Kinetic and Mechanistic Analysis of Dibenzothiophene

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Kinetic and Mechanistic Analysis of Dibenzothiophene Hydrodesulfurization on a Ti-SBA-15-NiMo Catalysts Almaz S. Jalilov, Abdulkadir Tanimu, Saheed Adewale Ganiyu, and Khalid Alhooshani Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02808 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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Kinetic and Mechanistic Analysis of Dibenzothiophene Hydrodesulfurization on a TiSBA-15-NiMo Catalysts Almaz S. Jalilov,† ,‡,* Abdulkadir Tanimu,† Saheed A Ganiyu,† and Khalid Alhooshani† †

Department of Chemistry and ‡Center for Integrative Petroleum Research, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia 31261 *Email: [email protected]

Graphical abstract

Abstract MoS2-based catalysts are the most commonly used systems for the hydrodesulfurization (HDS) process on an industrial scale. Different methods were employed to enhance the efficiency of the

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catalysts, which include changes in catalyst support and doping agents, as well as tuning the dispersion of MoS2 phase by using various synthetic techniques. In this work, we report the kinetic and mechanistic analysis of HDS of dibenzothiophene (DBT) for different NiMosupported Ti-SBA-15 catalysts prepared by (1) direct single-pot (SP) synthesis and (2) impregnations of NiMo phase into the Ti-SBA-15 support. Both methods were also repeated with and without the citric acid (CA) loading to achieve higher dispersion of the active metal sites. Kinetic modeling was performed to two “parallel-series” reactions of DBT, direct desulfurization (DDS) and hydrogenation (HYD), both leading to the final product of cyclohexylbenzene. Kinetic analysis reveals the considerable influence of the catalysts preparation methods to both steps of the HDS of DBT. We found that apparent rate constants for DDS are highly dependent on the employed synthetic method, while, the effect of CA has a high contribution to the apparent rate constants for the conversion of partially hydrogenated dibenzothiophene intermediate, .

Keywords:

hydrodesulfurization;

Ti-SBA-15-NiMo

catalyst;

kinetic

modeling;

dibenzothiophene; citric acid

Introduction Among various refinery processes, the hydrodesulfurization (HDS) is one of the largest in downstream petroleum industry.1 Reducing the sulfur content particularly from the heavy oil to ultra-low levels is challenging and environmentally important.2 Due to the strict regulations, the development and enhancing the performances of the HDS catalysts is crucial.3 Moreover, the


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complexity and the structural diversity of the sulfur content in the pertoleum oil has significant impact on the catalyst development.4 In general, sulfur exist as alkyl derivatives of thiophene, benzothiophene and dibenzothiophene (DBT), where DBT and its derivatives are known to be the most potent sulfur containing species to the HDS.5-8 Therefore, tuning the catalyst properties based on the HDS studies of the DBT and its derivatives is the common practice in catalyst development.9-11 Cobalt and nickel doped MoS2-based catalysts are the most efficient and commonly used catalyst in industry.12 Despite the several decade long studies of the catalytic activities of MoS2-based catalysts, the nature and the mechanistic details are still the subject of the vigorous debate.13 However, it is generally accepted in the literature that both HDS performances of the Co and Ni impregnated MoS2-based catalysts, on a thiophene derivatives, proceed through two competing pathways, the direct desulfurization (DDS) and the stepwise hydrogenation of the aromatic core followed by the subsequent sulfur removal (HYD).1,14,15 Various synthetic strategies have been developed in order to enhance the performances of the MoS2-based catalysts, which include tuning the porous catalyst support structures that exhibit higher Mo dispersions,16 doping of catalyst with heteroatoms that alter the acidity,17 as well as the enhancing of the metal support interactions.18 To study the mechanistic details of the catalytic HDS processes different spectroscopic, microscopic and kinetic studies have been employed.19-21 Recent work by Morales-Valencia et al.22 have shown that extensive kinetic modelling using Langmuir-Hinshelwood-Hougen-Watson (LHHW) formalism can result in accurate kinetic and thermodynamic parameters of the DDS and HYD pathways from the molecular level. In this work, we report the kinetic and mechanistic analysis of the HDS of DBT on a recently reported NiMo-supported Ti-SBA-15 catalysts. These solid catalysts prepared by two


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different methods: single-pot synthesis (SP) and impregnation method (Imp). In comparison to the SP synthesis procedure, during the Imp method the catalysts prepared by the subsequent impregnation of NiMo phase onto the porous support.23 The evaluation of the catalytic activity was done using the kinetic approach based on the LHHW formalism.

