characterization of co,al-mcm-41 and its activity in the t-butylation of phenol using isobutanol

Applied Catalysis A: General 268 (2004) 139–149

Characterization of Co,Al-MCM-41 and its activity inthet-butylation of phenol using isobutanol

M. Karthika, A.K. Tripathib, N.M. Guptab, A. Vinu c, M. Hartmannc,M. Palanichamya, V. Murugesana,∗

a Department of Chemistry, Faculty of Science and Humanities, Anna University, Chennai 600025, Indiab Applied Chemistry Division, BARC, Mumbai 400085, India

c Department of Chemistry, Chemical Technology, Kaiserslautern University of Technology, P.O. Box 3049, D-67653 Kaiserslautern, Germany

Received in revised form 3 March 2004; accepted 18 March 2004

Available online 26 April 2004

Abstract

Co,Al-MCM-41 catalysts with variousnSi/(nCo+nAl ) ratios were synthesized and extensively characterized by low-angle XRD, TGA/DTG,BET, AAS, DRIFT, UV-Vis DRS and ESR. UV-Vis DRS and ESR studies reveal that cobalt in Co,Al-MCM-41 is highly symmetrical andoccurs in tetrahedral coordination. Some of the cobalt atoms are transformed into the cobalt oxide form whennSi/(nCo + nAl ) is increased to20. t-Butylation of phenol with isobutanol was studied in the vapor phase as a model reaction at temperatures between 200 and 500◦C. Theproducts obtained wereO-tert-butyl phenol (OTBP), 2-tert-butyl phenol (2TBP) and 4-tert butyl phenol (4TBP).O-Butenyl phenol (OBP) and2-butenyl phenol (2BP) were also observed along with normal alkylated products. The phenol conversion drastically increased with temperatureover all the catalysts. The activity of the catalysts followed the order of Co,Al-MCM-41(20) > Co,Al-MCM-41 (50) > Co,Al-MCM-41(80) > Al-MCM-41 (23). The influences of various parameters such as temperature, reactant feed ratio and feed rate, time on stream onconversion and products selectivity were studied and the salient results are discussed.© 2004 Elsevier B.V. All rights reserved.

Keywords: Mesoporous materials; Co,Al-MCM-41;t-Butylation; Phenol; Isobutanol

1. Introduction

The M41S family of mesoporous materials has attractedsubstantial research attention since its report by Mobil Cor-poration in 1991[1,2]. The past decade has witnessed adramatic increase in the design, synthesis, characterizationand property evaluation of mesoporous molecular sieves forcatalysis, adsorption and separation of bulky molecules andenvironmental pollution control. MCM-41, the most impor-tant member of the M41S family, possesses a regular hexag-onal array of uniform pores with diameters in the range2–10 nm. These hexagonal MCM-41 molecular sieves haveattracted much research attention owing to their high surfacearea, thermal and hydrothermal stability and ordered meso-

∗ Corresponding author. Tel.:+91-44-22301168;fax: +91-44-22200660.

E-mail addresses: karthik [email protected] (M. Karthik),v [email protected] (V. Murugesan).

porous nature. They find potential applications as catalysts,adsorbents and hosts for different kinds of molecules[3–5].Pure siliceous mesoporous molecular sieves are of limiteduse as catalysts because of the presence of the neutral frame-work. In order to obtain materials for catalytic applications,one must modify the nature of the amorphous walls by in-corporation of heteroatoms. The isomorphous substitutionof Al3+ for some of Si4+ leads to negatively charges in theframework that are balanced by protons. The resulting acidsites can be formally represented as Si–O(H)–Al. The bron-sted acid sites in aluminosilicates are due to OH bridgedbetween Si and Al present in the framework[6,7]. MCM-41containing metal ions such as Al, V, Fe, Mn, B, Ga, Ti, Zr,Cs, Cu and Zn have been synthesized and their catalyticproperties have been reported[8–10]. The incorporation ofcobalt into the framework imparts both acidic and redoxproperties, which make such materials potentially interest-ing for catalytic applications[11]. It is interesting to notethat substitution of Co2+ into the framework of mesoporous

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2004.03.028

140 M. Karthik et al. / Applied Catalysis A: General 268 (2004) 139–149

MCM-41 leads to moderate acidity. Alkylation of phenol isan industrially important reaction because many alkylatedphenols have commercial applications.t-Butyl derivativesof phenol are the precursors for a number of commerciallyimportant antioxidants in the synthesis of various agrochem-icals, fragrants, thermoresistant polymers and protectingagents for plastics. The products likeO-butyl andO-butenylphenols are used in the synthesis of flavoring and fragrantcompounds, pharmaceutical intermediates, chromans, finechemicals and petrochemicals[12]. In general, this reactionis carried out over the conventional acid catalysts like AlCl3or sulfated zirconia in liquid phase[13,14], these catalystsare environmentally hazardous, require tedious work-upand cannot be used at higher temperatures. Zeolite catalystswith isobutanol/t-butanol in vapor phase have also beenattempted for this reaction, but it produces a mixture ofortho, para and meta butylated products or predominantlypara products and they undergo deactivation significantlywith time on stream[15]. However, t-butylation of phe-nol with isobutanol has not been studied over mesoporousAl-MCM-41 catalysts. In the present investigation, attempthas been made to synthesize mesoporous Co,Al-MCM-41(nSi/(nCo + nAl ) various ratios) by hydrothermal process.t-Butylation of phenol with isobutanol over these catalystswas studied extensively in the vapor phase at 200, 250,300, 350, 450 and 500◦C, the results are discussed in thispaper.

