Al Ratios on the Mesopore Distributions of

Dec 18, 2013 - The silicon/aluminum ratio is proved to be a key parameter for mesoporosity tuning in the synthesis of hierarchical MFI zeolites by org...
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Influences of Si/Al Ratios on the Mesopore Distributions of Hierarchical MFI Zeolites Synthesized by Organosilane Surfactant Lili Yu,†,‡ Shengjun Huang,*,† Shu Miao,† Xiangxue Zhu,† Shuang Zhang,† Zhenni Liu,† Wenjie Xin,† Sujuan Xie,† and Longya Xu*,† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China University of Chinese Academy of Sciences, Beijing 100049, China

‡

S Supporting Information *

ABSTRACT: The silicon/aluminum ratio is proved to be a key parameter for mesoporosity tuning in the synthesis of hierarchical MFI zeolites by organosilane surfactant. At the fixed synthesis temperature, the mesopore size distribution evolves from a bimodal type to a uniform mode with the increase of Si/Al ratios. Moreover, different Si/Al ratios result in opposite responses of mesopore distributions to the elevation of synthesis temperature. With the elevated temperature from 140 to 150 °C, the mesopore distribution of low-silica MFI zeolites (Si/Algel = 20) evolves from a bimodal type (centered at 4 and 10 nm) to a relatively uniform mode (centered at 8 nm), whereas the high-silica samples (Si/Algel = 40) exhibit a reverse transformation from a uniform mesopore distribution (centered at 3 nm) to a bimodal mode (centered at 4 and 20 nm). This work demonstrates alternative methods for the mesoporosity tuning of organosilane-directed hierarchical zeolites.

1. INTRODUCTION Microporous zeolites have found wide applications in various refining and chemical industries due to their crystalline framework, uniform pore size, high internal surface area, and tunable acidity. These features provide positive effects such as shape selectivity and environmental benignity. However, the small size of micropores (99.99%, 20 mL/min) at 350 °C for 1 h. After the pretreatment, the sample was cooled and IR spectra were acquired at 30 °C. Ammonia temperature programmed desorption (NH3-TPD) measurements were performed in a homemade flow apparatus using a U-shaped quartz microreactor (4 mm internal diameter). The NH3-TPD experiment was carried out in the range 150−600 °C at a ramp rate of 20 °C/min, and the desorbed ammonia was monitored by a gas chromatograph with a thermal conductivity detector (TCD). Pyridine adsorption measurements (Py-IR) were performed on a Thermo Nicolet Nexos 470 instrument. Prior to

measurement, a sample (10 mg) was pressed into a selfsupported wafer and evacuated in an IR cell (attached to a vacuum line) at 450 °C for 1.5 h up to 10−2 Pa. After the sample was cooled to room temperature, a spectrum was recorded as background. Subsequently, the wafer was exposed to pyridine vapor (equilibrium vapor at 0 °C) for 20 min, followed by outgas at 150 or 350 °C for 30 min. The IR spectrum was collected after cooling to room temperature.

3. RESULTS AND DISCUSSION 3.1. Influence of Si/Algel Ratios at the Same Synthesis Temperature. The influences of Si/Algel ratios are first studied at the synthesis temperature of 145 °C. The adsorption− desorption isotherms of the obtained Z5-x-145 series are shown in Figure 1a. Z5-20-145 shows a type IV isotherm with a narrow hysteresis loop in the range P/P0 = 0.40−0.60 and a wide one in the range P/P0 = 0.65−0.85, respectively. This corresponds to a bimodal-type mesopore distribution in the Barrett−Joyner−Halenda (BJH) curve, centered at 4 and 10 nm, respectively. The enhancement of Si/Algel ratios alters the porosities of Z5-29-145 and Z5-40-145. The narrow hysteresis loop in the range P/P0 = 0.40−0.60 is maintained, whereas the wide one in the higher P/P0 range is significantly reduced in the isotherms of Z5-29-145 and Z5-40-145. This corroborates the variations in their mesopore size distributions (Figure 1b). The typical bimodal-type mesopore distribution is still observed for Z5-29-145, which displays a sharp distribution centered at 4 nm and a diminished distribution centered at 11 nm, respectively. In contrast, the BJH curve of Z5-40-145 shows a relatively uniform mesopore distribution as indicated by its predominant distribution centered at 4 nm and a tiny peak at ∼25 nm. The crystallinity of the resulting hierarchical MFI zeolite also shows a dependence on the Si/Algel ratio. As shown in Figure 2, Z5-20-145 exhibits broad diffraction peaks at 7.99, 8.88, and 23.20° corresponding to the characteristic peaks of MFI zeolites. These broad peaks evolve into the sharp and intensive ones for Z5-29-145 and Z5-40-145, along with the appearance of diffraction peaks in the higher angle region (2θ = 23.37, 23.80, 24.0, and 24.50°). This indicates that the increase of the Si/Algel ratio leads to higher crystallinity at the same synthesis temperature of 145 °C. SEM images show the conspicuous change in morphology, from aggregated globular particles (ca. 1 μm) for Z5-20-145 and Z5-29-145 to large cubiclike particles (ca. 3 μm) for Z5-40-145 with rugged surfaces. TEM images in Figure 3 indicate that the obtained zeolites are of polycrystalline 694

