Design of an Enhanced Throughput Catalytic Test System Capable of Rapid Heating and Cooling

Introduction High-throughput techniques are used in combinatorial chemistry, for example to per-mit preparation and screening of hundreds of catalysts simultaneously [1]. The prin-ciple of conducting more than one experiment at the same time is generally desir-able. Here we present a system allowing three concurrent fixed bed reactor tests, to be conducted on the laboratory scale (2 ml bed volume). This enhancement in throughput is achieved without loss of reaction analysis information. System Requirements The system is designed to investigate low temperature alkane isomerization (butane, pentane) on sulfated zirconia catalysts and therefore must fulfill the following condi-tions: (I) Isothermal over a wide temperature range, from 0°C (n-pentane isomerization) to 650°C (in situ calcination of the catalyst material [2]) (II) Rapid heating and cooling to reduce time loss (III) Fast and quantitative gas phase analysis Design and Test Results The requirements led to the construction of a reaction vessel in which three U-shaped tubular quartz reactors (inlet Ø 12 mm, outlet Ø 6 mm) are positioned sym-metrically. These tubular reactors each contain a quartz frit in the inlet tube to hold the catalyst powders. They are fixed at the top by seals made of polytetrafluorethyl-ene. Cooling the lid by an air flow avoids thermolysis of the PTFE. Isothermal heating is possible using a fluidized sand bed. The bottom of the vessel is heated electrically. It contains a frit of metal wire that supports the sand (50-70 mesh, ca. 500 ml). The sand is fluidized by air flowing through the frit (ca. 12 l/min). A 25 K/min heating ramp is possible. For experiments below room temperature the air can be cooled, e.g. by liquid nitrogen. The reaction vessel is enclosed by a cylindrical shell that can be purged by air for cooling. Thus a fast return to lower temperatures after activa-tion/calcination is guaranteed (from 450°C to 50°C in ca. 45 min). The temperature of the reactor is monitored by a thermocouple positioned in the center of the vessel and controlled by a second thermocouple close to the heating wire. A four position valve selects the outlet of either one of the three reactors, or the bypass, for analysis. Analysis of the gas phase is performed using a Micro GC (Varian CP 4900) equipped with a thermal conductivity detector, which allows separation of n-butane and isobu-tane within ca. 1 min. Cross section through the reaction vessel containing the U-shaped reactors Reaction vessel without cylindrical shell References [1] A. Hagemeyer, B. Jandeleit, Y. Liu, D.M. Poojary, H.W. Turner, A.F. Volpe Jr, W.H. Weinberg, Appl. Catal. A, 2001, 221, 23-43. [2] A. Hahn, T. Ressler, R.E. Jentoft, F.C. Jentoft, Chem. Comm., 2001, 537-538

Promoting governability in small-scale capture fisheries in the Persian Gulf: The case of Qeshm Island

Source at http://jifro.ir/index.php?slc_lang=en&sid=1.The present study examines the fisheries governance status of small-scale capture fisheries in the northern Persian Gulf. Qeshm Island, which is selected as case study, plays a prominent role in fisheries in the Persian Gulf and territorial waters of the country. The research methodology included in-depth and semi-structured interviews with heads of fisheries cooperatives and fishers to deepen our understanding of the cultural and technical characteristics of local fisheries communities. Subsequently, data was drawn from 322 questionnaires, using a random sampling technique. The analyses indicate that fisheries co-management is at an interstitial situation, while the fishers are willing to cooperate with the government. A finding is also showed that literacy has a significant effect on fishers’ willingness to cooperate with government. There was also a considerable conflict of interest among the fisheries communities in the study area, which makes the implementation of rules difficult. Small-scale fishing communities are generally in a hard-pressed situation, which affects how fishers operate. Our study aims to contribute to improving the governance and governability of small-scale capture fisheries in the region

In-Situ Investigation of Gas Phase Radical Chemistry in the Catalytic Partial Oxidation of Methane on Pt

The catalytic partial oxidation of methane on platinum was studied in situ under atmospheric pressure and temperatures between 1000 and 1300 °C. By combining radical measurements using a molecular beam mass spectrometer and threshold ionization with GC, GC-MS and temperature profile measurements it was demonstrated that a homogeneous reaction pathway is opened at temperatures above 1100 °C, in parallel to hetero-geneous reactions which start already at 600 °C. Before ignition of gas phase chemistry, only CO, H2, CO2 and H2O are formed at the catalyst surface. Upon ignition of gas chemistry, CH3⋅ radicals, C2 coupling products and traces of C3 and C4 hydrocarbons are observed. Because the formation of CH3⋅ radicals correlates with the formation of C2 products it can be concluded that C2 products are formed by coupling of methyl radicals in the gas phase followed by dehydrogenation reactions. This formation pathway was predicted by numerical simulations and this work presents an experimental confirmation under high temperature atmospheric pressure conditions

