Article pubs.acs.org/IECR
MCM-41 Supported Phosphotungstic Acid for the Hydroxyalkylation of Phenol to Phenolphthalein Ajay Jha,† Ajit C. Garade,† Subhash P. Mirajkar,‡ and Chandrashekhar V. Rode*,† †
Chemical Engineering & Process Development Division ‡Inorganic and Catalysis Division National Chemical Laboratory, Pune 411008, India ABSTRACT: A novel application of phosphotungstic acid, H3PW12O40 (PWA) supported on MCM-41 molecular sieve as a solid acid catalyst has been demonstrated for the synthesis of phenolphthalein by hydroxyalkylation of phenol and phthalic anhydride. PWA/MCM-41 (20%) showed the highest activity as compared to that of parent PWA and MCM-41 individually, due to the excellent dispersion of PWA on MCM-41 leading to the redistribution of Bronsted and Lewis acid sites on MCM-41. The effect of PWA loading on phthalic anhydride conversion and phenolphthalein selectivity was also studied. All these prepared catalysts were characterized by XRD, N2 adsorption−desorption isotherm, pyridine-FTIR, and NH3-TPD. The effect of various reaction parameters, namely, mole ratios, catalyst concentration, temperature, reaction time, and percentage of PWA present in the catalysts on conversion and selectivity of products has been also investigated. The utility of 20% PWA/MCM-41 catalyst was established by its efficient activity for hydroxyalkylation of phenol and p-cresol with formaldehyde to the corresponding dihydroxydiarylmethane products.
1. INTRODUCTION Several organic transformations conventionally carried out using stoichiometric excess quantities of acid reagents are being replaced by solid acids to obviate toxic, corrosive, and waste disposal problems.1 Heteropoly acid is one of the best solid materials having strong acidity for catalyzing various organic reactions of practical interest.2 Two major drawbacks of heteropoly acids,3 namely, (i) very low surface area and (ii) their instability in polar solvents, can be easily overcome by dispersing them on a suitable support. This is highly desirable for their applicability in industrial processes such as hydroxyalkylation of phenol as reported previously by our group.4,5 In continuation of our efforts in this direction, we report here MCM-41-supported phosphotungstic acid (PWA) catalyzed synthesis of phenolphthalein which is a hydroxyalkylation-type reaction involving condensation of phenol and phthalic anhydride (Scheme 1). Phenolphthalein has wide ranging applications in polymer industry to make polycarbonates, polyester, and poly(arylene ether).6 An additional advantage of phenolphthalein into polymers backbone is that the heterocyclic pendant lactone provides chemically reactive sites for further derivatization or grafting suitable reactive moieties for making functional polymeric materials.7 Currently, multiblock poly(arylene ether sulfone) copolymers based on phenolphthalein are used in proton exchange membrane fuel cell technology.7 The available processes for phenolphthalein utilizing mineral or Lewis acids are time-consuming (15−20 h) and energy intensive due to tedious separation and recovery steps for pure products from homogeneous reaction crude. The most commonly used Lewis acid for phenolphthalein manufacture is ZnCl2 required in relatively large amounts (0.6 mol per mole of phthalic anhydride),8 which increases the viscosity with the progress of the reaction. © 2012 American Chemical Society
In our work, MCM-41 was chosen to impregnate the PWA since MCM-41 is mesoporous silica with a very high surface area (>1000 m2/g) and uniform large pore size (2−8 nm), which would allow the PWA cluster with ∼1.2 nm diameter to be introduced inside MCM-41 pores.9 We found that highly dispersed PWA on MCM-41 was an active and selective solid acid catalyst for condensation of phenol and phthalic anhydride to form phenolphthalein. The activity of supported PWA on MCM-41 has been also compared with those of different solid acid catalysts like PWA/Mont K10, and Mont KSF/O. The versatility of our catalyst system was shown by its activity for hydroxyalkylation of phenol and p-cresol to the corresponding dihydroxydiarylmethane (DAM) compounds.
