PLENARY ACCOUNT
Applications of Gas Chromatography in Catalysis V. R. Choudhary and 1. K. Doraiswamy‘ National Chemical Laboratory, Poona-X, India
L. K. DORAISWAMY was born on May 19, 1827, and received his MS i n chemical engineering from the University of Wisconsin i n 1960 and his P h D i n chemical engineering from the same University i n 1952. After working for a year with R. L. Car‘lisleChemical and !ManLaboratory at Poona, India, i n 1954. At present, he is Head, Division of Chemical Engineering and Process Development. H e has published about 60 papers, mostly in chemical reaction engineering and chemical engineering thermodynamics, and has been associated with the development of several processes at the National Chemical Laboratory which are now i n operation i n India.
VASANT R. CHOUDHARY, born Nov. 23, 1944, is a research fellow at the National chemical Laboratory, Poona, India. He received his bachelor of science (1965) and master of science (1967) demees in chemistrv” .from the University of Poona i n First Class. His .fields of interest are kinetics and heter
I”
T h e use of gas chromatography (gc) in chemical analysis is well known, b u t recent developments in the theory and technique of chromatography have made it possible to apply gassolid chromatography (gsc) and gas-liquid chromatography (glc) to a variety of physicochemical measurements (Khan, 1962b; Purnell, 1964; Kohayashi et al., 1967; Conder, 1967, 1968; Ionescu, 1967; Young, 1968; Trestiau, 1968). Apart from the simplicity and rapidity with which the required d a t a can he obtained, a particularly attractive feature of this tevhnique is t h a t most of the physicochemical properties of the solid catalyst can be studied under the operating conditions of the catalyst (in some cases during catalysis); and it also allows the simultaneous determination of the physicochemical properties and activity of the catalyst, which is useful in the direct correlation of these parameters to explain the exact mechanism of a surface catalyzed reaction, and ultimately in understanding the catalytic process. The theory of go has heen discussed in several hooks and reviews. The plate (or column) theory has heen discussed by Zhukhovitskii (1959), Kahn (1961, 1962a), Littlewood (1962), Purnell (1962), Giddings (1962a,b, 1967), Horvath (1967), Schupp (1968), and Walker and Palframan (1969). The theory of nonequilibrium chromatography has been reviewed hy Giddings (1965, 1967) and Schnpp (1968). The statisticalmoments theory of gsc has been discussed by Grubner (1968), and adsorption chromatography by Giles and Easton (1966), Snyder (1967) and Kiselev and Yashin (1969). A general theory of chromatography without carrier gas has also been developed (Zhukhovitskii e t al., 1967). Recent developments in gas chromatographic techniques, such as gc equipment, column preparation, high-temperature and high-pressure chromatography, continuous chromatography, temperature and flow programming, combination of gc with other analytical techniques, sampling and sample transfer and detectors, are discussed in books such as those by E t t r e and Zlatkins (1967) and Schupp (1968). Recently, E t t r e et al. (1969) have described in detail a technique for pressure (flow) programming in gc. I n the present paper the following aspects of gc as applied to catalysis are covered: experimental methods in catalysis, 1
218 Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
To whom correspondence should be addressed.
The survey has covered practically all the major publications that have appeared during the past 15-20 years, largely excluding those that have been covered in standard tests. As will be noticed from the list of references, almost 325 articles have been included. As such, it has obviously not been possible to explain some of the principles in detail. Nevertheless, mathematical equations pertaining to the important developments reported are given, and a connected account of gc is presented to include all its major aspects. By the very nature of the survey, it has become necessary to include in several places statements from original papers almost unaltered,
determination of the physicochemical properties of solid catalysts and adsorbents, catalyst evaluation and kinetics of catalytic reactions, and study of catalytic reactioiis under chromatographie conditions. Gc is 110 longer to be regarded merely as an analytical tool for the quick (and, if necessary, continuous) determination of product composition, but as an essential part of a n integrated program of kinetic analysis, including the determination of reaction paranietcx as well as diffusional coiistants. GAS CHROMATOGRAPHIC METHODS IN CATALYTIC STUDIES
Gc can be used in the study of catalysis in two ways. I n the first, the catalyst under study is packed in a chromabographic column, and the properties are estimated by the chromatographic parameters such as retention time, retent,ion volume, band width and shape, and behavior of the chromatographic peak; while in the second, a microreactor, in which a catalytic reaction or certain measurements on the catalyst are carried out, is directly connected to the chromatographic system whose function is to provide a rapid analj feed aiid products of t'he catalyt'ic process. GAS CH ROMATOGR APHlC TECHNIQUE : CATALYST PACKED IN CHROMATOGRAPHIC COLUMN
Gas chroiiiatographic data caii generally be obtained by four important basic procedures: elution chromatogriiphy, frontal method, combiiied frontal-clutioii metliods, :iiid displacement technique. Elution Chromatography
In this method, a discrete sample of material is introduced into the carrier gas stream. During p a s a g e through the column, a given cornpoiient is distributed iii a coiist'ant ratio between the gas and immobile phases. This ratio is governed by a fundamental physic:d quantity, the partition coefficient. If the various partitioil coefficients differ sufficiently, each compoiieiit of a mixture emerges from the column as a separate band. A t fixed conditions of temperature and flow rate, the time of emergence of a band is characteristic of the system. T h e repeated distributioii of material betweeii phases gives a more or less gaussian concentration profile along the column for each component, and this gives rise to the familiar elution chromatogram comprising a set of bell-shaped peaks. The retention volume, one of the most important parameters and usually applied in the physicochemical measurements of the catalyst and in qualitative analysis, is obtained by multiplying the retention time, t , (the time t h a t elapses between the injection of the sample aiid the reading of the peak maximum) by the flow rate of the carrier gas, F. T h e retention volume depends on the column conditions (Keulemans, 1959; Ambrose and Ambrose, 1961), and hence corrected espressioiis for retention volume, taking into account the pressure drop a t the inlet and outlet of the column, as well as the column temperature and the weight factor ( W / F ) , are used (Greens and Pust, 1958; RIisoiio e t al., 1965; Hansen et al.,
1964). -4typical coriected expression used by Greens and Pust (1958) is
Frontal Analysis
This is carried out by injecting into the carrier gas stream a continuous supply of the sample material. When the sample front enters the column, there is the recurring distribution between phases, but because the depletion of the gas phase is constaiit'ly made u p by incoming sample, peaks do not develop. Instead, the concentration prcfile takes the form of a plateau preceded by an S-shaped front. I n the region of the plateau the imniobile phase is saturated with respect to t h a t component, aiid fronts break through the column only d i e n the whole of it is saturated. The break-through times are again characteristic of the system, and the chromatogram takes the form of a series of steps. .4fter the kist component 11'1s emerged, the column serves oidy to delay the flow, and the input and output compositioiis are identical. The retention volume, in case of the frontal chromatogram, is given by the ratio of the amount of component adsorbed to the concentration of the componeiit per unit volume of the carrier gas. 1)etails of frontal analysis for two- and multicomponent systems are discussed by Keulemans (1959). Combined Elution-Frontal Method
An elegant alternative :ipproach which combines both elution and frontal methods has been suggested by Reilley e t al. (1962). Iii this, the columii is operated in the normal frontal manlier until the plateau breaks through. Then, discrete and minute volumes of either the sample or of t'he carrier gas are injected aiid eluted; tlie retention volumes are thus measured a t known concentrations in the immobile phase. Any carrier gas injected travels through t'he column as a negative sample. The injections may be repeated as ofteii as wished with any ~ n i o u n of t sample. .it the conclusioii of tlie experiment, a desorption may be carried out by suddenly stopping the sample supply and replaciiig i t with carrier gas. An illustrative chromatogram is s h o w i i i Figure 1. Displacement Method
The mixture to be analyzed is placed 011 the top of the column, and is then pushed through the coluniii by means of a
Time(t) or Volume flow (1F)
Figure
1
.
Combined elution-frontal chromatogram
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
219
displacer vapor, which is carried a t constant concentration in :Lstream of carrier gas. This displacer must be more strongly adsorbed than auy of the components of the mixture. Each component then forms a band of constant concentration on tlie column, and on leaving the column produces a step in the record of the detector. When these bands have been estab.lished, a component is adsorbed on a fraction cf the column eqLd to the time its step t'akes to leave the column (step length), divided by the break-through time of the displacer. The concentration of the vapor in equilibrium with the adsorbed component is the same as the concentration a t which the step is formed, and may be simply calculat'ed from the step length, the amount of the component in t'he mixture, and the carrier gas flow rate. The displacement method is not suitable in case of irreversible adsorption. Microcatalytic Reactor Combined with Gc
The microcatalytic reactor combined with a gas chromatogr:ipli has been found to be a very promising and rapid technique in the study of the physicochemical properties of catalysts, catalyst performance, activity and selectivity, and kinetics and mechanism of the catalytic reaction. It, may also be conveniently used in the study of the influence of esternal electric field, magnetic field, and the irradiations (X-ray and iluclear radiations such as y-rays, CY- and @-particles) on the adsorpt,ion properties of catalysts, their active centers, catalyst performance, and on tlie catalytic reaction. The microcatalytic technique and its applications in catalyt'ic reactions are discussed later. GAS CHROMATOGRAPHIC STUDY OF PHYSICOCHEMICAL PROPERTIES OF SOLID SUPPORTS AND CATALYSTS
In gc tlir retention time (or volume) and the shape and behavior of the clironiatographic peak owing to a given substance depend on the properties of the adsorbent, the substance adsorbed, the csperiment'al column conditions, and the process occurriiig in tlie column. By considering the reverse problem and by applying the chromatographic theories so far developed to account For the above factors, we call iiivestigate, on the bnsis of chromatographic data, most of the physicoc1iemic;il properties of adsorbents and solid catalysts under reaction conditions or any coiiventional conditions. 130th the gas chromat'ographic methods (catalyst packed in cliromutograpliic column and microcatalytic reactor combiiied nith :L gas chromatograph) are used in the physicocliemical measurements of a catalyst. Adsorption and Energetics of Adsorption
