Patent Application
20150246345
1. Field of the Invention
The invention relates generally to molecular sieve based-catalysts used in selectively converting nitrogen oxides (NOx) present in a gas stream to nitrogen using a nitrogenous reductant such as ammonia (NH3) or urea (CO(NH2)2) and in particular it relates to Fe-containing catalysts which are particularly active at relatively low temperatures in relation to conventional Fe zeolite catalysts. The molecular sieve in these catalysts is preferably a zeolite or a silicoaluminophosphate (SAPO).
2. Description of Related Art
Selective catalytic reduction (SCR) systems utilize NH3 as a reductant to reduce NOx to elemental nitrogen. A principal application of SCR technology is in the treatment of NOx emissions from internal combustion engines of motor vehicles, and especially lean-burn internal combustion engines. SCR systems are also applied to static sources of NOx, such as power plants.
One class of SCR catalysts is transition metal exchanged zeolites. Vanadium-based SCR catalysts are unsuited for higher temperature environment due to their thermal instability. This has led to the developments of copper and iron promoted zeolites. Copper zeolite catalysts achieve high NOx conversion (90% or more) at relatively low temperatures (from about 180° C. to about 250° C.), but they require the injection of greater amounts of urea to be effective at relatively higher temperatures (greater than about 450° C.). Conventional iron zeolite catalysts achieve high conversion (90% or more) of NOx at temperatures over 350° C., but at lower temperatures, more typical of normal diesel engine exhaust (about 180° C. to about 250° C.), high conversions (up to about 90%) are obtained only in the presence of high levels of NO2 (50% of the total NOx levels, i.e. 1:1 NO2:NO).
It would therefore be desirable to provide SCR catalysts having improved low temperature (from about 180° C. to about 300° C.) performance and/or improved resistance to ageing.
The invention reflects the inventors' surprising discovery that the presence of certain groups of organic compounds when iron is introduced into a molecular sieve, can improve the dispersion of the iron to the ion-exchange sites of the molecular sieve, and thereby improve the low-temperature performance and/or the ageing resistance of the molecular sieve. The molecular sieve in these catalysts is preferably a zeolite or a silicoaluminophosphate (SAPO).
Thus, in one aspect, the invention relates to a process for producing an SCR-active molecular sieve based-catalyst, comprising combining a molecular sieve, preferably a zeolite or a SAPO, with at least one ionic iron species and at least one organic compound to form a mixture; and calcining the mixture so as to remove the at least one organic compound. The removal of the at least one organic compound can occur through various processes, including combustion and decomposition.
The molecular sieve is preferably BEA (beta-zeolite), MFI (ZSM-5), FER (ferrierite), CHA (chabasite), AFX, AEI. SFW, SAPO-34, SAPO-56, SAPO-18 or SAV SAPO STA-7.
The organic compound is an oxygen-containing organic compound, such as one or more polycarboxylic acids, a nitrogen-containing compound, such as one or more tetraalkyl ammonium salts, or one or more trialkylamines, or mixtures thereof. Preferably, the organic compound is selected from the group consisting of L-ascorbic acid, citric acid, succinic acid, oxalic acid, sucrose, glucose, ethylene glycol, ethylenediamine, pyrrolidine, di-n-propylamine, diaminooctane, tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide, tetrapropylammonium bromide, adamantine-substituted tetraalkyl ammonium hydroxides, triethylmethyl ammonium salts, and tetra-n-propylammonium salts. These compounds are termed traditional organic compounds. The term organic compound, as used herein, also includes metal complexes or salts where one of the ions is an organic group. Preferably, the salt comprises iron and an ionic organic group, such as an acetate, citrate, succinate, gluconate, etc. The process can use a plurality of organic compounds, such as a traditional organic compound and an iron organic salt or metal organic complex, as described above.
The process comprises combining a molecular sieve, preferably a zeolite or a SAPO, with at least one ionic iron compound and at least one organic compound and introducing the iron compound to the molecular sieve via suitable catalyst preparation methods such as liquid phase ion-exchange, incipient wetness impregnation, wet impregnation, spray drying and solid-state mixing techniques. These solid-state techniques range from simple loose mixing and grinding through to high energy mixing methods, such as ball milling.
