MOF Material Testing

MOF material inspection mainly conducts scientific testing and analysis on the pore structure and specific surface area analysis, crystal structure and phase purity identification, thermal stability analysis, chemical composition and element analysis, morphology and micro-structure observation, and surface chemical properties and functional group analysis, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Pore structure and specific surface area analysis: This is the core of the MOF material property characterization. Mainly testing the specific surface area (BET method), pore volume, pore size distribution (DFT/NLDFT methods) and porosity. Through the static volumetric method adsorption/desorption isotherms of nitrogen (77K) or argon (87K), the data are obtained, and the adsorption capacity and pore channel accessibility of the material are evaluated. It is the key basis of judging whether it is suitable for the gas storage or separation.

Crystal structure and phase purity identification: Using the powder X-ray diffraction (PXRD) technology, the diffraction patterns of the samples are compared with the simulated or standard patterns, confirming their crystal structures, phase purity and crystallinity. This is the direct evidence of judging whether the MOF material is successfully synthesized, whether it contains the impurity phases or the structure collapse and amorphization occur.

Thermal stability analysis: Tested through the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Under the programmed temperature control, the mass changes and heat flow changes of the MOF material are measured, determining its decomposition temperatures, desolvation temperatures, phase transition temperatures and upper limits of the thermal stability, which is crucial for evaluating its service lives in the high-temperature application environments.

Chemical composition and element analysis: The methods such as the energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectrometry (ICP-OES) are adopted. Aiming to precisely determine the element types, contents, chemical states and ratios of the metal nodes and organic ligands in the MOF, verifying whether their stoichiometric ratios match the design.

Morphology and micro-structure observation: Mainly relying on the scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM is used to observe the overall morphologies, size distributions and surface roughness of the MOF particles; the TEM can provide the higher-resolution internal structures, lattice fringes and other information, assisting the judgement of the crystallinity.

Surface chemical properties and functional group analysis: The characteristic chemical bonds and functional groups in the material are detected through the Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. The infrared spectroscopy can identify the functional groups such as the carboxyl and amino groups in the organic ligands and their coordination states, and is the important means of verifying the successful connection of the ligands.

Testing Scope

Synthetic raw materials and intermediates: The testing scope covers the purities of the metal salts (such as the nitrates and chlorides), organic ligands (such as the carboxylic acids and nitrogen-containing heterocycles) and solvents used for synthesizing the MOF. The preliminary characterization of the intermediates can optimize the synthesis routes and ensure that the quality of the final products is consistent with the expected structures.

Raw powder materials: This is the most common testing object, referring to the MOF powders just synthesized without any post-treatments. The testing emphases are confirming their basic physical and chemical properties, such as the crystallinity, specific surface area and thermal stability, providing the baseline data for the subsequent application research.

Molded bodies and composites: The testing scope extends to the MOF molded bodies processed for the practical applications, such as the granules, pellets, films and fibers, as well as the composite materials formed by the MOF with the polymers and carbon materials. The mechanical strengths, pore retention rates after molding and interface bonding states need to be additionally evaluated.

In-service or failed samples: For the MOF materials after the cyclic uses such as the adsorption, catalysis and ion exchange, their structural integrity, active site changes, pollutant adsorption conditions and property attenuation causes are tested. This belongs to the failure analysis category and has the guiding significance for the material regeneration and process improvement.

Specific functionalized modified materials: For the MOF after the post-synthetic modifications, the types, amounts, distribution uniformity of the newly introduced functional groups and their effects on the original pore structures are tested. For example, the grafting rates and activities of the functionalized MOF such as the amine and sulfonic acid groups are tested.

Samples of the different batches and scale-up productions: In the process of the material from the laboratories to the industrialization, the systematic parallel testing needs to be carried out on the MOF produced in the different batches and at the different scales (such as from the gram level to the kilogram level), evaluating the consistency and repeatability of their properties and establishing the quality control standards.

Testing Methods

Gas adsorption analysis method: This is the standard method of evaluating the MOF porosity. Usually adopting the static volumetric method or gravimetric method, the adsorption isotherms of the nitrogen, argon or carbon dioxide are measured under the ultra-low pressures. The specific surface area is calculated with the BET model, and the pore size distributions of the micropores and mesopores are analysed with the density functional theory (DFT) or non-local density functional theory (NLDFT) models.

