# Characterization and trends in CO 2 adsorption of 3D structured Hofmann clathrate

Rizwan Minhas, Laurent Perrier, N Brefuel, Christelle Miqueu

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Rizwan Minhas, Laurent Perrier, N Brefuel, Christelle Miqueu. Characterization and trends in CO 2 adsorption of 3D structured Hofmann clathrate. 1st Mediterranean Conference, May 2023, Crete island, greece, Greece. hal-04387585

HAL Id: hal-04387585

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Submitted on 11 Jan 2024

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# Characterization and trends in CO₂ adsorption of 3D structured Hofmann clathrate

R. Minhas¹, L. Perrier¹, N. Brefuel², C. Miqueu¹

¹Université de Pau et des pays de l'Adour, E2S UPPA, CNRS, LFCR, Anglet, France

² CEMES CNRS, Toulouse, France

## 3D Hofmann Clathrate

<figure><img src="image_2.png" /><figcaption>Schematic Structure of 3D
FeNi(CN)4Pyrazine [1]</figcaption></figure>

To address the increasing energy consumption and serious environmental problems, it is critical to develop efficient and clean energy conversion and storage devices. In this context, among various categories of materials, a 3D structured Spin Crossover (SCO) MOF, [FeNi(CN)₄Pyrazine] [2], in the family of materials known as Hofmann clathrates, has been synthesized, characterized and its CO₂ and CH₄ sorption capacities are measured and analyzed at several temperatures and pressures.

## Spin-Crossover (SCO)

<figure><img src="image_3.png" /><figcaption>SCO temperature dependence [2]</figcaption></figure>

## CONTEXT AND OBJECTIVE

## Objective

The aim of this work, is firstly to synthesize the 3D Hofmann clathrate and then characterize it to identify its porous as well as physical properties. Once completely characterized, the CO₂ and CH₄ sorption capacities are measured and analyzed using well developed manometric and gravimetric techniques (at high pressure and several temperatures) and finally to study CO₂/CH₄ selectivity of this MOF.

5. IAST modeling of CO₂/CH₄ selectivity

1. Synthesis

2. Characterization

3D Hofmann
Clathrate
[FeNi(CN)₄Pyrazine]

4. CH₄ adsorption isotherms
(Gravimetric measurements)

3. CO₂ adsorption isotherms
(Manometric Technique)

## ADSORBENT SYNTHESIS

## ADSORBENT CHARACTERIZATION

### Chemical Reaction

The sample is synthesized using pyrazine ligands.

$$ \text{Fe}(\text{BF}_4)_2(\text{H}_2\text{O})_6 + 1(\text{Pyrazine}) + \text{K}_2\text{Ni}(\text{CN})_4 \rightarrow \text{FeNi}(\text{CN})_4\text{Pyrazine} + (\text{KBF}_4)_2 $$

Immediately precipitates as orange powder

Removal of KBF4 upon
washing with pure water
or using Centrifugator

<figure><img src="image_4.png" /></figure>

### Chemical composition of the adsorbent

Elemental composition of [FeNi(CN)₄](C₄H₄N₂)(H₂O)₂

<table><thead><tr><th>Element</th><th>Symbol</th><th>Atomic weight</th><th>Atoms</th><th>Mass percent</th></tr></thead><tbody><tr><td>Hydrogen</td><td>H</td><td>1.00794</td><td>8</td><td>2.4090</td></tr><tr><td>Oxygen</td><td>O</td><td>15.9994</td><td>2</td><td>9.5597</td></tr><tr><td>Carbon</td><td>C</td><td>12.0107</td><td>8</td><td>28.7057</td></tr><tr><td>Nitrogen</td><td>N</td><td>14.0067</td><td>6</td><td>25.1071</td></tr><tr><td>Iron</td><td>Fe</td><td>55.845</td><td>1</td><td>16.6838</td></tr><tr><td>Nickel</td><td>Ni</td><td>58.6934</td><td>1</td><td>17.5347</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Hydrated structure (as
synthesized)</figcaption></figure>

TGA/MS analysis to check the synthesis and
the thermal stability of the synthesized sample

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>IR Spectroscopy to check the
contaminants in the sample</figcaption></figure>

### Gas Porosimetry : Nitrogen & Argon Adsorption

<figure><img src="image_8.png" /></figure>

Meso-porosity : Due to
inter-grains (seen by SEM
images of the adsorbent)

<figure><img src="image_9.png" /></figure>

Comparison of hydrated and dehydrated sample

Hydrated sample

Dehydrated sample

160

328

## HIGH PRESSURE CO₂ and CH₄ ISOTHERMS : Measurements and Modelling

### Apparatus

<figure><img src="image_10.png" /></figure>

### High pressure isotherms (0-60 bar)

* Homemade Manometric setup coupled with DIC [3]

<figure><img src="image_11.png" /></figure>

### High pressure isotherms (0-150 bar)

* Magnetic Suspension Balance (MSB)

### CO₂ experimental adsorption isotherms

<figure><img src="image_12.png" /></figure>

* High pressure CO₂ isotherms are measured for the first time for this material and a sudden "jump" is observed at pressures higher than 20 bar (temperature dependent).

