1. INTRODUCTION:

Transition metal ferrites have attracted great attention because of their significant magnetic, electrical and catalytic properties [1-2]. The use of ferrite as heterogeneous catalysts over homogenous catalyst has led to enhanced reaction rates, better selectivity, high yield and simple workup[3-5] These catalyst are possessing remarkable magnetic properties and can be easily recovered from the reaction mixture. A variety of metal ferrite such as copper, nickel, chromium, cobalt, zinc and mixed metal ferrite have been developed and utilized in varias organic transformation for catalytic purposes [6-7]. The crystalline structure of metal ferrite M²⁺_tet.[Fe³⁺_octa]O₄ is spinel, where M is Co, Ni, Cr. The distributions of metal ion between tetrahedral and octahedral are influence by acido-basic properties of ferrospinels [8-9].

Furan and its derivatives are important heterocyclic compounds used as intermediates in drugs, pharmaceutical industries, research laboratories, etc. [10-11]. 2-actylfuran is an important intermediate to HIV-integrase S-1360 inhibitor [12]. Friedal-Craft acylation of furan using homogeneous catalytic system like AlCl₃, FeCl₃, TiCl₃, is the conventional and classical method for the synthesis of 2-actylfuran . Holdrich et al. reported the synthesis of 2-acetylfuran with 99% selectivity and 23% conversion of furan over Ce-doped boron zeolite [13]. Reddy et al. reported the acylation of furan with acetic anhydride over zeolites as heterogeneous catalytic system with 67.5% conversion of furan [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 synthesis of 2-actylfuran from furan and acetic anhydride using ferrites with improved yield reported compared to the literature.

2. EXPERIMENTAL:

2.1. Preparation of CoFe₂O₄ (NSF1)

Ferrites are prepared by using co-preparation method reported in the literature [15-17].

2.2. Preparation of other catalysts

The other catalysts were also prepared by following the same method as for CoFe₂O₄ (NSF1), there are variation in number of moles of metal. Ni_0.5Co_0.5Fe₂O₄ (NSF2) was prepared by taking 0.0375 mol of NiCl₂ and 0.0375 mol of CoCl_2, at the place of 0.075 mole of CoCl_2,and Co_0.5Cr_0.5Fe₂O₄(NSF3) was prepared by taking 0.0375 mol of CoCl₂ and 0.0375 mol of CrCl₂. Likewise, Ni_0.5Cr_0.5Fe₂O₄ (NSF4) was prepared by taking 0.0375 mol of NiCl₂ and 0.0375 mol of CrCl₂and Ni_0.33Co_0.33Cr_0.33Fe₂O₄(NSF5) was prepared by taking 0.025 mol of NiCl₂, 0.025 mol of CrCl₂and 0.025 mol of CoCl_2.[15-17]

2.3. Catalyst characterization, surface area and acidity measurements

Catalysts were characterized by FTIR, X-ray diffraction (XRD), BET surface Area and ammonia-TPD methods. FTIR spectra were recorded on Shimazdu 8000 spectrophotometer. Two major bands were observed around 700 cm^-1 and 500 cm^-1. These bands indicate that formation of spinel structure having tetrahedral and octahedral arrangement of ion. Absorption band around 700 cm^-1is attributed to the tetrahedral sites, and band observed around 500 cm^-1 is indicates octahedral sites which are in agreement with reported data in the literature [18]. X-ray diffraction of NSF1, NSF2, NSF3, NSF4 and NSF5 was recorded on a Rigaku diffractometer with Cu-Ka radiation and is reproduced. The observed X-ray peaks matches with the characteristic reflections of corresponding ferrites and confirm the phase purity of samples.

To determine the acidity values of all the ferrite catalysts temperature programmed ammonia desorption (NH₃-TPD) method was used. 1.0 g of the sample was taken in the tube and heated up to 673 K with under nitrogen atmosphere for 2 h followed by cooling down to 298 K. Then the sample was exposed to ammonia for 2 h and sample was flushed with nitrogen for 1 h to desorb the loosely bound ammonia molecules on the sample. The sample was again heated to 423 K, so that physically adsorbed ammonia get desorbed which corresponds to weak acidic sites. The temperature of the sample was then raised to 623 K and the amount of desorbed ammonia was estimated. The ammonia desorbed in this temperature range was considered to represent medium acidic sites. Further, this desorption process was repeated at 723 K; the ammonia desorbed in this region was considered to represent strong acidic sites. Quantitative estimation was made by volumetric analysis and results are presented in Table1. 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 depicted in Table-1.

2.4 Apparatus and procedure

On fixed-bed micro reactor having 0.45 m length and 13 mm diameter was used to determined catalytic activities. The lower half worked as reactor and upper half worked as preheater. The ferrite was packed between two plugs of glass wool. The catalyst was activated at 773 K with the flow of air for one hour. Then start the flow of nitrogen gas to down the desired temperature about 373K. Now set the flow of nitrogen gas of 30 mL/min and reactants were fed from the top. In condenser the cold water was used to condense gaseous products. To analyze the liquid product mixture we have to use gas chromatograph FID, SE-30 column. The activity of ferrites data comparison is made between different catalysts at 2h duration run. There was negligible thermal conversion when a blank run was taken without any catalyst.

