V O L U M E 2 4 , N O . 9, S E P T E M B E R 1 9 5 2 (7) Fassel, V. A,, Cook, H. D., Krote, L. C., and Kehres, P. W., Iowa State College, ISC-210 (March 1952); Spectrochim. A c t a , 5, KO.1 (1952). (8) Fassel, V. A., and Wilhelm, H. A,, J . Optical SOC.Am., 39, 18793 (1949). (9) Feldman, Cyrus, ASAL.CHEIM., 21,1041 (1949). (10) Gatterer, A., and Junkes, J., “Atlas der Restlinien. Spektren der Seltenen Erden,” Laboratorio Astrofisico della Specola Vaticana, 1945. (11) Gatterer, A,, and Junkes, J., Spectrochim. A c t a , 1, 31-46 (1939). (12) Harrison, G. R., Rosen, K.,and McNally, J. R., Jr., J . Optical SOC.AvL.,35,658-69 (1945). (13) Hilt, R. C., and Nachtrieb, hi. H., ASAL. CHEM.,20, 1077-8 (1948). (14) Hopkins, B. S., RlcCarty, C. K,,Scribner, L. R., and Lawrene, hI.,IND. ENG.CHEX.,ANAL.ED.,10, 184-7 (1938). (15) King, A. S., A s t r o p h y s . J., 72,221 (1930). (16) Lopez de Azcona, J. JI.,Bol. inst. geol. y minero Espaiia, 55,270433 (1941). (17) McClelland, J. A. C., A n a l y s t , 75, 392 (1950); 74, 529-36 (1949). (18) McCutchen, R. L., unpublished work, Oak Ridge National Laboratory, 1951. (19) McNally, J. R., Jr., and Harrison, G. R., J . Optica2 Soc. A m . , 35, 584-96 (1945). M o d e m Phys., 14, 96 (1942). (20) Neggers, W ,F., RED. (21) Meggers, W. F., and Scribner, B. F., J . Research S a t l . BUT. Standards, 5,73-81 (1930).
1403 Ibid., 19, 651-64 (1937). hloeller, T., and Brantley, J. C., “Observations on the Rare
Earths. Some Studies in the Absorption Spectra,’’ University of Illinois, Rept. NP-1105 (Sept. 1, 1949)- ANAL.CHEM., 22. 433-41 (1950). Moore, C. E., “Atomic Energy Levels,” Natl. Bur. Standards, Circ. 467, Vol. I (1949). Piccardi, G., Spectrochim. A c t a , 1, 249-69 (1939). Rosen, N., Harrison, G. R., and McNally, J. R., Jr., P h y s . REU., 60, 722-30 (1941). Russell, H. K.,Albertson, TT., and Davis, D. N., Ihid., 60,64156 (1941). Russell, R. G., University of Chicago, Rept. CC-2237 (December 1944). Scribnir, B. F., and Mnlhn. H. R., Katl. Bur. Standards R e p t . A-2907 (Sept. 14, 1945). Selwood, P. K., IND.ESG.CHEM.,ANAL.ED.,2,93-G (1930). . 20, 1093-6 Short, H. G., and Dutton, I T I,., A N ~ LCHEM.. (1948). Smith, D. RI., and Wiggins, G. SI., A n a l y s t , 74, 95-101 (1949). and Ziegler, IT. T., ANAL. CHEY.,21, 1422-3, Spicer, TI-. M,, (1949). I
,
RECEITED for review 1Iarch 5 , 19.52. Accepted J u n e 30, 1952. Presented a t t h e Pittsburgh Conference on Analytical Chemistry and dpplied Spectroscogy, Pittsburgh, P a . , March 5 t o ?, 1952. Based on work performed for t h e Atomic Energy Commission b y Carbide and Carbon Chemicals Co., a Division of Union Carbide and Carbon Corp., a t t h e Oak Ridge Xational Laborat0i.g.
