Indirect hydrodesulfurization of gasoline via sodium borohydride reduction with nickel catalysis under ambient conditions

Abstract

The present work introduced a novel indirect hydrodesulfurization (HDS) method for gasoline fuel at room temperature using an excellent reductant of sodium borohydride (NaBH₄), which produces active hydrogen with Ni–B catalysis. Then the active hydrogen can convert S-compounds into hydrogen sulfide (H₂S) or S²⁻, released from the gasoline. Under the optimized conditions the maximal desulfurization efficiencies of original and experimental gasoline can reach about 36.6% and 88%, respectively. It is a relatively clean and moderately indirect HDS method; after the process the Ni–B catalyst produced in the filtrate can be reused for the further desulfurization work. Moreover, the realization of the filtrate recycling, the evaluation of the hydrogen releasing and the exploration of catalyst development will be further helpful in cost reduction in future work.

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1. Introduction

Sulfur in transportation fuels (i.e. diesel, gasoline) remains a major source of air pollution and engine corrosion. Because of government mandates worldwide, increasingly stringent regulations are being imposed to reduce the S-content to a very low level, such as 10–20 ppm.^1,2 So refiners must put forward new requirements to produce increasingly cleaner fuels.³ The primary focus of the new regulations is the reduction of sulfur in gasoline and diesel.⁴ Removal of sulfur-containing compounds is an important operation in petroleum refining, and is achieved by catalytic processes operated at elevated temperatures (300–340 °C) and pressures (20–100 atm of H₂) by using the Co–Mo/Al₂O₃ or the Ni–Mo/Al₂O₃ catalyst.⁵

Hydrodesulfurization (HDS) is the essential process for removing sulfur-containing impurities from crude petroleum in order to obtain (i) cleaner fuels that minimize environmental pollution and (ii) cleaner chemical feedstocks that are less likely to poison the catalysts that are used for subsequent transformations.^6–8 Conventional desulfurization methods of transportation fuels are always catalytic HDS under the conditions of high temperature and high pressure. As such, costly reductant and catalysts, strict equipments and operation conditions have deeply restricted the development and utilization of them.^9,10

Recently, the renewed oxidative desulfurization (ODS) methods were studied, where ionic liquids (ILs) were used as extractants instead of organic solvents. It was found that the thiophenic S-compounds in fuels can be oxidized to their corresponding sulfone compounds, for example, dibenzothiophene sulfone (Scheme 1, DBTO₂).^11–22 The proposed method relies on adsorbents; however, the desulfurization function is not due to the adsorption of sulfur compounds, but to the conversion of them via a characteristic reaction occurring in adsorbents. However, ILs and adsorbents are so precious and excessive that they are difficult to be utilized practically on a large scale. Besides, the ODS method, which may destroy the heat value of fuels, becomes disabled in fuels containing too many aromatic components and/or dissolved water.²³

Scheme 1 Desulfurization of diesel fuel using ionic liquids.

In the previous studies, we have found that sodium borohydride (NaBH₄), owing to its excellent reductive property, appears to give a promising desulfurization efficiency of coal under ambient conditions.^9,24 If fine catalysts used, the hydrogen production and desulfurization efficiency can be significantly enhanced. However, indirect catalytic HDS via NaBH₄ reduction is rarely employed for transportation fuels (gasoline, diesel, etc.) desulfurization. In this sense, it is the first time a substitute for the conventional HDS methods has been investigated for gasoline using NaBH₄ catalytic (Ni) reduction under ambient conditions.

2. Experimental section

2.1. Gasoline and chemicals

Original gasoline (S-content: 40∼50 ppm) was obtained from an oil refinery in Shanghai, China. Organic sulfur (OS) exists in the real gasoline mainly in the forms of mercaptan, thioether, disulfide, thiophene and its derivates, which are shown in Fig. 1A. Mercaptan, thioether and benzothiophene (i.e. DBT) are not detected in the original gasoline. Since some other OS is omitted, the S-content of original gasoline is uniformly set as 40 ppm so that it is convenient for calculation in the following investigation. The sequence of hydrogenation activity about different types sulfides is:²⁵ thiophene < thioether < disulfide < mercaptan. Sodium borohydride (NaBH₄, 96%), NiCl₂·6H₂O (AR, >98%), NaOH (AR, >96%), commercial activated alumina (γ-Al₂O₃), and agar powder (BR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China), and stored in a dryer. Butyl sulfide (C₈H₁₈S, >95%, GC) used for preparing the higher S-content gasoline (experimental gasoline) was purchased from Fluka. All chemicals were used for experiments without any further purification.