Preferably, the at least one dissolved iron salt is one or more members selected from the group consisting of iron nitrate, iron sulfate, ammonium iron oxalate, iron chloride, iron acetate, iron ammonium sulfate, and iron ammonium citrate, where the iron can be Fe(II) or Fe(III), or a mixture thereof.
The at least one ionic iron species and the least one organic compound are present in a molar ratio from about 1:1 to about 1:10, preferably from about 1:2 to about 1:8, more preferably from about 1:3 to about 1:6, and most preferably about 1:4.
Calcining is performed at a temperature of about 400 to about 600° C. for a time of about 1 to about 3 hours.
In another aspect, the invention also relates to a process of making a catalyst module for abating nitrogen oxides in a gas stream by selective catalytic reduction. A catalyst module is a device containing a catalyst within a housing where the housing comprises one or more inlets for the gas stream to enter the housing, and one or more outlets for the gas to exit after passing through the catalyst in the housing. The process of making the catalyst module comprises combining a molecular sieve, preferably a zeolite or a SAPO, with at least one ionic iron species and at least one organic compound to form a mixture, calcining the mixture and removing the at least one organic compound, forming a catalyst structure by extruding the calcined mixture into a substrate or coating the calcined mixture onto a substrate and mounting the catalyst structure within a housing having one or more inlets for gas to be treated with a reductant such as ammonia or urea in selective catalytic reduction. A catalyst module can also be made by a process comprising preparing a washcoat by forming a mixture comprising a molecular sieve, preferably a zeolite or a SAPO, at least one ionic iron species and at least one organic compound, applying the washcoat to a substrate, calcining the coated mixture and removing the at least one organic compound to form a catalytic structure, and mounting the catalytic structure within a housing having one or more inlets for gas to be treated with a reductant such as ammonia or urea in selective catalytic reduction.
In yet another aspect, the invention relates to an iron-containing molecular sieve, preferably a zeolite or a SAPO, more preferably a ferrierite zeolite, wherein the iron-containing molecular sieve exhibits a selective catalytic reduction of nitrogen oxides with NH3 or urea of greater than about 25% conversion at 300° C. in exhaust gases prior to ageing or exposure to steam. Preferably, the iron-containing molecular sieve, preferably a zeolite or a SAPO, more preferably a ferrierite zeolite, provides for the conversion of nitrogen oxides at 300° C. that is greater than 30%, more preferably greater than 40%, even more preferably greater than 50%, most preferably greater than 60%.
The use of succinic acid in the manufacture of the catalysts improves NOx conversion of an iron-containing molecular sieve, preferably a zeolite or a SAPO, more preferably a ferrierite zeolite compared to an otherwise identical iron containing molecular sieve prepared without the use of succinic acid. At temperatures between 200° C. and 350° C., catalyst produced using succinic acid have approximately twice or greater NOx conversion compared to a similar catalyst produced without the use of an organic acid. At 300° C., the catalyst produced using succinic acid can have approximately three times the NOx conversion of the catalyst produced without the use of an organic acid.
The use of citric acid or oxalic acid in the manufacture of the catalysts improves NOx conversion of the iron-containing molecular sieve, preferably a zeolite or a SAPO, more preferably a ferrierite zeolite, compared to an otherwise identical iron containing molecular sieve prepared without the use of these acids. At 250° C., catalysts produced using citric acid or oxalic acid have NOx conversions greater than that of a comparable catalyst produced without the use of an organic acid. At 300° C. and 350° C., catalysts produced using citric acid or oxalic acid have NOx conversions of about two times greater than the conversion for a similar catalyst produced without the use of an organic acid.
