Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And additional al or si containing component
Patent
1992-02-28
1994-11-29
Dees, Carl F.
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And additional al or si containing component
502 60, 502 79, B01J 2910
Patent
active
053690702
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns catalytic compositions for the selective reduction by ammonia of nitrogen oxides (NO.sub.2) contained in gaseous effluents. It also concerns the preparation of these compositions and a purification process using them.
Among these processes in particular a process described in FR 2 568 789 is known; it proposes a reduction-denitrification process by ammonia of residual gases in the presence of a zeolite of a hydrogen and/or iron substitution type at a surface velocity of at least 5 m/h and with a ratio higher than 1.3 of the ammonia concentration to the nitrogen dioxide, NO.sub.2, concentration.
Applicant sought to reduce the nitrogen oxides contained within a wide range of concentration in oxygenated effluents by the use of lower concentration ratios of NH.sub.3 /NO.sub.x, in particular ratios of the order of 1.1, and has initially proposed catalytic compositions based on mordenite with a lower Na proportion, making it possible to obtain excellent results in the reduction by ammonia of NO.sub.x in oxygenated effluents at relatively elevated temperatures of 300.degree.to 375.degree. C. if the mordenite is in the ammonium form, 300.degree.to 550.degree. C. if in the acid form. It was later discovered that these temperatures may be considerably lowered by using as the active matter a mordenite exchanged to copper; the temperature is then of the order of 225.degree.to 400.degree. C. These results are complied in EP 0286507.
U.S. Pat. No. 4 052 337 describes the use of different catalysts of the zeolite type, in particular zeolite Y to reduce the NO.sub.x of NH.sub.3. This catalyst is intended for use at relatively high temperatures to reduce nitrogen oxides containing sulfur, is effective only if it is prepared by a process carrying out a preliminary exchange of zeolite with alkaline earth ions followed by an impregnation by metal ions, in particular cupric ions.
The present applicant has now discovered a catalytic system obtained by an exchange reaction with cupric ions of a (end of replacement page) zeolite Y selected as a function of its specific surface and perfectly effective in the reduction of NO.sub.x nitrogen oxides with ammonia, at temperatures lowered relative to those attained with catalysts based on mordenite exchanged to copper, described in EP 0286507, specifically between 200.degree.and 400.degree. C.
The initial zeolite used to prepare the catalytic compositions of the present invention is a NaY zeolite with a specific surface area of 750 to 950 m.sup.2 /g. This zeolite has a more open structure than mordenite. It contains the largest cavities known in the structures of the so-called supercage type zeolites, with a diameter around 13 .ANG.; these cavities communicate with each other through openings having diameters of the order of 8 to 9 .ANG. and their succession constitutes the pores of the lattice. The structure is cubic and the pores extend in the 3 perpendicular spatial directions and intersect at the supercages.
The initial NaY zeolite used in the preparation of the catalysts according to the invention has an atomic ratio of the Si/Al skeleton of 2 to 3.
The Si and Al analyses are carried out by X ray fluorescence.
The specific surface is calculated by the BET method from the adsorption-desorption isotherm effected at the temperature of liquid temperature after a pretreatment at 500.degree. C. under a flow of dry nitrogen.
The parameter of the crystalline lattice is generally between 24.60.times.10.sup.-10 m and 24.75.times.10.sup.-10 m. It is calculated from the x-ray diffraction diagram by the method described in ASTM D 3.942.80.
The present invention thus concerns novel catalytic compositions applicable to the reduction by NH.sub.3 of NO.sub.x nitrogen oxides and containing as the active matter a Y zeolite obtained from a NaY zeolite with a specific surface of 750 to 950 m.sup.2 /g preferably greater than 800 m.sup. /g, exchanged for cupric ions. Zeolites exchanged for cupric ions are defined in this context as both the products of a direct exchange for
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Descat M. Gilles
Hamon M. Christian
Dees Carl F.
Elf Atochem S.A.
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