Evaluation of Vanadium Oxide Nanocatalysts over Graphene for Propylene Production Through Oxidative Propane Dehydrogenations

Abstract

This study reports an efficient and low-cost hydrothermal method for synthesizing vanadium oxide/graphene nanocatalysts. Field-emission scanning electron microscopy (FESEM) revealed the formation of nanostructured catalysts with consistent and directional shapes, as confirmed by X-ray diffraction (XRD). Fourier transform infrared (FTIR) spectroscopy indicated the presence of V₂O₅ and graphene, highlighting their bonds and structures. Thermogravimetric analysis (TGA) identified three stages of weight loss in the nanocatalysts, corresponding to water molecule evaporation, decomposition of residual organics, and the formation of yellow vanadium pentoxide particles due to the oxidation of vanadium V⁴⁺. Gas chromatography analysis from 450 °C to 600 °C showed that ethylene selectivity increased with temperature, while propylene selectivity showed the opposite trend. The effectiveness of these nanocatalysts was assessed in the oxidative dehydrogenation of propane using temperature programmed reduction. The approach of graphene-based vanadium oxide nanostructures will open up a new insight into the fabrication of high-performance catalysts.

1. Introduction

Light olefins, predominantly produced through energy-intensive methods such as fluid catalytic cracking (FCC) and catalytic dehydrogenation of light hydrocarbons, are key in various industrial applications [1]. Additionally, propane can be synthesized from hydrogen-rich materials such as methanol [2]. Advancements in new technologies have led to the development of new methods for olefin production, including oxidative dehydrogenation (ODH), both with and without CO₂, and utilizing biomass as feedstock. Olefins play a significant role in the petrochemical industry, contributing to the production of polypropylene, phenol, and other valuable products [3]. However, the traditional catalytic dehydrogenation of propane to propylene is highly energy-demanding, typically requiring temperatures exceeding 600 °C. This high temperature often leads to rapid coke deposition and catalyst deactivation, necessitating the implementation of a catalyst regeneration strategy. The primary challenges in this process are its high energy requirements and the additional costs associated with the regeneration cycle [4,5]. Oxidative dehydrogenation presents a viable alternative pathway for the synthesis of olefins. The oxidative dehydrogenation of propane (ODHP) process operates at relatively lower temperatures while exhibiting exothermic characteristics, thereby substantially reducing the required energy and mitigating catalyst deactivation typically associated with higher operational temperatures [6]. However, the development of the ODH process encounters two principal challenges: the identification of an appropriate catalyst and the intricacies of reactor design and configuration. The presence of oxygen within the ODH process induces the formation of CO_x byproducts. Furthermore, the highly exothermic nature of the process poses a risk of hot spot formation, potentially compromising both catalyst integrity and reactor stability. To overcome these issues, it is imperative to employ an efficient catalyst capable of directing the reaction towards olefin production rather than CO_x formation. Additionally, an optimally configured reactor is critical to manage the thermal hot spots, thereby enhancing the selectivity towards the desired products. The oxidative dehydrogenation reaction proceeds through a series of oxidation reactions, both in series and parallel configurations, as presented in Table 1.

