Acetonitrile Combustion over Copper-Based Nanocatalysts: A Structure-Performance Relationship Study

Jéssica A. P. Ponciano , Marcelo S. Batista ^*

Federal University of São João del Rei, Campus Alto Paraopeba, highway MG 443, Km 5, Ouro Branco–MG 36420-000, Brazil

* Correspondence: Marcelo S. Batista

Academic Editor: Md Ariful Ahsan

Special Issue: Applications of Environmental Catalysis

Received: December 01, 2021 | Accepted: January 20, 2022 | Published: January 26, 2022

Catalysis Research 2022, Volume 2, Issue 1, doi:10.21926/cr.2201002

Recommended citation: Ponciano JAP, Batista MS. Acetonitrile Combustion over Copper-Based Nanocatalysts: A Structure-Performance Relationship Study. Catalysis Research 2022;2(1):12; doi:10.21926/cr.2201002.

© 2022 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

Acetonitrile (CH₃CN) is widely used as an industrial solvent, but it also finds applications in the production of acrylic plastic, acrylic fibers, resins, nitrile elastomers, and appears in numerous hazardous waste streams. Acetonitrile found in industrial exhaust gases is dangerous to humans and the environment; therefore, its abatement is extremely important [1,2,3]. Treatment of acetonitrile by traditional thermal combustion technique is controlled by chain-terminating reaction sequences at elevated temperatures (above 1000 °C) and often leads to the formation of undesirably large amounts of nitrogen oxides and HCN [4]. According to earlier studies [3,5,6,7], the selective catalytic combustion (SCC) of acetonitrile toward N₂ and CO₂ is the most efficient and environmentally benign route, owing to its high purification efficiency, low operation cost at lower temperatures, and fewer secondary pollutants.

Zhang’s group [8,9] studied the combustion of acrylonitrile and acetonitrile over various metals (M = Cu, Co, Fe, V, Mn, Pd, Ag, Pt), and reported good performance of copper-modified mesoporous material. Cu/SBA-15 exhibited a nearly complete conversion of acetonitrile and acrylonitrile together with achieving a good N₂ selectivity of 80% (>350 °C) and 64% (>400 °C), respectively. Nanba et al. [10] studied the SCC using catalysts of various metals (Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Ga, Pd, Ag, and Pt) supported on several frameworks (Al₂O₃, SiO₂, TiO₂, ZrO₂, MgO and ZSM-5). Among them, Cu-ZSM-5 achieved full conversion at above 350 °C associated with ∼80% N₂ selectivity. Although copper is considered an interesting transition metal for SCC, its conversion and N₂ selectivity depend very much on the catalyst structure.

La₂CuO₄ is a perovskite-like type mixed oxide and is an interesting catalyst in terms of its reactivity; it has versatility in both oxidation and reduction reactions. It has been demonstrated that NO-SCR and acrylonitrile-SCC performance, along with N₂ selectivity, is improved by the incorporation of Cu²⁺ into the lattice of perovskites [11,12]. The best performance was attributed to the redox properties inherent in the perovskite-type oxides. These features of perovskite-type materials inspired us to believe that these materials, especially using Cu²⁺, could offer promising performances in acetonitrile catalytic combustion.

The literature reports some studies discussing the performance of catalysts prepared via impregnation for the acrylonitrile-SCC [3,13,14] and rare works related to acetonitrile-SCC [7,8]. In terms of the zeolite topology, BETA was the most effective material among various commercial zeolites (MFI, FER, MOR, FAU) with similar Si/Al ratios for the decomposition of N₂O [15] and exhibited superior activity in comparison with ZSM-5 [16]. Recently, the influence of different active center structures over Cu-BETA was investigated for HCN-SCC [17]. DFT simulation results revealed that the geometry of the active centers greatly influences the final activity, selectivity, and even reaction mechanism. Therefore, in this study, to the best of our knowledge, a relationship was established between the activity and structure of Cu²⁺ in a different chemical environment using Cu-BETA, La₂CuO_4, and CuO catalysts, which were systematically investigated for acetonitrile-SCC. Various characterizations, including X-ray fluorescence (XRF), X-ray diffraction (XRD), temperature-programmed reduction by hydrogen (H₂-TPR), and UV-VIS spectroscopy, were employed.

