New Process for 2,6-Dimethylnaphthalene Synthesis by Using C10

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 12593−12601

pubs.acs.org/IECR

New Process for 2,6-Dimethylnaphthalene Synthesis by Using C10 Aromatics as Solvent and Transmethylation-Agentia: High-Efficiency and Peculiar Subarea-Catalysis over Shape-Selective ZSM-5/Beta Catalyst Junhui Li,*,†,‡ Qing Gong,† Hua Lian,† Zhonghua Hu,‡ and Zhirong Zhu*,‡ †

School of Chemical Engineering, Xiangtan University, Xiangtan 411105, China Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Siping Road 1239, Shanghai 200092, China

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‡

S Supporting Information *

ABSTRACT: A new methylation (2-methylnaphthalene (2MN) with methanol) process using low-value C10 aromatics (C10A) as solvent and transmethylation-agentia was developed for the lower-cost synthesis of 2,6-dimethylnaphthalene (2,6DMN), by using a ZSM-5/Beta composite catalyst. Especially, there is a peculiar subarea-catalysis over this catalyst due to both the reactant/product shape selectivity and the ingenious acidity design. Concretely, methylation (2-MN with methanol) and transmethylation (C10A with 2-MN) are catalyzed by the weak + medium + strong acid sites inside 12-ring pore and on the external surface of zeolites, while almost no transmethylation (but methylation and second shape-selective isomerization of fatter DMNs to β,β-DMN) occurs inside the smaller 10-ring pore (rejecting larger-molecular C10A) with mainly weak−medium acidity. Here, both the high-efficiency synergy of methylation with transmethylation and the meritorious isomerization incubated a 2-MN conversion of 52.98% and a 2,6-DMN yield of 11.01%. Our work may provide a promising model for the rational design of an excellent catalyst for a complicated reaction system.

1. INTRODUCTION

related to the differences in the molecular diffusion and the porous shape selectivity between large and medium pore.22−24 In order to enhance 2,6-DMN yield, various modifications for zeolites have been attempted. For example, the HMOR with treating-steam and subsequent acid-leaching exhibited increased 2,6-DMN yield and 2,6-/2,7-DMN ratio (2.4) in 2MN methylation with methanol.2 Hierarchical zeolites (HZSM-525−29 or HZSM-1230,31) showed improved 2-MN conversion. The steam-treating, Cu-modified, or hierarchical SAPO-1124,32−34 and the HZSM-5 modified with NH4F and SrO,35,36 Zr,37,38 or hydrothermally treated39 exhibited enhanced selectivity of 2,6-DMN. Especially, Wang et al.40 claimed that a conversion of 28% and a 2,6-DMN yield of 10.5% were obtained over a hollow HZSM-5 prepared through a one-step hydrothermal crystallization method with NaH2PO4 as the nucleation promoter. Zhang and Fang et al.41,42 pointed out that the tailoring of ZSM-5 acidity was more important than the spatial control for distinguishing between 2,6- and 2,7DMN. In addition, Tsutsui et al.43 obtained an enhanced

2,6-Dimethylnaphthalene (2,6-DMN) is considered to be the most desirable precursor for 2,6-naphthalene dicarboxylic acid used as a monomer of poly(ethylene naphthalate) (PEN).1−4 However, it now is mainly produced by the multistep B.PAmoco process which is complex and environmentally unfriendly.2 Many researchers also studied the conversion of DMNs to 2,6-DMN. But it is also complex and contains hydroisomerization, subsequent dehydrogenation, and another consecutive process with transmethylation, isomerization, demethylation, and methylation, for obtaining 2,6-DMN from the 10 DMN isomers.5 By contrast, methylation of 2methylnaphthalene (2-MN) or naphthalene (NA) with methanol over zeolite catalyst is regarded as a desirable process for the synthesis of 2,6-DMN. Various kinds of medium- or large-pore zeolites, such as HZSM-5, HSAPO-11, HZSM-11, HZSM-12, HBeta, HY, HMOR, HMCM-22 etc.,2,6−19 have been investigated. Generally, the large-porous zeolites show relatively high activity but low 2,6-DMN selectivity and 2,6/2,7-DMN ratio.2,13,15,20,21 On the contrary, medium-porous zeolites (especially HZSM-5 or HSAPO-11) exhibit the high selectivity of 2,6-DMN and β,β-DMN but the relatively low conversion of 2-MN.13,16−19,21 These are directly © 2019 American Chemical Society

Received: Revised: Accepted: Published: 12593

March 22, 2019 June 15, 2019 June 24, 2019 June 24, 2019 DOI: 10.1021/acs.iecr.9b01596 Ind. Eng. Chem. Res. 2019, 58, 12593−12601

