Raquel
Martínez-Franco
,
Cecilia
Paris
,
Marta E.
Martínez-Armero
,
Cristina
Martínez
,
Manuel
Moliner
* and
Avelino
Corma
*
Instituto de Tecnología Química (UPV-CSIC), Universidad Politécnica de Valencia, Consejo Superior de Investigaciones Científicas, Valencia, 46022, Spain. E-mail: acorma@itq.upv.es; mmoliner@itq.upv.es
First published on 8th October 2015
An efficient synthesis methodology to obtain homogeneous nanosized high-silica Beta zeolites (∼10–20 nm) with high solid yields (above 95%) using simple alkyl-substituted flexible dicationic OSDAs is described. These dicationic OSDAs allow the synthesis of nanosized Beta zeolites with different Si/Al ratios (15–30) in alkaline and fluoride media, resulting in nanocrystalline materials with different physico-chemical properties. These nanosized Beta zeolites show better catalytic behavior towards the industrially-relevant alkylation of benzene with propylene to obtain cumene compared with other commercially available nanosized Beta zeolites.
Beta is an industrially relevant zeolite with catalytic applications in petrochemistry,2 fine chemistry,3 biomass-transformations,4 and environmental chemistry.5 This broad applicability can be explained by the combination of a very open crystalline structure, formed of three-directional interconnected large pores,6 and the ability to be synthesized under very broad chemical compositions.6,7
The synthesis of nanocrystals of Beta zeolite that present large external surface area and low diffusion path lengths is of particular interest in order to improve the accessibility of reactants to the catalytic active sites.1 In this sense, nanocrystalline high-silica Beta zeolites with particle sizes below 100 nm have been described in the literature following different synthesis procedures. The first nanocrystalline high-silica Beta with crystal sizes in the 10 to 100 nm range was prepared by hydrothermal synthesis using tetraethylammonium hydroxide (TEAOH) as an organic structure directing agent (OSDA) in the absence of alkali metal cations.8 This methodology allows the synthesis of nanocrystalline Beta zeolites with different Si/Al ratios (from 6 to 50), but with a low solid yield (∼50%) for samples synthesized with Si/Al ratios above 10.8 Similar results have been described by other authors using TEAOH as an OSDA under related hydrothermal synthesis conditions.9 The synthesis of mesoporous high-silica Beta zeolites (Si/Al ∼ 8–30) formed by the assembly of uniform nanocrystals (20–100 nm) with high solid yields (∼80–90%) has also been described using TEAOH as the OSDA following steam-assisted conversion (SAC) procedures.10 However, SAC methodologies are difficult to scale-up for industrial applications. Recently, high-silica nanocrystalline Beta zeolites with intercrystalline mesoporosity have been described with good solid yields through hydrothermal synthesis methods,11 but they require the use of rigid and bulky organic compounds containing phenyl or biphenyl groups, such as 4,4′-trimethylenebis(N-methyl,N-benzyl-piperidinium),11a or 3,10-diazoniabicyclo[10.2.2]hexadeca-12,14,15-triene-3,3,10,10-tetramethyl-dichloride,11b,c and cationic polymers, such as poly diallyldimethylammonium chloride.11d Therefore, the efficient synthesis of high-silica nanocrystalline Beta zeolites in high-solid yield using simple OSDAs is still a challenging and relevant issue.
Here, we present an efficient synthesis procedure that affords nanosized high-silica Beta zeolites (∼10–20 nm) with solid yields above 95%, with simple alkyl-substituted flexible dicationic OSDAs (see Fig. 1) under hydrothermal synthesis conditions. Then, nanosized Beta zeolites with different Si/Al ratios (15–30) could be obtained with dicationic OSDAs in alkaline and fluoride media, and materials with different physico-chemical properties have been obtained. The catalytic behavior of the different nanosized Beta zeolites has been evaluated for an industrially-relevant chemical process, the alkylation of benzene with propylene to obtain cumene, revealing significantly improved catalytic activity compared to commercially available nanocrystalline Beta zeolites.
For its use in the synthesis of zeolites, the final product was ion exchanged to the hydroxide form using a commercially available hydroxide ion exchange resin (Dowex SBR).
For its use in the synthesis of zeolites, the final product was ion exchanged to the hydroxide form using a commercially available hydroxide ion exchange resin (Dowex SBR).
Finally, the gels were transferred to Teflon lined stainless autoclaves and heated at 150 °C for 10 days. The solids were recovered by filtration, extensively washed with distilled water, and dried at 90 °C overnight. The samples were calcined in air at 550 °C for 4 hours. The resultant solid yields have been calculated based on silica + alumina conversion.
The chemical analyses were carried out on a Varian 715-ES ICP-Optical Emission spectrometer, after solid dissolution in HNO3/HCl/HF aqueous solution.
The morphology of the samples was studied with field emission scanning electron microscopy (FESEM) using a ZEISS Ultra-55 microscope and with field emission transmission electron microscopy (TEM) using a JEM 2100F microscope.
Textural properties were obtained from the N2 adsorption–desorption isotherms measured at 77 K with Micromeritics ASAP 2020 apparatus.
