ZnAl2O4 catalysts

Applied Catalysis A: General 265 (2004) 221–227

Transformation of anisole over ZnAl2 O4 and Fe2 O3/ZnAl2 O4 catalysts Hanna Grabowska a,∗ , Mirosław Zawadzki a , Ludwik Syper b b

a Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-590 Wrocław 2, Poland Institute of Organic Chemistry, Biochemistry and Biotechnology, Technical University of Wrocław, Wybrze˙ze Wyspia´nskiego St. 27, 50-370 Wrocław, Poland

Received in revised form 29 September 2003; accepted 16 January 2004 Available online 14 March 2004

Abstract The gas-phase catalytic transformations of anisole on two catalytic systems: ZnAl2 O4 (A) and 8%Fe2 O3 /ZnAl2 O4 (B) have been studied. Catalyst A was synthesised at hydrothermal conditions from Zn and Al nitrates; catalyst B was made from catalyst A by additionally impregnating with aqueous solution of ferric nitrate. Experimental data show that the addition of iron oxide (Fe2 O3 ) into zinc aluminate improves the catalytic activity i.e. anisole conversion and total ortho-selectivity to ortho-cresol and 2,6-xylenol. On the surface of both catalysts, Lewis acid sites are present. Catalyst A is characterised by higher concentration of Lewis acidity. The proposal of the reaction mechanism in the presence of catalysts was presented. © 2004 Elsevier B.V. All rights reserved. Keywords: ZnAl2 O4 ; 8%Fe2 O3 /ZnAl2 O4 ; Gas-phase reaction; Anisole transformation

1. Introduction According to general experience on organic chemistry, ethers are unreactive molecules. Because their stability they are commonly used as solvents and occasionally as a medium for heat transfer. They are not used as a feed stock for the chemical industry. Simple ethers can be made reactive by using an acid with nucleophilic counterion (HBr or HI, for example) or reactive Lewis acids (BBr3 , BI3 , AlCl3 , Me3 SiI). But, every organic compound can be made reactive on appropriate heterogeneous catalyst. It is known that dimethyl ether is always formed during the methylation of aromatic compounds with methanol on acidic zeolites [1–3]. There are also indication that dimethyl ether may intervene in ring methylation under certain reaction conditions [1]. Dimethyl ether reacts to form formaldehyde, with high selectivity at 500–600 K on MoOx -ZrO2 catalyst [4]. Beside, our recent investigation concerning the phenol alkylation with methanol on the ZnAl2 O3 catalyst indicated that anisole is formed, simultaneously with ortho-methylated phenols (ortho-cresol and 2,6-xylenol) [5]. There was possibility that the anisole was not a side product, but the intermediate, because at higher temper∗ Corresponding author. Tel.: +48-71-343-5021; fax: +48-71-344-1029. E-mail address: [email protected] (H. Grabowska).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.01.021

atures its yield decreased. These considerations led us to examine the catalytic chemistry of anisole and, specifically its conversion to C-methylated phenols. The benign properties of ethers make them attractive as an alternate intermediate for the synthesis of chemicals, currently produced from methanol and phenol. From the practical point of view, methyl derivatives of phenol (especially at ortho-position to the phenolic hydroxyl group, i.e. ortho-cresol and 2,6-xylenol) belong to the most important substances for further syntheses. Methylation of phenol over oxides, mixed oxides, zeolites and spinel type compositions produces O- and C-methylated compounds [5–18]. Selectivity of the reaction depends on the acid–base properties of the catalysts [8,19]. In the presence of catalysts having basic properties, such as MgO [7,8,11,20] and Fe2 O3 -based [10–12,21], it is possible to obtain predominantly C-alkylated products with high ortho-selectivity. With increasing the acidity of the catalysts, for example zeolites [14,15], SiO2 -Al2 O3 [22] or ␥-Al2 O3 [9], the selectivity for ring-alkylated products decreased and products of O-alkylation of phenol are obtained. There was also found that alkylated phenol derivatives were obtained from anisole in the presence of zeolites [19,23–26]. In [11] are presented experimental results on the phenol methylation over iron oxide catalyst that contains additives, namely, Cr, Si and K oxides. In this case, the basic properties of the surface predominate [27].

