Infrared study of Ti-containing zeolites using CO as a probe molecule

H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.) Catalysis by Microporous Materials

Studies in Surface Science and Catalysis, Vol. 94 1995 Elsevier Science B.V.

163

Infrared study of Ti-containing zeolites using CO as a probe molecule O.V. Manoilova a , J. Dakka b , R.A. Sheldon b and A.A. Tsyganenko a alnstitute of Physics, St. Petersburg University, St.Petersburg, 198904. Russia bDelft University of Technology, Delft, The Netherlands Surface properties of Ti-containing adsorbents such as Ti-silica gel, Ti-AI beta zeolite, two samples of TS-I zeolite prepared in different ways, as well as of pure silicalite were studied by means of IR spectroscopy of CO adsorbed at low temperatures. Surface silanol groups, acidic bridged hydroxyls, coordinatively unsaturated Ti and AI atoms were characterized as adsorption sites. The capability of IR spectroscopy of adsorbed CO to distinguish between TS-1 samples of different origin is demonstrated. 1. INTRODUCTION The demonstration by Enichem workers [1] that titanium silicalite (TS-l) catalyzes a variety of synthetically useful oxidations with 30% aqueous hydrogen was a maior breakthrough in the field of zeolite catalysis [2]. The success of TS-I prompted a flourish of activity in the synthesis of other titanium-substituted molecular sieves, such as titanium silicalite-2 (TS-2) [3], Ti-ZSM-48 [4] Ti-Al-mordenite [5], Ti-Al-beta [6]and Ti-MCM-41 [7]. Moreover, this interest has also been extended to the synthesis of redox molecular sieves involving framework substitution by other metals, e.g. chromium, cobalt, vanadium, etc. [8]. As was recently noted by Notari [2] significantly different catalytic properties have been ascribed to titanium silicalites prepared in different laboratories. These differences can probably be attributed to different characteristics of the catalysts. In our experience we have also found that different samples of TS-I, which according to XRD and IR spectroscopy of the lattice vibrations were the same, showed vastly different catalytic properties. Poor catalytic performance was generally observed with catalysts made using a template (Pr4NOH) which contained traces of alkali metal impurities. It is of paramount importance, therefore, to have access to a rapid characterization method which can readily distinguish between active and inactive catalysts. Moreover, ideally such a method would also provide insights into the nature of the attachment of the redox metal to the molecular sieve framework. To this end we have examined some redox molecular sieves using low temperature IR spectroscopy of adsorbed CO. Vibrational frequency of this molecule was shown to be sensitive to the strength of both Lewis acid sites and proton donating centers of oxide adsorbents [9,10] and zeolites, in particular [11-13], while the integrated intensity of the bands, at least, for the systems without back donation of d-electrons, can be used as a measure of site concentration [141. In order to follow the influence of titanium insertion and the preparation procedure on the adsorbent properties, several various titanium containing systems, amorphous and crystalline,

