Ind. Eng. Chem. Res. 2006, 45, 3531-3536
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Methylation of 2-Methylnaphthalene with Methanol to 2,6-Dimethylnaphthalene over ZSM-5 Modified by Zr and Si Lijun Jin,† Haoquan Hu,*,† Xuyan Wang,† and Chang Liu‡ State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, Dalian UniVersity of Technology, 129 Street, Dalian 116012, People’s Republic of China, and Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, Dandong Rd., Fushun 113001, People’s Republic of China
ZSM-5 samples modified by isomorphic substitution of Si, Zr, or a combination thereof were characterized by FT-IR and UV spectroscopies, XRD, XRF spectroscopy, and NH3-TPD and used in the methylation of 2-methylnaphthalene (MN) to produce 2,6-dimethylnaphthalene (2,6-DMN). The results indicated that Si or Zr is inserted in the framework of ZSM-5 after modification and Zr is unstable and preferentially substituted by Si in the simultaneous presence of Zr and Al. Siliconization of ZSM-5 mainly improves its stability and is slightly preferable to enhancement of selectivity to DMNs, 2,6-DMN, and β,β-DMNs. A Zr-substituted sample can obviously improve the 2,6-DMN selectivity, yield, and ratio of 2,6- to 2,7-DMN. Co-modified ZSM-5 embodies the advantages of stable activity on Si-modified ZSM-5 and high selectivity on Zr-modified ZSM-5. The preferable catalyst was prepared by incorporation of Zr in silicated ZSM-5. About 52% 2,6DMN and 85% β,β-DMNs selectivity, over a 2.0 ratio of 2,6- to 2,7-DMN, and a 9% 2,6-DMN yield can be obtained. The superior methylation performance can be ascribed to the weakening of the acid strength and change in pore dimensions for different modifications. 1. Introduction ZSM-5 shows higher β-selectivity than large-pore zeolites in the methylation of 2-methylnaphthalene (MN) for shapeselective synthesis of 2,6-dimethylnaphthalene (DMN), as shown in Scheme 1, a preferred monomer for high-performance polymeric materials such as polyethylenenaphthalene (PEN) or liquid crystals.1-3 However, higher isomerization and a lower yield of 2,6-DMN and lower ratio of 2,6- to 2,7-DMN limit its practical utility. To date, the most promising way to improve catalyst performance is to insert other elements, especially transition metals into metallosilicates, because their acid strength can be controlled by changing the metal cation in the framework and they have the same structure as their aluminum analogue.4 It has been shown that the selectivities of 2,6-DMN and β,βDMNs, the ratio of 2,6- to 2,7-DMN, and the yield of 2,6-DMN are obviously enhanced on such modified ZSM-5 materials. Two techniques of isomorphic substitution of transition metals for Al in zeolite are usually used: directly atom-planting in the original synthesis5-7 and posttreatment of the parent catalyst with corresponding fluorides.8-10 Pu et al.5 reported a 2.9% conversion of 2-MN, about 48% 2,6-DMN selectivity, and a ratio of 2,6- to 2,7-DMN of 1.5 on Fe-MFI catalysts prepared by direct hydrothermal synthesis. Similarly, about 13% conversion of 2-MN, 49% selectivity to 2,6-DMN, and 1.7 ratio of 2,6- to 2,7-DMN with about 6% yield of 2,6-DMN were obtained by Komatsu et al.6 Song et al.8,9 investigated the methylation of 2-MN with methanol on modified ZSM-5 with transition metal fluorides by isomorphic substitution for Al in parent catalysts. About 20% conversion of 2-MN, 60% selectivity to 2,6-DMN, 1.8-2.2 ratio of 2,6- to 2,7-DMN, and about 7.5% 2,6-DMN yield were obtained on ZSM-5 modified with FeF3 and NH4HF2. They attributed the higher selectivity to 2,6* To whom correspondence should be addressed. Tel. and fax: +86411-88993966. E-mail:
[email protected]. † Dalian University of Technology. ‡ SINOPEC.
