Ammoxidation of alkyl aromatics and heteroaromatics to their corresponding aromatic nitriles has been considered another significant breakthrough in the chemical industry after the ammoxidation of propylene to acrylonitrile, and has attracted a great deal of attention [1-21] because the nitriles are very useful organic intermediates for the preparation of a good number of industrially important chemicals such as aromatic acids, amides, amines and so on, which provide raw materials for further production of pharmaceuticals, pesticides, pigments, dyes, rubbers, and optoelectronic materials. Gas-phase ammoxidation of 2, 6-dichlorotoluene (DCT) to 2, 6-dichlorobenzo- nitrile (DCBN) is of particular importance owing to the high industrial significance of DCBN for manufacturing many effective herbicides, fungicides, and various special kinds of engineering plastics [15-21]. Nevertheless, the realization of this reaction with high conversion and yield remains a challenging task and is much more difficult compared to the ammoxidation of various other alkyl aromatics and heteroaromatics due to steric hindrance of the reacting methyl group by two neighboring bulky chlorine atoms.

Martin et al. [18-21] have used vanadium phosphates (VPO) as catalysts to successfully ammoxidize DCT to DCBN with almost 99% conversion and ca. 70% yield at 400–420 ℃. Our group has been committed to developing highly active and selective catalysts and effective strategies for the ammoxidation of methyl aromatics to the corresponding aromatic nitriles [1-5, 15-17], and has studied the catalytic performances of silica-supported vanadium-phosphorus oxides (VPO/SiO₂) [15, 16] and vanadium-chromium oxides (VCrO/SiO₂) [17] for the ammoxidation of DCT to DCBN.

Vanadium-chromium composite oxides (VCrO) have been the subject of extensive research because of their use in a wide range of applications such as heterogeneous catalysis [1-9, 22, 23], gas sensing [24], and energy storage [25, 26]. They were traditionally prepared by high-temperature solid-state reactions between pure oxide components. Depending on the reaction conditions, various phases such as CrVO₄ [27], Cr(VO₃)₃ [28], Cr₄(V₂O₇)₃ [29], and Cr₂V₄O₁₃ [30] may occur. Among these, CrVO₄ has been widely used as an oxidation catalyst in some important industrial reactions, including oxidation of benzene to maleic anhydride or methanol to formaldehyde and the ammoxidation of aromatic compounds [1-9, 14, 31]. CrVO₄ has three different crystal structures, namely orthorhombic, monoclinic, and tetragonal [32]. Orthorhombic CrVO₄ has been obtained by heating an amorphous precursor, CrVO₄·3H₂O, at 450 ℃, whose precursor was synthesized by the reaction of solutions of ammonium vanadate (NH₄VO₃) and chromium nitrate [Cr(NO₃)₃·9H₂O] [33]. It could also be prepared by grinding a mixture of hydrous amorphous chromium oxide and vanadium oxide [34]. However, mixed phases of VCrO complexes were produced in these cases and the particle sizes were not well controlled. When used in an oxidation or ammoxidation reaction, a high temperature was required and resulted in the over-oxidation of the raw material as well as the deposition of carbon and loss of activity over time [27].

Recently, Huang et al. [6] reported the synthesis of sphere-like amorphous VCrO particles several micrometers in size and studied their catalytic performance for the ammoxidation of dichlorotoluenes. It was found that the size and morphology of the solid catalysts could influence the catalytic behaviors significantly. However, to the best of our knowledge, no literature about nano-sized crystalline CrVO₄ has been reported. Here, we present a facile solvothermal route to synthesize pure-phase nanocrystalline orthorhombic CrVO₄ for the first time, which exhibited excellent catalytic performance for the ammoxidation of DCT with high conversion and yield at a relatively low temperature.

All reagents used in the experiments were of analytical grade and purchased locally. In a typical synthesis, (1) 0.01 mol (1.82 g) of V₂O₅ powder, 0.02 mol (2.0 g) of CrO₃ powder, and 80 mL methanol were successively added to a 100-mL Teflon-lined autoclave, stirred and mixed thoroughly, and then heated at 180 ℃ for 24 h. After cooling to room temperature, the dark-green mixture was separated by filtration and washed with distilled water and absolute ethanol several times. The residue was vacuum dried at 60 ℃ for 5 h to afford a precursor. Appropriate amounts of the precursors were then placed in quartz boats and calcined in a tube furnace under nitrogen atmosphere (more than 99% pure nitrogen was introduced) for 2 h at 200, 300, 400, 500, 600, and 700 ℃, respectively. (2) Methanol was replaced with ethanol, isopropanol, or methanol/water (1:1 V/V) and the above procedure was repeated and the precursor was calcined at 400, 500, 600, and 700 ℃, respectively.

