SiO2-Al2O3 spent hydroprocessing catalysts for heavy oils

Catalysis Today 220–222 (2014) 89–96

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Characterization study of NiMo/SiO2 -Al2 O3 spent hydroprocessing catalysts for heavy oils C. Leyva a , J. Ancheyta a,∗ , L. Mariey b , A. Travert b , F. Maugé b a b

Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, Mexico City 07730, Mexico Laboratoire Catalyse et Spectrochimie, CNRS-ENSICAEN, Université de Caen, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex, France

a r t i c l e

i n f o

Article history: Received 1 March 2013 Received in revised form 1 October 2013 Accepted 2 October 2013 Available online 27 October 2013 Keywords: NiMo/SiO2 -Al2 O3 spent catalysts Carbon deposition Metal deposition CO adsorption Cumene cracking

a b s t r a c t NiMo/SiO2 -Al2 O3 spent catalysts were obtained after the hydroprocessing of Maya crude oil in a fixedbed up-flow reactor. The textural properties, metal content, carbon deposition and sulfide phase of spent catalysts were studied and compared with those of fresh catalysts. Scanning electron microscopy was used to study the deposit of nickel and vanadium, and the total metal content was determined by atomic absorption. The sulfide phase was analyzed with the use of infrared spectroscopy technique. Moreover, the catalytic activity of spent catalysts was evaluated with cumene hydrocracking and compared with the catalytic activities of fresh catalysts. The characterization results show that the silica-alumina NiMo catalysts are more sensitive to metal deposition than carbon deposition because of their porous structure which provokes pore mouth plugging and the decrease of the NiMoS active sulfide phase. The results show that optimizing the catalyst resistance to deactivation requires a trade-off between the support acidity and macro-mesoporosity. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, hydroprocessing plays an important role in the petroleum refining industry due to the increasing production of heavy and extra-heavy crude oils. It has become an essential process for conversion of petroleum fractions and residues to commercial fuels and other products [1–3]. However, in Mexico and in other countries the production of heavy and extra-heavy crude oils has started to be a problem, since either the international market buys heavy petroleum at low price or current refineries are not designed to process 100% heavy feeds. Hence, because the high content of impurities in these feedstocks [3,4], there are strong incentives to develop suitable catalysts for upgrading of heavy crude oils. It has been reported that NiMo/SiO2 -Al2 O3 mixed oxide supported catalysts constitute an attractive option for such a purpose, because of their acidic and textural properties [5]. However, these catalysts exhibit loss of activity during time-on-stream mainly due to coke formation, poisoning and/or sintering. The rate and extent of deactivation depend on several factors such as the nature and properties of crude oil, the operating conditions or the reactor design. The deactivation is faster when the crude oil has high content of asphaltenes and metals [6,7]. In order to understand the mechanisms of deactivation and to develop catalysts with

∗ Corresponding author. Tel.: +52 55 9175 8429; fax: +52 55 9175 8429. E-mail address: [email protected] (J. Ancheyta). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.10.007

appropriate life for commercial application, it is of great importance to characterize spent catalysts generated during hydroprocessing. The objective of the present study was to characterize a series of NiMo/SiO2 -Al2 O3 spent catalysts obtained from hydroprocessing of Maya crude oil, in order to elucidate the possible factors responsible of catalyst deactivation. This research is mainly focused on the changes in the porous structure, the catalytic active phase, and the deactivation by poisons such as carbon and metals deposits.

