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Novel mesoporous solid superacids for selective C-alkylation of m-cresol with tert-butanol
Microporous and Mesoporous Materials 89 (2006) 16–24 www.elsevier.com/locate/micromeso
Novel mesoporous solid superacids for selective C-alkylation of m-cresol with tert-butanol Ganapati D. Yadav *, Ganesh S. Pathre Department of Chemical Engineering, University Institute of Chemical Technology (UICT), University of Mumbai, Matunga, Mumbai 400 019, India Received 2 May 2005; received in revised form 30 July 2005; accepted 30 July 2005 Available online 18 November 2005
Abstract tert-Butylated phenols, which are important precursors in a variety of industries, are usually manufactured by reacting phenol in the presence of liquid acid catalysts, with pure isobutylene or C4 fraction from naphtha crackers containing isobutylene. These processes suffer from problems associated with the use of highly corrosive liquid acids and also the source of isobutylene. In the current study, alkylation of m-cresol was studied with tert-butyl alcohol (TBA) using sulphated zirconia and three novel superacidic catalysts, namely, UDCaT-4, 5 and 6. The new catalysts UDCaT-4, UDCaT-5 and UDCaT-6 are modified versions of zirconia. Amongst these, UDCaT-5, with 9% w/w sulphate content, was found to be the most active. Effects of different parameters such as, speed of agitation, catalyst loading, reactant ratio (mole ratio of m-cresol to that of TBA), effect of temperature and the recycle of catalyst are discussed. The conversion of m-cresol and the selectivity for mono C-alkylated product at a m-cresol to TBA mole ratio of 3:1, using 0.03 g/cm3 UDCaT-5 at 120 C under autogenous pressure, were 89% and 90%, respectively. The reaction was carried out without using a solvent to make the process greener and cleaner. The selectivity for the 2-tert-butyl-5-methylphenol was 93%. The reaction mechanism is discussed. A second order rate equation fits the data well. The apparent activation energy was determined as 9.04 kcal/mol. 2005 Elsevier Inc. All rights reserved. Keywords: Alkylation; m-Cresol; Mesoporic superacidic catalysts; UDCaT-4; UDCaT-5; UDCaT-6; Chlorosulphonic acid; Green process; Selectivity; 1-Cyclohexyloxybenzene
1. Introduction Friedel–Crafts reactions, in general, are carried out using highly corrosive liquid acids, which pose loss of yield, corrosion of equipment and post-treatment pollution problems. Alkylation reaction of m-cresol with tert-butyl alcohol is an important reaction both in organic synthesis and chemical manufacturing. The alkylated m-cresols are used as raw materials for the manufacturing of a variety of resins, durable sur* Corresponding author. Tel.: +91 22 2410 2121/2414 5616; fax: +91 22 2414 5614. E-mail addresses: [email protected], [email protected] (G.D. Yadav).
1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.07.047
face coatings, varnishes, wire enamels, printing inks, surface-active agents, rubber chemicals, antioxidants, fungicides, petroleum additives, ultraviolet absorbers, and heat stabilizers for polymeric materials [1–3]. tert-Butylated phenols are generally prepared by reacting phenol with pure isobutylene gas or C4 fraction of naphtha by using a liquid acid catalyst, giving wide product distribution. The use of highly corrosive and polluting liquid acids is a major environmental problem and therefore solid acids are much sought after. Both the formation of O- and C-alkylated products is possible depending on reaction conditions such as reaction temperature and type of catalyst used. Catalysts with strong acidic sites or reaction at high reaction temperature, typically over 80 C, lead to the formation
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
17
Nomenclature A aP B P Ci CV-Si D De Dij dP E K
TBA surface area of catalyst (cm) m-cresol isobutene concentration of species i (mol/cm3) concentration of vacant sites of type i tert-butylated m-cresol effectivity diffusivity (cm2/s) diffusivity of i in j (cm2/s) diameter of catalyst particle (cm) di-tert butyl ether adsorption/mass transfer/reaction rate constant
of C-alkylated products [4] while catalysts with weak acidic sites or reaction at low temperature lead to the formation of O-alkylated products [5,6]. 2-tert-Butyl5-methylphenol is a precursor for a number of commercially important antioxidants and a light protection agent of the bisphenol and thiobisphenol type. 2-tertButyl-5-methylphenol is also used to prepare important UV absorber, and musk ambrette which is a perfume fixation agent [2]. Stevens [7] prepared tert-butylated m-cresol from isobutene as an alkylating agent with cresol:isobutene mole ratio of 0.7:1, whereas Gehlawat and Sharma [8] reported the same reaction by using H2SO4 with a mole ratio of 1 where maximum yield of 80% was obtained. 2-tert-Butyl-5-methylphenol was also prepared by transbutylation of m-cresol with 4,6di-tert-butyl-3-methylphenol at 80–100 C in the presence of an acid treated clay with difficulty [9]. This isomer was also synthesized by controlled debutylation of 4,6-di-tert-butyl-3-methylphenol by heating with the phenolates of aluminium [10] and zirconia [11,12] (Table 1). There is a tremendous scope for devising a new catalytic process for the synthesis of tert-butylated cresols to replace conventional homogenously catalyzed, highly polluting processes. Besides, due to the problems associated with unavailability, transportation and handling of isobutylene, particularly for usage in low-tonnage fine and speciality chemical industry (typically 10–100 TPA production), it is advantageous
M ri RP S Shi t W Xi
mole ratio rate of reaction of species i (mol/cm3 s) radius of catalyst particle (cm) vacant catalyst sites Sherwood number of species i time (min) water conversion of species i
Subscripts SR surface reaction T total catalyst sites 0 initial concentration
to generate isobutylene in situ. Dehydration of tertbutanol is an attractive source for the same. Further, tert-butanol is available as a by-product in the ARCO process for propylene oxide which could be used effectively for this purpose. We have successfully carried out tert-butylation of several aromatic compounds by using tert-butanol, methyl-tert-butyl ether (MTBE) and isobutene as alkylating agents using ecofriendly solid acid catalysts [13–17]. The only problem which needs to be considered was activity and stability of solid catalysts in the presence of water. Our laboratory has been engaged in preparing several solid acid catalysts to make the processes environmental friendly by using them in industrially important reactions. In particular, sulphated zirconia has been extensively studied in a number of reactions [18–31] and it was believed that this solid superacid should be modified to bring in shape selectivity, mesoporosity and better acidity. Recently, we prepared novel solid superacids named as UDCaT-4, UDCaT-5 and UDCaT-6 which have found a great potential for industrially important reactions [29,32,33]. The acronym UDCaT symbolizes the University Department of Chemical Technology (UDCT), which is now renamed as University Institute of Chemical Technology (UICT), Mumbai. The work summarizes the investigation of activity and selectivity of these novel materials in alkylation of m-cresol with tert-butanol, including kinetics and mechanism.
Table 1 Published literature on preparation of 2-tert-butyl-5-methylphenol Reactants
Temperature, C
Catalyst
Mole ratio
Conversion, %
Ref.
