M.
Luisa Marin
ab,
Geniece L.
Hallett-Tapley
a,
Stefania
Impellizzeri
a,
Chiara
Fasciani
a,
Sabrina
Simoncelli
ac,
José Carlos
Netto-Ferreira
ad and
Juan C.
Scaiano
*a
aDepartment of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie, Ottawa, K1N 6N5, Canada. E-mail: scaiano@photo.chem.uottawa.ca
bInstituto Universitario Mixto de Tecnología Química (UPV-CSIC), Universitat Politècnica de València, Avenida de los Naranjos s/n, 46022 Valencia, Spain
cINQUIMAE and Departamento de Química Inorgánica, Analítica, y Química Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina
dDivisão de Metrologia Química, Instituto Nacional de Metrologia, Qualidade e Tecnologia-INMETRO, Duque de Caxias, 25250-020 Rio de Janeiro, Brazil
First published on 9th April 2014
Several forms of niobium oxide were prepared, including nanostructured mesoporous materials, and their acidity properties were comprehensively investigated and compared with commercially available materials. The composites were characterized by a variety of techniques, including XRD, TEM, N2 adsorption and Hammett acid indicator studies. The acidity of the niobium oxide derivatives was also investigated by the ability of the materials to successfully promote the halochromic ring-opening of an oxazine-coumarin probe that was specifically designed for use in fluorescence imaging studies. The ring-opening reaction was easily monitored using UV-visible, fluorescence and NMR spectroscopy. Single molecule microscopy was employed to gain a more in-depth understanding of the niobium oxide acid catalysis pathway. Using this technique, the rate of niobium oxide mediated protonation was estimated to be 1.8 × 10−13 mol m−2 s−1. Single molecule analysis was also used to obtain a detailed map of Brønsted acid sites on the niobium oxide surface. The active sites, located by multiple blinking events, do not seem to be localized on any area of the material, but rather randomly distributed throughout the solid state surface. As the reaction proceeds, the sites with the highest acidity and accessibility are gradually consumed, making the next tier of acid sites available for reaction. The phenomenon was more closely characterized by using time lapsed reactivity maps.
Due to the remarkable acidity of these materials, many opportunities exist for the use of this solid acid catalyst in a variety of systems where harsher chemicals are typically employed (e.g., concentrated H2SO4) and where the removal of excess acid following reaction is desired, but not always feasible. As such, the Brønsted acid character of Nb2O5·nH2O has been exploited in the catalysis of many industrially relevant reactions, including esterification,3 saccharide dehydration3,4 and hydrolysis.3,5 Nb2O5·nH2O holds several advantages over traditional acid catalysts; in addition to ease of removal, the acidity of niobium-based catalysts can be easily tailored using a variety of methodologies. Several studies have examined the effects of temperature on the acidity of Nb2O5·nH2O, where temperatures lower than 473 K are desired to achieve maximum Brønsted acidity; within conditions above this limit, Nb2O5·nH2O Brønsted acidity rapidly decreases due to the increasing crystalline character of the material.2,6,7 As a result, acid doping of hydrated niobium oxides (e.g.; H3PO4, HCl, HNO3)7,8 has been developed to allow for applications outside of such critical temperature ranges, and has also been exploited as a means of enhancing the acid catalytic capabilities of Nb2O5·nH2O.
In the current contribution, we explore the properties of a family of five Nb2O5·nH2O solids in order to compare the Brønsted acidity of these materials to commercially available alternatives. Initial trials were focused on the characterization of the materials. Later, an emissive halochromic oxazine-coumarin system (1) was used to probe the Nb2O5·nH2O acidic character with spectroscopic and single molecule microscopy techniques.
