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Evaluation of synthesized green carbon catalyst from waste date pits for tertiary butylation of phenol
⁎Corresponding author. muhtaseb@squ.edu.om (Ala'a H. Al-Muhtaseb)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The present study is intended to adopt a facile method for preparing a sulphonated green carbon catalyst from date pits biomass. Catalyst synthesis involves in situ carbonization and sulphonation and it has been characterized by following techniques such as XRD, SEM, EDX, TEM, FTIR, TGA, and BET. Surface and internal morphology results exhibited that the synthesized sulphonated carbon material possesses a mesoporous structure, while activated carbon possesses a microporous structure. Furthermore, the Fourier transform infrared (FTIR) spectra confirmed the presence of acidic groups (—OH, —COOH, and —SO3H) in synthesized sulphonated carbon material. Sulphonated carbon material exhibited high acidity (4.7 mmol/g) and good thermal stability. The application of this catalyst for the tertiary butylation of phenol without using any solvent has been investigated. The phenol alkylation reaction showed maximum conversion at reaction condition: temperature (140 °C) with 2 bar (nitrogen gas) pressure with maximum phenol conversion 79.27 wt%, with 68.01% selectivity towards 4TBP+2,4TBP, which is used as an intermediate in antioxidants. The catalyst exhibits comparable catalytic performance up to five reaction cycles. Thus it can be concluded that waste date pits can be successfully employed for green catalyst synthesis and used for reactions involving large molecules.
Keywords
Waste date seeds
Sulphonation
Carbonization
Butylation
Phenol
1 Introduction
Date palm is the main crop in Oman, and its production reached 3,08,000 tons in 2014 from 2,81,000 tons in 2013, according to the latest data released by the Ministry of Agriculture and Fisheries, Oman (Mabood et al., 2015; Jamil et al., 2016). This would deliver a large amount of residue, mainly their pits, which comprise almost 10–15 wt% of the total, and this huge amount of residue becomes an environmental problem, while the best solution for this is to produce biofuels and chemicals with that residue (Al-Badi and Al-Badi, 2012; Baliga et al., 2011; Amira et al., 2011). Date pits have been widely studied as adsorbent materials after carbonization to form activated carbon, and used further as an adsorbing agent due to their high wooden fibre content (Mahmudi et al., 2014; Halbus et al., 2013; Naushad et al., 2016; Allaboun and Al-Rub, 2016; Naushad et al., 2015; Sayğılı and Güzel, 2016).
In the last decades, an increased interest in biomass-derived carbon supported catalysts has been widely reported (Bagheri et al., 2015; Gupta and Paul, 2014; Rizhikovs et al., 2012; Yu et al., 2015). Due to a high density, mechanical strength, and porous nature with porous transport channels, activated carbon can be used as adsorbent and catalyst support for acid catalysed reactions (Jiang et al., 2016; Daud and Ali, 2004). There are several heterogeneous catalysts, such as metal oxides, metal sulphide, zeolite sieves, heteropoly acids, ion exchange resins, and clay, but none of these mentioned catalysts has been found suitable for acid catalysed reactions due to several facts, such as leaching, internal mass transfer limitation, and deactivation due to carbon deposition on sites (Gupta and Paul, 2014; Sun et al., 2015; Chen et al., 2014; Rafiee and Eavani, 2014; Wu et al., 2014). So, waste biomass-derived carbon catalysts are the best alternatives as they possess a large specific area, and a wide range of pore size. Activated carbon functionalized with sulphuric acid exhibits high acidity and appears to be a highly stable catalyst for liquid phase acid-catalysed reactions with noncorrosive and easily separable property (Hussein et al., 2000; Yang and Zou, 2014; Nandan et al., 2011).
Sulphonated activated carbon catalysts from waste bagasse involve carbonization at 375 °C for 30 min followed by sulphonation at 150 °C for 15 h, which gives an acidic strength as high as 1.06 mmol/g. Carbonization of corn straw for 1 h at 300 °C and sulphonation carried out in the presence of sulphuric acid fumes at 180 °C for 4 h gives a high acidic strength catalyst with an acid density of 2.64 mmol/g (Gergova et al., 1994). A comparison study for high acidic strength activated carbon catalysts from different waste biomass sources which involves sugar cane bagasse, coconut husk, and coffee grounds involves carbonization at 400 °C for 4 h, followed by sulphonation at 170 °C for 10 h in the presence of sulphuric acid. A maximum acidic strength was observed to be 4.9 mmol/g (Goncalves et al., 2013). (Suriapparaoa and Vinu) synthesized sulphonated carbon catalysts from char produced by pyrolysis of waste biomass for bio-oil production, and accredited it to low cost feedstock acid catalysts (Suriapparaoa and Vinu, 2015).
