An Overview of Catalysts for Ultra-deep Hydrodesulfurization of Diesel

Chen Yandie

(School of Public Security Information Technology and Intelligence,Criminal Investigation Police University of China,Shenyang 110854)

Abstract:Important progresses in hydrodesulfurization(HDS) field on HDS catalysts in recent years are reviewed,concluding the latest results about ultra-deep hydrodesulfurization of diesel oil over catalysts at home and abroad.Under mild HDS conditions,except conventional supported catalysts,bulk catalysts with high metal contents also exhibit excellent HDS performance and high hydrogenation activity.Moreover,the bulk catalysts can produce clean low-sulfur diesel oil to meet environmental legislation.

Key words:Multi-metallic catalysts;Deep hydrodesulfurization;Diesel;Activity

Recently,ultra-deep hydrodesulfurization of diesel has been an urgent issue to be solved worldwide.SOx from burning of organic sulfur-containing compounds in diesel oil cause acid rain and PM 2.5,and poison the three-way catalysts irreversibly in the tail gas clean-up system of engines.Besides,trace amounts of sulfur can poison the electrode catalysts in fuel cell and the noble metal catalysts in the reforming and transforming processes of fuels.Limiting sulfur contents in fuel oils to a ultra-low level has been receiving much more attention due to more stringent environmental regulations and increasing need for clean fuels[1].China has accelerated the pace of diesel quality upgrades and completed the upgrade of diesel standards from National IV to National VI in a relatively short time,effectively reducing the environmental pollution caused by diesel fuel[2].

Desulfurization technologies and processes include hydrodesulfurization (HDS),and nonhydrodesulfurization(NHDS) such as adsorption(ADS)[3],oxidation(ODS)[4],extraction(EDS)[5]and bioprocesses(BDS)[6]and so on.Hydrodesulfurization is a highly efficient process in refineries.Employing catalysts with high activity for hydrodesulfurization would be the less costly and the most effective way for the refineries,because it would allow avoiding modifications for the plant installations.In addition,the development of more active ultra-deep HDS catalysts of diesel has been an urgent subject.Based on current HDS technologies,deep hydrodesulfurization of diesel aims to remove sulfur-containing compounds in diesel less than 500 μg/g,and ultra-deep hydrodesulfurization refers to sulfur contents less than 50 μg/g.

1 Supported HDS Catalysts

Currently,the conventional commercial HDS catalysts,mainly CoMo(W)S/Al2O3or NiMo(W)S/Al2O3can remove most part of the sulfur from diesel.Under industrial conditions,the HDS reactivity of sulfur-containing compounds decreases as follows:thiophene (T) >benzothiophene (BT) >dibenzothiophene (DBT) >4,6-dimethyldibenzothiophene (4,6-DMDBT)[7].

The hydrodesulfurization of DBT and alkyl substituted DBTs proceeds mainly through two routes involving the hydrogenation (HYD) and the direct desulfurization (DDS)[8].

Except DDS route,the other desulfurization routes aim to reduce the steric hinderance of alkyl substituents at 4-or/and 6-position.The method generally used is to hydrogenate one benzene ring of molecular to reduce the steric hinderance[9].

In order to improve HDS performance of CoMo-and NiMotype hydrotreating catalysts,except increasing active metal contents,other methods were also used to promote dispersion and distribution of active metals,or enhance the number of active species.

One approach is using some additives (such as phosphorus,fluorine and boron) in the catalyst formulations.Phosphate has been in commercial use for many years,and fluorine is used for special hydrotreating processes[10].Moon et al.[11]reported that phosphorus addition increased the number of the active sites because of enhanced dispersion of the Mo species,via the interaction of the Mo species with the P-OH groups and the other hydroxyls on the catalyst surface.Phosphorus addition to the CoMo catalysts also increased the Bronsted acidity,and promoted migration of the methyl substituents in the aromatic ring.Over Mo/Al2O3catalyst,Prins et al.[12]found that adding fluorine did not change the intrinsic properties of the active sites,but changed the dispersion of the active sites on the surface.In another report,Wang's group[13]found that fluorine enhanced the acidity of titania by the formation of two kinds of fluorides.The result was consistent with that of alumina,namely,fluorine enhanced the acidity of alumina.

