Subscriber access provided by MacEwan University Libraries
Article
Effect of metal loading in NiMo/Al2O3 catalysts on Maya vacuum residue hydrocracking Holda Puron, José Luis Pinilla, J. Ascención Montoya de la Fuente, and Marcos Gabriel Millan Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Effect of metal loading in NiMo/Al2O3 catalysts on Maya vacuum residue hydrocracking Holda Purón†, José Luis Pinilla†,§, J A Montoya de la Fuente‡, Marcos Millán†, †
‡
Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK
Instituto Mexicano del Petróleo, Dirección de Investigación y Posgrado, Eje Central Lázaro Cárdenas 152, Mexico City 07730, Mexico §
Current Address: Instituto de Carboquímica. CSIC. C/ Miguel Luesma 4. 50018 Zaragoza, Spain
KEYWORDS: Hydrocracking, vacuum residue, NiMo catalysts, metal loading, asphaltenes.
Abstract
Hydrocracking catalysts with large porosity need to be developed to treat heavy oil feedstocks rich in large molecular weight components such as asphaltenes and withstand deactivation due to coke formation. In this work, catalysts were prepared by impregnation of varying NiMo loadings on a mesoporous Al2O3 support. The effect of metal loading on the hydrocracking of a vacuum residue at three temperatures (400, 425 and 450 °C) was studied in a batch microbomb reactor. Catalysts were reutilized in a second reaction with fresh feed to assess their activity following the initial period where results are dominated by a large carbon deposition. The textural properties and the coke content on the spent catalysts were evaluated after both reactions. It was found that the reaction temperature had an important effect on the conversion of the fraction with a boiling point above 450 °C whereas metal loading had minimal impact. On the other hand, metal loading had a significant effect on hydrodeasphaltenization (HDA); higher asphaltene conversions were obtained with higher metal loading. Reaction temperature had influence on HDA particularly for lower metal loadings, as catalysts with higher loadings showed significant activity at the lower temperatures studied. It was observed that coke deposits were mainly formed during the initial hour of reaction with little additional coke being formed in the reutilization of the catalysts. More deposits were obtained at lower reaction temperature, as coke precursors, polyaromatic hydrocarbons (PAH), polymerize into coke. No evidence of pore mouth plugging was observed, indicating that catalysts could accommodate coke while retaining most of their textural properties. Catalysts with higher metal loadings took longer to reach a stable amount of deposits but they stabilized at an overall smaller coke deposition and retained significant HDA activity. ACS Paragon Plus Environment
1
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 23
1. Introduction The share of heavy oils in refinery feedstock is increasing because of dwindling light oil reserves, which enhances the need for heavy oil upgrading processes that can deliver on the more stringent specifications on gasoline and diesel. Heavy oils tend to have a large amount of asphaltenes, which are coke precursors due to their heteroatoms and polycyclic aromatic hydrocarbon (PAH) structures. Therefore, catalysts need to withstand the detrimental effects these compounds have during operation, such as formation of coke and metal deposits1, which reduce catalytic activity and shorten catalyst lifecycle. Carbonaceous deposits adsorbed on the surface of the catalyst are the main cause of short-term deactivation and therefore heavy oil hydroprocessing catalysts need to accommodate more coke deposits while maintaining activity. The higher the asphaltene content of the feed, the higher wt % of carbon expected to form deposits on the catalysts. Earlier studies have found carbon contents of above 15 wt % in the first hours of reaction with an atmospheric residue as feed.2 Understanding how coke deposits affect catalytic activity and properties will help in the development of materials that better withstand deactivation from heavy feeds. Thermal and catalytic reaction pathways occur in parallel during hydrocracking of heavy feeds. Previous studies determined that catalytic activity mainly helped reduce coke formation.3 Bifunctional hydrocracking catalysts provide the necessary hydrogenation capacity, through its metal sites, to avoid or reduce coking.4 The cracking function is provided by an acidic support, such as alumina. The use of large porosity catalytic supports is beneficial to allow large PAH molecules to enter catalysts pores and diffuse to its active sites.5 In addition, large pores are fundamental in avoiding pore mouth blocking in the catalysts.6 The typical metal composition for hydroprocessing catalysts is Mo (or W) promoted by Ni (or Co); NiMo are more active with heavier feeds than CoMo catalysts.7 The metals are deposited on the supports in oxidic form and are afterwards sulfided to create the active phase of the catalyst, where Ni atoms decorate the edges of MoS2 slabs forming Ni-Mo-S species. The relationship between the Ni and Mo content in the catalysts is important; an atomic ratio of Ni/(Ni+Mo) close to 0.3 corresponds to the Ni and Mo sulfides optimal synergy, by having the maximum ratio of Ni to Mo atoms in the slab to generate coordinated unsaturated sites (CUS).8-12 Besides the Ni/(Ni+Mo) atomic ratio, the number of active sites available on the catalyst depends on metal loading. Metal sites need to be balanced against the cracking activity of the