DMN and 2,6-/2,7-DMN ratio to a weaker acid strength and greater percentage of weak acid sites on catalysts. Isomorphic substitution of silicon for aluminum by treatment with ammonium hexafluorosilicate [(NH4)2SiF6] results in the decrease in acid strength. Siliva et al.11 found that the selectivity to the para isomer was improved in the isomerization of m-xylene on Si-incorporated MFI. They attributed this result mainly to selective dealumination of the external surface of the crystallites. Han et al.12 found that the acidity, especially the surface acidity, of ZSM-5 can be selectively removed by (NH4)2SiF6 treatment. Our previous study showed that the methylation performance of microcrystalline ZSM-5 modified by partial isomorphic substitution of Zr or Si for Al in the framework was clearly enhanced:10,13 about 52% 2,6-DMN selectivity and the highest of about 3.0 ratio of 2,6- to 2,7-DMN were obtained on Zrinserted ZSM-5, but a 2-MN conversion of only 10% in the first 2 h and a rapid decrease with time limits its practical use. Si-incorporated ZSM-5 exhibits a constant activity, but its selectivity to 2,6-DMN is only slightly higher than that of the parent ZSM-5. It is expected that better performance catalyst can be obtained by combining the advantages of both Zr- and Si-modified ZSM-5 samples through co-modification of Zr and Si. In this article, submicrometer-sized ZSM-5 was chosen as the parent catalyst and modified with Zr or Si or their combination to prepare new catalysts by an isomorphic substitution method. The resultant catalysts were characterized by FT-IR and UV spectroscopies, XRD, XRF spectroscopy, and NH3-TPD and used to investigate the methylation performance of 2-MN with methanol. 2. Experimental Section 2.1. Catalyst Preparation. Na+-ZSM-5 with a SiO2/Al2O3 of 28 from Catalyst Plant of the Dalian University of Technology was transformed into H-ZSM-5 by quartic NH4+ exchange and subsequently calcined at 500 °C for 4 h in air. Zr- or Siincorporated samples were prepared by isomorphic substitution. Typically, an aqueous solution of (NH4)2ZrF6 or (NH4)2SiF6
10.1021/ie060075x CCC: $33.50 © 2006 American Chemical Society Published on Web 04/11/2006
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Scheme 1. Methylation of 2-Methylnaphthalene with Methanol
with a certain concentration was slowly added to 5 g of ZSM-5 in a 10 wt % slurry of water with rapid agitation in about 2 h at a constant temperature of 80 °C, and the mixture was refluxed for 10 h. The volume ratio of the salt solution to the weight of ZSM-5 was kept at 10:1. Here, the preferred molar concentration at which the prepared catalysts exhibit better performance is 0.056 M for both (NH4)2SiF6 and (NH4)2ZrF6. The corresponding catalysts are denoted as Si/(Al) ZSM-5 and Zr/(Al)ZSM-5, respectively. The mixture was filtered, washed with excess hot deionized water, dried at 120 °C for 2 h, and calcined at 500 °C for 4 h. The Si/(Al)ZSM-5 and Zr/(Al)ZSM-5 samples were further modified with (NH4)2ZrF6 and (NH4)2SiF6, respectively, to get co-modified samples, which are denoted as Zr-Si/(Al)ZSM-5 and Si-Zr/(Al)ZSM-5, respectively. The prepared catalyst samples were shaped with equivalent boehmite binder, dried at 120 °C for 2 h, calcined at 500 °C for 4 h, and then sieved to 20-40 mesh for use. 2.2. Catalyst Characterization. The crystal sizes of the ZSM-5 samples were determined on a JSM-6700F scanning electron microscope (SEM). X-ray diffraction (XRD) patterns of different catalyst samples were measured on a DMAX2400 diffractor equipped with Cu KR