Al2O3

Article pubs.acs.org/IECR

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* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 18 Fuxue Road, Beijing 102249, People’s Republic of China S Supporting Information *

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 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 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.

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 ultralow sulfur transportation fuels, highly refractory sulfur-containing compounds such as dibenzothiophene (DBT) and 4,6dimethyldibenzothiophene (4,6-DMDBT) shall be eliminated in the fuel.6−9 Therefore, it is necessary to develop novel and more effective hydrodesulfurization (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 ultradeep 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 published papers and the DBT and 4,6-DMDBT © 2017 American Chemical Society

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 NiMoS2/γ-Al2O3 catalysts by tuning different aging temperatures and Ni-MoS2/γ-Al2O3 catalyst at the aging temperatures of 90 °C 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 Received: Revised: Accepted: Published: 10018

July 14, 2017 August 16, 2017 August 23, 2017 August 23, 2017 DOI: 10.1021/acs.iecr.7b02897 Ind. Eng. Chem. Res. 2017, 56, 10018−10027

Article

Industrial & Engineering Chemistry Research

adsorption−desorption isotherm measurements, the samples were outgassed 4 h at 340 °C in a vacuum. 2.3. Evaluation of HDS Performance of Catalysts. 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 °C, 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 °C, 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.

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 intraparticle 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 intraparticle 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 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.

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

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

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). 3.2. N2 Adsorption−Desorption Pattern of the Catalysts. To further understand the structural features of assynthesized NiMo/Al2O3 series catalysts, N2 adsorption− desorption pattern is conducted, as shown in Figure 2A,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 2A). 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 2B). 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). 3.3. HDS Reaction Order of DBT and 4,6-DMDBT. 3.3.1. Removal of External Diffusion. External diffusion has the

2. EXPERIMENTAL SECTION 2.1. Synthesis of Alumina and the Corresponding NiMo Catalysts. 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 °C. Then, three mesoporous Al2O3 materials with different crystal forms were prepared at different calcination temperature from the boehmite sol, denoted as γ-30 (550 °C, 2 °C/min), δ-30 (900 °C, 2 °C/min) and θ-30 (1000 °C, 2 °C/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 performed at −196 °C with an automated gas adsorption− desorption analyzer (Quantachrome Autosorb-iQ, USA). Before 10019

DOI: 10.1021/acs.iecr.7b02897 Ind. Eng. Chem. Res. 2017, 56, 10018−10027

Article


Mears criterion, indicating that the effect of the external diffusion can be neglected in the experiments. 3.3.2. Elimination of Internal Diffusion. 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

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 (m2 g−1)

Vmes (cm3 g−1)

Vmic (cm3 g−1)

dBJH (nm)

NiMo/γ-30 NiMo/δ-30 NiMo/θ-30

163.8 126.3 118.6

0.39 0.48 0.46

0.005 0.004 0.004

6.3 19.2 20.5

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 −rapparentρb Rn Mears criterion: < 01.15 kcCAb (1)

Table 2. Properties of Catalysts with Different Particle Sizes

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 coefficient26 and CAb the bulk concentration (kmol·m−3). D where kc = AB Sh dp (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 Sc

(3)

where Re =

duρ μ

(4)

and Sc =

ν DAB

average pore size (nm)

range of particle size (mm)

NiMo/γ-30 (20−40 mesh) NiMo/γ-30 (40−60 mesh) NiMo/γ-30 (60−80 mesh) NiMo/γ-30 (80−100 mesh) NiMo/δ-30 (20−40 mesh) NiMo/δ-30 (40−60 mesh) NiMo/δ-30 (60−80 mesh) NiMo/δ-30 (80−100 mesh) NiMo/θ-30 (20−40 mesh) NiMo/θ-30 (40−60 mesh) NiMo/θ-30 (60−80 mesh) NiMo/θ-30 (80−100 mesh)

776 778 781 785 695 699 702 706 682 684 687 691

6.3 6.3 6.4 6.5 19.0 19.1 19.2 19.3 20.4 20.4 20.5 20.6

0.74−0.37 0.37−0.25 0.25−0.19 0.19−0.16 0.74−0.37 0.37−0.25 0.25−0.19 0.19−0.16 0.74−0.37 0.37−0.25 0.25−0.19 0.19−0.16

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 °C, 4.0 MPa, DBT @ 150 h−1, 4,6-DMDBT @ 80 h−1 and H2/oil ratios of 200 (v/v). 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

(5) 2

catalysts

bulk density (kg m−3)

−1

where DAB is the diffusion coefficient (m s ), 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

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. 10020


Article


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

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.

