Mejora de sensores vítreos sol-gel para la conservación preventiva de materiales históricos frente a la acidez

[ES] Los sensores a base de recubrimientos vítreos sol-gel dopados con ácido 2[4-(dimetil-amino) fenilazo] benzoico son capaces de cambiar su absorción óptica cuando se someten a distintas concentraciones de iones H3O+ y OH-. La respuesta de los sensores en ensayos de campo se estudió en Cracovia (Polonia) variando el procedimiento normal de uso, con el fin de mejorar su respuesta. Se midieron tanto los parámetros ópticos de los sensores como las condiciones ambientales (temperatura, humedad, presión y concentraciones de SO2 y de NOx). La respuesta de los sensores se analizó en términos de los cambios de su absorción visible. Dichos cambios se deben a reacciones locales de neutralización que tienen lugar en la superficie de los sensores, debido al efecto conjunto de los contaminantes de carácter ácido y a la humedad ambiental. Se establecieron correlaciones entre la concentración del contaminante principal (SO2) y la respuesta de los sensores para elaborar una calibración directa entre la absorción óptica y el pH ambiental. Los sensores pueden detectar y evaluar la acidez ambiental, así como alertar sobre la concentración de contaminantes ácidos que pueden dañar a la mayoría de los materiales históricos.[EN] Sensors based on sol-gel glassy coatings doped with 2[4-(dimethyl-amino) phenylazo] benzoic acid are able to change their optical absorption when they are submitted to different concentration of H3O+ and OH-. The sensors behaviour in field tests was studied in Cracow (Poland), varying the normal procedure of operation to improve their response. Both the sensors optical parameters and the environmental conditions (temperature, humidity, pressure, SO2 and NOx concentrations) were measured. The sensors response was analysed in terms of their visible absorbance changes, which are due to local neutralisation reactions in the sensors surface by the join effect of acid pollutants and humidity. Correlations between the main acid pollutant (SO2) concentration and the sensors response are established to provide a relation between the optical absorption and the environmental pH. The sensors are able to detect and monitorise environmental acidity, as well as to alert on the pollutant concentration that may damage most of the historical materials.The authors wish to acknowledge bilateral Polish-Spanish project Ref. PAN-CSIC 2003PL0011, European Marie Curie project Ref. MERG-CT-2004-516436 and Spanish project Ref. CICYT-MAT-2003-03231 for financing support. N.C. acknowledges CSIC-ESF for an I3P postdoctoral contract.Peer reviewe

Challenges of and Insights into Acid-Catalyzed Transformations of Sugars

The selective transformation of hexose and pentose sugars to intermediate platform chemicals, such as furans, is an essential step in the conversion of cellulosic and hemicellulosic biomass to biofuels and biochemicals. Yet, many challenges in achieving commercially viable processes remain. In this feature article, we outline challenges that need to be overcome to enable these transformations. Then, we present the newly introduced acid-catalyzed isomerization of aldose sugars to ketose sugars via a class of solid Lewis acid catalysts (e.g., Sn-Beta, Ti-Beta). We elucidate mechanistic insights arising from subnanometer cooperativity and solvent effects that can be controlled to tune reaction pathways and selectivity and draw parallels between heterogeneous and homogeneous Lewis acid catalysts. Subsequently, we discuss fructose dehydration to 5-hydroxyl-methylfurfural (HMF) via homogeneous and heterogeneous Brønsted acid-catalyzed chemistry. We show how fundamental insights arising from the combination of kinetics, spectroscopy, and multiscale simulations rationalize the improved yield of HMF in water–organic cosolvents. The stability of heterogeneous Lewis acid catalysts under low pH enables tandem Brønsted and Lewis acid-catalyzed reactions in a single pot that overcomes equilibrium limitations and gives a high HMF yield starting from sugar raw materials. Additionally, we provide an overview of multicomponent adsorption of biomass derivatives from solution in microporous materials and discuss how structure–property relations can lead to superior micro- and micromesoporous carbon adsorbents for reactive adsorption toward high HMF yield. Finally, we provide an outlook for the field

Tris-(hydroxyamino)triazines: high-affinity chelating tridentate O,N,O-hydroxylamine ligand for the cis-(VO2+)-O-V cation

The treatment of the trichloro-1,3,5-triazine with N-methylhydroxylamine hydrochloride results in the replacement of the three chlorine atoms of the triazine ring with the function -N(OH)CH3 yielding the symmetrical tris-(hydroxyamino) triazine ligand H-3 trihyat. Reaction of the ligand H-3 trihyat with (NaVO3)-O-V in aqueous solution followed by addition of Ph4PCl gave the mononuclear vanadium(V) compound Ph4P[(VO2)-O-V(Htrihyat)] (1). The structure of compound 1 was determined by X-ray crystallography and indicates that this compound has a distorted square-pyramidal arrangement around vanadium. The ligand Htrihyat(2-) is bonded to vanadium atom in a tridentate fashion at the triazine ring nitrogen atom and the two deprotonated hydroxylamido oxygen atoms. The high electron density of the triazine ring nitrogen atoms, which results from the resonative contribution of electrons of exocyclic nitrogen atoms, leads to a very strong V-N bond. The cis-[(VO2)-O-V(Htrihyat)](-) species exhibits high hydrolytic stability in aqueous solution over a wide pH range, 2.5-11.5, as was evidenced by potentiometry.</p