In another aspect of the invention, the temperature needed for the comparable conversion of NOx is reduced when the catalyst is prepared using an organic acid compared to a comparable catalyst that was prepared without using the organic acid. Temperatures needed for 10% NOx conversion were about 200, 250, 250 and 275° C. for catalysts prepared using succinic acid, oxalic acid, citric acid and without the use of an acid, respectively. Temperatures needed for 50% NOx conversion were about 300, 325, 325 and 375° C. for catalysts prepared using succinic acid, oxalic acid, citric acid and without the use of an acid, respectively. Temperatures needed for 90% NOx conversion were about 340, 375, 390 and 450° C. for catalysts prepared using succinic acid, oxalic acid, citric acid and without the use of an acid, respectively. In addition, the lowest temperatures at which maximum NOx conversion occurs is lower for catalysts prepared using succinic acid, oxalic acid, citric acid and without the use of an acid, with temperatures of about 360, 400, 425 and 475° C. respectively.
In still another aspect, the invention relates to an iron-containing molecular sieve, preferably a zeolite or a SAPO, more preferably a beta zeolite, wherein the molecular sieve exhibits (a) a first selective catalytic reduction of nitrogen oxides with NH3 or urea of at least 40%, preferably at least 45%, more preferably at least 50% conversion at 300° C. in exhaust gases after ageing for at least 20 hours at 700° C. in the presence of 10% H2O and (b) a second catalytic reduction of nitrogen oxides with NH3 or urea of at least 80% conversion at 400° C. in exhaust gases after ageing for 20 hours at 700° C. in the presence of 10% H2O. Preferably, the first selective catalytic reduction of nitrogen oxides with NH3 or urea is greater than 50%.
Other objects, features and advantages of the invention will become more apparent after reading the following detailed description of examples of the invention, given with reference to the accompanying drawings.
As used herein, the term “calcine”, or “calcination”, means heating the material to high temperatures in air or oxygen. This definition is consistent with the IUPAC definition of calcination. (IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi: 10.1351/goldbook.)
The term “template” refers to an agent that is added during the process of manufacturing molecular sieves to control the shape and size of pores in a molecular sieve. The use of templates in forming molecular sieves is known in the art.
As used herein, the term “about” means approximately. Approximating language, as used throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term such as “about” is not to be limited to the precise value specified. With regard to the use of the term “about” and specific numerical values encompassed by the term, the number of significant figures, the precision of the value and the context in which the term is used are important in determining the numerical values associated with the term. For example, if a series of measurements are taken over a temperature range from 300° C. to 500° C., where the measurements are made at 25° C. intervals, the term “about 400° C.” would encompass the range from 387° C. to 412° C., inclusive. When “about” is used in describing units of time in hours, the stated value includes a range of plus or minus 15 minutes, inclusive. For example, “about 2 hours” is meant to include time from 1 hour 30 minutes to 2 hours 30 minutes, inclusive. When “about” is used in describing the ratios of amounts of two components, the ratios include values that, when rounded, provide the stated ratio. For example, the term “about 1:4” is meant to include compositions having ratios of 1:3.5 to 1:4.4, inclusive.
The presence of certain types of organic material in a composition comprising a molecular sieve and an ionic iron compound during standard air calcination at temperatures of for example 500° C. can substantially improve the low temperature NH3 SCR activity of iron containing molecular sieve based catalysts. As discussed in greater detail herein, this effect has been observed for a number of organic molecules (e.g. citric acid, succinic acid, ascorbic acid, oxalic acid) and for both large pore zeolites such as BEA (beta zeolite) and MFI (ZSM-5) as well as medium pore zeolites such as FER (ferrierite) and is also expected to be applicable to small-pore zeolites such as CHA (chabasite), AFX, AEI and SFW. This effect is also expected to be applicable to other molecular sieves, including silicoaluminophosphates, such as SAPO-34, SAPO-56, SAPO-18 and SAV SAPO STA-7.
The effect is attributed to thermal redispersion of iron due to the exotherm generated during calcination with possibly a local reducing environment due to the presence of the organic. Some changes in the nature of the Fe sites, such as the Fe-zeolite interaction or Fe-organic interaction, may also contribute to the enhanced activity. This effect is also expected to be applicable to other molecular sieves, including silicoaluminophosphates, such as SAPO-34.
