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Synthesis, characterization and catalytic performance of well-ordered mesoporous Ni-MCM-41 with high nickel content
Microporous and Mesoporous Materials 208 (2015) 181e187
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis, characterization and catalytic performance of well-ordered mesoporous Ni-MCM-41 with high nickel content Jing Qin a, Baoshan Li a, *, Wen Zhang a, b, Wei Lv c, Chunying Han a, Jianjun Liu a a
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Yantai Valiant Fine Chemicals Co., Ltd, Yantai, Shandong 264006, PR China c China Huanqiu Contracting & Engineering Corporation, Beijing 100012, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2014 Received in revised form 29 January 2015 Accepted 3 February 2015 Available online 11 February 2015
The well-ordered Ni-MCM-41 with high nickel content was synthesized by a direct hydrothermal method using cetyltrimethylammonia bromide as the structure-directing agent in an ammonia aqueous solution. The resulting samples were characterized by X-ray diffraction, X-ray fluorescence spectroscopy, high-resolution transmission electron microscopy, nitrogen adsorptionedesorption isotherms, H2 temperature-programmed reduction analyses, Fourier-transform infrared spectroscopy, ammonia temperature programmed desorption, and X-ray photoelectron spectroscopy. The results indicated that the material maintained ordered physical mesostructure of MCM-41 with high nickel content, and the nickel atoms were mainly in the framework with tetrahedral coordination. The materials possessed high specific surface area (800e580 m2 g1), large total pore volume (1.05e1.22 cm3 g1) and pore diameter (2.77e3.16 nm). The material exhibited excellent catalytic performance for the hydrocracking of coker wax oil, which was improved with the increasing of Ni content in the framework of the molecular sieves. © 2015 Elsevier Inc. All rights reserved.
Keywords: Nickel-rich molecular sieve Mesoporous molecular sieve MCM-41 Direct hydrothermal method Hydrocracking of coker wax oil
1. Introduction MCM-41 has been extensively studied due to its high specific surface area, large pore volume, regular structure, uniform pore size and high thermal stability [1,2]. However, owing to the neutral character, the pure silica MCM-41 showed very limited catalytic activities. Therefore, many researchers are dedicated to incorporating different heteroatoms into the silica framework to create active sites for catalytic applications. Almost all kinds of transition metals [3-11] and some main group elements [2,10] have been incorporated into the mesoporous molecular sieve frameworks as the active sites to improve their catalytic activities by direct synthesis or impregnation methods [7,11-17]. The substitution of silicon atoms by heteroatoms can increase their Brønsted acid sites or cation exchange capacity [18]. The content of heteroatom incorporated in the silica framework will greatly influence the acidic strength, ion-exchange capacity [19] and the catalytic performance [20]. For instance, Chen et al. [21] reported a nickel-incorporated MCM-41 presented a good activity for the single-wall carbon nanotube synthesis by CO disproportionation. Wang et al. [22]
reported a direct synthesis method for preparing high nickelcontaining MCM-41-type mesoporous silica and the nickel content was up to 11.8 wt.%. In our previous study [23], the high content of 13.21 wt.% Febased MCM-41 in the framework with tetrahedral coordination was successfully prepared by a direct synthesized method. The material exhibited high catalytic performance for the oxidative desulfurization (ODS) of dibenzothiophene (DBT) from fuel oils. Nickel-containing molecular sieves have been used in catalytic fields, such as desulfurization [22], hydrogenation [23,24] and oxidation [7] etc. In the present work, a detailed synthesis procedure was reported for preparing high content nickel atoms NiMCM-41 mesoporous materials in the framework and the nickel content can be up to 16.3 wt.%. The prepared samples featured high specific surface area, large pore volume and pore diameter. The samples also exhibited excellent catalytic performance for the hydrocracking of coker wax oil. 2. Experimental 2.1. Materials
* Corresponding author. Tel./fax: þ86 10 64445611. E-mail address: [email protected] (B. Li). http://dx.doi.org/10.1016/j.micromeso.2015.02.009 1387-1811/© 2015 Elsevier Inc. All rights reserved.
