t-Butylation of toluene with t-butyl alcohol over mesoporous Zn–Al–MCM-41 molecular sieves

Microporous and Mesoporous Materials 85 (2005) 59–74 www.elsevier.com/locate/micromeso

t-Butylation of toluene with t-butyl alcohol over mesoporous Zn–Al–MCM-41 molecular sieves M. Selvaraj, T.G. Lee

*

Yonsei Center for Clean Technology, Yonsei University, Seoul 120-749, South Korea Received 5 October 2004; received in revised form 22 December 2004; accepted 27 May 2005 Available online 3 August 2005

Abstract Mesoporous Zn–Al–MCM-41 molecular sieves with Si/(Zn + Al) ratio equal to 75, 151, 228, 304 and 380 were synthesized using cetyltrimethylammonium (CTMA+) surfactants as template under hydrothermal conditions while the mesoporous materials were characterized using several instrumental techniques. The unitcell parameter, surface area, pore size, hydrothermal and thermal stability of Zn–Al–MCM-41 decreases with increasing metal-ions content. The acid sites in the Zn–Al–MCM-41 are higher than that of Al–MCM-41 [M. Selvaraj, A. Pandurangan, K.S. Seshadri, P.K. Sinha, K.B. Lal, Appl. Catal. A: Gen. 242 (2003) 347.] due to the aluminum ions non-framework by the introduction of zinc ions. The effect of reaction temperature, reaction time, t-butyl alcohol to toluene ratios and recyclability on the selectivity of 4-t-butyltoluene was studied. The Zn–Al–MCM-41(75) catalyst containing higher number of Bronsted acid sites was found to be more effective in the alkylation of alkyl aromatics, for example, in the production of 4-t-butyltoluene from toluene, using t-butyl alcohol as the alkylating reagent. The selectivity of 4-t-butyltoluene was obtained under varying experimental condition, viz. effect of reaction temperature, time and ratio of t-butyl alcohol to toluene for the various catalysts to follow the sequence Zn–Al–MCM-41(75) > Zn–Al–MCM-41(151) > Zn–Al–MCM-41(228) > Zn–Al– MCM-41(304) > Zn–Al–MCM-41(380).
2005 Elsevier Inc. All rights reserved. Keywords: Zn–Al–MCM-41 as catalysts; Acid sites; t-Butylation of toluene; Recyclability; Conversation of toluene; Selectivity of 4-t-butyltoluene

1. Introduction The alkylations of aromatic hydrocarbons with olefins/different type of alcohols are applied on a large scale in the chemical industry [1]. Among para-dialkylated aromatics, p-xylene, p-diisopropylbenzene, p-ethyltoluene, p-dietylbenzene, p- and m-cymenes and 4-t-butyltoluene are very important in fine chemical and petrochemical industries [2–6] and the above products are alkylated-aromatics products. 4-t-Butyltoluene is produced by alkylation of toluene with anyone alkylating agent like as isobutene, diisobutene, MTBE and t-butyl alcohol (t-BuOH), and the product has been used *

Corresponding author. Tel.: +82 2 2123 5751; fax: +82 2 312 6401. E-mail address: [email protected] (T.G. Lee).

1387-1811/$ - see front matter
2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.05.042

as intermediate product to produce 4-t-butylbenzoic acid and 4-t-butylbenzaldehyde while they are especially used in perfumery and in the field of plastics and resins [7,8]. Ioffe et al. [9] studied different catalytic systems as AlCl3, AlCl3–CH3NO2, sulphuric acid and polyphosphoric acid in the liquid phase for alkylation of toluene by C4-alcohols. By the above the reaction, low yield and selectivity of para isomer was obtained. While these alkylations are still performed with catalysts showing drawbacks in chemical industries. Often such catalysts are strong mineral acids or Lewis acids (e.g., HF, H2SO4 and AlCl3), which are highly toxic and corrosive. They are dangerous to handle and to transport as they corrode storage and disposal containers. Often, the products need to be separated from the acid with a

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difficult and energy consuming process. Finally, it occurs frequently that these acids are neutralized at the end of the reaction and, therefore, the correspondent salts have to be disposed. Similar problems arise when free bases are used as catalysts. In order to avoid these problems many efforts have been devoted to the search of solid acid catalysts more selective, safe, environmentally friendly, regenerable, reusable and which have not to be destroyed after reaction. So some solid acid catalysts used for t-butylation of toluene. t-Butylation of toluene reaction was carried out using toluene with MTBE, t-BuOH and t-butyl chloride in the presence of activated clay, silica-alumina and iron sulphate catalysts under liquid phase reaction conditions [8,10]. While the 4-t-butyltoluene was synthesized over NiY zeolite as catalyst, but, by the above reactions, the yield and selectivity of the products (p, m and o-isomers) were very low obtained because the activity of the catalysts is very low [11]. From the above solid acid catalysts, low yield and selectivity of para isomer was obtained, because the all catalysts are having small surface area along with different structures. While the performance of the zeolite is limited by diffusional constraints associated with smaller pores. While these materials suffer from limited thermal stability as well as negligible catalytic activity due to framework neutrality. Moreover, the need for present day heterogeneous catalysts in processing hydrocarbons with high molecular weights made researchers think for better systems. These limitations were overcome after the discovery of mesoporous materials [12,13]. The discovery of the mesoporous material MCM-41 [12,13] has greatly enlarged the window of porous materials applicable as catalyst for organic reactions. Unfortunately, the acid strength of MCM-41 resembles that of the amorphous silica aluminas rather than that of the more strongly acidic zeolites [14]. Although the material is valuable for many organic conversions [15,16], enhancement of its acidity is desirable for extension of its applicability. Several approaches aimed at increasing the acid strength of the material. Kozhevnkov et al. showed that heteropoly acids (HPA) supported on MCM-41 are excellent catalysts in several reactions [17–20]. However, the lack of stability of this catalyst in case polar or products are involved, concerning mainly the dispersion of the HPA [21] and leaching of the HPA the support, prevents its introduction in industrial applications. Another possibility is the use of sulfonic acid groups covalently attached to the MCM-41 pore wall via an organic spacer [22,23]. Selvaraj et al. [2,3,24–27] reported the details of synthesis and characterization of some mesoporous solid acid catalysts while they have been used for isopropylation of toluene, ethoxylation of b-naphthol, self-conden-

sation of acetophenone and intermolecular cyclization of ethanolamine, and, from those reaction, the good conversion and product selectivity are obtained. Particularly, alkylation of toluene was carried out over mesoporous solid acid catalysts by best candidates [3,5,6]. We have
first time applied the mesoporous catalysts for highly selective synthesis of 4-t-butyltoluene. In the present study, the synthesis of Zn–Al–MCM41 with different Si/(Zn + Al) ratios were synthesized using cetyltrimethylammoium bromide as template under hydrothermal conditions. The mesoporous materials were characterized using several techniques, e.g., ICP-AES, XRD, FTIR, and TG/DTA, nitrogen adsorption, 27Al-MAS-NMR, SEM, TEM, XPS and pyridine acidity (TPD and FTIR) measurements. The materials have been used as catalysts for the higher selectivity of 4-t-butyltoluene produced by the alkylation of toluene using t-BuOH as the alkylating reagent. Owing to its low cost, and extensive use in industries, t-BuOH was chosen instead of isobutene. The effect of reaction temperature, time and t-BuOH to toluene ratio on the selectivity of 4-t-butyltoluene was investigated.