Fig. 1 Schematic diagram of gasoline S-contents (A) and desulfurization setup (B).

Preparation of the KCl salt bridge. 3 g agar powder was added into 97 ml distilled water, and heated up to dissolve completely. Then 30 g KCl was added into the agar solution. Finally, the mixed solution was added dropwise or siphoned into the U-shaped glass tube. The salt-bridge can be used after agar condensation.

2.2. Apparatus and procedure

The reactions were carried out in a simple setup in Fig. 1B, which mainly consisted of the magnetic stirring apparatus, reactor (Erlenmeyer flask) and thermometer. Experimental reagents: aqueous solution/gasoline = 1/2 (volume ratio), NaBH₄/NiCl₂·6H₂O = 20 (mass ratio).

General procedure. Firstly, isopycnic distilled water (100 ml) and gasoline (100 ml) were added into the reactor in sequence. Then, the chemicals of NaBH₄ and NiCl₂·6H₂O were put into the reactor from the feedstock inlet. After that, the feedstock inlet and gas outlet were closed. With operating the magnetic stirring apparatus at a speed of 300 rpm, the reaction and timing were started. After 5 min the gas outlet was opened. The reaction was not finished until only a small amount of air bubbles were produced. And the produced tail gas (H₂S, etc.) was adsorbed by the alkaline solution (NaOH, etc.). The immediate formation of nickel boride (Ni₂B, black homogeneous precipitates) was observed, accompanied by the vigorous evolution of hydrogen (H₂, H*). Desulfurization occurred rapidly and the spent nickel boride was easily removed by filtration. Finally, the cleaner gasoline could be obtained by a separation method using the separating funnel. The separated aqueous solution containing Ni–B catalysts can be reused in the following experiments.

2.3. Analysis methods

Total sulfur (TS) in the gasoline and elements in the aqueous solution were determined by Inductively Coupled Plasma (Iris Advantage 1000, Thermo king-cord Co., USA). Various organic sulfurs (OS) were analyzed by Gas Chromatography (Shimadzu GC2010, Japan). GC operating conditions (injector temperature: 250 °C; carrier gas: helium; capillary column: SPB-5, bonded 5%, 30 m × 0.25 mm i.d. × 0.25 μm film thickness; flow rate: 1.5 ml min⁻¹; column oven: 50∼130 °C, 5 °C min⁻¹; detector type: FID; detector temperature: 250 °C; detecting time: 30 min) are given. And each GC analysis was repeated three times to check reproducibility. The desulfurization efficiency was calculated by the following equation,
where S₀ was the initial S-content and S₁ was the final S-content after HDS. And the data were presented as the average of two replicates in each treatment.

3. Results and discussions

3.1. Effect of reaction time

Time is an important technical parameter for chemical reactions, which reflects the chemical reaction efficiency and determines the experimental time. As a whole, the S-content in the original gasoline decreases with the increase of reaction time in Fig. 2(a). After 2 h, the S-content of real gasoline reduces from 40 ppm to 30 ppm (w/o Ni) and 25 ppm (with Ni), accompanied by the desulfurization efficiency increases by 25.5% and 36.6%, respectively (Fig. 2b). In the previous work, we have proposed the hydrolysis of NaBH₄ basically determining the treatment time.⁹ In Fig. 2(c), the desulfurization efficiency of real gasoline gradually increases with increase in reaction time as well, which accords with the linear eqn (1) (w/o Ni) and the first-order reaction eqn (2) (with Ni). Likewise, the S-content of experimental gasoline greatly reduces from 400 ppm to 190 ppm (w/o Ni) and 56 ppm (with Ni), accompanied by the desulfurization efficiency increases by 52% (w/o Ni) and 86% (w/o Ni), respectively. And the desulfurization efficiency increases with the increase of reaction time, which can also be fitted as eqn (III) (linear) and eqn (IV) (first-order) in Fig. 2(d). From the results, it is suggested that the increasing trend of S-removal recedes after 60 min, which may be caused by the hydrolysis rate slowing down. Addition of Ni catalyst can convert the hydrolysis kinetics of NaBH₄ from the linear reaction into a first-order reaction. Preliminarily, the catalytic reaction kinetics may be accorded with Langmuir–Hinshelwood kinetics. In the following investigation we use 60 min as the experimental time.

y = 0.2071x + 4.159 (linear, R_I² = 0.9089); (I)

y = −37.30exp^(−x/19.92) + 36.96 (first-order, R_II² = 0.998); (II)

y = 0.3845x + 11.70 (linear, R_III² = 0.8637); (III)

y = −79.99exp^(−x/14.61) + 80.84 (first-order, R_IV² = 0.9896), (IV)

where y is the desulfurization efficiency (%), x is the reaction time (0∼120 min), and R is the regression coefficient.

