Catalytic reactions are reactions in which a catalyst is used to change the reaction rate or reaction pathway so that the desired product can be obtained efficiently.
A catalyst is not consumed overall before and after the reaction and may greatly accelerate a reaction even when used in a small amount.
Catalytic reactions are important in a very wide range of fields, including organic synthesis, inorganic reactions, redox reactions, enzyme reactions, photocatalytic reactions, solid-catalyst reactions, and industrial chemistry.
In a discussion of a catalytic reaction, it is not sufficient simply to write that “the reaction became faster when a catalyst was added.”
It is necessary to explain why the catalyst increases the reaction rate, how the activation energy changes, how the selectivity toward the desired product changes, and how catalyst amount, surface area, temperature, pH, solvent, and reaction time affect the results.
Catalyst deactivation, catalyst poisoning, surface contamination, sintering, leaching, and reusability are also important points for discussion.
This article clearly explains, as examples of discussions that can be used in laboratory reports on catalytic reactions, the role of catalysts, reaction rate, activation energy, reaction selectivity, homogeneous catalysts, heterogeneous catalysts, catalyst amount, surface area, adsorption, catalyst deactivation, catalyst poisoning, reusability, yield, purity, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of catalytic reaction results obtained in basic chemistry experiments, organic chemistry experiments, inorganic chemistry experiments, physical chemistry experiments, and materials chemistry experiments at universities and similar institutions.
For the actual type of catalyst, reaction conditions, reaction mechanism, waste-liquid disposal, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is a Catalytic Reaction?
- Main Items to Include in the Results
- Why a Catalyst Increases the Reaction Rate
- Discussion of Activation Energy
- Catalysts and Reaction Selectivity
- Difference Between Yield and Selectivity
- Discussion of Homogeneous Catalysts
- Discussion of Heterogeneous Catalysts
- Effect of Catalyst Amount
- Effect of Catalyst Surface Area
- Relationship Between Adsorption and Reaction Rate
- Role of the Support
- Catalysts and Reaction Temperature
- Relationship Between Catalysts and pH
- Discussion of Enzyme Catalysis
- Discussion of Photocatalytic Reactions
- What Is Catalyst Deactivation?
- Effect of Catalyst Poisons
- Deactivation Caused by Surface Contamination
- Deactivation Caused by Sintering
- Deactivation Caused by Catalyst Leaching
- Catalyst Reusability
- Catalyst Recovery and Separation
- Catalytic Reactions and Side Reactions
- Reactant Concentration and Catalytic Reactions
- Effect of Stirring
- Methods for Confirming Catalytic Reactions
- Causes of Error in Catalytic Reactions
- When the Results Can Be Considered Good
- Example Discussions When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing Catalytic Reactions
- Summary
What Is a Catalytic Reaction?
A catalytic reaction is a reaction in which the reaction rate or reaction pathway is changed by a catalyst.
A catalyst interacts with reactants and may make the reaction easier to proceed by providing a reaction pathway different from the usual one.
Because the catalyst is regenerated before and after the reaction, it can theoretically function repeatedly even in a small amount.
A catalyst may not only accelerate the desired reaction but also play a role in selectively producing the desired product.
In other words, a catalyst affects both reaction rate and selectivity.
However, if the catalyst is contaminated with impurities or its structure changes during the reaction, its activity may decrease.
This is called catalyst deactivation.
Example Discussion:
The reaction proceeded faster under the condition in which the catalyst was added because the catalyst interacted with the reactants and provided a reaction pathway with a lower activation energy.
Because the catalyst is regenerated before and after the reaction, even a small amount can promote the overall reaction.
However, because catalyst activity is affected by reaction conditions and impurities, the amount of catalyst and the presence or absence of deactivation must also be considered.
Main Items to Include in the Results
In the results of a catalytic reaction, organize the type of catalyst, catalyst amount, amount of reactants, reaction time, reaction temperature, pH, solvent, stirring conditions, reaction rate, yield, selectivity, by-products, catalyst recovery rate, and whether the catalyst was reused.
Comparing conditions with and without the catalyst makes it easier to discuss the effect of the catalyst.
Main Items to Include in the Results
- Type of catalyst
- Amount of catalyst
- Shape and particle size of the catalyst
- Type and amount of reactants
- Type of solvent
- Reaction temperature
- Reaction time
- pH or acid-base conditions
- Stirring conditions
- Comparison with and without catalyst
- Change in reaction rate or reaction progress
- Yield
- Selectivity
- Presence or absence of by-products
- Whether the catalyst was recovered
- Activity when the catalyst was reused
- Possibility of catalyst deactivation
- Points for improvement
Example of How to Write the Results:
Under the condition in which the catalyst was added, the reaction proceeded faster than under the condition without the catalyst, and the desired product was obtained in a shorter time.
In addition, increasing the amount of catalyst increased the initial reaction rate, but above a certain amount, the increase in yield became small.
