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Catalytic Studies toward Synthesis of 2,6-Dimethylnaphthalene from 1-(p-Tolyl)-2-methylbutane Kari Vahteristo,* Kari-Matti Sahala, and Salme Koskimies† Lappeenranta UniVersity of Technology, Department of Chemical Technology, P. O. Box 20, FIN-53851 Lappeenranta, Finland
In this work experiments have been performed in order to study the synthesis of 2,6-dimethylnaphthalene (DMN) via dehydrocyclizing 1-(p-tolyl)-2-methylbutane over Pt/SiO2, V/Ca/Al2O3, and unimpregnated or potassium-impregnated Cr2O3/Al2O3 catalysts. The obtained liquid product mixture consisted of 1,6-cyclization products (dimethylnaphthalenes) and dealkylated 1,6-cyclization products (2-methylnaphthalene and naphthalene), 1,5-cyclization products (trimethylindanes and -indenes), and fragmentation products (mainly p-xylene and 3-(p-tolyl)-propylenes) and 1-(p-tolyl)-2-methylbutenes. Among 10 possible DMN isomers 1,5-, 1,6-, 2,6-, and 2,7-DMN were identified. Metal catalysts V/Ca/Al2O3 (5 wt % vanadium) and Pt/SiO2 (1 wt % platinum) were slightly more active than unimpregnated commercial Cr2O3/Al2O3. After 1 h on-stream time the conversion of 1-(p-tolyl)-2-methylbutane was 96 mol % when the artificial contact time (1/weight-hourly space velocity (WHSV)) was 5.7 h at 783 K over unimpregnated Cr2O3/Al2O3. Potassium impregnation decreased the conversion into 84 mol % because of the decreased Brunauer-Emmett-Teller surface area (from 97 m2/g unimpregnated to 70 m2/g potassium-impregnated catalyst). 1. Introduction Poly(ethylene-2,6-naphthalenedicarboxylate) (PEN) is an interesting polyester for industrial yarn and film applications, because some of its mechanical and barrier properties are better than those of the commonly used poly(ethyleneterephthalate) (PET). The cost of PEN is strongly dependent upon the price of 2,6-naphthalenedicarboxylic acid (NDC). The most common reactants for producing 2,6-NDC are 2-methylnaphthalene, 2,6diisopropylnaphthalene, and 2,6-dimethylnaphthalene (2,6DMN). According to a patent literature 2,6-DMN can be made via base-catalyzed side chain alkylation of alkyl aromatics followed by dehydrocyclization.1-14 In the synthesis of 2,6-DMN the reactant for a dehydrocyclization reaction leading to 2,6-DMN must be an alkylbenzene which has a side chain longer than three carbon atoms. While producing this reactant, it is very common to use xylenes1-3 as starting materials and alkylate them with olefins or diolefins over alkali metal catalyst. Alkali metals such as sodium, potassium, and rubidium are preferred as catalysts while producing 1-(p-tolyl)-2-methylbutane or -butenes by alkylating p-xylene with butenes or butadiene.6,7 Also toluene4 and 2,4,6octatriene5 have been used as a starting material. When C12-butyl- and butenylbenzenes are heated in the presence of a cyclization-dehydrogenation catalyst, both cyclization and dehydrogenation reactions proceed simultaneously. Depending on the position of the pendant methyl group, the product is 1,5-, 1,6-, 2,6-, or 2,7-dimethylnaphthalene.3,6 1-(pTolyl)-2-methylbutenes contain six isomers depending on the double bond position and geometry including cis and trans isomers.8 Any of these isomers alone or in the form of mixture can be used in the dehydrocyclization reaction. Several patents have been published concerning dehydrocyclization of C12-alkylbenzenes. The reactants most often used are 1-(p-tolyl)-2-methylbutane and -butenes9-12 and 5-(otolyl)pentane and -pentenes.13,14 In these studies 1-(p-tolyl)-2* To whom correspondence should be addressed. Fax: +358 05 6212199 E-mail:
[email protected]. † Present address: VTT, PL 1000. FIN-02044 VTT, Finland.
