Determining Surface Areas and Pore Volumes of Metal-Organic Frameworks

Our group focuses on synthesizing and characterizing porous materials for applications like water harvesting, air purification, and capturing carbon dioxide. What we're really interested in is understanding how the size, volume, connectivity, and chemistry of the pores impact the adsorption performance. And then using those insights to inform our work on materials design.

So robust characterization techniques are essential. Characterization is an essential aspect of any porous material research. X-ray diffraction, gas adsorption techniques, and thermal analysis techniques are among the top methods for characterization.

These can also be used to study structural stability, which is a crucial feature of any material used for real-world applications. Surface area and pore volume are two of the most important properties of porous materials. These properties can be measured using commercially available instruments that operate with established methods and software.

Nitrogen as an adsorbate is inexpensive, readily available, and safe. The measurements are quick and reliable. Begin by measuring the mass of an empty sample tube.

Then load 30 to 50 milligrams of the metal-organic framework or MOF UiO-66 into the sample tube, and measure the mass of the sample-loaded tube. Next, attach the sample tube to the sample preparation system while securing the seal with a 0.5 inch O-ring. Place the tube inside the heating mantle.

Set the temperature controller to the designated activation temperature of 120 degrees Celsius. Then open the valve connecting the system to the vacuum, and wait for the pressure to stabilize before leaving the sample for the designated activation time of 24 hours. After that, remove the tube from the heating mantle, and allow the sample to cool to room temperature.

Backfill the sample tube with nitrogen, and remove it from the preparation system. Finally, measure the mass of the sample tube along with the activated sample. Using the equation displayed, calculate the mass of the activated sample.

Begin by creating a sample file in the adsorption instrument software. To do so, open the instrument software, click File, and then click New Sample. Under the Sample Description tab, enter the sample name, sample mass, and sample density.

To set the analysis parameters open the Analysis Conditions tab, and select the adsorptive gas and nitrogen and the Analysis conditions as BET method. Then click on the Free Space button, and enter whether the free space is to be measured by the instrument, entered by the user, or calculated. Select whether the nitrogen dewar will be lowered during the measurement, and whether the system will perform a test for sample outgassing.

Next, select P0, and enter whether it will be measured by the P0 tube, entered by the user, or calculated. Input the P0 value if applicable. Select T, input the analysis temperature of 77 Kelvin, and click OK.In the next step, select Backfill, and set whether the sample will be backfilled before and after analysis.

If either is chosen, select the identity of the backfill gas as nitrogen, and click OK.In the Isotherm Collection section, select Target pressures, click Pressures. Then input the isotherm pressure values from a P over P0 between zero and one in intervals of 0.005, and click OK.Then click Options, and input the relative pressure tolerance of 5%before clicking OK.Finally, open the Report Options tab, and select the data analysis plots to be reported. Click Save As, name the file, and select a folder destination.

Begin by setting up the adsorption measurement instrument. To do so, first, slide the sample tubes into the isothermal sleeves and attach the tubes to the adsorption instrument while securing the seal with O-rings. Then, fill the dewar with liquid nitrogen while employing appropriate safety, and using proper personal protection equipment.

Place the dewar on the elevator below the sample. If using P0 tube, attach it and ensure it is configured to sit inside the dewar once the elevator is raised. After that, close the shield doors.

To run the experiment in the instrument software, click the name of the instrument followed by Sample Analysis. Click Browse and select the sample file. Ensure to match the analysis number with the number of the port where the sample is loaded, and then click Start.

For adsorption measurement, inject nitrogen into the sample tube until the first target pressure within the pressure tolerance range is reached. Leave the sample for the designated equilibration time until the pressure is stable. For desorption measurement, open the vacuum valve to desorb the nitrogen, until the first target pressure within the pressure tolerance range is reached, and leave the sample for the designated equilibration time until the pressure is stable.

Proceed to perform the data analysis after collecting all data points for the nitrogen adsorption experiment. In the adsorption instrument software, select File, followed by Export, and choose the experiment file. Enter the file destination and save the file as a spreadsheet.

Click OK.Next, following the displayed BET equation, use the isotherm data to create a BET plot with the appropriate x and y axes. Consider the linear range of the knee for a given isotherm. Using tools available to automatically detect the linear range for metal organic framework or MOF materials.

Ensure the linear range meets the Rouquerol criteria. Then, use the values of the BET plot's slope in the Y-intercept to calculate the BET constant and the monolayer capacity. Using the displayed equation, calculate the total surface area with the help of the monolayer capacity and adsorbate properties.

A typical type I nitrogen isotherm was obtained for UiO-66, which indicated a microporous structure and nitrogen monolayer formation. The key values obtained from the BET analysis indicated that an acceptable linear region was selected for the analysis.