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Identification of the basic sites on nitrogen-substituted microporous and mesoporous silicate frameworks using CO2 as a probe molecule Masaru Ogura, Shinya Fukuzawa, Seiichiro Fukunaga, Hiroshi Yamazaki, Junko N Kondo, Masafumi Morimoto, Remy Guillet-Nicolas, and Matthias Thommes Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03769 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Identification of the basic sites on nitrogen-substituted microporous and mesoporous silicate frameworks using CO2 as a probe molecule

Masaru Ogura1,2*, Shin-ya Fukuzawa1, Seiichiro Fukunaga1, Hiroshi Yamazaki3, Junko N. Kondo3, Masafumi Morimoto4, Remy Guillet-Nicolas5, and Matthias Thommes5

1: Institute of Industrial Science, The University of Tokyo, Komaba 4-6-1, Meguro, Tokyo 153-8505, Japan 2: Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan 3: Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan 4: Quantachrome Instruments Japan, KSP W311, Sakado, Takatsu, Kawasaki 213-0012, Japan 5: Quantachrome Corporation, Boynton Beach, Florida 33426, USA

* corresponding author: [email protected]

1 ACS Paragon Plus Environment

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Abstract Carbon dioxide was shown to identify surface basic properties of nitrogen-substituted microporous and mesoporous silicas, in addition to conventional basic oxides, by a detailed study using isotherm and heat of adsorption measurements as well as infrared spectroscopy. A hydrogen-bonded weak interaction was primarily observed between CO2 and silanol Si–OH and silamine Si–NH–Si groups. The heat of adsorption of CO2 demonstrated that the latter adspecies were formed preferentially over the former, although a much higher amount of linear CO2 adspecies were found on SBA-15 mesoporous silica due to the presence of a large quantity of silanol groups on its surface. Carbamate-type chemisorbed adspecies were not detected on silamino sites, while carbonate-type adspecies were formed on alkali ion exchanged zeolites and also residual sodium ions on the surface of the silicalite-1. CO2 was shown to be a successful probe molecule for identifying weakly interactive hydrogen-bonding sites, and it has potential as a surface probe for strongly interactive nucleophilic sites derived from alkaline ions or methylated silamino group, Si-N(CH3)-Si.


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Introduction The acid–base pair is one of the most important aspects in catalysis where redox reactions and complexation are also involved, in both liquid and gas phase reactions. A large number of acidor base-catalyzed reactions are known within the field of homogeneous catalysis, while limited reactions are found in the area of heterogeneous catalysis. Among these, as low as 10% are base-catalyzed reactions that are used in practical applications for petrochemical processes [1,2], even though environmental, economic, and ecological situations have changed a great deal over the last decade. Heterogeneous acid catalysis is recognized as an important catalytic process, especially in petroleum refinery and cracking, which are some of the most significant processes within the field of industrial chemistry. Extensive studies have been undertaken on acid-catalyzed reactions and acid catalysis since the 1950s, revealing the importance of acidity. Along with these experiments, a large amount of literature can be found regarding the characterization of such acidity in amorphous silica–alumina and crystalline zeolites, using NH3 or pyridine as probe molecules of acidity,

and

employing

analytical

methods

such

as

infrared

(IR)

spectroscopy

or

temperature-programmed desorption (TPD). In contrast, very little effort has been focused on the study of heterogeneous base catalysts. The first report on these was published by Pines et al. [3], where metallic Na dispersed on alumina acted as a catalyst for double bond migration in alkenes. This was followed by Sumitomo Chemical Co., Ltd., who developed its practical usage [4].


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Metallic Na has a tendency to donate its electron, providing a basic character. After the initial research in this field, several base catalysts have been reported and utilized for reactions such as hydrogenation [5], 1-butene isomerization [6], and side-chain alkylation [7]. Surface oxygen of oxide-type base catalysts is believed to be the active center for base-catalyzed reactions, mainly because its lone pair of electrons is available to interact with protons. In order to characterize such surface O or other sites for attracting protons, several different analytical techniques have been developed to date, including acid–base indicators, adsorption of acidic probe molecules, and assessment of other surface reactions. Density functional theory (DFT) modeling has also been recently adopted for investigating surface electron density [8]. TPD of CO2 is the most frequently used measurement technique for evaluating surface basicity [9]. The amount of basic sites and the basic strength are represented by the area of the desorption peak and the desorption temperature, respectively, in the profile of CO2-TPD, which is also commonly used in NH3-TPD when using the technique to characterize acidity. However, in contrast to NH3-TPD, CO2-TPD is not widely accepted as an accurate tool for estimating basicity, as the TPD profile from alkaline oxides is usually very broad. In this sense, utilization of probe molecules such as CO2 [10] and pyrrole [11], with detection using IR spectroscopy, is an alternative strong candidate for achieving accurate characterization. The shift in frequency of IR absorption spectral peaks reflects the polarizing effect of cations with high sensitivity. In the case of alkaline ion exchanged zeolites, CO2 is an adequately