Different types of catalysts have been studied for the ODHP process in recent years. Mitran et al. [7] prepared VO_x/SBA-15 catalysts for analyzing the effects of different amounts of vanadium loading on propylene selectivity. Fan et al. [8] analyzed a series of ordered mesoporous Ni-Mo-O catalysts and showed that this catalyst has high activity and propylene yield, the latter around 15.6%. Wang et al. [9] introduced a Co₃O₄ catalyst as suitable choice for oxidative dehydrogenation (ODH) due to its low temperature and high conversion, around 175 °C and 21% for propane. The study by Fattahi et al. [10] explored a vanadium–graphene catalyst, and they employed artificial neural networks (ANNs) for kinetic modeling. Their findings indicated that this catalyst achieved satisfactory conversion rates between 400 to 500 °C. Li et al. [11] analyzed the effect of Fe in catalysts on reaction performance by preparing FeV_xO_y catalysts. Their results demonstrated that the sum of propylene and CO selectivities exceeded 90% and that this catalyst could reduce CO₂ selectivity against increasing CO formation. Miranda et al. [12] examined catalysts with V-Mg-O compounds on an activated carbon base. They found that the stability of these catalysts was largely due to the presence of Mg-V connections on their surfaces. Additionally, MoO_x-modified nanodiamond (ND) catalysts, as investigated by Xiong and Sang [13], demonstrated significant stability in this process. The improved conversion and selectivity of these catalysts were attributed to the low MoO_x loading. Liu et al. [14] explored the use of vanadium-containing mesoporous silica nanoparticles as novel, multifunctional catalysts, noting their effectiveness in both ODH and direct dehydrogenation of propane. Zhang et al. [15] focused on (V–Sb–O/SiO₂) catalysts, recording a 65% selectivity for propylene and a yield of up to 26%, comparable to that of V-Mg-O catalysts. Rostom and de Lasa [16] introduced VO_x/ZrO₂–γAl₂O₃ catalyst with COx selectivity near to 2% as good choice for industrial application. A series of Cr-TUD-1 catalysts with various loadings of chromium were provided by Burri et al. [17] for ODH reactions with high propylene selectivity, around 75%. Wu et al. [18,19] proposed Fe–O–Al species as suitable active sites for ODH reaction catalysis, especially for N₂O-mediated catalysis. Moreover, they studied Fe-ZSM-5 catalysts, considering the effect of N₂O on reaction performance. They showed that the presence of nitrous oxide increased the propylene selectivity and led to lower deactivation. Balogun et al. [20] proposed a MoO₃/CaMnO₃ catalyst for chemical looping in the ODHP process. This catalyst exhibited remarkable stability and a high propylene selectivity, approximately 59%. The presence of free lattice oxygen in MoO₃ further amplified the selectivity towards the desired product. Zhou et al. [21] delved into the reaction mechanism of ODH of propane using boron phosphate catalysts, employing density functional theory for their analysis. Wang et al. [22] assessed the efficacy of MoO₃-Fe₂O₃ as a redox catalyst in the chemical looping process for converting propane to propylene. Their findings revealed that the catalyst not only maintained stability but achieved over 90% selectivity for propylene, attributable to the lattice oxygen mechanism. Monguen et al. [23] considered Cu-Cr double-layer thin film in their study. This catalyst showed good activity in low temperature and good propylene and propane conversion, near 87% and 92%, respectively. They suggested that active and efficient Cr₃₀Cu₁₂₀ could be a good potential catalyst for the petrochemical industry. Schumacher et al. [24] demonstrated that titania is suitable support for VO_x in ODH reactions. They concluded that this type of catalyst resulted in high propane selectivity and reduced CO_x formation. Dzwigaj et al. [25] conducted a thorough investigation into zeolite-supported catalysts for ODH, focusing particularly on the impacts of cobalt species addition on reaction efficiency. Moreover, Qiu et al. [26] introduced an innovative catalyst within the domain of zeolite-based catalysts. Their research centered around boron-incorporated zeolites, evaluating their efficacy in ODHP. Remarkably, they achieved a 20% enhancement in propylene selectivity compared with conventional zeolite-based catalysts. Further investigating in this field, Cao et al. [27] explored the intricacies of hexagonal boron nitrate catalysis, proposing indium oxide (In₂O₃) as a robust support for these catalyst types. Additionally, the multifaceted aspects of boron nitrate catalysts were comprehensively analyzed by Fu et al. [28], providing a broad perspective on their potential and challenges.

The support plays a pivotal role in influencing VO_x dispersion, redox behavior, and product selectivity [29,30,31,32,33,34,35,36,37]. It has been demonstrated that inert supports such as im-SiO₂ promote the formation of two-dimensional VO_x species while suppressing bulk V₂O₅ crystallization, resulting in improved catalytic activity and selectivity. High Lewis acidity has been reported to hinder propylene desorption and promote overoxidation, whereas low-acidity supports enable more efficient olefin release. The ability of vanadium oxides to catalyze ODHP at lower temperatures has been attributed to the participation of lattice oxygen in C–H bond activation. The influence of support surface structure on VO_x dispersion and redox cycling has also been emphasized. Graphene-based supports, particularly reduced graphene oxide, offer low acidity, high conductivity, and π-conjugated surfaces that stabilize VO_x species and enhance redox efficiency. Furthermore, improvements in ODHP performance have been linked to tailored surface functionalities, stabilized oxidation states, bifunctional site design, and the use of thermally stable nanostructured supports [29,30,31,32,33,34,35,36,37]. These combined effects justify the use of V₂O₅/graphene composites as highly efficient and selective catalysts for ODHP.