Tertiary butylation of phenol is of great industrial importance. Based on demand, an estimated amount of products from alkylation of phenol required annually by industry is more than 450,000 tons approximately (Krishnan et al., 2002; Modrogan et al., 2009). The products obtained from alkylation of phenol such as mono- and di-alkylated phenols have a huge demand for synthesizing UV absorbers, phenolic resins, and production of antioxidants. A solvent-free tertiary butylation of phenol usually shows less conversion (Dumitriu et al., 2005). Khatri et al. (2015) used sulphonated carbon composite (P—C—SO3H) for the alkylation of phenol and obtained up to 96% conversion of phenol to mono- and di-alkylated phenols. Similarly, Nandan et al. (2011) used sulphonated carbon prepared from coal tar for tertiary butylation of phenol and obtained 85% phenol conversion to 2-TBP, 2,4-TBP, and 2,4-DTBP. Moreover, Liu et al. (2010) also used sulphonated carbon catalysts for alkylation of hydroquinone with tertiary butanol and observed 73.3% conversion at 150 °C. So, based on the previous literature, it can be concluded that sulphonated carbon catalysts can best suit the alkylation of phenol.
In this paper, waste date pits sulphonated carbon catalysts by in situ carbonization and sulphonation method are synthesized and used for the tertiary butylation of phenol. The present study highlights the simple method for synthesis of acidic catalysts from date pits biomass and correlates the relationship between catalyst morphology and suitable catalytic applications involving bulky molecular transformations such as liquid phase butylation of phenol.
2 Experimental
2.1 Materials
Waste date pits were supplied from a local farm in Oman, while other chemicals e.g. H2SO4, NaOH, HCl, Tertiary butyl alcohol (TBA), and phenol were supplied by Merck chemicals (Germany).
2.2 Preparation of catalysts
In a typical procedure, 100 g of date pits was crushed and sieved to particle size less than 20 mm. Powdered date pits were dried at 100 °C for 8 h to remove the complete moisture. After that, the sample was washed with warm deionized water to remove the impurities and dried under vacuum at 100 °C for 3 h, and then it was used for carbonization and sulphonation. The carbonization process was used to convert the date pit biomass into activated carbon in a furnace at 500 °C for 5 h with a heating rate of 3 °C/min under N2 gas at the flow rate of 50 ml/min. In order to synthesize the sulphonated carbon material, the synthesis procedure followed the carbon material exchange with sulphuric acid (0.05 M) at 100 °C for 4 h. Afterwards, the mixture was washed with deionized water to remove the excess amount of sulphuric acid until the pH of the washing solution reached a neutral level. Finally, the prepared catalyst was dried at 120 °C under vacuum for 4 h. Furthermore, the carbonization of the resultant mixture was carried out in the nitrogen atmosphere at 300 °C for 4 h to facilitate the decomposition and transformation of the date pits carbon to hydrophobic carbon residue bearing sulphonyl groups. The resultant material was washed with warm deionized water to remove the weakly bound acid sites and carbon from the moiety surface and finally the sulphonated carbon material was dried and followed by calcination at 400 °C for 4 h to obtain porous activated sulphonated carbon material. The activated carbon and sulphonated carbon represents the coding ACT-C and SUL-C respectively.
