Utilization of waste vanadium-bearing resources in the preparation of rare-earth vanadate catalysts for semi-hydrogenation of α,β-unsaturated aldehydes

Abstract

Recycling industrial solid waste not only saves resources but also eliminates environmental concerns of toxic threats. Herein, we proposed a new strategy for the utilization of petrochemical-derived carbon black waste, a waste vanadium-bearing resource (V > 30000 ppm (10 ⁻⁶)). Chemical leaching was employed to extract metallic vanadium from the waste and the leachate containing V was used as an alternative raw material for the fabrication of vanadate nanomaterials. Through the screening of various metal cations, it was found that the contaminated Na⁺ during the leaching process showed strong competitive coordination with the vanadium ions. However, by adding foreign Ce³⁺ and Y³⁺ cations, two rare-earth vanadates, viz., flower-like CeVO₄ and spherical YVO₄ nanomaterials, were successfully synthesized. Characterization techniques such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, Fourier-transform infrared, and N₂ physisorption were applied to analyze the physicochemical properties of the waste-derived nanomaterials. Importantly, we found that rare-earth vanadate catalysts exhibited good activities toward the semi-hydrogenation of α,β-unsaturated aldehydes. The conversion of cinnamaldehyde and cinnamic alcohol selectivity were even higher than those of the common CeVO₄ prepared using pure chemicals (67.2% vs. 27.7% and 88.4% vs. 53.5%). Our work provides a valuable new reference for preparing vanadate catalysts by the use of abundant vanadium-bearing waste resources.

Key words： petrochemical solid wastes; vanadium recovery; resource utilization; nanomaterials; semi-hydrogenation

1 Introduction

With the exponential growth of the world population and rapid development of the global economy, the disposal of huge solid waste has become a challenging global issue [1]. The treatment and disposal of solid wastes have been causing heavy burdens on landfills and posing serious impacts on human health and the natural environment. An economically viable and sustainable solution to this problem is to recycle and reuse waste, which not only reduces environmental pollution but also creates benefits commercially [ 2]. Currently, the utilization of solid wastes has become a hot topic for research. Various solid wastes have been explored and utilized, such as food waste [3], biomass waste [4], plastic waste [5], e-waste [6], non-ferrous metal waste [7], oil fly ash [8].

Recently, the oil refinery industry has become an important component of the global chemical market. Carbon black waste (a petroleum coke), which is produced from partial oxidation processes for treating heavy residual oil, is one of the most prominent petrochemical solid wastes [9]. Since vanadium is the most abundant and troublesome metal present in crude oil (still in trace quantities), the generated petroleum coke is thus rich in metallic vanadium (e.g., V₂O₅) due to combustion [10]. The vanadium-bearing waste can impair antioxidant enzymatic activities of human cell lines, and it has been demonstrated that the petroleum coke may cause contamination of the groundwater, posing toxicity, erosion, and fouling concerns [2]. Therefore, there is an urgent demand for vanadium recovery from carbon black waste, whereby the process is considered to be environmentally friendly, energy-saving, and can also create considerable economic benefits.

Similar to other solid wastes, the valuable metals can be eluted and recovered from metal-bearing resources by many different techniques, such as chemical leaching [11], biological leaching [12,13], chlorination [14], electrodialysis [15], chemical precipitation [16], solvent extraction [17], ion exchange [18], membrane filtration [19], and ion flotation techniques [20]. Although these traditional methods show high recovery capacity for metals, the high operating costs, harsh reaction conditions, or complex operating procedures always hinder their industrial applications. Meanwhile, it has been reported recently that waste vanadium can be a potential raw material for the fabrication of value-added vanadate nanomaterials [21,22].

Accordingly, we recognize that vanadium extracted from oil refinery solid waste is an alternative resource for the preparation of vanadate nanomaterials, which would be a suitable technology for treating solid waste in oil refineries [23,24]. Vanadate materials have many unique chemical and physical properties [25]. In recent years, vanadate nanomaterials have shown their superior value in many industrial fields such as catalysis [26], photocatalysis [27], lithium batteries [28], chemical sensors [29], biomedicine [30], and other aspects. Among them, rare earth vanadates have received intense attention due to their excellent electrical, optical, and magnetic properties, high surface area, and good thermal stability [31]. For instance, Kumar et al. successfully prepared a flower-like CeVO₄ with good electrocatalytic activity for electrochemical detection of tryptophan in food and biological samples [32]. Mishra et al. synthesized CeVO₄ with a tetragonal shape which showed significant photocatalytic activity for dye degradation [33]. Perala et al. investigated multifunctional YVO₄ nanoparticles doped with Ho³⁺, Yb³⁺, and K⁺, which can be used as a chemical sensor for detecting uranyl ions at a low concentration [34].