This suggests that the catalyst improves the reaction rate, but there is a range in which the effect reaches a plateau as the catalyst amount increases.
Why a Catalyst Increases the Reaction Rate
The main reason a catalyst increases the reaction rate is that it lowers the activation energy required for the reaction to proceed.
A catalyst creates a state in which the reaction can proceed more easily by temporarily adsorbing reactants, stabilizing reaction intermediates, or bringing reactants closer together.
As a result, even at the same temperature, the proportion of molecules that exceed the energy required for the reaction increases, and the reaction rate becomes higher.
However, a catalyst does not fundamentally change the thermodynamic equilibrium of the reaction.
In an equilibrium reaction, the catalyst increases the rate at which equilibrium is reached but does not change the final product ratio determined by equilibrium.
It is important to distinguish between reaction rate and equilibrium in the discussion.
With catalyst: The reaction proceeds through a pathway with lower activation energy
Without catalyst: The activation energy is higher and the reaction is slower
Example Discussion:
The reaction rate increased when the catalyst was added because the catalyst lowered the activation energy of the reaction.
When the activation energy becomes lower, the proportion of molecules that can react at the same temperature increases, so the reaction proceeds faster.
Therefore, the increase in reaction rate caused by the addition of the catalyst can be explained by a change in the reaction pathway.
Discussion of Activation Energy
In a catalytic reaction, the reaction proceeds through a pathway with a lower activation energy than in the absence of a catalyst.
Activation energy is the energy barrier that must be overcome for reactants to change into products.
A catalyst increases the reaction rate by lowering this barrier.
According to the Arrhenius equation, the reaction rate constant depends on the activation energy and temperature.
When Ea becomes smaller because of a catalyst, the rate constant k becomes larger even at the same temperature.
Therefore, catalytic reactions may proceed even at lower temperatures.
k = A exp(-Ea / RT)
The smaller Ea is, the larger the reaction rate constant k tends to become.
Example Discussion:
The reaction became faster under the condition with the catalyst because the activation energy Ea of the reaction was considered to have decreased.
According to the Arrhenius equation, when Ea decreases, the reaction rate constant k increases.
Therefore, by using a catalyst, a reaction that is difficult to proceed without a catalyst was considered to have proceeded in a shorter time.
Catalysts and Reaction Selectivity
Catalysts affect not only reaction rate but also reaction selectivity.
When a reactant can react through multiple pathways, a catalyst may preferentially promote a specific reaction pathway.
As a result, the proportion of the desired product may increase or the amount of by-products may decrease.
However, some catalysts may promote not only the desired reaction but also side reactions.
The type of catalyst, acidity or basicity, metal species, ligands, surface structure, pore size, support, and other factors affect selectivity.
When evaluating a catalytic reaction, it is necessary to consider not only yield but also the selectivity toward the desired product.
Example Discussion:
If the proportion of the desired product increased under the condition using the catalyst, the catalyst was considered to have selectively promoted the pathway of the desired reaction.
Because a catalyst changes the adsorption state of reactants and the stability of intermediates, it may favor a particular pathway among multiple reaction pathways.
Therefore, in catalytic reactions, changes in selectivity are important in addition to reaction rate.
Difference Between Yield and Selectivity
In catalytic reactions, yield and selectivity must be distinguished.
Yield indicates the amount of desired product actually obtained relative to the amount theoretically obtainable.
Selectivity, on the other hand, is the concept of how much of the consumed reactant was converted into the desired product.
For example, even if the reactant is consumed well, selectivity toward the desired product is low if many by-products are formed.
Conversely, even if selectivity toward the desired product is high, the yield is low if the conversion of the reactant is low.
To evaluate catalyst performance, it is important to consider conversion, selectivity, and yield together.
| Item | Meaning | Point for Discussion |
|---|---|---|
| Conversion | How much of the starting material was consumed | Evaluate how far the reaction proceeded |
| Selectivity | Proportion of consumed starting material converted into the desired product | Evaluate how few side reactions occurred |
| Yield | Actual amount of desired product relative to the theoretical amount | Evaluate the overall outcome of the reaction |
Example Discussion:
If consumption of the starting material progressed under the condition using the catalyst but many by-products were also formed, the conversion is high but the selectivity is low.
To increase the yield of the desired product, it is necessary not only to accelerate the reaction but also to suppress side reactions and increase selectivity.
Therefore, when evaluating a catalyst, it is important to distinguish among conversion, selectivity, and yield.
Discussion of Homogeneous Catalysts
A homogeneous catalyst is a catalyst that exists in the same phase as the reactants.
For example, this applies when the reactants and catalyst are dissolved in the same liquid phase.
Because homogeneous catalysts mix well with reactants and act at the molecular level, they may exhibit high activity and selectivity.
On the other hand, homogeneous catalysts may be difficult to separate from the product after the reaction.
If the catalyst remains in the product, it may affect purity or subsequent reactions.
