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Microporous and Mesoporous Solids
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
Elaine A. Moore, Lesley E. Smart
Zeolites A and X are ‘aluminium-rich’ zeolites with Si/Al ratios of close to 1. These zeolites have very useful absorbent properties and are still used industrially, but are unstable and lose aluminium when attacked by acid, steam, or water. The natural zeolite mordenite, which is the most siliceous of the naturally occurring zeolite minerals, was found to be more stable and because it has an Si/Al ratio of >5, research turned to synthesising zeolites with higher ratios in the hope of greater stability. Many synthetic zeolites that have been developed for catalysis are thus highly siliceous: zeolite Y has an improved stability with a ratio of 2.25; zeolite ZK-4 (LTA), with the same framework structure as zeolite A, has a ratio of 2.5; and ZSM-5 (MFI) can have an Si/Al ratio that lies between 20 and ∞ (the latter, called silicalite [see above], being virtually pure SiO2), which far outstrips the ratio of 5.5 found in mordenite.
2 Separation from Natural Gas
Published in Zeinab Abbas Jawad, 2 Sequestration and Separation, 2019
A.K. Zulhairun, N. Yusof, W.N.W. Salleh, F. Aziz, A.F. Ismail
Silicalite-1 is an MFI-type zeolite which composed of pure SiO2 (Zhao et al. 2015, Wang et al. 2017). Silicalite-1 has the same crystalline structure as ZSM-5, but contains higher Si/Al ratio (> 1,000) while ZSM-5 ranges from 10 to infinity (Arudra et al. 2014, Zito et al. 2017). Silicalite-1 is favorable for the separation of gas mixtures containing non-polar molecules with similar physical properties due to the “smooth” adsorption surface containing few equilibrium ions (Wang et al. 2017). Despite its excellent selectivity and adsorption properties, the production of silicalite-1 is relatively costly due to exhaustive synthesis steps. The average time taken for synthesizing silicalite-1 is 4 to 12 days depending on the methods employed (Hu et al. 2017).
Silica and Silicates
Published in Shamil Shaikhutdinov, Introduction to Ultrathin Silica Films Silicatene and Others, 2022
In contrast to natural feldspars, zeolites may have very low Al:Si ratios. For example, in the synthetic Si-rich ZSM (Zeolite Socony Mobil) zeolites, this value can be as low as 10-4. The name silicalite has been introduced in order to highlight the fact that this is practically “all-Si” zeolite in nature.30 Although the lack of substitutional Al leads to catalytic or ion exchange activity much lower as compared to the regular zeolites, silicalites were found to selectively absorb small organic molecules and behave as a molecular sieve.
Feasibility of zeolites in converting butyric acid into propylene for biorefineries
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2018
Takashi Goshima, Yumi Isoda, Madoka Sakaguchi, Kenta Fukudome, Kei Mizuta, Susumu Nii
The effect of acid sites on catalytic conversion of butyric acid over H-ZSM-5 was shown in Figure 4. Reaction temperature and contact time were set at 500°C and 1.2 s, respectively. For H-ZSM-5, the ratio of CO + CO2 to other hydrocarbon products was about 1/3 regardless of their acidities. Thus, it was presumed that propylene is mainly generated by decarbonylation and dehydration of butyric acid like the case for SAPO-11. For H-ZSM-5 (27), the yields for propylene largely decreased, and propane and butane as well as ethylene and butene increased as compared with SAPO-11. However, for H-ZSM-5 (75, 140), the yields for butene and propylene increased, and the yields for propane and BTX decreased largely as compared with H-ZSM-5 (27). The finding indicates that hydrogen transfer reactivity at each acid site of H-ZSM-5 was equal regardless of their acidities. Therefore, it was considered that as the acidity of H-ZSM-5 decreased from 0.298 to 0.162 mmol/g, the distance between two acid sites on the surface and/or in the pores of H-ZSM-5 was larger than that required to transfer electrons between adsorbed molecules so that hydrogen transfer reaction was inhibited. Furthermore, for H-ZSM-5 (750) and silicalite, the yield for propylene increased and the yields for ethylene and butene rapidly decreased, compared to H-ZSM-5 (75, 140). Because the acidity of H-ZSM-5 decreased from 0.105 to less than 0.014 mmol/g, the net contact time decreased probably due to the suppression of contact efficiency between molecules and acid sites so that side reactions were inhibited. H-ZSM-5 zeolites whose acidity was less than 0.014 mmol/g enhanced the selectivity for propylene up to the same level as SAPO-11.