In this work, we reported the recovery and resource utilization of hazardous oil refinery solid waste through the preparation of rare-earth vanadate nanomaterials by using vanadium sourced solely from carbon black waste. As illustrated in , the preparation of vanadate nanomaterials follows two consecutive steps: (1) chemical leaching to obtain leachate containing vanadium and (2) chemical conversion of the liberated vanadium into value-added vanadate nanomaterials under hydrothermal treatments. Firstly, heavy metals (especially V, Fe, and Ni) were separated and extracted from the oil refinery solid waste by using alkaline solutions (i.e., NaOH) as leaching agents. After the screening of various foreign metal ions (Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺, In³⁺, Cu²⁺, Ce³⁺, Y³⁺, etc.), two rare-earth vanadates (CeVO₄ and YVO₄), were successfully obtained by using the base leachate solution. The effects of synthetic parameters were also investigated. The physicochemical properties of the waste-derived nanomaterials were systematically characterized. Furthermore, the catalytic performance of the obtained rare-earth vanadates was investigated towards the semi-hydrogenation of α,β-unsaturated aldehyde to unsaturated alcohol. Accordingly, we explored a more efficient and economical preparation process, which is quite different from the traditional solution-based synthesis using pure vanadium sources (such as NaVO₃, NH₄VO₃, or V₂O₅) as raw materials. The new process would provide an alternative way to recycle and comprehensively utilize waste vanadium-bearing resources.

Scheme1 The schematic illustration of the preparation routes of rare-earth vanadates from a vanadium-bearing waste and the catalytic application for semi-hydrogenation of cinnamaldehyde to cinnamic alcohol (COL).

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2 Experimental

2.1 Chemicals and materials

Vanadium-bearing waste (carbonaceous solid residue from gasification of crude oil bottoms) was collected from an oil refinery in Quanzhou, China. The following chemicals were used as received without further purification: NaOH (≥ 98%, Greagent), HNO₃ (65–68 wt %, Macklin), HCl (36–38 wt %, Macklin), Ce(NO₃)₃·6H₂O (99.99%, Adamas), Y(NO₃)₃·6H₂O (99.99%, Adamas), Cu(NO₃)₂·3H₂O (≥ 99%, Adamas), MnCl₂·4H₂O (99%, Adamas), Ni(NO₃)₂·6H₂O (99%, Adamas), Zn(CH₃COO)₂·2H₂O (99%, Adamas), CdCl₂ (99%, Macklin), InCl₃·4H₂O (≥ 99%, Macklin), FeCl₃ (98%, Adamas), Co(NO₃)₂·6H₂O (99%, Macklin), hexadecyltrimethylammonium chloride (CTAC, 99%, Adamas), phosphate buffer solution (PBS, 0.01 mol·L^–1, Solarbio), cell counting kit-8 (CCK-8, Solarbio), cinnamaldehyde (CAL, ≥ 99%, Adamas). All other chemicals are analytically pure grade and were used without further purification. The water used in these experiments was deionized using PURELAB water filtration technology (ELGA LabWater system) in our laboratory. Bioactive human umbilical vein endothelial cells (HUVEC) were purchased from Conservation Genetics CAS Kunming Cell Bank (Yunnan, China).

2.2 Chemical leaching of carbon black waste

The as-received wet carbon black was first dried in an oven at 150 °C for 12 h to completely remove the contained moisture. Then, 3.0 g of the dry solid waste (labeled original carbon black waste, CB-O) was dispersed in 60 mL NaOH (1 mol·L^–1) or 0.3 g in 60 mL of HNO₃ (0.5 mol·L^–1) and stirred for 3 h at room temperature. Next, the leaching solution was collected by using a suction filtration device with a mixed cellulose ester membrane (pore size: 0.22 μm, diameter: 47 mm). The treated carbon black waste samples were denoted as CB-A and CB-B, which are corresponding to the acid leaching sample and the base leaching sample, respectively.

2.3 Cytotoxicity assay of solid samples and leachates

The cytotoxicity of the carbon black samples before and after the chemical leaching experiment was compared. First, the three solid samples (i.e., CB-O, CB-A, and CB-B) were dispersed in deionized water with different concentrations, and then the supernatant (after mixing for 12 h) was separated and collected for testing. HUVEC cells were selected for cytotoxicity assay. The cell viability was evaluated using CCK-8. In the process, HUVEC cells were seeded into a 96-well plate at a density of about 10000 cells per well and incubated for 24 h. After the cells were fully attached to the bottom of the wells, the original medium was replaced with 200 μL of fresh medium containing the solid samples to investigate the possible toxic effects on HUVEC cells. When testing the toxicity of water leachate solutions, 20 μL of various concentrations of leachate and 180 μL of culture medium were added. After the medium was aspirated, followed by rinsing with PBS solution, 100 μL of medium containing 1% CCK-8 reagent was added to each well, and the cells were further incubated for 3 h. Subsequently, the UV-vis absorbance at 450 nm was measured with a microplate reader. Cell viability in each well was calculated as the ratio of absorbance to that of the control well. Four groups of parallel experiments were designed for each concentration under the same conditions.

