Article pubs.acs.org/JPCC
Characterization of Electron-Donor and Electron-Acceptor Sites on the Surface of Sulfated Alumina Using Spin Probes Alexander F. Bedilo,*,†,‡ Ekaterina I. Shuvarakova,†,‡ Alexandra A. Rybinskaya,†,§ and Dmitrii A. Medvedev†,∥ †
Boreskov Institute of Catalysis SB RAS, Prospekt Lavrentieva 5, Novosibirsk 630090, Russia Novosibirsk Institute of Technology, Moscow State University of Design and Technology, Krasny Prospekt 35, Novosibirsk 630099, Russia
‡
ABSTRACT: Electron-donor sites on the surface of γ-Al2O3 modified with different amounts of sulfates were characterized using 1,3,5-trinitrobenzene as a spin probe. The concentration of 1,3,5-trinitrobezene radical anions formed on the surface electron-donor sites was found to decrease when the sulfate concentration and the surface acidity increased. Electron-acceptor sites of different strengths were studied using the formation of radical cations after adsorption of donor aromatic molecules with different ionization potentials. Changes in the intensity of the EPR signal observed after adsorption of hexafluorobenzene, toluene, hexamethylbenzene, and anthracene on the surface of sulfated alumina samples with different sulfate concentrations were analyzed. Modification of γ-Al2O3 with sulfates was found to result in the formation of strong electron-acceptor sites capable of ionizing toluene and hexamethylbenzene to their radical cations. Such sites were observed on the samples with the sulfate concentrations 4 wt % or higher. Weak electron-acceptor sites tested using anthracene were present on the surface of pure Al2O3. Their concentration was found to grow substantially when the concentration of sulfates was increased. The intensity of the EPR signal was found to depend on time after adsorption. The mechanisms of processes leading to the formation of the EPR signal attributed to electron-acceptor sites and the possible nature of such sites are discussed. Suggestions concerning the use of spin probes for characterization of electron-acceptor sites are made.
■
strength.8,14 This fact attracts interest to detailed characterization of various active sites present on the surface of sulfated alumina and their dependence on the concentration of deposited sulfates. Electron-donor and electron-acceptor sites detected by EPR using suitable spin probes are well-known in the literature.6,15−19 The EPR spectroscopy can be used only for investigation of paramagnetic species. However, active sites of heterogeneous catalysts are mostly not paramagnetic. In such cases information on their properties and concentration can be often obtained using spin probes. This approach is based on the use of specific molecules (probes) that selectively interact with the surface sites yielding surface paramagnetic species observed by EPR. The properties of sites capable of generating radical ion species after adsorption of donor or acceptor molecules are particularly intriguing. Without any doubt they are very active at least in these reactions. Although their structure and role in catalytic reactions are still not sufficiently understood, recent data suggest that such sites may be actually active in catalytic reactions of general interest.7,8,20 Our recent data21 also suggest that weak electron-acceptor sites may account for the activity of
INTRODUCTION Alumina is widely used as a catalytic support. The surface of the low-temperature Al2O3 phases is known to have both acid and base sites of different types.1−3 Their effect on the Al2O3 adsorption and catalytic properties is extensively discussed in the literature.4,5 However, rarely direct correlations between the concentrations of different surface sites and the catalytic activity are observed. Recently we reported methods for quantitative characterization of electron-donor6,7 and weak electron-acceptor sites8 on the surface of Al2O3-based catalysts. An excellent correlation was observed between the concentration of the donor sites and the catalytic activity of Pd/Al2O3 in CO oxidation.7 Even more remarkable proportionality was found between the concentration of weak electron-acceptor sites and the rate of catalytic ethanol dehydration to ethylene.8 Deposition of sulfates on the γ-Al2O3 surface is known to lead to a substantial increase of its acidity and catalytic activity in acid-catalyzed reactions.8−10 It was even claimed that sulfated alumina might have superacid sites with H0 ≤ 14.5.9 The catalytic properties of sulfated alumina generally resemble those of sulfated zirconia, which is more widely known due to its higher acidity and catalytic activity.11−13 Meanwhile, low cost, high surface area, wide availability, and reasonable thermal stability make sulfated alumina an attractive catalyst for acidcatalyzed processes that do not require very high acid © 2014 American Chemical Society