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Zinc aluminate is known to be active in dehydrogenation of alkanes [28], isomerisation of cyclohexene [29], transformations of normal alcohols [30], synthesis of indenes and styrenes [31] and also in alkylation of phenolic compounds [5,32]. Basing on our earlier experience on the phenols methylation over the iron oxide catalyst [33], we supposed that an iron additive should improve the properties of ZnAl2 O4 catalyst in anisole transformation. The present study reports the preparation of two catalytic systems: • ZnAl2 O4 (catalyst A): synthesised by hydrothermal method followed by calcination at 673 K of the resulting gel; • 8% Fe2 O3 /ZnAl2 O4 (catalyst B): obtained by wet impregnation method of ZnAl2 O4 (catalyst A) with an appropriate amount of ferric nitrate solution. Both catalysts were characterised using powder X-ray diffraction, transmission electron microscopy and by textural measurements (SBET , Vp , pore size distribution). The gas-phase competitive dehydration/dehydrogenation reaction of cyclohexanol was carried out as a probe reaction to measure the surface acidity of the prepared samples. The concentration of Lewis and Brönsted acid sites on the surface of catalysts was determined using FTIR spectroscopy of adsorbed pyridine. Catalytic activity of catalysts A and B in anisole transformation was investigated.

2. Experimental 2.1. Preparation of catalysts The precursors for the hydrothermal preparation of zinc aluminate (catalyst A) were aluminium and zinc nitrates and ammonia as precipitating agent. The predetermined amounts of Al and Zn nitrates were dissolved in 800 cm3 of distilled water at room temperature (molar ratio of Al:Zn was 2:1 and the concentration of the mixture was below 10 wt.%). Then pH value was adjusted to 9.0 by the controlled addition (with stirring) of the desired amount of aqueous ammonia solution (25 wt.%) to the nitrates solution. The resulting mixture was stirred for 30 min. and the formed precipitate was filtered off and washed several times with distilled water until to the disappearance of nitrate ions. After that, suitable amount of distilled water was added to the residue to obtain slurry of the concentration ∼5 wt.%. Such prepared aqueous reaction medium was placed in a stainless steel autoclave of 1000 cm3 to fill 80% of total volume. The autoclave was sealed and maintained at 430 K for 5 h with continuous stirring and after that, cooled to room temperature naturally. The obtained product was water washed and condensed following evaporation at elevated temperature. The resulting gel thus prepared was forced out in a wire from 3 to 4 mm in diameter by extrusion, air-dried overnight to remove any moisture, then calcined at 873 K for 4 h, crushed and sieved

