汽车尾气脱除催化剂的表征 Yang Bo

Introduction The writer report the first study of the hydrolysis of tetraethyl ortho-silicate (TEOS) in an aqueous solution of N, N, N-trimethyl-1-adamantammonium (TMAda) hydroxide, the clear sol precursor for the preparation of the high-silica zeolite SSZ-13 (CHA). The initial stages of the hydrolysis of TEOS were monitored by quantitative Si and Al nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS). Si NMR allowed quantitative characterization of Si in nanoparticles and dissolved oligomers and measuring the average Si − O−Si connectivity. The writer try to elucidate the effect of the organocation TMAda + and the presence of Al on chemical composition, internal connectivity, and stability of precursor nanoparticles as well as soluble species.

Result and Discussion This has also been observed with the silicalite-1 and silicalite-2 systems when TPAOH and TBAOH are used as SDA, and thus, this seems to be a general pattern for alkylammonium-based silicate sols

Figure 2. (a) Si-distribution in TEOS ( ■ ), oligomers ( □ ), and nanoparticles ( ● ) during the hydrolysis of TEOS in TMAdaOH. (b) Comparison of the Si distribution for CHA,MEL, and MFI systems. TEOS decreases as a straight line similar to that in panel a. Si-distribution At this stage, the organic SDA acts as the hydroxide ion source adjusting the pH of the medium, and affecting the stability of the silicate species present in solut ion by shifting the hydrolysis, condensation, and precipitation equilibria.

Si-distribution In identifying the different Si species, we have employed the following chemical shift ranges: Q 0 = −71 to −72 ppm, Q 1 andQ 2Δ = − 79 to − 83 ppm, Q 2 and Q 3Δ = −86 to −91 ppm, Q 3 =− 92 to − 100 ppm, and Q 4 = −100 to −108 ppm. Figure 3. Si-distribution in (a) all oligomers and (c) nanoparticles during the hydrolysis of TEOS in TMAdaOH.

SAXS patterns

Hydrolysis of TEOS in the Aluminosilicate System Figure 7. Evolution of (a) 27 Al NMR spectra of clear sols/solutionswith the progress of TEOS hydrolysis. (b) Evolution of chemical shifts of q n Al sites with [Si]/[TMAdaOH] ratio. 77 to 73,72 to 67, 66 to 62, and 61 to 54 ppm, assigned, respectively, to q 0/ q 1 and q 2,q 3, and q 4

The role of the aluminum is to connect oligomers within the nanoparticles into core shell elements. As aluminum exhibits a lower activation energy than Si for metal oxygen bond breaking reformation, it catalyzes the core shell networking within the nanoparticles.

27 Al NMR The 27 Al spectra are only affected by the change in Si/TMAdaOH ratio and the connectivity of Si. This means that aluminate species behave like silicate species.

Transformation of LEV-type zeolite into less dense CHA- type zeolite Ikuhiro Goto, Masaya Itakura, Syohei Shibata, Koutaro Honda, Yusuke Ide, Masahiro Sadakane, Tsuneji Sano Microporous and Mesoporous Materials 158 (2012) 117–122

In general, medium/large pore size and high-silica zeolites are synthesized by hydrothermal treatment of amorphous aluminosilicate hydrogel as a starting material in the presence of organic structure- directing agents (OSDAs). The use of OSDAs is, however, undesirable from a practical point of view, because of their high cost as well as their large environmental impact. Hydrothermal conversion of LEV-type zeolite into CHA-type zeolite occurred in the absence of both anorganic structure-directing agent and a seed crystal. Introduction

Fig. 2. SEM images of (a) starting LEV, (b) CHA (Sample No. 1), (c) CHA (Sample No. 8), SEM Images Fig. 2 (b) and (c) show SEM images of the obtained CHA-type zeolites. The crystal morphology was cubic, and the crystals were 200–400 nm in size, which is smaller than the crystal size of the starting LEV-type zeolite, as shown in Fig. 2(a)

. When the hydrothermal conversion of LEV-type zeolite was carried out at 125–170 ℃ for 1.5 h, the LEV-type zeolite was transformed into CHA-type zeolite. At 200 ℃, pure ANA-type zeolite was ob- tained, suggesting that CHA-type zeolite transformed into the most stable zeolite. At 90 ℃, pure CHA zeolite was obtained when the synthesis time was prolonged to 12 h. The morphology and crystal size of the CHA-type zeolite crystals obtained at 90 ℃ were similarto those obtained at 125 ℃. Influence of the Synthesis Temperature

Influence of the Si/Al Ratio

LEV–CHA and LEV–LTA LEV- and CHA-type zeolites have similar composite building units because these two zeolites both belong to chabazite group, whereas there was less similarity between the composite units of LEV- and LTA-type zeolites. It was suggested that locally ordered aluminosilicate species (nanoparts) produced by decomposition/dissolution of the starting LEV-type zeolite contribute to the transformation process.

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