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.

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. 3.3.4. Determination of HDS Reaction Order. 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 °C, 4.0 MPa, and H2/Oil ratio of 200) are exhibited in Figures 5 and 6. The HDS efficiencies of DBT and 4,6-DMDBT increase sharply with the decreases of WHSVs over the three catalysts with

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 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 10021


Article


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.

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

40 and 60−80 mesh) can be calculated based on the following equation:21

different particle sizes. The DBT and 4,6-DMDBT HDS conversions follow the order of Ni−Mo/δ-30 > Ni−Mo/θ-30 > Ni−Mo/γ-30 at all WHSVs. The 4,6-DMDBT HDS efficiencies were lower than those of DBT. Many published papers23,29,30 have proposed that HDS reaction of a single reactant molecule follows pseudo-firstorder reaction model. The pseudo-first-order HDS reaction kinetic equation can be expressed as ln

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

kapparent = ηk instrinsic

(7)

For spheres or crushed catalysts, η=

3(Φcoth Φ − 1) 3 ⎛⎜ 1 1⎞ − ⎟= ⎝ ⎠ Φ tan Φ Φ Φ2

where Φ = L P (6)

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

and L P =

k instrinsic De

VP SP

⎛ Lp ⎞ De = k instrinsic⎜ ⎟2 ⎝Φ⎠

(8)

(9)

(10)

(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. Two typical feeds, DBT (with sulfur contents of 500 ppm) and 4,6-DMDBT (with sulfur 10022


Article


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.

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.

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.

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 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 4,6DMDBT 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 °C, 4.0 MPa and DBT @ 150 h−1, 4,6-DMDBT @ 80 h−1) are shown in Figures 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,

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 4,6DMDBT HDS on NiMo/Al2O3 catalysts (20−40 and 60−80 mesh) obtained at different WHSVs (340 °C, 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 Figures 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 10023


Article


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.

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.

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.

indicating that the HDS reaction rate of reactant molecules is promoted by the higher hydrogen concentration in the reaction system. 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 °C, 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 Figures 11 and 12, η values of DBT and 4,6DMDBT 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 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 °C. As shown in Figures 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 Figures 13 and 14. η values of DBT and 4,6DMDBT 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 4,6DMDBT HDS reaction increase with the increase of the reaction temperature. The movement of the reactant molecules increases 10024


Article


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.

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.

with the increasing temperature, which can intensify the mass transfer and diffusivity in the pore channel of the catalysts. 3.5.4. Influence of Reaction Pressure on η and De. The DBT and 4,6-DMDBT HDS activities are evaluated at 340 °C, 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 Figures S6 and S7 (in Supporting Information), the DBT and 4,6-DMDBT HDS efficiencies on NiMo/Al2O3 catalysts (20−40 and 60−80 mesh) increase slightly with the increase of the reaction pressure. 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 Figures 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 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. 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 °C, 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 apparent decline tendency with the increase of the molecular sizes, indicating that the diffusion

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.

resistance becomes larger with the increase of the reactant molecule sizes. 3.6. Restrictive Factor F(λ). Many researchers33,34 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 Dτ F(λ) = e = (1 − x)m D bεp (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 determined from the Stokes− Einstein equation:36 Db =

KT 6πμr

(13)

where K is the Boltzmann constant (1.38 × 10 J/K), μ the solvent viscosity (Pa·s). 23

10025



■

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

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b02897. Pyridine-FTIR spectra adsorbed on different catalysts, HDS activities of NiMo/Al2O3 series catalysts at different reaction conditions, the acid strength distribution and the acid quantities of NiMo/Al2O3 series catalysts and molecular structures and sizes of different model sulfides (PDF)

■

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

AUTHOR INFORMATION

Corresponding Authors

*A. Duan. E-mail: [email protected]. Tel.: +86 10 89732290. *C. Xu. E-mail: [email protected]. Tel.: +86 10 89733392. ORCID

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:

Aijun Duan: 0000-0001-5964-7544 Notes

The authors declare no competing financial interest.

■

3.42

For NiMo/γ‐30 catalyst: F(λ) = (1 − λ)

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

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.

