Hydrocracking of Maya Vacuum Residue with NiMo Catalysts

Article pubs.acs.org/EF

Hydrocracking of Maya Vacuum Residue with NiMo Catalysts Supported on Mesoporous Alumina and Silica−Alumina Holda Purón,† José Luis Pinilla,† César Berrueco,† J. A. Montoya de la Fuente,‡ and Marcos Millán*,† †

Department of Chemical Engineering, Imperial College London, London SW7 2AZ, U.K. Instituto Mexicano del Petróleo, Dirección de Investigación y Posgrado, Eje Central Lázaro Cárdenas 152, Mexico City 07730, Mexico

‡

S Supporting Information *

ABSTRACT: Hydrocracking of heavy oils poses greater challenges than that of lighter feeds in terms of catalyst properties and resistance to deactivation. The development of larger pore size catalysts may be beneficial to improve their effectiveness toward cracking the larger molecules present in these feedstocks. Mesoporous alumina and mesoporous silica−alumina (MSA) with different textural properties were used as supports to prepare NiMo-based catalysts. These catalysts were tested in the hydrocracking of a vacuum residue (VR) from Maya crude oil and compared against a commercial Al2O3 catalyst. The conversion of VR and asphaltenes into lighter hydrocarbons obtained with supports and catalysts was determined, and the coke deposition process was studied. The in-house developed catalysts were utilized for two consecutive runs with fresh feed to evaluate coke deposition during reutilization of the materials. It was found that coke deposition occurred mainly in the first run, with carbonaceous deposits stabilizing for all the catalysts during their reutilization. Reaction temperature had an important impact on conversions and product distributions, with higher reaction temperatures accounting for higher VR and asphaltene conversions at the expense of a large increase in gas yields. Although the NiMo/Al2O3 catalyst achieved similar VR conversion to the other catalysts, it displayed higher asphaltene conversion with lower coke deposition and a reduced gas yield. The effectiveness of this catalyst can be attributed to its larger pores that can allow better diffusion of asphaltene molecules than MSA or NiMo/MSA as well as the commercial catalyst.

1. INTRODUCTION Heavy oils (defined by API < 10) are growing in prevalence as a refining feedstock since light oil production is progressively declining. This increases the need for more efficient refining technologies, such as hydrocracking, that are capable of converting bottom-of-the-barrel fractions into lighter products of higher value. Hydrocracking catalysts lose their activity and selectivity more quickly during operation when heavy oil feeds are used. Unit turnaround including catalyst regeneration or replacement then results in low cost-benefit refinery economics. Hydrocracking catalysts undergo different types of deactivation during their life cycle, such as coke and metals deposition.1 Deposits of carbonaceous materials are the main cause of shortterm deactivation, occurring in the first hours of operation. Metals also cause deactivation by gradually accumulating on the catalyst active sites. Advances in the understanding of catalyst deactivation mechanisms in the presence of heavy oil and asphaltenic fractions can lead to improved catalysts with increased life cycles. Physical properties of the catalytic support, such as surface area and porosity, determine to a certain extent the capacity of the material to adsorb deposits. The presence of large pores is important in heavy feed processing1 as they allow heavy molecules to diffuse into the catalyst particles and reach the active sites, thereby enhancing catalytic activity. Heavy oils contain a larger share of compounds with polycyclic aromatic hydrocarbon (PAH) structures, mainly as part of asphaltene molecules, which tend to form carbonaceous deposits, or coke, on the catalysts.2 The formation of PAHs can also be promoted by high reaction temperatures that are needed to process © 2013 American Chemical Society

heavier feeds. Therefore, the ability of the catalyst to maintain its activity despite carbonaceous deposit formation remains a very important challenge for heavy oil hydrocracking catalysts. In heavy oil hydrocracking, it was reported that diffusion of large molecules such as asphaltenes through the catalyst pores to reach its active sites is the limiting step in the reaction.2−4 Therefore, the design of the pore size of the catalyst is crucial to improve the catalyst life cycle. Small pores not only hinder asphaltene access but also are preferential sites for coke deposition,2 leading to pore blockage and surface area loss. Metals present in the feed, mainly Ni and V, can also diffuse more easily into the inner surface through wide-pore catalysts and result in pore mouth plugging.5 It has been found6 that mesoporous materials favor the access of asphaltenes from heavier oils into the catalytic structure, promoting hydrodeasphalting (HDA) and hydrodemetalization (HDM) reactions. Hydrocracking catalysts are bifunctional catalysts containing hydrogenation-dehydrogenation and cracking functions. The former takes place on the metal sites of the catalysts, where coordinated unsaturated sites (CUS) are formed in the active state. The main metals that are used for hydrocracking catalysts are Mo or W, normally sulfided in situ and promoted by Ni or Co.1 Cracking occurs on an acidic support such as alumina, silica−alumina, or zeolites, and the cracking ability is proporReceived: April 8, 2013 Revised: June 5, 2013 Published: June 6, 2013 3952

