INTRODUCTION:

The acylation of aromatic compound has attracted increased attension because of its application in fine chemical industry due to its commercial importance [1-3]. A wide range of homogeneous catalyst such as FeCl_3,AlCl₃ and TiCl₄ etc, has been utilized for Friedel–Crafts acylation of various organic compounds. However these traditionally homogeneous catalysts are associated with many difficulties like they are corrosive in nature, expensive and non-regenerable. Thus, due to the environmental impact of heterogeneous catalysts the uses of theses catalysts has become attractive. Therefore the heterogeneous catalysts are replaced over homogeneous catalysts for acylation of organic compound such as sulphate-ion doped metal oxides, silica-alumina and zeolite have been investigated [4-11]. The Friedel–Crafts acylation of toluene is the traditional and most prevalent route for synthesis of methyl acetophenone (MAP) which are used as an intermediate in manufacture of drugs, cosmetics as well as perfumes. In literature different solid catalysed has been reported for the acylation of toluene e.g. over Beta zeolites [12].

Acylation of toluene over H-ZSM-5[13]. The acylation of toluene cation exchanged zeolite-Y and also be catalyzed by Brönsted acid sites [14]. The CeNa-Y catalyzed exhibited the acylation of toluene with C₁₂ and C₁₄ alkanoic acids provided the maximum yield [15]. Recently, the ferrites are successfully catalysts over homogenous catalyst using to their high thermal stability, reusability, better selectivity and high yield as well as eco-friendly nature. Magnetic, electrical and catalytic properties of these catalysts are well known. The spinel structure of ferrite is M²⁺_tet. [Fe³⁺_octa.]O₄. The ionic distribution of spinal structure is modified due to the addition of third metal. The distribution of metal ions among tetrahedral and octahedral are amiably influence by the acido-basic characterize of ferrospinels [16-20]. In view of the above, herein, we report the acylation of toluene with acetic anhydride using different ferrites. The present study based upon Ni²⁺/Co²⁺/Cr²⁺ionic distribution in spinal structure.

EXPERIMENTAL:

Catalyst preparation:

Preparation of NiFe₂O₄ (NSF-1):

Catalysts are prepared by using co- preparation method. The solution of 0.075 mol of NiCl₂ in 50 mL of water and solution of 1.2 mol of NaOH in 150 mL of water was allowed to react. The resulting solution was added to 0.15 mol of FeCl₃.6H₂O in 2.5 L of 0.6 M HCl. The resulting solution was stirred for 2 h. The mixture was further heated for 0.5 h at 60 ⁰C. The mixture was allowed to settle. And after that mixture was allowed to titrate with 2 M NaOH till a permanent phenolphthalein color was obtained. The product was washed (about 15 washings were required) with distilled water until the supernatant was free of Cl^-.Then allowed to filtere through a sintered glass filter, dried in an oven at 120 ⁰C and calcined at 500 ⁰C for 16 h. Final the product was strained through a 6/10 mesh size sieve [21-23].

Preparation of other catalysts:

Preparations of CrFe₂O₄ (NSF-2) and CoFe₂O₄ (NSF-3) were similar to that of NiFe₂O₄ (NSF-1) described above, except that, 0.075 mol of CrCl₂ or CoCl₂ were used in place of NiCl₂. Ni_0.5Co_0.5Fe₂O₄ (NSF-4) was prepared by taking 0.0375 mol of NiCl₂ and 0.0375 mol of CoCl_2, and Co_0.5Cr_0.5Fe₂O₄(NSF-5) was prepared by taking 0.0375 mol of CoCl₂ and 0.0375 mol of CrCl₂. Similarly, Ni_0.5Cr_0.5Fe₂O₄ (NSF-6) was prepared by taking 0.0375 mol of NiCl₂ and 0.0375 mol of CrCl₂and Ni_0.33Co_0.33Cr_0.33Fe₂O₄(NSF-7) was prepared by taking 0.025 mol of NiCl₂, 0.025 mol of CrCl₂and 0.025 mol of CoCl_2.

