina sol. Here we give the typical procedure for preparing a sample with 60% zeolite P as an example. A mixture of 3.46 g of y -AI2O3 (74% AI2O3 and 26% H2O) and 110 g of 0.05 N HNO3 was stirred for 40 min, resulting in clear alumina sol. 10 mL of water was added to 9.6 g of as-synthesized nanocrystalline zeolite colloidal solution (containing 38wt% of dry p zeolite). The as-prepared alumina sol was added rapidly to nanocrystalline zeolite p colloidal solution with agitation within about 10-20s. The mixture gelled to form a viscous liquid. The gel was placed in an oven and dried at 343 K for 2-3 days. The product was ground and then converted to the NH4^ form by ion exchange with 1.0 M NH4CI, and washed with deionized water. The last step was repeated 4 times. The material was then dried at 373 K.

A physical mixture of nanocrystalline zeolite P and alumina with the same composition, denoted as (p/Al203)inix, was prepared as a reference.

Pd'loaded zeolite P The ammonium-form of zeolite p was impregnated with Pd^% to prepare catalysts containing 0.5 wt%

Pd. The resulting catalysts were then dried and calcined in air at 550 °C for 3h. The catalysts Pd/pN, Pd/Pt, Pd/(p/Al203), Pd/(p/ Al203)inix wcrc pelletized and crushed to 40-60 mesh for catalytic tests.

Characterization The crystalline phases of the solid products were analyzed by powder X-ray diffraction (XRD) using a

D/MAX-llA diffractometer (CuK radiation) with a graphite monochromator. Crystal morphologies were observed by transmission electron microscopy (TEM) using a JEM-2000FX1I microscope. The Si02/Al203 ratio in the samples was determined by energy dispersive microanalysis (EDAX) using a JEM-2010F instrument. Surface area and total pore volume were obtained from low temperature N2 adsorption experiments using a Micromeritics ASAP2400 instrument. The acidity of the zeolites was also determined by temperature-programmed desorption (TPD) of the ammonium-form, where the desorbed ammonia was trapped in a buffer solution of boric acid and autotitrated with sulfamic acid.

243

Catalytic Tests

Acylation of 2-methoxynaphthalene (2MN) Acylation reactions were carried out in a sealed glass flask with a magnetic stirrer with heating under

constant pressure at 80 ^C using acetic anhydride as the acylating reagent and 1,2-dichloroethane as the solvent. Prior to use, 5.0 g of catalyst was calcined in air at 550 ^C. Aliquots of the reaction mixture were sampled and analyzed by a gas chromatograph equipped with an FID detector by GC-MS. The error in the experimental results was estimated to be in the range ± 7%.

Isomerization ofn-Octane The catalytic tests were carried out in a microreactor system consisting of a stainless-steel tube reactor of

1.2 cm LD., equipped with a flow controller and heated by an electrical heater. The catalysts were prereduced in situ with H2 at 648 K for 2 h. The isomerization of n-octane was then studied under the following conditions: temperature 473-513 K; pressure 0.6 MPa; WHSV=10 h"̂ ; H2/HC=1:1. The gas flow rate was such as to exclude the effect of external diffusion. During each run, products were collected every 4 h until the reaction reached a pseudo-stable state. Products were analyzed by GC. No significant deactivation of the catalyst was observed after a time on stream of 11 h.

RESULTS

Nanosized Zeolite P

XRD analysis The crystalline phases were identified by XRD analysis (Figure 1). The XRD patterns of as-synthesized

samples were similar to that of typical p zeolite reported in literature [1]. No peaks corresponding to impurities were observed. When the crystallinity is evaluated based on the intensity of the main peak of pL, the crystallinity of PM is 95%, and the crystallinity of PN is 74%. The peaks broadening were also observed from the full-width at half maximum (FWHM), indicating the crystal size decreased (Table 2).

2E

Figure 1. XRD patterns of zeolite p.

Table 1. The Crystallinity of P L' P M and p t

PL -•M PN

Cfystallinity/% 100 95 lA

F W H M 0.166 0.269 0.367

244

TEM studies The crystal size of all samples has been observed from TEM micrographs (Figure 2). While the samples

have a very small crystal size, the crystallites tend to aggregate making it difficult to determine a reliable crystal size distribution. Thus, the crystal sizes given here are estimated values. The crystal sizes of PL, PM and pN measured from TEM micrographs were 500, 200 and 50 nm respectively.

B ,, -200iim B„-50i ini

Figure 2. TEM Micrographs of Nanocrystalline Zeolite p Samples.

