Oxidation of Toluene on Supported and Unsupported LaCoO3 Perovskites Catalyst
H.S. Goswami, Shveta Acharya
Government College, Kota (Raj.) India.
*Corresponding Author E-mail: goswami.hs68@gmail.com, acharyashveta01@gmail.com
ABSTRACT:
Perovskite type oxide are known to be catalysts for a number of reactions such as total and partial oxidation, hydrocracking, hydrogeneous, hydrogenolysis and reduction etc. Efforts has largely been directed towards synthesis of unsupported and supported perovskites oxides of moderates or high specific area, their bulk and surface properties and their role in heterogenous catalysis. Oxidation of aromatic and aliphatic hydrocarbon over LaMO3 (M=Al, Ni, Mn, Co, Fe, Cr etc.) perovskites have been studied. Vapour phase catalytic oxidation of toluene over perovskites Viz., LaCoO3, LaCoO3/ SiO2 and LaCoO3/Al2O3 has been studied. The characterization of the catalyst was carried out using technique Viz. I.R., Surface area, Packing density. Surface acidity, Surface basicity. The surface area measurements in the temperature range 350șC to 600șC. The maximum surface area & maximum activity was observed at 450șC. The heterogeneous catalytic vapour phase oxidation of toluene give benzaldehyde, benzoic acid, maleic acid and CO2 as products over LaCoO3 and LaCoO3 supported on Al2O3 and SiO2 as catalyst. The LaCoO3 supported on Al2O3 has been found to be the most active and selective catalyst giving 84.0% selectivity for benzaldehyde at 450⁰C with surface area 78.9m2/g. The overall Kinetic analysis indicate that the oxidation of Toluene to benzaldehyde is first order. The order of catalytic reactivity is LaCoO3/Al2O3 > LaCoO3/ SiO2 > LaCoO3.
KEYWORDS: Oxidation, Toluene, Catalysis, Supported, Surface area, Benzaldehyde.
INTRODUCTION:
Perovskites type oxides are known to be catalyst for a number of reactions such as total partial Oxidation, Hydrocracking, hydrogenation, hydrogenolysis and reduction etc. Amongst the more important reactions in which these compounds have been used as catalyst are oxidation of CO1-4, CH45, NH36, Methanol7, Olefins8, Paraffin9-11, Aromatic compounds12-16, Hydrogenation17 and oxygenate18. The oxidation of light Paraffins as methane, propane and n-butane has been frequently taken as a test reaction for perovskites oxides.
Effort has largely been directed towards synthesis of an unsupported and supported Perovskites, Oxides of moderates or high specific surface area, their bulk and surface properties and their role in heterogeneous catalysis. The vapour phase oxidation of toluene over mixed oxide has been studied but work with perovskites is scanty. In this note we report the results of a systematic study of LaCoO3 and LaCoO3 supported on Al2O3 and SiO2 in the oxidation of toluene.
MATERIALS AND METHODS:
The LaCoO3 catalyst was prepared by the oxalate decomposition method19.
Preparation method: A concentration solution of oxalic acid was added to concentration solution of La(NO3)6.6H2O and Co(NO3)6.6H2O. The amorphous precussor was obtained by slow drying and finally heated in muffle furnace at 1000⁰C for 12 hours to get LaCoO3.
Table 1- Characteristics of LaCoO3, LaCoO3/ SiO2 and LaCoO3/Al2O3 Perovskite catalysts.
|
Catalyst |
Decomposition Temperature (șC) |
Packing density (g/cc) |
Acid strength n-butylamine titre, meq/q |
Base strength n-butylamine titre, meq/q |
I.R. frequency cm-1 |
|
|
PKa=6.8 (Neutral red.) |
PKa=4.8 (Methyl red.) |
Bromo thymol blue PK1=7.2 |
||||
|
LaCoO3 |
1000 șC |
0.965 |
0.096 |
0.248 |
NIL |
1180, 1100, 1060, 660, 590, 480, 390 |
|
20% LaCoO3/ SiO2 |
1000 șC |
1.11 |
0.091 |
0.241 |
2.9 |
|
|
20% LaCoO3/Al2O3 |
1000 șC |
0.99 |
0.096 |
0.216 |
1.9 |
|
Table 2- Activity and Selectivity data.