Cresol:isobutene Cresol:isobutene m-Cresol and 4,6-di-tert-butyl-3-methylphenol 4,6-Di-tert-butyl-3-methylphenol
80 80 80–100 280
H2SO4 H2SO4 Acid treated clay Phenolates of aluminium and zirconia
0.7:1 1:1 – –
– 80 – –
[7] [8] [9] [10–12]
18
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
2. Experimental 2.1. Chemicals and catalysts tert-Butanol and m-cresol were procured from E. Merck (India) Ltd., Mumbai. Zirconium oxychloride and aqueous ammonia solution were procured from M/s. s. d. Fine Chemicals Ltd., Mumbai. Hexadecyl amine and chlorosulphonic acid were purchased from Spectrochem Ltd., Mumbai, India. Tetraethyl orthosilicate (TEOS) was procured from Fluka, Germany. Sulphated zirconia [30,31], UDCaT-4 [32] and UDCaT-6 [33] were prepared by well-developed procedures and characterized in our laboratory. Since UDCaT-5 was the most efficient catalyst, details of its method of preparation and characterization are given. 2.2. Preparation of catalysts UDCaT-5 was prepared adding aqueous ammonia solution to zirconium oxychloride (ZrOCl2 Æ 8H2O) solution at pH of 9–10. The precipitated zirconium hydroxide so obtained was washed with deionized water until a neutral filtrate was obtained. The absence of chlorine ion was detected by AgNO3 tests. A material balance on Cl before and after precipitation and washing shows no retention of Cl on the solid. Zirconium hydroxide was dried in an oven for 24 h at 100 C and was crushed to 100 mesh size. Zr(OH)4 was then immersed in 15 cm3/ g of 0.5 M solution of chlorosulphonic acid and ethylene dichloride. All materials were immersed for 5 min in the solution and then without allowing moisture absorption were kept in an oven and then they were heated to evaporate the solvent, and then to a temperature of 120 C after about 30 min. These solids were kept in an oven at 120 C for 24 h and calcined at 650 C for 3 h to get the active catalyst UDCaT-5. 2.3. Characterization of UDCaT-5 catalyst UDCaT-5 is completely characterized by XRD, BET surface area and FTIR and the details were published recently by us [29]. Only a few salient features are reported here. Ammonia-TPD was used to determine the acid strength of UDCaT-5. Ammonia-TPD analysis shows that apart from intermediate and strong acidic sites present in sulphated zirconia, UDCaT-5 also contains superacidic sites. Elemental analysis shows that UDCaT-5 contains 9% sulphate content which is highest when compared to that reported so far. IR spectroscopy confirms that chlorosulphonic acid is decomposed during calcination at 650 C and sulpate ions are retained on the surface of UDCaT-5 and that is why sulphur content is higher in UDCaT5. XRD diffraction study proved that the tetragonal
phase in the UDCaT-5 is preserved. From BET surface area and pore size analysis, it was seen that surface area of UDCaT-5 decreases abruptly at maximum sulphur content. This is due to the migration of sulphate ions from bulk phase to zirconia matrix. Thus maximum sulphur present on the surface decreases the surface area of UDCaT-5. All this proves that UDCaT-5 is superacidic in nature due to maximum sulphur content present on the zirconia matrix with preservation of tetragonal phase of zirconia. 2.4. Apparatus and procedures All experiments were carried out in a 100 cm3 stainless steel Parr autoclave. A four bladed-pitched turbine impeller was used for agitation. The temperature was maintained at ±1 C of the desired value. Known quantities of reactants and catalyst were charged into the autoclave, the temperature raised to the desired value and agitation started. Then, an initial sample was withdrawn. Further samples were drawn at periodic intervals up to 2 h. A standard experiment consisted of 0.29 mol m-cresol, 0.097 mol TBA and a catalyst loading of 0.03 g/cm3 of total volume of the liquid. The temperature was maintained at 120 C and the speed of agitation at 1000 rpm. The reaction was carried out without using any solvent. The total volume of the liquid phase was 40 cm3. 2.5. Method of analysis Clear liquid samples were withdrawn at regular intervals by reducing the speed of agitation momentarily to zero and allowing the catalyst to settle at the bottom of the reactor. The samples were analyzed on a Chemito Model 8510 GC equipped with a 10% SE-30 column (3.175 mm diam. · 4 m length). A standard calibration method with synthetic mixtures was used for quantification of data. The reaction products were confirmed by GC–MS and authentic samples.
3. Result and discussion 3.1. Choice of catalysts Zirconia based solid acids are attracting much attention in recent years. Sulphated zirconia is proved to be a highly active versatile solid acid catalyst in a number of reactions such as isomerization [17,18], condensation [19], etherification [20], acylation [21], esterification [22], nitration [23], oligomerization [24] and alkylation [25–28]. But poor stability of sulphated zirconia in the presence of water, low surface area and a tendency to undergo deactivation easily are the major hurdles for its use in commercial processes. Several attempts
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
Fig. 1. Mechanism for tert-butylation of m-cresol in the presence of UDCaT-5.
on the organic volume of the reaction mixture was employed at 120 C and with a speed of 1000 rpm. Among these catalysts, UDCaT-5 showed higher conversions compared to other catalysts (Fig. 2). The molarity of the solution of chlorosulphonic acid in ethylene dichloride, used to get UDCaT-5 was the same as that of aqueous solution of sulphuric acid used to get S-ZrO2, but the presence of more sulphur content in UDCaT-5 as compared to that of sulphated zirconia made UDCat-5 more active. Hence the activity of UDCaT-5 was more as compared to the other catalysts.
120
100
Conversion (%)
were made to overcome these difficulties by modifying zirconia in the form of UDCaT series. Recently, we have extended this series by synthesizing several novel mesoporic superacidic catalysts named as UDCaT-4, UDCaT-5 and UDCaT-6. Several industrially important reactions were carried out by using these catalysts which possess a great potential to catalyse several reactions with high activity and selectivity. Hence we thought it exciting to use these catalysts for alkylation reactions using alcohol as an alkylating agent. Maximum sulphur content on the sulphated zirconia with tetragonal phase is essential for higher catalytic activity and is independent of the concentration of sulphuric acid used and method of preparation of sulphated zirconia. Hence, there was a scope to design a zirconia in such a way that it possesses high sulphur content than sulphated zirconia with pure tetragonal phase to catalyse the reactions where strong acidity is required. Yadav and Murkute [29] reported, for the first time, a preparation of sulphated zirconia with sulphur content as high as 9%, as compared to conventional sulphated zirconia (4% sulfur) with preservation of tetragonal phase by using chlorosulphonic acid as a new source for sulphate ion. It was designed as UDCaT-5. However, the BET surface area of the UDCaT-5 (83 m2/g) is much lower and hence there was a scope to extend its applications by modifying it, in order to get dual objectives, i.e., high acidity and larger pores. UDCaT-4 and UDCaT-6 are mesoporous catalysts which are prepared in order to expand the usefulness of zirconia based catalyst. These catalysts are prepared by loading 20% w/w zirconia and alumina/zirconia on the highly ordered structure of hexagonal mesoporous silica (HMS), respectively. The sulphating agents used for the UDCaT-4 and UDCaT-6 were ammonium persulphate and chlorosulphonic acid, respectively. Spectroscopic analysis proved that these metals are entrapped well inside the pores of HMS and the number of acidic sites present in UDCaT-5 are more as compared to UDCaT-4 and UDCaT-6. Several industrially important reactions have been carried out using these catalysts in order to test their efficacy. UDCaT-5 gave excellent activity and selectivity with better reusability. Further, UDCaT-5 was quite stable in the presence of water and HCl, which were one of the co-products of some of the reactions studied [29]. Hence, we thought it desirable to employ these catalysts in tert-butylation of m-cresol with an aim to obtain maximum selectivity for mono C-alkylated products (Fig. 1).
80
60
40
20
0 0
3.2. Efficacies of various catalysts UDCaT-4, UDCaT-4, UDCaT-4 and sulphated zirconia were used to assess their activity and selectivity in this reaction. A 0.03 g/cm3 loading of catalyst based
19
10
20
30 40 Time (min)
50
60
70
Fig. 2. Effect of various catalysts. m-Cresol: 0.32 mol, TBA: 0.064 mol, catalyst loading: 0.03 g/cm3, temperature: 120 C, speed of agitation: 1000 rpm, total volume: 40 cm3. (r) Sulphated zirconia, (j) UDCaT-4, (m) UDCaT-6, (d) UDCaT-5.
20
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24 4
120
Conversion (%)
Concentration (mol/lit)
100 3
2
80 60 40 20
1
0 0
0 0
10
20
30 40 Time (min)
50
60
70
Fig. 3. Concentration profile of various products in alkylation of mcresol with TBA. m-Cresol: 0.29 mol, TBA: 0.097 mol, UDCaT-5: 0.03 g/cm3, temperature: 120 C, total volume: 40 cm3. (r) TBA, (m) isobutylene, (·) C-alkylated products, (d) dialkylated product, (j) O-alkylated product (none).