2. Experimental

2.1. Preparation of the catalysts

Co,Al-MCM-41 samples with variousnSi/(nCo + nAl )ratios (thenCo/nAl was fixed to 1) were synthesized fromgels with the following gel composition: 10 SiO2:5.4C14TMABr:0.03–0.125 Al2O3:0.06–0.25 CoO:4.25 Na2O:1.3 H2SO4:480 H2O. The typical synthesis procedure forCo,Al-MCM-41 using tetradecyltrimethylammonium bro-mide was performed as follows: 32 g of tetradecyltrimethy-lammonium bromide and 115 g of water were mixed andstirred for 30 min. Thereafter, 37.4 g of sodium silicate so-lution was added drop wise to the surfactant solution undervigorous stirring. After stirring for another 30 min, the ap-propriate amounts of cobalt acetate tetrahydrate and sodiumaluminate for the desirednSi/(nCo + nAl ) ratio were dis-solved in 5 ml of water, added to the synthesis mixture andthe resulting gel was stirred for another 15 min. Then 2.4 gH2SO4 in 10 g of water was added to the above mixtureto reduce the pH to ca. 11.5. The stirring was continuedfor another 30 min, the resulting gel was transferred intoa polypropylene bottle and kept in an oven at 100◦C for24 h. After cooling to room temperature, the resultant solidwas recovered by filtration, washed with distilled water anddried in an oven at 100◦C for 6 h. Finally, the materialswere calcined in a muffle furnace at 540◦C for 10 h.

2.2. Characterization

The powder X-ray diffraction patterns of the Co,Al-MCM-41 materials were collected on a Siemens D5005diffractometer using Cu K� (λ = 0.154 nm) radiation. Thediffractograms were recorded in the 2θ range from 0.8 to10◦ with a 2θ step size of 0.01◦ and a step time of 10 s.High-resolution thermogravimetric analysis (SETARAMsetsys 16MS) was carried out under nitrogen atmospherein the temperature range 20–600◦C with a heating rate of5◦C/min. Nitrogen adsorption and desorption isothermswere collected at−196◦C on a Quantachrome Autosorb 1sorption analyzer. All the samples were outgassed for 3 h at250◦C under vacuum (P < 10−5 hPa) in the degas port ofthe adsorption analyzer. The specific surface area was calcu-lated using the BET model. The pore size distributions wereobtained from the desorption branch of the isotherm usingthe corrected form of the Kelvin equation by means of theBarrett–Joyner–Halenda method as proposed by Kruk et al.[16]. UV-Vis diffuse reflectance spectra were recorded witha Perkin-Elmer Lambda 18 spectrometer equipped with aPraying–Mantis diffuse reflectance attachment. BaSO4 wasused as reference. Elementary analysis was done by us-ing an Analyst AA 300 spectrometer. X-band EPR spectrawere recorded at−267◦C using an ESP 380E spectrom-eter (Bruker) equipped with an ER 041 XK-D microwavebridge (Bruker) operated with the resonance frequency of9.7 GHz and 100 KHz field modulation. DRIFT spectraof the samples were recorded on a Nicolet (Avatar 360)FT-IR spectrophotometer equipped with a high temperaturevacuum chamber. Approximately 30 mg of the sample wastaken in the sample holder and dehydrated at 400◦C for 6 hunder vacuum (10−5 mbar). The sample was then cooledto room temperature and the spectra were recorded. Thenpyridine was adsorbed at room temperature. The physicallyadsorbed pyridine was removed by heating the sample at150◦C under vacuum (10−5 mbar) for 30 min, after thesample was cooled to room temperature, the spectrum wasrecorded. The same sample was used to record the infraredspectrum in the range 1700–1400 cm−1 (pyridine adsorp-tion region). The number of Bronsted and Lewis acid siteswas calculated by measuring the integrated absorbance ofbands representing pyridinium ion formation and coordi-natively bonded pyridine[17]. The method developed forporous aluminosilicates by Emeis[18] was adopted for thedetermination of the acidity of the catalysts.

2.3. Catalytic studies

The reactor system was a fixed-bed, vertical, downward-flow type made up of a quartz tube of 40 cm length and2 cm i.d. The reactor was heated to the requisite temperaturewith the help of a tubular furnace controlled by a digitaltemperature controller cum indicator. The temperature wasmeasured by a chromal–alumel thermocouple placed insidethe furnace. About 0.5 g of the catalyst was placed in the

M. Karthik et al. / Applied Catalysis A: General 268 (2004) 139–149 141

middle of the reactor and supported on either side with a thinlayer of quartz wool and ceramic beads. The reactants werefed into the reactor using a syringe infusion pump that couldbe operated at different flow rates. The reaction was carriedout at atmospheric pressure. The liquid products collectedin the first 15 min were discarded and the analysis was madeonly with the products collected after this time. After eachcatalytic run, the catalyst was regenerated by passing mois-ture and carbon dioxide free air through the reactor for 6 hat 500◦C. The liquid products were analyzed using a Shi-madzu gas chromatograph (Model: GC-17A) using a DB-5capillary column (30 m) equipped with a flame ionization de-tector (FID) and nitrogen as a carrier gas. The identificationof products was also made using a GC-MS Perkin-ElmerAuto System XL gas chromatograph (Perkin-Elmer eliteseries PE-5 capillary column (30 m× 0.25 mm× 1�m)equipped with a turbo mass spectrometer (EI, 70 eV) withhelium as carrier gas at a flow rate of 1 ml/min.

3. Results and discussion

3.1. Characterization

The well-defined XRD pattern of Co,Al-MCM-41 inFig. 1 is typical of MCM-41 as described by Kresge et al.[2]. All the four XRD reflections (1 0 0), (1 1 0), (2 0 0) and(2 1 0) are well resolved and can be indexed to a hexagonallattice. The (1 1 0) and (2 0 0) reflections for Co,Al-MCM-41(20) are somewhat less well resolved. Upon calcinationof the as-synthesized materials, the XRD patterns ofCo,Al-MCM-41 become better resolved and the intensityof the XRD patterns increased significantly as a result ofremoval of the intercalated organic molecules (template).The unit cell parameters of Co,Al-MCM-41 are given inTable 1. It has been reported that the unit cell parameterincreases with increasing metal content[19,20]. However,we observe that unit cell parameters of all Co,Al-MCM-41samples, though similar to that of Si-MCM-41, are morethan that of Al-MCM-41 (23). The intensity of (1 0 0) peakdecreases with increasing Co and Al content. This may bedue to the formation of non-framework metal oxide speciesinside the mesopores upon calcination. Further, the broad-ening of XRD peaks is increased with increasing Co andAl content, suggesting a decrease in the structural integrity.

