Effect of Preparation Methods and Content of Boron on Hydrotreating

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Effect of Preparation Methods and Content of Boron on Hydrotreating Catalytic Activity S. K. Maity,* M. Lemus, and J. Ancheyta Instituto Mexicano del Petroleo (IMP), Eje Central Lazaro Cardenas Norte 152, Colonia San Bartolo Atepehuacan, Mexico D. F. 07730, Mexico ABSTRACT: The effects of boron content and preparation methods are studied in this present investigation. Three different methods are employed for the preparation of catalysts. The catalysts are characterized by pore size distribution (PSD), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Thiophene hydrodesulfurization (HDS), cumene hydrocracking (HC), gas oil HDS and Maya heavy crude oil HDS, and hydrodemetallization (HDM) activities are tested. The results show that the specific surface area and PSD are not changed with B loading. The enrichment of metal distribution on the alumina surface is observed from SEM experiments for the 0.6B
NiMo catalyst. More staging of MoS2 active phases is noticed in the B catalyst than the B-free catalyst. It can be stated that boron forms a monolayer on the alumina surface and reduces the metal
support interaction. It facilitates the formation of more staging of active phases. The thiophene HDS activity slightly increases with boron loading up to 0.6 wt % B, and after that, the activity becomes constant or slightly decreases. The cumene cracking of the NiMo
B catalyst shows the highest activity. The overall activity of boron catalysts was found to be the same or marginally higher than that of the boron-free catalyst. The presence of boron may help to reduce catalyst deactivation during hydrotreating of heavy crude oil.

1. INTRODUCTION The sulfur specifications in gasoline and diesel all over the world are becoming very stringent. A high and stable catalyst is required to cope with this stringent condition. The use of additives, such as phosphorus and boron, is one of the options to increase hydrotreating activity. The main idea behind the incorporation of additives (modification of support) is that they inhibit the strong interaction of active metal or promoter with support and, hence, the formation of hardly sulfidable NiAl2O4 (or CoAl2O4) and Al2(MoO4)3 type of species. In this respect, the addition of boron into the hydrotreating catalysts has been investigated. It has been found that boron mainly modified catalyst acidity and metal dispersion. How boron can modify the catalyst acidity is not so far clear from the reported investigations. Dalai1 and Lewandowski and Sarbak2 reported that boron increases intermediate acidity, whereas Flego and Parker3 and Ding et al.4 observed that total support acidity of the boron catalyst is Lewis type, which is strong in nature. On the other hand, Sato et al.5 reported strong Brønsted acidity of the B-hydrotreating catalyst. Ramirez et al.6 stated that with the addition of a little amount of boron (0.2 wt %), the surface acidity of alumina increases; however, with further addition, the total acidity decreases because of the condensation of H3BO3 on the alumina surface and/or the formation of Al
O
B bonds. They have also stated that boron not only modified the support acidity but also increased the acidity related to active metal sites.6 The incorporation of boron has also redistributed the active molybdenum and nickel oxide.7 Boron at lower loading formed a monolayer of boron species on the alumina surface, which prevents Ni and Mo atom from entering into alumina lattice. It leads to an increase in the total number of octahedral Ni species and, hence, the formation of highly active Ni
Mo
S species. An r 2011 American Chemical Society