Further, UDCaT-5 gave better reusability. The product selectivity for the C-alkylated products was 100% and o/p ratio was 11.7. Thus, UDCaT-5 was highly selective for 2-tert-butyl-5-methylphenol. Hence further experiments were conducted with UDCaT-5 due to its excellent activity, reusability and stability. The concentration profile of different products at 120 C with 0.03 g/cm3 catalyst loading and m-cresol/TBA mole ratio of 3 is shown in Fig. 3.
3.4. Effect of catalyst loading
20
30 40 Time (min)
50
60
70
Fig. 4. Effect of the speed of agitation. m-Cresol: 0.29 mol, TBA: 0.097 mol, UDCaT-5: 0.03 g/cm3, temperature: 120 C, total volume: 40 cm3. (r) 800 rpm, (j) 1000 rpm, (m) 1200 rpm.
0.05 g/cm3 on the basis of total volume of the reaction mixture. Fig. 5 shows the effect of catalyst loading on the conversion of TBA. The conversion increased with increase in catalyst loading, which is due to the proportional increase in the number of active sites. However, beyond a catalyst loading of 0.03 g/cm3, there was no significant increase in the conversion and hence all further experiments were carried out at this catalyst loading. By using Wiesz–Prater criterion, the reaction was free from external mass transfer effects [36]. A further proof of the absence of the intra-particle diffusion resistance was obtained through the study of the effect of temperature and it will be discussed later. 120
3.3. Effect of the speed of agitation
100
80 Conversion (%)
To assess the role of external mass transfer on the reaction rate, the effect of the speed of agitation (Fig. 4) was studied. The speed of agitation was varied from 800 to 1200 rpm. It was observed that the conversion of TBA was practically the same in all the cases. Thus, speed of agitation beyond 800 rpm ensured that the external mass transfer effects did not influence the reaction. Hence, all further reactions were carried out at 1000 rpm. By employing theoretical analysis [34,35] of the assessment of external mass transfer suggested that there was no effect of external mass transfer resistance.
10
60
40
20
0 0
10
20
30
40
50
60
70
Time (min)
In the absence of external mass transfer resistance, the rate of reaction is directly proportional to catalyst loading based on the entire liquid phase volume. The catalyst loading was varied over a range of 0.01–
Fig. 5. Effect of catalyst loading. m-Cresol: 0.29 mol, TBA: 0.097 mol, speed of agitation: 1000 rpm, temperature: 120 C, total volume: 40 cm3. (r) 0.01 g/cm3, (j) 0.02 g/cm3, (m) 0.03 g/cm3, (d) 0.05 g/ cm3.
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
3.5. Effect of mole ratio The mole ratio of m-cresol to TBA was varied from 5:1 to 1:1 (Fig. 6) under otherwise similar operating conditions. As the m-cresol/TBA ratio was increased, an increase in conversion of TBA and the rate of reaction with increase in selectivity for C-alkylated product was observed. There was an insignificant increase in conversion beyond the mole ratio 3:1. The mole ratio study indicates that using excess TBA results in fast deactivation of active sites, which may be attributed to passivation of active sites by strong adsorption of either excessive alcohol molecule or water produced [39]. There are three more reasons to carry out experiments at this mole ratio (i) to avoid the formation of large amounts of secondary products, such as the oligomers of isobutene and dialkylated products. (ii) To diminish the influence of the water formed by the dehydration of TBA in situ. (iii) Lower the concentration of TBA, more the adsorption of TBA to produce isobutene on the catalyst sites and therefore more availability of isobutyl cation to react with m-cresol. Thus all the subsequent reactions were carried out with a mole ratio of 3:1. With an increase in the amount of m-cresol, the activity of isobutene produced increases which results in increase in the rate of alkylation reaction. Also tert-butyl group is bulky and hence tert-butyl-m-cresol ether formed is unstable and it rearranged easily to C-alkylated products. Thus at the initial period of the reaction, a little amount of O-alkylated product was present, but no O-alkylated product was seen in the later part of the reaction. 3.6. Effect of temperature The alkylation of m-cresol with tert-butanol is highly temperature dependent. The temperature effect was
studied from 100 to 140 C (Fig. 7). With an increase in temperature, both the rate of reaction as well as selectivity for C-alkylated product increased. No O-alkylated product was seen after 1 h at this temperature range. The conversion of TBA was 89% with 100% selectivity for C-alkylated products. The selectivity for monoalkylated product was 90% with o/p ratio 11.7. Spectroscopy evidences proved that UDCaT-5 contains three types of acidic sites, namely, intermediate, strong and very strong. The selectivity for O-alkylated product was 16% at 120 C and 23% at 100 C after 5 min, which can be explained on the basis of the presence of intermediate sites. Strong acidic sites present in UDCaT-5 are more in number and hence C-alkylation was dominant. 3.7. Dehydration of TBA An independent dehydration study of TBA (Fig. 8) was made in the temperature range of 433–473 K to ensure there was no coke formation on the catalyst. Isobutylene was the only product formed in this reaction. No di-tert-butyl ether was formed. The nonappearance of di-tert-butyl ether is due to the fact that tert-butyl group is bulky and hence ether which was formed is unstable and breaks instantly into isobutylene. The rate of dehydration increased with increase in temperature. 3.8. Reaction kinetics and modeling Several linear as well as branched alcohols have been used by us along with ethers for alkylation of various aromatic compounds, wherein water is generated in situ as a co-product [32,37,38]. In some cases, there was formation of alkenes along with C- and O-alkylated products whereas isopropylation using isopropanol gave
100
120
100
80
80
Conversion (%)
Conversion (%)
21
60
40
60
40
20
20
0
0 0
10
20
30
40
50
60
70
Time (min)
Fig. 6. Effect of mole ratio. m-Cresol, TBA, UDCaT-5: 0.03 g/cm3, speed of agitation: 1000 rpm, temperature: 120 C, total volume: 40 cm3. (r) 1:1, (j) 3:1, (m) 5:1.
0
20
40 Time (min)
60
80
Fig. 7. Effect of temperature. m-Cresol: 0.29 mol, TBA: 0.097 mol, UDCaT-5: 0.03 g/cm3, speed of agitation: 1000 rpm, total volume: 40 cm3. (r) 100 C, (j) 110 C, (m) 120 C, (d) 140 C.
Conversion (%)
22
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24 120
The site balance for this case is:
100
C T-S1 ¼ C V-S1 þ C AS1 þ C ES1 þ C WS1 C T-S2 ¼ C V-S2 þ C AS2 þ C WS2
80
ð5Þ ð6Þ
The following adsorption equilibria for different species hold:
60
K 1- W W þ S1 ! WS1 K 2- W W þ S2 ! WS2 K 1- E E þ S1 ! ES1
40 20 0 0
20
40
60 80 Time (min)
100
120
140
Fig. 8. Effect of temperature on dehydration of TBA. UDCaT-5: 0.03 g/cm3, speed of agitation: 1000 rpm, total volume: 40 cm3. (r) 100 C, (j) 110 C, (m) 120 C, (d) 140 C.
