- Email: [email protected]
functionalized CMK-3 catalysts with superior hydrodesulfurization performance
Catalysis Communications 93 (2017) 25–28
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
A strategy for preparing highly dispersed Ni2P/functionalized CMK-3 catalysts with superior hydrodesulfurization performance Tingting Huang, Wenjin Shi, Jundong Xu, Yu Fan ⁎ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, PR China
a r t i c l e
i n f o
Article history: Received 18 November 2016 Received in revised form 11 January 2017 Accepted 17 January 2017 Available online 18 January 2017 Keywords: Ni2P Dispersion 4,6-Dimethyldibenzothiophene Hydrodesulfurization catalyst
a b s t r a c t A strategy is introduced for producing highly dispersed Ni2P/functionalized CMK-3 hydrodesulfurization catalysts via the anchoring effect of high-temperature stable groups activated by ammonia. In the proposed method, HNO3 acts as an oxidant that enables original CMK-3 to create more lactones and phenolic hydroxyls than acidified ammonium persulphate; these groups are activated with negative charges via the NH3 effect, and thereby [Ni(NH3)6]2+ ions can be anchored to the electronegative surface of functionalized CMK-3 via electrostatic adsorption. As a result, NiO and phosphatized Ni2P nanoparticles are highly dispersed on the HNO3-oxidized CMK-3 support, endowing the catalyst with excellent activity for 4,6-dimethyldibenzothiophene hydrodesulfurization. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Nickel phosphides have been much investigated because of their promising hydrogenation applications in petrochemical industries [1, 2]. A variety of methods have been developed for the synthesis of nickel phosphides, including the direct reaction of nickel salts and ammonium phosphate [3], the thermal decomposition of a single-source precursor (nickel hypophosphite) [4], the phosphidation of metals or metal oxides with PH3 [5]. However, these methods are restricted by extremely harsh conditions or high toxicity of PH3. In addition, these methods are difficult to prepare highly dispersed metal phosphides [3,4,6]. CMK-3 exhibits better thermal and chemical stability than conventional ordered mesoporous carbons (OMCs), which has the potential for broader applications [7–12]. Moreover, it is known that carbon surfaces can be made more hydrophilic by creating oxygen-containing groups (such as carboxyl, carbonyl, and phenol) on them using liquid or gas techniques [13], thus favoring the dispersion of supported metal species [14]. Herein, an appropriate oxidant was chosen to create a high concentration of functional groups with high-temperature resistance on CMK-3, thus enhancing the stable dispersion of active metal phases at high temperatures. These functional groups were activated with negative charges via the incorporation of ammonia, promoting the interaction between [Ni(NH 3 )6 ] 2 + ions and the negatively charged CMK-3 support. As a consequence, NiO particles and subsequent Ni2P particles were highly dispersed on CMK-3 due to the anchoring effect of high-temperature functional groups during the ⁎ Corresponding author. E-mail address: [email protected] (Y. Fan).
http://dx.doi.org/10.1016/j.catcom.2017.01.024 1566-7367/© 2017 Elsevier B.V. All rights reserved.
calcination and phosphorization processes. The novel as-prepared Ni2P/functionalized CMK-3 catalyst possessed excellent activity for 4,6-DMDBT hydrodesulfurization (HDS). 2. Experimental All experimental details are displayed in Supplementary information. 3. Results and discussion 3.1. XRD characterization In the wide-angle X-ray diffraction (WAXRD) patterns of NiO/C, NiO/ CN, and NiO/CS (Fig. S2), the broad diffraction peak at approximately 2θ = 25° is assigned to the carbon support [15]. A series of diffraction peaks at 2θ = 37.2°, 43.3°, 62.9°, and 75.4° are assigned to NiO (JCPDS, PDF no. 00-044-1159). The low NiO diffraction peaks of NiO/C are attributed to the lower loading of NiO (only 2.9 wt% determined by ICP). The two peaks at 2θ = 44.5° and 51.8° in the patterns of NiO/CN and NiO/CS are attributed to the metallic nickel phase (JCPDS, PDF no. 00-0040850). The mild reducibility of the carbon support converts a part of NiO into metallic Ni, and the formation of Ni can shorten the following reduction time at high temperature and improve the dispersion of active Ni2P phases [16,17]. NiO/CN has much weaker peaks at 37.2° and 43.3° than NiO/CS, indicating smaller NiO particles on NiO/CN than on NiO/CS in view of the same loading amount of NiO. The WAXRD patterns of the four supported Ni2P catalysts are shown in Fig. 1. The diffraction peaks at 2θ = 40.7°, 44.6°, 47.4°, and 54.2° are
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T. Huang et al. / Catalysis Communications 93 (2017) 25–28
Fig. 1. Wide angle XRD patterns of Ni2P/CN, Ni2P/CS, Ni2P/C and Ni2P/S catalysts.
indexed to Ni2P (JCPDS, PDF no. 01-089-4864). Ni2P/CS possesses the sharpest Ni2P diffraction peaks among the four catalysts, indicating that it has the largest Ni2P particles. Ni2P/S has relatively stronger Ni2P diffraction peaks compared to Ni2P/CN and Ni2P/C because the calcination and reduction at high temperature results in the generation of larger Ni2P particles. Compared with Ni2P/CS and Ni2P/S, Ni2P/CN and Ni2P/ C have much lower diffraction peaks of Ni2P. The low Ni2P diffraction peaks of Ni2P/C are attributed to the lower loading of Ni2P (only 3.0 wt%), while the low Ni2P diffraction peaks of Ni2P/CN with the loading amount of Ni2P at 13.5 wt% are due to smaller Ni2P particles. In the small-angle X-ray diffraction (SAXRD) patterns of SBA-15, CMK-3, and supported Ni2P catalysts (Fig. S3), three peaks of CMK-3 and SBA-15 at 0.5–5° are assigned to the (100), (110), and (200) diffractions of a 2D hexagonal (P6mm) structure, indicating long-range periodic order with hexagonal symmetry [18]. The four catalysts have a diffraction peak at approximately 2θ = 1.07°, attributed to the (100) diffraction of a 2D hexagonal (P6mm) structure, indicating their highly ordered pore structures.
