Activity of Aniline Methylation over Fe-Cu-Cr Ternary Spinel Systems

A series of spinels having the general formula CuCr2-xFexO4 with x = 0.25, 0.75, 1.25, 1.75 were prepared by co-precipitation method. The catalysts were characterized by various physico-chemical methods like XRD, BET, SEM, EDX and TPD. The reaction of aniline with methanol was studied in a fixedbed reactor system as a potential source for the production of various methyl anilines. It was observed that systems possessing low ‘x’ values are highly selective and active for N-monoalkylation of aniline leading to N-methylaniline. Reaction parameters were properly varied to optimize the reaction conditions for obtaining N-methylaniline selectively and in better yield. Among the systems CuCr1.75Fe0.25O4 is remarkable due to its very high activity and excellent stability. Under the optimized conditions Nmethylaniline selectivity exceeded 91%. CuCr1.25Fe0.75O4 gives better conversion than CuCr1.75Fe0.25O4 in CuCr2-xFexO4 series. The Lewis acid sites of the catalysts are mainly responsible for the good catalytic performance. © 2014 BCREC UNDIP. All rights reserved


Introduction
Metal oxides having the spinel structure show greater structural stability and catalytic activity.Spinels have different applications in the area of heterogeneous catalysts.Spinels are a class of ternary oxides with composition AB2O4 where A and B are divalent and trivalent cations respectively; A ions occupy tetrahedral sites and B ions occupy octahedral sites [1][2].The crystallographic structure of a spinel is packed face centered cubic close packed array of anions with holes partly filled by the cations.In a normal spinel structure the entire A atoms are tetrahedrally coordinated while B atoms are octahedrally surrounded by oxygen atoms.Chromite spinels have general formula MCr2O4; where M is a bivalent metal ion.All MCr2O4 chromites have normal spinel structure with M in tetrahedral sites because of strong preference of Cr 3+ ions for octahedral sites [3].This was confirmed by thermodynamic and quantum chemical calculations [4,5].
Metal chromites have been extensively used as catalyst in dehydrogenation [6][7][8], oxidation of hydrocarbon and dehydration of alcohols [9,10].Mixed oxides of chromium posses p-type semi conductivity [11,12].The catalytic activity increases with increasing p-type conductivity.Metal oxides with high conductivity have high activity regardless of their nature [13].p-type semiconductors that can be converted to n-type are effective catalysts [14].
CuCr2O4 is a normal spinel oxide with Cu as divalent ion and Cr as trivalent ion.Metal chromites have been extensively used as catalyst in dehydrogenation, oxidation of hydrocarbon and dehydration of alcohols.CuCr2O4 is a normal spinel and CuFe2O4 is a tetragonal spinel.Alkylation of aniline is an industrially outstanding reaction due to the numerous uses of alkylated products.Aniline methylation is a pseudo first order reaction with respect to aniline concentration N-methylation products (NMA and NNMA) are predominant in the temperature range 573-673 K. Selectivity to N, N dimethyl aniline is enhanced as reaction temperature and contact time increases.Schematic representation is given in Figure 1.
An-Nan Ko et al. [15] investigated the aniline methylation over g-alumina, which produced successively NMA and NNDMA.Narayanan et.al investigated the aniline methylation using transition metal oxides.Supported oxides are also used as catalyst for this type of reaction [16].
According to Elangovan et al. [17], weak and moderate acid sites are sufficient for Nalkylation where as strong acid sites are mandatory for C-alkylation.Aniline alkylation takes place on acidic [18] and on basic zeolites [19].The reaction performed on zeolites with redox properties.Vapour phase alkylation of aniline over zeolites and aluminophosphate depends on acid-base properties and shape selectivity [20].Selective N, N methylation over zeolites was reported.Mixture of N-and C-alkylated products can be obtained over more acidic form of the zeolite.Elangovan et al. also studied alkylation of aniline with methanol over various aluminophosphates in the vapour phase [17].The products formed were NMA, NNDMA, and NMT.Acidity of the catalyst plays an important role in the methylation of aniline.Vanadium incorporated aluminophosphate molecular sieves leads to the formation of NMA and NNDMA [17].High selectivity to NMA is obtained over metallosilicates.Bautista et al. reported such reactions using CrPO4 as catalyst [21].
Sugunan et al. calculated the kinetic parameters such as the activation energy and Arrhenius frequency factor for the N-methylation of aniline using methanol over Co ferrites [22,23].For these systems, maximum conversion obtained is 79.60% and NMA selectivity is 71.40% and suggests that surface basicity plays a dominating role in aniline methylation.Acidity and aniline alkylation activity of mixed Fe-Al pillared montmorillonites has been studied by sugunan et al. and suggest that weak and moderate acid sites influence the catalytic activity [24].The catalytic activity of Cr-Mn ferrospinels

Bulletin of Chemical Reaction Engineering & Catalysis, 9 (1), 2014, 40
Copyright © 2014, BCREC, ISSN 1978-2993 Figure 1.Scheme for aniline methylation in selective alkylation of aniline and related to their acid-base properties and also the cation distribution were reported [25].The vapourphase methylation of aniline by methanol has been extensively studied because its products are very essential intermediates in textile dye, perfumes, paper industry.
In present work spinels prepared via coprecipitation method have excellent conversion for aniline methylation due to their surface properties.In the present systems CFC 2 shows maximum conversion of 59.32% and 87.68% selectivity towards NMA.In our systems, the entire catalysts show good yield for NMA.It is observed that weak acid sites on the catalyst surface are favourable for the N-alkylation of aniline.