Received: April 10, 2014 Revised: July 4, 2014 Published: July 7, 2014 15779
dx.doi.org/10.1021/jp503523k | J. Phys. Chem. C 2014, 118, 15779−15794
The Journal of Physical Chemistry C
■
nanocrystalline oxides in destructive sorption of harmful halogenated compounds.21−25 The term “electron-donor site” was introduced in the 1960s for sites on the surface of oxide catalysts capable of reducing adsorbed molecules presumably due to single electron transfer from the surface. Aromatic nitro compounds are most frequently used as the probe molecules for investigation of the electron-donor sites due to their pronounced electron acceptor properties and formation of relatively stable radical products. Their reactions can be used to detect the surface electron-donor sites and determine their concentrations. Such sites were found on the surface of several typical oxide supports such as Al2O3, MgO, ZrO2, etc.6,26−29 Our recent results suggest that the formation of the paramagnetic species after adsorption of aromatic nitro compounds on γ-Al2O3 is a much more complex process than a simple charge transfer.6 It was suggested that this process involved separation of neutral radicals resulting from homolytic splitting in the activated complex similar to the mechanism proposed for the formation of O− radical anions on MgO30 rather than long-distance electron transfer leading to the formation of charged ion-radicals. Exact structure of the surface sites initiating this process is not known yet. It was proposed6 that they might be very basic isolated OH− groups that do not have a counterpart proton in the immediate vicinity. Such isolated groups are expected to have the properties similar to those experimentally observed for the electron-donor sites on the Al2O3 surface. Sites of the same type are likely to be responsible for the formation of O2− radical anions on ZrO2 treated with H2O2.31−34 Symmetrically, electron-acceptor sites are the sites capable of generating radical cation by abstracting an electron from donor molecules, usually aromatic.15 This process results in the formation of radical cations that can either be observed by EPR directly15,35,36 or participate in processes leading to the formation of secondary paramagnetic products on the surface.15,37,38 The strength of the surface acceptor sites can be qualitatively characterized by the donor properties of the molecules that can be ionized on such sites. Especially remarkable are strong sites capable of generating radical cations from benzene molecules with ionization potential 9.2 eV. The process seems to be direct electron transfer as it takes place immediately on contact even at relatively low temperatures.15,36 Such strong electron-acceptor sites are known on high-silica zeolites and sulfated oxides.15 Earlier we showed that sulfated zirconia possesses some of the strongest electron acceptor sites and extensively characterized them.15,36,39,40 In particular it was shown that the catalytic activity in butane isomerization correlated with the concentration of very strong acceptor sites capable of ionizing chlorobenzene.20,41 Strong electron acceptor sites were also noticed on sulfated alumina8,37 but were not studied in detail before. In the current study we characterized the electron-donor and electron-acceptor sites on the surface of sulfated alumina and changes of their properties depending on the concentration of deposited sulfates. The mechanisms of processes initiated on the electron-acceptor sites with different strength and the probable nature of such sites are discussed. Procedures for characterization of electron-acceptor sites with different strengths are suggested.
