Kinetic Study of Styrene Oxidation over Titania Catalyst Supported on Sulfonated Fish Bone-derived Carbon

The kinetic evaluation of titania supported sulfonated fish bone-derived carbon (TiO2/SFBC) as a catalyst in styrene oxidation by aqueous hydrogen peroxide was carried out. The catalysts were prepared by carbonation of fishbone powder at varying temperatures 500, 600 and 700 °C, respectively for 2 h, followed by sulfonation with sulfuric acid (1M) for 24 h and impregnated by varied titania concentration 500, 1000 and 1500 μmol. The physical properties of catalysts were characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Scanning Electron Microscope-Energy Dispersive X-Ray (SEM-EDX) and the nitrogen adsorptiondesorption analysis. The catalytic activity result showed that TiO2/SFBC can be used as a potential catalyst in styrene oxidation. Worth noting that the sulfonation process has not only transformed the TiO2/FBC particulates (without sulfonation) to cuboid-shaped TiO2/SFBC (with sulfonation) but also contributed to the high selectivity of benzaldehyde. On the other hand, carbonization at different temperatures has an indistinct effect on catalytic performance due to their similar surface areas. The styrene conversion rate responded positively with the increasing amount of titania in the functionalized composites. The styrene oxidation by aqueous H2O2 unraveled the firstorder reaction with the activation energy of ⁓63.5 kJ. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Introduction
In academic research and industrial fine chemicals synthesis, styrene oxidation is an in-epoxy resins, plasticizers, drugs, sweeteners, chiral pharmaceuticals, pesticides, and epoxy paints [1][2][3]. Due to these commercial commodities, innumerable researchers have taken vast efforts to find catalysts that could increase the product yield of benzaldehyde resulted from styrene oxidation. One of the extensively used catalysts is titania-based catalysts [4][5][6][7].
Titanium dioxide (TiO2) or titania is a very well-researched material that exhibits high efficiency in the oxidation reaction due to its structural stability, biocompatibility, optical and electrical properties. In nature, titania can be found in four polymorphs of minerals form such as rutile, anatase, and brookite and titanium dioxide (B) or TiO2 (B) [7,8]. Anatase and rutile are commonly used in the oxidation reaction due to commercial availability, large amounts of reactive oxygen species like hydroxyl (•OH) radicals, hydroperoxy radicals (•OOH) and superoxide (•O2 − ) radical anion onto TiO2 surface [9]. However, styrene oxidation using titania could only give low product yields if it is used without any catalyst supports [10,11].
Indonesia is the largest archipelagic country in the world with a sea area of 5.8 million km 2 [19] and the largest fishery potential in the world with a potential of 67 million tons/year [20]. In 2014, Indonesia was the second-largest producer of aquaculture in the world, just ranked behind China [21]. Regardless of the fisheries and aquacultures have vastly developed, the large production of fish waste could cause the environmental issue. By taking this into consideration, valorization of fish waste into value-added commodities, such as bioactive peptides, collagen, enzymes, chitosan, and so forth, has been advocated [22]. In recent years, fish waste has been used as catalysts in biodiesel production [23][24][25].
Prompted by simple preparation methods and cost-effective raw materials, our research team first adopted facile impregnation synthesis using carbonized fishbone waste as the sup-port for metals (Fe2O3/CFB, CuO/CFB, and TiO2/CFB, CFB = carbon-derived fish bone) in styrene oxidation [4]. In this research, we utilized a similar approach to synthesize the catalyst, whilst focusing more on the parameters, consisting of the temperature of carbonization, with and without the addition of H2SO4, and concentration of titania. In detail, the fishbone supports were prepared by carbonation for 2 hours at varying temperatures, 500, 600 and 700 °C; Subsequently, sulfonation (1 M H2SO4) to the carbonized powder for 24 h; Lastly, impregnation of titania (500, 1000 and 1500 µmol) with fishbone-derived hydroxyapatite. The effects of carbonization, sulfonation and titania concentration were studied to understand the structure-properties relationships of the catalyst. Since a catalyst can open a new reaction pathway with lower activation energy, the kinetic performance of the catalysts was evaluated via styrene oxidation in the presence of an aqueous H2O2 oxidant.

Carbonation Process
Carbon-containing hydroxyapatite was prepared from the waste of fish bones that was collected from many food companies around Samarinda, East Kalimantan, Indonesia. The fish bone powder was carbonized in a furnace at varied temperatures (500, 600, and 700 °C) for 2 h to form carbon. The fish bone-derived carbon is labeled as FBC(T), where T is the carbonation temperature. For example, FBC500 is the fish bone-derived carbon that is prepared via the carbonation process at 500 °C for 2 h.

Sulfonation Process
The sulfonation process to the FBC500 was carried out by adding 10 mL of sulfuric acid (1 M; JT Beker) per gram of sample. The mixture was stirred at room temperature for 24 h and washed with distillate water to remove any loosely bound acid and it was dried at 110 °C overnight. FBC500 which has sulfonated is indicated as sulfonated fish bone-derived carbon (SFBC500).

