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Synthesis of Hollow Nanocubes and Macroporous Monoliths of Silicalite-1 by Alkaline Treatment Chengyi Dai, Anfeng Zhang, Lingling Li, Keke Hou, Fanshu Ding, Jie Li, Dengyou Mu, Chunshan Song, Min Liu, and Xinwen Guo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm401739e • Publication Date (Web): 07 Oct 2013 Downloaded from http://pubs.acs.org on October 7, 2013
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Chemistry of Materials
Synthesis of Hollow Nanocubes and Macroporous Monoliths of Silicalite-1 by Alkaline Treatment Chengyi Dai,a Anfeng Zhang,a Lingling Li,a Keke Hou,a Fanshu Ding,a Jie Li,a Dengyou Mu,a Chunshan Song,a,b* Min Liua and Xinwen Guoa* a State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, P. R. China. Fax: +86-411-84986134; Tel: +86-411-84986133; E-mail:
[email protected] b EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States. Fax: +1-814-865-3573; Tel: +1-814863-4466; E-mail:
[email protected] KEYWORDS. hollow nanocubes, macroporous monoliths, silicalite-1, desilication, competitive adsorption, alkaline treatment. ABSTRACT: A simple strategy involving desilication and re-crystallization of silicalite-1 in tetrapropylammonium hydroxide (TPAOH) solution was successfully developed to prepare hollow zeolite nanocubes and three-dimensionally macroporous zeolite monoliths. Large voids were introduced to silicalite-1 crystals by controlled silicon leaching with OH- and thin intact shells were formed by the re-crystallization of silicon with TPA+. The size of nanocubes could be easily controlled from about 150 to 600 nm by simply adjusting the size of parent silicalite-1. Apart from template function to increase the yield of hollow silicalite-1, TPA+ adsorbed on the zeolite protects the parent crystal surface where the re-crystallization occurred. The size of the mesopores and/or macropores in the hollow zeolite shell can be controlled by varying the amount of competitive Na+ adsorbent added to the TPAOH solution. Furthermore, three-dimensional macroporous zeolite monoliths can be formed when an electrolyte, such as NaCl, was added to the TPAOH solution. When the sample was used as the support for iron based catalyst for hydrogenation of CO2 to hydrocarbons, both the conversion of CO2 and the selectivity of C5+ higher hydrocarbons were improved.
1. INTRODUCTION Hollow materials of nano- and microscopic sizes have recently attracted increasing attention due to their wide applications ranging from versatile microreactors to controlled storage and release containers1-3. The constituents of the hollow spheres include a variety of materials such as polymer4, silica5, 6 , metal7 and metal oxide8. Hollow spheres built of zeolites represent a special group of hollow structured materials because their purely microporous network of pores with molecular dimensions offering an ideal matrix for shape selectivity9, 10. One method for preparing hollow zeolite spheres is the layerby-layer assembly technique, which involves alternative deposition of oppositely charged zeolite nanoparticles and polyelectrolytes onto the polymer templating spheres to form a core-shell composite, followed by removal of the template core to create hollow structure11-13. Another method to synthesize hollow zeolites is alkaline treatment, by which the dissolution of zeolites first occurs at the center of the crystals in the alkaline solution14-16. However, the synthesis of highly uniform hollow zeolites, particularly ones with controlled size, is extremely difficult and remains a grand challenge. Zeolites as supports have attracted interest due to their unique properties (e.g., high thermal stability, regular microporosity, and unique shape selectivity) which can be widely applied in catalytic operations17-19. A major factor governing
