Benzylation of benzene over sulfated zirconia supported in MCM-41 using a single source precursor

Abstract

An array of highly active, thermally stable solid acid catalysts is reported by loading different weight fractions of sulfated zirconia on mesoporous MCM-41. Different weight fractions were used to optimize the sulfated zirconia content for active and selective catalysts in alkylation of benzene with benzyl ether. The supported catalysts were prepared by impregnation of an aqueous solution of the zirconium acetate sulfonate single-source precursor into the mesoporous support. The catalytic activity of the catalysts was stable with time on stream for all catalysts examined under the reaction conditions employed. The benzene to benzyl ether mole ratio was varied from 0.1 : 1 to 20 : 1. Benzyl ether conversion was found to increase with reaction temperature and catalyst loading, while diphenyl methane selectivity was observed to increase with increasing feed mole ratio. The used regenerated catalysts completely retained its activity after the first and second regeneration.

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1. Introduction

Friedel–Crafts alkylation is one of the most important reactions for production of fine chemicals and is a key reaction for all fields from petrochemicals to pharmaceutical chemicals. These reactions are usually catalysed by Lewis acids in the liquid phase.¹Diphenyl methane and its derivatives are widely used in pharmaceuticals, petrochemicals, cosmetics, dyes, fine chemicals, insulators, etc.^2–5 Commercially, a high yield of alkylated products is usually achieved using homogenous mineral acids such as sulfuric acid and HF.⁶ However, a number of problems concerning handling, safety, corrosion and waste disposal have to be overcome in order to replace these liquid acids. These issues encourage the development of solid acid catalysts which are suitable for alkylation reactions. Sulfated zirconia has attracted much attention as a promising catalyst thanks to its high activity and environmental friendliness.⁷ Sulfated zirconia has been used in alkylation reactions such as benzylation of benzene and benzene derivatives⁸ and in the alkylation of phenol with methyl tertiary butyl ether to produce tert-butyl phenols.⁹

Although sulfated zirconia is considered to be one of the important superacid catalysts, it is not widely applicable in industrial and commercial applications due to its low surface area and short lifetime of the catalysts. A composite material, which can combine the advantages of mesoporous molecular sieves and sulfated zirconia, should greatly expand the catalytic activities of the material, especially in applications as strong acid catalysts for reactions containing bulky molecules. Ordered mesoporous molecular sieves with very high surface area and well-ordered pores narrowly distributed between 20–100 Å, named as MCM-41 and MCM-48, were first reported by Mobil scientists.¹⁰ The mesoporous molecular sieve (MCM-41) used as a support in this present study, is known to have a uniform hexagonal array of mesopores and very high surface area (typically around 1000 m² g⁻¹ or higher). This material has been shown to be an excellent support for preparing supported catalysts with activities and selectivities superior to those over amorphous silica, alumina, and even zeolites.¹¹ Busto et al.¹² recently reported that the surface area and mesoporosity of sulfated zirconia do matter in order to achieve good catalytic activity. Lakshmi et al.¹³ reported that the deactivation of SZ can be attributed to sintering, sulfur migration into the metal oxide bulk, and coke formation. A family of these high surface area MCM materials was discovered in the early 1990's by a research group in the Mobile Corporation.^14,15 It has a unique ordered structure with a uniform arrangement of mesopores in a hexagonal lattice with narrow pore-size distribution, high thermal stability, and a large surface area.

Reports appeared on the alkylation of benzene and toluene by alkyl and benzyl halides, alcohols and alkenes, on montmorillonite doped with transition metal cations,^16,17 clays,¹⁸ cation exchanged resins,¹⁹niobium phosphate²⁰ and silica supported poly-tri-fluoromethane sulfosiloxane catalysed benzylation of benzene and substituted benzene.²¹Benzylation of aromatic compounds also appeared on Ga–Mg-hydrotalcite,²²AlCl_x-grafted Si-MCM-41,²³Fe-TUD-1,²⁴ and Al-SBA-15,²⁵ using benzyl chloride. Yadav et al.²⁶ investigated the application of S–ZrO₂ catalyst to the Friedel–Crafts alkylation of toluene with benzyl chloride, benzyl alcohol and benzyl ether. The authors reported that benzyl alcohol, in the presence of benzyl chloride, is preferentially adsorbed on to the catalyst site.

