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Restrictive Diffusion in the Hydrodesulfurization over Ni-MoS2/Al2O3 with different crystal forms Xilong Wang, Jinlin Mei, Zhen Zhao, Peng Zheng, Zhentao Chen, Jianmei Li, Jiyuan Fan, Aijun Duan, and Chunming Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02897 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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Industrial & Engineering Chemistry Research

Restrictive Diffusion in the Hydrodesulfurization over Ni-MoS2/Al2O3 with different crystal forms Xilong Wang †, Jinlin Mei †, Zhen Zhao †, Peng Zheng †, Zhentao Chen †, Jianmei Li †, Jiyuan Fan †, Aijun Duan *, †, Chunming Xu *, †

† State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 18 Fuxue Road, Beijing, P.R. China, 102249

Corresponding Author: *E-mail: [email protected]. Tel: +86 10 89732290. *E-mail: [email protected]. Tel: +86 10 89733392.

Abstract Al2O3 materials with different crystal forms were synthesized via the hydrothermal synthesis method using the low-cost raw material, and then the corresponding NiMo/Al2O3 catalysts were prepared by using Al2O3 materials with different crystal forms as the supports. The restrictive diffusion effects on the hydrodesulfurization (HDS) reaction of different reactants with different molecular sizes over the NiMo/Al2O3 catalysts with different crystal forms were investigated systematically for the first time. NiMo/δ-Al2O3 exhibited the highest values of the effective factor (η) and effective diffusion coefficient (De), which could be ascribed to its proper pore diameter and relatively concentrated pore diameter distribution. The hindered 1

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magnitudes of diffusion decrease in the order of NiMo/δ-30 (2.23) < NiMo/θ-30 (2.83) < NiMo/γ-30 (3.42), indicating that the restrictive diffusion effect of NiMo/δ-30 catalyst was weaker than those of the other two catalysts. Keywords Al2O3 material; Crystal forms; Effective factor; Effective diffusion coefficient; Restricted diffusion; Hydrodesulfurization

1. INTRODUCTION In recent years, removal of sulfur in gasoline and diesel fuels has been the object of intensive investigations due to the rigorous environmental regulations and the increasing requirement for high quality transportation fuels.1-5 To produce the ultra-low sulfur transportation fuels, highly refractory sulfur-containing compounds such as dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) shall be eliminated in the fuel.6-9 Therefore, it is necessary to develop novel and more effective HDS catalysts to improve plants efficiency and enable operation of older HDS units.10,

11

In general, γ-Al2O3 is used as industrial HDS catalyst supports

because of its economic performance and structural properties.12-15 However, the ultra-deep HDS of large molecular weight sulfur-containing compounds (DBT and 4,6-DMDBT) on the industrial catalysts (Ni-MoS2/γ-Al2O3 and CoMoS2/γ-Al2O3) is hard to achieve due to highly diffusion restrictiveness.16, 17 Therefore, in order to eliminate the diffusion restrictions of the large molecular weight compounds, large enough pore diameters of the novel catalysts are needed. Ni-MoS2/Al2O3 with different crystal forms were reported in our previous 2


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published papers and the DBT and 4,6-DMDBT HDS efficiencies were evaluated. The Ni-MoS2/δ-Al2O3 catalyst showed the best DBT and 4,6-DMDBT HDS performance than the other catalysts.18 Zhang et al.

19

synthesized different Al2O3

using AlCl3·6H2O and Al(NO3)3·9H2O as different alumina resources and evaluated the HDS and HDN activities with FCC diesel as the feed oil. The Ni-MoS2/δ-Al2O3 catalyst obtained from Al(NO3)3·9H2O presented the highest FCC diesel HDS and HDN conversions. Wang et al.

20

also prepared a series of Ni-MoS2/γ-Al2O3 catalysts

by tuning different aging temperatures and Ni-MoS2/γ-Al2O3 catalyst at the aging temperatures of 90 ºC exhibited the highest DBT and 4,6-DMDBT activities. Although the Al2O3 supports with open channels could eliminate the diffusion resistance to some extent, the diffusion restriction still exists under the HDS reaction conditions. If the diffusion rate of the sulfur-containing compounds is the rate control step compared to the HDS reaction, only the outer surface part of the catalysts will be used, the efficiency will decrease. Consequently, the comprehensive knowledge of the kinetics and effective diffusivities is imperative to utilize the catalyst more efficiently. Many researchers have investigated the restrictive diffusion effect with either full-size commercial catalysts or crushed catalyst particles. Chen et al.

21

studied the

intra-particle effect on DBT HDS reaction with a full-size commercial NiMo/Al2O3 and its crushed catalyst particles and found that the crushed catalyst particle exhibited less intra-particle diffusion resistance. Chen et al.

22

also investigated the inhibition

effects of H2S and NH3 on HDS efficiencies for sulfur compounds over NiMo/Al2O3 catalysts and discovered that H2S showed a stronger inhibition effect than NH3. Li et 3



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al. 23 investigated the influence of the ratio of the molecular size to pore size (λ) on the restrictive

diffusion

in

the

hydrotreating

of

heavy

residue

oils

over

CoMo/alumina-aluminum phosphate catalysts. The results indicated that De values decreased with the increase of λ for both HDS and hydrodemetallization reactions. Kabe et al.

