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Controlling hydrogenation selectivity with Pd catalysts on carbon nitrides functionalized silica
Journal of Catalysis 326 (2015) 38–42
Contents lists available at ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Research Note
Controlling hydrogenation selectivity with Pd catalysts on carbon nitrides functionalized silica Tao Yuan a, Haifeng Gong a, Kamalakannan Kailasam b, Yanxi Zhao a, Arne Thomas b, Junjiang Zhu a,⇑ a b
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs & Commission Ministry of Education, South-Central University for Nationalities, Wuhan 430074, China Department of Chemistry, Technische Universität Berlin, Berlin 10623, Germany
a r t i c l e
i n f o
Article history: Received 7 December 2014 Revised 13 March 2015 Accepted 14 March 2015
Keywords: Palladium Carbon nitrides SBA-15 1,5-Cyclooctadiene Selective hydrogenation
a b s t r a c t We report here the synthesis of Pd nanoparticles (NPs) supported on a carbon nitrides–silica composite (S-C_T) and its application as a catalyst in the hydrogenation of 1,5-cyclooctadiene. While the mesoporous silica provides a high surface area, the carbon nitrides (CNx) stabilize the Pd NPs, and the Pd particle size can be controlled by varying the treatment temperature (T) for the composite. The Pd catalysts show good conversion and selectivity for hydrogenation both at elevated pressure (50–80 °C, H2 pressure 2 MPa) and at atmospheric pressure, even at zero degrees, and the selectivity can be well controlled owing to the controlled particle size of the Pd NPs. Furthermore, after reusing the catalyst 6 times, no loss in selectivity is observed while the conversion is slightly decreased to about 15%. Thus, an active Pd catalyst with prolonged lifetime for hydrogenation is observed when carbon nitrides-silica composites are used as support. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction The need for sustainable and green chemical processes in the 21st century changes the criterion in the design of catalytic process from productivity to selectivity [1]. How to improve selectivity while keeping high conversion is a crucial issue in creating atom and energy efficient processes. Many attempts have been proposed to control the reaction selectivity by controlling the chemistry and structure of solid catalysts [2,3]. For instance, the control of pore size of porous catalysts can exclude large-size molecules from approaching the active site inside the pore, while small-size molecules can enter and react [4]. Also the control of particle size, particle shape, or exposure facets of metal nanoparticle (NP) catalysts can lead to varied reactivity to substrates and yield different products [5]. To control the particle size of catalytically active metal NPs, we recently reported a facile synthesis route introducing the coating of a layer of polymeric carbon nitrides (CNx) on a conventional silica support (e.g., SBA-15) before the metals are deposited [6]. The polymeric carbon nitride layer consists of amine bridged heptazine units and is easily prepared by heat treatment of nitrogen-rich precursors, such as cyanamide or melamine. This organic semiconductor has recently received enormous interest as a metal-free photocatalyst [7–12]. Because of the nitrogen-rich structure, CNx ⇑ Corresponding author. E-mail address: [email protected] (J. Zhu). http://dx.doi.org/10.1016/j.jcat.2015.03.007 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.
provides many adsorption sites to graft metal ions on its surface [13,14], while its polymeric character allows it to control the chemical structure of the support surface, and thus, the nature and number of adsorption sites of CNx by simply changing the treatment temperature. Indeed C/N values in polymeric carbon nitrides largely increase with temperature as the condensation of the triamino-heptazine units proceeds, and it was shown that the particle size of supported Au and Pt metals can be controlled by changing the treatment temperature for the carbon nitrides–silica (S-C_T) composites [6]. This offers a facile strategy to control the reaction selectivity by controlling the particle size. Hydrogenations are important organic reactions and are crucial for the synthesis of chemical intermediates in fine chemistry [15–17]. Wang et al. reported that Pd NPs@mpg-C3N4 (mesoporous carbon nitride) exhibits a high conversion for the direct hydrogenation of phenol to cyclohexanone and found a much higher catalytic performance than that for a comparable Pd NPs@carbon catalyst [18]. This significant enhancement was explained by electronic metal–semiconductor interactions, that is, formation of Mott–Schottky heterojunctions between the metal (Pd) and the semiconductor support (CNx) [19]. The selective hydrogenation of 1,5-cyclooctadiene (COD) to cyclooctene (COE) is a sustainable, green and important reaction [20,21], as the latter is an industrial intermediate product for manufacturing special polymers, for example, polyoctenamers, which are modifiers in rubbers and thermoplastics [22]. In addition, COD hydrogenation is also a good model reaction for estimating the selectivity of a catalyst as the
T. Yuan et al. / Journal of Catalysis 326 (2015) 38–42
two C@C bonds in the structure are similar in electronic environment. Indeed, the consecutive hydrogenation of COE to cyclooctane (COA) is a crucial problem for the reported catalysts (e.g., Pd/C, Pd/Al2O3), decreasing the selectivity to the monoolefine [20]. The search for highly active, selective, and reusable hydrogenation catalysts is currently a highly challenging and attractive topic. In this work, we present the preparation of Pd catalysts supported on S-C_T composites and their use for selective hydrogenation of COD to COE or COA. The results indicate that the particle size of Pd NPs depends intimately on the treatment temperature of the composite, and the resulting catalysts are very active and selective for COD hydrogenation. In particular, the selectivity to COE or COA can be well controlled by using catalysts with different Pd particle sizes or by changing the reaction temperature, while high COD conversion is even observed at low temperatures. At atmospheric conditions, the catalyst (Pd/S-C_300) shows 95% COD conversion and 99% COE selectivity after 90 min. Reusability tests demonstrate that the catalyst is keeping the high selectivity for at least 6 times, with COE selectivity >98% throughout the reaction, even though a decrease of 15% in the conversion is observed. 