Article
EXPERIMENTAL SECTION
Sulfated alumina samples with different sulfate concentrations were prepared by incipient wetness impregnation of a commercial γ-Al2O3 sample (Sasol, S.A. = 210 m2/g) with (NH4)2SO4 (Fisher, Certified A.C.S.) solutions. According to the chemical analysis data provided by the manufacturer, the γAl2O3 sample contained about 20 ppm of Na2O, 100 ppm Fe2O3 and 120 ppm of SiO2 as the main impurities. The concentrations of the solutions used for impregnation were calculated to obtain the concentration of sulfates equal to 1, 2, 4, 8, 12, and 16 wt % normalized to SO3. Then, the samples were dried in a muffle furnace at 120 °C overnight and calcined in air at 600 °C for 3 h. The surface areas of the sulfated samples were close to that of the original γ-Al2O3 support, going down to 180 m2/g for the sample with the highest sulfate loading. An ERS-221 EPR spectrometer working in the X-band (ν = 9.3 GHz) was used in the study. The EPR spectra were recorded at 20 dB attenuation with typical microwave power 3 mW. The frequency of the microwave irradiation and the magnetic field were measured using a ChZ-64 frequency meter and a Radiopan MJ-100 magnetometer, respectively. The spectrometer operation and the analysis of the obtained results were performed using a PC and the software package EPRCAD developed in our laboratory. The experimental installation was described in more detail earlier.42 Prior to adsorption, the catalyst surface was purified from water, CO2, and organic impurities by overnight activation in a quartz sample tube in air at the desired temperature, typically 500 °C. 1,3,5-Trinitrobenzene (TNB) was used for characterization of electron-donor sites. Aromatic donors with different ionization potentials (IP) were used for characterization of electron-acceptor sites with different strengths: hexafluorobenzene (HFB, IP = 9.9 eV), toluene (IP = 8.8 eV), hexamethylbenzene (HMB, IP = 7.8 eV), and anthracene (IP = 7.4 eV). All the used reagents were of the “chemically pure” grade. Prior to use, toluene and HFB were dried with activated alumina. Adsorption of the probes was carried out in air at room temperature immediately after the sample was taken from the furnace and cooled down. TNB and HMB were adsorbed from 2 × 10−2 M solutions in toluene. Anthracene was adsorbed from a 4 × 10−2 M solution in toluene. Earlier it was shown that such technique makes it possible to measure reproducibly most of the surface electron-acceptor sites capable of ionizing the corresponding probe.6,8 After an activated sample was covered with the solution, the sample tube was sealed and kept for several hours or days at room temperature or 80 °C. For investigation of detailed kinetics some samples were firmly placed in the spectrometer resonator, and their EPR spectra were automatically recorded at certain intervals. All the EPR spectra were recorded at room temperature. The concentrations of the paramagnetic species were routinely determined by numerical double integration with baseline compensation. A DPPH standard and a toluene solution of a stable nitroxyl radical were used for calibration. Attenuation was selected to make sure that the EPR signal was not saturated. Rigid sample placement in the spectrometer resonator and computer processing of the spectra allowed us to decrease typical errors in the determination of the relative radical concentrations to ±10%. Absolute radical concentrations were typically reproducible with ±20% error. 15780
dx.doi.org/10.1021/jp503523k | J. Phys. Chem. C 2014, 118, 15779−15794
The Journal of Physical Chemistry C
Article
formed on the electron-donor sites of γ-Al2O3 that were discussed in detail in our previous publication.6 The typical feature of their spectra is hyperfine splitting on a single nitrogen atom with strong anisotropy of the a-tensor. The Azz constant, which can be easily determined from the splitting between the extreme components of the spectra, was equal to 31 G for all the samples. The intensity of the EPR spectrum and, consequently, the concentration of the surface donor sites decrease when the concentration of sulfates increases (Figure 1). The shape of the EPR signal changes as well. A singlet line with g = 2.003 starts to predominate at high sulfate concentrations (Figure 1). Earlier we showed that such signals are typical for radical species formed by polycondensation of