2.3 Catalyst characterization
X-ray powder diffraction (XRD) patterns were measured on PANalytical Xpert PRO instrument, USA, equipped with a rotating anode and Cu Kα radiations. The measurements were conducted in a continuous θ/2θ scan refraction mode. The anode was operated at 30 kV and 15 mA, and the 2θ angles were measured 5–80° at the rate of 2°/min. Surface and crystal morphology of activated carbon and sulphonated carbon material samples was determined using Scanning Electron Microscope (SEM) and Transmission Electron Microscopy (TEM) techniques. SEM (JEOL JSM-7900F instrument, Japan) images were recorded for obtaining particle morphology while TEM (JEOL, JSM-2100F instrument, Japan) images were recorded for crystal morphology, which was operated at 200 kV and equipped with a microprobe system. A small amount of each sample was lightly ground and subsequently dispersed ultrasonically in ethanol. A drop of suspension was deposited on a carbon film supported on a copper grid for TEM measurement. The BET surface area, pore size, and pore volume measurements of both samples were carried out using a standard adsorption equipment (ASAP 2020, Micromeritics Instruments Inc., Norcross, GA, USA) using N2 gas (99.995% pure). A sample (0.2 g) was taken in a specially designed sample tube and degassed at 300 °C under vacuum of 1.3 × 10−6 bar for 4 h. A frit was attached to the mouth of the sample tube, so that when the sample tube was removed from the preparation mode, it would not have allowed the sample to get exposed to the atmosphere. The sample was cooled to room temperature (25 °C) under vacuum, and the sample tube is removed from the preparation port and attached to the analysis port of the instrument. For all the samples, N2 adsorption-desorption isotherms were obtained at −196 °C and the temperature was maintained at a constant using liquid nitrogen, whereas helium gas was used for measuring the dead space. The surface area, pore volume, and pore size distribution were obtained by measuring the volume adsorbed at different P/P0 values and by applying different methods. The total pore volume was estimated by measuring the volume of gas adsorbed at P/P0 of 0.99 whereas t-plot method was used to calculate the micropore surface area (0–20 Å) using the Harkins-Jura equation. The total micropore volume (0–20 Å) was obtained by applying the Horvath-Kawazoe method (H-K). A thermo gravimetric analysis (TGA) was done using the Perkin Elmer instrument. The analysis was done for a 10 mg sample which was heated from room temperature to 900 °C at the constant rate of 10 °C/min in the presence of nitrogen gas flowing at the rate of 40 ml/min. An FTIR was done by Perkin Elmer-2000 using the KBr pellet technique on spectrometer used for qualitative analysis of chemical structure and framework skeletal vibrations of activated carbon and sulphonated carbon samples. Acidity of both the synthesized materials activated carbon and acid-functionalized carbon material was determined by standard acid-base Boehm’s titration method. A standard base solution 0.1 N of NaOH was mixed with 0.1 g of the desired sample and allowed to react for 36 h at room temperature (25 °C) with continuous stirring. The solid was filtered and the remaining solution was titrated against the standard acidic solution of HCl (0.02 N) using phenolphthalein as an indicator to determine the total acidity. The CHNS analyser (CHN-2400 PerkinElmer, USA) was used to measure the weight percentage of the carbon, hydrogen, nitrogen, and sulphur in the samples. In order to check the leaching of sulphur in product Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) has been used. XPS spectra were obtained using photoelectron spectroscopy (Omicron Nanotechnology, Germany) with a monochromatic Al Kα radiation (hν = 1486.6 eV). XPS spectra were analysed using Casa XPS software (Casa Software Ltd). The binding energies of the obtained spectra were calibrated with respect to the intrinsic carbon C 1 s peak at 284.6 eV. In order to reduce charging effect, built-in neutralizer was used during the measurement.
2.4 Catalytic performance study
In general, the tertiary butylation of phenol is conventionally carried out at higher reaction temperature. The reaction of tertiary butyl alcohol (TBA) with phenol in the presence of solid acid catalyst mainly produces three products, namely 2,4-di-tertiary butyl phenol (2,4-DTBP), 4-tertiary butyl phenol (4-TBP), and 2-tertiary butyl phenol (2-TBP). In the present study, we have carried out liquid phase phenol butylation in Parr reactor autoclave. The synthesized material was then evaluated for solvent-free liquid phase alkylation reaction at a temperature range of 90–160 °C for 7 h reaction time, with phenol to TBA molar ratio 1:2.5 and 5 wt% catalyst (total reactant mixture (phenol + TBA)). The reactant mixture was transferred into the Parr reactor and the latter was pressurized up to 2 bar with nitrogen gas; then, the temperature increased by slow heating with PID controlled program up to the desired temperature. The reaction product was collected after 7 h and the used catalyst was separated by filtration, washed with ethanol, dried at 120 °C, and reused five times. The reaction product was analysed using a gas chromatograph (GC, Perkin Elmer) connected with a DB wax column (30 m length, 0.28 mm id, 0.25 μm film thickness).