2. Experimental 2.1. Materials The syntheses of Zn–Al–MCM-41 materials were carried out by hydrothermal method using sodium metasilicate (Na2SiO3 Æ 5H2O), cetyltrimethylammonium bromide (C16H33(CH3)3N+Br), zinc chloride (ZnCl2), aluminum sulphate (Al2(SO4)3 Æ 18H2O), sulfuric acid (H2SO4). In order to study the formation of 4-tbutyltoluene by t-butylation of toluene, the reagents t-BuOH ((CH3)3–C–OH), toluene (C7H8) and decane were used. The used of chemicals (AR grade) were purchased from M/s Aldrich & Co., USA. 2.2. Synthesis of Zn–Al–MCM-41 Zn–Al–MCM-41 material was synthesized according to the published procedure [3]. For the synthesis of the Zn–Al–MCM-41 (Si/(Zn + Al) = 75), 21.2 g (1 mol) sodium metasilicate (44–47% SiO2) dissolved in 50 g of deionized water was mixed with 0.832 g (0.0125 mol) of aluminum sulphate (dissolved in 10 g of deionized water) solution and 0.17 g (0.0125 mol) of zinc chloride (dissolved in 10 g of deionized water). This mixture was stirred for 30 min using a mechanical stirrer at a speed of about 250 rpm and in order to reduce the pH to 10.8, 1N of sulfuric acid was added with continuous stirring for another 30 min at a speed of about 250 rpm until the gel formation. After that, 9.1 g (0.25 mol) of cetyltrimethylammonium bromide was added dropwise (30 ml per hour) through the dual syringe pump so that the gel was

M. Selvaraj, T.G. Lee / Microporous and Mesoporous Materials 85 (2005) 59–74

changed into suspension. After further stirring for 1 h the resulting synthesis gel of composition 1SiO2/0.0125 ZnO/0.0125Al2O3/0.25CTMABr/100H2O, was transferred into Teflon-lined steel autoclave and heated at 165 C for 48 h. After cooling to room temperature, the material was recovered by filtration, washed with deionized water and ethanol and finally calcined in flowing air at 540 C for 6 h. The different Zn–Al–MCM-41 catalysts (Si/ (Zn + Al) = 151, 228, 304 and 380) were also synthesized in an above similar manner wherein only the ratio of sodium metasilicate, zinc chloride and aluminum sulphate were adjusted and the input in gel molar compositions 1SiO2/xAl2O3/yZnO/0.25CTMABr/100H2O; (x = 0.00625 0.0025, y = 0.00625 0.0025). 2.3. Hydrothermal treatment of Zn–Al–MCM-41 Zn–Al–MCM-41 samples were treated in boiling water in polypropylene bottles and retained at 100 C for l week in order to evaluate the hydrothermal stability of the mesoporous structure of MCM-41 materials. 2.4. Physico-chemical characterization 2.4.1. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) The zinc and aluminum content in Zn–Al–MCM-41 materials was recorded using ICP-AES with allied analytical ICAP 9000. 2.4.2. X-ray diffraction (XRD) The crystalline phase identification and phase purity determination of the calcined Zn–Al–MCM-41 samples were carried out by XRD (Philips, Holland) using nickel ˚ ). The samples filtered CuKa radiation (k = 1.5406 A were scanned from 1 to 5 (2h) angle in steps of 0.5, with a count of 5 s at each point. In order to protect the detector from the high energy of the incident and diffracted beam, slits were used in this work. 2.4.3. Fourier transform infra-red (FTIR) spectroscopy Infrared spectra were recorded with a Nicolet Impact 410 FTIR Spectrometer in KBr pellet (0.005 g sample with 0.1 g KBr) scan number 36, resolution 2 cm 1. The data was treated with Omnic Software. 2.4.4. Thermogravimetric-differential thermal analysis (TG-DTA) The weight loss, dehydration and dehyoxylation for as-synthesized Zn–Al–MCM-41 samples was evaluated by TG-DTA in a Rheometric scientific (STA 15H+) thermobalance. 10–15 mg of as-synthesized MCM-41 was placed in a platinum pan and heated from room temperature to 1000 C at a heating rate of 2 C/min in air with flow rate of 50 ml/min. For comparison

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experiments, samples were dried at 50 C for the same period. The data were collected at 30 s intervals using on-line PC. 2.4.5. N2-adsorption isotherm The surface area and pore properties of Zn–Al–MCM41 samples before and after hydrothermal treatments were analyzed using a NOVA-1000 (QUANTACHROME, version 5.01) sorptometer. The calcined samples were dried at 130 C and evacuated overnight for 8 h in flowing argon at flow rate of 60 ml min 1 at 200 C under vacuum. Surface area, pore size, pore volume and wall thickness was obtained from these isotherms using the conventional BET and BJH equations. 2.4.6. 27Al-MAS-NMR spectroscopy The 27Al-MAS-NMR spectra were recorded at a frequency of 75.512 MHz and a spinning rate of 8 kHz with a pulse length of 4 ls, a pulse interval of 1s and approximately 1000 scans. 27Al chemical shifts are reported relative to 1M Al(NO3)3. 2.4.7. Scanning electron microscope (SEM) The SEM microscope of a typical sample of Zn–Al– MCM-41 materials were obtained on a JEOL 2010 microscope operated at 200 KV from a thin film dispersed on a holly carbon grid in ethanol solvent. 2.4.8. Transmission electron spectroscope (TEM) TEM images were recorded using a JEOL JEM200CX electron microscope operating at 200 KV with modified spectrum stage with objective lens parameters Cs = 0.41 mm and Cc = 0.95 mm, giving an interpretable point resolution of 0.185 nm. Samples for analysis were prepared by crushing the particles between two glass slides and spreading them on a holly carbon film supported on a Cu grid. The samples were briefly heated under tungsten filament light bulb in air before transfer into the specimen chamber. The images were recorded at magnifications of 100,000·. 2.4.9. X-ray photoelectron spectroscopy (XPS) XPS data were recorded on a VG scientific photoelectron spectrometer using AlKa1.2 radiation (1486.8 eV) from an X-ray source operating at 13 KV and 10 Ma. Working pressure was kept below 2 · 10 8 Torr. The spectra were recorded at a photoelectron take-off angle of 45. Binding energies were referenced to the C 1s peak from adventitious carbon surface deposit at 284.8 ev. 2.4.10. Acidity measurements Temperature programmed desorption of pyridine was studied on Zn–Al–MCM-41 samples under chromatograph conditions [28]. IR spectra were recorded with JASCO FTIR-610 spectrometer, fully controlled by the OMNIC software,

