Partial oxidation of toluene by O2 over mesoporous Cr/AlPO

TL;DR: In this article, the prepn. of mesoporous Cr/AlPO was carried out under hydrothermal conditions and the catalyst was characterized by low angle XRD, N-adsorption, diffuse reflectance UV-visible spectroscopy, ESR and thermal anal.

About: This article is published in Catalysis Communications.The article was published on 2002-02-01 and is currently open access. It has received 34 citations till now. The article focuses on the topics: Partial oxidation & Catalysis.

Summary

1. Introduction

• The incorporation of transition metal ions into the framework sites of microporous aluminosilicates and aluminophosphates has been reported in the literature and resulting systems are potential catalysts for various selective redox reactions [1– 3].
• Cr– AlPO-5 was shown to be an active and recyclable catalyst for the oxidation of alkylbenzenes with either TBHP or O2 [7].
• The potential of chromium containing *Corresponding author.
• S1566-7367 (01 )00070-X catalysts can be increased by using molecular oxygen as oxidant and thereby carrying the reaction in vapour phase it is possible to control the leaching of chromium from the framework, also known as PII.

2.1. Synthesis

• Mesoporous chromiumaluminophosphateswere prepared using cetyltrimethylammoniumbromide (CTAB) as surfactant and with the following gel composition.
• Various sources of aluminium have been tried and aluminium hydroxide has been chosen as the source.
• Chromium nitrate was used as transition metal precursor.
• The pH of the gel was maintained at 9.5 with tetra methyl ammonium hydroxide, as the use of other sources like NaOH and NH4OH resulted only in the amorphous materials.
• The solidwas filtered,washed several times with deionisedwater and calcined at 773K for 6 h to remove the organic template.

2.2. Experimental setup and procedure

• The experimental setup consisted of three parts: the gas supply system, the reactor and the analysis system.
• The feed was regulated through mass flow controllers.
• The temperature was decreased and the flow was switched to the mixture of 2 vol% toluene plus 40-vol% O2 in Ar.

2.3. Characterisation of the catalyst and reaction products

• Various techniques have been used for the characterisation of the materials synthesised.
• Diffuse reflectance UV–VIS spectroscopy was carried out on a Cary 5E UV–VIS–NIR spectrophotometer.
• X-ray photoelectron spectroscopic measurements (XPS) for Cr2p were performed on a PHI-550 ESCA-System (Perkin–Elmer GmbH).
• A Blazers QMG-421 mass-spectrometer and a Perkin–Elmer Autosystem XL gas chromatograph were used for the gas phase analysis.
• Ar=O2, CO, CO2 and H2O were analysed in a Carboxen-1010 capillary column and analysed by a TCD.

3.1. XRD

• XRD patterns of as-synthesised and calcined AlPO, Cr–AlPO are shown in Fig. 1. XRD patterns of AlPO show low angle peaks typical for hexagonal phase.
• The as-synthesised Cr–AlPO material shows a maximum intense peak corre- sponding to ð100Þ reflection followed by weaker but clear peaks corresponding to ð110Þ and ð200Þ reflections that can be indexed on the basis of a hexagonal lattice.
• The sample retained its hexagonal structure after calcination.
• With NaOH and NH4OH only amorphous materials were formed.
• The function of organic ammonium cation from TMAOH is probably to modify the strength of the electrostatic interactions between the aluminophosphate species and the cationic surfactant micelle assembly to form the SþI =TMAþ ion pair.

3.2. Thermal analysis

• Thermogram of as-synthesised ALPO shows mainly two weight loss regions one corresponding to loss of physi-sorbed water below 373 K.
• The second and the main weight loss was observed in the temperature range 450–550 K corresponding to loss of organic template.

3.3. N2 adsorption

• Nitrogen adsorption–desorption isotherms of the Cr–AlPO are shown in Fig. 2.
• For all samples, the isotherms are similar having inflection around p=p0 ¼ 0:2–0.3 characteristic of mesoporous materials.
• BET surface area and BJH pore size distribution for various catalysts are given in Table 1.
• It was observed that Cr–ALPO exhibit type IV isotherm with a hysteresis characteristic of mesoporous material.

3.4. UV–VIS DRS

• UV–VIS spectroscopy is a technique for the characterisation of transition–metal-incorporated zeolites [13–15].
• Fig. 3 shows UV–VISDRS spectra of as-synthesised and calcined Cr–AlPO samples.
• These bands are characteristic of trivalent chromium in octahedral co-ordination.
• Due to the large difference in LFSE values of Cr(III) for tetrahedral (66.9 KJ/mol) and octahedral (224.5 KJ/mol) geometries, chromium atoms in as-synthesised material mainly occupy extra framework sites [16–18].

3.6. Catalytic activity

• Table 2 gives typical results for the oxidation of toluene in the temperature range 523–648 K over Cr–AlPO catalyst.
• Benzaldehyde, benzene, CO2 and CO are the reaction products.
• It was observed that with the increase of the temperature, the conversion of the toluene and selectivity of benzene increased.
• Coke formation was not detected to any significant extent.

4. Conclusions

• Cr–AlPO materials have been prepared by hydrothermal synthesis and found to have hexagonal MCM-41 like morphology.
• These materials have high surface area 500 m2=g and pores are in mesoporous range 29 A. UV–VIS and ESR techniques have confirmed the presence of Cr5þ=6þ in the framework.
• Cr–AlPO catalysts have been shown to be active for the vapour phase oxidation of toluene with molecular oxygen.
• It has been observed that in Cr–AlPO both acidity (due to Al3þ) and redox properties (due to Cr5þ=6þ) are competing leading to benzene and benzaldehyde, respectively.