support. Different optimal ratios between the number of acid sites and the number of hydrogenating sites have been reported, depending on the reaction conditions.12 If acid sites dominate the reaction, products are not fully hydrogenated and may polymerize into carbonaceous deposits. On the other hand, excess hydrogenation capacity causes a shortage in the availability of cracked molecules for reaction. The influence of metal loading on hydroprocessing catalysts has been studied in previous works on model compounds and heavy feeds. Higher NiMo content led to higher conversions in the hydrodesulfurization (HDS) of model compounds13 and atmospheric gas oil.14 However, the dispersion of the metals is also important, not only their concentration. It was found that when MoO3 crystallites are formed, which was observed above 18 wt % loading on SBA type mesoporous molecular sieves, catalytic activity for HDS of model compounds decreases.15 An optimal Mo content was also found during hydroprocessing of deasphalted vacuum bottoms with NiMo/alumina catalysts. Mo content was varied between 0 and 8.3 wt %, reaching an optimal hydrodemetallization (HDM) conversion at 4 wt %.16 Research on the impact of metal loading in hydrocracking catalysts for vacuum residue (VR) at different reaction temperatures is scarce, which prompted this study. The main aim of this work is to evaluate NiMo/Al2O3 catalysts with different metal loadings during hydrocracking of vacuum residue at three reaction temperatures. Mesoporous alumina was used as catalytic support because it can have a better dispersion of metal particles17 and enable larger molecules in the feed to reach the active sites. Mesoporous alumina seems to be a promising alternative to conventional γ-aluminas for hydrocracking heavy feeds. Three NiMo/Al2O3 catalysts with different ACS Paragon Plus Environment
2
Page 3 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
metal loading were developed maintaining the atomic ratio of Ni/(Ni+Mo) constant. These catalysts were then tested in the hydrocracking of a VR in a batch reactor at three temperatures: 400, 425 and 450 °C. The impact of metal content was analyzed in terms of conversions of the fraction with boiling point above 450 °C (>450 °C fraction) and asphaltenes, as well as product distribution, initial coke deposition and the textural properties of the spent catalysts. Catalysts were recovered and characterized by thermogravimetric analysis (TGA) to determine the amount of coke deposits and by N2 adsorption to explore the variation of their textural properties when compared to the fresh catalysts. The catalysts were reutilized under the same conditions with fresh VR as feed. Conversions of the fraction boiling above 450 °C and the hydrodeasphaltenization (HDA) were calculated. The spent catalyst characterization showed how the textural properties were affected by the initial reaction, and the changes that occurred after reutilization. 2. Experimental 2.1.
Support and catalyst synthesis
A full description of the synthesis of the alumina support and metal impregnation procedure is available elsewhere.18 Catalyst synthesis was carried out using a TECAN MPS9500 liquid handling robot with pH, agitation, and temperature controls. The system was controlled by Symyx software which enabled the use of planned concentrations. The atomic ratio of Ni/(Ni+Mo) for each catalyst was designed to be close to 0.3, with the following nominal metal oxide concentrations: 6 wt % MoO3 and 1 wt % NiO; 10 wt % MoO3 and 2 wt % NiO and 14 wt % MoO3 and 3 wt % NiO. The catalysts are named as NixMox where x is the loading of the corresponding metal oxide in wt %. 2.2.
Catalyst characterization techniques
Elemental analyses with energy dispersive X-Ray Fluorescence (XRF) were performed using a Bruker XRF-S2 Ranger with a Cu source. The number of metal sites (nM) were determined from the metal oxide content and are expressed in atoms of Ni + Mo available per gram of catalyst. Powder X-Ray Diffraction (XRD) patterns for the materials were carried out using a PANalytical diffractometer equipped with a secondary graphite monochromator, using a θ-2θ configuration. Diffraction patterns were measured between 10 and 80° in the 2θ range; counts were accumulated every 0.01º and the step time was 3 s. Measurements were taken at ambient temperature using CuKα radiation (λ=1.54 x 10-10 m). A Micromeritics PulseChemisorb 2700 was used to perform NH3 temperature programed desorption (TPD) studies. The sample was first outgassed in Ar flow from room temperature to 600 °C (10 °C·min-1) and then cooled down to 50 °C. After ammonia was fed into the system, the sample was heated to 600 °C at a rate of 10 °C·min-1 and the chemisorbed ammonia was determined with a thermoconductivity detector (TCD). The number of acid sites (nA) are reported as desorbed NH3 molecules per gram of catalyst. Temperature programed reduction (TPR) profiles were obtained using the same equipment. 200 mg of sample were heated with a rate of 10 °C·min-1 within a temperature range from 45 °C to 600 °C with a 10% H2/Ar flow of 50 mL·min-1; H2 consumption was determined with a TCD. The Brunauer–Emmett–Teller (BET) method was used to calculate the surface area and the pore size distribution (PSD) was calculated by the Barrett-Joyner-Halend (BHJ) method based on adsorptiondesorption isotherms. A Micromeritics Tristar analyser was used with approximately 200 mg of sample, dried previously at 110°C under N2 flow. N2 adsorption tests were performed at -196 °C.
