Multifunctional metal phosphonate frameworks for proton conductivity and photocatalysis

A lo largo de este trabajo de investigación han sido estudiadas las propiedades de conducción protónica, fotocatálisis y fotoluminiscencia de fosfonatos metálicos multifuncionales, preparados por incorporación de iones metálicos activos y/o funciones orgánicas específicas en diferentes sistemas estructurales. En primer lugar, se ha sintetizado una familia de derivados metálicos del ácido R,S 2-hidroxifosfonoacético (HPAA) con elementos alcalinos (Li+, Na+, K+ y Cs+) mediante cristalización a temperatura ambiente. Se ha investigado la relación directa existente entre el radio iónico y el incremento de la dimensionalidad, siendo 1D para (Li3(OOCCH(OH)PO3)(H2O)•H2O, Li-HPAA), láminas pilareadas para Na2(OOCCH(OH)PO3H)(H2O)4, Na-HPAA) y 3D para los sólidos K2(OOCCH(OH)PO3H)(H2O)2, K-HPAA y Cs(HOOCCH(OH)PO3H, Cs-HPAA. El Na-HPAA muestra la más alta conductividad protónica, σ=5.6 × 10-3 S•cm-1 a 24 °C y 98% de humedad relativa (HR). Adicionalmente para el Cs-HPAA se observó un inusual enriquecimiento del enantiómero (S-HPAA) del ligando a partir de una lenta cristalización a temperatura ambiente. Por otra parte, se ha sintetizado el FeII[HO3PCH(OH)COO]•2H2O, (FeHPAA), y se ha ensayado como catalizador foto-Fenton para la mineralización de fenol bajo radiación UVA obteniéndose, en las óptimas condiciones de reacción, un grado de mineralización del 90% a 80 minutos. También han sido estudiadas las propiedades de conductividad del sólido (FeHPAA), tanto para el compuesto de síntesis como tras su exposición a vapores de NH3 a diferentes tiempos de exposición. Pasando de σ=1.1 x 10-5 S•cm-1, para el sólido reciente sintetizado a σ=2.5 x 10-3 S•cm-1, para el sólido con el mayor contenido de NH3/H2O (24 h de exposición) a 80 ºC y 95% de HR. Los valores de las energías de activación (Ea) oscilan entre 0.31 a 0.36 eV. Se ha sintetizado el derivado cálcico del ácido 5-(dihidroxifosforil)isoftálico. Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H)(H2O)2]•5H2O, (Ca-PiPhtA-I), que presenta un entramado tridimensional abierto y con canales 1D ocupados por moléculas de agua las cuales interaccionan por enlaces de hidrógeno. La deshidratación parcial del Ca-PiPhtA-I a 75 ºC conduce a la formación de una nueva fase, Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H)(H2O)2], (Ca-PiPhtA-II). Cuando el compuesto Ca-PiPhtA-I se expone a vapores de amoniaco concentrado se obtiene un nuevo derivado, Ca-PiPhtA-NH3, siendo este derivado el que presenta un valor de conductividad más alto, σ= 6.6 × 10-3 S•cm-1a 24 ºC y 98% de humedad relativa, con una Ea=0,4 eV. Por otra parte, se ha obtenido una familia de derivados del ácido amino-tris-(metilenfosfónico) (H6AMP) por cristalización a temperatura ambiente. Estos materiales, con formula general Ln{[(HN(CH2PO3H)3(H2O)4]Cl}, (Ln-AMP) (Ln= La3+, Pr3+, Sm3+, Gd3+, Tb3+, Dy3+ y Ho3+) son isoestructurales y se caracterizan por presentan un entramado bidimensional, con láminas cargadas positivamente, y compensadas por la presencia de aniones Cl- y agua de cristalización entre las mismas. Estos sólidos han sido investigados como materiales fotoluminiscentes y conductores protónicos, obteniéndose un valor de conductividad de σ= 3.96 × 10-4 S•cm-1 a 80 ºC y 95% HR para el Gd-AMP, con una Ea= 0,20 eV. Finalmente, se han obtenido algunos resultados preliminares para el ácido sulfofosfónico (H2O3PCH2)2-N-(CH2)2SO3H, (DVb382) con cationes di- y trivalentes. Estudios realizados aplicando la metodología de síntesis en paralelo, denominada “High-Throughput” o mediante síntesis asistida por microonda

