Gas/solid structural differences

Phillips, J.A., Halfen, J.A., Wrass, J.P. et al. (2006) Large gas-solid structural differences in complexes of haloacetonitriles with boron trifluoride. Inorg. Chem., 45, 722-731. 

Burns, W.A. and Leopold, K.R. (1993) Unusually large gas-solid structure differences a crystallographic study of HCN-BF3 [hydrogen cyanide-boron trifluoride]. J. Am. Chem. Soc., 115, 11622-11623. 

Figure 5.2 Schematic models of different solid/gas interface structures.

As can be concluded from this short description of the factors influencing the overall reaction rate in liquid-solid or gas-solid reactions, the structure of the stationary phase is of significant importance. In order to minimize the transport limitations, different types of supports were developed, which will be discussed in the next section. In addition, the amount of enzyme (operative ligand on the surface of solid phase) as well as its activity determine the reaction rate of an enzyme-catalyzed process. Thus, in the following sections we shall briefly describe different types of chromatographic supports, suited to provide both the high surface area required for high enzyme capacity and the lowest possible internal and external mass transfer resistances. 

X-Ray studies on the phosphorus oxides led to similar observations (82-86) P4O7, for example, cannot be considered as made up of fragments taken from P4O8 and P40jq (85). They also reveal the effect of packing, as well as significant differences between the solid and gas phase structures (84,86). 

Both the quantity and quality of gas-phase experimental structural data rapidly diminish with incorporation of elements beyond the second row of the Periodic Table. Solid-phase structures abound, but differences in detailed geometries from gas-phase structures due to crystal packing may be significant and preclude accurate comparisons with the calculations. There are, however, sufficient gas-phase data primarily on very small molecules to enable adequate assessment to be made. 

G. C. Pimentel (Berkeley) In rebuttal of Dr. Rowlinson s remark that the gas-phase measurements should be given more weight than other types of measurements, we must remember that the measurements made in different phases may not give the same answer, nor may they refer to the same species. This is suggested by the unexpectedly high values of ZlH of H-bond formation shown by Dr. Davies for solid amides. While this AH may include certain interactions not present in the gas phase, and while it may be more difficult to interpret, nevertheless it is the AH appropriate to the solid structure and one cannot determine this value by gas-phase studies. Similarly, in the liquid phase there may be different processes or species than in the gas phase. Hence one cannot discard a measurement of AH of dimerization applicable to solution phase solely because it disagrees with measurements applicable to the gas phase. 

One of the big obstacles still existing in gas-solid chromatography is the lack of adequate adsorbent structure descriptions, as well as the distribution and dimensions of the pores. Another is the lack of reproducibility of adsorbents, not only among manufacturers (different products, presumably the same) but within the same manufacturer (different lots). 

Syntactic foamed materials are classified as foamed plastics because they are formally similar in structure to cellular gas-expanded plastics in that they are heterophase, gas-solid systems. In general, however, they differ from ordinary foamed plastics in that they are not binary but tertiary systems because the filler and binder are made usually from different materials 3 5). 

Washcoat thickness is analogous to particle size in that reactants must penetrate its pore structure and interact with the dispersed active sites. The products produced must diffuse through the structure and out into the bulk gas. This phenomenon differs from that involving a particle in that only the gas-solid washcoat surface is available since the other side is bonded to the wall of the monolith. 

In this chapter, the structures and textures of carbons at different scales are explained. The carbon materials are classified into four families, diamond, graphite, fullerene, and carbyne on the basis of hybridized sp3, flat sp2, curved sp2, and sp orbitals used, respectively. Each family has its own characteristic diversity in structure and also in the possibility of accepting foreign species. The formation of these carbon materials from organic precursors (carbonization) is shortly described by dividing the process into three phases (gas, solid, and liquid), based on the intermediate phases formed during carbonization. The importance of nanotexture, mainly due to the preferred orientation of the anisotropic BSU in the graphite family, i.e., planar, axial, point, and random orientation schemes, is particularly emphasized. 

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