aromatic molecules on the electron-acceptor sites of sulfated zirconia or sulfated alumina.8,15,20 A possible mechanism of such polycondensation process is discussed below. The appearance of this signal in our system is certainly related to the presence of strong electronacceptor sites in the samples with high sulfate concentrations. To differentiate the contributions of these two signals to the EPR spectrum, we used a method earlier successfully applied for quantitative analysis of electron-donor sites on carboncoated MgO.29 The intensities of the signal attributed to the TNB radical anions were compared using the leftmost line of their EPR spectrum. This line is practically not overlapped by the line belonging to the polycondensed structures formed on the electron-acceptor sites. The relative intensities of the signal of TNB radical anions on different samples were assumed to be proportional to the first integral of this spectral line. As the intensity of the singlet line on the γ-Al2O3 sample without sulfates is very low (see below), the absolute integral intensity of the EPR spectrum observed for this sample can be used for calibration of the obtained relative dependence. The results of such analysis are shown in Figure 2. The concentrations of the sites obtained by double integration of the whole EPR spectrum and by integration of the leftmost component after applying this calibration are very close for the samples with the sulfate concentrations 4% or lower. The concentration of the electron-donor sites goes down from 7 × 1018 g−1 for the samples with 0 or 1% SO3 to 1.5 × 1018 g−1 for
A different procedure was used in addition to the standard one for determination of the concentration of electron-donor sites on sulfated alumina samples in the presence of electronacceptor sites. First, the concentration of the signal obtained after TNB adsorption Al2O3 followed by heating at 80 °C was measured using the conventional procedure. This intensity was used for calibration of the signals obtained over sulfated alumina samples. As it is discussed below, it is mostly due to the radical anions formed on electron-donor sites. The contribution of electron-acceptor sites to this signal is lower by approximately 2 orders of magnitude. Then, the first integrals of the signal of the low-field components in the spectrum of TNB radical anions were determined for all the samples. Absolute concentrations of electron-donor sites on sulfated alumina samples were calculated by dividing the intensity of this low-field component on a sulfated alumina sample by its intensity observed after the TNB adsorption on Al2O3 followed by heating at 80 °C and multiplying by the absolute concentration of the latter signal.
■
RESULTS Effect of Sulfates on the Concentration of ElectronDonor Sites. Figure 1 presents the EPR spectra observed after
Figure 1. EPR spectra recorded at room temperature after TNB adsorption from toluene solution on alumina samples with different sulfate concentrations followed by heating at 80 °C for 3 h.
the TNB adsorption from toluene solution on the alumina samples with different sulfate concentrations. The samples were heated for 3 h at 80 °C before recording their EPR spectra at room temperature. We have earlier shown that this procedure can reveal most of the electron-donor sites that can generate TNB radical anions on the Al2O3 surface.6 The EPR spectra observed after TNB adsorption on the surface of the samples with low sulfate concentrations (4 wt % or lower) are similar to the spectra of the TNB radical anions
Figure 2. Total concentration of radicals obtained by double integration of the EPR spectra presented in Figure 1 (squares, solid line) and intensity of electron-donor sites obtained by integration of the left-most component in the spectra (circles, dashed line). 15781
dx.doi.org/10.1021/jp503523k | J. Phys. Chem. C 2014, 118, 15779−15794
The Journal of Physical Chemistry C
Article
Figure 3. Scheme of the TNB radical anion formation on an electron-donor site.
pounds that are solid under ambient conditions were adsorbed from toluene solutions. The EPR spectra observed after toluene adsorption on alumina samples modified with different concentrations of sulfates followed by heating at 80 °C for 3 h are shown in Figure 4. The experimental conditions were identical to those used for characterization of the electrondonor sites.
