Most crude oils contain traces of vanadium, which cause significant detrimental impact during catalytic conversion and combustion. The concentrations in bitumen and vacuum residue are generally much higher, which poses a problem for the economical upgrading of these feedstocks. These problems are relevant since the world reserves of conventional light oils are dwindling and being replaced by an increasing amount of heavier feedstocks. In this article, the current understanding of the distribution and form of vanadium compounds within vacuum residue is critically discussed. The implications of this chemistry on prospects for new separation methods, other than deasphalting, are considered. Although only a small fraction of the vanadium is contained within the polar resin fraction (i.e., maltenes), this fraction has been most often characterized to determine the chemical form of the vanadium compounds. Various spectroscopic techniques have been used to determine that these resin soluble vanadium compounds exist as metalloporphyrins characterized by their intense absorption of UV/visible radiation. In the case of the asphaltene bound metals, this UV/visible absorbance is not observed and has historically led to their distinction as “non-porphyrins”. However, more recent results using X-ray spectroscopies (EXAFS and XANES), as well as a more in depth analysis of the UV/vis response of metalloporphyrins, indicates that, although these asphaltene-bound vanadium compounds do not exhibit the characteristic UV/visible absorption, they are indeed still bound in a porphyrinic structure. The fact that the majority of the vanadium is contained within the highly aromatic, highly polar asphaltene fraction also poses additional roadblocks to their selective removal. This fraction has been shown to associate/aggregate significantly in most (if not all) solvents. Recent work on the nature and possible mechanisms of the molecular association of asphaltenes in solution can be extended to help elucidate the molecular interactions occurring between asphaltenes and metalloporphyrins, and hence the nature of the inclusion of metalloporphyrins within the asphaltene fraction. Many different solvent systems including aromatics, chloro-carbons, alcohols, ketones, as well as other polar solvents have been used to extract the metalloporphyrins from the asphaltene fraction with limited success. In most cases, the effect of asphaltene solubility and aggregation in the given solvent were not considered. The selective separation of metalloporphyrins is clearly hampered by gaps in the basic understanding of the metalloporphyrin properties and behavior in solution.

A variety of experimental techniques were applied to a single source asphaltene sample at the same experimental conditions in order to reveal the possible size distributions of asphaltene monomers and aggregates. The asphaltene sample was divided into solubility cuts by selective precipitation in solutions of heptane and toluene. Asphaltene self-association was assessed through a combination of density, vapor pressure osmometry (VPO), elemental analysis, Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry, and time-resolved fluorescence emission spectra measurements performed on each cut. The physical dimensions of the asphaltenes were assessed using SAXS, DLS, membrane diffusion, Rayleigh scattering, and nanofiltration measurements. Molecular and nanoaggregate dimensions were also investigated through a combination of interfacial tension, interfacial adsorption, and surface force measurements. All of the measurements indicated that approximately 90 wt % of the asphaltenes self-associated. Ultrahigh resolution spectrometry suggests that the nonassociated asphaltenes are smaller and more aromatic than bulk asphaltenes indicating that the associating species are larger and less aromatic. On the basis of VPO, the average monomer molecular weight was approximately 850 g/mol, while the molecular weight of the nanoaggregates spanned a range of at least 30000 g/mol with an average on the order of 10000 to 20000 g/mol. SAXS and DLS gave molecular weights 10 times larger. The physical dimensions of the nanoaggregates were less than 20 nm based on nanofiltration and with average diameters of 5 to 9 nm based on diffusion and Rayleigh scattering. SAXS and DLS gave average diameters of 14 nm and indicated that the nanoaggregates had loose structures. Film studies were consistent with the lower molecular weights and dimensions and also demonstrated that asphaltene monolayers swell by a factor of 4 in the presence of a solvent. The most consistent interpretation of the data is that asphaltenes form a highly polydisperse distribution of loosely structured (porous or low fractal dimension) nanoaggregates. However, the discrepancy between VPO and SAXS molecular weights remains unresolved.

