J. Phys. Chem. C 2008, 112, 1455-1462
1455
Mesoporous MCM-48 Aluminosilica Oxynitrides: Synthesis and Characterization of Bifunctional Solid Acid-Base Materials Yongde Xia and Robert Mokaya* School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, U.K.
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ReceiVed: September 20, 2007; In Final Form: October 26, 2007
The preparation, structural characterization, and catalytic evaluation of novel solid acid-base mesoporous aluminosilica oxynitride materials are presented in this report. The materials were prepared by subjecting both direct (mixed-gel) synthesized and postsynthesis Al-grafted aluminosilicate MCM-48 materials to hightemperature nitridation, i.e., treatment with ammonia at elevated temperature. The properties of the mesoporous aluminosilica oxynitrides are mainly dependent on the preparation method and Al content. In general, aluminosilicates with high Al content were more susceptible to structural degradation during nitridation. Directly synthesized samples suffered greater structural degradation compared to Al-grafted samples of similar Al content. Oxynitrides derived from directly synthesized Al-MCM-48 exhibited higher amounts of nitrogen compared to materials derived from Al-grafted MCM-48, and in all cases, the nitrogen was incorporated into the aluminosilicate frameworks to generate various NHx species, including terminal NH2 and bridging NH groups and ammonium ions. The relative amount of terminal NH2 and bridging NH groups was dependent on the Al content and preparation history. The aluminosilica oxynitride materials exhibit both acidic and basic surface functionalities. The amount of acid sites and basic sites can be adjusted by controlling the Al content and by tuning the nitridation conditions. Basic sites predominate in all the mesoporous MCM-48 aluminosilica oxynitride materials, and the basicity was demonstrated by catalytic activity for the Knoevenagel condensation reaction.
1. Introduction A plethora of researches on solid acid materials have been conducted, but only relatively few studies have focused on solid base materials despite their potential use as catalysts for important industrial processes.1,2 There is indeed an ever increasing need to develop novel basic materials which show structural chemistry similar to that of known porous framework topologies and with accessible catalytically active basic sites. Generally, basic porous materials can be obtained via cation exchange with alkali ions, impregnation with basic salts, or via grafting of organic bases onto the pore walls of porous materials such as zeolites and mesoporous materials.3-7 However, basic porous materials obtained via cation exchange, impregnation with alkali metal ions/basic salts, usually exhibit disadvantages such as low basic strength and low cation-exchange capacity, which limits their applicability in catalytic reactions. On the other hand, organic bases grafted onto zeolites and mesoporous materials suffer from leaching of the basic moiety especially during liquid-phase catalysis. Furthermore, in most cases structural degradation and a drastic decrease in textural properties (surface area, pore volume, and pore size) accompany the creation of basic sites5-7 due to the partial blocking of the pore channels. In contrast to conventional basic oxides such as MgO, which have oxygen atoms as the center of basic sites, nitride and/or oxynitrides are a class of nitrogen-containing solid base materials first developed in the 1990s, in which the basic sites are linked to the presence of nitrogen atoms. In general, amorphous porous silica,8-10 aluminum orthophosphate,11-15 zirconium phos* To whom correspondence should be addressed. E-mail: r.mokaya@ nottingham.ac.uk.