2.4 Synthesis of varied vanadate products

For the synthesis of CeVO₄ and YVO₄, 5 mL of the alkaline leaching solution was mixed with 5 mL of aqueous HCl solution (1 mol·L^–1) under stirring. Upon addition of HCl, the color of the solution was changed immediately from colorless and transparent to orange–yellow. Then, 78 mg of Ce(NO₃)₃·6H₂O or 69 mg of Y(NO₃)₃·6H₂O was added. The stoichiometry of the precursor solution was Ce:V = 1:1 (or Y:V = 1:1). During this process, orange–yellow precipitates were formed in the mixed solution. Upon vigorous stirring for 10 min, 1 mL of CTAC (5 wt %) was added to the mixed solution and the mixture was further stirred for 30 min at room temperature. The final solution was then transferred to a Teflon-lined steel autoclave (capacity: 25 mL) for hydrothermal treatment at 180 °C for 12 h. After the reaction, the vanadate products were collected by centrifugation and washed three times with water (or ethanol) to remove impurities. The solid product was finally dried at 60 °C for 4 h for future use.

2.5 Characterization methods

The morphologies and microstructures of the solid waste and vanadates samples were characterized with scanning electron microscopy (SEM, Hitachi, SU5500) and transmission electron microscopy (TEM, Thermo Fisher Scientific, Talos F200X G2). Energy-dispersive X-ray spectroscopy (EDX) was conducted by the same TEM system to analyze the compositional information of the materials. The crystallographic information of both raw materials and synthesized nanomaterials were analyzed by X-ray diffraction (XRD, Bruker D8 Advance) operated at 30 mA and 40 kV using Cu Kα radiation (λ = 1.54182 Å). The metal contents in solids and leachate solutions were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, PerkinElmer, Optima 7300DV). Surface chemical compositions of the samples were analyzed with an X-ray photoelectron spectroscopy (XPS, Thermo, ESCALAB 250XI). Thermogravimetric analysis studies were carried out on a thermal analyzer (TGA, Shimadzu DTG-60H) under air (or N₂) atmosphere at a temperature ramping rate of 10 °C·min^–1. Fourier-transform infrared (FTIR) spectra were collected on a spectrometer (Nicolet iS50, Thermo). The N₂ physisorption isotherms were carried out using a Quantachrome Autosorb-IQ system at 77 K.

2.6 Chemoselective hydrogenation of CAL

Selective hydrogenation of CAL was carried out in a high-pressure stainless reactor (HT-100YKC) with a Teflon liner (100 mL). Specifically, 30 mg of catalyst was dispersed into 20 mL of water under sonication. Upon even dispersion, 100 μL of CAL was added dropwise under stirring. The temperature and H₂ pressure in the reactor were monitored by a thermocouple and a pressure gauge, respectively. Unless specified otherwise, the reaction was carried out at 60 °C under 1 MPa (H₂ atmosphere) for 3 h, and the stirring speed was set to 500 r·min^–1. After the reaction was completed, the organic products were extracted with ethyl acetate and were analyzed using a gas chromatograph (Agilent 7890A) equipped with a flame ionization detector. In addition, the corresponding standard samples were also used to calculate the correction factor for each component, which was used to calculate the conversion and selectivity data of the catalytic reaction.

3 Results and discussion

3.1 Elemental analyses of the solid waste

As shown in Table S1 (cf. Electronic Supplementary Material, ESM), the CB-O sample contains a vanadium concentration of 30676 ppm (data measured by ICP), which is the highest element among the inorganic compounds. This provides feasibility for the subsequent recycling of vanadium for the preparation of vanadate nanomaterials. At the same time, Ni and Fe were the two other major metals in the solid waste with concentrations of 9941 and 1881 ppm, respectively. Additionally, alkali metals and alkaline earth metals such as K, Na, Ca, Mg, as well as transition metals (Co and Zn) were also present in the CB-O sample. However, the overall content of these elements was less than 2500 ppm.

The morphologies and structures of the CB-O, CB-B, and CB-A samples are shown in Fig.1. The SEM images of the samples are displayed in Fig.1(a–c), which show almost similar morphology. The carbonaceous particles are flocculent in appearance, and the porous structures consist of disordered stacking of granular particles. The TEM images in Fig.1(d–f) further confirm that carbon particles are spherical with both hollow and non-hollow structures. It should be noted that the carbonaceous particles mainly exist in the form of aggregates instead of primary particles. Although it is difficult to observe the isolated primary particles (especially for the CB-A sample), the size of some primary particles could be measured at around 38 nm in three samples. It should be noted that these fine carbon black powders with low density are easily suspended in the air. Thus, they may cause certain damage to human health. However, the dispersion of carbon black powders in water, acid solution, or alkaline solution can largely prevent the emissions of these particulates into the atmosphere.

Fig.1 Representative SEM images of (a) CB-O, (b) CB-B, (c) CB-A, and TEM images of (d) CB-O, (e) CB-B, (f) CB-A samples.

