CLAS SIFYING CATALYSTS Some Broad Principles How does one find, o r even begin t o look f o r , a
c a t a l y s t f o r a given reaction? Classification of c o n t a c t c a t a l y s t s based on electronic, steric,and geometric f a c t o r s may help answer this perpetually difficult question.
or many decades the development of a new catalyst
F has been equated in the public mind, to a large
extent, with lucky chance. I t may even be safe to state that some chemical engineers and chemists still look upon the selection of an effective contact catalyst as little more or less than a trial and error procedure carried out with minimal guidance from scientific principles. The reader with a strong interest in chemical technology might recall the tale of Groebe accidentally breaking a thermometer in a hot mixture of sulfuric acid and naphthalene, thereby bringing about the mercury catalyzed oxidation of naphthalene to phthalic anhydride, thus providing the starting point for the industrial synthesis of indigo. However, the same reader may not be fully aware of the detailed knowledge of catalytic phenomena that has been accumulated over the past few decades and, most important, the ordering of this knowledge which now provides a rather broad understanding of many heterogeneous catalytic phenomena, if not yet a complete scientific system encompassing all contact catalysts. I t is the purpose of this article to outline some of the broad principles which have evolved in heterogeneous catalysis and to present them in an organized form that might be applied in selecting contact catalysts for a research study directed toward a particular type of process. However, it is not intended as a detailed Robert W . Coughlin is Associate Professor of Chemical Engineering, Whitaker Laboratory, Lehigh University.
AUTHOR
literature review and many of the references given here are themselves reviews on particular aspects of catalysis, It should be stated at the outset that our detailed understanding of the intimate mechanisms of chemical reaction is far less precise in the case of the general heterogeneous catalytic reaction than in the case of the general homogeneous catalytic reaction. The reasons for this lie in the fact that the nature, properties, and concentrations of the chemical species are far more accessible to experiment when all are present within a single homogeneous fluid phase. The same cannot be said of the constituents of adsorbed species or the constituents of solids which play a major role in heterogeneously catalyzed reactions. However, the progress made in the physics and chemistry of surfaces and the solid state in recent years and the relationship between this work and contact catalysts and heterogeneous kinetics kindle the expectation that a large part of the understanding gap between homogeneous and heterogeneous catalysis will soon disappear. In principle, homogeneous catalysis is far more tractable than heterogeneous reactions, since the former is easier to carry out, more amenable to reaction mechanism elucidation, and not susceptible to “poisoning” owing to certain impurities in the reactants. Nevertheless, contact catalysis is one of the most effective processing techniques in the chemical industry and enjoys a continual increase in the number of its applications, and it is hardly imaginable that it will be eclipsed by homogeneous catalysis in the foreseeable future. VOL. 5 9
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-1 TABLE 1. COMPARISON OF CHAIN REACTION AND CATALYSIS TERMINOLOGIES*
I
RCactW"
Catalyllt-
Chin Rtaclion TminoropY
Chain initiation
C&3IVri$ Tmimlo~y
Activation and preparation of catalyst
,E:!;"
.46
INDUSTLIAL A N D ENGINEERING CHEMISTRY
Mechanism of Contact Cdalysir
The sequence of steps in a heterogeneously catalyzed reaction can be stated as:
I. Difurion of rewtatzt(s) to catalyst surface from fhe bulk fluid phase II. Chemica4 adsorption of one or more .reactant spccies the catalyst surfocafrom thefluidphase III. Reaction of thise adsorbed species either among themsdws, with physically adsorbed spe&s, or with species colliding, with the surface fiom the fluid Phase IV. Desorption of reaction products from the catalyst . 'surface V. D@usion of products from the surface into the 6ulk fluid phast The conceptual dirraence between physical and chemical- adsorption &I be expressed simply as k m l v i n g van der Waals-type forces with no electronic bonding in the case of physical adsorptiQn, whereas chemical adsorption takes place by virtue of the sharing or exchange of electrons to form a chemical .bond between adsorbate species and adsorbent. i n some very few catalytic reactions it is n k a r y that the reactant be only physically adsorbed. An enlightening.&alcgy between the m h a n i s m of contact catalysis and chain reactions haa been drawn by Burwell (8). This analogy is presented in Table I in which an active site on a catalyst surface reacts in an adsorption step and is subsequently regenerated in a d m r p t i w step. The fact that in contact catalysis all the chains. are begun at the same time (cadyst activation or intkluction) c6nstitutw a. departure from the usual chain rektion case. Here poisoning of the catalyst surface is explained quite simply as a chain termination reaction. . It is intuitively clear that for heterogeneous catalysis
ClarrMcaiion bf Cdalysh
It is possible to divide contact catalysts into categories according to their elecaical conductivity and to c l a d y the functions of these catalysts as shdwn in Table I1 wbete the,primary functions are shown in color i n d the le& important functions are shown in the black. In Table I1 hydrcgenolysis is considefed distinct from hydrogenation in that the former type of reaction causes the rupture of a bond, Usually a bond between carbon and nitrogen, oxygen, or another catbon atom. The term “reduction,” as used in t h i s table, applies pri&ily to reduction by oxygen transfer. Mast of the catalysti and reactions from the extensive listing of Innes (25) Fiprra 2. Actides of oxidcs in the fist long period, Do&
(78)