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sized molecule for investigating basicity in the micropores of the material. We have recently developed a series of novel basic catalysts centered around porous silica materials, in which nitrogen atoms are doped and substituted into the framework oxygen site [12, 13]. Nitrogen-substituted silicate materials were first reported in 1968 by Kerr and Shipman [14]. Nitrogen-substituted faujasite zeolite was carefully characterized by use of magic angle spinning (MAS) NMR in order to identify the basic nitrogen species [15], although energetic calculations carried out by the same group revealed that nitridation of the framework of the crystalline zeolite was quite difficult [16]. There has been a lot of recent research regarding nitrogen-substituted mesoporous silica [17-20], which has shown a base-catalytic function for C–C bond forming reactions such as the Knoevenagel condensation. From the viewpoint of base catalysis, a series of nitrogen-substituted silicates showed an interesting feature whereby the basicity required for a specific reaction was of a similar strength to conventional basic oxides such as Cs oxide. It should be noted that the nitrogen-substituted silicate did not require pretreatment prior to the catalytic reaction, while Cs oxide needed high-temperature treatment at over 873 K in order to recover its activity after poisoning by strong adhesion of CO2 and H2O molecules from the atmosphere [12]. Therefore, interaction of CO2 on the surface of nitrogen-substituted silicate appears to be much weaker than on alkaline materials, even though the basicity is strong enough to promote base-catalyzed reactions. In this study, we highlight the basicity of nitrogen-substituted mesoporous


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silica and zeolites using CO2 as an adsorption probe, with IR spectroscopy as a detection method, in order to elucidate the level of basicity and its origin, compared with conventional basic materials.

Experimental Preparation of nitrogen-substituted mesoporous silicas and silicalite-1 MCM-41 and SBA-15 mesoporous silicas were prepared according to the literature [21,22] using cetyltrimethyl ammonium bromide and P123 triblock copolymer composed of polyethylene oxide and polypropylene oxide, respectively, as supramolecular micelle templates. MFI-type, aluminum-free silicalite-1 was supplied by Tosoh Corp., Japan. Concurrently, sodium-free silicalite-1 was synthesized in-house using SBA-15 as the silica source and tetrapropyl ammonium hydroxide as the organic structure-directing agent for MFI by employing a solid phase conversion (SPC) technique [23]. Calcination at 773 K was carried out on all samples in order to remove organics occluding the pores of the structures. Nitridation was carried out using a previously reported methodology [12,13]. Pure NH3 was flowed at a rate of 1 L/min to effectively nitride the silicate samples at temperatures of at least 773 K for silicalite-1 so it did not lose its crystallinity, 973 K for MCM-41 so its mesostructure did not collapse, and 1173 K for stable mesoporous SBA-15 to achieve deep nitridation. Materials Characterization


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The crystallinity of the zeolites and the periodicity in the mesostructure of the mesoporous silicas were confirmed using powder X-ray diffraction (XRD, Rigaku Rint2100, Japan). The powder samples were tightly attached to a glass plate and then set vertically in the chamber. The samples were then irradiated with Cu Kα line X-rays at a power of 40 kV and 40 mA, and the diffractometer was run from 2θ = 0.7 to 8.0° at 1°/min in steps of 0.02° for the mesoporous silicas, and 2θ = 5 to 40° at 4°/min in steps of 0.05° for the zeolites. Framework composition, in particular, the nitrogen content in the samples, was determined using a CHNS analyzer (vario MICRO cube, Elementar Analysensysteme GmbH, Germany). Samples of 3–5 mg were accurately measured using a microbalance and taken in a Sn boat, which was then bent and tightly packed to remove air contamination. Combustion of the samples was carried out in a quartz cell at 1423 K under air flow and the resultant CO2 and N2 molecules were quantified using a thermal conductive detector attached in line after the combustion cell. N2 adsorption at 77 K was analyzed automatically using an Autosorb-1 instrument, (Quantachrome Instruments Co., USA) to measure adsorption and desorption isotherms for the mesoporous silicas and zeolites. Dehydration at 573 K for 3 h onto the zeolites and 423 K for 1 h onto the mesoporous silicas was conducted prior to an adsorption run of ca. 30 mg of sample. The surface area of the samples was calculated using the BET equation, pore volumes by the t-plot


