Al2

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Effect of Potassium Content on the Performance of CoMo/Al2O3-MgOK2O(x) Catalysts in Hydrodesulfurization of Dibenzothiophene D. Solís-Casados,*,† J. Escobar,‡ I. García Orozco,† and T. Klimova§ †

Centro Conjunto de Investigacion en Química Sustentable, Facultad de Química, Universidad Autonoma del Estado de Mexico, Km 14.5 Carretera Toluca-Atlacomulco, C.P. 50200, Toluca Estado de Mexico, Mexico ‡ Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, San Bartolo Atepehuacan, Gustavo A. Madero, Mexico D.F., Mexico, 07730 § Departamento de Ingeniería Química, Facultad de Química, Universidad Nacional Autonoma de Mexico, Cd. Universitaria, Coyoacan, Mexico D. F., 04510, Mexico ABSTRACT: A series of CoMo catalysts supported on Al2O3-MgO modified with different amounts of potassium were prepared with the aim to study the influence of the content of that basic modifier on the physicochemical characteristics of the materials and its correlation with the observed catalytic performance. It was assumed that potassium addition would enhance the textural and structural stability to the magnesium formulations and also decrease the number of acidic sites and their strength. Materials were characterized by N2 physisorption, X-ray powder diffraction, thermodesorption of pyridine, and scanning electron microscopy. Catalytic performance was evaluated in the hydrodesulfurization reaction (HDS) using dibenzothiophene (DBT) as a model molecule. Results show that the addition of a small amount of potassium (below 5 wt % of the catalytic formulation of potassium oxide) decreases acidity of the catalytic formulation and enhances the direct desulfurization pathway of dibenzothiophene HDS.

1. INTRODUCTION Recent development of new catalytically active supported CoMo formulations with decreased acidity has acquired great interest. These formulations can be used to remove sulfur from S-containing organic compounds preserving their initial structures without significant changes produced by their hydrogenation (HYD) and cracking (CK). Previously, it has been reported that the use of supports with low acidity enhances the hydrodesulfurization (HDS) activity and slightly decreases the hydrogenation functionality of the aforementioned catalysts.1-5 Zdrazil et al.1 reported that the addition of magnesia in supports for HDS catalysts promotes their activity without hydrogenation of unsaturated organic compounds. El-Shobaky et al.2 reported that the modification of NiO/MgO catalysts by some transition metal oxides (MnO2, CoO) brought about a considerable increase in their catalytic performance in hydrogen peroxide decomposition at low temperature (20-40 °C). These changes were ascribed to an increase in the concentration of active sites involved in H2O2 decomposition via creating new ion pairs. In other works of the researchers from the same group, it was reported that the effect of dopant (Li2O) addition in CoMo/ Al2O3 catalyst leads to an increase in the number of the active sites but does not change their energetic state.3,4 Blanchard et al.5 reported that the use of CoMo catalysts supported on lanthanum modified γ-alumina resulted in a decrease of the thiophene HDS conversion due to the preferential adsorption on the lanthanum hydroxyl group of the oxymolybdate species present in the impregnation solution as isolated monomeric entities, which are less easily sulfided, when deposited on the support, than the polyoxomolybdates. Bao et al.6 reported the use of combined potassium and phosphorus, r 2010 American Chemical Society

which are promising in improving the HDS selectivity of conventional hydrotreating catalysts. In our previous works,7,8 modification of the conventional CoMo/alumina catalyst by addition of magnesium oxide was proposed. Catalysts with good activity in thiophene hydrodesulfurization (HDS) and low hydrogenation ability were obtained. However, magnesia-containing supports of low acidity were found to be unstable in the presence of humidity and carbon dioxide. When exposed to environmental conditions, they suffered a strong decrease in textural properties and changed their chemical composition from MgO to Mg(OH)2 and MgCO3. Taking into account previous reports about low stability of MgO structure upon aqueous impregnation of catalytically active phases and exposition to environmental conditions, some attempts were made trying to stabilize magnesia-containing supports of low acidity. Zdrazil et al.9 reported that the nonaqueous impregnation could be an option to get more stable catalysts. However, their catalytic performance was not as good as it was expected. Klimova et al.8 reported that the use of Al2O3-MgO mixed oxide supports resulted in a decrease in the hydrogenation function of NiMo catalysts. However, the acidic sites were not deeply characterized in this work in order to correlate the acidic sites with the hydrogenation ability of the catalysts. There are some additional reports on the unstability of MgO support and the difficulties of using NiO as a catalytic promoter because of the Special Issue: IMCCRE 2010 Received: March 19, 2010 Accepted: July 21, 2010 Revised: July 20, 2010 Published: August 31, 2010 2755