Fig. 2. Comparison of CO2 desorption (a) and CO desorption (b) spectra of CMK-3-S and CMK-3-N.
3.4. XPS characterization 3.2. Boehm's titration The quantities of oxygenated surface groups on CMK-3, CMK-3-N, and CMK-3-S were measured by Boehm's titration (Table S1) [19]. The total acidities on CMK-3-N and CMK-3-S are much higher than on unfunctionalized CMK-3, with increases from 0.35 mmol/g in the original CMK-3 to 1.64 mmol/g in CMK-3-N and 1.15 mmol/g in CMK-3-S. Moreover, the amounts of carboxyl groups, lactonic groups, and phenolic groups on CMK-3-N and CMK-3-S increase evidently, compared to the original CMK-3. CMK-3-N has more carboxyl groups, lactonic groups, and phenol groups than CMK-3-S, indicating the advantage of HNO3 treatment in enhancing the oxygenated surface groups of CMK-3.
3.3. TPD characterization TPD experiments were carried out on functionalized CMK-3-N and CMK-3-S, and the CO2 and CO desorption curves are shown in Fig. 2. In the spectra of CO2 desorption, the peaks at approximately 300, 490, and 650 °C are assigned to unstable carboxyl groups, stable anhydrides, and stable lactones, respectively. In the spectra of CO desorption, the peak at approximately 800 °C is attributed to high-temperaturestabilized phenol groups [20]. Compared with CMK-3-S, CMK-3-N presents a higher peak intensity and larger area of CO2 and CO desorption, indicating that the HNO3 treatment produces more oxygenated surface groups than the acidified APS treatment, especially for lactonic and phenolic groups with high-temperature resistance.
Besides, the surface functional groups on the CMK-3, CMK-3-N, and CMK-3-S surfaces also studied by XPS survey spectra (Fig. S4). Binding energies in the range of 295–280 eV and 540–528 eV are ascribed to C1s and O1s spectra [21], respectively. CMK-3-N and CMK-3-S have a much higher peak intensity of O1s than CMK-3, and their O/C atomic ratios increase from 0.0207 for CMK-3 to 0.0839 for CMK-3-N and 0.0704 for CMK-3-S (Fig. S4a), further indicating that the HNO3 treatment introduces more oxygen-containing groups on the CMK-3 surface than the acidized APS treatment. According to the high-resolution scans and corresponding fitting curves of the O1s spectra (Figs. S4b–d), the relative concentrations of different functional groups were derived (Table 1) [21]. The oxygen-containing groups on the original CMK-3 surface are mainly quinone and lactone groups without carboxyl groups. And the relative concentrations of carboxyl groups (Peak IV) on CMK-3-N and CMK-3-S are 11.73% and 8.39%, respectively, demonstrating that the oxidation process with HNO3 or acidified APS
Table 1 Relative concentrations of functional groups in O1s and the O/C ratios from XPS spectra. Sample
O1s-relative atomic concentration (%)
O/C ratio
Peak I
Peak II
Peak III
Peak IV
CMK-3 CMK-3-N CMK-3-S
61.80 47.83 54.06
15.84 20.24 16.66
22.36 20.20 15.64
– 11.73 8.39
0.0207 0.0839 0.0704
T. Huang et al. / Catalysis Communications 93 (2017) 25–28
27
Fig. 3. HRTEM image (a) and typical particle size distribution (b) of Ni2P/CN catalyst.
introduces carboxyl groups onto the original CMK-3. In addition, CMK3-N has higher atomic concentrations of Peaks II and III than CMK-3-S, indicating that the HNO3 treatment produces more phenol and lactone groups with high-temperature resistance on the original CMK-3 surface. 3.5. TEM characterization To observe the structure of NiO particles more clearly, HAADF-STEM (Z contrast) measurements were performed on NiO/CN and NiO/CS (Fig. S5). The NiO particles supported on NiO/CN are highly dispersed and evenly distributed with a size of approximately 2.2 nm (Fig. S5a), while the NiO particles supported on NiO/CS are aggregated, some particles are larger than 20 nm (Fig. S5b). The above result demonstrates that when ammonia is used to regulate pH, CMK-3-N disperses NiO particles much better than CMK-3-S. This is because the concentration of functional surface groups on oxidized CMK-3 influences the dispersion of supported NiO. On the surface of the oxidized CMK-3, carboxyl groups with strong acidity interact with NiO precursors during impregnation, guaranteeing a high dispersion of NiO precursors on the support. However, the thermal stability of carboxyl groups is inferior, leading to their decomposition when calcined at 400 °C. As a consequence, the NiO particles immobilized by the carboxyl groups are released. Then, these free NiO particles are aggregated with larger particles during calcination. Therefore, high-temperature stable groups on functionalized CMK-3 play an important role in promoting the high dispersion of NiO. It is known that lactones and phenolic hydroxyls are wellpreserved up to 600–800 °C, due to their high-temperature stability [20]. These groups can serve as metal anchoring sites via their interaction with NiO particles during calcination. Thus, the supported NiO particles can be highly dispersed after calcination. According to the results of Boehm's titration, TPD, and XPS characterizations, HNO3 oxidation introduces more groups with high-temperature resistance groups on the original CMK-3 than acidified APS, endowing NiO/CN with smaller NiO particles and better NiO dispersion than NiO/CS. Representative HRTEM micrographs and the corresponding particle size histograms of supported Ni2P are shown in Fig. 3. The magnified