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and an all-glass high-vacuum system. Finely ground catalyst samples, equilibrated with water vapor at room temperature, were pressed in self-supporting thin wafers (10–15 mg cm 2), having a surface area of 1.0 cm2 on each face. Before pyridine adsorption, all wafers were pre-treated. Both clean sample reference spectra after pyridine adsorption were collected at room temperature, as an average of 60 runs with a 2 cm 1resolution. Preliminary investigations showed that these were the optimal recording conditions. In the series of experiments carried out to evaluate the proportionality coefficient between the integrated absorbance (A1) and the amount of adsorbed probe, accurately measured amounts (0.1–0.3 lmol) of pyridine vapor were successively adsorbed on the catalyst sample at 150 C. The amount of pyridine was determined volumetrically via the ideal gas law, after emission of pyridine vapor in a 492 cm3 compartment of the vacuum system. Complete adsorption was found to occur within a 5 min contact time. The pressure was measured by means of a barocel 600-capacitance type pressure-meter. The walls of the glass vacuum system were maintained at 150 C, in order to minimize pyridine adsorption on them. The total amount of pyridine adsorbed on the different catalysts was in the range of 1–3 lmol. Between each adsorption step, the samples were pumped off to 1.33 · 10 3 Pa and cooled down to room temperature, as usual, to collect the spectrum. After this preliminary calibration, FTIR analysis was carried out by equilibrating the catalyst wafers for 30 min at 150 C at a probe molecule vapor pressure of 500 Pa, followed by evacuation for 10 min at the same temperature, while maintaining the walls of the vacuum system at 150 C. Constant values of integrated absorbance were obtained by prolonging evacuation up to 500 min. Only the 1270–1700 cm 1 part of the IR difference spectra, obtained by subtracting the absorbance reference spectrum from the absorbance spectra recorded after probe adsorption, was considered. 2.5. t-Butylation of Toluene—experimental procedure for liquid phase catalytic reaction The Zn–Al–MCM-41 catalyst (0.2 g freshly calcined zeolite kept at 400 C was used) was added into a mixture of t-BuOH/toluene (various mmol ratios) with 100 ml of n-decane as solvent, and each reaction was carried out in a stirred batch autoclave reactor (100 ml, Autoclave Engineers) at reaction temperatures between 125 and 200 C for different times (h). The reactor was flashed twice with nitrogen to replace air. Alkylation reactions were carried out at the autogeneous pressure. The reactor was cooled down to 0 C and the reaction products were recovered from the reactor. The reaction was carried out with different Si/(Zn + Al) ratios of Zn–Al– MCM-41 catalysts at the same reaction conditions.

The samples of the reaction mixture were withdrawn periodically from the closed reactor and analysed on CHROMPACK 9002 gas chromatograph equipped with CP Sil 5 CB column (25 m · 0.53 mm) and FID detector. The temperature program was: 60 C (5 min), from 60 to 220 C with a slope of 5 C/min and at 220 C during 5 min isothermally. The products of the reaction were identified on GC/ MS QP5000 (Shimadzu) with EI and capillary column (HP-1, 50 m · 0.2 mm · 0.33 lm), carrier gas was helium (1 ml/min). Temperature program: from 50 C with gradient 5 C/min to 240 C was used.

3. Results and discussion 3.1. ICP-AES All Al–MCM-41 samples were analyzed by ICP-AES, which gave the corresponding Si/(Zn + Al) ratio of 75, 151, 228, 304 and 380 in bulk sample. The observed zinc and aluminum content in Zn–Al–MCM-41 has been presented in Table 1. 3.2. XRD Fig. 1 shows the X-ray powder diffraction patterns of calcined Zn–Al–MCM-41 samples. The X-ray diffractograms of Zn–Al–MCM-41, after calcinations in air at 540 C for 6 h, contain a sharp d100 reflection line in the range 2h = 1.95–2.33. These peaks have been attributed to the broadening effects of higher reflection lines due to small particles [29]. Physicochemical properties of these mesoporous materials are summarized in Table 1. The decrease in the unit-cell parameter on zinc-ion incorporation is probably in Al–MCM-41 materials due to the larger size of Zn2+ (radius 74 pm) compared with Al3+ (radius 53 pm) and Si4+ (radius 40 pm). It is also observed that the unit-cell parameter decreases with increasing zinc-ions content. These clear peaks indicate that the long-range order structure was achieved and the regular mesoporous structure was retained after the introduction of metals. These Bragg peaks were broadened and shifted slightly to higher angle with increasing metal-ions content, although the hexagonal structure still remained intact. These results suggest that the regularity of the mesoporous structure decreased with introduction of zinc and aluminum-ions content. Each diffraction pattern was the same as those of MCM-41 materials as described by Beck et al. [13]. 3.3. FT-IR Infra red spectroscopy had been used extensively for the characterization of transition-metal cation modified zeolites. The as-synthesized Zn–Al–MCM-41 samples

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63

Table 1 Physicochemical characterization of Zn–Al–MCM-41 Catalysts

Zn contentd (wt%)

Al contentd (wt%)

˚) d-spacinga (A

Unitcell parametera ˚) a0 (A

Surface areab (m2/g)

Pore sizeb ˚) D (A

Pore volumeb (cm3/g)

Wall thicknessc ˚) (A

Zn–Al–MCM-41(75) Zn–Al–MCM-41(151) Zn–Al–MCM-41(228) Zn–Al–MCM-41(304) Zn–Al–MCM-41(380)

0.123 0.060 0.040 0.031 0.020

0.220 0.123 0.079 0.060 0.049

37.91 38.41 38.91 39.98 41.09

43.77 44.35 44.92 46.15 47.44

820 867 912 970 1071

22.8 25.6 27.0 29.3 32.2

0.852 0.893 0.944 0.963 0.983

20.97 18.75 17.90 16.85 15.20

a b c d

Values obtained from XRD studies. Values obtained from N2-adsorption results. Wall thickness (t) = Unit cell parameter (a0)—pore size (D). The results obtained from ICP-AES.

a - Z n - A l- M C M - 4 1 ( 7 5 )

100 c - Z n - A l- M C M - 4 1 ( 7 5 )

Zn-Al-MCM-41(75)

a - Z n - A l- M C M - 4 1 ( 3 8 0 )

Transmittance

Intensity

Zn-Al-MCM-41(151)

Zn-Al-MCM-41(228)

c - Z n - A l- M C M -4 1 ( 3 8 0 )

965

Zn-Al-MCM-41(304)

1096

Zn-Al-MCM-41(380)

1500

4000

1400

1300

3000

1200

1100

1000

2000

Wave number

1

2

3

4

5

2θ Fig. 1. X-ray diffraction patterns of calcined Zn–Al–MCM-41 samples.

exhibit absorption bands around 2921 and 2851 cm 1 corresponding to n-C–H and d-C–H vibrations of the surfactant molecules (Fig. 2). The broad bands around 3500 cm 1 have concluded that water molecules are adsorbed on the surface silanol groups (Si–OH) but not on Si–O–Si groups [30]. It can therefore be reasoned that

900

800

700

1000

(cm-1)

Fig. 2. FTIR spectra of Zn–Al–MCM-41 samples (a—as-synthesized and c—calcined).

the hydrophobic character of the dehydroxylated SiO2 surface is due to the nonpolar Si–O–Si groups. Because the pores of mesoporous materials, which consist mainly silica, are constructed with tetrahedral SiO4, ideal pores do not possess silanol groups and should be highly hydrophobic. The hydrophobicity in Zn–Al–MCM41(75) is higher than that of other Zn–Al–MCM-41 due to the higher negative ions and highly exchangeable capacity within the pore by increasing of zinc and

