TiO2/SO42− Solid Superacid Catalyst Prepared by Recovered TiO2 from Waste SCR and Its Application in Transesterification of Ethyl Acetate with n-butanol

In this contribution, the recovered TiO2 from waste Selective Catalytic Reduction (SCR) was transformed into a solid superacid catalyst (TiO2/SO42−) modified by sulfuric acid (H2SO4). The results of XRD suggest that the crystal structures of TiO2 are not destroyed during the recovery and sulfation processes. The recovered TiO2-modified superacid catalyst has a greater surface area (42.84 m2/g) than TiO2/SO42− catalysts produced from pure TiO2 reported by previous researchers. The Barrett-Joyner-Halenda (BJH) pore size distribution confirms that the samples are essentially mesoporous structures. The NH3-TPD analysis demonstrated that the formation of the superacid sites occurs at a temperature ranging between 400 and 500 °C. The prepared TiO2/SO42− solid superacid catalyst exhibits good catalytic activity with a conversion above 92% in the transesterification of ethyl acetate and n-butanol.


Introduction
In recent years, solid superacid catalysts have received more attention due to their high catalytic activity, excellent thermal stability, strong acidity, reusability, and non-polluting nature [1][2][3].They have been used as catalysts for various organic reactions such as isomerizations, alkylations, catalytic reforming of alkanes, cracking, esterification, and transesterification [4][5][6][7], especially transesterification, which is the general term used to describe the important class of organic reactions where an ester is converted into another through the interchange of the alkoxy moiety [8,9] and widely used in the chemicals, pharmaceuticals, and biofuel industries [10,11].Contrasting to homogeneous catalysts [12,13], heterogeneously catalyzed transesterification has become a focus owning advantages, such as high activity and selectivity for target products, easy separation from the reaction medium, corrosion reduction, and benign reusability [14][15][16].Since solid superacid catalysts have higher stability than those solid base catalysts, they can be applied to feedstock with large amounts of free fatty acids without catalyst deactivation [17], among which the TiO 2 /SO 4 2− solid superacid catalysts have shown better performance than other metal oxides, and it becomes a research highlight.
Many studies have been carried out on pure TiO 2 -modified superacid catalysts applied in esterification and transesterification reactions, and these have shown excellent catalytic activities [18][19][20].Chen et al. synthesized a solid acid catalyst (1.5 SO 4 2− /TiO 2 ) from pure TiO 2 and evaluated the transesterification of Jatropha curcas L. seed oil and reached 85.3% of biodiesel production [21].Gardy et al. investigated simultaneous esterification and transesterification of cooking oil using sulfated titania solid acid nano-catalyst [Ti(SO 4 )O] prepared via pure TiO 2 , and they achieved 97.1% yield for the fatty acid methyl ester [22].Zhao et al. studied the catalytic activity of sulfated titania solid superacid (SO 4 2− /TiO 2 ) with exposed facets prepared from pure TiO 2 on the esterification of acetic acid with n-butanol and reported about 92.2% yield of butyl acetate, under optimal reaction conditions at 120 °C reaction temperature, 150 min reaction time, 1.8 g catalyst amount and a molar ratio of n-butanol to acetic acid of 1.2 [23].Unfortunately, pristine TiO 2 to produce solid superacid catalysts is expensive, and the resources manufacturing process is not sustainable and eco-friendly [24,25] because pure TiO 2 is mainly prepared with expensive chemical reagents, such as titanium tetrachloride (TiCl 4 ) [26], titanyl sulfate (TiOSO 4 ) [27], and titanium isopropoxide ([OCH(CH 3 ) 2 ] 4 ) [28,29].
In order to solve these problems, the potential production of solid superacid catalysts based on titanium dioxide recovered from different waste materials, such as Selective Catalytic Reduction (SCR) waste, could be a possible solution, etc.Indeed, TiO 2 has been recovered from waste SCR by different methods, such as Zhang et al. recovered TiO 2 from spent SCR using a NaOH molten salt [24].Ma et al. recovered TiO 2 from waste SCR through a roasting process combined with sodium salt [30].Zhang et al. recovered TiO 2 from waste SCR using the sodium moltensalt method [31].
But no attention has been focused on transforming the recovered titanium dioxide into a solid superacid catalyst.This technique can help to improve catalytic performance, protect titanium resources, minimize environmental impact, and reduce the cost of solid superacid catalyst production.
In this paper, a solid superacid catalyst (TiO 2 /SO 4 2− ) is prepared from TiO 2, which is recovered from the spent SCR catalyst.Its properties are analyzed and discussed by using XRD, N 2 -BET, FT-IR, and NH 3 -TPD results.The catalyst properties are compared with those reported from other work on pure TiO 2 .The catalytic performance is tested through the transesterification of ethyl acetate with n-butanol.