The catalyst may also decompose in the presence of air, moisture, acids, bases, or other substances.
For homogeneous catalysts, both activity and ease of separation should be considered.
Example Discussion:
In a homogeneous catalytic system, the catalyst exists in the same phase as the reactants, so it can readily contact the reactants and the reaction rate tends to be high.
On the other hand, it may be difficult to separate the catalyst from the product after the reaction.
Therefore, in reactions using homogeneous catalysts, not only reaction activity but also a decrease in purity caused by residual catalyst and the separation operation must be considered.
Discussion of Heterogeneous Catalysts
A heterogeneous catalyst is a catalyst that exists in a phase different from that of the reactants.
A typical example is the use of a solid catalyst in contact with liquid or gaseous reactants.
Because the reaction proceeds mainly on the catalyst surface, catalyst surface area, particle size, pore structure, adsorption properties, and stirring conditions affect the reaction rate.
Heterogeneous catalysts have the advantage of being relatively easy to recover after the reaction by filtration or centrifugation.
However, if the catalyst surface becomes contaminated or reactants or products adsorb too strongly, the activity decreases.
In addition, if diffusion of the reactants to the catalyst surface is slow, the intrinsic activity of the catalyst may not be fully expressed.
Example Discussion:
In a heterogeneous catalytic system, the reaction proceeds mainly on the catalyst surface, so catalyst surface area and adsorption of reactants affect the reaction rate.
The smaller the catalyst particles and the larger the surface area, the greater the number of active sites available for contact with reactants, making the reaction more likely to proceed rapidly.
On the other hand, if the catalyst surface is covered with impurities or products, the number of active sites decreases and catalyst deactivation may occur.
Effect of Catalyst Amount
Increasing the amount of catalyst may increase the reaction rate because the number of active sites available for the reaction increases.
Particularly in the initial stage of the reaction, the starting material may be consumed more rapidly as the catalyst amount increases.
However, increasing the catalyst amount does not cause the reaction rate or yield to increase without limit.
If the amount of reactant is insufficient, diffusion or mixing is rate-limiting, or the reaction is an equilibrium reaction, increasing the catalyst amount has only a small effect.
In addition, if too much catalyst is used, side reactions may increase, catalyst separation may become difficult, or the product may adsorb onto the catalyst and be lost.
There is an appropriate range for the catalyst amount.
Example Discussion:
The reaction rate increased when the catalyst amount was increased because the number of active sites available for the reaction increased.
However, above a certain amount, the increase in reaction rate and yield became small.
This may be because reactant concentration, diffusion, equilibrium, or other factors became limiting factors, so increasing the amount of catalyst no longer greatly changed the overall progress of the reaction.
Effect of Catalyst Surface Area
In solid catalysts, reactions proceed mainly at active sites on the surface.
Therefore, the larger the surface area of the catalyst, the greater the number of places where reactants can make contact, and the higher the reaction rate tends to become.
Catalysts with small particle sizes or porous catalysts tend to have larger surface areas.
However, if the particles are too fine, filtration and recovery may become difficult, or aggregation may reduce the surface area actually available.
In porous catalysts, whether the reactants can diffuse into the pores is also important.
A large surface area and the actual availability of active sites are not necessarily the same.
Example Discussion:
The reaction proceeded faster under the condition using a solid catalyst with a smaller particle size because the specific surface area increased and the number of active sites available for contact with reactants increased.
However, if catalyst particles aggregate, the effective surface area decreases and the activity may not become as high as expected.
Therefore, not only the catalyst surface area but also the state of dispersion and intrapore diffusion must be considered.
Relationship Between Adsorption and Reaction Rate
In heterogeneous catalysis, adsorption of reactants onto the catalyst surface may be the first step of the reaction.
When reactants adsorb appropriately onto the surface, the reactants are brought closer together and the reaction becomes easier to proceed.
Therefore, adsorption is an important factor that increases the reaction rate.
However, if adsorption is too strong, reactants or products become difficult to remove from the surface and the active sites become blocked.
If adsorption is too weak, reactants do not remain sufficiently on the catalyst surface and the reaction becomes difficult to proceed.
Catalytic reactions require an appropriate balance in the adsorption strength of reactants and products.
Example Discussion:
In a solid-catalyst reaction, adsorption of reactants onto the catalyst surface makes the reaction easier to proceed.
However, if products or impurities adsorb strongly, active sites become blocked and the reaction rate decreases.
Therefore, catalyst activity is considered to depend on whether adsorption onto the surface is appropriate.
Role of the Support
In solid catalysts, metals or active components may be dispersed on a support.
A support is a material that holds the catalytic component and is related to increasing surface area, dispersing the active component, improving thermal stability, and controlling adsorption of reactants.
Alumina, silica, activated carbon, zeolites, titanium oxide, and other materials may be used as supports.
The support itself may have acidic or basic sites and may affect reaction selectivity.
