Organic functionalization of mesopore walls in hierarchically porous zeolites

Abstract

Mesopore walls of hierarchically meso-/microporous zeolites (MFI, BEA and LTA) are covered with silanol groups, so that the zeolites can be functionalized with various organic groupsviasilylation; the organic-functionalized hierarchical zeolites exhibit hydrothermal stability and reusability in catalytic applications, as compared with organic-functionalized mesoporous silica.

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Organic functionalization of inorganic materials (e.g., mesoporous silica, silica gel and zeolite) has been extensively studied in heterogeneous catalysis for green chemistry during the last decade.^1–9 Organic-functionalized inorganic matrices can provide synergistic properties of organic and inorganic components, such as high functionalizability, accessible surface area and structural stability. Among the various inorganic supports, mesoporous (2 < pore diameter < 50 nm) silicas have attracted much attention in recent years because the large pore diameter allows grafting of bulky organic moieties.^1–6 However, one drawback of mesoporous silica is its low hydrothermal and chemical stabilities due to amorphous nature in the framework.^6,10 Crystalline zeolites are more stable, but in ordinary zeolites the solely microporous structure (pore diameter <2 nm) and lack of silanol (≡Si–OH) groups make the incorporation of bulky organic moieties difficult. Nanocrystalline zeolites⁸ and delaminated zeolites⁹ with enlarged surface area were used for functionalization.

Here we show that hierarchically meso-/microporous zeolites (‘hierarchical zeolites’ hereafter for brevity) can serve as a suitable inorganic support for organic functionalization. Hierarchical zeolites with various structures are currently available according to various synthesis routes developed recently, such as directly hydrothermal synthesis using organosilane surfactants as a mesopore generator,^11–13 desilication of pre-synthesized zeolites,^14,15 and synthesis using presynthesized solid template.^16,17 In the present work, we have chosen the synthesis route which adds organosilane surfactants into conventional zeolite synthesis compositions.^11–13 The hierarchical zeolites thus obtained exhibit uniform and tailorable mesopores. The mesopore walls are composed of microporous crystalline zeolite frameworks, which are terminated with silanol groups at the wall surface. Due to the large mesopore surface area, the amount of silanol groups is very high as compared with that on the external surface of conventional zeolite crystal. The silanol groups can readily be reacted with various alkoxysilanes (Scheme 1). Thus, the mesopore walls can be functionalized with various organic groups, to a high concentration that is comparable to functionalization of mesoporous silica SBA-15. Furthermore, the crystalline zeolite frameworks offer the advantage of improved chemical stabilities as compared with mesoporous silicas having amorphous frameworks.

Scheme 1 Organic functionalization of hierarchical zeolites.

Hierarchical zeolites with three types of framework (MFI, BEA and LTA) were synthesized following the reported procedure.^11,12 The zeolite samples are denoted by the 3-letter structural codes¹⁸ following ‘MP-’, where MP means ‘mesoporous’. Nitrogen adsorption–desorption isotherms and corresponding mesopore size distributions are presented in Fig. 1. As the isotherms show, there is a sharp increase in adsorption in 0.2 < P/P₀ < 0.8, which corresponds to the capillary condensation in mesopores. The pore size analysis by BJH algorithm gave a distribution curve centered at 3.4 nm for MP-MFI, 2.7 nm MP-BEA, and 9.9 nm MP-LTA. The mesoporous samples exhibited X-ray powder diffraction patterns similar to the corresponding conventional zeolites (ESI†). Due to the lack of structural order in mesoscale, the materials did not show XRD peaks in the low-angle regime.

Fig. 1 (a) N2 adsorption–desorption isotherms, and (b) BJH mesopore size distributions for hierarchical zeolites. The isotherm for MP-LTA was vertically offset by 300 ml g⁻¹.

Reactions between zeolite silanol groups and alkoxysilanes were carried out by refluxing in toluene solution containing trimethoxy or triethoxy silane, which are similar to the conventional functionalization of mesoporous silicas reported previously.^1–6 To prevent moisture contamination, zeolites were immediately used after calcination in air at 450 °C and anhydrous toluene (Aldrich, 99.9%) was used as a solvent. The organic content after functionalization was analyzed by thermogravimetric analysis (TGA), and confirmed by elemental analysis (EA). The organic content in hierarchical zeolites was compared with those of conventional zeolites and SBA-15¹⁹ (Table 1).

Table 1 Functionalization of hierarchical zeolites with oganosilanes, RC₃H₆Si(OCH₃)₃ or RC₃H₆Si(OC₂H₅)₃

Material	R	Organic content/mmol g^−1
a Reaction was carried out for 3 h using 1 g material, 5.0 mmol organosilane, and 10 ml toluene. Other samples were reacted for 12 h.
MP-MFI	NH2	1.13^a
MP-MFI	NH2	1.18
MP-BEA	NH2	1.10
MP-LTA	NH2	0.61
SBA-15	NH2	0.94
MFI	NH2	0.14
MP-MFI	Cl	1.20
MP-MFI	C6H4NH2	1.31
MP-MFI	NH(CH2)2NH2	1.29
MP-MFI	NHC(CH3)CHCOCH3	1.28
MP-MFI	NHC(C6H5)CHCOC6H5	1.13

