While reading the Hydrogenation processes i have observed that most of the time the Activity and Selectivity used for catalyst at the same time. I want to know about the main difference between these two terminologies. Why these are used and mostly In Hydrogenation Reactions?
Assume that you have an alkyne which is to be converted into an alkene by hydrogenation without going further to a fully saturated alkane. Assume also that the targeted alkene can be a cis isomer or a trans isomer.
A selective catalyst is one which will help in reaching the alkene stage only with specific configuration (e.g. trans-alkene).
An active catalyst is one which exhibits efficiency in increasing the rate of a reaction by providing an alternative lower energy mechanism. The repeated use of a catalyst, especially for organic reactions, will reduce its activity until it loses it completely (or it becomes de-activated). Therefore, there is regeneration of catalyst "a process by which the catalyst is cleaned & hence re-activated".
Hello,
The activity is depended on the ability to perform the reaction in small time....
But the selectivity is depended on the specialization to perform the reaction exp. the hydrogenation of any unsaturated hydrocarbon must needed special catalysts such as Pt, Pd or Ni
IUPAC recommedations:
selectivity
The term also sometimes refers to the ratio of products obtained from given reactants. This meaning is of importance for catalysts, which can have a wide range of selectivities. Selectivity is quantitatively expressed by ratios of rate constants for the alternative reactions, or by the decadic logarithms of such ratios.
The term "catalytic activity" is not well defined. I agree with a definition
" "catalytic activity": An increase in the rate of a chemical reaction produced by the presence of a substance that is not consumed in the net chemical reaction
These terms are used for any catalytic reactions but not for only Hydrogenation Reactions .
Activity: Activity is the ability of catalysts to accelerate chemical reaction. For example: reaction between H2 and O2 to form H2O in the presence of Pt as a catalyst takes place with explosive violence.
In absence of catalyst, H2 and O2 can be stored indefinitely without any reaction.
Selectivity : It is the ability of a catalyst to direct reaction to yield particular products (excluding others).
e.g., conversion of CO2 to CH4 (excluding CO, CH3OH, etc).
Dear Muhammad,
have you read my previous post?
The selectivity is NOT the ability of a catalyst. This is a number which characterizes the product distribution.
Are you sure that " reaction between H2 and O2 to form H2O in the presence of Pt as a catalyst takes place with explosive violence?" The "catalytic explosion" does not exist.
Catalysts are highly specific compounds. They have an ability to direct the reaction to yield a particular product. The reaction with same reactants but different catalyst may yield different products. This is termed as the selectivity of catalyst. Catalysts are highly selective in nature. They can accelerate a particular reaction while inhibit another reaction. Hence, we can say a particular catalyst can catalyse one particular reaction only. It may fail to catalyse another reaction of the same type.
Muhammad,
This is the plagiarism "Catalysts are highly specific compounds. They have an ability to direct the reaction to yield a particular product. The reaction with same reactants but different catalyst may yield different products. This is termed as the selectivity of catalyst."
this is copied from
https://byjus.com/chemistry/activity-selectivity-of-catalyst/
The term "selectivity of catalyst" does not make sense. When we say "selectivity", we mean the selectivity of a reaction. Surely, a catalyst may change a reaction selectivity.
What about a "catalytic explosion" of H2/O2 mixture?
As copied with little modifications.
Activity: Activity is a measure of the “effective concentration” of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.
Selectivity: Selectivity is the discrimination shown by a reagent in competitive attack on two or more substrates or on two or more positions in the same substrate. It is quantitatively expressed by ratios of rate constants of the competing reactions, or by the logarithms of these ratios
Dear Segun,
You gave a correct definition of the "thermodynamic activity."
see also IUPAC Gold Book
https://goldbook.iupac.org/
With activity and selectivity is so that everyone tries to adapt these terms to their needs. Therefore, some people, referring to selectivity, refer to the catalyst and others to the main product in a given system of chemical reactions. There is also a problem with the activity if, for example, chemical reactions are accompanied by the deactivation process of the catalyst. And besides, there are still non-ideal systems. So it's best to read the text carefully and try to figure out what the author meant. And strict definitions ........... Regards,
Precious metal based hydrogenation catalysts (Pd) are generally more active than base metal based hydrogenation catalyst (Ni or Cu), so the hydrogenation can proceed at with less catalyst, at lower temperature and/or at lower pressure. Selectivity refers to a catalyst tendency to hydrogenate specific sites on a molecule without hydrogenating other sites. Cu based catalysts are often more selective than higher activity catalysts, so produce more of the desired product and less of undesirable hydrogenation products.
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Difference between the term Activity and Selectivity. Activity means its reactivity where as selectivity means reactvity at a specific part of the molecule. i.e. hydrogenation reaction reaction addition of H2 will occur to C=C bond (reactvity) but addition of hydrogen occurs from same side of C=C bond(Cis addition), hence it is selectivity.
One can of course talk about activity or selectivity with regard to catalysts (solid or liquid) or enzymes. However, it is difficult then to assign specific numbers to this physical quantity. Therefore, when defining them, one should use measurable quantities such as mass (moles) change, conversion (reaction extent) or reaction rate. Regards,
I use the term activity as a measure of the total amount of reactant converted to products and selectivity to represent the portion of products that are the desired product.
