5706
J. Phys. Chem. B 2000, 104, 5706-5714
Photooxidation of Toluene and p-Xylene in Cation-Exchanged Zeolites X, Y, ZSM-5, and Beta: The Role of Zeolite Physicochemical Properties in Product Yield and Selectivity A. G. Panov, R. G. Larsen, N. I. Totah, S. C. Larsen,* and V. H. Grassian* Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52242 ReceiVed: March 2, 2000
The photooxidation of toluene and p-xylene with molecular oxygen and visible light has been investigated in several cation-exchanged zeolites. In general, the yield of the photooxidation products, for fixed irradiation time and intensity, was found to correlate with the electric field intensity at the cation sites within the zeolites. On the basis of measurements of CO vibrational frequencies, electric fields of approximately 3-7 V nm-1 are indicated for the cation-exchanged zeolites X, Y, ZSM-5, and Beta used in these studies. These large electric fields are thought to promote photooxidation by stabilizing an intermolecular charge transfer state (R+‚O2-) that is formed upon excitation with visible light. The measured electric field was found to correlate with the product yield and was highest in divalent cation-exchanged zeolites with high Si/Al ratios, such as BaZSM-5 and BaBeta. For zeolites containing the same cation, the selectivity of toluene to form benzaldehyde and p-xylene to form p-tolualdehyde was found to be higher in zeolites X and Y (g87%) compared to ZSM-5 and Beta ( 400 nm. The spectrum of benzaldehyde adsorbed in BaY zeolite is also shown.
CO (Matheson, 99.99%), propylene (Matheson, 99.6% purity) and O2 (Air Products, 99.6% purity) were also used without further purification. Results Photooxidation of Toluene in NaX, NaY, BaX, and BaY. Difference spectra following the room temperature visible light photooxidation of toluene in NaX, NaY, BaX, and BaY are shown in Figure 1. The spectra plotted in Figure 1 have been normalized to the amount of toluene initially present in the zeolite. The amount of toluene initially in the zeolite was determined from a calibration of the integrated absorbance of the toluene band at 1456 cm-1. Experiments are typically done under loading conditions of about 2 hydrocarbon molecules per super cage. Thus, the intensity of the positive bands in the spectra shown in Figure 1 correlate with the percent conversion of toluene to benzaldehyde. The amount of benzaldehdyde formed was determined from the 1313 cm-1 band of benzaldehyde and not the intense CdO stretching vibration because the intensity and shape of the band at 1313 cm-1 was not affected by the zeolite cation and gave a more reliable calibration for all of the zeolite hosts used in this study. Following photooxidation of toluene in BaY, negative features at 1600, 1595, 1466, and 1389 cm-1 and positive bands at 1677, 1654, 1593, 1580, 1456, 1392, and 1313 cm-1 are seen in the spectrum. The negative features are due to the loss of the parent toluene, and the positive bands are due to a new photoproduct. The spectrum of the photoproduct resembles the spectrum shown of benzaldehyde adsorbed in BaY. These results are in agreement with Sun et al.5 A similar albeit less intense FT-IR spectrum is obtained in BaX, indicating that toluene is being photooxidized to benzaldehyde in BaX but to a lesser extent compared to BaY. For NaY, the most intense band assigned to the CdO stretching vibration is shifted to higher wavenumber compared to that seen in the Ba2+-exchanged zeolites. For NaX, only a small amount of photoproducts are observed following visible-light irradiation of toluene in the presence of oxygen. The percent conversions of toluene to benzaldehyde were
Photooxidation in Cation-Exchanged Zeolites
J. Phys. Chem. B, Vol. 104, No. 24, 2000 5709
Figure 2. Bar graph of the electric fields for zeolites used in this study: NaX, BaX, NaY, BaY, BaZSM-5, and BaBeta. The electric fields were determined from νCO using eq 1.24
determined from the calibration curves for toluene and benzaldehyde and are 0.2, 0.8, 1.6, and 2.6% for NaX, BaX, NaY, and BaY, respectively.33 As discussed in the Introduction section, photooxidation of hydrocarbons in zeolites is thought to proceed through a hydrocarbon-O2 charge-transfer complex that is stabilized in the presence of the electric field of the cation sites in the zeolite. The electric fields in the zeolites were determined from the vibrational frequency of CO coordinated to the exchangeable cation in the zeolite. The CO frequencies (νCO, in wavenumber) of zeolites used in this study, measured or taken from the literature are 2164-NaX,17 2172-NaY,17 2170-BaX14, 2178BaY,14 2184-BaZSM-5,14 and 2184-BaBeta.24 The electric fields, as determined from eq 1 below,24 are plotted on a bar graph in Figure 2.
