INVESTIGATION OF THE ACTIVITY OF SULFIDE ZINC–COPPER METHANOL CATALYSTS

N. P. Kurin and I. V. Prorvin

Communication II

In the first communication¹, results were presented from an investigation of the activity of methanol catalysts whose initial form, in addition to ZnS and CuS, contained from 0 to 16% CuS*).

The investigation was carried out at temperatures from 250–400°C and at volume velocities of pure methanol vapor from 420 to 2100. In that work it was established that a catalyst obtained from pure CuS is completely inactive; however, catalysts made from pure ZnS, as well as its mixtures with CuS, proved to be considerably more active than zinc-oxide catalysts. For example, the temperature of 100% decomposition of methanol at a volume velocity of 420–600 was only 325–350°C. It was established that the activity of the mixed catalyst does not obey the rule of additivity and, with an increase in the content of copper sulfide in it, changes along a complex curve having, in the interval studied, 2 minima (0.5 and 8% CuS) and 1 maximum (4% CuS). The decomposition of methanol in the presence of these catalysts proceeds according to an apparent zero order.

Taking into account the nonobservance of the additivity rule between the content of inactive CuS in the catalyst and the catalytic properties of the latter, it was of interest to study the activity of the catalytic system obtained by reduction from ZnS—CuS at higher CuS contents (above 16%). It was also of interest to know whether the kinetic equation for the decomposition of methanol, previously found by one of us (N. P. Kurin), would prove valid in the presence of catalysts containing a larger amount of copper sulfide.

To eliminate the influence of the method of preparation on the activity of mixed catalysts, the latter were prepared by the procedure described in the 1st communication, i.e., by mixing ZnS and CuS precipitated with hydrogen sulfide from 0.5-normal aqueous solutions of zinc and copper nitrates. The fraction charged into the contact apparatus was, as before, that with grain sizes of 1.5–2 mm. The volume of catalyst was 5 ml.

The investigation of the catalysts was carried out on the previous apparatus (communication 1, Fig. 1), with the only difference that the gaseous products of methanol decomposition, after the refrigerator–condenser (12), bypassing the scrubbers (14) and (15), filled respectively with methanol and solid calcium chloride, were sent directly either to an aspirator (16), or were led to gas receivers (17) and a gas analyzer. This change was due to the desire to determine, with the aid of a modified Orsat apparatus, the total amount of products of side and intermediate reactions (for example, CH₂O, (CH₃)₂O, HCOOCH₃, etc.) in each experiment.

*) For the principal references to the literature, see communication 1.

The study of the catalyst consisted of two periods: the reduction period and the activity-testing period. Reduction began at a temperature of 250°C, and then the temperature was raised by 25°C increments until, at a space velocity of 600, 100% decomposition of methanol vapor was achieved. The duration of catalyst reduction at a given temperature was determined by the time after which the rate of gas formation, measured with an aspirator (16) (Communication I, Fig. 1), became constant; in addition, the liquid methanol collecting in the measuring burette of the cooler-condenser (12), when diluted with water, ceased to give a white precipitate (mercaptan) and acquired a normal odor.

During the catalyst-reduction period, upon transition from a lower temperature to a higher one, renewed evolution of mercaptans was observed; as the catalyst was treated with methanol vapor, this gradually decreased and then ceased to be noticeable. In parallel with the decrease in mercaptan evolution, the rate of methanol decomposition to gaseous noncondensing products (CO, H₂, etc.) increased until a steady state was established under the given conditions. Thus, the initial sulfide form of the catalyst is destroyed during the reduction process with removal of sulfur. In this period, in particular, the following reactions may occur:

\[ \begin{aligned} 1.\;& \mathrm{ZnS}\;(\mathrm{CuS}) + \mathrm{CH_3OH} \rightleftarrows \mathrm{CH_3SH} + \mathrm{ZnO}\;(\mathrm{CuO}).\\ 2.\;& \mathrm{ZnO}\;(\mathrm{CuO}) + \mathrm{CH_3OH} \rightleftarrows \mathrm{Zn}\;(\mathrm{Cu}) + 2\mathrm{H_2} + \mathrm{CO_2}.\\ 3.\;& \mathrm{CH_3OH} \rightleftarrows \mathrm{CH_2O} + \mathrm{H_2} \\ 4.\;& \mathrm{CH_2O} \rightleftarrows \mathrm{CO} + \mathrm{H_2} \end{aligned} \left\} \begin{array}{l} \text{on the catalyst surface}\\ \text{in the adsorption layer.} \end{array} \]

