Towards improvement of hydroprocessing catalysts - understanding the role of advanced mineral materials in hydroprocessing catalysts
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Abstract
Mineral materials play a pivotal in heterogeneous catalysts as active, support, or promoter components, with the oil refinery industry being one of the biggest beneficiaries. While conventional hydroprocessing catalysts have historically met the industry’s needs, the growing need to accommodate unique feedstocks, meet the increasing demand for environmentally acceptable products, obtain better product specifications, enhance selectivity for reactions to increase ratios for certain product cuts, and use more cost-effective and abundant mineral materials, has recently motivated for fresh considerations in the development of hydroprocessing catalysts. Based on periodic trends, noble metals possess the most desirable qualities, but their relative abundance in the Earth’s crust is too low to meet industry needs. They are costly and highly sensitive to sulfur poisoning. Mo and W lie in the sweet spot, but it is anticipated that they cannot meet the increasing demand. Investigations of electronic interactions of more economical and abundant metals, such as Nb, V, and Fe, with other elements and support materials have yielded a better understanding of synergistic effects that help to access noble metal-like qualities. This work contrasts conventional hydroprocessing catalysts and recently improved catalysts, detailing the chemistry considerations behind the selection of mineral materials used in the catalysts. It also explores how further manipulation of these mineral materials and synthesis approaches is driving toward more desirable properties. The work brings to the attention of the readers the challenges and opportunities for the further improvement of hydroprocessing catalysts to ensure environmental sustainability while meeting the industry’s growing needs.
Keywords
INTRODUCTION
The ability of humans to interact with their environment was greatly enhanced with the advent of exploiting mineral materials in our ecosystem during the stone, bronze, and iron ages, defining key functional points of the earliest evident civilizations based on the abilities to modify and add certain characteristics to these materials through various treatments. Human reliance on mineral materials continues, with most modern advancements, manufacturing, and processing relying on mineral material catalysts. The hydroprocessing industry is one such prominent industry where different kinds of catalysts are required at numerous stages to access various products[1,2]. The growth of the petroleum refinery is attributed to be the biggest driver of the growth of the global catalyst market, which is expected to grow to USD 9.5 billion by 2027[2]. These catalysts are mostly different combinations of transition metal silicates, phosphates, carbonates, sulfates, sulfides, nitrides, carbides, phosphides, borides, oxides, and hydrides, with various other elements such as phosphorous, boron, and fluorine being added to improve catalyst properties. Mineral materials, such as alumina, zeolites, titania, cerium, and zirconia, are also crucial as catalyst support materials[3]. Research for improving catalysts for these processes remains a prominent and active area of study[1]. Improved catalysts can reduce refinery costs, improve selectivity for certain products, allow non-conventional feedstock, such as creosotes, to be accommodated, and make them economically attractive fuel sources[4].
While the relative abundance of mineral materials used for current hydroprocessing catalysts is generally not a problem, the increasing demand for petroleum products and the need for more active and highly selective catalysts to improve refinery processes will change this situation. There is also a tendency to incline towards the noble metals, which have demonstrated superiority among all active elements. Research seeks to bridge the gap by manipulating properties of more abundant elements (e.g., iron) to achieve the same level of efficiency to ensure sustainability[5]. The pursuit of improvements in this field is broad, including changes in support materials, improved synthesis protocols, and the incorporation of multifaceted compositions that result in better catalyst properties. On the same note, the importance of recycling spent catalyst materials and the environmental concerns associated with the discarding of the modified mineral materials must also be stressed enough[6]. Sufficient studies are also required to understand the impact of modified mineral materials used in hydroprocessing catalysts on the environment.
Performances of conventional hydroprocessing catalysts used in typical hydroprocessing reactions are discussed, articulating the roles played by the various constituents of the catalysts that are obtained from a wide array of typical mineral materials. Chemistry aspects that motivated the combinations of choice mineral materials used in the conventional hydroprocessing catalysts are discussed. Their limitations (e.g., poor activity for some reactions, poor product selectivity, and catalyst deactivation) are also discussed, and we proffer solutions through this review work. The superiority of noble metals is identified throughout all the discussions, emphasizing the aim to manipulate combinations of more affordable and abundant mineral materials to achieve similar desirable properties. The current position in the development of catalysts to overcome challenges associated with the growing need to accommodate hydroprocessing of unconventional feedstocks, such as creosotes and bio-oils, with examples of promising catalysts, is also well covered.
HYDROPROCESSING IN GENERAL
Hydroprocessing reactions
Hydroprocessing reactions seek to modify the hydrocarbon composition to achieve desirable fuel specifications and eliminate heteroatoms to protect the environment and downstream processes[3,7]. Reactions such as the addition of hydrogen to carbon-carbon double bonds (hydrogenation), breaking of
Figure 1. Schematics showing (A) HDS and (B) HDN reactions of typical sulfur and nitrogen containing compounds as representative hydroprocessing reactions. DDS: Direct desulfurization; HDN: hydrodenitrogenation; HDS: hydrodesulfurization; HYD: hydrogenation.
Hydroprocessing reaction competitions
Hydroprocessing reactions occur simultaneously, competing with each other, usually causing a retarding effect on each other[4,10]. Equilibrium constants for the competition of catalyst sites by the heterocyclics generally increase in the order of aromatic hydrocarbons < sulfur compounds < oxygen compounds < nitrogen compounds. HDN is generally more difficult compared to other hydroprocessing reactions that occur simultaneously, with an inhibitory competitive effect, and can be used to dictate the hydrotreating operating conditions[11-14]. For example, high N content (> 60 ppm) negatively affects the ability to achieve deep HDS (less than 10 ppm wt. S)[13,15]. The competitive behavior in the adsorption of heterocyclic compounds on the active sites of hydroprocessing catalysts results in differences in selectivities and reaction depths of the catalysts[15-18]. Different sites for cleavage of C-S, C-O, C-N, and other reactions are also proposed[11]. The need to establish the different sites associated with specific reaction selectivities has created several research questions that have aided the conceptualization of various strategies for improving the catalysts based on composition or design for specific or multiple reactions[11]. Catalyst stacking technologies are also being used to maximize the complementary participation of different catalysts and their different active sites to obtain high performances for multiple reactions[19]. Improved catalysts can reduce the cost of refining crude oil, especially creosotes and other synfuels, making them economically attractive fuel sources[4].
Traditional hydroprocessing catalysts
Hydroprocessing catalysts are usually made up of different combinations of active metals, promoter metals, support materials, and other additives that help improve catalyst properties. Catalyst activity increases with an increasing percentage of active metals up to a certain point where the activity will start decreasing. As such, there is a need to balance between the increasing cost of catalysts as the percentage of active metal increases and the benefits of obtainable catalyst activity[11,20]. Transition metal sulfides are the most prominent materials in hydroprocessing catalysis[21]. The traditional catalysts for hydrotreating are the sulfided combinations of alumina-supported Mo or W promoted by Co or Ni; CoMo/Al2O3, NiMo/Al2O3, and Ni(Mo)W/Al2O3 being the most common commercial examples[19,22]. They are considered the baseline catalysts for benchmarking the performance of potential catalysts. Environmental concerns and the need to accommodate harsher feedstock and shape products have been the main drivers for the continuous development of hydroprocessing catalysts[19,23,24]. These subjects are discussed in greater depth in our previous review[25].
Periodic trends of metals in hydroprocessing
Electronic properties of the selected metals used in the catalysts lay the foundations for understanding catalyst activity and selectivity. Pecoraro and Chianelli[26-29] carried out transition metal tests on HDS activity [Figure 2A], while Eijsbouts[26-29] carried out similar tests on HDN [Figure 2B]. All the studies confirmed that the primary effect for activity is electronic and is related to the position the metals occupy in the periodic table. For both processes, transition metals occupying the first row in the periodic table are relatively inactive, while those occupying the second and third rows have high activity. HDS conversion levels are high for Ru, Os, Ir, Rh, and Re, respectively, while HDN conversion levels are high for Ir, Os, Re, Pt, and Rh, respectively. The HDN maxima were found to be at V, Rh, and Ir for the first-row, second-row, and third-row elements, respectively[29]. First-row carbon-supported transition metal sulfides had low quinoline conversions with a minimum at Mn/C and Fe/C and maxima at V/C and Ni/C, while the second- and third-row transition metal sulfides had their minimum with Mo/C and W/C, and the maxima at Rh/C and Ir/C. Noble metal catalysts (Rh/C, Pd/C, Os/C, Ir/C, and Pt/C) show the highest activities and selectivity for propylcyclohexane in quinoline HDN[28]. Raje et al. also reported similar volcano plots when they tested second-row unsupported transition metal sulfides: Zr, Nb, Mo, Ru, Rh, and Pd sulfides in the HDS, HDN, and HDO of coal-derived naphtha that contained significant amounts of S-, N-, and
Understanding periodic trends has motivated the manipulation of electronic properties of various active phases through the use of additives (halogens and phosphorus), chelating ligands, and promoters to facilitate catalyst preparation or performance, usually targeting the use of more affordable metals to obtain noble metal-like characteristics[20].
ROLES OF MINERAL MATERIALS AS SUPPORT MATERIALS IN HYDROPROCESSING CATALYSTS
Catalytic supports play a crucial role in dispersing deposited metals, increasing surface stabilization against deactivation processes such as agglomeration, improving the morphology of the active sulfide component[32], providing beneficial characteristics towards reactions such as defect sites, and enhancing the charge transfer ability and hydrogen spillover[33]. Catalyst supports can also provide additional functional sites and stabilize catalyst active sites. A new active phase on the surface of the catalyst may be formed via the interaction of the support with the catalyst. The following characteristics need to be considered when choosing hydroprocessing support materials: mechanical and thermal stability, surface area, acidity, and porosity[34,35]. The composition and distribution of Brønsted acid and Lewis acid sites are also important[36,37]. Brønsted acid sites are important for promoting the dispersion of the active metal, reducing the reduction temperature of the active metal, and modulating the interaction between the support and the metals. Lewis acid sites are regarded as adsorption sites of the heteroatomic compounds, and they play a role in selectivity via hydrogen spillover effects. Supports function differently depending on the strength of their acid sites, and optimal acidity is required for the activity and selectivity of reactions[38]. Metal oxides, such as Al2O3, SiO2, ZrO2, TiO2, and CeO2, and carbon materials have been investigated as hydroprocessing catalyst supports[32,35,39].
Support materials form the basis of hydrocracking reactions with the Brønsted acid sites and play a crucial role in the activity and selectivities of the catalysts[40]. Co-Mo/MCM-41 showed a high yield to the skeletal isomerization reactions because of its strong surface Brønsted acidity and relatively low hydrogenation activity compared to Ni-Mo/MCM-41 and Co-Mo/c-Al2O3, which exhibited the low selectivity to the isomerization reactions owing to their high hydrogenation activities[41]. Table 1 summarizes the properties of typical support materials that have been used in hydroprocessing catalysts.