E ) 6.1764 × 10-4 (νCO)2 - 2.4976 νCO + 2516
(1)
The bar graph clearly shows that the electric field increases with divalent cations versus monovalent cations for a given zeolite host. In addition, the electric field is also found to increase for zeolites with higher Si/Al ratios, for a given cation. For zeolites with cation sites that have large electric fields, a large stabilization of the hydrocarbon-oxygen charge transfer state is expected, causing a red shift of the charge-transfer transition, enabling excitation with visible photons (λ > 400 nm).4-7,9-13 Further, the red shift of the charge-transfer transition caused an improved overlap between the excitation spectrum and the broadband irradiation source (λ > 400 nm). Thus, more photons will have enough energy to access the charge transfer state for host zeolites with larger electric fields. For this reason, it is expected that product yield and electric field strength of the zeolite should vary, if not linearly, at least in an increasing monotonic fashion. Figure 3 shows a plot of percent conversion to benzaldehyde as determined from in-situ IR analysis versus electric field strength for the four zeolites shown in Figure 1. The graph in Figure 3 clearly shows that the electric field in faujasite zeolites plays a major role in the zeolite activity. Although the infrared spectra show that photooxidation of toluene with visible light produces benzaldehyde in high yield, overlapping vibrational bands of other possible products do not allow the determination of reaction selectivity to be determined by in-situ FT-IR spectroscopy. In-situ solid state 13C-H CP/ MAS NMR spectroscopy was also used to analyze product selectivity in these reactions. Figure 4 shows the 13C-H CP/ MAS NMR spectrum following photooxidation of toluene in BaY. As seen in Figure 4, the only peaks observed by solid state NMR are due to toluene (20 ppm) and benzaldehyde (199
Figure 3. Correlation of the electric field in the zeolite hosts NaX, BaX, NaY, and BaY with toluene percent conversion to benzaldehyde. Electric fields were calculated using eq 1.24
Figure 4. 13C-H CP/MAS NMR spectrum following the photooxidation of toluene-13CH3 and oxygen in BaY near room temperature with λ > 400 nm.
TABLE 2: GC Analysis of the Percent Product Distribution for Photooxidation of Toluene in NaY, BaX, and BaY Zeolite zeolite NaY BaXe BaY BaY-UVf
BZa 92 88 87 78
BAb 5 6 4 8
phenol 0.5 2 2 1
CONDc tr tr 2 4
d
p-xylene
cresol
tr 3 3 8
0.5 1 1 1
a BZ ) benzaldehyde. b BA ) benzyl alcohol. c COND ) condensation products. d tr ) trace amounts. e Extracted in ether. f Broadband irradation included UV wavelengths λ > 200 nm.
ppm). However, the relatively low signal-to-noise ratio in these experiments, even after 40 h of visible-light irradiation, indicates that any products on the order of 5-10% of that of the benzaldehyde would not be detected. Therefore, further analysis of the products with a more sensitive analytical technique was necessary. Ex-situ analysis of the products with GC was determined to be the best method for obtaining product distributions for the toluene photooxidation reaction in the zeolites. Table 2 gives the product distributions obtained from the GC data for NaY, BaX, and BaY following visible light excitation of toluene and molecular oxygen. Since the yield in NaX zeolite was very low, the product distribution in this zeolite was not analyzed by GC. For visible light photooxidation, Table 2 shows that benzaldehyde is formed with 87% or greater selectivity in these zeolites. NaY has the highest selectivity (92%) of the three zeolites. As can be seen in Table 2, the second major product is benzyl alcohol which accounts for approximately 5% of the products formed. Experiments were done to determine if benzyl alcohol was formed via a bimolecular reaction involving benzyl
5710 J. Phys. Chem. B, Vol. 104, No. 24, 2000
Panov et al.