\[ \begin{aligned} 5.\;& \mathrm{ZnS}\;(\mathrm{CuS}) + \mathrm{H_2} \rightleftarrows \mathrm{Zn}\;(\mathrm{Cu}) + \mathrm{H_2S}.\\ 6.\;& \mathrm{ZnS}\;(\mathrm{CuS}) + \mathrm{CO} \rightleftarrows \mathrm{Zn}\;(\mathrm{Cu}) + \mathrm{COS}.\\ 7.\;& \mathrm{CH_3OH} + \mathrm{H_2S} \rightleftarrows \mathrm{CH_3SH} + \mathrm{H_2O}. \end{aligned} \]

Thus, during catalyst reduction, sulfur may be removed in the form of various compounds (mercaptans, hydrogen sulfide, carbon oxysulfide). The amount of sulfur removed depends on the temperature and time of catalyst reduction. The repeated evolution of mercaptans upon transition, during reduction, to higher temperatures quite reliably indicates differences in the strength of the bonds by which the catalyst retains sulfur. Namely, at low temperatures sulfur held by relatively weak affinity forces is removed; at elevated temperatures more firmly bound sulfur is removed. The differing strength of sulfur bonding with the catalyst can be explained by recognizing the energetic heterogeneity of the surface of sulfide zinc-copper catalysts. As a rule, for the group of catalysts studied, the reduction time increases with increasing concentration of copper sulfide. Indeed, whereas for catalysts containing from 0 to 16% CuS the reduction time was 15–20 hours, increasing the copper sulfide content to 50–70% led to an increase in the reduction duration to 40 hours. In addition, reduction of this group of catalysts ended at higher temperatures (400, and for the catalyst with 70% CuS—425°C) than was the case for catalysts with a CuS content up to 16%. The catalyst with 20% CuS after reduction had a dark-gray color with a bright brown shade characteristic of reduced metallic copper. As the content of copper sulfide in the catalyst increased, this shade intensified. The reduction phenomenon observed by us is in agreement with the X-ray studies of Frolich, Fenske, and Davidson ⁴). The cited authors found that not only copper oxide, but also partially zinc oxide, during redu-

during reduction of the mixed catalyst (ZnO—CuO), can be reduced to the metals. Nussbaum and Frolich\(^5\), subjecting a catalyst of 58.3% ZnO—41.7% CuO to X-ray examination, observed the reduction of copper and zinc oxides to the metals with the formation of solid solutions, since the lattice constant of the catalyst is 3.634 Å, which exactly corresponds to the lattice of α-brass.

RESULTS OBTAINED AND THEIR DISCUSSION.

After reduction, the activity of catalysts was investigated whose initial form before reduction had the following compositions:

80% ZnS + 20% CuS; 75% ZnS + 25% CuS; 70% ZnS + 30% CuS;

65% ZnS + 35% CuS; 50% ZnS + 50% CuS; 30% ZnS + 70% CuS.

The activity study was carried out at space velocities of pure methanol vapors from 420 to 2100 and temperatures from 275 to 425. The results obtained are given in Table 1 and presented in Figs. 1–17.

From Table 1 and Figs. 1, 3, 5, 7, 9, and 11 it is seen that the observed dependence between the percentage of decomposition and the space velocity of gas passage for the catalysts described in Communication I is also retained for this group of catalysts. Namely, at constant temperature, as the space velocity increases, the percentage decomposition of methanol decreases; however, this decrease, as a rule, occurs more slowly than the space velocity increases, which generally leads to an increase in catalyst productivity with increasing space velocity. Thus, for example, for the catalyst that contained 25% CuS before reduction, at a temperature of 375°C, increasing the space velocity from 420 to 2100, i.e. by a factor of 5, leads to a decrease in the percentage decomposition of methanol only from 100 to 41.0%, i.e. by a factor of 2.44. In some cases the percentage decomposition of methanol vapors decreases at approximately the same rate as the space velocity increases, or even somewhat faster. This feature is observed, for example, at a temperature of 325°C, when an increase in space velocity from 420 to 2100, i.e. by a factor of 5, causes a decrease in the percentage decomposition of methanol in the presence of a catalyst containing 30% CuS from 12.5 to 1.7, i.e. by a factor of 7.3. However, the latter phenomenon apparently has an accidental character and is probably associated with the difficulty of establishing a stationary state between the concentrations of the components of the reacting gas mixture and the concentration of intermediate compounds or adsorption complexes formed on the surface of the solid catalyst during the transition from one space velocity to another.