dx.doi.org/10.1021/ef400623f | Energy Fuels 2013, 27, 3952−3960

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2. EXPERIMENTAL SECTION

tional to its acidity. The mechanisms for hydrocracking reactions have been discussed in detail elsewhere.7−9 Metals have to be present in a well dispersed manner within the support to balance the hydrogenation and cracking activities. When the severity, or temperature, of the process is increased, thermal cracking reactions compete with catalytic cracking reactions and can determine the cracking rates and product distributions.10 Conventional types of alumina, e.g. γ-alumina, have been widely used as a catalytic support for hydrotreating and hydrocracking processes because of their acidity and large surface areas. However, mesoporous aluminas − organized mesoporous materials that lack the ordering of conventional types − have received little recent attention for upgrading heavy oils despite having been used in some limited hydrotreating and hydrocracking studies.11−15 Mesoporous aluminas have a greater capability to disperse Mo species, compared to conventional aluminas, achieving higher activity during HDS reactions.12 Mesoporous alumina can have narrower pore size distribution than conventional aluminas.12 Mo catalysts supported on mesoporous aluminas of different porosity were prepared and tested in hydrocracking of residual oils.13 It was observed that conversions increased with increasing acidity of the aluminas and that higher yields of liquid products were obtained when aluminas with larger pores were used. NiMo catalysts supported on mesoporous silica− alumina had a higher stability and performed better than amorphous silica−alumina or USY zeolites during vacuum gas oil hydrocracking.14 Higher asphaltene conversions were obtained with CoMo supported on mesoporous silica−alumina when compared to conventional alumina-supported catalysts during the hydrocracking of Maya crude oil.15 Therefore, mesoporous supports seem to be a promising alternative to the conventional aluminas for the upgrading of heavy oil fractions. Batch and semibatch reactor configurations have been used to study vacuum residua hydrocracking at the laboratory scale.16 In the published works relevant to the present study, an autoclave continuously stirred tank reactor (CSTR) with constant H2 feed was used to convert Maya VR into lighter products using molybdenum hexacarbonyl as the catalyst precursor.17,18 It was concluded that the upgrading of Maya VR is largely thermally driven and that the main contribution of the catalyst was coke reduction. Sugimoto reported19 using a batch autoclave for hydrocracking atmospheric residue under constant gas feed conditions using a NiMo/Al2O3 catalyst. Their report suggested that catalysts main mode of action in heavy oil hydrocracking is hydrogenation and coke reduction, although thermal cracking dominated. The aim of this study is to explore the performance of catalysts supported on mesoporous materials in the upgrading of a highly complex and heavy feed. NiMo supported on mesoporous alumina and MSA catalysts were prepared and tested in the hydrocracking of Maya VR in a batch reactor. Catalysts were recovered and reutilized in a second run to evaluate the extension of further coke deposition and their longer term activity. Reactions were carried out at two different temperatures, 400 and 450 °C to observe its impact in the VR conversion. The asphaltene conversion and product distribution of the hydrocracking reactions were analyzed, and the effectiveness of the different catalysts was compared for activity and selectivity. The conversions are compared to a commercial NiMo alumina catalyst.

2.1. Support and Catalyst Synthesis. An alumina support with large surface area and pore volume was synthesized according to a procedure available in the literature.20 Briefly, aluminum isopropoxide was dissolved in ethanol and propanol with 1, 8, and 6 molar ratios, respectively. Afterward a nonionic surfactant (Pluronic F127) was added in a 0.01 molar ratio while stirring the mixture at 50 °C. Once dissolved, water was added in a 20:1 molar ratio, creating an emulsion. All molar ratios are expressed in reference to aluminum isopropoxide. The surfactant and solvents were later removed by subsequent drying steps at 150 and 350 °C and during calcination at 550 °C for 8 h. MSA was synthesized according to a literature procedure detailed elsewhere21 using aluminum isopropoxide dissolved in tetrapropylammonium hydroxide and later mixed with tetraethyl orthosilicate and ethanol. This mixture resulted in a Si/Al weight ratio of 9. The mixture was aged overnight at room temperature and later dried at 100 °C and calcined at 550 °C for 8 h. The metallic catalysts were prepared by the incipient wetness impregnation method with successive impregnation of the precursor salts, (NH4)2MoS4 and Ni(NO3)2, with intermediate drying steps at 70 °C for 12 h. The metal salts were diluted with diethylenetriamine (DETA) and water in a 10:1 vol/vol ratio. The dilution calculations were based on the support pore size and surface area for the following concentrations: 6 wt % MoO3 and 1 wt % NiO. A low metal concentration was chosen because hydroprocessing catalysts deactivate rapidly when upgrading heavy feeds.22 For catalyst synthesis, a TECAN MPS9500 liquid handling robot controlled by Symyx software, which enabled the use of planned library concentrations and liquid additions to the powder support, was employed. The calcination of the supports and catalysts was carried out in a tube muffle furnace under flowing air at 200 mL·min−1 at 500 °C for 4 h. A commercial NiMo catalyst (PBC-90) described elsewhere23,24 was used. 2.2. Catalyst Characterization Techniques. Elemental analyses with energy dispersive X-ray Fluorescence (XRF) were performed with a Bruker XRF-S2 Ranger with a Cu source. A Micromeritics Tristar analyzer was used to evaluate specific surface area and pore size distribution of the catalysts and supports, based on the standard method of nitrogen adsorption at −196 °C. Samples were dried at 125 °C under N2 flow prior to performing nitrogen adsorption tests. The BET method was used to calculate the surface area, and the pore size distribution was calculated using the BHJ method based on adsorption isotherms. X-ray powder diffraction (XRD) studies were carried out at ambient temperature in a Bruker AXS D8 Discovery with GADDS equipment using CuKα radiation (λ = 1.54 × 10−10 m). Diffraction patterns were measured between 7 and 70° in the 2θ range using two frames with a scanning time of 300 s. A Micromeritics PulseChemisorb 2700 was employed for Temperature Programmed Desorption (TPD), to determine the relative acidity of the samples. First, 200 mg of sample were outgassed in Ar flow at a rate of 10 °C·min−1 from room temperature to 600 °C and held at this temperature for 60 min to allow water and impurities to desorb from the sample. The sample was then cooled down to 50 °C, and ammonia was introduced into the system; the physically adsorbed ammonia was purged afterward. The system was heated at a rate of 10 °C·min−1 to 600 °C, and the chemisorbed ammonia was determined with a thermoconductivity detector (TCD). 2.3. Hydrocracking Experiments. A microbomb reactor described elsewhere25 was used for the hydrocracking reactions. Briefly, the reactor consisted of a 1/2 in. bored-through Swagelok union-T with both ends plugged and connected to a control line. During batch operation the reactor was placed inside a heated fluidized sand bath and connected to a reactor shaker assembly for stirring. The feed consisted of VR, of which 1 g was used in a 4:1 wt/wt ratio with the catalyst. Catalysts were subjected to two sets of reactions at the same conditions: a) the first reaction was conducted under conditions where high coking rates occur resulting in a stable amount of carbonaceous deposits on the catalyst; and the second experiment b) 3953