2. EXPERIMENTAL SECTION 2.1. Source of Chemicals and Materials. Cetyltrimethylammonium bromide (CTAB) and tetraethylorthosilicate (TEOS) were purchased from Aldrich Chemicals, Bangalore, India. Phosphotungstic acid and H2SO4 were procured from Thomas Baker, India. All chemicals were used as received without further purification. 2.2. Catalyst preparation. MCM-41 support was prepared by a hydrothermal synthesis method as described earlier.10,11 In a typical synthesis, 2.46 g of NaOH was dissolved in 146 mL of deionized water and stirred for 5 min. To this solution, 5.96 g of CTAB was added, and the mixture was stirred for 15 min. A 14 g portion of TEOS was added dropwise, and the pH of the solution was maintained in a range of 9−10 by adding dilute H2SO4. The resulting reaction mixture was stirred for 5 h at room temperature. Thereafter the entire reaction mixture along Received: Revised: Accepted: Published: 3916
September 2, 2011 January 12, 2012 February 10, 2012 February 10, 2012 dx.doi.org/10.1021/ie201989g | Ind. Eng. Chem. Res. 2012, 51, 3916−3922
Industrial & Engineering Chemistry Research
Article
Scheme 1. Synthesis of Phenolphthalein from Phenol and Phthalic Anhydride
and an arrangement for temperature control. In a typical experiment, phenol (34 mmol), phthalic anhydride (16 mmol), and catalyst (0.1 g/cm3) were added to the reactor, which was then heated to 423 K for 3 h. Conversion of phthalic anhydride and products selectivity were determined by a Hewlett-Packard model 1050 chromatograph equipped with an ultraviolet detector (λ max = 254 nm) on a 25 cm RP-18 column.
with the mother liquor was transferred into the Teflon-lined autoclave and heated under static conditions at 373 K for 48 h. The resulting solid product was recovered by filtration, washed several times with deionized water, dried at 353 K overnight, and calcined at 813 K for 6 h to remove the template from MCM-41. A series of PWA (10−30 wt %) impregnated on MCM-41 catalysts were prepared by a wet impregnation method. In a typical procedure, the calculated quantity of PWA was dissolved in 40 mL of methanol, which was stirred for 5 min, followed by the slow addition of MCM-41 support to the solution. The solution was kept for 6 h at room temperature under stirring conditions. The solvent was evaporated on a rotavap and then the catalysts were calcined at 573 K for 3 h. 2.3. Physico-chemical Characterization. Low angle powder X-ray diffraction patterns were collected on a Rigaku D MAX III VC diffraction system using Ni-filtered Cu Kα radiation (λ = 1.5404 Å) over the range 0.5−10° (2θ), with a scan rate of 2° per minute. Wide angle X-ray diffraction (WAXRD) were recorded on a PANalytical PXRD model X-Pert PRO-1712, using Ni filtered Cu Kα radiation (λ = 0.154 nm) as a source (current intensity, 30 mA; voltage, 40 kV) and a Xcelerator detector. Nitrogen adsorption/desorption isotherms were measured on a Quantachrome Autosorb-1C sorption unit. The Brunauer−Emmett−Teller (BET) method was used to calculate the multipoint BET surface area. The pore size distribution was derived from the desorption branches of the isotherms using the Barrett−Joyner−Halenda (BJH) method. The total pore volume, Vp, was estimated from the amount adsorbed at a relative pressure of p/po = 0.99. NH3-TPD measurements were performed on a Micromeritics AutoChem 2910 instrument. In a typical experiment, 0.1 g of catalyst was taken in a U-shaped, flow-thru, quartz sample tube. Prior to measurements, the catalyst was pretreated in He (30 cm3/min) at 773 K for 1 h. A mixture of NH3 in He (10%) was passed (30 cm3/min) at 323 K for 1 h. Then, the sample was subsequently flushed with He (30 cm3/min) at 373 K for 1 h. TPD measurements were carried out in the range 373−773 K at a heating rate of 10 K/min. Ammonia concentration in the effluent was monitored with a gold-plated, filament thermal conductivity detector. IR spectra of the PWA impregnated MCM-41 molecular sieves were collected with a FTIR (Spectrum 2000 Perkin-Elmer) instrument using the KBr pellet technique. About 10 mg of sample ground with 200 mg of spectral grade KBr to form a pellet under hydraulic pressure. Its IR spectrum was recorded in the range 4000−400 cm−1. Quantitative elemental analysis was done by inductively coupled plasma optical emission spectroscopy (ICP−OES) on a Spectro Arcos instrument equipped with winlab software. 2.4. Catalyst Activity Measurement. Phenolphthalein synthesis was carried out by condensation of phenol with phthalic anhydride using a catalyst in a magnetically stirred glass reactor (capacity 50 mL), fitted with a reflux condenser
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Low angle XRD diffractograms of pure MCM-41 and PWA/MCM-41 samples are shown in Figure 1. The pure MCM-41 sample (Figure 1 a)
Figure 1. Low angle XRD patterns of calcined materials: (a) MCM-41, (b) 10% PWA/MCM-41, (c) 20% PWA/MCM-41, and (d) 30% PWA/MCM-41.
