Labile sulfate species as key active components in sulfated zirconia for activating n-butane

Sulfated zirconia and other sulfated metal oxides have been studied for over 2 decades owing to their high catalytic activity for activation of short alkanes at low temperature. The surface structure of sulfated zirconia has been studied widely in order to elucidate the nature of active sites since the discovery of its catalytic property for alkanes conversion at low temperature. Nevertheless, no consensus has been reached so far. Here, we report that the labile sulfate, which can be removed from sulfated zirconia by water washing, acts as crucial component for the active site of sulfated zirconia. Experimental Sulfate-doped zirconium hydroxide was obtained from Magnesium Electron, Inc. (XZO 1077/01), which was heated up to 873 K with a ramp rate of 10 K/min in static air and kept at 873 K for 3 h, denoted as SZ. Water washing technique was applied to the above calcined sulfated zirconia. 20 g of SZ were suspended in 400 mL bi-distilled water and then filtered. Repeated the washing procedure for 3 times, then the cake was dried at room temperature. The resulting powder is denoted as SZ-WW. The materials were characterized by IR spectroscopy, (including the sorption of probe molecules such as pyridine and CO2), TAP measurements, XRD and the sulfate content was determined. n-Butane isomerization reactions were carried out in a quartz micro tube reactor under atmospheric pressure. Prior to the reaction, the catalyst was activated at 473 K for 2 h in He flow (10 ml/min). Results and Discussion The calcined sulfated zirconia, SZ, showed a catalytic activity of 0.015 mol/g/s for n-butane skeletal isomerization with an initial iso-butane selectivity of 96 % at 373 K. It is interesting to note that the removal of water soluble sulfate by water washing treatment of the parent sample resulted in an inactive sample (SZ-WW). The content of sulfate of the water washed sample (SZ-WW) is 0.25 mmol/g, which is much lower than that of the original calcined sulfated zirconia, 0.44 mmol/g. Thus, 43 % of the total sulfate of sulfated zirconia was removed by water washing. The water washing treatment not only removed the water soluble sulfate of SZ, but also the Brønsted acid sites leading to an increase of Lewis acid concentration. The IR spectra of water washed sulfated zirconia (SZ-WW) and sulfated zirconia (SZ) samples showed pronounced difference in the region OH and S=O vibrations. In the IR region of OH group above 3600 cm-1, water washing increased the intensity of the OH band at 3634 cm-1 and shifted it to higher frequency, 3661 cm-1. In addition, water washing reduced a fraction of sulfate groups at high frequency leading to sulfate stretching vibrations of water washed sample (SZ-WW) at 1391 cm-1 compared to the parent sample (SZ) at 1404 cm-1. Note that the wavenumber of the S=O stretching vibration is related to the SO bond order [ , ], indicating that fractions of highly covalent sulfate were removed. IR spectra recorded during adsorption of CO2 showed the formation of bicarbonate on the surface of the washed sample but not on the original sample. SZ-WW featured an about equal number of two different types of Lewis acid sites, while for SZ one type of Lewis acid sites was predominant. The data indicate that water washing produces domains of bare zirconia surface, free of sulfate. The results show for the first time that the water soluble sulfate species are responsible for the formation of highly covalent sulfates as well as the Brønsted acid sites, which are essential for the alkane isomerization reaction on sulfated zirconia at low temperature. Elementary steps are discussed based on steady state and transient kinetic measurements

Discrete populations of isotype-switched memory B lymphocytes are maintained in murine spleen and bone marrow

At present, it is not clear how memory B lymphocytes are maintained over time, and whether only as circulating cells or also residing in particular tissues. Here we describe distinct populations of isotype-switched memory B lymphocytes (Bsm) of murine spleen and bone marrow, identified according to individual transcriptional signature and B cell receptor repertoire. A population of marginal zone-like cells is located exclusively in the spleen, while a population of quiescent Bsm is found only in the bone marrow. Three further resident populations, present in spleen and bone marrow, represent transitional and follicular B cells and B1 cells, respectively. A population representing 10-20% of spleen and bone marrow memory B cells is the only one qualifying as circulating. In the bone marrow, all cells individually dock onto VCAM1+ stromal cells and, reminiscent of resident memory T and plasma cells, are void of activation, proliferation and mobility