164 were investigated after pretreatment at different temperatures, and the obtained spectra were compared with those of pure silicalite or with previously reported [9,15,16] results on CO adsorbed on titania. 2. EXPERIMENTAL The following samples were studied: TS-la (with Na + and K§ cations), TS-Ib (without cations), silicalite, amorphous Ti-silica gel, and Ti-Al-beta zeolite. Samples of about 80 mg of powder were pressed into the 10x30 mm pellets, which were then studied, typically, at three different stages of dehydration obtained by pretreatment in vacuum at 523, 673 and 973 K. To avoid the reduction of titanium-containing systems, heating in vacuum was always followed by exposure to oxygen which was changed several times before the final cooling in oxygen to room temperature. For better thermal contact of the sample with the cooled part of the cell, about 0.5 torr of helium was always inserted into the sample containing volume before running the spectra. CO was purified by passing through the trap with liquid nitrogen. Normally, the first spectrum was run when about 5 torr of a gas was let into the cell, then the excess of gas was removed by adsorption on zeolite cooled with liquid nitrogen, without pumping the helium, so that the temperature of the sample would be constant. Then, if necessary, the sample was evacuated at the desired temperature or heated in a closed cell, in order to observe the changes in the spectra of surface species without CO removal. Temperature was measured by a thermocouple inserted in the volume for coolant of the cell. The construction of the cell for studying the IR spectra of adsorbed species was described elsewhere [17]. Spectra were recorded by a scanning UR-20 (Carl Zeiss, Jena) IR spectrometer. 3. RESULTS Changes in the IR spectra of the studied systems on CO adsorption are illustrated in fig. 1. Data on the observed band positions for the surface OH groups and adsorbed CO molecules are summarized in the Table 1. 3.1. Silicalite In the initial spectrum of silicalite, a weak band of free OH groups was detected at 3750 cm-~. After CO adsorption at 77 K its intensity diminishes and a band of perturbed OH groups arises at 3660 cm1. In the CO stretching region two bands arise at 2162 and 2142 cm1, the intensity and position being almost the same for samples treated at 673 and 973 K. Almost the same bands were reported for CO adsorbed on amorphous aerosil samples and were attributed to molecules forming weak H-bonds with surface silanol groups and physically adsorbed CO, respectively. 3.2. Ti-silica gel In the spectrum of sample pretreated at 673K two bands arise in the CO region at 2142 and 2162 cm l, analogous to those of silicalite. For the sample treated at 973 K one more band appears at 2182 cm1. On rasing the sample temperature the 2142 cm1 band is the first to disappear, while that at 2182 cm~ is the most stable. In the OH stretching region CO adsorption manifestations are the same as for silicalite or amorphous silica: the band of

165 isolated silanols at 3750 cm ~ diminishes on CO addition, and is restored together with the disappearance of the CO band at 2162 cm 1 after CO removal or heating the sample. T,%

3 4

2 H

/

/

/t

1 I

3800

3700

3600

2200

2100

Wavenumbers, era- 1 Figure 1. IR spectra of CO (5 torr) adsorbed on the pretreated at 973 K samples of: 1- pure silicalite at 77 K, 2- Ti-silica gel at 77 K, 3- TS-la at 100 K, 4 - TS-lb at 100 K, 5 - beta-TiAI at 100 K. Dotted line- before adsorption, at 77 K. 3.3. TS- 1 a zeolite

For all the studied pretreatment temperatures three bands of adsorbed CO were observed at 2166, 2142 and 2126 cm 1. Changes in the spectra of the OH groups were much less pronounced as compared with those observed for silicalite or Ti-silica gel, that is why the band at 2166 cm I, whose maximum position does not coincide with that of molecules adsorbed on silanol groups, should rather be attributed to a new type of sites appearing due to the presence of titanium. 3.4. TS-1 b zeolite

Three bands were found in the spectrum of the sample preheated at 673 K, at 2162, 2142 and a weak band at 2098 cm 1 The latter is, evidently, due to the presence of 13CO of natural abundance (about 1%), for in the conditions of experiment the intensity of the band of physically adsorbed CO at 2142 cm 1 was very high. In the spectrum of OH groups the band at 3750 cm l diminishes and that of the perturbed OH group appears at 3660 cm ~ After pretreatment at 973 K one more band of CO was observed at 2182 cm ~

166 r,N

4

''''

3800

3600

' 0 ' ; 30 "

34 0

' 2200 Wavenumbers, cm- 1

' 2100

Fig.2.IR spectrum of Ti-Al-beta zeolite pretreated at 973K after addition of CO (5 torr) and removal the excess of CO at 77K (1), l13K (2), 133K (3), 173K (4), and after outgassing at 300K (5). T,~

2a 3

//

/

/ %

3700

3400

3100

2200

2100

Wavenumbers,cm-I

Fig.3. IR spectrum of Ti-Al- beta zeolite pretreated at 673K (a) and 973K(b). 1- at 77K before adsorption, 2- after addition of CO (5 torr), 3- after removal the excess of gas at about 100K.