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method, and pore size distribution by the non-local density functional theory (NLDFT) with the equilibrium branch kernel supposing cylindrical pore. The adsorption isotherm of CO2 at 273−293 K was obtained using an Autosorb-iQ instrument (Quantachrome Instruments Co) from a low pressure CO2 dosing. Dehydration was conducted carefully using the same conditions as for the N2 adsorption for ca. 100 mg of sample. The heat of adsorption of CO2 was calculated by using the isotherms obtained at temperatures of 263, 273, 283, and 293 K on the basis of the Clausius-Clapeyron equation under equilibrium between liquid and gaseous phases of adsorbate. IR spectra of adsorbed CO2 were measured using a JASCO 4100 FT-IR spectrometer (Japan) equipped with a mercury cadmium telluride (MCT) detector. Samples of 40 mg were pressed into a self-supporting disk of 20-mm diameter and placed in a quartz IR cell attached to a conventional closed gas circulation system. The sample was pretreated by evacuation at 573 K for 1 h. Each spectrum was collected at a resolution of 4 cm-1 with an average of 64 scans. The spectrum of the fresh disk, just after pretreatment, was recorded under evacuation as the background, with the spectra of the samples reported after subtraction of this background. Highly pure CO2 gas (99.995%, Taiyo Nippon Sanso Corp., Japan) was introduced into the cell at 195 K.


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Results and Discussion Preparation of nitrogen-substituted mesoporous silicas and silicalite-1 Figure 1 shows typical XRD patterns of MCM-41, SBA-15 mesoporous silicas, zeolitic silicalite-1, and their nitrogen-substituted analogs, obtained under conditions suitable for preserving each mesophase. MCM-41 needed a relatively low nitridation temperature of 973 K, compared to the 1173 K possible for SBA-15, because of loss of its mesostructure at higher temperatures. Temperatures lower than these were not sufficient for substituting framework oxygen by nitrogen, according to the nitridation mechanism reported by Chino and Okubo [19], where a silanol Si–OH group was involved in the first step of nitridation on the silica surface. SBA-15 has a preferable surface for nitridation as it has more silanol groups than MCM-41 owing to the mechanism of formation of the surfactant–silicate interface, with approximately 2 or 3 OH per nm2 of the surface [24]. Both XRD patterns after nitridation were shifted to a higher angle of diffraction, indicating that the density of the mesopore wall composed of amorphous silica increased with the high nitridation temperature in the reducing atmosphere. The less favorable nitridation of the MCM-41 did not pose a problem, however, as adequate mesostructures with a controlled level of nitridation could be achieved by varying the experimental conditions, as is discussed below.


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Figure 1. Powder XRD patterns of MCM-41, SBA-15, silicalite-1, and their nitrogen-substituted analogs. The same nitridation protocol was carried out for the crystalline silicalite-1. More than 340 kJ/mol of energy was required to replace framework oxygen by nitrogen on the surface of the aluminosilicate zeolite [16], a value which is of a similar magnitude to the energy of siloxane (Si–O–Si) bond formation; therefore, these bonds were easily cleaved, leading to the formation of defects and finally to collapse of the crystalline zeolite structure. For this reason, nitridation of silicalite-1 needed to be carried out at a lower temperature than those used for the mesoporous silicas. Here, nitrogen-substituted silicalite-1 was also prepared by a solid phase conversion (SPC)


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technique from nitrogen-substituted SBA-15 (NSBA-15) [12, 23], which is here designated N-silicalite-1 from NSBA-15, and by nitridation under mild conditions of 973 K, which is designated N-silicalite-1 from silicalite-1. Figure 2 shows the N2 adsorption isotherms of the prepared samples. The steep increment visible in the isotherm for SBA-15 (right) was decreased after nitridation, whereas the shape of the isotherm for MCM-41 (left) was not greatly changed. These isotherms clearly imply that the mesoporous structure could be retained even after severe nitridation. Table 1 summarizes the physicochemical properties along with the nitrogen content of the samples. The diameter of the mesopores and the surface area of SBA-15 were dramatically decreased due to a reduction in the amount of micropores that existed between the mesopores running through the grains, while the mesopore volume and mesostructure originating from the SBA-15 were somewhat retained after nitridation. The nitrogen content in the NSBA-15 suggests that not all the oxygen on the top surface of the mesopores of SBA-15 was converted to nitrogen; it required approximately 30 wt% to fully cover the surface with Si nitride. Owing to the milder conditions applied in this study, NMCM-41 showed similar porous properties to the unmodified MCM-41. For the N-silicalite-1 that was obtained from NSBA-15 by the SPC method, a silamino group, Si–NH–Si, underwent hydrolysis to release N as NH3, indicating that the opposite reaction to the nitridation of silica took place. Compared to the SPC method, direct nitridation was effective for obtaining the silamino group on


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crystalline silicalite-1, even though its content was much lower than for the nitrogen-substituted mesoporous silicas.