Incorporation of the organic compound to the molecular sieve may be via impregnation (using such methods as liquid phase ion-exchange, incipient wetness impregnation, wet impregnation, and spray drying), co-impregnation of the organic compound with the iron compound and physical mixing with the catalyst using solid-state mixing techniques. These solid-state techniques range from simple loose mixing and grinding through to high energy mixing methods, such as ball milling.
Mole ratios of iron to the organic compound of about 1:1 to about 1:10 are contemplated, preferably from about 1:2 to about 1:8, more preferably from about 1:3 to about 1:6 and more preferably about 1:4, are to be employed.
The iron may be incorporated into the molecular sieve by isomorphous substitution during synthesis of the molecular sieve, or, alternatively, the iron may be incorporated into the molecular sieve after it is formed, by the techniques described above. It is preferred to incorporate the iron after synthesis of the molecular sieve.
Framework iron resulting from isomorphous substitution is generally considered not to be catalytically active, as discussed for example in U.S. Pat. No. 6,890,501. The presence of iron in the crystal lattice of a molecular sieve might alter the quantity and arrangement of aluminium atoms in the lattice, which in turn could affect the performance of the molecular sieve in undesired ways. On the other hand, a molecular sieve that is first synthesized and then combined with an iron salt will contain substantially only extra-framework iron, with the techniques of the invention increasing the amount of that iron that is present at the catalytically active ion-exchange sites.
The compounds identified herein have been found to promote dispersion of the iron into the zeolite to be improved. A portion of the template used in producing the zeolite may still be present. This effect is also expected to be applicable to other molecular sieves including silicoaluminophosphates (SAPO), such as SAPO-34.
The organic compounds preferred for use according to the invention may also include those which are commonly used as structure directing agents (or templates) during synthesis of the molecular sieve, such as quaternary ammonium salts and hydroxides and alkylamines. The use of such compounds in the invention may have an advantage in that the template molecules used for synthesis of the molecular sieve may serve a dual purpose of directing the synthesis of the molecular sieve and also improving the dispersion of the iron according to the techniques described herein.
Examples of such template molecules include tetramethyl ammonium hydroxide, tetrapropylammonium bromide, adamantine-substituted tetraalkyl ammonium hydroxides and salts, ethylenediamine and other conventional structure-directing agents. The use of such compounds does not necessarily involve isomorphous substitution of iron into the lattice of the molecular sieve, because the iron salt can preferably be added after the molecular sieve has been synthesized, but before the template molecule has been eliminated by calcination. When the iron salt has been added after the molecular sieve has been synthesized, no significant framework iron remains.
Preferably the molecular sieves are small or medium pore. Small pore molecular sieves, including zeolites and silicoaluminophosphates, such as SAPO-34, or some medium pore molecular sieves, including zeolites, such as ferrierite, and silicoaluminophosphates, are advantageous due to their improved resistance to hydrocarbon adsorption. Hydrocarbon tolerance helps to avoid catalyst damage due to exotherms during filter regenerations and inhibition effects during SCR reaction at low temperature. The molecular sieves of the invention preferably display improved iron dispersion and performance at low temperatures (about 180° C. to about 300° C.).
Non-limiting examples of the types of exhaust gases that may be treated with the disclosed molecular sieve based-catalysts include automotive exhaust, including from diesel engines. The disclosed molecular sieves are also suitable for treating exhaust from stationary sources, such as power plants, stationary diesel engines, and coal-fired plants.
The iron-containing molecular sieves of the invention may be provided in the form of a fine powder which is admixed with, or coated by, a suitable refractory binder, such as alumina, bentonite, silica, or silica-alumina, and formed into a slurry which is deposited upon a suitable refractory substrate. The carrier substrate can have a “honeycomb” structure. Such carriers are well known in the art as having a many fine, parallel gas flow passages extending through the structure.