Table 1Synthesis and physicochemical parameters of pure siliceous MCM-41 and Co,Al-MCM-41

Catalyst nSi/nAl nSi/nCo ao (nm) ABET (m2/g) dp, BJH (nm) Pore volume (cm3/g)

Gel Product Gel Product

Si-MCM-41 – – – – 3.96 1273 2.40 0.85Al-MCM-41 (23) 23 22 – – 3.63 1210 2.43 0.80Co,Al-MCM-41 (80) 160 142 160 145 3.90 1147 2.49 0.77Co,Al-MCM-41 (50) 100 104 100 93 3.95 1099 2.49 0.74Co,Al-MCM-41 (20) 40 39 40 38 3.91 1015 2.44 0.66

Fig. 1. XRD powder patterns of calcined MCM-41 and Co,Al-MCM-41with different nSi/(nCo + nAl ) ratios: (a) MCM-41; (b) Co,Al-MCM-41(80); (c) Co,Al-MCM-41 (50); (d) Co,Al-MCM-41 (20).

Fig. 2. TGA and DTG analysis curves for as-synthesized purely siliceousMCM-41 and Co,Al-MCM-41 with differentnSi/(nCo + nAl ) ratios (—)MCM-41; (. . . ) Co,Al-MCM-41 (80); (– – –) Co,Al-MCM-41 (50); (–·· –)Co,Al-MCM-41 (20).

Fig. 2 shows TGA and DTG curves obtained for theas-synthesized pure siliceous sample and Co,Al-MCM-41samples with differentnSi/(nCo + nAl ) ratios. There arefour weight losses that appear in the TGA curve. The firstweight loss between 20 and 120◦C is attributed to the re-lease of physisorbed water molecules, it is very low forsiliceous MCM-41 (2.7%) compared to the correspondingmetal substituted analogue (4–5%). This indicates that thehydrophilicity increases with increase of metal incorpora-tion in the framework. The second and third weight losses,


which are centered around 210 and 265◦C, respectively, areattributed to the decomposition of loosely bound organictemplate within the framework. The fourth weight loss iscentered around 390◦C, it increases as thenSi/(nCo + nAl )ratio decreases and develops at the expense of the secondand third weight loss centered at 210 and 265◦C, respec-tively. This could be attributed to the strongly bonded tem-plate cation with the metallosilicate framework. As morealuminum and cobalt atoms are incorporated into the frame-work, more of occluded template molecule is protonated soas to balance the resulting negative charge. This indicatesthat most of the aluminum and cobalt atoms are incorporatedinside the framework of the materials.

The nSi/nAl and nSi/nCo molar ratios of all the samplesunder investigation are summarized inTable 1. It can be seenfrom the table that incorporation of Si, Co and Al into thesolid is in close agreement with the input gel composition.In the case of Co,Al-MCM-41 (80), the Co and Al content inthe solid phase is higher than that of the input gel, suggestingpreferential incorporation of both metal atoms compared tosilicon. Similar observations were previously reported in thecase of zeolites[21].

The nitrogen adsorption isotherms of purely siliceousMCM-41 and Co,Al-MCM-41 samples are shown inFig. 3.All the samples exhibit isotherms of type 1V of IUPACclassification, featuring a narrow step due to the capil-lary condensation of N2 in the primary mesopores. Thesamples containing Co and Al possess an often observedtype-H4 hysteresis loop atp/po between 0.5 and 1. Theassignment of hysteresis is made according to de Boer’sclassification. Such a hysteresis loop has already been ob-served in aluminum containing MCM-41 materials and isattributed to the capillary condensation of nitrogen withininterparticles and/or some impurity phases generated dur-ing synthesis[22]. The incorporation of Co and Al intothe walls of MCM-41 has a significant effect on the spe-cific surface area and specific pore volume of the materials

Fig. 3. Nitrogen adsorption isotherm (adsorption: closed symbols; des-orption: open symbols) of Co,Al-MCM-41 with differentnSi/(nCo + nAl )ratios: (�) Co,Al-MCM-41 (80); (�) Co,Al-MCM-41 (50); (�)Co,Al-MCM-41 (20).

(Table 1). With increasing metal content, the pore volumeis reduced from 0.85 to 0.66 cm3/g and the specific surfacearea declines from 1270 to 1015 m2/g. This is attributed toa slight reduction in the structural integrity of the sampleswith increasing metal content. A similar behavior has alsobeen observed in monometal-substituted MCM-41 materi-als [23,24]. Moreover, it is interesting to note that the porediameter of Co,Al-MCM-41 samples is larger than the puresilica MCM-41. As the unit cell parameter is almost identi-cal for both Si-MCM-41 and Co,Al-MCM-41 samples, theincrease in the pore diameter of Co,Al-MCM-41 samplesindicates the reduction in wall thickness. However, the porediameter of Co,Al-MCM-41 (20) is smaller than those ofCo,Al-MCM-41 (50) and (80). This could be due to theformation of more metal oxide clusters in the mesopores ofCo,Al-MCM-41 (20) than in (50) and (80).

The coordination geometry of cobalt incorporated inAl-MCM-41 is indicated by the UV-Vis absorption spec-tra. Fig. 4 shows the absorption spectra of calcinedCo,Al-MCM-41 samples. It exhibits a strong absorptionaround the 15 500–20 000 cm−1 region. This absorptionconsists of three components, with maxima at 19 685,17 241 and 15 384 cm−1. These bands can be assigned to4A2(F) → 4T1(P) transition of the tetrahedrally coordi-nated divalent cobalt species[25]. A broad band between20 800 and 22 000 cm−1 centered at 21 505 cm−1 is alsoobserved for all the samples. The intensity of this bandincreases monotonically with increasing Co content. Thisband could be assigned to the strong bonding of oxygenligands to Co2+ ions. A similar band has also been reportedin the zeolites with low Si/Al ratio[26]. However, the de-tailed geometry of the Co2+ ion is not clearly known[27].Moreover, a broad band in the UV region between 34 400and 45 500 cm−1 centered at 40 816 cm−1 is also observedfor all samples. This has been assigned to a low-energycharge transfer between the oxygen ligands and centralCo2+ ion in tetrahedral symmetry[28] or to self-absorptionof the molecular sieves[26,27]. All these results reveal that

Fig. 4. UV-Vis spectra of calcined Co,Al-MCM-41 with variousnSi/(nCo+nAl ) ratios: (a) Co,Al-MCM-41 (20); (b) Co,Al-MCM-41 (50);(c) Co,Al-MCM-41 (80).


