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FexAl1 − xPO4 solid acid catalysts for caprylic acid esterification
Catalysis Communications 99 (2017) 49–52
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Preparation and characterization of SO42 −/FexAl1 − xPO4 solid acid catalysts for caprylic acid esterification
MARK
Boliang Liu, Pingping Jiang⁎, Pingbo Zhang, Gang Bian, Mengtian Li Key Laboratory of Synthetic and Biological Colloids (Ministry of Education), School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: SO42 −/FexAl1 − xPO4 Characterization Esterification Caprylic acid Catalytic performance
A series of SO42 −/FexAl1 − xPO4 solid acid catalysts were prepared and characterized by means of XRD, FE-SEM, elemental analysis, N2 adsorption–desorption, XPS, NH3-TPD, pyridine adsorption FT-IR and ion exchange/ titration. Their catalytic performances were evaluated by the esterification of caprylic acid with ethanol. Experimental results revealed that the doping of iron can markedly enhance the acidic properties of the SO42 −/ AlPO4, thereby boosting its catalytic activity in esterification. The maximum caprylic acid conversion of 92.4% was achieved over the iron doped catalysts. Moreover, the incorporation of iron has also greatly elevated the stability of SO42 −/AlPO4 during repeated catalytic cycles.
1. Introduction The esterification of carboxylic acids with alcohols is a common and essential reaction in the synthesis of numerous fine chemicals and biofuels. Conventionally, industrial esterification processes are mainly catalyzed by homogeneous mineral acids (e.g. sulfuric acid). However, use of these homogeneous acid catalysts inevitably brings about some safety, economic, and environmental troubles such as toxicity, corrosivity, separation and contamination. In recent years, various solid acid catalysts have been explored for esterification reactions, including cation-exchange resins [1], zeolites [2], supported heteropolyacids [3], sulfate group promoted solid acids [4,5], oxides of elements with valence five or higher [6], acid activated clays [7] and carbon based solid acids [8]. Among them, sulfate group promoted solid acid catalysts (mostly sulfated metal oxides) have drawn considerable attention in this field [9–11]. Ascribed to its superior properties, aluminum phosphate is regarded as a high-quality support material and has been applied in a variety of catalytic systems [12,13]. The framework of AlPO4 consists of alternating oxygen sharing AlO4 and PO4 tetrahedron. Substituting Al3 + or P in the framework by suitable hetero-elements can obtain heteroatom substituted aluminum phosphate with varied properties, which may result in enhanced catalytic activities [14,15]. However, there are only a handful of reports about AlPO4 based solid acid catalyst [16]. Till now, investigations concerning sulfate anion promoted AlPO4 (and its heteroatom doped analogues) solid acids applied in esterification are rare. Accordingly, in this work, a series of SO42 −/FexAl1 − xPO4 (x: molar ⁎
Corresponding author. E-mail address: [email protected] (P. Jiang).
http://dx.doi.org/10.1016/j.catcom.2017.05.020 Received 14 February 2017; Received in revised form 4 May 2017; Accepted 21 May 2017 Available online 23 May 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
ratio of Fe) solid acid catalysts were prepared and characterized to investigate their physicochemical properties. The activities of these catalysts were evaluated by the esterification of caprylic acid with ethanol. The recyclability and stability of the catalysts were also discussed. 2. Experimental The catalysts were prepared through a co-precipitation method, and their catalytic activity measurements were carried out in a liquid phase batch reactor with 100 mL capacity. Techniques including XRD, FESEM, N2 adsorption–desorption, elemental analysis, XPS, NH3-TPD, pyridine adsorption FT-IR and ion exchange/titration were employed to investigate the physico-chemical properties of these catalysts. Detailed procedures are described in the supporting information. 3. Results and discussion 3.1. Catalysts characterization The XRD patterns of FexAl1 − xPO4 and SO42 −/FexAl1 − xPO4 (x = 0, 0.04, 0.08, 0.12) are presented in Fig. 1. As catalyst support, pure AlPO4 and its iron doped analogues (Fig. 1a) all exhibited broad peaks in the 2θ region of 15–35°, indicating an amorphous nature. In contrast, XRD patterns of all the sulfated catalysts after calcination at 500 °C (Fig. 1b) showed orthorhombic α-AlPO4 crystalline phase (2θ = 20.3°, 21.5°, 22.9° and 35.5°), reflecting the crystallization of these supports during calcination in sulfation procedure. Iron doped
Catalysis Communications 99 (2017) 49–52
B. Liu et al.
Table 1 Physico-chemical properties of the prepared catalysts.
(a)
Intensity (a.u.)