24

investigated the kinetics and mechanisms of DBT HDS over

CoMo/Al2O3 catalyst and discovered that the HDS reaction rates of DBTs had great relations with the adsorption ability of DBTs. However, there is no open report for the restrictive diffusion effect under HDS reactions on different crystal form alumina-supported catalysts. Therefore, a further study on the restrictive diffusion effect under HDS reactions on different crystal form alumina-supported catalysts is essential. In this work, different crystal form Al2O3 materials with different pore sizes have been synthesized based on our previous work

18

, and NiMo/Al2O3 catalyst was

prepared using Al2O3 as support and NiMo as active metals. The restrictive diffusion effect of different model sulfides (DBT and 4,6-DMDBT) on NiMo/Al2O3 with different pore sizes was investigated under industrial conditions in a fixed-bed reactor.

η and De values of the sulfur-containing model compounds with different structures and molecular sizes on NiMo/Al2O3 with different pore sizes are investigated systematically for the first time. Moreover, the relationship between the De and the ratio of the molecular diameter to pore diameter (λ) was further studied.

2. EXPERIMENTAL SECTION 2.1 Synthesis of alumina and the corresponding NiMo catalysts 4


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Three mesoporous Al2O3 materials with different crystal forms and the corresponding NiMo catalysts were synthesized according to our previous work

18

.

The boehmite sol was prepared by hydrothermal synthesis method using polyethylene glycol as the mesostructure directing agent at the aging temperature of 30 ºC. Then, three mesoporous Al2O3 materials with different crystal forms were prepared at different calcination temperature from the boehmite sol, denoted as γ-30 (550 ºC, 2 ºC/min), δ-30 (900 ºC, 2 ºC /min) and θ-30 (1000 ºC, 2 ºC /min), respectively. The corresponding NiMo catalysts were prepared by incipient wetness impregnation method

using

aqueous

solutions

of

heptamolybdate

tetrahydrate

((NH4)6Mo7O24·4H2O) and nickel nitrate (Ni(NO3)2·6H2O). The obtained catalysts were denoted as NiMo/γ-30, NiMo/δ-30, and NiMo/θ-30, respectively. In all catalysts, Mo and Ni contents were kept constants at 12 wt.% of MoO3 and 3 wt.% of NiO.

2.2 Characterization of Supports X-ray diffraction (XRD) patterns were taken on a Rigaku RINT D/Max-2500 powder diffraction system with a tube voltage of 40 kV and current of 40 mA. The samples were scanned in the 2 theta interval of 5 to 75°. N2 adsorption and desorption measurements

were

adsorption-desorption

performed analyzer

at

-196

ºC

(Quantachrome

with

an

Autosorb-iQ,

automated USA).

gas

Before

adsorption-desorption isotherm measurements, the samples were outgassed 4 h at 340 ºC in vacuum.

2.3 Evaluation of HDS Performance of Catalysts

5



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The HDS activities of DBT and 4,6-DMDBT were evaluated on a fixed-bed reactor with 0.5 g Ni-MoO3/Al2O3 catalyst. Prior to each experiment, the series oxide catalysts were presulfided with 2.5 wt.% CS2/cyclohexane solution at 340 ºC, 4.0 MPa, H2/Oil ratio of 600 mL·mL-1, and weight hourly space velocity (WHSV) of 8 h-1 for 4 h. After that, DBT (with sulfur content 500 ppm) and 4,6-DMDBT (with sulfur content 500 ppm) dissolved in cyclohexane were introduced at 320-380 oC, 3.0-6.0 MPa, H2/Oil ratio of 50-300 mL·mL-1, and WHSV of 2.0-150 h-1. After reaction, the sulfur contents of the liquid products were analyzed by RPP-2000SN sulfur and nitrogen testing instrument.

3. RESULTS AND DISCUSSION 3.1 XRD of the Supports Figure 1 shows the XRD patterns of the alumina supports with different crystal forms. γ-30 material presents four peaks at 2θ values of 37.5°, 39.5°, 45.9° and 66.9°, which are ascribed to the (311), (222), (400) and (440) diffractions of the γ-Al2O3 phase with face-centered cubic structure (ICDD file no.: 10-425). δ-30 support exhibits a series of characteristic peaks at 2θ values of 66.95°, 45.62°, 46.48°, 37.6° and 32.76°, which are attributed to the (440), (0012), (400), (311) and (220) planes of the well-crystallized δ-Al2O3 phase (ICDD file no.: 56-1186). θ-30 material presents typical diffraction peaks at 2θ = 31.05°, 33.05°, 37.15°, 39.52°, 45.10°, 48.0°, 60.20° and 67.58°, which are assigned to the (004), (200), (111), (104), (211), (116), (018) and (215) planes of θ-Al2O3 material with face-centered cubic monoclinic structure (ICDD file no.: 47-1771).

6


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Intensity, a.u.

