NATUREOF SILICA-ALUMINA CATALYSTS
Oct., 1957
1341
THE NATURE OF SILICA-ALUMINA CATALYSTS BYTHOMAS J. GRAY The Stale University of New York College of Ceramics at Alfred University, Alfred, New Yorlc Received February 86, 1067
Differential thermal anal sis and X-ray studies have been employed in a detailed investigation of extremely high purity silica-alumina specimens o f varying compositions to elucidate the nature of their catal tic activity. The disappearance of a unique alumina phase simultaneously with the disappearance of catalytic activity l a s led to the suggestion that the catalytically active species is a half-inverted spinel with protons in certain octahedral sites. The same spinel is responsible for the formation of mullite at very much reduced temperatures and might be regarded as an incipient mullite phase.
Introduction Many investigations have been made in endeavoring to elucidate the curious enhancement in catalytic activity occurring within a certain range of SiO2-Al208 compositions. A variety of explanations have been advanced to cover this important aspect, particularly as related to phase changes which occur in the material. Recently Barrett’ has indicated that certain phase changes can be characterized during the thermal deactivation of mixed compositions while other authors2 have stressed the accompanying collapse of the porous structure. In general, the activity of this type of catalyst has conventionally been related to Lewis acid sites3 although suggestions have been made that Bronsted acid sites may revert to a form of active site on h y d r a t i ~ n . ~ The more fundamental concept of an impurity defect of the same type as that responsible for the development of color centers in quartz has received but little attention.s It is probable that the intrinsic active sites can be explained readily on this basis although this does not infer any profound difference from the formal chemical concept of acid sites, but establishes a more accurate and valuable description of these sites. To this must be added consideration of the structure of alumina phases which are of spinel type with a strong probability of hydrated protons incorporated in octahedral coordination sites. The ambiguity which exists regarding the nature of the active phases made it desirable to establish more exact characteristics. This has been achieved by a combination of differential thermal analysis and X-ray techniques on catalyst materials of extremely high purity. By this means it has been possible to identify a single specific alumina phase which disappears simultaneously with the disappearance of most catalytic activity. Experimental Differential thermal analyses of a series of synthetic silicaalumina catalysts with residual metallic impurities below five parts per million were performed after the refined technique of West.6 All specimens showed less than ten parts per million inorgauic acid anions and were vacuum dehydrated at 450’. After initial investigation, detailed ex(1) W. T. Barrett, M. G. Sanchea and J. G. Smith, Advances in Culalgsis, Philadelphia Conferenoe, in press. (2) H. E. Reis, ibid., 1, 87 (1952). (3) A. G. Oblad, T. M. Milliken and G. A. Mills, ibid., 8, 221 (1951). (4) C. K. Thomas, I n d . Eng. Chen., 41, 2564 (1949). ( 5 ) T. J. Gray, “Defect Solid State,” Interscience Publishers, New York, N. Y.,1957. ,, (6) R. R. West, Defect Solid State,” Ed. Gray, Interscience Pub-
lishers, New York, N. Y.. 1857.
amination of D.T.A. peaks was combined with X-ray analysis of samples quenched from specific temperatures below, a t and above these peaks. The results of the differential thermal analysis investigation of the various specimens are shown in Fig. I, progressing from pure silica to pure alumina. The most significant aspect of these results is the appearance of the double peak in the region 970-1050’. The peak normally occurring at 980” is characteristic of the recrystallization of 7-alumina from quasi-amorphous alumina. It is noticeable that the presence of even a very small proportion of silica in alumina enhances this peak enormously. The upper peak of the doublet observed in the range 5-30% alumina has been identified with the appearance of mullite, the crystallization of which is not normally complete until about 1200”. Above about 30% alumina only a single peak is visible although the effect may persist for a somewhat greater range of compositions. X-Ray investigation shows that the starting materials are all quasi-amorphous in that no definite X-ray structure can be derived. On heating, r-a!umina can be identified a t relatively low temperatures for S i O ~ A l @mixtures ~ but not in the case of very pure alumina when it is observed only after heating to the 980’ recrystallization transformation. I n the case of mixtures containing between 3 and 30% alumina a further alumina phase (Alumina “X”) appears between 6-950” and has now been tentatively identified as the half-inverted aqua-aluminate spinel. The X-ray intensities observed are given in Table I.
