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Synthesis of ionic compounds

Synthesis of ionic compounds. Che5700 陶瓷粉末處理. Majority liquid phase method involves ionic compounds, calculation of supersaturation requires ionic products Ksp ; Another characteristics: charge neutrality

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Synthesis of ionic compounds

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  1. Synthesis of ionic compounds Che5700 陶瓷粉末處理 • Majority liquid phase method involves ionic compounds, calculation of supersaturation requires ionic products Ksp; • Another characteristics: charge neutrality • Ions in solution: often hydrated!, dehydration process is always involved; ions with slow dehydration – often form hydrated compounds. • In general: hydration number, for cations: 4-6; for anions: 0-2; • Interaction between ions and water molecule: electrostatic forces, thus electronic structure or charge density – main factor to determine this force • pH always an important factor: affects ion-pairing; complex ion formation etc.;

  2. Characteristics of ionic solutions Che5700 陶瓷粉末處理 • Solvent molecule: may adsorb on particle surface, thus shows effect, different solvents (or small additions, co-solvent) will affect nucleation and growth behavior; • Other factors: e.g. ultrasonic, electrical field, magnetic field, etc. may also show effects • Ion pairs formation  change distribution of species  CA = [A] + [AB] + 2 [A2B] + [AB2] + …. • driving force in ionic solution: (I.P) 1/2 – Ksp 1/2 (IP = ionic product; Ksp solubility product)

  3. Right figure: 5,6,7 region: suspended solution, 5,6 with charged, stable suspension

  4. Different pH: often obtain different ionic compounds, in theory, it is the one with the lowest solubility (thermodynamic reasons) Yet different ionic compounds involves complex chemistry, difficult to predict (kinetic reasons)

  5. Nielsen found: solubility of ionic compound related to its surface energy: /kT = 2.82 – 0.272 ln(Cs) Cs: mol/m3

  6. Metal Ions in Solution Che5700 陶瓷粉末處理 • Ionic radius Na+ < Ag+ < K+; number of salts containing water of crystallization  19 for Na+; 14 for K+; 4 for Ag+ • Metal ions: in addition to ion pair formation, may also form polynuclear complex (sort of precursor to nucleus) • All metal ions in solution are hydrated; when z/d increase, easier to release H+, or first hydrolysis constant • Bronsted acid: provide proton (proton donor) • Lewis acid: to provide empty electron orbital to accept electrons

  7. Classification of metal ions Che5700 陶瓷粉末處理 ======================

  8. Another classification Che5700 陶瓷粉末處理 Metal ion can associate with OH in water, to release H+, act like an acid

  9. Stability Sequence Che5700 陶瓷粉末處理 To the left: A type metal cations B type

  10. Stepwise Hydrolysis Reaction for Al 6 Al  8 Al ==

  11. Distribution of Fe Species Calculated based on stability constant

  12. From Fe(OH)3 to Fe2O3 • source: 李崧助MS thesis (NTHU, 2008) • Two transformation routes from hydroxide to oxide under boiling condition: (a) dissolution – re-crystallization; (b) in-situ solid state transformation • Case 1: adding Fe+2 as catalyst to accelerate dissolution and re-crystallization process; • Case 2: adding urea or NaHCO3 to accelerate solid state transformation process (possible via action of CO2 bubbles – need to be proven)

  13. Fe+2 concentration dropped at first, then gradually increase back to original concentration; • Fe+3 concentration increase with time, while pH of solution decrease with time • FeOOH formed immediately when Fe+2 was added;

  14. Some Possible Reactions • Fe(OH)3 + Fe+2 + H2O  FeOOH + Fe(OH)2 + H+ • Fe(OH)3 + H+ + e- Fe+2 + H2O (reductive dissolution) • e- may come from: • Fe+2  Fe+3 + e- or • Fe(OH)2 + OH-  Fe(OH)3 + e- • hydrolysis of Fe+3 and subsequent transformation into oxide • Fe+3 + 2H2O  Fe(OH)2+ + 2 H+ • 2 Fe(OH)2+  Fe2O3 + H2O + H+

  15. Initial pH = 9, t = 4 hours, still pure Fe3O4 phase; (final pH = 7.3) Initial pH = 7; t= 20, 50 & 90 minutes; (final pH = 5.4 (20 min; 3.1 (90 min))

  16. Possible Mechanism for Solid State Transformation • Dirk Walter (Thermochimica Acta, 445 (2006) 195) 含水氧化鐵 O2- H2O H+ and O2- combine together OH- continue to move toward interface Urea and sodium bicarbonate decompose to produce CO2, possibly exhibiting similar effects

  17. First Hydrolysis Constant * In figure d=M-O bond distance; four types of metal ions

  18. Solubility Product & Hydrolysis Constant • K = (*K1)z Kso • As shown in previous figure, according to its charge, four types can be found; • Often when the first water molecule left, the precipitation occurs; Overall reaction