Article

Industrial & Engineering Chemistry Research

2.2. Catalyst Preparation. The ZSM-5 and Beta zeolites, with the SiO2/Al2O3 molar ratios of 60 and 20 respectively, were purchased from Fuxu Company of Shanghai. First, they were converted to NH4-type by the conventional ion-exchange with aqueous NH4NO3 solution. Subsequently, they were dried in an oven at 393 K and subsequently calcined at 813 K for 1 h with a ramp of 3 K/min to obtain HZSM-5 and HBeta. The HZSM-5 powder was further modified by impregnation in an aqueous La(NO3)3·6H2O solution, dried at 393 K for 6 h, and calcined at 813 K for 1 h. Then, the zeolite powder (8.0 wt % La2O3−HZSM-5 or HBeta) was pressed, crushed, and sorted to get zeolite catalysts (particles of 12−20 mesh, denoted as La2O3−HZSM-5 or HBeta). The chemical liquid silica deposition was carried out by vacuum-impregnation of zeolite catalyst in liquid poly(dimethylsiloxane) (PDMS) dissolved in petroleum ether (20 wt %). After complete volatilization of petroleum ether at room temperature, the PDMS-modified catalyst was calcined at 813 K for 1 h. This procedure was repeated two times to obtain SiO2−La2O3− HZSM-5 or SiO2−HBeta. This procedure was repeated four times to obtain (IV)SiO2−La2O3−HZSM-5 or (IV)SiO2− HBeta. The extruded ZSM-5/Beta composite catalyst (cylinder, 1.5 mm diameter of cross section) was prepared with 63.0 wt % La2O3−HZSM-5, 30.0 wt % HBeta, and 7.0 wt % silica adhesive (silica sol) and subsequently calcined at 813 K for 1 h. The extruded La2O3−HZSM-5 or extruded HBeta was also prepared with 7.0 wt % adhesive (SiO2) and 93.0 wt % zeolite and subsequently calcined at 813 K for 1 h. 2.3. Catalyst Characterization. The crystal morphology of zeolites was observed by scanning electron microscope (SEM, such as S-4800 Hitachi). The crystal structure of samples was studied by X-ray diffraction analysis (XRD, D8ADVANCE) using Cu Kα radiation. The specific surface area and pore volume were measured by N2 adsorption− desorption at 77 K (NOVA 2200e, Quantachrome). NH3-TPD experiment was performed on a conventional setup equipped with a TCD (CHEMBET 3000). The amount of acidic sites was determined according to the temperature and the area of the NH3 desorption peaks.44 Infrared spectroscopy with pyridine adsorption (Py-IR) for sample was obtained respectively at 473 and 673 K (as the ordinary Py-IR spectrum of zeolite), with Pekin-Elmer 2000 FT-IR spectrometer and a self-supported wafer of sample. Here the adsorption of pyridine was carried out at 393 K, after evacuating the sample at 773 K for 3 h. The constraint index (CI) was determined with a fixedbed microreactor and hexane/3-methylpetane of 1/1 as the mixed reactants, 1.0 h−1 WHSV under total conversion 10− 60%, in helium flow at atmosphere pressure. The products were analyzed using online gas chromatography (HP5890) with a 50 m HP-PLOT/Al2O3 “S” capillary column and FID detector, and the constraint index (CI) was calculated according to the reported method.45 The probe reactions, cumene/1,3,5-triisopropylbenzene cracking and toluene disproportionation, were carried out in a fixed-bed reactor at 1.0 h−1 WHSV and atmospheric pressure with a 40 mL/min N2 flow. 2.4. Catalytic Reaction. The prepared catalyst (5.0 g) was packed into a stainless steel tubular fixed-bed reactor with 1.5 cm inner diameter and pretreated in high-purity nitrogen flow at 773 K for 1 h. After the temperature was decreased to 673 K, the reaction materials were pumped into the reaction equipment by a metering pump. The methylation using C10A as solvent and transmethylation-agentia was carried out at 2-

conversion of 2-MN without losing selectivity over HZSM-5 through a new unsteady-state reaction method with adsorption at a low temperature and subsequent flush at somewhat elevated temperature. On the whole, much progress has been achieved for 2-MN (or NA) methylation with methanol especially over HZSM-5 catalysts. However, the 2,6-DMN yield is still not high enough up to now. Moreover, a great deal of trimethylbenzene (not cheap, especially 1,3,5-trimethylbenzene) is commonly used as the solvent for this catalytic reaction, for weakening the conversion of methanol to hydrocarbons (MTH reaction) and making the experiment easy to operate. All these above result in the rather low economical efficiency of this synthetic route. In this work, a new methylation (2-MN with methanol) process using low-value C10 aromatics (C10A) as solvent and transmethylation-agentia was developed for the lower-cost synthesis of 2,6-DMN. Moreover, the used ZSM-5/Beta composite catalyst with reasonable acidity properties and two kinds of nanopores (10- and 12-ring) showed the reactant/ product shape selectivity and the high-efficiency and peculiar subarea-catalysis in this reaction system. As a result, an improved 2-MN conversion and an enhanced 2,6-DMN yield were obtained.

2. EXPERIMENTAL SECTION 2.1. C10 Aromatics. The C10A used in this work was obtained from Sinopec Shanghai Research Institute of Petrochemical Technology. As listed in Table 1, it mainly consisted of methylpropylbenzenes, dimethylethylbenzenes, and tetramethylbenzenes. Table 1. Composition of the C10A Used in This Work components

content (wt %)

C10 1-methylindene naphthalene

0.028 0.054 0.114 0.356 0.088 2.198 2.614 1.387 9.603 2.180 2.949 12.424 0.568 2.130 8.140 8.593 15.890 0.951 2.849 9.232 10.064 1.381 2.945 2.644 0.530 12594

