Thermal decomposition course of copper acetate monohydrate was monitored by combining diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) coupled with μ gas chromatography-mass spectrometry ( μGC-MS) with other analytical techniques (thermogravimetry analysis and in situ X-ray diffraction). Non-isothermal kinetic was examined in air and Ar. A complete analysis of the evolution of infrared spectra matched with crystalline phase transition data during the course of reaction allows access to significant and accurate information about molecular dynamics. While thermogravimetry gives broad conclusion about two steps reaction (dehydration and decarboxylation), in line approach ( in situ X-ray and in situ DRIFT coupled to μGC-MS) is proposed as an example of a new robust and forward-looking analysis. While decomposition mechanism of copper acetate monohydrate is still not well elucidated yet previously, the present in-line characterization results lead to accurate data making the corresponding mechanism explicit.
Continuous reaction and analysis is a relevant approach for sustainable chemical process [
Metal carboxylates are useful reagents particularly for organic synthesis [
The commercial crystalline copper (II) acetate mono-hydrate Cu(Ac)2∙H2O (Ac=CH3COO) was an ACS product of reagent grade (>98%) and used without further purification.
The thermal decompositions of copper acetate were analyzed by various follow-up techniques: in-situ X-ray diffraction (XRD), thermogravimetry analysis (TGA), in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) coupled to micro gas chromatography (µGC) and mass spectroscopy (MS).
The X-ray diffraction of copper acetate monohydrate was performed using TUR-M62 diffractometer with copper anticathode (λ = 1.54 Å), 34 kV voltage and 25 mA current. The XRD patterns were acquired for 2θ angles ranging from 10˚ to 70˚, with 0.03˚ steps. The XRD measurements were conducted in temperature programmed mode. For this purpose, Bruker D8 Advance diffractometer equipped with temperature programmed chamber was used. The XRD patterns were collected at temperatures ranging from 30˚C to 400˚C with heating rate of 10˚C/min flowing air or Ar. After the heating, the solid decomposition products were analyzed at room temperature into the chamber. Corresponding diffractograms during the acetate decomposition were collected at 30˚C, 110˚C, 170˚C, 190˚C, 250˚C and 370˚C. Thermal decomposition of sample was carried out in a TGA thermogravimetry (series Q5000) of TA Instruments in a flowing air or argon atmosphere (10 ml∙min−1). Approximately, 2 - 3 mg of sample was heated in an open platinum crucible at a rate of 15˚C/min up to 400˚C, incorporating a controlled rate thermal analysis program. To confirm the feasibility of comparing sample results by different techniques, TGA of Cu(Ac)2∙H2O and Cu(Ac)2∙H2O in KBr were tested and gave almost the same result for the same temperatures.
The in-situ experiments have been performed in a Vertex 70 Bruker IR spectrometer equipped with a one-pot sample compartment. The in situ setup provides all necessary gas in and outlets for flow control and allows temperature measurements. Mass flows were regulated using mass flow controllers (Bronkhorst). For DRIFTS, we used a controlled temperature reaction home-made chamber (7 ml), smaller than high-temperature chamber from Harrick, fitted with ZnSe windows and a Praying Mantis diffuse reflection accessory (Harrick). Temperatures were directly measured in the center of the catalytic bed by mean of a 0.75 mm * 150 mm type K thermo-couple. The apparatus originality is due to its coupling mode: it is an IR spectrometer coupled with a mass spectrometer and gas phase chromatograph (μGC-MS). This allows measurements in situ during a reaction under controlled atmosphere. The flow of gas is recovered at the outlet of the chamber and is brought to through a transfer rod to the μGC-MS switchgear. The experiments are performed by a new coupled instrumentation obtained by miniaturized cell, a DRIFT spectroscopy, 3 micro gas chromatographs and a mass spectrometer. One of the present technique advantages is to allow data acquisition throughout the transformation on the solid material and on the produced gases simultaneously. The coupling with gas chromatography (gas molecule identification by mass spectrometry) makes possible to monitor continuously the emission of gas. These experimental steps are systematically carried out during the sample preparation for DRIFTS analysis. The QMS data shown here were corrected by the following m/z values obtained by calibration: CO2 gas (m/z = 44), H2O (m/z = 18), acetic acid (m/z = 60) and acetone (m/z = 43). The spectra are measured from 600 to 4000 cm−1 with a resolution of 4 cm−1 (15 spectra/second) and are directly collected by the computer. The reaction chamber is equipped with ZnSe windows and a gas flow of 10 cm3∙min−1 across the sample. The experiments are carried out from room temperature to 600˚C (15˚C∙min−1), to avoid heating during high temperature studies, the chamber is equipped with a cooled double wall of water flow to control temperature. Samples are prepared by dilution (10 wt%) with dry powder of potassium bromide (KBr) or zinc selenide (ZnSe) as a diluent. Different conditions of test are made to show the influence of the presence of water interaction with the diluent during the decomposition steps. Powders are milled together and then placed on the sample holder in the chamber. This cell is closed and purged with helium (10 mL/min) for 5 minutes, sufficient time to purge the small chamber. The desired gas is then introduced, with a purge of 5 minutes for each experimental conditions variation. All gases were used without further purification (He:Alphagaz 2, >99.9999%; Ar:Alphagaz 2, >99.9999%; synthetic air Air Products, >99.9999%). No reabsorption, especially water, has been observed by leaving KBr sample overnight under flow.
