Selectivity improvement in xylene isomerization

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.

2169

S E L E C T I V I T Y I M P R O V E M E N T IN X Y L E N E I S O M E R I Z A T I O N Bauer, F., Bilz, E. and Freyer, A. Leibniz-lnstitut mr Oberflfichenmodifizierung, PermoserstrafSe 15, D-04303 Leipzig, Germany. E-mail: [email protected]

ABSTRACT To passivate the external surface of HZSM-5 crystallites the techniques of pre-coking and liquid phase deposition of tetraethoxysilane (TEOS) have been applied. After modification, both samples yielded higher selectivity in xylene isomerization, i.e., a decrease of undesired disproportionation products toluene and trimethylbenzenes. Compared to the coverage of 4 wt.-% silica, a more significant selectivation effect was achieved with 0.3 wt.-% carbonaceous deposits. 3~p MAS NMR experiments with adsorbed tributylphosphine oxide indicate an inefficient inertization of external surface acidity by silanization performed via water-induced decomposition of TEOS. However, experiments with C-14 labeled toluene confirm that disproportionation products enable a supplementary, intermolecular pathway of xylene isomerization via transmethylation on the medium pore zeolite ZSM-5. Keywords: xylene isomerization, HZSM-5, surface modification, pre-coking, silanization

INTRODUCTION To enhance the selectivity of zeolites in aromatic hydrocarbon processing various modification techniques have been suggested [1]. Due to the application of smaller crystallites and the corresponding increase in external surface area the inactivation of external active sites is crucial during a selectivation process. Chemical vapor/liquid deposition of organosilicon compounds and the pre-coking procedure are the most effective ways to deactivate non-selective acid sites present on the external surface of zeolite crystallites. In addition, both of the modification procedures may also have important secondary effects of narrowing or blocking of pore entrances thus modifying the diffusive properties of zeolites. In xylene isomerization, the modification effect should substantially suppress undesired disproportionation reactions which result in toluene and trimethylbenzenes (TMBs) as well as secondary isomerization steps of the target product p-xylene while maintaining a high catalyst activity. Furthermore, modern xylene isomerization technologies using m-xylene rich feedstocks of C8 aromatic cuts containing about 10 wt.-% ethylbenzene require ethylbenzene conversion in the range of 50-70% to avoid ethylbenzene accumulation during the recycle loop [2-4]. During the pre-coking treatment, e.g., applied in Mobil's Selective Toluene Disproportionation Process (MSTDpSM), the catalyst is typically contacted with the aromatic feedstock under high severity conditions during initial time-on-stream [5,6]. Coke contents exceeding 2 wt.-% on HZSM-5 has been found to be sufficient for enhancement of para-selectivity during toluene disproportionation [7]. However, the predominantly uncontrolled deposition of coke has the drawback of affecting active sites within the channels that, in turn, reduces the activity of the catalyst. Alternatively, pre-coking can be done preferentially on the external surface by using voluminous coke precursors whose molecular sizes are greater than the pore aperture of the zeolite. A more profitable route obtaining external surface coke deposition may be tapped by a post-treatment of conventionally pre-coked samples by hydrogen [8-10]. Such a hydrogen treatment at elevated temperatures initiates partial hydrocracking, rearrangement and/or migration of carbonaceous deposits. As a consequence, intracrystalline alkylaromatic coke may be effectively removed whereas bulkier polyaromatic coke remains on the external surface. As a result, a passivation of surface acid sites is achieved while improving catalyst activity [ 11 ]. In the silanization process, bulky alkoxysilanes such as tetraethoxysilane (TEOS) with a molecular diameter of about 0.96 nm interact with surface OH groups accompanied by the formation of the corresponding alcohol and the deposition of silica (after calcination) on the external surface. Chemical vapor deposition (CVD) in static vacuum systems or in vapor flow systems as well as chemical liquid deposition (CLD) have been reported [12-15]. For gas phase silanization, the effect of the deposition temperature on the