2. Experimental 2.1. Synthesis of S-C_T supports SBA-15 was synthesized using the general procedure reported by Zhao et al. [23]. The obtained SBA-15 was then impregnated with dicyandiamide in a water–ethanol mixed solution at 80 °C. After the solvents (water and ethanol) were removed, the resulting white solid was dried at 100 °C overnight and then thermally treated in N2 at 300, 400, and 500 °C, respectively, for 4 h at a heating rate of 2 °C/min. Based on the treatment temperature, the products are denoted S-C_T (T = 300, 400, 500). 2.2. Synthesis of Pd/S-C_T catalyst The catalysts were prepared by an impregnation method. 3.8 mL 0.01236 M pre-prepared PdCl2 solution (see the supplementary information, SI) was first diluted to 50 mL, to which 0.5 g of the above obtained S-C_T support was added. The resulting solution was then ultrasonicated for 10 min, followed by stirring for 4 h at room temperature. Thereafter, the sample was filtered, washed with distilled water and dried in a vacuum oven at 60 °C overnight, and finally reduced in H2 atmosphere at 300 °C for 3 h. Based on the S-C_T support, the catalyst was accordingly named as Pd/S-C_T. For comparison, Pd catalyst supported on SBA-15 functionalized with hydroxyl groups [24], Pd/SBA-15, was also prepared, applying the same procedure as for Pd/S-C_T. 2.3. Characterizations X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance X-ray diffractometer with Cu Ka (k = 1.5406 Å) irradiation. Transmission electron microscopy (TEM) images were obtained on a Tecnai G2 20 S-Twin apparatus with high-resolution transmission electron microscope (200 kV). N2 physisorption isotherms were measured on a TriStar II 3020 measurement at liquid nitrogen temperature ( 195.8 °C). X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Thermo Electron Corporation VG Multilab 2000 apparatus using a monochromatic Al Ka X-ray source (300 W) and analyzer pass energy of 25 eV. Binding energies were obtained by referencing to the C 1s binding energy of carbon taken at 284.6 eV. Ultraviolet–visible (UV–vis) DRS were collected on a Shimadzu UV-2550 spectrophotometer from 200 to 800 nm using BaSO4 as background.
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2.4. Catalytic tests 2.4.1. At elevated pressure The setup consists of a heating magnetic stirrer and a 50 mL titanium high pressure reactor (type: WCGF-50). 50 mg catalyst, 0.25 mL COD, 0.2 mL dodecane, and 30 mL n-heptane were charged in the reactor. The pressure was adjusted to 2 MPa, and the reactor heated to 50, 60, or 80 °C. After the reaction, the reaction mixture was centrifuged, filtered, and analyzed by an Agilent 7890 GC equipped with a FID detector and HP-5 column. 2.4.2. At atmospheric conditions The setup consists of a magnetic stirrer and a 50 mL threenecked flask, which is connected with a hydrogen balloon. 50 mg catalyst, 0.25 mL COD, 0.2 mL dodecane, and 30 mL n-heptane were added to the flask. The reaction was stirred (1000 rpm), and the reaction mixture was sampled at desired times for analysis. The catalytic performance was evaluated by two means: One is the COD conversion which is calculated from the ratio of reacted COD to initial COD, and the other is the turnover frequency (TOF) which is calculated by the moles of COD reacted per moles of surface metal used per second. To certify the lack of the mass diffusion limitations of COD in the reaction, the Weisz–Prater criterion, according to the method proposed in literature [25], was calculated, which equals to 7.05 10 4 and is far below 1, indicating that the mass diffusion limitation can be excluded at the present reaction conditions. Detailed procedures and parameters used in the synthesis, characterizations, and catalytic tests can be found in the experimental section of the SI. 3. Results and discussion The coating of CNx on SBA-15 was proven using small- and wide-angle XRD measurements. TEM images and N2 physisorption isotherms show also that the ordered structure of SBA-15 is not destroyed in the preparation process (see Fig. S1), in accordance with previous results [26,27]. TGA profiles (see Fig. S1D) show that the loading of CNx decreases with the increase of treatment temperature for S-C_T, implying that the composites treated at lower temperature (e.g., 300 °C) hold higher amounts of uncondensed – NH2 groups, which may be the adsorption site of metal NPs [13,14]. Fig. 1A shows the wide-angle XRD patterns of the supported Pd catalysts. By comparing with the standard PDF pattern, the diffraction peaks at 2h = 40.1°, 46.4°, and 67.9° can be assigned to the crystal face (1 1 1), (2 0 0), and (2 2 0) of metallic Pd0 (PDF #652867), indicating the formation of Pd NPs. The peak intensity decreases and the full-width at half-maximum (FWHM) of the peak at 2h = 40.1° increases when supports prepared with lower treatment temperature are used, indicating a decrease in Pd crystal size, which is in accordance with the change in particle size observed from TEM measurements (Fig. 2 below). XPS spectra reveal that the palladium exists mainly as metallic Pd0 (deconvoluted peak at BE = 335.3 eV), and only a small fraction of Pdd+ is found as PdO (deconvoluted peak at BE = 336.6 eV) [28], with an approximate Pd0/Pdd+ molar ratio of 7:1, Fig. 1B. Careful calculation indicates that the percentage of metallic Pd0 decreases slightly with increase of treatment temperature (see Table S1), which could be due to the slight increase in the particle size, as larger size particles have less exposed surface Pd atoms to be reduced during the H2 reduction process. As expected, TEM images show that the particle size of Pd NPs is influenced by the chemical nature of the supports and increases in the order of Pd/S-C_300 < Pd/S-C_400 < Pd/S-C_500, from 4.1 to 6.0 and to 8.2 nm, Fig. 2. This change can be ascribed to the change in the chemical nature of CNx on SBA-15. Thus, the S-C_300 with the
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T. Yuan et al. / Journal of Catalysis 326 (2015) 38–42
Fig. 1. (A) Wide angle XRD patterns and (B) XPS spectra for Pd/S-C_T (T = 300, 400, 500).