TABLE II. CLASSIFICATION OF CATALYSTS‘
to occur, chemisorption must perturb the adsorbate molecule somewha-but not to the extent of forming an extremely strong bond with the surface. In the latter h t a n c e one would expect further reaction to be energetically unfavorable if the strongly adsorbed mole cule is a reactant; an extremely strongly adsorbed re&tion product would introduce unfavorable kinetics in &e form of a slow step IV in the sequence stated above. An example of this is provided by the decomposition of g a i n m e on a germanium surface; it has been shown (55) that the Ge surface is covered by adsorbed hydrogen atoms, and Taylor has reported studies (57) in which the rate of -tion was increased fourfold by contacting an extended platinum surface to the Ge film-it is supposal that the Pt provides another pathway, in adaitioh to d m t desorption, for the hydrogen product & . e l the active Ge surface. Similar effects have been rep& by Turkevich (60) and Szabo and Solymosi
(541. Chemisorption is represented symbolically in Figure Here Case A illustrates the adsorption of an easily ibnizcd atom that contributes at least one of its electrons to the adsorbent. An example is the adsorption of Na &n W which leads, as would be expected, to increased elecirical cqpductivity and enhanced photoelectric sensitivity and w o n i c emission of the substrate. Case B preento the situation diametrically opposed, in which the electmjiric properties of the adsorbent change in a way exactly oppbsite t6 that of Case A. That is, Case B depicts adsgptioR accompanied by increased resistivity, deereapcd photoelectric knsitivity, and decreased therniiPnie:emission of the substrate. ,Case C depicts covalent b6nd formation which immobilizes the elec&ons of the adsorbent phase on the sutface, with the result that conductivity is decreased (as in Case B) but phot& electric and thermionic behavior is enhanced as in Case A. 1.
Hydmgenatioi (including ammonia synthcsia and FisherT----chreaetians) ~
,
.
Wv+w$pa!kn. Hydrogenoh .
..
t.
oxidation reduction
(including contact SO pmececs) ’ Reduction Isomerization Dehydmgenation Cracking lyclization pehydrntion ydrogenation Alkylation Hydrogen . , rranafcr Halogenation Dchabgenatian ,
e .4fW Bnd (6).
.
.
fit this classification. It is evident that the primacy catalytic functions of metals are hydrogenation and dehydrogenation; active metals catalyze these reactions by virtue of their ability properly to adsorb reactants and/or products. Thus the following reactions can be rationalized in terms of copper’s ability to w e d y adsorb hydrogen :
a. C k L O H + CHrCHO 2 C&OH
cu +
+ H, CHaCOOCrHa + 2 H,
whereas the same ethyl alcohol underg&.dehydration ovp alumina in accord with the latter’s ability to adsorb I water more readily than hydrogen:
V O L 5 9 NO. 9 S E P T E M B E R 1 9 6 7
47
I
The catalytic action of semiconducting metal oxides in oxidation and reduction reactions can be rationalized on a similar. basis. Whereas metal oxides of predominantly covalent character are always stoichiometric and &e therefore insulators which do not readily adsorb or desorb oxygen, their predominantly ionic cousins exoxygen easily. To this latter behavior are attributed their semiconducting properties, as well as their catalytic action in oxidation and reduction reactions. . A detailed discussion of the catalytic oxidation of hydrocarbons by metals and nonmetals has been given by Margolis (28). Whether a particular metal oxide or a particular elemental metal be specified for oxidation, reduction, hydrogenation, or dehydrogenation has a great deal to do with wheiher the catalyst will remain in the specified form.undk the conditions of the reaction in the presence of the reactants. Thus only noble metals like platinum and silver need be considered for ,oxidation teactions since moat others would be converted to the oxides under reaction. conditions. Likewise, it is an uncommon,hydrogenation reaction environment which W i l l not reauce a metal from its oxide to the elemental State. The polymerization, cracking, alkylation, and isomerization role of solid salts and acids is believed to be due to their ability to .form carbonium-ion intermediates with olefins. Indeed, the work of Weisz (65) and Mills, Heinemann, Milliiken, and Oblad (35) provides evidence that platinum 'supported on silica-alumina is really a dual-functional catalyst in reactions .like isomerization and hydrocracking .of alkanes. It appears that the alkanes are dehydrogenated on the metal portion of the catalyst and the resulting olefins migrate to acidic sites On the support and there react by carbonium ion mechanisms. Many of the other functions stated in Table I1 remain poorly understood, even in terms of a general rationale. Moreover, it should not be assumed that detailed mechanisms are thoroughly understood even when a general plausible' rationalization can be offered for the role ofithe catalyst as in the caw of dehydrogenation and '
1
Ti
V
Cr
Mn
h
Co
Ni
Cu
Figure 3. Achmfies of nutals in fha f i r f long w i n d , D&
dehydration mentioned above. These mechanisms the nature of the adsorbed intermediates are still subjects of much mearch in catalysis today. Ne thdess, the rudimentary classification given in table does provide a guide in selecting a suite of alysts as'potential candidates for a particul& kin, promising step. Mdallir Caialyrh
Since the sine qua non for any contact catalp that it chemically adgorbs at least one of &e reacti it is reasonable to approach catalyst classification f ' the point of view of propensity for chemisorption. is particularly inlormative in the case of metals only because of the amount of data available but cause it also illustrates the way understanding of tact catalysis has developed, as well as certain probl which continue to persist. I n 1953 Trapnell (59) organized information a1 &e activities of some 20 -metals for the chemiporp important gases and his classification is show of Table 111. In thb table, which has been modified expanded according to Bond (6)6 include more re data, a minus sign means that no adsorption is o h
six
.