methylbutane and -butenes were dehydrocyclized in the gas phase using a fixed bed tubular reactor and temperature range of 700-800 K. The catalyst systems typically used were platinum, palladium, or chromia on alumina or silica support. An alkali metal promoter (mostly a potassium salt) was used to control the acidity of the catalyst. Although the patent literature concerning the synthesis of 2,6DMN via dehydrocyclization of alkylbenzenes is broad, there are only few publications about the subject. The reasons for this may be that the needed alkylbenzene reactans are difficult to make, the product mixture is difficult to analyze, and the mechanisms for dehydrocyclization of n-butylbenzene are understood well enough to be used as a model reaction.15-20 In this paper dehydrocyclization of 1-(p-tolyl)-2-methylbutane with different dehydrogenation-hydrogenation catalysts have been studied by means of integral-rate measurements. 2. Experimental Section 2.1. Materials. 1-(p-Tolyl)-2-methylbutane was produced using p-xylene and 1-butene as reactants in alkylation with potassium on graphite.12 After distillation, 1-(p-tolyl)-2-methylbutane purity was over 97 wt %, and the major impurities were 1-(p-tolyl)-3-methylbutane and 1-(p-tolyl)pentane (about 3 wt %). The purities of carrier gases were as follows: nitrogen (AGA), 99.9%; hydrogen (AGA), 99.98%. A commercial Cr2O3/Al2O3 catalyst was impregnated (potassium content of 4.7 wt %) with aqueous K2CO3 solution using freeze-drying after impregnation. Then the catalyst was dried at 100 °C and calcined in air at 500 °C. The Pt/SiO2 catalyst was prepared by impregnating and thermal decomposition with Pt(acac)2. The amount of platinum in the catalyst was 1 wt %. Vanadium catalyst (V/Ca/Al2O3) was prepared by coimpregnating water solutions of ammonium vanadate (NH4VO3), oxalic acid, and calcium nitrate onto γ-aluminum oxide.11 The vanadium content of the catalyst was 5 wt %, and the aluminum oxide was still in the γ-form. 2.2. Apparatus. The experiments were carried out using a stainless steel fixed bed tube reactor (length, 40 cm; inner
10.1021/ie9008887 2010 American Chemical Society Published on Web 03/30/2010
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Figure 1. Reaction scheme when dehydrocyclizing 1-(p-tolyl)-2-methylbutane.
diameter, 9 mm). The catalyst bed (length, 2-6 cm) was positioned in the middle part of the reactor tube between two silicon carbide layers. Silicon carbide was also used to dilute the catalyst. The reactant was fed into the reactor with an HPLC pump (Waters 590), and the carrier gas was fed into the reactor by means of a mass flow controller (Brooks 5850E). The products obtained were collected in 4 mL glass vials. The temperature in the reactor was controlled in three points by measuring the outer surface temperature of the reactor tube. The temperature in the catalyst bed could be adjusted within (1 °C. The gas flow rate after sampling the liquid products was measured (Furness low-pressure transmitter FCO53) as well. 2.3. Product Analysis. The reaction products were analyzed with GLC (HP 5890 Series II Plus). The liquid reaction products were analyzed with a fused silica capillary column (cross-linked 5% PhMe silicone, HP-5). Conditions during the analysis were as follows: initial temperature, 100 °C; initial time, 3 min; heating rate, 3 °C/min; split-injector temperature, 280 °C; FIDdetector temperature, 300 °C. The split-ratio was 1:50. The peaks of toluene, ethylbenzene, 1-methyl-1-phenylethylene, i-propylbenzene, n-butylbenzene, 4-phenyl-1-butene, tetralin, naphthalene, and 2,6-DMN were identified by comparison with pure components which were also used for the calculation of the response factors. The response factors of other components were estimated according to their retention times using these calibration components. To analyze 2,6-DMN and 2,7-DMN, a polarized column (biscyanopropyl phenylcyanopropyl polysiloxane; J&W Scientific DB-DXN P/N 122-2461) had to be used, because an unpolarized column (HP-5) was not able to separate them. Some reaction products were hydrogenated with Pd/C in order to identify the unsaturated components. The gaseous reaction products were analyzed with a capillary alumina column (HP-PLOT/Al2O3). Conditions during the analysis were as follows: oven temperature, 100 °C; split-injector temperature, 250 °C; FID-detector temperature, 250 °C. The split ratio was