Figure 2. N2 adsorption isotherms at 77 K on pristine (△,▲) and nitrogen-substituted (○,●) MCM-41(left) and SBA-15(right). Table 1. Physicochemical properties of the samples used in this study.


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Determination of adsorption properties of CO2 by adsorption isotherms Adsorption isotherms of CO2 were collected at 273−293 K in order to investigate its weak interaction with the silicate wall. As illustrated in Figure 3, CO2 was adsorbed on SBA-15 and MCM-41 by way of almost linear, upward-convex type adsorption, where the adsorbed amount of CO2 increased with increasing the pressure. Nitridation decreased the amount of adsorbed CO2 to a level almost half that of the SBA-15, corresponding well with the differences in surface area between the two materials. These results indicate that CO2 adsorbed on SBA-15 mesoporous silica exhibited a dependency on surface area rather than the amount of specific basic sites such as silamine groups. This suggests that the CO2 was primarily physisorbed on SBA-15. Silicalite-1 demonstrated a similar but a closer type-I isotherm (Fig. 3). At a low pressure of CO2, a microporous structure seemed to be preferable to a mesoporous one, owing to the concentration of adsorbate in a much more narrower confined space of micropore of silicalite-1, 0.5 nm, as compared to SBA-15, 8 nm. The amount of adsorbed CO2 on the N-silicalite-1 was comparable to that on silicalite-1. Although N-silicalite-1 obtained by direct nitridation of silicalite-1 was also investigated, the isotherm appeared unchanged compared with the unmodified material, indicating that similar phenomena in the adsorption isotherms of CO2 occurred for silicalite-1 independent of the nitrogen content. It should also be noted that all of the adsorption and desorption branches of the isotherms were collected in the same line, so that the adsorption of CO2 was reversible.


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Figure 3. CO2 adsorption isotherms on pristine and nitrogen-substituted SBA-15, MCM-41, and


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silicalite-1.

Determination of adsorption properties of CO2 by the heat of adsorption Figure 4 summarizes the values for the heat of adsorption of CO2 on the prepared materials, which were calculated using the isotherms (adsorption points) at temperatures of 273, 283, and 293 K, appeared in Fig. 3, along with 263 K on SBA-15 series. It is interesting to note the apparently different trend in the heat of adsorption on the microporous and mesoporous materials. The heat of adsorption of CO2 on the SBA-15 and MCM-41 series decreased with increasing amount of adsorbed CO2, with nitrogen-substituted NSBA-15 showing a greater dependency on the

Figure 4. Heat of adsorption of CO2 on pristine (■) and nitrogen-substituted (○) SBA-15, MCM-41, and silicalite-1. 15 ACS Paragon Plus Environment

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volume of adsorbed CO2 than SBA-15. On the other hand, almost constant values of heat of adsorption were obtained for silicalite-1 and N-silicalite-1, respectively, which was not greatly altered on changing the adsorbed volume of CO2. The molecular size of CO2 is 0.3 nm [25], which is small enough to allow accumulation of CO2 molecules into an adsorption multilayer for the mesoporous SBA-15 and MCM-41 used in this investigation. When the amount of adsorbed CO2 is low, the interaction of CO2 with the mesopore wall is due to van der Waals forces. Such a weak force results in physical adsorption between the adsorbate CO2 molecules and the adsorbent surface of the silica [26]. As the amount of CO2 increases, adsorbate–adsorbate interactions govern the heat of adsorption, with values gradually becoming closer to the value of the condensation heat of CO2, which is approximately 17 kJ/mol under the conditions adopted here. The CO2 might eventually become liquefied in the mesopores under the high pressure produced by the confined space. In contrast, CO2 fits well in the micropores of silicalite-1, and monolayer adsorption is possible in the more confined space. The heat of adsorption was, accordingly, almost constant during the validation of P/P0 in this study, meaning that only the interaction of adsorbate and adsorbent was observed. It is worth noting that, at high pressure, the values of heat of adsorption were low for the amorphous SBA-15 series relative to those for the crystalline silicalite-1 series. The values for silicalite-1 might reflect the effect of micropore filling. In turn, at low pressure, the value became higher on SBA-15 compared to the silicalite-1, which might be caused by the basic silamine site, Si–NH–Si, located