Aidsorptioii of gases or vapors is usually determined by 5tatistical measurement's and is inconvenient a t elevated temperatures tilid pressures. Recent developmeiits in the field of gas cliromatograpliy have been concerned with the measurement of gas-solid adsorpt,ion both a t low (at or below atmoqpheric pressure) and high (up to 2000 psi) pressures. Adsorption at Low Pressures (below 1 Atm)
.Idsorption of gase5 and vapors on solid catalysts and ad-oibent,s at, low pressures can be used in the determination of t,he :idsor~~tioii-desor~~tion isotherm, isobar and isostere, lieat of adsorption, nature of adsorptioii, adsorptioii-desorptioii kiiietic*, tind iiiteractions between adsorbed molecules. Adsorption Isotherms, Isobars, and Isosteres. X a r t i n and dyiige (1941) developed a theory of chromatography in a f o r m applicable to cases of linear adsorption isotherms in 220
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
which effects of diffusion and of nonattainment of equilibrium, neglected by Wilson (1940), are taken into accouiit by dividing the column into theoretical plates similar to those of a distillation column. Lapidus and Amundson (1952) presented the most general solution to the elution gas-solid chromatographic (gsc) problem for a linear adsorption isotherm. Further, Kasten e t al. (1952) and Heister and Vermeuleri (1952) solved the mathematical model of Lapidus and Amundson with the added condition of noillinear adsorption. Several mathematical models have been described (Funk arid Horighton, 1960; Schay e t al., 1960), and the plate theory has bcen formulat'ed for the case where adsorption isotherms are noillinear (Robinson and Said, 1960). Said (1959) gave a theoretical treatment of elution chromatography. The concentration dependence of the elution curve was examined theoretically by Haarhoff and van der Linde (1966) from which it was established t h a t both adsorption and isotherm nonlinearity must be taken into account adequately to explain peak symmetry. Further, Locke (1965) described the theoretical aspects of gsc, taking into account the nonideality of carrier gases and nonlinear adsorption isotherms. Recently, tlie retention theory of gas chromatography has been extended to finite concentrations of solute by Conder and Purnell (1968, 1969). de Vault (1943) was among the first to direct attention to the possibility of obtaining the adsorption isotherm of a single solute from chromatographic results of the "diffuse" boundary, suggesting a differential equation for evaluating the measurement. There are many methods, summarized here, for the gas chromatographic determination of adsorption isotherms. Gluckauf's Method (1945, 1947a,b, 1949). This is the most common method, in which the adsorption isotherms are calculated from the shape of the diffuse rear boundary of a n elution peak (Sillen, 1950; Faucher e t al., 1952; Cremer, 1961; Cremer and Huber, 1961a) or a desorption curve (Gregg and Stock, 1958a). The mathematical basis of this technique was derived by Gluckauf (1945). 111 this method, a coiit'iiiuous stream of adsorbate is injected into the column until saturation, and the adsorbed material is then eluted by a pure carrier gas st'ream. The adsorption isotherm is calculated from the shape of the desorption curve by the principle developed by Wilson (1940), de Vault (1943), and Gluckauf (1945). The saturation technique of Gluckauf has the advantage that the entire isotherm can be determined from a single experiment (Gregg and Stock, 195%; Stock, 1961), and it can be used a t low concentrations. However, this method requires a knowledge of the area under the tailing portion of the chromatogram and thus cannot be used for small concentrations of polar solutes on a n active support, where the tailing portion of the curve approaches the base line asymptotically. From Frontal Analysis (Keulemans, 1969; Beebe et al., 1966). This method involves the determination of the retention volume of a given concentration of sample by suddenly switching a mixture of sample and carrier gas into a clean column and measuring the retention volume of the sample front. There are two somewhat different procedures used in obtaining adsorption isotherms from frontal analysis chromatography. One of these, which can only be used when the adsorbate front or the elution tail is not sharp, involves the quantitative measurenient of the shape of t'he front or tail. It is tlieii feasible to calculate a portion of t'he adsorption isotherm from a single chromatographic experiment (Gregg and Stock, 1958a; Eberly, 1961). The other procedure, which is used when the adsorbate front is relatively sharp, is to mea-
I
c,
0
a
d L
5 Y
u
a
0 VA
0
VI
0
Vn
Volume
Figure 2. Representative adsorption-desorption atogram (Parcher and Urone, 1966)
chrom-
sure the retention time of the front and then calculate the correspoiidiiig point on the isotherm. Detailed procedures for both the methods have been given by 13eebe e t al. (1966). Measurement of retention volumes from sample fronts, or break-through curves, requires a separate experiment for each point 011 the isotherm, and one is restricted t o rather large concentrations of the solute. Rigid control of experimental conditions is required. For surface-act'ive supports the most difficult experimental condition is the reproduction of the surface between runs. From Combined Front-Gluckauj' Method. The front-Gluckauf combined method for the determination of the isotherm from a n adsolption-desorItioii chromat'ograph lias been described by Parcher and Crone (1966) and is illustrated in Figure 2. The retention volume of the sample front gives one point on the isot,herm a t t,lie maximum concentration, C1. h s s balance requires t h a t t,he area CIVl be equal to the total area under the tailing portion of the desorption chromatogram. Measurement of CIVl removes the necessity of measuring the area uiider the tail. Only the area designated as Ajt has to be measured, and this can be accomplished accurately down to the coilcentration a t which tail approaches the base line. From initial saturation,
Q1
From the tailing portion, By mass balance, where
AQ
=
(2)
+ V26
= (M)~
0 = Ql -
= A(p)
CIV1 AQ
f V,(Ci -
(3)
(4)
6)
(5)
Substituting Equations 2 and 5 into Equation 4 gives us:
Q
=
CiVi - [V,(Ci - 6) f A(M)I
(6)
Thus, following from Equation 6, the points 011 the isotherm are determined. This method has d l the advantages of the Gluckauf method but does not require a measurement of the total area under the tailing portion of the chromatogram. The sorption isotherm obtained by this method shows good agreement with t h a t obtained by a front method alone. By Displacement Technique (James and Phillips, 1954). The adsorption isotherms of the gases or vapors on a solid support or catalyst can be obtained by altering the nature of the displacer or its concentr a t'1011. The displacement method cannot be used suitably to obtain isotherms in case of irreversible adsorption, aiid like the frontal method it caii be used only to determine the ascending isotherm. From Pcalc Jlazinza. method for the determination of sorptioii isotherm from the peak maxima has been reported by Kipping and Winter (1965). As noted from the chromatographic peaks reported by most of the workers in this field
(Huber aiid Keulemaiis, 1962; Kipping niid Jeffery, 1963), the retention volumes of the diffuse edges are generally independent of the aniouiit of adsorbate added, and in these cases the isotherm can be c:ilcuIated from a single injection. 13ut Kipping e t al. (1965) observed the dependence of diffuse edges on the amoiiiit of adsorbate, indicating t h a t the slope of the isotlierm would vary according t o the amount added arid t h a t a peak-maxima method should be used in prrfereiice to a singleiiijectioii procedure. In the peak-maxima method, the soiptioii isotherm is obtained from a series of injectioils, taking measurements a t peak maxima only and determiniiig the retention volume for zero partial pressure by extrapolation of the curve down through the maxima of the superimposed peaks. The isotherm obtained by this method slioivs good agreemriit with the isotherm obtained using tlie frontal techiiique. By Pulse Flow Technique (Eberly, Jr., 1961). 'l'he adsorption properties of solids a t high temperatures :ire convenieiitly studied b y allo\ving a pulse of adsorbate to be traiisported through a packed columii of adsorbent by an inert carrier gas stream. The concentration of adsorbate in the effluent stream is continuously measured by a sensitive thermal conductivity cell. Modification of such a tecliiiique permits the detection of the adsorptioii process atid enables one to determine its degree of reversibility. In addition, heats of adsorption can be determined readily by measuring the pulse retention times a t a series of temperatures. This flow method is particularly useful for studying adsorption a t high temperature coiiditioiis where static methods cannot be used because of the long coiitact time involved (which leads to decompositioii of the adsorbate). The sorption isothemis a t different temprrntures can be obtained by this method by changing the concentration of the adsorbate in the pulse, and the sorpt'ion isobars b y keepiiig the conceiitratioii of the adsorbate in the pulse constant, while varying the temperature. The pulsed flow technique has been used by many iiivestigators (Habgood, 1962; Gale and Heebe, 1964; Chrirnside and Pope, 1964; Kiselev e t al., 1964b; Ihozinger and Spannheimer, 1964) to study adsorption a t high temperatures, heat of adsorption, and adsorption constants. By Tracer Pulse Technique. The tracer pulse method has been proposed by Helfferich and Peterson (1963) for the accurate determiiiation of sorption isotherms for relat'ively high concentrations. Peterson e t nl. (1966) have reported a n apparatus to conduct trace pulse chromatography and have discussed its zipplicatioii to adsorption equilibrium isotherms. T o determine the partition coefficient, K O ,a t the mobile phase co~iceiitrationCo of the solute under iiivestigatioii, the column is brought to equilibrium with a coiitiiruous stream of the gas pliase a t conceiitratioii Co. Into this stream a small sample, also of coi~ceiitrationCo, is iiitroduced, but it is tagged with a detectable isobope of tlie substance uiider investigation; then the retention volume of the isotope is measured. Application of the equation
(7 1 with V R O as the adjusted retention volume of the isot'ope gives the distribution coefficient KO.T h e measurement is repeated nith a series of different coilcentrations CO to obtain the whole isotherm. The tracer pulse technique is general and can be applied to gas-solid, gas-liquid, and liquid-solid equilibria; it can also bc, applied \Tit11 any number of sorbable coniponeiits. B u t it is of course restricted to adr,orbates that c a n be tagged with deInd. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
221