  19. Solubility and supersaturation Che5700 陶瓷粉末處理 Due to changes in species, at different pH, solubility and corresponding supersaturation also changes

  20. Hydroxo, Oxo complexes from metal ions

  21. Start with simple hydration lost one H+, to become hydroxo  lost another H+ to become oxo complex

  22. Effects of anions Che5700 陶瓷粉末處理 • Mostly: chloride, nitrate, carbonate, sulfate, perchlorate, phosphate, etc; [counter-ions] • They will affect particle size, shape, composition, etc. • Anions entering particle: may not be easy to eliminate by calcination, may require higher temperature to lower to ppm level; • Different anions – different morphology, e.g. • Fe(OH)+2 + 2 FeSO4+ + 6 H2O  hexagonal Fe3(OH)5(SO4)2 2H2O + 4 H+ • 3 Fe(OH)+2 + Fe(SO4)+ + 4 H2O  monoclinic Fe4(SO4)(OH)10 + 4 H+

  23. Mixing Che5700 陶瓷粉末處理 • Cation and anions from two different sources, always require mixing, though very fast, still take several seconds, its effect may not be negligible. • Method of addition: direct strike, reverse strike (add alkaline to metal ion solution or reverse), it will affect reaction rate and thus rate of supersaturation. • Sequence of events: mixing  reaction  supersaturation  nucleation/growth/agglomeration  aging  final particle size distribution, shape, crystallinity;

  24. Co-precipitation Che5700 陶瓷粉末處理 • Classification:  Isomorphous mixed crystal formation: structure similar or formation of compounds • Anomalous mixed crystal formation: one of which should not precipitate, yet was removed from solution • Adsorption • Occlusion (inclusion): trapped during growth • Post-precipitation: after precipitation of the first compound, either independent or in coating form • distribution coefficient • (B/A) in whole crystal = D (B/A)sol • (B/A) on surface =  (B/A)sol • D: homogeneous distribution coefficient; • : logarithmic distribution coefficient (more often)

  25. Chemical Reactions • Examples • NiSO4 + 4 FeSO4 + 5 (NH4)2C2O4. H2O  (at 60oC)  5 Ni 0.2 Fe 0.8 C2O4. 2H2O + 10 NH4+ + 5 SO4-2 • Ba(OC3H7)2 + Ti(OC5H11)4 in alcohol solution + H2O  BaTiO3 + 2 C3H7OH + 4 C5H11OH

  26. Monodispersed Colloids Che5700 陶瓷粉末處理 • example: micelles, latex, micro-emulsion, photographic emulsion, ceramics, inorganic chemicals (pigment, pharmacy, catalyst, etc); stress micron or sub-micron size; now: nanosize • Applications: model system for fundamental studies; may have better packing and sintering results; • Formation: (a) separate nucleation and growth steps: best strong dependency of nucleation on supersaturation and low growth rate; (b) resist agglomeration (based on electrostatic force, surfactant, polymers, gel network etc.); (c) choice of growth modes: diffusion or surface reaction controlled

  27. Typical Monodispersed Systems • Homogeneous systems: • Redox reaction • Hydrolysis: such as alkoxide in alcohol; high temperature forced hydrolysis in acid; etc. • Decomposition of compounds: Cd ion + TAA  CdS • Reaction of chelates: M-EDTA complex • Heterogeneous systems: • Emulsion polymerization • Reaction in microemulsions • Reaction in aerosols • Recrystallization: involves dissolution

  28. Homogeneous system: depend on control of reaction rate, in a clean system, rapid nucleation to consume supersaturation, so particle grow to the same size in a short period

  29. Core may have time to become crystallized; material in diffusion – may be nanosized colloids

  30. ZnS colloids: use TAA, TAA = thioacetamide (S=C-(CH3)(NH2); it will slowly hydrolyze to release S-2, to do the precipitation reaction, slower than use of Na2S, with more uniform particle size但粒徑均勻.

  31. Reaction or Diffusion Control Che5700 陶瓷粉末處理 • Diffusion control or surface reaction control of growth rate; • Results show: diffusion control – size distribution become narrower with size; similar for reaction control (different scale)

  32. Results from semi-batch Stober process SiO2 (from 陳貞志)

  33. Hydrothermal Method Che5700 陶瓷粉末處理 • Higher reaction temperature (higher pressure), need to use autoclave, thus higher reaction rate. • To some strong acid solid, we can obtain oxide directly, or at least oxo-hydroxides (e.g. Zr, Zn…)

  34. Use hetereoaggregation to prepare core-shell particles example: latex particle + polyallylamine hydrochloride to make it positive charge, easy to adsorb negative charged CdTe colloids (from adv. Mater. 13(22), 1685, 2001

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