Spectroscopic Determination of Metals in Silica-Alumina Cracking Catalysts J. P. PAGLIASSOTTI
A”D
F. W. PORSCHE
Research Department, Standard Oil C o . (Indiana), Whiting, Ind.
I
N T H E catalytic cracking process, metallic contaminants
introduced into the catalyst during manufacture or from feed stocks impair catalytic activity ( 3 9). Accurate knowledge of the composition of cracking catalyst is therefore essential to the efficient operation of the process. Commercial catalltic cracking unite currently employ either naturally occurring or synthetic silica-alumina catalysts. The approximate composition of natural catalysts is: silica, 80%; alumina, 15%; magnesia, 2%; and total impurities, 2%. The synthetic niaterials have about the same silica and alumina content, but impurities, including magnesia, amount to about 0.5yo. +ln analysis for mrxtallic constituents should include iron, vanadium. nickel, copper, lead, calcium, magnesium, and sodium. Chemical methods for the determination of these elements are time-consuming, sometimes lack adequate sensitivity, and usually require a high degree of skill. They are therefore not well suited for routine control. Spectrographic methods (2, 6, 6, 17) generally do not mffer from these disadvantages. However, the spectrographic analysis of catalyst presents difficulties relative to ita mineralogical composition and chemical combination, the preparation of standards, hygroscopic activity, and the choice of internal-standard lines. Nonmetallic samples, particularly those of natural oi igin, vary in crystal structure. This fact, plus the amphoteric nature of some of the components of such materials and the number of possible combinations of anions and cations, makes the relative volatilization rates of the elements an important variable that affects spectral intensity. Certified standard samples are not available, so that spectroscopic standards must be prepared by adding to fresh catalysts the salts of the impurities sought. The preparation of kilogram quantities of homogeneous solid standards comparable to actual samples in physical and chemical form is a difficult problem. Difference in hygroscopic activity among catalysts and vary-
ing times from sample preparation to analysis may be other important variables. An ignition proccdure designed t o put all samples on a similar volatile-free basis prior to analysis is desirablp, although it may render the catalyst an active water adsoi ber. The internal-standard line may be chosen from the spectiuni of an element present in the catalyst in constant amount, or from the spectrum of an element added to each sample. Of the elements in cracking catalysts, silicon is present in relatively constant amount, but the simplicity of its spectrum and the remoteness of its lines from those of the element8 sought detract from its usefulness as an internal standard. All these problems can be minimized by working with solutions This technique produces a sample that is representative in (’omposition and uniform in physical form. Standards can be prepared from stocks of pure chemicals, and hygroscopic activity is of no serious consequence. Silicon cannot serve as the internalstandard element because it is evolved during the dissolution of the sample, but other elements can br readily incorporated These considerations have led to the development of a method in which the catalyst samples art1 dissolved with the aid of hydrofluoric acid, cobalt is added as the internal standard, and the spectra arc obtained by means of solutions excitation (IO, I f ) Sodium is determined bv mean? of the flame photometer. EQUIPMENT 4N1) REAGENTS
The spectrographic equipment includes a %meter grating spectrograph, an excitation unit providing both spark and arc excitation, a Universal arc-spark stand, a solutions-excitation apparatus, and a comparator-densitometer. This e uipment and the necessary apparatus for making commercia carbon dioxide or helium available a t the arc-spark stand have been described (IO). The flame-photometer apparatus is basically the Model 52A Perkin-Elmer instrument modified to use the Beckman atomizer
1404
ANALYTICAL CHEMISTRY
.kccurate knowledge of the concentration of certain metallic contaminants of cracking catalysts is essential because of their role as catalyst poisons. A spectroscopic method has therefore been developed for the rapid and accurate determination of metals in silica-alumina catalysts. After dissolution of the sample w-ithacid, the iron, vanadium, nickel, copper, lead, calcium, and magnesium in the resulting solution are determined spectrographically by either interrupted-arc or spark excitation. Sodium is determined by means of the flame photometer. The solution of the sample obviates the effects of crystal form and ion linkage. Comparison on a similar basis of naturally occurring minerals, s y n thetically prepared catalysts, and laboratory standards is therefore feasible. The method is capable of ready extension to the determination of additional metals as well as to the analysis of other solid-phase samples amenable to solution.