Structural and Active Site Characterization of Sulfated Zirconia Catalysts for Light Alkane Isomerization

Sulfated zirconia (SZ) is active for light alkane isomerization at temperatures as low as 373 K [1]. The material has been investigated extensively in the past 2 decades [2] but so far no convincing structure-activity relationship has been established. Here, we report on the investigation of two different SZ materials with an interesting combination of properties. Both materials have a sulfate content of 9 wt.%; however, the material with lower specific surface area (SZ-1, 90 m2og-1) displays a maximum n-butane isomerization rate (373-423 K, 1-5 kPa n-butane at 101.3 kPa total pressure) that is about one order of magnitude higher than that of the material with higher specific surface area (SZ-2, 140 m2og-1). Both materials were produced through precipitation from zirconyl nitrate solution, followed by aging of the precipitate either at 298 K for 1 h (SZ-1) or 373 K for 24 h (SZ-2). After drying, the samples were sulfated with ammonium sulfate and calcined for 3 h at 873 K. Scanning electron microscopy showed typical particle sizes of 5 to 20 µm for SZ-1, and of 1 to 5 µm for SZ-2. X-ray diffraction and Zr K-edge X-ray absorption spectra identified both materials as predominantly tetragonal ZrO2, but SZ-2 exhibited smaller crystalline domains than SZ-1 (7.5 vs. 10 nm). Diffuse reflectance IR spectra taken during catalyst activation (523 K, inert gas) suggest that the sulfate structures on the two materials rearrange in a slightly different way during dehydration. This is tentatively attributed to different sulfate group densities that result from the ratios of sulfate content to surface area. By ammonia adsorption/desorption, the concentration of acid sites was determined to be 0.52 and 0.48 mmolog-1 for SZ-1 and SZ-2, respectively; this result is not reflected by the catalytic activities. Temporal analysis of products measurements indicated that the residence times for n-butane were shorter than for i-butane, and SZ-1 retained both these molecules longer than SZ-2. The activation energy for n-butane desorption was equivalent for both samples, i.e., 40-41 kJomol-1. Calorimetric measurements of the adsorption of reactant and product at 313 K produced the following results. At 0.3 kPa alkane pressure, SZ-1 and SZ-2 adsorbed similar amounts of n-butane (20 and 25 µmol), but very different amounts of i-butane (80 and 25 µmol). At coverages below 2 µmol the differential heats of adsorption of n-butane were much higher on SZ-2 than on SZ-1, while at higher coverages the heats were nearly identical for both samples and decreased from 60 to 40 kJomol-1. The samples did not differ with respect to the strength of interaction with i-butane, the heats decreased with increasing coverage from 60 to 40 kJomol-1. The results demonstrate that (i) typical SZ catalysts have fewer than 100 µmolog-1 sites, rendering identification by spectroscopic techniques difficult, and (ii) product desorption is a critical factor for the catalytic performance. References: [1] M. Hino, K. Arata, J. Chem. Soc. Chem. Comm. (1980) 851. [2] X. Song, A. Sayari, Catal. Rev. Sci. Eng., 38 (1996) 32

Structural and Active Site Characterization of Sulfated Zirconia Catalysts for Light Alkane Isomerization

Two different sulfated zirconia catalysts were produced through precipitation from zirconyl nitrate solutions, followed by aging of the precipitate either at 298 K for 1 h (SZ-1) or 373 K for 24 h (SZ-2). After drying, the samples were sulfated with ammonium sulfate and calcined for 3 h at 873 K. SZ-1 had a smaller surface area (90 m2 g-1) than SZ-2 (140 m2 g-1) but displayed a one order of magnitude higher maximum n-butane isomerization rate (373–423 K, 1–5 kPa n-butane at 101.3 kPa total pressure). Both materials consisted predominantly of tetragonal ZrO2, contained 9 wt% of sulfate, and adsorbed about 0.5 mmol g-1 NH3. Measurements of adsorption isotherms and differential heats for propane and iso-butane at 313 K reveal a larger number of adsorption sites on SZ-1 than on SZ-2, extrapolated to 1 kPa, 42 vs. 20 µmol g-1 (propane) and 120 vs. 44 µmol g-1 (iso-butane). At coverages > 2 µmol g-1 the heats were similar for both samples with both probes and decreased from 60 to 40 kJ mol-1. Temporal analysis of products measurements indicated shorter residence times for n-butane than for iso-butane, and SZ-1 retained both of these molecules longer than SZ-2. The activation energy for n-butane desorption was 45 kJ mol-1 for both samples. Interaction with pulses of CO2 suggested that non-sulfated, basic ZrO2 surface is exposed on SZ-2, consistent with the larger surface area at the same sulfate content as SZ-1. The results suggest that only a fraction of the sulfate groups participates in adsorption and that product desorption may be of importance

