Showing papers by "Polytechnic University of Valencia published in 1997"

From Microporous to Mesoporous Molecular-Sieve Materials and Their Use in Catalysis

Abstract: It is possible to say that zeolites are the most widely used catalysts in industry They are crystalline microporous materials which have become extremely successful as catalysts for oil refining, petrochemistry, and organic synthesis in the production of fine and speciality chemicals, particularly when dealing with molecules having kinetic diameters below 10 A The reason for their success in catalysis is related to the following specific features of these materials:1 (1) They have very high surface area and adsorption capacity (2) The adsorption properties of the zeolites can be controlled, and they can be varied from hydrophobic to hydrophilic type materials (3) Active sites, such as acid sites for instance, can be generated in the framework and their strength and concentration can be tailored for a particular application (4) The sizes of their channels and cavities are in the range typical for many molecules of interest (5-12 A), and the strong electric fields2 existing in those micropores together with an electronic confinement of the guest molecules3 are responsible for a preactivation of the reactants (5) Their intricate channel structure allows the zeolites to present different types of shape selectivity, ie, product, reactant, and transition state, which can be used to direct a given catalytic reaction toward the desired product avoiding undesired side reactions (6) All of these properties of zeolites, which are of paramount importance in catalysis and make them attractive choices for the types of processes listed above, are ultimately dependent on the thermal and hydrothermal stability of these materials In the case of zeolites, they can be activated to produce very stable materials not just resistant to heat and steam but also to chemical attacks Avelino Corma Canos was born in Moncofar, Spain, in 1951 He studied chemistry at the Universidad de Valencia (1967−1973) and received his PhD at the Universidad Complutense de Madrid in 1976 He became director of the Instituto de Tecnologia Quimica (UPV-CSIC) at the Universidad Politecnica de Valencia in 1990 His current research field is zeolites as catalysts, covering aspects of synthesis, characterization and reactivity in acid−base and redox catalysis A Corma has written about 250 articles on these subjects in international journals, three books, and a number of reviews and book chapters He is a member of the Editorial Board of Zeolites, Catalysis Review Science and Engineering, Catalysis Letters, Applied Catalysis, Journal of Molecular Catalysis, Research Trends, CaTTech, and Journal of the Chemical Society, Chemical Communications A Corma is coauthor of 20 patents, five of them being for commercial applications He has been awarded with the Dupont Award on new materials (1995), and the Spanish National Award “Leonardo Torres Quevedo” on Technology Research (1996) 2373 Chem Rev 1997, 97, 2373−2419

Expression of the yeast trehalose-6-phosphate synthase gene in transgenic tobacco plants: pleiotropic phenotypes include drought tolerance.

Abstract: The yeast trehalose-6-phosphate synthase gene (TPS1) was engineered under the control of the cauliflower mosaic virus regulatory sequences (CaMV35S) for expression in plants. Using Agrobacterium-mediated transfer, the gene was incorporated into the genomic DNA and constitutively expressed in Nicotiana tabacum L. plants. Trehalose was determined in the transformants, by anion-exchange chromatography coupled to pulsed amperometric detection. The non-reducing disaccharide accumulated up to 0.17 mg per g fresh weight in leaf extracts of transgenic plants. Trehaloseaccumulating plants exhibited multiple phenotypic alterations, including stunted growth, lancet-shaped leaves, reduced sucrose content and improved drought tolerance. These pleiotropic effects, and the fact that water loss from detached leaves was not significantly affected by trehalose accumulation, suggest that synthesis of this sugar, rather than leading to an osmoprotectant effect, had altered sugar metabolism and regulatory pathways affecting plant development and stress tolerance.

Chrysanthemum chlorotic mottle viroid: Unusual structural properties of a subgroup of self-cleaving viroids with hammerhead ribozymes