2170 reaction mechanism and the influence of coadsorbed diluents on the uniformity of the silica coverage have been described. For example, adsorption of diluents such as water, methanol, and toluene, yields a more uniform silica covering [16]. Nevertheless, one-step gas phase deposition of TEOS typically results in a incomplete inertization of the external surface acidity. Therefore, a large-scale modification of zeolite catalysts may be cumbersome due to several cycles of silanization and intermittent calcination [14]. Chemical liquid deposition (CLD) has a considerable attraction because of its better adaptability to large scale preparation. In comparison to CVD, however, less attention has been paid to the liquid phase procedure. [15,17]. Liquid phase deposition of TEOS on zeolites, which is the focus of the present work, can be performed in an anhydrous environment or in water, and the choice of solvent greatly affects the resulting silanization coverage. For example, deposition in a hexane solvent resulted in a more complete inertization of the external surface acidity than the use of polar solvents such as ethanol and water [17]. But, water is known to hydrolyze the alkoxy groups of alkoxysilane to reactive silanols which are more vigorous in grafting reactions [ 18]. To date few investigators have studied in detail the effect of water in the silanization process. For CVD at temperatures above 200~ the alcohol released during the grafting of alkoxysilanes on the zeolite surface is dehydrated into an olefin and water. This in turn is proposed to hydrolyze the surface alkoxysilane species thus creating additional Si-OH for further silane attachments [19]. Physisorbed water present in no dried pellets or extrudate forms of zeolite catalysts may particularly affect the liquid phase procedure. With water-induced hydrolysis of TEOS, two aspects have to be considered. In an aqueous environment the alkoxy groups of organosilanes may undergo bulk hydrolysis and rapid condensation, forming voluminous polysiloxanes before depositing onto the zeolite. On the other hand, TEOS which is generally accepted not to enter the channels of HZSM-5 is converted into ethanol and silanols, i.e., the shielding effect of the bulky ethoxy groups is diminished to the size of small Si(OH)4 having a kinetic diameter of about 0.50 nm. As a consequence, a high degree of non-specific silica deposition inside the zeolite channel may be expected and thereby resulting in pore narrowing or blocking. In contrast to previous silanization investigations which operate with low or high amounts of water, we have added the stoichiometric amount of water to obtain a controlled hydrolysis of organosilanes similar to the surface modification of silica nanoparticles [20]. The amount of water is critical since too much water results in self-condensation of TEOS, whereas water deficiency may lead to an incomplete surface modification. Moreover, the use of hydrolyzed TEOS is expected to enhance the deposition rate. EXPERIMENTAL

SECTION

Catalyst modification Because pre-coking under flow conditions normally lead to axial heterogeneous distribution of coke within a fixed-bed reactor, methanol as coke precursor was adsorbed for 12 h at room temperature onto HZSM-5 (Si/AI = 12.5, 100 nm size, obtained from TRICAT, Germany) and on a Pt/HZSM-5 sample (pelletized with 70 wt.-% SiO2 and impregnated with 0.05 wt.-% Pt) prepared by KataLeuna GmbH (Germany). These samples were chosen to demonstrate that, in respect to coke selectivation, the modification treatment reported here is effective regardless of the presence of binder and trace amount of Pt in the catalyst. The incorporation of hydrogenating metal in the catalyst, a common practice for commercial catalysts, is crucial in improving both ethylbenzene conversion and catalyst stability during xylene isomerization. After methanol loading the samples were heated for 2 h at 723 K under static conditions and finally flushed with hydrogen at 773 K for 24 h. The coke content of both samples obtained by thermogravimetric analysis was about 0.3 wt.-%. For chemical liquid deposition of TEOS (equal to 4 wt.-% SiO2), the catalyst samples were suspended in acetone. Silane hydrolysis was accomplished by adding the stoichiometric amount of water acidified by maleic acid. The slurry was refluxed for 1 h. After the withdrawn of acetone, the sample was calcined by heating to 823 K.

Catalyst characterization The parent and modified HZSM-5 samples were characterized by surface area and porosity measurements carried out on a Micromeritics ASAP 2000 automated BET-sorptometer at 77.3 K using nitrogen as the analysis gas, by 29Si MAS NMR (Bruker MSL-500P) and by 31p MAS NMR (Bruker MSL-500P) after adsorbing tributylphosphine oxide. In addition, pre-coked and deactivated samples were studied by

2171 elementary analysis, temperature-programmed oxidation, and 13C CP MAS NMR (Bruker MSL-500P). Detailed descriptions of the related experiments have been described earlier [21 ].