Site Changes on Sulfated Zirconia during n-Butane Isomerization: Quasi-In-Situ Adsorption Calorimetry Study with Butanes as Probes

Introduction: Sulfated zirconia (SZ) changes its performance for n-butane isomerization considerably with time on stream (TOS). To probe the relevant sites on active SZ we interrupted the reaction at different stages (induction period, maximum conversion), removed weakly adsorbed species, and measured adsorption isotherms and differential heats of adsorption (qdiff) of butanes. Experimental: The calorimeter cell was used as a fixed bed flow reactor (0.5 g SZ, 378 K, 1 kPa n-butane in N2); the feed was introduced through a capillary. Conversion was monitored by on-line GC. The reaction was stopped after various TOS, the cell was evacuated at 378 K, and placed in a SETARAM MS 70 calorimeter [1]. Adsorption of n- or isobutane was performed at 313 K. Results and Discussion: The isotherms at TOS = 0 could not be fit with a 1st order Langmuir model, indicating a more complicated, maybe activated adsorption process. The differential heats for n- and isobutane adsorption on the unreacted catalyst were similar. The adsorption isotherms for n- and isobutane indicate a decrease of the number of sites for these molecules during the induction period and with further increasing TOS. Throughout the catalytic reaction, the shape of the isotherms changed and the apparent reaction orders decreased approaching 1. At the state of maximum activity, SZ adsorbed similar amounts of n-butane and isobutane (ca. 20 µmol/g at 6 hPa), and the majority of these sites (coverages > 2 micromol/g) produced qdiff ca 40-50 kJ/mol for both adsorptives. At coverages < 2 micromol/g, qdiff for n-butane was as high as 85 kJ/mol, while for isobutane it never exceeded 50 kJ/mol. Quasi-in-situ adsorption microcalorimetry with butanes as probe molecules revealed that only a small number of sites on SZ changes with the performance in n-butane isomerization. 1. L.C. Jozefowicz, H.G. Karge, E.N. Coker, J. Phys. Chem. 98 (1994) 8053

Two mechanisms drive pronuclear migration in mouse zygotes

A new life begins with the unification of the maternal and paternal chromosomes upon fertilization. The parental chromosomes first become enclosed in two separate pronuclei near the surface of the fertilized egg. The mechanisms that then move the pronuclei inwards for their unification are only poorly understood in mammals. Here, we report two mechanisms that act in concert to unite the parental genomes in fertilized mouse eggs. The male pronucleus assembles within the fertilization cone and is rapidly moved inwards by the flattening cone. Rab11a recruits the actin nucleation factors Spire and Formin-2 into the fertilization cone, where they locally nucleate actin and further accelerate the pronucleus inwards. In parallel, a dynamic network of microtubules assembles that slowly moves the male and female pronuclei towards the cell centre in a dynein-dependent manner. Both mechanisms are partially redundant and act in concert to unite the parental pronuclei in the zygote’s centre

Evolution of the electronic structure of Cs₂H₂PVMo₁₁O₄₀ under the influence of propene and propene/O₂