phate,16 and aluminum vanadate17,18 can be nitrided by being subjected to thermal treatment under a flow of ammonia at elevated temperatures. The nitridation process affords porous nitride or oxynitride materials via partial or complete displacement of the silanol groups and bridging oxygen in the precursor oxides. The resulting porous basic materials exhibit weak to medium strength basicity, and the number of basic sites depends on the nitrogen content. The nitrided amorphous materials can function as catalysts for base-catalyzed reactions due to the presence of nitrogen-containing species such as adsorbed ammonium ions and terminal NH2 and bridging NH groups as well as N3- moieties on the surface of the materials.12,13,16 They are considered to be more strongly basic than alkali ionexchanged zeolites and comparable to hydrotalcites or MgO.19 Crystalline aluminosilicate molecular sieves (such as zeolites) do not react as easily with ammonia as the amorphous oxides but can form crystalline nitride or oxynitride which exhibit relatively weak basicity.20-23 Mesoporous silica materials are currently the subject of intense research interest.3-5,24 The amorphous nature of the frameworks of mesoporous silicas renders them amenable to the introduction of nitrogen-containing species via nitridation processes involving high-temperature treatment in the presence of ammonia or nitrogen.25-33 Wan and co-workers25 obtained mesoporous silicon oxynitride materials with high nitrogen content by heat treatment of fresh SBA-15 mesoporous silica precursors in flowing ammonia at high temperatures, but no catalytic evaluation was reported. Kapoor and Inagaki26 reported the use of nitrogen for the nitridation of ethane-bridged mesoporous silica materials. However, the nitrogen content achieved in the nitrided materials was very low. Asefa et al.27
10.1021/jp077578g CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008
1456 J. Phys. Chem. C, Vol. 112, No. 5, 2008 reported that amine-containing functional groups can be introduced into methylene-bridged mesoporous materials via ammonolysis, but they did not perform any catalytic evaluation for the aminosilicas. El Haskouri et al.28 nitrided sodiumcontaining MCM-41 mesoporous silica in an ammonia atmosphere at high temperatures, but in some cases the hexagonal structural ordering of the resulting mesoporous silicon oxynitride materials was disrupted perhaps due to the detrimental effect of sodium ions on thermal stability.34 Liu and co-workers further expanded the nitridation process to incorporate nitrogen into mesoporous silica thin films to form mesoporous silicon oxynitride thin films.29,30 On the other hand, we have reported the introduction of NHx species into the framework of mesoporous silica MCM-48 or MCM-41, which results in highly ordered silicon oxynitride materials and generates basic sites capable of considerable catalytic activity for the Knoevenagel condensation reaction.31,32 The nitrogen content and basic catalytic activity were varied simply by changing the nitridation temperature and duration.32 We have reported preliminary data on the introduction of NHx species into the framework of Algrafted mesoporous aluminosilica MCM-41 materials.33 However, so far, there are no reports on the detailed characterization of mesoporous aluminosilica oxynitrides. The effect of the presence of Al on the nitridation process and subsequent formation of basic sites is not known. It is also not known how the mode of Al incorporation (i.e., direct synthesis or postsynthesis grafting) affects the nitridation process and generation of basic and acid sites. In this report, we present the synthesis, physicochemical characterization, and catalytic evaluation of the acid-base properties of sodium-free mesoporous MCM-48 aluminosilica oxynitride materials that were prepared by subjecting direct (mixed-gel) synthesized and postsynthesis Al-grafted aluminosilicate MCM-48 to nitridation, i.e., treatment with ammonia at high temperature. Powder X-ray diffraction (XRD) and nitrogen sorption studies were used to study the structural ordering of the aluminosilica oxynitride materials. Infrared (IR) spectroscopy and 29Si magic-angle spinning (MAS) NMR were used to probe the incorporation of N into the MCM-48 framework, and 27Al MAS NMR probed the nature of the Al before and after nitridation. Depending on the Al content and synthesis strategy, the aluminosilica oxynitride exhibit varied acidity and basicity, with the basicity as the predominant functionality. The ease of preparation and control of the surface functionality (basic and/or acid/base) of the mesoporous aluminosilica oxynitride materials makes them attractive as alternative solid/base catalysts especially for large-molecule transformations. 2. Experimental Section 2.1. Materials Synthesis. The directly synthesized AlMCM-48 materials were prepared as previously described,35 except that various amounts of Al were added to the synthesis gel and the time allowed for hydrothermal crystallization was fixed at 135 °C for 2 days. In a typical synthesis, 2.14 g of fumed silica and a calculated amount of aluminum isopropoxide were added to 30 g of 10 wt % cetyltrimethylammonium hydroxide (CTAOH) solution under stirring to give a gel mixture of molar ratio SiO2/0.28 CTAOH/xAl/42 H2O, where x ) 0.02, 0.04, or 0.1. After continuous stirring for 2 h at room temperature, the resulting gel mixture was transferred to a Teflon-lined autoclave and heated at 135 °C for 2 days. At the expiry of the hydrothermal synthesis period, the autoclave was cooled to room temperature, and the solid product was obtained