ACS Paragon Plus Environment
3
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
2.3.
Page 4 of 23
Hydrocracking experiments
A batch reactor, fully described elsewhere,18 was used for the reactions using VR. The reactor consisted of a ½ in. union-T with ends plugged and connected to a control line. 1 g of VR, with a boiling point above 535 °C and 33 wt % asphaltenes (full properties given elsewhere)18, was used in a 4:1 (wt/wt) ratio with the catalysts. The initial H2 pressure at the temperature of each experiment was 190 bar, typically falling to 180 bar due to H2 dissolution and reaction. A heated fluidized sand bath was used to heat up the reactor to reaction temperature, at which it was kept at for one hour. Then it was quenched in cold water to terminate reactions and depressurized. Catalysts containing coke deposits were reutilized in experiments at the same conditions using with fresh feed in order to establish the activity of the catalysts once they are fully sulfided and the amount of carbonaceous deposits has stabilised. CS2 (0.2 mL) was added for in-situ sulfiding during the first reaction. A solvent mixture (CHCl3/CH3OH 4:1 vol/vol) was used to recover the products from the reactor. The solid products were separated from the liquid by vacuum filtration using a Whatman PTFE membrane filter of 1 µm pore size. 2.4.
Product characterization techniques
The coke content on the catalysts was determined by TGA with a Perkin-Elmer TGA1. The sample was heated from 50 to 900 °C at a rate of 10 °C·min-1 under 40 mL·min-1 flow of air. The carbonaceous deposits were calculated as the difference between the initial stabilized weight (after 15 minutes) and the final stabilized weight (after 30 minutes). For reutilization reactions, coke deposits were reported as the newly formed coke, i.e. without taking into consideration the weight of coke formed in the initial reaction. The hydrocarbon products were separated by solubility in heptane and toluene into maltenes (heptane soluble) and asphaltenes (heptane insoluble, toluene soluble). The former were analyzed by simulated distillation (GC-SimDis) in a Perkin Elmer Clarus 500 Chromatographer fitted with a flame ionisation detector (FID) following the ASTM D2887 method. The product recovery procedure is described in detail elsewhere.17 Conversions of the VR to a fraction boiling below 450 °C (C>450°C) and the asphaltene fraction (HDA conversion) were calculated. As the initial carbon deposition is large, carbonaceous deposits are considered as part of the >450 °C materials and unconverted asphaltenes in order to distinguish between catalysts that were active in hydrocracking from those that led to larger coke deposition. C>450°C =
mF ⋅ x F >450°C − mLP ⋅ x LP >450°C − mSP ⋅ xSP −carbon mF ⋅ x F >450°C
HDA conversion =
m F ⋅ x F − A − m LP ⋅ x LP − A − m SP ⋅ x SP −carbon mF ⋅ xF − A
where mF, mLP, and mSP are the weight of the feed, liquid products and solid products recovered after reaction, respectively; xF>450°C and xLP>450°C are the mass fraction boiling above 450 °C for the feed (with a value of 1.0) and liquid products, correspondingly. xLP>450°C was calculated by adding the maltene fraction that did not elute in GC measurements, i.e. with a boiling point above 450 °C, to the asphaltene fraction of the sample. xSP-carbon is the amount of carbonaceous material deposited on the catalyst determined with TGA measurements as grams of carbon per gram of catalyst. xF-A and xLP-A are the asphaltene mass fractions in the feed and liquid products, respectively. Tests to assess experimental repeatability and to determine random error was carried out; liquid recovery had a ±2% error, the amount of carbonaceous material deposited on the catalyst had a ±1% error, and maltene to asphaltene fractionation had a ±2% error. ACS Paragon Plus Environment
4
Page 5 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
3. Results and Discussion 3.1. Catalyst characterization XRF was used to measure the metal oxide loadings on the calcined catalysts in powder form (Table 1), and it was observed that the nominal loadings were met. The atomic ratio of Ni/(Ni+Mo) was confirmed to be close to 0.3 for all materials. Acid sites measured in ammonia TPD studies are reported in Table 2 as weak (desorption below 200 °C), medium (200-400 °C) and strong (>400 °C). It was observed that an increase in NiMo loading resulted in an increase in weak acidity but a decrease in strong acidity. The increase in weak acidity upon NiMo loading might be related to the formation of new sites by NiMo species with weak acidic features. On the other hand, the decrease in strong acidity occurred when metals covered the surface of Al2O3 and blocked acid sites. For the hydrocracking reactions where catalysts are involved, more and stronger acid sites are correlated to higher cracking activities.19 The catalysts presented varying amounts of strong acidity, with a higher value for Ni1Mo6/Al2O3; thus, the cracking function can be considered stronger for this catalyst. The ratios of strong acid sites to metal sites (nA/nM) in the catalysts are included in Table 1. It was observed, as expected, that catalysts with higher metal content had a lower nA/nM ratio. The XRD diffraction patterns of the catalysts showed no Ni or Mo oxide crystallite reflections. This suggests