Alkylated phenols and their derivatives are important materials in both organic syntheses and chemical manufacturing. Mono-alkylphenols and di-alkylphenols are used as raw materials for the manufacture of diverse products such as resins, wire enamels, varnishes, printing inks, antioxidants, flame retardants, ultraviolet absorbers, fungicide, petroleum additives, and rubber chemicals [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. Friedel-Crafts alkylation of phenol using tert-butyl alcohol (TBA) generally produces 2-tert- butylphenol (2-TBP), 4-tert-butylphenol (4-TBP), 2,4-di-tert- butylphenol (2,4-DTBP), 2,6-di-tert-butylphenol (2,6-DTBP), and tert-butylphenol ether (TBPE), depending on the catalyst and reaction conditions. Based on previous studies, it is well known that moderate acid catalysts or high-temperature reactions lead to carbon-alkylated products, whereas weak acid catalysts produced TBPE as a major product. Product 2-TBP forms because of the presence of phenolic (-OH) groups on the aromatic ring that kinetically favor o-alkylation. However, because of steric hindrance, thermodynamically non-favored o-isomer 2-TBP readily isomerizes into the less hindered p-isomer (4-TBP), especially under moderate acidic media conditions. When strong acid catalysts are used in the alkylation reaction, 2,4-DTBP is the dominant product [17, 18]. 2,4-DTBP is used in the manufacture of its triphosphite derivatives and benzotriazole, which are employed as a co-stabilizer for PVC or UV absorbers in polyolefins [12, 13]. 2-TBP is used in pesticides, fragrances, and antioxidants [14]. High selectivity towards 4-TBP is preferred because this product imparts enhanced properties to the class of metallic detergents (phenates) that are used in lubricating oils [15]. Moreover, 4-TBP can be used in the manufacture of fragrances, phenol-formaldehyde resins, perfumes, antioxidants, ultraviolet absorbents, polymerization inhibitors, lube additives, oil field chemicals, and emulsifiers [1, 8]. Therefore, investigation of this alkylation reaction has great commercial interest as well as academic. Generally, Brnsted- and Lewis acid-type catalysts, such as AlCl3, ZnCl2, BF3, H3PO4, H2SO4, HF, and HClO4, have been used in Friedel- Crafts phenol alkylation [19]. However, these homogeneous catalysts have major drawbacks such as considerable pollution production, high waste disposal costs, poor selectivity, and tedious work-up [7, 20]. Gas phase alkylation of phenol has been investigated as an alternative because it can afford a high phenol conversion [9, 10, 11]. However, the process requires high temperature and pressure that lead to higher costs. Therefore, much attention has been devoted to heterogeneous solid acid catalysts as they have many merits, namely improved activity, high acid strength, enhanced selectivity, and easier post- reaction work-up [21]. The tert-butylation of phenol over various catalysts has been widely reported in the literature, and mainly involves heterogeneous catalysts such as clays and clay-based materials [4, 22], heteropoly acids [16, 23], cation-exchange resins [13], mixed oxides [11, 14], zeolites [6], microporous mesoporous materials [2, 3], molecular sieves [24], supercritical and near-supercritical water [1], ionic liquids [17, 20], MCMs [7, 8, 9, 10], sulfated zirconia [25], and iron-containing mesoporous aluminosilicate [26]. However, in some of these reported methods, low selectivity, by-product formation, long reaction times, organic solvents, and high temperature requirements remain the major disadvantages.