Fig. 2 Sulfur removal of gasoline (NaBH₄: 1 g L⁻¹, NiCl₂·6H₂O: 0.05 g L⁻¹).

3.2. Effect of NaBH₄ concentration

In the present work, NaBH₄ is the most important reactive chemical substance, which is the main resource of providing the protons. It can react with H₂O to produce amounts of active hydrogen. Meanwhile, the suitable concentration of NaBH₄ can reduce the excessive utilization. In that case, it is very necessary to investigate the NaBH₄ concentration in this reaction system. In Fig. 3(a), the desulfurization efficiency of gasoline increases with the increase of NaBH₄ concentration, while it is not evident. It can be concluded that the desulfurization efficiency is determined by the active hydrogen produced. However, the active hydrogen is determined by the two significant factors of NaBH₄ and catalyst (Ni) concentrations. If the ratio of C(NaBH₄)/C(Ni) is coordinating, it will produce more active hydrogen. Otherwise, it will bring about excessive NaBH₄ concentration. From Fig. 3(b), it can be found that the desulfurization efficiency is almost unchanged when the NaBH₄ concentration exceeds 2 g L⁻¹. Thus, the optimized condition of NaBH₄ concentration is 2 g L⁻¹.

Fig. 3 Effect of NaBH₄ concentration and reaction temperature on desulfurization efficiency.

3.3. Effect of reaction temperature

The previous experiments were carried on under room temperature (∼20 °C) conditions. However, reaction temperature has a significant effect on the chemical reaction. On one hand, the reaction temperature can affect the catalytic reaction rate. One the other hand, it can enhance the pressure of the reaction system, accordingly increasing the contact probability of S-compounds in gasoline with the reductants and catalysts in the aqueous phase. While gasoline is a volatile organic compound, whose boiling point is around 40–200 °C, a higher temperature can possibly lead to the volatilization of gasoline. Herein the effect of reaction temperature on the S-removal efficiency between 10 and 40 °C was investigated in this work. As Fig. 3(c) shows, the desulfurization efficiency gradually increases with increasing the reaction temperature from 10 to 40 °C. When the reaction temperature gets close to 40 °C, the maximal desulfurization efficiency of experimental gasoline can reach about 90%, while the desulfurization efficiency of real gasoline is almost unchanged. It presents that this indirect catalytic HDS is very difficult to remove sulfur in gasoline with an ultra-low S-content, i.e. below 15∼20 ppm. Since it is hard to control the temperature around so small a range, the desulfurization process could be carried out at room temperature.

3.4. Reuse of the filtrate

As we know, the catalytic chemical reaction and hydrogen release are happening in the aqueous medium. Namely, most of the chemicals still remain in the aqueous phase. These reaction products, such as Ni₂B, stably exist in the aqueous phase. Possibly, the part of the Ni₂B can be reused for further experiments. As Table 1 shows, the elemental concentration (i.e. B, Ni, and Na) in the filtrate is almost unchanged. Besides, the slight increase of sulfur (S) may be caused by HDS from the gasoline. In Fig. 4, we investigate the recycling utilization efficiency of the filtrate just by adding the same amount of NaBH₄. It can be found that the S-removal efficiencies of gasoline are 82% (initial S-content: 400 ppm) and 35% (initial S-content: 40 ppm) after 120 min. It demonstrates that the Ni₂B catalyst in the filtrate can also be reused for further desulfurization process. After the reaction, the pH value of the filtrate is slightly reduced, possibly due to the hydrolysis of H₂S.

Fig. 4 Sulfur removal efficiency of the gasoline using the filtrates.

Table 1 ICP analytical results of the filtrate^a

Element	Before reaction/mg L^−1 (pH = 9.80)	After reaction/mg L^−1
		O^b (pH = 9.26)	E^c (pH = 9.48)
a Process conditions: 60 min reaction time, 2 g L^−1 of NaBH4, and 0.1 g L^−1 of NiCl2. b O = original gasoline. c E = experimental gasoline.
S	0	3.62	6.80
B	277.90	275.30	271. 76
Ni	24.20	23.96	24.02
Na	592.05	591.40	590.78

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3.5. GC analysis

Fig. 5 shows the chromatogram of sulfur species distribution in original gasoline before and after the reaction. Also, the desulfurization performance values for main typical sulfur species are listed in Table 2. It can be found that thiophene and its derivates are significant components of OS in gasoline. This method is beneficial for 2-methylthiophene, 3-methylthiophene, and disulfide, etc. Moreover, the addition of thioether (higher concentration) can be greatly reduced, which is not shown in this figure. According to the peak areas before and after the reaction, we may estimate the approximate value of the S-removal efficiency (24.2%) using the following formula:
The estimated value of the desulfurization efficiency is just approaching the detected value (25.5%) as above. The produced error may be caused by the slightly reduced reaction time and ignoring some other little adsorption peaks of OS, which are not detected by GC.