Another approach is to employ new supports.Many new materials with high surface area and other properties suitable for support applications were developed.These include TiO2,ZrO2,SiO2,carbon,nanotubes,zeolites and molecular sieves etc.The mesoporous Al2O3with high surface area,large pore size,cylindrical pore shape,and surface defects,would lead to better Mo,Co dispersion and stabilization of the Co-Mo-S Type II phase[14].SBA-15 with ZrO2modified would improve the activity of the catalysts in the HDS of DBT-type compounds,which was correlated with the morphology of the active phase.

The third approach involves the impregnation procedure of active metals (e.g.,Mo or W and Co or Ni)[15].The impregnation step plays an important part in affecting the structure of the active phase,its dispersion and distribution.Different methods in literature described the impregnation of Mo and Co or Ni promoters on the support surface.The incipient wetness and the equilibrium adsorption methods were widely used.Incipient wetness involved contacting the support with a solution of the precursor salts of Mo,W,Co or Ni,in a predetermined volume of water enough to fill the pores.The equilibrium adsorption method consisted of adsorbing Co or Ni ions and molybdate from aqueous solutions of their salts over an extended period of time until equilibrium was attained followed by filtration of the leftover liquid.

The fourth approach is the addition of some chelating agents such as ethylenediaminetetraacetic acid (EDTA),citric acid (CA),nitrilotriacetic acid (NTA),1,2-cyclohexanediamine-N,N,N,Ntetraacetic acid (CyDTA) and ethylenediamine (EN) to the impregnation solution.These complexing compounds have been found to be highly efficient in the formation of highly active (Ni)CoMoS (type-II) species with extremely high selectivity by inhibiting Mo-support interactions.Cattaneo et al.[16]found that the role of the complexing ligand (e.g.,NTA) was to change the sulfidation mechanism of Ni,leading to a higher dispersion of promoter on the edges of MoS2slabs.Citric acid addition reduces the interaction between Co2+and the Al2O3surface by forming Co-CA complexes.These complexes dispersed well on the surface and retard Co sulfidation to the temperatures while Mo was sulfided completely.And a large fraction of the CoMoS phase was formed,which led to the increased HDS activity.

Fig.1 Various Ni species on NiW/Al2O3 catalysts at different stages of sulfidation[17]

The last approach is to change the sulfidation temperature and atmosphere of catalysts.Langeveld et al.[17]had described a detailed process of the sulfiding reactions on NiW/Al2O3and the species formed through a systematic study as shown in figure 1.They found there were four types of Ni present in the oxidic NiW/Al2O3catalysts before sulfidation.When sulfidation temperature was above 600 K,sulfided Ni was correlated to the formation of the NiWS phase.The formation of NiWS was related to the formation of WS2-like slabs.When sulfidation temperature was above 700 K,a complete decoration of the WS2edges with Ni sulfide was formed.Okamoto et al.[18]sulfided catalysts Co-Mo/SiO2under a stream of 10% H2S/H2or 10% H2S/He or their combinations.They found that the intrinsic HDS activity of catalyst was strongly affected by the sulfidation atmosphere.For the HDS reaction of thiophene and DBT,the presulfidation of Co-Mo-S in a strongly sulfiding atmosphere (H2S/He) would form the Co-Mo-S structure with a high intrinsic activity.They proposed a structure-reactivity relationship with the Co-Mo-S structure.They demonstrated that the intrinsic HDS activity of Co-Mo-S was strongly affected by the location and the local structure,which was equilibrated with the sulfidation atmosphere or reaction conditions,as well as by the MoS2-support interactions.

Both conventional supported catalysts and supported noble metal catalysts have been used in the HDS industrial units.Platinum catalysts exhibit an excellent hydrogenation promotion effect under moderate pressures.However,the expensive price and the lower sulfur-resist ability,limited the use of Pt catalysts.An effective method is adding a second noble metal into the catalyst to obtain a bimetallic catalyst,typically Pd-Pt catalysts.Zhao et al.[19]had synthesized catalysts Pt-Pd/Al2O3-SiO2-TiO2(AST) and Pt-Pd/Al2O3-TiO2(AT) by the sol-gel method.Through a 100 h test,the Pt-Pd/ATS catalyst exhibited excellent stability in HDS and HDA reactions.They concluded that the appropriate contents of Ti and Si incorporated into Al2O3increased the HDS and HDA activity and stability of the Pt-Pd catalyst.Moreover,ATS composite oxide was proved to be an excellent support candidate for Pt-Pd HDS/HDA catalysts.