that Ni and Mo oxides were highly dispersed in the supports and had a size of less than 4 nm, the detection limit of XRD technique.17, 20-23 The Al2O3 diffractogram showed no lines characteristic of an ordered oxide phase, indicative of a mesostructure with amorphous framework.24 Three broad peaks were observed, which match the diffractogram reported for alumina synthesis.25 These peaks are typically assigned to gamma alumina, suggesting some degree of ordering. The diffractograms are available in Supporting Information Figure SI-1. The first reduction peak for Ni1Mo6/Al2O3 in TPR (Figure 1) ranged from 250 to 700 °C with a maximum around 600 °C. This peak is likely to be the result of merging between the first Mo reduction from Mo+6 to Mo+4 (taking place between 400-500 °C) and the full reduction to metallic state at higher temperatures (near 700 °C). The maximum migrated to lower temperatures with increasing Ni and Mo content in Ni2Mo10/Al2O3 and Ni3Mo14/Al2O3. NiO species are typically reduced in a single step without intermediate formation into bulk Ni at around 400 °C, with a peak commonly shifting to lower temperatures with increasing Ni content.26 Higher Ni loadings led to an additional peak at around 350 °C, which may be ascribed to bulk NiO with a low interaction with the support.
3.2. Hydrocracking conversions and product distributions C>450°C achieved in hydrocracking reactions at 400, 425 and 450 °C using the fresh and reused NiMo/Al2O3 catalysts as well as without catalyst are shown in Figure 2. In all cases, including the experiment with no catalyst, C>450°C significantly increased with increasing reaction temperature. Hydrocracking experiments with alumina only as catalyst were reported in previous work 18 and showed significantly lower activity. However, no significant differences were observed between the conversions using no catalyst and those observed in catalytic experiments at 400 and 425 °C. At 450 °C, catalytic reactions led to only slightly higher conversions than the purely thermal reaction. Taken first use and reutilization reactions together, no clear enhancement in C>450°C occurred when metal content was increased, highlighting that thermal cracking was predominant over catalytic cracking, in line with literature reports.3, 27-29 Figure 3 shows HDA conversions for initial and reutilization reactions for Ni1Mo6/Al2O3, Ni2Mo10/Al2O3 and Ni3Mo14/Al2O3 along with purely thermal experiments (no catalyst). Reactions ACS Paragon Plus Environment
5
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 23
without catalyst showed little asphaltene upgrading. Given that no hydrogenation activity is available without a catalyst, asphaltenes were polymerized instead of converted into maltenes. By contrast with C>450°C, the effect of metal loading on HDA was clearly observed in initial reactions. An increase in metal loading led to higher HDA conversions, with the only exception of Ni2Mo10/Al2O3, which underperformed Ni1Mo6/Al2O3 in its 400 °C reaction but was very active at higher temperatures. All catalysts achieved higher HDA conversions during reutilization when compared to initial reactions. Firstly, it is likely that the activation process was completed in the first reaction leaving the catalyst fully active for the reutilization with fresh feed. Secondly, spent catalysts are less acidic than fresh ones and hence trigger less polymerization of PAH-containing species on their surface,30 allowing asphaltenes to follow upgrading reaction pathways instead of forming coke deposits. Asphaltene conversions for Ni1Mo6/Al2O3 and Ni2Mo10/Al2O3 reutilizations increased by 26 and 48%, correspondingly, when reaction temperature was raised from 400 to 450 °C. On the other hand, HDA conversion increased only 7% for Ni3Mo14/Al2O3. In summary, metal loading did not show great influence on C>450°C, which was dominated by thermal cracking, but was an important factor for asphaltene conversion. For all catalytic reactions, an increase in temperature resulted in an increase in HDA conversion, although this effect was smaller in the reutilization of Ni3Mo14/Al2O3, which led to stable high conversions across all reaction temperatures. In addition to the higher metal content leading to a lower nA/nM ratio, the higher activity of Ni3Mo14/Al2O3 at lower temperatures can be explained by the TPR study (Figure 1). It was observed that Ni3Mo14/Al2O3 was more easily reduced than the two other catalysts, thus it achieved a larger number of metal sites in their active form at 400 and 425 °C. The relationship between thermal and catalytic reactions was examined in more detail by analyzing the product distributions, shown in Tables 3 and 4 for initial and reutilization reactions, respectively. Reaction products are expressed as feed wt % and grouped into the following fractions: Gas, Maltenes450°C (maltenes with boiling points below and above 450 °C, respectively) Asphaltenes and Solids. Non-catalytic reactions (Table 3) led to a large gas production but did not generate any Maltenes450°C yield also varied significantly with temperature, decreasing from above 60 wt % at 400 °C to 20 to 30 wt % at 450 °C for all catalysts. The yield of Maltenes450°C. Coke deposits mainly formed in the initial reaction (Table 3). The Solids reported in reutilization reactions (Table 4) are those