Xylene isomerization Catalytic runs with an industrial, p-xylene depleted xylene feedstock (10 wt.-% ethylbenzene, 10 wt.-% p-xylene, 20 wt.-% o-xylene, 60 wt.-% m-xylene) or with pure o-xylene were carried out over Pt/HZSM-5 using a fixed-bed micro reactor at 523-623 K. Reaction products were analyzed on a Perkin Elmer Auto System XL GC with a 15.2 m x 0.51 mm ID Bentone 34/Didecylphthalate SCOT column (Supelco) using a temperature program of 10 K/min starting from 343 K (retained for 20 min) to 373 K (retained for 40 min).

RESULTS AND DISCUSSION

Characterization of pre-coked and silylated samples Pre-coking and CLD silanization of the HZSM-5 sample resulted in a decrease of <10% not only in the BET surface area but also in the micropore volume compared with the parent zeolite. In detail, BET surface areas of311 m2/g, 288 m2/g, and 284 m2/g and micropore volumes of 0.151 ml/g, 0.136 ml/g, and 0.141 ml/g were obtained for the parent, pre-coked, and silylated sample, respectively. These findings indicate that both modification procedures do not occur exclusively on the external surface of the zeolite crystals, but also influence the internal pores and the acid sites located in the pores. As expected, the deposition of carbonaceous residues cannot be excluded from interior pore structure. However, pre-coking hardly affects the pore size distribution (Fig. 1). Silanization using hydrolyzed TEOS yields a resultant pore size smaller than the parent sample, i.e. water-induced decomposition of alkoxysilanes results in undesired small silicon species that can enter the ZSM-5 channel system and thus alter the pore size. 0.30

i

0.25-

--- u~odifted saxx~ -4- silyla~< 4 wt.-%s io~ -~ precoked,0.3wt.-%ao~

-

_., 0.20-

0.150.100.05. 0.00.

6

8

iO

12

14

pore diameter(A) Figure 1. Horvath-Kawazoe pore size distribution of HZSM-5 before and after modifications. Solid-state 31p MAS NMR of adsorbed tributylphosphine oxide (TBPO) as a probe molecules has been shown to be an excellent technique for characterization of external acid sites in zeolites [22]. TBPO with a molecular diameter of about 0.82 nm is too large to penetrate into the ZSM-5 channels and hence can merely be adsorbed on acid sites located on the external surface of the crystallites. 3ip MAS NMR spectra obtained from the parent and modified samples are shown in Fig. 2. Note, signals at higher chemical shift would represent acid sites with higher acidic strength. Simulations by Gaussian deconvolution method indicate a significant decrease in the amount of strongest acid site (signals at 90 ppm) after pre-coking. Thus, the proportion of strong acid surface sites on the parent zeolite of about 26% is reduced to 3% on the pre-coked sample [23]. On the contrary, the silylated sample reveals only minor changes in the acid site distribution compared to the parent HZSM-5 zeolite, i.e., liquid phase silanization by hydrolyzed TEOS is less effective in inactivating strong acid surface sites.

2172

~'l

b)

| ....

, ....

.

, ....

| ....

37, /n~/~~

.

| ....

, ....

130

, ....

i ....

, ....

110

i ....

| ....

, ....

i ....

, ....

| ..................................

90

70

9

, ....

50

, .....

~

...............................

30

10

chemical shift (ppm) Figure 2.3~p MAS NMR spectra of TBPO adsorbed on parent (a) pre-coked (b) and silylated (c) HZSM-5 samples. (Asterisks indicate spinning sidebands). Effect of surface modification on selectivity

