Isopropanol oxidation in the liquid phase under atmospheric pressure and low temperature has been studied in the presence of transition metal oxides (Cr2O3 and Fe2O3) prepared by the precipitation method. These solids characterized by structural analyses (FTIR and XRD) and textural analysis (BET) have led to results in line with those reported in the literature. Chromium oxide has a much more developed texture, with a specific surface area and pore volume 5 times larger than iron oxide. Both of the solids show a good specific activity and led to acetone and carbon dioxide to be formed as the only oxidation products of isopropanol. However, chromium oxide is more active. The initial catalytic activity for the latter is varying between 4.87  10−6 and 5.79  10−6 mol·g−1·s−1 with temperature range from 40 to 80°C. Kinetic study shows that the reaction follows a successive scheme: isopropanol ⟶ acetone ⟶ CO2 involving a redox mechanism. The low value of the activation apparent energy (Ea.(Cr2O3) = 2.87 kJ·mol−1 < Ea. (Fe2O3) = 5.37 kJ·mol−1) justifies the relatively higher activity observed for chromium oxide.

1. Introduction

Nowadays, the preservation of air quality has become a priority, as evidenced by the attention and hope to the Copenhagen Conference on climate (2009), which brings together 192 countries. Emissions of pollutants involved in air pollution include volatile organic compounds (VOCs) [1].

Volatile organic compounds are the major air pollutants containing at least the element carbon and one or more other items [2, 3]. They are characterized by their large volatility (the transition from the liquid state to the gaseous state takes place under normal temperature and pressure) [4, 5]. Several techniques exist for the reduction of these pollutants: restoratives, which present the advantage of recycling (adsorption, condensation, absorption, and membrane technique) or destructive (catalytic oxidation, thermal oxidation, and biological destruction) [69].

The choice of the technique to be used is based on criteria linking at the same time, parameters of pollutants, and environment. When VOCs have weak concentrations, particularly in the aquatic environment, total catalytic oxidation has the advantage to limit the amount of energy spent while working at low temperature and to reduce the cost of pollutant destruction [1, 10]. This makes the catalytic oxidation the most promising in terms of energy saving.

The oxides of transition metals, such as Cr, Fe, Ni, Mn, Cu, Co, Zn, are well known as typical catalysts for hydrocarbons oxidation at high temperature (200–500°C) in the gas phase or in the liquid phase under high pressure and temperature above 100°C, but they have never been cited in the literature for the same reactions in the liquid phase, under atmospheric pressure and low temperature, below 100°C [11]. Thus, they can be very profitable for a complete oxidation of VOCs under these conditions.

The aim of this work is to test the catalytic efficiency of Fe2O3 and Cr2O3 in complete oxidation of isopropanol in the liquid phase at low temperature. The choice of isopropanol in this study is based on the classification of the latter among VOCs. So, it can contribute to pollution of environment and must be eliminated from liquid rejects.

2. Experimental Section

2.1. Catalyst’s Preparation

The solids were prepared by the precipitation method. In this process, the nitrates of metals are used as precursors and ammonia (NH4OH) as precipitation agent. The dried powders were calcined at 500°C for 3 h.

2.2. Catalyst Characterization
2.2.1. X-Ray Diffraction

Patterns were recorded using an X’PERT MPD-PRO wide angle X-ray powder diffractometer provided with a diffracted beam monochromator and Ni-filtered CuK ɑ radiation (λ = 1.5406 Å). The 2θ angle was scanned between 4° and 30° range with a counting time of 2.0 s at steps of 0.02° [12].

2.2.2. Fourier Transform Infrared

Fourier transform infrared (FTIR) spectra were obtained using a device (Shimadzu, JASCO 4100), in the range of 400–4000 cm−1 [13].

2.2.3. N2 Adsorption-Desorption Measurements

These measurements were performed at −196°C, using a Micromeritics ASAP 2010. Specific surface area and the average pore diameter were determined according to the standard BET and BJH (Barrett, Joyner, and Halenda) methods, respectively [13].

