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Development of bimetallic and trimetallic oxides doped on molybdenum oxide based material on oxidative desulfurization of diesel
⁎Corresponding author. Tel.: +60 137466213. wazelee@kimia.fs.utm.my (Wan Azelee Wan Abu Bakar),
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Catalytic oxidative desulfurization (Cat-ODS) activities of thiophenic sulfur were compared using alumina supported of mono-, bi- and trimetallic oxide molybdena based catalysts, prepared by incipient wetness impregnation. The aim of this study was to inquire on the possibility of supported trimetallic oxide catalysts for deep Cat-ODS process. The prepared catalysts were characterized by nitrogen adsorption, X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy, and tested in the ODS of model thiophene, dibenzothiophene and 4,6-dimethyldibenzothiphene, as well as on commercial and crude diesel. It was found that the addition of dopant and co-dopant toward MoO3/Al2O3 catalysts increased significantly the selectivity of catalytic performance in the order: mono- < bi- < trimetallic oxide. Under these optimum conditions: TBHP/sulfur ratio of 3, 2.13%V/4.08%W/16.07%MoO3/Al2O3 (0.2 g), temperature of 60 °C, 30 min of reaction, and complete removal of model organosulfur compounds were obtained. In addition, >90% of sulfur was removed in both commercial and crude diesel under similar reaction conditions. Reproducibility test showed that the catalyst has higher catalytic activity and could be repeatedly used with little change after five cycles.
Keywords
Catalytic oxidative desulfurization
Diesel fuel
Molybdena catalyst
Bimetallic oxide
Trimetallic oxide
1 Introduction
Sulfur compounds are the most notorious and undesirable petroleum contaminants and a large portion of these compounds would be transferred to diesel oil during refining process. In general, sulfur presents as hydrogen sulfide, organic sulfides and disulfides, benzothiophene, dibenzothiophene, and their alkylated derivatives (Song and Ma, 2003). Upon diesel combustion, sulfur compounds are converted to sulfur oxides (SOx) that contribute to acid rain and environmental pollution (Jose et al., 2011). Although environmental regulation has been applied in many countries to reduce the sulfur levels in diesel and other fuels (Babich and Mouljin, 2003), “sulfur removal” still represents a major operational and economic challenge for petroleum refining industry (Zhu et al., 2015a,b). The conventional catalytic hydrodesulfurization (HDS) process, operated at elevated temperature (>300 °C) and pressures (20–100 atm H2) over either Ni–Mo/Al2O3 or Co–Mo/Al2O3 catalyst, is highly efficient in removing low molecular weight of thiols, sulfides, disulfides and mercaptans. However, thiophenes and multi-ring aromatic compounds such as 4,6-dimethyldibenzothiophene and other alkyl-substituted derivatives of dibenzothiophene are relatively inert to HDS due to their steric hindrance that makes extremely difficult to achieve deep desulfurization by HDS process (Kedra-Krolik et al., 2011; Song, 2003). Thus, HDS causes an inappropriate process to produce diesel fuel that meets the current environmental regulations. Since then, catalytic oxidative desulfurization (Cat-ODS) has gained their attention and been documented in recent articles (Zhu et al., 2015a,b, 2014; Nava et al.; 2009; Cedeño-Caero et al., 2005). It is an excellent option after HDS since thiophene and its derivatives are easily oxidized at low temperature and pressure conditions in the presence of an oxidant, to their corresponding sulfones.
The use of molybdenum based catalysts was widely reported to be applicable in Cat-ODS systems using either H2O2 or organic hydroperoxide as the oxidizing agents. As example, Wang et al. (2003) reported that MoO3/Al2O3-TBHP performed the highest ODS activity as compared to other metal oxides. Garcia-Gutierrez et al. (2008) claimed 97.8% of model sulfur was removed utilizing Mo/Al2O3 and H2O2 oxidant under mild conditions. Ishihara et al. (2005) applied Mo and TBHP to treat light gas oil with a sulfone adsorption step while Jia et al. (2011) used Mo as both catalyst and adsorbent to process a low sulfur model diesel. A modification on Mo was done by Prasad et al. (2008), where Bi promoter was doped with Mo to increase the catalytic activity in desulfurization of light cycle oil. In addition, Chang et al. (2010) introduced a series of alkaline earth metal dopant on Mo and found out that Ca/Mo/Al2O3 showed the best catalyst toward sulfur removal. Han et al. (2013) modified MoO3/SiO2 with phosphorus to enhance the ODS activity using cumene hydroperoxide as oxidant. Meanwhile, Qiu et al. (2015) synthesized phosphomolybdic acid to remove benzothiophene and dibenzothiophene at 70 °C for 60 min of reaction conditions. However, regarding supported trimetallic oxide catalysts, there was no report done by the researchers.
