Hydrogenation multiphase

Fuel industry is of increasing importance because of the rapidly growing energy needs worldwide. Many processes in fuel industry, e.g. fluidized catalytic cracking (FCC) [1], pyrolysis and hydrogenation of heavy oils [2], Fischer-Tropsch (FT) synthesis [3,4], methanol and dimethyl ether (DME) synthesis [5,6], are all carried out in multiphase reactors. The reactors for these processes are very large in scale. Unfortunately, they are complicated in design and their scale-up is very difflcult. Therefore, more and more attention has been paid to this field. The above mentioned chemical reactors, in which we are especially involved like deep catalytic pyrolysis and one-step synthesis of dimethyl ether, are focused on in this paper. 

A multiphase system consisting of a hydrocarbon solvent, a strong alkaline solution, and a quaternary onium salt, in the presence of a Pd/C catalyst with hydrogen that was bubbled at atmospheric pressure through the organic phase, allows the rapid displacement of chlorine from polyhalogenated benzenes. The onium salt, insoluble in both phases, is localized in the interfaces, coats the Pd/C catalyst, and constitutes the phase in which the reaction takes... 

Metal NPs immobilized in ILs are highly active and recyclable for hydrogenation reactions in multiphase systems (see Scheme 1.2, Table 1.2, and Figure 1.5). 

Multiphase catalytic reactions, such as catalytic hydrogenations and oxidations are important in academic research laboratories and chemical and pharmaceutical industries alike. The reaction times are often long because of poor mixing and interactions between the different phases. The use of gaseous reagents itself may cause various additional problems (see above). As mentioned previously, continuous-flow microreactors ensure higher reaction rates due to an increased surface-to-volume ratio and allow for the careful control of temperature and residence time. 

Hessel V, Hofmann C, Lob P, Lohndorf J, Lowe H, Ziogas A (2005) Aqueous Kolbe-Schmitt synthesis using resorcinol in a microreactor laboratory rig under high-P,T conditions. Org Process Res Dev 9 479-489 Inoue T, Schmidt MA, Jensen KF (2007) Microfabricated multiphase reactors for the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Ind Eng Chem Res 46 1153-1160... 

Table 9.8 Hydrogenation of alkenes by Ru(0) nanoparticles under multiphase and solventless conditions (75°C and constant pressure of 4 atm, substrate/Ru = 500).

As with classical multiphase catalysis, the organometallic catalyst is retained here in a liquid phase that is immiscible with the second phase containing substrates and/or products. For hydrogenation, the liquid/SCF system is always biphasic, whereas conventional systems are usually triphasic (liquid-1 /liquid-2/ H2). The liquid phase must provide a stable environment for the organometallic catalyst and should be insoluble in the SCF phase. Water, ILs and PEG have been used successfully for this purpose, together with scC02 as the mobile phase. Again, the products must not be too polar in order to be effectively extracted if C02 is used as the SCF. 

The use of ionic liquids has been successfully studied in many transition metal-catalyzed hydrogenation reactions, ranging from simple alkene hydrogenation to asymmetric examples. To date, almost all applications have included procedures of multiphase catalysis with the transition-metal complex being immobilized in the ionic liquid by its ionic nature or by means of an ionic (or highly polar) ligand. 

New reactor technologies are currently under development, and these include meso- and micro-structured reactors or the use of membranes. Among meso-structured reactors, monolithic catalysts play a pre-eminent role in environmental applications, initially in the cleaning of automotive exhaust gases. Beside this gas-solid application, other meso-structures such as membranes [57, 58], corrugated plate or other arranged catalysts and, of course, monoliths can be used as multiphase reactors [59, 60]. These reactors also offer a real potential for process intensification, which has already been demonstrated in commercial applications such as the production of hydrogen peroxide. 

Multiphase homogeneous catalysis (continued) hydroformylation, 42 483-487, 498 hydrogenations, 42 488-491 metal salts as catalysis, 42 482-487 neutral ligands, 42 481 82 organic reactions, 42 495 0X0 synthesis, 42 483-487 ring-opening metathesis polymerization and isomerization, 42 492-494 telomerizations, 42 491-492 diols as catalyst phase, 42 496 fluorinated compounds as catalyst phase, 42 497... 

The hydrodehalogenation reaction of haloaromatics involved the substitution of halide atoms bound to the ring, with hydrogen. For example, tetrachloroben-zene could be reduced to benzene in 30 minutes, at 50°C, by bubbling H2 at atmospheric pressure in the multiphasic system constituted by isooctane, 50% aqueous KOH, 0.2 molar A336, in the presence of Pd/C (0.02 molar) (Figure h.lS)." ... 

As far as the metal catalyst was concerned, Raney-Ni and Pt/C were also investigated. Raney-Ni proved effective in the hydrodehalogenation reaction of dichloro- and dibromobenzenes with hydrogen in the multiphasic system... 

Dieldrin—which belongs to the 12 POPs banned by the Stockholm convention and is in the same class of other pesticides named drins, such as aldrin and endrin—possesses six aliphatic chlorine atoms on a polycyclic skeleton. The multiphasic dechlorination, in the presence of A336, isooctane, aqueous KOH, Pd/C, and hydrogen, proceeded with a different selectivity and degree of dechlorination, depending on the choice of catalyst system, and base concentration. It always required the base and was favored by the presence of A336. It produced a mixture of products derived from the subsequent removal of chlorines, up to a small percentage of monochlorinated derivative. ... 

As already shown in paragraph Section 6.2.2, the multiphasic conditions for hydrodechlorination, are also active for hydrogenation reactions, such as was the case of haloaromatic ketones, which could selectively be reduced to the alcohol." This reaction was investigated from the kinetic standpoint,... 

Fig 6-7. EUectrochemical cell composed of a silver-sOver chloride multiphase electrode of ion transfer and a normal hydrogen electrode Ag = silver metal AgCl = silver chloride. 

The investigations done by Claus and coworkers as part of the project Smart Solvents/Smart Ligands focussed on the selective hydrogenation of Q ,/3-unsaturated carbonyl compounds and showed the potential of aqueous multiphasic catalysis for the production of chemicals for fine chemistry, e.g., fragrance materials. 

As the next step in multiphasic hydrogenation, the design and implementation of a continuously driven loop reactor as a laboratory-scale plant model led to comparable selectivity applying the same water soluble ruthenium-based catalyst system. 

Keywords Aqueous multiphase catalysis Regioselective hydrogenation Hydrodynamics Mass transport Kinetic modelling... 

Metals, intermetallic compounds, and alloys generally react with hydrogen and form mainly solid metal-hydrogen compounds (MH ). Hydrides exist as ionic, polymeric covalent, volatile covalent and metallic hydrides. Hydrogen reacts at elevated temperatrrres with many transition metals and their alloys to form hydrides. Many of the MH show large deviations from ideal stoichiometry (n= 1, 2, 3) and can exist as multiphase systems. 

N. Kuiiyama, D. Chartouni, M. Tsukahara, K. Takahashi, H.T. Takeshita, H. Tanaka, L. Schlapbach, T. Sakai, I. Uehara, Scanning tunneling microscopy in situ observation of phase-selective cathodic hydrogenation of a V-Ti-Ni-based multiphase alloy electrode, Electrochem. Solid-State Lett. 1 (1998) 37-38. 

Hilger C, Stadler R. New multiphase architecture from statistical copolymers by cooperative hydrogen bond formation. Macromolecules 1990 23 2095-2097. 

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