R. Aiello

Kinetic study on the nucleation of (Na,TPA)-ZSM-5 zeolite

The autocatalytic model of nucleation is used to study the crystallization of zeolite ZSM-5 in five systems differing in the Al content of the starting gel. The crystallization kinetics and the nucleation kinetics are examined by SEM and X-ray diffraction. Experimental evidence is presented for the validity of the autocatalytic nucleation model. The two kinds of nucleation (heterogeneous and autocatalytic) processes adequately explain all the crystallization curves.

Effect of hydroxide on growth rate and morphology in silicalite synthesis

Crystallization kinetics and crystal morphology were determined for silicalite crystallized from two similar reaction batch mixtures. The batch compositions studied were xNa 2 O + 8 TPABr + 100 SiO 2 + 1000 H 2 O and xTPA 2 O + (8 − 2x)TPABr + 100 SiO 2 + 1000 H 2 O, where x was varied from 0.5 to 4.0. For the first mixture, sodium hydroxide was used as the base, while for the second reaction mixture, TPAOH was the alkali source. Reactions were carried out at 170°C in unstirred, Teflon-lined autoclaves. For all runs, silicalite was the only phase found to crystallize. Crystal morphology of silicalite produced from batches with different hydroxide contents were found to differ considerably. As the alkalinity of the reaction mixture was reduced, the aspect ratio (length/width) of the crystals increased from 0.9 for x = 4 to 6.7 for x = 0.5. For synthesis made without sodium, both nucleation and crystallization occurred more rapidly. From these results, some insights into the crystallization mechanism were proposed.

Zeolite crystallization from high-silica mono- or bicationic alkali (Li, Na or K) gels in presence and in absence of TPA ions

Abstract The synthesis of zeolites from high-silica systems in the presence of alkali cations such as lithium, sodium and potassium, with and without TPABr, has been systematically studied, contributing to the understanding of the specific role played by inorganic and organic cations on the synthesis of zeolite ZSM-5.

Synthesis of ZSM-5 zeolite from very dense systems: Formation of pelleted ZSM-5 zeolite from (Na, Li, TPA, Si, Al) hydrogels

(Li,Na)-, Na-, and (Na,K)ZSM-5 zeolites are readily prepared in hydrothermal conditions at 170 + 2 °C from highly dense gels of compositions aM 2 O b Na 2 O 2 b TPABr b Al 2 O 3 150 SiO 2 490 H 2 O with M = Li, Na, or K; 0.9 ⩽ a ⩽ 8.82 and 1.66 ⩽ b 15. In a quite narrow domain of composition, (Li,Na)ZSM-5 self-bonded pellets are formed from the same hydrogel with 8 ⩽ a ⩽ 9 and 14 ⩽ b ⩽ 16. The ca. 4 μm ZSM-5 crystallites are embedded in the remaining amorphous gel formed essentially by LiAlO 2 . The self-bonded pellets are also tested for attrition, pore-size distribution, and catalytic activity.

Synthesis of zeolite beta in presence of fluorides: Influence of alkali cations

The influence of alkali cations on the synthesis of zeolite Beta is systematically investigated in fluorine-containing media using the seeding method. Both initial gels and final crystalline samples were thoroughly characterized using XRD, chemical analysis, thermal analysis, and multi- n.m.r. ( 29 Si, 27 Al, and 13 C). The best crystalline samples were obtained after 15 days of crystallization at 170°C from gels of the following composition: 12.5 DABCO : 12.5 MA : x HF : y MF : z Al 2 O 3 : 25 SiO 2 : 500 H 2 O with x + y = 25 and M =NH 4 , Li, Na, K, and Cs. At high cation content ( y = 12.5), only NH 4 , Li, and Cs led to zeolite Beta. In the presence of Na and K fluorides, the amount of MF has to be decreased to synthesize pure zeolite Beta. From the analysis of both the induction and crystallization rates it was concluded that one of the main roles of the alkali cations is their electrostatic stabilization of the gel.

The influence of alkali cations on the synthesis of ZSM-5 in fluoride medium

Abstract The role of NH + 4 , Na + , K + and Cs + cations was investigated in the synthesis of ZSM-5 using fluoride as mineralizing agent. The amounts of aluminum and alkali cations were varied in gels of initial composition 10SiO 2 – x MF– y Al(OH) 3 –1.25TPABr–330H 2 O with M=NH 4 , Na, K and Cs, x =9, 15 and 24, y =0.16, 0.5 and 1.0 and TPABr=tetrapropylammonium bromide. The potassium-containing system is the most effective one with respect to incorporation of aluminum into the zeolite framework, while the ammonium-containing system is the least effective one. Crystallization patterns and crystal morphologies are strongly influenced by the nature of the fluoride salt added to the reaction mixture. The kinetic data show that the cations not only influence the reaction rates by their electrostatic interaction, but also due to their specific interaction with the fluoride anion in various siliceous and aluminosiliceous complexes.

Synthesis and characterization of Fe- and (Fe,Al )-MCM-22 zeolites

Abstract The synthesis of Fe-MCM-22 zeolite is reported for the first time. Its structure and characteristics are similar to those of the MCM-22 zeolite. The aluminum is in a framework tetrahedral position in the [Fe,Al]-MCM-22 samples. The white color of the samples, the increase of the 29 Si and 27 Al NMR linewidths with increasing Fe content and the constancy of the (Fe+Al)/u.c. values all suggest that Fe is also incorporated in tetrahedral position of the framework.

Formation of MFI crystalline zeosilites from fluoride-containing silicate gels

Abstract Silicalite-1 samples are readily prepared at 170°C from F− containing gels of composition a HF-b NaF-cNaCI-d TPABr-e TPAOH-10 SiO2-330 H2O with a + b = 1 (constant F− content) and d + e = 1.25 (constant TPA content). Large crystals of ca. 450 μm were found in some of the systems. The rate of length increase was proportional to the pH of the initial gels. For pH ≤ 2, no crystallization occurred. The aspect ratio (length width) of the crystals was found to increase as with crystal growth. Some 2 F/u.c. are incorporated into the products.

Synthesis of ZSM-5 Type Zeolites in the System (Na,K)2O-A12O3-SiO2-H2O Without and With Tpabr

The synthesis of zeolite ZSM-5 from Na,K cationic systems, both in the absence and in the presence of TPABr, has been systematically studied and the role played by alkaline and organic ions in the synthesis of this zeolite discussed.

Use of natural products for zeolite synthesis. V. Self-bonded zeolite pellets from rhyolitic pumice

A method for preparing self-bonded zeolite pellets by hydrothermal treatment of preformed pumice-alkali-water mixtures is described. Zeolite crystallization in this particular system is discussed. Formation of these crystals, together with a gel-like phase, is shown to be responsible for the hardening of the pellets. Basic characterization of self-bonded pellets, including attrition resistance and water vapour diffusion, has been performed in comparison with commercial zeolite pellets.