formed in this reaction only, without taking into account coke deposits from the first reaction. The most important change that occurred to the product yield when catalysts were reutilized at any temperature studied was the decline in coke build-up (Table 3 and 4). It is well accepted that coke deposition reaches steady state after increasing rapidly in the first hours of contact with hydrocarbon feeds.34, 35 In batch reactors there is a reduction in coke precursors in the system as these react, contrary to the steady supply in continuous reactors.36 Additional solid production occurred for most reutilization reactions, nevertheless the yields were significantly lower than for initial reactions. These trends were in agreement with literature 37-39 where the same approach of catalyst reutilization was employed. An increase in Asphaltenes yield was observed for some reutilization reactions at the lower temperatures, compared to the initial ones, which could be attributed to asphaltenes that did not polymerize to form coke but remained unreactive. However, the combined ACS Paragon Plus Environment
6
Page 7 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
yields of asphaltenes and solids always decreased with the reutilization, showing that stabilization of the catalysts allowed selectivity into desirable products (Maltenes) to increase. In agreement with this trend, the yield of Maltenes>450°C increased in all but one reutilization reactions. Figure 4 shows the maltene to asphaltene (M/A) ratio in the reutilization reaction products. This ratio was calculated as the total maltene yields (Maltenes450°C) to Asphaltene yield. Higher hydrogenation capacity and lower nA/nM ratio of the catalysts reduced the asphaltene yield. For Ni1Mo6/Al2O3 a slight rise in the M/A ratio was observed with increasing temperature. Ni2Mo10/Al2O3 substantially gained activity at higher temperature and its products presented an increase in their M/A ratio from 4.0 at 400 °C to 9.9 at 450 °C, due to a 50% reduction in Asphaltenes yield. On the other hand, Ni3Mo14/Al2O3 presented a fairly high M/A ratio constant with temperature, indicating that it was active even at lower temperatures. The HDA conversion and M/A ratio were higher when using Ni2Mo10/Al2O3 than Ni3Mo14/Al2O3 at 450 °C. This could relate to the relatively high increase in coke deposits of Ni3Mo14/Al2O3 in its reutilization reaction, as coke formation had not stabilized in this run, and therefore asphaltenes being converted to coke rather than maltenes.
3.3. Analysis of spent catalysts High levels of coke deposits were obtained in this study, as expected for a heavy feed such as VR. Table 5 shows the amount of coke deposited on the catalysts (as gram of coke per gram of catalyst) at all reaction conditions studied and relates to the Solids yield (Tables 3 and 4). Coke deposits decreased with increasing reaction temperature and were significantly affected by catalyst metal loading. The hydrogenation capacity of the catalysts, quantified by their nA/nM ratio, was important in reducing the amount and rate of formation of coke deposits on the fresh catalysts. The nA/nM ratio of the catalysts, which followed the order: Ni1Mo6/Al2O3 > Ni2Mo10/Al2O3 > Ni3Mo14/Al2O3 (Table 1), changed the balance between cracking and hydrogenation activity. A rise in metal loading implied a larger amount of available metal sites for hydrogenation reactions and therefore smaller nA/nM ratios led to smaller extents of coke deposits at all temperatures. The fresh Ni1Mo6/Al2O3 had less hydrogenation activity, which was reflected in higher cracking rate and higher Solids yield. However, coke stabilized earlier for this catalyst, as shown by the little additional carbon picked up in the reutilization reaction. On the other hand, there was further coke deposition during Ni2Mo10/Al2O3 and Ni3Mo14/Al2O3 reutilization, indicating that they had not stabilized yet. The large further deposition in reutilization reactions with Ni2Mo10/Al2O3 at 400 °C and Ni3Mo14/Al2O3 450 °C contributed to the lower HDA conversion at these conditions, which departs from the trends, as shown in Figure 3. A higher metal loading, i.e. reduced acidity, slows down coke deposition,40 taking longer for the amount of deposits to stabilize. However, stabilization takes place with an overall smaller amount of deposits formed. As the catalysts became more stable after the first reaction, asphaltenes and heavy maltenes were able to follow reaction pathways leading to lower boiling point materials in the catalyst reutilization. The textural properties of the fresh and spent catalysts are listed in Table 6. For Ni1Mo6/Al2O3 and Ni2Mo10/Al2O3, a reduction in surface area when compared to the Al2O3 support (341.2 m2/g) was observed. However, SBET for Ni3Mo14/Al2O3 was similar to the Al2O3 support. Some authors have reported the increase in the surface area of the catalyst as a function of metal loading 41, 42.This fact can be explained by two different roles of metal oxides on the catalyst structure: metal oxide provoke a partial covering of pores at relatively low metal loadings. As the metal loading increased, surface area of catalysts increased. This is probably related to the formation of a porous metal oxide structure at higher loadings that increased the surface area of the Ni3Mo14/Al2O3 catalyst. All catalysts experimented a reduction in SBET, pore volume and average pore diameter (APD) after the initial reaction as a consequence of the large coke deposition, in line with literature reports.5, 37 Following reutilization, two different patterns emerged. For Ni1Mo6/Al2O3 at 400 and 450 °C, SBET increased to larger values than ACS Paragon Plus Environment