Larval fish abundance, composition and distribution at Senghor Seamount (Cape Verde Islands)

The Cape Verdean Senghor-Seamount rises up to 93 m below the surface, and lies within the Cape Verde Frontal Zone (CVFZ), and within the southwestward North Equatorial Current. The seamount and its oceanic surroundings were surveyed with Multiple-Opening-Closing Net (MCN) fish larval catches, Issacs-Kidd midwater trawl (IKMT) micronekton hauls and an analysis of some conductivity–temperature–depth data (RV Poseidon cruise no. POS 320/2). The thermal and saline stratifications showed widely symmetrical uplifts near the summit. The larval fish community was diverse (H′ = 2.656) and composed mainly of larvae of meso- to bathypelagic species (91.5%). In IKMT, 44.1% of the specimens originated from demersal parents (H′ = 3.296), and mostly, albeit not entirely, from West African coastal waters, after advection along the CVFZ to and across this potential “stepping stone”. Gross larval fish abundance (median 35.5 specimens/1 m2) and composition agreed well with historical literature data from adjacent waters and seamounts north of the CVFZ, whereas south of the CVFZ and towards NW-Africa reported abundances were higher. Vertical distributions of larvae which generally live at greater depths showed a rise above the seamount, following the hydrographic uplift, accompanied by “thinning-out effects” through bathymetric disturbance. The extent to which findings at Senghor Seamount are representative for small, shallow and steep seamounts in the tropical NE-Atlantic is discussed

Kinetik mittels IR-Spektroskopie: Aktivierung von H2 und D2 an Ag/SiO2-Hydrierkatalysatoren

Kinetik mittels IR-Spektroskopie: Aktivierung von H2 und D2 an Ag/SiO2-Hydrierkatalysatoren F.C. Jentoft,1 J. Kröhnert,1 K. Klaeden,1 R. Schlögl1 M. Bron,2 P. Claus2 1Abteilung Anorganische Chemie, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin 2Ernst-Berl-Institut für Technische und Makromolekulare Chemie, TU Darmstadt, Petersenstr. 20, 64287 Darmstadt SiO2-geträgerte Ag-Katalysatoren können für die Selektivhydrierung von α,β-ungesättigten Aldehyden zu ungesättigten Alkoholen eingesetzt werden [1,2]. Es ist noch ungeklärt, an welchen Zentren und auf welche Weise der Wasserstoff aktiviert wird. Wir haben die Reaktion von H2 und D2 mit dem reinen Träger SiO2 und einem 9 Gew.% Ag enthaltenden Katalysator IR-spektroskopisch verfolgt. Freitragende Preßlinge wurden bei 325°C im H2-Strom aktiviert, im Vakuum auf Reaktionstemperatur (100-250°C) abgekühlt und 100 mbar D2 (H2) ausgesetzt. Zeitaufgelöst aufgenommene Transmissions-IR-Spektren zeigten, daß die OH-Gruppen des SiO2 zu OD-Gruppen austauschten. Der Austausch ging für die Ag-haltige Probe stets schneller vonstatten. Die Kinetik der Reaktion ließ sich aus der am Anfang linearen Zunahme der OD-Banden verfolgen. Aus der Temperaturabhängigkeit der scheinbaren Geschwindigkeitskonstanten errechneten sich Aktivierungsenergien von ca. 28 kJ/mol für Ag/SiO2 und ca. 38 kJ/mol für SiO2. Der Rücktausch der OD-Gruppen mit H2 zu OH-Gruppen war erheblich langsamer. Für SiO2 bei 200°C betrug das Verhältnis der Geschwindigkeitskonstanten kD/kH ≈ 2. Dieser kinetische Isotopeneffekt weist darauf hin, daß die Spaltung der OH-Gruppe und nicht die Spaltung des Wasserstoffmoleküls geschwindigkeitsbestimmend ist. Die Untersuchungen liefern das etwas überraschende Ergebnis, daß Wasserstoff unter den Bedingungen der Hydrierkatalyse an SiO2 aktiviert (gespalten) werden kann. Der genaue Einfluß des Silbers auf diese Reaktion und die Aktivierung des ungesättigten Aldehyds sind Gegenstand weiterer Untersuchungen. [1] P. Claus, H. Hofmeister, J. Phys. Chem. B 103 (1999) 2766-2775. [2] P. Claus, P.A. Crozier, P. Druska, Fresenius J. Anal. Chem. 361 (1998) 677-679

