Chemistry
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Temperature-programmed Deoxygenation of Acetic Acid on Molybdenum Carbide Catalysts
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Summary February 7th, 2017
Presented here is a protocol for the operation of a micro-scale temperature-programmed reactor for evaluating the catalytic performance of molybdenum carbide during acetic acid deoxygenation.
Transcript
The overall goal of this temperature-programmed reaction method is to determine the deoxygenation performance of molybdenum carbide catalyst from the pyrolysis oil model compound acetic acid. This method can help answer key questions in the field of vapor phase upgrading using heterogeneous catalysts, such as those regarding catalytic activity and selectivity from pyrolysis oil model compounds. The main advantages of this technique is the ability to test catalytic performance over a range of temperatures in a single experiment.
While this method could provide insight into catalytic performance with acetic acid vapors, it can also be applied to other pyrolysis oil model compounds such as acid aldehyde, ethanol, methanol, and anisole. First, remove a clean, dry quartz U-tube reactor from a drying oven. Once the reactor has cooled to room temperature, load a small amount of quartz wool into it using the minimum amount needed to support the catalyst bed.
Using a clean, stiff metal wire packing word, gently push the wool down the quartz tube to the end of the straight section. Next, mix 50 milligrams of molybdenum carbide catalyst with 200 milligrams of sieved calcined quartz chips to prevent channeling and to ensure consistent temperature through the catalyst bed. Using weigh paper and a clean funnel, pour the catalyst and quartz chip mixture into the reactor such that the catalyst bed sits on top of the quartz wool support.
Then, gently push a second small piece of quartz wool on top of the catalyst bed, securing the catalyst bed in place. It's important to ensure that the catalyst-quartz chip mixture is homogenous. Inhomogeneous mixtures can lead to channeling, temperature gradients, and an excessive pressure drop across the catalyst bed.
Install the reactor by first connecting and tightening the fitting on the upstream side of the catalyst bed, which is the straight section of the reactor tube. Then, connect and tighten the fitting on the downstream side of the catalyst bed. Next, adjust the reactor thermocouple position by pushing the thermocouple through a bored-through fitting so that the tip of the thermocouple sits on the top edge of the catalyst bed.
Install insulation around the upper section of the reactor, and then carefully raise the furnace to the highest level allowed by the U-tube reactor such that the top edge of the furnace does not touch the reactor. Begin purging the system by flowing ultra-high purity helium at 40 sccm through the calibrated mass flow controller, or MFC. Following this, open the mass spectrometer, or MS orifice valve, to allow gas to reach the MS ion source.
After opening the hydrogen tank cylinder and regulator needle valves, begin flowing ultra-high purity hydrogen, but opening the system line shutoff valve immediately downstream of the hydrogen MFC and setting the MFC to flow at 1.3 sccm. Then, adjust the helium flow to 36 sccm, such that the gas mixture is 3.5%hydrogen. Once the hydrogen and helium gas phase concentrations have stabilized, enter the temperature program into the furnace controller, then begin the temperature program.
For the temperature-programmed reaction, begin the flow of acetic acid vapors through the reactor by opening the inlet and outlet saturator valves and subsequently closing the saturator bypass valve. Route the gas flow to the microgas chromatograph, or micro GC, by switching the three-way valve downstream of the reactor to the micro GC position. After allowing the system pressure of acetic acid gas phase concentration to stabilize, program the furnace temperature controller to ramp from room temperature to 600 degrees Celsius at ten degrees Celsius per minute.
Now begin collecting micro GC samples as frequently as possible. Begin the temperature program immediately after starting to collect the micro GC samples. When the temperature program has completed, stop the micro GC samples.
Using the computer software associated with the MS, turn the MS ion source off, then close the one micrometer MS orifice valve. After the last micro GC sample has completed, load a method that sets the column temperatures to the maximum allowable limit as recommended by the manufacturer. Shut off the hydrogen flow by setting the hydrogen MFC to zero sccm and closing the hydrogen shutoff valve.
Shut off the flow of acetic acid vapors to the reactor by opening the saturator bypass valve and closing the inlet and outlet saturator valves. After allowing the reactor to cool to ambient temperature, turn off the helium flow by setting the helium MFC to zero sccm, then close the shutoff valve located immediately downstream of the helium MFC. Once the system has reached ambient pressure, route the gas to the local exhaust ventilation using the three-way valve.
Uninstall the reactor by first disconnecting the fitting on the downstream side of the reactor, which is the fitting connected to the flexible stainless steel tubing. Finally, disconnect the fitting on the upstream side of the reactor. As shown here, many of the common products from acetic acid temperature-programmed reaction experiments are characterized by common mass to charge signals in mass spectrometry, and thus signal deconvolution is required.
Following deconvolution, the normalized and corrected MS data maybe used semiquantitatively to gather information, such as reactive conversion and relative product concentration as a function of reaction temperature. Representative data from a study comparing the deoxygenation performance of several molybdenum carbide catalysts in an acetic acid temperature-programmed reaction is shown here. A templated nanoparticle molybdenum carbide catalyst demonstrated greater acetic acid turnover rate compared to its untemplated counterpart in similar acetic acid turnover rates compared to the bulk molybdenum carbide catalysts below 400 degrees Celsius.
Above 400 degrees Celsius, the templated catalysts demonstrated a greater acetic acid turnover rate than any of the other catalysts studied. The hydrogen turnover rates were lower in the nanoparticle catalysts than on the bulk catalysts at all temperatures. Both nanoparticle materials demonstrated higher selectivity to ketonization products above 400 degrees Celsius than their bulk counterparts, which was attributed to an increase in the fraction of strong acid sites relative to the bulk materials.
Furthermore, the ratio of acid sites to hydrogen activation sites was identified as a key property in determining acetic acid deoxygenation performance. This technique can be performed in less than four hours, which makes the temperature-programmed reaction method an effective tool for screening the performance of catalytic materials and their various reaction conditions. After watching this video, you should have a good understanding of how to safely and effectively operate a temperature-programmed reaction using acetic acid vapors.
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