Oxidative Desulfurization of Hydrotreated Gas Oil using Fe₂O₃ and Pd Loaded over Activated Carbon as Catalysts

Introduction

Refining factories play an important rule to transfer coat oil to more valuable products such as liquid petroleum gas, jet fuel, gasoline, and diesel by using various techniques like distillation, extraction, reforming, hydrogenation, and cracking¹.

Millions of barrels of fuels are consumed daily in the worldwide road transportation therefore; increasing attention is being focus to the composition of fuel processing.  The big challenge that faces the refineries is the harmful sulfur oxides which liberate to the environment from the combustion of petroleum fuel that contained high sulfur ^2,3. Sulfur exists in hydrotreated petroleum as hydrogen sulfide, disulfides, organic sulfides, benzothiophene, dibenzothiophene, and their alkylated derivatives ^4,5. These compounds are unwanted and detrimental pollutants that converted to gaseous sulfur oxides (SO_X) upon fuel combustion which causes an acidic rain and the contamination of the atmosphere ⁶.

Oxidative desulfurization (ODS) is a very effective and considers a promising method for removing the dibenzothiophene derivatives from liquid fuel under moderate conditions, as low temperature, pressure and the cost of operation is less compared to HDS ^7,10.The OSD method is a kind of technology using oxidants to oxidize organic sulfur in two steps: the first step; oxidation of the sulfur atoms to strong polarity species and these polar products can be effectively separated by liquid extraction in the final step. In OSD the sulfur containing compounds in fuels are oxidized using different oxidizing agents such as nitric acid, nitrogen dioxide, organic hydro peroxides, hydrogen peroxide, peroxy acids, and ozone in the presence of a catalyst to produce sulfoxides and sulfones compounds respectively ^9,14. By that means, the oxidation of sulfides to sulfoxides and sulfones is generally accomplished by reaction with peroxy acid which generated by the reaction between peroxide and appropriate carboxylic acid. These oxidized molecules due to the strong polarity of them compared to the respective sulfides and the liquid extraction can be more efficient way to remove selectively the oxidized sulfur compounds from oil layer ^15,16.

This work contributes to a growing interest to produce low or no sulfur containing compounds in gas oil. The oxidation of gas oil by hydrogen peroxide using the catalysts ferric oxide Fe₂O₃/ Ac, palladium Pd/ Ac prepared in this work.

Experimental

Chemicals and Reagents

The analytical grade chemicals and reagents used in this study are ferric nitrate Fe(NO₃)₃.9H₂O, sodium hydroxide, and hydrochloric acid purchased from Fluka, palladium chloride PdCl₂ and charcoal from Sigma company. Also, barium chloride BaCl₂, sodium bicarbonate NaHCO₃, sodium carbonate Na₂CO₃, and acetonitrile CH₃CN were supplied from BDH. In addition, the oxidizing agent for ODS procedure is hydrogen peroxide H₂O₂ (30%w/w) and glacial acetic acid C₂H₄O₂ 16N, nitric acid HNO₃, 12N, were equipped from Scharlau. While, the hydrotreated gas oil used in this study provided from Midland Refineries Company in Iraq which analyzed using X-ray fluorescence to determine  total sulfur content (wt (%)) equal to 1.477 (1477ppm), API-Gravity= 31.9, and specific gravity= 0.85, distilled water used prior the experiments.

Preparation of Palladium, and Ferric Oxide Loaded Over Activated Carbon

The catalysts in this study were prepared according to the ratio cat.:Ac (20:80). Fe₂O₃/ Ac catalyst was prepared as reported by Hussein H. A. ¹⁷ as follow: a calculated weight of Fe(NO₃)₃.6H₂O was dissolved individually in 40ml distilled water. The precipitation of metal achieved by adding 1.0M NaHCO₃ to the solution gradually until pH of the solution reached to 8-9 and the content was continuously stirred at 60C° for 3h. After that, the mixture was filtered through a Buchner funnel in vacuum and washed using distilled water to get rid of sodium ions. To make sure that the precipitate was free of sodium ions few drops of 0.1M BaCl₂ was added to the filtrate as filtrate turbidity means the presence of sodium ions so continued in the washing with water until it is free from sodium ions. The precipitate cake was dried in an oven at 120°C for 24h to remove water.