ð6aÞ ð6bÞ ð6cÞ
Thus the rate of formation of isobutylene is r0P ¼
K SR1 K 1-A C A K 2-A C A C TS1 C TS2 ð1 þ K 1-A C A þ K 1-W C W Þð1 þ K 2-A C A þ K 2-W C W þ K 2-E C E Þ
ð7Þ When the adsorption constants of all species are very weak, Eq. (7) is reduced to:
diisopropyl ether (DIPE) and this ether further acted as an alkylating agent [37]. The rate of alkylation was higher using ether than alcohol [37]. Thus, the alkylation with alcohols does not follow a simple reaction pathway and our current rate data using a new catalyst needed testing of different hypotheses. Several models were tried to fit the experimental data which were collected in the absence of any external mass transfer and intra-particle diffusion limitations. The model was expected to obey a first order kinetics for weakly adsorbed species but contrary to this, the model followed a second order kinetics. The analysis of earlier published reports on the dehydration of ethanol and isopropanol [40,41] suggests that there is a production of ether at lower temperature and also two types of sites should be involved in the reaction. Thus, a model based on two catalytic sites was proposed according to which TBA (A) gets adsorbed onto two different sites, S1 and S2
K2
A þ S2 AS2
ð9Þ
Writing in terms of conversion, and further integration results in the following equation: XA ¼ k P C A0 t 1 XA
ð10Þ
Thus a plot of XA/(1 XA) against t (Fig. 9) was made at different temperature to get an excellent fit, thereby supporting the model. This is an overall second order reaction for weak adsorption of TBA. 3.8.2. Case 2—tert-butylation of m-cresol Various models were tried including typical first order (weak adsorption of TBA and strong adsorption
14
ð2Þ
12
3.8.1. Case 1—dehydration of TBA In this case, the rates determining step is the reaction of AS1 and AS2, to form di-tert-butyl ether and water on the surface (ES1) and (WS2). ES1 being unstable, subsequently decomposes instantly into isobutylene (P) in the gas phase as shown below: k SR1
k P ¼ K SR1 K 1-A K 2-A C T-S1 C T-S2
ð1Þ
These two species participate in the reaction. Two different cases are considered, (i) dehydration of TBA gives isobutylene and water, and (ii) alkylation of m-cresol with TBA.
ð8Þ
where
10 XA/(1-XA)
K1
A þ S1 AS1
r0P ¼ k P C 2A
8 6 4 2 0 0
50
100
150
Time (min)
AS1 þ AS2 ! ES1 þ WS2
ð3Þ
ES1 ! 2P þ WS1
ð4Þ
Fig. 9. Validation of the mathematical model for dehydration of TBA. (r) 100 C, (j) 110 C, (m) 120 C, (d) 140 C.
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
of m-cresol) and second order kinetics (weak adsorption of TBA and m-cresol). However, none of these fitted the data. The second order dehydration model was taken as a basis. m-Cresol is adsorbed on sites S2, and it reacts with ES1, which is formed due to the surface reaction (3), to give the mono-alkylated m-cresol (D) as follows: B þ S2 ! BS2
3
2
BS2 þ AS1 ! DS2 þ WS1
ð12Þ
Analogously, the site balance can be written to obtain:
lnk
ð11Þ
k SR2
rA ¼
23
1
k SR2 C T-S1 C T-S2 K 1-A C A K 2-B C B ð1 þ K 1-A C A þ K 1-W C W þ K 1-E C E Þð1 þ K B C B þ K D-S C D Þ
ð13Þ With weak adsorption of all species, it gives the following: r0A
¼ kC A C B
ð14Þ
0 0.0024
0.0025
0.0026 1/T, K
where k ¼ k SR2 C T-S1 C T-S2 K 1-A K 2-B
Fig. 11. Arrhenius plot of ln k vs 1/T.
ð15Þ
Writing in terms of conversion, and further integration results into M XA ð16Þ ln ¼ ðM 1ÞkC A0 t M ð1 X A Þ MX A Thus, a plot of ln½Mð1X against t at different temperaAÞ tures is shown in Fig. 10. The values of the rate constant at different temperatures were calculated and an Arrhenius plot (Fig. 11) was used to estimate activation energy of alkylation of m-cresol. The activation energy was found to be 9.04 kcal/mol.
0.0027
-1
Table 2 Effect of catalyst reusability Run no.
% conversion of TBA
Fresh First run Second run
89 82 78
m-Cresol: 0.29 mol; TBA: 0.097 mol; UDCaT-5: 0.03 g/cm3; temperature: 120 C; speed of agitation: 1000 rpm; total volume: 40 cm3.
3.9. Reusability of catalyst
y = 0.0503x 2 R = 0.9698
M(1-X)/(M-X)
3
y = 0.0325x 2 R = 0.9785
2
y = 0.0238x 2 R = 0.9974
1 y = 0.0141x 2 R = 0.9901
0 0
10
20
30
40
50
60
70
Time, min
Fig. 10. Validation of mathematical model for tert-butylation of mcresol with TBA. UDCaT-5: 0.03 g/cm3, speed of agitation: 1000 rpm, total volume: 40 cm3. (r) 100 C, (j) 110 C, (m) 120 C, (d) 140 C.
Reusability of UDCaT-5 was tested by conducting two runs (Table 2). After the reaction the catalyst was filtered and then refluxed with 50 cm3 TBA for 30 min in order to remove any adsorbed material from the catalyst surface and pores and dried at 120 C. There was only a marginal decrease in conversion. There was no effect on selectivity of the products. 4. Conclusion The alkylation of m-cresol was studied over different catalysts which are modified versions of zirconia named as UDCaT-4, UDCaT-5 and UDCaT-6 with tert-butanol as alkylating agents in an autoclave at 120 C. UDCaT-5 was found to be the most active for this alkylation reaction. No di-tert-butyl ether as well as oligomerization of isobutene was formed at standard reaction conditions and only C-alkylated products were obtained exclusively. The ratio of o/p isomer was 11.7. The effects of various parameters on
24
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rates and selectivities over UDCaT-5 were discussed. The kinetics of the reaction is also reported.
[15] [16] [17] [18]
Acknowledgments
[19] [20]
G.S.P. acknowledges the recipient of Junior Research Fellowship (JRF) by the Council of Science and Industrial Research (CSIR), Government of India, New Delhi. G.D.Y. received support from the Darbari Seth professorship endowment.
[21] [22] [23]
References [1] J. Kirk, K. Othmer, Encyclopedia of Chemical Technology, third ed., Wiley-Interscience, New York, 1981. [2] H. Fiege, A.G. Bayer, Leverkusen, Ullmanns Encyclopedia of Industrial Chemistry, fifth ed., VCH, 1991. [3] H.G. Franck, J.W. Stadelhofer, Industrial Aromatic Chemistry, Springer, Berlin, 1988. [4] C. DeCastro, E. Sauvage, M.H. Valkenberg, W.F. Hoelderich, J. Catal. 196 (2000) 86. [5] Y. Ono, B. Imelik, et al., Catalysis by Zeolites, Elsevier, Amsterdam, 1980. [6] G.D. Yadav, P. Kumar, Appl. Catal. A: Gen. 286 (2005) 61. [7] D.K. Stevens, Ind. Eng. Chem. Res. 35 (1943) 655. [8] J.K. Gehlawat, M.M. Sharma, J. Appl. Chem. 20 (1970) 93. [9] J. Atkinson, D. Ball, ICI, DE-05 2423 356, 1973. [10] G. Leston, Koppers Co. US 3091 646, 1960. [11] G. Leston, Koppers Co. US 3 346 649, 1963. [12] G. Leston, Koppers Co. US 1 230 432, 1963. [13] G.D. Yadav, N.S. Doshi, J. Mol. Catal. A: Chem. 194 (2003) 195. [14] G.D. Yadav, N.S. Doshi, Appl. Catal. A: Gen. 236 (2002) 129.
