Patent Application: US-38213203-A

Abstract:
the present invention provides metal - exchanged hexaaluminate catalysts that exhibit good catalytic activity and / or stability at high temperatures for extended periods with retention of activity as combustion catalysts , and more generally as oxidation catalysts , that make them eminently suitable for use in methane combustion , particularly for use in natural gas fired gas turbines . the hexaaluminate catalysts of this invention are of particular interest for methane combustion processes for minimization of the generation of undesired levels of nox species . metal exchanged hexaaluminate oxidation catalysts are also useful for oxidation of volatile organic compounds , particularly hydrocarbons . metal exchanged hexaaluminate oxidation catalysts are further useful for partial oxidation , particularly at high temperatures , of reduced species , particularly hydrocarbons .

Description:
this invention relates to hexaaluminate catalysts prepared by metal exchange reactions employing an alumoxane precursor . alumoxanes are inorganic polymers consisting of carboxylic acids covalently bound to a boehmite core ( kareiva et al . 1996 , kareiva et al . 2001 , harlan et al . 1997 ). the invention is based at least in part , on the discovery that hexaaluminates made by this method are very stable at high temperatures compared to hexaaluminates prepared by other methods , such as coprecipitation . the use of alumoxanes as precursors have several synthetic advantages over other methods for preparing hexaaluminate catalysts . first , the boehmite particles are very small , between 10 and 100 nm . in addition , after the boehmite particles are bound with carboxylic acids , they are also water - soluble . but , perhaps the most significant advantage alumoxanes have over traditional precursors is that the metal exchange ( of al for one or more other selected metals ) can be accomplished using the acetylacetonate ( acac ) of the desired metal and therefore , the metal exchange can be made at the atomic level . there are several significant advantages to this preparation technique . first , because all of the metals to be incorporated in the hexaaluminate lattice can be exchanged directly into these extremely small particles , the diffusion distances are greatly reduced . rather than being required to diffuse over micron ranges as would be expected for coprecipitated particles , atoms incorporated into the boehmite lattice may only have to diffuse a distance of several atoms , or on the order of angstroms . this greatly reduces the time required for the hexaaluminate phase to form . second , by adjusting the relative amounts and concentrations of the alumoxane and metal - acac compounds , catalyst precursor particles can be prepared that are exchanged at the atomic level producing a known and well - controlled stoichiometry . in addition , the exchange can be carried out with two or more metals . thus , this synthetic procedure can be used to rapidly prepare materials for testing as catalysts with varying metal content , each containing well - defined concentrations of heteroatoms . there are two primary steps in the preparation of the hexaaluminate catalysts . the first step is the preparation of the alumoxane precursor . the alumoxane precursor is synthesized by attaching carboxylic acid ligands to a boehmite particle . the conversion of boehmite into a water - soluble alumoxane ( referred to as carboxylato - alumoxane ) compound is illustrated in fig2 . we used a 12 : 1 molar ratio of boehmite to carboxylic acid , and found that this formed a stable alumoxane sol . the next step in the process is the introduction of metal cations into the boehmite core of the carboxylato - alumoxane ( fig3 ). we carried out this step by stirring the aqueous alumoxane solution with selected amounts of selected metal acetylacetonates . the solution is usually heated for a short period of time , about four hours . after heating , we dried the mixture in an oven at 80 ° c ., and finally , heated it at a rate of 3 ° c ./ min to a temperature of about 800 ° c . where it was maintained for about 1 h . the resulting compound ( a metal - exchanged alumoxane ) was removed from the oven , ground to a fine powder , and then heated to about 1300 ° c . at 3 ° c ./ min . the samples were maintained at this temperature for about 1 h . [ 0067 ] 27 al nmr studies indicate that during the exchange reaction aluminum cations ( al 3 + ) are extracted from the lattice as al ( acac ) 3 and replaced by the metal cation contained in the exchange reagent acac complex . the very