Fuel Processing: Regenerable Sulfur Adsorbent for Liquid

Phase JP-8 Fuel Using Gold/Silica Based Materials

by Dat T. Tran, Zachary Dunbar, and Deryn Chu

ARL-TR-5678 September 2011

Approved for public release; distribution unlimited.

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Army Research Laboratory Adelphi, MD 20783-1197

ARL-TR-5678 September 2011

Fuel Processing: Regenerable Sulfur Adsorbent for Liquid Phase JP-8 Fuel Using Gold/Silica Based Materials

Dat T. Tran, Zachary Dunbar, and Deryn Chu

Sensors and Electron Devices Directorate, ARL Approved for public release; distribution unlimited.

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Fuel Processing: Regenerable Sulfur Adsorbent for Liquid Phase JP-8 Fuel Using Gold/Silica Based Materials

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U.S. Army Research Laboratory ATTN: RDRL-SED-C 2800 Powder Mill Road Adelphi, MD 20783-1197

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ARL-TR-5678

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14. ABSTRACT

Applications requiring hydrogen fuel, including portable, mobile, and stationary fuel cells for power generation, are increasing. The conversion of JP-8 to hydrogen offers an energy dense feedstock for hydrogen production through fuel reformation. Unfortunately, organic sulfur compounds in logistical fuels, even at parts per million levels, can poison reformer and fuel cell catalysts. In this work, adsorbents based on silica supported gold ions and gold nanoparticles were synthesized and evaluated for the adsorptive desulfurization of JP-8 jet fuel. The adsorbents were evaluated with JP-8 fuel containing 430 ppmw sulfur under ambient conditions. The preparation, as well as the sulfur removal and adsorption characteristics for two adsorbents, are described.

15. SUBJECT TERMS

Regenerable adsorbent; Fuel processing; JP-8 fuel; Logistical fuels; Gold/silica; Organic sulfur compounds

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Dat T. Tran a. REPORT

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iii

Contents

List of Figures iv 

List of Tables iv 

Acknowledgments v 

1.  Introduction 1 

2.  Materials and Methods 2 

2.1  Materials ..........................................................................................................................2 

2.2  Synthesis of Au Ions on Silica Particles ..........................................................................2 

2.3  Au Nanoparticles Coated on Silica Particles ...................................................................3 

2.4  Materials Characterization ..............................................................................................3 

2.5.  Fixed Bed Adsorption Experiments ................................................................................3 

3.  Results and Discussion 4 

3.1  Preparation and Morphology of the Adsorbents .............................................................4 

3.2.  Adsorption Characteristics of Adsorbents .......................................................................4 

4.  Conclusions 7 

5.  References 8 

List of Symbols, Abbreviations, and Acronyms 9 

Distribution List 10

iv

List of Figures

Figure 1. Logistic fuel processing scheme. .....................................................................................1 

Figure 2. Photos of (a) pure silica powder, (b) Au-ions on silica powder, and (c) Au NPs on silica powder. .............................................................................................................................4 

Figure 3. SEM micrographs of (a) pure silica powder, (b) Au-ions on silica powder, and (c) Au NPs on silica powder............................................................................................................4 

Figure 4. JP-8 fuel desulfurization at room temperature of (A-1, blue) Au ions/SiO2 adsorbent and (A-2, red) AuNPs/SiO2 adsorbent. .....................................................................5 

Figure 5. Sulfur removal from JP-8 fuel by (A-1, blue) fresh adsorbent and (red) sulfur removed after one regeneration of A-1 using isooctane as the regenerating agent. ..................6 

Figure 6. Temperature effect for JP-8 fuel desulfurization of (blue) A-1 adsorbent at room temperature and (red) A-1 adsorbent at 80 °C. ..........................................................................7 

List of Tables

Table 1. BET surface areas for SiO2, Au ions/SiO2 (A-1), and Au NPs/SiO2 (A-2). .....................6 

v

Acknowledgments

The U.S. Army Research Laboratory is acknowledged for financial support of this research.

vi

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1. Introduction

The production of hydrogen for fuelling advanced power sources can be completed with a variety of hydrogen-rich feedstocks, which are often favorable in terms of safety, logistics of storage and transportation, and specific energy. These sources may include any number of traditional hydrocarbons, as well as routes that involve the splitting of water. One of the more common approaches is the thermochemical reformation and processing of fossil fuels and biomass, such as coal, natural gas, propane, petroleum, methane, and alcohols. Power generated from solar, wind, and nuclear sources may also be used to electrolytically split water, and solar energy may be used to photolytically split water using photoelectrochemical and photobiological processes (1).