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3.2 Chemical leaching of carbon black waste

The leaching efficiency of heavy metals from the CB-O sample was studied by using NaOH (1 mol·L^–1) and HNO₃ (0.5 mol·L^–1) as leaching agents. The two filtrated solid samples were labeled as CB-B and CB-A, respectively. Actually, the ICP analysis concluded that the resultant CB-B solid sample mainly contains Ni and Fe, which were 9299 and 1698 ppm, respectively. However, the V content was greatly decreased to 2377 ppm, accounting for a leaching efficiency of 92.3%. This result indicates that alkaline solution has a unique leaching selectivity for vanadium. By comparing with CB-O, the concentrations of V (1320 ppm), Ni (1148 ppm), and Fe (392 ppm) in the CB-A solid sample were significantly lesser, indicating that most of the three heavy metals could be dissolved in the acid leaching solution. As can be seen in Table S2 (cf. ESM), HNO₃ was an effective leaching agent for removing heavy metals (V, Ni, and Fe) in carbon black waste. The leaching efficiency of V was about 95.7%, while 88.5% of Ni and 79.2% of Fe could be eluted under the same conditions.

It could be observed that the color of the CB-A leachate solution was blue, while the CB-B leachate solution was colorless, suggesting different chemical compositions in the two leachate solutions. The presence of the blue color of the acid leaching solution can be attributed to the coexistence of VO²⁺, Ni²⁺, and Fe²⁺ ions [35]. Despite HNO₃ (0.5 mol·L^–1) can effectively extract various metals from carbon black waste, it is not conducive to the effective separation of these metals. Thus, the acid leaching solution was not suitable for further selective conversion of vanadium ions to vanadate nanomaterials. Furthermore, as shown in Table S2, V with a concentration of 1848 ppm was detected in the leachate solution when NaOH was used as the leaching agent, but Ni and Fe were hardly extracted under the same conditions. This is because metallic compounds (such as NiO, Ni(OH)₂, and FeO) might be insoluble in alkaline solution but soluble in acid solution [23,24,36]. In the alkaline leaching solution, vanadium is mainly in the form of monomeric anions VO₄^3–, which led to the colorless appearance of the solution. Therefore, based on this result, NaOH (1 mol·L^–1) was chosen as the leaching agent to carry out the recovery and transformation of vanadium in the followed experiments.

3.3 TGA characterization of solid waste before and after leaching treatments

The thermal behaviors of the CB-O, CB-B, and CB-A samples under different atmospheres (air or N₂) were studied by the TGA technique (Fig.2). The weight loss (about 2.5%) observed before 100 °C can be attributed to the removal of moisture and any other volatile impurities. As shown in Fig.2(a), it was found that about 90 wt % weight loss occurred in the range between 350 °C and 700 °C, which can be assigned to the oxidative decomposition of carbon. At the final temperature (800 °C), CB-O had the highest residual weight (7.3 wt %), which was due to the possibility of heavy metals remaining in the resulting ash [37,38]. Accordingly, the relative carbon content of CB-B and CB-A samples can be determined as 92.45 and 94.75 wt %, respectively. However, as shown in Fig.2(b), the three samples demonstrated very different degradation behaviors in a N₂ atmosphere. The weight loss curve shows not only the carbon oxidation reaction but also other reactions such as the decomposition of hydroxides and carbonates. The CB-O showed the largest weight loss, which may be due to the thermal decomposition of carbonaceous materials promoted by the metal components.

Fig.2 TGA profiles of CB-O, CB-B, and CB-A in (a) air and (b) N₂ atmospheres.

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3.4 Cytotoxicity evaluation of the solid waste

The toxicity of the discarded black carbon waste is a great concern before conducting resource utilization research. Herein, the toxicity tests were performed on HUVEC cells mixed with carbon black waste samples (CB-O, CB-B, and CB-A) and the aqueous leachate solutions of the three samples (Fig.3). As shown in Fig.3(a), the CB-O sample was highly toxic to HUVEC cells with cell viability of less than 18%. As compared to that of CB-O, the vanadium removal rate of the CB-B sample was about 92.3%. Under the same sample concentration, the cell survival rate was nearly doubled. In the case of the CB-A sample, most of the metals were removed (95.7% V, 88.5% Ni, and 79.2% Fe), resulting in a nearly twofold increase in cell survival as compared to being exposed to untreated samples. Accordingly, it can be concluded that the cytotoxicity of the solid waste can be decreased to a certain extent after chemical leaching treatments. Since carbon black is a carbon-based material, it can be speculated that the carbonaceous material might adsorb part of the nutrients in the medium, which affect the cell viability and lead to the death of some HUVEC cells due to lack of nutrients [24].

Fig.3 Viability of HUVEC cells in different concentrations of waste samples, (a) solid samples of (A) CB-O, (B) CB-B, (C) CB-A, and (b) leachate solutions of (D) CB-O, (E) CB-B, (F) CB-A. All data were conducted through four independent parallel experiments. The data points in the figures were expressed as the average value with a standard deviation.