[24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]
G.D. Yadav, T.S. Thorat, Ind. Eng. Chem. Res. 35 (3) (1996) 721. G.D. Yadav, A.A. Pujari, A.V. Joshi, Green Chem. 1 (1999) 269. G.D. Yadav, D.V. Satoskar, J. Am. Oil Chem. Soc. 4 (1997) 397. G.D. Yadav, J.J. Nair, J. Chem. Soc. Chem. Commun. (1998) 2369. P.S. Kumbhar, G.D. Yadav, Chem. Eng. Sci. 44 (1989) 2535. G.D. Yadav, M.S. Krishnan, Ind. Eng. Chem. Res. 37 (1998) 3365. G.D. Yadav, A.A. Pujari, Green Chem. 1 (1999) 69. G.D. Yadav, P.H. Mehta, Ind. Eng. Chem. Res. 33 (1994) 2198. G.D. Yadav, M.S. Krishnan, A.A. Pujari, N.S. Doshi, M.S.M. Mujeebur Rahuman, US Patent 211-499, 1998. G.D. Yadav, J.J. Nair, Catal. Lett. 62 (1999) 49. G.D. Yadav, T.S. Thorat, Tett. Lett. 37 (1996) 5405. G.D. Yadav, T.S. Thorat, P.S. Kumbhar, Tett. Lett. 34 (1993) 529. G.D. Yadav, N.S. Doshi, Org. Proc. Res. Dev. 6 (2002) 263. G.D. Yadav, P. Ramesh, Can. J. Chem. Eng. 78 (2000) 323. G.D. Yadav, A.D. Murkute, J. Catal. 224 (2004) 218. G.D. Yadav, S. Sengupta, Org. Proc. Res. Dev. 6 (2002) 256. G.D. Yadav, B. Kundu, Can. J. Chem. Eng. 79 (2001) 805. G.D. Yadav, A.D. Murkute, Langmuir 20 (2004) 11607. G.D. Yadav, A.D. Murkute, J. Phys. Chem. A: Gen. 108 (2004) 9557. G.D. Yadav, S.P. Nalawade, Chem. Eng. Sci. 58 (2003) 2573. G.D. Yadav, M.S. Krishnan, Org. Proc. Res. Dev. 2 (1998) 86. H.S. Fogler, Elements of Chemical Reaction Engineering, Prentice-Hall, New Delhi, 1995. G.D. Yadav, S.S. Salgaokar, Micropor. Mesopor. Mater. 80 (2005) 129. G.D. Yadav, A.D. Murkute, Adv. Synth. Catal. 346 (2004) 389. S. Song, S.K. Kirsy, Micropor. Mesopor. Mater. 2 (1994) 467. A. Rouge, B. Spoetzl, K. Gebauer, R. Schenk, A. Renken, Chem. Eng. Sci. 56 (2001) 1419. S. Golay, O. Wolfrath, R. Doepper, A. Renken, in: G.F. Froment, K.C. Waugh (Eds.), Dynamics of Surface and Reaction Kinetics in Heterogeneous Catalysis, vol. 109, Elsevier, Amsterdam, 1997, p. 295.
Novel mesoporous solid superacids for selective C-alkylation of m-cresol with tert-butanol Ganapati D. Yadav *, Ganesh S. Pathre Department of Chemical Engineering, University Institute of Chemical Technology (UICT), University of Mumbai, Matunga, Mumbai 400 019, India Received 2 May 2005; received in revised form 30 July 2005; accepted 30 July 2005 Available online 18 November 2005
Abstract tert-Butylated phenols, which are important precursors in a variety of industries, are usually manufactured by reacting phenol in the presence of liquid acid catalysts, with pure isobutylene or C4 fraction from naphtha crackers containing isobutylene. These processes suffer from problems associated with the use of highly corrosive liquid acids and also the source of isobutylene. In the current study, alkylation of m-cresol was studied with tert-butyl alcohol (TBA) using sulphated zirconia and three novel superacidic catalysts, namely, UDCaT-4, 5 and 6. The new catalysts UDCaT-4, UDCaT-5 and UDCaT-6 are modified versions of zirconia. Amongst these, UDCaT-5, with 9% w/w sulphate content, was found to be the most active. Effects of different parameters such as, speed of agitation, catalyst loading, reactant ratio (mole ratio of m-cresol to that of TBA), effect of temperature and the recycle of catalyst are discussed. The conversion of m-cresol and the selectivity for mono C-alkylated product at a m-cresol to TBA mole ratio of 3:1, using 0.03 g/cm3 UDCaT-5 at 120 C under autogenous pressure, were 89% and 90%, respectively. The reaction was carried out without using a solvent to make the process greener and cleaner. The selectivity for the 2-tert-butyl-5-methylphenol was 93%. The reaction mechanism is discussed. A second order rate equation fits the data well. The apparent activation energy was determined as 9.04 kcal/mol. 2005 Elsevier Inc. All rights reserved. Keywords: Alkylation; m-Cresol; Mesoporic superacidic catalysts; UDCaT-4; UDCaT-5; UDCaT-6; Chlorosulphonic acid; Green process; Selectivity; 1-Cyclohexyloxybenzene
1. Introduction Friedel–Crafts reactions, in general, are carried out using highly corrosive liquid acids, which pose loss of yield, corrosion of equipment and post-treatment pollution problems. Alkylation reaction of m-cresol with tert-butyl alcohol is an important reaction both in organic synthesis and chemical manufacturing. The alkylated m-cresols are used as raw materials for the manufacturing of a variety of resins, durable sur* Corresponding author. Tel.: +91 22 2410 2121/2414 5616; fax: +91 22 2414 5614. E-mail addresses: [email protected], [email protected] (G.D. Yadav).
1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.07.047
face coatings, varnishes, wire enamels, printing inks, surface-active agents, rubber chemicals, antioxidants, fungicides, petroleum additives, ultraviolet absorbers, and heat stabilizers for polymeric materials [1–3]. tert-Butylated phenols are generally prepared by reacting phenol with pure isobutylene gas or C4 fraction of naphtha by using a liquid acid catalyst, giving wide product distribution. The use of highly corrosive and polluting liquid acids is a major environmental problem and therefore solid acids are much sought after. Both the formation of O- and C-alkylated products is possible depending on reaction conditions such as reaction temperature and type of catalyst used. Catalysts with strong acidic sites or reaction at high reaction temperature, typically over 80 C, lead to the formation
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
17
Nomenclature A aP B P Ci CV-Si D De Dij dP E K
TBA surface area of catalyst (cm) m-cresol isobutene concentration of species i (mol/cm3) concentration of vacant sites of type i tert-butylated m-cresol effectivity diffusivity (cm2/s) diffusivity of i in j (cm2/s) diameter of catalyst particle (cm) di-tert butyl ether adsorption/mass transfer/reaction rate constant
of C-alkylated products [4] while catalysts with weak acidic sites or reaction at low temperature lead to the formation of O-alkylated products [5,6]. 2-tert-Butyl5-methylphenol is a precursor for a number of commercially important antioxidants and a light protection agent of the bisphenol and thiobisphenol type. 2-tertButyl-5-methylphenol is also used to prepare important UV absorber, and musk ambrette which is a perfume fixation agent [2]. Stevens [7] prepared tert-butylated m-cresol from isobutene as an alkylating agent with cresol:isobutene mole ratio of 0.7:1, whereas Gehlawat and Sharma [8] reported the same reaction by using H2SO4 with a mole ratio of 1 where maximum yield of 80% was obtained. 2-tert-Butyl-5-methylphenol was also prepared by transbutylation of m-cresol with 4,6di-tert-butyl-3-methylphenol at 80–100 C in the presence of an acid treated clay with difficulty [9]. This isomer was also synthesized by controlled debutylation of 4,6-di-tert-butyl-3-methylphenol by heating with the phenolates of aluminium [10] and zirconia [11,12] (Table 1). There is a tremendous scope for devising a new catalytic process for the synthesis of tert-butylated cresols to replace conventional homogenously catalyzed, highly polluting processes. Besides, due to the problems associated with unavailability, transportation and handling of isobutylene, particularly for usage in low-tonnage fine and speciality chemical industry (typically 10–100 TPA production), it is advantageous
M ri RP S Shi t W Xi
mole ratio rate of reaction of species i (mol/cm3 s) radius of catalyst particle (cm) vacant catalyst sites Sherwood number of species i time (min) water conversion of species i
Subscripts SR surface reaction T total catalyst sites 0 initial concentration
to generate isobutylene in situ. Dehydration of tertbutanol is an attractive source for the same. Further, tert-butanol is available as a by-product in the ARCO process for propylene oxide which could be used effectively for this purpose. We have successfully carried out tert-butylation of several aromatic compounds by using tert-butanol, methyl-tert-butyl ether (MTBE) and isobutene as alkylating agents using ecofriendly solid acid catalysts [13–17]. The only problem which needs to be considered was activity and stability of solid catalysts in the presence of water. Our laboratory has been engaged in preparing several solid acid catalysts to make the processes environmental friendly by using them in industrially important reactions. In particular, sulphated zirconia has been extensively studied in a number of reactions [18–31] and it was believed that this solid superacid should be modified to bring in shape selectivity, mesoporosity and better acidity. Recently, we prepared novel solid superacids named as UDCaT-4, UDCaT-5 and UDCaT-6 which have found a great potential for industrially important reactions [29,32,33]. The acronym UDCaT symbolizes the University Department of Chemical Technology (UDCT), which is now renamed as University Institute of Chemical Technology (UICT), Mumbai. The work summarizes the investigation of activity and selectivity of these novel materials in alkylation of m-cresol with tert-butanol, including kinetics and mechanism.