and heated spray chamber ( 4 ) . The I'erkin-Elmer burner has been modified to provide for oxygen enrichment of the flame and to include t,wo pressure-release valves as safety devices in the event of backfire. Propane is used as the fuel. Calibration standards were prepared by dissolving reagentgrade nickelic oxide, sodium carbonate, lead oxide, calcium carbonate, standard iron wire, and high-purity copper in perchloric acid. Reagent-grade vanadium pentoxide was dissolved separately in perchloric acid to which about 10% by volume of 50% hydrofluoric acid reagent had been added to aid in solution. The hydrofluoric acid was driven off by evaporating the solution to perchloric acid fumes, and the t x o solut,ions were combined, Each of the chemical compounds was analyzed chemically prior t o use. The final catalyst-impurity standard contained 1.25% by volume of perchloric acid and the following concentrations by weight of impurity elements: Iron, 0.03%; vanadium, O.Oqj%; nickel, 0.005%; copper, 0.001%; lead, 0.01%; and calcium, 0.04%. The catalyst-impurity reagent was stored in So-Sol-Vit glass bottles. Spectrographic calibration standards xere then obtained by adding known amounts of this solution to portions of fresh catalyst as each standard was needed. The fresh catalyst contained small but significant amounts of iron, copper, lead, and calcium. The concentration of each of these elements was determined from spectra obtained by the resent method and computed by the addition method described gy ilhrens ( 1 ) . The necessary corrections were then applied t'o the catalyst standards. Calibration standards for magnesium and sodium were based upon chemical analysis. For magnesium, standards were obtained by adding weighed quantities of analyzed magnesium carbonate to fresh solid catalyst. Standards for the flamephotometric determination of sodium were prepared by adding knon-n quantities of analyzed sodium chloride to sulfuric acid solutions of high-purity aluminum. Cobalt and lithium internal-standard solutions were prepared for spectrographic analyses and flame analyses, respectively. High-purity cobalt oxide was dissolved in perchloric acid and diluted with distilled water; the completed Eolution contained 0.iO mg. of cobalt per ml. and 15% of perchloric acid by volume. Reagent-grade lithium carbonate was dissolved in 65% by volumc sulfuric acid to yield a solution containing 1.5 mg. of lithium per ml. A N A LYTlCA L PROCEDURE
Sample Preparation. One gram, &0.001 gram, of catalyst t h a t has been ignited to 1500' F. for 1 hour is weighed into a platinum dish, and 5 ml. of the cobalt internal-standard solution are added. (If the sample is to be a standard, the required volume of the catalysbimpurity standard is introduced at this point.) Several drops of hydrofluoric acid are added cautiously. If no evolution of silicon tetrafluoride takes place within a ferr seconds, the dish is heated carefully until evolution begins. The additions of hydrofluoric acid are repeated and heating is continued as necessary until no further evolution takes place. An excess of 1 to 2 ml. of hydrofluoric acid is then added and the solution is evaporated until the evolution of perchloric acid fumes begins. The decomposition of the catalyst Pample will require
approximately 10 minutes. The residue in the dish is taken u with 10 ml. of dilute hydrochloric acid, transferred to a smaE beaker, and boiled gently until the solution clears. The hot solution is filtered into another small beaker, 2 ml. of perchloric acid are added, and the solution is evaporated until the evolution of perchloric acid fumes begins. The sample is then diluted with 5 ml. of distilled water and 0.25 + 0.01 gram of lithium carbonate is added to neutralize most of the excess acid. Sulfuric acid may be substitut,ed for perchloric acid in the preceding analytical procedure, if the determination of lead and calcium is not to be included in the analytical scheme and if some loss in sensitivity can be tolerated. Spectrographic Technique. A portion of the prepared sample is transferred to a small combustion boat and placed in position for exeitation. The disk is rotated a t 5 r.p.m. Excitation may be by interrupted-arc or spark discharge; in either case, the rotating electrode is made positive. For interrupted-arc excitation, the source settings are 20 microfarads, 480 microhenries, and 50 ohms, and the output voltage is adjusted to 940 volts. The resistance in the igniter circuit is adjusted to 10 ohms. These discharge condit.ions are the