1:15. The peaks of methane, ethylene, and 1-butene were identified by comparison with pure components. Other peaks were identified using the test report of the column. Few liquid samples were analyzed with GC-MS (JEOL JMS-AX505WA mass spectrometer). 2.4. Procedure. Before each experiment the catalyst was reduced with hydrogen for 1 h at 510 °C. The duration of the experiments varied from 1.5 to 5 h. After each experiment the catalyst was regenerated with air (10 cm3 min-1) at 510 °C and with nitrogen (10 cm3 min-1) for more than 12 h. 3. Results 3.1. Product Mixture. Liquid product mixture consisted of fragmentation products (mainly p-xylene and 3-(p-tolyl)propylenes), 1-(p-tolyl)-2-methylbutenes (olefins), 1,5-cyclization (1,5) products (trimethylindanes and -indenes), 1,6-cyclization (1,6) products (dimethylnaphthalenes), and dealkylated 1,6-cyclization products (2-methylnaphthalene (MN) and naphthalene (N)). Fragmented gaseous products were methane, ethane, ethene, propane, propene, n-butane, trans-2-butene, cis-2-butene, 1-butene, isobutylene, and hydrogen. The reaction products are depicted in Figure 1. All different 1-(p-tolyl)-2-methylbutene isomers seemed to reach their thermodynamical equilibrium because their relative amount did not change much during experiments. These relative amounts were calculated theoretically using Benson group contribution method21 in order to estimate standard enthalpies and entropies of formation, and ideal-gas heat capacities at constant pressure for each 1-(p-tolyl)-2-methylbutenes. Calculated relative amounts were rather near experimental ones (see Table 1). The identification of 1-(p-tolyl)-2-methylbutene isomers is based on these thermodynamical equilibrium calculations only.
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Table 1. Thermodynamical Equilibrium Composition of 1-(p-Tolyl)-2-methylbutenes in Different Experiments Cr2O3/Al2O3
theoretical thermodynamic equilibrium
compound (mol %)
723 K
783 K
Pt/SiO2783 K
723 K
783 K
trans-1-(p-tolyl)-2-methyl-1-butene cis-1-(p-tolyl)-2-methyl-1-butene trans-1-(p-tolyl)-2-methyl-2-butene cis-1-(p-tolyl)-2-methyl-2-butene 3-(p-tolyl)-2-ethyl-1-propylene 4-(p-tolyl)-3-methylbutene
28.5 26.5 17.6 16.7 10.7 ∼0
28.7 25.3 17.5 15.8 12.7 ∼0
26.8 27.7 17.7 16.5 11.3 ∼0
41 28.5 17.1 11.9 1.3 0.2
38.7 28.1 18.2 13.3 1.5 0.2
Among 10 possible dimethylnaphthalene isomers, 1,5-, 1,6-, 2,6-, and 2,7-DMN were identified. Because 2,7-DMN is not the member of the isomerization tendency group22 (1,5-, 1,6-, and 2,6-DMN), it is plausible to assume that one impurity of reactant, 1-(p-tolyl)-3-methylbutane, dehydrogenated into 2,7DMN.3 The amount of 2,7-DMN was about 2 wt %. Main product, 2,6-DMN probably isomerized into 1,5- and 1,6-DMN. It is also possible that impurity of reactant 1-(p-tolyl)pentane has dehydrogenated into 1,5-DMN. The amounts of 1,5- and 1,6-DMN were negligible. 3.2. Catalyst Deactivation. Potassium-impregnated Cr2O3/ Al2O3 catalyst (4.7 wt %) deactivated more rapidly than the unimpregnated one. After 2 h on-stream time the conversion of 1-(p-tolyl)-2-methylbutane decreased from almost 100 mol % into 88 mol % when using unimpregnated Cr2O3/Al2O3 catalyst. When using potassium-impregnated catalyst, the conversion decreased into 71 mol % (see Figure 2).
During 1 h on-stream time both unimpregnated and impregnated Cr2O3/Al2O3 catalysts changed completely. At the beginning both catalysts had high fragmentation activity and the impregnation of potassium even increased this property. After 1 h the selectivity of fragmentation products decreased into 28 mol %, and the selectivity of 1,6-cyclization products increased to 60 mol % when using unimpregnated Cr2O3/Al2O3 catalyst. When using impregnated catalyst, the selectivity of fragmentation products decreased almost from 100 to 73 mol %, and the selectivity of 1,6-cyclization products increased from almost 0 to 19 mol %, respectively (see Figures 3 and 4). After reduction of V/Ca/Al2O3 catalyst using hydrogen (1,6cyclization selectivity was 24 mol %) or carbon monoxide (28 mol %), essential differences were not detected. When the catalyst was not reduced, the selectivity of 1,6-cyclization products was 29 mol %. The fragmentation selectivity decreased from about 80 to 45 mol % during 2 h on-stream time. Even
Figure 2. Deactivation of different catalysts during on-stream time at 783 K.
