Crystallization initiation reaction time

PA-6,6 is made from the relatively expensive materials hexamethylene diamine and adipic acid. An alternative synthesis of PA-6,6 from adiponitrile and hexamethylene diamine utilizing water is under investigation.16 PA-6 can be synthesized in a continuous process at atmospheric pressure, but reaction times are very long as the ring-opening initiation step is particularly slow. The reaction time can be shortened considerably by carrying out prepolymerization in the presence of excess water at pressure however, this makes the continuous polymerization process more complex. Copolymers with amide units of uniform length (diamides) are relatively new the diamide units are able to crystallize easily and have a thermally stable crystalline structure. 

To a solution of 13.2 g (0.054 mole) of 9-bromofluorene in 200 ml of acetonitrile was added 3.1 g (0.018 mole) of l,6-bis(dimethylamino)hexane. Heat was evolved and solid began to precipitate immediately. After the initial reaction had subsided the mixture was refluxed for 3.5 h on a steam bath. The solid was collected and recrystallized three times from propanol to give 10.66 g (89% of the theoretical yield based on alkylenediamine) of hexamethylene bis(9-fluorenyldimethylammonium) dibromide, as white crystals melting at 188°-189°C. 

The reaction conditions are constrained. In other words, there is usually a strict upper and lower limit for each reaction parameter. In the case of the synthesis described above, for example, the lower temperature is set by the need to provide sufficient thermal energy to initiate the reaction and the upper temperature by the need to remain below the decomposition temperature of the glue (see Section 2). The lower and upper limits on the total flow rate meanwhile are determined, respectively, by the maximum length of time one is prepared to allow for a single reaction and the minimum reaction time needed to produce crystals of nanometer dimensions. In this work, we select minimum and maximum total flow rates of 2 and 40 il min 1 which, for the typical chip volumes we use ( 16.6 il), correspond to average residence times of about 500 and 25 s, respectively. 

The pH of mother liquor was measured for all three experimental runs. These data are plotted on Figure 5. The alkalinity of the reacting solution was found to decline with time. The pH of the initial mixture was 11.85 declining to less than 10.00 upon completion of the crystallization. The changes in pH with time were found to exhibit minima. For all three cases, the lowest pH values were measured for the reaction times which produced the largest crystals. Under normal gravity, minimum pH values were noted at one and seven days. These times correspond to the formation of the two maximum crystal sizes. Similar trends are observed for the elevated gravity synthesis. 

The progress of the epoxidation can be followed by measuring periodically the optical density of the reaction mixture at 295 m/x. The reaction time and temperature specified in the procedure were found to reduce the optical density of the reaction mixture at 295 m/x (and, accordingly, the /rans-stilbene concentration) to less than 3% of its initial value. If more than this amount of unchanged fraws-stilbene remains in the crude product, it cannot be removed by recrystallization from either methanol or hexane. Even after repeated crystallization the melting point of the product does not rise above 66-67°. Pure fraws-stilbene oxide can be isolated from such a mixture if the mixture is treated with additional peracetic acid to convert the remaining fraras-stilbene to fraws-stilbene oxide. 

Main factors which affect a hydrothermal reaction are the initial eomposition, reaction temperature and time. In mild hydrothermal synthesis, reaction temperatures lower than 240 °C are respected for both safety of high pressure in normal autoclaves and protection of softness of Teflon line. In our specific synthesis system, high temperature favorites the reaction and the most important factor was the base concentration in the initial reaction mixtures. The reaction time associated with reaction temperature affected the reaction. Crystallization kinetic experiment for a typical reaction showed that a reaction time more than lOh gave well-crystallized product and the further crystal growth needed additional time. Table 1 lists the starting reaction compositions and phase identification of products obtained at 240 °C for lOh. 

The Initial turnover frequencies observed are comparable with those reported by Somorjai et al on unsupported stepped platinum crystal surfaces and by Boudart et al on supported platinum and decrease with reaction time and reaction temperature. 

The MTO and DTO initial reaction rates were identical, but the maximum MTO rate was three times higher than the maximum DTO rate. Olefin distributions were almost identical for MTO and DTO. These results indicate that methanol and DME are probably converted to olefins through the same reaction mechanism. However, the DME conversion can be retarded by such effects as slower diffusion into the pores of SAPO-34 compared with methanol. Experiments with SAPO-34 of different crystal sizes have confirmed that... 

Isothermal (593 to 693 K) ar-time curves for the decomposition of lanthanum oxalate [81] were sigmoid but asymmetrical with the decay period being the more pronounced. The Prout-Tompkins equation applied in two linear regions with , = 132 kJ mol. A branching chain mechanism was proposed during which channels fi om the surface penetrate the crystals as reaction proceeds. The predominant initial product was carbon monoxide, which disproportionated to yield carbon dioxide, and the residual solid contained carbonate and finely divided carbon. 

A time resolved crystallographic experiment has three key components reaction initiation, reaction monitoring and X-ray data acquisition and analysis. The X-ray aspects of such experiments are feasible now. However, the real challenge lies in devising suitable ways of reaction initiation and monitoring in the crystal. Progress to date will now be covered. Table 10.8 provides details of examples of time resolved crystallography. 

After reaction initiation, X-ray diffraction data can be recorded. If the crystal at any time t after initiation contains several conformers, can these structural states be extracted Moffat (1989a) outlines an approach to this problem. A series of experiments will yield several X-ray data sets along a time pathway. Each data set will correspond to several conformers of different occupancy and these occupancies will vary with time. 

In this specific case, seeding was crucial to the success of the process. If the system was not seeded, the initially crystallized BCSA oxazinone was 90% de. The major factor in the length of reaction time was the slow turnover of the undesired diastereomer that had crystallized. A three hour addition of BCSA solution to a solution of racemic oxazinone 1 that contained 10% seed of BCSA oxazinone avoided supersaturation problems and high diastereopurity of the crystallized solids was maintained throughout the process. With this modified procedure, since very little of the undesired diastereomer was formed early in the process, the time cycle was reduced from 7 days to 2 days... 

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