Alongside the development of efficient catalysts, researchers have focused on mathematical modeling of ODH reactors in recent years. They have suggested different types of reactors, such as fixed-bed, fluidized-bed, chemical looping, and microreactors. Darvishi et al. [38] studied a multitube fixed-bed reactor of propane to propylene over a V₂O₅/γ-Al₂O₃ catalyst. They investigated the effect of injection oxygen between catalytic beds on the amount of CO_x production. They showed that oxygen could increase propylene selectivity. Moreover, Kotanjac et al. [39] studied a packed-bed membrane reactor with a distributed oxygen feed. They demonstrated the reaction order of oxygen for side reactions was higher than for the main reaction. They showed the benefits of a packed-bed membrane reactor for the ODH process over a Ga₂O₃/MoO₃ catalyst. Chen et al. [40] conducted an analysis of the extinction–ignition curve for the oxidative dehydrogenation of ethane process in an autothermal adiabatic reactor. Their research focused on evaluating the reactor’s performance under varying conditions, including different bed lengths, feed temperatures, and pressure. In a separate study [41], a comparative analysis of autothermal reactors and cooled tubular reactors, specifically for ODH processes, was conducted. Fazlinezhad et al. [42] conducted a multiobjective optimization for ODHP for maximizing propylene yield and minimizing CO_x yield over a V₂O₅/graphene catalyst. They demonstrated a maximum propylene yield of ~40% and tested the optimum operational point for safety analysis by the use of a flammability diagram. Cai et al. [43] studied numerical modeling of the chemical looping of the ODH reaction. They optimized the operational parameters and obtained above 67% yield for olefin production over redox catalysts. Geng et al. [44] conducted CFD analysis and multiscale modeling of the oxidative dehydrogenation of ethane process from mesoscale to macroscale. Reyes-Antonio et al. [45] conducted a comprehensive study on the application of packed bed-reactors for ODHP, utilizing computational fluid dynamics (CFD) analysis. They achieved increases in both propane conversion and propylene selectivity, comparable to those of membrane reactors, by implementing a 3D model and integrating various physical aspects within the reactor. Khraibet et al. [46] investigated the performance of bubbling fluidized-bed reactors, assessing the impact of feed temperature, pressure, and superficial velocity on both conversion efficiency and propylene selectivity. Rostom and de Lasa [47] explored the use of downer fluidized-bed reactors for ODHP, demonstrating that employing a CFD particle model could reduce propane conversion by 20% while maintaining a propylene selectivity of around 94%. Li et al. [48] examined the efficacy of spouted-bed reactors (SBRs) in the ODHP reaction under two different configurations: with and without an internal draft tube. They observed that higher propane conversion rates were achieved in SBRs lacking draft tubes, whereas enhanced propylene selectivity was noted in SBRs equipped with a draft tube.

Recent developments in catalyst design for light olefin production have emphasized the strategic engineering of active sites and interfaces to overcome selectivity and stability challenges in ODHP. Zhang et al. [49] reported a TiO₂-based catalyst embedded with isolated Fe species (Fe₁–TiO₂) synthesized via a cation-exchange strategy, achieving a notable 38.2% olefin yield at 520 °C with high long-term stability. The enhanced performance was attributed to the formation of isolated Fe sites that promoted a low-barrier Fe–H dehydrogenation pathway, as confirmed by in situ FT-IR and DFT calculations. Complementarily, Lu et al. [50] developed heterogeneous binuclear-site catalysts (HBSCs) by anchoring dual metal atoms (e.g., Pt₁Pd₁) on titania through a vacancy-pair capturing approach. These HBSCs outperformed single-atom counterparts in acetylene hydrogenation via a bifunctional mechanism involving spatially separated adsorption and hydrogen migration steps. These studies highlight the importance of tailoring catalytic interfaces and site architectures to enhance activity and selectivity. Graphene offers a high surface area, excellent electrical conductivity, and strong metal–support interactions, which collectively enhance catalytic activity and stability. These features promote better dispersion of active sites and inhibit sintering, making graphene a superior support to conventional materials. In this context, our vanadium over graphene system aligns with these principles by leveraging redox-active vanadium sites anchored on a high-surface-area graphene support, facilitating efficient charge transfer, dispersion of VO_x species, and enhanced selectivity toward propylene in ODHP.

Table 1. The sequence of the catalytic oxidative dehydrogenation reaction [40].