into the 0.6–1.2 mm particles (catalyst A). The part of such obtained material was impregnated, after determination of its total water absorptivity (0.37 cm3 H2 O/g), with suitable amount of water solution of ferric nitrate nonahydrate equal to the pore volume. The amount of Fe(NO3 )3 ·9H2 O used for impregnation was adjusted to yield ZnAl2 O4 containing 8 wt.% of Fe2 O3 . After impregnation the sample was kept in ammonia atmosphere during 24 h. After drying, the obtained material was finally calcined in air at 743 K/5 h to obtain 8% Fe2 O3 /ZnAl2 O4 (catalyst B). 2.2. Characterisation of catalysts The catalysts prepared as above were examined by the following conventional techniques. X-ray diffraction patterns for the catalysts were recorded on a DRON-3 X-ray powder diffractometer. The XRD was carried out employing Ni-filtered Cu K␣ radiation as the X-ray source. A scan rate of 0.5◦ /min. was used to record the patterns in the 2Θ range 20–80◦ . The average crystallite size of zinc aluminate phase was calculated from the broadening of the X-ray line (3 1 1) using Schererr’s equation. Transmission electron microscopy images and selected area diffraction’s patterns were taken with a Tesla BS 500 TEM using an accelerating voltage of 90 kV and Philips CM20 SuperTwin HRTEM instrument at 200 kV. The specimens were prepared by ultrasonic dispersion in methanol, evaporating a drop of the resultant suspension onto a holey carbon support gird. The average size of the catalysts particles were measured from TEM or HRTEM microphotographs. The textural properties of catalysts (surface area, pore size distribution and pore volume) were determined by nitrogen adsorption–desorption isotherms at liquid nitrogen temperature by using an automatic volumetric apparatus (FISONS Sorptomatic 1900). Prior to measurements samples were degassed at 523 K for 5 h and 10−3 Torr. Specific surface areas, SBET , were calculated by the BET method. The pore distribution was analysed following the Dollimore–Heal method [34], which was applied to the desorption branch of each isotherm. The surface acidity, i.e. the concentration of Lewis and Brönsted centres on the surface of the catalysts, was established by means of IR spectroscopy in accordance with a procedure described in the literature by Kung, using pyridine as the probe molecule [35]. Transmission IR spectra were recorded at room temperature with a Specord M80 spectrometer. Self-supporting wafers of each sample (ca. 10 mg) were calcined in air at 773 K in the IR cell. The vacuum tight cell was evacuated and the wafers were exposed to pyridine vapours for 30 min (T = 298 K, p = 939 Pa), thereafter wafers were evacuated at 423 K for 90 min. The catalytic conversion of cyclohexanol may give dehydrated product (cyclohexene) and/or dehydrogenated (cyclohexanone) depending on the acid–base nature of the catalyst. Some authors [36,37] used the conversion of cyclohexanol as a measure for the catalyst acidity. The test reactions were

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performed in the gas phase at 573 K at atmospheric pressure over both studied catalysts. 2.3. Catalytic test The course of anisole transformation in the presence of catalysts A and B was studied at the same conditions. Experimental runs were performed in the gas phase at atmospheric pressure in an electrically heated standard quartz down-flow tubular reactor of i.d. 6.0 mm containing of 3 cm3 of the catalyst in both cases placed in the middle of the reactor. The catalyst B, contained iron oxide, was first reduced at 473 K for 2 h with methanol. During the reduction, ␣-Fe2 O3 was transformed into active magnetite. The space below catalyst bed was filled with quartz wool. The temperature was controlled with thermocouple placed in the centre of the catalytic bed. The feed (anisole as sole reagent—commercial material of analytical grade) was delivered from the top of reactor to pre-heating zone with aid of a syringe micropump with a flow intensity of 3.0 cm3 of liquid per hour (load 1.0 h−1 ) after reaching the predetermined temperature. No carrier gas was used. The reactions started at 533 K and their dependence on increasing temperature, up to 663 K, was followed. At the temperatures, when the activity reached a steady state, the condensed reaction products were collected directly below the catalyst bed and identified by the comparison of their retention times to the standards, and analysed quantitatively with GC analysis, using HP 6890 gas chromatograph equipped with an FID detector and an HP-5 capillary column (30 m × 0.32 mm × 0.25 ␮m) filled with 5% phenyl methyl silicone. Helium was used as a carrier gas.

