The pyrolysis of asphaltenes under hydrothermal environments covering the subcritical and supercritical regions of water was applied, and the influence of the presence of H-donors was surveyed by a reaction kinetics analysis based on the lumping approach. Under hydrothermal environments, the pyrolysis of asphaltenes consisting mainly of condensation to coke and decomposition to maltenes is significantly faster than that under a N2 environment. The H-donors introduced, decalin or maltenes, may provide nonaromatic H atoms, capping the carbon radicals essential to pyrolysis. Accordingly, the apparent activation energies of the condensation and the decomposition of asphaltenes both increase to varying degrees. The pyrolysis of asphaltenes in the presence of a small quantity of decalin is seriously retarded in subcritical water but recovers rapidly in supercritical water owning to the promoted initiation efficiency at high temperature. Accompanied by a large amount of maltenes, the decomposition to maltenes involved in the original pyrolysis network of asphaltenes can be neglected.

Under hydrothermal environments covering the subcritical and supercritical regions of water, the involvement of the carbonium mechanism in the dealkylation of aromatics and its resulting influence on the pyrolysis of heavy oil were surveyed. α-Olefin groups, as either a part of straight chain hydrocarbons or the terminal of alkyl substitutes of aromatics, are protonated spontaneously by hydronium ions into carboniums, followed by β-scission with similar reaction kinetic characteristics. The probability of the protonation of α-olefins under hydrothermal environments depends on the ionic product of water, so the occurrence of the β-scission in the carbonium mechanism is related to the thermodynamic state of water. With the aid of the carbonium mechanism at increasing water density, a recovered conversion rate and an increasing ratio of propene to ethylene in the product occur during the pyrolysis of a model α-olefin of dodecene under hydrothermal environments. Also, the dealkylation involved in the pyrolysis of maltenes is accelerated.

Asphaltenes derived from the condensation of maltenes under high-pressure N2 and supercritical water (SCW) environments were characterized with various approaches. The reaction kinetics of the condensation of asphaltenes under hydrothermal environments were also measured. By improving the diffusivity of hydrocarbon species and the isolation of aromatic carbon radicals from hydrogen donors in the SCW phase, the dealkylation of alkyl substitutes and the condensation of aromatic rings involved in the condensation of maltenes are promoted. Consequently, asphaltenes formed in the SCW phase (As-SCW) have a higher aromaticity and lower alkyl and naphthenic fractions than asphaltenes formed in the oil phase (As-oil). Nevertheless, As-SCW and As-oil both present a cokelike supermolecular structure with the stacking of polycyclic aromatic rings. Benefitting from the highly fused aromatic molecular structure and the cokelike supermolecular structure, the condensation of As-SCW to coke occurs readily under hydrothermal environments, significantly faster than that of As-oil.

The initiated pyrolysis of heavy oil in the presence of near-critical water (near-CW) was investigated with density functional theory (DFT) calculation and experimental characterization. Theoretical calculation indicated the thermodynamic feasibility of forming hydrocarbon radicals in heavy oil with the aid of appropriate radical initiators. By introducing ditertbutyl peroxide (DTBP) into heavy oil, the H-abstraction of Hβ atoms distributed mainly in the fractions of saturates, resins, and asphaltenes occurs, forming hydrocarbon radicals located on aliphatic chains with priority. At the temperature of 653 K and water density of 0.30 g/cm3, it was experimentally confirmed that the introduced DTBP was capable of initiating the pyrolysis of heavy oil. After 15 min's reaction, the pyrolysis products centered toward the fraction of aromatics whose weight proportion in the liquid product increased drastically by ca. 25.0 wt.%. Meanwhile, only a negligible coke yield of 0.5 wt.% was collected.With the introduction of DTBP into heavy oil, the H-abstraction of Hβ atoms distributed mainly in the fractions of saturates, resins, and asphaltenes occurs, by which the pyrolysis of heavy oil in the presence of near-critical water is effectively initiated at relatively lower temperatures.Highlights► The initiated pyrolysis of heavy oil in the presence of near-CW was investigated. ► Hydrocarbon radicals can be formed by introducing DTBP into heavy oil. ► Pyrolysis of heavy oil in near-CW is effectively initiated by the introduced DTBP. ► The initiated pyrolysis of heavy oil can be run under mild condition.