lwIUUTORi
CdO Cr203
AN
48
INDUSTRIAL
4w
_il-
A N D ENGINEERING C H E M I S T R Y
2
6w
HOMOGENEOUS DECOMPOSITION
800
abovc the temperatures at which physical adsaption begins; the plus sign means that chemisorption takes place wef a large part of the surface with great speed. The
leder
I
TABLE 111. CLASSIFICATION OF METALS AS TO CHEMISORPTION*
A through F divide the metals into group in of decreasing activity. Much activity is a850
ciattd with transition metals, although there are a few ex6eptions. This activity riaea with increasing atomic n u m b as a transition period is approached and entered and decrease as the period ends. Manganese is the only transition metal in the less active group, C-F,while the moat active groups, A and B. contain only transition metals, with the exception of Ca, Sr, and Ba. It is suspected that the last-mentioned three elements have some electrons in the d-bands wben in the crystalline metallic state and thus they &are with the transition elements the property of partly 6Ued &bands. From t h i s type of reasoning mmea the notion of partly filled d-bands as responsible fD1.ewdent bondii to metal surfacea in chemisorption and cadpis. Indeed, the experimental facts are that d y t k and chemisorption activity is confined largely to thu transition metals. As a matter of fact the three common metal catalysts are probably Pt, Ni,and Fe. Pt is always in the elemental state and the other ~ W an D used in the oxide form as well as in the metallic ate.
innes (25)has assembled some pertinent information for the activity of a number of metals for reactions involving hydrogen. This is shown in Table IV. It is evidslt that, in general, the strength of hydrogen ad@on incnascS with the number of vacant d-orbitals +mi mrehcs a maximum at about one vacant d-orbital (GO,Ni,Rh, Pd, Ir, Pt). There is also some evidence that the rare earth metals are unusually catalytically active, presumably becaw of f-band vacancies arising in's way dmilar to the &band vacancies of the tranailion m e d 8 as deduced from the Aufbau principle of atomic physics. Much information remains to be ded o @ as to the catalytic activity of the rare earth BLI well BS the leap common transition metals. uptake of oxygen on all metals except gold can be explained as a m u l t of oxidation which involves the breaking of metal-metal bonds rather than of true chemknption. H w ~ , Schuit and wworkers (45)have argued that metal-metal bonds may indeed mpture upon chemisorption. The wideapread admrptive paver of Cu is thought to be due to the ability d. k t metal to easily promote an electron out of its I-W or the ability of oxygen and other impurities y t on the surface to n m w e such a d-electron. In +.of this rationalization for the activity of Cu, the PCJorly understood synergistic role played by catalyst nrpports and pmmota~should come as no surprise. Thae are many other exceptions to the d-band vacancy concept but this does not seriously hinder its general -pd&lyfor dassification purposes. * .kd&hd evidence for the d-band hypothcsii is
ILE IV. ACTlU m 3 l O N S INW
31 9
-2.9
Vel$ low
28 27 Low
Low 47
43
VOL 5 9
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SEPTEMBER 1 9 6 7
49
provided by the well established role of d-orbitais in the coordination chemistry of the transition metals and the formation of many Organometallic complexes .and compounds. It, therefore, &. not surprising that dcomplexes occupy a central portion of the atage in presentday reseanh in homogeneous catalysis. Organ; ometallic suifate intermediates were first proposed in the 1930's but remained suspect &til the 1950%when analogous molecular c o m p o d s of the transition el* ments became more fully appreciated and understood. The general idea of surface organometallicintermediates was first pmposed by Sabatie (42) whose work remains one ofthe empirical co&rstones of modem hetercgeneous catalysis. Smllconduelon
For semiconducting oxides or Sulfides, two types of adsorption center suggest themSelves, namely +e metallic and'nonmetallic ions. On oxygen ions (03, COS would be Urpected to chemisorb as carbonate: co*
+ 0-
-*
c0,a
whereas water might be expected to sorb & hydroxide and SO2 as suffite. Oxides enjoy widespread use for the oxidation .of hyd&en and hydrocarbons, and the compounds of transition metals of variable valency .(including their sulfides and halides) are among the most effective redox catalysts, kpecially th& of &e elenwits of the first long period. The active solids are often nonstoichiometiic and d-bands are &aught to play a vital role ,in the bonding of Surhce intkrniediates. Dowden (78-20) has compiled data on the intrinsic activity of some oxides of m e m h of the first long period for .,hydrogen-deut+m exchange, propane dehydrogenation, and cycloheiene ,disproportionation; this is.shown in Figure 2. Here hydrogen adsorption thought to' involve the anions &.well as &e cations. Figure 3, +a after -Bowden, show% the.acti&ti& of the same imnsition metals in elemental form for-theinterconversioti of 0- &d p-hydrogen. Similar khavior han been observed for the HrD, 'exchange over these metals ( 6 6 5 ) : There is a basic s i m i h y betwekn Figum 2 i n d 3 i n that manganese r e p e n t s an S a n d of low catalytic activi* as compared to its ilnmediate neighbors in the transition series which show considerable . activity. Although a decline in catalytic activitL to the .right d nickel dciea seem to be an evident similkty between the elemental transition metal catalyits 'and their oxidi counterparts, there is no such similarity f& elements to ~
