Benzoylation of xylenes using zeolitic catalysts

J. Weitkamp, H.G. Karge, H. Pfeifer and W. HBlderich (Eds.) Zeolires and Related Microporous Marerials: &are of rhe A n 1994 Studies in Surface Science and Catalysis, Vol. 84 0 1994 Elsevier Science B.V. All rights reserved.

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Benzoylation of xylenes using zeolitic catalysts R. Fang, H.W.Kouwenhoven and R. Prins

Laboratorium fur Technische Chemie, Eidgenossische Technische Hochschule, 8092 Ziirich, Switzerland The effect of post-synthesis treatments of zeolite Y on the acylation of xylenes by benzoyl chloride has been investigated. The most active catalyst is a highly crystalline material having a meso-pore structure and containing non-framework aluminium species, which shows an even higher initial activity than proton superacids. rn-Xylene is the most active substrate among the three xylene isomers. Polar solvents such as sulfolane and nitrobenzene increase the reaction rate by several times, and non-polar solvents slow down the reaction. 1. INTRODUCTION

The Friedel-Crafts acylation of aromatics by acyl chloride is of considerable technical importance, and is industrially carried out using Lewis acid catalysts such as AlC1,. The use of zeolites which are reusable and can be easily tailored to fit desired reactions could solve some of the problems related to the waste management of the industrial processes. Acylation of aromatics over zeolitic catalysts has been described in the literature [I-41. Y and Beta are among the promising candidates due to their relative large pore size enabling accommodation of aromatic rings. For example, HY and HBeta were found to be effective for the acylation of anisole by phenylacetyl [ 1,2] and phenylpropanyl chlorides [ 11 and by acetic anhydride [2], CeY catalyzed the acylation of toluene with a series of straight-chain carboxylic acids [3] and the acylative cyclization of phenol [4]. The benzoylation of xylenes with benzoyl chloride on conventional Lewis acid catalysts was extensively studied in the literature [5-81. In this work, we investigated the acylation of xylenes with benzoyl chloride using zeolitic catalysts. The post-synthesis treatment of the zeolite and the influence of solvent on this reaction were examined. 2. EXPERIMENTAL 2.1 Modification of Zeolite Y

A series of modified Y zeolites were prepared based on a commercial synthetic NaY (PY43-Na) provided by CU Chemie Uetikon, Switzerland. The NaY was subjected to a triple ion-exchange with 1N NH4N0, at refluxing temperature with a solution to zeolite ratio 10 (mug). The resultant suspension was filtered and washed with distilled water (20 mug). The remaining solid (NH4Y) was partially dried at 120°C for three hours before

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being subjected to a deep-bed calcination under self steaming conditions at 450°C (3 hours), giving 1YL or at 750°C (2 hours) resulting in 1YH. The ion-exchangekalcination procedure was repeated one time and two times with lYL, giving 2YL and 3YL respectively, and also with 1YH yielding 2YH and 3YH respectively. 3YH was further leached with 2N HNO, (10 mllg, 85°C 4 hours), giving 3YHA. The procedures are summarized in Table 1. Table 1: Preparation procedures for vxious modified Y zeolites Sample Code Parent Zeolite

Treatment

1YL 2YL 3YL 1YH 2YH 3YH 3YHA

A

PY43-Na 1YL 2YL PY43-Na 1YH 2YH 3YH

A A

B B B 2N HNO, (85"C, 4 hours)

Treatment A: 3x(lN NH,NO, ion-exchanged, 1 hour at refluxing temperature), dried for 3 hours at 120"C, and deep-bed calcined for 3 hours at 450°C; Treatment B: the same as Treatment A except that the deep-bed calcination was carried out for 2 hours at 750°C. a

All the samples were characterized by A A S (Varian SpectrAA. 10 spectrometer), XRD (Siemens D-500 diffractometer, CuKa radiation), and N2 adsorptioddesorption (ASAP 2000M Micromeritics). For the calculation of unit cell parameter, Si powder was used as an internal standard. Their characteristics are reported in Table 2. Nay, 3YL, 3YH and 3YHA were additionally examined by "Al and "Si MAS NMR (Bruker AMX 400). Table 2: Analytical data of the modified Y zeolites