radiation (40 kV, 100 mA). Relative crystallinities (CXR) were determined by comparing the integrated XRD intensities for the modified samples with that of the parent ZSM-5, which was considered as 100%. Chemical compositions of the samples were determined by X-ray fluorescence (XRF) spectroscopy on an SRS3400X instrument. The framework IR and UV-visible spectra were recorded on EQUINOX55 FT-IR and UV-550 spectrophotometers, respectively. Nitrogen adsorption measurements were performed using a Quantachrome AUTOSORB-1 adsorption analyzer. The samples were outgassed at 300 °C for 4 h prior to adsorption. The acidic properties were examined by temperature-programmed desorption of preadsorbed NH3 (NH3-TPD). TPD profiles were obtained in a conventional flow system equipped with a thermal conductivity detector (TCD) from 120 to 600 °C at a constant heating rate of 10 °C/min in a 60 mL/min flow of helium. 2.3. Methylation. The methylation of 2-MN with methanol was carried out in a fixed-bed, continuous-flow microreactor of 10-mm i.d. under atmospheric pressure. The catalyst was placed in the reactor, activated at 500 °C for 2 h in situ under a N2 flow for dehydration, and then cooled to 400 °C. The liquid reactant, including 2-MN, methanol, and mesitylene (solvent) in a molar ratio of 1:5:3, was introduced into the reaction system by a metering pump and was preheated before being passed into the reactor in a 10 mL/min N2 flow. The weight hourly space velocity (WHSV) of 2-MN was 0.5 h-1 in all experiments. The products were analyzed by FID-type GC7890F gas chromatogram with a 30-m TCIcaps DMN capillary column.
3. Results and Discussion 3.1. Characterization. A typical SEM image of the ZSM-5 used in the experiment is shown in Figure 1. It can be seen that the crystallite is almost uniform and the particle size is about 100 nm. Figure 2 shows the XRD patterns of the parent ZSM-5 and samples modified with Zr, Si, and combinations thereof, indicating that very similar XRD patterns including all of the major peaks but relative intensities and diffraction angles of 2θ ) 7.90° and 23.06° are obtained. The intensities of the main peaks increase after modification, indicating that treatment with Si or Zr compounds causes a slight increase in crystallinity, as reported in the Table 1. The increase of crystallinity in the order ZSM-5 < Zr/(Al)ZSM-5 < Si-Zr/(Al)ZSM-5 < Zr-Si/(Al)ZSM-5 < Si/(Al)ZSM-5 means that (NH4)2ZrF6 is less effective in Al leaching than (NH4)2SiF6. From the enlarged inset in Figure 2, the interplanar d spacings of Zr/(Al)ZSM-5 shift to higher values upon incorporation of larger zirconium ions, which
Figure 1. SEM image of parent ZSM-5.
Figure 2. XRD profiles of parent and modified ZSM-5 samples.
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3533 Table 1. Physicochemical Characteristics of Parent and Modified ZSM-5
sample
a
unit cell parameters (Å) b
ZSM-5 Si/(Al)ZSM-5 Zr/(Al)ZSM-5 Zr-Si/(Al)ZSM-5 Si-Zr/(Al)ZSM-5
19.86 19.79 19.89 19.84 19.80
20.12 20.07 20.17 20.10 20.10
c
unit cell volume (Å3)
CXR (%)
peak Ia (°C)
peak IIb (°C)
13.40 13.35 13.44 13.38 13.35
5362 5348 5385 5359 5352
100 113 105 111 107
235.3 207.4 219.1 226.3 216.6
406.6 396.3 345.8 338.9 393.0
BET surface area (m2/g) total micropore
pore volume (mL/g) total micropore
384.0 390.4 393.7 398.4 401.2
0.248 0.261 0.275 0.258 0.264
258.8 248.0 263.8 262.2 244.8
0.106 0.102 0.107 0.107 0.101
a Temperature corresponding to the first peak in NH -TPD profile shown in Figure 4. b Temperature corresponding to the second peak in NH -TPD 3 3 profile shown in Figure 4.
