Supercritical Carbon Dioxide as a Reaction Medium for the Zeolite

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Ind. Eng. Chem. Res. 2003, 42, 6294-6302

Supercritical Carbon Dioxide as a Reaction Medium for the Zeolite-Catalyzed Alkylation of Naphthalene Roger Gla1 ser and Jens Weitkamp* Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany

The alkylation of naphthalene with methanol and 2-propanol over the zeolite catalyst LaNaY73 has been conducted in supercritical carbon dioxide as the reaction medium at 250 °C. Catalytic experiments under flow conditions have been performed at high dilution of the reactants (mole fractions < 0.05) and in the pressure range from 1 to 400 bar. Catalyst deactivation is strongly reduced at supercritical conditions as compared to the one in the gas phase. A pressure increase at supercritical conditions results in a higher catalyst activity. The yield ratio of the major reaction products, i.e., mono- and dialkylnaphthalenes, mainly depends on the alcohol/ naphthalene ratio in the feed and is independent of the reaction pressure and the degree of catalyst deactivation. The distribution of isomers within the mono- and dialkylated products changes with time on stream, according to a shift from thermodynamically to kinetically controlled product formation, the extent being dependent on the reaction pressure and the state of the fluid. The results are discussed in terms of the pressure-dependent properties of the supercritical reaction phase. It is proposed that the properties of the supercritical reaction medium mainly affect the processes taking place on the outer surface of the zeolite crystallites rather than those occurring inside the zeolite pores. For a systematic comparison between the experiments in the gas phase and those in the supercritical phase, the influence of the modified residence time, the carrier gas flow rate, the catalyst mass, and the concentration of the reactants in the feed were studied in naphthalene alkylation with methanol at ambient pressure. Adsorption of naphthalene on the zeolite was conducted in the gas phase at 1 bar and at supercritical conditions at 200 bar. Introduction Because of its favorable physical and chemical properties, carbon dioxide is among the supercritical fluids most frequently used as a solvent for extractions and chemical reactions. Supercritical carbon dioxide also offers interesting opportunities as a chemically inert reaction medium for heterogeneously catalyzed conversions. Above all, it is the combination of the tunable (liquidlike) density and the (gaslike) viscosity/diffusivity of supercritical fluids that can be fruitfully exploited in heterogeneous catalysis.1-6 Supercritical fluids have also been successfully applied to heterogeneous catalysis by zeolites.1,2,5,7-10 In many of the zeolite-catalyzed conversions in supercritical fluids reported in the literature, the supercritical reaction medium contained the reactants in high concentrations (y > 0.1) or consisted of the reactant (mixture) itself in the supercritical state. Only a few investigations have been published in which supercritical carbon dioxide was the reaction medium for a conversion catalyzed by an acidic zeolite.2,5,11 For instance, the usefulness of supercritical carbon dioxide has been demonstrated in the alkylation of isobutane with 1-butene over zeolite H-USY. As was initially shown by Clark and Subramaniam12 and recently readdressed by Santana and Akgerman,13 catalyst deactivation could be almost completely suppressed by reducing the critical temperature of the reaction mixture through dilution with an excess of carbon dioxide. As a continuation of previously published work,14,15 we report here on the alkylation of naphthalene with * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +49/711/685-4065.

methanol or 2-propanol over the Brønsted acidic zeolite LaNaY-73 as a catalyst in supercritical carbon dioxide as the reaction medium. Naphthalene alkylation provides important products to the chemical industry, such as 2-methylnaphthalene as an intermediate in the synthesis of the vitamins K1 and K3 or 2,6-dialkylated naphthalene that can be oxidized to 2,6-naphthalenedicarboxylic acid. The latter serves as a building block for liquid-crystalline polymers and heat-resistant polyester fibers.16 In this study, the results from naphthalene alkylation at supercritical conditions are compared to those obtained in the gas phase. A supercritical alcohol/CO2 mixture was used not only as a reaction medium but also as a solvent for solid naphthalene. The low reactant concentrations in the catalytic experiments facilitate the observation of effects on conversion and selectivity of the catalytic reaction due to changes in solvent properties and the state of the reaction medium by pressure variations. This is additionally facilitated because shape selectivity effects are known to be absent in naphthalene conversion with both methanol and 2-propanol over faujasite-type zeolites in the gas phase.17,18 Moreover, acidic faujasite-type catalysts are known to deactivate rapidly in the conversion of naphthalene with either methanol or 2-propanol in the gas phase at ambient pressure.17,18 Therefore, this reaction is also a suitable test for evaluating the potential of supercritical carbon dioxide to extend the catalyst lifetime by the in situ extraction of coke precursors. There is disagreement in the literature as to whether coke precursors formed within the pores of zeolite catalysts can be dissolved by supercritical reaction media. Manos and Hofmann19 conclude that the pore

