The overall goal of this experiment is to obtain high temperature absorption isotherms of gases on Zeolite H-ZSM-5, or other microporous materials, using an absorption measurement device based on a langatate crystal microbalance. This method can help answer key questions in the field of high temperature gas absorption measurement of microporous materials, such as how to synthesize microporous materials on our high frequency oscillating microbalance. The main advantage of a method based on high frequency oscillating microbalance is that it is more affordable than a commercialized formatic and a gravimetric methods.
The main advantage of a langatate crystal microbalance over a quads crystal microbalance is that it's not limited to temperatures below 80 degrees Celsius. The target reach in here is 200 to 300 degrees Celsius. To begin the procedure, thoroughly wash two LCMs with deionized water.
Then clean the LCMs in an ultrasound bath with deionized water and dry them in an oven at 80 degrees Celsius for several minutes. Meanwhile, clean the LCM holder, the O rings and the sample chamber with acetone and compressed air. Place both clean, dry LCMs on the clean, dry LCM holder and designate one LCM as the reference and one as the sample position.
Then, connect the LCM holder to the oscillator by high temperature resistant electric cables. Pay special attention to the positions of the LCMs on the holder since only the appropriate positions ensure that resonance frequencies can be detected. Finally, turn on the oscillator.
Using the oscillator software, check for the presence of resonance frequencies for both LCMs. Close, insulate, and evacuate the sample chamber. Evacuate and purge the chamber with nitrogen gas several times and then set the chamber pressure to the lowest pressure to be tested.
Once the pressure has stabilized, set the chamber temperature to the lowest temperature to be tested. Once the temperature has stabilized, measure the resonance frequencies of both LCMs using a Butterworth Van Dyke equivalent circuit model fitted by the oscillator software. Determine and record the F-0 value.
Then, slowly add nitrogen gas to pressurize the sample chamber to the next pressure to be tested. Wait for the pressure to stabilize before obtaining the delta F-0 value. Repeat this process for every pressure to be tested.
Obtain delta F-0 values for each temperature for which an isotherm will be determined in this way. Prepare a Zeolite H-ZSM-5 synthesis mixture as described in the text protocol. Once prepared, the mixture must be used within five hours.
With a pipette, carefully place several drops of Zeolite mixture on the center gold electrode of the sample LCM. Do not let the mixture contact the oscillator connection points on the outer gold electrodes. Dry the loaded LCM in an oven at 80 degrees Celsius for two hours to yield a viscous gel on the electrode.
Place 10 milliliters of deionized water in a PTFE lined autoclave reactor. Then, place a PTFE holder in the reactor. Place the loaded LCM on the holder, ensuring that the LCM does not contact the liquid water, and close the autoclave reactor.
Carefully transfer the autoclave reactor to an oven. Heat the reactor at 150 degrees Celsius for 48 hours to perform the steam assisted crystallization. Once crystallization is complete, wash the loaded LCM with deionized water and dry it in an oven.
Then, allow the LCM to cool to room temperature. Place the loaded LCM in an oven and at a rate of three Kelvin per minute, heat the resonator to 450 degrees Celsius for calcination. Program a holding time of four hours at 450 degrees Celsius, followed by a cooling rate of likewise, there Kelvin per minute to room temperature.
Then, soak the loaded LCM in one millimolar ammonium chloride to perform ion exchange. Repeat the process once to complete the ion exchange. Calcine the loaded LCM once more by the same method to obtain the LCM loaded with H-ZSM-5 crystals.
First, place both the reference and sample LCMs in the holder and ensure that the resonance frequencies can be detected. Then, close and evacuate the sample chamber. Heat the LCMs overnight under vacuum to activate the Zeolite.
Set the temperature in the sample chamber to 50 degrees Celsius for the absorption measurements and wait for the temperature to stabilize. Connect the sample LCM to the oscillator and measure its resonance frequency to obtain the value of the resonance frequency of the loaded sample LCM. Then, connect the oscillator to the reference LCM and measure its resonance frequency to obtain the value of the resonance frequency of the reference LCM.
Using the Sauerbrey equation and the measured differences in resonance frequencies, determine the mass of the H-ZSM-5 deposited on the sample LCM. To begin the as absorption measurements, slowly fill the sample chamber with the selected gas to the first desired pressure. Once the temperature has stabilized, measure and record the resonance frequencies of the reference and sample LCMs.
Repeat these measurements at several different pressures. For each measurement, calculate the combined mass of H-ZSM-5 and the absorbed gas using the Sauerbrey equation and then subtract the deposited mass of H-ZSM-5. The difference is the mass of the absorbed gas.
Use the calculated masses of the absorbed gas at each pressure to determine the gas absorption isotherm at that temperature. The carbon dioxide absorption isotherms determined by this procedure are fitted by a least squares method to a single site Langmuir isotherm model, from which the absorption equilibrium constants can be calculated for each temperature. These equilibrium constants are used in a Van't Hoff plot, which provides the absorption enthalpy and absorption entropy for carbon dioxide in H-ZSM-5.
The absorption parameters of carbon dioxide in H-ZSM-5 using this LCM based technique are close to literature values reported for carbon dioxide absorption in other MFI type Zeolites. Once mastered, this technique can be done in 30 minutes for one measurement if it is performed properly. While attempting this procedure, it is important to remember that good temperature control of the sample by good insulation of the sample chamber and the careful preheating of the feet gas is crucial for high measurement equality.
After watching this video, you should have a good understanding of how to synthesize the Zeolite crystals on the desired surface of the LCM and use the LCM device for high temperature gas absorption measurements. Don't forget that working with toxic, flammable, or explosive materials can be extremely hazardous and precautions, such as a fume hold or a glove box should be always taken while performing this procedure. After its development, this technique paved a way for researchers in the field of catalysis or microprosolids to explore multicomponant gas absorption, especially at high temperature and pressure conditions.