Solvation of methyl radicals in subcritical and supercritical water was investigated with the ab initio MD simulation to increase the understanding of the thermal cracking of hydrocarbons under the severe hydrothermal environments. The calculation results show that water clusters around the radical could be formed with the following prerequisites: the bulk density of water is close to liquid phase, and the state point of water on its phase diagram is far away from the critical point and from the vapor–liquid equilibrium boundary. The occurrence of water clusters superimposes a negative influence on the originally depressed diffusivity of the radical under the dense hydrothermal environments, and the interference from the immediately adjacent water molecules with the frontier orbitals of the radical results in randomly reduced activity of the radical. Regardless of whether there are water clusters around the radical or not, in subcritical and supercritical water the bimolecular reactions participating via hydrocarbon radicals should be partially suppressed by the reduced diffusivity and lower activity of the radical.Graphical abstractHighlights► Solvation of methyl radicals in sub-CW and SCW was characterized by ab initio MD. ► Solvent clusters around the radical occur under specific hydrothermal environments. ► Formation of solvent clusters results in depressed diffusivity of the radical. ► Activity of the hydrocarbon radical is suppressed by hydrothermal environments.

In this work, hydrophilic property of Ru (0 0 0 1) with and without H adatoms was theoretically studied to increase the understanding of the partial hydrogenation of benzene to cyclohexene over ruthenium catalysts. Density functional theory based calculations suggest that formation of hydrogen bonding among the adsorbed H2O molecules results in a weakened interaction between the water adlayer and Ru (0 0 0 1), while the calculated heat of adsorption containing the contribution of hydrogen bonding is no longer suitable for evaluating the hydrophilic property of the metal surface. The presence of H adatoms exerts an electrostatic repulsion on the adsorbed H2O molecules; thereby the latter can be in the state of chemisorption or physisorption on the Ru metal depending on the number of immediately adjacent H adatoms and on the relative distance to the H adatoms. By tuning the coverage of H adatoms, the surface hydrophilic/hydrophobic balance of the Ru metal can be effectively adjusted, which provides an approach to improve the selectivity to cyclohexene in the partial hydrogenation of benzene.

In this work, the hydrogenation of cyclohexene over Ru–Zn/Ru(0 0 0 1) surface alloy was investigated by a DFT study so as to improve the understanding of the catalytic mechanism of the partial hydrogenation of benzene to cyclohexene over Ru–Zn alloy catalyst. Calculation results show that the presence of Zn atoms on the surface alloy results in not only a direct decrease in sites for the chemisorption of cyclohexene but also a depressed adsorption capability of the neighboring surface Ru sites. For an adsorbed cyclohexene molecule, whether the subsequent hydrogenation can be readily performed actually is determined by the relative position among the Zn atom, the H atom, and the adsorbed cyclohexene molecule. In most cases, the hydrogenation is forbidden because of the repulsion from Zn atoms to the nearby H atoms. Only in the specific situations in which the H atom participating in the reaction is not immediately close to the Zn atom, can the hydrogenation be accomplished with a relatively lower activation energy compared with the reactions on the Ru(0 0 0 1) surface. From the perspectives of adsorption and reaction kinetics, Ru-based catalyst modified by metallic Zn is no longer suitable for the hydrogenation of cyclohexene, which is supposed to be crucial to the improvement of cyclohexene yield in the partial hydrogenation of benzene over Ru–Zn alloy catalyst.The hydrogenation of cyclohexene over Ru–Zn/Ru(0 0 0 1) surface alloy was investigated by a DFT study. Both the chemisorption and hydrogenation kinetics of cyclohexene on the surface alloy were found to be depressed, which is supposed to be crucial to the improvement of cyclohexene yield in the partial hydrogenation of benzene over Ru–Zn alloy catalyst.