In addition, interactions between the support and active metal may change catalyst activity.
In discussions of catalytic reactions, the properties of the support are important as well as those of the active component.
Example Discussion:
In a supported catalyst, dispersion of the active component over the support surface increases the effective catalytic surface area.
As a result, reactants can more readily contact the active sites, and the reaction rate was considered to have increased.
In addition, the acidity, basicity, or pore structure of the support may have affected reaction selectivity.
Catalysts and Reaction Temperature
Using a catalyst may allow a reaction that requires a high temperature without a catalyst to proceed at a lower temperature.
This is because the catalyst lowers the activation energy.
If the reaction can proceed at a lower temperature, thermal decomposition and side reactions may be suppressed.
However, temperature is also important in catalytic reactions.
If the temperature is too low, the reaction rate is insufficient, while if it is too high, structural changes in the catalyst, sintering, decomposition, or side reactions may occur.
In catalytic reactions, it is necessary to select a temperature range in which catalyst activity is high and deactivation and side reactions are limited.
Example Discussion:
The reaction proceeded even at a relatively low temperature when the catalyst was used because the catalyst lowered the activation energy.
Performing the reaction at a low temperature may suppress thermal decomposition and side reactions.
However, because catalyst deactivation and side reactions may proceed if the temperature is too high, catalytic reactions also have an optimum temperature.
Relationship Between Catalysts and pH
In reactions using acid or base catalysts, pH has a large effect on reaction rate and selectivity.
In acid catalysis, protonation may activate the reactant, while in base catalysis, deprotonation or changes in nucleophilicity may make the reaction easier to proceed.
If the pH is inappropriate, the catalytic effect may become weak.
On the other hand, if the acid or base is too strong, side reactions such as hydrolysis, decomposition, dehydration, or polymerization may proceed.
In enzyme catalysis, pH changes the charge state and three-dimensional structure of the active site, greatly affecting activity.
pH is related to both catalyst activity and side reactions.
Example Discussion:
The reaction proceeded more readily under acid-catalyzed conditions because the reactant was protonated and became more reactive.
However, if the acidity is too strong, side reactions such as decomposition or hydrolysis of the desired product may occur.
Therefore, in reactions using acid or base catalysts, the pH must be adjusted while considering the balance between reaction rate and side reactions.
Discussion of Enzyme Catalysis
Enzyme catalysts are highly selective catalysts that function in living organisms.
Enzymes bind specifically to substrates and carry out reactions at their active sites.
In enzyme reactions, temperature, pH, substrate concentration, inhibitors, and enzyme concentration have large effects on the reaction rate.
Because enzymes are proteins, they may denature and lose activity at high temperatures or extreme pH values.
In addition, the reaction rate increases as substrate concentration increases, but once the active sites of the enzyme are saturated, increasing the substrate concentration further does not greatly increase the reaction rate.
In enzyme catalysis, it is important to consider active sites and deactivation.
Example Discussion:
In enzyme catalysis, the reaction proceeds when the substrate binds to the active site of the enzyme.
Enzyme activity is high within an appropriate range of temperature and pH, but at high temperatures or extreme pH values, the enzyme denatures and its activity decreases.
Therefore, in enzyme reactions, conditions must be set while considering not only reaction rate but also the structural stability of the enzyme.
Discussion of Photocatalytic Reactions
In a photocatalytic reaction, the catalyst absorbs light and becomes excited, generating electrons and holes that drive oxidation-reduction reactions.
In photocatalysts such as titanium dioxide TiO2, irradiation with light may cause decomposition of organic substances or oxidation-reduction reactions to proceed.
In photocatalysis, light wavelength, irradiation intensity, irradiation time, catalyst amount, oxygen concentration, and surface adsorption are important.
In photocatalysis, reactants adsorb onto the catalyst surface, and electrons and holes generated by light participate in the reaction.
However, if the electrons and holes recombine, they are not used in the reaction and the catalytic efficiency decreases.
In discussing photocatalytic reactions, the light irradiation conditions should be related to reactions occurring on the catalyst surface.
Example Discussion:
The reaction proceeded under light irradiation because the photocatalyst absorbed light and became excited, generating electrons and holes.
These participated in oxidation-reduction reactions and promoted the desired reaction or decomposition reaction.
However, reaction efficiency is considered to vary depending on electron-hole recombination and the adsorption state on the catalyst surface.
What Is Catalyst Deactivation?
Catalyst deactivation is a phenomenon in which the activity of a catalyst decreases during a reaction or during reuse.
Although a catalyst is theoretically regenerated before and after a reaction, in practice its activity may decrease because of adsorption of impurities, contamination of the catalyst surface, structural changes, leaching, sintering, changes in oxidation state, or other factors.
When catalyst deactivation occurs, the reaction rate decreases and starting material tends to remain even after the same reaction time.
Selectivity may also change, and the amount of by-products may increase.