On the basis of previous studies, we report results from a comparison study of MoO3/Al2O3 catalysts with various dopants and co-dopants such as V, Ti, and W to improve the removal of sulfur compounds from diesel oil under mild reaction conditions. Bimetallic and trimetallic oxide catalysts were characterized and tested in model diesel containing thiophene, dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The potential catalyst was further investigated using commercial and crude diesel. The reusability of the catalyst was also studied.
2 Experimental
2.1 Materials
All materials that are purchased from Sigma–Aldrich were used without further purification. Tert-butyl hydroperoxide (aqueous, 70 wt.% TBHP) and N,N-dimethylformamide (99%, DMF) were used as oxidizing agent and extraction solvent. N-octane (99%) was used as solvent for sulfur compounds of thiophene (Th, 99%), dibenzothiophene (DBT, 97%) and 4,6-dimethyldibenzothiophene (4,6-DMDBT, 95%). Model diesel was prepared with 600 ppmw of sulfur; Th (200 ppmw), DBT (200 ppmw) and 4,6-DMDBT (200 ppmw), respectively. Commercial and crude diesels were obtained from Petronas, Malaysia.
2.2 Preparation of catalysts
Incipient wetness impregnation (IWI) method was used in the production of all catalysts referring to the previous works (Bakar et al., 2012, 2015). MoO3/Al2O3 catalyst was initially prepared as reference. Alumina beads were dipped into aqueous solution containing the desired amount of ammonium heptamolybdatetetrahydrate (99%) for 30 min, aging at 80 °C for 24 h followed by calcination process at 600 °C for 5 h under air atmosphere with a ramp of 5 °C min−1. Then, bimetallic oxides catalysts were prepared using three different dopants. Vanadium-, tungsten- and titanium-doped MoO3/Al2O3 catalysts were prepared by dissolving (NH4)6Mo7O24·4H2O with different precursors of ammonium metavanadate (NH4VO3, 99.9%), ammonium metatungstate ((NH4)6H2W12O40·H2O, 99.9%) and ammonium titanyl oxalate monohydrate ((NH4)2TiO(C2O4)2·H2O, 99.9%) with deionized water. For trimetallic oxide catalyst, three types of precursors were used to fabricate the solutions which were, ammonium metatungstate as dopant and ammonium metavanadate or titanyl oxalate as co-dopant. Further preparation steps were similar to monometallic oxide catalysts with different ratio of dopants and co-dopants.
2.3 Characterization of catalyst
The morphology of the surface catalyst was analyzed by field emission scanning electron microscopy (FESEM), using Zeiss Supra 35VP and transmission electron microscopy (TEM) using JEOL-JEM 2100. The XRD patterns of the powder samples were recorded at 2θ between 10° and 80° using a Bruker Advance D8 with Siemens 5000 diffractometer. The Cu Kα radiation operates at 40 kV and 40 mA with λ = 1.5418 Å. Textural properties were obtained by N2 adsorption/desorption isotherms, using Micromeritics ASAP 2010 volumetric adsorption analyzer at −196 °C and degassed the catalyst at 200 °C for an hour while elemental composition was determined by energy dispersive X-ray spectroscopy (EDX).