dx.doi.org/10.1021/ie100682x | Ind. Eng. Chem. Res. 2011, 50, 2755–2761

Industrial & Engineering Chemistry Research formation of a NiO-MgO solid solution under aqueous impregnation. Ancheyta et al.10 reported that the use of another preparation technique for deposition of catalytically active phases on MgO could enhance their catalytic performance decreasing the hydrogenation function. However, catalytic formulations prepared by such a way did not show the expected performance in hydrodesulfurization of gasolines preserving their octane number. In the present work, we report results obtained with low acidity CoMo/Al2O3-MgO catalysts doped with different amounts of potassium oxide in order to achieve more stable catalytic formulations prepared in a basic medium. It was expected that potassium addition would further preserve the low hydrogenation functionality of the catalysts without affecting their ability for sulfur removal from dibenzothiophene through the direct desulfurization route. The main aim of the present work was to study the effect of K2O content on the acidic sites and their strength correlating them with the catalytic performance of the CoMo catalysts supported on alumina-magnesia. We also tried to determine the best K2O content leading to a high activity in hydrodesulfurization of the model molecule (DBT) and a low hydrogenation of desulfurized products.

2. EXPERIMENTAL SECTION 2.1. Precursors. Magnesium nitrate hexahydrated (Mg(NO3)2 3 6H2O, Sigma, ACS reagent), AlOOH (pseudoboehmite Catapal B TM), formic acid (Sigma, ACS reagent, g96.0%), citric acid (Aldrich, ACS reagent, g99.5%), potassium hydroxide (Sigma-Aldrich, ACS reagent, g 85%, pellets); CoCO3 monohydrated (Aldrich, 99.998% trace metals basis) and MoO3 (Fermont) were used as precursors without further purification. Citric and formic acids were used to stabilize CoMo impregnation solutions and to get the gelation of AlOOH to be used as a binder for pelletize the mixtures. 2.2. Al2O3-MgO Support. The Al2O3-MgO support was prepared from the mechanical mixture of magnesium nitrate and pseudoboehmite Catapal B as precursors using a binder of boehmite (AlOOH) to join both precursors. The binder was obtained from aluminum hydroxide powder (AlOOH) by gelation using an aqueous solution of formic acid (5 vol %) as a peptizing agent. Obtained gel helps to join precursors of the Al2O3-MgO in a ratio of 95:5, which is the optimum magnesium content reported in the past.7,8 The obtained mixture of alumina and magnesia precursors and boehmite binder was extrudated by using a syringe. Extrudates were dried at 60 °C overnight and thermally treated in an air convection oven at 400 °C for 4 h to obtain the Al2O3-MgO support. 2.3. Al2O3-MgO Supports Modified by K2O. Al2O3-MgO extrudates were impregnated by the pore volume technique with an aqueous solution of potassium hydroxide (KOH). Solutions with different concentrations of KOH were prepared to obtain formulations with theoretical K2O loadings of x = 1, 3, 5, 10, and 20 wt %. Potassium-containing supports were calcined at 400 °C for 4 h in an air convection oven. Hereinafter, modified supports will be referred to as Al2O3-MgO-K2O(x), where x is K2O loading in the support (wt %). 2.4. Preparation of CoMo Catalysts. The impregnation of Co and Mo precursors over Al2O3-MgO-K2O(x) supports was made simultaneously by impregnation of a complex solution containing both metals in an atomic ratio of Co/(Co þ Mo) = 0.3 and the amount of Mo corresponding to the nominal

