Liquid catalyst

Figure 4-8 shows a continuous reactor used for bubbling gaseous reactants through a liquid catalyst. This reactor allows for close temperature control. The fixed-bed (packed-bed) reactor is a tubular reactor that is packed with solid catalyst particles. The catalyst of the reactor may be placed in one or more fixed beds (i.e., layers across the reactor) or may be distributed in a series of parallel long tubes. The latter type of fixed-bed reactor is widely used in industry (e.g., ammonia synthesis) and offers several advantages over other forms of fixed beds. 

Details of two related patents for the alkylation of aromatic compounds with chloroaluminate(III) ionic or chlorogallate(III) ionic liquid catalysts have become available. The first, by Seddon and co-workers [81], describes the reaction between ethene and benzene to give ethylbenzene (Scheme 5.1-51). This is carried out in an... 

Acidic chloroaluminate ionic liquids have already been described as both solvents and catalysts for reactions conventionally catalyzed by AICI3, such as catalytic Friedel-Crafts alkylation [35] or stoichiometric Friedel-Crafts acylation [36], in Section 5.1. In a very similar manner, Lewis-acidic transition metal complexes can form complex anions by reaction with organic halide salts. Seddon and co-workers, for example, patented a Friedel-Crafts acylation process based on an acidic chloro-ferrate ionic liquid catalyst [37]. 

Temperature-dependent phase behavior was first applied to separate products from an ionic liquid/catalyst solution by de Souza and Dupont in the telomerization of butadiene and water [34]. This concept is especially attractive if one of the substrates shows limited solubility in the ionic liquid solvent. 

To produce reliable data on the lifetime and overall activity of the ionic catalyst system, a loop reactor was constructed and the reaction was carried out in continuous mode [105]. Some results of these studies are presented in Section 5.3, together with much more detailed information about the processing of biphasic reactions with an ionic liquid catalyst phase. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

Figure 5.3-3 Example of an extraction method for product separation from ionic liquid/catalyst...

The use of acidic chloroaluminates as alternative liquid acid catalysts for the allcy-lation of light olefins with isobutane, for the production of high octane number gasoline blending components, is also a challenge. This reaction has been performed in a continuous flow pilot plant operation at IFP [44] in a reactor vessel similar to that used for dimerization. The feed, a mixture of olefin and isobutane, is pumped continuously into the well stirred reactor containing the ionic liquid catalyst. In the case of ethene, which is less reactive than butene, [pyridinium]Cl/AlCl3 (1 2 molar ratio) ionic liquid proved to be the best candidate (Table 5.3-4). 

During the continuous reaction, alkene, CO, H2, and CO2 were separately fed into the reactor containing the ionic liquid catalyst solution. The products and uncon-... 

Figure 5.4-3 shows the results of a lifetime study for Wilke s catalyst dissolved, activated, and immobilized in the [EMIM][(CF3S02)2N]/compressed CO2 system. Over a period of more than 61 h, the active catalyst showed remarkably stable activity while the enantioselectivity dropped only slightly. These results clearly indicate - at least for the hydrovinylation of styrene with Wilke s catalyst - that an ionic liquid catalyst solution can show excellent catalytic performance in continuous product extraction with compressed CO2. 

In comparison with catalytic reactions in compressed CO2 alone, many transition metal complexes are much more soluble in ionic liquids without the need for special ligands. Moreover, the ionic liquid catalyst phase provides the potential to activate and tune the organometallic catalyst. Furthermore, product separation from the catalyst is now possible without exposure of the catalyst to changes of temperature, pressure, or substrate concentration. 

The cationic nature of the copper(I) catalyst means that it is immobilized in the ionic liquid. This permits the PMMA product to be obtained, with negligible copper contamination, by a simple extraction procedure with toluene (in which the ionic liquid is not miscible) as the solvent. The ionic liquid/catalyst solution was subsequently reused. 

The plant was able to operate continuously. The continual and controlled state of turbulence in the bed assured close intermixing between solids and vapors and an even distribution of thermal energy throughout the bed, and the liquid catalyst flowed smoothly and rapidly from one vessel to the next. 

A fluidized catalyst behaves like a liquid. Catalyst flow occurs in the direction of a lower pressure. The difference in pressure between any two points in a bed is equal to the static head of the bed between these points, multiplied by the fluidized catalyst density, but only if the catalyst is fluidized. 

The most important biphasic liquid systems are probably those that combine a conventional organic phase with another type of solvent, such as water, a fluorous organic solvent, or an ionic liquid [3]. In those cases the solvent can be considered as the support for the catalyst phase and we have therefore limited the examples in this review to those where the recycled liquid catalyst phase is recovered as a whole. 