Nickel nitrate hexahydrate [Ni(NO3)2$6H2O] (A.R., Beijing Yili Fine Chemical Product Limited Company, China) was used as Ni
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source. Cetyltrimethylammonium bromide (CTAB, A.R., Tianjin Jinke Fine Chemical Institute, China) was used as the structural template. Tetraethyl orthosilicate (TEOS, A.R., Beijing Chemical Factory, China) was used as Si source and ammonia solution (25%, Beijing Beihua Fine Chemical Product Limited Company, China) was used as a ligand. All solvents and reactants are commercially available and were used without further purification. 2.2. Synthesis of the materials The Ni-MCM-41 mesoporous material was prepared through a direct hydrothermal synthesis method, and the detailed process is as following: 2.35 g of CTAB was dissolved in 100 ml of deionized water in a flask with stirring vigorously for 30 min to get a clear solution. A certain amount of Ni(NO3)2$6H2O was dispersed in 20 ml of deionized water in a beaker and 15 ml of ammonia solution (25%) was added to obtain a complex Ni(NH3)2þ 6 solution. Then, the solution was added into the flask. After stirring vigorously for 30 min, 10 ml of TEOS was dropped into the mixture and stirred for another 30 min. The pH was adjusted to 10.0 by using an ammonia solution, and the final mixture was stirred for 5 h. The molar composition of the mixture was 1.0 (TEOS):0.152 (CTAB):2.8 (NH3):x (Ni):141.2 (H2O). Then, the final mixture was transferred into a Teflon-lined stainless steel autoclave under the autogenous pressure at 383 K for 48 h. The samples were filtered, washed with deionized water, dried for overnight at 323 K and finally calcinated at 823 K in air for 6 h. The result Ni-MCM-41samples are designated A, B, C and D with the Si/Ni molar ratio of 5, 10, 20 and 30, respectively. The pure silica MCM-41, designated E was synthesized with the same procedure only without adding Ni(NO3)2$6H2O. The impregnated Ni/MCM-41 (Si/Ni ratio is 5) was prepared according to the following procedure: 1.0 g of pre-synthesized E powder was dispersed into the nickel nitrate solution to obtain a suspension, which was kept stirring at 323 K to improve the mixing and to make the water evaporation in a very slow rate. The resulting material was dried at 323 K overnight and calcinated in air at 823 K for 6 h. The final products were designated as A*. 2.3. Characterization of the samples X-ray diffraction (XRD) characterization was performed with a Rigaku D/Max 2500 VBZþ/PC diffractometer using Cu-Ka radiation (40 kV, 200 mA) in both low angle (2q range 0.5e10 ) and wide angle (2q range 10e80 ). Chemical compositions of the synthesized samples were determined by X-ray fluorescence analysis spectroscopy (XRF) on a Philips Magix-601 X-ray fluorescence spectrometer and the samples were pressed into pellets beforehand. Pore morphology of the samples was examined by high-resolution transmission electron microscopy (HRTEM) on a Jem-3010 with an accelerating voltage of 200 kV. The samples were prepared by ultrasonic dispersion, using absolute alcohol as solvent and copper grids as support membranes. Nitrogen adsorptionedesorption isotherms were determined on a Micromeritics ASAP 2020 M volumetric adsorption analyzer. Before nitrogen adsorption, each sample was degassed in a vacuum at 473 K for 6 h. The pore size distribution was carried out on desorption branch of the isotherm using BarretteJoynereHalenda (BET) method and the specific surface area was calculated using BrunauereEmmetteTeller (BET) equation. H2 temperature-programmed reduction (H2-TPR) analyses was finished using a thermal conductivity detector (TCD) in 10 vol.% of hydrogen in argon in the temperature range 323e1273 K with the heating rate of 10 K min1 . Before testing, the samples were pretreated in argon at 573 K for 2 h. Fourier-transform infrared (FT-IR) spectra were scanned in KBr pellets on a Bruker
VECTOR 22 spectrometer in the range 4000e400 cm1. Acid properties of the samples were determined by ammonia temperature programmed desorption (NH3-TPD) in a TP-5080 system using He as carrier gas. Previously, the samples were pretreated at 573 K for 1 h in He flow. After cooling to 300 K, an ammonia flow of 35 ml min1 was passed through the sample for 0.5 h, and then the gaseous and weakly adsorbed NH3 was purged with He. The TPD was conducted from 300 K to 1000 K at a heating rate of 10 K min1. X-ray photoelectron spectroscopy (XPS) analyses were conducted on an ESCALAB 250 spectrometer equipped with an Al-Ka X-ray source. The carbon 1s peak at 284.6 eV was used as the reference for binding energies. 2.4. Catalytic performance test Catalytic hydrocracking performance of the materials were conducted in a 250 ml high pressure reactor with a magnetic drive stirring machine, with the coker wax oil as the raw material. The reaction temperature was set at 573 K and the hydrogen pressure was 3.0 MPa. The catalyst dosage was 0.5 g per 50 g of oil, and the reaction time was 60 min, the system was cooled and separated into gaseous and liquid products, using a gas-collector and a glass liquid collector, respectively. The products were separated by distillation for getting different fractions. For analyzing the coke yield, the residue after the distillation was washed with toluene and the catalyst was dissolved in a HF solution at room temperature. After the catalyst solved, the coke was filtrated, washed with deionized water, dried in an oven at 373 K, and then weighed to calculate the rate of the residual carbon. 3. Results and discussion The low-angle XRD patterns of the samples are shown in Fig. 1. It is clear that the sample A shows a strong (1 0 0) peak, and the sample B shows a broad one. For the other samples (C, D, and E), another two diffraction peaks (2 0 0) and (2 1 0) appear, indicating that the samples possess relative perfect structure of MCM-41. With the increase of nickel content, the diffraction peaks representing the ordered mesoporous structure are widened and shifted to the
Fig. 1. Low-angle XRD patterns of the sample A, B, C, D, and E.
