MECHANISM O F THE CATALYTIC DEHYDRATION O F METHANOL AND SOME PROPERTIES O F T H E HYDROUS ALUMINIUM OXIDE CATAIiY ST“ BY H. C . HOWARD, J R .
Our present knowledge in regard to the mechanism of catalysis can be accurately classified under three general heads:. ( I ) those cases in which intermediate compound formation has been definitely proven; ( 2 ) those cases in which absence of compound formation has been demonstrated; and (3) a type of catalytic reaction in which the matter of the formation of an intermediate chemical compound is still controversial, or merely a matter of definition, On this question of intermediate compound formation in catalysis, Sabatier states that “the idea of a temporary, unstable, intermediate compound has been the beacon light that has guided all my work on catalysis.” Consequently, he explains such a catalytic reaction as the dehydration of an alcohol over hydrous aluminium oxide, by assuming a mechanism analogous to that assigned to the Williamson reaction. Aluminium is an amphoteric element, so that its compounds will possess both basic and acidic properties, and the analogy of the role played by the alumina catalyst, in the dehydration of an alcohol, to that of the sulfuric acid in the Williamson reaction, is most clearly brought out if the reaction over the alumina catalyst is pictured as taking place by the addition of a mol of the alcohol to the acidic form of the aluminium hydroxide, H,AlOa, or its partial dehydration product, HA102, as shown in the following equations: CH 30H HA102+CH3A102 HOH CH3A102+ CH30H+(CH3) 2 0 +HA102 Such a mechanism lays emphasis upon the acid properties of the oxide, and the following facts indicate that such properties are of importance.
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( I ) Only amphoteric or acidic oxides have been shown to be capable of acting as dehydration catalysts in such a reaction; that is, no exclusively basic oxide has been shown to function in this manner.
The effectiveness of a given oxide as a dehydration catalyst appears (2) in many cases to be related to its “acidity.” The compounds formed by the hydration of such strongly acidic oxides as SO3and P205 are excellent catalysts of dehydration. The hydrous forms of weakly acidic oxides such as A1203 are fairly active, while those of the very weakly acidic oxides such as Fe2O8 exhibit catalytic dehydrating power to only a very limited degree, indeed. ( 3 ) Those methods of preparation which would be expected to yield a catalyst in which the acidic form of the hydrated oxide predominated, are, in general, the methods which furnish the most active dehydration catalysts. *Contribution from the Chemical Laboratory, University of Missouri.
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(4) Charioul has shown that adsorbed lime on an alumina catalyst decreases very markedly the activity of such a catalyst in the dehydration of ethyl alcohol to ether, and Taylor2 has pointed out that Adkins’ data show the adverse effect of ammonia in catalytic dehydration.
Adkins3 questions the formation of a compound or compounds between the alcohol and the catalyst because such an hypothesis necessitates the assumption of two different types of compounds in the case of oxides which are capable of inducing two different types of catalysis. In every case it will be noted that it is an amphoteric oxide which is capable of exercising this dual function. Hence it appears possible to the writer that in the case of oxides of the amphoteric elements we do have txyo or more foyms present, the relative amount, of each form being determined by the method of preparation. It is possible to make a partial test, at least, of Sabatier’s hypothesis, by an application of the Phase Rule, and in this investigation such an application has been made. If, in a system consisting of methyl alcohol and hydrous aluminium oxide, the alcohol forms a compound with the oxide and the properties of this compound are such as to render it a separate phase, me will be dealing with a three-phase, two-component system, a system having one degree of freedom. If, on the other hand, no compound is formed, or a compound which dissolves in the alumina to form a single phase, then we will have a two-phase, twocomponent system and two degrees of freedom. The methods used in distinguishing between these two types of system are discussed in the following paragraphs,
T y p e A . Assume a system such as CnH,,A10~+HA102 (in which the compound C,H,,A102 constitutes a separate phase), in s n atmosphere of an inert gas, such as n i t r ~ g e nand , ~ assume the total pressure on the system to be 760 mm. In such a system, at any definite temperature, there will be a certain equilibrium concentration of CnH,,A10~+HAl02 and C,H,,. If the temperature be raised, more C,H,,A102 will dissociate, with the formation of HA102 and C,H,,, and the pressure in the system will tend to increase, but up to a certain temperature the total pressure in the system can be held at 760 mm. (the initially chosen value) by allowing the C,H,, and the nitrogen to expand, and more C,H,,A102 to d-issociate, since by this means the partial pressure of C,H,, in the gas phase can be increased, Ultimately, however, if the temperature is increased sufficiently, the dissociation pressure of the compound C,H,,A102 will become greater than the possible attainable partial pressure of C,H,, in the gas phase, (in this case 760 mm. will be the limiting Compt. rend. 180. 213 (1925).