Synthesis of chromite spinels
Solutions of metal nitrates are required for the preparation.Stoichiometric masses of the metal nitrates were accurately weighed out and dissolved in distilled water to get 10% solutions.To the metal nitrate solution, 10% NaOH solutions are added drop-by-drop and stirred well by using a mechanical stirrer.pH was adjusted between 9 and 10.The precipitation is carried out at a temperature of 80 °C.The precipitate was kept overnight for ageing and then washed several times with distilled water until free from nitrate ions and alkali.It was filtered, dried in an oven at 80 °C for 24 h and the dried materials were powdered and sieved below 75 μm mesh.The powdered samples were calcined at 500 °C for 6 h to achieve complete spinel phase formation.Synthesized samples are summarized in Table 1.

Characterization of synthesized materials
All the prepared catalysts were characterized by different physico-chemical techniques viz X-ray Diffraction (XRD), Energy Dispersive X-ray Fluorescence Analysis (EDX), Scanning Electron Microscopy (SEM), Surface area measurements (BET) and acidity measurement by Temperature programmed desorption of ammonia.Micromeritics Gemini surface area analyzer was used to determine the surface area with nitrogen as adsorbate.EDX spectra of the samples were recorded in an EDX-JEM-35 instrument (JEOL co-link system AN-1000 Si detector).The XRD pattern of catalyst was taken by using Philips diffractometer (PW 1710).To determine the acidity of the catalysts, ammonia TPD measurements in the range 100-600 ⁰C were performed in a conventional flow-type apparatus at a heating rate of 20 ⁰C min -1 and in a nitrogen atmosphere.

Aniline Methylation
Aniline methylation was carried out in a self assembled fixed bed flow reactor made up of a glass tube at atmospheric pressure.0.5 g of the catalyst was kept at the centre of the reactor which is packed with glass wool.The reactor was heated with help of a furnace.A solution of aniline in methanol was introduced to the reactor using a syringe pump at the required flow rate and reaction temperature.Analysis of products was done by Gas chromatograph (Chemito GC 1000, flame ionization detector) equipped with a SE-30 capillary column.

X-Ray Diffraction (XRD) Analysis
A sharp peak observed at 35° is due to the spinel phase formed.By the addition of iron, lattice parameter decreases.This is due to the addition of Fe 3+ ions into octahedral sites.The XRD peak for substituted Cu-Cr spinels shows the presence of spinel phase formation.It can be observed that, spinel phase was formed at 500 °C hence it was taken as optimum calcination temperature for the prepared chromites.Table 2 shows XRD data for CuCr2-xFexO4 series.Figure 2 represents the X-ray diffractogram of CuCr2-xFexO4 series.The results confirm the crystal structure of the systems.CF has lowest crystallite size and CFC 4 has highest crystallite size in the CuCr2-xFexO4 series.

Energy Dispersive X-ray fluorescence (EDX) analysis
The elemental composition of chromite spinels determined by EDX analysis is given in Table 3.It is seen that the theoretical values are in accordance with the experimental value is almost all cases.The difference in values is due to leaching of metal in contact with mother liquor and also due to experimental error.

Scanning Electron Microscopy (SEM)
Scanning electron micrographs of some representative systems are given below.SEM analysis gives us an idea about the surface topography of the catalyst.Figure 3 represents the micrographs of CC, CFC 2 and CF.The particle size is observed to be larger in the case of CFC 2. Aggregate nature was observed for all samples.

BET Surface area and pore volume measurements
The surface area data is shown in the Table 4. Surface area and pore volume increases the amount of Fe content increases but after CFC3 both are decreases.It is observed that CFC3 is having the highest surface area among the Fe-chromite spinels.

Surface Acidity measurements by TPD -NH3
The TPD of ammonia was used to characterize the acid site distribution and furthermore to obtain the quantitative amount of acid site in the specified temperature range.Ammonia is an excellent probe molecule as it allows the determination of both the protonic and cationic acid centers.In this method, the interaction of acid sites and basic probe molecule (ammonia) is studied to determine the amount and strength of acid sites.The acid site distribution pattern can be classified into weak (desorption at 100-200 °C), medium (201-400 °C) and strong (401-600 °C) acid sites.The amount of ammonia desorbed at 100˚C may contain some

Activity of different catalytic systems
A comparison of the catalytic activity of various chromite spinel catalysts in vapour phase aniline conversion is discussed in the following section.All the reactions are carried out at a temperature of 300 °C for 2 h that a flow rate of 4 ml/h.The results are given in the Table 6.The result shows that CFC2 has high aniline conversion in CuCr2-xFexO4 series.Acidity and surface area has directly influences aniline methylation.From Table 4 and Table 5 it is clear that CFC 2 has highest surface area and acidity among the samples.
Ammonia is used to determine the acid properties.Acidity has a direct influence on aniline conversion.Weak and moderate acid sites are sufficient for N-alkylation, where as strong acid sites are required for C-alkylation.In CuCr2-xFexO4 series CFC 2 has more weak acid sites and hence it has more N-alkylated product.There is a good correlation between the weak acidity obtained from ammonia TPD and aniline conversion .

Conclusions
Chromite spinel was prepared by the low temperature co-precipitation techniques produced homogeneous and very fine particles with high surface area.Powder XRD pattern shows the characteristic peaks of spinel indicating the phase formation.Ammonia TPD establishes the acidity distribution at various temperatures.Optimization of reaction parameters for aniline methylation has been carried out on CFC 2 and the selected the conditions are: temperature of 300 °C, flow rate of 4 ml/h

Table 3 .
EDX data for chromite spinels

Table 6 .
Conversion and selectivity for aniline methylation

Table 4 .
Surface areas and total pore volume of different chromite spinels