Figure 3. Effect of deactivation on the selectivity at 783 K over Cr2O3/Al2O3 when WHSV was 0.18 h-1.
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Figure 4. Effect of deactivation on the selectivity at 783 K over potassium impregnated Cr2O3/Al2O3 (K, 4.7 wt %) when WHSV was 0.18 h-1.
Figure 5. 1,6-/1,5-cyclization ratio as a function of conversion at 783 K during deactivation procedure in different experiments.
though Pt/SiO2 deactivated fast after 1 h on-stream time, the selectivities of major compounds were rather constant during deactivation procedure. The selectivities of fragmentation and 1,6-cyclization products were 34 and 17 mol % after 2 h onstream time. During fast deactivation period the 1,6-/1,5-cyclization ratio changed radically when using Cr2O3/Al2O3 or potassiumimpregnated Cr2O3/Al2O3 catalyst at high conversion level. At the conversion level of almost 100 mol %, the 1,6-/1,5cyclization ratio was over 40, but it decreased to 10 at the conversion level under 80 mol % when unimpregnated Cr2O3/ Al2O3 catalyst was used. When Pt/SiO2 and V/Ca/Al2O3 catalysts were used, the ratio was under 5 at the conversion level of 40-90 mol % (see Figure 5). 3.3. Acidity of Cr2O3/Al2O3. Ammonia adsorption measurements show that potassium impregnation decreased the acidity of Cr2O3/Al2O3 catalyst. Unimpregnated Cr2O3/Al2O3 adsorbed ammonia 120 µmol/g, and the adsorption decreased into 22 µmol/g when Cr2O3/Al2O3 was impregnated using potassium (4.7 wt %). The decrease of acidity caused increase in the 1,6-/ 1,5-cyclization ratio at high conversion level, because 1,5cyclization is an acid-catalyzed reaction15 (Figure 5). 3.4. Activity of Catalysts. In these experiments metal catalysts V/Ca/Al2O3 (5 wt % vanadium) and Pt/SiO2 (1 wt % platinum) were slightly more active than unimpregnated commercial Cr2O3/Al2O3. After 1 h on-stream time the conversion of 1-(p-tolyl)-2-methylbutane was 96 mol % when the artificial contact time, 1/WHSV was 5.7 h at 783 K when unimpregnated Cr2O3/Al2O3 was used. Potassium impregnation decreased the conversion into 84 mol %, because the BET surface decreased
from 97 (unimpregnated) to 70 m2/g (potassium-impregnated). Reduction using carbon monoxide or hydrogen decreased the activity of V/Ca/Al2O3 only slightly. The conversion of nonreduced V/Ca/Al2O3 was 69 mol % when 1/WHSV was 1.3 h at 783 K. When using Pt/SiO2 or unimpregnated or potassiumimpregnated Cr2O3/Al2O3, the conversions were 64, 55, or 39 mol %, respectively (see Figure 6). 3.5. Effect of Temperature. According to experiments the highest selectivity to 1,6-cyclization products can be reached with a temperature range between 750 and 800 K when using unimpregnated Cr2O3/Al2O3. A temperature change did not affect much the selectivity of 1,5-cyclization products, but the selectivity of fragmentation products increased from 32 mol % at 783 K into 53 mol % at 833 K. At 723 K the fragmentation selectivity was as high as 36 mol %, which also indicates that the optimum 1,6-cyclization/fragmentation ratio was between 750 and 800 K (see Figure 7). 3.6. Integral-Rate Experiments. Kinetic modeling was performed using a program package which consisted of routines for least-squares minimization and integration of ordinary differential equations.23 Numerical integration of the rate equations was performed by using fifth-order Cash-Karp Runge-Kutta method. Kinetic parameters for the pseudohomogenous model were fitted using the downhill simplex method of Nelder and Mead in order to minimize least-squares. Reactions of 1-(p-tolyl)-2-methylbutane over unimpregnated and potassium-impregnated Cr2O3/Al2O3 and Pt/SiO2 were modeled using a pseudohomogeneous model following the reaction scheme (see Figure 1). Kinetic parameters were fitted using experimental data determined for 1-(p-tolyl)-2-methylbu-
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Figure 6. Conversions of performed experiments as a function of 1/WHSV after 1 h on-stream time when dilution N2:1-(p-tolyl)-2-methylbutane was 2 at 783 K (Pt/Silica H2:1-(p-tolyl)-2-methylbutane, 3.3). Table 2. Apparent Reaction Rate Constants and Orders (Equation 1) for Different Reaction Types (Figure 1) at 783 K over Different Catalysts when Dehydrocyclizing 1-(p-Tolyl)-2-methylbutane Pt/SiO2
Cr2O3/Al2O3 Cr2O3/Al2O3 (K, 4.7 wt %)
Figure 7. Effect of temperature on the selectivity over acidic Cr2O3/Al2O3 when temperature (K)/on-stream time (min)/conversion (mol %) was 723/ 60/55.2, 783/60/55.1, and 833/30/51.2.
