10316
J. Phys. Chem. 1996, 100, 10316-10322
Characterization of Titanium Species Incorporated into Dealuminated Mordenites by Means of IR Spectroscopy and 18O-Exchange Technique Peng Wu, Takayuki Komatsu, and Tatsuaki Yashima* Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152, Japan ReceiVed: January 31, 1996; In Final Form: April 13, 1996X
Titanium mordenite, Ti-M, was prepared by the solid-gas reaction (atom-planting method) between highly dealuminated mordenites and TiCl4 vapor at elevated temperatures (473-873 K), and the coordination state of the incorporated Ti was studied comparing with that in titanosilicate with MFI structure (TS-1). The amount of Ti incorporated leveled off at 1.7 kPa of TiCl4 vapor for a process time of 1 h, and only trace amount of Al and Si was released during the treatment. IR spectra confirmed that TiCl4 reacted with internal SiOH groups in hydroxyl nests. The concentration of oxygen atoms bound to the framework Ti atoms in both TS-1 and Ti-M zeolites was measured by the 18O-exchange reaction between C18O2 and the zeolite. The number of oxygen atoms bound to a Ti atom was ca. 4 for TS-1 zeolites and varied from 5 to 4 for Ti-M zeolites with increasing temperature of the TiCl4 treatment. From these results, a reaction mechanism was proposed for the reaction of TiCl4 with hydroxyl nests in highly siliceous mordenites. Ti-M exhibited an IR band at 963 cm-1, while it was observed at 960 cm-1 for TS-1. The relationship between the relative intensity of the 963 cm-1 band and the Ti content also depended on the treatment temperature. The 18O exchange with C18O2 shifted the 963 cm-1 band for Ti-M to 928 cm-1. On the basis of the 18O exchange and IR measurements, we assign the characteristic IR band around 960 cm-1 for titanium zeolites to the stretching vibration of Si-O-Ti-bonds.
Introduction Titanium silicalite with MFI structure (TS-1) has received considerable attention during the last decade. Since its hydrothermal synthesis was first reported by Taramasso et al. in 1983,1 subsequent studies have proved that TS-1 is an effective catalyst for the selective oxidation of a large family of organic substrates under mild conditions with aqueous hydrogen peroxide as an oxidant.2-5 The successes on TS-1 have promoted intensive studies on the isomorphous substitution of Ti atoms into zeolites other than MFI-type and led to the synthesis of medium-pore TS-2 (MEL structure)6 and Ti-ZSM-48,7 recently, large-pore Ti-β,8-10 and TAPSO-5.11 A variety of studies have also been devoted to clarify the nature of Ti species and their coordination state using various techniques including XRD,2 UV-vis,12 IR,13-15 EXAFS,16-20 Raman,21 ESR,22 and XPS.18 It is believed consistently that the active sites are isolated Ti species located in the framework sites. Several alternatives of configuration and coordination of Ti atoms in TS-1 have been proposed. The Ti sites in TS-1 were once postulated to have a titanyl structure (TidO).4 A characteristic IR band around 960 cm-1 which is always observed for calcinated TS-1 was thus assigned to TidO bonds. Boccuuti et al. adopted UV-vis spectroscopy to deny the TidO structure and to propose that Ti species are tetrahedrally coordinated TiO4 units.12 Therefore, the 960 cm-1 band was attributed to a stretching vibration of SiO4 tetrahedra bound to the Ti atoms, that is, Si-O-Ti bonds. Bellussi et al. provided further support for the later assignment on the basis of the fact that the 960 cm-1 band was shifted to lower frequencies by 17O or 18O-labeled water but not affected by deuterium oxide.23 Recently, Camblor et al.10 and Khouw et al.14,15 gave a new assignment for the 960 cm-1 band to the stretching vibration * To whom correspondence should be addressed. Tel: +81-3-5734-2236. Fax: +81-3-5734-2758. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00307-3 CCC: $12.00
of Si-OH groups. As a result, the assignment of the 960 cm-1 band is still not completely established. The relevant aspects related to the nature of the novel titanium sites given by X-ray absorption techniques (XANES and EXAFS) are also not definitive at present.24 Behrens et al.16 suggested that Ti atoms in TS-1 are mainly octahedrally coordinated, even though a small proportion of tetrahedral or square-pyramidal Ti species were also detected. Lopez et al.17 showed that Ti atoms in TS-1 are essentially either octahedral or tetrahedral, depending on its hydration or dehydration state. A detailed EXAFS analysis given by Trong On et al.18,19 suggested that TS-2 contains double Si-O-Ti bridges formed between the tetrahedral TiOx species and the SiO4 framework tetrahedral units, and the double bridges can be partially hydrolyzed in the presence of water to yield 5- and 6-coordinated Ti sites. The coordination of Ti sites in Ti-β was also shown to vary depending on calcination or hydration.9 Based on their EXAFS data, Pei et al. reported that Ti atoms in TS-1 were neither 5- nor 6-coordinated, but 4-coordinated species including TiOx units sharing