2 Platinum-like HDS Catalysts

Transition metal nitrides and carbides are a kind of interstitial compounds with metallic property,consisting of N or C atoms into lattice of transition metals.The compounds own all properties of covalence compound,ion crystal and transitional metals,exhibiting special physical and chemical properties[20].In many reactions catalyzed by noble metals,especially in the reactions of hydrocarbons dehydrogenation,hydrogenolysis and isomerization,both nitrides and carbides display excellent catalytic performance.Therefore,it is thought that transition metal nitrides and carbides are“Quasiplatinum catalysts”.Compared with conventional hydrotreating catalysts,nitrides and carbides exhibit higher catalytic activity,higher selectivity,and lower H2consumption.Transition metal phosphides also possess similar physical properties to nitrides and carbides.Phosphides with excellent HDS,HDN activity and selectivity have attracted more attention in new catalytic materials field[21].

3 Bimetallic Bulk HDS Catalysts

Whether phosphides,nitrides,carbides,or improved conventional sulfided catalysts,all could not improve the HDS activity greatly.For conventional supported catalysts,limited by supports,HDS activity could not be improved by increasing the contact area between reactants and support or increasing synergy between support and active metals.And the strong interaction between supports and active species may reduce activity.In comparison,bulk catalysts with high HDS activity are considered as new HDS catalysts due to high metal contents,more active components and no support[22].

The preparation methods of bulk catalysts include ultrasound[23],in situ thermal decomposition of thiosalts[24],hydrothermal[25],solgel[26],chemical vapor deposition(CVD)[27],co-precipitation[28],coimpregnation[29]and solution synthesis[30].Genuit et al.[31]reported a reaction of (NH4)2MoS4and Ni or Co salts in solution,and the bulk catalyst Ni(Co)-Mo-S was prepared via thermal decomposition.They found that catalysts prepared using mixture of organic solvents and surfactant exhibited much higher HDS activity of 4,6-DMDBT.Zhang et al.[32]had reported preparation of bulk catalysts NiW using (NH4)2WO2S2via liquid-liquid,solid-solid and liquid-solid methods.Catalyst prepared by liquid-solid method exhibited higher HDS and HDN activity than that of commercial catalysts.Jiang et al.[33]prepared a series of nanosized sulfided catalysts in solution.Taking preparation of NiMoS precursor for example,a certain amount of ammonium heptamolybdate,nickel nitrate,ethylenediamine tetraacetic acid (EDTA) and S8were added to glycol under N2atmosphere,and after a certain temperature and time filtered the catalyst precursor.The HDS evaluation showed that the activity of catalyst was 4-5 times higher than that of commercial catalysts.ExxonMobil Research Engineering Company[34]had developed a hydrotreating bimetallic bulk catalyst composed of Ni(Co) and W(Mo) via a solid-slurry reaction of nickel (cobalt) carbonate and tungstic acid (MoO3).During reaction,the first organic compound such as ethylenediamine or the second organic compound like citric acid could be added to the precursor at different steps.Via thermal decomposition under different atmospheres,the catalysts showed different XRD patterns and hydrogenation activities.Chevron Corp.[35]also had provided a single metal slurry catalyst for upgrade of heavy oil.The slurry catalyst catalysts were prepared by sulfiding a primary metal precursor (water soluble salts) using sulfiding agents such as dimethyl sulfide (DMS),then mixing the sulfided metal precursor with a hydrocarbon diluent to form the slurry catalyst in the presence of reducing agent.The primary metals were Mo (W) compounds or Ni (Co) compounds or mixture of the two.The promoter metals like Ti could be added during the reaction.The particle size of the slurry catalyst ranged from 1 to 300 μm,and the average particle size was less than 50 μm.

4 Multi-metallic Bulk HDS Catalysts

Compared to bimetallic bulk catalysts,multi-metallic bulk HDS catalysts could integrate HDS performance of various active metals to achieve maximum impact.At present,except some patents[36-39,43-59],there is much less research reporting multimetallic bulk HDS catalysts.