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3.4. Thermal analysis

TGA

Weight loss (%)

aluminum ions. However, if some Si4+ ions becomes partly substituted by Al3+ and Zn2+ ions, a negative charge would result on the oxygen atom of (AlO4) and (ZnO4)2 respectively. Each Zn-ion in Zn–Al– MCM-41 is donated two negative ions but, the Al-ion in Al–MCM-41[3] is donated one negative ion on the inner-side pores of silica surface. While deformational vibrations of adsorbed molecules cause the absorption bands at 1623–1640 cm 1. The substitution of silicon by zinc and aluminum causes shifts of the lattice vibration bands to higher wave numbers. Compared to the Si-MCM-41, the wave number of the anti-symmetric Si–O–Si vibration band of Zn–Al–MCM-41 samples increases from 1096 to 1099 cm 1 (Fig. 2a). These shifts should be due to the increase of the mean Si–O distance in the walls caused by the substitution of the small silicon by the larger size of Al3+ [31]. The observed shifts, which depend as well on the change in the ionic radii as on the degree of substitution, are comparatively small. Therefore, only degrees of substitution of metalions (Zn- and Al-ions) are suggested. Interestingly, the wave number of the anti-symmetric Si–O–Zn vibration bands of Zn–Al–MCM-41 samples increases from 965 to 967 cm 1 (Fig. 2). However the ionic radius of silicon is smaller than those of aluminum. Upon introduction of higher zinc and aluminum content, most of the bands shifted to higher wave numbers, consistent with their incorporation in lattice positions. This is generally considered to be a proof of the incorporation of the heteroatom into the framework. Cambler et al. [32] have reported similar stretching vibrations of Si–OH groups present at defect sites. By the disappearing peaks at 2851 and 2921 cm 1, one could conclude that calcination of the original framework was complete and the identity of organic molecule completely disappeared from the calcined Zn–Al–MCM-41 (Fig. 2).

Z n -A l-M C M -4 1 ( 7 5 )

Z n -A l-M C M -4 1 ( 1 5 1 )

Z n -A l-M C M -4 1 ( 2 2 8 )

Z n -A l-M C M -4 1 ( 3 0 4 )

Z n -A l-M C M -4 1 ( 3 8 0 )

200

400

600

800

1000

Temperature (0˚C)

DTA

Z n -A l-M C M -4 1 ( 7 5 )

∆T (micro volt)

64

Z n -A l-M C M -4 1 ( 1 5 1 )

Z n -A l-M C M -4 1 ( 2 2 8 )

Z n -A l-M C M -4 1 ( 3 0 4 )

Z n -A l-M C M -4 1 ( 3 8 0 )

200

400

600

800

1000

Temperature (0˚C)

Thermal analysis of the catalysts show distinct weight losses that depend on framework composition. Representative thermograms are given in Fig. 3. Generally, when the zinc and aluminum content increases, there is a decrease in organic content and increase in water content. Three distinct regions of weight loss are noted in the temperature range 50–150, 150–350 and 350– 550 C. The first weight loss (4.6–16.61%) corresponds to the desorption and removal of the water and/or ethanol molecules physisorbed on the external surface of the crystallites or occluded in the macropores and mesopores present between the crystallites aggregates. A second weight loss (38.58–40.5%), between 150 and 350 C is attributed to the removal of the organic template. Finally, a third weight loss (1.51–1.9%) between 350 and 550 C is related to water loss from the condensation of adjacent silanol groups to form siloxane bond

Fig. 3. TG/DTA plots of as-synthesized Zn–Al–MCM-41 samples.

[33]. The total weight loss at 1000 C of the Zn–Al– MCM-41 samples are in the 44.69–59.01% range. However, the distribution of successive weight loss depends on the framework or substituted Si/(Zn + Al) ratios [34]. Thus, weight loss was higher in the low zinc and aluminum contents of MCM-41 materials than that in the high zinc and aluminum contents of MCM-41 materials. 3.5. Adsorption isotherm of nitrogen Fig. 4 shows the isotherm of nitrogen adsorption by calcined Zn–Al–MCM-41 measured at liquid nitrogen temperature (77 K). Each isotherm showed type IV

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400

3

Volume adsorbed (cm /g STP)

500

300

adsorption desorption

200

100

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

a 600

400

3

Volume adsorbed (cm /g STP)

500

adsorption desorption

300

200

100

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

b 700

500

3

Volume adsorbed (cm /g STP)

600

400

adsorption desorption

300 200 100 0 0.0

c

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Fig. 4. N2-adsorption isotherms of calcined (a) Zn–Al–MCM-41(75), (b) Al–MCM-41(122) and (c) Al–MCM-41(380).

character, which is a typical shape for mesoporous MCM-41. The adsorbed amount increased gradually with increasing relative pressure by multilayer adsorption [35]. A steep rise in the adsorbed amount was ob-

65

served at a relative pressure in the range of 0.21–0.48 being caused by capillary condensation of nitrogen in ˚ diameter in the mesopores in the range of 22.8–32.2 A the Zn–Al–MCM-41 samples. This rise became more gentle and was shifted to lower relative pressure with increasing zinc and aluminum content, which suggests that the pore size was narrowed and distributed. In the figures, the desorption phenomena can also be seen along with the adsorption isotherms of each sample. The specific surface area values of samples determined by the BET method lie in the range of 820–1071m2/g for Zn–Al–MCM-41. The surface area, pore size, pore volume and wall thickness are given in Table 1. It is observed that as the Si/(Zn + Al) ratio decreases from 380 to 75 (the concentration of zinc and aluminum ion increases accordingly), the surface area, pore diameter and pore volume decrease while the wall thickness increases for Zn and Al incorporation. This may be due to the creation of negative charge raised on the surface of pore walls by increasing Zn2+ and Al3+ ions. The pore diameter became linearly narrowed by increasing the zinc and aluminum ions incorporation, which was coincident with decreasing the d-value along the (1 0 0) reflection. The remarkable improvement of hydrothermal stability due to the higher metal-incorporation is further evidenced by the measurement of N2-adsorption. The surface areas of all the MCM-41 materials were measured after they had been treated in boiling water for 1 week. The surface areas slightly decreased from 1071 to 1020 m2/g in Zn–Al–MCM-41(380), but the surface area does not decrease in other Zn–Al–MCM-41 samples [3]. The Si–O–Al and Si–O–Zn bonds are relatively stable to further attack from boiling water [36]; the presence of Al3+and Zn2+ creates a negative charge on the surface of the pore walls, repelling OH ions and therefore, preventing the hydrolysis of siloxane bonds and also resulting in an increase in the number of acid sites. The surface area in Zn–Al–MCM-41(380) samples decreased due to low metal incorporation/non-framework as it does not create sufficient negative charges on the surface of the pore walls. But the surface area of other Zn–Al–MCM-41 samples does not decrease due to repelling OH ions higher on the pore walls. Hence, it is concluded that Zn is irreversibly incorporated into the structure of Zn–Al–MCM-41 samples. Thus the hydrothermal stability is higher in Zn–Al–MCM41(75) than that in other Zn–Al–MCM-41 samples, and therefore, preventing the hydrolysis of siloxane bonds and also resulting in an increase in the number of Bronsted acid sites. 3.6. Solid state NMR spectroscopy Fig. 5 shows solid-state 27Al-MAS spectra of Zn–Al– MCM-41 samples. Each chemical shift peak spectrum in

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Si/Zn+Al = 75

Intensity

Si/Zn+Al = 151

Si/Zn+Al = 228

Fig. 6. SEM images for Zn–Al–MCM-41(75).

Si/Zn+Al = 304

3.8. TEM

Si/Zn+Al = 380

10

8

6

4

5

0

-2

-4

-6

-8

-10

Chemical shift / ppm Fig. 5.