The required weight of active carbon to achieve the ratio of cat.: Ac (20: 80) dried at 120°C for 24h and then wetted with 10ml of distilled water. The calculated weight of the catalyst prepared earlier dissolved individually in (5ml concentrated HCl: 40ml distilled water) and mixed with continuous  stirring with the calculated weight of active carbon and the mixture left for 24h at room temperature to allow the most of metal solution to be loaded on the activated carbon. The dried catalyst is calcinated for 2h at 400°C in muffle furnace and the purpose of this step is to load the metal salts on the active carbon and converted them to their corresponding metal oxides leading to deposit of active metal oxide over the support. Also, in the calcination step generate strong interaction between metal oxide and the support (Activated carbon). While, the preparation of Pd/C include dissolving of a calculated amount of PdCl₂ in hydrochloric acid (PdCl₂ doesn’t dissolve in water).

The impregnation step of Pd on carbon has been done by adding (10ml) of  formaldehyde CH₂O to PdCl₂ solution to reduce palladium ions Pd⁺² to Pd as metal precipitated on active carbon. After that, neutralization the solution by adding sodium hydroxide with continuous stirring and remove chlorides ions by washing the precipitate with distilled water. The drying step was important to get rid of water and moisture at 120°C. Before the impregnation of Fe₂O₃ and Pd on activated carbon, charcoal was mixed with nitric acid HNO₃ at room temperature for 24h to increase reactivity and porosity of carbon. After 24h, the resultant active carbon filtered, washed many times with distilled water to remove the residual of acid in its pores and then dried at 120°C for 2h.

Procedure of Oxidative Desulfurization Experiments

The ODS experiments were performed in a water bath at 40°C with 5ml of glacial acetic acid, 10ml of H₂O₂ mixed with 100ml of gas oil and 3gm of the catalyst in a three necks round bottom flask with. The mixture was stirred for 3h using a mechanical stirrer at 1000rpm, during the stirring process a solution of 2gm Na₂CO₃ dissolved in 20ml of distilled water was added drop by drop to the mixture. After the oxidation was finished the mixture attained phase separation, the oxidized sulfur was extracted with acetonitrile at room temperature. The acetonitrile and gas oil ratio was 1:1 by volume then the gas oil phase was separated and analyzed using X-ray fluorescence spectrometer ARL 8410 (ASTM D4294 method). The removal efficiency of sulfur compounds is calculated using the following equation:

Efficiency of desulfurization= (s^_° – s₁/ s^_°)*100

Where S₀ is the initial S-content and S₁ is the final S-content after (ODS). The results indicate that the catalyst is efficient to remove sulfur from gas oil with the catalyst

Instruments used in the Structural Characterization

Crystalline structure of the catalysts was examined by X-ray diffraction (Shimadzua-XRD-6000, Japan) equipped with Cu Ka radiation (ƛ= 1.5406A°) with a scanning speed 8° per min., 40Kv and 30mA, 2θ from 10° to 80°. Atomic absorption spectroscopy type (Shimadzu Europe AA-6300) used to determine the percentage of elements Pd and Fe in the prepared catalysts Pd/Ac and Fe₂O₃/Ac respectively. HORIBA/SA9600 series was used to measure the surface area and pore volume.

Also, thermal gravimetric analysis was performed to determine the thermal stability of oxide was performed by TGA-4000 instrument from Perkin-Elmer/Holland.

X-ray fluorescence analyzer (Lab X-300) used to calculate the content of sulfur in gas oil before and after treatment.