X-ray powder diffraction (XRD) was performed on a Bruker D8 X-ray diffractometer with Cu K_α radiation, λ = 0.154 nm. X-ray photoelectron spectroscopy (XPS) was conducted on a VG Multilab 2000 X XPS spectrometer. The morphology of the powder particles was examined using a TECNAI G2 20 S-TWIN transmission electron microscope (TEM).

The catalytic reactions were conducted in a fixed-bed micro-reactor equipped with a 30-mm inner diameter glass tube loaded with 10 g of nanocrystalline CrVO₄. DCT was injected into the vaporizer by a micro-injection pump, and the vaporized reactant was then mixed with ammonia and air and fed into the reactor. The outlet stream was cooled and the products were condensed to solid in a condensing apparatus. Test runs were carried out at different conditions by varying the temperature, space velocity of the reactant, and the molar ratios of ammonia and air to DCT. The product steam was collected every 30 min after attaining steady state conditions and then analyzed off-line by gas chromatograph. The chemical reaction of DCBN synthesis by the ammoxidation of DCT is shown in Scheme 1.

The precursor prepared by the solvothermal reaction of V₂O₅, CrO₃, and methanol at 180 ℃ for 24 h was characterized by XRD. There were no diffraction peaks in the XRD patterns, suggesting that no crystal phases were formed at this stage. Replacing methanol with ethanol or isopropanol, extending the reaction time to 48 h, or raising the solvothermal temperature to 200 ℃ did not alter the amorphousness of the precursors (powder XRD patterns not shown).

The methanol-reduced vanadium-chromium oxide precursor was placed in quartz boats and calcined in a tube furnace under nitrogen atmosphere for 2 h at 200, 300, 400, 500, 600, and 700 ℃, and their XRD patterns are shown in Fig. 1. Diffraction peaks began to appear at 400 ℃ (Fig. 1(3)), indicating the formation of crystal phases. At 500 ℃, sharp peaks corresponding to Cr₂V₄O₁₃ (Fig. 1(4), black dot indicated) and tetragonal CrVO₄ (Fig. 1(4), star indicated) could be distinguished. At 600 ℃, significant amounts of orthorhombic CrVO₄ (Fig. 1(5), black triangle indicated) were produced, and a small amount of tetragonal CrVO₄ phase was still present. The assignment of these peaks was referred to PDF cards PDF#51-0445, PDF#15-0296, and PDF#38-1376, respectively. At 700 ℃, however, only well-resolved orthorhombic CrVO₄ peaks were observed (Fig. 1(6)), suggesting the production of pure-phase crystalline orthorhombic CrVO₄. The diffraction pattern well matches the PDF card (#38-1376) of orthorhombic CrVO₄ and the peaks were indexed accordingly.

Precursors using ethanol and isopropanol as solvothermal media were subsequently produced and calcined at 400, 500, 600, and 700 ℃. Crystal phases began to appear at 400 ℃, and at 700 ℃, the major products were still orthorhombic CrVO₄; however, a small amount of another phase, probably CrVO₃, still remained (Fig. 2). In contrast, replacing methanol with mixtures of methanol and distilled water (volume ratio of 1:1) produced pure-phase orthorhombic CrVO₄ (Fig. 2(3)).

The amorphous state of the precursor and the crystalline nature of the orthorhombic CrVO₄ powder were substantiated by TEM measurement. Fig. 3(a), (b), (c) and (d) show the TEM micrographs of the methanol-reduced precursor and calcined powders, respectively. The TEM image of the precursor (Fig. 3(a)) is typical of an amorphous solid, with a particle size of around 20 nm. When calcined at 500 ℃, regularly shaped rod-like and cube-like nanocrystals around 100 nm in size were formed (Fig. 3(b)). Raising the calcination temperature to 600 ℃ and 700 ℃ led to further growth of the crystallites to about 200 and 500 nm, respectively (Fig. 3(c) and (d)).

The addition of water to methanol produced well-crystalline CrVO₄, and greatly reduced the size of the CrVO₄ nanocrystals. Moreover, the size could be effectively tuned by the methanol/water ratio (Fig. 3(e)–(h)). When a 10:1 ratio was used, well-crystalline particles with ~50-nm diameter were produced (Fig. 3(e)). The particle sizes further decreased to ~30, ~20, and ~10 nm, respectively, when the volume ratios decreased to 5:1, 1:1, and 1:5 (Fig. 3(f), (g), and (h)). The addition of water to the hydrothermal medium is expected to decrease the concentration of methanol as well as its reducing ability; thus, the growth of crystals may be impeded, resulting in the production of finer nanoparticles. Ethanol or mixtures of ethanol/water as a reducing reagent to synthesize transition metal oxides have occasionally been reported, and it was found that the ratio of ethanol/H₂O had significant effects on the morphology and composition of the as-obtained products [35].