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As the untreated carbon black waste is usually disposed into landfills or incineration plants, heavy metals may be partially extracted through rainwater and pollute groundwater, causing irreversible damage to the natural ecology. Thus the toxicity test was carried out on the water leachate solutions of CB-O, CB-B, and CB-A (Fig.3(b)). It was found that with the increase in leachate concentration of the three samples, CB-O leachate is increasingly toxic to HUVEC cells. However, due to the low concentration of soluble toxic heavy metals in CB-A and CB-A leachate, there was insignificant cytotoxicity, and the cell survival rate could achieve more than 80%. Again, the chemical leaching treatment lowers the risk of such carbon black samples during resource utilization. Therefore, it is suggested that the chemical leaching treatment not only recovers the vanadium ions from solid waste but also lowers the cytotoxicity. Upon removal of hazardous heavy metals, the carbonaceous materials might be used as safe conductive or adsorbent materials for other studies.

3.5 Synthesis and characterizations of various vanadates nanomaterials

After adding HCl into the base leachate solution (initial pH of 14.25), the mixed solution immediately turned from colorless to orange–yellow with a final pH value of 8.35. Due to the strong complexing ability of vanadium, polyanionic aggregates were formed, which led to the color change of the solution as the solution pH decreased [39]. Vanadium mainly exists in the valence of +5 at a pH of 8.35, even without adding any other oxidants. With the change in vanadium concentration and solution pH, the chemistry of the vanadium compounds in the solution may contain [(V(OH)_h(OH₂)_6−h]^(5−h)+, V₃O₉^3–, V₁₀O₂₈^6–, VO₂⁺, and other polyvanadate anions [39,40].

As the NaOH was used during the chemical leaching process, the resultant base leachate solution inevitably contains a large amount of Na⁺. SEM images and XRD patterns of the corresponding products are shown in Fig.4. Without the introduction of foreign metal ions, HNaV₆O₁₆·4H₂O crystal was produced after a hydrothermal treatment of the leachate solution (180 °C for 12 h) [41]. As shown in Fig.4(a), the product exhibited a flower-like shape with a hierarchical porous structure. The average particle size was about 10 μm. XRD pattern confirmed the chemical composition (JCPDS card No. 49-0996, Fig.4(f)). Moreover, when CTAC surfactant was added to the mixed solution before the hydrothermal treatment, the product showed a clear nanoribbon morphology with a length of about 3 μm (Fig.4(b)). It is suggested that the complexation of CTA⁺ in the polyionic solution profoundly affects the nanostructures of the product [42].

Fig.4 Representative SEM images of vanadium-containing products by adding different amounts of Cu(NO₃)₂·3H₂O: (a) control experiment without CTAC, (b) 0 g, (c) 0.05 g, (d) 0.075 g, (e) 0.1 g, and (f) XRD patterns of corresponding products.

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Next, we attempted to introduce Cu(NO₃)₂ to the base leachate solution to check if copper vanadate can be produced. It is obvious that the concentration of Cu(NO₃)₂ greatly affected the morphology of the product (Fig.4(b–e)). It is also worth noting that by adding a small amount of Cu(NO₃)₂·3H₂O (0 or 0.05 g), the expected copper vanadate was not able to be obtained. However, only rod-shaped HNaV₆O₁₆·4H₂O was produced instead (Fig.4(c)), which was due to the inability of low concentration Cu²⁺ to be combined with vanadium oxyanions for the formation of copper vanadate. XRD confirmed that the sample was assigned to HNaV₆O₁₆·4H₂O phase (Fig.4(f)). As the amount of Cu(NO₃)₂ was increased to 0.075 and 0.1 g, it was found that the products were copper pyrovanadate (Cu₃(OH)₂V₂O₇·2H₂O, JCPDS card No. 46-1443) in the shapes of polyhedra (Fig.4(d)) and rods (Fig.4(e)), respectively. The rod-like structure of Cu₃(OH)₂V₂O₇·2H₂O might be due to the anisotropic growth following the “Osterwald ripening” process at the expense of smaller particles [43]. It has been demonstrated that copper nitrate, rather than copper acetate, is favorable for fabricating one-dimensional Cu₃(OH)₂V₂O₇·2H₂O crystals [43]. Although traces of other metal ions might occur in the leachate solution after the leaching process, they can be largely removed from the product by centrifugation and washing treatments after the hydrothermal reactions.

When HCl is gradually added to the alkali leaching solution, various polymerization transformations of vanadic acid radicals will occur, forming a complex ionic environment of vanadium-containing solution [44]. From a charge balance point of view, vanadium polyanions could be bonded to any metal cations to form precipitate materials under the hydrothermal treatment. Therefore, we screened other different metal cations in the alkaline leachate to check the phase, size, and morphology of the obtained solid products (Fig.5). As the contaminated Na⁺ showed a strong competitive relationship in the coordination process, the formation of HNaV₆O₁₆·4H₂O seems to be inevitable. During this process, many metal ions (e.g., Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺, In³⁺, etc.) showed poor combinability with the vanadium polyanions. The results showed that it was difficult for these metal ions to coordinate with vanadium polyanions to form corresponding metal vanadates (Fig.5(a)). As shown in Fig.5(b), the XRD peaks of the products are basically consistent with the standard crystalline HNaV₆O₁₆·4H₂O. Amorphous vanadate crystals may be formed during the reaction period, but they are unstable and would be decomposed in the final product. Moreover, in the cases of Fe³⁺ and Co²⁺, the chemical compositions of the obtained products cannot be determined by our current characterization techniques (see Figs. S1 and S2 (cf. ESM), respectively). When Fe³⁺ was used as the foreign metal cation, the possible components of the obtained product were V, Na, Fe, and O. The concentrations of the V and Fe elements were as high as 244976 and 245316 ppm, respectively, by ICP analysis (Table S3, cf. ESM). However, the content of the Co element was only 996 ppm in the product by using Co²⁺ as the foreign metal cation (Table S4, cf. ESM).