Fig. 2. TEM images of (A) Pd/S-C_300; (B) Pd/S-C_400; (C) Pd/S-C_500; and (D) the corresponding particle size for them (about 750 particles are counted for each sample).
Fig. 3. COD conversion and COE/COA selectivity obtained from (A) Pd/S-C_T at 80 °C and (B) Pd/S-C_300 at 50, 60 and 80 °C. Reaction conditions: 50 mg catalyst; 2 MPa H2 pressure; 0.25 mL COD; 30 mL n-heptane; 0.2 mL dodecane; reaction time, 90 min.
largest amount of lower condensed CNx provides most sites for metal adsorption and stabilization, resulting in the smallest Pd particle size, as was already observed for Pt and Au NPs [6]. It can be also seen that the ordered pores of SBA-15 are maintained in the Pd/S-C_T samples. The actual Pd loading is measured to be 0.94, 0.91, and 0.89 wt.%, respectively (see Table S1), which is slightly lower than the theoretical value (1 wt.%) from the preparation process. This proves again that the CNx has a high ability to adsorb Pd ions from solution. Based on the Pd loading and the particle size, the Pd dispersion is calculated to be 0.27, 0.19, and 0.14 for Pd/S-C_300, Pd/S-C_400, and Pd/S-C_500, respectively. Fig. 3A shows the COD conversion and selectivity to COE or COA measured over the catalysts at 80 °C and a H2 pressure of 2 MPa. The catalysts are active in COD hydrogenation, with 100% COD conversion within a reaction time of 90 min. Notably, the selectivity to
COE and COA depends intimately on the catalyst’s structure, with 94% selectivity to COA over Pd/S-C_300 and 89% selectivity to COE over Pd/S-C_500. That is, small Pd NPs lead to the formation of COA and large Pd NPs to COE. This can be explained by the higher reactivity of small Pd NPs in comparison to bigger ones, and thus, the consecutive hydrogenation reaction to COA occurs much faster. The selectivity however can be well controlled by changing the reaction temperature. For example, the selectivity of Pd/S-C_300 to COA is changed from 94% to 8% when the temperature is decreased from 80 to 50 °C, while the selectivity to COE changes from 4% to 90%, Fig. 3B, indicating that also the reaction temperature has a significant influence on the selectivity. On the other hand, for the given reaction conditions, full COD conversion is observed also for decreasing reaction temperature, that is, 50 °C.
T. Yuan et al. / Journal of Catalysis 326 (2015) 38–42
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Fig. 4. (A) The TOF of Pd/S-C_T for COD hydrogenation at atmospheric conditions; (B) stability tests of Pd/S-C_300 for COD hydrogenation at atmospheric conditions. Reaction conditions: 50 mg catalyst; atmospheric H2 pressure; 0.25 mL COD; 30 mL n-heptane; 0.2 mL dodecane.
Since a lower temperature can improve the selectivity of Pd/SC_300 to COE with still high COD conversion, in the following we attempt to conduct the reaction at less demanding conditions, that is, at room temperature and atmospheric H2 pressure. Fig. 4A shows that the catalysts are also active at these conditions (details see Fig. S2), with TOF value of above 0.5 s 1 in the first 10 min (note: The selectivity to COE is always above 99%, not shown here). The TOF decreases in order of Pd/S-C_300 > Pd/S-C_400 > Pd/SC_500, indicating that the catalyst with smaller Pd particle size is more active for the reaction, as expected. Further studies show that Pd/S-C_300 is also active for the reaction even at zero degrees (temperature controlled by ice–water mixture), with a TOF of 0.19 s 1 and a COE selectivity above 98% at 90 min (see Fig. S3B). A blank experiment (without Pd NPs, see Fig. S3A) indicates that the S-C_T support is inactive in the reaction, and no appreciable COD conversion is observed even at 90 min, suggesting that the reaction is mainly catalyzed by the Pd NPs. It should be further noted that when an as-made SBA-15, without any CNx coating, was used as support, Pd NPs cannot be easily deposited on the silica surface. Indeed, when exactly the same preparation protocol was used no Pd particles can be observed within the porous silica from TEM images. However, to elucidate the role of the CNx in the reaction, a hydroxyl functionalized SBA-15 was prepared on which small Pd NPs could be immobilized (4.9 nm, Pd/SBA-15). This catalyst shows comparable COD conversion and COE selectivity to Pd/S-C_300 (see Fig. S4) and therefore suggests that an additional effect of the CNx on the catalytic reaction can be excluded. However, the CNx enables the dispersion of very small Pd NPs on the silica surface, as seen above, and this has been also observed for Au and Pt NPs [6]. On the other hand, CNx also stabilizes the Pd NPs during the reaction, as verified from the change in the particle size by comparing with the Pd NPs on the hydroxyl functionalized SBA-15. Indeed, for the latter catalyst a significant increase in the Pd particle size, from 4.9 to 7.5 nm, was observed after the reaction (see Fig. S5), while only a slight increase, from 4.1 to 4.6 nm, was observed for