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composition of 12 wt % MoO3. The impregnation solution was prepared in an ammoniacal medium. For this, first 7.5 g of MoO3 were dissolved with stirring at 60 °C in 30 mL of NH4OH, then 10.7 g of CoCO3 H2O were added, and the solution was stirred until its complete dissolution. After that, CoMo solution was acidified and stabilized in pH of 8.0 by using approximately 7.5 g of citric acid. The obtained solution was spread above the thermally treated Al2O3-MgO-K2O(x) pellets by using a rotatory chamber to homogeneously deposit the solution containing the CoMo phases. Extrudates were thermally treated at 400 °C for 4 h to obtain the catalytic formulations CoMo/Al2O3-MgO-K2O(x). Pure Al2O3 was also impregnated following the same procedure to prepare the conventional CoMo/Al2O3 catalyst used as a reference in the present work. 2.5. Characterization of Supports and CoMo Catalysts. Supports and prepared CoMo/Al2O3-MgO-K2O(x) catalysts were characterized by different techniques. Textural properties were characterized by N2 physisorption measurements using ASAP 2000 equipment from Micromeritics. The surface area values were determined using the Brunnauer-Emmet-Teller (BET) equation. Adsorption isotherms were used to determine pore size distributions. The pore diameter was determined by using the Barret-Joyner-Halenda (BJH) equation. The total pore volume was determined at relative pressure (P/P0) equal to 0.98. All samples were degassed out at 200 °C for 3 h previously to N2 physisorption measurements. The concentration of pyridine (Py) adsorbed on samples was used as a measure of the total number of acid sites and their strength. This technique allows one to determine both the Lewis and Br€onsted acid sites on the support and the catalyst surface. Before Py adsorption, samples were cleaned out in O2 flow overnight and after in high vacuum upon thermal treatment at 450 °C to remove water and carbon dioxide adsorbed from the environment. Infrared spectroscopy monitoring the adsorbed pyridine is an established tool to characterize the acidity of the solid acid catalysts. This technique let to distinguish Br€onsted (B) and Lewis acid sites (L) because pyridine adsorbed on different types of acid sites presents different characteristic signals in the infrared spectrum. The maximum acid strength (MAS) distribution on the sites can be followed by monitoring the thermodesorption of Py from room temperature up to 400 °C. The characteristic band of Py adsorbed on the B site, attributed to the piridinium ion, is located at 1545 cm-1, whereas the band of Py adsorbed on the L site is located at 1455 cm-1. The difference spectra were obtained by subtrating the spectrum of the dehydrated sample from the spectra obtained after pyridine absorption. Calculation of the concentration of the adsorbed pyridine was obtained as it was reported previously by Emeis.11 Emeis reported that the integrated molar coefficients extinction could be applied to other catalysts, however, it remain for research the accuracy of these values. For calculation, the integrated absorbance of the wavelength corresponding to B and L sites was used and the eqs 1 and 2 proposed previously by Emeis. C ðB sitesÞ ¼ 1:88IAðBÞR 2 =W

ð1Þ

C L sitesÞ ¼ 1:42IAðLÞR 2 =W

ð2Þ

C = concentration of pyridine (millimole/gram); where IA = integrated absorbance of L or B band (wavenumber), 2756

dx.doi.org/10.1021/ie100682x |Ind. Eng. Chem. Res. 2011, 50, 2755–2761

Industrial & Engineering Chemistry Research

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Scheme 1. Schematic Diagram of the Hydrodesulphurization Routes

Figure 1. FT-IR spectra after pyridine thermodesorption of (a) Al2O3MgO and (b) Al2O3-MgO-K2O(5).

