An Experimental Protocol for Studying Mineral Effects on Organic Hydrothermal Transformations

Podsumowanie

Earth-abundant minerals play important roles in the natural hydrothermal systems. Here, we describe a reliable and cost-effective method for the experimental investigation of organic-mineral interactions under hydrothermal conditions.

Streszczenie

Organic-mineral interactions are widely occurring in hydrothermal environments, such as hot springs, geysers on land, and the hydrothermal vents in the deep ocean. Roles of minerals are critical in many hydrothermal organic geochemical processes. Traditional hydrothermal methodology, which includes using reactors made of gold, titanium, platinum, or stainless-steel, is usually associated with the high cost or undesired metal catalytic effects. Recently, there is a growing tendency for using the cost-effective and inert quartz or fused silica glass tubes in hydrothermal experiments. Here, we provide a protocol for carrying out organic-mineral hydrothermal experiments in silica tubes, and we describe the essential steps in the sample preparation, experimental setup, products separation, and quantitative analysis. We also demonstrate an experiment using a model organic compound, nitrobenzene, to show the effect of an iron-containing mineral, magnetite, on its degradation under a specific hydrothermal condition. This technique can be applied to study complex organic-mineral hydrothermal interactions in a relatively simple laboratory system.

Wprowadzenie

Hydrothermal environments (i.e., aqueous media at elevated temperatures and pressures) are ubiquitous on Earth. The hydrothermal chemistry of organic compounds plays an essential role in a wide range of geochemical settings, such as organic sedimentary basins, petroleum reservoirs, and the deep biosphere^1,2,3. Organic carbon transformations in hydrothermal systems occur not only in pure aqueous medium but also with dissolved or solid inorganic materials, such as Earth-abundant minerals. Minerals have been found to dramatically and selectively influence the hydrothermal reactivity of various organic compounds,^1,4,5 but how to identify the mineral effects in complex hydrothermal systems still remains as a challenge. The goal of this study is to provide a relatively simple experimental protocol for studying mineral effects on hydrothermal organic reactions.

The laboratory studies of hydrothermal reactions traditionally use robust reactors that are made of gold, titanium, or stainless steel^6,7,8,9. For example, gold bags or capsules have been favorably used, because gold is flexible, and it allows the sample pressure to be controlled by pressurizing water externally, which avoids generating a vapor phase inside the sample. However, these reactors are expensive and could be associated with potential metal catalytic effects¹⁰. Hence, it is imperative to find an alternative method with low cost but high reliability for these hydrothermal experiments.

In recent years, reaction tubes made of quartz or fused silica glass have been more frequently applied to hydrothermal experiments^11,12,13. Compared to precious gold or titanium, quartz or silica glass is considerably cheaper but also the strong material. More importantly, quartz tubes have shown little catalytic effects and can be as inert as gold for the hydrothermal reactions^11,14. In this protocol, we describe a general method for conducting small-scale hydrothermal organic-mineral experiments in thick-walled silica tubes. We present an example experiment using a model compound (i.e., nitrobenzene) in the presence/absence of an iron-oxide mineral (i.e., magnetite) in a 150 °C hydrothermal solution, in order to show the mineral effect, as well as to demonstrate the effectiveness of this method.

Protokół

1. Prepare the Sample for Hydrothermal Experiment

1. Choose the size of the quartz or silica glass tubes, e.g., 2 mm inner diameter (ID) x 6 mm outer diameter (OD) or 6 mm ID x 12 mm OD, and determine the amounts of organic compounds and minerals to use. In this work, the amounts of nitrobenzene and magnetite (Fe₃O₄) to load into the silica tube (e.g., 2 mm ID x 6 mm OD) are 3.0 µL and 13.9 mg, respectively.
   NOTE: The large diameter tubes allow easier loading of the materials but require more efforts of tube sealing.
2. Cut the clean silica glass tubing into small pieces with ~30 cm in length using a tube cutter. Seal one end of the tube closed using an oxyhydrogen torch with an appropriate flame head.
   CAUTION: Follow the safety procedures for using the oxyhydrogen torch.
3. Weigh the predetermined amount of the starting organic compound on a 0.1 mg-scale balance (if it is solid) and transfer it into the silica glass tube using a weighing paper. If the compound is liquid (e.g., nitrobenzene in this case), use a microliter syringe (e.g., 10 µL) to transfer it into the small silica tube. Add the weighed minerals into the silica tube through a Pasteur pipette, and then add deionized and deoxygenated water (e.g., 0.3 mL). Use 18.2 MΩ·cm deionized water and deoxygenate it by sonication.
4. Connect the silica tube to a vacuum line (~1 cm ID) with a closed valve. Immerse the tube in a Dewar flask filled with liquid nitrogen for ~3 min until the organics and water are completely frozen.
   CAUTION: Follow the safety procedures for transferring and using liquid nitrogen.
5. When the tube remains immersed in liquid nitrogen, open the vacuum valve and remove the air from the headspace of the tube.
   NOTE: This process should last until the pressure drops below 100 mtorr on the pressure gauge of the vacuum pump.
6. Switch off the valve, remove the tube from the liquid nitrogen, and let the tube warm up to room temperature. Gently tap the bottom of the tube to release the remaining air bubbles from solution to headspace.
7. Repeat the above freeze-pump-thaw cycle for two more times and keep the tube in liquid nitrogen before sealing the other end of the tube. Close the vacuum line and use the oxyhydrogen flame to make the entire tube closed.
   NOTE: When the tube undergoes hydrothermal experiments, the headspace volume of the tube will decrease due to liquid water expansion. For example, the density of water reduces about 30% from room temperature to 300 °C. Calculate and leave enough headspace volume when sealing the tube.