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smaller angles, i.e. the 2q degree of diffraction peak (1 0 0) was shifted to 1.79 , 1.97, and 2.22 for the sample A, B, and C, respectively, relative to the sample D and E at 2.26 (pure silica MCM-41). This is because the increase of the pffiffiffiinterplanar spacing d100 and cell parameter values a0 (a0 ¼ 2d100 3) (listed in Table 1) induced by the bigger Ni atoms substitute for the smaller Si atoms, which is normally attributed to the incorporation of Ni atoms into the silica frameworks [25,26]. The wide-angle XRD patterns of the samples A and A* are shown as Fig. 2. It is clear that Ni/MCM-41 displays the well-resolved characteristic diffraction pattern of nickel oxide, indicating that a lot of nickel oxide particles are located [27] in the A* sample. However, the sample A has not showed these peaks, illustrating the phase separation between NiO and SiO2 did not happen, while a little insignificant asymmetric reflection of nickel phyllosilicate [28] appeared. These results illustrate that the nickel oxide particles were formed on the sample A* and there was no these particles in the sample A except a little amount of nickel species were in the form of nickel phyllosilicate. This indicates that the Ni atoms were highly dispersed in the silica framework for the sample A. The chemical composition of the samples was measured by XRF, and the results are listed in Table 1. The results indicated that the actual content of nickel in each of the samples is 16.3%, 8.8%, 4.7%, 3.8%, and 15.8 wt. % for the sample A, B, C, D, and A*, corresponding to the molar ratio of Si/Ni ¼ 5, 10, 20, 30, and 5, respectively. This illuminates that the content of Ni in the framework of Ni-MCM-41 can be in a wide range. Fig. 3 shows the micrographs of the samples A, B, C, D, E and A*. The well-oriented mesoporous channels are observed clearly in all the micrographs, indicating that the samples retain highly ordered two-dimensional hexagonal structure, even with the high Ni content, which is consistent with the wide-angle XRD result, suggesting that the phase separation between NiO and SiO2 did not happen and the nickel atoms are mostly introduced into the silica framework [29] in Ni-MCM-41 samples. However, there are large NiO particles on the surface of sample A* that is very different from the others. It is also clear that the regularity of samples is decreased with the increase of nickel incorporation, which is in agreement with the results of the low-angle XRD results. As the nickel content increasing, more silicon atoms are substituted for nickel atoms, which lead to the degradation of the pore system. The N2 adsorptionedesorption isotherms are shown in Fig. 4a. It shows that the isotherms of the samples are type IV and possess type H1 hysteresis loops. It is indicated that the samples are typical mesoporous materials, exhibiting a sharp characteristic of capillary condensation at intermediate partial pressures (0.3 < P/P0 < 0.5). In the relative pressure range 0.90e0.99, all samples show another sharp with the increase of the nitrogen adsorption volume, resulting from filling the macropores formed by interparticle spaces and untransformed amorphous silica [30]. The texture properties of the samples are summarized in Table 1. It is clear that the pore diameter (Dp), pore wall thickness (Wt) and total pore volume (Vp) increase with the progressive incorporation
Table 1 Physical structural data of the samples. Sample Si/Ni Ni/wt% d100/nm a0/nm SBET/m2 g1 Dp/nm Wt/nm Vp/m3 g1 A B C D A* E
5 10 20 30 5 e
16.3 8.8 4.7 3.8 15.8 0
4.92 4.48 3.98 3.88 e 3.04
5.68 5.17 4.60 4.48 e 3.51
580 748 774 800 553 1082
3.16 3.08 2.77 2.77 3.01 2.53
2.52 2.09 1.83 1.71 e 0.98
Dp: calculated using the BJH method; Wt: calculated as Wt ¼ a0 Dp.
1.10 1.22 1.11 1.05 0.56 0.97
183
Fig. 2. Wide-angle XRD patterns of the sample A and A*.
of nickel atoms into the silica framework of MCM-41. Meanwhile, the expansion of unit cell is just because of the substitution of smaller silicon atom (Si4þ ion radius, 0.41 A ) with bigger nickel atom (Ni2þ ion radius, 0.69 A ) [27], which are in good agreement with the as-mentioned XRD results. On the contrary, the specific surface area (SBET) of the sample decreased with the nickel content increases, which was induced mainly by the crescent density with the heavy Ni atoms introduced. In addition, the pore size distributions were widened with the Ni content increase (Fig. 4b), illustrating a relative loss of order and regularity in the pore system, which is in good agreement with the XRD and HRTEM results above. H2-TPR is a very convenient way to study the reduction behavior of supported oxide catalysts. The H2-TPR curves of the samples are shown as Fig. 5. It is obviously that there was a large reduction stage over the range 930e1155 K in all curves, suggesting that the Ni species were mainly existence in the framework with high stability, indicating that most of Ni atoms were incorporated into the framework of the materials. In addition, for the sample A*, two large reduction peaks are obtained over the range 600e720 K and 840e930 K centers at 658 K and 873 K, respectively. The lowtemperature reduction peak coincides with the reduction of bulk NiO [27,31]. The higher temperature reduction peak is associated with the presence of very small NiO particles strongly interacted with SiO2 framework formed nickel silicate during the calcination process [24,25,28]. Two weak reduction pecks over the range 600e720 K and 840e930 K also appearance in the H2-TPR curves of the A and B samples, indicating that there is a very low content of bulk NiO and nickel silicate. For the samples C and D, the lowtemperature reduction peak at ca. 658 K was absent, indicating that the bulk NiO was not present. But a weak reduction peak over the higher temperature range 840e930 K (ca. 880 K) also appears in the curve of sample C, indicating that there was a tiny of nickel silicate in the sample. And for the sample D, the peak was absent indicating there were no NiO and nickel silicate present in the sample D. The results illuminate that the Ni atoms can be highly dispersed mainly in the framework of Ni-MCM-41, which is agreement with the XRD results. FT-IR spectra of the samples are presented in Fig. 6 (as-MCM-41 is the pure silica MCM-41 before calcination). The broad
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Fig. 3. HRTEM images of the samples A, A*, B, C, D and E.