Pd/S-C_300 (see Fig. S6). This is also supported by the following stability tests which were performed to study the reusability of Pd/S-C_300 for COD hydrogenation at atmospheric conditions. After reaction, the catalyst is filtered and dried at 100 °C for 6 h before the next run. Fig. 4B shows that no lessening of TOF is observed for the first cycle, and only a decrease of 15% in the TOF is observed after the fifth cycle; meanwhile, the selectivity to COE remains unchanged, with above 98% throughout the tests. This indicates that the catalyst is not only active and selective but also relatively stable to the reaction. In comparison, a decrease of 10% and 29% in the COD conversion is observed for Pd/SBA-15 in the first and the fifth run (see Fig. S4B), due to the significant aggregation of Pd NPs (from 4.9 to 7.5 nm). Characterization of the used catalysts by XRD, TEM, and
XPS (see Fig. S6) shows that the structure of the Pd/S-C_300 catalyst is slightly changed, as a decrease of Pd loading (8% Pd loss) and a minor increase in the average Pd NPs size (from 4.1 to 4.6 nm) are observed for an used sample. To test whether leached Pd NPs can catalyze the reaction by homogeneous catalysis, we performed an additional experiment by filtering the catalyst after running the reaction for 30 min, and then keeping the filtered solution reacting for another 30 min. No appreciable difference (less than 3%) in the COD conversion was observed before and after the filtration of catalyst, indicating that the small amount of leached Pd does not contribute to the reaction and the reaction is catalyzed by heterogeneous catalysis. To provide further information and understanding on the reaction, the effect of solvent, COD concentration, and Pd loading on the catalytic performance of Pd/S-C_300 for the reaction was also studied. The results are presented in Table S2, Fig. S7A and S7B, respectively, showing that (1) n-heptane is a more suitable solvent for the reaction than toluene, methanol, ethanol, and tetrahydrofuran; (2) the optimum capacity of Pd/S-C_300 for the COD hydrogenation is conducted at 0.5 mL (or 0.08 mol) COD per 50 mg catalyst. Further increase of COD amount leads to a decreased conversion, possibly as the Pd active site is mostly covered by COD, limiting the access of H2 to its surface; (3) the increase of Pd loading improves the COD hydrogenation reaction, as more Pd active sites are exposed on the surface and are accessible to the reactants. Finally, the applicability of Pd/S-C_300 for hydrogenation of other substrates, especially aromatic compounds, conducted at atmospheric pressure was also tested, showing that the catalyst is also active for the hydrogenation of –NO2, –Cl groups, and even the benzene ring at atmospheric conditions, see Table S3. To justify the good catalytic performances of Pd/S-C_300, the conversion of Pd/S-C_300 was compared with that of Pd/SiO2, prepared by laser vaporization deposition of bulk Pd metal [29], for o-chloronitrobenzene (o-CNB) hydrogenation. It was found that the former shows 99.9% conversion at 90 min, while the latter shows only 85.8% conversion even at 180 min with more severe reaction conditions (5 g o-CNB, 90 mg catalyst, 60 °C, and H2 pressure 1 MPa). 4. Conclusions In summary, we have shown that Pd NPs with controlled particle size can be synthesized by regulating the treatment temperature for S-C_T composites and are very active and stable for hydrogenation reactions, either at elevated pressure (50–80 °C, H2 pressure of 2 MPa) or at atmospheric conditions, and even at zero degrees. The catalyst is also active in the activation of –NO2, –Cl and even benzene at atmospheric conditions. The outstanding performance of Pd/S-C_300 for hydrogenation reactions observed
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in this work motivates a deeper investigation of S-C_T supports and is a highly promising starting point to prepare novel catalysts of high industrial relevance. Acknowledgments Finance support from the National Science Foundation of China (Nos. 21203253, 21203254), the Scientific Research Foundation for Returned Scholars, Ministry of Education of China (No. BZY13005), the Science and Technology Activities of Overseas Personnel Preferential Funding Project (No. BZY14038), the National Demonstration Center of Experimental Teaching on ethnical pharmacy, South-Central University for Nationalities, as well as the Cluster of Excellence ‘‘Unifying Concepts in Catalysis-UniCat’’ from the German Science Foundation (DFG), is gratefully appreciated. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2015.03.007. References [1] Y. Zhu, F. Zaera, Catal. Sci. Technol. 4 (2014) 955. [2] I. Lee, F. Delbecq, R. Morales, M.A. Albiter, F. Zaera, Nat. Mater. 8 (2009) 132. [3] A. Solhy, B.F. Machado, J. Beausoleil, Y. Kihn, F. Gonçalves, M.F.R. Pereira, J.J.M. Órfão, J.L. Figueiredo, J.L. Faria, P. Serp, Carbon 46 (2008) 1194. [4] A. Corma, Chem. Rev. 97 (1997) 2373.