•

•• •

10

20

30

∗

∗

γ-30 δ-30

∗

∗ ∗∗ ∗

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40 50 60 2θ, degree

θ-30 70

80

Figure 1. The wide-angle XRD patterns of the synthesized Al2O3 materials with different crystal forms

3.2 N2 adsorption-desorption pattern of the catalysts To further understand the structural features of as-synthesized NiMo/Al2O3 series catalysts, N2 adsorption-desorption pattern is conducted, as shown in Figure 2 A and B. The NiMo/Al2O3 series catalysts exhibit type IV isotherms with a clear H2 type hysteresis loop, which suggests the presence of the representative curve characteristics of ordered mesopores (Figure 2 A). And the BJH pore diameter distributions of NiMo/γ-30, NiMo/δ-30 and NiMo/θ-30 show that the concentration degree of mesopores changes in the sequence of NiMo/γ-30 > NiMo/δ-30 > NiMo/θ-30 (Figure 2 B). The corresponding textural properties of the series catalysts are listed in Table 1. As can be seen, the pore sizes of the series catalysts follow the order of NiMo/γ-30 (6.3 nm) < NiMo/δ-30 (19.2 nm) < NiMo/θ-30 (20.5 nm).

7



(A)

NiMo/γ-30 NiMo/δ-30 NiMo/θ-30

0.0

(B)

dV/dD, cm3·g-1·nm-1

Volume, m3·g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NiMo/γ-30 NiMo/δ-30 NiMo/θ-30

1.0 0

0.2 0.4 0.6 0.8 Relative Pressure, P/P0

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20

40 60 80 Pore diameter, nm

100

Figure 2 (A) N2 adsorption-desorption isotherms and (B) BJH pore diameter distributions patterns of NiMo/Al2O3 catalysts with different crystal forms.

Table 1. Textural properties of series NiMo/Al2O3. Samples

SBET 2

-1

Vmes

Vmic

3

3

-1

dBJH -1

(m g )

(cm g )

(cm g )

(nm)

NiMo/γ-30

163.8

0.39

0.005

6.3

NiMo/δ-30

126.3

0.48

0.004

19.2

NiMo/θ-30

118.6

0.46

0.004

20.5

3.3 HDS reaction order of DBT and 4,6-DMDBT 3.3.1. Removal of external diffusion External diffusion has the significant effects on the DBT and 4,6-DMDBT HDS reaction, Mears criterion is often used to distinguish whether the external diffusion effects can be negligible.25 Mears criterion:

−r

apparent

ρ b Rn

< 0.15

(1)

k c C Ab

Where rapparent is apparent reaction rate, ρb the catalyst bulk density (kg·m-3), R the catalyst particle radius (m), n the reaction order, kc the mass transfer coefficient 26 and CAb the bulk concentration (kmol·m-3). 8


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D AB Sh dp

kc =

where

(2)

where Sh is Sherwood number (or nusselt number)，the relationship of Sh with Reynold number (Re) and schmidt number (Sc)：

Sh = 2 +

Re 3 S c

S

c

(3)

du ρ

Re =

where

(4)

µ

=

and

ν D

(5) AB

where DAB is the diffusion coefficient (m2 s-1), d the catalyst particle diameter (m), u the flow velocity (m s-1), µ the viscosity (Pa s), and ν the dynamic viscosity (ν = µ/ρ, m2 s-1). Substituting these values into the above equations, the value is approximately 0.07 < 0.15, the experiments were satisfied in Mears criterion, indicating that the effect of the external diffusion can be neglected in the experiments.

3.3.2. Elimination of internal diffusion

(40-60 mesh) (20-40 mesh) 0

10

20 30 40 50 Pore diameter, nm

60

70 0

(80-100 mesh) (60-80 mesh) (40-60 mesh) (20-40 mesh) 20

40 60 80 Pore diameter, nm

(C)


(80-100 mesh) (60-80 mesh)

(B)


(A)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 0

(80-100 mesh) (60-80 mesh) (40-60 mesh) (20-40 mesh) 10 20 30 40 50 60 70 80 Pore diameter, nm

Figure 3. BJH pore diameter distributions of NiMo/Al2O3 catalysts with different meshes. (A) NiMo/γ-30, (B) NiMo/δ-30, (C) NiMo/θ-30.

9



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Calculating the intrinsic reaction rate constant is the key to determine η and De of the model sulfide reactant molecules on the surface of the catalyst. Based on the above analysis, the external diffusion is neglected. To calculate the intrinsic reaction rate constant, the influence of the internal diffusion resistance on the reaction is also need to be considered. Catalyst particle size is the main factors influencing the internal diffusion resistance. The internal diffusion resistance decreases with the reduction of the catalyst particle size. In order to study the influence of internal diffusion on DBT and 4,6-DMDBT HDS reaction, different crystal form NiMo/Al2O3 catalysts are sieved into 4 groups of different size ranges (20-40, 40-60, 60-80 and 80-100 mesh). The pore diameter distribution for different crystal form NiMo/Al2O3 catalysts with different particle size ranges are shown in Figure 3. It can be seen from Figure 3, the pore size distributions of different crystal form NiMo/Al2O3 catalysts are becoming more concentrated with the increase of the particle sizes. The physicochemical properties of NiMo/Al2O3 with different size ranges are summarized in Table 2. The bulk densities and the average pore sizes of series NiMo/Al2O3 increase with the decrease of catalyst particle sizes. The series NiMo/Al2O3 catalysts with different catalyst particle sizes were evaluated by the catalytic HDS reaction of DBT and 4,6-DMDBT at 340 oC, 4.0 MPa, DBT @ 150 h-1, 4,6-DMDBT @ 80 h-1 and H2/oil ratios of 200 (v/v).