Bachari and Cherifi²⁷ studied the benzylation of benzene and substituted benzenes reaction employing benzyl chloride as the alkylating agent over a series of tin-containing mesoporous silica. The authors reported that the mesoporous tin-containing materials showed both high activity and high selectivity for benzylation of benzene. Lei et al.²⁸ synthesized supported sulfated zirconia over MCM-41 by two-step impregnation methods. They reported that the acidity increased with increasing zirconium oxide content over the support and that the mesoporous framework was destroyed by supporting more than 30% wt/wt ZrO₂. Xia et al.²⁹ prepared supported sulfated zirconia over MCM-41 by chemical deposition of zirconium propoxide in hexane. This was followed by hydrolysis in an aqueous solution of sodium chloride and sulfation using sulfuric acid. They achieved highly acidic supported catalysts which exhibited high catalytic activity for the isomerization of n-pentane. Khatri et al.³⁰ studied the liquid phase benzylation of benzene and toluene with benzyl chloride over a fly ash supported sulfated zirconia catalyst. The authors reported that the chemical activation of fly ash by acid leaching results in increased silica content and thus surface hydroxyl contents, which are responsible for efficient loading of zirconia on the fly ash support. Mantri et al.³¹ investigated the alkylation of aromatics with benzyl alcohol as an alkylating agent over rare earth metal triflates supported on MCM-41. They found Sc(OTf)₃ supported on MCM-41 to be the best catalyst for the benzylation of benzene among the four types of metal triflates on MCM-41. It was reported that the catalytic activity of Sc(OTf)₃ was enhanced by being loaded on MCM-41 because of increased dispersion. Also, alkylation reactions over Al-MCM-41 have been reported by several researchers.^32–35

Generally speaking, a survey of Friedel–Crafts alkylation of aromatics over supported sulfated zirconia over the last decade has shown that these reactions have received the most attention over alkylating agents like benzyl chloride and benzyl alcohols, as compared with benzyl ether. Benzyl ether on the other hand, is an important intermediate for the formation of diphenylmethane by consecutive mechanism.³¹ Therefore, the present study is aimed at investigating the benzylation of benzene with benzyl ether over supported sulfated zirconia, using zirconium acetate sulfonate as a single source precursor. The study will focus on the effect of reaction conditions (time and temperature) on benzyl ether conversion, diphenylmethane selectivity and the effect of varying the reactants feed molar ratio on the product selectivity.

2. Experimental

2.1 Catalyst preparation

All the starting materials were purchased from Aldrich Chemical Company and used without further purification. A sample of 10.7 g (50 mmol) zirconium acetate was dissolved in 40 ml of distilled water in a 250 ml beaker. 23.5 g (150 mmol) of ethane sulfonic acid (70 wt% solution in water) was dissolved in 30 ml of de-ionized water. The ethane sulfonic acid solution was added slowly to the zirconium acetate solution. The ethane sulfonate replaced the acetate groups to release acetic acid in the solution. The precursor was obtained by evaporation of the water and acetic acid from the solution by rotary evaporation followed by drying under vacuum for 12 h. A more detailed method of preparation and characterization of the single source precursor will be a subject of future work. The MCM-41 support was synthesized by preparing a solution of tetraethyl orthosilicate (TEOS) solution in ethanol and isopropanol with a mole ratio of 1 : 6 : 1, TEOS, ethanol, and isopropanol, respectively. A solution of hexadecylamine was added to the TEOS solution and the resulting mixture was stirred continuously for 1 h and then left for 12 h without stirring at room temperature. The resulting solid was filtered off and washed with deionized water to remove all residual template and solvents. The solid was dried under vacuum at room temperature and calcined at 500 °C for 12 h. The supported sulfated zirconia was synthesized by addition of the required amount of MCM-41 support to an aqueous solution of the single precursor prepared earlier. This was stirred for 3 h at room temperature to ensure the impregnation of the zirconium salt into the pores of the silica support. The water in the solution was removed by rotary evaporation, and the samples were dried under vacuum. The supported sulfated zirconia was obtained by calcination of the supported precursors at 650 °C.