Titania Impregnation
The titania impregnation process was obtained from the previous research [4]. Every 1 gram carbon was impregnated by titanium(IV) isopropoxide (500 µmol, Sigma Aldrich) that was immersed in 10 mL acetone (Merck) and stirred until all of the acetone solvents completely evaporated. The residual acetone was removed from the carbon sample by washing with ethanol (Merck) and subsequently dried at 110 °C overnight. The carbon sample was impregnated and labeled as TiO2(x)/FBC(T), x = concentration of TiO2 and T = temperature of carbonization. For example, TiO2(500)/FBC500 was generated from fish bone-derived carbon that was prepared by carbonation process at 500 °C for 2 h and the resultant was impregnated by titanium(IV) isopropoxide (500 µmol). The codes of all samples can be listed in Table  1.

Samples Characterization
A series of samples were characterized, inc l u d i n g T i O 2 ( 5 0 0 ) / F B C 5 0 0 , TiO2(500)/SFBC500, TiO2(500)/SFBC600, TiO2(500)/SFBC700, TiO2(1000)/SFBC500 and TiO2(1500)/SFBC500, to investigate the structure-activity relationships between the catalysts and the styrene oxidation. The samples were characterized by using FTIR, XRD, SEM-EDX and BET adsorption-desorption. FTIR spectrometer (IR−Prestige−21 Shimadzu) with a spectral resolution of 2 cm −1 , scans 10 s, at temperature 20 °C and range of wavenumber from 400 to 4000 cm −1 was used to identify the functional groups in the catalyst. The crystallinity and phase content of the catalyst was analyzed using XRD instrument (Phillips PANalytical X'Pert PRO) with the Cu-Kα (λ = 1.5406 Å) radiation as the diffracted monochromatic beam at 40 kV and 40 mA and the pattern was scanned in the 2θ ranges between 7° and 60° at a step 0.03° and step time 1 s. The SEM-EDX (FEI Inspect S50) instrument with an accelerating voltage of 15 kV was used to determine the surface morphology and element containing the catalyst. The nitrogen adsorption-desorption isotherms were measured at bath temperature 77.3 K and outgas tempera-ture 300.0 °C using a Quantachrome NovaWIn instrument version 11.0.

Catalytic Activity Test
The catalysts were tested by styrene oxidation reaction with aqueous hydrogen peroxide (H2O2) as an oxidant. The catalytic reactions procedure was conducted according to the previous research. All reactions were performed by mixing styrene (Merck; 5 mmol), 30% aqueous H2O2 (Merck; 5 mmol), acetonitrile (Merck; 4.5 mL), and catalyst (100 mg) with stirring for 24 h at room temperature. The products were then separated from the catalysts by centrifugation. A portion of the resulting liquid mixture was withdrawn and analyzed by GC-2010 Shimadzu-gas chromatograph equipped with a SH-Rxi-5ms column (30 m x 0.25 mmID x 0.25 µm df), serial 1652111, a flame ionization detector (FID) and nitrogen as the carrier gas. The temperatures of the injector and detector were programmed at 250 and 260 °C, respectively. The temperature of the column oven was programmed to increase from 80 to 140 °C, at a rate of 10 °C/ min.
The sulfonation process to catalyst support can increase the styrene conversion and benzaldehyde selectivity, which proven by styrene conversion and benzaldehyde selectivity over TiO2(500)/SFBC500) was 17.1% and 86.0%, respectively; while TiO2(500)/FBC500 gave only 5.3% and 3.0%, respectively. A sulfonate group is a polar group for H2O2 adsorption while carbonized hydroxyapatite is a nonpolar group for styrene adsorption. By lowering the mass transfer resistance, titania (the catalytic active sites) could convert styrene to benzaldehyde after both styrene and H2O2 adsorbed nearby. The varying carbonization temperature of catalyst support showed similar catalytic conversion in this study, evidenced by TiO2(500)/SFBC500, TiO2(500)/SFBC600, and TiO2(500)/SFBC700 gave 17.1%, 18.5%, and 16%, respectively. An insignificant increase of surface area resulted in less impregnation of titania active sites onto the support surface. The effect of the varying amount of metal titanium that impregnated onto catalyst support can be investigated based on the styrene conversion w h e n T i O 2 ( 5 0 0 ) / S F B C 5 0 0 , TiO2(1000)/SFBC500 and TiO2(1500)/SFBC500 were used as catalysts. The increasing of the amount of metal titanium as the active site from 500 to 1500 µmol in the catalyst can in-crease styrene conversion from 17.1 to 51.1%. This result is in agreement with the previous research when styrene oxidation with H2O2 as an oxidant was catalyzed with titanium dioxide-supported sulfonated low-rank coal [18].
In order to check the reusability and stability of the catalyst which have been created, TiO2(1500)/SFBC500 catalyst was selected for the assessment. TiO2(1500)/SFBC500 catalyst was recovered and recycled for further reaction. The spent catalyst was washed with ethanol and then centrifuged thrice and drying at 110 °C in a vacuum oven overnight. The styrene conversion was used as a basis to compare every reaction cycle. As shown in Figure 6, it is seen that the decreasing of the styrene conversion for the respective first, second and third reaction cycles, i.e. 51.1, 48.4 and 40.8%. The decreasing of the styrene conversion might be due to the physical detachment of some catalyst powder during mechanical stirring. Moreover, another possible explanation for decreasing of the activity is the leaching out of titania active sites of the pores during the washing process by ethanol solvent [11,39]. The selectivity of benzaldehyde for the first, second, and third cycles are almost similar, i.e. 70.8, 70.1, and 75.6%, respectively. It is because the sulfonate groups of TiO2(1500)/SFBC500 are still accessible for the high selectivity of benzaldehyde at room temperature. The effectiveness of sulfonate group to achieve high selectivity of benzaldehyde is evidenced in Figure 5(d), i.e. ~100% benzaldehyde selectivity. This can be explained that low concentration of titania en-  hances benzaldehyde selectivity. Nonetheless, the role of titania cannot be neglected as it could increase the conversion rate of styrene.

