Three-step activation

The acetoxylation reaction is carried out at 70°C under 70 atm pressure by reacting 1,3-butadiene and acetic acid in the presence of air and a small amount of polymerization inhibitor. A special three-step activation (reduction-oxidation-reduction), also used in regeneration of the used catalyst, ensures high activity and selectivity. Regeneration of the catalyst is necessary after about one year of operation. 

All biotin-requiring enzymes follow the same three steps activation of bicarbonate by ATP, reaction of activated bicarbonate with biotin to form carboxybiotin, and transfer of the carboxyl group from carboxybiotin to the substrate. 

A mathematical model of Silicaiite crystallization from clear solution was developed and solved numerically. Crystallization was considered to be solution-mediated, and to involve three categories of silica species and three steps activation, nucleation, and crystal growth. Nucleation was represented by a condensation reaction between the hydroxyl groups present on the surface of 10 nm amorphous silica particles and the soluble silica species, producing activated complexes, which transform to crystalline nuclei. Both nucleation and crystal growth were considered to be reaction controlled. The model simulated well the in situ experimental data. This suggests that the hypothesized nucleation mechanism can be used to qualitatively describe the nucleation event during clear solution Silicaiite crystallization. 

In addition, the separation of FAZ from the partially activated fly ash (the impurities) remains a bug bear to the researchers and industrialists. With all such challenges in view, this book is the first time presenter of a novel technique (three-step activation, TSA), which stresses for recycling of the fly ash-NaOH-water reactions by-products till formation of zeolitic residue. The FAZ (the final residue after third step reactions) synthesized by this technique exhibits exceptionally high cation exchange capacity, specific surface area and pore area. 

In order to synthesize zeolites from fly ash by its activation with NaOH, attempts have been made to identify a suitable fly ash out of its two disposal sites (viz., dry site at the electrostatic precipitator and wet site at the lagoons in the thermal power plants) for conventional (i.e., one step) hydrothermal activation technique [1-10]. Subsequently, the fly ash ascertained to exhibit improved zeolitization potential has been prefered to undergo novel hydrothermal treatment processes (viz., three step activation by hydrothermal technique and three step fusions) to activate the fly ash significantly for synthesis of fly ash zeolites with high cation exchange cqjadfy [11-15]. The details of both the types of alkali activations (viz., conventional with the two ashes and three step activations with the superior ash) are presented in the following. 

In view of the above, this technique focuses to recycle/resuse initial products (i.e., the residue, AAF and the supernatant, ST) up to three steps of treatment, which has been termed as three step activation (TSA) by hydrothermal technique [12-15]. Moreover, uniform conditions of various parameters (viz., L/S, duration of treatment, temperature and pressure) were maintained during each step of the treatment. As such, with an intention to investigate the relative variation in the characteristics (viz., chemical, mineralogical and morphological) of the products of several recyclings, TSA techniques for advanced characterization of the end products have also been devised. This is a major modification in the process of zeolitization of fly ash, by the hydrothermal technique in this study, over conventional techniques demonstrated by the earlier researchers [2, 16]. 

Fig. 5.2 The flowchart for three step activation of the fly ash by hydrothermal technique...

Based on the findings presented in this chapter, it can be opined that the hydrothermal treatment of the fly ash collected from the hopper yields much better zeolites as compared to its counterpart (the fly ash collected from the lagoon). The three-step activation of the hopper ash with NaOH results in several changes (viz.,... 

Jha, B., Singh, D.N. Formation of meso- and micro-pores in fly-ash zeolites using three step activation. Acta. Geotech. Slov. 1, 63-69 (2014)... 

Major Findings of the Three-Step Activation Technique... 

Keywords Fly ash Three step activation Hydrothermal Fusion Zeolite Quantification Transition Purification Pores... 

Synthesis of zeolites from fly ash by resorting to hydrothermal treatments of the (i) fly ash-NaOH mixture, in one step and the (ii) supernatant, the liquid by-product of such treatment, and NaAlOa mixture, in two steps, is well established. However, the presence of residual fly ash in such zeolites is responsible for their impurities (i.e., presence of Quartz and MuUite) and their low grade, in terms of the cation-exchange capacity and crystallinity. To overcome these issues, a novel methodology, three-step activation (TSA) of the fly ash by hydrothermal method has been developed and its details are presented in this section. It has been demonstrated that this method results in high grade polycrystalline zeolites, which... 

A correlation of transitions of pH (refer Fig. 6.19) and electrical conductivity, EC (refer Fig. 6.20) elucidates some interesting effects of the three-step activation process. These figures exhibit that a reduction in both pH and EC of the supernatant influences some critical residues (viz., 0.5-PT-12, 1.5-PT-12, 3.0-PT-12 and 2.0-PT-36) of Step-1, 0.5-R1-12, 0.5-R2-12 and 1.5-R1-12 of Step-2, 1.5-R2-12,... 

Accordingly, a general relationship developed between Si/CT and Al/CT, is noticeable, as depicted in Fig. 6.25c. This figure reveals that AFCT increases with an increase in Si/CT. However, from Fig. 7.25c most of the zeolites (designated in Tables 6.9 and 6.10) are observed to have Si/CT and AFCT ranging from 0-4 and 0-2.5, respectively, which are significantly below those for the fly ash, RFA. This shows high effectiveness of the adopted three-step activation process, a remarkable improvement over the previous researchers. 

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