Table 1 Published literature on preparation of 2-tert-butyl-5-methylphenol Reactants
Temperature, C
Catalyst
Mole ratio
Conversion, %
Ref.
Cresol:isobutene Cresol:isobutene m-Cresol and 4,6-di-tert-butyl-3-methylphenol 4,6-Di-tert-butyl-3-methylphenol
80 80 80–100 280
H2SO4 H2SO4 Acid treated clay Phenolates of aluminium and zirconia
0.7:1 1:1 – –
– 80 – –
[7] [8] [9] [10–12]
18
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2. Experimental 2.1. Chemicals and catalysts tert-Butanol and m-cresol were procured from E. Merck (India) Ltd., Mumbai. Zirconium oxychloride and aqueous ammonia solution were procured from M/s. s. d. Fine Chemicals Ltd., Mumbai. Hexadecyl amine and chlorosulphonic acid were purchased from Spectrochem Ltd., Mumbai, India. Tetraethyl orthosilicate (TEOS) was procured from Fluka, Germany. Sulphated zirconia [30,31], UDCaT-4 [32] and UDCaT-6 [33] were prepared by well-developed procedures and characterized in our laboratory. Since UDCaT-5 was the most efficient catalyst, details of its method of preparation and characterization are given. 2.2. Preparation of catalysts UDCaT-5 was prepared adding aqueous ammonia solution to zirconium oxychloride (ZrOCl2 Æ 8H2O) solution at pH of 9–10. The precipitated zirconium hydroxide so obtained was washed with deionized water until a neutral filtrate was obtained. The absence of chlorine ion was detected by AgNO3 tests. A material balance on Cl before and after precipitation and washing shows no retention of Cl on the solid. Zirconium hydroxide was dried in an oven for 24 h at 100 C and was crushed to 100 mesh size. Zr(OH)4 was then immersed in 15 cm3/ g of 0.5 M solution of chlorosulphonic acid and ethylene dichloride. All materials were immersed for 5 min in the solution and then without allowing moisture absorption were kept in an oven and then they were heated to evaporate the solvent, and then to a temperature of 120 C after about 30 min. These solids were kept in an oven at 120 C for 24 h and calcined at 650 C for 3 h to get the active catalyst UDCaT-5. 2.3. Characterization of UDCaT-5 catalyst UDCaT-5 is completely characterized by XRD, BET surface area and FTIR and the details were published recently by us [29]. Only a few salient features are reported here. Ammonia-TPD was used to determine the acid strength of UDCaT-5. Ammonia-TPD analysis shows that apart from intermediate and strong acidic sites present in sulphated zirconia, UDCaT-5 also contains superacidic sites. Elemental analysis shows that UDCaT-5 contains 9% sulphate content which is highest when compared to that reported so far. IR spectroscopy confirms that chlorosulphonic acid is decomposed during calcination at 650 C and sulpate ions are retained on the surface of UDCaT-5 and that is why sulphur content is higher in UDCaT5. XRD diffraction study proved that the tetragonal
phase in the UDCaT-5 is preserved. From BET surface area and pore size analysis, it was seen that surface area of UDCaT-5 decreases abruptly at maximum sulphur content. This is due to the migration of sulphate ions from bulk phase to zirconia matrix. Thus maximum sulphur present on the surface decreases the surface area of UDCaT-5. All this proves that UDCaT-5 is superacidic in nature due to maximum sulphur content present on the zirconia matrix with preservation of tetragonal phase of zirconia. 2.4. Apparatus and procedures All experiments were carried out in a 100 cm3 stainless steel Parr autoclave. A four bladed-pitched turbine impeller was used for agitation. The temperature was maintained at ±1 C of the desired value. Known quantities of reactants and catalyst were charged into the autoclave, the temperature raised to the desired value and agitation started. Then, an initial sample was withdrawn. Further samples were drawn at periodic intervals up to 2 h. A standard experiment consisted of 0.29 mol m-cresol, 0.097 mol TBA and a catalyst loading of 0.03 g/cm3 of total volume of the liquid. The temperature was maintained at 120 C and the speed of agitation at 1000 rpm. The reaction was carried out without using any solvent. The total volume of the liquid phase was 40 cm3. 2.5. Method of analysis Clear liquid samples were withdrawn at regular intervals by reducing the speed of agitation momentarily to zero and allowing the catalyst to settle at the bottom of the reactor. The samples were analyzed on a Chemito Model 8510 GC equipped with a 10% SE-30 column (3.175 mm diam. · 4 m length). A standard calibration method with synthetic mixtures was used for quantification of data. The reaction products were confirmed by GC–MS and authentic samples.
3. Result and discussion 3.1. Choice of catalysts Zirconia based solid acids are attracting much attention in recent years. Sulphated zirconia is proved to be a highly active versatile solid acid catalyst in a number of reactions such as isomerization [17,18], condensation [19], etherification [20], acylation [21], esterification [22], nitration [23], oligomerization [24] and alkylation [25–28]. But poor stability of sulphated zirconia in the presence of water, low surface area and a tendency to undergo deactivation easily are the major hurdles for its use in commercial processes. Several attempts
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
Fig. 1. Mechanism for tert-butylation of m-cresol in the presence of UDCaT-5.
on the organic volume of the reaction mixture was employed at 120 C and with a speed of 1000 rpm. Among these catalysts, UDCaT-5 showed higher conversions compared to other catalysts (Fig. 2). The molarity of the solution of chlorosulphonic acid in ethylene dichloride, used to get UDCaT-5 was the same as that of aqueous solution of sulphuric acid used to get S-ZrO2, but the presence of more sulphur content in UDCaT-5 as compared to that of sulphated zirconia made UDCat-5 more active. Hence the activity of UDCaT-5 was more as compared to the other catalysts.
120
100
Conversion (%)
were made to overcome these difficulties by modifying zirconia in the form of UDCaT series. Recently, we have extended this series by synthesizing several novel mesoporic superacidic catalysts named as UDCaT-4, UDCaT-5 and UDCaT-6. Several industrially important reactions were carried out by using these catalysts which possess a great potential to catalyse several reactions with high activity and selectivity. Hence we thought it exciting to use these catalysts for alkylation reactions using alcohol as an alkylating agent. Maximum sulphur content on the sulphated zirconia with tetragonal phase is essential for higher catalytic activity and is independent of the concentration of sulphuric acid used and method of preparation of sulphated zirconia. Hence, there was a scope to design a zirconia in such a way that it possesses high sulphur content than sulphated zirconia with pure tetragonal phase to catalyse the reactions where strong acidity is required. Yadav and Murkute [29] reported, for the first time, a preparation of sulphated zirconia with sulphur content as high as 9%, as compared to conventional sulphated zirconia (4% sulfur) with preservation of tetragonal phase by using chlorosulphonic acid as a new source for sulphate ion. It was designed as UDCaT-5. However, the BET surface area of the UDCaT-5 (83 m2/g) is much lower and hence there was a scope to extend its applications by modifying it, in order to get dual objectives, i.e., high acidity and larger pores. UDCaT-4 and UDCaT-6 are mesoporous catalysts which are prepared in order to expand the usefulness of zirconia based catalyst. These catalysts are prepared by loading 20% w/w zirconia and alumina/zirconia on the highly ordered structure of hexagonal mesoporous silica (HMS), respectively. The sulphating agents used for the UDCaT-4 and UDCaT-6 were ammonium persulphate and chlorosulphonic acid, respectively. Spectroscopic analysis proved that these metals are entrapped well inside the pores of HMS and the number of acidic sites present in UDCaT-5 are more as compared to UDCaT-4 and UDCaT-6. Several industrially important reactions have been carried out using these catalysts in order to test their efficacy. UDCaT-5 gave excellent activity and selectivity with better reusability. Further, UDCaT-5 was quite stable in the presence of water and HCl, which were one of the co-products of some of the reactions studied [29]. Hence, we thought it desirable to employ these catalysts in tert-butylation of m-cresol with an aim to obtain maximum selectivity for mono C-alkylated products (Fig. 1).
