Molecular Sieves - Science and Technology - Vol. 6 - Characterization II / 06-Isomorphous Substitution in Zeolites

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.al et Nagy.B .J

2.1.2 [Ga]-MFI

The initial gels have a general composition of 10SiO2 – xMF – yGa(NO3)3 – 1.25TPABr – 330H2O where M = NH4, Na, K or Cs with x = 9, 18 and y = 0.1 and 0.3. The reactants were mixed in the following order: MF (M = NH4, Na, K, Carlo Erba and M = Cs, Sigma), distilled water, Ga(NO3)3 (Aldrich), tetrapropylammonium bromide (TPABr, Fluka) and fumed silica (Sigma) [183, 184].

2.1.3 [Fe]-MFI

2.1.3.1 Fluoride Route

The reactants were ammonium fluoride (Carlo Erba, RPE), sodium fluoride (Carlo Erba, RPE), potassium fluoride (Carlo Erba, RPE), cesium fluoride (Aldrich, purum), tetrapropyl ammonium bromide (TPABr, Fluka, purum), iron nitrate (Fe(NO3)3 · 9H2O, Merck, purum), fumed silica (Sigma) and distilled water [185, 186]. The molar composition of the starting mixture was:

10SiO2 – xFe(NO3)3 · 9H2O – yMF – 1.25TPABr – 300H2O

where 0.1 ≤ x ≤ 0.3, y = 3 – 24 and M = NH4, Na, K or Cs.

The starting mixtures were prepared by adding the fluoride salt, iron nitrate, TPABr and the fumed silica to the distilled water in this order. After complete homogenization the resulting gels were placed in PTFE-lined 25 cm3 stainless-steel autoclaves. The samples were obtained by hydrothermal synthesis at 170 ◦C for predetermined times. After quenching the autoclaves, the products were recovered, filtered, washed with distilled water and dried at 80 ◦C overnight. The crystallization time was ca. 7 days for the sample with the smallest crystallization rate.

2.1.3.2 Alkaline Route

Batch composition: 1.0SiO2 – (xAl2O3 + yFe2O3) – 0.13Na2O – 52.0H2O – 0.125 template for [Fe,Al]-ZSM-5: x = 0.0025; y = 0.0025, 0.005, 0.01 and 0.002, for [Al]-ZSM-5: x = 0.01; y = 0, for [Fe]-ZSM-5: x = 0; y = 0.01 and 0.01333 [187–192].

Source materials: distilled water; waterglass (Aldrich, vide supra); colloidal silica (Aldrich, Ludox 40); iron(III) oxide (Aldrich, 99.98%); iron(III) oxide enriched in 57Fe to 80%; iron(III) nitrate nonahydrate (Aldrich > 98%); aluminum powder (Aldrich, 20 micron, > 99%); aluminum oxide trihydrate

(Hungarian origin); sodium hydroxide (Reanal, reagent grade); TPA bromide as template (Aldrich, 98%); perchloric acid, 70% (Aldrich, reagent grade); sodium perchlorate (Aldrich, 99%); hydrochloric acid 37% (Reanal, reagent grade); sulphuric acid 97% (Reanal, reagent grade).

Batch preparation recipe (for about 12–14 g volatile-free zeolite): Prepare stock-solutions of iron (dissolve known amount of iron (III) oxide, or iron tracer in concentrated HCl at 120 ◦C in a closed Teflon autoclave; or dissolve iron(III) nitrate in acidified water) and aluminum (dissolve aluminum powder in HCl diluted to 1 : 1). Save the Feand Al-contents, respectively, per gram of solution.

In order to simplify the description of recipes the exact amounts for the variable components will be summarized in Table 4 for various Si/Fe, etc., ratios.

Sixteen grams Aldrich-type waterglass (see Aldrich catalogue) + 140.9 g distilled water; stir and keep it in refrigerator overnight (solution 2). Blend appropriate amounts of stock-solution for iron (or aluminum, or iron and aluminum) with 5.8 g of 70% perchloric acid; keep in refrigerator overnight (solution 3). Under vigorous mixing add to (solution 3) so much of (solution 2) to reach pH = 4.5 (use pH paper). Admix to remaining (solution 2) 19.2 g cool Ludox 40 and continue the neutralization of (solution 3) while maintaining agitation. The meanwhile solidified gel should be broken up. Add 6.7 g TPA bromide, 2 g sodium chloride as solid and 0.2 g ZSM-5 seed (the best choice is silicalite-1 in the as-synthesized (AS) state, ground carefully in a porcelain mortar using a few drops of water).

The pH of the slurry should be between 10.5 and 11.0. Adjust the pH by either a few drops of perchloric acid or 50% sodium hydroxide. In a stainlesssteel shaker, mill the slurry for at least 5 h using similar balls. Check (and adjust if necessary) the pH.

Crystallization vessel: Teflon-lined autoclave(s); temperature: 160 ◦C; time: depending on the degree of substitution 6–12 h, without agitation. Product recovery: after cooling, filter and wash with warm water (to pH = 10.5). Dry

Table 4 Amounts (g) of Al and/or Fe per batch at various slurry compositions

in air thermostat at 110 ◦C. Save the zeolite product as such or apply “caring calcination” [193]. Besides the zeolitic crystals, the cool mother liquor contains 2–3 cm-long needle-like crystals of sparingly soluble TPA perchlorate. It is easy to separate them by a tea-strainer. Another method of preparation was used in the alkaline route [188].