10


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Table 2. Properties of catalysts with different particle sizes. Bulk density

Average pore size

Range of particle size

(kg m-3)

(nm)

(mm)

NiMo/γ-30 (20-40 mesh)

776

6.3

0.74-0.37


778

6.3

0.37-0.25


781

6.4

0.25-0.19

NiMo/γ-30 (80-100 mesh)

785

6.5

0.19-0.16

NiMo/δ-30 (20-40 mesh)

695

19.0

0.74-0.37


699

19.1

0.37-0.25


702

19.2

0.25-0.19

NiMo/δ-30 (80-100 mesh)

706

19.3

0.19-0.16

NiMo/θ-30 (20-40 mesh)

682

20.4

0.74-0.37


684

20.4

0.37-0.25


687

20.5

0.25-0.19

NiMo/θ-30 (80-100 mesh)

691

20.6

0.19-0.16

Catalysts

90

70

(A) NiMo/δ-30

HDS conversion, %

100

HDS conversion, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 NiMo/θ-30

70 60 50

NiMo/γ-30

40 30

20-40

40-60

60-80

80-100

Catalyst pellet size, mesh

(B) NiMo/δ-30

60 50 40

NiMo/θ-30 NiMo/γ-30

30 20 20-40

40-60 60-80 80-100 Catalyst pellet size, mesh

Figure 4. Relationship between desulfurization efficiency and particle size (A) HDS of DBT; (B) HDS of 4,6-DMDBT

The HDS conversions of DBT and 4,6-DMDBT over the three catalysts with different particle sizes are displayed in Figure 4. The HDS conversions increase with the increasing catalyst particle sizes from 0.74 to 0.16 mm, demonstrating that the internal diffusion effect in the HDS reaction system gradually decrease with the decrease of particle sizes. When the particle size is less than 0.19 mm, the DBT and 11



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4,6-DMDBT HDS conversions show little change, indicating that the effect of the internal diffusion can be eliminated. Moreover, the pressure drop of the bed increases with the decrease of the catalyst particle size. Smaller catalyst particle sizes may cause catalyst bring out of the fixed-bed reaction system by hydrogen flow, and the blockage of the pipeline system. Thus, in the HDS reaction system, the catalyst particle sizes were kept for 0.19-0.25 mm to analyze the intrinsic reaction rate constant and avoid the pressure drop in the fixed-bed reactor.

3.3.3. Elimination of other influencing factors The HDS reactions of reactant molecules are influenced by dispersion, surface effect and channeling effect in the mass transfer process.27 Doraiswamy et al.

28

proposed that if the ratio of the catalyst bed layer to the catalyst particle diameter > 4, the influence of channeling effect and heat transfer effect on HDS reaction can be ignored; and if the ratio of the catalyst bed height to the catalyst particle size > 30, the influence of the axial diffusion effect and the axial heat conduction effect on HDS reaction can be ignored. In this HDS reaction system, the maximal catalyst diameter is 0.74 mm (20 mesh), the catalyst bed diameter is 6 mm, the catalyst bed height is 150 mm. Substituting these values into the above definitions, the ratio of the catalyst bed layer to the catalyst particle diameter is approximately 8.1 > 4, and the ratio of the catalyst bed height to the catalyst particle size is 202.7 > 30, indicating that the channeling effect, heat transfer effect, axial diffusion effect and axial heat conduction effect can all be ignored in this DBT and 4,6-DMDBT HDS reaction systems.

12


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3.3.4. Determination of HDS reaction order

90

(A)

80 70 60

NiMo/θ-30 NiMo/δ-30 NiMo/γ-30

50 40 160 140 120 100 80 60 WHSV, h-1

100 90 (B) 80 70 60 NiMo/δ-30 50 40 NiMo/γ-30 NiMo/θ-30 30 20 10 160 140 120 100 80 60 WHSV, h-1

HDS conversion, %

HDS conversion, %

100

40

20

40

20

Figure 5. HDS reaction results of NiMo/Al2O3 (20-40 mesh) catalysts with different crystal forms at different WHSVs. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

100 (A) 90 80

NiMo/θ-30 NiMo/δ-30

70 NiMo/γ-30 60 50 160 140 120 100 80 60 WHSV, h-1

100 (B) 90 80 70 NiMo/δ-30 60 50 NiMo/θ-30 40 NiMo/γ-30 30 20 160 140 120 100 80 60 40 WHSV, h-1

HDS conversion, %

HDS conversion, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

20

20

Figure 6. HDS reaction results of NiMo/Al2O3 (60-80 mesh) catalysts with different crystal forms at different WHSVs. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

20-40 and 60-80 mesh catalysts were tested on HDS reaction to investigate the HDS reaction order of DBT and 4,6-DMDBT. The results of DBT and 4,6-DMDBT HDS on NiMo/Al2O3 catalysts with 20-40 and 60-80 mesh obtained at different WHSVs (340 oC, 4.0 MPa, and H2/Oil ratio of 200) are exhibited in Figure 5 and 6. The HDS efficiencies of DBT and 4,6-DMDBT increase sharply with the decreases of WHSVs over the three catalysts with different particle sizes. The DBT and 4,6-DMDBT HDS conversions follow the order of Ni-Mo/δ-30 > Ni-Mo/θ-30 >