Typical hydrotreating supports
| Support | Synthesis methods | Mass composition | Surface area | Acidity composition (@200 oC) | Comments | Ref. | |
| LASs | BASs | ||||||
| SBA-16 and Al-Ti-SBA-16 | Two-step method | Al2O3 (0%-10%) and TiO2 (10%) | 975.7 and 969.6 | 21.4 and 83.7 | 21.9 and 26.7 | Highly ordered structure and enhancing acidity. | [42] |
| TiO2-Al2O3 (Ti/Al 0-4) | Pre-hydrolysis co-precipitation method | Ti/Al ~0, 0.96, 1.93, 3.88 | 250.8 | Weak acid sites (~65.3), moderate acid sites (~58.5), and strong acid sites (~12.4) | - | Ti modification can modulate metal support interaction, Al-O-Ti bonds formed, and reduced acid sites. Ti enhanced HYD and CUS. | [34] |
| TiO2-Al2O3 | Solvent evaporation-induced self-assembly method | Ti ~0, 10, 20, 40% | 428, 391, 365, 296 | Weak acid sites (µmol·g-1) (~196) and strong acid sites (~68) | - | 20% support increase of TiO2 increased pore size, decreased strong acid sites. | [43] |
| TiO2-Al2O3 | EISA method | Ti/Al 0, 0.2, 0.4, 0.6 | 241, 273, 284, 207 | Weak acid sites (mmol·g-1) ~0.082 Medium and strong acid sites ~0.022 | - | Highly ordered structure with high surface area, narrow pore size distribution, and high stability, Ti-O-Al bonds formed, Ti and Al homogeneously distributed. | [44] |
| -γ-Al2O3-HZSM-5 -γ-Al2O3-MCM-41 | Physically blending method | HZSM-5 (5%, 10%, 20%), MCM-41 (5%, 10%, 15%) | 215 | 0.33 (µmol·m-2) | 0.49 | Higher surface area, increased BAS/LAS obtained. | [45] |
| C@γ-Al2O3 | Chemical vapor deposition | - | 259, 249, 207 | Total acidity 315 (µmol·g-1) | - | Higher dispersion of the active metal and promotion of MoS2 by Co. | [46] |
| NPC@γ-Al2O3 | Pyrolysis of phytic acid and urea impregnated on γ-Al2O3 | 0.7 g of phytic acid, 0.7 g of urea | 220 | 57 (µmol·g-1) | 0.48 | High electron donating effect of N and P; decreased acid sites on the support. | [47] |
| Zeolite-activated carbon (ZAC50%) | Reflux | 50wt.% activated carbon | 379 | - | - | Uniform mesoporous structure, moderate acidity, and better active metal dispersion. | [48] |
| SBA-15, activated carbon, mesoporous alumina | So-gel, commercial activated carbon | - | 573, 447, 276 | - | - | SBA-16, AC showed a high surface area; the mesoporous catalyst showed higher hydroprocessing and hydrocracking activity due to higher pore size and dispersion. | [49] |
| Hexagonal mesoporous silica [Ti-HMS (Si/Ti 20, 40, 60)] | Post-synthesis method | - | 285 | - | - | Ti-incorporation improved the catalytic activity, and | [50] |
| Pseudoboehmite | Eco-friendly technology (reprecipitation) and hydrothermal treatment | - | 192 | - | - | Bimodal pore distribution. Catalyst supported on H1 has more CoMoS phase. | [35] |
| Mesostructured titania | - | - | - | - | - | Higher specific surface area. | [51] |
| Al2O3-CeO2 | Sol-gel | CeO2 loadings (0wt.%, 5wt.%, 10wt.%, and 15wt.% | - | - | - | 15wt.% CeO2 loading resulted in 97% DBT HDS conversion and inhibited the formation of NiAl2O4 spinnel and increased BAS. | [52] |
Alumina supports
Alumina supports are the most widely used in industrial applications since they are inexpensive and readily available, with good chemical and thermal stability and excellent surface area. However, the main challenge is its strong interactions with the active metals, which negatively affects catalytic activity[51,52]. During the synthesis of the catalysts, the impregnated metal precursors, Co/Ni(Mo/W), interact with the support, and the high affinity of Ni2+/Co2+ ions coordinating with the Al3+ surface vacancies can lead to the formation of strongly bonded spinel-like NiAl2O4 structures, resulting in low catalytic activity due to limited availability of Ni ions to form the active phases Co(Ni)Mo(W)S[52]. Mo2C, MoS2, PtO2, Pt/C, Pd/C, Ru/C, and Rh/C, sulfided Pt/C and sulfided Pt/γ-Al2O3 were compared in the HDN of pyridine, and Pt/γ-Al2O3 was the most active[53]. The catalysts followed the trend: Pt/C > Pd/Al2O3 > Pd/C > Ni2P > Pt20Ru10/C > Pt/Al2O3 in the conversion of pyridine[54]. HDO activity using Pt was influenced by the type of support used, showing the trend: Pt/Al2O3 > Pt/ASA > Pt/TiO2 > Pt/CeZrO2 > Pt/ZrO2 > Pt/CeO2[55]. However, coke formation was cited as the biggest challenge with these catalysts[56]. While the traditional alumina-supported catalysts do well for HDO, alumina is unstable in the presence of high levels of water and is highly vulnerable to coking due to the acidic sites[24].
Mesoporous silica supports
Mesoporous silicas have excellent physicochemical properties, such as high surface area, large pore size, and structural order[39,42,57]. Silicas provide moderate metal-support interaction and well-dispersed active metals, resulting in good catalyst performance[58]. The textural properties of SBA-15 are desirable for HDS of refractory sulfur compounds, including stability under harsh hydrotreatment conditions[57]. Liu et al. investigated NiMo catalysts supported on SBA-15 and -Al2O3 for the HDS of 4,6-DMDBT and found good dispersion of active metals and high HDS activity compared to -Al2O3-supported catalysts[58]. SBA-16 promoted good dispersion of active metals and is reported to be good for hydrogenation catalysts with large pores that promote mass transfer[42]. Hydrocracking properties of Pd (1%) loaded mesoporous silicas (MCM-41, MCM-48, SBA-15 ASA) were investigated, and a cracked product distribution was found to decrease with loss of mesopores while low metal dispersion resulted in overcracking due to long poor residence time of olefins[59]. However, the low acidity of mesoporous silicas limits their range of application as catalytic supports[57].
Titania, zirconia and ceria supports
Titanium dioxide has been widely used in hydroprocessing catalysts and is reported to promote better dispersion (e.g., of MoS2) and increased acidity[36,56]. However, conventional TiO2 has not penetrated commercial applications as much as alumina because it suffers low mechanical strength and specific surface area[51,60]. Several strategies have been developed to obtain TiO2 with better pore sizes and specific surface area, but crystallization during preparation leads to the collapse of the porosity[46]. Better HDS reactivity by weakening the geometric constraint through dealkylation and/or isomerization has been reported through TiO2 and ZrO2[61]. TiO2 and ZrO2 have been reported to be ideal as HDO catalysts. Sudhakar et al. found that these supports provide additional HDO sites, stabilize the catalyst active sites, and enhance resistance to coking[22]. Cerium dioxide (CeO2) is one of the most abundant rare earth metals and has attracted attention due to its properties such as chemical stability. It shares many similar properties with TiO2[62]. CeO2 has good chemical stability, can play a role as a promoter, can activate H2, and promotes the effective dispersion of the active metal. Rare earth metals generally have less carbon deposition, and CeO2 has a lower tendency for coke formation[63,64]. It is particularly valuable for HDO due to its ability to generate oxygen vacancies and activate H2 and C=O bonds[56]. However, it is worth noting that CeO2 is relatively expensive and has low surface area and poor thermal and mechanical properties[39]. Good acidity for C=O bond activation has been reported in TiO2, ZrO2, and CeO2, and the order of their acidity follows the order TiO2 > ZrO2 > CeO2[56]. Small surface area, lack of porosity, and deactivation at high temperatures have been cited as challenges in these metal oxides, encouraging the use of aluminosilicates[56].
Zeolite supports
Hydrated aluminosilicates (zeolites), such as NaY, SAPO-11(34), USY, and ZSM, are gaining more attention as supports due to their good balance of Bronsted and Lewis acid sites, good crystalline structure and controlled porosity, high thermal and mechanical resistance, ease of dispersing active phases optimally, and large surface area[65]. Much attention towards them has increased due to their high isomerization and cracking activities[34,66]. They play an important role in the morphology of the active phase[66]. Y-zeolites are more dominant in commercial hydrocracking applications, while ZSM-5-type materials are preferred for hydrodewaxing or shape-selective cracking[67]. Zeolite desilication was found to improve the performance of a PtPd/HY catalyst in the hydrocracking of a plastic pyrolysis oil/vacuum gas oil blend[68]. Drawbacks with zeolites include limited pore accessibility and distribution, diffusion barriers, and deactivation by coking[65,69]. Sometimes, strong Bronsted acid sites in zeolites lead to unwanted over-cracking of hydrocarbons and carbon deposition. To mitigate these challenges, various metals, such as Ni, Pt, Rh, Re, La, Bi, Mg, K, and Mg non-metals, such as P and B, have been incorporated in the support[43,66,70-74]. Ga was found to be the most suitable[43,61,75]. Zhou et al. demonstrated that Ga weakens the acid site strength on the surface of Y zeolites by acting as a weak electron acceptor compared to Al[66]. Zeolites have also been successfully employed in HDO but also show dealumination and high coke formation[76]. A high acidity and well-balanced ratio of micro- and meso-porosity was found at a higher SiO2/Al2O3 ratio, giving high activity in a Ni-W/Zeolite-Y catalyst[77].
Carbon material supports
Carbon materials show good thermal conductivity, textural and structural characteristics, weak interaction with active metals, higher dispersion of active metals, higher resistance to coke deposition due to the absence of acidity, lower tendency of nitrogen poisoning, wide transport pores that limit the formation of condensation products during hydrotreating reactions, and good electron transport capability[46,78,79]. However, it is important to note that different types of carbon materials have different characteristics. Pure carbon materials and activated carbon materials have poor mechanical properties, low density, and microporosity, while mesoporous carbon material is known to have a low surface area, low mechanical strength, and bulk density[46]. Multi-walled carbon nanotubes (MWCNTs) have high surface area, high pore volume, relatively high mesopore diameter, and almost complete absence of micropores. MWCNT-supported catalysts were credited to having a low crystallinity that increases the ability to adsorb hydrogen and act as a hydrogen transfer agent during the hydrogen spillover from the active sites to the adsorbed feedstock molecules, resulting in increased HDS and hydrogenation (HYD) activity and reduced N-containing compound inhibition[32]. Carbon-supported catalysts have lower HDO activity for carbonyl, carboxyl, and methoxyl groups compared to alumina-supported catalysts[24].