50
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
ym Irhlkrn llllD L1 I Y 4 t b / b l F t M 5. Efmt of W@la&@ on n&ng ocrivity, T m k (56). The indicord cakdysls haw h forrowing pacdnrogs of A1,O.: A, O.fZ%; E, OX?& C, 1.0%; D, 2~25%; E, 3.56%;
F, IO.% the left of vanadium. In the latter instance it may be possible to attribute the decline in oxide activity to the generally lower catalytic activity of oxide catalysts as compared to metals. The similarities between these two figurea suggest that valency considerationscan apply to metal oxides as well as to the pure metals. More recmtly h d e n (21) has attempted to explain this twin-peaked dependence of catalytic activity on the &upancy of d-orbitals by using crystal field theory. He was motivated by the fact that these activity patterns suggest the stability of the half filled (d' for manganese) d-shell which is ala0 evident from other aSpeets of inorganic chunistry. Dmvden's approach was to adsume that the crystallites of solid catalysts urpose the dominant cleavage planes and the moat densely pacLeed lattice planes, which continue to preserve the intaior lattice structure except for some distortion. When there is a transition metal atDm at the surface in this structure, its five d-orbitals will not be degenerate as in the case. bf a free ion-rather they will be split by the crystal field. Upon chemisorption of a molecule upon this transition metal ion there will be a wordination change involving its &orbitals and thereby a chmge in the stabilization energy AE,. Considering the many difTerent kinds of pomible k variacoordination change, Dmvden showed that t tion in aE, as one proceeds from dp to d"' displays the same kind of twin-peaked appearance for almost all the different kinds of coordination change upon &misorption that can be considered. Dowden points out that his data obtained from crystal field theory suggest that the semiconducting properties of oxides and sulfides may also be controlled by the same enpar a m e m involved in change of coordination d aaurface metallic ion due to adsorption or desorption. On this basis, the crystal field approach could well be in acw?d with the band theory approach to catalysis over semiconductors. The latter has been the more common theoretical abstraction for them reactions. Catalytic t h e
TABLE V. BEHAVIOR OF NOnSTOICHIOMETRIC SEMICONDUCTOR OXIDES'
P-TW Qxidca m* intaatIhal ions
ZnO, CdO
@Et of adding
FeO Increaas conductivity conductivity* IncreaKs Decrrases conductivity conductivity Increars conductivity conductivity
I
conductivity Positiw holea
I
I
-
I
- S i - O - S i - ~ - S i -
I
DCCnCW
conductivity
I
Si-O-Ssi-~-~si
I
UOr
CutO,NiO, of adding
-
- s
I
I
0
0
I
I
- 0 - s,
-0
-u
0 -0
-si
I
-
I I
/ Bt(R0IkamM uplll YDI
-
Figure 6. Possibb mid Cnuns an a sit&n-ahina cdfuIyd
ries based on the more conventional concepts of semiwductors are outlined in the followingparagraphs. In principle, oxides and sulfides should behave more as ineulatom than semiconductorsinasmuch as one would expect few electrons to be available to carry electric cun'ent. However, in fact, most oxides and sulfides wi* bonding of partial ionic character display semioosdrretivity and this is attributed to structural defects from impurities and/or nonstoichiometry. Z i c oxide, perhaps an extreme example because it can show conductivity approaching that of metals, owes its n-type d d u c t i v i t y to exzinc in the lattice. Interstitial zinc atom can ionize very easily, thereby acting IU d o n m of electrons. Oxygen chemisorbed on the zinc oxide c a w the resistivity to rise by acting as an dectron acceptor in accord with the reaction: Zn+
+ 0%z$Zn+* + 01-
However, in some e m , chemisorbed hydrogen causes the resistance to fall by donating electrons; this suggests thereaction : 1 Zn* 0-* - H t z$ Zn* OH-
+
+2
+
In the c e of an oxygen-rich oxide l i e NiO, #-type aemitonauctivity can be explained as due to unoccupied nickel-ion site in the lattice. To preserve electrical neutrality two existing nickel cations must exist in the Ni* state for every Ni+' &ing from the lattice; the
