Spray dried VCrO/SiO2 micro-spheroidal catalyst for the ammoxidation of p-chlorotoluene

Formulation and shaping of heterogeneous catalysts are vital in the successful industrial application. Here micro-sized vanadium chromium composite oxides catalysts with the spherical shape were prepared via spray drying with colloidal silica as a binder material. The physicochemical properties of catalysts with different Cr/V molar ratios were characterized by XRD, XPS, FT-IR, TPR, and particle size distribution analysis. It was revealed that the addition of Cr inhibited the formation of the crystalline phase V2O5 and decreased the reduction temperature of pentavalent vanadium species, and also resulted in the formation of monoclinic CrVO4 and a highly dispersed state of vanadia species. VCrO/SiO2 particles with various Cr/V atomic ratio were studied as catalysts for p-chlorotoluene ammoxidation to p-chlorobenzonitrile, in which the catalyst with Cr/V ratio of 1 exhibited the best catalytic performance. When the Cr/V ratio was less than 1, mixed phases of orthorhombic CrVO4 and monoclinic Cr2V4O13 were formed and resulted in a low catalytic activity. With the increase of Cr/V ratio, the content of monoclinic CrVO4 in the catalysts increased, resulting in the catalytic activity of the catalysts improved. However, too large an amount of Cr led to the formation of highly oxidizing hexagonal-Cr2O3 phase, which reduced the selectivity of the catalytic reaction.


Introduction
As an important type of intermediates for organic reactions [1,2], aromatic nitriles are widely used in dyes, medicine, pesticides, perfumes and plastics industries [3,4], and the demand for various aromatic nitriles has increased year by year in the international market.Many methods have been developed to prepare aromatic nitriles over the past 100 years , among which direct ammoxidation of methyl aromatics is the most economical and environmentally friendly method and is widely used for the production of aromatic nitriles in industry [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27].The ammoxidation catalyst plays a key role in the synthesis of aromatic nitriles through ammoxidation of substituted toluenes and methyl heteroaromatics.Bulk V 2 O 5 usually exhibits high catalytic activity for ammoxidation reactions but gives low selectivity due to its deep oxidation property [17], making it difficult to be used as catalyst alone.Thus, it is necessary for vanadia to be supported or promoted with other elemental oxides to improve the catalytic performance.Different composite metal oxides like VPO, VAlO, VWO and VCrO have been developed and showed excellent catalytic performance in various ammoxidation applications.Martin found that VPO supported on γ-Al 2 O 3 was an efficient catalyst for the ammoxidation of 2,6-dichlorotoluene [18].The added P could inhibit the deep oxidation reactions and improve the selectivity of catalytic reactions, while the increase of P/V ratio reduced the catalytic activity.For the ammoxidation of 3-methylpyridine, Jiang prepared the VCrO/SiO 2 catalyst by an impregnation method [19].It was found that the active phases such as V 2 O 5 and CrVO 4 were dispersed in an amorphous form on the surface of silica gel and the amount of Cr greatly affected the catalytic performance.Goto synthesized a WVO catalyst by a solvothermal method and found that the addition of W significantly improved the selectivity of 3-methylpyridine during ammoxidation [20].Babu found that the addition of Mo, W, and La could improve the reducing ability and acidity of the catalyst when V 2 O 5 /Al 2 O 3 was used as catalysts for the ammoxidation of o-chlorotoluene [21].We also prepared some VCrO/SiO 2 or VPO/SiO 2 catalysts through impregnation methods and investigated their catalytic performances for the ammoxidation of methylaromatics [22][23][24][25][26][27].
Nano-or micro-sized V-based catalysts, due to their high surface effect, usually have better performance than that of the bulk analogue [28].Many methods, including hydrothermal, solvothermal, sol-gel [29], and spray drying methods [30], have been developed for the design and synthesis of nano-or micro-sized catalysts.We have prepared a series of VCrO nanoparticles [31][32][33] and VMoO [34] microcuboids by hydrothermal method, and found they exhibited excellent performance for the ammoxidation of chlorotoluenes.Huang and Li also produced some VCrO [16], VWO [12] and VTiO [14] micro-or nano-structures through hydrothermal synthesis and investigated their catalytic performance for ammoxidation of dichlorotoluenes.However, hydrothermal method cannot be used in large-scale industrial 1 3 Spray dried VCrO/SiO 2 micro-spheroidal catalyst for the… production of catalysts.Furthermore, the unsupported catalysts are not suitable for industrial production of aromatic nitriles due to their high cost and low strength.
The spray drying method is well known as a simple and economical technique to prepare highly uniform micro-nano particles [35].Spray drying process is versatile to shape the catalysts and to control the particle size and morphology of final products, and has been widely used in the fields of catalysis and materials chemistry [36,37].In the present study, SiO 2 -supported VCrO catalyst was prepared by spray drying method.The effect of Cr/V ratio on the structure was examined by particle size distribution (PSD), polarized light microscope (PLM), X-ray powder diffraction (XRD) and hydrogen temperature programmed reduction (H 2 -TPR).The performance of the catalyst was evaluated for the ammoxidation of p-chlorotoluene to p-chlorobenzonitrile.