Catalyst characterization, acidity and surface area measurements:

Catalysts were characterized by BET surface area, X-ray diffraction and NH₃-TPD methods. X-ray diffraction of NSF-1, NSF-2, NSF-3, NSF-4, NSF-5, NSF-6 and NSF-7 was recorded on a Rigaku diffracto meter with Cu-Ka radiation and is reproduced. The peaks in the pattern match well with the characteristic of corresponding ferrites and confirm the samples phase purity. Temperature programmed ammonia desorption (NH₃-TPD) method was used to measure the acidity values of all the ferrite catalysts. Take 1.0 g of the sample was introduced into the sample tube. It was heated to 300 ⁰C with nitrogen flow for 2 h. Then temperature of the sample brought down to 25 ⁰C. And the sample at this temperature was exposed to ammonia for 2 h.

The sample was flushed with nitrogen for 1 h to desorb the loosely bound ammonia molecules on the surface of catalyst. The temperature of the sample was then raised to 150 ⁰C the physically adsorbed ammonia was desorbed. Ammonia desorbed in this temperature range corresponds to weak acidic sites. The temperature of the sample was then raised to 250 ⁰C and the amount of desorbed ammonia was predicted. We observed the ammonia desorption in different temperature range (150-450 ⁰C) was considered to represent medium of strong and weak acidic sites. Quantitative estimation was made by volumetric analysis; results are depicted in Table.1.

Fig.1. X-ray diffractograms of (a) NSF-1 (b) NSF-2 (c) NSF-3 (d) NSF-4 (e) NSF-5 (f) NSF-6 (g) NSF-7

Apparatus and procedure:

By using a tubular fixed-bed micro reactor of 0.45 m length and 13 mm diameter the catalytic activities were determined. The upper half worked as preheater and lower half worked as reactor. The catalyst was packed between two plugs of glass wool and was activated at 500 ⁰C under a flow of air. The flow of nitrogen gas was started to down the desired temperature. The reactants were fed from the top with the flow of nitrogen gas of 30 mL/min. By using a cold water condenser the gaseous products were condensed. A gas chromatograph with FID, SE-30 column was used to analyze the liquid product mixture. The catalytic activity data comparison is made between different catalysts at 2h duration run. When a blank run was taken without any catalyst there was negligible thermal conversion. Quantitative estimation was made with the help of volumetric analysis and the results are depicted in Table 1. The BET surface areas of ferrites were determined on Quantachrome Autosorb Automated Gas Sorption System Report Autosorb 1 for Windows 1.55 instrument; the results are presented in Table 1. Surface area and acidity data of various ferrite catalysts are presented in table 1. The acidity of catalyst increased from NSF1 to NSF7.

Table 1

Catalysts	150-250 ⁰C	250-350 ⁰C	350-450 ⁰C	Total acidity mmol/g	BET Surface area (m²/g)
NSF1	0.39	0.36	0.34	1.09	62.45
NSF2	0.40	0.39	0.38	1.17	60.73
NSF3	0.42	0.40	0.39	1.21	58.11
NSF4	0.43	0.41	0.40	1.24	57.80
NSF5	0.44	0.43	0.40	1.27	48.85
NSF6	0.46	0.42	0.41	1.29	46.36
NSF7	0.47	0.44	0.42	1.33	39.91

Catalytic acidity at different temperatures and BET surface areas (m²/g).

Table 2

Catalyst	Toluene conversion (%)	Para -MAP (%)	ortho- MAP (%)
NSF1	81.6	97.3	2.7
NSF2	81.9	98.0	2.0
NSF3	82.3	98.4	1.6
NSF4	82.6	98.6	1.4
NSF5	84.0	96.1	3.9
NSF6	85.7	98.2	1.8
NSF7	85.9	98.4	1.4

Variation of catalyst:

Effect of variation of catalyst on acylation activity:

All the tested ferrites are found to be active catalysts for toluene acylation with acetic anhydride under optimized conditions from the data table 2. The order of catalytic activity of different catalysts were found to be NSF7>NSF6>NSF5>NSF4>NSF3>NSF2>NSF1. The highest yield of acylated products obtained was 85.9 % of p- MAP with selectivity 98.4 % on NSF-7 catalyst. The other products formed in negligible amounts on optimized condition (molar ratio 1:3, WHSV 0.4h^-1and temperature 300 ⁰C. The Ni²⁺/Co²⁺/Cr⁺²ionic distribution in the spinel lattice of catalyst NSF-7 plays an important role for the high yield and selectivity.