tane, 1-(p-tolyl)-2-methylbutenes, fragmentation products, 1,5cyclization, 1,6-cyclization, and dealkylated 1,6-cyclization product groups after precoking 1 h on-stream time. The amount of hydrogen and C1-C4 gases were calculated stoichiometrically. 1-(p-Tolyl)-2-methylbutenes have been assumed to fragment 7 times faster than 1-(p-tolyl)-2-methylbutane, and the reaction orders have been taken from initial-rate data obtained using n-butylbenzene as a reactant over Cr2O3/Al2O3.20 Concerning Pt/SiO2 the assumption of the similarities of the apparent orders is not very plausible, because Pt/SiO2 has different adsorption properties than Cr2O3/Al2O3. This assumption has been done because it makes it easier to compare the apparent reaction rate constants and because of the lack of the differentialrate data. The production rate for each group of components rj in Figure 1 was calculated using rj )
dn˙j dn˙j 1 ) ) kj dmC d(1/WHSV) m ˙F
∏ (p /p )
θ R(j,i)
i
(1)
i
Here kj is the apparent reaction rate constant of component j, pi is a partial pressure of component i, pθ is the standard pressure (101.3 kPa), R is the apparent reaction order with respect to component i, n˙j is a molar flow rate of component j, mC is a
fragmentation 0.66 kAF, RAF kEF, REF 4.6 dehydrogenation kAE, RAE 1.3 hydrogenation kEA, REA 2.4 R H2 1,5-cyclization 0.067 kAI, RAI kEI, REI 1.5 1,6-cyclization kED, RED 75 dealkylation of DMNs 0.19 kDDD, RDDD dealkylation 50 kID, RID a
-6
10
mol gcat.
R
ka
R
ka
R
1.1 1.1
1.2 8.5
1.1 1.1
1.2 8.5
1.1 1.1
0.53
0.75
0.53
4.1
0.53
0.8 0.1
9.7
0.8 0.1
0.18 1.1
0.05 0.86
0.18 1.1
ka
reaction type
-1
1.5 1 1.4
80 0.67 67
1.5 1 1.4
28
0.8 0.1
0.017 0.18 3.0 1.1 62 4.6 10
1.5 1 1.4
-1
s .
mass of the catalyst, m ˙ F is a mass flow rate of a reactant, and WHSV is the weight-hourly space velocity. Apparent reaction rate constants and orders are given in Table 2, and experimental and fitted results are depicted in Figures 8 and 9 for potassium impregnated Cr2O3/Al2O3. Potassium impregnation did not affect 1,6-cyclization reaction, because the apparent reaction rate constant changed only slightly after potassium impregnation. The apparent reaction rate constant even increased from 75 × 10-6 (unimpregnated) to 80 × 10-6 mol gcat.-1 s-1 (potassium-impregnated). When using Pt/SiO2, the apparent reaction rate constant of 1,6-cyclization products was 62 × 10-6 mol gcat.-1 s-1. Potassium impregnation even doubled the apparent reaction rate constant of 1-(p-tolyl)2-methylbutane fragmentation products from 0.66 × 10-6 to 1.2 × 10-6 mol gcat.-1 s-1. The corresponding figure for Pt/ SiO2 was 1.2 × 10-6 mol gcat.-1 s-1. 1,5-Cyclization products seemed to react further even though the reaction to 1,6cyclization should be improbable. Therefore it was assumed that 1,5-cyclization products polymerized and formed coke.20 Potassium impregnation decreased the apparent reaction rate constant
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Figure 8. Partial pressure of major components when dehydrocyclizing 1-(p-tolyl)-2-methylbutane over potassium impregnated Cr2O3/Al2O3 (K, 4.7 wt %) at 783 K. Modeling results have been depicted using lines.