DOI: 10.1021/acs.iecr.9b01596 Ind. Eng. Chem. Res. 2019, 58, 12593−12601

Article


Table 2. Results of the Catalytic Reaction over the La2O3−HZSM-5, the HBeta, or the Composite ZSM-5/Beta Catalysta La2O3−HZSM-5

catalyst raw material

conversion (%) selectivity (%) NA 1-MN ENs DMNs poly-MN DMN distribution (%) 2,6-DMN 2,7-DMN 1,7-DMN 1,6-DMN 1,3-DMN 2,3-DMN β,β-DMN other DMNs 2,6/2,7-DMN 2,6-DMN yield (%)

b

c

2-MN + CH3OH + C10A 22.63

2-MN + CH3OH + 1,3,5-triMB

20.33

2.21 37.31 2.16 53.08 5.24

39.83 25.02 1.05 12.60 12.44 2.89 67.74 6.17 1.59 4.78

HBeta d

2-MN + C10A

b

c

composite ZSM-5/Beta catalyst c

30.64

2-MN + CH3OH + C10A 52.98

32.21

24.12

16.39 13.45 1.62 49.82 18.72

14.63 26.21 1.58 43.67 13.91

3.17 25.26 2.59 57.85 11.13

5.12 24.32 1.81 62.33 6.42

5.84 41.13 1.59 42.01 9.43

22.46 19.56 0.93 20.33 17.76 5.79 44.45 16.53 1.15 3.53

24.67 19.59 0.66 26.36 13.32 4.03 48.29 11.37 1.25 3.30

35.92 26.51 0.92 13.23 11.87 4.52 66.95 7.03 1.35 11.01

34.08 26.53 0.98 13.33 11.92 3.59 64.20 9.57 1.29 6.84

26.23 18.89 0.74 28.14 14.32 2.45 47.57 9.23 1.39 2.66

5.92

2-MN + CH3OH + C10A 54.12


31.51

3.54 30.32 1.78 61.09 3.27

4.71 57.68 1.29 34.15 2.17

13.51 15.98 2.22 45.01 23.28

43.42 26.23 1.01 10.12 8.97 2.55 72.20 7.70 1.65 5.39

35.40 22.54 1.24 14.56 15.73 2.18 60.12 8.35 1.57 0.72

23.37 19.64 0.96 23.02 15.92 6.83 49.84 10.26 1.19 5.69

d

2-MN + C10A

b


d

2-MN + C10A

a Conversion: 2-MN conversion, EN: ethylnaphthalene, Poly-MN: poly(methylnaphthalene); 1,3,5-triMB: 1,3,5-trimethylbenzene. bMethylation using C10A as solvent and transmethylation-agentia. cConventional methylation process using 1,3,5-trimethylbenzene as solvent. d Transmethylation. The adhesive (SiO2) used for the extruded ZSM-5/Beta composite catalyst is inactive to both methylation and transmethylation (as shown in Table S1).

2,6-DMN yield of 2.66% over this composite catalyst. This proves there is also transmethylation between C10A with 2-MN in this new methylation process. That is because there is much richer methyl in the C10A molecules, such as tetramethylbenzenes, dimethylethylbenzenes, and methylpropylbenzenes. In other words, the C10A are not only the solvent here but also the transmethylation-agentia. Significantly, a 2-MN conversion of 52.98% and a 2,6-DMN yield of up to 11.01% were obtained over the composite ZSM-5/Beta catalyst when using the raw material containing both C10A and methanol. 3.2. Physicochemical Properties of the Zeolite Samples. To reveal the catalytic behaviors in the new methylation system on the ZSM-5/Beta composite, the two active components in this catalyst of La2O3−HZSM-5 and HBeta zeolites were further modified by liquid silica deposition respectively to eliminate only their external acidity. Then, the obtained SiO2−zeolites, the parent HZSM-5, the La2O3− HZSM-5, and the HBeta were characterized and also tested in the new methylation process and the transmethylation of 2MN with C10A respectively, together with the conventional methylation (except SiO2−zeolites). Figure 1 shows the SEM images for La2O3−HZSM-5 and HBeta. As can be seen, this modified HZSM-5 has the diamond-shaped crystals with an approximate size of ∼1.0 μm, while HBeta shows the 0.2−0.5 μm spheroidicity particles. The XRD patterns for the prepared samples are displayed in Figure 2. It can be seen that they showed the typical characteristic peaks of ZSM-5 or/and Beta zeolite. The peaks of La2O3- or/ and SiO2-modified zeolites were very similar to their precursors, and no new diffraction peak appeared. This suggests that the structure of molecular sieves was not destroyed and no new phase was produced during liquid silica deposition or/and La2O3 loading.

MN:methanol:C10A = 1:4:4 (mol/mol/mol), 673 K, 1.0 h−1WHSV (2-MN) and atmospheric pressure with a 40 mL/ min N2 flow. The conventional methylation process using 1,3,5-trimethylbenzene as solvent was performed at 2MN:methanol:1,3,5-trimethylbenzene = 1:4:4 (mol/mol/ mol), 673 K, 1.0 h−1 WHSV (2-MN), and atmospheric pressure with a 40 mL/min N2 flow. The transmethylation of 2-MN with C10A was carried out at 2-MN:C10A = 1:4 (mol/ mol), 673 K, 1.0 h−1WHSV (2-MN), and atmospheric pressure with a 40 mL/min N2 flow. The reaction products were analyzed by a HP-5890 gas chromatograph equipped with a FID detector and a 50 m HP-FFAP capillary column.