PXRD and PDF analysis of multifunctional lanthanide nitrilotris-methylphosphonate-based proton conductors

Metal phosphonates are multifunctional solids which possess tunable properties, such as H-bond networks, while exhibiting high chemical and thermal stability. Depending on the protonation of the ligand, different crystalline phases can be obtained. Here, we report three different families of proton conductors based on lanthanide nitrilotrismethylphosphonates. Compounds having cationic layers compensate by chloride or sulfate anions were isolated: [Ln(H4NMP)(H2O)2]Cl·2H2O and Ln(H5NMP)]·SO4·4H2O [H6NMP = nitrilotris(methylphosphonic acid)]. The crystal structure of Gd-(H5NMP)]·SO4·4H2O was solved ab initio from synchrotron powder diffraction data (λ=0.4124 Å, beamline BL04-MSPD ALBA) and refined by the Rietveld method. Chloride containing phases show two irreversible solid state transformations take place: (1) a crystalline-to-crystalline phase transition, {Ln-H4NMP → [Ln2(H3NMP)2(H2O)4]·4.5H2O for Ln= La, Pr}, and (2) crystalline-to-amorphous phase transition, {LnH4NMP → [Ln(H3NMP)]·1.5H2O for Ln= Gd - Ho}, both implies the loss of HCl and structural rearrangements of the frameworks. Variations in average and local structure have been monitored by high resolution powder diffraction and PDF analysis, upon exposure the samples at high relative humidity and temperature (95% RH and 80 ºC), in order to understand their behavior as proton conductors.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Proton conductivity of lanthanide nitrilotris-methylphosphonates

Multifunctional metal phosphonates are acidic coordination polymers (CPs) with remarkable stability and proton conducting properties owing to their structure is usually composed of extended hydrogen-bond networks that favor proton transfer pathways [1]. In this communication, three different families of proton conductors based on lanthanide nitrilotris-methylphosphonates are examined. Compounds were isolated by crystallization at room temperature at pH <0.8 in the presence of. When chloride is presented in solution two families of compounds were isolated, depending on the concentration of chloride in solution: free-chloride 1D solids with formula Ln2(H3NMP)2(H2O)4]·4.5H2O [Ln= La3+] [2] or layered chloride-containing Ln(H4NMP)(H2O)2]Cl·2H2O [Ln= La3+ - Ho3+] materials [3]. In absence of chloride, a third series of compounds was obtained. This structural versatility leads to a wide range of proton conductivity varying between 3 × 10−4 S·cm−1 and 2 × 10−3 S·cm−1 as measured at 80 °C and 95% RH.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Structural study of the local order in ammonia-modulated FE(II) hydroxyphosphonoacetate proton conductors

Layered Fe(II) carboxiphosphonate, Fe-HPAA·2H2O, is a crystalline multifunctional coordination polymer exhibiting properties as photocatalyst and proton conductor. Postsynthesis modification by ammonia/water adsorption strongly enhances its proton conductivity. However, this process entails a progressive amorphization but in no case intercalation of the guest species was detected. Understanding the mechanism involved in this increased conductivity is crucial to develop novel high performance proton conductors for PEMFCs. Thus, total scattering and PDF study has been carried out to explore the mechanism of ammonia adsorption and subsequent amorphization. Different lenght scales have been investigated to characterize the average and local structure at variable ammonia loaded in order to ascertain posible structural modifications after gas/solid reactions. While significant short range order (from 1.4 to 10 Å) variations were observed even for low loadings, the average structure seems to be basically preserved except for the highest ammonia/water contents.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Properties and Applications of Metal Phosphates and Pyrophosphates as Proton Conductors