Xia and Mokaya by filtration followed by repeated washing with a large amount of distilled water and drying at room temperature to yield the as-synthesized Al-MCM-48 samples. The as-synthesized samples were calcined at 550 °C for 4 h to remove the surfactant. The samples prepared at x ) 0.02, 0.04, and 0.1 were designated as Al-MCM-48A, Al-MCM-48B, and Al-MCM-48C, respectively. A pure silica material, Si-MCM-48, was also prepared using the same procedure described above but in the absence of aluminum isopropoxide. Al-grafted MCM-48 was prepared using a “dry” grafting procedure.36,37 Typically, 1.0 g of calcined Si-MCM-48 was added to 50 mL of n-hexane containing the required amount of aluminum isopropoxide followed by stirring for 24 h at room temperature. The resulting powder was recovered by filtration and thoroughly washed with dry hexane, dried at room temperature, and calcined at 550 °C for 4 h. The resulting Al-grafted MCM-48 samples were designated as GAl-MCM-48A, GAlMCM-48B, and GAl-MCM-48C corresponding grafting gel Si/Al ratio of 25, 15, and 8, respectively. For nitridation, the MCM-48 samples were subjected to hightemperature treatment in ammonia atmosphere.31-33 Typically, 0.3 g of MCM-48 was placed in an alumina boat and inserted into a flow-through tube furnace. Prior to thermal treatment, the tube furnace was purged by N2 for 30 min, followed by further purging with NH3 for another 30 min. With an NH3 flow rate of 100 mL/min, the temperature of the furnace was raised, at a ramp rate of 5 °C/min, to 950 °C and maintained for 20 h under the NH3 atmosphere. The furnace was then cooled to ca. 100 °C under NH3, and then the gas flow was switched to N2 until it cooled to room temperature. Nitrided samples obtained from Si-MCM-48, directly synthesized and Al-grafted AlMCM-48 were designated as N-MCM-48, NAl-MCM-48, and NGAl-MCM-48, respectively. 2.2. Characterization. Powder XRD analysis was performed using a Philips PW1830 diffractometer with Cu KR radiation (40 kV, 40 mA) in 0.02° step size and 2 s step time. Nitrogen sorption isotherms and textural properties of the materials were determined at -196 °C using nitrogen in a conventional volumetric technique by a Micromeritics ASAP 2020 sorptometer. Before analysis the samples were oven dried at 150 °C and evacuated for 12 h at 200 °C under vacuum. The surface area was calculated using the Brunauer-Emmett-Teller (BET) method based on adsorption data in the relative pressure (P/P0) range of 0.05-0.2, and total pore volume was determined from the amount of the nitrogen adsorbed at P/P0 ) ca. 0.99. Elemental compositions (Si/Al ratio) were determined using a Philips MiniPal PW4025 X-ray fluorescence (XRF) instrument. The nitrogen content of the nitrided samples was obtained using a CHNS analyzer (Fishons EA 1108). Infrared spectra were recorded using a Perkin-Elmer 2000 FTIR spectrometer on selfsupporting sample wafers in a Pyrex vacuum IR cell. Prior to the collection of spectra the samples were heated in the cell for 2 h at the desired temperature, after which the spectra were recorded at room temperature. Magic-angle spinning NMR spectra were acquired at the EPSRC Solid-State NMR Service (Durham) on a Varian Unity Inova 300 MHz spectrometer. 29Si MAS NMR spectra were acquired using a 7.5 mm probe with silicon-29 frequency of 59.56 MHz, pulse angle of 90°, acquisition time of 30-50 ms, recycle delay between 60 and 300 s, total spectral width of 30 kHz, and a MAS rate of 5.1 kHz. 27Al MAS NMR spectra were acquired using a 4.0 mm probe at a frequency of 78.12 MHz,
Mesoporous MCM-48 Aluminosilica Oxynitrides
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1457
Figure 1. Powder XRD patterns of (A) pure silica Si-MCM-48 (a) and directly synthesized Al-MCM-48 samples (b) Al-MCM-48A, (c) Al-MCM-48B, and (d) Al-MCM-48C and (B) silicon oxynitride N-MCM-48 (a) and aluminosilica oxynitrides (b) NAl-MCM-48A, (c) NAl-MCM-48B, and (d) NAl-MCM-48C.
Figure 2. Nitrogen sorption isotherms of (A) pure silica Si-MCM48 (a) and directly synthesized Al-MCM-48 samples (b) Al-MCM48A, (c) Al-MCM-48B, and (d) Al-MCM-48C and (B) silicon oxynitride N-MCM-48 (a) and aluminosilica oxynitrides (b) NAlMCM-48A, (c) NAl-MCM-48B, and (d) NAl-MCM-48C.