increase of metal dispersion of the B catalyst has also been supported by Ding et al.4 The results obtained by these authors from X-ray photoelectron spectroscopy (XPS) showed that the ratio of Mo/Al and Ni/Al increases with boron, indicating that boron increases metal dispersion on an alumina support. Usman et al.8 reported that boron may change low active Co
Mo
S type I sites to high Co
Mo
S type II sites. On the other hand, Morishige and Akai9 reported that boron decreases dispersion of molybdenum on an alumina support. In general, two types of boron-modified hydrotreating catalysts have been reported. One uses boric acid for impregnation into the support and/or catalyst,1,4,6,8,10,11 such as active metal. Another way is the modification of the alumina support by co-precipitation from aluminum and boron salt solution.12
14 Hydrodesulfurization (HDS), hydronitrogenation (HDN), hydrogenation (HYD), and hydrodearomatization (HDA) reactions have been studied on B-containing hydrotreating catalysts. HDN activity of B-free and B content NiMo catalysts was compared by Dalai and his co-workers.1,11 The surface area of the boron catalyst (at high loading) decreases with an increasing boron loading. The HDN activity increases with a decreasing of the surface area of the catalysts. Therefore, it explains that the higher activity of the BNiMo catalyst may be due to the lower surface area (particularly at higher loading). Another explanation is the influence of catalyst acidity by boron. They noticed that boron increases both weak and intermediate acidity and helps to increase HDN activity. It is also in line with the results found by Lewandowski and Sarbak.2 They have investigated the boron Received: April 1, 2011 Revised: May 31, 2011 Published: June 01, 2011 3100

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Energy & Fuels effect on HDN of the coal liquid, and it was noticed that, with an increasing boron concentration, the HDN activity increases. It is also concluded by the authors that borate ions may increase the resistance of catalyst deactivation. Ramirez et al.6 used boron for modification of an alumina support. The thiophene activity of the CoMo/Al2O3
B catalyst has been tested. The HDS activity increases with an increasing boron content up to 0.8 wt % B, and with a further increase of it, the activity decreases. It is also reported that HYD sites are promoted by boron addition. The alumina-borate-supported CoMo catalysts showed substantially higher HDS activity of Kuwait heavy oils than that for the boronfree catalyst.12 The activity reaches a maximum at 4 wt % B2O3 content. It is concluded that the higher activity of the B catalyst is due to the higher dispersion of the active sulfide phase, increase of active sites, and more hydrogenation ability of active phases.12 From the above discussion, it is observed that boron increases HDS and HDN activities of the hydrotreating catalyst. However, there are others who noticed a negative effect of boron on the catalyst.2,8,10,11 The HDS activity of thiophene does not significantly change with boron contents.8,11 Lewandowski and Sarbak2 reported that HDS of coal liquid decreases with an increasing B concentration. The effect of boron on thiophene HDS has been investigated in detail by Okamato and co-workers.8,15,16 It is observed that boron addition decreases thiophene HDS activity.8 However, it is also found that the promotional effect of Co is more on boroncontaining catalysts compared to the B-free catalyst. It is noticed that B may block and, hence, decrease the Co coverage on MoS2 edges. Although boron has a negative effect on HDS, the same boron has a great promotional effect when it was used with citric acid (CA).15,16 It is observed that the thiophene HDS activity of CA/CoMo/B/Al is higher than that of CA/CoMo/Al. It is sited that boron reduces the metal
support interaction, which leads to the facilitation of the formation of CoMoO4 phases. These oxide phases easily converted to the CA
Co complex, which is dispersed inside the pores of the catalyst. However, CoAl2O3-like species is formed in the boron-free CoMo/Al catalyst. These phases are inactive toward CA.15 This is the reason why CA has a promotional effect on the boron-containing catalyst than the B-free catalyst. Moreover, the effect of CA on HDS is more when the catalyst was prepared by post-treatment (CA/CoMo/B/Al) than that of simultaneous impregnation (CoMo
CA/B/Al). Although there are several studies of the boron effect on the hydrotreating catalysts, it is not clear so far how boron really works on these catalysts. Therefore, in this work, a systematic approach is adapted to investigate the effect of the B content and preparation method on catalyst activity tested with model compound, gas oil, as well as heavy crude oil. In this regard, three different preparation methods are employed to prepare catalysts having 0.2
1.2 wt % B using an alumina support. The catalysts have also been characterized by various techniques.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Several catalysts having different concentrations of boron were prepared by the wetness impregnation method. In this method, appropriate amounts of salts were dissolved into the predetermined distilled water. The prepared solution was impregnated into the dry support. Ammonium heptamolybdate (AHM), nickel nitrate, and boric acid were used for impregnation of molybdenum, nickel, and boron, respectively. To prepare cobalt-promoted catalyst, cobalt nitrate was used instead of nickel nitrate in a similar way as described above.