As a heterogeneous green solid acid catalyst, α-zirconium phosphate (ZP), Zr(HPO4)2·H2O, is one of the most important inorganic materials that has been prepared by various methods [27, 28]. Crystalline ZPs are an important class of layered multi-functional materials with a well-ordered structure. The layered structure of ZPs consists of zirconium ions in a semi- planar arrangement, located slightly above and below the mean plane, and each Zr4+ ion is connected via the oxygen atoms of the phosphate groups from above and below the mean plane. Three of the four oxygen atoms in the phosphate groups are bonded to three different zirconium atoms. The fourth oxygen atom of the phosphate groups that bonds to a proton, free -OH group, is oriented towards the interlayer region. These hydroxyl groups are responsible for the Brnsted acidity of ZP. Hence, each zirconium is octahedrally coordinated to six oxygens of six different phosphate groups [27, 28, 29, 30]. The P-OH groups on the surface of ZP serve as adsorption sites for various organic functional groups (acidic, polar, and hydrophobic), thereby allowing control over the reactivity and selectivity of the reaction. Previous studies have shown potential applications of ZP in many fields [31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42]. ZP has attracted enormous interest as a low-cost, efficient, thermally stable, acidic, eco-friendly, and reusable catalyst both in organic syntheses and chemical manufacturing [34, 35, 36, 37, 38]. It is an important ion exchanger [31] with high water tolerance ability and ease of sedimentation [33]. Moreover, it is used in nuclear waste management [32], drug delivery, and immobilization of biological materials [39, 40, 41, 42]. Recently, much attention has been focused on the development of preparation methods to produce different ZP nano compounds such as nanoparticles [35, 36, 43], nanoplates [28, 41], and nanocomposites [44, 45, 46]. Polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA) are both white, water-soluble, odorless, and nontoxic polymers. New PVA- and PVP-based precursor methods to produce nanocomposites and nanoparticles are available [44, 45, 46, 47, 48, 49, 50].
To the best of our knowledge, there are no literature reports on the use of ZP nanoparticles as a catalyst for the tert- butylation of phenol, and only a few studies on the solvent-free alkylation of phenol with TBA have been published [9, 51, 52]. Therefore, in continuation of our studies introducing efficient and green catalysts [53, 54, 55, 56, 57, 58], herein we investigated the alkylation of phenol and some substituted phenols with TBA over ZP nanoparticles under solvent-free conditions. The reaction conditions, such as the amount of catalyst, reaction time, temperature, and molar ratio, were investigated in detail. Additionally, we investigated the alkylation process over catalysts P2O5/ Al2O3, P2O5/SiO2, and α-ZrP (prepared in the absence of the polymers), and the results were compared with those obtained from the studies using ZPA and ZPP. In comparison with the other reported methods, the following advantages were achieved in the current study: (1) higher phenol conversion, (2) higher selectivity towards 4-TBP, and (3) ease of recovery of the product and catalyst.
All chemicals and solvents were purchased from Sigma- Aldrich and Merck and used without further purification.
A 10% solution of PVA (Mw = 70000) was prepared by dissolving PVA in deionized hot water (95 °C). The pH of the solution was adjusted to ~3 by adding HCl. To prevent thermal decomposition and monitor the temperature of the PVA solution, a water bath was used. Then, 50 mL of a 1 mol/L solution of ZrOCl2·8H2O was added dropwise to the precursor solution and heated at 60 °C for 2 h with constant stirring using a magnetic stirrer that affords better dispersion of the Zr4+ ions into the polymer chains. After a homogeneous solution was obtained, 50 mL of 2 mol/L H3PO4 was added dropwise to the solution at 50-60 °C for 2 h, assisted by sonication. Upon addition of phosphoric acid, Zr4+ ions reacted with PO43- ions almost immediately to form ZP. The resulting mixture was allowed to age for 6 h, and then the precipitate was filtered, washed with distilled water several times, and dried in an oven at 80 °C overnight. The obtained white fluffy precursor was ground and calcined at 550 °C for 4 h to remove the organic matrix. The final product, pure ZP nanocrystals, is denoted as ZPA. In another synthesis, a 10% solution of PVP (Mw = 40000) was prepared by dissolving PVP in deionized water. Subsequently, the same procedure, as described above, was used to produce ZP nanoparticles. The final product is denoted as ZPP. The summarized procedure is presented in Scheme 1.
A detailed procedure for the preparation of α-ZrP can be found in the literature [28]. P2O5/Al2O3 [54], P2O5/SiO2 [55], 1-H-3-methyl-imidazolium bisulfate ([Hmim]HSO4) [56], morpholinium bisulfate ([morH]HSO4) [57], and N-(4-sulfonic acid) butyl triethyl ammonium hydrogen sulfate ([TEBSA]HSO4) [58] were prepared according to the previously reported procedures by our research team.