Fe0.12Al0.88PO4 Fe0.08Al0.92PO4
Fe0.04Al0.96PO4
AlPO4
10
20
30
40
50
60
SO42 −/ FexAl1 − xPO4
x=0
x = 0.04
x = 0.08
x = 0.12
BET surface area (m2/g) Total pore volume (cm3/g) Crystallite size (nm)a Sulfur content (wt%) Total acid sites (mmol/g) Acid site density (mmol/m2) B/L acid ratiod
22.45
43.35
46.78
47.86
0.071
0.103
0.112
0.128
48 3.2 2.009b (1.988c) 0.089b (0.088c) 1.17
29 4.8 3.086b (3.003c) 0.071b (0.069c) 4.57
26 4.9 3.187b (3.106c) 0.068b (0.066c) 5.45
25 4.1 2.612b (2.537c) 0.055b (0.053c) 4.90
a
o
2 Theta ( )
b c d
(b)
α
α β
standard) from the XPS instrument itself. The P 2p peaks (Fig. S2b) with binding energy around 134.5 eV are ascribed to pentavalent phosphorus of PO4 tetrahedral unit in the phosphates skeleton [21]. In case of SO42 −/AlPO4, the peak assigned to Al 2p locates at binding energy of 75.3 eV (Fig. S2c), suggesting the presence of Al3 + cation that results from the bonding of surface sulfate groups [22]. XRD patterns of the catalysts also verified this interaction. The O 1s peak centered at 532.3 eV (Fig. S2d) can be attributed to oxygen with valence of − 2, which exist in the phosphate matrix as well as surface sulfate groups [23]. However, for the Fe doped catalyst, binding energies of Al 2p and O 1s slightly shift to 75.6 eV and 532.7 eV, respectively. These slight variations imply that the incorporation of iron into the AlPO4 lattice could change the chemical environment of the relevant atoms, which may be a factor influencing the acidic properties and catalytic activities of the solid acid. The peaks (Fig. S2e) at 712 and 726 eV correspond to binding energies of Fe 2p3/2 and Fe 2p1/2, respectively, indicating the existence of Fe3 + in this catalyst [24]. Meanwhile, peaks at 169.6 eV are detected in the S 2p spectra for the two samples (Fig. S2f), which is assigned to sulfur with oxidation state of +6 contained in sulfate groups [25]. These electron-drawing sulfur species are doubtlessly critical in the formation of strong acid sites [26]. The acidic properties of the prepared catalysts were evaluated by NH3-TPD measurements and their desorption profiles were shown in Fig. 2. Various peaks were identified in the measurement region of 100–850 °C, corresponding to ammonia desorption from surface acid sites of different strength. There is a positive correlation between
2-
α
SO4 /Fe0.12Al0.88PO4
Intensity (a.u.)
β
α
2-
SO4 /Fe0.08Al0.92PO4 2-
SO4 /Fe0.04Al0.96PO4 α--AlPO4 β --Al2(SO4)3 2-
SO4 /AlPO4 10
20
30
40
50
By Scherrer formula from the characteristic peak (2θ = 21.5°) in powder XRD. By ion-exchange/titration mothod [19]. By NH3-TPD. Calculated by molar extinction coefficient method (spectra recorded at 150 °C) [20].
60
o
2 Theta ( ) Fig. 1. XRD patterns of (a) phosphate supports and (b) their corresponding catalysts.
supports and their corresponding catalysts revealed little difference in the diffraction patterns compared with AlPO4 and SO42 −/AlPO4, respectively. This implies that the Fe3 + ions are well incorporated and highly dispersed into the AlPO4 lattice. It is worth to note that the intensities of characteristic peaks of crystalline AlPO4 exhibited by SO42 −/FexAl1 − xPO4 (x = 0.04, 0.08, 0.12) decreased as compared to SO42 −/AlPO4, suggesting that the iron doped supports have lower crystallinities after sulfating treatment. Besides, Al2(SO4)3 crystalline phases (2θ = 15.1°, 25.4°) are observed in all samples of these sulfated catalysts, which might be attributed to the interaction between excess SO42 − and metal ions during calcination procedure. Similar phenomena were also observed in those SO42 −/MxOy solid acids [17,18]. The textural properties of the catalysts based on N2 physical adsorption-desorption measurements are summarized in Table 1. It can be observed that these SO42 −/FexAl1 − xPO4 (x = 0.04, 0.08, 0.12) catalysts all have larger specific surface areas and pore volumes than the AlPO4 based solid acid. This could be ascribed to the relatively low crystallinities of these iron doped catalysts (evidenced by XRD patterns). For the catalysts concerned, crystallinity reflects the extent of sintering of the supports. Furthermore, the crystallite sizes (calculated by Scherrer equation, listed in Table 1) of these iron doped catalysts are all smaller as compared to the SO42 −/AlPO4, and their particle sizes observed from SEM images (Fig. S1) also manifested a similar tendency. All these account for the relatively higher surface areas of iron doped catalysts as well. The XPS scan spectra of SO42 −/AlPO4 and SO42 −/Fe0.08Al0.92PO4 catalysts are depicted in Fig. S2. The two survey scan spectra (Fig. S2a) reveal clear peaks of Al, P, S, C, and O elements. Thereinto, the observed peaks of C 1s (285 eV) are due to carbon tape (i.e. internal
strong
TCD signal (a.u.)
weak
super
moderate
(4) (3) (2) (1)
100
200
300
400 500 600 o Temperature ( C)
700
800
Fig. 2. NH3-TPD profiles of the catalysts: (1) SO42 −/AlPO4; (2) SO42 −/Fe0.04Al0.96PO4; (3) SO42 −/Fe0.08Al0.92PO4; (4) SO42 −/Fe0.12Al0.88PO4.