This is one of the shorter definitions of selectivity. I use a similar one, however, instead of the words "desired product", I use the words "main (direct translation of Polish word) product / s" - but "desired" seems better. "Products" because there may be a few "desired products" and not one. For example, we may be interested in two isomers and the third not. Regards,
Rick,
"a measure of the total amount of reactant converted to products" is called TON (TurnOver Number). TOF (TurnOver Frequency) would better describe the "speed" of a catalyst. For simplicity, TOF = TON/(reaction time)
Yurii, This time I will stand with Rick. In publications in such journals as "Applied Catalysis A - General" "Journal of Catalysis, ACS Catalysis etc, the conversion of the substrate at a given time or at a given residence time (space time) is used to compare the "activity" of catalysts and on this basis it determines which catalyst is more active. Also selectivity, defined as above, is used there. Regards,
Thank you @Muhammad Zahid for the good question.
As the question attracted many experts, I will try to make use of that to learn from them. Discussing the following issues may help in writing accurate definitions for the activity and selectivity.
1- Is it correct to give one definition for the “activity” for the homogeneous, heterogeneous, and enzyme catalysts?
As we know, in the case of heterogeneous reactions, there are many steps, rather than the surface reaction, including the internal diffusions, the external diffusions, the adsorption of the reactants, and desorption of the products. In such case, there will be a global rate for the catalytic reaction which is NOT a purely chemical event as it includes many physical events.
2- Is it correct to use the “total amount of converted reactants” to describe the activity?
Suppose there are two possible catalytic reactions, using the same catalyst, for given reactants; one reaction produces the desired products and the other produces undesired products. Shall we describe the catalyst as “active” when the total conversion of the reactants is high even if the products contain, say for example, 90% of the undesired products and only 10% of the desired products?
3- Regarding the selectivity of the heterogeneous catalyst (after the permission of Yurii to use the selectivity as a property for the catalyst), we know that the undesired reactions are catalyzed on the surface of the catalyst, however, are they catalyzed on the same active sites on which the desired reaction is catalyzed?
Hope to hear from the experts regarding these issues.
I think that, for the heterogeneous catalyst, the activity measures two things:
a) The suitability of the given active sites to catalyze a given desired reaction (chemical activity: that depends on the type of the active site).
b) The availability of these active sites (physical activity: that depends on the number of the active sites per unit area and the texture of the catalyst).
On the other hand, the selectivity measures:
a) The unsuitability of that given active sites to catalyze the undesired reactions.
b) The unavailability of “secondary active sites” that catalyze the undesired reactions.
Miroslaw (and Rick), we all agree with the definition of "selectivity." You definitions are rewording of the IUPAC definition.
"Activity." You wrote:" the conversion of the substrate at a given time or at a given residence time (space time) is used to compare the "activity" of catalysts and on this basis it determines which catalyst is more active. " I just pointed out that there exists the term TOF, which is actually similar to your definition of a catalyst activity, but TOF takes into account the number of catalytic cites. You can't compare the activity of two systems with different amount of catalysts.
This is a review on TOF
Chem. Rev. 1995, 95, 661-666
Turnover Rates in Heterogeneous Catalysis
M. Boudart
Imad,
" Is it correct to give one definition for the “activity” for the homogeneous, heterogeneous, and enzyme catalysts?"
Theoretically yes, this would be TOF. TOF is more precisely defined as the maximum number of catalytic cycles, which a catalytic cite can perform in a unit time
Moroslaw and Yuri, I think we are in close agreement on the chemistry and our differences are mostly about vocabulary.
To Imad Disher: There is no problem with selectivity. There is a strict definition that allows you to calculate its value on the basis of changes in the number of moles (mass) or on the basis of the progress of lineary independent chemical reactions. However, it must be remembered that selectivity depends on time (space time). Transport phenomena also affect selectivity.
There is a problem with the activity. This quantity appears in relation to the comparison of catalyst activity from the point of view of substrate/s conversion or product/s yield (for a single reaction, conversion and yield are the same). It is different, however, with the activity of the deactivating catalyst, where it is assumed that for fresh catalyst a = 1. In turn, the activity itself is defined as the ratio of the number of deactivated active sites to all active sites. Until the end, however, it is not known what the deactivated active sites mean, eg in regards to various types of deactivation: fouling, poisoning, sintering, aging etc. Regards,
Miroslaw and Rick,
We are all in agreement and well understand all nuances. Each time when you use whatever term, you must clearly define what you mean.
In my posts I attempted to give as simple as possible definitions for an average level of readers.
Yurii
PS Looking forward for another fruitful discussion.
And again, I have to write: Oj Jerzy, Jerzy (now in Polish). To compare anything, the conditions must be comparable. I really know it! I would also add that I never needed TOF and TON in my works. Regards,
With all these concepts, terms and quantities , it's like with concentrations. As one know and understand the definition of a particular concentration, it's easy to convert into other ones. If not, students cry that they can not, or use converters and, if they do not understand anything, they still do not understand. It is similar with conversion, selectivity, yield or other similar measures. As one know and understand stoichiometry, it's no problem. However, it is the hardest to guess what a given author meant by using one measure or another. So first of all correct definitions! Regards,
activity is that it can reduce the group
selectivity is reduction will be enantioselective
usually reduction is not selective
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