Fig. 5. ESR spectra of calcined Co,Al-MCM-41 with variousnSi/(nCo+nAl ) ratios: (a) Co,Al-MCM-41 (20); (b) Co,Al-MCM-41 (50);(c) Co,Al-MCM-41 (80).

the majority of the cobalt ions in the Co,Al-MCM-41 sam-ples occupy framework positions in the surface layer of theCo,Al-MCM-41 channel walls.

ESR spectroscopy is a sensitive technique to studythe coordination of high spin Co2+ ions in molecularsieves. The X-band ESR spectrum at−267◦C of calcinedCo,Al-MCM-41samples synthesized with differentnSi/nCoratios in the synthesis gel is shown inFig. 5. The spectrumshows two major components, atg = 5.4 and 2.05. WithincreasingnSi/nCo ratio from 160 to 40, the correspond-ing spectra show ESR signal increases in intensity. TheESR signal atg = 5.4 and 2.05 are attributed to Co2+ inan elongated tetrahedral environment[29]. Similar spectrawere also reported in cobalt-substituted aluminophosphatemolecular sieves[30,31]. It is important to note that the in-tensity of ESR signal in the calcined material is higher thanin the as-synthesized material. The increase in the intensityof ESR signal in the calcined materials indicates that thereis no oxidation of Co2+ to Co3+ during calcination[31].

Fig. 6. DRIFT spectra of pyridine adsorption region of mesoporous materials: (a) Co,Al-MCM-41 (20); (b) Al-MCM-41 (23); (c) Co,Al-MCM-41 (50);(d) Co,Al-MCM-41 (80); (L: Lewis acid; B: Bronsted acid; L+ B: Lewis and Bronsted acid sites).

Table 2Bronsted and Lewis acidity values for Al-MCM-41 and Co,Al-MCM-41

Catalyst Bronsted (B) acidsite (mmol/g)

Lewis (L) acidsite (mmol/g)

B/L acidsite ratio

Al-MCM-41 (23) 0.114 0.121 0.942Co,Al-MCM-41 (20) 0.190 0.284 0.669Co,Al-MCM-41 (50) 0.117 0.107 1.093Co,Al-MCM-41 (80) 0.057 0.026 2.192

The in situ DRIFT spectra of pyridine adsorbed on meso-porous Al-MCM-41 and Co,Al-MCM-41 molecular sievesare shown inFig. 6. The acidity of the catalysts was cal-culated using extinction coefficients of the bands of Bron-sted and Lewis acid sites adsorbed pyridine[18] and theacidity values are given inTable 2. The DRIFT spectra ofchemisorbed pyridine have shown that Co,Al-MCM-41 andAl-MCM-41 contain both Bronsted and Lewis acid sites.The existence of Bronsted acid sites in the samples is clearlyshown by the bands at 1545 and 1636 cm−1 due to ring vibra-tions of pyridine bound to Bronsted acid sites[32,33]. Thebands at 1451, 1493 and 1617 cm−1 are assigned to pyridineassociated with Lewis acid sites[34]. The band at 1493 cm−1

is attributed to pyridine chemisorbed on both Bronsted andLewis acid sites[32,35]. The band at 1400 cm−1 is attributedto pyridine hydrogen bonded to defective Si–OH groups[17,36]. The peak at 1230 cm−1 is assigned to the asymmet-ric T–O–T vibration of the framework[37,38]. Comparisonof spectra reveals that Co,Al-MCM-41 (20) has a greateramount of acid sites (Bronsted and Lewis) than Al-MCM-41(23) as the corresponding bands in the former are more in-tense than those in the latter. The acid sites are found to besame in both Co,Al-MCM-41 (50) and Al-MCM-41 (23),whereas Co,Al-MCM-41 (80) has a smaller amount of acidsites than either Co,Al-MCM-41 (50) or Co,Al-MCM-41(20). The intensity of the bands due to Lewis acid sites inCo,Al-MCM-41 (20) is more than in either Co,Al-MCM-41


(50) or Co,Al-MCM-41 (80). This is ascribed to the forma-tion of more cobalt oxide in Co,Al-MCM-41 (20) upon cal-cination. It is obvious that, with increasing cobalt contentin Al-MCM-41, the number of surface acid sites increases.This study reveals that the acidity of the catalysts decreasesin the order Co,Al-MCM-41(20) > Al-MCM-41 (23) ≥Co,Al-MCM-41 (50) > Co,Al-MCM-41 (80). Hence, it isconcluded that the isomorphic substitution of Co2+ into theframework of mesoporous Al-MCM-41 creates more acidsites (Bronsted and Lewis) and enhances the catalytic activ-ity of Co,Al-MCM-41.

3.2. Catalytic activity

Alkylation of phenol with isobutanol was studied overCo,Al-MCM-41 (20, 50 and 80) and Al-MCM-41 (23) at200, 250, 300, 350, 400, 450 and 500◦C by co-feedingphenol and isobutanol in the feed ratio of 1:3 and feedrate of 3 ml/h. The plot of conversion versus temperatureis shown in Fig. 7. There is a nearly linear increase ofphenol conversion with temperature over all the catalysts.It can be seen from the figure that the butylation activityin Co,Al-MCM-41 (20) is more than in other catalysts.Al-MCM-41 (23) possesses less density of acid sites thanCo,Al-MCM-41 (20), as isomorphic substitution of Al3+by Co2+ would provide two Bronsted acid sites, and hencethe former shows less activity than the latter. As the reac-tion requires formation of isobutyl cation in order to haveelectrophilic attack on either the chemisorbed phenol or thefree phenol, the density of Bronsted acid sites, which arethe active sites in this reaction, is important. As the densityof acid sites in Co,Al-MCM-41 decreases in the followingorder: Co,Al-MCM-41 (20) > Co,Al-MCM-41 (50) >

