Catalyst Design and Preparation Quiz

Catalyst Synthesis

• Catalyst synthesis involves creating a catalyst, a substance that speeds up a chemical reaction without being consumed.
• The process typically involves several steps:
  • Preparation of the primary solid: Includes various methods (e.g., deposition/impregnation, co-precipitation, gel formation, etc.)
  • Processing/Treatment of the primary solid: Involves techniques for treatment of the primary solid(s) (e.g., drying, thermal decomposition of salts, calcination)
  • Activation of the precursor: This encompasses procedures of activating the catalyst (e.g., reduction, sulfonation, deammoniation, etc.)
• Catalyst properties and characteristics are important:
  • Texture and surface characteristics (number and strength of acidic/basic sites, isoelectric point)
  • The presence of exchangeable ions
  • Chemical properties (e.g., reactivity in acidic or basic solutions)
• The types of products and properties of the catalyst depend on:
  • The reactants themselves
  • Reaction conditions (e.g., temperature, pressure, and time)
• Key criteria for a good catalyst:
  • High activity
  • High selectivity
  • High thermal and mechanical strength
  • Low cost
• Catalyst formation methods (e.g.: powder, spray drying, oil drop, granulation, tableting, extrusion)

Types of Catalyst Impregnation

• Impregnation: involves contacting a solid with a liquid or gas containing the components to be deposited.
• Factors affecting impregnation rates:
  • Selective adsorption
  • Coulombic forces
  • Van der Waals forces
  • Hydrogen bonding
  • Ion exchange
  • Polymerization/depolymerization
  • Partial dissolution
• Different impregnation types exists:
  • Soaking method
  • Dry or pore volume impregnation
  • Incipient wetness impregnation
  • Deposition by selective reaction
  • Co-impregnation
  • Successive impregnation
  • Impregnation by percolation

Precipitation & Co-precipitation

• Precipitation: One or more metals are precipitated together with the support to create the catalyst.
• Factors that influence precipitation reactions:
  • Order and rate of adding solutions
  • Mixing techniques
  • Control of pH during the process
• Important processes in precipitation:
  • Nucleation (high supersaturation)
  • Growth (approaching equilibrium)
• Co-precipitation: Process in which normally soluble compounds are carried out of solution by a precipitate.
• Co-precipitation types: surface adsorption, mixed-crystal formation, occlusion, mechanical entrapment
• Co-precipitation affects particle size and purity.

Other Deposition Methods

• Gas-phase deposition is a method of catalyst deposition via adsorption or reaction from a gas phase. It improves dispersion and distribution control.
• Solid-solid reactions are also suitable methods to create specific catalysts using solid salts. This process is beneficial for industrial production but poses challenges regarding reproducibility in laboratories.
• Wash coat is an approach used to coat monoliths using slurry, a mixture of small particles. This process enables the particles to adhere, and evaporation of water results in adhesion to the monolith.

Catalyst Stability

• Common contaminants to catalysts during handling and storage include water, CO2, hydrocarbons, and poisons/inhibitors.
• Water, corrodes and causes hydrolysis.
• CO2 causes carbonate formation and texture changes.
• Hydrocarbons can have uncontrolled reactions during treatment/activation.
• Poisons/inhibitors can alter reactions.

Catalyst Protection

• Methods for protecting catalysts from alteration include Passivation, Inert gas protection, protection by a condensed phase.
• Passivation: controlled exposure to air to reduce rapid exothermic reactions and surface layer inhibition
• Inert gas protection: using an inert atmosphere to prevent reactions
• Condensed phase protection: using a wax or other solid material to protect the catalyst

Catalyst Forming Methods

• Methods for creating various shapes/sizes:
  • Powder: preparation, crushing, grinding
  • Spray drying
  • Oil drop
  • Granulation
  • Tableting
  • Extrusion

Gel Formation

• Gels are solid-like substances that retain active elements and eliminate reaction byproducts through evaporation/sublimation.
• Methods for gel formation include chemical reactions (like polymerization), complexation, freeze drying, and gum/gelling addition.

Selective Removal

• Some catalysts use selective removal of a component in an alloy to create a highly dispersed form.
• Raney Ni is an example of a catalyst resulting from selective removal of one element.

Pillared Layer Method

• This method uses layered structures (like clays), and adjustments in charge balance (often using Na) to create slit-shaped pores.

• Catalyst loading in the reactor is another part of the catalyst creation process, which involves methods to insert prepared catalysts into the reactor.

Zeolite Synthesis via Precipitation/Co-precipitation

• Zeolites are prepared via crystallization under hydrothermal conditions.
• High temperatures and pressure are generally used.
• Both batch and continuous synthesis methods are applicable.

Other important information

• A catalyst can be described as a material that increases the rate of a reaction, usually by creating a lower activation energy pathway, without being consumed in the overall reaction. A catalyst will increase the rate of the reaction.