The pore size of the mesoporous materials (T-III and T-IV) was assessed using nitrogen adsorption isotherms due to the distinctive nitrogen uptake as a result of the capillary condensation of nitrogen inside of the mesopores. Table S1 (ESI†) presents the average surface area and pore size as calculated from the isotherm data for the three niobium oxide materials with mesoporous structure and demonstrates the porosity of the solids. Importantly, the overall surface area and pore size variations are not only dependent on the method of preparation, but also tend to vary from batch to batch within the same synthetic methodology, as shown for T-IIIa and T-IIIb. Both of these mesoporous niobium oxides were prepared using P-123 as a templating agent; nonetheless, the surface area (528 m2 g−1 and 295 m2 g−1, respectively) and pore size (24.4 Å and 9.57 Å) are significantly different, trends that are not uncommon in mesopore synthesis.14 As such, both solids were employed for further testing as Brønsted acid catalysts.
Indicator | Max. pKa | Comm. Nb2O5 | T-I | T-II | T-IIIa | T-IIIb | T-IV |
---|---|---|---|---|---|---|---|
a (+) = observable colour change; (−) = no observable colour change. | |||||||
Methyl red | +5.0 | − | + | + | + | + | + |
Methyl yellow | +3.3 | − | + | + | + | + | + |
Crystal violet | +0.8 | − | + | + | + | + | + |
Dicinnamal-acetone | −3.0 | − | − | + | + | + | + |
Chalcone | −5.6 | − | − | + | + | + | − |
Anthraquinone | −8.6 | − | − | − | − | − | − |
H 0 | > +5.0 | −3.0 to +0.8 | −8.6 to −5.6 | −8.6 to −5.6 | −8.6 to −5.6 | −5.6 to −3.0 |
Fig. 2 Thermogravimetric analysis of (a) comm. Nb2O5·nH2O, (b) T-I, (c) T-II, (d) T-IIIa, (e) T-IIIb and (f) T-IV. |
The absorption spectrum of 1 in CH3CN shows a band centered at 410 nm. The addition of acid, such as trifluoroacetic acid (TFA), opens the oxazine ring and permanently generates compound 2 (Fig. 3). Within this transformation, the coumarin functionality is brought in conjugation with the cationic fragment of the generated isomer, shifting its absorption band bathochromically by ca. 180 nm. A fluorescence band centered at 645 nm can then be observed by selectively exciting 2 at λex of 530 nm. Therefore, the transformation of 1 into 2, encouraged by the addition of acid, can be exploited to activate fluorescence and allows the investigation of materials with distinctive acidic properties with relatively simple experimental setups.
The use of an emissive probe with switchable character offers several advantages with respect to other pH-sensitive fluorophores (e.g. coumarin-6) which are strongly emissive either before or after the addition of acid and show only a discrete shift of the emission wavelength in response to protonation.18 The observation of stimuli-generated fluorescence in a dark background (such is observed for 1), rather than the shifting of the emission of a fluorescent substrate (coumarin-6), offers the opportunity to image fluorescent elements with improved signal to noise ratio, allowing the detection and identification of structural features with higher precision and the investigation of Nb2O5·nH2O-mediated acid catalysis at the molecular level.
To further investigate the niobium oxides discussed herein as Brønsted acid catalysts, 2 mg of each solid were placed in 3 mL of a 7.5 μM CH3CN solution of compound 1. In all cases, the reaction mixture rapidly changed colour (from pale yellow to blue/purple) upon the addition of the five synthesized Nb2O5·nH2O materials (Fig. 4), indicating the formation of 2 upon ring-opening reaction, while the addition of commercial Nb2O5 resulted in no observable colour change. These results support the absence of Brønsted acidity in the commercial niobium sample. Trifluoroacetic acid was also used as a control and yielded positive results.
Fig. 4 Photographs illustrating the colour change observed upon ring opening of 1 in the presence of Brønsted acidic Nb2O5·nH2O. |
The halochromic opening of 1 was monitored using both absorption and fluorescence spectroscopy. Fig. 5 and S4 (ESI†) present the UV-visible spectra obtained upon addition of TFA or the Nb2O5·nH2O catalysts discussed in Table 1. TFA was first added to a CH3CN solution of oxazine-coumarin 1 as a control experiment. The reaction immediately turned deep blue in colour and was left to equilibrate for 1 h. Subsequent spectroscopic measurements revealed a shift in the absorption spectrum from 410 nm to 595 nm (Fig. S4A†), due to the formation of 2, which slowly continued over the monitoring period of 5 days. Similarly, the niobium oxide catalysts presented in Table 1 were added to a fresh solution of 1. Implementing commercial Nb2O5 as a heterogeneous Brønsted acid catalyst afforded no appreciable change in the absorption maximum (Fig. 5A), indicating no conversion to the ring-opened isomer 2. The results obtained using the commercial sample further suggest that this Nb2O5·nH2O is void of any Brønsted acid properties.