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was conducted whereby the coked catalysts were reutilized with fresh feed to assess their remaining activity. In the first runs, 0.2 μL of CS2 was used for the in situ sulfiding of the catalysts. Hydrocracking reactions were carried out at 185 bar of H2 pressure and at temperatures of 400 or 450 °C with contact time (defined as the holding time at the desired reaction temperature) of one hour. The reactor was quickly immersed in cold water to ambient temperature to quench the reactions followed by reactor depressurization. Samples were carefully extracted from the reactor with a solvent mixture of CHCl3/CH3OH 4:1 vol/vol. The catalysts were separated from the products by filtration with Whatman PTFE membrane filters of 1 μm pore size. The solids were washed with solvent to ensure that all the soluble materials were removed followed by drying in vacuo at 60 °C for 3 h. The liquid products were dried to constant weight. The remaining traces of solvent were removed with a N2 flow. The coked catalysts were subsequently utilized for a second run with fresh feed, maintaining the feed to catalyst weight ratio without added CS2. The same procedure was followed to recover catalysts and liquid products after the second run. The commercial catalyst was used in a single reaction at 400 and 450 °C. The experimental error was determined by performing repeats; liquid recovery had a ±2.15% error, and maltene to asphaltene fractionation had a ±2.02% error. 2.4. VR and Product Characterization Techniques. Liquid products were recovered after each reaction and fractioned into maltenes (heptane soluble) and asphaltenes (heptane insoluble, toluene soluble). A 100 mg sample was mixed with 6 mL of heptane for 2 h and then centrifuged at 2000 rpm for 20 min. The supernatant was separated, and heptane was added to the rest of the sample. The procedure was repeated at least three times or until the supernatant was clear. Subsequently, toluene was added to the solids, mixed for 2 h, and centrifuged. Both the heptane soluble and toluene soluble fractions were filtered and then dried with N2 flow to constant weight to obtain the maltene/asphaltene weight ratio. Afterward, the maltene fraction was analyzed by gas chromatography (GC), and the molecular weight distribution of the asphaltene fraction was examined by size exclusion chromatography (SEC). The asphaltene fraction did not elute from the GC column, as its boiling point is above 450 °C, beyond the limit of the equipment for this type of sample. A Perkin-Elmer Clarus 500 Chromatographer fitted with a flame ionization detector (FID) was used to quantify the boiling point distribution below 450 °C in the maltene fraction. The GC was equipped with a SGE capillary column (HT-5, 25 m long, 0.1 μm film thickness) and was operated in split mode (split ratio 1:20) with helium as a carrier gas using the ASTM 2887 method. A calibration to evaluate the percentage of elution of the material was performed using Standard Gas Oil (Sigma Aldrich). Another calibration was developed, using the same Standard Gas Oil, to obtain the column retention times that corresponds to the following boiling fractions: heavy naphtha (425 °C). This calibration was used to calculate the concentration of these fractions in the maltenes. SEC was performed on the asphaltene fraction in a system consisting of a “Mixed D” packed column with polystyrene/ polydivinylbenzene (Polymer Laboratories, UK) operated with Nmethyl-2-pyrrolidone (NMP) and chloroform mixtures (6:1 vol/vol) as the mobile phase in conjunction with a Perkin-Elmer LC 290 variable wavelength UV-absorbance detector operating at 300 nm. This setup has previously been used to determine the molecular weight distribution of petroleum derived materials and described in detail elsewhere.26,27 Calibration was performed with polystyrene standards as well as PAH structures, obtaining two curves: one for materials with large molecular mass (600 to 52,000 u) and another one for others with lower molecular mass (80 to 530 u). The exclusion limit of the column for polystyrene is around 200,000 u. Ultraviolet fluorescence (UV−F) spectra were obtained with a Perkin-Elmer LS50 luminescence spectrometer in the static cell mode with chloroform as solvent. Only synchronous UV−F spectra obtained with a 20 nm difference between excitation and emission wavelength are shown in this work.

The amount of carbonaceous deposits on the catalysts after hydrocracking reactions was determined with a Perkin-Elmer TGA1 thermogravimetric analyzer (TGA). Samples of approximately 3 mg were combusted under 40 mL·min−1 of air flow from 50 to 900 °C at 10 °C·min−1. The carbonaceous deposits were calculated as the difference between the initial weight and the final stabilized weight. The VR conversion was defined as the reduction in material boiling above 450 °C and accounted for the carbonaceous deposits on the catalyst as unconverted feed. This enables a distinction between active catalysts and catalysts that merely lead to a large carbon deposition.23 The conversion was therefore calculated with the following expression

VR Conversion mfeed (f> 450 ° C )feed − mLproducts(f> 450 ° C )Lproducts − mSproducts(fcarbon ) = mfeed (f> 450 ° C )feed where mfeed, mLproducts, and mSproducts are the weight of the feed, liquid products, and solid products recovered after reaction, respectively; (f>450 °C)feed and (f>450 °C)Lproducts are the fraction boiling above 450 °C (450 °C+) for the feed (with a value of 1.0) and liquid products, correspondingly. The (f>450 °C)Lproducts was calculated by adding the maltene fraction that did not elute in GC measurements, given that its boiling point was above 450 °C, plus the asphaltene fraction of the sample. ( fcarbon) is the amount of carbonaceous material deposited on the catalyst during reaction determined with TGA measurements. The conversion of the asphaltene fraction of the liquid products was defined as the reduction in asphaltene content between the feed and the products, assuming that the carbonaceous deposits on the catalyst were generated from unconverted asphaltenes. The conversion was calculated with the following expression Asphaltene Conversion =

mfeed (fA )feed − mLproducts(fA )Lproducts − mSproducts(fcarbon ) mfeed (fA )feed

where ( fA)feed and ( fA)Lproducts are the fractions of asphaltenes in the feed and liquid products, respectively. Gases were not collected, but their yield was calculated from a mass balance according to