Abstract: The causal agent of chrysanthemum chlorotic mottle (CChM) disease has been identified, cloned, and sequenced. It is a viroid RNA (CChMVd) of 398–399 nucleotides. In vitro transcripts with the complete CChMVd sequence were infectious and induced the typical symptoms of the CChM disease. CChMVd can form hammerhead structures in both polarity strands. Plus and minus monomeric CChMVd RNAs self-cleaved during in vitro transcription and after purification as predicted by these structures, which are stable and most probably act as single hammerhead structures as in peach latent mosaic viroid (PLMVd), but not in avocado sunblotch viroid (ASBVd). Moreover, the plus CChMVd hammerhead structure also appears to be active in vivo, because the 5′ terminus of the linear plus CChMVd RNA isolated from infected tissue is that predicted by the corresponding hammerhead ribozyme. Both hammerhead structures of CChMVd display some peculiarities: the plus self-cleaving domain has an unpaired A after the conserved A9 residue, and the minus one has an unusually long helix II. The most stable secondary structure predicted for CChMVd is a branched conformation that does not fulfill the rod-like or quasi-rod-like model proposed for the in vitro structure of most viroids with the exception of PLMVd, whose proposed secondary structure of lowest free energy also is branched. The unusual conformation of CChMVd and PLMVd is supported by their insolubility in 2 M LiCl, in contrast to ASBVd and a series of representative non-self-cleaving viroids that are soluble under the same high salt conditions. These results support the classification of self-cleaving viroids into two subgroups, one formed by ASBVd and the other one by PLMVd and CChMVd.

Synthesis of nanocrystalline zeolite beta in the absence of alkali metal cations

Abstract: Contrarily to previous reports, pure aluminosilicate zeolite Beta can be synthesized in the complete absence of alkali cations without the use of seeds. Thus, the presence of alkali cations is not necessary for the nucleation and crystal growth to occur, although they greatly affect the crystallization kinetics. By this synthesis method stable suspensions of colloidal zeolite Beta particles in the nanoscale size (100 to 10 nm) were prepared. It is also possible to synthesize 100% crystalline zeolite Beta with up to 7.6 framework Al atoms per unit cell (Si/Al=7.4).

Solid acid catalysts

Abstract: Using qualitative and quantitative models, including both short and long range effects, progress has been made in understanding acidity in zeolites, but limitations that still exist in measuring that acidity by use of probe molecules. The benefits and limitations of enlarging zeolite pores by means of MCM-41 structures have been outlined. Major advances have been made in improving the catalytic activity of heteropolyacids and ZrO2/So42−, as well as tackling the mechanism through which ZrO2/SO42− catalyzes hydrocarbon reactions. Finally, new advances have also been made in the preparation and potential as acid catalysts of organic—inorganic composites (Zr phosphates and phosphonates, polysiloxane, nafion—silica).

Adsorption of Heavy Metals from Aqueous Solutions onto Activated Carbon in Single Cu and Ni Systems and in Binary Cu–Ni, Cu–Cd and Cu–Zn Systems

Abstract: Single copper and nickel adsorption from aqueous solutions onto a granular activated carbon is reported. Metal removals increase on raising pH and temperature, and decrease on raising the initial metal concentration at constant carbon dose. The adsorption processes are modelled using the surface complex formation (SCF) Triple Layer Model (TLM) with an overall surface bidentate species. A dependence of the SCF constant on pH, initial molar metal/carbon ratio and temperature is observed, and a correlation for log Kads is determined. The SCF model successfully predicts copper and nickel removals in single metal solutions. Adsorption in the binary metal systems copper–nickel, copper–cadmium and copper–zinc is also reported, showing competitive adsorption effects. © 1997 SCI.

Preparation and catalytic properties of new mesoporous materials

Abstract: The development of ordered mesoporous solids is revised. Acid, base, and redox catalytic properties are presented together with their application for a series of organic reactions of fundamental and practical interest. Finally it is explained that it is possible to make use of the high surface and regular porosity of MCM–41 to prepare supported metals, and bifunctional catalysts. The above properties together with the presence of a large number of surface silanol groups allow the grafting of catalytically active transition metal complexes.

Spin Crossover in Novel Dihydrobis(1-pyrazolyl)borate [H(2)B(pz)(2)]-Containing Iron(II) Complexes. Synthesis, X-ray Structure, and Magnetic Properties of [FeL{H(2)B(pz)(2)}(2)] (L = 1,10-Phenanthroline and 2,2'-Bipyridine).

