Total Oxidation of Isopropanol in the Liquid Phase, under Atmospheric Pressure and Low Temperature, on Transition Metal Oxides Catalysts Cr2O3 and Fe2O3

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.


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]. ey 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) [6][7][8][9]. e 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]. is makes the catalytic oxidation the most promising in terms of energy saving. e 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]. us, they can be very profitable for a complete oxidation of VOCs under these conditions. e aim of this work is to test the catalytic efficiency of Fe 2 O 3 and Cr 2 O 3 in complete oxidation of isopropanol in the liquid phase at low temperature. e 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.

Catalyst's Preparation.
e solids were prepared by the precipitation method. In this process, the nitrates of metals are used as precursors and ammonia (NH 4 OH) as precipitation agent. e dried powders were calcined at 500°C for 3 h.

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 Nifiltered CuK ɑ radiation (λ �1.5406Å). e 2θ angle was scanned between 4°and 30°range with a counting time of 2.0 s at steps of 0.02° [12].

N 2 Adsorption-Desorption Measurements.
ese 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].

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. e 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). e 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. e chromatographic analysis was conducted using a device (SHIMADZU CR15A) fitted with a column under the following conditions. Column temperature is 80°C, P N2 � 24.525 kPa, P H2 � 39.24 kPa, pair � 49.05 kPa, and carrier gas flow right: D � 40 mL·mn −1 .

Catalyst Characterization
3.1.1. X-Ray Diffraction. Samples of solids, Cr 2 O 3 and Fe 2 O 3 , calcined at 500°C for 3 hours have been analyzed by diffraction of X-rays. e diffractograms obtained are grouped in Figure 1. Figure 1 shows the X-ray diffraction patterns (XRD) of the prepared materials. e 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 Cr 2 O 3 [14]. e XRD iron oxide spectrum lets peaks appear corresponding to α-Fe 2 O 3 . ey were observed at 24 e obtained spectra are presented in Figure 2. Figure 2 corresponding to the calcined Cr 2 O 3 sample spectrum shows bands located at 956, 901, 833, 796, 563, and 500 cm −1 characterizing Cr 2 O 3 . It can also be noted the presence of other bands located at 2423 and 1386 cm −1 attributed to the vibration of CO 2 adsorbed form and which comes from the ambient air. On the same spectrum, we note the presence of a band in 1637 cm −1 . is band corresponds to the vibration of water's hydroxyl [16]. e Fe 2 O 3 spectrum lets a relatively strong band appear centered at 500, 630, 800, and 890 cm −1 . ese 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 CO 2 and hydroxyl of adsorbed water [15].

Adsorption-Desorption of N 2 -BET.
Pore volume distribution and nitrogen adsorption/desorption isotherms of the synthesized catalysts (Cr 2 O 3 and Fe 2 O 3 ) are illustrated in Figure 3. e isotherms are of type IV with an H3 hysteresis loop according to the IUPAC classification. e 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. e 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]. e 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. is implies particles with small diameter and therefore a better solid division state.

Catalytic Tests: Catalytic Oxidation of Isopropanol.
e 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. e tests without the catalyst in the same conditions of pressure and temperature showed the absence of any activity in homogenous phase [18].

Catalyst Amount Effect on the Specific Activity.
e specific activity is defined as the number of moles of isopropanol disappeared by unit time and unit mass of catalyst [11,19]: e 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. is means that under these conditions, the catalyst operates in chemical regime.

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 20   Journal of Chemistry operating conditions, make it possible to completely convert isopropanol into CO 2 , after a period of 60-110 minutes, depending on the catalyst and the reaction temperature. is 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]. e difference can be related to the solid's nature and its well-developed texture (Table 1).

Variation of TT ac and TT CO2 versus TT G .
TT g is the global conversion of isopropanol: TT G � (n 0 − n)/n 0 ; TT act � n ac /n 0 ; and TT CO2 � TT g − TT ac . n and n ac are, respectively, the moles number of isopropanol acetone at time t. n 0 is the moles number of isopropanol at time t � 0. Figure 6 curves represent evolution of acetone yield TT act and CO 2 yield TT CO2 versus TT G at all temperatures tested in case of Cr 2 O 3 catalyst [21]. ese 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. e secondary product formed is acetone, which passes throwà 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.
is implies the conversion of acetone into CO 2 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]:    isopropanol + oxidized catalyst ⟶ reduced catalyst Moreover, given the conversion of acetone into carbon dioxide over time, discussed above, this suggests a scheme of successive reaction as follows [24][25][26][27][28][29][30][31][32]:

Apparent Activation
Energy. e 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 (E a ) of the oxidation reaction. e specific activity can be expressed as a s � A exp (−E a /RT)· f, where A is the preexponential Arrhenius factor and f is a function of active constituents' concentrations and catalyst mass. en, at constant concentrations and variable temperature, a leaner expression of temperature inverse can be written as follows [18]:

Conclusion
e catalytic solids, Cr 2 O 3 and Fe 2 O 3, 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. e 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
e authors declare that they have no conflicts of interest.