asymmetric yas(SieO) vibration band at around 1084 cm1, which is characteristic peak of the silica network, and can be observed in all the samples. The band at about 460 cm1 can be attributed to the bending vibrations of SieOeSi groups. The band at about 550 cm1 can be attributed to the bending vibrations of eNieOeSie [32,33], which is the one evidence for the heteroatom constructed into the framework of MCM-41. In addition, the sample A has two small peaks at about 710 cm1 and 670 cm-1, which can be assigned to the dOH vibration of isolated eOH surrounded by three Ni atoms and a tetrahedral SiO mode [34], respectively. The two bands also were considered as the nickel silicate formation [35], and the very weak peaks indicate the presence of nickel silicate with a very small amount, which is agreement with the XRD and H2-TPR results. Except the as-MCM-41, the bands at about 2925 cm1, 2854 cm1 and 1480 cm1 were absent in all the Ni-CMM-41 and Ni/CMM-41 samples, illuminating that the organic template has been effectively removed [29,36]. Acidity of the samples was characterized by NH3 - TPD and the results are shown in Fig. 7. It can be seen that the sample A, B, C, D and A* all show a peak at low temperature about 430e470 K, which
corresponds to the weak acid sites. In addition, the pure Si-MCM-41 has no desorption peak [37], which in agreement with the test results, so this kind of acid sites can be attributed to the nickel atoms in the sample. The curves A, B, C and D also show a wide peak at relative higher temperature range about 470e650 K, which can be designated to the middle acid sites. This peak is attributed to the nickel atoms introduced in the framework. The total acidity was mainly calculated based on the amount of strongly adsorbed ammonia molecules and the values were found to be 0.71, 0.43, 0.32, 0.26 and 0.14 mmol/g for sample A, B, C, D and A*, respectively. Additionally, it is obvious that the total acid concentration of the material was increased with the increase of Ni content. The acid strength and concentration determined the catalytic activity and selectivity. Fig. 8 shows the XPS spectra of samples A and A*, and the inset is the de-convoluted Ni 2p3/2 spectra. As shown in Fig. 8A, there is a predominant peak centered at 857.4 eV, indicating the strong interaction between nickel and the silica base, suggesting a large amount of nickel incorporating into the silica framework, which corroborates the previous results; But for Fig. 8A*, a peak centered
Fig. 4. Nitrogen adsorptionedesorption isotherms (a) and BJH pore diameter distributions (b) of the samples.
J. Qin et al. / Microporous and Mesoporous Materials 208 (2015) 181e187
185
Fig. 7. NH3-TPD curves of the samples.
shown in Table 1. At the same time, too much nickel atoms incorporated would destroy the mesostructure in a certain extent. Fig. 5. H2-TPR curves of the samples.
at 855.1 eV was appearance, which is the characteristic peak of NiO, indicating the present of NiO in the sample A*. Based on the above characterizations, the formation mechanism of the samples was conformed to the mechanism SþIeIþI, which is raised by Guo et al. [5]. And the preparation method takes advan4 þ þ tage of using CTAþ, SiO44, Ni(NH3)2þ 6 , and SiO4 as S , I , I , and I of the proposed SþIIþI mechanism, respectively. The synthetic mechanism can be revealed in Scheme 1. In the mixture solution with plenty of ammonia, added TEOS hydrolyzes immediately and polymerize, followed by the formation of mesostructured materials. Ni2þ ions polymerize much more weakly than Si species; therefore, more Si species polymerize to stabilize the resulting mesostructure. With the increase of nickel content, a lot of Ni(NH3) 62þ weakened the electric interaction between CTAþ and inorganic species SiO44, leading to thicker pore wall and bigger pores, just as
3.1. Catalytic performance of the samples The catalytic performance of the materials for hydrocracking of coker wax oil was investigated. All the results are shown in Table 2. It is clear that the liquid products were just two fractions which was the low temperature fraction (70e120 C) and the high temperature fraction (120e160 C) respectively. Comparing with the Ni/ MCM-41 and Si-MCM-41, the samples Ni-MCM-41 exhibited excellent conversion, higher selectivity for gasoline and lower coke yield. The good catalytic activity is attributed to the sample possessing uniform channels, appropriate pore diameter and active sites caused by introducing a large number of Ni atoms into the silica framework. It is clear that the conversion is increasing with the increase of Ni content. As we have known, there are a lot of alkaline nitrogen compounds existing in the oil sample, and they are more attractive to the active sites of catalyst than polycyclic aromatic hydrocarbons. So sample A possesses most active sites and exhibited the best conversion, which is strongly influenced by the nickel content. Meanwhile, the selectivity of the samples is also related to the amount of nickel atoms introduced into the samples and the uniformity of the channels. The low temperature fraction yield of sample A is higher than other catalysts, and the selectivity of high temperature fraction is almost zero, which means that it is very active in hydrocracking of coker wax oil into low temperature fraction. Furthermore, gas and coke come from the secondary reactions, due to the limited outward diffusion of the produce molecules [38]. The gas and coke yield of this reaction is because the mesopores in the catalysts can accelerate the diffusion of the product molecules, and then the secondary reactions can lead to gas and coke formation are decreased. The slightly higher gas and coke yield of sample A is because the mesopores in the catalyst is a little more irregular than the other catalysts, which can be improved by the pore diameter distribution curves. From the results of hydrocracking, the Ni-MCM-41 mesoporous molecular sieves with high nickel contents have good catalytic performance for hydrocracking of the coker wax oil. 4. Conclusion
Fig. 6. FT-IR spectra of the samples and as-MCM-41.
In summary, high content nickel mesoporous molecular sieves Ni-MCM-41 was successfully prepared with a novel direct
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Fig. 8. XPS spectra of the sample A and A*.
Scheme 1. The supposed synthetic mechanism of Ni-MCM-41.