tectable isotopes aiid requires elaborate equipment for counb iiig the radioactivity of the tagged sample. I n this case, also, experiments must be run for each point 011 the isotherm. Recently, R rapid gas chromatographic method has beeri proposed by Fedorov et a1. (1968) for the determination of desorption isotliernis by gradual displacement of adsorbed nitrogeii wit11 a noiindsorbable carrier gas-e.g., helium--under dynamic coiiditioiis. The methods presented above have been employed b y several iiivestigators (Schay e t al., 1957, 1958; Afanasev and Rubiiistein, 1958; Toth and Graf, 1960; Roginskii et al., 1960; Crcmer mid FIuber, 196lb; Eberly, Jr., and Iiimberlin, Jr., 1961; Oldeiikaiiip : i d Houglitoii, 1963; Owxis e t al., 1964 ; Urooks, 1965; Saint, 1965; Felt1 e t al., 1965; Gavrilova and Kiselev, 1965; Heger aiid Scliny, 1965; Komers and Kocliloefl, 1967; lkljakova e t al., 1967; Figuertis e t al., 1967; Fink aiid ~ ~ ~ ~ l l e . ~ c h c 1968; i i s k y Padberg , a d Smith, 1968; Sc1i:iiiel e t :iL, l9GS; Ilrljakova et d,1968; Alereen e t nl., 19GS; Ivtiiiovn :inti %liukliovitskii, 1969; Schanel e t al., 1969) for tlic determiiiatioii of the adsorption isothernis and isobars of gases and vapors 011 solid supports aiid catalysts. Nature of Adsorption. Threc types of adsorption are considered: Reversible and Irreversible Adsorption [I:'berlg, Jr., f96f 1. l'lie pulse techiiique caii be used to detect the occurrence of irrcversiblr :idsorptioii, provided the process occurs nt sucli a ixte that :I iiiensurablc number of molecules are adsorbctl witliiii the contact t'ime allotted. For thrse experiments, a pulsc coiit:iiiiiiig a mixture of helium and the adsorbnte under iiivestigation is injected into the helium stream. Thus, the coiirluctivity cell will respond only to the presence of the adsorbate. The area of the efflueirt pulse i.; directly proportioiial to the nmouiit of adsorbate issuing from the colunnn. If the 1)ulsc area obtained when flowing tlit.ougli the packed bed iq lcss t l i n i i tlwt with a completely empty column, irreversible adsoi,ption litis occurred aiid the extent of the irrevrrsible :idsorptioii can also be det'erniined. This is a rapid tccliiiique for the study of irreversible adsorptioii and can be used advantageously in the study of catalyst poisoning due to irreversible adsorption of the trace impurities or poisoiis present in tlie feed and products of the catalytic process. .slightly I differeiit technique is used to detect a reversible :idsorption process. 111this case, a pulse consisting of a mixture of argon slid the adsorbate is injected into tlie helium stream. The coiiductivity cell will now respond either to argon (a lionadsorbable gas) or the adsorbate. If only oiie effluent pulse is recorded, then most of the adsorbate molecules have traveled through the column a t the same rates as argon, and hence 110 :idsorptio~ihas occurred. If, on the other hand, the adsorbate molecules are retarded in their passage by adsorption on the surface, two peaks will result, the first being t h a t of argon and the second that of the adsorbate. 111 this niaiiner, reversible ttdsorptioii caii be observed for the systems exhibiting a very sinall adsorption capacity. Ozaki and co-wor1rei.s have studied the reversible adsorption of hydrogcii over nickel (Ozaki e t al., 1967b; Ozaki and Shigehitr:t, 1967), cobalt and copper (Shigehara and Ozaki, 1967b), iroii (Ozaki et d,, 1967a), aiid palladium (Shigehara and Ozaki, 1967a) by the gas-liquid chromatographic technique. Phgsicnl ildsorption and Chemisorption. The nature of adsorptioii, whether i t is physical or chemical, can be detected by tlie gas chromatographic technique by determining the esteiit of :idsorptioii as a fuiictioii of temperature and the energetics of tlie adsorption. Geiierally, physical adsorption decreases as the temperature 222
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
-5T
'K-
Figure 3. Variation of retention volume with reciprocal of temperature for oxygen on hopcalite (Ivanova and Zhukhovitskii, 1967)
incremes (the reverse is also true), while chemisorption goes through maxima (Ivanova and Zhukhovitskii, 1967) as the temperature increases. Moreover, the heat of chemisorption is generally much higher than that for physical adsorption. These two criteria caii very well establish the physical or chemical nature of the adsorption. Yakerson (1967) studied the adsorption of water on y-alumina in the temperature range 16O-45O0C and concluded that the effluent curve and experimentally determined heat of adsorption of H20 on 7-alumina (1.5 kcal/mol) correspond to only physical and not chemical adsorption. Activated iidsorption (Ivanova and Zhukhovitskii, 1967). The retention volumes of the gas samples introduced are the main source of iiifor mation about the activated process. Figure 3 represents the principal results of the activated adsorption of oxygen 011 iiidustrial hopacalite. A t low temperatures the data for all the rates lie on a single liiie, ab, describing the decrease of retention volume with temperature, and correspond to niolecular sorption-Le., physical adsorption. B u t as the temperature is increased, there is a sharp increase in retention volume, represeiited by the line, bc, aiid then again there is a decrease in retention volume with temperature, represented by cd. The iiicrease of the reteiitioii volume ~ i t hincrease in temperature is associated with an activated process involving chemical forces. The heat of molecular sorption aiid activated reversible adsorption can be derived as follows: Heat of molecular sorption: AH = 2.303 R X slope of ab Heat of activated reversible adsorption: A U = 2.303 R X slope of cd The activation energy for reversible sorption call be estimated from the slope of curve ln K vs. l/Tc where K is the adsorption rate constant, obtained from the expression,
K
= 1
t
In
(z)
where eo/Co is the observed degree of elution of oxygen. H e a t of Adsorption. T o determine the heat of adsorption from chromatographic data, use is made (Eberly, Jr., and Spencer, 1961) of the following relationship:
p = - trn% L
(9,
As proviously reported (Greens and Pust, 1958), the constant, p , can be treated as thermodynamic equilibrium constant and by making the appropriate substitution, the following equation can be obtained:
log t,'
=
AH
Q
- ___
2.303 R
(')
lTc
(10)
where
The coiistaiit, a, in Equation 10 is a function of the entropy of adsorption, the dimensions of t h e column, a i d the carrier gas flow rate. If these factors are kept coiistaiit, then a plot of the logarithm of the corrected retention time, t?,;', against the reciprocal of the absolute column temperature, T,, gives normdly a straight line, the slope of mliicli is proportional to the heat of adsorption, A l l . Several papers have been published coiifirmiiig the validity of the above equation used for the determinatioii of lieat of ndsorptioii (Greeiis mid I'ust, 1958; Eberly, Jr., 1961; Eberly, Jr., and Kimberlin, Jr., 1961; Eberly, Jr., and Speiicei,, 1961 ; Ross et al., 1962). Knozinger and Spannlieimer (1964) have described :I gas clironiatograpli in detail u i t h which heats of adsorption can be determined. Gale and Beebe (1964) obtained the heat of adsorptioii by the eluted pulse technique using the following espreqsioii: obt'aiiied by proper substitution in the espressioii developed by Habgood and Haiilan (1959) :
where XO
[(fm
=
-
fa) F.273]'(TT7T/)
(13)
If the flow rate is held coiistaiit and the variation iii 2'1 duc to changed in the room temperature is negligible, it is oiily iiecessary to plot log (t, - i d ) agniiist 1 T , from the slope of n-liicli tlie heat of adsorption caii be determilied. Arita e t al. (1965) estimated the heats of :idsorptioii from the retention volume data 11)- usiiig the relation, Icg
v
RO
-a--
2.i:R
($)
Thus, b y plotting log of corrected retention volume ( V n a ) against the reciprocal of the column temperat'ure, the heat of adsorptioii is obtained from tlie slope of the line given by the plot. Beebe et al. (1966) obtained tlid lieat of adsorption from tlie adsorption isotherm by frontal aiialysis. The iwsteric lieat of adsorption, [q,], is defined by the equatioii, qo
=
- R [b 111 p , O ( l / T c ) ] v o ~
(15)
Thus by plotting 111 p , vs. l('Tc, q, caii be det'ermiiied. The relation bet'weeii the retention time, f v l ' , and the free energy change AG has been obtained by Cremer [1959d] as follows: AG
= GI
- GZ
=
RT 111 [ t ' m ( i ) l t ' m ( 2 ) ]
Since the apparent retention volume, V R = F.t,', AG is then espressed as AG
=
RT
111
(16)
V,, can be written as
(VR(~>O/T'R~)')
coiiceriiiiig such topics as the estimation of heat's of adsorption and entropy of adsorption by gas chromatography liave been published (Cremer and Prior, 1951; Schay and Szekely, 1954; "0th and Graf, 1960; Carberry, 1961; Grubiier, 19Glb; Ikebe and Emmett, 1961; Eberly, Jr., 1962; Petrovn et al., 1962; Kiselev aiid Yasliin, 1964; Eberly, 1964; 13eljakovx et al., 1964a,b; Piringer aiid Tataru, 1964; Kiselev et al., 1965a; Kiioziiiger aiid Spaiiiilieimer, 1965; S1xiiinlieinier aiid Knozinger, 1966; Grab aiid Weiiiert, 1966; Gillcsliie et d , 1966; 3ln~ukan.aet al., 1967; dinariglio, 19G8; 8 t r m d et al., 1968; Jlimitrov e t al., 1969). Escellent :igreemeiit lias been found between tlie calorimetric method and the gas chromatographic method in the determiiiatioii of heats of adsorption 1962; Arita et al., 1965). (Greeiis and Pust, 1958; Ross e t d., T h e gas chromatographic method is superior to tlit calorimetric method iii that the thrrniodyiiamic data oil hetcrogeiieous solid catalysts can be obtained at, the renctioii coiiditions (Kubasov et al., 1964; Avgul et al., 1965). Carberry (1961) has established tlie limitatioiis of the determiiintioii of heat of adsorptioii b\- tlie gab c1iroiiintogr~ii)liic method aiid has pointed out t h a t the isotherm caii give the lieat' of :rd?orptioii (from reteiitioii data) only ivheii tlie coiiceiitrutioii falls in the linear portioii of tlie isotherni- i.e., a t low surface coverage. Interaction Energy between Adsorbed Molecules. Eeljakovrt et al. (1964a,b) have determilied the eiiergy of t h r hydrogen bonds i i i t,he adsorbed layer? of ~ilcoliolsby determiiiiiig the heat of adsorption a t high surface covernge (by colorimetric method) which includes the eiicrgy of mutual interaction betxeeii adsorbed molecules. They have also determined the actual heat of adsolptioii a t low coverage (by gas chromatographic method) iiicludiiig oiily the lieat of adsorptioii as the intermolecular distance. I n caw of Ion. siirface coverage, it is not sufficient t o promote interactioiis betwccii molecules through hydrogen hoiidiiig. ?liiis, tlie m w g y o E iiiteractioii betn-eeii adsorbed molecules caii he o1)tniiied Gnply by subtracting tlie heat of :idsorl)tioii (determilied by the gas chromatographic method a t low .surface coverage) from the value obtained by colorimetric nieasuremeiits a t high surface coverage a t room temperature. For the alcohols adsorbed on graphit'ized carbon black, the mutiial interaction energy due to liydrogeii bonding was detemined (13cljakov:i et al., 1964a) t'o be 5 kcal !niol. Kinetics of Adsorption. If t h e maiii p r o c e ~ saffecting t,he width and emergence time of the adsorbate pulse i3 adsorption rather tliaii diffusion, theii it is espei~imeiitally ~iossibleto study adsorptioii kinetics and tliermodyiianiici by gsc. llabheniatical solutions for peak broadeiiiiig due solely to adsorption have beeii developed by 1;btrIy aiid Speiicei. (1961). From the analysis of the shape of the effluent pulse, t'he magnit'ride of the rate constants can lie deterrniiied. h theoretical treatment of the kinetics of ndqorptioii for n pulse of solut'e passing through the coluniii 11as been proposcd by Eberly (1964) : The rate at which molecules eiiter the particles is represented by olnvo,'4, and the rate a t which molecule. leave the particles by d 0 . The overall rate is then espi,essed by
(17)
The numbers in parelltheses show the kind of adsorbate. The free energy change of the adsorption of several gases on the same adsorbent n-as calculated by Arita e t al. (1965) by using Equation 17. Coiider (1969) obtained thermodynamic information from asymmetrical chromatographic peak. h p a r t from the literature discussed above, numerous papers
At equilibrium, this equation reduces to v, =
(7)
ne
Fyliicli describes a system having a linear adqorption isotherm, Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
223
. I mas5 conservation equation over section br of the adsorbent column can be written as
The solution of the partial differential Equations 18 and 20 is expressed in terms of the following four dimensionless quantities:
0.u = IJ,cO/A' T.M= tTn w =
PL/OuF,
(22) (23) (24)
