Organic-inorganic layered hydroxides for applications in catalysis and adsorption

La presente Tesis Doctoral abarca la síntesis, caracterización y aplicaciones de HDLs. Como resultado de esta investigación se han obtenido diferentes materiales laminares utilizando diferentes métodos de síntesis con aplicaciones en adsorción y catálisis. En el trabajo “Vibrational spectroscopic study of sol-gel layered double hydroxides containing different tri-and tetravalent cations” se sintetizaron cinco materiales tipo HDL a partir de etóxido de magnesio en presencia de acetilacetonato de aluminio, galio, indio, estaño y circonio utilizando la técnica solgel. Las suspensiones coloidales obtenidas inicialmente fueron gelificadas y finalmente separadas por centrifugación. La caracterización mediante XRD confirmó la estructura tipo hidróxido doble laminar de los materiales y los espectros IR y Raman nos proporcionaron información sobre las diferencias entre los sólidos. En el trabajo “Use of Raman spectroscopy to assess the efficiency of MgAl mixed oxides in removing cyanide from aqueous solutions” se estudió la capacidad de actuación de los HDLs como remediadores ambientales, concretamente en la purificación de agua conteniendo cianuro. El proceso se basa en el “efecto memoria” de los HDL. La cinética del proceso se siguió por espectroscopia Raman. Se demostró que la relación metálica del HDL tiene una influencia crucial en la capacidad de adsorción del óxido mixto resultante tras la calcinación. La caracterización mediante XRD confirmó la estructura laminar de los HDLs y la estructura de tipo periclasa de los óxidos mixtos obtenidos tras calcinación. En este trabajo se utilizó por primera vez la espectroscopia Raman para monitorizar el proceso de adsorción, obtenido como resultado un proceso eficaz y expeditivo para el propósito previsto, permitiendo la monitorización in situ del proceso de adsorción En el trabajo “Microwave-assisted synthesis of hybrid organo-layered double hydroxides containing cholate and deoxycholate” se sintetizaron materiales hidróxidos dobles laminares (HDLs) orgánico-inorgánicos mediante intercalación de los aniones colato y desoxicolato. Se empleó un método de síntesis asistido por microondas. La caracterización mediante difracción de rayos X y espectroscopia Raman muestra que la intercalación de los aniones orgánicos se produce después de un tratamiento de 1h bajo microondas, lo que supone acortar los tiempos de síntesis frente a otros métodos más convencionales de obtener este tipo de sólidos. En ambos órgano-HDLs el espaciado basal nos indica que los aniones orgánicos se sitúan en la región interlaminar sin entrecruzamiento. Esta distancia interlaminar ha podido ser confirmada con las micrografías de HR-TEM. Asimismo, se ha comprobado que la temperatura de descomposición del anión orgánico aumenta sensiblemente al ser intercalado en el HDL. Esta observación se ha realizado por medidas termogravimétricas y se ha confirmado por espectroscopia Raman, a través de una monitorización de la señal del anión orgánico durante el período de calefacción. En el trabajo “Microwave-assisted synthesis of basic mixed from hydrotalcites” se sintetizaron tres hidróxidos dobles laminares (HDLs) empleando diferentes métodos de irradiación con microondas: un procedimiento de coprecipitación, este mismo procedimiento, pero en presencia de un agente director de estructura como es el Pluronic P123 y un procedimiento de precipitación homogénea con urea. Estos tres HDLs, tras ser calcinadas a 450 °C formaban de fases de óxidos mixtos de composición MgAlOx, cuyas propiedades texturales y químico-texturales dependían directamente del método de síntesis empleado. El óxido mixto obtenido a partir del HDL sintetizado por el método de coprecipitación homogénea es el que presentaba los valores más elevados de superficie específica y de basicidad. Asimismo, presentaba una elevada microporosidad. Los