Results and discussion

X-ray Diffraction

X-ray diffraction (XRD) gives crystallographic and amorphous information of the prepared catalysts loaded over an active carbon. Fig. 1 and 2 show the XRD patterns for the supported catalysts prepared in this work. The diffractogram of the calcined Fe₂O₃/ C at 400C° shows the main diffraction peaks at 33.34°, 35.68°, 49.44°, 54.12°, and 62.54° which are related to Iron oxide in the form of hematite Fe₂O₃ and magnetite Fe₃O₄. It is assumed that the Iron oxide in the form of magnetite particles are formed mainly inside the support pores in the calcination step after the impregnation of ferric nitrate over the support. The formation of magnetite is attributed to the reduction some of Fe⁺³ to Fe⁺² by carbon particles in the presence of high temperature (calcination step). Reduction agent in the carbon is the carbon monoxide CO resulting from burning of the activated carbon, carbon monoxide dose not has the ability to access to inside of the catalyst so, magnetite is formed on the surface. Therefore, the final result particles formed are hematite the central part surrounded by magnetite ¹⁸.

The XRD investigation for Pd/ C reveals three diffraction peaks at 40.32°, 46.6° and 68.3° related to the Pd face-centered cubic crystal structure as in Fig. 2. Also, there is a characteristic values for 2θ at 24.25° and 25.13° attributed to carbon particles which supported Fe₂O₃ and Pd respectively. These results match with the earlier studies with simple variation due to the difference in the calcination temperature ^19,21.

The peaks of the patterns of Fe₂O₃/C and Pd/C catalysts are in good agreement with those of the standard XRD JCPDS data: PDF number 39-1346 and PDF-number 046-1043 respectively.

Figure 1: XRD of Fe2O3/ Ac   Click here to View figure

Figure 2: XRD of Pd/C   Click here to View figure

 

Atomic Absorption Spectroscopy

Atomic absorption spectroscopy technique utilizes to determine the percentage of metal ions presented in the catalysts. Table (1) reveals that the prepared catalysts containing on 1.1149% and 5.8708% of Pd and Fe in their composition respectively.

Table 1: Percent of metal and metal oxide in the prepared catalyst.

Catalyst	Metal (%)	Metal oxide (%)	Weight of carbon (gm)
Fe2O3/ Ac	5.8708	16.79	16
Pd/ Ac	1.1149	–	16

 

Determination of surface area and pore volume

Also, determination of the surface area and pore volume of the catalysts are fundamental aspect, since such properties can influence the catalytic activity and mass transport of reactants and products²². The data in Table (2) exhibit surface area and pore volume of Fe₂O₃/C and Pd/C.

Table 2: Surface area and pore volume of Fe₂O₃ and Pd loaded over carbon

Catalyst	Surface Area m2/gm	Pore Volume cm3/gm
Fe2O3/ Ac	44.260	0.233
Pd/ Ac	70.179	0.200

 

Thermal Analysis

Thermal gravimetric analysis studied for Fe₂O₃/C as in Fig. 3. The TGA curve showed, there was losing of weight at two points; first one at 271C° which was around 8.197%. After TGA calculation, the compound suffered losing in weight around 13.4 which could be equivalent to the notional value. The second point occurred at 595.5 and has the percent 13.351% and practical value was 2.57 which is parallel with the theoretical value. The weight losing of ferric oxide at the two points have small value. This can be attributed to loss of water molecules that oxide absorbed it from the surrounding ²⁵. The second prospect for losing in weight can be represented to transform the oxide from γ formula to the more stable formula (α) especially that α formula of ferric oxide is prepared at 700 ᵒC and while in this work (α) formula was prepared at 400C°¹¹.