Experimental Section Four catalysts were used in this study. Both Ti-SBA-15-NiMo catalysts denoted as TSMN(CA)-SP300 and TSMN-SP300 were prepared by using the direct single-pot synthesis method at 300 °C, with and without the citric acid at Mo:CA ratio of 1:1, respectively.23 The other two Ti-SBA-15-NiMo catalysts denoted as TSMN(CA)-Imp300 and TSMN-Imp300 were prepared by using the impregnation method at 300 °C, with and without the citric acid at Mo:CA ratio of 1:1, respectively. Briefly, TSMN-SP300 was prepared by adding 4.2 g of tetraethylorthosilicate (Sigma-Aldrich) to the stirring solution of 2 g of pluronic P123 in 60 g of 2 M HCl and 15 g of deionized water. To the resultant mixture titanium isopropoxide in 10:1 (Si:Ti) molar ratio was added and stirred for another 20 h. Ammonium molybdate (VI) tetrahydrate in 13 wt% and nickel nitrate hexahydrate in 3 wt% was added to the final mixture and stirred for another 3 h at ambient temperature before transferring the final reaction mixture to Teflon-lined autocalve for hydrothermal treatment at 100 °C for 24 h. The final solid product washed rigorously with DI water and dried at 100 °C before treating at 300 °C in a furnace with the heating rate of 10 °C/min. TSMN(CA)-CP300 was prepared by identical method with only addition of citric acid in ratio of CA:Mo at 1:1 at the same time with the addition of Mo precursor. Details of the synthesis procedure for TSMN-Imp300 and TSMN(CA)-Imp300


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together with the characterization procedures by physisorption and chemisorption, elemental analysis as well as the spectroscopic and microscopic analyses are reported in a recently published work.23,24 EPR Spectroscopy. EPR spectra of Ti-SBA-15-NiMo catalysts were recorded on an Adani EPR Spectrometer SPINSCAN X at room temperature. The following parameters were used: center field 324 mT, sweep width 256 G, microwave frequency 9.5 GHz, microwave power 1 mW, modulation frequency 100 kHz, and modulation amplitude 1.0 G. The solid samples were filled in a capillary tubes and directly used for measurements. Catalyst Evaluation. Before the test of the HDS performances of the catalysts, each catalyst sieved into 300-500 µm sizes, reduced with 5% H2/He at 400 °C for 2 h with the flow rate of 60 ml/min, and presulfidized for 5 hours with aqueous solution of CS2 (2 wt%) in a quartz tube at 350 °C overnight. Catalytic testing was done in a high-pressure batch reactor at 325 °C, 350 °C and 375 °C under the reaction pressure at 5MPa of H2 and stirring rate of 300 rpm. About 0.25 g of catalyst was loaded to 100ml, DBT (1000 ppm-S) solution in dodecane. Reaction products for sulfur and hydrocarbon contents were analyzed in the gas chromatographs with three different detectors sulfur chemiluminiscence detector (SCD), the mass spectrometry (MS) and the flame ionization detector (FID). Kinetic Modelling Based on the experimental observations, the catalytic HDS of DBT for this work involved two parallel-series reaction, DDS pathways and the HYD of aromatic ring pathways as shown in Figure 1. DDS involves the direct desulfurization step to form biphenyl (BP) followed


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by the hydrogenation of one phenyl group to yield the final product of cyclohexylbenzene (CHB). HYD involves the partial hydrogenation of DBT to form tetrahydrodibenzothiophene (4HDBT) followed by the cleavage of C-S bond to yield CHB. Two different reaction pathways are deduced from the existence of the two different active sites for DDS and HYD, often reported as σ and π, respectively.1