Improved support materials
Various strategies have been used to improve support materials and enhance the performance of hydroprocessing catalysts[32]. Changing the support preparation method, use of novel precursors, additives, and chelating agents, blending different support materials, and other modification methods have been used to overcome challenges such as strong metal-support interactions[80]. Various support materials have been mixed together to explore the combination of positive characteristics from these individual materials and overcome their drawbacks. These combinations include TiO2-Al2O3, Al2O3-SiO2, TiO2-ZrO2, TiO2-SiO2, MCM-41-Al2O3, USY, ZSM-5, Ti-SBA-15, MCM-41, Ti-HMS, TiO2-MgO, Al2O3-MgO, CeO2-Al2O3,
Silica-alumina-supported NiMo, CoMo, and NiW catalysts show higher HDN, hydrocracking, and hydrodearomatization activities compared to alumina[84]. Much higher hydrocracking activity has been reported through amorphous oxide supports such as amorphous silica-alumina compared to crystalline supports such as zeolites[67]. Reducible metal oxides (TiO2, CeO2) have been used as electronic and structural promoters to improve the catalytic activity, selectivity, and thermal stability of the catalysts[63]. Incorporation of Ti can improve the dispersion of the active metals; it can donate electrons to the conductive band of the active metal, weakening the metal-oxide interaction and making it easier to sulfide, and also weakens the metal-sulfur bond energy forming more coordinatively unsaturated sites (CUS) that increase catalytic activity[42,85,86]. The incorporation of Al and Ti in mesoporous silica SBA-15 improves the acidity on the surface[57]. Increasing Si/Al molar ratios (up to 20) in Al-modified SB-15 supported NiMo catalyst resulted in increasing 4,6-DMDBT HDS activity owing to good dispersion of active metals[36,42,87]. TiO2-Al2O3 has superior HDN catalytic performance due to excellent hydrogen transfer capacity and suitable acid properties[51,88]. Better dispersion of CoMoS was obtained on a TiO2-SBA-15 composite, resulting in higher DBT and 4,6-DMDBT selectivity and conversion[51]. Ceria has been incorporated into alumina to produce a material with a higher surface area, increased thermal resistance, reduced carbon deposition, improved dispersion of metals on the surface, and better interaction between the active metal and the support[64]. CeO2 promotes the HDN activity of Ni2P[64]. Xia et al. conducted a study on CoMo/γ-Al2O3 catalysts modified with Ce, and they concluded that the introduction of Ce adjusted the acidity properties of the support, formed new BAS and increased the BAS/LAS ratio, resulting in excellent isomerization performance[85]. A carbon/zeolite composite with attractive properties from both materials was reported[38]. The addition of halides (especially F and Cl), P, and B to support materials has also been reported, usually to improve support acidity[9,89]. Promotional attributes are also given to these additives, with improved HDS and HDN activity in conventional catalysts and enhanced hydrogenolysis and cracking in hydrocracking catalysts[90]. Rare earth elements (La, Ce, Nd, and Pr) have been used to modify the acidity of Y zeolites (SiO2/Al2O3 ≈ 25)[89].
Considerations for catalyst preparation methods and the nature of the precursor components also need to be made. He et al. employed three different synthesis methods (impregnation, co-precipitation, and sol-gel methods) to obtain TiO2-Al2O3 binary oxide composites and found that the catalyst synthesized using the co-impregnation method possessed the highest specific surface area, while the one by the sol-gel method showed higher pore volume and better mesopore distribution[86]. Yan et al. demonstrated through
ROLES OF MINERAL MATERIALS AS ACTIVE METALS AND PROMOTERS IN HYDROPROCESSING CATALYSTS
Active and promoter mineral materials in hydrodesulfurization catalysts
Conventional hydrodesulfurization catalysts
Conventional HDS catalysts are obtained through sulfidation of metal oxide precursors, i.e., Mo or W oxides (active metal) promoted by Co and/or Ni supported on a support such as Al2O3 and TiO2. Upon sulfidation, MoS2 or WS2 with Ni or Co promoter at the edges are generated as the active phases[38]. Various models are used to describe the active phases and are accorded to belong to either Type I [Ni(Co)MoS I] or Type II [Ni(Co)MoS II]. Type I contains the monolayer MoS2 that maintains a strong interaction with Al2O3 via chemical bonds such as (Ni)Mo-O-Al, forming tetrahedral Mo oxides that are difficult to reduce and sulfide. Type II represents weaker interactions with Al2O3 via van der Waals-type interactions with suppressed dispersion, MoS2 layers with better stacking, increased cathedral Mo oxides that are easier to reduce, and complete sulfidation that leads to better HDS activity[38]. The role of the promoter is to donate electrons, promote dispersion, and reduce the interaction between the support and the metal (active metal)[82]. With a better understanding of the Co(Ni)Mo(W)S structure, it has been widely recognized that the HDS activity can be promoted by tuning the morphology of the active phase, the interaction between the active metal and the support, adjusting the properties of the supports (surface area and porosity, acidity), and using additives and chelating ligands[36].
Metal precursors have been varied during synthesis of HDS catalysts to come up with enhanced catalytic activity. Traditional metal salts are being replaced by other metal precursors, such as ammonium heptamolybdates, and cobalt salts, such as heteropoly compounds (HPCs). Dawson, Anderson, Keggin, and Strandberg HPCs as precursors for CoMo catalysts were found to produce catalysts with higher activity due to good homogenous distribution of the active phases on the surface of the support and more metal loading compared to conventional compounds[92]. Anderson-type HPCs {Mo12O30(μ2-OH)10H2[Co(H2O)3]4} contain both Co and Mo atoms in their molecular structure, and using them as precursors leads to a higher concentration of active CoMoS phase on the support material and increased HDS activity due to intimate contact between Co and Mo atoms[92]. Various additives have also been investigated to improve the activity and selectivity of conventional HDS catalysts; P, F, B, Cl, Zn, Mg, Li, Na, K, Ca, and rare-earth metals, with P and F being the most prominent[90,93,94].
Improved hydrodesulfurization catalysts
HDS has never been a challenge due to the high reactivity of most S-containing compounds until recently due to the need to achieve deep desulfurization through improved desulfurization of refractory S-containing compounds and the need to accommodate harsher feedstock that have higher concentrations of
We recently reported higher HDS of DBT due to a good synergistic effect between Rh promoter and Mo and more MoS2/RhMoS phases when we tested a series of RhMo-x/Al2O3 catalysts with different chelating ligand ratios (x = ethylene diamine, tetraacetic acid, acetic acid, citric acid)[104,105]. We attributed high activity to electron donation from Rh to the Mo conducting band, weakening Mo-S bonds and facilitating the formation of CUS [Figure 3]. However, the high cost and vulnerability of noble metals discourage their industrial application as HDS catalysts.
More cost-effective and abundant metals to replace the conventional active and promoter metals, especially Mo and W, are also being sought due to the increasing demand, low crust abundance, high mining cost, and environmental toxicity of these metals[36,38,106]. Ti, V, Nb, and Fe have been tested as potential HDS catalysts[36]. Iron has attracted the most attention due to its low cost, high crust abundance, and environmental friendliness[36]. Fe has some level of hydrogenation activity as it is known to have lattice structures and an unfilled d-electron layer, similar to metal species that are used in conventional hydrotreating catalysts (Co, Ni, Mo, Pt, and Pd)[36]. Although Fe is common in processes such as ammonia synthesis, direct coal liquefaction, Fischer-Tropsch synthesis, and HDO, there are few studies where Fe was tested as an active metal in HDS catalysts. Iron-based catalysts are generally considered to have poor HDS activity, with iron sulfide having much lower than Mo and W sulfide, which is associated with Pauling d% of metals that lead to the different strength of the metal-sulfur covalent bond and sulfidation degree of TMS[36]. Li et al. reported improved activity of Fe-based catalysts after incorporating zinc as a promoter[107]. The presence of Zn in the catalyst attributed excellent electron donation to Fe, thereby forming a new active phase (FeZnSx)[95,107]. The addition of Zn also resulted in the generation of more sulfur vacancies[37]. The study also emphasized the major role played by suitable support materials in enhancing the catalytic activity of Fe-based catalysts. It was found that the incorporation of nitrogen on the carbon support provides unique electronic properties such as promoting the dispersion of Fe, improving its electron-donating capabilities to the active metal (Fe), greatly weakening the Fe-S bond and promoting the formation of CUS, thereby enhancing the HDS activity[95,107]. Typical catalysts that have been developed for HDS are presented in Table 2.
