Synthesis induction period

After reduction and surface characterization, the iron sample was moved to the reactor and brought to the reaction conditions (7 atm, 3 1 H2 C0, 540 K). Once the reactor temperature, gas flow and pressure were stabilized ( 10 min.) the catalytic activity and selectivity were monitored by on-line gas chromatography. As previously reported, the iron powder exhibited an induction period in which the catalytic activity increased with time. The catalyst reached steady state activity after approximately 4 hours on line. This induction period is believed to be the result of a competition for surface carbon between bulk carbide formation and hydrocarbon synthesis.(6,9) Steady state synthesis is reached only after the surface region of the catalyst is fully carbided. 

Isonitriles, Anthranilates, Tetraethylammonium chloride Bossio, R. et al., J. Heterocycl. Chem., 1995, 32 (4), 1115 A one pot synthesis of heterocycles involving the above reagents and catalyst sometimes showed an induction period followed by violent reaction. 

After an inordinately long induction period, organopalladium chemistry has finally been embraced by synthetic organic chemists. Currently, it is being utilized across the spectrum of organic synthesis, from applications to complex natural product syntheses to the synthesis of polymers. A substantial portion of organopalladium methodology has been developed in the context of heterocyclic chemistry and applications to heterocyclic syntheses abound. 

The initial stages of the DBX-1 synthesis using Cun-purified NaNT in place of raw NaNT produced similar observations as described above. Upon addition of the initial dose of sodium ascorbate, a gel-like material was formed with no evidence of crystalline DBX-1. After an apparent induction period of several minutes, a very different phenomenon was noticed. These events included a visual change in reaction mixture turbidity (it became clear) and an almost instantaneous drop in the counts of fine particles (see Figure 1). As the fines dropped the PVM almost instantaneously identified crystalline DBX-1 (see Figure 2). The event only lasted seconds and produced the beautiful rust colored crystals indicative of DBX-1 product. The second dose of reducing agent was started (denoted by the second start on the x axis) after formation of the DBX-1 crystals was identified. 

In most oases, the fluoro Pummerer reaction can be carried out with 1.33 to 2.0 equiv of DAST and a "catalytic" amount of antimony trichloride in either refluxing methylene chloride or chloroform at room temperature or 50°C. In the synthesis of fluoromethyl phenyl sulfide, however, the induction period makes room temperature conditions the preferred method for large scale synthesis. 

Experimental work using a pulse-quench catalytic reactor650 to probe transition between induction reactions and hydrocarbon synthesis on a working H-ZSM-5 catalyst has resulted in the suggestion that stable cyclopentenyl cations are formed during the induction period from small amounts of olefins formed in an induction reaction 647 One study reports a surprising observation, namely, enhanced aromatic formation over the physical mixture of Ga203 and H-ZSM-5 (1 1) (18.2% of benzene and methylbenzenes).651... 

Synthesis of Zeolite X. Zeolite X was crystallized from a batch of overall composition 4 Na20-Al203-5 SiO2-200 H20 at 90° C (8). The crystallization curve is shown in Figure 7. After an induction period of 2.4 hours, zeolite X was formed rapidly as a single phase at a conversion... 

Figure 1 shows that the phase transformation, the crystallization rates as well as the length of the induction periods, depends drastically on the kind of template and the alkalinity. One has to take into account that all other synthesis parameters have been kept as constant as possible. 

Cumene has a potent narcotic action characterized by a slow induction period. It is a depressant to the central nervous system. The long duration of its action indicates a possible slow rate of elimination, meaning that cumulative effects must be considered. Cumene is thought to have a greater acute toxicity than either benz or toluene (Ref 4) Cumene is used in org synthesis as a solvent, diluent, and as an additive to aviation gasoline... 

Figure 4.4 shows the crystallinity of the TS-1 and silicalite-1 samples synthesized under different conditions in the YNU and method A systems (the Si/Ti ratio in the synthesis gel was 50 for both series of samples). Clearly, the crystallization rate of TS-1 was much slower in the YNU system than that in the method A system and the presence of Ti in the gel not only severely retarded the crystallization process by prolonging the induction period but also reduced the crystallinity of most of the samples. Nevertheless, higher crystallinity of TS-1 was finally achieved in the YNU system than in the method A system. 

A one pot synthesis of heterocycles involving the above reagents and catalyst sometimes showed an induction period followed by violent reaction. 

A desired level of protein synthesis should be obtainable within a short time after induction. Rapid induction is desirable for two reasons First, a short induction period reduces the time for each round of screening. Second, and most important, when the cells are kept in an induced state for a shorter period, Darwinian selection of faster growing non-target clones is minimized. 

Initiation of Zeolite Crystallization. An induction period (tj) is always observed at zeolite crystallization, and is accepted to be the period necessary for the formation of zeolite nuclei (20). The inverse value of induction period, 1/t, is called the rate of nucleation. The crystallization curves were plotted as the degree of crystallization (estimated by the XRD method) versus synthesis time. The values of tj were obtained by extrapolating the time when crystal growth started. 

The treatment combination "be" of Table VI was chosen in order to monitor the growth kinetics of NH4-ZSM-5. Table VII summarizes the results and compares the growth rates with the data found by various other investigators with different systems. Strong influences of OH" concentration on growth rates have already been observed by Hou et al. for NH -TPA mixtures (17) and by Lowe ( 0) and Hayhurst et al. (18) for Na-TPA systems. The latter group recorded even faster crystallization and shorter induction periods in Na+-free synthesis... 

Uniform and small crystallites could be rapidly synthesized by milling of crystal precursor. The size and concentration profiles of precipitated particles affected not only the induction period and the rate of crystallization, but also the crystal structure and distribution of the acid sites. Accordingly, if the crystal precursor is as small and uniform as possible, the synthesis of a very small and uniform crystallites and very rapid crystallization would be expected. Such small particles would play the role of nuclei of the crystals. Therefore, in this study, the precipitated gel was milled by a motor driven mortar. The liquid phase separated from the gel by the progress of milling was removed twice by centrifuge (Method 3) ... 

Phenomenology of the crystallization. The conversion versus time curves obtained at three different temperatures are shown in Figure 1. With the synthesis procedure used, the sigmoid curves were characterized by shorter induction periods than the traditional method (11,12). As expected, temperature had a strong effect on the rate of crystallization. The overall crystallization rates may be approximated by the reciprocal of the times of half conversion. From these values an apparent activation energy of 22 1 kcal/mol was obtained. With respect to literature data, this value exceeds that reported, for instance, for zeolite Na-X (1,4) but compares well with the 19.8 kcal/mol found for ZSM-11 (13). 

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