7
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 23
the spent catalysts from the first reaction. This can be attributed to coke being reactive and evolving into different configurations, becoming more aromatic with time.43, 44 Since coke deposits were permeable and reactive, they originally blocked most of the surface area and then reacted to form more compact coke in the subsequent reaction. As in these reutilization reactions no fresh deposition took place, this change in coke structure is the predominant phenomenon. All other reutilization runs showed new formation of deposits that led to a further decrease in SBET of the catalysts. Similar trends to SBET were observed in pore volume and pore diameter. The spent catalysts from reutilization reactions had smaller pore volume than those from initial reactions. The reutilizations of Ni1Mo6/Al2O3 at 400 and 450 °C were again the only exceptions to this behaviour as at these conditions coke deposits were stable (Table 4). The formation of deposits typically decreased the pore volume but coke preferentially blocked small pores, either by formation in the small pores or by migration into small pores of coke already deposited on the catalyst, resulting in the shift of APD to larger values. Changes in the pore size distribution profiles for Ni1Mo6/Al2O3, Ni2Mo10/Al2O3 and Ni3Mo14/Al2O3 are shown in Supporting Information (Figures SI-26 to SI-4). These figures revealed that lower levels of coke deposits resulted in a smaller effect on PSD profile; the PSD curve had pore volumes in decreasing order for reutilizations at: 450, 425 and 400 °C. Even though pore volume decreased for all pore sizes, no evidence of pore mouth plugging was observed. These materials appeared to accommodate large amounts of coke deposits while maintaining their textural properties. Catalysts with higher metal loading could better retain their textural properties than those with lower metal content, even with comparable coke deposits. 4. Conclusions Three hydrocracking catalysts were prepared by impregnation of varying Ni and Mo loadings on a mesoporous Al2O3. The mesoporosity was retained after metal impregnation and allows the macromolecules in heavy feedstocks to reach the catalyst active sites. All catalysts presented acidity above 400 °C, which promoted cracking at reaction conditions. The influence of metal loading in NiMo/Al2O3 catalysts was studied for the initial stages of hydrocracking reactions at 400, 425 and 450 °C. A large coke build-up occurred within the first hour of reaction. Therefore, catalysts were recovered and reused with fresh feed in order to provide an insight on catalyst performance once the initial deposition had taken place. Metal loading proved to be little influential in the conversion of the vacuum residue into a fraction boiling below 450 °C, in which thermal reactions were predominant. However, its effect was very significant in HDA conversion, with higher metal loadings leading to higher asphaltene conversions. This is due to a decrease in the strong acidity, leading to lower nA/nM ratios, and in the reduction temperature of Ni and Mo oxides, which makes more metal active sites available for the reaction using Ni3Mo14/Al2O3. Hence, Ni3Mo14/Al2O3 showed less dependence on reaction temperature, with a good performance even at the lower temperatures tested. Higher HDA conversions were obtained when the catalysts were reutilized. The relatively large amount of deposits on the catalysts led to a further reduction in acidity but not to significant deactivation. This allowed the heavier fractions undergo reaction pathways different from polymerization to coke, enhancing the yield of maltene fractions. The catalysts presented large amounts of coke deposits, but higher metal loadings slowed down coke deposition, leading to smaller extents of deposits but also longer times to reach a stable amount of deposits. By contrast, Ni1Mo6/Al2O3 underwent strong deposition but it was completed after one hour of reaction. Although spent catalysts suffered a reduction in their surface area, pore volume and average ACS Paragon Plus Environment
8
Page 9 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
pore diameter, no evidence of pore mouth plugging was observed, indicating that these mesoporous catalysts can accommodate coke without altering their textural properties and sustain a significant HDA performance.
ACKNOWLEDGMENTS HP thanks CONACYT Mexico for partial funding towards her PhD. JLP thanks the Spanish Economy and Competitiveness Ministry (MINECO) for his Ramon y Cajal research contract (RYC-2013-12494).
ACS Paragon Plus Environment
9
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 23
AUTHOR INFORMATION Corresponding Author
Corresponding author:
[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. Supporting Information Available
ACS Paragon Plus Environment
10
Page 11 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
REFERENCES 1.