Using N2 adsorption-desorption isotherms at liquid nitrogen temperature, the specific surface areas of the samples were determined by the Brunauer-Emmett-Teller (BET) method on a Quantachrome ChemBET 3000 instrument. Prior to analysis, each sample was degassed at 400 °C for 2 h to remove any adsorbed species on the surface. Pyridine adsorption was performed to determine the acid sites using Fourier transform infrared spectroscopy (FTIR). Prior to the measurements, 20 mg of the catalyst was compacted into a self-standing disc and placed in the IR cell attached to a vacuum line at 400 °C for 4 h. The adsorption of pyridine was performed at 150 °C for 30 min. The excess probe molecules were evacuated at 150 °C for 0.5 h. The adsorption-evacuation process was repeated several times until no changes in the spectra were observed. Total acidity of the samples was determined by temperature-programmed desorption of ammonia (NH3-TPD) on a Quantachrome ChemBET 3000. Before the adsorption of NH3, the samples were pre-treated in He at 250 °C for 30 min, then 350 °C for 1 h, and cooled to 100 °C. Then, NH3 was adsorbed onto the samples for 1 h. NH3-TPD was carried out between 100 and 880 °C at 10 °C min-1 and the desorbed NH3 was continuously monitored using a thermal conductivity detector (TCD). Zr and P contents were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Perkin Elmer ICP-OES 7300 dv spectrometer. FTIR measurements were carried out using a JASCO FT/IR (680 plus) spectrophotometer. The spectra were recorded in the range of 400-4000 cm-1 using the KBr technique. The crystallinity of ZP after calcination at 550 °C was analyzed by X-ray powder diffraction (XRD, Philips X’pert). The samples were scanned in the 2θ range of 0°-50°. The morphology of the samples was investigated on a scanning electron microscope (SEM, Philips XL). Transmission electron microscopy (TEM) images were collected using an LEO912-AB microscope.
The alkylation reaction was carried out in a 25-mL two-neck round bottom flask equipped with a magnetic stirrer and water condenser under solvent-free conditions. The water in the condenser was chilled to 2-3 °C in ice. To minimize evaporation of TBA, a pump was used to circulate the chilled water through the condenser. Phenol (10 mmol) was transferred to the round bottom flask containing 50 mg of the catalyst (pre- activated at 400 °C for 2 h and cooled in a desiccator to minimize moisture content). To minimize TBA vaporization and side reactions such as dehydration of TBA to 2- methylpropene, TBA (20 mmol) was added to the reaction mixture after the reaction temperature reached 80 °C. The optimized reaction conditions were achieved by investigating the phenol-to-TBA molar ratio, reaction time, amount of catalyst, and temperature. Samples were collected periodically and analyzed by gas chromatography (GC). Following reaction completion, the reaction mixture was cooled, and the catalyst was recovered by centrifugation and regenerated by a simple regeneration method for reuse. The reaction mixture was diluted with n-hexane, dried over MgSO4, and then subjected to gas chromatography mass spectroscopy (GC-MS, Agilent 5975C). Alkylation of phenol generated a mixture of three different products: 2-TBP, 4-TBP, and 2,4-DTBP (Scheme 2).
Additionally, alkylation reactions involving catalysts α-ZrP (50 mg, pre-activated at 110 °C for 2 h), P2O5/Al2O3 and P2O5/SiO2 (10 mol%, pre-activated at 120 °C for 2 h), [Hmim]HSO4 (20 mol%), [morH]HSO4 (10 mol%), and [TEBSA]HSO4 (5 mol%) were performed as described above. The conversion of phenol was calculated as the mole percent change in phenol content in the initial and final reaction mixture, and the selectivity of the product was calculated on a mole percent basis using GC data as follows: selectivity = (GC peak area % of the desired product)/(total GC peak area % of all products) × 100%.
To examine the recyclability of the catalyst, the spent ZPA was recovered from the reaction media and reused. For recycling, after the first use, the catalyst was separated from the reaction mixture by centrifugation, washed with ethanol, and refluxed with 15% (w/v) H2O2 for 2 h. Then, it was washed with water, dried at 120 °C for 2 h, and finally activated at 450 °C for 2 h [34, 36]. The regenerated catalyst was used under the established optimum reaction conditions to study the performance of the recycled catalyst. After each reaction cycle, the catalyst was recovered, washed, and activated prior to testing.