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Table 2 Catalytic performances of the prepared samples.
2-
SO4 /Fe0.08Al0.92PO4
100
2-
Yield of ethyl caprylate (%)
Conversion of caprylic acid (%)
SO42 −/AlPO4 SO42 −/Fe0.04Al0.96PO4 SO42 −/Fe0.08Al0.92PO4 SO42 −/Fe0.12Al0.88PO4 AlPO4 Fe0.04Al0.96PO4 Fe0.08Al0.92PO4 Fe0.12Al0.88PO4 None
75.8 90.1 91.9 89.1 5.7 6.1 6.2 5.8 4.8
76.3 90.7 92.4 89.5 5.7 6.1 6.3 5.9 4.8
SO4 /AlPO4 Conversion (%)
Sample
20 1
desorption temperature and acid strength of the solid acid. It can be observed that the desorption curves of iron doped catalysts exhibit peaks at higher temperature in the strong and super acidic territories, indicating stronger acidities as compared to the SO42 −/AlPO4. Besides, the results of total acid sites (Table 1) showed that these iron doped catalysts possess more acid sites than the SO42 −/AlPO4. A similar tendency in sulfur content (Table 1) also can be observed for the prepared catalysts. It is probably that the relatively larger surface areas of these iron doped catalysts (Table 1) enable them to bond more sulfate groups on the surface, thus forming more acid sites. Consequently, these iron doped solid acids manifested higher catalytic activities (see Table 2) as compared to the SO42 −/AlPO4 catalyst. FT-IR spectra of pyridine adsorbed onto a solid surface can provide a further insight into the nature of the surface acid sites. The recorded spectra for the prepared catalysts (room temperature) are presented in Fig. 3. All samples show a variety of bands in the region from 1700 to 1300 cm− 1. Covalently bonded pyridine (corresponding to Lewis acid sites) exhibits peaks at 1450 and 1616 cm− 1 (shoulder peaks) [27]. Bands at 1540 and ~1640 cm− 1 are assigned to pyridinium ion (pyridine adsorbed onto Brönsted acid sites) [28]. The bands observed at 1490 cm− 1 in all spectra represent combined pyridine adsorbed on both Brönsted and Lewis acid sites [29]. Furthermore, the peaks around 1576 cm− 1 can also be attributed to pyridine adsorbed onto Lewis acid sites [30]. These spectra in Fig. 4 unambiguously reveal that all the catalysts investigated contain both Brönsted and Lewis acid sites. It is worth noting that the spectra of these iron doped catalysts possess evidently larger peak areas as compared to the SO42 −/AlPO4. This means the surface of SO42 −/FexAl1 − xPO4 (x = 0.04, 0.08, 0.12) catalysts can accommodate more acid sites, which is also confirmed
B+L
Absorbance (a.u.)
L
5
Esterification of caprylic acid with ethanol was used to assess the catalytic activities of the prepared catalysts and the results are listed in Table 2. For comparison, esterifications without any catalyst present and with only the phosphate supports were also performed. Under the employed reaction conditions, ethyl caprylate was the predominant product with its selectivity above 99%. It was found that the conversion of caprylic acid were extremely low (less than 7%) in the absence of catalyst and in the presence of these supports, indicating that this reaction is efficiently catalyzed by strong Brönsted acids owing to surface bonded sulfate groups. Under the same conditions, these iron doped solid acids all markedly outperformed the SO42 −/AlPO4 in esterification, proving that Fe-doping is an efficient method to improve the catalytic activity of sulfated AlPO4. The NH3-TPD profiles and FT-IR spectra of adsorbed pyridine reveal that these iron doped catalysts display stronger acidities, possess more acid sites and present more evident Brönsted acidities. Besides, the relatively lower acid site densities of these iron doped catalysts as compared with the SO42 −/ AlPO4 (see Table 1) imply that total acid sites scattered on larger surface area generate more accessible acid sites. It is believed that the dispersion of acid sites and the accessibility of the relevant organic substrate also play important roles in determining the activity [31]. All these abovementioned are accountable for the comparatively higher activities of the iron doped catalysts. To evaluate the stability of the prepared catalysts, SO42 −/AlPO4 as a baseline sample and SO42 −/Fe0.08Al0.92PO4, were subject to successive reaction cycles. After each batch reaction cycle, the spent catalyst was separated by centrifugation, washed with ethanol, dried at 100 °C in air, and then reused in the next run under the same reaction conditions. The results obtained are presented in Fig. 4. As can be noted, the SO42 −/AlPO4 catalyst underwent a significant decline in activity after five catalytic cycles (caprylic acid conversion: 76.3% → 45.6%), whereas the SO42 −/Fe0.08Al0.92PO4 only showed slight loss of its initial activity after the same number of cycles (caprylic acid conversion: 92.4% → 80.5%). To further probe into the catalyst deactivation, acid site contents of the spent catalysts retrieved after five repeated cycles were also determined (by ion-exchange/titration method). The spent SO42 −/AlPO4 only retained an acid site content of 0.813 mmol/g, far less than the fresh catalyst (2.009 mmol/g). How-
B
(3) (2) (1) 1650
4
3.2. Catalytic performance and reusability
B
1500 1550 1600 -1 Wavenumber (cm )
3 Runs of cycles
by the NH3-TPD profiles. In addition, the calculated Brönsted/Lewis acid site ratios (1540 cm− 1 for B and 1450 cm− 1 for L, spectra at 150 °C, Fig. S3) of these iron doped catalysts are also significantly higher than that of the SO42 −/AlPO4 (Table 1), exhibiting obvious Brönsted acidities on them.