⎛ mfeed − mLproducts − mSproducts(f )⎞ carbon ⎟ Gas Yield = ⎜⎜ ⎟ mfeed ⎠ ⎝

3. RESULTS AND DISCUSSION 3.1. Vacuum Residue Properties. The VR sample utilized was obtained from Maya crude oil, a heavy oil with a large heteroatom content. The main physicochemical properties of the VR are described in Table 1. Vacuum residue properties worth noting include a low API gravity and a large Conradson carbon number that is indicative of the coke forming precursors. Since the relative reactivity of asphaltenes in Maya Table 1. VR Properties

a

3954

parameter

value

boiling point interval, °C specific gravity at 15 °C °API gravity viscosity at 100 °C, cSt sulfur content, % wt Conradson carbon, % wt asphaltenes, % wt Ni, ppm w/w V, ppm w/w

540+ 1.08 −0.34 265546 7.02 25.54 33.63a 102 169

Determined with the same method as reaction products. dx.doi.org/10.1021/ef400623f | Energy Fuels 2013, 27, 3952−3960

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VR is very low compared to other residua,1 this feed was challenging to upgrade. The VR was used as received without dilution (solid at ambient conditions). Figure 1 presents the SEC and UV−F profiles of the VR and its fractions. In the SEC chromatogram (Figure 1a) a bimodal

Table 3. Acidity of Catalysts and Supports Measured by TPD of Ammoniaa 350−400 °C 400−450 °C 450−500 °C

Al2O3

NiMo/Al2O3

MSA

NiMo/MSA

94.82 74.08 56.75

92.21 69.79 53.30

5.54 3.62 7.80

7.72 2.53 1.43

a

Ammonia concentrations on the catalyst measured by TCD are calculated in three temperature intervals, expressed as (mmol NH3)·(g catalyst)−1.

acidity. All the catalysts retained some acidity above 450 °C. One should expect that the more acidic catalysts would have a stronger cracking function, which could lead to higher coking rates.29 The textural properties for the materials are detailed in Table 4. Both supports presented a mesoporous structure. The Table 4. Textural Properties of Catalysts and Supports Obtained from N2 Adsorption-Desorption Isotherms BET surface area (m2·g−1) pore volume (cm3·g−1) pore diameter (nm)

Figure 1. SEC chromatogram and UV−F spectra of VR (solid black), the VR maltenes fraction (gray) and the VR asphaltenes fraction (dash black).

Al2O3

NiMo/Al2O3

MSA

NiMo/MSA

276 1.42 15.9

233 0.23 11.5

723 0.45 4.7

580 0.15 4.7

Al2O3 had a pore size distribution centered at 15.6 nm, and its surface area of 276 m2·g−1 was reduced by about 15% when the metals were impregnated on the support. The MSA presented a mesoporous structure with the pore diameter centered at 4.7 nm and a surface area of 723 m2·g−1, that was diminished by 20% once metals were deposited. The smaller pore mouth diameter for MSA relative to the alumina might be expected to result in pore mouth plugging. The pore size distribution curves of the supports are presented in Figure 2. The pore size of this alumina is greater than others reported in the literature,11 whereas the MSA has pore sizes similar to other reported MSA supports.14 Figure 3 displays isotherms and BJH adsorption pore size distribution for the supports, Al2O3 and MSA. The Al2O3 exhibited a Type IV isotherm with a hysteresis loop H3 that matches materials formed by platelike particles assembled in parallel. In contrast, the MSA support also had a Type IV isotherm but with a hysteresis loop H2, which has been

distribution was observed where the earlier-eluting peak corresponds to higher apparent mass material (labeled VR asphaltenes) that does not enter the column porosity and therefore elutes faster. However, others have proposed that molecular conformation rather than molecular weight27 may play a predominant role in these molecules being excluded from the column porosity. The second peak, referred here as “retained”, corresponds to material able to penetrate into the column pores. This peak in the VR SEC chromatogram presented a maximum at around 20 min, corresponds to 435 u according to the polystyrene calibration, and a range expanding into several thousand u. The VR asphaltenes and maltenes showed elution times shifted toward higher and lower molecular weight distributions, respectively. UV−F spectroscopy provided information on the extent of aromatic conjugation, showing signal at higher wavelengths for samples containing molecules with larger fused-ring aromatics. The UV−F spectra (Figure 1b) are displayed peak normalized. It was expected that the asphaltenes would be represented by larger polyaromatic ring systems compared to the maltenes based on previous studies.28 3.2. Catalyst Characterization. XRF spectroscopy was used to measure the metal loadings present in the supports. Quantified values for the nominal loading of metal oxides were confirmed (Table 2). Results for acidity studies performed using an ammonia desorption technique are presented in Table 3. This table shows that both the Al2O3 support and NiMo/ Al2O3 had higher acidities than the MSA across several temperature ranges. All catalysts presented strong acidity (above 400 °C). MSA and NiMo/MSA presented very low Table 2. Metal Loadings of the Catalysts, Measured by XRF

MoO3 wt % NiO wt %

NiMo/Al2O3

NiMo/MSA

5.9 1.1

6.0 1.0

Figure 2. Adsorption desorption isotherms and pore size distribution curves (inset) for Al2O3 and MSA. 3955


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Table 5. VR Conversions As a Function of Temperature for Initial and Catalyst Reutilization Reactionsa VR conversion catalyst none PBC-90 Al2O3 NiMo/ Al2O3 MSA NiMo/ MSA

initial reaction 400 °C

reutilization 400 °C



0.21 0.33 0.18 0.23

0.20 0.16

0.68 0.68 0.72 0.64

0.72 0.81

0.21 0.14

0.34 0.23

0.66 0.76

0.77 0.58

a

Experiments were carried out at 185 bar H2 pressure and 1 h contact time.

of the 450 °C+ fraction, although, as discussed below, there were marked differences in terms of asphaltene conversion. The product distributions for the initial and reutilization reactions are reported in Tables 6 and 7, respectively. Products