* J. Phys. Chem. 30, 167 (1926). J. Phys. Chem., 30, 167 (1926). The use of nitrogen introduces another component and another degree of freedom in each ease, but the method of distinguishing between the two types of system is still valid. The second system still has one more degrre of freedom than the first.
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value) and dissociation will then continue as long as any of the C,H,,AlOa phase remains. In this type of system the vapor pressure is not a function of the concentration. Type B. Assume a system similar in all respects to that described under Type A, except that the alcohol either does not react to form a compound, or if it does form a compound, the latter does not exist as separate phase. In this case it will be possible to maintain an equilibrium at any chosen pressure and temperature, merely by allowing the products of the dissociation
FIG.I
to expand, by increasing the volume of the system. In other words, with this type of system there is no temperature a t which dissociation is continuous. In this case the dissociation pressure is a function of the concentration. Experimental The methyl alcohol used in this work was supplied by a large manufacturer of dimethyl aniline. It gave negative tests for aldehydes and ketones and boiled 64.4oo-64.55OC. at 747 mm. (Bureau of Standards certified thermometer). It was dried in the usual manner over calcium oxide. The alumina catalysts were prepared according to the directions of Sabatier, by precipitating a solution of C. P. aluminium nitrate with a slight excess of ammonium hydroxide, washing thoroughly and drying at 3ooOC. The dried material was ground and screened and the particles passing a U. S. Standard screen
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No. 8, and retained on a KO. 16, were employed in this work. This catalytic material contained about I ;C;; water, determined by ignition a t IOOOOC. The nitrogen was prepared in the usual manner from sodium nitrite and ammonium chloride. Carbon dioxide and oxides of nitrogen were removed by sodium hydroxide, and oxygen by hot copper. It was washed with water and finally dried over phosphorous pentoxide. The apparatus used in this study is shown in Fig. I . It consisted essentially of a bulb “A”, communicating through a capillary “C” with a weighing burette “B”, The burette communicated with a small mannometer “D” and the other leg of this manometer opened into a compensating bulb “E”. The latter could be connected to vacuum or pressure, or to the large constant volume manometer “G”, by means of the stop-cock “F”. The vertical capillary tube “H” communicated, through the three-way cock “I”, either to vacuum or pressure (through the mercury trap “J”) or the supply of nitrogen and methyl alcohol vzpor. The baths “K” and “L” were electrically heated and fitted with automatic temperature controls. “K” was a bath of potassium acid phosphate and phosphoric acid and could be maintained at any temperature desired ketneen 180’ and 400OC. “L” was a glycerine bath and was kept a t IOOOC.Only about I O em. of the communicating capillary “C” was not immersed in either bath and this part was lagged with asbestos string.