'I'he :idsorption equilihriurn constaiit, p, can be evaluated from c1irorii:itograI)liic data by using Equation 9. This conat:iiit is directly proportional t'o the slope of the adsorption isotherm atid is a true constant only for those having linea,. adsorption isotherms. Since the partial pressures normally involved in gsc :ire quite sinall, the value of fl as calculnted by Eqriatioii 9 should be expected to correspond to the liniitiiig slope of the isotherms i n the low-pressure 1,egion (Eberly, Jr., nnd Spencer, 1961). T o obt:iin information about rates of :idsorpt'ion in the column, liowevcr, it is necessary to measure the height of the :idsorption peak as well. From the numericd iutegratioii data on pulse flow process, the following relationship between the dimensioiiless quantities T,,r, Q.,,, and w h:~s been obt'iined (Eherly, Jr., and Spencer, 1961; Eberly, 1964):
where
Q MT.M= IJmctm/d' w =
Tu
+ 0.20
(26) (27)
Thris, by measuring the height, ( f J , ) , area ( A ' ) ,and retentioii time (t,) of the adsorbate pulse and Itnowing the chart speed, (c), 8 can be evaluated from Equations 25 and 26 and the definitioii of T , in Equation 23. Then W , p , and the product aS can be evaluated from Equations 21, 24, arid 27. Eberly (1964) st,udied the kinetics of n-butane adsorption on silica gel as n function of particle size, flow rate, and temperature by using the principles discussed above, and reported good agreenient bet,ween the fl values obtained by pulse flow experiments and those obtained from the initial slopes of the :idsorption isotherms. In addition, the kinetic constant', 8, showed the s:ime dependence upon temperature as the corrected time, ( t m ' ) j giving nearly the same value of the heat of adsorption. Piringer and 'l'ataru (1964) have also described a method for stitdying the kinetics of chemisorption of hydrogen on metal surfaces by means of the gas chromatographic technique. The amounts of hydrogen adsorbed during specific time intervals are determined through subsequent desorption ill ; i i i inert gas stream, and kinetic data are calculated. Shigrliara and Ozaki (1969) determined the rate of reversible :idsorption of hydrogen on a nickel catalyst. Recently, Pulberg and Smith (1968) have described a cliroinatogra1)hic method in which a n isotopic tracer has been used for the measuremcnt of t,he chemisorption rates. The method consists of transporting the pulses of deuterium into the carrier hydrogen strcam; the chemisorption rates are then caleiilated from the nioiiicnts of the chromatographic peaks. 224
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
State of the Adsorbed Molecules. T h e understanding of the nature and the state of the adsorbed molecules-Le., whether it is in the form of charged species, a free radical, or a complex with atoms of catalyst-is very important in formulating the mechanism of the catalysis, though the exact role of the "adsorbed state" in catalysis still is not fully under1966). stood (Discuss. Faraday SOC., The measurement of the electrical conductivity of the solid catalyst pellet or particle during adsorption helps in understanding the nature and the state of the adsorbed moleculeswhether the adsorption is physical or chemical. I n the case of physical adsorption there will be no appreciable change in the electrical properties of the catalyst owing to adsorption, but in the case of chemisorption (involving chemical forces batween the catalyst and the adsorbed molecule due to electrostatic attractioii or certain electronic processes occurring during adsorption), there will certainly be a change in the eleetrical conductivity or even in the type of conductivity (in the case of semiconductors). The charge species formed on the cat>alystsurface may also largely affect the overall electrical conductivity of the catalyst because of surface conductivity. Further, :i study of adsorption under the influence of the external electrical field (for the case of charged species formed on the surface) may also throw light on the adsorbed state and t,lie role of the adsorbed state in catalysis. Such a study can be very conveniently carried out in a reactor equipped with a n arrangement for the measurement of electrical conductivity of the catalyst particles or pellet as a function of temperature. It is also possible to apply an external electric or magnetic field to a catalyst placed in the reactor connected to the gas chromatograph, as in the case of the microcatalytic technique. Thus a direct correlation of the adsorption properties of the solid catalyst with its electrical properties can be obtained. The following procedure can be adopted. First, the electrical conductivity of the solid catalyst is measured separately with and without the influence of the external electric or magnetic field in the presence of a pure inert carrier gas-e.g., helium-while varying the temperature. Then a continuous flow of the adsorbate in the carrier gas is started, and the simultaneous measurement of electric conductivity of the catalyst with or without the influence of the external electric or magnetic field is done a t different partial pressures of the adsorbate in the carrier gas. The extent of adsorption for both the cases-Le., with or without the iiifluence of cxternal fields-is determined by the frontal method or the' saturation method of Gluckauf. S o doubt, siich a study is not easy, and many complications rnay arise in the simultaneous measurements, but it is essential iri the cme of catalytic reactions on semiconductors and metal catalysts whose catalytic activities are interpreted in terms of their electrical properties. -4dsorption during catalysis as initiated by Tamaru (1964) will be discussed later. Adsorption at High Pressures (up to 2000 Psi)
The importance of adsorption measurements a t high pressures and the different techniques applied for the purpose have been discussed recently by Menon (1968). h gas chromatographic apparatus modified for high-pressure operation has been described by Stalkup and Kobayashi (1963). A simple and more straightforward gas chromatographic flow method (Mason and Cooke, 1966) is based on a material balance as the components are transported by a carrier gas through a n adsorbent packed column. The column is packed with a weighed quantity of adsorbent that has been
heated to the required temperature for 2-3 hr. After the coluinii is purged with a carrier gas and the pressure is adjusted, flow it; diverted to a bypass line while the composition of the inlet stream is adjusted. When the derived composition is reached, tlie flow is turned on into the adsorption column. Samples of the outlet gas from the colunin are withdrawn periodically and analyzed b y gc until their composition bccomes steady and equal to t h a t of the inlet gas. An adsorption breakout curve is obtained from :L plot of composition vs. time. For desorption, pure carrier gas is passed through the column m t i l the exit gas analysis shows no trace of the component gases under study. Thus, a desorption break-through curve is ob taiiied. The area behind the break-out curve is proportional to the quantity of the component adsorption a t the concentration of the inlet gas, and the temperature and pressure of the column. Thus, the adsorption of all components, other than the carrier, can be determined in any gas mistnre. For biliary gas mixtures, Eberly (1961) has shown t h a t the values of adsorption for the heavier component a t all levels of concentration up to t h a t of the mixture can be calculat’ed from a single desorption break-out curve. This method has been followed b y Nason and Cooke (1966) for measuring adsorption of ethane, propane, n-butane, isobutaiie, n-pentane, isopentane, and n-liesane in binary mixtures with methane on silica gel. This gas chromatogrnphic technique has becn used to the measure vapor-solid distribution coefficient, or K Ovalues of a solute distributed between a gas phase and an adsorbed pliase at essentially infinite dilutions. Gilmer and Kobayashi (1964) did the work for ethane, propane, and n-butane in the methane-silica-gel system u p to 2000 psi. The method has teeii extended to a study of multicomponeiit ga at high pressures (Gilmer and Kobayashi, 1965) with tlie methane-proparie-silica gel system up to a pressure of 1000 psi. The total adsorption, component adsorption, aiid the K O value for each component are related to tlie retention volume for the cornlionelits. Radioactive labeled hydrocarbons are used to obtain the appropriate retention volumes. Other papers concerned with adsorption a t high pre.ssures by gc have been presented by Gilmer (1963) aiid Haj-del and Kobayashi (1966, 1967). Gas-Solid Interaction Potentials
Hanlan and Freeman (1959) have shown t h a t the first-order gas-sclid interactioii coefficient can be obtained from gas chromatographic retention volumes, and t h a t the potential energy of gas-solid interactions can therefore be obtained from the temperature depeiideiice of these retention volumes. Thus,
If tlie logarithm of the left side of Equation 28 is plotted against 1/T, the resulting plot is nearly linear (Hansen, 1959) over most of the temper:tture range for which the left side can be accurately established. Haiisen and Murphy (1963) have derived a low-temperature asymptotic espression for the firstorder gas-solid interaction applicable to general potential forms. Tlir final form of the asymptotic espression, :ipplic:il)le to the esperimental data, has been given by Haliseii e t i d . (1964) :is
where
To
=
t(z)/k (iiiteraction potential)
(30)
assumiiig the gas-solid inter:iction potential to be of the 3: 9 form. In zeroth approsimatioii, the terms in T / T oaiid ( T / T O ) * are neglected and the left side of Equation 29 plotted. agaiiist 1/T to get ‘1 zeroth order v d u e of To.This value will providc values of the small terms in T / T , ]:uid ( T / T o ) *correct to zeroth order, and they caii now be sulitrnctcrl from both sides of the equation. l’he resultiiig left side can I I O W be plotted against l / T to establish values of TOaiid S,xO corrwt,ed to first-order. Yo further interaction is necessary for over raiiges of To/T commoiily studied and to which Equntion 29 npplics. Results of further iiiteraction will substaiiti:illy r o i i i d e with the first-order value. Thus a t high values of l / T , the slopc of tlic plot must bccome constant and equal to T o ; so the 1)nr:imcter To, from which the interaction potent,ial e(z) can be calculated by Equation 30, can be uiianibiguously deteimiined. Hansen e t al. (1964) detei,miiied the dependence of retention volume 011 temperature for argon, nitrogen, carbon monoxide, methane, etliaiie, propane, and propylene on Columbia-activated carbon from 300-700°K and also evaluated the interaction parameters To = e ( r ) / k :tnd S,xO for carbon monoxide by both asymptotic and exact methods aiid found good agreement between theni. \17aksinuiitlzki e t al. (1966) applied the method of Crerner ( 1 9 5 9 ~ to ) determine tlie :idsorption potential of C‘S, on silica gel. Harper (1964) used gas ndsorption chromatography to study the interactioii of a series of gases nith a Columbia-activated cli:ircoal surface. Kiselev (1964) tiiscussed the varicus kinds of intermolecu1:ir interactions involved in gc and ,suggested tli:it 311 improved quantum mechanical formulation of the interaction poteiitials between adsorbate and the solute are needed. Surface Properties of Solid Catalysts and Adsorbents
in which N moles of gas in volume V are in contact n-it11 S, cmz solid surface a t an equilibrium pressure P , and temperature T , and e(z) is the potential energy of gas-solid interaction at distance z from the surface (assumed to be plane). The temperature dependence of the left-hand side of Equation 28 gives the function e ( x ) and, with further assuniptioiis to be discussed, the surface area, S,.The basic theory of this method and the most accurate supporting csperimental work are tlie work of Steele and Halsey (1954, 1955). 13arker and Everett (1962) have recast Halscy’s theory in a form inore clearly indicating its relation to adsorption; they also developed the method of surface-area interfereiice based oii the second-order gas-solid interaction coefficient.