2.2 Characterization

The prepared catalysts samples were characterized using several techniques. The specific surface area was obtained by conventional Brunauer–Emmett–Teller (BET) multilayer nitrogen adsorption methods using a Quantachrome Nova 1200 instrument.

Thermogravimetric analyses were performed using a Seiko EXSTAR 6000 TG/DTA 6200 instrument under flowing of air (50 ml min⁻¹). Scanning Electron Micrographs were obtained using a JEOL JXM 6400 SEM. Infrared spectra were collected using a Nicolet Magna-IR 750 spectrometer, and the data were collected by diffuse reflectance of a ground powder diluted with potassium bromide. X-Ray powder diffraction patterns were obtained using a Bruker AXS D8 advance diffractometer using copper Kα radiation with a wavelength of 1.5418 Å. The mean crystallite size of the oxide samples was estimated by X-ray diffraction using a line broadening method.

X-Ray photoelectron spectroscopy (XPS) results were obtained on a PHI Quantera SXM photoelectron spectrometer using Al Kα radiation. Elemental analysis was determined using Siemens model SRS 3000 wavelength dispersive X-ray fluorescence. Carbon, hydrogen, nitrogen and sulfur analyses were performed using a Vario ELMake (CHNS) Elemental Analyzer. The acidity of the supported and unsupported zirconium dioxide prepared was determined by the Langmuir adsorption isotherm method using cyclohexylamine as an adsorbate^36,37 and using Hammett indicators.

2.3 Reaction procedure

The alkylation reaction of benzene with benzyl ether was carried out in an autoclave Teflon-lined batch reactor in a constant temperature oil bath with magnetic stirring. The reaction was carried out at different temperatures ranging from 100 to 180 °C and time ranging from 30 min to 35 h. Samples were collected from the reactor at different reaction times and diluted with methylene chloride for analysis. (GC/MS) was performed on a Hewlett Packard G1800A instrument equipped with a 30 m × 0.25 mm HP5 column (crosslinked 5% PhME silicone). The ether conversion was calculated as follows: ether conversion (%) = (Cn/Co) × 100, where Cn and Co are the molar ether reactant converted, and moles of ether in feed, respectively.

3. Results and discussion

3.1 XRD and infrared spectroscopy

Fig. 1 shows the IR spectra for the supported samples taken in the region of 3500 cm⁻¹ to 4000 cm⁻¹ after calcination at 650 °C. The MCM support showed a sharp peak with medium intensity at about 3750 cm⁻¹ corresponding to the O–H bond vibration of the free Si–O–H groups on the support surface. The intensity of this peak was obviously reduced with the increase of the amount of the sulfated zirconia on the support surface. The XRD pattern for the supported samples gives an indication of the dispersion of the sulfated zirconia over the MCM-41 support surface. Fig. 2 shows that there are no diffraction peaks corresponding to the tetragonal zirconia when the zirconia contents reached 30%. At about 40% zirconia load over the support, a weak tetragonal phase was observed indicating that up to 40% ZrO₂ load, the zirconia is still well-dispersed in the pores of the silica. Above that concentration (about 70% ZrO₂/MCM) a strong tetragonal phase peak at 2θ = 30° was observed, which indicated that the support pores were packed and the zirconia started to build on the amorphous silica walls. The uniform decrease of the surface area with increasing the ZrO₂ content (Table 1) also confirmed the blockage of the pores with small crystalline zirconia. Fig. 2 shows that the tetragonal phase, which is known to be the more catalytically active phase^38,39 was observed for the supported samples when calcined at 750 °C. However, the XRD profile for the unsupported calcined precursor showed the appearance of the monoclinic phase along with the dominant tetragonal phase. These results are similar to that obtained by Huang et al.⁴⁰ for the zirconium sulfate supported over silica gel. The crystallite sizes obtained by Scherrer's equation were 4 and 7 nm for the supported samples containing 40% and 70% zirconia, respectively, while the size of the unsupported zirconia was 14 nm. This showed clearly that the crystallite size was increasing with increasing the zirconia contents over the support surface.