2. Experimental 2.1. Spent catalyst Four supports were synthesized by homogeneous coprecipitation method varying the composition of silica and the four respective nickel-molybdenum supported catalysts were prepared by the incipient wetness co-impregnation method; more details about operating conditions and synthesis of materials can be found elsewhere [5]. The NiMo/SiO2 -Al2 O3 spent catalysts were obtained after the hydroprocessing of Maya crude oil (21.31◦ API, 12.7 wt% asphaltenes, 322 ppm Ni + V, 3.52 wt% sulfur) carried out in a stainless steel tubular fixed-bed up-flow reactor (1.3 cm, inner diameter). The operating conditions used during the reaction were: temperature, 380 ◦ C; pressure, 5.4 MPa; LHSV, 1.0 h−1 ; H2 -to-oil, 356 m3 /m3 ; time-on-stream, 204 h. The spent catalysts were unloaded from the reactor after hydroprocessing reaction and washed by Soxhlet method with a mixture of toluene/heptane

90

C. Leyva et al. / Catalysis Today 220–222 (2014) 89–96

(2:1) at 96 ◦ C during 8 h. Hereafter, the spent catalysts are denoted as NiMoSA-5-S, NiMoSA-10-S, NiMoSA-25-S and NiMoSA-50-S, where the number (5, 10, 25, 50) represents the composition of silica in the support. 2.2. Catalyst characterization 2.2.1. Deposit of metal on spent catalysts by electron microscopy of the energy dispersive X-ray (SEM-EDX) The dispersion of nickel and vanadium in spent catalysts as well as the content of carbon were determined by a SEM-FIB analytical instrument xT Nova NanoLab 200, using SEM-EDX analysis. 2.2.2. Textural properties Nitrogen adsorption–desorption analysis at liquid nitrogen temperature (−196 ◦ C) were obtained with a Quantachrome 4000 unit. Prior to analysis, the pellets were out-gassed in vacuum at 350 ◦ C for 3 h. BET specific surface area (SSA), total pore volume (TPV), and pore size distribution (PSD) were obtained by the BJH method [8]. 2.2.3. Atomic absorption (AA) Nickel and vanadium contents were measured by atomic absorption using a SOLAAR AA Series Spectrometer analyzer. Solid samples were heated to 550 ◦ C for elimination of possible organic material. After that, the samples were digested in a mixture of acids (HCl/HNO3 ) with heating until total dissolution. Finally, they were filtered and analyzed. 2.2.4. Fourier transformed infrared spectroscopy (FT-IR) (a) Pyridine adsorption on SiO2 -Al2 O3 supports SiO2 -Al2 O3 supports were studied by IR spectroscopy following adsorption of pyridine (1 Torr at equilibrium) and desorption under evacuation at increasing temperatures. Concentrations of Lewis and Brønsted acid sites were estimated from the area of IR bands at ca. 1450 and 1540 cm−1 , respectively, using the integrated molar extinction coefficients given by Emeis [9]. (b) CO adsorption on catalysts Freshly sulfided and spent catalysts were studied using CO adsorption at low temperature (−196 ◦ C) with increasing amounts of small CO doses up to an equilibrium pressure of 133 Pa. Spectra were scanned using a Nicolet 710 FT-IR spectrometer equipped with MCT detector with a resolution of 4 cm−1 . For comparison, all the spectra were normalized to a disk of 10 mg/cm2 . More detail about the procedure of catalyst sulfidation is described in a previous work [5]. 2.3. Hydrocracking of cumene The cumene hydrocracking reaction was conducted in a continuous flow glass reactor operating at atmospheric pressure and 400 ◦ C, in which 200 mg of catalyst were loaded. Prior to reaction, the catalysts were sulfided at 400 ◦ C for 3 h in a flow of 50 mL/min of hydrogen saturated with CS2 . 3. Results and discussion 3.1. Scanning electron microscopy (SEM-EDX) The SEM-EDX mapping of NiMoSA-5 and NiMoSA-10 catalyst extrudates is illustrated in Fig. 1, where the NiK␣ and VK␣ distribution reveals how the nickel and vanadium were deposited on theses extrudates. While nickel is deposited along the extrudate, vanadium is deposited on the surface of the catalyst, which confirms that the pores are plugged by this latter metal [10–13]. This