Reaction	Description of Reaction
${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2⟷{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}+{\mathrm{H}}_2\mathrm{O}$	The main reaction
${\mathrm{C}}_3{\mathrm{H}}_8+{\mathrm{O}}^{\mathrm{*}}⟷{\mathrm{C}}_3{\mathrm{H}}_8{\mathrm{O}}^{\mathrm{*}}$	Propane absorption on lattice oxygen
${\mathrm{C}}_3{\mathrm{H}}_8{\mathrm{O}}^{\mathrm{*}}+{\mathrm{O}}^{\mathrm{*}}⟷{\mathrm{C}}_3{\mathrm{H}}_7{\mathrm{O}}^{\mathrm{*}}+{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{\mathrm{*}}$	Scission of a sigma carbon–hydrogen bond and formation of a propyl intermediate
${\mathrm{C}}_3{\mathrm{H}}_7{\mathrm{O}}^{\mathrm{*}}⟷{\mathrm{C}}_3{\mathrm{H}}_6+{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}$	Reaction of propyl with the surface oxygen and formation of propylene
${\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{\mathrm{*}}+{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{\mathrm{*}}⟷{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}^{\mathrm{*}}+{\mathrm{V}}_{\mathrm{r}\mathrm{e}\mathrm{d}}^{\mathrm{*}}$	Formation of water and reduced vanadium site
${\mathrm{O}}_2+2{\mathrm{V}}_{\mathrm{r}\mathrm{e}\mathrm{d}}^{\mathrm{*}}⟷{\mathrm{O}}^{\mathrm{*}}+{\mathrm{O}}^{\mathrm{*}}$	Reoxidation of the catalyst

Table 1. The sequence of the catalytic oxidative dehydrogenation reaction [40].

Reaction	Description of Reaction
${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2⟷{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}+{\mathrm{H}}_2\mathrm{O}$	The main reaction
${\mathrm{C}}_3{\mathrm{H}}_8+{\mathrm{O}}^{\mathrm{*}}⟷{\mathrm{C}}_3{\mathrm{H}}_8{\mathrm{O}}^{\mathrm{*}}$	Propane absorption on lattice oxygen
${\mathrm{C}}_3{\mathrm{H}}_8{\mathrm{O}}^{\mathrm{*}}+{\mathrm{O}}^{\mathrm{*}}⟷{\mathrm{C}}_3{\mathrm{H}}_7{\mathrm{O}}^{\mathrm{*}}+{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{\mathrm{*}}$	Scission of a sigma carbon–hydrogen bond and formation of a propyl intermediate
${\mathrm{C}}_3{\mathrm{H}}_7{\mathrm{O}}^{\mathrm{*}}⟷{\mathrm{C}}_3{\mathrm{H}}_6+{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}$	Reaction of propyl with the surface oxygen and formation of propylene
${\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{\mathrm{*}}+{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{\mathrm{*}}⟷{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}^{\mathrm{*}}+{\mathrm{V}}_{\mathrm{r}\mathrm{e}\mathrm{d}}^{\mathrm{*}}$	Formation of water and reduced vanadium site
${\mathrm{O}}_2+2{\mathrm{V}}_{\mathrm{r}\mathrm{e}\mathrm{d}}^{\mathrm{*}}⟷{\mathrm{O}}^{\mathrm{*}}+{\mathrm{O}}^{\mathrm{*}}$	Reoxidation of the catalyst

In this research, keeping in view the above-mentioned shortfalls, some studies were conducted to investigate the catalytic performance of vanadium oxide nanocatalysts in the oxidative propane dehydrogenation reaction. In this regard, four distinct nanocatalysts were synthesized using a hydrothermal method under varied conditions, utilizing graphene as a support. The characterization of these nanocatalysts encompassed their structure, efficiency, and selectivity. This was achieved through a suite of analytical techniques including field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectrometry (FTIR), temperature programmed reduction (TPR) analysis, and gas chromatography (GC). Subsequently, a comparative evaluation of the performance of these nanocatalysts were conducted.

2. Results and Discussion

2.1. FE-SEM

Figure 1a–d illustrate that the nanostructured vanadium oxide/graphene catalysts exhibited regular and directional shapes, with a uniform distribution of V₂O₅ and graphene particles, indicating a well-organized and homogeneous morphology. The hydrothermal method employed in sample preparation effectively dispersed vanadium oxide particles within the graphene structures, resulting in the formation of an integrated and homogeneous morphology. The uniform appearance observed in the SEM images suggests that the synthesis process produced well-organized composites with no visible structural defects or irregularities. A release of energy accompanied the synthesis of vanadium oxide catalysts based on graphene due to the interactions between reactive particles, which prevented the accumulation of V₂O₅ particles and encouraged the formation of bar-like graphene structures instead of flat ones. Comparing the flat morphology of the prepared nanocatalysts in Figure 1c,d, it can be concluded that an increase in the time of the hydrothermal process broke the nanosheets, resulting in needle-like and long tubular structures. These observations are depicted in Figure 1a,b, corresponding to hydrothermal durations of 48 h and 24 h, respectively. Comparing the flat morphology of the nanocatalysts in Figure 1c,d, it can be concluded that increasing the hydrothermal synthesis time promoted the transformation of nanosheets into needle-like and tubular structures, as observed in Figure 1a,b for 48 h and 24 h synthesis times, respectively. This morphological evolution is attributed to time-dependent dissolution and recrystallization processes, where high-energy crystal facets dissolve and reprecipitate on more stable, low-energy surfaces, driving anisotropic growth. Moreover, prolonged hydrothermal exposure enhances Ostwald ripening and oriented attachment, favoring the formation of elongated structures. Similar transformations under hydrothermal conditions have been reported in vanadium oxide systems and other layered materials [51,52,53]. Furthermore, as well-documented in the open literature [54], the growth of V₂O₅ particles on carbon nanotube and graphene by the hydrothermal method leads to the formation of flexible nanostructured materials, depending on the concentration of the surface V₂O₅ particles, in which the dots represent vanadium atoms, while the underlying support is graphene or carbon nanotubes. In contrast, our hydrothermal strategy produced uniformly dispersed needle-like and tubular V₂O₅ morphologies, distinct from the commonly reported nanoparticle or flake-like structures, potentially enhancing structural integrity and interfacial contact [55,56]. This study synthesized all samples with the same molar ratio of vanadium oxide and graphene (i.e., 1:1) to reach comparable samples [57].

