An-Qi Wang,
Jun-Xia Wang*,
Hui Wang,
Ya-Nan Huang,
Ming-Liang Xu and
Xiu-Ling Wu*
Faculty of Materials Science and Chemistry, Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, China. E-mail: wjx76@sina.com; xlwu@cug.edu.cn
First published on 2nd March 2017
A series of novel sulfated ZnAl2O4–TiO2 (SO42−/ZnAl2O4–TiO2) composite solid acids were synthesized for the esterification of biomass derived acids. The properties were characterized by XRD, FT-IR, XPS, NH3-TPD, acid–base titration, TG and FE-SEM. The experimental results showed that the introduction of ZnAl2O4 spinel oxide played a key role in stabilizing the structure of the composite catalysts as well as the surface sulfate groups. The composition of SO42−/TiO2–ZnAl2O4 catalysts significantly influenced their catalytic properties and performance in esterification reactions. The optimal SO42−/ZnAl2O4–TiO2 (6:4) catalyst with the largest number of acid sites showed superior conversion of 95.8% in oleic acid esterification and 98.5% conversion in acetic acid esterification. Moreover, the SO42−/ZnAl2O4–TiO2 (6:4) catalyst showed excellent reusability in these reactions, which was ascribed to its stable structure and good resistance of the surface sulfate groups. The efficient and reusable SO42−/ZnAl2O4–TiO2 composite solid acid coupled with spinel modification is expected to bring new opportunities in the design of favorable acid catalysts.
Currently, a number of solid acids have been investigated in esterification, such as cation-exchange resins, clays, sulfated metal oxide (SO42−/MxOy), activated carbons, zeolites, MOFs.4–6 The SO42−/TiO2 catalyst, a representative SO42−/MxOy solid acid, is used extensively in esterification because of its outstanding performance of high catalytic activity, good selectivity, easy separation and so on.7 However, pure SO42−/TiO2 catalysts usually suffer from rapid deactivation and low life despite their high initial activities, which is mainly due to the loss of sulfur species and carbon deposition.8,9 Therefore, development of acid catalysts for liquid reaction is still a challenge from the view of not only high activity but also stability and life. To date, several MxOy oxides, including ZrO2, Al2O3, Fe3O4, etc. have been studied widely in the synthesis of SO42−/TiO2–MxOy composite solid acids because of the apparent improvement in catalytic activities and reusability.8–11 In our previous study, the ZnAl2O4 spinel-type oxide was used successfully to obtain the sulfated ZnAl2O4 solid acid which has the advantage of easy preparation, single crystal shape, stable structure and good performance in acetic esterification.12–14 It is therefore expected that using the ZnAl2O4 spinel-type oxide as MxOy to synthesize the SO42−/TiO2–MxOy composite may result in interesting properties and catalytic activities. In addition, it is generally accepted that metal oxides (MxOy) have their own superior catalytic crystallographic form.15,16 For example, titania was reported to have three crystallographic forms, including anatase, rutile and brookite. Among them, the anatase form of titania was regard as the most active form.15 Similarly, it was reported that spinel was the favorable form to perform catalytic activities in reactions for sulfated ZnAl2O4 oxides.12 Hence, it is necessary to maintain the superior crystallographic forms of TiO2 and ZnAl2O4 in the composite catalysts in this study.