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Three different methods were employed for impregnation of boron. In the first method, boron was impregnated into the support and then the impregnated samples were dried at 393 K for 6 h and calcined at 773 K for 5 h. Active metal molybdenum and promoter nickel were then impregnated into the dry sample by co-impregnation. These series of catalysts are assigned as XB
NiMo, where X is the weight percentage of B and “
” represents dry and calcination. In the second method, the solution of AHM, nickel nitrate, and boric acid was impregnated into the dry support by co-impregnation (XBNiMo). In the third method, molybdenum and nickel are impregnated first and then boron. In between two impregnations, samples were dried and calcined (NiMo
XB). It is worth mentioning that impregnation of a higher percentage of boron is not possible because of the lower solubility of boric acid into water. Finally, all catalysts were calcined at 773 K for 5 h. γ alumina was used as a support material for the preparation of all hydrotreating catalysts. 2.2. Characterization of Catalysts. Specific Brunauer
Emmett
Teller (BET) surface area, pore volume, and pore size distribution (PSD) of the catalysts were measured by nitrogen adsorption at 77 K (Quantachrome Nova 2000). X-ray diffractograms were recorded on a Siemens D-500 model using Cu KR radiation. The metal distribution in the catalysts was measured by a scanning electron microscope, model XL30ESEM, Philips. Transmission electron microscopy (TEM) was performed on a Tecnai G2 F30 S-Twin operated at 300 kV. The microscope is equipped with a Schottky-type field emission gun and a S-Twin objective lens. The sulfide catalyst was powdered and sonicated in ethanol. One drop was taken with a micropipet and placed on a copper grid coated with a sputtered carbon polymer. The total amount of metals in the feed and products were measured by atomic absorption (Thermoelectron model Solaar AA). Sulfur was analyzed by X-ray fluorescence (HORIBA model SLFA-2100). 2.3. Catalyst Activity Test. The activities of thiophene HDS and cumene hydrocracking were tested in a microplant. In this plant, 0.5 g of catalyst was sulfided in situ into a glass-tubular reactor at atmospheric pressure and 673 K temperature. For sulfidation, hydrogen was passed through a container having CS2. The saturated mixture of CS2 and hydrogen passed through the reactor. Hydrogen flow was 50 mL/min, and duration of sulfidation was 2 h. After sulfidation, the catalyst was flushed at this temperature by H2 until no CS2 could be detected in the effluent gas. Thiophene or cumene feed was introduced through the gas bubblers. The H2 flow rate was 50 mL/min. Reaction products were analyzed by online gas chromatography using a flame ionization detector (FID). A total of 5 g of fresh catalyst was sulfided ex situ for each experiment to study hydrotreating activities of straight run gas oil (SRGO) and Maya crude oil. An atmospheric unit was used for sulfidation. In this unit, hydrogen was passed through a container having CS2. The saturated mixture of CS2 and hydrogen passed through the reactor. The sulfiding conditions for the sulfidation of catalyst were temperature of 673 K, atmospheric pressure, duration of sulfidation of 3 h, and hydrogen flow of 40 mL/min. The catalytic activities with SRGO and Maya heavy crude oil were studied in a batch reactor. An appropriate amount of feed (SRGO or Maya heavy crude oil) was taken into the reactor vessel (1 L capacity). The sulfided catalyst was transferred into the reactor in nitrogen atmosphere very quickly, so that the catalyst would not contact air for a long time. The reactor vessel was properly tight and checked for leakage. The reactor vessel was then purged 2
3 times with hydrogen gas, so that there was no air left inside the reactor. Heating was started from room temperature to the required temperature at the rate of 3 K/min. Stirring was started when the temperature reached the set point (reaction temperature), and the time was noted as the beginning of the reaction at this point. Hydrotreated products of SRGO were collected every hour, and its sulfur content was analyzed. However, for hydrotreating of Maya crude, only one product 3101