holes characteristic of )-type semiconductors are attributed to this Ni-. Semiconductivity can ala0 he produced by impurities as well as nonstoichiometry and this provides the possibility of controlling the phenomenon. Examples are: (1) dissolution of LirO in NiO thereby substituting Li+ ions for Ni" ions in the lattice and producing controlled p-type semiconductivity; and (2) dissolving A I 4 8 in ZnO BO that the necessary electrons be furnished for controlled n-type semiconductivity. The general behavior of nonstoichiometric semiconductor oxides is outlined in Table V. The general electronic theory of catalysis by semiconducton has been discussed by Wokenstein (69). The decomposition ofN*O, CO oxidation, and HTDI exchange have been employed to illustrate the chargetransfer theory of contact catalysis on semiconductors as recently d m u d by Lee and Mason (27). Molinari and Parravano (36) observed that the activity of ZnO for H r D s exchange was increased by doping with Gat08 to produce n-type behavior and decreased by doping with LirO to produce p-type behavior. Similar behavior was noted (40)for the activity of NiO in CO oxidation; here doping with Li& again produced less catalytically active p-type behavior, whereas Cr& increased the activity by making more electrons available. On the other hand, the decomposition of nitrous oxide is favored by p-type behavior as shown by the experiments of Dell, Stone, and Tiley (75) and o h m V O L 5 9 NO. 9 S E P T E M B E R 1 9 6 7
51
The oxidation role of semiconductor catalysts is related t o their ability toadsorboxygen
beat known insulator catalysts .are alumina, silicaalumina, silica-magnesia, silica g& phosphates like A l p 0 4 and certain clays that have been activated by appropriate chemical treatment. All these catalysts exhibit acidic Sites on their surfaces. These Sites are often OH p u p which can function M proton donon Br6nsted acids); an0the.r plausible mechanism of acidic behavior in some casts is electron pair aesamilation by a catalyst site (Lewis acid). Thae "wid-site" catalysts strongly chemisorb bases and fnquently contain a residue of structural water. Their eatalytk action is attributed to their acidic properties, and the mechanism are akin to the of general acid-base 'catalysis. Tamele, Schlafkr, and Johnson (56) have h e dcmormtrated that the cracking activity of silica'dumina catalysts depends strongly on surface acidity an &nvn in F i W 5. 'Reactions6vei acidic catalysts 'are discussed at length by Olah (38)and Pined (41). Carbonium-ion intermediates are thought to be involved in isomerization, alkylation, polymerization, and crackiig .dhytlroearbons Ow h l 4 W . W y s t S . HoweVer, u n ~ t y ~ p . t o t h e t r u e ' h a ~ ~ ( l e w i a h t a j d the &vc a M sib, w i i both typca f d i i tin &iny'&~tdysti. Poesibt6 & r ~ M , f ~ + g d d M t e r s iuishavn in Figure 6 : T h e s ~ ~ ~ ~and ~ tIdcntiure #&:t$n (d .& flw & ~ . 4 v. d-' have been d d in detail bg''.&hm (5) ind. 'the genwal chemical propntieb of &&bg catalysts by OBIad; Milliten, and MitL (37).''ln;recCnt studies Sat0 and 'xworked .(d3) f b d , Uaaag &aalumjna and d-a-boria w t a ,:;that &id bitci's&me&k be &ponsibie fOr pelymaieathd &tins, the Cracking of cumene and the di?qnapartlailatfon of tohene to tienZene and ~+4cnewliile mnptonic acid' si* seemed to be active for the dcwinpbi6CMi of that the &dM* isobutane; they also the protonic Sit=. for . ~ ~ t i WM l j inscMitivc n to catalyat composition but the wnversc appeared to be thle f& c r a h ~ gand dispropOttion&%ni I n ' a i t b t b reci)nt paper of interest, Swab andS-ICrhl(50) suggest that, besides the ordinarily neabpary acidie.'(redLing sites, basic points the %tal* surface also play a rok in c r a c k i i and that this implies that k c k i n g .isfavored by "polarity within the surface." "' 'Con&tualrj, tbe.%rmatfon of a carbonium ion is m&t reprcaditcd'ai proton.additid te an OMMC
a
.Mi.'' '.,
~.
I
,.: .
.~
. ~,
. .
"GdHR
.
+ H++ CHIC'HR
For more complex olefins that ion is formal which has the most alkyl groups attached to the carbonium carbon as discussed by Whitmore (68). Subsequent polymerization can be represented as :
CH3C+HR
+ H2C=CHR
--t
R
R
I
I
H3C-C-CHt-C+
I
I
H
H
and subsequent alkylation of R H as:
+ CH3C+HR R + + HzC=CH2 RC+HCH3 + R H
RH
+ CH3CHzR
+ R+
+ RC'HCH3
+ RCH2CH3
+ R+
and isomerization of a carbonium ion as: CH3
CH3
I CH3-C-CH2-CfHz
bond. This is further evidence that the solid acidsite catalysts function in somewhat the same way as conventional acids. I n general, the refractory oxides are most useful catalysts and are easily prepared in a form possessing very high specific surface area. For the latter reason they are commonly employed as supports for other specific catalysts, especially metals. The salts classified under the insulator heading in Table I1 are undoubtedly also acid-site catalysts in many instances. The first acid catalyst was probably anhydrous A1203 and other Friedel-Crafts-type catalysts, like BF3, have also been used for cracking, polymerization, and isomerization as well as for the typical FriedelCrafts reactions like alkylation and acylation. Halide salts are often one of the ingredients in Ziegler- and Alfin-type catalysts for stereospecific polymerization, but here the mechanisms remain more widely disputed.