Table 2 Results of hydrocracking of the coker wax oil over the samples. Catalysts
A B C D Ni/MCM-41 Si-MCM-41
Conversion/%
83.0 78.3 70.8 70.0 45.9 36.3
Product yield/% Gas
70e120 C
120e160 C
Coke
13.1 12.5 12.4 11.9 9.7 6.9
79.2 74.4 60.1 57.6 20.3 64.1
e 5.9 21.2 23.4 61.4 21.4
7.7 7.2 7.3 7.1 8.6 7.6
hydrothermal method. The synthesized samples possess the ordered mesostructure of MCM-41 with high nickel content. The results indicating that the large amount of nickel atoms were incorporated into the silica framework. The materials exhibit excellent catalytic performance for the hydrocracking of coker wax oil and possess good selectivity for low temperature fraction. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21271017). References [1] C. Kresge, M. Leonowicz, W. Roth, J. Vartuli, J. Beck, Nature 359 (1992) 710e712.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis, characterization and catalytic performance of well-ordered mesoporous Ni-MCM-41 with high nickel content Jing Qin a, Baoshan Li a, *, Wen Zhang a, b, Wei Lv c, Chunying Han a, Jianjun Liu a a
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Yantai Valiant Fine Chemicals Co., Ltd, Yantai, Shandong 264006, PR China c China Huanqiu Contracting & Engineering Corporation, Beijing 100012, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2014 Received in revised form 29 January 2015 Accepted 3 February 2015 Available online 11 February 2015
The well-ordered Ni-MCM-41 with high nickel content was synthesized by a direct hydrothermal method using cetyltrimethylammonia bromide as the structure-directing agent in an ammonia aqueous solution. The resulting samples were characterized by X-ray diffraction, X-ray fluorescence spectroscopy, high-resolution transmission electron microscopy, nitrogen adsorptionedesorption isotherms, H2 temperature-programmed reduction analyses, Fourier-transform infrared spectroscopy, ammonia temperature programmed desorption, and X-ray photoelectron spectroscopy. The results indicated that the material maintained ordered physical mesostructure of MCM-41 with high nickel content, and the nickel atoms were mainly in the framework with tetrahedral coordination. The materials possessed high specific surface area (800e580 m2 g1), large total pore volume (1.05e1.22 cm3 g1) and pore diameter (2.77e3.16 nm). The material exhibited excellent catalytic performance for the hydrocracking of coker wax oil, which was improved with the increasing of Ni content in the framework of the molecular sieves. © 2015 Elsevier Inc. All rights reserved.
Keywords: Nickel-rich molecular sieve Mesoporous molecular sieve MCM-41 Direct hydrothermal method Hydrocracking of coker wax oil
1. Introduction MCM-41 has been extensively studied due to its high specific surface area, large pore volume, regular structure, uniform pore size and high thermal stability [1,2]. However, owing to the neutral character, the pure silica MCM-41 showed very limited catalytic activities. Therefore, many researchers are dedicated to incorporating different heteroatoms into the silica framework to create active sites for catalytic applications. Almost all kinds of transition metals [3-11] and some main group elements [2,10] have been incorporated into the mesoporous molecular sieve frameworks as the active sites to improve their catalytic activities by direct synthesis or impregnation methods [7,11-17]. The substitution of silicon atoms by heteroatoms can increase their Brønsted acid sites or cation exchange capacity [18]. The content of heteroatom incorporated in the silica framework will greatly influence the acidic strength, ion-exchange capacity [19] and the catalytic performance [20]. For instance, Chen et al. [21] reported a nickel-incorporated MCM-41 presented a good activity for the single-wall carbon nanotube synthesis by CO disproportionation. Wang et al. [22]
reported a direct synthesis method for preparing high nickelcontaining MCM-41-type mesoporous silica and the nickel content was up to 11.8 wt.%. In our previous study [23], the high content of 13.21 wt.% Febased MCM-41 in the framework with tetrahedral coordination was successfully prepared by a direct synthesized method. The material exhibited high catalytic performance for the oxidative desulfurization (ODS) of dibenzothiophene (DBT) from fuel oils. Nickel-containing molecular sieves have been used in catalytic fields, such as desulfurization [22], hydrogenation [23,24] and oxidation [7] etc. In the present work, a detailed synthesis procedure was reported for preparing high content nickel atoms NiMCM-41 mesoporous materials in the framework and the nickel content can be up to 16.3 wt.%. The prepared samples featured high specific surface area, large pore volume and pore diameter. The samples also exhibited excellent catalytic performance for the hydrocracking of coker wax oil. 2. Experimental 2.1. Materials
* Corresponding author. Tel./fax: þ86 10 64445611. E-mail address: [email protected] (B. Li). http://dx.doi.org/10.1016/j.micromeso.2015.02.009 1387-1811/© 2015 Elsevier Inc. All rights reserved.
Nickel nitrate hexahydrate [Ni(NO3)2$6H2O] (A.R., Beijing Yili Fine Chemical Product Limited Company, China) was used as Ni