Nanosized Zeolite p Composites

XRD analysis Figure 3 shows that zeolite p phase was imbedded in composites, and no peaks corresponding to

impurities were observed. The content of zeolite p can be calculated by the result of peak area (Table 3). It was also found that the zeolite starts to agglomerate and gel of p / AI2O3 cannot be formed within a short time (10-20s) at higher zeolite loading. The maximum zeolite loading is 85%.

Figure 3. XRD patterns of p / AI2O sComposite

Table 3. The content of Zeolite p in composites.

^ SOvrtSSp/AlA 60wt35p/AlA 70 wtSSp/AtO:

CtystaUinity 4307.5 2151.8

Content of |?/o 100 50

2571.4

59.7

3019.9

70.1

245

Distribution of zeolite P within the composites We have employed ED AX analysis to investigate the distribution of the zeolite nanocrystallites within the

composites. EDAX analysis of the Si02/Al203 ratio was performed by moving the electron spot along the sample [11]. When the spot diameter is substantially larger than the dimension of the zeolite nanocrystals, the Si02/Al203 ratio is the average value of the bulk composition of the pellet. When the spot diameter is reduced close to the size of the zeolite nanocrystals, and the spot hits a zeolite nanocrystal, a higher Si02/Al203 ratio is measured. A lower Si02/Al203 ratio is obtained when the diameter of the electron spot was 50nm and 12.5nm. Significant jumps in the measured Si02/Al203 ratio when the diameter of the electron spot was decreased to around 5 nm, indicating that this is the approximate particle diameter of the zeolite phase in the P/AI2O3 composite. The results were shown in Figure4.

16

o

1 2 3 4 5 6

Different position on the surface of the conposite Figure 4. Effect of Spot Diameter on the Si02/Al203 Ratio as Measured by EDAX in 60wt% P/AI2O3 Composite.

Surface area and pore volume of the composites Surface area and pore volume data for the composites with different content of p zeolite are shown in

Table 4. Embedding the zeolite crystals in AI2O3 resulted in a material with a greater external surface area and mesoporus volume than those possessed by the corresponding mechanical mixture (p/ Al203)mix.

Table 4. N2 Adsorption Properties of Samples.

Sample

50 wt%p/ AI2O3

60 wt%p/Al203

75 wt%p/ AI2O3

50 wt%(p/Al203W

60 Wt%(p/Al203)™x

75 wt%(p/Al203U

Surface area / m^.g'̂ Stotal

428

451

476

358

441

494

^internal

218

249

349

181 294

382

^external

210

201

127

177

147

112

Pore volume/mL.g"^ V micro

0.101

0.115

0.163

0.084

0.136

0.179

*^meso

0.759

0.792

0.643

0.402

0.388

0.375

Acidity of the composites The acidities of the samples were measured by the temperature-programmed desorption (TPD) of

ammonia. The amount of ammonia evolved is considered to be a measure of the number of acid sites present. The results shown in Table 5 suggest that the P/AI2O3 composite has a lower acid content than the corresponding mechanical mixture. This is caused by the interaction between the matrix hydroxyl groups and the zeolite during the sol-gel process [12].

Table 5. Acid Content of Samples.

Sample Acid

content/mmol.g"^ 0.86

60 Wt%P/ AI2O3 60 Wt%(p/ Al203)n, 0.56 0.66

246

Catalytic performance

A cylation of 2-Methoxynaphthalene Friedel Crafts acylation of aromatics is one of the main ways of producing aromatic ketones, which are

useful intermediates for the synthesis of a number of fine chemicals [13]. The acylation of 2-methoxynaphthalene (2MN) is a key step in the synthesis of the pharmaceutical compound, (S)-Naproxen. In the acylation of 2MN, l-acetyl-2-methoxy naphthalene (1,2-AMN) and 2-acetyl-6-methoxynaphthalene (2,6-AMN) are obtained. 2,6-AMN is the intermediate required for (S)-Naproxen production. However, the undesired 1,2-AMN isomer generally predominates. This is because acylation at the a-position is kinetically favored. The reactions are shown in Scheme 1.

-OOCHj, +H

CO' .OCH,

(ca,co)jO

2MN U-AMN

OCHa CH3COOH

.OCHa

IJ + CH3COOH

C H Q

2,6-AMN

Scheme 1. Acylation of 2-MN.