27

Al–MAS–NMR spectra of calcined Zn–Al–MCM-41.

the MCM-41 shows a strong and sharp signal at 0 ppm regardless of the Al content. The intensities of these signals increased with increasing Zn2+ and Al3+ ions content or with low Si/(Zn + Al) ratio. From the above results, it is inferred that when the Zn2+ ion is incorporated in the Al–MCM-41, the Al3+ ion is non-framework while Zn2+ ion is produced two negative ions with tetrahedral environment on the silica surface under ion-exchange method. Thus the Bronsted and Lewis acidity in the Zn–Al–MCM-41 [3] should be associated with the presence of tetrahedral Zn and octahedral Al -ions respectively.

The TEM image of calcined Zn–Al–MCM-41 (Fig. 7 for Zn–Al–MCM-41(304)) exhibits ordered hexagonal arrays of mesopores with uniform pore size [13]. The corresponding electron diffraction pattern also shows reflections contrast in the TEM image of the sample, the distance between mesopores are estimated, in good agreement with the value determined from XRD and N2-adsorption measurements values. Both pore channels and hexagonal symmetry can be clearly identified in the TEM image for a large number of Zn–Al–MCM-41 samples which indicates that the MCM-41 have only one uniform phase as inferred from the XRD results. Interestingly, the TEM image viewed down the d110 direction shows that pore channels are not straight, they are arc-like. In contrast, the TEM image recorded along the d110 direction of MCM-41 shows horizontal channels. The are-like channels may result from the unique preparation of the Zn–Al–MCM-41

3.7. SEM All Zn–Al–MCM-41 materials are having micellar rod-like shape hexagonal or spherical edges, as shown in the Fig. 6 (Zn–Al–MCM-41(75)). Each rod, itself transformed into the MCM-41 hexagonal-phase mesostructure. Because all materials have been synthesized using cetyltrimethyammonium bromide as surfactant in the liquid crystal template mechanism, Steel et al. [37] postulated that CTMABr surfactant molecules assembled directly into the hexagonal liquid crystal phase upon addition of the silicate species, based on 14 NMR spectroscopy.

Fig. 7. TEM images for Zn–Al–MCM-41(304).

M. Selvaraj, T.G. Lee / Microporous and Mesoporous Materials 85 (2005) 59–74

appears to be rather moderate as nearly all the pyridine desorbed below 300 C. The acidities (mmol/g) based on the pyridine desorbed by the samples beyond 100 C are presented in Table 3. As the zinc and aluminum content increases in the Zn–Al–MCM-41 samples, the total acidity increases in the order Si/(Zn + Al) ratios 75 > 151 > 228 > 304 > 380.

Table 2 Binding energies for Zn–Al–MCM-41 samples Samples

Binding energies/eV Si 2p

Zn 2p

Al 2p

O 1s

Zn–Al–MCM-41(75) Zn–Al–MCM-41(151) Zn–Al–MCM-41(228) Zn–Al–MCM-41(304) Zn–Al–MCM-41(380)

289.1 288.8 288.5 288.3 288.1

1024.1 1023.7 1023.5 1023.3 1023.0

74.9 74.8 74.5 74.4 74.3

103.7 103.5 103.3 103.1 103.0

67

samples. The pore sizes as observed by TEM analysis are in agreement with the results obtained from N2adsorption measurements. 3.9. XPS XPS was performed so as to determine the distribution of Zn and Al, and also to characterize the Al and Zn species on the external surface of the MCM-41 sample. The surface Si/(Zn + Al) ratio and binding energies (BE) for Zn–Al–MCM-41 samples with different Si/ (Zn + Al) ratios along with data for related reference materials are given in Table 2. The surface Si/(Zn + Al) ratio of the metal incorporated samples indicate that the surface/ near surface region has high metal content while Zn-ion penetrates the small particles to a greater extent. This is consistent with the incorporating procedure in which metals first come in contact with the outer surface of the silica matrix before being distributed into the bulk. The metal content increases with increasing binding energies of Zn 2p, Al 2p, Si 2p and O 1s. 3.10. Acidity of Zn–Al–MCM-41 The acidities of the Zn–Al–MCM-41 samples were characterized by the TPD-pyridine and FTIR-pyridine. 3.10.1. Total acidity strength The acidities of the Zn–Al–MCM-41 samples were characterized by the TPD of pyridine. The acid strength

3.10.2. Bronsted and Lewis acidity As one of the major fields of application of Zn–Al– MCM-41 are used as acid catalysts. Characterization of the acidity of the above catalysts is very important. The infrared spectra region of the pyridine adsorbed on the Zn–Al–MCM-41 are in the region 1700– 1270 cm 1. FTIR spectra were recorded at room temperature. All the samples show the expected bands due to hydrogen bonded pyridine (1444 and 1597 cm 1), Lewis-bound pyridine (1455, 1576 and 1623 cm 1), pyridine bound on Bronsted acid sites (1546 and 1638 cm 1), and a band at 1496 cm 1 attributed to pyridine associated with both Lewis and Bronsted (B + L) acid sites [38]. The Bronsted acid sites and Lewis acid sites with various temperatures in different Si/(Zn + Al) ratio containing MCM-41 samples are listed in Table 3. In the Zn–Al–MCM-41, the Bronsted acid sites and Lewis acid sites are same. Both the acid sites increases with increasing zinc and aluminum content due to replacing the tetrahedral aluminum-ions by the introduction of zinc ions under ion-exchange method, however, the zinc ion produced two negative ions at the tetrahedral position, the replaced aluminum is non-framework on the silica surface. The Zn–Al–MCM-41 sample exhibits absorption bands around 1444 and 1597 cm 1 due to hydrogen-bonded pyridine in the temperature range from room temperature to 300 C. Both Lewis and Bronsted acid (L + B) sites decreases with increasing temperature due to the desorbed pyridine molecule and as shown in Fig. 8 for the Zn–Al–MCM-41(151). The Bronsted acid sites increase in order to Zn–Al– MCM-41(75) > Zn–Al–MCM-41(151) > Zn–Al–MCM-41

Table 3 Bronsted acidity and Lewis acidity of the Zn–Al–MCM-41 samples measured by FTIR spectroscopy combined with pyridine adsorption and desorption at different temperatures; total acidity measured by TPD-pyridine Catalysts

Zn–Al–MCM-41(75) Zn–Al–MCM-41(151) Zn–Al–MCM-41(228) Zn–Al–MCM-41(304) Zn–Al–MCM-41(380)

Total acidity from TPD-pyridine studies (mmol pyridine g 1)

0.103 0.095 0.076 0.055 0.035

R—Room temperature. a Values obtained from FTIR-pyridine studies.