In discussing a catalytic reaction, check whether the rate changed between the early and late stages of the reaction and whether activity decreased upon reuse.
Example Discussion:
If the reaction rate decreased as the reaction time progressed, catalyst deactivation may have occurred.
If the catalyst surface is covered with products or impurities, reactants become less able to contact the active sites, and catalyst activity decreases.
As a result, the reaction becomes more difficult to proceed even with the same amount of catalyst, which may have led to a decrease in yield.
Effect of Catalyst Poisons
A catalyst poison is a substance that binds strongly to active sites and interferes with catalytic action.
Sulfur compounds, phosphorus compounds, halides, some metal ions, strongly adsorbing organic substances, and other materials may act as catalyst poisons.
When a catalyst poison is present, even a small amount may greatly decrease catalyst activity.
In deactivation caused by catalyst poisoning, the reaction may not proceed sufficiently even if the catalyst amount is increased.
Impurities contained in starting materials, solvents, glassware, or the reaction system may also act as catalyst poisons.
If the reaction is slower than expected or reproducibility is poor, contamination by catalyst poisons can be considered.
Example Discussion:
One possible reason why the reaction did not proceed sufficiently even after adding the catalyst is that a catalyst poison adsorbed onto the active sites and inhibited the catalytic action.
A catalyst poison can block active sites even in a small amount and prevent reactants from reacting on the catalyst surface.
Therefore, in catalytic reactions, impurities in starting materials and solvents must be reduced and contamination by components that act as catalyst poisons must be avoided.
Deactivation Caused by Surface Contamination
In solid catalysts, products, by-products, unreacted materials, or impurities may adsorb onto the catalyst surface during the reaction and block active sites.
This can be considered surface contamination or fouling.
When the catalyst surface is covered, reactants cannot approach the active sites, and the reaction rate decreases.
In decomposition reactions of organic substances and high-temperature reactions, carbonaceous deposits may accumulate on the catalyst surface.
Precipitates or salts may also adhere to the catalyst surface.
Activity lost through surface contamination may sometimes be partially recovered by washing, calcination, or regeneration treatment.
Example Discussion:
One possible reason for the decrease in activity when the catalyst was reused is that reaction products or by-products adsorbed onto the catalyst surface and covered the active sites.
When the active sites are blocked, the number of places where reactants can react on the catalyst surface decreases.
Therefore, washing or regeneration treatment of the catalyst surface may partially restore activity.
Deactivation Caused by Sintering
Sintering is a phenomenon in which catalyst particles or metal particles aggregate and become larger under high-temperature conditions.
When the catalyst particles become larger, the surface area decreases and the number of available active sites becomes smaller.
As a result, catalyst activity decreases.
Sintering tends to occur in high-temperature reactions or calcination treatments and may be a problem with metal catalysts and supported catalysts.
Measures to prevent sintering include avoiding excessively high temperatures, dispersing metal particles on a support, and using a stable catalyst structure.
If catalyst activity decreases under high-temperature conditions, sintering can be considered.
Example Discussion:
If catalyst activity decreased after high-temperature treatment, the catalyst particles may have sintered and the surface area may have decreased.
Because reactions on solid catalysts proceed at surface active sites, a decrease in surface area reduces the reaction rate.
Therefore, when using a catalyst under high-temperature conditions, attention must be paid to the decrease in activity caused by sintering.
Deactivation Caused by Catalyst Leaching
In solid catalysts and supported catalysts, the active component may dissolve into the solution during the reaction.
This is called leaching.
If the active component is lost from the catalyst surface, the amount of material functioning as the catalyst decreases, and the reaction rate decreases.
Catalyst leaching may occur more readily under acidic or basic conditions, in the presence of complexing agents, or under strong oxidation-reduction conditions.
In addition, if the leached metal component contaminates the product, it may affect purity and safety.
If activity decreases when the catalyst is recovered and reused, leaching can also be considered.
Example Discussion:
One possible reason for the decrease in reaction rate when the catalyst was reused is that active components in the catalyst leached into the reaction solution.
When the amount of active component decreases, the number of active sites available for the reaction decreases and catalyst activity falls.
In addition, because leached components may contaminate the product and reduce its purity, it is desirable to check for metal components or similar substances in the reaction solution.
Catalyst Reusability
Because catalysts are regenerated after a reaction, they can theoretically be used repeatedly.
Particularly for heterogeneous catalysts, the catalyst may be recovered by filtration or centrifugation, washed and dried, and then reused.
Reusing a catalyst can reduce costs and waste.
However, activity may decrease upon reuse.
Possible causes include contamination of the catalyst surface, leaching of active components, particle aggregation, structural changes, and loss during recovery.
In reuse experiments, the stability of the catalyst is discussed by comparing the reaction rate and yield in the first use with those in the second and subsequent uses.
Example Discussion:
If the yield decreased under conditions in which the catalyst was reused, catalyst deactivation or catalyst loss during recovery may have been the cause.