The main economic objective in modification of commercial isomerization catalysts is to reduce the loss of xylenes due to disproportionation reactions of xylenes into toluene and TMBs. The extent of the undesired toluene formation obtained by the parent and modified samples at different residence times is shown in Fig. 3. For both of the modified samples, a substantial reduction in xylene loss has been achieved. Compared with CLD of TEOS depositing 4 wt.-% silica on the catalyst, just a small amount of coke (0.3 wt.-%) has been required to obtain a far better xylene selectivity. This mean, the deposition of coke takes place very selectively even on external acid sites. As indicated by 3~p MAS NMR experiments with adsorbed TBPO, the higher disproportionation activity of the silylated sample may result from inefficient silica deposition on the external crystallite surface. In addition, amounts of TEOS may also be consumed by coating non-acid OH groups on the SiO2 binder. 2.5 -4,- ammdifiocl samph 4 - si].ylat~d,4 wt.-% S iO~ - ~ la'ecoked, 0.3wt.-%~h~

o~,~ 2.0

~

1.5

~l.0 ~3 "~ [].5 .=.~--

~0

i

I

01

012

03

014

0.5

1/WHSV (h) Figure 3. Effect of surface modification and WHSV on toluene yield during xylene isomerization at 673 K on Pt/HZSM-5. Unfortunately, selectivity enhancement in xylene isomerization cannot be separated from ethylbenzene conversion. Fig. 4 clearly shows the opposite effect of contact time on ethylbenzene conversion and on formation of disproportionation products such as TMBs. Whereas the intramolecular 1,2 methyl group shift even takes place at low acid sites the scission of C-C bonds during dealkylation of ethylbenzene demands stronger acid sites which are present on the parent sample. Therefore, a compromise has to be found between high ethylbenzene conversion and low xylene disproportionation. For example, pre-coked samples with

2173 excellent low xylene loss requires longer contact times (or higher temperatures) to obtain the desired degree of ethylbenzene conversion.

12.01

tm.m~d~i~ d ~ m p le dlyht~d, 4 wt.-% SiO~

o~" i0'~"~'--, , I ~-.\. ~,,

~

.

.

.

l:~recok~d,0.3wt.-%coke . . . . .

oo

0

0.I

0.2

0.3

0.4

0.5

1~sv

0.6

0.?

0.8

0.9

1.0

(h)

Figure 4. Effect of surface modification and WHSV on ethylbenzene conversion (closed symbols) and TMBs formation (open symbols) during xylene isomerization at 673 K on Pt/HZSM-5.

1.2

..4- umrodKa~lsa~l~ --.- silylat~d, 4 wb% S iO~

',/

1.0 |

"-" 0.8 e~ [?.6 .,-..i

o 0.4

....=,

0.2

0

i6

~ 36 ~ 5J ethylbenzene conversion (%)

&

7o

Figure 5. Effect of surface modifications on ethylbenzene conversion and toluene formation during xylene isomerization at 673 K on Pt/HZSM-5. To compare the performance of catalysts modified by different techniques the ratio (~) of ethylbenzene conversion vs. xylene loss is an excellent criterion. Previous pilot-scale studies with Pt/HZSM-5 selectivated by the combined pre-coking/hydrogen treatment resulted in ~ values of about 50 at 55% ethylbenzene conversion, whereas samples pre-coked by the conventional (high severity) method led to ~ values of about 25 [11 ]. For comparison, the patent literature reported an average ~ value of 35 at about 50% ethylbenzene conversion on a modified HZSM-5 catalyst comprising 0.1 wt.-% Pt and 2.4 wt.-% Mg [24]. Fig. 5 visualizes the relationship between ethylbenzene conversion and undesired xylene disproportionation on the parent, pre-coked, and silylated sample. Obviously, the pre-coked sample is more effective in ethylbenzene cracking at a low disproportionation rate. For example to obtain a ethylbenzene conversion of about 50%, the parent and silylated samples yield similar high proportions of toluene and TMBs. The nearly identical behavior of the parent and silylated sample may be explained by a comparable distribution of external acidic strength observed by 3~p MAS NMR (see Fig. 2). In other words, the bimolecular disproportionation of xylenes predominantly takes place on the external crystallite surface at strong acid sites which remain accessible after the particular applied silanization technique; and, which are mainly poisoned after pre-coking.

2174

70-

~-

o~d~o xylene

-

tlt~ta X ylen~

"__ _

lO~a xylene

~60-

orthoxylene meta xylene p~a xylene

20.