acquisition time of 20 ms, recycle delay of 0.5 s, pulse tip angle of 20.5°, spectral width of 50 kHz, and a MAS rate of 12.0 kHz. 2.3. Evaluation of Acidity and Basicity. The acid content was determined using established procedures that employ thermal desorption of cyclohexylamine (CHA) as previously described.37-40 Prior to thermogravimetric analysis (TGA), the samples were exposed to liquid CHA at room temperature overnight followed by pretreatment of the CHA-containing samples at 80 °C for 2 h. The samples were cooled to room temperature under nitrogen, and their TGA curves were obtained using a Perkin-Elmer Pyris 6 TGA analyzer with a heating rate of 20 °C/min under nitrogen flow of 20 mL/min. The weight loss due to amine desorption from acid sites between 300 and 450 °C was used to quantify the total acidity (in mmol of H+) assuming that each CHA molecule interacts with one Brønsted acid site.37-40 To determine what may be regarded as the medium to strong acidity, the CHA-containing samples were first heat-treated at 80 °C for 2 h followed by further thermal treatment at 250 °C for 2 h prior to thermogravimetric determination of the acidity. To evaluate the basicity of the oxynitride materials, Knoevenagel condensation test reactions were performed under an inert atmosphere (N2), in a flask that was fitted with a reflux condenser. The flask containing a mixture of redistilled benzaldehyde (10 mmol), malononitrile (10 mmol), and 40 mL of toluene was immersed in an oil bath, and the reaction mixture was magnetically stirred. Once the mixture reached the reaction temperature (40 °C), 0.03 g of catalyst, which was predried at 150 °C for several hours, was added into the flask. Samples of the reaction mixture were then periodically withdrawn by a filtering syringe and analyzed by a GC-17A gas chromatography to obtain the extent of conversion (%) as a function of reaction time.
TABLE 1: Elemental Composition and Textural Properties of Mesoporous Silica, Silicon Oxynitride, Aluminosilicate, and Aluminosilica Oxynitride Materials
3. Results and Discussion 3.1. Mesoporous Aluminosilica Oxynitrides Derived from Directly Synthesized Al-MCM-48. Figure 1A shows powder XRD patterns of pure silica Si-MCM-48 and directly synthesized Al-MCM-48 materials used as starting materials for nitridation. The XRD pattern of the pure silica MCM-48 exhibits four peaks, i.e., (211), (220), (420), and (332), which can be indexed to a structurally well-ordered cubic phase (Ia3d) mesoporous material. On the introduction of Al into the
sample Si-MCM-48 N-MCM-48 Al-MCM-48A Al-MCM-48B Al-MCM-48C NAl-MCM-48A NAl-MCM-48B NAl-MCM-48C GAl-MCM-48A GAl-MCM-48B GAl-MCM-48C NGAl-MCM-48A NGAl-MCM-48B NGAl-MCM-48C
Si/Al ratio
N content (wt %) 15.96
33.1 20.5 10.1 41.4 23.6 10.0 23.8 16.0 8.9 26.6 16.3 9.0
15.21 15.16 11.97
3.81 2.63 2.81
a0 (Å)a 92.1 77.5 94.5 96.1 97.2 80.3 82.4 85.2 84.0 82.4 75.0 76.2
surface area (m2/g)
pore volume (cm3/g)
1291 1191 1250 1110 1129 1139 478 90 806 762 712 454 325 214
1.31 0.73 1.44 1.22 1.41 0.72 0.71 0.47 0.69 0.63 0.60 0.37 0.32 0.24
a a0 is the lattice parameter obtained from XRD data using the formula a0 ) x6d211.
framework, the resulting Al-MCM-48 samples display XRD patterns that indicate a lower level of structural ordering; the Al-MCM-48 samples mainly show the basal (211) peak; the (220) peak is not observed, and the (420) and (332) peaks merge into a broad peak. This indicates that the incorporation of Al via direct mixed-gel synthesis diminishes long-range structural ordering.41 Figure 2A shows the nitrogen sorption isotherms of the pure silica Si-MCM-48 and directly synthesized Al-MCM48 materials. All the samples exhibit type IV isotherms with a pore filling step into uniform mesopores in the relative pressure (P/P0) range of 0.2-0.4. This indicates that the samples possess good mesostructural ordering with relatively narrow pore size distribution (Supporting Information Figure 1). In Figure 2A, a hysteresis loop above P/P0 ) 0.4 is observed for the pure silica Si-MCM-48 sample. The hysteresis loop is due to capillary condensation in secondary mesopores.42 This type of hysteresis loop has previously been observed to be present even for highly ordered MCM-48 materials prepared with C16 surfactants such as the CTAOH used in our study.42 The secondary mesopores may arise from interparticle voids.42 The textural properties of Si-MCM-48 and the directly synthesized Al-MCM-48 materials are summarized in Table 1. All of the samples have high surface area and pore volume (Table 1), which along with pore size slightly decrease at higher Al content. The lattice parameter