If products or impurities remain on the catalyst surface after the reaction, active sites are blocked and the reaction rate decreases.
Therefore, to evaluate catalyst reusability, the washing and drying conditions after the reaction and the reaction rate during reuse must be compared.
Catalyst Recovery and Separation
In heterogeneous catalytic systems, the catalyst can be recovered after the reaction by filtration or centrifugation.
If recovery is insufficient, the catalyst may remain in the product, causing a decrease in purity or overestimation of mass.
In addition, if the catalyst cannot be recovered, evaluation of reusability becomes difficult.
If the catalyst particles are fine, they may pass through the filter paper or remain dispersed with the product, making separation difficult.
If the catalyst adsorbs the product, discarding the catalyst may also cause loss of the desired product.
Catalyst separation affects both yield and purity.
Example Discussion:
The catalyst was removed by filtration after the reaction, but if the catalyst particles were fine, some catalyst may have contaminated the product.
Residual catalyst causes the product mass to be measured too high and also decreases purity.
In addition, if the desired product is adsorbed onto the catalyst surface, the desired product may also be lost when the catalyst is removed, so the recovery operation is considered to affect the yield.
Catalytic Reactions and Side Reactions
A catalyst may accelerate the desired reaction while also promoting side reactions.
Particularly when a catalyst exhibits strong acidity or basicity, decomposition, dehydration, polymerization, isomerization, or other reactions besides the desired reaction may proceed.
With metal catalysts, undesired oxidation or reduction may also occur.
When side reactions proceed, selectivity and yield of the desired product decrease.
In catalytic reactions, it is important not only to accelerate the desired reaction but also to select a catalyst and conditions that suppress side reactions.
The presence or absence of by-products can be checked by TLC, GC, HPLC, NMR, or other methods to discuss selectivity.
Example Discussion:
If the amount of by-products increased under the condition using the catalyst, the catalyst may have promoted not only the desired reaction but also side reactions.
Depending on the acidity or basicity of the catalyst or the properties of the metal active sites, decomposition or excessive reaction may proceed.
Therefore, in catalytic reactions, conditions with high selectivity that suppress side reactions must be selected in addition to improving the reaction rate.
Reactant Concentration and Catalytic Reactions
Reactant concentration affects the rate of catalytic reactions.
As the reactant concentration increases, more reactant may reach the catalyst surface or catalytic center, increasing the reaction rate.
However, once all catalyst active sites are occupied by reactants, increasing the concentration further may produce little additional increase in reaction rate.
In addition, if the reactant concentration is too high, side reactions, polymerization, precipitation, increased viscosity, or diffusion limitations may occur.
In catalytic reactions, not only catalyst amount but also reactant concentration affects reaction rate and selectivity.
Example Discussion:
Increasing the reactant concentration may increase the reaction rate because more reactant reaches the catalyst active sites.
However, once the active sites become saturated, further increases in concentration cause only a small increase in reaction rate.
In addition, because side reactions or diffusion limitations may occur under high-concentration conditions, reactant concentration is also an important factor in catalytic reactions.
Effect of Stirring
In heterogeneous catalytic reactions, stirring may have a large effect on the reaction rate.
Because the reaction cannot proceed unless reactants move to the catalyst surface, insufficient stirring slows mass transfer.
As a result, the reaction rate may remain low even when a sufficient amount of catalyst is present.
Stronger stirring improves contact between the reactants and catalyst and promotes diffusion to the catalyst surface.
However, excessively vigorous stirring may cause crushing or scattering of catalyst particles or adhesion to the apparatus.
Stirring conditions are also related to the reproducibility of catalytic reactions.
Example Discussion:
Under conditions with insufficient stirring, the reactants may not have been supplied sufficiently to the catalyst surface, and the catalyst activity may not have been fully expressed.
Because heterogeneous catalytic reactions proceed on the catalyst surface, contact between the reactants and catalyst is important.
Therefore, to evaluate the reaction rate correctly, not only the catalyst amount but also the stirring conditions must be kept constant.
Methods for Confirming Catalytic Reactions
A basic method for confirming the effect of a catalyst is to compare conditions with and without the catalyst.
If the starting material is consumed faster, the product forms sooner, the yield is higher, or fewer by-products are formed in the presence of the catalyst, a catalytic effect can be judged to have occurred.
Depending on the reaction, the effect can be confirmed by TLC, GC, HPLC, absorbance, pH change, gas generation, current change, or other methods.
To confirm catalyst deactivation, changes in reaction rate over time or decreases in yield upon reuse are investigated.
For solid catalysts, color, mass, particle size, surface condition, XRD, SEM, BET surface area, and other properties before and after the reaction may also be compared.
In discussing catalytic reactions, it is important to clarify which data demonstrate the catalytic effect.
Example Discussion:
Because the spot of the starting material decreased more quickly and the peak of the desired product increased under the condition with the catalyst, the catalyst was considered to have increased the reaction rate.