3. RESULTS AND DISCUSSION 3.1. The New Methylation Process for 2,6-DMN Synthesis by Using C10A as Solvent and Transmethylation-Agentia. In the previous research, the expensive trimethylbenzene was commonly used as the solvent for the 2-MN alkylation with methanol, directly resulting in the high cost for 2,6-DMN synthesis. Here we overcome this problem by demonstrating a new methylation (2-MN with methanol) process over a composite ZSM-5/Beta catalyst extruded with La2O3−HZSM-5, HBeta, and inactive adhesive (SiO2), by using C10A (low-value byproducts from catalytic reforming and cracking) as the solvent. For comparison, the conventional process using 1,3,5-trimethylbenzene as solvent was also investigated over this catalysts. The results are listed in Table 2. As can be seen, the new process with lower cost exhibited both higher conversion of 2-MN and higher 2,6DMN yield than the conventional process, besides some differences in the product distribution. Besides, when using the raw material only consisting of 2-MN and C10A (as shown in Table 2), the 2-MN conversion was also up to 24.12% with a 12595


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Table 3. Specific Surface Areas and Pore Volumes of HZSM5, La2O3−HZSM-5, HBeta, SiO2-Modified Zeolites, and Composite ZSM-5/Beta Catalyst

catalyst HZSM-5 La2O3−HZSM-5 SiO2−La2O3−H ZSM-5 HBeta SiO2−HBeta ZSM-5/Beta composite

specific surface area/m2 g−1

pore volume/mL g−1

total (BET)

mesopore micropore

external intrapore

339.8 321.9 282.6

70.6 64.8 29.7

269.2 257.1 252.9

0.052 0.048 0.025

0.134 0.126 0.124

592.7 552.8 394.8

69.5 32.8 85.1

523.2 520.0 309.7

0.049 0.027 0.104

0.273 0.269 0.157

pores.46,47 Significantly, the external surface area and mesopore volume greatly decreased for SiO2-modified zeolites, while their internal surface area and micropore volume almost kept constant. This indicates that silica deposition occurred only on the external surface of La2O3−HZSM-5 and HBeta.46−48 Because the siliceous products (SiO2 precursors) formed from PDMS decomposition are too large in molecular size to enter the pores and to effect the internal surface of microporous zeolites.46−50Besides, owing to the introduction of adhesive (silicon dioxide), the composite ZSM-5/Beta catalyst showed increased external surface area and mesoporous volume, while its internal surface area and micropore volume remained at the normal value. Cumene can diffuse into the 10- and 12-ring micropores and on the external surface of zeolites, while 1,3,5-triisopropylbenzene stays only on the external surface owing to its too large molecular size.51,52 Thus, the cracking reactions of cumene/ 1,3,5-triisopropylbenzene were used to evaluate the changes of the internal and external surface acidity of SiO2-modified zeolites here. As shown in Figure 3, 1,3,5-triisopropylbenzene cracking almost could not take place over SiO2−La2O3− HZSM-5 and SiO2−HBeta, while the conversion of cumene still remained at a very high level. These suggest that silica deposition selectively eliminated most of the external acid sites but without obvious influence on the internal acidity.

Figure 1. SEM images of La2O3−HZSM-5 (a) and HBeta (b).

Figure 2. XRD patterns of HZSM-5, La2O3−HZSM-5, HBeta, SiO2modified zeolites, and composite ZSM-5/Beta catalyst.

The specific surface areas and pore volumes of the prepared samples are listed in Table 3. As can be seen, compared with the parent HZSM-5, La2O3−HZSM-5 showed decreased external surface, internal surface, mesopore volume, and micropore volume, implying that La2O3 species was loaded not only on the external surface but also into the 10-ring

Figure 3. Results of cracking reactions of cumene/1,3,5-triisopropylbenzene over the prepared samples: 1,3,5-triisopropylB, 1,3,5triisopropylbenzene; cumene/1,3,5-triisopropylbenzene cracking was carried out in a fixed-bed reactor at 673 K, 1.0 h−1 WHSV, and atmospheric pressure with a 40 mL/min N2 flow. Data were obtained at 1.0 h. 12596


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Table 5. Py-IR Results of HZSM-5, La2O3−HZSM-5, HBeta, SiO2-Modified Zeolites, and Composite ZSM-5/Beta Catalysta

Nevertheless, the cracking of both cumene and 1,3,5triisopropylbenzene powerfully occurred over the composite ZSM-5/Beta catalyst due to the normal acidity on its internal and external surfaces. Figure 4, Table 4, and Table 5 display the results of NH3TPD and Py-IR. As can be seen, La2O3-modification resulted

B acid sites

L acid sites

catalyst

473 K

673 K

473 K

673 K

strong/weak

HZSM-5 La2O3−HZSM-5 SiO2−La2O3−HZSM-5 HBeta SiO2−HBeta ZSM-5/Beta composite

6.84 5.33 4.45 7.31 6.65 5.87

2.30 0.91 0.68 2.68 2.23 1.38

1.65 1.31 1.11 1.89 1.68 1.44

0.51 0.24 0.19 0.78 0.59 0.40

0.49 0.21 0.19 0.60 0.51 0.32

a The peaks at around 1540 and 1450 cm−1 of Py-IR spectra are respectively assigned to Bronsted (B) acid sites and Lewis (L) acid sites, according to the reported method.50,53 Here the number of acid sites is a relative value of Brønsted acid sites to Lewis acid sites, estimated by the corresponding calibrated peak area. In addition, the other peaks such as at around 1490 and 1620 cm−1 in the spectra are also closely related to the framework −OH group of zeolites and are assigned to the pyridinium H-bonded with pyridine as well as zeolites.54,55

