Three Sources of Catalyst Deactivation and How To Mitigate Them

ChemCatBio 2023 Technology Brief

This ChemCatBio Technology Brief reviews current knowledge on catalyst deactivation and mitigation for thermo-catalytic processes in biomass conversion, detailing three primary causes of catalyst deactivation and key strategies for mitigating them.

To reach commercial viability, biomass conversion industries need catalysts to be stable enough to operate for months or even years. But catalyst deactivation—a chemical or mechanical issue that limits or prevents desired chemical reactions—can greatly reduce the lifetime of catalysts and the durability of the process. This ChemCatBio Technology Brief highlights three common sources of catalyst deactivation: (1) structural damage by water, (2) poisoning by contaminants, and (3) fouling by coke. It also outlines key strategies for mitigating catalyst deactivation, equipping researchers with information and resources for designing more stable, resilient, and economical catalytic conversion processes.

What Mechanisms Drive Catalyst Deactivation?

Three Sources of Catalyst Deactivation From Biomass-Derived Feedstocks

Case Study: Potassium Deactivation of Pt/TiO2 Catalysts

Four Ways To Mitigate Catalyst Deactivation

Related ChemCatBio Capabilities

Supporting Resources and Publications

"Activity, selectivity, and stability are the three 'virtues' of catalyst performance. However, stability (or catalyst lifetime) is the least explored, especially for early-stage research and development."

What Mechanisms Drive Catalyst Deactivation?

Over time, deactivation is inevitable for most catalytic processes, which can greatly affect their research, development, design, and operation. In general, catalyst deactivation can be chemical or mechanical in nature.

Six Common Deactivation Mechanisms

Three Sources of Catalyst Deactivation From Biomass-Derived Feedstocks

[CHART]
Connections between biomass feedstock characteristics and associated catalysts with potential catalyst deactivation modes and mitigation approaches. Arrow colors represent different feedstock characteristics and different catalyst deactivation modes. Arrow thickness indicates the significance of connection.

Catalysts Can Be Deactivated by Water and Steam

Water—present in biomass-derived feedstocks or generated as a reaction product—can deactivate catalysts via the difficult-to-reverse mechanisms of deconstruction, leaching, and sintering. The presence of water cannot be avoided in biomass conversion processes, so improving the hydrothermal stability of the catalyst is the most promising way to address deactivation by water.

Strategies for Mitigating Water Deactivation

  • Improve the Catalyst:
    • Reduce the number of weak points by adjusting formulation or healing them.
    • Protect the surface with an overcoating (e.g., carbon, hydrophobic organic group).
    • Stabilize metal particles by embedding or adding dopant.
    • Increase hydrophobicity to reduce water near active sites or weak points.
  • Improve the Process:
    • Reduce water content in feed.
    • Lower the reaction temperature.

Catalysts Can Be Poisoned

Nonmetal contaminants (e.g., sulfur, nitrogen, phosphorus, chlorine) poison metal catalysts, which are usually used in hydrogenation reactions, via strong adsorption on metal sites. Metal contaminants (predominantly alkali and alkaline earth metals) mainly harm acidic catalysts (e.g., zeolites) by neutralizing strong acid sites. In addition, the interaction between nonmetal anions and metal cations can result in salt deposits (e.g., calcium phosphate), which fouls or poisons catalysts.

Strategies for Mitigating Poisoning

  • Develop Regeneration Methods: It is not realistic to completely remove contaminants from biomass feedstocks, but catalyst poisoning can be mitigated by regenerating the catalyst.
  • Develop Contaminant-Tolerant Catalysts: Some catalysts can be designed to resist contamination. Introducing additives can scavenge impurities.
  • Introduce Upstream Separations: Impurities can be removed prior to contacting the catalyst bed.

Catalysts Can Be Fouled

In the catalytic processing of biomass-derived feedstocks, heavy carbonaceous species—often called “coke”—are formed through the polymerization and condensation of reactive oxygenated compounds, such as sugars, aldehydes, ketones, carboxylic acids, phenolics, and furanics. Coke can foul the voids and surface of catalysts, blocking the access to active sites.

Strategies for Mitigating Fouling

  • Condition the Feedstock: The reactivity of coke precursors can be reduced through chemical conversion (such as hydrogenation of aldehydes and ketones to alcohols). Alternatively, the concentration of coke precursors can be reduced by separation.
  • Modify the Catalyst Design: Coke precursors can be converted to less reactive compounds by incorporating new active sites in catalysts, therefore competing against polymerization and condensation reactions.
  • Regenerate the Catalyst: As with catalyst poisoning, regenerating the catalyst can help remove coke and extend its lifetime.

Case Study: Potassium Deactivation of Pt/TiO2 Catalysts

Trend lines
[CHART]
The impact of potassium (K) content on the properties and catalytic activity of the Pt/TiO2 catalyst.

The Challenge: Potassium Accumulates on Pt/TiO2, a Catalyst Used in Catalytic Fast Pyrolysis

During catalytic fast pyrolysis, potassium—an abundant metallic contaminant in woody biomass—can deposit on the surface of the catalyst, platinum on titanium dioxide (Pt/TiO2), at the atomic level. The effect on the catalyst active sites and therefore catalyst stability was unclear.

The Approach: Combine Detailed Characterization and Catalytic Activity Measurements To Probe the Influence of Potassium

Researchers simulated potassium accumulation on Pt/TiO2. They then characterized the properties of the catalyst with various techniques, gathering kinetic measurements of several reactions to establish the correlation of potassium distribution with the change of catalytic sites.

The Findings

  • Poisoning Occurs at Lewis Acid Sites: The Pt/TiO2 catalyst was deactivated by potassium via the poisoning of Lewis acid Ti sites, both on the TiO2 support and at the metal−support interface. The metallic Pt clusters remained largely uncontaminated.
  • Potassium Poisoning Is Reversible: Water washing can successfully remove the accumulated potassium and recover the catalyst’s activity.

Four Ways To Mitigate Catalyst Deactivation

  1. Deal with it early: Consider deactivation during early catalyst research and development. Fully assess the actual biomass-derived feedstock to identify properties that may cause catalyst failure at industrially relevant conditions.
  2. Understand it better: Conduct research for a deeper understanding of catalyst deactivation mechanisms during biomass conversion. In situ and operando characterization methods can probe changes in catalyst active sites and the formation of surface species during reactions. Also, perform extended-duration experiments to evaluate catalysts after their initial “break-in” period.
  3. Measure it correctly and quickly: Study catalyst deactivation under kinetically-controlled conditions. Try to quantify deactivation, such as measuring the loss rate of active sites. To save time and money, develop accelerated catalyst aging processes to simulate catalyst deactivation. Multiscale, sophisticated, and realistic computational models can help demystify catalyst deactivation and, more importantly, predict catalyst stability for long-term operation.
  4. Look beyond the catalyst: Take a holistic approach to address catalyst deactivation by both improving catalysts and process design. Techno-economic analysis can provide powerful insights into economic feasibility and key impact factors related to catalyst stability with more rational and rigorous assumptions on catalyst lifetime.

 

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