In examples 1-5, powder samples of the catalysts were obtained by pelletizing the original samples, crushing the pellets and then passing the powder obtained through a combination of 255 and 350 micron sieves to obtain a composition having particle sizes between 255 and 350 microns. The powder samples were loaded into a Synthetic Catalyst Activity Test (SCAT) reactor and tested using the following synthetic diesel exhaust gas mixture (at inlet) including nitrogenous reductant: 500 ppm NO, 550 ppm NH3, 12% O2, 4.5% H2O, 4.5% CO2, 200 ppm CO, balance N2 at a space velocity of 330 liters per gram of powder catalyst per hour. The samples were heated ramp-wise from 150 to 550° C. at 5° C./min and the composition of the off-gases detected and the activity of the samples to promote NOx reduction was thereby derived.
The low temperature activity of an iron zeolite catalyst can be enhanced by addition of organic acids during the impregnation of iron into the catalyst. The improvement can be attributed to improved iron exchange and redispersion due to an exotherm effect during calcination and possibly creating a locally reducing environment.
Modified 3 wt % Fe/ferrierite catalysts were prepared by impregnating a commercially available ferrierite zeolite with a solution of iron (III) nitrate and an organic acid (citric, succinic or oxalic acid). The molar ratio of Fe:organic acid was 1:4. The samples were dried at 105° C. overnight and then calcined for 2 hours at 500° C.
The powder samples were analyzed by diffuse-reflectance UV-Vis in a Perkin-Elmer Lambda 650S spectrometer equipped with an integrating sphere using BaSO4 as a reference. The samples were placed and packed in a holder. The scan interval was set to 1 nm from 190 to 850 nm, the response time was 0.48 sec and a 10% beam attenuator was used in the reference beam. The data was converted to Kubelka-Munk and normalised to 5 to the maximum ordinate. The resulting spectra (See
Selected powder samples were also analyzed by Mössbauer spectroscopy. 57Fe Mössbauer spectroscopy was performed at room temperature using a Wissel constant acceleration spectrometer in transmission mode using a 57Co source in a rhodium matrix. The spectrometer was calibrated relative to α-Fe. The samples were dried and placed in a holder that was glued closed. Mössbauer data were collected over a velocity range of +/−6 mm s−1 and for different periods of time depending on the sample. A calibration run was performed on an α-Fe foil over the same velocity range. All isomer shift values were reported relative to α-Fe and spectra were analysed using the Lorentzian line-shapes facility of RECOIL software [Lagarec K and Rancourt D G, Recoil: Mössbauer spectral analysis software for Windows. http://www.isapps.ca/recoil/].
As shown in
These results demonstrate that the use of the organic acids in preparing the catalyst results in significantly higher NOx conversion compared to a comparable catalyst that did not use an organic acid during the preparation of the catalyst. Catalysts produced using the organic acids convert NOx at lower temperatures compared to a comparable catalyst that did not use an organic acid during the preparation of the catalyst.
Succinic acid was selected as the organic acid to study the effect of different molar ratios of iron to organic acid.
Modified 3 wt % Fe/ferrierite catalysts were prepared by impregnating a commercially available ferrierite zeolite with a solution of iron(III) nitrate and different amounts of succinic acid so that the molar ratio of Fe:organic acid was 1:2, 1:4 and 1:8. The control sample did not have any succinic acid added. The samples were dried at 105° C. overnight and then calcined for 2 hours at 500° C.
As shown in
These results demonstrate that the use of the organic acid in differing molar amounts relative to the amount of iron present in preparing the catalyst results in significantly higher NOx conversion compared to a comparable catalyst that did not use an organic acid during the preparation of the catalyst. These results also indicate that catalysts produced using organic acids in amounts such that the molar ratio of iron to organic acid ranges from 1:2 to 1:8 could convert NOx at lower temperatures compared to a comparable catalyst that did not use an organic acid during the preparation of the catalyst. Among the tested mole ratios, a ratio of 1:4 of iron:organic acid was found to be optimal.
Succinic acid was selected as the organic acid to study the effect of different iron salts on the catalytic activity of the catalyst.