SCHEME 1
hydroperoxide and unreacted toluene. In experiments with BaY, higher loadings of toluene were used in order to determine if the benzyl alchol yield would increase relative to benzaldehyde formation. It was found that when the toluene to Ba2+ ratio was doubled from 1.5 to 3 toluene molecules per supercage the amount of benzyl alcohol increased from approximately 5 to 10% of the products formed. These data are consistent with the formation of benzyl alcohol via the reaction between benzyl hydroperoxide and toluene. The reaction mechanisms for the formation of the two major oxidation products, benzaldehyde and benzyl alcohol are shown in Scheme 1 and are consistent with the data presented here. Other products that are present at low levels include p-xylene, phenol, cresol, and condensation products. The condensation products are defined as compounds that result from secondary reactions between two aromatic molecules. The identity of the condensation products and mechanisms for their formation, as well as for some of the other nonselective products, are discussed in more detail in the next section. In addition to visible light excitation, the effect of wavelength on the product selectivity was investigated. GC analysis following broadband excitation using the full Hg arc (λ > 200 nm) of toluene in the presence of O2 in BaY is also given in Table 2. Irradiation times on the order of 4 h were used in these experiments. These shorter times for broadband irradiation with λ > 200 nm gave the same yield as the visible light excitation done for approximately four times as long. The selectivity with respect to benzaldehyde formation decreased to 78% from 87% and the yield of benzyl alcohol doubled when UV broadband irradiation was used. Photooxidation of Toluene with Visible Light in CaY, BaZSM-5, NaZSM-5, BaBeta and BaHY. Difference spectra following the room temperature visible light photooxidation of toluene in CaY, BaZSM-5, NaZSM-5, and BaBeta are shown in Figure 5. These resulting spectra significantly differ from the ones shown in Figure 1. The spectral features are broader with the most prominent positive peaks near 1711, 1630-1650, 1585-1595, and 1511 cm-1. The appearance of the peaks at 1711 and 1511 cm-1 as well as the broadening of the spectra suggest that there is a mixture of products with overlapping vibrational bands. The physicochemical properties of these zeolite hosts have little in common, e.g., they differ in chemical composition (Si/Al ratio and exchangeable cation) and framework topology, yet all exhibit a significant loss of selectivity. As will be shown, a common feature for these zeolites is that they contain Brønsted acid sites. In the case of BaHY zeolite, acid sites were intentionally introduced into the zeolite in order to determine if the selectivity would substantially decrease. It can be seen from the infrared spectra that this is indeed the case. Ex-situ GC analysis for the photooxidation of toluene with
Figure 5. Difference FT-IR spectra of toluene photooxidation in CaY, BaZSM-5, NaZSM-5, BaBeta, and BaHY zeolites near room temperature with λ > 400 nm. The spectra no longer look like that of benzaldehyde alone, and the bands are quite broad in these spectra compared to those seen in the NaY, BaX, and BaY spectra shown in Figure 1.
TABLE 3: GC Analysis of the Percent Product Distribution for Photooxidation of Toluene in BaHY, BaZSM-5, and BaBeta zeolite BaHY BaZSM-5 BaBeta
BZa BAb phenol CONDc p-xylene cresol p-TALDd othere 56 31 27
4 1 trf
7 23 24
20 15 21
1 1 1
3 8 10
1 4 10
8 17 7
a BZ ) benzaldehyde. b BA ) benzyl alcohol. c COND ) condensation products. d p-TALD ) p-tolualdehyde. e Other ) doubly oxygenated products including benzoic acid and methylbenzoquinone. ftr ) trace amounts.
