Solution-mediated transformation, process

There is a lot of evidence that the crystallization of zeolites from aluminosilicate gels is a solution-mediated transformation process in which the amorphous phase is a precursor for silicate, aluminate and/or aluminosilicate species needed for the growth of the crystalline phase (1-9). Generally, it is well known that the kinetics of most gel-zeolite and zeolite-zeolite transformations can be expressed mathematically by the simple kinetic equation (1,2, 10-12),... 

Our earlier studies of zeolite-zeolite (10,26) and gel-zeolite (11, 12) transformations have shown that, under the assumption that the crystallization of zeolite is a solution-mediated transformation process (1-9) and that the crystal growth is size-independent (5,6, 12-16), the crystallization (transformation) kinetics can generally be expressed as ... 

The presence of the solvent does not change the thermodynamics and stability relationship, unless a solvate/hydrate is formed with the solvent. However, owing to the much higher mobility in the solution state than in the solid, transformation to the stable phase is much faster. This process is analogous to the effect of catalysts for chemical reactions (Zhang et al. 2009). The schematic representation of concentrations in the solution, as well as the solid compositions as a function of time, is shown in Fig. 15.3 for a typical solution-mediated transformation process (Zhang et al. 2009). 

A rate enhancement effect due to secondary nucleation has been identified in the solution-mediated transformation of the 7-phase of (i)-glutamic acid to its / -phase [82]. In this study, the kinetics of the polymorphic transition were studied using optical microscopy combined with Fourier transform infrared, Raman, and ultraviolet absorption spectroscopies. The crystallization process of n-hexatriacontane was investigated using micro-IR methodology, where it was confirmed that single... 

Secondary processing does not always lead to phase transformations, as was shown during studies of the polymorphs of ranitidine hydrochloride [92]. No solid-solid transformation could be detected during either the grinding or compression of metastable Form I, stable Form II, or of a 1 1 mixture of these forms. The dissolution rates of both forms were found to be equivalent, and the solution-mediated transformation of Form I to Form II was observed to be slow. 

These examples are all solution-mediated transformations, which depend upon the solution phase to provide the mobility necessary to rearrange in the most stable form. Solid-state transformations are also possible when temperature and pressure changes can move the system across a phase boundary. These conditions often favor the metastable phase, however, unless there is a temperature reduction. Solid-state relaxation is more likely to be a slow (kinetically controlled) process, which is also possible during storage. 

The process of solution mediated transformation can be considered the result of two separate events, (a) dissolution of the initial phase, and (b) nucleation/growth of the final, stable phase. If crystals do not grow as expected from a saturated solution, the interior of the vessel can be scratched with a glass rod to induce crystallization by distributing nuclei throughout the solution. Alternatively, crystallization may be promoted by adding nuclei, such as seed crystals of the same material. For example, Suzuki showed that the a-form of inosine could be obtained by crystallization from water, whereas isolation of the 3-form required that seeds of the 3-form be used [13]. 

Characterization of solution-mediated transformations in the amorphous state can give an insight into amorphous crystallization (Zhang et al. 2(X)9). The importance of the phase transition kinetics, molecular interpretations, and process implications has been emphasized in numerous studies (Cardew and Davey 1985 Davey et al. 1986, 1997a, 1997b Rodriguez-Homedo et al. 1992 Blagden et al. 1998). 

Now, following the assumption that the crystallization of zeolite from gel is a solution-mediated gel-zeolite transformation (1-8) and the Kacirek s and Lechert s conclusion (34) that the growth of crystalline particles inside the gel matrix is considerably blocked and that they can grow only in the full contact with the solution phase, the rate dN-p/dTof the production of nuclei-II is assumed to be proportional to the amount of the gel dissolved during the crystallization process (2) and hence, also proportional to the amount of the crystalline phase formed during the same time (11). For the described mechanism of the production... 

The resulting dynamic aminonitrile systems were first subjected to lipase mediated resolution processes at room temperature. A-Methy] acetamide was observed as a major product from the lipase amidation resolution. In this case, free methylamine A was generated during the dynamic transimination process and transformed by the lipase. To avoid this by-reaction, the enzymatic reaction was performed at 0 °C, and the formation of this amide was thus detected at less than 5% conversion. To circumvent potential coordination, and inhibition of the enzyme by free Zn(II) in solution [54], solid-state zinc bromide was employed as a heterogeneous catalyst for the double dynamic system at 0 °C. The lipase-catalyzed amidation resolution could thus be used successfully to evaluate /V-substituted a-aminonitrile substrates from double dynamic systems in one-pot reactions as shown in Fig. 7d. Proposedly, the heterogeneous catalyst interfered considerably less or not at all in the chemo-enzymatic reaction because the two processes are separated from each other. Moreover, the rate of the by-reaction was reduced due to strong chelation between the amine and zinc bromide in the heterogeneous system. 

Another name for solid hydrogel transformation mechanism is solid-phase mechanism, while solution-mediated transport mechanism is also called liquid-phase mechanism. The main difference in explaining the formation process of zeolites by these two mechanisms lies in whether the liquid component is involved during the crystallization of zeolites. The views of these two mechanisms are opposite to each other and have their own experimental supporting evidence. To date, the liquid-phase mechanism has more experimental support than does the solid-phase mechanism. 

Throughout the history of the synthesis of zeolite-like materials, there has been much debate about the location of the structure-forming activity [28,36,39,50]. At one extreme, there is the possibility of solution-mediated crystallisation. In this view, the reactants dissolve (to a greater or lesser extent) in the reaction mixture to afford active species, from which the product is formed as it crystallises from the solution. In the opposite view, the solution is seen as more or less inert with the product being formed within the gel phase by a process described as a solid state transformation . Whereas the solution-mediated route is well known in the science of crystallisation, the alternative is a somewhat shadowy phenomenon for which no chcmically-specific mechanism has ever been published. 

Theoretical and experimental studies of the role of solvent on polymorphic crystallization and phase transformations abound in the literature of the last few years and some pertinent examples are described here. For solvent-mediated transformations, the driving force is the difference in solubility between different polymorphs. An important earlier paper on the kinetics of such phase transformations [51 ] described a model featuring two kinetic processes in sohd to solid phase changes via a solution phase, namely dissolution of the metastable phase and growth of the stable one. 

Much more attention was paid to the transformation process of vaterite to calcite. Upon heating at 730 K, vaterite irreversibly transforms into calcite. This transformation in aqueous solution at ambient conditions was a solution-mediated process [42]. All of the experimental results and the data analysis indicated that the transformation took place through dissolution of vaterite, followed by the crystallization of calcite. 

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