3. Results and discussion Fig. 1 illustrates the XRD patterns of the hydrothermally synthesised catalysts A and B. In the case of catalyst A, the XRD profiles with rather broad diffraction peaks show a single-phase cubic spinel-type structure and all the peaks could be indexed as the spinel ZnAl2 O4 (JCPDS PDF no. 05-0669). The average size of the zinc aluminate particles determined by X-ray peak broadening analysis of the (3 1 1) reflection was about 10 nm. For catalyst B with 8% loading of Fe2 O3 and calcined at 743 K, besides the zinc aluminate as a main phase additionally the iron oxide could be poorly noted. However, the presence of ␣-Fe2 O3 could not be established unequivocally, because of relatively high background of the diffractogram. For catalyst B heated at higher temperature (1050 K) diffraction lines provide the evidence of that oxide. The size and shape of the prepared catalyst’s particles were investigated by TEM. In the case of both hydrothermally prepared catalysts, the morphology is quite uniform—they consist of approximate spherically shaped ZnAl2 O4 particles of an average size consistent with those obtained for the XRD patterns. ␣-Fe2 O3 particles were observed for catalyst B only after the heat treatment at higher

Fig. 1. Powder X-ray diffraction patterns: catalyst A (ZnAl2 O4 ); catalyst B (8% Fe2 O3 /ZnAl2 O4 ); catalyst B/1050 K (8% Fe2 O3 /ZnAl2 O4 calcined at 1050 K).

temperatures; otherwise, iron oxide was in a very dispersed state. Reported in Fig. 2 is an example of the morphology of catalyst A; selected area diffraction pattern confirms the structure of ZnAl2 O4 . The morphology of catalyst B additionally heated at 1050 K is shown in Fig. 3. A lot of aggregated particles can be observed with the lattice fringes of distances about 0.28 nm or 0.24 nm. Unfortunately, ZnAl2 O4 and ␣-Fe2 O3 particles have very similar lattice fringe spacings (lattice distances corresponding to the most X-ray intensities are 0.2861 nm or 0.2438 nm for ghanite and 0.2700 nm or 0.2519 nm for hematite—in accordance with JCPDS PDF no. 05-0669 and 33-0664). So, it is very difficult to univocally distinguish both structures however it is believed that some particles are iron oxide like this one indicated in Fig. 3 with the lattice fringes of distance of 0.272 nm which may correspond to the (1 0 4) lattice plane of ␣-Fe2 O3 . The marked particle is about 7 nm in size and is smaller than zinc aluminate particles (average size of about 10 nm). The adsorption–desorption isotherms obtained at temperature of liquid nitrogen and pore size distribution for catalysts A and B are displayed in Fig. 4. According to the BDDT classification, the shapes of the nitrogen adsorption– desorption isotherm are clearly of type IV, characteristic of the presence of mesopores. The isotherm hysteresis loops are well developed and can be classified into type A

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Fig. 2. TEM image of hydrothermally synthesised catalyst A.

(“cylindrical” pores) in the de Boer classification. It could be noted that hysteresis loop for catalyst A is a little shifted to a lower P/P0 ratio indicating a shift of the pore size distribution to lower values. The pore size distribution analysis (inset in Fig. 4) showed monomodal pore radius centred at about 2.7 and 2.8 nm for catalysts A and B, respectively. Some properties for catalysts A and B are collected in

Fig. 3. HRTEM image of catalyst B heated at 1050 K. The lattice fringes of distance of 0.272 nm on marked particle can be attributed to the (1 0 4) lattice plane of ␣-Fe2 O3 .

Table 1. The data indicate a decrease of surface area accompanied by a decrease of the total pore volume and a slight change of the mean pore radius for catalyst B, being a consequence of the sintering which occurs during second calcination. The gas-phase competitive reaction (dehydration/dehydrogenation) of cyclohexanol over catalysts A and B was studied as a test for determination the acid–base character of both studied solids. It is commonly known that the highest activities in the dehydration to cyclohexene are related to acidic catalysts, while for dehydrogenation of cyclohexanol to cyclohexanone take place in the presence the basic ones. The results indicated that catalyst A is an acidic system, giving nearly 98% selectivity for cyclohexene, while

Fig. 4. Nitrogen adsorption–desorption isotherms and pore size distribution obtained for catalysts A and B.