In the same Table 1, and also in Figs. 2, 4, 6, 8, 10, and 12, one can see the effect of temperature on the activity of the catalysts, measured by the percentage decomposition of methanol. For the group of catalysts discussed in the present communication, containing before reduction from 20 to 70% CuS, the temperature dependence of the percentage decomposition generally has the same character as was the case for the catalysts described in Communication I, i.e., at a constant gas space velocity the percentage decomposition of methanol increases with increasing temperature; moreover, the intensity of this increase in activity for all catalysts of this group rises as the space velocity decreases, which is characterized by a flatter arrangement of the curves of decomposition percentages = f(t°C) at higher space velocities. Measurable decomposition of methanol vapors with the formation of noncondensable gaseous products (CO, H₂, etc.) is observed, as Table 2 shows, for catalysts with 20 and 50% CuS at a temperature of 275°C, while for all the others this temperature rises to 300°C.

Table 1

Percentage decomposition of methanol as a function of temperature and space velocity in the presence of catalysts whose initial form consisted of:
80% ZnS + 20% CuS

75% ZnS + 25% CuS

70% ZnS + 30% CuS

65% ZnS + 35% CuS

50% ZnS + 50% CuS

30% ZnS + 70% CuS

The initial temperature of the region in which the sharpest increase in activity occurs is 325°C, while the final temperature of this region rises with increasing CuS content in the catalyst from 375°C (catalyst with 20% CuS) to 425°C (catalyst with 70% CuS). Thus, the temperature interval in which the sharpest increase in activity occurs broadens as the CuS content in the initial form of the catalyst increases. Indeed, if this value for the catalyst with 20% CuS is 50° (see Table 2), then for the catalyst with 70% CuS

Fig. 1. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: 80% ZnS + 20% CuS.

Fig. 2. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: 80% ZnS + 20% CuS.

Fig. 3. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: \(75\%\ \mathrm{ZnS} + 25\%\ \mathrm{CuS}\).

Fig. 4. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: \(75\%\ \mathrm{ZnS} + 20\%\ \mathrm{CuS}\).

Fig. 5. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: 70% ZnS + 30% CuS.

Fig. 6. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: 70% ZnS + 30% CuS.

Fig. 7. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: 65% ZnS + 35% CuS.

Fig. 8. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: 65% ZnS + 35% CuS.

Fig. 9. Dependence of methanol decomposition on space velocity and temperature. Catalyst composition before reduction: 50% ZnS + 50% CuS.

Fig. 10. Dependence of methanol decomposition on space velocity and temperature. Catalyst composition before reduction: 50% ZnS + 50% CuS.

Fig. 11. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: 30% ZnS + 70% CuS.

Fig. 12. Dependence of the percentage decomposition of methanol on space velocity and temperature. Catalyst composition before reduction: 30% ZnS + 70% CuS.

it already increases to 100°C. It should further be noted that the temperature of 100% decomposition of methanol, when the latter is passed over the catalyst at a space velocity of 420–600, is the same for the first four catalysts and is equal to 375°C; for catalysts containing 50 and 70% CuS it rises, respectively, to 400 and 425°C.

Table 2

For a more graphic demonstration of the influence of the composition of the catalysts on their activity, Figs. 13, 14, 15, 16, and 17 were constructed from the data of Table 1.

From the graphs presented it is seen that a complex dependence exists between the initial composition of the catalyst and its activity. With an increase in the CuS content from 16 (Communication I) to 20%, the activity of this catalytic system decreases; in the CuS concentration range from 20 to 50% it fluctuates, with the appearance, at elevated temperatures and space velocities, of a certain maximum at a CuS content in the catalyst of 30%. A further increase in the concentration of copper sulfide invariably leads to a decrease in activity, and in this region the curves (percentage decomposition) \(v,t^\circ = f(\% \mathrm{CuS})\) have a smooth character, with no minima or maxima.