Figure 3: TGA curve of Fe2O3/C   Click here to View figure

Oxidative desulfurization of gas oil using Pd/ C & Fe₂O₃/ C

Oxidative desulfurization based on the use of gas oil was conducted as mentioned in literature ²⁰ using hydrotreated gas oil (sulfur content 1477ppm, 100ml) with 3gm of the prepared catalyst mixed with 30% H₂O₂ as an oxidizing agent followed by adding 5ml of CH₃COOH as a co-catalyst. The mixture refluxed for 3h at 40°C using a mechanical stirrer at 1000rpm and while stirring 2gm of Na₂CO₃ dissolved in 20 ml of DW added to the mixture. At the end of reaction, the mixture left to settle for 30min to attain phase separation. The biphasic mixture was separated by extraction with acetonitrile and 2% barium chloride. Clearly a white precipitate of barium sulfate BaSO₄ was formed in the aqueous layer.

Figure 4: Experimental flow chart for desulfurization procedure   Click here to View figure

Results of the oxidation of gas oil containing on sulfur compounds in the presence and absence of acetic acid as a co-catalyst are shown in Table.(3). It can be seen that the OXD process is more effective in the presence of the acetic acid than without it. In the same table the effect of hydrogen peroxide and Na₂CO₃ on the OXD process also studied. The removal of sulfur was 71.20% and 66.0% for Fe₂O₃/Ac and Pd/Ac respectively. H₂O₂ is considered as a green reagent which is oxidizing organic sulfur compounds to the corresponding sulfones at ambient temperature.

Table 3: Efficiency removal of sulfur compounds in the presence and absence of CH₃COOH and different amount of H₂O₂ and Na₂CO₃ at 40C° and 3h the time of reaction.

No. Exp.	Catalyst	Vol. of CH3COOH (ml)	Vol. of H2O2 (ml)	Amounts of Na2CO3 (gm)	Removal Efficiency (%)
1	Fe2O3/ Ac	0.0	10	0.0	12.10
2	Pd/ Ac	0.0	10	0.0	10.40
3	Fe2O3/ Ac	5.0	10	2	71.2
4	Pd/ Ac	5.0	10	2	66.0
5	Fe2O3/ Ac	5.0	15	2	17.03
6	Pd/ Ac	5.0	15	2	13.24
7	Fe2O3/ Ac	0.0	10	0.0	16.10
8	Pd/ Ac	0.0	10	0.0	18.24

 

The results in Table (3) indicates that the higher removal efficiency of sulfur from hydrotreated gas oil was achieved when using 10ml of H₂O₂ as oxidizing agent, 5ml of CH₃COOH as co-catalyst, and 2gm of Na₂CO₃.

Mechanism of the oxidation reaction

A reasonable mechanism of oxidative desulfurization process can be suggested as an active catalyst species can be formed by the nucleophilic attack of hydrogen peroxide on metal atoms of the catalyst. Electrons are evacuated from the peroxyl ions, because of that increasing the electrophilic character of the peroxidic oxygen. The ODS moves by nucleophilic attack of an active catalyst (oxidants) and convert sulfur containing compounds in fuels to form sulfones which are much more polar species. Fig.5 represents the mechanism of ODS in liquid phase ^{20, 25}.

Figure 5: Mechanism of ODS in liquid phase     Click here to View figure

 

Conclusion

In this study, Fe₂O₃/ Ac and Pd/ Ac were prepared by precipitation method in ratio Cat.:Ac equal to 20:80 and used as catalysts for the oxidation of sulfur compounds in gas oil. It was found that the catalytic activity at selected parameters: 100 ml of gas oil mixed with 3gm of the catalyst in the presence of H₂O₂ as oxidizing agent and CH₃COOH and Na₂CO₃ as a co-catalyst. Results obtained in this study for gas oil samples showed that the use of the prepared catalysts achieved an efficiently removal of sulfur in gas oil using eco friendly reagents reached to 71.20% and 66.0% for Fe₂O₃/ Ac and Pd/Ac respectively.

Acknowledgement

The authors are grateful to the Head department of Chemistry, College of Sciences, Diyala University and the Head of Chemical Engineering, College of Engineering, Diyala University for providing the necessary support and technical assistant to accomplish this study.

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