Figure 9. Partial pressure of minor components when dehydrocyclizing 1-(p-tolyl)-2-methylbutane over potassium impregnated Cr2O3/Al2O3 (K, 4.7 wt %) at 483 K. Modeling results have been depicted using lines.
of 1,5-cyclization products from 1.5 × 10-6 to 0.86 × 10-6 mol gcat.-1 s-1. 4. Discussion In dehydrocyclizing 1-(p-tolyl)-2-methylbutane the reactions monitored or postulated were as follows: for major reactions, (1) fragmentation of 1-(p-tolyl)-2-methylbutane, (2) fragmentation of 1-(p-tolyl)-2-methylbutenes, (3) dehydrogenation of 1-(ptolyl)-2-methylbutane, (4) double bond transfer of 1-(p-tolyl)2-methylbutenes, (5) hydrogenation of 1-(p-tolyl)-2-methylbutenes, and (6) 1,6-cyclization of 1-(p-tolyl)-2-methylbutenes; for minor reactions, (7) 1,5-cyclization of 1-(p-tolyl)-2-methylbutenes, (8) 1,5-cyclization of 1-(p-tolyl)-2-methylbutane, and (9) methyl group fragmentation of 1,6-cyclization products.
There were two reaction types which could not be analyzed: skeletal isomerization of 1-(p-tolyl)-2-methylbutane and -butenes and further reaction of 1,5-cyclization products. It would be plausible to assume that 1-(p-tolyl)-2-methylbutane and -butenes undergo skeletal isomerization at least with acidic catalyst.18 However this reaction type could not be monitored quantitatively, because after distillation the side chain alkylation product contained already traces of 1-(p-tolyl)-2methylbutane isomers, and these isomers can also react further to dimethylnaphthalene isomers.3 The skeletal isomerization of 1-(p-tolyl)-2-methylbutane and -butenes is one reason for the amount of 2,7-dimethylnaphthalene (about 2 wt %) in the product mixture.
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It could be assumed that 1,5-cyclization products reacted further to 1,6-cyclization products, especially 2,6-DMN, because this kind of mechanism has been suggested when dehydrocyclizing butenylbenzenes over silica-alumina.19 Even though in this case only plausible triindane or -indene isomers would be 2,3,5-triindane or 2,3,5-tri-1-indene as a reactants for 1,6cyclization products. Also the fragmentation and further polymerization of 1,5-cyclization products is one possible explanation.20 Among six possible 1-(p-tolyl)-2-methylbutene isomers,8 five were detected. The double bond transfer reaction was fast enough, and it reached thermodynamical equilibrium, because in all experiments in spite of the conversion level and the catalyst, the relative amounts of different 1-(p-tolyl)-2-methylbutene isomers were about the same. The experimental relative amounts of isomers agree rather nicely with theoretically calculated ones (see Table 1). According to these results the isomer which was not detected was 4-(p-tolyl)-3-methylbutene, which agrees with the experimental thermodynamical equilibrium composition of butenylbenzenes.17 During the precoking period (1 h) product selectivity changed radically when using Cr2O3/Al2O3 or 4.7 wt % potassiumimpregnated Cr2O3/Al2O3 as a catalyst, but it remained rather stable after 1 h on-stream time. At the beginning of the precoking period fragmentation activity was high, and against our expectations, it even increased when potassium-impregnated Cr2O3/Al2O3 was used, because fragmentation is an acidcatalyzed reaction.24 Potassium impregnation also diminished the Brunauer-Emmett-Teller (BET) surface, and according to our experiments fragmentation occurred without catalyst. These two facts probably explain the increased fragmentation activity compared to other reactions while using