In the case of the five synthetically prepared Nb2O5·nH2O catalysts, a distinct variation in the UV-visible spectrum was detected upon addition of each heterogeneous material to a solution of the oxazine-coumarin probe 1. Following addition of T-I and T-IIIa as Brønsted catalysts (Fig. 5B and C), a subsequent shift in the absorption spectrum was observed, similar to that seen in TFA control studies, indicative of oxazine ring-opening to form compound 2. In the case of catalyst T-I, the formation of 2 slowly progresses over 5 days, whereas when catalyst T-IIIa was employed, rapid formation of 2 was preceded by a gradual decrease of the absorption band ascribed to ring-opened isomer 2 (the decrease in the observed absorbance is only attributed to adsorption on the catalyst surface rather than hydrolysis of 2 as is illustrated by the control experiments presented in Fig. S5†).
Moreover, after 5 days, both catalysts appeared blue in colour, implying that adsorption of the probe molecule onto the acidic sites of the Nb2O5·nH2O catalysts is integral in the catalytic ring-opening process. The more rapid decline in the absorption of 2 in the presence of T-IIIa may be simply due to the mesoporous nature of this material and the more rapid and favourable adsorption of the probe molecule into the support matrix.
Results similar to T-IIIa were obtained for the remaining three Nb2O5·nH2O solids (T-II, T-IIIb and T-IV) and are presented in Fig. S4.† In all cases, the solid catalyst turned blue after 5 days of reaction and the absorption spectra attributed to ring-opened oxazine 2 decreased as a function of monitoring time, again suggesting that adsorption of the probe onto the catalyst surface may constitute a fundamental step in the catalysis pathway.
The formation of the emissive isomer 2 was also monitored via fluorescence spectroscopy. An appearance of fluorescence at 645 nm following the addition of TFA (Fig. S6A†) resulted from protonation of compound 1 to generate 2, which continued to form over the timescale of the experiment (4 days). Fig. 6 presents the emission spectra obtained when commercial Nb2O5·nH2O, T-I and T-IIIa were added as heterogeneous Brønsted acid catalysts. These trends closely mimic those observed for UV-visible spectroscopy, where no changes in the fluorescence spectra were observed when the commercial material was used (over a monitoring period of 4 days). On the other hand, an emission band at 645 nm was detected after 1.5 h following addition of both T-I and T-IIIa. The emission at 645 nm slowly increased as a function of time in the presence of T-I. On the contrary, rapid protonation of 1, followed by an ensuing decrease in intensity of the 645 nm emission over 4 days, was observed for catalyst T-IIIa.
Fig. S6† presents the remaining fluorescence spectra for Nb2O5·nH2O T-II, T-IIIb and T-IV. In all cases, protonation of 1 manifests as the growth of a fluorescence band at 645 nm due to formation of compound 2; however, the emission progressively decreases over 4 days, likely due to adsorption of ring-opened oxazine 2 onto or into the support matrix.