80
60
40
20
0 0
3.2. Efficacies of various catalysts UDCaT-4, UDCaT-4, UDCaT-4 and sulphated zirconia were used to assess their activity and selectivity in this reaction. A 0.03 g/cm3 loading of catalyst based
19
10
20
30 40 Time (min)
50
60
70
Fig. 2. Effect of various catalysts. m-Cresol: 0.32 mol, TBA: 0.064 mol, catalyst loading: 0.03 g/cm3, temperature: 120 C, speed of agitation: 1000 rpm, total volume: 40 cm3. (r) Sulphated zirconia, (j) UDCaT-4, (m) UDCaT-6, (d) UDCaT-5.
20
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24 4
120
Conversion (%)
Concentration (mol/lit)
100 3
2
80 60 40 20
1
0 0
0 0
10
20
30 40 Time (min)
50
60
70
Fig. 3. Concentration profile of various products in alkylation of mcresol with TBA. m-Cresol: 0.29 mol, TBA: 0.097 mol, UDCaT-5: 0.03 g/cm3, temperature: 120 C, total volume: 40 cm3. (r) TBA, (m) isobutylene, (·) C-alkylated products, (d) dialkylated product, (j) O-alkylated product (none).
Further, UDCaT-5 gave better reusability. The product selectivity for the C-alkylated products was 100% and o/p ratio was 11.7. Thus, UDCaT-5 was highly selective for 2-tert-butyl-5-methylphenol. Hence further experiments were conducted with UDCaT-5 due to its excellent activity, reusability and stability. The concentration profile of different products at 120 C with 0.03 g/cm3 catalyst loading and m-cresol/TBA mole ratio of 3 is shown in Fig. 3.
3.4. Effect of catalyst loading
20
30 40 Time (min)
50
60
70
Fig. 4. Effect of the speed of agitation. m-Cresol: 0.29 mol, TBA: 0.097 mol, UDCaT-5: 0.03 g/cm3, temperature: 120 C, total volume: 40 cm3. (r) 800 rpm, (j) 1000 rpm, (m) 1200 rpm.
0.05 g/cm3 on the basis of total volume of the reaction mixture. Fig. 5 shows the effect of catalyst loading on the conversion of TBA. The conversion increased with increase in catalyst loading, which is due to the proportional increase in the number of active sites. However, beyond a catalyst loading of 0.03 g/cm3, there was no significant increase in the conversion and hence all further experiments were carried out at this catalyst loading. By using Wiesz–Prater criterion, the reaction was free from external mass transfer effects [36]. A further proof of the absence of the intra-particle diffusion resistance was obtained through the study of the effect of temperature and it will be discussed later. 120
3.3. Effect of the speed of agitation
100
80 Conversion (%)
To assess the role of external mass transfer on the reaction rate, the effect of the speed of agitation (Fig. 4) was studied. The speed of agitation was varied from 800 to 1200 rpm. It was observed that the conversion of TBA was practically the same in all the cases. Thus, speed of agitation beyond 800 rpm ensured that the external mass transfer effects did not influence the reaction. Hence, all further reactions were carried out at 1000 rpm. By employing theoretical analysis [34,35] of the assessment of external mass transfer suggested that there was no effect of external mass transfer resistance.
10
60
40
20
0 0
10
20
30
40
50
60
70
Time (min)
In the absence of external mass transfer resistance, the rate of reaction is directly proportional to catalyst loading based on the entire liquid phase volume. The catalyst loading was varied over a range of 0.01–
Fig. 5. Effect of catalyst loading. m-Cresol: 0.29 mol, TBA: 0.097 mol, speed of agitation: 1000 rpm, temperature: 120 C, total volume: 40 cm3. (r) 0.01 g/cm3, (j) 0.02 g/cm3, (m) 0.03 g/cm3, (d) 0.05 g/ cm3.
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
3.5. Effect of mole ratio The mole ratio of m-cresol to TBA was varied from 5:1 to 1:1 (Fig. 6) under otherwise similar operating conditions. As the m-cresol/TBA ratio was increased, an increase in conversion of TBA and the rate of reaction with increase in selectivity for C-alkylated product was observed. There was an insignificant increase in conversion beyond the mole ratio 3:1. The mole ratio study indicates that using excess TBA results in fast deactivation of active sites, which may be attributed to passivation of active sites by strong adsorption of either excessive alcohol molecule or water produced [39]. There are three more reasons to carry out experiments at this mole ratio (i) to avoid the formation of large amounts of secondary products, such as the oligomers of isobutene and dialkylated products. (ii) To diminish the influence of the water formed by the dehydration of TBA in situ. (iii) Lower the concentration of TBA, more the adsorption of TBA to produce isobutene on the catalyst sites and therefore more availability of isobutyl cation to react with m-cresol. Thus all the subsequent reactions were carried out with a mole ratio of 3:1. With an increase in the amount of m-cresol, the activity of isobutene produced increases which results in increase in the rate of alkylation reaction. Also tert-butyl group is bulky and hence tert-butyl-m-cresol ether formed is unstable and it rearranged easily to C-alkylated products. Thus at the initial period of the reaction, a little amount of O-alkylated product was present, but no O-alkylated product was seen in the later part of the reaction. 3.6. Effect of temperature The alkylation of m-cresol with tert-butanol is highly temperature dependent. The temperature effect was
studied from 100 to 140 C (Fig. 7). With an increase in temperature, both the rate of reaction as well as selectivity for C-alkylated product increased. No O-alkylated product was seen after 1 h at this temperature range. The conversion of TBA was 89% with 100% selectivity for C-alkylated products. The selectivity for monoalkylated product was 90% with o/p ratio 11.7. Spectroscopy evidences proved that UDCaT-5 contains three types of acidic sites, namely, intermediate, strong and very strong. The selectivity for O-alkylated product was 16% at 120 C and 23% at 100 C after 5 min, which can be explained on the basis of the presence of intermediate sites. Strong acidic sites present in UDCaT-5 are more in number and hence C-alkylation was dominant. 3.7. Dehydration of TBA An independent dehydration study of TBA (Fig. 8) was made in the temperature range of 433–473 K to ensure there was no coke formation on the catalyst. Isobutylene was the only product formed in this reaction. No di-tert-butyl ether was formed. The nonappearance of di-tert-butyl ether is due to the fact that tert-butyl group is bulky and hence ether which was formed is unstable and breaks instantly into isobutylene. The rate of dehydration increased with increase in temperature. 3.8. Reaction kinetics and modeling Several linear as well as branched alcohols have been used by us along with ethers for alkylation of various aromatic compounds, wherein water is generated in situ as a co-product [32,37,38]. In some cases, there was formation of alkenes along with C- and O-alkylated products whereas isopropylation using isopropanol gave
100
120
100
80
80
Conversion (%)
Conversion (%)
21
60
40
60
40
20
20
0
0 0
10
20
30
40
50
60
70
Time (min)
Fig. 6. Effect of mole ratio. m-Cresol, TBA, UDCaT-5: 0.03 g/cm3, speed of agitation: 1000 rpm, temperature: 120 C, total volume: 40 cm3. (r) 1:1, (j) 3:1, (m) 5:1.