Table 2. Chemical Analyses of Different Catalyst Samples by XRF Spectroscopy
a
catalyst
SiO2
composition (wt %) Al2O3
ZrO2
SiO2/Al2O3
ZrO2/(ZrO2 + Al2O3) (%)
Si/uca
Zr/uca
ZSM-5 Si/(Al)ZSM-5 Zr/(Al)ZSM-5 Zr-Si/(Al)ZSM-5 Si-Zr/(Al)ZSM-5
94.21 97.55 93.60 97.90 97.57
5.79 2.45 3.68 1.97 2.37
0 0 2.72 0.13 0.06
28 68 43 86 70
0 0 38.13 5.25 2.07
89.53 93.24 90.53 93.72 93.31
1.28 0.06 0.02
Moles per unit cell.
Figure 3. Framework (a) FT-IR and (b) UV-visible spectra of ZSM-5 and Zr/(Al)ZSM-5.
is consistent with the results for Al-free zirconium silicates with MFI structure obtained through hydrothermal synthesis by Rakshe et al.14 In contrast, the incorporation of Si results in a decrease in interplanar d spacings. From the change in unit cell parameters and unit cell volume, it is found that the Si treatment leads to a decrease, whereas the Zr treatment results in an increase of unit cell parameters and volume, suggesting that the pore dimensions are enlarged after modification with Zr. It is reasonable to consider that the crystal structure of the zeolitic material remains intact after modification and Si or Zi is incorporated into the framework. A comparison of the chemical compositions determined by XRF spectroscopy (Table 2) reveals that the SiO2/Al2O3 ratios in the modified samples have increased to a certain extent and fluorine is not present. For the same exchangeable amount of (NH4)2SiF6, almost the same percentage of SiO2 (about 97.5% compared to 94.2% in the parent ZSM-5) is obtained in Si/ (Al)ZSM-5 or co-modified Zr-Si/(Al)ZSM-5 and Si-Zr/(Al)ZSM-5. After ZSM-5 or Si/(Al)ZSM-5 is zirconated, the percentage of Al2O3 decreases and that of ZrO2 increases while the amount of SiO2 remains almost unchanged, which indicates that Al rather than Si in the framework has been substituted by Zr. In this case, about 38% and 5% of Al2O3 in ZSM-5 and Si/(Al)ZSM-5, respectively, is substituted by ZrO2. However, after siliconization of Zr/(Al)ZSM-5, the decrease in ZrO2, from 2.72% to 0.06%, is much more obvious than the decrease in Al2O3, from 3.68% to 2.37%, suggesting that Zr be unstable and preferentially substituted for Al by Si in the simultaneous presence of Zr and Al. The FT-IR and UV-visible spectra shown in Figure 3 can further confirm the incorporation of Zr into the framework. The
Figure 4. NH3-TPD profiles of different catalysts.
slight decrease from 1101 to 1097 cm-1 of Zr/(Al)ZSM-5 in Figure 3a indicates the incorporation of Zr, which is in accord with the findings of Szostak et al.15 of a nearly linear decrease in the position of the main T-O-T vibrations (νas) at about 1100 cm-1 with increasing Zr content in the MFI framework. Likewise, the UV-visible spectrum of Zr/(Al)ZSM-5 in Figure 3b shows the presence of a characteristic absorption at about 212 nm attributed to charge-transfer transitions involving the Zr(IV) sites.14 A change in acidic properties is usually evaluated by the temperature-programmed desorption of ammonia, and the profiles can be differentiated both in the integral area of the profiles and in the shift of peak temperature. The former corresponds to the amount of acid sites, and the latter indicates the strength of the acid sites. NH3-TPD profiles of different catalyst samples are illustrated in Figure 4, and the temperatures for peaks I and II are listed in Table 1. It can be seen that the
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Figure 5. (a) Conversion of 2-MN, (b) selectivity to DMNs, and (c) isomerization vs TOS on different catalysts.