Abstract: The first example of spin crossover iron(II) complexes based on dihydrobis(1-pyrazolyl)borate are presented here. The complexes {Fe[H(2)B(Pz)(2)](2)(phen)} (phen = 1,10-phenanthroline), 1, and {Fe[H(2)B(Pz)(2)](2)(bipy)} (bipy = 2,2'-bipyridine), 2, have been synthesized and their structures determined by X-ray diffraction methods. Crystals 1 and 2 are monoclinic, space group C2/c, Z = 4 with a = 17.448(4) A, b = 16.101(4) A, c = 10.611(2) A, and beta = 112.47(2) degrees for 1 and a = 16.307(2) A, b = 15.075(4) A, c = 11.024(4) A, and beta = 114.95(5) degrees for 2 at 293 K. The crystal structure of 2 was also determined at 139 K in order to detect the structural changes associated with the S = 0 S = 2 spin conversion. 2 retains the same space group upon spin conversion with a = 16.086(6) A, b = 14.855(6) A, c = 10.812(2) A, and beta = 114.18(3) degrees. The structures of 1 and 2 are made up of mononuclear neutral species where the positive charge of iron(II) is neutralized through the coordination of two chelate bidentate dihydrobis(pyrazolyl)borate anions, and phen or bipy neutral ligands are used to fill the iron(II) coordination sphere. The molecular structures for both compounds are very similar, with Fe-N bond lengths in the 2.212-2.158 A range for the high-spin phase. The structural modifications associated with the spin change in 2 mainly consist of a large reorganization of the metal environment: the Fe-N decreases by 0.15 A (mean value) when the temperature is lowered from 290 to 139 K and a more regular shape of the [FeN(6)] octahedron is achieved through a slight modification of the trigonal deformation angle from 5.3 degrees to 3.2 degrees along with remarkable variations of the N-Fe-N angles. The thermodynamic model of Slichter and Drickamer was applied to account for the magnetic data. The intermolecular interaction parameter and the enthapy and entropy changes associated with the spin transition were estimated as Gamma = 3.3 kJ mol(-)(1), DeltaH = 13.4 kJ mol(-)(1), and DeltaS = 81.9 J mol(-)(1) K(-)(1) for 1 and Gamma = 1.7 kJ mol(-)(1), DeltaH = 13.4 kJ mol(-)(1), and DeltaS = 83.9 J mol(-)(1) K(-)(1) for 2, respectively.

Continuously variable true time-delay optical feeder for phased-array antenna employing chirped fiber grating

Abstract: In this paper, we present and demonstrate a novel approach of a true time-delay (TTD) optical feeder for phased-array antennas (PAAs). A continuously variable TTD is achieved by employing tunable lasers and a wide-bandwidth chirped fiber Bragg grating (FBG) as the dispersive element. The results show that a very high resolution performance (equivalent to a 6-b microwave phase shifter) is obtained for an L-band PAA employing narrow-tuning bandwidth lasers with a wavelength stability of 0.005 nm and a 4-nm bandwidth chirped grating with a dispersion of 835 ps/nm.

Dielectric relaxation spectroscopy of polyethylene terephthalate (PET) films

Abstract: The dielectric properties of biaxially stretched polyethylene terephthalate (PET) films of thickness and 68% degree of crystallinity were investigated by means of dielectric relaxation spectroscopy in the frequency range Hz and the temperature range C. Differencial scanning calorimetry (DSC), in the range C, was employed to investigate the thermal properties of the PET samples. Besides measuring the relaxation associated with the glass transition and the secondary relaxation, special attention has been paid to the investigation of DC-conductivity-related effects. They give rise to high dielectric permittivity values and dielectric losses at low frequencies and high temperatures. The results are analysed within the complex permittivity formalism and discussed in terms of interfacial Maxwell - Wagner - Sillars polarization, the peak, conductivity relaxation, space-charge polarization, electrode polarization and DC conductivity. DC conductivity values determined from frequency plots of the AC conductivity follow the Vogel - Tamman - Fulcher equation at temperatures higher than the glass transition temperature, indicating that the charge-carrier transport mechanism is governed by the motion of the polymeric chains. On the basis of the temperature dependence of the DC conductivity PET is classified as a fragile system.

Isolation and transcript analysis of gibberellin 20-oxidase genes in pea and bean in relation to fruit development

Abstract: PCR was used with degenerate primers based on conserved amino acid sequences in gibberellin (GA) 20-oxidases to isolate cDNA clones for these enzymes from young seeds of pea (Pisum sativum) and developing embryos of French bean (Phaseolus vulgaris). One GA 20-oxidase cDNA (Ps27-12) was obtained from pea and three (Pv 15-11, Pv73-1 and Pv85-26) from bean. Their identities were confirmed by demonstrating that fusion proteins expressed in Escherichia coli exhibited GA 20-oxidase activity, converting [14C]GA12 to [14C]GA9. The intermediates in this three-step reaction, GA15 and GA24, were also identified as products. The expression proteins from three of the clones (Ps27-12, Pv15-11 and Pv73-1) were also shown to convert GA53 to GA20, as effectively as they did GA12. On the basis of transcript levels measured by northern blot analysis, the pea GA 20-oxidase gene is most highly expressed in young leaves, fully expanded internodes, very young seeds (until 4 days after anthesis) and expanding pods (from 3 days after anthesis at least until day 6). Expression in pods from 3-day-old unpollinated ovaries is higher than in those from pollinated ovaries. Treatment of unpollinated ovaries with GA3 to induce parthenocarpic fruit-set severely reduced the amount of GA 20-oxidase mRNA, whereas treatment with 2,4-D, although inducing fruit-set, did not reduce the levels of these transcripts. Plant decapitation above an unpollinated ovary resulted in very high levels of GA 20-oxidase mRNA in the pod. The three GA 20-oxidase genes from French bean showed very different patterns of expression: Pv 15-1 was expressed in the roots, young leaves, and developing seeds, but most highly in immature cotyledons, while Pv73-1 has a similar expression pattern to Ps27-12, with transcripts found only in young seeds and young leaves, where it was particularly abundant. Transcripts corresponding to Pv85-26 were detected in developing seeds, and just traces in the young leaves. Southern blot analysis indicated that the bean GA 20-oxidases are each encoded by single-copy genes, whereas one more gene, homologous to Ps27-12, could also exist in pea.

