Effect of metal oxide contents on the structure and performance of spray dried CrVO4/SiO2 catalysts for the ammoxidation of chlorotoluenes

doi:10.21203/rs.3.rs-4373906/v1

This work aims at developing a customized ammoxidation catalyst through optimizing the metal oxide content in CrVO₄/SiO₂ catalysts. Various amounts of Cr-V-O with 1:1 molar ratio of Cr/V were successfully loaded on SiO₂ via spray drying method with colloidal silica as the binder. Polarized light microscope (PLM) illustrated that the CrVO₄/SiO₂ particles presented as well dispersed microspheres in the micro size (20-80 μm). X-ray diffraction spectroscopy (XRD) confirmed the formation of monoclinic CrVO₄ phase wherein the metal oxide content was ranged from 30 to 70 weight percent. FT-IR spectra indicated that the introduction of metal oxide had no obvious effects on the skeletal structure of SiO₂. X-ray photoelectron spectroscopy (XPS) analysis revealed that the valence state of V and Cr was hardly affected with increasing the metal oxide content. Performances of the obtained CrVO₄/SiO₂ catalysts with different metal oxide contents were investigated via the ammoxidation of a model compound namely; para-chlorotoluene. As a result of catalytic test, the CrVO₄/SiO₂ catalyst with 60% metal oxide content exhibited the best catalytic performance. In addition, this catalyst was an efficient and selective catalyst toward ammoxidation reactions of toluene, ortho- and meta-chlorotoluenes.

Heterogeneous catalyst

Ammoxidation

Monoclinic CrVO4

Spray drying method

Chloro-substituted benzonitriles are important chemical intermediates for the synthesis of various value-added chemicals such as herbicides [1], pesticides, pharmaceuticals [2], and so on. The preparation of these benzonitriles via a catalytic ammoxidation process is of particular interest [3]. Among various ammoxidation catalysts, vanadium (V) oxides-based catalysts were industrially important and have been widely implemented for practical use [4]. Usually, vanadium oxide catalysts were promoted by other metal components (e.g. Cr, Ti, Mo or P) on carriers (e.g. silica or alumina) or as bulk systems [5–15]. V-Cr-O based composite oxides are known to be highly efficient for selective gas-phase ammoxidation of various chloro-substituted toluenes. Nano- or micro-sized V-Cr-O catalysts have been prepared by several methods such as solid-state reaction [5], co-precipitation [6], hydrothermal [7–9] and solvothermal routes [10]. Spray drying method has the characteristics of fast heat transfer, large evaporation rate, short drying time and low equipment cost [16]. Comparing with the above methods, the spray-drying process is known to be simple, low cost, reliable, and easy to control the particle size and morphology [17–20]. Thus, the spray drying method is suitable for large scale production of micro catalytic particles. Our group focuses on developing various high-efficiency V-Cr-O based catalysts and their application in different ammoxidation reactions. Very recently, we prepared VCrO/SiO₂ micro-spheroidal catalyst via spray drying and demonstrated that 1:1 ratio of Cr/V resulted in the formation of monoclinic phase of CrVO₄, which exhibited the best catalytic performance for ammoxidation of p-chlorotoluene [21]. In this process, silica binder was also used as a support material, helping to increase the catalyst’s mechanical strength, to maintain the chemical and physical stability, and to lower production cost. However, the effect of SiO₂ content on the structural and catalytic behavior of CrVO₄/SiO₂ catalysts was imperceptible. Increasing the SiO₂ content could modify the surface structure [22, 23], change heat and oxygen transfer capacity [24], and lessen the content of active metal oxides. Therefore, a systematic investigation should be processed to optimize the SiO₂ content for higher catalytic activity.

In the present work, we prepared CrVO₄/SiO₂ catalysts with different metal oxide contents via spray-drying process. The catalysts were characterized by X-ray diffraction (XRD), polarized light microscope (PLM), X-ray photoelectron spectroscopy (XPS) and hydrogen temperature programmed reduction (H₂-TPR) to determine the nature of the active species. In addition, their catalytic activity in the ammoxidation of chlorotoluenes was also examined along with the effect of V-Cr-O loading ratio.