8

815. 1

0,0

a) 0 0,2

~

0,4 0,6 1/WHSV 0a)

0,8

b)

1,0

10 0,0

0,2

0,4 0,6 1/WHSV 0a)

0,8

1,0

Figure 6. Effect of contact time on m-xylene conversion over parent (a) and pre-coked (b) Pt/HZSM-5 at 673 K.(- data fit with consecutive reaction scheme). Due to the slight decrease of activity after pre-coking and silanization procedures, both modified samples need longer contact times to obtain a particular m-xylene conversion in comparison to the parent sample (Fig. 6). Nevertheless, mean residence times >0.3 h'gcat/g are not required for approaching the equilibrium of xylene isomerization. For comparison, large-scale isomerization plants operate at 2.5-2.8 LHSV which correspond to contact times of 0.26-0.30 h'gcat/g [2]. Assuming an intramolecular 1,2-methyl group shift model for the isomerization of the m-xylene rich industrial feed,

kpm " kxa~

ortho-xylene.,

kxa~ " kpm

meta-xylene ~

para-xylene

the kinetic constants k~ of the consecutive reaction scheme have been estimated by data fitting (Table 1). Table 1. Kinetic constants (+ 95% confidence limits) of consecutive reaction scheme for xylene isomerization at 673 K. kinetic constants [h- 1]

parent

l~x

1.39 +0.54

km

0.62 •

k~ 1S~

4.25 • 9.54 +0.56

k~/(k~+k~)

0.87

Pt/HZSM-5 sample pre-c oke d 0.43 0.21 2.60 5.81

• • • •

silylate d 0.25 0.13 2.67 6.03

0.93

• • • •

0.95

In particular, the ratio of p-xylene formation to m-xylene conversion, i.e., s kmp/(kmo+kmp), can be taken as a measure of para-selectivity. S-values of 0.87, 0.93, and 0.95 for the unmodified, pre-coked, and silylated sample, resp., revealed an increase in para-selectivity due to zeolite modification. However, this enhancement in para-selectivity may originate from lower catalyst activity. For silanization that is even more para-selective than pre-coking, another explanation has to be considered because of the high activity in xylene disproportionation. In accordance with sorption measurements and 31p MAS NMR results of adsorbed TBPO, liquid phase deposition of hydrolyzed TEOS yields a SiO2 coverage on the zeolite surface. But, the aforementioned water-induced CLD technique is assumed to lead effectively to a narrowing of pore size or pore openings and, to a lesser extent, to an inertization of acid sites. =

2175 I00

9O

~ ortho xylene --n-- meta xyter~

80 ~

~~',~o6070 o~.<

__--'~l~ at'-x~ene -

N Jo b40 03

o

30

o 20 I0 .

0,0

i

0,2

.

.

I

.

0,4 IfWHSV

I

0,6

'

I

0,8

10

(h)

Figure 7. Product distribution observed in o-xylene isomerization on PT/HZSM-5 at 673 K. (- data fit with consecutive reaction scheme).

Xylene isomerization via transmethylation reactions As shown in Fig. 6, the isomerization of a m-xylene rich feed on Pt/HZSM-5 can be sufficiently described by the consecutive reaction scheme ortho ~ meta ~ para. However, o-xylene isomerization under similar reaction conditions reveals a significant misfit especially at short contact times between experimental and theoretical data predicted by the consecutive scheme (Fig. 7). The discrepancy in p-xylene profile can be overcome by the assumption of a direct conversion of o-xylene into p-xylene. This ortho ~ p a r a isomerization can be related either to diffusion limitations and/or to an intermolecular pathway via transmethylation reactions. The intermolecular isomerization mechanism requires the presence of methyl group transferring agents such as trimethylbenzenes (TMBs) formed by xylene disproportionation [25, 26]. Via an intermolecular shift of a methyl group TMBs can interact with xylene molecules yielding another xylene isomer: TMBs + xylene ~ x y l e m + TMBs. Thus, p-xylene can be directly formed from o-xylene via 1,2,4-TMB:

Hence, the undesired disproportionation leads to a supplementary formation of the requested p-xylene when transmethylation reactions are much faster than xylene disproportionation. For HFAU and mesoporous MCM-41, the intermolecular isomerization mechanism has been shown to be the predominant one [27, 28]. On zeolite ZSM-5 with smaller pores and channel intersection, the disproportionation/isomerization ratio is claimed to approach zero, i.e., xylene isomerization should only occur through the intramolecular mechanism [26]. Moreover, toluene, the second product of xylene disproportionation, has been disregard in the intermolecular isomerization mechanism although toluene have more free attachment sites at the aromatic ring for methyl group transfer compared with TMBs. To elucidate the role of toluene transmethylation within the isomerization process a small amount of ~4C-toluene (labeled in the ring positions) has been added to the industrial xylene feed. A typical product distribution is shown in Fig. 8a. The radioactivity detection of the HPLC eluents clearly indicates that toluene is converted into benzene, xylenes and TMBs (Fig. 8b). These findings are contrary to the theoretical expectations on exclusive intramolecular xylene isomerization on HZSM-5 [26]. Note, no radioactivity has been observed in ethylbenzene (Table 2). Depending on the catalyst activity after modification (see unconverted content of ethylbenzene), the conversion of 14C-toluene varies between 24 % and 5 % on the parent and pre-coked sample, respectively.

2176

Izimethyl-

toluene ~ ]il

.,=i

=.

0

'

lb

'

m

'

m

I

'

~

'

m

'

|

~0

retentionlime [rain]

Figure 8. Radio-HPLC chromatogram of a C-14 toluene spiked xylene feed after isomerization on Pt/HZSM-5 at 673K and 1 g/(gcat'h). Table 2. Effect of surface modifications on radioactivity distribution of a C-14 toluene spiked xylene feed after isomerization on Pt/HZSM-5 at 673 K and 1 g/(gcat'h).

benzene toluene xylenes Etbenzene TMBs

parent sample mass (~/o) radio act. (%) 7.3 1.0 6.1 76.4 84.0 20.7 0.4 2.2 1.9

silylated sample mass (%) radio act. (~/o) 6.8 1.3 4.7 83.2 85.4 11.0 1.7 1.4 4.5

pre-coked sample mass (%) radio act. (~/o) 4.5 0.3 1.0 94.1 88.8 3.4 5.5 0.2 2.2

Based on data of specific molar radioactivity (not shown), toluene takes part to an extent of about 3 % in intermolecular transmethylation reactions forming xylenes and TMBs. Because toluene as well as TMBs participate in the intermolecular xylene isomerization on zeolite ZSM-5 (see Fig. 10) the proportion of the bimolecular mechanism via toluene/TMBs transmethylation is estimated to be at least 5 %. The pathway to the reaction products labeled in the aromatic ring can be described by methyl group transfer with m-xylene as the main feed component:

Whereas transmethylation reactions forming labeled xylenes only alter the isomeric distribution of xylenes, labeled benzene is the product of disproportionation reactions. Because of the kinetic features of parallel reactions the ratio of ~4C-xylenes v s . ~4C-benzene directly indicates that transmethylation are about 10 times faster than disproportionation (Table 2). The more selective formation of labeled TMBs on the modified samples, having lower acidity, reveals that transmethylation reactions take place even on weaker acid sites in comparison with disproportionation. Moreover, high proportions of labeled TMBs compared with labeled benzene may imply that the rate of transmethylation increases with the number of methyl groups attached to the aromatic ring.

2177 elispropor- F ~ tionafion / ~

intram01 ecular methyl group shif~

~

7

~-~ /

]

(~9s%) ,

Figure 9. Mono- and bimolecular mechanism of xylene isomerization on HZSM-5.

CONCLUSIONS Modifications of HZSM-5 by deposition of silica or coke and their effects on selectivity in xylene isomerization has been studied. A combined pre-coking/hydrogen treatment was found to facilitate selective passivation of external acid sites which are responsible for undesired xylene disproportionation. Liquid phase deposition of hydrolyzed TEOS (initiated by addition of stoichiometric amounts of water) resulted in inefficient inertization of external surface acidity. For the silylated sample, the observed enhancement in para-selectivity is assumed to be the consequence of a narrowing of pore openings. Detailed features of both surface modification techniques have been obtained by combining the results of kinetic studies with that of zeolite characterization, e.g., 31p MAS NMR of adsorbed phosphine oxide probe molecules. Furthermore, experiments with 14C-toluene confirmed that the undesired products of xylene disproportionation (toluene and trimethylbenzenes) lead to a supplementary intermolecular pathway of xylene isomerization on HZSM-5 because transmethylation reactions are faster than xylene disproportionation.