high stability of the aluminum acac compound serves as the driving force for the reaction and makes the substitution reaction very predictable and reproducible . in the metal - exchanged aluminates of this invention at least about { fraction ( 1 / 12 )} of the al cations of the alumoxane are exchanged for other metal ions . the al may be exchanged for one or more other metal cations . in exemplified embodiments the al is exchanged or one other metal ion , two other metal ions , or three other metal ions . hexaaluminate catalysts are formed by sintering powders of the metal - exchanged alumoxanes that result from the metal exchange reaction . the hexaaluminate catalysts of this invention exhibit increased stability at high temperatures compared to hexaaluminate catalysts prepared by other methods . stability is assessed as retention of catalytic activity as a function of aging of the catalyst at a given high temperature , e . g ., subjecting the hexaaluminate to a prolonged period of heating , particularly under environmental conditions ( humidity , presence of co 2 , etc ) which simulate the environment in which the catalysts will be used . more specifically , one means for assessing catalysts activity is by measurement of the temperature corresponding to conversion of ½ of the methane in a feed stream ( t ½ ). alternative , assessments of activity measure the temperature corresponding to conversion of 10 % of the methane in a feed stream ( t10 %). comparisons made using measurements of t ½ or t10 % should provide similar results , t ½ measurements are used herein . these temperature measurements can provide a single figure of merit , which can be used to compare the activity of a number of catalyst samples of different composition . alternatively , because catalyst activity as a combustion catalysts is generally directly proportional to catalyst surface area , relative activity of a given catalyst can be assessed by measurement of changes in catalyst surface area , e . g ., as % increases or decreases in surface area . thus , changes in the surface area of a given catalyst as a function of its exposure to an adverse environment ( e . g ., high temperatures ) can be assessed by following changes in catalyst surface area . metal - exchanged hexaaluminate catalysts of this invention have exhibited high thermal stability with retention of about 50 % or more of their initial surface area after about 50 h of aging at 1300 ° c . under simulated combustion conditions . preferred metal - exchanged hexaaluminate catalysts of this invention for use particularly in high temperature applications over about 700 ° c . exhibit less than about a 75 % decrease in surface area ( compared to their initial “ as prepared ” surface area ) on heating at 1300 ° c . under simulated combustion conditions . more preferred metal - exchanged hexaaluminate catalysts of this invention for use particularly in high temperature applications over about 700 ° c . exhibit less than about a 50 % decrease in surface area on heating at 1300 ° c . under simulated combustion conditions . yet more preferred metal - exchanged hexaaluminate catalysts of this invention for use particularly in high temperature applications over about 700 ° c . exhibit equal to or less than about a 35 % decrease in surface area on heating at 1300 ° c . under simulated combustion conditions . metal exchanged hexaaluminate catalysts of this invention have exhibited t ½ as low as about 360 ° c .- 420 ° c . even after aging at 800 ° c . for up to 50 h . preferred metal exchanged hexaaluminate catalysts of this invention exhibit t ½ equal to or less than about 500 ° c . even after aging at 800 ° c . for up to 50 h . more preferred metal exchanged hexaaluminate catalysts of this invention exhibit t ½ equal to or less than about 450 ° c . even after aging at 800 ° c . for up to 50 h . yet more preferred metal exchanged hexaaluminate catalysts of this invention exhibit t ½ equal to or less than about 400 ° c . even after aging at 800 ° c . for up to 50 h . metal exchanged hexaaluminate catalysts of this invention have exhibited initial surface areas of 10 m 2 / g or more “ as prepared .” preferred hexaaluminate catalysts are those that exhibit the highest initial surface area and which exhibit the lowest % decrease in surface area on heating at 800 ° c . or 1300 ° c . it is generally the case , that a catalyst will exhibit a higher % decrease in surface area if it is aged at a higher temperature . as noted above , aging is intended to demonstrate the stability of a catalyst under the conditions to which it is expected to be subjected during use . catalyst stability or activity could be assessed under environmental conditions other than those specifically employed in examples herein that would be more representative of its potential application . metal exchanged hexaaluminate catalysts of this invention are particularly useful as oxidation and combustion catalysts . in this regard , catalysts of this invention can be employed in art - known combustion methods which employ catalysts to increase efficiency , or decreased undesired by - products of combustion . metal - exchanged hexaaluminate catalysts of this invention can , for example be employed in methods describe in u . s . pat . nos . 