edges with SiO4 units.20 In addition, Tuel et al. supported the tetrahedral coordination of Ti in TS-1 with ESR spectra measured after reducing the Ti4+ into ESR-active Ti3+ species.22 The oxidation state of Ti atoms in TS-1 was clarified by XPS analyses to be +4, as the binding energy of Ti(2p) measured for TS-1 was identical to that of Ti(4+) in TiO2-SiO2.18 Ti-containing zeolite with mordenite (MOR) structure has been prepared seldom by the usual hydrothermal synthesis. A solid-gas reaction between highly siliceous zeolites and metal chloride vapor has been proved to be an effective way for incorporating Al, Ga, Sb, As, and In into the zeolite framework to prepare MFI-type metallosilicates.25-28 This new method was named as “atom-planting” method.27 A definite study using TiCl4 vapor reported the introduction of Ti atoms into vacant framework positions of highly dealuminated ZSM-5.29 Recently, we applied this atom-planting method to dealuminated © 1996 American Chemical Society
Titanium Species Incorporated into Dealuminated Mordenites mordenites and succeeded in incorporating Al and Sb atoms into the framework.30-32 According to the detailed studies on the alumination of highly siliceous HZSM-5 or mordenite with AlCl3 vapor, we proposed that the incorporation of Al atoms into zeolite framework occurred through the reaction of AlCl3 molecules with the hydroxyl nest formed with internal silanols.26,28,31 Endoh et al. reported an 18O-exchange technique which could estimate the concentration of oxygen atoms on the defect sites in highly siliceous HZSM-5 and HZSM-11 by measuring the reaction rates between the framework oxygen and 18O-labeled carbon dioxide.33 Recently, Yamagishi et al.28 further confirmed the validity of the application of this method to distinguish quantitatively three kinds of lattice oxygen atoms having different reactivity with C18O2. In this study, we have carried out the atom-planting treatment with TiCl4 vapor at elevated temperatures on dealuminated mordenites to prepare MOR-type titanium zeolites. The purpose is to clarify the coordination state of the Ti atoms thus incorporated by comparing with that of Ti atoms in TS-1 using the 18O-exchange technique. As the assignment of the characteristic IR band at 960 cm-1 observed for titanium zeolites is still not well addressed, we will also deal with this issue based on the IR spectra of 18O-labeled samples. Experimental Section Materials. H-mordenites M(11) and M(71) (both provided by Tosoh Co. Ltd., framework Si/Al atomic ratios of 11 and 71, respectively) were used as starting materials together with a hydrothermally synthesized H-mordenite, M(8.2), for the dealumination to obtain various dealuminated mordenites. The dealumination was carried out by the calcination in air at 973 K followed by HNO3 reflux, as described in detail previously.31 Dealuminated mordenites, M(123) and M(195) were prepared from M(11), and M(169) from M(71), while a highly siliceous mordenite, M(300) was obtained from M(8.2) by repeating the above treatments. No significant collapse of the crystal structure was observed as indicated by XRD and the measurements of specific surface area. MFI-type titanium-silicalites TS-1(50), TS-1(70), and TS-1(104) (Si/Ti atomic ratios of 50, 70 and 104, respectively), were hydrothermally synthesized according to the patent1 using tetraethyl orthotitanite (TEOT) and tetraethyl orthosilicate (TEOS) as Ti and Si sources, respectively. TiCl4 Treatment. The procedure of TiCl4 treatment, that is, atom planting, was similar to that of the alumination with AlCl3 vapor.26,28,31 Dealuminated mordenite powder (2 g) was placed in a vertical quartz tubular reactor and dried at 773 K for 4 h under a stream of dry helium. The reactor was then brought to reaction temperature (473-873 K). The helium stream was diverted through an anhydrous TiCl4 liquid contained in a glass bubbler. The bubbler was maintained at 273 K with an ice bath to achieve a partial pressure of TiCl4 of 1.7 kPa. The helium carrier containing TiCl4 vapor flowed through the zeolite bed at the reaction temperature for a prescribed process time (5 min to 4 h). The sample was purged with pure helium at the same temperature for 1 h to remove any residual unreacted TiCl4 from the zeolite. After cooling to room temperature in helium, the treated zeolite was washed with 2000 cm3 of deionized water and dried in air at 383 K for 24 h. The mordenite thus treated with TiCl4 at 1.7 kPa for 2 h of process time was denoted as Ti-M(n)-T, where n and T are the Si/Al atomic ratio of the parent dealuminated mordenite and the treatment temperature, respectively. Analytical and Characterization Methods. The Ti content was determined by inductively coupled plasma (ICP). Atomic absorption spectrophotometry (AAS) on a Shimadzu AA-640-
J. Phys. Chem., Vol. 100, No. 24, 1996 10317
Figure 1. Effect of process time on the TiCl4 treatment at various temperatures: blank, bulk Ti; solid, bulk Al and released Si. M(300) was treated with TiCl4 vapor at 1.7 kPa.
