Porphyrin metal compounds, petroleum

B. Metal-Bearing Compounds in Petroleum 1. Porphyrin Metal Compounds... 

In addition to hydrocarbons, petroleum also contains compounds that consist of nitrogen, oxygen, and sulfur (in the minority) as well as trace amounts of metals such as vanadium, nickel, iron, and copper. Porphyrins, the major organometaUic compounds present in petroleum, are large, complex cyclic carbon structures derived from chlorophyll and characterized by the ability to contain a central metal atom (trace metals are commonly found within these compounds). 

The amount of metal porphyrin in a petroleum varies considerably in terms of both absolute concentration and as a percentage of metal compounds among different petroleums. A comparison of the total metals content and of the percentage of the metals occuring as porphyrins is shown in Table II for various petroleums. 

Some researchers have doubted whether nonporphyrin metal compounds exist at all in petroleum (Goulon et al., 1984 Berthe et al., 1984). This speculation is based on the similarities in spectral features of petroleum-bound metals and model-metal porphyrin compounds from selected petroleums under the examination of x-ray absorption, photoelectron, and laser desorption spectroscopies. Interpretation of spectra from these methods, however, is uncertain and difficult given the lack of sufficient data on model compounds, particularly nonporphyrin metals and in a petroleum environment or matrix. 

A spectrum of metal compound reactivities in petroleum could arise for several reasons. Nickel and vanadium exist in a diversity of chemical environments. These can be categorized into porphyrinic and non-porphyrinic species vanadyl and nonvanadyl or associated with large asphaltenic groups and small, isolated metal-containing molecules. Each can be characterized by unique intrinsic reactivity. Reaction inhibition which occurs between the asphaltenes and the nonasphaltenes, as well as between Ni and V species, can also contribute to reactivity distributions. The parallel reaction interpretation of the observed reaction order discrepancy is therefore compatible with the multicomponent nature of petroleum. Data obtained at low conversion could appear as first order and only at higher conversions would higher-order effects become obvious. The... 

Some of the metallic constituents were somewhat soluble in the propane-oil portion, but all tended to be concentrated in the asphaltic portion. Although vanadium (0.02% by weight in the original crude petroleum) was present in all fractions, the greater part was found in the cyclohexane and benzene fractions. From the similarity of the absorption spectra of the vanadium concentrates from petroleum and those of synthetic vanadium porphyrin complexes, Skinner arrived at the conclusion that vanadium compounds from the petroleum of the Santa Maria Valley Field in California exist as porphyrin complexes. Additional metallic constituents were detected by Skinner as these became concentrated in the various solvent fractions, including aluminum, titanium, calcium, and molybdenum. 

Model compound studies have shown the importance of porphyrin macrocycle basicity, resulting from electron-withdrawing substituents and metal ligands, on the reducibility and susceptibility of the central metal to reaction. Similar insight into the differences in relative basicity of vanadium- and nickel-containing complexes found in petroleum may therefore be valuable in rationalizing the observed effects and predicting demetallation activity. 

Vanadium is present in crudes mainly in the +4 state (58). In fact, up to 50% of the total vanadium in crude oil can be found as V02+ in organometallic compounds such as porphyrins and naphthenates (59-63). During the cracking reaction in a FCCU, these compounds deposit V (probably in the form of VO+2 cations) on the catalyst surface. Then, after steam-stripping and catalyst regeneration, formation of V+5 surface phases occur. The effects of vanadium on FCC properties are more severe than any of the other metals present in petroleum feedstocks. In fact, vanadium causes an irreversible loss of cracking activity which is the result of a decrease in crystallinity, pore volume and surface area of the catalyst, Figure 5. 

Most of the measurements on diffusivities of compounds in the liquid phase, in catalyst pores, have been made with small molecules. Recently Chantong and Massoth [26] measured diffusivities of non-metallic porphyrin molecules. Baltus and Anderson [27] made diffusivity measurements using narrow molecular weight fractions of a petroleum residuum. In their work tetrahydrofuran asphaltenes were separated into five fractions by gel permeation chromatography. The polystyrene equivalent average molecular weights they reported were 3000, 6000, 12000, 24000, and 48000. 

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