In order to determine the influence of temperature, space velocity, and catalyst composition on the composition of the gaseous products of decomposition of methanol vapor, analyses of the gas phase were carried out for each catalyst at temperatures of 350–375 and 400°C, at the lowest (420) and the highest of the space velocities used (2100). In this part of the work we were interested first of all in how close the ratio \(\frac{\mathrm{H}_2}{\mathrm{CO}}\) comes to the stoichiometric ratio determined by the equation \(\mathrm{CH_3OH} \rightarrow 2\mathrm{H}_2 + \mathrm{CO}\), and also in how large the formation of methane and other organic compounds is. The results obtained are presented in Table 3.

From the table presented it is seen that, in the decomposition of methanol vapor in the presence of the catalysts we investigated, a gas of good composition is obtained.

The carbon monoxide content ranges from 26–27 to 30%; the hydrogen concentration lies within 66–72%.

If methanol decomposition proceeded exclusively according to the equation \(\mathrm{CH_3OH} \rightarrow 2\mathrm{H}_2 + \mathrm{CO}\), then in this case the stoichiometric concentrations of CO and \(\mathrm{H}_2\) would be respectively equal to 33.34 and 66.66% (by volume), and the ratio of hydrogen to carbon monoxide would be 2. As the last column in Table 3 shows, the actual ratio of hydrogen to carbon monoxide is somewhat higher than the theoretical ratio; namely, it fluctu-

Fig. 13. Dependence of the percentage decomposition of methanol on the CuS content in the catalyst before reduction. Space velocity 420.

Fig. 14. Dependence of the percentage decomposition of methanol on the CuS content in the catalyst before reduction. Space velocity 600.

Fig. 15. Dependence of the percent decomposition of methanol on the CuS content in the catalyst before reduction. Space velocity 1200.

Fig. 16. Dependence of the percent decomposition of methanol on the CuS content in the catalyst before reduction. Space velocity 1800.

ranges from 2.13 to 3.5. However, in fact, the overwhelming majority of analyses (76%) showed that $\dfrac{\mathrm{H_2}}{\mathrm{CO}}$ varies within narrower limits, namely between 2.2 and 2.6. A certain exception is the composition of the gas obtained in the presence of the last catalyst; in this case the ratio $\dfrac{\mathrm{H_2}}{\mathrm{CO}}$ is higher.

A positive feature of the catalysts we studied is the low methane content in the gaseous products of methanol decomposition. Only one gas analysis gave a methane content of 2%. In the overwhelming majority of analyses, the methane content is considerably less than 1%.

The content of organic substances absorbed from the gas by sulfuric acid, such as $\mathrm{CH_2O}$, $(\mathrm{CH_3})_2\mathrm{O}$, etc., varies for the first five catalysts from 0.4 to 3.6%. Carbon dioxide is present in approximately the same amounts. For the first five catalysts the gas composition proved stable and depended little on space velocity and temperature. A certain exception is the catalyst with 70% CuS. In the presence of this substance, methanol decomposition gave a gas of somewhat poorer and less stable composition.

Fig. 17. Dependence of the percentage decomposition of methanol on the CuS content in the catalyst before reduction. Space velocity 2100.

In particular, the carbon monoxide content varied within the limits from 20.1 to 27.59. The concentration of organic substances absorbed by sulfuric acid and the concentration of carbon dioxide varied, respectively, from 0.5 to 6.7% and from 0.9 to 6.3%. However, as for the other catalysts of this group, the methane content in the gas was low.

Thus, from all the foregoing it is clear that the decomposition of methanol vapors in the presence of the catalysts we studied closely follows the overall stoichiometric equation $\mathrm{CH_3OH \rightarrow 2H_2 + CO}$.

Catalysts containing, before reduction, from 20 to 70% CuS proved under the conditions studied to be considerably less active than catalysts of the same type described in Communication I and containing up to 16% copper sulfide. Indeed, if in the presence of catalysts containing, in their initial form, up to 16% CuS the temperature at which methanol decomposition began was 250–275°C, then increasing the copper sulfide in the catalytic system under consideration to 20–70% leads to an increase of this temperature to 275–300°C. A similar picture is also observed for the temperature of 100% methanol decomposition. At a methanol vapor space velocity of 420–600, this value in the presence of cataly-

The temperature for catalysts of the first group is 325–350°C (with the exception of the catalyst with 8% CuS), while at the same space velocities, but on catalysts containing from 20 to 70% CuS, this value increases to 375–425°C. Thus, the catalytic system consisting, before reduction, of zinc and copper sulfides, with a CuS content from 0 to 70% and with the selected methods of catalyst preparation and reduction, exhibits the highest—

Table 3.