Fig. 1 The IR spectra for the supported samples, the stretching frequencies of the Si–OH groups of the MCM support.

Fig. 2 X-Ray diffraction pattern for the supported samples with different zirconia contents calcined at 750 °C.

Table 1 The pore volume and elemental analysis results for the supported sulfated zirconia samples

Catalyst sample	S BET/m^2 g^−1	Pore volume^b/ml g^−1	Elemental analysis by XRF^d (%)				Sulfur contents by CHNS^c analyzer (%)
			Zr	Si	S	O
a S–Zr = unsupported sulfated zirconia. b Total pore volume was measured at P/Po = 0.9929. c CHNS = carbon hydrogen nitrogen sulfur. d XRF = X-ray florescence.
MCM-41	1175	1.1	—	45.3	—	52.4	—
S–Zr^a	50	0.08	73.1	—	0.41	26.4	0.38
15% S–Zr/MCM	1037	1.03	16.6	35.1	0.95	47.35	0.633
30% S–Zr/MCM	714	0.92	32.2	25.1	1.08	41.6	0.754
40% S–Zr/MCM	545	0.75	43.9	16.3	2.28	37.5	1.76
70% S–Zr/MCM	274	0.35	61.1	7.14	0.78	30.8	0.979

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3.2 Bulk and surface analysis

The bulk elemental analysis and pore volume results of the synthesized supported catalytic systems are summarized in Table 1. The maximum sulfur contents observed were for the 40% S–Zr/MCM sample. This indicates that the support stabilized the coordinated sulfate groups on the surface. Additionally, the pore volume and specific surface area data are shown in Table 1. The results showed a slight and uniform decrease on both the surface area and the pore volume with an increase of the zirconium oxide contents over the MCM support surface. These results showed that most of the ZrO₂ contents are available inside the pores and well dispersed on the surface. At samples with sulfate zirconia content higher than 40%, sharper decrease of the pore volume and surface area occurred. In this case the decrease in the pore volume was observed markedly and the sulfur contents as well decreased. This decrease in the sulfur contents may be attributed to the migration of most of the sulfate groups from the MCM pores to the surface which facilitate the decomposition during thermal calcination. This strongly indicated that the support stabilized the coordinated sulfated groups. The content of 40% of sulfate zirconia on the MCM surface is a threshold beyond which the properties of the system are dramatically changed.

The XPS data show a strong influence of the support on the surface chemistry of the zirconia. Below the maximum loading (where crystalline ZrO₂ forms) the surface is rich in sulfate while once the ZrO₂ starts to crystallize, the surface sulfur concentration drops. The XPS surface analysis results (Table 2) showed that the Zr and S atomic percentage on the surface increased with increasing of the sulfated zirconia content on the MCM support. However, the results showed that the S/Zr atomic ratio on the surface decreased with increasing zirconium content. For 15 wt% ZrO₂/MCM, the surface atomic ratio of S/Zr was higher than the ratio of Zr : S in the precursor. With further increase in the amount of sulfated zirconia on the support more than 40 wt%, the atomic ratio of S/Zr of the surface decreases remarkably. The results in Table 2 show also the S/Zr ratio obtained from the XRF bulk elemental analysis. The S/Zr ratio decreased with increasing amount of sulfated zirconia on the MCM-41 support. These results are similar to the results obtained from the XPS analysis. However, the S/Zr ratio obtained from bulk analysis is much lower (10–15 times lower) than that derived from the XPS surface analysis. These results strongly support a very important conclusion that the sulfur species are mostly available on the surface of the oxide rather than in the bulk.

