The postulates of the correlation are consistent with the reaction mechanism previously proposed in the literature. To study the activity and selectivity as a function of the catalyst state, a quantitative correlation between the acid-base distributions and the. The estimate of the effective location density for each. group by combination of appropriate acid-base site pairs is in accordance with the mechanism proposed by Figueras et al. 1971) • The values of the specific rates for olefin and ether indicate that although strongly acidic sites catalyze the formation of both these products, the former is more affected by it.
This difference in the behavior of the basicity distribution significantly affects the selectivity of the catalyst. The effect of intrinsic kinetics is shown in Figures 3 and 4 in terms of the p ratio. It appears that selP.activity varies between people as much as twenty-fold. Previous ~result~ and especially directional effects and. The plots in p and cr show that the kinetics of alcoholic dehydration are inseparable from intoxication.
First of all, it should be noted that KA and 'Kw', although related to adsorption-desorption equilibria, should not be interpreted as equilibrium constants of Langmuir-type isotherms in view of the strong interaction effects of adsorbed methanol and water. The present work deals with the characterization of acid-base catalysts in terms of their acidity and basicity distribution and attempts to correlate their acid-base characteristics with their activity for the dehydration of methanol and ethanol. Ammonia adsorptive capacity: Since the dehydration activity of the tested catalysts is known to be impaired by chemisorption of ammonia and.
A representative set of differential heat of absorption curves is shown in Figure 1 for some of the catalysts. A correlative best fit using this particular combination of basicity and acidity suggests that although a catalyst surface may exhibit stronger basic sites than (B .. 2), the latter are optimal for alkoxide formation as well as the reaction following with the intermediate formed in Ai and (B4 + . s5) the combination of the pair of pages. The specific rates for ethylene and ether increase with the acid strength of the group.
ALC O AHC
AHC AHC+M
KSFO+ M ALC
Gavalas (1974b) describing the activity of fresh catalysts in terms of their acidity and basicity distribution was used to describe the activity of poisoned catalysts. It includes a study of the effects of various poisons on the acidity and basicity distribution of the catalyst and attempts to quantitatively explain the activity and selectivity of alcohol dehydration reactions in terms of these distributions. These results indicate that the effect of poisoning on the selectivity of the catalyst depends on the strength of the poison as well as on the nature of the surface of the catalyst in the fresh state.
The basic distributions of the fresh and poisoned states of the three catalysts are shown in Figures 1-3 in terms of their differential heats of adsorption curves. A division of the differential heat curves into groups of basalities as indicated in Figure 2 leads to the basality distributions given in Table 4. A comparison of the basicity distributions for fresh and poisoned catalysts indicates that the adsorption of a strong base on an acidic site results. in an increase in the strength of the neighboring basic sites for all three catalysts tested.
The basicity distributions of the poisoned catalysts show an increase in the sites of the s0 group at the expense of the sites in Bp B2 and B3 gro1.1ps. The acidity and basicity distributions of the fresh catalysts have been used in an earlier paper (Bakshi and Gavalas, 1974b) to develop a quantitative correlation for the rate of dehydration reactions. The details of the group analysis are shown in Table 7 in terms of the actual contribution of each group for all catalysts.
Upon poisoning with n-butylamine, part of the acidity in this group is lost with a simultaneous shift in the basicity distribution, as shown in Table 4. Model B thus obtains the acid-base distributions of the state of the poisoned catalyst without actual titration. The basicity distributions of all poisoned catalysts show an increase in the number density of the s0 group at the expense of the B1, B2 and B3 groups.
The importance of the change in basidity distribution due to induction in correlating catalyst activity for dehydration reactions is clear. In contrast, the acidity distribution of the fresh ALC catalyst shows a higher number density in A3 compared to A2. The evaluation of sij based on specific models involving acid-base site pairs with specific strengths assumes a random distribution of the acid and base sites on the catalyst surface.
Acidity Curves
Current investigation of different aluminas and silica-alumina shows that these catalysts exhibit considerable variation in their acid-base distributions, which in turn control their activities and selectivities for dehydration of primary alcohols. Success of the group analysis in correlating the observed activities indicates that the catalyst states can be operationally defined in terms of their corresponding acid-base distributions. As illustrated in this work, the transformation of the catalyst state upon partial poisoning can also be followed by the resulting acid–base distributions.
These results suggest further investigation into the use of selective poisoning to improve product distribution in other industries. In addition to poisoning by feed impurities, commercial catalysts undergo transformation in their states due to a number of processes such as variations in catalyst pretreatment, inclusion of moderators or promoters;. Each of these transformations and their effects on catalyst activity can be studied by following their resultant acid-base distributions.
Future research may include a study of the kinetics of these transformation processes using acid-base distributions as a measure to follow the reaction pathway. It should be noted here that although this operational definition of catalyst state appears to be useful in applied catalysis, it is not sufficient to obtain a complete understanding of the processes leading to changes in catalyst state. A clear understanding of the coke process, for example, would require investigating the structure of coke and. its effect on the catalyst as well.
Incorporation of results from more sophisticated analyzes such as IR 1 and NMR should be of great help in obtaining a clear understanding of the catalyst state. Effects of mass transfer limitations on the observed kinetic data must therefore be determined before assigning the observed kinetic behavior to the surface reaction alone. a) Effect of gas phase mass transfer:. For F49 catalyst the following kinetic data for methanol dehydration are observed: .. average particle size of the catalyst: 0.25 ll1TI.
From the above conditions, the Reynolds number, Re, based on catalyst particles is estimated to be 2.288. Now, if an additional assumption is made that the gas phase mass transfer mimics rate 1, the surface concentration of alcohol is negligible due to rapid reaction. Therefore, 6P is equal to the parial pressure of alcohol in the gas phase, or 0.091 atm in our case.
The rate of mass transfer in the gas phase is then estimated from (1) to be 7.81 mol/hour.g. Comparing this with the observed rate shows that the above assumption is incorrect and that under the experimental conditions the mass transfer in the gas phase is not rate limiting. Thus, the above considerations indicate that the kinetic data obtained for this system represent true reaction kinetics and are not affected by mass transfer limitations.