Fibre bragg gratings tuned and chirped using magnetic fields

Abstract: The authors report on the use of magnetic fields in conjunction with magnetostrictive materials for tuning and chirping optical fibre Bragg gratings. The Bragg wavelength shifts as consequence of the strain induced in the fibre by a magnetostrictive rod when a magnetic field is applied. A tuning range of 1.1 nm has been achieved by a magnetic field of 103 mT and the grating has been chirped by applying non-uniform magnetic fields.

Mechanisms of salt tolerance conferred by overexpression of the HAL1 gene in Saccharomyces cerevisiae.

Abstract: Overexpression of the HAL1 gene improves the tolerance of Saccharomyces cerevisiae to NaCl by increasing intracellular K+ and decreasing intracellular Na+. The effect of HAL1 on intracellular Na+ was mediated by the PMR2/ENA1 gene, corresponding to a major Na+ efflux system. The expression level of ENA1 was dependent on the gene dosage of HAL1 and overexpression of HAL1 suppressed the salt sensitivity of null mutants in calcineurin and Hal3p, other known regulators of ENA1 expression. The effect of HAL1 on intracellular K+ was independent of the TRK1 and TOK1 genes, corresponding to a major K+ uptake system and to a K+ efflux system activated by depolarization, respectively. Overexpression of HAL1 reduces K+ loss from the cells upon salt stress, a phenomenon mediated by an unidentified K+ efflux system. Overexpression of HAL1 did not increase NaCl tolerance in galactose medium. NaCl poses two types of stress, osmotic and ionic, counteracted by glycerol synthesis and sodium extrusion, respectively. As compared to glucose, with galactose as carbon source glycerol synthesis is reduced and the expression of ENA1 is increased. As a consequence, osmotic adjustment through glycerolsynthesis, a process not affected by HAL1, is the limiting factor for growth on galactose under NaCl stressed.

Structure of ITQ-4, a New Pure Silica Polymorph Containing Large Pores and a Large Void Volume

Abstract: The new pure silica polymorph ITQ-4, structure code IFR, Si32O64, monoclinic I2/m a = 18.65243(13), b = 13.49597(8), c = 7.63109(6) A, β = 101.9781(5)°, is a zeolitic material containing a large sinusoidal channel with 12-member ring windows as minimum apertures. Its void volume (0.21 cm3/g) is surpassed only by that of pure silica Beta among the known pure silica polymorphs. The success of the ab initio structure solution from synchrotron powder diffraction data was greatly assisted by the absence of Al and Si−OH defect groups, which generates a very high degree of crystallographic order.

Activation of Molecules in Confined Spaces: An Approach to Zeolite−Guest Supramolecular Systems

Abstract: Ab initio calculations showing frontier molecular orbital energy modifications of a guest molecule when located inside a microporous zeolitic cavity are presented. Micropores of zeolites Beta, ZSM-12, and ZSM-5 have been modeled using all silica clusters from which it is found that the highest occupied molecular orbital (HOMO) energy of toluene increases when going from the gas phase state to restricted microporous environments. The influence of the zeolite cavity size and its chemical composition on the toluene HOMO energy are discussed. Regarding the size of the confining environment, it is found that the smaller the zeolite cavity the higher is the rise in toluene HOMO energy. The effect of the chemical composition on the toluene HOMO energy was tested in the ZSM-5 zeolite by varying the Al content. The results obtained showed that the frontier orbital energy increases upon decreasing the Al content of the cluster. As a consequence of the confinement effect, toluene reactivity in zeolite catalyzed reac...