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mainly in the mesopore of NSBA-15. It should be addressed that the values of the heat of adsorption of CO2 became higher on NSBA-15 than on SBA-15 at a much lower level of adsorbate pressure, corresponding to the expected stronger interaction caused by the basic silamine site. Quite interestingly, in the low volume range, the heat of adsorption of CO2 on NSBA-15 exceeded the one on SBA-15, meaning that nitridation gives us a small number of strong affinity site for CO2, even though enough basic silamine site existed on NSBA-15. This might be caused by the closure of micropores existed in the pristine SBA-15 during nitridation at higher temperatures, where much more amount of silamine site is expected to be located. This phenomenon never be observed on NMCM-41 and N-silicalite-1 which in the range always gave us stronger CO2 adsorption sites. NMCM-41 provided a larger heat of adsorption compared with MCM-41 over the whole range of pressures tested. N-silicalite-1 showed a similar trend compared to silicalite-1. Those mean that the heat of adsorption on Si-NH-Si is higher than that on Si-OH, while the population of the OH group on the surface determines the heat on the material per mole of CO2, especially on SBA-15. Furthermore, the siliceous SBA-15 exhibited a higher heat of adsorption than the siliceous MCM-41, even though the walls of the mesopores were amorphous in both cases. The amount of nitrogen in the silicate framework of each porous material was re-evaluated; nitrogen in NMCM-41 was 4.8 mmol-N/g, 17 mmol-N/g in NSBA-15, and 2.0


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mmol-N/g in N-silicalite-1 from silicalite-1. On the contrary, the amount of CO2 adsorbed on those materials from the maximum values of x-axis in Fig. 4 were; 0.63 mmol-CO2/g on NMCM-41, 0.89 mmol-CO2/g on NSBA-15, and 1.3 mmol-CO2/g on N-silicalite-1, where CO2 was physically adsorbed on those materials, that could be judged by the heat of adsorption nearly equally to that of condensation of CO2. A quite limited number of CO2 could be like a chemisorbed state on the nitrogen site substituted in the silicate framework, otherwise the heat of adsorption of CO2 on such nitrogen site is quite similar to the one for physisorption of CO2. From the results obtained in this study, the amount of a surface basic point could not be determined. Exceptionally high heat of adsorption of CO2 on NSBA-15 might be due to the nitrogen site in the micropore of NSBA-15 that could be survived in a quite small extent. Compared with those mesoporous N-silicate, microporous N-silicate showed as much CO2 content as the amount of nitrogen at the same order, but almost a constant value of the heat of adsorption was seen in Fig. 4, meaning that the adsorption of CO2 on/in the micropore is majorly governed by micropore filling.

Identification of adsorption site of CO2 by IR spectroscopy Figure 5 shows typical IR spectra of SBA-15 before and after dehydration pretreatment at 423 K. In the region of the OH band, a sharp absorption at 3739 cm-1 assignable to the vibration mode of silanol SiOH, ν(O–H), was clearly detected even before dehydration, and a largely broad


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band centered at 3400 cm-1 can be seen in the spectra prior to dehydration due to ν(O–H) of H2O. The deformation peak of H2O, δ(Η–O–H), was also detected at 1631 cm-1 prior to dehydration. After the heat treatment, the δ(Η–O–H) band disappeared, and the broad ν(O–H) band drastically decreased, accompanied by the generation of a new band at 3450 cm-1, corresponding to ν(O–H) of a geminal SiOH group. As previously described, SBA-15 possesses many SiOH groups in the micropores rather than in mesopores, in accordance with the mechanism of formation using triblock copolymer P123 [22]. This makes the initiation of nitridation easy on the silica surface [19].

Figure 5. IR spectra of SBA-15 before (a) and after (b) dehydration at 423 K. Spectra were measured at room temperature. The SBA-15 was subsequently exposed to a small amount of CO2, and the gas pressure in the IR measurement cell was gradually increased, as shown in Figure 6. The spectra are reported after subtraction of a background spectrum of SBA-15 taken prior to the introduction of the gaseous 19 ACS Paragon Plus Environment

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CO2; therefore, a positive peak corresponds to a newly formed species on the surface, with a negative peak corresponding to a species already existing on the surface that underwent interaction with the introduced gas molecules. Occasionally, as the partial pressure of CO2 increased, a ν3(C–O) band due to weakly adsorbed, linear type CO2 [32] was observed at 2345 cm-1, with a small peak at 2279 cm-1 as a satellite of ν3(C–O), and a negative peak at 3731 cm-1 due to ν(O–H) for SiOH increasing in intensity. Furthermore, a new positive band corresponding to an induced shift of the SiOH peak by interaction with gaseous CO2 took place at 3700 to 3600 cm-1. This provided important information that indicated that silanol could interact with CO2 via a weakly H-bonded, linear-shaped mode of adsorption.

Figure 6. IR spectra of CO2-dosed SBA-15 at 195 K. The pressure of CO2 was increased from 5 to 1000 Pa.