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Table 1. Reaction Conditions for Hydrotreating of Different Feeds microplant

batch

thiophene

cumene

0.3

0.5

catalyst weight (g) feed weight (g)

SRGO 5

Maya 5

300

200

temperature (K) pressure (MPa)

673 0.1

673 0.1

633 5.9

653 9.8

duration (h)

4

4

6

6

Table 2. Properties of SRGO and Maya Heavy Crude Oil properties

SRGO

Maya

American Petroleum Institute (API) gravity (deg)

29.67

20.89

sulfur (wt %)

1.67

3.59

nitrogen (wppm)

589

3011

Ramsbottom carbon (wt %)

10.98

asphaltenes (in C7) (wt %)

12.11

Ni (wppm)

50

V (wppm)

260

was collected at the end of the reaction. Products were separated from the catalyst after the reaction, and metal and sulfur contents of the products were analyzed. The experimental conditions for the microplant and batch reactor are given in Table 1.

3. RESULTS 3.1. Physicochemical Properties of Feed and Catalysts. In Table 2, the properties of SRGO and Maya heavy crude oil are presented. Maya heavy crude oil contains 3.59 wt % sulfur and 310 wppm metals (Ni þ V). All catalysts have a fixed amount of MoO3 (10 wt %) and NiO (3 wt %), but the B content is varied from 0.2 to 1.2 wt %. The BET specific surface areas (SSAs) of B-containing catalysts are presented in Figure 1. The surface area does not change with B loading up to 0.8 wt %, but it starts to decrease slightly with a further increase of boron in the case of B
NiMo and BNiMo catalysts. However, there is no change of the surface area throughout the boron loading in NiMo
B catalysts. In Table 3, the textural properties and PSD have also been given. Dalai and his co-workers1,11 have also reported that the surface area does not change significantly with the addition of a small amount of boron (1.1 wt % B); however, with the addition of a higher concentration, the surface area decreases. A decrease of the surface area at higher boron loading was also reported by others.17 The NiMo
B catalyst is prepared by first co-impregnation of the salt solution of Ni and Mo, and then the impregnated sample was dried and calcined. On the calcined sample, boric acid was impregnated and, finally, the sample was calcined. There is no change of surface area of these catalysts, indicating that most of the alumina surface was covered by molybdenum and nickel. There is no alumina surface available to see the effect of boron. It is also important to mention that boron loading is low in our study. It is also possible that boron may preferably sit on metallic sites rather than the alumina surface. No drastic change of the PSD is observed from Table 3. However, several researchers have observed that the surface area in general decreases with the

Figure 1. Effect of the boron loading on BET SSA.

addition of boron (at high loading), particularly when boron was added before active metal. Maity et al.18 reported that, when other additive P is added to the alumina support, the surface area contributed from micropores decreases with an increasing concentration of P. It indicates that phosphorus ions are deposited on the pore mouth particularly on the pore diameter below 200 Å, and it causes the loss of the total surface area. Therefore, in this case, the results indicate that boron may not specifically cover the pore mouth; rather, it may form a monolayer, particularly at a lower loading. 3.2. X-ray Diffraction (XRD). The X-ray diffractogram of NiMo and XBNiMo catalysts is presented in Figure 2. The X-ray diffractogram shows that the support used in this study is γ alumina, and molybdenum phases are well-dispersed up to 1.2 wt % B loading. The peaks at 2θ = 25
27° for crystalline molybdenum oxide have not been observed, even though there is no peak because of borate. We have also studied XRD of the B catalysts prepared by different ways. We do not observe any other peak, except high-intensity peaks for γ alumina (the diffractograms are not presented). The XRD results indicate that active metal molybdenum and promoter are well-dispersed into the alumina support. It is reported in the literature1 that, with the addition of a small amount of boron into the alumina support, the intensity of alumina decreases and may be due to the formation of the highly dispersed microcrystalline aluminum borate phase on the catalyst surface. Moreover, the crystalline borate as well as MoO3 phases are observed at a higher boron (1.1
1.7 wt %) loading. In this case, well dispersion of molybdenum and boron is noticed. It is worth mentioning that, in this present work, a high surface area of alumina (>350 m2/g) and a lower loading of molybdenum oxide and boron (maximum of 1.2 wt %) are used. 3.3. Scanning Electron Microscopy (SEM). The morphologies of B-free and B-containing catalysts have also been studied by SEM. The images of NiMo and 0.6B
NiMo show that molybdenum atoms are evenly distributed on the support surface, indicating the well dispersion of metals. An energy-dispersive X-ray (EDX) is used to examine the metal radial distribution through a catalyst particle from one end to other, taking concentrations of metals at different points along the diameter of the catalyst particle. The molybdenum radial distribution is presented in Figure 3, where it is observed that, in the boron-free catalyst, molybdenum is more evenly distributed throughout the catalyst particle. However, a little more molybdenum atoms are concentrated near the periphery of the boron catalyst. It is particularly observed when boron is impregnated first and then active metals (0.6B
NiMo). However, in the NiMo
0.6B catalyst (where B is impregnated after active metal), molybdenum 3102