tres óxidos mixtos han sido utilizados como catalizadores en la reacción de Meerwein-Ponndorf-Verley de benzaldehído y 2-butanol, pudiéndose certificar que la actividad catalítica es directamente proporcional a la población de centros básicos superficiales.This Doctoral Thesis covers the synthesis, characterization and applications of LDHs. As a result of this research, different layered materials using different synthesis methods with applications in adsorption and catalysis have been obtained. In the paper “Vibrational spectroscopic study of sol-gel layered double hydroxides containing different tri-and tetravalent cations”,ffive different layered double hydroxides were synthesized from magnesium ethoxide in the presence of aluminium, gallium, indium, tin and zirconium acetylacetonates by using the sol–gel technique. The colloid suspensions initially obtained were gelled and separated by centrifugation. XRD diffraction patterns confirmed that the five solids thus obtained possessed a layered double hydroxide structure. Also, IR and Raman spectra revealed differences between the solids. In the paper “Use of Raman spectroscopy to assess the efficiency of MgAl mixed oxides in removing cyanide from aqueous solutions” the ability of LDHs as environmental remediators was studied, specifically in the purification of water containing cyanide. The process is based on the “memory effect” of LDHs. The kinetics of the process was examined by Raman spectroscopy. The metal ratio of the LDH was found to have a crucial influence on the adsorption capacity of the resulting mixed oxide. The characterization by XRD confirmed the layered structure of the LDHs and the periclase-like structure of the mixed oxides obtained by calcination. In this work, Raman spectroscopy was for the first time used to monitor the adsorption process. Based on the results, this technique is an effective, expeditious choice for the intended purpose and affords in situ monitoring of the adsorption process. In the paper “Microwave-assisted synthesis of hybrid organo-layered double hydroxides containing cholate and deoxycholate” Organic–inorganic layered double hydroxides (LDHs) were synthesized by intercalation of the cholate and deoxycholate ion. A microwave-assisted synthesis method was used. The characterization by X-ray diffraction and Raman spectroscopy for the resulting LDHs showed that a treatment time of only 1 h sufficed to ensure complete intercalation of the organic anions. This makes the proposed synthetic method more expeditious than existing alternatives for the same purpose. Based on the baseline spacing for the organo-LDHs, the organic anions were intercalated with no cross-over between their molecular chains. The interlayer distance of the solids was confirmed by high-resolution transmission electron micrographs (HR-TEM). As revealed by thermogravimetric monitoring measurements, and confirmed by Raman spectra, the decomposition temperature for the LDHs increased considerably upon intercalation of the organic anion. In the paper “Microwave-assisted synthesis of basic mixed from hydrotalcites” three layered double hydroxides (LDHs) were prepared by using three different microwave irradiation methods, namely: coprecipitation in the absence and presence of Pluronic P123 as template, and homogeneous precipitation in the presence of urea. Calcination at 450 ºC of the three hydrotalcites gave MgAlOx mixed oxides. Their textural and surface chemical properties were found to depend on the particular synthetic method used. The mixed oxide obtained from the hydrotalcite prepared by homogeneous coprecipitation exhibited the highest specific surface area and basicity, in addition to a high microporosity. The three mixed oxides were used as catalysts in the Meerwein–Ponndorf–Verley reaction of benzaldehyde with 2-butanol, where the activity was found to be directly proportional to the population of basic surface sites