visible light in several of these zeolites (BaBeta, BaZSM-5, and BaHY) is given in Table 3. The product distribution for toluene photooxidation in BaBeta, BaZSM-5, and BaHY as analyzed by GC, and GC/MS for further identification of products, clearly shows a significant loss of selectivity for benzaldehyde compared to the zeolites NaY, BaX, and BaY. Other major products now include phenol, condensation products, i.e., products that contain more than one aromatic ring, cresol and p-tolualdehyde, and other products identified as doubly oxygenated ring compounds, such as methyl-substituted benzoquinone and benzoic acid. In addition, there is an overall decrease in the benzyl alcohol product. As will be shown, this is due to the reactivity of benzyl alcohol in these zeolites to form some of the other products identified in Table 3. As noted above, the commonality between the zeolites that are less selective (i.e., CaY, BaZSM-5, NaZSM-5, and BaBeta) compared to those that are selective (i.e., NaY, BaX, and BaY) is that the zeolites which exhibit lower selectivity have a higher content of Brønsted acid sites. The colorimetric test described by Ramamurthy and co-workers was utilized to detect Brønsted acid sites in the zeolites used in this study.29,30 The colorimetric test is based on differences in the electronic absorption properties of protonated and unprotonated forms of probe molecules, such as retinol and retinyl acetate. Both retinol and retinyl acetate form a blue retinyl cation in an acid solution. The zeolites were
Photooxidation in Cation-Exchanged Zeolites SCHEME 2
J. Phys. Chem. B, Vol. 104, No. 24, 2000 5711 SCHEME 3
SCHEME 4
activated by heating to 300 °C under vacuum for approximately 12 h. Samples of BaBeta, BaZSM-5,34 and CaY turned dark blue, NaZSM-5 turned blue, and BaY turned light blue when a dilute solution of retinol was added to the activated zeolite. Activated samples of BaX, NaX, and NaY exhibited no color change when a dilute solution of retinol was added to the zeolite. The intensity of blue coloration was taken to indicate the relative amount of Brønsted acid sites. The results of the tests suggest the following acidity trend: BaBeta > BaZSM-5, CaY > NaZSM-5 > BaY > BaX > NaX. This is qualitatively consistent with the observed trends in product distributions for toluene photooxidation. Additional support for the existence of Brønsted acid sites in zeolite samples was obtained using FT-IR spectroscopy. Brønsted acid sites in zeolites are known to catalyze the polymerization of propylene. Therefore, the polymerization of propylene as monitored by FT-IR spectroscopy can be used detect Brønsted acid sites in zeolites.15,31,32 The small size of propylene relative to the pore sizes of the zeolites used in these studies ensures that the internal and external surface of the zeolites are probed. In zeolites X and Y, the strong intensity of the CdC stretching mode in the FT-IR spectrum of propylene adsorbed in these zeolites suggests that very little propylene polymerizes. However, in cation-exchanged zeolites ZSM-5 and Beta, there is a substantial decrease in the intensity of the Cd C stretching mode in the propylene FT-IR spectrum, indicating that propylene has polymerized at acid sites. The data shown in Tables 2 and 3 indicate that the presence of phenol and condensation products correlates with the presence of acid sites in the zeolites. Mechanisms can then be proposed for the formation of these products in the presence of acid sites. In this regard, it is well-known that alcohols and phenols can be produced from hydroperoxides in the presence of acid sites.35 Scheme 2 shows a mechanism for the formation of phenol from benzyl hydroperoxide in the presence of acid sites. This mechanism also predicts that formaldehyde will be formed in equimolar amounts. Because of the low molecular weight and volatility of this compound, it is difficult to quantify the amount of formaldehyde using GC analysis. As