Manual Zeolite Microporous Solids: Synthesis, Structure, and Reactivity

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Many commercial washing powders thus contain substantial amounts of zeolite. Commercial waste water containing heavy metals, and nuclear effluents containing radioactive isotopes can also be cleaned up using such zeolites. Wales Zeolites contribute to a cleaner, safer environment in a great number of ways. In fact nearly every application of zeolites has been driven by environmental concerns, or plays a significant role in reducing toxic waste and energy consumption.

In powder detergents, zeolites replaced harmful phosphate builders, now banned in many parts of the world because of water pollution risks. Catalysts, by definition, make a chemical process more efficient, thus saving energy and indirectly reducing pollution. Moreover, processes can be carried out in fewer steps, miminising unecessary waste and by-products.

Introduction

As solid acids, zeolites reduce the need for corrosive liquid acids, and as redox catalysts and sorbents, they can remove atmospheric pollutants, such as engine exahust gases and ozone-depleting CFCs. Zeolites can also be used to separate harmful organics from water, and in removing heavy metal ions, including those produced by nuclear fission, from water. Zeolite science and technology has traditionally been very strong in the UK. Partly this has been due to the scientific legacy of the late Professor Richard Barrer, the "father of zeolite science" who, during a career of over 50 years in various British universities, laid the foundations for the study of zeolites and discovered many of their important properties.

The prominence of the UK has also been associated with the strength of its chemical industry, particularly in areas where zeolites have applications, such as petrochemicals, detergents, fine chemical synthesis and nuclear processing.

Bell, May Your browser version is outdated. We recommend that you update your browser to the latest version. What are zeolites? Framework Structure Zeolite tetraeder A defining feature of zeolites is that their frameworks are made up of 4-connected networks of atoms. Catalysis Zeolites have the ability to act as catalysts for chemical reactions which take place within the internal cavities.

Figure 1 shows the catalytic activity as a function of time on stream for the samples under study.


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It can be observed that a low rare earth content has little influence on the activity and the stability of sample REUSY when compared to USY, no matter how much acid sites density is reduced. However, the presence of a high rare earth content sharply decreases cracking activity, thus increasing the catalytic stability, as shown in Table 2. Indeed, the sample with the highest rare earth content presents the lowest deactivation coefficient. Murta Valle et al.

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These results can be associated with the decrease in acidity density of sites in the samples as rare earth content increases, thus decreasing cracking by means of a bimolecular mechanism which predominates in large pore zeolites. A decrease in catalytic activity due to the presence of rare earth elements was reported by Camorim et al. In both cases, the decrease in acid sites density was also used to explain the observed trends.

Reaction selectivity is not significantly influenced by the content of rare earth elements since the cracking product distribution is similar for the three samples.

Zeolite Microporous Solids: Synthesis, Structure, and Reactivity | E.G. Derouane | Springer

This fact was reinforced by the low values lower than 1 for the C. Figure 1: Catalytic activity as a function of time on stream n-heptane cracking at o C. Figure 2: Distribution of initial reaction products n-heptane cracking at o C. Table 2: Deactivation coefficients n. Sample n USY 0. The results presented in Table 3 also show that, independently of the range of conversion, a low rare earth content REUSY sample has no significant effect on reaction selectivity. Lemos et al. The greater the rare earth cation content, the lower the coking rate and the total coke content, thus depicting the negative effect of rare earth on coke formation.

This fact may be attributed to a reduction in both acid sites density and mesoporosity. A decrease in acid sites density hinders bi and polimolecular reactions involved in coking, while the absence of a significant secondary mesoporous system limits the room available for voluminous coke precursor molecules. Similar results, reported by Henriques et al. It can be seen that the TPO peak shifts toward higher temperatures with a decrease in rare earth content. This can be associated with both the greater rare earth content in the CREY sample, which could promote coke oxidation, and to the differences in coke composition since different coke contents are being compared 8.

Moljord et al. This was not observed in the present work, since coke formed in USY high acid sites density was more difficult to burn than that formed in CREY low acid sites density. The Journal of Physical Chemistry B , 10 , Tatiana Y.

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Zeolite Microporous Solids: Synthesis, Structure, and Reactivity

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