The constraint index (CI) is an effective method to determine the microporous diffusion constraint (size of pore or pore opening) and the shape selectivity of zeolite, based on the cracking reaction of probe molecules.48,56,57 Thus, the prepared samples in this work were characterized by CI determination. As listed in Table 6, La2O3−HZSM-5, SiO2− La2O3−HZSM-5, or SiO2−HBeta showed a very close CI value to their precursors (compared with the significantly increased CI value for (IV)SiO2-modified zeolites), indicating their similar pore-opening size. Besides, as an industrial process for the highly selective production of p-X (para-xylene), shapeselective toluene disproportionation has been fully understood over modified HZSM-5 catalyst. The high p-X selectivity is due to the following: (i) the narrowed pore openings restrict the meta-xylene (m-X) and orth-xylene (o-X) isomers diffusing out, and (ii) the passivation of external surface ensures that the p-X formed inside channels rarely isomerizes after diffusing out.58 Therefore, the toluene disproportionation (under the nearly same conversion) was used to evaluate the changes of the pore-opening size and the external surface acidity for the SiO2modified zeolites. As can be seen in Table 6, the p-X selectivity (in xylene) was only 23−24%, close to that for a thermodynamic equilibrium distribution, over HZSM-5, La2O3−HZSM-5, or HBeta. Also, the SiO2−La2O3−HZSM-5 and SiO2−HBeta showed a very slightly increased paraselectivity, owing to the elimination of the most of external acidic sites.59 However, up to 98.9% and 40.6% para-selectivity was obtained over the (IV)SiO2−La2O3−HZSM-5 and (IV)SiO2−HBeta respectively, due to both the passivation of external surface and the narrowing of pore openings.46,50,58 The above phenomena indicate that four-cycle silica deposition actually narrowed the pore opening of these two kinds of zeolites, while two-cycle silica deposition hardly did. It is in accordance with the conclusion from CI determination. Therefore, in the next section, the SiO2−La2O3−HZSM-5 and SiO2−HBeta were used for investigation to help to understand the catalytic behavior inside the 10- and 12-ring channels of the composite ZSM-5/Beta catalyst. 3.3. Reactant/Product Shape Selectivity and HighEfficiency Subarea-Catalysis over the Composite ZSM5/Beta Catalyst. The results of the transmethylation of 2-MN

Figure 4. NH3-TPD patterns of HZSM-5, La2O3−HZSM-5, HBeta, SiO2-modified zeolites, and composite ZSM-5/Beta catalyst.

Table 4. Acid Site Amounts Determined from NH3-TPD Experiments desorbed NH3/mmol g−1

catalyst HZSM-5 La2O3− HZSM-5 SiO2−La2O3− HZSM-5 HBeta SiO2−HBeta ZSM-5/Beta composite

393−623 K (weak + medium acid sites)

623−773 K (strong acid sites)

strong/(weak + medium)

0.69 0.65

0.35 0.16

0.51 0.25

0.51

0.14

0.27

0.70 0.64 0.65

0.45 0.38 0.25

0.64 0.59 0.38

in the sharp decrease of the strong-acid density on HZSM-5 but very slight decrease for weak + medium acid sites. This means the basic La2O3 selectively neutralized most of the strong acid sites on both external and internal surface. Besides, HBeta showed much higher acidic density and strength than the La2O3−HZSM-5 with mainly weak and medium acid sites. As a result, the composite ZSM-5/Beta catalyst had stronger acidity than La2O3−HZSM-5, although the former contained a small quantity of adhesive. Here this trend of acid strength is also in accordance with that for the activity of cumene/1,3,5triisopropylbenzene cracking over the four above-mentioned samples (in Figure 3). In addition, although SiO2-modified zeolites also showed decreased acidic density owing to the significant loss of external acid sites, their total acidic density was still at a high level due to the retention of internal acidity. 12597


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Table 6. CI Values and the para-Selectivity (in Toluene Disproportionation) for HZSM-5, La2O3−HZSM-5, HBeta, SiO2Modified Zeolites, and Composite ZSM-5/Beta Catalysta zeolites

HZSM-5

La2O3− HZSM-5

SiO2−La2O3− HZSM-5

(IV)SiO2−La2O3−H ZSM-5

HBeta

SiO2− HBeta

(IV)SiO2− HBeta

ZSM-5/Beta composite

CI value DIS para-selec.

6.6 23.7

6.7 23.8

7.0 25.1

11.9 98.9

2.2 23.3

2.3 23.9

8.5 40.6

4.4 23.7

a

DIS para-selec.: para-xylene selectivity (in xylene) in toluene disproportionation under the nearly same conversion. Toluene disproportionation was carried out in a fixed-bed reactor at 1.0 h−1 WHSV and atmospheric pressure with a 40 mL/min N2 flow. Data were obtained at 0.5 h. The reaction temperatures for HBeta, SiO2−HBeta, (IV)SiO2−HBeta, HZSM-5, La2O3−HZSM-5, ZSM-5/Beta composite, SiO2−La2O3−HZSM-5, and (IV)SiO2−La2O3−HZSM-5 were 653, 673, 693, 660, 673, 673, 693, and 708 K, respectively.