To prevent particle aggregation and control the average particle size and shape, polar polymers such as PVP and PVA can be used. The PVP (or PVA) shell encapsulates the nanoparticles and weakens interactions between neighboring nanoparticles, thereby hindering unwanted precipitation of the zirconium phosphates used in this work. It has been reported that the PVA and PVP functional groups are responsible for the formation of hydrogen bonds with the hydroxyl groups on the particle surface [44, 45, 46, 47, 48, 49, 50]. Moreover, very strong interactions between the PVA matrix and ZP nanofillers have been reported [45, 46]. Upon addition of phosphate ions to the PVA/Zr4+ or PVP/Zr4+ solutions, ZP formed immediately. This suggests that hydrogen bonds formed between the anchored polar groups of PVA or PVP, and P-OH groups of ZP play an important role in improving the dispersion of ZP (Scheme 3) [45, 46, 49, 50]. After calcination of PVA/ZP or PVP/ZP at 550 °C for 4 h, instigating the decomposition of the organic matrix, pure ZP nanoparticles with a hexagonal shape were obtained. Alkylation of aromatics over solid catalysts is known to occur either by adsorption of both the aromatic and the alkylating agent molecules on the surface, i.e. via a Langmuir-Hinshelwood mechanism, or by reaction of the adsorbed alkylating agent with aromatic molecules in the gas phase, i.e. via an Eley-Rideal mechanism [59, 60, 61, 62].
TBA and phenol adsorption on the ZPA (or ZPP) surface appears to proceed via hydrogen bonding. In the presence of ZPA (or ZPP) as a solid acid catalyst, TBA converts to tert- butyloxonium, which is very reactive towards nucleophiles such as phenol, subsequently producing 2-TBP, 4-TBP, and 2,4-DTBP. The main advantage of ZPA (or ZPP) as an alkylating catalyst in comparison with other catalysts is that ZPA (or ZPP) can significantly direct the product selectivity owing to its uniform structure (Scheme 4).
ZPA and ZPP feature comparable elemental composition based on characterization studies. Table 1 shows the ICP data whereby the P/Zr ratios for ZPA and ZPP were 1.76 and 1.89, respectively [35, 36].
The BET surface area was estimated from the data obtained from Fig. 1 that shows the N2 adsorption-desorption isotherm of ZPA, as a typical representative, in the relative pressure range (p/p0) of 0.05-1.0. The surface areas of ZPA and ZPP were 118.2 and 119.4 m2/g, respectively. The isotherm shows three adsorption stages. The first stage is observed at p/p0 < 0.35, the second stage is observed in the range of 0.35 < p/p0 < 0.85, and the third stage is observed at the higher relative pressures (p/p0 > 0.85). The nitrogen adsorption-desorption isotherm corresponds to a type-IV isotherm with a distinct hysteresis loop, characteristic of a mesoporous material [63]. The hysteresis loop (type H2) is associated with the occurrence of capillary condensation in the mesopores, indicating the presence of a mesoporous structure in the calcined ZP. The increase in adsorption at the higher p/p0 indicates the presence of larger-sized mesopores in the sample [37, 64, 65].
The nature of the acid sites was studied by IR spectroscopy using pyridine as a probe molecule, and the resulting IR spectra are shown in Fig. 2. The origin of Brnsted acidity of the samples is due to the presence of P-OH groups [35, 36, 37, 65, 66, 67]. The main bands observed in the samples were assigned according to the literature data [66, 67]. The pyridine-desorbed FTIR spectra of ZPA and ZPP showed strong bands at 1632 and 1541 cm-1, typical of pyridinium ions. This result confirms the presence of Brnsted sites [35, 36, 37, 65]. The band at 1488 cm-1 indicates the presence of a mixture of Brnsted and Lewis acid sites that can be respectively assigned to the bands at 1550 and 1444 cm-1 [36, 37].