ACKNOWLEDGEMENTS The support of this work by Deutsche Forschungsgemeinschafi, Germany, and National Science Council, Taiwan (ROC), is gratefully acknowledged. We are particularly obligated to Dr. H. John (KataLeuna GmbH) for preparing the Pt/H-ZSM-5 catalyst and for helpful discussions. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Tsai, T.C., Liu, S.B., Wang, I., Appl. Catal. A, 181 (1999), 355-398. John, H.H., Neubauer, H.D., Birke, P., Catalysis Today, 49 (1999), 211-220. Benazzi, E., De Tavernier, S., Beccat, P., Joly, J.F., Chemtech, 24/10 (1994), 13. Sic, S.T., De Vries, A.F., Mesters, C.M.A.M., Boon, A.Q.M., Bottenberg, K., Trautrims, B., Erd61 Erdgas Kohle, 112 (1996), 463-466. Venkat, C., 19th Dewitt Petrochemical Review, 22-24 March 1994, A1-A11. Low, C.D., Lawson, R.J., Kuchar, P.J., Gray, G.L., US Patent No. 5 321 184 (1994) assigned to UOP. Haag, W.O., Olson, D.H., US Patent No. 4 117 026 (1978) assigned to Mobil Oil Corporation. Haag, W.O., Olson, D.H., Rodewald, P.G., US Patent No. 4 508 836 (1985) assigned to Mobil Oil Corporation. Bauer, F., Ernst, H., Geidel, E., Sch6del, R., J. Catal., 164 (1996), 146-151. Pradhan, A.R., Lin, T.S., Chen, W.H., Jong, S.J., Wu, J.F., Chao, K.J., Liu, S.B., J. Catal., 184 (1999), 29-38. Bauer, F., Chen, W.H., Zhao, Q., Freyer, A., Liu, S. B., Micropor. Mesopor. Mater., 47 (2001), 67-77. Kim, J.H., Kunieda, T., Niwa, M., J. Catal., 173 (1998), 433-439. Impens, N.R.E.N., van der Voort, P., Vansant, E.F., Micropor. Mesopor. Mater., 28 (1999), 217-232. O'Connor, C. T., Moller, K. P., Manstein, H., Cattech, 5/3 (2001), 172-182.

2178 15. Zheng, S., Heydenrych, H., ROger, H.P., Jentys, A., Lercher, J.A., Stud. Surf. Sci. Catal., 135 (2001), 214. 16. Bhat, Y.S., Das, J., Halgeri, A.B., J. Appl. Catal., 115 (1994), 257-267. 17. Weber, R., MOiler, K., Unger, M., O'Connor, C., Micropor. Mesopor. Mater., 23 (1998), 179-187. 18. Brand, M., Frings, A., Jenkner, P., Lehnert, R., Metternich, H.J., Monkiewicz, J., Schramm, J., Z. Naturforsch. 54b (1999), 155-164. 19. Kim, J., Ishida, A., Okajima, M., Niwa, M., J. Catal., 161 (1996) 387-392. 20. Bauer, F., Ernst, H., Decker, U., Findeisen, M., Glgsel, H.J., Hartmann, E., Langguth, H., Mehnert, R., Peuker, Ch. Macromol. Chem. Phys., 201 (2000), 2654-2659. 21. Bauer, F., Freyer, A., Stud. Surf. Sci. Catal., 135 (2001), 307. 22. Zhao, Q., Chen, W.H., Huang, S.J., Wu, Y.C., Lee, H.K., Liu, S.B. Stud. Surf. Sci. Catal., 141 (2002), 453-458. 23. Conf., Cape Town, 25-30 April 2004, H-478-A-PP. 24. Chu, Y.F., Smith, F.A., Chester, A.W., US Patent No. 4 482 773 (1984) assigned to Mobil Oil Corporation. 25. Corma, A., Sastre, E., J. Cata|., 129 (1991), 177. 26. Guisnet, M., Gnep, N.S., Morin, S., Micropor. Mesopor. Mater., 35-36 (1998), 47-59. 27. Morin, S., Ayrau|t, P., Gnep, N.S., Guisnet, M., Appl. Catal. A, 166 (1998), 281-292. 28. Morin, S., Ayrault, P., El Mouahid, S., Gnep, N.S., Guisnet, M., Appl. Catal. A, 159 (199), 317-331.