Reaction of Surface Deposits on Deactivated Sulfated Zirconia with O2 and H2O Monitored by In Situ DR UV-vis Spectroscopy and Mass Spectrometry

Introduction Unsaturated surface deposits have been detected by in situ UV-vis spectroscopy on tetragonal [1] and mesoporous sulfated zirconia (tSZ and mpSZ) during n-butane and n-pentane isomerization. The different bulk structures of the two materials appear to lead to a different deactivation behavior. In order to further identify the nature of these deposits on tSZ and mpSZ and to identify a procedure for catalyst regeneration, the deactivated materials were reacted at 298 K first with oxygen and then with water vapor. Changes to the catalyst and the deposits during the treatment with both reagents were monitored (in situ) by DR UV-vis spectroscopy, and the effluent stream was analyzed by MS. Experimental tSZ was obtained by calcination of a commercial precursor (MEL Chemicals) at 823 K, mpSZ was synthesized as reported in the literature [2, 3]. For in situ spectroscopy, a fixed bed flow reactor was placed in the sample position at the integrating sphere of a Lambda 9 spectrometer (PerkinElmer). Spectra were recorded using a scan speed of 240 nm min-1, a slit width of 5 nm, and with Spectralon® as a white standard. n-Butane (5 vol%, 50 cm3 min-1) isomerization was conducted at 378 K (tSZ) or 453 K (mpSZ), and n-pentane (1 vol%, 50 cm3 min-1) isomerization at 298 K (tSZ) or 323 K (mpSZ); and the gas phase was analyzed by on-line GC. After 16 h on stream, the samples were cooled to 298 K in He, treated first with 20 vol% O2 in He (50 cm3 min 1) for 1.5 h, and then with water vapor in He (50 cm3 min-1) for 1.5 h. The effluent gas stream was analyzed using a Pfeiffer Thermostar mass spectrometer. Results and discussion During n-butane and n-pentane isomerization, unsaturated surface deposits (absorption band at 300-330 nm, allylic-type species [1]) were formed on the surface of tSZ and during n-pentane isomerization on mpSZ, while the catalysts deactivated rapidly. Only during n-butane isomerization on mpSZ were nearly no changes in the UV-vis spectra with time on stream observed, and deactivation was slow. The spectra of the SZ samples with allylic type deposits showed only minor changes in the oxygen stream. Oxygen treatment of the mpSZ sample caused an overall intensity decrease between 250 and 450 nm within the first 5 min but no further changes. Nevertheless, fragments of hydrocarbons and oxygenated derivatives were registered in the mass spectra of the effluent stream for all samples. Fig. 1: UV-vis spectra of tetragonal SZ During subsequent water vapor treatment of the SZ samples with allylic deposits, intense bands at about 380, 455-460, and 550-560 nm developed and the original band at 310-330 nm was reduced in intensity (Fig. 1). Bands at 430-455 nm have been assigned to quinone-type compounds [4], the other features are not yet explained. The spectrum of the mpSZ sample that had been deactivated in n-butane became similar to the spectrum of the activated state of this sample, with recovery of the overall intensity and the presence of absorption bands at 280 and 320 nm. The mass spectra of the gas phase during the water treatments showed the same fragments as during the oxygen treatment but with much lower intensity. The nature of the surface deposits on tSZ and mpSZ can be different depending on reactant and conditions. Surface deposits formed during alkane isomerization react with the components of air and are partially volatilized. Color changes consistent with the UV-vis spectra in Fig. 1 have been observed when taking deactivated samples out of the reactor. Surface deposits must thus be studied in situ. 1. R. Ahmad, J. Melsheimer, F.C. Jentoft, R. Schlögl, J. Catal., 218 (2003) 365. 2. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schüth, Angew. Chem., 108 (1996) 597. 3. X. Yang, F.C. Jentoft, R.E. Jentoft, F. Girgsdies, T. Ressler, Catal. Lett., 81 (2002) 25. 4. D. Spielbauer, G.A.H. Mekhemer, E. Bosch, H. Knözinger, Catal. Lett., 36 (1996) 59

Quasi in-situ Adsorptive Microcalorimetric Characterization of Sulfated Zirconia Catalyst for n-Butane Isomerization with n-Butane and Isobutane as Probe Molecules