Solid NMR spectra were recorded at room temperature with a Bruker AV 400 MAS spectrometer. 27Al MAS NMR spectra were recorded at 104.2 MHz with a spinning rate of 10 kHz and a 9° pulse length of 0.5 μs with a 1 s repetition time. 27Al chemical shift was in reference to Al3+(H2O)6.
Infrared spectra were measured with a Nicolet 710 FT IR spectrometer. Pyridine adsorption–desorption experiments were made on self-supported wafers (10 mg cm−1) of original samples previously activated at 673 K and 10−2 Pa for 2 hours. After wafer activation, the base spectrum was recorded and pyridine vapor (6.5 × 102 Pa) was admitted into the vacuum IR cell and adsorbed onto the zeolite. Desorption of pyridine was performed under vacuum over three consecutive one-hour periods of heating at 423, 523 and 623 K, each of them followed by an IR measurement at room temperature. The spectra were scaled according to the sample weight.
As seen in Fig. 2a, OSDA-C4 allows the preferential crystallization of ZSM-5 materials under most of the studied conditions, especially for high Si/Al ratios (∼30), and diluted gels (H2O/Si ∼ 30). Similar ZSM-5 materials have been recently reported by Burton under analogous synthesis conditions.13 Interestingly, Beta zeolite material was crystallized at Si/Al, OSDA/Si and H2O/Si molar ratios of 15, 0.4 and 10, respectively in the gel (see Beta-15-OH in Fig. 2a). The as-prepared Beta-15-OH zeolite shows a Si/Al ratio of 15.3 (see Table 1) and 27Al MAS NMR spectroscopy reveals that all the aluminum is tetrahedrally coordinated within the zeolitic framework (see Fig. 4a). Moreover, the solid yield obtained is above 95%, according to the initial oxide sources used in the synthesis.
Sample | Si/Al |
---|---|
Beta-15-OH | 15.3 |
Beta-30-OH | 30.6 |
Beta-15-F | 16.0 |
Beta-30-F | 29.8 |
CP811 | 13.0 |
The low-intense and broad diffraction peaks observed in the PXRD pattern of this Beta zeolite suggest that this material should be in the form of small crystallites (see Beta-15-OH in Fig. 3a). FE-SEM microscopy confirms the formation of very small homogeneous crystals (see Beta-15-OH in Fig. 5), and TEM microscopy reveals that the average particle size is ∼10 nm (see Beta-15-OH in Fig. 6).
The N2 adsorption–desorption isotherms of Beta-15-OH (see Fig. 7) after calcining the sample at 550 °C in the presence of air show a steep rise uptake at low P/P0 pressure, which is typical for microporous materials, and an inflection and hysteresis loop at P/P0 between 0.6–0.9, which can be explained by a large interparticle capillary condensation due to the presence of small crystal sizes. The measured BET surface area is 757 m2 g−1, with an external surface area and a micropore volume of 440 m2 g−1 and 0.15 cm3 g−1, respectively (see Table 2).
Sample | Area BET (m2 g−1) | Ext. surface area (m2 g−1) | Micr. area (m2 g−1) | Micr. vol. (cm3 g−1) |
---|---|---|---|---|
Beta-15-OH | 757.4 | 439.9 | 317.5 | 0.15 |
Beta-30-OH | 738.2 | 428.7 | 309.5 | 0.14 |
Beta-15-F | 719.8 | 396.2 | 323.6 | 0.15 |
Beta-30-F | 568.5 | 241.1 | 327.4 | 0.16 |
CP811 | 580.0 | 203.1 | 378.4 | 0.18 |
For comparison purposes, a commercially available nanocrystalline Beta zeolite (CP811, Zeolyst) with a similar Si/Al ratio (∼13) has also been characterized. This nanosized Beta zeolite shows larger particle sizes (∼20–25 nm, see TEM image for CP811 in Fig. 8) and lower BET and external surface areas (580 and 203 m2 g−1, respectively, see Table 2). Thus, the nanosized high-silica Beta zeolite with a Si/Al ratio of 15 prepared using OSDA-C4 (see Fig. 1) shows improved physico-chemical properties compared to the commercially available nanocrystalline Beta zeolite.
However, attempts to synthesize nanosized Beta zeolites with higher Si/Al ratios using OSDA-C4 as the OSDA resulted in the crystallization of ZSM-5 materials (see Fig. 2a).
To favor the nucleation and crystallization of the nanosized large pore Beta instead of the medium pore ZSM-5 zeolite at higher Si/Al ratios, we thought to use a dicationic OSDA molecule similar to OSDA-C4 but presenting larger heterocyclic end-groups (see OSDA-C6 in Fig. 1). We based this hypothesis on the fact that larger heterocyclic end-groups would hardly allow the nucleation of the medium pore ZSM-5 zeolite due to host–guest size constraints, favoring the crystallization of the large pore Beta zeolite. Thus, the OSDA-C6 molecule was tested as an OSDA under the same synthesis conditions used before for OSDA-C4. As can be seen in Fig. 2a, OSDA-C6 allows the preferential formation of Beta zeolite under a wider range of conditions, achieving the crystallization of a pure Beta zeolite with a Si/Al ratio of 30 in the synthesis medium (see Beta-30-OH in Fig. 2a). This as-prepared sample shows a Si/Al ratio of 30.6 in the final solid (see Table 1) and the 27Al MAS NMR spectrum only shows a band at ∼54 ppm, associated with aluminum in tetrahedral coordination (see Fig. 4a). The solid yield achieved is above 95%, based on the initial oxide sources.