Composition of the gaseous products of methanol decomposition as a function of temperature, space velocity, and the initial composition of zinc–copper methanol catalysts

... activity in the case where it contains from 1 to 4% CuS, and therefore, as before, for the synthesis of methanol, zinc–copper catalysts with a content of 1 to 4% CuS should be recommended first of all as very promising catalysts.

Despite the lower activity of the catalysts we investigated with a CuS content of from 20 to 70% in comparison with catalysts with a low percentage of CuS, nevertheless they (this applies especially to the catalyst containing 30% CuS) are still considerably more active than zinc-oxide catalysts. Thus, for example, according to Dolgov, at a feed rate of 0.15 cm³ of liquid methanol per minute through 25 cm³ of catalyst (space velocity 200), methanol is completely decomposed only at a temperature of 400°C.

In evaluating the quality of a given catalyst, the selectivity of its action is of great importance, i.e., in the present case, its ability to decompose methanol only to CO and H₂ with a minimum content of other gases that are products of undesirable side reactions. From this point of view the selectivity of this group of catalysts is considerably higher than the selectivity of action of the zinc–copper sulfide catalysts described in Communication I, with a CuS content in their initial form not exceeding 16%. Indeed, in the majority of cases the ratio

$$ \frac{\mathrm{H}_2}{\mathrm{CO}} $$

for catalysts with a copper sulfide content of from 20 to 70% is much closer to the stoichiometric ratio than is the case for catalysts containing up to 16% CuS. As a rule, the content of carbon dioxide in the gaseous decomposition products for the catalysts we investigated is considerably lower than the analogous values for the first group of catalysts. Especially noteworthy is the low methane content. As already indicated above, the methane concentration in the presence of these catalysts is considerably less than 1%, whereas for the first group of catalysts, for example, at a temperature of 300°C and \(V = 600\), the methane content ranges from 3.3 to 7%.

For comparison of the selectivity of action of the sulfide zinc–copper catalysts we investigated with zinc-oxide catalysts, one may, for example, use the data of Dohse³). The latter found that the products of methanol decomposition in the presence of ZnO I in the temperature interval 225.3–316.8°C contain CO from 22 to 26.8%. The concentration of carbon monoxide in the gases obtained in the presence of ZnO II and at temperatures of 315.9–350.5°C and at 343.7–383.6°C ranges, respectively, from 22.3 to 27.2 and from 28 to 29.2%.

Thus, our catalysts, with the exception of the last one, gave a gas of the same quality as the zinc-oxide catalysts. The composition of the gas obtained in the presence of the catalysts we investigated is incomparably better in quality than the composition of the gas found by Dolgov, Karpinskii, and Silina²) in the decomposition of methanol over sulfur catalysts. According to the data of these authors, the average analysis of the gaseous products of methanol decomposition gives the content \((\mathrm{CH}_3)_2\mathrm{O}\)—0.5–10%; CO₂—8–9%; CO—8–25%; H₂—6–75%; CH₄—0–7%.

What has been set forth here indicates that zinc–copper sulfide catalysts with a content in the initial form of from 20 to 70% CuS possess high selectivity of action and, from this point of view, as well as in activity measured by the temperature of 100% methanol decomposition at space velocities of 420–600, they approach zinc-oxide catalysts.

To answer the question of whether the catalysts we investigated are also capable of being poisoned, the catalysts were subjected to the action of hydrogen sulfide...

catalyst, which before reduction had contained 70% CuS. Hydrogen sulfide was passed for 10 minutes at a rate of 30 cm³/min at a temperature of 400°C. The supply of hydrogen sulfide was then stopped, and vapors of pure methanol were passed through the poisoned catalyst at the same temperature and at a space velocity of 1800. It turned out that if, before treatment of the catalyst with hydrogen sulfide, gas formation was equal to 155 cm³/min, then in the first minutes after poisoning it decreased to 100.3 cm³/min. Then, as vapors of pure methanol were passed through, the activity continuously increased and after 1 hour 15 minutes rose to its initial value. Thus, catalysts obtained by reducing zinc and copper sulfides with a high CuS content are capable of being poisoned by hydrogen sulfide; however, this poisoning is reversible in character.