* The "jump" does not correspond to a LS -> HS change as evidenced by no color change in the sample (example of LS -> HS change shown below).

* It is related to the sample as no jump observed when the sample is submitted to a long evacuation under primary vacuum.

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

Low spin
Mean = 189.35
STD = 17.38

High spin
Mean = 192.34
STD = 18.57

Color Histogram

### CH₄ experimental adsorption isotherms

<figure><img src="image_15.png" /></figure>

* No "jump" observed whatever the evacuation of the sample.

* Unlike CO₂, adsorption is reversible.

### Comparison and modeling of CO₂

#### and CH₄ isotherms at 30°C

<figure><img src="image_16.png" /></figure>

<table><thead><tr><th rowspan="2">q<sub>m</sub> (mmol/g)</th><th colspan="3">Sips Modeling Parameters</th></tr><tr><th>CH4 at 30°C</th><th>CO2 at 30°C (Before Jump)</th><th>CO2 at 30°C (After Jump)</th></tr></thead><tbody><tr><td>b (bar) - 1</td><td>0.33</td><td>0.84</td><td>0.98</td></tr><tr><td>n</td><td>1.12</td><td>1.17</td><td>0.93</td></tr></tbody></table>

## IAST MODELLING FOR CO₂/CH₄ SEPARATION

### Selectivity of CO₂/CH₄ at 1 bar and 30°C

<figure><img src="image_17.png" /></figure>

Comparison of CO₂/CH₄ Selectivity at 50% composition

<figure><img src="image_18.png" /></figure>

Sips model is a classic model used in the modeling of gas separation by adsorption processes.

Modeling of CO₂ is being divided into two parts : one before the observed jump, secondly after the observed jump (including

desorption curve). The selectivity of CO₂ upon CH₄ at low pressure (before the step) is approx. 7, while at high pressure (after

the step), the selectivity increased a great deal to approx. >30.

### Selectivity of CO₂/CH₄ at 30°C (before jump)

<figure><img src="image_19.png" /></figure>

### Selectivity of CO₂/CH₄ at 30°C (after jump)

<figure><img src="image_20.png" /></figure>

## CONCLUSIONS

1. The success of [FeNi(CN)₄Pyrazine] synthesis is confirmed by the results of the characterization.

2. The gas porosimetry results confirm the microporosity of the adsorbent while the irregular mesoporosity is merely due to the inter-grains (seen by the SEM images).

3. The complete removal of water molecules (at a temperature of 120°C, confirmed by TGA analysis) from the cavities of sample is necessary for the guest adsorption. A home-made manometric setup is built to analyze different gas adsorptions at high pressure and at several temperatures while simultaneously visualizing the sample by means of a camera placed near the sapphire window.

4. A pronounced jump is observed for the first time for CO₂ capture at high pressure. It can be attributed to changes in the framework structure while not being a classical spin-state change. The pressure of this structure change depends on temperature.

5. IAST modeling shows an impressive selectivity of CO₂ thanks to this change of structure.

## REFERENCES

[1] V. Niel, J. M. Martinez-Agudo, M. C. Muñoz, A. B. Gaspar, and J. A. Real, "Cooperative Spin Crossover Behavior in Cyanide-Bridged Fe(II)-M(II) Bimetallic 3D Hofmann-like Networks (M = Ni, Pd, and Pt)," *Inorg. Chem.*, vol. 40, no. 16, pp. 3838-3839, Jul. 2001, doi: 10.1021/ic010259y.

[2] P. D. Southon et al., "Dynamic Interplay between Spin-Crossover and Host-Guest Function in a Nanoporous Metal-Organic Framework Material," *J. Am. Chem. Soc.*, vol. 131, no. 31, pp. 10998-11009, Aug. 2009, doi: 10.1021/ja902187d.

[3] L. Perrier, F. Plantier, and D. Grégoire, "A novel experimental setup for simultaneous adsorption and induced deformation measurements in microporous materials," *Rev. Sci. Instrum.*, vol. 88, no. 3, p. 035104, Mar. 2017, doi: 10.1063/1.4977595.

E2S UPPA hub Newpores supported by the "Investissements d'Avenir" French programme managed by ANR (ANR-16- IDEX-0002) is acknowledged for the funding of this project and R. Minhas PhD grant. TEOT is acknowledged for financial support.