Kinetic Model: the Power-rate Law
The power-rate law is chosen as a kinetic model to determine the rate constant (k) and reaction behavior of a reaction using a function of temperature and concentration. A series kinetic experiment of styrene oxidation by H2O2 was performed at various temperatures (303, 313 and 323 K) with acetonitrile as solvent. The power-rate law can be represented as [40,41]: where, ri is the reaction rate of the styrene oxidation (mol.cm −3 .s −1 ); k is the reaction rate constant (min −1 ); Ci is styrene concentration after oxidation time t (mol.cm −3 ); [catalyst] is the concentration of TiO2(1500)/SFBC500 catalyst and K1 and K2 are pre-equilibrium constants of the step in the Schema.
If the total catalyst concentration is expressed as [catalyst]T and considering the steady-state approach, which includes the con-centration of all the intermediate catalyst species, the power-rate law can be given as: The value of rate constant k was determined of the intercept from the plot of [catalyst]T/ri vs 1/[styrene].
The kinetic data obtained from styrene oxidation with H2O2 as oxidant onto TiO2(1500)/SFBC500 are shown in Table 4. Base on the experiment results, the styrene oxidation over TiO2(1500)/SFBC500 catalyst follows first-order reaction model. The rate constant (k) at 303, 313, and 323 K were reported as 0.000512, 0.00299, and 0.00339 min −1 , respectively. The value of the rate constant can be used to predict the rate of reaction. Higher the rate constant, faster is the reaction rate. In details, it can be concluded that the rate of styrene oxidation increases with the reaction temperature.
The activation energy (E) of styrene oxidation with H2O2 as oxidant in the presence of TiO2(1500)/SFBC500 was investigated with the Arrhenius law, which the equation is written as [42] where k, A and exp[−E/(RT)] are rate constant, frequency factor and the Boltzmann expression for the fraction of systems having energy in excess of the value E (the fraction of the reactant molecules), respectively. Equation (7) can be rearranged to linear form as: The plotting ln k against 1/T with the slope is −E/R, so that the activation energy can be calculated. The activation energy of styrene oxidation with H2O2 as oxidant onto TiO2(1500)/SFBC500 was 63.485 kJ.
It is interesting to compare the activation energy of titania sulfonated fish bone-derived carbon with the results of other studies. In general, the activation energy of chemical reactions in the presence of a solid catalyst in liquid-phase oxidation of styrene with hydrogen peroxide is in the range of 20-80 kJ [43]. The results obtained in this research work are in agreement with these results, suggesting that a low-cost fish bone-derived carbon can be used as a catalyst.

Conclusions
The styrene oxidation by H2O2 oxidant over titania fish bone-derived carbon has been demonstrated in this research. The sulfonation with H2SO4 (1 M) can transform the particulate shape of FBC500 into the cuboid shape of SFBC500. The catalysts surface area was not the determinant in this catalytic reaction. Regardless of the smaller surface area of TiO2/SBFC, the titania impregnation and fish bone-derived carbon sulfonation improved styrene conversion and gave high benzaldehyde selectivity. The impregnation titania (TiO2) onto fish bone-derived carbon changed the physical properties and catalytic activity. The amorphous structure of FBC and SFBC were changed to crystalline structure after the impregnation of titania. Higher the amount of titania impregnated onto sulfonated fish bonederived carbon, higher the catalytic oxidation of styrene by H2O2. Increasing of the carbonization temperature of sulfonated fish bonederived carbon barely affected the catalytic activity. The kinetic data of styrene oxidation by H2O2 followed the first-order reaction model, whereby the reaction rate increases with increasing of reaction temperature. The activation energy of the styrene oxidation reaction over TiO2(1500)/SFBC500 catalyst was reported as 63.5 kJ, falling within the range of other typical catalysts. To advocate the green materials synthesis and economic sustainability, this research work successfully demonstrated the styrene oxidation by engineered low-cost fish bone catalysts. [4] Nurhadi, M., Kusumawardani, R., Wirawan, T., Sumari, S., Yuan, L.S., Nur