Fig. 2. Effect of variation of catalyst on acylation activity.

Effect of temperature on acylation:

The acylation of toluene with acetic anhydride was studied over (NSF-7) in the temperature range 200-600 ⁰C, WHSV 0.4h^-1 and molar ratio 1:3, the results are shown in fig.3. Therefore the analysis of acylation of toluene was restricted to only 300 ⁰C, where the yield of methyl-acetophenone is high in comparison to the values reported in literature in each acylation reaction. It was found that there are decrease in the yield of the acylated product at higher temperatures is due to the charring. The present study concludes that the strong and acidic medium sites favor vapor phase acylation of toluene.

Fig. 3. Effect of temperature on acylation

Variation of feed with respect to molar ratio:

As seen from the results in fig.4., the molar ratio of toluene : acetic anhydride varied from 1:2 to 1:5 and the vapor phase acylation of toluene was carried out over NSF-7 at temperature 300 ⁰C and WHSV 0.4 h^-1. Toluene conversion and selectivity of p-MAP increased with increase in the toluene -to-acetic anhydride molar ratio, reaching a maximum at 1:3. At higher molar ratio, the selectivity and yield of p-MAP decreases. This was probably due to the absence of active sites for toluene on catalyst surface, because of competition between toluene and acetic anhydride adsorption.

Fig. 4. Variation of feed with respect to molar ratio

Variation of weight hour space velocity (WHSV):

The acylation of toluene was studied over (NSF-7) catalyst at a temperature of 300 ⁰C, at 1:3 molar ratio of toluene to acetic anhydride and WHSV (0.2, 0.3, 0.4 and 0.5 h^-1). The results are presented in fig.5. Toluene conversion improved as the WHSV increased from 0.2 to 0.4 h^-1 and decreased thereafter. The high contact time causes charring over active sites, thereby decreasing the yield of the main product. Below 0.4 h^-1 WHSV toluene conversion was decreased due to the low contact time with the catalyst.

Fig. 5. Variation of weight hour space velocity (WHSV).

Effect of time on stream (TOS):

On all the ferrites the reaction was carried out for 10 h and the best result was given by catalyst NSF-7, under optimized conditions i.e. WHSV 0.4 h^-1, molar ratio 3 and temperature 300 ⁰C. The continuous decrease of ferrite catalytic activity has been observed. The results are shown in fig. 6. The conversion with time on stream decreased gradually but selectivity is not affected very much. This might be due to coke induced selectivity or may be absence of side products.

4. Mechanism

Reaction:-

CONCLUSIONS:

In summary, for the acylation of toluene the catalytic activity experiment reveals that strong and acidic sites medium are suitable and the ferrites activity was dependent on the reaction parameters. The vapor phase acylation of toluene occurred more on NSF-7. The high yield and selectivity depends on Ni²⁺/ Co²⁺ /Cr⁺² ionic distributions on spinel lattice of ferrite. The maximum yield of acylated product obtained was 85.9%, and the selectivity of p-MAP was 98.4% and o-MAP was 1.4%, over NSF-7 ferrite and acetic anhydride at molar ratio 1:3, WHSV 0.4 h^-1 and temperature 300 ⁰C. It was found to be gradually decreasing with the time on stream as observed for the period of 10 h.

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Received on 29.01.2016 Modified on 17.02.2016

Asian J. Research Chem. 9(2): Feb., 2016; Page 77-81

DOI: 10.5958/0974-4150.2016.00014.6