X-ray diffraction method: The powder X-ray diffraction is the primary method of identifying the MOF crystal structures. Through the comparison of the sample diffraction angles with the theoretical patterns calculated by the Bragg equation, the qualitative phase analysis is carried out. The in-situ PXRD can also be used to monitor the dynamic structural changes of the material during the gas adsorption, heating or chemical reactions.

Spectroscopic analysis method: It is a collection of a series of methods. The infrared spectroscopy is used for the qualitative analysis of the functional groups; the XPS is used for the quantitative analysis of the surface element chemical states; the nuclear magnetic resonance spectroscopy (solid-state NMR) can be used to study the ligand connection modes and local chemical environments; the UV-Vis diffuse reflectance spectroscopy is used to evaluate the optical band gaps of the semiconducting MOF.

Thermal analysis method: The thermogravimetric analysis is usually carried out in the inert or air atmospheres to determine the thermal decomposition temperatures, thermal stability ranges and possible stepwise weight losses (such as the guest molecule desorption) of the MOF. The combined technologies such as the TGA-MS (mass spectrometry) can synchronously analyse the released gas compositions.

Electron microscopy: The scanning electron microscopy provides the morphology information from the micron to nanometre scales, usually equipped with the EDX for the micro-area element analysis. The transmission electron microscopy, especially the high-resolution TEM, can directly observe the lattice images, used to analyse the crystal defects, grain sizes and orientations. The sample preparation needs to pay attention to avoiding the electron beam damage.

Other physical property testing methods: Including the mercury intrusion porosimetry (for the macropore analysis), positron annihilation lifetime spectroscopy (for the sub-nanometre defect detection), and chemical adsorption instruments (for determining the amounts and strengths of the active sites), etc. These methods are selectively used according to the specific application goals of the MOF.

Testing Standards

Domestic Standards:

1. GB/T 6682-2008, Water for analytical laboratory use — Specification and test methods

2. GB/T 19587, Determination of the specific surface area of solids by gas adsorption using the BET method

3. GB/T 21650.1, Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption — Part 1: Mercury porosimetry

International Standards:

1. ISO 9277, Determination of the specific surface area of solids by gas adsorption — BET method

2. ISO 15901-2, Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption — Part 2: Analysis of mesopores and macropores by gas adsorption

3. ISO 15901-3, Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption — Part 3: Analysis of micropores by gas adsorption

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Specific surface area and pore analyser: The core equipment is the physical adsorption instruments, such as the products of the brands Micromeritics and Quantachrome. The instruments are equipped with the high-precision pressure sensors, liquid nitrogen dewars and vacuum systems, which can automatically complete the measurement of the gas adsorption/desorption isotherms and have the built-in multiple calculation models for the pore structure analysis.

X-ray diffractometer: The modern powder X-ray diffractometers adopt the Cu target or Mo target X-ray tubes, equipped with the high-speed array detectors. To adapt to the sensitivity of the MOF to the air or water vapor, the in-situ sample chambers, low-temperature attachments or atmosphere control attachments are often equipped, realizing the structure analysis under the special conditions.

Thermal analysis system: Mainly including the thermogravimetric analysers and differential scanning calorimeters. The high-end models can be combined with the mass spectrometry and infrared spectroscopy, realizing the simultaneous thermal analysis-evolved gas analysis. The instruments need to have the precise temperature control programs (usually from the room temperature to above 1000 ℃) and stable gas flow control.

Electron microscope: The scanning electron microscopes need to be equipped with the field emission electron guns to obtain the higher resolutions, and integrate the EDX energy spectrometers. For the insulating and easily damaged samples such as the MOF, the low accelerating voltages are often adopted combined with the gold or carbon coating treatments. The transmission electron microscopes are required to have the high-resolution imaging capabilities and possible STEM modes.

Spectroscopic analysis instruments: The Fourier transform infrared spectrometers are usually equipped with the diffuse reflectance or attenuated total reflectance attachments to adapt to the powder samples. The X-ray photoelectron spectrometers need to be equipped with the monochromatic Al Kα sources to improve the resolutions, and equipped with the argon ion sputtering guns for the depth profiling. The solid-state nuclear magnetic resonance spectrometers are also the powerful tools of studying the local structures of the MOF.

Element and composition analysis instruments: The inductively coupled plasma optical emission spectrometers or mass spectrometers are used to precisely determine the contents of the metal elements in the MOF. The element analysers determine the contents of the organic elements such as C, H, N and S through the high-temperature combustion method. The combination of the two can precisely calculate the chemical formulas of the material.