Catalyst preparation
V 2 O 5 (99%), CrO 3 (99%), oxalic acid (99%) and colloidal silica were purchased from Aladdin and used as received.The solution of colloidal silica has 30 wt% 10-20 nm SiO 2 dispersed in aqueous media.Take the Cr/V ratio of 1:1 for example, H 2 C 2 O 4 •2H 2 O (101.86 g) was dissolved in deionized water.The temperature was set at 80 °C.After dissolution, V 2 O 5 (16.35 g) was added.The mixture under continuous stirring turned dark blue, then CrO 3 (17.96g) was slowly added.Large numbers of bubbles were formed during the mixing process.After cooling to room temperature, the obtained dark green solution was diluted to 100 mL.Solution of colloidal silica (30 wt%, 100 mL) was added to the above solution and the resulting mixture was spray-dried at the outlet temperatures of 180 °C in a centrifugal spray dryer.The spray dried powder was placed in a muffle furnace.Operating temperature rose from room temperature to 300 °C at a heating rate of 5 °C/min, kept at 300 °C for 2 h, then rose from 300 to 550 °C at a heating rate of 5 °C/min and kept at 550 °C for 4 h.The total loadings of vanadium chromium composite oxides in catalyst were 50%.The samples prepared without Cr, without V, with a Cr/V ratio of 0.5:1, 1:1, 1.5:1 and 2:1 were denoted as Cat-V, Cat-Cr, Cat-VCr0.5,Cat-VCr1, Cat-VCr1.5 and Cat-VCr2, respectively.

Catalyst characterization
The morphology of the spray dried powders was investigated with polarizing microscopy (Polarizing Microscope Axioskoppo, Zeiss, Germany).The particle size was evaluated with a particle size distribution analyzer (Zetasizer Nano ZS90, Malvern, U.K.).The crystalline structure was characterized using a Bruker Advanced D8 X-ray powder diffractometer (Bruker, Germany) with nickel-filtered Cu-Kα radiation (λ = 1.54060Å, operating at 40 kV/40 mA).A Nexus 470 Fourier infrared spectrometer (ThermoNicolet, USA) was used to record IR spectra in KBr plates.X-ray photoelectron spectroscopy (XPS) data was collected using a MULTI-LAB2000 X-ray photoelectron spectrometer (VG, USA).A physisorption analyzer (JW-BK132F) was used for to measure the specific surface areas of the catalyst.The temperature-programmed reduction (TPR) experiments were performed on an AMI-200 Catalyst Multifunctional Characterization Analyzer (Zeten Altamira, USA).

Catalytic performance
The catalyst activity test was performed in a fixed-bed reactor, which was made of a quartz tube (about 50 cm in length and about 3.0 cm in inner diameter, placed in an electrically heated furnace).The 20 g catalyst powder diluted with quartz sand was filled at the middle of the tube reactor.NH 3 and air were quantified by the glass rotor flow-meter.p-Chlorotoluene was accurately metered by a micro-injection pump and injected into the vaporizer, mixed with NH 3 and air, and fed into the reactor.The product was trapped in a spherical condenser behind the reactor.The collected solid products were washed, suction filtered, dried, and weighed.All samples were analyzed by gas chromatography.The conversion of p-chlorotoluene, the yield and the selectivity of p-chlorobenzonitrile were calculated with the follow equations: Conversion rate = (1−Amount of unreacted raw material / Amount of raw material entered) × 100% Molar yield = (Amount of product obtained / Amount of due product) × 100% Selectivity = (Conversion rate / Molar yield) × 100%.

