1. INTRODUCTION:

The aromatic ketones are synthesized by the acylation of aromatic compounds, which are highly used in fine chemical and pharmaceutical industry [1]. Acylated compounds are highly used for production of fragrances, drugs, plastisizers, pharmaceuticals, dyes, paint additives, photoinitiators and other many products [2]. From last many years homogenous catalysts were used for acylation. Due to some limitation of these homogenous catalysts are replaced by heterogeneous catalysts. These are mainly solid acid catalysts such as oxides, metal oxide, sulfate-ion-doped, zeolite, ferrite, silica–alumina, heteropoly acids and their salts [3] Main advantage of these catalysts because of their high thermal stability, reusability, easy separation and their eco-friendly nature. Ferrites are also having great attention in field of heterogeneous catalysts because of their significant electrical and magnetic catalytic properties .The use of ferrites has led to enhanced reaction rate, high yield, better selectivity and simple workup with low cost [4].

From literature survey, Friedel-Crafts acylation of anisole was studied with propionic anhydride over H-Beta zeolite, synthesised directly from crystalline rice husk ash in differents gel ratios of SiO₂/Al₂O₃. The main products was obtain under the optimum acylation condition was p-methoxy propiophenone. The highest activity,with anisole conversion of 88.9% and product selectivity toward p-methoxy propiophenone of 75.3% [5]. The electrophilic acylation of anisole with acetyl chloride [6] and acylation with acetic anhydride [7] also it has been reported that acylation of anisole with phenylpropionyl chloride, phenyl acetate and phenyl propionic, phenylacetyl chloride [8]. Synthesis of 4-methoxy propiophenone was found at anisole to propionic anhydride molar ratio 8:1, at 100⁰C. The conversion of propionic anhydride to 4-methoxy propiophenone was found to be 44.7% [9]. Acylation of anisole with benzoic acid over AlPW₁₂O₄₀at temperature 120⁰C the yield was 92% [10]. Acylation of anisole with heptanoic acid over Eu(NTf₂)₃at temperature 250⁰C yield was 87% [11]. Acylation of anisole octanoic acid over Cs_2.5H_0.5PW₁₂O₄₀ at 110⁰C yield 45% [12]. The acylation of anisole with acetic anhydride in the presence of mixed oxides WO₃/ ZrO₂catalyst. These catalysts having both Bronsted acid and Lewis acid sites [13]. Deutch et al have been reported that the acylation of anisole with carboxylic anhydrides and chlorides over SZ treated solid oxide catalysts. SZ shows greater activity than HBEA. The yield of the product 88-96% and 94-99% selectivity was observed [14].

In view of the above and in confirmation of our continuing investigation of ferrite ferrospinels as catalytic system, herein we report the vapor phase acylayion of anisole with acetic anhydride by using ferrites with improved yield reported compared to the literature.

2. EXPERIMENTAL:

2.1 Catalyst preparation

2.1.1 Preparation of NiFe₂O₄(NF), Ni_0.5Co_0.5Fe₂O₄ (NCF), Cr_0.5Ni _0.5 Fe₂O₄(CNF), and Cr_0.33Ni_0.33Co_0.33Fe₂O₄(CNCF), these all ferrite are prepared by using low temperature co- precipitation method [15].

2.1.2 Apparatus and Procedure

Anisole was acylated with acetic anhydride in a continuous fixed bed down flow reactor. The catalyst was sieved through a sieve of size 6/10 mesh and placed on the glass wool bed in the middle of the reactor. The upper halves worked as pre-heater and lower half as reactor. The current of N₂ gas @ 30 mL/min. The reactants were fed from the top of reactor. The heating of heater was controlled by pyrometer. The gaseous products were collected at the bottom of reactor with the help of ice cold water coiled condenser. Gas Chromatograph with flame ionization detector having fused silica based column (SE-30) was used to determine the composition of the product mixture.