unmodified ZSM-5 exhibits very strong acid strength and more acidity. When ZSM-5 is modified by (NH4)2ZrF6 or (NH4)2SiF6, the population and acid strength are obviously reduced. Moreover, different treatments lead to marked differences in acidity and acid strength. Siliconization tends to decrease the percentage and strength of weak acid sites; zirconated samples prefer to decrease the amount and strength of strong acidic sites. For the co-modified samples, Si-Zr/(Al)ZSM-5 has a lower weak acid strength and higher strong acid strength than Zr/(Al)ZSM-5, whereas Zr-Si/(Al)ZSM-5 exhibits the opposite trend with respect to Si-Zr/(Al)ZSM-5, i.e., higher weak acid strength and lower strong acid strength than Si/(Al)ZSM-5. The pore structures of different catalyst samples were characterized by N2 adsorption isotherms. The BET surface areas and pore volumes of the catalysts are listed in Table 1. It can be seen that, when parent ZSM-5 was modified by Si, Zr, or both, the BET surface area and pore volume showed a slight increase. This can be attributed to the partial removal of extraframework Al species in the parent ZSM-5 as well as better crystallization, which is in agreement with the relative crystallinity indicated by the XRD spectra. 3.2. Conversion of 2-MN. The methylation of 2-MN with methanol over different catalysts was carried out, and Figure 5a shows the conversions of 2-MN with time on stream (TOS) for different catalysts. As shown, the durability of the parent catalyst is poor, and the conversion of 2-MN decreases quickly despite the high initial activity. When Si or Zr is incorporated into ZSM-5, the initial activity is lowered, but the deactivation of the catalysts is obviously retarded for the Si-inserted samples. The conversion of 2-MN on Si/(Al)ZSM-5 decreases from 27% at the beginning to 23% at 10.25 h, in comparison to the decrease from 33% to 21% on the parent catalyst. However, the conversion on Zr/(Al)ZSM-5 is maintained above 20% for only the first 2.25 h and then decreases similarly to that on parent ZSM-5. In the case of co-modified samples, the results show that the activity on Si-Zr/(Al)ZSM-5 remains constant and about 21% of 2-MN conversion is obtained at 10.25 h. The conversion of 2-MN on Zr-Si/(Al)ZSM-5 is maintained over
18% for 5 h under the same conditions as used for Zr/(Al)ZSM-5. Therefore, it can be concluded that siliconization can retard deactivation of the catalyst whereas the incorporation of Zr can stabilize activity for only a short time. The selectivity to DMNs and isomerization before and after modification is shown in Figure 5b and c, respectively. From Figure 5b, in the first 2.25 h, incorporation of Si or Zr is preferable in improving the selectivity to DMNs. Especially on Zr/(Al)ZSM-5 and Zr-Si/(Al)ZSM-5, the selectivity to DMNs increases significantly, from about 45% to 80% at 2.25 h, and remains at a level of 82% until the end of the experiment. For Si/(Al)ZSM-5, the selectivity to DMNs is slightly higher for the first 2.25 h and then becomes lower than that on the unmodified sample with TOS. The sequence of incorporation of Si and Zr in the catalyst samples has a significant influence on the