12 spectrophotometer was applied to determine the Al content. The silicon released during the TiCl4 treatment was collected by passing the effluent helium through a trap containing 100 cm3 of 1 M NaOH and was also analyzed with AAS. IR spectra were recorded on a Shimadzu FTIR-8100 spectrometer at a spectral resolution of 2 cm-1. The sample was pressed into a self-supported wafer with 4.8 mg cm-2 thickness. The wafer was set in a quartz IR cell which was sealed with KBr windows and connected to a vacuum system. The sample was heated under vacuum from 298 to 773 K at a heating rate of 8 K min-1, and dehydrated at 773 K for 1.5 h. After cooling to room temperature, a spectrum was measured in the absorbance mode. The absorbance values were normalized with the 1882 cm-1 band corresponding to the Si-O overtone of zeolite framework.25,31 In IR measurements of samples labeled with 18O, the wafer was contacted with C18O (2.67 kPa) at 773 K 2 for a prescribed time (0.5-18 h). 18O Exchange. The 18O-exchange reaction between C18O 2 and zeolite was carried out with a static system similar to that reported by Takaishi et al.33,34 The procedures were same as those described for the exchange reaction between C18O2 and highly siliceous H-ZSM-5 zeolites.28 C18O2 (98 atom % 18O) was used as supplied. A 30 mg zeolite sample was dehydrated at 773 K under a vacuum of 10-4 Pa for 15 h before the exchange reaction. The exchange was then carried out at 773 K and at an initial pressure of C18O2 of 2.73 kPa. C18O2 (m/e ) 48), C18O16O (m/e ) 46), and C16O2 (m/e ) 44) were monitored periodically with a quadrupole mass spectrometer (Ulvac, Massmate-100) by passing the sampling gas through a glass capillary leak into the ionization region of the spectrometer. Results and Discussion Preparation of Titanium Mordenites. A highly siliceous mordenite M(300) (Al content of 0.020 mmol g-1) was used as parent sample for investigating the effects of treatment conditions, reaction temperature, and process time on the Ti incorporation with the partial pressure of TiCl4 vapor at 1.7 kPa. Figure 1 shows the dependence of the amount of incorporated Ti on the process time of TiCl4 treatment at various temperatures. At every treatment temperature, the Ti amount increased rapidly on prolonging the process time to 1 h and then did not increase significantly with process time longer than 1 h. At the saturated stage, the Ti amount decreased with increasing reaction temperature from 473 to 873 K. The effect of the TiCl4 treatment on the amount of bulk Al and Si released is also given in Figure 1 for the treatment at 873 K. No significant loss of bulk Al was observed even after 4 h of the treatment. The amount of Si released increased slightly with the process time but was negligibly small compared with that of Ti incorporated.
10318 J. Phys. Chem., Vol. 100, No. 24, 1996
Wu et al.
Figure 3. IR spectra (broken lines) of silicalite (A), M(71) (B), and M(300) (C) and difference spectra of TS-1 (A), Ti-M(71) (B), and TiM(300) (C). The spectra were recorded after an evacuation at 423 K for 0.5 h. TiCl4 treatment conditions: TiCl4 vapor pressure, 1.7 kPa; temperature, 673 K; process time, 5 min-2 h.
Figure 2. IR spectra of Ti-M(300) prepared by the treatment with TiCl4 at various temperatures: (a) M(300); (b) Ti-M(300)-473; (c) TiM(300)-573; (d) Ti-M(300)-673; (e) Ti-M(300)-773; (f) Ti-M(300)873. TiCl4 treatment conditions: TiCl4 vapor pressure, 1.7 kPa; process time, 2 h.
Considering the sites where Ti atoms were inserted, the above results strongly suggest that the isomorphous substitution of Ti for the framework Al or Si should be ruled out. The phenomena of the TiCl4 treatment reflected in Figure 1 are very similar to those observed for the alumination of dealuminated mordenites with AlCl3 vapor reported in a previous paper,31 where we proposed by means of IR and MAS NMR spectroscopies that the incorporation of Al atoms into the framework proceeded not through the exchange between Al and Si but through the insertion of Al atoms into defect sites, such as hydroxyl nests formed with four internal silanols. We will provide here some evidence for the reaction of TiCl4 vapor with internal silanols using IR spectroscopy. Figure 2 shows the IR spectra in the hydroxyl stretching region for the parent M(300) and titanium mordenites prepared by the TiCl4 treatment at different temperatures for a process time of 2 h. The parent mordenite (a) exhibits a band at 3745 cm-1 due to terminal silanol groups,31 two bands at 3700 and 3500 cm-1 due to internal silanol groups,28,31 and also a very weak band at 3610 cm-1 attributed to structural Si(OH)Al groups31 indicating that M(300) is highly siliceous. The internal silanols corresponding to the 3700 and 3500 cm-1 bands not observed for the as-synthesized mordenite were generated by the dealumination and were believed to be clustered to form hydroxyl nests as reported in the previous alumination study.31 With the TiCl4 treatment at various temperatures (b-f), the 3745 and 3610 cm-1 bands did not change. In the alumination studies, the reaction of AlCl3 vapor with the terminal silanols was assumed to result in extraframework Al species.26,28 The independence of the 3745 cm-1 band from the TiCl4 treatment indicates that the extraframework Ti species generated through the reaction with terminal silanols would be negligible. In fact, Ti-M zeolites thus prepared showed in UV-vis spectra only an absorption at 220 nm due to tetrahedral framework Ti atoms, but neither absorption at 330 nm due to anatase TiO2 nor absorption at ∼270 nm due to extraframework Ti species.35 Moreover, the fact that the 3610 cm-1 band remained after the TiCl4 treatment is in accordance with no release of Al during the treatment shown in Figure 1 to suggest that the isomorphous substitution of Ti for the framework Al is also negligible. After the TiCl4 treatment at 473 K (b), the 3700 and 3500 cm-1 bands decreased in intensity compared with those for the