10.1021/ie000153v CCC: $25.00 © 2003 American Chemical Society Published on Web 04/25/2003

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structure of the faujasite-type zeolite offers enough space for the reaction mixture to dissolve coke precursors formed within the zeolitic pores. A corresponding extraction of coke precursors from the medium-pore zeolite ZSM-5 could, however, not be achieved.20 Similarly, Moser’s group applied in situ cylindrical internal reflectance Fourier transform infrared (CIR-FTIR) spectroscopy to study n-heptane cracking over a faujasitetype zeolite catalyst and found that, at supercritical conditions, a denser phase exists inside the pores than outside the pores, which was made responsible for the extraction of coke.21-23 At variance to these conclusions, Wang and Li24 observed that the deactivation of a ZSM-5 zeolite used as the catalyst in toluene disproportionation could only be slightly reduced at supercritical conditions relative to the one in the gas phase. However, because deactivation could be completely avoided at supercritical conditions with a macroporous γ-Al2O3 catalyst, it was concluded that the micropores of ZSM-5 are too narrow to be accessible for the supercritical reaction phase. More recently, Kuo and Tan25 reported on the alkylation of toluene with propene over zeolite H-ZSM-5 in supercritical carbon dioxide. As opposed to the results of the conversion in the gas phase, the catalyst did not deactivate at supercritical conditions. Likewise, Tiltscher and Hofmann26 found a lower deactivation rate in the supercritical phase than in the gas phase in the alkylation of benzene with propene over acidic zeolite catalysts. These authors interpreted their results in terms of two coking mechanisms: intercrystalline coking, which can be mitigated by extraction with a supercritical fluid, and intracrystalline coking, which cannot be avoided. The results presented in this paper are meant to contribute to this issue, and they will be discussed with respect to processes occurring on the outer surface of the zeolite crystallites and on the walls inside the zeolite pores. Experimental Section Zeolite LaNaY-73 (73 mol % of the Na+ ions are replaced by the corresponding amount of La3+ ions) was prepared from NaY (nSi/nAl ) 2.6; Union Carbide, Tarrytown, NY) by a two-step ion exchange with an aqueous solution of La(NO3)3 at 80 °C. The solutions were 1 N in the first exchange step and 0.5 N in the second exchange step. Between the two steps, the zeolite was calcined at 150 °C for 3 days. After its preparation and drying, zeolite LaNaY-73 had a specific surface area of 753 m2/g as determined from the Langmuir isotherm for nitrogen adsorption at 77 K on a micromeritics ASAP 2010 instrument. The crystallite size of the zeolite as determined by scanning electron microscopy was 100 bar), the experiments were performed at constant reaction temperature, nMeOH/nNp ratio, and constant modified residence time W/FNp. To keep the mole fraction of naphthalene in the reactant mixture yNp constant as well, either the flow rate of the carrier gas or the catalyst mass would have had to be adapted to match the range of parameters that can be handled without significant loss of experimental accuracy in the flow-type apparatus used in this study (cf. Figure 3, bottom part). Conducting the experiments at the same flow rate of the carrier gas as that in the high-pressure experiments would have required an unacceptably low catalyst mass (W < 5 mg), and using the same mass of catalyst as that in the high-pressure experiments would have afforded a carrier gas flow rate too high for reproducible sampling. Therefore, the effects of the catalyst mass, flow rate, and mole fraction of naphthalene in the feed mixture were determined separately. Starting from the reaction conditions for the conversion at ambient pressure shown previously for comparison with the one at 200 bar (Figure 3), the flow rate of the carrier gas was varied, resulting necessarily in a variation of the modified residence time (Figure 8). At the low flow rate of 2.9 cm3/min, an adsorption phase similar to the one observed at supercritical conditions is observed (Figure 8, left part). Although the modified residence time at this flow rate is nearly increased by an order of magnitude, neither the conversion nor the yields of the alkylation products reach a higher level than those at the lower modified residence time. Obviously, the increased modified residence time only reduces the rate of catalyst deactivation, which is complete after about 20 h of time on stream. The same results are obtained when the mole fractions of naphthalene