shown previously, formaldehyde exhibits a band near 1711 cm-1 in cationexchanged zeolites.15 Therefore, the band near 1711 cm-1 in the FT-IR spectra shown in Figure 5 is evidence for the presence of formaldehyde in the zeolites. Other products in the spectrum can also be attributed to the presence of acid sites. For example, the major condensation products identified include diphenyl methane derivatives. Although diphenyl methane derivatives have been shown to form from the reaction of toluene with aldehydes, such as formaldehyde and benzaldehyde, in zeolites with strong acid sites, i.e., USY,36 only a minor amount of these condensation products
were formed when benzaldehyde and toluene were allowed to react in the zeolite BaHY (60 °C). However, the thermal reaction of benzyl alcohol and toluene in the presence of acid sites (BaHY, 60 °C) did result in the formation of isomers of monomethyl-substituted diphenyl methane as the major products. A likely mechanism for this reaction is shown in Scheme 3. Thus, the decrease in the amount of benzyl alcohol observed in these less selective zeolites is consistent with its further reaction to form condensation products. The thermal reaction between formaldehyde and toluene in the zeolites that contained acid sites (BaHY, 60 °C) was also shown to produce diphenyl methanes. Formaldehyde is known to be more reactive than benzaldehyde for acid-catalyzed condensation reactions with aromatic compounds.36 Scheme 4 shows a mechanism for the formation of condensation products that is consistent with these data. As discussed in the next section, additional products, such as cresol and p-tolualdehyde, most likely arise from the photooxidation of small quantities of p-xylene that form in these zeolites. As predicted from the electric field strength determined for BaBeta and BaZSM-5, the conversion of toluene to benzaldehyde is greater in these two zeolites than in BaX and BaY. The electric field strength determined from νCO of 2184 cm-1 for both zeolites and eq 1 is 7.3 V nm-1, and the conversions to benzaldehyde were estimated to be on the order of 5%.33 Interestingly, the conversion of toluene to benzaldehyde in BaHY was similar to that for BaY, on the order of 1-2%. Photooxidation of p-Xylene with Visible Light in Zeolites X, Y, and ZSM5. Difference spectra following the roomtemperature visible light photooxidation of p-xylene in X, Y, and ZSM-5 are shown in Figure 6. The reactivity trends observed for p-xylene photooxidation in these zeolites are similar to those observed for toluene. The difference spectra for BaX and BaY zeolites exhibit bands at 1705, 1662, 1598, and 1578 cm-1 that correspond to the formation of p-tolualdehyde. A spectrum of p-tolualdehyde in BaY is shown for comparison. In the case of BaHY and BaZSM5 zeolites, spectral features due to p-tolualdehyde are seen, as well as other bands near 1712 and 1493 cm-1. In addition, the vibrational bands are broader in the BaHY and BaZSM5 spectra compared to the BaX and BaY spectra indicating that additional products are formed that have overlapping vibrational bands.
5712 J. Phys. Chem. B, Vol. 104, No. 24, 2000
Panov et al. as compared to BaHY. These results suggest that the topology of the zeolites also play a role in formation of the higher molecular weight condensation products. Discussion
Figure 6. Difference FT-IR spectra following the photooxidation of p-xylene in BaZSM-5, BaHY, BaX, and BaY zeolites near room temperature with λ > 400 nm. The spectrum of p-tolualdehyde adsorbed in BaY zeolite is also shown.
TABLE 4: GC Analysis of the Percent Product Distribution for Photooxidation of p-Xylene in NaY, BaY, BaHY, and BaZSM-5 zeolite NaY BaY BaHY BaZSM-5
p-TALDa 89 76 39 32
p-MBAa
cresol
CONDc
otherd
10 10 1 2
tre
tr 3 21 10
1 4 8 16
7 31 40
a p-TALD ) p-tolualdehyde. b p-MBA ) p-methylbenzyl alcohol. COND ) condensation products. d Other ) double oxygenated products including p-toluic acid. etr ) trace amounts.