Gc has proved to be a n escellent tool for the rapid evaluation of the surface properties of solid catalysts and :idsorbents, such :is specific surface area, active surface area, surface coverage during catalysis and the nature of the complex surface, whether it is uniform or noiiriiiiforin. Specific Surface Area
At first, in most of the studies dealing with the deteriiiiii:ttioii of the surface area of adsorbents and catalysts hy g ~ i s cliromatogrq,liy, use made of the coiiditioiis of equilibrium chroniatogi,apliy (Gluckauf, 1945; Iiogiiiskii et’ al., 1960; Gregg and Stock, 1961). Hut recentla-, nietliodi which do iiot require the coiiditioii of equilibrium chrornatogi~:ii)li~ havt: been developed. Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
225
Selseii and Eggertsen (1933) developed a rapid and accuriite flow method-in which the conditions of equilibrium cliroinatography are not required-for the determination of surface area. The principle is based on adsorption of nitrogel) by the solid from a fixed composition Ife/Kz stream a t liquid nitrogen temperature, followed b y desorption upon removal of the liquid nitrogen coolant’. The amount of nitrogeli adsorbed a t the corresponding relative pressure is determined from the peak eluted as a result of the desorptioli process. Tlie amount of nitrogen adsorbed for a monolayer coverage is the11 determined from three such determilintions a t different, S 2 partial pressures from a BET plot, and hence the surface area. Good agreement has been obt’ained between conventioiial pressure-volume and chromatographic methods. The original method of Kelsen and Eggertsen was modified by many investigators (Roth and Ellwood, 1959; Lee and Stross, 1959; Derby aitd L a l I o n t , 1960; hlkiiis, 1963) to improve the convenience and accuracy of the method. Further, Ellis e t al. (1960) modified this method to measure surface areas of the materials as low as 0.005 m2/g, aiid E t t r e (1960) has described a comniercial apparatus for the purpose. An important’ extension is that of Haley (1963) in n-hich nitrogen, adsorbed or desorbed by increasing or decreasing the pressure in the sainple tube, is measured continuously a t atmospheric pressure, with a reference strean1 of 10% nitrogen in helium. -1dsorptioll or desorption isothernis deteriniiied by this method shon- good agreement with those determilied by the conventioiial volumetric adsorption measurements. Stock (1961) has described a method of determining surface areas of adsorbents similar to t h a t of Kelse~iand Eggertseil, but with a slight modification which permits the determination of surface area from a single elution esperinieiit, and has also discussed the procedure to obt:iin mi adsorptioii isothe1.m from :i siiigle elution esperirnent. Cremer (1959b) derived ail equation relating the retention time to tlie “effective” surface area and also studied the effect of decrease in surface area on the catalytic activity. Grubller (1962) applied a perturbation method to determine the metal dispersion or specific metal surface area of niulticompoiieiit, catalyst d i o s e metal area corresponds to only about O . l - l ~ o of the total surface area of the catalyst. Further, Smolkova et al. (1965) described a gsc method for the determination of specific sui,face areas of low surface area (0.01-0.05 m2/g) substances using organic vapors such as benzene, n-hesane, aiid n-hept,aiie on powdered aluniiiia, magnesium oxide, titanium dioxide, mid zinc oxide. A method lias also beeii described (Crenier, 1959a,c; Rolf and Beger, 1959; Gaziev e t XI., 1960b) for the determination of the relative surface area based on the determination of the specific retention volume, V R ,a t low concentrations, which is proportional to the specific surface, S ,
s, = AO’VR
(31)
where A. is a proportionality coefficient, which depends 011 the iiature of the adsorbent, adsorbate system, and temperature. Equation 31 has been tested for a large number of catalysts, and the method is promising for measurement of the variation of the catalyst surface during its operation and also for tlie rapid estimation of the surface areas of adsorbents and catalysts of the same chemical type. Kiselev and co-workers (1 964a,b) suggested t h a t the retention volume of a given substance, referred to unit area of the adsorbent, be used a s a characteristic of a n adsorbentadsorbate system. Thus. 226 Ind. Eng. Chern. Prod. Res. Develop., Vol. 10, No. 3, 1971
v s = vR/sg
(32)
where V , is independent of the adsorbent surface area and reflects many properties per unit surface. Gavrilova and Kiselev (1965) described a simple method for the rapid determination of the specific scrface area of an adsorbent. Recently, Buyaiiova et :d. (1967a, 1969b) criticized the different original methods suggested by Cremer (1959d), Rogiiiskii et al. (1960), Grubner (1961b), and Schay and Szekely (1954) for the determination of specific surface area and concluded t h a t Grubner’s heat desorption method has many advantages over frontal methods. A number of other publications have also appeared on specific surface-area measurements by gsc methods (Grubner, 1961a; Roginskii e t al., 1961; Walton, 1961; Leuteritz, 1961; Ettre e t al., 1962; Hughes e t al., 1962; Gazeeyer and Feelemovskii, 1963; Thibault et al., 1964; X n g Chu, 1965; Kuge and Yoshikawa, 1965; Frost, 1969; Scharf e t al., 1969; and Vespalec, 1969). Surface Area Measurement under Reaction Conditions
Recently, the advantages of studying a catalyst under conditions approaching as closely as possible those used in the actual reaction have been emphasized. Study of surface properties a t high temperatures by adsorption of gases is difficult in static s y s t e m because most of the adsorbates decompose oiviiig to large contact time. Hence flow systems have been recommended to decrease the contact time and thus minimize decomposition. Folloiviiig the principles of Wilson (1940), de Vault (1943), Gluckauf (1947a), and Fberly (1961) developed a continuous flow method to determine adsorption isotherms a t high temperature. I n this method, a coiitinuous stream of adsorbate is transported through a column of initially unsaturated adsorbent by means of ail inert nonadsorbable carrier gas, and after saturation, the adsorbed material is eluted by the pure carrier gas. From an analysis of the composition of the effluent stream as a function of time, the adsorption isotherm can be evaluated if the conditions are satisfied that’ (a) diffusional effects are kept to a minimum and (b) the adsorption equilibrium is rapidly established. At high temperature, these conditioiis can be espected to be essentially fulfilled. The continuous flow method yields sufficiently accurate adsorption isotherms, and many points 011 the isotherm can be determined from only a single flow curve (Stock, 1961; Eberly, 1961). The above technique was applied (Eberly, 1961) to measure butane isotherms 011 silica gel a t 48°C covering a a i d e range of partial pressures and ammonia isot,herms on both fresh and steam-deactivated silica-alumiiia cracking catalyst, silica gel, and 7-alumina a t 260-482°C. Active Surface Area by Chemisorption
Generally, chemisorption methods are used for the determiiiation of the active surface area (which is a fraction of the total surface area of the catalyst involved in chemisorption and can also be expressed as number of active sites on the catalyst). Volumetric and gravimetric gas adsorption methods are commonly employed for chemisorption measurements, b u t their use is limited for high-temperature measurement due to decomposition of chemisorbed adsorbate. To avoid this, use of gas chromatographic prucedures has been made successfully by many investigators (Grubner, 1962; Hughes, e t al., 1962; Piringer aiid Tataru, 1964; Roca e t al., 1968). Recently, Iirooks and Kehrer (1969) have developed a pulse chromatographic procedure for the measurement of the surface areas of catalytic metals by carbon monoxide chemisorp-
tion. I n this method, exposure of the catalyst to carbon monoxide pulses is repeated until the cumulative saturation value, corresponding to irreversible adsorption a t a particular temperature, is established. The metallic or active surface area is then calculated from the amount of carbon monoxide adsorbed and the effective cross-sectional area of carbon monoxide. Active surface, as expressed in terms of total number of active centers on the catalyst as a function of temperature, can be calculated from the number of adsorbate molecules chemisorbed, assuming t h a t only one molecule is chemisorbed on a single site. By the application of the above technique, correction for reversible physical adsorption can be avoided. Good agreement between volumetric and pulse chromatographic procedures has been observed (Brooks and Kehrer, 1969) b y iiidependent measurement on unsupported platilium and nickel metal powders, and on supported nickel catalyst for the determination of carbon monoxide chemisorption. Buyanova e t al. (1969~)have also described chromatographic apparatus for the determination of the active surface area of a catalyst b y tlie chemisorption of oxygen. The area of the complex surface of platinum on alumina has been determined b y a differential chroniatographic method by Buyanova e t al. (1969a). Surface Area Measurement during Catalysis
Tamaru (1957, 1958) was among the first to iiiitiate adsorption ineasurements during surface catalysis with simultaneous measurement of reaction rate. The gas chromatographic method (Tamaru, 1959; Kakanishi and Taniaru, 1963) is the most coiivenient of the methods suggested by Tamaru (1964) for the determination of adsorption and subsequently the surface area of the catalyst during catalysis. The apparatus used is similar to that employed in gc. Instead of the inert carrier gas, the reacting gas or gas misture is used as a carrier gas and the catalyst is placed in a n adsorption column which is maintained a t reaction temperature so that a stationary state of the reaction is established. The gases which participate in the reaction are introduced into the system a t the top of the adsorption column as gas samples, and the extent of their adsorption on the catalyst surface in its working state can be measured simultaneously b y analyzing the product in the exit gas. Thus, the adsorption on the catalyst surface in its working state and the react’ioii rate can be studied simultaneously under various reaction conditions. The necessary requirement of this technique is that the reaction should proceed slowly enough to keep the composition of the reacting gas as well as the surface condition of the catalyst virtually the same throughout the catalyst colunin; adsorption should be reversible, rapid, and moderate. I n this case the retention time t, is given by
t, = A x / A N (33) where Ax is the increase in adsorption correspoliding to increase of concentration of gas samples by A N . When the sample gas is one of the reactants, its adsorption also takes place from the carrier gas. When the surface is saturated with a sample gas from the carrier gas, the retention time of the sample is zero, and no further adsorption takes place. Accordingly, a small value of A x / A N is attributed to weak adsorption or to a strong aiid nearly saturated one from the cawier gas. I n the case of a weak adsorption, Ax/A-V should increase as temperature is descreased, while in the case of strong alld nearly saturated adsorption, Ax/AN should decrease, as the adsorption from the carrier gas approaches saturation, so t h a t A x / A N decreases, provided the adsorption is equilibrated.