the performance of catalytically active particles supported on a zeolite carrier is the degree of dispersion19, 20. A smaller particle size is important as the number of active sites per gram of metal is increased, which in turn, in many cases, leads to enhanced catalytic performance. The functionality of mesoporous zeolites to increase dispersion was established by Christensen et al21. They showed that the introduction of noncrystallographic mesopores into zeolite single crystals (silicalite-1, ZSM-5) may increase the degree of particle dispersion. An attractive method of inducing mesopores is by incorporation of hard or soft template during the synthesis of zeolite22-24, but this method can hardly be applied to zeolite production on large scale25. Desilication is an efficient methodology to create intracrystalline mesoporosity in zeolites14, 15. This method uses pore-directing agents, such as metal complexes or tetraalkylammonium cations, to introduce hierarchical porosity in zeolite in alkaline medium26. However, a common problem is that, after alkaline treatment, the yield of solid crystals and their crystallinity are reduced. Re-crystallization of the zeolite in the presence of basic and organic molecules is a promising and practical method to solve this problem, as illustrated in this work. In the present work, a series of hollow silicalite-1 nanocubes which range from about 150 to 600 nm in size have been synthesized, and different sizes of pores have been introduced in the shell or wall of hollow zeolite nanocubes. Fur-
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thermore, 3D macroporous silicalite-1 monoliths can be synthesized by this new route. The hollow silicalite-1 or macroporous silicalite-1, when used as supports of iron based catalyst, can enhance the dispersion of active phase and increase the surface basicity, which increase the conversion of CO2 and the selectivity of C5+ higher hydrocarbons. 2. EXPERIMENTAL SECTION 2.1 Parent zeolite Silicalite-1 (referred to as P) with various particle sizes was synthesized from the clear solution method27. Typically, 15.4mL of tetraethyl orthosilicate (TEOS) was mixed with a certain amount of tetrapropylammonium hydroxide (TPAOH) solution. The molar composition of the synthesis mixture was 1SiO2 : xTPAOH : 4EtOH : 46H2O (x=0.17-0.27). After being stirred for 5 h at 308 K, the gel was transferred into a 100 mL Teflon-lined steel autoclave and crystallized at 443 K for 3 days. The product was recovered by centrifugation and dried overnight at 373 K. Finally, the template was removed by calcination in static air at 813 K for 6 h. 2.2 Alkaline treatments Alkaline treatments were performed under the conditions outlined in Table 1. In a typical experiment, a 4.0 g zeolite sample was added to 40 mL alkaline solution. The mixture was mixed and heated in an autoclave at 443 K under stirring conditions for 72 h. Afterwards, the resulting solid was recovered by centrifugation, washed with distilled water, and dried overnight at 373 K. Finally, the template was removed by calcination in static air at 813 K for 6 h. Table 1 Notation of the samples and treatment conditions. Sample code
Reagent
C[M]
AT-1
TPAOH
0.20
AT-2
TPAOH
0.30
AT-3
TPAOH
0.50
AT-4
TPAOH+NaOH
0.25+0.05
AT-5
TPAOH+NaOH
0.20+010
AT-6
TPAOH+NaOH
0.15+0.15
AT-7
TPAOH+NaOH
0.10+0.20
AT-8
TPAOH+NaOH
0.05+0.25
AT-9
TPAOH+NaCl
0.30+0.10
AT-10
TPAOH+NaCl
0.30+0.30
AT-11
TPAOH+NaCl
0.30+0.50
AT-12
TPAOH+NaCl
0.30+0.70
Treatment conditions: temperature 443 K, time 72 h. 2.3 Preparation of Ag/solid silicalite-1 and Ag/hollow silicalite-1 Ag/solid silicalite-1 was prepared by the incipient-wetness impregnation method of aqueous solution of AgNO3 (99.0%) onto solid silicalite-1 sample at room temperature for 12 h and the content of Ag in the Ag/solid silicalite-1 sample is 2 wt %. The sample was dried at 373 K overnight, and calcined in a
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muffle furnace at 813 K for 4 h. Then, the sample was treated by 0.3 M TPAOH at 443 K for 72 h, which resulted in the sample of Ag/hollow silicalite-1. 2.4 Preparation of silicalite-1 supported catalysts The sample of P, AT-2 and AT-11 were used as the support materials. The FeK/S-1 catalyst was prepared by the incipientwetness impregnation method using aqueous solutions of iron and potassium nitrates (Fe(NO)3.9H2O and KNO3), to abtain 15 wt % of Fe and 5 wt % of K loading. The catalyst was obtained after drying at 373 K overnight followed by calcination in air at 813 K for 4 h to decompose iron and potassium nitrates, for which the heating rate was 2 K min-1. 