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type. Z5-20-140 displays a sharp mesopore distribution centered at 4 nm and a shoulder peak around 10 nm (Figure 5), respectively. The increase of synthesis temperature induces a growth of the shoulder peak for Z5-20-145, and eventually results in the merge of two peaks in the bimodal distribution into a single peak centered at around 8 nm for Z5-20-150. The mesopore distribution of Z5-40-y samples, however, demonstrates a reverse evolution from the uniform type to the bimodal mode with the elevated synthesis temperature. As shown in Figure 4, the isotherm for Z5-40-140 is characteristic of type IV with a single horizontal hysteresis loop in the range P/P0 = 0.40−0.90. Significant variation is observed in the isotherm for Z5-40-150, which shows a narrow hysteresis loop in the range P/P0 = 0.50−0.75 and a pronounced hysteresis loop in the range P/P0 = 0.85−0.95, respectively. The BJH pore size curves confirm the development of a bimodal type in the mesoporosity of Z5-40-y series. As depicted in Figure 5, only one sharp mesopore peak centered at 3 nm is observed for Z540-140. In the case of Z5-40-145, this sharp mesopore distribution shifts to 4 nm, and a second small bump appears around 25 nm. Further increased synthesis temperature gives rise to a mesopore distribution centered at 4 nm and an obvious shoulder at 20 nm, respectively. The variation of mesopore size distribution is closely related to the assembling state of TPHAC ([(CH3O)3SiC3H6N(CH3)2C16H33]Cl) moieties. The diameters of TPHAC micelle and TPHAC oligomers are ca. 4 and 10 nm, respectively.14 In our opinions, both the synthesis temperatures and Si/Al ratios influence assembling states of the surfactant. Higher synthesis temperatures improve the formation of oligomers for larger mesopores (>4 nm), whereas higher Si/Al ratios result in the preferential formation of TPHAC micelle for small mesopores (4 nm). The final mesoporosity distribution mode is the overall consequence of these two synthesis parameters. Irrespective of the apparent transitions in mesoporosities as discussed above, the overall textural parameters of low-silica Z5-20-y samples are relatively stable as a function of elevated synthesis temperature. For the high-silica counterparts,

Figure 2. XRD patterns of hierarchical MFI zeolites with different Si/ Algel ratios.

framework, and the micrographs with higher magnifications (Supporting Information, Figure S1) prove the presence of mesopores in these samples. 3.2. Influence of Si/Algel Ratios at Different Synthesis Temperatures. Hierarchical MFI zeolites with different Si/ Algel ratios (20 versus 40) were synthesized at the temperatures of 140, 145, and 150 °C, respectively. The mesopore distributions of Z5-20-y and Z5-40-y show opposite variation trends with the elevated synthesis temperature. As shown in the N2 adsorption−desorption isotherms (Figure 4), Z5-20-140 exhibits a characteristic type IV isotherm with a narrow loop in the range P/P0 = 0.40−0.60 and a pronounced loop in the range P/P0 = 0.70−0.90, respectively. With the increase of synthesis temperature for Z5-20-y, the narrow loop becomes reduced whereas the wide loop undergoes further enlargement. Finally, the isotherm is transformed into a typical type IV isotherm with a wide and steep hysteresis loop in the range P/ P0 = 0.70−0.90. The BJH pore size distribution also evidences a transition from a bimodal mesopore distribution to a relatively uniform

Figure 3. (a−c) SEM and (d−f) TEM images of hierarchical MFI zeolites with different Si/Algel ratios. (a, d) Z5-20-145; (b, e) Z5-29-145; (c, f) Z540-145. 695

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Figure 4. N2 adsorption−desorption isotherms of hierarchical MFI zeolites (Si/Al = 20 and Si/Al = 40) synthesized in the temperature range 140− 150 °C.