By a slight modification of the above technique, it is possible to study reactions such as decompositioii, isomerization, or polymerization b y using the reactant as a sample gas and an inert gas as :Lcarrier gas, keeping the catalyst column a t reaction temperature. The exit pulse of the snniplc gas gives its reteiitioii time, and its area the amount of reactant uiireacted in passing through the columii. Bassett and Habgood (1960) applied this gas chromatographic metliod, in additioii to the microcatalytic chromatographic method, to the isomerizatioii of cyclopropane on molecular sieves and were able to assess t’he lieats of adsorption of the reactant, the activation energy, and the order of tlie reaction. Ozaki et al. (1962) have also studied the “rapid and reversible” part of the adsorption of hydrocarboii on nicltel-Irieselgiilir (50 n.t o/o) by this techiiique. Nature of the Surface
Recently, Dzhavadov e t :d. (1967) have studied tlie iionuniformity of the surfaces of three different types of silicaalumina catalysts by the gas chromatogral)liic technique. h differential gas chroniatograpliic study of the coiiiples c a t a lyst surface of nickel 011 a support 1ia.q also been rej)oi,ted by Buyanova e t al. (1967a). Pore Structure and Pore Size Distribution
Lin Cliung Haui e t al. (1964) suggested the possibility of estimating by the chromatographic metliod the nature and distribution of the pores lvith respect to radius by using the adsorbate substances of different moleculai, sizes. ’l‘liis method is particularly useful in tlie epluatioli of pore size disti,ihutioii in solids below the range 10 A (diameter), which i b difficult to obtain from the usual methods, such as h i g l i - ~ ~ r e s s uporo~~e someter (Darke, 1949), sorption isothernis of iiitrogeii a t liquid nitrogeii temperature ( I h r r e t t et al., 1951), and lowangle X-ray scattering measurements (Shull and Rosses, 1947; Ritter and Erich, 1948). The latter are generally w e d in tlie pore size distribution studies. The pore size of the difftrent types of niolecular sieves, generally ranging from 3-15A, can be estimated by gs or Isc by the selective adsorption of the adsorbate molecules of different molecular size from the bulk phase (gas or liquid) to the solid phase, while making the correctiolis due to the adyorption a t tlie external surface of the solid p a h c l e s . The extent of the adsorption of a particular adsorbate is co~~ti~olled by the selective diffusion of t h a t adsorbate molecule which ultimately depends on the pore size of the particle. Conversely, gc helps to determine the pore size dist1,ibutioii from the sorption isotherms of iiitrogeii measui,ed a t liquid nitrogen temperature, also obtained by gas chromatogrn1)hic measurement. The pore size distribution can be computed from tlie adsorption or desorption branch of the isotherm by differelit methods ( I k r r e t t e t al., 1951; Cranston and Inkley, 1957; Innes, 1957; Aiidersoii, 1964; Dollimore and Heal, 1964; Lippens et al., 1964; I3riinauer et al., 1967; Eming mid Hofman, 1967; Lester, 1967, Viswv:inathaii and Sastri, 1967; Brockhoff and de Boer, 1967, 196s). Recently, a rapid gas chromatographic method has been proposed by Fedorov e t al. (1968) for the determination of desorption isotherm by gradual displacement of adsorbed nitrogen with a nonadsorbable carrier gas-e.g., lieliuniunder dynamic conditions from which the specific sm,face area, pore size distribution with respect to radius, aiid the total pore volume of the solid sorbents and catalysts have beeii determined. Many investigators (Hiiley) 1062; Lunge, 1962; Cahen and Fripial, 1965; Winter, 1969) have described rapid flow methods for the pore size distribution in solid catalyst. Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
227
Further, a n attempt was also made by Saha and Giddings (1965) t o correlate the measured liquid-phase nonequilibrium term of glc with phyuical properties and the structure of the support, in particular with the support's pore size distribution. 1)iffusion measurements (Fatt, 1959) have also beell found to provide the information about the pore structure (tortuosity factor) in sintered glass. Gas Chromatographic Study of Transport Properties
Recent developments in the theory of chromatography have made it possible to predict in an exact manner the kinetics of :idsorptiori-desorptioii and the diffusion coefficients in both h l k phase aud phases contained within the tube (Giddings, 1959, 1960a,b, 196la,b). Effective Diffusivity of Catalyst
'The effective tliffusi\.ity of a catalyst (De) can be determined (Leffler, 1966) by measuring the pulse broadening of a iioncheinicnlly interacting gas a t various flow velocities that occur in a column packed with catalyst and using the value obtained to calculate the constant' in the van Deemster equation
l1EPT
=
II
=
.I
+ B / u + CU
(34)
is the f l o ~velocity. The C term in Equation 34, due truisfer between gas and solid phases, is a function of the effective diffusivity of the gas i n the porous solid. Habgood and Haiihn (1959) have given the following equation for C: where
Mass and Heat Transfer, Intraparticle, and Surface Diffusion Coefficients
The gas phase mass transfer terms in the van Deemter equation have been discussed in detail by Giddings (1962b). Grubner et al. (196613) st'udied mass transfer phenomena in gas-solid systems with special emphasis on the internal porosity of the immobile phase. The mass transfer coefficients were d s o calculated from more exact treatment of gsc (Grubner et al. 1966a). Recently :L new method has been described by Schneider aiid Smith (1968) for determining adsorption equilibrium constants, rate constants, mass transfer coefficients, axial or longitudinal diffusivity, intraparticle diffusivity, aiid surface diffusivity from gas chroniatographic d a h The method is based upon the theory of chromatography developed by Kubin (1965) and Kucera (1965) for relating the moment of the effluent concentratioii wive from a bed of adsorbent particle to the rate constants associated with various steps in the overall adsorption process. X temperature pulse method has also been developed (Sagara e t al., 1970) for evaluating heat t,ransfer parameters in packed beds of solid particles.
ZL
The plate height, I I , can be measured by using nitrogen as the pulsed ineaqiii ing gas mid helium aq the carrier gas and by employing the ielation
I1
=
(;>'I,
When we use the values of I1 as a fuiiction of flow velocity, the term C (and also A mid B ) of the van Deemter equation can be calculated by plotting the plate height', H , against the flow velocity; the slope of the linear portion of the curve in the s corresponds to the value of the region of high g ~ velocities term C. The distribution coefficient,, K O ,in Equation 35 is calculated from the relation,
K O
=
1:'[E
+ (t,
-
fd)UZ.'1/FJ,I
(37)
'Thus, hitice :dl the terms in Equation 35 except De can be determined experimentally, the effective diffusivity of the catalyst can be calculated accurately. Detailed procedures for the deterininntioil of the experimental values of the terms in this expression are discussed by Leffler (1966). The average pore radius>r p , can be estimated (if desired) from the relntioiiship proposed by Satterfield a i d Shernood (1964) :
Davis tiid Scott (1965), using t'he gc met'hod, reported good agreement i i i the values of De with steady-st'ate results for solids which are reasonably homogeneous. Recently, with the pulse flow technique, Eberly, Jr. (1969) has studied the diffusioii of iiiert gaFes (argon, krypton, and sulfur hexafluoride) in a series of Xn- and H-mordenites, Sa-faujasite and amorphous silic~i-:~liiri~iii~i catalyst. This gc method of determining De ha3 the inherent advantage that fairly large and representa228
tive samples of the solid can be studied a t temperatures npproaching those used in the commercial process.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
Gas Chromatographic Study of Active Sites on Catalyst
The gas chromatographic technique and the microcatalytic reactor combined with gc provide excellent information on the nature of the active sites on the solid catalyst, total number of active sites and their strength distribution on the surface, and the factors influencing t'heni a t t,he operating conditions of the catalyst. Estimation of Total Number of Active Sites
The number of catalytically active centers and their relative strengths a t operating conditions can be determined b y a pulse microcatalytic technique (Kokes et al., 1955) which consists of introducing a pulse of a n infinitesimal amount of substrate into an iiiert gas stream flowing over t'he catalyst in a niicroreactor and analyzing the resultant pulse via a relay line of a gas chromatographic column into pulses of products and unchauged reactant. When a pulse of substrate is altered Fvith a pulse of poisoii-e.g., quinoline or pyridine-the poison pulse is irreversibly adsorbed, blocking an active catalyst site. With each successive pulse the reactivity of the catalyst is decreased. In this way the nuniber of poison molecules is determined n-hicli just decreases the catalytic act'ivity to zero. Assuming that oiie poison molecule is adsorbed a t one active site oiily, the total number of active sites on t8hecatalyst can be calculated. A similar pulse poisoning technique has been used by Turkevich et al. (1965) aud RIurakanii (1968) for the study of active sites on solid catalysts. h displacement technique may also be used for the titrat'ion of the specific active centers responsible for a particular catalytic reaction, because, whether a particular adsorbate is adsorbed weakly or strongly depends on the nature and strength of t,he active site and the properties of the adsorbate. Therefore it would be possible to determine the specific active sites by displacing the weakly adsorbed adsorbate from the active sites by the adsorbate which is comparatively strongly adsorbed on those sites. The displaced adsorbate is then measured quantitatively by gc and the number of the specific active sites can be calculated. For example, a-sites on alumina and silica-alumina catalysts, supposedly responsible for isomerization reactions, can be estimated by displacing the
carbon dioxide, weakly adsorbed on a-sites, by ammonia which is strongly adsorbed on a-sites. Surface Acidity and Acid Strength Distribution
Sortoii (1962) used inicroreactor glc t o show the presence of solid acid sites 011 syiithetic zeolite. The total number of acid sites and their acid strength can be found from t,he chemisorption of the ammonia and volatile organic bases, such as aliphatic amine-e.g., methylamines and ethylaminespyridine, and quinoline, which can be readily studied by R frontal gas c1irom:rtograpliy (Misono et al., 1965; 13uyaiiova et al., 1965). The effect of surface acidity on the adsorptioii of hydrocarboils aiid on catalytic activity has also been investigated b y gas chromatographic methods (hlisono e t al., 1965; Mosely and hrchibald, 1963). Recently, 1Iisoiio et til. (1965) proposed a gas chromatographic method for the siinultaiieous determination of tlie distribution of acid centers n i t h respect to the strength of their acidity and catalytic activity under the operat,iiig conditions of tlie catalyst. The method coiisists in determining the retention volume and the lieat of adsorptioii of benzene (a weak base) on the catalyst a t different surface coverages of tlie latter with pyridiiie (a strong base). d continuous flow of pyridine in a carrier gas is iiitroduceti to the column packed Ivith catalyst a t the operating teniperature of tlie c a t d y s t , t'lieii the amount of pyridine adsorbed, B , is obtained by froiital aiialysi fter t'he adsorption equilibrium of pyridine is established, benzene ( a weak base) is injected, aiid its retention volume, Vx,is measured. From these values the acid strength distribution is calculated on the assumpt,ioii that adsorbed pyridine occupies acid sites successively from stronger to weaker ones as :I fuiictioii of pyridine adsorbed, B , by the relationship:
By plotting the left side of Equation 39 a t a certain value of B against l / T G , we obtain a n isosteric heat of adsorption, q , which is a measure of the acid strength, the temperature dependence of log (f'/p,T,) beiiig neglected for the narrow temperat'ure range. The surface acid strength can be directly correlated to tlie activit'y of tlie catalyst for a particular reaction. If, iiistead of benzene, actual reactants are admitted to the catalyst column, it is possible t o determine on the basis of the react,ion products corresponding t'o different degrees of poisoniiig of acid sites b y pyridine, the activity of the catalyst in relatioii to different types of reactions as a function of acid strength distribution by simultaneously det'erniining the acid strength distribution of the acid sites on tlie surface aiid the activity aiid selectivity of the cat'alyst. Thus, it is possible to investigate strength of the acid sites responsible for the activity and the selectivity of the catalyst for a particular reuctioii by blocking the uiidesired sites-Le., b y successive poisoning of the catalyst. This method may also be used to study the effect of the presence of proniot~ers,which modify certain properties of t,he catalyst, on the nature and the strength diyt'ributioii of the acid sites. For example, the effect' of fluorine coiiceiitratioii in e.g., :ilumiiia and silica-alumilia-caii ing the vapors of the compounds coiitainiiig fluorine-e.g., HF, CHF,, CH2F2-over tlie catalyst i i i the column follonecl b y its acid strength distribution measurement.: and activity test,. Similarly the effect of surface 1iydr:itioii or dehydration on the acid strength distribution and the activity of t'lie catalyst may nlso be evaluated. Micro-
catalytic pulse technique can d s o be used to ev:ilu:ite the effect of poisons and promoters on the number of acid sites and their strength distribution and ultimately on the act'ivity of the catalyst. Similarly it' may also be possible to determine the surface basicity and base strength distribution of the basic solid catalyst b y tlie adsorption of weak acid on the catalyst a t different surface coverages of the latter t~itli vapors of strong orgaiiic acids. The residence time of pulses :it infinite dilution iii :I carrier gas has been used to calculate adsorption site eiiergy distribution (Snyder, 1961) :tiid relative :~dsoi~b:ibility of hydrocarbons on acid catalysis (LIosely :tiid .\rchibald, 1063). Study of Effects of External Electric Field, Magnetic Field, and Irradiation on Active Sites