same as those upcd in the analysis of blood serum (14). The exposure conditions are 25-second prespark and 75-second exposure. A carbon dioside atmosphere is maintained at the analytical gap by regulating the pressure in the transfer line to 7 inches of mercury. For spark excitation, the source is adjusted to 100 volts in the primary circuit and a capacitance of 0.007 microfarad and an inductance of 360 microhenries in the secondary. The exposure conditions are 30-second prespark and 25-second exposure. A helium atmosphere is maintained a t the analytical gap by regulating the pressure in the transfer line to 4 inches of mercury. The discharge is focused on the grating by a 5-inch cylindrical lens with its axis in the horizontal plane, and a slit width of 50 microns is used. 3Ietallized quartz filters having nominal transmissions of 50% are positioned in the camera so as to interlines. Spectrum cept the calcium 3158 A , and the copper 3274 -4. Analysis KO.1 film is used and calibrated with an iron spectrum photographed through a ralibrated two-step filter. Filmprocessing, photometry, and calculation techniques are conventional (10). Film background is negligible and is ignored. The spectral lines used and relat:.d data for each of the two sets of excitation conditions are s h o x r :ii Tnhle I.
Table I. .Snalytical Line. and Related Data Fleinent
Length,
A.
Interrripted-Xic Indexa, at.
%?E'
Spark Range, wt. % wt. %
Indexa,
Int 3044 0 Iron 2599 4 0 015-( 0.06 2607 0.12 0 03 -0.30 ... ,... .. Iron Iron 2621 , ', . . . . . ., . . 0.60 0.06 -0.6 Iron 3008.1 0.42 0.10 - 1 . 5 1.00 0.50 -1.5 0.005-0.05 Vanadiiim 3185.4 0.018 0.005-0.05 0.014 0 . 0 3 -0.15 ... .... . ., Vanadium 2952. 1 0.08 Kickel 3050.8 0.010-0.10 0.07 0 . 0 1 -0.10 0.04 3274.0 0 . 006 0.002-0.015 0.013 0.003-0.020 2833.1 0 . 0 1 -0.'; 0.16 0.045 0 . 0 3 -0.30 0.10 -: Calcium 3 158.9 0.35 1.40 0.30 -: Magnesium 2781,4 Concentration a t which intcnsitirs of a n a l p i s line and standard line are equal.
Cobalt
:::CY (I
Flame-Photometric Determination of Sodium. For the determination of sodium, 2 st 0.001 grams of ignited sample are weighed into a platinum dish. Following the addition of 5 ml. of the lithium internal-standard solution, decomposition of the sample proceeds in the manner previously described, except that heating is continued until sulfuric acid fumes are no longer evolved. The evolution of fluoride ion must be more complete than is necessary for the spectrographic determination of the other contaminants, because remaining fluoride may attack the glass atomizer. The flame-analysis technique and calculation^ follow the conventional procedure ( 1 2 ) . DISCUSSIOE
Data showing the precision of the method arc presented in Table 11. The precision for each metal is wfficiently high to allow the reliable use of a single analysis. In order to appraise the accuracy of the method, experiments were conducted wherein known amounts of various impurity elements were added to portions of fresh catalyst. Table I11
V O L U M E 2 4 , NO. 9, S E P T E M B E R 1 9 5 2 ~
~
_ ~
__
_
_____
1405 ~_
obtained with varialile magnesium content by each of the two excitation conditions are shown in Table IV. Yo interferences m r e noted in thtl Average flame-analysis procedure for sodium. deviation The data in Table I indicate that the method % 2.7 has high spectral sensitivity. The sensitivitb- de... 1 9 pends in part on the use of special atmospheres to .2 .. 4. suppress background spectra. Rare g a m (gj, 3.0 ox)-gen (ILi), carbon dioxide ( 7 ) ,and stclam (13j16) 1.9 have been used t o minimize cyanogcin bands or air spectra. The present method makes use of carbon , . 1 5 dioxide to suppress the background contiriuum in __ the arc and helium to suppress air sprxt,ra in thv spark. While helium is also effective in suppressing the background continuum in the arc, it docr not appear to offer any advantage over the cheaper carbon dioxidc. The conversion of the sample t o a solution of its perchlorate salts also coiiti,ibutes to the sensitivity of the method. The high solubility of metal perchlorates allows high salt concentrations in the test solut,ion. Furthermore, the volatility of perchlorates favors high spectral sensitivity. The ignition procedure, included in order to put d l samples on a comparable volatile-free basis, was checked for possible loss of impuritjy elements. Table V demonsti.;ctcs that no such 109s was incurred Kith t,\vo catalyst ~ : ~ i i i ~ i\r-ithtlr:ia-n Iw from (>omniercial cata1i.t ic cracking unit h .