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source. Cetyltrimethylammonium bromide (CTAB, A.R., Tianjin Jinke Fine Chemical Institute, China) was used as the structural template. Tetraethyl orthosilicate (TEOS, A.R., Beijing Chemical Factory, China) was used as Si source and ammonia solution (25%, Beijing Beihua Fine Chemical Product Limited Company, China) was used as a ligand. All solvents and reactants are commercially available and were used without further purification. 2.2. Synthesis of the materials The Ni-MCM-41 mesoporous material was prepared through a direct hydrothermal synthesis method, and the detailed process is as following: 2.35 g of CTAB was dissolved in 100 ml of deionized water in a flask with stirring vigorously for 30 min to get a clear solution. A certain amount of Ni(NO3)2$6H2O was dispersed in 20 ml of deionized water in a beaker and 15 ml of ammonia solution (25%) was added to obtain a complex Ni(NH3)2þ 6 solution. Then, the solution was added into the flask. After stirring vigorously for 30 min, 10 ml of TEOS was dropped into the mixture and stirred for another 30 min. The pH was adjusted to 10.0 by using an ammonia solution, and the final mixture was stirred for 5 h. The molar composition of the mixture was 1.0 (TEOS):0.152 (CTAB):2.8 (NH3):x (Ni):141.2 (H2O). Then, the final mixture was transferred into a Teflon-lined stainless steel autoclave under the autogenous pressure at 383 K for 48 h. The samples were filtered, washed with deionized water, dried for overnight at 323 K and finally calcinated at 823 K in air for 6 h. The result Ni-MCM-41samples are designated A, B, C and D with the Si/Ni molar ratio of 5, 10, 20 and 30, respectively. The pure silica MCM-41, designated E was synthesized with the same procedure only without adding Ni(NO3)2$6H2O. The impregnated Ni/MCM-41 (Si/Ni ratio is 5) was prepared according to the following procedure: 1.0 g of pre-synthesized E powder was dispersed into the nickel nitrate solution to obtain a suspension, which was kept stirring at 323 K to improve the mixing and to make the water evaporation in a very slow rate. The resulting material was dried at 323 K overnight and calcinated in air at 823 K for 6 h. The final products were designated as A*. 2.3. Characterization of the samples X-ray diffraction (XRD) characterization was performed with a Rigaku D/Max 2500 VBZþ/PC diffractometer using Cu-Ka radiation (40 kV, 200 mA) in both low angle (2q range 0.5e10 ) and wide angle (2q range 10e80 ). Chemical compositions of the synthesized samples were determined by X-ray fluorescence analysis spectroscopy (XRF) on a Philips Magix-601 X-ray fluorescence spectrometer and the samples were pressed into pellets beforehand. Pore morphology of the samples was examined by high-resolution transmission electron microscopy (HRTEM) on a Jem-3010 with an accelerating voltage of 200 kV. The samples were prepared by ultrasonic dispersion, using absolute alcohol as solvent and copper grids as support membranes. Nitrogen adsorptionedesorption isotherms were determined on a Micromeritics ASAP 2020 M volumetric adsorption analyzer. Before nitrogen adsorption, each sample was degassed in a vacuum at 473 K for 6 h. The pore size distribution was carried out on desorption branch of the isotherm using BarretteJoynereHalenda (BET) method and the specific surface area was calculated using BrunauereEmmetteTeller (BET) equation. H2 temperature-programmed reduction (H2-TPR) analyses was finished using a thermal conductivity detector (TCD) in 10 vol.% of hydrogen in argon in the temperature range 323e1273 K with the heating rate of 10 K min1 . Before testing, the samples were pretreated in argon at 573 K for 2 h. Fourier-transform infrared (FT-IR) spectra were scanned in KBr pellets on a Bruker
VECTOR 22 spectrometer in the range 4000e400 cm1. Acid properties of the samples were determined by ammonia temperature programmed desorption (NH3-TPD) in a TP-5080 system using He as carrier gas. Previously, the samples were pretreated at 573 K for 1 h in He flow. After cooling to 300 K, an ammonia flow of 35 ml min1 was passed through the sample for 0.5 h, and then the gaseous and weakly adsorbed NH3 was purged with He. The TPD was conducted from 300 K to 1000 K at a heating rate of 10 K min1. X-ray photoelectron spectroscopy (XPS) analyses were conducted on an ESCALAB 250 spectrometer equipped with an Al-Ka X-ray source. The carbon 1s peak at 284.6 eV was used as the reference for binding energies. 2.4. Catalytic performance test Catalytic hydrocracking performance of the materials were conducted in a 250 ml high pressure reactor with a magnetic drive stirring machine, with the coker wax oil as the raw material. The reaction temperature was set at 573 K and the hydrogen pressure was 3.0 MPa. The catalyst dosage was 0.5 g per 50 g of oil, and the reaction time was 60 min, the system was cooled and separated into gaseous and liquid products, using a gas-collector and a glass liquid collector, respectively. The products were separated by distillation for getting different fractions. For analyzing the coke yield, the residue after the distillation was washed with toluene and the catalyst was dissolved in a HF solution at room temperature. After the catalyst solved, the coke was filtrated, washed with deionized water, dried in an oven at 373 K, and then weighed to calculate the rate of the residual carbon. 3. Results and discussion The low-angle XRD patterns of the samples are shown in Fig. 1. It is clear that the sample A shows a strong (1 0 0) peak, and the sample B shows a broad one. For the other samples (C, D, and E), another two diffraction peaks (2 0 0) and (2 1 0) appear, indicating that the samples possess relative perfect structure of MCM-41. With the increase of nickel content, the diffraction peaks representing the ordered mesoporous structure are widened and shifted to the
Fig. 1. Low-angle XRD patterns of the sample A, B, C, D, and E.