Table 2 Surface atomic percentage extracted from XPS analysis

Sample	Atomic%				Bulk S/Zr ratio	XPS S/Zr atomic ratio
	C	Zr	S	O
S–Zr = unsupported sulfated zirconia.
15 wt% S–Zr/MCM	6.2	4.39	4.21	85.16	0.057	0.959
40 wt% S–Zr/MCM	8.7	8.89	6.25	76.19	0.052	0.703
70 wt% S–Zr/MCM	15.4	17.35	8.38	58.84	0.013	0.453
S–Zr	14.8	27.87	1.33	56.25	0.006	0.05

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It was determined by XPS that if the MCM-41 silica was treated with ethanesulfonic acid followed by calcination at 650 °C, it will not contain a considerable amount of sulfate species on the surface. This clearly demonstrates that the sulfate groups are strongly coordinated to the zirconium metal centers not to the silica of the MCM-41 support (Si : S atomic ratio = 200 : 1). Therefore, it can be concluded that the zirconium oxide attaches to the surface of the MCM-41 support by condensation with reactive silanol groups and the silicate groups are bonded to the surface of the zirconia.

3.3 Acidity property

The total acidity measurements were performed using cyclohexylamine as a titrant (Table 3). The cyclohexylamine adsorption results showed that the acidity of the supported sulfated zirconia catalysts increased with increasing amount of sulfated zirconia over the MCM support. It was calculated that about 0.6 g of sulfated zirconia per 1 g of MCM-41 will form a monolayer, which corresponds to approximately 37%. It can be concluded that a monolayer is formed after loading the MCM-41 with about 30–40 wt% sulfated zirconia. A slight deviation in the acidity was noticed after the support was loaded with 70 wt% sulfated zirconia which is much higher than the required amount to form a monolayer. The acidity strength measurement using the Hammett indicators method showed that the supported sulfated zirconia samples exhibited high acid strength. The 40 wt% ZrO₂/MCM protonated p-nitrotoluene (pK_a = −11.4) caused the indicator to change from colorless to yellow. The zirconias failed to react with 2,4-dinitrotoluene which has a pK_a of −13.75 demonstrating that sulfated zirconias pK_a are in the range of −11.3 to −13.75. In order to test the influence of the support, both the supported (40 wt% ZrO₂) and unsupported precursors were calcined at 950 °C for 8 hours. In contrast to the unsupported precursor, the supported samples calcined at high temperature retained its acidity strength. This was demonstrated by the use of Hammett indicators. The supported sample gave a yellow coloration with both anthraquinone (pK_a = −8.2) and p-nitrotoluene (pK_a = −11.4), while no color change occurred with both indicators over the unsupported samples. Additionally, the high temperature calcined supported sample gave a very dark red coloration with dicinnamalacetone (pK_a = −3), while a very light orange color was noted with unsupported sample calcined at 950 °C. It is known that the acid-form color of dicinnamalacetone is red and the neutral-form color is yellow.⁴¹ Therefore, the results clearly indicate that the acid strength and number of acid sites of the unsupported oxide drop when heated to high temperature, presumably due to the loss of the active sites in the form of SO₂ or SO₃. However, the supported sample retained acidity due to strong support/catalyst interaction, which plays an important role in the (thermal stability of supported samples) delay of decomposition and loss of sulfur.

Table 3 Acidity measurement for the supported sulfated zirconia

Catalyst sample	S BET/m^2 g^−1	Cyclohexylamine adsorption
		μmol g^−1	μmol m^−2
S–Zr = unsupported sulfated zirconia.
MCM-41	1175	1733	1.475
S–Zr	50	277	5.601
15% S–Zr/MCM	1037	1737	2.440
30% S–Zr/MCM	714	1469	2.652
40% S–Zr/MCM	545	1213	2.954
70% S–Zr/MCM	274	1464	7.102

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3.4 Surface morphology and thermal analysis

The SEM data for the supported catalyst with 15% and 70% zirconia over the MCM support are shown in Fig. 3. A tubular-like morphology with 70% zirconia contents over the support is formed. The micrograph showed a uniform diameter of the tubes with an approximate diameter between 100 nm and 200 nm which was larger than the average diameter of the MCM pore channels (3–30 nm). This indicated that the zirconium precursor, at some point, is over loaded in terms of the support pores available. Upon thermal calcination, zirconia started to crystallize outside the pore channels of the support. Hence agglomeration of the particles to form a larger crystallite size occurred with maintaining the tubular shape. The mechanism of formation of the zirconia fibers will be the subject of a further paper.

Fig. 3 Scanning Electron Micrographs for the supported sulfated zirconia (a) with 15% zirconia and (b) with 70% zirconia.