As previously discussed, the change in colour of the synthetically-prepared Nb2O5·nH2O catalysts, in conjunction with the observed variations in both the UV-visible and fluorescence spectra over time, seems to indicate a permanent adsorption of the probe onto the support surface. The rate at which this process occurs was found to be dependent upon the catalyst used, where the mesoporous Nb2O5·nH2O (T-IIIa, T-IIIb and T-IV) demonstrated more rapid adsorption, likely due to incorporation of the molecule into the porous structure. In order to further investigate this process, time-lapsed 1H NMR was performed on a solution of 1 following the addition of T-IIIa in CD3CN. Although the solution coloured instantaneously, the 1H NMR spectrum (Fig. S8a, ESI†) shows broadened peaks that can be only attributed to the starting material 1 (Fig. S7†), and not to its protonated form 2 (chemical shifts from 2 can be observed by adding 10 equivalents of TFA to a solution of 1 in CD3CN, Fig. S7†). The spectra in Fig. S8† represent only the 8.5 to 4 ppm region of the NMR to allow for a more specific comparison of the peaks that are relevant to this discussion. Further confirmation of the exclusive presence of 1 is provided by the analysis of the diastereotopic protons Hy and Hz, which are known to shift from 4.7 ppm in 1 to 5.8 ppm in 2 (Fig. S7 and S8†). In addition, the signals decrease in intensity and lose definition over time (Fig. S8b–d), in agreement with the previous observation of increasing colouration of the solid material. The solid catalyst was separated by centrifugation and analyzed by diffuse reflectance and fluorescence spectroscopy; the corresponding spectra (Fig. S9†) illustrate the presence of a broad band centered at ~630 nm and a broad emission centered at 720 nm, respectively, further confirming the adsorption of compound 2 onto the support surface. Broadening of absorption and emission curves when measured in the solid state is commonly observed due to differing dielectric constants of the solvent and solid state media.19
In order to test the proposal that the halochromic probe was, in part, incorporated into the mesopores of T-IIIa (as suggested by absorption and emission spectroscopy), time-controlled 1H NMR analysis of a mixture of 1 and the non-mesoporous T-I was also carried out for comparison. In the case of T-I, broad signals were also detected that can be attributed to 1, together with the progressive diminution of their intensity (Fig. S10†) and the contemporaneous colouration of the solid catalyst. However, no noteworthy variations in the NMR spectrum could be observed 2 h after catalyst addition (Fig. S10b†), while 1H signals are already significantly different from the initial recordings (Fig. S8b†) 2 h after the addition of T-IIIa. These results confirm that compound 1 adsorbs differently on the catalyst surface depending on the type of Nb2O5·nH2O used, as was also observed during spectroscopy measurements (Fig. 5, 6, S4 and S6†). Furthermore, measurements performed using T-IIIa show that the chemical shifts remained constant for the duration of the experiment (up to 96 hours, with signals that are barely visible), whereas after 4 days from the initial addition of T-I to 1, the 1H NMR peaks shift to resemble those of 2 (Fig. S10c;† as compared with the TFA control). This comparative study suggests that in the case of T-IIIa, product 2 has likely been incorporated into the pores of the material.
Based on these observations, compound 1 is believed to be initially adsorbed onto the surface of the catalyst (due to the total absence of 1H NMR signals from 2) and, once adsorbed, to undergo the proposed Brønsted acid catalyzed ring-opening reaction directly on the surface to generate 2. The instantaneous purple/blue colouration of the solution when the niobic acid catalysts are added to 1 is attributed to an initial stage of the reaction, where the blue colouration is likely the result of a combined effect that includes the reaction of molecules in close proximity to the most exposed acidic sites on the catalyst surface, and the large absorption extinction coefficient of product 2.9
Recyclablity of the Nb2O5·nH2O catalysts was also briefly examined using catalyst T-I as the probe. This particular composite was chosen due to its lack of mesoporosity, thus eliminating consideration of substrate adsorption into the pores of the material. As is shown in Fig. S11,† the amount of ring-opened product 2 obtained following one reuse of the material suggests good catalyst reusability and minimal loss of the Nb2O5·nH2O acidic properties.
Fig. 7 Transmission (left) and TIRF (center) images of the same T-I particle immobilized on a cover glass and immersed in a 0.2 nM solution of compound 1 with 633 nm laser excitation (see ESI† Video S1). (Right) 3D representation of the accumulated spot intensity over the same particle reconstructed from a movie of 500 frames in length (frame time = 100 ms). |
Typical trajectories at selected bright spots are shown in Fig. 8A. Characterization of bright spots was performed by measuring the emission spectra in situ (Fig. 8C), ascribed to product 2. No fluorescent events were observed in control experiments performed without catalyst or without reagent or in the surrounding solution when compound 1 was added, indicating that the formation of the fluorescent product 2 can only be attributed to the Brønsted acidity of the T-I catalyst. Counting the single “turn ON” events of fluorescent molecules over the catalyst allows for an estimation of the reaction rate constant for the Brønsted acid-catalyzed ring opening of compound 1 and was found to be ~1.8 × 10−13 mol m−2 s−1, as illustrated in Fig. 8B.