parent M(300) (a). They further decreased in intensity at 573 K (c), 673K (d), and 773 K (e). After the treatment at 873 K, these two bands were almost not observed (f). These results suggest that TiCl4 vapor has reacted with the internal silanols, which may result in the incorporation of Ti atoms into the framework sites in the same manner as alumination of highly siliceous HZSM-528 or H-mordenite.31 As depicted in Figure 1, the TiCl4 treatment for 2 h is sufficient to make the amount bulk Ti saturated; that is, TiCl4 could react with all of the available internal silanol to a maximum extent. The fact that the amount of internal silanols consumed by the TiCl4 treatment depended on the reaction temperature (Figure 2), therefore, may not be due to that higher temperature prefers to fill up nearly all of the hydroxyl nests in the dealuminated mordenite but may be due to the presence of alternative configuration and environment of Ti atoms incorporated at different temperatures, which will be discussed in detail latter. Characterization of Ti Sites with IR Spectroscopy. IR spectra in the structural vibration region are widely used to characterize titanium zeolites, as they exhibit a characteristic IR band at approximately 960 cm-1. This band is regarded as fingerprint of the framework Ti. IR spectra in this region are usually recorded in the transmittance mode using diluted wafers with KBr. For the purpose of determining accurate absorbance and exact position of the IR absorption, we measured the spectra using self-supported wafers in the absorbance mode after the dehydration under vacuum, which can eliminate the contamination of H2O and KBr as suggested by Zecchina et al.21 Figure 3 shows the difference IR spectra of TS-1 and Ti-M samples prepared from the parents of M(71) and M(300) by the TiCl4 treatment at 673 K. The spectra drawn with broken lines were that of silicalite, M(71), and M(300) and were used as original spectra for obtaining the difference ones. These samples showed a cutoff for the strong absorption due to structural Si-O bonds at 1000-1200 cm-1. TS-1 zeolites with various Ti content (A) all exhibited an expected symmetric band at 960 cm-1, which has been well reported to be related with the framework Ti for various titanium zeolites.1,6,10 The treatment with TiCl4 vapor developed a similar band not observed for the parent dealuminated mordenites at 963 cm-1 for both Ti-M(71) (B) and TiM(300) (C). The intensity of this band increased with the process time of the treatment, while its position was not affected. The bands of Ti-M are slightly broader than those of TS-1, which may be due to the difference of framework structure or crystal size between these two types of zeolites.23 Similar spectra were also obtained for Ti-M prepared from other parents at various treatment temperatures. The relative intensity of the 963 cm-1 band (960 cm-1 for TS-1) for Ti-M prepared at various temperatures and TS-1 is depicted in Figure
Titanium Species Incorporated into Dealuminated Mordenites
J. Phys. Chem., Vol. 100, No. 24, 1996 10319
Figure 5.
Figure 4. Dependence of the absorbance of the 963 cm-1 IR band on the Ti content of TS-1 and Ti-M zeolites. TiCl4 treatment conditions: TiCl4 vapor pressure, 1.7 kPa; temperature, 473-873 K; process time, 5 min-2 h.
4 against their titanium content. Interestingly, from the relationships between these two variables, the samples fell into three groups; that is, Ti-M(n)-773 and Ti-M(n)-873 series are in the first group with TS-1, and Ti-M(n)-673 series are in the second group individually, while the Ti-M(n)-473 and Ti-M(n)-573 series comprise the third group. For each group, the absorbance of the 963 cm-1 band is proportional to the titanium content. The linear variation of the relative intensity has already been reported for TS-11 to show the titanium incorporation into the framework in tetrahedral sites. Thus, the coordination state and environment of the Ti atoms in Ti-M zeolite from the first group are assumed to be the same as those in TS-1, while Ti-M zeolites from the other two groups may contain Ti species located in different states within the crystals, which will be discussed later. Determination of Concentration of Oxygen Atoms Bound to Ti. As mentioned earlier, although the nature of Ti sites has been widely studied with various techniques, various forms of Ti sites have been reported for different types of titanium zeolites. In this study, we tried to deal with this issue by measuring the concentration of oxygen atoms in the neighborhood of Ti atoms with an 18O-exchange technique. Endoh et al.33 reported that the concentration of oxygen atoms on the defect sites in a highly siliceous HZSM-5 zeolite can be measured by an 18O-exchange reaction between C18O2 and the zeolite. Recently, this technique was used practically to study the alumination mechanism for highly siliceous HZSM-5 zeolites.28 This technique has also been applied to characterize the framework Ga atoms in galliated zeolites prepared by the galliation with NaGaO2 solution.33 As the Ga atom has a larger radius than the Si atom, the incorporation of Ga atoms into the framework would distort neighboring oxygen atoms and make them more reactive to isotope exchange than the oxygen atoms bound to Si. Therefore, these two kinds of oxygen atoms are quantitatively distinguishable. Reasonably, we apply this method here to TS-1 zeolites in which the oxygen atom in Ti-O bond (1.9 Å) is expected to have higher reactivity with C18O2 than that in Si-O (1.6 Å). The exchange reaction was carried out, in all cases, at the ratio of the amount of oxygen in the gas phase to that in the solid was ca. 0.2, which was reported to be convenient for the exchange reaction over a wide range in its degree of advancement.28 Figure 5 shows the degree of oxygen exchange between C18O2 and TS-1(50) or TS-1(70). The exchange reaction occurred rapidly in the initial stage, and then proceeded slowly with the reaction time. After reaction for about 60 h, the mole fraction of 16O atom in CO2 was ca. 35%. TS-1(50) containing more amount of Ti showed higher exchange ability with C18O2 than TS-1(70) as expected.
18O
exchange between C18O2 and TS-1 zeolites at 773 K.