13



Ni-Mo/γ-30 at all WHSVs. The 4,6-DMDBT HDS efficiencies were lower than those of DBT.

ln[1/(1-X)]

10 8 6

NiMo/δ-30 y=213.56x R2=0.999

3.5

NiMo/θ-30 y=163.11x R2=0.998

3.0 2.5 NiMo/γ-30 y=57.35x R2=0.998

4 2

ln[1/(1-X)]

12 (A)

2.0

(B) NiMo/δ-30 y=53.68x R2=0.994

NiMo/θ-30 y=51.66x R2=0.993

1.5

NiMo/γ-30 y=17.89x R2=0.993

1.0 0.5

0 0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.00 0.01 0.02 0.03 0.04 0.05 0.06 1/WHSV, h 1/WHSV, h

Figure 7. Obtained curves by plotting ln [1/(1-X)] vs 1/WHSV of NiMo/Al2O3 (20-40 mesh) catalysts with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT 5

15 (A)

9 6

NiMo/θ-30 y=229.06x NiMo/δ-30 R2=0.994 y=282.94x NiMo/γ-30 R2=0.997 y=92.65x R2=0.998

4

ln[1/(1-X)]

12

ln[1/(1-X)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 32

3

(B)

NiMo/θ-30 y=74.12x NiMo/δ-30 R2=0.995 y=84.95x R2=0.998

2 1

3

NiMo/γ-30 y=49.68x R2=0.999

0 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.00 0.01 0.02 0.03 0.04 0.05 0.06 1/WHSV, h 1/WHSV, h

Figure 8. Obtained curves by plotting ln[1/(1-X)] vs 1/WHSV of NiMo/Al2O3 (60-80mesh) catalysts with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

Many published papers

23, 29, 30

have proposed that HDS reaction of a single

reactant molecule follows pseudo-first-order reaction model. The pseudo-first-order HDS reaction kinetic equation can be expressed as:

ln

1 c0 k1 = − ln(1 − X ) = ln = − X LHSV 1 c1

(6)

where c0 is the sulphur content in feed, c1 the sulphur content in product, X the percentage of sulphur removal and k1 is the reaction rate constant.

14


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The relationships between HDS catalyst conversions and WHSVs are plotted to the ln[1/(1-X)]-1/WHSV (20-40 and 60-80 mesh) diagram, respectively, as shown in Figure 7 and 8. A good linear relationship between ln[1/(1-X)] and 1/WHSV is obtained based on the experimental data points in DBT and 4,6-DMDBT HDS on NiMo/Al2O3 (20-40 and 60-80 mesh) with different crystal forms. The fitting straight lines indicate that the DBT and 4,6-DMDBT HDS reactions under the operating conditions used also follow the pseudo-first order kinetic in this research. Moreover, all the linear slopes of DBT and 4,6-DMDBT HDS on NiMo/Al2O3 (20-40 and 60-80 mesh) follow the order of Ni-Mo/δ-30 > Ni-Mo/θ-30 > Ni-Mo/γ-30.

3.4 Calculation method of η and De in HDS reactions As validated in the previous section, the kinetics of the DBT and 4,6-DMDBT HDS reactions follow the pseudo-first order rule under the operating conditions used in this study. Therefore, the catalyst effectiveness factors (η) with different particle sizes (20-40 and 60-80 mesh) can be calculated based on the following equation 21:

k apparent = η k intrinsic

(7)

For spheres or crushed catalysts,

η=

where

and

3 1  Φ  tanh Φ

Φ = LP LP =

1  3( Φ coth Φ − 1) = Φ Φ2

(8)

k intrinsic De

(9)

VP SP

(10) 15



 Lp  D e = k intrinsic    Φ 

2

(11)

where kapparent is the apparent reaction rate constant, kintrinsic the intrinsic reaction rate constant, Φ the thiele modulus, Lp the feature size of the catalyst, Vp the volume of the catalyst, and Sp the catalyst surface area. The values of kintrinsic and kapparent can be calculated based on the data obtained from DBT and 4,6-DMDBT HDS conversions on different catalysts (20-40 and 60-80 mesh) with different crystal forms. Substituting the data into the previous equation, the η and De values of the DBT and 4,6-DMDBT HDS reaction systems can also be obtained. Thus, η and De values of the DBT and 4,6-DMDBT HDS on NiMo/Al2O3 with different crystal forms under different reaction conditions are discussed systematically below.

