CO2 utilization as a soft oxidant for the synthesis of styrene from ethylbenzene over Co3O4 supported on magnesium aluminate spinel: role of spinel activation temperature

Magnesium aluminate spinel (MgAl2O4) supported Co3O4 catalysts are synthesized and tested for the oxidative dehydrogenation (ODH) of ethylbenzene using CO2 as a soft oxidant. The effect of spinel calcination temperature on the catalytic performance has been systematically investigated. With an increase in the activation temperature from 600 to 900 °C, the active presence of a single-phase MgAl2O4 spinel is observed. A catalyst series consisting of MgAl2O4 spinel with varying Co loadings (10–20 wt%) were prepared and systematically distinguished by ICP, XRD, BET, TPR, NH3-TPD, UV–Vis DRS, FT-IR, XPS, SEM, and TEM. Among the tested cobalt catalysts, 15Co/800MA sample derived by calcination of MgAl2O4 support at 800 °C exhibits the most excellent catalytic performance with the maximum ethylbenzene conversion (≥ 82%). Also, high yields of styrene (≥ 81%) could be consistently achieved on the same active catalyst. Further, the catalyst exhibited almost stable activity during 20 h time-on-stream with a slow decrease in the ethylbenzene conversion from 82 to 59%. However, the selectivity of styrene (98%) stayed almost constant during the reaction. Activation of the MgAl2O4 spinel at 800 °C facilitates a dramatic chemical homogeneity for the alignment of Co3O4 nanoparticles on the surface of the active catalyst. Moreover, the isolated Co3O4 clusters have a strong chemical/electronic interaction with the Mg2+ and Al3+ ions on the support perform a crucial role to achieve the maximum catalytic activity.


Experimental
Catalyst preparation. Magnesium aluminate spinel (MgAl 2 O 4 ) with Mg/Al molar ratio of 1:2 was prepared by the co-precipitation method at a p H of 8.9-9.0, using 5% NH 4 OH solution as a precipitating agent. The precipitate obtained was filtered, gently washed with high purity millipore water followed by oven drying at 100 °C for 12 h. The obtained solid was calcined at different temperatures like 600, 700, 800, and 900 °C for 5 h labeled as 600MA, 700MA, 800MA, and 900MA respectively. A series of cobalt catalysts are prepared by the impregnation method using MgAl 2 O 4 as support and cobalt nitrate as a metal precursor. The catalysts are calcined at 600 °C for 5 h with a heat ramp of 10 °C/min, designated as x Co 3  Characterization of catalyst samples. The XRD experiments for all catalyst samples are carried out in an ULTIMA-IV X-Ray Diffractometer (M/s. Rigaku Instruments, Japan) with Cu Kα monochromatic radiation operating at 40 kV voltage and a current of 30 mA. The crystalline phase of catalysts is determined by continuous scan mode with the 2θ range of 10°-80° at a sampling pitch of 0.07 and 4° min −1 scan rate. Reduction patterns of calcined catalysts are investigated by TPR on a self-made reactor system 19 . Typically, 50 mg of the catalyst is loaded in a small quartz reactor and placed in the micro furnace. The reactor is heated linearly up to 800 °C at a ramping rate of 10 °C min −1 while flowing 5%H 2 /Ar gas (30 mL/min). After reaching the maximum temperature, the specimen is maintained under isothermal conditions for another 60 min. The amount of H 2 consumption is monitored by online TCD in-built GC (M/s. CIC Instruments, India) 19 . The amount of cobalt content present in the catalysts was accurately estimated using iCAP 6500 duo ICP-OES Analyzer (M/s. Thermo Fisher Scientific, USA). Before the ICP analysis, the specific cobalt catalyst was dissolved in a stock solution of aquaregia.
Scientific Reports | (2020) 10:22170 | https://doi.org/10.1038/s41598-020-79188-z www.nature.com/scientificreports/ FTIR spectra of the samples were recorded on a spectrum-GX spectrometer in the scan range of 4000 to 400 cm −1 . UV-Vis diffused reflectance spectra (DRS) of samples are investigated on a UV Winlab spectrometer in the UV-Vis region of 200-800 nm. BET principle is employed to investigate the specific surface area of all the catalysts. Before the N 2 sorption analysis, all samples are degassed under vacuum at 350 °C for 4 h to eliminate physically absorbed gases and moisture. The surface morphology of samples is explored by high-resolution TEM (TECHNAI G2, Japan), operated at an acceleration voltage of 200 kV. A carbon-coated grid is used to disperse the powder which has been mixed with ethanol under sonication. The elementary composition is identified by EDX analysis from the emitted X-rays of the specimen during TEM analysis.