[5] Y. Gong, P. Zhang, X. Xu, Y. Li, H. Li, Y. Wang, J. Catal. 297 (2013) 272. [6] P. Xiao, Y.X. zhao, T. Wang, Y.Y. Zhan, H.H. Wang, J.L. Li, A. Thomas, J.J. Zhu, Chem. Eur. J. 20 (2014) 2872. [7] Y. Zheng, L. Lin, X. Ye, F. Guo, X. Wang, Angew. Chem. Int. Ed. 53 (2014) 11926. [8] X. Zhang, H. Wang, H. Wang, Q. Zhang, J. Xie, Y. Tian, J. Wang, Y. Xie, Adv. Mater. 26 (2014) 4438. [9] M. Zhang, X. Wang, Energy Environ. Sci. 7 (2014) 1902. [10] H. Shi, G. Chen, C. Zhang, Z. Zou, ACS Catal. 4 (2014) 3637. [11] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76. [12] K. Kailasam, J.D. Epping, A. Thomas, S. Losse, H. Junge, Energy Environ. Sci. 4 (2011) 4668. [13] X. Tuaev, J.P. Paraknowitsch, R. Illgen, A. Thomas, P. Strasser, Phys. Chem. Chem. Phys. 14 (2012) 6444. [14] J. Zhu, P. Xiao, H. Li, S. Carabineiro, ACS Appl. Mater. Interfaces (2014). [15] Y. Nakagawa, M. Tamura, K. Tomishige, ACS Catal. 3 (2013) 2655. [16] F.A. Harraz, S.E. El-Hout, H.M. Killa, I.A. Ibrahim, J. Catal. 286 (2012) 184. [17] E.E. Gonzo, M. Boudart, J. Catal. 52 (1978) 462. [18] Y. Wang, J. Yao, H. Li, D. Su, M. Antonietti, J. Am. Chem. Soc. 133 (2011) 2362. [19] X.-H. Li, M. Antonietti, Chem. Soc. Rev. 42 (2013) 6593. [20] A. Schmidt, A. Wolf, R. Warsitz, R. Dittmeyer, D. Urbanczyk, I. Voigt, G. Fischer, R. Schomäcker, AICHE J. 54 (2008) 258. [21] A. Schmidt, R. Schomäcker, Ind. Eng. Chem. Res. 46 (2007) 1677. [22] G. Oenbrink, T. Schiffer, Cyclododecatriene, cyclooctadiene, and 4vinylcyclohexene, in: Ullmann’s Encyclopedia of Industrial Chemistry, WileyVCH Verlag GmbH & Co. KGaA, 2000. [23] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [24] Z. Chen, L. Zhou, F. Zhang, C. Yu, Z. Wei, Appl. Surf. Sci. 258 (2012) 5291. [25] S.T. Oyama, X. Zhang, J. Lu, Y. Gu, T. Fujitani, J. Catal. 257 (2008) 1. [26] X. Chen, J. Zhang, X. Fu, M. Antonietti, X. Wang, J. Am. Chem. Soc. 131 (2009) 11658. [27] Y.S. Jun, W.H. Hong, M. Antonietti, A. Thomas, Adv. Mater. 21 (2009) 4270. [28] S.P. Chenakin, G. Melaet, R. Szukiewicz, N. Kruse, J. Catal. 312 (2014) 1. [29] L. Jiang, H. Gu, X. Xu, X. Yan, J. Mol. Catal. A: Chem. 310 (2009) 144.
Contents lists available at ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Research Note
Controlling hydrogenation selectivity with Pd catalysts on carbon nitrides functionalized silica Tao Yuan a, Haifeng Gong a, Kamalakannan Kailasam b, Yanxi Zhao a, Arne Thomas b, Junjiang Zhu a,⇑ a b
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs & Commission Ministry of Education, South-Central University for Nationalities, Wuhan 430074, China Department of Chemistry, Technische Universität Berlin, Berlin 10623, Germany
a r t i c l e
i n f o
Article history: Received 7 December 2014 Revised 13 March 2015 Accepted 14 March 2015
Keywords: Palladium Carbon nitrides SBA-15 1,5-Cyclooctadiene Selective hydrogenation
a b s t r a c t We report here the synthesis of Pd nanoparticles (NPs) supported on a carbon nitrides–silica composite (S-C_T) and its application as a catalyst in the hydrogenation of 1,5-cyclooctadiene. While the mesoporous silica provides a high surface area, the carbon nitrides (CNx) stabilize the Pd NPs, and the Pd particle size can be controlled by varying the treatment temperature (T) for the composite. The Pd catalysts show good conversion and selectivity for hydrogenation both at elevated pressure (50–80 °C, H2 pressure 2 MPa) and at atmospheric pressure, even at zero degrees, and the selectivity can be well controlled owing to the controlled particle size of the Pd NPs. Furthermore, after reusing the catalyst 6 times, no loss in selectivity is observed while the conversion is slightly decreased to about 15%. Thus, an active Pd catalyst with prolonged lifetime for hydrogenation is observed when carbon nitrides-silica composites are used as support. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction The need for sustainable and green chemical processes in the 21st century changes the criterion in the design of catalytic process from productivity to selectivity [1]. How to improve selectivity while keeping high conversion is a crucial issue in creating atom and energy efficient processes. Many attempts have been proposed to control the reaction selectivity by controlling the chemistry and structure of solid catalysts [2,3]. For instance, the control of pore size of porous catalysts can exclude large-size molecules from approaching the active site inside the pore, while small-size molecules can enter and react [4]. Also the control of particle size, particle shape, or exposure facets of metal nanoparticle (NP) catalysts can lead to varied reactivity to substrates and yield different products [5]. To control the particle size of catalytically active metal NPs, we recently reported a facile synthesis route introducing the coating of a layer of polymeric carbon nitrides (CNx) on a conventional silica support (e.g., SBA-15) before the metals are deposited [6]. The polymeric carbon nitride layer consists of amine bridged heptazine units and is easily prepared by heat treatment of nitrogen-rich precursors, such as cyanamide or melamine. This organic semiconductor has recently received enormous interest as a metal-free photocatalyst [7–12]. Because of the nitrogen-rich structure, CNx ⇑ Corresponding author. E-mail address: [email protected] (J. Zhu). http://dx.doi.org/10.1016/j.jcat.2015.03.007 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.