NH3-TPD analysis provides a quantitative estimation of the total number of acid sites and the degree of acid strength. The peak area associated with the NH3 desorption profile corresponds to the amount of acid sites, whereas the position of the peak, i.e. the desorption temperature, indicates the strength of the acid sites. The higher the temperature of desorption, the stronger the acidity of the sites. ZPA desorbed NH3 across a wide range of temperatures from 100 to 880 °C, which corresponds to the presence of medium and strong acid sites. The characteristic features of the TPD curves of both samples are very similar. The NH3-TPD curve of ZPA (Fig. 3) suggests the presence of significant amounts of strong acid sites in ZPA. Desorption of NH3 starts at ~190 °C, and peak appears at ~423 °C, with a small shoulder at ~333 °C, before decreasing at increasing temperatures. However, desorption is not complete even at 850 °C. The acid strength of the catalyst is attributed to the presence of free P-OH groups on the ZP layers, thus affording ZP as a suitable solid acid catalyst [34, 35, 36, 37, 38]. The desorption amount of NH3 was calculated as ~2.1 mmol NH3/g of catalyst. The obtained results were compared with previous reports [29, 36, 65].
Table 2 shows the specific surface area and total acidity of the solid acid catalysts. As observed, both ZPA and ZPP are more acidic and feature higher surface areas when compared with those of other prepared catalysts.
The FTIR spectra of the calcined ZPs are shown in Fig. 4. The data were compared with those obtained from earlier reports and showed comparable results [65, 68, 69, 70]. The main characteristic bands of ZPA and ZPP (data shown in brackets) are 3366 (3360), 1626 (1621), 1062 (1042), and 597 (594) cm-1. The strong band with a peak maxima at 1062 (1042) cm-1 corresponds to P-O symmetrical stretching vibration of PO43-. The broad peak with a peak maxima at 3366 (3360) cm-1 and the sharp peak at 1626 (1621) cm-1 are attributed to surface asymmetric OH stretching and bending of water molecules, respectively. Individual peaks corresponding to the stretching of different types of P-OH are masked by the broad band at ~3400 cm-1 [65]. The spectral band at 597 (594) cm-1 was assigned to Zr-O bonds.
Figure 5 shows the powder XRD patterns of the prepared ZPs. Some characteristic reflections in the 2θ range of 0°-50° that are representative of the structure of ZP are presented. The diffraction peak at 12° corresponds to a d002 basal spacing of 0.76 nm that is consistent with that of the refined crystal structure of ZP reported in the literature [27, 28, 68, 69, 70]. This confirmed the formation of ZP.
The morphology of the prepared ZP nanoparticles was characterized by SEM and TEM, as conventionally conducted in the respective literature [65, 68, 69, 70] and [36, 43, 65]. The SEM micrographs of ZPs reveal the presence of well-defined hexagonal disks with a very smooth surface (Fig. 6). The morphology of both solid samples is comparable in terms of shape with the layers aligned parallel to each other, indicating good crystallinity. Some small plates and oval-shaped particles were observed.
The TEM images (Fig. 7) show that the particles are hexagonal with smooth surfaces. The average diameter of the ZP nanoparticles is about 60 nm. TEM analysis revealed that both ZPA and ZPP featured particles of comparable shapes and average diameters. However, SEM analysis revealed that ZPA featured bigger hexagonal plates when compared with ZPP. The slight discrepancy was attributed to the onset of condensation reactions between the hydroxyl groups of adjacent particles during aging or calculation, whereby some particles adhered to each other, producing agglomerates. Regardless, the SEM and TEM results of the ZPs samples are consistent with the results reported in the literature [36, 43, 65, 68, 69, 70].
Tert-butylation of phenol using TBA was performed over ZPA nanoparticles, and the reaction parameters such as the amount of catalyst, reaction time, temperature, and reactant mole ratio were varied. No significant differences were observed between the alkylation of ZPA and ZPP (see Section 3.4). The results were based on the ZPA catalytic activity towards the tert-butylation of phenol. The obtained products were 4-TBP, 2-TBP, and 2,4-DTBP. Scheme 2 shows the reaction and the detected products under the established optimized reaction conditions. 2,6-DTBP and TBPE were not detected.