(lmol pyridine g 1)a Bronsted acidity

Lewis acidity

Temperature (0 C)

Temperature (0 C)

R

100

200

300

R

100

200

300

27.5 23.4 19.5 17.4 14.3

20.3 19.2 17.3 15.4 13.2

18.2 16.3 14.2 12.4 9.5

13.5 12.4 10.3 6.7 5.3

40.3 38.3 25.4 20.6 17.7

35.6 30.3 24.5 20.4 15.3

30.5 27.3 24.3 20.3 14.5

25.3 23.5 20.2 18.3 15.6

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M. Selvaraj, T.G. Lee / Microporous and Mesoporous Materials 85 (2005) 59–74

acts with removal water over the catalyst to form alcohols (Eq. (7)). The above all products (all equations) are obtained with respect to the catalytic properties along with optimal reaction conditions. The reaction of toluene with t-butanol is given in Fig. 9. In all cases, the main reaction products have been identified as 4-t-butyltoluene and 3-t-butyltoluene. 2-tButyltoluene was present in the reaction products only in the trace amounts. The main product is logically 4-t-butyltoluene because para-position is favored by the influence of steric hindrance of the methyl group on one side and voluminous t-butyl group due to proper structure of mesoporous Zn–Al–MCM-41 catalysts. The formation of 2-t-butyltoluene is hindered by ortho-position of methyl and voluminous t-butyl group. The same steric effect allows only the formation of 3,5di-t-butyltoluene, where all alkyl groups are in the meta-position. 3,5-di-t-Butyltoluene was also found in the reaction products, but only in the trace amounts obtained over Zn–Al–MCM-41 catalysts. The other products have been identified by GC-MS as alkyltoluenes with longer alkyl chain. These products were formed by alkylation of toluene with lower oligomers of isobutene (preferentially by dimer). The effects of various parameters on the t-butylation of toluene reaction are discussed later. Fig. 8. FTIR spectra of pyridine adsorbed on Zn–Al–MCM-41(151).

(228) > Zn–Al-MCM-41(304) > Zn–Al–MCM-41(380) due to the replacing tetrahedral aluminum by zinc under ion-exchange conditions and the double negative ions produced by the introduction of Zn-ions. 3.11. t-Butylation of toluene The t-butylation of toluene with t-BuOH is an electrophilic substitution reaction on the aromatic ring. tButylation reactions catalyzed by acids or solid acid zeolites are commonly considered to proceed via carbonium ion mechanisms [3]. t-BuOH reacts with solid acid catalyst to form isobutene along with removal of water (Eq. (1)). Isobutene is protonated by the catalyst to form t-butyl carbocation (Eq. (2)). The carbocation further reacts with toluene in the presence of the catalyst to form 4-t-butyltoluene and 3-t-butyltoluene (Eq. (3)). Either 4-t-butyltoluene or 3-t-butyltoluene reacts with the carbocation over the catalyst to form 3,5-di-t-butyltoluene (Eq. (4)). Excess isobutene further reacts with each other over the catalyst to form oligomers (Eq. (5)) while the oligomers (R-alkyl groups) react with excess toluene in the presence of the catalyst to form alkyltoluene with longer alkyl chain (Eq. (6)). The remain of oligomers further re-

3.11.1. Selectivity of 4-t-butyltoluene The reaction of t-butylation of toluene was carried out in the presence of various Si/(Zn + Al) ratios of Zn–Al–MCM-41 catalysts. Maximum conversion of toluene to the extent of was 90.4% and 96.90% 4-t-butyltoluene selectivity was obtained when the reaction was carried out in the presence of Zn–Al–MCM-41(75). The conversion of toluene and selectivity of 4-t-butyltoluene in the presence of Zn–Al–MCM-41(75) are the higher due to increasing zinc and aluminum-ions content, the higher hydrothermal stability and also due to the higher number of Bronsted acid sites by creation of negative charge on the pore walls, which is attributed to the incorporation of Zn-ions in place of tetrahedral Al-ions in the structure. The number of acid sites for the different catalysts follow the order Zn–Al–MCM41(75) > Zn–Al–MCM-41(151) > Zn–Al–MCM-41(228) > Zn–Al-MCM- 41(304) > Zn–Al–MCM-41(380) as obtained from TPD and FTIR-pyridine treatment. This reaction is activated on the m- and the p-positions by the presence of the methyl group. While the selectivity of 4-t-butyltoluene is higher than that of 3-t-butyltoluene, 2-t-butyltoluene and 3,5-di-t-butyltoluene due to steric hindrance of the methyl group at the para-position. The m-t-butyltoluene and 3,5-di-t-butyltoluene is thermodynamically more stable [3]. Thus the conversion of toluene and selectivity of 4-t-butyltolueneare higher in Zn–Al–MCM-41(75) than those of the other

M. Selvaraj, T.G. Lee / Microporous and Mesoporous Materials 85 (2005) 59–74

69

Fig. 9. Reaction scheme of t-butylation of toluene.

Zn–Al–MCM-41 catalysts and the results are shown in Table 4. 3.11.2. Variation of reaction time with different Si/(Zn + Al) ratios of Zn–Al–MCM-41 The liquid phase reaction of t-butylation of toluene was carried out at various reaction times with 2:1 mmol ratio of t-BuOH to toluene and 100 ml of decane as solvent at 175 C reaction temperature in the presence of Zn–Al–MCM-41 with different Si/(Zn + Al) ratio cata-

lysts. Lower reaction time (<1 h) do not favor the formation of 4-t-butyltoluene because surface activity of the catalysts is insufficient to react with reactants. Then conversion of toluene, yield and selectivity of t-butyltoluene increases with increasing reaction time upto 2 h over different Zn–Al–MCM-41 (different Si/(Zn + Al) ratios) catalysts in the same reaction conditions. But the conversion, yield and selectivity decreases with increasing Si/(Zn + Al) ratios because the Bronsted acid sites on the catalyst surface decreased with decreasing

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Table 4 t-butylation of toluene: variation of reaction time with different Si/(Zn + Al) ratios of Zn–Al–MCM-41 Catalysts

Time (h)

Conversion of toluene (%)

Yield of the products (%)

4-t-BT selectivity

4-t-BT

3-t-BT

Others

Zn–Al–MCM-41(75)

1 2 4 8

78.5 90.4 97.6 99.45

73.3 87.6 64.7 60.4

3.8 1.3 31.1 37.05

1.4 1.5 1.8 2.0

93.37 96.90 66.29 60.73

Zn–Al–MCM-41(151)

1 2 4 8

73.4 85.2 92.9 94.5

67.8 80.1 59.2 54.9

3.7 1.25 29.7 35.2

1.9 3.85 4.00 4.4

92.37 94.01 63.72 58.09

Zn–Al–MCM-41(228)

1 2 4 8

67.3 79.1 86.0 87.5

61.3 72.6 53.7 49.4

3.6 1.20 28.2 33.4

2.4 5.3 4.1 4.7

91.08 91.78 62.44 56.45

Zn–Al–MCM-41(304)

1 2 4 8

60.2 71.8 78.7 81.2

52.8 64.1 48.2 43.9

3.5 1.14 26.1 31.3

3.9 6.56 4.4 6.0

87.70 89.27 61.24 54.06

Zn–Al–MCM-41(380)

1 2 4 8

52.1 63.6 70.4 72.8

44.3 53.8 42.7 38.4

3.30 1.12 22.3 25.40

4.5 8.68 5.4 9.0

85.02 84.59 60.65 52.74

Reaction conditions: 0.2 g of catalyst; reaction temperature (T) = 175; 2:1 mmol ratio of t-BuOH to toluene; 100 ml of decane; 4-t-BT, 4-t-butyltoluene; 3-t-BT, 3-t-butyltoluene; others—2-t-butyltoluene, 2,5-di-t-butyltoluene and oligomers.