Direct desulfurization (DDS) k1 - H2S

S

BP

DBT

k3

k2 k4 - H2S

S

4HDBT

CHB

Hydrogenation (HYD)

Figure 1. Reaction pathways for HDS of DBT. The rate of formation of products and the mass balance equations for each of the four steps are

−

DBT = r1 + r2

(1)

= r1 ‒ r3

(2)

= r1 ‒ r2

(3)

= r3 + r4

(4)


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To estimate the kinetic parameters of each separate steps, the model that was employed by Farag was used to compare the different catalysts.25 The kinetic model can accurately estimate the individual apparent rate constants for different catalysts and their contribution to the two parallel pathways. Based on the assumptions listed below the Langmuir-Hinshelwood kinetic model was applied to fit the experimental data: constant hydrogen concentration across the reactor, existence of the two different active sites for DDS and HYD, there are only three observed products in addition to H2S that exist from the parallel reaction network as shown in figure 1, surface reactions are the only rate-limiting steps and irreversible, and all of the reaction products compete for the active sites with the magnitude of the product of equilibrium constants and the concentrations are being negligible with respect to unity. The last approximation also would imply that the inhibition by the products are small and negligible. Details of the adsorption properties of the reaction components and their impacts on the total HDS of DBT is beyond the scope of this work, therefore the adsorption equilibrium constants will be incorporated in the final apparent rate constants. Using the above assumptions and the mass balance rate equations based on the LangmuirHinshelwood model for disappearance of DBT, formations of BP, 4HDBT and CHB can be expressed as follows:

−

DBT = r1 + r2 = + ⋯ ⋯

= r1 ‒ r3 = ‒ ⋯ ⋯


(5)

(6)

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= r2 ‒ r4 = ‒ ⋯ ⋯

(7)

= r3 + r4 = + ⋯ ⋯

(8)

Where k1, k2, k3 and k4 are the intrinsic rate constants, K1, K2, K3 and K4 are the adsorption equilibrium constants and CDBT, CBP, C4HDBT and CCHB are the reaction concentrations for DBT, BP, 4HDBT and CHB, respectively. Since the denominator is negligible with respect to unity the rate expression can be further simplified by integrating to: "

"

DBT = DBT° (! # )

BP =

4HDBT =

°!" " & (!" #" )

"

(9) "

"

[ (! # ) − & ]

°#" " - (!" #" )

"

"

(10)

"

[ (! #) − - ]

(11)

where CDBT° is the initial concentration of DBT and the apparent rate constants k1K1, k2K2, k3K3 and k4K4 for simplicity are replaced by , , and respectively. The concentrations of species CDBT, CBP, and C4HDBT during the reaction course were calculated using the equations 11-13 and fits with experimental values were estimated using LevenbergMarquardt algorithm for non-linear regression according to Eq. 12: ../ = ∑89:(1231 + 567 )

(12)

where SSE is the sum of the squared errors, Ccalc estimated values and the Cexp is the experimental values for concentrations of species.


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The calculated values for , and can be further used to estimate the concentration of CHB (CCHB) using the integrated form of the equation 8: BP = ( BP + 4HDBT )

(13)

thereby, doubly checking the accuracy of the used kinetic model.

Results and Discussion Catalysts Characterizations. Detailed synthetic procedure and the additional characterization data for the catalyst used in this work has been previously reported.23 Here we report the selected data summarized in Table 1 that have close relevance to the kinetic and mechanistic analysis of the HDS of DBT. Particularly, the surface characteristics of the catalysts show the generally higher surface areas and the pore volumes for SP catalysts. This also reflected on the temperature-programmed desorption (TPD) of NH3 data that reveal the surface acidity. Therefore, the acidity of the catalysts is monodisperse and linearly dependent to the surface area and pore volume. Thus the different preparation methods have minimal effect on altering the surface acidity of the catalysts. However, the temperature-programmed reduction by H2 (TPRH2) shows rather indirect correlation with the structural characteristics. TSMN(CA)-Imp300 shows higher chemisorbed H2 uptake, although the surface area is relatively small. TPR-H2 values is better representation of the active metal centers, hence indicating that complexing agent has a better impact on the dispersion of the active sites. Additionally, increasing the calcination temperature from 300 °C to 550 °C during the preparation of the catalysts decreased the HDS activity.23