The framework oxygen atoms in a highly siliceous HZSM-5 zeolite have been classified into three kinds, that is, oxygen atoms from terminal Si-OH groups, from internal Si-OH forming defect sites of hydroxyl nests, and from Si-O-Si in the framework, respectively.28,33 According to this classification, the framework oxygen atoms in TS-1 may be also classified into three kinds, that is, the most reactive oxygen atoms in the terminal Si-OH groups, medium ones in Si-O-Ti bonds 20 or in Si-O-Ti-OH‚‚‚HO-Si defect sites,15 and the least reactive ones in Si-O-Si bonds. On the basis of the Mckay’s law, Takaishi et al.34 have derived the following equation for the exchange reaction of the three kinds of oxygen atoms in zeolite with CO2 in gas phase:
ng dyg/dt ) -ngyg
∑
ki(1 - yi)ni + ng(1 - yg)
i)1-3
∑
i)1-3
kiyini (1)
where the subscripts 1-3 indicate the three kinds of oxygen atoms, the subscripts g and i denote the gas-phase carbon dioxide and the framework oxygen, respectively, t is the reaction time, k is the rate constant, n is the amount of oxygen atoms in a specified phase, and y is the mole fraction of 18O. Additionally, we have the material balance equation:
ng(yg0 - yg) )
∑
niyi
(2)
i)1-3
where yg0 is the initial value of yg at t ) 0. If the rate constants of the exchange reaction between C18O2 and three kinds of 16O atoms vary greatly from each other, the following approximate solution can be made:
k1 . k2 . k3
(3)
In the last stage of the exchange reaction, O1 and O2 atoms are in isotopic equilibrium with the gas phase. Therefore, the rate of exchange rate is determined by the reaction of O3 atoms. By assuming yg ) y1 ) y2, an approximate equation can be derived from eqs 1 and 3 as follows:
(
ln yg -
) ( )
ngyg0 ngyg0 ) ln n1 + n2 + n3 + ng n1 + n2 + ng 0
ngyg - k3(n1 + n2 + n3 + ng)t (4) n1 + n2 + n3 + ng Plots of ln(yg - [ngyg0/n1 + n2 + n3 + ng]) against t are depicted in Figure 6. The plots of data points become linear after t ) 15 h. As the total amount of oxygen atoms, n1 + n2 + n3, is known, the values of k3, n3, and n1 + n2 are calculated from the slope of the linear part and its extrapolation to the ordinate. Now, consider the intermediate stage of the exchange reaction, where O1 atoms are in isotopical equilibrium state with gas phase and only a negligible amount of O3 atoms participate in the reaction due to their very small rate constant. The
10320 J. Phys. Chem., Vol. 100, No. 24, 1996
Wu et al. SCHEME 1 Si O Si O Ti O Si O Si (I)
Figure 6. Plots of the degree of exchange against reaction time, based on eq 4. yg0 ) 0.98; amount of oxygen atoms in gas (ng), 0.206 mmol for TS-1(50) and 0.220 mmol for TS-1(70); in solid (ns), 0.912 mmol for TS-1(50) and 0.922 mmol for TS-1(70).
Figure 7. Plots of the degree of exchange against reaction time, based on eq 5.
exchange rate is determined dominantly by the reaction of O2 atoms. Thus, eq 4 is replaced by the following equation:
) (
(
)
ngyg0 ngyg0 ngyg0 ) ln ln yg n1 + n2 + ng n1 + ng n1 + n2 + ng k2(n1 + n2 + ng)t (5) where the value of n1 + n2 has already been calculated from Figure 6. Figure 7 shows the plots using eq 5. The unknown parameters n1, n2, and k2 are determined from the slope and the intersection of the linear part except for the initial stage. In the initial stage of the reaction, only O1 takes part in the exchange. Similarly, the unknown parameter k1 is determined. The values thus obtained for TS-1(50) and TS-1(70) together with TS-1(104) are listed in Table 1. The values of k1, k2, and k3 varied little with Ti content. For each sample, k1 was larger than k2 by a factor of 102 and k2 was larger than k3 by a factor of 103, which satisfies the assumption k1 . k2 . k3 (eq 3).
The oxygen atoms (O2) with medium reactivity are mainly generated in the vicinity of Ti atoms due to the incorporation of Ti atoms of large covalent radius into the framework. Therefore, the coordination state of Ti atoms can be deduced from number of O2 generated by a Ti atom. As shown in Table 1, the ratios of n2 to nTi were ca. 4 for the three TS-1 zeolites studied. This fact indicates the framework Ti atoms incorporated during the hydrothermal synthesis are mainly located in a tetrahedrally coordinated state and exist as the species I described in Scheme 1. The species I is fully consistent with the UV-vis experimental evidence by Boccuti et al., who found that TS-1 only exhibited a band at 220 nm due to isolated framework titanium in the tetrahedral coordination.12 Recently, Pei et al.20 and Tuel et al.22 provided further support for the presence of tetrahedral Ti atoms in TS-1 with EXAFS and ESR spectra, respectively. The 18O-exchange technique was also used to investigate the nature of Ti atoms incorporated into the mordenite framework by the TiCl4 treatment. The case of Ti-M is slightly different from that of TS-1 due to the presence of residual Al. The oxygen atoms in Si-O-Al bonds are expected to show relatively high reactivity to the isotope exchange to make the matter complex. The exchange reactivity of oxygen atoms