I -+
CH3-C-C+H-CHa
I
I
CH3
CH3 4 CH3
+ I
CH3-C-CH-CH3
I CH3 Cracking of carbonium ions occurs by scission at a bond in the @-positionto the positive charge: RCHzCHzC+HCH3 + RC+H2
+ CH2=CHCH%
Thus, given the carbonium ion, all of the above reactions can be rationalized. The unanswered questions about these catalysts relate to the exact method of formation of the carbonium ion and how it is bound as a chemisorbed species to the catalyst surface. The exact nature and structure of the active sites on the catalyst surface also remain the object of research. Recently the use of electron spin resonance (ESR) has attracted widespread attention in catalysis research as a means of studying the nature of active sites and adsorbed species on the surfaces of catalysts. We can expect information about chemisorbed carbonium ions eventually to be developed in this way. O'Reilly (39) has reviewed ESR techniques, and Voevodskii (63) has presented and discussed some recent results of ESR studies in catalysis. The acid groups are also no doubt active in dehydration reactions. Thus gaseous t-butanol is dehydrated homogeneously to butene by the presence of HC1 or HBr, or heterogeneously over H2S04 or H3P04in porous solids or over alumina, silica-alumina, or aluminum phosphate. In addition, aqueous solutions of strong acid can catalyze reactions of olefins in which they undergo various additions, isomerizations, or hydrations depending on operating conditions. All of these reactions are supposed to proceed through carbonium ion intermediates formed by protonation of the olefinic
Steric Considerations
The discussion up to now has neither touched on the geometrical properties of reactant molecules nor on the crystal habit of contact catalysts. Considerations such as these are not implied in the classification basis used above but such considerations are very important and may be thought to constitute an additional basis for catalyst classification. The earliest and perhaps the most convincing evidence for the importance of steric factors in catalysis has been obtained for hydrogenation reactions and therefore appears to be confined largely to metal catalysts. This information shows quite clearly that d-band vacancy alone is not in itself sufficient cause always to expect catalytic activity and that geometrical criteria are important in chemisorption and catalysis in addition to electronic considerations. For example, Trapnell (58) has presented evidence that only elements with internuclear distances between 2.48 A. and 2.77 A. will catalyze the dehydrogenation of cyclohexane. This information is summarized in Table VI. One of the earliest comprehensive theories of catalysis-the so-called multiplet theory of Balandin (2, 3)-was based almost exclusively on purely structural and geometrical ideas and data like these. Similarly, today, olefin hydrogenation and double-bond migration are thought to involve as intermediates either a twopoint, a-diadsorbed species or a a-adsorbed olefin. A a-diadsorbed olefin is supposed to be formed by rehybridization of the carbon atoms of the olefinic bond to sb3 hybridization involving two a-bonds between the carbons and two metal atoms: CH3CH=CHz
+ 2*
Ni -+
CHaCHCHz
I / * *
The original evidence for this is based on mechanistic studies with deuterium by Conn and Twigg (12, 61). This work led Twigg and Rideal (62) to point out that these results implied important geometric considerations, viz., that if the short Ni-Ni distance be taken as VOL. 5 9
NO. 9 S E P T E M B E R 1 9 6 7
53
*_-
-,;
--
I
t
' I
.l
-1
F i p n 7. Bonding in an cihylcne T-cornplcx, Dnum (76)
TABLE VI. STRUCTURE AN0 ATOMIC RADII OF SOME M E T I I S
Na
Cc
Li
Th Pb
186 1.52 Ta 1.43 W 1.36 Mo 1.36 V 1.30 & 1.23 a+ 1.24
4 Au
AI Pt" Pdb
I+ Rb"
1.83 1.80 1.75 1.44 1.44 1.43 1.38 1.37
Cd
1.49
TI 1.46 0.'
1.35 1.33 Rub 1.33 &Cd 1.26 Be 1.12 znl
1.35
1.34 1.28 a-Cd 1.26 Nib 1.24 Cub
54
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
2.47 A,, and C-C distance be taken as 1.54 A. (as in ethylene), and the N i x bond be assumed equal to 1.82 A. (as in nickel carbonyl), t h m the chemisorbed carbon-carbon skeleton will fit on the Ni-Ni spacing with only a very slight distortion of the N i - G - C angle from the value of 109' 28' for the tetrahedral bond. Indeed, this holds true for metal surfaces where the interatomic distance is between about 2.4 A. and 2.8 A. as in the case of Ni, Co, Cu, F't, and Pd, all of which catalyze the hydrogenation ofthe olefinic double bond. The r-complexes that have been suggested as an alternative type of 'bonding between olefin and catalyst involve a single atomic center of catalyst and are analogous to the r-complexes known to form between olefins and transition elements (%77). According to Dewar (76), the metal-olelin bonding is brought about by the overlap of the *-electron density of the olefin with an acceptor orbital of the metal atom as well as the simultaneous donation of electrons from filled metal dworbitals into the antibonding orbitals on the carbon atoms. This is shown schematically in Figure 7. Bond and Wells (7) have distinguished another form of radsorbed olefin involving only one metal atom in which the olefin l o w an a-methylenic hydrogen atom to form a r-allyl-adsorbed species similar to the r-allyl complexes of palladium salts (24). The relationship between stereochemical considerations and the mechanism of olefin hydrogenation over heterogeneous catalysts has been reviewed in detail by Siege1 (47) and by Bond and W e b (7). One of the earliest investigations of steric or geometric factors in chemisorption and catalysis is the work of Sherman and Eyring (46). They argued that ease of chemisorption of diatomic molecules is strongly influenced by lattice spacing and reasoned that at v q large catalyst atom separations, a diatomic molecule must be dissociated before complete chemisorption can occur and, conversely, an extremely densely packed lattice will again hinder chemisorption, largely owing to replusion forces. Figure 8 illustrates this situation in terms of a minimum in the plot of activation energy for chemisorption of hydrogen as a function of adsorbent lattice spacing. I t is clear that steric considerations become important, particularly when a catalyst calls into play more than one bond of the same molecule. In such a case, ehemisorp tion may involve not one but several chemical bonds between the adsorbate molecule and the individual surface group (or orbitals) of the solid. I n the m o b cule these interatomic distances may be strictly set, while in the solid they may vary somewhat because of different allotropic structurea, dislocations, and other lattice imperfections. Thus, the lattice structure and perfection of contact
.