Three zeolite p samples with varying crystallite sizes, as described above, were investigated as catalysts for the acylation of 2MN. Table 6 shows that the conversion of 2MN increases as the crystal size decreases. Accordingly, the yield of 2,6-AMN is the highest for the zeolite with the smallest crystal size. This may be due to the decreased difftision limitations found in the nanometer size zeolite having a greater external surface area. A similar result was reported by G.Harvey et al. [2]. Additionally, dealumination of the zeolite after reaction for 8 h is observed, as shown in Table 6. The dealumination is likely to be related to the acetic acid formed during the conversion.

The 60%p/Al2O3 composite has a lower activity than the zeolite P samples due to the low amount of zeolite in the composite. The physical mixture (60%p/Al2O3)mix is inactive in the acylation of 2MN.

Table 6. Acylation of 2MN.

Sample

3L

PM

PN 60%p/Al2O3 (60%p/Al2O3)mix

Si/AP

11.8 11.8 12.8

— —

Si/Al''

12.4 12.2 ^ 13.3

— —

Crystalsize/nm

500 200 50

— —

Conversion of2MN/%

21.0 28.3 51.3 9.1

—

Yield/%

1,2-AMN

17.4 23.8 38.7 4.4

—

2,6-AMN

3.5 4.5 12.5 4.7

—

Selectivity /%

1,2-AMN

83.1 84.1 75.6 48.7

—

2,6-AMN

16.9 15.9 24.4 51.3

— ' Measured before reaction. Measured after reaction for 8 h.

The zeolite samples became darker in color after the reaction indicating coke formation on the acid sites. The stability of two zeolite samples (PM and PN) was compared in Figure 5. Unfortunately, Zeolite PN was found to exhibit more rapid deactivation in the reaction.

247

Times

Figure 5. Activity of Aacylation of 2MN.

Table 7. Comparison of fresh and regenerated zeolite (3 catalysts.

Sanple Fnesh catalyst

C onversion/yo Selectivity to

2,6-AIvIN

After regeruetation

CcMmversion/yo Sdectivi^ to

2,6-AMN

^

28.3

51.3

15.9

24.4

24P

43.6

146

17.7

Regeneration of the deactivated zeolite samples was carried out by calcining in air at 550 °C for 3 h. The regenerated samples were tested under similar conditions as the fresh catalyst. Conversion of 2MN with the recovered catalysts was >85 % of that of the fresh catalyst and the selectivity to 2,6-AMN was decreased after regeneration (Table 7). The results suggest that regeneration of the deactivated zeolite P by oxidation in air is practical.

n-Octane Isomerization Isomerization of n-octane in the presence of H2 was investigated over samples of Pd/^L, P^/PN,

Pd/(60%p/Al2O3)mix and Pd/(60%p/Al2O3) containing 0.5 wt%Pd. Catalytic activities as a function of reaction temperature in the range 280-340^C are shown in Figure 6. The activity of Pd/(60%p/Al2O3) and Pd/(60%p/Al2O3)mix were higher than that of and Pd/Pn and Pd/Pt- It is may be not only due to mesopores but also due to higher metal dispersion. Pd/(60%p/Al2O3) showed higher activity than Pd/(60%p/Al2O3)mix. Pd/(60%p/Al2O3) with its large pore volume and nanocrystalline zeolite component may be an attractive isomerization catalyst. As shown in Table 8, however, the crystal size has no significant influence on the yield of isooctanes at 30.8% conversion.

Table 8. Yield of Cg isomers over different catalysts (at 30.8% conversion).

Sample Pdypn Pdypi Pd(60^%|3/A1A) Pd^dO wt%((3/Al203i,i5

Yield of iso-Cg% 9.05 7.8 10.04 7.4

248

t5

O

Temperature/* C

Figure 6. Catalytic activities as a function of reaction temperature.

DISCUSSION

The key problem of synthesis of nanosized zeolite is how to control the size of zeolite crystals. Zeolites follow the usual laws of crystal growth. In batch crystallization, crystal size is a function of the ratio between rate of nucleation and rate of growth. Generally, smaller crystals are observed at higher ratio. Both rates depend on supersaturation in zeolite synthesis [14]. The supersaturation can be controlled by several parameters in the synthesis system. These parameters are classfied into crystallization conditions (temperature, time, stirring. Seeding, Gel aging) and composition-dependent parameters (alkalinity, template concentration, additive). Generally, at lower temperature, a greater number of nuclei may be formed from the same reactant mass, and smaller crystallites are obtained. Due to the Ostwald ripening phenomenon, large crystallites will grow at the expense of smaller crystallites with lengthened crystallization time. Therefore, nanosized zeolite ^ (sample denoted PN) can be prepared by lowering the temperature and decreasing the time of crystallization (Table 1).

