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Understanding the Pore Structure of MOFs

insights from industryFrancisco SotomayorHead of Product ManagementAnton Paar

In this interview, AZoM talks to Francisco Sotomayor about gas sorption analysis of metal organic frameworks (MOFs) and the importance of understanding their pore structure.

Why is it important to understand the pore structure of MOFs?

Many metal organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are candidates for gas storage and gas separation applications as well as applications in catalysis, drug delivery, and sensing due to their tunable pore structures and controllable surface functionality. Although MOFs are crystalline, various factors can contribute to deviations from perfect crystalline structure and expected performance in end applications.

In practice, MOFs may have reduced pore volumes and sizes due to non-volatile reactants in pores, partial collapse, or other activation-related problems. Additionally, many of these MOF applications take place under conditions where small amounts of moisture may be present. Depending on their metal and organic linker components, many MOFs are unstable in the presence of moisture, preventing their use in these end applications.

An advanced physical characterization is therefore crucial for accurately assessing the effective pore size, the apparent surface areas of MOF materials, and their interaction with application contaminants such as water vapor.

Understand the Pore Structure of MOFs

Image Credit: Anton Paar GmbH

What are the most common techniques for this kind of research?

Due to their crystalline nature, the overall MOF structure and theoretical porosity can be calculated from X-Ray diffraction and other scattering techniques.

In addition, because of their micro- and mesoporous structures, gas sorption techniques are well-suited and often used for the characterization of MOFs.

Some benefits of using gas sorption, either independently from or in conjunction with scattering techniques, are that gas sorption can determine the true probe accessible porosity of a material, a material’s affinity for various adsorbates, and can uncover structural changes in non-rigid MOFs. As such, gas sorption can confirm the suitability of candidate materials in applications such as gas storage.

Understand the Pore Structure of MOFs

Image Credit: Anton Paar GmbH

Are there any official standards or recommendations for gas sorption analysis?

In the case of the structural characterization of polar microporous materials, such as MOFs, Argon adsorption at 87 K (liquid argon temperature) is the IUPAC-recommended adsorptive. Coupled with state-of-the-art density functional theory (DFT) methods, it is possible to calculate the pore size and volume distributions for these materials and compare those results to crystallographic data.

How does gas sorption analysis work?

When it comes to gas sorption analysis of MOFs, samples are degassed under high-vacuum conditions for an extended period on degassing stations prior to measurement. After degassing, the samples are backfilled with inert gas, cooled to room temperature, weighed, and moved to the analysis stations.

The argon adsorption of the samples is then measured over a wide range of relative pressures (from as low as 10-7 to as high as 1) while the sample is submerged in a cryogenic bath at 87 K. The resulting adsorption isotherm can be used to calculate various parameters, including the apparent BET surface area and pore size distribution of the MOF sample.

Anton Paar has a new gas adsorption analyzer, the Autosorb 6100. Did you have MOF materials in mind when designing the new instrument, and are there any features that make it particularly suited for the analysis of MOFs?

Anton Paar’s new Autosorb 6100 is a customizable high-vacuum gas sorption analyzer designed for the most challenging measurements of surface area and pore size distributions in the nanometer range on MOFs and other microporous materials.

For material characterization laboratories focused on MOF analysis, the Autosorb 6100 offers best-in-class accuracy, agility, accessibility, adaptability, and assurance.

What exactly does this mean for users of this instrument in their daily work?

At each step in the measurement process, the Autosorb 6100 offers a number of exciting features for the characterization of MOFs.

Starting from degassing, the Autosorb 6100 has six integrated high-vacuum degassing stations featuring high-temperature mantles, pressure-controlled heating (to protect delicate MOF structures from steaming-induced damage), and a test for completion routines to ensure samples have been fully degassed and are ready for analysis.

Moving to analysis, every section of the instrument has been refined for accuracy and performance. For high-accuracy gas sorption measurements, temperature control is one of the most important factors.

With the Autosorb 6100, the manifold temperature can be controlled between 35 °C and 50 °C with stability better than 0.05 °C, making measurements highly reproducible independent of fluctuating environmental conditions. The analysis temperature is also strictly controlled, even over very long analysis times, with our unique TruZone active coolant level control system.

In addition, the Autosorb 6100 offers significant measurement flexibility and agility. The Autosorb is able to simultaneously analyze up to three different samples with three different analysis gases at three different temperatures with independent analysis stations. It is perfect for rapidly screening the suitability of your MOFs for a wide range of end applications.

What else makes the Autosorb unique?

The updated and streamlined Kaomi software is accessible to all levels of users and makes the whole measurement process easier than ever before.

The instrument is also adaptable to the specific characterization challenges a scientist faces. You can select the number of stations (one, two, or three) depending on throughput.

Furthermore, a built-in vapor generator and recirculating Dewar can be added to enable precise measurements of water and organic vapor sorption. If your needs change, these optional features can be added to the instrument after purchase in a future upgrade.

Understand the Pore Structure of MOFs

Image Credit: Anton Paar GmbH

Understand the Pore Structure of MOFs

Image Credit: Anton Paar GmbH

Is the Autosorb compliant with the most important industry standards?

The instrument complies with 15 ASTM, DIN, and ISO standards for surface area, pore size, and pore volume measurements.

What are other common applications for gas sorption analysis?

This instrument is definitely most interesting for universities and research laboratories. However, we also work with R&D labs in the industry and chemical manufacturers.

Besides MOFs, typical samples include novel catalysts, carbons, zeolites, and even novel microporous or low surface area materials for next-generation battery materials.

The Autosorb 6100 is perfect for the structural characterization of any porous monolith, pellet, powder, or thin films, especially those materials being considered for applications in gas storage and gas separation, catalysis, drug delivery, and sensing. 

About Dr. Francisco SotomayorBio image

Dr. Francisco Sotomayor is Head of Product Management for Anton Paar’s Surface and Pores product lines and has more than a decade of academic and industrial experience in the physical characterization of porous solids.

He manages a wide range of products, including gas pycnometers, gas sorption analyzers, mercury porosimeters, and capillary flow porometers. Prior to becoming Head of Product Management at Anton Paar QuantaTec (formerly Quantachrome Instruments), he worked as a product manager and application scientist for specialty gas sorption products and traveled extensively, providing worldwide support and customer training on specialized gas sorption topics, including advanced physisorption techniques, vapor sorption, chemisorption, and high-pressure analyses. Dr. Sotomayor obtained his Ph.D. in Environmental Engineering from the University of Michigan in 2016 and was an Oak Ridge Institute for Science and Education Fellow at the U.S. Environmental Protection Agency.

This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.

For more information on this source, please visit Anton Paar GmbH.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

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