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smaller angles, i.e. the 2q degree of diffraction peak (1 0 0) was shifted to 1.79 , 1.97, and 2.22 for the sample A, B, and C, respectively, relative to the sample D and E at 2.26 (pure silica MCM-41). This is because the increase of the pffiffiffiinterplanar spacing d100 and cell parameter values a0 (a0 ¼ 2d100 3) (listed in Table 1) induced by the bigger Ni atoms substitute for the smaller Si atoms, which is normally attributed to the incorporation of Ni atoms into the silica frameworks [25,26]. The wide-angle XRD patterns of the samples A and A* are shown as Fig. 2. It is clear that Ni/MCM-41 displays the well-resolved characteristic diffraction pattern of nickel oxide, indicating that a lot of nickel oxide particles are located [27] in the A* sample. However, the sample A has not showed these peaks, illustrating the phase separation between NiO and SiO2 did not happen, while a little insignificant asymmetric reflection of nickel phyllosilicate [28] appeared. These results illustrate that the nickel oxide particles were formed on the sample A* and there was no these particles in the sample A except a little amount of nickel species were in the form of nickel phyllosilicate. This indicates that the Ni atoms were highly dispersed in the silica framework for the sample A. The chemical composition of the samples was measured by XRF, and the results are listed in Table 1. The results indicated that the actual content of nickel in each of the samples is 16.3%, 8.8%, 4.7%, 3.8%, and 15.8 wt. % for the sample A, B, C, D, and A*, corresponding to the molar ratio of Si/Ni ¼ 5, 10, 20, 30, and 5, respectively. This illuminates that the content of Ni in the framework of Ni-MCM-41 can be in a wide range. Fig. 3 shows the micrographs of the samples A, B, C, D, E and A*. The well-oriented mesoporous channels are observed clearly in all the micrographs, indicating that the samples retain highly ordered two-dimensional hexagonal structure, even with the high Ni content, which is consistent with the wide-angle XRD result, suggesting that the phase separation between NiO and SiO2 did not happen and the nickel atoms are mostly introduced into the silica framework [29] in Ni-MCM-41 samples. However, there are large NiO particles on the surface of sample A* that is very different from the others. It is also clear that the regularity of samples is decreased with the increase of nickel incorporation, which is in agreement with the results of the low-angle XRD results. As the nickel content increasing, more silicon atoms are substituted for nickel atoms, which lead to the degradation of the pore system. The N2 adsorptionedesorption isotherms are shown in Fig. 4a. It shows that the isotherms of the samples are type IV and possess type H1 hysteresis loops. It is indicated that the samples are typical mesoporous materials, exhibiting a sharp characteristic of capillary condensation at intermediate partial pressures (0.3 < P/P0 < 0.5). In the relative pressure range 0.90e0.99, all samples show another sharp with the increase of the nitrogen adsorption volume, resulting from filling the macropores formed by interparticle spaces and untransformed amorphous silica [30]. The texture properties of the samples are summarized in Table 1. It is clear that the pore diameter (Dp), pore wall thickness (Wt) and total pore volume (Vp) increase with the progressive incorporation
Table 1 Physical structural data of the samples. Sample Si/Ni Ni/wt% d100/nm a0/nm SBET/m2 g1 Dp/nm Wt/nm Vp/m3 g1 A B C D A* E
5 10 20 30 5 e
16.3 8.8 4.7 3.8 15.8 0
4.92 4.48 3.98 3.88 e 3.04
5.68 5.17 4.60 4.48 e 3.51
580 748 774 800 553 1082
3.16 3.08 2.77 2.77 3.01 2.53
2.52 2.09 1.83 1.71 e 0.98
Dp: calculated using the BJH method; Wt: calculated as Wt ¼ a0 Dp.
1.10 1.22 1.11 1.05 0.56 0.97
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Fig. 2. Wide-angle XRD patterns of the sample A and A*.
of nickel atoms into the silica framework of MCM-41. Meanwhile, the expansion of unit cell is just because of the substitution of smaller silicon atom (Si4þ ion radius, 0.41 A ) with bigger nickel atom (Ni2þ ion radius, 0.69 A ) [27], which are in good agreement with the as-mentioned XRD results. On the contrary, the specific surface area (SBET) of the sample decreased with the nickel content increases, which was induced mainly by the crescent density with the heavy Ni atoms introduced. In addition, the pore size distributions were widened with the Ni content increase (Fig. 4b), illustrating a relative loss of order and regularity in the pore system, which is in good agreement with the XRD and HRTEM results above. H2-TPR is a very convenient way to study the reduction behavior of supported oxide catalysts. The H2-TPR curves of the samples are shown as Fig. 5. It is obviously that there was a large reduction stage over the range 930e1155 K in all curves, suggesting that the Ni species were mainly existence in the framework with high stability, indicating that most of Ni atoms were incorporated into the framework of the materials. In addition, for the sample A*, two large reduction peaks are obtained over the range 600e720 K and 840e930 K centers at 658 K and 873 K, respectively. The lowtemperature reduction peak coincides with the reduction of bulk NiO [27,31]. The higher temperature reduction peak is associated with the presence of very small NiO particles strongly interacted with SiO2 framework formed nickel silicate during the calcination process [24,25,28]. Two weak reduction pecks over the range 600e720 K and 840e930 K also appearance in the H2-TPR curves of the A and B samples, indicating that there is a very low content of bulk NiO and nickel silicate. For the samples C and D, the lowtemperature reduction peak at ca. 658 K was absent, indicating that the bulk NiO was not present. But a weak reduction peak over the higher temperature range 840e930 K (ca. 880 K) also appears in the curve of sample C, indicating that there was a tiny of nickel silicate in the sample. And for the sample D, the peak was absent indicating there were no NiO and nickel silicate present in the sample D. The results illuminate that the Ni atoms can be highly dispersed mainly in the framework of Ni-MCM-41, which is agreement with the XRD results. FT-IR spectra of the samples are presented in Fig. 6 (as-MCM-41 is the pure silica MCM-41 before calcination). The broad
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Fig. 3. HRTEM images of the samples A, A*, B, C, D and E.
