Cu-BTC, also known as HKUST-1, is a widely studied metal-organic framework, or MOF. Its porous framework is constructed from dicopper paddlewheel units connected by trimesic acid linkers. This combination of porosity and tunable functionality makes Cu-BTC relevant to research involving adsorption, separation and catalysis. Because the copper centers are closely associated with the chemical and functional properties of the material, understanding their oxidation state and immediate coordination environment is an important part of Cu-BTC characterization.
Conventional X-ray diffraction provides valuable information about long-range crystal structure. However, changes occurring at a specific metal site may not always create a significant change in the material's average crystal structure. X-ray Absorption Spectroscopy, commonly abbreviated as XAS, addresses this challenge by providing element-specific information about the electronic state and local structural environment of a selected element.
XAS data can be interpreted through complementary regions of the absorption spectrum. X-ray Absorption Near Edge Structure, or XANES, is sensitive to the electronic state of the absorbing atom. When measurements are performed at the Cu K-edge, the position and shape of the absorption edge can therefore help investigate the oxidation state of copper.
Extended X-ray Absorption Fine Structure, or EXAFS, focuses on oscillations that arise from the local environment surrounding the absorbing atoms. Analysis of these oscillations can provide information about neighboring atoms, interatomic distances and coordination numbers. For Cu-BTC, this makes EXAFS suitable for investigating the oxygen atoms surrounding copper within the characteristic paddlewheel units.
This application note examines Cu-BTC using an Empyrean diffractometer equipped with an XAS module, an Ag LFF HR X-ray tube and a 1Der Advanced detector. The sample was measured in transmission geometry at the Cu K-edge. Metallic copper foil was used for energy calibration, while copper foil and CuO provided reference measurements.
The study combines XANES and EXAFS to investigate two connected questions:
The XANES spectrum is evaluated against metallic Cu and CuO references to assess the copper chemical state. The EXAFS data are analyzed using an HKUST-1 structure model and fitted in Artemis software to quantify the first-shell Cu–O contribution. The resulting interpretation is considered alongside information available from pair distribution function analysis and X-ray diffraction.
The strength of this approach lies in the complementary information supplied by each technique. XANES investigates copper oxidation state and electronic environment. EXAFS examines copper coordination and Cu–O distance. PDF and XRD provide atomic pair correlations and broader structural information. Used together, these methods connect the local chemistry of individual copper sites with the wider structure of the metal-organic framework.
This application note is intended for scientists and technical professionals working with metal-organic frameworks, coordination materials, catalysts and other materials in which the local chemistry of a specific metal can influence performance. It also provides a practical example of laboratory XAS measurements performed alongside established diffraction capabilities.
Register for full access to review the complete experimental setup, Cu K-edge spectra, EXAFS fitting parameters and detailed interpretation of the Cu-BTC copper environment.
Cu-BTC (HKUST-1) was investigated using laboratory X-ray Absorption Spectroscopy (XAS) on an Empyrean diffractometer. Cu K-edge XANES was used to investigate the oxidation state of copper, while EXAFS provided information about its local coordination environment.
The Cu-BTC spectrum is consistent with predominantly Cu2+ copper. EXAFS analysis revealed a first-shell Cu-O distance of 1.938(9) Å with a coordination number of 4.0(4), consistent with the local coordination expected for the copper paddlewheel units. The XAS results complement PDF and XRD characterization by providing element-specific information about the copper sites.
| Cu oxidation state | Cu-O distance | Coordination number |
|---|---|---|
| Consistent with Cu2+ | 1.938(9) Å | 4.0(4) |
Metal-organic frameworks have attracted significant attention because of their exceptional porosity and tunable functionality. Cu-BTC, also known as HKUST-1, is a widely studied MOF for adsorption, separation and catalytic applications. Its framework consists of dicopper paddlewheel units interconnected through trimesic acid linkers.
The copper centers are closely related to the chemical and functional properties of Cu-BTC, making their oxidation state and local coordination environment important characteristics to investigate. Local changes at a specific metal site may not necessarily produce a significant change in the average crystal structure.