(4)
1450
2
Fig. 4. Catalytic activities of SO42 −/AlPO4 and SO42 −/Fe0.08Al0.92PO4 during five repeated cycles of caprylic acid esterification (reaction conditions: the same as Table 2).
L
1400
60
40
Reaction conditions: catalyst amount 1.5 wt%, ethanol/caprylic acid molar ratio 6:1, reaction conducted at 75 °C for 4 h, stirring speed 600 rpm.
L
80
1700
Fig. 3. FT-IR spectra of pyridine adsorbed on (1) SO42 −/AlPO4; (2) SO42 −/ Fe0.04Al0.96PO4; (3) SO42 −/Fe0.08Al0.92PO4; (4) SO42 −/Fe0.12Al0.88PO4 (room temperature).
51
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ever, total acid site present on the SO42 −/Fe0.08Al0.92PO4 experienced a more moderate decrease from 3.187 to 2.796 mmol/g. The lessening in acid site content probably results from the elution of surface sulfate groups, which is responsible for the deactivation of solid acids. These above results demonstrate that doping of Fe into AlPO4 support can reinforce the bonding of surface sulfate groups, thus enhancing the stability of its sulfated solid acid.
[3] M. Kuzminska, T.V. Kovalchuk, R. Backov, E.M. Gaigneaux, J. Catal. 320 (2014) 1–8. [4] G.C. Chen, H.B. Qiao, J.K. Cao, Z.C. Wang, M.F. Ye, C.Y. Guo, P.F. Ding, X.M. Wen, Fuel 163 (2016) 41–47. [5] M.Z. Wu, J.M. Guo, Y. Li, Y.J. Zhang, Ceram. Int. 39 (2013) 9731–9736. [6] G. Mitran, T. Yuzhakova, I. Popescu, I.-C. Marcu, J. Mol. Catal. A Chem. 396 (2015) 275–281. [7] M.J.C. Rezende, A.C. Pinto, Renew. Energy 92 (2016) 171–177. [8] Y. Zhou, S.L. Niu, J. Li, Energy Convers. Manag. 114 (2016) 188–196. [9] K. Saravanan, B. Tyagi, R.S. Shukla, H.C. Bajaj, Fuel 165 (2016) 298–305. [10] K. Saravanan, B. Tyagi, H.C. Bajaj, Appl. Catal. B Environ. 192 (2016) 161–170. [11] C.C. Huang, C.J. Yang, P.J. Gao, N.C. Wang, C.L. Chen, J.S. Chang, Green Chem. 17 (2015) 3609–3620. [12] Y. Danjo, I. Kikuchi, Y. Ino, M. Ohshima, H. Kurokawa, H. Miura, React. Kinet. Mech. Catal. 105 (2012) 381–389. [13] S.C. Kishore, A. Pandurangan, Chem. Eng. J. 222 (2013) 472–477. [14] D. Chakrabortty, J.N. Ganguli, C.V.V. Satyanarayana, Microporous Mesoporous Mater. 137 (2011) 65–71. [15] A.V. Vijayasankar, N. Nagaraju, C. R. Chim. 14 (2011) 1109–1116. [16] W.L. Xie, D. Yang, Bioresour. Technol. 119 (2012) 60–65. [17] H.P. Yan, Y. Yang, D.M. Tong, X. Xiang, C.W. Hu, Catal. Commun. 10 (2009) 1558–1563. [18] K.H. Jiang, D.M. Tong, J.Q. Tang, R.L. Song, C.W. Hu, Appl. Catal. A Gen. 389 (2010) 46–51. [19] H. Yu, S. Niu, C. Lu, J. Li, Y. Yang, Energy Convers. Manag. 126 (2016) 488–496. [20] K. Saravanan, B. Tyagi, R.S. Shukla, H.C. Bajaj, Appl. Catal. B Environ. 172–173 (2015) 108–115. [21] R. Fu, L. Liu, W. Huang, P. Sun, J. Appl. Polym. Sci. 87 (2003) 2253–2261. [22] A.Q. Wang, X.L. Wu, J.X. Wang, H. Pan, X.Y. Tian, Y.L. Xing, RSC Adv. 5 (2015) 19652–19658. [23] Q. Yu, X.J. Yao, H.L. Zhang, F. Gao, L. Dong, Appl. Catal. A Gen. 423–424 (2012) 42–51. [24] C.Z. Liu, D.W. Meng, H.X. Pang, X.L. Wu, J. Xie, X.H. Yu, L. Chen, X.Y. Liu, J. Magn. Magn. Mater. 324 (2012) 3356–3360. [25] Q. Yi, J. Zhang, X. Zhang, J. Feng, W. Li, Fuel 143 (2015) 390–398. [26] C.W. Dunnill, Z.A. Aiken, A. Kafizas, J. Pratten, M. Wilson, D.J. Morgan, I.P. Parkin, J. Mater. Chem. 19 (2009) 8747–8754. [27] M.J. López-Muñoz, R. van Grieken, J. Aguado, J. Marugán, Catal. Today 101 (2005) 307–314. [28] R.V. Sharma, C. Baroi, A.K. Dalai, Catal. Today 237 (2014) 3–12. [29] B. Mallesham, P. Sudarsanam, G. Raju, B.M. Reddy, Green Chem. 15 (2003) 478–489. [30] V.C. dos Santos, K. Wilson, A.F. Lee, S. Nakagaki, Appl. Catal. B Environ. 162 (2015) 75–84. [31] K. Wilson, A.F. Lee, D.J. Macquarrie, J.H. Clark, Appl. Catal. A Gen. 228 (2002) 127–133.
