DD261747A1 - PROCESS FOR PREPARING ALUMINUM-FREE PENTASIL ZEOLITHES AS CATION EXCHANGERS - Google Patents

Abstract

The Erfndung relates to a process for the preparation of aluminum-free, ZSM-5 similar Pentasilzeolithen as an ion exchanger for mono- or polyvalent cations. Due to their performance properties, such as temperature stability up to 1 100C, Saureestabilitaet and radiation resistance they are suitable for Ionenaustauschvorgaenge under complicated conditions. The essence of the invention is that crystals are generated from a gel with silica sol and tetrapropylammonium bromide by shaking in an autoclave, which are treated with hydrochloric acid.

Description

Field of application of the invention

The invention relates to a process for the preparation of aluminum-free, ZSM-5-like pentasil zeolites as ion exchangers for mono- and polyvalent cations. In contrast to the inorganic ion exchangers of the faujasite type in the organic exchange resins, the aluminum-free pentasils have such outstanding performance properties such as temperature resistance up to 1100 ⁰ C, acid stability against concentrated mineral acids such. B. 5n HNO3 and radiation resistance. The aluminum-free pentasilizeolites can therefore be used advantageously for ion exchange processes under these conditions. One application is the removal of radioactive metal ions.

Characteristic of the known state of the art

Main field of application for inorganic ion exchangers, in particular the zeolites, was water softening. In the field of water treatment, however, the zeolitic exchangers are today almost completely replaced by organic exchange resins. This also applies to their use in the food and beverage industry, since it is possible to produce polymerization resins without taste-impairing substances.

A renaissance experienced synthetic inorganic ion exchangers on the detergent sector. Zeolites of the NaA and NaX type are used here to a large extent as cation exchangers in detergents (eg DE 3,301,577, EP 11,963, US 4,438,012). There is a Na ⁺ -Ca ⁺⁺ cation exchange to soften the wash liquor.

Despite their comparatively high exchange capacities, A and X zeolites have found little use as ion exchangers beyond their use as detergent additives. There are the following causes:

a) A- and X-zeolites, like all other inorganic cation exchangers, tend to disintegrate, both in acidic waters (pH <6.5) and in alkaline waters (pH> 8.5) and in waters with a low silica or salt content with release of silica.

b) A and X zeolites are not temperature resistant, at temperatures above 600 ⁰ C, the crystal structure is destroyed, it forms new phases and amorphous products, quartz, Christobalite or nepheline.

c) Faujasites, ie zeolites of type A and X, with an Si / Al ratio <1.7 can not be converted to a stable Η form by H ⁺ or NHj exchange. Therefore, in the application as a cation exchanger only metal ions can be exchanged.

d) In contrast to the polymeric exchange resins, the zeolite type synthetic molecular sieves already have an increased hardness due to their crystal structure. However, this is still insufficient for many applications. With increasing SiO ₂ content, the strength increases further, but the exchange capacity decreases.

Although the disadvantages mentioned could be overcome by using highly silicic zeolites with a Si / Al ratio which would have to be more than 100, such virtually aluminum-free zeolites have hardly any exchange equivalents (see Table 1), since these are normally used in the production of Zeolites are closely related to the aluminum content. The reason for this is that in zeolitic exchangers the crystal structure carries a negative framework charge to Si lattice sites through the incorporation of trivalent metal atoms, especially Al ³⁺ , which is compensated by counterions such as K ⁺ , Na ⁺ or Ca ²⁺ .

Table 1 Exchange capacities of selected zeolites at 27 ⁰ C and pH = 7 for Na ⁺ ions.

zeolite Si / Al ratio exchange capacity meq / g Zeolite A 1.0 7.0 Zeolite X 1.3 6.4 Natrolith 1.0 6.1 Analcim 2.0 5.2 Chabasit 2.0 5.0 Zeolite Y 2.4 4.4 Zeolite L 3.0 3.8 Heulandith 3.5 3.3 clinoptilolite 5.0 2.6

The ion exchange properties of the zeolites are used in quite a different sense to modify zeolites for use as adsorbents and catalysts. So z. For example, in the case of zeolite A, the replacement of the sodium ions by the large volume potassium cations reduces the effective pore diameters and the zeolite X becomes a selective adsorbent (e.g., GB 1,574,915, DE 3,035,482). On the other hand, by exchanging the sodium ions for calcium ions, the pore diameter of A zeolites can be increased, since the double-positive charge of the calcium makes certain cation positions in the access pores of the faujasites unoccupied (DD 140,450, US 4,354,929). Such pore changes form the basis for the technical use of zeolites as selective adsorbents. Even as catalysts, zeolites are used almost exclusively after prior ion exchange. For the catalytic activity in particular the acidic centers and the electrostatic field inside the pores are responsible. The latter can be influenced and controlled by the exchange of differently charged ions (eg US 4,329,531, US 4,326,947). In addition, platinum or palladium complexes in ionic form are readily exchanged into the zeolite and reduced in situ to the hydrogenation-active metals (e.g., US 4,276,151, GB 2,042,490).

Object of the invention

The aim of the invention is to develop a new or improved process for the synthesis of a zeolitic ion exchanger, which is acid stable and temperature resistant and has a reasonable exchange capacity. In question come only aluminum-free zeolites. The sites required for ion exchange can be created by the targeted generation of structural defects in the form of internal silanol groups. These silanol groups must have exchange capacity.