1458 J. Phys. Chem. C, Vol. 112, No. 5, 2008 values in Table 1 indicate that higher Al content expands the lattice. This expansion is consistent with previous reports5,7 and is usually ascribed to the formation of Al-O bonds, which are longer than Si-O bonds. Overall, the directly synthesized AlMCM-48 samples exhibit textural properties that are typical for well-ordered mesoporous materials and are therefore suitable as “starting” materials for nitridation. Figure 1B shows the powder XRD patterns of N-MCM-48 and NAl-MCM-48 oxynitrides derived from pure silica and directly synthesized Al-MCM-48, respectively. Only the silicon oxynitride (N-MCM-48) and aluminosilica oxynitride sample with low Al content (NAl-MCM-48A) exhibit the main basal (211) XRD peak. At higher Al content, the XRD patterns of aluminosilica oxynitrides exhibit a shoulder peak (sample NAlMCM-48B), which virtually disappears for sample NAl-MCM48C sample, indicating a much lower level of structural ordering. Indeed for sample NAl-MCM-48C, no ordering is evident. The basal (d211) spacing (and consequently the lattice parameter, a0) of the nitrided samples (Table 1) shows a reduction of ca. 17% compared with the corresponding parent Al-MCM-48 materials. This may be attributed to a greater extent of condensation within the MCM-48 framework due to exposure to the high nitridation temperature, which results in the contraction of the lattice. Figure 2B shows the nitrogen sorption isotherms of N-MCM48 and NAl-MCM-48 oxynitrides. Only the silicon oxynitride (N-MCM-48) and aluminosilica oxynitride sample with low Al content (i.e., NAl-MCM-48A) exhibit a type IV sorption isotherm with well-defined pore filling step into uniform pores. The isotherm for sample NAl-MCM-48B exhibits a rather broad pore filling step, whereas the isotherm for sample NAlMCM-48C has no pore filling step at P/P0 < 0.9. This indicates that the structural ordering of nitrided samples decreases at higher Al content and mesostructural ordering is virtually lost at Si/Al ratio of 10. It is also apparent that at higher Al content, the pore filling step, which is in the partial pressure range of 0.15-0.25 for N-MCM-48, shifts to the lower partial pressure range for samples NAl-MCM-48A and NAl-MCM-48B, which implies a reduction in pore size for the more aluminous samples. This may be caused by contraction of the mesoporous network due to thermal effects of the high nitridation temperature (950 °C). It is noteworthy that the hysteresis loop above P/P0 ) 0.4, which is observed for the pure silica Si-MCM-48 sample, is retained in the nitrided N-MCM-48 sample. This suggests that any secondary pores (interparticle voids) in the Si-MCM-48 are not affected by the nitridation process. As shown in Table 1, the silicon oxynitride and parent Si-MCM48 have comparable surface area and pore volume, whereas for NAl-MCM-48 oxynitrides, the textural properties are dependent on the Al content. At low Al content (Si/Al of ca. 40, sample NAl-MCM-48A), nitridization does not cause any change in the textural properties. At higher Al content (Si/Al of ca. 20, sample NAl-MCM-48B), nitridization causes a large (at least 50%) reduction in textural properties. Nitridization virtually destroys the most aluminous sample (Si/Al ) 10), which is accompanied by a decrease in surface area of >90%. Although the more aluminous Al-MCM-48 samples start from a lower level of mesostructural ordering (Figures 1 and 2), we nevertheless attribute the severe degrading effects of nitridization to the destabilizing (to thermal treatment) effect of larger amounts of Al. The elemental composition (i.e., Si/Al ratio and nitrogen content) of the Al-MCM-48 and NAl-MCM-48 aluminosilica oxynitride samples is shown in Table 1. For most of the samples, the Si/Al ratios are close to the target values for both sets of
Xia and Mokaya
Figure 3. Infrared spectra of aluminosilica oxynitride (sample NAlMCM-48C) derived from directly synthesized Al-MCM-48, as a function of evacuation temperature. The spectra are displayed in two regions: (A) 3800-2800 cm-1 and (B) 1800-1300 cm-1.