R = radius of catalysts disks (centimeter), and W = weight of disk (milligram). Scanning electron microscopy images and elemental analysis (SEM-EDS) were acquired for the catalysts in order to observe the differences in morphology and the elemental composition in a semiquantitative way by using a JEOL LSV4500 microscope. To identify the crystalline phases and corroborate the absence of the unstable periclase crystalline phase in catalytic formulations, powder X-ray diffraction (XRD) was carried out in a Bruker D8 advance diffractometer equipped with a Linxeye detector, using Ni-filtered CuK R radiation. Tube conditions were voltage 30 kV, current 25 mA; 2θ range 5-80°; step size 0.05°; time per step 106 s. The crystalline phases were analyzed using EVA software in conjunction with the PDF-2 database. 2.6. Catalytic Evaluation of CoMo Catalysts. Oxidic precursors were sulfided at 400 °C (1 h) under a H2S/H2 (10 v/v %, Praxair) stream at 4 L/h constant flow rate. HDS activity was studied in a triphasic slurry batch reactor (Parr 4562 M). The reaction mixture was prepared by dissolving ∼0.3 g of DBT in 0.1 L of n-hexadecane (both from Aldrich) and adding ∼0.2 g of sieved catalyst (80-100 Tyler mesh, 0.165 mm average particle diameter). The operating conditions (carefully chosen to avoid external and/or internal diffusional limitations) were PH2 = 5.59 ( 0.03 MPa, TR = 593 ( 2 K, and a 1000 rpm (∼105 rad/s) mixing speed. Samples were taken periodically and analyzed in a gas chromatograph Perkin-Elmer AutoSystem XL equipped with flame ionization detector and Econopac-5 capillary column (Altech). The products obtained from the main reaction routes (direct desulfurization and hydrogenation routes) were biphenyl (BP) and cyclohexylbenzene (CHB) (Scheme 1). The amount of these products at different reaction times was followed by gas chromatography analysis. The deactivation of the catalysts was not studied. However, it was supposed that catalysts continue being active even after the final reaction time if the CoMoS active phase was still remaining.

3. RESULTS AND DISCUSSION In previous work, it has been reported that MgO addition to the alumina support resulted in a decrease in the Lewis acid sites.7,8 It is known that the Br€onsted acid sites are absent on the Al2O3-MgO supports. The infrared spectra of adsorbed Py and after Py desorption at different temperatures were acquired and

analyzed to elucidate changes induced by the potassium oxide addition on the total number of acid sites and their strength. Before Py adsorption, samples were cleaned out, which was corroborated by taking the IR spectrum. The absence of bands located at 1640, 1439, 1526, and 1375 cm-1 evidenced that the samples were cleaned properly. These bands are assigned to the physisorbed water, the ionic CO32-, physisorbed CO2, and monodentated -O-CO2 carbonate, as it was reported before.8 Figure 1 shows the spectra of supports: (a) Al2O3-MgO and (b) the potassium modified Al2O3-MgO support (1% K2O). In these spectra, an infrared band located at 1450 cm-1 was attributed to the aromatic ring vibration when pyridine is absorbed on the Lewis acidic sites. A decrease in the intensity of this band can be observed after potassium addition, which reflects the decrease in the number of Lewis acid sites in the presence of potassium. The band located at 1540 cm-1, corresponding to the Br€onsted acidic sites, was of small intensity, like a shoulder. However, it was considered to estimate a number of B acidic sites. The concentration of the adsorbed pyridine was calculated using the equations reported by Emeis11 previously. The pyridine desorption was followed through temperature increments at intervals of 100 °C. Figure 2 shows the IR spectra for the CoMo/Al2O3-MgO-K2O(1) formulation after Py desorption at temperature increments of 100 °C (from 100 until 400 °C). It can be observed that the concentration of Py chemically adsorbed on Lewis acid sites decreases considerably, and the Br€onsted acidic sites were also quantified when it was possible. Concentration of Py adsorbed on Lewis, and the Br€onsted acidic sites were also quantified by the method reported by Emeis.11 Results from Table 1 show that the most acidic catalyst is the conventional CoMo/Al2O3, which contains 0.017 mmol Py/ gram of catalyst after Py desorption at 200 °C. Addition of 5 wt % MgO into the catalytic formulation resulted mainly in a slight decrease (11.8%) in the total amount of Py adsorbed on Lewis 2757



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Table 2. Textural Properties of the As-Prepared Catalysts surface catalysts

total pore

mean pore

area (m2/g) volume (cm3/g) diameter (Å)

CoMo/Al2O3

248

0.37

CoMo/Al2O3-MgO(5)

192

0.36

59 72

CoMo/Al2O3-MgO-K2O(1)

200

0.33

78


180

0.31

65


185

0.32

60


190

0.32

50


200

0.33

45

Figure 2. FT-IR Spectra after pyridine thermodesorption of CoMo/ Al2O3-MgO-K2O(1%) at several desorption temperatures: (a) 25 °C, (b) 100 °C, (c) 200 °C, (d) 300 °C, and (e) 400 °C.