~

~~~

NaY 1YL 2YL 3YL 1YH 2YH 3YH 3YHA

2.2 2.3 2.5 2.6 2.2 3.2 5.8 28.2

24.64 24.62 24.58 24.55 24.49 24.39 24.35 24.25

790 781 792 154 714 720 734 757

44 47 50 63 70 80 101 146

0.301 0.297 0.300 0.279 0.26 1 0.259 0.257 0.248

0.039 0.043 0.056 0.089 0.095 0.172 0.256 0.342

Measured by AAS. N, adsorption. ' Obtained from de Boer t-plots [9]. * Calculated from Barrett-Joyner-Halenda analysis of desorption branches [lo].

a

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The !NO acidic ion exchangers, Amberlyst 15 and Nafion NR50 were bought from Fluka Chemie AG, Switzerland. Before use, they were subjected to overnight drying at 120°C. 2.2 Catalysis Experiment

Batch experiments were conducted at 130°C under atmospheric pressure. In a typical run, 1.0 g freshly activated zeolite (calcined at 450°C for 3 hours) was added to the reaction mixture (usually 0.10 mole substrate and 0.05 mole acylating agent). Samples were taken periodically and were analysed by GC (HP 589011, cross-linked methyl silicone column), using cool on-column injection. All the chemicals ( A R grade) were obtained from Fluka Chemie AG andor Aldrich, Switzerland and were used without further processing.

3. RESULTS AND DISCUSSION Post-synthesis modification of zeolites offers an opportunity to tailor their acid activity and/or shape selectivity. Table 3 reports the conversions and selectivities of the systematically modified Y samples described in Table 1 for the benzoylation of rn-xylene and shows that the catalytic activity of zeolite Y was strongly dependent on the postsynthesis treatment procedure. 2YH and 3YH were about I0 times as active as 2YL and 3YL, i.e. the high temperature deep-bed calcined Y was about 10 times more active than low temperature deep-bed calcined Y. It is quite interesting to note this, since 3YL has more acid sites than 3YH, as was confirmed by base titration of the two solids. Table 3: The effect of pretreatment procedures for Y on benzoylation of rn-xylene with benzoyl chloride Cumulative Conversiona (%) Catalyst

1YL 2YL 3YL 1YH 2YH 3YH 3YHA

Time (hr)

Selectivityb (moVmo1)

2.0

6.0

2.0

6.0

2 1 1 1 11 11 7

3 2 2 2 16 18 12

95 92 91 89 90 88 92

95 92 91 89 90 89 92

Conversions based on m-xylene consumed. The maximum theoretical conversion is 50%. Selectivity to 2,4-dimethylbenzophenone based on m-xylene, the only other product observed presumably is 2,6-dimethylbenzophenone. Conditions: m-Xylene (O.lmo1); benzoyl chloride (0.0Smol); catalyst ( l g ) ; temperature

a

(130°C).

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n

350

M

300

\

250 a,

24 200 (d 3

a

3 150 I N 0.0

z

I

0.5

I

1.o

Relative Pressure

Figure 1: Isotherms of N2 adsorptioddesorption of Y pretreated by high temperature deepbed calcination. To better understand the difference in activities for benzoylation between the low and high temperature deep-bed calcined samples, we Fist look at the analytical data (Table 2). The decrease in unit cell parameter with calcination temperature and number of calcinations is consistent with the concept of an increasing formation of non-framework aluminium (NFAL) species followed by structural healing. That means that the degree of dealumination is in line with the severity of the treatment. In this case, high temperature deep-bed calcination removed more aluminium from the framework than calcination at low temperature, and zeolites calcined three times contained less lattice aluminium than those calcined twice. The formation of NFAL species is further confirmed by 29Siand 27AlNMR analysis. The spectra are not reported here as they have been well established in the literature [e.g. 11,121. The progressive decrease of micropore volume and increase of meso-pore volume demonstrate that the dealumination process is accompanied by a loss in crystallinity and the generation of mesopority. The nitrogen adsorptioddesorption isotherms plotted in Figure 1 show more pronounced hysteresis for the severely dealuminated samples. Accordingly, the higher activity of 3YH compared to 3YL may be attributed to the formation of meso-pores, to the lower number of acidic sites, or to the NFAL species. In order to clarify this, zeolite 3YH was further leached with 2N HN03 for 4 hours at 85 "C.As a result, both the meso-pore structure of the zeolite was further enhanced as shown by the more pronounced hysteresis (Figure 1) of 3YHA in nitrogen adsorption experiment and a further unit cell shrinkage was observed in the XRD measurement (Table 2). The NFAL species are washed away during the process, indicated