F. Edward, in Stud. Surf. Sci. Catal., ed. F. Edward, Elsevier, 2007, vol. Volume 169, pp. 23-41.
2.
H. Beuther, O. A. Larson and A. J. Perrotta, Stud. Surf. Sci. Catal., 1980, 6, 271-282.
3.
Y. Miki, S. Yamadaya, M. Oba and Y. Sugimoto, J. Catal., 1983, 83, 371-383.
4.
R. M. Gray, Upgrading Petroleum Residues and Heavy Oils: 56 (Chemical Industries), CRC Press, 1 edition edn., 1994.
5.
C. Song, T. Nihonmatsu and M. Nomura, Ind. Eng. Chem. Res., 1991, 30, 1726-1734.
6.
J. Ancheyta, M. S. Rana and E. Furimsky, Catal. Today, 2005, 109, 3-15.
7.
M. Egorova and R. Prins, J. Catal., 2006, 241, 162-172.
8.
S. P. Ahuja, M. L. Derrien and J. F. Le Page, Industrial and Engineering Chemistry Product Research and Development, 1970, 9, 272-281.
9.
P. Grange, Catalysis Reviews, 1980, 21, 135-181.
10.
S. Kasztelan, H. Toulhoat, J. Grimblot and J. P. Bonnelle, Applied Catalysis, 1984, 13, 127-159.
11.
S. Houssenbay, S. Kasztelan, H. Toulhoat, J. P. Bonnelle and J. Grimblot, The Journal of Physical Chemistry, 1989, 93, 7176-7180.
12.
J. L. Lemberton, M. Touzeyidio and M. Guisnet, Appl. Catal., A, 1991, 79, 115-126.
13.
N. Kagami, B. M. Vogelaar, A. D. v. Langeveld and J. A. Moulijn, Appl. Catal., A, 2005, 293, 11-23.
14.
H. Qabazard, F. Abu-Seedo, A. Stanislaus, M. Andari and M. Absi-Halabi, Fuel Science and Technology International, 1995, 13, 1135-1151.
15.
O. Y. Gutiérrez, F. Pérez, G. A. Fuentes, X. Bokhimi and T. Klimova, Catal. Today, 2008, 130, 292-301.
16.
M. Núñez and M. Villamizar, Appl. Catal., A, 2003, 252, 51-56.
17.
L. Kaluža, M. Zdražil, N. Žilková and J. Čejka, Catal. Commun., 2002, 3, 151-157.
ACS Paragon Plus Environment
11
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
18.
Page 12 of 23
H. Purón, J. L. Pinilla, C. Berrueco, J. A. Montoya De La Fuente and M. Millán, Energy Fuels, 2013, 27, 3952-3960.
19.
A. Corma, A. Martinez, V. Martinezsoria and J. B. Monton, J. Catal., 1995, 153, 25-31.
20.
M. S. Rana, B. N. Srinivas, S. K. Maity, G. Murali Dhar and T. S. R. Prasada Rao, J. Catal., 2000, 195, 31-37.
21.
G. M. Dhar, B. N. Srinivas, M. S. Rana, M. Kumar and S. K. Maity, Catal. Today, 2003, 86, 4560.
22.
P. Kim, Y. Kim, H. Kim, I. K. Song and J. Yi, Appl. Catal., A, 2004, 272, 157-166.
23.
S. K. Maity, J. Ancheyta and M. S. Rana, Energy Fuels, 2005, 19, 343-347.
24.
Z. Zhang and T. J. Pinnavaia, J. Am. Chem. Soc., 2002, 124, 12294-12301.
25.
C. Lesaint, W. R. Glomm, Ø. Borg, S. Eri, E. Rytter and G. Øye, Appl. Catal. A-Gen., 2008, 351, 131-135.
26.
C. Li and Y.-W. Chen, Thermochim. Acta, 1995, 256, 457-465.
27.
F. Khorasheh, H. A. Rangwala, M. R. Gray and I. G. Dalla Lana, Energy Fuels, 1989, 3, 716722.
28.
C. E. Galarraga and P. Pereira-Almao, Energy Fuels, 2010, 24, 2383-2389.
29.
M. R. Gray, F. Khorasheh, S. E. Wanke, U. Achia, A. Krzywicki, E. C. Sanford, O. K. Y. Sy and M. Ternan, Energy Fuels, 1992, 6, 478-485.
30.
M. Absi-Halabi, A. Stanislaus and D. L. Trimm, Applied Catalysis, 1991, 72, 193-215.
31.
S. n. Dehkissia, F. ç. Larachi and E. Chornet, Fuel, 2004, 83, 1323-1331.
32.
M. S. Rana, J. Ancheyta, S. K. Maity and P. Rayo, Catal. Today, 2008, 130, 411-420.
33.
G. Gualda and S. Kasztelan, J. Catal., 1996, 161, 319-337.
34.
J. M. Oelderik, S. T. Sie and D. Bode, Applied Catalysis, 1989, 47, 1-24.
35.