the sample with 4% SO3. At higher sulfate concentrations the contribution of the singlet line attributed to the polycondenced aromatics formed on the electron-acceptor sites becomes significant and grows with an increase of the sulfate concentration. Meanwhile, the concentration of the donor sites continues to decrease, going down to 1.5 × 1017 g−1 for the sample with 16% SO3. Thus, the concentration of the electron-donor sites on the surface of this sample is only about 2% of the maximum concentration observed in the absence of sulfates. So, the electron-donor sites are apparently related to surface basic sites, which are consumed by modification with sulfates. The structure of electron-donor sites and mechanism of the TNB radical anion formation on them is still a matter of discussion. However, the strongest electron-donor sites exist on basic oxides, such as CaO or MgO. On these oxides TNB radical anions are generated immediately after adsorption.21,29 The sites on the surface of alumina are weaker. So, generation of TNB radical anions on Al2O3 is indirect and requires prolonged storage at room temperature or heating at 80 °C.6 Suggested mechanism for the formation of TNB radical anions is shown in Figure 3. Its left section contains a hypothetical surface active site with an adsorbed TNB molecule and a hydroxyl group. The layer of aluminum and oxygen atoms in the bottom schematically represents the alumina surface. Under real experimental conditions the surface of oxides contains some adsorbed hydroxyl groups even after activation at high temperatures.43 We think that such activation results in separation of some H+ and OH− groups resulting from water chemisorption on the surface. Such sites act as strong acid and base sites, respectively. Note that the overall structure is electrically neutral but the basic active site where a TNB radical is formed has a local negative charge. The electron transfer to TNB leads to desorption of a neutral •OH radical. In this case, the TNB molecule acquires the negative charge and the spin density forming a radical anion existing as a part of an ion pair that remains overall electrically neutral. Detailed structure of a surface donor site that could initiate such process is not known yet. Our DFT calculations suggest that surface direct process shown in Figure 3 is energetically unfavorable for the simplest structures representing the aluminum oxide. The exact role of the solvent molecules is also not fully understood. The experimentally observed dependence of the process efficiency on the solvent donor properties indicates that the solvent acts as an electron donor.6 Most, likely, it helps to take the •OH radicals from the surface site and reacts with them undergoing partial oxidation. Characterization of Electron-Acceptor Sites Using Toluene as a Donor. The effect of sulfate deposition on the concentration of electron-acceptor sites was studied using four aromatic probe molecules: hexafluorobenzene, toluene, hexamethylbenzene, and anthracene. The latter two com-
Figure 4. EPR spectra recorded at room temperature after toluene adsorption on alumina samples with different sulfate concentrations followed by heating at 80 °C for 3 h.
A singlet line with g = 2.003 attributable to polycondensed structures formed on electron-acceptors sites was the only feature observed in the EPR spectra of all the samples. Its intensity was found to grow with the sulfate concentration. However, a weak signal was observed even on pure Al2O3. The signal results from toluene polycondensation on weak electronacceptor sites as it is discussed below. This fact should be taken into account during the quantitative analysis of the concentration of electron-donor sites using the method described above. Fortunately, on the samples not doped with sulfates its integral intensity is weaker by more than an order of magnitude than that of the signal attributed to TNB radical anions. So, its contribution can be neglected in the first approximation. The obtained dependences of the concentrations of the radicals formed from toluene on the concentration of sulfates are presented in Figure 5. The dependence of the concentration of paramagnetic species observed after toluene adsorption followed by heating at 80 °C for 3 h (Figure 5, line 2) is not 15782
dx.doi.org/10.1021/jp503523k | J. Phys. Chem. C 2014, 118, 15779−15794
The Journal of Physical Chemistry C
Article
Figure 6. Dependence of the integral concentration of paramagnetic species on time after toluene adsorption on alumina samples with different sulfate concentrations at room temperature.
Figure 5. Effect of the sulfate concentration on the concentrations of paramagnetic species generated from toluene on alumina samples with different sulfate concentrations: after TNB adsorption from toluene solution followed by heating at 80 °C for 3 h (1); after toluene adsorption followed by heating at 80 °C for 3 h (2).