Materials
The spent SCR catalyst was provided by Shenfu Honghan Power Plant (China).Sodium hydroxide (NaOH) and Na 3 PO 4 •H 2 O were purchased from Shanghai Macklin Biochemical lo., Ltd.Sulfuric acid (H 2 SO 4 ) and n-butanol were purchased by Sinopharm Chemical Reagent Co. Ltd (China).Ethyl acetate was supplied by Beijing Chemical Factory (China).Deionized water was manufactured by the experimental center of the University of Science and Technology Beijing (China).

Recovery TiO 2 Process
The TiO 2 has been recovered from the waste SCR catalyst by using the alkali leaching method in which 3 g of spent SCR catalyst was dissolved in 30 ml of 5 M alkali solution (NaOH) under continuous stirring, and 0.465 g of Na 3 PO 4 •H 2 O was added to the solution.After leaching for 3 h at 100 ℃, the mixture was filtered, then the filter residues were washed thoroughly and dissolved in 30 ml of sulfuric acid solution (H 2 SO 4 , 5.5%) under continuous stirring for 2 h.The filter residues were washed several times with deionized water, and the recovered TiO 2 was dried at room temperature.

Catalyst Preparation
The solid superacid was prepared according to the literature procedure [21].Recovered TiO 2 was transformed into a superacid catalyst under the following conditions, such as, 1 g of recovered TiO 2 was impregnated for 4 h in 15 ml of 0.5 M H 2 SO 4 under continuous stirring.Then the obtained suspension was filtered and washed with deionized water several times.Afterward, the mixture of TiO 2 and H 2 SO 4 was dried at 100 °C for 8 h to remove water contents and then calcined at 500 °C for 3 h to obtain the TiO 2 /SO 4 2− solid superacid catalyst.

X-Ray Diffraction
The crystallinities of samples were characterized by X-ray diffraction (XRD) (D/MAX-RB, Rigaku, Japan diffractometer), with Cu-Kα radiation (λ = 0.15406 nm) at a scanning rate of 0.2°/min in a 2θ range of 5° to 80°.The crystal size was calculated based on the Debye-Scherrer equation from the full width at half maximum of anatase (101 where K is the particle form factor, λ is the wavelength of the CuK α radiation, β is the physical broadening of the (101) diffraction peaks, and θ is the incident angle of the X-rays.

Surface Area Measurement
Nitrogen physio sorption (N 2 -adsorption) measurements were carried out by an automated surface area and porosimetry system (ASAP 2600, Micro-metrics, USA) at the temperature of liquid nitrogen of − 196 °C.The specific surface area, pore size distributions, and pore volume of the samples were calculated using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods.

Surface Acidity Measurements
Temperature-programmed desorption of ammonia (NH 3 -TPD) was conducted by an (AutoChem1 II 2920) (1) with a thermal conductivity detector used to monitor desorbed ammonia continuously.The solid acid catalyst sample (0.100 g) was pretreated for 1 h at 550 °C in ultrapure He gas flow (50 ml/min) and then saturated with ammonia gas for 30 min at 50 °C.After sweeping with He gas (50 ml/ml) at 50 °C for 1 h to remove the physical absorbed ammonia, the sample was heated from 50 to 550 °C, with a heating rate of 10 °C/min.

Catalytic Evaluation
The catalytic performance was evaluated in a 100 ml round-bottom flask equipped with a reflux condenser to avoid any loss of volatile compounds.The reaction consisted of 10 ml of ethyl acetate, 10 ml of n-Butanol (molar ratio 1:1), and a solid catalyst of 4 wt.%.The reaction mixtures were incubated at 100 °C with stirring at 400 rpm.The transesterification was carried out for 3 h.The autoclave was closed, followed by setting the temperature and the rotating rate on the control board.The samples were separated from the reaction mixture using a centrifuge.The conversion of n-butanol was calculated based on the GC analyses using the following equation [32]:

Analytical Method
The samples were analyzed by gas chromatography (GC-2014-SHIMADZU) equipped with a flame ionization detector (FID) and a capillary column RTX-5 (30.0 m in length, 0.25 mm of internal diameter, and 0.25 mm film thickness).The temperature of the detector was 240 °C, the temperature of the injector was 200 °C, and the oven temperature-programmed ranged from 40 to 200 °C with a rate of 10 °C/min.The gas flow was 17.5 ml/min for N 2, and the sample amount was 0.2 μL.200), ( 105), ( 211), ( 204), ( 116), (220), and (2015), respectively, for the tetragonal titanium dioxide anatase phase according to JCPDS card number 021-1272 [33,34].Recovered TiO 2 and TiO 2 /SO 4 2− show similar diffraction patterns.However, X-Ray Diffraction peaks of sulfur compounds have not been found, and this might be due to the high dispersion of sulfate ions (SO 4 2− ) in the catalysts [35].For a better understanding of the effect of the recovering process and treatment with sulfuric acid on the TiO 2 crystal structure, the average particle sizes were calculated perpendicular to the (101) plane for each sample from XRD patterns using the Debye-Scherrer method (Eq. 1) [19] and the results are 10.26 nm and 10.30 nm, respectively.These results demonstrate that the crystal structures are not overly affected during the recovery of TiO 2 and the sulfation process.

BET-Specific Surface Area
The N 2 adsorption-desorption isotherms and pore size distribution of samples are shown in Fig. 2. The recovered TiO 2 and TiO 2 /SO 4 2− catalyst can be classified as type IV isotherms with a type (H3) hysteresis loop at a relative pressure (P/P° = 0.8) (Fig. 2a), according to the IUPAC classification [39].Moreover, the BJH pore size distribution between 30.04 and 18.41 nm (Fig. 2b) confirms that the samples  ) into recovered TiO 2 .This is due to the aggregation of very small crystallites and also the blockage of pores by sulfate species [36,39].Hung et al. reported a 60% reduction in the surface area of commercial TiO 2 after treatment of 0.5 M H 2 SO 4 [37], and A. V. Nakhate et al. (2016) observed a reduction of surface area after the sulfation of TiO 2 [41].While on the other hand, the solid superacid catalyst (TiO 2 / SO 4 2− ) prepared from recovered TiO 2 has a higher surface area (42.84 m 2 /g) and large porosity (0.30 cm 3 /g) rather than reported by other researchers made from pure TiO 2 in the same condition as shown in Table 1.Hence, high specific surface area and large porosity are the key factors in understanding the catalytic activity of the catalyst for esterification and transesterification reactions [42].

Fourier Transform Infrared Spectroscopy (FT-IR)
The FT-IR spectra of recovered TiO 2 and TiO 2 /SO 4 2− superacid catalyst are shown in Fig. 3. Infrared absorption spectra bands observed between 3600 and 3000 cm −1 describe both hydroxyl group's symmetric and asymmetric stretch vibrations [43].The peak at 3412.31 cm −1 is associated with a hydroxyl group (OH), and the bend vibrations peak at 1636.93 cm −1 is allied with water (H-O-H) molecules [28].However, the bands of solid superacid catalysts in the region between 1300 and 980 cm −1 are attributed to the vibrational modes of sulfate [44].The peaks at 1148.56 and 1045.96cm −1 are assigned due to the asymmetric and symmetric stretching frequencies of the S=O and S-O bonds of the solid superacid [45,46].The intense band between 1000 and 400 cm −1 corresponds to the Ti-O and Ti-O-TiO stretch modes [47,48].The bands aligned to SO 4 2− vibrations can be observed for recovered TiO 2 sulfated, while the peaks of recovered TiO 2 are not observed.Therefore, the results confirmed that the (SO 4 2− ) group was bidentate coordinated with TiO 2 , as shown in Scheme 1

Surface Acidity Measurements
Temperature-programmed desorption of ammonia (NH 3 -TPD) analysis was conducted to estimate the strength of TiO 2 /SO 4 2− solid super acid catalyst.The intense band NH 3 -desorption temperature lies between 400 °C and 500 °C, implying the formation of superacid sites.These results were in agreement with those reported in the literature [44,49,50].The number of acid sites is expressed as the amount of NH 3 desorbed (mmol/g) calculated from the area below the peak of these profiles [49], and the total amount of ammonia desorbed was 0.41 mmol/g for recovered TiO 2 and 0.87 mmol/g for TiO 2 /SO 4 2− .However, the strong acidity of the solid superacid catalysts was usually determined by the total amount of ammonia (NH 3 ) desorbed.Cho et al. synthesized an SO 4 2− /SnO 2 solid superacid catalyst and observed the total ammonia desorbed was 760.2 mol /g (0.7602 mmol/g) [51].Hossain et al. also prepared an S-ZrO 2 /SBA-15 solid super-acid catalyst, and they reported the total amount of ammonia desorbed was 0.36 mmol/g [52].Compared to the above mentioned literature, the total ammonia desorbed of the recovered TiO 2 -modified solid superacid catalyst was 0.87 mmol/g, which suggests it is a stronger superacid and can promote transesterification reactions [53].