■

REFERENCES

(1) Liu, H.; Bao, S.; Cai, Z.; Xu, T.; Li, N.; Wang, L.; Chen, H.; Lu, W.; Chen, W. A novel methodfor ultra-deep desulfurization of liquid fuels at room temperature. Chem. Eng. J. 2017, 317, 1092−1098. (2) Ganiyu, S.; Alhooshani, K.; Sulaiman, K.; Qamaruddin, M.; Bakare, I.; Tanimu, A.; Saleh, T. Influence of aluminium impregnation on activated carbon for enhanced desulfurization of DBT at ambient temperature: Role of surface acidity and textural properties. Chem. Eng. J. 2016, 303, 489−500. (3) Wang, X.; Fang, H.; Zhao, Z.; Duan, A.; Xu, C.; Chen, Z.; Zhang, M.; Du, P.; Song, S.; Zheng, P.; Chi, K. Effect of promoters on the HDS activity of alumina-supported Co-Mo sulfide catalysts. RSC Adv. 2015, 5, 99706−99711. (4) Vatutina, Y.; Klimov, O.; Nadeina, K.; Danilova, I.; Gerasimov, E.; Prosvirin, I.; Noskov, A. Influence of boron addition to alumina support by kneading on morphology and activity of HDS catalysts. Appl. Catal., B 2016, 199, 23−32. (5) Chen, W.; Nie, H.; Li, D.; Long, X.; van Gestel, J.; Maugé, F. Effect of Mg addition on the structure and performance of sulfide Mo/Al2O3 in HDS and HDN reaction. J. Catal. 2016, 344, 420−433. (6) Sánchez-Minero, F.; Ramírez, J.; Cuevas-Garcia, R.; GutierrezAlejandre, A.; Fernández-Vargas, C. Kinetic Study of the HDS of 4,6DMDBT over NiMo/Al2O3-SiO2(x) Catalysts. Ind. Eng. Chem. Res. 2009, 48, 1178−1185. (7) Ho, T.; McConnachie, J. Ultra-deep hydrodesulfurization on MoS2 and Co0.1MoS2: Intrinsic environmental factors. J. Catal. 2011, 277, 117−122. (8) Wang, X.; Du, P.; Chi, K.; Duan, A.; Xu, C.; Zhao, Z.; Chen, Z.; Zhang, H. Synthesis of NiMo catalysts supported on mesoporous silica FDU-12 with different morphologies and their catalytic performance of DBT HDS. Catal. Today 2017, 291, 146−152. (9) Liu, K.; Ng, F. Effect of the nitrogen heterocyclic compounds on hydrodesulfurization using in situ hydrogen and a dispersed Mo catalyst. Catal. Today 2010, 149, 28−34.

4. CONCLUSION Al2O3 materials with different crystal forms were 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 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 decreases 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 on 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. 10026


Article


(30) Sun, Y.; Prins, R. Mechanistic studies and kinetics of the hydrodesulfurization of dibenzothiophene on CoMoS2/γ-Al2O3. J. Catal. 2009, 267, 193−201. (31) Gao, W.; Robinson, R.; Gasem, K. Solubilities of Hydrogen in Hexane and of Carbon Monoxide in Cyclohexane at Temperatures from 344.3 to 410.9 K and Pressures to 15 MPa. J. Chem. Eng. Data 2001, 46, 609−612. (32) Park, J.; Robinson, R.; Gasem, K. Solubilities of hydrogen in heavy normal paraffins at temperatures from 323.2 to 423.2 K and pressures to 17.4 MPa. J. Chem. Eng. Data 1995, 40, 241−244. (33) Tsai, M.; Chen, Y.; Li, C. Restrictive diffusion under hydrotreating reactions of heavy residue oils in a trickle bed reactor. Ind. Eng. Chem. Res. 1993, 32, 1603−1609. (34) Kathawalla, I.; Anderson, J. Pore size effects on diffusion of polystyrene in dilute solution. Ind. Eng. Chem. Res. 1988, 27, 866−871. (35) Lee, S.; Seader, J.; Tsai, C.; Massoth, F. Solvent and temperature effects on restrictive diffusion under reaction conditions. Ind. Eng. Chem. Res. 1991, 30, 607−613. (36) Li, C.; Chen, Y.; Tsai, M. Highly restrictive diffusion under hydrotreating reactions of heavy residue oils. Ind. Eng. Chem. Res. 1995, 34, 898−905.

