Theoretical model for mass transport and adsorption of gases in porous solids based on the frequency response method

Detailed knowledge of mass transport and adsorption is one of the key factors in the development of novel high-performance porous materials for a wide range of technical applications. In the course of an optimization process, a quick conclusion on the properties of the pore system and its accessibility for certain sample molecules is crucial. On the other hand, predictions about the pore system can save steps in material development

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

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

Interaction between Sulfated Zirconia and Alkanes: Prerequisites for Active Sites – Formation and Stability of Reaction Intermediates

Two sulfated zirconia catalysts were prepared via sulfation and calcination at 873 K of zirconium hydroxide aged at room temperature for 1 h (SZ-1) or aged at 373 K for 24 h (SZ-2). SZ-1 was active for n-butane isomerisation at 373 K; SZ-2 reached similar performance only at 473 K. Both materials contained about 9 wt% sulfate and were tetragonal. Due to a BET lower surface area (105 m2/g vs. 148 m2/g) SZ-1 featured a higher sulfate density, and XRD and EXAFS analysis showed larger (ca. 10 nm) and more well ordered crystals than for SZ-2. n-Butane TPD on SZ-1 showed a butene desorption peak at low temperature, whereas, no obvious butene desorption was observed with SZ-2, suggesting that SZ-1 has a higher oxidizing power at low temperature than SZ-2. The number of sites capable of dehydrogenation are less than 5 µmol/g, because the differential heats of n-butane adsorption as measured by microcalorimetry were 45–60 kJ/mol for higher coverages, indicating weak and reversible interaction. TAP experiments describe the adsorption and desorption behavior of n-butane at different activity states and are the basis for a simple adsorption model. Reactant pulses and purge experiments show that the active species, presumably formed in an oxidative dehydrogenation step, are stable at the surface under reaction conditions

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

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

Activation and Isomerization of n-Butane on Sulfated Zirconia Model Systems - An Integrated Study Across the Materials and Pressure Gaps

Butane activation has been studied using three types of sulfated zirconia materials, single-crystalline epitaxial films, nanocrystalline films, and powders. A surface phase diagram of zirconia in interaction with SO3 and water was established by DFT calculations which was verified by LEED investigations on single-crystalline films and by IR spectroscopy on powders. At high sulfate surface densities a pyrosulfate species is the prevailing structure in the dehydrated state; if such species are absent, the materials are inactive. Theory and experiment show that the pyrosulfate can react with butane to give butene, H2O and SO2, hence butane can be activated via oxidative dehydrogenation. This reaction occurred on all investigated materials; however, isomerization could only be proven for powders. Transient and equilibrium adsorption measurements in a wide pressure and temperature range (isobars measured via UPS on nanocrystalline films, microcalorimetry and temporal analysis of products measurements on powders) show weak and reversible interaction of butane with a majority of sites but reactive interaction with < 5 µmol/g sites. Consistently, the catalysts could be poisoned by adding sodium to the surface in a ratio S/Na=35. Future research will have to clarify what distinguishes these few sites

Acid-Base Catalyzed Activation of n-Alkanes: Isomerization of n-Butane

Due to its unique activity for skeletal isomerization of short alkanes at low temperature, sulfated zirconia (SZ) is generally recognized as the most promising alternative for the zeolite based hydroisomerization catalysts. However, despite the large amount of investigations, several important topics related to SZ are still discussed controversially. Here we report on our detailed investigation of the mechanism of butane skeletal isomerization on SZ. Typically, SZ had an induction period followed by a period of virtually constant activity. The selectivity to isobutene was higher than 96%, the byproducts being propane and pentanes. The induction period can be related to the formation and accumulation of reactive intermediates on the catalyst surface. We show that the alkane activation is initiated via stoichiometric oxidative dehydrogenation of butane by sulphate species to butane, water and SO2. For the first time, direct experimental evidence is given for all reaction products formed by oxidative dehydrogenation. In situ IR spectroscopy and density functional calculations indicate that pyrosulfate or re-adsorbed SO3 species are the active species for the oxidation. Butene formed interacts with Bronsted acid sites and forms sec-butoxy groups which isomerize mono-molecularly to tert-butoxy groups, as deduced from the 100% selectivity to isobutane at zero conversion. The tert-butoxy group undergoes hydride transfer from n-butane, forming a new sec-butoxy group and isobutane. The lower selectivity to isobutane with increasing conversion is explained by the higher isobutene concentration which triggers a bimolecular pathway. Note that isobutane is kinetically a primary product, while propane and pentanes are secondary products formed in sequential reactions. The larger amount of propane with respect to pentanes for conversion above 40% is attributed to multiple alkylation reactions followed by cracking. Transient experiments showed conclusively that the isomerization of the carbenium ion is the rate-determining step in the chain sequence and that hydride transfer is in quasi equilibrium. A kinetic model for butane isomerization under differential conditions is presented showing that the overall rate of butane conversion is proportional to the rate constant of the monomolecular isomerization of the carbenium ion, the concentration of Bronsted acid sites, the partial pressure of the alkane and the concentration of the labile sulfate-based redox sites. We show here that the key to successful catalysts for skeletal isomerization does not lie in high acid strength, but that a subtle balance between redox and acid sites is necessary

A Cross-Sectional Study of HPV Vaccine Acceptability in Gaborone, Botswana

Background Cervical cancer is the most common cancer among women in Botswana and elsewhere in Sub-Saharan Africa. We sought to examine whether HPV vaccine is acceptable among parents in Botswana, which recently licensed the vaccine to prevent cervical cancer. Methods and Findings We conducted a cross-sectional survey in 2009, around the time the vaccine was first licensed, with adults recruited in general medicine and HIV clinics in Gaborone, the capital of Botswana. Although only 9% (32/376) of respondents had heard of HPV vaccine prior to the survey, 88% (329/376) said they definitely will have their adolescent daughters receive HPV vaccine. Most respondents would get the vaccine for their daughters at a public or community clinic (42%) or a gynecology or obstetrician\u27s office (39%), and 74% would get it for a daughter if it were available at her school. Respondents were more likely to say that they definitely will get HPV vaccine for their daughters if they had less education (OR = 0.20, 95% CI = 0.07–0.58) or lived more than 30 kilometers from the capital, Gaborone (OR = 2.29, 95% CI = 1.06–4.93). Other correlates of acceptability were expecting to be involved in the decision to get HPV vaccine, thinking the vaccine would be hard to obtain, and perceiving greater severity of HPV-related diseases. Conclusions HPV vaccination of adolescent girls would be highly acceptable if the vaccine became widely available to the daughters of healthcare seeking parents in Gaborone, Botswana. Potential HPV vaccination campaigns should provide more information about HPV and the vaccine as well as work to minimize barriers