Due to the availability, ease of transportation, safety, and high energy density, JP-8 fuel is the primary focus of the U.S. Army’s fuel processing research efforts. Commercial interest in reforming JP-8 to hydrogen does not appear to have as high of a priority; however, the principles and practices may be transferable to other hydrocarbon fuels as merited. One of the most significant challenges in logistic fuel processing to produce useable hydrogen is liquid phase desulfurization (figure 1). The difficulty in the desulfurization of the fuel results from the complexity of JP-8. JP-8 consists of a mixture of hundreds of distinct hydrocarbon chains. Any organic sulfur compounds that are not removed can poison the reformer and fuel cell catalysts, even at parts per million concentration. While gas phase desulfurization technologies exist, these systems tend to be bulky and process the fuel stream post-reformation. Therefore, the reformation catalysts are not protected from potential poisoning (2). These factors are critical when considering the portable fuel cell systems of interest to the Army.

Figure 1. Logistic fuel processing scheme.

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Therefore, sorbents for the processing of liquid JP-8 are recognized as a key technology that requires development before portable, mobile, and stationary hydrogen generation from logistical fuel feedstocks can become a reality. The ultimate target for JP-8 desulfurization is to reduce sulfur levels to 1 ppmw or less.

Recent works have reported an adsorptive desulfurization method using ion exchange zeolite- and nickel (Ni)-based adsorbents (3–6). These studies, which have considered transportation fuels and JP-8, have been found to be promising approaches relative to more traditional non-hydrodesulfurization-based desulfurization technologies, such as charge-transfer complex formation, extraction using ionic liquids, bio-catalytic treatment, electrochemical catalytic oxidation, oxy-desulfurization, etc. Yang and co-worker reported the desulfurization of a model jet fuel by adsorption using carbon-based sorbents (7). The advantage of this method is that adsorption could be accomplished at ambient temperature and pressure.

In this report, we describe the synthesis of gold (Au) on silica (SiO2) support in both reduction and non-reduction form as sorbents for organic sulfur removal from liquid phase JP-8 fuel at ambient conditions using the adsorptive desulfurization approach.

2. Materials and Methods

2.1 Materials

All chemicals were purchased from Sigma-Aldrich and used as received:

• chloroauric acid [HAuCl4]·3H2O, 99.9+% metal basis

• silica gel impaq RG 1080 sil

• 3-aminopropyltriethoxysilane (APS), 99%

• ethanol, reagent, denatured, spectrophometric grade

• 2,2,4-trimethylpentane, anhydrous, 99.8%

Silica particles have an average pore diameter of 100 Å. JP-8 fuel was obtained from Fort Belvoir, VA, and had a sulfur concentration of 430 ppmw.

2.2 Synthesis of Au Ions on Silica Particles

Silica particles were functionalized with APS. Au ions were subsequently loaded on the functionalized particles via electrostatic interactions. Specifically, 1.4 mL of APS (0.2 M) was dissolved in 28.6 mL of ethanol and magnetically stirred for 10 min. Next, 1 g of SiO2 was added to this solution and the mixture was continuously stirred for 2 h. The solid was then collected by vacuum filtration and dried at 110 °C for 1 h to remove excess APS. The amino-

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functionalized SiO2 particles were then added to 5 mM of HAuCl4 (aqueous) and stirred for 2 to 3 h. The final product of AuCl4

¯ ions coated on silica support was filtered by vacuum suction and air dried under vacuum before use.