indicates an internal diffusion limitation of these two spent catalysts that are characterized by small pore diameter and low silica content. SEM-EDX analysis for NiMoSA-25-S and NiMoSA-50-S spent catalysts are shown in Fig. 2. The large pore diameter NiMoSA-50-S catalyst leads to higher amounts of metal deposit. The profiles of both metals are homogeneous along the extrudate, confirming that macropores promote the diffusion of large molecules into the pores. NiMoSA-25-S has an intermediate behavior between the small pores catalysts (NiMoSA-5-S and NiMoSA-10-S) and the large pore catalyst: the distribution of nickel is homogeneous along the extrudate and while vanadium is more concentrated at the outer surface, its internal concentration is significantly higher than that of NiMoSA-5-S or NiMoSA-10-S catalysts. These results indicate that the diffusion of Ni containing molecules is larger than that of vanadium ones, which could be due to its molecular size and/or intrinsically slower de-nickelation kinetic rate. It is also reported that the Ni complex molecule is more refractory than vanadium [1,14].

3.2. Textural properties A comparison of the textural properties of fresh and spent catalysts is presented in Table 1. The decrease of specific surface area of NiMoSA-5-S and NiMoSA-10-S is large (70 and 80%, respectively), this is mainly due to the deposit of coke and metals on the catalyst surface and the presence of small diameter pores (5.6–5.8 nm). For high silica content catalysts (NiMoSA-25-S and NiMoSA-50-S) spent catalyst surface area decreases by 58 and 56%, respectively. These two catalysts were relatively less affected than low silica content catalysts, which could be explained by their macroporosity, that allows large molecules to diffuse into the macropores instead of blocking up micro/mesopore mouths. The porous structure of spent catalysts exhibits an increase in the fraction of micro-pores, which could be explained by substantial meso- and macro-pore blocking caused by the carbon and metal deposit during the reaction. The mesopores decrease for all catalysts in the following order: NiMoSA-25-S, NiMoSA-5-S, NiMoSA-10-S and NiMoSA-50-S, which could be related to the metal deposited on the surface of the catalyst [5]. Pore size distribution of spent and fresh catalysts are shown in Fig. 3. A large decrease in small pore diameter (<5 nm) is shown for the NiMoSA-5-S and NiMoSA-10-S samples. This confirms the deactivation at the pore mouth, which is affecting the maximum utilization of internal surface of the catalyst, and consequently, blocks the active sites. This figure also indicates that the deposition of metal around the pore mouth increases with the decrease of pore diameter. The PSD for NiMoSA-25-S shows a significant decrease in the small pores in the range of 5–10 nm, around 85% with respect to the fresh catalyst. On the other hand, NiMoSA-50-S was less affected due to its macroporosity structure, larger pore diameter has lower pore mouth plugging and better tolerance for metal retention. The most common deactivation is either due to the blocking of catalytic sites by deposit of species or due to the reactant diffusion limitation, which is more severe for small pore diameter catalysts than high silica content catalysts. Adsorption–desorption isotherms of spent and fresh catalysts are also shown in Fig. 3. A large increase in the hysteresis loop is shown for the NiMoSA-5-S and NiMoSA-10-S samples. The comparison of isotherms indicates that the placed species are either totally close to the pore or partially for these two catalysts. This confirms the deactivation at the pore mouth, which is affecting the maximum utilization of internal surface of the catalyst, and consequently, blocks the active sites. This figure also indicates that

C. Leyva et al. / Catalysis Today 220–222 (2014) 89–96

Fig. 1. SEM-EDX profiles of low silica content spent catalysts: V and Ni deposited on NiMoSA-5-S, V and Ni deposited on NiMoSA-10-S.

Fig. 2. SEM-EDX profiles of high silica content spent catalysts: V and Ni deposited on NiMoSA-25-S, V and Ni deposited on NiMoSA-50-S.