TABLE I
d
Relative intensities Alumina “X“
Relative intensities “&Ah mina” (after Stumpf. et al.7)
4.57 2.85 2.74 2.58 1.91 1.80
35 80 100 25 35 15
medium medium strong medium strong weak medium weak weak
There is good agreement with so-called “8-alumina” but it is suggested that this compound is probably the proton analog of lithium aluminate, LiA1608,which has been identified by Verwey’ and by Brauns as having a partially inverse cubic spinel structure with long range ordering in the octahedral coordination sites. Lithium aluminate, LiA1608, exhibits a cubic spinel structure with some super-structure lines which persist to temperatures of 850-900”. It is a partially inverse spinel with long range ordering on octahedral sites with space groups P433 or P413. The analogous ferrite, LiFes08, has very marked similarity of structure and similar super-structure lines. These characteristics are also found in Y-FezOa. On extraction with water, lithium ferrite gives “aqua-ferrite’’ which is considered to be the v t o n analog H.Fe608. It is important to observe that erweya states that y-FezOa cannot be freed from water without conversion to a-FezOa. Welch10 also has found that (7) H. C. Stumpf, e t a l . Ind. Eng. Chem., 62, 1398 (1950). (8) E. J. W. Verwey and E. L. Heilmann, J . Chem. Phys., lS, 174 (1947). (9) P. Braun, Nature, 170, 1123 (1952). (10) A. J. E. Welch and I. David, Trans. Taradoy SOC.,68, 1642 (1856).
i342
THOMAS J. GRAY
Vol. 61
which may be tentatively designated aqua-aluminate. Alternatively or additionally this might be regarded as an incipient mullite phase. The disappearance of this phase on heating exactly coincides with the disappearance of the major portion of catalytic activity even in specimens in which there still remains significant amounts of y-alumina. It is probable that the active sites are directly related to the peculiarity of the spinel structure where some alumina is bonded in a partially covalent manner with either a free electron in the vicinity of a hydrogen or other cation relatively loosely attached to the site with a bond energy considerably reduced below normal proton bonding. On the basis of Ols exchange many authors” have considered Lewis acid sites comprise a type of
Weight ’%ratio A129/ SlOZ 0 Pure Si02
0.0983 0.3452 0.6662 1.0152 2.002 2.927 10.041
I l
edge site -A1-OH.
However, much better evi-
dence is available in the work of O’Brien12 and others on the nature of color centers in quartz which indicates very clearly that the nature of these sites is more probably
co
Pure A1208
800 1000 1200 1400 Temperature, ’ C. Fig. 1.-Differential thermal analysis of silica-alumina mixtures. 600
7-Fe2Oa cannot be prepared from carefully dehydrated magnetite although it can be prepared if that material is not completely dehydrated. From X-ray studies i t has been suggested (ref. 8) that y-FeiOa and 7-A120sare cation deficient spinels with some 2.67 cation sites vacant. Since the presence of an hydrated proton would not be observed in an X-ray study, this leaves open an alternative suggestion that these 2.67 sites are occupied by protons. Although there is a discrepancy between this value and the four sites called for in the formuIa H.A1608or H.Fe608the agreement is as good as could be expected from the experimental data. There must still exist some conjecture as to the exact nature of the species Alumina “X,” although its separate identity becomes abundantly clear through the disappearance of all traces of this species with the formation of mullite and the simultaneous destruction of catalytic properties. It can be anticipated definitely that its structure is related to the spinel structure common to both r-AlgOs and mullite but without neutron diffraction studies its exact constitution cannot, as yet, be determined. By anaIogy with lithium aluminate the proton aluminate compound may well be the phase frequently designated ‘‘&alumina.” However, from the catalytic point of view great importance attaches to the proton and the designation “aqua aluminate” is a more accurate description. Since this species is found in the range 3-30% A 1 2 0 3 but has not so far been observed in this series of experiments at higher concentrations of alumina, it could be inferred that the particular structure requires the stabilization due to the presence of silica. Although its structure is entirely distinct from that of mullite, possible alternate explanation might be found in a type of incipient mullite phase particularly in interface regiona. Catalytic activity was compared on a qualitative basis by the cracking of methylnaphthalene under standard conditions and was made on an equivalent surface area basis. The catalysts before heating possessed a surface area of about 450 m.2/g. by Krypton B.E.T. measurement and decreased rapidly on heating. The most important observation was the disappearance of catalytic activity on heating corresponds exactly with the disappearance of Alumina
,h----
I
0
’
1 1 1 ’ 1
-0-Si-9-Al
I I
0
$,
I
H+ 0
\I
le
s.O
‘,
I I
t; O-Si-0-
I---
rl ,’
0
I
whit: “X.