Co,Al-MCM-41 (80), the conversion also follows the sametrend as that shown inFig. 7. The density of acid sites are

Fig. 7. Effect of temperature on conversion of phenol. Phenol:isobutanol—1:3; feed rate—3 ml/h; catalyst weight—0.5 g. Conversionof phenol—(�) Co,Al-MCM-41 (20); (�) Co,Al-MCM-41 (50); (�)Co,Al-MCM-41 (80); (�) Al-MCM-41 (23).

in good agreement with DRIFT (pyridine adsorption) mea-surements. The activity of Co,Al-MCM-41 (80) is expectedto be less than Al-MCM-41 (23), but the results indicate thereverse trend. Hence, in addition to acidity, the associatedhydrophilic and hydrophobic properties of the catalysts arealso important factors to account for the difference in theactivity of the catalysts. Though Al-MCM-41 (23) pos-sesses more density of acid sites, it is less hydrophobic thanCo,Al-MCM-41 (80). Hence, the adsorption of hydropho-bic isobutanol on the active sites is to be less and for thisreason the former gives less conversion than the latter. Thistrend is similar to the previously reported hydrophilic andhydrophobic properties of molecular sieves in acetalyzation[35] and isopropylation ofm-cresol[37]. In a similar way,Co,Al-MCM-41 (20) can be expected to give less conversionthan even Al-MCM-41 (23), but it gives more conversion.There might be yet another route to yield isobutyl carbo-nium ion in addition to the above Bronsted acid-assistedroute. The formation of non-framework cobalt oxide in thechannels of MCM-41 may provide an alternate route to yieldtert-butyl cations as shown in reaction (Scheme 1). Suchdissociative adsorption of phenol on the Lewis acid sites,so that the released proton can rest on the adjacent basicsite, has already been proposed by many workers[39,40].Isobutyl alcohol should be chemisorbed on the Bronstedsite in order to yield isobutyl cation. This cation without orwith rearrangement is to react with the adjacent phenoxideto yield O-or C-alkylated product, as shown in the reac-tion (Scheme 1(a)–(c)). Based on the conversion, the orderof activity of Co,Al-MCM-41 catalysts is Co,Al-MCM-41(20) > Co,Al-MCM-41 (50) > Co,Al-MCM-41 (80),which is also the order of the amount of non-frameworkcobalt oxide in the pores of MCM-41. These particles arealso expected to be of nanodimensions close to 1 nm, so thatthe reactants and products can very well diffuse in and out ofthe pores of the catalysts without any diffusional constraints.

In general, alkylation of phenol is a reaction sensitive tothe acid–base properties of the catalysts employed. It hasbeen observed thatO-alkylation of phenol is favored bystrong acid sites[41], while C-alkylation is favored by weakacidic (or) strong basic sites[42]. Tleimaat-Manzaliji et al.[43] and Marczewski et al.[44] have claimed that weakacids sites favorC-alkylation, while Velu and Swamy[45]reported thatC-alkylation occurred due to the presence ofhigher acidity. The formation ofO-alkylated products de-pends on the intrinsic properties of the alcohol and acid–baseproperties of the catalysts[46]. Based on the above obser-vations, we have also tried to explain the formation of prod-ucts as a function of acidity and activity of the catalysts forthe t-butylation of phenol.

Table 3shows the selectivity of the products obtained overthe catalysts in the temperature range 350–500◦C. The prod-ucts were identified by GC and GC-MS. The main productsof the catalytic butylation of phenol wereO-tert butyl phe-nol (OTBP), 2-tert-butyl phenol (2TBP), 4-tert-butyl phe-nol (4TBP),O-butenyl phenol (OBP) and 2-butenyl phenol


Scheme 1.

Table 3Effect of temperature ont-butylation of phenol over Al-MCM-41 and Co,Al-MCM-41

Catalysta Temperature (◦C) Conversion (wt.%) Selectivity of products (wt.%)

OTBP 2TBP 4TBP OBP 2BP Others

Al-MCM-41 (23) 350 13.54 31.21 50.31 10.03 – – 7.21400 19.43 26.60 53.43 10.47 – – 9.40450 24.32 25.43 47.21 13.04 – – 14.23500 37.52 19.57 44.72 10.10 – – 25.01

Co,Al-MCM-41 (20) 350 32.71 43.24 14.26 12.10 30.20 – –400 44.34 37.65 13.40 13.43 20.24 9.10 6.06450 50.24 30.72 14.01 12.12 18.31 10.03 14.08500 48.98 17.21 22.09 11.01 16.03 13.12 20.21

Co,Al-MCM-41 (50) 350 21.22 56.81 14.06 5.62 22.98 – –400 34.14 42.02 18.79 8.74 20.18 7.68 2.24450 42.08 31.41 20.12 9.55 22.10 8.01 8.21500 46.19 20.67 24.22 10.52 18.24 10.21 16.14

Co,Al-MCM-41 (80) 350 19.94 80.24 6.92 – 12.14 – –400 26.82 69.85 11.58 – 18.26 – –450 34.28 52.26 12.01 6.94 19.24 2.10 7.12500 43.34 39.52 13.44 10.22 20.52 4.84 10.75

Phenol:isobutanol feed ratio: 1:3; feed rate: 3 ml/h.a Catalyst weight: 0.5 g.


Scheme 2.