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Table 3. Textural Properties of Fresh and Spent Catalysts Supported on Alumina NiMo properties

0.6B
NiMo

0.6BNiMo

NiMo
0.6B

fresh

spent

fresh

spent

fresh

spent

fresh

spent

SSA (m /g)

345

303

357

325

360

288

351

293

TPV (mL/g)

0.79

0.49

0.80

0.66

0.81

0.48

0.79

0.49

APD (Å)

92

64

90

81

90

67

90

67

1000 Å

0.09

0.16

0.09

0.12

0.03

0.10

0.05

0.12

2

PSD (vol %)

Figure 2. X-ray diffractograms of NiMo/Al2O3 and XBNiMo/Al2O3 catalysts.

Figure 3. Radial distribution of molybdenum of NiMo/Al2O3, 0.6B
NiMo/Al2O3, and NiMo
0.6B/Al2O3 catalysts.

atoms are evenly distributed, similar to the boron-free catalyst. Similarly, the preferential deposition of nickel near the periphery of the catalyst particle has also been observed from Figure 4. It is also worth mentioning that this type of favored deposition of MoO3 and NiO on the periphery of the catalyst particle is marginal. Elemental compositions are also measured at different points on the surface of a catalyst particle. It is found that the concentrations

Figure 4. Radial distribution of nickel of NiMo/Al2O3, 0.6B
NiMo/ Al2O3, and NiMo
0.6B/Al2O3 catalysts.

of MoO3 and NiO are 11.76 and 4.76% for NiMo catalysts, whereas these are 27.02 and 6.6 wt %, respectively, for the 0.6B
NiMo catalyst. These weight percentages of MoO3 and NiO are the average concentrations measured at different points on the support. The results indicate that, when boron is impregnated first, there is a high possibility for the formation of a layer of boron and it inhibits metals to enter into the pore cavity deeply; hence, the enrichment of metals on the surface is observed. Similar phenomena have also been noticed by other.6 The formation of the monolayer of boron on the alumina surface also diminishes the metal
support interaction and leads to the formation of multilayer molybdenum oxide or sulfide phases, which is observed from our TEM studies. When this phase grows more, there is a high possibility of the agglomeration of active phases.19 However, molybdenum distribution of the boron content catalyst does not indicate any agglomeration of this phase. Even from our XRD experiment, the well distribution of the active phase is observed. The formation of crystalline MoO3 was observed at a higher loading of boron (1.1
1.7 wt %) by Ferdous et al.11 3.4. High-Resolution Transmission Electron Microscopy (HRTEM). The images of TEM of NiMo/Al2O3 and 0.6BNiMo/ Al2O3 sulfided catalysts are presented in Figure 5(a) and Figure 5(b), respectively. It is observed from Figure 5a that MoS2 fringes having a maximum of four slabs are formed. Mostly the staging of the MoS2 fringes is two to three. The large size of a single slab of MoS2 is also 3103

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Figure 7. Effect of boron loading on cumene hydrocracking activity (T = 673 K and P = 0.1 MPa).