Method of Manipulation The bulb “A” was filled with a sample of the catalyst and sealed to the communicating caFillary “C” a t some point such as “M”.Bath “K” was then brought into position and maintained a t I X O O C . Bath “L” was brought and the whole system evacuated through cocks “F” and “I” until to IOOOC. the pressure fell to a few mm. The mixture of methyl alcohol vapor and the nitrogen (the nitrogen was saturated n-ith methyl alcohol at 2 j°C) was then introduced through “I” until the Fressure in the system was about I O cm. less than atmospheric, air being introduced simultaneously through “F,” to maintain the pressure on both sides of the mannometer “D”, approximately the same. Cock “I” w.as now cautiously turned to communicate with the mercury trap “ J ” , which \vas open to atmospheric pressure, and mercury allowed to flow up into the capillary “H”, until the point “X” was reached, Stopcock “I” was then shut off, the column of mercury in “H” effectively sealing it. The pressure in the system was now adjusted1 (by means of the burette “B”, the weighing bottle “P” and leveling bulb “Q”) to some definite value, by the indicating point shown a t “0”. Several weighings of‘ the mercury and weighing bottle were made and the average recorded. The temperature was increased by increments of approximately 2o°C. Time was given for the attainment of temperature equilibrium at the end of each increment, and several w-eighings of the weighing For a detailed description cf the method of operation of this type of burette, see J. Phys. Chem. 28, 1c82-1c95 (1924).
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bottle were made at definite time intervals. Data, typical of many measurements are presented in Fig. 2 in the form of a curve. Temperatures of the catalyst bulb are plotted as ordinates and weights of the weighing bottle as abscissa. The weights of the weighing bottle, or rather the changes in weight of the weighing bottle, are evidently inversely proportional to the changes in the amount of vapor in the bulb containing the catalyst and, because of the high density of mercury, very precise measurements of volume changes in the catalyst bulb could readily be made. The time interval between readings was approximately fifteen minutes.’
FIG.2
At first sight it appears that this curve meets the requirements set forth under Type “A” very exactly, that is, there is a certain temperature range over which the arbitrarily chopen pressure can be maintained, merely by allowing the gas phase to expand into the burette “B”, and ultimately a temrerature is reached at which dissociation appears to be continuous A more careful analysis indicates, however, that this is a faulty conclusion. The actual change in volume over the 1 5 minute period at the temperature of 283’C. is approximately 0.480 cc. (the volume of 60 gm. of mercury at IOO C.). It is evident that this much vapor could evaporate from the catalyst without changing appreciably the concentration of the methyl alcohol in the alumina. That is, a flat portion to such a curve, corresponding to such a relatively An interesting feature of this curve is the evidence of “drift,” or slow penetration of the gas phase into the catalyst, a t t h e lower temperature. This “drift” is of about the same magnitude whether the gas phase 17 pure nitrogen or a mixture of nitrogen and methyl alcohol vapor. Thus it is evidentiy not FpeciEc in nature.
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small evolution of vapor, is not conclusive evidence that we have to deal with the dissociation of a separate phase of constant composition. That this latter is the correct conclusion mas indicated by later experiments in which the temperature at which dissociation appeared to be continuous was maintained constant over a period of several hours, when a very gradual, but distinct decrease in the rate of vapor evolution was observed. In order to fix this point still more definitely, however, the following experiment was carried out. The catalyst and the communicating system mere evacuated at 250°C until the pressure fell to a few mm. The mixture of methanol and
I
VOLUME O F VRPOR REMOEO- CCS. at /OO”C.
FIG.3
nitrogen was then admitted until a pressure of about 5 j o mm. was reached, The pressure remained approximately constant at this value for about thirty minutes, Now, with the temperature constant, vapor was removed from the catalyst bulb by allowing mercury to flow out of the burette “B”. The ef‘fect of this withdrawal of vapor, upon the pressure in the system, is shown in Fig. 3. It is evident from this experiment that the pressure in the system falls steadily, if any appreciable quantity of vapor is withdrawn, that is, the vapor pressure of the system is a function of the concentration of the methyl alcohol vapor. This system then evidently falls in the class of those described under “B” and hence we can state definitely that no compound, constituting a separate phase, is formed. Compound formation with subsequent solution in the catalyst (as in the case of the Williamson Reaction) is, of course, not excluded.