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Table 1. Structural characteristics, chemisorption properties and elemental composition of the catalysts. Elemental Composition

SBET (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

NH3 uptake (mmol/g)

H2 uptake (mmol/g)

TSMN-Imp300

146

0.16

0.011

0.158

29.49

2.95

TSMN-SP300

403

0.66

0.014

0.198

44.22

TSMN(CA)-Imp300

160

0.19

0.007

0.138

TSMN(CA)-SP300

352

0.51

0.013

0.235

Catalyst

a

Ni(%)a

Mo(%)a

Ni(%)b

Mo(%)b

S(%)b

12.90

0.06

4.04

3.49

1.63

12.46

0.06

3.96

3.09

44.14

2.96

13.01

0.07

3.40

2.83

43.74

1.39

11.92

0.06

3.30

2.93

from ICP-OES data, b from XPS data.

Surface and structural complexity of the catalysts is also noticeable from the difference in the elemental compositions by the Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) data and the X-ray Photoelectron Spectroscopy (XPS) data. While surface composition is better exhibited by the XPS, ICP-OES data show the composition of the bulk material. Having much smaller Mo and Ni composition on the surface demonstrate the existence of the surface Mo species mainly in the oxidized and sulfided forms on the surface. EPR spectra for the catalysts are shown in Figure 2. EPR signals at g = 1.928 is attributed to Mo(V) species that are formed from Mo(VI) during the reductive sulfidization of the catalysts.26 The EPR signal intensities of Mo(V) species correlate well with the TPR-H2 data, unlike Mo-XPS data that indicates that the majority of the Mo species on the surface of TSMNImp300 are not in a reduced Mo(V) form.


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Figure 2. EPR spectra of catalysts. The EPR signal at g = 1.999 correspond to the sulfur radicals (attached to the Mo(V)), and their intensity correlates with the surface area and the pore volume of the catalysts. Similar to the Mo(V) EPR peak intensities, poor correlation with the XPS data is observed for the EPR peak intensity of the sulfur radicals,27 pointing to the existence of the active (in the form of -S-Mo(V)S-) sites for TSMN-SP300, TSMN(CA)-Imp300 and TSMN(CA)-SP300 in higher content in comparison to TSMN-Imp300. HDS of DBT and estimation of kinetic parameters. The detailed results on the comparison of catalysts prepared at different calcination temperatures, the catalytic performances of the pure substrates, as well as the effect of catalyst weight can be found in previous works.23,24


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Figure 3 shows the representative comparison of the experimental and calculated concentrations of species during the HDS of DBT over the catalysts at 325 °C. All of the HDS test were performed in an identical condition in order to investigate the effects of catalyst preparation methods and the complexing agent on the catalyst performance. In all of the cases three primary products were identified as BP, 4HDBT and CHB. The kinetic profiles clearly reveal the increase in the catalytic activity for TSMN-SP300 in comparison to the TSMNImp300. TSMN-SP300 showed twice as high conversion compared to TSMN-Imp300 after 2 h of reaction time. Both TSMN(CA)-Imp300 and TSMN(CA)-SP300 showed similar conversion profiles. However, both CA treated catalyst show higher CHB formation relative to CA untreated catalysts. Figure 3 also reveals that the relative formation concentrations for CHB is higher for CA untreated catalysts. This suggest that conversion of 4HDBT on the CA treated catalyst are more efficient which explains the higher formation of CHBs on a CA untreated catalyst. CA treatment clearly does correlate with the rate of the 4HDBT conversion. In general, all catalyst shows similar reaction profiles with respect to the reactant conversion and the product formations, that differs significantly on the kinetic profiles. Therefore, the kinetic profiles indicate that mechanism for HDS of DBT proposed in Figure 1 could be applied for all catalysts.