91

92

C. Leyva et al. / Catalysis Today 220–222 (2014) 89–96

Table 1 Textural properties of fresh (F) and spent (S) NiMoSA supported catalysts. Catalyst

NiMoSA-5

SSA, m2 g−1 TPV, cm3 g−1 APD, nm Micropore, %a Mesopore, %a Macropore, %a

NiMoSA-10

NiMoSA-25

NiMoSA-50

F

S

F

S

F

S

F

S

327 0.48 5.8 1.1 98.3 0.59

89.6 0.16 7.0 16.2 79.5 4.4

349 0.49 5.6 1.6 97.7 0.66

65.5 0.14 8.4 13.0 81.2 5.8

255 0.54 8.5 1.2 93.3 5.5

106.0 0.27 10.1 4.9 46.8 48.3

167 0.58 14.0 1.5 73.8 24.7

73.1 0.30 16.1 2.3 65.2 32.5

19.7 31.2 33.1 10.5 4.3 1.2

2.4 2.9 14.3 32.1 27.5 20.8

P.S.D. (Pore size distribution), % <5 nm 5–10 nm 10–25 nm 25–50 nm 50–100 nm >100 nm

43.0 47.8 7.5 1.02 0.4 0.19

27.4 48.8 13.7 5.7 2.3 2.1

46.8 44.4 7.0 1.1 0.38 0.28

22.1 37.9 26.9 7.3 3.9 1.9

9.0 15.8 27.8 22.7 15.9 8.8

8.5 11.4 24.8 22.8 20.5 12.0

F, fresh catalyst; S, spent catalyst. a Micropore <2 nm, mesopore 2–50 nm, macropore >50 nm.

the deposition of metal around the pore mouth increases with the decreasing the pore diameter. The isotherms for NiMoSA-25-S and NiMoSA-50-S catalysts show a slight increase as compared with the isotherms of fresh catalyst. This suggests that macropore structure is less affected; larger pore diameter has lower pore mouth plugging and better tolerance for metal retention. The increase in isotherm area is due to

0.30

A)

0.25

0.25

0.20

0.20

Dv (log d), cc/g

Dv (log d), cc/g

0.30

the change in cylindrical pores into the “ink-bottle” types of pores [15,16]. The “ink-bottle” pores are expected difficult to be desorbed until relative pressure is quite low to allow the physisorbed nitrogen from the narrow neck of pores. The most common deactivation is either due to the blocking of catalytic sites by deposit of species or due to the reactant diffusion limitation, which is more severe for small pore diameter catalysts than high silica content catalysts.

0.15 0.10 0.05 0.00

0.15 0.10 0.05 0.00

1

0.10

10 100 Pore diameter, nm

1000

1

0.10

C)

0.08

10 100 Pore diameter, nm

1000

D)

0.08 Dv (log d), cc/g

Dv (log d), cc/g

B)

0.06 0.04 0.02

0.06 0.04 0.02 0.00

0.00 1

10 100 Pore diameter, nm

1000

1

10 100 Pore diameter, nm

1000

Fig. 3. Pore size distribution and nitrogen adsorption–desorption isotherms of fresh (solid line) and spent (dotted line) catalysts. (A) NiMoSA-5, (B) NiMoSA-10, (C) NiMoSA-25 and (D) NiMoSA-50.

C. Leyva et al. / Catalysis Today 220–222 (2014) 89–96

3.0

illustrated in Fig. 5. The presence of Brønsted acid sites is evidenced by the band 1540 cm−1 characteristic of pyridinium species (BPy), whereas pyridine coordinated on Al3+ Lewis acid sites in tetrahedral and octahedral environments lead to the bands at 1621 cm−1 and 1616 cm−1 (LPy) [17]. SA-5 and SA-10 present high amount of Lewis acid sites and almost no Brønsted acidity. When silica content increases, Lewis acid site concentration decreases whereas Brønsted acidity develops. The band at 1492 cm−1 corresponds to the hydrogen bond with pyridine (HPy) as well as the protonation (BPy) and coordination (LPy) of pyridine, respectively. Pure alumina is well-known to have high concentration of Lewis acid sites resulting from coordinative unsaturated Al3+ , and no Brønsted acidic sites detected by pyridine. Hence, low-silica containing ASA samples present characteristics close to those of alumina. High silica-containing supports, develop Brønsted acid sites that were previously assigned to the formation of Si OH Al linkages [18,19].