I , . . ?
....
''&w#'S. 'Adsation fahvdrogan chnn*aption 0s a funcrion &&n%m spacing of the carbon a&arb&, Shmm Md Erring (46)
~-
ts may in.such c a m take on special importance. F&mps one of the most dramatic aspects of steric and g b o n h c faaorS can be found in the realm of stereoc plynmizarion in which the catalyst directs thc '%&donnection of molecular units all conforming &:r&'llfreci&d orientation thereby forming a linear, reg~ M * ~ ~ t h d i i ostructure. n a I In recent years akdt'..t000 patents on stereospecific . catalyib have a p peared and it'has become possible to "tailor make" a'pulymer to possess specific ehbmfcal, physical, and M i d propertie?. Here, as is usual in catalytic W M n e n a , the technology leids understanding but thtchhms have been suggesad (14,23,26). ..!
:
..
-an.rcnuG.skllvm Another important basis for classification bf solid us catalpti is&& of grolar-physical structure, specific surface,aiea- and port-siZe dis: That these propkies can be controlled to ' h e extent in the mamdaeture of solid catalysts does hcitlesse~~ their importance. For practical utility, it ,*odd be possible .%oprodude the chemital compound ,chk&'as catalyst',in S form m i n g not only the dcsiied spec& d a c e &a But the proper pore sizca
Thio consideration can be particularly important for reactions involving some of the larger organic molecules. Diffusion to the active, internal surface of a catalyst can often be slower than the uninhibited (intrinsic) surface reaction; this happins when the fluid phase is a liquid or when the intrinsic reaction is very fast. In such a case, part of the interior surface remains ineffective and, in addition, mass transport mechanisms can thoroughly becloud reaction rate constants, activation energies, phenomenological reaction order, and selectivity as deduced from the measurements of concentrations in the fluid phase. Discussion of the details of these phenomena is outside the scope of this paper. Thorough elementary treatmcnts have been presented by Wheeler (66,69, and pore structure, diffusion in pores and the interaction of t h i s with adsorption mechanisms, and thermal effects continue to be popular topics for research work reported in the current catalysis and chemicalengineering litexature. The size of the catalyst pellets and the distribution of pore sizes within the pellet then become extremely important parameters to be measured and adjusted. This is necessary not only to ensure that reactants reach the active surface of the catalyst but also in order to compare experimental results obtained with different catalysts and yet be sure one is comparing actual reaction kinetics parameters and not transport effects. T o be certain of this, experimental studies should be carried Out with catalyst pellets of different sizes and only those sizes sufficiently small to show no internal diffusion effects would be considered as yielding unambiguous results. Transport rate from the bulk phase to the pellet exterior is increased by varying external flow conditions until bulk $ransport is sufficiently fast not to influencekinetic results. Polroni
Ps:W&. '
.'Thatthe activity of unit mass of catalyst increases with
Ils specit% surface area eail betaken as a rather general W t h e n t ' b f truth; The actidty of a catalyst depends .4?&$&a ' bFty ty c+misorb m a least,one reactant, and 3b rtsamvity w l l inmease as its surface e a increases. RW& reason, industriahatalysts are usually porous &dV& specific surface a k l y i n g between about 100 sq, m. 'per gram. In heat catalysts almost %% t?& a d a c e drea'is found within the catdyst pellet W C W 4~' frequently a compaction of small particles. @ti611to take plaee, the Mctants must diffuse to the pellet and through its pore structure to the active ktxHW &n. 'Thus, for a-catalyst pellet to be efWW&all, t its pores mwt be at leist larger than the h , ' a f , t h e molecuka of the .react;tnts and products. '
e:&
No discussion of the classification of catalysts and catalytic phenomena would be complete without brief mention of that class of substance called catalytic poisons. These species act by forming abnormally strong adsorptive bonds with the surface of a catalyst and thereby a very small amount of poison can interfere with adsorption between catalyst and reactants. Poisons are usually thought of as foreign to the reacting system; for exampk, when a reaction product desorbs slowly from rhe catalyst surface and thereby interferes with the reaction, it is ordinarily said to be an inhibitor and not called a poison. In spite of the pejorative name, poisons can sometimes act in a beneficial way, particularly in cases when a reaction intermediate is a desired product. Thus, the beneficial role of the poison VOL 5 9
NO. 9
SEPTEMBER 1967
55
-
VII.