Solar-energy-driven conversion of CO 2 into hydrocarbon fuels can simultaneously generate chemical fuels to meet energy demand and mitigate rising CO 2 levels. The utilization of the clean and renewable solar power resource is, on a longterm basis, an essential component of solutions to address growing global energy demand, which is projected to be 40 TW by 2050. [1] This is because the solar energy received on the earths surface in one hour exceeds current total global energy demand. From the perspective of climate change, expansion of the current fossil fuel-based energy infrastructure to meet the projected energy demand is predicted to add 2986-7402 Gt of CO 2 to the atmosphere by 2100, resulting in a mean rise in global temperature of 2.4-4.5 8C. [2] Since the discovery of the photoreduction of carbon dioxide to form organic compounds using semiconductors by Fujishima and co-workers in 1979, [3] a growing interest in the development of catalysts that are capable of solar-based capture and storage of CO 2 has evolved. [4] Titanium dioxide, which is a cost-efficient, non-toxic and abundant n-type semiconductor, has been widely considered in the solar-driven reduction of CO 2 . Owing to its large band-gap energy of 3-3.2 eV, TiO 2 without doping or post-synthesis treatments can absorb only the ultraviolet portion of the solar spectrum. To narrow the band-gap of TiO 2 and improve its photocatalytic performance, strategies such as compositional doping or deliberately introducing disorder in crystalline TiO 2 are being investigated. [5] Herein, we demonstrate an approach that is able to achieve high-rate sunlight-driven conversion of diluted CO 2 to light hydrocarbons in which an optimized combination of a Cu-Pt coating and modulated-diameter TiO 2 nanotubes are used as the photocatalyst. We demonstrate at least a fourfold improvement in CO 2 conversion rates over prior art [6] by using a catalyst consisting of coaxial Cu-Pt bimetallic shells supported on a periodically modulated double-walled TiO 2 nanotube (PMTiNT) array core. The photocatalytic reaction occurs at room temperature and generates CH 4 , C 2 H 4 , and C 2 H 6 as reaction products. Under AM 1.5 one-sun illumination, using 99.9 % CO 2 , we obtained a hydrocarbon production rate of 3.51 mL g À1 h À1 or 574 nmol cm À2 h À1 . A key novelty is the effectiveness of our photocatalyst for the photoreduction of unconcentrated CO 2 . When the Cu 0.33 -Pt 0.67 /PMTiNT heterogeneous catalyst was utilized for the photoreduction of diluted CO 2 (0.998 % in N 2 ) at 25 8C, we found an average hydrocarbon production rate of 3.7 mL g À1 h À1 or 610 nmol cm À2 h À1 . The periodic modulation of the diameters of the nanotube arrays increased the surface area and improved the utilization of light while the bimetallic coating increased catalyst activity and specificity. Our version of a highly active CO 2 reduction system does not require reactant gases with high purities and could potentially be used to photocatalytically capture CO 2 directly from air or from flue gas...

Although the aggregation of petroleum asphaltenes has been measured by a variety of methods, little is known about the rate of exchange between aggregated and dissolved components. This work studied the diffusion of asphaltenes from several heavy oils and bitumens in dilute toluene solutions using a stirred diffusion cell equipped with ultrafiltration membranes (Ultracel YM and Anopore). The pore sizes were varied between 3 and 20 nm to retain aggregates while allowing free molecules to diffuse. The permeate was continuously monitored using in situ UV−vis spectroscopy. These experiments demonstrated that the sizes of the asphaltene aggregates at a concentration of 1 g/L in toluene at 25 °C were between 5 and 9 nm and that rates of exchange of material between the aggregates and free solution were extremely low. An increase in the temperature results in an increase in asphaltene mobility but does not reduce the size of the asphaltene structures below 5 nm. Likewise, a decrease in the concentration to 0.1 g/L did not result in a decrease in size. The origin of the asphaltenes (Athabasca, Safaniya, and Venezuela) did not have a significant impact on the observed behavior; therefore, the above observations are widely applicable to C7 asphaltenes. The diffusion of vanadyl porphyrins (vanadyl meso-tetraphenylporphyrin, vanadyl octaethylporphyrin, and native petroporphyrins) in the presence of asphaltenes showed that the native petroporphyrins were larger than the model vanadyl porphyrins based on the appearance of hindered diffusion within smaller pores. An increase in the temperature resulted in an increase in petroporphyrin mobility as per the Stokes−Einstein relationship, although decreasing the asphaltene concentration did not. The mobility of the vanadyl petroporphyrins varied with the origin of the sample (Safaniya, Venezuela, and Athabasca) and is therefore not universal.

Asphaltenes exist in the form of a colloidal dispersion in crude oils and solvents. Even in a good solvent such as toluene, asphaltene aggregates persist at the nanoscale. In this study, the impact of Rayleigh scattering on the apparent absorption of visible radiation by asphaltene aggregates in solution was assessed. Recent work with a stirred diaphragm diffusion cell indicates that membranes with pore sizes less than 5 nm are capable of removing the species responsible for the absorption of visible light with wavelengths >550 nm. A further analysis of the spectra of the whole asphaltene samples in toluene indicates that the absorbance of visible light with wavelengths >600 nm follows a λ–4 dependence for asphaltenes from a range of sources over a wide range of concentration. This functional dependence is consistent with Rayleigh scattering, rather than a mixture of colored components or chromophores. Rayleigh scattering equations were combined with experimental visible spectra to estimate the average nanoaggregate sizes, which were in a very good agreement with the sizes reported in the literature by other methods. Various additives, solvents, and ultrasound and heat treatments were employed in an attempt to completely disaggregate the asphaltene nanoaggregates in solution at room temperature. None of the treatments eliminated nanoaggregration, but some treatments increased absorption due to formation of larger aggregates, as confirmed by acoustic spectroscopy.