3 Results and discussion
3.1 Catalyst characterization
XRD patterns for activated carbon and sulphonated carbon are shown in Fig. 1. The broad diffraction peak 10–35° for activated and sulphonated carbon reveals the ordered amorphous nature of carbon structures. The XRD patterns for carbon material at 2θ are reported to give weak and broad peak between 10° and 40° angles, which is related to graphite structure. There is no noticeable difference in the XRD patterns between ACT-C and SUL-C catalysts, which clearly indicates that treatment with sulphuric acid does not affect the microstructure of the carbon materials. The morphology by SEM images of the ACT-C and SUL-C catalysts at various scales is shown in Fig. 2. SEM images for both ACT-C and SUL-C catalyst suggested that the white dots represent the inorganic composition. It can be revealed from the images that activated carbon possesses irregular compact structure while particles appear to be like grains with smooth surfaces. Fig. 3 shows the EDX plots for both ACT-C and SUL-C catalysts, and the presence of sulphur in the SUL-C plot was observed. Moreover, there were some extra peaks in plots for ACT-C and SUL-C which refer to impurities, as the carbon source was waste biomass due to which these impurities might have been present. The morphology and structure for activated and sulphonated carbon were analysed by TEM as shown in Fig. 4. The TEM image of the ACT-C catalyst shows the presence of only micropores while the SULC catalyst shows the micropores along with the mesopores, which were revealed due to sulphonation on carbon material, creating the mesopores in the carbon material.
The N2 physiosorption analysis was carried out to find out the pore volume and surface area as listed in Table 1, and to check the adsorption-desorption behaviour of ACT-C and SUL-C catalysts as shown in Fig. 5. From Fig. 5 it can be observed that as the relative pressure was increased for ACT-C till 0.45 and for SUL-C till 0.05, isotherms exhibited the microporous nature of the material, but by further increasing the relative pressure, a hysteresis loop appeared due to capillary condensation which represents the mesoporous material. Meanwhile, it can also be seen that after sulphonation, the meso/macro pore volume of SUL-C increased to 0.0757 cm3/g which was also supported by N2 physiosorption isotherms that revealed the presence of big hysteresis loop for SUL-C as compared to ACT-C. It can be further observed that after the sulphonation, the surface area and pore volume of the catalyst decreased from 432.14 m2/g and 0.2234 cm3/g to 272.09 m2/g and 0.1878 cm3/g respectively. It is clear that during sulphonation, the grafted SO3H groups have occupied a small part of the pore space. The pore diameter did not change considerably during the sulphonation.
| Properties | ACT-C | SUL-C |
|---|---|---|
| SBET (m2/g) | 432 | 272 |
| Micro pore area (m2/g) | 388.7 | 219.9 |
| External surface area (m2/g) | 62.7 | 52.7 |
| Total pore volume (cm3/g) | 0.223 | 0.188 |
| Micro pore volume (cm3/g) | 0.207 | 0.112 |
| Meso/macro pore volume (cm3/g) | 0.017 | 0.076 |
| Median pore diameter (Å) | 7.48 | 7.55 |
ACT-C: Activated carbon catalyst sample; SUL-C: sulphonated carbon catalyst sample.
The thermal behaviour of ACT-C and SUL-C catalysts was determined by TGA as shown in Fig. 6. The TGA plots for both the materials present weight loss by increasing the temperature under N2 atmosphere by a heating rate of 10 °C. The thermal decomposition behaviour of activated carbon material attributes to a marginal weight loss at 110 °C, which is probably due to the thermo-desorption of the physical adsorb material, which is basically the moisture contents. Similarly, the marginal weight loss occurred for SUL-C sample at 110 °C due to desorption of water vapours. Meanwhile, for sulphonated carbon, it can be observed that it is more stable than activated carbon, and can bear temperatures up to 430 °C as it can be seen from the plot which shows that there is almost plateau between 140 and 430 °C due to vey less weight loss which normally depicts the decomposition of SO3H for sulphonated carbon. Further on, from 250 °C till 830 °C for activated carbon, a continuous weight loss can be observed from a plot which refers to thermal decomposition of weak acidic sites and strong Bronsted acidic sites. As for sulphonated carbon, loss of weak and strong Bronsted acidic sites was observed to be from 430 °C to 830 °C. TGA analysis exhibits that the sulphonated carbon material was thermally highly stable as compared to the activated carbon material. Elemental analysis and total acid capacity for both ACT-C and SUL-C are mentioned in Table 2. The Boehm titration experiments used to determine the total acidic capacity for sulphonated carbon are 4.7 mmol/g. The acidic capacity represents the total number of acidic functional groups, which includes —SO3H, —COOH, and —OH present in sulphonated carbon material (Yang and Zou, 2014). However, these acidic groups are hydrophilic in nature, which helps to promote the alkylation of phenol, that involves tertiary butanol and phenol, which are hydrophilic in nature, and reaction among them is enhanced in the presence of synthesized sulphonated carbon as a catalyst.