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Table 4: Benzoylation of xylenes with benzoyl chloride and/or benzoic anhydride over various solid acids Relative Rate

Initial Reaction Ratea (moYg/min x106)

Catalyst

p-Xyleneb

3YH Amberlyst Nafion 3YHA

100 33 79

o-Xyleneb

61 48 56

m-Xyleneb

209 146' 106 95' 194 213' 76'

m-Xylene"

66

47'

175 69

222' 54'

m:pb

m.:ob

2.1 3.2 2.5

3.4 2.2 3.5

Average reaction rate based on xylene consumed in the first 30 minutes-on-stream. Benzoylating agent: benzoyl chloride. Benzoylating agent: benzoic anhydride. reaction rate based on m-xylene consumed in the first 60 minutes-on-stream. Conditions as for Table 3. a

o - Xylene

40 n

+

P-Xylene

Average

-I 40

!R

W

F:

30

130

20

20

10

10

0

.A

Fc

Q)

>

F: 0

u

0

0

2

4

6

2

4

6

2

4

6

Time (hrs) Figure 2: Benzoylation of xylenes with benzoyl chloride over various heterogenous solids. Conditions as for Table 3. Conversions based on xyiene consumed. Keys: 3YH; Amberlyst;

* Nafion

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by the drastic increase of the bulk SUAl ratio in 3YHA compared with 3YH. Nevertheless, it was found that 3YHA is less active than 3YH (Table 3). Accordingly the NFAL species in 3YH must be related to the higher activity of 3YH compared to 3YHA for the benzoylation of m-xylene with benzoyl chloride. On the other hand, when employing benzoic anhydride as benzoylating agent, no appreciable difference in activity was observed over 3YH and 3YHA (Table 4). Using 3YH as a catalyst, the reaction rate using benzoyl chloride as acylating agent was about three times as much as that using benzoic anhydride as benzoylating agent. In contrast, on 3YHA, there is no substantial difference in reaction rate using the two benzoylating agents. The same applies for the super acid Nafion, i.e. the reaction rate is about the same regardless of which acylating agent being used. Therefore the observed superior activity with 3YH must be attributed to the interaction of chloride in the reaction mixture with NFAL species present in the zeolite, generating a kind of strong acid sites. In the 27Al NMR spectra, we did observe three crystallographically different aluminium species, at about 59, 29, and 0 ppm, corresponding to framework tetrahedral aluminium, pentacoordinated non-framework aluminium, and non-framework octahedral aluminium. It has been proposed in the literature [e.g. 13,141 that the aluminium species may include AI(OH),+, Al(OH)’+, Al,O,, polymeric oxides, etc., the exact nature of these NFAL species being far from clear. We have tried y-Alz03as a catalyst for the benzoylation of m-xylene with benzoyl chloride, but no reaction was observed. Therefore, chlorided NFAL species in zeolite differ from chlorided alumina, presumably due to the more disperse nature of NFAL species. The nature of the interaction of the NFAL species and chloride is not clear.

In addition, the benzoylation of the other two xylene isomers was studied, and the results are shown in Table 4. The cumulative conversions in benzoylation of xylenes with benzoyl chloride over 3YH and the two acidic ion-exchangers versus time-on-stream are plotted in Figure 2. Table 4 and Figure 2 clearly show that: (i) In the benzoylation of xylenes with benzoyl chloride, the initial reaction rates of 3YH are higher than those of Amberlyst, and slightly higher than those of Nafion; (ii) In the benzoylation of m-xylene with benzoic anhydride, Nafion is more active than 3YH; (iii) In all cases, the benzoylation rates for the three xylene isomers over the heterogenous solids with benzoyl chloride are of the same order. In this respect, the present catalysts are different from the conventional Lewis acids such as AICl,. For instance, compared to p-xylene, the relative benzoylation rate of mxylene with benzoyl chloride at 25°C using AlCl, as a catalyst is 27.5 in nitrobenzene solution [5], 28.1 in benzoyl chloride solution [6], 16.2 in ethylene dichloride [7]. Compared to o-xylene, the relative rates of benzoylation of m-xylene under the same conditions are 2.9, 3.5, and 2.8 respectively 15-71; (iv) The observed similar patterns in benzoylation rate and selectivity over the three heterogeneous solids indicate that their catalytic properties are similar, and that 3YH behaves as a proton acid; (v) The observed selectivities (to 2,4-dimethylbenzophenone) of these catalysts infer that the reaction is not shape selective over zeolite; (vi) 3YH tends to deactivate more rapidly than Nafion, but at a rate comparable to Amberlyst. Polar solvents are known to enhance the acylation reaction over conventional catalysts [15]. Accordingly we investigated the effect of solvent polarity on the benzoylation of mxylene over zeolitic and protonic catalysts. Results in Table 5 clearly demonstrate that