Table 5 t-butylation of toluene: variation of reaction temperature with different Si/(Zn + Al) ratios of Zn–Al–MCM-41 Catalysts

Temperature (C)

Conversion of toluene (%)

Yield of the products (%) 4-t-BT

3-t-BT

4-t-BT selectivity Others

Zn–Al–MCM-41(75)

125 150 175 200

70.9 79.5 90.4 94.5

64.4 74.3 87.6 67.4

1.1 1.2 1.3 18.2

5.4 4.0 1.5 8.9

90.83 93.45 96.90 71.32

Zn–Al–MCM-41(151)

125 150 175 200

66.6 75.3 85.2 90.1

58.7 68.7 80.1 61.6

0.95 1.15 1.25 16.7

6.95 5.45 3.85 11.80

88.13 91.23 94.01 68.36

Zn–Al–MCM-41(228)

125 150 175 200

60.1 69.0 79.1 83.5

51.0 61.1 72.6 53.8

0.75 1.10 1.20 14.3

8.35 6.80 5.3 15.4

84.85 88.55 91.78 64.43

Zn–Al–MCM-41(304)

125 150 175 200

52.35 61.7 71.8 76.0

41.3 51.5 64.1 44.2

0.15 1.05 1.14 12.6

10.9 9.15 6.56 19.2

78.89 83.46 89.27 58.15

Zn–Al–MCM-41(380)

125 150 175 200

45.3 54.4 63.6 68.2

31.9 41.9 53.8 34.5

0.25 1.00 1.12 11.7

13.15 11.50 8.68 22.0

70.41 77.02 84.59 50.58

Reaction conditions: 0.2 g of catalyst; reaction time (T) = 2 h; 2:1 mmol ratio of t-BuOH to toluene; 100 ml of decane. 4-t-BT, 4-t-butyltoluene; 3-tBT, 3-t-butyltoluene; others—2-t-butyltoluene, 2,5-di-t-butyltoluene and oligomers.

zinc and aluminum content and the results are shown in Table 4. When the reaction time is increased (>3h), the

conversion of toluene increased but the yield and selectivity of 4-t-butyltoluene decreased because 4-t-butyltol-

M. Selvaraj, T.G. Lee / Microporous and Mesoporous Materials 85 (2005) 59–74 100

95

90

Conversion of toluene (%)

uene is gradually transformed into 3-t-butyltoluene while the other byproducts namely 2-t-butyltoluene, 3,5-di-t-butyltoluene and oligomers slightly increased. The conversion of toluene and selectivity of 4-t-butyltoluene are higher in Zn–Al–MCM-41(75) than those of other Zn–Al–MCM-41 catalysts due to the greater chemisorption of reactants on the catalyst surface pores by the higher of number of Bronsted acid sites at 175 C for 2 h, and the results are shown in Table 4. So the optimum reaction time was found to be 2 h for the highly selective synthesis of 4-t-butyltoluene. Hence a higher yield and selectivity of 4-t-butyltoluene and conversion of toluene using Zn–Al–MCM-41(75) depicts its superiority in performance compared to other Zn–Al–MCM41.

71

o

125 C o

150 C

85

o

175 C o

200 C

80

75

70

65 0

1

2

3

4

5

6

7

8

9

10

8

9

10

Reaction time (h)

a 100

90

o

125 C Selectivity of 4-t-butyltoluene (%)

3.11.3. Variation of reaction temperature with different Si/(Zn + Al) ratios of Zn–Al–MCM-41 The t-butylation of toluene was carried out at various reaction temperatures with 2:1 mmol ratio of t-BuOH to toluene and 100 ml of decane as solvent for 2 h reaction time in the presence Zn–Al–MCM-41 with different Si/ (Zn + Al) ratio catalysts and the results are shown in Table 5. By increasing temperature upto 175 C at the same reaction conditions, the conversion of toluene, yield and selectivity of 4-t-butyltoluene increased. After the reaction temperature 175 C, the conversion increases, but the yield and selectivity of 4-t-butyltoluene decreased, because the selectivity of 3-t-butyltoluene and other byproducts namely 2-t-butyltoluene, 3,5-dit-butyltoluene and oligomers increase by transalkylating of 4-t-butyltoluene. Because the 4-t-butyltoluene has low thermodynamical stability in higher reaction temperature (>175 C). The conversion of toluene, yield and selectivity of 4-t-butyltoluene are higher in Zn–Al– MCM-41(75) than those of other Zn–Al–MCM-41catalysts due to increasing the catalytic activity along with the higher number of Bronsted acid sites on the surface of the catalyst while the most of converted reactants are favored to 4-t-butyltoluene at 175 C for 2 h. So the optimum reaction temperature was found to be 175 C for the highly selective synthesis of 4-t-butyltoluene. Further the reaction temperature is increased to 200 C, the conversion of toluene and yield and selectivity of 4-t-butyltoluene decreased because of debutylation of 4-t-butyltoluene with formation of toluene and isobutene. The liquid phase reaction of t-butylation of toluene was carried out at various reaction times with 2:1 mmol ratio of t-BuOH to toluene and 100 ml of decane as solvent at different reaction temperatures in the presence Zn–Al–MCM-41(75). By increasing reaction time to 2 h at the temperature from 125 to 175 C in the above same reaction conditions, the conversion of toluene, yield and selectivity of 4-t-butyltoluene increases, but, after 2 h, the yield and selectivity decreases and the re-

o

150 C 80

o

175 C o

200 C 70

60

50

40

30 0

b

1

2

3

4

5

6

7

Reaction time (h)

Fig. 10. Variation of reaction time with (a) conversion of toluene (%) and (b) selectivity of 4-t-butyltoluene (%) at the different temperature over Zn–Al–MCM-41(75).

sults are shown in Fig. 10. When the reaction time (>4 h) along with temperature (>175 C) are increased, selectivity of 3-t-butyltoluene increased and other byproducts as 2-t-butyltoluene and 3,5-di-t-butyltoluene also increased because of debutylation of 4-t-butyltoluene with formation of toluene and isobutene. 3.11.4. Variation with t-butanol to toluene ratio The t-butylation of toluene was carried out at 175 C reaction temperature with various mmol ratio of t-BuOH to toluene and 100 ml of decane as solvent for 2 h reaction time in the presence Zn–Al–MCM-41 with different Si/(Zn + Al) ratio catalysts and the results are shown in Table 6. The conversion of toluene, yield and selectivity of t-butyltoluene decreased at

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Table 6 t-butylation of toluene: variation of mmol ratio (t-BuOH/toluene) with different Si/(Zn + Al) ratios of Zn–Al–MCM-41 Catalysts

t-B/toluene mmol ratio

Conversion of toluene (%)

Yield of the products (%)

4-t-BT selectivity

4-t-BT

3-t-BT

Others

Zn–Al–MCM-41(75)

1:1 2:1 1:2 1:4

46.6 90.4 96.1 78.5

38.7 87.6 72.6 57.8

7.1 1.3 18.5 11.3

0.8 1.5 5.0 9.4

83.04 96.90 75.54 73.63

Zn–Al–MCM-41(151)

1:1 2:1 1:2 1:4

41.3 85.2 91.0 67.1

33.8 80.1 68.0 48.2

6.9 1.25 18.1 11.1

0.6 3.85 4.9 7.8

80.38 94.01 74.72 71.83

Zn–Al–MCM-41(228)

1:1 2:1 1:2 1:4

36.7 79.1 84.9 55.7

29.5 72.6 63.3 38.6

6.7 1.20 17.3 10.5

0.5 5.3 4.3 6.6

80.38 91.78 74.55 69.29

Zn–Al–MCM-41(304)

1:1 2:1 1:2 1:4

29.3 71.8 77.7 44.3

23.2 64.1 57.8 29.0

5.9 1.14 15.8 10.1

0.2 6.56 4.10 5.2

79.18 89.27 74.38 65.46

Zn–Al–MCM-41(380)

1:1 2:1 1:2 1:4

27.2 63.6 69.4 31.9

21.4 53.8 50.6 19.4

5.7 1.12 14.7 9.5

0.1 8.68 4.1 4.0

78.67 84.59 72.91 60.81

Reaction conditions: 0.2 g of catalyst; reaction temperature (T) = 175; reaction time = 2 h; 100 ml of decane. 4-t-BT, 4-t-butyltoluene; 3-t-BT, 3-tbutyltoluene; others—2-t-butyltoluene, 2,5-di-t-butyltoluene and oligomers.