2.5 Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2400 diffractometer with Cu Ka radiation (λ=1.5406 Å) source. The spectra were recorded from 2θ=50 to 500 (or 800) with 0.02 step size. The crystallite phases were identified by comparing the diffraction patterns with the data of the Joint Committee on Powder Standards (JCPDS). Transmission electron microscopy (TEM) images were taken on Tecnai G2 20 S-twin instrument (FEI Company) with an acceleration voltage of 200 kV. The samples for TEM analysis were prepared by dipping the carbon-coated copper grids into the ethanol solutions of the samples and drying at ambient conditions. Scanning electron microscopy (SEM) images were obtained on a Hitachi S-5500 instrument with an acceleration voltage of 3 kV. Some samples were sputtered with a thin film of gold. N2 isotherms at 77 K were measured in a Quantachrome autosorb-iQ2 gas adsorption analyzer. Prior to the measurement, the samples were degassed in low vacuum (p/p0＜10-4) at 573 K for 6 h, followed in high vacuum (p/p0＜10-7) at 573 K for 4 h. The Brunauer-Emmett-Teller (BET) method was applied to calculate the total surface area, while the t-plot method was used to distinguish between micro- and mesoporosity. In the tplot, the reported mesopore surface area (Smeso) consists of contributions from the outer surface of the particles as well as mesopores and macropores. Solid-state NMR spectra were recorded on a Bruker AvanceIII 600 spectrometer. 1H→13C CP/MAS NMR spectra were acquired at 150.9 MHz and a spinning rate of 6 kHz, with a contact time of 3 ms, a recycle delay of 2 s, using a 4 mm MAS probe. The chemical shifts were referenced to adamantane with the upfield methine peak at 29.5 ppm. H2-TPR measurements were carried out with ChemBET Pulsar TPR/TPD equipment (Quantachrome, USA) to analyze the reducibility of the calcined catalysts. Prior to the reduction, the calcined sample (0.10 g) was placed in a quartz tube in the interior of a controlled oven. The sample was flushed with high purity argon at 573 K for 8 h to remove water and other contaminants then cooled down to room temperature. A gas mixture containing 5 vol% H2 in Ar was passed through the sample at a total flow rate of 30 ml min-1 with a heating rate at 10 K min-1 up to 1152 K. A cooling trap was placed between the sample and the detector for removal of released water formed during the reduction process.
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CO2-TPD measurements were conducted in the same equipment as TPR with helium as the carrier gas. About 0.20 g of catalyst was placed in the reactor. The sample was reduced with 5% H2/95% Ar (30 ml min-1) at temperature of 773 K for 2 h. The catalyst was subsequently flushed at the same temperature with helium gas (30 ml min-1) for 30 min. After re-
duction, the sample was cooled to 303 K, and then CO2 flow (30 ml min-1) was continued for 30 min at 303 K. After adsorption, the system was purged with helium gas (30 ml min-1) for 30 min to remove the weakly adsorbed species. The CO2TPD profile was monitored using TCD while the temperature was increased from 303 to 825 K at a rate of 10 K min-1.
Table 2 Treatment yields and properties of samples Sample
Yield[a] [%]
Crystallinity[b] [%]
Smeso[c] [m2g-1]
SBET[d] [m2g-1]
Vmicro[c] [cm3g-1]
Vpore[e] [cm3g-1]
P
100
100
186
482
0.15
0.55
AT-2
90
84
182
479
0.15
0.91
AT-4
91
81
142
409
0.14
0.77
AT-5
90
73
154
393
0.13
0.72
AT-6
92
80
168
388
0.12
0.65
AT-7
93
76
170
393
0.12
0.62
AT-8
92
75
175
345
0.09
0.62
AT-9
92
59
166
402
0.13
0.91
AT-10
93
81
140
335
0.11
0.70
AT-11
91
73
131
375
0.13
0.78
AT-12
90
85
176
389
0.12
0.82
15Fe5K/P
--
71
88
226
0.07
0.28
15Fe5K/AT-2
--
69
76
233
0.08
0.35
15Fe5K/AT-11
--
65
74
223
0.07
0.37
[a] Yield with respect to the parent zeolite; [b]Determined by X-ray diffraction (XRD); [c] t -plot method; [d] BET method; [e] Volume adsorbed at P/P0= 0.995. 2.6 Catalytic testing The catalytic hydrogenation of carbon dioxide was carried out in a pressurized fixed-bed flow reactor where 1.0 g catalyst was pretreated by reduction with pure H2 at 773 K overnight. After the reduction, the feed gas was changed to the mixture of carbon dioxide and hydrogen under the reaction conditions of n (H2)/n (CO2) =3.0 (molar ratio); P=3.0 MPa; T=673 K and the space velocity was 1800 mlg-1h-1. The products were analyzed on-line by a gas chromatograph (FULI GC 97). Carbon monoxide, carbon dioxide and methane were analyzed on a carbon molecular sieve column with a thermal conductivity detector (TCD) while methane and C2C8 hydrocarbons were analyzed with a flame ionization detector (FID) with a HayeSep Q column. Chromatograms were correlated through methane and product selectivity was obtained based on carbon.