selectivity to DMNs. Siliconization before Zr incorporation in the framework, i.e., Zr-Si/(Al)ZSM-5, exhibits similar selectivity to DMNs as Zr/(Al)ZSM-5. However, siliconization after Zr incorporation, Si-Zr/(Al)ZSM-5, shows an obviously low DMNs selectivity compared to that of Zr/(Al)ZSM-5. Isomerization of 2-MN to 1-MN and initially produced 2,6DMN to other DMNs is undesirable during the methylation of 2-MN for the synthesis of 2,6-DMN. Hereafter, the selectivity of isomerizaton is expressed by the percentage of 1-MN in the products. In Figure 5c, one can see that a decrease in isomerization can be accomplished by substitution of Al by Si or Zr, and the effect of Zr incorporation is obviously superior to that of Si incorporation. Compared to the parent ZSM-5, lower isomerization selectivity on Si/(Al)ZSM-5 is obtained only during the first 2.25 h. However, isomerization is seriously depressed on Zr/(Al)ZSM-5, from 62.6% to 38.4% during the initial 0.25 h, and it rapidly decreases and remains below 10% after 1 h. Zr-Si/(Al)ZSM-5 also shows low isomerization, similar to that of Zr/(Al)ZSM-5. After siliconization of Zr/(Al)ZSM-5, however, the 1-MN selectivity is obviously enhanced and slowly decreases with TOS, exhibiting a performance similar to that of Si/(Al)ZSM-5. It is thought that this change is related to the weakening of the acid strength on the catalyst.10 Komatsu et al.4 proposed that alkylation is usually catalyzed by weak acid sites. From Figure 4, the strong acid strength is in the order ZSM-5 > Si/(Al)ZSM-5 ≈ Si-Zr/(Al)ZSM-5 >Zr/ (Al)ZSM-5 ≈ Zr-Si/(Al)ZSM-5. The reaction data show that the increase in selectivity to DMNs and the decrease in that to 1-MN are in agreement with the weakening of acid strength. 3.3. β-Selectivity. The selectivity to 2,6-DMN is usually low because of secondary isomerization to other DMN isomers on unmodified ZSM-5.5,6 Figure 6 shows the variation of the 2,6DMN and β,β-DMNs selectivities and the 2,6-/2,7-DMN ratio over different catalysts with TOS. Owing to the short pore channel of the submicrometer ZSM-5 used in the experiments, a low selectivity to 2,6-DMN, about 22%, is obtained. The selectivity to 2,6-DMN on Si/(Al)ZSM-5 is somewhat higher in the initial time and almost the same as that of parent ZSM-5 at 10.25 h. Zr/(Al)ZSM-5 exhibits a high selectivity to 2,6DMN and still remains at a level of 45% at 10.25 h. In the case of ZSM-5 co-modified with Si and Zr, different sequences of siliconization and zirconization result in obviously different effects on 2,6-DMN selectivity. When Zr/(Al)ZSM-5 is silicated, the 2,6-DMN selectivity decreases dramatically from 38% to 27% in the initial reaction period and increases gradually with TOS to about 36% at 10.25 h, whereas when Zr is inserted into Si/(Al)ZSM-5, the 2,6-DMN selectivity obviously increases, from 30% to 52% at a reaction time of 10.25 h, giving a notably higher value than that on individually modified Zr/(Al)ZSM-5.
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Figure 7. Yield of 2,6-DMN vs TOS on different catalysts.
Figure 6. Selectivities to (a) 2,6-DMN and (b) β,β-DMNs and (c) ratio of 2,6- to 2,7-DMN vs TOS on different catalysts.