Figure 8. Gas oil HDS of CoMo, NiMo, and different boron-containing catalysts with contact time (T = 633 K and P = 5.9 MPa).

Figure 5. TEM images of (a) NiMo and (b) 0.6BNiMo catalysts.

Figure 6. Effect of boron loading on thiophene HDS activity (T = 673 K and P = 0.1 MPa).

noticed (arrow). Slightly more MoS2 fringes are found in the 0.6BNiMo sulfide catalyst (Figure 5b). The average staging of this catalyst is also higher, and it is three to four slabs. The boroncontaining catalyst may form a monolayer on the alumina surface, and it diminishes the metal
support interaction. This leads to the

formation of high MoS2 slabs. On the other hand, the single large size MoS2 slab in the NiMo catalyst (strong interaction of metal
support) indicates more dispersed phases of MoS2 active sites. 3.5. Thiophene HDS Activity. Thiophene HDS activity of CoMo, NiMo, and B-containing catalysts is studied, and the results are presented in Figure 6. This figure shows that the CoMo catalyst has lower HDS activity than NiMo- and B-containing catalysts. It is also observed that, with an increasing B concentration, the activity increases slowly and the optimum activity is noted at 0.6 wt % B loading. With a further increase of the boron loading, the activity decreases. A similar tendency is observed for all catalysts, irrespective of the preparation methods. This is in good agreement with the results reported by Usman et al.,8 who have found that thiophene activity increases with increasing B loading (0.3
0.6 wt %) and then sharply decreases with a further increase of B. It was explained by the authors that the increment of HDS activity with 0.6 wt % boron addition may be due to the weak interaction of MoS2 with Al2O3. This weak interaction leads to a change of a lower active CoMoS type I site to a more active CoMoS type II site. However, the dispersion of MoS2 on alumina decreases at a higher loading of B. In this case, when the activity is compared to the boron-free catalyst, the increment is very insignificant. This is in line with the results found by others.1,2,10,11 3.6. Cumene HC Activity. The hydrocracking activity of cumene is studied, and the results are presented in Figure 7. The same activity of the boron-free NiMo catalyst has also been compared. The activity is increased with an increasing concentration of B loading up to 1 wt % loading. With a further 3104

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Figure 11. PSDs of fresh (F) and spent (S) 0.6B
NiMo catalyst. Figure 9. Effect of catalyst preparation methods on HDS and HDM activities of Maya heavy crude oil (T = 653 K and P = 9.8 MPa).

Figure 10. PSDs of fresh (F) and spent (S) NiMo catalyst.

loading, the activity decreases. This trend is observed for all boron catalysts, irrespective of the preparation methods. In comparison to the NiMo catalyst, the cracking activity of the B
NiMo and BNiMo catalysts is lower. Only near 1 wt % of boron loading, HC activity of these two catalysts is slightly higher than that of the NiMo catalyst. It is also surprising to note that the same activity of the NiMo
B catalyst is higher than that of NiMo and the other two series (B
NiMo and BNiMo) of catalysts. 3.7. Gas Oil HDS Activity. In Figure 8, gas oil HDS activities of CoMo, NiMo, and B-containing catalysts are presented. In this figure, conversions are plotted against contact time, which is calculated for a batch reactor by the following equation:20 contact time ¼

ðcatalyst weightÞðtotal reaction timeÞ feed weight

It is observed that with a catalyst contact time, the HDS activity increases as expected. Similar to thiophene HDS, the gas oil activity of the CoMo catalyst is also lower than the other catalysts. The HDS activity of boron free and with boron is almost the same. The NiMo
0.6B catalyst shows a slightly higher activity than 0.6B
NiMo and 0.6BNiMo catalysts. From our cumene cracking studies, we also noticed that this NiMo
0.6B catalyst shows higher cracking performance, indicating that this catalyst may have more acid sites. These acid sites may help to remove more refractory sulfur compounds from SRGO. It is worth mentioning that the cumene cracking activity of this catalyst is very high compared to that of the other two

