Hydrogenation Behavior of Bicyclic Aromatic Hydrocarbons in the

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Hydrogenation behavior of bicyclic aromatic hydrocarbons in the presence of dispersed catalyst Wenan Deng*, Jing Lu, Chuan Li State Key Laboratory of Heavy Oil, China University of Petroleum, Qingdao, Shandong 266580, PR China Wenan Deng, E-mail: [email protected]

ABSTRACT:

2,6-Diisopropylnaphthalene(2,6-DIPN),

1-Methylnaphthalene(1-MN)

and

Biphenyl(BP) were selected as model compounds in order to investigate the hydrogenation behavior of bicyclic aromatics. The experiments were conducted in micro-autoclave in the presence of dispersed catalyst at temperature from 380 °C to 430 °C and pressure from 3 MPa to 15 MPa. Through the analysis of the reaction products by gas chromatography- mass spectrum (GC-MS) and gas chromatography (GC), the results revealed that hydrogenation of aromatics can be promoted significantly in the presence of dispersed catalyst. High selectivity towards monocyclic aromatics products was observed in hydrogenation of these model compounds, and its behavior was affected by the presence of substituents and π-conjugated bridge on aromatic ring. The yield of hydro-bicyclic aromatics increased with temperature and hydrogen pressure, while sometimes ring hydrogenation was influenced by thermodynamics properties of aromatics.

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Reaction pathways for hydrogenation of 2,6-DIPN, 1-MN and BP in the presence of dispersed catalyst were proposed.

Keywords：： Hydrogenation; dispersed catalyst; model compound 1. Introduction Heavy and extra heavy oils has been explored and processed as replacements of light petroleum reserves, owning to the increasing demand of fuel energy and depletion of conventional crude oil. The overwhelming advantages in acquisition of high quality distillate and prohibition of coke formation contribute to the increasingly attention to the development of slurry phase hydrocracking technology of heavy oil. Originally exploited in the coal hydrogenation industry, it has now been promisingly utilized in upgrading and recycling of heavy fuels in recent years.

1-7

The slurry phase process is operated in the presence of fine

dispersed metal sulfide catalyst in typical amounts from 50 to 1000 ppm, 3 which provides active hydrogen radicals and coke formation center that pull together polymerized asphaltenes and toluene insoluble ingredients. 8,9 Previous literatures 6,10-12 proved that the existence of catalyst is assumed to be functioning in inhibition of coke formation and hydrofining of compounds with heteroatoms such as sulfur, nitrogen and oxygen. Moreover, the activation of hydrogen radicals are estimated to be a hindrance of achieving equal products yield as thermal cracking, since it prohibits intermediate cracking to light product like gases and naphtha. 13-15 The aromaticity and polymeration degree of feedstock have been considered to be in close relationship with contents of resins and asphaltenes of heavy oil, 16,17 whereas researches into the hydrogenation reactivity of aromatics are faced with great difficulties owing to the complex structure of aromatics exist in heavy oil. For years it has been widely accepted to use model


2

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compounds to study the structure and reactivity of aromatics in heavy oil process 22-27

liquefaction,

18-21

and coal

and aliphatic substituent and π-conjugated bridge on aromatic ring are

considered to be important geometric features of bicyclic aromatics. Heavy oil contains significant amount of aromatic compounds, such as Naphthalene, Pyrene, and Anthracene.

28

Among different kinds of aromatics, bicyclic aromatic hydrocarbons possess the simplest structures among all different kinds of condensed aromatics and diarylalkanes compounds, and are considered to follow the same mechanism as polycyclic aromatics in hydrogenation reactions.

29

Consequently, in order to investigate the influence of the presence of substituents

and π-conjugated bridge on aromatic ring on the reactivity of aromatics and selectivity of dispersed catalyst, multi-substituted bicyclic aromatics with isomerized side chain, monosubstituted bicyclic aromatics with short side chain and biaryl were selected as model compounds of bicyclic aromatics, the representatives of which were 2,6-DIPN, 1-MN and BP. The aim of present work is to probe into the hydrogenation behavior of bicyclic aromatics under slurry bed hydrocracking. Reactions would be carried out mainly in catalytic hydrocracking system, and also in thermal cracking and hydrothermal cracking for comparison to explore reaction pathway of bicyclic aromatics. Meanwhile, the function of dispersed catalyst would be concluded based on the product distribution under different conditions including reaction temperature and hydrogen pressure. 2. Experimental Section 2.1 Materials 2,6DIPN, 1-MN and BP were purchased from J&K Chemical Ltd, Aladdin’s reagent Co, Ltd and Sinopharm Chemical Reagent Co, Ltd, respectively. Toluene (AR), benzene (AR), n-