Heating at 80 °C can significantly accelerate the process. Additional experiments showed that this was the optimum temperature giving the highest concentrations of the paramagnetic species. The maximum concentrations of the radical cations obtained during storage at room temperature and after heating for several hours are close (compare data in Figure 5 and 6). We carried out several sets of identical experiments to determine whether the heat treatment procedure gives comparable information on the concentrations of weaker electron-acceptor sites as the storage at room temperature. The results of another series of experiments are reported in Figure 7. Comparison of these data with the results reported
linear. At sulfate concentrations 2% or lower the concentration of the paramagnetic species remains very low and does not grow much with the deposition of sulfates. The main growth is observed in the sulfate concentrations range between 2% and 8%. Further increase of the sulfate concentration leads to a very small growth of the radical concentration. The concentration of the same paramagnetic polycondensed species formed after adsorption of the TNB solution in toluene under the same conditions was substantially lower, on the average by a factor of 2 (Figure 5, line 1). This concentration was determined by subtracting the concentration of the TNB radical anions from the total intensity of the EPR signal. Here the concentration of the TNB radical anions was determined by integrating the leftmost component of its spectrum. Apparently, the presence of TNB adsorbed on the catalyst surface partially hinders polycondensation of the solvent molecules decreasing the concentration of the radicals formed from them. To analyze changes in the concentration of paramagnetic species taking place with time, we monitored radicals formed on the surface of alumina samples with different sulfate concentrations after toluene adsorption for 1 week at room temperature (Figure 6). The first spectrum was registered in less than 1 h after adsorption. The following spectra were recorded every day with approximately 24 h intervals. For the samples with the sulfate concentrations 12 and 16% the kinetic curve passes through a maximum on the second or third day after the toluene adsorption. The intensities of the signals observed on the samples with 4 and 8% SO3 level out in approximately the same time and do not change much thereafter. Meanwhile, the intensities of the signals observed on the samples with the sulfate concentrations 2% or lower keep increasing for the whole week. So, one has to be very cautious when comparing the concentrations of the radicals formed over different samples only at one time after adsorption. The obtained kinetic data (Figure 6) prove that the concentration of the observed paramagnetic species substantially increases after storage for several days. This is a standard procedure used earlier by many researchers. Unfortunately, it is very slow. In addition, the maximum concentrations are obtained at different times depending on the catalyst acidity.
Figure 7. Comparison of the EPR spectra intensities observed on alumina samples with different sulfate concentrations immediately after toluene adsorption, after heating at 80 °C for 20 h and maximum concentration achieved at room temperature.
above show that the concentrations of the EPR spectra can be reproduced reasonably well. In all cases the concentrations of the sites tested after heating at 80 °C were somewhat higher than those reached at room temperature, except for the sample with the highest concentration of sulfates. Both were significantly higher than the concentration of the EPR signal observed immediately after toluene adsorption. On the samples with 2% SO3 or less the radical cations were not formed from toluene immediately after adsorption 15783
dx.doi.org/10.1021/jp503523k | J. Phys. Chem. C 2014, 118, 15779−15794
The Journal of Physical Chemistry C
Article
suggesting that such strong acceptor sites were not present on these samples in measurable concentrations. It seems that exceeding the threshold at about 2% is necessary for the formation of strong electron-acceptor sites. However, the obtained data do not imply that strong acceptor sites do not exist at all on the surface of sulfated alumina with 2% SO3. We can only claim that their concentration is below our detection limit. In any case it appears that high concentration of sulfates is essential for the formation of strong electron-acceptor sites capable of ionizing toluene after adsorption. This result suggests that the strong sites may originate from the interaction of two weaker sites. Note that typically a considerable fraction of the surface of sulfated zirconia catalysts above one-half of the monolayer has to be covered with sulfates before their catalytic activity in isomerization of alkanes increases.44−46 The origin of this effect may be the same as that of the dependence of the strong electron-acceptor site concentration on the sulfur loading found in this study. Comparison of the polycondensation rates at room temperature and 80 °C showed that the rates of the secondary processes leading to the appearance of the spectra attributed to polycondensed species substantially increases with temperature. Unfortunately, our attempts of estimating the activation energy of this process were not successful. The obtained data could not be approximated in the Arrhenius coordinates. Most likely, the