2. Materials and Methods

2.1 Catalyst Preparation

The Cu-BETA catalyst was prepared by three consecutive ion-exchange reactions employing a commercial NH₄-BETA zeolite (BETA Products Standard from Tricat, SiO₂/Al₂O₃ molar ratio of 24.5) and an aqueous solution of cupric nitrate at room temperature for 12 h. After each step, the solid was filtered, washed with distilled water, and dried at 110 °C. Finally, the Cu-BETA catalyst was obtained after final calcination at 650 °C for 2 h. CuO catalyst was prepared by calcination of cupric nitrate at 650 °C for 2 h at the rate of 10 °C/min.

La₂CuO₄ catalyst was prepared by the polymerization method (also known as Citrate or Pechini). Copper and lanthanum nitrates were used as the initial reactants for the synthesis of perovskite-type oxide. These nitrates contain cations in stoichiometric quantity. They were dissolved in water, followed by the addition of citric acid and ethylene glycol in equimolar amounts. The mixture was stirred and the excess water was evaporated at 80 °C. A resin-like material was formed after being dried at 110 °C for overnight. The material was deagglomerated and calcined at 550 °C for 3 h and 800 °C for 5 h (heating rate 5 °C/min) in static air.

2.2 Catalyst Characterization

The catalysts were characterized using a couple of techniques, including X-ray fluorescence (XRF), X-ray diffractometry (XRD), temperature-programmed reduction by hydrogen (H₂-TPR), and UV-VIS spectroscopy. XRF analyses were carried out in the equipment Shimadzu EDX 720/800HS with 200 mg of catalyst and a flow of 200 mL/min of He. XRD analyses were performed by adopting the powder method using a Rigaku diffractometer (Miniflex 600) with Cu tube, Ni-filtered, operating at 40 kV, 15 mA, and employing CuKα radiation. The speed of the goniometer was 2°(2θ)/min; the 2θ angle ranges between 5 and 80°. The average particle size was calculated using the half-width at half-height of the most intense peak of CuO (2θ = 38.7°) and the Debye-Scherrer equation (D_hk1 = Kλ/βcos(θ)).

H₂-TPR analyses were performed on SAMP3 apparatus (Termolab Equipment, Brazil) equipped with a thermal conductivity detector (TCD). A trap was used to remove the water stemming due to reduction before the gas of the reactor outlet was sent to the TCD. TPR started with a ramp of 10 °C/min from 50 to 1000 °C. A flow rate of 30 mL/min from a high purity mixture of 2 vol.% H₂ in Ar and 100 mg of Cu-BETA, 50 mg La₂CuO_4, and 10 mg CuO were used.

The diffuse reflectance spectra were recorded employing a UV–VIS spectrophotometer (Shimadzu UV 2700). BaSO₄ was used as the reference material. Before the measurement, the samples were dried at 120 °C for 2 h to remove any water molecules or OH groups. Samples were scanned in the range of 200–800 nm. The reflectance data were converted in terms of the Schuster-Kubella-Munk function, F(R) = (1-R)²/2R, where R represents the diffuse reflectance obtained directly from the spectrometer.

2.3 Catalytic Evaluation

The catalysts were evaluated in the selective combustion reaction of acetonitrile (CH₃CN-SCC) using a quartz wool bed "U" reactor. The sample contained 8.6 mg of copper fed with a continuous flow of 40 mL/min of a mixture containing 2.8 vol.% CH₃CN in synthetic air. The reactor operated at atmospheric pressure, and the reaction temperature varied from 100 to 600 °C. The reactor was coupled in line with a mass spectrometer (THERMO) for gas analysis: N₂ (m/z = 28), O₂ (m/z = 32 and 16), NO + N₂O (m/z = 30), NH₃ (m/z = 17), HCN (m/z = 27), NO₂ (m/z = 46), CH₃CN (m/z = 41) and CO₂ (m/z = 44). Conversion of CH₃CN was calculated using Equation 1. The turnover frequency (TOF) was calculated in terms of the mole of reagent consumed per unit time and mole of copper.

\[ \text { Conversion }(\%)=\left(\frac{\mathrm{CH}_{3} \mathrm{CN}_{(\mathrm{in})}-\mathrm{CH}_{3} \mathrm{CN}_{(\text {out })}}{\mathrm{CH}_{3} \mathrm{CN}_{(\mathrm{in})}}\right) \times 100 \tag{1} \]