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Table 1 Some textural properties of A and B catalysts Catalyst

SBET (m2 /g)

Vp a (cm3 /g)

rp b (nm)

Acid sites concentration (␮mol Py/g) Lewis

Brönsted

A (ZnAl2 O4 ) B (Fe2 O3 /ZnAl2 O4 )

99 74

0.16 0.13

2.7 2.8

86 33

0 0

a b

Total pore volume. Mean pore radius.

Tables 2 and 3, respectively. The comparison of results (obtained in the same temperature range) for catalyst A, with those obtained in the presence of B, show its lower conversion and its lower activity. In the case of both catalysts at higher temperature the conversion of anisole increases. Besides, unreacted anisole in the liquid reaction products phenol, methylanisoles, cresols (mainly ortho-cresol), xylenols and trimethylphenols were present. Higher substituted methylphenols and small amount of other compounds, which were not identified, were also found. In the case of both catalysts, phenol was produced in substantial amounts; its yields are almost as high as yields of ortho-cresol, and increase with temperature. The yields of cresols, xylenols and trimethylphenols also increase as a function of temperatures. The more efficient catalyst for conversion of anisole into methylated phenol at ortho-positions is that containing iron oxide, in which acidic and basic sites are presented as was confirmed in cyclohexanol test. For instance at 623 K the total ortho-selectivities to ortho-cresol and 2,6-xylenol were 37.7 and 41.7% for catalysts A and B, respectively. The results in these investigation indicate that anisole is a reactive molecule under the influence of the investigated catalysts. Generally, anisole undergoes the rearrangement, giving ortho-cresol, and phenol as a demethylation product; those are two main products. We anticipate that the transfer of the methyl group from the oxide to the ortho-position of the phenol ring is a two step process. Anisole, being weak Lewis base, adsorbs on the acidic centre of the catalyst,

Fig. 5. Temperature dependence of the conversions of anisole obtained in the presence of catalysts A and B.

the B catalyst is more basic (70% selectivity to cyclohexene and up to 30% to cyclohexanone). It was confirmed by IR spectroscopy measurements of adsorbed pyridine. The concentration of acid sites for the catalysts A and B are presented in Table 1. Both catalysts have only Lewis acidity. Catalyst A is characterised by higher concentration of acid sites. Catalysts A and B do not indicated Brönsted acidity. The catalytic activities in conversion of anisole on the applied catalysts as a function of temperature and distribution of obtained products are presented in Fig. 5, and Table 2 Results of anisole transformations over catalyst A (load 1.0 h−1 ) Temperature (K)

533 563 583 603 623 633 648 663 a b c d e f

Composition of liquid products (%) Ana

PhOHb

Me-Anc

o-Crd

m-Cr

2,6-Xyle

2,5-Xyl

2,3-Xyl

2,4,6-TMPf

2,3,5-TMP

Others

85.4 72.9 58.5 49.2 40.0 37.2 33.4 33.8

6.7 12.8 18.8 22.5 24.9 25.5 26.0 26.0

0.9 0.8 0.7 0.7 0.5 0.4 0.3 0.3

3.8 6.6 9.5 12.3 15.4 17.1 18.6 19.1

– Traces 0.5 0.7 1.0 1.2 1.3 1.6

1.9 3.4 4.8 6.0 7.2 7.7 7.9 7.6

0.2 Traces 1.1 1.7 2.2 2.6 2.9 3.1

0.1 Traces 0.4 0.6 0.8 1.0 1.0 1.3

– – 0.5 0.8 1.0 1.0 1.1 1.0

0.4 1.0 1.7 2.0 2.3 2.5 2.6 2.2

0.6 2.5 3.5 3.5 4.7 3.8 4.9 4.0

An: anisole. PhOH: phenol. Me-An: methylanisole. Cr: cresol. Xyl: xylenol. TMP: trimethylphenol.