TG analysis was performed on the samples in order to estimate the suitable calcination temperature for preparation of the zirconium oxide from the precursors. Fig. 4 shows the TGA profiles for the different supported sulfated zirconia catalysts. The profiles suggest that the precursor thermally decomposed in a stepwise manner. In the first step, the weakly bonded water molecules were lost, and with further heating, a decomposition of the coordinated ligands occurred to give a volatile organic byproducts, carbon dioxide and water.

Fig. 4 Thermogravimetric analysis profiles for the supported sulfated zirconia with different zirconia concentrations.

4. Catalytic performance

The reactivity of the supported and unsupported sulfated zirconia was investigated in the alkylation of benzene with benzyl ether. Alkylation of benzene with benzyl ether produced mainly diphenylmethane. The alkylation reaction was carried out at 100 °C, 150 °C, and 180 °C over 10 wt% of supported sulfated zirconia catalysts with respect to the total weight of reactants with a constant benzene to benzyl ether molar ratio of 10 : 1. The mechanism for the benzylation of benzene with benzyl ether is better illustrated in Scheme 1. Protonation of the benzyl ether is caused by the supported sulfated zirconia catalystvia adsorption of the benzyl ether on acid sites. As a result, polarization of the C–O bond of the ether will occur to give an adsorbed benzyloxy species on the catalyst surface with concomitant formation of a benzyl carbocation. An electron pair from the benzene ring subsequently attacks the carbocation forming a C–C bond. The resulting cationic intermediate undergoes proton transfer to give a neutral alkylated substitution product and generate the catalyst.

Scheme 1 Benzylation of benzene with benzyl ether over supported sulfated zirconia catalysts.

4.1 Effect of the reaction temperature on benzyl ether conversion and product distribution

The benzyl ether conversion over 10 wt% supported sulfated zirconia catalysts with respect to the reactants increased with increasing temperature and contact time. Benzyl ether conversion of approximately 20, 54.1, and 100% was achieved at 100 °C, 150 °C and 180 °C, respectively, after 25 hours reaction time, as shown in Fig. 5. The major product of the alkylation reaction over the supported sulfated zirconia catalyst is diphenyl methane. After about 10 hours time-on-stream, the conversion of benzyl ether was observed to ∼100% at a reaction temperature of 180 °C. Thereafter, the benzyl ether conversion was noticed to be stable all through the reaction time, even after 24 hours, at a reaction temperature of 180 °C. Fig. 6 presents the effect of reaction temperature on the product selectivities after 5 hours and 24 hours, using a constant benzene to benzyl ether molar ratio of 10 : 1. The selectivity of diphenyl methane increased with increasing reaction temperature, after 5 hours time-on-stream. Diphenyl methane selectivity increased from ∼68% at 100 °C to ∼97% at 180 °C, representing an increase of about 43%, after 5 hours time-on-stream. It was noticed that the selectivity of the side products like benzyl alcohol and benzaldehyde was suppressed after 24 hours, as compared to the selectivity of these products observed after 5 hours time-on-stream (Fig. 6).

Fig. 5 Alkylation of benzene with benzyl ether at different reaction temperatures (reaction conditions: benzene : benzyl ether mole ratio = 10 : 1, catalyst loading with respect to benzyl ether = 10 wt%, (△) 180 °C, (●) 150 °C, (□) 100 °C).

Fig. 6 Effect of the reaction temperature on the product selectivity. A = 5 hours and B = 24 hours (reaction conditions: benzene : benzyl ether mole ratio of 10 : 1, catalyst loading with respect to benzyl ether = 10 wt%). BzOH = benzyl alcohol and PhCHO = benzaldehyde and DPM = diphenylmethane.

Fig. 7 shows a plot of the natural logarithm of the concentration of benzyl etherversus time. The activation energy of the reaction was determined from a slope of the plot of the natural logarithm of benzyl ether conversion rate versus the inverse of the temperature according to Arrhenius equations. The resulting activation energy was ∼62 kJ mol⁻¹.

Fig. 7 A plot of the natural logarithm of benzyl ether concentration versus time ((△) 100 °C, (○) 150 °C, (⋄) 180 °C).