Figure 5. BJH pore size distribution curves of hierarchical MFI zeolites (Si/Algel = 20 and Si/Al = 40) synthesized in the temperature range 140−150 °C.

to 261 m2/g for Z5-40-150, which counteracts the slight enhancement in microporous surface area and leads to a net decrease in BET surface area. In contrast, the mesopore volume increases steadily from 0.41 cm3/g for Z5-40-140 to 0.52 cm3/g for Z5-40-150. The different Si/Al ratios lead to obvious differences in the crystallinities of the resulting MFI zeolites. As shown in Figure 6, all Z5-20-y and Z5-40-y samples show the major characteristic peaks (2θ = 7.99, 8.88, 23.20, and 23.80°) of MFI zeolites in the XRD patterns, with obvious differences in peak width and intensity. Compared to the wide and broad diffraction peaks for the Z5-20-y sample, much sharper and more intensive ones are indicated for the Z5-40-y series, corroborating higher crystallinities and larger particle sizes as shown in their SEM images (Figure 7). TEM images (Figure 8) reveal the polycrystalline frameworks of Z5-20-y and Z5-40-y series, and their micrographs with higher magnifications (Supporting Information, Figure S2) prove the formation of a highly mesoporous structure. 3.3. Acidity of HZ5-20-y and HZ5-40-y. The abovementioned series of Z5-20-y and Z5-40-y samples are

however, significant variations in textural parameters are observed along with the presence of substantially developed large-sized mesopores. As shown in Table 1, the mesoporous surface area reduces substantially from 346 m2/g for Z5-40-140 Table 1. Textural Parameters of Hierarchical MFI Zeolites Synthesized with Various Conditionsa sample

SBET (m2/g)

Smic (m2/g)

Smeso (m2/g)

Vmic (cm3/g)

Vmeso (cm3/g)

Z5-20-145 Z5-29-145 Z5-40-145 Z5-20-140 Z5-20-150 Z5-40-140 Z5-40-150

512 546 553 523 503 553 491

209 221 217 196 214 207 230

303 325 336 327 289 346 261

0.10 0.10 0.10 0.09 0.10 0.09 0.11

0.56 0.45 0.45 0.55 0.58 0.41 0.52

a

SBET, apparent BET surface area; Sext, external surface area evaluated from the t-plot method; Smic, micropore surface area evaluated from the t-plot method; Vmic, micropore volume calculated from the t-plot method; Vmeso, mesopore volume. 696

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Figure 6. XRD patterns of hierarchical MFI zeolites (a, Si/Algel = 20; b, Si/Algel = 40) with various hydrothermal temperatures.

Figure 7. SEM images of hierarchical MFI zeolites at various hydrothermal temperatures. (a) Z5-20-140; (b) Z5-20-150; (c) Z540-140; (d) Z5-40-150.

Figure 8. TEM images of hierarchical MFI zeolites at various hydrothermal temperatures (a) Z5-20-140; (b) Z5-20-150; (c) Z5-40140; (d) Z5-40-150.

3610 and 3660 cm−1 are barely influenced in HZ5-40-145 and HZ5-40-150 as a function of synthesis temperature. Further information of the acidities is provided by NH3-TPD profiles and Py-IR spectra. As shown in Figure 10a, HZ5-20-y samples display similar profiles with an intensive peak centered at 250 °C and a weak one centered at 425 °C, respectively. In contrast, HZ5-40-y samples display two comparable peaks centered at 250 and 425 °C, respectively, and the latter increases gradually with raised synthesis temperature. The comparison of NH3-TPD profiles suggests that HZ5-40-y samples hold higher portions of strong acid sites than HZ5-20-y samples. As depicted in Figure 11a, pyridine adsorption on the HZ5-20-y samples gives rises to absorption bands at 1540 and 1450 cm−1, characteristic of pyridinium ions interacted with Brønsted acid sites and coordinatively bounded pyridine on Lewis acid sites, respectively. For the HZ5-40-y samples, three absorption bands of pyridine are presented after desorption at 150 °C: the band at 1540 cm−1 for pyridinium ions and two overlapping bands at 1454 and 1446 cm−1,19,20 respectively. The absorption bands at 1446 cm−1 are assigned to the pyridine interacting with silanols, which disappear in the spectra after