showed three (100), (110), and (200) diffraction peaks below 10° (2θ), which indicate the formation of an ordered hexagonal mesoporous structure.12,13 Interestingly, 10%, 20%, and 30% PWA/MCM-41 samples also showed XRD peaks (Figure 1b− d) similar to that of parent MCM-41; however, the intensity of (100) reflection peak of the MCM-41 diminished with an increase in PWA loading from 10 to 30% (Figure 1). This decrease in the intensity of (100) peaks could be due to the beginning of disorder in the hexagonal structure of MCM-41 support with the increase in PWA loading.14 Wide angle X-ray diffraction patterns of MCM-41, PWA/ MCM-41, and bulk PWA are shown in Figure 2. The parent MCM-41 shows a broad band in the range of 2θ = 15−40°, which is a characteristic of siliceous material.15 All PWA/MCM41 samples (10−30% PWA loading on MCM-41) showed XRD patterns (Figure 2b −d) similar to that of parent MCM-41. No separate crystal phase characteristic of bulk PWA exists in the 10%, 20%, and 30% PWA/MCM-41 samples due to the larger surface area of MCM-41. This also confirms the high dispersion of PWA on MCM-41 support. The acidic properties of the samples with PWA loading ranging from 10 to 30% were investigated by FT-IR 3917
dx.doi.org/10.1021/ie201989g | Ind. Eng. Chem. Res. 2012, 51, 3916−3922
Industrial & Engineering Chemistry Research
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PWA was dispersed on MCM-41, both Brönsted as well as Lewis acid sites were observed. The acidity increased gradually with increasing PWA loading, as revealed by the acidity measurement (NH3-TPD) shown in Figure 4. Parent MCM-41
Figure 2. Wide angle XRD patterns of (a) MCM-41, (b) 10% PWA/ MCM-41, (c) 20% PWA/MCM-41, (d) 30% PWA/MCM-41, and (e) bulk PWA.
spectroscopy of adsorbed pyridine and NH3-TPD measurements. FT-IR spectra of chemisorbed pyridine showed both Brönsted as well as Lewis acid sites in these samples (Figure 3).
Figure 4. NH3-TPD profiles of (a) MCM-41, (b) 10% PWA/MCM41, (c) 20% PWA/MCM-41, and (d) 30% PWA/MCM-41 catalysts.
showed (Figure 4 a) a very small hump indicating low acidity which was in accordance with the results of pyridine IR (Figure 3) while PWA/MCM-41 samples with various PWA loadings, showed broad desorption peaks near 573 K (Figure 4b−d). The concentration of acid sites (in terms of NH3 desorbed) of parent MCM-41 increased from 0 to 0.9 μmol S−1 with an increase in PWA loading from 0 to 30% (Table 1). Table 1. Textural Properties of the Catalysts catalysts MCM-41 10% PWA/MCM41 20% PWA/MCM41 30% PWA/MCM41 bulk PWA 20% PWA/Mont K10 montmorillonite KSF/O
surface area (m2/g)
pore size BJHDES (nm)
pore volume BJHDES (cc/g)
NH3 desorbed (μmol/S)
1250 1050
3.98 3.84
1.28 0.98
nil 0.2
1036
3.72
0.96
0.4
619
3.57
0.55
0.9
8.3 130
93.6 4.8
128.4
6
Nitrogen adsorption−desorption measurements were used to study the textural properties of the prepared catalysts. A typical isotherm of the MCM-41 and 20% PWA/MCM-41 samples are shown in Figure 5 curves a and b, respectively. Isotherms of MCM-41 and 20% PWA/MCM-41 showed a type IV isotherm having a hysteresis loop at p/po = 0.3−0.4, which is a characteristic of mesoporous materials.18 With increasing PWA loading, the hysteresis loop became gradually shorter corresponding to a reduction in the pore volume. A bend in the adsorption−condensation region, at p/po = 0.3−0.4 for the 20% PWA/MCM-41 sample could be due to dispersion of PWA over the walls of MCM-41.19 Table 1 shows values of BET surface area, pore size, and pore volume for MCM-41 and supported PWA catalysts. It was observed that parent MCM-41 had the highest surface area and pore volume among the
Figure 3. FT-IR pyridine spectra of MCM-41 and 10−30% PWA/ MCM-41 catalysts.