Fig. 5 Chromatogram of sulfur species distribution in gasoline.

Table 2 Desulfurization performance for typical sulfur species in gasoline^a

Peak no.	Sulfur species	Before reaction		After reaction
		Peak area, P(a)	Percent. (%, total: 100)	Peak area, P(b)	Percent. (%)	Removal (%)
a Process conditions: 60 min of reaction time, 2 g L^−1 of NaBH4, and 0.1 g L^−1 of NiCl2.
1	Dibenzothiophene	3 622 975	32.9	3 434 111	31.2	5.2
2	2-Methylthiophene	2 382 634	21.6	1 410 902	12.8	40.7
3	3-Methylthiophene	2 995 981	27.2	1 798 030	16.3	40.1
4	Disulfide	924 349	8.4	586 631	5.3	36.9
5	Dimethylthiophene	1 046 519	9.5	1 051 124	9.5	0
6	Isopropylthiophene	42 276	0.38	71 062	0.65	—

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3.6. Possible mechanism of the indirect catalytic HDS

From the above analytical results, we herein can find that the increase of desulfurization efficiency and the decrease of treatment time are attributed to the addition of Ni catalyst. It is possible that the S atom in gasoline contains lone-pair electrons, and Ni²⁺ has an empty “d” orbital, so S react easily with Ni²⁺. In the Ni based catalyst, the stabilizing agent (NaOH) promoted the production of hydrogen. Furthermore, NaBH₄ can undergo fast reaction with NiCl₂ in the transformation of Ni₂B at the beginning, dispersing in the aqueous solution. Under Ni₂B catalysis, NaBH₄ can immediately react with H₂O, producing amounts of H₂ and active hydrogen. In the reaction, the activated hydrogen is adsorbed on the surface of nickel boride (Ni–B), forming into a kind of Ni–MH intermediate product. Then high activated hydrogen attacks S bonded with α-C, resulting in C–S bond cleavage. Meanwhile, the Ni–B produced by the reaction of NiCl₂ and deoxidizer has a high Ni–B/Ni rate, so it is beneficial for the activated adsorption of in situ hydrogen and organic sulfides (OS), which possess a higher desulfurization activity to aromatic organic sulfides.^26–30 Nickel boride (Ni₂B) is a fine and stable black solid that is easily prepared by the reduction of Ni²⁺ salts with NaBH₄, usually in protic solvents as shown in eqn (1).²⁷ Using Ni catalysts can greatly improve the desulfurization efficiency of organic sulfurs. The possible desulfurization mechanisms are presented as the following eqn (1)–(11).

Step 1 (catalyst production)

4NaBH₄ + 9H₂O + 2NiCl₂ → Ni₂B↓ (black precipitates) + 3H₃BO₃ + 4NaCl + 12.5H₂↑ (1)

Step 2 (active hydrogen production)

NaBH₄ + 2H₂O → NaBO₂ + 4H₂↑ (w/o Ni catalysis, slow reaction) (2)

NaBH₄ + 2H₂O → NaBO₂ + 4H₂↑ (8H*) (with Ni catalysis, fast reaction) (3)

4NaBO₂ + 11H₂O → Na₂B₄O₅ (OH)₄·8H₂O (sodium borate) + 2NaOH (4)

Step 3 (S-removal)

2H* + R–SH → R–H + H₂S↑ (5)

4H* + R–S–R¢ → R–H + R¢–H + H₂S↑ (6)

6H* + R–S–S–R¢ → R–H + R¢–H + 2H₂S↑ (7)

Special example

4H* + C₄H₉–S–C₄H₉ → 2C₄H₁₀ + H₂S↑ (Fig. 6A, very effective) (8)

H₂S + 2NaOH → Na₂S + 2H₂O (10)

Also, the schematic diagram of indirect catalytic HDS is shown in Fig. 6.

Fig. 6 Schematic diagram of indirect catalytic HDS.