X-ray diffraction provides information about the long-range crystal structure, while XAS provides complementary, element-specific information about the electronic and local structural environment of a selected element.
![[AN261005-figure1.png] AN261005-figure1.png](https://dam.malvernpanalytical.com/496f6b4f-5975-4709-a16f-b4da0058eebe/AN261005-figure1_Original%20file.png)
X-ray Absorption Spectroscopy measurements were performed using an Empyrean diffractometer equipped with the XAS module, an Ag LFF HR X-ray tube and the 1Der Advanced detector. The Cu-BTC sample was measured in transmission geometry at the Cu K-edge.
A metallic Cu foil was used for energy calibration, while Cu foil and CuO were measured as reference materials. The Empyrean configuration enables Cu K-edge XAS measurements alongside the platform's established diffraction capabilities.
| Parameter | Measurement condition |
|---|---|
| Diffractometer | Empyrean |
| X-ray source | Ag LFF HR tube |
| Detector | 1Der Advanced |
| Measurement geometry | Transmission |
| Absorption edge | Cu K-edge |
| Energy calibration | Metallic Cu foil |
| Reference materials | Cu foil and CuO |
The XANES region of the Cu K-edge is sensitive to the electronic state of copper and can therefore be used to investigate its oxidation state.
![[AN261005-figure2.png] AN261005-figure2.png](https://dam.malvernpanalytical.com/4c7ef11e-048f-4fb9-90ce-b4da0058ebdf/AN261005-figure2_Original%20file.png)
The Cu-BTC XANES spectrum is consistent with predominantly Cu2+ species, as expected for the copper paddlewheel structure. The absorption edge position is comparable to that of the CuO reference, supporting this assignment.
XANES therefore provides direct, element-specific information about the chemical state of copper that complements the structural information obtained from diffraction.
The EXAFS region provides information about the atoms surrounding the absorbing copper atoms and their distances from the copper center.
The Cu-BTC EXAFS spectrum shows well-defined oscillations associated with the local environment around copper. Fourier transformation of the EXAFS data reveals a clear first-shell contribution associated with oxygen atoms surrounding copper.
Using the HKUST-1 structure model (Bae J. et al., 2022, CSD Communication, private communication), the Cu-BTC EXAFS spectrum was fitted using Artemis software with the disorder-reduced HKUST-1 structure.
The analysis revealed a first-shell Cu–O distance of 1.938(9) Å with a coordination number of 4.0(4). These values are consistent with the four equatorial oxygen atoms surrounding copper in the Cu paddlewheel units.
Only the first Cu–O single-scattering path was required to fit the first coordination shell.
![[AN261005-figure3.png] AN261005-figure3.png](https://dam.malvernpanalytical.com/db4ca11a-0504-43fd-b702-b4da0058eb83/AN261005-figure3_Original%20file.png)
| Parameter | Value |
|---|---|
| S02 | 0.782 (obtained from EXAFS of CuO) |
| Coordination number, N | 4.0(4) |
| Cu-O distance, R | 1.938(9) Å |
| σ2 | 0.005(1) Å2 |
The different techniques provide complementary levels of structural information:
| Measurement | Information obtained |
|---|---|
| XANES | Cu oxidation state and electronic environment |
| EXAFS | Cu coordination and Cu-O distance |
| PDF / XRD | Atomic pair correlations and broader structural information |
For Cu-BTC, XANES identifies the chemical state of copper, while EXAFS quantifies its immediate coordination environment. PDF and XRD provide complementary information about the material's structure.
Together, these measurements connect the local environment of the Cu sites with the broader structure of the MOF. This is particularly relevant when changes at specific metal sites may occur without producing a large change in the average crystal structure.
Cu-BTC was successfully characterized using Cu K-edge XAS on an Empyrean diffractometer. The XANES spectrum is consistent with Cu2+, while EXAFS quantifies the Cu-O coordination environment with a Cu-O distance of 1.938(9) Å.
These results demonstrate how XAS adds element-specific information about the local chemical environment to the structural information obtained from PDF and XRD. The Empyrean XAS configuration therefore provides a laboratory approach for investigating materials in which the chemistry of specific metal sites and the broader material structure need to be considered together.