Modified 3 wt % Fe/ferrierite catalysts were prepared by impregnating a commercially available ferrierite zeolite with a solution of succinic acid and iron (III) nitrate, iron (II) acetate or iron (II) sulphate to give a molar ratio of Fe:organic acid of 1:4. Control samples did not have any succinic acid added. The samples were dried at 105° C. overnight and then calcined for 2 hours at 500° C.
As shown in
Temperatures for 90% NOx conversion were about 310 to about 340° C. for catalysts prepared using acetate, acetate plus succinic acid and nitrate plus succinic acid and were about 360 to about 415° C. for catalysts produced using nitrate, sulphate or sulphate plus succinic acid. The lowest temperatures for maximum NOx conversion were about 330 to about 360° C. for catalysts prepared using acetate, acetate plus succinic acid and nitrate plus succinic acid and were about 370 to about 450° C. for catalysts produced using nitrate, sulphate or sulphate plus succinic acid.
These results demonstrate that the iron salt used in preparing the catalyst can result in widely differing amounts of NOx conversion.
The low temperature activity of an iron zeolite catalyst can be enhanced by addition of organic acids or bases during the impregnation of iron into the catalyst. The improvement can be attributed to improved iron exchange and redispersion due to an exotherm effect during calcination that creates a locally reducing environment.
Modified 5 wt % Fe/Beta catalysts were prepared by impregnating a commercially available Beta zeolite with a solution of iron(III) nitrate and either citric acid, succinic acid or ethylenediamine (EDA) to give an Fe:organic additive molar ratio of 1:4. The samples were dried at 105° C. overnight and then calcined for 2 hours at 500° C.
Diffuse-reflectance UV-Vis was applied to powder samples and the data was normalised to the maximum ordinate. Diffuse-reflectance UV-Vis shows (See
As shown in
These results demonstrate that the use of an organic acid or base in preparing the catalyst results in significantly higher NOx conversion compared to a comparable catalyst that did not use an organic acid or base during the preparation of the catalyst. Catalysts produced using the organic acids or bases convert NOx at lower temperatures compared to a comparable catalyst that did not use an organic acid or base during the preparation of the catalyst.
Citric acid was selected to study the effect of different iron salt precursors on SCR activity when adding an organic acid to iron Beta.
Modified 5 wt. % Fe/Beta catalysts were prepared by impregnating a commercially available Beta zeolite with a solution of citric acid and either iron (III) nitrate, iron (II) acetate or iron (II) chloride, to give an Fe:organic acid molar ratio of 1:4. The control sample did not have any citric acid added. The samples were dried at 105° C. overnight and then calcined for 2 hours at 500° C.
As shown in
These results demonstrate that the use of different iron salts with organic acid in preparing the catalyst results in significantly higher NOx conversion compared to a comparable catalyst that did not use organic acid during the preparation of the catalyst. These results also indicate that catalysts produced using iron salts with organic acids in amounts such that the molar ratio of iron to organic acid was about 1:4 could convert NOx at lower temperatures compared to a comparable catalyst that did not use an organic acid during the preparation of the catalyst.
The techniques described herein have also been found to improve the resistance of iron-containing zeolites to hydrothermal ageing, in addition or alternatively to the improved low temperature performance.
Iron (III) nitrate was dissolved in deionized water, to which L-ascorbic acid was then added, followed by mixing for 30 min. A commercially available beta zeolite powder was then added to the slurry and mixed for a further three hours. Colloidal silica and boehmite alumina powder were added to the slurry while mixing, followed by scleroglucan to thicken the slurry, followed by another one hour of mixing. The resulting slurry was then coated on a catalyst substrate, and subjected to hydrothermal ageing at 700° C. and 10% H2O for 20 hours. A similar catalyst was prepared without the addition of L-ascorbic acid.
The NOx conversion of these two catalysts was evaluated at SCR inlet temperatures between 150° C. and 500° C. using the method described above.
It will be understood that the foregoing description and specific examples shown herein are merely illustrative of the invention and the principles thereof, and that modifications and additions may be easily made by those skilled in the art without departing from the spirit and scope of the invention, which is therefore understood to be limited only by the scope of the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| 61946075 | Feb 2014 | US |