bound to Al atoms were measured on two synthesized Hmordenites, M(8.2) and M(11). They exhibited no 3700 and 3500 cm-1 band in their IR spectra, indicating that they contained few defect sites such as hydroxyl nests. A similar treatment for these mordenites to that for TS-1 zeolites gave the amounts of the three kinds of oxygen atoms and their corresponding rate constants (Table 1). The ratio of n2 to nAl is about 2, indicating that only half of four oxygen atoms bound to a framework Al atom contribute to n2. The other half behaved like oxygen in Si-O-Si bonds contributing to n3, probably because of the relatively small radius of Al atom compared with Ti. The values of k3 for M(8.2) and M(11) were 1 order larger than those for TS-1 zeolites, which may be another indication of the contribution of two oxygen atoms bound to an Al atom to n3. Moreover, the ratio of n2 to nAl decreased from 2.2 for
TABLE 1: Values of n1, n2, n3, k1, k2, and k3 Determined by the 18O-Exchange Technique for TS-1, M(n), and Ti-M(n)-T Zeolites
samplea
bulk Tib (nTi)/mmol g-1
bulk A1c (nA1)/mmol g-1
TS-1(50) TS-1(70) TS-1(104) M(8.2) M(11) M(300) Ti-M(300)-873 Ti-M(300)-773 Ti-M(300)-673 T8-M(300)-573 Ti-M(300)-473
0.314 0.228 0.151 0 0 0 0.131 0.142 0.148 0.160 0.172
0 0 0 1.650 1.167 0.020 0.018 0.018 0.020 0.020 0.020
amount of O atomsd/mmol g-1 n1 n2 n3 0.133 0.067 0.057 0.867 1.233 0.033 0.100 0.123 0.133 0.133 0.133
1.300 0.967 0.645 3.633 2.333 0.500 0.533 0.589 0.667 0.800 0.867
30.400 30.734 30.898 29.500 28.033 33.433 32.567 32.431 32.333 32.233 32.167
rate const k/mmol-1 h-1 k1 k2 k3 137 123 106 183 174 167 152 136 147 135 126
1.99 1.96 2.01 0.78 0.91 1.16 1.22 1.18 1.10 1.09 1.04
4.60 × 10-3 4.62 × 10-3 4.68 × 10-3 1.25 × 10-2 1.16 × 10-2 2.74 × 10-3 2.76 × 10-3 2.80 × 10-3 2.81 × 10-3 2.83 × 10-3 2.79 × 10-3
n2/nTi
n2/nA1
4.14 4.24 4.27 2.20 2.00 25.00 4.07 4.15 4.45 5.00 5.04
a Ti-M(300)-T samples were prepared by treating M(300) with TiCl vapor (1.7 kPa) for 2 h. b Determined by ICP. c Determined by AAS. d Due 4 to the difference in the amount of absorbed water and that in the Ti content, the total amount of oxygen atoms per gram varied between the samples.
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Figure 8. 18O exchange between C18O2 and M(300) or Ti-M(300) at 773 K. Ti-M(300) zeolites were prepared at 1.7 kPa of TiCl4 vapor for 2 h.
SCHEME 2
M(8.2) to 2.0 for M(11). This fact suggests that the oxygen atoms around Al atoms tend to be less active to isotope exchange with decreasing the Al content. In fact, Y-type zeolite was reported to be less active for isotope exchange than X-type zeolite.34 Based on the above results and discussion, the influence of residual Al atoms in the highly siliceous mordenite, M(300) and Ti-M(300) zeolites on the determination of the value of n2 can be neglected. Thus, the oxygen atoms in M(300) are classified into three kinds in the same way for highly siliceous HZSM-5 zeolites;28,32 particularly the second kind of oxygen atoms is related to internal silanols, while the classification for oxygen atoms in Ti-M(300) zeolites prepared by the TiCl4 treatment is treated in the same way for those in TS-1. Figure 8 depicts the degree of exchange between C18O2 and M(300) or Ti-M(300) at 773 K. Ti-M(300) zeolites showed higher reactivity than the parent M(300), and the reactivity of Ti-M(300) zeolites slightly increased with decreasing the treatment temperature from 873 to 473 K, that is, with increasing the Ti content. Similar to the procedure obtaining Figures 6 and 7, the exchange degree was plotted against reaction time based on eqs 4 and 5. The values of ni and ki were calculated from the plots and listed in Table 1. Again, it is obvious that the values of k1, k2, and k3 satisfy eq 3. The value of n2 for M(300) exceeded greatly the number of oxygen atoms directly bound to Al atoms, which is evident from n2/nAl ratio of 25. This indicates that just like the case of highly siliceous HZSM-5 zeolites,28,33 n2 for M(300) mainly accounts for the oxygen atoms in internal SiOH groups on defect sites. Indeed, the existence of internal Si-OH groups in M(300) has been revealed by IR (Figure 2a) and a previous 1H-29Si CP MAS NMR study.31 The value of n2 for M(300) was comparable to that for Ti-M(300)-873 and Ti-M(300)-773 which gave the n2/nTi ratio of ca. 4. It is concluded from these results that at the TiCl4 treatment temperatures of 873 and 773 K, Ti atoms are dominantly incorporated into the hydroxyl nests in M(300) to form tetrahedral framework titanium species with structure I. The values of n2 for Ti-M(300)-473 and Ti-M(300)573 were more than that for M(300) and gave the n2/nTi ratio of ca. 5. A framework Ti atom with five oxygen atoms of
Figure 9. IR spectra of Ti-M(300)-873: (a) Ti-M(300)-873 after an evacuation at 773 K for 1.5 h; (b) as (a) after an exposure to C18O2 (2.67 kPa) at 773 K for 1 h; (c) for 5 h; (d) for 18 h; (e) as (d) after an evacuation and exposure to C18O2 (2.67 kPa) for 2 h; (f) as (e) after exposure to C16O2 (13.33 kPa) for 3 h.