Typical catalysts for HDS reactions
| Catalyst | Composition | Feedstock | Reaction conditions | Comments | Ref. | ||
| Active material(s) | Promoter(s)/Additive(s) | Support | |||||
| MoS2 | MoS2 | - | - | DBT and/or Q in decane | Batch, 340 °C, 3 MPa H2 pressure, copper scrubber for H2S | Strong hydrogenation function, quinoline inhibitory on HDS via hydrogenation pathway. | [108] |
| CoMo/Al2O3 | Mo | 0.57wt.%, 1.21wt.%, 2.18wt.%, 2.68wt.%, 3.35wt.% Co, 8.17wt.%, 8.27wt.%, 8.35wt.%, 8.37wt.%, 8.45wt.% Mo | Al2O3 | DBT | Batch Parr reactor, 593 K, 1,250 psi, Time on stream = 4 h | Increased activity in Co-promoted catalysts due to cobalt sulfide upon addition of Co/Al2O3 to Mo/Al2O3 or CoMo/Al2O3 catalysts. Both MoS2 and CoMoS phases were promoted. | [109] |
| CoMo/Al2O3-TiO2 | MoS2 | 13wt% of MoO3 and 3.3wt% of CoO | Al2O3-TiO2 | Pyrolysis gasoline | Fixed-bed, 400 °C, 1 atm | Ti increased the surface acidity and improved the reductive behavior of the tetrahedral and octahedral Mo species. CoMo/Al2O3-TiO2 with 10wt.% titania removed over 96% of sulfur-containing molecules in Pygas with improved product quality. | [60] |
| CoMoP/γ-Al2O3 | Mo | 12.5wt.% ± 0.2wt.% of Mo, 3.5wt.% ± 0.1wt.% of Co and 1,5wt.% ± 0,1wt.% of P | γ-Al2O3 | DBT, quinoline and naphthalene | Flow glass reactor, 280 °C, 3.5 MPa, WHSV = 80 h-1 | Hydrothermal treatment of a flash calcined gibbsite makes it possible to obtain hydrotreating catalysts with the highest or the same activity in the HDS and HDN of fuel mixture compared to catalyst supported on alumina obtained through the precipitation of pseudoboehmite from aluminum nitrate. | [35] |
| CoMo/MWCNT-citr, CoMoS/Al2O3-citr | Mo | 12.0wt.% of Mo and 3.2wt.% of Co, Ctr | Al2O3 and MWCT | DBT | Fixed bed, 280 °C, 3.5 MPa, LHSV 20.0 h-1 | CoMo/MWCNT-citr catalyst had the highest HDS activity due to higher sulfidation degree, higher CoMoS phase content, and improved dispersion of sulfide components. | [79] |
| CoMoS/C@γ-Al2O3 | MoS2 | 3.2wt.% Co and 12.0wt.% Mo | C@γ-Al2O3 | DBT | Fixed bed flow, 270 °C, 3.5 MPa, LHSV = 20.0 h-1 | Higher dispersion of sulfided components and enhanced promotion of MoS2 by Co. | [46] |
| CoMoS2/ZSM-5 | MoS2 | Co, 10.7wt% Mo, EDTA | ZSM-5 | 4,6-DMDBT | High-pressure stirred reactor, 573-593 K, 2-5.0 MPa | Multi-stacked MoS2 nanocrystallites on CoMoS2/MZSM-5 provide more available sulfur vacancies to facilitate the DDS pathway and catalyze efficiently the hydrogenation. | [110] |
| CoMo/NDC@Al2O3 | Mo | 18% (mass) MoO3 and 4% (mass) CoO | NDC@Al2O3 | DBT, 4,6-DMDBT, and Dagang diesel | Batch reactor, 300 and 320 °C, 6 and 7 MPa, LHSV = 1 h-1, H2/Oil = 200:1 | The introduction of NDC reduced the interaction between the support and active components. Improved dispersion of active phase and sulfidation degree of Mo by 21.8% compared to the CoMo/γ-Al2O3. | [111] |
| Sulfided NiMo/γ -Al2O3 | 8wt.% Mo | 3wt.% Ni on | γ-Al2O3 (Condea, pore volume 0.5 cm3·g-1, specific surface area 230 m2·g-1) | 4,6-DMDBT vs. presence of 2-methylpyridine and 2-methylpiperidine | Fixed bed, 340 °C and 5 MPa | DDS and HYD inhibited, 2-methylpiperidine > 2-methylpyridine, no significant contribution from change to aromatic solvent | [112] |
| NiMo/Al2O3 | MoS2 | 3.0wt.% NiO and 13.0wt.% MoO3 | Al2O3 | DBT | Fixed-bed, 240, 260, and 280 °C, 4 MPa, LHSV = 20 h-1 | Ni-Al2O3 interaction enhances the availability of surface nickel atoms to form more NiMoS phase and improves the microstructure of MoS2. | [36] |
| NiMo | MoS2 | Ni | - | DBT | Batch reactor, 340 °C, 50 bar | Both catalysts exhibited good performance (lp-NiMoS, 95.8%, and hp-NiMoS, 98.6% DBT conversion), with hp having the highest due to the formation of more NiMoS. | [113] |
| NiMo/SBA-15 | Mo (6.09wt.%, 10.53wt.%, 11.08wt.%, 15.75wt.%) | Ni (1.65wt.%, 3.04wt.%, 2.59wt.%, 2.63wt.%) | SBA-15 | DBT | Batch reactor, 320 °C, 3 and 6 MPa | One-pot synthesis promoted the generation of dispersed shorter MoS2 slabs with desirable average stacking numbers. | [114] |
| NiW/Al2O3 | WS2 | Ni, P WO3 (20w.t%), NiO(4wt.%) and P2O5(2wt.%) | Al2O3 | DBT | Fixed-bed, 220-280 °C, 4.0 MPa, LHSV = 10-60 h-1 | Addition of P improved the availability of surface Ni atoms, P weakened metal support interaction, and enhanced the promotion of NiWS. | [80] |
| NiMoW/Ti- HMS | MoS2 and WS2 | 3.8wt.%, 13.8wt.% and 17.3wt.% of NiO, MoO3 and WO3 | Ti-HMS mesoporous | DBT | Batch reactor, 320 °C, 58.6 bar | More CUS formed, increased both DDS and HYD and high dispersion. | [50] |
| NiTMo-x/Al2O3 | Mo | Ni, tetradecylamine; for NiO and MoO3 (3.6wt.% and 16.9wt.%, and 3.5wt.% and 16.8wt.%) | Al2O3 | 4,6-DMDBT | Fixed-bed, 360 °C, 4.0 MPa, LHSV = 15.0 h-1, time on stream = 6 h | NiTMo-2.0/Al2O3 had the most corner and edge S vacancies, exhibited the highest reaction rate constant, and had the highest DDS ratio of 99.5% in HDS of 4,6-DMDBT. | [86] |
| Ni2P@C, Ni2Si@C, and NiS2@C | - | P, Si | Carbon | DBT | Trickle fixed-bed, 260 and 360 °C, 3.0 MPa | HDS activity: Ni2P@C > Ni@C ≈ Ni2Si@C > NiS2@C. Ni2P@C shows outstanding stability during the long-term reaction. | [95] |
| Ni2-xRuxP/SiO2 | - | Ru, Ni | SiO2 | 4,6-DMDBT | Fixed bed, 533-593 K, 3.0 MPa | Ni1.85Ru0.15P/SiO2 catalyst 15 and 6 times more active (on a mass basis) than Ni2P/SiO2 and Ru2P/SiO2 catalysts. Ru resulted in P surface enrichment and lowered the temperature of precursor reduction. | [97] |
| Rh-M-P (M = Ru, Pt, Ir) | Rh (5.0 and 4.85wt.%), Pt | Ru (0.15wt.%), Pt (0.28wt.%), Ir, P (5.0wt.%, 1.51wt.%, and 3.0wt.%) | SiO2 | Th and 4,6-DMDBT | Fixed bed, 350 °C, 0.1 and 4.0 MPa | Rh-Pt-P catalyst had the highest HDS activity among Rh-M-P catalysts. | [115] |
| Pt-Co-Mo | Mo | Pt, 16wt.% Mo and 3.5wt.% Co | - | Diesel | Fixed bed, 355 °C, 30 bar, LHSV = 1.5 h-1, Time on steam = 24 h | The addition of ppm levels of Pt to a conventional Co-Mo-S catalyst boosted HDS activity by up to 46%. Pt is suggested to reduce the sulfur binding energy and increase the abundance of CUS. Pt is speculated to create sites for favorable adsorption of sterically hindered molecules. | [104] |
Active and promoter mineral materials in hydrodenitrogenation catalysts
Conventional hydrodenitrogenation catalysts
The C(sp2)-N bond is very strong and makes hydrogenation of at least the N-heterocyclic ring in aromatic nitrogen-containing heterocycles a prerequisite to achieving C-N bond breaking, unlike in HDS where pre-hydrogenation of the S-ring is a possible pathway but not a prerequisite for C-S bond breaking[93,116-118]. As such, HDN catalysts need to be bifunctional, having both hydrogenation sites and hydrogenolysis sites. The most common HDN catalysts are the conventional Mo and W sulfides promoted by Ni sulfides, e.g.,
Improved hydrodenitrogenation catalysts
Various transition metal oxides, sulfides, phosphides, borides, nitrides, and carbides have been investigated as HDN catalysts[21,123,124]. Sulfides, nitrides, and carbides show good potential for HDN due to their ability to exchange S, N, or C, and they can adapt their chemical composition to that of the feed during reactions to form highly active phases[125]. Al-Megren et al. demonstrated using a pyridine stream that the HDN activities of bimetallic (CoMo) carbide, oxide, nitride, and sulfide catalysts are of the order: CoMoCx ~ CoMoNx ~ CoMoOx > CoMoSx, while their stability follows the order: CoMoCx > CoMoNx > CoMoOx with decrease in activities over time[124]. Qiu et al. found the α-phase of Mo2C to have the highest selectivity and conversion of quinoline compared to the β-phase[126]. Better HDN potential has been demonstrated using metal phosphides, borides, and nitrides. Fang et al. investigated the effect of different metals, metal loading, ratio of metal to phosphorous, and other catalyst preparation methods on the activities of transition metal phosphides[21]. The performance of transition metal phosphides was found to be significantly higher than metal sulfides. High activity was obtained from a NiP catalyst, with further improvement of activity being obtained with the addition of small amounts of Co or Fe and a change of support materials[21]. This was attributed to Ni preferentially occupying the active hydrogenation square pyramidal M(2) site[21]. During simultaneous HDS and HDN, the overall activity of metal phosphides followed the order of Ni2P > WP > MoP > CoP > Fe2P[13]. Transition metal phosphides were tested for HDS (dibenzothiophene) and HDN (quinoline), and their activities followed the order Fe2P < CoP < MoP < WP < Ni2P. Ni2P/SiO2 had higher activity compared to the commercial Ni-Mo-S/Al2O3 (HDS 98% vs. 78% and HDN 80% vs. 43%) and commercial Co-Mo-S/Al2O3 (HDS 85% vs. 80%)[23]. It was also demonstrated that HDN using Ni2P involves activation at carbon positions α and β to nitrogen atoms in contrast to sulfides where activation only occurs at the β position. HDN activities of silica-supported phosphide catalysts (Co2P/SiO2, MoP/SiO2, WP/SiO2, CoMoP/SiO2, and NiMoP/SiO2) were tested using
Based on periodic trends in Figure 2B, noble metals are the best candidates for HDN activity. Their high HDN activity is accompanied by a lower tendency for coke formation at milder conditions compared to sulfided CoMo catalysts[24]. Additionally, synthesis of noble metal catalysts with non-alumina supports is easy, where alumina has to be avoided[24]. Noble metals are, however, very expensive, and they are vulnerable to inhibition and/or poisoning[120]. For example, good HDN activity has been reported for a Pt catalyst, but sulfur inhibition has been cited as a major challenge[8]. Contrastingly, alumina-supported metallic Ni, Rh, and Ru showed high HDN activity but suffered deactivation due to H2S, except for Pt, which maintained suitable activity[133]. HDN of quinoline was very high using 1% Ir in Al- and Ga-modified silica catalysts due to Ga and Al having Bronsted and Lewis acid sites that work in synergy to give high activity alongside the good dispersion of the catalytic sites[134]. However, appreciable deactivation was observed in the catalysts. On the other hand, ReS2 was among the most active transition metal sulfides, but its activity was not always better than industrial CoMo/Al2O3 and NiMo/Al2O3, and the promoting effect of Co and Ni was found to be low compared to the Mo catalysts[135]. The addition of Ru3(CO)12, a precursor to catalysts that cleave C-N bonds in amine transalkylation reactions, to a commercial CoMo HDN catalyst resulted in significantly improved activity[4]. Carbon and nitrogen can be incorporated in interstitial positions (lattice) of early transition metals (groups 4 to 6), resulting in some pure transition metal carbides and nitrides (e.g., Mo and W carbides and nitrides) that have metallic properties that resemble noble metals[93,99]. The addition of carbon was reported to cause higher hydrogenation properties compared to the addition of nitrogen[99]. Transition metal oxynitrides and oxycarbides are reported to act as bifunctional catalysts through their metallic and acidic active sites, and HDN of indole can occur without pre-hydrogenation of the aromatic ring. They are efficient at activating dihydrogen and N-containing molecules[125].
Physical properties of the catalysts, e.g., particle size and porosity, also have an influence on catalyst performance[11]. The morphology of the sulfide material (e.g., curvature and stacking of layers) was found to influence the catalyst activity in bulk metal sulfides such as MoS2 and WS2[121]. The order of impregnation of the metal precursors on the support (e.g., Mo then Co or Ni, or vice versa) and calcination was found to have a significant influence on the activity of the final sulfided catalyst. Impregnating with Mo first was found to give catalysts with an invariably higher activity[11,93]. Mixing metals by closely interlinking them using sulfur bonds in HDN catalysts results in a significant increase in activity compared to analogous conventionally prepared catalysts[136]. Table 3 comprises a range of catalysts that have been developed for HDN.
