Metal-supported activated carbon catalysts

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. 

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. 

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. 

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]. 

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. 

Nitrogen adsorption experiments showed a typical t)q5e I isotherm for activated carbon catalysts. For iron impregnated catalysts the specific surface area decreased fix>m 1088 m /g (0.5 wt% Fe ) to 1020 m /g (5.0 wt% Fe). No agglomerization of metal tin or tin oxide was observed from the SEM image of 5Fe-0.5Sn/AC catalyst (Fig. 1). In Fig. 2 iron oxides on the catalyst surface can be seen from the X-Ray diffractions. The peaks of tin or tin oxide cannot be investigated because the quantity of loaded tin is very small and the dispersion of tin particle is high on the support surface. 

Another part of our investigation deals with the effect of heat treatment on the leaching behavior of palladium on activated carbon catalysts. Heat treatment is a known technique to increase the performance of catalysts. (3) Therefore, standard carbon supported palladium catalysts were exposed to different temperatures ranging from 100 to 400 °C under nitrogen. The catalysts were characterized by metal leaching, hydrogenation activity and CO-chemisorption. 

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]. 

Liquid phase carbonylation of methanol to acetic acid with a rhodium complex catalyst is a well known process (ref. 1). The authors have found that group 8 metals supported on carbonaceous materials exhibit excellent activity for the vapor phase carbonylation of methanol in the presence of iodide promoter(ref. 5). Especially, a nickel on active carbon catalyst gave acetic acid and methyl acetate with the selectivity of 95% or higher at 100% methanol conversion under 10 atm and 250 °C. In the present study it has been found that a small amount of hydrogen which is always contained in the commercially available CO and requires much cost for being removed completely, accelerates greatly the carbonylation reaction. 

Figure 15.4 Transmission electron micrograph of palladium on activated carbon catalyst (a) the metal crystallites are located on the edge of the carbon support (b) uniform distribution of the crystallites. In both cases the crystallites are 2 to 3 nm in size.

Catalysts were prepared by impregnation of Pt inside the pore structure of carbon fibers. Care was taken to eliminate the active metal from the external surface of the support. A very high dispersion of Pt was measured. Four reactions were carried out in a fixed-bed reactor competitive hydrogenation of cyclohexene and 1-hexene, cyclization of 1-hexene, n-heptane conversion and dehydrogenation of cyclohexanol. Three types of carbon fibers with a different pore size and Pt-adsorption capacity along with a Pt on activated carbon commercial catalyst were tested. The data indicate a significant effect of the pore size dimension on the selectivity in each system. The ability to tailor the pore structure to achieve results drastically different from those obtained with established supports is demonstrated with heptane conversion. Pt on open pore carbon fibers show higher activity with the same selectivity as compared with Pt on activated carbon catalysts. 

Besides oxide supported Sn-Ru catalysts, carbon supported catalysts also find application in hydrogenation reactions. Sn Mossbauer spectroscopy was used to investigate the tin component of ruthenium and tin supported on activated carbon catalysts containing 2 wt. % ruthenium and having Sn/(Sn-f Ru) ratios between zero and 0.4. Four major components in the Sn Mossbauer spectra were attributed to both Sn(II) and Sn(IV) oxides and to Ru-SnOx species formed on the surface of ruthenium metal particles. In addition to this " Sn spectra reveal the presence of minor amounts of Ru3Sn7 alloy phase. ... 

Pd showed significantly promotional effect than Pt and Ru for Co/Si02 catalyst in hydroformylation of 1-hexene [4], Cobalt free lwt% Ru, Pt and Pd supported on active carbon were prepared and tested in hydroformylation reaction. As shown in Table 1, these kinds of catalysts showed very low activity for 1-hexene hydroformylation. For the Ru catalyst, the 1-hexene eonversion was 81.3%, but the 1-hexene was only converted to isomerization produets. For the Pt and Pd catalysts, both the 1-hexene conversion and oxygenates selectivity were very low. Results show in Table 1 indicate that the noble metal/active carbon catalysts themselves had no catalytic activity of 1-hexene hydroformylation to form oxygenates. 

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). 

K. Aika et al. [1] studied alkali metal/transition metal/active carbon catalysts in ammonia synthesis. Authors postulated that carbon support enables electron transport from alkali metal... 

Palladium supported on alumina or active carbon catalysts were prepared using ultrasound during the preparation steps. A large increase in the metal dispersion and in the catalytic activity of the samples, tested during the reduction of acetophenone with flowing hydrogen, was found. 

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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