Figure 3. BJH desorption pore size distribution curves for fresh and spent NiMo/Al2O3.

reported for materials with “ink bottle” pores.30 The isotherms of both materials matched the ones reported in the literature that describes their respective synthesis.20,21 XRD diffraction patterns (available in the Supporting Information) of the catalysts showed no Mo or Ni metal oxide crystallite reflections. The diffraction patterns corresponded only to the Al2O3, which is similar to γ-Al2O3, or MSA microcrystalline materials, suggesting that the Mo and Ni metal oxide crystallites were highly dispersed in the supports and had a size of less than 4 nm.12,15,31,32 3.3. VR Conversion. In the present study VR was hydrocracked at 400 and 450 °C, and the performance of the catalysts was evaluated at both temperatures. The in-house developed catalysts and supports were recovered and reutilized as a strategy to assess their effectiveness when coated with carbon deposits. Catalyst reutilization simulates the contact between fresh feed and a coked catalyst found in continuous operation. A first reaction was carried out to assess the initial coke deposition on the catalysts as well as the product yields, after which the catalysts were recovered and used again to study their performance in the presence of carbon deposits. This experimental approach9,23,24 was necessary since the first reaction conversions were masked by a large deposition of coke. Coke deposition reaches steady state after increasing rapidly in the first hours of contact with hydrocarbon feeds.5 The conversion data of the VR obtained from the first and subsequent reutilization reactions using all the materials (thermal, PBC-90, both supports and catalysts) is reported in Table 5. Runs with only support, Al2O3 and MSA, have been included to evaluate the hydrogenating activity of the metals. It was observed that VR conversions ranged from a) 0.18 to 0.34 for the 400 °C reactions and b) 0.58 to 0.81 for the 450 °C reactions. Thus an important thermal effect was observed in the VR conversion. A significant extent of thermal cracking was observed in the hydrocracking runs without catalyst, with conversion increasing at the higher temperature. For catalysts and supports, the increase in reaction temperature from 400 to 450 °C caused VR conversions to double or even triple. Conversion values are in agreement with others obtained for Maya vacuum residue,33,34 highlighting the predominant role of thermal over catalytic conversion. Catalysts containing Ni and Mo did not necessarily outperform their respective supports in terms of the conversion

Table 6. Product Distribution Obtained after the Initial Reactions Using Supports and Catalystsa % wt of products for initial reactions maltenesb catalyst none none PBC-90 PBC-90 Al2O3 Al2O3 NiMo/ Al2O3 NiMo/ Al2O3 MSA MSA NiMo/ MSA NiMo/ MSA

reaction temp (°C)

gas

450 °C−

450 °C+

asphaltenes

solids

400 450 400 450 400 450 400

10.2 43.5 10.3 34.6 0.0 37.3 0.0

0.0 0.0 11.2 16.2 10.1 18.8 16.3

61.7 26.4 54.8 24.8 51.5 18.2 61.7

26.4 21.2 17.7 13.5 28.5 18.2 12.9

1.7 8.9 6.0 10.9 9.9 7.5 9.1

450

28.0

20.6

31.5

11.5

8.4

400 450 400

1.0 36.2 0.0

12.6 17.5 11.6

47.4 7.2 43.8

31.1 12.6 42.4

7.9 26.5 2.2

450

46.3

8.9

30.1

11.1

3.6

a

The liquid products were fractioned into maltenes and asphaltenes. Carbonaceous deposits are reported as solids and were determined by TGA. Gas yields were calculated by mass balance. bMaltenes were analyzed by GC. Only species with boiling point below 450 °C (450 °C−) eluted from the column and were observed. The value of ( f>450 °C)Lproducts included in the VR conversion calculations was composed of maltenes 450 °C+ and asphaltenes.

were divided into the following: gas, maltenes boiling below 450 °C (450 °C−), maltenes boiling above 450 °C (450 °C+), asphaltenes, and solids. When comparing the initial and reutilization runs, it can be observed that the main changes in the products occurred in the solid yields because most of the coke deposits were formed in the first reaction as discussed above. The Al2O3 and MSA, since they did not present hydrogenation activity, had a small increase in solid yield in their reutilization, with higher coke deposits in the Al2O3 which were promoted by its higher acidity. The significant amount of coke deposited on MSA at the initial 450 °C reaction resulted in the smallest conversion value for this set of reactions. Solids 3956


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of Maya crude oil on CoMo supported on a mixture of zeolite and alumina.37 Gas production (calculated from the Gas Yield equation) was lower in experiments with NiMo/Al2O3 than in those with the Al2O3 itself. The Al2O3 support had the highest acidity among all these catalysts, and it was not balanced by a hydrogenating function. The support showed a slight increase in coke formation when compared to the NiMo/Al2O3, as the presence of the hydrogenation function in the latter stabilized reactive cracking products in an intermediate boiling point range, leading to a reduction in gas yield and asphaltene content accompanied by an increase in 450 °C− materials. On the other hand, the MSA support produced lower gas yield at 450 °C than the NiMo/MSA and a higher coke formation than any other material tested. The high surface area and relatively small pore volume of MSA suggest that it was prone to coke deposition and reduced hydrogenating activity resulting in PAHs conversion into coke. Its lack of hydrogenation function was also observed in the minimal yield of products in the 450 °C− range when the reaction took place at 450 °C. Even though the use of NiMo/MSA led to lower conversions than MSA at 400 °C, its coke deposition was lower because the metal sites in the NiMo/MSA provided hydrogenation activity. The reduced acidity of the material did not trigger cracking reactions. Increasing the temperature also favored a decrease in the 450 °C+ materials. In the case of MSA, even though there was a reduction in the asphaltene content, all the high boiling point hydrocarbon materials were converted into coke. Comparing the performance of MSA and Al2O3 supports, it was observed that the Al2O3 materials show higher coke deposits than the MSA materials in the catalyst reutilization reactions. The difference between catalytic hydrocracking and thermal cracking lies in the quantity of coke generated; thermal cracking produces a higher weight yield of coke than liquid products and a high gas yield.38 In this work the catalytic effect can be observed from the reduction in coke deposits along with higher conversion into material boiling below 450 °C (450 °C−) and less gas formation. PBC-90 had high VR conversion values at both reaction temperatures. From Table 6 it was observed that gas production was higher at 400 °C than for NiMo/Al2O3, influencing the VR conversion. At the 450 °C reaction, higher gas and solids yields than for NiMo/Al2O3 were obtained as well. The loss of textural properties, possibly by pore mouth plugging, for NiMo/MSA (Figure 4) were reflected in the product yields from Tables 6 and 7. Spent MSA had a similar behavior. When MSA was reutilized at 450 °C the maltenes 450 °C− yield was 0.3, compared to 9.9 for Al2O3. For NiMo/MSA the maltenes 450 °C− yield for the initial reaction at 450 °C was 8.9, compared to 20.6 for NiMo/Al2O3; for the reutilization at the same temperature 7.1 of maltenes 450 °C− were obtained contrasted to 40.0 for NiMo/Al2O3. At the latter conditions a yield of 23.3 of asphaltenes was observed, which was double the yield than for NiMo/Al2O3 (11.0). Therefore the loss of textural properties accounted for a reduction in the catalytic activity of MSA materials. On the other hand, Figure 4 shows that NiMo/Al2O3 presented a reduction in pore volume throughout the total pore size range, but the effects of coke on the textural properties were significantly less than for NiMo/ MSA.

