Chapter 6 catalysis

Measurement of ‘texture properties’
• The nature and structure of the support is of vital importance to the efficiency of the catalytic reaction.
The reactants are in a different phase to the catalyst. Diffusion: pore structure Adsorption: the surface nature and area
Chapter 6
Catalysis and Green Chemistry
Mike Lancaster
• 6.1 Introduction to Catalysis
• 6.2 Heterogeneous Catalysts
• 6.3 Homogeneous Catalysis • 6.4 Phase Transfer Catalysis • 6.5 Biocatalysis • 6.6 Photocatalysis
Other classifications important to green chemistry
• Asymmetric catalysts:
rare in industrial processes important in the pharmaceuticals for producing enantiomerically pure compounds.
6.2 Heterogeneous Catalysts
Heterogeneous catalysts have been used industrially for well over 100 years. Amongst the first processes was the catalytic hydrogenation of oils and fats to produce margarine using finely divided nickel.
• 6.2.4 Catalytic Converters
6.2.1 Basics of Heterogeneous Catalysis
• Compositions of the Heterogeneous catalyst:
Active components: such as transition metal (often Pd, Pt, Ni or Co) Support: inorganic or polymeric (inert or acid or basic functionality)
Table 6.1 Comparison of heterogeneous and homogeneous catalysts Heterogeneous Homogeneous
Usually distinct solid phase
Readily separated Readily regenerated and recycled Rates not usually as fast as homogeneous May be diffusion limited Quite sensitive to poisons Lower selectivity Long service life Often high-energy process Poor mechanistic understanding
Same phase as reaction medium
Often difficult to separate Expensive/difficult to recycle Often very high rates Not diffusion controlled Usually robust to poisons High selectivity Short service life Often takes place under mild conditions Often mechanism well understood
• Shades of greenness for some catalysts based on heavy metals.
Figure 6.1 Activation energy for catalysed and uncatalysed ammonia synthesis
Introduction to Catalysis
Measurement of ‘texture properties’
• Surface area:
Specific surface area: physical adsorption of N2(cross-sectional area 0.162 nm2) internal surface, external surface active metal surface area: chemical adsorption of H2
Three important parameters on both the commercial viability and the inherent greenness of a particular catalyst: • 1. Selectivity
S 100
the amount of substrate converted to the desired product total consumed substrate
• Homogeneous catalysts:
Catalyst and substrates are in the same phase The catalyst being dissolved in the reaction medium The reaction takes place within the liquid equency – efficiency, activity of the catalyst
moles of product /(moles of catalyst · second) • 3. Turnover number – lifetime of the catalyst
Heterogeneous Catalysts Homogeneous Catalysts
• Heterogeneous catalysts:
Surface catalysts or contact catalysts The solid catalyst with the gaseous or liquid reactants The actual reaction takes place on the external surface, and more effectively, internal pores surface of the catalyst.
the reactants are in two separate phases transporting the reactants from one phase into another.
harness energy from the sun to carry out chemical transformations beneficial in destroying harmful waste and for water clean-up.
• 6.2.1 Basics of Heterogeneous Catalysis
• 6.2.2 Zeolites and the Bulk Chemical Industry
• 6.2.3 Heterogeneous Catalysis in the Fine Chemical and Pharmaceutical Industries
• Biocatalysts:
• Phase transfer catalysts: • Photocatalysts:
enzymes transformations under mild conditions valuable for enantiomerically pure materials problems of isolating and screening enzymes
• Why catalysts can be considered to be inherently green?
Increasing the rate and selectivity Reducing the energy requirement Reducing waste by replacing stoichiometric reagents.
the amount of product / per mole of catalyst
Comparison of Catalyst Types
• Catalysts are commonly divided into two basic types depending on their state relative to the reaction medium.
• Specific surface area: inorganic supports 100 to >l000 m2· -1 g
• Average pore size: pore size distribution (broader or narrower) nm, Å • Pore Volume: cm3· -1 g
Comparison of Catalyst Types
The ultimate goal is to combine the best characteristics of both types of catalyst: attempts to heterogenize a homogeneous catalyst.