4.1 NEBULA Multi-metallic Bulk Catalysts

In 2001,ExxonMobil,Albemarle and Nippon Ketjen had jointly developed a novel bulk HDS catalyst—“NEBULA”.NEBULA catalyst consisted of Ni,Mo and W,and the activity was at least three-fold of other catalysts[40].Refineries can reduce sulfur contents in diesel less than 15 μg/g without extra investment employing NEBULA at local hydrotreating units.

Alonso et al.[41-42]reported the preparation of bulk catalysts NiMoW via in-situ an ex-situ thermal decomposition of MoWS organic quaternary ammonium modified by Ni.They studied the effect of various vulcanizer on morphologies of catalysts.It was found that Ni/[N(CH3)4]4MoWS8exhibited the highest HDS activity,and hydrogenation selectivity of DBT would increase with increasing carbon chain of organic amine.Using dimethyl disulfide (DMDS) as vulcanizer,bulk catalyst NiMoW showed the highest activity and better morphology.Nevertheless,the preparation process was complex and material costs were high,limiting further application on catalysts.To synthesize bulk NiMoW catalysts,Demmin et al.[43]changed the feeding order,chose various basic precipitant,and changed the temperature and pH values.After drying and calcination,the catalyst exhibited excellent HDS activity,HDN activity and hydrodearomatization (HDA) activity,far more than that of the conventional supported catalysts.Xu et al.[44]improved the mentioned precipitation method.Firstly,NiW composite precursor was synthesized and then the precursor was recombined with MoO3and adhesive.The prepared NiMoW catalysts displayed higher activity and greater intensity than that of catalysts from co-precipitation.Li et al.[45]also used home-made Ni precursor and added surfactant or glycol or a mixture of surfactant and glycol to prepare bulk catalyst NiMoW by solid state reaction.The catalyst exhibited high HDS activity,which was six times of the commercial catalyst.

Recently,Chevron Corp.had developed a series of bulk multimetallic catalysts[46],which own macropores at least of 95%,a total pore volume at least of 0.08 cm3/g,and surface area at least of 150 m2/g.Under hydrotreating conditions of heavy oil,the catalyst could remove sulfur content from 31 135 μg/g to 10 μg/g or less,and the nitrogen content could be reduced from 31 230 μg/g down to 25 μg/g or less,displaying excellent HDS and HDN performances.After that,the corporation disclosed a multi-metallic catalysts for hydrotreating hydrocarbon feed-stock[47].The mixed metal sulfide catalyst consisted of five phases:a molybdenum sulfide phase,a tungsten sulfide phase,a molybdenum tungsten sulfide phase,an active nickel phase and a nickel sulfide phase.HDS and HDN evaluation of heavy coker gas oil over test of catalysts showed that both conversion of the HDS and HDN were above 95%.In order to improve HDA,HDN and HDS activity and to yield higher value products,Chevron Crop.also developed a bulk multi-metallic catalyst Ni(Co,Zn,or Sn)-Mo-W[48].Using a vacuum gas oil for HDS and HDN evaluation over catalyst NiMoW-maleate,the nitrogen removal was at least 70%.Kuperman et al.[49]improved the subsequent synthesis of NiMoW bulk catalysts.The catalyst exhibited good catalytic activity for the process of heavy oil such as vacuum gas oil (VGO),and the nitrogen conversion is 90%.In another patent,Kuperman et al.[50]used metal precursors M and L,sulfiding agent,sulfur additives,surfactant and so on to prepare bulk catalysts.Metal precursors M and L could be organic or inorganic metal precursors (eg.NiO,ZnO,Fe-DTC),and the sulfiding agent included CS2,DMS,DMDS,Na2S2O3and so on.The average catalyst particle size was about 0.3-20 μm with the total surface area at least of 100 m2/g.Although the catalysts couldn’t be employed for diesel hydrofining,the special preparation method still provides a new idea for synthesizing novel catalysts.

