Crystalline MOPs

The formation of supported MoP can be explained as follows. First, PH3 decomposes on the surface of the Mo particles to form P atoms, which chemisorb on the Mo particles and form a thin MoP layer. At the low phosphiding temperature (523 K), the P atoms cannot migrate easily into the metallic particles and will remain on the outer surface of the Mo particles. When the sample is heated to 723 K, the P atoms migrate into the Mo particles and ciystallization take place, resulting in the formation of crystalline MoP. The incorporation of phosphorus into the Mo particles resulted in an increase in the particle size from 9 to 13 nm on silica and from 6 to 8 nm on alumina, as shown by XRD (Figs. 1 and 2). Considering that the lattice volumes of Mo and MoP are 31.2 and 29.0 respectively and that there are two Mo atoms and one MoP per unit cell, the diameter of a MoP particle must be 1.23 times larger than the diameter of the Mo particle for the same number of Mo atoms. This value is close to the value we obtained. 

It cannot be said that one particular strategy - order versus disorder - is better than the other, and the choice will depend on the desired application. There are a number of potential advantages for crystalline MOPs. Structural uniformity makes it possible to design materials with very narrow pore size distributions and thus, conceivably, gain some of the advantages associated with zeolites [1] - for example, molecular specificity in catalysis. Crystalline MOPs can also be characterized structurally at the molecular level by X-ray diffraction [14,15, 20] in a manner that is not possible for amorphous materials. At the time of writing, the highest apparent BET surface areas reported for MOPs (up to 4,210 m g ) were found for ordered, crystalhne COFs [20]. 

Nitrides and Oxynitrides. Crystalline films of NbN and Nb Nj have been prepared. Electron-diffraction studies showed that the former has a NaCl-type structure and the latter is isomorphous with Ti405 and Ta Nj. " The phase 0-TaN, 0 has been prepared by nitriding Ta metal powder with ammonia gas at 850—1200°C. Although it could not be obtained pure, the phase was characterized by X-ray diffraction studies. The free energy of formation of TajNj has been determined as -272 42 kJ mol and that of TaO, 05N0.95 as -396 21 kJ moP. ... 

Unprotected MOP glycosyl donors are easily prepared in multigram scale from the corresponding peracetyl glycosyl halides, followed by de-O-acetylation. They are usually crystalline and have excellent shelf life. MOP donors 2,13,18,19, 20, and 22, shown in Scheme 5, are typical examples. 

Molybdenum Phosphide, MoP, is formed when molybdenum trioxide is fused -with phosphoric acid and a little chalk, and the grey crystalline metallic mass is extracted first with hydrochloric acid and then with caustic soda. It has density 6-17, and is readily oxidised on heating in air, treating with nitric acid, or fusing with potassium nitrate. 

From the enthalpy of formation of crystalline imidazole (61 3 kJ moP ), the heat of sublimation (67 4 kJ mol ) and the heat of formation of gaseous imidazole (128 8 kJ mol ), a resonance energy value of 59 kJ mol can be calculated. While this is considerably lower than that of benzene, it still indicates considerable stabilization. 

The XRD patterns of all the synthesized bulk phosphides are in agreement with literature data [12]. Small amounts of an unidentified crystalline phase accompanied CoMoP, while NiMoP contained small amounts of MoP. In all the other cases, only one ciystalline phase was detected. Fig. la shows the XRD pattern observed for WP and the calculated pattern, while Fig. lb represents the corresponding crystallographic structure. 

In the same reference the authors prepared crystalline K2Se20s(cr) from KHSe03(cr) by heating. The specimen was reacted with a solution of Pb(N03)2 in a calorimeter with formation of crystalline PbSeO ,. The calorimetric data were used to calculate the enthalpy change of the reaction K2Sc205(cr) + 2Pb + H20(l) —> 2PbSe03(cr) + 2K" + 2H in Appendix A to be (34.40 3.00) kJ-moP. The standard enthalpy of formation of K2Se205(cr) is calculated from this datum to the selected value ... 

The authors also prepared anhydrous aluminium selenate by heating AIH(Se04)2 H20 to 600 K. The product will be denoted as solid only, as a check of its crystalline state is not mentioned. The enthalpy of dissolution of the two aluminium selenates in 0.5 M KOH to the same unspecified concentration was measured to be - (178.90 + 0.92) kJ-mol and - (442.30 1.02) kJ-mof with the result for the anhydrous compound mentioned last. From these data the standard enthalpy of formation of Al2(Se04)3(s) is calculated to be-(2561.68 17.11) kJ-moP. ... 

Phase effects in the oligomerization of l-aIIyl-l,2-dicarba-doso-dodecaborane by electron radiolysis have been investigated. Liquid state radiolysis indicates an activation energy of ca. 5 kcal moP for the formation of oligomer, and ca. 0 kcal mol for the formation of unsaturated dimers. For the plastic crystalline phases, negative activation energies were observed, indicating the existence of a complex reaction sequence." ... 

The conformational behaviour of saturated thiols and thioethers is rather similar to that of the oxygen analogues, allowing for the long C—S bonds (1.825 A). Methanethiol has a rotational barrier of 1.27 kcal moP (Kojima, 1960), and dimethyl sulphide 2.13 kcal mol" (Kadzhar et al., 1968) according to microwave studies. IR spectra in glass, in solution, and in the crystalline state show ethanethiol [24] to prefer the gauche form to the anti. 

Unambiguous classification is challenging for MOPs, even when the subject is deUnealed as above. We have divided materials into amorphous (Sect. 2) and crystalline (Sect. 3) groups since this distinction is clear-cut. Within amorphous MOPs, classification is more problematic and several classes overlap. For example, many network-type MOPs may be classified in principle as hypercrosslinked, although this term has been confined in the literature to only a few polymers (Sect. 2.1). 

Covalent organic frameworks (COFs Sect. 3.1) 711-4,210 0.32-1.66" High thermeil stability porosity sensitive to air for B-conteiining COFs Highest MOP surface areas crystalline tunable pores [14, 20, 67]... 

The role of solvent and other additives in pore formation in MOPs is likely to be very important but is poorly understood. Templating effects are commonly invoked for microporons MOFs and similar considerations would apply to crystalline microporons COFs [14, 20], for example. Likewise, the influence of solvents and other species as porogens in porous polymer synthesis is well-known [26] but mostly understood at a qualitative level. Indeed, there is relatively little detailed information to hand for MOPs and this issue requires further attention. 

---