Catalyst preparation

VCrO/SiO₂ catalysts was prepared by spray-drying method. The following precursors were used for the preparation of CrVO₄/SiO₂ catalysts: V₂O₅ (99%), CrO₃ (99%), oxalic acid (99%), and colloidal silica (30%). Appropriate amount of H₂C₂O₄·2H₂O was dissolved in deionized water. V₂O₅ (1.82 g) and CrO₃ (1.98 g) were added in sequence under vigorous stirring, forming a homogeneous mixture. After adding a certain volume of colloidal silica (30wt.%) to the above solution, the resulting mixture was spray-dried at 180°C in a centrifugal spray dryer. The spray dried powder was heated at 300°C for 2 h in a box furnace and then calcined at 550°C for 4 h. The obtained CrVO₄/SiO₂ catalysts were labeled as Cat-MOx, where x represents the weight contents of metal oxides in the synthetic catalysts and x = 20, 30, 40, 50, 60, 70 and 100%.

The Cat-PTVCrO with 20% content of metal oxides was prepared for comparison through wetness impregnation method with formed silica sphere as support.

Catalyst characterization

The morphology of Cat-MOx catalysts was investigated with polarizing microscopy (Polarizing Microscope Axioskoppo, Zeiss, Germany). The particle size distribution profile was determined with a particle size distribution analyzer (Zetasizer Nano ZS90, Malvern, U.K.). Powder X-ray diffraction (XRD) patterns of Cat-MOx catalysts were recorded on a Bruker Advanced D8 X-ray diffractometer using Cu Kα radiation (λ = 1.54178 Å) operating at 40 kV and 40 mA. Data were recorded in the range of 2θ = 10 − 90° with a scan step width of 0.0163°. Fourier transform infrared spectra (FT-IR) were measured using a NEXUS470 FTIR spectrometer (Thermo Nicolet, USA) following the KBr pellet method. Nitrogen adsorption experiments were performed at 77 K using a JW-BK132F gas adsorption analyzer. Before the measurement, the samples were degassed at 200°C for 4 h. X-ray photoelectron spectroscopy (XPS) data were obtained using a MULTILAB2000 X-ray photoelectron spectrometer (VG, USA) with an Al Kα X-ray beam as the excitation source. The binding energies (BE) were calibrated against the C 1s peak at 284.6 eV. Temperature-programmed reduction (TPR) experiments were carried out on an AMI-200 Catalyst Multifunctional Characterization Analyzer (Zeton Altimira, USA).

Catalytic performance

Chlorotoluene ammoxidation runs were performed in a fixed-bed reactor. About 20 g of catalyst was loaded in the middle of the tube reactor, which was built with pyrex glass with an inner diameter of 30 mm and heated in an electric furnace. Both NH₃ and air were quantified using gas flow-meters, while quantified chlorotoluene was injected into the vaporizer by an infusion pump. NH₃, air and chlorotoluene were mixed and fed into the reactor. The product was collected behind the reactor and was dissolved in ethanol. All the products and unconverted reactants were analyzed by a Shimadzu GC-2010 Plus gas chromatography using a capillary column and an FID detector.

Optical microscope

The size and morphology of Cat-MOx particles were witnessed via an optical microscope. The magnification was 200× and the scale length was 100 µm. Light micrographs of Cat-MOx particles are showed in Fig. 1. Cat-MOx particles were well dispersed, and no significant aggregations were observed.The particles had spherical shape in the size range from 20 to 80 µm. The particle size decreased with the increase of metal oxide content. Disintegration of some granules was observed, which was caused by the decomposition of oxalate precursors at high temperature.

Particle size distribution

The particle size distribution (PSD) profiles of spray-dried Cat-MOx catalysts with various metal oxide contents were measured and the results were showed in Fig. 2 and Table 1. The PDS profiles of Cat-MO20, Cat-MO30 and Cat-MO40 samples featured a wide particle size distribution in the range of 20–120 µm. Cat-MO50 and Cat-MO60 samples exhibited a less wide particle size distribution in the range of 20–80 µm. Unsupported Cat-MO100 sample had a particle size distribution in the range of 10–50 µm. These results indicated that higher metal oxide content led to narrower particle size distributions and smaller diameters.