provides many adsorption sites to graft metal ions on its surface [13,14], while its polymeric character allows it to control the chemical structure of the support surface, and thus, the nature and number of adsorption sites of CNx by simply changing the treatment temperature. Indeed C/N values in polymeric carbon nitrides largely increase with temperature as the condensation of the triamino-heptazine units proceeds, and it was shown that the particle size of supported Au and Pt metals can be controlled by changing the treatment temperature for the carbon nitrides–silica (S-C_T) composites [6]. This offers a facile strategy to control the reaction selectivity by controlling the particle size. Hydrogenations are important organic reactions and are crucial for the synthesis of chemical intermediates in fine chemistry [15–17]. Wang et al. reported that Pd NPs@mpg-C3N4 (mesoporous carbon nitride) exhibits a high conversion for the direct hydrogenation of phenol to cyclohexanone and found a much higher catalytic performance than that for a comparable Pd NPs@carbon catalyst [18]. This significant enhancement was explained by electronic metal–semiconductor interactions, that is, formation of Mott–Schottky heterojunctions between the metal (Pd) and the semiconductor support (CNx) [19]. The selective hydrogenation of 1,5-cyclooctadiene (COD) to cyclooctene (COE) is a sustainable, green and important reaction [20,21], as the latter is an industrial intermediate product for manufacturing special polymers, for example, polyoctenamers, which are modifiers in rubbers and thermoplastics [22]. In addition, COD hydrogenation is also a good model reaction for estimating the selectivity of a catalyst as the
T. Yuan et al. / Journal of Catalysis 326 (2015) 38–42
two C@C bonds in the structure are similar in electronic environment. Indeed, the consecutive hydrogenation of COE to cyclooctane (COA) is a crucial problem for the reported catalysts (e.g., Pd/C, Pd/Al2O3), decreasing the selectivity to the monoolefine [20]. The search for highly active, selective, and reusable hydrogenation catalysts is currently a highly challenging and attractive topic. In this work, we present the preparation of Pd catalysts supported on S-C_T composites and their use for selective hydrogenation of COD to COE or COA. The results indicate that the particle size of Pd NPs depends intimately on the treatment temperature of the composite, and the resulting catalysts are very active and selective for COD hydrogenation. In particular, the selectivity to COE or COA can be well controlled by using catalysts with different Pd particle sizes or by changing the reaction temperature, while high COD conversion is even observed at low temperatures. At atmospheric conditions, the catalyst (Pd/S-C_300) shows 95% COD conversion and 99% COE selectivity after 90 min. Reusability tests demonstrate that the catalyst is keeping the high selectivity for at least 6 times, with COE selectivity >98% throughout the reaction, even though a decrease of 15% in the conversion is observed. 2. Experimental 2.1. Synthesis of S-C_T supports SBA-15 was synthesized using the general procedure reported by Zhao et al. [23]. The obtained SBA-15 was then impregnated with dicyandiamide in a water–ethanol mixed solution at 80 °C. After the solvents (water and ethanol) were removed, the resulting white solid was dried at 100 °C overnight and then thermally treated in N2 at 300, 400, and 500 °C, respectively, for 4 h at a heating rate of 2 °C/min. Based on the treatment temperature, the products are denoted S-C_T (T = 300, 400, 500). 2.2. Synthesis of Pd/S-C_T catalyst The catalysts were prepared by an impregnation method. 3.8 mL 0.01236 M pre-prepared PdCl2 solution (see the supplementary information, SI) was first diluted to 50 mL, to which 0.5 g of the above obtained S-C_T support was added. The resulting solution was then ultrasonicated for 10 min, followed by stirring for 4 h at room temperature. Thereafter, the sample was filtered, washed with distilled water and dried in a vacuum oven at 60 °C overnight, and finally reduced in H2 atmosphere at 300 °C for 3 h. Based on the S-C_T support, the catalyst was accordingly named as Pd/S-C_T. For comparison, Pd catalyst supported on SBA-15 functionalized with hydroxyl groups [24], Pd/SBA-15, was also prepared, applying the same procedure as for Pd/S-C_T. 2.3. Characterizations X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance X-ray diffractometer with Cu Ka (k = 1.5406 Å) irradiation. Transmission electron microscopy (TEM) images were obtained on a Tecnai G2 20 S-Twin apparatus with high-resolution transmission electron microscope (200 kV). N2 physisorption isotherms were measured on a TriStar II 3020 measurement at liquid nitrogen temperature ( 195.8 °C). X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Thermo Electron Corporation VG Multilab 2000 apparatus using a monochromatic Al Ka X-ray source (300 W) and analyzer pass energy of 25 eV. Binding energies were obtained by referencing to the C 1s binding energy of carbon taken at 284.6 eV. Ultraviolet–visible (UV–vis) DRS were collected on a Shimadzu UV-2550 spectrophotometer from 200 to 800 nm using BaSO4 as background.