| Catalyst | Carbon (wt%) | Hydrogen (wt%) | Sulphur (wt%) | Oxygen (wt%) | TADa (mmol/g) |
|---|---|---|---|---|---|
| ACT-C | 76.24 | 6.91 | 0.10 | 16.75 | 0.28 |
| SUL-C | 73.15 | 5.23 | 3.29 | 18.33 | 4.70 |
FTIR spectra for activated and sulphonated carbon are shown in Fig. 7. The peaks appearing to be at the 3400 cm−1 wave number are referring to the —OH stretching (Sharma et al., 2017; Alqadami et al., 2016) for both activated carbon and sulphonated carbon while the definite peaks present at 2900 cm−1 and 2850 cm−1 are due to the C—H stretching vibrations for both materials. Moving further, the small peak appearing at 1805 cm−1 represents —C⚌O stretching, and the prominent peak appearing at 1650 cm−1 is referred to as a bending of —OH group present in both materials. Furthermore, the difference in spectra as compared to activated and sulphonated materials is shown from 1500 cm−1 to 900 cm−1, the peaks present in sulphonated material but not for activated carbon material, such as the peak present at 1295 cm−1 are due to —SO3H stretching, while the peak present at 1010 cm−1 is referred to as O⚌S⚌O stretching while these are not present in activated carbon which shows the presence of sulphur group in sulphonated carbon. After reviewing the complete spectra of IR for both materials, it can be concluded that the presence of —COOH, —OH, and —CH in both materials while due to the presence of some definite peaks between 1200 cm−1 and 1000 cm−1 for sulphonated carbon represents the presence of —SO3H group. Ngaosuwan et al. (2016) have reported a similar kind of spectra for sulphonated and activated carbon. Based on the textural properties of ACT-C (activated carbon) and SUL-C (sulphonated carbon) catalysts, it is clear that the presence of mesopores in sulphonated carbon makes it suitable for organic reactions. The presence of mesopores in sulphonated carbon revealed that in situ sulphonation with carbonization of carbon material leads to a suitable technique. The pore size in sulphonated carbon generally increases due to the treatment of carbon with H2SO4. However, in general, for both materials activated and sulphonated carbon after modification leads to destruction of smooth surfaces and forms cleavages, while large particles are transformed into small ones with an irregular structure. Moreover, images (TEM) show the hierarchical porous structure of sulphonated carbon material and the presence of porous carbon matrix due to interconnected particles. Therefore, it can be concluded that in situ sulphonation and carbonization leads to mesoporous acidic catalysts from waste carbon material with interconnected carbon particles with porous matrix which can be highly efficient for phenol alkylation as it involves bulky molecule products.