Labile sulfate species as key active components in sulfated zirconia for activating n-butane

Sulfated zirconia and other sulfated metal oxides have been studied for over 2 decades owing to their high catalytic activity for activation of short alkanes at low temperature. The surface structure of sulfated zirconia has been studied widely in order to elucidate the nature of active sites since the discovery of its catalytic property for alkanes conversion at low temperature. Nevertheless, no consensus has been reached so far. Here, we report that the labile sulfate, which can be removed from sulfated zirconia by water washing, acts as crucial component for the active site of sulfated zirconia. Experimental Sulfate-doped zirconium hydroxide was obtained from Magnesium Electron, Inc. (XZO 1077/01), which was heated up to 873 K with a ramp rate of 10 K/min in static air and kept at 873 K for 3 h, denoted as SZ. Water washing technique was applied to the above calcined sulfated zirconia. 20 g of SZ were suspended in 400 mL bi-distilled water and then filtered. Repeated the washing procedure for 3 times, then the cake was dried at room temperature. The resulting powder is denoted as SZ-WW. The materials were characterized by IR spectroscopy, (including the sorption of probe molecules such as pyridine and CO2), TAP measurements, XRD and the sulfate content was determined. n-Butane isomerization reactions were carried out in a quartz micro tube reactor under atmospheric pressure. Prior to the reaction, the catalyst was activated at 473 K for 2 h in He flow (10 ml/min). Results and Discussion The calcined sulfated zirconia, SZ, showed a catalytic activity of 0.015 mol/g/s for n-butane skeletal isomerization with an initial iso-butane selectivity of 96 % at 373 K. It is interesting to note that the removal of water soluble sulfate by water washing treatment of the parent sample resulted in an inactive sample (SZ-WW). The content of sulfate of the water washed sample (SZ-WW) is 0.25 mmol/g, which is much lower than that of the original calcined sulfated zirconia, 0.44 mmol/g. Thus, 43 % of the total sulfate of sulfated zirconia was removed by water washing. The water washing treatment not only removed the water soluble sulfate of SZ, but also the Brønsted acid sites leading to an increase of Lewis acid concentration. The IR spectra of water washed sulfated zirconia (SZ-WW) and sulfated zirconia (SZ) samples showed pronounced difference in the region OH and S=O vibrations. In the IR region of OH group above 3600 cm-1, water washing increased the intensity of the OH band at 3634 cm-1 and shifted it to higher frequency, 3661 cm-1. In addition, water washing reduced a fraction of sulfate groups at high frequency leading to sulfate stretching vibrations of water washed sample (SZ-WW) at 1391 cm-1 compared to the parent sample (SZ) at 1404 cm-1. Note that the wavenumber of the S=O stretching vibration is related to the SO bond order [ , ], indicating that fractions of highly covalent sulfate were removed. IR spectra recorded during adsorption of CO2 showed the formation of bicarbonate on the surface of the washed sample but not on the original sample. SZ-WW featured an about equal number of two different types of Lewis acid sites, while for SZ one type of Lewis acid sites was predominant. The data indicate that water washing produces domains of bare zirconia surface, free of sulfate. The results show for the first time that the water soluble sulfate species are responsible for the formation of highly covalent sulfates as well as the Brønsted acid sites, which are essential for the alkane isomerization reaction on sulfated zirconia at low temperature. Elementary steps are discussed based on steady state and transient kinetic measurements