2.3. Catalytic Tests

In a three-necked flask of 250 ml capacity, used as a reactor, 0.15 mL of isopropanol (reactive), 120 mg of catalysts, and 200 mL of distilled water are introduced successively. The temperature of water bath heating (reaction temperature) is adjusted to the desired value (40°C, 60°C, and 80°C) before plunging the reactor in the bath. Air (oxidizing reagent) is then admitted in the reactor using a lateral tubing under a flow of the order 40 mL/min (more than enough to make the solution constantly saturated with oxygen). The reaction mixture is stirred continuously for the duration of the test in order to facilitate the accessibility of catalysts’ grains to the reagent and to ensure the operation of the catalyst in chemical regime. It should be recalled that each catalytic test was performed with a new test portion of catalysts. The chromatographic analysis was conducted using a device (SHIMADZU CR15A) fitted with a column under the following conditions.

Column temperature is 80°C, PN2 = 24.525 kPa, PH2 = 39.24 kPa, pair = 49.05 kPa, and carrier gas flow right: D = 40 mL·mn−1.

3. Results and Discussion

3.1. Catalyst Characterization
3.1.1. X-Ray Diffraction

Samples of solids, Cr2O3 and Fe2O3, calcined at 500°C for 3 hours have been analyzed by diffraction of X-rays. The diffractograms obtained are grouped in Figure 1.

Figure 1 shows the X-ray diffraction patterns (XRD) of the prepared materials. The XRD diagram (Figure 1(a)) shows peaks at 2θ = 24.5, 33.5, 36.5, 41.5, 50.5, 54.9, and 65.5 corresponding, respectively, to the reticular planes (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), and (3 0 0) of the hexagonal structure of Cr2O3 [14]. The XRD iron oxide spectrum lets peaks appear corresponding to α-Fe2O3. They were observed at 24.1, 33.1, 35.6, 49.5, 54.1, 58.2, 62, and 64°, which corresponds, respectively, to reticular plans (012), (104), (110), (024), (116), (214), (118), and (441) of hematite [15].

3.1.2. FTIR: Fourier Transform Infrared Spectroscopy

Cr2O3 and Fe2O3 solids calcined at 500°C for 3 hours have been characterized by FTIR spectroscopy. The obtained spectra are presented in Figure 2.

Figure 2 corresponding to the calcined Cr2O3 sample spectrum shows bands located at 956, 901, 833, 796, 563, and 500 cm−1 characterizing Cr2O3. It can also be noted the presence of other bands located at 2423 and 1386 cm−1 attributed to the vibration of CO2 adsorbed form and which comes from the ambient air. On the same spectrum, we note the presence of a band in 1637 cm−1. This band corresponds to the vibration of water’s hydroxyl [16]. The Fe2O3 spectrum lets a relatively strong band appear centered at 500, 630, 800, and 890 cm−1. These bands are attributed to the vibrations of the hematite network. We also note the presence of other strips located at 1350, 1570, and 1650 cm−1, which can be attributed to the vibrations of CO2 and hydroxyl of adsorbed water [15].

3.1.3. Adsorption-Desorption of N2-BET

Pore volume distribution and nitrogen adsorption/desorption isotherms of the synthesized catalysts (Cr2O3 and Fe2O3) are illustrated in Figure 3. The isotherms are of type IV with an H3 hysteresis loop according to the IUPAC classification. The hysteresis loop was observed at a high relative pressure between 0.8 and 1, which indicates the presence of pores with large diameters on the prepared samples. The presence of hysteresis loop in the isotherms is associated with the secondary process of capillary condensation, which results in the complete filling of the mesopores at relative pressure P/P 0 below 1 [17]. The results are regrouped in Table 1.

It is important to note the great difference between the textural properties of the two solids. Chromium oxide has a much more developed texture, with a specific surface area 5 times larger than iron oxide, as attested by the results in Table 1, and the X-ray diffractogram (Figure 1), which shows a large continuous background and low intensity of diffraction peaks, indicating that the chromium oxide sample is not well crystallized. This implies particles with small diameter and therefore a better solid division state.

3.2. Catalytic Tests: Catalytic Oxidation of Isopropanol

The chromatograph must be calibrated by each reaction constituent in order to quantify them at any time during the reaction. For this, a calibration curve was traced for each constituent.

3.2.1. Blanck Test

The tests without the catalyst in the same conditions of pressure and temperature showed the absence of any activity in homogenous phase [18].

3.2.2. Catalyst Amount Effect on the Specific Activity

The specific activity is defined as the number of moles of isopropanol disappeared by unit time and unit mass of catalyst [11, 19]:

The influence of the catalyst amount on the specific activity is shown in Figure 4.

It appears that, at the chosen reaction temperature (T = 80°C), the initial specific activity relative to the disappearance of isopropanol keeps a constant value whatever the amount of catalyst. This means that under these conditions, the catalyst operates in chemical regime.

