Zeolites nitrogen-containing compounds

The pyrolysis gas chromatogram of ABS at 550°C changes considerably when the pyrolysis products are passed over zeolite catalysts. The specific activity towards certain reactions, e.g., cycliza-tion, aromatization, or chain cleavage is somewhat dependent on the nature of the individual zeolite. In general, enhanced benzene, toluene, ethylbenzene at the cost of dimer, trimer formation is observed. Nitrogen containing compounds do not appear in the pyrolysis oil after catalytic conversion. However, the product gas is rich in nitriles (132). 

General studies on the EPR of copper-exchanged zeolites and on the interpretation of the spectra of nitrogen-containing compounds adsorbed on the same systems have been published by Carl and Larsen. A computational fitting procedure to obtain Cu + spin-Hamiltonian parameters in the case of the simultaneous presence of similar species and of relevant g and A strains is proposed. ... 

The basis of the demonstration can be based on already published data on the surface reaction between NOz and adsorbed organic compounds. Yokoyama and Misono have shown that the rates of N02 reduction over zeolite or silica are proportional to the concentration of adsorbed propene [29], whereas Il ichev et al. have demonstrated that N02 reacts with pre-adsorbed ethylene and propylene on H-ZSM-5 and Cu-ZSL-5 to form nitro-compounds [30], Chen et al [2-4] have observed the same nitrogen-containing deposits on MFI-supported iron catalysts. The question on the pairing of nitrogen atoms is not considered here. 

Any basic or alkaline material can react with a zeolite to effectively neutralize the acidic active sites, which generally results in irreversible loss of catalyst activity. Basic compounds found in the ethylene or benzene feedstocks can include amines, amides, nitriles, and trace metal cations such as sodium and potassium. Of particular concern are nitrogen-containing organic compormds typically present in the benzene feed. 

Zeolites are known to catalyze the formation of various nitrogen-containing aromatic ring systems. Examples include the synthesis of pyridines by dehydrogenation / condensation / cyclization of C -Cg precursors [1], the formation of methylpyridines by high-temperature isomerization of anilines [2], the amination of oxygen-containing heterocyclic compounds [3] and the Fischer indole synthesis [4,5]. The latter synthesis consists (see Scheme 1) of a condensation towards a phenylhydrazone followed by an acid-catalyzed cyclization with elimination of ammonia. The two reaction steps are usually combined in a one-pot procedure. 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

Some studies have been performed to develop new sorbents to remove the nitrogen compounds from fossil fuels prior to HDS process (Table 3). Hemandez-Maldonado and Yang [17] showed that Cu(I)-Y zeolite can effectively remove nitrogen from a commercial diesel fuel that contains 83 ppmw nitrogen to well below 0.1 ppmw nitrogen at a sorbent capacity of 43 cm diesel per g of sorbent. This corresponds to a very high and practical sorbent capacity of 3mg N/g sorbent. 

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