In addition, because the reaction barely proceeded over the same period under the condition without the catalyst, the effect of the catalyst could be confirmed.
Comparing conditions with and without the catalyst is important for evaluating the catalytic effect.
Causes of Error in Catalytic Reactions
Causes of error in catalytic reactions include errors in measuring the catalyst amount, insufficient catalyst dispersion, insufficient stirring, temperature nonuniformity, contamination of the catalyst surface, contamination by catalyst poisons, catalyst deterioration, loss during recovery, variation in reactant concentration, and deviations in pH.
Because a catalyst can greatly affect a reaction even in a small amount, slight differences in conditions may produce large differences in the results.
In solid catalysts, differences in particle size and surface area, moisture, and storage conditions also have an effect.
In homogeneous catalysts, decomposition of the catalyst and sensitivity to air or moisture may be problems.
In catalytic reactions, it is important to consider not only the reaction conditions but also the condition of the catalyst itself as a cause of error.
Example Discussion:
Possible causes of variation in the catalytic reaction results include errors in measuring the catalyst amount, insufficient catalyst dispersion, and differences in stirring conditions.
In solid catalysts, the reaction proceeds through contact of the reactants with the catalyst surface, so the state of dispersion and stirring greatly affect the reaction rate.
In addition, if the catalyst surface was contaminated with impurities, the number of active sites may have decreased and the reaction may have become slower.
When the Results Can Be Considered Good
A catalytic reaction can be considered to have produced good results when the reaction rate clearly increases under the condition with the catalyst, the yield and selectivity toward the desired product improve, and few by-products are formed.
Furthermore, if the decrease in activity is small even when the catalyst is recovered and reused, the stability of the catalyst can also be judged to be high.
However, a catalyst cannot be considered good simply because the reaction is fast.
Improvement is necessary if there are many side reactions, catalyst remains in the product, the catalyst deactivates quickly, or recovery is difficult.
Catalyst performance is evaluated comprehensively in terms of activity, selectivity, stability, reusability, and ease of separation.
Example Discussion:
Under the condition using the catalyst, the reaction proceeded faster than without the catalyst, and the yield of the desired product also increased.
In addition, because few by-products were formed, the catalyst was considered to have selectively promoted the desired reaction.
Furthermore, if the decrease in activity upon reuse was small, the stability of the catalyst could also be judged to be relatively high.
Example Discussions When the Experiment Did Not Go Well
When a catalytic reaction does not go well, possible causes should be considered from results such as failure of the reaction to proceed, no increase in rate even after adding the catalyst, formation of many by-products, decreased activity upon reuse, inability to recover the catalyst, or contamination of the product with catalyst.
Organizing the causes according to catalyst amount, catalyst condition, deactivation, reaction conditions, stirring, and separation operations makes the discussion easier.
Example Discussion:
Possible reasons why the reaction rate did not increase sufficiently even after adding the catalyst include an insufficient amount of catalyst or catalyst deactivation.
If the catalyst surface is covered with impurities, reactants cannot contact the active sites and the catalytic effect becomes weaker.
Therefore, the storage condition and pretreatment of the catalyst and the presence or absence of catalyst poisons during the reaction must be checked.
Another Example Discussion:
If many by-products were formed under the condition using the catalyst, the catalyst may have promoted not only the desired reaction but also side reactions.
If acidic sites or metal active sites on the catalyst promote excessive reaction, decomposition, or isomerization, the selectivity toward the desired product decreases.
Therefore, the type of catalyst, reaction temperature, and reaction time must be reviewed to identify conditions with higher selectivity.
Another Example Discussion:
Possible reasons why the yield decreased when the catalyst was reused include adsorption of products onto the catalyst surface, leaching of active components, and catalyst loss during recovery.
When the number of active sites decreases, the reaction rate decreases and unreacted material tends to remain after the same reaction time.
To improve catalyst reusability, washing, drying, and regeneration treatment after the reaction must be performed appropriately.
How to Write Points for Improvement
In a discussion of catalytic reactions, writing not only about the catalytic effect and causes of deactivation but also about how the experiment can be improved makes the report easier to organize.
Points for improvement can be organized according to catalyst selection, catalyst amount, pretreatment, reaction conditions, dispersion, reuse, and analytical confirmation.