3. Results and Discussion

Crystalline phases obtained after synthesis were identified by XRD analysis. Figure 1 depicts the diffractogram of Cu-BETA, CuO, and La₂CuO₄ catalysts. The diffractogram of CuO catalyst exhibited the characteristic peaks of copper oxide (PDF#41–0254). Cu-BETA presented diffraction peaks corresponding to zeolite BETA (PDF#48–0074). In addition, Cu-BETA also showed peaks corresponding to CuO (2θ= 35.5, 38.7, and 48.7°), indicating that copper species underwent precipitation during the ion exchange reaction. La₂CuO₄ showed reflection lines for orthorhombic perovskite structure (PDF#70–0449) and also for CuO, suggesting that the copper oxide was formed together with La₂CuO₄. The relative amount of perovskite (87%) and copper oxide (13%) phases present in the samples was determined using the integrated intensity of the most intense peak of the corresponding phases [18,19]. No evidence of La₂O₃ peaks (PDF#02–0688) or Cu₂O peaks (PDF#35–1091) could be gathered by XRD. These results are in good agreement with the literature data [12,18], suggesting that after calcination, a small amount of CuO is observed along with the La₂CuO₄ phase. However, the existence of tiny La₂O₃ or Cu₂O species cannot be excluded as their quantity present in the sample might be below the XRD detection limit.

Figure 1 XRD patterns of CuO, Cu-BETA, and La₂CuO_4, in which the symbol (o) indicates the peaks characteristic of copper oxide, (*) BETA zeolite, and (■) La₂CuO₄.

Table 1 shows the average crystallite size of CuO, Cu-BETA, and La₂CuO₄ catalysts. The crystallite size was determined from the XRD data of the samples using the Scherrer equation. The average crystallite size for CuO, La₂CuO_4, and Cu-BETA catalysts is estimated to lie between 15 and 20 nm. Note that Cu-BETA contains a small crystallite size of CuO formed over the large surface area of BETA zeolite (~ 600 m²/g). This resulted in more Cu²⁺ cations on the surface and exhibited better redox properties. On the other hand, the average crystallite size of CuO and La₂CuO₄ catalysts was 20 nm. It is a known fact that larger CuO particles are reduced at higher temperatures [20]. Therefore, it is interesting to compare the reducibility of these copper-containing catalysts.

Table 1 Chemical composition, crystallite size, and reduction degree of the catalysts.

H₂-TPR profiles of the CuO, La₂CuO_4, and Cu-BETA catalysts are presented in Figure 2. For all samples, the total hydrogen consumption showed H₂/Cu molar ratio close to one (Table 1), which is in agreement with the theoretical value corresponding to the reduction of Cu²⁺ to Cu⁰ (H₂/Cu = 1). The color change from dark blue or black to red-purple after TPR analysis also confirms that complete reduction of Cu²⁺ to Cu° has occurred. Cu-BETA showed peaks at 229 and 356 °C. The peak at 229 °C corresponds to the reduction of dispersed Cu²⁺ to Cu⁺ and of CuO nanoparticles (observed by XRD) to Cu⁰ [10,14,21,22,23]. The peak at 356 °C can be attributed to the successive reduction of Cu⁺ to Cu⁰ [21,22,23]. Thus it can be suggested that Cu-BETA contains both dispersed Cu²⁺ ions and CuO nanoparticles after preparation.

Figure 2 H₂-TPR profiles of the CuO, Cu-BETA, and La₂CuO₄ catalysts.

The data given in Table 1 shows that the copper content of Cu-BETA (19.7 wt.%) exceeds the entire ion exchange capacity of the BETA zeolite (3.9 wt.%), with the assumption that each Cu²⁺ cation needs two ionic exchange sites. Thus, considering the areas under the peaks at 229 °C (A1) and 356 °C (A2) in Figure 2, the distribution of the exchangeable Cu-species (%Cu²⁺ = 2 × (A2/A1+A2) × 19.7) and CuO (%CuO = ((A1-A2)/(A1+A2)) × 19.7) for Cu-BETA (3.0 wt.% of Cu²⁺ and 16.70 wt.% of CuO) can be estimated.

As shown in Figure 2, the reduction of CuO corresponds to a single reduction peak at 296 °C. TPR studies of the reduction of bulk CuO have typically shown a direct transformation to Cu°. The reduction of bulk copper species requires higher temperature in comparison to surface copper species [24]. La₂CuO₄ shows two main reduction peaks: the first peak appears at 287 °C together with a slight shoulder (at 333 °C) assigned to reduction of Cu²⁺ (present in CuO and La₂CuO₄) to Cu⁰; another reduction peak at 401 °C is due to the reduction of Cu⁺ to Cu⁰, which was not reduced in the previous step [25,26].

