Mechanism of Hydrocarbon Cracking in the Presence of Aluminum Silicate as Catalyst SIR: Sumerous investigators, pubAND lishing their articles in INDUSTRIAL ENGINEERING CHEMISTRY, have stated that aluminum silicate is one of the best cracking catalysts and suggested that this is due to the active acid centers present in the catalyst (7-3, 9). T o explain this fact, the structure of A1203, whose catalytic activity, generally speaking, is connected with its y form as active transitory phase with structural defects, should be examined first. T h e active r-A12O3 may be represented most simply by its radical structure ( 4 ) in the form of the model formula:
O=AL-0-AL=O T h e radical structure of heterogeneous catalysts, first presented by me in 1936 ( 5 , 8 ) , has found its adherents (70) and has been used to explain the mechanism of many catalytic reactions (7). Returning to aluminum oxide (which, as a matter of fact, is a macromolecular compound), and considering, which version of the (A1203)% structure should be chosen as appropriate, only two tvpes of radicals can be deduced-namely, the electron donor R=A1-, and the electron acceptor R=A1-0-. T h e last has the function of a potential anion and, according to Lewis, it is an “acid.” The radical R=Almay become a cation; according to Lewis, it is a “base.” M y radical hypothesis of active metal oxides is in principle in agreement with modern theory of semiconductors (70). If the cracking catalyst has to be active, the “acid” radical R=Al-0should be present for attacking the hydrocarbon, but the radical R=Al--, when possible, should be blocked or, a t least, weakened. Therefore Si02 is introduced into the catalyst. T h e advantage is that silicon can form compounds with hydrogen (hydrides), the radical aluminum silicate getting a positive charge. Although the so-called carbonium cation (7-3, 9 ) is not necessarily formed, the hydrocarbon radical left after giving up one atom
1358
of hydrogen (passing into the H- anion) undergoes a strong polarization, which facilitates its tautomeric transformation. The H- anion can transfer its one electron [i] to the radical O=A-0and turn into a hydrogen atom: which a t last can find its place in the transformed hydrocarbon. The mechanism of this reaction, taking place on the surface of the catalyst, can be represented by the following equations : 0
+ Sios
O=X-
--t
0
O=.A1-0-si~ -CLH1,
H-
+
o=/i-O-siH
\
+ c~H,~ +
\
Pentane (0=.41-O-Si=O)+
+ -O-A1=0 +
-C~HI~ H 0=.\1-0[~] 0=241-0-
(1)
-
+
H- (2) + 0=.41-0 [i] €3 (3) CjH12 (isopentane or propene ethane) (4)
+
+ + (O=Al-O--Si=O)* +0>Si-0-.41=0,
+
etc.
(reaction chain)
(5)
So in this system a migration of electrons (6) takes place. Interesting is the observation that the aluminum silicate catalyst is poisoned by alkalies ( 7-3, 9). This can be also explained by means of the radical structure of this compound:
o=.u-oXaOH
0
+
)\si-O-Al=o
+
+
O=Al-0-Na
O~Si-O--Al=O HO/
+ (6)
X a O H blocks both radicals or active centers of the catalyst, which can be unblocked under the influence of acids and by the calcination of the hydroxide formed : O=Al-ONa
INDUSTRIAL AND ENGINEERING CHEMISTRY
+
Ob-O-Al=O HO/
HC1+ O=Al-OH
+
+
0 ~Si-O-.Al=O HO/
O=Al-OH
+ NaCl
+ Ho,Si-O-.41=0 \ o
o==A~-o-
(7) +
+ o ~ ~ i - o - ~ l =+o
’
H,O
(8)
I n the system described above different reactions may occur, as well as byreactions, depending on which of the hydrocarbon atoms approaches with its binding sphere the active centers (radicals) of the catalyst, whose composition and steric properties are important. The velocity of single stages of the catalytic reaction should also be taken into account, as it can complicate the catalytic reaction. If, for example, one molecule of hydrocarbon loses its end hydrogen atoms, cyclization may occur. But if two molecules of hydrocarbon lose one end hydrogen atom each, polymerization may occur. The remaining hydrogen atoms could eventually cause socalled hydrogenating cracking. literature Cited (1) Good, G. M., Voge, H. H., Greensfelder, B. S., IND.Esc. CHEM.39, 1032
(1947).
(2) Greensfelder, B. S., Voge, G. M., Ibid., 41, 2573 (1949). (3) Hansford, R. C., Ibzd., 39, 849 (1947). (4) Krause, A., Bull. soc. amzs sci. lettres de Poznan, Ser. B, XII, 67 (1953); Il’aturc (London) 183, No. 4675, 1615 (1959). (5) Krause, A , , in J. Alexander “Colloid
Chemistrv.” Vol. VII. P. 175, Van Nostrand; h e w York, 1950: (6) Krause, A., Poznanskie Towarzystwo Przyjaci61 ,\‘auk, Prace Komisji mat.przyrodniczej, Ser. A . V . 3, 163 (1948). (7) Krause, A , , Przemysl. chem. 28, 267 (1949). (8) Krause, A , , Krach, H., Ber. deut. chem. Ges. 69, 2709 (1936). (9) Thomas, C. L., IND.ENG. CHEM.41, 2564 (1949). (10) Wolkenstein, T., J . chin. phys. 1957, 181 ; Chem. Technik 11, 8, 103 (1959). ALFONS KRAUSE Institute for Inorganic Chemistry
of the University
Poznafi, Poland