2.4 Catalytic experiments
In a typical reactive cycle, the oil bath was first heated up and stabilized to 60 °C. After that, 20 mL of simulated diesel, commercial diesel or crude diesel, TBHP oxidant (O/S = 3) and catalyst (0.2 g) were introduced into round-bottom flask and refluxed under atmospheric pressure with vigorous stirring (1000 rpm) for 30 min. After the reaction, the oxidized diesel fuel was extracted by solvent extraction (DMF) with DMF to diesel ratio of 1.0 for 30 min at temperature of 30 °C. The mixture of diesel and solvent extraction was vigorously stirred (1000 rpm) followed by phase separation in a separating funnel, and left until two separate layers of diesel and solvent formed (Mokhtar et al., 2015). The reaction progress was monitored for sulfur concentration measurement using Gas Chromatography-Flame Photometric Detector (GC-FPD). A 50 m × 0.25 mm inner diameter × 0.25 lm film thickness HP-1 capillary column was used for separation. High purity helium and hydrogen were used as carrier and fuel for FPD, respectively. Both injection and detection temperatures were maintained at 250 °C. Sample volume of 1.0 mL was injected without any solvent dilution. The removal percentage of sulfur compounds was calculated as follows: where Co is the initial concentration of total sulfur in the diesel solution, and Ct is the total sulfur concentration in the treated diesel after reaction t min.
3 Results and discussion
3.1 Catalyst characterization
X-ray diffraction patterns of bimetallic and trimetallic oxides of MoO3/Al2O3 catalysts are shown in Fig. 1. The diffraction patterns of bimetallic oxide catalyst of MoO3/Al2O3 with tungsten (Fig. 1(b)) and vanadium (Fig. 1(c)) as dopants showed similar trend but higher degree of amorphous property than the undoped MoO3/Al2O3 catalyst (Fig. 1(a)). The amorphous structure was shown by a marked decrease and broader intensity of MoO3 (orthorombic) peaks observed for bimetallic oxide catalysts compared to undoped MoO3/Al2O3 catalyst. Compared to W/MoO3/Al2O3 and V/MoO3/Al2O3 catalysts, the Ti/MoO3/Al2O3 showed a different diffraction patterns since the reflection of MoO3 phase almost undetected. Some MoO3 peaks in XRD pattern were disappeared possibly due to the incorporation of Mo species into the bulk matrixes in the presence of titanium as dopant towardMoO3/Al2O3 (Said, 1994). No reflections belonging to W, V or Ti oxides were observed in the XRD patterns emphasized the good dispersion of the deposited metal oxide species on the catalyst surface. Similar situation was observed on Ti/W/MoO3/Al2O3 and V/W/MoO3/Al2O3 where the catalysts were dominated by the alumina support. This result suggested that the addition of tungsten and vanadium as dopants and co-dopants into alumina supported molybdena catalyst contributes to the remarkable effect of Mo dispersion on the alumina support.
XRD diffractogram patterns of mono-, bi- and trimetallic oxide catalysts of (a) MoO3/Al2O3, (b) WO3/MoO3 (15:85)/Al2O3, (c) V2O5/MoO3 (15:85)/Al2O3, (d) TiO2/MoO3 (15:85)/Al2O3, (e) Ti/WO3/MoO3 (5:15:80)/Al2O3 and (b) V/WO3/MoO3 (5:15:80)/Al2O3 calcined at 600 °C for 5 h.
Fig. 2(a)–(c) shows the comparison of the effect of tungsten loading on the molybdena based catalyst. As shown in the micrographs, molybdena catalyst with 4.26 wt.% tungsten species obtained from 15:85 of W:Mo ratio showed well-dispersed homogeneous structure (Fig. 2(b)) and the particles of each element are distributed uniformly on the alumina surface. Fig. 2(d) and (e) shows the comparison of the co-dopants as the trimetallic molybdena based catalysts which are titanium (Fig. 2(d)) and vanadium (Fig. 2(e)) prepared from 5:15:80 of Ti/V:W:Mo ratios after calcination of the precursor at 600 °C.
FESEM micrographs of (a) W/MoO3 (5:95)/Al2O3, (b) W/MoO3 (15:85)/Al2O3, (c) W/MoO3 (25:75)/Al2O3, (d) Ti/WO3/MoO3 (5:15:80)/Al2O3 and (e) V/WO3/MoO3 (5:15:80)/Al2O3 calcined at 600 °C for 5 h.