Table 1

Particle size characteristics of catalysts with different metal oxide contents
Cat-MOx	MO20	MO30	MO40	MO50	MO60	MO70	MO100
D50/µm	51.61	46.84	42.70	38.41	38.11	40.04	24.87

The PSD profiles of Cat-PTVCrO catalyst and the colloidal silica support were presented in Fig. 3. Both the PSD profiles had a particle size distribution in the range of 50–300 µm. The introduction of metal oxides had no obvious effect on the particle size distribution due to the low loading of metal oxides over colloidal silica.

X-ray powder diffraction (XRD)

XRD characterization was performed to check the effect of metal oxide contents on the structure of Cat-MOx catalysts. The XRD patterns of Cat-MOx (x = 20–100) samples were shown in Fig. 4 for comparison. The broad peak around 2θ = 23° corresponded to the amorphous structure of SiO₂ support. For the samples of 30 ≤ x ≤ 70, the XRD patterns agreed well with PDF#51 − 0031, which corresponded to monoclinic-CrVO₄ [21], and no diffraction peaks of impurity could be observed. The intensities of XRD peaks gradually increased with the increase of x. It was also found that Cat-MO20 and Cat-MO100 samples were indexed to be dominant monoclinic-CrVO₄ (PDF#51 − 0031) phase. For the Cat-MO20 sample, diffraction peaks for hexagonal-Cr₂O₃ (PDF#38-1479) phase were traced, suggested that in the low loading catalyst, the interaction between metal oxide specimen and the silica support could hinder the formation of CrVO₄ phase and promote the formation of amorphous V⁵⁺ phase and Cr₂O₃ phase. On the high loading sample, however, the metal oxide species could easily be precipitated next to the silica surface and be aggregated together in the phase of monoclinic-CrVO₄, due to the relatively weak interaction with the silica support. Compared with standard card of monoclinic-CrVO₄, the XRD pattern of Cat-MO100 appeared weak miscellaneous peaks around 24.296°, 26.223°, 32.053°, 35.684°, 36.440° and 64.171°, which could be ascribed to orthorhombic-CrVO₄ (PDF#38-1376). For the Cat-PTVCrO sample, the possible CrVO₄ phase was not formed, and instead, only the phase of hexagonal-Cr₂O₃ appeared. Cat-PTVCrO was prepared via an incipient wetness impregnation method with a low vanadium-chromium loading. This result suggested that the supported vanadium-chromium oxide component was mostly in amorphous form.

Fourier transform-IR spectra (FT-IR)

FT-IR spectroscopy was used to verify the change in the skeletal structures of SiO₂ resulting from loading the metal oxides. The FTIR spectra of Cat-MOx (x = 20, 50 and 60) samples were shown in Fig. 5. In the fingerprint region, there are one absorption band at 956 cm^− 1 attributed to V⁵⁺=O bond stretching, two absorption bands at 869 and 548 cm^− 1 assigned to V−O−V bending vibration and stretching vibration, and two bands at 1250 and 654 cm^− 1 due to the vibrations of Cr-O-Cr and Cr-O. The band peaked at 1116 cm^− 1 was related to the antisymmetric stretching vibration of Si-O-Si, suggesting that the skeletal structures of SiO₂ was not damaged as the metal oxides were loaded.

X-ray photoelectron spectroscopy (XPS)

The XPS analysis was carried out to elucidate the chemical states of V, Cr and O presented in the surface region of Cat-MOx (x = 20–100) samples. The survey XPS spectrum of the Cat-MO50 sample contained the Cr, V, O, Si, and C peaks (Fig. 6a). The C 1s peak was originated from the organic residues of the XPS instrument itself. The high-resolution Cr 2p spectrum (Fig. 6b) was resolved into two peaks located at 576.6 and 586.6 eV, assigning to the binding energies of Cr 2p 3/2 and Cr 2p 1/2 [25]. The V 2p spectrum (Fig. 6c) exhibits two peaks at 516.6 and 523.2 eV (V 2p1/2 and V 2p3/2) ascribed predominantly to V⁵⁺ oxidation state [26]. Additionally, the O 1s peak could be attributed to the oxygen bonds with Cr or V in the CrVO₄ lattice.