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heptane (AR), absolute ethyl alcohol, sublimed sulfur and carbon disulfide was purchased from Sinopharm Chemical Reagent Co, Ltd. 2.2 Procedures Hydrocracking (catalytic or non-catalytic) and thermal cracking reactions were carried out in a 9 mL micro batch autoclave reactor without mixing unit. Soluble Molybdenum Naphthenate liquid (Molybdenum content of 6.4 %) was applied as hydrogenation catalyst. The principle of catalytic hydrogenation in slurry bed hydrocracking is considered to be the metallic catalyst precursor being vulcanized by sulfur or other sulphur-containing substance before the reaction begins. The active phase of catalyst is molybdenum disulfide. With temperature above 350°C, the vulcanization process is considered to be of good effect. The autoclave was charged with 0.128 g of model compound, 0.2 g of toluene as solvent and oil soluble catalyst (molybdenum content 600 ppm), and pressurized with hydrogen or nitrogen from 2 to 12 MPa. Tin bath was used to heat the autoclave and the top of thermocouple was touched with the bottom of autoclave to measure the temperature during the reaction. When the tin bath was heated to the fixed temperature, put the reactor in tin bath (The pressure in the reactor would rise up to 3 to 15MPa). Start counting when the temperature of autoclave reached the desirable temperature. The reaction was allowed to proceed at desirable temperature from 370 to 430 °C for fixed time (40min or 120min). During the reaction, the temperature of tin bath was kept to be within ±2°C of reaction temperature. After the reaction, the reactor was immediately cooled to room temperature by soaking in ice water to terminate the reaction. The liquid products were analyzed by gas chromatography (GC). 2.3 Analytical Methods


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Products were qualitatively analyzed using a gas chromatograph (GC HP5890 SERIES) and a mass selective detector (MS, Hewlett-Packard 5970) employing a fused silica capillary column (OV-1, 0.25 mm x 50 m). The quantitative analysis of liquid product was carried out by GC (Agilent 6890N) with a SE-54 capillary column (0.25 mm× 30.0 m× 0.25 µm). Since the coking amount is too small to analysis in experiment conditions, and the total mol content of reactant changes little throughout the reaction, product yield (mol %) is defined as the mol content of the liquid product fraction divided by the moles of total products obtained in percentage. X=

C frac Ctotal

× 100%

(1)

In the above equation, Cfrac stands for the concentration (mol %) of liquid product fraction, Ctotal stands for the concentration (mol %) of total reactants which was calculated as the total concentration (mol %) of unreacted model compound and total products. 3. Results and Discussion 3.1 Hydrogenation Behaviors of Model Compounds under Different Reaction Temperature 3.1.1 Hydrogenation behavior of 2,6-DIPN Table

1

reveals

that

only

2,5-Diisopropylnaphthalene

(2,5-DIPN)

and

2,7-

Diisopropylnaphthalene (2,7-DIPN) were detected after thermal cracking reaction while no dealkylation product was observed. Yield of 2,6-DIPN isomers increased with temperature, which indicates Caryl–Caliphatic bond could be activated to transfer among different positions on aromatic ring at high temperature. Table 1. Relationship between temperature and product distribution of 2,6-DIPN under thermal cracking


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Product distribution(%)

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Temperature(°C) 370

380

390

400

410

2,5-DIPN

0.16

0.32

0.52

0.65

0.97

2,7-DIPN

0.89

1.31

1.83

2.17

2.91

Conversion

1.05

1.64

2.35

2.82

3.97

Reaction conditions: N2:10 MPa, Time: 40 min. Table 2 shows that 2,6-Diisopropyltetralin (2,6-DIPT), 2-Isopropylnaphthalene (2-IPN) and 2,6-DIPN isomers were the main products. 2,6-DIPT is probably produced by the ring hydrogenation of 2,6-DIPN through step 1 of Scheme 1. As shown in Table 2, yield of 2,6-DIPT increased with temperature, which is probably because the increasing of temperature raised the rate constant of reactions. Consequently, ring hydrogenation of 2,6-DIPN and cracking of hydrogen molecules were promoted at higher temperature. Table 2. Relationship between temperature and product distribution of 2,6-DIPN under hydrothermal cracking Product distribution(%)