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Figure 3. Concentrations as a function of time of species during the HDS of DBT over the (a) TSMN-Imp300, (b) TSMN-SP300, (c) TSMN(CA)-Imp300 and (d) TSMN(CA)-SP300 at 325 °C and 5 MPa of H2. In a previous work we have concluded that SP method significantly enhances the DDS pathway in comparison to impregnation.23 Also the CA as a complexing agent improves the activity of the catalyst prepared by impregnation. However, the impact of the used synthetic method and the complexing agent on each step were not thoroughly addressed.23,28 As shown in Figure 4, the predicted by the kinetic model species concentrations during the course of the reaction compares closely with the experimental data. In this work we aimed to calculate rate


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constants for separate steps of the reaction network shown in Figure 1 and to compare the catalyst properties with respect to synthesis and reaction temperature.

Figure 4. Parity diagrams for the comparison of experimental with calculated concentrations of species during the HDS of DBT over the (a) TSMN-Imp300, (b) TSMN-SP300, (c) TSMN(CA)Imp300 and (d) TSMN(CA)-CP300 at 325 °C and 5 MPa of H2.


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Table 2 summarizes the kinetic parameters calculated using the model. The total apparent rate constant (k0 = + ) for the conversion of DBT is clearly increased for SP catalyst in comparison with the impregnation catalyst. The difference in the apparent rate constants and for the TSMN-Imp300 and TSMN-SP300 further reveal in higher contribution for SP catalyst than the . The difference of the apparent rate constants for the subsequent step, i.e. and were rather marginal for TSMN-Imp300 and TSMN-SP300. Therefore, we conclude that the main difference in the higher activity of the SP catalyst over the impregnation catalyst is the higher rates for the first DDS step of the reaction network. This also support the EPR data that show higher dispersion of active sulfur sites. The effect of the complexing agent (CA) for the catalytic activity clearly reveal higher contribution to TSMN(CA)-Imp300 and TSMN(CA)SP300 through the and . Although the complexing agent has lesser effect on of TSMN(CA)-SP300 in comparison to TSMN(CA)-Imp300, has dramatically increased upon addition of complexing agent. Complexing agent clearly has increased the rate of the DDS of 4HDBT by orders of magnitude. Thus, the complexing agent does enhance the catalytic activity of HDS of DBT but more through the effect on the DDS of 4HDBT, which also explains the higher formations of CHB for CA treated catalyst. Eventually through the HYD pathway of DBT rather than DDS pathway. The activity of the catalysts was also tested at different reaction temperatures, in order to estimate the activation energies of each step using the Arrhenius equation. Figure 5 shows the representative conversion profile for DBT over the TSMN(CA)-SP300 at temperature range of 325-375 °C. The rate of conversion of DBT increases with temperature nearing the complete conversion within 2 h of reaction time at 375 °C. Overall the temperature effect is similar for all catalysts. Higher conversion and formation rates were observed for all catalysts with increasing


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the reaction temperature. Details of the estimated activation energies for each step, Eai, together with the frequency factors Ai for each step of the reaction network are summarized in Table 2.

Figure 5. (a) Concentrations of DBT as a function of time on a TSMN(CA)-SP300 at different reaction temperatures. (b) Arrhenius plots of HDS apparent rate constants of DBT on a TSMN(CA)-SP300. As it is shown in Table 2, all of the catalysts perform HDS of DBT with differing Ea values for each step of the reaction network. Ea1, Ea2 and Ea4 values for TSMN-Imp300 are higher than for TSMN-SP300 implying the higher activation barrier for all three steps for TSMN-Imp300. However, Ea3 value is rather small and negative for TSMN-Imp300, which also can be explained by a favorable adsorption equilibrium constant for BP on a TSMN-Imp300. On the other hand, Ea values for TSMN-SP300 are all positive and are relatively small, with Ea1 having a smallest value, that explains the higher DDS/HYD selectivity observed in previous work.23 The effect of complexing agent on the HYD pathway is clear from the Table 1. Comparing the impregnation catalysts, Ea1 is not effected by CA treatment, however, Ea2 has drastically decreased from 118 kJ/mol to ‒13 kJ/mol. This indicates that increase in the HDS performance of the CA treated