1:1 mmol ratio of t-BuOH to toluene because the t-butanol is insufficient to react with toluene. As the 2 mmol of t-BuOH is increased to 1 mmol of toluene, the conversion of toluene, yield and selectivity of 4-tbutyltoluene increases because of equilibrating of both reactants on the Bronsted acid sites of the inner side surface of catalyst. As the 2 mmol of toluene is increased to 1 mmol of t-BuOH, the conversion increased, but the yield and selectivity of 4-t-butyltoluene the 3-t-butyltoluene decreases while other byproducts slightly increases. By increasing 4 mmol of toluene with 1 mmol of t-BuOH, the conversion of toluene, yield and selectivity of 4-t-butyltoluene and 3-t-butyltoluene decreases to compare with 1:2 mmol ratio of t-BuOH to toluene, but the other products of oligomers increases. In all the cases, 4-t-butyltoluene was obtained as the major product along with small amounts of 3t-butyltoluene products. In addition, trace amount of other alkylated like as 3,5-di-t-butyltoluene, 2-t-butyltoluene and oligomers products was also observed. It is observed that at reaction temperature at 175 C and reaction time for 2 h over different Si/(Zn + Al) ratios of Zn–Al–MCM-41 catalysts, the highest conversion of toluene and selectivity of 4-t-butyltoluene were obtained at t-BuOH to toluene ratio of 2:1 while the conversion and yield and selectivity increased with high zinc and aluminum-ions content and the results are shown in Table 6. Generally, as the molar ratio of toluene is increased with t-BuOH, the conversion of tolu-

ene decreased, and as the molar ratio of t-BuOH is increased with toluene, conversion of toluene decreased because of coking of the catalyst by unsaturation of reactant and catalyst pores while there would be fast diffusion without reaction from the catalyst active sites. Hence the optimal molar ratio of t-BuOH to toluene is 2:1. Thus, the optimum conditions for obtaining maximum conversion of toluene (90.4%) and highest selectivity of 4-t-butyltoluene (96.90%) can be summarize as follows: Catalyst, Zn–Al–MCM-41 with Si/(Zn + Al) = 75; reaction temperature = 175 C, time = 2 h and t-BuOH to toluene ratio = 2:1. The liquid phase reaction of t-butylation of toluene was carried out at various reaction times with different mmol ratio of t-BuOH to toluene and 100 ml of decane as solvent at 175 C in the presence Zn–Al–MCM41(75) and the results are given in Fig. 11. The conversion of toluene and selectivity of 4-t-butyltoluene decreases with different time in the series 1:1 < 1:4 < 1:2 < 2:1 mmol ratios of t-BuOH to toluene. The conversion and selectivity are higher in 2:1 mmol ratios for 2 h than those of other mmol ratios due to equilibrium of the reactants with the greater chemisorption on the Bronsted acid sites of catalysts surface. The other products as oligomers in 1:1 mmol ratio, 3-t-butyltoluene and 2-5-di-t-butyltoluene in 1:2 and 1:4 are slightly higher than those of 2:1 mmol ratio due to favorable the catalyst surface and reaction conditions.

M. Selvaraj, T.G. Lee / Microporous and Mesoporous Materials 85 (2005) 59–74 100

73

100

90 90

Converstion of toluene (%)

Conversion of toluene (%)

80 80

1:1 2:1 1:2 1:4

70

60

70

60

50

Zn-Al-MCM-41(75) Zn-Al-MCM-41(151) Zn-Al-MCM-41(228) Zn-Al-MCM-41(304) Zn-Al-MCM-41(380)

40 50

30 40

20 0

1

2

3

4

5

6

7

8

9

10

1

a

Reaction time (h)

a 100

2

3

4

Run of the catalysts

100

Selectivity of 4-t-butyltoluene (%)

Selectivity of 4-t-butyltoluene (%)

90

1:1 2:1 1:2 1:4

80

70

60

90

80

70

Zn-Al-MCM-41(75) Zn-Al-MCM-41(151) Zn-Al-MCM-41(228) Zn-Al-MCM-41(304) Zn-Al-MCM-41(380)

60 50

0

b

1

2

3

4

5

6

7

8

9

50

10

1

Reaction time (h)

Fig. 11. Variation of reaction time with (a) conversion of toluene (%) and (b) selectivity of 4-t-butyltoluene (%) on different mmol ratio (tBuOH/toluene) over Zn–Al–MCM-41(75).

3.11.5. Recyclability All Zn–Al–MCM-41 catalysts was reused for the tbutylation of toluene at 175 C with 2 h reaction time and 2:1 mmol ratio of t-BuOH to toluene was carried out for the highly selective synthesis of t-butyltoluene, and the results have been depicted in Fig. 12. No loss of catalytic activity was observed after 4 runs. Instead, its conversion of toluene, yield and selectivity of t-butyltoluene increased with each cycling in Zn–Al–MCM41(75). But, the conversion, yield and selectivity decreased in other Zn–Al–MCM-41 catalyst at the same reaction conditions and the results are shown in Fig. 12. This may be due to decreasing the catalytic activity along with dehydration of Bronsted acid sites on the sur-

b

2

3

4

Run of the catalysts

Fig. 12. Variation with run of the catalysts with (a) conversion of toluene (%) and (b) selectivity of 4-t-butyltoluene (%) in the presence of different Zn–Al–MCM-41.

face of the catalyst because of the catalysts having less aluminum-ions content.

4. Conclusions Mesoporous Zn–Al–MCM-41 with Si/(Zn + Al) ratio equal to 75, 151, 228, 304 and 380 were synthesized using sodium metasilicate, aluminum sulfate as the reagents in sulfuric acid medium and cetyltrimethylammonium bromide as the template under hydrothermal conditions. From the physico-chemical characterization of the materials it was concluded that all the synthesized catalysts have good Zn–O–Si and Al–O–Si framework,

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high surface area, good thermal, hydrothermal stability and more Bronsted acid sites. From the various catalytic reaction experiment carried out using the synthesized Zn–Al–MCM-41 catalysts with different Si/(Zn + Al) ratio, it could be inferred that at molar ratio of 2:1 of t-BuOH to toluene at 175 C reaction temperature and at 2 h reaction time, there was high selectivity of 4-tbutyltoluene (96.90%) for Zn–Al–MCM-41(75) under liquid-phase reaction conditions while the conversion and selectivity of t-butyltoluene are higher in recyclable Zn– Al–MCM-41(75) than those of other Zn–Al–MCM-41 catalysts due to no loss of catalytic activity after the recycling process. Thus, a higher yield and selectivity of 4-t-butyltoluene and conversion of toluene using Zn–Al–MCM-41(75) depicts its superiority in performance compared to other Zn–Al–MCM-41 catalysts.

Acknowledgement The authors gratefully acknowledge the KOSEF through National Core Research Center for Nanomedical Technology for sponsoring this work (R15-2004024-00000-0).

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