Figures

Fig. 1. XRD pattern of Cr–AlPO: (a) uncalcined, (b) calcined.

Fig. 2. N2 adsorption–desorption isotherms of Cr–AlPO.

Table 2 Catalytic activity of Cr–AlPO for the oxidation of toluene with molecular oxygen

Fig. 3. UV–VISDRS of Cr–AlPO: (a) uncalcined, (b) calcined.

Fig. 4. ESR spectra of Cr–AlPO: (a) uncalcined, (b) calcined.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Partial oxidation of toluene by o2 over mesoporous cr–alpo" ?

Viswana et al. this paper used mesoporous Cr-AlPO for partial oxidation of toluene with molecular oxygen in vapour phase. 

Q2. What is the reaction sequence of the catalyst?

The formation of benzene, as a result of dealkylation reaction predominates on acid ðAl3þÞ sites, whereas on redox sites ðCr5þ=6þÞ oxidation of toluene is taking place leading to benzaldehyde. 

Q3. What is the potential of chromium containing solid catalysts?

The incorporation of transition metal ions into the framework sites of microporous aluminosilicates and aluminophosphates has been reported in the literature and resulting systems are potential catalysts for various selective redox reactions [1– 3]. 

Q4. What is the function of the organic ammonium cation from TMAOH?

The function of organic ammonium cation from TMAOH is probably to modify the strength of the electrostatic interactions between the aluminophosphate species and the cationic surfactant micelle assembly to form the SþI =TMAþ ion pair. 

Q5. What is the potential of chromium containing solid catalysts in the industrial phase?

But for the large-scale production of fine chemicals, replacement of conventional homogeneous systems by heterogeneous catalysts will be advantageous inthe sense of catalyst recovery and reduction of undesirable side reactions. 

Q6. What is the potential of chromium containing solid catalysts in liquid phase?

With the recent discovery of mesoporous materials, the activities of chromium containing MCM-41, MCM-48, HMS have been tested for the oxidation of alkylbenzenes with peroxides as oxidants [8–10]. 

Q7. What is the effect of NaOH on the cationic surfactant?

If either NaOH or NH4OH is used, the smaller cations Naþ, NHþ4 compete with the aluminophosphate species and thus restrict the interaction with the positively charged cationic surfactant. 

Q8. What is the reason for the decrease in surface area of the Cr–AlPO compared?

The decrease in the surface area of the Cr– AlPO compared to AlPO could be due to partial loss of crystallinity, which is in agreement with the observation from XRD. 

Q9. What is the PII of the chromium catalyst?

PII: S1566-7367 (01 )00070-Xcatalysts can be increased by using molecular oxygen as oxidant and thereby carrying the reaction in vapour phase it is possible to control the leaching of chromium from the framework. 

Q10. What is the g value of the calcined Cr–AlPO?

ESR spectra of calcined Cr–AlPO shows g value at 1.95 characteristic of pentavalent chromium in tetrahedral or distorted tetrahedral co-ordination. 

Q11. What was the pH of the gel?

The pH of the gel was maintained at 9.5 with tetra methyl ammonium hydroxide, as the use of other sources like NaOH and NH4OH resulted only in the amorphous materials. 

Q12. What is the effect of calcination on the aluminophosphate?

As stated earlier, aluminium hydroxide may form a less polymerised aluminophosphate with many hydroxyl groups and favour the assembly of the mesostructure compared to other aluminium sources [11,12]. 

Q13. What is the potential of chromium containing porous solid catalysts in liquid phase?

the potential of chromium containing porous solid catalysts in liquid phase is limited because of the leaching of the active metal from framework. 

Q14. What is the morphology of the AlPO materials?

Pore size ( A) Pore volume (cc/g)AlPO 34.5 33.0 38.10 685 28 0.65 Cr–AlPO 35.0 33.4 38.56 490 29 0.51Cr–AlPO materials have been prepared by hydrothermal synthesis and found to have hexagonal MCM-41 like morphology. 

Q15. What is the composition of the gel?

Mesoporous chromiumaluminophosphateswere prepared using cetyltrimethylammoniumbromide (CTAB) as surfactant and with the following gel composition. 

Q16. What was the diffraction pattern of the sample?

The low angle X-ray diffraction pattern of the sample was recorded on a Siemens D 500 ðh=2hÞ using monochoromatised Cu-Ka radiation ðk ¼ 1:5406 AÞ with a scan speed of 1 /min over the range 2 < 2h < 10 . 

Q17. What is the pore size of the AlPO?

In both the cases the pore size is around 29 A.UV–VIS spectroscopy is a technique for the characterisation of transition–metal-incorporated zeolites [13–15]. 

Q18. What is the g value of the as-synthesised Cr–AlPO?

4. As-synthesised material shows a broad singlet with a g value of 1.98 indicating Cr3þ ions in octahedral co-ordination [20,21]. 

Q19. What is the chemical composition of the chromiumaluminophosphate?

In this communication, the synthesis, characterisation and catalytic activity of mesoporous Cr–AlPO for toluene oxidation with molecular oxygen are reported. 

Q20. What is the XRD pattern of the AlPO?

CO, CO2 and H2O were analysed in a Carboxen-1010 capillary column and analysed by a TCD.XRD patterns of as-synthesised and calcined AlPO, Cr–AlPO are shown in Fig. 1. XRD patterns of AlPO show low angle peaks typical for hexagonal phase.