The reaction was carried out with different amounts of catalyst within the range of 10-60 mg under the following conditions: temperature, 80 °C; reaction time, 210 min; and phenol:TBA molar ratio, 1:1. The results are presented in Fig. 8. As observed, the percentage conversion of phenol increased with increasing amounts of catalyst. When the catalyst mass increased from 10 to 50 mg, the conversion of phenol increased from 40% to 86%. This increase was attributed to the corresponding increase in the number of active sites. The increase in the amount of catalyst not only enhanced the percentage conversion of phenol, it also increased the selectivity of the para- product. By increasing the catalyst mass from 10 to 50 mg, the percentage selectivity of 4-TBP increased from 67% to 83%.
Acid-catalyzed alkylation by alcohol or alkene is known to occur by nucleophilic attack of the aromatic substrate by carbonium ion intermediates. Carbonium ion intermediates are generated by protonation and subsequent dehydration of alcohol or by direct protonation of alkene [71]. Because the ZP surface contains numerous hydroxyl groups (P-OH), effective interactions between both phenol and TBA and the hydroxyl groups, via hydrogen bonding, are very likely [72, 73]. The tertiary carbocation appears to interact with the initially adsorbed phenol. This can lead to specific orientations and rationalize such product distribution observed in Scheme 4. Steric hindrance is believed to lower the degree of selectivity for 2-TBP (Fig. 8). Moreover, no 2,6-DTBP was detected. When the catalyst amount was increased to 60 mg, the conversion of phenol increased slightly to 90%, but the selectivity for 4-TBP decreased to 80%. This may be due to the increased availability of acid sites, so that either 2-TBP or 4-TBP can undergo secondary alkylation to produce 2,4-DTBP. To evaluate the role of ZPA, we studied the alkylation of phenol in the absence of catalysts. No products were observed. Thus, the optimal catalyst amount was 50 mg.
Figure 9 shows the phenol conversion and product selectivity over the ZP catalyst as a function of time for the alkylation of phenol with TBA at 80 °C; the molar ratio of phenol to TBA used was 1:2. The percentage conversion of phenol increased with reaction time up to about 210 min. In the early stages of the reaction, the first 60 min, phenol conversion was 30% and increased slightly up to 47% after 120 min. We believe that this may be because tert-butanol is consumed, fewer molecules remained in the reaction mixture, thus more time is required to generate the carbocation intermediate. Furthermore, the initial rate of desorption of the product from the catalyst surface was high, and as the concentration of the product increased in the reaction mixture, the rate became slower. During the first 2 h, only the formation of 2-TBP and 4-TBP was observed, which may be due to the increased availability of phenol around the Brnsted acid sites to react with the tertiary carbocation intermediate. Subsequently, a decrease in the selectivity towards 2-TBP was observed; and an increase in the selectivity towards 4-TBP and 2,4-DTBP, attributed to isomerization and dialkylation of the initial product, respectively, was observed [3, 20, 24, 52]. The conversion of phenol was 86% after 210 min, and the selectivity towards the products 2-TBP, 4-TBP, and 2,4-DTBP was 8%, 83%, and 9%, respectively. It is clear from Fig. 9 that unlike some other reported catalysts [24, 43], the catalytic activity of the ZP nanoparticles remained mostly constant for several hour, and subsequently decreased. This could be due to the formation of water molecules during the reaction that may block Brnsted sites [74, 75]. Prolonging the reaction time was not beneficial to the selective formation of 4-TBP; however, conversion of phenol increased further to 94%. Therefore, an optimal reaction time of 210 min was chosen for the following studies.
The effect of reaction temperature on the phenol conversion and product selectivity was studied in the range of 40-100 °C. To prevent excessive vaporization of TBA, the reaction temperature remained below 100 °C, water in the condenser was chilled, and TBA was added to the reaction media after reaching the desired temperature. The results are summarized in Fig. 10. The conversion of phenol increased with increasing reaction temperatures. Maximum phenol conversion was observed at 80 °C. At 40 °C, phenol conversion was 35% and increased to 86% at 80 °C. Further increases in the temperature did not significantly affect the conversion; however, a slight decrease in phenol conversion was observed. The decrease in the conversion of phenol could be due to the dealkylation of tert-butyl phenol to phenol at the elevated temperatures or the reduced availability of TBA as it was consumed during the alkylation of the mono-alkylated products to form 2,4-DTBP [8, 18, 24].