Improvements to Catalyst Conditions
- Adjust the catalyst amount appropriately
- Disperse the catalyst sufficiently
- Keep the catalyst particle size and surface area consistent
- Perform catalyst pretreatment
- Remove impurities that may act as catalyst poisons
- Confirm interactions between the catalyst and desired product
- Change the support or catalyst type and compare selectivity
Improvements to Reaction Conditions
- Optimize the reaction temperature
- Use an appropriate reaction time
- Adjust the pH or acid-base conditions
- Stir sufficiently
- Adjust the reactant concentration
- Select a solvent that produces fewer side reactions
- Avoid the effects of oxygen and moisture
Improvements to Prevent Catalyst Deactivation
- Wash the catalyst thoroughly after the reaction
- Dry and store the catalyst appropriately
- Avoid sintering caused by high temperatures
- Prevent contamination of the catalyst surface
- Regenerate the catalyst when necessary
- Compare activity after each reuse
- Check the catalyst structure and surface condition after the reaction
Example of How to Write Points for Improvement:
To improve the reproducibility of the catalytic reaction, the catalyst amount, particle size, dispersion state, and stirring conditions must be kept constant.
In addition, to prevent catalyst deactivation, it is important to wash and dry the catalyst after the reaction and remove products and impurities adsorbed on the surface.
Furthermore, comparison of conditions with and without the catalyst and reuse experiments make it possible to evaluate catalyst activity, selectivity, and stability more clearly.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of a catalytic reaction, simply writing that “the reaction became faster when the catalyst was added” or “the catalyst deteriorated” results in a superficial discussion.
A good discussion relates activation energy, active sites, selectivity, catalyst amount, surface area, deactivation, catalyst poisoning, and reusability.
| Superficial Discussion | Good Discussion |
|---|---|
| The reaction became faster because the catalyst was added. | The reaction rate was considered to have increased at the same temperature because the catalyst provided a reaction pathway with a lower activation energy. |
| Increasing the catalyst amount is good. | Increasing the catalyst amount increases the number of active sites, but once reactant concentration or diffusion becomes the limiting factor, the effect becomes smaller above a certain amount. |
| By-products were formed. | The catalyst may have promoted side reactions as well as the desired reaction, reducing the selectivity toward the desired product. |
| The catalyst was deactivated. | Catalyst activity may have decreased because products or impurities adsorbed onto the catalyst surface and blocked active sites, or because active components leached out. |
| The yield decreased upon reuse. | During reuse, effective active sites may have decreased because of catalyst surface contamination, catalyst loss during recovery, sintering, or leaching, reducing the reaction rate and yield. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of catalytic reactions.
Adjust the necessary parts according to your own experimental results.
- A catalyst lowers the activation energy of a reaction and increases the reaction rate.
- Under the condition with the catalyst, the starting material was consumed faster than under the condition without the catalyst.
- A catalyst affects not only the reaction rate but also the selectivity toward the desired product.
- Increasing the catalyst amount increases the number of active sites and may increase the initial reaction rate.
- Above a certain catalyst amount, reactant concentration or diffusion becomes rate-limiting and the improvement in reaction rate becomes small.
- In heterogeneous catalysts, catalyst surface area and particle size affect the reaction rate.
- If the catalyst surface is covered with products or impurities, the number of active sites decreases and catalyst deactivation occurs.
- Catalyst poisons adsorb strongly onto active sites and can reduce catalyst activity even in small amounts.
- If activity decreases when the catalyst is reused, surface contamination, leaching, sintering, or recovery loss may be possible causes.
- Catalyst performance must be evaluated comprehensively in terms of activity, selectivity, stability, and reusability.
Points to Check When Discussing Catalytic Reactions
Checking the following points before writing the report makes the discussion easier to write.
- Is the role of the catalyst explained?
- Are activation energy and reaction rate related in the discussion?
- Are conditions with and without catalyst compared?
- Has the effect of catalyst amount been considered?
- Are selectivity and yield distinguished?
- Are the differences between homogeneous and heterogeneous catalysts considered?
- For solid catalysts, are surface area and adsorption considered?
- Are the effects of the support and particle size considered?
- Have the causes of catalyst deactivation been considered?
- Are catalyst poisoning and surface contamination considered?
- Are reusability and recovery rate considered?
- Do the points for improvement correspond to the causes of error?
Summary
In catalytic reactions, a catalyst lowers the activation energy of the reaction and increases the reaction rate.
A catalyst can promote a reaction even in a small amount and, depending on the conditions, may also increase selectivity toward the desired product.
However, because a catalyst may promote side reactions as well as the desired reaction, selectivity is important in addition to reaction rate.
In heterogeneous catalysts, catalyst surface area, particle size, adsorption, stirring, support, and the number of active sites affect the reaction rate.
In homogeneous catalysts, miscibility with the reactants and difficulty of separation are important.
In addition, catalysts may become deactivated during the reaction because of surface contamination, catalyst poisoning, sintering, leaching, and other factors, so decreases in activity during reuse also require attention.
In a report, rather than simply writing that “the reaction became faster when the catalyst was added,” organize and discuss activation energy, reaction rate, selectivity, catalyst amount, surface area, adsorption, support, catalyst deactivation, catalyst poisoning, reusability, effects on yield and purity, causes of error, and points for improvement.
Discussion of catalytic reactions is important for understanding the conditions that allow a reaction to proceed efficiently and the conditions that allow a catalyst to function stably.