Sulfated zirconia (SZ with 0.94 mmol/g sulfur and 100 m2/g) is an active catalyst for the industrially important low temperature (373 K) isomerization of light alkanes, e.g. of n-butane [1]. The catalytic activity exhibits a multicomponent profile along time-on-stream (TOS) including an activation period and a maximum followed by rapid deactivation. This observation suggests a continuous change of the catalyst under reaction conditions, especially, of the sites that gain activity during the induction period. It has been found that a freshly prepared catalyst shows some characteristics that are far from those of the catalyst during reaction. Also it is known that probe molecules such as NH3 and pyridine, which are often used for catalyst characterization by adsorptive microcalorimetry, interact with the active sites much stronger than the reactant. It is hence compulsory to do characterization in-situ using probe molecules with the same characteristics as the reactant. Therefore, we developed a quasi in-situ adsorptive microcalorimetric method in order to characterize the active sites on SZ. The calorimeter cell was used as a fixed bed flow reactor, in which the catalytic reaction of n-butane isomerisation was carried out (0.5 g SZ pellets, 378 K, 1 kPa n-butane in N2). The feed was introduced through a capillary. Conversion was monitored on-line by GC. The reaction was stopped after various TOS, the cell was evacuated at 378 K, and placed in a SETARAM MS 70 calorimeter equipped with a volumetric system that allows dosages of < 0.01 µmol [2]. Adsorption of n- or isobutane was performed at 313 K [3]. The n- and isobutane adsorption isotherms of the SZ catalyst at different TOS indicate that the number of sites interacting with educt- (n-butane) and product (isobutane) molecules decreases with TOS, especially, during the induction period. Only a modified and not the simple Langmuir model fites these adsorption isotherms [4]. The order of adsorption decreases with the increasing catalytic activity, e.g. n-butane adsorption from 1.7 (TOS = 0) to 1.1 (TOS = 120 min). This indicates a more complicated, maybe activated adsorption process. Differential heats of butanes adsorption at coverages > 2 mmol/g show that the majority of sites produce 40 – 50 kJ/mol. These stable sites are probably related to the steady state activity of SZ beyond the activity maximum. Differential heats at < 20 mmol/g show that only a minority (< 2% of sulfur species) of sites change their character during the induction period. It seems that only these sites determine the induction process. The state of highest activity is characterized by a strong interaction of n-butane with the active sites (75 kJ/mol at < 2 mmol/g). However, the weak interaction of isobutane (50 kJ/mol at < 2 mmol/g) indicates an increasing easiness of product-desorption from the surface sites. The adsorbed amount of n-butane and isobutane is comparable (ca. 20 mmol/g at 6 mbar). In the state of highest activity the catalyst reacted with n-butane in the calorimeter cell (additional heat evolution, gas phase products). [1] M. Hino, S. Kobayashi, K. Arata, J. Am. Chem. Soc., 101 (1979) 6439. [2] L.C. Jozefowicz, H.G. Karge, E.N. Coker, J. Phys. Chem. 98 (1994) 8053. [3] S. Wrabetz, X. Yang, F.C. Jentoft, R. Schlögl, in preparation. [4] I. Langmuir, J. Am. Chem. Soc. 38 (1916) 2221

Dynamic nature of surface sites on VxOy/SBA-15 catalysts

A series of SBA-15 supported VxOy catalysts was prepared and characterized by N2 adsorption and Raman, UVvis, and XP spectroscopies. The surface sites were probed by propane adsorption after various treatments. IR spectra indicate several types of OH groups, some of which are dehydroxylated at increasing temperature, leading to a stronger interaction of the remaining OH groups with propane. These results are corroborated by the heats of adsorption of propane, which reach 60, 80, and 170 kJ/mol after activation at 373, 573, and 673 K. The surface can be rehydroxylated, and it is proposed that the steam necessary for the conversion of propane to acrylic acid also adjusts the surface acidity

Structure of Molybdenum Oxide Supported on Silica SBA-15 Studied by Raman, UV-Vis and X-Ray Absorption Spectroscopy

The structure of molybdenum oxide supported by silica SBA-15 has been studied by visible Raman spectroscopy, diffuse reflectance UV–Vis spectroscopy and X-ray absorption spectroscopy in the dehydrated state obtained after thermal treatment at elevated temperatures (≥350 °C). No dependence of the molybdenum oxide structure on preparation procedure or loading has been observed within the range of loadings studied in detail (0.2–0.8 Mo/nm2). X-ray absorption spectroscopy (XAS) reveals that the dehydrated state consists of a mixture of monomeric and connected molybdenum oxide centres. While the presence of crystalline MoO3 can be excluded by Raman spectroscopy, tetrahedrally and octahedrally coordinated MoO4 and MoO6 units are identified by XAS. The MoO6 units possess connectivity similar to that of MoO3 building blocks, whereas the MoO4 units are isolated or connected to other MoxOy units. These results are supported by UV–Vis spectra showing intensity at wavelengths (>300 nm) typical for dimeric and/or oligomeric species