167

Table 1

IR band positions of surface OH groups and adsorbed CO molecules, c m "1. sample

T(K) of pretr.

free OH

pert. OH

silicalite

673 973

3750 3750

Ti-silica gel

673 973

TS-1 a

TS-I b

Beta-Ti-A1

phys. ads.

on OH groups

3660 3660

2142 2142

2162 2162

3750 3750

3680 3680

2142 2142

2162 2162

673 973

3750 3750

3660 3660

673 973

3750 3750

3660 3660

2142 2142

2162 2162

673

3750, 3620, 3540 3750, 3620

3660, 3320

2146, z. " 140

2180

3660, 3330

2146, 2140

2180 2160

973

on redu ced sites

2126 2126

on Lsites

2182 2166 2166

2142 2142

2182

2160

2232 2190

3.5. Ti-AI-Beta zeolite

The spectrum of this zeolite is more complicated. In the OH stretching region three bands were observed after pretreatment at 573 or 673 K, at 3750, 3620 and 3540 cm 1, the former two resist pumping at 973 K, when the last one disappears. After CO adsorption at 77 K (fig. 2.) these bands diminish while two bands of perturbed hydroxyls show up at 3660 and 3330 cm -~. That at 3660 cm ~ disappears after pumping off the gaseous CO at 77 K together with the band of adsorbed CO at 2160 cm ~ and the restoration of the 3750 c m 1 band, however, to remove all the CO bands and to recover the initial OH group spectrum, evacuation at room temperature is needed. Fig.3. illustrates the influence of CO adsorption on the spectra of samples pretreated at 673 and 973 K. For both the pretreatment temperatures in the region of physisorbed CO instead of a single band, a doublet appears with an extra band at 2146 cm ~. Strong CO bands occur at 2160 and 2180 cm -1 with a shoulder at about 2190 cm -1 and one more band at 2232 cm 1. After pretreatment at 973 K the shoulder is more pronounced and the intensity of the 2232 cm -1 band is higher. No new bands indicative for the appearance of any other surface sites were detected. 4. DISCUSSION Comparison of the data obtained shows that some bands have the same position for different samples. This enables us to assign most of the bands, and to discuss the dissimilarity between the two TS-1 samples.