Typical hydrodenitrogenation catalysts
| Catalyst | Composition | Feedstock | Reaction conditions | Comments | Ref. | ||
| Active material(s) | Promoter(s)/Additive(s) | Support | |||||
| CoMo/Al2O3 | 12.5wt.% Mo | 3.5wt.% Co 1wt.% P 1wt.% B | Boehmite HQ102 B produced in China (292 m2·g-1; pore volume 1,02 cm3·g-1; pore diameter 140 Å) | Straight-run diesel fuel: 0.383% of S; 192 ppm of N; density of 0,861 g/cm3; boiling range = 207-380 °C (“GazpromneftOmsk NPZ”, Omsk, Russia) | 31 cm3 catalyst mixed 1:2 with SiC, 360 and 370 °C, 3.8 MPa, Н2/feed = 544 nm3/m3, LHSV = 2.5 h-1 | CoMoS phase content increased with increasing concentration of P, but more than 2wt.% of P leads to a decrease of SSA and pore volume. Increased activity is attributed to increased surface concentrations of P(OH) or B-OH groups, which adsorb N compounds before they migrate to the sulfide active component. | [137] |
| Nitrided 12.5% Mo/Al2O3 | Mo | - | γ-Alumina xerogel | 0.25wt.% carbazole in xylene | 2.0 g catalyst in a stainless steel fixed-bed microreactor, feed flow = 20 mL·h-1, H2 flow = 74.4 µmol·s-1, 573 K, 10.1 MPa | High hydrogenation activity in carbazole HDN. The activity of the nitrided catalysts for the hydrogenation in carbazole HDN is not related to surface acidity but rather to the reduced molybdenum ions Mo2+ and Mo0 on the surface of the molybdenum nitrided catalysts. | [138] |
| Bulk molybdenum carbide (Mo2C) | Mo | - | - | 2.5wt.% of indole dissolved in decaline or 0.4wt.% carbazole in o-xylene | 0.8 g of catalyst in SiC (SiC/Mo2C = 5:1), 613 K, 60 bar, H2/feed ratio of 600 and contact times (tc) 0.13-1.07 s | High HDN is maintained even in the presence of small amounts of S (50 ppm). Bulk Mo2C behaves similarly to that of a noble metal catalyst. Direct HDN is predominant during the early conversion percentages, while at high conversions, the HYD route is followed. | [129,139] |
| α-MoC1-x | Mo | - | Specific surface area (107.6 m2·g-1) | - | - | Higher SSA, activity, and selectivity than β-Mo2C | [126] |
| Molybdenum carbide (Mo2C) vs. Tungsten carbide (W2C) | Mo or W | - | - | 0.4wt.% of carbazole in o-xylene | Downflow fixed bed microreactor, 553 and 653 K, 6 MPa, H2/feed ratio of 600, contact times (tc) = 0.07-0.8 s | HDN via hydrogenation for both catalysts. Both are bifunctional catalysts (hydrogenation and isomerization). Both reactions are more significant in W2C, but the activity of Mo2C is higher than that of W2C. | [140] |
| Silica-supported nickel phosphide (Ni2P) and Ni-rich bimetallic phosphide catalysts [25wt.% Ni2P/SiO2, Fe0.03Ni1.97P/SiO2 and Co0.1Ni1.9P/SiO2] | Ni or Fe or Co | P (ratio 0.9 to 1.0 with metal) | Silica (Cab-O-Sil, M7D) | 1,000 ppm carbazole + 500 ppm dodecane in 39.85wt.% p-xylene/decane or 3,000 ppm benzothiophene + 1,000 ppm carbazole + 500 ppm dodecane in 39.55wt.% p-xylene/decane. | 0.25 g catalyst mixed with quartz flakes to a total volume of 5 mL in a fixed bed continuous flow reactor, 548-673K, 3.0 MPa, feed flow = 5.4 mL·h-1, H2 flow = 50 mL·min-1 | Higher carbazole HDN conversion in all catalysts compared to commercial Ni-Mo/Al2O3. Metal phosphides are much higher (92%-98%) vs. Ni-Mo/Al2O3 (75%-84%). Carbazole HDN on metal phosphides inhibited by DBT co-feeding Ni2P/SiO2 and Ni-rich bimetallic phosphide catalysts maintained their activities. Metal phosphides favored ring-opened and ring-contracted products, indicating the presence of metal and Brönsted acid sites on the catalyst surfaces. | [141] |
| Transition-metal phosphides (Co2P, Ni2P, MoP, WP, CoMoP, NiMoP) | Ni or Mo or W | Ni or Co, P | - | o-propylaniline in n-octane | 0.05-0.4 g catalyst diluted with 8 g SiC, continuous-flow microreactor, 643 K, 3 MPa | MoP and WP > Ni2P and NiMoP > Co2P and CoMoP. All catalysts except for WP lost their activity in the presence of H2S, being irreversible in the case of Ni2P. | [142] |
| CoMoP/Al2O3 and NiMoP/Al2O3 | Mo | Co or Ni | Al2O3 | - | - | NiMoP has stronger hydrogenating power than CoMoP. | [143] |
| Zeolite supported Nickel phosphides (NixPy) | 5wt.% Ni | P (3:0 P/Ni) | Commercial Na+Y and NH4+Y zeolites (Molecular sieves, powder, Si/Al ratio: 2.56) or zeolite (SiO2/Al2O3 mol ratio: 30:1, powder form) | 1 mL 10vol.% (14.1wt.%) quinoline in dodecane solution | 50 mg catalyst in 1 g of silica gel in 6 mL stainless steel batch reactors, 400 °C, 40 bar | HDN activity: Ni2P > Ni0 > Ni12P5. The activity of catalysts comparable to commercial 5wt.% Pd/C, NiCoMo/Al2O3 and NiMo/CeO2 catalysts. | [128] |
| Ni2P/MgAlO | 60% Ni | Phosphorous | MgAlO | 1.72% DBT (3,000 ppm S), 0.185% quinoline (200 ppm N), 5% tetralin and 0.5% n-octane in n-tetradecane | Fix-bed reactor, 513 to 613 K, 3.1 MPa, LHSV = 2 h-1, H2/oil ratio = 1,500 (v/v) | MgO/Al2O3 ratios had no influence on the intrinsic activity of Ni2P. Ni2P/MgAlO with the MgO/Al2O3 ratio of 3 showed the highest activities. 92.6% conversion of DBT at 513 K Ni2P/MgAlO catalysts. 67.6% conversion of tetralin at 613 K, significantly higher than other catalysts (46.1%-58.6%). | [144] |
| Ni-rich bimetallic phosphides | Mo and Ni | P | - | 2wt.% quinoline and 1wt.% DBT in decalin | Simultaneous HDN (quinoline) and HDS (DBT) in a fix-bed reactor. 1 g catalyst mixed with 9 mL quartz sand, 320-380 °C, 3 MPa, WHSV = 6 h-1, H2/feed flow ratio of 500:1 | Increase in HDN and HDS conversions with increasing temperature, approaching 90% and 100% at 380 °C, higher than the commercial MoNiW/Al2O3 catalyst. Mo0.05Ni1.95P showed the highest activity in simultaneous HDS and HDN compared to Co0.05Ni1.95P, Mo0.05Ni1.95P, Fe0.05Ni1.95P and W0.05Ni1.95P. Catalyst activity is greatly reduced by quinoline. | [21] |
| NiB alloy | 87.2% of nickel | 7.1% of boron | - | 0.08% carbazole (100 ppm N) or 0.08% carbazole and 0.17% 4,6-DMDBT (300 ppm S) in o-xylene. | Down-flow fixed-bed microreactor, simultaneous HDN/HDS (0.8 g catalyst) or HDN alone (0.2 g catalyst) in SiC (1:5), H2 flow = 60,360 cm3·min-1 | Carbazole HDN takes place mainly on the Ni metallic phase, with bicyclohexyl as the main product. Decomposition of catalyst noted during the 100 h of use. Catalyst activity is lower for HDS/HDN due to partial sulfidation of the catalyst. | [129] |
| W/Al2O3 | 20wt.% WO3 | 6wt.% P2O5 | Al2O3 GirdlerT-126, surface area 188 m2·g-l, pore volume 0.39 cm3·g-l, particle size 0.15-0.25 mm | Gas oil and pyridine | Flow trickle bed microreactor, 3.5 g catalyst in SiC, LHSV = 8.8 h-1, H2/liquid feed of 400 v/v; 598, 623, and 648 K | Simultaneous HDS/HDN. P2O5 increases the amount of reducible tungsten oxide species by promoting the formation of octahedral polytungstates, has little effect on the lateral growth of the WS2 slabs but significantly increases the average number of layers, enhances both HDS and HDN activities but more effectively for HDS than HDN. | [20] |
| Platinum-doped tungsten carbide (W2C-Pt) | W | 0.3wt% Pt | - | 0.17% 4,6-DMDBT (300 ppm S) and 0.08% carbazole (100 ppm N) in o-xylene | 350 °C, 6.0 MPa, molar H2/feed ratio = 600 | Simultaneous HDS and HDN. W2C, Pt-W2C, W2C-Pt compared. W2C-Pt has the highest conversions 55% and 100% of conversion for HDS and HDN, while W2C, Pt-W2C are not far at around 27% and 90% for HDS and HDN. Pt increases HDN activity of W2C-Pt. Increased HDS conversion after tc > 0.31 s when HDN is complete is attributed to earlier completion of HDN. | [15] |
| W2C, Pt-W2C and W2C-Pt | W2C and Pt | - | W2C synthesized from temperature programmed reaction of WO3 (Fluka 99.9% pure) with 10vol.% CH4/H2 gas mixture. | 0.17% 4,6-DMDBT (300 ppm S) and 0.08% carbazole (100 ppm N) in o-xylene | 1:5 catalyst SiC, 0.25 g catalyst, 350 C, 6.0 MPa, molar H2/feed ratio of 600 | Simultaneous HDS and HDN. W2C exhibits both HDS and HDN activity. Addition of Pt showed no effect on HDS activity at short contact time (tc < 0.21 s), unlike HDN activity, where a significant increase was observed. HDS only increased towards the completion of HDN, indicating possible inhibition of N on Pt. W2C and Pt-W2C had similar activity (up to 27% HDS and 90% HDN activity) compared to W2C-Pt (55% HDS and 100% HDN activity). The addition of Pt prior to W2C synthesis results in no increased activity. | [15] |
| Re/Al-SBA-15 and Ni-Re/Al-SBA-15 | 5wt.% Re | 1wt.% Ni | SBA-15 572 m2/g, 0.5 cm3/g, Dp 6.96 nm | 2.5wt.% of o-toluidine in cyclohexane | Fixed-bed flow reactor system, 4 cm3 catalyst, 673-723 K, H2 flow = 50 cm3·min-1, atmospheric pressure | The highest HDN activity in 5wt% Re/Al-SBA-15(10) is attributed to the fine dispersion of Re over the support and successful incorporation of Al into SBA-15, which enhances the strong metal-support interactions and formation of moderate acid sites. Ni-Re/Al-SBA-15 had higher activity due to Ni promoter effects. | [145] |
| Mo-Ir/Al2O3 | Mo | 0.34wt.%-0.53wt.% Ir | γ-alumina CS 331-1 SA 255 m2·g-1, pore volume 0.76 mL·g-1, particle size of 0.16-0.32 mm | 220 ppm of pyridine and 240 ppm of thiophene. | Fixed bed, 0.05-0.2 g catalyst, 32 °C, 20 bar, H2 flow = 0.4 mol·h-1 | Simultaneous HDN/HDS. The addition of Ir to Mo/Al2O3 led to increased reducibility of the Mo phase and enhancement of HDN and HDS activities, especially HDN, where a synergetic effect resulted in 3 times more activity at loadings of about 0.34wt.%-0.53wt.% Ir. HDN/HDS selectivity of modified catalysts was higher than the selectivities of individual Ir and Mo catalysts. | [146] |
| Pt/A12O3 and Pt/SiO2-A12O3 | Pt | - | Alumina (Condea, Sp - 180 m2·g-1) or silica alumina (Condea, 40% of SiO2, Sp - 374 mE·g-1) | - | - | Five times more active than NiMo/Al2SO3, sulfur inhibition poses a challenge. | [8] |
Active and promoter mineral materials in hydrodeoxygenation catalysts
Conventional hydrodeoxygenation catalysts