2.3 Au Nanoparticles Coated on Silica Particles

Au nanoparticles (NPs) were synthesized by sodium borohydride reduction of HAuCl4 in the presence of sodium citrate as a stabilizing reagent. The synthesis of the Au NPs was performed in aqueous solution, where the stoichiometry of the HAuCl4/sodium citrate/sodium borohydride (NaBH4) had a molar ratio of 1:1:5. A solution of 1 mM of HAuCl4 and 1 mM of sodium citrate was prepared by dissolving HAuCl4 in water followed by addition of sodium citrate under continuous stirring. NaBH4 dissolved in water was rapidly added to the solution during vigorously stirring. After 1 h of continuous stirring at room temperature, a dark red solution was formed.

The pH of the as-synthesized solution of Au NPs was adjusted to 5 by adding dilute hydrogen chloride (HCl). This converted carboxylate to carboxylic acid as protection groups on the Au NPs. A certain amount of SiO2 particles were added to this pH-adjusted solution of Au NPs, and the mixture was stirred for several hours. The Au NPs-loaded SiO2 particles were then filtered and dried in vacuum, and the final product was then calcinated at 200 or 300 ºC for several hours and cooled to room temperature.

2.4 Materials Characterization

The morphology of the silica and coated materials were examined using a scanning electron microscope (SEM) (FEI NOVA NANOSEM 600) at an accelerating voltage of 5 kV. The samples were sputter-coated to reduce charging before SEM operation. Brunauer, Emmet, and Teller (BET) surface areas of the samples were additionally measured by physical adsorption of N2 at 77 K using Micromeritics Accelerated Surface Area and Porosimetry (ASAP) 2010 analyzer.

2.5. Fixed Bed Adsorption Experiments

The synthesized materials were packed in a 4.6-mm inner diameter (ID) and 50-mm length column (Chromtech) to perform fixed bed adsorption experiments. JP-8 fuel containing an initial sulfur concentration of 430 ppmw was pumped into the column using a Shamatzu high performance liquid chromatography (HPLC) pump. The liquid fuel flow rate was operated at 0.5 mL/min.

Sample effluent from the packed column was collected in 1-mL aliquots, which were subsequently analyzed for total sulfur concentration using an ultraviolet ( UV) total sulfur analyzer (multi EA 3100, Analytikjena) with the detection limit set at less than 0.5 ppm.

The major organic sulfur contaminants in military jet fuel have previously been analyzed in our laboratory and reported within the literature (10).

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3. Results and Discussion

3.1 Preparation and Morphology of the Adsorbents

The pure silica can be seen in figure 2a while the as-synthesized adsorbents are depicted in figures 2b and 2c. The Au ions coated on silica particles are shown after the addition of the amino-functionalized silica particles to the HAuCl4 solution, where the amino groups on silica have been protonized to positive ammonium ions. The AuCl4

¯ ions were loaded on silica particles through electrostatic interactions between negative gold ions and positive ammonium ions. Figure 3 shows the morphologies and sizes of pure silica particles, gold ions, and gold NPs coated on silica particles. The scale bar is 100 μm.

Figure 2. Photos of (a) pure silica powder, (b) Au-ions on silica powder, and (c) Au NPs on silica powder.

Figure 3. SEM micrographs of (a) pure silica powder, (b) Au-ions on silica powder, and (c) Au NPs on silica powder.

3.2. Adsorption Characteristics of Adsorbents

In this study, Au ions and Au NPs coated on silica particles were used as adsorbents A-1 and A-2, respectively. Our method for examining these adsorbents was to test their ability to directly

(a) (b) (c)

5

remove organic sulfur compounds in the JP-8 fuel at ambient conditions. This approach used an actual JP-8 sample, whereas many reports in the literature limit their analyses to model fuels (e.g., isooctane) at moderate temperature, and/or containing a single known sulfur compound (e.g., benzothiophene or dibenzothiophene) (8, 9).

Fixed bed adsorption experiments were performed. The A-1 and A-2 samples exhibited significant sulfur adsorption at room temperature. The pure silica did not show significant adsorption of sulfur compounds. As shown in figure 4, the adsorption characteristic of A-1 performance was better than A-2. The performance of sulfur removal from JP-8 fuel for A-1 was initially above 80% before dropping to 60–70% and subsequent decay. The adsorption performance of A-2 was approximately 80% at the onset; however, it quickly dropped to approximately 30%. It subsequently saturated and fell to 0%.