Table 1. Br€ onsted and Lewis Acidic Sites Quantified by FT-IR Lewis acidity

Br€onsted acidity

(mmol Py/g

(mmol Py/g

of catalyst 200 °C

of catalyst) 200 °C

CoMo/Al2O3 CoMo/Al2O3-MgO(5)

0.017 0.015

0.04 0.04

catalysts


0.010

0.01


0.009

0.00


0.010

0.00


0.008

0.00


0.008

0.00

acid sites. For this catalytic formulation, no changes were observed in the number of the B acidic sites with MgO addition in the catalytic formulation. As it was expected, the K2O addition in the catalytic formulation showed a great decrease in the concentration of adsorbed Py that can be clearly observed. This decrease represents the 41.2% in the L acidic sites and the decrease in 75% of the B acidic sites. The concentration of adsorbed pyridine decreases 6% more in the number of L acidic sites with the increase in K2O content, reaching the minimum value of 0.008 mmol Py/g of catalyst for the formulations containing 10 and 20 wt % of K2O. It is necessary to mention that the above quantitative results show a clear decrease in the acidity, in spite of a percent of error of about 10%, due to the small intensity of the peaks observed in the IR spectra, and to the discrepancies about the accuracy of the calculation method by Emeis described in the Experimental Section. The results showed the decrease in the total number of L and B acidic sites with MgO and K2O addition and that the B acidic sites disappear with the increment in K2O content. In addition to the changes in the acidic properties, it is important to consider that the addition of MgO and K2O to the catalytic formulations can also produce some changes in their textural properties, since a low stability of formulations containing MgO was observed and reported previously.7,8 Table 2 shows the results obtained from N2 physisorption measurements (specific surface area, mean pore diameter, and total pore volume).

Figure 3. X-ray diffraction lines for (a) CoMo/Al2O3, (b) CoMo/ Al2O3-MgO, (c) CoMoAl2O3-MgO-K2O(1), (d) CoMo/Al2O3-MgOK2O(5), (e) CoMo/Al2O3-MgO-K2O(10), and (f) CoMoAl2O3-MgOK2O(20).

It is important to note the considerable decrease (22.6%) in the surface area of the conventional CoMo/Al2O3 catalyst after addition of MgO in the catalytic formulation. An increase in the K2O content does not produce important systematic changes in the specific surface area values, which vary around 180-200 m2/g; the experimental error in this technique is almost 3% ((7 m2/g). Some changes can be also observed in the mean pore diameter, which increases from 59 to 72 Å when MgO is added to the catalytic formulation, which represents an 18% change in the pore diameter. The addition of a small amount of potassium (1 wt % K2O) produces a slight increase in the pore diameter (from 72 to 78 Å). However, a further increase of the K2O content results in a progressive decrease in the pore diameter up to 45 Å in the sample with 20 wt % of K2O (Table 2). Probably, this decrease is due to the blockage of mesopores by the deposited oxide metal species (K, Co, and Mo ones). In addition, the above changes in the pore diameter almost do not affect the specific surface area values of the catalytic formulations. This result seems to points out the textural stability of the potassium-modified catalysts. In order to prove catalyst stability, the crystallite phases present on the catalysts were also studied in the present work by X-ray powder diffraction analysis (XRD). The obtained results are shown in Figure 3. According to the XRD results, it appears that the incorporation of magnesia to the pure alumina slightly decreases its crystallinity, since three main diffraction lines of the γ-Al2O3 crystalline phase can still be observed in 2758



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Figure 4. Electron micrographs of the catalysts: (a) CoMo/Al2O3, (b) CoMo/Al2O3-MgO, (c) CoMoAl2O3-MgO-K2O(1), (d) CoMo/Al2O3-MgOK2O(20).