Conventional alumina-supported CoMo and NiMo catalysts are usually applied in HDO at moderate temperatures (300-600 °C) and high pressure (7-20 MPa)[147]. The oxygen is removed through a series of reactions, such as decarboxylation, saturation of unsaturated compounds, and C=O via hydrogenation, cleavage of C-O bonds via hydrogenolysis, and cleavage of heavy molecular weight compounds to lighter molecular weight compounds via hydrocracking[148-150] Hydrocarbon percentage yields were similar in the HDO of waste cooking oil using conventional sulfided catalysts (NiMO/Al2O3, CoMo/Al2O3, and
Improved hydrodeoxygenation catalysts
New non-conventional hydroprocessing bimetallic catalysts, such as alumina-supported Ni-Fe and Ni-Cu, have been reported as promising HDO catalysts[148,162]. Ni-Fe/Al2O3 was tested on three model compounds of bio-oil: furfuryl alcohol, benzene alcohol, and ethyl oenanthate at 400 °C, and the conversion was found to be 100%, 95.48%, and 97.89%, respectively. The pathway of C-O cleavage was followed[148]. A synergistic effect of Ni and Cu was attributed to the high HDO efficiency obtained through Cu-Ni/ZrO2 compared to the monometallic Ni and Cu catalyst[149]. Although monometallic catalysts show a promising route for HDO, bimetallic and trimetallic catalysts are more promising as they have been shown to inhibit the sintering of the active phase due to their exceptional electronic and dispersion properties[149]. HDO and decarboxylation products are obtained in the HDO of vegetable oils when using monometallic Mo/SiO2 and Ni/SiO2, respectively, while a mixture of decarboxylation and HDO products are obtained through bimetallic NiMo/SiO2 with overall catalytic activity following the order Ni/SiO2 < Mo/SiO2 < NiMo/SiO2[154]. Similar observations were made in the HDO of rapeseed oil using NiMo/Al2O3 > Co/Al2O3 > Ni/-Al2O3[154]. Xue et al. obtained the highest HDO activity on bioderived phenol through the trimetallic alumina-supported
Numerous HDO catalysts that migrate from traditional alumina supports have been tested. A carbon-supported CoMo catalyst has been studied for the HDO reaction with 4-methyl-acetophenol, guiacol, and ethyl decanoate as a feedstock below the typical HDO temperature range (280 °C) and 7 MPa[164]. Successful HDO of phenols in the temperature range of 300-450 °C and 5 MPa using a MgO-supported CoMo catalyst has been reported[165]. NiW with C support has also been employed for the HDO of phenols at a very low pressure of 1 MPa at a temperature of 250-300 °C, which is lower than the typical HDO temperature range[166]. Yoosuk et al. reported an amorphous unsupported NiMoS catalyst with high HDO activity[167]. Unsulfided catalysts on neutral supports, such as activated carbon and SiO2, have also been explored to address the challenges of very low sulfur bio-oils[155-160].
Noble metals have also been investigated as HDO catalysts, and these are mostly hosted on other supports besides alumina[168]. Pd/Al2O3 was reported to be suitable for the in-situ HDO of phenol, o-cresol, and p-tert-butylphenol with high selectivity of cyclohexanone and had better conversions compared to Raney Ni catalysts[169]. A C/alumina-supported Ru catalyst was tested for the HDO of pyrolysis oil at 350 °C and
Typical hydrodeoxygenation catalysts
| Catalyst | Composition | Feedstock | Reaction conditions | Comments | Ref. | ||
| Active material(s) | Promoter(s)/Additive(s) | Support | |||||
| CoMo/Al2O3 | Sulfided CoMo | - | Al2O3 | o, m, p-cresols | Autoclave, 360 °C, 7 MPa and 60 min | The efficiency of the HDO reaction of ortho < meta < para. P-cresol reported 95% conversion, while o-cresol had 87% conversion. | [147] |
| CoMo/Al2O3 | 4% CoO, 15% MoO, 235 m2·g-1 Al2O3 | - | Al2O3 | Phenols (methyl substituted) | Microflow reactor, 300 °C, 2.85 MPa | Two pathways were observed DDO (leading to the formation of aromatics) and HYD (formation of cyclohexane). | [9] |
| NiMo | Unsupported NiMO sulfide | - | - | Phenol | Parr reactor, 350 °C, 2.8 MPa, 60 min, 150 rpm | NiMo sulfide followed HYD pathway, forming aliphatic hydrocarbons with 96.2% conversion due to the addition of Ni, while Mo sulfide followed DDO pathway. | [167] |
| NiFe/Al2O3 | 15wt.% Ni, 5wt.% Fe | - | Al2O3 | Furfuryl alcohol, benzene alcohol, and ethyl oenanthate | Tubular reactor, 300-400 °C, atmospheric pressure | A 100%, 95.48%, and 97.89% conversion for furfuryl alcohol, benzene alcohol, and oenanthate was observed, respectively. The byproducts of the reaction include toluene, 2-methylfuran, and heptance. The conversion of oenanthate is more affected by temperature. | [148] |
| Ni-Cu-Co/Al2O3 | 20Ni-5Cu-5Co/Al, 20Ni/Al, 20Ni-5Cu/Al | - | Al2O3 | Bio-derived phenol | 240 °C, 4 MPa, 6 h | Ni-Cu-Co/Al2O3 performs better (100% conversion) than monometallic and bimetallic catalysts (< 80% conversion). The main products of the reaction included cyclohexane and cyclohexanol. Both HDO pathways were illustrated as there was a sign of DDO pathway indicated by the formation of benzene, which is unlikely in situ HDO. | [150] |
| Cu/Al2O3, Ni/Al2O3, Cu-Ni/Al2O3 | 60wt.% Cu/Al2O3, 60wt.% Ni/Al, 20wt.% Cu-40wt.% Ni/Al | - | Al2O3 | Furfural | Microbatch reactor, 300 °C, 10 bar/hydrogen donor (methanol/isopropyl) | Bimetallic catalyst Cu-Ni/Al2O3 had a better performance when compared to the monometallic analogs of Ni and Cu. Isopropranol is a better hydrogen donor and yields higher conversions compared to methanol. | [149,162] |
| Ni/Ce-SBA-15 | 5wt.% Nickel | Cerium 0.08 Ce:Si ratio | Ce-SBA-15 696 m2/g, 0.98 cm3/g, pore diameter 9.3 nm | 4wt.% anisole in heptane | Fixed-bed tubular reactor, 200 mg catalyst in SiC, 7 bar, 290 °C | Improved catalytic activity and selectivity towards benzene (double compared to Ni/SBA-15). Ce promotion is more efficient for intermediate Ce content Ce/Si = 0.03. Better performance is also attributed to better dispersion of the metallic phase and formation of specific active sites in the Ni particles in contact with Ce-containing surface species. | [172] |
| Pd/Al2O3vs. Raney Ni | 3wt.% Pd, Raney Ni: 9:11 (Ni:Al) | - | Pd/Al2O3: 250 m2·g-1 of Al2O3, 80-120 mesh | Phenol, o-cresol, p-tert-butylphenol | Tubular reactor, 470-490 K, 3.5 MPa | Phenol HDO Pd/Al2O3 cyclohexanone selectivity: 96.1%, 92.4%, and 71.1%, with methanol, ethanol, and H2 as hydrogen sources, respectively. HDO of phenol Raney Ni cyclohexanone selectivity: 20.5%, 27.4%, and 12.6%, with methanol, ethanol, and H2 as hydrogen sources, respectively. Conversions using Pd/Al2O3; H2 > EtOH > MeOH, phenol (82.5%) > o-cresol > p-tert-butylphenol (0.65%). Better HDO of o-cresol with EtOH as hydrogen donor as compared to Raney Ni. | [169] |
| Ru/C, Ru/TiO2, Ru/Al2O3, Pt/C, Pd/C | 5wt.% of active metal | - | C | Bio-oil | Batch autoclave, two-stage operating conditions: 250 and 350 °C, 100 and 200 bar, 4 h | Ru/C showed a higher yield of 60wt.% and had better HDO performance (90wt.%) than other noble metal catalysts. Ru/C yields better-upgraded oil than traditional CoMo/Al2O. Pd/C had higher yields than Ru/C, but oil had a high oxygen content. | [171] |
| Pt/Al2O3, Pt/TiO2, Pt/ZrO2, Pt/SiO2-Al2O3 | - | - | Al2O3, TiO2, ZrO2, SiO2, SiO2-Al2O3 | Bio-oil | Autoclave, 350 °C, 6.9 bar | Pt/Al2O3 showed better performance compared to other Pt catalysts, reducing oxygen content from 41.4wt.% to 2.8wt.%. Supports influence how well the catalyst performs in HDO | [55] |
Active and promoter mineral materials in hydrodemetallization catalysts
Conventional hydrodemetallization catalysts
While alkali and alkali earth metal salts in feedstock can be easily removed by washing before distillation, heavy metals can only be effectively removed through HDM using catalysts and hydrogen at high temperatures[7,173]. Conventional alumina-supported NiMo, CoMo, and NiW hydrotreating catalysts have been reported for HDM[173,174]. Nickel removal from bitumen-derived and Conradson carbon residue oils was investigated using a commercial NiMo/Al2O3 catalyst; high Ni removal was achieved under mild conditions[175]. Ancheyta-Juárez et al. studied the HDM of Maya heavy crude using Ni-Mo/Al2O3 catalysts, cited diffusion limitation to the effective processing of large molecules, and recommended high catalyst pore volumes in the range 100-250 Å to be optimal for HDM[176]. Similar pore size ranges were also reported by Liu et al. when they tested Ni-Mo/Al2O3 catalysts on the HDM of Saudi Arabia vacuum residuum[177]. They indicated high HDM activity for the smaller and middle range Ni and V compounds, but effective HDM of the larger compounds required macropores (> 100 Å)[177]. HDM is easy, but the deposition of metal products is a challenge that necessitates considerations for catalyst metal retention capacity, pore structures, size distributions, and acid-base properties of the support and catalyst morphology[7,178]. Organometallic compounds of high molecular weight and dimensions that approach or exceed catalyst pore size also cause fouling of catalysts[179-181]. Metals cause or promote catalyst deactivation as they accumulate on catalyst surfaces through deposition (e.g., as metal sulfides)[181,182]. Fe bound by naphthenates causes plugging when it forms FeS in catalyst beds and filters. FeS also enhances coking reactions[7]. Catalyst activity, selectivity for desired products, and lifespan are influenced. Heavy metals also restrict conditions for the disposal of spent catalysts[181]. Heavy metals are commonly found in porphyrin rings and are mostly concentrated in high boiling point fractions (e.g., heavy oil residues, asphaltenes, and resins), with V and Ni being the most abundant in most oils[7,178,181]. These aspects motivate the continuous development of HDM catalysts, especially since HDM is important as a first step to protect downstream catalysts for cracking, HDS, HDN, and HAD[7,173,174].