2.2. XRD Analyses

As shown in Figure 2a–d, the X-ray diffraction analysis of samples V-G-1, V-G-2, V-G-3, and V-G-4 revealed the presence of graphene structures and V₂O₅ catalysts. These samples, porous graphene and V₂O₅, demonstrated distinct diffraction patterns indicative of their composition. These diffractions were in accordance with the standard (JCPDS No. 96-901-1016), and in all of them, vanadium pentoxide corresponded to monoclinic crystal structure and graphene to orthorhombic crystal structure. Crystallite sizes were estimated using the Scherrer equation based on the FWHM of the most intense diffraction peak, without correction for instrumental broadening, and no Rietveld refinement was performed in this study. According to Scherrer’s equation, the average size of nanoparticles for compounds V-G-1, V-G-2, V-G-3, and V-G-4 was 2.90, 9.70, 4.40, and 2.13 nm, respectively.

In Figure 2a, which pertains to the V-G-1 structure, diffraction of graphene was observed at 2θ angles of 26° and 35°, corresponding to planes (001) and (002), and diffraction of V₂O₅ was observed at angles of 31°, 47°, 51°, 61°, and 71°, associated with surfaces (202), (−601), (112), (−512), and (424). In Figure 2b, focusing on the V-G-2 structure, graphene showed diffraction at 2θ angles of 26°, 33°, and 35°, linked to planes (001), (002), and (020), while V₂O₅ displays diffraction at 22°, 31°, 47°, 52°, and 62°, corresponding to planes (−201), (202), (−601), (112), and (−512). Figure 2c details the V-G-3 structure, where graphene diffraction was evident at 2θ angles of 26° and 35°, for planes (001) and (020), and V₂O₅ diffraction occurred at angles of 31°, 47°, 52°, and 62°, pertaining to planes (202), (−601), (112), and (−512). Lastly, in Figure 2d, related to the V-G-4 structure, graphene was diffracted at 2θ angles of 26°, 32°, 35°, and 71°, associated with planes (001), (002), (020), and (3–60), and V₂O₅ showed diffraction at angles of 23°, 47°, 52°, and 62°, linked to planes (−201), (−601), (112), and (−512).

In the X-ray diffraction spectra of these samples, additional subtle diffraction patterns were noted, potentially attributable to lower oxidation states of V₂O₅, such as VO₂ or V₂O₃. The formation of intermediate vanadium oxides such as V₂O₃ and VO₂ may be inferred from the thermal behavior, though further confirmation through techniques such as XPS or Raman spectroscopy would be needed to support this observation. These compounds, when formed under hydrothermal conditions, tend to develop superficially. This superficial formation results in less distinct and identifiable diffractions in the spectra, making their precise identification challenging. Because of the average sizes of the synthesized catalysts being below 10 nm, the dispersion of VO_x was predominantly influenced by the synthesis temperature, the acidity of the support, and the vanadium precursors [58]. Consequently, in this study, vanadium oxides performed their traditional role in facilitating C-H bond rupture.

The formation of vanadium oxide/graphene heterostructures typically involves depositing vanadium precursors onto graphene, followed by thermal treatment. Functional groups on graphene facilitate the anchoring of vanadium species, leading to uniform dispersion. Upon calcination, graphene is reduced to reduced graphene oxide, and vanadium converts into VO_x phases. XRD analysis confirmed the presence of crystalline VO_x and the preserved graphene structure, while SEM images showed well-dispersed VO_x particles on the graphene sheets, indicating strong interfacial bonding [59,60]. This interfacial architecture improves redox behavior and electronic conductivity, which are crucial for ODHP. In this reaction, VO_x species act as redox-active sites, while graphene enhances charge transfer and prevents catalyst sintering. The VO_x/graphene heterostructure exhibits superior selectivity toward propylene and greater thermal stability due to the synergistic interaction between the two components [60,61,62]. These features make VO_x/graphene catalysts promising for selective light alkane dehydrogenation under oxidative conditions.