|
Catalyst |
Surface area m2/g |
Reaction Temp. șC |
Conversion % to |
Total Conversion |
% selectivity to BzH |
|||
|
BzH |
BzA |
MA |
CO2 |
|||||
|
LaCoO3 |
6.7 |
350 |
7.2 |
2.8 |
2.4 |
3.0 |
15.4 |
46.7 |
|
8.4 |
400 |
8.2 |
2.3 |
2.1 |
2.3 |
14.9 |
53.0 |
|
|
12.4 |
450 |
11.2 |
1.6 |
1.1 |
1.9 |
15.8 |
70.8 |
|
|
11.0 |
550 |
9.4 |
2.0 |
1.7 |
2.8 |
15.9 |
59.1 |
|
|
8.0 |
600 |
7.2 |
2.4 |
2.0 |
2.1 |
13.7 |
52.5 |
|
|
LaCoO3/SiO2 (20% LaCoO3) |
50.1 |
350 |
9.0 |
2.6 |
1.9 |
2.4 |
15.9 |
56.6 |
|
54.2 |
400 |
10.6 |
2.4 |
1.9 |
2.0 |
16.9 |
62.7 |
|
|
61.1 |
450 |
15.1 |
1.0 |
1.8 |
1.1 |
19.0 |
79.4 |
|
|
56.4 |
550 |
9.8 |
2.1 |
1.7 |
3.2 |
16.8 |
58.3 |
|
|
59.0 |
600 |
9.0 |
2.9 |
2.0 |
2.6 |
16.5 |
54.5 |
|
|
LaCoO3/Al2O3 (20% LaCoO3) |
70.4 |
350 |
10.4 |
3.2 |
1.9 |
2.0 |
17.5 |
59.4 |
|
74.0 |
400 |
13.4 |
1.0 |
2.2 |
2.9 |
19.5 |
68.7 |
|
|
78.9 |
450 |
17.9 |
1.2 |
1.1 |
1.1 |
21.3 |
84.0 |
|
|
71.0 |
550 |
12.0 |
3.0 |
2.4 |
2.0 |
19.4 |
61.8 |
|
|
61.0 |
600 |
9.6 |
3.1 |
2.0 |
2.4 |
17.1 |
56.1 |
|
Supported LaCoO3 on Alumina and Silica as supported were prepared. A general method for preparing 20% LaCoO3 on Alumina and Silica are follows-Aqueous solution of La(NO3)3.6H2O(7.2gm), Co(NO3)2.6H2O(4.73gm), Concentrate oxalic acid and Alumina/Silica (20gm) were mixed in a beaker and then transferred to china dish. The contents were heated at 1000⁰C in a muffle furnace to get the desired catalyst.
The surface area of catalyst was determined using ethylene glycol monoethylether (EGME) adsorption method20,21 at different temperature. The surface acidity and surface basicity of perovskites catalyst were determined by n-butylamine titration22 and phenol adsorption method23 respectively. The experimental setup and the methods for the analysis of the products such as benzaldehyde, benzoic acid, maleic acid and carbon dioxide were the same as described elsewhere24-27. The I.R. spectra of the perovskites was recorded in KBr using Perkin-Elmer 883 spectrophotometer.
The oxidation was studied at space velocities (2998) temperature 350șC, 400șC, 450șC, 550șC and 600șC at air toluene ratio (121) respectively.
RESULTS AND DISCUSSION:
The characterization of the catalyst was carried out using techniques Viz; I.R., Surface area, packing density, surface acidity and surface basicity. The result of these studies has been incorporated in table I and II. The surface area measurements in the temperature range 350șC to 600șC (Table I) shows that surface area of the catalyst increases with increase in temperature up to 450șC but on further increase in temperature the surface area decreases. The specific surface area for different catalyst was found to follow the order LaCoO3/Al2O3> LaCoO3/SiO2 > LaCoO3. The surface acidity and basicity measurements show that LaCoO3/Al2O3 and LaCoO3/SiO2 perovskite have both acidic and basic sites but unsupported LaCoO3 have only acid sites on the surface of the catalyst. The LaCoO3/SiO2 is more basic than LaCoO3/Al2O3.