Morphology and microstructure
Colloidal silica was used as a binder during the shape forming process.The size and morphology of spray-dried powders were observed under a polarized light microscope (PLM) and the images were represented in Fig. 1, with the photo taken at 200× magnification.From Fig. 1, it could be found that the particles were smooth faced highly dispersed microspheres with a size of 20-80 μm, and no obvious aggregations were observed.Particles with higher Cr content displayed a narrower particle size distribution.Some broken particles could be seen, due probably to the decomposition of the oxalate precursor at high temperature and the disintegration of spherical particles.This suggested that some microporous structures could be formed during the spray drying process.

Particle size distribution
The particle size distributions of spray-dried catalysts with different Cr/V molar ratios were tested by laser particle size analyzer and the results were presented in Fig. 2 and Table 1.Cat-V and Cat-VCr0.5 samples exhibited a wide particle size 1 3 Spray dried VCrO/SiO 2 micro-spheroidal catalyst for the… Fig. 1 Optical microscopy images of shaped catalysts synthesized with different Cr/V ratios Fig. 2 Particle size distribution of catalysts with different Cr/V ratio distribution profile in the range of 20-120 μm and a large mean diameter.The size distribution profile of Cat-VCr1 featured a sharp peak.Cr-rich samples including Cat-VCr1.5,Cat-VCr2 and Cat-Cr had narrower particle size distribution, shifting toward smaller diameters, as compared to the V-rich samples such as Cat-V and Cat-VCr0.5.The higher Cr/V ratio resulted in more homogeneous particles with smaller diameters, which was consistent with the observation from optical microscopy.

X-ray powder diffraction (XRD)
The XRD patterns of precursors with various ratios of Cr/V were showed in Fig. 3.All the XRD patterns were identical, presenting only one broad peak assigned to amorphous SiO 2 around 2θ = 23°.The characteristic diffraction peaks of V 2 O 5 , Cr 2 O 3 , or the oxalate can hardly be observed over these precursors.XRD result indicated the amorphous nature of the precursors.
Figure 4 showed the XRD patterns of catalysts synthesized with various Cr/V ratios.In the XRD pattern of the Cat-V sample, only orthorhombic V 2 O 5 was detected.The crystal phases in the Cat-VCr0.5sample were mainly composed of monoclinic CrVO 4 (PDF#83-0761), orthorhombic CrVO 4 (PDF#75-1613) and monoclinic-Cr 2 V 4 O 13 .The Cat-VCr1 sample with Cr/V ratio of 1 was mono-phased and its XRD pattern could be well indexed to the monoclinic CrVO 4 structure.But in the Cat-VCr1.5sample, monoclinic CrVO 4 was the primary phase along with a small  ).Using Scherrer's equation, the average sizes of the crystals were estimated to be 141.8 nm (Cat-VCr1), 164.9 nm (Cat-VCr1.5)and 179.5 nm (Cat-VCr2).This result indicated that the average crystallites size of monoclinic CrVO 4 increased with the increase of Cr/V ratio.