To develop a catalyst with superior acid properties and stability, SO42−/TiO2–ZnAl2O4 solid acids were synthesized by compositing TiO2 and ZnAl2O4 oxides. The main aim was to maintain both the catalytic crystallographic forms of respective TiO2 and ZnAl2O4 components, which was a unique advantage compared to other SO42−/TiO2–MxOy composite solid acids. Moreover, this study modulated the acidic properties of SO42−/TiO2–ZnAl2O4 catalysts by adjusting the mass ratio of TiO2/ZnAl2O4 components. The obtained acid catalysts with different compositions were evaluated in the liquid-phase esterification of acetic acid with acetic acid with n-butanol and oleic acid with methanol. The esterification of oleic acid with methanol has attracted great interest in biodiesel production, as the oleic acid is a common type of free fatty acids (FFAs) existing in plant oils.17,18 The esterification of acetic acid, as a representative acid in initial bio-oils, can efficiently improve the stability and heating value of bio-oils.19,20 The product from the esterification of acetic acid with n-butanol is a common green chemical. In addition, it also discussed the roles of respective components present in the SO42−/ZnAl2O4–TiO2 composite catalyst. In the meantime, we assumed the relationships between catalyst composition, catalytic performances structure and acidic properties. To date, there are few studies on SO42−/ZnAl2O4–TiO2 solid acid catalysts, which are regarded as a referential study for the modification of SO42−/MxOy solid acid catalyst.
The esterification of oleic acid with methanol was carried out in 50 mL autoclave immersed in an oil bath. The reactors were initially loaded with the reaction mixture consisting of a 5 wt% (based on the oleic acid) of catalysts and a 1:10 molar ratio of oleic acid and methanol. The autoclave was sealed and immersed in the oil bath at 100 °C for 2 h.
The conversion of acids can be calculated by titration according to the report of Pires et al.23 and the method of GB1668-81 using the following equation:
The used SO42−/ZnAl2O4, SO42−/ZnAl2O4–TiO2 (6:4) and SO42−/TiO2 catalysts were recovered by filtering and drying, after completing each reaction.
Fig. 1 XRD patterns of (1) SO42−/ZnAl2O4, (2) SO42−/ZnAl2O4–TiO2 (8:2), (3) SO42−/ZnAl2O4–TiO2 (6:4), (4) SO42−/ZnAl2O4–TiO2 (4:6), (5) SO42−/ZnAl2O4–TiO2 (2:8) and (6) SO42−/TiO2 catalysts. |
The surface sulfate groups are responsible for inducing acidity in the catalyst. It has been assumed that the sulfate group is bonded covalently to the oxide.24 All the catalysts shown in Fig. 2 exhibited a broad peak between 900 and 1400 cm−1, which was assigned to sulfate groups bonded covalently to the surface metal oxides (Scheme 1a and b).25,26 The sulfate group belonged to C2v symmetry can be bonded to surface metals by two oxygen atoms forming bridge (Scheme 1a) or chelate structures (Scheme 1b). The broad peaks of all catalysts could be divided into three peaks at 1227, 1139 and 1060 cm−1. Peaks at 1139 and 1060 cm−1 were ascribed to asymmetric and symmetric stretching of S–O bonds, respectively, and the others at 1398 and 1227 cm−1 were due to asymmetric and symmetric stretching of the SO.26,27 The peaks at 1227 cm−1 indicated that bonded sulfate groups were formed with the chelate structure (Scheme 1b).28 In addition, the peaks that belonged to Al–O, Zn–O and Al–O vibrations for ZnAl2O4 spinel were observed at 669, 561 and 505 cm−1 in all SO42−/ZnAl2O4–TiO2 catalysts.29 The existence of a spinel phase in mixed oxides catalysts was consistent with the results of XRD. The broad band in the range of 400–800 cm−1 occurred in SO42−/TiO2 was regarded as the stretching vibration of Ti–O–Ti. The broad band was not detected clearly in SO42−/ZnAl2O4–TiO2, which might be due the band being covered by the intense peaks of Zn–O and Al–O vibrations.30 In addition, peaks at 1621 and 3400 cm−1 were associated with the bending mode and stretching mode of the OH group of water molecules, respectively.27