c
GC and GC/MS analysis was carried out on reaction mixtures extracted from zeolites NaY, BaY, BaHY, and BaZSM-5. The results of these analyses are shown in Table 4. NaY and BaY show the greatest selectivity for p-tolualdehyde, 89 and 76%, respectively. The product yields for these two zeolites were determined from analysis of the infrared data and show that the conversions are approximately double for the p-xylene photooxidation reaction compared to what they are for the toluene photooxidation reaction in these zeolites. The second most abundant product in these selective zeolites is p-methylbenzyl alcohol. In analogy to the toluene photooxidation mechanisms, Scheme 1 with RdCH3 summarizes the reaction mechanisms operative for the two major products for p-xylene photooxidation. Similar to the case for toluene, photooxidation of p-xylene in the zeolites BaHY and BaZSM-5 shows a much lower selectivity. Loss of the selectivity in these more acidic zeolites is largely due to the formation of cresol and various condensation products via mechanisms analogous to those proposed for toluene, as described in Schemes 2, 3, and 4 with RdCH3. It should be noted, that despite higher cresol yields, the relative amount of condensation products formed is lower in BaZSM-5
Correlation of Electric Field Strength and Product Yield in Zeolites with Different Chemical Composition and Topology. The cation effect (NaY vs BaY) and thus the electric field dependence has been demonstrated previously by Frei and coworkers in their studies of the photooxidation of hydrocarbons in zeolites.5,7,8,13 Here we have examined how the electric field changes in different zeolite hosts by examining zeolites that differ in chemical composition (both exchangeable cation and Si/Al ratio) and framework topology. As shown in Figure 2, it was found that for the faujasite zeolites, X and Y, the electric fields at the cation sites were not only a function of the exchangeable cations but also the Si/Al ratio of the zeolite hosts. It was also found that for a given cation, higher fields are obtained in the zeolite with the higher Si/Al ratio as discussed by Bordiga et al.17 The impact of the electric field value on the photooxidation reactions was demonstrated by the correlation shown in Figure 3 between the percent yield to benzaldehyde at low conversions and the electric field value measured for each zeolite. Thus, for the cation exchanged faujasite zeolites examined, high product yields are favored by divalent cations compared to monovalent cations and by zeolite Y compared to zeolite X. For the zeolites with other structures, i.e., BaZSM-5 and BaBeta, it was determined that even higher electric fields are present in these cation-exchanged zeolites compared to the faujasite zeolites. This is due to the even higher Si/Al ratio in these zeolites and the zeolite structure. Importantly, in the case of BaHY, there was no increase in percent yield from that of BaY indicating that the electric field in the zeolite is determined mostly by the presence of the exchangeable cation and not the presence of acid sites. It was also observed that there is a greater conversion for the p-xylene photooxidation reaction compared to the toluene photooxidation reaction. p-Xylene has a lower ionization potential (IP ) 8.44 V) compared to toluene (IP ) 8.82 V); therefore, the charge-transfer band will be lower in energy for p-xylene‚O2 complexes compared to toluene‚O2 complexes. Thus, for broadband visible irradiation, λ > 400 nm, more photons will be effective in exciting the p-xylene‚O2 complex to the charge-transfer state, R+O2- than will be for the toluene‚ O2 complex, assuming similar hydrocarbon-oxygen dynamics within the zeolite. From the charge transfer state, stable products are formed, and thus an increase in the concentration of the initial charge transfer state will lead to an increase in the yield of stable products. Loading, UV Light, and Zeolite Topology Dependence on Reaction Selectivity. Several other factors besides electric field are found to effect zeolite selectivity in these photooxidation reactions. These other effects include hydrocarbon loading, the use of higher energy UV photons, and zeolite topology. It is important to note that in the previous studies of Frei and coworkers, product yield and selectivity were determined by insitu FT-IR spectroscopy of the zeolite adsorbed reactant and products. While in-situ FT-IR spectroscopy is a valuable technique for monitoring zeolite reactions, the use of FT-IR spectroscopy for determining detailed product distributions is less conventional. More typically, products and reactants are removed from the zeolite by heating or extraction methods and the reaction mixtures are then analyzed using analytical