2.3. TGA Analyses

The TGA spectra of the synthesized nanocatalysts in Figure 3a–d showed a weight loss of 5% in the temperature range of 50–100 °C as a consequence of the decomposition, evaporation, and outflow of water molecules. Additionally, weight loss of around 10–12% in the temperature range of 100–430 °C was observed owing to the decomposition of remaining organic molecules. At the end of the graph, there was a weight loss (approximately 2%) at 450 °C, which represents the oxidation process of vanadium V⁴⁺ and the formation of yellow vanadium pentoxide particles according to the following formula. This phase transformation is significant, as crystalline V₂O₅ (yellow) is known to contribute actively to redox processes in oxidative dehydrogenation reactions [63].

$$2\mathrm{V}{\mathrm{O}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2⟷{\mathrm{V}}_2{\mathrm{O}}_5$$

(1)

2.4. FTIR Analyses

The FTIR spectra of the prepared nanocatalysts in Figure 4a–d confirmed the formation of V-O catalytic bonds via the presence of specific peaks between 450 and 950 cm⁻¹, which were ascribed to V-O vibrations. The broad absorption of the spectrum in the range of 528 to 541 cm⁻¹ was related to the symmetric and asymmetric stretching vibration of V-O-V groups in VO_x layers. Furthermore, the detected signal ranging from 915 to 924 cm⁻¹ suggested the presence of vanadyl groups (V=O). A weak band observed between 1003 cm⁻¹ and 1008 cm⁻¹ was indicative of the initial phase of octahedral rearrangement (phase B) in VO₂. This peak signified that the layered vanadium structure was reorganized upon the carbon nanostructure templates.

Moreover, the peaks located between 900 and 1050 cm⁻¹ represented the intermediate oxidation of V⁵⁺ and V⁴⁺, which were attributed to the stretch of short bonds (V=O) appearing from VO₂ in phase B. Stretching bands of C-C in the range between 1550 and 1650 cm⁻¹ were related to carbon nanotubes. The low-intensity bands observed around 3400 cm⁻¹ showed that water molecules and OH groups were adsorbed on the surface of the prepared catalysts. It is worth mentioning that the interaction between the carbon nanostructures and vanadium species shifted all the peaks to smaller wavelengths.

2.5. TPR Analyses

The quantity of hydrogen chemically released from the synthesized vanadium pentoxide catalysts was quantified using temperature programmed reduction (TPR). The spectrum of the emitted hydrogen was captured using a gas chromatograph. The ensuing data, derived from the integral area under the curve and the positions of the peaks, are displayed in Figure 5a–d.

The area under the figures shows the amount of hydrogen consumed and amount of catalyst reduction. Figure 5a–d show this value around 8405 mm², 3892 mm², 3270 mm², and 3138 mm². The superior regeneration capacity of the V-G-1 catalyst, as reflected in its stable selectivity and conversion across multiple cycles, aligns with previous reports on vanadium-based catalysts supported on carbon materials, which emphasize the importance of strong metal–support interactions in sustaining catalytic performance during repeated use. Based on these measurements, the V-G-1 catalyst exhibited higher values than the others, indicating a more efficient regeneration process. The enhanced regenerative capacity of the V-G-1 sample may be attributed to the presence of various types of VO_x on its surface, facilitating regeneration over different time periods. Consequently, the V-G-1 sample emerged as the most effective catalyst among those synthesized.

2.6. Catalyst Evaluations

Although this study focuses on the enhanced catalytic performance of the vanadium oxide/graphene heterostructure, it is important to acknowledge the catalytic behavior of the individual components. Bare vanadium oxide, while active in ODHP, is often associated with low propylene selectivity due to overoxidation and excessive CO_x formation under oxidative conditions [64]. On the other hand, pristine graphene lacks intrinsic catalytic activity for ODHP because of the absence of redox-active surface sites [65]. The superior performance of the VO_x/graphene system can therefore be attributed to the synergistic interaction at the oxide–carbon interface, where VO_x provides the active redox centers and the graphene matrix facilitates electron transport and thermal conductivity while preventing VO_x aggregation. While individual control experiments using bare VO_x and graphene were not included in this work, their limited performance is well-documented.