4.2 Effect of the benzene to benzyl ether molar ratio on the catalytic activity

The effect of the amount of benzene on the benzylation over the supported sulfated zirconia catalyst is presented in Table 4. The reaction was carried out by varying the mole ratio of benzene to benzyl ether from 0.1 to 20. After 15 hours time-on-stream, the benzyl ether conversion was noticed to increase with increasing the feed mole ratio of benzene to benzyl ether from 0.1 to 10. But beyond the ratio of benzene to benzyl ether of 10, a significant decline of the conversion of benzyl ether was observed. Similar to the decrease in benzyl ether conversion observed after 15 hours time-on-stream was also noticed after 35 hours time-on-stream (Table 4). The drop in conversion of benzyl ether noticed beyond the benzene to benzyl ether molar ratio of 10 can be as a result of lesser availability of the alkylating agent in the feed. The reduction in the conversion of benzyl ether noticed by increasing the feed mole ratio of benzene to benzyl ether beyond 10 can also be due to the large excess of aromatic reactant injected into the system, thereby blocking most of the active sites of the catalyst surface. With an increase in the ratio of benzene to benzyl ether, the selectivity for diphenyl methane is improved at the expense of benzyl alcohol, benzaldehyde, benzyl diphenyl methane and dibenzyl diphenyl methane. Side products such as toluene, benzyl alcohol and benzaldehyde formed when benzyl ether was present in high concentration in the feed are likely formed via a disproportionation reaction that occurs between two adjacent benzyloxy groups adsorbed on the surface via direct hydrogen transfer from one surface molecule to the other to yield toluene and benzaldehyde. This is analogous to the mechanism of the disproportionation of benzyl alcohol over acidic alumina.⁴² Part of the benzyl alcohol is oxidized to benzaldehyde and most of the toluene formed was alkylated by benzyl ether to give 2-methyldiphenyl methane (2-MDPM) and 4-methyldiphenyl methane (4-MDPM), as shown in Scheme 2.

Table 4 Product distributions for alkylation of benzene with benzyl ether over supported sulfated zirconia at 150 °C and reaction time of 15 and 35 hours, for benzene to benzyl ether molar ratios of 0.1 to 20

BE : Bz mole ratio	Reaction time/h	%BE conversion	% Products selectivity
			BzOH + PhCHO	DPM	BDPM + DBDPM	Others
BE and Bz are dibenzyl ether and benzene, respectively; BzOH and PhCHO are benzyl alcohol and benzaldehyde, respectively; DPM is diphenylmethane; BDPM and DBDPM are benzyl diphenyl methane and diphenyl methane, respectively. Reaction conditions = (reaction temp. = 150 °C, catalyst 10 wt% with respect to reactants).
1 : 20	15	24.9	0	100.0	0	0
	35	41.4	5.0	95.0	0	0
1 : 15	15	25.5	8.1	90.2	1.1	0.6
	35	40.7	6.2	91.8	1.3	0.7
1 : 10	15	59.7	2.5	96.8	0	0.6
	35	61.1	4.1	91.9	3.4	0.6
1 : 0.2	15	16.1	28.4	6.9	64.7	1.6
	35	23.4	22.6	4.3	67.9	5.2
1 : 0.1	15	4.63	35.9	3.2	59.3	1.6
	35	10.6	19.9	2.0	75.3	2.8

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Scheme 2 Possible secondary products formed from alkylation reaction of benzene with benzyl ether.

4.3 Effect of the support on the catalytic activity

Alkylation of benzene with benzyl ether was performed over a pure MCM-41 support for more than 40 hours at 150 °C. No reaction took place, indicating that the acid sites of the support were not capable of performing the alkylation reaction. Fig. 8 shows a plot of the natural logarithm of the benzyl ether concentration versus time over supported sulfated zirconia catalysts, ranging from 15 wt% to 100 wt%. The slopes of the resulting straight lines gave the rate of the alkylation reaction. Fig. 9 shows a plot of the rate of reaction versus the amount of zirconia. This result reveals that the alkylation activities increased with increasing sulfated zirconia contents over the support to a maximum 40 wt% zirconia/MCM-41 where the reaction was almost three times faster (conversion is greater) compared to the unsupported sulfated zirconia.