We review the progress in metal phosphate structural chemistry focused on proton conductivity properties and applications. Attention is paid to structure–property relationships, which ultimately determine the potential use of metal phosphates and derivatives in devices relying on proton conduction. The origin of their conducting properties, including both intrinsic and extrinsic conductivity, is rationalized in terms of distinctive structural features and the presence of specific proton carriers or the factors involved in the formation of extended hydrogen-bond networks. To make the exposition of this large class of proton conductor materials more comprehensive, we group/combine metal phosphates by their metal oxidation state, starting with metal (IV) phosphates and pyrophosphates, considering historical rationales and taking into account the accumulated body of knowledge of these compounds. We highlight the main characteristics of super protonic CsH2PO4, its applicability, as well as the affordance of its composite derivatives. We finish by discussing relevant structure–conducting property correlations for divalent and trivalent metal phosphates. Overall, emphasis is placed on materials exhibiting outstanding properties for applications as electrolyte components or single electrolytes in Polymer Electrolyte Membrane Fuel Cells and Intermediate Temperature Fuel Cells.This research was funded by PID2019110249RB-I00 (MICIU/AEI, Spain) and PY20-00416 (Junta de Andalucia, Spain/FEDER) research projects. M.B.-G. thanks PAIDI2020 research grant (DOC_00272 Junta de Andalucia, Spain) and R.M.P.C. thanks University of Malaga under Plan Propio de Investigación for financial support

Structure-properties correlations in divalent metal phosphonates

Crystalline metal phosphonates may offer acidic sites, structural flexibility and guest molecules (H2O, heterocyclics, etc.) which can act as proton carriers. In addition, some frameworks are also amenable for post‐synthesis modifications in order to enhance desired properties [1,2]. In this work, we present the synthesis and structural characterization of two hydroxyphosphonoacetates hybrids based on magnesium, [Mg5(O3PCHOHCOO)2(HO3PCHOHCOO)2·8H2O] [Mg5(HPAA)2(H1HPAA)2·8H2O], and zinc, [Zn6K(O3PCHOHCOO)4(OH)·6.5H2O] [Zn6K(HPAA)4(OH)·6.5H2O]. Both solids present three-dimensional frameworks and their crystal structures were solved ab initio from X-ray powder diffraction. The proton conductivity of [Zn6K(HPAA)4(OH)·6.5H2O] as well as ammonia derivatives of M(II)(HO3PCHOHCOO)·2H2O [M(II)=Zn, Mg] will be reported and discussed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. FQM-1656; MAT2013-41836-

Synthesis and proton conduction properties of lanthanide amino-sulfophosphonates

Crystalline acid-functionalized metal phosphonates are potential candidates as proton conducting electrolytes. Their frameworks can be chemically modified to contain proton carriers such as acidic groups (P-OH; -SO3H, -COOH,…) and guest molecules (H2O, NH3,…) that generates hydrogen bond networks stable in a wide range of temperature [1,2]. In this work, focus is laid on properties derived from the combination of lanthanide ions with the amino-sulfophosphonate ligand (H2O3PCH2)2-N-(CH2)2-SO3H. Hightrough-put screening was followed to reach the optimal synthesis conditions under solvothermal conditions at 140 ºC. Isolated polycrystalline solids, Ln[(O3PCH2)2-NH-(CH2)2-SO3H].2H2O (Ln= La, Pr and Sm), crystallize in the monoclinic (La) and orthorhombic (Pr and Sm) systems with unit cell volume of ~2548 Å3. Preliminary proton conductivity measurements for Sm derivative have been carried out between 25º and 80 ºC at relative humidity (RH) values of 70 % and 95 %. The sample exhibits enhanced conductivity at high RH and T (Figure 1) and constant activation energies of 0.4 eV, typical of a Grothuss mechanism of proton.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. FQM-1656; MAT2013-41836-