W/MoO3/Al2O3 catalyst with vanadium species as co-dopants showed some degree of dispersion with homogeneous structure. Meanwhile, for W/MoO3/Al2O3 catalyst with titanium species as co-dopants, the particles formed agglomeration probably due to the incorporation of Mo species into the bulk matrixes, which lowered the surface contact area of active sites on the surface catalyst. This result was in parallel with XRD analysis obtained, which showed that the addition of titanium toward W/MoO3/Al2O3 caused the reflection of MoO3 species almost undetectable due to the incorporation of Mo species into the bulk matrixes.
Fig. 3 shows the TEM images of the Ti/W/Mo/Al2O3 and V/W/Mo/Al2O3 catalysts and calcined at 600 °C for 5 h. The polycrystalline structure could be correlated with γ-Al2O3. Ti/W/Mo/Al2O3 catalyst with molybdenum content of 16.07 wt.% has large crystalline particles with spherical structure and composition appeared with size off the dark crystal was approximately 11–12 nm. Meanwhile, for the V/W/Mo/Al2O3 catalyst, smaller particles with spherical structure were also appeared with approximately 4–5 nm.
TEM micrographs of (a) Ti/WO3/MoO3 (5:15:80)/Al2O3 and (b) V/WO3/MoO3 (5:15:80)/Al2O3 calcined at 600 °C for 5 h.
Results from textural characterization of MoO3/Al2O3 and prepared bi- and trimetallic oxide catalysts are summarized in Table 1. Meanwhile, nitrogen adsorption/desorption isotherms of the catalysts can be seen in Fig. 4. From the isotherm plots, results showed that V/W/MoO3/Al2O3 catalyst possessed higher surface area compared to W/MoO3/Al2O3 and MoO3/Al2O3 catalysts. It was clearly explained that by promoting the vanadium in MoO3/Al2O3 catalyst gave a significant effect, where the catalyst led to the production of new active sites (refer Fig. 2). On the other hand, the degree of mesoporosity of Mo based catalysts decreased in the order of monometallic > bimetallic > trimetallic oxide catalysts, indicated by the type of hysteresis loop. Monometallic catalyst (MoO3/Al2O3) showed isotherm characteristic of Type IV with an H2 hysteresis loop, indicating the bottle neck shape with narrow necks and wide body. However, the presence of dopants and co-dopants on the Mo catalyst exhibited similar isotherm characteristic, but in type H3 hysteresis loop with the presence of slit shape pores.
Catalyst/Al2O3
Catalyst ratio (wt.%)
SBETa (m2 g−1)
VTotala (cm3 g−1)
Wda (nm)
Loading (wt.%)
V
W
Mo
Al
O
Mo
–
209
0.30
5.76
–
–
16.14
39.92
43.92
W/Mo
5:95
223
0.35
6.33
–
2.71
16.63
39.12
41.53
15:85
216
0.33
6.35
–
4.26
16.12
38.87
40.75
25:75
209
0.32
6.38
–
5.87
15.96
37.63
40.30
V/W/Mo
5:15:80
238
0.36
6.63
2.13
4.08
16.07
37.58
40.14
10:15:75
230
0.34
6.66
3.37
4.11
16.13
37.29
39.12
15:15:70
228
0.31
6.67
4.16
3.95
15.88
36.82
39.20
Isotherm plots of (a) MoO3/Al2O3, (b) W/MoO3 (15:85)/Al2O3 and (c) V/W/MoO3 (5:15:80)/Al2O3 after calcined at 600 °C for 5 h.
3.2 Catalytic evaluation in diesel fuel
In this section, the MoO3-based bimetallic/trimetallic catalysts were investigated using different transition metal dopants/co-dopants (e.g., tungsten, vanadium and titanium) and dopant/co-dopant loadings in the Cat-ODS.