The coke content present in the used catalysts is estimated by TG analysis on TA instruments Q-500 using alumina sample holder heated from 30 to 800 °C at a ramping of 5 °C min −1 under O 2 flow. The NH 3 -TPD studies are performed using ASAP 2020 (M/s. Micrometrics, USA). Typically, 5%NH 3 balanced He gas is adsorbed at 100 °C for 30 min, then purged with ultra-pure He (20 mL/min) to eliminate the physisorbed water and other gases. The TPD patterns were recorded by desorption of NH 3 from 100 to 800 °C at a heat ramp of 5 °C min −130 . The XPS analysis is carried out on a Thermo Scientific K-Alpha Spectrometer operated with Al Kα as X-ray radiation (E photon = 1486.7 eV) at room temperature. The B.E. values of all the elements are accurately calculated by adopting C 1s as a standard reference peak.

Results and discussion
Powder X-ray diffraction. The crystallographic structure and active phase of cobalt catalysts with different calcination temperatures of MgAl 2 O 4 are typically displayed in Fig. 1. The direct XRD reflections corresponding to the lattice planes at (111), (220), (311), (222), (400), (422), (511), and (440) confirm the active presence of single-phase MgAl 2 O 4 spinel (JCPDS No # 04-008-1061) 31 . As shown in Fig. 1a, the peak intensity of the MgAl 2 O 4 spinel increases linearly with the gradual rise in calcination temperature, eventually, noticeable growth in crystallite size is typically observed. The 600MA sample exhibited very low-intensity broad peaks due to the partial formation of the MgAl 2 O 4 spinel phase along with γ-Al 2 O 3 active phase. The MgO formed by the decomposition of Mg(OH) 2 , reacts instantly with Al(OH) 3 to form MgAl 2 O 4 spinel since the Al(OH) 3 phase is stable up to 600 °C 32 . Conversely, the 700MA sample represents significant growth in the signal intensity, and the unique formation of the MgAl 2 O 4 spinel phase is observed 33 . It should be carefully noted that when the activation temperature is 800 °C (800MA), a highly pure MgAl 2 O 4 spinel phase was typically formed. Consequently, a high calcination temperature of 900 °C promoted drastic acceleration in the crystallite size due to severe sintering of MgAl 2 O 4 spinel particles and typically exhibited an extremely sharp reflection with strong intensity. In summary, calcination in the temperature between 700 to 900 °C is typically preferred for the proper formation of single-phase MgAl 2 O 4 spinels.
The XRD patterns of MgAl 2 O 4 supported Co 3 O 4 catalysts are as shown in Fig. 1b. All the diffractions can be assigned to Co 3 O 4 (JCPDS No # 00-042-1467) with lattice planes of (111), (220), (311), (222), (400), (422), (511), and (440) respectively 34 . No other reflections are observed, indicating the presence of an active phase with high purity. However, the diffraction lines of Co 3 O 4 and MgAl 2 O 4 are not well-resolved which can be attributed to the similar 2θ values of the corresponding active phases 33 . Besides, 15Co/600MA, 15Co/700MA, and 15Co/800MA catalysts display XRD lines with similar intensity while the 15Co/900MA catalyst maintains extraordinary intensity. The spent 15Co/800MA catalyst displayed intact XRD peaks as observed for fresh catalysts even after several hours of activity studies (Fig. 1b). Also, the intensity of direct XRD reflections of 15Co/600MA is higher than the bare 600MA spinel due to the formation of bigger crystallites of Co 3 O 4 after doping. Similarly, the micropores present in the 900MA spinel are covered by the large Co 3 O 4 crystals in the 15Co/900MA catalyst, consequently,   Table 1, the measured surface area of 15Co/600MA and 15Co/700MA catalysts are 146 and 121 m 2 /g respectively. The smooth decrease in the surface area probably due to the partial filling of Co 3 O 4 species in the micropores of the MgAl 2 O 4 spinel. Remarkably, the high surface area and pore-size of 15Co/800MA catalyst can be explained based on the fine dispersion of Co 3 O 4 particles on the 800MA support. Likewise, 10Co/800MA and 20Co/800MA catalysts promptly follow a similar trend and typically exhibited a more specific surface area as stated in Table 1.