(2BP). Small amounts of other products were also detected,which were unidentified polymerized products. The forma-tion of products witht-butyl group evidently proves rapidisomerization of isobutyl cation intot-butyl cation on thecatalyst surface. The formation of OBP and 2BP has alsobeen reported by earlier workers[47]. The proposed routeas shown inScheme 2, which is a little different from thatgiven by previous workers. But isobutyraldehyde was notobserved in present study. It is visualized that, as and whenisobutyraldehyde is formed, it may react immediately withphenol to yield the products. Selectivity to ring alkylationover Al-MCM-41 (23) is found to be more than that toO-alkylation. Generally, the electrophilic substitution maytake place in theortho andpara positions of the phenyl ring

[46,48]. As there is more probability forortho position thanpara position for electrophilic reactions between phenol inthe vapor phase andtert-butyl cation adsorbed on the cat-alyst surface, more selectivity to 2TBP is observed. More-over, the presence of phenolic group kinetically favorsorthoalkylation[49]. It is clearly seen fromTable 3that OTBP isone of the main products of the reaction between phenol andisobutanol over Co,Al-MCM-41 catalysts. Co,Al-MCM-41(80) gives about 80% selectivity to OTBP. This suggests thatScheme 1(a) is the most probable one for the high selectiv-ity of OTBP. When phenol is dissociatively adsorbed on theLewis acid site, the plane of the aromatic ring is to be in-clined towards the catalyst surface. Hence, if the ring is ro-tated about the cobalt-phenol oxygen bond axis and brought


close to the adjacently adsorbedtert-butyl cation, there couldbe much steric hindrance forpara position. Hence, in theLewis acid-adsorbed state of phenol, the phenoxide is to re-act nucleophilically withtert-butyl cation to yield OTBP.The selectivity of OTBP over Co,Al-MCM-41 (80) is higherthan that over Co,Al-MCM-41 (50) and Co,Al-MCM-41(20) due to lower density of acid sites. However, a decreasein the selectivity of OTBP is observed over all the catalystswith an increase in temperature, as it is thermally unstable.

It can be seen fromTable 3that the selectivity to 2TBPincreases with increase in temperature over all the Co,Al-MCM-41 catalysts. But the selectivity over Co,Al-MCM-41(80) is much less than over other catalysts, this may be dueto the much lower density of Bronsted acid sites availableon Co,Al-MCM-41 (80). Similarly, Co,Al-MCM-41 (80)with a lower density of Bronsted acid sites gives less yieldof 4TBP, whereas Co,Al-MCM-41 (50) and Co,Al-MCM-41 (20) give more selectivity due to a greater density of acidsites. The selectivity to OBP increases with decrease in thenSi/(nCo+ nAl ) ratio of the catalysts. But this product is notobserved over Al-MCM-41 (23). Hence, the dehydrogena-tion of isobutanol to isobutyraldehyde and its subsequentreaction with free phenol to give OBP and/or 2BP dependson the amount of non-framework cobalt oxide inside thepores of the catalysts (Scheme 2). Formation of this prod-uct only over Co,Al-MCM-41 and not over Al-MCM-41evidently proves the reaction is Lewis-acid dependent; theformer catalysts have been proved to have non-frameworkcobalt oxide. It has also been reported in the literature thatthe modes of adsorption of phenol on catalytic surfaces alsoplay an important role in the formation ofO-butenyl phenol[50].

Table 4Effect of feed rate ont-butylation of phenol over Al-MCM-41 and Co,Al-MCM-41

Catalysta Feed rate (ml/h) Conversion (wt.%) Selectivity of products (wt.%)


Al-MCM-41 (23) 1 9.04 37.21 59.63 3.16 – – –2 15.21 31.91 53.74 9.82 – – 4.213 19.43 26.60 53.43 10.47 – – 9.404 12.86 20.58 47.98 19.62 – – 11.32

Co,Al-MCM-41 (20) 1 34.42 43.03 10.08 10.14 24.19 12.17 –2 40.18 40.08 11.21 12.10 22.84 10.72 2.643 44.30 37.65 13.43 13.41 20.24 9.10 6.064 32.81 30.82 12.10 21.16 18.89 10.21 6.12

Co,Al-MCM-41 (50) 1 28.74 47.16 12.21 6.18 22.28 11.87 –2 30.12 44.12 15.81 7.91 21.88 10.21 –3 34.14 42.02 18.79 8.74 20.18 7.68 2.244 23.16 35.72 17.81 17.10 20.02 7.90 1.04

Co,Al-MCM-41 (80) 1 18.42 71.21 7.51 – 19.24 2.04 –2 22.18 70.77 7.83 – 18.42 2.98 –3 26.82 69.85 11.58 – 18.26 – –4 15.46 69.62 12.07 – 18.21 – –

Phenol:isobutanol feed ratio: 1:3; temperature: 400◦C.a Catalyst weight: 0.5 g.

Fig. 8. Effect of feed ratio and feed rate on conversion of phenol overCo,Al-MCM-41 (20). Catalyst weight—0.5 g; temperature—400◦C. Con-version of phenol in the feed ratio (phenol:isobutanol)—(�) 1:2; (�)1:3; (�) 1:4.

The effects of feed ratio and feed rate on conversion werestudied at 400◦C over Co,Al-MCM-41 (20). The feed ratioswere kept at 1:2, 1:3 and 1:4. The variation in conversionwith feed rate for each feed ratio is illustrated inFig. 8. In-crease in conversion with increase in feed rate up to 3 ml/hfor feed ratios 1:2 and 1:3 may be attributed to the force ex-erted on the reactants to move closer to the channel surface,which could facilitate their chemisorption. Above this feedrate (3 ml/h) the conversion decreases, which may be due tothe fast diffusion of the reactants. The feed ratio 1:4 gives agradual decrease in conversion with increase in feed rate.

Table 4 shows the effect of feed rate on phenol con-version and product selectivity over Al-MCM-41 and


Table 5Effect of time on stream ont-butylation of phenol over Al-MCM-41 and Co,Al-MCM-41

Catalysta Time on stream (h) Conversion (wt.%) Selectivity of products (wt.%)


Al-MCM-41 (23) 1 19.43 26.60 53.43 10.47 – – 9.402 15.82 24.26 47.21 13.13 – – 15.393 10.14 20.14 44.80 15.81 – – 22.514 8.26 15.82 40.28 18.20 – – 25.645 7.12 13.21 37.12 19.70 – – 29.96

Co,Al-MCM-41 (20) 1 44.34 37.65 13.40 13.43 20.24 9.10 6.062 39.45 33.12 12.22 16.08 18.22 10.12 10.213 30.47 29.34 11.36 16.89 17.81 13.24 11.364 25.89 22.83 9.98 24.17 15.88 15.16 11.985 20.84 19.16 9.02 29.54 14.02 15.82 12.44

Co,Al-MCM-41 (50) 1 34.14 42.02 18.77 8.74 20.18 7.68 2.242 34.10 40.16 17.24 13.14 18.36 7.92 3.183 31.68 38.44 15.98 15.09 17.12 8.14 5.234 28.36 35.28 13.22 17.83 16.82 8.97 7.885 23.65 31.17 12.81 22.70 15.72 9.39 8.21

Co,Al-MCM-41 (80) 1 26.82 69.85 11.58 – 18.26 – –2 26.76 69.81 11.02 – 18.21 – –3 26.05 69.68 10.96 – 18.20 – –4 25.86 68.94 10.54 – 18.00 – 2.525 25.34 68.51 10.06 – 17.98 – 3.00

Phenol:isobutanol feed ratio: 1:3; feed rate: 3 ml/h; temperature: 400◦C.a Catalyst weight: 0.5 g.