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polar solvents appreciably enhanced the reaction. For example, with 20 ml sulfolane as solvent, the reaction approached completion within 6 hours, and the activity achieved was even higher than that of the superacid Nafion (cf. Table 6 ) . Similarly, nitrobenzene assisted the reaction, in which case the 3YH functioned almost as well as Nafion. It appears that polar solvents increase the reaction rate by stabilizing ionic species and lowering the rate of deactivation by formation of deposits on the catalyst surface. However, the exact role of the polar solvent in the system is not clear from the present experimental results. It is interesting to note that sulfolane does not induce any solvent promoting effects when superacids are used. Therefore, the actual active sites in 3YH may be similar, but certainly not identical to the proton in Nafion. In the same reaction system, when non-polar solvents are used, the reaction activity is greatly reduced. Table 5: Effect of solvent on benzoylation of m-xylene with benzoyl chloride over zeolite 3YH Cumulative Conversionb (%)

Solvent" Time (hr) Decahydronaphthalene n-Decane Nitrobenzene Sulfolane

Selectivity" (mol/mol)

1.0

6.0

1.o

6.0

3 5 9 17 28

10 18 37 47

5

91 89 89 90 94

90 88 89 91 94

When applicable 20 ml solvent was used; Conversions based on m-xylene consumed. The maximum theoretical conversion is 50%. " Selectivity to 2,4-dimethylbenzophenone based on mxylene, the only other product observed presumably is 2,6-d1methylbenzophenone. Conditions as for Table 3. a

Table 6: Effect of sulfolane on benzoylation of rn-xylene with benzoyl chloride over nonzeolitic solids ~

Cumulative Conversionb (a)

Catalyst Time (hr) Nafion

without sulfolane with sulfolane" Amberlyst without sulfolane with sulfolane"

~~

Selectivity" (mol/mol)

1.0

6.0

1.0

6.0

13 12 7 3

42 40 17 6

93 94 91 93

93 94 91 93

a 20 ml sulfolane was used. Conversions based on rn-xylene consumed. The maximum theoretical conversion is 50%. Selectivity to 2,4-dimethylbenzophenonebased on rn-xylene, the only other product observed presumably is 2,6-dimethylbenzophenone. Conditions as for Table 3.

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4. CONCLUSIONS

This work shows that zeolitic catalysts could be used in Friedel-Craft benzoylation reaction as workable alternatives to conventional catalysts. It has been established that very ultra-stabilized Y (VUSY) is very promising, it is at least comparable to, if not better than, the superacid. The VUSY can be prepared by conventional ion-exchange techniques with continued self-steaming/deep-bed calcination. The activity of the as-prepared VUSY can be enhanced greatly by the use of polar solvents such sulfolane and nitrobenzene, making it comparable in activity and selectivity to Nafion. ACKNOWLEDGMENTS Dr. G. Harvey-Estermann is gratefully acknowledged for her assistance in the NMR and XRD measurements. This work was supported by KWF project no. 1814.1 "Zeolites as Catalysts", in collaboration with CU Chemie Uetikon. REFERENCES 1. A. Corma, M.J. Climent. H. Garcia, and J. Primo, Appl. Catal., 49, 109 (1989). 2. G . Harvey, A. Vogt, H.W. Kouwenhoven, and R. Prins, Proc. 9th Int. Zeolite Con$, R. von Ballmmos, J.B. Higgins and M.M.J Freacy (eds.), Butterworth, Boston, Vol 11, 363 (1992). 3. B. Chiche, A. Finiels, C. Gauthier, P. Geneste, J. Graille, and D. Pioch, J. Org. Chem., 51, 2128 (1986). 4. Y.V. Subba Rao, S.J. Kulkarni, M. Subrahmanyam, and A.V. Rama Rao, J. Chem. SOC., Chem. Commun., 1456 (1993). 5 . H.C. Brown, B.A. Bolto, and F.R. Jensen, J. Org. Chem., 23, 417 (1958). 6. H.C. Brown and F.R. Jensen, J. Am. Chem. SOC., 80, 2296 (1958). 7. H.C. Brown and G. Marino, J. Am. Chem Soc., 81, 3308 (1959). 8. J.P. Morley, J. Chem. Soc., Perkin II, 601 (1977). 9. B.C. Lippens, B.G. Linsen, and J.H. de Boer, J. Cutul., 3, 32 (1964). 10. E.P. Barrett, L.G. Joyner, and P.P. Halenda, J. Am. Chem. SOC., 73, 373 (1951). 11. G. Engelhardt and D. Michel, High-Resolution Solid-state NMR of Silicates and Zeolites, John Wiley & Sons, New York, (1987). 12. G.J. Ray and A. Samoson, Zeolites, 13, 410 (1993). 13. N.P. Rhodes and R. Rudham, J. Chem. SOC., Furuduy Trans., 89(14), 2551 (1993). 14. R. Szostak, Stud. Sur$ Sci. Catul., 58, 153 (1991). 15. G.A. Olah, Friedel-Cruj& Chemistry, Interscience, New York, (1973).