3.2.1 Effect of dopant on MoO3/Al2O3
Fig. 5 shows the effect of different dopants on Mo/Al2O3 catalyst in desulfurization of diesel fuel. Three types of dopants with the ratio of 15 wt.% have been investigated and the results (Fig. 5) demonstrate that the catalyst reactivity decreased in the order: W/Mo > V/Mo > Ti/Mo > Mo > without catalyst. As displayed in Fig. 5, W/MoO3/Al2O3 catalyst showed a highest conversion of sulfur removal: 88.7% (Th), 95.6% (DBT) and 94.2% (4,6-DMDBT) respectively. The addition of W is believed to increase the porosity of the catalyst structure, which could enhance the reaction of the substrate contact area and enhance the catalytic efficiency. This finding is in agreement with Dieterle et al. (2001) who stated that tungsten acts as a promoter and potentially stabilizes the intermediate Mo oxide by forming M–O bonds. On the contrary, V/MoO3/Al2O3 catalyst has a slight decrease in the desulfurization yield. This result is in contradiction with Juniora et al. (2005) who reported that V was a promising dopant for catalyst which mainly acted as a textural promoter which can provide higher specific area and thus favoring the successive oxidation due to large substrate contact area. However, the introduction of Ti in MoO3/Al2O3 led to significant decrease in catalytic activity. According to Cheng et al. (2006), the hydrophilic character of Ti-based catalyst caused water to absorb strongly on the catalyst surface, which inhibited the adsorption, and hence resulted in a remarkable decrease in the oxidation activity. On the other hand, alumina support showed a similar trend of desulfurization performance with the oxidative reaction in the absence of catalyst.
The effect of different dopants on MoO3/Al2O3 catalyst on the performance of sulfur removal in Cat-ODS process.
3.2.2 Effect of dopant ratio
The potential W/MoO3/Al2O3 catalyst was further investigated on different W ratio and its catalytic performance is shown in Fig. 6. Result showed that 4.26%W/16.12%MoO3/Al2O3 catalyst prepared in the ratio of 15:85 (W/Mo) contributed to the highest removal of DBT (95.6%), 4,6-DMDBT (94.2%) and thiophene (88.7%), respectively. The addition of tungsten (4.26 wt.%) to the catalyst increased the conversion percentage and further increment of tungsten loading has no additional advantage on the catalytic activities. These results suggested that tungsten, which acts as promoter was unnecessary required in large quantity. Furthermore, this finding can be supported by FESEM micrograph (Fig. 2), where an excessive of tungsten in the catalyst caused the agglomeration of molybdenum oxide on the catalyst surface which lowered the degree of dispersion of molybdenum, decreased the number of coordinatively unsaturated sites, and thus possess weak catalytic activity. In view of these results, it can be concluded that the presence of tungsten at 4.26 wt.% is sufficient to serve as good structural promoter which could significantly enhance the catalytic activity and selectivity of the catalyst.
Effect of tungsten dopant ratios on MoO3/Al2O3 catalyst toward catalytic performance of sulfur removal in Cat-ODS process.
3.2.3 Effect of co-dopant
Table 2 summarizes the catalytic oxidative desulfurization of thiophene, DBT and 4,6-DMDBT over trimetallic catalyst by different co-dopants and co-dopant ratios. From the results obtained, it can be seen that the V/W/MoO3/Al2O3 performed a complete removal of DBT and 4,6-DMDBT, while for thiophene 94.5% was removed. Nevertheless, the elimination of thiophene, DBT and 4,6-DMDBT was slightly lower with 89.7%, 94.2% and 93.2% respectively when utilizing Ti/W/MoO3/Al2O3 catalyst. The potential V/W/MoO3/Al2O3 catalyst was then further investigated on different co-dopants ratios and the result showed that increase in the amount of vanadium toward W/MoO3/Al2O3 decreased the efficiency of the sulfur removal.
Catalyst/Al2O3
Catalyst ratio
Sulfur removal (%)
Th
DBT
4,6-DMDBT
MoO3
–
81.3
84.7
86.6
W/MoO3
15:85
88.7
95.6
94.2
Ti/W/MoO3
5:15:80
89.7
94.2
93.2
V/W/MoO3
5:15:80
94.5
100
100
V/W/MoO3
10:15:75
92.9
97.8
95.7
V/W/MoO3
15:15:70
89.5
92.2
91.4
3.2.4 Effect of catalyst dosage
Fig. 7 represents the effect of the dosage of 2.13%V/4.08%W/16.07%MoO3/Al2O3 catalyst on the oxidative desulfurization of commercial diesel and crude diesel. A set of five different dosages of catalysts 0.10, 0.15, 0.20, 0.25 and 0.30 g were tested to determine the optimum amount of catalyst for oxidation reaction. Based on the results obtained, the catalyst dosage of 0.20 g has the highest sulfur removal compared to other catalyst loadings for both types of diesels. Interestingly, when the catalyst dosage was increased up to 0.3 g, there was no significant difference in sulfur removal for both types of diesels. This was probably due to the catalyst reached its maximum catalyst loadings for the reaction and agglomeration of the catalyst particles, which led to decrease in the contact area between catalyst and diesel phase. Therefore, the sulfur removal of the diesel exhibited almost similar results even though the catalyst loadings were increased.