Synthesis and proton conduction properties of lanthanide amino-sulfophosphonates

Acidic groups-containing metal phosphonates exhibit a wide range of proton conductivity depending on the water content and functionality. Moreover, this property can be enhanced by appropriate post-synthesis chemical and/or thermal treatments [1,2]. In this work, focus is laid on properties derived from the combination of lanthanide ions with the amino-sulfophosphonate ligand (H2O3PCH2)2-N-(CH2)2-SO3H. Highthrough-put screening was used to reach the optimal synthesis conditions under hydrothermal conditions at 140 ºC. Isolated polycrystalline solids, Ln[(O3PCH2)2-NH-(CH2)2-SO3H]·2H2O (Ln= La, Pr, Sm, Eu, Gd, Tb and Er), crystallize in the monoclinic (La and Er) and orthorhombic (Pr, Sm, Eu, Gd and Tb) systems with unit cell volume of ~1200 and 2548 Å3 respectively. Their crystal structures, solved ab initio from X-ray powder diffraction data, correspond to different layered frameworks depending on the lanthanide cation size. Thus, compounds with orthorhombic symmetry show free acidic sulfonic pointing to the interlayer space, while La- and Er- derivatives display layered structures where both phosphonate and sulfonated groups are coordinated to the metal, leaving free P-OH groups. As consequence of this structural variability, different H-bond networks and proton transfer pathways are generated. Preliminary proton conductivity measurements have been carried out between 25 and 80 ºC at 70-95 % relative humidity. The sample exhibits conductivities near to 3.10-3 S.cm-1 and activation energies characteristics of a Grotthuss-type mechanism of proton transfer.Proyectos de investigación del ministerio MICINN, Españam(MAT2016-77648-R), Proyectos de la Junta de Andalucía (P12-FQM-1656), Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Ammonia effects on proton conductivity properties of coordination polymers

Crystalline metal phosphonates are referred to as a type of structurally versatile coordination polymers [1]. Many of them contain guest molecules (H2O, heterocyclics, etc.), acidic sites and, furthermore, their structure can be also amenable for post‐synthesis modifications in order to enhance desired properties [2]. In the present work, we examine the relationships between crystal structure and proton conductivity for several metal phosphonates derive from multifunctional ligands, such as 5-(dihydroxyphosphoryl)isophthalic acid (PiPhtA) [3] and 2-hydroxyphosphonoacetic acid (H3HPAA). Crystalline divalent metal derivatives show a great structural diversity, from 1D to 3D open-frameworks, possessing hydrogen-bonded water molecules and acid groups. These solids present a proton conductivity range between 7.2·10-6 and 1.3·10−3 S·cm-1. Upon exposure to ammonia vapor, from an aqueous solution, solid state transformations are observed accompanied of enhance proton conductivities. The stability of these solids under different environment conditions (temperature and relative humidities) as well as the influence of the ammonia adsorption on the proton conduction properties of the resulting solids will be discussed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Photocatalytic behavior of phosphonate-based hybrid materials on dyes and phenols degradation

Comunicación presentanda al congreso EUROMAT2013, del 8 al 13 de Septiembre, Sevilla.There is increasing interest in using heterogeneous catalysis for mineralization of organic pollutants. Within Advanced Oxidation Processes (AOPs), Photo-Fenton reaction is one of the most efficient methodologies. To date, most of heterogeneous iron catalysts studied was based on oxides or hydroxides. We extend here our previous studies on phenol photodegradation [1] by exploring the photocatalytic activity of various hybrid MII phosphonates (MII = Mn, Fe, Cu) for several organic pollutants. Synthesis conditions, pre-activation, H2O2 concentration, and surface characteristic have been studied/optimized. For dyes, decolouring and mineralization degrees up to 90% and 45%, respectively, were attained. Chemical analysis and X-ray photoelectron spectroscopy revealed the dynamic character of the photocatalyst surface upon reaction.MAT2010-15175; FQM-11