HRTEM images (Figs. S6e and f) of selected frames from Figs. 3a and S6a show d-spacing values of 0.206 and 0.194 nm for the (201) and (210) crystallographic planes of Ni2P, confirming the existence of Ni2P crystallites in the catalysts. The EDS results show that the measured P/ Ni ratios on Ni2P/CN, Ni2P/S and Ni2P/CS are 0.54, 0.66, and 0.55, respectively (Fig. S7), which are close to the ratios of 0.52, 0.62, and 0.54 determined by the ICP measurement (Table S2). The Ni2P particles supported on Ni2P/CN are highly dispersed, with an average particle size of 4.8 nm (Fig. 3b). Ni2P/S possesses an average Ni2P particle size of 8.1 nm (Fig. S6d), which is larger than that of Ni2P/CN. According to the concentration of metal sites, the calculated particle sizes of Ni2P/CN and Ni2P/S are 4.6 and 7.2 nm, which are very close to the particle sizes determined by the TEM analysis (Table S4). Ni2P/S was synthesized using an impregnation and TPR method, in which the reduction and calcination at high temperatures lead to the generation of large Ni2P particles with a low dispersion. The Ni2P particles supported on Ni2P/CS are severely aggregated (Fig. S6c), with an average particle size of 15.6 nm (Table S4). Compared with the CMK-3-N support of Ni2P/CN, the CMK-3-S support of Ni2P/CS has a much lower concentration of thermally stable groups (Table S1), leading to a larger size and lower dispersion of Ni2P particles on Ni2P/CS than on Ni2P/CN. Although Ni2P/C has a smaller particle size of 3.8 nm with a higher dispersion than in Ni2P/CN (Fig. S6b), its low Ni2P-loading limits its applications in ultra-deep HDS of diesel. The highly dispersed Ni2P/CN catalyst is obtained by the electrostatic adsorption (EA) method. In this method, the surface groups on functionalized CMK-3 are deprotonated by the effect of NH3, making these groups electronegative. Thus, [Ni(NH3)6]2+ ions can be anchored on the electronegative surface of functionalized CMK-3 via electrostatic adsorption. During the Ni2P/C preparation, the ammonia used to control the solution pH value is eliminated, resulting in an acidic environment (pH = 5.8); this acidic environment restrains the deprotonation of oxygen-containing groups on CMK-3-N, and therefore, the numbers of Ni species and following Ni2P on CMK-3-N decrease remarkably. To further investigate the effect of ammonia on metal loading, the pH values of the impregnating solution were correlated with the loading of Ni2P (Fig. S8). The amount of Ni2P supported on CMK-3-N increases with
Table 2 4,6-DMDBT HDS results on the supported Ni2P catalysts. Catalyst
Ni2P/CN Ni2P/S Ni2P/CS a b c
CO uptakea (μmol g−1)
142 125 90
kHDSb (10−7 mol g−1 s−1)
TOFc (10−3 s−1)
2.39 1.76 1.08
1.11 0.99 0.83
Product selectivityb (%)
Product ratiob
TH + HH
MCHT
DMBCH
DMBP
MCHT/DMBP
25.26 39.06 7.45
46.32 43.75 59.26
23.16 10.94 8.60
5.26 6.25 24.69
8.81 7.00 2.40
Stoichiometry of CO/active site is assumed to be one for the tested samples. Determined at 50% of the total 4,6-DMDBT conversion by changing liquid hourly space velocity. TH, 4,6-THDMDBT; HH, 4,6-HHDMDBT. Obtained at 3.0 MPa, 340 °C, with the total conversion of 20% by changing liquid hourly space velocity.
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T. Huang et al. / Catalysis Communications 93 (2017) 25–28
the increase in pH value, indicating the promoting effect of ammonia on Ni2P loading. At the initial pH value of 5.8 without ammonia, the loading of Ni2P on CMK-3-N is only 3.0 wt% because this pH value is close to the point of zero charge (PZC) of oxidized CMK-3 (2.3). However, the loading of Ni2P with the pH value of 12.0 is as high as 13.5 wt%. At a high pH value caused by alkalinity, the oxygen-containing groups on functionalized CMK-3 become deprotonated and negatively charged, greatly improving the loading of nickel ammine and subsequently, Ni2P on oxidized CMK-3. 3.6. Catalytic activity The 4,6-DMDBT HDS results of the four catalysts showed that the HDS ratio decreases in the order Ni2P/CN N Ni2P/S N Ni2P/CS N Ni2P/C, at a high velocity (13.0 h− 1), as well as at a low velocity (7.0 h−1) (Table S5). The reaction rate constant kHDS and TOF values of Ni2P/CN are higher than those of Ni2P/S and Ni2P/CS (Table 2), indicating the better catalytic activity of Ni2P/CN. This is mainly consistent with the Ni2P dispersion and CO uptake on the three catalysts (Table 2) [22]. Compared with Ni2P/CS, Ni2P/CN has more lactonic and phenolic groups with high-temperature resistance (Figs. 2 and S4), and therefore possesses smaller Ni2P nanoparticles with better dispersion after calcination and phosphorization. Furthermore, XPS analyses indicate a higher proportion of active phases (Niδ + and Pδ −) on Ni2P/CN (Fig. S3 and Table S3) than on Ni2P/CS and Ni2P/S. As a consequence, Ni2P/CN presents a higher HDS activity than Ni2P/CS and Ni2P/S. The Ni2P/S catalyst was prepared using the conventional impregnation and TPR method, in which calcination and phosphorization at a very high temperature result in the aggregation of Ni2P nanoparticles, with an uneven distribution. The P/Ni molar ratio of Ni2P/S is higher than those of Ni2P/CN and Ni2P/CS because of the formation of excess phosphorus species in the conventional TPR method. These excess phosphorus species decrease the dispersion of supported Ni2P particles and are detrimental to the HDS performance of Ni2P catalysts [23]. Thus, Ni2P/S has a lower HDS activity than Ni2P/CN. The assessment results (Table S5) show that among the four catalysts, Ni2P/C has the worst HDS activity due to its minimal loading of Ni2P. It has been proposed that 4,6-DMDBT HDS mainly involves two parallel reaction pathways [24]. One is the direct desulfurization (DDS) route to produce 3,3″-dimethylbiphenyl (DMBP), and the other is the hydrogenation route (HYD) to generate tetrahydro-4,6-dimethyl dibenzothiophene (4,6-THDMDBT), hexahydro-4,6-dimethyl dibenzothiophene (4,6-HHDMDBT), 3,3″-methylcyclohexyltoluene (MCHT), and 3,3″-dimethlybicyclohexyl (DMBCH). The 4,6-DMDBT HDS selectivities of the catalysts are shown in Table 2. The HYD route is the dominant pathway on the three catalysts, and the HYD/DDS ratio (MCHT/DMBP) decreases in the order Ni2P/CN N Ni2P/S N Ni2P/CS. This is because the 4,6-DMDBT HDS activities of the Ni2P catalysts are closely correlated with the size of supported Ni2P nanoparticles. It is known that there are two types of sites in the Ni2P crystal structure, tetrahedral Ni (1) sites coordinated with four phosphorus atoms and square pyramidal Ni (2) sites connected with five phosphorus atoms [25]. The DDS route occurs on the Ni (1) site, while the HYD route takes place on the Ni (2) site. With the decrease in the size of Ni2P crystallites, the number of Ni (2) sites increases and the number of Ni (1) sites is unchanged [25], leading to the increase in the HYD/DDS ratio on the supported Ni2P catalysts. Thereby, Ni2P/CN with smaller Ni2P nanoparticles and higher Ni2P dispersion possesses more hydrogenation sites than Ni2P/S and Ni2P/CS, improving its HYD activity and selectivity.