5 , 823 , 761 ; 5 , 830 , 822 ; 5 , 899 , 679 ; 5 , 915 , 951 ; 6 , 298 , 664 and 6 , 334 , 987 . we have examined two different types of catalysts that will be useful in methane combustion systems , including natural gas fired turbines , for example in a first and a second stage of a combustor . the catalyst used in the first stage should be sufficiently active that combustion begins at approximately 400 ° c . as the reaction proceeds and the temperature of the feed mixture increases to approximately 700 ° c ., the gas mixture is directed over the second catalyst , which is used to complete the combustion reaction . the two catalysts used in a two - stage combustion configuration have different requirements . the catalyst used in the first stage is preferably very active ; however it only has to be stable at temperatures up to 800 ° c . ( about 100 ° c . greater than the maximum temperature to which it will be exposed ). the second stage catalyst must have excellent stability at temperatures up to 1300 ° c ., however , it does not have to be as active as the first stage catalysts ( although it is preferably as active as possible ), because the reactants will contact second stage catalysts at temperatures in excess of 700 ° c . to measure the catalyst activity for methane combustion , we placed approximately 0 . 5 g sample in a test apparatus , flowed a mixture of 3 % methane in air over the catalyst at a pressure of 75 psi and a flow rate corresponding to 17 , 000 cc feed per cc of catalyst per hour or a gas hourly space velocity ( ghsv ) of 17 , 000 h − 1 . we then monitored co 2 production as a function of temperature and calculated the temperature corresponding to conversion of ½ of the methane feed stream ( t ½ ). this value represents a single figure of merit , which can be used to compare the activity of a number of catalyst samples . we measured activity for methane combustion on the samples after preparation and also after aging . the first stage catalysts were aged at 800 ° c . for both 16 and 50 hours while the second stage catalysts were aged for 50 hours at 1300 ° c . in all cases , we simulated a combustion environment during the aging process by passing air containing 6 % co 2 and 6 % h 2 o over the catalysts . we prepared one series of catalysts of the stoichiometry sr 1 − x pd x al 11 o 18 where x = 0 . 5 , 0 . 25 , 0 . 125 , 0 . 0625 , 0 . 0312 . this series produced palladium loadings ranging from 7 . 8 wt % at x = 0 . 5 to 0 . 25 wt % at x = 0 . 0312 . the results of tests performed on the “ as prepared ” and aged samples ( aged at 800 ° c .) in this group are shown in fig4 . the figure shows that the t ½ values for pd loadings of 1 wt % and higher are no greater than 450 ° c . considering that oxidation activity begins at temperature of at least 50 ° c . lower than the t ½ values , these catalysts appear to have the sufficient initial activity that would be suitable in a gas turbine . in addition the data presented in this figure shows that all catalysts in this group are very stable . in all cases except the lowest palladium loading , there is very little change in the t ½ value between the “ as prepared ” and “ aged ” samples . aging at 800 ° c . under combustion conditions does not significantly affect catalyst activity except at low loading of pd these results particularly for the catalysts containing 0 . 5 , 1 . 0 and 2 . 0 wt percent pd , are very encouraging for several reasons . first , the activities of these materials are very high ( t ½ of about 400 ° c .) under these test conditions . in addition , the figure shows that the activity of three of these catalysts ( 0 . 5 , 1 . 0 and 2 . 