S. M. Richardson, H. Nagaishi and M. R. Gray, Industrial & Engineering Chemistry Research, 1996, 35, 3940-3950.
ACS Paragon Plus Environment
12
Page 13 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
36.
Energy & Fuels
D. S. Thakur and M. G. Thomas, Industrial & Engineering Chemistry Product Research and Development, 1984, 23, 349-360.
37.
M. Millan, C. Adell, C. Hinojosa, A. A. Herod, D. Dugwell and R. Kandiyoti, Energy Fuels, 2007, 21, 1370-1378.
38.
M. Millan, C. Adell, C. Hinojosa, A. A. Herod and R. Kandiyoti, Oil and Gas Science and Technology, 2008, 63, 69-78.
39.
M. Marafi and A. Stanislaus, Pet. Sci. Technol., 2001, 19, 697-710.
40.
P. E. Savage, M. T. Klein and S. G. Kukes, Energy Fuels, 1988, 2, 619-628.
41.
A. E. Aksoylu, A. N. Akin, Z. I. Önsan and D. L. Trimm, Appl. Catal., A, 1996, 145, 185-193.
42.
Ş. Özkara, A. N. Akin, Z. Misirli and A. E. Aksoylu, Turk. J. Chem., 2005, 29, 219-224.
43.
A. Hauser, A. Stanislaus, A. Marafi and A. Al-Adwani, Fuel, 2005, 84, 259-269.
44.
K. Matsushita, A. Hauser, A. Marafi, R. Koide and A. Stanislaus, Fuel, 2004, 83, 1031-1038.
ACS Paragon Plus Environment
13
Energy & Fuels
200
Ni1Mo6/Al2O3
150 100 50
H2 consumption (µmol H2 g-1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 23
0 200
Ni2Mo10/Al2O3
150 100 50 0 200
Ni3Mo14/Al2O3
150 100 50 0 200
400
600
800
Temperature (°C)
Figure 1. TPR profiles for Ni1Mo6/Al2O3, Ni2Mo10/Al2O3, and Ni3Mo14/Al2O3.
ACS Paragon Plus Environment
14
Page 15 of 23
0.9 450 °C
0.8 0.7
>450°C
C>450°CCConversion
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
0.6 425 °C
0.5 0.4 0.3
400 °C
0.2 0.1 1 No catalyst
2 3 4 Ni1Mo6/Al O Ni2Mo10/Al O Ni3Mo14/Al O 2 3 2 3 2 3 Initial Initial Initial Reused Reused Reused
Figure 2. C>450°C conversion for experiments with and without catalysts. Reactions were carried out at an initial 190 H2 pressure (at the experiment temperature) for 60 minutes.
ACS Paragon Plus Environment
15
Energy & Fuels
0.9 0.8 0.7 450 °C
HDA
HDA Conversion
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 16 of 23
450 °C
0.6
425 °C 425 °C
0.5
400 °C 400 °C
0.4 0.3 0.2 0.1 1 No catalyst
2 3 4 Ni1Mo6/Al O Ni2Mo10/Al O Ni3Mo14/Al O 2 3 2 3 2 3 Initial Initial Initial Reused Reused Reused
Figure 3. HDA conversion for experiments with and without catalysts. Reactions were carried out at an initial 190 H2 pressure (at the experiment temperature) for 60 minutes.
ACS Paragon Plus Environment
16
Page 17 of 23
Ni1Mo6/Al2O3
12
Maltene/Asphaltene ratio in products
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
VR Thermal
Ni2Mo10/Al2O3 Ni3Mo14/Al2O3
10
8
6
4
2
400
425
450
Temperature (°C)
Figure 4. Maltene to asphaltene ratio for reaction products. Reactions were carried out at an initial 190 H2 pressure (at the experiment temperature) for 60 minutes.
ACS Paragon Plus Environment
17
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 18 of 23
Table 1. Fresh catalyst properties. Metal loadings measured by XRF, acidity determined by NH3-TPD. __Ni__
nM
nA*
nA/nM
MoO3 wt %
NiO wt %
(Ni+Mo)
Ni1Mo6/Al2O3
5.96
1.22
0.28
3.66
1.96
0.54
Ni2Mo10/Al2O3
9.54
2.11
0.30
6.19
0.77
0.12
Ni3Mo14/Al2O3
13.88
2.98
0.29
9.29
0.52
0.06
Catalyst
[(Ni+Mo)·g-1]·10-20 (NH3·g-1)·10-20
*NH3 data calculated from integrated areas from NH3-TPD in the 400 to 600 °C range; acidity values calculated with an ammonia concentration calibration and taking into account the sample mass.
ACS Paragon Plus Environment
18
Page 19 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Table 2. Acidity measurements calculated from NH3-TPD studies. Acid sites* (µmol NH3·gcatalyst-1) Catalyst
Weak
Strong (>400 °C)
Total
(