3.2.3. Evolution of Isopropanol Concentration over Time

Catalytic performance tests for the total oxidation of isopropanol were carried out at different temperatures, such as 40, 60, and 80°C, both on iron oxide and chromium oxide. Results obtained, represented in terms of moles number of isopropanol over time, are shown in Figures 5 and 6. It is important to note that these catalysts, used under the same operating conditions, make it possible to completely convert isopropanol into CO2, after a period of 60–110 minutes, depending on the catalyst and the reaction temperature. This time decreases with temperature increasing and at constant temperature, from iron oxide to chromium oxide; so, it is possible to conclude from analysis of Figure 5 curves that chromium oxide has a best potential for catalytic total oxidation of isopropanol. Indeed, whatever the reaction temperature, the specific activity is larger for chromium oxide (Table 2) [19, 20]. The difference can be related to the solid’s nature and its well-developed texture (Table 1).

3.3. Kinetics and Mechanism of the Reaction
3.3.1. Variation of TTac and TTCO2 versus TTG

TTg is the global conversion of isopropanol: TTG = (n0 − n)/n0; TTact = nac/n0; and TTCO2 = TTg − TTac. n and nac are, respectively, the moles number of isopropanol acetone at time t. n0 is the moles number of isopropanol at time t = 0.

Figure 6 curves represent evolution of acetone yield TTact and CO2 yield TTCO2 versus TTG at all temperatures tested in case of Cr2O3 catalyst [21].

These curves show that oxidation of isopropanol into carbon dioxide is complete at all working temperatures 40, 60, and 80°C for a reaction time not exceeding 110 min. The secondary product formed is acetone, which passes throw à maximum for all temperatures. It should be noted that the remarkable selectivity initial extrapolated to time 0 (100%), which falls to 0 when conversion is close to unity. This implies the conversion of acetone into CO2 over time, verified in this study. Furthermore, the absence of isopropylene as reaction product indicates the absence of acidic sites at a solid surface. It seems, taking account of all these findings, that isopropanol oxidation takes place according to a redox mechanism [22], which alternates catalyst oxidation and reduction steps as a general scheme [23]:

Moreover, given the conversion of acetone into carbon dioxide over time, discussed above, this suggests a scheme of successive reaction as follows [2432]:

3.4. Apparent Activation Energy

The conversion evolution curves of isopropanol in time, at different temperatures, obtained previously, can be used to determine kinetic parameters, such as the constant speed k and apparent activation energy (Ea) of the oxidation reaction.

The specific activity can be expressed as as = A exp (−Ea/RT)·f, where A is the preexponential Arrhenius factor and f is a function of active constituents’ concentrations and catalyst mass.

Then, at constant concentrations and variable temperature, a leaner expression of temperature inverse can be written as follows [18]:

Figure 7 represents ln (as) = f (1/T) right. The slope of this line led to a value of activation apparent energies: 5.32 kJ·mol−1 in case of iron oxide and 2.87 kJ·mol−1 for chromium oxide. These low values explain the relatively high values of activity observed (Table 2). The comparison of the different values of the apparent activation energies relating to disappearance of isopropanol for the two catalysts studied shows that Ea (Cr2O3) < Ea (Fe2O3) [33].

4. Conclusion

The catalytic solids, Cr2O3 and Fe2O3, were prepared by the precipitation method and characterized by X-ray diffraction and infrared spectroscopy; their structure matches well with those mentioned in the literature and present a good texture.

Catalytic tests conducted in the presence of these solids, in the mild temperature and pressure conditions, show a relatively high catalytic activity with respect to the total oxidation of isopropanol and a good stability over time during catalytic tests.

The two solids accuse a similar comportment catalytic with a great catalytic activity for chromium oxide related to its best texture.

Furthermore, kinetic of catalytic oxidation in the liquid phase, carried out isopropanol, showed that this reaction follows a successive mechanism: whatever the catalyst used. It should also be noted that the low value of the apparent activation energy, associated with the disappearance of isopropanol, in line with a value of the catalytic activity, is sufficiently high, in the field of temperature studied.

In sum, based on all these results, it can be concluded that oxides of chromium and iron can be used in isopropanol removal from waste water by a process involving total catalytic oxidation, which can be extended to other light alcohols.

Data Availability

All data underlying the finding of this study are fully available without restriction.

Conflicts of Interest

The authors declare that they have no conflicts of interest.