materials. However, there is an apparent increase in Si/Al ratio for samples with lower Al content, after nitridation. The cause of this apparent change in Al content is not clear but may be related to the structural stability of the aluminosilicate framework during the high-temperature nitridation.41 Structural stability of Al-MCM-48 is known to be dependent on the Al content.41 More importantly, the nitrogen content of the NAlMCM-48 oxynitrides (11.97-15.21 wt %) is close to that of the silicon oxynitride (15.96 wt %), and compares well with previous data.31-33 The N content appears to decease at higher Al content. We attribute the loose inverse relationship between N content and Al content to the presence of Si-O-Al bonds in the more aluminous parent Al-MCM-48C sample, which reduces the proportion of silanol groups capable of interacting with NH3 during nitridation. This assumption is confirmed by the IR spectra of representative aluminosilica oxynitride shown in Figure 3 (and Supporting Information Figure 2); the spectra do not show any silanol peak at 3750 cm-1. Indeed, the IR spectra for aluminosilica oxynitrides differ significantly from those of silicon oxynitride.31,32 We have previously shown that the IR spectra of mesoporous silicon oxynitrides display bands at 3505 and 1555 cm-1 due to symmetric bending vibrational frequency of -NH2 groups (δs(HNH) in free Si-NH2) and stretching NH bands (υNH in free Si-NH2).32 Mesoporous silicon oxynitrides also show bands at 3396 and 3137 cm-1 due to stretching NH bands (υNH), along with bands at 1401 and 1639 cm-1 due to δas(NH4+) of NH3 bonded to Si-OH groups and adsorbed NH3 or/and water, respectively.31 As shown in Figure 3, the stretching vibration frequency of N-H bands in NH2 groups (expected at ca. 3500 cm-1) is not observed in the NAl-MCM-48 aluminosilica oxynitride samples; only a weak symmetric bending vibration of NH2 is observed at ca. 1560 cm-1. This implies that the number of terminal NH2 groups on the surface of NAl-MCM-48 samples is low. The bands at 3400 and 3150 cm-1 are stretching vibration modes of bridged NH groups from Si-NH-Si and υNH stretching mode from NH4+.18,19,32,43 All the observed terminal NH2 and bridging NH species (Figure 3) are stable up to evacuation temperature of 600 °C, implying high thermal stability of the NHx species. The band at 1400 cm-1 may be assigned to the asymmetric bending vibration from NH4+, which was formed from NH3 adsorbed on acidic Al sites. Since the acid-base reaction produces neutral salt, the resulting NH4+ species are relatively thermally stable,
Mesoporous MCM-48 Aluminosilica Oxynitrides
Figure 4. (A) 29Si and (B) 27Al MAS NMR spectra of aluminosilica oxynitrides derived from directly synthesized Al-MCM-48. A representative 29Si spectrum of the parent Al-MCM-48 (sample Al-MCM48C) is shown for comparison.
which contrasts with the low thermal stability of NH4+ species on silicon oxynitrides where the NH4+ is generated from NH3 molecules adsorbed on silanol groups.32 It is also likely that any NH3 desorbed during the evacuation may react with Al sites to generate more NH4+, which might explain the apparent increase in the intensity of the band at 1400 cm-1 at temperatures of up to 400 °C (Figures 3 and 9, and Supporting Information Figures 2 and 4). We, however, do not rule out other possibilities such as isolated Al sites reacting with NH3 molecules to form NH4+ species, which can also contribute to the IR band at 1400 cm-1. 29Si MAS NMR spectroscopy was used to confirm the presence of N in the framework of the NAl-MCM-48 oxynitride samples. Figure 4A shows the 29Si MAS NMR spectra for NAl-MCM-48 samples, and for comparison, the spectrum of a representative parent Al-MCM-48 sample is also shown. We first note that Al-MCM-48 samples exhibit a broad resonance, in the range of -100 to -110 ppm, which arises from Si(OSi)3OH (Q3) and Si(OSi)4 (Q4) silicon environments. Such spectra are typical for calcined mesoporous aluminosilicas. After nitridation, the intensity of the resonances at -100 to -110 ppm reduces drastically. The spectra of the NAl-MCM48 aluminosilica oxynitrides is dominated by resonances at lower ( 0.4 is retained from the parent Al-grafted samples. The isotherms indicate that the extent of mesoporosity gradually decreases at higher Al content, accompanied with a decrease in pore size. As shown in Table 1, both the surface area and pore volume for the NGAl-MCM-48 oxynitride samples decrease significantly compared with the corresponding parent Al-grafted MCM-48 materials. Indeed, the surface area and pore volume for nitrided sample NGAl-MCM-48A (Si/Al ratio of 26.6), NGAl-MCM-48B (Si/Al ratio of 16.3), and NGAl-MCM-48C (Si/Al ratio of 9.0) reduced by 45%, 50%, and 60%, respectively, compared to their parent Al-grafted MCM-48 materials. When oxynitride samples with similar Al content are compared, the Al-grafted sample NGAl-MCM-48C has much higher surface area (214 m2/g) compared to directly synthesized NAl-MCM48C sample (90 m2/g). This implies that the preparation method (i.e., direct synthesis or grafting) and the structural ordering of the parent aluminosilicate MCM-48 samples influence the properties of the resulting nitrided materials. The Si/Al ratio of the Al-grafted aluminosilicate and aluminosilica oxynitride samples are close to the target values (Table
Mesoporous MCM-48 Aluminosilica Oxynitrides
Figure 9. Infrared spectra of aluminosilica oxynitride (sample NGAlMCM-48C) derived from Al-grafted MCM-48, as a function of evacuation temperature. The spectra are displayed in two regions: (A) 3800-2800 cm-1 and (B) 1800-1300 cm-1.