overall process consists of multiple different simultaneous reactions with different rates and activation energies, making it impossible to determine single activation energies at different temperatures. Note that primary toluene radical cations could not be observed even on sulfated alumina samples. Earlier we failed to detect them on sulfated zirconia as well. However, we managed to observe and study in detail the formation of secondary radicals with characteristic well-resolved EPR spectra, which are formed on strong acceptor sites from the primary radical cations.36 No such radicals were observed after the toluene adsorption on sulfated alumina either. The most probable reason for such difference in the observed spectra is related to substantial hyperfine splitting on27Al nuclei, which have nuclear spin unlike predominating zirconia isotopes. Such splitting may result in the widening of the EPR spectrum leading to the observed unresolved signals. Characterization of Electron-Acceptor Sites Using Hexamethylbenzene as a Donor. Meanwhile, relatively stable primary radical cations were registered on sulfated zirconia after adsorption of hexamethylbenzene.15,35 We studied the formation of HMB radical cations from toluene solution on γ-Al2O3 samples with different sulfate concentrations. No signals attributable to HMB radical cations were observed on the samples with the sulfate concentrations 2% or lower (Figure 8). A typical EPR signal with nearly isotropic hyperfine splitting with a = 6.05 G attributed to primary HMB radical cations with 18 equiv protons was observed on the samples with higher concentrations of sulfates. Although we observed only 13 or 11 lines instead of the expected 19, a good correlation between their experimental and theoretical intensities proved that it belonged to HMB radical cations.35 The other hyperfine lines were too weak to be experimentally detected. We have shown earlier that only HMB radical cations are observed in the EPR spectra after its adsorption on sulfated zirconia from benzene or toluene solutions.15,35 The same
Figure 8. EPR spectra observed after hexamethylbenzene adsorption from toluene solution on alumina samples with different sulfate concentrations.
result was obtained in this study on sulfated alumina. These results are due to fast electron exchange reactions apparently typical for radical cations formed on the surface of sulfated oxides. As a result, only the spectra of the cation radicals formed from a substance with the lowest ionization potential are observed by EPR. The line width of the central narrowest lines in the spectrum of HMB radical cations observed in this study was close to 1 G. This value exceeds that observed on sulfated zirconia approximately by a factor of 2. Most likely, very small unresolved hyperfine splitting on27Al nuclei makes the lines on γ-Al2O3 wider. The signal of the HMB radical cations appeared immediately after adsorption without any noticeable activation energy. Then its intensity only decreased. Such kinetics is very different from that of the other studied radical species. It is very likely that such primary radical cations are formed by direct electron transfer to the electron-acceptor site with sufficient strength. This process does not seem to involve any other stages or chemical reactions, except for the above-mentioned possible involvement of the solvent radical cations as intermediates in the electron transfer. Relatively high stability of HMB radical cations originates from the lack of hydrogen atoms attached directly to the aromatic ring. It is much more difficult to substitute CH3 groups attached to the aromatic ring in HMB than hydrogen atoms. As a result, the rates of various reactions involving substitution at the aromatic ring, such as the polycondensation processes discussed in the manuscript, are much slower. So, we can readily observe primary HMB radical cations formed on electron acceptor sites at room temperature unlike most other aromatic radical cations, including those of anthracene and toluene. The obtained results indicate that sufficiently strong electronacceptor sites capable of direct ionization of HMB molecules with ionization potential 7.85 eV are formed on sulfated alumina only starting from the sulfate concentration 4 wt %. Then, their concentration grows when the concentration of sulfates increases. On the samples where these radical cations are formed, the concentration of sites able to ionize HMB immediately after adsorption is higher than that of the sites ionizing toluene. However, both sites appear in measurable 15784
dx.doi.org/10.1021/jp503523k | J. Phys. Chem. C 2014, 118, 15779−15794
The Journal of Physical Chemistry C
Article
amounts at the same sulfate concentration of 4 wt % and are not observed on the samples with lower sulfate concentrations. Characterization of Electron-Acceptor Sites Using Anthracene as a Donor. Donors with even lower ionization potentials have to be used to study weaker sites. Earlier we successfully used anthracene to test weak electron-acceptor sites active in ethanol dehydration.8 Indeed, anthracene adsorption from toluene solution led to immediate appearance of a singlet line on all the studied samples (Figure 9). The
Figure 10. Comparison of the EPR spectra intensities observed on alumina samples with different sulfate concentrations immediately after anthracene adsorption, after heating at 80 °C for 20 h and maximum concentration achieved at room temperature.