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Figure 7 shows the IR spectra of NSBA-15 before and after dehydration. An interesting peak appeared at 3368 cm-1 that was not present in the spectra of SBA-15, which was attributed to the vibration mode of a bridged NH group, ν(N–H), in a silamino group, Si–NH–Si, on the surface of the SBA-15. It was repeatedly noted that nitridation provided a hydrophobic surface; even before dehydration, the NH band could be clearly observed without any interference with a band due to adsorbed H2O, as was seen in the SBA-15 before heat treatment. Again, the nitridation mechanism should be taken into consideration. First of all, silanol is attacked by gaseous NH3 to give a Si–NH2 group with corresponding consumption of silanol [19]. Therefore, it was concluded that silanol initiated the nitridation of silica, producing a hydrophobic surface. A distinct signal at 1550 cm-1 due to the deformation mode of NH, δ(N–H), further supports the presence of –NH– groups.

Figure 7. IR spectra of NSBA-15 before (a) and after (b) dehydration at 423 K. Spectra were measured at room temperature. 21 ACS Paragon Plus Environment

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Exposure to CO2 resulted in an important finding for elucidating the form of the interaction of CO2 with the surface of nitrogen-substituted silica. As shown in Figure 8, highly similar positive and negative peaks could be detected for the SBA-15 and NSBA-15, with a large negative peak at 3376 cm-1. This could be assigned to the ν(N–H) band due to interaction of a silamino group with gaseous CO2. As the partial pressure of CO2 increased, a linear mode adsorption at 2343 cm-1 increased positively, and the interacting SiOH and Si–NH–Si increased negatively. At a low pressure of 5 Pa (Figure 8b), only the negative band corresponding to the silamino group interacting with gaseous CO2 was detected. Further increase in CO2 pressure up to 500 Pa induced the interaction of CO2 with silanol, generating a negative peak at 3760 cm-1. This phenomenon indicated that silamine is the preferable site for CO2 adsorption compared to silanol. However, the linear CO2 band at 2343 cm-1 did not split into two peaks, which implies that the weak interaction of CO2 with silamine and silanol did not provide other types of CO2-derived adspecies. It should be noted that carbonate or carbamate-like adspecies in particular, produced by strong interaction of CO2 with oxygen or nitrogen atoms on the surface through a chemical adsorption/bonding mode [27], were not detected in any of the experiments on nitrogen-substituted mesoporous silica. This is in contrast to previous studies that have demonstrated carbamate-like adspecies on nitrogen-substituted MOF [28], as well as amine-anchored SBA-15 [29]. Here, in the case of silamine on the porous silica surface, CO2 could probe the physically adsorptive point via


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hydrogen-bonding [30], which is a very weak interaction, and not by acid–base interaction, nor by an electrically positive–negative Coulomb interaction, as is frequently observed for alkaline oxides, because CO2 has a strong quadrupole moment.

Figure 8. IR spectra of CO2-dosed NSBA-15 at 195 K. The pressure of CO2 was increased from 5 to 1000 Pa. The compared spectra were at the CO2 pressure of 5 (a) and 500 Pa (b).

Figure 9 shows the typical spectroscopic data from NMCM-41 before and after dehydration. Compared with NSBA-15, two specific points were identified: a smaller NH band, and the almost complete disappearance of the silanol OH after nitridation. As previously mentioned, the number of silanol groups on pristine MCM-41 was found to be much lower than on SBA-15, which is due to the mechanism of formation and the nature of the aqueous solution from which each mesophase is deposited. In this sense, it could be speculated that insertion of nitrogen via a silanol group on MCM-41 is more difficult than for SBA-15, which would result in less silamine and much 23 ACS Paragon Plus Environment

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less silanol on MCM-41. It is interesting to note that even though the content of silamine was lower, the band corresponding to the bending δ(N–H) at 1551 cm-1 appears much sharper than that in the spectrum of NSBA-15. This might be caused by the lower density of silamino groups on the NMCM-41 as the silamine exists in an isolated state on the mesopore surface of MCM-41. The CO2 adspecies on NMCM-41 is shown in Figure 10. As can be clearly seen, CO2 interacted with silamine, with a negative peak visible at 3438 cm-1, while the negative peak corresponding to the interaction of CO2 with silanol was barely observed, even though the positive peak corresponding to the interactive shift band ν(O–H) in silanol was clearly present.

Figure 9. IR spectra of NMCM-41 before (a) and after (b) dehydration at 423 K. Spectra were measured at room temperature.


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Figure 10. IR spectra of CO2-dosed NMCM-41 at 195 K. The pressure of CO2 was increased from 5 to 1000 Pa.