0
20
40 Time (min)
60
80
Fig. 7. Effect of temperature. m-Cresol: 0.29 mol, TBA: 0.097 mol, UDCaT-5: 0.03 g/cm3, speed of agitation: 1000 rpm, total volume: 40 cm3. (r) 100 C, (j) 110 C, (m) 120 C, (d) 140 C.
Conversion (%)
22
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24 120
The site balance for this case is:
100
C T-S1 ¼ C V-S1 þ C AS1 þ C ES1 þ C WS1 C T-S2 ¼ C V-S2 þ C AS2 þ C WS2
80
ð5Þ ð6Þ
The following adsorption equilibria for different species hold:
60
K 1- W W þ S1 ! WS1 K 2- W W þ S2 ! WS2 K 1- E E þ S1 ! ES1
40 20 0 0
20
40
60 80 Time (min)
100
120
140
Fig. 8. Effect of temperature on dehydration of TBA. UDCaT-5: 0.03 g/cm3, speed of agitation: 1000 rpm, total volume: 40 cm3. (r) 100 C, (j) 110 C, (m) 120 C, (d) 140 C.
ð6aÞ ð6bÞ ð6cÞ
Thus the rate of formation of isobutylene is r0P ¼
K SR1 K 1-A C A K 2-A C A C TS1 C TS2 ð1 þ K 1-A C A þ K 1-W C W Þð1 þ K 2-A C A þ K 2-W C W þ K 2-E C E Þ
ð7Þ When the adsorption constants of all species are very weak, Eq. (7) is reduced to:
diisopropyl ether (DIPE) and this ether further acted as an alkylating agent [37]. The rate of alkylation was higher using ether than alcohol [37]. Thus, the alkylation with alcohols does not follow a simple reaction pathway and our current rate data using a new catalyst needed testing of different hypotheses. Several models were tried to fit the experimental data which were collected in the absence of any external mass transfer and intra-particle diffusion limitations. The model was expected to obey a first order kinetics for weakly adsorbed species but contrary to this, the model followed a second order kinetics. The analysis of earlier published reports on the dehydration of ethanol and isopropanol [40,41] suggests that there is a production of ether at lower temperature and also two types of sites should be involved in the reaction. Thus, a model based on two catalytic sites was proposed according to which TBA (A) gets adsorbed onto two different sites, S1 and S2
K2
A þ S2 AS2
ð9Þ
Writing in terms of conversion, and further integration results in the following equation: XA ¼ k P C A0 t 1 XA
ð10Þ
Thus a plot of XA/(1 XA) against t (Fig. 9) was made at different temperature to get an excellent fit, thereby supporting the model. This is an overall second order reaction for weak adsorption of TBA. 3.8.2. Case 2—tert-butylation of m-cresol Various models were tried including typical first order (weak adsorption of TBA and strong adsorption
14
ð2Þ
12
3.8.1. Case 1—dehydration of TBA In this case, the rates determining step is the reaction of AS1 and AS2, to form di-tert-butyl ether and water on the surface (ES1) and (WS2). ES1 being unstable, subsequently decomposes instantly into isobutylene (P) in the gas phase as shown below: k SR1
k P ¼ K SR1 K 1-A K 2-A C T-S1 C T-S2
ð1Þ
These two species participate in the reaction. Two different cases are considered, (i) dehydration of TBA gives isobutylene and water, and (ii) alkylation of m-cresol with TBA.
ð8Þ
where
10 XA/(1-XA)
K1
A þ S1 AS1
r0P ¼ k P C 2A
8 6 4 2 0 0
50
100
150
Time (min)
AS1 þ AS2 ! ES1 þ WS2
ð3Þ
ES1 ! 2P þ WS1
ð4Þ
Fig. 9. Validation of the mathematical model for dehydration of TBA. (r) 100 C, (j) 110 C, (m) 120 C, (d) 140 C.
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
of m-cresol) and second order kinetics (weak adsorption of TBA and m-cresol). However, none of these fitted the data. The second order dehydration model was taken as a basis. m-Cresol is adsorbed on sites S2, and it reacts with ES1, which is formed due to the surface reaction (3), to give the mono-alkylated m-cresol (D) as follows: B þ S2 ! BS2
3
2
BS2 þ AS1 ! DS2 þ WS1
ð12Þ
Analogously, the site balance can be written to obtain:
lnk
ð11Þ
k SR2
rA ¼
23
1
k SR2 C T-S1 C T-S2 K 1-A C A K 2-B C B ð1 þ K 1-A C A þ K 1-W C W þ K 1-E C E Þð1 þ K B C B þ K D-S C D Þ
ð13Þ With weak adsorption of all species, it gives the following: r0A
¼ kC A C B
ð14Þ
0 0.0024
0.0025
0.0026 1/T, K
where k ¼ k SR2 C T-S1 C T-S2 K 1-A K 2-B
Fig. 11. Arrhenius plot of ln k vs 1/T.
ð15Þ
Writing in terms of conversion, and further integration results into M XA ð16Þ ln ¼ ðM 1ÞkC A0 t M ð1 X A Þ MX A Thus, a plot of ln½Mð1X against t at different temperaAÞ tures is shown in Fig. 10. The values of the rate constant at different temperatures were calculated and an Arrhenius plot (Fig. 11) was used to estimate activation energy of alkylation of m-cresol. The activation energy was found to be 9.04 kcal/mol.
0.0027
-1
Table 2 Effect of catalyst reusability Run no.
% conversion of TBA
Fresh First run Second run
89 82 78
m-Cresol: 0.29 mol; TBA: 0.097 mol; UDCaT-5: 0.03 g/cm3; temperature: 120 C; speed of agitation: 1000 rpm; total volume: 40 cm3.
3.9. Reusability of catalyst
y = 0.0503x 2 R = 0.9698
M(1-X)/(M-X)
3
y = 0.0325x 2 R = 0.9785
2
y = 0.0238x 2 R = 0.9974
1 y = 0.0141x 2 R = 0.9901
0 0
10
20
30
40
50
60
70
Time, min
Fig. 10. Validation of mathematical model for tert-butylation of mcresol with TBA. UDCaT-5: 0.03 g/cm3, speed of agitation: 1000 rpm, total volume: 40 cm3. (r) 100 C, (j) 110 C, (m) 120 C, (d) 140 C.
Reusability of UDCaT-5 was tested by conducting two runs (Table 2). After the reaction the catalyst was filtered and then refluxed with 50 cm3 TBA for 30 min in order to remove any adsorbed material from the catalyst surface and pores and dried at 120 C. There was only a marginal decrease in conversion. There was no effect on selectivity of the products. 4. Conclusion The alkylation of m-cresol was studied over different catalysts which are modified versions of zirconia named as UDCaT-4, UDCaT-5 and UDCaT-6 with tert-butanol as alkylating agents in an autoclave at 120 C. UDCaT-5 was found to be the most active for this alkylation reaction. No di-tert-butyl ether as well as oligomerization of isobutene was formed at standard reaction conditions and only C-alkylated products were obtained exclusively. The ratio of o/p isomer was 11.7. The effects of various parameters on
24
G.D. Yadav, G.S. Pathre / Microporous and Mesoporous Materials 89 (2006) 16–24
rates and selectivities over UDCaT-5 were discussed. The kinetics of the reaction is also reported.
[15] [16] [17] [18]
Acknowledgments
[19] [20]
G.S.P. acknowledges the recipient of Junior Research Fellowship (JRF) by the Council of Science and Industrial Research (CSIR), Government of India, New Delhi. G.D.Y. received support from the Darbari Seth professorship endowment.
[21] [22] [23]
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