medium reactivity in the neighborhood is assumed to have the structure II illustrated in Scheme 2. The Ti species (II) was reported for TS-1 and TS-2 when tetrahedral Ti sites were hydrated.14,19 On the other hand, the n2/nTi ratio for Ti-M(300)673 was 4.45. Ti atoms incorporated at 673 K, therefore, are expected to have both structure I and II. From these results, we propose the incorporation mechanism of Ti atoms into the mordenite framework graphically in Scheme 2, where the reaction of a TiCl4 molecule with a hydroxyl nest generates a tetrahedral Ti atom with different environment depending on the reaction temperature. The SiOH group in species II may account for the remaining 3700 and 3500 cm-1 IR bands observed for Ti-M(300) zeolites prepared at relatively lower temperatures (Figure 2). IR Study on 18O-Labeled Ti-M. The exchange reaction of C18O2 with terminal and internal SiOH groups was studied on highly siliceous HZSM-5 zeolites by IR spectroscopy.28 The present study provides some evidence for the exchange between C18O2 and the oxygen atoms bound to the framework Ti atoms using IR spectra of 18O-labeled samples. IR spectra in the structural vibration region for Ti-M(300)-873 before and after the exchange with C18O2 at 773 K are depicted in Figure 9. In the spectrum of Ti-M(300)-873 (a), the characteristic IR band due to the framework Ti was observed at 963 cm-1. When Ti-M(300)-873 was contacted with C18O2 (2.67 kPa) for 1 h, the 963 cm-1 band decreased in intensity, while a weak band at 928 cm-1 appeared (b). The 928 cm-1 band further increased after the reaction for 5 h (c). It is clear that the 16O-related band at 963 cm-1 shifted to 18O-related band at 928 cm-1. These two bands did not vary further as shown by the spectrum measured after 18 h (d). When the IR cell was evacuated and
10322 J. Phys. Chem., Vol. 100, No. 24, 1996 C18O2 (2.67 kPa) was added again, the 928 cm-1 band increased to have an intensity comparable to the initial 963 cm-1 band, while the 963 cm-1 band turned into a small shoulder (e). These facts indicate that the exchange between C18O2 and the oxygen atoms around Ti sites reached the equilibrium within 5 h, which is in agreement with the intermediate stages of the isotope exchange depicted in Figure 7. When the 18O-exchanged sample was exposed to C16O2 (13.33 kPa) containing an excess amount of 16O atoms at 773 K for 3 h, the 928 cm-1 band decreased in intensity greatly and the 963 cm-1 band restored to about 80% of the original intensity (f), indicating the exchange is reversible. The vibration bands of TidO and Si-O-Ti bonds, and SiOH groups are expected to occur in the frequency range around 960 cm-1. The 960 cm-1 band observed for TS-1 was once assigned simply to TidO bonds.4 This assignment, however, was denied by Boccuti et al. who assigned the 960 cm-1 band to a stretching vibration of Si-O-Ti bonds in a simplified model.12 Recently, Camblor et al.10 observed that the intensity of the 960 cm-1 band for as-synthesized TS-1 and Ti-β was strengthened upon calcination. Khouw et al.14,15 found that the 960 cm-1 band disappeared with Na exchange using NaOH solution and was restored reversibly with acid treatment. Therefore, a completely different assignment was proposed for the 960 cm-1 band to the stretching vibration of Si-OH groups formed along with the incorporation of Ti into the framework. Thus, the exact assignment of the 960 cm-1 band is still uncertain. If the 963 cm-1 band observed for Ti-M(300)-873 is due to the stretching vibration of TidO bonds, the isotope exchange with C18O2 would shift this band to 922 cm-1 due to Tid18O according to the calculation from the square root of the reduced mass ratio for the harmonic vibrational model of titanyl.35 The difference between the calculated value and the actual value of 928 cm-1 given in Figure 11 is far in excess of the spectral resolution of 2 cm-1. Therefore, the assignment of the 963 cm-1 band to TidO bonds can be ruled out. The parent dealuminated mordenite, M(300), exhibited an IR band at 959 cm-1 due to internal Si-OH forming hydroxyl nests in its hydrated form (not shown). This band was broader than the 963 cm-1 band in Ti-M zeolites, suggesting that the former is more water affected. The silanol band was removed completely by a dehydration at 423 K for 30 min under vacuum (Figure 3 C). Thus, the band due to silanol groups can only be observed for hydrated samples. From this fact, it is hard to assign the 963 cm-1 band observed for dehydrated Ti-M zeolites to Si-OH groups, although the nature of Si-OH forming hydroxyl nests in dealuminated mordenites may be different from that of SiOH generated as a result of titanium incorporation into the framework. Moreover, Figure 2 showed that more SiOH groups corresponding to the 3700 and 3500 cm-1 bands remained in Ti-M zeolites