Fig.5 (a) SEM images, and (b) XRD patterns of the products obtained by introducing different types of metal cations (e.g., Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺, or In³⁺) into the alkaline leachate solutions.

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In addition, the orthovanadate ion (VO₄^3–) is a very suitable host for the rare-earth ions due to the relatively simple Crystal Field splittings [45]. Thus, we introduced two rare-earth ions (Ce³⁺ and Y³⁺, at a stoichiometric ratio) to the leaching solution to check the possible products. It was found that the obtained orthovanadates were CeVO₄ and YVO₄ with uniform sizes and morphologies. In addition, the preparation process of the rare-earth vanadates had high reproducibility, which is suitable for large-scale synthesis. It should be noted that during the hydrothermal crystallization process, CeV₃O₉·H₂O and some other decavanadates may appear. However, these precipitations will eventually decompose to form orthovanadates after the hydrothermal process (at 12 h) [44].

The crystal composition, morphology, porosity, and size of the obtained rare earth vanadates were systematically analyzed by XRD, SEM, TEM, and EDX techniques (Fig.6). As evident from the SEM (Fig.6(a) and 6(b)) and TEM images (Fig.6(c) and 6(d)), the CeVO₄ product exhibits pencil-shaped nanorods with a length of 1–2 μm and a diameter of 300–400 nm. The nanorods as building blocks were assembled into a flower-like structure. From the high resolution transmission electron microscope (HRTEM) image (Fig.6(e)), the d-spacing for the adjacent lattice of 0.372 nm is in accordance with the interplanar space of the (200) lattice plane of the CeVO₄ [46]. The homogeneous distribution of V, Ce, and O elements in the entire structure was further validated by the EDX elemental maps (Fig.6(f)).

Fig.6 Characterizations of CeVO₄ sample, (a, b) SEM images, (c, d) TEM images, (e) high-resolution TEM image, and (f) the corresponding EDX elemental maps.

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Furthermore, XRD patterns and FTIR spectra were also investigated (Fig.7). As shown in Fig.7(a), the phase purity and crystalline structure of the tetragonal CeVO₄ product were determined by XRD (JCPDS card No. 12-0757) [46]. The FTIR spectra further confirmed the chemical structure of the CeVO₄ materials (Fig.7(c)), wherein, the major IR bands found at 792 and 445 cm^–1 can be attributed to the V–O stretching vibration and Ce–O stretching vibration, respectively. The observed broad IR bands at around 3417 and 1629 cm^–1 are assigned to the stretching vibration and bending vibration of the O–H group, respectively, belonging to the physically adsorbed water on the sample surface [46,47].

Fig.7 (a, b) XRD patterns and (c, d) FTIR spectra of the synthesized CeVO₄ and YVO₄ samples.

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Similarly, the characterizations of the YVO₄ product prepared from the base leachate solution of the solid waste are presented in Fig.8. As can be seen from the SEM and TEM images (Fig.8(a–d)), the product consists of uniform and nearly spherical particles with an average particle size of 4.5 μm. The HRTEM image (Fig.8(e)) shows clear lattice fringes with d-spacing of approximately 0.264 nm which can be assignable to the (112) crystal plane of YVO₄ [48]. As shown from the EDX elemental maps in Fig.8(f), the Y, V, and O elements were distributed uniformly in the synthesized YVO₄ particles. Furthermore, Fig.7(b) depicts the XRD pattern of the sample which was found to be well-matched with the tetragonal YVO₄ phase (JCPDS card. No 17-0341) [49]. It is noteworthy that no other impurities were detected in the XRD patterns, indicating the high purity of the obtained YVO₄ material. The chemical bonds of YVO₄ were characterized by FTIR, as shown in Fig.7(d). A strong IR band at 810 cm^–1 and a weak IR band at 453 cm^–1 were observed, which can be attributed to the V–O and the Y–O stretching vibrations, respectively. Similarly, the IR bands observed at around 3428 and 1630 cm^–1 are characteristic of the O–H groups caused by the surface hydration [50,51].

Fig.8 Characterizations of YVO₄. (a, b) SEM images, (c, d) TEM images, (e) high-resolution TEM image, and (f) the corresponding EDX elemental maps.

