METODOLOGÍA DE DISEÑO DE AASHTO 93 PAVIMENTOS RÍGIDOS

Models of adsorption consider an equilibrium situation where adsorbate molecules freely access the adsorption surface (see Section 2.3.1). However, the structure of microporous solids can restrict access to sites of adsorption, so it is necessary to consider the diffusion process. Diffusion occurs either by activated jumps between neighbouring adsorption sites or movements across pores.[10] The rate is dependent upon the strength of the interaction between adsorbate and surface, the distance between sites to complete the jump and the amount of site and pore blocking due to defects or other adsorbates.

Two modes of diffusion occur simultaneously within micropores: self-diffusion and transport diffusion. Self-diffusion is diffusion in the absence of a chemical- potential gradient[177], i.e. at equilibrium, whereas transport diffusion is a non- equilibrium process in which a chemical-potential causes diffusion.

As measurement of diffusion with nanoporous materials is experimentally challenging, little is currently known about diffusion within MOFs and almost

Chapter 2. Synthesis and Characterisation Techniques

all the information available has come from molecular simulations.[165] Diffusion through micropores has been modelled by combined molecular dynamics (MD) and GCMC methods. Xu et al.[178] used transport diffusion MD studies in micropores to show how loading affects diffusivity within a pore. D¨uren et al. simulated transport with a variety of pore geometries and surface types.[179] It was found that diffusion is independent of pore surface structure, for systems with strong adsorbate- surface interactions, compared to adsorbate-adsorbate interactions. In such systems, diffusing molecules have a weak interaction with the adsorbate covered surface, and so move relatively freely through the pores. D¨uren et al. suggest that in systems where the interaction between adsorbate and surface is weaker, a different transport profile would be expected and surface structure may become important in transport.

Chapter 3

Synthesis of Phosphonic Acid

Ligands

A wide range of coordination polymers have been prepared using bisphosphonic acid ligands as the linker — examples have been reviewed elsewhere in this work (Section 1.4). Phosphonic acids may be regarded as organic derivatives of phosphoric acid (H3PO4) in which one of the OH groups is substituted by an organic group (R).

Phosphonic acids have the general formula RPO(OH)2. The P atom issp3hybridised

and therefore substituents are arranged in a tetrahedral geometry about the central atom (Fig. 3.1).

Synthetically the challenge in preparing a phosphonic acid is the formation of the C-P bond. A wide range of synthetic approaches to this problem has recently been reviewed.[180] The most commonly applied approaches are the Michaelis- Arbuzov reaction, catalytic cross-coupling reactions and the modified Mannich reaction, originally reported by Moedritzer and Irani.[181] The Michaelis-Arbuzov and catalytic cross-coupling reactions are quite closely related to one another. In the Michaelis-Arbuzov reaction, a primary alkyl halide reacts directly with trialkylphosphite to form a phosphonium halide salt. Aryl and alkenyl halides have less electrophilic C-X bonds and therefore either a Ni(II) or Pd(II) catalyst is needed

Chapter 3. Synthesis of Phosphonic Acid Ligands

Figure 3.2: Michaelis-Arbuzov reaction mechanism for the preparation of phosphonic acids from primary alkyl halides. For aryl or alkenyl halides, a Ni(II)or Pd(II) catalyst is needed to first form the phosphonium halide salt.

to form the salt. Once generated, the salt eliminates an alkyl halide to leave a dialkyl alkyl phosphonate which may be converted to the acid by hydrolysis (Fig. 3.2).

In the modified Mannich reaction α-aminomethylphosphonic acids are prepared from primary or secondary amines. The reaction proceeds by nucleophilic attack of the amine on formaldehyde, activated by protonation of the formaldehyde O atom by the acid catalyst. Water is then eliminated, forming an electrophilic iminium cation which is attacked by phosphorous acid (H3PO3), forming the phosphonic

acid following loss of a proton (Fig. 3.3).

All ligands in this work were prepared through the modified Mannich reaction. In order to prepare bisphosphonic acid ligands suitable for forming extended networks, cyclic diamines were used as the substrate. This also has the advantage that, provided the reaction goes to completion, a single component product is obtained as each amine may only be mono-methylenephosphonylated. It was also found that phase pure products (according to1_H,13_{C and}31_{P solution-state NMR spectroscopy)}

were obtained directly from syntheses, from the ligand which crystallised on cooling of the reaction mixture. Syntheses were adapted from those reported by Mowat[182, 111] with all ligands fully characterised by NMR for the first time, with further characterisation by elemental analysis (where possible). A typical synthesis using piperazine as the substrate is given (Section 3.1). The synthesis of six bisphosphonic acid ligands has been attempted (H4L, H4L0, R-H4L0, H4L00,

H4LL and H4LC3L) and the results of these syntheses and characterisation of each

ligand is reported. In addition, a carboxyphosphonic acid (H3LC) and a mono-

Chapter 3. Synthesis of Phosphonic Acid Ligands

Figure 3.3: Modified Mannich reaction for the preparation of α-aminomethylphosphonic acids, first reported by Moedritzer & Irani.[181]

reaction and for comparison of crystal structures.

3.1

Typical Synthesis of Bis-α-aminomethyl-

phosphonic acid Ligand

In a typical synthesis of N,N0-piperazinebis(methylenephosphonic acid) (H4L)

piperazine (Sigma) (7.75 g, 0.09 mol) was dissolved in distilled water (70 ml) with phosphorous acid (Sigma) (19.19 g, 0.23 mol) and hydrobromic acid (Sigma, 48 wt.% aqueous solution) (74 ml, 0.65 mol). Formaldehyde (Alfa Aesar, 35 wt.% aqueous solution) (38.5 ml, 0.52 mol) was added to the reaction dropwise over 20 minutes. The reaction was then refluxed at 120°C for 20 hours. On cooling a white precipitate was obtained. Solid was separated by filtration, washed with ethanol-water solution (90:10, 3× 30 ml) to remove unreacted phosphorous acid and HBr residue, and dried overnight at 40°C. The volume of the filtrate was then reduced by∼70 % and the remaining solution placed in the fridge overnight to crystallise any remaining ligand. Solids obtained from the filtrate were also collected by filtration, washed with ethanol-water solution and dried overnight. Products were characterised by elemental analysis1H, 13C,31P solution-state NMR spectroscopy, powder and single crystal X-ray diffraction.

Syntheses of the other ligands followed similar synthetic procedures — any differences in the syntheses will be described. In all reactions a stoichiometric ratio of 0.09 : 0.23 : 0.52 (amine N atom : H3PO3 : CH2O) was used. Although only

Chapter 3. Synthesis of Phosphonic Acid Ligands

catalytic, HBr molar concentration was also scaled with other reagents (Zo´n et al.

indicate that a molar excess of acid is important).[180] It was not possible to obtain single crystals suitable for diffraction for all ligands. Discrepancies in elemental analysis results are thought to be due to residual HBr coordinated to the ring N atoms and inclusions of solvent in the samples (for example, NMR shows evidence of ethanol and water may also be incorporated).