The comparison reveals that the co-modified Zr-Si/(Al)ZSM-5 catalyst exhibits the best effect on the selectivity to 2,6-DMN. For the selectivity to β,β-DMNs (2,3-, 2,6-, and 2,7-DMN), the same tendency as for 2,6-DMN on modified samples is obtained, as shown in Figure 6b. Si/(Al)ZSM-5 exhibits a slightly higher selectivity than parent ZSM-5 until 4 h of reaction. Zr/(Al)ZSM-5 has an advantage over Si-incorporated samples in β,β-DMNs selectivity, especially on Zr-Si/(Al)ZSM-5, for which >85% β,β-DMNs selectivity is obtained and maintained until the end of the experiment, i.e., 10.25 h, whereas Si-Zr/(Al)ZSM-5 exhibits an obviously lower selectivity to β,βDMNs than Zr/(Al)ZSM-5. The ratio of 2,6-DMN to 2,7-DMN is very important in the purification of 2,6-DMN because a eutectic mixture will be formed at the 2,6-/2,7-DMN ratio of 0.7.16,17 A higher ratio of 2,6- to 2,7-DMN can facilitate subsequent separation. Figure 6c gives the ratios of 2,6- to 2,7-DMN in the methylation of 2-MN with methanol on different catalysts. On the parent ZSM5, the ratio of 2,6-/2,7-DMN is only about 1.0, near the thermodynamic value,5,18 and increases little with TOS. The 2,6-/ 2,7-DMN ratio can be improved when Si or Zr is incorporated individually or in combination into the framework in place of Al. Incorporation of Zr leads to a higher ratio of 2,6- to 2,7DMN than incorporation of Si. For example, no more than a 1.3 ratio of 2,6- to 2,7-DMN is obtained on Si/(Al)ZSM-5; however, on Zr/(Al)ZSM-5, the ratio of 2,6- to 2,7-DMN is above 1.6. ZSM-5 co-modified with Si and Zr shows different effects. Si-Zr/(Al)ZSM-5 gives a lower ratio of 2,6- to 2,7DMN than Zr/(Al)ZSM-5, whereas Zr-Si/(Al)ZSM-5 has a higher 2,6-/2,7-DMN ratio than Si/(Al)ZSM-5. A maximum 2,6-/2,7-DMN ratio of 2.1 is obtained at 10.25 h on Zr-Si/ (Al)ZSM-5, obviously higher than that on Zr/(Al)ZSM-5. It is thought that the differences in selectivity to 2,6-DMN and β,β-DMNs and 2,6-/2,7-DMN ratio on different catalysts are mainly attributed to two aspects: the change in acid properties and the pore dimensions. According to frontier molecular orbital theory,19 in the case of electrophilic substitution reactions such as the alkylation studied here, the electron
density in the highest occupied molecular orbital (HOMO) is a measure of the reactivity at a specific position. For 2-MN, the electron density at C-6 is significantly higher than that at C-7, which indicates a higher reactivity of the C-6 position than the C-7 position.20 Therefore, weaker acid sites prefer to produce 2,6-DMN than 2,7-DMN. Figure 4 and Table 1 indicate that Zr/(Al)ZSM-5 and Zr-Si/(Al)ZSM-5 show weaker acid strengths and lower strong acid amounts than the other catalysts; correspondingly, a higher 2,6-DMN selectivity and 2,6-/2,7DMN ratio are obtained. In addition, because 2,6-DMN is somewhat larger than 2,7-DMN,11,20 2,6-DMN suffers more diffusion resistance when diffusing from the pore channel of ZSM-5. From Table 1, it can be seen that Si incorporation makes the unit cell volume decrease, whereas the incorporation of Zr results in an increase of the unit cell volume from 5362 Å3 in parent the ZSM-5 to 5385 Å3 in Zr/ZSM-5. Such an enlargement of the pore dimensions might reduce the negative “product selectivity” between 2,6-DMN and 2,7-DMN as mentioned above, i.e., the incorporation of Zr in place of Al in the framework makes the pore dimensions of Zr/(Al)ZSM-5 more suitable for 2,6-DMN synthesis than those of the parent material, but siliconization is spatially unprofitable for the production of 2,6-DMN. It is thought that the improvement in selectivity to 2,6-DMN and the ratio of 2,6- to 2,7-DMN on Si-modified catalysts is mainly attributable to the effect