3.2 Performance of the catalysts towards tertiary butylation of phenol
The synthesized activated and sulphonated carbon was utilized for alkylation of phenol by t-butyl alcohol. The obtained products were 2-TBP, 4-TBP, and 2,4 TBP. The reaction scheme for alkylation of phenol by t-butyl alcohol is given in Fig. 8. Table 3 shows the conversion of phenol into mono- and di-alkylated products with respect to a temperature range of 90–160 °C, and it has been revealed that very less conversion occurred at low temperature for both synthesized catalysts, but as the temperature increased, the conversion of phenol increased until it was 140 °C. The ACT-C and SUL-C catalysts synthesized in the present study indicated promising catalytic functionality. The maximum conversion of phenol is achieved at 140 °C. A further increase in the temperature results in a decrease in the phenol conversion, and this can be attributed to thermodynamics of alkylation and de-alkylation reaction at higher temperatures (Nandan et al., 2011). Furthermore, it can be also related to the fact that at a higher temperature, several parallel reactions start occurring which consume olefins in side reactions resulted from dehydration of tertiary-butanol, such as oligomerization, alkylation, and cracking (Suriapparaoa and Vinu, 2015). While it is a sure fact that cracking is dominant at high temperatures, this causes a decrement in phenol conversion (Suriapparaoa and Vinu, 2015). On ACT-C catalyst, it was found that 23.38% and 76.62% of 2-TBP and 4-TBP were formed respectively as shown in Table 3 at 140 °C. The SUL-C catalyst exhibits enhanced performance in terms of phenol conversion (79.27 wt%) as well as alkylated product selectivity as 31.99%, 53.15%, and 14.86% to 2-TBP and 4-TBP, and 2,4TBP respectively at 140 °C. Furthermore, a high 4TBP+2,4TBP selectivity 53.15% and 14.86% respectively at 79.27% phenol conversion obtained in this reaction on SUL-C catalyst is suitable for its applications in the production of value added intermediates of antioxidants. It can be observed that 2,4TBP was not observed while using ACT-C as a catalyst, which was attributed due to the presence of micropores and small pore volume of activated carbon catalysts which does not allow the formation of 2,4TBP. Meanwhile due to the mesoporous nature of SUL–C, it can be seen that from 120 to 160 °C, the formation of 2,4TBP was observed, which can be ascribed to the coexistence of hydrophilic group (—COOH and —OH), —SO3H and the hydrophobic carbon support, high acid density, and porous nature of sulphonated carbon.
| Temperature (°C) | Conversion of phenol (wt%) | Selectivity of product (%) | ||
|---|---|---|---|---|
| 2-TBP | 4-TBP | 2,4-DTBP | ||
| ACT–C catalyst⁎ | ||||
| 90 | 9.59 | 24.95 | 75.05 | – |
| 120 | 29.98 | 25.72 | 74.28 | – |
| 140 | 36.55 | 23.38 | 76.62 | – |
| 160 | 30.26 | 26.52 | 73.48 | – |
| SUL–C catalyst⁎ | ||||
| 90 | 15.59 | 20.12 | 79.98 | – |
| 120 | 73.48 | 36.21 | 61.23 | 2.56 |
| 140 | 79.27 | 31.99 | 53.15 | 14.86 |
| 160 | 76.87 | 37.08 | 47.76 | 10.68 |
3.3 Reusability of catalyst
The activity in reusability of the catalyst (SUL-C) is synthesized by filtering the reaction solution, washing with ethanol, and drying at 120 °C between five consecutive reaction cycles as shown in Fig. 9. Hydrophobic surface properties helped to resist the deactivation of sulphonated moieties of SUL-C. Meanwhile, higher performance of the catalyst observed in the present study can be ascribed to the presence of highly active sites in the sulphonated carbon material obtained in the single step carbonization and sulphonation adopted during the synthesis. Thus results obtained from ICP analysis of product gave very low concentration of sulphur (91 ppb), which confirmed the presence of active sites still in used catalyst due to very less leaching. In order to determine the physiochemical properties of used catalyst BET analysis was conducted and results are reported in Table 4. Thus, it can be observed that for used catalyst the surface area along with pore diameter and volume, varied slightly from fresh catalyst.
| Properties | Used SUL-C |
|---|---|
| SBET (m2/g) | 291 |
| Micro pore area (m2/g) | 243.1 |
| External surface area (m2/g) | 55.3 |
| Total pore volume (cm3/g) | 0.196 |
| Micro pore volume (cm3/g) | 0.115 |
| Meso/macro pore volume (cm3/g) | 0.081 |
| Median pore diameter (Å) | 7.57 |
In order to determine the behavioural changes for fresh and used catalysts, phenol conversion with respect time was considered as shown in Fig. 10. It can be observed that initially for both catalysts the phenol conversion was low and increased gradually with the reaction time. It was noticed, that when the time increased beyond 420 min, as for fresh catalyst conversion starts decreasing; however, for used catalyst it was stable first and then faced the decrement. Thus, the difference in trends can be attributed to the fact that it might be due to changed physiochemical properties as shown in Table 4, that might allow the availability of active sites of catalyst for reaction and desorption of products in different scenarios for both fresh and used catalysts. Further on, when structure is referred it can be seen from SEM images (irregular particular structure same as fresh catalyst) and XRD plot in Figs. 11 and 12 respectively that no significant changes were observed for used catalyst. Hence, the structure and nature of the catalyst is retained after tertiary butylation of phenol due to which phenol conversion is not much affected when spent catalyst is used for next run.