2. Set Up the Hydrothermal Experiment

1. After the sealing steps, put the silica tube into a small steel pipe (~30 cm in length and 1.5 cm in diameter) with loose screw caps, in order to prevent damage from any pressure building or tube failure inside the pipe.
2. Place the pipe in a well temperature-controlled furnace or oven and heat it up to the desired temperature (e.g., 150 °C in this work). Use a thermocouple inside the oven to monitor the temperature through the hydrothermal reaction.
3. As soon as the reaction time is reached (e.g., 2 h in this work), quench the silica tube by quickly putting the pipe into an ice water bath.
   NOTE: The quenching process takes less than 1 min to cool down to room temperature, which avoids potential retrograde reactions.

3. Analyze the Sample after the Experiment

1. Open the silica tube using a tube cutter, and quickly transfer all the products (e.g., ~0.3 mL in small silica tube) into a 10 mL glass vial using a Pasteur pipette.
2. Extract the organic products with 3 mL dichloromethane (DCM) solution that contains 8.8 mM dodecane as an internal standard for gas chromatography (GC). Cap the vial and shake it by hands for 2 min and vortex it for 1 min.
   NOTE: This helps to facilitate the extraction of organic products into the organic phase. Also, rinse the transferring pipet and inside walls of the silica tube with DCM to ensure products recovery. For samples with high mineral contents, sonicate them in the DCM solution for better extraction.
3. Allow the mineral particles to settle down in the extraction solution (i.e., DCM with dodecane) for 5 min. Use a Pasteur pipette to carefully transfer ~1 mL of the sample from the DCM layer (i.e., the bottom layer) into a GC vial.
4. Analyze the organic product distribution using GC with a poly-capillary column (e.g., 5% diphenyl/95% dimethylsiloxane) and a flame ionization detector. Set up the GC oven with a program to start at 50 °C and hold for 8 min, increase at 10 °C/min to 220 °C and hold for 10 min, increase at 20 °C /min to 300 °C and hold for 5 min. Set the injector temperature to 300 °C.
   NOTE: The GC program needed to be changed based on the type of organic compounds being analyzed.
5. Build the GC calibration curves by plotting the peak area ratio of the analyte to the internal standard versus the concentration of the analyte.
6. Calculate the reaction conversion based on the concentrations of the starting organic material before and after the reaction, i.e., conversion% = ([initial] – [final])  ⁄  [initial] ×100%. Use the conversions to determine if the mineral facilitates or slows down the hydrothermal organic transformations.

Wyniki

To demonstrate how to use this approach to study hydrothermal organic-mineral interactions, a simple experiment using a model compound, nitrobenzene, was conducted with mineral magnetite (Fe₃O₄) at a hydrothermal condition of 150 °C and 5 bars for 2 h. To show the mineral effect, an experiment of nitrobenzene without mineral was also performed under the same hydrothermal condition. As shown in Figure 1a, two silica tubes were made f...

Dyskusje

In this study, we used nitrobenzene with mineral magnetite as an example to demonstrate how to evaluate mineral effects on hydrothermal organic reactions. Although the experiments are carried out in small silica glass tubes, highly reproducible results are observed in the magnetite experiments, i.e., 30.3 ± 1.4% in nitrobenzene conversion, which suggests the effectiveness and the reliability of this hydrothermal protocol. In the no-mineral experiments, the conversion of nitrobenzene is 5.2 ± 2.1%, whic...

Materiały

Name	Company	Catalog Number	Comments
Chemicals:
Dichloromethane	VWR	BDH23373.400
Dodecane	Sigma-Aldrich	297879
Nitrobenzene	Sigma-Aldrich	252379
Fe2O3	Sigma-Aldrich	310050
Fe3O4	Sigma-Aldrich	637106
Supplies:
Silica tube
Vacuum pump	WELCH	2546B-01
Vacuum line
Oven	Hewlett Packard	5890
Thermocouple	BENETECH	GM1312
Gas chromatography	Agilent	7820A