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Moreover, the porous structure and thermal behaviors of the rare-earth vanadate samples were investigated (Fig.9). N₂ physisorption isotherms and pore size distributions of CeVO₄ and YVO₄ samples are shown in Fig.9(a) and 9(b), respectively. The specific BET surface area (S_BET), pore volume, and average pore diameter of the samples are summarized in Tab.1. The N₂ physisorption isotherm of CeVO₄ can be assigned to type-IV with a narrow hysteresis loop at high relative pressure, indicating the existence of mesopores due to the inter-particle space [52]. The specificS_BET of CeVO₄ was 36 m²·g^–1, which was much higher than that of YVO₄ (0.33 m²·g^–1). Meanwhile, the pore volume in YVO₄ material became even smaller (0.0034 vs. 0.18 cm³·g^–1). The average pore size of the CeVO₄ sample was about 3.06 nm.

Tab.1 Textural properties of CeVO₄ and YVO₄ samples

Sample	BET surface area a)/ (m2·g–1)	Pore diameter b)/ nm	Pore volume c) / (cm3·g–1)
CeVO4	36	3.06	0.18
YVO4	0.33	n.a.	0.0034

(a) Determined by the multi-point BET method; (b) obtained from the BJH method by using the data in the desorption branch; (c) calculated at the relative pressure of 0.990 (the highest P/P0 point).

Fig.9 (a) N₂ physisorption isotherms, (b) the corresponding pore size distributions (based on the Barrett–Joyner–Halenda (BJH) method), and (c, d) TGA profiles in air and N₂ atmospheres of CeVO₄ and YVO₄ samples.

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The thermal behaviors of the rare-earth vanadates under different atmospheres (air or N₂) were studied by the TGA technique (Fig.9(c) and 9(d)). In both cases of air and N₂ atmospheres, the first weight loss observed at temperatures higher than 100 °C is related to the removal of adsorbed water molecules on the surface of CeVO₄ and YVO₄ samples. More importantly, the weight loss of both materials was found to be less than 5% as the temperature was increased to 800 °C, indicating the good thermal stability of the prepared rare-earth vanadate nanomaterials.

Furthermore, in order to study the chemical states and surface compositions of CB-O and the obtained vanadate materials, XPS analysis was conducted and the results are depicted in Fig.10. As shown in Fig.10(a), the core level binding energies of V 2p_3/2 and V 2p_1/2 of the CB-O sample were found at 517.2 and 524.6 eV, respectively, indicating a valence state of +5 for V (e.g., V₂O₅). However, for the vanadate samples (Cu₃(OH)₂V₂O₇·2H₂O, HNaV₆O₁₆·4H₂O, CeVO₄, and YVO₄), the V 2p XPS spectra can be deconvoluted into two spin-orbit doublets (Fig.10(b–e)). The peaks can be attributed to high-valent V⁵⁺ and low-valent V³⁺ species, suggesting the rich surface chemistry of the as-synthesized vanadates [53]. Both V⁵⁺ and V³⁺ species were found over the surface of as-synthesized vanadates, suggesting that the hydrothermal process may be accompanied by the redox reaction from V⁵⁺ to V³⁺. In addition, the amount of trivalent state V³⁺ was highly related to the existence of defects (e.g., oxygen vacancy) and active sites in the vanadates [54]. In addition, for the CeVO₄ sample, as shown in Fig.10(f), the binding energies of Ce 3d_5/2 are at 881.6 and 885.7 eV, and those of Ce 3d_3/2 are at 900.0 and 904.1 eV, which can be assigned to the Ce³⁺ valance states in the sample [46]. Fig.10(g) displays the XPS spectrum of Y 3d, wherein the two strong peaks located at the binding energies of 157.8 and 159.8 eV can be ascribed to the Y 3d_5/2 and Y 3d_3/2 of Y³⁺, indicating the existence of Y³⁺ in YVO₄ [48]. Especially, the O 1s peak is quite asymmetric and can be fitted with two peaks. The peak at 529.9 (Fig.10(h)) and 530.1 eV (Fig.10(i)) can be attributed to the lattice oxygen of CeVO₄ and YVO₄, respectively. The other peaks at 531.6 (Fig.10(h)) and 532.2 eV (Fig.10(i)) may correspond to the adsorbed oxygen species on the surface oxygen vacancy [46].

Fig.10 XPS spectra in the V 2p region of (a) CB-O, (b) Cu₃(OH)₂V₂O₇·2H₂O, (c) HNaV₆O₁₆·4H₂O, (d) CeVO₄, and (e) YVO₄, (f) Ce 3d XPS spectra of CeVO₄, (g) Y 3d XPS spectra of YVO₄, and (h, i) O 1s XPS spectra of rare-earth vanadates.