potassium-impregnated Cr2O3/Al2O3. However the amounts of fragmentation products in the other experiments show the emphasis on the temperaturecatalyzed nature of fragmentation reactions. During the precoking period at high conversion level it was possible to get almost 100 mol % selectivities of fragmentation products while using potassium-impregnated Cr2O3/Al2O3. Some acidity was also left, after potassium impregnation, because the catalyst adsorbed 22 µmol/g ammonia. Hence, one possible explanation for increased fragmentation is that the ratio of weak/ strong acidic sites of Al2O3 increased substantially as a consequence of potassium impregnation. Strong acidic sites can be regarded as the Lewis sites, because it has been claimed that alkali promoters eliminate Lewis acidity by forming superficial chromate which is not acidic.25 Those weak acidic sites (Broensted sites) caused the major part of fragmentation, and they deactivated at the beginning of the precoking period. Not until after that were other reactions possible. According to the modeling results potassium impregnation did not affect the apparent reaction rate constant of 1,6cyclization products, or it even promoted 1,6-cyclization slightly (Table 2). Instead, the apparent reaction rate constant of 1,5cyclization products decreased as a factor of almost 0.5 because of the decrease of acidity as a consequence of potassium impregnation.15 Concerning fragmentation, Cr2O3/Al2O3 and V/Ca/Al2O3 catalysts functioned similarly during the precoking period, because of the acidic Al2O3 support. Because Pt/SiO2 had neutral SiO2 support, the fragmentation activity was lower, and product selectivities remained rather constant during experiments although the conversion was high. The 1,6-/1,5-cyclization ratio remained about the same no matter which of the metal catalysts was used, but it increased slightly when using Cr2O3/Al2O3. In
addition the Pt/SiO2 catalyst seemed to dealkylate 1,6-cyclization products more efficiently than Cr2O3/Al2O3. Acknowledgment TEKES (Finnish Funding Agency for Technology and Innovation, Finland) and Neste Oy are gratefully acknowledged for financial support. Authors also want to thank Markku Laatikainen, Sigmund M. Csicsery, Matti Lindstro¨m, Juha Jakkula, Vesa Niemi, Erkki Halme and OPTATECH CORPORATION for their interest and advices. Nomenclature DMN ) dimethylnaphthalene k ) apparent reaction rate constant, mol s-1 gcat.-1 mC ) mass of the catalyst, g m ˙ F ) mass flow rate of a reactant, g s-1 MN ) 2-methylnaphthalene N ) naphthalene NDC ) naphthalenedicarboxylic acid n˙ ) molar flow rate, mol s-1 olefins ) 1-(p-tolyl)-2-methylbutenes PEN ) poly(ethylene-2,6-naphthalene-dicarboxylate) p ) pressure, kPa r ) reaction rate, mol s-1 gcat.-1 WHSV ) weight hourly space velocity, h-1 R(j,i) ) apparent order with respect to reactant i in reaction j 1,5 ) 1,5-cyclization products 1,6 ) 1,6-cyclization products Subscripts and Superscripts AE ) dehydrogenation of 1-(p-tolyl)-2-methylbutane to -butene AF ) 1-(p-tolyl)-2-methylbutane fragmentation AI ) 1-(p-tolyl)-2-methylbutane 1,5-cyclization DDD ) dealkylation of dimethylnaphthalenes EA ) hydrogenation of 1-(p-tolyl)-2-methylbutenes to -butane ED ) 1-(p-tolyl)-2-methylbutene 1,6-cyclization EF ) 1-(p-tolyl)-2-methylbutene fragmentation EI ) 1-(p-tolyl)-2-methylbutene 1,5-cyclization H2 ) hydrogen ID ) dealkylation of 1,5-cyclization products θ ) standard state (101.3 kPa)