As the temperature increased from 40 to 80 °C, a decrease in the selectivity towards 2-TBP from 44% to 8% and an increase in the selectivity towards 4-TBP from 56% to 83% were observed. These results could be ascribed to the conversion of 2-TBP into 4-TBP at relatively high temperatures [5]. 4-TBP is the thermodynamically stable product, and its formation is favored at high temperatures. Therefore, an increase in the reaction temperature resulted in an increase in the selectivity towards 4-TBP up to 80 °C (Fig. 10). The selectivity towards 2,4-DTBP increased at reaction temperatures above 70 °C. This may be due to the dialkylation of the mono-alkylated products at the higher temperatures. Based on the phenol conversion and product distribution data, the optimum reaction temperature was 80 °C.
Figure 11 shows the effect of the molar ratio of phenol to TBA on the conversion and selectivity of the catalytic reaction carried out with 50 mg of catalyst for 210 min at 80 °C. The phenol:TBA molar ratio was varied between 3:1 and 1:4. As observed, the conversion of phenol changed gradually. Generally, the phenol conversion increased with increasing amounts of TBA. The maximum conversion of phenol, 88%, was obtained when the molar ratio of phenol:TBA was 1:3. It has been shown that polar molecules, such as methanol and higher alcohols, compete with phenol for adsorption sites, and thus an increase in the molar content of alkylating agent results in an increase in the conversion of phenol [24, 76], as observed in the present study. However, phenol conversion decreased to 73% when the phenol:TBA molar ratio reached 1:4. The significant decrease in the conversion could be attributed to the relative dilution of phenol by TBA upon increase in the concentration of TBA, thereby preventing the chemisorption of phenol [24, 76, 77, 78, 79]. During the reaction, tert-butyl cations are first generated on the active sites and then react with phenol molecules adsorbed onto nearby sites. The active sites on the catalyst may be blocked because of the presence of excess TBA molecules and the onset of water adsorption. This will reduce chances of adsorption of phenol onto the catalyst, consequently leading to a reduced phenol conversion. Moreover, as observed in Fig. 11, the conversion of phenol decreased when the molar ratio of phenol:TBA increased. The data are in agreement with the literature [10, 20, 25].
Furthermore, the selectivity towards the mono-alkylated products decreased when the TBA concentration increased. The selectivity towards 4-TBP was only 60% when the phenol:TBA molar ratio was 1:4. This finding shows that higher concentrations of TBA as the alkylating agent promote the formation of the tert-butyl cations in the reaction medium that are available for possible secondary alkylation of the initially formed mono-alkylated products, 2-TBP or 4-TBP, to produce additional 2,4-DTBP. Figure 11 clearly shows that excess amounts of phenol (phenol:TBA molar ratio > 1) can minimize the formation of 2,4-DTBP. The maximum selectivity for 4-TBP, 83%, was obtained when the phenol:TBA molar ratio was 1:2. A similar observation was reported previously [25, 79]. Based on the phenol conversion and product distribution data, the optimum phenol:TBA molar ratio was 1:2.
The activity of the regenerated catalyst following several catalytic runs under the established optimum reaction conditions is presented in Fig. 12. The spent catalyst generated similar product yields to those produced by the fresh catalyst over the five catalytic cycles studied. The conversion of phenol markedly decreased from 86% to 67% after the fifth cycle.
The observed reduced activity of the spent catalyst confirms the occurrence of catalyst deactivation during the reaction. The regenerated catalysts were characterized in terms of their chemical composition by elemental analysis (Table 1). No significant changes in the composition or chemical environment of the phosphate groups in the catalyst were observed following regeneration over five catalytic cycles. As observed from Fig. 12, ZPA suffered no major activity decrease when used repeatedly over four catalytic runs, though refluxing and thermal activation of the regenerated ZPAs induced increased agglomeration that could consequently reduce the catalyst activity. Figure 13 shows the SEM and TEM images of ZPA after the fifth catalytic run. As mentioned before, agglomeration may be due to the condensation reactions between hydroxyl groups of adjacent particles. As observed, the agglomeration of the plates led to larger particles (about 85-110 nm) and reduced catalyst surface area (down to 70.1 m2/g). Moreover, condensation between hydroxyl groups decreased the number of acid sites on the catalyst (1.0 mmol NH3/g of catalyst after the fifth cycle).