3.5 Influence of reaction conditions on η and De 3.5.1. WHSVs influence on η and De 80

90

(B)

(A) NiMo/δ-30 η, %

80 70

NiMo/γ-30

NiMo/δ-30

70

η, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 32

NiMo/θ-30

60

NiMo/θ-30

50 NiMo/γ-30 40

60

30 20

40

60 80 100 120 140 160 WHSV, h-1

20 40 60 80 100 120 140 160 WHSV, h-1

Figure 9. Influence of WHSV on η over NiMo/Al2O3 catalysts supported on the series Al2O3 supports with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

16


80 (A) NiMo/δ-30 60 NiMo/θ-30 40 20

NiMo/γ-30

18.0 (B) 15.0 12.0 NiMo/δ-30

De× × 1010, m2·S-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


De× × 1010, m2·S-1

Page 17 of 32

9.0 6.0 3.0

NiMo/γ-30

NiMo/θ-30

0.0

0 20 40 60 80 100 120 140 160 WHSV, h-1

20 40 60 80 100 120 140 160 WHSV, h-1

Figure 10. Influence of WHSV on De over NiMo/Al2O3 catalysts supported on the series Al2O3 supports with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

Two typical feeds, DBT (with sulfur contents of 500 ppm) and 4,6-DMDBT (with sulfur contents of 500 ppm), are chosen as probe molecules to study the influence of different reaction conditions on η and De under NiMo/Al2O3 series catalysts. The results of DBT and 4,6-DMDBT HDS on NiMo/Al2O3 catalysts (20-40 and 60-80 mesh) obtained at different WHSVs (340 oC, 4.0 MPa, and H2/Oil ratio of 200) are discussed in the §3.3.4 section. And the influence of different WHSVs on η and De under the DBT and 4,6-DMDBT HDS reactions on NiMo/Al2O3 catalysts with different crystal forms are shown in Figure 9 and 10. The η and De of DBT and 4,6-DMDBT HDS reaction increase with the increase of WHSVs over the three catalysts with different crystal forms. The η and De values all follow the order of NiMo/δ-30 > NiMo/θ-30 > NiMo/γ-30 at various WHSVs, indicating that the Ni-Mo/δ-30 catalyst imposes a less diffusion resistance on the reactant molecules than the other two catalysts. This is because the NiMo/δ-30 (19.2 nm, 126.3 m2 g-1, 0.48 cm3 g-1) support not only has larger pore size than NiMo/γ-30 (6.3 nm), but also possesses the higher BET surface area and pore volume than NiMo/θ-30 (118.6 m2 g-1, 0.46 cm3 g-1), which are beneficial to the diffusion of reactants. The η and De 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

values of DBT HDS reaction are higher than those of 4,6-DMDBT, indicating that the small reactant molecules of DBT suffer less restrictions than 4,6-DMDBT with large molecule sizes in HDS reaction. Moreover, η and De increase with the increase of WHSVs, indicating that the catalysts show lower intermolecular diffusion hindrance at higher flow velocity of reactant molecules.

3.5.2. Influence of H2/oil ratio on η and De The HDS reaction results of DBT and 4,6-DMDBT on NiMo/Al2O3 catalysts (20-40 and 60-80 mesh) obtained at different H2/oil ratios (340 oC, 4.0 MPa, and DBT @ 150 h-1, 4,6-DMDBT @ 80 h-1) are shown in Figure S2 and S3 (in supporting information). The DBT and 4,6-DMDBT HDS efficiencies on NiMo/Al2O3 catalysts (20-40 and 60-80 mesh) with different crystal forms increase with the increase of H2/oil ratios, indicating that the HDS reaction rate of reactant molecules is promoted by the higher hydrogen concentration in the reaction system.

18


Page 18 of 32

Page 19 of 32

NiMo/δ-30

90

90

(A)

NiMo/δ-30

80 70

η, %

η, %

80 NiMo/θ-30

(B)

70 NiMo/γ-30

60

NiMo/θ-30

50 60 50

NiMo/γ-30

40 30

50

100

150 200 250 H2/Oil, v/v

300

50

100

150 200 250 H2/Oil, v/v

300

Figure 11. Influence of H2/Oil on η over NiMo/Al2O3 catalysts supported on the series Al2O3 supports with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT 120

(A)

30

NiMo/δ-30

25 De× × 1010, m2·S-1

100 De× × 1010, m2·S-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

80

NiMo/δ-30

15

60 40

(B)

10


20

5


0

0 50

100

150 200 250 H2/Oil, v/v

300

50

100

150 200 250 H2/Oil, v/v

300

Figure 12. Influence of H2/Oil on De over NiMo/Al2O3 catalysts supported on the series Al2O3 supports with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

The influence of H2/oil ratio on η and De under the DBT and 4,6-DMDBT HDS reaction on NiMo/Al2O3 series catalysts is studied at the conditions of 340 oC, 4.0 MPa, DBT @ 150 h-1, 4,6-DMDBT @ 80 h-1 and different H2/oil ratios of 50-300 (v/v). As shown in Figure 11 and 12, η values of DBT and 4,6-DMDBT HDS reaction decrease with the increase of H2/oil ratio. The hydrogen partial pressure increases with the increase of H2/oil ratio when the total pressure remains 4.0 MPa. The hydrogen partial pressure has a greater influence on the intrinsic reaction rate than 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

that of the apparent reaction rate. However, the De values of DBT and 4,6-DMDBT HDS reaction increase with the increase of H2/oil ratio, indicating that higher hydrogen partial pressures can accelerate the diffusion behavior of the reactant molecules in the catalyst pore channels 31, 32.