X-ray photoelectron spectroscopy (XPS)
The chemical state of V, Cr and O in the catalysts was characterized using X-ray photoelectron spectroscopy (XPS).Figure 5a presented the full survey spectra of Cat-VCr1 and signals of Cr, V, O and Si elements were displayed.The binding energies of Cr 2p, V 2p, and O 1 s signals were investigated using high resolution XPS spectra.The Cr 2p spectra in Fig. 5b showed two peaks (Cr 2p 1/2 and Cr 2p 3/2 ) at the binding energies of 586.6 and 576.6 eV, respectively.The V2p region in Fig. 5c can be fitted into two peaks, which can be ascribed to V 2p 1/2 (522.8eV) and V 2p 3/2 (516.6 eV), respectively.According to the literature, the O 1 s peak at 532.8 eV in Fig. 5d was assigned to the lattice oxygen on the catalyst surface [39].As shown in Fig. 6 and in Table 2, vanadium existed in the pentavalent state in Cat-V, and chromium was trivalent in Cat-Cr [40,41].In samples with different Fig. 4 XRD patterns of catalysts with different Cr/V ratios Cr/V ratios, the Cr 2p XPS spectra (Fig. 6a) showed two sets of doublets (Cr 2p 1/2 and Cr 2p 3/2 ) ascribed predominantly to Cr 3+ oxidation state [42,43].Vanadium mainly existed in the pentavalent state in Cat-VCr0.5 and Cat-VCr1, and in the tetravalent state in Cat-VCr1.5 and Cat-VCr2, as shown in Fig. 6b.According to the bifunctional catalysis mechanism [44], VO x provided redox sites, while Cr supplied appropriate acidic adsorption sites for NH 3 .The addition of Cr could stabilize the V 4+ state in the catalyst.The ratio of V 4+ /V 5+ increased with the increase of the Cr/V ratio.Since high V 4+ /V 5+ ratio leads to high catalytic activity, the catalytic performance can be improved by adjusting the V 4+ /V 5+ ratio in the catalyst.
The high chromium content not only resulted in the high Cr/V ratio on the surface of the catalysts, but also promoted the dispersion of vanadium, which was conducive to improving the catalytic activity.Thus, the catalytic performance could be improved by adjusting the Cr/V ratio.In addition, a certain degree of vanadium enrichment on the surface of the catalyst would reduce the exposure of metal chromium, which was not advantageous to redox reactions and catalytic cycles.

Fourier transform infrared spectrum (FTIR)
The effect of loaded active components on the chemical structure of catalysts was further studied using FTIR spectroscopy.The FTIR spectra of catalysts with  different Cr/V ratios were shown in Fig. 7. Cat-VCr0.5,Cat-VCr1, Cat-VCr1.5, and Cat-VCr2 showed similar profiles.The broad band resulting from stretching vibrations of the hydroxyl groups (O-H) appeared at 3443 cm −1 .The absorption band occurred at 1631 cm −1 originated from the bound water present on the surface of catalysts.The absorption bands at 1113 cm −1 and 808 cm −1 were attributed to the anti-symmetric stretching vibration of Si-O-Si and the symmetrical stretching vibration of Si-O bond, respectively.The absorption band peaked at 1037 cm −1 was attributed to the stretching vibration of the V 5+ =O bond.This absorption band became obscure and slightly shifted to a lower wave number with increasing the Cr/V ratio, indicating the weakening of the V 5+ =O bonds [45].This is mutually confirmed by the XPS analysis results.The absorption peaks at 869 cm −1 and at 546 cm −1 are attributed to the bending and stretching vibrations of V-O-V, respectively.The absorption peaks at 1263 cm −1 and 655 cm −1 are attributed to the Cr-O-Cr and Cr-O bonds, respectively.The appearance of characteristic bands of the active components suggests that Cr-V-O composite oxides are successfully supported on silica.

Hydrogen program temperature reduction (H 2 -TPR)
Temperature-programmed reductions (TPR) of Cat-V, Cat-VCr0.5,Cat-VCr1, Cat-VCr1.5, and Cat-VCr2 catalysts were carried out under a H 2 flow and the TPR profiles of catalysts with different Cr/V ratios were illustrated in Fig. 8. Apparently, different Cr/V ratios led to emergence of different reduction peaks.The Cat-V catalyst exhibited one H 2 consumption peaks at around 630 °C, corresponding to the reduction of V 2 O 5 [46,47].For Cat-VCr0.5, Cat-VCr1, Cat-VCr1.5, and Cat-VCr2 catalysts, the reduction of V 5+ → V 4+ [48,49] occurred at 508 °C, 494 °C, 488 °C, Fig. 7 FTIR spectra of catalysts with different Cr/V ratios and 482 °C, respectively, which was more than 100 °C lower compared with that of Cat-V catalyst.The existence of Cr made V 5+ reducible at comparatively lower temperature.Thus, the addition of Cr made V 5+ species more prone to reduction reactions and significantly improved the catalytic activity.