Evolution of the Electronic Structure of Cs2H2PVMo11O40 under the Influence of Propene and Propene/O2 J. Kröhnert, F.C. Jentoft, J. Melsheimer, R. Ahmad, M. Thiede, G. Mestl, and R. Schlögl Fritz-Haber-Institut der Max-Planck-Gesellschaft, 14195 Berlin, Faradayweg 4-6, Germany Changes in the electronic and vibrational spectra of Cs2H2PVMo11O40 in the presence of propene (1) or propene/O2 (2) were followed by in situ UV/Vis/NIR diffuse reflectance spec-troscopy. (1) At 298 K propene leads to reduction as indicated by a broad absorption band extending from the Vis to the NIR range. Iso-propanol was detected at 323 K and the maxi-mum of the broad band shifted from 740 to 700 nm. At higher temperatures the visible ab-sorption band shifted back about 25 nm. (2) Under conditions of catalytic oxidation a propene conversion of ca. 4% was detected with acrolein and CO as major products (670 K). Although the absorption band in the Vis range is less pronounced than in the presence of propene only at the same temperature, the catalyst is not restored to its fully oxidized state. The evolution of a band at 680-700 nm at 620-670 K indicates the formation of a structure with reduced and oxidized metal sites next to each other. This maybe related to the observation of molydenyl and vanadyl species in post mortem Raman spectra. 1. Introduction Cs salts of the vanadomolybdophosphoric acid are, for example, applied as catalysts for oxidative dehydrogenation of isobutyric acid to methacrylic acid [1-3]. The sensitivity of the catalyst under industrial operation suggests that the nature of the active phase may not be identical to the structurally well-defined salts which are molecular solids composed of Keggin ions, Cs cations, and water. Interestingly, the light-off temperature for oxidation reactions coincides with the temperature for the loss of constitutional water [4]. It is thus hypothesized that the water loss is connected to the formation of the active phase, whereby the electronic state of the active phase evolves in an atmosphere that contains both oxidative (O2) and re-ductive (hydrocarbon) components at the same time. In situ UV/Vis/NIR diffuse reflectance spectroscopy offers the unique possibility to si-multaneously investigate electronic features such as d-d transitions, intervalence charge trans-fers (IVCT), and ligand-to-metal charge transfers (LMCT) as well as the vibrational overtones and combination modes of water. From preliminary UV/Vis/NIR experiments, as from other methods (e.g., TG-DTA experiments), it has become clear that catalysts of the type CsxH4-xPVMo11O40 with x = 0-2 are already thermally unstable in the presence of an inert gas. This instability is expressed by the appearance and disappearance of absorption bands. The goal of this work was to investigate the loss of crystal and subsequently constitutio-nal water, and possible concomitant electronic changes of Cs2H2PVMo11O40 under inert, oxi-dative, and reductive conditions over a wide temperature range, as well as under the conditi-ons of oxidation catalysis. Propene was selected as a reactant and the gas phase was monito-red in order to correlate catalytic performance with spectroscopic data. 2. Experimental A Perkin-Elmer Lambda 9 spectrometer with an enlarged integrating sphere was used for in situ UV/Vis/NIR diffuse reflectance spectroscopy on different dilute catalyst samples. So-lutions of Cs2CO3 and heteropoly acid were used for the preparation of the Cs2H2PVMo11O40 samples. Approximately 110 mg of the catalyst (7-10 wt%) were mixed with SiO2 (Heraeus, 0.1-0.4 mm) and placed in a microreactor of in-house design operating under continuous gas flow. Sequential spectroscopic measurements were carried out with a scan speed of 240 nm/min, a slit width of 5.0 nm, and a response time of 0.5 s. Spectralon® was used as a refe-rence. The apparent absorption was evaluated from the diffuse reflectance data using the for-mula 1-Rmixture/RSiO2. The feed mixture was 10 vol-% propene in helium or 10 vol-% propene plus 10 vol-% oxygen in helium with a total gas flow of 71 or 74 ml/min, respectively. The gases were analyzed with two gas chromatographs (Perkin Elmer), equipped with heated au-tomatic gas sampling valves, an FFAP column (Macherey-Nagel) and a packed Carboxen-1000 column using FID and TCD in both GCs. Series A experiments (10% propene): The temperature was held constant for 2 h at room temperature (RT), and then the temperature was increased at a rate of 1 K/min to 323 K, and spectra were recorded over a period of ca. 5 hours. Series B experiments (10% propene): The temperature was increased from RT to 323 K and then to 670 K in steps of ~ 50 K (5 K/min heating rate), with a 2 h isothermal period after each step. Series C experiments (10% propene, 10% O2): The temperature was increased as in Series B with extended isothermal periods of 9 h at 413 K and 19 hours at 670 K. 3. Results The Series A spectra show a strong increase in apparent absorption already at RT. After 40 min on stream (RT3 in Fig. 1) a visible absorption band formed at ~ 740 nm and this band underwent a blue shift to 700 nm when the temperature was increased to 319 K. In contrast to similar experiments using