Fig. 2 FT-IR spectra of (1) SO42−/ZnAl2O4, (2) SO42−/ZnAl2O4–TiO2 (8:2), (3) SO42−/ZnAl2O4–TiO2 (6:4), (4) SO42−/ZnAl2O4–TiO2 (4:6), (5) SO42−/ZnAl2O4–TiO2 (2:8) and (6) SO42−/TiO2 catalysts. |
Scheme 1 Structure of bonded sulfate group with C2v bridge (a) and bonded sulfate ion with C2v chelate (b). |
In the XPS spectra for the SO42−/ZnAl2O4–TiO2 (6:4), the peaks for Zn 2p, Al 2p, Ti 2p, O 1s and S 2p were presented in Fig. 3. The spectra centered at 169.4 eV, corresponding to S 2p binding energy, was typical of the S6+ high oxidation state. This high oxidation state is essential evidence of formation of bidentate sulfate group, indicating that catalyst possessed acid sites.30 In addition, the O 1s spectra in Fig. 3c can be decomposed into three peaks at 530.4 eV, 531.4 eV and 532.3 eV, which were assigned to lattice oxygen of metal oxide, bridged oxygen of surface hydroxyl groups and the oxygen of sulfate group, respectively.31 For the oxidation state of the metal elements, the peaks at 1045.8 eV and 1022.7 eV shown in Fig. 3d belonged to Zn 2p1/2 and Zn 2p3/2 of Zn2+ ion.32 Fig. 3e shows the Al 2p peak of the catalyst at 75.2 eV, which was assigned to the Al3+ ion.33 The binding energy of Ti 2p3/2, shown in Fig. 3f, was 459.1 eV and that of Ti 2p1/2 was 464.5 eV, revealing that Ti was in the 4+ oxidation state.34
Fig. 3 XPS spectra of the (a) SO42−/ZnAl2O4–TiO2 (6:4) catalysts and the high-resolution photoelectron spectra of (b) Zn 2p, (c) Al 2p, (d) S 2p, (e) O 1s and (f) Ti 2p. |
NH3-TPD was used to estimate the amount and strength of acid sites formed on the surface of the catalysts. All the SO42−/ZnAl2O4, SO42−/ZnAl2O4–TiO2 and SO42−/TiO2 catalysts contain a certain amount of acid sites, indicating both TiO2 and ZnAl2O4 acted as active components to form the acid sites on the surface by sulfation. The broadened peaks, shown in Fig. 4, were divided into peaks assigned to weak acid sites (below 200 °C), middle strength acid sites (200–400 °C) and strong acid sites (above 400 °C).35 These profiles showed that the catalysts contained various amounts of acid sites and different acid strengths. Compared to SO42−/ZnAl2O4 and SO42−/TiO2, the SO42−/ZnAl2O4–TiO2 contained the largest number of acid sites with a clear increase in weak and middle strength acid sites. This may be explained that the synergistic interaction of two active components improved the generation of acid sites, which was commonly observed in the modified SO42−/TiO2–MxOy composite catalysts.35,36 Moreover, the acid site density of the catalysts with different compositions was studied by the acid–base titration, as shown in Table 2. The mass ratio of ZnAl2O4 and TiO2 had a strong effect on the acid site density of the catalysts. With increasing TiO2 component, the acid site density was improved gradually. SO42−/ZnAl2O4–TiO2 (6:4) showed a higher density of acid sites than SO42−/ZnAl2O4 and SO42−/TiO2, which is in agreement with NH3-TPD. However, excess TiO2 had a negative influence in acid site density. This may be due to the accumulation of TiO2 on the outer surface of ZnAl2O4, which can be seen in other composite catalysts.37 It was suggested that the acidic properties of SO42−/ZnAl2O4–TiO2 can be tuned by the mass ratio of the ZnAl2O4 and TiO2 components and the acidic property was maximized with the optimal ratio of 6:4.