prepared at lower temperatures (473 or 573 K). These samples, however, showed less intensive 963 cm-1 band compared with Ti-M zeolites prepared at higher temperatures at a same level of titanium content (Figure 4). This result also suggests that the 963 cm-1 band cannot be associated with Si-OH groups. The assignment of the 963 cm-1 band to Si-OH groups is also denied by the fact that it was only isotopically shifted to lower frequency region by 17O or 18Olabeled water, but not affected by deuterium oxide.23 Combining with these results with those in Figure 2 and Figure 4, we assign the band at ∼960 cm-1 to the stretching vibration of Si-O-Ti bond for both TS-1 and Ti-M. Due to the fact that the average number of Si-O-Ti bonds generated following the incorporation of a Ti atom into a hydroxyl nest varies from 3 (species II) to 4 (species I), Ti-M zeolites prepared at various temperatures fall into three groups (Figure 4). Thus,
Wu et al. the isotope exchange with 18O would reasonably make the vibration band of Si-16O-Ti shift to the lower frequency region. Conclusion Ti atoms are incorporated into mordenite framework sites by the TiCl4 treatment at elevated temperatures. The 18O-exchange technique clarifies that Ti atoms in TS-1 are tetrahedral species with structure (I), while Ti atoms in Ti-M prepared by the TiCl4 treatment have both structure (I) and (II) (Scheme 2) depending on the temperature of TiCl4 treatment. The characteristic IR band around 960 cm-1 observed for titanium zeolites is assigned to the stretching vibration of Si-O-Ti bond. The isotope exchange with 18O shifts this band to 928 cm-1. References and Notes (1) Tramasso, M.; Perego, G.; Notari, B. U.S. Patent 4,410,501, 1983. (2) Perego, G.; Bellussi, G.; Corno, C.; Taramasso, M.; Buonomo, F.; Esposito, A. Stud. Surf. Sci. Catal. 1986, 28, 129. (3) Bellussi, G.; Rigutto, M. S. Stud. Surf. Sci. Catal. 1994, 85 , 177. (4) Notari, B. Stud. Surf. Sci. Catal. 1988, 37 , 413. (5) Notari, B. Catal. Today 1993, 18, 163. (6) Reddy, J. S.; Kumar, R.; Ratnasamy, P. Appl. Catal. 1990, 58, L1. (7) Serrano, D. P.; Li, H.-X., Davis, M. E. J. Chem. Soc., Chem. Commun. 1992, 745. (8) Camblor, M. A.; Corma, A.; Martı´nez, A.; Pe´rez-Pariente, J. J. Chem. Soc., Chem. Commun. 1992, 589. (9) Camblor, M. A.; Corma, A.; Pe´rez-Pariente, J. J. Chem. Soc., Chem. Commun. 1993, 1557. (10) Camblor, M. A.; Corma, A.; Pe´rez-Pariente, J. J. Chem. Soc., Chem. Commun. 1993, 557. (11) Tuel, A. Zeolites, 1995, 15, 228. (12) Boccuti, M. R.; Rao, K. M.; Zecchina, A.; Leofanti, G.; Petrini, G. Stud. Surf. Sci. Catal. 1988, 48 , 133. (13) Huybrechts, D. R. C.; Buskens, P. L.; Jacobs, P. A. J. Mol. Catal. 1992, 71, 129. (14) Khouw, C. B.; Dartt, C. B.; Labinger, J. A.; Davis, M. E. J. Catal. 1994, 149, 195. (15) Khouw, C. B.; Davis, M. E. J. Catal. 1995, 151, 77. (16) Behrens, P.; Felsche, J.; Vetter, F.; Schulz-Elkoff. G.; Jaeger, N. I.; Niemann, W. J. Chem. Soc., Chem. Commun. 1991, 678. (17) Lopez, A.; Kessler, H.; Guth, J. L.; Tuilier, M. H.; Popa, J. M. In Proceedings of the 6th International Conference on X-ray Absorption Fine Structure, York, U. K., 1990; Elsevier: Amsterdam, 1990; p 548. (18) Trong On, D.; Bonneviot, L.; Bittar, A.; Sayari, A.; Kaliaguine, S. J. Mol. Catal. 1992, 74, 233. (19) Bonneviot, L.; Trong On, D.; Lopez, A. J. Chem. Soc., Chem. Commun. 1993, 685. (20) Pei, S.; Zajac, G. W.; Kaduk, J. A.; Faber, J.; Boyanov, B. I.; Duck, D.; Fazzini, D.; Morrison, T. I.; Yang, D. S. Catal. Lett. 1993, 21, 333. (21) Zecchina, A.; Spoto, G.; Bordiga, S.; Ferrero, A.; Petrini, G.; Leofanti, G.; Padovan, M. Stud. Surf. Sci. Catal. 1991, 69, 251. (22) Tuel. A.; Taarit, Y. B. Appl. Catal. 1993, A102, 69. (23) Bellussi, G.; Carati, A.; Clerici, M. G.; Maddinelli, G.; Millini, R. J. Catal. 1992, 133, 220. (24) Clause, O.; Bonneviot, L.; Che, M.; Dexpert, H. J. Catal. 1991, 130, 21. (25) Chang, C, D.; Chu, C. T.-W.; Miale, J. N.; Bridger, R. F.; Calvert, R. B. J. Am. Chem. Soc. 1984, 106, 8143. (26) Yamagishi, K.; Namba, S.; Yashima, T. J. Catal. 1990, 121, 47. (27) Yamagishi, K.; Namba, S.; Yashima, T. Stud. Surf. Sci. Catal. 1991, 60, 171. (28) Yamagishi, K.; Namba, S.; Yashima, T. J. Phys. Chem. 1991, 95, 872. (29) Kranshaar, B.; Van Hoof, J. H. C. Catal. Lett. 1988, 1, 81. (30) Wu, P.; Nakano, T.; Komatsu, T.; Yashima, T. Stud. Surf. Sci. Catal. 1994, 90, 295. (31) Wu, P.; Komatsu, T.; Yashima, T. J. Phys. Chem. 1995, 99, 10923. (32) Wu, P.; Komatsu, T.; Yashima, T. J. Chem. Soc., Faraday Trans., in press. (33) Endoh, A.; Nishimiya, K.; Tsutsumi, K.; Takaishi, T. Stud. Surf. Sci. Catal. 1989, 46, 779. (34) Takaishi, T.; Endoh, A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 411. (35) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley & Sons: New York, 1980; p 697.
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