In contrast, the low surface area of the 900MA spinel could not provide enough external surface space for the sheer distribution of Co 3 O 4 clusters. Therefore, the 15Co/900MA catalyst has typically shown an extremely low specific surface area in comparison with the other samples. As per the above discussion, 15Co/800MA represents the best catalyst for the titled reaction with a high specific surface area and large pore size.  Fig. 2a. It has been extensively investigated that the reduction pattern of Co 3 O 4 could be influenced by several factors like preparation method, metal precursor, nature of the support, and activation treatment. The TPR profiles of CA, CM, Co/MA (i.e., 15Co/700MA, and 15Co/900MA) catalysts exhibit the reduction peak with high intensity at elevated temperatures compared to those present in low-temperature reduction signals. This allows a scope to think that the reduction process of Co 3 O 4 phase in 15Co/γ-Al 2 O 3 , 15Co/MgO, 15Co/700MA, and 15Co/900MA catalysts proceed in two steps as follows: Thereby, H 2 consumption could be higher for 15Co/900MA catalyst because of larger crystallite size than 10Co/800MA, 15Co/800MA and 20Co/800MA catalysts. The H 2 -TPR of 15Co/700MA catalyst displayed two reduction signals one at a lower temperature (500-600 °C) with less intensity and another peak at a high (1) www.nature.com/scientificreports/ temperature (600-900 °C) with more intensity. The low-temperature signal can be ascribed to the reduction of uniformly distributed Co 3 O 4 particles. Whereas, the high-temperature peak is attributed to the complex reduction pattern of strongly interacted Co 3 O 4 species. Besides, the 15Co/γ-Al 2 O 3 catalyst exhibited two major reduction peaks at high temperature due to the strong interaction of Co 3 O 4 species with γ-Al 2 O 3 support 35 . It is reported that transition metal (M 2+ ) ions preferentially occupies tetrahedral vacancies of γ-Al 2 O 3 , so it is difficult to get metallic Co, compared to those occupied in octahedral vacancies of γ-Al 2 O 3 36 . In the case of Co/ MgO catalyst three different reduction signals are observed with different peak intensities as described in Fig. 2a. Amongst, the first reduced signal centered at 300 °C is ascribed to the direct transition of Co 3 O 4 into metallic Co species, while second reduced signal about T max 400 °C is illustrated the transformation of strongly support interacted Co 3 O 4 species into CoO and metallic Co. Then, reduction pattern at high temperature (600 °C) may probably represent the formation of some solid solution species such as MgCo 2 O 4 and CoO-MgO species in Co 3 O 4 /MgO catalyst 37 .
However, reduction patterns of 10Co/800MA, 15Co/800MA, and 20Co/800MA catalyst are quite different from Co/MgO, Co/γ-Al 2 O 3 , 15Co/700MA, and 15Co/900MA catalyst. As a result, 10Co/800MA, 15Co/800MA, and 20Co/800MA catalysts displayed facile reduction patterns owing to synergistic interface between Co 3 O 4 and MgAl 2 O 4 support. Therefore, 15Co/800MA catalyst is exhibited low-intense reduced signals compared to those present in Co/MgO, Co/γ-Al 2 O 3 , 15Co/700MA, 20Co/800MA, and 15Co/900MA catalysts. Furthermore, the surface textural properties of 15Co/800MA catalyst is better in line with smaller crystallite size, and high surface area facilitates the easy reduction of Co 3 O 4 species as evidenced from H 2 -TPR patterns (Fig. 2a). In the present investigation, reduction patterns of 10Co/800MA and 20Co/800MA catalysts are almost identical with the 15Co/800MA catalyst. In notable addition, highly intense TPR signals are observed for 20Co/800MA catalyst due to the reduction of bulk Co 3 O 4 species. While 10Co/800MA catalyst displayed very low intensity TPR signals owing to the low cobalt oxide content. Similarly, the 15Co/600MA catalyst contain two reduction peaks as observed in 10Co/800MA catalyst, which is in agreement with the reported literature 35,38 . UV-Vis DRS. The UV-Vis spectra of MgAl 2 O 4 spinel activated at various temperatures are presented in  31 .