asymmetric yas(SieO) vibration band at around 1084 cm1, which is characteristic peak of the silica network, and can be observed in all the samples. The band at about 460 cm1 can be attributed to the bending vibrations of SieOeSi groups. The band at about 550 cm1 can be attributed to the bending vibrations of eNieOeSie [32,33], which is the one evidence for the heteroatom constructed into the framework of MCM-41. In addition, the sample A has two small peaks at about 710 cm1 and 670 cm-1, which can be assigned to the dOH vibration of isolated eOH surrounded by three Ni atoms and a tetrahedral SiO mode [34], respectively. The two bands also were considered as the nickel silicate formation [35], and the very weak peaks indicate the presence of nickel silicate with a very small amount, which is agreement with the XRD and H2-TPR results. Except the as-MCM-41, the bands at about 2925 cm1, 2854 cm1 and 1480 cm1 were absent in all the Ni-CMM-41 and Ni/CMM-41 samples, illuminating that the organic template has been effectively removed [29,36]. Acidity of the samples was characterized by NH3 - TPD and the results are shown in Fig. 7. It can be seen that the sample A, B, C, D and A* all show a peak at low temperature about 430e470 K, which
corresponds to the weak acid sites. In addition, the pure Si-MCM-41 has no desorption peak [37], which in agreement with the test results, so this kind of acid sites can be attributed to the nickel atoms in the sample. The curves A, B, C and D also show a wide peak at relative higher temperature range about 470e650 K, which can be designated to the middle acid sites. This peak is attributed to the nickel atoms introduced in the framework. The total acidity was mainly calculated based on the amount of strongly adsorbed ammonia molecules and the values were found to be 0.71, 0.43, 0.32, 0.26 and 0.14 mmol/g for sample A, B, C, D and A*, respectively. Additionally, it is obvious that the total acid concentration of the material was increased with the increase of Ni content. The acid strength and concentration determined the catalytic activity and selectivity. Fig. 8 shows the XPS spectra of samples A and A*, and the inset is the de-convoluted Ni 2p3/2 spectra. As shown in Fig. 8A, there is a predominant peak centered at 857.4 eV, indicating the strong interaction between nickel and the silica base, suggesting a large amount of nickel incorporating into the silica framework, which corroborates the previous results; But for Fig. 8A*, a peak centered
Fig. 4. Nitrogen adsorptionedesorption isotherms (a) and BJH pore diameter distributions (b) of the samples.
J. Qin et al. / Microporous and Mesoporous Materials 208 (2015) 181e187
185
Fig. 7. NH3-TPD curves of the samples.
shown in Table 1. At the same time, too much nickel atoms incorporated would destroy the mesostructure in a certain extent. Fig. 5. H2-TPR curves of the samples.
at 855.1 eV was appearance, which is the characteristic peak of NiO, indicating the present of NiO in the sample A*. Based on the above characterizations, the formation mechanism of the samples was conformed to the mechanism SþIeIþI, which is raised by Guo et al. [5]. And the preparation method takes advan4 þ þ tage of using CTAþ, SiO44, Ni(NH3)2þ 6 , and SiO4 as S , I , I , and I of the proposed SþIIþI mechanism, respectively. The synthetic mechanism can be revealed in Scheme 1. In the mixture solution with plenty of ammonia, added TEOS hydrolyzes immediately and polymerize, followed by the formation of mesostructured materials. Ni2þ ions polymerize much more weakly than Si species; therefore, more Si species polymerize to stabilize the resulting mesostructure. With the increase of nickel content, a lot of Ni(NH3) 62þ weakened the electric interaction between CTAþ and inorganic species SiO44, leading to thicker pore wall and bigger pores, just as
3.1. Catalytic performance of the samples The catalytic performance of the materials for hydrocracking of coker wax oil was investigated. All the results are shown in Table 2. It is clear that the liquid products were just two fractions which was the low temperature fraction (70e120 C) and the high temperature fraction (120e160 C) respectively. Comparing with the Ni/ MCM-41 and Si-MCM-41, the samples Ni-MCM-41 exhibited excellent conversion, higher selectivity for gasoline and lower coke yield. The good catalytic activity is attributed to the sample possessing uniform channels, appropriate pore diameter and active sites caused by introducing a large number of Ni atoms into the silica framework. It is clear that the conversion is increasing with the increase of Ni content. As we have known, there are a lot of alkaline nitrogen compounds existing in the oil sample, and they are more attractive to the active sites of catalyst than polycyclic aromatic hydrocarbons. So sample A possesses most active sites and exhibited the best conversion, which is strongly influenced by the nickel content. Meanwhile, the selectivity of the samples is also related to the amount of nickel atoms introduced into the samples and the uniformity of the channels. The low temperature fraction yield of sample A is higher than other catalysts, and the selectivity of high temperature fraction is almost zero, which means that it is very active in hydrocracking of coker wax oil into low temperature fraction. Furthermore, gas and coke come from the secondary reactions, due to the limited outward diffusion of the produce molecules [38]. The gas and coke yield of this reaction is because the mesopores in the catalysts can accelerate the diffusion of the product molecules, and then the secondary reactions can lead to gas and coke formation are decreased. The slightly higher gas and coke yield of sample A is because the mesopores in the catalyst is a little more irregular than the other catalysts, which can be improved by the pore diameter distribution curves. From the results of hydrocracking, the Ni-MCM-41 mesoporous molecular sieves with high nickel contents have good catalytic performance for hydrocracking of the coker wax oil. 4. Conclusion
Fig. 6. FT-IR spectra of the samples and as-MCM-41.
In summary, high content nickel mesoporous molecular sieves Ni-MCM-41 was successfully prepared with a novel direct
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Fig. 8. XPS spectra of the sample A and A*.
Scheme 1. The supposed synthetic mechanism of Ni-MCM-41.
Table 2 Results of hydrocracking of the coker wax oil over the samples. Catalysts
A B C D Ni/MCM-41 Si-MCM-41
Conversion/%
83.0 78.3 70.8 70.0 45.9 36.3
Product yield/% Gas
70e120 C
120e160 C
Coke
13.1 12.5 12.4 11.9 9.7 6.9
79.2 74.4 60.1 57.6 20.3 64.1
e 5.9 21.2 23.4 61.4 21.4
7.7 7.2 7.3 7.1 8.6 7.6
hydrothermal method. The synthesized samples possess the ordered mesostructure of MCM-41 with high nickel content. The results indicating that the large amount of nickel atoms were incorporated into the silica framework. The materials exhibit excellent catalytic performance for the hydrocracking of coker wax oil and possess good selectivity for low temperature fraction. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21271017). References [1] C. Kresge, M. Leonowicz, W. Roth, J. Vartuli, J. Beck, Nature 359 (1992) 710e712.
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