Figure 9. EPR spectra observed after anthracene adsorption from toluene solution on alumina samples with different sulfate concentrations. Figure 11. Changes of the EPR spectra intensity during the first 30 min after anthracene adsorption on selected sulfated alumina samples.
intensity increased with the growth of the sulfate concentration until 12% SO3. This means that electron-acceptor sites capable of ionizing anthracene (IP = 7.4 eV) after adsorption are present even on pure alumina. Their concentration increases with SO3 loading in the range of 0−12% from 0.2 to 1.8 × 1018 g−1 (Figure 10). Special experiments demonstrated that even sulfate concentrations as low as 0.1 wt % SO3 could be distinguished using anthracene based on the increase of the acceptor site concentration. The intensities of the EPR signals observed on the samples with SO3 concentrations 0−8 wt % slowly increased with time. Meanwhile, on the samples with higher sulfate concentrations they decreased, so that the highest intensities were observed directly after adsorption. As a result, the values in the first two columns in Figure 9 are the same for the samples with 12 and 16 wt % SO3. This decrease was especially fast over the sample with the highest SO3 concentration. It may be the reason why the intensity of the EPR signal observed after anthracene adsorption on the sample with 16% SO3 was lower than on the sample with 12% SO3. It is notable that both for anthracene and toluene, the integral intensities did not change by more than several percent in the first 30 min after adsorption on all samples, except for the anthracene adsorption on the samples with 12 or 16% SO3, where the intensity rapidly decreased (Figure 11). So far, we did not observe a system, where the intensity of the EPR signal
would quickly grow after adsorption. In the case of HMB, the intensity of the resolved spectrum belonging to primary radical cations decreased with time. The decrease was faster on the samples with lower sulfate concentrations. Meanwhile the integral intensity again did not change much, proving that most of the HMB radical cations were converted to some oligomeric species rather than disappear. The initial intensity also did not significantly depend on the adsorption temperature. These observations seem to indicate that the EPR signal observed immediately after adsorption reflects the concentrations of electron-acceptor sites capable of fast direct electron transfer. Later oligomerization or back-transfer processes are much slower. For samples with SO3 concentrations 0−8 wt % the maximum integral intensities of the EPR spectra were reached after waiting for 2−3 days. They exceeded the initial intensities observed after adsorption by a factor of 2−2.5. Very similar values were again obtained after heating at 80 °C for 20 h (Figure 10). As anthracene is the strongest donor with the lowest ionization potential, which was used in this study, such procedure revealed the weakest electron-acceptor sites. Their concentration varied from 0.5 × 1018 g−1 for Al2O3 to 4 × 1018 g−1 for 8% SO3/Al2O3. There is little doubt that even higher 15785
dx.doi.org/10.1021/jp503523k | J. Phys. Chem. C 2014, 118, 15779−15794
The Journal of Physical Chemistry C
Article
Table 1. Concentrations of Electron-Acceptor Sites Revealed after Adsorption of Different Donors on Alumina Samples with Different Sulfate Concentrations experimental conditions
immediately after adsorption at room temperature
aromatic donor
HFB
toluene
ionization potential, eV sample Al2O3 2% SO3−Al2O3 4% SO3−Al2O3 8% SO3−Al2O3 12% SO3−Al2O3 16% SO3−Al2O3
9.9
8.8
