Metal-supported activated carbon

Y. Uemichi, Y. Makino and T. Kanazuka, Degradation of polyethylene to aromatic hydrocarbon over metal-supported activated carbon catalysts. J. Anal. Appl. Pyrolysis, 14, 331-334 (1989). 

On the other hand, activated carbon may be considered as a catalyst in the cracking of waste plastics. This is because it is a neutral catalyst with a high surface area and, therefore, it might be more resistant to impurities and coke formation. It has been reported that Pt-, Fe- and Mo-supported activated carbon catalysts were effective for the pyrolysis ofPEandPP [11,14,15]. Use of metal-supported activated carbon catalysts has enhanced the formation of aromatics via dehydrocychzation of straight- or branched-chain radicalic intermediates. 

Pyrolysis of polyethylene on catalysts of metal-supported activated carbon changes the composition of pyrolysates with the formation of aromatic compounds. Different metals lead to different yield of aromatic components, the presence of Pt leading to the highest, and Fe and Cu leading to the lowest content of aromatics [63]. 

Figure 5.12 Relationship between contact time (W/F) and aromatic yield in the catalytic degradation of PE in a fixed bed reactor over metal-supported activated carbons 21 O C, Mo/C, 0 Ni/C, Zn/C, A Fe/C, Cu/C, Pt/C, Co/C.

Table 12.3 BET Surface Areas (Abet) and Micropore (Vmi) and Mesopore (V meso) Volumes of Virgin and Metal-Supported Activated Carbon Specimens...

Metal-supported activated carbon catalysts are active for aromatics formation [395, 396], Pt having a high catalytic activity for toluene demethylation. Zeolites are active in cracking and ZSM-5 has a selective activity due to the pore structure [334, 397-399], giving a high content of aromatics, while MeAlCl4 catalysts are selective for C4 hydrocarbon formation [401]. A detailed study of the influence of the PO structure on catalytic decomposition over silica alumina indicated that LDPE and LLDPE produced only smaller waxlike compounds compared with HDPE and XPE. 

Hydrogenation. Hydrogenation is one of the oldest and most widely used appHcations for supported catalysts, and much has been written in this field (55—57). Metals useflil in hydrogenation include cobalt, copper, nickel, palladium, platinum, rhenium, rhodium, mthenium, and silver, and there are numerous catalysts available for various specific appHcations. Most hydrogenation catalysts rely on extremely fine dispersions of the active metal on activated carbon, alumina, siHca-alumina, 2eoHtes, kieselguhr, or inert salts, such as barium sulfate. 

The effect of reaction conditions (temperature, pressure, H2 flow, C02 and/or propane flow, LHSV) and catalyst design on reaction rates and selectivites were determined. Comparative studies were performed either continuously with precious-metal fixed-bed catalysts in a trickle-bed reactor, or batchwise in stirred-tank reactors with supported nickel or precious metal on activated carbon catalysts. Reaction products were analyzed by capillary gas chromatography with regard to product composition, by titration to determine iodine and acid value, and by elemental analysis. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Table 20. Ammonia synthesis activity of metals supported on carbon with potassium metal promotion (ml Nib,/ mL catalyst, 573 K, 1283.13 mbar. H N = 3 1...

Figure 1 Relationship between the average size (dn,dy) of Pd particles in Pd/C catalysts and the equilibrium constant (K2) of formation of the metal precursors (surface 71-complexes of PdCb with the A2 sites). (From Ref. 16.) Pd/C catalysts are prepared by reduction of PdCb/C in flowing H2 at 250 °C for 3 h the metal loading is 1 pmol/m ( S phenoi)- Supports. Active carbons Eponit 113H (1), PN (2), AR-D (3). Activated pyrocarbon Sibunit (4,6,7). Carbon blacks PME-800 (5), PM-105 (8).

Carbon-supported (Activated carbon SXplus supplied by NORIT, Sbet = 750 m2.g-l, particle size 0.2-0.1mm) bimetallic and monometallic catalysts were prepared by deposition from a suspension of carboxylate particles in n-heptane chosen as inert organic solvent. Precursors used for the incorporation of the metals were either, palladium(II) acetate (ACROS) and bismuth(lll) oxoacetate, Bi0(02CCH3) (synthesized as described elsewhere [8]), or diammine(pyrazine-2,3-dicarboxylato-N,0)palladium(II) [12] and tris(monohydrogenopyrazine-2,3-dicarboxylato)bismuth(III) (noted Bi(2,3-pzdcH>3) [13]. 

Because catalysts to be employed in the fine-chemical industry must be resistant against (weakly) acid and alkaline liquids, active carbon is the support of choice for precious metal catalysts. Activated carbon is a relatively complicated support... 

The transmission electron micrograph represented in Figure 1 shows a typical carbon-supported precious metal catalyst. Activated carbon contains very small clustered particles beside carbon flakes. A flake is indicated in the micrograph. Precious metal particles are usually present only at the edges of the flakes. On the small carbon particles fairly uniform distribution of small precious metal particles has been achieved. Two clusters of platinum particles can also be seen in the micrograph of Figure 1. 

Hydrogenation is one the oldest and most widely used apphcations for supported catalysts. The usual metals are Co, Cu, Ni, Pd, Pt, Re, Rh, Ru, and Ag. There are munerous catalysts for special apphcations. Most hydrogenation catalysts consist of an extremely fine dispersion of the active metal on activated carbon, AI2O3, aluminosilicates, zeolites, kieselguhr, or inert salts such as BaS04 [22]. An example is the selective hydrogenation of chloronitrobenzene (Eq. 4-75). 

In past years, metals in dilute sulfuric acid were used to produce the nascent hydrogen reductant (42). Today, the reducing agent is hydrogen in the presence of a catalyst. Nickel, preferably Raney nickel (34), chromium or molybdenum promoted nickel (43), or supported precious metals such as platinum or palladium (35,44) on activated carbon, or the oxides of these metals (36,45), are used as catalysts. Other catalysts have been suggested such as molybdenum and platinum sulfide (46,47), or a platinum—nithenium mixture (48). 

Cost. The catalytically active component(s) in many supported catalysts are expensive metals. By using a catalyst in which the active component is but a very small fraction of the weight of the total catalyst, lower costs can be achieved. As an example, hydrogenation of an aromatic nucleus requires the use of rhenium, rhodium, or mthenium. This can be accomplished with as fittie as 0.5 wt % of the metal finely dispersed on alumina or activated carbon. Furthermore, it is almost always easier to recover the metal from a spent supported catalyst bed than to attempt to separate a finely divided metal from a liquid product stream. If recovery is efficient, the actual cost of the catalyst is the time value of the cost of the metal less processing expenses, assuming a nondeclining market value for the metal. Precious metals used in catalytic processes are often leased. 

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