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catalysts is primarily due to enhancing the rate of the HYD pathway. Moreover, Ea4 is also decreased significantly upon the CA treatment. Contrarily, the SP catalysts activity slightly decreased by the complexing agent, mainly due to increased values of Ea1 and Ea2. This indicates that complexing agent has better effect on the dispersion of active sites during the preparation of the catalyst by the impregnation method by assisting the dissolution of metal ions in order to freely penetrate them through the porous Ti-SBA-15 substrate. On the other hand, SP catalyst are prepared in a single-pot by dissolving all of the components at ones where the formation of components such as porous catalyst support and the active sites require different timing for formation. Table 2. The values of the kinetic and thermodynamic parameters Parameter

TSMN-Imp300

TSMN-SP300

TSMN(CA)-Imp300 TSMN(CA)-SP300

A1 (s-1g cat-1)

9.7×103

2.6×10-2

5.3×103

3.0×103

A2 (s-1g cat-1)

2.4×105

3.9×10-3

3.1×10-6

3.5

A3 (s-1g cat-1)

3.9×10-13

3.1×10-3

2.1×10-3

4.7×10-4

A4 (s-1g cat-1)

2.6×1013

3.2×109

2.9×102

1.7×105

Ea1 (kJ mol-1)

86.7

8.4

80.0

77.1

Ea2 (kJ mol-1)

117.5

25.6

‒13.3

56.3

Ea3 (kJ mol-1)

‒95.4

21.6

19.0

11.8

Ea4 (kJ mol-1)

202.4

154.4

59.2

91.9

k0, ×10-5 (s-1g cat-1)a,b 30.7

50.4

83.4

62.3

, ×10-5 (s-1g cat-1)a 27.9

48.1

64.4

51.5

, ×10-5 (s-1g cat-1)a 1.4

2.0

5.5

3.9

, ×10-5 (s-1g cat-1)a 8.0

3.75

4.6

5.2


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, ×10-5 (s-1g cat-1)a 5.2

6.7

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249.6

128.2

at 325 °C, b here k0 denoted as the sum of the and referring to the total apparent rate constant for conversion of DBT.

a

Figure 6 shows the TPR-H2 uptake values corresponding to the reduced actives cites, and TPD-NH3 uptake values corresponding to the surface acidity of the catalyst as a function of total apparent rate constants k0 in HDS of DBT.

Figure 6. Total apparent rate constants for HDS of DBT at 325 °C as a function of H2 uptake (TRP-H2) and NH3 uptake (TPD-NH3) values for various catalysts.


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Increased activity of the catalysts correlates better with the TPR-H2 uptake values in comparison to the TPD-NH3 uptake values. This is specifically noticeable for TSMN(CA)-Imp300, which also has lower dispersion of the sulfur active sites estimated using the EPR spectra.

Conclusion In summary, the kinetic analysis and further catalyst characterization reveal additional interpretation of the physical and chemical phenomena of the catalysts activity for HDS of DBT, with respect to the synthetic procedure and the consequence of the complexing agent treatment. A Langmuir-Hinshelwood kinetic model was used in order estimate the kinetic parameters ( , , , Ea1, Ea2, Ea3 and Ea4) that permitted the better view of the experimental results in terms

of the parallel-series reaction network and the affect by the different catalysts on the intrinsic steps. Thus, the current work emphasizes the important role of the synthetic method and the complexing agent that are able to change the distribution and chemical environment around the active sites, thereby controlling the mechanism of HDS of DBT. We believe that our results bring the additional insights to the existing HDS catalyst development processes that can be applied for predicting the additional catalytic properties of the NiMo-based hydrodesulfurization catalysts and the improved method of preparations. Particularly, TPR-H2 uptake values in combination with the EPR spectroscopy seem to provide better correlation with the catalyst activity.

Acknowledgements


Energy & Fuels 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

The authors thank the financial support by King Fahd University of Petroleum and Minerals for funding this work. A. S. J. supported though project DSR SR171007 and K.A. supported though project DSR NUS15105.

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