The study of the nctive centers of a cat:ily.st under tlie iiiflueiice of external electric field, mngnetic field, :ind irradiation throws light on the esact nature of the active ceiiter.5 aiid their role in catalysis. Such a st'udy is particularly useful i n the case of conductor-Le., metallic or *upported metal-aiid semiconductor-type catalysts where the catalj associated with electron trawfer processes aiid, to some estent, in the case of poorly coiiductiiig catnlyits-i.e., iiisukitors-also. If, however, an arrangement is made to determine the electrical conductivity of the catalyst (this can he done by using a single pellet of the catalyst with a meaiis to niea.wre electric conduct,ivity instead of the pondered c a t u l p t i i i the niicroreactor), the activity of the catalyst caii be directly correlated to its electrical conductivity. The nature of the act,ive centers in the c a t d y s t c a n be established by studying tlie influence of esteriial electrical aiid niagiietic fields o n the adsorptive properties of the catalyst. Generally there will be a far greater effect oii tlie adsorptive liropert'ies of metals and seniicoiiductors b y the esternal electric or magnetic fields, while iii care of iiiwlators or poorly conducting catalysts, the effect will lie iiegligible provided the electrostatic fields produced in some of the c:it:ilysts due t o the presence of highly charged c:itions or anioii?-i.e., localization of the electric charge-are not the sites for adsorption. Though the study of active centers aiid of catalytic reactioiis under the influence of a n external electric field, magnetic field, or coiitinuoiis irradiation is of coiisider;il~leiiiterept, no at,tentioii has been given to t'liis ;ipproacli. Study of Catalyst Deactivation by Gc
The deactivation of a catalyst is caused by niaiiy factors, such as by coke formation, which iiivolves tlic blockiiig of the pore mouths by coke, reeultiiig i i i a decrease in tlie iiiteriial surface area of the catalyst; by the poisoniiig of the active sites which arises from the irreversible :idhorl)tioii of the poisons on the active sites of the catalyst; and by the destruction of the active sites due to the structural breakdown of the catalyst such as by sintering and fouliiig. T h e rate of coke formation on tlie catalyst during reaction may be determined by a sudden oxidation of the coke 011 tlie catalyst with oxygen as a fuiictioii of the coke formation time and by determining the resulting oxides of c:ti,boii by gc. The decrease in catalytic activity us a fiiiictioii of coke fomutioii can also be determined. Poisoning of the active sites during c a t a l p i s is niuiiily due to the irreversible adsorptioii of tlie trace impurities p m e i i t in the reactants aiid to some extent' due to the coiltaminations of the catalyst with reactor material or foreign impurities Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
229
during catalyst handliiig. If the trace impurities in the reactants :ire known, then it is possible to study the adsorption propert'ies of these impurities-whether they are adsorbed reversibly or irrcvewihly. The poisoning effect on the catalyst activity for the irreversibly adsorbed trace impurity can then be estimated by the same Iirccedure as discussed iii tlie earlier sections. ociatetl with catalyst deactivation is tlic decrease iii active sites by s t l u c t u r d hreakdowii of the catsl y d , such :is sintering and fouling. Siiitering is :i fuiictioii of temperature : i d time. The cliaiige in the active ceiiters 01' the activity of the cotillj.st due to siiiteriiig c a n be studied by est'imatiiig the active site< or the activity of tlic catalyst n t its oper:itiiig coiiditioiiq :is :I friiictioii of its prehextiiig temper:itiire :~iidtiine i i i :i microcntal>~ticrenctoi,. l'he detwmiiis'I t 1011 ' of tlic riitc of 4iitciiiig :it i i particul:ii. teiiiperatiire by studying t'lie cli:iiiges i i i the activity of the cataly,\t is coniplicnted by the decrease iii the :ictivit>-of tlie cnt:ilyst also by coke formatioti :itid poisoniiig, a i i t l hence it' caiiiiot be easily determined by activity studies. This caii be doiie by studying the rate of crystal-phase tmiisforiii:ition as a fuiictioii of temperature by 1iigli-teiiil)er:~tLireX-ray measuremeiits followed by the activity test. Thuu, by detertiiiiiiiig the rate of coke forniatioii, poisoniiig effects of the tr:ice iriii)urities preseiit i i i the reactants, mid the rtite of siiiteriiig of the catalyst for a particular reaction a t its oiwatiiig coiiditioiis, it is ~)ossibleto estimate the probablc life of a catalyst. Thermochromatographic Study of Catalyst Formation
Receiitly, Golosiinaii et al. (1 9693) applied thermochromatogrnphic iiiethods for stud1,iiig the catalyst formation. Catalysts ~)re]iaredfrom oxides of C'r! % i i , Si, mid .\I were formed i i i a special apparatus i i i which a programmed hcat,ing schedule was followd and the efflueiit gases \vere analyzed by gc. 'I'he tlierriiochroiiiatogral,hic method was further ai)plied by Golosiiiaii et al. (1969b) to the determiiiatioii of liouiid water! iioiistoichiometric 0, and C'r-oxide content in commercial iiittal oxide catalysts cont,aiiiing %u aiid Cr or Zn-Cr oxide and Cu oxide.;itlvtiiit:ige of this tcchiiiquc ib that the result.: obtaiiied c n i i i i o t tle;ci,ihe the i)rocc.>- :IC. a ivholc, ailti therefore care muit lie takcii i i i sclectiiig :I c:italy>t 011 the basis of such data done. Ho\\-c\-er, the iiiicrocatnlytic reactor with chromatograph can he iihed .:ucce>sfully iii the study of catalyst deactivatioii b\, I)oi>oniiig and the inechaiiism of catalytic procews. Kinetics and Mechanism of Catalytic Reaction
t and Habgood ( I 960. 1961) have giveii a quantitatmeiit to the kiiietirs of a fii,st-ortlcr m f a c e catalyzed renctions (isomeiizatioii of cyc1oI)rop:uic) whcii the catnlytic re:ictioii is c:irrictl out by 11 irig a imct:iiit 1)i11.e over the catalyst placed i i i a Inicroreactor. T h e extent of' titlmrptioii of the reactmt oii the catalyst mv cqtiinated by the retelitioii volume for the peak of reactant oii the cat:ilyqt column. They showed that it ii: poszible t o detenniiic the activntioii eiiei from the rate based oii the pas pha.:e 1):irtial pwsui'e, the heat of adsorptioii, mid t,he activatioii ciicl'g!- for t h e surface itelis. The rate equation for a first-order w f a c e c:it:il!-zcti reaction, when carried out by u G i i i g thc pii1.e ii1ic.i'ocntal~tictccliiiique, is reprewited as L'O
l'he apparelit actiratioii eiierg!-. Fd'zilmi!' l x olitaiiieti from the slope of the ldot of 111 [ k ' / j ]v>. 1 7': : i i i t l :it coiihtaiit floiv rate [Po], this is equivalent to t h c ])lot of 111 [ I (1 - S ) ]v'. I , 2'. Froin a plot of 111 p vs. 1 T , the heat of atlsorptioii, A I I ] may be obtained. Thus the activatioii energy of the SUI'face reaction E , = Ea f AI1 ma\. he detemiiietl. Hightower aiid Hall (1968) have a1q)lied the LaiigmuirHiiishelwood scheme t o microcatalytic d a h niid tiei,ived a rate equation for the first-order reactioii as Ind. Eng. Chern. Prod. Res. Develop., Vol. 10, No. 3, 1971
231
1
1
Ti
’
where
I!: = (V’/k’Q,WRT)
(43)
For a given catalyst and temperature, D (Equation 42) and E (Equation 43) will be constants, and the initial rate will be a function of the partial pressure of the reactant, P’, after a part of the pulse has been physically adsorbed. Recently, Yushcheiiko and Xiitipina (1 96913) applied pulse microcatalytic technique for studying the kinetics of cumene cracking oii zerolite N C and of dehydration of isopropanol 011 7 - a h inina, assuming the Langmuir adsorption. Recently some attempts have also been made to use the pulse technique to get additional information on the kinetic 1i:mmeters of the catalytic reactioii which can oidy be got with difficulty by the flow technique (Saito et al., 1965; Murakami and Hattori, 1967). For t’his, sufficient kiloidedge of the charactcristics of the pulse technique is required. quuititative study of first-order catalytic reactions can e:isily be done with the pulse microcatalytic technique, since under certain conditions for a first-order reaction, the conversion is independent of the peak shape (Hassett and Habgood, 1960; Gaziev et :iL, 1963), but complications arise for reactioiis other thaii first-order. Gaziev et al. (1963) attempted to cxteiid the usefulness of this techiiique to kinetic equations other than first-order by solving analytically the case of square-topimi :ind triangular input peaks with no axial dispersion. Howevcr, the iioiidispersed square-topped peak is ~iiathennnticall~ identical to the steady-state case, and the triangulnr l)ecik is difficult to obtain esperimeiititlly. Ai more gcner:il treatment has beeii given by I>e:tiis et al. (196’7), but, solutiolis so far :ire available only for lincar kiiretics. Colliiis and 1)eans (1968) have investigated pulsed reactors with axial disimsion for the case of reaction a t equilibrium usiiig a radio tracer technique, but here also the kinetics are linearized. Recently, I3ett and Hall (196’7) have shown how the pulsed microcatalytic technique can be used to complement the steady-state flow technique for very strongly adsorbed products. Hattori and hlurakami (1968n,b) and I3ett and Hal1 (1968) have given special attention to the reversible reactioiis. llurakanii and Hattori (1 967) and Hattori and hlurakami (1968n,b) have carried out a theoretical investigat,ioii of the pulse-reactioii technique on several reaction models [irreversible, reversible, and con*ecutive reactions] and compared the conversion of reactants arid yield of products with those from the contiiiuous-flow techiiique. They showed that the results from the pulse techiiique agree with those from the flow techiiique it1 the liiiear reactions, excluding the rat,e processes other than first-order, while in the nonlinear reactions there are some times quite remarkable differences between the two techniques (Saito et al., 1965; Hattori aiid Nurakami, 1968a). I n all reaction models, t’he coiiversioiis from the pulse techiiique were larger t,han those from the flow technique and increased wit’h a decrease in the pulse width. The high conversion i-: attributed to the lowering of concentration due to the broadeiiiiig of peaks and the separation between the comlionelits in a reaction of the type,
A e R + S 232
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10,
No. 3, 1971
The effect of the pulse width on the catalytic cracking of cumene was studied using a rectangular pulse, and the activation energy of the forward reaction was determined @Iurakami e t al., 1968). Further, Blaton e t al. (1968) extended the usefulness of the pulsed-flow technique to the more general cases of nonfirstorder kinetics. Numerical solutions were obtained for several rate expressions assuming a gaussian input pulse for the case of a concentration pulse which does not broaden in the reactor. The rate constants of a reaction can also be determined from the shape of the concentration pulse obtained a t a single carrier gas-flow rate, on the assumption that under the experimental conditions the adsorption isotherm of the product on the catalyst is linear (Filinovskii e t al., 1965; Blaton e t al., 1968). Chromatographic data obtained from very small samples of the reactant give accurate velocity constants for the surface catalyzed reaction, and the true activation energies can be calculated. Where the reacting molecules give only one product, a normal elution technique is used. A stopped-flow technique has been developed by Phillips et al. (1967) for reactions where several products are produced, where more than one reaction may occur, or where consecutive reactions occur. In stopped-flow chromatography, the gas flow through the column is shut off a t calculated time intervals during the passage of the reactant through the column. The products produced during these intervals appear as sharp peaks superiniposed on the continuous chromatogram. Study of reaction kinetics by distortion of chromatographic elution peak has been discussed in detail by van Sway (1969). Apart from the literature discussed above, several papers (Guerin, 1965; Cher and Hollingsworth, 1966; Giordano e t al., 1966; v a i i Dyke e t al., 1966; Johnson, 1967; Sethi and I>evaprabhakara, 1969) have appeared utilizing the gas chromatographic techiiique iir the kinetic study of gas-phase re:ictions and gas-solid reactions. Purnell (1967) summarized the various approaches to the study of complexing reaction by gc and developed a generalized retention theory for each. The gas chromatographic methods of studying reaction kiiietics have been limited to fast processes and low partial pressures of the reactaiit,s. Complications arise if the adsorption and desorption steps are slow relative to t’he rate of surface reaction. In some cases extensive tailing of the reactant peak iiidicates strong surface heterogeneity with slow desorption from active sites of the catalyst. The results are seldom directly comparable with reactions in steady-flow reactors under working conditions. However, for studying the mechanism of catalytic reactions, the simplification of kinetics, and the ability to measure the adsorption and reaction steps separately, are real advantages. The microcatalytic technique has also been useful in the study of the molecular mechanism of catalytic reactions by using the reactants labeled with radioactive isotopes, since the study of the process in a microreactor requires only small amounts of the reactants, which are very costly when labeled. The products of the react’ion are separated and analyzed chromatographically with two detectors connected in seriesfor example, a katharometer aiid a radiometric cell. Comparison of the chromatograms obtained with the aid of these two detectors shows which product can be obtained from the substance labeled with a radioactive isotope (Kokes et al., 1955; Xleksandrov and YanolTskii, 1961). Then, the mechanism and the kinetics of the individual stages of the reaction are established by Seiman’s method (Seiman, 1954; Derbentsev et al., 1964; Dulton and Mounts, 1964).