The LaCoO3 perovskite catalyst (Unsupported) shows absorption band at 1180, 1100, 1060, 600, 620, 590, 480 cm-1 (Table 1) respectively. In pure CO3O4 oxide bands are observed in the region 660 cm-1 and 580 cm-1. In the perovskites oxide shows that it has characteristics different than that observed with CO3O4. The order of packing density for catalyst is LaCoO3/SiO2> LaCoO3/Al2O3> LaCoO3.
Unsupported LaCoO3 and Supported LaCoO3Viz LaCoO3/Al2O3 and LaCoO3/SiO2 were subjected to toluene oxidation. The oxidation of toluene gave benzaldehyde (BzH), benzoic acid (BzA), maleic acid (MA) and CO2 as the products.
The formation of benzaldehyde as a function of temperature and aerial activity of LaCoO3, LaCoO3/SiO2 and LaCoO3/Al2O3 at 350șC, 400șC, 450șC, 550șC and 600șC are presented in Table II. The rate of formation of BzH in the beginning is higher on all catalysts. An initial increase in temperature from 350șC to 450șC shows an increase in the percentage conversion of BzH from 46.7% to 70.8% on unsupported LaCoO3 (Specific surface area of LaCoO3 increases from 6.7 m2/g to 12.4 m2/g). On LaCoO3/SiO2 from 56.6% to 79.4% (Specific surface area of LaCoO3/SiO2 increases from 50.1 m2/g to 61.1 m2/g). On LaCoO3/Al2O3 from 59.4% to 84.0% (Specific surface area of LaCoO3/Al2O3 increases from 70.4 m2/g to 78.9 m2/g)
Further increase in temperature from 450șC to 600șC decreases the percent conversion of benzaldehyde (Table 2). Thus in the present investigation the catalyst have been found to be active as well as selective at 450șC for the partial oxidation of toluene. The increase in activity up to 450șC can be ascribed to increasing removal of trace surface contaminant such as adsorbed gases, hydroxy species of adsorbed water27-28 and to the generation of stoichiometric or structural defects such as anion vacancies or disorders and exposed metal ions, which serves as catalytic sites29. The decrease in activity observed above 450șC may be due to a decrease in surface disorder due to the relatively high mobility of O2- ions in the lanthanide sesquioxides30 and resulting in the formation of low surface area at 600șC.
From the result it has been seen that the catalyst is highly select in the oxidation of toluene. The selectivity and activity of catalyst is correlated to surface area values. The most selective and active catalyst is LaCoO3 supported on Al2O3 catalyst because it has more surface area value than unsupported LaCoO3 and LaCoO3/SiO2 catalyst.
The difference in catalytic activity of these Perovskites heated at different temperature can also be related to the different degrees of heterogenecity of the surface of these oxide31,32. This is caused by terraces, steps, Kinks, Vacancies etc., having atoms with different degrees of unsaturated and with unusual oxidation states which may play an important role in catalysis.
Voorhoeve et al33 have suggested that the catalytic activity of LaCoO3 for CO Oxidation increases with increasing Co2+ content in the sample. Bhide et al.34 have shown that in the temperature range employed in the present study LaCoO3 contains Co2+ and Co4+ besides low and high spin Co3+ and their relative concentrations depend on the temperature.
Based on Haber et at35: It can be suggested that toluene activated by the abstraction of hydrogen atom is attacked by a nucleophilic O2- ion. It can further be suggested that benzaldehyde appears when O2- ion approaches the CH2 group from the direction perpendicular to the benzene ring. This is precisely the direction from which nucleophilic addition of a surface O2- ion of an oxide catalyst could be expected if the toluene molecule were adsorbed side on at an site of the surface through its π electron system. It can thus be concluded that oxidation of toluene to benzaldehyde at the surface of a catalyst is a nucleophilic oxidation.
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Received on 01.04.2021 Modified on 26.04.2021
Accepted on 16.05.2021 ©AJRC All right reserved
Asian Journal of Research in Chemistry. 2021; 14(4):282-284.
DOI: 10.52711/0974-4150.2021.00048