From a stability perspective, graphene’s unique structure and high thermal conductivity can help suppress coke formation by facilitating heat dissipation and reducing localized hot spots. Moreover, its chemically inert surface and strong interaction with active species may inhibit the polymerization of hydrocarbon intermediates, thereby improving catalyst stability. In our system, these properties contributed to the observed resistance to deactivation during ODHP. Oxidative dehydrogenation of light alkanes to olefins, characterized by its exothermic nature and reduced coke formation, presents a promising alternative to direct dehydrogenation, operating at lower temperatures without requiring frequent catalyst regeneration [66,67,68].

The profiles of conversion; ethylene, propylene, and carbon dioxide selectivities; and propylene yield chart for the synthesized nanocatalysts obtained from the GC analysis after 6 h from propane injection are presented in Figure 6a–d for V-G-1 through −4, respectively. The catalytic performance was evaluated within the temperature range of 450–600 °C, selected based on previous studies [69,70] indicating optimal activity and selectivity for vanadium-based catalysts in ODHP within this window. The most striking observation was that in all samples, increasing the temperature from 450 to 600 °C enhanced ethylene selectivity, while selectivity towards propylene was reduced. It is worth mentioning that at lower temperatures, there is insufficient energy to drive the oxidative dehydrogenation reaction effectively. The temperature enhancement leads to the oxidation of propane and further oxidation of the formed propylene, resulting in decreased selectivity towards propylene and increased CO_x formation. Furthermore, the conversion percentage and efficiency were directly related to temperature changes. According to Figure 6, selectivity towards ethylene and CO_x increased with rising temperature. Unlike propylene selectivity, which increased with decreasing temperature, selectivity towards CO_x increased with increasing temperature because of the higher burning of propane or formed propylene. Both propane conversion and propylene selectivity depend on the type of basic structure, and the impurities on the surface of the catalysts may act as the sites’ diluents, enhancing the dehydrogenation reaction’s selectivity. Carbon bases function as inert particles, enhancing the selectivity towards propylene.

As shown in Figure 6, a general decrease in propylene selectivity and increase in ethylene selectivity were observed with decreasing reaction temperature from 600 °C to 450 °C. This behavior can be attributed to multiple factors involving cracking pathways and the dynamic nature of VO_x active sites. At higher temperatures (600–550 °C), the VO_x/graphene oxide catalysts promoted the selective dehydrogenation of propane to propylene because of the enhanced activation energy available for C–H bond cleavage, with minimal C–C bond cracking. However, as the temperature decreased, the reduced reaction rate limited dehydrogenation efficiency, and the competitive formation of ethylene, likely through secondary cracking of propylene or heavier intermediates, increased. Additionally, lower temperatures may alter the oxidation state or dispersion of VO_x species, affecting the selectivity-determining steps by favoring C–C bond scission over selective C–H activation.

2.7. Comparison of Present Study with Open Literature

The performances of different types of vanadium catalysis in the present work are compared in

Table 2. Comparison of the performances of the synthesized catalysts in this study with similar studies in the literature.

Catalyst/Oxidizing Agent	T (°C)	Propane Conversion (%)	Selectivity (%)			C3H6 Yield (%)	Reference
			COx	C3H6	C2H4
V2O5/O2	500	6.73	-	25.26	73.70	1.69	[71]
V2O5/N2O	500	4.30	-	72.25	27.21	3.11	[71]
V2O5/Air	460	3.50	-	15.3	89.7	0.54	[72]
Al2O3/(O2 and H2S)	700	24.2	13.9	71.3	5.7	17.2	[73]
Al2O3/O2	700	26.8	34.8	44.5	3.1	11.9	[73]
V-Al2O3/(O2 and H2S)	700	53.7	25.1	56.5	1.0	30.4	[73]
V-Al2O3/O2	700	22.8	29.9	25.0	22.3	5.7	[73]
CeO2	450	7.25	0.0	18.14	82.21	1.32	[74]
V2O5-CNT/Air	500	17.21	3.45	51.02	47.24	8.78	[75]
V2O5-Benzylamine/Air	500	73	34.5	47.59	10.8	34.74	[75]
V2O5-Dodecyl Amine/Air	500	50.44	17.44	41.03	41.28	20.69	[76]
V2O5-Aniline-Graphene/Air	500	50.65	21.49	51.86	25.75	26.27	[76]
V-G-1	600	71.20	26.72	42.25	30.50	30.08	Present study
V-G-1	500	55.10	20.68	54.38	24.32	30.10	Present study

. Table 2 indicates that in this study, the optimal propane conversion occurred at 600 °C. Moreover, at 500 °C, the catalyst exhibited lower CO_x selectivity compared with similar works in the field. Notably, the propylene yield at 500 °C in this study was 30%, surpassing the yields reported in another study. This highlights the advantages of the catalyst developed herein.