0 wt % pd ) did not change significantly after being exposed to a temperature at least 100 ° c . higher than would be encountered under operating conditions for a period of 50 hours . finally , the catalysts having the best combination of activity and stability included those that contained no more than about 2 wt % pd , which suggests that these catalysts would not be too expensive to be used in a combustor . we have also identified catalysts that would be suitable for use in the second stage of the combustion reactor where much higher temperatures would be encountered . we prepared several groups of catalysts substituted with three metals , la , sr , and mn . we synthesized samples of the stoichiometry la x sr y mn z al 11 o 18 − α ( in which we set z equal to 0 . 2 , 0 . 4 , 0 . 6 , 0 . 8 , where a is a number that makes the compound charge neutral , and for each value of z , we prepared samples where x / y = to 0 . 25 , 0 . 5 , 1 . 0 , and 2 . 0 , while maintaining the sum of x + y equal to 1 − z . our initial results indicated that the best results were obtained for z = 0 . 4 and x / y equal to 1 . we then prepared additional samples where z was equal to 0 . 4 and varied the ratio of x / y from 0 . 5 to 1 . 5 in increments of 0 . 1 . the t ½ values obtained for the as prepared and aged samples of this group are shown in fig5 . this figure shows that the as prepared t ½ values for this group of catalysts ( white bars ) vary from 544 ° c . to about 575 ° c . again this group of catalysts has acceptably high initial catalyst activity . ( the fluid temperature likely will be in excess of 650 ° c . before it contacts the second stage catalyst ). moreover , the results following aging at 1300 ° c . for a 50 hour period , also included ( as the black bars ) indicate that several catalysts in this group have excellent thermal stability as evidenced by little or no change in t ½ compared to the as prepared values . an increase in t ½ indicates a decrease in activity and a decrease in t ½ indicates an increase in activity . for example the post aging t ½ value for catalysts with la / sr of 0 . 5 , 0 . 6 , 0 . 8 and 1 . 1 are all slightly lower ( within experimental error ) compared to the “ as prepared ” values , indicating that exposure to the severe conditions ( 1300 ° c . with 6 % water ) caused no loss in combustion activity . further , even the catalysts that lose activity ( e . g ., those with la / sr of 1 . 2 - 1 . 5 ) do not undergo severe losses . the largest increase observed in t ½ was about a 45 ° c . increase representing an increase of about 8 % ( 45 / 545 ). typically , the t ½ increases by only 30 - 40 ° c . following the aging step . considering that the sintering process occurs most rapidly in early periods of exposure to extreme conditions , the results of our testing demonstrate that hexaaluminate catalysts with these formulations have excellent thermal stability . the surface areas obtained for this group of catalysts were measured by the bet method and are shown in fig6 . overall these results are consistent with the activity measurements presented in fig5 . the data show that the surface areas for catalysts with la / sr ranging from 0 . 5 to 0 . 8 change very little following aging . for la / sr = 0 . 8 we see that the surface area is about 10 m 2 / g both before and following aging . in addition catalysts with la / sr = 0 . 7 and 1 . 1 exhibit only about 10 % losses in total surface area . there is relatively good correlation between surface area and catalyst activity . the catalyst with la / sr = 0 . 7 exhibited a small loss in activity following aging as evidenced by an increase in t ½ from 575 to 590 ° c . however catalysts with la / sr = 0 . 8 and 1 . 1 exhibited no increase in activity following aging , which is consistent with surface area data presented here . we performed a direct comparison of two catalysts prepared by the alumoxane / metal - exchange technique to those of identical stoichiometry prepared by coprecipitation methods . we measured the surface area of each catalyst as prepared and then aged each sample at 1300 ° c . for 50 hours in an environment containing 6 % water and 6 % co 2 in order to simulate a combustion environment . we first prepared a stock solution of alumoxane , specifically 2 ( 2 -( 2 - methoxyethoxy ) ethoxy ) acetato alumoxane ( meea - alumoxane ). we combined 7000 g of a commercially available boehmite sol ( disperal sol p2 ) solution with 1050 g of psuedo boehmite ( catapala ) ( alooh ·× h 2 o ) and 472 . 5 g meea . the solution was maintained at 94 ° c . for 24 hours . to prepare the la 0 . 27 sr 0 . 33 mn0 . 4al 11 o 18 hexaaluminate ( la / sr = 0 . 8 ), we combined 50 g of the meea - alumoxane with 0 . 979 g manganese acetylacetate [ mn ( acac ) 2 ], 1 . 136 g lanthanum acetylacetate [ la ( acac ) 3 ], and 0 . 922 g strontium acetylacetate [ sr ( acac ) 2 ]. for the la 0 . 30 sr 0 . 30 mn 0 . 