In the previous communication it was shown that the decomposition of methanol vapors in the presence of sulfide zinc–copper catalysts proceeds according to a reaction law having an apparent zero order. Naturally, the question arises whether this regularity is also preserved in the presence of sulfide zinc–copper catalysts with a considerably higher CuS content. For this purpose, the results we obtained were calculated using the kinetic equation proposed by one of us (N. P. Kurin). (For the derivation of the equation, see communication 1.) This equation, in its final form after integration, has the following form:

\[ K = V(1+\alpha)\frac{T}{273}\,\alpha, \tag{1} \]

where \(K\) is the rate constant of the methanol decomposition reaction,
\(T\) is the catalysis temperature in °K,
\(\alpha\) is the degree of methanol decomposition, in fractions of unity, at the given space velocity.

The principal criterion for the applicability of this equation, derived on the basis of the assumption that methanol vapors decompose in the presence of catalysts according to a law of apparent zero-order reactions (i.e., \(\frac{dx}{dt}=K\)), must be the constancy of the rate constants calculated from equation (1) at constant temperature but at different space velocities.

The results of calculations by equation (1) for the catalysts we studied are presented in Table 4. As can be seen from this table, the constancy of the rate constants at different space velocities required by equation (1) (\(t^\circ\mathrm{C}=\mathrm{Const}\)) is observed quite satisfactorily, especially if one takes into account the difficulties in measuring the rate of methanol decomposition, which arise from the need to establish a new steady state when passing from one space velocity to another, even if the temperature remains constant. Thus, in the presence of the group of catalysts we investigated, characterized by an increased CuS content (from 20 to 70%), decomposition of methanol vapors is governed by the same zero-order law as was the case for catalysts with a CuS content from 0 to 16%.

However, in some cases the rate constants calculated from equation (1) do not remain constant and undergo, for example, a systematic decrease, as is the case for catalysts containing, before reduction, 50 and 70% CuS at a temperature of 325°C. Apparently, in these cases methanol decomposition follows a somewhat different kinetic regularity.

As is known, the dependence of the rate constant of a reaction on temperature is given by the Arrhenius equation, which, after integration under constancy of the activation energy, has the following form:

Table 4

Rate constants of the vapor-phase methanol decomposition reaction in the presence of sulfide catalysts having different degrees of reduction of zinc and copper sulfides, calculated by the equation of N. P. Kuranov.

Rate constants in the presence of sulfide catalysts having a decreased composition

Continuation of Table 4

\[ \log K = -\frac{E}{2.303 \cdot R}\left(\frac{1}{T}\right)+\mathrm{Const}, \tag{2} \]

where \(K\) is the reaction-rate constant,
\(R\) is the gas constant, expressed in calories,
\(T\) is the temperature in °K,
\(E\) is the activation energy in calories per mole.

Using the experimental data, the activation energy can be calculated from equation (2). For this purpose, in Fig. 18, \(\left(\frac{1}{T}\right)\) was plotted against \(\log K\), and if the activation energy were constant over the entire temperature interval studied, a straight line should have been obtained for each catalyst. However, as is evident from Fig. 18, for all the catalysts studied, not straight lines but broken lines with rectilinear branches were obtained. On each branch the points fit a straight line quite satisfactorily. The temperatures at which the straight lines \(\log K = f\left(\frac{1}{T}\right)\) break are 375°C for the first two catalysts, while for the last four (with a CuS content from 30 to 70%) they lie in the range 350–360°C.

From the slopes of the straight lines \(\log K = f\left(\frac{1}{T}\right)\), the apparent activation energies of the methanol-vapor decomposition reaction were calculated for the two temperature regions. The results of these calculations are given in Table 5.

Fig. 18.