This comparison demonstrates that the V-G-1 catalyst developed in this study was highly competitive with, and in some cases superior to, other reported vanadium-based systems, particularly at moderate temperatures. At 500 °C, the catalyst achieved a propylene yield of approximately 30%, which was higher than that of most comparable systems in the literature. Notably, some reported catalysts require higher temperatures (around 700 °C) to reach similar yields, whereas the V-G-1 catalyst delivered comparable performance at just 600 °C, highlighting the advantages of the proposed graphene-supported configuration. Furthermore, the V-G-1 system exhibited higher carbon conversion and maintained a reasonable balance between propylene selectivity and CO_x suppression. While a few reported systems may offer lower CO_x selectivity, the present catalyst combined improved olefin production with controlled combustion by-products. These benefits were likely due to better VO_x dispersion and redox behavior promoted by the graphene support. To further enhance catalytic efficiency, future studies could focus on optimizing VO_x loading, tailoring graphene surface functionalities, and introducing dopants to fine-tune the electronic properties and oxygen mobility for industrially relevant ODHP processes.

3. Methodology

3.1. Materials

All chemicals in this research work, unless noted otherwise, were of analytical grade and used without further purification. Vanadium pentoxide (V₂O₅, 89%), ethanol (87%), and n-hexane (99%) were purchased from Merck, Darmstadt, Germany, and mesoporous graphene (99%) was acquired from the Nanotechnology Research Institute, where its synthesis procedure and characterization are patented [77].

3.2. Preparation of Vanadium Oxide/Graphene Nanocatalysts

Various nanocatalysts were synthesized in different retention times between 2 to 48 h, as listed in

Table 3. Characteristics of the prepared nanocatalysts in this study.

Sample	V2O5/Graphene Molar Ratio	Hydrothermal Conditions
		Time (h)	Temperature (°C)
V-G-1	1/1	48	200
V-G-2		24
V-G-3		10
V-G-4		2

. In a typical experiment, vanadium oxide and porous graphene with a molar ratio of 1:1 were mixed in 200 mL ethanol under the magnetic stirrer at 80 °C for 2 h. Afterward, the prepared solution, after being combined with 50 mL of deionized water, was subjected to treatment in an autoclave at 200 °C for varying durations ranging from 2 to 48 h. Upon completion of the aforementioned procedures, the contents of the autoclave were subjected to multiple washes using n-hexane and ethyl alcohol. Subsequently, the mixture was dried in a vacuum oven at 80 °C for a duration of 24 h. This process concluded in the collection of vanadium oxide/graphene powders.

3.3. Characterization

The detailed morphology and crystal structure of the samples were characterized with field-emission scanning electron microscopy (FE-SEM, JSM 6400, JEOL Ltd., Tokyo, Japan) and X-ray diffraction (XRD, D6792 X’Pert MPD model, Philips, Amsterdam, The Netherlands, CuK_α radiation, λ = 0.15418 nm), respectively. Fourier transform infrared spectra (FTIR) spectra were recorded on a PerkinElmer spectrophotometer (Waltham, MA, USA) with KBr pellets in the 4000–400 cm⁻¹ region. The nanocatalysts were measured in the range of 500–4000 cm⁻¹ with a resolution of 1 cm⁻¹. Thermogravimetric analysis (TGA, TA Instruments SDT-Q600, New Castle, DE, USA) was employed to investigate the thermal behavior of the prepared powders. The catalytic activities of the samples were studied by temperature programmed reduction (TPR) isotherms and gas chromatography (GC, ASAP-2010, Shimadzu, Kyoto, Japan).

4. Conclusions

In summary, various vanadium oxide nanocatalysts were synthesized based on porous graphene using a hydrothermal method. FE-SEM and XRD analyses showed that nanostructured catalysts with regular and directional shapes were formed, which changed to needle-like and long tubular structures with increased hydrothermal time. The prepared structures comprised V₂O₅ and graphene particles, as demonstrated by FTIR. TGA analysis showed that the samples had three weight-loss stages corresponding to the outflow of water molecules, the decomposition of remaining organics, and the formation of vanadium pentoxide particles. The catalytic performance of the nanocatalysts was evaluated in the oxidative dehydrogenation of propane using TPR. In the temperature range of 450 to 600 °C, the prepared nanocatalysts demonstrated a higher selectivity for ethylene over propylene, as verified by gas chromatography.