Table 11. Precision of Determination of Seven Elements Interrupted-A.r
Iron Iron iron Iron 1.3 ria diu ni Vanadium Sickel r:opper Lead Calcium Ilagneeium
8
2599.4 2607,l 2621.7 3008.1 3185.4 20.52, 1 3050 S 3274 0 2833 1 3158.9 2781.4
18
0.07 0.14
2.4 1.8
20 26 8 26 26 26 26 6
0.33 0.037 0.11 0.05 0.010 0.11 0.60 1 . bo
...
..
.~
... -.
'ypical ialysis, vt. % 0.07
7
...
2.4 2.1 1.3 2.1 2.1 2.3 3 7
i6
0.31
24 7 24 24 24
0.11 0.024 0.005 0.068
1 2
6
...
..
0:0is
, . .
1.50
.. .
presents the data obtained. The errors in rtxoveries were all .mall. Because the solutions-excitation technique permits the preparation of stmdards t h a t closely duplicate solutions of the samples, and bcrause the use of solutions avoids any effect due t o crystal structure or ion linkage, these studirs provide R reliahlc index of the validity of the method. ~_________
~
Table 111. .iccirracy of Determination of Seren Elements ~y~~~ Length, Elemcnt
A.
Iron Iron Vanadium Vanaditi in Nickel Copper Lead Calcium Magnesiun
3008.1 2607.1 3185.4 2952.1 3050.8 3274.0 2833.1 3158 9 2781.4
Interrupted-Arc Added, Found,
mt. %
Wt. %
Spnrk Added, Found, wt. % ut. 7c
0.25
0.25
0.030 0.050 0.037 0.0070 0.11 0 68 1 50
0.029 0.048 0.036 0.0071 0.11 0.69 1.41
0.25 0.25 0.030 0.050 0.037 0.0070 0.11 1
io
0.27 0.28 0.030 0.053 0.039 0.0071 0.11
1 4;
z
~
. .
Table I V . l I g , Wt c L 0.02" 0.5 1.5
3.0
Efl'ect of \-ariable fiIagnesium Content on .ipparent Composition Fe 0.095 0.005 0.100 0.082
0.02O 0.095 0.5 0.093 1.5 0,107 3.0 0.115 Fresh catalyst.
Apparent Composition, We:eht C; Y S I cu Ph Interrupted-Arc Excitation 0.0024 0,018 0.012 0.015 0.0025 0.020 0.012 0.014 0.012 0.014 0.0025 0 020 0,0027 0.021 0.011 0.016 Spark Excitation 0.0048 0.057 0.019 0.024 0.066 0.018 0.026 0.0048 0.070 0.026 0.0048 0.019 0.018 0.029 0.0055 0.088
Ca
0.27 0.20 0.27 0.30 0.20 0.36 0.33 0.35
~
~~~
__
.