3. RESULTS AND DISCUSSION 3.1 Synthesis of hollow Silicalite-1 nanocubes with different sizes Figure 1 presents the TEM images of the parent and alkaline-treated silicalite-1. The parent samples are monodisperse solid crystals, which have regular morphology and uniform dispersion, with dimensions of 120×180 nm (Figure 1 P). After alkaline treatment, a large regular void is created in the interior of the silicalite-1 and the thickness of the shell is about 10 nm. The hollow silicalite-1 synthesized (such as Figure 1 AT-1) can be called nanocube. Although the concentration of TPAOH used in the alkaline treatment increased from 0.2 M to 0.5 M, the morphologies of hollow zeolite nanocubes did not change, and the shell was not damaged (Figure 1 AT1-3). Furthermore, the zeolite nanocubes are still silicalite-1 (Figure S2) and the thickness of the zeolite shell can be controlled by regulating the re-crystallization of the zeolite (Figure S3).
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Figure 3 SEM images of the parent silicalite-1 with different sizes, a) 156±26 nm, b) 263±31 nm, c) 338±32 nm, d) 517±48 nm. The average sizes were obtained from SEM images by measuring at least 100 particles.
Figure 1 TEM images of Silicalite-1 after treatment with 0.20 M (AT-1), 0.30 M (AT-2), and 0.50 M (AT-3) TPAOH. The N2 adsorption-desorption isotherm shows the presence of a H2 hysteresis loop with an abrupt step around P/P0=0.45 in the desorption branch (Figure 2). H2-type hysteresis is a result of the voids that can only be accessed via entrances smaller than 4 nm, indeed pointing toward a relatively intact outer surface14. The enhanced N2 uptake at high P/P0 is associated with capillary condensation of nitrogen within the large voids of the zeolite nanocubes15. The calculated specific surface area and pore volume of the nanocubes and parent zeolites are listed in Table 2. The BET surface area and the micropore volume of the sample have almost no change after the alkaline treatment in 0.3 M TPAOH solution, while the pore volume increases from 0.55 to 0.91 cm3g-1. Although there is a large void in the interior of the crystal, the yield of AT-2 is 90%, much higher than the treatment by inorganic bases, because the formation of large voids in the crystals involves both dissolution and re-crystallization of the zeolite in the presence of TPAOH16. 800 700
3
Vads ( cm /g)
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600 500 400
AT-2
300
P
200 100 0 0.0
0.2
0.4
0.6
0.8
Figure 4 SEM images of hollow silicalite-1 with different sizes (AT-2), a) 182±23 nm, b) 303±21 nm, c) 351±30 nm, d) 535±47 nm. The average sizes were obtained from SEM images by measuring at least 100 particles. By the method of alkaline treatment with TPAOH, a series of hollow silicalite-1 nanocubes with different sizes can be synthesized by adjusting the parent particle size. Figure 3 presents the SEM images of the parent silicalite-1 with different sizes. The average particle size of parent silicalite-1 can be controlled from about 130 to 570 nm by simply adjusting the amount of TPAOH during the synthesis process. After alkaline treatment, all of the crystals become nanocubes and no solid silicalite-1 is observed in the sample (Figure S4 and Figure S6). The morphologies of nanocubes are more regular and the sizes are larger than that of the parent samples (Figure 4), which indicates the re-crystallization occurs at the surface of silicilite-1crystals (Figure 5).
