Catalysts separation

Recent advances in Eischer-Tropsch technology at Sasol include the demonstration of the slurry-bed Eischer-Tropsch process and the new generation Sasol Advanced Synthol (SAS) Reactor, which is a classical fluidized-bed reactor design. The slurry-bed reactor is considered a superior alternative to the Arge tubular fixed-bed reactor. Commercial implementation of a slurry-bed design requires development of efficient catalyst separation techniques. Sasol has developed proprietary technology that provides satisfactory separation of wax and soHd catalyst, and a commercial-scale reactor is being commissioned in the first half of 1993. 

Because of its volatility, the cobalt catalyst codistills with the product aldehyde necessitating a separate catalyst separation step known as decobalting. This is typically done by contacting the product stream with an aqueous carboxyhc acid, eg, acetic acid, subsequently separating the aqueous cobalt carboxylate, and returning the cobalt to the process as active catalyst precursor (2). Alternatively, the aldehyde product stream may be decobalted by contacting it with aqueous caustic soda which converts the catalyst into the water-soluble Co(CO). This stream is decanted from the product, acidified, and recycled as active HCo(CO)4. 

Meth5l-l,3-propanediol is produced as a by-product. The hydroformylation reaction employs a rhodium catalyst having a large excess of TPP (1) and an equimolar (to rhodium) amount of 1,4-diphenylphosphinobutane (DPPB) (4). Aqueous extraction/decantation is also used in this reaction as an alternative means of product/catalyst separation. 

Future Trends. In addition to the commercialization of newer extraction/ decantation product/catalyst separations technology, there have been advances in the development of high reactivity 0x0 catalysts for the conversion of low reactivity feedstocks such as internal and a-alkyl substituted a-olefins. These catalysts contain (as ligands) ortho-/-butyl or similarly substituted arylphosphites, which combine high reactivity, vastiy improved hydrolytic stabiUty, and resistance to degradation by product aldehyde, which were deficiencies of eadier, unsubstituted phosphites. Diorganophosphites (28), such as stmcture (6), have enhanced stabiUty over similarly substituted triorganophosphites. 

The solubility of the resulting product may dictate the choice of solvent. Reductive alkylation of norepinephrine with a series of keto acids proceeded smoothly over platinum oxide in methanol-acetic acid mixtures. However, when n = 4 or 5, the product tended to precipitate from solution, making catalyst separation difficult. The problem was circumvented by using glacial acetic acid as solvent 38). 

Different Technical Solutions to Catalyst Separation through the Use of Ionic Liquids... 

BP Chemicals studied the use of chloroaluminates as acidic catalysts and solvents for aromatic hydrocarbon allcylation [41]. At present, the existing AICI3 technology (based on red oil catalyst) is still used industrially, but continues to suffer from poor catalyst separation and recycling [42]. The aim of the work was to evaluate the AlCl3-based ionic liquids, with the emphasis placed on the development of a clean... 

Figure 8-5. The Hoechst AG and Rhone Poulenc process for producing butyraldehydes from propene (1) reactor, (2) catalyst separation, (3) stripper (using fresh syngas to strip unreacted propylene to recycle), (4) distillation.

After exiting the riser, catalyst enters the reactor vessel. In today s FCC operations, the reactor serves as a housing for the cyclones. In the early application of FCC, the reactor vessel provided further bed cracking, as well as being a device used for additional catalyst separation. 

The general picture of the relative merits of homogeneous and heterogeneous processes has not yet emerged clearly. The homogeneous catalyst system may offer advantages in chemical efficiency but lead to difficulties of catalyst separation and recovery, or catalysts may tend to plate out in the reactor due to slight instability. Materials of construction may have to be different for the two rival plants. All these factors will have to be considered in an economic assessment and detailed studies made of the complete process networks in both cases. 

When the desired hydrogen uptake had been achieved, the vessel was opened, catalyst separated by filtration, and the reaction solution analysed by chiral gas chromatography (column Cydex B, 50 m, SGE Ltd). Analysis gave conversion and enantiomeric excess Enantiomeric excess is defined as IR - SI /(R+S). 

In the sixties of past century, a few patents issued to Bergbau Chemie [5,48,49] and to Mobil Oil [50-52], respectively described the use of CFPs as supports for catalytically active metal nanoclusters and as carriers for heterogenized metal complexes of catalytic relevance. For the latter catalysts the term hybrid phase catalysts later came into use [53,54], At that time coordination chemistry and organo-transition metal chemistry were in full development. Homogeneous transition metal catalysis was expected to grow in industrial relevance [54], but catalyst separation was generally a major problem for continuous processing. That is why the concept of hybrid catalysis became very popular in a short time [55]. 

As mentioned earlier, a major cause of high costs in fine chemicals manufacturing is the complexity of the processes. Hence, the key to more economical processes is reduction of the number of unit operations by judicious process integration. This pertains to the successful integration of, for example, chemical and biocatalytic steps, or of reaction steps with (catalyst) separations. A recurring problem in the batch-wise production of fine chemicals is the (perceived) necessity for solvent switches from one reaction step to another or from the reaction to the product separation. Process simplification, e.g. by integration of reaction and separation steps into a single unit operation, will provide obvious economic and environmental benefits. Examples include catalytic distillation, and the use of (catalytic) membranes to facilitate separation of products from catalysts. 

D. R. Bryant, In Catalyst Separation, Recoveiy and Recycling (eds. D. J. Cole-Hamilton, R. Tooze), Springer, Dordrecht, 2006, Ch. 2. 

The use of thermomorphic systems has recently been studied as a way of achieving catalyst separation in homogeneous catalysis. For example, a biphasic hydroformylation catalyst system was developed to take advantage of the unusual solvent characteristics of perfluorocarbons combined with typical organic solvents (4). Fluorous/organic mixtures such as perfiuoromethylcyclohexane... 

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

The classical HCK mechanism on bifunctional catalysts separates the metallic action from that of the acid by assigning the metallic function to the creation of an olefin from paraffin and the isomerization and cracking of the olefins to the acid function. Both reactions are occurring through carbenium ions [102],... 

The second important piece in the process development is the separation scheme. Several methods were suggested, such as decanting, water extraction or fractional distillation, use of hydrocyclones, hydrophobic membrane filters, etc. In the early work at EBC, many of its patents refer to facilitating catalyst separation via immobilization, although no mention is given on how activity was impacted by that immobilization. Furthermore, there were no details on how immobilization was achieved and which were the preferred means and techniques. 

D.J Cole-Hamilton and R. P. Tooze (eds.), Catalyst Separation, Recovery and Recycling, 1-8. 

In this book, we report on the state of the art of methods for catalyst separation recovery and recycling, not just describing the chemistry, but also discussing the process design that would be required to put the processes into practice. 

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