4. Conclusions A strategy was introduced herein for the synthesis of highly dispersed Ni2P nanoparticles on HNO3-oxidized CMK-3. Compared to oxidation by acidified ammonium persulphate, HNO3 oxidation creates more high-temperature-stabilized lactones and phenolic hydroxyls groups on CMK-3, which can be activated with negative charges via the NH3 effect. Therefore, [Ni(NH3)6]2 + ions can be anchored on the electronegative surface of functionalized CMK-3 via electrostatic adsorption. As a consequence, NiO nanoparticles and phosphatized Ni2P nanoparticles are highly dispersed on the HNO3-oxidized CMK-3 support due to the anchoring effect of the functional groups with hightemperature resistance. The corresponding Ni2P catalyst supported on HNO3-oxidized CMK-3 has excellent activity for 4,6-DMDBT HDS due to its smaller Ni2P nanoparticles with better dispersion than the Ni2P/ SBA-15 prepared by the conventional impregnation and TPR method. Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 21076228 and U1162116). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.01.024. References [1] F. Sun, W. Wu, Z. Wu, J. Guo, Z. Wei, Y. Yang, Z. Jiang, F. Tian, C. Li, J. Catal. 228 (2004) 298–310. [2] D. Kanama, S.T. Oyama, S. Otani, D.F. Cox, Surf. Sci. 552 (2004) 8–16. [3] S. Oyama, J. Catal. 216 (2003) 343–352. [4] Q. Guan, W. Li, M. Zhang, K. Tao, J. Catal. 263 (2009) 1–3. [5] S. Yang, C. Liang, R. Prins, J. Catal. 237 (2006) 118–130. [6] H.P. Andaraarachchi, M.J. Thompson, M.A. White, H.J. Fan, J. Vela, Chem. Mater. 27 (2015) 8021–8031. [7] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 10712–10713. [8] S. Che, K. Lund, T. Tatsumi, S. Iijima, S.H. Joo, R. Ryoo, O. Terasaki, Angew. Chem. Int. Ed. 42 (2003) 2182–2185. [9] R. Ryoo, S.H. Joo, S. Jun, T. Tsubakiyama, O. Terasaki, Stud. Surf. Sci. Catal. 135 (2001) 150. [10] P.A. Bazuła, A. Lu, J.J. Nitz, F. Schüth, Microporous Mesoporous Mater. 108 (2008) 266–275. [11] M.J. Lázaro, L. Calvillo, E.G. Bordejé, R. Moliner, R. Juan, C.R. Ruiz, Microporous Mesoporous Mater. 103 (2007) 158–165. [12] C. Liang, Z. Li, S. Dai, Angew. Chem. Int. Ed. 47 (2008) 3696–3717. [13] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Órfão, Ind. Eng. Chem. Res. 46 (2006) 4110–4115. [14] Z.H. Zhu, L.R. Radovic, G.Q. Lu, Carbon 38 (2000) 451–464. [15] J.R.C. Salgado, F. Alcaide, G. Álvarez, L. Calvillo, M.J. Lázaro, E. Pastor, J. Power Sources 195 (2010) 4022–4029. [16] J. Wang, H. Chen, Y. Fu, J. Shen, Appl. Catal. B 160–161 (2014) 344–355. [17] J. Wang, Y. Fu, H. Chen, J. Shen, Chem. Eng. J. 275 (2015) 89–101. [18] R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677–681. [19] N. Prabhu, A.K. Dalai, J. Adjaye, Appl. Catal. A 401 (2011) 1–11. [20] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Órfão, Ind. Eng. Chem. Res. 46 (2007) 4110–4115. [21] J.H. Zhou, Z.J. Sui, J. Zhu, P. Li, D. Chen, Y.C. Dai, W.K. Yuan, Carbon 45 (2007) 785–796. [22] S.T. Oyama, X. Wang, Y.-K. Lee, K. Bando, F.G. Requejo, J. Catal. 210 (2002) 207–217. [23] S.J. Sawhill, K.A. Layman, D.R. Van Wyk, M.H. Engelhard, C. Wang, M.E. Bussell, J. Catal. 231 (2005) 300–313. [24] R. Wang, K.J. Smith, Appl. Catal. A 361 (2009) 18–25. [25] S.T. Oyama, Y.-K. Lee, J. Catal. 258 (2008) 393–400.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
A strategy for preparing highly dispersed Ni2P/functionalized CMK-3 catalysts with superior hydrodesulfurization performance Tingting Huang, Wenjin Shi, Jundong Xu, Yu Fan ⁎ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, PR China
a r t i c l e
i n f o
Article history: Received 18 November 2016 Received in revised form 11 January 2017 Accepted 17 January 2017 Available online 18 January 2017 Keywords: Ni2P Dispersion 4,6-Dimethyldibenzothiophene Hydrodesulfurization catalyst
a b s t r a c t A strategy is introduced for producing highly dispersed Ni2P/functionalized CMK-3 hydrodesulfurization catalysts via the anchoring effect of high-temperature stable groups activated by ammonia. In the proposed method, HNO3 acts as an oxidant that enables original CMK-3 to create more lactones and phenolic hydroxyls than acidified ammonium persulphate; these groups are activated with negative charges via the NH3 effect, and thereby [Ni(NH3)6]2+ ions can be anchored to the electronegative surface of functionalized CMK-3 via electrostatic adsorption. As a result, NiO and phosphatized Ni2P nanoparticles are highly dispersed on the HNO3-oxidized CMK-3 support, endowing the catalyst with excellent activity for 4,6-dimethyldibenzothiophene hydrodesulfurization. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Nickel phosphides have been much investigated because of their promising hydrogenation applications in petrochemical industries [1, 2]. A variety of methods have been developed for the synthesis of nickel phosphides, including the direct reaction of nickel salts and ammonium phosphate [3], the thermal decomposition of a single-source precursor (nickel hypophosphite) [4], the phosphidation of metals or metal oxides with PH3 [5]. However, these methods are restricted by extremely harsh conditions or high toxicity of PH3. In addition, these methods are difficult to prepare highly dispersed metal phosphides [3,4,6]. CMK-3 exhibits better thermal and chemical stability than conventional ordered mesoporous carbons (OMCs), which has the potential for broader applications [7–12]. Moreover, it is known that carbon surfaces can be made more hydrophilic by creating oxygen-containing groups (such as carboxyl, carbonyl, and phenol) on them using liquid or gas techniques [13], thus favoring the dispersion of supported metal species [14]. Herein, an appropriate oxidant was chosen to create a high concentration of functional groups with high-temperature resistance on CMK-3, thus enhancing the stable dispersion of active metal phases at high temperatures. These functional groups were activated with negative charges via the incorporation of ammonia, promoting the interaction between [Ni(NH 3 )6 ] 2 + ions and the negatively charged CMK-3 support. As a consequence, NiO particles and subsequent Ni2P particles were highly dispersed on CMK-3 due to the anchoring effect of high-temperature functional groups during the ⁎ Corresponding author. E-mail address: [email protected] (Y. Fan).
http://dx.doi.org/10.1016/j.catcom.2017.01.024 1566-7367/© 2017 Elsevier B.V. All rights reserved.
calcination and phosphorization processes. The novel as-prepared Ni2P/functionalized CMK-3 catalyst possessed excellent activity for 4,6-DMDBT hydrodesulfurization (HDS). 2. Experimental All experimental details are displayed in Supplementary information. 3. Results and discussion 3.1. XRD characterization In the wide-angle X-ray diffraction (WAXRD) patterns of NiO/C, NiO/ CN, and NiO/CS (Fig. S2), the broad diffraction peak at approximately 2θ = 25° is assigned to the carbon support [15]. A series of diffraction peaks at 2θ = 37.2°, 43.3°, 62.9°, and 75.4° are assigned to NiO (JCPDS, PDF no. 00-044-1159). The low NiO diffraction peaks of NiO/C are attributed to the lower loading of NiO (only 2.9 wt% determined by ICP). The two peaks at 2θ = 44.5° and 51.8° in the patterns of NiO/CN and NiO/CS are attributed to the metallic nickel phase (JCPDS, PDF no. 00-0040850). The mild reducibility of the carbon support converts a part of NiO into metallic Ni, and the formation of Ni can shorten the following reduction time at high temperature and improve the dispersion of active Ni2P phases [16,17]. NiO/CN has much weaker peaks at 37.2° and 43.3° than NiO/CS, indicating smaller NiO particles on NiO/CN than on NiO/CS in view of the same loading amount of NiO. The WAXRD patterns of the four supported Ni2P catalysts are shown in Fig. 1. The diffraction peaks at 2θ = 40.7°, 44.6°, 47.4°, and 54.2° are
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T. Huang et al. / Catalysis Communications 93 (2017) 25–28
Fig. 1. Wide angle XRD patterns of Ni2P/CN, Ni2P/CS, Ni2P/C and Ni2P/S catalysts.
indexed to Ni2P (JCPDS, PDF no. 01-089-4864). Ni2P/CS possesses the sharpest Ni2P diffraction peaks among the four catalysts, indicating that it has the largest Ni2P particles. Ni2P/S has relatively stronger Ni2P diffraction peaks compared to Ni2P/CN and Ni2P/C because the calcination and reduction at high temperature results in the generation of larger Ni2P particles. Compared with Ni2P/CS and Ni2P/S, Ni2P/CN and Ni2P/ C have much lower diffraction peaks of Ni2P. The low Ni2P diffraction peaks of Ni2P/C are attributed to the lower loading of Ni2P (only 3.0 wt%), while the low Ni2P diffraction peaks of Ni2P/CN with the loading amount of Ni2P at 13.5 wt% are due to smaller Ni2P particles. In the small-angle X-ray diffraction (SAXRD) patterns of SBA-15, CMK-3, and supported Ni2P catalysts (Fig. S3), three peaks of CMK-3 and SBA-15 at 0.5–5° are assigned to the (100), (110), and (200) diffractions of a 2D hexagonal (P6mm) structure, indicating long-range periodic order with hexagonal symmetry [18]. The four catalysts have a diffraction peak at approximately 2θ = 1.07°, attributed to the (100) diffraction of a 2D hexagonal (P6mm) structure, indicating their highly ordered pore structures.
