Hybrid crystals

Separation Processes for PX. There are essentially two methods that are currendy used commercially to separate and produce high purity PX (/) crystallization and (2) adsorption. A third method, a hybrid crystallization /adsorption process, has been successfiiUy field-demonstrated and the first commercial unit is expected in the near future. 

Hybrid Crystallization/Adsorption Process. In 1994, IFP and Chevron announced the development of a hybrid process that reportedly combines the best features of adsorption and crystallization (59,99). In this option of the Eluxyl process, the adsorbent bed is used to initially produce PX of 90—95% purity. The PX product from the adsorption section is then further purified in a small single-stage crystallizer and the filtrate is recycled back to the adsorption section. It is reported that ultrahigh (99.9+%) purity PX can be produced easily and economically with this scheme for both retrofits of existing crystallization units as well as grass-roots units. A demonstration plant was built at Chevron s Pascagoula refinery in 1994. 

The structure of the host crystal before and after treatment with vinyl bromide is shown in Figure 7.44 and Hybrid crystals can be isolated by stopping the process part-way. Guest absorption by the calixarene was also monitored by DSC which showed surprisingly that the process is endothermic, and hence... 

Their Tammann temperatures are low. As a consequence, these lattices are in a metastable state, near these temperatures. If two structures are closely related, as we shall see in the next section, the two solids will be able to form either coherent interfaces or solid solutions. In these cases, "hybrid crystals, in which microdomains of both phases coexist, can be formed according to UBBELOHDE s theory (43), which considers strain energy E and internal surface energy ri (44). The free enthalpy of a domain (1) in a matrix of structure (2) is given by ... 

An interesting class of crystals are the hybrid crystals, in which an organic partner is built into an inorganic crystal lattice in a stoichiometric ratio. A nice example of this are the lead halides. 

By now, you should have some appreciation for the different types of crystalline lattices and have some idea of the difficulty in predicting the exact structure that a compound will form. There are a variety of factors involved in the process of crystallization, including (but not limited to) closest packing of spheres, closest packing of polyhedra, electrostatic interactions, hydrogen bonding, hybridization, crystal field effects, and the degree of polarization. When it comes to the solid state, simple stoichiometries do not necessarily imply simple structures. 

Pure crystals of cellulose IV have been prepared by deacetylation and precipitation of cellulose acetate of a low degree of polymerization at ISO— 160 C. At temperatures between 90 and 150 °C, hybrid crystals are obtained having cellulose II and cellulose IV, domains cocrystallizing in syntaxy. When a solution leading to cellulose IV was seeded with native cellulose fibrils, a shish-kebab structure of microfibi s with cellulose II lamellae was obtained. 

Demadis K. D., Mantzaridis C., Raptis R. G., Mezei G. Metal-Organotetraphosphonate Inorganic-Organic Hybrids Crystal Structure and Anticorrosion Effects of Zinc Hexa methylenediaminetetrakis(methylenephosphonate) on Carbon Steels Inorganic Chemistry, Vol. 44, No. 13 (2005). P. 4469-4471. 

Of course such criteria break down if we move to the nanometer scale. Nevertheless it is assumed that a hybrid crystal in which two polymorphs coexist allows for separation in principle. 

---