From these results, the adspecies derived from CO2 can be classified into two types: a H-bonded weak interaction with silamine, (Si-)2-NH•••OCO, and a H-bonded weak interaction with silanol, Si-OH•••OCO. The former can be concluded to be a common adspecies on a nitrogen-substituted silica surface, where H remains at the N site. The latter was detected not only on SBA-15 but also on NSBA-15, where residual silanol exists on the surface after nitridation. The difference in the heat of adsorption of CO2 on NSBA-15 and NMCM-41 can be explained by the adsorption sites. Judging from the results shown in Figure 4, CO2 adsorption on NSBA-15 occurred


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mainly on silanol, even though sufficient nitrogen species was present on the surface. The nitrogen atom in the silamino group remained chemically unreacted with CO2 in the range of adsorption tested. Gaseous CO2 condensed on the surface of pristine MCM-41, where siloxane Si–O–Si was mainly exposed on the top surface with a relatively weak interaction with existing CO2. Taking into consideration the potential of alkaline oxide-loaded or alkali ion exchanged zeolites to be used for CO2 capture and storage [31], the materials that contain nitrogen in the skeletal framework of silica have a unique property. This provides the expectation that they would be repeatable adsorbents, where CO2 would be weakly adsorbed, but slightly stronger than the physically adsorbed and condensed CO2. Filling of the micropores of N-silicalite-1 with CO2 is also expected as a different characteristic of this series of materials. However, the silamino group obtained in the experiments described here was found to be highly sensitive to H2O vapor, easily undergoing hydrolysis during processes such as crystallization into zeolite via dry gel conversion, where the presence of H2O vapor is inevitable [12]. As suggested by Figure 11, N-silicalite-1 prepared from NSBA-15 contained much fewer silamino groups, and silanol groups were barely observed in the spectrum even after dehydration. This is one of the important features of the zeolite obtained via dry gel conversion and SPC, as has been previously reported [23]. Figure 12 shows the spectra for N-silicalite-1 prepared from silicalite, produced at a milder temperature than the nitrogen-substituted mesoporous silicas. Even though the nitrogen


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Figure 11. IR spectra of N-silicalite-1 prepared from NSBA-15 before (a) and after (b) dehydration at 573 K. Spectra were measured at room temperature.

Figure 12. IR spectra of N-silicalite-1 prepared from silicalite-1 before (a) and after (b) dehydration at 573 K. Spectra were measured at room temperature.


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content of N-silicalite-1 (2.8 wt%) was much lower than for the nitrogen-substituted mesoporous silicas, a sharp absorption band assignable to ν(N–H) in silamine was detected at 3403 cm-1 along with a small peak from δ(N–H) at 1550 cm-1. The wavenumber observed here was larger than those for the nitrogen-substituted mesoporous silicas owing to the difference in the surrounding environment: crystalline or amorphous. This might explain the different catalytic activities reported for crystalline and amorphous nitrogen-substituted porous silica for base-catalyzed reactions [12]. The broad band corresponding to adsorbed H2O seen in the area around 3200 cm-1 was more apparent than for the N-silicalite-1 from NSBA-15 obtained via SPC, which was due to the slight hydrophilic nature of the parent silicalite supplied from Tosoh Corp., and it was caused by a small amount of sodium ion, less than 0.05 wt%, remaining on the surface. The sodium ion also worked as an adsorption site for CO2, as weakly detected in Figure 13, as a carbonate species at 1688 and 1545 cm-1, ν(C–O or C=O), as well as CO2 adspecies on silamine and silanol, observed at 3391 (ν(N–H)), 3698 (ν3+2ν2(O–H)), and 3596 (ν3+ν1(O–H)) cm-1, respectively, as the interactive, positive peak of adsorption. It should be noted that the carbonate species was already detected at a shoulder peak in the same position after dehydration pretreatment. Adsorbed CO2 on basic zeolites is well-known to be difficult to desorb, and high-temperature pretreatment above 873 K is necessary in order to use these materials as basic catalysts. This result also indicates that CO2 is weakly adsorbed on silamine and silanol as linearly adsorbed adspecies, while it is strongly adsorbed on the


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residual sodium ion of silicalite as a chemisorbed carbonate species.

Figure 13. IR spectra of CO2-dosed N-silicalite-1 prepared from silicalite-1 at 195 K. The pressure of CO2 was increased from 5 to 1000 Pa.