and methanol in the feed are reduced by a factor of 10 and the carrier gas flow rate is increased to again obtain a modified residence time of W/FNp ≈ 2400 g‚h/mol. Apparently, the specific value of the carrier gas flow rate and the mole fractions of the reactants do not affect the results of the conversion, as long as a constant mass flow of naphthalene, i.e., feed rate FNp, is established. Similarly, the same changes of the product mixture with time on stream as shown in Figure 3, bottom part, were observed upon a reduction of both the catalyst mass and the mole fractions of naphthalene and methanol by a factor of 2. This again demonstrates that the naphthalene mole fraction is unimportant in obtaining the naphthalene mass flow rate. Finally, a catalyst mass comparable to that used in the conversion at 200 bar was chosen (W ) 1450 mg) for an experiment at ambient pressure. For maintaining a constant modified residence time of W/FNp ) 280 g‚h/mol, both the carrier gas flow rate and the naphthalene mole fraction in the feed were increased accordingly (V˙ carrier ) 53 cm3/min and yNp ) 4.0%). Still the changes of the product mixture with time on stream were the same as those in the experiment with the lower catalyst mass and the lower reactant concentrations. Because this fourfold increase in the naphthalene concentration in the gas phase does not change the rate of catalyst deactivation (at constant modified residence time and nMeOH/nNp ratio), it is not likely that the rate of coke formation and the rate of formation of the methylation products depend differently on the reactant concentration. Therefore, the increased activity observed in the supercritical phase at 400 bar with respect to the one at 200 bar and the same methanol nMeOH/nNp ratio (Figure 6, upper and bottom right parts) is presumably due to the enhancement of the solvating aptitude of the higher density reaction medium for higher molecular coke precursors rather than to a mere increase of the reactant concentration by a density increase at constant mole fraction. Conclusions Friedel-Crafts alkylations of aromatics with alcohols catalyzed by an acid zeolite can be successfully conducted in supercritical carbon dioxide as a reaction medium. Additionally, the supercritical medium can be applied as a powerful solvent for higher boiling reactants that are not accessible in sufficiently high concentration in a gas-phase process. Although the reaction conditions necessary for appreciable activity of zeolite catalysts may be considerably remote from the critical point, the increased density of the supercritical reaction phase can be successfully exploited to reduce or even avoid catalyst deactivation that might occur rapidly in the gas phase. If, thus, the pore entrances are not blocked by coke deposits, shape-selectivity effects, which represent a key feature in catalysis by zeolites, can be exploited to direct the product distribution. Furthermore, the results of the present study indicate that mainly the processes occurring on the outer surface of the zeolite crystallites are affected by the physical properties of the reaction medium. Therefore, the wellknown principles of shape selectivity that are based on local constraints inside the zeolite pores can be directly applied to reactions at supercritical conditions. Consequently, the use of supercritical fluid reaction media offers the opportunity to consider zeolite-catalyzed conversions for practical application, although rapid deactivation of the catalyst may occur in the gas phase.

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Furthermore, the reaction pressure provides an additional variable to influence selectivity of products with similar physical properties such as mono- or dialkylnaphthalene isomers. This pressure tuning in supercritical reaction media may be especially useful for zeolite-catalyzed conversions where the product distribution is not controlled by shape-selectivity effects. Investigations are actively underway in our laboratory to extend the use of supercritical fluids as reaction media to conversions of other higher molecular weight aromatics and to study the role of mass transfer within the pores of zeolite catalysts relative to the mass transfer from the supercritical fluid to the outer surface of the catalyst particles. Acknowledgment The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft, Max-BuchnerForschungsstiftung, and Fonds der Chemischen Industrie. R.G. thanks the Dr. Leni-Scho¨ninger-Stiftung for a stipend. Nomenclature DM-Np ) dimethylnaphthalene DIP-Np ) diisopropylnaphthalene F ) feed rate of the reactant, mol/h IP-Np ) isopropylnaphthalene IPOH ) 2-propanol MeOH ) methanol M-Np ) methylnaphthalene n ) number of moles Np ) naphthalene W ) mass of the catalyst, g X ) conversion Y ) yield y ) mole fraction

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Resubmitted for review January 17, 2003 Revised manuscript received January 17, 2003 Accepted January 21, 2003 IE000153V