Table 7. Results of the Transmethylation and Methylation Using C10A as Solvent and Transmethylation-Agentia) over SiO2Modified Zeolites, the Methylation over SiO2-ZSM-5/Beta Composite, and the 1,6-Dimethylnaphthalene (1,6-DMN) Isomerization over SiO2−La2O3−HZSM-5a SiO2−La2O3−HZSM-5

catalyst

SiO2−HBeta

SiO2−ZSM-5/Beta composite

reaction

transmethylation

methylation

1,6-DMN isomerization

transmethylation

methylation

methylation

conversion (%) selectivity (%) NA 1-MN EN DMNs poly-MN DMN distribution (%) 2,6-DMN 2,7-DMN 1,7-DMN 1,6-DMN 1,3-DMN 2,3-DMN β,β-DMN other DMNs 2,6/2,7-DMN 2,6-DMN yield (%)

1.32

22.73

31.50

25.93

42.85

39.91

1.8 90.79 0.00 7.38 0.03

2.71 45.32 1.64 47.32 3.01

0.24 0.56 0.00 98.34 0.12

15.82 30.23 1.47 42.50 9.98

12.11 21.32 2.01 51.32 13.24

3.66 33.52 1.97 55.73 5.12

36.87 23.30 0.00 22.88 16.24 0.09 60.26 0.62 1.58 0.04

45.30 28.12 1.07 9.45 8.30 3.92 77.34 3.84 1.61 4.87

54.18 30.98 1.40 7.15 3.07 88.23 3.22 1.74 -

26.12 20.48 0.58 23.32 11.65 5.25 51.85 12.60 1.28 2.88

25.22 21.05 0.88 19.45 14.67 7.56 53.83 11.17 1.20 5.55

42.01 29.10 0.94 10.08 8.43 5.26 76.37 4.18 1.44 9.34

SiO2−ZSM-5/Beta composite: the extruded catalysts prepared with 63.0 wt % SiO2−La2O3−HZSM-5, 30.0 wt % SiO2−HBeta, and 7.0 wt % silica adhesive (silica sol) and subsequently calcined at 813 K for 1 h. Conversion: conversion of 2-MN or conversion of 1,6-DMN; EN: ethylnaphthalene; poly-MN: poly(methylnaphthalene); methylation: 2-MN + methanol + C10A; transmethylation: 2-MN + C10A. The 1,6-DMN isomerization was carried out at 673 K, 1.0 h−1WHSV and atmospheric pressure with a 40 mL/min N2 flow (1,6-DMN:1,3,5-trimethylbenzene = 1:4 (mol/mol)). a

with C10A and the methylation of 2-MN with methanol (by using C10A as solvent and transmethylation-agentia) over the SiO2-modified zeolites are listed in Table 7. As can be seen, the transmethylation hardly occurred over SiO2−La2O3−HZSM-5 with the elimination of most external acid sites. However, the reactivity of 2-MN methylation with methanol (using C10A) was still high over this catalyst, and the conversion of 2-MN and the selectivity of DMNs in products were up to 22.73% and 47.32% respectively. These indicate that the molecules of methanol and 2-MN can diffuse into the 10-ring pores of La2O3−HZSM-5 and alkylate on the weak and medium acid sites there,60 while C10A molecules are too large in size to enter ZSM-5 channels. That is to say the weak assisted transmethylation of C10A with 2-MN over La2O3−HZSM-5 (listed in Table 2) is catalyzed by only the external acid sites, owing to the reactant shape selectivity of 10-ring channel. Moreover, the content of naphthalene in the product for La2O3−HZSM-5 or SiO2−La2O3−HZSM-5 (with mainly weak + medium acid sites) was very low, implying the very weak demethylation or disproportionation of methylnaphthalene which was catalyzed by strong acid sites (for instance, violently occurring over parent HZSM-5 with much more strong acid sites, in Table

S2). The high selectivity of 1-MN indicates the preferential isomerization of 2-MN over modified ZSM-5. This is because the strength of acidity required for catalyzing the reactions mentioned above follows an order: methylation < isomerization < dealkylation < disproportionation.47,61 Interestingly, SiO2−HBeta still showed high reactivity for both methylation (using C10A) and transmethylation. Especially, a conversion of 42.85% and a DMNs selectivity of 51.32% were obtained in the new methylation and were obviously higher than those for transmethylation (25.93% and 42.50% respectively). This indicates the 12-ring pore of HBeta has no shape-selective rejection to C10A in reactants. Not only 2-MN and methanol can enter the larger pore for methylation catalyzed by the weak and medium acid sites there, but also C10A can diffuse to be adsorbed on the relatively strong acid sites62 on the internal surface and then lead to the transalkylation with 2-MN. Besides, compared with HBeta zeolite (Table 2), SiO2− HBeta exhibited obviously decreased both 2-MN conversion and 2,6-DMN yield in either methylation or transmethylation. The same phenomenon was also observed for the transmethylation over ZSM-5-based samples. These indicate the external acid sites of zeolites also showed an important 12598


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Figure 5. Schematic diagram for the reactant/product shape selectivity and high-efficiency and peculiar subarea-catalysis over the composite ZSM5/Beta catalyst.