380

390

400

410

420

430

2-IPN

0.00

0.00

0.00

0.18

0.83

1.66

2.05

2,6-DIPT

0.69

0.74

0.79

1.05

1.10

1.13

1.16

2,6-DIPN Isomers

0.24

0.48

0.71

0.98

1.23

1.48

2.24


6

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Conversion

0.94

1.23

1.50

2.23

3.17

4.28

5.46

Reaction conditions: H2: 10 MPa, Time: 40 min. Comparing to ring hydrogenation, high bond dissociation energy is required in cleverage of Caryl-Caliphatic since the reaction could only be achieved above 400°C. With no 2-IPN detected in products of thermal cracking, it is deducible that the cracking of 2,6-DIPN was activated by hydrogen molecule. It is noted that the cracking of hydrogen molecule can be facilitated by the increment of temperature. Consequently, dissociation of Caryl-Caliphatic was supposed to be favored by hydrogen radicals since the yield of 2-IPN (i.e. product of dealkylation) increased with temperature. Catalytic hydrocracking of 2,6-DIPN was carried out under hydrogen pressure of 10MPa and reaction time of 40min, with temperature ranging from 370 to 420°C. It can be seen from Figure 1 that comparing to hydrothermal cracking, the hydrogenation of 2,6-DIPN was enormously promoted by soluble molybdenum naphthenate liquid since not only conversion rise was achieved in catalytic hydrogenation but also the reaction depth was upgraded. Complex products of chain scission and aromatic saturation reaction 6-Ethyltetralin (6-ET) and 1,1,6Trimethyltetralin (1,1,6-TMT) were detected after reaction, and the reaction pathway is demonstrated in Scheme 1.


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Scheme 1. The possible hydrogenation pathways of 2,6-DIPN

Figure 1. Relationship between temperature and product distribution of 2,6-DIPN under catalytic hydrocracking. (■)1,1,6-TMT, (●) 2-Ethyltetralin (2-ET), (▲) 2-IPN, (▼) 2,6-DIPT, (◆) 2,6-DIPN Isomers, (★) Conversion.


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As shown in Figure 1, 2,6-DIPN is prior to be hydrogenated into 2,6-DIPT rather than cracked into 2-IPN. Therefore, the hydrogenation of some of 2,6-DIPN towards 2,6-DIPT appears to be the initial step in complex reaction. The yield of 2,6-DIPT increased with temperature, which indicates that catalytic hydrogenation of aromatic ring was controlled by reaction kinetics below 390 °C. On the contrary, the yields of 2,6-DIPT decreased from 15.73 % to 10.39 with temperature increase from 390 to 420 °C. The result suggests that the ring hydrogenation of 2,6-DIPN was disfavored at higher temperature, which was in consistent with the endothermic nature of ring hydrogenation. Overall, the hydrogenation reactivity of 2,6-DIPN was promoted significantly with dispersed catalyst. Though more kinds of products were detected in catalytic hydrocracking than hydrothermal cracking, the reaction was in essence the same, which can be increased by rise of temperature and addition of dispersed catalyst. In summary, 2,6-DIPN was experimented under different reaction systems as catalyzed hydrocracking, hydrothermal cracking and thermal cracking, and the comparison of product distribution is demonstrated in Table 3. Based on the above analysis of reaction pathway, the mechanism of 2,6-DIPN hydrogenation is concluded in Fig. 2 and reaction products of 2,6-DIPN under catalytic hydrocracking are classified in: (i) cracking products, i.e., 2-IPN resulting from thermal cracking or hydrogenolysis, (ii) aromatic ring saturation product 2,6-DIPT, (iii) combination of cracking and aromatic saturation products 2-ET, 1,1,6-TMT and (iv) isomerization products, i.e., 2,5-DIPN, 2,7-DIPN. Table 3. Comparison of product distribution of 2,6-DIPN under different conditions Product distribution(%)

Thermal cracking

Hydrothermal

Catalytic

cracking

hydrocracking


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1,1,6-TMT

-

-

1.06

2-ET

-

-

0.38

2-IPN

-

0.18

3.09

2,6-DIPT

-

1.05

14.80

2,6-DIPT Isomers

2.82

0.98

2.85

Conversion

2.82

2.23

22.26

Reaction conditions: Reaction pressure:10 MPa, Time: 40 min.

