Page 120 - Principles of Catalyst Development
P. 120
108 CHAPTER 6
compounds. This is because the defect spinel structure of y-AI 20]
(6.4 )
favors solid solution with transition oxides such as FeO, CoO, NiO, and
CuO. Cations, M2+, do not necessarily occupy the octahedral vacancies but
migrate to positions with higher stabilization energy. Thus we tind
AI[AIFe]04, Co[AI 2]04, AI[AINij04, and Cu[AI2J04' The cations occupy
stable positions and are difficult to reduce. These same procedures are used
to prepare catalysts for low temperature shift and alcohol synthesis reactions,
CuO-ZnO-AI 20,. In this case, mixed gels of CuO-AI 20J are easily precipi-
tated, with ZnO added at this or some later point. (166)
Coprecipitated oxides are the precursors to metal catalysts in Table
6.1 that are produced by extraction.
6.3.3. Zeolites
Zeolite synthesis was first attempted by duplicating the hydrothermal
genesis of naturally occurring minerals. Mostly these were unsuccessful.
Breakthroughs came with the realization that cogels of aluminosilicates,
like those in SiOrAI,Ol preparations, are depolymerized by OH- ions to
yield crystalline nucleii. Depending upon composition, temperature, press-
ure, and pH, these nuclcii develop into a vast number of zeoli tic types.( 11'.136)
With correct conditions for supersaturation, crystalline seeds grow into
microcrystalline powders of appropriate material. (351 For example, Faujasite
Y is made by a procedure similar to that for silica-alumina. Sodium silicate
and sodium aluminate solutions in the correct proportions (SiO l / AI 20 l =
3-30) are mixed, not with an acid, but with sodium hydroxide at a pH of
12. Finely divided silica is added as a seeding agent, and the temperature
raised from 100 to 400°C, with pressure necessary at higher temperatures.
Nucleation and crystallization commence at rates determined by the tem-
perature and pressure. This digestation continues until the reaction is
complete, taking a period of time that may vary from hours to days.,lb71
Other zeolites are made in similar ways, with essential differences in starting
conditions and compositions. I 1381
6.4. DEPOSITION OF THE ACTIVE COMPONENTS
Dispersion of oxides on high-area supports is carried out by one of
four methods: (J) precipitation, (2) adsorption, (3) ion exchange, and (4)
impregnation. Each technique has advantages and disadvantages. Often
preference for one method over another is a matter of compromise. The
supports are either in powder or particle form. Depositing active components