very little methylnaphthalene and naphthalene in product for the 1,6-DMN isomerization over SiO2−La2O3−HZSM-5 (as shown in Table 7) and the much higher 2,6-DMN and β,βDMN selectivity in DMNs over the SiO2-ZSM-5/Beta composite sample (compared with SiO2−HBeta). In addition, for the transmethylation using the large-sized aromatic molecules with obvious steric hindrance (especially C10A) as reactants, we guess the formation of the aromatic H-bond of C10A or 2-MN molecule (the surplus negative charge for aromatic ring) with the −OH group of zeolite framework may be beneficial to the subsequent activation of these large molecules for forming carbocations and/or even their reaction with the activated molecules on the neighboring acid sites. Based on the above findings, it can be confirmed that there are peculiar subarea-catalysis reactions in the methylation system using C10A as solvent and transmethylation-agentia over the ZSM-5/Beta composite. As the schematic diagram shown in Figure 5, both methylation and transmethylation occur inside the 12-ring pore of HBeta and on the external surface of zeolites, while almost no transmethylation (but methylation) inside the smaller ZSM-5 pore with the shapeselective rejection to C10A. Moreover, the second isomerization of the fatter DMNs to produce β,β-DMN also takes place inside 10-ring pore. Here the high-efficiency transmethylation occurs on the relatively strong acid sites, but both methylation and DMNs isomerization were powerfully catalyzed by weak and medium acid sites while avoiding side reactions. Just the good operation of this catalytic system including the synergy of methylation with transmethylation and the second shape-selective isomerization of byproducts ensures the superior reaction results over the composite ZSM5/Beta catalyst.

contribution to the catalytic reactions. This also was powerfully proved by the results of methylation using C10A over a SiO2ZSM-5/Beta composite sample prepared with 63.0 wt % SiO2−La2O3−HZSM-5, 30.0 wt % SiO2−HBeta, and 7.0 wt % silica adhesive. As listed in Table 7, this sample showed some improved 2,6-DMN selectivity in DMNs (because the second isomerization of β,β-DMN on external surface was suppressed) but much lower 2-MN conversion and 2,6-DMN yield than the ZSM-5/Beta composite catalyst, owing to the elimination of most external acid sites on zeolite components. In addition, here SiO2−La2O3−HZSM-5 showed the approximate conversion to La2O3−HZSM-5 may due to the reasonable strength of internal acidity and the good matching between the size of 10-ring pore and the methylation of 2-MN with methanol. Although HBeta (Table 2) and SiO2−HBeta with higher density of acid sites showed the higher activity than modified ZSM-5 zeolites in the methylation using C10A, the selectivity of 2,6-DMN and β,β-DMN in DMNs over the formers is rather lower. This is because the 10-ring pore shows the stronger diffusion constraint to DMNs than 12-ring pore, and ZSM-5 channel exhibits the obvious shape selectivity to β,β-DMN depending on the wide difference in diffusion rate between the β,β-DMN with thinner molecule and the other fatter isomers there. Significantly, the composite ZSM-5/Beta catalyst containing 30.0 wt % HBeta and 63.0 wt % La2O3−HZSM-5 showed not only high activity (52.98%, approximate to that for HBeta) but also much higher selectivity (close to those for La2O3−HZSM-5) of 2,6-DMN (35.92%) and β,β-DMN (66.95%). This is attributed to both the excellent synergy of methylation with transmethylation occurring at the reasonable position on this composite catalyst and the second shapeselective conversion of DMNs inside the channels of La2O3− HZSM-5. On the one hand, both methylation and transmethylation highly effectively converted 2-MN into DMNs. On the other hand, the fatter DMN isomers formed in the 12ring pore and on the external surface of zeolites can diffuse into the 10-ring pore, and a part of them was further converted into β,β-DMN depending on both the isomerization catalyzed by weak and medium acid sites and the porous shape selectivity there, effectively preventing the demethylation or disproportionation of DMNs and methylnaphthalene. These are also powerfully proved by a high conversion of up to 31.5% but

4. CONCLUSIONS A new methylation (2-MN with methanol) process using lowvalue C10A as solvent and transmethylation-agentia was developed for the synthesis of 2,6-DMN, by using a ZSM-5/ Beta composite as catalyst which shows the high-efficiency and peculiar subarea-catalysis due to its reactant/product shape selectivity and ingenious acidity design. In this catalytic system, both methylation (2-MN with methanol) and transmethylation (C10A with 2-MN) are catalyzed by the weak + medium + 12599