Photooxidation in Cation-Exchanged Zeolites techniques with higher sensitivity and resolution than in-situ FTIR. Our approach is to combine in-situ FTIR spectroscopy with ex-situ product analysis by GC or solution NMR.15 This approach has led to a greater understanding of the product distributions resulting from photooxidation in zeolite hosts. First of all, it should be noted that the level of selectivity achieved in the photooxidation of toluene to form benzaldehyde and p-xylene to form p-tolualdehyde is not 100%, even in the most highly selective zeolites, such as NaY and BaY. Instead, selectivities to form aldehyde products in BaY are approximately 87% (benzaldehyde) and 76% (p-tolualdehyde), for the roomtemperature photooxidation of toluene and p-xylene, respectively. In these cases, benzyl alcohol and p-methylbenzyl alcohol account for approximately 4% (for toluene) and 10% (for p-xylene) of the products. The limited hydrocarbon loading data presented here suggests that the alcohol products are formed via reaction of the parent toluene or p-xylene with the corresponding hydroperoxide as shown in Scheme 1. This is similar to the chemistry observed for 1-alkenes where the parent molecule can also react with the hydroperoxide that forms from the initial charge transfer state.9-12,15 In the case of the 1-alkenes, this reaction leads to two additional oxidation products, an epoxide and an alcohol,12,15 whereas for toluene and p-xylene, there is only one additional oxidation product, an alcohol, formed from the reaction of the hydroperoxide with the parent molecule. A decrease in the alcohol byproduct, benzyl alcohol or p-methylbenzyl alcohol, and thus an increase in the selectivity of benzaldehdye and p-tolualdehyde formation may occur at lower loadings, for example, if the toluene or p-xylene loading was limited to less than one per zeolite supercage. If diffusion between cages is slower than the decomposition of the hydroperoxide to the aldehyde, the selectivity of the aldehyde should increase with decreasing loading. Photooxidation of toluene in BaY using broadband irradiation containing UV wavelengths (λ > 200 nm) also decreased the selectivity of the photooxidation reactions with respect to benzaldehyde formation. The amount of benzyl alcohol doubled, and there was also an increase in the formation of products, such as p-xylene and condensation products, that were present in trace amounts or at the 1% level when visible wavelengths were used. The formation of condensation products according to Schemes 3 and 4 is enhanced in the larger pore acidic zeolites, such as BaHY and BaBeta, compared to BaZSM-5, which has a smaller pore size. In addition, the relative ratios of the three isomers of the condensation products that are formed in Schemes 3 and 4 are different in BaHY, BaBeta, and BaZSM-5. These results suggest that shape selectivity can play an important role in determining product selectivities in these side reactions. Loss of Selectivity Due to the Presence of Acid Sites. It is clear from the data presented here that the greatest loss of selectivity is observed in zeolites that contain acid sites. As stated earlier, the acid site test of Ramamurthy et al. indicated the presence of acid sites in all zeolites that showed a loss of selectivity; these include CaY, BaHY, BaZSM-5, NaZSM-5, and BaBeta.29 The causal role of acid sites in opening nonselective reaction pathways is clearly demonstrated by comparing the results obtained with BaY and BaHY. Although BaY zeolite exhibited high selectivity, the intentional addition of acid sites by partial exchange of about a third of the barium cations with protons caused a loss of selectivity in the BaHY system. These results confirm that the presence of acid sites is sufficient to cause a loss of selectivity. Some possible acid-catalyzed reaction
J. Phys. Chem. B, Vol. 104, No. 24, 2000 5713 pathways are given in Schemes 2-4 for the major nonselective products as observed by GC product analysis. In the case of zeolites, such as CaY and BaZSM-5, in which acid sites are not intentionally added, one source of Brønsted acid sites for divalent cation-exchanged zeolites is the following reaction,
M2+ + H2O + Si-O-Al f M(OH)+ + Si-O(H+)-Al, where Si-O-Al represents a part of the zeolite framework.37 This reaction is thought to occur during pretreatment or activation at elevated temperatures, and the concentration of the acid sites increases with increasing temperatures. It has also been shown that Brønsted acid sites can be present in low concentrations even in alkali-metal zeolites. For example, Ramamurthy et al. have established the presence of low levels of Brønsted acidity in NaY and NaX zeolites using the color change of a base indicator.29,30,37 These studies showed that the presence of small quantities of acid sites can alter the reactivity of various alkenic substrates. In another study using solid-state NMR spectroscopy, the