1.0
p/p0 Figure 2 N2 adsorption/desorption isotherms at 77 K of Silicalite-1 nanocubes (AT-2) and parent zeolites. The isotherms of AT-2 have been shifted upwards by 150 cm3 g-1.
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Figure 5 TEM images of hollow silicalite-1 with different sizes (AT-2), a) 190 nm, b) 310 nm, c) 380 nm, d) 700 nm. Direct evidence that the re-crystallization occurs at the surface of silicilite-1 crystals was obtained and shown in Figure 6. Ag/solid silicalite-1 has been synthesized by the incipientwetness impregnation method, the content of Ag in the sample is 2 wt % (Figure 6 a). After treatment by 0.3 M TPAOH at 443 K for 72 h, TEM images of Ag/hollow silicalite-1 nanocube at several tilting angles are shown in Figure 6 b, c, d. By rotating one hollow silicalite-1 nanocube around one axis and taking a series of images, whether or not the particles are sitting on the surface or inside the shell could be able to determined28. From Figure 6 a, some particles protrude from the solid silicalite-1. However, there is no particles protrude from the hollow silicalite-1 nanocube (Figure 6 b, c, d). Therefore, after treatment, most particles of Ag are in the shell of hollow silicalite-1 nanocube, which demonstrates that the recrystallization of the silicate-1 takes place on the parent crystal surface.
Figure 7 SEM images of silicalite-1 after alkaline treatment with different TPA+/Na+, AT-2: 0.3M TPAOH, AT4-8: TPA+/Na+=5, 2, 1, 0.5, 0.2. 3.2 Hollow silicalite-1 with mesopores and/or macropores in the shell To investigate the reason why the re-crystallization of silicalite-1 occurs on the surface of the parent crystals, the alkaline solutions, with fixed OH- concentration but different TPA+/Na+, were used to treat the parent sample. The SEM images of the final products are shown in Figure 7. It is clear that pores with different sizes have been created in the shell of hollow silicalite-1 while the MFI-type framework structure of sample is unchanged (Figure S8). The size of the pores increases with the increase of Na+, and most amount of mesopores and/or macropores were obtained in the case of TPA+/Na+=1. When the TPA+/Na+ is greater than 2, most of the pores are smaller than 50 nm, and when the TPA+/Na+ is less than 0.5, there are a lot of hemispherical zeolites in the sample.
Figure 6 TEM images of Ag/solid silicalite-1 (a) and Ag/hollow silicalite-1 nanocube at several tilting angles (b, c, d).
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interconnected with each other, and it also confirms the recrystallization of the sample. 1000 900 800 700 600 500 400 300 200 100 0
AT-8
3
Vads ( cm /g)
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AT-6
AT-4
0.0
0.2
0.4
0.6
0.8
1.0
p/p0 Figure 8 N2 adsorption/desorption isotherms of silicalite-1 after alkaline treatment. AT-4, 6, 8: TPA+/Na+=5, 1, 0.2. The isotherms of each sample are upset vertically by 300 cm3g-1. Figure 8 shows N2 adsorption/desorption isotherms of silicalite-1 after alkaline treatment with different TPA+/Na+. The isotherms change from a type H2 to a type H1 hysteresis loop with an increasing amount of macropores in the shell of nanocubes, and all of the yields of the sample after treatment are more than 90% (Table 2).
80
70 60
50
40
30
20
10
0
-10
ppm Figure 10 1H-13C CP/MAS NMR spectrum of silicalite-1 after alkaline treatment (AT-11) Figure 10 shows the 1H-13C CP/MAS NMR spectrum. The spectrum is the same as those recorded on silicalite-1 samples obtained by direct synthesis. In particular, the signal of the methyl group is split into two components, which means the two different environments for the propyl chains in the MFI framework. As TPA+ cations are too large to penetrate the channels of silicalite-1, organic molecules were necessarily occluded in the framework during crystallization of the zeolite16, which demonstrates that re-crystiallization also occurs in the mixed solution of NaCl and TPAOH.