Effect of catalyst dosage on Cat-ODS reaction (reaction conditions: Oxidant = TBHP, T = 60 °C, time = 30 min, solvent = DMF, extraction T = 30 °C, mixing time = 30 min and no. of extraction stage = 1).
3.2.5 The series of oxidative desulfurization treatment on real diesel
Fig. 8 shows the series of oxidative desulfurization on real diesel comprising crude diesel and commercial diesel which contain 8269 ppmw and 550 ppmw of sulfur respectively. After a series of Cat-ODS process under optimum condition reaction, the sulfur content for both diesels achieved up to Green Diesel (10 ppmw) for commercial diesel fuel and Euro IV (<500 ppmw) standard for crude diesel fuel according to International Standard Diesel Fuel. In case of commercial diesel, there was about 75% of sulfur removal after single Cat-ODS process. In order to achieve Green Diesel (10 ppmw), second extraction has been conducted to remove the remaining sulfur in the diesel using other fresh solvent. With this second extraction, another 22.4% of sulfur was diminished which showed that Cat-ODS was a successful method to produce Green Diesel with 10 ppmw of sulfur content. Meanwhile using similar approach, crude diesel was treated up to four cycles of extraction with double oxidation process in order to achieve Euro IV (<500 ppmw) standard. It is because the sulfur content in the crude diesel was very high with various types of sulfur content which led to difficulties in removing most sulfur compound in the diesel fuel.
The series of oxidative desulfurization on real diesel (DMF to diesel volume ratio: 1.0 on each batch).
3.2.6 Reproducibility test of Cat-ODS process
The reusability of 2.13%V/4.08%W/16.07%MoO3/Al2O3 catalyst was tested for five consecutive Cat-ODS cycles and is presented in Fig. 9. The catalyst was tested in a series of oxidation of commercial diesel by similar aforementioned conditions. For each run (30 min), more than 75% of the sulfur compounds were oxidized, and revealed the higher activity and selectivity of the catalyst toward sulfur compounds in diesel fuel. After each used, the catalyst was rinsed with methanol to remove any adsorbed sulfur compounds, followed by drying at 80 °C and re-used for other reactions. The removal of sulfur during the reaction appeared to little changes through usage of five cycles. Results from EDX analysis over the used 2.13%V/4.08%/16.07%MoO3/Al2O3 catalyst, proved the high reusable ability of the catalyst due to little change in elemental composition presented after usage of five cycles of the catalyst in Cat-ODS compared to fresh catalyst.
Reproducibility test of Cat-ODS process over 2.13%V/4.08%W/16.07%MoO3/Al2O3 catalyst using commercial diesel. (Reaction conditions: Oxidant = TBHP, T = 60 °C, time = 30 min, catalyst dosage = 0.1 g, solvent = DMF, extraction T = 30 °C, mixing time = 30 min and No. of extraction stage = 1).
4 Conclusion
Trimetallic oxide catalyst of molybdena based on alumina support showed a great performance in desulfurization of model diesel and commercial diesel and also for crude diesel. The addition of tungsten and vanadium in an appropriate ratio onto the molybdena based catalyst led to a significant increase in the oxidation activity of the Cat-ODS process. These results indicate that 2.13%V/4.08%W/16.07%MoO3/Al2O3 catalyzed on Cat-ODS system has the great potential to be employed in industrial to meet the future regulation of sulfur in diesel fuels.
Acknowledgments
The authors would like to acknowledge the Ministry of Higher Education (MOHE) and Universiti Teknologi Malaysia (UTM) for the financial support under GUP Grant of 11H99 and FRGS Grant of 4F740.
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