168 The band at 2142 cm~ is present in the spectra of all the studied samples and has the same position as that of free CO in gas phase (2143 cm~). It belongs, evidently, to CO molecules physically adsorbed at the surface. The band disappears immediately after CO removal from the gas phase, and its presence could be used as an indication about saturation of all more or less strong surface sites by CO. The band at about 2162 cm~ appears together with the perturbation of surface silanol groups, that is diminishing of the band at about 3750 c m l and growth of that of hydroxyls perturbed by adsorbed CO molecules, and is due, thus, to molecules hydrogen bonded to the surface silanol groups. This kind of specific CO adsorption is the only one for pure silica samples like amorphous aerosil [18] or silicalite, and could be detected spectroscopically for most of the other silica-containing systems. The band at 2166 cm~ observed in the spectrum of TS-la could also be attributed to Hbonding, but its higher wavenumber is indicative of stronger acidity of the hydroxyls and should be accompanied by larger frequency shift of the OH stretching vibration. However, the position of the perturbed OH band is the same as for silica (3660 cm~), and the relative decrease in the OH band intensity is comparatively small. That is why it should rather belong to CO molecules adsorbed on weak Lewis acid sites, or to the superposition of the band of the latter with that due to CO adsorption on silanol groups. Adsorption on more acidic OH groups could be illustrated by the spectra of Ti-A1- beta zeolite. For this system one more band of perturbed hydroxyls at 3330 cm 1 could be associated with the diminution of that at 3620 cm~, the frequency shift of almost 300 cm~ indicating the extremely high Bronsted acidity. Such shifts were observed for the bridged AI-OH-Si hydroxyls, typical of the N-containing zeolites [11-13, 19, 20]. It is not easy to find the corresponding band of CO molecules assignable to this form of adsorption. That at 2180 cm ~ is in fact the only band observed simultaneously in the CO vibration region. However, the shift by 37 cm~ with respect to gaseous CO is too high as compared with the reported 32-35 cm1 values [11, 19, 20] corresponding to that of 290-340 c m 1. for the OH groups. Moreover, the band at almost exactly the same frequency was detected for the TS-lb, Ti- silica gel, or pure titania samples containing no aluminium at all. Perhaps, the band of H-bonded CO is superimposed here with another intense band of chemisorbed molecules. CO molecules adsorbed on strong L-sites, that is on the coordinatively unsaturated Ti ions, evidently, account for the band at 2182 cm~, which appear in the spectrum of TS-1 b sample after evacuation at 973 K. Absence of this band for lower temperature of pretreatment is in accordance with the notions that coordination sphere of titanium is completed by adsorbed water, which could be removed by thermoevacuation [2,21,22], however, the temperature range of 673-973 K seems to be too high for the desorption of coordinatively bonded molecular water. The same form of adsorption was found for Ti-silica gel (band at 2182 cm~), and for pure TiO2 (anatase) strong band of coordinatively adsorbed CO molecules occurs at 2196- 2179 cm1 [9, 15,16], in agreement with the above assignment. The high-frequency CO bands in the spectrum of Ti-Al-beta zeolite with the intensity also increasing after vacuum treatment at high temperatures should also be assigned to molecules adsorbed on Lewis sites. That at 2190 cm~ is, evidently, due to adsorption on Ti cations, while the position of another one at 2232 cm ~ is too high for titanium, and it should be attributed to CO molecules bound to strongly coordinatively unsaturated, perhaps, threecoordinated AI ions, that account for the band at 2242-2215 cm1 in the spectra of CO adsorbed on alumina [9,23,24]

169 One more band was found in TS- 1a sample at 2126 cm~ below that of flee CO in the gas phase. Frequency lowering is characteristic of d-electron back donation to the antibonding orbitals of CO, that is why this band can be considered as an evidence for the reduced transition metal sites on the surface. Comparison of the data on the two TS-1 samples reveals great dissimilarity.i) The intensity of the OH group bands after pretreatment at the same conditions differs essentially and is much higher for TS-lb. Great number of hydroxyls corresponds in the spectrum of adsorbed CO to the intense band of H-bonded molecules, ii) It was only for the TS-lb sample that the band of CO on strong Lewis sites was found at 2183 cm J after high-temperature treatment. TS-la, on the contrary, displays adsorption on weaker electron-accepting centers with a band of CO at 2166 cm~. iii) Even after heating and cooling the sample in oxygen, the band at 2126 cm1, evidently due to CO adsorbed on reduced sites, arise for TS-1 a, but not for TS- 1b sample. It is noteable that the spectra of samples calcined at 923 K, when the zeolite framework could well be irreversibly sintered, do not differ too much from those for the same zeolites after mild pretreatment. As a rule, the same sites were detected, and only the concentration of the most coordinatively unsaturated atoms increases after calcination, while the number of hydroxyl groups diminishes. Apparently, the method is sensitive to the local arrangement, that does not change greatly even if the lattice symmetry is lost. Adsorption on the defect sites arising after high temperature treatment can supply supplementary" information about the composition of material or about the state of incorporated metal atoms before the calcination. 5. CONCLUSIONS Low temperature IR spectroscopy of CO as a test molecule provides means to display the difference of surface properties of Ti-containing adsorbents. Two kinds of TS-1 zeolite samples, which have identical structure and slightly different amount of residual alkali cations, characterized by this method, show great dissimilarity in the amount of acidic OH groups, Lewis acid sites of different strength, or of the reduced Ti atoms. Thus, the method could really be used as a rapid test for the surface properties of metal containing zeolites.

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