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As the result with 3-aminopropylsilane shows, the grafting reaction was almost completed within 3 h. Ordinary MFI zeolites having solely microporous structure showed negligible organic content due to the small area of the external surface. In contrast, the functionalization of hierarchical zeolites led to a similar or even higher amount of organic groups than SBA-15 mesoporous silica, as compared with the same weight of samples. For example, in the case of 3-aminopropyl functionalization, the MP-MFI zeolite contained 25% more organic groups than SBA-15. Note that the SBA-15 sample has approximately 2 times larger mesopore surface area (683 m² g⁻¹) than the MP-MFI zeolite (350 m² g⁻¹), according to the crude estimation using t-plot analysis. Such a t-plot analysis can underestimate mesopore surface area in the case of the aluminosilicate zeolite. It is thus reasonable that the surface functionalization density would be at least 2 times higher at the mesopore walls of MFI than SBA-15. The high surface functionalization indicates that more silanol groups per surface area were available in the case of hierarchical zeolite. This seemed to be due to the presence of silanol (≡Si–OH) groups on the external surface (i.e., mesopore surface) of the zeolite framework, where the fully crosslinked Si–O–Si linkage could not allow further condensation. TGA analysis showed that the hierarchical zeolite lost physisorbed water during 200–300 °C, but no further weight losses indicating silanol condensation were detected thereafter. In contrast, SBA-15 (atomically disordered, and partially condensed Si–O–Si network) showed continuous decreases in weight even in the region of 300–800 °C. It was therefore reasonable that many of the SBA-15 silanol groups could be lost during the calcination at 450 °C.

To explore the catalytic applicability of organic-functionalized hierarchical zeolites, we have investigated catalytic activities in the Sonogashira coupling reaction, after grafting dichloro-β-oxoiminato-palladium (Scheme 2, and ESI† for details). The Pd-catalyzed Sonogashira cross-coupling reaction of terminal alkynes is one of the most powerful methods for synthesizing arylated alkynes.²⁰ A major challenge in Sonogashira reaction has been the recycling of expensive Pd catalyst. The reaction is carried out in the presence of Na₂CO₃, as a base to prevent Pd leaching by the product HCl. Nevertheless, low hydrothermal stability of the support is still a major problem for recycling, particularly in the case of silica-supported catalyst. As our results show in Table 2, the Pd complexes grafted on hierarchical zeolites had similar catalytic activities to those on SBA-15. It should be noted that the MP-MFI-based catalyst exhibited only a slight decrease in catalytic activity (from 96 to 91% in yield) during recycling for five times. In comparison, SBA-15 exhibited the decrease in catalytic conversion from 84 to 38%. Thus, the hierarchical zeolites exhibited much higher catalyst durability in the basic medium than SBA-15 did. The high recyclability of the hierarchical zeolite may be attributed to the enhanced hydrothermal stability, due to the crystalline nature of the zeolite framework. Pore size analysis of the used catalyst by N₂ isotherm showed that the functionalized MP-MFI maintained the original mesoporosity up to 24 h under the basic reaction condition at 90 °C, whereas the functionalized SBA-15 completely lost the mesoporosity in 12 h. MP-LTA indicated even higher stability than MP-MFI. The Al-rich LTA zeolite maintained their mesoporosity and catalytic activity after the hydrothermal treatment for 4 d. The hierarchical zeolites, functionalized with similar Pd-complexes, were also effective for other C–C coupling reactions such as Suzuki–Miyaura, Stille and Heck of aryl chlorides. Further studies of these coupling reactions are currently in progress.

Scheme 2 Grafting of dichloro-β-oxoiminato-Pd on mesopore walls of hierarchical zeolites, and their catalytic application in Sonogashira reaction.

Table 2 Sonogashira coupling of chlorobenzene with terminal alkynes^a

Catalyst	R^1	Temp./°C	Time/h	Yield (%)^b
a Reaction conditions: chlorobenzene (1.0 mmol), alkynes (1.2 mmol), CuI (1 mol%), Na2CO3 (2.0 mmol), solvent (0.7:0.7 ml = H2O:DMA) and Pd catalyst (1 mol%). b GC yield was determined using n-dodecane as an internal standard. Isolated yield is given in parenthesis.
MP-MFI	Ph	70	6	96(93)
MP-BEA	Ph	70	6	89(84)
MP-LTA	Ph	70	6	91
SBA-15	Ph	70	6	76
MP-MFI	CH2OH	90	6	92(89)
MP-MFI	n-C4H9	90	8	86
MP-MFI	n-C8H17	90	8	48
MP-BEA	n-C8H17	90	8	29
MP-LTA	n-C8H17	90	8	88(85)
SBA-15	n-C8H17	90	8	67

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In conclusion, hierarchically meso-/microporous zeolites (MFI, BEA and LTA) were functionalized by grafting organosilanes onto the silanol groups located at the surface of mesopore walls. Whereas the solely microporous zeolites could not be functionalized due to the lack of free silanol groups inside micropores, the hierarchical zeolites could be functionalized at the mesopore surface by using high density of silanol groups. The organic-functionalized hierarchical zeolites exhibited significantly enhanced hydrothermal stability and catalytic reusability as compared with mesoporous silica SBA-15 having amorphous framework. It was also remarkable that the surface functionalization density was higher than that of SBA-15, although the total amounts of functional groups were similar if based on weight. We only demonstrated catalytic application, but we believe that organic-functionalized hierarchical zeolites might provide new opportunities in the design of catalyst, sensor, adsorbent and ion-exchange materials. In particular, it would be interesting to design bifunctional materials that can combine the acid and ion-exchange properties of zeolite with various functions provided by grafted organic moieties.

This work was supported by the National Honor Scientist Program of the Ministry of Education, Science and Technology in Korea.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: General experimental procedures, synthesis and characterization details. See DOI: 10.1039/b815540b

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