45.0 V

Ni

40.0

C

35.0 2.5

30.0

2.0

25.0

1.5

20.0 15.0

1.0

10.0 0.5

Coke content , wt%

Metal content (Ni and V), wt%

3.5

5.0 0.0

0.0

93

NiMoSA-5-S NiMoSA-10-S NiMoSA-25-S NiMoSA-50-S Fig. 4. Metal and carbon depositions on spent catalysts.

3.3. Atomic absorption The nickel and vanadium contents deposited on the catalysts obtained by atomic absorption are presented in Fig. 4, coke deposition is also shown in the same figure. All the values correspond to fresh basis, i.e., grams of metal/100 g of fresh catalyst, and nickel amount of the fresh catalysts has been discounted. On the one hand, a trend is observed between the amount of deposited metals and the acidity and mesoporosity of the NiMoSA-5-S, NiMoSA-10S and NiMoSA-25-S catalysts. This fact is rather normal due to the presence of wide pores as the silica content increases. On the other hand, NiMoSA-50-S has lower amount of deposited metals than NiMoSA-25-S but higher than NiMoSA-5-S and NiMoSA-10-S, which can be attributed to its highest acidity that provokes higher carbon deposits [5], causing those pores available for metal deposition be first occupied by coke. Also mesoporosity of this catalyst, particularly its pores of less than 10 nm, is about a half of that of NiMoSA-25-S catalyst.

3.4.2. Carbon deposition The deactivation of heavy oil hydroprocessing catalysts is not only due to deposition of metals such as nickel and vanadium, but also to coke deposit. To elucidate the species of carbon deposited on the surface, the spent catalysts were characterized by FT-IR using CO as molecule probe for studying the sulfided phase after reaction. Figs. 6 and 7 illustrate the carbonaceous species present on the surface of the catalyst (C C stretching region and C H stretching region, respectively). According to the literature, the 1377 cm−1 band is characteristic of the catalysts poisoned by coke and is ascribed to the symmetric bending CH vibration of CH3 groups [14] and characterized as “soft coke”. No specific band at 1580 cm−1 could be observed. The latter is usually characteristic of crystalline graphite (hard coke). This shows that no significant amount of hard coke was deposited. Since hard coke is expected to build up at long time-on-stream, this absence might be due to the short time-onstream (204 h) used in the experiments. The frequency band of CH2 and CH3 groups are related to the 1500–1400-cm−1 region [20]. The vibration mode of methyl/methylene groups in long aliphatic chains are characterized by the bands located near 1456 cm−1 which corresponds to the asymmetric bending C H vibration bond and the bands in the region of 1375 cm−1 are due to the

3.4. Fourier transformed infrared spectroscopy (FT-IR) 3.4.1. Pyridine adsorption–desorption on supports The evolution of Lewis and Brønsted acid sites after the thermodesorption of pyridine at 50 ◦ C for the four Si-Al supports is

1450 SA-5 SA-10 SA-25 SA-50 1622

1640

1597

1492 1577

0.02 1545

1750

1700

1650

1600

1550

1500

1450

1400

Wavenumber, cm-1 Fig. 5. IR spectra of adsorbed pyridine on SiO2 -Al2 O3 supports (SA-5, SA-10, SA-25, SA-50, the number indicates the amount of SiO2 in the support). Thermodesorption at 50 ◦ C.

94

C. Leyva et al. / Catalysis Today 220–222 (2014) 89–96 Aromatic and graphite

35 Aliphatic

Olefinic

1621

12924 cm -1

30

1456

22954 cm -1

0.1 1734

Area, cm-1

1580

1690

1377

NiMoSA-5-S

NiMoSA-10-S

25 20 15 10 5

NiMoSA-25-S

0 0

NiMoSA-50-S

1700

1600

1500

1400

1300

0.1

0.2 0.3 Si/(Si+Al)

0.4

0.5

Fig. 8. Integrated intensities of the bands at 2924 cm−1 (CH2 groups) and 2954 cm−1 (CH3 groups) in the spectra of spent catalysts as a function of Si/(Si + Al) ratio.