Recently, Yaiiovskii and Gaziev (1968) have described in detail the application of the microcatalytic reactor in the study of the isomerization and dehydrogenation of butenecontaining labeled atoms. Further developmeiit in the separation by gc of deuterated compounds from the uiideuterated ones will make it possible to use the microreactor coupled with chromatograph to study the catalytic reaction mechanism by using the reactants labeled with deuterium, otherwise the products of the process are generally analyzed b y mass spectrometry. CATALYTIC REACTIONS UNDER CHROMATOGRAPHIC CONDITIONS
Chemical reactions carried out in a chromatographic columii under chromatographic conditions have recently been used as a very accurate method for obtaining kinetic data. A catalytic reaction is allowed to occur in a pulse of reactants traveling through a chromatographic column. Such a reaction differs from those usually encountered in that, under chromatographic conditions, the intermediate and final products are separated from the starting materials and therefore the interactions between them (and the reverse reactions) are almost completely escluded. Hence there is a sharp increase in the conversion. Further, the characteristic feature of the chromatographic conditions makes it possible to apply this method to irivestigate the reactions of two or more start'ing materials in an inert carrier gas, since, as a result of the chromatographic separation of the initial misture, the reactant molecules do not meet either in the gas phase or on the catalyst surface. Giddings (1959) has derived a general theory of chromatography which takes into accourit the individual react,ion steps occurring in a chromat,ograph. Klikenberg (1 961) made a theoretical study of the reversible reaction,
A G B occurring on a chromatographic column and calculat'ed the shape and retention time of the eluted peak. Keller and Giddings (1960) have analyzed similar cases in which the reaction rate is slow. Roginskii and co-workers (1960) studied the dehydrogenation of cyclohexane pulses under irreversible and reversible conditions. A mathematical treatment (Roginskii et al., 1962) was also presented for simple irreversible reactions. Kallen and Heilbronner (1960) have calculated the shapes of the chromatographic peaks resulting from the irreversible transformation on the column of one substance into another with different retention characteristics. hlagee (1963) has proposed a simple model and mathematical description of the course of a reaction in a chromatographic column operated as a pulse reactor. Masten e t al. (1965) have studied the reversible reaction of the type,
A=B+C with a very low equilibrium constant and demonstrated t h a t the reaction can be conducted in a chromatographic column so that the products can be separated during the course of reaction. Higher conversions can result from this type of operation than in a conventional equilibrium reactor. This is a potentially attractive mode of operation for such equilibrium-limited reactions, arid the possibility has already been recognized in the patents by Ilinwiddie (1961) and Magee (1961), and a mathematical analysis of such a reactor has been presented b y Masten e t al. (1953).
Reactions of the type,
A+B+C+D under chromatographic conditions have been described b y Zimiii et al. (1965). One of the starting materials is either continuously passed through the catalyst column i n the stream of the carrier gas or introduced as a pulse, mixed with the other component. Probably in this case the start,ingmaterials are not separated in the reactor :uid therefore reactioii between them is possible. The gas chromatographic reaction can be advantageous only under a well-defined and limited set of coiiditioiis (Masten et al., 1965) such as: the equilibrium constant^ for the reaction must bc snxill reaction rates should be high enough so t'hat the separation of products rather than rate of the reaction limits the estent of reaction at least two products must be formed whirh are chronlatographically separated in the reactor reactants must iiot be separated in the re:ictor c d r i i n l l It would, seem, therefore, that oiie n.oultl he limitctl to a single reactant, or to two reactaiits where one also serves as the carrier gas. Though t'he mathematical model of AIagee (1963) mas related qualitatively to the experimental data (1Ia~terIet :&] 1953), the conditions of stoichiometry and elutioii velocity would iiot allow qutiiititative comparison. Thus the com1)les theoretical problems of the chromatographic reactor have not yet been solved completely (AIastcn e t al., 1965). Recclitlp, Langer et al. (1969) have described i n detail the use of gas chromatographic column as a chemical reactor. Catalytic reactions under c,hrorn:ttoglni)hic coiiditioiis are more specific, and the results of the study of a catalytic process under chromatographic conditions provide adtlitioiial information both about the process ithelf and about the state of the catalyst. This mrthod has been used (Roginskii e t al., 1962; Gaziev et al., 1963; Rogiiiskii et d j 1963; Kogiiiskii and Rosetal, 1964; Masten e t al., 1965) in the study of various types of catalytic reactions aiitl of the changes in the catalyst such as poisoiiiiig aiitl regelleratioil. A microcatalytic reactor can also be used in studyiiig the catalytic reactions under chrornatogral)hic coirditions. For this, the starting material is introduced as a separute sain1)le pulse into the reactor and is transferwd by the inert, cwrier gas or by a gaseous reactant to a layer of c:italyst where the react'ion products and the starting materials are separated 011 the catalyst itself and are then analyzed a t the outlet from the reactor by rneiins of a suitable detector. A brirf thcory has been presented by Yanovskii and Gaziev (1968) for a procesh involving simultaneous chromatography ailti catalytic conversion. Conclusion
The survey cited above highlights the importance a ~ i duses of gc in catalysis. While several aspects of catalysis of iiiterest to the physical chemist are brought out in the survey, it is particularly noteworthy that gc call be a very valuable tool iii chemical engineering esperimentation also. This is clem from the fact that studies in physicochemical properties, reactioii kinetics, and catalyst evaluation can be carried out with ease and speed wit'h the help of gc. Nomenclature
A A'
time and flow velocity-iiidel)eiideiit term in Equatiori 34 = area of effluent pulse, r n m 2 =
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
233
Iiroliortioiiality coefficient (Equatioii 31) coiistant of Equations 10, 12, 14 = time-deliciideiit term in Equation 34 = aniouiit of adsorbed Iiyridine, mol,'g = \elocit!.-deliendciit term in Equation 34 = coiicentration of adsorbate i i i iiiobile phase, mol/ml = amouiit of a d . ; o h t e in stationary phase, niol/ml = roii(witratioii of sarnple iii gas phaw, ml/ml (Fig11re 2) = cliart 1)al)rrq)cecl of recorclcr, nllii /set = cffertivr tiiffusivity of catalyst, cm*,'sec = avcr:ige Iinrticle tlianietcr, cm = alqxirciit activatioii eiieigy, cal;'mol = wrface activatioii eiier = flow rate, nil 'see = coiwcted flow rate cqual to (273/T)F', ml/sec = void friicatioii in 1)aekiiig = -olitl fixctioii i i i packiiig (1 - 1'1) = l~aititi(iii fuiictioii of ad-orbed beiizeiic = f1,cc ciiergy cllallgc, cal '"I< 'mol = height cquivaleiit of theoretical lilate, III:TP = height of effluent 1iuI1c iiiasiiiy! miii = heat of adsorptioii, c d mol = adwrptioii ratr coiistant, 1,RCC = di;tril)utioii coefficiciit = Ihltz111:~1 coiistaiit, erg "I< = 1,c:aCtioii rate coiistant = length of picked coluniii, rni = iiiolccrilai. vciglit = coiiceiitratioii iii gas, mol ~ c m 3 = cquilibrium gas-eoiiceiitratioii, inol '(71113 = mole. of gas in volume 1' = initial partial preswre of the reactant after a part of pulse has liecii adsorlieti, 111111 I Tg = rquilibi~ium11rcsaurc, nim Hg = iii1t.t I)rrliwrr of the colun~ii,111111 IIg = outlet prrwiirr of the colunin, n i n i H g = ])or? volulne, 1111lg = quantity defiiicd by Equntioii 22 = s~~ecific moiiola!.er rapacity of adsoi,bent, nil t'g = ainoiuit of sample adsoi~hcdon the solid supliort, nil ' g (Figure 2) = quaiitity ticfinctl b y Equation 5 = ismtcric hrnt of ad.orptioii, cal 'in01 = $215 con~tnllt,ea11 oK~'lilol = average pore radius, cm = e s t e r i d particle surface per ~ 1 1 of 1 ~ packed column, =
IT7
=
X
c 1112 specific surface area, m2/g = temp, "I( = qiiaiitity tlcfinrd by Equation 30 = colunin temperature, O K = tciiip of the flow meter, ' K = quaiitit!. defined by Equation 23 = tinie, see = reteiitioii tinie of a pulse masimuiii, see = ccii,rected retelltioil times, Equitioii 11 , sec = retciitioii tinie of iioiiadsorbatc, scc = iiitcwtitial carrier gas velocity, cm,'sec = superficial linear gas velocity, cm,'sec = vol~inicof gn. containing S moles, nil = \-oluiiic of carrier gas measured from the air peak,
=
1111, 'g
uiifilletl volume of catalyst bed, nil 'cin = volume of t h e qtatioiiary phase, in1 = sl)ccific retention volume, nil ' g = correct,ed retention volume, Equatioii 1, ml/g = reteiition volumc of beiizene oii partially poisoned catalyst, nal ,jg = rctciitioii volunie of a given substance referred to uiiit a i w i of the ndsorbent, Equation 32, ml/m2 = a \ w a g c tlieriiinl velocity of gas molecules, cm/sec =
234
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
XO 2: 20
= = = = =
weight of the catalyst or adsorbent in column, g fractional conversion quantity defined by Equation 13 coordinate in the direction of the surface flux distalice from the surface to minimum in potential well, cm
Greek Letters CY
=
P
=
Y
=
ye
e(z)
= = =
0
=
( P ) ~
=
e
A(p)
=
w
= =
7
probability of a molecular collision with external surface of the particle resulting in adsorption adsorption equilibrium constant number of molecules adsorbed per em3 of column divided by external surface area, em2,of the particle per em3 column equilibrium concentration of adsorbed molecules pellet porosity potential energy of gas solid interaction a t a distance, 2 , from the surface av residence time of adsorbed molecule in given solid particle, see area under tlic diffuse rear portion of the chromatogram, nillg (Figure 2) area under the curve defined, ml/g (Figure 2) quantity defined by Equation 24 standard deviation, sec
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