In situ XRD profiles of Cu(Ac)2∙H2O under heat treatment at 30˚C, 110˚C, 170˚C, 190˚C, 250˚C and 370˚C are showed by
TGA/TGD curves of Cu(Ac)2∙H2O are shown in
Step | Temperature range (˚C) | Mass change (%) | Theoretical mass change (%) | |
---|---|---|---|---|
Air | 1 | 100 - 170 | −9.0 | −9.0 |
2 | 230 - 310 | −53.5 | −55.1 | |
3 | 310 - 600 | + 1.6 | +4 | |
Ar | 1 | 80 - 140 | −8.9 | −9.0 |
2 | 200 - 275 | −68.6 | −65.3 |
to Cu(Ac)2 decomposition since the experimental value of mass loss under air for this step is consistent with the theoretical one (−55.1%) [
1) Cu(CH3COO)2∙H2O ® CuO under air;
2) Cu(CH3COO)2∙H2O ® Cu under Ar.
In this study, DRIFT analysis monitor phenomena occurring at the molecular scale during thermal decomposition reaction. To achieve this goal, main spectral regions were identified then we focused on a limited number of spectra for Cu(Ac)2∙H2O decomposition at specific temperature. The in situ DRIFT spectra of Cu(Ac)2∙H2O from room temperature to 410˚C in air and in He, are showed in
As shown in
DRIFT domain spectrum between 1150 and 1850 cm−1 contained characteristic peak of bidentate/bridged copper acetate bonds vibration mainly. During decomposition course, we observed intensity variation and shift position of peak intensities in that spectra region (
Wavenumber (cm−1) | Assignment |
---|---|
3585 | O?H stretching in acetic acid/CO2 gas |
3478 - 3373 | O?H stretching in water |
3000 [ | CH3 asymmetric stretch in acetone |
2989, 2942 | C?H stretching in methyl |
2360 | CO2 gas |
1715 [ | C=O stretching in acetone |
1722 [ | C=O stretching in acetic acid |
1608 | C=O asymmetric stretching |
1620 | O-H in H2O, |
1613 [ | Cu?H stretching in CH3-CuH− |
1580 - 1550 | C=O asymmetric stretching |
1573 [ | O-H stretching in chemisorbed H2O |
1450 - 1430 | C=O symmetric stretching |
1375 (1362) [ | C=O symmetric stretching |
1355 | C?H bending in methyl |
1243 [ | C-OH stretching in acetic acid |
1203 [ | CH3-Cu, CH3 deform |
1051 - 1033 | C?CH framework vibration |
944 | C?O stretching in acetic acid |
1012 [ | CH3-CuH, CH3 deform |
668 - 682 | CO2 gas |
648 [ | CH3-Cu, CH3 rock |
of hydrated Cu(Ac)2 (
Gaseous products as CO2, acetic acid, acetone and ethanol are also detected in the infrared chamber. These volatile products have been proposed in literature [
According to the quantification by injection pulses (
oxidized under air but the decomposition of acetate involves the creation of intermediate compounds which is mainly acetic acid under Ar (acetone and ethanol are formed at minor). According to DRIFT (
Scheme 1 (in annex section) describes mechanistic dynamic steps during thermal decomposition under air and Ar leading to CuO and Cu respectively as solid with emission of H2O, CO2, CH3COOH, H2 [
2Cu ( CH 3 COO ) 2 ⋅ H 2 O → 2Cu ( CH 3 COO ) 2 + 2H 2 O { 2 0 ˚ C - 17 0 ˚ C under air 2 0 ˚ C - 14 0 ˚ C under Ar (1)
According to TGA data, thermal decomposition of Cu(Ac)2∙H2O ends at 310˚C under air and 275˚C under Ar. Nevertheless, Cu(Ac)2∙H2O signal on XRD patterns falls before, in the range of 190˚C and 250˚C (before 190˚C under Ar) with appearance of broad peaks at 2θ ranging from 35˚ to 40˚ (