transformed into the protonic form to study their acidities. DRIFT IR spectra (Figure 9) indicate their differences in the OH stretching region. As shown in Figure 9a, HZ5-20-140 displays an intensive band at 3740 cm−1, which is characteristic of isolated silanol groups.15,16 This band becomes even stronger in HZ5-20-145 and HZ5-20-150, indicating that the neutral SiOH groups prevail on the surface of HZ5-20-y samples. Although no distinct bands associated with acidic groups are discerned in the spectra, it should be noted that the intensive 3740 cm−1 band may impose a masking effect for the reorganization of other bands. As a comparison, DRIFT IR spectra of HZ5-40-y samples in Figure 9b are more informative. Besides the band of isolated silanols at 3740 cm−1, HZ5-40-140 also shows the characteristic wide bands of hydroxyl nests around 3500 cm−1.17 Moreover, the adsorption bands at 3610 and 3660 cm−1 are attributed to Brønsted acidic bridging hydroxyl groups and OH groups bonded to extralattice aluminum species, respectively.18 A decrease of the broad 3500 cm−1 band is observed from HZ5-40-140 to HZ5-40-150, reflecting partial removal of the hydroxyl nests at the elevated synthesis temperature. Meanwhile, the adsorption bands at 697

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Figure 9. DRIFT IR spectra in OH stretching region of (a) HZ5-20-y and (b) HZ5-40-y..

Figure 10. NH3-TPD profiles of (a) HZ5-20-y and (b) HZ5-40-y.

Figure 11. Py-IR spectra of (a) HZ5-20-y and (b) HZ5-40-y samples after desorption at 150 °C (marked with uppercase letters in solid lines) and 350 °C (marked with lowercase letters in dotted lines). a, A, HZ5-20-140; b, B, HZ5-20-145; c, C, HZ5-20-150; d, D, HZ5-40-140; e, E, HZ5-40145; f, F, HZ5-40-150.

extinction coefficients of the bands at 1540 and 1450 cm−1 has been reported to be about 0.75.21 HZ5-20-y samples just show slightly higher concentrations of total acid sites (0.38−0.33 mmol/g) than those of HZ5-40-y samples (0.30−0.32 mmol/ g) despite their obvious difference in nominal Si/Al ratios. This may be due to the higher crystallinites of HZ5-40-y samples as indicated in XRD patterns (Figure 6). In particular, the concentration of strong Brønsted acid sites on HZ5-40-145 and

desorption at 350 °C. For both HZ5-20-y and HZ5-40-y series, the similarities between their Py-IR spectra after desorption at 150 and 350 °C indicate the presence of strong acid sites on all these samples. Combined with the concentration of strong acid sites determined by NH3-TPD (350−600 °C), the integral intensities of the Py-IR bands at 1540 and 1450 cm−1 after desorption at 350 °C were used to estimate the concentration of strong Brønsted and Lewis acid sites, and the results are summarized in Table 2. The ratio between the integrated molar 698

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Table 2. Concentration of Strong Brønsted and Lewis Acid Sites Determined by Py-IR and NH3-TPD concn strong Brønsted and Lewis acid sites (mmol/g)

