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

Formation of planar and spiral Ca2+ waves in isolated cardiac myocytes.

A novel Nipkow-type confocal microscope was applied to image spontaneously propagating Ca2+ waves in isolated rat ventricular myocytes by means of fluo-3. The sarcolemma was imaged with di-8-ANEPPS and the nucleus with SYTO 11. Full frame images in different vertical sections were obtained at video frame rate by means of an intensified CCD camera. Three types of Ca2+ waves were identified: spherical waves, planar waves, and spiral waves. Both spherical waves and spiral waves could initiate a planar wave, and planar waves were not influenced by the presence of a nucleus. Spiral waves, however, were consistently found adjacent to a nucleus and displayed a slower propagation rate and slower rate of increase in Ca2+ concentration in the wave front than did spherical and planar waves. The planar waves were apparent throughout the vertical axis of the cell, whereas spiral waves appeared to have a vertical height of approximately 3 microm, less than the maximum thickness of the nucleus (5.0 +/- 0.3 microm). These results provide experimental confirmation of previous modeling studies which predicted an influence of the nucleus on spiral-type Ca2+ waves. When a spontaneous Ca2+ wave is small relative to the size of the nucleus, it appears that the Ca2+ buffering by the nucleus is sufficient to slow the rate of spontaneous propagation of the Ca2+ wave in close proximity to the nucleus. These findings thus support the idea that the nucleus can influence complex behavior of Ca2+ waves in isolated cardiac myocytes

Comparison of Tetragonal and Ordered Mesoporous Sulfated ZrO2: Alkane Isomerization Studied by In Situ DR UV-vis Spectroscopy

Introduction: The role of the ZrO2 bulk structure in sulfated zirconia (SZ) catalysts is unclear. We compare SZ of tetragonal structure (tSZ) to ordered mesoporous SZ of MCM-41 structure (omSZ), focussing on performance and deactivation behavior in n-butane and n-pentane isomerization. Experimental: tSZ was obtained by calcination of a commercial precursor, omSZ in a procedure based on the description by Ciesla [1,2]. In situ DR-UV-vis-NIR spectra were recorded using a lambda 9 spectrometer (PerkinElmer) with integrating sphere and a fixed bed flow reactor [3]. Products of n-butane (5 kPa in He) and n-pentane (1 kPa) isomerization were analyzed by on-line GC. Results and Discussion: tSZ exhibited a higher maximum activity than omSZ; to obtain comparable rates, conditions were varied. For n-butane isomerization catalyzed by tSZ at 378 K, the conversion increased over 50 min and then declined in the next 100 min to a steady state. Despite a higher reaction temperature of 453 K, omSZ, which passed through a 100 min induction period, deactivated only slowly. Both catalysts produced propane and pentanes as side products, suggesting a similar mode of operation. During the deactivation phase, which stretched over 100 min for tSZ and over more than 900 min for omSZ, bands developed in the UV-vis spectra. A band at 310 nm was detected for tSZ and assigned to allylic cations; for omSZ, the band was positioned at 285 nm. In the reaction with n-pentane, the materials produced more isobutane than isopentane within the observation span of 15 h. Both catalysts deactivated rapidly while bands at 330 nm (tSZ, 298 K) and 285 nm with a shoulder at ca. 330 nm (omSZ, 323 K) formed. Unsaturated species are formed on both materials but they are either differently polarized by the surface or differently structured, indicating an influence of the nature of the ZrO2 structure. 1. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schüth, Angew. Chem. 108 (1996) 597. 2. X. Yang, F.C. Jentoft, R.E. Jentoft, F. Girgsdies, T. Ressler, Catal. Lett. 81 (2002) 25. 3. M. Thiede, J. Melsheimer, Rev. Sci. Inst. 73 (2002) 394

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

Interaction of VxOy/SBA-15 surface sites with the reactant propane observed by microcalorimetry and FTIR spectroscopy

Vanadium is an important component in oxidation catalysts [1]. Vanadium supported on mesoporous silica SBA-15 is active for the partial oxidation of propane to acrylic acid in the presence of steam [2]. The goal of this work is to characterize the surface sites that interact with propane by investigating: i) number of sites and strength of interaction (isotherms, differential heats), ii) nature of the adsorbate complexes (IR spectroscopy), iii) influence of vanadium loading, and iv) role of H2O