Fig. 2. Comparison of CO2 desorption (a) and CO desorption (b) spectra of CMK-3-S and CMK-3-N.
3.4. XPS characterization 3.2. Boehm's titration The quantities of oxygenated surface groups on CMK-3, CMK-3-N, and CMK-3-S were measured by Boehm's titration (Table S1) [19]. The total acidities on CMK-3-N and CMK-3-S are much higher than on unfunctionalized CMK-3, with increases from 0.35 mmol/g in the original CMK-3 to 1.64 mmol/g in CMK-3-N and 1.15 mmol/g in CMK-3-S. Moreover, the amounts of carboxyl groups, lactonic groups, and phenolic groups on CMK-3-N and CMK-3-S increase evidently, compared to the original CMK-3. CMK-3-N has more carboxyl groups, lactonic groups, and phenol groups than CMK-3-S, indicating the advantage of HNO3 treatment in enhancing the oxygenated surface groups of CMK-3.
3.3. TPD characterization TPD experiments were carried out on functionalized CMK-3-N and CMK-3-S, and the CO2 and CO desorption curves are shown in Fig. 2. In the spectra of CO2 desorption, the peaks at approximately 300, 490, and 650 °C are assigned to unstable carboxyl groups, stable anhydrides, and stable lactones, respectively. In the spectra of CO desorption, the peak at approximately 800 °C is attributed to high-temperaturestabilized phenol groups [20]. Compared with CMK-3-S, CMK-3-N presents a higher peak intensity and larger area of CO2 and CO desorption, indicating that the HNO3 treatment produces more oxygenated surface groups than the acidified APS treatment, especially for lactonic and phenolic groups with high-temperature resistance.
Besides, the surface functional groups on the CMK-3, CMK-3-N, and CMK-3-S surfaces also studied by XPS survey spectra (Fig. S4). Binding energies in the range of 295–280 eV and 540–528 eV are ascribed to C1s and O1s spectra [21], respectively. CMK-3-N and CMK-3-S have a much higher peak intensity of O1s than CMK-3, and their O/C atomic ratios increase from 0.0207 for CMK-3 to 0.0839 for CMK-3-N and 0.0704 for CMK-3-S (Fig. S4a), further indicating that the HNO3 treatment introduces more oxygen-containing groups on the CMK-3 surface than the acidized APS treatment. According to the high-resolution scans and corresponding fitting curves of the O1s spectra (Figs. S4b–d), the relative concentrations of different functional groups were derived (Table 1) [21]. The oxygen-containing groups on the original CMK-3 surface are mainly quinone and lactone groups without carboxyl groups. And the relative concentrations of carboxyl groups (Peak IV) on CMK-3-N and CMK-3-S are 11.73% and 8.39%, respectively, demonstrating that the oxidation process with HNO3 or acidified APS
Table 1 Relative concentrations of functional groups in O1s and the O/C ratios from XPS spectra. Sample
O1s-relative atomic concentration (%)
O/C ratio
Peak I
Peak II
Peak III
Peak IV
CMK-3 CMK-3-N CMK-3-S
61.80 47.83 54.06
15.84 20.24 16.66
22.36 20.20 15.64
– 11.73 8.39
0.0207 0.0839 0.0704
T. Huang et al. / Catalysis Communications 93 (2017) 25–28
27
Fig. 3. HRTEM image (a) and typical particle size distribution (b) of Ni2P/CN catalyst.
introduces carboxyl groups onto the original CMK-3. In addition, CMK3-N has higher atomic concentrations of Peaks II and III than CMK-3-S, indicating that the HNO3 treatment produces more phenol and lactone groups with high-temperature resistance on the original CMK-3 surface. 3.5. TEM characterization To observe the structure of NiO particles more clearly, HAADF-STEM (Z contrast) measurements were performed on NiO/CN and NiO/CS (Fig. S5). The NiO particles supported on NiO/CN are highly dispersed and evenly distributed with a size of approximately 2.2 nm (Fig. S5a), while the NiO particles supported on NiO/CS are aggregated, some particles are larger than 20 nm (Fig. S5b). The above result demonstrates that when ammonia is used to regulate pH, CMK-3-N disperses NiO particles much better than CMK-3-S. This is because the concentration of functional surface groups on oxidized CMK-3 influences the dispersion of supported NiO. On the surface of the oxidized CMK-3, carboxyl groups with strong acidity interact with NiO precursors during impregnation, guaranteeing a high dispersion of NiO precursors on the support. However, the thermal stability of carboxyl groups is inferior, leading to their decomposition when calcined at 400 °C. As a consequence, the NiO particles immobilized by the carboxyl groups are released. Then, these free NiO particles are aggregated with larger particles during calcination. Therefore, high-temperature stable groups on functionalized CMK-3 play an important role in promoting the high dispersion of NiO. It is known that lactones and phenolic hydroxyls are wellpreserved up to 600–800 °C, due to their high-temperature stability [20]. These groups can serve as metal anchoring sites via their interaction with NiO particles during calcination. Thus, the supported NiO particles can be highly dispersed after calcination. According to the results of Boehm's titration, TPD, and XPS characterizations, HNO3 oxidation introduces more groups with high-temperature resistance groups on the original CMK-3 than acidified APS, endowing NiO/CN with smaller NiO particles and better NiO dispersion than NiO/CS. Representative HRTEM micrographs and the corresponding particle size histograms of supported Ni2P are shown in Fig. 3. The magnified