The basicity derived from silamine was found to be much weaker than that from alkali ion exchanged in the zeolite and also from the residual sodium ion on the supplied silicalite sample. The former forms weak, hydrogen-bonded CO2 adspecies, while the latter involves strong, chemisorptive carbonate. The silamino group was also found to be a preferable hydrogen-bonding


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site to the silanol group. The interaction between silamine and gaseous CO2, as demonstrated by the heat of adsorption values, was found to be relatively strong compared to the silanol, which was caused by the greater polarizing ability of silamine [30, 32, 33]. However, this interaction does not reach over long distances. This characterization of CO2 adsorption enabled us to evaluate the basic characters of these materials as catalysts for the Knoevenagel condensation, which has been reported separately by our group [12, 13]. It is generally accepted that base catalysis works by promoting proton subtraction from a reactant substrate, meaning that base strength relates to proton affinity. In the Knoevenagel condensation, proton subtraction from an active methylene compound such as malononitrile, ethyl cyanoacetate, or diethyl malonate is the rate-determining step of the reaction with benzaldehyde [13]. The required base strength depends on the substrate; therefore, there have been few reports on the use of a heterogeneous base catalyst for promoting the condensation of diethyl malonate. Among the materials tested, the base catalytic properties were found to be in the order: N-siliceous zeolite > N-mesoporous silica > alkali oxide, alkali ion >> silica. Through the investigation of CO2 adsorption, it was clarified that physisorbed CO2 could be detected on non-modified siloxane, that more linear CO2 adspecies could be found on silamine relative to silanol, and that carbonate-type chemisorbed adspecies could form in the presence of alkali ions. The detection of such species had a clear relationship with basic strength, although the trend


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involving the basic catalysis is not entirely consistent with the sequence. Linear-type adspecies required H-bonding to protons, indicating a weak affinity to CO2. The initial aim of this study was to determine the suitability of CO2 for use as a probe of basic sites on a catalyst surface. Carbonate species are known to be formed through a reaction of CO2 with a lone pair on basic sites such as oxygen adjacent to an alkali ion exchanged in a zeolite [32]. In the case of silamine, this corresponds to a carbamate species, formed with a lone pair on a nitrogen site in the silicate framework. Thus, the nucleophilicity of the lone pair is likely to have an influence. We recently developed a simple modification of the basic character of nitrogen-substituted mesoporous silica by methylation of the nitrogen species [13], where the proton on the nitrogen atom in the silicate framework was isomorphously substituted with a methyl group. Methylation has been revealed to enhance the basicity of NSBA-15, and Knoevenagel condensation of benzaldehyde with diethyl malonate was successfully catalyzed on the surface [13]. Furthermore, a nucleophilic reaction was catalyzed on the methylated NSBA-15 [34]. Such a highly basic catalyst possessed carbamate adspecies when the surface was exposed to CO2 (data not shown). This demonstrates that CO2 could be an effective probe molecule of nucleophilic sites, which is one of the strong basic characteristics where proton subtraction takes place. It should be stressed that linear-type adspecies of CO2 could be an identification of a weak affinity to protons, not only on silamine but also on silanol, regardless of basicity.


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Conclusions CO2 is a strong candidate for use as a probe molecule for identification of surface basic properties of recently-developed nitrogen-substituted microporous and mesoporous silica, and conventional basic oxides. Using adsorption isotherms, the interaction of CO2 was identified to be dependent on porosity, with a microporous structure undergoing a monolayer filling with CO2, while mesoporous materials gave multilayered adsorption of CO2 at an amount that correlated well with the surface area. Studies on the heat of adsorption of CO2 revealed that the silamino group, Si–NH–Si, had a stronger interaction with CO2, more than 30 kJ/mol, compared to the silanol group, Si–OH, which was 25−30 kJ/mol, with the latter adspecies mainly formed on SBA-15 that possesses a large amount of silanol. MCM-41 type mesoporous silica exhibited an interaction with siloxane, ~25 kJ/mol, which was as weak as physical adsorption and condensation of CO2. These values were much smaller than those derived from chemisorption on conventional basic oxides and alkali ion exchanged zeolites, ca. 40–50 kJ/mol. IR spectroscopy provided direct evidence of such interactions with silamine and silanol groups on nitrogen-substituted porous silica via hydrogen-bonding. Linearly-adsorbed CO2 was formed preferentially on silamine rather than silanol, while chemically interactive carbamate species did not form on any of the nitrogen-substituted materials. Carbonate was seen to form in the presence of alkali ions that existed as a residue on the surface of the silicalite-1 material. These results indicate that nitrogen-substituted porous silica has


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a weaker basic site for proton subtraction than a conventional basic oxide. Further methylation of nitrogen in the silicate framework enhanced the nucleophilic interaction with CO2, facilitating the formation of a carbamate species, and leading to high catalytic performance for reactions that required stronger basicity and nucleophilicity.

Acknowledgement A part of this work was performed under a management of “Elements Strategy Initiative for Catalysts & Batteries (ESICB)” supported by MEXT program “Elements Strategy Initiative to Form Core Research Center” since 2012, MEXT; Ministry of Education, Culture, Sports, Science and Technology, Japan. MO would like to express appreciation to Editage for providing editorial assistance.


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