3.7. Further R&D for indirect catalytic HDS

3.7.1. Catalyst (Ni–B/γ-Al₂O₃) for indirect HDS. Since the Ni–B catalyst is beneficial for hydrogen production by NaBH₄ reduction and S removal from gasoline, it can be loaded on the surface of activated aluminium oxide (γ-Al₂O₃),⁵ mesoporous silica (SiO₂)^31,32 or TiO₂ mesocrystals³³ in the form of the Ni–B/γ-Al₂O₃ supported amorphous catalyst, Ni–B/SiO₂ or Ni–B/TiO₂ (converted from dispersion to non-dispersion), which is easier to be prepared and recycled. Meanwhile, metallic compounds loaded in the surface pores of γ-Al₂O₃, just like Co–Mo/Al₂O₃ or Ni–Mo/Al₂O₃ catalyst,⁵ can enhance the catalytic activity of HDS. Fig. 7A presents the general preparation process of the Ni–B/γ-Al₂O₃ supported catalyst. And catalyst samples before and after reaction are given in Fig. 7B. From the XPS analysis, one can conclude that Ni in the Ni–B amorphous alloy is electron-rich and that the number of active centers decreases after reaction (Fig. 7C). It is suggested that loaded Ni–B is slightly reduced due to hydrogen (H₂) production and the HDS process. As Fig. 7D shows, the S-removal efficiency and H₂ yield increase rapidly with increasing the reaction time under the certain optimized conditions (details of optimizing experiments not shown here), while the increase trend becomes weaken after 120 min.

Fig. 7 Catalyst (Ni–B/γ-Al₂O₃) test for indirect HDS of gasoline.

3.7.2. Electrochemical hydrodesulfurization (EHDS) and electrochemical oxidative desulfurization (EODS). According to previous reports in the literatures and experimental theory analysis,^34,35 the ideal equation can be got as Fig. 8A. It is noteworthy that the removed S from gasoline in the form of gaseous H₂S in the cathodic slot can be further directly electrochemically oxidized into the element sulfur or absorbed by sodium hydroxide (NaOH) in the anodic slot. So it may be a good conception to combine chemical with electrochemical methods of NaBH₄ reduction for the desulfurization of fuels. It can resolve the problems of deoxidizer costs and realize the filtrate recycling. Besides, this kind of clean reduction method only uses electricity and green solutions, so it reduces the additional pollution, realizing reasonable application and sustainable development of resources. Fig. 8B shows the schematic diagram of the EHDS process for gasoline. General procedure: step 1. EHDS process in the cathodic slot; step 2. EODS process in the anodic slot. Fig. 8C presents the FT-IR analysis of gasoline before and after treatment, and gives the removal mechanism of OS (i.e. dibenzothiophene, DBT) in the anode. Meanwhile, employing of the optimized conditions (details of optimizing experiments and reductive mechanism not shown here), the desulfurization efficiency rapidly increases with increasing of the electrolytic time from 4 to 8 h (Fig. 8D). Also, the hydrogen yield during the EDS process improves remarkably with the electrolytic time as well from 1 to 5 h, according with first-order reaction (Fig. 8E). The further investigations of filtrate recycling and hydrogen yield will be helpful for cost reduction and sustainable development. Significantly, the developed method makes the most of electric energy to realize the cathodic reduction and anodic oxidation, converting OS into the inorganic S from gasoline to the aqueous phase.

Fig. 8 Combination EHDS with EODS for gasoline.

4. Conclusions

Herein, the presented indirect catalytic HDS process for gasoline fuel, which is an efficient and convenient method, can be operated under ambient conditions. Under the optimized conditions of room temperature, reaction time (60 min), NaBH₄ (2 g L⁻¹), NiCl₂ (0.1 g L⁻¹) and stirring rate (300 rpm), the maximal desulfurization efficiency can reach about 86% when the initial S-content of gasoline is 400 ppm. It is highly efficient in removing OS in gasoline, especially for the thioether added in. Meanwhile, the produced Ni₂B catalyst in the filtrate and waste aqueous solution can be reused for further desulfurization work. Furthermore, the environmentally friendly recycling of the Ni catalyst and aqueous solution will greatly decrease the expense of the desulfurization process. Significantly, the realization of the filtrate recycling, the evaluation of the hydrogen (H₂) release and the exploration of the catalyst development will be further helpful in cost reduction in the future work.

In addition, the developed Ni–B/γ-Al₂O₃ supported amorphous catalyst shows excellent performance for hydrogen production from NaBH₄ hydrolysis (improving the utilization ratio of NaBH₄), and exhibits a high S-removal efficiency of gasoline by NaBH₄ reduction. And the further employed EHDS can accomplish filtrate and boron (B) recycling for the first step, accordingly cutting down the secondary pollution and realizing economically sustainable development.

Acknowledgements

This work is financially supported by the National High Technology Research and Development Program (“863” program, Grant No. 2009AA062603). And the insightful comments and suggestions provided by reviewers and editor are greatly appreciated.

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