In all cases a transicis selectivity of around 7/3 is obtained Numbers separated by dashes indicate results in successive reuses Bromine-free ionic liquid Catalyst concentration 25 mM... 

During the last decade many industrial processes shifted towards using solid acid catalysts (6). In contrast to liquid acids that possess well-defined acid properties, solid acids contain a variety of acid sites (7). Sohd acids are easily separated from the biodiesel product they need less equipment maintenance and form no polluting by-products. Therefore, to solve the problems associated with liquid catalysts, we propose their replacement with solid acids and develop a sustainable esterification process based on catalytic reactive distillation (8). The alternative of using solid acid catalysts in a reactive distillation process reduces the energy consumption and manufacturing pollution (i.e., less separation steps, no waste/salt streams). 

Liquid catalyst Low thermal stability Soluble in water Low activity per weight Small pore size Low activity Deactivates in water, but not in organic phase Medium activity... 

Zheng, X.X., Luo, S.Z., Zhang, L. and Cheng, J.P. (2009) Magnetic nanoparticle supported ionic liquid catalysts for CO2 cydoaddition reactions. Green Chemistry, 11 (4), 455 158. 

Since char formation results from dehydrogenation and condensation, a reduction in conversion temperature (which is accessible only with liquid catalysts) will allow lower hydrogen pressures to be utilized without threat of char formation. 

Since the focus of this contribution is clearly on catalysis and catalyst recycle using the ionic liquid methodology it is not possible to go into more detail on the materials science aspects of ionic liquids. However, it should be clearly stated that at least some understanding of the ionic liquid material is a prerequisite for its successful use as a liquid catalyst support in catalysis. Therefore, the interested reader is strongly encouraged to explore the more specialized literature [28],... 

During a 33 h continuous hydroformylation run using this set-up, no catalyst decomposition was observed and Rh leaching into the scC02/product stream was less than 1 ppm. The selectivity for the linear nonanal was found to be stable over the reaction time with n/iso = 3.1. During the continuous reaction, alkene, CO, H2 and C02 were separately fed into the reactor containing the ionic liquid catalyst solution. Products and unconverted feedstock dissolved in SCCO2 were removed from the ionic liquid. After decompression the liquid product was collected and analysed. 

To our knowledge, none of the developed SLP and SAP catalysts made their way into a technical process. Obviously, the possibility of using a supported liquid catalyst in a continuous liquid phase reaction is generally very restricted. The reason is that a very low solubility of the liquid in the feedstock/product mixture is enough to remove the catalyst from the surface over time (due to the very small amounts of liquid on the support). Even worse, the immobilised liquid film can be removed from the support physically by the mechanical forces of the continuous flow even in the case of complete immiscibility. 

Therefore, the most promising area of application for a supported liquid catalyst is a continuous gas phase process. In this context, the SILP concept offers very important advantages that make a reinvestigation of supported liquid catalysis with these unique liquids highly promising. 

Table 41.17 Comparative hydrogenation studies using supported ionic liquid catalysts, biphasic catalyst systems and the classical homogeneous catalyst systems [116].a) ... 

Beside SILP experiments with silica as support material, reports have also been made on the use of membranes coated with ionic liquid catalyst solution for the hydrogenation reaction of propene and ethene. The membranes were obtained by supporting various ionic liquids, each containing 16 to 23 mmol Rh(I) complex Rh(nbd)(PPh3)2 (nbd=norbornadiene), in the pores of poly(vinylidene fluoride) filter membranes [118]. 

Since the discovery of alkylation, the elucidation of its mechanism has attracted great interest. The early findings are associated with Schmerling (17-19), who successfully applied a carbenium ion mechanism with a set of consecutive and simultaneous reaction steps to describe the observed reaction kinetics. Later, most of the mechanistic information about sulfuric acid-catalyzed processes was provided by Albright. Much less information is available about hydrofluoric acid as catalyst. In the following, a consolidated view of the alkylation mechanism is presented. Similarities and dissimilarities between zeolites as representatives of solid acid alkylation catalysts and HF and H2S04 as liquid catalysts are highlighted. Experimental results are compared with quantum-chemical calculations of the individual reaction steps in various media. 

C 4 g catalyst, H20/EtOH = 3 Liquid Catalysts were prepared by impregnation using nickel oxalate as a Ni precursor, flow rate = 0.05 ml/min The activity, stability and H2 selectivity decreased in the order Ni/La203 > Ni/... 

Ionic liquids in which the anion is composed of an active transition metal catalyst providing essentially a liquid catalyst may be prepared, although it is too early to comment whether these will offer any advantages over catalysts dissolved in ionic liquids [23],... 

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