Catalytic performance
The effect of Cr/V ratio on the catalytic performance was evaluated in the ammoxidation of p-chlorotoluene.The catalytic tests were performed in fix-bed mode and the reaction conditions were fixed as follows: 400 °C, 0.13 g/gCat•h −1 ammonia ratio Fig. 8 TPR plots of catalysts with different Cr/V ratios Fig. 9 Effect of Cr/V ratio on the catalytic performance for the ammoxidation of chlorotoluene (Reaction conditions: t = 400 °C, load 0.13 g/gCat•h −1 , ammonia ratio 3, air ratio 15) of 3, and air ratio of 15. Figure 9 summarized the conversion of p-chlorotoluene, the yield and selectivity of p-chlorobenzonitrile.The active components in Cat-V and Cat-Cr were orthorhombic-V 2 O 5 and hexagonal-Cr 2 O 3 , respectively.Both Cat-V and Cat-Cr catalysts exhibited low selectivity and low yield of p-chlorobenzonitrile due to the deep oxidation of p-chlorotoluene.As discussed above, the addition of Cr promoted the elimination of the oxidizing V 2 O 5 , and made V 5+ species more prone to reduction reaction, leading to an enhancement of selectivity and yield of p-chlorobenzonitrile.The conversion of p-chlorotoluene over Cat-VCr0.5 was much lower than that over Cat-VCr1, Cat-VCr1.5 and Cat-VCr2, due to the low catalytic activity of orthorhombic-CrVO 4 [31] and monoclinic-Cr 2 V 4 O 13 [32] in Cat-VCr0.5.The ratio of V 4+ /V 5+ in the catalyst increased with an increase of Cr/V ratio, thus the redox process in the catalyst was accelerated and the catalytic activity was improved.When the ratio of Cr/V was 1, the catalyst was a single monoclinic-CrVO 4 crystal phase and exhibited the highest catalytic activity and selectivity.As depicted in Fig. 4, monoclinic CrVO 4 was the main phase in Cat-VCr1.5 and the addition of Cr increased the degree of crystallization, leading to an increase of the catalytic activity.However, the selectivity and yield of p-chlorobenzonitrile decreased slightly due to the emergence of oxidizing hexagonal Cr 2 O 3 in Cat-VCr1.5.A large amount of hexagonal-Cr 2 O 3 appeared in the Cat-VCr2 catalyst and an obvious decrease of catalytic performance could be observed.It has been reported that micro-cuboid MoV 2 O 8 catalyst exhibited high activity and selectivity in the ammoxidation of p-chlorotoluene to p-chlorobenzonitrile [34], with a near 100% conversion and 83.6% yield at the temperature of 440 °C.Thus, Cat-VCr1 and Cat-VCr1.5 catalysts were equally effective for the selective ammoxidation of p-chlorotoluene.The reaction temperature was set at 400 °C, which was obviously lower than that for micro-cuboid MoV 2 O 8 catalysts.

Conclusions
In this work, a spray drying technique was developed for the preparation of microparticulate V-Cr-O catalysts with different Cr/V ratios, with the size in the range of 20-80 μm.It was found that the Cr/V ratio had a great influence on the structure and catalytic performance of the catalysts.The addition of Cr effectively inhibited the formation of the oxidizing crystalline phase V 2 O 5 , and greatly decreased the reduction temperature of pentavalent vanadium species.When the Cr/V ratio was less than 1, the predominant orthorhombic-CrVO 4 and monoclinic-Cr 2 V 4 O 13 species exhibited low catalytic activity.At the Cr/V ratio of 1, the catalyst exhibited the highest catalytic activity and selectivity, suggesting that monoclinic CrVO 4 was the most effective catalyst among Cr-V-O composite oxides.However, further addition of Cr led to the formation of hexagonal-Cr 2 O 3 crystal phase, which reduced the selectivity and yield of the catalytic ammoxidation.

Fig. 3 3
Fig. 3 XRD patterns of precursors with different Cr/V ratios

Table 1
Particle size characteristics of catalysts with different Cr/V ratios

Table 2
XPS results of Cr-V-O catalysts with different Cr/V ratios