Efficient Removal of Nonylphenol Isomers from Water by Use of Organo-Hydrotalcites

The presence of potent organic endocrine-disrupting chemicals (EDCs) in natural aquifers can have adverse impacts on public health and the environment. 4-nonylphenol, one such EDC, can be efficiently removed from water by adsorption onto a clayey material. In this work, we created an effective sorbent for this purpose by using co-precipitation and subsequent ion-exchange to intercalate the organic anion deoxycholate into a Mg/Al hydrotalcite. Intercalating deoxycholate ions increased the organophilicity of the hydrotalcite surface. The solid was used to adsorb 4-nonylphenol at different pollutant concentrations and temperatures. The adsorption process was subjected to a kinetic study. Based on the results, the EDC was adsorbed by chemisorption. In addition, based on the equilibrium isotherms used for the process, the Freundlich model was the most accurate in reproducing the adsorption of 4-nonylphenol onto deoxycholate-intercalated hydrotalcite

Three-Dimensional Hierarchical Hydrotalcite–Silica Sphere Composites as Catalysts for Baeyer–Villiger Oxidation Reactions Using Hydrogen Peroxide

The development of effective, environmentally friendly catalysts for the Baeyer–Villiger reaction is becoming increasingly important in applied catalysis. In this work, we synthesized a 3D composite consisting of silica spheres coated with Mg/Al hydrotalcite with much better textural properties than its 2D counterparts. In fact, the 3D solid outperformed a 2D-layered hydrotalcite as catalyst in the Baeyer–Villiger reaction of cyclic ketones with H2O2/benzonitrile as oxidant. The 3D catalyst provided excellent conversion and selectivity; it was also readily filtered off the reaction mixture. The proposed reaction mechanism, which involves adsorption of the reactants on the hydrotalcite surface, is consistent with the catalytic activity results

Cobaloxime tethered pyridine-functionalized ethylene-bridged periodic mesoporous organosilica as efficient HER catalyst

An efficient cobaloxime hydrogen production catalyst has been synthesized through coordination of a cobalt complex (Co(dmgH2)(dmgH)Cl2) on an ethylene-bridged periodic mesoporous organosilica (PMO) containing pyridine moieties. The effective assembly of cobaloxime units through cobalt-pyridine axial bond on the porous channels of the PMO was clearly evidenced by different techniques, including 13C NMR, Raman, IR and XPS. The catalyst was investigated for the hydrogen evolution reaction in a visible-light activated system in the presence of a photosensitizer (eosin Y) and a sacrificial electron donor (TEOA). It showed a good photocatalytic performance on the HER with a TON of 119 at 6 h, largely exceeding the catalytic activity of the homogeneous counterpart, Co(dmgH)2pyCl, under the conditions studied. The process was proven to be photocatalytic and heterogeneous. The studied system has the cobaloxime catalyst with the highest turnover reported to date for a heterogeneous catalyst under photocatalytic conditions. This catalytic system maintained its activity after two recycling experiments, previous activation of the catalyst

An evaluation of peri-implant marginal bone loss according to implant type, surgical technique and prosthetic rehabilitation: a retrospective multicentre and cross-sectional cohort study

To evaluate implant loss (IL) and marginal bone loss (MBL); follow-up period of up to 10 years after prosthetic loading. Retrospective multi-centre cross-sectional cohort study. Double analysis: (1) all the implants (n = 456) were analysed; (2) to allow for possible cluster error, one implant per patient (n = 143) was selected randomly. Statistical analysis: Spearman's correlation coefficient; Kruskal-Wallis (post-hoc U-Mann-Whitney); Chi-square (post-hoc Haberman). (1) Analysing all the implants (456): IL was observed in patients with past periodontitis (6 vs. 2.2%, p < 0.05), short implants (12 vs. 2.8%, p < 0.001) and when using regenerative surgery (11.3 vs. 2.9%, p < 0.001); greater MBL was observed among smokers (0.39 ± 0.52 vs. 0.2 ± 0.29, p < 0.01), maxillary implants (0.28 ± 0.37 vs. 0.1 ± 0.17, p < 0.0001), anterior region implants (0.32 ± 0.36 vs. 0.21 ± 0.33, p < 0.001), external connection implants (0.2 ± 0.29 vs. 0.63 ± 0.59, p < 0.0001), and 2-3 years after loading (p < 0.0001). (2) analysing the cluster (143): IL was observed in smokers (18.8 vs. 3.5%, p < 0.05), splinted fixed crowns (12.9%, p < 0.01), short implants (22.2 vs. 4.0%, p < 0.01) and when using regenerative surgery (19.2 vs. 3.4%, p < 0.01); greater MBL was observed in maxillary implants (0.25 ± 0.35 vs. 0.11 ± 0.18, p < 0.05), in the anterior region (p < 0.05), in the first 3 years (p < 0.01), in external connection implants (0.72 ± 0.71 vs. 0.19 ± 0.26, p < 0.01) and in short implants (0.38 ± 0.31 vs. 0.2 ± 0.32, p < 0.05). There is greater risk in smokers, patients with past periodontal disease, external connection implants, the use of short implants and when regenerative techniques are used. To prevent MBL and IL, implantologists should be very meticulous in indicating implants in patients affected by these host factors