It may be argued that this formulation is superficially not very different from the earlier chemical representation; however, the energy state implications are profoundly significant and readily afford a field for development and a better basis for the explanation of more complicated features. Furthermore, the existence of such sites is fully substantiated by physical measurements. That these sites exist even in the purest silica and even after firing to high temperature as evidenced by infrared adsorption and nuclear magnetic resonance, emphasizes the necessity for their consideration, particularly in relation to the special phase alumna “X,” which disappears with decrease in catalytic activity and the appearance of mullite. The particular significance of the spinel structure already has been emphasized13 but the detailed appreciation of surface bond contributions is difficult t o formulate other than as a qualitative model. If the NBel or Zener theories of ferrimagnetism are accepted, then in the surface of a spinel not only are there sites of partial covalent bonding propensity but these sites are particularly related by electron spin designation and interaction. This interaction will differ from normal to inverse spinels and where long range ordering in octahedral sites occurs, as in the present instance. This correlation between spinel structure and catalytic reactions has been challenged by certain critics on the basis of the relatively small variation of catalytic activity on
Discussion It is apparent from this brief survey that the special catalytic activity of silica-alumina is directly related to the existence of a spinel phase
(11) J. D. Danforth, “International Congress on Catalysis,” Aoademic Press, New York, N. Y., 1956. (12) M. C. O’Brien, Proc. R o y . Xoc. (London), 231, 404 (1955). (13) T. J. Gray, Seminars (1955), Solid State Symposium. Alfred, 1956.
Oct., 1957
’
NATURE OF SILICA-ALUMINA CATALYSTS
1343
passing through a magnetic transformation. How- indicate a contribution from long range ordering in ever, this criticism is not valid since it is the speci- the octahedral sites and on this basis incipient forficity and product distribution which would be ex- mation of mullite from the aqua-aluminate phase pected to vary and not necessarily the activity. would appear logical. It has been suggested that completely dehyThese features re-emphasize the necessity for a drated “y-alumina” does not exist but that the new and more detailed study of the important cornormal variety contains hydrated protons in co- relation between the structure of surfaces, the ordination sites. This contention is supported by nature of the chemical bond within the solid in the nuclear magnetic resonance evidence for the exist- vicinity of the surface and the surface reactions ence of protons in LLy-alumina”at all times. It which these conditions support and assist. may well be that the significant differencebetween Investigations employing in combination the this form of alumina and the form which, by this techniques of nuclear and paramagnetic resonance, and other studies, has been related t o the high cat- measurements of semi-conducting and magnetic alytic activity, is the difference between a normal properties should, by correlation with chemical in (“ y-alumina”) and a partially inverse spinel vestigation of simple catalytic processes and neu(“aqua-aluminate”) with additional ordering in the tron diffraction determination of structure, lead octahedral sites. t o a more satisfactory appreciation of the chemicalConclusion physics of surfaces. Although the structure of mullite is not acAcknowledgment.-The assistance of Professor curately determined, it is known to be a ~pine1.I~R. R. West in performing and interpreting the The inherent stability of mullite might be taken t o differential thermal analyses and X-ray investigation is gratefully acknowledged. (14) E. C. Shears and W. A. Arahibald, Iron and Steel, 63 (1954).