Various substituted phenols were also alkylated by ZPA under solvent-free conditions. The results are presented in Table 3. The presence of electron-donating substituents (-CH3, -OCH3, -OH) on the aromatic ring substantially increased the rate of alkylation (Table 3, entries 1-6), i.e., shorter reaction times were observed, whereas electron-withdrawing groups (NO2, Cl, Br) decreased the rate of reaction (Table 3, entries 7-9). Higher reaction temperatures were required for reactions that involved electron-withdrawing substituents on the phenol. Moreover, the conversion for reactions involving the electron-withdrawing groups was significantly lower than that involving electron-donating groups. The low reaction yield obtained in the presence of electron-withdrawing substituents (Table 3, entries 7-9) may be due to the strong deactivating influence of the chlorine, bromine, and nitro substituents on the aromatic ring.
The performance results of the alkylation of phenol with TBA over P2O5/Al2O3, P2O5/SiO2, α-ZrP (prepared in the absence of the polymers), and a series of ionic liquids ([Hmim]HSO4, [morH]HSO4, [TEBSA]HSO4, and [Hcpy]HSO4) are summarized in Table 4. Aside from α-ZrP, the activity of the catalysts in different organic reactions was reported in our previous studies [54, 55, 56, 57, 58]. The results were compared with those obtained for reactions involving ZPA and ZPP. The activities of all the catalysts were tested under similar conditions. α-ZrP, P2O5/Al2O3, and P2O5/SiO2 showed low conversion of phenol, which reflects their lower activity (reduced surface area and acidity) when compared with that of ZPA and ZPP. However, they showed comparable product selectivity, which may be attributed to the presence of P-OH groups on their surface. Similarly, ionic liquids showed low conversion of phenol but higher selectivity towards 2-TBP. The observed ortho-selectivity can be correlated to the non-selective surface reactions of ionic liquids [5]. These results clearly show that both ZPA and ZPP are good catalysts for this alkylation reaction with excellent selectivity towards the para-product. We believe that the high activity of our catalysts is due to their high surface areas, small nanoparticles, and the presence of well-ordered P-OH groups on the surface (increased availability of acid sites). As noted, ZPP showed a slightly higher catalytic activity (higher phenol conversion by 2%) than ZPA but comparable selectivity towards the para-product (Table 4).
Table 5 shows a comparison of the results obtained for the catalytic reaction involving the ZPA nanoparticles and other reported catalysts under optimum conditions. Despite the different reaction conditions employed across the literature, the catalysts (ZPA and ZPP) used in this study can be classified among high-performance catalysts with regards to the low reaction time and temperature, high phenol conversion, high selectivity towards 4-TBP, and ease of regeneration that the catalysts offer. The main advantages of ZPA as an alkylating catalyst are that it can significantly direct product selectivity, owing to its uniform structure, and afford milder reaction conditions when compared with those of other catalysts (Scheme 4).
ZPA (ZPP) is an efficient and recyclable catalyst for the alkylation of phenol at low temperatures. Hexagonal zirconium phosphate nanoparticles were synthesized and characterized by various methods. The catalyst displayed a high selectivity towards 4-TBP that was generated as the major product along with smaller amounts of 2-TBP and 2,4-DTBP. The following reaction conditions, i.e. temperature 80 °C, reaction time 210 min, molar ratio of phenol:TBA = 1:2, and ZPA amount, 50 mg, afforded a phenol conversion of 86% and a 4-TBP selectivity of 83%. The presence of hydroxyl groups on the catalyst surface appears to play an important role in directing the selectivity of the products through interactions of the hydroxyl groups of TBA with phenol. The catalysts were successfully regenerated without significant losses in their catalytic activity and could be reused after a simple work-up using a mild regeneration technique.
We gratefully acknowledge the funding support received for this project from the Isfahan University of Technology (IUT), IR Iran. Further financial support from the Center of Excellence in Sensor and Green Chemistry Research (IUT) is gratefully acknowledged.
2014, Vol. 35 Issue (7): 1136-1147
PDF (641KB)
2014, Vol. 35 Issue (7): 1136-1147
PDF (641KB)