of acid strength. In the case of co-modified Zr-Si/(Al)ZSM-5 with the highest 2,6DMN selectivity, it might be a result of the lower acid amounts on the outer surface resulting from the substitution of Si for Al in the process of siliconization,13 in addition to electronic and spatial effects. Figure 7 shows the variation of the 2,6-DMN yield with TOS on different catalysts. As shown, Si/(Al)ZSM-5 is ineffective in improving the 2,6-DMN yield. Individual Zr modification or co-modification with Si and Zr can obviously improve the yield. A 2,6-DMN of about 9% yield is obtained on Zr-Si/ (Al)ZSM-5 at about 3 h, which, to the best of our knowledge, is the highest yield on modified ZSM-5 that has ever been reported. It can also be seen that the 2,6-DMN yield on ZrSi/(Al)ZSM-5 is higher than that on the individually Zr-modified catalyst. 4. Conclusions In summary, the methylation performance of ZSM-5 for 2,6DMN production from 2-MN and methanol can be improved through modification by partially incorporating Si or Zr in the framework by the isomorphic substitution method. Compared to the parent ZSM-5, siliconization of ZSM-5 mainly improves its stability at high levels of conversion of 2-MN and is slightly preferable for the enhancement of the selectivities to DMNs, 2,6-DMN, and β,β-DMNs. Zr-inserted ZSM-5 can obviously improve the selectivity to 2,6-DMN, the ratio of 2,6- to 2,7DMN, and the yield of 2,6-DMN. Co-modified samples embody the advantages of stable activity on Si/(Al)ZSM-5 and high
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selectivity to DMNs and 2,6-DMN, high 2,6-/2,7-DMN ratio, and high 2,6-DMN yield on Zr/(Al)ZSM-5. Zr-Si/(Al)ZSM-5 exhibits the best performance in β-selectivity: about 52% 2,6DMN and 85% β,β-DMNs selectivities, >2.0 ratio of 2,6- to 2,7-DMN, and the highest yield of 2,6-DMN (9.0%) are obtained. Acknowledgment This work was financially supported by the National Natural Science Foundation of China under Contracts 20276011 and 20376012. Literature Cited (1) Anunziata, O. A.; Pierella, L. B. Transalkylation of Naphthalene with Mesitylene over H-ZSM-11 Zeolite. Catal. Lett. 1997, 44, 259. (2) Frankel, D.; Cherniavsky, M.; Ittah, B.; Levy, M. Shape-selective Alkylation of Naphthalene and Methylnaphthalene with Methanol over H-ZSM-5 Zeolite Catalysts. J. Catal. 1986, 101, 273. (3) Inui, T.; Pu, S.; Kugai, J. Selective Neutralization of Acid Sites on the external Surface of HZSM-5 Crystallites by a Mechanochemical Method for Methylation of Methylnaphthalene. Appl. Catal., A 1996, 146, 285296. (4) Komatsu, T.; Kim, J. H.; Yashima, T. MFI-Type Metallosilicates as Useful Tools to Clarify What Determines the Shape Selectivity of ZSM-5 Zeolites. In Shape-SelectiVe Catalysis; Song, C., Garce´s, J. M., Sugi, Y., Eds.; American Chemical Society: Washington, DC, 2000; pp 162-179. (5) Pu, S. B.; Inui, T. Synthesis of 2,6-Dimethylnaphthalene by Methylation of Methylnaphthalene on Various Medium and Large-Pore Zeolite Catalysts. Appl. Catal., A 1996, 146, 305-316. (6) Komatsu, T.; Araki, Y.; Namba, S.; Yashima, T. Selective Formation of 2,6-Dimethlnaphthalene from 2-Methylnaphthalene on ZSM-5 and Metallosilicates with MFI Structure. Stud. Surf. Sci. Catal. 1994, 84, 1821. (7) Xu, H.; Song, C.; Zhao, J.; Wang, L.; Gan, W. Study on Methylation of 2-Methylnaphthalene with Methanol over MFI Type Zeolite Catalyst. Acta Pet. Sin. 1999, 15, 52-55. (8) Lillwita, L. D.; Song, C. Selective Methylation Catalyst, Method of Catalyst Manufacture and Methylation Process. World Patent WO 02,060,581, 2002.
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ReceiVed for reView January 17, 2006 ReVised manuscript receiVed March 18, 2006 Accepted March 18, 2006 IE060075X