TOXICITY OF METALLIC IONS-
-
Li+,Bc" Na+,%+',A+' K+,Ca" Rbf Sr+', Zr* cr+, Ba", La*, cc' Th* I
&+, Zn+' &+' Ag+,
a+$, In+'
Sn+'
Au+, He+'
Hs+ TI+,Pb*, E+' Q+' Q*
4 r 0 1 Tmlc 3 1 Q8QQ0 4 5 0 Tmlc =dQQQ@Q
4dQQQQ69 4 1 QQQQQ
5 5
0
Toxic
Tdc Q 5d$869886 5 0 Toxic 5 d Q@@@@ 6 5 0 Toxic TO& 9 d BDQQQ8 65 Q Nontoxic 3 1 00000 4 5 0 3 d 00000 4 5 0 Nontoxic 5 5
,Mn* FC+'
a+' Ni"
can be to minimize the production of decomposition
products or side reactions and thereby enhance catalyst selectivity. An example is the hydrogenation of benzoyl chloride in the presence of palladium. Here poisoning the palladium with various compounds can enhance formation of an intermediate product, benzaldehyde, whenas under ordinary conditions the hydrogenation proceeds all theway to toluene as principle product. The very strong bond by means of which a poison is held to a catalyst is thought to be of a highly specific and chemical nature, with the formation of such bonds d+dent on definite typed of electronic configuration both in the catalyst and in the poison. In a review of catalytic poisoning phenomena Maxted (29) classified poisons for metal catalysts into three groups:
I. Molecules containing elements of Groups Vb and VIb of the periodic table 11. Compoundsand ions ofcertain metals 111. Certainmolecules containing multiple bonds 36
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
In the 6rst of Maxted's categories can be found certain molecules containing atoms like N, P, As, Sb, 0,S, Se, and Te. Maxted and Morrish (32, 33) have shown that the toxic compounds of these elements possess an unshared pair of valence electrons which presumably are involved in chemisorptive bonding to the catalyst surface. Indeed magnetic susceptibility measurements (77) have shown that electrons from dimethyl sulfide entered the d-band of palladium to form a dative bond upon chemisorption of the sulfide on that metal. Moreover, Maxted and Morrish have also shown (32,33) that the toxic character of thene molecules disappears when the effective atom's normal valency orbitals are completely occupied by stable bonding to other elements. For example, they showed that sulfate, sulfonate, selenate, tellurate, or phosphate does not poison platinum but that the analogous compounds containing an unshared electron pair (phosphite, hypophosphite, selenite, tellurite, sulfide, thiosulfate, and tetrathionate) are intensive poisons. Although Maxted and Morrish worked with Pt catalysts, it may be inferred, in the absence of other data, that platinum is representative of most metal catalysts as regards susceptibility to poisoning. Metals are the most easily poisoned form of catalyst. Maxted's second category of poisons for Pt is made up of metals or metal ions which contain at least five electrons within the d-shell. As shown by the work of Maxted and Ball (m), alkali and alkaline earth ions with a completely empty d-shell, or those with only three (or fewer) electrons in the d-shell are nontoxic. Theae data have been further extended (70) and are presented here in Table VII. It is clear from this table that no toxicity was observed where the potential metallic poison possessed unoccupied d-orbitals or, alternatively, if no d-orbitals were possible as in the case of lighter metah in the ground state. These data, although again pertaining only to a platinum catalyst, suggest that d-electrons are in some way involved in the intermetallic bond between toxic metal and catalyst. The importance of the d-electmns here e.eb this instance apart from the first category mentioned above: the nonmetallic poisons containing elements of Groups Vb and VIb. In the latter case, strong bonds to metallic catalyst apparently involve the s- or p-valency electrons of the toxic element in the poison molecule. The third and last category of poison for metallic catalysts cited by Maxted refera to molecules containing a suitable type of unsaturated bond. Thus some un-
saturated molecules interfere with the heterogeneously catalyzed hydrogenation of others and this appears to depend upon the relative tenacity with which different kinds of unsaturated linkages are adsorbed by the catalyst. Because of this wide variation in strength of the chemisorption bond, if two kinds of unsaturated molecules are present in a hydrogenation system, the observed inhibition may range from mere competitive hydrogenation to almost complete suppression of one reaction by the presence of extremely small amounts of competing unsaturated substance. Examples are the poisoning of platinum and nickel hydrogenation catalysts by carbon monoxide and cyanide ions and the retardation of cyclohexene hydrogenation by small amounts of benzene. In the case of acid-site catalysts for cracking, isomerization, polymerization, and alkylation, basic substances appear to be significant poisons as might be expected. Thus, Mills, Boedeker, and Oblad (34)found that organic nitrogen compounds of basic character were severe poisons for SiOZ-Al203, SiOz-Zr03, and SiOz-MgO catalysts. On silica-alumina the poisoning effect increased in the order aniline, decylamine, piperidine, pyrrole, quinoline, quinaldine, as measured by the catalysis of cumene dealkylation. Danforth (73) has also shown that alkali metal ions can poison cracking catalysts and the poisoning effect seems to be related to the number of acid sites that can be covered by large alkali ions. An extensive discussion of the poisoning of acid-site catalysts has been given by Oblad, Milliken, and Mills (37). Summary
The paragraphs above are an attempt to organize and classify some well known information about heterogeneous catalysts. Evidently it is not yet possible to specify with certainty a particular catalyst which would be expected to accelerate a particular chemical reaction known only to be thermodynamically possible. Nevertheless, there are broad classifications of catalysts and catalyst behavior which can be invoked in an attempt to narrow the search for a useful catalyst-beyond this, however, the frontier of empiricism remains. There seems little doubt that the broad unifying theme of catalysis is electronic, at least from the point of view of understanding mechanism and explaining fundamental action and behavior. However, steric and geometric factors can also be of importance not only with regard to the lattice structure of solid crystallites in a catalyst but with regard to pore structure of catalyst pellets as well. Except for pore structure, steric and geometric factors may also be considered to be electronic in nature. Much remains to be explained and investigated. Questions still persist as to the mechanism of organic chlorination reactions over chloride salts, the catalytic cyclization reactions in general, the catalytic action of bases, why molybdenum catalysts seem to be particularly effective in opening rings and removing heterocyclic sulfur during hydrodesulfurization-to list only a few unanswered questions.
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