a

sample

concn total acid sitesa (mmol/g), 150−600 °C

concn strong acid sitesb (mmol/g), 350−600 °C

CLewis

CBrønsted

HZ5-20-140 HZ5-20-145 HZ5-20-150 HZ5-40-140 HZ5-40-145 HZ5-40-150

0.38 0.36 0.33 0.30 0.32 0.32

0.19 0.16 0.16 0.17 0.19 0.19

0.07 0.07 0.06 0.06 0.07 0.07

0.11 0.09 0.10 0.11 0.12 0.12

The integrated areas between 150 and 600 °C of NH3-TPD curves. bThe integrated areas between 350 and 600 °C of NH3-TPD curves. (3) Perez-Ramirez, J.; Christensen, C. H.; Egeblad, K.; Christensen, C. H.; Groen, J. C. Hierarchical zeolites: Enhanced Utilisation of Microporous Crystals in Catalysis by Advances in Materials Design. Chem. Soc. Rev. 2008, 37, 2530. (4) Vinu, A.; Murugesan, V.; Bohlmann, W.; Hartmann, M. An Optimized Procedure for the Synthesis of AlSBA-15 with Large Pore Diameter and High Aluminum Content. J. Phys. Chem. B 2004, 108, 11496. (5) Sakthivel, A.; Dapurkar, S. E.; Gupta, N. M.; Kulshreshtha, S. K.; Selvam, P. The Influence of Aluminium Sources on the Acidic Behaviour as well as on the Catalytic Activity of Mesoporous HAlMCM-41 Molecular Sieves. Microporous Mesoporous Mater. 2003, 65, 177. (6) Karlsson, A.; Stocker, M.; Schmidt, R. Composites of Micro- and Mesoporous Materials: Simultaneous Syntheses of MFI/MCM-41 Like Phases by a Mixed Template Approach. Microporous Mesoporous Mater. 1999, 27, 181. (7) Xia, Y. D.; Mokaya, R. On the Synthesis and Characterization of ZSM-5/MCM-48 Aluminosilicate Composite Materials. J. Mater. Chem. 2004, 14, 863. (8) Huang, L. M.; Guo, W. P.; Deng, P.; Xue, Z. Y.; Li, Q. Z. Investigation of Synthesizing MCM-41/ZSM-5 Composites. J. Phys. Chem. B 2000, 104, 2817. (9) Kim, J.; Choi, M.; Ryoo, R. Effect of Mesoporosity Against the Deactivation of MFI Zeolite Catalyst during the Methanol-tohydrocarbon Conversion Process. J. Catal. 2010, 269, 219. (10) Park, H. J.; Heo, H. S.; Jeon, J. K.; Kim, J.; Ryoo, R.; Jeong, K. E.; Park, Y. K. Highly Valuable Chemicals Production from Catalytic Upgrading of Radiata Pine Sawdust-derived Pyrolytic Vapors over Mesoporous MFI Zeolites. Appl. Catal., B 2010, 95, 365. (11) Inayat, A.; Knoke, I.; Spiecker, E.; Schwieger, W. Assemblies of Mesoporous FAU-Type Zeolite Nanosheets. Angew. Chem., Int. Ed. 2012, 51, 1962. (12) Li, X. F.; Prins, R.; van Bokhoven, J. A. Synthesis and Characterization of Mesoporous Mordenite. J. Catal. 2009, 262, 257. (13) Majumdar, P.; Lee, E.; Gubbins, N.; Christianson, D. A.; Stafslien, S. J.; Daniels, J.; VanderWal, L.; Bahr, J.; Chisholm, B. J. Combinatorial Materials Research Applied to the Development of New Surface Coatings XIII: An Investigation of Polysiloxane Antimicrobial Coatings Containing Tethered Quaternary Ammonium Salt Groups. J. Comb. Chem. 2009, 11, 1115. (14) Cho, K. G.; Cho, H. S.; Menorval, L.-C.; Ryoo, R. Generation of Mesoporosity in LTA Zeolites by Organosilane Surfactant for Rapid Molecular Transport in Catalytic Applications. Chem. Mater. 2009, 21, 5664−5673. (15) Kolyagin, Y. G.; Ordomsky, V. V.; Khimyak, Y. Z.; Rebrov, A. I.; Fajula, F.; Ivanova, I. I. Initial Stages of Propane Activation over Zn/ MFI Catalyst Studied by in situ NMR and IR Spectroscopic Techniques. J. Catal. 2006, 238, 122. (16) El-Malki, E. M.; van Santen, R. A.; Sachtler, W. M. H. Introduction of Zn, Ga, and Fe into HZSM-5 Cavities by Sublimation: Identification of Acid Sites. J. Phys. Chem. B 1999, 103, 4611. (17) Gallas, J. P.; Goupil, J. M.; Vimont, A.; Lavalley, J. C.; Gil, B.; Gilson, J. P.; Miserque, O. Quantification of Water and Silanol Species

HZ5-40-150 are obviously higher than those on HZ5-20-145 and HZ5-20-150.

4. CONCLUSIONS In the synthesis of hierarchical MFI zeolites by dual templating of TPABr−TPHAC, the influence of Si/Algel ratios on the mesoporosity is studied in the synthesis temperature range 140−150 °C. At a fixed synthesis temperature of 145 °C, the mesopore size distribution of hierarchical MFI zeolites transforms from a bimodal type into a relatively uniform mode with the increase of Si/Algel ratios from 20 to 40. Moreover, opposite mesoporosity developments are observed for the hierarchical zeolites with different Si/Algel ratios from 140 to 150 °C. In the case of low-silica MFI zeolites (Si/Algel = 20), the mesopore distribution evolves from a bimodal type to a relatively uniform mode, whereas the high-silica MFI zeolites (Si/Algel = 40) exhibit a reverse transformation from a uniform mesopore distribution to a bimodal mode. This work demonstrates that the Si/Al ratio can be used as an effective parameter for tuning the mesoporosity of zeolites synthesized by the dual-templating (organosilane−zeolitic SDA) approaches.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1, TEM images of hierarchical MFI zeolites at different Si/Algel ratios; Figure S2, TEM images of hierarchical MFI zeolites at various hydrothermal temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Action Plan of CAS to Support China’s New and Strategic Industries with Science and Technology (2012−2014) and the Knowledge Innovation Program of the Chinese Academy of Sciences (S201041) for financial support.

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REFERENCES

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