From the adsorption point of view, partial hydrogenation of benzene to cyclohexene over the metallic Zn modified Ru-based catalyst was experimentally and theoretically investigated. A decreased hydrogenation activity but increased selectivity to cyclohexene over the prepared Ru-Zn/ZrO2 catalyst was observed in the partial hydrogenation of benzene. Theoretical calculations suggest that the above phenomena are mainly resulted from the depression of the chemisorption of benzene and cyclohexene on the modified catalyst, especially for the latter. Undesired deep hydrogenation from cyclohexene to cyclohexane in the middle and late reaction stage therefore is effectively retarded, by which an improved cyclohexene yield is guaranteed. An optimal Zn content of 2.72 wt.% in the Ru-based catalyst was proposed by both the experiment and calculation for the partial hydrogenation of benzene, and a cyclohexene yield up to 44% was obtained over Ru-Zn/ZrO2 catalyst.From the adsorption point of view, partial hydrogenation of benzene over metallic Zn modified Ru-based catalyst was investigated. Theoretical calculation suggests that chemisorption of benzene and cyclohexene on the modified catalyst is both restrained, and the latter is much more serious than the former. A reduced catalytic activity but improved selectivity to cyclohexene therefore is experimentally observed.

With the view of increasing the understanding to the anti-scaling mechanism in the membrane process, zeta potential on the anti-scalant (Calgon and PAA) modified sub-micro calcite surface was measured and was further characterized with a molecular simulation. Experimental results show that the original positively charged calcite in an aqueous solution is reversed to be negatively charged by introducing anti-scalants at a dosage of ppm level. On the basis of theoretical calculation, the observed reversal is resulted from the statistically predominant orientation of negative potential-determining groups in the adsorbed anti-scalant molecules, i.e. PO and CO, stretching against the calcite surface. A negative zeta potential prevailing on the calcite surface can effectively prevent the calcite from precipitating on the membrane surface. In the presence of anti-scalants, zeta potential on the sub-micro calcite surface is a sensitive function of calcite content, anti-scalant concentration, salt concentration, and pH of the suspension.

For the sake of improving the performance of Ru-based catalyst used in partial hydrogenation of benzene to cyclohexene, the effect of Zn2+/Zn layer on the adsorption and dissociation of H2 on Ru (0001) surface was investigated by applying density function theory (DFT) calculations. Calculation results show that the dissociation of H2 occurs only after its being chemisorbed horizontally at atop site. Because of the influence of Zn2+ on the electron delocalization between molecular orbit of H2 and valence orbits of atop Ru atom, a remarkable increase in the H2 dissociation barrier is noticed, which results in zones of sparse chemisorbed H around Zn2+. Adsorbed Zn2+ can be reduced by chemisorbed H, and the H2 dissociation kinetics varies little in the presence of Zn atoms at adjacent sites. Split zones of chemisorbed H are formed at a high coverage of Zn layer. The consecutive or synchronous hydrogenation of benzene is disturbed when benzene is adsorbed in zones of sparse chemisorbed H or split zones of chemisorbed H. It is therefore deducted that a high coverage layer of some transition metal atoms on catalyst surface should be helpful for maintaining the hydrogenation activity of the catalyst and improving the cyclohexene yield.

WOx/ZrO2 catalyzed hydration of cyclohexene in sub-critical water was experimentally investigated. The migration of reaction zone into sub-critical water makes it possible to run the hydration in a single liquid phase, and the reaction is free from the limitation of liquid–liquid phase mass transfer to hydration kinetics. The severe hydrothermal environments favor the transformation of surface active sites on WOx–ZrO2 catalysts to Brønsted acid centers of stronger acidity, which are highly effective for the hydration of cyclohexene to the desired product cyclohexanol.WOx–ZrO2 catalysts present a highly selective activity to the hydration of cyclohexene to cyclohexanol conducted in sub-critical water, and the reaction free from the liquid–liquid mass transfer is greatly accelerated.Download full-size imageResearch Highlights► WOx-ZrO2 catalyzed hydration of cyclohexene is operated in a single liquid phase. ► Surface WOx species of stronger acidity are formed in the hydrothermal environments. ► WOx-ZrO2 is effective to the hydration of cyclohexene to cyclohexanol in sub-CW.