Differential Thermal Analysis N e w Technique for Testing Silica-Alumina Catalysts ROBERT L. STONE, Robert 1. Stone Co., 2607 Hillview Road, Austin, Tex. HOWARD
F.
RASE, University o f Texas, Austin, Tex.
b A differential thermal analyzer can b e used to measure activity of fluid cracking catalyst in 5 minutes. The adsorption (below the decomposition temperature of the gas) of an active gas on a catalyst sample is compared simultaneously with adsorption of the same gas on a physically similar but catalytically inert material. The difference in temperature rise between the catalyst and the inert material is proportional to the number of active sites and thus to catalyst activity. This temperature difference, platted L activity, against the Jersey D shows correlation of temperature difference to activity. The differential thermal analyzer consists of a doublecavity sample holder, a heater, a system for injecting gases into the samples, and suitable instrumentation. The active gas should b e some strongly chemisorbed material; for fluid catalyst testing, water, ammonia, and organonitrogen compounds are satisfactory. Ammonia was most convenient, because it did not condense a t any point in the apparatus and is obtainable in gaseous form. Other uses of the analyzer include determinations of carbon content of catalysts, studies on catalyst poisoning, measurement of heats of chemisorption, and studies of chemical and physical properties of catalysts.
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A
differential thermal analyzer makes it possible to measure the activity of a fluid cracking catalyst in 5 minutes. The method for mineral identification and study utilizes the energy evolved or absorbed by characteristic reactions when the test mineral is in a n environment of rising temperature. The temperature rise or fall caused by these reactions is measured by thermocouples and the electromotive force of the thermocouples is recorded by sensitive instruments giving peaks or loops corresponding to the reactions. A modification of the method can be applied to catalyst testing, by making use of the heats of adsorption of catalyst poisons a t approximately constant temperature. The height of the peak KEWLY DESIGSED
obseryed is a function of the amount of chemisorption, which in turn is related to activity. FUNDAMENTAL CONSIDERATIONS
Common Activity Tests. Satisfactoiy operation of a catalytic cracking unit depends largely on maintaining a constant-activity catalyst. When the activity declines, as shown by periodic tests. fresh catalyst is added. At present the most reliable methods for testing fluid catalysts ( 1 ) employ pelletized or fluidized samples in a bench-scale cracking unit through which a standard gas oil is passed a t prescribed conditions of temperature and space velocity. The quantities of gas and liquid products are measured, and the liquid is analyzed by distillation. These tests are time-consuming and expensive. Simpler tests (3, 4, 8) yield data that are functions of total surface area. These data, however, may not always be proportional to activity, because it is the number and activity of active sites, not surface area, that determine catalytic activity. Chemisorption and Activity. lIi11s3 Boedpker, and Oblad ( 2 ) have shon n that the \\eight of a strongly adsorbed material such as quinoline on silicaalumina catalyst correlates very \\ ell with catalyst activity. Quinoline and other organonitrogen compounds such as pyridine and piperidine poison the silica-alumina catalyst. Such poisons are qtrongly chemisorbed by a catalyst surface, and thus rapidly cover all the active sites. Xlthough it is difficult to draw a fine line between chemisorption and physical adsorption. it can be definitely stated that chemisorption involves forces of much greater strength than the ordinary physical \'an der Kaals forces. These stronger forces cause higher heat. of adsorption and sloiyer desorption. Hence. chemisorption may be characterized as that portion of adsorption that exhibits high heat of adsorption. The chemisorbant is difficult to remove, compared to the material physically adsorbed. It is reasonable to assume that the amount of chemisorbed material is proportional to the number and the
activity of active sites and should tlierefore correlate with the catalyst activity. This was found to be the cahe b y 1Iills and associates, who determined the amount of Chemisorbed gas by subtracting from the total absorbed gas the portion that was easi]!- removed (physically adsorbed gas.), These quantities were determined gravimetrically; although this test is wiieirliat more rapid than standard catalyst actiyity tests. it is tedious and not readily applicable to routine testing. Thermal Measurements of Chemisorption. Thermal chanqt.s accompanying chemisorptioti are perhaps more significant,in rplatinn to catalyst activity t h a n t h e actual w i g h t s of the chemisorbed material. 