4. Conclusion Iron was doped into AlPO4 matrix via a simple co-precipitation method to prepare a series of SO42 −/FexAl1 − xPO4 solid acid catalysts. It is corroborated that incorporation of Fe into the AlPO4 skeleton can change the chemical state of the exterior atom and brought about prominent enhancements in acidic properties (acid strength, acid site amount and brönsted/lewis acid site ratio) of the corresponding catalysts. Accordingly, these iron doped solid acids exhibited higher catalytic activities in the esterification of caprylic acid with ethanol as compared to the SO42 −/AlPO4. In addition, the reusability tests proved that doping of iron can also upgrade the stability of SO42 −/AlPO4 catalyst in successive reaction cycles. Acknowledgements This work was financially supported by the Fundamental Research Funds for the Central Universities (JUSRP51623A). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.05.020. References [1] N. Boz, N. Degirmenbasi, D.M. Kalyon, Appl. Catal. B Environ. 165 (2015) 723–730. [2] A.M. Doyle, T.M. Albayati, A.S. Abbas, Z.T. Alismaeel, Renew. Energy 97 (2016) 19–23.
52
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Preparation and characterization of SO42 −/FexAl1 − xPO4 solid acid catalysts for caprylic acid esterification
MARK
Boliang Liu, Pingping Jiang⁎, Pingbo Zhang, Gang Bian, Mengtian Li Key Laboratory of Synthetic and Biological Colloids (Ministry of Education), School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: SO42 −/FexAl1 − xPO4 Characterization Esterification Caprylic acid Catalytic performance
A series of SO42 −/FexAl1 − xPO4 solid acid catalysts were prepared and characterized by means of XRD, FE-SEM, elemental analysis, N2 adsorption–desorption, XPS, NH3-TPD, pyridine adsorption FT-IR and ion exchange/ titration. Their catalytic performances were evaluated by the esterification of caprylic acid with ethanol. Experimental results revealed that the doping of iron can markedly enhance the acidic properties of the SO42 −/ AlPO4, thereby boosting its catalytic activity in esterification. The maximum caprylic acid conversion of 92.4% was achieved over the iron doped catalysts. Moreover, the incorporation of iron has also greatly elevated the stability of SO42 −/AlPO4 during repeated catalytic cycles.
1. Introduction The esterification of carboxylic acids with alcohols is a common and essential reaction in the synthesis of numerous fine chemicals and biofuels. Conventionally, industrial esterification processes are mainly catalyzed by homogeneous mineral acids (e.g. sulfuric acid). However, use of these homogeneous acid catalysts inevitably brings about some safety, economic, and environmental troubles such as toxicity, corrosivity, separation and contamination. In recent years, various solid acid catalysts have been explored for esterification reactions, including cation-exchange resins [1], zeolites [2], supported heteropolyacids [3], sulfate group promoted solid acids [4,5], oxides of elements with valence five or higher [6], acid activated clays [7] and carbon based solid acids [8]. Among them, sulfate group promoted solid acid catalysts (mostly sulfated metal oxides) have drawn considerable attention in this field [9–11]. Ascribed to its superior properties, aluminum phosphate is regarded as a high-quality support material and has been applied in a variety of catalytic systems [12,13]. The framework of AlPO4 consists of alternating oxygen sharing AlO4 and PO4 tetrahedron. Substituting Al3 + or P in the framework by suitable hetero-elements can obtain heteroatom substituted aluminum phosphate with varied properties, which may result in enhanced catalytic activities [14,15]. However, there are only a handful of reports about AlPO4 based solid acid catalyst [16]. Till now, investigations concerning sulfate anion promoted AlPO4 (and its heteroatom doped analogues) solid acids applied in esterification are rare. Accordingly, in this work, a series of SO42 −/FexAl1 − xPO4 (x: molar ⁎
Corresponding author. E-mail address: [email protected] (P. Jiang).