Wavenumber, cm -1 Fig. 6. FT-IR spectra of NiMoSA spent catalysts, (CO) region.

2924 2954

0.1 2854 2873

NiMoSA-5-S NiMoSA-10-S NiMoSA-25-S NiMoSA-50-S 3000

2950

2900 Wavenumber, cm -1

2850

2800

Fig. 7. FT-IR spectra of NiMoSA spent catalysts, (OH) region.

SEM-EDX analysis (Fig. 4) NiMoSA-50-S resulted to have the highest coke deposition, while from FT-IR results (Table 2) NiMo-SA-25-S exhibited coke deposition a little bit higher than NiMoSA-50-S catalyst. This different behavior is because FT-IR results correspond only to soft coke. In the CH stretching region (3000–2800 cm−1 , Fig. 7), four bands were detected by infrared spectroscopy. The band at 2924 cm−1 corresponds to the asymmetric CH2 stretching vibrations while the band at 2954 cm−1 is attributed to asymmetric CH3 stretching vibrations. The 2873 cm−1 and 2854 cm−1 bands are related to CH stretching bond in both CH3 and CH2 groups. The broad adsorption bands in the range of 3000–2800 cm−1 are assigned to aliphatic moieties containing CH3 and CH2 groups [24], the relative intensity of the CH2 and CH3 characteristics bands may give some indications on the amount and chain length or branching of aliphatic moieties. The computed areas are shown in Fig. 8. These results indicate that the amount of both CH3 and CH2 decreases as the silica content increases. In parallel, both the amount of deposited carbon (Fig. 4) and soft coke (Fig. 6 and Table 2) increase. This trend could be correlated to the effect of silica acidity which may favor both aromatic coke deposition and cracking of aliphatic chains.

Table 2 Soft coke deposited on spent catalysts. Catalyst

Area, a.u.

NiMoSA-5 NiMoSA-10 NiMoSA-25 NiMoSA-50

1.13 0.98 1.43 1.33

symmetrical bending mode [20,21]. The band at 1734 cm−1 could be related to the (C O) vibration of the carboxylic acids [22]. The appearance of bands between 1500 and 1640 cm−1 can be ascribed to aromatics C C stretching vibrations over polyaromatic compounds [23] and/or to vibration of C O (at around 1620–1550 cm−1 ) due to the presence of oxygenated molecules. The similarity of the spectra indicates that the four spent catalysts have the same type of carbon species. The area of the bands characteristic of the soft coke present on spent catalysts is reported in Table 2. It shows that the amount of soft coke deposited on the four catalysts is of the same order of magnitude. The deposit on high silica content catalysts, however, is significantly higher than that on low silica content catalysts, which can be related to acidity and the specific surface area, since high silica content catalysts possess higher acidity but much lower specific surface area. The trend of results of Table 2 matches well with those plotted in Fig. 4, except for the highest silica content catalyst. From

3.4.3. Sulfide phase of spent catalyst The difference between the fresh and spent catalysts is illustrated in Fig. 9, which shows the spectra of CO adsorbed at saturating conditions (1 Torr in equilibrium at −196 ◦ C of CO) on both types of catalysts. The sulfide phase of spent catalysts was analyzed after the thermo-evacuation of the samples at high temperature (400 ◦ C) in order to remove some weakly adsorbed carbon species of the surface and to analyze them with the adsorption of CO at low temperature. The IR spectra obtained on fresh catalysts display bands at ∼2190 cm−1 (Al3+ Lewis acid sites), ∼2155 cm−1 (surface silanol and hydroxy groups of the support) and bands at lower frequencies characteristic of non promoted molybdenum sites and promoted sites of the NiMoS (2127 cm−1 ) sulfide phase [5,25]. The spectra obtained for the spent catalysts show a strong change in the amount of surface sites. In particular, the area of the CO bands characteristic of NiMoS species and unpromoted molybdenum are shifted toward lower frequencies, indicating an increase of the sulfide phase electron density [25]. It is worth noticing that such a frequency shift also leads to an increase of the CO absorption coefficient [26], indicating that the total amount of sulfide phase sites is lower on spent catalysts as compared with fresh catalysts. Similarly, surface OH groups and Lewis acidic sites from the support exhibit a considerably decrease. This shows that the deposit of coke and metals causes the poisoning of the support surface