The band at 1545 cm−1 (pyridinium ion) was assigned to Brönsted acid sites, whereas bands at 1610 and 1450 cm−1 (pyridine coordinated to Lewis acid sites) were assigned to Lewis acid sites. The band at 1490 cm−1 (hydrogen bonded pyridine) is common to both types of acidic sites.16,17 The surface of MCM-41 contained only Lewis acid sites, but as 3918
dx.doi.org/10.1021/ie201989g | Ind. Eng. Chem. Res. 2012, 51, 3916−3922
Industrial & Engineering Chemistry Research
Article
Figure 5. Adsorption isotherm of (a) MCM-41 and (b) 20% PWA/ MCM-41 catalysts.
Figure 7. Catalysts screening for synthesis of phenolphthalein. Reaction conditions: phenol, 34.0 mmol; phthalic anhydride, 16.2 mmol; catalyst concentration, 0.1 g/cm3; temperature, 423 K; time, 3 h; mole ratio of phenol to phthalic anhydride, 2:1.
prepared catalysts,20 while an increase in PWA loading caused a reduction in the pore volume and notable compression of the pore size distribution, which seems logical as the impregnated PWA was dispersed and deposited on the support surface, decreasing the pore diameter and thus diminishing the surface area.21 These observations were also consistent with the low angle XRD results, which showed a decrease in intensity of the (100) diffraction peak due to diminishing hexagonal order structure with PWA loading. However, the average pore size distribution of the prepared catalysts was centered at about 24.5 Å, the same as that of parent MCM-41 (Figure 6).
dispersed on MCM-41, a number of active sites on the surface increased considerably leading to higher activity of supported PWA catalysts as compared to bulk PWA. In the case of 20% PWA/Mont K10 and Mont KSF/O, conversion of phthalic anhydride was less than that of PWA supported catalysts but higher than bulk PWA. This result could be due to the lower surface areas of 20% PWA/Mont K10 and Mont KSF/O catalysts as compared to the PWA/MCM-41catalysts. The catalyst performance of 20% PWA/MCM-41 was also evaluated for the hydroxyalkylation of phenol and p-cresol with formaldehyde (Schemes 2 and 3, respectively) to give corresponding DAM. For the hydroxyalkylation of phenol, 20% PWA/MCM-41 showed 31% conversion of phenol with 94 and 6% selectivity to bisphenol F and trimers, respectively. While for the p-cresol hydroxyalkylation reaction, it gave 34% conversion with 97 and 3% selectivity to DAM and trimer, respectively. The catalyst activity and reusability results of 20% PWA/MCM-41 for the hydroxyalkylation of phenol and pcresol are discussed in later section (Figure 13a,b). The effects of the mole ratio of phthalic anhydride to phenol on conversion and phenolphthalein selectivity were studied in the range of 1:1 to 1:3 for 20% PWA/MCM-41 catalyst by varying the concentration of phenol at a constant phthalic anhydride concentration, and the results are presented in Figure 8. The conversion of phthalic anhydride increased from 17 to 40% as the mole ratio of phthalic anhydride to phenol increased from 1:1 to 1:2. Since the formation of phenolphthalein involves the condensation of two moles of phenol with one mole of phthalic anhydride, conversion of the latter was facilitated with an increase in phenol amount from 1:1 to 1:2. With a further increase in the mole ratio from 1:2 to 1:3, the conversion of phthalic anhydride decreased from 40 to 32%, while selectivity to phenolphthalein remained almost constant (91%). This could be explained based on the competition between phenol and phthalic anhydride molecules for active acid sites of the catalyst. Since nucleophilicity of the phenol molecule is higher than that of phthalic anhydride, causing better adsorption of the former rather than the latter on active acid sites resulted in lower conversion of phthalic anhydride. The effect of the catalyst concentration in the range of 0.02− 0.1 g/cm3 on the conversion of phthalic anhydride and phenolphthalein selectivity was also studied, and the results are
Figure 6. Pore size distribution of MCM-41 and 20% PWA impregnated MCM-41 catalysts.
3.2. Catalytic Activity. Phenolphthalein synthesis is an acid-catalyzed condensation reaction of phenol and phthalic anhydride, for which various solid acid catalysts were screened (Figure 7). Among these, 20% PWA/MCM-41 showed better activity and selectivity to phenolphthalein. Almost negligible activity of MCM-41 (not shown in the figure) was due to its extremely low concentration of acidic sites (Figure 4) which was a more predominant factor than its surface area (Table 1). On the other hand, bulk PWA showed lowest activity as compared to the supported PWA catalysts despite its strong Brönsted acidity. The very low surface area (