As mentioned by GC-MS data measurements (
2Cu ( CH 3 COO ) 2 → ( CuO ) 2 ( 1-CO ( CH 3 ) CH 2 ) 2 + 2CO 2 { 300 ˚ C - 480 ˚ C under air 25 0 ˚ C - 45 0 ˚ C under Ar (2)
Air media promotes complete oxidation of acetate ligands to CO2 and H2O (transition 5 ® 6’). We note an involving rate of adsorbed acetic acid on Cu2O (
The transition 2 ® 3’’ on Scheme 1 is proposed as one step decomposition of Cu(Ac)2∙xH2O (x ≠ 0) to acetic acid and Cu under Ar without CO2 desorption and copper oxide intermediates. H2O is essential for this transition that its equilibrium is rather shifted for compound 2.
A notable difference is identified for thermal decomposition under Ar after 300˚C compared to the same one under air. Acetic acid is detected by GC-MS (
To assess the influence of adsorbed water in mechanism reaction, two types of diluent were tested under Ar: KBr on one hand with hygroscopic property and ZnSe on the other hand. When the diluent is hygroscopic, C=O bands shifted during decomposition process (
Transition 7 ® 8 illustrates copper methyl formation (CuCH3) and adsorbed acetic acid after nucleophilic substitution by H2O. Concerning adsorbed acetic acid, DRIFT spectra shows (
Last steps of mechanism reaction are resumed by the following reactions equations system (3), (4) and (5) under air and Ar:
( CuO ) 2 ( 1-CO ( CH 3 ) CH 2 ) 2 → 2CuO + 6CO 2 + 5H 2 O 370 ˚ C - 600 ˚ C under air (3)
( CuO ) 2 ( 1-CO ( CH 3 ) CH 2 ) 2 → 2CH 3 COOH + 2Cu 2 O + 2C + 3H 2 30 0 ˚ C - 40 0 ˚ C under Ar (4)
2CH 3 COOH + 2Cu 2 O + 2C + 3H 2 → 2Cu + 1 2 CO 2 40 0 ˚ C - 60 0 ˚ C under Ar (5)
The present work deals with thermal decomposition of copper acetate monohydrate under air and Ar. A complete analysis of the evolution of infrared spectra matched with crystalline phase transition data allows to underline detailed steps for reaction mechanism. In-line µGC-MS results about chronologic desorption of major gaseous products (H2O, CO2 and CH3COOH) correlate favorably with DRIFT spectra observations and provide accurate information about intermediates structure of copper acetate during heat treatment. While TGA gives broad conclusion about two steps reaction (dehydration and decarboxylation), in line approach (in situ X-ray and in situ DRIFT coupled to µGC-MS) was used to assess H2O influence on reaction mechanism under Ar. In fact, H2O desorption is incomplete during dehydration step as defined by TGA and residual moieties are essential to conclude CH3COOH acetic acid formation, main gas product under Ar. The present in line approach is proposed as an example of a new robust and forward-looking analysis to highlight crucial information about reaction selectivity, conversion and product structure. This tool could be performed also to predict mechanism path way according to reaction media for a large numbre of products and precursors.
The authors declare no conflicts of interest regarding the publication of this paper.
Youssef, I., Sall, S., Dintzer, T., Labidi, S. and Petit, C. (2019) Forward Looking Analysis Approach to Assess Copper Acetate Thermal Decomposition Reaction Mechanism. American Journal of Analytical Chemistry, 10, 153-170. https://doi.org/10.4236/ajac.2019.105014