Figure 2. Proposed reaction pathways of 2,6-DIPN hydrogenation in the presence of dispersed catalyst 3.1.2 Hydrogenation behavior of 1-MN The product selectivity of 1-MN thermal cracking is presented in Table 4. Both 2Methylnaphthalene (2-MN, i.e. product of isomerization) and Naphthalene (i.e. product of dealkylation) were detected in thermal cracking products, though at very low content.


10

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Additionally, different from former literature, only trace amount of methylated dimers were detected in product, which might be triggered by the insufficiency of reaction time for condensation of 1-MN. Consequently, condensation of 1-MN would not be discussed in the research due to the low content of methylated dimers. As shown in Table 4, yield of Naphthalene increased with temperature, indicating Caryl-Caliphatic bond could be activated at higher temperature to transfer among different positions on aromatic ring, parts of which could also be dissociated to produce naphthalene. Table 4. Relationship between temperature and product distribution of 1-MN under thermal cracking Product distribution(%)


390

400

410

420

Naphthalene

0.11

0.30

0.10

0.32

0.23

2-MN

0.79

0.85

0.85

0.72

0.79

Conversion

0.91

1.16

0.96

1.05

1.02

Reaction conditions: N2: 10 MPa, Time: 40 min. Product distribution of hydrothermal cracking in the same reaction (Table 5) reflected the effect of temperature on the performance of dispersed catalyst for the conversion of 1-MN. By increasing temperature from 380 °C to 420 °C, the yields of hydrogenation products were all elevated in different extents. The relatively high selectivity of 5-Methyltetralin (5-MT) and 1Methyltetralin (1-MT, i.e. products of ring hydrogenation) suggests that hydrothermal cracking of 1-MN was dominated by ring hydrogenation and it is mainly due to the relatively low active


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energy of ring hydrogenation. Unlike hydrogenolysis, more consumptions of hydrogen radicals were required in aromatic hydrogenation, which resulted in the low activity of ring hydrogenation at low temperature, since the bond dissociation of hydrogen molecular could hardly be motivated. Slight amount of 1-Ethyl-2-methylbenzene (1E-2MB), 1-Ethyl-3methylbenzene (1E-3MB) and 1-Ethyl-4-methylbenzene (1E-4MB) were also detected in the product, which suggests the existence of hydrogen radicals could activate the cracking of C-C bond, thus promoting the ring opening of 1-MN. Table 5. Relationship between temperature and product distribution of 1-MN under hydrothermal cracking Product distribution(%)


390

400

410

420

1E-2MB

0.56

0.79

1.11

1.10

1.11

1E-3MB

0.78

0.91

1.04

0.95

0.89

1E-4MB

0.84

1.10

1.36

1.30

1.21

Tetralin

0.00

0.00

0.00

0.00

0.03

Naphthalene

0.08

0.13

0.21

0.22

0.70

1-MT

0.72

1.23

1.94

2.04

2.24

5-MT

1.24

1.80

2.92

3.27

3.48

Conversion

4.25

5.98

8.61

8.91

9.67


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Reaction conditions: H2:10 MPa, Time: 40 min. In comparison with the reactions without dispersed catalyst, considerable increases in the conversion and yields of hydrogenation products were observed in catalytic hydrocracking of 1MN (Table 6). As shown in Table 6, the yields of the liquid products increase with the increment of temperature, but the effect of temperature on the distribution of products is different. The possible hydrogenation pathways are proposed to explain this phenomenon, through which the relationship between the distribution of products and pyrolysis temperature will also be discussed. Products of catalytic hydrocracking of 1-MN share the same classis with hydrothermal cracking, indicating the main function of dispersed catalyst is activating hydrogen molecules to crack into more hydrogen radicals rather than motivate the hydrogenation process in different mechanism. Table 6. Relationship between temperature and product distribution of 1-MN under catalytic hydrocracking Product distribution(%)