The high-resolution XPS spectra of the Cr and V + O regions were presented in Figs. 7a and 7b, respectively. The detailed results were summarized in Table 1. The binding energies (BE) of Cr 2p, V 2p and O 1s showed no appreciable differences when the metal oxide content was raised from 20–100%. The results suggested that oxidation states of Cr and V remained constant. Meanwhile, the surface atomic Cr/V ratios were close to 1:1, which was the nominal value of Cr/V molar ratio in CrVO₄, suggesting a well dispersion of metal oxides on the surface of catalysts.

Table 2

XPS results of the Cat-MOx (x = 20–100) samples
MO content	20%	30%	40%	50%	60%	70%	100%
Sample	MO20	MO30	MO40	MO50	MO60	MO70	MO100
V 2p_3/2	517.4	517.1	517.2	516.6	516.9	516.9	516.9
V 2p_1/2	523.1	524.1	524.4	523.2	524.1	524.2	524.1
O 1s	533.0	532.7	532.8	532.8	532.5	532.7	532.7
Cr 2p_3/2	577.3	577.0	577.2	576.6	576.9	576.9	576.7
Cr 2p_1/2	587.0	586.5	586.8	586.6	586.4	586.3	586.5
Ratio of Cr/V	0.96:1	1:1	0.90:1	1.14:1	1:1	1.20:1	2.13:1

BET surface area

BET gas sorptometry measurement was used to investigate the surface areas and the porosity nature of Cat-MOx samples. The N₂ adsorption-desorption isotherms for the Cat-MOx (x = 20, 50, 60, 100) samples were presented in Fig. 8. The data of BET surface areas, the average pore volumes and sizes were given in Table 3. The N₂ adsorption-desorption isotherms for Cat-MOx samples displayed a type-IV hysteresis loop, characteristic of micro/mesoporous materials [27]. As can be seen in Table 3, the surface area of the unsupported catalyst (Cat-MO100) was 18.123 m²g^− 1 and the pore volume was 0.079 cm³ g^− 1, which were much lower than those of the supported Cat-MOx (x = 20, 50, 60) samples. The surface area and pore volume of the supported catalyst increased firstly, reached the highest values of 73.574 m²g^− 1 and 0.267 cm³ g^− 1 at x = 0.50, and then decreased with increasing CrVO₄ loading from x = 20 to 60. However, the pore size was increased continuously from 8.324 to 11.075 nm, as could be seen in Table 3. The loaded CrVO₄ species could plug the pore mouth, resulting in those pores inaccessible for N₂ adsorption and consequently a decrease in the surface area and pore volume. Moreover, some larger pores were formed on the higher loading Cat-MOx sample, due probably to the generation of large volume gas products during thermal decomposition of the oxalate precursor at high temperature. Thus, the contents of SiO₂ support and oxalate precursor jointly determined the surface area and porosity of Cat-MOx sample.

Table 3

Pore structure parameters of Cat-MOx catalyst
Sample	Specific surface (m²g^− 1)	Pore volume (cm³g^− 1)	Pore size (nm)
Cat-MO20	66.893	0.244	8.324
Cat-MO50	73.574	0.267	10.383
Cat-MO60	50.612	0.214	11.075
Cat-MO100	18.123	0.079	9.437

H₂-TPR spectroscopy

The H₂-TPR profiles of Cat-MOx (x = 20–100) samples are shown in Fig. 9. Pure Cat-MO100 showed one H₂ consumption peak at ∼442 °C, which corresponded to the reduction of V⁵⁺ to V³⁺. The additional SiO₂ support significantly modified the reduction profiles and causes a shift in the main reduction peak to higher temperatures, which was certainly due to the interaction between the metal oxides and the SiO₂ support.