2.2 Catalyst characterization, acidity and surface area measurements

Ferrites were characterized by, X-ray diffraction, BET surface area, SEM, X- Ray florescence (XRF) and NH₃-TPD methods. X-ray diffraction of different ferrites, the peaks pattern was well match with the characteristic of corresponding ferrites and confirms the samples phase purity. X-ray diffraction was observed with the help of Rigaku diffractometer with Cu-Ka radiation. SEM of the ferrite was recorded on Instrument-SEMTRAC MINI. The sample of ferrite should be freshly calcined and nanopowders. SEM is shown in Figure 1. (a), (b), (c) and (d).

Infrared spectra of ferrites were recorded on Shimadzu IR Affinity 8000 FT-IR spectrometer. The FTIR-spectrum consists of two major bands. Due to stretching vibration of the M-O bond at the tetrahedral site the band arise around 700 cm^-1 that is higher band and the lower band around 500 cm^-1 which corresponds to stretching vibration of the M-O bond at the tetrahedral site. [16]. The band position difference is due to the difference in metal-oxygen bond distance at tetrahedral and octahedral sites of the spinel.

To measure the acidity NH₃-TPD method was used. To measure the acidity of all ferrite catalysts, take 1.0 gm of the sample was introduced into the sample tube. With the flow rate of N₂gas at pressure 30 mL/min for 2 h with heating at 500 ⁰C. After that the temperature of the reactor brought down to 25 ⁰C. And at this temperature by removing nitrogen gas follow, exposed to ammonia for 2 h. The sample was flushed with nitrogen for 1 h to desorbed the loosely bound ammonia molecules on the surface of catalyst. The physically adsorbed ammonia was desorbed when temperature of the sample was then raised up to 150 ⁰C. Ammonia desorbed in this temperature range corresponds to weak acidic sites. 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. BET surface areas are also reported in table no.1.

Table 1 BET surface areas (m²/g) and catalytic acidity.

Catalysts	Acidity mmol/g	BET Surface area (m²/g)
NF	1.09	62.45
NCF	1.24	57.80
CNF	1.29	46.36
CNCF	1.33	39.91

BET surface areas (m²/g) and catalytic acidity at different temperatures [15].

Table 2 Metal Concentration (wt %)

Chemical Analysis of Ferrite Catalysts by XRF
Metal Concentration (wt %)
Catalysts	Cr	Ni	Co	Fe
NF	-------------	33.05 (34.45)	---------------	64.76 (65.55)
NCF	--------------	16.67 (17.21)	16.92 (17.28)	65.10 (65.50)
CNF	15.13 (15.56)	17.59 (17.56)	--------------	66.30 (66.87)
CNCF	10.21 (10.30)	11.48 (11.62)	12.13 (11.67)	65.17 (66.39)

Table 3 Relative molar ratio of ferrite catalyst

Chemical Analysis of Ferrite Catalysts by XRF
Relative molar ratio of ferrite catalysts
Catalysts	Cr	Ni	Co	Fe
NF	------------	0.96 (1.0)	------------	1.94 (2.0)
NCF	------------	0.48 (0.5)	0.49 (0.5)	1.98 (2.0)
CNF	0.48 (0.5)	0.50 (0.5)	-------------	1.98 (2.0)
CNCF	0.32(0.33)	0.32 (0.33)	0.34 (0.33)	1.96 (2.0)

(a) (b)

(c ) (d)

Figure 1. SEM (a) NF, (b) NCF (c) CNF (d) CNCF

3. EFFECT OF REACTION PARAMETER.

3.1. Variation of catalyst

Effect of variation of catalyst was tested for the selection of most appropriate catalytic system and the results are depicted in table 4.

Table 4

Catalysts	% Conversion	% Yield	Selectivity
NF	85.56	85.29	99.68
NCF	88.76	88.46	99.66
CNF	90.46	90.33	99.85
CNCF	98.50	97.76	99.24

The acylation of anisole with acetic anhydride under optimized conditions was found on all the tested ferrites are shown in table 4. It was observed that the order of catalytic activity of different ferrites was NF< NCF< CNF< CNCF. The maximum yields of acylated product obtained 97.76 % of 4-MAP with selectivity 99.24 % on CNCF catalyst. The other products formed in negligible amounts on optimized condition (molar ratio 1:1, WHSV 0.4h^-1and temperature 350 ⁰C). The Ni²⁺/Co²⁺/Cr⁺²ionic distributions in the spinel lattice of catalyst plays important role for high yield and selectivity.