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Table 3 Results of anisole transformations over catalyst B (load 1.0 h−1 ) Temperature (K)

533 563 583 603 623 648 663 a b c d e f

Composition of liquid products (%) Ana

PhOHb

Me-Anc

o-Crd

m-Cr

2,6-Xyle

2,5-Xyl

2,3-Xyl

2,4,6-TMPf

2,3,5-TMP

Others

76.4 48.4 22.9 11.3 6.9 4.4 3.4

10.1 21.7 30.9 33.9 34.5 33.5 33.0

1.8 1.4 0.7 0.3 0.1 – –

5.5 12.2 19.2 23.7 25.7 26.9 27.8

0.3 0.5 0.8 1.3 1.9 2.9 4.8

3.5 7.8 12.1 13.3 13.1 12.1 10.1

– 0.6 1.8 2.9 3.8 5.3 6.7

– 0.3 0.6 0.9 1.1 1.4 1.7

– 0.4 1.8 1.8 2.0 1.8 1.5

0.6 1.8 3.1 3.3 3.1 3.0 1.8

1.8 4.9 6.1 7.3 7.8 8.7 9.2

An: anisole. PhOH: phenol. Me-An: methylanisole. Cr: cresol. Xyl: xylenol. TMP: trimethylphenol.

becomes prone towards the nucleophilic attack. The only reactive nucleophile is a surface O atom bounded to Zn(II), it is more reactive and softer than O atom bounded to Al(III). This O atom attacks the methyl group of the nearest adsorbed anisole molecule. It results in methoxide and phenoxide formation. The phenoxide is strongly bounded to the surface and is very reactive nucleophile. At high temperature, methoxide bounded to Zn(II) migrates to the nearest Al(III). Methoxide as hard base prefers to be bounded to harder Al(III). This process makes the methyl group of methoxide more reactive electrophile. In the subsequent step the methyl group of the methoxide attacks the ortho-position of the nearest phenoxide species. Along the reaction coordinate a carbenium ion CH3 + is formed, but it is not an intermediate but a transition state, stabilised by interaction with the surface atoms. Basing on this mechanism, the ortho-selectivity is easily understandable. The aromatic ring of the phenoxy species is electron rich, and the surface of the catalyst, composed mainly of oxygen atoms is also negatively charged, as a result of Coulombic interaction, the aromatic ring should be vertically or obliquely positioned towards the surface. As the reaction takes place within the adsorbed complex, i.e. all reacting molecules are strongly bounded to the surface, the reaction is an intramolecular type, or rather an intra-cluster type. In this case only ortho-positions of the phenoxy ring are accessible to the electrophilic attack, meta-positions, intrinsically less reactive, are less accessible, and a para-position is completely unaccessible. The formation of phenol is also understandable. It is well known, that methoxide bounded to transition metal or on basic surface of heterogeneous catalysts is chemically unstable [38]. ␤-Hydride elimination is an extremely common surface reaction of adsorbed alkyl and alkoxy groups [39]. The reaction in alkoxy groups generates aldehydes or ketones. In our case, the methoxy species bound to Zn(II) or Fe(III) decomposes to formaldehyde, which desorbs readily, or further decomposes giving CO and H2 . This reaction consumes methoxide species, which are needed for methylation of the phenoxide species.

4. Conclusions ZnAl2 O4 (catalyst A)—prepared by hydrothermal synthesis, and Fe2 O3 /ZnAl2 O4 (catalyst B)—made from catalyst A by wet impregnation method with ferric nitrate were investigated in the gas-phase transformation of anisole. Both catalysts were active in studied reaction. The catalysts surface properties can explain the catalytic behaviour. Fe3+ cations in the ZnAl2 O4 structure reduce the acidity of the surface and result in the increase of the observed catalytic activity. Addition of Fe2 O3 causes the increase of conversion of anisole as well as yields of ortho-methylated phenol derivatives.

Acknowledgements The authors thank Dr. Leszek Kˆepiñski for HRTEM studies and Dr. Janusz Trawczyñski for his help in generating acidity data.

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