Catalytic converters zeolites

The second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

FIGURE 9.2 This high-resolution electron micrograph shows the unique pore structure of the ZSM-5 zeolite catalyst. Molecules such as methanol and hydrocarbons can he catalytically converted within the pores to valuable fuels and lubricant products. Courtesy, Mobil Research and Development Corporation. 

The HYSEC Process was developed by Mitsubishi Kakoki K. Ltd. and The Kansai Coke Chemicals Company. It has basically the same PSA unit as the UCC Process. It has prefilter beds with activated carbon that remove dirty components. After the main PSA beds, trace amounts of remaining oxygen are removed by a deoxo catalytic converter followed by a zeolitic dehumidifier. A Ni-LaaOj-Rh catalyst, supported on silica, could lower the reaction temperature to about 30°( a. 

The catalyst chamber is the heart of the Thermofuel process and is directly responsible for the high quality of the output fuel from this process. The technology in and around this unit is highly proprietary since competitive processes do not have this type of longlife catalytic converter. Many other pyrolysis processes add zeolite catalysts directly to the pyrolysis chamber, however, these are expensive and quickly become fouled and deactivated. 

A number of two-step processes (thermal and catalytic treatments) have been reported in the patent literature for the conversion of plastic wastes. Fukuda et a/.48 have proposed the thermal degradation of polyolefinic plastics in a stirred reactor at 420-470 °C, the volatile products being subsequently passed through a fixed bed reactor at 250-340 °C, containing a ZSM-5 zeolite catalyst. The process developed by Lechert et al49 also consists of a two-reactor system plastics are first pyrolysed above 600 °C in a sand fluidized bed reactor, and the gases produced are catalytically converted over ZSM-5 zeolite at 350-410 °C in a fixed bed reactor to increase the overall yield of liquid products. 

The development of catalytic converters has recently encompassed the use of zeolites, e.g. Cu-ZSM-5 (a copper-modified ZSM-5 system), but at the present time, and despite some advantages such as low light-off temperatures, zeolite-based catalysts have not shown themselves to be sufficiently durable for their use in catalytic converters to be commercially viable. 

Zeolites are unique as shape-selective catalysts. Mass transport shape selectivity is a consequence of transport restrictions allowing some species to diffuse more rapidly than others in zeolite pores. Small molecules enter the pores and are catalytically converted, but larger molecules may pass through a flow reactor unconverted because they do not fit into the pores, where almost all the catalytic sites are located. Similarly, product molecules formed inside a zeolite may be so large that their diffusion out of the pores may be so slow that they are largely converted into other products before escaping into the product stream. Mass transport selectivity is illustrated by toluene disproportionation catalysed by HZSM-5 13 (figure C2.7.13). The desired product is industrially valuable/ -xylene. 

H2O, CO2 and N2 removed from air by non-cryogenic pressure swing adsorption over LiX zeolite, leaving > 95% O2 High silica zeolites adsorb unburnt hydrocarbons and desorb them as the engine and the catalytic converter warm up Shape selective Ca-A zeolite (5A) adsorbs linear but not branched hydrocarbons Shape selective MFI type zeolites (silicalite) adsorb para- but not ortho- or meta-xylenes. FAU type zeolites are also effective for this separation under simulated moving bed conditions... 

Chatterjee D, Burkhardt T, Weibel M, Nova I, Gtossale A, Tronconi E (2007) Numerical Simulation of Zeolite- and V-Based SCR Catalytic Converters. SAE paper 2007-01-1136... 

Table 19.8 NOx removal efficiency and percentage of ammonia used to reduce NOx in a smart catalytic converter based on Pt-Ba-NSR and Fe-zeolite SCR catalyst...

As presented in Scheme 4,5-HMF is prone to rehydration and rearrangement to levulinic acid and formic acid. Formic acid may be regarded as a hydrogenation agent or an H equivalent, while levuliiuc acid is a particularly useful chemical intermediate that can be catalytically converted to a variety of useful chemicals and fuels [112], Although direct reaction of fructose to levuUnic acid has not been very successful with soluble acids and commercial resins in early days [103], the use of zeolite was proposed, as the pores and cages could trap the intermediate 5-HMF [105,106,113], and subsequently converting 5-HMF further to levulinic acid with the... 

Often, there is a correlation between acidic/basic or red-ox properties of some solid material and its ability to adsorb or catalytically convert certain pollutant. For example, acidity of different zeolites and mesoporous materials is important for their ability to adsorb aldehydes and ketones (from the gas phase) or phenol and nicotine (from the aqueous phase) [40, 43]. Red-ox properties are often correlated with catalysts efficiency for example, red-ox features of ceria-based mixed oxides are of importance for their ability to catalyse direct conversion of methane to synthesis gas, and they can be affected by incorporation of Zr02 [44]. The oxidability of mixed oxides is an important feature, which determines the possibility of their use as catalysts for combustion reactions and it can be also determined using TPR-TPO techniques [45]. 

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