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(6) Wen, J.; Zhao, L. F.; Wei, W. Synthesis of 2,6-Dimethyl Naphthalene over SAPO-11 Zeolite. Petrochem. Technol. 2012, 41, 1282. (7) Wu, G.; Wu, W.; Wang, X.; Zan, W.; Wang, W. J.; Li, C. Nanosized ZSM-5 Zeolites: Seed-Induced Synthesis and the Relation between the Physicochemical Properties and the Catalytic Performance in the Alkylation of Naphthalene. Microporous Mesoporous Mater. 2013, 180, 187. (8) Wang, X. X.; Zhang, W.; Guo, S. Q.; Zhao, L. F.; Xiang, H. W. Optimization of the Synthesis of SAPO-11 for the Methylation of Naphthalene with Methanol by Varying Templates and Template Content. J. Braz. Chem. Soc. 2013, 24, 1180. (9) Wang, X. X.; Guo, S. Q.; Zhang, W.; Zhao, L. F. Effect of Synthesis Conditions on the Crystallinity and Catalytic Performance of SAPO-11 Molecular Sieves. J. Mol. Catal.(Chin.) 2013, 27, 295. (10) Zhang, Y.; Feng, J. P.; Lyu, Z. J.; Li, X. K. Improved Stability and Shape Selectivity of 2,6-Dimethylnaphthalene by Methylation of Naphthalene with Methanol on Modified Zeolites. Mod. Res. Catal. 2014, 3, 19. (11) Lou, N. J.; Cao, F. H.; Fang, D. Y. Researches in the Catalysts for Synthesis of 2,6-Dimethylnaphthalene. Ind. Catal. 2006, 14, 7. (12) Li, S. X.; Yuan, X. D.; Qi, Y. T.; Zhang, Y. R.; Shen, J.; Sun, M. Z. Synthesis of 2,6-Dimethylnaphthalene over β Zeolite. Petrochem. Technol. 2002, 31, 811. (13) Pu, S. B.; Inui, T. Synthesis of 2,6-Dimethylnaphthalene by Methylation of Methylnaphthalene on Various Medium and LargePore Zeolite Catalysts. Appl. Catal., A 1996, 146, 305. (14) Wen, J.; Wang, G. Y.; Zhang, Y.; Qiu, Z. G.; Zhao, L. F. ShapeSelective Catalytic Methylation of Naphthalene with Methanol over SAPO-11 Molecular Sieve Catalyst. Petrochem. Technol. 2010, 39, 487. (15) Klein, H.; Fuessa, H.; Ernst, S.; Weitkamp, J. Localization of Naphthalenes in Zeolite HZSM-5 by X-Ray Powder Diffraction and Molecular Mechanics Calculation. Microporous Mater. 1994, 3, 291. (16) Fraenkel, D.; Cherniavsky, M.; Ittah, B.; Levy, M. ShapeSelective Alkylation of Naphthalene and Methylnaphthalene with Methanol over H-ZSM-5 Zeolite Catalysts. J. Catal. 1986, 101, 273. (17) Weitkamp, J.; Neuber, M. Shape Selective Reactions of Alkylnaphthalenes in Zeolite Catalysts. Stud. Surf. Sci. Catal. 1991, 60, 291. (18) Komatsu, T.; Araki, Y.; Namba, S.; Yashima, T. Selective Formation of 2,6-Dimethylnaphthalene from 2-Methylnaphthalene on ZSM-5 and Metallosilicates with MFI Structure. Stud. Surf. Sci. Catal. 1994, 84, 1821. (19) Inui, T.; Pu, S. B.; Kugai, J. I. Selective Neutralization of Acid Sites on the External Surface of H-ZSM-5 Crystallites by a Mechanochemical Method for Methylation of Methylnaphthalene. Appl. Catal., A 1996, 146, 285. (20) Park, J. N.; Wang, J.; Lee, C. W.; Park, S. E. Methylation of Naphthalene with Methanol over Beta, Mordernite, ZSM-12 and MCM-22 Zeolite Catalysts. B. Kor. Chem. Soc. 2002, 23, 1011. (21) Liu, M.; Wu, W.; Kikhtyanin, O. V.; Xiao, L. F.; Toktarev, A. V.; Wang, G. L.; Zhao, A. J.; Smirnova, M. Yu.; Echevsky, G. V. Alkylation of Naphthalene with Methanol over SAPO-11 Molecular Sieve Synthesized by Different Crystallization Methods. Microporous Mesoporous Mater. 2013, 181, 132. (22) Millini, R.; Frigerio, F.; Bellussi, G.; Pazzuconi, G.; Perego, C.; Pollesel, P.; Romano, U. A Priori Selection of Shape-Selective Zeolite Catalysts for the Synthesis of 2,6-Dimethylnaphthalene. J. Catal. 2003, 217, 298. (23) Bobuatong, K.; Probst, M.; Limtrakul, J. Structures and Energetics of the Methylation of 2-Methylnaphthalene with Methanol over H-Bea Zeolite. J. Phys. Chem. C 2010, 114, 21611. (24) Wang, X. X.; Liu, Z. M.; Guo, S. Q.; Gao, F.; Yu, Y.; Duan, X. Q.; Mao, S. H.; Wang, R. P.; Zhao, L. Y.; Wang, M.; Wei, S. X.; Zhang, P.; Xue, Y. B.; Wang, Y. Y. Methylation of Naphthalene with Methanol Catalyzed by Cu Modified SAPO-11 Zeolite. J. Mol. Catal. 2016, 30, 435.

strong acid sites inside 12-ring pore and on the external surface of zeolites, while no transmethylation (but methylation and second shape-selective isomerization of fatter DMNs to β,βDMN) takes place inside the smaller 10-ring pore with mainly weak−medium acidity. On the one hand, the synergy of methylation with transmethylation effectively enhanced the 2MN conversion and the DMNs content in products. On the other hand, the second isomerization ensured the high selectivity of 2,6-DMN and β,β-DMN, while avoiding side reactions (demethylation and disproportionation). As a result, a 2-MN conversion of up to 52.98% and a significantly enhanced yield of 2,6-DMN (11.01%) were obtained.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01596. Results of the methylation and the transmethylation over the extruded La2O3−HZSM-5 sample, the extruded HBeta sample, adhesive (SiO2), or the parent HZSM-5 zeolite (Table S1 and Table S2) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-18121186687. Fax: +86-21-65981097. E-mail: [email protected]. *Tel.: +86-21-65982563. Fax: +86-21-65981097. E-mail: [email protected]. ORCID

Junhui Li: 0000-0002-0127-2484 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (21802115), Science and Technology Program of Hunan Province of China (2017XK2048 and 2018JJ3501), and Research Start-Up Fund of Xiangtan University (17QDZ13). The authors also gratefully acknowledge the support from National & Local Joint Engineering Research Centre of Chemical Process Simulation and Enhancement.

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REFERENCES

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New Process for 2,6-Dimethylnaphthalene Synthesis by Using C10

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