In comparison, calcination of the respective oxalates, i.e., using V₂O₅, CrO₃, and H₂C₂O₄ solutions as the starting materials by a similar hydrothermal approach produced micrometer-sized crystals (~1 μm in diameter, Fig. 3(i)).

The chemical states of the elements in the precursor and V–Cr–O complexes calcined at various temperatures were studied by XPS. The spectra were taken on a VG Multilab 2000 X XPS spectrometer and calibrated using the C 1s peak (284.6 eV) arising from carbon contamination. As shown in the survey spectrum (Fig. 4), the CrVO₄ nanocrystal showed peaks at about 586, 577, 530, 517, and 285 eV corresponding to Cr 2p_1/2, Cr 2p_3/2, O 1s, V 2p_3/2, and C 1s, respectively.

The high-resolution spectra of the Cr and V + O regions are presented in Figs. 5 and 6, respectively. The data were processed using the XPS Peak41 package as shown in Fig. 7, with the detailed results summarized in Table 1. The fittings of V 2p are combined with the O 1s region as G. Silversmit et al. [36] suggested because this fit model is significantly better than fitting without the O 1s region included. The binding energies (BE) of Cr 2p_1/2 and Cr 2p_3/2 showed no appreciable differences and appeared at 586.2 and 576.6 eV, respectively, well consistent with the Cr³⁺ oxidation state [37, 38]. The V 2p_1/2 and V 2p_3/2 signals were observed at ~522 and ~516 eV initially and shifted to 526.5 and 516.9 eV, respectively, when the calcination temperature was raised to 700 ℃. The results suggested that chromium was reduced to Cr³⁺ after the solvothermal reaction, but vanadium did not fully convert to V⁵⁺ oxidation state until calcination at 700 ℃ [36-38].

The possible solvothermal reaction mechanism is shown in Scheme 2. Methanol reduced the chromium from sexavalent to trivalent state to form nanocrystalline orthorhombic CrVO₄ with vanadium oxides.

DCBN is an important fine chemical and can be prepared by the gas-phase ammoxidation of DCT. However, only a few papers were dedicated to this route and low yields were often obtained due to the low reactivity of the methyl group from the steric hindrance of the two neighboring bulky and electron-withdrawing chlorine atoms. Most of the catalysts for the ammoxidation of DCT were focused on VPO [15, 16, 18-21]. In this work, nanocrystalline CrVO₄ produced by solvothermal reaction was found to have interesting catalytic performance for the ammoxidation of DCT to DCBN. The findings along with some typical literature results are listed in Table 2.

From Table 2, it was found that the composition and structure of the catalysts could exert great influence on the catalytic behavior. For example, unsupported VPO catalysts showed weak catalytic activity toward ammoxidation of DCT even at very high temperatures (Entry 1 and 3, Table 2), while the activity could be considerably improved as the VPO catalysts were loaded on supports such as SiO₂, Al₂O₃, or TiO₂ (Entry 2, 4, and 5, Table 2), probably owing to the larger specific surface areas and more exposed active centers. Furthermore, some micro- or nano-sized composite oxides may exist on the supports.

The nano-CrVO₄ exhibited excellent catalytic activity for the ammoxidation of DCT even at relatively low temperatures. At 335 ℃, the conversion of DCT is 84%, and the yield and selectivity of DCBN were 75% and 89%, respectively (Entry 7, Table 2). The nano-CrVO₄ catalyst exhibited (5–6)% higher conversion and yield with similar selectivity compared with those of the micro-sized V–Cr–O catalysts reported by Huang's group (Entry 6, Table 2). When the temperature was raised to 390 ℃, the conversion of DCT and the yield of DCBN increased to 99% and 81%, respectively (Entry 8, Table 2), which was almost the highest activity for ammoxidizing DCT to DCBN among the reported bi-component composite oxides. In contrast, the micro-CrVO₄ that we prepared by solvothermal reaction using H₂C₂O₄ solution as a reducing agent showed a similar catalytic activity compared to the micrometer-sized V–Cr–O reported by Huang's group (Entry 9 vs. 6, Table 2). The excellent catalytic performances of the nanostructured VCrO were thus attributable to the very small particle sizes, which led to high surface areas and exposed a large amount of active centers.

Pure-phase nanocrystalline orthorhombic CrVO₄ was obtained through calcination of an amorphous precursor, which was produced from the solvothermal reaction using V₂O₅ and CrO₃ as raw materials and either methanol or mixtures of methanol and water as the reducing agent. When using mixtures of methanol and water, CrVO₄ crystals with much smaller size were produced and the size could be effectively tuned from 10 to 50 nm by changing the methanol/water volume ratio. The nano-CrVO₄ could catalyze the ammoxidation of DCT to produce DCBN with high catalytic activity at relatively low temperatures owing to its very small particle sizes and large specific surface areas.