Figure 9 SEM images of silicalite-1 after alkaline treatment. 3.3 Synthesis of 3D macroporous silicalite-1 It is found that the nanocubes grow together during recrystiallization when an electrolyte, such as NaCl, was added in the solution of TPAOH (Figure 9 AT-9). When the concentration of NaCl is 0.3 M, macropores with sizes between 50 and 100 nm can be found in the shell of aggregated nanocubes (Figure 9 AT-10). And when the concentration of NaCl is higher than 0.5 M, most of the nanocubes have been transformed into a 3D ordered macroporous monolith (Figure 9 AT11-12). All of the samples keep MFI structure. Figure S9 shows the SEM images of the 3D macroporous zeolite monoliths at different magnifications. From the low magnification images (Figure S9 a, b), it can be seen that the macroporous zeolite is polycrystalline. From the high magnification images (Figure S9 c, d), it is evident that most of the macropores are
Scheme 1 Mechanism of silicalite-1 nanocube formation 3.4 Mechanism of controlling the location of desilication and re-crystiallization Zeolite single-crystals contain many Si-OH groups on their surfaces29, which could lose protons in alkaline medium, resulting in highly negatively charged surfaces (-35 mV at pH around 13 for silicalite-1). TPA+ ions are preferentially attached on the surface of zeolite crystals with Si-O- via electrostatic interactions, and reduce the dissolution of the zeolites external surface. Simultaneously, the silicate oligomers are leached from the interior of the crystal, where crystallization was not fully reached and therefore is not protected by TPA+.
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The leached silicate oligomers, then begin to interact with TPA+ aggregates on zeolite surfaces to form a silicate/TPA intermediate state, and re-crystallize at 443 K. Continued desilication and re-crystallization promote the synthesis of hollow silicalite-1 nanocubes (Scheme 1).
Scheme 4 Mechanism of 3D macroporous silicalite-1 formation
Scheme 2 Formation mechanism of silicalite-1 nanocubes with mesoporous and/or macroporous in the shell
Scheme 3 The adsorption of Na+ (a) and TPA+ (b) on the surface of silicalite-1 The concentration and location of TPA+ on the zeolite surface are responsible for the formation of an intact shell. When other positive ions, such as Na+, are present in the alkaline solution, competitive adsorption will occur on the zeolite surface (Scheme 2). There will be some Na+ attached on the surface where the silicate oligomers will not re-crystallize because of the absence of TPA+, and excessive dissolution of the surface will occur. DFT calculations were performed to compare the binding energies of [zeo]- with Na+ and TPA+ at B3LYP/6-311+g(d, p) level including the zero point energy correction by Gaussian03 package30. The binding energy of [zeo]- with Na+ and TPA+ is -130.3 kcal mol-1 and -81.3 kcal mol-1, respectively, which implies a possible interaction between the two positive ions and the [zeo]- (Scheme 3). The electrolyte could make the surface of nanoparticles electrically neutral. When NaCl was added in the solution of TPAOH, silicalite-1 will grow together in the alkali treatment process, and 3D macroporous zeolite monoliths will be synthesized following the desilication and re-crystallization, as illustrated in Scheme 4.
3.5 Hollow and/or 3D macroporous silicalite-1 as the support of Fe-based catalyst for the hydrogenation of carbon dioxide Figure 11 shows the catalytic performances of the silicalite1 supported FeK catalysts (Fe loading 15 wt % and K loading 5 wt %) for CO2 hydrogenation. The 15Fe5K/P (solid silicalite-1 as the support) catalyst provided a CO2 conversion of 39% and selectivities for CO, CH4, C2-C4 and C5+ of 47%, 31%, 19% and 2%, respectively. The use of hollow silicalite-1 (or macroporous silicalite-1) as the support significantly increased the conversion of CO2, decreased the selectivity to CO and increased the selectivity to C5+ higher hydrocarbons. The catalysts were characterized by TEM, XRD, H2-TPR and CO2TPD. The results show that when hollow silicalite-1 or macroporous silicalite-1 were used as supports for iron based catalysts, they can enhance the dispersion of iron oxide which could affect the catalytic performances of FeK/S-1 catalysts. Figure 12 shows that, for the sample with solid silicalite-1 as the support, the metal oxide particles are of 20 nm size and located at the outer surface of the zeolite crystals (Figure 12 a), whereas using hollow and/or macroporous silicalite-1 as the support, the metal oxide particles are dispersed better with a size of