C. Leyva et al. / Catalysis Today 220–222 (2014) 89–96 2156

Sulfidephase 2127 2111 2135

a)

95

Sulfidephase

b)

2095 2078

2056 2077

2056 0.02

0.02

2195

2128 2110

2200

2150

2100

2050

2000

2200

2150

Wavenumber, cm -1

2100

2050

Sulfidephase

21112194

c)

2076

2000

1950

Wavenumber, cm -1 Sulfidephase

2057

2129

d)

2127

2109

0.01

0.02

2095 2075 2051

2200

2150

2100

2050

2000

1950

2200

2150

Wavenumber, cm -1

2100

2050

2000

Wavenumber, cm -1

Fig. 9. FT-IR spectra of CO (1 Torr) adsorbed at −196 ◦ C on fresh (—) and spent (—) catalysts: (a) NiMoSA-5, (b) NiMoSA-10, (c) NiMoSA-25 and (d) NiMoSA-50.

sites. In general, the deactivation is due to the strongly adsorbed species, for example, N-compounds, coke, metal, which occupies active sites, resulting in a loss of catalytic activity. These results also show that the nature of the sulfide phase could be affected, in particular the promoted NiMoS sites which show enhanced electron density. While the origin of this enhancement is not definitely established, it can be tentatively assigned to the deposit of electron rich deposits (e.g. coke or reduced metals) in contact with the sulfide phase.

4.0 3.5

Fresh catalysts Spent catalysts

0.6

3.5. Cumene hydrocracking activity The decrease of catalytic acid sites, which are selectively poisoned by carbon deposition on Brønsted sites of silica-alumina, was confirmed by cumene hydrocracking as is illustrated in Fig. 10. It shows a drastic decrease of catalyst activity of spent catalysts as compared with fresh catalysts. Moreover, while in the case of fresh catalysts, activity increases according to the increasing acidity of the catalyst, the spent catalysts show the opposite trend, i.e. decrease of the catalytic activity with the silica content. This fact is related mainly to the coke and metals deposited on the surface of the most acidic catalysts (NiMoSA-25-S and NiMoSA-50-S). It is observed that the NiMoSA-10-S has a slightly higher catalytic activity among all catalysts, this fact can be attributed to the lower carbon deposition on its surface.

0.5

-1 -1

2.0

0.3

1.5

0.2

3

rHCR (mol h g )*10

0.4

-1 -1

2.5

rHCR (mol h g )*10

3

3.0

1.0 0.1 0.5 0.0 0.0 NiMoSA-5

NiMoSA-10

NiMoSA-25

NiMoSA-50

Fig. 10. Comparison of hydrocracking activity of cumene for fresh and spent catalysts.

4. Conclusions The silica-alumina NiMo catalysts for hydrocracking of heavy crude oils are strongly affected by carbon and metal deposition. Due to the short time-on-stream used in the experiments, only soft coke was detected by FT-IR, which was found to be in the same order of magnitude for the four spent catalysts. Hard coke is more common at long time-on-stream that is why no specific band was detected. The deactivation rate increases with silica content of the catalyst, the acidity favors the accumulation of aromatic carbon on the surface strongly affecting the high silica content catalyst, while lower silica content catalysts are relatively less affected by carbon deposition. Low silica content catalysts are more sensitive to metal deposition because of their porous structure, which provokes pore mouth plugging. Another important aspect is the decrease of the