3.5.3. Influence of reaction temperature on η and De The DBT and 4,6-DMDBT HDS conversions on NiMo/Al2O3 catalysts (20-40 and 60-80 mesh) are evaluated at the reaction conditions of 4.0 MPa, DBT @ 150 h-1, 4,6-DMDBT @ 80 h-1, H2/Oil ratio of 200 and different reaction temperatures of 320-380 oC. As shown in Figure S4 and S5 (in supporting information), the DBT and 4,6-DMDBT HDS efficiencies on NiMo/Al2O3 catalysts (20-40 and 60-80 mesh) with different crystal forms increase with the increase of the reaction temperature. The number of the activated molecules increases with the increase of the reaction temperature, resulting in the improvement of DBT and 4,6-DMDBT HDS reaction rates. The influence of reaction temperature on η and De of the DBT and 4,6-DMDBT HDS reaction on NiMo/Al2O3 series catalysts is exhibited in Figure 13 and 14. η values of DBT and 4,6-DMDBT HDS reaction decrease with the increase of the reaction temperature, demonstrating that there exists a great influence on intrinsic reaction rate achieved over the apparent reaction rate at higher temperature. However, De values of DBT and 4,6-DMDBT HDS reaction increase with the increase of the

reaction temperature. The movement of the reactant molecules increases with the

20


Page 20 of 32

Page 21 of 32

increasing temperature, which can intensify the mass transfer and diffusivity in the pore channel of the catalysts.

1.0 0.9

(A)

0.9

0.8

0.8

0.7

η, %

η, %

NiMo/δ-30

NiMo/θ-30

0.7

0.6

(B)

NiMo/δ-30

NiMo/γ-30 NiMo/θ-30

0.5 0.6

0.4

NiMo/γ-30

0.5

0.3 320

340 360 Temperature, oC

320

380


380

Figure 13. Influence of reaction temperature on η over NiMo/Al2O3 catalysts supported on the series Al2O3 supports with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

(A)

25

NiMo/δ-30

100

De× × 1010, m2·S-1

120 De× × 1010, m2·S-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40


20

(B)

NiMo/δ-30

20 15

NiMo/θ-30

10

NiMo/γ-30

5 0

0 320


320

380


380

Figure 14. Influence of reaction temperature on De over NiMo/Al2O3 catalysts supported on the series Al2O3 supports with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

3.5.4. Influence of reaction pressure on η and De The DBT and 4,6-DMDBT HDS activities are evaluated at 340 oC, DBT @ 150 h-1, 4,6-DMDBT @ 80 h-1, H2/Oil ratio of 200, and different reaction pressures of 3.0-6.0 MPa. As shown in Figure S6 and S7 (in supporting information), the DBT and

21



4,6-DMDBT HDS efficiencies on NiMo/Al2O3 catalysts (20-40 and 60-80 mesh) increase slightly with the increase of the reaction pressure. 90 (A)

(B)

NiMo/δ-30

80 70 60

NiMo/θ-30

70 60

NiMo/γ-30 NiMo/θ-30

50 40

NiMo/γ-30

30 3.0

NiMo/δ-30

80

η, %

η, %

90

4.0 5.0 Pressure, MPa

6.0

3.0


6.0

Figure 15. Influence of reaction pressure on η over NiMo/Al2O3 catalysts supported on the series Al2O3 supports with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

30

(A)

25

80 60

NiMo/δ-30

40 NiMo/γ-30

NiMo/θ-30

20

De× × 1010, m2·S-1

100

De× × 1010, m2·S-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

(B)

NiMo/δ-30

20 15 10

NiMo/γ-30

5

NiMo/θ-30

0

0 3.0


6.0

3.0


6.0

Figure 16. Influence of reaction pressure on De over NiMo/Al2O3 catalysts supported on the series Al2O3 supports with different crystal forms. (A) HDS of DBT; (B) HDS of 4,6-DMDBT

The influence of the reaction pressure on η and De under the DBT and 4,6-DMDBT HDS reaction on NiMo/Al2O3 series catalysts is exhibited in Figure 15 and 16. η values decrease with the increase of the reaction pressure, manifesting that the intrinsic reaction rate are affected more than the apparent reaction rate at higher temperature. Moreover, the concentration of hydrogen on catalyst surface increases at 22


Page 23 of 32

higher pressure, which results in an major increasing of the intrinsic reaction rate in the catalyst channel. However, De values increase with the increase of the HDS reaction pressure, since the reactants concentration and hydrogen solubility increases with the increase of the reaction pressure, which can reduce the viscosity of the feeds and the products.31, 32 The diffusion resistance of the reactant and product molecules in the pore channels decreases with the reduction of the viscosity.

3.5.5. Influence of molecular sizes on η and De

80

NiMo/δ-30 (A)

70 60

NiMo/θ-30

50 40 30

NiMo/γ-30

De× ×1010,m2·S-1

90

η, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5 0.6 0.7 0.8 0.9 1.0 1.1 Molecular diameter, nm

70 60 50 40 30 20 10 0

NiMo/δ-30

(B)

NiMo/θ-30 NiMo/γ-30 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Molecular diameter, nm

Figure 17. Influence of molecular diameter on η (A) and De (B) over NiMo/Al2O3 catalysts supported on the series Al2O3 supports with different crystal forms

The effect of different molecular sizes (T, BT, DBT, and 4,6-DMDBT) on the η and De over NiMo/Al2O3 catalysts with different crystal forms is investigated under the reaction conditions of 340 oC, 4.0 MPa, 80 h-1, and the H2/oil ratio of 200. The molecular structures and molecular sizes of different reactant molecules as listed in Table S2 (in supporting information). As shown in Figure 17, η values of HDS reaction decrease with the increase of the molecular sizes. De values also show an

23



apparent decline tendency with the increase of the molecular sizes, indicating that the diffusion resistance becomes larger with the increase of the reactant molecule sizes.