Bone loss around narrow implants versus standard diameter implants : retrospective 2-years case-control study

The objectives were to evaluate the bone loss (BL) around narrow diameter implants (3.3 mm) 2 years after implant loading and compare with the bone loss around conventional-diameter implants (4.1 mm), as well as with clinical and anatomical variables. 2-years follow-up. Cases: 20 patients either gender-age, narrow implants (Straumann TM-SLA, diameter 3.3 mm); Control: 20 patients matching for gender-age, conventional implants (Straumann TM-SLA, diameter 4.1). Total 82 implants (31 narrow implants and 51 conventional implants) in 40 patients. To avoid statistical bias, a cluster of one implant per patient was randomly selected (20 narrow implants and 20 conventional implants). To evaluate changes resulting from bone loss around the implants, a total of 80 panoramic radiographs were taken of all 40 patients; the first panoramic image was taken at the time of implant loading and the second one 2 years later. Clinical and demographic variables were obtained from the patients? medical records. Statistical method: Spearman?s correlation coefficient, chi-squared (Haberman?s post hoc), Mann-Whitney U and Kruskal-Wallis tests. Statistical significance p< 0.05. No significant differences in bone loss around were found around narrow implants versus conventional implants. Differences linked to tobacco use were found after studying one implant per patient (p< 0.05). With the limitations of the present study, no significant differences in BL were found when comparing narrow implants with conventional implants after 2 years of implant loading. There were also no differences found when accounting for other demographic and clinical variables, with the exception of tobacco use

Zn-Cr layered double hydroxides for photocatalytic transformation of CO2 under visible light irradiation: the effect of the metal ratio and interlayer anion

Carbon dioxide is the main gas responsible for the greenhouse effect. Over the last few years, the research focus of many studies has been to transform CO2 into valuable products (CO, HCOOH, HCHO, CH3OH and CH4), since it would contribute to mitigating global warming and environmental pollution. Layered double hydroxides (LDHs) are two-dimensional materials with high CO2 adsorption capacity and compositional flexibility with potential catalytic properties to be applied in CO2 reduction processes. Herein, Zn-Cr LDH-based materials with different metal ratio and interlayer anions, i.e., chloride (Cl−), graphene quantum dots (GQDs), sodium dodecyl sulfate (SDS) and sodium deoxycholate (SDC), have been prepared by a co-precipitation method and characterized by different techniques. The influence of the interlayer inorganic and organic anions and the metal ratio on the application of Zn-Cr LDHs as catalysts for the photocatalytic CO2 reduction reaction under visible light irradiation is unprecedentedly reported. The catalytic tests have been carried out with Ru(bpy)32+ as photosensitizer (PS) and triethanolamine as sacrificial electron donor (ED) at λ = 450 nm. All LDHs materials exhibited good photocatalytic activity towards CO. Among them, LDH3-SDC showed the best catalytic performance, achieving 10,977 µmol CO g−1 at 24 h under visible light irradiation with a CO selectivity of 88%. This study provides pertinent findings about the modified physicochemical features of Zn-Cr LDHs, such as particle size, surface area and the nature of the interlayer anion, and how they influence the catalytic activity in CO2 photoreduction