Homogeneous Catalysis - Advantages and Problems.- Classical Homogeneous Catalyst Separation Technology.- Supported Catalysts.- Separation by Size-Exclusion Filtration.- Biphasic Systems: Water - Organic.- Fluorous Biphasic Catalysis.- Catalyst Recycling Using Ionic Liquids.- Supercritical Fluids.- Areas for Further Research.
CHAPTER 2 CLASSICAL HOMOGENEOUS CATALYST SEPARATION TECHNOLOGY (p. 9)
DAVID R. BRYANT
2.1 Coverage of Chapter
When considering a separation technique for a homogeneous catalytic process, one must realize that catalyst/product/byproduct separation is an integral part of the entire process. The selection and design of the separation technology goes hand-in-hand with catalyst design, often in an iterative fashion.
That is, a catalyst is selected and tested in a continuous unit, with recycle of streams, to discover if there are problems that will necessitate redesign of the catalyst. Redesign is more often the fact than the exception. The objective of this chapter is to detail considerations that must be addressed in order to successfully marry a catalyst technology with catalyst/product separation technology. The focus of this chapter is hydroformylation, but the general principles should apply to many homogeneous precious-metal catalyzed processes.
2.2 General Process Considerations
There are four principal factors that are paramount in selecting the best separation technique. They are the energy required for the separation, the capital required for the equipment used in the separation, the efficiency/effectiveness of the separation, and the vitality of the catalyst after the separation. General process considerations include: Transitions of any type including temperature, pressure or phase changes should be minimized.
Cooling below 40 degrees Celsius becomes more expensive (river water cannot be used). Vacuum below 20 mm Hg is challenging. Byproduct formation should be minimized. Single product processes are better. A distillation column, or other step, will be required for each material in the mixture.
Everything feasible should be recycled so as to minimize waste. Pressures should be kept below 35 bar, at least below 100 bar, to minimize costs and because most process design experience is here. The use of rotating equipment such as compressors or centrifuges should be minimized to minimize maintenance costs.
Corrosive materials, particularly chloride, should be avoided. Batch operations should be avoided. The handling of solids should be avoided.
2.3 Everything is a Reactor
This may be a good time to introduce a very simple principal of process chemistry, but one that is not widely recognized. It is taught in chemical engineering that the only things in chemistry that matter are temperature and concentration. Every other variable can be reduced to these two. For example, time is simply a reflection of changing concentration.
Now a corollary: since every piece of process equipment has associated with it temperature and concentration, all pieces of process equipment are reactors. Stated differently, everything is a reactor.
There is a tendency to think that once the catalyst is removed from the reactor, all chemistry ceases. Chemistry is occurring throughout the process, and that is why separation of products cannot be viewed in isolation from the process that made them.
2.4 Overview of Separation Technologies
2.4.1 TRADITIONAL COBALT WITH CATALYST DECOMPOSITION
Traditional cobalt hydroformylation separations will not be covered in detail since they have been described in many excellent references.