TG analysis of the SO42−/ZnAl2O4, SO42−/ZnAl2O4–TiO2 (6:4) and SO42−/TiO2 catalysts was carried out, as shown in Fig. 5. The initial weight loss (around 100–200 °C) on the TG profiles was assigned to the desorption of physically adsorbed water.37 Further increasing the temperature from 200 °C to 550 °C led to a gradual weight loss due to the removal of surface hydroxyl groups.38 The third weight loss that started at about 550 °C was associated with the decomposition of surface sulfate groups, which was a typical feature in the TG profiles of SO42−/MxOy.38 Based on the TG profiles, the weight percentages of the sulfate groups eliminated from SO42−/ZnAl2O4, SO42−/ZnAl2O4–TiO2 (6:4) and SO42−/TiO2 were estimated to be 13.1%, 20.2% and 16.5%, respectively, as shown in Table 1. This also proved that both TiO2 and ZnAl2O4 acted as active components to generate surface sulfate groups after sulfation. Moreover, more the weight losses, more the sulfate groups that exist.13,38 Thus, it was revealed that the SO42−/ZnAl2O4–TiO2 (6:4) catalyst contained more sulfate groups over their pure components, which may be due to the cooperation of ZnAl2O4 and TiO2. This result was consistent with that of NH3-TPD. In addition, the decomposition temperatures of sulfate groups were 660, 610 and 550 °C for SO42−/ZnAl2O4, SO42−/ZnAl2O4–TiO2 (6:4) and SO42−/TiO2 respectively, indicating that the sulfate groups on ZnAl2O4-based catalysts showed better thermal stability than that of SO42−/TiO2 catalysts. This suggests that the ZnAl2O4 component might help stabilize surface sulfate groups by enhancing its thermal stability.
Fig. 6 shows FE-SEM images of SO42−/ZnAl2O4, SO42−/ZnAl2O4–TiO2 and SO42−/TiO2 catalysts. The SO42−/ZnAl2O4 catalyst, shown in Fig. 6a, had a near spherical particle appearance. However, Fig. 6b revealed plate-like morphologies of pure SO42−/TiO2. As shown in Fig. 6c, the morphology of SO42−/ZnAl2O4–TiO2 (6:4) showed that both ZnAl2O4 and TiO2 kept up their unique morphologies. Moreover, spherical ZnAl2O4 particles were dispersed on or crossed through plate-like TiO2 bulk. This further confirmed that the two components were composited in SO42−/ZnAl2O4–TiO2 (6:4) catalyst.
Fig. 6 FE-SEM micrographs of (a)SO42−/ZnAl2O4, (b) SO42−/TiO2 and (c) SO42−/ZnAl2O4–TiO2 (6:4) catalysts. |
On the basis of the abovementioned characterization, the possible structures are shown in Fig. 7. The XPS corroborating FT-IR analysis gave the possible chelate structure of sulfate groups bonded on the surface metal oxides of catalysts. FE-SEM showed that spherical ZnAl2O4 particles were dispersed on or crossed through plate-like TiO2 particles, and the M–O–Ti linkages were expected on the interface, resulting from the cooperation of two components, which may have improved the acidic properties of the composite catalysts.
Fig. 7 Possible structures of the sulfate groups formed on the surface of SO42−/ZnAl2O4–TiO2 catalysts. |
Fig. 8 Esterification of (a) acetic acid with n-butanol and (b) oleic acid with methanol over varied catalysts. |
Catalysts | Acid sites (mmol g−1) | Conv. (%) | TOFa (min−1) | ||
---|---|---|---|---|---|
Acetic acid (20 min) | Oleic acid (30 min) | Acetic acid | Oleic acid | ||
a TOF = Macid/Msite, where Macid and Msite are mole of the initial acid consumed in the reaction per unit time (min) and mole of active sites, respectively. Msite is obtained from the result of acid–base titration for each catalyst. | |||||
SO42−/ZnAl2O4 | 1.84 | 45.3 | 50.2 | 2.81 | 0.57 |
SO42−/ZnAl2O4–TiO2 (8:2) | 2.21 | 55.8 | 55.2 | 2.88 | 0.52 |
SO42−/ZnAl2O4–TiO2 (6:4) | 2.31 | 63.8 | 75.8 | 3.15 | 0.68 |
SO42−/ZnAl2O4–TiO2 (4:6) | 1.62 | 40.2 | 52.6 | 2.83 | 0.67 |
SO42−/ZnAl2O4–TiO2 (2:8) | 1.69 | 36.6 | 63.1 | 2.47 | 0.78 |
SO42−/TiO2 | 2.16 | 69.8 | 70.9 | 3.69 | 0.68 |
It is generally accepted that different esterification reactions have their preferential acid sites in terms of acidic strength.39,40 In this study, the amounts of acid sites with different acidic strengths were calculated based on the NH3-TPD profiles. Fig. 9 shows that the SO42−/ZnAl2O4–TiO2 (6:4) catalyst showed different acidic properties from SO42−/ZnAl2O4 and SO42−/TiO2 catalysts, which may indicate their different catalytic activities. It was found that the trend of acetic acid conversion corresponded to the number of strong acid sites. On the other hand, the change in catalytic activities in oleic acid esterification increased in line with middle strength acid sites or weak acid sites. These two reactions may have their own favorable acid strength of acid sites. The relationship between acidic properties and catalytic activities gave a referential study for the design of catalysts for these reactions.