~
Tahle \-. Effect of Ignition on 31etals Analysis .ipparrnt Composition, \$-eight % . 1'e Txi 1'1) 0.36 0.013 0,020 0.081 0.36 0.013 0.020 0.062 ~~~
Unignit?ii Ignited Urnignited Ignited
The t x o sets of iexcitation conditions were studied concurrently (luring the course of the development of the method and both Ivere found t o be satisfactory. The iderrupted-arc discharge is preferred brcause the shhility of its working curves is somcwhat better. It is possible to determine sodium spectrographically with the (~tliercontaminant elements, although not with the accuracy j)nqsible with the flame phot ome t er. Precision and accuracy of the flame-photometric determination r ~ t ' sodium are bot,h xvithin &1y0of the amount present. The precision was nieasured by making replicate determinations on single saniple containing 0.040% sodium. The accuracy was measured hy adding kno1T-n quantities of sodium chloridt, to 5tandards of linown composition. Possible interference effects were studied by varying in turn the concentration of each of the element's. I n each case, the miitration of the element varied was raised t o a level appreciably higher than is expected in actual catalyst samples. S o signifie m t interference effect was noted except for a m:iynesiuni influence on the lead analysis with spark excitation. The data
.
1B 0.llH 0 118
...
..
0.38 0.36
~
0.013 0 012
0 019 0.019
0.065 0 066
~~
~-
Cll
0,0024 0.0024 0.0023 0 0026
The solutions method has supplanted the solid-phase catalystanalysis trchnique formerly used in this laboratory. Alinor modificationc: have found valuable application for the analysis of othcr catalvtic materiale The solutions method is particularly attractive because it IS independent of other methods for the analysib of standaids. The method is recommended as a proceduic having ycncrnl utility n-hcncvcr solid qamples are amenalile to ~ o l u t ~ o i i LITER-iTURE CITED
(1) .ihreiia, L. €I.. "Spectrochemical Ailalysis." p. 135, Cambridge, Mass., ;\tldiaoii-~TesleyPress, 1050. ( ? H E M . , 19, 238 (1 947). (2) Burrlrtt. 13. .I.,aud .Jones. I.. C.. ISAI.. ( 3 ) Duffy. H. J.. and Hart, H. )I., C h ~ m E . N Q .Progress, 48, 344 (1952). Jr.. Halves. R. C . , aiid Ueckniaii. A , O., .ISAL. (4) Gilbert. I-'. T,, CHEX,22, 772 (1950). (5) Guiin, E. L.. I h i d . , 21, 598 (1949). (6) Harmon, D. D.. arid Russell. R. G . , Ibid,, 23, 12.5 (1951). ( 7 ) Harvey. C'. E.. "Spectrochemical Procedures," p. 275, Glendale, Calif.. -Lodied Research Laboratories. 1950. ( 8 ) AIansfield,-k.0 . . .Jr.. Fuhrmeister, .J. I?,, and F r y , D. L., J . Optiatl Yoc. A m . , 41, 412 (1951). (9) Mills, G. A , . I n d . Eng. Chem., 42, 182-7 (1950). (10) Pagliassotti, ,J. I'.. atid Porsche, F.IT-., -4x.t~.CHEM.,23. 198 (1951). (11) Ibid., p. 1820. (12) Perkin-Elmer C'orp.. "Instruction hlnnual, Flame Photometer Y o d e l 52A." 1949. (13) Smith, D. hl., and Wiggins, G . AI., Spectrochim. A c t n , 3 , 325 (1948). (14) S p e c t r o g r a p h u ' s S e w s Letter, 2, S o . 10, Glendale, Calif., Applied Research Laboratory (1949). (15) Steadman. C. T., Phys. Rev., 63, 322 (1943). (16) Wiggins, G. h l . , A n a l y s t , 74, 101 (1949). (17) Zogg, R. E., Soc. A p p l i e d SpectroscoplJ BuZZ.. 5, S o . 3 (1951). RECEIVED for review 3Iarch 6. 1952.
Accepted July 9.1952.