Improved hydrodemetallization catalysts
Alumina-supported HDM catalysts are not efficient as they suffer deactivation through the deposition of metals on active sites and pore plugging arising from accumulations of metal sulfides[180]. Rana et al. used carbon as a support for HDN and observed that carbon support leads to weak metal support interaction, resulting in more Type II active sites that increase the deposition of metal on the carbon surface, preventing overlaying on existing MoS2 sites[178]. Rana et al. also investigated the incorporation of TiO2 into alumina to produce a CoMo/TiO2-Al2O3 and obtained better textural properties and higher HDS activity but a lower HDM activity, indicating the need for higher average pore diameter and macropore size distribution to achieve higher HDM activity[183]. Rana et al. attributed much of the HDM catalysis activity to Mo sites when they tested the promoter effect of Fe, Co, and Ni, observing the trend of Fe > Co > Ni for the promoters[178]. They also investigated the effect of mixing peptized alumina with activated carbon to reduce diffusion limitations of larger compounds such as asphaltenes and enhance the metal storage capacity of the catalysts. Patents are also available for Mo, W, and Fe (with Co, Ni, or Fe promoters) and 1:1 alumina to carbon extrudate support with a bimodal type pore size distribution (i.e., both meso- and macro-porosity), and a magnesium aluminosilicate clay as potential HDM catalysts[184,185]. Typical catalysts that have been developed for HDM are summarized in Table 5.
Typical hydrodemetallization catalysts
| Catalyst | Composition | Feedstock | Reaction conditions | Comments | Ref. | ||
| Active material(s) | Promoter(s)/Additive(s) | Support | |||||
| Commercial NiMo/Al2O3 | Mo | Ni | Al2O3 | bitumen-derived heavy oils and Conradson carbon residue | 625-685 K, LHSV = 0.14-0.80 h-l, 13.7 MPa), H2/oil ratio = 890 m3·m-3 | High Ni removal under mild conditions with minor improvements with increasing severity of conditions. | [175] |
| Ni-Mo/Al2O3 | Mo | Ni | Al2O3 | Vacuum residue | 410 °C, LHSV = 0.3 h-1, 15 MPa, H2 to liquid was 600 | The percentage of light impurities easier to remove on spherical catalysts (78.20% and 39.43% in HDM and HDCCR reactions, respectively) is higher than 65.20% and 17.50% on cylindrical catalysts. | [186] |
| Commercial NiMo/Al2O3 (B), and NiMo/Al2O3 (A) {has high metal content} | Mo | Ni | Al2O3 | VO-TPP in o-xylene, thiophene | 600 K, LHSV = 0.2 h-1, 8.0 MPa | The model results showed that the pore blocking rate in catalyst B was lower than in catalyst A. These data confirm that it is a durable high-performance HDM catalyst. | [187] |
| Co-Mo/γ Al2O3 | Mo | Co | γ Al2O3 | Asphaltenes, diesel, extra heavy oil | 360-410 °C, LHSV = 1.0 h-1, 4.0 MPa, H2/oil ratio = 600 nL·L-1 | The catalyst synthesized on a carrier with cylindrical pores exhibited higher catalytic activity in sulfur, heavy metals, and asphaltenes removal reactions that are synthesized on a carrier with slit-like pores. | [188] |
| FeMo, CoMo, NiMo | Mo | Fe, Co, Ni | (0%-75% activated carbon in alumina | residual oil and heavy crude oil (Ku crude) | 380-400 °C, LHSV = 1.0 h-1, 120 bar, H2/Oil = 680 | Carbon in support reduced coke and metal depositions on pore-mouth and catalytic sites. The optimum textural and mechanical properties of the support are obtained at a 1:1 carbon-alumina weight ratio. FeMo-supported catalyst composition (10wt.%-75wt.% carbon) had the best HDM results for an optimum pore diameter. | [178] |
Active and promoter mineral materials in hydrocracking catalysts
Conventional hydrocracking catalysts
There are two extreme hydrocracking catalytic functions: an acidic functionality and a hydrogenation/dehydrogenation functionality[189]. The acidic functionality constitutes bond cleavages, isomerizations, and intermolecular and intramolecular skeletal rearrangements, including cyclizations. The acidic functionality is provided by support materials such as silica-alumina, zeolites, or alumina[9,179,190]. Cracking reactions usually require protonic (Brønsted) acid sites[179,190]. The hydrogenation/dehydrogenation functionality has a hydrogenolysis character that cleaves bonds of reactants into fragments, dehydrogenates saturated reactants to produce reactive olefin intermediates, hydrogenates the unsaturated products from cracking, and prevents catalyst deactivation by hydrogenating coke precursors. These two extremes result in different product distributions. The extent of each of these reactions varies from catalyst to catalyst. Hydrocracking catalysts of commercial importance possess dual functionality, with reactions being primarily catalyzed by a strong acidic functionality and a weak hydrogenation/dehydrogenation functionality with minimal hydrogenolysis.
Conventional sulfided hydrotreating catalysts, such as CoMo/Al2O3 and NiMo/Al2O3, do not have sufficient protonic acidity to afford good cracking functionality even at high temperatures (400 °C)[179,190]. However, feed pre-treatment using these catalysts is essential to reduce the heteroatomic impurities since hydrocracking catalysts are sensitive to inhibition and/or poisoning[9,191,192]. This pre-treatment also helps with saturation of olefins and aromatics. One of the early successful hydrocracking catalysts was pelleted WS2[193]. There is also a wide body of literature on hydrocracking tests using traditional hydrotreating NiMo, CoMo, and NiW catalysts[194]. Hydrocracking of poly-aromatic hydrocarbons to mono-aromatic hydrocarbons has been reported using NiMo/Al2O3-HY and NiMo/Beta catalysts[195]. The use of CoO-MoO3/Al2O3 for hydrocracking of heavy feedstock is common, and promotion of the catalyst with Na, K, or Li has been reported, with Li showing the best results[194].
Improved hydrocracking catalysts
Improved hydrocracking through modified conventional hydrotreating catalysts has been reported[89,196,197]. A Ni (1wt.%, 2wt.%, and 3wt.%) modified sulfated zirconia (SO4/ZrO2) catalyst was investigated for hydrocracking activity and selectivity, and the catalyst loaded with 1% Ni had the highest acidity producing the highest activity and selectively for the gasoline range (70.28%)[198]. Bimetallic catalysts have been reported to show better activity[77,199]. Bimetallic CoWS2 catalysts were found to have higher hydrocracking activity compared to monometallic Co9S8 and WS2 catalysts[199]. Bimetallic Ni-W catalysts showed high catalytic activity (78%-91% conversion) compared to monometallic Ni catalysts (74.2%-82.7% conversion)[77]. Hydrocracking of waste plastic pyrolysis oil and vacuum gas oil using a NiW/HY catalyst and plastic pyrolysis oils using a Ni/SBA-15 have been reported[200]. Catalysts constituting multiple metals to fine-tune the bifunctionality have also been reported. Liu et al. carried out hydroconversion of polyolefins using Pt/WO3/ZrO2 and HY zeolites and reported an 85% yield at 225 °C[201]. Pt had the role of activating the polymer followed by cracking on WO3, ZrO2, and HY zeolites, which are acid functions ending. WO3 and ZrO2 sites were responsible for isomerization, and Pt was responsible for hydrogenation of the olefin intermediates. There is a special category of hydrocracking catalysts called shape-selective catalysts that utilize crystalline aluminosilicates with pore structures of specific geometrical characteristics that restrict access of reactants and products to particular sizes and shapes[189]. A novel class of promising hydrocracking catalysts based on stable single-atom Mo has recently been reported by Sun et al.[202]. Good hydrogenation activity, high liquid oil yields, and reduced tendency of coke formation were observed when tested on slurry phase hydrocracking of vacuum residue.
For the hydrogenation/dehydrogenation functionality, metals from Group VIII and VIb are selected based on their hydrogenation capabilities, and noble metals, such as Pt, Pd, and Pt/Re, and transition metals, such as nickel, cobalt, molybdenum, and tungsten, have been reported to be among the best performers[9,67,189]. Pt and Pd have good hydrogenating functions but must be used in an environment that does not have sulfides[203]. Higher hydrogenation and less cracking activity by Pt compared to NiMoS-supported bifunctional catalysts were observed by Brito et al. during the hydrocracking of octylcyclohexane[203]. The use of chromium, niobium, and Sn has also been reported but not as extensive[9,67]. Nanoparticles of noble metals and transition metal oxides, nitrides, and carbides have also been reported for the hydrogenation component of hydrocracking catalysts[204,205]. Typical catalysts developed for hydrocracking are presented in Table 6.