Table 7. Product Distribution Obtained after Reutilization Reactions Using Supports and Catalystsa % wt of products for reutilization reactions maltenesb catalyst Al2O3 Al2O3 NiMo/ Al2O3 NiMo/ Al2O3 MSA MSA NiMo/ MSA NiMo/ MSA

reaction temp (°C)

gas

450 °C−

400 450 400

0.4 41.6 2.5

16.1 9.9 11.4

48.6 34.2 66.2

30.8 12.2 19.9

4.1 2.1 0.0

450

32.0

40.2

16.7

11.0

0.1

400 450 400

0.0 52.1 3.4

33.9 0.3 16.5

42.5 38.3 61.0

22.9 9.3 19.1

0.7 0.0 0.0

450

31.9

7.1

37.7

23.3

0.0

450 °C+ asphaltenes

solids

a

The liquid products were fractioned into maltenes and asphaltenes. Additional carbonaceous deposits formed in these runs are reported as solids and were determined by TGA. Gas yields calculated by mass balance. bMaltenes were analyzed by GC. Only species with boiling point below 450 °C (450 °C−) eluted from the column and were observed. The value of ( f>450 °C)Lproducts included in the VR conversion calculations was composed of maltenes 450 °C+ and asphaltenes.

varied with reaction temperature, with higher yields at lower temperatures for NiMo/Al2O3 and Al2O3. This was consistent with results reported35 for the hydrocracking of atmospheric residue with NiMo/alumina catalyst in a batch reactor. The authors explained this observation via a reduction of coke precursors in the system, which does not happen in continuous feeding systems because the concentration of these precursors is constant. Most of the coke was deposited on the catalysts in the first run. When reusing the catalysts only marginal coke formation occurred, resulting in stabilization or even a decrease in the amount of carbonaceous deposits. These trends are in agreement with the literature23,24,36 where the same approach of catalyst reutilization was employed. Coke deposits were taken into account as unconverted feed thereby affecting conversion negatively. Their stabilization contributed to an increase in conversion for most materials, in line with observations reported in other hydrocracking studies.24 In the 400 °C reused catalyst experiments, the conversions were similar to those of the initial run, while at 450 °C an increase in conversion was observed in the run with catalyst reutilization. The only exception to this behavior was NiMo/MSA. Despite the fact that NiMo/MSA had no incremental coke deposition at 450 °C, it did not maintain its activity during the second run where the catalyst was reused. This is likely the result of pore blockage, given that pores of NiMo/MSA are smaller than those of NiMo/Al2O3 as observed in Figure 2 and account for more coke in the first reaction (Table 4). It can be observed in Table 6 that the reaction temperature affected gas production in a substantial manner, indicating that a significant amount of thermal cracking took place at 450 °C. This was confirmed by the large amount of gas products (44 wt %) observed in the thermal run, without catalytic material, which led to a small reduction in asphaltene content. This increase in gas production with reaction temperature is well documented in the literature, for example during hydrocracking 3957


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on the catalysts were unconverted asphaltenes. Table 8 shows the asphaltene conversions for the initial and reutilization Table 8. Asphaltene Conversions As a Function of Temperature for Initial and Reutilization Reactions in the Hydrocracking of Maya VR asphaltene conversion catalyst PBC-90 Al2O3 NiMo/ Al2O3 MSA NiMo/ MSA

Figure 4. BJH desorption pore size distribution curves for fresh and spent NiMo/MSA.