HRTEM images (Figs. S6e and f) of selected frames from Figs. 3a and S6a show d-spacing values of 0.206 and 0.194 nm for the (201) and (210) crystallographic planes of Ni2P, confirming the existence of Ni2P crystallites in the catalysts. The EDS results show that the measured P/ Ni ratios on Ni2P/CN, Ni2P/S and Ni2P/CS are 0.54, 0.66, and 0.55, respectively (Fig. S7), which are close to the ratios of 0.52, 0.62, and 0.54 determined by the ICP measurement (Table S2). The Ni2P particles supported on Ni2P/CN are highly dispersed, with an average particle size of 4.8 nm (Fig. 3b). Ni2P/S possesses an average Ni2P particle size of 8.1 nm (Fig. S6d), which is larger than that of Ni2P/CN. According to the concentration of metal sites, the calculated particle sizes of Ni2P/CN and Ni2P/S are 4.6 and 7.2 nm, which are very close to the particle sizes determined by the TEM analysis (Table S4). Ni2P/S was synthesized using an impregnation and TPR method, in which the reduction and calcination at high temperatures lead to the generation of large Ni2P particles with a low dispersion. The Ni2P particles supported on Ni2P/CS are severely aggregated (Fig. S6c), with an average particle size of 15.6 nm (Table S4). Compared with the CMK-3-N support of Ni2P/CN, the CMK-3-S support of Ni2P/CS has a much lower concentration of thermally stable groups (Table S1), leading to a larger size and lower dispersion of Ni2P particles on Ni2P/CS than on Ni2P/CN. Although Ni2P/C has a smaller particle size of 3.8 nm with a higher dispersion than in Ni2P/CN (Fig. S6b), its low Ni2P-loading limits its applications in ultra-deep HDS of diesel. The highly dispersed Ni2P/CN catalyst is obtained by the electrostatic adsorption (EA) method. In this method, the surface groups on functionalized CMK-3 are deprotonated by the effect of NH3, making these groups electronegative. Thus, [Ni(NH3)6]2+ ions can be anchored on the electronegative surface of functionalized CMK-3 via electrostatic adsorption. During the Ni2P/C preparation, the ammonia used to control the solution pH value is eliminated, resulting in an acidic environment (pH = 5.8); this acidic environment restrains the deprotonation of oxygen-containing groups on CMK-3-N, and therefore, the numbers of Ni species and following Ni2P on CMK-3-N decrease remarkably. To further investigate the effect of ammonia on metal loading, the pH values of the impregnating solution were correlated with the loading of Ni2P (Fig. S8). The amount of Ni2P supported on CMK-3-N increases with
Table 2 4,6-DMDBT HDS results on the supported Ni2P catalysts. Catalyst
Ni2P/CN Ni2P/S Ni2P/CS a b c
CO uptakea (μmol g−1)
142 125 90
kHDSb (10−7 mol g−1 s−1)
TOFc (10−3 s−1)
2.39 1.76 1.08
1.11 0.99 0.83
Product selectivityb (%)
Product ratiob
TH + HH
MCHT
DMBCH
DMBP
MCHT/DMBP
25.26 39.06 7.45
46.32 43.75 59.26
23.16 10.94 8.60
5.26 6.25 24.69
8.81 7.00 2.40
Stoichiometry of CO/active site is assumed to be one for the tested samples. Determined at 50% of the total 4,6-DMDBT conversion by changing liquid hourly space velocity. TH, 4,6-THDMDBT; HH, 4,6-HHDMDBT. Obtained at 3.0 MPa, 340 °C, with the total conversion of 20% by changing liquid hourly space velocity.
28
T. Huang et al. / Catalysis Communications 93 (2017) 25–28
the increase in pH value, indicating the promoting effect of ammonia on Ni2P loading. At the initial pH value of 5.8 without ammonia, the loading of Ni2P on CMK-3-N is only 3.0 wt% because this pH value is close to the point of zero charge (PZC) of oxidized CMK-3 (2.3). However, the loading of Ni2P with the pH value of 12.0 is as high as 13.5 wt%. At a high pH value caused by alkalinity, the oxygen-containing groups on functionalized CMK-3 become deprotonated and negatively charged, greatly improving the loading of nickel ammine and subsequently, Ni2P on oxidized CMK-3. 3.6. Catalytic activity The 4,6-DMDBT HDS results of the four catalysts showed that the HDS ratio decreases in the order Ni2P/CN N Ni2P/S N Ni2P/CS N Ni2P/C, at a high velocity (13.0 h− 1), as well as at a low velocity (7.0 h−1) (Table S5). The reaction rate constant kHDS and TOF values of Ni2P/CN are higher than those of Ni2P/S and Ni2P/CS (Table 2), indicating the better catalytic activity of Ni2P/CN. This is mainly consistent with the Ni2P dispersion and CO uptake on the three catalysts (Table 2) [22]. Compared with Ni2P/CS, Ni2P/CN has more lactonic and phenolic groups with high-temperature resistance (Figs. 2 and S4), and therefore possesses smaller Ni2P nanoparticles with better dispersion after calcination and phosphorization. Furthermore, XPS analyses indicate a higher proportion of active phases (Niδ + and Pδ −) on Ni2P/CN (Fig. S3 and Table S3) than on Ni2P/CS and Ni2P/S. As a consequence, Ni2P/CN presents a higher HDS activity than Ni2P/CS and Ni2P/S. The Ni2P/S catalyst was prepared using the conventional impregnation and TPR method, in which calcination and phosphorization at a very high temperature result in the aggregation of Ni2P nanoparticles, with an uneven distribution. The P/Ni molar ratio of Ni2P/S is higher than those of Ni2P/CN and Ni2P/CS because of the formation of excess phosphorus species in the conventional TPR method. These excess phosphorus species decrease the dispersion of supported Ni2P particles and are detrimental to the HDS performance of Ni2P catalysts [23]. Thus, Ni2P/S has a lower HDS activity than Ni2P/CN. The assessment results (Table S5) show that among the four catalysts, Ni2P/C has the worst HDS activity due to its minimal loading of Ni2P. It has been proposed that 4,6-DMDBT HDS mainly involves two parallel reaction pathways [24]. One is the direct desulfurization (DDS) route to produce 3,3″-dimethylbiphenyl (DMBP), and the other is the hydrogenation route (HYD) to generate tetrahydro-4,6-dimethyl dibenzothiophene (4,6-THDMDBT), hexahydro-4,6-dimethyl dibenzothiophene (4,6-HHDMDBT), 3,3″-methylcyclohexyltoluene (MCHT), and 3,3″-dimethlybicyclohexyl (DMBCH). The 4,6-DMDBT HDS selectivities of the catalysts are shown in Table 2. The HYD route is the dominant pathway on the three catalysts, and the HYD/DDS ratio (MCHT/DMBP) decreases in the order Ni2P/CN N Ni2P/S N Ni2P/CS. This is because the 4,6-DMDBT HDS activities of the Ni2P catalysts are closely correlated with the size of supported Ni2P nanoparticles. It is known that there are two types of sites in the Ni2P crystal structure, tetrahedral Ni (1) sites coordinated with four phosphorus atoms and square pyramidal Ni (2) sites connected with five phosphorus atoms [25]. The DDS route occurs on the Ni (1) site, while the HYD route takes place on the Ni (2) site. With the decrease in the size of Ni2P crystallites, the number of Ni (2) sites increases and the number of Ni (1) sites is unchanged [25], leading to the increase in the HYD/DDS ratio on the supported Ni2P catalysts. Thereby, Ni2P/CN with smaller Ni2P nanoparticles and higher Ni2P dispersion possesses more hydrogenation sites than Ni2P/S and Ni2P/CS, improving its HYD activity and selectivity.