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2.4. Catalytic tests 2.4.1. At elevated pressure The setup consists of a heating magnetic stirrer and a 50 mL titanium high pressure reactor (type: WCGF-50). 50 mg catalyst, 0.25 mL COD, 0.2 mL dodecane, and 30 mL n-heptane were charged in the reactor. The pressure was adjusted to 2 MPa, and the reactor heated to 50, 60, or 80 °C. After the reaction, the reaction mixture was centrifuged, filtered, and analyzed by an Agilent 7890 GC equipped with a FID detector and HP-5 column. 2.4.2. At atmospheric conditions The setup consists of a magnetic stirrer and a 50 mL threenecked flask, which is connected with a hydrogen balloon. 50 mg catalyst, 0.25 mL COD, 0.2 mL dodecane, and 30 mL n-heptane were added to the flask. The reaction was stirred (1000 rpm), and the reaction mixture was sampled at desired times for analysis. The catalytic performance was evaluated by two means: One is the COD conversion which is calculated from the ratio of reacted COD to initial COD, and the other is the turnover frequency (TOF) which is calculated by the moles of COD reacted per moles of surface metal used per second. To certify the lack of the mass diffusion limitations of COD in the reaction, the Weisz–Prater criterion, according to the method proposed in literature [25], was calculated, which equals to 7.05 10 4 and is far below 1, indicating that the mass diffusion limitation can be excluded at the present reaction conditions. Detailed procedures and parameters used in the synthesis, characterizations, and catalytic tests can be found in the experimental section of the SI. 3. Results and discussion The coating of CNx on SBA-15 was proven using small- and wide-angle XRD measurements. TEM images and N2 physisorption isotherms show also that the ordered structure of SBA-15 is not destroyed in the preparation process (see Fig. S1), in accordance with previous results [26,27]. TGA profiles (see Fig. S1D) show that the loading of CNx decreases with the increase of treatment temperature for S-C_T, implying that the composites treated at lower temperature (e.g., 300 °C) hold higher amounts of uncondensed – NH2 groups, which may be the adsorption site of metal NPs [13,14]. Fig. 1A shows the wide-angle XRD patterns of the supported Pd catalysts. By comparing with the standard PDF pattern, the diffraction peaks at 2h = 40.1°, 46.4°, and 67.9° can be assigned to the crystal face (1 1 1), (2 0 0), and (2 2 0) of metallic Pd0 (PDF #652867), indicating the formation of Pd NPs. The peak intensity decreases and the full-width at half-maximum (FWHM) of the peak at 2h = 40.1° increases when supports prepared with lower treatment temperature are used, indicating a decrease in Pd crystal size, which is in accordance with the change in particle size observed from TEM measurements (Fig. 2 below). XPS spectra reveal that the palladium exists mainly as metallic Pd0 (deconvoluted peak at BE = 335.3 eV), and only a small fraction of Pdd+ is found as PdO (deconvoluted peak at BE = 336.6 eV) [28], with an approximate Pd0/Pdd+ molar ratio of 7:1, Fig. 1B. Careful calculation indicates that the percentage of metallic Pd0 decreases slightly with increase of treatment temperature (see Table S1), which could be due to the slight increase in the particle size, as larger size particles have less exposed surface Pd atoms to be reduced during the H2 reduction process. As expected, TEM images show that the particle size of Pd NPs is influenced by the chemical nature of the supports and increases in the order of Pd/S-C_300 < Pd/S-C_400 < Pd/S-C_500, from 4.1 to 6.0 and to 8.2 nm, Fig. 2. This change can be ascribed to the change in the chemical nature of CNx on SBA-15. Thus, the S-C_300 with the
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Fig. 1. (A) Wide angle XRD patterns and (B) XPS spectra for Pd/S-C_T (T = 300, 400, 500).
Fig. 2. TEM images of (A) Pd/S-C_300; (B) Pd/S-C_400; (C) Pd/S-C_500; and (D) the corresponding particle size for them (about 750 particles are counted for each sample).
Fig. 3. COD conversion and COE/COA selectivity obtained from (A) Pd/S-C_T at 80 °C and (B) Pd/S-C_300 at 50, 60 and 80 °C. Reaction conditions: 50 mg catalyst; 2 MPa H2 pressure; 0.25 mL COD; 30 mL n-heptane; 0.2 mL dodecane; reaction time, 90 min.
largest amount of lower condensed CNx provides most sites for metal adsorption and stabilization, resulting in the smallest Pd particle size, as was already observed for Pt and Au NPs [6]. It can be also seen that the ordered pores of SBA-15 are maintained in the Pd/S-C_T samples. The actual Pd loading is measured to be 0.94, 0.91, and 0.89 wt.%, respectively (see Table S1), which is slightly lower than the theoretical value (1 wt.%) from the preparation process. This proves again that the CNx has a high ability to adsorb Pd ions from solution. Based on the Pd loading and the particle size, the Pd dispersion is calculated to be 0.27, 0.19, and 0.14 for Pd/S-C_300, Pd/S-C_400, and Pd/S-C_500, respectively. Fig. 3A shows the COD conversion and selectivity to COE or COA measured over the catalysts at 80 °C and a H2 pressure of 2 MPa. The catalysts are active in COD hydrogenation, with 100% COD conversion within a reaction time of 90 min. Notably, the selectivity to
COE and COA depends intimately on the catalyst’s structure, with 94% selectivity to COA over Pd/S-C_300 and 89% selectivity to COE over Pd/S-C_500. That is, small Pd NPs lead to the formation of COA and large Pd NPs to COE. This can be explained by the higher reactivity of small Pd NPs in comparison to bigger ones, and thus, the consecutive hydrogenation reaction to COA occurs much faster. The selectivity however can be well controlled by changing the reaction temperature. For example, the selectivity of Pd/S-C_300 to COA is changed from 94% to 8% when the temperature is decreased from 80 to 50 °C, while the selectivity to COE changes from 4% to 90%, Fig. 3B, indicating that also the reaction temperature has a significant influence on the selectivity. On the other hand, for the given reaction conditions, full COD conversion is observed also for decreasing reaction temperature, that is, 50 °C.
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Fig. 4. (A) The TOF of Pd/S-C_T for COD hydrogenation at atmospheric conditions; (B) stability tests of Pd/S-C_300 for COD hydrogenation at atmospheric conditions. Reaction conditions: 50 mg catalyst; atmospheric H2 pressure; 0.25 mL COD; 30 mL n-heptane; 0.2 mL dodecane.