Literature Cited (1) Taniguchi, M.; Matsuoka, H. 2-Methyl-1-tolylbutane (in Japanese). Jpn. Kokai Tokkyo Koho JP 50,093,925, 1975. (2) Abe, T.; Ebata, S.; Machida, H.; Kida, K. Process for production of 2,6-dimethylnaphthalene. Eur. Pat. Appl. EP 362,651, 1990. (3) Shimada, K. Preparing of alkenylbenzenes (in Japanese). Jpn. Kokai Tokkyo Koho JP 52,137,91, 1993. (4) Abe, T.; Uchiyama, S.; Ojima, T.; Kida, K. Process for production of 2,6-dimethylnaphthalene. Eur. Pat. Appl. EP 362,507, 1990. (5) Miyamoto, S.; Taniguchi, K.; Teranishi, K. Naphthalenes (in Japanese). Jpn. Kokai Tokkyo Koho JP 48,048,458, 1973. (6) Mitsui Petrochemical Industries Ltd. Process for preparing dimethylnaphthalenes. GB 1448136, 1973. (7) Kiricsi, I.; Koskimies, S.; Csicsery, S. Preparation of 1-(2-methylbutyl)-4-methylbenzene. PCT Int. Appl. WO 9724305, 1997. (8) Inamasi, K.; Fushimi, N.; Takagawe, M. Process for producing 2,6dimethylnaphthalene by cyclization dehydrogenation of methyl(tolyl)butenes and -butanes using palladium-based catalysts. Eur. Pat. Appl. EP 557 1993, 722. (9) Inamasi, K.; Fushimi, N.; Takagawe, M. Process for production of 2,6-dimethylnaphthalene. Eur. Pat. Appl. EP 546,266, 1993. (10) Takagawa, M.; Yamagishi, K.; Yoshihara, J.; Inamasa, K.; Watabe, K. Alumina-supported lead or indium oxide catalyst for dehydrogenation and cyclization of 2-methyl-1-(4-methylphenyl)butene or -butane. Eur. Pat. Appl. EP 430,714, 1991.
Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010 (11) Jakkula, J.; Niemi, V.; Vahteristo, K.; Sahala, K.-M. Preparation of dimethylnaphthalene by cyclization. PCT Int. Appl. WO 9700012, 1997. (12) Vahteristo, K.; Halme, E.; Koskimies, S.; Csicsery, S. M.; Laatikainen, M.; Niemi, V. Preparation of 2,6-dimethylnaphthalene. PCT Int. Appl. WO 9702225, 1997. (13) Ozawa, S.; Takagawa, M.; Fujimori, T. Process for producing dimethyl naphthalene. Eur. Pat. Appl. EP 554,704, 1993. (14) Sikkenga, D.; Lamb, J.; Zaenger, I.; Williams, G. Preparation of dimethyltetralins. U.S. Patent 5,073,670, 1991. (15) Csicsery, S. Reactions of n-butylbenzene over supported platinum catalysts. J. Catal. 1967, 9, 336. (16) Csicsery, S. The reactions of N-butylbenzene over nonacidic chromia-alumina. J. Catal. 1968, 9, 416. (17) Csicsery, S. Catalytic reactions of phenylbutenes. J. Catal. 1968, 12, 183. (18) Pines, H.; Goetschel, C. Alumina: Catalyst and support XXX. Dehydrogenation and skeletal isomerization of butylbenzenes over chromiaalumina catalysts. J. Catal. 1966, 6, 371. (19) Arnaudov, S.; Dimitrov, C. Catalytic conversion of 1-phenyl-butene3, 1-phenylbutene-2 and 1-phenylbutene-1 on silica-alumina catalyst (in Bulgarian). Annu. UniV. Sofia, Fac. Chim. 1969/1970, 64, 343.
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(20) Laatikainen, M.; Vahteristo, K.; Saukkonen, S.; Lindstro¨m, M. Kinetics of N-butylbenzene dehydrocyclization over chromia-alumina. Ind. Eng. Chem. Res. 1996, 35, 2103. (21) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill:: New York, 1987. (22) Takagawa, M.; Shigematsu, R.; Nagagata, K. Method for isomerizing dimethylnaphthalene. Eur. Pat. Appl. EP 618,180, 1994. (23) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, V. T. Numerical recipes; The Art of Scientific Computing; Cambridge University Press: Cambridge, U.K., 1986. (24) Poole, C. P.; Mac Iver, D. S. The physical-chemical properties of chromia-alumina catalysts. AdV. Catal. 1967, 17, 223. (25) Damon, J. P.; Scokart, P. O. Acid-base properties of alkali promoted chromia-alumina catalysts. Chem. Lett. 1980, 327.
ReceiVed for reView May 30, 2009 ReVised manuscript receiVed March 5, 2010 Accepted March 14, 2010 IE9008887