1). The nitrogen content of the NGAl-MCM-48 materials (2.63.8 wt %) is relatively low compared to that of the silicon oxynitride (15.96 wt %), NAl-MCM-48 (directly synthesized) samples (12-15.2 wt %), or aluminosilica oxynitrides derived from Al-grafted MCM-41.31-33 The low N content of the present NGAl-MCM-48 materials is a consequence of fewer silanol (Si-OH) groups on the surface of the parent Al-grafted MCM48 due to the fact that a significant proportion of the silanols will already have reacted with Al during the grafting process. Although Si-O-Si linkages can also react with ammonia to produce bridged NH species during nitridation, the formation of such bridging NH groups is energetically less favorable compared to generating terminal NH2 from Si-OH groups. It is also possible that acidic (Al) sites on the surface of the Algrafted MCM-48 can interact with NH3 molecules to form neutral salt that may generate additional NHx species. However not all Al sites are available to adsorb NH3, and therefore the overall outcome is a low content of N retained on the Al-grafted MCM-48. Two observations emerge from the N content data. The first is that Al grafting is less favorable (compared to direct synthesis) for the incorporation of large amounts of N via nitridation. This may be explained by considering that although Si-O-Al bonds are formed in both directly synthesized AlMCM-48 and Al-grafted MCM-48, a significant percentage in the former occur within the pore wall framework, and thus a significant proportion of silanol groups still exist on the surface of Al-MCM-48 samples. The surface silanol groups readily react with NH3 to form terminal -NH2 species during nitridation. This contrasts with the Al-grafted MCM-48 surface, which is depleted of silanol groups. The second observation is that, for similar Al content, Al-grafted MCM-48 incorporates less N compared to Al-grafted MCM-41.33 Although this may be due to differences in the concentration of silanol groups, a more likely explanation is the relative stability of the MCM-41 and MCM-48 frameworks to Al grafting and nitridation. It appears that MCM-41 retains a much higher level of structural ordering through the two processes,33 which translates to higher incorporation of nitrogen. Structural collapse of MCM-48 may hinder the incorporation of N during nitridation. Representative IR spectra of NGAl-MCM-48 materials are shown in Figure 9 (and Supporting Information Figure 4). Similar to directly synthesized NAl-MCM-48 materials, no peak is observed at 3750 cm-1, implying the depletion of silanol
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Figure 10. (A) 29Si and (B) 27Al MAS NMR spectra of aluminosilica oxynitrides derived from Al-grafted MCM-48. A representative 29Si spectrum of the parent Al-grafted MCM-48 (sample GAl-MCM-48C) is shown for comparison.
