Gel aging, in zeolite crystallization

KATOVIC ETAL. Role of Gel Aging in Zeolite Crystallization... 

Subotic B (1989) Influence of autocatalytic nucleation on zeolite crystallization processes. In OcceUi ML, Robson HE (eds) Zeolite synthesis. American Chemical Society, Washington DC, p 110, and Katovic A, Subotic B, Smit I, Despotovic LjA, Curie M (1989) Role of gel aging in zeolite crystallization. Ibid, p 124... 

Zeolites. A large and growing industrial use of aluminum hydroxide and sodium alurninate is the manufacture of synthetic zeoHtes (see Molecular sieves). ZeoHtes are aluminosiHcates with Si/Al ratios between 1 and infinity. There are 40 natural, and over 100 synthetic, zeoHtes. AH the synthetic stmctures are made by relatively low (100—150°C) temperature, high pH hydrothermal synthesis. For example the manufacture of the industriaHy important zeoHtes A, X, and Y is generaHy carried out by mixing sodium alurninate and sodium sHicate solutions to form a sodium alurninosiHcate gel. Gel-aging under hydrothermal conditions crystallizes the final product. In special cases, a small amount of seed crystal is used to control the synthesis. 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

The analysis of the kinetics of crystallization of different types of zeolites from aluminosilicate gels points to the conclusion that the crystallization takes place by the simultaneous growth of the constant number N0 of nuclei-I present in the system at the very start of the crystallization process and the number Na of nuclei-II released from the gel disolved during the crystallization process. Some characteristics of the crystallization systems such as the duration of the "induction period", the shortening of the "induction period" and the increase of the crystallization rate, respectively, with the gel ageing and the bimodal size distributions in the specific cases have been discussed and explained in relation to the ratio Na/N0 of particles (nuclei)-II and particles (nuclei)-I present in the crystallizing systems. 

It is well known that the low-temperature ageing of aluminosilicate gel precursor markedly influences the course of zeolite crystallization at the appropriate temperature (1-10). The primary effects of the gel ageing are the shortening of the induction period and the acceleration of the crystallization process (1-5), but in some cases the gel ageing also influences the type(s) of zeolite(s) formed (1,6,7,10). 

Thus, in many syntheses the gel ageing (8-11) or the addition of the "crystal direction agent" (aged, X-ray amorphous aluminosilicate gel) (7,12-14) is a necessary step needed for the obtaining of the desired type of zeolite at the desired reaction rate. 

Figure 1 shows the kinetics of the crystallization of zeolite X (Figure 1A) and zeolite Na-Pc (Figure 1B), respectively, at 80°C from the gels aged at 25°C for 1, 3, 5, 7 and 10 days. In all cases, zeolite X appears as the first crystalline phase, thereafter zeolite Na-Pc co-crystallizes with zeolite X. After the maximal yield of zeolite X crystallized has been attained, the fraction f of zeolite X slowly decreases as the consequence of the spontaneous transformation of zeolite X into more stable zeolite Na-Pc (17). 

Figure 2. The changes in A. mass fractions of zeolite X (o), fpc of zeolite Na-Pc ( ) and fg of gel (A), B. concentrations C [(L) of aluminum ( ) and Cgj(L) of silicon (o) in the liquid phase and C. molar ratio [ Si/Al]s in the solid phase (o), with the crystallization time tc, during the crystallization from the gel aged for ta = 1 d.

Similar process of the increase in the number of nuclei-I during the gel ageing was observed in zeolite A crystallizing system (5). 

Na = rtf ia the number of particles-II (formed by the growth of the particles of quasicrystalline phase released from the gel during the crystallization process), both contained in a unit mass of zeolite formed at the end of the crystallization process, Kg is the constant of the linear growth rate of zeolite particles and 0 = 6/(q+1)(q+2) (q+3). The numerical values of the constants K0 = G N0Kg and Ka = G BNaK, calculated by the procedure described earlier (25), as well as the ratios Na/ N0 = Ka/0 K0, are listed in Table IV. as functions of time ta of the gel ageing. 

The increase in the yield of zeolite X crystallized, with the gel ageing, is the consequence of the increase in the total number of nuclei of zeolite X at constant, or even decreasing total number of zeolite P, during the gel ageing. 

The primary reagents are combined in an OH medium and the gel, so-formed, held at defined temperatures and pressures for the crystallization period (Figure 16). Gel ageing prior to this crystallization time can be needed. More recent syntheses have shown that high-silica zeolites prepared with a F mineralizer are almost fault-free, unlike those prepared in OH media. 

---