Wang et al.[60]made substantial progress in characterizing and preparing catalysts.The NiMoW-1,NiMoW-2 and NiMoW-3 catalysts were prepared by co-precipitation method,using various surfactants,solvents and Ni precursors.During HDS reaction of diesel oil,NiMoW-3 could remove sulfur content from 1400 μg/g to less than 1μg/g,while just to 57 μg/g for the commercial catalyst under the same experiment conditions.The sulfur conversion over NiMoW-3 (95%) is higher than that of the commercial catalyst (95%).By series of characterization,the author proposed the model of catalysts and the processes for the formation of sulfided catalysts.Ni precursor retained the layered structure in NiMoW.After sulfurization,the layered Ni3S2behaved as the support,and Mo,W sulfides were deposited on the surface and interlayer galleries.The layered structure led to the interaction between the active metal sulfides and resulted in the high HDS activity.The high activity of catalysts was ascribed to high active metal contents and the layered structure.Considering lowering active metal contents and influence of layered structure on activity,multi-metallic bulk catalysts NiAlMoW with layered structure were developed[61].By optimizing different pH value and Ni/Al molar ratios,catalyst Ni3AlMoW-5 with active metal contents of 48.1 wt% exhibited high HDS activity.The intrinsic HDS activity of Ni3AlMoW-5 was still 2 times higher than that of the commercial catalyst.

After that,Chen et al.[62]synthesized a layered multi-metallic bulk catalyst NixZnyMoW with layered structure by co-precipitating method.The HDS activity of 4,6-DMDBT on catalysts was found to be much higher than that of the commercial catalyst CoNiMoW/Al2O3.And the sulfur content in the model diesel could be reduced from 517 μg/g to less than 10 μg/g under mild reaction conditions.Much higher specific HDS activity of catalysts NixZnyMoW could be ascribed to high contents of active metals and the layered structure.Meanwhile,it was found that the synergism between Ni/Zn and Mo/W played an important role in achieving the excellent HDS performance.Chen et al.[63]further reduced the active metal content of NiMoW catalysts by introducing the second promoter Zn and Al,and the catalyst NiZnAlMoW could still maintain high HDS activity.Through the HDS evaluation,it was found that the catalyst NiAlMoW with Zn-doping could enormously improve the HDS activity.The HDS conversion of the catalyst NiZnAlMoW was 10% higher than that of the commercial catalyst CoNiMoW/Al2O3.High HDS activity of catalysts was ascribed to the high active metal content,the layered structure,and the positive promoter effect of Zn.Zn-doping reduced the apparent activation energy of catalysts and resulted in larger pore volume and larger pore size for catalysts.

4.2 Industrialization of NEBULA Multi-metallic Bulk Catalyst

Bulk catalysts are still in development at present,and catalysts NEBULA have made its way into industrial application.In the past twenty years,industrial application of NEBULA is increasing rapidly.Duo to high hydrogen consumption and loading costs,on the basis of high activity,it has developed a process that KF760 STARS and NEBULA catalyst were mixed to fill.The NEBULA+KF760 filling process exhibits the same stability and hydrogen consumption as the single KF760 system.But the desired temperature to produce 8 μg/g sulfur diesel was 15 ℃ lower than that of the KF760[64].

The second-generation catalyst NEBULA-20 was simultaneously developed with NEBULA industrialization,and NEBULA-20 improved HDS activity of heavy oil.The hydrogen consumption of NEBULA-20 decreased obviously while still maintaining high activity.It enables refiners to produce ultra-low sulfur diesel oil.

5 Outlook

Decreasing petroleum reserves makes the low-quality crude oil a major source.Therefore,it is necessary to develop a novel HDS catalyst to meet more stringent legislations.Multi-metallic bulk catalysts possess more advantages compared to the conventional supported catalysts.The bulk catalysts eliminate the strong interaction between supports and active species,and it is clearer to observe the structure and morphology.However,there still exist several defects for multi-metallic bulk catalysts,such as high costs,complex preparation methods,low utilization of active metals and so on,limiting their large-scale industrial applications.What’s more,the studies on active species and structure of catalysts are still in the initial stage.

For increasing demands of low sulfur diesel,the bulk catalysts with super high HDS activity would be expected to be applied in refineries.Apart from that,it is also necessary to develop hydrotreating catalysts which could meet requirements of more refinery processing,as HDS process also combines other processing.

6 Acknowledgment

This work was financially supported by National Nature Science Foundation of China (NSFC grant No.21173214).The authors thanks National Nature Science Foundation of China.Profound gratitude also goes to the editor for providing an opportunity to summarize our research progress in hydrodesulfurization of diesel oil.