96

C. Leyva et al. / Catalysis Today 220–222 (2014) 89–96

total amount of NiMoS sites due to the accumulation of electron rich deposits of carbon and reduced metals, which is more severe for high silica content catalysts. Hence, in order to minimize the deactivation of silica alumina supported catalysts it is required a trade-off between acidity and meso-macroporosity, which both increase with the silica content. Acknowledgements The authors thank Instituto Mexicano del Petróleo and Consejo Nacional de Ciencia y Tecnología (CONACyT) for financial support by means of Programa de Cooperación de Posgrado México-Francia (Project PCP 03/07). References [1] J. Ancheyta, Modeling of Processes and Reactors for Upgrading of Heavy Petroleum, CRC Press Taylor and Francis Group, Boca Raton, 2013. [2] I. Sandrea, R. Sandrea, J. Oil Gas 105 (2007) 1–4. [3] Oil: Regional Consumption-by product Group, Statistical Review, 2009, www.bp.com [4] J.W. Gosselink, CatTech 2 (1998) 127. [5] C. Leyva, J. Ancheyta, A. Travert, F. Maugé, L. Mariey, J. Ramírez, M.S. Rana, Appl. Catal. A: Gen. 145/146 (2012) 1–12. [6] C.H. Bartholomew, Mechanisms of catalyst deactivation, Appl. Catal. A: Gen. 212 (2001) 17–60.

[7] M. Marafi, A. Stanislaus, E. Furimsky, Handbook of Spent Hydroprocessing Catalysts, Elsevier, Amsterdam, 2010, pp. 17–49 (Chapter 3). [8] Manual Quantochrome, Nova 2000 equipment. [9] C.A. Emeis, J. Catal. 141 (1993) 347–352. [10] S.K. Maity, E. Blanco, J. Ancheyta, F. Alonso, H. Fukuyama, Fuel 100 (2012) 17–23. [11] R.A. Ware, J. Wei, J. Catal. 93 (1985) 100–121. [12] R.A. Ware, J. Wei, J. Catal. 93 (1985) 122–134. [13] R.A. Ware, J. Wei, J. Catal. 93 (1985) 135–151. [14] A. Guo, Z. Ren, L. Tian, Z. Wang, K. Li, J. Fuel Chem. Technol. 35 (2) (2007) 169–175. [15] M.S. Rana, J. Ancheyta, S.K. Maity, P. Rayo, Catal. Today 104 (2005) 86– 93. [16] F. Rojas, I. Kornhauser, C. Felipe, J.M. Esparza, S. Cordero, A. Domínguez, J.L. Ricardo, Chem. Chem. Phys 4 (2002) 2346–2355. [17] G. Busca, in: J.L.G. Fierro (Ed.), Metal Oxides: Chemistry and Applications, CRC Press, Boca Raton, FL, USA, 2005, pp. 247–318. [18] G. Crepeau, V. Montouillou, A. Vimont, L. Mariey, T. Cseri, F. Mauge, J. Phys. Chem. B 110 (2006) 15172–15185. [19] M. Rodriguez-Delgado, C. Otero-Arean, Energy 36 (2011) 5286. [20] L.P.A.F. Elst, S. Eijsbouts, A.D. van Langeveld, J.A. Moulijn, J. Catal. 196 (2000) 95–103. [21] S.H. Wang, P.R. Griffiths, Fuel 64 (1985) 229–236. [22] L. Mariey, J. Lamotte, T. Chevreau, L.C. Lavalley, React. Kinet. Catal. Lett. 59 (1996) 241–446. [23] P. Magnoux, H.S. Cerqueira, M. Guisnet, Appl. Catal. A: Gen. 235 (2002) 93–99. [24] J.M. Chalmers, P.R. Griffiths, Handbook of Spectroscopy, Sample Characterization and Spectral Data Processing, Vol. 3, John Wiley & Sons, Chichester, U.K., 2002. [25] A. Travert, C. Dujardin, F. Mauge, E. Veilly, S. Cristol, J.F. Paul, E. Payen, J. Phys. Chem. B 110 (2006) 1261–1270. [26] F. Maugé, J.-C. Lavalley, J. Catal. 137 (1992) 69–76.