Since a lower temperature can improve the selectivity of Pd/SC_300 to COE with still high COD conversion, in the following we attempt to conduct the reaction at less demanding conditions, that is, at room temperature and atmospheric H2 pressure. Fig. 4A shows that the catalysts are also active at these conditions (details see Fig. S2), with TOF value of above 0.5 s 1 in the first 10 min (note: The selectivity to COE is always above 99%, not shown here). The TOF decreases in order of Pd/S-C_300 > Pd/S-C_400 > Pd/SC_500, indicating that the catalyst with smaller Pd particle size is more active for the reaction, as expected. Further studies show that Pd/S-C_300 is also active for the reaction even at zero degrees (temperature controlled by ice–water mixture), with a TOF of 0.19 s 1 and a COE selectivity above 98% at 90 min (see Fig. S3B). A blank experiment (without Pd NPs, see Fig. S3A) indicates that the S-C_T support is inactive in the reaction, and no appreciable COD conversion is observed even at 90 min, suggesting that the reaction is mainly catalyzed by the Pd NPs. It should be further noted that when an as-made SBA-15, without any CNx coating, was used as support, Pd NPs cannot be easily deposited on the silica surface. Indeed, when exactly the same preparation protocol was used no Pd particles can be observed within the porous silica from TEM images. However, to elucidate the role of the CNx in the reaction, a hydroxyl functionalized SBA-15 was prepared on which small Pd NPs could be immobilized (4.9 nm, Pd/SBA-15). This catalyst shows comparable COD conversion and COE selectivity to Pd/S-C_300 (see Fig. S4) and therefore suggests that an additional effect of the CNx on the catalytic reaction can be excluded. However, the CNx enables the dispersion of very small Pd NPs on the silica surface, as seen above, and this has been also observed for Au and Pt NPs [6]. On the other hand, CNx also stabilizes the Pd NPs during the reaction, as verified from the change in the particle size by comparing with the Pd NPs on the hydroxyl functionalized SBA-15. Indeed, for the latter catalyst a significant increase in the Pd particle size, from 4.9 to 7.5 nm, was observed after the reaction (see Fig. S5), while only a slight increase, from 4.1 to 4.6 nm, was observed for Pd/S-C_300 (see Fig. S6). This is also supported by the following stability tests which were performed to study the reusability of Pd/S-C_300 for COD hydrogenation at atmospheric conditions. After reaction, the catalyst is filtered and dried at 100 °C for 6 h before the next run. Fig. 4B shows that no lessening of TOF is observed for the first cycle, and only a decrease of 15% in the TOF is observed after the fifth cycle; meanwhile, the selectivity to COE remains unchanged, with above 98% throughout the tests. This indicates that the catalyst is not only active and selective but also relatively stable to the reaction. In comparison, a decrease of 10% and 29% in the COD conversion is observed for Pd/SBA-15 in the first and the fifth run (see Fig. S4B), due to the significant aggregation of Pd NPs (from 4.9 to 7.5 nm). Characterization of the used catalysts by XRD, TEM, and
XPS (see Fig. S6) shows that the structure of the Pd/S-C_300 catalyst is slightly changed, as a decrease of Pd loading (8% Pd loss) and a minor increase in the average Pd NPs size (from 4.1 to 4.6 nm) are observed for an used sample. To test whether leached Pd NPs can catalyze the reaction by homogeneous catalysis, we performed an additional experiment by filtering the catalyst after running the reaction for 30 min, and then keeping the filtered solution reacting for another 30 min. No appreciable difference (less than 3%) in the COD conversion was observed before and after the filtration of catalyst, indicating that the small amount of leached Pd does not contribute to the reaction and the reaction is catalyzed by heterogeneous catalysis. To provide further information and understanding on the reaction, the effect of solvent, COD concentration, and Pd loading on the catalytic performance of Pd/S-C_300 for the reaction was also studied. The results are presented in Table S2, Fig. S7A and S7B, respectively, showing that (1) n-heptane is a more suitable solvent for the reaction than toluene, methanol, ethanol, and tetrahydrofuran; (2) the optimum capacity of Pd/S-C_300 for the COD hydrogenation is conducted at 0.5 mL (or 0.08 mol) COD per 50 mg catalyst. Further increase of COD amount leads to a decreased conversion, possibly as the Pd active site is mostly covered by COD, limiting the access of H2 to its surface; (3) the increase of Pd loading improves the COD hydrogenation reaction, as more Pd active sites are exposed on the surface and are accessible to the reactants. Finally, the applicability of Pd/S-C_300 for hydrogenation of other substrates, especially aromatic compounds, conducted at atmospheric pressure was also tested, showing that the catalyst is also active for the hydrogenation of –NO2, –Cl groups, and even the benzene ring at atmospheric conditions, see Table S3. To justify the good catalytic performances of Pd/S-C_300, the conversion of Pd/S-C_300 was compared with that of Pd/SiO2, prepared by laser vaporization deposition of bulk Pd metal [29], for o-chloronitrobenzene (o-CNB) hydrogenation. It was found that the former shows 99.9% conversion at 90 min, while the latter shows only 85.8% conversion even at 180 min with more severe reaction conditions (5 g o-CNB, 90 mg catalyst, 60 °C, and H2 pressure 1 MPa). 4. Conclusions In summary, we have shown that Pd NPs with controlled particle size can be synthesized by regulating the treatment temperature for S-C_T composites and are very active and stable for hydrogenation reactions, either at elevated pressure (50–80 °C, H2 pressure of 2 MPa) or at atmospheric conditions, and even at zero degrees. The catalyst is also active in the activation of –NO2, –Cl and even benzene at atmospheric conditions. The outstanding performance of Pd/S-C_300 for hydrogenation reactions observed
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in this work motivates a deeper investigation of S-C_T supports and is a highly promising starting point to prepare novel catalysts of high industrial relevance. Acknowledgments Finance support from the National Science Foundation of China (Nos. 21203253, 21203254), the Scientific Research Foundation for Returned Scholars, Ministry of Education of China (No. BZY13005), the Science and Technology Activities of Overseas Personnel Preferential Funding Project (No. BZY14038), the National Demonstration Center of Experimental Teaching on ethnical pharmacy, South-Central University for Nationalities, as well as the Cluster of Excellence ‘‘Unifying Concepts in Catalysis-UniCat’’ from the German Science Foundation (DFG), is gratefully appreciated. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2015.03.007. References [1] Y. Zhu, F. Zaera, Catal. Sci. Technol. 4 (2014) 955. [2] I. Lee, F. Delbecq, R. Morales, M.A. Albiter, F. Zaera, Nat. Mater. 8 (2009) 132. [3] A. Solhy, B.F. Machado, J. Beausoleil, Y. Kihn, F. Gonçalves, M.F.R. Pereira, J.J.M. Órfão, J.L. Figueiredo, J.L. Faria, P. Serp, Carbon 46 (2008) 1194. [4] A. Corma, Chem. Rev. 97 (1997) 2373.
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