As observed above for the Beta-15-OH sample, the PXRD pattern of Beta-30-OH also shows the presence of broad diffraction peaks, which are typical of zeolites with very small crystal sizes (see Fig. 3a). SEM and TEM images confirm the formation of nanosized Beta crystals, with an average size comprised between 10–15 nm (see Beta-30-OH in Fig. 5 and 6). In addition, N2 adsorption–desorption characterization shows a BET surface area of 738 m2 g−1, with an external surface area and a micropore volume of 428 m2 g−1 and 0.14 cm3 g−1, respectively (see Table 2).
The results obtained could be an indication that this type of dicationic OSDA would be the origin of the small crystallites obtained. If this was so, they may generate nanocrystalline Beta even during the synthesis in the presence of F−, where large crystal sized zeolites are most generally obtained. Then, the synthesis of high-silica nanosized Beta zeolites was attempted using dicationic OSDA-C4 and OSDA-C6 molecules under fluoride media (see Fig. 2b). As can be seen in Fig. 2b, the crystallization of the Beta zeolite in the presence of fluoride anions in the synthesis media could be accomplished with two different Si/Al ratios, 15 and 30 (see Beta-15-F and Beta-30-F in Fig. 2b). These as-prepared solids show similar Si/Al ratios to the ones introduced in the synthesis gels (16 and 29.8 for Beta-15-F and Beta-30-F, respectively, see Table 1), where all the aluminum is tetrahedrally coordinated within the zeolitic frameworks (see the 27Al MAS NMR spectra in Fig. 4a). The solid yields were also above 95% in both cases.
TEM images reveal crystallites with average particle sizes of 10 and 30–50 nm for Beta-15-F and Beta-30-F, respectively (see Fig. 6). In fact, the smaller crystal sizes observed for the Beta-15-F sample results in higher BET and external surface areas (720 and 396 m2 g−1, respectively, see Table 2) compared to Beta-30-F (568 and 241 m2 g−1, respectively, see Table 2). Notice that the materials obtained present much smaller crystallites than the reported synthesis of Beta in fluoride media with sizes within several microns.14
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Fig. 9 Transmission FTIR spectra in the stretching C–C region of the different nanosized Beta zeolites after adsorbing pyridine followed by desorption at 150, 250, and 350 °C. |
Sample | Acidity (μmol pyr per g) | |||||
---|---|---|---|---|---|---|
B150 | B250 | B350 | L150 | L250 | L350 | |
Beta-15-OH | 146 | 130 | 94 | 68 | 64 | 52 |
Beta-30-OH | 110 | 98 | 47 | 84 | 82 | 71 |
Beta-15-F | 163 | 144 | 88 | 87 | 75 | 71 |
Beta-30-F | 71 | 53 | 23 | 24 | 21 | 19 |
CP811 | 220 | 165 | 85 | 211 | 205 | 193 |
The catalytic activity of these nanosized Beta zeolites has been studied for the alkylation of benzene with propylene to produce cumene, which is an important industrial intermediate product to obtain phenol and acetone.16 This industrial process is mainly operated in the liquid phase using large pore zeolites as heterogeneous acid catalysts.16 However, the design of large pore acid zeolites presenting nanosized crystals is convenient for this catalytic process, to reduce the restriction of cumene diffusion and to decrease the catalyst deactivation by olefin oligomerization and pore occlusion by multi-alkylated subproducts. As can be seen in Fig. 10, the two nanosized Beta zeolites with a Si/Al ratio of 15, Beta-15-OH and Beta-15-F, not only show better initial catalytic activity as compared to the commercial nanocrystalline Beta zeolite (CP811) despite the lower acidities of the former, but also a remarkable decrease of the catalyst deactivation with time on stream (TOS). In addition, the yield to the desired cumene is also significantly higher for Beta-15-OH and Beta-15-F zeolites (see Fig. 11a).
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Fig. 10 Propylene conversion with TOS for the liquid phase alkylation of benzene with propylene using nanosized Betas as catalysts. |
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Fig. 11 Selectivity to cumene (a) and di-isopropylbenzene (b) with TOS for the liquid phase alkylation of benzene with propylene using nanosized Betas as catalysts. |
With respect to the samples synthesized with a Si/Al ratio of 30, the Beta-30-OH shows an intermediate catalytic activity between Beta-15-F and the commercial CP811 zeolite (see Fig. 10 and 11). In this sense, the lowest catalytic activity of Beta-30-OH could be explained by its lower Brønsted acidity compared to Beta-15-F (see Table 3), but its high external surface area decreases the catalyst deactivation compared to CP811 (see Table 2 and Fig. 10).
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