From the table presented it is evident that the decomposition of methanol vapor in the presence of the catalysts studied by us proceeds, depending on the temperature region, with different activation energies. Specifically, the region of lower temperatures is characterized by high energies (from 44,400 to 67,000 cal/mole), while the region of high temperatures by low energies (from 17,000

Table 5

Values of the apparent activation energies of the methanol decomposition process in the presence of zinc–copper catalysts.

up to 31,100 cal/mol) activation. The transition from high activation energies to lowered ones for the catalysts studied occurs in the region of 350—375°C. The apparent activation energies, both in the region of low and in the region of high temperatures, depend on the composition of the catalyst. Namely, for both temperature regions, at first, with an increase in the percentage content of copper sulfide in the catalyst from 20 to 30%, the activation energy increases, and then, with a further increase in CuS, it decreases. The maximum value of the activation energy at lowered and elevated temperatures is observed for the catalyst with 30% CuS.

The mechanism of the process of decomposition of methanol vapor in the presence of the catalysts studied by us is the same as for zinc-copper sulfide catalysts with a CuS content from 0 to 16% and described in Communication I. Namely, methanol vapors, owing to intensive adsorption, almost completely cover the reactive surface of the catalysts studied by us. As a result, the concentration of the reacting substance in the adsorption layer does not depend on its concentration in the gas phase interacting with the solid contact. Under the latter condition, the rate of the decomposition process in the adsorption layer under isothermal conditions will likewise be a constant quantity and independent of the concentration or partial pressure of methanol vapor in the gas phase, which, as was indicated above, found expression in the applicability of the kinetic equation of apparent zero order (2). In the preceding communication, the true process of decomposition of methanol in the adsorption layer on catalysts consisting, prior to reduction, of zinc and copper sulfides was represented in the form of a consecutive reaction consisting of two stages:

\[ \mathrm{CH_3OH \longrightarrow 2H_2 + CO,} \tag{1} \]

\[ \mathrm{CH_2O \longrightarrow CO + H_2.} \tag{2} \]

In kinetic analysis, evidence for the occurrence of a consecutive reaction is usually provided by the presence of a break in the straight line \(\log K = f\left(\frac{1}{T}\right)\) and by a flatter position of the upper straight line compared with the lower one. In Communication I, on the basis of the systematic deviation from the straight line \(\log K = f\left(\frac{1}{T}\right)\) of the points at high temperatures, the assumption was made that, when the temperature interval of the study is extended toward higher temperatures, a flatter part of the straight line should also be observed. The correctness of this assumption was fully confirmed for the catalysts studied by us. As Fig. 18 shows, plotting \(\log K\) against \(\left(\frac{1}{T}\right)\) indeed gives a broken line, and, as expected, the break is directed upward. Thus, the true process of methanol decomposition in the adsorption layer is in fact a consecutive reaction consisting of two stages.

Dohse³) showed in his studies that, in the presence of ZnO, the higher activation energy pertains to the decomposition of formaldehyde. In accordance with this, both for catalysts containing from 0 to 16% CuS and for the catalysts studied by us with a higher content of copper sulfide (20—70%), the decomposition of methanol vapor proceeds by two successive reactions (1) and (2), with reaction (1) corresponding to low activation energies and reaction (2) to high activation energies (correspond-

respectively, curves 5 and 3 of Table 5). It is known that the overall rate of a process consisting of several consecutive reactions is determined by the rate of the slowest reaction. In our case, the overall rate of decomposition of methanol in the presence of the catalysts we studied, in the region of low temperatures (from 275) up to the temperature at which a break is observed in the straight line \(\log K = f\left(\frac{1}{T}\right)\), the decomposition of methanol to formaldehyde and hydrogen according to reaction (1) proceeds relatively rapidly, and the limiting stage of the process is the decomposition of formaldehyde to carbon monoxide and hydrogen, accompanied by the expenditure of increased activation energies (from 44,400 to 67,000 cal/mol). At higher temperatures, corresponding to the gently sloping straight lines in Fig. 18, the decomposition of formaldehyde into hydrogen and carbon monoxide is accelerated to such an extent that reaction (1), according to which methanol decomposes into \(\mathrm{CH_2O}\) and \(\mathrm{H_2}\), proves to be the limiting stage of the process. At the points of break in the straight lines \(\log K = f\left(\frac{1}{T}\right)\) (Fig. 18), the rates of these two reactions are equal to each other. Along with reactions (1) and (2), leading to the formation of CO and \(\mathrm{H_2}\), in the presence of the catalysts we studied, side reactions also proceed, which explains the presence in the gas obtained as a result of the decomposition of methanol vapor of such components as \(\mathrm{CO_2}\), \(\mathrm{CH_4}\), etc.