Typical hydrocracking catalysts
| Catalyst | Composition | Feedstock | Reaction conditions | Comments | Ref. | ||
| Active material(s) | Promoter(s)/Additive(s) | Support | |||||
| NiMo/Al2O3-HY and NiMo/Beta | Mo (and 15wt.% MoO3) | Ni (5wt.% NiO) | Al2O3-HY and Beta | Tetralin or 1-MN | Fixed-bed reactor, 320-420 °C, 6 MPa WHSV = 2 h-1 | NiMo/Beta(25) possessed high Brønsted acidity and high MoS2 dispersion and can facilitate the conversion of intermediates of 1-MN hydrocracking (mainly methyl tetralins and indanes) into MAHs. NiMo/A30Y50 - NiMo/Beta(25) (up to 99.9% conversion of 1-MN and 63.8% selectivity of MAHs). NiMo/A30Y50 with 30wt.% Al2O3 showed the best performance in selective hydrocracking reactions owing to the high density of MoS2 and BAS. | [196] |
| NiMo/γAl2O3, NiMo/γAl2O3 + Beta | Mo | Ni | γAl2O3 and γAl2O3 + Beta | Naphthalene | Fixed bed reactor, 400 °C, 4 MPa | Catalytic performance is dependent on the coupled hydrogenation ability of NiMo/γ-Al2O3 and the acidity of Beta zeolite. The highest BTX selectivity (62.8%) at 98% naphthalene conversion in Ni(2)Mo(13.2)/γ-Al2O3 + Beta(20) due to well-matched HYD and acid function. | [197] |
| NiMo/REY + Al2O3 | Mo | Ni | REY + Al2O3 | Vacuum gas oil | Fixed bed reactor, 390 and 410 °C, 16.0 MPa, LHSV = 0.7 h-1 | A high yield of middle distillates fraction (50.5wt.% at 410 °C) was achieved. Increase in selectivity to middle distillates selectivity can be explained by a decrease in the strong acid site density that suppresses overcracking reactions to produce naphtha. | [89] |
| NiW/Y-ASA-Al2O3 | W | Ni | Y-ASA-Al2O3 | Straight-run gas oil, dimethyl disulfide, and aniline, vacuum gas oil, unconverted oil | Fixed bed reactor, 340-410 °C, 16.0 MPa, LHSV = 1.4 h-1 | Increase of zeolite content in the catalysts leads to an increase of activity and a decrease of selectivity to diesel in second-stage hydrocracking. Increased activity is attributed to the increasing BASs. | [198] |
| CoWS2 | W | Co | - | Vacuum residue | Autoclave batch reactor, 693 K, 10 MPa | Bimetallic CoWS2 catalysts have higher activity than monometallic Co9S8 and WS2 catalysts in regard to hydrocracking TOF and C7-ASP conversion for vacuum residue hydrocracking. CoWS2 has well-dispersed mono-slabs with a better dispersion of 11.1 nm and stability. | [199] |
| Ni/Zeolite-Y, Ni-W/Zeolite-Y and NiW/HY | W (22.7wt.% WO3) | Ni (4.54wt.% NiO) | Zeolite-Y and HY | Heptane | Batch reactor, 400-440 °C, 80 bar | Higher catalytic activity (conversion, 78%-91%) Ni-W catalysts than Ni-based monometallic catalysts (conversion, 74.2%-82.7%), with NiO-WO3-ZY30(SiO2/Al2O3 ratio equal to 30) exhibiting the highest conversion. | [77] |
| Pd/x (where x = SBA-15, ASA, MCM-41, and MCM-48) | 1wt.% Pd | - | SBA-15, ASA, MCM-41, and MCM-48) | n-hexadecane | Packed bed, 473 K, 60 bar WHSV = 10 gn-C16·gcat-1·h-1 | All materials (including ASA) exhibited low acidity compared to zeolites. Increasing Al content reduced the order of mesopores. Ideal hydrocracking operation is approached for ASA, MCM-48, and SBA-15 prepared at high pH contained disordered mesopores. | [59] |
SUMMARY AND PERSPECTIVES
Conventional hydroprocessing catalysts [Co(Ni)Mo/Al2O3 and Ni(Mo)W/Al2O3] have met the hydroprocessing industry catalyst needs, but better hydroprocessing catalysts are now being highly sought after to meet the increasing demand for petroleum products, achieve stricter fuel specifications, and selectively target individual hydroprocessing reactions to accommodate unique feedstocks and/or obtain specific products[206,207]. For example, selective coordination and saturation of N-heteroaromatics are important to minimize saturation of unsubstituted aromatics and the need for additional reforming steps to achieve high-octane fuel[119]. There is an increasing need to accommodate feedstocks, such as sour and heavy crudes, bio-oils, waste plastic, biomass, and coal pyrolysis oils, to meet the increasing demand for petroleum products and as alternative sustainable sources, especially biomass and bio-oils. These products come with heavier concentrations of heteroatomic compounds, especially the refractory compounds[7]. They require hydrocracking, yet hydrocracking catalysts are vulnerable to inhibition and poisoning by heteroatoms[207-210]. Considerations for catalyst deactivation (poisoning by intermediates or products, sintering of support materials or metallic components) and coke deposition need to be made when designing catalysts and choosing operating conditions for these feedstocks[9,209-213]. For example, specific catalysts need to be designed for processing feeds with large concentrations of oxygen-containing compounds since higher amounts of water are produced, which leads to dealumination of alumina supports, and HDO being a highly exothermic process also means increased HDO of higher oxygen content feeds can result in heat build-up in reactors[9,119,211].
The periodic trends for the active metals are well understood, and the high catalytic activities and selectivities of noble metals are not contested. Noble metals are required in small quantities. High HDN activity and HDN/HDS selectivity were obtained through 1% Ir supported on various materials compared to the conventional NiMo system[214]. Nevertheless, the relative abundance of noble metals in the Earth’s crust is too low to meet current needs; they are expensive and sensitive to sulfur poisoning[215]. Solutions to increasing the resistance of noble metals to poisoning by sulfur have been explored[52]. They are usually employed as second-stage catalysts in pre-treated feeds that have reduced sulfur[38]. They are useful in shaping the finer properties of oils, e.g., deep desulfurization and hydrogenation. The relative abundance of the non-noble metals that are most active is also much lower than that of the less active ones. Although Mo and W lie in the sweet spot, their abundance is also low and cannot be projected to meet the increasing demand[37,107]. The drive to use other metals, such as Nb, V, and Fe, recently attracted extensive attention, especially iron (Fe). Fe is one of the metals of interest to be considered for substituting the conventionally used catalysts due to its high crust abundance, low cost, and environmental non-toxicity[37,107]. A synergistic effect between Fe and Mo, Ni, or V has been studied for HDS[207]. Although it has been widely considered that the Fe-based catalysts exhibit poor HDS performance, the addition of Zn was found to significantly enhance HDS activity[216]. The promotional effect of Zn comes from the formation of an active FeZnS phase that triggers a strong electron-donating effect from Zn to Fe species and the generation of more sulfur vacancies that change the electronic state of Fe[37,63]. Sudhakar et al. patented Fe-Mo sulfide catalysts that are as highly active for HDN as they are for HDS[22]. HDO using Ni-Fe and Ni-Cu has been reported[148,162].
Several efforts are also being made to migrate from traditional sulfides. Various transition metal carbides, nitrides, phosphides, borides, hydrides, oxycarbides, and oxynitrides have been notably tested to modulate the characters of transition metals and obtain noble metal-like properties and even obtain bifunctionality in processes such as hydrocracking[93]. Good HDS and HDN activities have been reported through transition metal carbides[15]. Ternary catalysts for HDS, HDO, HDN, and HDC have also been explored[97]. The metal ratios in the ternary catalysts were found to be important for the concentrations of active phases, dispersion of active phases, and the textural characteristics that influence accessibility of reactive surfaces[121].
Aside from the active metal phase, the catalytic support also has a great influence on the performance of hydroprocessing catalysts[37,63]. The role played by supports is now understood, and modification of supports is now considered a crucial component to achieving high catalyst activity and selectivity. Alumina supports are the most prominent in hydroprocessing catalysts. Their functionality in certain processes is limited due to their strong interaction with the active metals, which impedes sulfidation, resulting in poor catalytic activity. The strong acid sites also promote undesirable reactions that lead to higher coke formation and catalyst deactivation[51,217]. There are various reports on modification of the alumina supports to control the acidity, completely migrating to other support materials such as zeolites or making composite materials to benefit from the attractive properties of multiple support materials[36,76]. For example, chelating agents are used to cover the surface of Al2O3 with a carbon layer to reduce the metal support interaction and improve resistance to deactivation by coke deposition. Al2O3-carbon composites combine the positive characteristics of the individual components to give optimum support. Mendes et al. investigated a Ni2P catalyst supported by an Al2O3-carbon composite and reported reduced interactions of phosphorus with alumina and minimal formation of inactive P deficient phases, Ni12P5 and AlPO4[215]. The biggest challenge observed when making support material composites was the difficulty in controlling the composition and distribution of mesopores and micropores, which, in turn, affects accessibility and effective promotion of the active metals. The methods used for support preparation are also crucial to get the right catalyst properties such as optimum acidic nature, selectivity, optimal metal-support interaction, and textural properties[35]. The effect of impurities in catalysts also needs to be taken into consideration, and the choice of starting material can be of importance in that regard, as observed in a study where the influence of the Ir precursor: [Ir(AcAc)3,
Overall, multiple factors work in synergy to provide the optimum catalyst performance with suitable properties that allow for a long catalyst lifespan. A balance needs to be struck depending on the feedstocks or expected products.
CONCLUSIONS AND OUTLOOKS
Heterogeneous hydroprocessing catalysis relies on various mineral materials. Wide strides have been made in improving the catalysts following a better understanding of the interactions between the various mineral materials used as supports, active metals, promoters, additives, etc. The attractive properties of noble metals (e.g., high activities and selectivities) are identified throughout all the processes discussed in contrast to their vulnerability to poisoning under hydroprocessing conditions (e.g., where sulfur is present) and the fact that they are expensive and found in small quantities. Catalysts with properties that resemble those of noble metal catalysts have been obtained through well-thought combinations of cheaper mineral materials (active and promoter metals and supports), using suitable precursor mineral materials, and understanding synthesis protocols and modifications. Perfecting the catalytic properties using abundant metals such as iron will ensure sustainability, benefitting the hydroprocessing industry and environment in many regards. Better catalysts will also enable the utilization of unconventional feedstocks to meet the increasing demand for particular petroleum products. The review shows that there is a large pool of literature to be repurposed to advance commercialization of technologies involving unconventional feedstocks such as pyrolysis oils, bitumen, and shale-derived oils.
DECLARATIONS
Acknowledgments
We are grateful to Nelson Mandela University for providing the facilities for us to carry out the research.
Authors’ contributions
Data curation, interpretation, writing, and editing: Majodina S, Poswayo O, Dembaremba TO
Conception, administrative, technical, material support, and editing: Tshentu ZR
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by NRF-Sasol Grant (UID138605).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2023.
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Majodina S, Poswayo O, Dembaremba TO, Tshentu ZR. Towards improvement of hydroprocessing catalysts - understanding the role of advanced mineral materials in hydroprocessing catalysts. Miner Miner Mater 2023;2:13. http://dx.doi.org/10.20517/mmm.2023.23
AMA Style
Majodina S, Poswayo O, Dembaremba TO, Tshentu ZR. Towards improvement of hydroprocessing catalysts - understanding the role of advanced mineral materials in hydroprocessing catalysts. Minerals and Mineral Materials. 2023; 2(4): 13. http://dx.doi.org/10.20517/mmm.2023.23
Chicago/Turabian Style
Majodina, Siphumelele, Olwethu Poswayo, Tendai O. Dembaremba, Zenixole R. Tshentu. 2023. "Towards improvement of hydroprocessing catalysts - understanding the role of advanced mineral materials in hydroprocessing catalysts" Minerals and Mineral Materials. 2, no.4: 13. http://dx.doi.org/10.20517/mmm.2023.23
ACS Style
Majodina, S.; Poswayo O.; Dembaremba TO.; Tshentu ZR. Towards improvement of hydroprocessing catalysts - understanding the role of advanced mineral materials in hydroprocessing catalysts. Miner. Miner. Mater. 2023, 2, 13. http://dx.doi.org/10.20517/mmm.2023.23
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