http://dx.doi.org/10.1016/j.catcom.2017.05.020 Received 14 February 2017; Received in revised form 4 May 2017; Accepted 21 May 2017 Available online 23 May 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
ratio of Fe) solid acid catalysts were prepared and characterized to investigate their physicochemical properties. The activities of these catalysts were evaluated by the esterification of caprylic acid with ethanol. The recyclability and stability of the catalysts were also discussed. 2. Experimental The catalysts were prepared through a co-precipitation method, and their catalytic activity measurements were carried out in a liquid phase batch reactor with 100 mL capacity. Techniques including XRD, FESEM, N2 adsorption–desorption, elemental analysis, XPS, NH3-TPD, pyridine adsorption FT-IR and ion exchange/titration were employed to investigate the physico-chemical properties of these catalysts. Detailed procedures are described in the supporting information. 3. Results and discussion 3.1. Catalysts characterization The XRD patterns of FexAl1 − xPO4 and SO42 −/FexAl1 − xPO4 (x = 0, 0.04, 0.08, 0.12) are presented in Fig. 1. As catalyst support, pure AlPO4 and its iron doped analogues (Fig. 1a) all exhibited broad peaks in the 2θ region of 15–35°, indicating an amorphous nature. In contrast, XRD patterns of all the sulfated catalysts after calcination at 500 °C (Fig. 1b) showed orthorhombic α-AlPO4 crystalline phase (2θ = 20.3°, 21.5°, 22.9° and 35.5°), reflecting the crystallization of these supports during calcination in sulfation procedure. Iron doped
Catalysis Communications 99 (2017) 49–52
B. Liu et al.
Table 1 Physico-chemical properties of the prepared catalysts.
(a)
Intensity (a.u.)
Fe0.12Al0.88PO4 Fe0.08Al0.92PO4
Fe0.04Al0.96PO4
AlPO4
10
20
30
40
50
60
SO42 −/ FexAl1 − xPO4
x=0
x = 0.04
x = 0.08
x = 0.12
BET surface area (m2/g) Total pore volume (cm3/g) Crystallite size (nm)a Sulfur content (wt%) Total acid sites (mmol/g) Acid site density (mmol/m2) B/L acid ratiod
22.45
43.35
46.78
47.86
0.071
0.103
0.112
0.128
48 3.2 2.009b (1.988c) 0.089b (0.088c) 1.17
29 4.8 3.086b (3.003c) 0.071b (0.069c) 4.57
26 4.9 3.187b (3.106c) 0.068b (0.066c) 5.45
25 4.1 2.612b (2.537c) 0.055b (0.053c) 4.90
a
o
2 Theta ( )
b c d
(b)
α
α β
standard) from the XPS instrument itself. The P 2p peaks (Fig. S2b) with binding energy around 134.5 eV are ascribed to pentavalent phosphorus of PO4 tetrahedral unit in the phosphates skeleton [21]. In case of SO42 −/AlPO4, the peak assigned to Al 2p locates at binding energy of 75.3 eV (Fig. S2c), suggesting the presence of Al3 + cation that results from the bonding of surface sulfate groups [22]. XRD patterns of the catalysts also verified this interaction. The O 1s peak centered at 532.3 eV (Fig. S2d) can be attributed to oxygen with valence of − 2, which exist in the phosphate matrix as well as surface sulfate groups [23]. However, for the Fe doped catalyst, binding energies of Al 2p and O 1s slightly shift to 75.6 eV and 532.7 eV, respectively. These slight variations imply that the incorporation of iron into the AlPO4 lattice could change the chemical environment of the relevant atoms, which may be a factor influencing the acidic properties and catalytic activities of the solid acid. The peaks (Fig. S2e) at 712 and 726 eV correspond to binding energies of Fe 2p3/2 and Fe 2p1/2, respectively, indicating the existence of Fe3 + in this catalyst [24]. Meanwhile, peaks at 169.6 eV are detected in the S 2p spectra for the two samples (Fig. S2f), which is assigned to sulfur with oxidation state of +6 contained in sulfate groups [25]. These electron-drawing sulfur species are doubtlessly critical in the formation of strong acid sites [26]. The acidic properties of the prepared catalysts were evaluated by NH3-TPD measurements and their desorption profiles were shown in Fig. 2. Various peaks were identified in the measurement region of 100–850 °C, corresponding to ammonia desorption from surface acid sites of different strength. There is a positive correlation between
2-
α
SO4 /Fe0.12Al0.88PO4
Intensity (a.u.)
β
α
2-
SO4 /Fe0.08Al0.92PO4 2-
SO4 /Fe0.04Al0.96PO4 α--AlPO4 β --Al2(SO4)3 2-
SO4 /AlPO4 10
20
30
40
50
By Scherrer formula from the characteristic peak (2θ = 21.5°) in powder XRD. By ion-exchange/titration mothod [19]. By NH3-TPD. Calculated by molar extinction coefficient method (spectra recorded at 150 °C) [20].