He, the crystal water bands at 1430 and 1925 nm already disappear after 70 min on stream (Figure 1). Formation of iso-propanol was detected at 319 K. Series B spectra showed similarly strong changes in apparent absorption with a red shift of ca. 25 nm for the visible absorption band and the appearance of an additional band in the NIR (at ~ 1050 nm). The NIR band (appearing above 560K) is broad and overlaps with the visible band (Fi-gure 2). The visible band increases with increasing temperature until a single broad visi-ble/NIR band forms. For Series C, increasing temperature leads to a decrease in the intensity of the absorption bands, particularly the NIR band (Figure 3). However, the visible band be-comes clearly recognizable again at 563 K; it is possible that a catalytic reaction begins to occur at this temperature. The products acrolein, propionic acid, acrylic acid and water were first detected at 603 K. At 670 K in addition to these products we also detected propionalde-hyde, acetone, CO and acetic acid, with the conversion of propene being ca. 4% and that of O2 ca. 12 %, and the highest selectivities being for acrolein and CO. In the Series C spectra the defined feature in the UV region does not disappear as it did in the Series B spectra at higher temperatures. Under catalytic reaction conditions above 563 K one observes an increase in the intensity of the shifted visible absorption band at 680-700 nm with increasing temperature (=620 K) and time on stream (Figure 4). 4. Discussion The water bands disappear much more readily in the presence of propene than in inert gas, and at the same time, isopropanol is formed. These observations can be explained by an addition of water from the catalyst to propene, a typical acid-catalyzed reaction. Propene thus appears to draw the crystal water from the catalyst, and when the crystal water is gone the constitutional water is removed as well. The sample apparently underwent considerable re-duction even at the relatively low temperature of propene hydration, which corresponds to the observations in inert gas at higher temperature, and reduction generally seems to accompany the water loss. Hence, water, which is added in the industrial oxidation process, may play an essential role in maintaining a certain, i.e. active, state of the catalyst which is different from a van-der-Waals solid built of isolated Keggin units. The electronic structure change in the pre-sence of propene is dramatic; the defined LMCT band is obscured by an intense, almost con-tinuous absorption which is even more pronounced at higher temperatures (up to 670 K). The catalyst sample was black after treatment with the propene atmosphere, in contrast to He-treated catalyst samples that were blue [5]. In the presence of propene and oxygen, the initial reduction at 555 K is partly reversed at 620-670K; however, although excess oxygen is available the catalyst remains in a reduced state. The decrease in the intensity of the visible absorption band below the catalytic reaction temperature (603K) may be attributed to an oxidation of some Mo5+ and V4+ centers by the gas phase oxygen. Above this temperature the absorption band increases with rising tempera-ture through the stronger reduction of the catalyst and at the same time the conversion also increases. The blue shifted absorption band at ca. 680 nm that was observed at 670K could indicate oxygen vacancies that are important for the oxidation reactions. These species may be the same as a species observed in post mortem Raman analysis of these samples that was charac-terized by a shoulder at about 1002 cm-1 and was interpreted as molybdenyl species [6]. Un-der the same conditions, the free acid H4PVMo11O40 showed a blue shift up to 660 nm [5], which might indicate the presence of molybdenyl and vanadyl species in the catalyst sample, since Raman bands were in turn detected at 1008 and 1030 cm-1 [6]. In summary, the changes in electronic structure appear too dramatic to be just a conse-quence of a partial reduction of the Keggin ion; rather it seems that the geometric structure is partially dissolved leading to a transformation from a molecular solid to more condensed oxi-dic species with semiconducting character. The availability of relatively free electrons that is suggested by the continuous character of the UV/Vis spectra at high temperatures is a prere-quisite for the activation of molecular oxygen and thus for the redox catalytic activity. The structural changes are too severe to allow the restoration of the heteropolyacid through the water that is formed in the propene oxidation; and acidic properties also no longer play a role for the product distribution under these conditions. References 1. M. Misono, N. Nojiri, Appl. Catal., 64 (1990) 1. 2. Th. Ilkenhans, B. Herzog, Th. Braun and R. Schlögl, J. Catal., 153 (1995) 275. 3. L. Weismantel, J. Stöckel and G. Emig, Appl. Catal., 137 (1996) 129. 4. S. Berndt, Dissertation, TU Berlin, 1999. 5. J. Kröhnert, O. Timpe, J. Melsheimer, F.C. Jentoft, G. Mestl and R. Schlögl, to be pub-lished. 6. G. Mestl, T. Ilkenhans, D. Spielbauer, M. Dieterle, O. Timpe, J. Kröhnert, F.C. Jentoft, H. Knözinger and R. Schlögl, Appl. Catal. A, submitted