catalysts. However, the gas oil HDS activity of the NiMo
0.6B catalyst is not so high. Thus, there may be other reasons behind it. Therefore, a further investigation is required to make a definite conclusion. 3.8. HDS and HDM Activities of Maya Heavy Crude Oil. The effect of B on HDS and HDM activities of Maya heavy crude oil is also studied, and the results are given in Figure 9. It shows that the CoMo catalyst has lower activity than the NiMo catalyst. In comparison to boron-free NiMo, the B-containing catalyst shows either equal or slightly higher HDS and HDM activities. The HDS activity of Maya heavy crude oil of NiMo is the same as that of the 0.6B
NiMo catalyst; however, the latter shows marginally higher HDM activity. The highest HDS and HDM activities were found for the 0.6BNiMo catalyst among all catalysts studied in this work. Higher activities of HDS and HDM of heavy crude oils have also been reported by Chen and co-workers.12,13 The alumina-borate-supported CoMo catalysts showed substantially higher HDS activity of Kuwait heavy oils than the boron-free catalyst.12 The activity reaches a maximum at 4 wt % B2O3 (∼1.2 wt % B) content. However, with an increase above this concentration, the activities decrease sharply. 3.9. Characterization of Spent Catalysts. The PSDs of fresh and spent NiMo and 0.6B
NiMo catalysts are presented in Figures 10 and 11, respectively. The PSDs of the spent catalysts are changed with respect to their fresh catalysts. The change of PSD is mainly due to the coke formation during hydrotreating of Maya heavy crude oil. The changes are more prominent in the B-free catalyst. It suggests that B inhibits coke formation. The carboneous material on the spent NiMo catalyst is 12.7 wt %, whereas it is 10.5 wt % for the 0.6B
NiMo catalyst. The PSDs of other boron-containing spent catalysts have also been compared to their respective fresh catalyst in Table 3. During early stages of hydrotreating of Maya heavy crude oil, coke is predominantly deposited on the external surface of the catalyst extrudates. Because boron has a monolayer over the alumina surface, it leads to the reduction of the coke formation on this catalyst.

4. DISCUSSION In this study, three different methods are employed for the preparation of boron NiMo/Al2O3 catalysts. The B
NiMo and NiMo
B catalysts containing 0.2
1.2 wt % were prepared by the subsequent impregnation method, whereas BNiMo catalysts were prepared by co-impregnation. It has been described earlier that boron mainly modified two principal characteristics of the hydrotreating catalyst: (1) The modification of alumina support acidity. Several researchers noticed that boron (impregnated first 3105