Explanation of the essence of the invention

The object of the invention is to produce an aluminum-free ZSM-5-like zeolite of the pentasil type, which has a high concentration of internal silanol groups as cation exchange centers. According to the invention, the aluminum-free, ZSM-5-like zeolites of the pentasil type having a high concentration of internal silanol groups are obtained by crystallization from a source gel of a given chemical composition by continuous shaking of the autoclave during the crystallization process with shaking frequencies between 0.01 Hz and 0.5 Hz , For the preparation of the starting gel, use is made of a silica sol solution (20% to 50% SiO ₂ ) and a 0.5m to 2 m solution of tetrapropylammonium bromide in water. With the aim of saving this expensive tetrapropylammonium salt, a comparatively high ratio of SiO ₂ to tetrapropyiammonium bromide of from 10 to 50 can be selected. With 15 to 35 H ₂ O per SiO ₂ , the starting gel is a relatively low-condensed phase. The starting gel is stirred for 10 minutes and filled into autoclaves. The crystallization takes place at temperatures between 140 ⁰ C and 210 ⁰ C, preferably at 170 ⁰ C, in crystallization times between 1 and 8 h, preferably 5 h, the autoclave continuously with frequencies between 0.01 Hz and 0.5 Hz, preferably 0.2Hz, shaking. The crystalline product obtained is washed, 6h calcined at 500 ⁰ C in air, and finally by two times the H ⁺ - ion exchange transferred with 0.5M HCI in excess at 27 ⁰ C in the active for the ion exchange Η-shape. As a result of the X-ray determination of the crystallinity, in each case crystallinities exceed 93%. For this purpose, the Cu-K _a radiation is used; according to the summation method, the peak intensities in the diffractogram between 20 = 26 ° and 20 = 29 ° are evaluated and compared with standard samples. The 29 Si solid state nuclear resonance gives a typical spectrum for pentasils with numerous structural defects. With the high-resolution proton solid-state nuclear resonance, the ¹ H-MAS NMR, the internal silanol groups are determined quantitatively in the zeolite prepared according to the invention. It is found the surprisingly high number of up to 15 x 10 ²⁰ silanol groups prog. These silanol groups (^ Si-OH) are located at defect centers in the interior of the zeolite, where the siloxane bonds (= Si-O-Si =) were not built up in a controlled manner by the production process according to the invention. In the treatment of the Η-form of the pentasil zeolite with sodium hydroxide solution, these silanol groups surprisingly showed ion-exchange properties: = Si-OH + NaOH - * = Si-ONa + H ₂ O. In the opposite case, the Na form of the zeolite can be mixed with HCl in the Η- Be recycled form: = Si-ONa + HCI -> = Si-OH + NaCl.

embodiments example 1

From a silica solution (30% SiO ₂ ), a starting gel of the composition 70SiO ₂ .12.5Na ₂ O .3.5TPABr. 1 650H ₂ O is formed. This will be after 10min. Stirring in autoclave filled. Crystallization is carried out at 170 ⁰ C with constant shaking of the autoclave at a frequency of 0.2 Hz. After 5 h, the crystalline zeolite material is removed, washed several times with distilled water, calcined at 500 ⁰ C for 6 h and transferred by ion exchange with O, 5n HCI in excess at 27 ° C in the ion exchange active Η-form. X-ray diffractograms and ²⁹ Si NMR characterize the resulting material as an aluminum-free ZSM-5-like pentasil zeolite of orthorhombic symmetry with numerous structural defects. The crystal size is 13pm. With X-ray methods no amorphous parts can be detected. The ¹ H-MAS NMR allows the statement that a total of 5 x 10 ²⁰ silanol groups per g are present.

Number of silanol groups / 10 ²⁰ g ~ ¹ calcination of the zeolites at 550 ⁰ C, 6 h 2.2

Preparation of the calcined form H ⁺ form with 0.1 mHCl at 27 "C, 6 h 5.0

Preparation of the Na ⁺ Form by Exchanging the H ⁺ Form

0.1M NaOH at 27 ⁰ C, 6 h 2.0

Reverting the Na ⁺ - FormindieH ⁺ Form to 0.1 mHCl

27 ° C, 6h 5.0

Example 2

An aluminum-free pentasil zeolite prepared according to the invention has 8.5.10 ²⁰ silanol groups per g. This Pentasilzeolith is introduced at 95 ° Cfürfor 5h in unpressed, ie powder form, in 0.1 η CsOH, the suspension is stirred. From the decrease of the Cs concentration in the solution and its increase in the zeolite (chemical digestion), it can be concluded that the zeolite after the exchange contains 4.2 χ 10 ²⁰ Cs prog. By treatment of the zeolite with 0.5 η HCl, the Cs is extracted, while the zeolite retains its original exchange capacity in the form of 8.5 χ 10 ²⁰ silanol groups per g and is ready for the next ion exchange cycle.

Claims (3)

1. A process for preparing aluminum-free pentasil zeolites as cation exchangers, characterized in that from a silica sol solution and a solution of tetrapropylammonium bromide containing gel shaking in an autoclave with shaking frequencies between 0.01 Hz and 0.50 Hz, preferably 0.2 Hz, in crystallization times between 1h and 8h, preferably in 5h, at temperatures between 14O ⁰ C and 200 ⁰ C, preferably 170 ⁰ C, generated crystals with structural defects, washed the resulting crystalline products, calcined in air and by treatment with hydrochloric acid in crystals with silanol groups be transferred. 2. The method according to claim 1, characterized in that a gel is used which contains a silica sol solution with 20% to 50%, preferably 30%, SiO ₂ and a 0.5 m to 2.5 m solution of tetrapropylammonium bromide in water wherein the ratio of SiO ₂ to tetrapropylammonium bromide is between 10 and 50, preferably about 20, and the ratio of water to SiO _{2 is} between 15 and 35, preferably about 25. 3. The method according to claim 1 and 2, characterized in that the washed crystalline product is 6h calcined at 500 ⁰ C in air and converted by treatment with 0.5m hydrochloric acid in excess at 27 ⁰ C in crystals with silanol groups.