3.6 Restrictive factor F(λ) Many researchers

33, 34

studied the restrictive diffusion processes in order to

obtain the relationship between the effective diffusivity and the ratio of the molecular size to pore size (λ). A general formula was proposed and widely used as shown below 35.

F (λ ) =

Deτ = (1 − λ ) m Dbε p

(12)

where m is the hindered magnitude of diffusion, εp the porosity of catalyst, λ the ratio of the molecular size to pore size, τ the tortuosity factor of catalyst, and Db the bulk diffusivity, which is determind from the Stokes- Einstein equation 36:

Db =

KT 6πµ r

(13)

where K is the Boltzman constant (1.38×1023 J/K), µ the solvent viscosity (Pa·s).

Ln(De/Db)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

-1.2 -1.3 -1.4 -1.5 -1.6 -1.7 -1.8 -1.9 -2.0

y=2.23x-1.10 NiMo/δ-30 y=2.83x-1.12 NiMo/θ-30 y=3.42x-1.32 NiMo/γ-30 -0.18 -0.16 -0.14 -0.12 -0.10 -0.08 Ln(1-λ)

Figure 18. Restrictive factors as a function of the size/diameter ratio of reactant molecules to catalyst pore. 24


Page 25 of 32

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The relationships between De/Db and λ are plotted to the ln[De/Db]-ln[1-λ] diagram, as shown in Figure 18. The linear fitting curves were also given for NiMo/Al2O3 catalysts with different crystal forms. Thus the restrictive factor of F(λ) can be described as follows: For NiMo/γ-30 catalyst: F(λ)=(1-λ)3.42; For NiMo/δ-30 catalyst: F(λ)=(1-λ)2.23; For NiMo/θ-30 catalyst: F(λ)=(1-λ)2.83. The hindered magnitude of diffusion for the HDS reaction on NiMo/Al2O3 catalysts with different crystal forms decrease in the following order: NiMo/δ-30 (2.23) < NiMo/θ-30 (2.83) < NiMo/γ-30 (3.42), indicating that the diffusion restriction effect of the reactant molecules on NiMo/δ-30 catalyst is weaker than those of the other two catalysts. The relatively lower hindered magnitude of diffusion on NiMo/δ-30 catalyst ensures the reactant and product molecules can diffuse in and out of the mesochannels easily and have higher conversion.

4. CONCLUSION Al2O3 materials with different crystal forms was synthesized via the hydrothermal synthesis method using the low-cost raw material. The synthesized Al2O3 series materials exhibit the pore diameters with a wide range from 6.5 to 21.5 nm. The influence of different reaction conditions on η and De values of DBT and 4,6-DMDBT molecules on HDS reactions over NiMo/Al2O3 series catalysts was studied systematically for the first time. The η and De of DBT and 4,6-DMDBT molecules in HDS reactions over NiMo/Al2O3 catalysts with different crystal forms increase in the order of NiMo/δ-30 > NiMo/θ-30 > NiMo/γ-30. Furthermore, η and De 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

values of different reactant molecules (including T, BT, DBT, and 4,6-DMDBT with different molecular sizes) over NiMo/Al2O3 catalyst in the HDS reaction decrease with the increase of the reactant molecular sizes. The correlations of the restrictive factors were also investigated and the hindered magnitude of diffusion decrease in the order of NiMo/δ-30 (2.23) < NiMo/θ-30 (2.83) < NiMo/γ-30 (3.42), indicating NiMo/δ-30 showed less restrictive diffusion resistance due to its relatively open pore channel, concentrated pore distribution and relatively large surface area. The information of the restrictive diffusion effect on NiMo/Al2O3 catalysts with different crystal forms provided the theoretical guidance for the design of hydrotreaters and the synthesis of the superior catalysts.

ASSOCIATED CONTENT Supporting Information. Pyridine-FTIR spectra adsorbed on different catalysts (Figure S1), HDS activities of NiMo/Al2O3 series catalysts at different reaction conditions (Figure S2-S7), the acid strength distribution and the acid quantities of NiMo/Al2O3 series catalysts (Table S1) and molecular structures and sizes of different model sulfides (Table S2).

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No.21676298, U1463207 and 21503152), CNOOC project (CNOOC-KJ 135 FZDXM 00 LH 003 LH-2016), Opening Project of Guangxi Key Laboratory of

26


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Petrochemical Resource Processing and Process Intensification Technology (2015K003), CNPC Key Research Project and KLGCP (GCP201401).

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NiMo/γ-Al2O3 catalysts for the hydrodesulfurization of DBT and 4, 6-DMDBT. Fuel Process. Technol. 2017, 161, 52-61.

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Al2O3

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