Further information on the copper species present in the CuO, La₂CuO_4, and Cu-BETA catalysts was studied by UV-VIS spectroscopy, as shown in Figure 3. Cu-BETA shows absorption bands located at 217 and 255 nm corresponding to charge transfer (CT) transition from the oxygen atoms to exchanged Cu²⁺ in a single site on the zeolite structure [27]. There are also broad absorptions observed in the region of d–d transitions at 400–800 nm corresponding to the octahedral Cu²⁺ in the crystalline structure of CuO [28,29]. According to the intensity of relative bands, the weak band at about 450 nm characteristic of (Cu-O-Cu)²⁺ or di-copper species indicates the prevailing existence of Cu²⁺, which compensates for the negative charge of the zeolite framework [27,30,31]. These results are consistent with the XRD and TPR measurements done on these catalysts. As for the UV–VIS spectra of CuO and La₂CuO₄ catalysts (Figure 3), the shapes of curves are similar to each other, with some minor differences in absorbance. The similarity of the UV–VIS spectra may indicate the presence of the same type of Cu-O bonds and crystallite sizes (see Table 1) among all these catalysts [32]. Figure 3 shows absorption bands located at 330 and 682 nm related to the d-d electronic transitions of copper in CuO and La₂CuO₄ catalysts. A small band at 580 nm can be assigned to Cu²⁺…Cu⁺ intervalence transition (IVT) [28,33].

Figure 3 UV-VIS spectra of Cu-BETA, CuO, and La₂CuO₄ catalysts.

In summary, the UV-VIS results show that exchanged and octahedral species of Cu²⁺ coexist in Cu-BETA, while octahedral Cu²⁺ is the predominant species in both CuO and La₂CuO₄ catalysts. Exchanged copper in zeolites has more accentuated CT bands compared to CuO and La₂CuO₄ (Figure 3). These dispersed single Cu²⁺ occupying cationic sites and CuO nanoparticles could also be detected in Cu-zeolites [34].

Figure 4 shows the CH₃CN conversions on Cu-BETA, CuO, and La₂CuO₄ catalysts as a function of reaction temperature. BETA zeolite without copper has practically no activity under the studied conditions (not shown). The copper species are active sites for the CH₃CN-SCC reaction, and the comparison of the catalysts was performed with the same mass of copper introduced into the reactor. It was found that the CH₃CN initial conversion rises rapidly upon increasing the temperature and attains a stable state at a temperature range of 400–600 °C for all catalysts. Cu-BETA has well-dispersed Cu-species, as confirmed by H₂-TPR and UV-VIS analyses, leading to much higher CH₃CN conversion. This attributes to the higher activity of Cu-BETA catalysts than CuO and La₂CuO₄ catalysts, showing that copper species on the zeolite surface are more active due to the lower activation energy barrier for the CH₃CN-SCC reaction. It has been reported that well-dispersed CuO species exhibit much higher oxidation activity and reducibility compared to the exchanged Cu ions [34]. Thus, the Cu-O bonding as it occurs in CuO dispersed over BETA zeolite is the reason for its easy reducibility and high oxidation catalytic activity. As already discussed, the UV-VIS results showed that octahedral Cu²⁺ species are dominant in both CuO and La₂CuO₄ catalysts (Figure 3). However, octahedral Cu²⁺ species in CuO undergo reduction at a lower temperature than La₂CuO₄ (Figure 2). This greater ease of reduction is a favorable factor that can contribute to the improvement of catalytic performances [35,36,37,38]. The oxy-reduction properties of the catalysts play a significant role in the CH₃CN-SCC reaction. According to the aforementioned results, the Cu-BETA catalyst had a lower reduction temperature and hence higher CH₃CN conversion rate.

Figure 4 Acetonitrile conversion over Cu-BETA, La₂CuO_4, and CuO catalysts using 2.8 vol.% CH₃CN in airflow (40 mL/min) and 8.6 mg of copper in the sample.