Table 3. Elemental Analysis from the EDS Spectra chemical element

reference K2O(0) K2O(1) K2O(5) K2O(10) K2O(20)

Co

1.77

1.36

1.53

2.17

1.59

Mo

1.71

4.31

2.11

1.79

1.89

0.50 1.67

Al

29.88

27.82

26.23

26.31

27.03

25.89

O

66.64

66.38

67.53

66.98

67.10

69.31

Mg

0.00

0.13

2.33

2.49

2.08

2.20

K

0.00

0.00

0.27

0.26

0.31

0.43

Figure 3 (*γ-Al2O3 JCPDS 10-0425). The diffraction lines of the periclase (MgO) or brucite (Mg(OH)2) crystallite phases were absent in all XRD patterns shown in Figure 3. The absence of diffraction lines at 2θ deg of 18, 38, and 52°, which are attributed to the brucite crystalline phase (JCPDS 7-239) is an indication of the support structural stability after impregnation of Co and Mo precursors. No evidence of the transformation of periclase into brucite crystalline phase was observed even after exposure of samples to the environment. In addition, the formation of any Co or Mo-containing crystalline phase was not observed, probably because of a high dispersion of these active phase precursors on the Al2O3-MgO-K2O(x) supports or due to the small crystallite sizes, which must be less than 40 Å. Once the acidity of the prepared catalysts was found to be low and they showed textural and structural stability, scanning electron microscopy (SEM) images were taken to analyze changes in the morphology of the samples after MgO and the K2O addition. This study was undertaken in order to detect any possible morphological changes induced in each impregnation (Mg, K, Co, and Mo), which were not evidenced by previous characterization of the crystal structure and textural properties.

Figure 4a-d shows the SEM images, on which the morphology of the surface and the pore structure can be observed. No changes in morphology were observed, that is the reason for showing only a few images. When MgO and different amounts of K2O were added in the catalytic formulations, the morphology observed by SEM was the same. As expected, the chemical analysis by EDS showed that elemental composition of the prepared catalysts changed. Thus, the experimentally determined percentage of K increases in catalytic formulations when the amount of potassium precursor added was increased, being in line with the theoretically expected values (Table 3). Mapping was made to observe each element (Co, Mo, K, O, and Mg) and its dispersion through the whole catalytic formulation as can be observed in Figure 5a-c. Mapping shows that each element has a good distribution, especially the K, Co, and Mo elements, which do catalytic homogeneous formulations. Hydrodesulfurization of dibenzothiophene was carried out to evaluate catalytic performance of the prepared formulations with low acidity. Results from the catalytic activity tests shown in Figure 6 indicate that the conversion of dibenzothiophene increases in 10-30% due to the addition of small amounts of K2O to the conventional CoMo catalyst (1-5 wt %). However, further increase in the K2O content to 10 and 20 wt % does not enhance the catalytic performance more. In this case, the dibenzothiophene conversion is almost the same as obtained with the conventional CoMo catalyst. This indicates that there is an optimal quantity of K2O which promotes hydrodesulfurization activity. This optimal amount of K2O is below 5 wt %, regardless of the further decrease in acidity at higher K2O contents. Selectivity of the catalysts in hydrodesulfurization of DBT was studied by following the main desulfurized reaction 2759



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Figure 5. Elemental mapping on the (a) CoMo/Al2O3, (b) CoMo/Al2O3-MgO, and (c) CoMoAl2O3-MgO-K2O(20) catalysts.

products: cyclohexylbenzene (CHB) formed in the hydrogenation route and biphenyl (BP) formed in the direct desulfurization pathway (Scheme 1) by gas chromatography. The ratio of these two products, namely, the CHB/BP ratio, reflects the ability of the catalyst for hydrogenation of the initial DBT molecule and of other unsaturated and aromatic compounds during the HDS process. In general, these side reactions of hydrogenation lead to a loss of the fuel octane number and decrease of its quality. Because of this, for many practical applications it is desired to considerably decrease the hydrogenation ability of the catalyst that will enable desulfurization of dibenzothiophene to biphenyl with the lowest hydrogen consumption and without the loss of quality of the final product. In other words, in the present study,

it was expected that the addition of potassium in the catalyst formulation will enhance the direct desulfurization route, whereas the hydrogenation activity will decrease. Table 4 summarizes the selectivity results and shows the CHB/BP ratio at 30 and 60% of DBT conversion. It can be observed that at both conversion values, K2O addition to the catalytic formulation produces a slight decrease in the CHB/BP ratio.