The diffusion coefficient of a small molecule in water, for example of fluorescein, is in the order of 5 × 10−6 cm2 s−1,20 thus, within the TIRF excitation region (~200 nm) the diffusion time is on the μs timescale, being much shorter than the 100 ms imaging frame of our experiments. However, the ON time distribution histogram (not shown) shows an average ON time of around 2.5 s. This observation leads to the assumption that molecules of product 2 reside over the Nb2O5 catalyst sites and are observable before photobleaching occurs.
Deactivation of catalyst activity is an important concern for industry applications of any type of catalytic reaction and therefore using microscopic techniques for analyzing adsorption of the product is of wide importance.
In order to establish how robust the catalytic sites are, we carried out a study in which a sequence of 500 images was treated as in Fig. 7 (right), but accumulating separately the first 250 images from the subsequent set of 250 images. This corresponds to two sets of 25 seconds each as shown in Fig. 9, for a data set different from that in Fig. 7 (maps for the spot in Fig. 7 are available in Fig. S13†). Subtraction of the two sets gives the image shown in Fig. 9C and from a side-view in Fig. 9D, where positive signals represent loss of activity and negative signals activity gained. We were initially puzzled by the latter observation, as we did not expect additional acid sites to appear or be released with time. Interestingly, statistics calculated from Fig. 9D show that the average of all peaks is approximately zero within experimental error; this means that active sites that become inactive are replaced by new sites nearby. As we are using diffraction-limited optical methods, this effectively means new sites are ~300 nm or more from sites losing activity. Sites that appear at the same location will not be counted in this method as their bright blinks simply replace the previous ones. Visual analysis of enlarged images suggest that new sites are not very far from old ones. The simplest interpretation is that early acidity maps reveal hot spots on the catalyst, where ‘hot spots’ are characterized by their high acidity and easy accessibility; as these are used up by the chemistry of Fig. 3, the next tier of sites becomes active, effectively appearing in these maps as if acid sites had been created, when in fact they were available all along, they simply become active as their better competitors are used. We find that the time-lapse maps of Fig. 9 are a convenient representation of what otherwise can only be examined from TIRF videos.
Fig. 9 Protonation activity as shown by the conversion of 1 → 2 and detection of the fluorescence from 2. Surface maps A and B show the accumulated signals for 25 seconds and the subsequent 25 seconds, while map C shows the difference of A minus B expanded approximately ×2 (see colour coded scale). Panel D shows a different view of panel C, showing positive and negative peaks. Note that this figure is for a different spot than that in Fig. 7. |
Footnote |
† Electronic supplementary information (ESI) available: TEM images of commercial Nb2O5·nH2O, T-I, T-II, T-IIIa, T-IIIb and T-IV; XRD spectra of T-II, T-IIIa, T-IIIb and T-IV; pore size and surface area data as determined by nitrogen adsorption isotherms; nitrogen adsorption isotherms of commercial Nb2O5·nH2O, T-IIIa, T-IIIb and T-IV; UV-visible spectra monitoring the Brønsted acid ring opening of 1 to 2 using TFA, T-II, T-IIIb and T-IV as heterogeneous catalysts; fluorescence spectra monitoring the Brønsted acid ring opening of 1 to 2 using TFA, T-II, T-IIIb and T-IV as heterogeneous catalysts; 1H NMR spectra and peak attribution of 1 and 2 in CD3CN; 1H NMR spectra of 1 in CD3CN after the addition of T-IIIa; diffuse reflectance and solid state fluorescence spectra of product 2 adsorbed onto T-IIIa; 1H NMR spectra of 1 in CD3CN after the addition of T-I; experimental set up for single-molecule fluorescence observation under TIRF illumination for the Brønsted acid-catalyzed isomerization of 1 to 2. See DOI: 10.1039/c4cy00238e |
This journal is © The Royal Society of Chemistry 2014 |