Apart from the high conversion rate of CH₃CN, the high yield of N₂ and CO₂ is another important criterion for choosing the ideal catalyst considering the possibility of releasing other undesirable by-products (NH₃, N₂O, HCN, NO, and NO₂) during CH₃CN combustion. Figure 5 shows the measure of N-containing products (N₂, NH₃, N₂O, HCN, NO, and NO₂) on Cu-BETA, CuO, and La₂CuO₄ catalysts as a function of reaction temperature. It is worth noting that acetonitrile was oxidized directly to CO₂ by all the catalysts. In addition, CH₃CN was largely transformed into N₂ and other undesirable by-products (N₂O and NO) above 320 °C and 360 °C over CuO and La₂CuO₄ catalysts, respectively. The relatively greater amount of N₂O could be tackled up to 550 °C followed by slightly declining temperatures for CuO (Figure 5a) and La₂CuO₄ (Figure 5b). These undesirable by-products and the lower N₂ yield observed for the CuO and La₂CuO₄ catalysts are very similar to the reported outcome of CuO supported on the SBA-15 catalyst [8]. Therefore, CH₃CN was largely transformed into N₂ and undesirable by-products (N₂O + NO) on octahedral Cu²⁺ species present in CuO and La₂CuO₄ catalysts (Figure 3). On the other hand, Cu-BETA (Figure 5c) showed the highest N₂ yield, which started from 300 °C, increasing progressively with the reaction temperature, and almost remaining stable at a temperature range of 400–600 °C. Thus, the easy reducibility of the highly dispersed Cu-species and the small crystallite size contributed to the better performance of the Cu-BETA catalyst. The discussion suggests that the high conversion of acetonitrile and the highest N₂ yield make Cu-BETA an excellent candidate for catalyzing CH₃CN-SCC.

Figure 5 Catalytic performance as a function of temperature during CH₃CN combustion over: (a) CuO; (b) La₂CuO₄; (c) Cu-BETA. Conditions: 2.8% CH₃CN, 20.4% O₂, 8.6 mg of copper in the sample.

Moreover, the stability test of Cu-BETA was performed at 600 °C (Figure 6). It could be seen that Cu-BETA showed a very stable conversion rate (nearly 100%) with a 10 h reaction time (Figure 6a), along with maintaining the desired high yield of N₂ and CO₂ throughout CH₃CN combustion (Figure 6b). Such results affirmed that the Cu-BETA catalyst maintained excellent catalytic activity and N₂ selectivity. Results from the present study could surpass other works reported in the literature [7,8,39], where the N₂ selectivity decreases as the reaction proceeds [40]. Cu-BETA also showed exceptionally high turnover frequencies (TOF = 5.3–11.7 × 10^–4 s^–1) at a temperature range of 300–600 °C compared to CuO (TOF = 0.5–2.3 × 10^–4 s^–1) and La₂CuO₄ (TOF = 0–7.8 × 10^–4 s^–1) catalysts. These results show the influence of the catalyst structure on acetonitrile combustion and N₂ selectivity.

Figure 6 Stability curves of (a) catalytic oxidation of acetonitrile over the Cu-BETA catalyst at 600 °C and (b) measure of products during the reaction.

4. Conclusions

Copper-based nanocatalysts were used for acetonitrile combustion and were subjected to the formation of N-containing products. According to the study, Cu-BETA has both exchanged and octahedral species, while octahedral species are predominant in CuO and La₂CuO₄. CuO, Cu-BETA, and La₂CuO₄ catalysts achieved high conversion rates of acetonitrile, which increased rapidly with temperature. However, CuO and La₂CuO₄ nanocatalysts resulted in the formation of undesired products, N₂O and NO. On the other hand, Cu-BETA nanocatalysts showed excellent activity and high N₂ yield in CH₃CN combustion. The easy reducibility of the highly dispersed Cu-species and its small crystallite size primarily enhance the oxidation activity of acetonitrile as well as N₂ yield. Furthermore, Cu-BETA has shown exceptional stability. Therefore, the high conversion rate of acetonitrile and the high N₂ selectivity make Cu-BETA a potent candidate for C₃CN-SCC.

Acknowledgments

We are grateful to the Prof. Luiz Carlos A. Oliveira for collaboration with UV-VIS spectroscopy analysis.

Author Contributions

Mrs. Jéssica A. P. Ponciano performed experiments and analyses. Prof. Marcelo S. Batista supervised the research.

Funding

We are grateful to the FAPEMIG for the scholarships.

Competing Interests

The authors have declared that no competing interests exist.