380

390

400

410

420

1E-2MB

0.97

1.03

1.28

1.19

1.42

1.22

1E-3MB

0.81

0.92

0.93

0.88

1.01

0.88

1E-4MB

1.12

1.23

1.33

1.25

1.44

1.27

Tetralin

0.00

0.00

0.06

0.08

0.14

0.27

Naphthalene

0.17

0.19

0.31

0.38

0.55

0.76


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1-MT

5.06

5.74

8.58

8.65

9.63

11.66

5-MT

7.36

8.21

11.90

12.26

13.88

17.79

Conversion

15.50

17.33

24.42

24.72

28.10

33.88

Reaction conditions: H2:10 MPa, Time: 40 min. Notably, cracking of 1-MN in hydrogen atmosphere was more favored compared with thermal cracking: both the yield of Naphthalene and 1E-2MB Isomers (i.e. ring opening product naphthalene) were higher in hydrogen atmosphere and increased with temperature. Same as hydrothermal cracking, from Table 6 it can also be detected that both the yield of Naphthalene and 1E-2MB isomers were promoted in catalytic hydrocracking. However, former literature

1

concluded that thermal cracking would be hampered by hydrogen radicals in that the main function of hydrogen radical is to combine with macroalkyl radicals for inhibition of coke formation and with smaller alkyl radicals to prevent overcracking. The result may be attributed to that the bond rupture of Caryl-Caliphatic can be achieved either with or without the favor of hydrogen radicals. Conventionally, the high reactivity of alkanes in thermal cracking derives from the relatively low C-C bond dissociation enthalpies, which facilitates the chain propagation reaction, whereas some theoretically feasible thermal cracking of aromatics can hardly happen due to the instability of leaving groups. For dealkylation of 1-MN, the reaction pathway in catalytic hydrocracking is demonstrated in Scheme 2.


14

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Scheme 2. The possible hydrogenation pathways of 1-MN In hydrothermal cracking, free radicals produced by step 1 would be terminated by hydrogen radicals so that the yield of Naphthalene would decrease with hydrogen pressure rise. Nevertheless, the yield of Naphthalene increased with pressure, which suggests Naphthalene was produced through step 2 with the favor of hydrogen radicals. In summary, 1-MN was experimented under different conditions as catalyzed hydrocracking, hydrothermal cracking and thermal cracking, and the comparison of product distribution is demonstrated in Table 7. Based on the above analysis of reaction pathway, the mechanism of 1-MN hydrogenation in the presence of dispersed catalyst is proposed in Figure 3 and four kinds of products were concluded in catalytic hydrocracking: (i) ring opening products 1E-2MB, 1E-3MB and 1E-4MB, (ii) dealkylation products Naphthalene, (iii) ring hydrogenation products 1-MT and 5-MT, complex of dealkylation and ring hydrogenation products Tetralin. Table 7. Comparison of product distribution of 1-MN under different conditions Product distribution(%)

1E-2MB

Thermal cracking

-

Hydrothermal

Catalytic

cracking

hydrocracking

1.11

1.19


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1E-3MB

-

1.04

0.88

1E-4MB

-

1.36

1.25

Tetralin

-

0.00

0.08

Naphthalene

0.10

0.21

0.38

1-MT

-

1.94

8.65

5-MT

-

2.92

12.26

Conversion

0.96

8.61

24.72


Figure 3. Proposed reaction pathways of 1-MN hydrogenation in the presence of dispersed catalyst 3.1.3 Hydrogenation behavior of BP Table 8 shows the reactivity of BP in thermal cracking and hydrothermal cracking. From the above conclusion, ring saturation of bicyclic aromatic hydrocarbons takes more priority in


16

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hydrothermal cracking than any other reaction. However, no ring hydrogenation product was detected in hydrothermal cracking of BP, which means the ring hydrogenation of biaryl compounds is more difficult than condensed aromatics. Moreover, the low conversion of thermal cracking and hydrothermal cracking of BP also indicates the strong Caryl–Caryl bond in BP. Since only trace amount of terphenyl was detected in the product of thermal cracking, condensation of BP would not be discussed in the research. Benzene was considered to be the product of thermal cracking and hydrothermal cracking. As shown in Table 8, yield of Benzene increased with temperature both in in nitrogen and hydrogen atmosphere. The relatively higher yield of benzene under hydrothermal cracking suggests that the bond dissociation of Caryl - Caryl of BP could be motivated by hydrogen radicals. Scheme 3 shows the reaction pathway of BP under thermal cracking and hydrothermal cracking. As demonstrated above, the cracking reaction of BP followed step 1. In hydrothermal cracking, the yield of benzene was largely promoted by the step 2, which can be classified as hydronolysis of Caryl - Caryl. Table 8. Relationship between temperature and conversion of BP under thermal cracking and hydrothermal cracking Conversion (%)


390

400

410

420

430

440

0.10

0.09

0.09

0.09

0.13

0.14

0.17

Hydrothermal 0.16

0.18

0.22

0.31

0.57

0.71

0.93

Thermal cracking

cracking Reaction conditions: Reaction pressure;10 MPa, Time: 120 min.