Co,Al-MCM-41. The reaction was carried out at differentfeed rates at 400◦C with feed ratio 1:3. An appreciableincrease in phenol conversion is observed up to the feedrate of 3 ml/h, above which the conversion decreases dueto the fast diffusion of the reactants through the catalyst[51,52]. On the other hand, the low conversion at low feedrate could be attributed to coke formation due to more con-tact time of the catalyst[51]. It is observed that increase inthe feed rate decreases the selectivity of OTBP, while the4TBP selectivity increases. A similar increase in selectivityof 4TBP has been reported in thetert-butylation of phenolover H-AlMCM-41 [51]. The selectivity values of OBP and2BP are observed to remain the same with increase in thefeed rate.

The time on stream study was conducted to assess itseffect on conversion and selectivity of the products. Thetemperature was maintained at 400◦C and the feed ra-tio was set at 1:3 with a feed rate of 3 ml/h. Al-MCM-41 (23), Co,Al-MCM-41 (20), Co,Al-MCM-41 (50) andCo,Al-MCM-41 (80) were examined; the results are pre-sented inTable 5. There is a decrease in conversion overAl-MCM-41 (23), Co,Al-MCM-41 (20) and Co,Al-MCM-41 (50) with increase in the stream. This is attributed tothe formation of coke and subsequent blocking of activesites. Co,Al-MCM-41 (20) and Al-MCM-41 (23) are morerapidly deactivated than Co,Al-MCM-41 (50) due to moreacid sites[53,54]. Furthermore, the product selectivity alsochanges as a consequence of deactivation. With increase instream, the selectivity of 4TBP and 2BP gradually increases,

which is accompanied by decreases in the selectivity ofOTBP, 2TBP and OBP. The increase in the selectivity of4TBP with increase in stream has already been reported in[55,56]. The increase in the selectivity of 2BP with increasein stream is attributed to enhancement in the transport ofisobutanol to cobalt oxide particles of specific dimension.It is assumed that only such particles encourage dehydro-genation of isobutanol to isobutyraldehyde, which resultsin high selectivity of 2BP. The deactivation rate of the cat-alysts is in the order Co,Al-MCM-41(20) > Al-MCM-41(23) > Co,Al-MCM-41 (50) > Co,Al-MCM-41 (80). Thecatalytic activity of Co,Al-MCM-41 (80) is not affectedeven at the end of 5 h, due to a lower density of acid sites.

4. Conclusion

Cobalt containing mesoporous aluminoslicate molec-ular sieves that contain tetrahedrally coordinated cobaltatoms has been hydrothermally synthesized. A series ofCo,Al-MCM-41 with nSi/(nCo + nAl ) ratios from 20 to 80was synthesized successfully and characterized by physic-ochemical methods such as XRD, nitrogen adsorption,UV-Vis DRS, ESR, AAS, DRIFT (pyridine adsorption) andTGA/DTG. UV-Vis DRS and ESR studies reveal that cobaltoccurs in Co,Al-MCM-41 is highly symmetrical tetrahedralcoordination and that some of the cobalt atoms are trans-formed into the oxide form when thenSi/(nCo+nAl ) ratio isincreased to 20. The adsorbed pyridine measured by FT-IR


indicates the presence of both Bronsted and Lewis acid sties.Alkylation of phenol with isobutanol over Al-MCM-41 (23)and Co,Al-MCM-41 catalysts shows an increase in conver-sion with increase in temperature. Isomerization of isobutylcation to tert-butyl cation is very evident in this study.Co,Al-MCM-41 (20) is more active than other catalysts.The activity of Al-MCM-41 and Co,Al-MCM-41 catalystsfollows the order: Co,Al-MCM-41(20) > Co,Al-MCM-41(50) > Co,Al-MCM-41 (80) > Al-MCM-41 (23). Theselectivity ofO-tert-butyl phenol is found to be more thanthat of other products. Formation of non-framework cobaltoxide, which is evident in the spectral studies, is expectedto give more selectivity toO-butenyl phenol. The interest-ing observation with Co,Al-MCM-41 (80) is its uniformactivity with increase of time.

Acknowledgements

The authors gratefully acknowledge the financial supportfrom BRNS-DAE, Mumbai (Sanction No. 98/37/23/BRNS/Cell/720), for this research work. The authors M.K. andA.V. are thankful to BRNS-DAE, Mumbai, for the researchfellowship.

References

[1] J.S. Beck, US Patent 5,057,296 (1991).[2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck,

Nature 359 (1992) 710.[3] X.S. Zhao, G.Q. Lu, G.J. Millar, Ind. Eng. Chem. Res. 35 (1996)

2075.[4] L. Mercier, T.J. Pinnavaia, Adv. Mater. 9 (1997) 500.[5] A. Corma, Chem. Rev. 97 (1997) 2373.[6] J.M. Thomas, Faraday Discuss. 105 (1996) 1.[7] H. Knozinger, S. Huber, J. Chem. Soc., Faraday Trans. 94 (1998)

2047.[8] P.T. Tanev, M. Chibwe, T.J. Pinnavaia, Nature 368 (1994) 321.[9] A. Tuel, Micropor. Mesopor. Mater. 27 (1999) 151.