Catalytic Performance
In order to study the catalytic performance of the TiO 2 / SO 4 2− catalyst on the transesterification of ethyl acetate and n-butanol (Scheme 2), the various reaction parameters (catalyst loading, reaction temperature, and reaction time) were investigated.The concentration of the products was analyzed by gas chromatography.The degree of conversion of n-butanol was calculated through Eq. 2. The results are shown in Fig. 5. Figure 5a represents the effect of catalyst amount on the n-butanol conversion of the transesterification reaction; when the variation of catalyst amount ranges between 0.1 and 0.4 g, the conversion of n-butanol increases from 79 to 90% because of the accessibility of more active sites [36].However, when the catalyst concentration exceeds 0.4 g, the conversion of n-butanol decreases.Extra catalysts disturb the reaction mixture, which causes diffusion problems [54].Therefore, 0.4 g of catalyst was chosen as the optimum value.The plot of n-butanol conversion versus reaction temperatures (15-120 °C) is shown in Fig. 5b.The conversion of n-butanol rises from 48.9 to 64.0% when the reaction temperature increases the scope from 15 to 100 °C.Once the reaction temperature is beyond 100 °C, the conversion of n-butanol is almost constant.So, the reaction temperature near the boiling point of the alcohol is recommended for a faster conversion reaction [55].Thus, 100 °C is selected as the reaction temperature for the transesterification process.Figure 5c shows the effect of reaction time on the n-butanol conversion.The other reaction conditions were kept constant, such as molar ratio ethyl acetate to n-butanol 1:1, catalyst amount 4 wt%, and reaction temperature 100 °C.It can be accredited that when the reaction time changes from 15 to 180 min, the conversion of n-butanol increases.Nevertheless, the long reaction time may promote the growth of the conversion of n-butanol.But for economic reasons, the reaction time of 3 h was chosen as an efficient reaction condition for this experiment.
Indeed, a catalyst amount of 4 wt%, a reaction temperature of 100 °C, ethyl acetate to the n-butanol molar ratio of 1:1, and a reaction time of 3 h are considered as optimal conditions for the transesterification reaction in this study.

Conclusions
This contribution reveals that the recovered TiO 2 -modified solid superacid catalyst potential can be used as a solid superacid catalyst for the transesterification reaction.XRD results (10.26 nm and 10.30 nm) demonstrated that the crystalline structure is not destroyed by the recovery process of TiO 2 after sulfuric acid treatment.The BET and BJH results of recovered solid superacid (TiO 2 /SO 4 2− ) of 42.84 m 2 /g and 0.30 cm 3 /g indicate higher surface area and larger porosity, respectively.The FT-IR results of band absorption at 1148.56 and 1045.96cm −1 showed that the SO 4 2− group was bidentate coordinated with TiO 2 .The temperature of NH 3 -desorption was between 400 and 500 °C, confirming the formation of superacid sites.The catalytic activity tests show that the recovered TiO 2 /SO 4 2− catalyst exhibits higher catalytic activity with a 92% conversion of n-butanol.

Fig. 1 XFig. 2 N 2 -
Fig. 1 X-ray diffraction patterns of recovered TiO 2 and TiO 2 /SO 4 2− Figure 4 shows the NH 3 -TPD profiles for recovered TiO 2 and TiO 2 /SO 4 2− .The principal desorption peaks were detected at 198 °C, 285 °C, 425 °C, and 478 °C corresponding to the weak, middle, and strong acid sites.It can be perceived that recovered TiO 2 contains weak and middle acid sites while TiO 2 / SO 4 2− superacid catalyst contains only stronger acid sites.

Fig. 5
Fig. 5 Effects of reaction amount (a), reaction temperature (b), and reaction time (c) in the transesterification reaction

Table 1
The results of the specific surface area, pore volume, and particle size calculated with the BET and BJH methods are reported in Table1.They indicate that the recovered TiO 2 exhibits a large surface area (79.23 m 2 /g).However, the specific surface area of BET decreases after incorporating sulfate groups (SO 4