60
o
2 Theta ( ) Fig. 1. XRD patterns of (a) phosphate supports and (b) their corresponding catalysts.
supports and their corresponding catalysts revealed little difference in the diffraction patterns compared with AlPO4 and SO42 −/AlPO4, respectively. This implies that the Fe3 + ions are well incorporated and highly dispersed into the AlPO4 lattice. It is worth to note that the intensities of characteristic peaks of crystalline AlPO4 exhibited by SO42 −/FexAl1 − xPO4 (x = 0.04, 0.08, 0.12) decreased as compared to SO42 −/AlPO4, suggesting that the iron doped supports have lower crystallinities after sulfating treatment. Besides, Al2(SO4)3 crystalline phases (2θ = 15.1°, 25.4°) are observed in all samples of these sulfated catalysts, which might be attributed to the interaction between excess SO42 − and metal ions during calcination procedure. Similar phenomena were also observed in those SO42 −/MxOy solid acids [17,18]. The textural properties of the catalysts based on N2 physical adsorption-desorption measurements are summarized in Table 1. It can be observed that these SO42 −/FexAl1 − xPO4 (x = 0.04, 0.08, 0.12) catalysts all have larger specific surface areas and pore volumes than the AlPO4 based solid acid. This could be ascribed to the relatively low crystallinities of these iron doped catalysts (evidenced by XRD patterns). For the catalysts concerned, crystallinity reflects the extent of sintering of the supports. Furthermore, the crystallite sizes (calculated by Scherrer equation, listed in Table 1) of these iron doped catalysts are all smaller as compared to the SO42 −/AlPO4, and their particle sizes observed from SEM images (Fig. S1) also manifested a similar tendency. All these account for the relatively higher surface areas of iron doped catalysts as well. The XPS scan spectra of SO42 −/AlPO4 and SO42 −/Fe0.08Al0.92PO4 catalysts are depicted in Fig. S2. The two survey scan spectra (Fig. S2a) reveal clear peaks of Al, P, S, C, and O elements. Thereinto, the observed peaks of C 1s (285 eV) are due to carbon tape (i.e. internal
strong
TCD signal (a.u.)
weak
super
moderate
(4) (3) (2) (1)
100
200
300
400 500 600 o Temperature ( C)
700
800
Fig. 2. NH3-TPD profiles of the catalysts: (1) SO42 −/AlPO4; (2) SO42 −/Fe0.04Al0.96PO4; (3) SO42 −/Fe0.08Al0.92PO4; (4) SO42 −/Fe0.12Al0.88PO4.
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Table 2 Catalytic performances of the prepared samples.
2-
SO4 /Fe0.08Al0.92PO4
100
2-
Yield of ethyl caprylate (%)
Conversion of caprylic acid (%)
SO42 −/AlPO4 SO42 −/Fe0.04Al0.96PO4 SO42 −/Fe0.08Al0.92PO4 SO42 −/Fe0.12Al0.88PO4 AlPO4 Fe0.04Al0.96PO4 Fe0.08Al0.92PO4 Fe0.12Al0.88PO4 None
75.8 90.1 91.9 89.1 5.7 6.1 6.2 5.8 4.8
76.3 90.7 92.4 89.5 5.7 6.1 6.3 5.9 4.8
SO4 /AlPO4 Conversion (%)
Sample
20 1
desorption temperature and acid strength of the solid acid. It can be observed that the desorption curves of iron doped catalysts exhibit peaks at higher temperature in the strong and super acidic territories, indicating stronger acidities as compared to the SO42 −/AlPO4. Besides, the results of total acid sites (Table 1) showed that these iron doped catalysts possess more acid sites than the SO42 −/AlPO4. A similar tendency in sulfur content (Table 1) also can be observed for the prepared catalysts. It is probably that the relatively larger surface areas of these iron doped catalysts (Table 1) enable them to bond more sulfate groups on the surface, thus forming more acid sites. Consequently, these iron doped solid acids manifested higher catalytic activities (see Table 2) as compared to the SO42 −/AlPO4 catalyst. FT-IR spectra of pyridine adsorbed onto a solid surface can provide a further insight into the nature of the surface acid sites. The recorded spectra for the prepared catalysts (room temperature) are presented in Fig. 3. All samples show a variety of bands in the region from 1700 to 1300 cm− 1. Covalently bonded pyridine (corresponding to Lewis acid sites) exhibits peaks at 1450 and 1616 cm− 1 (shoulder peaks) [27]. Bands at 1540 and ~1640 cm− 1 are assigned to pyridinium ion (pyridine adsorbed onto Brönsted acid sites) [28]. The bands observed at 1490 cm− 1 in all spectra represent combined pyridine adsorbed on both Brönsted and Lewis acid sites [29]. Furthermore, the peaks around 1576 cm− 1 can also be attributed to pyridine adsorbed onto Lewis acid sites [30]. These spectra in Fig. 4 unambiguously reveal that all the catalysts investigated contain both Brönsted and Lewis acid sites. It is worth noting that the spectra of these iron doped catalysts possess evidently larger peak areas as compared to the SO42 −/AlPO4. This means the surface of SO42 −/FexAl1 − xPO4 (x = 0.04, 0.08, 0.12) catalysts can accommodate more acid sites, which is also confirmed
B+L
Absorbance (a.u.)