[10] S. Lim, G.L. Haller, Appl. Catal. A: Gen. 188 (1999) 277.[11] D.B. Akolekar, Catal. Lett. 28 (1994) 249.[12] B. Elvers, S. Hawkins, G. Schulz (Eds.), Ullmann’s Encylopedia of

Industrial Chemistry, 5th ed., A.19, (1991) 325–341.[13] V.A. Koshchii, B.Ya. Kozlikovskii, A.A.Zh. Matyusha, Org. Khim.

24 (7) (1989) 1508.[14] A.R. Rajadhyaksha, D.D. Chaudhari, Ind. Eng. Chem. Res. 26 (7)

(1987) 1276.[15] H.C. Kuizhang, H. Zhang, S. Xiang, S. Liu, D. Xu, H. Li, Appl.

Catal. A: Gen. 166 (1998) 89.[16] M. Kruk, M. Jaroniec, A. Sayari, Langmuir 13 (1997) 6267.[17] E.P. Parry, J. Catal. 2 (1963) 371.[18] C.A. Emeis, J. Catal. 141 (1993) 347.[19] C.F. Cheng, J. Klinowski, J. Chem. Soc., Faraday Trans. 92 (1996)

289.[20] N. Ulagappan, C.N.R. Rao, J. Chem. Soc. Chem. Commun. (1996)

1047.

[21] E.G. Derouane, S. Detremmerie, Z. Gabelica, N. Blom, Appl. Catal.1 (1981) 201.

[22] Q. Huo, R. Leon, P.M. Petroff, G.D. Stucky, Science 268 (1995)1324.

[23] M. Kruk, M. Jaronice, A. Sayari, Langmuir 15 (1999) 5683.[24] Y.W. Chen, Y.H. Lu, Ind. Eng. Chem. Res. 38 (1999) 1893.[25] J. Šponer, J. Èejka, J. Dı̀deèek, B. Wichterlová, Micropor. Mesopor.

Mater. 37 (2000) 117.[26] J. D̀ıdeèek, D. Kaucky, B. Wichterlova, Micropor. Mesopor. Mater.

35/36 (2000) 483.[27] D. Kaucky, J. D̀ıdeèek, B. Wichterlova, Micropor. Mesopor. Mater.

31 (1999) 75.[28] R.A. Schoonheydt, in: F. Delannay (Ed.), Diffuse Reflectance Spec-

troscopy Characterization of Catalysis, Marcel Dekker, New York,1984, p. 220.

[29] S. Thomson, V. Luca, R. Howe, Phys. Chem. Chem. Phys. 1 (1999)615.

[30] B.M. Weckhuysen, A.A. Verberckmoes, M.G. Uytterhoeven, F.E.Mabbs, D. Collison, E.D. Boer, R.A. Schoonheydt, J. Phys. Chem.B 37–42 (2000) 104.

[31] V. Kurshev, L.Kevan, D. Parillo, G.T. Kokotailo, R.J. Gorte, J. Phys.Chem. 98 (1994) 10160.

[32] A. Corma, V. Fornes, M.T. Navarro, J. Perez-Pariente, J. Catal. 148(1994) 569.

[33] B. Chakraborty, B. Viswanathan, Catal. Today 49 (1999) 253.[34] M.L. Occelli, S. Biz, A. Aurous, G.J. Ray, Micropor. Mesopor. Mater.

26 (1998) 193.[35] M.J. Climent, A. Corma, S. Iborra, S. Miquel, J. Primo, F. Rey, J.

Catal. 76 (1999) 783.[36] A. Corma, Chem. Rev. 95 (1995) 559.[37] V. Umamaheswari, M. Palanichamy, V. Murugesan, J. Catal. 210

(2002) 367.[38] S. Biz, M.L. Occelli, Catal. Rev. Sci. Eng. 40 (3) (1998) 329.[39] M. Bowker, R.W. Pelts, K.C. Waugh, J. Catal. 99 (1986) 33.[40] T. Yashima, H. Suzuki, N.J. Hara, J. Catal. 33 (1974) 486.[41] V.V. Rao, K.V.R. Chary, V. Durga Kumari, S. Narayanan, Appl.

Catal. 61 (1990) 89.[42] V. Durga Kumari, G. Sreekanth, S. Narayanan, Res. Chem. Intermed.

14 (1990) 223.[43] R. Tleimaat-Manzaliji, D. Bianchi, G.M. Pajonk, Appl. Catal. A:

Gen. 101 (1993) 339.[44] M. Marczewski, J.P. Bodibo, G. Perot, M. Guisnet, J. Mol. Catal.

50 (1989) 211.[45] S. Velu, C.S. Swamy, Appl. Catal. A: Gen. 145 (1996) 141.[46] E. Dumitriu, V. Hulea, J. Catal. 218 (2003) 249.[47] A.H. Padmasri, V. Durga Kumari, P. Kanta Rao, Recent

Trends in Catalysis, Narosa Publishing House, New Delhi, 1999,pp. 189–196.

[48] R.T. Parton, J.M. Jacobs, D.R. Hoybrechts, P.A. Jacobs, Stud. Surf.Sci. Catal. 46 (1998) 163.

[49] A.J. Kolka, J.P. Napolitano, G.G. Elike, J. Org. Chem. 21 (1956) 712.[50] A.H. Padmasri, V. Durga Kumari, P. Kanta Rao, Stud. Surf. Sci.

Catal. 113 (1998) 563.[51] A. Sakthivel, S.K. Badamali, P. Selvam, Micropor. Mesopor. Mater.

39 (2000) 457.[52] J.W. Yoo, C.W. Lee, H.C. Jeong, Y.K. Park, S.E. Park, Catal. Today

60 (2000) 255.[53] V. Umamaheswari, M. Palanichamy, B. Arabindoo, V. Murugesan,

Indian J. Chem. 39A (2000) 1241.[54] Z. Liu, P. Moreau, F. Fajula, Appl. Catal. A 159 (1997) 305.[55] S.K. Badamali, A. Sakthivel, P. Selvam, Catal. Lett. 65 (2000)

153.[56] A. Sakthivel, N. Saritha, P. Selvam, Catal. Lett. 72 (2001) 225.

characterization of co,al-mcm-41 and its activity in the t-butylation of phenol using isobutanol

Documents