Fig. 9 Conversion of different acids and intensity of acidic amount as a function of SO42−/ZnAl2O4, SO42−/TiO2 and SO42−/ZnAl2O4–TiO2 (6:4) catalysts with different acidic strengths. |
In order to develop a green and reusable catalyst, the capability of the catalyst to be recovered and reused should be considered seriously in the study of SO42−/MxOy solid acids. Thus, SO42−/ZnAl2O4, SO42−/TiO2 and optimal SO42−/ZnAl2O4–TiO2 (6:4) solid acid catalysts were recycled to study the stability of catalytic activity. As shown in Fig. 10, spinel-based catalysts outperformed SO42−/TiO2 catalyst in terms of the reusability for both reactions. This advantage was evidently manifested in the SO42−/ZnAl2O4–TiO2 (6:4) catalysts. Fig. 10a shows that SO42−/ZnAl2O4–TiO2 (6:4) can maintain the conversion of acetic acid above 85% in the fifth cycle, while SO42−/TiO2 showed a clear deactivation (∼80%) after two repeated cycles. The catalytic activities for the esterification of oleic acid with methanol was ∼40% and ∼8% lower in reused SO42−/TiO2 and SO42−/ZnAl2O4–TiO2 (6:4), respectively, in the second cycle (Fig. 10b). These results show that the SO42−/ZnAl2O4–TiO2 (6:4) catalyst as a reusable catalyst can be recycled several times.
Fig. 10 Reusability of SO42−/ZnAl2O4, SO42−/TiO2 and SO42−/ZnAl2O4–TiO2 (6:4) catalysts for esterification of (a) acetic acid with n-butanol and (b) oleic acid with methanol. |
To study the stability and the reason for the deactivation, the used catalysts after two repeated cycles were characterized by XRD, FT-IR and TG, as shown in Fig. 11. According to FT-IR and XRD analysis, no evident change in the structure of the used SO42−/ZnAl2O4–TiO2 (6:4) catalyst was observed by comparing to the fresh catalyst. This indicated that SO42−/ZnAl2O4–TiO2 (6:4) had good structural stability after recycling. The IR spectra, shown in Fig. 11b, also gave broadened peaks of surface sulfate group in the range of 900–1400 cm−1. The peaks of sulfate groups almost disappeared in the IR spectra of used SO42−/TiO2, while they were kept relatively better in the spectra of used SO42−/ZnAl2O4–TiO2. SO42−/ZnAl2O4–TiO2 had superior resistance to the loss of sulfur species, which might interpret its better reusability. The TG curves in Fig. 11c were also performed to estimate the amount of residual sulfate groups on the surface of the used catalysts. As shown in Table 1, the weight percentages of surface sulfate groups on the used SO42−/ZnAl2O4, SO42−/ZnAl2O4–TiO2 (6:4) and SO42−/TiO2 were estimated to be 10.9%, ∼16.2% and ∼7.8%, respectively. ZnAl2O4-based catalysts showed relatively minimal sulfur loss particularly in the SO42−/ZnAl2O4–TiO2 (6:4) catalysts. Accordingly, it was suggested that the ZnAl2O4 component played a significant role in stabilizing sulfate groups on the surface of the SO42−/ZnAl2O4–TiO2 catalyst and enhance its resistance of sulfur loss, which was also approved in IR analysis. Based on IR and TG analysis, a decrease in the intensity of peaks and the number of sulfate groups was observed in all used catalysts. This indicated that the catalysts inevitably suffered from the loss of sulfur species. The loss of surface sulfate groups is one of the main reasons for the deactivation of these catalysts in recycling reaction, which is common in other sulfated catalysts.40–42