Figure 4. JP-8 fuel desulfurization at room temperature of (A-1, blue) Au ions/SiO2 adsorbent and (A-2, red) AuNPs/SiO2 adsorbent.

The difference in adsorption behavior between samples A-1 and A-2 can be explained by the loading of gold on the silica substrate in the respective samples. This can be shown using BET surface area measurements, which are provided in table 1. The total areas determined by BET of pure silica and the A-1 and A-2 adsorbents are provided. The area of Au ions loaded on the silica, per gram of sample, is considerably larger than the Au NPs, corresponding to a greater mass of gold deposited on A-1. The surface areas of the samples ranges from 364 m2/g for initial pure silica to 264 m2/g for A-1 and to 361 m2/g for A-2. Therefore, we attribute a quick

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Sulfur Removed (%)

JP‐8 Proccessed (mL)

A‐1: Au‐Ion/SiO2

A‐2: AuNPs/SiO2

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saturation of A-2 to the relatively low quantity of gold deposited on the surface that is available for sulfur adsorption compared to A-1. We note that there is only a slight change in total surface area between pure silica and A-2 adsorbent, which explains why the A-2 adsorbent saturated at lower volumes of JP-8 processed.

Table 1. BET surface areas for SiO2, Au ions/SiO2 (A-1), and Au NPs/SiO2 (A-2).

SiO2 Au Ions/SiO2 Au NPs/SiO2 Micropore area (m2/g) 34.6 2.7 32.7 External surface area (m2/g) 329.7 261.6 328.9 Total SA by BET (m2/g) 364.3 264.2 361.6

To restore adsorptive performance to the A-1 adsorbent, pure isooctane was pumped through the reactor column. This regeneration process caused adsorbed organic sulfur compounds to desorb and diffuse into the clean isooctane stream. Figure 5 shows that after exposure to isooctane adsorptive performance was restored to nearly 100% original adsorption capacity.

Figure 5. Sulfur removal from JP-8 fuel by (A-1, blue) fresh adsorbent and (red) sulfur removed after one regeneration of A-1 using isooctane as the regenerating agent.

In addition to adsorbent regeneration, the tests of the effects of temperatures on the JP-8 sulfur removal performance of the A-1 adsorbent were performed. Figure 6 shows adsorptive behavior

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0 1 2 3 4 5 6 7 8 9 10 11

Sulfur Removed (%)

JP‐8 Processed (mL)

Fresh Adsorbent

Regen One

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of A-1 adsorbent at room temperature and 80 °C. The increase in temperature to 80 °C improved the adsorbent’s sulfur capacity. This phenomenon is not completely understood, but is believed to be caused by an increase in the strength of the interaction between sulfur and gold (compared to hydrocarbon and gold) with respect to temperature. Therefore, as temperature increases, the number of gold sites available for sulfur absorption increases due to reduced competition with hydrocarbon.

Figure 6. Temperature effect for JP-8 fuel desulfurization of (blue) A-1 adsorbent at room temperature and (red) A-1 adsorbent at 80 °C.

4. Conclusions

Liquid phase desulfurization is a critical technology requiring adsorbent development for portable, mobile, and stationary hydrogen generation from logistic fuel feedstocks to become a practical reality. In this work, two adsorbents for liquid phase organic sulfur removal from military JP-8 fuel were synthesized and characterized. The Au ions/SiO2 adsorbent (A-1) demonstrated considerable sulfur adsorption capability and room-temperature regeneration without a loss in capacity. The increase of the adsorptive temperature from room temperature to 80 °C showed improved sulfur removal. Our focus on continued development of this technology will help facilitate hydrogen to power generation for the Army of the future.

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Sulfur Removed (%)

JP‐8 Proccessed (mL)

Room Temp

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5. References

1. Department of Energy, Hydrogen Program. http://www.hydrogen.energy.gov/production.html (February 2011).

2. Karanjikar, M.; Chang, B-K; Lu, Y.; Tatarchuk, B. Logistic Fuel to Hydrogen – Fuel Processing and Sorbents for PEM Fuel Cell. Prepr Pap – Am Chem Soc, Div Fuel Chem 2004, 49, 910–11.

3. Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Desulfurization of Transportation Fuels with Zeolites Under Ambient Conditions. Science 2003, 301, 79–81.

4. Velu, S.; Ma, X.; Song, C. Selective Adsorption for Removing Sulfur from Jet Fuel Over Zeolite-based Adsorbents. Ind Eng Chem Res 2003, 42, 5293–5304.

5. Velu, S.; Song, C.; Engelhard, M. H.; Chin, Y-H. Adsorptive Removal of Organic Sulfur Compounds from Jet Fuel Over K-exchanged NiY Zeolites Prepared by Impregnation and Ion Exchange. Ind Eng Chem Res 2005, 44, 5740–49.

6. Velu, S.; Ma, X.; Song, C.; Namazian, M.; Sethuraman, S.; Venkataraman, G. Desulfurization of JP-8 Jet Fuel by Selective Adsorption Over a Ni-based Adsorbent for Micro Solid Oxide Fuel Cells. Energy & Fuels 2005, 19, 1116–25.

7. Wang, Y.; Yang, R. T. Desulfurization of Liquid Fuels by Adsorption on Carbon-based Sorbents and Ultrasound-assisted Sorbent Regeneration. Langmuir 2007, 23, 3825–31.

8. Cychosz, K. A.; Wong-Foy, A. G.; Matzger, A. J. Liquid Phase Adsorption by Microporous Coordination Polymers: Removal of Organosulfur Compounds. J Am Chem Soc 2008, 130, 6938–39.

9. Sami, A. H.; Hamad, D. M.; Albusairi, B. H.; Fahim, M. A. Removal of Dibenzothiophenes from Fuels by Oxy-desulfurization. Energy Fuels 2009, 23, 5986–94.

10. Lee, I. C.; Ubanyionwu, H. C. Determination of Sulfur Contaminants in Military Jet Fuels. Fuel 2008, 87, 312–18.

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List of Symbols, Abbreviations, and Acronyms

APS 3-aminopropyltriethoxysilane

ASAP Accelerated Surface Area and Porosimetry

Au gold

BET Brunauer, Emmet, and Teller

HAuCl4 chloroauric acid

HCL hydrogen chloride

HPLC high performance liquid chromatography

ID inner diameter

NaBH4 sodium borohydride

Ni nickel

NPs nanoparticles

SEM scanning electron microscope

SiO2 silicon dioxide

UV ultraviolet

10

NO. OF COPIES ORGANIZATION 1 DEFENSE TECHNICAL (PDF INFORMATION CTR only) DTIC OCA 8725 JOHN J KINGMAN RD STE 0944 FORT BELVOIR VA 22060-6218 1 DIRECTOR US ARMY RESEARCH LAB IMNE ALC HRR 2800 POWDER MILL RD ADELPHI MD 20783-1197 1 DIRECTOR US ARMY RESEARCH LAB RDRL CIO LL 2800 POWDER MILL RD ADELPHI MD 20783-1197 1 DIRECTOR US ARMY RESEARCH LAB RDRL CIO MT 2800 POWDER MILL RD ADELPHI MD 20783-1197 3 DIRECTOR US ARMY RESEARCH LAB ATTN RDRL SED C DAT T. TRAN (10 COPIES) ZACHARY DUNBAR DERYN CHU 2800 POWDER MILL RD ADELPHI MD 20783-1197

1 US ARMY TARDEC NON-PRIMARY POWER SYSTEM JP8 FUEL CELL APU SYSTEM ATO AMSRD-TAR-R/MS233 ATTN KEVIN CENTECK ATTN JEFF RATOWSKI

6501 E. ELEVEN MILE RD WARREN, MI 48397 BLDG 200D, 2ND FLOOR

NO. OF COPIES ORGANIZATION

1 US ARMY CERDEC FUEL CELL TECHNOLOGY CERDEC

COMMAND AND CONTROL DIRECTORATE

ATTN SHAILESH SHAH 5100 MAGAZINE ROAD

ABERDEEN PROVING GROUND, MD 21005-1852

TOTAL: 9 (1 ELEC, 8 HCS)