As shown in Fig. 3a, a split in the absorption bands below 300 nm observed for Co 3 O 4 supported catalysts placed at 214 nm (λ 1 ) and 240 nm (λ 2 ) corresponding to the CT transition of O −2 → Al 3+ and O −2 → Co 3+ respectively 39,40 . The broad absorption signals centered at 430 (λ 3 ) nm and 720 nm (λ 4 ) among all the cobalt catalysts indicate the presence of a large amount of Co atoms in the octahedral and tetrahedral symmetry     Fig. 2b. The resulting basic sites analyzed from TPD patterns of samples are further classified into three categories (i.e., weak, moderate, and strong) assigning to their desorption strength. However, the bare 800MA support contains a lesser number of strong acidic sites, although the uniform distribution of weak and moderate acidic sites is investigated because of balanced Al 3+ and Mg 2+ ions. In the CM catalyst, weak and moderate acidic sites are massive in number, whereas strong acidic sites are relatively lesser compared to that of CA catalysts. It is majorly owing to the more significant number of Mg 2+ species present in CM catalyst; consequently, diminish the surface acidic nature of the catalyst. Moreover, the formation of some extent solid solution species like MgCo 2 O 4 also decreases the surface acidic nature of CM catalyst. Therefore, CM catalyst afforded a low conversion of EB but promoted high styrene selectivity as illustrated in Table 3.
In the case of CA, 10Co/800MA, 15Co/800MA, and 20Co/800MA catalysts strong acidic site distribution and the total acidity are relatively higher than CM, 15Co/900MA, and 800MA samples as mentioned in Table 2. The 10Co/800MA catalyst displayed a lower fraction of moderate and strong acidic sites, because of less cobalt www.nature.com/scientificreports/ oxide content. In contrast, 20Co/800MA catalyst with more cobalt oxide containing a more considerable fraction of strong acidic sites accordingly severe decrease of styrene monomer selectivity (Table 3). Whereas, a more significant quantity of weak and moderate acidic sites along with strong acidic sites exist in the CA catalyst is responsible for poor catalytic activity. In a similar approach, less acidity present in the 800MA and CM samples is unfavorable for high EB conversion and ST selectivity ( Table 3). The 15Co/900MA catalyst despite containing less acidic sites did not yield more ST owing to the sintering of active cobalt oxide particles at high calcination temperatures. The number of weak and moderate acidic sites generated in 15Co/800MA catalyst is considerably higher in number due to the uniform alignment of Co 3 O 4 species, but strong acidic sites are noticeably low compared to CA and 20Co/800MA catalyst. Typically, based on NH 3 -TPD patterns neither a low fraction of acidic sites nor strong acidic sites generated are suitable for potential catalytic activity. Therefore, balanced weak and moderate acidic nature along with adequate strong acidic sites possessed 15Co/800MA catalyst represented a significant role in getting maximum EB conversion and styrene selectivity.  Fig. 5b, 15Co/800MA catalyst displayed homogeneously distributed nano-sized cobalt oxide particles environment on the high surface area of MA spinel. Similarly, the bare 800MA spinel (Fig. 5c) is prominently displayed a lesser number of aggregated species with related lattice fringes substantially similar to the 15Co/800MA catalyst. As shown in Fig. 5d, the lattice resolved HRTEM image of 15Co/800MA catalyst confirm the uniformly    Fig. S7 (supplementary information). It can be distinguished that a more considerable number of isolated and uniformly distributed Co 3 O 4 species on the surface of the MgAl 2 O 4 support. All these consistent findings are precisely following the HRTEM morphology results procured as illustrated in Fig. 5.

Thermogravimetric analysis. TG analysis of spent cobalt catalysts collected after 20 h of EB dehydro-
genation activity is graphically displayed in the supplementary information (Fig. S8). As shown in the figure, TGA patterns of 15Co/700MA, 15Co/800MA, and 15Co/900MA catalysts exhibit a characteristic signal in the temperature range of 400-500 °C in all the catalysts. This can be attributed to the decomposition of deposited hydrocarbon moiety 28 . The 15Co/800MA and 15Co/700MA catalysts have a low intense endothermic signal beyond 450 °C with a weight loss of 10% and 11.5% results from the catalytic oxidation of deposited carbon (coke). While 15Co/900MA catalyst has a steep decomposition peak at 470 °C with a significant weight loss of 14.5% owing to excessive decomposition of EB molecules on the cobalt oxide particles with large crystallize size. Also, elemental analysis results are in line with the TGA analysis concerning coke deposition on spent catalysts. As described in TGA patterns, less coke deposition is investigated on the surface of 15Co/800MA spent catalysts because of efficient oxidation of deposited carbon into CO 2 molecules in the course of ODH of EB. Therefore, one can expect that the 15Co/800MA catalyst would typically exhibit a steady catalytic activity compared to 15Co/700MA and 15Co/900MA catalyst.