Figure 2. Variation of molar ratio.

3.2. Variation of molar ratio.

From fig.2., the molar ratio of anisole : acetic anhydride varied from 1:1 to 1:4 and the vapor phase acylation of anisole was carried out over CNCF at temperature 350 ⁰C and WHSV 0.4 h^-1. Anisole conversion and selectivity of 4-MAP decrease with increase in the anisole-to-acetic anhydride molar ratio. At higher molar ratio, the selectivity and conversion of 4-MAP was reduced. This was probably due to the unavailability of active sites for anisole.

Figure 3: Effect of temperature on acylation of anisole.

3.3 Effect of temperature.

From fig.3., the acylation of anisole with acetic anhydride was studied over CNCF catalysts at temperature range 200-400 ⁰C, WHSV 0.4 h^-1 and molar ratio 1:1; Therefore the investigation of acylation of anisole was restricted to only 350 ⁰C, where it was found that the yield of 4-MAP is high in comparison to the values reported in literature. This may be due to the generation of strong Lewis acid sites at high reaction temperatures. These strong Lewis acid sites enhance the formation of 4-MAP. It was also found that the decrease in the yield of the acylated product at much higher temperatures is due to the charring of some reactants. These observations draw our attention to conclude that the strong and medium acidic sites favor vapor phase acylation of anisole. The activity of catalyst goes decrease with increase the temperature.

Figure 4: Effect of WHSV on acylation of anisole

3.4. Variation of WHSV.

It was experimentally observed that in acylation of anisole over Cr²⁺ / Ni ²⁺ /Co ²⁺ ionic distribution in the spinel lattice influences catalyst at a temperature of 350 ⁰C, at 1:1 molar ratio of anisole : acetic anhydride and WHSV (0.2, 0.3, 0.4 and 0.5 h^-1). The effect of WHSV results are shown in fig.4. anisole conversion increased as the WHSV increased from 0.2 to 0.4 h^-1 and decreased thereafter. The more contact time of catalysts and feed causes charring over active sites, hence decreasing the anisole conversion. Below 0.4 h^-1 WHSV anisole conversion was decreased due to the low contact time of feed and the catalyst. Hence the best result was obtained on 0.4 h^-1, that is optimum weight hour space velocity.

Figure 5: Effect of time on stream (TOS).

3.5. Effect of time on stream (TOS)

Under the optimized condition the reaction was carried out for 10 h. The acylation of anisole highest conversion was obtained on catalyst CNCF at temperature 350 ⁰C, WHSV 0.4 h^-1 and molar ratio 1:1. It was observed that there was continuing decrease of catalytic activity with increasing time, it may be due decrease in acidic active site of catalysts and also may be due to coke formation on the surface of catalysts or due to deactivation of ferrite. The results are shown in fig. 5. The acylated products decreased gradually with time on stream, but selectivity is not very much affected.

5. CONCLUSIONS:

Acylation of anisole under optimized condition, In conclusion, catalysts NF, NC, CN and CNCF prepared via low temperature co-precipitation method are utilized for acylation of anisole with high yields of 4-MAP compared to the literature. The catalytic activity experiment reveals that strong and medium acidic sites are suitable for the acylation of anisole. The catalytic activity of the system was dependent on the reaction parameters and maximum yield of 4-MAP was achieved on CNCF catalytic system. The maximum yield of acylated product obtained was 97.76 % of 4-MAP with 99.24 %.selectivity, with negligible side products over CNCF ferrite and acylating agent is acetic anhydride, at molar ratio 1:1, WHSV 0.4 h^-1 and temperature 350 ⁰C.

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