Table 1

Catalysts	423-523K	523-623K	623-723K	Total acidity mmol/g	BET Surface area (m²/g)
NSF1	0.42	0.40	0.39	1.21	58.11
NSF2	0.43	0.41	0.40	1.24	57.80
NSF3	0.44	0.43	0.40	1.27	48.85
NSF4	0.46	0.42	0.41	1.29	46.36
NSF5	0.47	0.44	0.42	1.33	39.91

BET surface areas (m²/g) and catalytic acidity at different temperatures. [19]

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 2.

Table 2

Catalyst	% Conversion	% Yield	Selectivity
NSF1	84.76	69.12	98.01
NSF2	91.80	83.32	99.03
NSF3	86.34	72.67	98.90
NSF4	87.40	75.43	99.10
NSF5	94.12	89.07	99.71

From the data of table 2, all the tested ferrites are found to be active catalysts for furan acylation with acetic anhydride under observed conditions. The catalytic activity of different catalysts were found to be in order NSF5>NSF2>NSF4>NSF3> NSF1. The maximum yields of acylated products obtained was 89.07 % of 2-actyle furan with selectivity 99.71 % on NSF-5 catalyst .The other products formed in negligible amounts on optimized condition (molar ratio 1:4, WHSV 0.3 h^-1and temperature 573 K). The Ni²⁺/Co²⁺/Cr⁺²ionic distribution in the spinel lattice of catalyst NSF-5 plays important role for high yield and selectivity.

Table 3

Effect of temperature on acylation of furan (catalyst NSF5, WHSV 0.3 h^-1 and molar ratio 1:4)

Temperature (K)	% Conversion	% Yield	Selectivity
473	34.97	27.14	86.18
523	92.62	76.81	97.93
573	94.12	89.07	99.71
623	91.42	80.54	98.46
673	75.11	62.31	98.01

3.2 Effect of temperature.

An effect of temperature on acylation of furan acetic anhydride was studied over (NSF-5) in the temperature range 473-673 K, WHSV 0.3 h^-1 and molar ratio 1:4; the results are shown in table 3. Therefore the investigation of acylation of furan was restricted to only 573 K, where the yield of 2-actylfuran is high in comparison to the values reported in literature in each acylation reaction. It was also found that the decrease in the yield of the acylated product at 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 furan. The activity of catalyst goes decrease with increase the temperature.

Fig. 1: Effect of WHSV on acylation of furan (catalyst NSF5, temperature 573 K and molar ratio is 1:4)

3.3. Variation of WHSV.

The effect of WHSV on acylation of furan was studied over (NSF-5) catalyst at a temperature of 573K, at 1:4 molar ratio of furan : acetic anhydride and WHSV (0.2, 0.3, 0.4, 0.6 and 0.8 h^-1). The results are shown in fig.2. Furan conversion increased as the WHSV increased from 0.2 to 0.3 h^-1 and decreased thereafter. The more contact time of catalysts and feed causes charring over active sites, hence decreasing the furan conversion. Below 0.3 h^-1 WHSV furan conversion was decreased due to the low contact time of feed and the catalyst. Hence the best yield of acylated products on 0.3 h^-1, that is optimum weight hour space velocity.

Fig. 2: Effect of molar ratio on acylation of furan (catalyst NSF5, temperature 573 K and 0.3 h^-1WHSV)

3.4. Variation of molar ratio.

As seen from the results in fig.2., the molar ratio of furan : acetic anhydride varied from 1:2 to 1:8 and the vapor phase acylation of furan was carried out over NSF-5 at temperature 573 K and WHSV 0.3 h^-1. Furan conversion and selectivity of 2-actyle-furan increased with increase in the furan-to-acetic anhydride molar ratio, reaching a maximum at 1:4. At higher molar ratio, the selectivity and conversion of 2-actylefuran was reduced. This was probably due to the unavailability of active sites for furan over the catalyst surface, because of competition between furan and acetic anhydride adsorption

Fig. 3: Effect of time on stream (TOS)

3.5. Effect of time on stream (TOS)

The acylation of furan was carried out for 10 h on all the ferrites. Under optimized conditions the highest conversion was obtained on catalyst NSF-5 at temperature 573 K, WHSV 0.3 h^-1 and molar ratio 1:4. It has been observed that there was continuing decrease of catalytic activity with increasing time. The results are shown in fig. 3. The acylated products decreased gradually with time on stream, but selectivity is not very much affected. This may be due to coke formation on the surface of catalysts or due to deactivation of ferrite.

4. MECHANISM:

The aromatic five-membered heterocyclic compounds all undergo electrophilic substitution. Electrophilic substitution at 2- or 3-position is readily likely to occur. Further, quantitative results derived by molecular orbital method applied to furan show greater p-electron density at 2-position than at 3-position. Dewar gave a qualitative explanation to this difference in reactivity [20-22].

5. CONCLUSIONS:

In conclusion, catalysts NSF1, NSF2, NSF3, NSF4 and NSF5 prepared via low temperature co-precipitation method are utilized for acylation of furan with high yields of 2-aceylfuran compared to the literature. The catalytic activity experiment reveals that strong and medium acidic sites are suitable for the acylation of furan. The catalytic activity of the system was dependent on the reaction parameters and maximum yield of 2-acetylfuran was achieved on NSF-5 catalytic system. The maximum yield of acylated product obtained was 89.07 % of 2-actylefuran with 99.71% selectivity, with negligible side products over NSF-5 ferrite and acylating agent is acetic anhydride, at molar ratio 1:4, WHSV 0.3 h^-1and temperature 573 K.

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Received on 01.03.2016 Modified on 04.04.2016

Asian J. Research Chem. 2016; 9(5): 200-204

DOI: 10.5958/0974-4150.2016.00034.1