Catalytic activity studies. The CO 2 assisted ODH of EB over cobalt catalysts takes place in two steps as shown in Eq. (3) and (4). In the first step, EB dehydrogenated into ST with the liberation of H 2 . Afterward, the H 2 formed will typically react with CO 2 via reverse water-gas shift (RWGS) reaction. Therefore, the general reaction comprises a combination of EB dehydrogenation and RWGS reaction as displayed in Eq. (5).
Influence of temperature on the ODH of EB activity. The influence of reaction temperature on the EB conversion (X EB ) was investigated in the presence of N 2 and CO 2 as oxidants over Co/MA catalysts as depicted in Fig. 6a,b respectively. Typically, ODH of EB is endothermic thereby mostly depends on the reaction temperature 47 . Therefore, with a rise in the temperature from 450 to 650 °C, X EB increased gradually to a maximum extent. As evident from Fig. 6, it can be observed that the remarkable activity of CO 2 assisted ODH of EB in comparison with the EB dehydrogenation under N 2 flow.
Among all the catalysts, 15Co/800MA catalyst performed outstanding activity than other samples (Fig. 6b), where the X EB increased progressively from 8 to 82% with a change in the temperature from 450 to 600 C. Further www.nature.com/scientificreports/ increase in the reaction temperature to 650 °C, which showed a maximum X EB (93%) but the selectivity of ST decreased to significant levels (not shown in Fig. 6). It can be observed that all the samples afforded very mild activity of EB at low reaction temperatures (450-550 °C), and then considerable improvement in X EB is observed at 600 °C. After recognizing the optimum temperature, further CO 2 assisted ODH of EB reactions were carried out over all the cobalt catalysts including the MA support and the results are illustrated in Table 3.
MA spinel activated at low calcination temperature (600MA) exhibits a very low X EB (29%) and S ST (92%), which can be ascribed to the partial formation of single-phase MgAl 2 O 4 spinel. In contrast, improved X EB of 700MA (35%), and 800MA (42%) owing to the complete formation of single-phase MA spinel. However, the X EB of 900MA spinel remained at 23% owing to the sintering of Mg 2+ and Al 3+ ions result in the formation of bigger crystallites with low specific surface area as depicted in Table 1.
After the incorporation of the optimal amount Co 3 O 4 onto the 800MA spinel support, the catalytic activity increased two folds from 42 to 82%. Thus, cobalt oxide nanoparticles played an active role via synergistic interaction with the MgAl 2 O 4 spinel support. Moreover, homogeneously distributed Co 3 O 4 species on 800MA spinel with adequate surface acidic-basic sites has shown high EB conversion and ST selectivity. Among all, 15Co/600MA catalyst displayed the lowest X EB and S ST because of the lesser number of Co 3 O 4 clusters on the MgAl 2 O 4 spinel support and the presence of γ-Al 2 O 3 phase as described in XRD (Fig. 1a). Besides, 15Co/700MA and 20Co/800MA catalysts with almost similar physicochemical characteristics (Table 1) afforded some extent equal X EB at all reaction temperatures. The high surface area, thermal stability, and optimum surface acidic nature of the 15Co/800MA sample provide an enhanced chemical homogeneity towards the uniform distribution of Co 3 O 4 nanoparticles. As a result, less coke deposition is observed on the external surface of the 15Co/800MA spinel catalyst even after several hours of catalytic activity study when compared to 15Co/700MA and 15Co/800MA catalysts, further evidenced from TGA analysis as displayed in Fig. S8 (supplementary information). It can be observed that neither low acidic strength of 10Co/800MA catalyst nor strong acidic sites of 20Co/800MA catalyst are suitable for achieving optimum catalytic activity (Table 3). Similarly, a very low specific surface area, and large crystallite size 15Co/900MA catalyst is also undesirable for the higher activity of CO 2 assisted ODH of EB.

Time-on-stream (TOS) study.
To distinguish the long-term stability for the ODH of EB, TOS studies are performed over 15Co/800MA and 15Co/900MA catalysts in the presence of N 2 and CO 2 atmosphere as drawn in Fig. 7. It is evident from Fig. 7a, with an increase in the reaction time from 1 to 20 h, the X EB decreases from 60 to 37% over the 15Co/800MA catalyst. Similarly, X EB decreases from 43 to 23% over the 15Co/900MA catalyst with passing time. However, the S ST stayed constant at 90% in both the samples during the reaction. Typically, the generated ST undergoes self-polymerization to yield polystyrene, which is crucial for coke deposition on the active surface of the catalyst 12 . In the presence of N 2 , the polystyrene will transform into graphitic carbon at elevated temperature (> 500 °C), consequently, rapid catalyst deactivation takes place (Fig. 7a).