Table 3 lists these two types of esterifications over various catalysts, such as modified SO42−/TiO2, MCM-41, MOFs, and H2SO4, and compares the reported catalysts with catalysts obtained in this study. The results showed that the high conversion in the esterification of acetic acid n-butanol over SO42−/ZnAl2O4–TiO2 (6:4) was comparable to that of modified SO42−/TiO2 and Al–MCM-41 mesoporous catalysts presented in the literature, whereas the relatively milder reaction condition was applied in this study.9,43,44 For the esterification of oleic acid with methanol, the SO42−/ZnAl2O4–TiO2 (6:4) catalyst with a smaller catalyst amount and shorter reaction time exhibited a higher catalytic activity (95.8%) than [TMEDAPS][HSO4] and MOF-808-2.5SO4.6,45 Although a homogeneous acid, i.e., H2SO4, showed high conversion (91%) in low molar ratio of alcohol to acid and at low temperature,46 the use of eco-friendly heterogeneous catalysts was more advantageous due to the easy separation and re-use of the catalyst. This suggested that the SO42−/ZnAl2O4–TiO2 (6:4) is more or less superior compared to the reported catalysts in terms of the high acid conversion with short reaction times and mild conditions, easy recoverability and high recyclability.
Catalyst | Reaction conditions | Conv./% | Ref. | |||||
---|---|---|---|---|---|---|---|---|
Amounta/wt% | Acid | Alcohol | Molar ratiob | Temp./°C | Time/h | |||
a Based on the acid.b The molar ratio of acid to alcohol.c Present study.d n([TMEDAPS][HSO4]):n(Methanol). | ||||||||
SO42−/TiO2–Zr–La | 14.3 | Acetic acid | n-Butanol | 1.56:1 | 98–112 | 0.5 | 85 | 9 |
SO42−–Ce0.02/TiO2 | 12 | Acetic acid | n-Butanol | 3/1 | 145 | 0.75 | 97.1 | 43 |
Al–MCM-41 (Si/Al = 25) | 1.67 | Acetic acid | n-Butanol | 1/2 | 125 | 6 | 87.3 | 44 |
SO42−/ZnAl2O4–TiO2 (6:4) | 7.3 | Acetic acid | n-Butanol | 3/1 | 115 | 2 | 98.5 | —c |
MOF-808-2.5SO4 | 20 | Oleic acid | Methanol | 70/1 | 65 | 6 | 80 | 6 |
H2SO4 | — | Oleic acid | Methanol | 3/1 | 80 | 6 | 91 | 46 |
[TMEDAPS][HSO4] | 0.2:1.8d | Oleic acid | Methanol | 1.8/1 | 70 | 6 | 95 | 45 |
SO42−/ZnAl2O4–TiO2 (6:4) | 5 | Oleic acid | Methanol | 10/1 | 100 | 2 | 95.8 | —c |
This study suggested that the ZnAl2O4 spinel oxide can be used as a novel component to modify the SO42−/MxOy catalyst. The spinel-type component in composites plays a role in tuning the catalytic properties and stabilizing the structure and surface sulfate groups.
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