4. Conclusions A strategy was introduced herein for the synthesis of highly dispersed Ni2P nanoparticles on HNO3-oxidized CMK-3. Compared to oxidation by acidified ammonium persulphate, HNO3 oxidation creates more high-temperature-stabilized lactones and phenolic hydroxyls groups on CMK-3, which can be activated with negative charges via the NH3 effect. Therefore, [Ni(NH3)6]2 + ions can be anchored on the electronegative surface of functionalized CMK-3 via electrostatic adsorption. As a consequence, NiO nanoparticles and phosphatized Ni2P nanoparticles are highly dispersed on the HNO3-oxidized CMK-3 support due to the anchoring effect of the functional groups with hightemperature resistance. The corresponding Ni2P catalyst supported on HNO3-oxidized CMK-3 has excellent activity for 4,6-DMDBT HDS due to its smaller Ni2P nanoparticles with better dispersion than the Ni2P/ SBA-15 prepared by the conventional impregnation and TPR method. Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 21076228 and U1162116). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.01.024. References [1] F. Sun, W. Wu, Z. Wu, J. Guo, Z. Wei, Y. Yang, Z. Jiang, F. Tian, C. Li, J. Catal. 228 (2004) 298–310. [2] D. Kanama, S.T. Oyama, S. Otani, D.F. Cox, Surf. Sci. 552 (2004) 8–16. [3] S. Oyama, J. Catal. 216 (2003) 343–352. [4] Q. Guan, W. Li, M. Zhang, K. Tao, J. Catal. 263 (2009) 1–3. [5] S. Yang, C. Liang, R. Prins, J. Catal. 237 (2006) 118–130. [6] H.P. Andaraarachchi, M.J. Thompson, M.A. White, H.J. Fan, J. Vela, Chem. Mater. 27 (2015) 8021–8031. [7] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 10712–10713. [8] S. Che, K. Lund, T. Tatsumi, S. Iijima, S.H. Joo, R. Ryoo, O. Terasaki, Angew. Chem. Int. Ed. 42 (2003) 2182–2185. [9] R. Ryoo, S.H. Joo, S. Jun, T. Tsubakiyama, O. Terasaki, Stud. Surf. Sci. Catal. 135 (2001) 150. [10] P.A. Bazuła, A. Lu, J.J. Nitz, F. Schüth, Microporous Mesoporous Mater. 108 (2008) 266–275. [11] M.J. Lázaro, L. Calvillo, E.G. Bordejé, R. Moliner, R. Juan, C.R. Ruiz, Microporous Mesoporous Mater. 103 (2007) 158–165. [12] C. Liang, Z. Li, S. Dai, Angew. Chem. Int. Ed. 47 (2008) 3696–3717. [13] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Órfão, Ind. Eng. Chem. Res. 46 (2006) 4110–4115. [14] Z.H. Zhu, L.R. Radovic, G.Q. Lu, Carbon 38 (2000) 451–464. [15] J.R.C. Salgado, F. Alcaide, G. Álvarez, L. Calvillo, M.J. Lázaro, E. Pastor, J. Power Sources 195 (2010) 4022–4029. [16] J. Wang, H. Chen, Y. Fu, J. Shen, Appl. Catal. B 160–161 (2014) 344–355. [17] J. Wang, Y. Fu, H. Chen, J. Shen, Chem. Eng. J. 275 (2015) 89–101. [18] R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677–681. [19] N. Prabhu, A.K. Dalai, J. Adjaye, Appl. Catal. A 401 (2011) 1–11. [20] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Órfão, Ind. Eng. Chem. Res. 46 (2007) 4110–4115. [21] J.H. Zhou, Z.J. Sui, J. Zhu, P. Li, D. Chen, Y.C. Dai, W.K. Yuan, Carbon 45 (2007) 785–796. [22] S.T. Oyama, X. Wang, Y.-K. Lee, K. Bando, F.G. Requejo, J. Catal. 210 (2002) 207–217. [23] S.J. Sawhill, K.A. Layman, D.R. Van Wyk, M.H. Engelhard, C. Wang, M.E. Bussell, J. Catal. 231 (2005) 300–313. [24] R. Wang, K.J. Smith, Appl. Catal. A 361 (2009) 18–25. [25] S.T. Oyama, Y.-K. Lee, J. Catal. 258 (2008) 393–400.