Fig. 8 Effect of the sulfated zirconia concentration over MCM-41 on the rate of the alkylation reaction of benzene with benzyl ether ((◆) 15%, (□) 30%, (△) 40%, (○) 70%, (—) 100%).

Fig. 9 A plot of the rate of alkylation reaction versus the zirconia content over the MCM-41 (reaction conditions: benzyl ether : benzene mole ratio = 1 : 10; reaction temperature: 150 °C).

Although, 15 wt% supported sulfated zirconia has twice as wide surface area as the 40 wt% S–Zr/MCM-41 has, the catalytic activity of these samples cannot be interpreted simply by surface area. It is obvious from the result presented in Table 2 that, 15 wt% supported sulfated zirconia contains more sulfur on the surface as compared with other samples reported in this present study. The surface sulfur content noticed over these samples may be an important factor other than acidity and surface area in generating higher benzylation activity. Also, a higher amount of acid sites was noticed in the 15 wt% supported sulfated zirconia as compared to the acidities of all other samples reported. However, it is evident from Fig. 9 that the rate of alkylation reaction is apparently not proportional to the amount of acid sites on the catalysts.

4.4 Catalyst regeneration

Deactivation of the catalysts occurred after being used for several hours during the alkylation reaction of benzene with benzyl ether. The deactivation was presumably due to the adsorption of benzyloxy groups which cover the active sites on the surface. Deactivation of supported sulfated zirconia catalysts might also be due to strong adsorption of hydrocarbons polycyclic or even benzene on the catalyst surface. The IR spectra for both fresh and used supported catalyst samples dried at 170 °C under vacuum are presented in Fig. 10. The carbon–carbon double bonds stretching frequencies at 1600 cm⁻¹, as well as the sp² C–H stretching frequency in the range between 3000 cm⁻¹ and 3100 cm⁻¹, clearly indicate that some aromatic species are adsorbed on the catalyst surface, probably benzyloxy and other aromatic species.

Fig. 10 Infrared spectra for the fresh and used catalysts.

The used catalyst was regenerated several times by heating at 500 °C for 4 hours in the flow of air. The results of the used regenerated catalysts showed that the catalyst completely retained its activity after the first and second regeneration (Fig. 11). This implies that the deactivation is mainly attributed to the carbon deposition, and the loss of sulfur during the reaction is ruled out. A significant loss in the activity was observed after the third regeneration. The conversion of benzyl ether dropped from 81% for the fresh sample to 60% after the third regeneration. This drop in conversion after several regenerations may be due to either the loss of the very small amount of active sites in the form of SO₂ or from migration of the sulfur into the bulk of the zirconium oxide. Similar loss in activity was observed by Li and Gonzalez^43,44 during isomerization of n-butane, when the used catalyst was regenerated in nitrogen environment.

Fig. 11 Regeneration of the used samples at 500 °C in air for 4 hours (reaction conditions: sulfated zirconia loading = 20% w/w; benzyl ether : benzene mole ratio = 1 : 10; reaction temperature: 150 °C).

5. Conclusions

The following conclusions can be drawn from the alkylation of benzene with benzyl ether over the supported sulfated zirconia catalysts:

• Catalyst loading and reaction temperature plays a major role in benzyl ether conversion.

• Acidity measurement showed that the acidity of the supported sulfated zirconia catalysts increased with increasing amount of sulfated zirconia over the MCM support.

• The maximum benzyl ether conversion of 100%, gave a diphenyl methane selectivity of ∼95% at 180 °C after 24 hours time-on-streams for the 10 : 1 (benzene/benzyl ether) molar ratio.

• The conversion of benzyl ether shows a significant increase with increasing feed mole ratio of benzene to benzyl ether from 0.1 to 10. The selectivity for diphenyl methane improved with an increase in the ratio of benzene to benzyl ether.

• The results of the used regenerated catalysts showed that the catalysts completely retained its activity after the first and second regeneration.

Acknowledgements

The authors would like to express their appreciation to SABIC for their financial support.

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