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Scheme 3. The possible hydrogenation pathways of BP Table 9 shows the product distribution of BP hydrogenation in the presence of dispersed catalyst. Comparing with hydrothermal cracking, hydrogenation of BP was promoted with dispersed catalyst since both the yield Benzene and Cyclohexylbenzene (CHB, i.e. product of ring hydrogenation) had been elevated. It is suggested in literature 30 that the hydroconversion of aromatic hydrocarbons over supported catalyst engages in two processes: skeletal isomerization of six-member naphthenic ring yielding into five-member ring contraction product on acid sites, followed by hydrogenolysis of those isomers on metal sites. Comparatively, in slurry phase hydrocracking, the first isomerization is not likely to happen due to the lack of acidic catalyst. As isomerization product of CHB, (Cyclopentylmethyl) cyclohexane (CMCH) were likely to be produced in Scheme 4, and the slight amount of alkylbenzene detected after reaction were supposed to be the ring opening products of CMCH. Since it couldn’t be proved whether the isomerization of 2,6-DIPN and 1-MN could be favored by hydrogen radicals, there is no striking proof that the isomerization of CHB could be facilitated with hydrogen radicals. However, it still can be concluded that yield of CMCH is positively related with increment of temperature, which suggests ring opening activity of bicyclic aromatics with π-conjugated bridge.


18

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Table 9. Relationship between temperature and product distribution of BP under catalytic hydrocracking Product

Temperature(°C)

distribution(%)

380

390

400

420

440

Benzene

0.39

0.39

0.46

1.03

1.32

CMCH

0.07

0.11

0.25

0.43

1.05

CHB

3.78

5.19

5.34

6.40

10.88

Conversion

4.25

5.69

6.04

7.85

13.24

Reaction conditions: H2;10 MPa, Time: 120 min. In summary, BP was experimented under different conditions as catalyzed hydrocracking, hydrothermal cracking and thermal cracking, and the comparison of product distribution is demonstrated in Table 10. Based on the above analysis of reaction pathway, the mechanism of BP hydrogenation in the presence of dispersed catalyst is proposed in Figure 4 and three kinds of products were concluded in catalytic hydrocracking: (i) aromatic ring saturation product, i.e. CHB, (ii) chain scission product, i.e. Benzene and (iii) combination of ring saturation and isomerization product, i.e. CMCH. Table 10. Comparison of product distribution of BP under different conditions Product distribution(%)

Thermal cracking

Hydrothermal

Catalytic

cracking

hydrocracking


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Benzene

0.09

0.22

0.46

CMCH

-

-

0.25

CHB

-

-

5.34

Conversion

0.09

0.22

6.04


Figure 4. Proposed reaction pathways of BP hydrogenation in the presence of dispersed catalyst 3.2 Hydrogenation Behaviors of Model Compounds under Different Reaction Pressure 3.2.1 Hydrogenation behavior of 2,6-DIPN To further investigate the influence of hydrogen pressure on product selectivity of 2,6-DIPN hydrogenation, experiments were carried out both in catalyst and non-catalyst system. The hydrogenation of 2,6-DIPN was complex under ordinary reaction condition of slurry bed hydrocracking, while under moderate reaction conditions, it would be easier to analysis the product distribution and reaction pathways of hydrogenation. Accordingly, the experiment was carried out under temperature of 380°C and reaction time of 40min. Table 11 shows the yield of 2,6-DIPT (i.e. product of ring hydrogenation) and 2-IPN (i.e. product of dealkylation ) s under hydrothermal cracking . With pressure ranging from 3 MPa to 15 MPa and temperature keeping