Catalyst activity

Activities of Cat-MOx catalysts were investigated for the vapor phase ammoxidation of p-chlorotoluene (PCT) to p-chlorobenzonitrile (PCBN). All the reaction conditions including reaction temperature, n(NH₃)/n(PCT), n(air)/n(PCT), and loading of metal oxides were assessed in the ammoxidation of PCT over Cat-MO60 catalyst. The influence of reaction temperature on the activity, selectivity and yield behavior of Cat-MO60 catalysts is shown in Fig. 10a. It was observed that the conversion of PCT increased from 98.5% to almost 100% upon a temperature increase from 380 ^◦C to 420 ^◦C. The selectivity and yield of PCBN were increased initially and then remained constant upon an increase in the temperature. The maximum selectivity and yield of PCBN observed at 410 ^◦C were 87.61% and 87.38%, respectively. The selectivity and yield of PCBN decreased at the reaction temperature higher than 410^◦C due to the thermal decomposition or deep oxidation of PCBN.

As shown in Fig. 10b, the molar ratio of n(NH₃)/n(PCT) had a highly pronounced promotional effect on the ammoxidation of PCT. As the n(NH₃)/n(PCT) ratio was increased from 1 to 3, the conversion of PCT was observed to increase initially and then reached maximum at around 99.5%. The selectivity and yield of PCBN had the same trend. On the surface of Cat-MO60 catalyst, increasing the NH₃ content led to an increase of M = NH species, which were formed via the condensation reaction between terminal M = O with NH₃ along with the loss of water [28]. Further increase in the NH₃ content led to a reduction of the O₂ supply and a decrease in the yield and selectivity.

The evolution of the catalytic behavior as a function of the ratio of n(air)/n(PCT) in the range of 11–21 was shown in Fig. 10c. The conversion of PCT kept constant with increasing the ratio of n(air)/n(PCT), while the selectivity and yield of PCBN increased firstly to a maximum at 15 and then decreased. Obviously, high ratio of n(air)/n(PCT) was benefit from the recovery of lattice oxygen. However, excess oxygen content might lead to the deep oxidation of PCBN.

The effect of reaction load over Cat-MO60 catalyst bed was studied and the results are shown in Fig. 10d. With an increase in the reaction load, the conversion of PCT slightly decreased due to a reduction in contact time between the reactant and active sites. Lower reaction load prolonged the contact time, leading to somewhat lower yield and selectivity due to the deep oxidation of PCBN over the catalyst. The best yield and selectivity were obtained at a reaction load of 0.13 g/gCat·h.

As discussed above, the optimal reaction conditions could be determined as follows: T = 410 ^oC, PCT:NH₃:Air = 1:3:15; reaction load = 0.13 g/gCat·h. The activity of VCat-MOx catalysts for the ammoxidation of PCT was evaluated. During the ammoxidation of PCT over these catalysts, PCBN was the desired product and other possible by products, such as benzonitrile, chlorobenzene and toluene, were not detected. The variation in the catalytic activity was observed with change in the CrVO₄ loading in the catalysts and the results are depicted in Fig. 11. The VCat-MO20 catalyst exhibited only 57.14% conversion of PCT and 43.55% yield of PCBN. When the metal oxides loading increased up to 60 wt%, the conversion of PCT and the yield of PCBN increased up to 99.62% and 86.52%, respectively. However, for VCat-MO100 catalyst, which contains no SiO₂ support, the conversion of PCT and the yield of PCBN were with levels of 98.62% and 78.30%, respectively.

Since silica support showed no catalytic activity under the same experimental conditions, the performance measured should be merely attributed to the supported CrVO₄ species. Generally, VCat-MO100 showed lower activity than the supported catalyst. The inner CrVO₄ species was not accessible in the bulk phase of CrVO₄. If the CrVO₄ phase was dispersed on the support with high surface area, more active species exposed and the number of available reactive sites increased. On the contrary, in high level of SiO₂ supported samples, CrVO₄ species could be heavily embedded, leading to a decrease of the catalytic activity. Notice in Fig. 11 that the VCat-MO60 catalyst showed the highest PCT conversion and PCBN yield in comparison with the other supported or pure catalysts.