Two different hydrogel systems were studies. The first system was prepared from the following initial reaction mixture:

(A) xNa2O–yTPABr–zAl2O3 –SiO2 –qFe2O3 –pHA–20H2O,

where x = 0.1 – 0.32; y = 0.02 – 0.08; z = 0 – 0.05; q = 0.005 – 0.025; the ratio p/q = 3 and HA stands for H2C2O4 or H3PO4.

The second system was prepared from the following initial reaction mixture:

(B) 0.16Na2O–xTPABr–yEG–zAl2O3 –SiO2 –qFe2O3 –pHA–10H2O,

where x = 0 – 0.08, y = 0 – 6.0; z = 0 – 0.05; q = 0.005 – 0.1; the ratio p/q = 3 and HA stays for H2C2O4 or H3PO4.

2.1.4 [Fe]-BEA

The following reagents were used: 20% tetraethylammonium hydroxide (TEAOH), in H2O (Aldrich), 25% TEAOH, in methanol (Aldrich), tetraethyl orthosilicate (TEOS, Aldrich) as silicon source, Fe(NO3)3 · 9H2O (Merck), Al(OH)3 (Pfaltz & Bauer), NaOH (Carlo Erba) and ultradistilled water [194]. The following general composition was used for [Fe]-BEA:

40SiO2 – xFe(NO3)3 · 9H2O – yTEAOH – 4NaOH – 676H2O, with x = 0.38, 0.40, 0.44, 0.45, 0.49, 0.51, 0.60, 1.02, and y = 10.88, 13.6, 16.3 and 19.04.

The experimental procedure was carried out as follows: solution A was prepared by adding TEOS to aqueous solution of 20% TEAOH under magnetic stirring during 1 h. Solution B was prepared by adding NaOH to 20% aqueous TEAOH solution under magnetic stirring: Solution C was obtained by introducing Fe(NO3)3 · 9H2O into ultradistilled water under magnetic stirring during 10 min.

Solution A is added dropwise to solution C. It is important to fully dissolve solution A in solution C. The obtained solution is solution D. Finally, solution D is added slowly to solution B under magnetic stirring leading to a light yellow gel: The system is stirred for 24 h. This allowed the ethanol formed during the hydrolysis of TEOS to evaporate.

2.1.5 [Fe]-MOR

The zeolite samples [Al]-BEA and [Fe,Al]-BEA were also prepared starting from the following molar composition [195]:

(B) xNa2O–0.2TEAOH–yAl2O3 –zFe2O3 –wC2H2O4 –SiO2 –20H2O,

where TEAOH stands for tetraethylammonium hydroxide, C2H2O4 is oxalic acid; 0.1 ≤ x ≤ 0.15; 0.015 ≤ y ≤ 0.02; 0 ≤ z ≤ 0.015, w/z was constantly equal to 3.

The zeolite samples of [Al]-MOR and [Fe,Al]-MOR were prepared starting from the following molar composition:

(A) xNa2O–0.2TEABr–yAl2O3 –zFe2O3 –wC2H2O4 –SiO2 –20H2O,

where TEABr stands for tetraethylammonium bromide, C2H2OO4 is oxalic acid; 0.1 ≤ x ≤ 0.27; 0.015 ≤ y ≤ 0.025; 0 ≤ z ≤ 0.025, w/z was constantly equal to 3.

The hydrogels were obtained by adding precipitated silica (BDH) to a previously prepared homogeneous solution consisting of the organic compound (Fluka), sodium hydroxide (Carlo Erba), sodium aluminate (Carlo Erba) and distilled water. In another beaker, a solution of an iron complex with oxalic acid was prepared, starting from iron nitrate (Carlo Erba) and oxalic acid (Carlo Erba). This solution was slowly added to the hydrogel, which, after 1 h of homogenization, was transferred into autoclaves.

2.1.6 [Co]-MFI

Two different gels were prepared [196, 197]. For the first series of gels (A), sodium silicate was used and the final batch compositions were: 35SiO2 – xNa2O – yCo(CH3COO)2 · 4H2O – 3.4TPABr – 8.4H2OSO4 – 808H2O with x = 11, 12.9 and 15 and y = 0.5, 1, 1.5 and 2. The second type of gels (B) was prepared using colloidal silica with a global composition: 35SiO2 – xNa2O – Co(CH3COO)2 – 3.4TPABr – 808H2O with x = 0, 3, 6, 9 and 12. The gels, after complete homogenization, were placed in PTFE-lined 25 cm3 stainless-steel autoclaves. The Co-containing samples were obtained by hydrothermal synthesis at 170 ◦C after 2 days. After quenching the autoclaves, the products were recovered, filtered, washed with distilled water and finally dried at 80 ◦C overnight. The samples were characterized by various physico-chemical techniques such as XRD, chemical and thermal analysis, XPS and diffuse reflectance UV-visible spectroscopy.