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at 380°C, no 2-IPN was detected in products of hydrothermal cracking while the yield of 2,6DIPT increased from 0.03 to 0.81%, which indicates higher energy was required in bond dissociation of Caryl-Caliphatic than ring hydrogenation. Comparatively, in catalytic hydrocracking, yield of 2-IPN increased from 4.36 to 28.12 % with pressure rise from 3 MPa to 15 MPa, which also suggests that the bond dissociation of Caryl-Caliphatic was promoted dynamically by hydrogen pressure. Table 11. Relationship between pressure and product distribution of 2,6-DIPN under hydrothermal cracking Product distribution(%)

Pressure(MPa) 3

6

8

10

13

15

2-IPN

0.00

0.00

0.00

0.00

0.00

0.00

2,6-DIPT

0.03

0.07

0.23

0.48

0.65

0.81

2,6-DIPN Isomers

0.13

0.38

0.52

0.47

0.44

0.49

Conversion

0.17

0.45

0.75

0.96

1.09

1.31

Reaction conditions: Temperature: 380°C, Time: 40 min. It can be seen from Figure 5 that as hydrogen pressure ranging from 3 to 15 MPa, the yield of 2-IPN increased linearly while the growth of 2,6-DIPT shows a regional pattern: though positively related, the velocity of yield increase was higher in the middle section of pressure scope. It is deduced that hydrogen molecules couldn’t be activated sufficiently by dispersed catalyst with pressure below 6 MPa, and when pressure surmounted 12 MPa, thermodynamic equilibrium of hydrogenation takes priority over hydrogen radical density to influence the


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reaction. Due to the incapability of solvent (toluene) in transferring hydrogen radicals to reactant, it is also deducible that hydrogen radicals catalyze the hydrogenolysis of Caryl-Caliphatic bond directly other than transfer from solvent.

Figure 5. Relationship between pressure and product distribution of 2,6-DIPN under catalytic hydrocracking. (■)1,1,6-TMT, (●) 2-ET, (▲) 2-IPN, (▼) 2,6-DIPT, (◆) 2,6-DIPN Isomers, (★) Conversion. 3.2.2 Hydrogenation behavior of 1-MN Figure 6 shows that yield of 5-MT and 1-MT increased dramatically with hydrogen pressure rise whereas the yield of 1E-2MB Isomers and Naphthalene didn’t change obviously even under extremely high hydrogen pressure. The ring hydrogenation of 1-MN can be motivated by pressure in two steps: (1) the reaction that hydrogen molecules homolysis into more hydrogen radicals can be facilitated by pressure rise which improves hydrogenation dynamically; (2) large consumption of hydrogen radicals in ring hydrogenation making the reaction facilitated with pressure increment thermodynamically. Previously it has been mentioned that 1E-2MB Isomers are product of hydrogenolysis of 1-MN while Naphthalene is produced by the combination of


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thermal cracking and hydrogenolysis. Hydrogenolysis could be favored dynamically with increase of hydrogen radical density. Nevertheless, thermal cracking of 1-MN could be inhibited by pressure rise in consideration of thermodynamics equilibrium, which also explained why yield of Naphthalene didn’t increase much as in catalytic hydrocracking.

Figure 6. Relationship between pressure and product distribution of 1-MN under catalytic hydrocracking. (■) 1E-2MB Isomers, (●) Tetralin, (▲) Naphthalene, (▼) 1-MT, (◆) 5-MT, (★) Conversion. Selectivity of hydrogenation product 1-MN shows a regular connection with pressure rise. As typical mono substitute polycyclic hydrocarbon with strong aromaticity, 1-MN has been widely used as standard compound to inspect catalytic activity of both metallic and metal sulfide catalyst. The selectivity of 5- MT is higher than 1-MT due to the steric hindrance of methyl side chain. In our work, S is defined as the selectivity of 5-MT in aromatic saturation and the equation is listed as follows: S=

W5− MT × 100% W5− MT + W1− MT


(2)

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In fixed bed reactor, the selectivity of 5-MT usually surmounts 80 % sometime even reaches100 % because during reaction 1-MN is absorbed on surface of metallic catalyst to be hydrogenated, benefiting reactions with bigger equilibrium constant. The relationship between pressure and structural product selectivity of aromatic saturation is presented in Figure 7 based on experiments carried on in hydrogen atmosphere with or without dispersed catalyst. Generally, the selectivity of 5-MT decreased with hydrogen pressure rise under circumstance both with and without catalyst. When reacting in catalytic atmosphere, the selectivity decreases apparently, indicating the hydrogenation of aromatics in dispersed catalyst benefited more to reactions with smaller equilibrium constant.