The performance of Cat-PTVCrO catalyst was also tested in the same conditions for comparison and the results were depicted in Fig. 12. Although the conversion of PCT was close to 100%, both the selectivity and yield of PCBN were obviously below than those obtained over VCat-MO60 catalyst. Furthermore, the reaction load was quite low. All these results suggested that the spray drying technique showed significant advantages in preparing ammoxidation catalysts.

In addition, the catalytic behavior of the Cat-MO60 catalyst was also investigated in the ammoxidation reaction of toluene and other chloro-substituted toluenes such as m-chlorotoluene (MCT) and o-chlorotoluene (OCT), and the results were depicted in Fig. 13. The present catalyst exhibited equally excellent catalytic ability in these reactions. Meanwhile, the position of Cl-substituent had an obvious influence on the catalytic activity of ammoxidation reaction. The Cl-substituent had a weak electron-withdrawing effect. Meanwhile, the Cl-substituent in para position had almost no steric hindrance in activating the methyl group. For PCT the process got the highest conversion of PCT and yield of PCBN, but the selectivity was slightly lower than that for toluene due to the deep oxidation of PCBN at high temperature. OCT had the electronic similarity with PCT but more steric hindrance, leading to obviously lower activity. Due to the serious electron-withdrawing effect of Cl-substituent in the meta position, the ammoxidation process exhibited the lowest conversion of MCT and the lowest yield and selectivity of MCBN. These results were consisted with that reported previously [29, 30].

We synthesized CrVO₄/SiO₂ catalysts with different metal oxides content, characterized their physicochemical properties, and conducted ammoxidation reactions. The catalysts were synthesized by spray drying method, i.e., wet mixing Cr and V precursors with colloidal silica, spray drying the mixture and calcining at 550°C for 4 h. The spray dried CrVO₄/SiO₂ microspheres were high dispersed with diameter about 20–80 µm. The as-synthesized CrVO₄/SiO₂ catalysts showed the same monoclinic-CrVO₄ phase as the weight contents of metal oxides located in the range of 30%-70%. Among them, CrVO₄/SiO₂ catalysts with metal oxides of 60% exhibited the best performance in p-chlorotoluene ammoxidation reaction. This study may pave the way for the large-scale preparation of supported catalysts used in the vapor phase ammoxidation reactions.

Ethical approval Not applicable.

Competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Authors’ contributions All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Jiale Tong and Y. Huang. The first draft of the manuscript was written and revised by W. Tang. The manuscript was revised by Q. You, and all authors commented on previous versions of the manuscript. G. Xie did project administration, supervised the work, revised, and completed the manuscript. All authors read and approved the final manuscript.

Funding This work was supported by the National Natural Science Foundation of China (Grant No. 21172269), the Fundamental Research Funds for the Central Universities, South-Central Minzu University (Grant No. CZY22010), the Major bidding projects of provincial and ministerial scientific institutions, South-Central Minzu University (Grant No. PTZD22007), and the Opening Project of Key Laboratory of Optolectronic Chemical Materials and Devices of Minstry of Education, Jianghan Univeristy (Grant No. JDGD-202220).

Availability of data and materials The data and materials can be accessed from the manuscript for the current study.

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No competing interests reported.

GraphicAbstract.png

Effect of metal oxide contents on the structure and performance of spray dried CrVO4/SiO2 catalysts for the ammoxidation of chlorotoluenes

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Materials and methods

Catalyst preparation

Catalyst characterization

Catalytic performance

Results and discussion

Optical microscope

Particle size distribution

X-ray powder diffraction (XRD)

Fourier transform-IR spectra (FT-IR)

X-ray photoelectron spectroscopy (XPS)

BET surface area

H₂-TPR spectroscopy

Catalyst activity

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

Effect of metal oxide contents on the structure and performance of spray dried CrVO4/SiO2 catalysts for the ammoxidation of chlorotoluenes

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Materials and methods

Catalyst preparation

Catalyst characterization

Catalytic performance

Results and discussion

Optical microscope

Particle size distribution

X-ray powder diffraction (XRD)

Fourier transform-IR spectra (FT-IR)

X-ray photoelectron spectroscopy (XPS)

BET surface area

H2-TPR spectroscopy

Catalyst activity

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

H₂-TPR spectroscopy