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In order to learn the behavior of an alumina catalyst in the presence of an inert gas alone, experiments were carried out similar in every way to those already described, except that the catalyst bulb was filled with catalyst and dry nitrogen only. To our surprise, in all cases exactly the same type of curve was obtained as that shown in Fig. 2 . At first this was thought t o be due to traces of CH,OH or water remaining in the catalyst from previous experiments, so that a fresh sample of catalytic material was prepared, placed in a new bulb and sealed into position. Again, however, exactly the same type of curve was obtained. It was now observed that if several successive measurements, using dry nitrogen, and with intervening evacuation, were made upon
FIG.4
the same sample of catalyst, that the temperature required to bring about rapid dissociation (indicated by the flat portion of the curve) became higher and higher. In many cases samples after having had vapor pressure measurements made upon them, were tested, qualitatively, for catalytic activity, and it was always observed that the catalysts showing a high "break" temperature were those requiring a high temperature to initiate the catalytic dehydration of methyl alcohol. The catalytic dehydration of methyl alcohol, like many other catalytic reactions is markedly affected by rather small temperature changes. Qualitative determinations with our catalytic material indicated that below 200' the reaction is very slow, while at 250' to 275' it becomes very rapid. It 'was thought probable that this was due to a considerable variation, with temperature, of the vapor pressure of this aluminium oxide-water complex. To test this point the following experiment was carried out. The sample was evacuated a t 197°C. until the pressure in the system had fallen to 9 mm. Water
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vapor was now admitted until the pressure reached 98 mm. The supply of water vapor was shut off and the temperature raised to 2 j j o C. In 2 j minutes the pressure had risen to 420 mm., and a t the end of 40 minutes the pressure was 568 mm., where it remained approximately constant for the next thirty minutes. The extraordinarily steep slope of the vapor preesure curve of this hydrous aluminium oxide catalyst is evident when comparison is made, as in Fig. 4, with the corresponding curve for water over the same temperature range. These data show the enormous effect of temperature on the vapor pressure of an alumina catalyst, and indicate that the temperature of catalysis is determined by the vapor pressure of this complex; when a temperature is reached such that the vapor-pressure of the complex is sufficiently great so that water can evaporate from the catalyst as fast as it is formed in the reaction, we have catalysis. These experiments indicate that the ca talytic dehydration of an alcohol over alumina takes place in some way through a water-aluminium oxide complex. Khether this be A1(OH)3.or HA102, a solid solution of water in & 0 3 , or adsorbed water, it is impossible to say from the present data. Khatever it may be, it can be stated definitely that this complex does not constitute a separate phase. I t also appears probable that the sensitiveness t o heat treatment shown by alumina catalysts, is not due, primarily, to sintering and consequent decrease of surface (change in degree of unsaturation or change in spacing) but to destruction of this complex.
It is proposed to study further the properties of alumina catalysts and to
. determine, if possible, the nature of this Al2O3H20complex, which appears to be so essential for dehydration catalysis. In some cases alumina catalysts prepared from aluminates have been reported to be superior t o those prepared in any other way. This indicates that there may be a real difference in structure between Al(OH)3 prepared by precipitation by an alkali from an aluminium salt, and that produced by the acidification, or hydrolysis, of an aluminate. It seems certain a t least that very different impurities would be present in the catalyst depending upon the method of preparation used.
Summary ( I ) By an application of the Phase Rule to a system containing hydrous aluminium oxide and methyl alcohol, it has been shown that over the temperature range a t which the oxide functions as a catalyst, no compound between the alcohol and the oxide, constituting a separate phase, is formed. The possibility of formation of a compound, with subsequent solution in the catalyst is not excluded. Hence a mechanism such as that suggested by Sabatier, analogous to the Williamson reaction, is possible. ( 2 ) A close relation has been found between the temperature at which the evolution of water vapor from a sample of hydrous aluminium oxide be-
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H. C . HOWARD, JR.
comes rapid, and the temperature at which catalytic dehydration of alcohol takes place over such an oxide. This relation shows the intimate connection between the presence of water in the oxide before catalysis and the process of catalysis. (3) The magnitude of the variation of the vapor pressure of a hydrous aluminium oxide catalyst with temperature, has been determined, and the relation between this quantity, and the effect of temperature on the catalytic dehydration of an alcohol over such a catalyet has been pointed out. Columbia, Missouri Mamh 24, 1926.