0.29 0 0.35

0 0.41

0.27 0.24 0.41

0.57 0.67

0 0

0.30 0.43

0 0.56

0.72 0.31

catalyst experiments at 400 or 450 °C. In the 400 °C runs using NiMo/Al2O3 there was a notably higher asphaltene conversion than for the other materials. The use of Al2O3 and MSA led to negligible asphaltene conversion, as could be expected since they lacked a hydrogenation function and had strong acidity. This might have caused cracking and subsequent polymerization and coke formation, resulting in a small reduction in the asphaltenic content. It was observed that the reutilization reactions produced a higher asphaltene conversion because most coke deposits reached a steady state during the first reactions. Therefore asphaltenes were transformed into lighter products rather than coke, although this included formation of gases as well as desirable products. The commercial catalyst, PBC-90, had a better performance than the supports or NiMo/ MSA, but at both reaction temperatures the asphaltene conversion was higher for NiMo/Al2O3. The pore volume available in PBC-90 was lower than for NiMo/Al2O3, showing that not only the pore size is important but also the volume of pores. Asphaltene conversion was heavily influenced by reaction temperature in a similar manner to VR conversion. The conversion values for reactions at 450 °C were higher for all catalysts, except for the MSA which had a low performance in the reactions when the catalyst was first used because a significant amount of coke was deposited on it. In the subsequent catalyst reutilization reaction the asphaltene conversion was higher as coke deposits had stabilized. In addition, deposition may generate a reduction in the acidity of the catalysts or supports, which then would be better balanced with the hydrogenation activity. These results are consistent with the reports in the literature showing a significant increase in conversion of asphaltenes in hydrocracking above 425 °C for Middle East VR,39 hydroprocessing above 430 °C for heavy feeds,29 and hydrotreating of Maya crude over 440 °C.40 The latter feed only contained 13 wt % of asphaltenes compared to the 33 wt % content of the Maya VR used in these studies. The former crude was less viscous, thereby facilitating H2 transfer into the feed as well as diffusion of the VR into the catalyst pores. Asphaltene conversion values compared well with hydrocracked Maya crude oil at high temperatures with CoMo supported on zeolite and alumina.37 Figure 6 presents the SEC chromatograms for asphaltenes in the VR and reaction products of the NiMo/Al2O3 catalyst. It can be identified that even those asphaltenes not converted into a different fraction during these reactions underwent molecular

The 450 °C− fraction of the products corresponds to a lower boiling point syncrude with reduced asphaltene content, which in turn would be suitable for further upgrading. The product distributions for the 450 °C− fraction are detailed in Figure 5.

Figure 5. Product distribution obtained by GC on the 450 °C− fraction obtained after reutilization reactions using supports and catalysts.

A large portion of the sample had a boiling point above 425 °C, corresponding to atmospheric residue. Figure 5 reveals that the naphtha yields are very low, and the proportion of kerosene, diesel, and gas oil are similar for all reactions despite the fact that the conversion values of VR (reported in Table 5) differ between catalysts and supports. The MSA at 400 °C displays better yields of lower boiling point material, although the content of asphaltenes in the sample was very high. 3.4. Asphaltene Conversion. The Maya VR used for these studies was highly asphaltenic, and the significant formation of coke deposits on the catalysts during reaction was expected. The porosity of the catalysts affects the diffusion of the hydrocarbons into the active sites, which is the limiting step in the hydrocracking reaction.4 Therefore, the ability of the catalysts to minimize coke production and upgrade asphaltenes was evaluated. The conversion of asphaltenes in the VR hydrocracking runs discussed above was calculated assuming that the coke deposits 3958


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without significantly affecting the size of the fused-ring clusters themselves. This could also explain the large production of gases, which could be generated from cleaving aliphatic side chains.

4. CONCLUSIONS Novel mesoporous NiMo catalysts were prepared with Al2O3 and silica−alumina as supports and tested in hydrocracking reactions at temperatures between 400 and 450 °C and 185 bar H2 pressure. A heavy and viscous VR from Maya crude oil was used for hydrocracking reactions, evaluating the conversion of the VR itself along with the conversion of asphaltenes with the novel materials. The catalysts were reutilized to observe their coking propensity and longer term stability. When the catalysts and supports were utilized in a first reaction with VR, an important amount of carbonaceous deposits was formed. Afterward, when the coked catalysts and supports were reutilized with fresh VR, the NiMo/Al2O3 and the NiMo/ MSA had a negligible increase in coke content, whereas the supports still showed some additional coke deposition. This suggests that the catalysts reached a balance between their cracking activity, provided by the acid supports, and their hydrogenation-dehydrogenation activity, present in their metal sites. NiMo/MSA and MSA proved to be poor materials for upgrading heavy residue since their pores were almost completely blocked by coke deposits in the first hour of reaction. NiMo/Al2O3 could maintain its textural properties despite coke formation. Both VR and asphaltene conversions were highly driven by reaction temperature. Significant asphaltene conversions were maintained in the reutilization reaction for the metal catalysts, with the NiMo/Al2O3 performing better, due to its larger pores. Since the supports alone did not have a hydrogenation capability, the cracked hydrocarbons suffered condensation reactions into asphaltenes, PAH, and coke instead of lighter compounds. NiMo/Al 2 O 3 was the only material that maintained asphaltene conversion in the reutilization reaction. SEC chromatograms identified that asphaltenes underwent molecular size reduction during the reactions, with further decrease in size with an increase in reaction temperature. However, the size of the aromatic ring system did not appear to change considerably. This indicates that cracking mostly took place by cleaving of side chains in the structure being responsible for size reduction. A fraction of the products with boiling point below 450 °C and reduced asphaltene content was obtained.

Figure 6. SEC of asphaltenes in VR and reutilization reaction products obtained with NiMo/Al2O3 as catalyst.

size reduction. The shift in elution times was greater for the 450 °C sample, indicating that the severity of the process impacts the size of the asphaltenes. The SEC of products obtained with the other catalysts tested (not shown) followed the same pattern as NiMo/Al2O3, showing a decrease in asphaltene size with reaction temperature from 400 to 450 °C. In Figure 7 the UV−F spectra of the asphaltene products are presented and compared to the spectrum of the VR

■

Figure 7. UV−F spectra of asphaltenes in VR and reutilization reaction products obtained with NiMo/Al2O3 and Al2O3 as catalyst.