Figure 7. Relationship between pressure and structural product selectivity of aromatic saturation reaction. (■) Hydrothermal cracking, (●) Catalytic hydrocracking. With 2,6-DIPN and 1-MN selected typical Naphthalene derivatives with aliphatic substituents, consideration was also given to the influence of the presence of substituents on aromatic ring on the activity and selectivity. Comparing Figure 5 and Figure 6, it can be detected that, classified as ring hydrogenation products, yield of 1-MT and 5-MT are higher than 2,6-


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DIPT in same condition .It can be explained by a combination of electronic and geometry effects of aliphatic substituents: the steric hindrance and electrophilic properties of aliphatic group inhibit hydrogen radicals reacting on aromatic rings.

3.2.3 Hydrogenation behavior of BP Figure 8 exhibits the relationship between pressure and product distribution of BP. Monocyclic aromatics are hard to saturate in catalytic hydrocracking. Yield of Toluene hydrogenation for 2 h with dispersed catalyst proved to be 1.78 %. However, yield of the cracking products of BP was 3 times as much as Toluene in the same condition, owing to the πconjugated bridge on aromatic ring. With pressure rising from 3 to 13 MPa, both the yield of cracking and aromatic saturation products was increased. Like hydrogenation of 2,6-DIPN and 1MN, insufficient hydrogen source limited the hydrogenation reactivity, while pressure rise up to 6 MPa, yield of CHB was promoted dramatically compared with benzene.

Figure 8. Relationship between pressure and productivity of BP under catalytic hydrocracking. (■) Benzene, (●) (Cyclopentylmethyl) benzene, (▲) CHB, (★) Conversion.


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Table 12 shows the product distribution of BP under hydrothermal cracking and catalytic hydrocracking. With increment of reaction pressure from 3 MPa to 13 MPa, yield of CHB was increased from 0.1 to 6.81, whereas yield of bridge rupture product increased from 0.25 to 0.51.Combining with hydrogenation rules of 2,6-DIPN and 1-MN, it can be concluded that elevation of hydrogen pressure is more effective in ring hydrogenation than bridge rupture and ring opening.

Table 12. Relationship between pressure and product distribution of BP under catalytic hydrocracking Pressure(MPa)

Product distribution(%)

3

6

8

10

13

0.20

0.22

0.23

0.25

Hydrothermal cracking Benzene/Conversion 0.11

Catalytic hydrocracking Benzene

0.25

0.34

0.40

0.45

0.51

CMCH

0.00

0.00

0.19

0.25

0.30

CHB

0.10

1.25

4.05

5.34

6.81

Conversion

0.35

1.59

4.64

6.04

7.62

Reaction conditions: Temperature;400°C, Time: 120 min.

4. Conclusion


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2,6-DIPN, 1-MN and BP were selected as model compounds in order to investigate the hydrogenation behavior of bicyclic aromatics. The results revealed that hydrogenation of aromatics can be promoted significantly in the presence of dispersed catalyst. High selectivity towards monocyclic aromatics products was observed in hydrogenation of these model compounds. Consideration was also given to the influence of the presence of substituents and πconjugated bridge on aromatic ring on the activity and selectivity, and the result indicates the characteristics of substituents groups and π-conjugated bridge could influence the way hydrogen radicals react with aromatic ring, which can be explained by a combination of electronic and geometry effects. The yield of hydro-bicyclic aromatics increased with temperature and hydrogen pressure, while sometimes ring hydrogenation was influenced by thermodynamics properties of aromatics. In summary, this experiment demonstrates the ability of dispersed catalyst to achieve the transformation of bicyclic aromatics into monocyclic aromatics. Reaction pathways for hydrogenation of 2,6-DIPN, 1-MN and BP in the presence of dispersed catalyst were proposed. However, further approach is expected on kinetics to deeply understand the mechanism of hydrogenation of bicyclic aromatics.

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Acknowledgement This work was supported by National Natural Science Foundation Young Investigator Grant Program of China (No. 21106186).


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Hydrogenation Behavior of Bicyclic Aromatic Hydrocarbons in the

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