’ CONCLUSIONS Magnesia and potassium addition promotes the decrease of the number of L acidic sites, and the B acidic sites disappear when 2760



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’ ACKNOWLEDGMENT D. Solís-Casados acknowledges the financial support and equipment to finish this work, FQ-UAEM for the equipment employed through the Centro Conjunto de Investigacion en Química Sustentable to finish this work and the SIEA-UAEM for financial support through the 2638 UAEM Project. ’ REFERENCES

Figure 6. Dibenzothiophene conversion against reaction time for the as-obtained catalysts: (a) CoMo/Al2O3, (b) CoMo/Al2O3MgO, (c) CoMoAl2O3-MgO-K2O(1), (d) CoMo/Al2O3-MgO-K2O(5), (e)CoMo/Al2O3-MgO-K2O(10), and (f) CoMoAl2O3-MgO-K2O(20).

Table 4. Hydrodesulphurization Selectivity of the As-Obtained Catalysts catalysts

CHB/BF ratio

CHB/BF ratio

conversion 30%

conversion 60%

CoMo/Al2O3

0.094

0.167


0.051

0.051


0.072

0.072


0.050

0.059

CoMo/Al2O3-MgO-K2O(10) CoMo/Al2O3-MgO-K2O(20)

0.043 0.053

0.074 0.080

(1) Klicpera, T.; Zdrazil, M. J. Catal. 2002, 206, 315. (2) El-Shobaky, G. A.; El-Molla, S. A.; Ali, A. M. I. Appl. Catal., A: General 2003, 253, 417–425. (3) El-Shobaky, G. A.; Ghozza, A. M.; Mohamed, G. M. Appl. Catal., A: General 2003, 241, 235. (4) El-Shobaky, G. A.; Abdalia, F. F.; Hamed, M. N.; El-Molla, S. A. Colloids Surf., A: Physicochem. Eng. Aspects 2002, 211, 1. (5) Blanchard, P.; Payen, E.; Grimblot, J.; Bihan, L.; Poulet, O.; Loutaty, R. J. Mol. Catal., A: Chem. 1998, 135, 143. (6) Xiaojun, B. Chem. Mater. 2008, 110, 128. (7) Klimova, T.; Solis-Casados, D.; Ramirez, J. Catal. Today 1998, 43, 135. (8) Solís, D.; Klimova, T.; Ramirez, J.; Cortez, T. Catal. Today 2004, 98, 99–108. (9) Zdrazil, M. Catal. Today 2003, 86, 151. (10) Caloch, B.; Rana, M. S.; Ancheyta, J. Catal. Today 2004, 98, 91–98. (11) Emeis, C. A. J. Catal. 1993, 141, 347–354.

potassium precursor was increased in the catalytic formulation to theorical values of 10% of K2O. The acidity strength in the prepared formulations is mostly weak or medium, which is assumed on the basis of the pyridine desorption analysis, where almost all chemisorbed pyridine was removed at temperatures as low as 250 °C. Only a small amount of pyridine was still remaining at higher temperature of desorption. Catalytic formulations prepared in the present study showed high textural and structural stability, which was attributed to the preparation method used and especially to the addition of K2O, which promotes the stability on the MgO-containing catalysts. It was observed an important decrease on hydrogenation functionality when MgO and K2O were added as was observed in Table 4. This decrease could be convenient because it allows one to reduce olefin hydrogenation leading to a loss of the octane number of hydrotreated gasolines. When K2O was added, a decrease in the catalyst’s acidity was observed. However, an increase in the hydrodesulfurization activity was detected only at low K2O loadings (below 5 wt %). The optimum catalytic performance in HDS of DBT was obtained at low K2O contents (between 1 and 5 wt %). These catalysts showed an increase in both the HDS activity and the selectivity toward the direct desulfurization of a model molecule such as DBT.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: (525) 722 2 17 38 90. E-mail: [email protected]. 2761


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