The new composites of nanocrystalline zeolite p dispersed in a support were prepared using a two-step sol-gel technique. The two steps refer to the initial prehydrolysis of the matrix precursors, followed by a second base treatment step, which accelerates the condensation leading to gelation. The gelation rate can be controlled by altering the pH and the concentration of the matrix. There are four attractive features of the sol-gel method: 1) The matrix sol is used as a 'glue' for preparation of the composite gels. As the gels form, they incorporate all of the dispersed second phase in their three-dimensional network. 2) The particle size of the zeolite phase can be measured accurately in a simple way, without the necessity to use complicated and expensive instruments, such as cryo-TEM or HR-TEM [11]. 3) After gelation, the composite is easy to separate by centrifugation. Neither repeated washing nor supercritical drying leaches the immobilized particles out of the matrix structure and even particles as small as 5 nm are retained in the gel. 4) Since the zeolite phase is dispersed in the continuous mesoporous network of the matrix, gas- or liquid-phase molecular reactants can rapidly enter and react with the active centers. The sol-gel method improves the catalyst efficiency significantly.

Five zeolite p catalysts were used in acylation of 2MN and four Pd-impregnated catalysts were employed for isomerization of n-octane in the presence of hydrogen. The results show that activity of nanocrystalline PN in the two probe reactions is higher. This is due to the fact that the smaller the crystallite size, the shorter the length of the pores and consequently the faster the diffusion of reactants and products. Unfortunately, the influence of the crystal size on the selectivity of acylation of 2MN cannot be delineated, since the same degree of conversion with different catalysts is difficult to achieve in the batch reactor. The variety of crystal size did not change the selectivity of iso-octane significantly. The P/AI2O3 composite exhibits a higher conversion of n-octane than (P/Al203)niix, because the diffusion limitation decreases within the mesoporous network of the sol-gel composite. In acylation of 2MN, P/AI2O3 composite possesses a little activity, whereas (P/Al203)mix hinders mass transport of reactants to the active center of zeolite and shows no activity.

249

CONCLUSION

Nanocrystalline zeolite PN can be synthesized at low temperature with short crystallization time. A new type of composite material has been prepared by a two-step sol-gel method, which solves the problems of separation and aggregation of nanocrystalline zeolites. The results with two probe reactions show that nanocrystalline zeolite PN exhibits the highest activity in acylation of 2MN, and that the composite material P/AI2O3 is a suitable support for palladium in the isomerization of n-octane in the presence of hydrogen.

ACKNOWLEDGEMENT

Financial support from the State Key Basic Research Projects scheme (2000) is acknowledged.

REFERENCES

1. R.L. Wadlinger, G.T. Kerr, and E.J. Rosinski, US Pat. 3308069, (1964) 2. G. Harvey, G. Binder, R. Prins, Stud. Surf Sci. Catal., (94), 397-403, (1995) 3. A. ChicaandA. Corma, J.Catal.,(187), 167-176,(1999) 4. M.A. Camblor, A. Corma, A. Mifsud, J. Perez-Pariente aand S. Valencia, Stud. Surf Sci., Catal., (105),

341-348,(1997) 5. Verduijn Johannes Petrus, WO 93/08125, (1993) 6. P.R. Hari Prasad Rao and M. Matsukata, J.Chem. Soc, Chem. Commun., 1441-1442, (1996) 7. Tver Schmidt, Glaus Madsen, and Glaus J.H. Jacobsen, Inorg. Ghem., (39), 2279-2283, (2000) 8. Valentin Valtchev, Svetlana Mintova, Micro. Meso. Mater., (43), 41-49, (2001) 9. Limin Huang, Zhengbao Wang, Jinyu Sun, Lei Miao, Quanzhi Li, Yushan Yan, and Dongyuan Zhao,

et al., J.Am.Ghem. Soc, (122), 3530-3531, (2000) 10. Liu Guanhua, He Mingyuan, Shu Xingtian, Zuo Lihua, Gao yang, GN 1154341A, (1997) 11. ] M.V. Landau, D. Tavor, O. Regev, M.L. Kaliya, and M. Herskowitz, Ghem. Mater., (11), 2030-2037,

(1999). 12. Istvdn Pdlinko, Bela Torok, G.K.Surya Prakash, George A.Olah, Appl. Gatal., (174), 147-153, (1998) 13. J.A. Horseley, GHEM TEGH, 45-49, (1997) 14. Francesco Di Renzo, Gatal. Today., (41), 37-40, (1998)