As was already indicated in Communication I, the activity of the catalytic system we studied and the activation energy of the methanol-decomposition process in its presence depend on the quantitative ratio between zinc and copper sulfides in the catalyst before reduction. This can be explained by the fact that the rate of reduction and the physicochemical structure of the reduced catalyst depend on the ratio of ZnS to CuS; in particular, this ratio determines the size and structure of the finest particles from which the catalyst is built, as well as the development and degree of shielding of its active surface by foreign substances. A detailed discussion of the relationship between the activity, activation energy, and composition of this catalytic system will be considered in one of the following communications.

Conclusions

1. The catalytic activity of methanol zinc–copper sulfide catalysts was investigated; before reduction, these catalysts had the following composition:

\[ \mathrm{ZnS} + 20\%\,\mathrm{CuS};\quad \mathrm{ZnS} + 25\%\,\mathrm{CuS};\quad \mathrm{ZnS} + 30\%\,\mathrm{CuS};\quad \mathrm{ZnS} + 35\%\,\mathrm{CuS};\quad \mathrm{ZnS} + 50\%\,\mathrm{CuS}\ \text{and}\ \mathrm{ZnS} + 70\%\,\mathrm{CuS}. \]

The activity study was carried out on the apparatus described in Communication I, in the temperature range 250–425°C and at methanol-vapor space velocities from 420 to 2100.

1. The reduction time of catalysts containing 20–70% CuS is, first, greater than the analogous value characteristic of catalysts with a low CuS content from 0 to 16%; and, second, it increases with an increase in the CuS content in the catalyst from 15–20 hours (for 16% CuS) to 40 hours (catalyst with 70% CuS).

2. For the indicated group of catalysts, as well as for catalysts with a lower CuS content, at constant temperature the percentage decomposition of methanol decreases with increasing space velocity; however, this decrease in the percentage decomposition, as a rule, lags behind the increase in space velocity. At constant space velocity, the percentage decomposition of methanol increases with increasing temperature.

1. The group of catalysts studied proved to be less active in comparison with catalysts containing a low percentage of CuS (up to 16%). The temperatures at which decomposition begins and the temperatures for 100% decomposition of methanol vapors at a space velocity of 420–600 are, respectively, 275–300° and 375–425°C, i.e., they considerably exceed the corresponding values for catalysts with a lower CuS content.

2. This group of catalysts possesses increased selectivity of action. Under the conditions investigated, the gas composition depends little on the composition of the catalyst, the temperature, and the space velocity, and is characterized by a low content of methane, carbon dioxide, and organic impurities.

3. The decomposition of methanol proceeds according to a law characteristic of reactions having an apparent zero order. The rate constants were calculated from Kurin’s equation.

4. When plotting the logarithm of the rate constant against $\left(\dfrac{1}{T}\right)$ for all catalysts of this group, in the temperature range 350–375°C an upward break is observed in the straight line

$$ \log K = f\left(\dfrac{1}{T}\right), $$

indicating the presence of two consecutive reactions. The activation energies of these two processes, depending on the composition of the catalyst, were found to be 17,000–31,100 and 44,400–67,000 cal/mol.

1. It is still assumed that the processes occurring in the adsorption layer consist of the decomposition of methanol vapors into formaldehyde and hydrogen, followed by the decomposition of formaldehyde into carbon monoxide and hydrogen. Moreover, the rate-limiting stage of the process in the region of reduced temperatures from 275° to the temperature of the break in the straight line

$$ \log K = f\left(\dfrac{1}{T}\right) $$

is the decomposition of formaldehyde into CO and H$_2$, whereas at elevated temperatures (above the temperature of the break in the straight line) it is the decomposition of methanol into formaldehyde and hydrogen.

References

1. N. P. Kurin and N. N. Tantsov—Izvestiya Tomskogo industrial’nogo instituta im. S. M. Kirova. Vol. 59, issue III, 1941.

2. Dolgov, Karpinskii, and Silina—Khimiya tverdogo topliva, 5, 470, 1934.

3. Dohse, Z.—Phys. Chem., 8, 3, 159, 1930.

4. Frolich, Fenske, and Davidson—Ind. Eng. Chem. 21, 109, 1929.

5. Nussbaum and Frolich—Ind. Eng. Chem., 23, 1387, 1931.

1. Communication 1. ↩