It can be observed the X EB decreases from 82 to 59% over the 15Co/800MA catalyst the 15Co/900MA catalyst follows a similar trend where the X EB declined from 58 to 37%. However, the S ST of 15Co/800MA (98%) and 15Co/900MA (95%) remains constant throughout the catalytic run. It is supposed to be due to the promotion of the RWGSR phenomenon in the presence of the CO 2 environment, which played a key role in superior catalytic activity. Besides, a greater number of uniform Co 3 O 4 oxide particles on the 800MA spinel results in the high specific surface area and optimum crystallite size responsible for the enhancement in the catalytic activity. Nevertheless, the coke formation can be removed effectively by the oxidation of CO 2 as a soft oxidant. Hence, CO 2 assisted ODH of EB has a potential role to improve the catalytic activity as evident from Fig. 7b. www.nature.com/scientificreports/ In the case of 10Co/800MA catalyst, a mild fraction of Co 3 O 4 oxide particles leads to the marginal conversion of EB but no major changes are observed in the ST selectivity. In contrast, high Co oxide content possessed by 20Co/800MA catalyst influence the ST selectivity due to the high density of surface acidic sites as evidenced from NH 3 -TPD analysis ( Table 2). For convenience, we have not included the TOS study of 10Co/800MA and 20Co/900MA catalysts in the manuscript, however, corresponding X EB and S ST are listed in Table 3. In contrast, 15Co/900MA catalyst accomplished insignificant X EB during the time-on-stream owing to the bigger crystallite size and subsequently displayed low surface area as listed in Table 1.
The regeneration of the used catalyst was performed on 15Co/800MA and 15Co/900MA samples, the corresponding results are as shown in. Typically, before the reusability studies, the specimens were activated in the airflow (30 mL/min) for 1 h. There is no observable change in the activity is observed and so we can assume that the 15Co 3 O 4 /MgAl 2 O 4 catalyst possesses excellent thermal stability. The enhanced catalytic activity in the active presence of CO 2 is due to the potential elimination of H 2 formed in the dehydrogenation via reverse water-gas shift (RWGS) reaction. The amount of CO liberated during ODH of EB was accurately measured and graphically represented in Fig. S9 (supplementary information). High CO yield (≥ 9%) is obtained on 15Co/800MA catalyst compared to H 2 evolution, which specifies the great participation of H 2 in the RWGS reaction. While low yield of CO (6.4%) on 15Co/900MA catalyst indicates the low RWGS activity which is responsible for the mild X EB (54%) as illustrated in Fig. 8. Accordingly, more fractions of carbonaceous material deposition tend to oxidize for the generation of excess CO 2 molecules on the surface of 15Co/800MA catalyst compared to that of 15Co/900MA catalyst.  www.nature.com/scientificreports/ the 15Co/800MA achieved a remarkable enhancement in the EB dehydrogenation activity with a maximum X EB of 82% and S ST of 98% (Table 4, entry 10), which is much superior and better than the other catalysts stated.

Conclusions
In summary, 15Co 3 O 4 /MgAl 2 O 4 spinel is found to be an efficient catalyst system for the ODH of EB in the presence of CO 2 as a soft oxidant. Besides, the catalyst possesses remarkable stability with a prolonged activity during 20 h TOS study. A gradual decrease in the conversion from 82 to 59% is anticipated for the mild coke formation. However, the selectivity of the styrene monomer (98%) has stayed almost the same throughout the reaction. The significant improvement can be ascribed to the calcination of MgAl 2 O 4 spinel at 800 °C, which facilitates a chemical uniformity for the distribution of Co 3 O 4 nanoparticles on the active surface of the catalyst. To be specific, a greater number of uniform Co 3 O 4 oxide particles on the 800MA spinel results in the high specific surface area and optimum crystallite size responsible for the enhancement in the catalytic activity. Furthermore, the coke formation was suppressed effectively by the oxidation of CO 2 as a soft oxidant. It is supposed to be due to the promotion of RWGSR in the CO 2 environment, which performed a crucial role to achieve the maximum catalytic activity.

Data availability
The datasets generated and analyzed during the current study are included in this article and also it is available from the corresponding author on reasonable request.