groups during both the Al-grafting process (as anchoring sites for the Al) and by replacement with terminal NH2 species during the subsequent nitridation process. Neither the stretching vibration frequency (expected at ca. 3500 cm-1) nor symmetric bending vibration of NH2 (expected at about 1560 cm-1) are observed, suggesting the presence low amounts of terminal NH2 groups on the surface. The bands at 3030 and 1400 cm-1 may be assigned to the stretching vibration and asymmetric bending vibration from NH4+, which was formed from NH3 adsorbed on acidic Al sites. The bands at 3400 and 3150 cm-1 are stretching vibration mode of NH group from Si-NH-Si and NH4+, respectively. All the observed terminal NH2 and bridging NH species are stable up to evacuation temperature of 600 °C, indicating high thermal stability of the NHx species, which is in general agreement with previous observations.31-33 The 29Si MAS NMR spectra for NGAl-MCM-48 samples and, for comparison, the spectrum of a representative parent GAl-MCM-48 sample, are shown in Figure 10A. As expected, the spectra of the Al-grafted samples is typical for calcined mesoporous aluminosilicas and exhibits a broad resonance, in the range of -100 to -110 ppm arising from Si(OSi)3OH (Q3) and Si(OSi)4 (Q4) silicon environments. After nitridation, the silicon environments are significantly altered as indicated by the shift of the observed resonances to lower ppm values. It is also clear that the intensity of the resonances at -100 to -110 ppm (arising from Q3 and Q4 sites) reduces significantly, although the reduction is not as large as that observed for NAlMCM-48 materials. The spectra of NGAl-MCM-48 materials exhibit a main peak at ca. -92 ppm which can be attributed to SiNO3 and a shoulder peak at -72 ppm which arises from SiN2O2. It is noteworthy that the SiN3O and SiN4 peaks expected at ca. -60 and -50 ppm, respectively,32 are not clearly observed. This is due to the lower amount of N in NGAlMCM-48 materials (Table 1). Figure 10B shows 27Al MAS NMR spectra of the NGAl-MCM-48 materials. The spectra exhibit resonances at 52 and 0 ppm arising from tetrahedrally and octahedrally coordinated Al, respectively. In all cases, the intensity of the tetrahedrally coordinated Al is higher, which confirms that Al is retained in framework positions after nitridation. The NGAl-MCM-48 materials possess low amounts of acidity as shown in Figure 5B. The acidity is slightly higher for oxynitrides with lower Al content. The basicity of the Algrafted aluminosilica oxynitrides was evaluated using the
1462 J. Phys. Chem. C, Vol. 112, No. 5, 2008 Knoevenagel condensation between benzaldehyde and malononitrile as a test reaction. As shown in Figure 6B, the NGAlMCM-48 materials were active catalysts for the test reaction. The conversion was best for samples with low Al content. The catalytic activity of the NGAl-MCM-48 aluminosilica oxynitrides shows no systematic correlation with the N content. It is interesting to note that, despite a lower N content, the conversion achieved for grafted NGAl-MCM-48 samples is generally comparable to that of directly synthesized aluminosilica oxynitrides. For example, grafted sample NGAl-MCM-48A (N content of 3.81 wt %) has very similar catalytic activity to directly synthesized sample NAl-MCM-48B (N content of 15.16 wt %). Although the two samples have similar surface area, it is more likely that the similarity in their catalytic activity is related to the N species they contain. The IR spectra of the samples have little evidence of the presence of large amounts of NH2 groups. Indeed, the IR spectra (Figures 3 and 9 and Supporting Information Figures 2 and 4) are dominated by bands at 3400 and 3150 cm-1, which are the stretching vibration modes of bridged NH groups from Si-NH-Si and Si-NH-Al, respectively, and bands at 3030 and 1400 cm-1 due to the stretching vibration and asymmetric bending vibration from NH4+. Since NH4+ is not intrinsically basic, the catalytically active basic sites are likely to be NH groups from Si-NH-Si and Si-NH-Al. The catalytic data, when compared to the N content of the oxynitrides, suggests the presence of both “active” and “inactive” N. It appears that directly synthesized aluminosilica oxynitrides have a large proportion of “inactive” N perhaps in the form of NH4+ ions generated when NH3 reacts with Brønsted acid sites during nitridation. 4. Conclusions Bifunctional mesoporous aluminosilica oxynitride materials that exhibit acid and base properties may be prepared by subjecting direct (mixed-gel) synthesized or postsynthesis Algrafted aluminosilicate MCM-48 materials to high-temperature nitridation. The preparation conditions (i.e., mode of Al incorporation and the nitridation process) and the Al content determine the properties of the mesoporous aluminosilica oxynitrides. In general, aluminosilicate MCM-48 with high Al content suffers greater structural degradation during the hightemperature nitridation process. However, careful control of the Al content enables preparation of well-ordered materials with considerable nitrogen content. Nitrogen is incorporated into the aluminosilicate frameworks to generate a variety of NHx species, including terminal NH2 and bridging NH groups and adsorbed NH4+ ions. The aluminosilica oxynitride materials exhibit both acidity and basicity, with a higher relative concentration of basic sites. The basicity was amply demonstrated by catalytic activity for the Knoevenagel condensation reaction. The ease of preparation and the ability to control the surface acidic and basic functionalities of the mesoporous aluminosilica oxynitride materials make them attractive as alternative solid acid-base catalysts especially for large-molecule transformations. Acknowledgment. The authors thank EPSRC for financial support. Supporting Information Available: Four additional figures: pore size distribution (PSD) curves of directly synthesized
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