ASSOCIATED CONTENT

S Supporting Information *

asphaltenes. The Al2O3 reaction product spectra were very similar to the VR asphaltenes, although shifted to shorter wavelengths, which indicates certain reduction in aromatic group sizes in the asphaltenic fraction. It is likely that this shift is at least partly due to removal of the larger asphaltenic structures as coke, which was produced in large amounts in runs with these catalysts. The spectra of the NiMo/Al2O3 reaction products appeared at slightly longer wavelengths than the VR asphaltenes. Overall, the UV−F spectra were not very different between feed and products, showing that hydrocracking reactions did not affect chromophore sizes greatly even though significant changes in the size of the product molecules were observed by SEC. This could be attributed to breakage of side chains or chains between aromatic ring clusters

XRD patterns for the fresh catalysts and supports are provided in Figure 8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 3959


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(33) Niemann, K.; Wenzel, F. Fuel Process. Technol. 1993, 35 (1−2), 1−20. (34) Wenzel, F. Proceedings of the International Symposium on Heavy Oil and Residue Upgrading and Utilization; Fushun, China, 1992; pp 185−201. (35) Gualda, G.; Kasztelan, S. J. Catal. 1996, 161 (1), 319−337. (36) Richardson, S. M.; Nagaishi, H.; Gray, M. R. Ind. Eng. Chem. Res. 1996, 35 (11), 3940−3950. (37) Rana, M. S.; Ancheyta, J.; Maity, S. K.; Rayo, P. Catal. Today 2008, 130 (2−4), 411−420. (38) Gray, M. R. Extraction and Upgrading of Oil Sands Bitumen: Intensive short course; Edmonton, 2001. (39) Gauthier, T.; Danial-Fortain, P.; Merdrignac, I.; Guibard, I.; Quoineaud, A. A. Catal. Today 2008, 130 (2−4), 429−438. (40) Ancheyta, J.; Centeno, G.; Trejo, F.; Marroquín, G. Energy Fuels 2003, 17 (5), 1233−1238.

ACKNOWLEDGMENTS H.P. thanks CONACYT Mexico for partial funding towards her PhD. J.L.P. acknowledges the Spanish Ministry of Education for his postdoctoral research grant (ref EX2009-0088).

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REFERENCES

(1) Furimsky, E. Elsevier Science: 2007. (2) Thakur, D. S.; Thomas, M. G. Appl. Catal. 1985, 15 (2), 197− 225. (3) The Chemistry and Technology of Petroleum, 3rd ed.; James, G. Ed.; Speight, Mercel Dekker, Inc.: New York, NY, 1999. (4) Song, C.; Hanaoka, K.; Nomura, M. Energy Fuels 1992, 6, 619− 628. (5) Oelderik, J. M.; Sie, S. T.; Bode, D. Appl. Catal. 1989, 47 (1), 1− 24. (6) Ancheyta, J.; Rana, M. S.; Furimsky, E. Catal. Today 2005, 109 (1−4), 3−15. (7) Degnan, T. F.; Kennedy, C. R. AIChE. J. 1993, 39 (4), 607−614. (8) Sanford, E. C. Ind. Eng. Chem. Res. 1994, 33 (1), 109−117. (9) Sanford, E. C. Energy Fuels 1995, 9 (3), 549−559. (10) Khorasheh, F.; Rangwala, H. A.; Gray, M. R.; Dalla Lana, I. G. Energy Fuels 1989, 3 (6), 716−722. (11) Č ejka, J. Appl. Catal., A 2003, 254 (2), 327−338. (12) Kaluža, L.; Zdražil, M.; Ž ilková, N.; Č ejka, J. Catal. Commun. 2002, 3 (4), 151−157. (13) Looi, P. Y.; Mohamed, A. R.; Tye, C. T. Catal. Commun. 2012, 181−182, 717−724. (14) Corma, A.; Martinez, A.; Martinezsoria, V.; Monton, J. B. J. Catal. 1995, 153 (1), 25−31. (15) Maity, S. K.; Ancheyta, J.; Rana, M. S. Energy Fuels 2005, 19 (2), 343−347. (16) Reynolds, M. A.; Milam, S. N.; Wellington, S. L. Process for Treating a Hydrocarbon-Containing Feed. U.S. Patent Application No. US 2011/0174687, Filed January 21, 2011. (17) Heck, R. H.; Rankel, L. A.; DiGuiseppi, F. T. Fuel Process. Technol. 1992, 30 (1), 69−81. (18) Heck, R. H.; DiGuiseppi, F. T. Energy Fuels 1994, 8 (3), 557− 560. (19) Miki, Y.; Yamadaya, S.; Oba, M.; Sugimoto, Y. J. Catal. 1983, 83 (2), 371−383. (20) Lesaint, C.; Glomm, W. R.; Borg, Ø.; Eri, S.; Rytter, E.; Øye, G. Appl. Catal., A 2008, 351 (1), 131−135. (21) Perego, C.; Amarilli, S.; Carati, A.; Flego, C.; Pazzuconi, G.; Rizzo, C.; Bellussi, G. Microporous Mesoporous Mater. 1999, 27 (2−3), 345−354. (22) Rana, M. S.; Ancheyta, J.; Maity, S. K.; Rayo, P. Catal. Today. 2005, 104 (1), 86−93. (23) Millan, M.; Adell, C.; Hinojosa, C.; Herod, A. A.; Dugwell, D.; Kandiyoti, R. Energy Fuels 2007, 21 (3), 1370−1378. (24) Millan, M.; Adell, C.; Hinojosa, C.; Herod, A. A.; Kandiyoti, R. Oil Gas Sci. Technol. 2008, 63 (1), 69−78. (25) Zhang, S. F.; Xu, B.; Herod, A. A.; Kandiyoti, R. Energy Fuels 1996, 10 (3), 733−742. (26) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2004, 18 (3), 778−788. (27) Berrueco, C.; Venditti, S.; Morgan, T. J.; Á lvarez, P.; Millan, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2008, 22 (5), 3265−3274. (28) Suelves, I.; Islas, C.; Millan, M.; Galmes, C.; Carter, J.; Herod, A.; Kandiyoti, R. Fuel 2003, 82 (1), 1−14. (29) Absi-Halabi, M.; Stanislaus, A.; Trimm, D. L. Appl. Catal. 1991, 72 (2), 193−215. (30) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57 (4), 603−619. (31) García, M. F. Supported metals in catalysis; Imperial College Press: 2005. (32) Kim, P.; Kim, Y.; Kim, H.; Song, I. K.; Yi, J. Appl. Catal., A 2004, 272 (1−2), 157−166. 3960


Hydrocracking of Maya Vacuum Residue with NiMo Catalysts

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