Tris-(hydroxyamino)triazines: high-affinity chelating tridentate O,N,O-hydroxylamine ligand for the cis-(VO2+)-O-V cation

The treatment of the trichloro-1,3,5-triazine with N-methylhydroxylamine hydrochloride results in the replacement of the three chlorine atoms of the triazine ring with the function -N(OH)CH3 yielding the symmetrical tris-(hydroxyamino) triazine ligand H-3 trihyat. Reaction of the ligand H-3 trihyat with (NaVO3)-O-V in aqueous solution followed by addition of Ph4PCl gave the mononuclear vanadium(V) compound Ph4P[(VO2)-O-V(Htrihyat)] (1). The structure of compound 1 was determined by X-ray crystallography and indicates that this compound has a distorted square-pyramidal arrangement around vanadium. The ligand Htrihyat(2-) is bonded to vanadium atom in a tridentate fashion at the triazine ring nitrogen atom and the two deprotonated hydroxylamido oxygen atoms. The high electron density of the triazine ring nitrogen atoms, which results from the resonative contribution of electrons of exocyclic nitrogen atoms, leads to a very strong V-N bond. The cis-[(VO2)-O-V(Htrihyat)](-) species exhibits high hydrolytic stability in aqueous solution over a wide pH range, 2.5-11.5, as was evidenced by potentiometry.</p

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H₂bihyat. Water-Assisted Activation of the Mo^VIO Bond and Reversible Dimerization of <i>cis</i>-[Mo^VIO₂(bihyat)] to [Mo^VI₂O₄(bihyat)₂(H₂O)₂]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat) with <i>cis</i>-[Mo^VIO₂(acac)₂] in tetrahydrofuran resulted in isolation of the mononuclear compound <i>cis</i>-[Mo^VIO₂(bihyat)] (<b>1</b>). The treatment of Na₂Mo^VIO₄·2H₂O with the ligand H₂bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>2</b>) and <i>trans</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>3</b>) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum­(VI) compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat^2– ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo₂^VIO₂(μ₂-O₂)}⁴⁺ moiety. The potentiometry revealed that the Mo^VIbihyat^2– species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3–5. A subtle change in the coordination environment of the five-coordinate compound <b>1</b> with ligation of a weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic than that in <b>1</b>, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (<b>1</b> → <b>2</b>/<b>3</b>). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum­(VI) species, vibrational spectra, and energetics of the metal–ligand interaction for the three molybdenum­(VI) compounds <b>1</b>–<b>3</b> have been studied by means of density functional theory calculations

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H₂bihyat. Water-Assisted Activation of the Mo^VIO Bond and Reversible Dimerization of <i>cis</i>-[Mo^VIO₂(bihyat)] to [Mo^VI₂O₄(bihyat)₂(H₂O)₂]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat) with <i>cis</i>-[Mo^VIO₂(acac)₂] in tetrahydrofuran resulted in isolation of the mononuclear compound <i>cis</i>-[Mo^VIO₂(bihyat)] (<b>1</b>). The treatment of Na₂Mo^VIO₄·2H₂O with the ligand H₂bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>2</b>) and <i>trans</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>3</b>) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum­(VI) compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat^2– ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo₂^VIO₂(μ₂-O₂)}⁴⁺ moiety. The potentiometry revealed that the Mo^VIbihyat^2– species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3–5. A subtle change in the coordination environment of the five-coordinate compound <b>1</b> with ligation of a weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic than that in <b>1</b>, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (<b>1</b> → <b>2</b>/<b>3</b>). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum­(VI) species, vibrational spectra, and energetics of the metal–ligand interaction for the three molybdenum­(VI) compounds <b>1</b>–<b>3</b> have been studied by means of density functional theory calculations

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H₂bihyat. Water-Assisted Activation of the Mo^VIO Bond and Reversible Dimerization of <i>cis</i>-[Mo^VIO₂(bihyat)] to [Mo^VI₂O₄(bihyat)₂(H₂O)₂]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat) with <i>cis</i>-[Mo^VIO₂(acac)₂] in tetrahydrofuran resulted in isolation of the mononuclear compound <i>cis</i>-[Mo^VIO₂(bihyat)] (<b>1</b>). The treatment of Na₂Mo^VIO₄·2H₂O with the ligand H₂bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>2</b>) and <i>trans</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>3</b>) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum­(VI) compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat^2– ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo₂^VIO₂(μ₂-O₂)}⁴⁺ moiety. The potentiometry revealed that the Mo^VIbihyat^2– species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3–5. A subtle change in the coordination environment of the five-coordinate compound <b>1</b> with ligation of a weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic than that in <b>1</b>, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (<b>1</b> → <b>2</b>/<b>3</b>). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum­(VI) species, vibrational spectra, and energetics of the metal–ligand interaction for the three molybdenum­(VI) compounds <b>1</b>–<b>3</b> have been studied by means of density functional theory calculations