L
5
Esterification of caprylic acid with ethanol was used to assess the catalytic activities of the prepared catalysts and the results are listed in Table 2. For comparison, esterifications without any catalyst present and with only the phosphate supports were also performed. Under the employed reaction conditions, ethyl caprylate was the predominant product with its selectivity above 99%. It was found that the conversion of caprylic acid were extremely low (less than 7%) in the absence of catalyst and in the presence of these supports, indicating that this reaction is efficiently catalyzed by strong Brönsted acids owing to surface bonded sulfate groups. Under the same conditions, these iron doped solid acids all markedly outperformed the SO42 −/AlPO4 in esterification, proving that Fe-doping is an efficient method to improve the catalytic activity of sulfated AlPO4. The NH3-TPD profiles and FT-IR spectra of adsorbed pyridine reveal that these iron doped catalysts display stronger acidities, possess more acid sites and present more evident Brönsted acidities. Besides, the relatively lower acid site densities of these iron doped catalysts as compared with the SO42 −/ AlPO4 (see Table 1) imply that total acid sites scattered on larger surface area generate more accessible acid sites. It is believed that the dispersion of acid sites and the accessibility of the relevant organic substrate also play important roles in determining the activity [31]. All these abovementioned are accountable for the comparatively higher activities of the iron doped catalysts. To evaluate the stability of the prepared catalysts, SO42 −/AlPO4 as a baseline sample and SO42 −/Fe0.08Al0.92PO4, were subject to successive reaction cycles. After each batch reaction cycle, the spent catalyst was separated by centrifugation, washed with ethanol, dried at 100 °C in air, and then reused in the next run under the same reaction conditions. The results obtained are presented in Fig. 4. As can be noted, the SO42 −/AlPO4 catalyst underwent a significant decline in activity after five catalytic cycles (caprylic acid conversion: 76.3% → 45.6%), whereas the SO42 −/Fe0.08Al0.92PO4 only showed slight loss of its initial activity after the same number of cycles (caprylic acid conversion: 92.4% → 80.5%). To further probe into the catalyst deactivation, acid site contents of the spent catalysts retrieved after five repeated cycles were also determined (by ion-exchange/titration method). The spent SO42 −/AlPO4 only retained an acid site content of 0.813 mmol/g, far less than the fresh catalyst (2.009 mmol/g). How-
B
(3) (2) (1) 1650
4
3.2. Catalytic performance and reusability
B
1500 1550 1600 -1 Wavenumber (cm )
3 Runs of cycles
by the NH3-TPD profiles. In addition, the calculated Brönsted/Lewis acid site ratios (1540 cm− 1 for B and 1450 cm− 1 for L, spectra at 150 °C, Fig. S3) of these iron doped catalysts are also significantly higher than that of the SO42 −/AlPO4 (Table 1), exhibiting obvious Brönsted acidities on them.
(4)
1450
2
Fig. 4. Catalytic activities of SO42 −/AlPO4 and SO42 −/Fe0.08Al0.92PO4 during five repeated cycles of caprylic acid esterification (reaction conditions: the same as Table 2).
L
1400
60
40
Reaction conditions: catalyst amount 1.5 wt%, ethanol/caprylic acid molar ratio 6:1, reaction conducted at 75 °C for 4 h, stirring speed 600 rpm.
L
80
1700
Fig. 3. FT-IR spectra of pyridine adsorbed on (1) SO42 −/AlPO4; (2) SO42 −/ Fe0.04Al0.96PO4; (3) SO42 −/Fe0.08Al0.92PO4; (4) SO42 −/Fe0.12Al0.88PO4 (room temperature).
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ever, total acid site present on the SO42 −/Fe0.08Al0.92PO4 experienced a more moderate decrease from 3.187 to 2.796 mmol/g. The lessening in acid site content probably results from the elution of surface sulfate groups, which is responsible for the deactivation of solid acids. These above results demonstrate that doping of Fe into AlPO4 support can reinforce the bonding of surface sulfate groups, thus enhancing the stability of its sulfated solid acid.
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4. Conclusion Iron was doped into AlPO4 matrix via a simple co-precipitation method to prepare a series of SO42 −/FexAl1 − xPO4 solid acid catalysts. It is corroborated that incorporation of Fe into the AlPO4 skeleton can change the chemical state of the exterior atom and brought about prominent enhancements in acidic properties (acid strength, acid site amount and brönsted/lewis acid site ratio) of the corresponding catalysts. Accordingly, these iron doped solid acids exhibited higher catalytic activities in the esterification of caprylic acid with ethanol as compared to the SO42 −/AlPO4. In addition, the reusability tests proved that doping of iron can also upgrade the stability of SO42 −/AlPO4 catalyst in successive reaction cycles. Acknowledgements This work was financially supported by the Fundamental Research Funds for the Central Universities (JUSRP51623A). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.05.020. References [1] N. Boz, N. Degirmenbasi, D.M. Kalyon, Appl. Catal. B Environ. 165 (2015) 723–730. [2] A.M. Doyle, T.M. Albayati, A.S. Abbas, Z.T. Alismaeel, Renew. Energy 97 (2016) 19–23.
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