PROCESS FOR PREPARING MORDENITE

Abstract: A process for preparing mordenite having in its anhydrous condi­ tion the formula: Me₂O.Al₂O₃. x SiO₂ wherein Me is an alkali metal and x is between 10 and 20 is performed by contacting in an aqueous medium sodium silicate with an aluminium compound, the molar ratio of SiO₂ to Al₂O₃ being from 10 to 200, in the presence of at least one alkali metal hydroxide, the molar ratio of Me⁺ to SiO₂ being higher than 0,6, in the presence of a mineralizing agent, at a tempera­ ture between 20°C and 100°C, checking that the molar ratio OH⁻ to SiO₂ of the resultant mixture is comprised between 0,2 and 0,8, and crystallizing the mixture for a time from a few hours to 40 days at a temperature between 100°C and 200°C.

Description: This invention relates to a process for producing mordenite, which is a porous crystalline zeolite, and is a division of EP-A-98 641. Generally speaking, zeolites having a high SiO₂ to Al₂O₃ ratio of 10 and over are characterized by a high thermal stability, which makes it possible to use such zeolites for a number of practical applications. These zeolites are characterized by structural sub-units consisting of 5-tetrahedron rings: among these, the zeolites of the mordenite family are notable, and they encompass ferrierite, dachiardite, epistilibite and bikitaite. A survey of the prior art shows that US-A 4 175 114 discloses that a mixture having a molar ratio of OH⁻ to SiO₂ between 0,05 and 0,3 produces ZSM-5 zeolites rather than mordenite, the more so that it could be presumed, on the basis of the teachings of EP-A-40104 that the OH:SiO₂ ratio may vary between 0,2 and 0,8. The teachings of these two prior art disclosures are thus mutually conflicting, as table C of US-A 4 175 114 clearly establishes that, to obtain ZSM-5 and not mordenite, the OH⁻/SiO₂ ratio may vary between 0,1 and 0,8, a still preferred range lying between 0,2 and 0,75. Therefore, the technical teaching of US-A 4 175 114 is that the OH⁻:SiO₂ ratio, in EP-A-40104, which is adapted to produce mordenite, must be outside the range between 0,07 and 1, or, at the most, outside the range between 0,2 and 0,75. The EP-A-30811 proposes to synthesize zeolites with a silica: alumina ratio greater than 12 without using organic compounds such as alcohols, tetraalkylammonium ions or alkylene oxides. US-A-4 257 885 describes a synthetic zeolite prepared in the absence of organic substances or ammoniacal substances, by using colloidal silica as the silicon source. It has been found that, by controlling certain critical factors, such as the SiO₂:Al₂O₃ ratio and the molar ratio OH⁻:SiO₂ of the hydroxyl concentration (determined by titration) to the silica, it is possible to prepare large-pore mordenites of a high silica content. Therefore, according to the invention, there is provided a process for the preparation of mordenite having in its anhydrous condition the formula: Me₂O.Al₂O₃. x SiO₂ wherein Me is an alkali metal and x is comprised between 10 and 20, characterized by comprising the steps of contacting in an aqueous medium sodium silicate with an aluminium compound, with a molar ratio SiO₂ to Al₂O₃ comprised between 10 and 200, in the presence of one or more alkali metal hydroxides, with a molar ratio Me⁺:SiO₂ higher than 0,6, and also in the presence of a mineralizing agent, working at a temperature comprised between 20°C and 100°C, checking that the molar ratio OH⁻:SiO₂ of the resultant mixture is comprised between 0,2 and 0,8, said resultant mixture being caused to crystallize with said hydroxyl to SiO₂ ratio in an enclosure for a period of from a few hours to 40 days at a temperature comprised between 100°C and 200°C. Preferably, the molar ratio Me⁺:SiO₂ is comprised between 0,66 and 0,96. A still preferable combination of conditions is that in which the molar ratio SiO₂:Al₂O₃ is between 15 and 200 and the molar ratio OH⁻:SiO₂ is comprised between 0,2 and 0,50. The crystallization temperature is preferably comprised between 150°C and 180°C. The mineralizing agents are preferably selected among sodium sulphate, potassium sulphate, sodium phosphate, potassium prosphate, sodium chloride and potassium chloride. The alkali metal hydroxides are selected from among LiOH, NaOH and KOH. Finally, the aluminium compound is selected from among sodium aluminate, aluminium sulphate, aluminium chloride and trihydrous alumina. It is actually surprising that, by varying the OH⁻:SiO₂ ratio properly, it becomes possible to aim at a process for preparing a zeolite preferentially over another. The invention will now be illustrated in more detail with the aid of a few illustrative examples. EXAMPLES 1-14 The reaction mixture is prepared from sodium silicate (26.03 weight% SiO₂; 8.97 weight% Na₂; 65 weight% H₂O) and from aluminium sulphate Al₂ (So₄)₃.16H₂O (Al₂O₃ = 17 weight%). The sodium silicate is dissolved in distilled water containing the required quantity of NaOH to obtain the required OH⁻/SiO₂ ratio, and the aluminium sulphate is separately dissolved in water (possibly containing sulphuric acid). The second solution is added to the first under vigorous agitation, and the mixture heated to 80°C. The gel obtained in this manner is transferred to an autoclave and kept at a temperature of 176°C under agitation. The reagent composition, the crystallisation time and the nature of the zeolite produced are given in Table 1. The molar SiO₂/Al₂O₃ ratios of the mordenites obtained vary between 16 and 19. In all the described examples, the salt added as mineralising agent is sodium sulphate, but in Example 8 a mixture of sodium and potassium is used. In Examples 1, 3 and 5, the addition of different zeolite nuclei equally gave rise to the formation of mordenite, whereas in Example 8 mordenite nuclei were added. In all cases the zeolite obtained is that corresponding to the OH⁻/SiO₂ ratio, and not that corresponding to the nuclei.