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3.6 Catalytic performance of the rare earth vanadates

The semi-hydrogenation of α,β-unsaturated aldehyde to unsaturated alcohols is an important industrial process for the production of pharmaceutical intermediates and fine chemicals [55]. However, the hydrogenation of the C=O bonds is thermodynamically and kinetically unfavorable for the hydrogenation of the C=C bonds, leading to a low selectivity toward unsaturated alcohols [56,57]. The typical reaction pathway for the hydrogenation of CAL is shown in . Due to the concomitant C=C and C=O groups in CAL, three products might be obtained, including the target product COL, hydrocinnamaldehyde (HCAL), and hydrocinnamyl alcohol (HCOL) [58]. The catalytic performance data of the rare-earth vanadate catalysts are shown in Fig.11. We first investigated the effect of the amount of CeVO₄ on the catalytic performance. As shown in Fig.11(a), the CAL conversion was found to increase from 40.2% to 67.2% as the catalyst dosage was increased from 10 to 30 mg. Meanwhile, the selectivity of the target product COL was found to increase significantly from 47.0% to 88.4%. As the catalyst amount was further increased to 50 mg, the selectivity of COL was decreased to 73.4%, which might be due to the over-hydrogenation reaction. Meanwhile, the selectivity of HCOL increased rapidly from 7.7% to 23.4%.

Scheme2 Reaction pathway for the hydrogenation of CAL to COL, HCAL, and HCOL products.

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Fig.11 Catalytic performance of rare-earth vanadates for the chemoselective hydrogenation of CAL under different conditions: (a) the effect of the amount of CeVO₄ catalyst, (b) the effect of the reaction temperature, (c) the effect of the H₂ pressure, and (d) comparison of catalytic performance with YVO₄ and c-CeVO₄ catalysts.

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It can be seen from Fig.11(b) that the conversion of CAL increased from 8.3% to 72.7% with the increase in reaction temperature from 40 to 70 °C. The COL selectivity increased slightly to 88.9% at 70 °C. In general, high-pressure hydrogen can improve the solubility of H₂, making hydrogen molecules easily accessible to substrates and active sites [59]. Thus, we studied the effect of H₂ pressure on catalytic performance (Fig.11(c)). As expected, an increase in H₂ pressure from 0.5 to 3 MPa led to a linearly increased CAL conversion from 54.2% to 98.1%. The COL selectivity increased slightly from 82.3% to 88.4% as the H₂ pressure was increased from 0.5 to 1 MPa. However, the COL selectivity decreased sharply with a further increase in H₂ pressure due to the hydrogenation of the C=C bond to form HCOL [60]. At the H₂ pressure of 3 MPa, the COL selectivity was found to drop to 3.7%. Interestingly, as shown in Fig.11(d), the catalytic performance of YVO₄ showed similar activity to CeVO₄. Moreover, we also prepared a comparison c-CeVO₄ catalyst by a co-precipitation method using pure chemicals (that is, Ce(NO₃)₃·6H₂O and NH₄VO₃, refer to the supplemental information for the synthetic details) rather than the waste vanadium source. The SEM image and XRD pattern of the c-CeVO₄ are shown in Fig. S3 (cf. ESM). It can be seen that the product morphology was observed to be noneven, with the coexistence of both nanoparticles and rods. All the XRD diffraction peaks can be readily indexed to the pure phase of CeVO₄ (JCPDS card No. 12-0757). However, the conversion of CAL and selectivity of COL over c-CeVO₄ catalyst were much lower than waste-derived CeVO₄ catalyst (27.7% vs. 67.2% and 53.5% vs. 88.4%). This indicated that the c-CeVO₄ exhibited poorer performance as compared to the waste-derived CeVO₄.

4 Conclusions

In summary, we demonstrated a new recycling strategy for hazardous oil-refinery solid waste by extraction of vanadium ions and the transformation to value-added rare earth vanadates (CeVO₄ and YVO₄). At the same time, the combination manners between different metal cations and vanadium ions were also investigated. During the facile hydrothermal treatment, many metal cations (e.g., Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺, In³⁺, etc.) showed poor combinability (or affinity) with the vanadium polyanions, leading to the formation of crystalline HNaV₆O₁₆·4H₂O rather than the respective metal vanadates. It is obvious that the concentration of Cu(NO₃)₂ greatly affected the morphology of the Cu₃(OH)₂V₂O₇·2H₂O product. On the other hand, two rare-earth vanadates (CeVO₄ and YVO₄) with high purity, good crystallinity, and uniform morphology (flower-like and spherical shapes, respectively) were achieved by adding the rare-earth cations, which are suitable for large-scale production with high reproducibility and low raw material cost. Moreover, the obtained rare-earth vanadates could serve as effective catalysts for the selective hydrogenation of cinnamaldehyde to COL. Under optimal conditions, the utilization efficiency of waste vanadium to vanadate catalyst was higher than 80%. The highlight of this work is the successful realization of the resource utilization of low-cost vanadium-bearing waste in the petrochemical industry.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. U21A20324 and 21908073), the Natural Science Foundation of Fujian Province (Grant Nos. 2019J01074 and 2021J06026). We also thank Mr. Pingping Chen from Fujian Refining & Petrochemical Company for the helpful discussions on this project and thank the Analysis and Testing Center of Huaqiao University for providing part of the characterizations.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://dx.doi.org/10.1007/s11705-022-2191-x and is accessible for authorized users.

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