presence of Brønsted acid sites, as well as other types of sites, were identified in the alkaline earth zeolite CaY.37 The presence and concentration of acid sites was found to depend on whether the zeolite was activated in air or under vacuum. These studies indicate that the reactivity and properties of cation-exchanged zeolites can change from zeolite source to zeolite source and from the use of different pretreatment conditions.30 Further Efforts to Improve Selectivity in the Most Active Zeolites. It is therefore desirable to find ways to increase the selectivity of the photooxidation reactions of hydrocarbons in the most active zeolites, i.e., those zeolites that have the largest electric fields, such as divalent cation-exchanged ZSM-5 and Beta. Several directions are being pursued in this regard. First, it has been determined by propylene adsorption and the colorimetric acid site tests that ZSM-5 zeolites synthesized using a template-free technique as described in the Experimental Section contained fewer and weaker acid sites than those found in commercially available ZSM-5. Commercially prepared zeolites are typically synthesized using an organic amine template. Heating the zeolite to the high temperatures necessary to decompose the template often introduces acid sites into the zeolite. The photooxidation of p-xylene was examined using templatefree Ba-ZSM-5-TF as the host. The selectivity to p-tolualdehyde increased from 32 to 57% using commercial and templatefree preparations, respectively. Correspondingly, the percentage of cresol formed also decreased from 40% to 10% for commercial and template-free preparations, respectively. The substantial increase in selectivity to p-tolualdehyde is presumed to be due to the reduction of acid sites in ZSM-5-TF compared to commercial ZSM-5. However, it should be noted that the product yield decreases in ZSM-5-TF relative to commercial ZSM-5. Scanning electron microscopy indicates that the particle size for commercial ZSM-5 (Zeolyst) is much smaller than the particle size for ZSM-5-TF. The difference in particle sizes could lead to reduced intraparticle diffusion for ZSM-5-TF. It should also be noted that even the most selective ZSM-5-TF results are still less selective than zeolites such as NaY and BaY. Additional strategies to increase selectivity by eliminating acid sites are currently being pursued including synthetic methods, postexchange occlusion of base,38 and postactivation treatments with acid-coordinating molecules.
5714 J. Phys. Chem. B, Vol. 104, No. 24, 2000 Conclusions The photooxidation of methyl-substituted benzenes, toluene and p-xylene, has been demonstrated in a variety of zeolite hosts including NaX, BaX, NaY, CaY, BaY, BaHY, NaZSM-5, BaZSM-5, and BaBeta. The product yield for these reactions correlates with the electric field at the cation site within the pores of the zeolite. The use of ex-situ GC analysis allowed for the product distribution to be investigated with high sensitivity and high chemical resolution and thus provided a better evaluation of the selectivity. For zeolites NaY, BaX, and BaY, the reaction of toluene to form benzaldehyde was near 90%. Fairly high selectivity is also observed in these zeolites for the photooxidation of p-xylene to form p-tolualdehyde. A small decrease in selectivity is observed when UV light is used and the loading of the hydrocarbon is increased. Much more importantly is the effect of residual acid sites in these zeolites. For zeolites that contained Brønsted acid sites, the selectivity decreased significantly down to approximately 30-40%. Methods to decrease the concentration of acid sites should be pursued if the selectivity of the most active zeolites, i.e., those with the highest measured electric fields, can be used as hosts for these reactions. Acknowledgment. Although the research described in this article has been funded wholly or in part by the Environmental Protection Agency through grant number R825304-01-0 to S.C.L. and V.H.G., it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. The authors thank Professor John Wiencek for use of an ICP-AES instrument. References and Notes (1) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992. (2) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; 2nd ed ed.; Wiley: New York, 1992. (3) Weissermel, K.; Arpe, H.-J. Industrial Organic Chemistry; Second ed.; VCH: Weinheim, 1993. (4) Sun, H.; Blatter, F.; Frei, H. J. Am. Chem. Soc. 1996, 118, 68736879. (5) Sun, H.; Blatter, F.; Frei, H. J. Am. Chem. Soc. 1994, 116, 79517952. (6) Sun, H.; Blatter, F.; Frei, H. SelectiVe oxidation of small hydrocarbons by O2 in zeolites with Visible light; Presented at the ACS National Meeting, New Orleans, March, 1996. (7) Sun, H.; Blatter, F.; Frei, H. Catal. Lett. 1997, 44, 247-253.
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