1 1 1 the active sites on the catalyst wrface are not alike; some :ire .stronger than others. When a gas is rhemisorbed. the portion adsorbing o n the niore active sit,es produce5 larger heats of adsorption than that occurring on the less active sites. Heats ot' chemisorption per unit weight of catalyht observed for samples exposed to gases n.hich are strongly chemisorbed nhoiild compare with not only the iiuniher hut also the activity of the active 4te; and thus produce a good correlation of catalyst activity. Even for strongly chenii~orbetlm i teriala such as poisoiiz. the Iveight' of gas chemisorbed is less than that physically adsorbed. In any gnvimetric measurement this smaller amount must be determined from the iiifference in two large numbers, totu! :idsorption minus physical adsorption. I11 thermal measurement, o n the other hand. the heats of chemisorption are m u c h greater than those of phyr;icai :dmrption. Even t,hough the arnountr of chemisorbed material are imuli, the heat effects are large chompareil to those of the phyFicnlly :idmhcrl material. Hence a useful activity te-t might be developed by measuring the total heat of chemisorption per unit weight of catalyst or, preferably, w i l e easily measurable functions of this heat. A special differential thermal analyzer was designed and constructed (Robert L. Stone Co., Austin. Tex.) to accomplish such measurement. The unit II-W first VOL. 29, NO. 9, SEPTEMBER 1957
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developed for and used in the study of clay minerals and carbonates (5-7). In designing the machine and developing the tests, the following principles were considered. The test gas should he some strongly chemisorbed material such as an organonitrogen compound, ammonia, or steam. These gases will produce the highest heats of adsorption and thus increase the accuracy of measurement. The test must be rapid and simple and readil adaptable to routine testing procedures. Heat losses and other effects commonly requiring corrections should be eliminated by simultaneous comparison of the test sample and an inactive reference sample. APPARATUS A N D PROCEDURE
Apparatus. Figures 1 and 2 show the controlled pressure and controlled atmosphere DTA apparatus, which can he used to study many physical chemical changes other than those occurring in catalysts. Only the controlled atmosphere portion of the apparatus was used in the catalyst evaluation study reported herein.
forced through the powders; hence the name "dynamic gas method." The path of the gas is shown by the arrows. The test involves first passing an inactive gas, nitrogen, through the powders, then quickly displacing the inactive gas with a catalyst poison and observing the heat effects when the poison is injected. After the maximum effect of the poison has been reached, the sample is purged with nitrogen and the heat ahsorption reaction is observed. I n the apparatus illustrated, the sample holder is 1.5 inches in diameter by 0.75 inch thick. The cavities are 0.25 inch in diameter by 0.375 inch deep. Inconel is used for the sample holder and platinum-platinum, 10% rhodium for the thermocouples. The inside diameter of the furnace is 3.0 inches and the inside height is 2.5 inches. A selector valve, to which two gas sources a= connected, makes possible changing from one gas to another simply by turning the valve. In the present study, nitrogen was used as the inert gas (port labeled Gas l), and the test gases were passed through the port labeled Gas 2. The apparatus is equipped with a vapor generator which was used to produce superheated water vapor and piperidine vapor. Ammonia was obtained from a steel cylinder. One bead of the differential thermocouple is located in the reference cell and one in the test cell. This couple is connected to the upper recorder shown in Figure 1. This recorder automa% ically-produces a trace of any electromotive force output of the difEerentia1 couple resulting from reaction occurring in the system. The reference-temperature recorder and controller system maintain the desired temperature in the furnace. In catalyst testing, the furnacesample holder system is maintained a t a temperature of 330' f 10" F., chosen b e cause it is above the temperature a t which mechanically held water is r e moved, yet low enough to permit comfortable and safe working conditions for the operator. The reactions being measured, particularly with water vapor, are insensitive to temperature in this region; hence the test temperature can vary considerably without affecting the results. The thermocouple bead used for measuring and controlling the temperature of the system is located in the reference cavity. DEVELOPMENT
Figure 1. Typic01 assembly of controlled atmosphere and controlled pressure differential thermal analysis apparatus
Figure 3 illustrates the dynamic gas method of testing. The test powder is placed in the X cavity of the sample holder and the reference or standard material is placed in the S cavity. Gas (or vapor) of known, desired composition is carried by tubes to the test cavities and is 1274
ANALYTICAL CHEMISTRY
OF A CORRELATION
Nomenclature
C
= heat capacity of catalyst and
reference material MrD = mass of test gas physically adsorbed by reference Mtc = mass of test gas chemisorbed by catalyst being tested Me, = mass of test gas physically adsorbed by catalyst being tested W , = mass of reference material Wi = mass of catalyst being tested AH, = heat of chemisorption per unit mass of gas chemisorbed
Figure 2. Pressure chamber and furnace
4Hn = heat of physical adsorption per un i t mass of gas adsorbed 4, = teinrler ature rise in reference
,"..1.I
= =
temperature rise in test cell difference between temperature rise in test cell and reference cell, A, - A,
= rliffmanw
=
hetween "l"..""..
tomnma"..~ - . ^
ture drop in test cell and reference cell, A't - A', A-A'
r h e Correlation
When the test gas is admitted to the Lpparatus, heat liberated by noncata,. ~I~ , ~1 ~(no I~~ . cnaImsurp~,~.~~~