Fig. 13 shows the high resolution XPS spectra of S 2p peaks for fresh and used sample catalysts. The S 2p peaks comprised of two main components namely S 2p3/2 and S 2p1/2 respectively. For fresh sample, binding energy of S 2p3/2 and S 2p1/2 can be observed at 168.8 eV and 170 eV while 169.3 eV and 170.7 eV for used catalyst sample. The slight higher binding energy shift of ΔBE ∼ 0.7 eV was observed for used catalyst compared with fresh one. In this case, sulphur atoms on the catalyst surface were interacted with reactants which might lose electron during reaction time resulted in the binding energy shift to higher values. Similar case has been observed by Lim et al. where there was charge transferred from Ag to C where positive binding energy shift was observed (Ngaosuwan et al., 2016). This process led to the shifting of higher binding energy ca. 0.7 eV in used catalyst.
Thus, based on detailed study for used catalyst it can be concluded that reaction occurred within pores which can be attributed from BET analysis (variation in surface area and pore size); meanwhile, the reaction also took place on surface due to the presence of active sites and it can attributed from XPS results (shift in binding energy for S2p for fresh and used catalysts). Meanwhile, the surface morphology and structure were retained for used catalyst. So, it can be concluded that synthesized catalyst is highly active and reusable and can be used on commercial scale.
The comparison for the conversion of phenol by using the catalysts synthesized with several catalysts previously used for tertiary butylation of phenol is shown in Table 5. It can be observed that sulphonated carbon material gave better conversion at low temperature conditions. That might be due to the high acidity of SUL-C catalyst. The high acidity of the SUL-C is due to the coexistence of the hydrophilic group (—COOH and —OH), —SO3H, and the hydrophobic carbon support.
| Catalyst | Temperature (°C) | Phenol conversion (%) | References |
|---|---|---|---|
| SUL-C | 140 | 79.27 | Present |
| ACT-C | 140 | 36.55 | Present |
| β-zeolite | 70 | 54.2 | Ngaosuwan et al. (2016) |
| Zirconium phosphate | 80 | 86 | Lim et al. (2007) |
| Sulphated ZrO2 | 175 | 57.8 | Dumitriu and Hulea (2003) |
| SAPO-11 | 175 | 59 | Hajipour and Karimi (2014) |
| USY | 70 | 37.7 | Ngaosuwan et al. (2016) |
| Al-SBA | 165 | 62.3 | Sakthivel et al. (2001) |
4 Conclusions
The findings of the present study demonstrated a novel approach for the synthesis of a potential sulphonated carbon catalyst (SUL-C) prepared by comprehensive technique of in situ carbonization and sulphonation from date pits biomass waste. The method is cheaper and produces thermally stable material suitable for catalytic applications involving bulky organic transformations. The synthesized sulphonated carbon material was mesoporous with a mesopore volume of 0.0757 cm3/g and a total acidic capacity of 4.7 mmol/g, which attributed to its novel characteristics. It was thermally stable up to 430 °C and the FTIR analysis confirmed the presence of acidic groups such —SO3H, —OH and —COOH. The present study showed a maximum phenol conversion of 79.27 wt% with 68.01% selectivity towards 4TBP+2,4TBP which is used for synthesizing various antioxidants, UV absorbers, and phenolic resins. Moreover, the active material does not undergo deactivation, and can be efficiently reused in five consecutive catalytic cycles. Furthermore, the highly mesoporous material obtained is expected to be an efficient catalyst for several other bulky-molecules involved reactions of industrial importance.
Acknowledgements
The authors would like to thank “The Research Council” of Oman for their generous financial support under grant number ORG/EI/13/013 and for the generous financial support of the Oman India Fertiliser Company S.A.O.C. One of the authors (Mu. Naushad) is grateful to the Researchers Supporting Project number (RSP-2019/8), King Saud University, Riyadh, Saudi Arabia for the support.
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