4 al 11 o 18 , hexaaluminate , we combined 50 g of the meea - alumoxane with 0 . 979 g manganese acetylacetate [ mn ( acac ) 2 ], 1 . 265 g lanthanum acetylacetate [ la ( acac ) 3 ], and 0 . 828 g strontium acetylacetate [ sr ( acac ) 2 ]. we mixed the solutions at 60 ° c . for four hours and then dried the mixture for 16 hours at 120 ° c . after the material was dry , we heated it at 3 ° c ./ min to 800 ° c . and held the material at this temperature for one hour before cooling it to ambient temperature . we then removed the material , ground it to a fine power , and calcined it again . for the second calcination , we heated it at 3 ° c ./ minute to a maximum temperature of 1300 ° c . and maintained the material at this temperature for one hour before cooling . we prepared the coprecipitated la 0 . 30 sr 0 . 30 mn 0 . 4 al 11 o 18 sample ( la / sr = 1 . 0 ) by dissolving the 82 . 5 g al ( no 3 ) 3 · 9 h 2 o , 2 . 597 g la ( no 3 ) 3 · 9 h 2 o , 1 . 270 g sr ( no 3 ) 2 and 1 . 423 g mn ( no 3 ) 2 in 100 ml water . we then heated the solution and slowly added a base made by dissolving 40 g ammonium carbonate in 200 ml water , until the ph reached a value of 10 , where precipitation was complete . after filtering the mixture , we dried the solid residue overnight at 120 ° c . and calcined the material at 1300 ° c . for one hour . we prepared the coprecipitated la 0 . 27 sr 0 . 33 mn 0 . 4 al 11 o 18 ( la / sr = 0 . 8 ) sample by dissolving the 82 . 5 g al ( no 3 ) 3 · 9 h 2 o , 2 . 340 g la ( no 3 ) 3 · 9 h 2 o , 1 . 397 g sr ( no 3 ) 2 and 1 . 423 g mn ( no 3 ) 2 in 100 ml water and followed the same precipitation procedure described above . we compared the surface areas of the two hexaaluminate catalysts prepared by the alumoxane method to those prepared by coprecipitation both after preparation and after aging at 1300 ° c . in a simulated combustion environment for 50 hours . the results of this comparison are shown in fig7 . the figure shows that the two catalysts prepared by the alumoxane method have higher initial surface areas , 9 . 7 and 9 . 2 m 2 / g for la / sr = 1 . 0 and 0 . 8 respectively , compared to those of the corresponding compositions prepared by coprecipitation ( 2 . 9 and 2 . 8 m 2 / g , respectively ). in addition , the coprecipitated samples lose a much higher percentage of surface area after aging compared to the catalysts prepared by the alumoxane / metal exchange method . after 50 hours in a simulated combustion environment the coprecipitated samples have surface areas of 0 . 15 and 0 . 11 m 2 / g for compositions la / sr = 1 . 0 and 0 . 8 respectively . these values represent losses of 95 and 96 % of the original surface areas of these coprecipitated samples . on the other hand the data gathered shows that catalysts prepared by the alumoxane have much greater thermal stability . for the sample where la / sr = 1 . 0 the surface area following aging is 4 . 9 m 2 / g , which is 51 % of the surface area prior to aging . for the sample where la / sr = 0 . 8 , the surface area after aging is 5 . 9 m 2 / g which is 64 % of the original value . this comparison demonstrates that the use of the alumoxane / metal exchange method produces hexaaluminate catalysts that are significantly more thermally stable that those prepared by other methods , particularly by coprecipitation . it is believed that the metal exchange method results in a hexaaluminate with a more homogeneous structure . we also conducted aging studies on selected catalyst for periods of up to 300 hours to determine the effect of stoichiometry and calcination conditions on the thermal stability of the hexaaluminate materials . we prepared five different catalyst samples by the alumoxane method . three samples had the composition la 0 . 27 sr 0 . 33 mn 0 . 4 al 11 o 18 ( la / sr = 0 . 8 ); two of these were calcined at 1300 ° c . for one hour and the other was calcined at 1400 ° c . for one hour . the fourth sample had the composition la 0 . 30 sr 0 . 30 mn 0 . 4 al 11 o 18 ( la / sr = 1 ) and was calcined at 1300 ° c . for one hour , while the fifth sample had the composition la 0 . 23 sr 0 . 37 mn 0 . 4 al 11 o 18 ( la / sr = 0 . 6 ) and was calcined at 1400 ° c . for one hour . all samples were aged at 1300 ° c . in an environment containing 6 % water and 6 % co 2 . the results of these tests are shown in fig8 . the data collected shows that the samples tend to lose surface area for the first 100 hours of aging , but after this period the rate of loss is greatly reduced . in most cases , there is very little loss in surface area after 200 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