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Energy & Fuels into the support or prepared homogeneously with alumina) increases weak or medium acidity on the support surface. (2) The modification by boron is the change of catalyst morphologies, such as metal dispersion, reducibility, change of active NiMoS phases, etc. In our present experimental conditions, the addition of boron does not greatly influence the HDS activities of thiophene, gas oil, and Maya heavy crude oil. The apparent activities of NiMo- and B-containing catalysts are almost the same. Usman et al.8 have reported that thiophene HDS performance has not been changed greatly with the addition of boron at low loading. Moreover, with higher loading, the HDS activity decreases with loading. This is in line with the results found by Lewandowski and Sarbak,2 who investigated that HDS of coal liquid is lower or similar in the BNiMo catalyst than that of the NiMo catalyst. It is explained that, at lower boron contents, borate species slightly cover the alumina surface and the active centers are mainly associated with alumina. However, at higher loading, borate ions form a linear polymeric monolayer and impregnated molybdenum is linked with B3þ ions and forms sites that are more active than the ordinary molybdenum sites, such as those with alumina.2 The addition of boron may also decrease Co coverage of the MoS2 edge and cause lower thiophene HDS of the B-containing catalyst.8 The HDS activity depends upon several physiochemical properties of the catalyst, such as surface area, metals dispersion, type of NiMoS active sites, reducibility of active sites, and also to some extent, the support acidity. The presence of higher hydrogenation functionality of the B hydrotreating catalyst responsible for the increase of HDS activity of 4,6-dimethyldibenzothiophene (4,6-DMDBT) has been proven by Ding et al.,4 who also found that this B catalyst has more HDN and HDA activities. The improvement of hydrogenation sites has also been confirmed by Ramirez et al.6 Chen and his group12,13 have also observed the increment of HDS and HDM activities with the addition of extra boron on the hydrotreating catalyst. They mostly agree that the synergic influence of boron on hydrotreating activity is due to the enhancement of dispersed MoS2 active sites, increment of hydrogenation sites, and change of the reducibility of active sites. However, other groups1,2,8,10,11,15,16 reported that boron does not have great influence on HDS reactions. These ambiguous results suggest that there may be differences on the catalyst preparation methods, catalyst activation and reaction conditions, etc. We have observed little complicated results. In this work, three different methods are employed for the preparation of B-containing catalysts, and all of the catalysts show different HDS activity tendency with respect to thiophene, gas oil, and Maya heavy crude oil. How boron influences the hydrotreating catalyst is not very clear. In this study, HDS activity of the boron catalyst is either the same or marginally higher than that of the NiMo catalyst. However, the cumene cracking activity of NiMo
B is very high, although two other series of catalysts, B
NiMo and BNiMo, show almost equal activity of the NiMo (at 1 wt % B loading) catalyst. The literature studies show that strong acidity is required for cumene cracking, and boron does not increase strong acidity of the support. Therefore, our result suggests that boron may increase some acidity of active sites (S
H group)21,22 and may not be linked with support acidity. If boron changes the support acidity, we should observe a higher cracking activity of the B
NiMo/Al2O3 catalyst, where boron is impregnated first on the support and then active metals. On the other reaction, the slight increment of HDS (Maya crude oil) activity of the boron catalyst could be explained from our SEM and TEM results. The

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results found in the SEM experiment show that more active metals are accessible on the surface, whereas TEM results indicate the presence of a little bit higher staging of active MoS2 sites compared to those for the boron-free NiMo catalyst. Hence, more edge and corner sites are available for hydrotreating reactions. It is also important to mention that, in our study, the boron contents are limited to 1.2 wt % only. Therefore, the overall results reveal that boron may form a monolayer over the alumina surface and inhibit active metals to enter into the core of the alumina support. This leads to the formation of more staging of MoS2 slabs and, hence, more activity. However, the activity of boron catalysts is marginal with the boron-free catalyst. Moreover, because of the formation of the borate monolayer, the coke formation is also restricted on the boron catalysts. It also supports that boron may not increase support acidity on higher acid strength, which provokes the coke formation. In the previous studies, boron is generally used for modification of the alumina support, and metals are then impregnated on this modified alumina. However, in this study, we have used three possible impregnation methods and verified which methods could be appropriate. The B
NiMo/Al2O3 catalyst (where the alumina support was first modified by boron) does not show higher HDS activity for any of the four reactions tested in this work. Therefore, our results (with others) suggest that boron in general does not have a great promotional effect on HDS activity.

5. CONCLUSION The addition of boron in the hydrotreating catalyst may form a monolayer of borate on the alumina surface, and it leads to the formation of more staging of MoS2 phases on the support surface. SEM studies shows that more metals are on the surface compared to the boron-free catalyst. The thiophene HDS activity increases with the increasing concentration of boron into the catalysts, and it reaches a maximum at 0.6 wt % B loading. The cumene cracking of the NiMo
B catalyst shows the highest activity. The overall activity of boron catalysts is the same or marginally higher than that of the boron-free catalyst. It is confirmed that boron inhibits coke formation on the catalysts during hydrotreating of heavy oil. ’ AUTHOR INFORMATION Corresponding Author

*Fax: (52)55-91758429. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank Instituto Mexicano del Petroleo (IMP) for financial support. The authors also thank the Microscopy Division of IMP for their help with SEM and TEM. ’ REFERENCES (1) Dalai, A. K. Prepr.
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