(10) La Vopa, V.; Satterfield, C. Poisoning of thiophene hydrodesulfurization by nitrogen compounds. J. Catal. 1988, 110, 375−387. (11) Stanislaus, A.; Marafi, A.; Rana, M. Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production. Catal. Today 2010, 153, 1−68. (12) Prins, R.; Bussell, M. Metal phosphides: preparation, characterization and catalytic reactivity. Catal. Lett. 2012, 142, 1413−1436. (13) Li, X.; Chai, Y.; Liu, B.; Liu, H.; Li, J.; Zhao, R.; Liu, C. Hydrodesulfurization of 4, 6-dimethyldibenzothiophene over CoMo catalysts supported on γ-alumina with different morphology. Ind. Eng. Chem. Res. 2014, 53, 9665−9673. (14) Nikulshin, P.; Mozhaev, A.; Maslakov, K.; Pimerzin, A.; Kogan, V. Genesis of HDT catalysts prepared with the use of Co2Mo10 HPA and cobalt citrate: study of their gas and liquid phase sulfidation. Appl. Catal., B 2014, 158, 161−174. (15) Laurenti, D.; Phung-Ngoc, B.; Roukoss, C.; Devers, E.; Marchand, K.; Massin, L.; Lemaitre, L.; Legens, C.; Quoineaud, A.; Vrinat, M. Intrinsic potential of alumina-supported CoMo catalysts in HDS: Comparison between γc, γT, and δ-alumina. J. Catal. 2013, 297, 165− 175. (16) Fan, Y.; Xiao, H.; Shi, G.; Liu, H.; Qian, Y.; Wang, T.; Gong, G.; Bao, X. Citric acid-assisted hydrothermal method for preparing NiW/ USY-Al2O3 ultra deep hydrodesulfurization catalysts. J. Catal. 2011, 279, 27−35. (17) Zhang, D.; Duan, A.; Zhao, Z.; Xu, C. Synthesis, characterization, and catalytic performance of NiMo catalysts supported on hierarchically porous Beta-KIT-6 material in the hydrodesulfurization of dibenzothiophene. J. Catal. 2010, 274, 273−286. (18) Wang, X.; Zhao, Z.; Zheng, P.; Chen, Z.; Duan, A.; Xu, C.; Jiao, J.; Zhang, H.; Cao, Z.; Ge, B. Synthesis of NiMo catalysts supported on mesoporous Al2O3 with different crystal forms and superior catalytic performance for the hydrodesulfurization of dibenzothiophene and 4,6dimethyldibenzothiophene. J. Catal. 2016, 344, 680−691. (19) Zhang, M.; Fan, J.; Chi, K.; Duan, A.; Zhao, Z.; Meng, X.; Zhang, H. Synthesis, characterization, and catalytic performance of NiMo catalysts supported on different crystal alumina materials in the hydrodesulfurization of diesel. Fuel Process. Technol. 2017, 156, 446− 453. (20) Wang, X.; Zhao, Z.; Chen, Z.; Li, J.; Duan, A.; Xu, C.; Gao, D.; Cao, Z.; Zheng, P.; Fan, J. Effect of synthesis temperature on structureactivity-relationship over NiMo/γ-Al2O3 catalysts for the hydrodesulfurization of DBT and 4, 6-DMDBT. Fuel Process. Technol. 2017, 161, 52−61. (21) Chen, J.; Yang, H.; Ring, Z. Study of intra-particle diffusion effect on hydrodesulphurization of dibenzothiophenic compounds. Catal. Today 2005, 109, 93−98. (22) Chen, J.; Ring, Z. HDS reactivities of dibenzothiophenic compounds in a LC-finer LGO and H2S/NH3 inhibition effect. Fuel 2004, 83, 305−313. (23) Li, C.; Chen, Y.; Tsai, M. Highly restrictive diffusion under hydrotreating reactions of heavy residue oils. Ind. Eng. Chem. Res. 1995, 34, 898−905. (24) Kabe, T.; Akamatsu, K.; Ishihara, A.; Otsuki, S.; Godo, M.; Zhang, Q.; Qian, W. Deep hydrodesulfurization of light gas oil. 1. kinetics and mechanisms of dibenzothiophene hydrodesulfurization. Ind. Eng. Chem. Res. 1997, 36, 5146−5152. (25) Mears, D. Tests for transport limitations in experimental catalytic reactors, Ind. Eng. Chem. Ind. Eng. Chem. Process Des. Dev. 1971, 10, 541−547. (26) Thoenes, J.; Kramers, H. Mass transfer from spheres in various regular packings to a flowing fluid. Chem. Eng. Sci. 1958, 8, 271−276. (27) Deen, W. Hindered transport of large molecules in liquid-filled pores. AIChE J. 1987, 33, 1409−1425. (28) Doraiswamy, L.; Tajbl, D. Laboratory catalytic reactors. Catal. Rev.: Sci. Eng. 1974, 10, 177−219. (29) Texier, S.; Berhault, G.; Pérot, G.; Harlé, V.; Diehl, F. Activation of alumina-supported hydrotreating catalysts by organosulfides: comparison with H2S and effect of different solvents. J. Catal. 2004, 223, 404− 418. 10027


Al2O3

Recommend Documents