Claims: A process for the preparation of a mordenite having in its anhydrous condition the formula: Me₂O.Al₂O₃. x SiO₂ wherein Me is an alkali metal and x is comprised between 10 and 20, characterized by comprising the steps of contacting in an aqueous medium sodium silicate with an aluminium compound, with a molar ratio SiO₂ to Al₂O₃ comprised between 10 and 200, in the presence of one or more alkali metal hydroxides, with a molar ratio Me⁺:SiO₂ higher than 0,6, and also in the presence of a mineralizing agent, working at a temperature comprised between 20°C and 100°C, checking that the molar ratio OH⁻:SiO₂ of the resultant mixture is comprised between 0,2 and 0,8, said resultant mixture being caused to crystallize with said hydroxyl to SiO₂ ratio in an enclosure for a period of from a few hours to 40 days at a temperature comprised between 100°C and 200°C. Process according to claim 1, characterized in that the molar ratio Me⁺:SiO₂ is comprised between 0,66 and 0,96. Process according to claim 1, characterized in that the molar ratio SiO₂:Al₂O₃ is comprised between 15 and 200 and the molar ratio OH⁻:SiO₂ is comprised between 0,25 and 0,50. Process according to claim 1, characterized in that the crystallization temperature is comprised between 150°C and 180°C. Process according to claim 1, characterized in that the mineralizing agents are selected from among sodium sulphate, potassium sulphate, sodium phosphate, potassium phosphate, sodium chloride and potassium chloride. Process according to claim 1, characterized in that the alkali metal hydroxide are selected from among LiOH, NaOH and KOH. Process according to claim 1, wherein the aluminium compound is selected from among sodium aluminate, aluminium sulphate, aluminium chloride and trihydrous alumina.

Inventors: BELLUSSI, GIUSEPPE, MANARA, GIOVANNI, NOTARI, BRUNO, TARAMASSO, MARCO
Applicants: SNAMPROGETTI S.P.A.
Application year: 1983
Main classification: C01B33
IPC codes: C01B39